Pathomorphology of the immune system.

Pathomorphology of the immune system.

Processes of adaptation and indemnification.

Regeneration and reparation

 

General Features of the Immune System

 

Although vital to survival, the immune system is similar to the proverbial two-edged sword. On the one hand, immunodeficiency states render humans easy prey to infections and possibly tumors; on the other hand, a hyperactive immune system may cause fatal disease, as in the case of an overwhelming allergic reaction to the sting of a bee. In yet another series of derangements, the immune system may lose its normal capacity to distinguish self from non-self, resulting in immune reactions against one’s own tissues and cells (autoimmunity). This chapter considers diseases caused by too little immunity as well as those resulting from too much immunologic reactivity. We also consider amyloidosis, a disease in which an abnormal protein, derived in some cases from fragments of immunoglobulins, is deposited in tissues. First, we review some advances in the understanding of innate and adaptive immunity and lymphocyte biology, then give a brief description of the histocompatibility genes because their products are relevant to several immunologically mediated diseases and to the rejection of transplants.

 

INNATE AND ADAPTIVE IMMUNITY

 

The physiologic function of the immune system is to protect individuals from infectious pathogens. The mechanisms that are responsible for this protection fall into two broad categories. Innate immunity (also called natural, or native, immunity) refers to defense mechanisms that are present even before infection and have evolved to specifically recognize microbes and protect multicellular organisms against infections. Adaptive immunity (also called acquired, or specific, immunity) consists of mechanisms that are stimulated by (adapt to) microbes and are capable of also recognizing nonmicrobial substances, called antigens. Innate immunity is the first line of defense, because it is always ready to prevent and eradicate infections. Adaptive immunity develops later after exposure to microbes and is even more powerful in combating infections. By convention, the term “immune response” refers to adaptive immunity.

 

Innate and adaptive immunity. The principal mechanisms of innate immunity and adaptive immunity are shown.

 

The major components of innate immunity are epithelial barriers that block entry of environmental microbes, phagocytic cells (mainly neutrophils and macrophages), natural killer (NK) cells, and several plasma proteins, including the proteins of the complement system. Phagocytes are recruited to sites of infection, resulting in inflammation, and here the cells ingest the microbes and are then activated to destroy the ingested pathogens. Phagocytes recognize microbes by several membrane receptors. These include receptors for mannose residues and N-formyl methionine-containing peptides, which are produced by microbes but not by host cells, and a family of receptors that are homologous to a Drosophila protein called Toll.¹ Different Toll-like receptors (TLRs) are involved in responses to different microbial products. Upon recognition of the relevant microbial structure, the TLRs signal by a common pathway that leads to the activation of transcription factors, notably NF-κB (nuclear factor κB). NF-κB stimulates production of cytokines and several proteins that are responsible for the microbicidal activities of the phagocytes. Phagocytes internalize microbes into vesicles, where the microbes are destroyed by reactive oxygen and nitrogen intermediates and hydrolytic enzymes.

 

Toll-like Receptors

The Toll-like receptors (TLRs) are membrane proteins that recognize a variety of microbe-derived molecules and stimulate innate immune responses against the microbes. The first protein to be identified in this family was the Drosophila Toll protein, which is involved in establishing the dorsal-ventral axis during embryogenesis of the fly, as well as mediating antimicrobial responses. Ten different mammalian TLRs have been identified based on sequence homology to Drosophila Toll, and they are named TLR1-10. All these receptors contain leucine-rich repeats flanked by characteristic cysteine-rich motifs in their extracellular regions, and a conserved signaling domain in their cytoplasmic region that is also found in the cytoplasmic tails of the IL-1 and IL-18 receptors and is called the Toll/IL-1 receptor (TIR) domain. The TLRs are expressed on many different cell types that participate in innate immune responses, including macrophages, dendritic cells, neutrophils, NK cells, mucosal epithelial cells, and endothelial cells.

Mammalian TLRs are involved in responses to widely divergent types of molecules that are commonly expressed by microbial but not mammalian cells (see Figure). Some of the microbial products that stimulate TLRs include Gram-negative bacterial lipopolysaccharide (LPS), Gram-positive bacterial peptidoglycan, bacterial lipoproteins, the bacterial flagellar protein flagellin, heat shock protein 60, unmethylated CpG DNA motifs (found in many bacteria), and double-stranded RNA (found in RNA viruses). The specificity of TLRs for microbial products is dependent on associations between different TLRs and non-TLR adapter molecules. For instance, LPS first binds to soluble LPS-binding protein (LBP) in the blood or extracellular fluid, and this complex serves to facilitate LPS binding to CD14, which exists as both a soluble plasma protein and a glycophosphatidylinositol-linked membrane protein on most cells. Once LPS binds to CD14, LBP dissociates, and the LPS-CD14 complex physically associates with TLR4. An additional extracellular accessory protein, called MD2, also binds to the complex with CD14. LPS, CD14, and MD2 are all required for efficient LPS-induced signaling, but it is not yet clear if direct physical interaction of LPS with TLR4 is necessary.

Signaling by TLRs results in the activation of transcription factors, notably NF-κB (see Figure). Ligand binding to the TLR at the cell surface leads to recruitment of cytoplasmic signaling molecules, the first of which is the adapter protein MyD88. A kinase called IL-1 receptor associated kinase (IRAK) is recruited into the signaling complex. IRAK undergoes autophosphorylation, dissociates from MyD88, and activates another signaling molecule, called TNF-receptor (TNF-R) associated factor-6 (TRAF-6). TRAF-6 then activates the I-κB kinase cascade, leading to activation of the NF-κB transcription factor. In some cell types certain TLRs also engage other signaling pathways, such as the MAP kinase cascade, leading to activation of the AP-1 transcription factor. Some TLRs may use adapter proteins other than MyD88. The relative importance of these various pathways of TLR signaling, and the way the “choice” of pathways is made, are not well understood.

 

The genes that are expressed in response to TLR signaling encode proteins important in many different components of innate immune responses. These include inflammatory cytokines (TNF, IL-1, and IL-12), endothelial adhesion molecules (E-selectin), and proteins involved in microbial killing mechanisms (inducible nitric oxide synthase). The particular genes expressed will depend on the responding cell type.

A, Different TLRs are involved in responses to different microbial products. B, Signaling by a prototypic TLR, TLR4, in response to bacterial LPS. An adapter protein links the TLR to a kinase, which activates transcription factors such as NF-κB and AP-1. TIR, Toll/IL-1 receptor domain.

Complement proteins, are some of the most important plasma proteins of the innate immune system. Recall that in innate immunity, the complement system is activated by binding to microbes using the alternative and lectin pathways; in adaptive immunity, it is activated by binding to antibodies using the classical pathway. Mammalian cells express regulatory proteins that prevent inappropriate complement activation. Other circulating proteins of innate immunity are mannose-binding lectin and C-reactive protein, both of which coat microbes for phagocytosis and complement activation. Lung surfactant is also a component of innate immunity, providing protection against inhaled microbes.

The adaptive immune system consists of lymphocytes and their products, including antibodies. The receptors of lymphocytes are much more diverse than those of the innate immune system, but lymphocytes are not inherently specific for microbes, and they are capable of recognizing a vast array of foreign substances. In the remainder of this introductory section we focus on lymphocytes and the reactions of the adaptive immune system.

 

CELLS AND TISSUES OF THE IMMUNE SYSTEM

 

There are two main types of adaptive immunity-cell-mediated (or cellular) immunity, which is responsible for defense against intracellular microbes, and humoral immunity, which protects against extracellular microbes and their toxins. Cellular immunity is mediated by T (thymus-derived) lymphocytes, and humoral immunity is mediated by B (bone marrow-derived) lymphocytes and their secreted products, antibodies. All these mechanisms of adaptive immunity are capable of causing injury to the host and subsequent disease.

Humoral and cell-mediated immunity.

 

T Lymphocytes

T lymphocytes are generated from immature precursors in the thymus. Mature, naive T cells are found in the blood, where they constitute 60% to 70% of lymphocytes, and in T-cell zones of peripheral lymphoid organs, such as the paracortical areas of lymph nodes and periarteriolar sheaths of the spleen. The segregation of naive T cells to these anatomic sites is because the cells express receptors for chemoattractant cytokines (chemokines) that are produced only in these regions of lymphoid organs. Each T cell is genetically programmed to recognize a specific cell-bound antigen by means of an antigen-specific T-cell receptor (TCR). In approximately 95% of T cells, the TCR consists of a disulfide-linked heterodimer made up of an α and a β polypeptide chain, each having a variable (antigen-binding) and a constant region. The αβ TCR recognizes peptide antigens that are displayed by major histocompatibility complex (MHC) molecules on the surfaces of antigen-presenting cells. (The function of the MHC is described later.) T cells (in contrast to B cells) cannot be activated by soluble antigens; therefore, presentation of processed, membrane-bound antigens by antigen-presenting cells is required for induction of cell-mediated immunity.

 

Histology of a lymph node. A, The organization of the lymph node, with an outer cortex containing follicles and an inner medulla. B, The location of B cells (stained green, using the immunofluorescence technique) and T cells (stained red) in a lymph node. C, A germinal center.

 

Each TCR is noncovalently linked to a cluster of five polypeptide chains, three of which form the CD3 molecular complex and two are a dimer of the ζ chain. The CD3 and ζ proteins are invariant. They do not bind antigen but are involved in the transduction of signals into the T cell after the TCR has bound the antigen. T-cell receptors are capable of recognizing a very large number of peptides; each T cell expresses TCR molecules of one structure and specificity. TCR diversity is generated by somatic rearrangement of the genes that encode the TCR chains. As might be expected, every somatic cell has TCR genes from the germ line. Rearrangements of these genes occur only in T cells during their development in the thymus; hence the presence of TCR gene rearrangements demonstrated by molecular analysis is a marker of T-lineage cells. Such analyses are used in classification of lymphoid malignancies. Furthermore, because each T cell has a unique DNA rearrangement (and hence a unique TCR), it is possible to distinguish polyclonal (non-neoplastic) T-cell proliferations from monoclonal (neoplastic) T-cell proliferations.

