GENERAL MORPHOLOGY OF INFLAMMATION. EXUDATIVE INFLAMMATION.
PROLIFERATIVE INFLAMMATION: NONSPECIFIC AND SPECIFIC. MORPHOLOGY OF GRANULOMA
Inflammation is a complex reaction to injurious agents such as microbes and damaged, usually necrotic, cells that consists of vascular responses, migration and activation of leukocytes, and systemic reactions. The unique feature of the inflammatory process is the reaction of blood vessels, leading to the accumulation of fluid and leukocytes in extravascular tissues.
The inflammatory response is closely intertwined with the process of repair. Inflammation serves to destroy, dilute, or wall off the injurious agent, and it sets into motion a series of events that try to heal and reconstitute the damaged tissue. Repair begins during the early phases of inflammation but reaches completion usually after the injurious influence has beeeutralized. During repair, the injured tissue is replaced through regeneration of native parenchymal cells, by filling of the defect with fibrous tissue (scarring) or, most commonly, by a combination of these two processes.
Inflammation is fundamentally a protective response, the ultimate goal of which is to rid the organism of both the initial cause of cell injury (e.g., microbes, toxins) and the consequences of such injury (e.g., necrotic cells and tissues). Without inflammation, infections would go unchecked, wounds would never heal, and injured organs might remain permanent festering sores. Inflammation and repair may be potentially harmful, however. Inflammatory reactions, for example, underlie common chronic diseases, such as rheumatoid arthritis, atherosclerosis, and lung fibrosis, as well as life-threatening hypersensitivity reactions to insect bites, drugs, and toxins. Repair by fibrosis may lead to disfiguring scars or fibrous bands that cause intestinal obstruction or limit the mobility of joints. For this reason, our pharmacies abound with anti-inflammatory drugs, which ideally would control the harmful sequelae of inflammation yet not interfere with its beneficial effects.
The inflammatory response consists of two main components, a vascular reaction and a cellular reaction. Many tissues and cells are involved in these reactions, including the fluid and proteins of plasma, circulating cells, blood vessels, and cellular and extracellular constituents of connective tissue .The circulating cells include neutrophils, monocytes, eosinophils, lymphocytes, basophils, and platelets. The connective tissue cells are the mast cells, which intimately surround blood vessels; the connective tissue fibroblasts; resident macrophages; and lymphocytes. The extracellular matrix, consists of the structural fibrous proteins (collagen, elastin), adhesive glycoproteins (fibronectin, laminin, nonfibrillar collagen, tenascin, and others), and proteoglycans. The basement membrane is a specialized component of the extracellular matrix consisting of adhesive glycoproteins and proteoglycans.
Inflammation is divided into acute and chronic patterns. Acute inflammation is rapid in onset (seconds or minutes) and is of relatively short duration, lasting for minutes, several hours, or a few days; its main characteristics are the exudation of fluid and plasma proteins (edema) and the emigration of leukocytes, predominantly neutrophils. Chronic inflammation is of longer duration and is associated histologically with the presence of lymphocytes and macrophages, the proliferation of blood vessels, fibrosis, and tissue necrosis. Many factors modify the course and morphologic appearance of both acute and chronic inflammation, and these will become apparent later in this chapter.
The vascular and cellular reactions of both acute and chronic inflammation are mediated by chemical factors that are derived from plasma proteins or cells and are produced in response to or activated by the inflammatory stimulus. Such mediators, acting singly, in combinations, or in sequence, then amplify the inflammatory response and influence its evolution. Necrotic cells or tissues themselves-whatever the cause of cell death-can also trigger the elaboration of inflammatory mediators. Such is the case with the acute inflammation after myocardial infarction
Inflammation is defined as the local response of living tissues to injury caused by any agent. It is body defense reaction in order to eliminate or limit the spread injurious agent.
Causes. The agents causing inflammation may be follows:
1. Physical agents (heat, cold, radiation, mechanical injury).
2. Toxic chemical agents (organic and inorganic poisons).
3. Microbiological agents (bacteria, viruses, parasites, fungi).
4. Immunological agents (cell-mediated, immune complex and antigen-antibody reactions).
Clinico-morphological signs of inflammation. There are 5 main clinico-morphological signs of inflammation: rubor (redness); tumor (swelling); calor (heat), dolor (pain) and functio laesa (loss of function).
The word “inflammation” means burning. This nomenclature has its origin in old times but now we know that burning is only one of the signs of inflammation. The condition develops on the histion. T’pes of inflammation
There are 3 phases in inflammation: alteration, exudation and proliferation. The first phase is alteration, degeneration and necrosis of the cells, tissue. The second phase is exudation, formation of exudate. There are several stages in exudation: a) micro- circulation reaction with disturbance of blood rheology, b) increased vascular permeability, c) exudation of main blood components, d) emigration of blood cells, e) phagocytosis, f) formation of exudation and g) development of inflammatory infiltration.
According to prevailing one of these phases, inflammation is classified into 2 groups. We distinguish exudative and proliferative inflammations.
Depending upon the defense capacity of the host and duration of the response, inflammation can be classified as acute and chronic.
Exudative inflammation usually develops as acute inflammation, proliferative inflammation develops as chronic one.
Acute inflammation is of short duration and represents the early body reaction and is usually followed by repair.
Serous rhinitis in allergic nasal polyp; note the severe edematous swelling of the stroma (arrow).
Pseudomembranous enteritis (serofibrinous exudate) in small intestine of baby with staphylococcal food poisoning; note the loose yellowish membranes covering the mucosa (arrow).
Chronic inflammation is of longer duration and occurs either after the causative agent of acute inflammation persists for a long time, or the stimulus is such that it induces chronic inflammation from the beginning. Cells involved in inflammation Neutrophils (also known as polymorphonuclear neutrophils), are the predominant cells in acute inflammation as well as in abscess formation, connective tissue proliferation, and empyema. They are the white blood cells (WBCs) most responsible for the leukocytosis that occurs in response to an inflammatory or infectious crisis.
Suppurative microcarditis with abscess formation and bacterial colonies, gross
Microscopic; note the well-circumscribed yellow necroses (arrow) and fine granular bacterial colonies (arrow).
ACUTE INFLAMMATION
Acute inflammation is a rapid response to an injurious agent that serves to deliver mediators of host defense-leukocytes and plasma proteins-to the site of injury. Acute inflammation has three major components: (1) alterations in vascular caliber that lead to an increase in blood flow; (2) structural changes in the microvasculature that permit plasma proteins and leukocytes to leave the circulation; and (3) emigration of the leukocytes from the microcirculation, their accumulation in the focus of injury, and their activation to eliminate the offending agent
Certain terms must be defined before specific features of inflammation are described. The escape of fluid, proteins, and blood cells from the vascular system into the interstitial tissue or body cavities is known as exudation. An exudate is an inflammatory extravascular fluid that has a high protein concentration, cellular debris, and a specific gravity above 1.020. It implies significant alteration in the normal permeability of small blood vessels in the area of injury. In contrast, a transudate is a fluid with low protein content (most of which is albumin) and a specific gravity of less than 1.012. It is essentially an ultrafiltrate of blood plasma that results from osmotic or hydrostatic imbalance across the vessel wall without an increase in vascular permeability. Edema denotes an excess of fluid in the interstitial or serous cavities; it can be either an exudate or a transudate. Pus, a purulent exudate, is an inflammatory exudate rich in leukocytes (mostly neutrophils), the debris of dead cells and, in many cases, microbes
As in the preceding diagram, here PMN’s that are marginated along the dilated venule wall (arrow) are squeezing through the basement membrane (the process of diapedesis) and spilling out into extravascular space.
STIMULI FOR ACUTE INFLAMMATION
Acute inflammatory reactions are triggered by a variety of stimuli:
· Infections (bacterial, viral, parasitic) and microbial toxins
· Trauma (blunt and penetrating)
· Physical and chemical agents (thermal injury, e.g., burns or frostbite; irradiation; some environmental chemicals)
· Tissue necrosis (from any cause)
· Foreign bodies (splinters, dirt, sutures)
Immune reactions (also called hypersensitivity reactions)
Each of these stimuli may induce reactions with some distinctive features, but all inflammatory reactions share the same basic features. We first describe the characteristic reactions of acute inflammation, and then the chemical mediators responsible for these reactions
VASCULAR CHANGES
Since the two major mechanisms of host defense against microbes-antibodies and leukocytes-are normally carried in the bloodstream, it is not surprising that vascular phenomena play a major role in acute inflammation. Normally, plasma proteins and circulating cells are sequestered inside the vessels and move in the direction of flow. In inflammation, blood vessels undergo a series of changes that are designed to maximize the movement of plasma proteins and circulating cells out of the circulation and into the site of injury or infection
Changes in Vascular Flow and Caliber
Increased Vascular Permeability (Vascular Leakage)
A hallmark of acute inflammation is increased vascular permeability leading to the escape of a protein-rich fluid (exudate) into the extravascular tissue. The loss of protein from the plasma reduces the intravascular osmotic pressure and increases the osmotic pressure of the interstitial fluid. Together with the increased hydrostatic pressure owing to increased blood flow through the dilated vessels, this leads to a marked outflow of fluid and its accumulation in the interstitial tissue.The net increase of extravascular fluid results in edema.
The vasculitis shown here demonstrates the destruction that can accompany the acute inflammatory process and the interplay with the coagulation mechanism. The arterial wall is undergoing necrosis, and there is thrombus formation in the lumen.
Normal fluid exchange and microvascular permeability are critically dependent on an intact endothelium. How then does the endothelium become leaky in inflammation? The following mechanisms have been proposed
· Formation of endothelial gaps in venules. This is the most common mechanism of vascular leakage and is elicited by histamine, bradykinin, leukotrienes, the neuropeptide substance P, and many other classes of chemical mediators. It occurs rapidly after exposure to the mediator and is usually reversible and short-lived (15 to 30 minutes); it is thus known as the immediate transient response. Classically, this type of leakage affects venules 20 to 60 μm in diameter, leaving capillaries and arterioles unaffected. (The precise reason for this restriction to venules is uncertain; it may be because there is a greater density of receptors for the mediators in venular endothelium. Parenthetically, many of the later leukocyte events in inflammation-adhesion and emigration-also occur predominantly in the venules in most organs. Binding of mediators, such as histamine, to their receptors on endothelial cells activates intracellular signaling pathways that lead to phosphorylation of contractile and cytoskeletal proteins, such as myosin. These proteins contract, leading to contraction of the endothelial cells and separation of intercellular junctions. Thus, the gaps in the venular endothelium are largely intercellular or close to the intercellular junctions. Cytokines such as interleukin-1 (IL-1), tumor necrosis factor (TNF), and interferon-γ (IFN-γ) also increase vascular permeability by inducing a structural reorganization of the cytoskeleton, such that the endothelial cells retract from one another. In contrast to the histamine effect, the cytokine-induced response is somewhat delayed (4 to 6 hours) and long-lived (24 hours or more).
· Direct endothelial injury, resulting in endothelial cell necrosis and detachment. This effect is usually encountered iecrotizing injuries and is due to direct damage to the endothelium by the injurious stimulus, as, for example, in severe burns or lytic bacterial infections. Neutrophils that adhere to the endothelium (discussed below) may also injure the endothelial cells. In most instances, leakage starts immediately after injury and is sustained at a high level for several hours until the damaged vessels are thrombosed or repaired. The reaction is known as the immediate sustained response. All levels of the microcirculation are affected, including venules, capillaries, and arterioles. Endothelial cell detachment is often associated with platelet adhesion and thrombosis.
· Delayed prolonged leakage. This is a curious but relatively common type of increased permeability that begins after a delay of 2 to 12 hours, lasts for several hours or even days, and involves venules as well as capillaries. Such leakage is caused, for example, by mild to moderate thermal injury, x-radiation or ultraviolet radiation, and certain bacterial toxins. Late-appearing sunburn is a good example of a delayed reaction. The mechanism of such leakage is unclear. It may result from the direct effect of the injurious agent, leading to delayed endothelial cell damage (perhaps by apoptosis), or the effect of cytokines causing endothelial retraction, as described earlier.
· Leukocyte-mediated endothelial injury. Leukocytes adhere to endothelium relatively early in inflammation. As discussed later, such leukocytes may be activated in the process, releasing toxic oxygen species and proteolytic enzymes, which then cause endothelial injury or detachment, resulting in increased permeability. In acute inflammation, this form of injury is largely restricted to vascular sites, such as venules and pulmonary and glomerular capillaries, where leukocytes adhere for prolonged periods to the endothelium.
