CONGENITAL AND ACQUIRED IMMUNODEFICIENCY STATES, THEIR MANIFESTATIONS IN THE MAXILLOFACIAL AREA

The immunological aspects of autoimmune diseases. Immunology of tumors.

 

The autoimmune process (autoimmune response) is a form of the immune response, induced by the autoantigenic determinants under the normal conditions and pathology; it is one of the mechanisms of homeostasis maintenance.

The autoantibodies in comparatively low titers are revealed in healthy people, the frequency of positive results constantly increases with age, approximately up to 60-70. The formation of autoantibodies and development of the autoimmune diseases is more frequently observed in women than in men.

The autoimmune disease is a disease of the immune system associated with the disturbance of formation or maintenance of the immunological tolerance, which is manifested in the form the clinically manifested immune-causing self-destruction of the organs and tissues of organism. It is based on the organism loss of the immunological tolerance to the antigens of its own tissues. If the autoantibodies react with the components of one organ, then the pathologic process is of the local nature. In the systemic processes they react with the components of many tissues of organism.

Signs, by which this or that disease may be referred to the autoimmune one, are formulated by L. Vitebski (1961).

1. Presence of the autoantibodies or cytotoxic T-lymphocytes, directed at the antigen, associated with the present disease.

2. Identification of the autoantigen, at which the immune response is directed.

3. Transfer of the autoimmune process with the aid of the serum, which contains antibodies or cytotoxic T-lymphocytes.

4. Possibility of creation of the experimental model of the disease with the aid of introduction of the autoantigen with development of the corresponding morphological disturbances, characteristic of the disease.

Basic theories of the pathogenesis of the autoimmune diseases:

1. Theory of “forbidden” clones (most popular today) – at some stages of maturation of the immune system elimination of T- and B-lymphocytes occurs, which possess autoreactivity (producing autoAB). However, if total elimination does not occur, tolerance failure is possible in future.

2. Theory of the sequestered antigens – the definite tissues are protected by the histohematic barriers (sexual glands, the eye tissues, brain, thyroid gland and others).  During damage of the histohematic barrier these tissues will be recognized as foreign.

3. Theory of disorder of the immunological regulation: reduction in the function of T -suppressors; an increase in the function of T- helpers.

4. Theory of the polyclonal activation of the B-lymphocytes – in polyclonal activation of the B-lymphocytes autoreactive B-lymphocytes are activated.

5. Theory of development of autoimmunity under the effect of superantigens – a number of bacteria produce super-AG (enterotoxins A, B, C for Staphylococcus aureus, erythrogenic toxin for streptococcus, etc), which can activate the autoreactiveT-l and B- l or the antigen-presenting cells.

6. Theory of genetic predisposition – there is a the genetically determined predisposition to development of the autoimmune diseases, controlled by minimum six genes on different chromosomes (their major portion is in the main complex of the HLA histocompatibility of man); majority of the autoimmune diseases are associated with the presence of AG DR2, DR3, DR4 and DR5 in HLA- phenotype of man.

7. Theory of molecular mimicry – the similarity of AG of some infectious agents and autoAG may lead to development of the autoimmune diseases (classical poststreptococcal glomerulonephritis and others).

Mechanisms and causes

The autoimmune disease is a pathologic process, and the autoantibodies and/or cellular autoimmune response play an important role in pathogenesis of it. In the autoimmune response the B-lymphocytes develop autoantibodies against the background of increase in the activity of T- helpers. The factors providing the development of these clones of lymphocytes include: genetic factors, interfering microbial antigens, disturbance in the cytokine network of regulation and factors of the environment. The probability of failure of the mechanisms of the immunological tolerance grows with age.

There are 8 possible versions of development of the autoimmune response (there may be observed 2 of them or more simultaneously):

1.                 Intracellular virus infection (viruses of Epstein – Barr and others) – “its own” cell, which carries foreign antigens, may be destroyed together with them.

2.                 Medicines and other factors attached to the cells (penicillin; malarial agent and others) – “its own” cell, which carries foreign antigens, may be destroyed together with them.

3.                 Interfering antigens (streptococci of group A, B; spirochaeta; tripanosoms and others) – in presence of the autoreactive B-lymphocytes the invasive microorganisms having antigenic determinants common with the host, are capable of causing the production of autoantibodies to “their” antigens.

4.                 Interfering idiotops – in presence of the autoreactive B-lymphocytes the invasive microorganisms having antigenic determinants common with the host, are capable of causing the production of autoantibodies to “their” antigens.

5.                 Late developed or sequestered antigens (lens; sperm) – the antigens, which arose at the later stage of development or which were released from the sequestered tissues, in contact with the immune system are perceived as “strangers”.

6.                 Anomalous representation of the antigen (thyroid and pancreases) – the presentation of the antigen by the cells, which are not specialized for this function, may lead to autoreactivity.

7.                 Polyclonal activation (viruses of Epstein – Barr; malaria; tripanosoms; “the reaction: transplant against the host”) – Autoreactive B-lymphocytes may be stimulated by directly polyclonal activators bypassing the normal conditions for activation.

8.                Insufficiency of regulation – disturbance of regulation in the idiotype- anti-idiotypic network, in the system of cells – suppressors can lead to the fact that the autoimmune reaction becomes the cause of the disease.

The genetic factors

There is a strict correlation between the autoimmune diseases and specific HLA.

Antigen HLA-B27: the only antigen, which plays a role in diagnostics of the autoimmune diseases. It is frequently revealed in juvenile rheumatoid arthritis, chronic inflammatory diseases of the bowels, reactive arthritis.

Figure VII.1. Associations of HLA serotype and of sex with susceptibility to autoimmune disease. The relative risk’ for an HLA allele in an autoimmune disease is calculated by comparing the observed number of patients carrying the HLA allele with the number that would be expected, given the prevalence of the HLA allele in the general popu­lation. For insulin-dependent diabetes mellitus, the association is in fact with the HLA-DQ gene, which is tightly linked to the DR genes but is not detectable by serotyping. Some diseases show a significant bias in the sex ratio; this is taken to imply that sex hormones are involved in pathogenesis. Consistent with this, the difference in the sex ratio in these diseases is greatest between the menarche and the menopause, when levels of such hormones are highest (Charles A. Janeway et al., Immunobiology, 1999).

 

Antigen HLA-DR4: in carriers of this antigen rheumatoid arthritis is more frequently accompanied by severe affection of the joints and extraarticular manifestations and has less favorable outcome than in the remaining patients with rheumatoid arthritis.

 

Autoimmune diseases classified by the mechanism of tissue damage

(Charles A. Janeway et al., Immunobiology, 1999)

 

 

Classification of the autoimmune diseases

At present the autoimmune diseases include all diseases associated with formation of the autoantibodies, with exception of those cases, when it is possible to assert that the immunological phenomena bear the clearly expressed second nature.

The autoimmune diseases are divided into two groups:

1) organ-specific – for example severe myasthenia, Hashimoto’s thyroiditis, Grave’s disease (thyrotoxicosis with diffuse goiter) and others;

2) systemic – for example systemic lupus erythematosis, rheumatoid arthritis and others

Autoimmune diseases are also divided by the types of the autoreactive T- helpers:

1) Predominantly T- helpers of 1 type (rheumatoid arthritis, reactive arthritis, Wegener’s granulematoz, lime- arthritis, gigantocellular arthritis);

2) T- helpers of 2 type (syndrome of Charge- Strosa);

3) Predominantly T- helpers of 2 type (SLE, dermatomyositis, systemic scleroderma, Sjogren’s syndrome).

 

Basic autoimmune diseases:

1. Collagenoses (SKV, rheumatoid arthritis, scleroderma, dermatomyositis)

2. Diseases of the skin (Sjogren’s syndrome, psoriasis, vitiligo, herpetiform dermatitis, pemphigus, bullous pemfigoid)

3. Neurologic diseases (disease of Bekhterev, multiple sclerosis, acute postinfection polyneuritis (syndrome of Guillain- Barre), myasthenia).

4. Pathology of the endocrine system (Hashimoto’s thyroiditis, Grave’s disease (thyrotoxicosis with diffuse goiter), insulin-dependant diabetes mellitus (I type), the autoimmune affection of the adrenal glands (Addison disease), autoimmune polyendocrinopathy)

5. Sarcoidosis

6. Idiopathic pulmonary fibrosis

7. Diseases of the digestion organs (nonspecific ulcerous colitis, Crohn’s disease, autoimmune gastritis, primary billiary cirrhosis, chronic active hepatitis, autoimmune enteropathy, celiacia)

8. Diseases of the kidneys (glomerulonephritis, Goodpasture’s syndrome)

9. Diseases of the sexual sphere (autoimmune orchitis, autoimmune infertility, primary syndrome of antiphospholipid antibodies)

10. Diseases of the eyes (autoimmune uveitis, sympathetic ophthalmia, autoimmune conjunctivitis)

11. Diseases of the blood vessels (nodular periarteritis, gigantocellular granulomatous arteritis)

12. Diseases of the blood (autoimmune hemolytic anemia, autoimmune thrombocytopenia, autoimmune neutropenia and others)

 

Diagnostics of the autoimmune diseases

Most significant for making a diagnosis is development of the autoantibodies to the tissue antigens (SLE – to native DNA, RNA, mitochodria; multiple sclerosis – to the myeline; arthritis, vaskulitis – to IgG; thyreoditis – to the thyroglobulin; glomerulonephritis, Goodpasture’s syndrome – to the basal lamina of the renal glomerules; insulin-dependant diabetes mellitus – to insulin or receptors of insulin and so forth).

Immunolaboratory diagnostics:

1. Presence of specific autoantibodies;

2. Presence of the specific cellular sensitization (it is revealed with the aid of the reaction and test of the inhibition of the leukocyte migration in presence of the corresponding autoantigen);

3. Increase in the level of gamma globulin and/or IgG;

4. Change in the quantity of T- helpers and T -suppressors, which leads to an increase in the immunoregulator index;

5. Reduction in the level of C3 and C4 in the affected tissues (IgG, IgM, C3, C4 and fibrin);

7. Lymphoid- cellular infiltration of the affected tissues;

8. Determination of HLA- phenotype.

 

Some of the autoantibodies to the tissue antigens

 

The determination of cryoglobulins – immunoglobulins of the sera, which reversibly precipitate at a temperature below 37⁰C.

The precipitates, which contain both monoclonal (for example, the rheumatoid factor) and polyclonal (for example, IgG) antibodies, are called mixed cryoglobulins. Mixed cryoglobulinaemia is usually manifested by vasculites of the skin. In this case the areas of body subjected to the action of cold, are most frequently affected. Mixed cryoglobulinaemia is characteristic of the autoimmune diseases. It is observed in SLE, nodular periarteritis, syndrome of Sjogren and disease of Kawasaki. Hepatitis A, B and  C are always accompanied by cryoglobulinaemia. Cryoglobulins are also revealed in hemoblastoses, chronic infections and sarcoidosis. When cryo-precipitates contain only monoclonal antibodies, myelomatosis and macroglobulinemia of Waldenstrem are excluded.

Study of the synovial fluid. On suspicion of rheumatoid arthritis and SLE the hemolytic activity of complement and the rheumatoid factor are determined in the synovial liquid. In  rheumatoid arthritis and LE the complement hemolytic activity in the synovial fluid of the affected joint is usually reduced and makes less than 30% of the normal level in the serum. In majority of other diseases with affection of the joints, the hemolytic activity of complement corresponds to the normal value in the serum or exceeds it. The rheumatoid factor. Any particles, covered with IgG, can be agglutinated by the rheumatoid factor. Initially the erythrocytes of the ram covered with antibodies were used for detection of the rheumatoid factor. Recently, nephelometry has been used as the method of determination of the rheumatoid factor (it evaluates an increase in the serum turbidity after the addition of IgG to it). The rheumatoid factor is revealed in the autoimmune diseases, which are accompanied by affection of the joints, infectious endocarditis and some chronic diseases of the liver. Predominantly IgM to IgG are revealed with the aid of the nephelometric reaction. Besides IgG and IgA to IgG can also be revealed in the serum. In certain cases the rheumatoid factor is determined only in the synovial liquid, and it is absent in the serum. The development of the rheumatoid factor in the synovial liquid of the affected joints allows to confirm the diagnosis of seronegative rheumatoid arthritis.

