Tolerance

1. Tolerance Is specific unresponsiveness i.e., an immune response to certain antigen (or epitope) does not occur, although the immune system is other wise functioning normally. -In general, antigens that are present during embryonic life are considered “Self” and do not stimulate an immunologic response. -The lack of immune response in the fetus is caused by the deletion of self reactive T-cell precursors in the Thymus.

2. On the other hand, antigens that are not present during the process of maturation but encountered later when body is immunologically mature are considered as “non-self”. • These usually elicit an immunologic response. If there is no immunological response to any antigen it is called as tolerance. • Both T & B cells participate in tolerance but T-cell tolerance plays a major role. T-cell tolerance- Main process here occurs in the fetal thymus • Ability to distinguish “self” & “non-self” process is called CLONAL DELETION. • -It involves killing of T-cells (by negative selection) that react against antigens (mainly self MHC proteins) present in the fetus at that time.

3. Eg. exogenous substances injected into the fetus early in development are treated as self. • Self reactive T-cells die by a process of APOPTOSIS. • T-cell tolerance can be 1) Central tolerance –acquired with in thymus 2) Peripheral tolerance- acquired outside the thymus. • Peripheral tolerance is necessary because some self reactive T cells are not killed in the thymus.

4. Mechanisms- Some self reactive T-cells are killed, some are inhibited, others are suppressed by regulatory T-cells producing inhibitory cytokines. • Clonal anergy-self reactive T-cells that are not activated due to improper co-stimulation. • -Clonal ignorance-self reactive T-cells that ignore self antigens. These self reactive T-cells are kept ignorant- by physical separation from target antigen • e.g. blood brain barrier • or ignore self antigens because of very small amount of antigen.

5. Though some of these T-cells, that are clonally anergiccells, non-functional, they can become functional & initiate an autoimmune disease if conditions change later in life. • Mechanism of Clonal energy-Due to inappropriate presentation of antigen, leading to failure of IL-2 production. This inappropriate presentation is due to failure of’costimulatory signals’

6. Eg. 1. Sufficient an out of IL-1 might not be made 2.Cell surface proteins, CD 28 on T cell and B7 on B cell might not interact properly leading to farther of signal transduction by ras proteins. 3. Further more, B7 is an inducible protein, and failure to induce it in sufficient amounts can lead to anergy. 4. In addition, co-stimulatory proteins CD40 on the B cell and CD40L on helper T cell may fail to interact properly.

7. The failure of Co-stimulatory signals most aften accurse when there insufficient inflammatory response at the site of infection. • The presence of microbes typically stimulates the production of pro inflammatory cytokines like TNF & IL-1. • How ever if this inflammatory response is insufficient meaning adjuvant effect of cytokines is inadequate, the T cell will die instead of being activated.

8. B cell tolerance – B cell becomes tolerance to self by 2 mechanisms. 1. Clonal deletion, probably while the B-cell precursors are in the bone marrow and 2. Clonal anergy of B cells in the periphery. But the tolerance in B cell is less complete than in T cell. This observation is supported by the finding that most autoimmune diseases are mediated by antibodies.

9. Induction of tolerance: Whether an antigen will induce tolerance rather than immunologic response is mainly determined by • Immunologic maturity of the host. Eg. Neonatal animals do not respond well to foreign antigens because of them being immunologically immature. Neonates will accept allograft that would be rejected by mature animals.

10. b. The structure and dose of antigen. Eg. A very simple molecule induces tolerance more readily than a complex one, very high or very low doses of antigen may result in tolerance instead of immune response. Purified polysaccharides or amino acid copolymers injected in very large doses result in ‘Immune paralysis’ – Lack and response.

11. Others – in induction maintenance of tolerance: a. T cell become tolerant more reading and remain tolerant longer than B cell. b. Administration of a cross reacting antigen tends to terminate tolerance. c. Administration of immunosuppressive drugs enhances tolerance eg. In patients who have received organ transplants. d. Tolerance is maintained best if the antigen to which immune system is tolerant continues to be present.

12. Auto immune diseases: • The adult host exhibits tolerance to tissue antigens present during fetal life that are recognized as ‘self’. However in certain circumstances tolerance may be host and immune reactions to host antigens may develop resulting in autoimmune disease. • The most important step is activation of self reactive helper [CD4] T cells. • These Th-1 or Th-2 cells can induce either CMI or AMI autoimmune reaction. But most autoimmune diseases are antibody mediated.

