Autoimmunity

Author : Dr. Saurav K Sarkar

Discipline : B. Tech (Biotech)

Designation : Lecturer

Module : BT 501 IV

Semester : V

Topic : AUTOIMMUNITY

Objective: The student should understand : 1. What is Autoimmunity 2. immune mechanisms involved in autoimmunity 3. factors initiating the autoreactivity 4. Difference between autoimmunity & autoimmune disease 5. Characteristics of autoimmune diseases What is Autoimmunity Although the immune system has an elaborate system of checks and balances to ensure self tolerance, occasionally this system breaks down. When the immune system attacks host components causing pathological change, this is called autoimmunity. Many people experience an autoimmune reaction during their lifetime. Mostly these are short-lived, self-resolving sequelae of infection. However in some 5% of individuals the reaction is chronic, debilitating and even (rarely) life-threatening. It is these latter conditions where serious immunopathology occurs which are usually considered autoimmune disease. Autoimmunity can affect any part of the body, and according to specificity these disorders form a spectrum. At one end, there are organ-specific diseases in which immune responses are directed to a target Ag unique to a single organ or gland, so that the manifestations are largely limited to a single organ or gland (e.g., Hashimoto's disease or thyroiditis). At the other end of the spectrum are the systemic or non-organ-specific diseases, where immune responses are directed toward a broad range of target Ags and involve a number of organs and tissues. Tissue damage is widespread, both from cell-mediated immune response and from direct action of auto-Abs or by accumulation of immune complexes. Systemic autoimmune diseases are usually due to defect in immune regulation that results in hyperactive T and B cells. (e.g., systemic lupus erythematosus where both lesions and anti-nuclear Abs are not confined to any one organ and pathological changes are widespread). In the intermediate range are disorders where the lesion are localized in one or two organs. In Goodpasture's disease, auto-Abs are deposited on the basement membrane of kidney glomeruli and lung parenchyma, causing the chronic glomerulonephritis and pulmonary haemorrhages. Autoantibodies - cause or effect? Almost all patients presenting with autoimmune conditions have some autoantibodies present in their serum. However they also have autoreactive T cells present (though these are far harder to demonstrate experimentally). It is not always known whether the autoantibodies play an important role in the disease or are a secondary result of the tissue damage which has been caused by the disease process itself. This is problem is particularly difficult in many organspecific conditions. A useful example of the contrast between diseases whose destructive mechanism is well understood and a similar condition in which it is much less well understood is Graves' disease and Hashimoto's thyroiditis.

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Author : Dr. Saurav K Sarkar

Discipline : B. Tech (Biotech)

Designation : Lecturer

Module : BT 501 IV

Semester : V

Topic : AUTOIMMUNITY

Both diseases affect the thyroid gland specifically, in Graves' the thyroid is hyperactive whereas Hashimoto's results in thyroid hypoactivity. Graves Disease This is a rare example of an autoimmune disease which can be transferred with IgG antibodies. Firstly passive transfer of IgG from patients to rats often produces similar symptoms transiently in the animals. Secondly babies born to mothers with Graves' have shown transient symptoms of hyperthyroidism which disappear with catabolism of the maternal IgG (transferred via the placenta) and are relieved by plasma exchange.The disease causing antibodies can be shown to recognise the thyroid stimulating hormone (TSH) receptor and to stimulate thyrocytes in vitro. Hashimoto's Thyroiditis This disease is characterised by an intense mononuclear cellular infiltrate into the thyroid and by the presence of autoantibodies primarily directed at thyroglobulin and thyroid peroxidase. There are a number of theories about the mechanism of pathogenic damage to the tissue. •

Autoreactive T cells (TH1) may cause tissue damage by release of cytokines, either directly (eg TNF) or by recruiting and activating macrophages, which subsequently mediate tissue destruction.

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Autoreactive antibodies, whose production requires the help of autoreactive T cells, may be directly responsible for the pathology, by for example interfering with iodine uptake and binding by thyroglobulin.