 

 

A minority of mature T cells express another type of TCR composed of γ and δ polypeptide chains. The γδ TCR recognizes peptides, lipids, and small molecules, without a requirement for display by MHC proteins. γδ T cells tend to aggregate at epithelial surfaces, such as the mucosa of the respiratory and gastrointestinal tracts, suggesting that these cells are sentinels that protect against microbes that try to enter through these epithelia. However, the precise functions of γδ T cells are not known. Another small subset of T cells expresses markers that are found on natural killer (NK) cells; these cells are called NK-T cells. NK-T cells express a very limited diversity of TCRs, and they recognize glycolipids that are displayed by the MHC-like molecule CD1. The functions of NK-T cells are also not well defined.

 

The T-cell receptor (TCR) complex. A, Schematic illustration of TCRα and TCRβ chains linked to the CD3 complex. B, Recognition of MHC-associated peptide displayed on an antigen-presenting cell (top) by the TCR. Note that the TCR-associated ζ chains and CD3 complex deliver signals (signal 1) upon antigen recognition, and CD28 delivers signals (signal 2) upon recognition of costimulators (B7 molecules).

 

In addition to CD3 and ζ proteins, T cells express a number of nonpolymorphic, function-associated molecules, also called accessory molecules, including CD4, CD8, CD2, integrins, and CD28. CD4 and CD8 are expressed on two mutually exclusive subsets of αβ T cells. CD4 is expressed on approximately 60% of mature CD3+ T cells, whereas CD8 is expressed on about 30% of T cells. These T-cell membrane-associated glycoproteins serve as coreceptors in T-cell activation. During antigen presentation, CD4 molecules bind to the nonpolymorphic portions of class II MHC molecules expressed on antigen-presenting cells. In contrast, CD8 molecules bind to class I MHC molecules. CD4 and CD8 are required to initiate signals that activate T cells that recognize antigens. Because of this requirement for coreceptors, CD4+ helper T cells can recognize and respond to antigen only in the context of class II MHC molecules, whereas CD8+ cytotoxic T cells recognize cell-bound antigens only in association with class I MHC molecules. It is now well established that T cells need two signals for activation. Signal 1 is provided when the TCR is engaged by the appropriate MHC-bound antigen, and the coreceptors CD4 and CD8 bind to MHC molecules. Signal 2 is delivered by the interaction of the CD28 molecule on T cells with the costimulatory molecules B7-1 (CD80) and B7-2 (CD86) expressed on antigen-presenting cells. The importance of costimulation by this pathway is attested to by the fact that, in the absence of signal 2, the T cells fail to respond, undergo apoptosis, or become unreactive. When T cells are activated by antigen and costimulators, they secrete locally acting proteins called cytokines (described below). Under the influence of a cytokine called interleukin-2 (IL-2), the T cells proliferate, thus generating a large number of antigen-specific lymphocytes. Some of these cells differentiate into effector cells, which perform the function of eliminating the antigen that started the response. Other activated cells differentiate into memory cells, which are long-lived and poised to respond rapidly to repeat encounters with the antigen.

CD4+ and CD8+ T cells perform distinct but somewhat overlapping effector functions. The CD4+ T cell can be viewed as a master regulator-the conductor of a symphony orchestra, so to speak. By secreting cytokines, CD4+ T cells influence the function of virtually all other cells of the immune system, including other T cells, B cells, macrophages, and NK cells. The central role of CD4+ T cells is tragically illustrated when the human immunodeficiency virus cripples the immune system by selective destruction of this T-cell subset. In recent years, two functionally distinct populations of CD4+ helper cells have been recognized on the basis of the different cytokines they produce.¹¹ The T-helper-1 (T_H1) subset synthesizes and secretes IL-2 and interferon-γ (IFN-γ) but not IL-4 or IL-5, whereas T_H2 cells produce IL-4, IL-5 and IL-13 but not IL-2 or IFN-γ. This distinction is significant because the cytokines secreted by these subsets have different effects on other immune cells. The T_H1 subset is involved in facilitating delayed hypersensitivity, macrophage activation, and synthesis of opsonizing and complement-fixing antibodies, such as IgG2a in mice, all of which are actions of IFN-γ. The T_H2 subset aids in the synthesis of other classes of antibodies, notably IgE (mediated by IL-4 and IL-13) and in the activation of eosinophils (mediated by IL-5). CD8+ T cells function mainly as cytotoxic cells to kill other cells but, similar to CD4+ T cells, they can secrete cytokines, primarily of the T_H1 type.

 

B Lymphocytes

B lymphocytes develop from immature precursors in the bone marrow. Mature B cells constitute 10% to 20% of the circulating peripheral lymphocyte population and are also present in peripheral lymphoid tissues such as lymph nodes, spleen, or tonsils and extralymphatic organs such as the gastrointestinal tract. In lymph nodes, they are found in the superficial cortex. In the spleen, they are found in the white pulp. At both sites, they are aggregated in the form of lymphoid follicles, which on activation develop pale-staining germinal centers. B cells are located in follicles, the B-cell zones of lymphoid organs, because the cells express receptors for a chemokine that is produced in follicles.

Structure of antibodies and the B-cell antigen receptor. A, The B-cell receptor complex composed of membrane IgM (or IgD, not shown) and the associated signaling proteins Igα and Igβ. CD21 is a receptor for a complement component that also promotes B-cell activation. B, Crystal structure of a secreted IgG molecule, showing the arrangement of the variable (V) and constant (C) regions of the heavy (H) and light (L) chains. (Courtesy of Dr. Alex McPherson, University of California, Irvine, CA.)

 

B cells recognize antigen via the B-cell antigen receptor complex. Immunoglobulin M (IgM) and IgD, present on the surface of all naive B cells, constitute the antigen-binding component of the B-cell receptor complex. As with T cells, each B-cell receptor has unique antigen specificity, derived in part from somatic rearrangements of immunoglobulin genes. Thus, the presence of rearranged immunoglobulin genes in a lymphoid cell is used as a molecular marker of B-lineage cells. After antigenic stimulation, B cells form plasma cells that secrete immunoglobulins, which are the mediators of humoral immunity. In addition to membrane immunoglobulin, the B-cell antigen receptor complex contains a heterodimer of nonpolymorphic transmembrane proteins Igα and Igβ. Similar to the CD3 proteins of the TCR, Igα and Igβ do not bind antigen but are essential for signal transduction through the antigen receptor. B cells also express several other nonpolymorphic molecules that are essential for B-cell function. These include complement receptors, Fc receptors, and CD40. It is worthy of note that complement receptor-2 (CD21) is also the receptor for the Epstein-Barr virus (EBV), and hence EBV readily infects B cells.

B lymphocytes may be activated by protein and nonprotein antigens. The end result of B-cell activation is their differentiation into antibody-secreting cells, called plasma cells. Antibody-secreting cells reside in lymphoid organs and mucosal tissues, and some plasma cells may migrate to the bone marrow and live for many years in this tissue. Secreted antibodies enter mucosal secretions and the blood and are able to find, neutralize, and eliminate antigens. B-cell responses to protein antigens require help from CD4+ T cells. Helper T cells activate B cells by engaging CD40, a member of the tumor necrosis factor (TNF)-receptor family, and by secreting cytokines. Activated helper T cells express CD40 ligand, which specifically binds to CD40 expressed on B cells. This interaction is essential for B-cell maturation and secretion of IgG, IgA, and IgE antibodies. Patients with mutations in the CD40 ligand have an immunodeficiency disease called X-linked hyper-IgM syndrome, described later. Different cytokines stimulate B cells to produce different antibody classes, which perform distinct functions.

 

Macrophages

 

Macrophages are a part of the mononuclear phagocyte system; their origin, differentiation. Here we need only to emphasize that macrophages play important roles both in the induction and in the effector phase of immune responses.

·               Macrophages that have phagocytosed microbes and protein antigens process the antigens and present peptide fragments to T cells. Thus, macrophages are involved in the induction of cell-mediated immune responses.

·               Macrophages are important effector cells in certain forms of cell-mediated immunity, such as the delayed hypersensitivity reaction. As mentioned earlier, macrophages are activated by cytokines, notably IFN-γ produced by the T_H1 subset of CD4+ cells. Such activation enhances the microbicidal properties of macrophages and augments their ability to kill tumor cells.

·               Macrophages are also important in the effector phase of humoral immunity. Macrophages phagocytose microbes that are opsonized (coated) by IgG or C3b.

Dendritic Cells

 

The morphology and functions of dendritic cells (DC). A, The morphology of cultured dendritic cells. (Courtesy of Dr. Y-J. Liu, M. D. Anderson Cancer Center, Houston.) B, The location of dendritic cells (Langerhans cells) in the epidermis. (Courtesy of Dr. Y-J. Liu, M. D. Anderson Cancer Center, Houston.) C, The role of dendritic cells in capturing microbial antigens from epithelia and transporting them to regional lymph nodes.

 

There are two types of cells with dendritic morphology that are functionally quite different. Both have numerous fine dendritic cytoplasmic processes, from which they derive their name. One type is called interdigitating dendritic cells, or just dendritic cells. These cells are the most important antigen-presenting cells for initiating primary immune responses against protein antigens. Several features of dendritic cells account for their key role in antigen presentation. First, these cells are located at the right place to capture antigens-under epithelia, the common site of entry of microbes and foreign antigens, and in the interstitia of all tissues, where antigens may be produced. Immature dendritic cells within the epidermis are called Langerhans cells. Second, dendritic cells express many receptors for capturing and responding to microbes (and other antigens), including TLRs and mannose receptors. Third, in response to microbes, dendritic cells express the same chemokine receptor as do naive T cells and are thus recruited to the T-cell zones of lymphoid organs, where they are ideally located to present antigens to recirculating T cells. Fourth, dendritic cells express high levels of MHC class II molecules as well as the costimulatory molecules B7-1 and B7-2. Thus, they possess all the machinery needed for presenting antigens to and activating CD4+ T cells.