13. Many factors – • Genetic factors – Many autoimmune disease exhibit a marked familial incidence suggesting genetic predisposition. • Strong association of some diseases with certain HLA specificities especially class II gene. Eg. Rh arthritis – individuals carrying HLA – DR4 gene. • Ankylosingspondylitis is 100 times more likely in people with HLA –B27 a class I gene. • Likely explanation – a. genes those encode class I or class II MHC proteins that present auto-antigens more efficiently than those MHC proteins not associated with disease.

14. ii. Auto reactive T cells escape negative selection in thymus because they bind poorly to those class I or class II MHC proteins on the surface of thymic epithelium – during thymic education. • But whether a person carrying these genes may develop autoimmune disease or not is clearly multifactorial – many reasons. • Why? – those carrying these genes may not develop the autoimmune disease. Eg. Many people with HLADR4 gene may not develop Rheumatoid arthritis. So HLA genes appear to be necessary but not sufficient to cause autoimmune diseases.

15. In general class II MHC related diseases. Eg. RA, Grave’s disease and SLE – more commonly in women. Class I MHC related ankylosingspondylitis, Reiter’s syndrome more commonly in men. • Hormonal factors: Approx 90% of all autoimmune disease occur in women – explanation unclear. Some evidence from animal models suggest that estrogen can alter B cell function and enhance the formation of antibody to DNA.

16. Clinically – SLE either appears or exacerbates during pregnancy or immediately post partum supports this hypothesis that hormones play a important role in predisposing women to autoimmune diseases. • Environmental factors: Many environmental agents, most of them bacteria and viruses trigger auto immune disease. Eg. Pharyngitis caused by Streptopyogenespredisposes to rheumatic fever. • Certain infections cause autoimmune disease in animals. Eg. Coxsackie virus infection in mice leads to type I DM – not established in humans.

17. Others – drugs – procainamide – SLE • Gold, Hg cause autoimmune disease in animals. • 2 main mechanisms – i)is molecular mimicry which proposes that infectious agents possess antigen that elicit an immune response that cross react with component of human cells. ii) tissue injury releases intracellular [sequestrated] antigens that elicit immune response.

18. Summary – of current model of auto immune disease. Occurs in people i). with a genetic predisposition as determined by their MHC genes. • ii). Who are exposed to an environmental agent which triggers a cross reacting immune response against some components of normal tissue. • iii). autoimmune diseases increase in number with advancing age. So a decline in no. of regulatory T cell allows any surviving auto reactive T cell to proliferate and cause disease. • iv). Elevated Ig levels, auto antibodies demonstrable, deposition of Ig at site, lymphocyte, plasma cell infiltration at site, temporary or lasting benefit after immunosuppressive therapy.

19. Some mechanisms which explain auto immunity. a. Molecular mimicry – Various viruses and bacteria are implicated as a source of cross reacting antigens that trigger the activation of auto reactive T cell or B cells. Eg. Reiter’s syndrome occurs following – Shigellosis or chlamydial infections. • GullianBarre Syndrome – campylobacters. • This concept of molecular mimicry is used to explain these phenomena. Environmental trigger resembles or mimics a component of the body sufficiently and the immune attack is directed against the cross reacting body component.

20. Best characterized - M.protein of Streptopyogensand myosin of cardiac muscle. Antibody against M. protein cross – react with cardiac myosin – Rheumatic fever. • Additional evidence of this concept is identical amino acid sequences in certain viral proteins and certain human proteins. eg. Identical 6 amino acid sequence in Hep. B viral polymerase and human myelin basic protein. b. Alteration of normal proteins – Drugs can bind to normal protein and make them immunogenic. Eg. Procainamide induced SLE.

21. c. Release of sequestrated antigens – certain tissues. Eg. Sperm, CNS, uveal tract of eye are sequestrated [hidden] so that their antigen are not exposed to immune system. These are called immunologically privileged sites. If these antigen enter circulation accidentally eg. After damage they can induce humoral and CMI response. Result – aspermatogenesis, encephalitis, endopthalmitis. Sperm should be well sequestrated because they develop after immunological maturity has been reached and yet normally not subject to immune attack. • Intracellular Antigen eg. DNA, histones and mitochondrial enzymes are normally sequestrated, however bacterial or viral infection may damage cells and cause release of these sequestrated antigen – elicit immune response.

