Mucosal Immunity PDF

Mucosal Immunity Mucosal Immunology BDS2 Dr. David Moyes Mucosal Surfaces Exposed to the environment > 1 Ton of nutrients pass through the adult gut /year Large surface area specialised for absorption. Gut ~ 400m2 Skin ~2m2 Systemic vs. Mucosal Immunity Mucosal tissues Cells in mucosal immunity Lymp...

Mucosal Immunity Mucosal Immunology BDS2 Dr. David Moyes Mucosal Surfaces Exposed to the environment > 1 Ton of nutrients pass through the adult gut /year Large surface area specialised for absorption. Gut ~ 400m2 Skin ~2m2 Systemic vs. Mucosal Immunity Mucosal tissues Cells in mucosal immunity Lymphocyte homing B cell function T cell function Oral Tolerance Systemic vs. Mucosal Systemic environment Mucosal environment Contained Exposed Sterile Non-sterile Encounters undefined antigens rarely Encounters undefined antigens continuously Resident micro flora at most mucosal epithelia Main route of entry for infectious micro organisms 1 The importance of Mucosal Immunity Mucosal Tissues Protection against pathogens Prevention of hypersensitivity to foods, commensal organisms/microbiota etc Immunopathology – Crohn’s disease, Coeliac disease, lung inflammation. Vaccine development Lymphocyte development. ? The mucosal barrier Non-immunological: Natural barriers (e.g. stomach acid) Mucin Peristalsis Proteolysis Microvillus membrane Immunological: Secretory IgA/IgM (IgG) IELs Phagoctyes Regionalised secondary mucosal immune tissues There are specific ‘inductive’ and ‘effector’ sites at mucosal surfaces, much as for systemic immunity. The sites are called Mucosal Associated Lymphoid Tissue (MALT) Each mucosal surface has it’s own – e.g. NALT (Nasal), GALT (gut) etc. 2 Common Mucosal Immune System The Mucosal Immune system is a connected series of tissues. In other words, all the different mucosal surfaces of the body are part of a ‘common’ mucosal immune system. GALT organisation Peyer’s Patches Sub-epithelial follicles first described in 1677 by JC Peyer Located throughout the small intestine. In man there about 200 Patches contain 50% B cells, 30% T cells and 8% macrophages. There are very few plasma cells. Inductive sites of GALT. They connect to the lymphatic system only by efferent lymph vessels, they have no afferent vessels B lymphocytes from the Peyer’s patch give rise to IgA producing plasma cells which home to all mucosal sites 3 Peyer’s patches Peyer’s patches Peyer’s patches and ILFs Isolated Lymphoid Follicles Isolated lymphoid follicles (ILF) are induced by products of commensal gut microorganisms. Like PPs, mainly B cells, some T cells and dendritic cells. More numerous than PP e.g mice PP ~10/ ILF ~150 4 M cells M cell function M (Microfold) cells Unique epithelial cell subset Specialised in uptake and transepithelial transport of particulate antigens No brush border/microvilli Located in the epithelial layer above Peyer’s patches and ILFs Peyer’s patch function Particles and macromolecules: eg cholera toxin, latex particles, horseradish peroxidase, ferritin. Viruses: eg poliovirus, HIV In Peyer’s patches: – T and B cells become fully activated (also in mesenteric lymph nodes) – T & B cells switch expression from L-selectin & CCR7 to a4b 7 integrin and CCR9 – B cells class switch to IgA Subesquently traffic to circulation and home back to the mucosal tissue. Parasites: eg Cryptosporidium Bacteria: eg Cholera, salmonella, Campylobacter Yersinia, Shigella, E. coli M cell targets 5 Cells of mucosal immune system Conventional cells Dendritic Cells Macrophages Neutrophils Mast cells Eosinophils Basophils T cells B cells Specific cells Epithelial cells M cells IntraEpithelial Lymphocytes gd T Cells Lamina Propria T cells Mucosal compartments Found underlying the epithelial layer Majority are CD4+ (CD4+:CD8+ = 2:1) Most express a4b7 integrin (ligand = MadCAM) They produce cytokines dependent on their subset Lamina Propria T cell subsets Tregs act to regulate/supress other immune cells via IL-10 and TGFb Th1 act to activate macrophages via IFNg Th2 act to induce B cell class-switching to IgA via IL-5 Th17 act to maintain epithelial barrier function via IL-22 and activate neutrophils via IL-17 6 IntraEpithelial Lymphocytes (IELs) Lymphocyte homing All are T cells – The majority are CD8+ (a large number CD8aa) gd T cells may be frequent >70% in certain species or neonatal/germfree Express specific aEb 7 integrin (ligand E-cadherin on enterocytes/epithelial cells) Unresponsive to TCR stimulation Many extrathymically derived Effector T and B cells activated in mucosal tissues specifically recognise MadCAM and CCL25 and home to mucosal tissue STEP 1: CHEMOATTRACTION STEP 2: ROLLING ADHESION Chemokines e.g. IL-8, TECK recruit lymphocytes and induce expression of adhesion molecules Lymphocyte binds loosely to endothelial cell and rolls along L-SELECTIN LYMPHOCYTE (C-TYPE LECTIN) LYMPHOCYTE MAdCAM-1 (Mucosal addressin cell adhesion molecule 1) ENDOTHELIAL CELL ENDOTHELIAL CELL 7 STEP 3: ARREST OF ROLLING LYMPHOCYTE STEP 3: TIGHT ADHESION a4b7 INTEGRIN BINDS MAdCAM-1 LFA-1 (αLβ2) STEP 3: Tight adhesion. Chemokine-mediated activation of a4b7 Integrin BINDS ICAM-1 AND ICAM-2 α4β7 integrin binds to MadCAM-1 LYMPHOCYTE ENDOTHELIAL CELL ENDOTHELIAL CELL Migration to epithelial cell surface Further migration through lamina propria to epithelial cell surfaces occurs through secretion/recognition of chemokines Epithelial cells release specific chemokines – e.g. CCL25 and CCL20 These are recognised by specific chemokine receptors, e.g. CCR9 and CCR6 (respectively) Adhesion/recognition is through recognition of epithelial Ecadherin by lymphocyte αEβ7 integrin. STEP 4: DIAPEDESIS LYMPHOCYTE ENDOTHELIAL CELL 8 Mucosal cell function: B cells Secretory IgA Primary function of mucosal B cells is to produce IgA. Produced at mucosal sites Secreted across epithelial surfaces Under the influence of TGFb and IL-5, they switch production from IgM to IgA. Has different functions/activities to IgG Most highly produced Ig Production of IgA Lymphoid organs 2.3 x1010 cells (Bone marrow, spleen and lymph nodes) Mucosal Tissues >6x1010cells (GI tract, Resp. tract, GU tract, Salivary, lacrymal and mammary glands. – Found in all mucosal secretions – e.g. saliva Serum IgG v Secretory IgA antibodies Serum Antibody Light Chain Heavy Chain Secretory Antibody Ig IgA 30% 80% IgG 52% 6% IgM 17% 13% Joining Chain Secretory component 9 Mechanisms of action of antibodies IgG sIgA Virus neutralisation Virus neutralisation Enzyme and toxin neutralisation Enzyme and toxin neutralisation Inhibition of adherence Inhibition of adherence Agglutination Agglutination Complement activation Opsonisation Immune exclusion Intra-cellular neutralisation Virus excretion Interactions with non-specific factors lysozyme, lactoferrin, peroxidases IgA protection at mucosal surfaces Mucosal Cell function: T cells Provide/maintain an environment that: – Doesn’t respond to the commensal microbiota – Doesn’t respond to food/ingested antigens – Does respond to pathogenic microbes They regulate responses in conjunction with epithelial cells. Key in this process are the cytokine TGF-b and retinoic acid (a metabolite of vitamin A) - These factors control immune responses by: -controlling mucosal homing -controlling dendritic cell activation/induction of Treg Woof and Mestecky 2005 Imm Rev 206 10 Mucosal Cell function: T cells IELs have a further function. – IEL protect and maintain the epithelial barrier layer, protecting against infection Oral Tolerance State of immune non-responsiveness to antigen induced by feeding. It is a feature of the common immune system. Can also be induced by ‘immunisation’ at other mucosal sites – i.e. Intranasal. Features of Oral Tolerance Normal immune function Tolerance can be local or systemic It requires a functional immune system Symbiosis - in the absence of commensals, a poor immune response develops and oral tolerance cannot be induced 11 General properties of Oral Tolerance: General properties of Oral Tolerance Antigen specific. Often partial (eg. antibodies inhibited, but T cell responses may remain). Not complete (eg. may be a quantitative reduction in antibody levels). Wanes with time. Easier to abrogate a response than reduce and established response. Good immunogens are better at inducing tolerance (adjuvant…) Dose and route dependent. Breakdown of oral tolerance Immune responses to food – leads to food intolerance – eg coeliac disease Immune responses to commensal bacteria Mechanisms of oral tolerance Subject of intense research Proposed mechanisms involve: – activity of Tregs – Induction of anergy – leads to inflammatory bowel disease (IBD) – eg Crohn’s disease, ulcerative colitis 12 Induction of Treg is a likely mechanism of oral tolerance Vitamin A Epithelium TGFb RETINOIC ACID DC NAÏVE CD4 Treg iTreg Induction of anergy in oral tolerance GUT LUMEN LAMINA PROPRIA CCR9 a4b7 FoxP3 Some epithelial cells in the gut and lung normally express class II MHC, but not costimulatory molecules Result is activation of TCR without the second (CD28) signal – leads to anergy of responsive T cells. CCR9 a4b7 (FoxP3) Key Points: Mucosal surfaces: large, exposed to external environment, major route of entry of infectious microorganisms. Mucosal (secretory) immune system is targeted by Lymphocyte homing and is the site of transport of sIgA. Mucosal immunity involves a wide variety of cells. Immunoregulation critical to avoid damaging inflammatory responses to commensal flora or food antigens (both high load). General unresponsiveness maintained by Treg. ❖ TGFb and Retinoic acid may be major factors. Tolerance to an antigen can be induced by immunisation at a mucosal surface. 13