Module 8 Humoral Immunity PDF

Module 8 Chapter 13: Effector Mechanisms of Humoral Immunity 3 Humoral immunity overview Mediated by an1bodies Target is extracellular pathogens Transferrable (passive) produced by plasma cells in secondary (peripheral) lymphoid organs, inflamed 1ssues, and bone marrow Reside in luminal spaces a...

Module 8 Chapter 13: Effector Mechanisms of Humoral Immunity 3 Humoral immunity overview Mediated by an1bodies Target is extracellular pathogens Transferrable (passive) produced by plasma cells in secondary (peripheral) lymphoid organs, inflamed 1ssues, and bone marrow Reside in luminal spaces and blood 4 Func>ons of An>body Isotypes 5 Func>ons of An>body Isotypes, Cont. 6 Neutraliza>on of Microbes and Microbial Toxins An1bodies that bind to microbial structures interfere with the ability of the microbes to interact with cellular receptors by means of steric hindrance and thus prevent infec1on requires only the an1gen-binding regions of the an1bodies Do not require Fc region to be effec1ve Blood = IgG; Gut/respiratory tract = IgA 7 Neutraliza>on of microbes and toxins by an>bodies 8 An>body-Mediated Opsoniza>on and Phagocytosis IgG an1bodies coat (opsonize) microbes and promote their phagocytosis by binding to Fc receptors on phagocytes. enhance the ability of phagocytes to engulf extracellular pathogens express receptors for the Fc por1ons of IgG an1bodies à opsoniza1on 9 Leukocyte Fc Receptors, 1 10 Leukocyte Fc Receptors, 2 Fc receptors are op1mally ac1vated by an1bodies bound to their an1gens and not by free, circula1ng an1bodies Fcγ receptorsàmost important for opsoniza1on 11 Leukocyte Fc Receptors, 3 Cross-linking of the ligand-binding α chains of an FcR results in 10/22/24 1 10 11 Leukocyte Fc Receptors, 3 Cross-linking of the ligand-binding α chains of an FcR results in signal transduc1on on leukocytes transcrip1on of genes encoding: cytokines inflammatory mediators microbicidal enzymes mobiliza1on of the cytoskeletonàphagocytosis 12 Clinical Correla>on: IVIG FcγRIIB receptor = inhibitory receptor for Ab expressed on B cells, DCs, neutrophils, macrophages, and mast cells  Involved in Ab nega1ve feeback (Ch 12) Can help “dampen” Ab driven autoimmune disease through passive ar1ficial immunity (IVIG) by reducing B cell/ neutrophil ac1vity E.g. mul1ple sclerosis, Kawasaki disease, chronic inflammatoryc demyelina1ng polyneuropathy 13 An>body-Dependent Cell-Mediated Cytotoxicity NK cells and other leukocytes bind to an1body-coated cells by Fc receptors and destroy these cells Mediated in NK cells by FcγRIIIA (CD16) ADCC occurs only when the target cell is coated with an1body molecules 14 An>body-Mediated Clearance of Helminths Helminths (worms) are too large to be engulfed by phagocytes’ killed by a toxic ca1onic protein, major basic protein (MBP), present in the granules of eosinophils An1bodies that coat helminths, especially IgG, can bind to Fc receptors on eosinophils and cause the degranula1on of the eosinophil 15 The Complement System 16 Overview of complement system major effector mechanisms of humoral and innate immunity serum and cell surface proteins that interact with one another and with other molecules of the immune system in a highly regulated manner to generate products that func5on to eliminate microbes. 17 15 16 with other molecules of the immune system in a highly regulated manner to generate products that func5on to eliminate microbes. 