22. Once autoantibodies formed, subsequent release of sequestrated antigens leads to formation of immune complexes and the symptoms of autoimmune disease. • Other than that – radiation, chemicals can also damage cells and release sequestrated antigen of the cell. Eg. Sunlight exacerbates skin rashes in SLE. • Reason is UV radiation damages cells – release sequestrated DNA and histones – major antigen of SLE.

23. d. Epitope spreading – Is the term used to describe the now exposure of sequestrated of auto antigens as a result of damage of cells caused by viral infection. • These exposed auto antigens stimulate auto reactive T cells and auto immune disease results. • In an animal model – multiple sclerosis like disease was caused be infection with encephalomyelitis virus. • The self reactive T cells here were directed not against the virus but against the cellular antigen.

24. e. Others : a. Polyclonal activation of B cells by direct action of mitogen. Eg. LPS or secretion of factors by T cells that are stimulated by mitogen. Eg. Phytohemagglutinin, concanavalin A. b. Regulatory by pass of immune system i.e., failure of suppressor cells. c. Inappropriate expression of class II MHC molecules on surfaces of cells.

25. Diseases: Diseases involving primarily one type of cell or Organ a. Allergic encephalitis- Clinically seen in people injected with Rabies vaccine made in rabbits is a rare but serious disease. So this type of vaccine is discontinued in developed countries. Here there is immune response against foreign myelin protein in vaccine cross react with human myelin-inflammation of brain. Also can occur following viral infection or immunization against them e.g. measles, influenza-basis of autoimmunity is uncertain. Allergic encephalitis can be reproduced in the laboratory by injecting basic myelin protein into rodent brain which lead to CMI response & demyelination.

26. b. Multiple sclerosis: Auto reactive T-cells & activated macrophages cause demyelination of white matter of brain. Trigger thought to be viral infection but no virus has been implicated till now. The finding in multiple sclerosis typically wax &wane ,it effects both sensory &motor functions. -MRI of brain reveals plaques in white matter.Oligoclonal bands of IgG are found in spinal fluid of the patients. Immunosuppressive drugs prednisone, methotrexate or β-interferon effective to reduce severity of the symptoms.

27. c. Chronic thyroiditis: When animals areinjected with thyroid material they develop CMI & AMI against thyroid antigen leading to chronic thyroiditis. Humans with Hoshimoto’s chronic thyroiditis have antibodies to thyroglobulin-maybe this provokes inflammatory process leading to fibrosis of the gland. d. Hemolytic anemias,thrombocytopenias, granulocytopenias: Various forms of these diseases attributed to attachment of auto antibodies to cell surface & subsequent destruction of cell.-Pernicious anemia-Antibody to intrinsic factor(parietal cells secretion favoring B12 absorption).

28. ITP-Antibody against platelet, Platelets coated with auto antibody is destroyed in spleen or lysed by MAC of complement. Several drugs acting as haptens bind to platelet membrane & form neo antigens which induces cytotoxic antibodies-platelet destruction. Penicillins, cephalosporins, tetracycline, sulphonamide INH, rifampicin & other antimicrobials can cause autoimmune hemolytic anemia by same mechanism. e. IDDM: The main antigen which the T-cell directs its attack is the islet cell enzyme glutamic acid decarboxylase. Infection with Coxasackie B4 virus has been shown to trigger IDDM in mice-not proved in humans. Here there is commonality of 6-aminoacid sequence between virus and glutamic acid decarboxylase.-Antibody against various antigen of β cells also produced but major damage is T-cell mediated.

29. f. Insulin resistant diabetes, Myasthenia gravis & hyperthyroidism(Grave’s disease):Antibody to receptors play a major role, • i)Insulin resistant diabetes mellitus-Interference with insulin binding by antibody to insulin receptors (antibody has been demonstrated). • ii) Myasthenia gravis-antibody to acetyl choline receptors in the neuromuscular junction found in the serum. Severe muscular paralysis results in this condition. • iii)Lambert Eaton Syndrome-Antibody to calcium channel receptors leads to muscular weakness.

30. iv) In some Grave’s disease –circulating antibodyto thyrotrophin receptors seen. This when bound to receptors resemble thyrotrophin in activity-stimulate more thyroxine production. g. Gullian- Barre Syndrome: Most common cause of acute paralysis. Follows variety of infectious diseases –Upper respiratory viral infection, HIV, mononucleosis by EBV, CMV, diarrhea by campylobacters. There is a demyelination polyneuropathy due to antibody against myelin protein. • Rapidly progressing ascending paralysis. • Treatment – Plasmapheresis or IV Immunoglubulin • H.Pemphigus – skin disease characterized by bullae [blisters].