The study of the antinuclear antibodies by the method of immunofluorescence is observed to give the spotty staining of the tissue sections. In some patients there are signs of several autoimmune diseases, but, in contrast to the patients with the mixed disease of the connective tissue, there are no antibodies to the ribonucleoprotein. In this case, if the existing signs meet the criteria of several autoimmune diseases, the diagnosis of the cross syndrome is made, but if there are no signs the diagnosis of the undifferentiated disease of connective tissue is made. The signs, which allow to make the diagnosis of this or that disease, usually develop subsequently: rheumatoid arthritis, SLE, systemic scleroderma, etc.

Principles of treatment of the autoimmune diseases

The methods of modern therapy consist in directed change in various stages of the immune response and include the directed effect on the metabolic reaction of a number of organ-specific diseases and the use of antipyretic substances (corticosteroids, salicylates, penicillamine, salts of gold, antimalarial medicines) and immunodepressants. In some diseases (thrombocytopenic purpura, myasthenia gravis) it is effective to give i/v introduction of immunoglobulin preparations. Plasmaferesis gives a temporary improvement in the state of patients.

 

Figure VII.2. Current and potential treatment of autoimmune disease. Current treatments for arresting the pathological developments in autoimmune disease are given in blue boxes, and those that may become practicable in green boxes. Anti­mitotic drugs are given in severe cases of SLE or chronic active hepatitis, and anti-inflammatory drugs are widely prescribed in rheumatoid arthritis. Organ-specific disorders (e.g. primary myxoedema) can be treated by supplying the defective component (e.g. thyroid hormone). When a live graft is necessary, immunosuppressive therapy can protect the tissue from damage (I.Roitt et al., Immunology, 2001).

 

The main of diseasses

Rheumatoid arthritis is an autoimmune disease that causes chronic inflammation of the joints. Rheumatoid arthritis can also cause inflammation of the tissue around the joints, as well as in other organs in the body. While rheumatoid arthritis is a chronic illness, meaning it can last for years, patients may experience long periods without symptoms. However, rheumatoid arthritis is typically a progressive illness that has the potential to cause joint destruction and functional disability. The cause of rheumatoid arthritis is unknown. It is believed that the tendency to develop rheumatoid arthritis may be genetically inherited. It is also suspected that certain infections or factors in the environment might trigger the activation of the immune system in susceptible individuals. The symptoms of rheumatoid arthritis come and go, depending on the degree of tissue inflammation. During remissions, symptoms of the disease disappear, and people generally feel well. When the disease becomes active again (relapse), symptoms return. When the disease is active, symptoms can include fatigue, loss of energy, lack of appetite, low-grade fever, muscle and joint aches, and stiffness. Muscle and joint stiffness are usually most notable in the morning and after periods of inactivity. Also during flares, joints frequently become red, swollen, painful, and tender. In rheumatoid arthritis, multiple joints are usually inflamed in a symmetrical pattern (both sides of the body affected). The small joints of both the hands and wrists are often involved. Since rheumatoid arthritis is a systemic disease, its inflammation can affect organs and areas of the body other than the joints. Inflammation of the glands of the eyes and mouth can cause dryness of these areas and is referred to as Sjogren’s syndrome. Rheumatoid inflammation of the lung lining (pleuritis) causes chest pain with deep breathing, shortness of breath, or coughing. The lung tissue itself can also become inflamed, scarred, and sometimes nodules of inflammation (rheumatoid nodules) develop within the lungs. Inflammation of the tissue (pericardium) surrounding the heart, called pericarditis, can cause a chest pain that typically changes in intensity when lying down or leaning forward. The diagnosis will be based on the pattern of symptoms, the distribution of the inflamed joints, and the blood and X-ray findings. Blood tests: rheumatoid factor; citrulline antibody (also referred to as anticitrulline antibody, anticyclic citrullinated peptide antibody, and anti-CCP);  antinuclear antibody” (ANA); tests called the sedimentation rate and C-reactive protein tests. Joint X-rays may be normal or only show swelling of soft tissues early in the disease. As the disease progresses, X-rays can show bony erosions typical of rheumatoid arthritis in the joints. Treatment: “first-line” medications (nonsteroidal anti-inflammatory drugs and corticosteroids); “second-line” or “slow-acting” drugs – disease-modifying anti-rheumatic drugs or DMARDs (hydroxychloroquine, sulfasalazine, gold salts – gold thioglucose, gold thiomalate, auranofin, D-penicillamine, immunosuppressive medicines); the types of joint surgery range from arthroscopy to partial and complete replacement of the joint.

Systemic lupus erythematosis is an autoimmune disease, that characterized by acute and chronic inflammation of various tissues of the body. Both discoid and systemic lupus are more common in women than men (about eight times more common). The disease can affect all ages but most commonly begins from 20-45 years of age. The precise reason for the abnormal autoimmunity that causes lupus is not known. Inherited genes, viruses, ultraviolet light, and certain medications may all play some role. More than 90% of cases of “drug-induced lupus” occurs as a side effect of one of the following six drugs: hydralazine; quinidine and procainamide; phenytoin; isoniazid and d-penicillamine. But drug-induced Systemic lupus erythematosis usually resolves when the medications are discontinued. Common complaints and symptoms include fatigue, low-grade fever, loss of appetite, muscle aches, arthritis, ulcers of the mouth and nose, facial rash (“butterfly rash”), unusual sensitivity to sunlight (photosensitivity), inflammation of the lining that surrounds the lungs (pleuritis) and the heart (pericarditis), and poor circulation to the fingers and toes with cold exposure (Raynaud’s phenomenon). The skin rash in discoid lupus often is found on the face and scalp. It usually is red and may have raised borders. Discoid lupus rashes are usually painless and do not itch, but scarring can cause permanent hair loss (alopecia). The hair loss can be patchy or diffuse and appear to be more like hair thinning. Over half of the people with Systemic lupus erythematosis have a characteristic red, flat facial rash over the bridge of their nose. Most people with Systemic lupus erythematosis will develop arthritis during the course of their illness. Arthritis in Systemic lupus erythematosis  commonly involves swelling, pain, stiffness, and even deformity of the small joints of the hands, wrists, and feet. More serious organ involvement with inflammation occurs in the brain, liver, and kidneys. Inflammation of muscles (myositis) can cause muscle pain and weakness. Vasculitis is characterized by inflammation with damage to the walls of various blood vessels. Inflammation of the lining of the lungs (pleuritis) and of the heart (pericarditis) can cause sharp chest pain. The chest pain is aggravated by coughing, deep breathing, and certain changes in body position. Kidney inflammation in SLE can cause leakage of protein into the urine, fluid retention, high blood pressure, and even kidney failure. Involvement of the brain can cause personality changes, thought disorders (psychosis), seizures, and even coma. Damage to nerves can cause numbness, tingling, and weakness of the involved body parts or extremities. Brain involvement is referred to as lupus cerebritis. The diagnosis The 11 criteria used for diagnosing systemic lupus erythematosus (American Rheumatism Association): malar (over the cheeks of the face) “butterfly” rash; discoid skin rash (patchy redness with hyperpigmentation and hypopigmentation that can cause scarring); photosensitivity (skin rash in reaction to sunlight [ultraviolet light] exposure); mucous membrane ulcers (spontaneous ulcers of the lining of the mouth, nose, or throat); arthritis (two or more swollen, tender joints of the extremities); pleuritis or pericarditis (inflammation of the lining tissue around the heart or lungs, usually associated with chest pain upon breathing or changes of body position); kidney abnormalities (abnormal amounts of urine protein or clumps of cellular elements called casts detectable with a urinalysis); brain irritation (manifested by seizures and/or psychosis); blood-count abnormalities (low counts of white or red blood cells, or platelets, on routine blood testing); immunologic disorder (abnormal immune tests include anti-DNA or anti-Smith antibodies, falsely positive blood test for syphilis, anticardiolipin antibodies, lupus anticoagulant, or positive LE prep test); antinuclear antibody (positive ANA antibody testing – antinuclear antibodies in the blood). Addition criterias: tests called the sedimentation rate and C-reactive protein, blood-chemistry testing, direct analysis of internal body fluids, and tissue biopsies. Ttreatment: nonsteroidal anti-inflammatory drugs; corticosteroids hydroxychloroquine; chloroquine or quinacrine; dapsone and retinoic acid; immunosuppressive medicines; mycophenolate mofetil; plasmapheresis; rituximab.

Myasthenia gravis is an autoimmune disease that causes muscle weakness. Disease affects the neuromuscular junction, interrupting the communication betweeerve and muscle, and thereby causing weakness. A person with Myasthenia gravis  may have difficulty moving their eyes, walking, speaking clearly, swallowing, and even breathing, depending on the severity and distribution of weakness. Increased weakness with exertion, and improvement with rest, is a characteristic feature of Myasthenia gravis. The earliest symptoms of Myasthenia gravis often result from weakness of the extraocular muscles, which control eye movements. Symptoms involving the eye (ocular symptoms) include double vision (diplopia), especially wheot gazing straight ahead, and difficulty raising the eyelids (ptosis). A person with ptosis may need to tilt their head back to see. Eye-related symptoms remain the only symptoms for about 15% of Myasthenia gravis patients. Another common early symptom is difficulty chewing and swallowing, due to weakness in the bulbar muscles, which are in the mouth and throat. Choking becomes more likely, especially with food that requires extensive chewing. Weakness usually becomes more widespread within several months of the first symptoms, reaching their maximum within a year in two-thirds of patients. Weakness may involve muscles of the arms, legs, neck, trunk, and face, and affect the ability to lift objects, walk, hold the head up, and speak. Myasthenia gravis is often diagnosed accurately by a careful medical history and a neuromuscular exam, but several tests are used to confirm the diagnosis. The diagnosis Hysical examination. A thorough investigation includes: looking upward and sidewards for 30 seconds: ptosis and diplopia; looking at the feet while lying on the back for 60 seconds; keeping the arms stretched forward for 60 seconds; 10 deep knee bends; walking 30 steps on both the toes and the heels; 5 situps, lying down and sitting up completely; “Peek sign”: after complete initial apposition of the lid margins, they quickly (within 30 seconds) start to separate and the sclera starts to show. Blood tests: test is for antibodies against the acetylcholine receptor and the MuSK protein. Spirometry. Electromyogram. A chest CT-scan showing a thymoma. Treatment: edrophonium; acetylcholinesterase inhibitors (neostigmine and pyridostigmine); corticosteroids; azathioprine and cyclosporine; thymectomy.

 

Autoimmune responses are directed against self antigens.

Autoimmune disease occurs when a specific adaptive immune response is mounted against self antigens. The normal consequence of an adaptive immune response against a foreign antigen is the clearance of the antigen from the body. Virus-infected cells, for example, are destroyed by cytotoxic T cells, whereas soluble antigens

are cleared by formation of immune complexes of antibody and antigen, which are taken up by cells of the mononuclear phagocytic system such as macrophages. When an adaptive immune response develops against self antigens, however, it is usually impossible for immune effector mechanisms to eliminate the antigen

completely, and so a sustained response occurs. The consequence is that the effector pathways of immunity cause chronic inflammatory injury to tissues, which may prove lethal. The mechanisms of tissue damage in autoimmune diseases are essentially the same as those that operate in protective immunity and in hypersensitivity diseases. Some common autoimmune diseases are listed.

 

Autoimmune diseases classified by the mechanism of tissue damage.

