Diagnostic Immunology Topic: Immunological Tolerance Objectives: Define Immunological tolerance Define Immunological tolerance Discuss mechanism of tolerance induction Discuss mechanism of tolerance induction Know types of tolerance Know types of tolerance Central thymic Central thymic Peripheral post thymic Peripheral post thymic Explain B cell tolerance to self antigens Explain B cell tolerance to self antigens Discuss artificial induction of tolerance Discuss artificial induction of tolerance Know therapeutic applications of tolerance Know therapeutic applications of tolerance

Immune tolerance or immunological tolerance is the process by which the immune system does not attack an antigen. is the process by which the immune system does not attack an antigen.immune systemantigenimmune systemantigen It can be either : It can be either : 1) Natural' or 'self tolerance', where the body does not mount an immune response to self antigens. 2) Induced tolerance', where tolerance to external antigens can be created by manipulating the immune system. It occurs in three forms: central tolerance, peripheral tolerance and acquired tolerance. It occurs in three forms: central tolerance, peripheral tolerance and acquired tolerance.

Definitions: Tolerance is a state of unresponsiveness that is specific for a particular antigen Tolerance is a state of unresponsiveness that is specific for a particular antigen Self tolerance is the mechanisms by which the body is prevented from mounting an immune response against its own tissues. Self tolerance is the mechanisms by which the body is prevented from mounting an immune response against its own tissues. Self reactivity is prevented by processes during development rather than being programmed. Self reactivity is prevented by processes during development rather than being programmed.

Factors influencing tolerance Molecule structure Molecule structure Stage of differentiation when lymphocyte first encounter the epitopes Stage of differentiation when lymphocyte first encounter the epitopes The site of the encounter The site of the encounter The nature of the cell presenting the epitopes The nature of the cell presenting the epitopes Number of responding lymphocytes Number of responding lymphocytes Q. What is an epitope? Q. What is an epitope?

History Trub (1938): Trub (1938): Inutero mice against choriomeningitis virus Medawar (1953) Medawar (1953) Induced immunological tolerance to skin allograft Burnet (1957) Burnet (1957) Clonal selection theory: Particular immunocyte is selected by an antigen and then divides to give rise to clone of daughter cells with the same specificity

Leaderburg (1959) Modification of Clonal selection theory ( Stage of cell maturation) Key discoveries of (1960) a) Role of thymus in development of the immune system a) Existence of two interacting subsets of lymphocytes

Types of Tolerance Central Tolerance : Central Tolerance : It occurs during lymphocyte development. [Thymus or Bone marrow] Peripheral Tolerance : Peripheral Tolerance : Occurs after lymphocytes leave the primary organs.

Central thymic Tolerance to Self Antigens The immune system generates a vast array of TCRs The immune system generates a vast array of TCRs T cells are not only effector cells but are also regulators of the immune system T cells are not only effector cells but are also regulators of the immune system T cells become “ educated ” in the thymus T cells become “ educated ” in the thymus They become dependant on self MHC for survival They become dependant on self MHC for survival

III-T cell development is subjected to several checkpoints β selection checkpoint: β selection checkpoint: Only cells with a rearranged β chain mature from double negative to double positive cells. Independent on MHC α selection checkpoint: α selection checkpoint: Cells expressing an αβ complex must interact with MHC molecules to survive

Lineage commitment checkpoint: Lineage commitment checkpoint: Cells are instructed to repress expression of either CD4 or CD8 and to develop into single positive cells Negative selection checkpoint: Negative selection checkpoint: Cells that interact strongly with MHC and antigen in the thymus are deleted

Peripheral or Post-Thymic Tolerance to Self Antigens Many auto-reactive T cells escape central tolerance due to: a) Antigens are absent b) Antigens are insufficient However, tolerance is maintained by mechanisms in peripheral lymphoid organs

1-Sequestration of antigens in some tissues: Many self antigens are hidden in tissues that are anatomically located away from T lymphocytes 2- Privileged sites are protected by regulatory mechanisms: Privileged sites include brain, testes and anterior champers of the eye In these sites lymphocytes are controlled by apoptosis or cytokines such as TNFβ and IL10 These mechanisms include:

3- T cell death can be induced by persistent activation or neglect: - Cells possess Fas receptor, when it cross-links with its ligand FasL it promote apoptopic cell death. - Activated lymphocyte will die by passive cell death (PCD) when deprived from its antigen 4- Regulation by suppressive cytokines IL-2 and IL-2 receptor regulate sensitivity to Fas mediated apoptosis

Summary of peripheral post thymic mechanisms of tolerance: Thymus DeletionEscape Sequestration Antigen hidden from immune system Privileged sites Prevention by Fas and cytokines Deletion By activation- induced Cell death Immune regulation By Suppressive cytokines

B- Cell Tolerance to Self Antigens High affinity IgG production is T cell dependent High affinity IgG production is T cell dependent Lack of T cell leads to non-self reactivity by B cells Lack of T cell leads to non-self reactivity by B cells Self reactive B cells either: Self reactive B cells either: a) Edit their receptors to become non-reactive b) Die by the process of apoptosis In peripheral B cell tolerance, self reactive cells are removed by negative selection in the spleen in a process that is similar to T cell removal in the thymus. In peripheral B cell tolerance, self reactive cells are removed by negative selection in the spleen in a process that is similar to T cell removal in the thymus.

B cell tolerance in the BM During B cell development in the bone marrow: The complete antigen receptor (IgM) is first expressed on 'immature' B cells The complete antigen receptor (IgM) is first expressed on 'immature' B cells If those cells encounter their target antigen in a form which can cross-link their IgM then such cells are programmed to die If those cells encounter their target antigen in a form which can cross-link their IgM then such cells are programmed to die The requirement for cross-linking means that the antigen has to be polyvalent The requirement for cross-linking means that the antigen has to be polyvalent

Potential clinical applications for tolerance : Understanding of tolerance is valuable in many ways : Understanding of tolerance is valuable in many ways : Promote tolerance to tissue graft Promote tolerance to tissue graft Control damaging immune response in hypersensitivity Control damaging immune response in hypersensitivity Control damaging in autoimmune disease Control damaging in autoimmune disease Limit tumor growth Limit tumor growth