31. Antibody against desmoglein – protein in desmosomes that form tight junction between epithelial cells in skin. When tight junction is disrupted fluid fills in the spaces between cells and forms the bullae. One form is pemphigusfolliaceum – endomic in rural S.America – evidence of infection is environmental trigger for the disease.

32. Diseases involving multiple organs [Systemic diseases] : a. SLE – Autoantibodies against DNA, histones, nucleolar proteins and other components of cell nucleaus. Antibody against ds DNA is hall mark of SLE. • Women 20-60 years, HLA DR 2 or DR3 gene predisposition. • Agent inducing autoantibodies not known in most patients. • However 2 drugs – procainamide and hydralazine – known to cause SLE. • C/F – caused by immune complexes that activate complement and consequently cell damage. • Characteristic rash on cheek – vasculitis caused by immune complex deposition.

33. Arthritis and Glomerulonephritis – also due to immune complex deposition. • Immune complex found in glomeruli contain IgG, IgM or IgA and C3 component of complement but not fibrinogen. • Cytotoxicantibodies result in anemia, leucopenia and thrombocytopenia of SLE. But not immune complex. • Diagnosis – Detecting ANA – supports. • Fluorescent antibody technique. • ELISA – anti double standard DNA antibodies. • Reduced level of complement. Antibody to several other cellular components.

34. Treatment – depend on severity of disease and organ affected. • Aspirin, NSAID, Corticosteroids – commonly used. b. Rhematoid arthritis. – Autoantibodies found against IgG called RA factor are IgM class. • Women 30-50 years, HLADR4 predisposed. • Agent inducing not known. • Inflamed joints – synovial membrane infiltrated with T cells, plasma cells and macrophages – synovial fluid as high level of macrophage induced cytokines like TNF, IL-1 and IL-8. • Clinical features inflammation of small joints of hand and feet.. • Other organs – pleura, pericardium, skin can also be involved. • Reason activation of complement by immune complexes and damage.

35. Diagnosis – supported by detecting RA factor in serum. Confirmation by waaler – Rose type of test. • Treatment – aspirin, NSAID, immunosuppressive drugs, corticosteroids. • Anticytokine therapy :- Fusion proteins of TNF receptor and FC fragment of human IgG [Etanercept, Enbrel] also available. • Soluble TNF receptor neutralizes TNF - important mediator of inflammation. • Etanercept + methotrexate – help patient with persistent active RA. • Monoclonal antibody – infliximab used. • Inflixlimab neutralizes TNF decresedjoint inflammation.

36. c. Rheumatic fever : Post streptococcal sequelae antibody against M protein cross react with myocin of cardiac muscle and joint tissue. d. Reiters syndrome – triad of arthritis, conjunctivitis, urethritis – culture of effected organs do not reveal causative agent. Infection by Shigella, Salmonella, Yersinia, campylobacter, chalamydia predispose, patients mostly men with HLAB27 positive. Others : Good pasture syndrome, Wegner granulomatosis, Collegen vascular diseases – Ankylosing spondylitis, polymyositis-dermatomyositis, scleroderma, Periarteritisnodosa, Sjogren’s syndrome.

37. Treatment of autoimmune disease: Basis is reduce patients immune response to eliminate symptoms. • Corticosteroids eg. Prednisone are mainstay with antimetabolites – azothioprine, methotrexate added. Always caution – because of opportunistic infection. • Two approaches to therapy that do not involve systemic suppression of immune system is antibody to TNF and soluble receptor for TNF that acts as decoy. Bothe infliximab [antibody to TNF] and etanercept [TNF receptor] shown to reduce joint inflammation in RA.

38. Certain antibody mediated autoimmune disease – Gullain Barre, Myasthenia Gravis – ameliorated either by Plasmapheresis which reduces autoantibodies or with high doses of IgG pooled from human donors. • High dose IgG bids to Fc receptors on surface of neutrophils and blocks attachment of immune complexes that activate the neutrophils. • Excess of IgG saturates the FcRn receptor on the surface of vascular endothelial cells accelerating catabolism of IgG reducing level of autoimmune antibody.