Disorders of the immune system: Tolerance & Auto-immunity

1. Immunology Disorders of the immune system: Tolerance & Auto-immunity 1

2. Objectives • Define and discuss the general characteristics of tolerance • Define the main factors that influence the development of tolerance • Identify the main mechanisms of tolerance induction in T and B cells • Identify the mechanisms involved in the development of autoimmunity • Identify and discuss 3 animal models of autoimmune diseases • Approach to treatment of autoimmune diseases 2

3. Key Definitions • Tolerance: A type of specific unresponsiveness to an antigen induced by the exposure of the specific lymphocyte to that antigen. • Self Tolerance: Immunologic unresponsiveness to self antigens • Tolerogens: Antigens that induce tolerance • Immunodeficiency: Developmental or maturational defects in lymphocyte populations and/or function • Immunosuppression: Active or passive inhibition of immune response in an otherwise immunologically competent host 3

4. The Clonal Selection of Lymphocytes (Burnett, 1950)

5. The two-signal requirement for lymphocyte activation • Requirements: Two signals • Signal 1: specific recognition of antigen • TCR-Peptide-MHC • BCR-Native antigen • Signal 2: Non-specific • Microbial-induced molecules on/from APC • Microbial molecule (LPS, CpG etc) • Signal 1 alone leads of unresponsiveness • Anergy, Deletion, Apoptosis

6. Consequences of the encounter of lymphocytes with an antigen 6

7. Self/non-self discrimination • Property of the adaptive immune system to mount specific/targeted responses to foreign antigens without responding to self • Achieved by early and continuous presence of self-antigens • Important for self tolerance and control of autoimmunity

8. Danger vs non-Danger model • Immune system does not discriminate b/w self and non-self • Only interested in responding to dangers elicited via recognition of danger signals • What makes an antigen dangerous??

9. MHC Restriction: • T cell receptors recognize antigenic peptide/MHC complexes • CD4+ T cells: restricted by class II • CD8+ T cells: restricted by class I

10. Self Tolerance • Immunologic unresponsiveness to self antigens • Self tolerance develops very early in life • Chimeras • Medawar’s classical graft experiment Mouse Strain A skin >>>> Strain B mouse >>>>>>>Rejection Mouse Strain A cells >>>Neonate Strain B mouse wait 6 months + Strain A skin >>>>>>>>>>>Accepted Mouse Strain A cells >>>Neonate Strain B mouse wait 6 months + Strain C skin >>>>>>>>>>>Rejection

11. General Characteristics of Tolerance • Antigen-specific immunological unresponsiveness. • The immune response is affected only to antigens used to induce tolerance. • Tolerance may be transient or permanent. • Different from immuno-suppression (usually transient) • Must be established at clonal level (T and /or B clones specific). • Tolerance may be induced to self antigens or induced to foreign antigen 11

12. Things to remember about Tolerance • Tolerance results from recognition of antigen by specific lymphocytes; mostly in the absence of signal two • It is antigen-specific • Normal individuals are usually tolerant to their own antigens = self tolerance • Tolerance is inducible: Foreign antigens may be administered in a way that induces tolerance • Tolerance can be broken

13. Factors that influence development of tolerance • Host: status of immune system • Immaturity, pharmacological immunosuppression, irradiation. • Form of antigen: soluble vs aggregates • Size of the antigen • Immunogenicity • Route of administration: intravenous, oral. • Dose of antigen: high or low dose of antigen. 13

14. Tolerogenic self antigens vs immunogenic (foreign) antigens 14

15. Mechanisms of Tolerance: • Central Tolerance • Negative selection • Development of regulatory T cells • Receptor editing (B cells) • Peripheral Tolerance • Regulatory cells and suppression • Anergy/Ignorance • Exclusion from lymphoid follicles (B cells)

16. Central T lymphocyte Tolerance • Location: Generative lymphoid organ (Thymus). • Targets: >> Immature lymphocytes • Mechanisms: >> Negative selection >> Deletion >> Regulatory cells 16

17. Positive and negative selection • Positive selection • Double negative (DN) thymocytes: no expression of CD4, or CD8. • DN can proliferate and differentiate into thymocytes that express a TCR and both CD4/CD8: double positive (DP) thymocytes. • DP thymocytes moved toward the deeper portion of the thymus die unless their TCR recognize MHC Class I or II expressed by dendritic cells in the cortex 17

18. Positive and negative selection • Negative selection • DP thymocytes that showed strong interaction with self peptides presented by MHC molecules in the thymus undergo programmed cell death • Only molecules of self origin are dispalyed on thymic APCs 18

19. Positive and Negative Selection

20. Peripheral T cell Tolerance • Location: peripheral tissues (secondary lymphoid organ). • Targets: mature lymphocytes. • Mechanisms: >> Anergy (functional inactivation) >> Deletion (activation-induced cell death) >> Immunosuppression >> Regulatory T cells 20

21. Peripheral T cell Tolerance: Anergy • Anergy: functional inactivation of T lymphocytes. • recognition of antigen without adequate levels of the second signal. • Lack of co-stimulatory molecules in APC (B7). • Expression of inhibitory CTLA4 (CD152). 21

22. Deletion: activation induced cell death (AICD) Repeated activation of mature T lymphocytes by self antigen without second signals. 22

23. Immune suppression • Most regulatory T cells: »CD4+CD25+Foxp3+ • »Thymus • » Peripheral lymphoid tissue • Treg mediates their function through: • » soluble factors (IL-10, TGF): inhibition of the effector phase • » direct interaction: inhibition of activation and differentiation of naïve T cells 23