· Increased transcytosis across the endothelial cytoplasm. Transcytosis occurs across channels consisting of clusters of interconnected, uncoated vesicles and vacuoles called the vesiculovacuolar organelle, many of which are located close to intercellular junctions. Certain factors, for example, vascular endothelial growth factor (VEGF) (appear to cause vascular leakage by increasing the number and perhaps the size of these channels. It has been claimed that this is also a mechanism of increased permeability induced by histamine and most chemical mediators.
Leakage from new blood vessels. During repair, endothelial cells proliferate and form new blood vessels, a process called angiogenesis. New vessel sprouts remain leaky until the endothelial cells mature and form intercellular junctions. In addition, certain factors that cause angiogenesis (e.g., VEGF) also increase vascular permeability. and endothelial cells in foci of angiogenesis have increased density of receptors for vasoactive mediators, including histamine, substance P, and VEGF. All these factors account for the edema that is characteristic of the early phases of healing that follow inflammation .
Although these mechanisms are separable, all may play a role in response to one stimulus. For example, at different stages of a thermal burn, leakage results from chemically mediated endothelial contraction, direct and leukocyte-dependent endothelial injury, and regenerating capillaries when the injury begins to heal. The vascular leakage induced by all these mechanisms accounts for the life-threatening loss of fluid in severely burned patients.
In summary, in acute inflammation, fluid loss from vessels with increased permeability occurs in distinct phases: (1) an immediate transient response lasting for 30 minutes or less, mediated mainly by the actions of histamine and leukotrienes on endothelium; (2) a delayed response starting at about 2 hours and lasting for about 8 hours, mediated by kinins, complement products, and other factors; and (3) a prolonged response that is most noticeable after direct endothelial injury, for example, after burns.
At higher magnification, vasculitis with arterial wall necrosis is seen. Note the fragmented remains of neutrophilic nuclei (karyorrhexis). Acute inflammation is a non-selective process that can lead to tissue destruction.
CELLULAR EVENTS: LEUKOCYTE EXTRAVASATION
AND PHAGOCYTOSIS
A critical function of inflammation is to deliver leukocytes to the site of injury and to activate the leukocytes to perform their normal functions in host defense. Leukocytes ingest offending agents, kill bacteria and other microbes, and get rid of necrotic tissue and foreign substances. A price that is paid for the defensive potency of leukocytes is that they may induce tissue damage and prolong inflammation, since the leukocyte products that destroy microbes and necrotic tissues can also injure normal host tissues
The sequence of events in the journey of leukocytes from the vessel lumen to the interstitial tissue, called extravasation, can be divided into the following steps
ü In the lumen: margination, rolling, and adhesion to endothelium. Vascular endothelium normally does not bind circulating cells or impede their passage. In inflammation, the endothelium has to be activated to permit it to bind leukocytes, as a prelude to their exit from the blood vessels.
ü Transmigration across the endothelium (also called diapedesis)
ü Migration in interstitial tissues toward a chemotactic stimulus
Iormally flowing blood in venules, erythrocytes are confined to a central axial column, displacing the leukocytes toward the wall of the vessel. Because blood flow slows early in inflammation (stasis), hemodynamic conditions change (wall shear stress decreases), and more white cells assume a peripheral position along the endothelial surface. This process of leukocyte accumulation is called margination. Subsequently, individual and then rows of leukocytes tumble slowly along the endothelium and adhere transiently (a process called rolling), finally coming to rest at some point where they adhere firmly (resembling pebbles over which a stream runs without disturbing them). In time, the endothelium can be virtually lined by white cells, an appearance called pavementing. After firm adhesion, leukocytes insert pseudopods into the junctions between the endothelial cells, squeeze through interendothelial junctions, and assume a position between the endothelial cell and the basement membrane. Eventually, they traverse the basement membrane and escape into the extravascular space. Neutrophils, monocytes, lymphocytes, eosinophils, and basophils all use the same pathway to migrate from the blood into tissues. We now examine the molecular mechanisms of each of the steps
Leukocyte Adhesion and Transmigration
Leukocyte adhesion and transmigration are regulated largely by the binding of complementary adhesion molecules on the leukocyte and endothelial surfaces, and chemical mediators-chemoattractants and certain cytokines-affect these processes by modulating the surface expression or avidity of such adhesion molecules. The adhesion receptors involved belong to four molecular families-the selectins, the immunoglobulin superfamily, the integrins, and mucin-like glycoproteins
Neutrophils are granular leukocytes with a polymorphonucleus and fine cytoplasmic granules that stain readily with neutral dyes. In the inflammatory response, neutrophils are the first cells to arrive at the injured area. The major activity of neutrophils is phagocytosis of invading bacterial cells, with subsequent destruction of the cells through the release of lysosomal enzymes.
Eosinophils (eosinophilic granulocytes) have a characteristic bilobate nucleus and cytoplasmic granules that stain orange with Romanovsky’s stain and red-orange with eosin. The granules contain hydrolytic enzymes (e.g., histaminase, which inactivates histamine; arylsulfatase B, which inactivates S kS-A). The granules also contain a poorly understood major basic protein. Although they also can be found in peripheral blood, a number of the body’s eosinophils exist in hypersensitivity sites within the tissues, where they can abort hypersensitivity reactions. Eosinophils are increased in the peripheral blood in the presence in the allergy and parasitic infestation. Eosinophils are readily chemotactic upon the Bronchopneumonia (hemorrhagic release of eosinophil chemotactic factor (ECF) from IgE-sensitized mast cells—an occurrence in anaphylaxis. Eosinophils are also phagocytic, although phagocytosis is a minor function.
Bronchopneumonia (hemorrhagic), gross
and microscopic ; note the prominent extravasation of erythrocytes
Basophils (basophilic granulocytes) contain granules that stain blue with Wright’s stain. The granules contain histamine, heparin, and slow-reacting substance of anaphylaxis. Basophils are involved in type I immediate, or immunoglobulin E (IgE)mediated hypersensitivity reactions. When an IgE specific antigen enters the body, basophils stimulate the formation of IgB, which binds to the surface of the antigen. The basophilic granules then release histamine and other vasoactive substances to produce anaphylactic reactions in susceptible persons. Basophils also play a role in type IV (i.e., delayed) hypersensitivity reactions, such as contact dermatitis.
Necrotizing pneumonia, microscopic view; note the pale granular destruction of lung tissue (arrow).
Macrophages. The mononuclear phagocyte system (also known as the monocyte-macrophage system and reticuloendothelial system) is an extensive network of macrophages that exists throughout the body. Pulmonary alveolar macrophages, Pleural and peritoneal macrophages, Kupffer cells of the liver, Histiocytes of mesenchymal and connective tissue, Mesangial cells of the kidney. Both fixed and mobile macrophages in the lymph nodes, spleen, and bone marrow. Macrophages in the body tissues develop from monocytes that have left the peripheral blood. The monocytes originally derive from bone marrow precursors. Monocytes in the bone marrow and the peripheral blood can be converted rapidly into additional macrophages when needed.
Granulomatous (fungal) pneumonitis, gross
and microscopic with fungal organisms {histoplasma sp. red in PAS stain) in giant cells (arrows).
Macrophages dispose of noxious matter within tissues, for example, microorganisms and necrotic tissue or other debris. Macrophages also appear to serve in tumor cell killing. In phagocytosis, the cytoplasmic membrane extends around particles and engulfs them, forming an intracellular vacuole. In microcytosis, the cell membrane engulfs extracellular fluid along with the particles. The lysosomes of macrophages contain degradative substances similar to those ieutrophils. Macrophages have surface receptors for the Fc segment of the immunoglobulin U (IgG) molecule and for complement component C3b. These aid the macrophage in phagocytosis of opsonized microorganisms.
Chronic (lymphocytic) gastritis in autoimmune disease; note the interstitial lymphoplasmacytic infiltration of the mucosa (arrow).
Lobar pneumonia
Granulation tissue (skin wound) preceding repair with fibrosis; note the edematous stroma with mixed inflammatory infiltration and proliferation of capillaries (arrow).
Macrophages are important components of the immune system. Their involvement begins with the initiation of the immune response, and they interact closely with T-lymphocytes. B-cell activation requires IL-i, which is secreted by macrophages (and some other cells). B-cell activation also requires that antibody on the B-cell surface match its specific antigen. Antigen on the macrophage surface can serve this purpose.
Mast cells resemble basophils in both structure and function. Whereas basophils are present mainly in the peripheral blood and at sites of inflammation, mast cells are connective tissue cells found close to small blood vessels. Mast cells contaiumerous granules that stain metachromatically with basic dyes. Like basophilic granules, mast cell granules release histagune, heparin, and SRS-A during type I reactions. In addition, mast cell granules release. Agents that cause inflammation (e.g., physical factors, drugs, immunoglobulins, complement components C3a and C5a, cationic proteins) may cause histamine release from mast cells.
Lymphocytes and their derivatives are found in the tissues in all types of inflammation, especially after the acute ingress of neutrophils. All lymphocytes are: derived from bone marrow stem cells. Stem cells differentiate into lymphocytes in the primary lymphoid organs (thymus and bone marrow). From these1 locations, some lymphocytes migrate — via the circulation—to secondary lymphoid organs, namely, the spleen, lymph nodes, and lymphoid germinaL centers throughout the body. Lymphocytes are divided into two types —i T-cells and B-cells — which serve different functions (See Immunopathology).
Chronic atrophic enteritis (Crohn’s) with mucosal atrophy in a patient with Crohn’s disease; note the fibrous thickening of the terminal ileum with loss of mucosal structure (arrow).
Of course, inflammatory reactions are not neatly categorized by cell type. A variety of inflammatory cell types may be present, though one may predominate. Seen here are mainly neutrophils, but there are also plasma cells, lymphocytes, and macrophages. Macrophages can phagocytoze other cells as well as cellular debris. One macrophage here has “pigged out” by consuming a neutrophil, a red blood cell, and a nuclear fragment.
Here is simple edema, or fluid collection within tissues. This is “pitting” edema because, on physical examination, you can press your finger into the skin and soft tissue and leave a depression.
This example of edema with inflammation is not trivial at all: there is marked laryngeal edema such that the airway is narrowed. This is life-threatening. Thus, fluid collections can be serious depending upon their location.
Here is an example of fluid collection into a body cavity, or an effusion. This is a right pleural effusion (in a baby). Note the clear, pale yellow appearance of the fluid. This is a serous effusion. Extravascular fluid collections can be classified as follows:
Exudate: extravascular fluid collection that is rich in protein and/or cells. Fluid appears grossly cloudy.
Transudate: extravascular fluid collection that is basically an ultrafiltrate of plasma with little protein and few or no cells. Fluid appears grossly clear.
Effusions into body cavities can be further described as follows:
Serous: a transudate with mainly edema fluid and few cells.
Serosanguinous: an effusion with red blood cells.
Fibrinous (serofibrinous): fibrin strands are derived from a protein-rich exudate.
Purulent: numerous PMN’s are present. Also called “empyema” in the pleural space.
Here is an example of bilateral pleural effusions. Note that the fluid appears reddish, because there has been hemorrhage into the effusion. This is a serosanguinous effusion.
The other integrins are expressed on platelets and other cell types, and bind to extracellular matrix proteins as well as proteins involved in coagulation.
The milky white fluid shown here in the peritoneal cavity represents a chylous ascites. This is an uncommon fluid accumulation that can be due to blockage of lymphatic drainage, in this case by a malignant lymphoma involving the mesentery and retroperitoneum.
The next step in the process is migration of the leukocytes through the endothelium, called transmigration or diapedesis. Chemokines act on the adherent leukocytes and stimulate the cells to migrate through interendothelial spaces toward the chemical concentration gradient, that is, toward the site of injury or infection. Certain homophilic adhesion molecules (i.e., adhesion molecules that bind to each other) present in the intercellular junction of endothelium are involved in the migration of leukocytes. One of these molecules is a member of the immunoglobulin superfamily called PECAM-1 (platelet endothelial cell adhesion molecule) or CD31. Leukocyte diapedesis, similar to increased vascular permeability, occurs predominantly in the venules (except in the lungs, where it also occurs in capillaries). After traversing the endothelium, leukocytes are transiently retarded in their journey by the continuous basement membrane of the venules, but eventually the cells pierce the basement membrane, probably by secreting collagenases. The net result of this process is that leukocytes rapidly accumulate where they are needed.
Once leukocytes enter the extravascular connective tissue, they are able to adhere to the extracellular matrix by virtue of β1 integrins and CD44 binding to matrix proteins. Thus, the leukocytes are retained at the site where they are needed.