The other type of cells with dendritic morphology is present in the germinal centers of lymphoid follicles in the spleen and lymph nodes and is hence called follicular dendritic cells. These cells bear Fc receptors for IgG and receptors for C3b and can trap antigen bound to antibodies or complement proteins. Such cells play a role in ongoing immune responses by presenting antigens to B cells and selecting the B cells that have the highest affinity for the antigen, thus improving the quality of the humoral immune response. Follicular dendritic cells also play a role in the pathogenesis of the acquired immunodeficiency syndrome (AIDS) and are discussed in this context later in the chapter

 

Natural Killer Cells

 

NK cells make up approximately 10% to 15% of the peripheral blood lymphocytes and do not bear T-cell receptors or cell surface immunoglobulins. Morphologically, NK cells are somewhat larger than small lymphocytes, and they contain abundant azurophilic granules. Hence, they are also called large granular lymphocytes. NK cells are endowed with an innate ability to kill a variety of tumor cells, virally infected cells, and some normal cells, without previous sensitization.¹⁶ These cells are part of the innate immune system, and they may be the first line of defense against viral infections and, perhaps, some tumors. NK cells do not rearrange T-cell receptor genes and are CD3 negative. Two cell surface molecules, CD16 and CD56, are widely used to identify NK cells. CD16 is the Fc receptor for IgG and it endows NK cells with another function, the ability to lyse IgG-coated target cells. This phenomenon, known as antibody-dependent cell-mediated cytotoxicity, is described in greater detail later.

 

A highly activated natural killer cell with abundant cytoplasmic granules. (Courtesy of Dr. Noelle Williams, Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX.)

 

Schematic representation of NK-cell receptors and killing. NK cells express activating and inhibitory receptors; some examples of each are indicated. Normal cells are not killed because inhibitory signals from normal MHC class I molecules override activating signals. In tumor cells or virus-infected cells, there is increased expression of ligands for activating receptors, and reduced expression or alteration of MHC molecules, which interrupts the inhibitory signals, allowing activation of NK cells and lysis of target cells. KIR, killer cell Ig-like recepors.

 

The functional activity of NK cells is regulated by a balance between signals from activating and inhibitory receptors. The activating receptors stimulate NK cell killing by recognizing ill-defined molecules on target cells, some of which may be viral products; the inhibitory receptors inhibit the activation of NK cells by recognition of self-class I MHC molecules. The class I MHC-recognizing inhibitory receptors on NK cells are aptly called killer inhibitory receptors. They are biochemically distinct from T-cell receptors. It is believed that NK cells are inhibited from killing normal cells because all nucleated normal cells express self-class I MHC molecules. If virus infection or neoplastic transformation perturbs or reduces the expression of class I MHC molecules, inhibitory signals delivered to NK cells are interrupted, and lysis occurs. However, merely the absence of inhibition is not sufficient for NK cell-mediated killing, and NK cell-mediated killing requires triggering of activating receptors in conjunction with release of inhibitory receptors. Several types of activating receptors have been discovered, including members of the NKG2D family and some Ig-like receptors. The NKG2D receptors recognize stress-induced proteins that are normally expressed by only a few cells in the gut epithelium but whose expression increases on many cells following viral infection or neoplastic transformation. Other activating receptors recognize viral proteins that are structurally similar to class I MHC molecules. Thus, NK cells are activated by contact with virus-infected and tumor cells, both of which often express reduced levels of class I MHC molecules and therefore do not engage inhibitory receptors.

NK cells also secrete cytokines, such as IFN-γ, TNF, and granulocyte macrophage colony-stimulating factor (GM-CSF). IFN-γ activates macrophages to destroy ingested microbes, and thus NK cells provide early defense against intracellular microbial infections. IFN-γ also promotes the differentiation of naive CD4+ T-cells into T_H1 cells. Thus, activation of NK cells early in the immune response can favor induction of delayed hypersensitivity and secretion of opsonizing antibodies by promoting the development of T_H1 cells. The activity of NK cells is regulated by many cytokines, including IL-2, IL-15, and IL-12. IL-2 and IL-15 stimulate proliferation of NK cells, whereas IL-12 activates killing and secretion of IFN-γ.

 

CYTOKINES: MESSENGER MOLECULES OF THE IMMUNE SYSTEM

 

The induction and regulation of immune responses involve multiple interactions among lymphocytes, monocytes, inflammatory cells (e.g., neutrophils), and endothelial cells. Many such interactions depend on cell-to-cell contact; however, many interactions and effector functions are mediated by short-acting soluble mediators, called cytokines. This term includes the previously designated lymphokines (lymphocyte-derived), monokines (monocyte-derived), and several other polypeptides that regulate immunologic, inflammatory, and reparative host responses. Molecularly defined cytokines are called interleukins, implying that they mediate communications between leukocytes. Most cytokines have a wide spectrum of effects, and some are produced by several different cell types.

A large number of cytokines have been identified by molecular cloning, and the list continues to grow. Below we summarize the main cytokines whose functions are well established. It is convenient to classify these mediators into distinct functional classes, although many belong to multiple categories.

·               Cytokines that mediate innate (natural) immunity. Included in this group are IL-1, TNF (tumor necrosis factor, also called TNF-α), type 1 interferons, and IL-6. Some cytokines, such as IL-12 and IFN-γ, are involved in both innate and adaptive immunity against intracellular microbes. Certain of these cytokines (e.g., the interferons) protect against viral infections, whereas others (e.g., IL-1 and TNF) promote leukocyte recruitment and acute inflammatory responses.

·               Cytokines that regulate lymphocyte growth, activation, and differentiation. Within this category are IL-2, IL-4, IL-12, IL-15, and transforming growth factor-β (TGF-β). IL-2 is an important growth factor for T-cells, IL-4 stimulates differentiation to the T_H2 pathway and acts on B cells as well, IL-12 stimulates differentiation to the T_H1 pathway, and IL-15 stimulates the growth and activity of NK cells. Other cytokines in this group, such as IL-10 and TGF-β, down-regulate immune responses.

·               Cytokines that activate inflammatory cells. In this category are IFN-γ, which activates macrophages; IL-5, which activates eosinophils; and TNF and lymphotoxin (also called TNF-β), which induce acute inflammation by acting oeutrophils and endothelial cells.

·               Cytokines that affect leukocyte movement are also called chemokines. Most fall into two structurally distinct subfamilies, referred to as C-C and C-X-C chemokines, on the basis of the position of cysteine (c) residues. The C-X-C chemokines are produced mainly by activated macrophages and tissue cells (e.g., endothelium), whereas the C-C chemokines are produced largely by T cells. Different chemokines recruit different types of leukocytes to sites of inflammation. Chemokines are also normally produced in tissues and are responsible for the anatomic localization of different cell types, for example, the location of T and B cells in distinct regions of lymphoid organs.

·               Cytokines that stimulate hematopoiesis. Many cytokines derived from lymphocytes or stromal cells stimulate the growth and production of new blood cells by acting on hematopoietic progenitor cells. Several members of this family are called colony-stimulating factors (CSFs) because they were initially detected by their ability to promote the in vitro growth of hematopoietic cell colonies from the bone marrow. Some members of this group (e.g., GM-CSF and G-CSF) act on committed progenitor cells, whereas others, exemplified by stem cell factor (c-kit ligand), act on pluripotent stem cells.

General Properties of Cytokines

 

Although cytokines have many diverse actions, all of them share some important properties.

·               Many individual cytokines are produced by several different cell types. For example, IL-1 can be produced by virtually any cell leukocytes, endothelial cells, and fibroblasts.

·               The actions of cytokines are pleiotropic, meaning that any one cytokine may act on many cell types and mediate many effects. For example, IL-2, initially discovered as a T-cell growth factor, is known to affect the growth and differentiation of B cells and NK cells as well. Cytokines are also often redundant, meaning that different cytokines may stimulate the same or overlapping biologic responses.

·               Cytokines induce their effects in three ways: (1) They act on the same cell that produces them (autocrine effect), such as occurs when IL-2 produced by antigen-stimulated T cells stimulates the growth of the same cells; (2) they affect other cells in their vicinity (paracrine effect), as occurs when IL-7 produced by bone marrow or thymic stromal cells promotes the maturation of B-cell progenitors in the marrow or T-cell precursors in the thymus, respectively; and (3) they affect many cells systemically (endocrine effect), the best examples in this category being IL-1 and TNF, which produce the systemic acute-phase response during inflammation.

·               Cytokines mediate their effects by binding to specific high-affinity receptors on their target cells. For example, IL-2 activates T cells by binding to high-affinity IL-2 receptors (IL-2R). Blockade of the IL-2R by specific antireceptor monoclonal antibodies prevents T-cell activation. This observation is the basis for the use of anti-IL-2R antibodies to control undesirable T-cell activation, as in transplant rejection.

The knowledge gained about cytokines has practical therapeutic ramifications. First, by inhibiting cytokine production or action, it may be possible to control the harmful effects of inflammation or tissue-damaging immune reactions. Patients with rheumatoid arthritis often show dramatic responses to TNF antagonists, an elegant example of such therapy. Second, recombinant cytokines can be administered to enhance immunity against cancer or microbial infections (immunotherapy).

 

STRUCTURE AND FUNCTION OF HISTOCOMPATIBILITY MOLECULES

Although originally identified as antigens that evoke rejection of transplanted organs, histocompatibility molecules are now known to be extremely important for the induction and regulation of the immune response. The principal physiologic function of the cell surface histocompatibility molecules is to bind peptide fragments of foreign proteins for presentation to antigen-specific T cells. Recall that T cells (in contrast to B cells) can recognize only membrane-bound antigens, and hence histocompatibility molecules are critical to the induction of T-cell immunity. Here we summarize the salient features of human histocompatibility molecules, primarily to facilitate understanding of their role in rejection of organ transplants and in disease susceptibility.

In humans, the genes encoding the most important histocompatibility molecules are clustered on a small segment of chromosome 6, the major histocompatibility complex, or the human leukocyte antigen (HLA) complex in humans, so named because MHC-encoded antigens were initially detected on leukocytes. The HLA system is highly polymorphic, meaning that there are many alleles of each MHC gene in the population and each individual inherits one (often unique) set of these alleles. This, as we see subsequently, constitutes a formidable barrier in organ transplantation.

On the basis of their chemical structure, tissue distribution, and function, the MHC gene products are classified into three categories. Class I and class II genes encode cell surface glycoproteins involved in antigen presentation. Class III genes encode components of the complement system, and are not discussed in this section.

 

 

The HLA complex and the structure of HLA molecules. A, The location of genes in the HLA complex is shown. The sizes and distances between genes are not to scale. B, Schematic diagrams and crystal structures of class I and class II HLA molecules. (Crystal structures are courtesy of Dr. P. Bjorkman, California Institute of Technology, Pasadena, CA.)

 

Antigen processing and recognition. The sequence of events in the processing of a cytoplasmic protein antigen and its display by class I MHC molecules are shown at the top. The recognition of this MHC-displayed peptide by a CD8+ T cell is shown at the bottom.