24. Over view of Central and Peripheral Tolerance From A. K. Abbas and A. Lichtman

25. Central B cell tolerance • Elimination of lymphocytes with high affinity receptors for abundant, widely expressed , cell membrane or soluble self antigens. • Involves immature B cell in the bone marrow » Apoptosis » Receptor editing : expression of new antigen receptor. 25

26. Peripheral B lymphocytes tolerance Major mechanisms: A. Anergy B. Exclusion 26

27. Why do we need B cell tolerance? • Accommodate T independent antigens • Self polysaccharides • Lipids • Nuclei acids 27

28. Factors that regulate tolerance • Generally, tolerance is regulated by signals • Signal strength is dependent on • Avidity of interaction b/w Ag and Ag receptor • Affinity • Costimulation • Inhibitory signals e.g. CD25 for B cells

29. Reasons to Induce Tolerance • Prevent the rejection of organ allografts or xenografts • Treat autoimmune diseases • Treat allergic diseases

30. Reasons to Terminate Tolerance • To treat cancer: enhance first or second signals • Treat infectious diseases

31. Termination of tolerance • Clonal regeneration. • Tolerogenic dose is not maintained. • Cross-immunization. • Cross reacting antigen • Co-stimulation of anergic clones. • Infectious agents. 31

32. Autoimmunity • Refers to failure of an organism to recognize its own constituent parts as self • Allows an immune response against self cells, organs and tissues • Tissue damage usually resulting from inflammatory responses

33. What Triggers an Autoimmune Response? • Cryptic Antigen? • Somatic hypermutation? • Th-cell By-pass? • Genetic Defects? • Tr-cell Defect? • Danger Signals? 33

34. Types of Autoimmune Diseases • Systemic Autoimmunity: >> Autoimmune diseases involving several organs and tissues >> Most involve both humoral and cell-mediated immunity • Organ-specific: >> Immune response directed to specific organs leading to cellular damage and organ destruction

35. Mechanisms for developing autoimmunity • Molecular mimicry: cross-reaction of foreign and self epitopes with the same antigen receptors. • Polyclonal B cell or T cell activation: superantigens (EBV, toxins). • Cytokines imbalance (Th-1 versus Th-2). • Loss of suppression: regulatory T cells mediated (age related). • Epitope spreading • activation of bystander self reactive lymphocytes • Responses originally directed against infectious agents can create conditions leading to subsequent autoimmune response 35

36. Factors influencing autoimmune diseases development • Genetic. HLA, FcgR, FAS/FASL, Complement proteins. • Environmental: microbes • Gender: females greater than males. • Others: injury 36

37. Animal models of autoimmunity I- Experimental allergic encephalomyelitis (EAE) • Organ specific • Immunization with myelin basic protein (MBP) and adjuvant. • Perivascular inflammation (CD4+ T cells ); phagocytes recruitment. • Enzymes release and demyelination. • Formation of auto Abs to MBP and proteolipid protein (PLP) • Disease can be induced by adoptive transfer of CD4+ T cells • Human: multiple sclerosis • Symptoms: shaky movements of the limbs, defects in speech… 37

38. Animal models of autoimmunity-contd. II- Non obese diabetic mouse (NOD) • Spontaneous. • H2 Linkage. • In human: Insulin Dependent Diabetes Mellitus (IDDM): HLA-DR-3, and others. • T cell mediated insulitis evolving to diabetes. • CD4+ and CD8+ specific for pancreatic islet beta cells. • Defects in deleting auto-reactive clones during T cell differentiation. 38

39. Animal models of autoimmunity III-Systemic Lupus Erythrematosus (SLE) • (NZBXNZW)F1 female • Glomerulonephritis, hemolytic anemia and others • Alterations in B/T cell and macrophage functions; cytokines release. • Hyper-activation of the humoral immune system. • Anti-Nuclear Abs (ANA). • MRL-Ipr/Ipr: fas deficient, gld mice : fas ligand -Defective deletion of autoreactive B and T cells 39

40. Diagnosis of autoimmunity • Clinical symptoms • Autoantibodies detection: IF • Autoimmune T cells detection: proliferation, cytotoxicity assay 40

41. Treatment of autoimmune diseases • Conventional therapies • Anti-inflammatory agents • Immunosuppressive agents • Lymphocyte specific Abs • New Therapies • TNFR blocker (Embrel) • Anti-CD20 (depletes B cells for SLE treatment) • Experimental Therapies • Induction of Tolerance • Blocking peptides • TRAIL/TRAILR (induction of apoptosis)

42. Summary • Self reactive B-cells & T-cells are normally present but are anergic or under check by regulatory T cells • Several factors can induce an Autoimmunity • Genetic • Tissue damage & release of cryptic e.g. • Somatic mutation in Ig V-genes • TH by-pass, e.g. by Ag mimicry • Treg defects • Danger signals

43. General Summary: • Tolerance is specific unresponsiveness, is inducible, highly specific and may be broken • Two major mechanisms of Tolerance • Central and Peripheral Tolerance • Both B and T cells undergo central and peripheral tolerance • Deletion, Anergy, immunosuppression, exclusion, receptor editing • Failure of Tolerance results in Autoimmune diseases • Several factors contribute to autoimmune diseases • Genetic, environment, infections, mimicry, etc • Approaches to treatment of autoimmune diseases are varied

44. Recall Objectives: • Define and discuss the general characteristics of tolerance • Define the main factors that influence the development of tolerance • Identify the main mechanisms of tolerance induction in T and B cells • Identify the mechanisms involved in the development of autoimmunity • Identify and discuss 3 animal models of autoimmune diseases • Approach to treatment of autoimmune diseases