The most telling proof of the importance of adhesion molecules is the existence of genetic deficiencies in the leukocyte adhesion proteins, which result in impaired leukocyte adhesion and recurrent bacterial infections. In leukocyte adhesion deficiency type 1 (LAD1), patients have a defect in the biosynthesis of the β2 chain shared by the LFA-1 and Mac-1 integrins. Leukocyte adhesion deficiency type 2 (LAD2) is caused by the absence of sialyl-Lewis X, the fucose-containing ligand for E-selectin, owing to a defect in a fucosyl transferase, the enzyme that attaches fucose moieties to protein backbones. In addition, antibodies to adhesion molecules abrogate leukocyte extravasation in experimental models of acute inflammation, and gene knockout mice deficient in these molecules show defects in leukocyte adhesion and extravasation
The type of emigrating leukocyte varies with the age of the inflammatory response and with the type of stimulus. In most forms of acute inflammation, neutrophils predominate in the inflammatory infiltrate during the first 6 to 24 hours, then are replaced by monocytes in 24 to 48 hours. Several reasons account for this sequence-neutrophils are more numerous in the blood, they respond more rapidly to chemokines, and they may attach more firmly to the adhesion molecules that are rapidly induced on endothelial cells, such as P- and E-selectins. In addition, after entering tissues, neutrophils are short-lived; they undergo apoptosis and disappear after 24 to 48 hours, whereas monocytes survive longer. There are exceptions to this pattern of cellular exudation, however. In certain infections-for example, those produced by Pseudomonas organisms-neutrophils predominate over 2 to 4 days; in viral infections, lymphocytes may be the first cells to arrive; in some hypersensitivity reactions, eosinophilic granulocytes may be the main cell type
Chemotaxis
After extravasation, leukocytes emigrate in tissues toward the site of injury by a process called chemotaxis, defined most simply as locomotion oriented along a chemical gradient. All granulocytes, monocytes and, to a lesser extent, lymphocytes respond to chemotactic stimuli with varying rates of speed. Both exogenous and endogenous substances can act as chemoattractants. The most common exogenous agents are bacterial products. Some of these are peptides that possess an N-formyl-methionine terminal amino acid. Others are lipid iature. Endogenous chemoattractants, which are detailed later, include several chemical mediators: (1) components of the complement system, particularly C5a; (2) products of the lipoxygenase pathway, mainly leukotriene B4 (LTB4); and (3) cytokines, particularly those of the chemokine family (e.g., IL-8).
How does the leukocyte sense the chemotactic agents, and how do these substances induce directed cell movement? Although not all the answers are known, several important steps and second messengers are recognized. All the chemotactic agents mentioned above bind to specific seven-transmembrane G-protein-coupled receptors (GPCRs) on the surface of leukocytes. Signals initiated from these receptors result in recruitment of G-proteins and activation of several effector molecules, including phospholipase C (PLCγ) and phosphoinositol-3 kinase (PI3K), as well as protein tyrosine kinases. PLCγ and PI3K act on membrane inositol phospholipids to generate lipid second messengers that increase cytosolic calcium and activate small GTPases of the Rac/Rho/cdc42 family as well as numerous kinases. The GTPases induce polymerization of actin, resulting in increased amounts of polymerized actin at the leading edge of the cell. The leukocyte moves by extending filopodia that pull the back of the cell in the direction of extension, much as an automobile with front-wheel drive is pulled by the wheels in front .Actin reorganization may also occur at the trailing edge of the cell. Locomotion involves rapid assembly of actin monomers into linear polymers at the filopodium’s leading edge, followed by cross-linking of filaments, and disassembly of such filaments away from the leading edge. A number of actin-regulating proteins, such as filamin, gelsolin, profilin, and calmodulin, interact with actin and myosin in the filopodium to produce contraction
Leukocyte Activation
Microbes, products of necrotic cells, antigen-antibody complexes, and cytokines, including chemotactic factors, induce a number of responses in leukocytes that are part of the defensive functions of the leukocytes (neutrophils and monocytes/macrophages) and are referred to under the rubric of leukocyte activation (Activation results from several signaling pathways that are triggered in leukocytes, resulting in increases in cytosolic Ca2+ and activation of enzymes such as protein kinase C and phospholipase A2. The functional responses that are induced on leukocyte activation include the following
Leukocytes express a number of surface receptors that are involved in their activation. These receptors include the following:
· Toll-like receptors (TLRs), which are homologous to a Drosophila protein called Toll, function to activate leukocytes in response to different types and components of microbes. To date, 10 mammalian TLRs have been identified, and each appears to be required for responses to different classes of infectious pathogens .Different TLRs play essential roles in cellular responses to bacterial lipopolysaccharide (LPS, or endotoxin), other bacterial proteoglycans, and unmethylated CpG nucleotides, all of which are found only in bacteria, as well as double-stranded RNA, which is produced only by some viruses. These receptors function by receptor-associated kinases to stimulate the production of microbicidal substances and cytokines in the leukocytes.
· Different seven-transmembrane G-protein-coupled receptors recognize microbes and some mediators that are produced in response to infections and tissue injury. These receptors have a conserved structure with seven transmembrane α-helical domains; are found oeutrophils, macrophages, and most other types of leukocytes; and are specific for diverse ligands. Receptors of this class recognize short peptides containing N-formylmethionyl residues, as well as chemokines, chemotactic breakdown products of complement such as C5a, and lipid mediators of inflammation, including platelet-activating factor, prostaglandin E, and LTB4. Because all bacterial proteins and few mammalian proteins (only those synthesized within mitochondria) are initiated by N-formylmethionine, this receptor allows neutrophils to detect and respond to bacterial proteins. Binding of ligands, such as microbial products and chemokines, to the G-protein-coupled receptors induces migration of the cells from the blood through the endothelium and production of microbicidal substances by activation of the respiratory burst. In a resting cell, the receptor-associated G-proteins form a stable inactive complex containing guanosine diphosphate (GDP) bound to Gα subunits. Occupancy of the receptor by ligand results in an exchange of GTP for GDP. The GTP-bound form of the G-protein activates numerous cellular enzymes, including an isoform of phosphatidylinositol-specific phospholipase C that functions to degrade inositol phospholipids and ultimately to increase intracellular Ca2+ and activate protein kinase C. The G-proteins also stimulate cytoskeletal changes, resulting in increased cell motility.
· Phagocytes express receptors for cytokines that are produced during immune responses. One of the most important of these cytokines is IFN-γ, which is secreted by natural killer (NK) cells during innate immune responses and by antigen-activated T lymphocytes during adaptive immune responses. IFN-γ is the major macrophage-activating cytokine.
· Receptors for opsonins promote phagocytosis of microbes coated with various proteins and deliver signals that activate the phagocytes. The process of coating a particle, such as a microbe, to target it for phagocytosis is called opsonization, and substances that do this are opsonins. These substances include antibodies, complement proteins, and lectins. One of the most efficient systems for opsonizing particles is coating the particles with IgG antibodies, which are termed specific opsonins and are recognized by the high-affinity Fcγ receptor of phagocytes, called FcγRI. Components of the complement system, especially fragments of the complement protein C3, are also potent opsonins, because these fragments bind to microbes and phagocytes express a receptor, called the type 1 complement receptor (CR1), that recognizes breakdown products of C3 (discussed later). These complement fragments are produced when complement is activated by either the classical (antibody-dependent) or the alternative (antibody-independent) pathway. Many bacteria can activate the alternative pathway and produce complement proteins that efficiently opsonize the bacteria in the absence of antibody molecules. A number of plasma proteins, including mannose-binding lectin (MBL), fibronectin, fibrinogen, and C-reactive protein, can coat microbes and are recognized by receptors on phagocytes. For example, a macrophage cell surface receptor called the C1q receptor binds microbes opsonized with plasma MBL, and integrins bind fibrinogen-coated particles.
Phagocytosis and the release of enzymes by neutrophils and macrophages are responsible for eliminating the injurious agents and thus constitute two of the major benefits derived from the accumulation of leukocytes at the inflammatory focus. Phagocytosis involves three distinct but interrelated steps (1) recognition and attachment of the particle to be ingested by the leukocyte; (2) its engulfment, with subsequent formation of a phagocytic vacuole; and (3) killing or degradation of the ingested material.
Recognition and Attachment. Although neutrophils and macrophages can engulf bacteria or extraneous matter (e.g., latex beads) without attachment to specific receptors, typically the phagocytosis of microbes and dead cells is initiated by recognition of the particles by receptors expressed on the leukocyte surface. Mannose receptors and scavenger receptors are two important receptors that function to bind and ingest microbes. The mannose receptor is a macrophage lectin that binds terminal mannose and fucose residues of glycoproteins and glycolipids. These sugars are typically part of molecules found on microbial cell walls, whereas mammalian glycoproteins and glycolipids contain terminal sialic acid or N-acetylgalactosamine. Therefore, the macrophage mannose receptor recognizes microbes and not host cells. Scavenger receptors were originally defined as molecules that bind and mediate endocytosis of oxidized or acetylated low-density lipoprotein (LDL) particles that cao longer interact with the conventional LDL receptor. Macrophage scavenger receptors bind a variety of microbes in addition to modified LDL particles. Macrophage integrins, notably Mac-1 (CD11b/CD18), may also bind microbes for phagocytosis.
The efficiency of phagocytosis is greatly enhanced when microbes are opsonized by specific proteins (opsonins) for which the phagocytes express high-affinity receptors. As described above, the major opsonins are IgG antibodies, the C3b breakdown product of complement, and certain plasma lectins, notably MBL, all of which are recognized by specific receptors on leukocytes.
Engulfment. Binding of a particle to phagocytic leukocyte receptors initiates the process of active phagocytosis of the particle. During engulfment, extensions of the cytoplasm (pseudopods) flow around the particle to be engulfed, eventually resulting in complete enclosure of the particle within a phagosome created by the plasma membrane of the cell. The limiting membrane of this phagocytic vacuole then fuses with the limiting membrane of a lysosomal granule, resulting in discharge of the granule’s contents into the phagolysosome. During this process, the neutrophil and the monocyte become progressively degranulated.
The process of phagocytosis is complex and involves the integration of many receptor-initiated signals with the coordinated orchestration of membrane remodeling and cytoskeletal changes. Phagocytosis is dependent on polymerization of actin filaments; it is, therefore, not surprising that the signals that trigger phagocytosis are many of the same that are involved in chemotaxis. (In contrast, fluid phase pinocytosis and receptor-mediated endocytosis of small particles involve internalization into clathrin-coated pits and vesicles and are not dependent on the actin cytoskeleton.)
Killing and Degradation. The ultimate step in the elimination of infectious agents and necrotic cells is their killing and degradation withieutrophils and macrophages, which occur most efficiently after activation of the phagocytes. Microbial killing is accomplished largely by oxygen-dependent mechanisms. Phagocytosis stimulates a burst in oxygen consumption, glycogenolysis, increased glucose oxidation via the hexose-monophosphate shunt, and production of reactive oxygen intermediates (ROIs, also called reactive oxygen sepecies).
The generation of reactive oxygen intermediates is due to the rapid activation of an oxidase (NADPH oxidase), which oxidizes NADPH (reduced nicotinamide-adenine dinucleotide phosphate) and, in the process, reduces oxygen to superoxide anion. Superoxide is then converted into hydrogen peroxide (H2O2), mostly by spontaneous dismutation. Hydrogen peroxide can also be further reduced to the highly reactive hydroxyl radical (OH). Most of the H2O2 is eventually broken down by catalase into H2O and O2, and some is destroyed by the action of glutathione oxidase.
NADPH oxidase is an enzyme complex consisting of at least seven proteins. In resting neutrophils, different NADPH oxidase protein components are located in the plasma membrane and the cytoplasm. In response to activating stimuli, the cytosolic protein components translocate to the plasma membrane or phagosomal membrane, where they assemble and form the functional enzyme complex. Thus, the reactive oxygen intermediates are produced within the lysosome where the ingested substances are segregated, and the cell’s own organelles are protected from the harmful effects of the ROIs. A similar enzyme system generates reactive nitrogen intermediates, notably nitric oxide, which also helps to kill microbes.
The H2O2 generated by the NADPH oxidase system is generally not able to efficiently kill microbes by itself. However, the azurophilic granules of neutrophils contain the enzyme myeloperoxidase (MPO), which, in the presence of a halide such as Cl–, converts H2O2 to hypochlorite (HOCl). The latter is a potent antimicrobial agent that destroys microbes by halogenation (in which the halide is bound covalently to cellular constituents) or by oxidation of proteins and lipids (lipid peroxidation). The H2O2-MPO-halide system is the most efficient bactericidal system ieutrophils. MPO-deficient leukocytes are capable of killing bacteria (albeit more slowly thaormal cells), by virtue of the formation of superoxide, hydroxyl radicals, and singlet-oxygen.