 

Autoimmune diseases can be grouped in the same way as hypersensitivity reactions, according to the type of immune response and the mechanism by which it damages tissues. The immunopathological mechanisms are as illustrated for the

hypersensitivity reactions, with the exception of the type I IgE-mediated responses, which are not a known cause of autoimmune disease. Some additional autoimmune diseases in which the antigen is a cellsurface receptor, and the pathology is due to altered signaling, are listed later. Several immunopathogenic mechanisms operate in parallel to cause many autoimmune diseases. This is illustrated in the case of rheumatoid arthritis, which appears in more than one category of immunopathological mechanism (VIDEO).

 

There are four types of hypersensitivity reaction mediated by immunological mechanisms

that cause tissue damage.

 

Types I III are antibody-mediated and are distinguished by the different types of

antigens recognized and the different classes of antibody involved. Type I responses are mediated by IgE, which induces mast-cell activation, whereas types II and III are mediated by IgG, which can engage Fc-receptor and complement-mediated effector mechanisms to varying degrees, depending on the subclass of IgG and the

nature of the antigen involved. Type II responses are directed against cell-surface or matrix antigens, whereas type III responses are directed against soluble antigens, and the tissue damage involved is caused by responses triggered by immune complexes. Type IV hypersensitivity reactions are T cell-mediated and can be subdivided into three groups. In the first group, tissue damage is caused by the activation of macro-phages by TH1 cells, which results in an inflammatory response. In the second, damage is caused by the activation by TH2 cells of inflammatory responses in which eosinophils predominate; in the third, damage is caused directly by cytotoxic T cells (CTL).

Adaptive immune responses are initiated by the activation of antigen-specific T cells, and it is believed that autoimmunity is initiated in the same way. T-cell responses to self antigens can inflict tissue damage either directly or indirectly. Cytotoxic T-cell responses and inappropriate activation of macrophages by TH1 cells can cause extensive tissue damage, whereas inappropriate T-cell help to self-reactive B cells can initiate harmful autoantibody responses. Autoimmune responses are a natural consequence of the open repertoires of both Bcell and T-cell receptors, which allow them to recognize any pathogen. Although these repertoires are purged of most receptors that bind with high affinity to self antigens encountered during development, they still include receptors of lower affinity reactive to some self antigens. It is not known what triggers autoimmunity, but both environmental and genetic factors, especially MHC genotype, are clearly important. Transient autoimmune responses are common, but it is only when they are sustained and cause lasting tissue damage that they attract medical attention. In this section, we will examine the nature of autoimmune responses and how autoimmunity leads to tissue damage. In the last section of this chapter, we will examine the mechanisms by which selftolerance is lost and autoimmune responses are initiated.

Specific adaptive immune responses to self antigens can cause autoimmune disease.

Early in the study of immunity it was realized that the powerful effector mechanisms used in host defense could, if turned against the host, cause severe tissue damage; Ehrlich termed this horror autotoxicus. Healthy individuals do not mount sustained adaptive immune responses to their own antigens and, although transient responses to damaged self tissues occur, these rarely cause additional tissue damage. But although selftolerance is the general rule, sustained immune responses to self tissues occur in some individuals, and these autoimmune responses cause the severe tissue damage that Ehrlich predicted.

In certain genetically susceptible strains of experimental animals, autoimmune disease can be induced artificially by injection of ‘self’ tissues from a genetically identical animal mixed with strong adjuvants containing bacteria. This shows that autoimmunity can be provoked by inducing a specific, adaptive immune response to self antigens and forms the basis for our understanding of how autoimmune disease arises. In humans, autoimmunity usually arises spontaneously; that is, we do not know what events initiate the immune response to self that leads to the autoimmune disease. There is evidence, as we will learn in the last part of this chapter, that some autoimmune disorders, such as rheumatic fever, may be triggered by infectious agents. There is, however, also evidence, particularly from animal models of

autoimmunity, that many autoimmune disorders occur through internal dysregulation of the immune system without the participation of infectious agents.

Autoimmune diseases can be classified into clusters that are typically either organ-specific or systemic.

The classification of disease is an uncertain science, especially in the absence of a precise understanding of causative mechanisms. This is well illustrated by the difficulty in classifying the autoimmune diseases. It is useful to distinguish two major patterns of autoimmune disease, the diseases in which the expression of

autoimmunity is restricted to specific organs of the body, known as ‘organ-specific’ autoimmune diseases, and those in which many tissues of the body are affected, the ‘systemic’ autoimmune diseases. Examples of organspecific autoimmune diseases are Hashimoto’s thyroiditis and Graves’ disease, each predominantly affecting the

thyroid gland, and type I insulin-dependent diabetes mellitus (IDDM), which affects the pancreatic islets.

Examples of systemic autoimmune disease are systemic lupus erythematosus (SLE) and primary Sjögren’s syndrome, in which tissues as diverse as the skin, kidneys, and brain may all be affected.

The autoantigens recognized in these two categories of disease are themselves respectively organ-specific and systemic. Thus, Graves’ disease is characterized by the production of antibodies to the thyroid-stimulating hormone (TSH) receptor in the thyroid gland; Hashimoto’s thyroiditis by antibodies to thyroid peroxidase; and

type I diabetes by anti-insulin antibodies. By contrast, SLE is characterized by the presence of antibodies to antigens that are ubiquitous and abundant in every cell of the body, such as anti-chromatin antibodies and antibodies to proteins of the pre-mRNA splicing machinery the spliceosome complex within the cell.

It is likely that the organ-specific and systemic autoimmune diseases have somewhat different etiologies, which provides a biological basis for their division into two broad categories. Evidence for the validity of this classification also comes from observations that different autoimmune diseases cluster within individuals and within families. The organ-specific autoimmune diseases frequently occur together in many combinations; for example, autoimmune thyroid disease and the autoimmune depigmenting disease vitiligo are often found in the same person. Similarly, SLE and primary Sjögren’s syndrome can coexist within a single individual or among different members of a family.

These clusters of autoimmune diseases provide the most useful classification into different subtypes, each of which may turn out to have a distinct mechanism. A working classification of autoimmune diseases based on clustering is given in. It can be seen that the strict separation of diseases into ‘organ-specific’ and ‘systemic’ categories breaks down to some extent. Not all autoimmune diseases can be usefully classified in this manner. Autoimmune hemolytic anemia, for example, sometimes occurs as a solitary entity and could be classified as an organ-specific disease. In other circumstances it may occur in conjunction with SLE as part of a systemic autoimmune disease.

 

Some common autoimmune diseases classified according to their ‘organ-specific’ or ‘systemic’ nature.

 

Diseases that tend to occur in clusters are grouped in single boxes. Clustering is defined as more than one disease affecting a single patient or different members of a family. Not all autoimmune diseases can be classified according to this scheme. For example, autoimmune hemolytic anemia may occur in isolation or in association with systemic lupus erythematosus (SLE).

Although anyone can, in principle, develop an autoimmune disease, it seems that some individuals are more at risk than others of developing particular diseases. We will first consider those factors that contribute to susceptibility.

Susceptibility to autoimmune disease is controlled by environmental and genetic factors, especially MHC genes.

The best evidence in humans for susceptibility genes for autoimmunity comes from family studies, especially studies of twins. A semiquantitative technique for measuring what proportion of the susceptibility to a particular disease arises from genetic factors is to compare the incidence of disease in monozygotic and dizygotic twins. If a disease shows a high concordance in all twins, it could be caused by shared genetic or environmental factors. This is because both monozygotic and dizygotic twins tend to be brought up in shared environmental conditions. If the high concordance is restricted to monozygotic rather than dizygotic twins, however, then genetic factors are likely to be more important than environmental factors.

Studies with twins have been undertaken for several human diseases in which autoimmunity is important, including type I IDDM, rheumatoid arthritis, multiple sclerosis, and SLE. In each case, around 20% of pairs of monozygotic twins show disease concordance, compared with fewer than 5% of dizygotic twins. A similar technique is to compare the frequency of a disease such as diabetes in the siblings of patients who have diabetes with the frequency of that disease in the general population. The ratio of these two frequencies gives a measure of the heritability of the disease, although shared environmental factors within families could also be at least partly responsible for an increased frequency.

Results from both twin and family studies show an important role for both inherited and environmental factors in the induction of autoimmune disease. In addition to this evidence from humans, certain inbred mouse strains have an almost uniform susceptibility to particular spontaneous or experimentally induced autoimmune diseases, whereas other strains do not. These findings have led to an extensive search for genes that determine susceptibility to autoimmune disease.

So far, susceptibility to autoimmune disease has been most consistently associated with MHC genotype. Human autoimmune diseases that show associations with HLA type are shown. For most of these diseases, susceptibility is linked most strongly with MHC class II alleles, but in some cases there are strong associations with particular MHC class I alleles.

 

Associations of HLA serotype and sex with susceptibility to autoimmune disease.

 

The ‘relative risk’ for an HLA allele in an autoimmune disease is calculated by comparing the observed number of patients carrying the HLA allele with the number that would be expected, given the prevalence of the HLA allele in the general population. For type I insulin-dependent diabetes mellitus (IDDM), the association is in fact with the HLA-DQ gene, which is tightly linked to the DR genes but is not detectable by serotyping. Some diseases show a significant bias in the sex ratio; this is taken to imply that sex hormones are involved in pathogenesis.

Consistent with this, the difference in the sex ratio in these diseases is greatest between the menarche and the menopause, when levels of such hormones are highest.

The association of MHC genotype with disease is assessed initially by comparing the frequency of different alleles in patients with their frequency in the normal population. For IDDM, this approach originally demonstrated an association with HLA-DR3 and HLA-DR4 alleles identified by serotyping. Such studies also showed that the MHC class II allele HLA-DR2 has a dominant protective effect; individuals carrying HLA-DR2, even in association with one of the susceptibility alleles, rarely develop diabetes. Another way of determining whether MHC genes are important in autoimmune disease is to study the families of affected patients; it has been shown that two siblings affected with the same autoimmune disease are far more likely than expected to share the same MHC haplotypes.

 

Population studies show association of susceptibility to IDDM with HLA genotype.

 

The HLA genotypes (determined by serotyping) of diabetic patients (bottom panel) are not representative of those found in the population (top panel). Almost all diabetic patients express HLA-DR3 and/or HLA-DR4, and HLADR3/ DR4 heterozygosity is greatly overrepresented in diabetics compared with controls. These alleles are linked tightly to HLA-DQ alleles that confer susceptibility to IDDM. By contrast, HLA-DR2 protects against the development of IDDM and is found only extremely rarely in diabetic patients. The small letter x represents any allele other than DR2, DR3, or DR4.

 

Family studies show strong linkage of susceptibility to IDDM with HLA genotype.

 

In families in which two or more siblings have IDDM, it is possible to compare the HLA genotypes of affected siblings.

Affected siblings share two HLA haplotypes much more frequently than would be expected if the HLA genotype did not influence disease susceptibility.

As HLA genotyping has become more exact through the sequencing of HLA alleles, disease associations that were originally discovered through HLA serotyping using antibodies have been defined more precisely. For example, the association between IDDM and the DR3 and DR4 alleles is now known to be due to their tight genetic linkage to DQβ alleles that confer susceptibility to disease. Indeed, disease susceptibility is most closely associated with polymorphisms at a particular position in the DQβ amino acid sequence. The most abundant DQβ amino acid sequence has an aspartic acid at position 57 that is able to form a salt bridge across the end of the peptidebinding cleft of the DQ molecule. By contrast, the diabetic patients in Caucasoid populations mostly have valine, serine, or alanine at that position and thus make DQ molecules that lack this salt bridge.

The nonobese diabetic (NOD) strain of mice, which develops spontaneous diabetes, also has a serine at that position in the homologous MHC class II molecule, known as I-Ag7.

 

Amino acid changes in the sequence of an MHC class II protein correlate with susceptibility to and protection from diabetes.