 

Class I MHC molecules are expressed on all nucleated cells and platelets. They are encoded by three closely linked loci, designated HLA-A, HLA-B, and HLA-C. Each of these molecules is a heterodimer, consisting of a polymorphic α, or heavy, chain (44-kD) linked noncovalently to a smaller (12-kD) nonpolymorphic peptide called β₂-microglobulin, which is not encoded within the MHC. The extracellular region of the heavy chain is divided into three domains: α₁, α₂, and α₃. Crystal structure of class I molecules has revealed that the α₁ and α₂ domains form a cleft, or groove, where peptides bind to the MHC molecule. Biochemical analyses of several different class I alleles have revealed that almost all polymorphic residues line the sides or the base of the peptide-binding groove. As a result, different class I alleles bind and display different peptide fragments. In general, class I MHC molecules bind and display peptides that are derived from proteins, such as viral antigens, synthesized within the cell. The generation of peptide fragments within the cells, and their association with MHC molecules and transport to the cell surface, is a complex process. Involved in this sequence are proteolytic complexes (proteasomes), which digest antigenic proteins in the cytoplasm into short peptides, and transport proteins, which ferry peptide fragments from the cytoplasm to the endoplasmic reticulum. Within the endoplasmic reticulum, peptides bind to the antigen-binding cleft of newly synthesized class I heavy chains, which then associate with β₂-microglobulin to form a stable trimer that is transported to the cell surface for presentation to CD8+ cytotoxic T lymphocytes. In this interaction, the TCR recognizes the MHC-peptide complex, and the CD8 molecule, acting as a coreceptor, binds to the nonpolymorphic α₃ domain of the class I heavy chain. CD8+ cytotoxic T cells can recognize viral (or other) peptides only if presented as a complex with self-class I antigens, and therefore CD8+ T cells are said to be class I MHC-restricted. In the eyes of T cells, self-MHC molecules are those that they “grew up with” during maturation within the thymus. Because one of the important functions of CD8+ T cells is to eliminate viruses, which may infect any nucleated cell, it makes good sense to have widespread expression of class I HLA molecules.

Class II MHC molecules are coded for in a region called HLA-D, which has three subregions: HLA-DP, HLA-DQ, and HLA-DR. Each class II molecule is a heterodimer consisting of a noncovalently associated α chain and β chain. Both chains are polymorphic, and each of the three HLA-D subregions encodes one α chain and one β chain. The extracellular portions of the α and β chains have two domains each: α₁, α₂ and β₁, β₂. Crystal structure of class II molecules has revealed that, similar to class I molecules, they have an antigen-binding cleft facing outward. In contrast to class I molecules, however, the antigen-binding cleft is formed by an interaction of the α₁ and β₁ domains of both chains, and it is in this portion that most class II alleles differ. Thus, it seems that, as with class I molecules, polymorphism of class II molecules is associated with differential binding of antigenic peptides. The nature of peptides that bind to class II molecules is different from that of peptides that bind to class I molecules. In general, class II molecules present exogenous antigens (e.g., extracellular microbes, soluble proteins) that are first internalized and processed in the endosomes or lysosomes. Peptides resulting from proteolytic cleavage then associate with class II heterodimers that were assembled in the endoplasmic reticulum and transported into the vesicles. Finally, the peptide-MHC complex is transported to the cell surface, where it can be recognized by CD4+ helper T cells. In this interaction, the CD4 molecule acts as the coreceptor. Because CD4+ T cells can recognize antigens only in the context of self-class II molecules, they are referred to as class II MHC-restricted. In contrast to class I molecules, the tissue distribution of MHC class II molecules is largely restricted to antigen-presenting cells (macrophages, dendritic cells, and B cells). Expression of class II molecules can be induced on several other cell types, however, including endothelial cells and fibroblasts, by the action of IFN-γ.

MHC molecules play key roles in regulating T cell-mediated immune responses in two ways. First, because different antigenic peptides bind to different class II gene products, it follows that an individual mounts a vigorous immune response against an antigen only if he or she inherits the gene(s) for those class II molecule(s) that can bind the antigen and present it to helper T cells. The consequences of inheriting a given class II gene depend on the nature of the antigen bound by the class II molecule. For example, if the antigen is a peptide from ragweed pollen, the individual who expresses class II molecules capable of binding the antigen would be genetically prone to allergic reactions against pollen. In contrast, an inherited capacity to bind a bacterial peptide may provide resistance to disease by evoking a protective antibody response. Second, during their maturation in the thymus, only T cells that can recognize self-MHC molecules are selected for export to the periphery. Thus, the type of MHC molecules that T cells encounter during their development influences the reactivity of mature peripheral T cells.

 

HLA and Disease Association

 

A variety of diseases have been found to be associated with certain HLA alleles. The best known is the association between ankylosing spondylitis and HLA-B27; individuals who inherit this allele have a 90-fold greater chance (relative risk) of developing the disease than those who are negative for HLA-B27. The diseases that show association with the HLA locus can be broadly grouped into the following categories:

1.           Inflammatory diseases, including ankylosing spondylitis and several postinfectious arthropathies, all associated with HLA-B27

2.           Inherited errors of metabolism, such as 21-hydroxylase deficiency (HLA-BW47) and hereditary hemochromatosis (HLA-A)

3.           Autoimmune diseases, including autoimmune endocrinopathies, associated mainly with alleles at the DR locus.

The mechanisms underlying these associations are not fully understood. In some cases (e.g., 21-hydroxylase deficiency), the linkage results from the fact that the relevant disease-associated gene, in this case the gene for 21-hydroxylase, maps within the HLA complex. Similarly, in hereditary hemochromatosis, a gene that is mutated, called HFE, maps within the HLA locus. HFE resembles MHC molecules structurally, but its function is not in the presentation of antigens to T cells but in the regulation of iron transport. In the case of immunologically mediated disorders, it seems likely that the role of HLA class II molecules in regulating immune responsiveness may be relevant.

 

Disorders of the Immune System

 

 

 

Having reviewed some fundamentals of basic immunology, we caow turn to general features of immunologic tissue injury and immunopathology, and some specific immunologic diseases. Our discussion is divided into four broad headings:

• Hypersensitivity reactions, which give rise to immunologic injury in a variety of diseases, discussed throughout this book

• Autoimmune diseases, which are caused by immune reactions against self

• Immunologic deficiency syndromes, which result from genetically determined or acquired defects in some components of the normal immune system

• Amyloidosis, a poorly understood disorder having immunologic association

MECHANISMS OF HYPERSENSITIVITY REACTIONS

Humans live in an environment teeming with substances capable of producing immunologic responses. Contact with antigen leads not only to induction of a protective immune response, but also to reactions that can be damaging to tissues. Exogenous antigens occur in dust, pollens, foods, drugs, microbiologic agents, chemicals, and many blood products used in clinical practice. The immune responses that may result from such exogenous antigens take a variety of forms, ranging from annoying but trivial discomforts, such as itching of the skin, to potentially fatal diseases, such as bronchial asthma. The various reactions produced are called hypersensitivity reactions, and tissue injury in these reactions may be caused by humoral or cell-mediated immune mechanisms.

Injurious immune reactions may be evoked not only by exogenous environmental antigens, but also by endogenous tissue antigens. Some of these immune reactions are triggered by homologous antigens that differ among individuals with different genetic backgrounds. Transfusion reactions and graft rejection are examples of immunologic disorders evoked by homologous antigens. Another category of disorders, those incited by self-, or autologous, antigens, constitutes the important group of autoimmune diseases (discussed later). These diseases arise because of the emergence of immune responses against self-antigens.

Hypersensitivity diseases can be classified on the basis of the immunologic mechanism that mediates the disease. This classification is of value in distinguishing the manner in which the immune response ultimately causes tissue injury and disease, and the accompanying pathologic alterations. Prototypes of each of these immune mechanisms are presented in the subsequent sections.

• In immediate hypersensitivity (type I hypersensitivity), the immune response releases vasoactive and spasmogenic substances that act on vessels and smooth muscle and pro-inflammatory cytokines that recruit inflammatory cells.

• In antibody-mediated disorders (type II hypersensitivity), secreted antibodies participate directly in injury to cells by promoting their phagocytosis or lysis and injury to tissues by inducing inflammation. Antibodies may also interfere with cellular functions and cause disease without tissue injury.

• In immune complex-mediated disorders (type III hypersensitivity), antibodies bind antigens and then induce inflammation directly or by activating complement. The leukocytes that are recruited (neutrophils and monocytes) produce tissue damage by release of lysosomal enzymes and generation of toxic free radicals.

• In cell-mediated immune disorders (type IV hypersensitivity), sensitized T lymphocytes are the cause of the cellular and tissue injury

 

 

Most hypersensitivity diseases show a genetic predisposition. Modern methods of mapping disease-associated susceptibility genes are revealing the complex nature of these genetic influences. Many susceptibility loci have been identified in different diseases. Among the genes known to be associated with hypersensitivity diseases are MHC genes, but many non-MHC genes also play a role.

 

Immediate (Type I) Hypersensitivity

Immediate, or type I, hypersensitivity is a rapidly developing immunologic reaction occurring within minutes after the combination of an antigen with antibody bound to mast cells in individuals previously sensitized to the antigen. These reactions are often called allergy, and the antigens that elicit them are allergens. Immediate hypersensitivity may occur as a systemic disorder or as a local reaction. The systemic reaction usually follows injection of an antigen to which the host has become sensitized. Often within minutes, a state of shock is produced, which is sometimes fatal. The nature of local reactions varies depending on the portal of entry of the allergen and may take the form of localized cutaneous swellings (skin allergy, hives), nasal and conjunctival discharge (allergic rhinitis and conjunctivitis), hay fever, bronchial asthma, or allergic gastroenteritis (food allergy). Many local type I hypersensitivity reactions have two well-defined phases. The immediate, or initial, response is characterized by vasodilation, vascular leakage, and depending on the location, smooth muscle spasm or glandular secretions. These changes usually become evident within 5 to 30 minutes after exposure to an allergen and tend to subside in 60 minutes. In many instances (e.g., allergic rhinitis and bronchial asthma), a second, late-phase reaction sets in 2 to 24 hours later without additional exposure to antigen and may last for several days. This late-phase reaction is characterized by infiltration of tissues with eosinophils, neutrophils, basophils, monocytes, and CD4+ T cells as well as tissue destruction, typically in the form of mucosal epithelial cell damage.