Bacterial killing can also occur by oxygen-independent mechanisms, through the action of substances in leukocyte granules. These include bactericidal permeability increasing protein (BPI), a highly cationic granule-associated protein that causes phospholipase activation, phospholipid degradation, and increased permeability in the outer membrane of the microorganisms; lysozyme, which hydrolyzes the muramic acid-N-acetyl-glucosamine bond, found in the glycopeptide coat of all bacteria; lactoferrin, an iron-binding protein present in specific granules; major basic protein, a cationic protein of eosinophils, which has limited bactericidal activity but is cytotoxic to many parasites; and defensins, cationic arginine-rich granule peptides that are cytotoxic to microbes (and certain mammalian cells). In addition, neutrophil granules contain many enzymes, such as elastase, that also contribute to microbial killing (discussed later in the chapter).
After killing, acid hydrolases, which are normally stored in lysosomes, degrade the microbes within phagolysosomes. The pH of the phagolysosome drops to between 4 and 5 after phagocytosis, this being the optimal pH for the action of these enzymes.
Release of Leukocyte Products and Leukocyte-Induced Tissue Injury
During activation and phagocytosis, leukocytes release microbicidal and other products not only within the phagolysosome but also into the extracellular space. The most important of these substances ieutrophils and macrophages are lysosomal enzymes, present in the granules; reactive oxygen intermediates; and products of arachidonic acid metabolism, including prostaglandins and leukotrienes. These products are capable of causing endothelial injury and tissue damage and may thus amplify the effects of the initial injurious agent. Products of monocytes/macrophages and other leukocyte types have additional potentially harmful products, which are described in the discussion of chronic inflammation. Thus, if persistent and unchecked, the leukocyte infiltrate itself becomes the offender, and leukocyte-dependent tissue injury underlies many acute and chronic human diseases. This fact becomes evident in the discussion of specific disorders throughout this book.
Regulated secretion of lysosomal proteins is a peculiarity of leukocytes and other hematopoietic cells. (Recall that in most secretory cells, the proteins that are secreted are not stored within lysosomes.) The contents of lysosomal granules are secreted by leukocytes into the extracellular milieu by diverse mechanisms. Release may occur if the phagocytic vacuole remains transiently open to the outside before complete closure of the phagolysosome (regurgitation during feeding). If cells are exposed to potentially ingestible materials, such as immune complexes deposited on immovable flat surfaces (e.g., glomerular basement membrane), attachment of leukocytes to the immune complexes triggers leukocyte activation, but the fixed immune complexes cannot be phagocytosed, and lysosomal enzymes are released into the medium (frustrated phagocytosis). Cytotoxic release occurs after phagocytosis of potentially membranolytic substances, such as urate crystals, which damage the membrane of the phagolysosome. In addition, there is some evidence that proteins in certain granules, particularly the specific (secondary) granules of neutrophils, may be directly secreted by exocytosis.
After phagocytosis, neutrophils rapidly undergo apoptotic cell death and are ingested by macrophages.
From the preceding discussion, it is obvious that leukocytes play a central role in host defense. Not surprisingly, therefore, defects in leukocyte function, both genetic and acquired, lead to increased vulnerability to infection. Impairments of virtually every phase of leukocyte function-from adherence to vascular endothelium to microbicidal activity-have been identified, and the existence of clinical genetic deficiencies in each of the critical steps in the process has been described. These include the following:
· Defects in leukocyte adhesion. We previously mentioned the genetic deficiencies in leukocyte adhesion molecules (LAD types 1 and 2). LAD 1 is characterized by recurrent bacterial infections and impaired wound healing. LAD 2 is clinically milder than LAD 1 but is also characterized by recurrent bacterial infections.
· Defects in phagolysosome function. One such disorder is Chédiak-Higashi syndrome, an autosomal recessive condition characterized by neutropenia (decreased numbers of neutrophils), defective degranulation, and delayed microbial killing. The neutrophils (and other leukocytes) have giant granules, which can be readily seen in peripheral blood smears and which are thought to result from aberrant organelle fusion. In this syndrome, there is reduced transfer of lysosomal enzymes to phagocytic vacuoles in phagocytes (causing susceptibility to infections) and abnormalities in melanocytes (leading to albinism), cells of the nervous system (associated with nerve defects), and platelets (generating bleeding disorders). The gene associated with this disorder encodes a large cytosolic protein that is apparently involved in vesicular traffic but whose precise function is not yet known. The secretion of granule proteins by cytotoxic T cells is also affected, accounting for part of the immunodeficiency seen in the disorder.
· Defects in microbicidal activity. The importance of oxygen-dependent bactericidal mechanisms is shown by the existence of a group of congenital disorders with defects in bacterial killing called chronic granulomatous disease, which render patients susceptible to recurrent bacterial infection. Chronic granulomatous disease results from inherited defects in the genes encoding several components of NADPH oxidase, which generates superoxide. The most common variants are an X-linked defect in one of the plasma membrane-bound components (gp91phox) and autosomal recessive defects in the genes encoding two of the cytoplasmic components (p47phox and p67phox).
· Clinically, the most frequent cause of leukocyte defects is bone marrow suppression, leading to reduced production of leukocytes. This is seen following therapies for cancer (radiation and chemotherapy) and when the marrow space is compromised by tumor metastases to bone.
Although we have emphasized the role of leukocytes recruited from the circulation in the acute inflammatory response, cells resident in tissues also serve important functions in initiating acute inflammation. The two most important of these cell types are mast cells and tissue macrophages. Mast cells react to physical trauma, breakdown products of complement, microbial products, and neuropeptides. The cells release histamine, leukotrienes, enzymes, and many cytokines (including TNF, IL-1, and chemokines), all of which contribute to inflammation. Macrophages recognize microbial products and secrete most of the cytokines important in acute inflammation. These cells are stationed in tissues to rapidly recognize potentially injurious stimuli and initiate the host defense reaction.
CHEMICAL MEDIATORS OF INFLAMMATION
Having described the events in acute inflammation, we caow turn to a discussion of the chemical mediators that are responsible for the events. Many mediators have been identified, and how they function in a coordinated manner is still not fully understood. Here we review general principles and highlight some of the major mediators.
· Mediators originate either from plasma or from cells. Plasma-derived mediators (e.g., complement proteins, kinins) are present in plasma in precursor forms that must be activated, usually by a series of proteolytic cleavages, to acquire their biologic properties. Cell-derived mediators are normally sequestered in intracellular granules that need to be secreted (e.g., histamine in mast cell granules) or are synthesized de novo (e.g., prostaglandins, cytokines) in response to a stimulus. The major cellular sources are platelets, neutrophils, monocytes/macrophages, and mast cells, but mesenchymal cells (endothelium, smooth muscle, fibroblasts) and most epithelia can also be induced to elaborate some of the mediators.
· The production of active mediators is triggered by microbial products or by host proteins, such as the proteins of the complement, kinin, and coagulation systems, that are themselves activated by microbes and damaged tissues.
· Most mediators perform their biologic activity by initially binding to specific receptors on target cells. Some, however, have direct enzymatic activity (e.g., lysosomal proteases) or mediate oxidative damage (e.g., reactive oxygen and nitrogen intermediates).
· One mediator can stimulate the release of other mediators by target cells themselves. These secondary mediators may be identical or similar to the initial mediators but may also have opposing activities. They provide mechanisms for amplifying-or in certain instances counteracting-the initial mediator action.
· Mediators can act on one or few target cell types, have diverse targets, or may even have differing effects on different types of cells.
· Once activated and released from the cell, most of these mediators are short-lived. They quickly decay (e.g., arachidonic acid metabolites) or are inactivated by enzymes (e.g., kininase inactivates bradykinin), or they are otherwise scavenged (e.g., antioxidants scavenge toxic oxygen metabolites) or inhibited (e.g., complement regulatory proteins break up and degrade activated complement components). There is thus a system of checks and balances in the regulation of mediator actions.
· Most mediators have the potential to cause harmful effects.
We now discuss some of the more important mediators of acute inflammation.
Extensive acute inflammation may lead to abscess formation, as seen here with rounded abscesses (the purulent material has drained out after sectioning to leave a cavity) in upper lobe and lower lobe.
The two amines, histamine and serotonin, are especially important because they are present in preformed stores in cells and are therefore among the first mediators to be released during inflammation.
Histamine is widely distributed in tissues, the richest source being the mast cells that are normally present in the connective tissue adjacent to blood vessels It is also found in blood basophils and platelets. Preformed histamine is present in mast cell granules and is released by mast cell degranulation in response to a variety of stimuli: (1) physical injury such as trauma, cold, or heat; (2) immune reactions involving binding of antibodies to mast cells (3) fragments of complement called anaphylatoxins (C3a and C5a); (4) histamine-releasing proteins derived from leukocytes; (5) neuropeptides (e.g., substance P); and (6) cytokines (IL-1, IL-8).
In humans, histamine causes dilation of the arterioles and increases the permeability of venules (it, however, constricts large arteries). It is considered to be the principal mediator of the immediate transient phase of increased vascular permeability, causing venular gaps, as we have seen. It acts on the microcirculation mainly via binding to H1 receptors on endothelial cells.
Serotonin (5-hydroxytryptamine) is a preformed vasoactive mediator with actions similar to those of histamine. It is present in platelets and enterochromaffin cells, and in mast cells in rodents but not humans.
Release of serotonin (and histamine) from platelets is stimulated when platelets aggregate after contact with collagen, thrombin, adenosine diphosphate (ADP), and antigen-antibody complexes. Platelet aggregation and release are also stimulated by platelet activating factors (PAF) derived from mast cells during IgE-mediated reactions. In this way, the platelet release reaction results in increased permeability during immunologic reactions. As discussed later, PAF itself has many inflammatory properties.
A variety of phenomena in the inflammatory response are mediated by plasma proteins that belong to three interrelated systems, the complement, kinin, and clotting systems.
The complement system consists of 20 component proteins (and their cleavage products), which are found in greatest concentration in plasma. This system functions in both innate and adaptive immunity for defense against microbial agents. In the process of complement activation, a number of complement components are elaborated that cause increased vascular permeability, chemotaxis, and opsonization.
Complement proteins are present as inactive forms in plasma and are numbered C1 through C9. Many of these proteins are activated to become proteolytic enzymes that degrade other complement proteins, thus forming a cascade capable of tremendous enzymatic amplification. The critical step in the elaboration of the biologic functions of complement is the activation of the third (and most abundant) component, C3. Cleavage of C3 can occur by one of three pathways: the classical pathway, which is triggered by fixation of C1 to antibody (IgM or IgG) combined with antigen; the alternative pathway, which can be triggered by microbial surface molecules (e.g., endotoxin, or LPS), complex polysaccharides, cobra venom, and other substances, in the absence of antibody; and the lectin pathway, in which plasma mannose-binding lectin binds to carbohydrates on microbes and directly activates C1. Whichever pathway is involved in the early steps of complement activation, they all lead to the formation of an active enzyme called the C3 convertase, which splits C3 into two functionally distinct fragments, C3a and C3b. C3a is released and C3b becomes covalently attached to the cell or molecule where complement is being activated. C3b then binds to the previously generated fragments to form C5 convertase, which cleaves C5 to release C5a. The remaining C5b binds the late components (C6-C9), culminating in the formation of the membrane attack complex (MAC, composed of multiple C9 molecules).
The biologic functions of the complement system fall into two general categories: cell lysis by the MAC, and the effects of proteolytic fragments of complement. Complement-derived factors mediate a variety of phenomena in acute inflammation:
· Vascular phenomena. C3a, C5a, and, to a lesser extent, C4a are split products of the corresponding complement components that stimulate histamine release from mast cells and thereby increase vascular permeability and cause vasodilation. They are called anaphylatoxins because they have effects similar to those of mast cell mediators that are involved in the reaction called anaphylaxis C5a also activates the lipoxygenase pathway of arachidonic acid (AA) metabolism in neutrophils and monocytes, causing further release of inflammatory mediators.
· Leukocyte adhesion, chemotaxis, and activation. C5a is a powerful chemotactic agent for neutrophils, monocytes, eosinophils, and basophils.
· Phagocytosis. C3b and its cleavage product iC3b (inactive C3b), when fixed to the bacterial cell wall, act as opsonins and favor phagocytosis by neutrophils and macrophages, which bear cell surface receptors for these complement fragments.
