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) autoimmunity Suppression (tolerance) 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. Immunological Ignorance Important features of immunoregulation: Normal response 1. Antigen specific; affects T or B lymphocytes 2. Tolerance vs. activation? Determined by the nature of antigen and associated stimuli, and when and where the antigen is encountered Mechanisms of unresponsiveness Mechanisms of autoimmunity Disease models Proliferation and differentiation Antigen/lymphocyte barrier Tissue abnormalities contributing to release and presentation of self antigens. Sympathetic ophthalmia, experimental allergic encephalomyelitis (EAE) Central tolerance in B and s (I): Clonal Deletion Lymphoid precursor Self antigen presented in generative lymphoid organs Deletion of immature lymphocytes strongly recognizing self antigens present in generative organs Survival of clones which are only moderately responsive to self antigens present in generative organs; forms repertoire 1
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 B follicle pre-b bone marrow zone 2% κ rearrangement immature B further rearrangement 10% B Maturation of clones: Non-reactive to soluble- low-affinity to soluble- self-reactive to monovalentantigens in bone marrow B220 Moma-1 TCR 90% Self antigens Spleen artery 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, s. - Rapid elimination depends on the presence of a normal repertoire of B cells competition between B cells (for limited survival factors?) 2
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. Peripheral tolerance in B cells (II): Anergy Immunogenic signaling Acute antigens CD40L LPS Chronic antigens Tolerogenic signaling BCR CD40 TLR4 BCR Fcγ2b CD40 TLR4 Ca ++ NFκB Ca ++ NFκB Growth genes Inhibitory genes Anergic B cells can respond to Stronger antigens The two-signal requirement for activation Oligovalent self antigens Constitutively exposed Multivalent foreign antigens Acutely exposed Signal 1 TCR Microbial antigen presented by APC MHC APC Signal 2 Anergic B cell Anergic B cell Costimulatory Receptor () Remains anergic Activated The role of co-stimulation in activation Antigen recognition Resting APC: (costimulatordeficient) Activated APC: increased expression of costimulators, secretion of cytokines Activation of APC Innate immune response response No response proliferation And differentiation Two- signal requir em ent f or T l ym p ho c yte ac t iva ti on N a ïv e T l ympho c y t e s ne e d t w o s ign al s to ini ti ate respon ses S ign al 1: a n t ig e n re c ogn i t i on En s ur e th at th e i m m un e re s pon s e is an ti gen- s p e c i f i c S ign al 2: mi c r obe s or s ub st an c e s produ c ed dur i ng i nna t e imm une r es pon s e s t o m ic robe s En s ur e th at th e i m m un e sy ste m r es pond s to mi c r obe s and no t t o ha r m les s a n t ig e n i c s ub st anc e ( S e c ond s ign al s f or T c e l l s a re co stim ul at or s on A PC s a nd cy tok ine s produ c ed by A PC s. ) 3
Peripheral tolerance in s (II): Anergy Molecular basis of anergy in T lymphocytes Pretreatment of s Stimulation with antigens response TCR P CD3ζ Zap 70 PROXIMAL EVENTS: - Reduced tyrosine phosphorylation - Reduced Ca ++ influx GROWTH FACTORS CELL CYCLE NUCLEAR EVENTS: - no induction of NFκB JNK activities Co-stimulatory pathways interacts with CD80 (-1) CD86 (-2) to initiate responses. Preferentially expressed in naive s ICOS ( homolog) stimulate effector responses. Preferentially expressed in activated s Peripheral tolerance in s (II): Anergy Pretreatment of s Stimulation with antigens response primed CTLA-4 and PD-1 negatively regulate activation tolerated CTLA-4 -/- s resist tolerance induction How do s choose between and CTLA-4? Primed Tolerated Level of expression on APCs: low levels favor CTLA- 4 engagement (high affinity receptor) Kinetics: on APCs engages early, CTLA-4 late in responses Days 4
Regulation of homeostasis during immune responses Pathways of apoptosis in s Magnitude of response activation expansion CTLA-4 Activated s express CTLA-4 Apoptosis Activated s are deprived of antigen and other stimuli Time after antigen exposure Anergy Surviving memory cells IL-2 Elimination of Antigen and other signals Persistence of antigen, repeated stimulation Fas Release of mitochondrial cytochrome c, activation of caspase-9 proliferation FasL Activation of caspase-8 Passive cell death (death by neglect) Activation induced cell death mediated suppression MBP-TCR In Rag -/- MBP MBP-TCR In wildtype MBP MBP-TCR In Rag -/- MBP CD4 + CD25 + s Science (2003) 299:1057 Foxp3 is expressed specifically in T R Regulatory s (T R ) 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 s exist (including CD8 + ) Expression of Foxp3 converts naïve CD4 + s to T R Mechanisms of action: Antigen-induced suppression; secrete immunosuppressive cytokines, trigger inhibitory cell surface molecules (CTLA-4 expressed on T R ). Prevent the activation of s; suppress cell proliferation and IL-2 production 5
APC Cytokines produced by suppressor s Role of cytokines in suppression of cell-mediated immune responses Antigen recognition IL-12 Naïve IL-10 inhibits Functions of APCs: IL-12 secretion, expression proliferation and differentiation Effector s (T H 1) IFN-γ TGF-β inhibits proliferation Effector functions of s IL-4 inhibits action of IFN-γ Activated macrophages IL-10, TGF-β inhibit macrophage activation Suppressor s - unresolved issues How many types of suppressor s 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)? Suppressor s 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 Central tolerance: Deletion; Receptor editing Peripheral tolerance: Clonal ignorance; Clonal deletion; Anergy; Suppression 6