17 Overview of complement system, Cont. Ac1va1on factors Microbes; an1bodies and lec1ns bound to microbes involves the sequen1al proteolysis of proteins to generate enzyme complexes with proteoly1c ac1vity biologically ac1ve cleavage products of complement ac1va1on become covalently ajached to microbial cell surfaces inhibited by regulatory proteins that are present on normal host cells and absent from microbes 18 Ac>va>on of Complement three pathways Classical Alterna1ve Lec1n central event in complement ac1va1on is proteolysis of the complement protein C3 All pathways differ in ini1a1on Synchronize at ac1va1on of C3 convertaseàC3bàC5 convertase (C3bBb3b in AP and C4Bb2aC3b in CP/LP) à C5a + C5b + membrane ajack complex (MAC) 19 3 pathways of complement ac>va>on 20 Alterna>ve Pathway Spontaneous hydrolysis of plasma C3 leads to the forma1on of a fluid-phase C3 convertase and the genera1on of C3b in the process called C3 5ckover ac1vated hydrolyzed C3, indicated as C3∗, is deposited on microbial surfaces, where it binds Factor B and forms the alterna1ve pathway C3 convertase. C3 convertase (C3bBb) cleaves C3 to produce more C3bàbinds to the microbial surface and par1cipates in the forma1on of a C5 convertase. 21 the microbial surface and par1cipates in the forma1on of a C5 convertase. C5 convertase, C3bBbC3b, cleaves C5 to generate C5bàprogress to late steps of complement ac1va1on. 21 Alterna>ve Pathway – key proteins Factor B serine protease and the ac1ve enzyme of the C3 and C5 convertases. Factor D Plasma serine protease cleaves factor B when it is bound to C3b Properdin stabilizes C3 convertases (C3bBb) on microbial surfaces. 22 The Classical Pathway ini1ated by binding of the complement protein C1 to the CH2 domain of IgG or the CH3 domain of IgM molecules that have bound an1gen C1 is mul1meric C1q binds the immunoglobulin IgM>IgG3>IgG1>IgG2 binding affinity C1r + C1s = serine proteases C1r cleaves and ac1vates C1s Ac1vated C1s cleave C4 and C2à C4a2b (C3 convertase) 23 The Classical Pathway, Cont. Comparison to alterna5ve pathway C3 in the alterna1ve pathway is homologous to C4 in the classical pathway, Factor B is homologous to C2 net effect of the mul1ple enzyma1c steps and amplifica1on is that millions of molecules of C3b can be deposited within minutes on the cell surface where complement is ac1vated 24 The Classical Pathway – Fc requirements C1 must bind to two or more Fc por1ons to ini1ate the complement cascade adjacent IgG Fc por1ons can bind and ac1vate C1 25 24 cascade adjacent IgG Fc por1ons can bind and ac1vate C1 The Fc por1ons of soluble pentameric IgM are not accessible to C1 Aper IgM binds to surface- bound an1gens, it undergoes a shape change that permits C1 binding and ac1va1on 25 Lec>n Pathway Similar to classical pathway Ac1vated in ABSENCE of an1body Mannose Binding Lec1n (MBL) is similar to C1q MASP1/MASP2 similar to C1r and C1s Cleaves C4 and C2 to make the 3 convertaseàC4b2a Progresses through C5 convertase in fashion similar to classical pathway 26 Late Steps of Complement Ac>va>on Once C5 is madeàprogress to late steps of complement ac1va1on and the forma1on of the membrane ajack complex MAC is formed by C5b – C8 + Poly C9 – creates pore in membrane to allow for cell lysis C5-8 makes an “unstable” pore; stabilized by C9 C9 is similar to perforin 27 Comparison of complement pathways 28 Complement receptors 29 Regula>on of complement Complement ac1va1on must be limited Specialized plasma proteins keep complement ac1va1on in check given the spontaneous ac1va1on at rest 30 Regula>on of complement – C1 INH C1 inhibitor 31 serine protease inhibitor mimics the normal substrates of C1r and C1s C1r2-C1s2 tetramer dissociates from C1q, thus stopping ac1va1on by the classical pathway 30 C1r2-C1s2 tetramer dissociates from C1q, thus stopping ac1va1on by the classical pathway 31 Clinical Correla>on: Hereditary Angioedema Disease characterized by recurrent episodes of angioedema WITHOUT ur1caria (hives) or pruri1s (itching) Defect in C1 INH gene (SERPING1) on chromosome 11 Most commonly autosomal dominant inheritance pajern Angioedema results from excess bradykinin – mediator of vasodila1on (not histamine) 32 Clinical Correla>on: Hereditary Angioedema, Cont. Presenta1on: facial swelling, hand swelling, asymmetric swelling, recurrent projec1le emesis (GI swelling) Laryngeal edema may be fatal Lasts 24-72 hours 3 type  T1 HAE: low C1 INH, low complement T2 HAE: nml C1 INH, but doesn’t work, low complement T3 HAE: nml complement and C1 INH (hormonal related?) Disease can be mimicked by angiotensin conver1ng enzyme inhibitors E.g lisinopril Treat with complement replacement 33 Clinical Correla>on: Complement deficiencies Leads to impaired humoral immune func1on and the persistence of immune complexes Pa1ents with muta1ons in the classical pathway are more suscep1ble to extracellular pathogens due to a lack of clearance of opsonized bacteria Failures in the early complement componentsàinability to clear immune complexes. Complement components ajach to soluble immune complexes and allow them to be transported, ingested and degraded by cell with complement receptors Immune complexes deposit into 1ssues and ac1vate phagocytes, causing inflamma1on and 1ssue damage Failed membrane ajack complex forma1on leads to impaired defense against Neisseria spp Chapter 15 Immunologic Tolerance and Autoimmunity Immunologic tolerance, 1 unresponsiveness to an an1gen that is induced by exposure to that an1gen An1gens that induce tolerance are called tolerogens failure of self-tolerance results in immune reac1ons against self (autologous) an1gens = autoimmunity 4 Immunologic tolerance, 2 The mechanisms of tolerance eliminate and inac1vate lymphocytes that express high-affinity receptors for self an1gens Tolerance is an1gen specific, resul1ng from the recogni1on of an1gens by individual clones of lymphocytes. Self-tolerance may be induced in immature self-reac1ve lymphocytes in the genera1ve lymphoid organs (central tolerance) or in mature lymphocytes in peripheral sites (peripheral tolerance) 5 Immunologic tolerance, 3 central tolerance: immature lymphocytes specific for self an1gens may encounter these an1gens in the genera1ve lymphoid organs self reac1ve lymphocytes change their specificity (B cells only), or (in the case of CD4+ T cells) develop into regulatory lymphocytes (Tregs)  Peripheral Tolerance: self-reac1ve lymphocytes enter peripheral 1ssues and may be inac1vated or deleted by an encounter with self an1gens in these 1ssues or are suppressed by the regulatory T cells (Tregs, peripheral tolerance) 6 Central Tolerance occurs during a stage in the matura1on of lymphocytes when an encounter with an an1gen may lead to cell death or replacement of a self-reac1ve an1gen receptor with one that is not self-reac1ve Ag in thymus or BM are self Ag Mature lymphocytes that recognize self an1gens in peripheral 1ssues 78 1 10/23/24 78 9 10 Mature lymphocytes that recognize self an1gens in peripheral 1ssues become incapable of ac1va1on by re-exposure to that an1gen or die by apoptosis T Lymphocyte Tolerance Central T Cell Tolerance, 1 Developing T cells are exposed to self an1gen in the thymus If the T cell binds strongly to self an1gen Apoptosis (nega1ve selec1on) If the T cell binds moderately to self an1gen T reg can be formed Central T cell tolerance, 2 Nega1ve selec1on occurs in double-posi1ve T cells in the thymic cortex and newly generated single-posi1ve T cells in the medulla Central T cell tolerance, 3 medullary thymic epithelial cells (MTECs) Express many peripheral self Ags for T cell to sample Autoimmune Regulatory (AIRE) gene expressed by mTECs Promote self an1gens only found in non-lymphoid 1ssue (ie insulin from pancrea1c beta cells) High concentra5on of self-ag and high affinity promote nega5ve selec5on 11 1 2 12 Clinical Correla>on – APECED/APS-1 Aka autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy or autoimmune polyglandular syndrome type 1. T cells not deleted that are self reac1ve to