Among the complement components, C3 and C5 are the most important inflammatory mediators. In addition to the mechanisms already discussed, C3 and C5 can be activated by several proteolytic enzymes present within the inflammatory exudate. These include plasmin and lysosomal enzymes released from neutrophils (discussed later in this chapter). Thus, the chemotactic effect of complement and the complement-activating effects of neutrophils can set up a self-perpetuating cycle of neutrophil emigration.
The activation of complement is tightly controlled by cell-associated and circulating regulatory proteins.The presence of these inhibitors in host cell membranes protects the host from inappropriate damage during protective reactions against microbes.
The kinin system generates vasoactive peptides from plasma proteins, called kininogens, by the action of specific proteases called kallikreins. Activation of the kinin system results in the release of the vasoactive nonapeptide bradykinin. Bradykinin increases vascular permeability and causes contraction of smooth muscle, dilation of blood vessels, and pain when injected into the skin. These effects are similar to those of histamine. It is triggered by activation of Hageman factor (factor XII of the intrinsic clotting pathway; see later) upon contact with negatively charged surfaces, such as collagen and basement membranes. A fragment of factor XII (prekallikrein activator, or factor XIIa) is produced, and this converts plasma prekallikrein into an active proteolytic form, the enzyme kallikrein. The latter cleaves a plasma glycoprotein precursor, high-molecular-weight kininogen, to produce bradykinin. High-molecular-weight kininogen also acts as a cofactor or catalyst in the activation of Hageman factor. The action of bradykinin is short-lived because it is quickly inactivated by an enzyme called kininase. Any remaining kinin is inactivated during passage of plasma through the lung by angiotensin-converting enzyme. Kallikrein itself is a potent activator of Hageman factor, allowing for autocatalytic amplification of the initial stimulus. Kallikrein has chemotactic activity, and it also directly converts C5 to the chemoattractant product C5a.
The clotting system and inflammation are intimately connected processes. The clotting system is divided into two pathways that converge, culminating in the activation of thrombin and the formation of fibrin .The intrinsic clotting pathway is a series of plasma proteins that can be activated by Hageman factor (factor XII), a protein synthesized by the liver that circulates in an inactive form until it encounters collagen or basement membrane or activated platelets (as occurs at the site of endothelial injury). Factor XII then undergoes a conformational change (becoming factor XIIa), exposing an active serine center that can subsequently cleave protein substrates and activate a variety of mediator systems (see later).
The Complement System in Health and Disease
The activation of the complement cascade may be divided into early and late steps. In the early steps, three different pathways lead to the proteolytic cleavage of C3. In the late steps, all three pathways converge, and the major breakdown product of C3, C3b, activates a series of other complement components.
The Early Steps of Complement Activation
The pathways of early complement activation are the following (see Figure): The classical pathway is triggered by fixation of C1 to antibody (IgM or IgG) that has combined with antigen, and proteolysis of C2 and C4, and subsequent formation of a C4b2b complex that functions as a C3 convertase. The alternative pathway can be triggered by microbial surface molecules (e.g., endotoxin, or LPS), complex polysaccharides, and cobra venom. It involves a distinct set of plasma components (properdin, and factors B and D). In this pathway, the spontaneous cleavage of C3 that occurs normally is enhanced and stabilized by a complex of C3b and a breakdown product of Factor B called Bb; the C3bBb complex is a C3 convertase. In the lectin pathway, mannose-binding lectin, a plasma collectin, binds to carbohydrate-containing proteins on bacteria and viruses and directly activates C1; the remaining steps are as in the classical pathway. The C3 convertases break down C3 into C3b, which remains attached to the surface where complement is activated, and a smaller C3a fragment that diffuses away.
The Late Steps of Complement Activation
The C3b that is generated by any of the pathways binds to the C3 convertase and produces a C5 convertase, which cleaves C5. C5b remains attached to the complex and forms a substrate for the subsequent binding of the C6-C9 components. Polymerized C9 forms a channel in lipid membranes, called the membrane attack complex, which allows fluid and ions to enter and causes cell lysis.
Collection of fluid in a space is a transudate. If this fluid is protein-rich or has many cells then it becomes an exudate. The large amount of fibrin in such fluid can form a fibrinous exudate on body cavity surfaces. Here, the pericardial cavity has been opened to reveal a fibrinous pericarditis with strands of stringy pale fibrin between visceral and parietal pericardium.
Regulation of Complement Activation
To prevent inappropriate activation of complement and to limit its activation even when appropriate, mammals express many regulatory proteins. (These proteins are generally absent from microbes, which explains why complement is activated by and functions against microbes.) Complement activation can be controlled at several steps:
· Regulation of C3 and C5 convertases. Since the formation of C3 convertase and the generation of C3b are the central features of all complement pathways, it is not surprising that many of the regulatory proteins are directed at controlling these activities. These regulators function by enhancing the dissociation (decay acceleration) of the convertase complex (e.g., decay-accelerating factor [DAF]) or by proteolytically cleaving C3b (e.g., factor I).
· Binding of active complement components by specific proteins in the plasma. The first step in the classical pathway, which is triggered by C1 binding to an immune complex, is blocked by a plasma protein called C1 inhibitor (C1INH). C1INH interferes with the enzymatic activity of two of the proteins in the C1 complex. Excessive complement activation is also prevented by a number of proteins that act to inhibit MAC formation (e.g., CD59, also called membrane inhibitor of reactive lysis).
Microscopically, the fibrinous exudate is seen to consist of pink strands of fibrin jutting from the pericardial surface at the upper left. Below this, there are a few scattered inflammatory cells.
Anti-inflammatory therapy can be directed at many targets along the eicosanoid biosynthetic pathways:
· Cyclooxygenase inhibitors include aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), such as indomethacin. They function by inhibiting prostaglandin synthesis; aspirin does this by irreversibly acetylating and inhibiting cyclooxygenase. COX-2 inhibitors are a newer class of these drugs. The finding that COX-2 is inducibly expressed only in response to inflammatory stimuli was the impetus for developing antagonists against this enzyme to reduce inflammation without interfering with the physiologic functions of AA metabolites. COX-2 inhibitors are now widely used as anti-inflammatory drugs and generally produce less toxicity than the older COX-1 inhibitors.
· Lipoxygenase inhibitors. 5-lipoxygenase is not affected by NSAIDs, and many new inhibitors of this enzyme pathway have been developed. Pharmacologic agents that inhibit leukotriene production or block leukotriene receptors (CysLT1 and CysLT2) have been found useful in the treatment of asthma.
· Broad-spectrum inhibitors include glucocorticoids. These powerful anti-inflammatory agents may act by down-regulating the expression of specific target genes, including the genes encoding COX-2, phospholipase A2, proinflammatory cytokines (such as IL-1 and TNF), and nitric oxide synthase (iNOS) (see later). Glucocorticoids also up-regulate genes that encode potent anti-inflammatory proteins, such as lipocortin 1. Lipocortin 1 inhibits release of AA from membrane phospholipids.
· Another approach to manipulating inflammatory responses has been to modify the intake and content of dietary lipids by increasing the consumption of fish oil. The basis for this approach is that fish oil fatty acids serve as poor substrates for conversion to active metabolites by both the cyclooxygenase and the lipoxygenase pathways.
ACUTE INFLAMMATION
Although all acute inflammatory reactions are characterized by vascular changes and leukocyte infiltration, the severity of the reaction, its specific cause, and the particular tissue and site involved introduce morphologic variations in the basic patterns. Several types of inflammation are recognized, which vary in their morphology and clinical correlates.
Morphological manifestations of inflammation depend upon a number of factors and processes. They ire factors of the organisms and the host, type of exudation, cellular proliferation.
Factors involving the organisms:
1. Type of injury and infection. For example, skin reacts to herpes simplex infection by formation of a vesicle and to streptococcal infection by formation of a boil; lung reacts to pneumococci by occurrence of lobar pneumonia while to tubercle bacilli it reacts by granulomatous inflammation.
2. Virulence. Many species and strains of organisms may have varying virulence e.g. the three strains of C. diphtheriae (gravis, intermedius and mitis) produce the same diphtherial exotoxin but in different amount.
3. Dose. The concentration of organism in small doses produces usually local lesions while a larger dose results in more severe spreading infections.
4. Portal of entry. Some organisms are infective hanly if administered by particular route, e.g. Vibrio ho lerae is not pathogenic if injected subscutaneously but causes cholera if swallowed.
5. Product of organisms. Some organisms produce enzymes that help in spread of infections, e.g. hyaluronidase by Cl., streptokinase by Streptococci, staphylokinase and coagulase by Staphylococci.
Factors involving the host
1. General health of host. For example, starvation, hemorrhagic shock, chronic debilitating diseases like diabetes mellitus, alcoholism, etc. render the host more susceptible to infections.
2. Immune state of host. Immunodeficiency helØs in spread of infections rapidly, e.g. in AIDS.
3. Leukopenia. Patients with low WBC count with neutropenia or agranulocytosis develop spreading infection.
4. Site or type of tissue involved. For example, the lung has loose texture as compared to bone and thus both tissues react differently to acute inflammation.
5. Local host factors. For instance, ischemia, presence of foreign bodies and chemicals cause necrosis and are thus harmful.
Type of exudation. The appearance of escaped plasma determines the morphological type of inflammation. These are:
1. Serous, when the fluid exudate resembles serum or is watery, e.g. pleural effusion in tuberculosis, blister formation in burns.
2. Fibrinous, when the fibrin content of the fluid exudate is high, e.g. in pneumococcal and rheumatic pericarditis.
Two types may be croupous and diphtheria fibrinous inflammation.
3. Purulent or suppurative exudate is formation ot creamy pus as seen in infection with pyogenic bacteria, e.g. abscess, acute appendicitis, phlegmon.
4. Hemorrhagic, when there is vascular damage, ‘g. acute hemorrhagic pneumonia in influenza.
5. Catarrhal, when the surface inflammation of epithelium produces increased secretion ofmucus, e.g. common cold.
SEROUS INFLAMMATION
Serous inflammation is marked by the outpouring of a thin fluid that, depending on the size of injury, is derived from either the plasma or the secretions of mesothelial cells lining the peritoneal, pleural, and pericardial cavities (called effusion). The skin blister resulting from a burn or viral infection represents a large accumulation of serous fluid, either within or immediately beneath the epidermis of the skin.
With more severe injuries and the resulting greater vascular permeability, larger molecules such as fibrinogen pass the vascular barrier, and fibrin is formed and deposited in the extracellular space. A fibrinous exudate develops when the vascular leaks are large enough or there is a procoagulant stimulus in the interstitium (e.g., cancer cells). A fibrinous exudate is characteristic of inflammation in the lining of body cavities, such as the meninges, pericardium and pleura. Histologically, fibrin appears as an eosinophilic meshwork of threads or sometimes as an amorphous coagulum. Fibrinous exudates may be removed by fibrinolysis and clearing of other debris by macrophages. As mentioned above, the process of resolution may restore normal tissue structure, but when the fibrin is not removed, it may stimulate the ingrowth of fibroblasts and blood vessels and thus lead to scarring. Conversion of the fibrinous exudate to scar tissue (organization) within the pericardial sac leads either to opaque fibrous thickening of the pericardium and epicardium in the area of exudation or, more often, to the development of fibrous strands that reduce and may even obliterate the pericardial space.
Cellular proliferation. Variable cellular proliferalion is seen in different types of inflammations.
1. There is no significant cellular proliferation in acute bacterial infections except in typhoid fever in which there is intestinal lymphoid hyperplasia.
2. Viral infections have the ability to stimulate cellular proliferation, e.g. epidermal cell proliferation in herpes simplex, chickenpox and smallpox.
Chronic abscess of liver
Necrosis. The extent and type of necrosis in inflammation is variable. In gas gangrene, there is extensive necrosis with discolored and foul smelling tissues. in acute appendicitis, there is necrosis as a result of vascular obstruction. In chronic inflammation such as tuberculosis, there is characteristic caseous necrosis. Morphology of acute inflammation Inflammation of an organ is usually named by adding the suffix “itis” so its Latiame e.g. appendicitis, hepatitis, cholecystitis, meningitis, etc. A few morphologic varieties of acute inflammation are described below:
1. Catarrhal inflammation. A surface inflammation associated with greatly increased secretion of clear mucus. Later, polymorphs appear (common cold and some forms of colitis).
2. Hemorrhagic inflammation. Where the damage is severe, actual rupture of all blood vessels occurs, with hemorrhage the most striking feature (acute hemorrhagic pneumonia occasionally occurring in fatal cases of influenza).
3. Suppuration. There are several types of suppuration: an abscess, phlegmon, furuncle, carbuncle, cellulitis, bacterial infections of the blood.
