Lecture outline Self-tolerance: concept, significance Mechanisms of central and peripheral tolerance: deletion, anergy, regulatory T cells Pathogenesis of autoimmunity: roles of susceptibility genes and environmental factors

The immunological equilibrium: balancing lymphocyte activation and control Effector and memory T cells Tolerance Regulatory T cells Normal: reactions against pathogens Pathologic: inflammatory disease, e.g. caused by reactions against self No response to self Controlled response to pathogens

The problem of self-nonself discrimination The immune system responds to many foreign (microbial) antigens but not to self antigens Developing lymphocytes express a large number of antigen receptors, not biased by specificity Therefore, all individuals produce lymphocytes with the ability to recognize self antigens Self antigens have access to the immune system Therefore, self-reactive lymphocytes must be selected against (eliminated or inactivated) to prevent autoimmunity

Immunological tolerance Definition: specific unresponsiveness to an antigen that is induced by exposure of lymphocytes to that antigen (implies antigen specificity, in contrast to “non-specific immunosuppression”) Significance: All individuals are tolerant of their own antigens (self-tolerance); breakdown of self-tolerance results in autoimmunity Therapeutic potential: Inducing tolerance may be exploited to prevent graft rejection, treat autoimmune and allergic diseases, and prevent immune responses in gene therapy, perhaps stem cell transplantation

Central and peripheral tolerance The principal fate of lymphocytes that recognize self antigens in the generative organs is death (deletion), BUT: Some B cells may change their specificity (called “receptor editing”) Some T cells may differentiate into regulatory (suppressor) T lymphocytes

Mechanisms of unresponsiveness to self antigens Central tolerance: Immature self-reactive lymphocytes that recognize self antigens in generative (“central”) lymphoid organs die by apoptosis; other fates Peripheral tolerance: Mature self-reactive lymphocytes that recognize self antigens in peripheral tissues are inactivated (anergy), killed (deletion) or suppressed “Clonal ignorance”: Mature self-reactive lymphocyte clones do not encounter or respond to self antigens In normal individuals it is not known which self antigens induce tolerance by which mechanism

Central T cell tolerance

What self antigens are seen in the thymus? Ubiquitous cell-associated and circulating proteins The thymus has a special mechanism for displaying peripheral tissue antigens in thymic medullary epithelial cells, where they signal self-reactive thymocytes for death

Consequences of AIRE mutation Human disease: autoimmune polyendocrinopathy with candidiasis and ectodermal dysplasia (APECED), also called autoimmune polyendocrine syndrome (APS-1) Associated gene identified by positional cloning, named AIRE (“autoimmune regulator”) Mouse knockout: autoantibodies against multiple endocrine organs, retina Failure to express many self antigens in the thymus --> failure of negative selection

Deletion of self-reactive T cells in the thymus: how are self antigens expressed in the thymus? AIRE (autoimmune regulator) is a regulator of gene transcription that stimulates thymic expression of many self antigens which are largely restricted to peripheral tissues Discovered as the genetic cause of a human autoimmune disease (APS-1)

Central tolerance: fates of immature self-reactive lymphocytes Induced by antigen in generative lymphoid organs (thymus for T cells, bone marrow for B cells), and high-affinity (“strong”) recognition of the antigens Immature lymphocytes undergo apoptosis upon encounter with antigens (negative selection) Eliminates high-affinity self-reactive (potentially most dangerous) lymphocytes Some self-reactive T cells that encounter self antigens in the thymus develop into regulatory T cells and immature B cells in the bone marrow change their receptors (rendered harmless)

Peripheral tolerance

Peripheral tolerance Costimulation (signal 2) Immunogenic antigen (microbe, vaccine) APC Naïve T cell TCR Antigen (peptide + HLA): signal 1 Effector and memory cells Tolerogenic antigen (e.g. self) Tolerance: functional inactivation or cell death, or sensitive to suppression

T cell anergy

T cell anergy (“clonal anergy”) Induced by self antigens that are displayed to the immune system without inflammation or innate immune responses (prolonged signal 1, i.e. antigen, inadequate signal 2) Recognition of such antigens may lead to signaling block and/or engagement of inhibitory receptors Role of anergy in self-tolerance in humans is unclear; therapeutic potential?