Immediate hypersensitivity. A, Kinetics of the immediate and late-phase reactions. The immediate vascular and smooth muscle reaction to allergen develops within minutes after challenge (allergen exposure in a previously sensitized individual), and the late-phase reaction develops 2 to 24 hours later. B, C, Morphology: The immediate reaction (B) is characterized by vasodilation, congestion, and edema, and the late phase reaction (C) is characterized by an inflammatory infiltrate rich in eosinophils, neutrophils, and T cells. (Courtesy of Dr. Daniel Friend, Department of Pathology, Brigham and Women’s Hospital, Boston, MA.)

 

Because mast cells are central to the development of immediate hypersensitivity, we first review some of their salient characteristics and then discuss the immune mechanisms that underlie this form of hypersensitivity.²² Mast cells are bone marrow-derived cells that are widely distributed in the tissues. They are found predominantly near blood vessels and nerves and in subepithelial sites, where local immediate hypersensitivity reactions tend to occur. Mast cells have cytoplasmic membrane-bound granules that contain a variety of biologically active mediators. In addition, mast-cell granules contain acidic proteoglycans that bind basic dyes such as toluidine blue. Because the stained granules often acquire a color that is different from that of the native dye, they are referred to as metachromatic granules. As is detailed next, mast cells (and basophils) are activated by the cross-linking of high-affinity IgE Fc receptors; in addition, mast cells may also be triggered by several other stimuli, such as complement components C5a and C3a (anaphylatoxins), both of which act by binding to their receptors on the mast-cell membrane. Other mast-cell secretagogues include macrophage-derived cytokines (e.g., IL-8), some drugs such as codeine and morphine, adenosine, mellitin (present in bee venom), and physical stimuli (e.g., heat, cold, sunlight). Basophils are similar to mast cells in many respects, including the presence of cell-surface IgE Fc receptors as well as cytoplasmic granules. In contrast to mast cells, however, basophils are not normally present in tissues but rather circulate in the blood in extremely small numbers. (Most allergic reactions occur in tissues, and the role of basophils in these reactions is not as well established as that of mast cells.) Similar to other granulocytes, basophils can be recruited to inflammatory sites.

Pathogenesis of immediate (type I) hypersensitivity reaction. The late-phase reaction is dominated by leukocyte infiltration and tissue injury. T_H2, T-helper type 2 CD4 cells.

 

Most immediate hypersensitivity reactions are mediated by IgE antibodies. IgE-secreting B cells differentiate from naive (membrane IgM and IgD-expressing) B cells, and this process is dependent on the activity of CD4+ helper T cells of the T_H2 type. Hence, T_H2 cells are pivotal in the pathogenesis of type I hypersensitivity. The first step in the synthesis of IgE is the presentation of the antigen to naive CD4+ helper T cells by dendritic cells that capture the antigen from its site of entry. In response to antigen and other stimuli, including cytokines produced at the local site, the T cells differentiate into T_H2 cells. The newly minted T_H2 cells produce a cluster of cytokines upon subsequent encounter with the antigen; as we mentioned earlier, the signature cytokines of this subset are IL-4, IL-5, and IL-13. IL-4 is essential for turning on the IgE-producing B cells and for sustaining the development of T_H2 cells. IL-5 activates eosinophils, which, as we discuss subsequently, are important effectors of type I hypersensitivity. IL-13 promotes IgE production and acts on epithelial cells to stimulate mucus secretion. In addition, T_H2 cells and epithelial cells produce chemokines that attract more T_H2 cells, as well as eosinophils and occasionally basophils, to the reaction site.

Mast cells and basophils express high-affinity receptors for the Fc portion of IgE, and therefore avidly bind IgE antibodies. When a mast cell, armed with cytophilic IgE antibodies, is re-exposed to the specific allergen, a series of reactions takes place, leading eventually to the release of a variety of powerful mediators responsible for the clinical expression of immediate hypersensitivity reactions. In the first step in this sequence, antigen (allergen) binds to the IgE antibodies previously attached to the mast cells. Multivalent antigens bind to more than one IgE molecule and thus cross-link adjacent IgE antibodies and the underlying IgE Fc receptors. The bridging of IgE molecules activates signal transduction pathways from the cytoplasmic portion of the IgE Fc receptors. These signals initiate two parallel and interdependent processes-one leading to mast cell degranulation with discharge of preformed (primary) mediators that are stored in the granules, and the other involving de novo synthesis and release of secondary mediators. These mediators are directly responsible for the initial, sometimes explosive, symptoms of immediate hypersensitivity, and they also set into motion the events that lead to the late-phase response. In addition to inducing mediator release and production, signals from IgE Fc receptors promote the survival of mast cells and can enhance expression of the Fc receptor, providing an amplification mechanism.

 

Primary Mediators. Primary mediators contained within mast-cell granules can be divided into three categories:

• Biogenic amines. The most important vasoactive amine is histamine. Histamine causes intense smooth muscle contraction, increased vascular permeability, and increased secretion by nasal, bronchial, and gastric glands.

• Enzymes. These are contained in the granule matrix and include neutral proteases (chymase, tryptase) and several acid hydrolases. The enzymes cause tissue damage and lead to the generation of kinins and activated components of complement (e.g., C3a) by acting on their precursor proteins.

• Proteoglycans. These include heparin, a well-known anticoagulant, and chondroitin sulfate. The proteoglycans serve to package and store the other mediators in the granules.

Activation of mast cells in immediate hypersensitivity and release of their mediators. ECF, eosinophil chemotactic factor; NCF, neutrophil chemotactic factor; PAF, platelet-activating factor.

 

Secondary Mediators. Secondary mediators include two classes of compounds (1) lipid mediators and (2) cytokines. The lipid mediators are generated by sequential reactions in the mast-cell membranes that lead to activation of phospholipase A₂, an enzyme that acts on membrane phospholipids to yield arachidonic acid. This is the parent compound from which leukotrienes and prostaglandins are derived by the 5-lipoxygenase and cyclooxygenase pathways.

• Leukotrienes. Leukotrienes C₄ and D₄ are the most potent vasoactive and spasmogenic agents known. On a molar basis, they are several thousand times more active than histamine in increasing vascular permeability and causing bronchial smooth muscle contraction. Leukotriene B₄ is highly chemotactic for neutrophils, eosinophils, and monocytes.

• Prostaglandin D₂. This is the most abundant mediator derived by the cyclooxygenase pathway in mast cells. It causes intense bronchospasm as well as increased mucus secretion.

• Platelet-activating factor (PAF). PAF is produced by some mast-cell populations. It causes platelet aggregation, release of histamine, bronchospasm, increased vascular permeability, and vasodilation. In addition, it has important pro-inflammatory actions. PAF is chemotactic for neutrophils and eosinophils. At high concentrations, it activates the newly recruited inflammatory cells, causing them to aggregate and degranulate. Because of its ability to recruit and activate inflammatory cells, it is considered important in the initiation of the late-phase response. Although the production of PAF is also triggered by the activation of phospholipase A₂, it is not a product of arachidonic acid metabolism.

• Cytokines. Mast cells are sources of many cytokines, which play an important role in the late-phase reaction of immediate hypersensitivity because of their ability to recruit and activate inflammatory cells. The cytokines include TNF, IL-1, IL-3, IL-4, IL-5, IL-6, and GM-CSF, as well as chemokines, such as macrophage inflammatory protein (MIP)-1α and MIP-1β. Mast cell-derived TNF and chemokines are important mediators of the inflammatory response seen at the site of allergic inflammation. Inflammatory cells that accumulate at the sites of type I hypersensitivity reactions are additional sources of cytokines and of histamine-releasing factors that cause further mast-cell degranulation.

The development of immediate hypersensitivity reactions is dependent on the coordinated actions of a variety of chemotactic, vasoactive, and spasmogenic compounds. Some, such as histamine and leukotrienes, are released rapidly from sensitized mast cells and are responsible for the intense immediate reactions characterized by edema, mucus secretion, and smooth muscle spasm; others, exemplified by cytokines, set the stage for the late-phase response by recruiting additional leukocytes. Not only do these inflammatory cells release additional waves of mediators (including cytokines), but they also cause epithelial-cell damage. Epithelial cells themselves are not passive bystanders in this reaction; they can also produce soluble mediators, such as IL-6, IL-8, and GM-CSF.

 

 

 

Among the cells that are recruited in the late-phase reaction, eosinophils are particularly important. They are recruited to sites of immediate hypersensitivity reactions by chemokines, such as eotaxin and others, that may be produced by epithelial cells under the influence of mediators such as TNF, T_H2 cells, and mast cells. The survival of eosinophils in tissues is favored by IL-3, IL-5, and GM-CSF, and IL-5 is the most potent eosinophil-activating cytokine known. These cytokines, as mentioned earlier, are derived from T_H2 cells and mast cells. The armamentarium of eosinophils is as extensive as that of mast cells, and in addition they produce major basic protein and eosinophil cationic protein, which are toxic to epithelial cells. Activated eosinophils and other leukocytes also produce leukotriene C₄ and PAF and directly activate mast cells to release mediators. Thus, the recruited cells amplify and sustain the inflammatory response without additional exposure to the triggering antigen. It is now believed that this late-phase inflammatory response is a major cause of symptoms in some type I hypersensitivity disorders, such as allergic asthma. Therefore, treatment of these diseases requires the use of broad-spectrum anti-inflammatory drugs, such as steroids.

A final point that should be mentioned in this general discussion of immediate hypersensitivity is that susceptibility to these reactions is genetically determined. The term atopy refers to a predisposition to develop localized immediate hypersensitivity reactions to a variety of inhaled and ingested allergens. Atopic individuals tend to have higher serum IgE levels, and more IL-4-producing T_H2 cells, compared with the general population. A positive family history of allergy is found in 50% of atopic individuals. The basis of familial predisposition is not clear, but studies in patients with asthma reveal linkage to several gene loci. Candidate genes have been mapped to 5q31, where genes for the cytokines IL-3, IL-4, IL-5, IL-9, IL-13, and GM-CSF are located, consistent with the idea that these cytokines are involved in the reactions. Linkage has also beeoted to 6p, close to the HLA complex, suggesting that the inheritance of certain HLA alleles permits reactivity to certain allergens. Another asthma-associated locus is on chromosome 11q13, the location of the gene encoding the β chain of the high-affinity IgE receptor, but many studies have failed to establish a linkage of atopy with the FcεRI β chain or even this chromosomal region.

