Mucosal Immunity Lecture Notes PDF

Learning Objectives Identify the types of mucosal-associated lymphatic tissue 1. (MALT) and their roles in the mucosal immune response. Identify the cells of mucosal immunity and describe their 2. specific roles within the mucosal immune response. Describe the specific...

Learning Objectives Identify the types of mucosal-associated lymphatic tissue 1. (MALT) and their roles in the mucosal immune response. Identify the cells of mucosal immunity and describe their 2. specific roles within the mucosal immune response. Describe the specific defenses associated with mucosal 3. immunity, including cells, molecules, enzymes, and types of antibodies. Describe how the mucosal immune system differs from 4. humoral and cell-mediated immune systems. Describe how cells and antigens are trafficked within the 5. mucosal immune system.  Mucosal Immunity European Respiratory Journal Apr 2016, 47 (4) 1244-1260 2  Mucosal Immunity 75% of our lymphocytes are in mucosal tissues The mucosal immune system is distinct o different antibodies o most of our γδ T cells are in mucosal tissues o different physical structure & architecture of the lymphoid tissues in mucosae o different mechanisms of antigen uptake o different means of inducing (inductive sites) & carrying out immune functions (effector sites) 3 Anatomy of MALT 4  Anatomy of MALT There are two types of MALT (mucosa- associated lymphoid tissue) o O-MALT (O = organized) o D-MALT (D = diffuse) O-MALT is the immune response is induced (inductive sites) D-MALT is the effector site where antibodies are secreted (effector sites) 5 https://www.researchgate.net/publication/267952599 6  O-MALT D-MALT https://www.researchgate.net/publication/268793243 7  Mucosal Immunity Inductive sites (O-MALT) o antigen is taken up o antigen is presented o lymphocytes are activated o B cells undergo isotype switching & SHM Effector sites (D-MALT) o lymphocytes home back to these sites o lymphocytes secrete effector molecules 8  O-MALT Has follicles & germinal centers (less structured than lymph node or spleen, but still has B cell zones & T cell zones) Loosely divided into 4 major anatomical areas o GALT – Peyer’s patches, appendix o NALT – tonsils, adenoids o BALT – less developed in humans o RALT – reproductive & urogenital 9  D-MALT Not organized – no follicles or germinal centers o diffuse, disorganized Spread throughout mucosal tissues Lamina propria, salivary glands, lacrimal glands O-MALT D-MALT 10  O-MALT vs. D-MALT Abbas: Cellular and Molecular Immunology, 2018 SIgA p D-MALT O-MALT D-MALT 11 Defenses in MALT 12  Innate Immune Features The gut mucosa protects us from infections in the following ways o mucus ▪ thick layer of mucus keeps pathogens from reaching the surface of the gut epithelium ▪ the mucus layer is comprised of mucin molecules produced by goblet cells ▪ the mucin molecules turn over every 6-12 hrs, taking with them what they have bound ▪ we make several L of mucus/day in the gut ▪ various cytokines induce the production of mucin molecules 13  Innate Immune Features The gut mucosa protects us from infections in the following ways o glycocalyx ▪ membrane-bound mucin molecules combined with glycolipids ▪ the glycocalyx is a thick layer which provides a physical barrier that prevents microbes from making contact with the epithelial layer https://www.news-medical.net/health/What- 14 is-Mucosal-Immunity.aspx  Innate Immune Features The gut mucosa protects us from infections in the following ways o antimicrobial peptides (small intestine) ▪ the main antimicrobial peptides in the small intestine are α-defensins, produced by Paneth cells ▪ α-defensins permeabilize microbial membranes ▪ these antimicrobial peptides do not affect our natural gut bacteria & help maintain gut microbial homeostasis 15  Innate Immune Features The gut mucosa protects us from infections in the following ways o antimicrobial peptides (large intestine) ▪ the main antimicrobial peptides in the large intestine are β-defensins, produced by absorptive epithelial cells (some are produced constitutively, others in response to IL-1 or the recognition of invasive bacteria) ▪ neutrophilic granules also contain α- defensins which help protect the colon Chron’s disease is known to include a defect in defensin production – however, it is not known if this defect causes the disease, or the disease 16 causes the defect  Innate Immune Features The gut mucosa protects us from infections in the following ways o C-type lectins ▪ Paneth cells also secrete C-type lectins called regenerating islet-derived proteins (REG III) ▪ These REG III lectin proteins do 2 things 1. block bacterial colonization of the mucosal epithelial surface 2. have bactericidal effects against G+ bacteria 17  Innate Immune Features The gut mucosa protects us from infections in the following ways o localized PRRs ▪ TLR & NOD proteins are expressed only in certain cells/areas of the gut so they can be activated upon bacterial invasion e.g. TLR5, which recognizes flagellin, is expressed on the basolateral