 

The HLA-DQβ1 chain contains an aspartic acid (Asp) at position 57 in most

people; in Caucasoid populations, patients with IDDM more often have valine, serine, or alanine at this position instead, as well as other differences. Asp 57, shown in red on the backbone structure of the DQβ chain, forms a salt bridge (shown in green in the center panel) to an arginine residue (shown in pink) in the adjacent α chain (gray). The change to an uncharged residue (for example, alanine, shown in yellow in the bottom panel) disrupts this salt bridge, altering the stability of the DQ molecule. The nonobese diabetic (NOD) strain of mice, which develops spontaneous diabetes, shows a similar replacement of serine for aspartic acid at position 57 of the homologous I-Aβ chain, and NOD mice transgenic for β chains with Asp 57 have a marked reduction in diabetes incidence.

The association of MHC genotype with autoimmune disease is not surprising, because autoimmune responses involve T cells, and the ability of T cells to respond to a particular antigen depends on MHC genotype. Thus the associations can be explained by a simple model in which susceptibility to an autoimmune disease is determined by differences in the ability of different allelic variants of MHC molecules to present autoantigenic peptides to autoreactive T cells. This would be consistent with what we know of T-cell involvement in particular diseases. In diabetes, for example, there are associations with both MHC class I and MHC class II alleles and this is consistent with the finding that both CD8 and CD4 T cells, which respond to antigens

presented by MHC class I and MHC class II molecules, respectively, mediate the autoimmune response.

An alternative hypothesis for the association between MHC genotype and susceptibility to autoimmune diseases emphasizes the role of MHC alleles in shaping the T-cell receptor repertoire. This hypothesis proposes that self peptides associated with certain MHC molecules may drive the positive selection of developing thymocytes that are specific for particular autoantigens. Such autoantigenic peptides might be expressed at too low a level or bind too poorly to self MHC molecules to drive negative selection in the thymus, but be present at a sufficient level or bind strongly enough to drive positive selection. This hypothesis is supported by observations that I-Ag7, the disease-associated MHC class II molecule in the diabetes-prone NOD mice, binds many peptides very poorly and may therefore be less effective in driving intrathymic negative selection of T cells that bind self peptides.

However, MHC genotype alone does not determine genetic susceptibility to disease. Identical twins, sharing all of their genes, are far more likely to develop the same autoimmune disease than MHC-identical siblings, demonstrating that genetic factors other than the MHC also affect whether an individual develops disease.

Recent studies of the genetics of autoimmune diabetes in humans and mice have shown that there are several independently segregating disease susceptibility loci in addition to the MHC.

There is also evidence that variation in the level of a potential autoantigen within the thymus can influence disease development. In the case of human insulin, which can act as an autoantigen in type I IDDM, the level of transcription of the insulin gene shows genetic variation between individuals; this is associated with a polymorphic minisatellite sequence located upstream of the gene. Gene variants that are transcribed at a high level in the thymus tend to protect against the development of diabetes, whereas variants transcribed at a lower level are associated with disease susceptibility. This is because the expression of high levels of insulin in the thymus may cause the deletion of T cells specific for the insulin peptides.

The genes that have been associated with the development of systemic lupus erythematosus provide important clues to the etiology of the disease.

The major serological abnormality in SLE is the presence of autoantibodies to ubiquitous and abundant intracellular antigens, such as chromatin. How is tolerance broken to such all-pervasive self antigens? A number of genes have been implicated in the etiology of SLE in humans and mice. These can be classified into three categories on the basis of their physiological function. The first comprises genes whose products are active in the body’s mechanisms for disposing of dead and dying cells, which could provide a source of autoantigens. Genetic knockout in mice of four genes in this category has produced animal models of SLE. One of these genes codes for the complement protein C1q, which, together with other complement proteins, is involved in the effective clearance of immune complexes and apoptotic cells. A second gene in this category encodes serum amyloid P component, which binds chromatin and may mask it from the immune system. Its deletion results in the development of antibodies against chromatin and development of glomerulonephritis caused by deposition of immune complexes of these antibodies in the kidney. Third, deletion of DNase I, an enzyme that digests extracellular chromatin, results in the development of anti-chromatin antibodies and glomerulonephritis. Fourth, a similar phenotype has been seen in mice in which the secretory portion of the immunoglobulin μ chain is deleted, and which thus lack secreted IgM, which may have an important role in the clearance of effete cells. However, the majority of cases of spontaneous SLE are likely to be influenced by far more complex genetic factors than these single-gene defects.

 

The HLA-DQβ1 chain contains an aspartic acid (Asp) at position 57 in most people; in Caucasoid populations, patients with IDDM more often have valine, serine, or alanine at this position instead, as well as other differences. Asp 57, shown in red on the backbone structure of the DQβ chain, forms a salt bridge (shown in green in the center panel) to an arginine residue (shown in pink) in the adjacent α chain (gray). The change to an uncharged residue (for example, alanine, shown in yellow in the bottom panel) disrupts this salt bridge, altering the stability of the DQ molecule. The nonobese diabetic (NOD) strain of mice, which develops spontaneous diabetes, shows a similar replacement of serine for aspartic acid at position 57 of the homologous I-Aβ chain, and NOD mice transgenic for β chains with Asp 57 have a marked reduction in diabetes incidence.

The second category of disease susceptibility genes for SLE includes those encoding proteins that regulate the thresholds for tolerance and activation of T and B lymphocytes, such as Fas, Fas ligand, the signaling molecule SHP-1, the B-cell inhibitory receptor CD22, FcγRIIB, and the cell-cycle inhibitor p21. The third category of genes encode proteins that could modify the expression of SLE in individual organs by their involvement in immune complex-mediated inflammation. Examples are the polymorphic genes for FcγRIIa and FcγRIII, where the variant proteins are thought to differ in their ability to bind immune complexes and are associated with the presence of nephritis in SLE.

A further very important factor in disease susceptibility to SLE is the hormonal status of the patient. Indeed, many autoimmune diseases show a strong sex bias. Where a bias towards disease in one sex is observed in experimental animals, castration or the administration of estrogen to males usually normalizes disease incidence between the two sexes. Furthermore, many autoimmune diseases that are more common in females show peak incidence in the years of active child bearing, when production of the female sex hormones estrogen and progesterone is at its greatest. A thorough understanding of how these genetic and hormonal factors contribute to disease susceptibility might allow us to prevent the autoimmune response.

Antibody and T cells can cause tissue damage in autoimmune disease.

Tissue injury in autoimmune disease results because the self antigen is an intrinsic component of the body and, consequently, the effector mechanisms of the immune system are directed at the body’s own tissues. Also, because the adaptive immune response is incapable of removing the offending autoantigen from the body, the immune response persists, and there is a constant supply of new autoantigen, which amplifies the response. An important exception to this rule is type I IDDM, in which the autoimmune response destroys the target organ completely. This leads to a failure to produce insulin one of the major autoantigens in this disease. Lack of insulin is in turn responsible for the phenotype of diabetes mellitus.

The mechanisms of tissue injury in autoimmunity can be classified according to the scheme adopted for hypersensitivity reactions. As with the hypersensitivity reactions, tissue damage can be mediated by the effector actions of both T cells and B cells. The antigen, or group of antigens, against which the autoimmune response is directed, and the mechanism by which the antigen-bearing tissue is damaged, together determine the pathology and clinical expression of the disease.

Autoimmune diseases differ from hypersensitivity responses in that type I IgE-mediated responses do not seem to have a major role. IgE autoantibodies have, however, been found in autoimmune disease, and although there is no proof that they mediate any autoimmune disease, there are diseases where this may be so. For example, asthma and eosinophilia  are found in a rare autoimmune vasculitis, an inflammatory disease of blood vessels that is known as Churg-Strauss vasculitis.

By contrast, autoimmunity that damages tissues by mechanisms analogous to type II hypersensitivity reactions is quite common. In this form of autoimmunity, IgG or IgM responses to autoantigens located on cell surfaces or extracellular matrix cause the injury. In other cases of autoimmunity, tissue damage can be due to type III responses, which involve immune complexes containing autoantibodies to soluble autoantigens; these autoimmune diseases are systemic and are characterized by autoimmune vasculitis. Finally, in a number of organ-specific autoimmune diseases, T-cell responses are directly involved in causing the tissue damage. In most autoimmune diseases, several mechanisms of immunopathogenesis operate. Examples include SLE, type I IDDM, and rheumatoid arthritis, in which there is evidence that both T-cell and antibody-mediated pathways cause tissue injury. We will examine how autoantibodies cause tissue damage, before ending with a consideration of self-reactive T-cell responses and their role in autoimmune disease.

Autoantibodies against blood cells promote their destruction.

IgG or IgM responses to antigens located on the surface of blood cells lead to the rapid destruction of these cells. An example of this is autoimmune hemolytic anemia, where antibodies against self antigens on red blood cells trigger destruction of the cells, leading to anemia. This can occur in two ways. Red cells with bound IgG or IgM antibody are rapidly cleared from the circulation by interaction with Fc or complement receptors, respectively, on cells of the fixed mononuclear phagocytic system; this occurs particularly in the spleen. Alternatively, the autoantibody-sensitized red cells are lysed by formation of the membrane-attack complex of complement. In autoimmune thrombocytopenic purpura, autoantibodies against the GpIIb:IIIa fibrinogen receptor on platelets can cause thrombocytopenia (a depletion of platelets), which can in turn cause hemorrhage.

 

Antibodies specific for cell-surface antigens can destroy cells.

 

In autoimmune hemolytic anemias, red cells coated with IgG autoantibodies against a cell-surface antigen are rapidly cleared from the circulation by uptake by Fc receptor-bearing macrophages in the fixed mononuclear phagocytic system (left panel). Red cells coated with IgM autoantibodies fix C3 and are cleared by CR1- and CR3-bearing macrophages in the fixed mononuclear phagocytic system (not shown). Uptake and clearance by these mechanisms occurs mainly in the spleen. The binding of certain rare autoantibodies that fix complement extremely efficiently causes the formation of the membrane-attack complex on the red cells, leading to intravascular hemolysis (right panel).

Lysis of nucleated cells by complement is less common because these cells are better defended by complement regulatory proteins. These proteins protect cells against immune attack by interfering with the activation of complement components and their assembly into a membrane-attack complex. Although the activation of complement by the bound autoantibody can proceed to a limited degree, nucleated cells are able t resist lysis by exocytosis or endocytosis of parts of the cell membrane bearing the membrane-attack complex.

Nevertheless, nucleated cells targeted by autoantibodies are still destroyed by cells of the mononuclear phagocytic system. Autoantibodies against neutrophils, for example, cause neutropenia, which increases susceptibility to infection with pyogenic bacteria. In all of these cases, accelerated clearance of autoantibodysensitized cells is the cause of their depletion in the blood. One therapeutic approach to this type of autoimmunity is removal of the spleen, the organ in which the main clearance of red cells, platelets, and leukocytes occurs.

The fixation of sublytic doses of complement to cells in tissues stimulates a powerful inflammatory response.

The binding of IgG and IgM antibodies to cells in tissues causes inflammatory injury by a variety of mechanisms. One of these is fixation of complement. Although nucleated cells are relatively resistant to lysis by complement, the assembly of sublytic amounts of the membrane-attack complex on their surface provides a powerful activating stimulus. Depending on the type of cell, the interaction of sublytic doses of the membraneattack complex with the cell membrane can cause cytokine release, generation of a respiratory burst, or the mobilization of membrane phospholipids to generate arachidonic acid the precursor of prostaglandins and leukotrienes (lipid mediators of inflammation).

Most cells in tissues are fixed in place and cells of the inflammatory system are attracted to them by chemoattractant molecules. One such is the complement fragment C5a, which is released as a result of complement activation triggered by autoantibody binding. Other chemoattractants, such as leukotriene B4, can be released by the autoantibody-targeted cells. Inflammatory leukocytes are further activated by binding to autoantibody Fc regions and fixed complement C3 fragments on the tissue cells. Tissue injury can then result from the products of the activated leukocytes and by antibody-dependent cellular cytotoxicity mediated by natural killer (NK) cells.

