The Immune System Second Edition Chapter 11 Disruption of Healthy Tissue by the Immune Response Copyright © 2005 by Garland Science Publishing Peter Parham

Autoimmune diseases Autoimmunity or autoimmune disease - the process of developing an immune response to self antigens (autoantigens). leads to chronic inflammatory damage to tissues. Classified based on the types of immune responses with which you are already familiar (i.e., hypersensitivity reactions, types II, III and IV). (autoimmune diseases are never caused by IgE, the source of type I hypersensitivity reactions).

Antibody responses against red blood cells. Autoimmune hemolytic anemia. Antibodies are raised against cell surface antigens on red blood cells, resulting in destruction of red cells and anemia. two primary antigens are recognized: an IgG response against Rh antigens an IgM response against I antigens present in glycophorin The IgM response leads to a version of hemolytic anemia termed cold hemagglutinin disease (CHAD). The antibodies agglutinate cells with increasing strength as the temperature drops. Extensive hemolysis occurs when arms, legs and extremities drop below 37 o C in cold weather. Type II hypersensitivity-related autoimmune diseases

Graves’ disease. Antibodies developed against receptors for thyroid- stimulating hormone (TSH)

Myasthenia gravis. Autoantibodies against acetylcholine receptors at the neuromuscular junction.

Type III hypersensitivity-related autoimmune diseases (Immune-complex mediated autoimmune disease). Systemic lupus erythematosus (SLE). Circulating IgG antibodies against common cellular constituents like cell surface components and constituents of cytoplasm and nucleus (e.g., double- stranded DNA, nucleosomes (DNA-histone complexes), spliceosomes, and a small cytoplasmic ribonucleoprotein complex. Y Y Soluble immune complexes Poor at fixing complement Y Not clearedblood vessel walls Y Joints + Complementlarge aggregatesRenal glomerulus Activated neutrophils inflammation and tissue damage

T-cell-mediated autoimmune diseases. Resulting from T cells specific for self antigens- (like type IV hypersensitivity reactions) Multiple sclerosis. Autoimmune response against the myelin sheath of nerve cells. Involves demyelination of central nervous system tissue resulting in sclerotic plaques of demyelinated tissue. The initial lesion development appears dependent on T cell infiltration into the CNS. Best guess is activated T H 1 CD4 cells that secrete interferon- , which activates macrophages, which are the direct cause of demyelination.

. Model for multiple sclerosis: experimental allergic encephalomyelitis (EAE) caused by immunizing mice or rats with myelin basic protein. Develop T cells specific for myelin basic protein, a protein found in the myelin sheath that surrounds nerve cell axons in the brain and spinal cord. Mouse – inject with MBP + adjuvant (heat-killed bacteria) paralysis mediated by MBP-specific TH1 cells (inflammatory T cells). MBP is processed for presentation by MHC class II molecules to CD4+ T cells. Transfer of CD4+ T cells to another inbred mouse with same MHC causes transfer of the disease as well.

TH1 IFN- , TNF-  MHCII similar to T cell-mediated delayed-type hypersensitivity or type IV hypersensitivity. When T H 1 cells encounter MBP, can produce inflammation.

Type I diabetes mellitus (insulin-dependent diabetes mellitus, IDDM). Destruction of the insulin-producing  cells of the pancreas by CD8 T cells (cytotoxic T cells) that recognize peptides from a  cell specific protein and kill the  cell. cells make: glucagon insulin somatostatin Islet cell type  express on surface different cell-specific peptides.

Effector CD8 T cell Candidate autoantigens : glutamic acid decarboxylase, tyrosine phosphatase IA-2, SOX13 transcription factor, and insulin or proinsulin itself.

Antibody and T cell-mediated autoimmune disease. Rheumatoid arthritis. Chronically inflamed joints infiltrated by multiple immune cells. T H 1 cells recognizing a specific antigen present within the joint triggers them to release inflammatory cytokines to initiate local inflammation plasma cells make a IgM, IgG and IgA that binds to the Fc region of patients’ own IgG - called rheumatoid factor (RF). These are deposited in the joints and activate complement cascade to increase inflammatory response.

All autoimmune diseases involve a breach of the normal mechanisms of self tolerance. Self-reactive B cells are eliminated during maturation in the bone marrow Self reactive B cells in the periphery die by apoptosis or become anergic.

Autoimmune diseases require a breakdown in T cell tolerance Negative selection of self-reactive T cells occurs during development in the thymus epithelial cells in the thymus express lots of relatively rare proteins under the control of a transcription factor known as the autoimmune regulator (AIRE). defective alleles of AIRE lead to incomplete negative selection and inherited autoimmune polyglandular disease. Self-reactive (naive) T cells that do escape negative selection are usually not activated by binding cells expressing specific peptide-MHC complexes because these don’t express B7 (needed for co-stimulatory signal by binding CD28). Induces anergy.

Genetic predisposition to development of autoimmune disease. Certain inbred strains of mice reliably develop spontaneous autoimmune diseases. 20% of monozygotic twins show disease concordance; <5% of dizygotic twins show concordance. genetic studies indicate the inheritance of susceptibility is polygenic – meaning several independently segregating disease susceptibility loci exist. The genes most consistently associated with susceptibility to autoimmune diseases are those of the MHC or HLA (human MHC) complex.

Environmental contributions to loss of self-tolerance. genetically predisposed individuals at highest risk develop disease at a maximum frequency of about 20%. trauma can cause disruption of cell or tissue barrier allowing a formerly sequestered antigen to become recognized. An example of this is sympathetic ophthalmia.

Infectious agents. Infections trigger an autoimmune response. Molecular mimicry. Antibodies or T cells generated in response to an infectious agent cross-react with self antigens.

chronic autoimmune diseases caused by autoreactive T cells that arise in the course of combating infection. Most tissue specific antigens are not presented by APC’s and are not, therefore, on cells that express B7 co-stimulatory molecules (induces anergy). However, perhaps dead or infected cells are taken up by APC’s, which present self- antigens and activate effector T cells.

Inflammatory cytokines like IFN-  can induce some cells to express HLA class II molecules.

Epitope spreading in SLE

Senescence of the T cell population

The hygiene hypothesis arises again! Incidence of autoimmune disease is increasing in developed countries. Lack of exposure of children to pathogens alters the way in which the immune system develops so they are less skilled in attacking pathogens while maintaining T cell tolerance.