Chronic abscess
When acute bacterial infection is accompanied by intense neutrophilic infiltrate in the inflammed tissue, it results in tissue necrosis. A cavity is formed which is called an abscess and contains purulent exudate or pus and the process of abscess formation is known as suppuration. The bacteria which cause suppuration are called pyogenic. Pus is creamy or opaque in appearance and is composed of numerous dead as well as living neutrophils, some red cells, fragments of tissue debris and fibrin. In old “pus” inacrophages and cholesterol crystals are also present. The wall of abscess is called pyogenic membrane. An abscess may be discharged to the surface due to increased pressure inside or may require drainage by the surgeon. Due to tissue destruction, resolution does not occur but instead healing by fibrous bearing takes place.
Here is a purulent exudate in which the exuded fluid also contains a large number of acute inflammatory cells. Thus, the yellowish fluid in this opened pericardial cavity is a purulent exudate.
SUPPURATIVE OR PURULENT INFLAMMATION
Suppurative or purulent inflammation is characterized by the production of large amounts of pus or purulent exudate consisting of neutrophils, necrotic cells, and edema fluid. Certain bacteria (e.g., staphylococci) produce this localized suppuration and are therefore referred to as pyogenic (pus-producing) bacteria. A common example of an acute suppurative inflammation is acute appendicitis. Abscesses are localized collections of purulent inflammatory tissue caused by suppuration buried in a tissue, an organ, or a confined space. They are produced by deep seeding of pyogenic bacteria into a tissue . Abscesses have a central region that appears as a mass of necrotic leukocytes and tissue cells. There is usually a zone of preserved neutrophils around this necrotic focus, and outside this region vascular dilation and parenchymal and fibroblastic proliferation occur, indicating the beginning of repair. In time, the abscess may become walled off and ultimately replaced by connective tissue.
A purulent exudate is seen beneath the meninges in the brain of this patient with acute meningitis from Streptococcus pneumoniae infection. The exudate obscures the sulci.
The abdominal cavity is opened at autopsy here to reveal an extensive purulent peritonitis that resulted from rupture of the colon. A thick yellow exudate coats the peritoneal surfaces. A paracentesis yielded fluid with the properties of an exudate: high protein content with many cells (mostly PMN’s).
Phlegmon is unbounded purulent inflammation in which pus spreads diffusely between different components of tissue owing to fusion and tissue lysis. Phlegmon frequently occurs along the muscular bands, tendons, fascias, vascular-nerves bands and in subcutaneous fat. Two types of phlegmon have been described: sofi and dense.
If purulent exudate appears in the human cavities ‘is called empyema.
Furuncle is an acute inflammation via hair flicles in the dermal tissues.
Carbuncle is seen in untreated diabetics and occurs as a located abscess in the dermis and soft tissues of the neck.
Cellulitis. It is a diffuse inflammation of soft tissues resulting from spreading effects of substances like hyaluronidase released by some bacteria.
Bacterial infections of the blood. This includes the following 3 conditions: bacteremia, septicemia, pycmia.
Bacteremia is defined as presence of small number ol bacteria in the blood which don’t multiply significantly. They are commonly not detected by direct microscopy. Blood culture is done for their detection, e.g. infection with Salmonella typhi, Escherichia colt, Streptococcus viridans.
Septicemia means presence of rapidly multiplying, highly pathogenic bacteria in the blood, e.g. pyogenic cocci, bacilli of plague, etc. Septicemia is generally accompanied by systemic effects like toxemia, multiple small hemorrhage, neutrophilic leucocytosis and disseminated intravascular coagulation (DIC).
Pyemia is the dissemination of small septic thrombi in the blood which cause their effects at the site where they are lodged. This can result in pyemic abscesses or septic infarcts. Pyemic abscesses are multiple small abscesses in various organs such as in cerebral cortex, myocardium, lungs and renal cortex, resulting from very small emboli fragmented from septic thrombus. Microscopy of pyemic abscess shows a central zone of necrosis containing numerous bacteria, surrounded by a zone of suppuration and an outer zone of acute inflammatory cells. Septic infarcts result from lodgment of larger fragments of septic thrombi in the arteries with relatively larger foci of necrosis, suppuration and acute inflammation, e.g. septic infarcts of the lungs, liver, brain, and kidneys from septic thrombi of leg veins or from acute bacterial endocarditis.
4. Serous inflammation. Serous inflammation is marked by the outpouring of a thin fluid that, depending 1pericardial space.
One the size of injury, is derived from either the blood serum or the secretions of mesothelial cells lining the )erltoneal, pleural, and pericardial cavities. The skin blister resulting from a bum or viral infections represents a ‘arge accumulation of serous fluid, either within or immediately beneath the epidermis of the skin.
Acute glomerulonephritis
5. Fibrinous inflammation. With more severe injuries and the resulting greater vascular permeability, larger molecules such as fibrin pass the vascular barrier. A fibrinous exudate develops when the vascular leaks are large enough or there is a pro- coagulant stimulus in the interstitium (e.g., cancer cells). A fibrinous exudate is characteristic of inflammation in body cavities, such as the pericardium and pleura. Microscopically, fibrin appears as an eosinophilic meshwork of threads, or sometimes as an amorphous coagulum. Fibrinous exudates may be removed by fibrinolysis, and other debris by macrophages. This process, called resolution, may restore normal tissue structure, but when the fibrin is not removed it may stimulate the ingrowth of fibroblasts and blood vessels and thus lead to scarring. (‘onversion of the fibrinous exudate to scar tissue (organization) within the pericardial sac will lead either to opaque fibrous thickening of the pericardium and epicardium in the area of exudation or, more often, to the development of fibrous strands that bridge. According to the type of epithelium on which inflammatory process develops and depth of necrosis there are two types of fibrinous inflammation croupous and diphtheroid fibrinous inflammation. Usually croupous inflammation develops on the columnar epithelium. In this case the fibrinous membranes unfix easily, without any effort. Diphtheroid fibrinous inflammation develops on the squamous or intermediate epithelium, when the fibrinous membranes unfix with difficulties.
The lung fibrinoid inflammation
6. Pseudomembranous inflammation. It is inflammatory response of mucous surface (oral, respiratory, bowel) to toxins of diphtheria or irritant gases. As a result of denudation of epithelium, plasma exudes on the surface where it coagulates, and together with necrosed epithelium, forms false membrane that gives this type of inflammation its name.
7. Ulcer. Ulcer is a local defect on the surface of an organ produced by inflammation. In the acute stage, there is infiltration by polymorphs with vasodilatation while long-standing ulcers develop infiltration by lymfhocytes, plasma cells and macrophages with5 associated fibroblastic proliferation and scarring.
An ulcer is a local defect, or excavation, of the surface of an organ or tissue that is produced by the sloughing (shedding) of inflammatory necrotic tissue. Ulceration can occur only when tissue necrosis and resultant inflammation exist on or near a surface. It is most commonly encountered in: (1) inflammatory necrosis of the mucosa of the mouth, stomach, intestines, or genitourinary tract; and (2) subcutaneous inflammation of the lower extremities in older persons who have circulatory disturbances that predispose to extensive necrosis.
Ulcerations are best exemplified by peptic ulcer of the stomach or duodenum, in which acute and chronic inflammation coexist. During the acute stage, there is intense polymorphonuclear infiltration and vascular dilation in the margins of the defect. With chronicity, the margins and base of the ulcer develop fibroblastic proliferation, scarring, and the accumulation of lymphocytes, macrophages, and plasma cells.
The lung fibrinoid inflammation with necrosis
The appendix purulent inflammation
The acute inflammatory process can culminate one of the following 4 outcomes: resolution, healin by scarring, progression to suppuration, progressio to chronic inflammation.
Outcome. This means complete return to norma tissue following acute inflammation. It occurs when issue changes are slight and the cellular changes are reversible, e.g. resolution in lobar pneumonia.
Healing by scarring. This takes place when the issue destruction in acute inflammation is extensive so that there is no tissue regeneration but actually there is healing by fibrosis.
Progression to suppuration. When the pyogenic bacteria causing acute inflammation result in severe lissue necrosis, the process progresses to suppuration. Initially, there is intense neutrophilic infiltration. Subsequently, mixture of neutrophils, bacteria, Iiagments of necrotic tissue, cell debris and fibrin comprise pus which is contained in a cavity to form an abscess. The abscess, if not drained, may get organized by dense fibrous tissue, and in time, get caftified.
Progression to chronic inflammation. Acute inflammation may progress to chronic one in which the processes of inflammation and healing proceed side by side.
Summary of Acute Inflammation
Now that we have described the components, mediators, and pathologic manifestations of acute inflammatory responses, it is useful to summarize the sequence of events in a typical response of this type. When a host encounters an injurious agent, such as an infectious microbe or dead cells, phagocytes that reside in all tissues try to get rid of these agents. At the same time, phagocytes and other host cells react to the presence of the foreign or abnormal substance by liberating cytokines, lipid messengers, and the various other mediators of inflammation. Some of these mediators act on endothelial cells in the vicinity and promote the efflux of plasma and the recruitment of circulating leukocytes to the site where the offending agent is located. The recruited leukocytes are activated by the injurious agent and by locally produced mediators, and the activated leukocytes try to remove the offending agent by phagocytosis. As the injurious agent is eliminated and anti-inflammatory mechanisms become active, the process subsides and the host returns to a normal state of health. If the injurious agent cannot be quickly eliminated, the result may be chronic inflammation.
The different components of the inflammatory response are mediated by different signals and serve distinct (and overlapping) functions. The vascular phenomena of acute inflammation are characterized by increased blood flow to the injured area, resulting mainly from arteriolar dilation and opening of capillary beds induced by mediators such as histamine. Increased vascular permeability results in the accumulation of protein-rich extravascular fluid, which forms the exudate. Plasma proteins leave the vessels, most commonly through widened interendothelial cell junctions of the venules. The redness (rubor), warmth (calor), and swelling (tumor) of acute inflammation are caused by the increased blood flow and edema. Circulating leukocytes, initially predominantly neutrophils, adhere to the endothelium via adhesion molecules, transmigrate across the endothelium, and migrate to the site of injury under the influence of chemotactic agents. Leukocytes that are activated by the offending agent and by endogenous mediators may release toxic metabolites and proteases extracellularly, causing tissue damage. During the damage, and in part as a result of the liberation of prostaglandins, neuropeptides, and cytokines, one of the local symptoms is pain (dolor).
Fibrinopurulent pericarditis.
In patients with acute viral pericarditis or uremia, the exudate typically is fibrinous, imparting an irregular, shaggy appearance to the pericardial surface (so-called “bread and butter” pericarditis). In acute bacterial pericarditis, the exudate is fibrinopurulent (suppurative), often with areas of frank pus; tuberculous pericarditis can exhibit areas of caseation. Pericarditis due to malignancy often is associated with an exuberant, shaggy fibrinous exudate and a bloody effusion; metastases can be grossly evident as irregular excrescences or may be grossly inapparent, especially in the case of leukemia. In most cases, acute fibrinous or fibrinopurulent pericarditis resolves without any sequelae. With extensive suppuration or caseation, however, healing can result in fibrosis (chronic pericarditis).
Chronic pericarditis may be associated with delicate adhesions or dense, fibrotic scars that obliterate the pericardial space. In extreme cases, the heart is so completely encased by dense fibrosis that it cannot expand normally during diastole—resulting in the condition known as constrictive pericarditis.
Describe the macro-specimen, characterize the surface of the epicardium. What is the descriptive name for the specimen? Indicate the causes and I outcome; possible clinical determination of pericarditis.
Chronic inflammation is defined as prolonged process in which tissue destruction and inflammation occur at the same time. Although difficult to define precisely, chronic inflammation is considered to be inflammation of prolonged duration (weeks or months) in which active inflammation, tissue destruction, and attempts at repair are proceeding simultaneously. Although it may follow acute inflammation, as described earlier, chronic inflammation frequently begins insidiously, as a low-grade, smoldering, often asymptomatic response. This latter type of chronic inflammation is the cause of tissue damage in some of the most common and disabling human diseases, such as rheumatoid arthritis, atherosclerosis, tuberculosis, and chronic lung diseases.
The neutrophils are seen infiltrating the mucosa and submucosa of the gallbladder in this patient with acute cholecystitis and right upper quadrant abdominal pain with tenderness on palpation.
CAUSES OF CHRONIC INFLAMMATION
· At medium power magnification, numerous neutrophils fill the alveoli in this case of acute bronchopneumonia in a patient with a high fever. Pseudomonas aeruginosa was cultured from sputum. Note the dilated capillaries in the alveolar walls from vasodilation with the acute inflammatory process.