A probable example of this type of autoimmunity is Hashimoto’s thyroiditis, in which autoantibodies against tissue-specific antigens such as thyroid peroxidase and thyroglobulin are found at extremely high levels for prolonged periods. Direct T cell-mediated cytotoxicity, which we will discuss later, is probably also important in this disease.

Autoantibodies against receptors cause disease by stimulating or blocking receptor function.

A special class of type II hypersensitivity reaction occurs when the autoantibody binds to a cell-surface receptor. Antibody binding to a receptor can either stimulate the receptor or block its stimulation by its natural ligand. In Graves’ disease, autoantibody against the thyroid-stimulating hormone receptor on thyroid cells stimulates the excessive production of thyroid hormone. The production of thyroid hormone is normally controlled by feedback regulation; high levels of thyroid hormone inhibit release of thyroid-stimulating hormone (TSH) by the pituitary. In Graves’ disease, feedback inhibition fails because the autoantibody continues to stimulate the TSH receptor in the absence of TSH, and the patients become hyperthyroid

 

Feedback regulation of thyroid hormone production is disrupted in Graves’ disease.

 

Graves’ disease is caused by autoantibodies specific for the receptor for thyroidstimulating hormone (TSH). Normally,  thyroid hormones are produced in response to TSH and limit their own production by inhibiting the production

of TSH by the pituitary (left panels). In Graves’ disease, the autoantibodies are agonists for the TSH receptor mand therefore stimulate production of thyroid hormones (right panels). The thyroid hormones inhibit TSH production in the normal way but do not affect production of the autoantibody; the excessive thyroid hormone

production induced in this way causes hyperthyroidism.

In myasthenia gravis, autoantibodies against the α chain of the nicotinic acetylcholine receptor, which is present on skeletal muscle cells at neuromuscular junctions, can block neuromuscular transmission. The antibodies are believed to drive the internalization and intracellular degradation of acetylcholine receptors

Patients with myasthenia gravis develop potentially fatal progressive weakness as a result of their autoimmune disease. Diseases caused by autoantibodies that act as agonists or antagonists for cell-surface receptors are listed

 

Autoantibodies inhibit receptor function in myasthenia gravis.

 

In normal circumstances, acetylcholine released from stimulated motor neurons at the neuromuscular junction binds to acetylcholine receptors on skeletal muscle cells, triggering muscle contraction (left panel). Myasthenia gravis is caused by autoantibodies against the α subunit of the receptor for acetylcholine. These autoantibodies bind to the receptor without activating it and also cause receptor internalization and degradation (right panel). As the number of receptors on the muscle is decreased, the muscle becomes less responsive to acetylcholine.

 

Autoimmune diseases caused by autoantibodies against cell-surface receptors.

 

These antibodies produce different effects depending on whether they are agonists (which stimulate) or antagonists (which inhibit) the receptor. Note that different autoantibodies against the insulin receptor can either stimulate or inhibit signaling.

Autoantibodies against extracellular antigens cause inflammatory injury by mechanisms akin to type II and type III hypersensitivity reactions.

Antibody responses to extracellular matrix molecules are infrequent, but can be very damaging when they occur. In Goodpasture’s syndrome, an example of a type II hypersensitivity reaction, antibodies are formed against the α3 chain of basement membrane collagen (type IV collagen). These antibodies bind to the basement membranes of renal glomeruli  and, in some cases, to the basement membranes of pulmonary alveoli, causing a rapidly fatal disease if untreated. The autoantibodies bound to basement membrane ligate Fcγ receptors, leading to activation of monocytes, neutrophils, and tissue basophils and mast cells. These release chemokines that attract a further influx of neutrophils into the glomeruli, causing severe tissue injury. The autoantibodies also cause local activation of complement, which may amplify the tissue injury.

 

Autoantibodies reacting with glomerular basement membrane cause the inflammatory glomerular disease known as Goodpasture’s syndrome.

 

The panels show sections of renal glomeruli in serial biopsies taken from patients with Goodpasture’s syndrome. Panel a, glomerulus stained for IgG deposition by

immunofluoresence. Anti-glomerular basement membrane antibody (stained green) is deposited in a linear fashion along the glomerular basement membrane. The autoantibody causes local activation of cells bearing Fc receptors, complement activation, and influx of neutrophils. Panel b, hematoxylin and eosin staining of a

section through a renal glomerulus shows that the glomerulus is compressed by formation of a crescent (C) of proliferating mononuclear cells within the Bowman’s capsule (B) and there is influx of neutrophils (N) into the

glomerular tuft.  Immune complexes are produced whenever there is an antibody response to a soluble antigen  Normally, they are cleared efficiently by red blood cells bearing complement receptors and by phagocytes of the mononuclear phagocytic system that have both complement and Fc receptors, and such complexes cause little tissue damage. This clearance system can, however, fail in three circumstances. The first follows the injection of large amounts of antigen, leading to the formation of large amounts of immune complexes that overwhelm the normal clearance mechanisms. An example of this is serum sickness, which is caused by injection of large amounts of serum proteins. This is a transient disease, lasting only until the immune complexes have been cleared. The second circumstance is seen in chronic infections such as bacterial endocarditis, where the immune response to bacteria lodged on a cardiac valve is incapable of clearing infection. The persistent release of bacterial antigens from the valve infection in the presence of a strong antibacterial antibody response causes widespread immune-complex injury to small blood vessels in organs such as the kidney and the skin.

The third type of failure to clear immune complexes is seen in SLE. This is an immune complex-mediated disease characterized by chronic IgG antibody production directed at ubiquitous self antigens present in all nucleated cells. In SLE, a wide range of autoantibodies are produced to common cellular constituents. The main antigens are three intracellular nucleoprotein particles the nucleosome, the spliceosome, and a small cytoplasmic ribonucleoprotein complex containing two proteins known as Ro and La (named after the first two letters of the surnames of the two patients in which autoantibodies against these proteins were discovered). In order for these autoantigens to participate in immune-complex formation, they must become extracellular. The autoantigens of SLE are exposed on dead and dying cells and released from injured tissues. In SLE, large quantities of antigen are available, so large numbers of small immune complexes are produced continuously and are deposited in the walls of small blood vessels in the renal glomerulus, in glomerular basement membrane , in joints, and in other organs. This leads to activation of phagocytic cells through their Fc receptors. The consequent tissue damage releases more nucleoprotein complexes, which in turn form more immune complexes. Eventually, the inflammation induced in small blood vessel walls, especially in the kidney, can cause sufficient damage to kill the patient. Mice that lack FcγRIII illustrate the importance of Fc receptors in causing the inflammatory response to immune complexes. Such mice do not develop glomerulonephritis, despite deposition of immune complexes and C3, demonstrating the dominant role of Fc receptors in the autoimmune effector mechanisms of SLE.

 

Panel a, a section through a renal glomerulus from a patient with SLE, shows that

the deposition of immune complexes has caused thickening of the glomerular basement membrane, seen as the  clear ‘canals’ running through the glomerulus. Panel b, a similar section stained with fluorescent antiimmunoglobulin, reveals immunoglobulin deposits in the basement membrane. Panel c, by electron microscopy the immune complexes are seen as dense protein deposits between the glomerular basement membrane and the renal epithelial cells. Polymorphonuclear neutrophilic leukocytes are also present, attracted by the deposited

immune complexes. 

Environmental cofactors can influence the expression of autoimmune disease.

The presence of an autoantibody by itself is not sufficient to cause autoimmune disease. For disease to occur, the autoantigen must be available for binding by the autoantibody. Two examples illustrate how the availability of autoantigens and the resulting expression of disease can be modulated by environmental cofactors. In untreated Goodpasture’s disease, autoantibodies against type IV collagen typically cause a fatal glomerulonephritis. Type IV collagen is distributed widely in basement membranes throughout the body, including those of the alveoli of the lung, the renal glomeruli, and the cochlea of the inner ear. All patients with Goodpasture’s disease develop glomerulonephritis, about 40% develop pulmonary hemorrhage, but none become deaf.

This pattern of disease expression was explained when it was discovered that pulmonary hemorrhage was found almost exclusively in those patients who smoked cigarettes. What differs between basement membrane in glomeruli, alveoli, and the cochlea is the availability of the antigen to antibodies. The major function of glomerular basement membrane is the filtration of plasma, and the endothelium lining glomerular capillaries is fenestrated to allow access of plasma to the basement membrane. Glomerular basement membrane is therefore immediately accessible to circulating autoantibodies. In the alveoli, in contrast, the basement membrane separates the alveolar epithelium from the capillary endothelium, whose cells are joined together by tight junctions. Injury to the endothelial lining of pulmonary capillaries is therefore necessary before antibodies can gain access to the basement membrane. Cigarette smoke stimulates an inflammatory response in the lungs, which damages alveolar capillaries and exposes the autoantigen to antibody. Finally, in the inner ear, the cochlear basement membrane seems to remain inaccessible to autoantibodies at all times.

A second example of the importance of environmental influences on the expression of autoimmunity is the effect of infection on the vasculitis associated with Wegener’s granulomatosis. This disease, which is characterized by a severe necrotizing vasculitis, is strongly associated with the presence of autoantibodies to a granule proteinase of neutrophils; the antibodies are known as anti-neutrophil cytoplasm antibodies (commonly abbreviated as ANCA). The autoantigen is proteinase-3, an abundant serine proteinase of neutrophil granules. Although there is a general correlation between the levels of ANCA and the expression of disease, it is quite common to find patients with high levels of ANCA who remain asymptomatic. If such an individual develops an infection, however, this frequently induces a severe flare-up of the vasculitis.

 

Serum from patients with Wegener’s granulomatosis contains autoantibodies reactive with neutrophil cytoplasmic granules.

 

Normal neutrophils with permeabilized cell membranes have been incubated with serum from a patient with Wegener’s granulomatosis. IgG antibodies in the serum reactive with cytoplasmic granules are detected by addition of fluorescein-conjugated antibodies against IgG.

It is thought that the reason for this is that resting neutrophils do not express proteinase-3 on the cell surface, and so in the absence of infection the antigen is inaccessible to anti-proteinase-3 autoantibodies. After infection, a variety of cytokines activate neutrophils, with translocation of proteinase-3 to the cell surface. Antiproteinase-3 antibodies caow bind neutrophils and stimulate degranulation and release of free radicals. In parallel, activation of vascular endothelial cells by the infection causes the expression of vascular adhesion molecules, such as E-selectin, which promote the binding of activated neutrophils to vessel walls with resultant injury. In this way, a variety of nonspecific infections can exacerbate an autoimmune disease.

The pattern of inflammatory injury in autoimmunity can be modified by anatomical constraints.

We have seen that the distribution of organ injury in Goodpasture’s syndrome can be explained by the accessibility of basement membrane collagen to autoantibodies and that environmental factors can influence the availability of antigen in different organs. Another example of how the expression of autoimmune inflammation can be modified by anatomical factors is seen in membranous glomerulonephritis. In this disease, patients develop heavy proteinuria (the excretion of protein in the urine), which can cause life-threatening depletion of plasma protein levels. Biopsy of an affected kidney reveals evidence of deposition of antibody and complement beneath the basement membrane of the glomerulus but, in contrast to Goodpasture’s syndrome, there is no significant influx of inflammatory cells. The autoantigen in this disease has not been characterized. However, an excellent rodent model of membranous glomerulonephritis is Heymann’s nephritis, in which autoantibodies against a glycoprotein on the surface of tubular epithelial cells of the kidney are induced by injection of tubular epithelial tissue. The proteinuria can be abolished by depletion of any of the proteins of the membrane-attack complex of complement but is unaltered by depletion of neutrophils. This shows that the antibodies deposited beneath the glomerular basement membrane in this disease cause tissue injury by activation of complement, but the glomerular basement membrane acts as a complete barrier to inflammatory leukocytes.