endocrine self an1gens Significant endocrinopathies due to autoimmune ajack on various organs – parathyroid (Ca++ regula1on), adrenal gland (low cor1sol), 12 13 Significant endocrinopathies due to autoimmune ajack on various organs – parathyroid (Ca++ regula1on), adrenal gland (low cor1sol), pancreas (diabetes) 2 HusebyeESetal.NEnglJMed2018;378:1132-1141 Clinical Correla>on – APECED/APS-1 cont’d 14 15 16 17 1 Open presents with difficult to treat thrush (candidiasis), nail and teeth hypoplasia 2/2 lack IL-17, IL-22, IFNs due to autoan1body to these cytokines Autosomal recessive inheritance pajern Peripheral T cell Tolerance mechanisms of peripheral tolerance are anergy (func1onal unresponsiveness) suppression by Tregs dele1on (cell death) Lack of secondary signal or cos1mula1on produces anergy of peripheral T cells self an1gens are con1nuously displayed to specific T cells in the absence of innate immunity and strong cos1mula1on Peripheral T cell Tolerance, Cont. sAg recogni1on without cos1mula1on ac1vate cellular ubiqui1n ligases, which ubiqui1nate TCR- associated proteins and target them for proteoly1c degrada1on in proteasomes or lysosomes Peripheral T cell Tolerance: Regula>on of T Cell Responses by Inhibitory Receptors CTLA-4 compe11ve inhibitor of CD28 reduces the availability of B7 for the CD28 receptor Peripheral T cell Tolerance: Regula>on of T Cell Responses by Inhibitory Receptors, Cont. CTLA-4 compe11ve inhibitor of CD28 reduces the availability of B7 for the CD28 receptor Outcompetes B7 in absence of innate immune response PD-1 Recognizes PD-L1/2 18 PD-1 Recognizes PD-L1/2 ITIM mo1f Ac1vates phosphatase SHP2 17 CTLA-4 and PD-1 18 Suppression by Regulatory T Cells express high levels of the IL-2 receptor α chain (CD25) and the transcrip1on factor called FOXP3 Express CD4, CD25 + FOXP3 Tolerance/regula1on is maintained by IL-10 and TGFβ 19 Mechanisms of Ac>on of Regulatory T Cells inhibit the s1mulatory ability of dendri1c cells suppressing T cell ac1va1on CTLA-4–mediated inhibi1on of cos1mula1on Produc1on of the immunosuppressive cytokines IL-10 and TGF-β. Consump1on of IL-2. 20 Clinical Correla>on: IPEX syndrome Immune Dysregula1on Polyendocrinopathy Enteropathy X-linked syndrome Clinical Presenta1on: Infant with derma11s (eczema), intractable diarrhea (failure to thrive), diabetes and hypothyroidism Defect in FOXP3 gene Tx: Stem Cell Transplant 21 Transforming Growth Factor-β inhibits the prolifera1on and effector func1ons of T cells and the ac1va1on of macrophages regulates the differen1a1on of func1onally dis1nct subsets of T cells. Dichotomy with ability to promote Th17 cells s1mulates produc1on of immunoglobulin A (IgA) an1bodies by inducing B cells to switch to this isotype 22 s1mulates produc1on of immunoglobulin A (IgA) an1bodies by inducing B cells to switch to this isotype promotes 1ssue repair aper local immune and inflammatory reac1ons subside. 22 IL-10 inhibitor of ac1vated macrophages and DCs inhibits the produc1on of IL-12, leads to less IFN-γ nega1ve feedback regulator inhibits the expression of cos1mulators and class II MHC molecules on DCs and macrophages. 23 Dele>on of T Cells by Apopto>c Cell Death Apoptosis of T cells Bind to self Ag with strong affinity repeatedly s1mulated by an1gens  2 major pathways of Apoptosis Intrinsic Extrinsic 24 mitochondrial (or intrinsic) pathway Self Ag repeatedly encountered by T cell ac1vates Bim (pro-apopto1c member of Bcl-2 family) Bim binds Bak & Bax: pro-apopto1c effector proteins Insert into mitochondrial membraneàleaks out cytochrome Cà ac1vates caspase 9 à nuclear DNA fragmenta1on 25 death receptor (or extrinsic) pathway Fas upregulated on repeatedly s1mulated T cells. Interacts with Fas Ligand may delete T cell àac1vates caspase 8 26 Factors That Determine the Immunogenicity and Tolerogenicity of