Chronic inflammation can be caused by one of the following 3 ways:
1. Chronic inflammation following acute inflammation — when the tissue destruction is extensive, or the bacteria survive and persist in small numbers at the site of acute inflammation, e.g. in osteomyehtis, pneumonia terminating in lung abscess.
2. Recurrent attacks of acute inflammation — when repeated bouts of acute inflammation culminate in chronicity of the process, e.g. in recurrent urinary tract infection leading to chronic pyelonephritis, repeated acute infection of gall bladder leading to chronic cholecystitis.
3. Chronic inflammation starting de novo — when the infection with organisms of low pathogenicity is chronic from the beginning, e.g. infection with Mycobacterium tuberculosis.
Though there may be differences in chronic inflammatory response depending upon the tissue involved and causative organisms, there are some basic similarities amongst various types of chronic inflammation.
These general features characterize any chronic inflammation.
In contrast to acute inflammation, which is manifested by vascular changes, edema, and predominantly neutrophilic infiltration, chronic inflammation is characterized by:
1. Mononuclear infiltration. Chronic inflammatory lesions are infiltrated by mononuclear inflammatory cells like phagocytes and lymphoid cells. Phagocytes are represented by circulating monocytes, tissue macrophages, epithelioid cells and sometimes, multinucleated giant cells. The macrophages comprise the most important cells in chronic inflammation.
2. Tissue destruction and necrosis. Tissue destruction and necrosis are common in many chronic inflammatory lesions and are brought about by activated macrophages by release of a variety of biolotucally active substances.
· 3. Proliferative changes. As a result of necrosis, proliferation of small blood vessels and fibroblasts is stimulated resulting in formation of inflammatory granulation tissue. Eventually, healing by fibrosis and coflagen laying takes place. Attempts at healing by connective tissue replacement of damaged tissue, accomplished by proliferation of small blood vessels (angiogenesis) and, in particular, fibrosis.
MONONUCLEAR CELL INFILTRATION
The macrophage is the dominant cellular player in chronic inflammation, and we begin our discussion with a brief review of its biology. Macrophages are one component of the mononuclear phagocyte system). The mononuclear phagocyte system (sometimes called reticuloendothelial system) consists of closely related cells of bone marrow origin, including blood monocytes and tissue macrophages. The latter are diffusely scattered in the connective tissue or located in organs such as the liver (Kupffer cells), spleen and lymph nodes (sinus histiocytes), and lungs (alveolar macrophages). Mononuclear phagocytes arise from a common precursor in the bone marrow, which gives rise to blood monocytes. From the blood, monocytes migrate into various tissues and differentiate into macrophages. The half-life of blood monocytes is about 1 day, whereas the life span of tissue macrophages is several months or years. The journey from bone marrow stem cell to tissue macrophage is regulated by a variety of growth and differentiation factors, cytokines, adhesion molecules, and cellular interactions.
As discussed previously, monocytes begin to emigrate into extravascular tissues quite early in acute inflammation, and within 48 hours they may constitute the predominant cell type. Extravasation of monocytes is governed by the same factors that are involved ieutrophil emigration, that is, adhesion molecules and chemical mediators with chemotactic and activating properties. When the monocyte reaches the extravascular tissue, it undergoes transformation into a larger phagocytic cell, the macrophage. Macrophages may be activated by a variety of stimuli, including cytokines (e.g., IFN-γ) secreted by sensitized T lymphocytes and by NK cells, bacterial endotoxins, and other chemical mediators (Activation results in increased cell size, increased levels of lysosomal enzymes, more active metabolism, and greater ability to phagocytose and kill ingested microbes. Activated macrophages secrete a wide variety of biologically active products that, if unchecked, result in the tissue injury and fibrosis characteristic of chronic inflammation
In short-lived inflammation, if the irritant is eliminated, macrophages eventually disappear (either dying off or making their way into the lymphatics and lymph nodes). In chronic inflammation, macrophage accumulation persists, and is mediated by different mechanisms
1. Recruitment of monocytes from the circulation, which results from the expression of adhesion molecules and chemotactic factors. Most of the macrophages present in a focus of chronic inflammation are recruited from circulating monocytes. The process of monocyte recruitment is fundamentally similar to the recruitment of neutrophils, described earlier Chemotactic stimuli for monocytes include chemokines produced by activated macrophages, lymphocytes, and other cell types (e.g., MCP-1); C5a; growth factors such as platelet-derived growth factor and transforming growth factor-α (TGF-α); fragments from the breakdown of collagen and fibronectin; and fibrinopeptides. Each of these may play a role under given circumstances; for example, chemokines are major stimuli for macrophage accumulation in delayed-hypersensitivity immune reactions.
2. Local proliferation of macrophages after their emigration from the bloodstream. Once thought to be an unusual event, macrophage proliferation is now known to occur prominently in some chronic inflammatory lesions, such as atheromatous plaques
3. Immobilization of macrophages within the site of inflammation. Certain cytokines and oxidized lipids can cause such immobilization.
The products of activated macrophages serve to eliminate injurious agents such as microbes and to initiate the process of repair, and are responsible for much of the tissue injury in chronic inflammation. Some of these products are toxic to microbes and host cells (e.g., reactive oxygen and nitrogen intermediates) or extracellular matrix (proteases); some cause influx of other cell types (e.g., cytokines, chemotactic factors); and still others cause fibroblast proliferation, collagen deposition, and angiogenesis (e.g., growth factors). This impressive arsenal of mediators makes macrophages powerful allies in the body’s defense against unwanted invaders, but the same weaponry can also induce considerable tissue destruction when macrophages are inappropriately activated. Thus, tissue destruction is one of the hallmarks of chronic inflammation.
A variety of substances in addition to the products of macrophages may contribute to tissue damage in chronic inflammation. Necrotic tissue itself can perpetuate the inflammatory cascade through the activation of kinin, coagulation, complement and fibrinolytic systems, the release of mediators from leukocytes responding to the necrotic tissue, and liberation of substances like uric acid from dying cells. In cellular immune reactions, T lymphocytes may directly kill cells. Thus, ongoing tissue destruction can activate the inflammatory cascade by diverse mechanisms, so that features of both acute and chronic inflammation may coexist in certain circumstances.
OTHER CELLS IN CHRONIC INFLAMMATION
Other cell types present in chronic inflammation include lymphocytes, plasma cells, eosinophils, and mast cells:
· Lymphocytes are mobilized in both antibody-mediated and cell-mediated immune reactions and even ionimmune inflammation. Antigen-stimulated (effector and memory) lymphocytes of different types (T, B) use various adhesion molecule pairs (predominantly the integrins and their ligands) and chemokines to migrate into inflammatory sites. Cytokines from activated macrophages, mainly TNF, IL-1, and chemokines, promote leukocyte recruitment, setting the stage for persistence of the inflammatory response.
· Lymphocytes and macrophages interact in a bidirectional way, and these reactions play an important role in chronic inflammation . Macrophages display antigens to T cells, and produce membrane molecules (costimulators) and cytokines (notably IL-12) that stimulate T-cell responses. Activated T lymphocytes produce cytokines, and one of these, IFN-γ, is a major activator of macrophages. Plasma cells develop from activated B lymphocytes and produce antibody directed either against persistent antigen in the inflammatory site or against altered tissue components. In some strong chronic inflammatory reactions, the accumulation of lymphocytes, antigen-presenting cells, and plasma cells may assume the morphologic features of lymphoid organs, particularly lymph nodes, even containing well-formed germinal centers. This pattern of lymphoid organogenesis is often seen in the synovium of patients with long-standing rheumatoid arthritis.
· Eosinophils are abundant in immune reactions mediated by IgE and in parasitic infections . The recruitment of eosinophils involves extravasation from the blood and their migration into tissue by processes similar to those for other leukocytes. One of the chemokines that is especially important for eosinophil recruitment is eotaxin. Eosinophils have granules that contain major basic protein, a highly cationic protein that is toxic to parasites but also causes lysis of mammalian epithelial cells. They may thus be of benefit in controlling parasitic infections but they contribute to tissue damage in immune reactions
· Mast cells are widely distributed in connective tissues and participate in both acute and persistent inflammatory reactions. Mast cells express on their surface the receptor that binds the Fc portion of IgE antibody (FcεRI). In acute reactions, IgE antibodies bound to the cells’ Fc receptors specifically recognize antigen, and the cells degranulate and release mediators, such as histamine and products of AA oxidation . This type of response occurs during anaphylactic reactions to foods, insect venom, or drugs, frequently with catastrophic results. When properly regulated, this response can benefit the host. Mast cells are also present in chronic inflammatory reactions, and may produce cytokines that contribute to fibrosis.
Although neutrophils are characteristic of acute inflammation, many forms of chronic inflammation, lasting for months, continue to show large numbers of neutrophils, induced either by persistent microbes or by mediators produced by macrophages and T lymphocytes. In chronic bacterial infection of bone (osteomyelitis), a neutrophilic exudate can persist for many months. Neutrophils are also important in the chronic damage induced in lungs by smoking and other irritant stimuli.
Much smaller abscesses are seen here. These could be termed “microabscesses” due to their small size. Abscesses can come in a variety of sizes. Perhaps the most common abscess is the pimple on the face of a teenager.
An abscess is a localized collection of PMN’s. Here is a microabscess in the myocardium. The irregular dark purple center is a collection of bacteria that are the cause for this abscess.
One consequence of acute inflammation is ulceration. This occurs on epithelial surfaces. Here the gastric mucosa has been lost, or ulcerated. A larger ulcer and several adjacent smaller ones with surrounding erythema appear at the left of center.
Main classifications of chronic inflammation
Conventionally, chronic inflammation is subdivided into 2 types:
1. Nonspecific, when the irritant substance produces a non-specific chronic inflammatory reaction with formation of granulation tissue and healing by Jibrosis, e.g. chronic osteomyelitis, chronic ulcer.
2. Specific, when the injurious agent causes a characteristic histologic tissue response, e.g. tuberculosis, leprosy, syphilis.
However, for a more descriptive classification, histological features are used for classifying chronic inflammation into 3 corresponding types.
1. Chronic nonspecific interstitial inflammation. This is characterized by nonspecific inflammatory cell infiltration, e.g. chronic osteomyelitis, lung abscess. A variant of this type of chronic inflammatory response is chronic suppurative inflammation in which infiltration by polymorphs and abscess formation are additional features, e.g. actinomycosis. The inflammatory cell infiltration consist of lymphocytes, monocytes, plasmocytes, eosinophils and other cells.
2. Chronic nonspecific interstitial inflammation with formation of polyps and pointed condyloma. It occurs on the mucous membranes and in the areas borderline with squamous epithelium.
Polyps are the end result of prolonged chronic irritation. Nasal, cervical, colorectal polyps are common. Macroscopically they are gelatinous masses with smooth and shining surface. Microscopically they are composed of loose edematous connective tissue containing some mucous glands and varying number of inflammatory cells (lymphocytes, plasmocytes, eosinophils).
Condyloma is commonly located on the coronal sulcus on the penis or the perineal area.
Popov’s granuloma
3. Chronic granulomatous inflammation. This is characterized by formation of granulomas, e.g. tuberculosis, leprosy, syphilis, actinomycosis, sarcoidosis, etc. Granuloma is defined as a circum-scribed, tiny lesion, about
Miocarditis
GRANULOMATOUS INFLAMMATION
Granulomatous inflammation is a distinctive pattern of chronic inflammatory reaction characterized by focal accumulations of activated macrophages, which often develop an epithelial-like (epithelioid) appearance. It is encountered in a limited number of immunologically mediated, infectious and some noninfectious conditions. Its genesis is firmly linked to immune reactions.. Tuberculosis is the prototype of the granulomatous diseases, but sarcoidosis, cat-scratch disease, lymphogranuloma inguinale, leprosy, brucellosis, syphilis, some mycotic infections, berylliosis, and reactions of irritant lipids are also included . Recognition of the granulomatous pattern in a biopsy specimen is important because of the limited number of possible conditions that cause it and the significance of the diagnoses associated with the lesions.
Chronic inflammation is more difficult to understand, because it is so variable. Seen here is chronic endometritis with lymphocytes as well as plasma cells in the endometrial stroma. In general, the inflammatory infiltrate of chronic inflammation consists mainly of mononuclear cells (“round cells”): lymphocytes, plasma cells, and macrophages.
Here is chronic cervicitis. Prolonged acute inflammation or repeated bouts of acute inflammation may lead to the appearance of more mononuclear cells, and chronic inflammation. In this case the inflammation is severe enough to produce mucosal damage with hemorrhage.