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Inflammation may cause tissue damage by triggering apoptosis in thyrocytes by inducing expression of a 'death' receptor (Fas, a molecule which triggers apoptotic death). Unusually the ligand for this 'death' receptor appears to be constitutively expressed by thyrocytes. It is also expressed by activated but not resting T cells. While it is clear that activation of autospecific T cells is a prerequisite for this disease it is far from clear what the significance of the autoantibodies is.

Genetic Factors Autoimmune disorders tend to be more frequent in certain families, indicating certain genetic basis for the diseases. Genetic susceptibility plays an important role in almost all autoimmune conditions. First degree relatives, sibs, parents and children, of patients with Hashimoto's disease show a high incidence of anti-thyroid Abs and of overt and sub-clinical thyroiditis. A high incidence of Abs to gastric parietal cells in the relatives of those with pernicious anemia has also been reported. Chances of getting affected by thyrotoxicosis are more in identical twins than in non-identical twins. Thyroid auto-Abs are more prevalent in individuals with ovarian dysgenesis having X-chromosome aberrations like XO. Studies have shown that within the human population there are multiple genetic loci involved in susceptibility to common autoimmune conditions like Insulin Dependent Diabetes Mellitus (IDDM) and rheumatoid arthritis. Strong associations between several autoimmune diseases and particular HLA specificities have been established. A good number of patients with different autoimmune diseases like Sjogren's syndrome, Addison's disease, insulin-dependent diabetes or thyrotoxicosis were found to have HLA-DR3 specificity. HLA-DR2 in Goodpasture's syndrome, HLA-DR4 in rheumatoid arthritis and HLA-DR5 in Hashimoto's thyroiditis have been recorded with significant frequency. It is likely that most autoimmune conditions have some genetic component. No single predisposing allele has to be present for the disease to occur, rather the presence of combinations of susceptibility alleles significantly increases the probability of that individual developing the specific disease. Demonstrating the existence and approximate location of such susceptibility genes is now routine. Endocrine Factors

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Author : Dr. Saurav K Sarkar

Discipline : B. Tech (Biotech)

Designation : Lecturer

Module : BT 501 IV

Semester : V

Topic : AUTOIMMUNITY

Most autoimmune disease do not occur with equal frequency in males and females. For example Graves' and Hashimoto's are 4-5 times, and SLE 10 times, more common in females while Ankylosing Spondylitis is 3-4 × more frequent in males. These differences are believed to be the result of hormonal influences A second well documented hormonal effect is the marked reduction in disease severity seen in many autoimmune conditions during pregnancy. Rheumatoid arthritis is perhaps the classic example of this effect. In some cases there is also a rapid exacerbation (rebound) after giving birth. Environment Environmental factors also play a role in autoimmune disease. If we examine how frequently identical twins both develop a disease (the concordance rate), it is only about 20-40% for common autoimmune diseases such as diabetes, SLE and rheumatoid arthritis. This makes it highly likely that environmental factors must also be important. While we might expect factors such as diet to play a role, we can postulate that infectious organisms are the most significant environmental factor. In a few cases, we have evidence for a direct link between a specific infection and an autoimmune disease. The classical example is that of rheumatic fever following Streptococcal infection. More recently, evidence implicates infection with a variety of organisms (Yersinia, Shigella ,Chlamydia) and reactive arthritis (not rheumatoid). Age and Sex : The incidence of cases with demonstrable auto-Abs increases steadily with age. In general, auto-Abs and autoimmune diseases are found more frequently in women than in men. Women are at least tenfold more likely to develop SLE. The female predominance for autoimmunity is also true of most animal models, it is suggested that hormonal influences may play a significant role in this regard. Mechanisms of autoimmune pathology There are three general types of mechanisms : •

Direct antibody mediated effects

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T cell mediated effects (cellular immune)

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Immune complex mediated effects

