Immune Regulation and Tolerance

Immunoregulation: A balance between activation and suppression of effector cells to achieve an efficient immune response without damaging the host. Activation (immunity) Suppression (tolerance) autoimmunity immunodeficiency Significance: The induction of tolerance may be exploited to prevent graft rejection, to treat autoimmune and allergic diseases, and to prevent immune responses in gene therapy.

Important features of immunoregulation: Antigen specific; affects T or B lymphocytes Tolerance vs. activation? Determined by the nature of antigen and associated stimuli, and when and where the antigen is encountered

Immunological Ignorance Mechanisms of unresponsiveness: Immunological Ignorance Normal response Proliferation and differentiation Mechanisms of unresponsiveness Antigen/lymphocyte barrier Mechanisms of autoimmunity Tissue abnormalities contributing to release and presentation of self antigens. Disease models Sympathetic ophthalmia, experimental allergic encephalomyelitis (EAE)

Central tolerance in B and T cells (I): Clonal Deletion Mechanisms of unresponsiveness: Central tolerance in B and T cells (I): Clonal Deletion Self antigen presented in generative lymphoid organs Deletion of immature lymphocytes strongly recognizing self antigens present in generative organs Lymphoid precursor Survival of clones which are only moderately responsive to self antigens present in generative organs; forms T cell repertoire

AIRE: Autoimmune regulator. Transcription factor. Expressed at a high level by thymic medullar epithelium cells. Autosomal recessive mutation leads to autoimmune polyendocrine syndrom - type 1 (APS-1) Inactivation of aire abolishes expression of tissue specific genes in thymic medulla.

aire -/- WT

Central tolerance in B cells (II):Receptor editing Mechanisms of unresponsiveness: Central tolerance in B cells (II):Receptor editing pre-B bone marrow B rearrangement 10% Maturation of clones: Non-reactive to soluble- low-affinity to soluble- self-reactive to monovalent- antigens in bone marrow further rearrangement immature B

B220 Moma-1 TCR B follicle T cell 2% zone Spleen artery Self antigens 90%

Mechanisms of unresponsiveness: Peripheral tolerance in B cells (I): Follicular exclusion - B cells binding to autoantigens in the periphery may be excluded from follicles - Excluded B cells undergo apoptosis mechanisms for elimination is independent of Fas, T cells. - Rapid elimination depends on the presence of a normal repertoire of B cells competition between B cells (for limited survival factors?)

BAFF: TNF family cytokine, critical B cell survival factor. BAFF reduction leads to a decrease in peripheral B cell numbers; Over-expression of BAFF results in autoimmune diseases (SLE). BAFF receptor: expressed on mature B cells.

Mechanisms of unresponsiveness: Peripheral tolerance in B cells (II): Anergy Immunogenic signaling Tolerogenic signaling Acute antigens BCR CD40 CD40L TLR4 LPS NFkB Ca++ Growth genes Chronic antigens BCR CD40 TLR4 NFkB Ca++ Inhibitory genes Fcg2b

Anergic B cells can respond to “Stronger” antigens Remains anergic Oligovalent self antigens Constitutively exposed Activated Multivalent foreign antigens Acutely exposed

The two-signal requirement for T cell activation Microbial antigen presented by APC TCR MHC Signal 1 APC Signal 2 B7 Costimulatory Receptor (CD28)

Innate immune response The role of co-stimulation in T cell activation Antigen recognition T cell response CD28 Resting APC: (costimulator- deficient) No response Activation of APC Innate immune response Activated APC: increased expression of costimulators, secretion of cytokines CD28 B7 T cell proliferation And differentiation

Mechanisms of unresponsiveness: Peripheral tolerance in T cells (II): Anergy Pretreatment of T cells Stimulation with antigens T cell response CD28 CD28 B7 18-24 hr CD28 B7 CD28 B7 18-24 hr

Molecular basis of anergy in T lymphocytes TCR CD3z P PROXIMAL EVENTS: - Reduced tyrosine phosphorylation - Reduced Ca++ influx Zap 70 GROWTH FACTORS NUCLEAR EVENTS: - no induction of NFkB JNK activities CELL CYCLE

Co-stimulatory pathways CD28 interacts with CD80 (B7-1) CD86 (B7-2) to initiate T cell responses. Preferentially expressed in naive T cells ICOS (CD28 homolog) stimulate effector T cell responses. Preferentially expressed in activated T cells CTLA-4 and PD-1 negatively regulate T cell activation

Mechanisms of unresponsiveness: Peripheral tolerance in T cells (II): Anergy Pretreatment of T cells Stimulation with antigens T cell response 18-24 hr CD28 B7 primed CD28 18-24 hr B7 tolerated

CTLA-4-/- T cells resist tolerance induction Primed Tolerated Days

How do T cells choose between CD28 and CTLA-4? Level of B7 expression on APCs: low levels favor CTLA-4 engagement (high affinity receptor) Kinetics: B7 on APCs engages CD28 early, CTLA-4 late in T cell responses

Regulation of T cell homeostasis during immune responses Anergy T cell expansion B7 CTLA-4 Magnitude of T cell response Activated T cells express CTLA-4 Apoptosis Surviving memory cells B7 CD28 Activated T cells are deprived of antigen and other stimuli T cell activation Time after antigen exposure

Pathways of apoptosis in T cells Release of mitochondrial cytochrome c, activation of caspase-9 Elimination of Antigen and other signals “Passive” cell death (death by neglect) CD28 B7 IL-2 T cell proliferation FasL Persistence of antigen, repeated stimulation Fas Activation of caspase-8 Activation induced cell death

T cell mediated suppression MBP-TCR In Rag-/- MBP MBP-TCR In wildtype MBP MBP-TCR In Rag-/- MBP CD4+CD25+ T cells

Science (2003) 299:1057

Foxp3 is expressed specifically in TR Expression of Foxp3 converts naïve CD4+ T cells to TR

Regulatory T cells (TR) in self tolerance Phenotype and ontogeny: CD4+cells (most are CD25+, some are CD25-) , develop in the thymus Recognize self antigens? Other populations of regulatory T cells exist (including CD8+) Mechanisms of action: Antigen-induced suppression; secrete immunosuppressive cytokines, trigger inhibitory cell surface molecules (CTLA-4 expressed on TR). Prevent the activation of T cells; suppress cell proliferation and IL-2 production

Role of cytokines in suppression of cell-mediated immune responses Antigen recognition T cell proliferation and differentiation Effector functions of T cells IL-12 Effector T cells (TH1) Activated macrophages APC Naïve T cell IFN- Cytokines produced by suppressor T cells IL-10 inhibits Functions of APCs: IL-12 secretion, B7 expression TGF- inhibits T cell proliferation IL-4 inhibits action of IFN- IL-10, TGF- inhibit macrophage activation Suppressor T cells

Suppressor T cells - unresolved issues How many types of suppressor T cells  cellular markers? How do they develop  positive/negative selection? What are their physiological ligands? What are their target cells? What is their mechanisms of actions? Are they beneficial (for the prevention of autoimmunity, allergy and graft rejection)? Are they harmful (in terms of their effects on tumor immunity, immune response to chronic infections and weak vaccines)?

Conclusions: Tolerance vs. Immunity Immune responses are the outcome of a balance between the need to make a protective response and the need to maintain self-tolerance Mechanisms of unresponsiveness: Central tolerance: Deletion; Receptor editing Peripheral tolerance: Clonal ignorance; Clonal deletion; Anergy; Suppression