In other autoimmune diseases, high levels of autoantibodies against intracellular antigens can be found in the

absence of any evidence of antibody-induced inflammation. One such example is a rare myositis (inflammation of muscle) associated with pulmonary fibrosis. Most patients with this disease have high levels of autoantibodies reactive with aminoacyl-tRNA synthetases, the intracellular enzymes responsible for loading tRNAs with amino acids. Addition of these autoantibodies to cell-free extracts in vitro stops translation and protein synthesis completely. There is, however, no evidence that these antibodies cause any injury in vivo, where it is unlikely that they can enter living cells. In this disease, the autoantibody is thought to be a marker of a particular pattern of tissue injury, and does not contribute to the immunopathology of the myositis. Other examples of autoantibodies that are useful diagnostic markers of the presence of disease, but that might play no part in causing organ injury, are antibodies against mitochondrial antigens associated with primary biliary cirrhosis and antibodies against smooth muscle antigens in chronic active hepatitis.

The mechanism of autoimmune tissue damage can often be determined by adoptive transfer.

To classify a disease as autoimmune , one must show that an adaptive immune response to a self antigen causes the observed pathology. Initially, the demonstration that antibodies against the affected tissue could be detected in the serum of patients suffering from various diseases was taken as evidence that the diseases had an autoimmune basis. However, such autoantibodies are also found when tissue damage is caused by trauma or infection, though these are typically of much lower affinity than those associated with autoimmune disease.

This suggests that autoantibodies can result from, rather than be the cause of, tissue damage. Thus, one must show that the observed autoantibodies are pathogenic before classifying a disease as autoimmune.

It is often possible to transfer disease to experimental animals through the transfer of autoantibodies, causing pathology similar to that seen in the patient from whom the antibodies were obtained. This does not always work, however, presumably because of species differences in autoantigen structure. Some autoimmune diseases can also be transferred from mother to fetus  and are observed in the newborn babies of diseased mothers. When babies are exposed to IgG autoantibodies transferred across the placenta, they will often manifest pathology similar to the mother’s. This natural experiment is one of the best proofs that particular autoantibodies exert pathogenic effects. The symptoms of the disease in the newborn typically disappear rapidly as the maternal antibody is catabolized, although they may cause chronic organ injury, such as damage to the heart in babies of mothers with SLE or Sjögren’s syndrome. The clearance can be speeded up by a complete exchange of the infant’s blood or plasma (plasmapheresis), though this is of no clinical use after permanent injury has occurred, as in congenital heart block.

 

Serum from some patients with autoimmune disease can transfer the same disease to

experimental animals.

 

When the autoantigen is very similar in humans and mice or rats, the transfer of

antibody from an affected human can cause the same symptoms in an experimental animal. For example, antibody from patients with Graves’ disease frequently produces thyroid activation in rats.

 

Some autoimmune diseases that can be transferred across the placenta by pathogenic IgG autoantibodies.

 

These diseases are caused mostly by autoantibodies to cell-surface or tissue-matrix molecules.

This suggests that an important factor determining whether an autoantibody that crosses the placenta causes disease in the fetus or newborn baby is the accessibility of the antigen to the auto-antibody. Autoimmune congenital heart block is caused by fibrosis of the developing cardiac conducting tissue, leading to slowing of

the heart rate (bradycardia), and there is evidence that this expresses abundant Ro antigen.

Ro protein is a constituent of an intracellular small cytoplasmic ribonucleo-protein. It is not yet known whether it is expressed at the cell surface of cardiac conducting tissue to act as a target for autoimmune tissue injury.

 

Antibody-mediated autoimmune diseases can appear in the infants of affected mothers as a consequence of transplacental antibody transfer.

 

In pregnant women, IgG antibodies cross the placenta and accumulate in the fetus before birth. Babies born to mothers with IgG-mediated autoimmune disease therefore frequently show symptoms similar to those of the mother in the first few weeks of life.

Fortunately, there is little lasting damage as the symptoms disappear along with the maternal antibody. In Graves’ disease, the symptoms are caused by antibodies against the thyroid-stimulating hormone receptor (TSHR). Children of mothers making thyroid-stimulating antibody are born with hyperthyroidism, but this can be corrected by replacing the plasma with normal plasma (plasmapheresis), thus removing the maternal antibody.

 

Immunoglobulin isotypes are selectively distributed in the body.

 

IgG and IgM predominate in plasma, whereas IgG and monomeric IgA are the major isotypes in extracellular fluid within the body. Dimeric IgA predominates in secretions across epithelia, including breast milk. The fetus receives IgG from the mother by transplacental transport. IgE is found mainly associated with mast cells just beneath epithelial surfaces (especially of the respiratory tract, gastro-intestinal tract, and skin). The brain is normally devoid of immunoglobulin.

T cells specific for self antigens can cause direct tissue injury and have a role in sustained autoantibody responses.

Activated effector T cells specific for self peptide:self MHC complexes can cause local inflammation by activating macrophages or can damage tissue cells directly. Diseases in which these actions of T cells are likely to be important include type I IDDM, rheumatoid arthritis, and multiple sclerosis. Affected tissues in patients with these diseases are heavily infiltrated with T lymphocytes and activated macrophages. These autoimmune diseases are mediated by T cells specific for the autoantigen presented by self MHC. T cells are, of course, also required to sustain all autoantibody responses.

It is much more difficult to demonstrate the existence of autoreactive T cells than it is to demonstrate the presence of autoantibodies. First, autoreactive human T cells cannot be used to transfer disease to experimental animals because T-cell recognition is MHC-restricted and animals and humans have different MHC alleles.

Second, it is difficult to identify the antigen recognized by a T cell; for example, autoantibodies can be used to stain self tissues to reveal the distribution of the autoantigen, whereas T cells cannot. Nevertheless, there is strong evidence for the involvement of autoreactive T cells in several autoimmune diseases. In type I IDDM,

the insulin-producing β cells of the pancreatic islets are selectively destroyed by specific T cells. When such diabetic patients are transplanted with half a pancreas from an identical twin donor, the β cells in the grafted tissue are rapidly and selectively destroyed by CD8 T cells. Recurrence of disease can be prevented by the immunosuppressive drug cyclosporin A, which inhibits T-cell activation. Progress towards identifying the targets of such autoreactive T cells and proving that these cells cause disease will be discussed

 

Autoantibodies can be used to identify the target of the autoimmune process.

Autoantibodies can be used to purify an autoantigen so that it can be identified. This approach is particularly useful if the autoantibody causes disease in animals, from which large amounts of tissue can be obtained.

Autoantibodies can also be used to examine the distribution of the target antigen in cells and tissues by immunohistology, often providing clues to the pathogenesis of the disease.

The identification of a critical autoantigen can also lead to the identification of the CD4 T cells responsible for stimulating autoantibody production. As we learned in Chapter 8, CD4 T cells selectively activate those B cells that bind epitopes that are physically linked to the peptide recognized by the T cell. It follows that the proteins or protein complexes isolated by means of autoantibodies should contain the peptide recognized by the autoreactive CD4 T cell. For example, in myasthenia gravis the autoantibodies that cause disease bind mainly to the α chain of the acetylcholine receptor and can be used to isolate the receptor from lysates of skeletal muscle cells. CD4 T cells that recognize peptide fragments of this receptor subunit can also be found in patients with myasthenia gravis. Thus, both autoreactive B cells and autoreactive T cells are required for this disease.

 

Autoimmune disease caused by antibodies also requires autoreactive T cells.

 

Autoantibodies from the serum of myasthenia gravis patients immunoprecipitate the acetylcholine receptor from lysates of skeletal muscle cells (top panels). To be able to produce antibodies, the same patients should also have CD4 T cells that respond to a peptide derived from the acetylcholine receptor. To detect them, T cells from myasthenia gravis patients are isolated and grown in the presence of the acetylcholine receptor plus antigen-presenting cells of the correct MHC type (bottom panels). T cells specific for epitopes of the acetylcholine receptor are stimulated to proliferate and can thus be detected.

The same phenomenon is seen in SLE. Tissue damage in this disease is caused by immune complexes of autoantibodies directed against a variety of nucleoprotein antigens. These autoantibodies show a high degree of somatic hypermutation, which has all the hallmarks of being antigen-driven  , and the B cells that produce  them can be shown to have undergone extensive clonal expansion. Thus, the autoantibodies have the characteristic properties of antibodies formed in response to chronic stimulation of B cells by antigen and specific CD4 T cells, strongly suggesting that they are produced in response to autoantigens containing peptides recognized by specific autoreactive CD4 T cells. Further evidence for this comes from the collective autoantibody specificities observed in individual patients. The autoantibodies in any one individual tend to bind all constituents of a particular nucleoprotein particle; this strongly suggests that there must be CD4 T cells present that are specific for a peptide constituent of this particle. A B cell whose receptor binds a component of this particle will internalize and process the particle, present the peptide to these autoreactive T cells, and receive help from them. Such B-cell-T-cell interactions initiate the antibody response and promote clonal expansion and somatic hypermutation, thus accounting for the observed characteristics of the autoantibody response as well as the clustering of autoantibody specificities in individual patients. This allows the spreading of the autoimmune response to different components of multimolecular complexes, known as antigen spreading or determinant spreading.

 

Autoreactive helper T cells of one specificity can drive the production of autoantibodies

with several different specificities, in a phenomenon known as antigen spreading.

 

In an SLE patient, a B cell specific for the H1 histone protein iucleosomes, for example, will bind and internalize the whole nucleosome, and present peptides derived from H1 histone as well as other peptides. This B cell can receive help from a T cell specific for one of the peptides derived from H1 (top panels). A B cell that recognizes the DNA in the nucleosome can also internalize the nucleosome, process it, and present the H1 peptide to that T cell and be activated by it (center panels). Thus, a single auto-reactive helper T cell can stimulate a diverse antibody response, but the antibodies will be restricted to those specific for the constituents of a single type of particle. B cells able to bind ribosomes, for example, do not present the H1 peptide and so will not be activated to produce anti-ribosomal antibodies in this patient (bottom panels).

 

The target of T cell-mediated autoimmunity is difficult to identify owing to the nature of T-cell

ligands.

Although there is good evidence that T cells are involved in many autoimmune diseases, the T cells that cause particular diseases are hard to isolate, and their targets are difficult to identify. Also, the cells are hyporesponsive and thus difficult to assay. It is also difficult to assay the T cells for their ability to cause disease, because any assay requires target cells of the same MHC genotype as the patient. This problem becomes more tractable in animal models. As many autoimmune diseases in animals are induced by immunization with self tissue, the nature of the autoantigen can be determined by fractionating an extract of the tissue and testing the fractions for their ability to induce disease. It is also possible to clone T-cell lines that will transfer the disease from an affected animal to another animal with the same MHC genotype.

This has made it possible to identify the autoantigens recognized by T cells in many experimental autoimmune diseases; they are individual peptides that bind to specific MHC molecules. In some cases, the peptide antigen, when made immunogenic, is able to elicit disease symptoms in animals of the appropriate MHC genotype. An example of such an experimental autoimmune disease is experimental allergic encephalomyelitis (EAE), which can be induced in certain susceptible strains of mice and rats by injection of central nervous system tissue together with Freund’s complete adjuvant. This disease resembles human multiple sclerosis, in which characteristic plaques of tissue injury are disseminated throughout the central nervous system. Plaques of active disease show infiltration of nervous tissue by lymphocytes, plasma cells, and macrophages, which cause destruction of the myelin sheaths that surround nerve cell axons in the brain and spinal cord.