Direct antibody mediated pathology : In Graves' disease, stimulatory antibodies dysregulate thyroid function. A counter effect is seen in Myasthenia Gravis, where, auto-Abs to the Acetylcholine receptor block neuromuscular transmission from cholinergic neurons by blocking the binding of acetylcholine and by causing downregulation of its' receptor. Rheumatic fever is an example of direct tissue pathology following antibody binding. T cell mediated damage : Here, recognition of auto-Ag by T cells leads to tissue destruction without requiring the production of autoantibody. This may occur through the following ways : 1. Direct T cell cytotoxicity via CD8+ CTL 2. Self-destruction of tissue cells induced by cytokines, eg, TNFa 3. Recruitment and activation of macrophages leading to bystander tissue destruction 4. Induction of target tissue apoptosis by the T cell membrane protein FasL In most cases, we do not know what the relative contribution of these factors is, though mechanism 4 would have to be confined to those tissues which constitutively express the 'death' protein Fas or which express it as a consequence of inflammation. Immune-complex mediated pathology Immune-complexes are frequently implicated in autoimmune pathology. Systemic diseases such as SLE and vasculitis almost certainly result from autoantibody-antigen complexes and their consequences. Certain organs are especially sensitive to immune complex deposition particularly the kidney. SLE patients possess a 3

Author : Dr. Saurav K Sarkar

Discipline : B. Tech (Biotech)

Designation : Lecturer

Module : BT 501 IV

Semester : V

Topic : AUTOIMMUNITY

wide variety of autoantibodies to both cytoplasmic and nuclear antigens. Presence of IgG anti dsDNA is characteristic of this condition IgM anti-dsDNA is NOT pathogenic. Two significant facts point to the role of immune complexes in SLE. First, patients demonstrate significant depletion of complement (C3) and neutrophils resulting from activation by the complexes. Second, complement deficiencies which impair IC clearance (C1,C2 or C4, see lecture 10) are very strong predisposing factors for SLE. How are autoimmune reactions initiated? 1. Release of sequestered antigen : Many self-Ags, for example Ags expressed on eye lens, spermatozoa, brain tissue, are normally confined anatomically to such sites that do not allow their access to lymphocytes and thus natural tolerance to these self-Ags does not develop. Clinical incidents may release such an Ag which then readily induces immune responses to the cells bearing the Ag. Abs to heart muscle have been found after myocardial infarction, and Abs to eye Ags appear after the trauma of an eye injury. An autoimmune condition can be experimentally produced by removing such an Ag and injecting it back into the same animal. When cells obtained from a testicle removed from a rabbit are injected back to the animal, the other testicle gradually atrophies due to an autoimmune reaction. 2. Defective Production of T Cells and Emergence of "Forbidden" Clones : In the clonal selection hypothesis, lymphocytes reactive to auto-Ags are assumed to be somehow eliminated. A defect in the hypothetic elimination or suppression mechanism could cause the emergence of autoreactive forbidden clones of lymphocytes. This is supported by certain clinical reports; an individual suffering from one autoimmune disorder is likely to have signs of other autoimmune diseases. For instance, a high percentage of persons with autoimmune thyroiditis have pernicious anaemia, another autoimmune disorder. Neonatal thymectomy exacerbates autoimmune haemolytic anaemia and gbmerulonephritis in NZB mice. The onset of autoimmune disorders in these mice can be mostly prevented by repeated injections of normal thymocytes from young unaffected animals and conversely spleen cells transferred from diseased to young normal NZB mice continue to synthesize auto-Abs for a short time unless the recipients are immunosuppressed at the beginning. Thus, it seems that normal T cells exert a controlling influence on the autoantibody producing B cells. 3. Modified Forms of Self-Ag : The thyroid autoantigen thyroglobulin (Tg), leaves via the thyroid lymphatics reaching the serum in normal human at a concentration of approximately 0.01-0.05 µg/ml. Such a concentration produces low-zone tolerance by affecting T cells. Hence anti self-Tg B cells are probably present but unable to respond to self-Tg due to the absence of cooperation from T cells tolerant to self-Tg. If a foreign or new antigenic determinant appears on Tg due to some modification, then it will stimulate a different set of TH cells specific for the new determinant. These TH cells can then cooperate with the existing, quiescent anti-Tg B cells allowing them to secrete anti-Tg Abs. Rabbits injected with chemically modified rabbit Tg, or with cross-reacting hog Tg, develop thyroiditis, with production of Ab to native rabbit Tg. Similarly, encephalitis occasionally develops in people injected with rabies vaccine made from infected rabbit brain cells, due to heterologous Ags from brain tissue. Inappropriate expression of MHC class II on epithelial cells may also act as auto-Ags to induce autoimmunity. Thyroid epithelium is normally class II Ag-, but when induced to express the class II Ag, autoimmunity follows. Thymocytes of patients with Hashimoto's thyroiditis express class II products. IFN-γ induces class II Ag expression on thymocytes in vitro. Probably IFN is produced in response to local infection or triggered by other agents. In case of diabetes, pancreatic β cells selectively express class II molecules. Some bacteria carry Ag determinants resembling those of human and these cross-reacting Ags may induce autoimmunity. In rheumatic fever antibodies produced to group A streptococci also cross-react with human heart tissue. Colon antibodies present in ulcerative colitis cross-react with Escherichia coli 014. In addition, in some bacterial infections certain bacterial products, such as glycolipids, endotoxins, produce adjuvant effects by providing the second, non-specific activating signal for B cell stimulation, thus bypassing the need for T cell help. This might help in developing an autoimmune response. 4