You can find both acute and chronic inflammation here. This type of mixed inflammation is typical of repeated or recurrent inflammation. Examples of this process include diagnoses such as “acute and chronic cholecystitis” or “acute and chronic cervicitis”.
Microscopically, this abscess has a mixture of inflammatory cells, but the wall of the abscess is “organizing” with ingrowth of capillaries (filled with red blood cells) and fibroblasts. As organization continues there is resolution with decreasing size of the abscess, until only a scar remains. If the body’s defensive systems cannot contain the agent causing the abscess, then the process may continue and even spread.
A granuloma is a focus of chronic inflammation consisting of a microscopic aggregation of macrophages that are transformed into epithelium-like cells surrounded by a collar of mononuclear leukocytes, principally lymphocytes and occasionally plasma cells. In the usual hematoxylin and eosin stained tissue sections, the epithelioid cells have a pale pink granular cytoplasm with indistinct cell boundaries, often appearing to merge into one another. The nucleus is less dense than that of a lymphocyte, is oval or elongate, and may show folding of the nuclear membrane. Older granulomas develop an enclosing rim of fibroblasts and connective tissue. Frequently, epithelioid cells fuse to form giant cells in the periphery or sometimes in the center of granulomas. These giant cells may attain diameters of 40 to 50 μm. They have a large mass of cytoplasm containing 20 or more small nuclei arranged either peripherally (Langhans-type giant cell) or haphazardly (foreign body-type giant cell) . There is no known functional difference between these two types of giant cells, a fact that does not deter students from remembering the morphologic differences!
There are two types of granulomas, which differ in their pathogenesis. Foreign body granulomas are incited by relatively inert foreign bodies. Typically, foreign body granulomas form when material such as talc (associated with intravenous drug abuse) sutures, or other fibers are large enough to preclude phagocytosis by a single macrophage and do not incite any specific inflammatory or immune response. Epithelioid cells and giant cells form and are apposed to the surface and encompass the foreign body. The foreign material can usually be identified in the center of the granuloma, particularly if viewed with polarized light, in which it appears refractile.
Immune granulomas are caused by insoluble particles, typically microbes, that are capable of inducing a cell-mediated immune response. This type of immune response does not necessarily produce granulomas but it does so when the inciting agent is poorly degradable or particulate. In these responses, macrophages engulf the foreign material and process and present some of it to appropriate T lymphocytes, causing them to become activated. The responding T cells produce cytokines, such as IL-2, which activates other T cells, perpetuating the response, and IFN-γ, which is important in activating macrophages and transforming them into epithelioid cells and multinucleate giant cells.
Necrosis may be a feature of some granulomatous conditions, e.g. centr & caseous necrosis of tuberculosis, so called because of cheese-like appearance and consistency of necrosis.
Fibrosis is due to proliferation of fibroblasts at the periphery of granuloma.
The following two factors favour the formation of granulomas:
1. Presence of poorly digestible irritant which may be organisms like Mycobacterium tuberculosis, particles of talc, etc.
2. Presence of cell-mediated immunity to the irritant, implying thereby the role of hypersensitivity in granulomatous inflammation.
A fully-developed tubercle is about
Granulomatous inflammation is typical of reaction to poorly digestible agents elicited by tuberculosis, leprosy, fungal infectiohs, schistoso-miasis, foreign particles, etc.
The outcomes of chronic inflammation depend on the type of inflammation, morphofunctional characteristic of the definite organ or tissue, where inflammation develops. Frequently sclerosis and hyalinosis may develop.
Trichinelosis
The system of lymphatics and lymph nodes filters and polices the extravascular fluids. Together with the mononuclear phagocyte system, it represents a secondary line of defense that is called into play whenever a local inflammatory reaction fails to contain and neutralize an external agent, such as a microbe.
Lymphatics are delicate channels that are difficult to visualize in ordinary tissue sections because they readily collapse. They are lined by continuous, thin endothelium with loose, overlapping cell junctions; scant basement membrane; and no muscular support except in the larger ducts. In inflammation lymph flow is increased and helps drain the edema fluid from the extravascular space. Because the junctions of lymphatics are loose, lymphatic fluid eventually equilibrates with extravascular fluid. Not only fluid, but also leukocytes and cell debris may find their way into lymph. Valves are present in collecting lymphatics, allowing lymph content to flow only proximally. Delicate fibrils, attached at right angles to the walls of the lymphatic vessel, extend into the adjacent tissues and serve to maintain patency of the lymphatic channels.
In severe injuries, the drainage may transport the offending agent, be it chemical or microbial. The lymphatics may become secondarily inflamed (lymphangitis), as may the draining lymph nodes (lymphadenitis). Therefore, it is not uncommon in infections of the hand, for example, to observe red streaks along the entire arm up to the axilla following the course of the lymphatic channels, accompanied by painful enlargement of the axillary lymph nodes. The nodal enlargement is usually caused by hyperplasia of the lymphoid follicles as well as by hyperplasia of the phagocytic cells lining the sinuses of the lymph nodes. This constellation of nodal histologic changes is termed reactive, or inflammatory, lymphadenitis
The system of lymph nodes sometimes contains the spread of the infection, but in severe infections the organisms gain access to the vascular circulation, thus inducing a bacteremia. The phagocytic cells of the liver, spleen, and bone marrow constitute the next line of defense, but in massive infections, bacteria seed distant tissues of the body. The heart valves, meninges, kidneys, and joints are favored sites of implantation for blood-borne organisms, and when this happens endocarditis, meningitis, renal abscesses, and septic arthritis may develop.
Systemic Effects of Inflammation
Anyone who has suffered a severe sore throat or a respiratory infection has experienced the systemic manifestations of acute inflammation. The systemic changes associated with inflammation, especially in patients who have infections, are collectively called the acute phase response, or the systemic inflammatory response syndrome (SIRS). These changes are reactions to cytokines whose production is stimulated by bacterial products such as LPS and by other inflammatory stimuli. The acute phase response consists of several clinical and pathologic changes:
· Fever, characterized by an elevation of body temperature, usually by 1° to
· Acute-phase proteins are plasma proteins, mostly synthesized in the liver, whose plasma concentrations may increase several hundred-fold as part of the response to inflammatory stimuli. Three of the best-known examples of these proteins are C-reactive protein (CRP), fibrinogen, and serum amyloid A protein (SAA). Synthesis of these molecules by hepatocytes is upregulated by cytokines, especially IL-6 (for CRP and fibrinogen) and IL-1 or TNF (for SAA). Many acute-phase proteins, such as CRP and SAA, bind to microbial cell walls, and they may act as opsonins and fix complement. They also bind chromatin, possibly aiding in the clearing of necrotic cell nuclei. During the acute phase response, serum amyloid A protein replaces apolipoprotein A, a component of high-density lipoprotein particles. This may alter the targeting of high-density lipoproteins from liver cells to macrophages, which can utilize these particles as a source of energy-producing lipids. The rise in fibrinogen causes erythrocytes to form stacks (rouleaux) that sediment more rapidly at unit gravity than do individual erythrocytes. This is the basis for measuring the erythrocyte sedimentation rate (ESR) as a simple test for the systemic inflammatory response, caused by any number of stimuli, including LPS. Acute-phase proteins have beneficial effects during acute inflammation, but as we shall see prolonged production of these proteins (especially SAA) causes secondary amyloidosis in chronic inflammation. Elevated serum levels of CRP are now used as a marker for increased risk of myocardial infarction in patients with coronary artery disease. It is believed that inflammation involving atherosclerotic plaques in the coronary arteries may predispose to thrombosis and subsequent infarction, and CRP is produced during inflammation. On this basis, anti-inflammatory agents are being tested in patients to reduce the risk of myocardial infarction.
· Leukocytosis is a common feature of inflammatory reactions, especially those induced by bacterial infection. The leukocyte count usually climbs to 15,000 or 20,000 cells/μl, but sometimes it may reach extraordinarily high levels of 40,000 to 100,000 cells/μl. These extreme elevations are referred to as leukemoid reactions because they are similar to the white cell counts obtained in leukemia. The leukocytosis occurs initially because of accelerated release of cells from the bone marrow postmitotic reserve pool (caused by cytokines, including IL-1 and TNF) and is therefore associated with a rise in the number of more immature neutrophils in the blood (shift to the left). Prolonged infection also induces proliferation of precursors in the bone marrow, caused by increased production of colony stimulating factors (CSFs). Thus, the bone marrow output of leukocytes is increased to compensate for the loss of these cells in the inflammatory reaction. (See also the discussion of leukocytosis in Neutrophilia refers to an increase in the blood neutrophil count. Most bacterial infections induce neutrophilia. Viral infections such as infectious mononucleosis, mumps, and German measles produce a leukocytosis by virtue of an absolute increase in the number of lymphocytes (lymphocytosis). In an additional group of disorders, which includes bronchial asthma, hay fever, and parasitic infestations, there is an absolute increase in the number of eosinophils, creating an eosinophilia. Certain infections (typhoid fever and infections caused by viruses, rickettsiae, and certain protozoa) are associated with a decreased number of circulating white cells (leukopenia). Leukopenia is also encountered in infections that overwhelm patients debilitated by disseminated cancer or rampant tuberculosis.
· Other manifestations of the acute phase response include increased pulse and blood pressure; decreased sweating, mainly because of redirection of blood flow from cutaneous to deep vascular beds, to minimize heat loss through the skin; rigors (shivering), chills (search for warmth), anorexia, somnolence, and malaise, probably because of the actions of cytokines on brain cells.
· In severe bacterial infections (sepsis), the large amounts of organisms and LPS in the blood stimulate the production of enormous quantities of several cytokines, notably TNF and IL-1. As a result, circulating levels of these cytokines increase and the form of the host response changes. High levels of TNF cause disseminated intravascular coagulation (DIC). Thrombosis results from two simultaneous reactions: LPS and TNF induce tissue factor (TF) expression on endothelial cells, which initiates coagulation; the same agents inhibit natural anticoagulation mechanisms, by decreasing the expression of tissue factor pathway inhibitor (TFPI) and endothelial cell thrombomodulin. Cytokines cause liver injury and impaired liver function, resulting in a failure to maintaiormal blood glucose levels due to a lack of gluconeogenesis from stored glycogen. Overproduction of NO by cytokine-activated cardiac myocytes and vascular smooth muscle cells leads to heart failure and loss of perfusion pressure, respectively, resulting in hemodynamic shock. The clinical triad of DIC, hypoglycemia, and cardiovascular failure is described as septic shock;. Multiple organs show inflammation and intravascular thrombosis, which can produce organ failure. Tissue injury in response to LPS can also result from the activation of neutrophils before they exit the vasculature, thus causing damage to endothelial cells and reduced blood flow. The lungs and liver are particularly susceptible to injury by neutrophils. Lung damage in the systemic inflammatory response, commonly called the adult respiratory distress syndrome (ARDS), results when neutrophil-mediated endothelial injury allows fluid to escape from the blood into the airspace The kidney and the bowel are also injured, largely due to reduced perfusion. This condition is often fatal.
Consequences of Defective or Excessive Inflammation
Now that we have described the process of inflammation and its outcomes, it is helpful to summarize the clinical and pathological consequences of too much or too little inflammation.
· Defective inflammation typically results in increased susceptibility to infections and delayed healing of wounds and tissue damage. The susceptibility to infections reflects the fundamental role of the inflammatory response in host defense, and is the reason why this response is a central component of the defense mechanisms that immunologists call innate immunity . Delayed repair is because the inflammatory response is essential for clearing damaged tissues and debris, and provides the necessary stimulus to get the repair process started.
· Excessive inflammation is the basis of many categories of human disease. It is well established that allergies, in which individuals mount unregulated immune responses against commonly encountered environmental antigens, and autoimmune diseases, in which immune responses develop against normally tolerated self-antigens, are disorders in which the fundamental cause of tissue injury is inflammation. But recent studies are pointing to an important role of inflammation in a wide variety of human diseases that are not primarily disorders of the immune system. These include cancer, atherosclerosis and ischemic heart disease, and some neurodegenerative diseases such as Alzheimer disease. In addition, prolonged inflammation and the fibrosis that accompanies it are responsible for much of the pathology in many chronic infectious, metabolic and other diseases. The specific diseases are discussed in relevant chapters later in the book. Since these disorders are some of the major scourges of mankind, it is not surprising that the normally protective inflammatory response is being called the “silent killer”.
Although our discussion of the molecular and cellular events in acute and chronic inflammation is concluded, we still need to consider the changes induced by the body’s attempts to heal the damage, the process of repair. As described next, the repair begins almost as soon as the inflammatory changes have started and involves several processes, including cell proliferation, differentiation, and extracellular matrix deposition.