To summarize, immediate (type I) hypersensitivity is a complex disorder resulting from an IgE-mediated triggering of mast cells and subsequent accumulation of inflammatory cells at sites of antigen deposition. These events are regulated in large part by the induction of T_H2-type helper T cells that promote synthesis of IgE and accumulation of inflammatory cells, particularly eosinophils. The clinical features result from release of mast-cell mediators as well as the accumulation of an eosinophil-rich inflammatory exudate. With this consideration of the basic mechanisms of type I hypersensitivity, we turn to some conditions that are important examples of IgE-mediated disease.

 

Systemic Anaphylaxis

Systemic anaphylaxis is characterized by vascular shock, widespread edema, and difficulty in breathing. In humans, systemic anaphylaxis may occur after administration of foreign proteins (e.g., antisera), hormones, enzymes, polysaccharides, and drugs (such as the antibiotic penicillin). The severity of the disorder varies with the level of sensitization. Extremely small doses of antigen may trigger anaphylaxis, for example, the tiny amounts used in ordinary skin testing for various forms of allergies. Within minutes after exposure, itching, hives, and skin erythema appear, followed shortly thereafter by a striking contraction of respiratory bronchioles and respiratory distress. Laryngeal edema results in hoarseness. Vomiting, abdominal cramps, diarrhea, and laryngeal obstruction follow, and the patient may go into shock and even die within the hour. The risk of anaphylaxis must be borne in mind when certain therapeutic agents are administered. Although patients at risk can generally be identified by a previous history of some form of allergy, the absence of such a history does not preclude the possibility of an anaphylactic reaction.

 

Local Immediate Hypersensitivity Reactions

 

Local immediate hypersensitivity, or allergic, reactions are exemplified by so-called atopic allergy. About 10% of the population suffers from allergies involving localized reactions to common environmental allergens, such as pollen, animal dander, house dust, foods, and the like. Specific diseases include urticaria, angioedema, allergic rhinitis (hay fever), and some forms of asthma, all discussed elsewhere in this book. The familial predisposition to the development of this type of allergy has been mentioned earlier.

Antibody-Mediated (Type II) Hypersensitivity

Type II hypersensitivity is mediated by antibodies directed toward antigens present on cell surfaces or extracellular matrix. The antigenic determinants may be intrinsic to the cell membrane or matrix, or they may take the form of an exogenous antigen, such as a drug metabolite, that is adsorbed on a cell surface or matrix. In either case, the hypersensitivity reaction results from the binding of antibodies to normal or altered cell-surface antigens. Most of these reactions involve the effector mechanisms that are used by antibodies, namely the complement system and phagocytes.

Opsonization and Complement-and Fc Receptor-Mediated Phagocytosis

The depletion of cells targeted by antibodies is, to a large extent, because the cells are coated (opsonized) with molecules that make them attractive for phagocytes. When antibodies are deposited on the surfaces of cells, they may activate the complement system (if the antibodies are of the IgM or IgG class). Complement activation generates byproducts, mainly C3b and C4b, which are deposited on the surfaces of the cells and recognized by phagocytes that express receptors for these proteins. In addition, cells opsonized by IgG antibodies are recognized by phagocyte Fc receptors, which are specific for the Fc portions of some IgG subclasses. The net result is the phagocytosis of the opsonized cells and their destruction. Complement activation on cells also leads to the formation of the membrane attack complex, which disrupts membrane integrity by “drilling holes” through the lipid bilayer, thereby causing osmotic lysis of the cells.

Antibody-mediated destruction of cells may occur by another process called antibody-dependent cellular cytotoxicity (ADCC). This form of antibody-mediated cell injury does not involve fixation of complement but instead requires the cooperation of leukocytes. Cells that are coated with low concentrations of IgG antibody are killed by a variety of effector cells, which bind to the target by their receptors for the Fc fragment of IgG, and cell lysis proceeds without phagocytosis. ADCC may be mediated by monocytes, neutrophils, eosinophils, and NK cells. Although, in most instances, IgG antibodies are involved in ADCC, in certain cases (e.g., eosinophil-mediated cytotoxicity against parasites), IgE antibodies are used. The role of ADCC in hypersensitivity diseases is uncertain.

Clinically, antibody-mediated cell destruction and phagocytosis occur in the following situations: (1) transfusion reactions, in which cells from an incompatible donor react with and are opsonized by preformed antibody in the host; (2) erythroblastosis fetalis, in which there is an antigenic difference between the mother and the fetus, and antibodies (of the IgG class) from the mother cross the placenta and cause destruction of fetal red cells; (3) autoimmune hemolytic anemia, agranulocytosis, and thrombocytopenia, in which individuals produce antibodies to their own blood cells, which are then destroyed; and (4) certain drug reactions, in which antibodies are produced that react with the drug, which may be attached to the surface of erythrocytes or other cells.

 

Complement-and Fc Receptor-Mediated Inflammation

When antibodies deposit in extracellular tissues, such as basement membranes and matrix, the resultant injury is because of inflammation and not because of phagocytosis or lysis of cells. The deposited antibodies activate complement, generating byproducts, such as C5a (and to a lesser extent C4a and C3a), that recruit neutrophils and monocytes. The same cells also bind to the deposited antibodies via their Fc receptors. The leukocytes are activated, they release injurious substances, such as enzymes and reactive oxygen intermediates, and the result is damage to the tissues. It was once thought that complement was the major mediator of antibody-induced inflammation, but knockout mice lacking Fc receptors also show striking reduction in these reactions. It is now believed that inflammation in antibody-mediated (and immune complex-mediated) diseases is because of both complement and Fc receptor-dependent reactions.

Antibody-mediated inflammation is the mechanism responsible for tissue injury in some forms of glomerulonephritis, vascular rejection in organ grafts, and other diseases. As we shall discuss in more detail below, the same reaction is involved in immune complex-mediated diseases.

 

Antibody-Mediated Cellular Dysfunction

In some cases, antibodies directed against cell-surface receptors impair or dysregulate function without causing cell injury or inflammation. For example, in myasthenia gravis, antibodies reactive with acetylcholine receptors in the motor end-plates of skeletal muscles impair neuromuscular transmission and therefore cause muscle weakness. In pemphigus vulgaris, antibodies against desmosomes disrupt intercellular junctions in epidermis, leading to the formation of skin vesicles. The converse (i.e., antibody-mediated stimulation of cell function) is noted in Graves disease. In this disorder, antibodies against the thyroid-stimulating hormone receptor on thyroid epithelial cells stimulate the cells, resulting in hyperthyroidism.

 

Immune Complex-Mediated (Type III) Hypersensitivity

 

Schematic illustration of the three major mechanisms of antibody-mediated injury. A, Opsonization of cells by antibodies and complement components and ingestion by phagocytes. B, Inflammation induced by antibody binding to Fc receptors of leukocytes and by complement breakdown products. C, Antireceptor antibodies disturb the normal function of receptors. In these examples, antibodies against the thyroid stimulating hormone (TSH) receptor activate thyroid cells in Graves disease, and acetylcholine (ACh) receptor antibodies impair neuromuscular transmission in myasthenia gravis.

 

Antigen-antibody complexes produce tissue damage mainly by eliciting inflammation at the sites of deposition. The toxic reaction is initiated when antigen combines with antibody within the circulation (circulating immune complexes) and these are deposited, typically in vessel walls, or the complexes are formed at extravascular sites where antigen may have been deposited previously (in situ immune complexes). The mere formation of antigen-antibody complexes in the circulation does not imply the presence of disease; immune complexes are formed during many immune responses and represent a normal mechanism of antigen removal. The factors that determine whether the immune complexes formed in circulation will be pathogenic are not fully understood, but some possible influences are discussed later.

 

Two general types of antigens cause immune complex-mediated injury: (1) The antigen may be exogenous, such as a foreign protein, a bacterium, or a virus; or (2) Under some circumstances, the individual can produce antibody against self-components-endogenous antigens. The latter can be circulating antigens present in the blood or, more commonly, antigenic components of one’s own cells and tissues.

 

 

 

Immune complex-mediated diseases can be generalized, if immune complexes are formed in the circulation and are deposited in many organs, or localized to particular organs, such as the kidney (glomerulonephritis), joints (arthritis), or the small blood vessels of the skin if the complexes are formed and deposited locally. These two patterns are considered separately.

 

Systemic Immune Complex Disease

 

 

Acute serum sickness is the prototype of a systemic immune complex disease; it was at one time a frequent sequela to the administration of large amounts of foreign serum (e.g., immune serum from horses used for passive immunization.) The occurrence of diseases caused by immune complexes was suspected in the early 1900s by a physiciaamed Clemens von Pirquet. Patients with diphtheria infection were being treated with serum from horses immunized with the diphtheria toxin. Von Pirquet noted that some of these patients developed arthritis, skin rash, and fever, and the symptoms appeared more rapidly with repeated injection of the serum. Von Pirquet concluded that the treated patients made antibodies to horse serum proteins, these antibodies formed complexes with the injected proteins, and the disease was due to the antibodies or immune complexes. He called this disease “serum disease”; it is now known as serum sickness. In modern times the disease is infrequent, but it is an informative model that has taught us a great deal about systemic immune complex disorders.

For the sake of discussion, the pathogenesis of systemic immune complex disease can be divided into three phases: (1) formation of antigen-antibody complexes in the circulation; (2) deposition of the immune complexes in various tissues, thus initiating; and (3) an inflammatory reaction at the sites of immune complex deposition. The first phase is initiated by the introduction of antigen, usually a protein, and its interaction with immunocompetent cells, resulting in the formation of antibodies approximately a week after the injection of the protein. These antibodies are secreted into the blood, where they react with the antigen still present in the circulation to form antigen-antibody complexes. In the second phase, the circulating antigen-antibody complexes are deposited in various tissues.

The factors that determine whether immune complex formation will lead to tissue deposition and disease are not fully understood, but two possible influences are the size of the immune complexes and the functional status of the mononuclear phagocyte system:

• Large complexes formed in great antibody excess are rapidly removed from the circulation by the mononuclear phagocyte system and are therefore relatively harmless. The most pathogenic complexes are of small or intermediate size (formed in slight antigen excess), which bind less avidly to phagocytic cells and therefore circulate longer.

• Because the mononuclear phagocyte system normally filters out the circulating immune complexes, its overload or intrinsic dysfunction increases the probability of persistence of immune complexes in circulation and tissue deposition.