Author : Dr. Saurav K Sarkar

Discipline : B. Tech (Biotech)

Designation : Lecturer

Module : BT 501 IV

Semester : V

Topic : AUTOIMMUNITY

4. Chronic Viral infection : Chronic infection of a virus causes the host to produce anti-viral Abs that form virus-Ab complexes without neutralizing the infectivity of the virus. The binding of anti-viral Abs or specific effector T cells to budding virions on infected cells, or to the viral Ags adsorbed on RBCs or other cells may lead to chronic disorders, such as hemolytic anemia. Circulating immune complexes of viral Ags and anti-viral Abs may be deposited in blood vessels and along the glomerular basement membrane, thus sometimes causing glomerulonephritis. In polyarteritis nodosa, an autoimmune disease in which the hepatitis B virus has been implicated, deposits of viral Ag-Ab complexes and complement have been detected in the walls of the affected blood vessels. Sub-acute sclerosing panencephalitis, a degenerative brain disease of children, is related to measles virus infection. Similarly scrapie, a degenerative brain disease of sheep, is another example of slow virus disease of the central nervous system. The Epstein-Barr (EB) virus is a herpes-like DNA virus strongly implicated in the pathogenesis of infectious mononucleosis (IM) in which auto-Ab and splenomegaly may be significant features. Apparently, in IM the EB virus infects B lymphocytes and its expression on these cells causes a profound host T cell response to the new cell surface Ags. This is a self-limiting disease, and most patients recover. However, the EB virus is also Infection with EB virus results in a permanent malignant transformation of B cells associated with the malignant lymphoproliferative disease called Burkitt’s lymphoma. Viral infection has also been implicated in Systemic Lupus Erythematosus (SLE), an acute or chronic inflammatory disease. 5. Breakdown of T Cell Anergy : Breakdown in peripheral tolerance mechanisms may be responsible for the emergence of autoimmunity in adult. Autoimmune diseases typically do not occur in neonatal period. Certain changed conditions might activate tissue APCs for enhanced expression of costimulatory molecules and production of cytokines. These in turn may stimulate T cells which remained anergic so far. These T cells now proliferate and differentiate into pro-inflammatory effector cells causing autoimmune reactions against the tissue. APC activation may be due to infections, tissue necrosis or local inflammation. In experimentally induced T cell mediated autoimmune diseases, such as encephalomyelitis and thyroiditis, the diseases develop only if the self-Ag, myelin basic protein (MBP) and thyroglobulin (Tg) respectively, are administered in strong adjuvant. Such adjuvant may activate macrophages to express costimulatory molecules like B7-1 and B7-2 which in turn help in breakdown of anergy and development of effector T cells reactive with the self-Ag. Self-Ag with adjuvant may also overcome clonal ignorance, if the self-Ags are the ones that are normally ignored by self-reactive lymphocytes. Τ cell anergy may also fail because of abnormalities of the cell surface regulatory molecules. In CTLA-4 gene knockout mice, fatal autoimmunity with Τ cell infiltration and tissue destruction in heart, pancreas and other organs takes place. In the absence of CTLA-4, Τ cell anergy cannot develop, rather autoimmunity sets in. 6. Failure of Activation-Induced Cell Death : The failure of activation induced cell death may result in autoimmunity. Two homozygous inbred mouse strains, called lpr/lpr (for lymphoproliferation) and gld/gld (for generalized lymphoproliferative disease) die by the age of 6 months from severe systemic autoimmune disease with multiple autoantibodies and nephritis just like human SLE. Lymphoadenopathy and splenomegaly accompany this disease in mouse. The lpr disease occurs due to defect in the gene encoding Fas molecule responsible for inducing cell death. The gld defect is due to a point mutation in Fas ligand that abrogates the signaling capacity of this molecule. Defects in Fas or in Fas ligand do not allow activation induced cell death in mature CD4+ Τ cells. Thus, some self reactive ΤH cells escape the deletion mechanism that normally induce peripheral tolerance. Some of the anergic ß cells are normally eliminated by Fas dependent death usually after interactions with Τ cells in peripheral lymphoid tissues, and this pathway of Β cell deletion also becomes defective in lpr and gld homozygous mice. Abnormalities in both helper Τ cells and Β cells, thus, contribute to autoAb production. Defects in Fas-mediated death pathway and in consequence a failure of activation-induced cell death are known to induce phenotypically similar disease in children. In the last lecture we mentioned that some antigens are hidden away from the immune system in immunologically privileged sites. When trauma or other events cause damage to the barriers which protect 5