Further analysis of EAE showed that injection with various purified components of the myelin sheath, notably myelin basic protein (MBP), proteolipid protein (PLP), and myelin oligodendroglial protein (MOG), can induce EAE. The disease can be transferred to syngeneic animals by using cloned T-cell lines derived from animals with EAE. Many of these cloned T-cell lines are stimulated by peptides of MBP. EAE can be caused by injection of these MBP peptides into animals that possess MHC alleles capable of presenting such peptides to T cells. Activated T cells specific for myelin proteins have also been identified in patients with multiple sclerosis. Although it has not yet been proved that these cells cause the demyelination in multiple sclerosis, this finding suggests that animal models might provide clues to the identity of autoantigenic proteins in human disease.

 

T cells specific for myelin basic protein mediate inflammation of the brain in experimental

autoimmune encephalomyelitis (EAE).

 

This disease is produced in experimental animals by injecting them

with isolated spinal cord homogenized in complete Freunds’ adjuvant. EAE is due to an inflammatory reaction in the brain that causes a progressive paralysis affecting first the tail and hind limbs (as shown in the mouse on the left of the photograph, compared with a healthy mouse on the right) before progressing to forelimb paralysis and eventual death. One of the autoantigens identified in the spinal cord homogenate is myelin basic protein (MBP). Immunization with MBP alone in complete Freund’s adjuvant can also cause these disease symptoms.

Inflammation of the brain and paralysis are mediated by TH1 cells specific for MBP. Cloned MBP-specific TH1 cells can transfer symptoms of EAE to naive recipients provided that the recipients carry the correct MHC allele. In this system it has therefore proved possible to identify the peptide:MHC complex recognized by the TH1 clones that transfer disease. Other purified components of the myelin sheath can also induce the symptoms of EAE, so there is more than one autoantigen in this disease.

A variety of inflammatory autoimmune diseases can be mediated by TH1 cells responding to self antigens.

EAE, for example, can be caused by TH1 cells specific for MBP, as shown by the ability of specific clones of TH1, but not TH2, cells to cause disease on adoptive transfer. Although MBP is an intracellular protein, it is processed for presentation by the vesicular pathway and thus its peptides are presented by MHC class II molecules and recognized by CD4 T cells. Another inflammatory autoimmune disease, rheumatoid arthritis, may be caused by TH1 cells specific for an as yet unidentified antigen present in joints. Engagement with this antigen triggers the T cells to release lymphokines that initiate local inflammation within the joint. This causes swelling, accumulation of polymorphonuclear leukocytes and macrophages, and damage to cartilage, leading to the destruction of the joint. Rheumatoid arthritis is a complex disease and also involves antibodies, often including an IgM anti-IgG autoantibody called rheumatoid factor. Like the SLE autoantibodies described the rheumatoid factors isolated from the joints of patients with rheumatoid arthritis show evidence of a T-cell dependent, antigen-driven B-cell response against the Fc portion of IgG. Some of the tissue damage in this disease is caused by the resultant IgM:IgG immune complexes.

Autoantigens recognized by CD4 T cells can be identified by adding cell extracts to cultures of blood mononuclear cells and testing for recognition by CD4 cells derived from an autoimmune patient. If the autoantigen is present in the cell extract, it should be effectively presented, as phagocytes in the blood cultures can take up extracellular protein, degrade it in intracellular vesicles, and present the resulting peptides bound to MHC class II molecules. Identification of autoantigenic peptides is, however, particularly difficult in autoimmune diseases caused by CD8 T cells, as autoantigens recognized by CD8 T cells are not effectively presented in such cultures. Peptides presented by MHC class I molecules must usually be made by the target cells themselves; intact cells of target tissue from the patient must therefore be used to study autoreactive CD8 T cells that cause tissue damage. Conversely, the pathogenesis of the disease can itself give clues to the identity of the antigen in some CD8 T cell-mediated diseases. For example, in type I IDDM, the insulin-producing β cells of the pancreatic islets of Langerhans seem to be specifically targeted and destroyed by CD8 T cells. This suggests that a protein unique to β cells is the source of the peptide recognized by the pathogenic CD8 T cells. Studies in the NOD mouse model of type I diabetes have shown that peptides from insulin itself are recognized by pathogenic CD8 cells, confirming the role of insulin as one of the principal autoantigens in type I diabetes.

 

Selective destruction of pancreatic β cells in insulin-dependent diabetes mellitus (IDDM)

indicates that the autoantigen is produced in β cells and recognized on their surface.

 

In IDDM, there is highly specific destruction of insulin-producing β cells in the pancreatic islets of Langerhans, sparing other islet cell types (α and δ). This is shown schematically in the upper panels. In the lower panels, islets from normal

(left) and diabetic (right) mice are stained for insulin (brown), which shows the β cells, and glucagon (black), which shows the α cells. Note the lymphocytes infiltrating the islet in the diabetic mouse (right) and the selective loss of the β cells (brown) whereas the α cells (black) are spared. The characteristic morphology of the islet is also disrupted with the loss of the β cells. Photographs courtesy of I. Visintin.

CD4 T cells also seem to be involved in type I IDDM, consistent with the linkage of disease susceptibility to particular MHC class II alleles. Identifying the autoantigen recognized by CD4 T cells in these diseases is an important goal. Not only might it help us to understand disease pathogenesis but it might also result in several innovative approaches to treatment.

 

Clinical examples

 

Rheumatoid arthritis (RA) is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks flexible (synovial) joints. The process involves an inflammatory response of the capsule around the joints (synovium) secondary to swelling (hyperplasia) of synovial cells, excess synovial fluid, and the development of fibrous tissue (pannus) in the synovium. The pathology of the disease process often leads to the destruction of articular cartilage and ankylosis (fusion) of the joints. Rheumatoid arthritis can also produce diffuse inflammation in the lungs, membrane around the heart (pericardium), the membranes of the lung (pleura), and white of the eye (sclera), and also nodular lesions, most common in subcutaneous tissue. Although the cause of rheumatoid arthritis is unknown, autoimmunity plays a pivotal role in both its chronicity and progression, and RA is considered a systemic autoimmune disease.

About 1% of the world’s population has rheumatoid arthritis, women three times more often than men. Onset is most frequent between the ages of 40 and 50, but people of any age can be affected. In addition, individuals with the HLA-DR1 or HLA-DR4 serotypes have an increased risk for developing the disorder. It can be a disabling and painful condition, which can lead to substantial loss of functioning and mobility if not adequately treated. It is a clinical diagnosis made on the basis of symptoms, physical exam, radiographs (X-rays) and labs, although the American College of Rheumatology (ACR) and the European League Against Rheumatism (EULAR) publish classification criteria for the purpose of research. Diagnosis and long-term management are typically performed by a rheumatologist, an expert in joint, muscle and bone diseases.

Various treatments are available. Non-pharmacological treatment includes physical therapy, orthoses, occupational therapy and nutritional therapy but these do not stop the progression of joint destruction. Analgesia (painkillers) and anti-inflammatory drugs, including steroids, are used to suppress the symptoms, while disease-modifying antirheumatic drugs (DMARDs) are required to inhibit or halt the underlying immune process and prevent long-term damage. In recent times, the newer group of biologics has increased treatment options. Clinical trials have shown that consumption of fish oil reduces the number of swollen joints for people with rheumatoid arthritis, provides a beneficial anti-inflammatory effect, and provides a protective effect for occlusive cardiovascular disease, for which people with RA are at risk.

 

 

In 2010 the 2010 ACR / EULAR Rheumatoid Arthritis Classification Criteria were introduced. These new classification criteria overruled the “old” ACR criteria of 1987 and are adapted for early RA diagnosis. The “new” classification criteria, jointly published by the American College of Rheumatology (ACR) and the European League Against Rheumatism (EULAR) establish a point value between 0 and 10. Every patient with a point total of 6 or higher is unequivocally classified as an RA patient, provided he has synovitis in at least one joint and given that there is no other diagnosis better explaining the synovitis. Four areas are covered in the diagnosis:

 

 

–         joint involvement, designating the metacarpophalangeal joints, proximal interphalangeal joints, the interphalangeal joint of the thumb, second through fifth metatarsophalangeal joint and wrist as small joints, and shoulders, elbows, hip joints, knees, and ankles as large joints:

1.     Involvement of 1 large joint gives 0 points

2.     Involvement of 2-10 large joints gives 1 point

3.     Involvement of 1-3 small joints (with or without involvement of large joints) gives 2 points

4.     Involvement of 4-10 small joints (with or without involvement of large joints) gives 3 points

5.     Involvement of more than 10 joints (with involvement of at least 1 small joint) gives 5 points

–         serological parameters – including the rheumatoid factor as well as ACPA – “ACPA” stands for “anti-citrullinated protein antibody”:

1.     Negative RF and negative ACPA gives 0 points

2.     Low-positive RF or low-positive ACPA gives 2 points

3.     High-positive RF or high-positive ACPA gives 3 points

–         acute phase reactants: 1 point for elevated erythrocyte sedimentation rate, ESR, or elevated CRP value (c-reactive protein)

–         duration of arthritis: 1 point for symptoms lasting six weeks or longer

 

There is no known cure for rheumatoid arthritis, but many different types of treatment can alleviate symptoms and/or modify the disease process. Recommendations of the American College of Rheumatology (ACR), published in 2008, followed a trend in supporting earlier, more aggressive treatment of RA, and reflected heightened expectations of treatment effectiveness, including remission or substantial alleviation of symptoms for a rising percentage of patients.

The goals of treatment include minimizing clinical symptoms such as pain and swelling, as well as preventing bone deformity and radiographic damage (for example, bone erosions visible in X-rays), and maintaining the quality of life in terms of day-to-day activities. These goals can be achieved using the following two main categories of pharmacological drugs: analgesics and NSAIDS, and DMARDS. ACR recommends that RA should generally be treated with at least one specific anti-rheumatic medication. ACR also recommends different combinations or DMARDs depending on the duration of disease from onset, prognosis (based on radiographic images and laboratory results), and activity of the disease.

 

Systemic lupus erythematosus, often abbreviated to SLE or lupus, is a systemic autoimmune disease (or autoimmune connective tissue disease) that can affect any part of the body. As occurs in other autoimmune diseases, the immune system attacks the body’s cells and tissue, resulting in inflammation and tissue damage. It is a Type III hypersensitivity reaction in which antibody-immune complexes precipitate and cause a further immune response.

 

 

 

Summary.

For a disease to be defined as autoimmune, the tissue damage must be shown to be caused by an adaptive immune response to self antigens. Autoimmune diseases can be mediated by autoantibodies and/or by autoreactive T cells, and tissue damage can result from direct attack on the cells bearing the antigen, from immunecomplex formation, or from local inflammation. Autoimmune diseases caused by antibodies that bind to cellular receptors, causing either excess activity or inhibition of receptor function, fall into a special class. T cells can be involved directly in inflammation or cellular destruction, and they are also required to sustain autoantibody responses. Similarly, B cells may be important antigen-presenting cells for sustaining autoantigenspecific T-cell responses. The most convincing proof that the immune response is causal in autoimmunity is transfer of disease by transferring the active component of the immune response to an appropriate recipient.

The immediate challenge is to identify the autoantigens recognized by T cells in autoimmunity, and to use this information to control the activity of these T cells, or to prevent their activation in the first place. The deeper, more important question is how the autoimmune response is induced. Much has been learned about the induction of immune responses to tissue antigens by examining the response to nonself tissues in transplantation experiments. We will therefore examine the immune response to grafted tissues in the next part of the chapter before turning to the problem of how tolerance is normally maintained, and why immune responses to self antigens occur to cause autoimmune disease.

 

Tumor Immunology

 

The proliferation of normal cells is carefully regulated. However, such cells when exposed to chemical carcinogens, irradiation and certain viruses may undergo mutations leading to their transformation into cells that are capable of uncontrolled growth, producing a tumor or neoplasm.

A tumor may be

1) Benign, if it is not capable of indefinite growth and the host survives.