Regulatory T cells

Regulatory T cells Regulatory T cells are CD4+ cells that express high levels of CD25 (IL-2 receptor a chain) Generated by self antigen recognition in the thymus or peripheral tissues Generation requires a transcription factor called Foxp3 (mutations in Foxp3 are the cause of a severe autoimmune disease in humans and mice)

Mechanism of action: may be multiple Regulatory T cells Regulatory T cells are CD4+ cells that express high levels of CD25 (IL-2 receptor a chain) Mechanism of action: may be multiple Secretion of immune-suppressive cytokines CTLA-4 on Tregs blocks B7 on APCs Significance for self-tolerance: Some autoimmune diseases may be associated with defects in regulatory T cells or resistance of responding cells to suppression Therapeutic potential of cellular therapy (autoimmune diseases, graft rejection, etc)

“Activation-induced cell death”: death of mature T cells upon recognition of self antigens

Deletion (activation-induced cell death) Stimulation of T cells by self antigen triggers apoptosis by engagement of death receptors (“death receptor pathway”) or imbalanced expression of pro-apoptotic proteins (“mitochondrial pathway”) Evidence for the importance of AICD in maintenance of self-tolerance: Mice with mutations in Fas or Fas ligand develop a lupus-like autoimmune disease Humans with mutations in Fas or enzymes involved in death receptor-induced apoptosis (caspases): the autoimmune lymphoproliferative syndrome (ALPS) Eliminating both death pathways in mice --> “spontaneous” systemic autoimmune disease

Tolerance in B lymphocytes Central tolerance: Deletion of immature cells by high-affinity antigen recognition in the bone marrow Some immature cells may change their antigen receptors when they encounter antigens in the bone marrow (“receptor editing”) Peripheral tolerance: Anergy Exclusion from lymphoid follicles, death because of loss of survival signals Engagement of inhibitory receptors

Autoimmunity Definition: immune response against self (auto-) antigen, by implication pathologic General principles: Pathogenesis: The development of autoimmunity reflects a combination of susceptibility genes and environmental triggers (usually infections) Different autoimmune diseases may be systemic or organ-specific; may be caused by different types of immune reactions (antibody- or T cell-mediated)

Autoimmunity Definition: immune response against self (auto-) antigen General principles: Pathogenesis: The development of autoimmunity reflects a combination of susceptibility genes and environmental triggers (usually infections) Different autoimmune diseases may be systemic or organ-specific; may be caused by different types of immune reactions Challenges in understanding pathogenesis of human autoimmune diseases: Failure to identify target antigens, heterogeneous disease manifestations, disease may present long after initiation Recent advances: identifying self antigens (MS, type 1 diabetes); genetic analyses; improved methods for studying immune system of patients

Pathogenesis of autoimmunity

Genetic basis of autoimmunity -- 1 Genetic predisposition of autoimmune diseases Increased incidence in twins (more in monozygotic than in dizygotic twins) Identification of disease-associated genes by breeding and genomic approaches Multiple genes are associated with autoimmunity Most human autoimmune diseases are multigenic Single gene mutations and mouse knockouts reveal critical pathways

Genetic basis of autoimmunity -- 2 MHC genes Major genetic association with autoimmune diseases (relative risk of disease in individuals with particular HLA haplotypes) Disease-associated alleles are present in normal individuals Non-MHC genes: Many gene variants identified by genome-wide association and linkage studies Many of these variant genes individually have small effects on disease susceptibility, may influence disease when present in combination Interactions with environmental factors difficult to define

Genetic basis of autoimmunity -- 3 Genome wide association studies are revealing genetic polymorphisms associated with autoimmune diseases Crohn’s disease: NOD-2: microbial sensor in intestinal epithelial and other cells IL-23 receptor: involved in TH17 responses Rheumatoid arthritis, others: PTPN-22 (tyrosine phosphatase): may control kinase-dependent lymphocyte activation Multiple sclerosis, others: CD25 (IL-2 receptor): role in T cell activation and maintenance of regulatory T cells

Genetics of autoimmunity: challenges Difficult to relate complex genotypes to phenotypic and functional abnormalities, to better understand pathogenesis Limitations of GWAS: detects frequency of common variants, misses rare mutations Identified disease-associated polymorphisms have small effects, therefore little predictive value Because of small effects of any one gene, targeting these genes therapeutically is unlikely to have significant benefit

Infections predispose to autoimmunity Genes encoding antigen receptor specific for a myelin antigen Transgenic mouse with myelin-specific T cells Normal mouse colony Pathogen-free mouse colony CNS disease No disease

Infections and autoimmunity Infections trigger autoimmune reactions Clinical prodromes, animal models Autoimmunity may develop after infection is eradicated (i.e. the autoimmune disease is precipitated by infection but is not directly caused by the infection) Some autoimmune diseases are prevented by certain infections Type 1 diabetes, multiple sclerosis, others? -- increasing incidence in developed countries Mechanism unknown; the “hygiene hypothesis”

Mechanisms by which infections may promote autoimmunity

Other environmental influences Hormones Gender bias of autoimmune diseases Mechanisms still not defined UV exposure SLE

Understanding autoimmunity Experimental models have been very valuable for defining pathways of normal and abnormal immune responses, BUT they may have limited value for understanding human diseases Need technologies for studying patients Emphasis should be on antigen-specific immune responses