Protein An>gens 27 B Lymphocyte Tolerance 28 Central B cell tolerance, 1 B lymphocytes first express IgM as their an1gen receptor in BM If exposed to self Ag and binds can undergo: Receptor Edi1ng 29 27 28 If exposed to self Ag and binds can undergo: Receptor Edi1ng Reac1va1on of RAG1/RAG2 Kappa over lambda Apoptosis (nega1ve selec1on) Anergy 29 Central B Cell Tolerance, 2 B lymphocytes recognize self an1gens in the bone marrow with high affinity, they either change their specificity or are deleted 3 outcomess Receptor edi1ng Apoptosis (neg selec1on) Anergy 30 Receptor edi>ng Recall from Ch 8 immature B cells reac1vate RAG1 and RAG2 genes Reini1ate light chain recombina1on Kappa over lambda 31 Central B cell Tolerance, 3 32 Peripheral B cell Tolerance Occurs in peripheral lymph 1ssues when a naïve mature B cell encounters self an1gen 3 outcomes: Apoptosis (dele1on) Anergy Repeated s1mula1on Outcompeted for BAFF by pro-inflammatory B cells Chronic Ag s1m leads to downregula1on of CXCR5 – less homing to T cells Regula1on by inhibitors 33 Summary 34 Mechanisms for Autoimmunity 35 Mechanisms of autoimmunity Autoimmune diseases are the result of an1bodies and effector T cells that ajack healthy cells and 1ssue as though they were infected with a pathogen and produce reac1ons resembling the type II, III, and IV 34 35 that ajack healthy cells and 1ssue as though they were infected with a pathogen and produce reac1ons resembling the type II, III, and IV hypersensi1vity reac1ons. 2 main factors: Environmental Gene1c 36 General Features of Autoimmune Disorders Autoimmune diseases may be systemic or organ specific, depending on the distribu1on of the autoan1gens that are recognized. tend to be chronic, progressive, and self-perpetua1ng self an1gens that trigger these reac1ons are persistent epitope spreading – 1ssue injury may result in the release and altera1ons of other 1ssue an1gens, ac1va1on of lymphocytes specific for these other an1gens, and exacerba1on of the disease 37 Immunologic Abnormali>es Leading to Autoimmunity Inadequate elimina1on or regula1on of T or B cells, leading to an imbalance between lymphocyte ac1va1on and control, is the underlying cause of all autoimmune diseases Major overlying factors Defects in dele1on (nega1ve selec1on) of T or B cells or receptor edi1ng in B cells during the matura1on of these cells in the genera1ve lymphoid organs Defec1ve numbers or func1ons of regulatory T lymphocytes Defec1ve apoptosis of mature self-reac1ve lymphocytes Inadequate func1on of inhibitory receptors 38 Gene>c Basis of Autoimmunity There are certain gene1c factors that make individuals MORE suscep1ble to development of autoimmunity. Complex polygenic traits Require environmental factors as well MHC alleles portray greatest inheritable risk factors 39 Ex: Ankylosing spondyli1s is 100 1mes more likely to be associated with HLA-B27 Ex: Ankylosing spondyli1s is 100 1mes more likely to be associated with HLA-B27 39 Gene>c Basis of Autoimmunity con>nued nucleo1de polymorphisms (variants) of several genes are associated with autoimmune diseases Ex: NOD2 polymorphism in Crohn’s disease Unregulated response to commensal gut bacteria Single gene inheritance pajerns Discussed throughout this course AIRE: autoimmune polyendocrine syndrome FAS/FASL: autoimmune lymphoprolifera1ve syndrome FOXP3: IPEX 40 Role of Infec>ons in Autoimmunity Viral and bacterial infec1ons may contribute to the development and exacerba1on of autoimmunity result from host immune responses that may be triggered or dysregulated by the microbe. Ac1va1on by APCs from infec1on can lead to aberrant self Ag presenta1on Molecular Mimicry: Infec1ous microbes may contain an1gens that cross-react with self an1gens Ex: Rheuma1c Fever with streptococcal cell-wall an1gens cross- react with an1gens on heart 1ssue