2) Malignant, if the tumor continues to grow indefinitely and spreads (metastasizes), eventually killing the host.

This uncontrolled growth may be due to upregulation of oncogenes (cancer-inducing genes) and/or downregulation of tumor suppressor genes (that normally inhibit tumor growth often by inducing cell death).

Evidence for existence of an immune response against tumors

The following criteria serve as evidence that tumors can elicit an immune response.

1. Certain tumors regress spontaneously (e.g., melanomas, neuroblastomas).suggesting an immunological response.

2. Tumors that have severe mononuclear cell infiltration have a better prognosis than those that lack it.

3. Some tumor metastases regress after removal of primary tumor which reduces the tumor load, thereby inducing the immune system to kill the residual tumor..

4. Although chemotherapy leads to rejection of a large number of tumor cells, the few tumor cells that evade the action of the drugs can outgrow and kill the host. However, the immune system may be able to mount an attack against the few tumor cells that are spared by the chemotherapeutic agent.

5. There is an increased incidence of malignancies in immunodeficient patients such as AIDS patients who are susceptible to Kaposi sarcoma and transplant patients who are susceptible to Epstein Barr virus (EBV)-induced lymphoma.

6. Tumor-specific antibodies and T lymphocytes (detected in cytotoxicity and proliferative response assays) have been observed in patients with tumors.

7. The young and the old population have an increased incidence of tumors. These members of the population often have an immune system that is compromised.

8. Hosts can be specifically immunized against various types of tumors demonstrating tumor Ags can elicit an immune response.

Tumor antigens

Tumorigenesis may lead to expression of new antigens or alteration in existing antigens that are found oormal cells. These antigens may include membrane receptors, regulators of cell cycle and apoptosis, or molecules involved in signal transduction pathways.

There are 2 main types of tumor antigens.

1. Tumor-specific transplantation antigens (TSTA) which are unique to tumor cells and not expressed oormal cells. They are responsible for rejection of the tumor.

2. Tumor associated transplantation antigens (TATA) that are expressed by tumor cells and normal cells.

Although chemical- , UV- or virus-induced tumors express neo-antigens, majority of the tumors are often weakly immunogenic or non-immunogenic. In most cases, tumor-specific transplantation Ags cannot be identified easily. Also, some of these antigens may be secreted while others include membrane-associated molecules.

Tumor associated transplantation antigens (TATA)

The majority of tumor Ags are the tumor associated transplantation antigens (TATA). They may be expressed at higher levels on tumor cells when compared to normal cells. Alternatively, they may be expressed only during development of cells and lost during adult life but re-expressed in tumors. These include the tumor-associated developmental Ags (TADA) and tumor-associated viral Ags (TAVA).

Tumor-associated developmental Ags (TADA) or Onco-fetal antigens

These include alpha-fetoprotein (AFP) and carcino-embryonic antigen (CEA) found secreted in the serum.

AFP is found in patients with hepatocellular carcinoma whereas CEA is found in colon cancer. These are important in diagnosis.

Virus-induced tumors:

Viruses that cause tumors include

DNA viruses:

1. Papova (papilloma, polyoma) viruses. Ex. Papilloma virus causes cervical cancer.

2. Hepatitis virus: Hepatitis B virus causes hepatocellular cancer.

3. Adenoviruses

RNA viruses:

Retroviruses: Human T-lymphotropic viruses (HTLV-I and HTLV-II) causes Adult T cell leukemia.

Virus-induced tumors express tumor-associated viral Ags (TAVA). These are cell surface antigens that are

distinct from antigens on the virion itself. However, these transplantation-associated viral Ags are shared by all tumors induced by the same virus, regardless of tissue origin of the tumor or animal in which the tumor exists.

Chemically-induced tumors

Chemically-induced tumors are different from virally-induced tumors in that they are extremely heterogeneous in their antigenic characteristics. Thus, any two tumors induced by the same chemical, even in the same animal, rarely share common tumor specific antigens. These unique antigens on chemically-induced tumors are referred

to as tumor- specific transplantation antigens (TSTA).

Syngeneic, Allogeneic and Xenogeneic Tumors:

A tumor that grows in an animal strain will also grow in another animal belonging to the same inbred strain obtained by repeated brother-sister matings. These animals express the same MHC molecules and are referred to as syngeneic. However, most normal animal populations are allogeneic and have various MHC haplotypes.

Thus, a tumor transferred from one animal to another animal belonging to an outbred strain is rejected because of the allo-MHC rather than the TSTA. A tumor transferred from an animal belonging to one species to another animal belonging to a different species is rapidly rejected because the animals are xenogeneic.

Immune response to tumors:

Evidence for immunity against malignancy comes mostly from experimental studies, wherein mice were immunized by administering irradiated tumor cells or following removal of a primary tumor challenged with the same live tumor. These animals were found to be resistant to rechallenge with the same live tumor. While Abs may develop against few cancers, cell-mediated immunity plays a critical role in tumor rejection. Thus, immunity can be transferred, in most cases, from an animal, in which a tumor has regressed, to a naive syngeneic recipient by administration of T lymphocytes. The T helper (Th) cells recognize the tumor Ags that may be shed from tumors and internalized, processed and presented in association with class II MHC on antigen 3

presenting cells. These Th cells when activated will produce cytokines. Thus, the Th cells provide help to B cells in Ab production. The cytokines such as IFN-γ may also activate macrophages to become tumoricidal.

Furthermore, the Th cells also provide help to tumor-specific cytotoxic T cells (CTL) by inducing their proliferation and differentiation. The CTL recognize tumor Ags in the context of class I MHC and mediate tumor cell lysis. In tumors that exhibit decreased MHC Ags, natural killer (NK) cells are important in mediating tumor rejection.

How tumors evade immune system:

According to the Immune Surveillance Theory, cancer cells that arise in the body are eliminated by the immune system. However, due to impaired immune reactivity, the cancer cells escape destruction.

Tumors evade immune recognition by several mechanisms. Some tumors may evade the immune system by secreting immunosuppressive molecules such as interleukin-10 (IL-10) or transforming growth factor-beta (TGF-β) and others may induce regulatory cells particularly the CD4+

CD25+

 FoxP3+

 T regulatory cells or myeloid derived suppressor cells (MDSC) which have both granulocyte and macrophage markers (Gr-1+ CD11b+). Also, some tumors may shed their antigens which in turn may interact and block antibodies and T cells from reacting with the tumor cells. Tumors may not express neo-antigens that are immunogenic or they may fail to express co-stimulatory molecules required for the activation of T cells. In addition, certain tumors are known to lack or be poor expressers of MHC antigen. Such tumors are however, susceptible to NK cell cytotoxicity. Another reason for failure of immune surveillance may be the fact that in the early development of a tumor, the amount of antigen may be too small to stimulate the immune system (low dose tolerance) or due

to the rapid proliferation of malignant cells (high dose tolerance), the immune system is quickly overwhelmed.

Tumor cells may express the death inducing ligand, FasL (CD95L) whereas the T cells express the death receptor, Fas (CD95), thereby leading to killing of the T cells. However, CTL have been shown to express FasL and some tumors may express Fas.

Immunotherapy

Immunotherapy has been used as a novel mode to treat cancer. Both active and passive means of stimulating the non-specific and specific immune system have been employed, in some cases with significant success.

1) Active Immunotherapy: Wherein the host actively participates in mounting an immune response

a) Specific activation using vaccines:

i) Hepatitis B vaccine useful against development of hepatocellular cancer.

ii) Human Papilloma virus (HPV) vaccine (Gardasil) has been successfully used to prevent cervical cancers

b) Nonspecific activation which results in stimulation of generalized immune response is achieved by immunization with:

i) Bacillus Calmette-Guerin (BCG) mycobacteria.

ii) Corynebacterium parvum

These microbes lead to activation of macrophages which are tumoricidal.

2) Passive Immunotherapy: This involves transfer of preformed Abs, immune cells and other factors into the hosts.

a) Specific: Preformed Abs or CTL directed against tumor Ags are used in the treatment of tumors

i) Antibodies against tumor Ags (e.g. Abs against Her2/Neu for treatment of breast cancer)

ii) Abs against interleukin-2 receptor (IL-2R) are used in the treatment of Human T lymphotropic virus

(HTLV-1)-induced adult T cell leukemia as this virus infects T cells and leads to production of IL-2 that binds to IL-2R and induces the T cell proliferation.

iii) Abs against CD20 expressed on all B cells are used in the treatment of non Hodgkin’s B cell lymphoma.

4 These Abs bind to tumor Ags on the cell surface and activate complement (C’) to mediate tumor cell lysis. In addition, Fc receptor bearing cells such as NK cells, macrophages and granulocytes may bind to the Ag-Ab complexes on tumor cell surface and mediate tumor cell killing through Ab-dependent cell-mediated cytotoxicity. iv) Abs conjugated to toxins, radioisotopes and anti-cancer drugs have also been used. These enter the cells and inhibit protein synthesis because of the toxin. e.g. anti-CD20 conjugated to Pseudomonas toxin or ricin toxin has been used in the treatment of B cell tumors.

There are several problems with the use of Abs

(1) Abs are not efficient because the tumor Ags are associated with class I MHC Ags.

(2) The tumors may shed Ag or Ag-Ab complexes. Thus, immune cells cannot mediate tumor destruction.

(3) Some Abs may not be cytotoxic.

(4) Abs may bind nonspecifically to immune cells expressing the Fc receptors which include NK cells, B cells, macrophages and granulocytes without binding to tumor cells.

v) Dendritic cells pulsed with tumor Ags may be administered which can present tumor Ags in the context of class II MHC to tumor-specific Th cells. As tumor Ags are usually not known, tumor lysates are used. The Th cells may in turn produce cytokines which lead to development of CTL activity.

On the other hand, the dendritic cells may be transfected with the gene for tumor Ags, in which case, the Ags will associate with the Class I MHC and elicit a CTL response.

b) Nonspecific:

i) Adoptive Transfer of lymphocytes:

(1) Lymphokine-activated killer (LAK) cells which are IL-2 activated T cells and NK cells can be used in the treatment of melanoma and renal cell carcinoma

(2) Tumor-infiltrating lymphocytes (TIL) include T cells and NK cells. While the infiltrating NK cells will kill tumors nonspecifically, the CTL will be able to kill specific tumor targets.

ii) Cytokines

(1) Interleukin-2 (IL-2): Activates T cells/NK cells which express IL-2 receptors and leads to their proliferation. Used in the treatment of renal cell carcinoma and melanoma,

(2) Interferon-alfa (IFNα): Activates NK cell activity against tumors and also used in the treatment of Kaposi sarcoma, renal cell carcinoma and melanomas.

(3) IFN-γ: Ιncreases class II MHC expression; used in the treatment of ovarian cancers.

(4) Tumor necrosis factor (TNF)-α: Kills tumor cells.

(5) Granulocyte-macrophage colony stimulating factor (GM-CSF): Useful in overcoming neutropenia due to chemo- or radiation therapy

iii) Cytokine gene transfected tumor cells may also be used which can activate T or LAK cells that can mediate anti-tumor immunity.

 

 

References.

1.    Stephen Holgate. Martin Church. David Broide Fernando Martinez,  Allergy Hardbound, Published: November 2011.- 432 Pages

2.  Mark Peakman. Diego Vergani. Basic and Clinical Immunology with STUDENT. – Imprint: Churchill Livingstone Published: – April 2009

3.    Roderick Nairn, Matthew Helbert.  Immunology for medical students /  Hardboun – 2012 – p. 326

4.    Web -sites:

a)    http://emedicine.medscape.com/

5. Linda Cox.  Allergen Immunotherapy, An Issue of Immunology and Allergy Clinics.- / Published: May 2011.-  Hardbound, – 312 p.

6. Dédée Murrell. Autoimmune Diseases of the Skin, An Issue of Immunology and Allergy Clinics. –  Imprint: Saunders.- Published: May 2012.