In addition, several other factors, such as charge of the immune complexes (anionic versus cationic), valency of the antigen, avidity of the antibody, affinity of the antigen to various tissue components, three-dimensional (lattice) structure of the complexes, and hemodynamic factors, influence the tissue deposition of complexes. Because most of these influences have been investigated with reference to deposition of immune complexes in the glomeruli. In addition to the renal glomeruli, the favored sites of immune complex deposition are joints, skin, heart, serosal surfaces, and small blood vessels. For complexes to leave the microcirculation and deposit in the vessel wall, an increase in vascular permeability must occur. This is believed to occur when immune complexes bind to inflammatory cells through their Fc or C3b receptors and trigger release of vasoactive mediators as well as permeability-enhancing cytokines. Mast cells may also be involved in this phase of the reaction.

Once complexes are deposited in the tissues, they initiate an acute inflammatory reaction (third phase). During this phase (approximately 10 days after antigen administration), clinical features such as fever, urticaria, arthralgias, lymph node enlargement, and proteinuria appear.

Wherever complexes deposit, the tissue damage is similar. Two mechanisms are believed to cause inflammation at the sites of deposition: (1) activation of the complement cascade, and (2) activation of neutrophils and macrophages through their Fc receptors. Complement activation promotes inflammation mainly by production of chemotactic factors, which direct the migration of polymorphonuclear leukocytes and monocytes (mainly C5a) and by release of anaphylatoxins (C3a and C5a), which increase vascular permeability.

 

Schematic illustration of the three sequential phases in the induction of systemic immune complex-mediated disease (type III hypersensitivity).

 

Pathogenesis of immune complex-mediated tissue injury. The morphologic consequences are depicted as boxed areas.

 

The leukocytes that are drawn in by the chemotactic factors are activated by engagement of their C3b and Fc receptors by the immune complexes. This results in the release or generation of a variety of pro-inflammatory substances, including prostaglandins, vasodilator peptides, and chemotactic substances, as well as several lysosomal enzymes, including proteases capable of digesting basement membrane, collagen, elastin, and cartilage. Tissue damage is also mediated by oxygen free radicals produced by activated neutrophils. Immune complexes have several other effects, including aggregation of platelets and activation of Hageman factor; both of these reactions augment the inflammatory process and initiate the formation of microthrombi. The resultant inflammatory lesion is termed vasculitis if it occurs in blood vessels, glomerulonephritis if it occurs in renal glomeruli, arthritis if it occurs in the joints, and so on.

It is clear from the foregoing that complement-fixing antibodies (i.e., IgG and IgM) and antibodies that bind to leukocyte Fc receptors (some subclasses of IgG) induce the pathologic lesions of immune complex disorders. Because IgA can activate complement by the alternative pathway, IgA-containing complexes may also induce tissue injury. The important role of complement in the pathogenesis of the tissue injury is supported by the observations that during the active phase of the disease, consumption of complement decreases the serum levels, and experimental depletion of complement greatly reduces the severity of the lesions.

Morphology. The morphologic consequences of immune complex injury are dominated by acute necrotizing vasculitis, with necrosis of the vessel wall and intense neutrophilic infiltration. The necrotic tissue and deposits of immune complexes, complement, and plasma protein produce a smudgy eosinophilic deposit that obscures the underlying cellular detail, an appearance termed fibrinoid necrosis. When complexes are deposited in kidney glomeruli, the affected glomeruli are hypercellular because of swelling and proliferation of endothelial and mesangial cells, accompanied by neutrophilic and monocytic infiltration. The complexes can be seen on immunofluorescence microscopy as granular lumpy deposits of immunoglobulin and complement and on electron microscopy as electron-dense deposits along the glomerular basement membrane.

If the disease results from a single large exposure to antigen (e.g., acute serum sickness and perhaps acute poststreptococcal glomerulonephritis), the lesions tend to resolve, owing to catabolism of the immune complexes. A chronic form of serum sickness results from repeated or prolonged exposure to an antigen. Continuous antigenemia is necessary for the development of chronic immune complex disease because, as stated earlier, complexes in antigen excess are the ones most likely to be deposited in vascular beds. This occurs in several human diseases, such as systemic lupus erythematosus (SLE), which is associated with persistent antibody responses to autoantigens. In many diseases, however, the morphologic changes and other findings suggest immune complex deposition but the inciting antigens are unknown. Included in this category are membranous glomerulonephritis, many cases of polyarteritis nodosa, and several other vasculitides.

Immune complex vasculitis. The necrotic vessel wall is replaced by smudgy, pink “fibrinoid” material. (Courtesy of Dr. Trace Worrell, Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX.)

 

Mechanisms of T cell-mediated (type IV) hypersensitivity reactions. A, In delayed type hypersensitivity reactions, CD4+ T cells (and sometimes CD8+ cells) respond to tissue antigens by secreting cytokines that stimulate inflammation and activate phagocytes, leading to tissue injury. B, In some diseases, CD8+ cytolytic T lymphocytes (CTLs) directly kill tissue cells. APC, antigen-presenting cell.

 

Local Immune Complex Disease (Arthus Reaction)

The Arthus reaction is a localized area of tissue necrosis resulting from acute immune complex vasculitis, usually elicited in the skin. The reaction can be produced experimentally by intracutaneous injection of antigen in an immune animal having circulating antibodies against the antigen. As the antigen diffuses into the vascular wall, it binds the preformed antibody, and large immune complexes are formed locally, which precipitate in the vessel walls and trigger an inflammatory reaction. In contrast to IgE-mediated type I reactions, which appear immediately, the Arthus lesion develops over a few hours and reaches a peak 4 to 10 hours after injection, when it can be seen as an area of visible edema with severe hemorrhage followed occasionally by ulceration. Immunofluorescent stains reveal complement, immunoglobulins, and fibrinogen deposited in the vessel walls, usually the venules, and histologically the vessels show fibrinoid necrosis and inflammation. Thrombi are formed in the vessels, resulting in local ischemic injury.

 

Cell-Mediated (Type IV) Hypersensitivity

The cell-mediated type of hypersensitivity is initiated by antigen-activated (sensitized) T lymphocytes. It includes the delayed type hypersensitivity reactions mediated by CD4+ T cells, and direct cell cytotoxicity mediated by CD8+ T cells. It is the principal pattern of immunologic response not only to a variety of intracellular microbiologic agents, such as Mycobacterium tuberculosis, but also to many viruses, fungi, protozoa, and parasites. So-called contact skin sensitivity to chemical agents and graft rejection are other instances of cell-mediated reactions. In addition, many autoimmune diseases are now known to be caused by T cell-mediated reactions. The two forms of T cell-mediated hypersensitivity are described next.

 

 

 

Delayed hypersensitivity in the skin. A, Perivascular infiltration by T cells and mononuclear phagocytes. B, Immunoperoxidase staining reveals a predominantly perivascular cellular infiltrate that marks positively with anti-CD4 antibodies. (Courtesy of Dr. Louis Picker, Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX.)

 

Delayed Type Hypersensitivity

The classic example of delayed hypersensitivity is the tuberculin reaction, which is produced by the intracutaneous injection of tuberculin, a protein-lipopolysaccharide component of the tubercle bacillus. In a previously sensitized individual, reddening and induration of the site appear in 8 to 12 hours, reach a peak in 24 to 72 hours, and thereafter slowly subside. Morphologically, delayed type hypersensitivity is characterized by the accumulation of mononuclear cells around small veins and venules, producing a perivascular “cuffing”. There is an associated increased microvascular permeability caused by mechanisms similar to those in other forms of inflammation. Not unexpectedly, plasma proteins escape, giving rise to dermal edema and deposition of fibrin in the interstitium. The latter appears to be the main cause of induration, which is characteristic of delayed hypersensitivity skin lesions. In fully developed lesions, the lymphocyte-cuffed venules show marked endothelial hypertrophy and, in some cases, hyperplasia. Immunoperoxidase staining of the lesions reveals a preponderance of CD4+ (helper) T lymphocytes.

With certain persistent or nondegradable antigens, such as tubercle bacilli colonizing the lungs or other tissues, the initial perivascular lymphocytic infiltrate is replaced by macrophages over a period of 2 or 3 weeks. The accumulated macrophages often undergo a morphologic transformation into epithelium-like cells and are then referred to as epithelioid cells. A microscopic aggregation of epithelioid cells, usually surrounded by a collar of lymphocytes, is referred to as a granuloma. This pattern of inflammation that is sometimes seen in type IV hypersensitivity is called granulomatous inflammation.

A section of a lymph node shows several granulomas, each made up of an aggregate of epithelioid cells and surrounded by lymphocytes. The granuloma in the center shows several multinucleate giant cells. (Courtesy of Dr. Trace Worrell, Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX.)

 

The sequence of cellular events in delayed hypersensitivity can be exemplified by the tuberculin reaction. When an individual is first exposed to protein antigens of tubercle bacilli, naive CD4+ T cells recognize peptides derived from these antigens in association with class II molecules on the surface of antigen-presenting cells. This initial encounter drives the differentiation of naive CD4+ T cells to T_H1 cells. The induction of T_H1 cells is of central importance because the expression of delayed hypersensitivity depends in large part on the cytokines secreted by T_H1 cells. Why certain antigens preferentially induce the T_H1 response is not entirely clear, but the cytokine milieu in which naive CD4+ T cells are activated seems to be relevant, as discussed subsequently. Some of the T_H1 cells enter the circulation and may remain in the memory pool of T cells for long periods, sometimes years. On intracutaneous injection of tuberculin in an individual previously exposed to tubercle bacilli, the memory T_H1 cells recognize the antigen displayed by antigen-presenting cells and are activated. These T_H1 cells secrete cytokines, mainly IFN-γ, which are responsible for the expression of delayed-type hypersensitivity. Cytokines most relevant to this reaction and their actions are as follows:

• IL-12, a cytokine produced by macrophages and dendritic cells, is critical for the induction of the T_H1 response and hence delayed hypersensitivity. On initial encounter with a microbe, the macrophages and dendritic cells that are presenting microbial antigens secrete IL-12, which drives the differentiation of naive CD4+ helper cells to T_H1 cells. These, in turn, produce other cytokines, listed below. IL-12 is also a potent inducer of IFN-γ secretion by T cells and NK cells. IFN-γ further augments the differentiation of T_H1 cells.