Author : Dr. Saurav K Sarkar

Discipline : B. Tech (Biotech)

Designation : Lecturer

Module : BT 501 IV

Semester : V

Topic : AUTOIMMUNITY

such special sites this can lead to the release of novel autoantigens and the production of autoantibodies. In the case of autoimmune sympathetic opthalmia, damage to one eye leads to subsequent autoimmune attack on the contralateral eye. 7. T cell bypass : Autoimmunity can arise as a result of T cell tolerance being bypassed. This might occur in a number of ways : •

Modification: When a small molecule (eg a drug) binds to a protein and alters an MHC- binding peptide so that it becomes a neoantigen recognized by T cells. This provides T cell help, through linked recognition, for Ab production which need not be (and usually is not) directed against a neodeterminant.

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Inflammation: During an inflammatory response, an immunostimulatory environment is created by the release of cytokines which recruit and activate professional APC and provide support for T cell activation rather than anergy. As a result autoreactive T cells which were anergic or ignorant may become activated. This concept is central to Matzinger's Danger Hypothesis.

Molecular mimicry: An epitope of an invading microorganism may cross-react with a self protein. The T cell help provided by the other microbial Ags permits the activation of B cells which make an crossreactive Ab which either escaped tolerance or which acquires sufficient self reactivity through somatic mutation and selection driven by the cross-reactive Ag. •

Immunoregulation A number of induced autoimmune diseases resolve themselves. This has revealed the potential for the immune system to return to a tolerant state by some form of regulation. The regulatory cells are CD4+ T cells and can protect naive mice in transfer studies. Idiotype (Id) and anti-idiotype mechanisms to cause autoimmunity : (A) Microbial Ag or Ab to microbe cross reacts with autoreactive lymphocyte Id. (B) Antibody to virus generates anti-Id which is autoantibody to the cells bearing virus receptors. (C) Anti-hormone Ab may generate anti-Id to react with hormone receptors on the target cell.

The TH1 /TH2 paradigm How these regulatory T cells work has yet to be shown. A simple model is that the disease process requires the bias of the immune response to be either 'TH1 type' or 'TH2 type' (depending on the disease) and that anything which stimulates the opposite environment may protect. This may be the explanation for the protection of NOD mice by infection with Schistosoma (or administration of derived antigens) which establishes a strong Th2 bias. However many situations are likely to be more complex.

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