• IFN-γ has many effects and is the key mediator of delayed-type hypersensitivity. Most importantly, it is a powerful activator of macrophages. Activated macrophages are altered in several ways: their ability to phagocytose and kill microorganisms is markedly augmented; they express more class II molecules on the surface, thus facilitating further antigen presentation; they secrete several polypeptide growth factors, such as platelet-derived growth factor (PDGF), which stimulate fibroblast proliferation and augment collagen synthesis; they secrete TNF, IL-1, and chemokines, which promote inflammation; and they produce more IL-12, thereby amplifying the T_H1 response. Thus, activated macrophages serve to eliminate the offending antigen; if the activation is sustained, continued inflammation and, ultimately, fibrosis result.

• IL-2 causes autocrine and paracrine proliferation of T cells, which accumulate at sites of delayed hypersensitivity; included in this infiltrate are some antigen-specific CD4+ T_H1 cells and many more bystander T cells that are recruited to the site.

• TNF and lymphotoxin are two cytokines that exert important effects on endothelial cells: (1) increased secretion of prostacyclin, which, in turn, favors increased blood flow by causing local vasodilation; (2) increased expression of P-E-selectins, adhesion molecules that promote attachment of the passing lymphocytes and monocytes; and (3) induction and secretion of chemokines such as IL-8. Together, all these changes in the endothelium facilitate the extravasation of lymphocytes and monocytes at the site of the delayed hypersensitivity reaction. The steps in this process are initial rolling on the endothelium, followed by activation of integrins and firm adhesion and, ultimately, transmigration through the vessel wall.

• Chemokines produced by the T cells and macrophages recruit more leukocytes into the reaction site. This type of inflammation is sometimes called “immune inflammation.”

Schematic illustration of the events that give rise to the formation of granulomas in cell-mediated (type IV) hypersensitivity reactions. Note the role played by T cell-derived cytokines.

 

T cell-mediated hypersensitivity is a major mechanism of defense against a variety of intracellular pathogens, including mycobacteria, fungi, and certain parasites, and is also involved in transplant rejection and tumor immunity. In addition to its beneficial, protective role, delayed type hypersensitivity can also be a cause of disease. Contact dermatitis is a common example of tissue injury resulting from delayed hypersensitivity. It may be evoked by coming in contact with urushiol, the antigenic component of poison ivy or poison oak, and manifests in the form of a vesicular dermatitis. The basic mechanism is similar to that described for tuberculin sensitivity. On repeat exposure to the plants, the sensitized CD4+ cells of the T_H1 type first accumulate in the dermis, then migrate toward the antigen within the epidermis. Here they release cytokines that damage keratinocytes, causing separation of these cells and formation of an intraepidermal vesicle. Type I diabetes and multiple sclerosis are two diseases involving different organs in which tissue injury is caused by delayed type hypersensitivity reactions against autologous tissue antigens, mediated by the T_H1 type of CD4+ T cells. In these examples of T_H1-mediated autoimmune disease, there is some evidence that CD8+ cells may also be involved. In certain other forms of delayed hypersensitivity reactions, especially those that follow viral infections, cytokine-producing CD8+ cells may be the dominant effector cells.

 

T Cell-Mediated Cytotoxicity

Contact dermatitis showing an epidermal blister (vesicle) with dermal and epidermal mononuclear infiltrates. (Courtesy of Dr. Louis Picker, Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX.)

 

In this variant of cell-mediated hypersensitivity, sensitized CD8+ T cells kill antigen-bearing target cells. Such effector cells are called cytotoxic T lymphocytes (CTLs). Tissue destruction by CTLs may be an important component of many T cell-mediated diseases. CTLs directed against cell surface histocompatibility antigens play an important role in graft rejection, to be discussed next. They also play a role in resistance to virus infections. In a virus-infected cell, viral peptides associate with the class I molecules within the cell, and the two are transported to the cell surface in the form of a complex that is recognized by the TCR of cytotoxic CD8+ T lymphocytes. The lysis of infected cells leads, in due course, to the elimination of the infection. It is believed that many tumor-associated antigens may also be similarly presented on the cell surface, and CTLs are therefore also involved in tumor immunity.

 

Much has been learned about the mechanisms by which CTLs kill their targets, and this knowledge may be of value in therapeutic modulation of T cell-mediated cytotoxicity in the settings of some autoimmune diseases. Two principal mechanisms of T cell-mediated damage have been discovered: (1) perforin-granzyme-dependent killing, and (2) Fas-Fas ligand-dependent killing.

Perforins and granzymes are preformed mediators contained in the lysosome-like granules of CTLs. As its name indicates, perforin can perforate the plasma membranes of the target cells that are under attack by CD8+ lymphocytes. At first, CD8+ T cells come in close contact with the target cells; this is followed by polymerization of the released perforin molecules and their insertion into the target cell membranes, thus “drilling holes” into the membrane. The CTL granules contain proteases called granzymes, which are delivered into the target cells via the perforin-induced pores. Once within the cell, granzymes activate caspases, which induce apoptosis of the target cells. In addition, the perforin pores allow water to enter the cells, thus causing osmotic lysis. Fas-dependent killing also induces apoptosis of the target cells but by a different mechanism. Activated CTLs express.

 

Transplant Rejection

Transplant rejection is discussed here because it involves several of the immunologic reactions discussed earlier. A major barrier to transplantation is the process of rejection, in which the recipient’s immune system recognizes the graft as being foreign and attacks it. One of the important goals of present-day immunologic research is successful transplantation of tissues in humans without rejection. Although the surgical expertise for the transplantation of skin, kidneys, heart, lungs, liver, spleen, bone marrow, and endocrine organs is now well in hand, it outpaces thus far the ability to confer on the recipient permanent acceptance of foreign grafts.

 

Mechanisms Involved in Rejection of Kidney Grafts

As stated above, graft rejection depends on recognition by the host of the grafted tissue as foreign. The antigens responsible for such rejection in humans are those of the HLA system. Because HLA genes are highly polymorphic, any two individuals (other than identical twins) will express some HLA proteins that are different. Thus, every individual will recognize some HLA molecules in another individual as foreign (allogeneic) and will react against these. This reaction is the basis of rejection of grafts from one individual to another. Rejection is a complex process in which both cell-mediated immunity and circulating antibodies play a role; moreover, the relative contributions of these two mechanisms to rejection vary among grafts and are often reflected in the histologic features of the rejected organs.

T Cell-Mediated Reactions. The critical role of T cells in transplant rejection has been documented both in humans and in experimental animals. T cell-mediated graft rejection is called cellular rejection, and it is induced by two mechanisms: destruction of graft cells by CD8+ CTLs and delayed hypersensitivity reactions triggered by activated CD4+ helper cells. The recipient’s T cells recognize antigens in the graft (the allogeneic antigens, or alloantigens) by two pathways, called direct and indirect.

• In the direct pathway, T cells of the transplant recipient recognize allogeneic (donor) MHC molecules on the surface of an antigen-presenting cell in the graft. It is believed that dendritic cells carried in the donor organs are the most important immunogens because they not only richly express class I and II HLA molecules but also are endowed with costimulatory molecules (e.g., B7-1 and B7-2). The T cells of the host encounter the dendritic cells either within the grafted organ or after the dendritic cells migrate to the draining lymph nodes. Both the CD4+ and the CD8+ T cells of the transplant recipient are involved in this reaction. CD8+ T cells recognize class I HLA antigens and differentiate into mature CTLs. This process of CTL differentiation is complex and incompletely understood. It appears to be dependent on the release of cytokines, such as IL-2, from CD4+ helper cells and CD40 ligand on the helper cells activating antigen-presenting cells to promote the differentiation of CTLs. Once mature CTLs are generated, they kill the grafted tissue by mechanisms already discussed. The CD4+ helper T-cell subset is triggered into proliferation and differentiation into T_H1 effector cells by recognition of allogeneic class II molecules. As in delayed hypersensitivity reactions, cytokines secreted by the activated CD4+ T cells cause increased vascular permeability and local accumulation of mononuclear cells (lymphocytes and macrophages), and activate the macrophages, resulting in graft injury. The direct recognition of allogeneic MHC molecules seems paradoxical to the rules of self-MHC restriction: If T cells are normally restricted to recognizing foreign peptides displayed by self-MHC molecules, why should these T cells recognize foreign MHC? Such recognition has been explained by assuming that allogeneic MHC molecules, with their bound peptides, resemble, or mimic, the self-MHC-foreign peptide complexes that are recognized by self-MHC-restricted T cells. The structural basis of such mimicry is not entirely clear.

• In the so-called indirect pathway of allorecognition, recipient T lymphocytes recognize antigens of the graft donor after they are presented by the recipient’s own antigen-presenting cells. This process involves the uptake and processing of MHC molecules from the grafted organ by host antigen-presenting cells. The peptides derived from the donor tissue are presented by the host’s own MHC molecules, like any other foreign peptides. Thus, the indirect pathway is similar to the physiologic processing and presentation of other foreign (e.g., microbial) antigens. The indirect pathway generates CD4+ T cells that enter the graft and recognize graft antigens being displayed by host antigen-presenting cells that have also entered the graft, and the result is a delayed hypersensitivity type of reaction. However, CD8+ CTLs that may be generated by the indirect pathway cannot directly recognize or kill graft cells, because these CTLs recognize graft antigens presented by the host’s antigen-presenting cells. Therefore, when T cells react to a graft by the indirect pathway, the principal mechanism of cellular rejection may be T-cell cytokine production and delayed hypersensitivity. It is postulated that the direct pathway is the major pathway in acute cellular rejection, whereas the indirect pathway is more important in chronic rejection. However, this separation is by no means absolute.

Schematic representation of the events that lead to the destruction of histoincompatible grafts. In the direct pathway, donor class I and class II antigens on antigen-presenting cells in the graft (along with B7 molecules, not shown) are recognized by CD8+ cytotoxic T cells and CD4+ helper T cells, respectively, of the host. CD4+ cells proliferate and produce cytokines that induce tissue damage by a local delayed hypersensitivity reaction and stimulate B cells and CD8+ T cells. CD8+ T cells responding to graft antigens differentiate into cytotoxic T lymphocytes that kill graft cells. In the indirect pathway, graft antigens are displayed by host APCs and activate CD4+ T cells, which damage the graft by a local delayed hypersensitivity reaction. The example shown is of a kidney allograft.