surface of intestinal epithelial cells to recognize invasive bacteria (a place where normal gut bacteria are not found) ▪ TLR signaling results in the secretion of defensins, REG III protein lectins, & SIgA 18  Innate Immune Features The gut mucosa protects us from infections in the following ways o ILCs (innate lymphoid cells) ▪ innate lymphoid cells secrete cytokines in response to alarmins ▪ alarmins are produced by epithelial cells in response to injury or pathogenic microbes ▪ the response to the alarmins vary depending on the alarmin, location within the gut, type of ILC activated, & which cytokines they produce in response to that alarmin signal examples: production of IL-17, IL-22, IL-23, increased production of mucus or defensins, enhanced function of tight junctions 19  Innate Immune Features The gut mucosa protects us from infections in the following ways o inhibition of inflammation ▪ some gut mΦs have a unique phenotype that secrete anti-inflammatory cytokines (e.g. IL- 10) ▪ TLR4 expression is reduced to limit inflammation that could arise in response to commensal bacteria (many of which are G- & contain LPS, which is recognized by TLR4) 20  Adaptive Immune Features The gut mucosa includes the following adaptive immune features o pIgA → SIgA o IgG o pIgM → SIgM o specialized anatomy ▪ inductive sites & effector sites are separated o specialized cells ▪ M cells, DCs, CD8+ IELs, CD4+ TH & TREG o strong homing & lymphocyte trafficking 21  Oral tolerance & TREG cells M cells are specialized cells in mucosal immunity that capture antigens & deliver the antigens to mucosal DCs o “antigen delivery cells” These mucosal DCs carry food antigens to CD4+ TREG cells o this allows the TREG cells to become “conditioned” to not react & downregulate the inflammatory response resulting in tolerance to food antigens 22  Antibodies of mucosal immunity SIgA antibodies o pIgA is produced by pIgA SIgA plasma cells in the lamina propria & transcytosed through mucosal epithelium to the Abbas: Cellular and Molecular Immunology, 2018 lumen of the gut via Plasma cells in the lamina The J chain of the pIgA is Transcytosis of the pIgA Part of the pIgR remains the pIgR propria produce bound by the pIgR on the through the mucosal attached to the pIgA dimeric pIgA basolateral epithelium to (called the o this creates SIgA with a J chain surface of the mucosal the lumen of the gut secretory piece) (dimeric IgA with epithelial cells creating SIgA the secretory piece) 23  IgA Plasma Cells B cells which are activated in the Peyer’s patches undergo isotype switching to IgA transcytosis The plasma cells home back to the lamina propria where they secrete pIgA The pIgA is taken up (via Abbas: Cellular and Molecular Immunology, 2018 the J chain) by the pIgR & transcytosed into the lumen → SIgA 24  Antibodies of mucosal immunity IgG o plays an important role in mucosal immunity of the oral cavity o less predominant in the GI tract pIgM & SIgM o pIgM pentamer with a J chain o transcytosed via the pIgR → SIgM o found in oral cavity & the GI tract 25  Peyer’s Patches Found throughout GI tract, especially in the small intestine These are O-MALT (inductive sites) of the mucosal immune response in the gut o have a structure similar to a LN follicle o antigens are presented o lymphocytes are activated o very little antibody is produced Covered with different types of cells than the rest of the small intestine 26  Peyer’s Patches Covered by a dome area which lies beneath the follicle associated epithelium (FAE) Antigens enter the Peyer’s patches via transcytosis through M cells o M cells are a type of cell that are part of the FAE Contain large numbers of APCs 27  Peyer’s Patches Under the dome (FAE) area is the corona which is where the naïve B cells are located Follicular zone lies beneath the corona & contains activated B cells (i.e. the germinal FAE centers) Peyer’s Patch Dome Corona Germinal center 28  Lamina Propria Not organized into specialized lymphoid structures – D-MALT (effector site) Where most gut pIgA is produced which will be transcytosed by pIgR to become SIgA Populated with o memory mIgA+ B cells o IgA-secreting plasma cells o memory TH cells o DCs, mΦs, mast cells 29  Salivary Glands Part of the D-MALT Main source of IgA in the oral cavity o produced by both major & minor salivary glands o most is produced as (dimeric) pIgA which, when secreted via pIgR & becomes SIgA 30 Cells of the MALT 31  Mucosal Epithelium pIgR on the basolateral surface binds to pIgA (mostly) and pIgM to transcytose & secrete these antibodies across the epithelial layer Organized into villi & crypts Lymphocytes, DCs, & mΦs are interspersed between & among the epithelial cells 32 Abbas: Cellular and Molecular Immunology, 2018 Intraepithelial Lymphocytes (IEL’s) Mature, differentiated T cells that are interspersed among the mucosal villous epithelial cells o most are αβ memory CTLs o about 10% are γδ T cells ▪ while only 10% of these cells are γδ T cells, this is a much higher proportion of γδ T cells than are found throughout the rest of the body ▪ γδ T cells play an important role in recognizing lipid antigens in mucosal immunity 33  Intraepithelial Lymphocytes Abbas: Cellular and Molecular Immunology, 2018 34  Intraepithelial Lymphocytes The γδ T cells have a protective role similar to NK cells o when a mucosal epithelial cell becomes infected, injured, or stressed, it expresses certain proteins on it’s basolateral surface (MIC-A/MIC-B) o the γδ T cells bind to these receptors & kill the cell o MIC-A & MIC-B are also recognized by the NKG2D (NK group 2 member D) receptors on NK cells & IELs 35 Follicle Associated Epithelium Specialized FAE epithelial layer that covers the Peyer’s patches (i.e. follicles) 36  Follicle Associated Epithelium Contains specialized non-cilliated cells called M cells Villi & Crypts Abbas: Cellular and Molecular Immunology, 2018 37  M cells Specialized antigen uptake cells of the FAE overlying the O-MALT tissues Main function is uptake of luminal antigens o actively pinocytic Sauls RS, Taylor BN. Histology, M Cell. [Updated 2022 Nov 14]. In: StatPearls [Internet]. Treasure Island (FL): o have Fcα receptors to StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534232/ bind SIgA A reduction in M cells occurs with aging & Ingested antigen is plays a role in reduced immunity in older adults transcytosed unaltered to Some pathogens (Shigella, Salmonella, Listeria, Yersinia) use M cells to gain the Peyer’s patch beneath access to sub-epithelial tissues 38  Goblet cells Secrete mucin molecules that form a thick layer of mucus protecting the intestinal epithelium Located at the top of the villi & in the FAE In the large & small intestines Can participate in antigen uptake similar to M cells 39  Paneth cells Secrete α-defensin anti-bacterial peptides There are approx. 5-10 Paneth cells at the bottom of each crypt The cells contain granules with lysozyme, defensins & phospholipase enzymes 40 Abbas: Cellular and Molecular Immunology, 2018 Lymphocyte Trafficking 41  Lymphocyte trafficking An important characteristic of mucosal immunity is homing of the activated lymphocytes (which is lacking in systemic immunity) O-MALT & D-MALT are linked by this homing ability o lymphocytes activated in the inductive sites of the O-MALT home back to the effector sites of the D-MALT o integrins on the lymphocytes bind to the addressins on the vascular endothelial cells 42  Lymphocyte trafficking When the lymphocyte is activated, it changes its cell surface proteins o they downregulate the ligands that allow them to enter peripheral lymphoid organs o they upregulate ligands that allow them to enter mucosal tissues (e.g. CCR9, MAdCAM- Abbas: Cellular and Molecular Immunology, 2018 1, α4β7 integrins) 43 Antigen Flow of MALT 44  Antigen flow Antigen uptake is different in the mucosal immune system o antigen must be taken across the epithelial barrier by active transepithelial transport o antigen is not phagocytosed by an APC until after it crosses the epithelial barrier o APC does not migrate to a lymph node – it presents antigen in local O-MALT tissues (e.g. Peyer’s patch, tonsils) 45  Antigen flow in GALT Uptake of antigen by the M cell o transcytosed (unaltered) to basolateral surface An APC beneath the M cell (usually a DC) takes up the antigen & presents it to T Abbas: Cellular and Molecular Immunology, 2018 cells in the dome area of the Peyer’s patch 46  Antigen flow in GALT In the Peyer’s patch o APC which has captured antigen, moves to the T cell zone for presentation to T cells o T cell is activated o T cell-B cell interactions activate the B cell o cytokines produced by the activated T cell cause isotype switching to IgA o B cell, in Peyer’s patch follicle, undergoes isotype switching & SHM 47  Antigen flow in GALT In the lymph node o B cells with mIgA leave the Peyer’s patch & travel to mesenteric lymph node o proliferation of these B cells occurs in the lymph node germinal center o B cells differentiate to memory IgA B cells Activated B cells do not undergo clonal proliferation in the Peyer’s patch because that would result in swelling, obstructing the lumen of the small intestine – the activated, isotype switched B cells travel to the mesenteric lymph node before undergoing clonal proliferation 48  Antigen flow in GALT In the lamina propria o memory mIgA B cells exit the mesenteric lymph nodes, re-enter circulation & home back to the lamina propria D-MALT o memory B cells are activated, differentiate into plasma cells & secrete pIgA which is transcytosed by the pIgR & becomes SIgA 49  Antigen flow in GALT Peyer’s patches (Inductive site) o Antigen presentation & activation o Isotype switching o SHM & affinity maturation Mesenteric lymph nodes* o Proliferation o Differentiation into memory cells Lamina propria (Effector site) o Polymeric antibody production o Secretion of antibody via pIgR 50 * For oral cavity this would be the cervical LN’s  Antigen flow in GALT 1 Antigen presentation T cell activation 2 B cell activation Isotype switching Affinity maturation 4 Activation of memory B cells Differentiation to plasma cells 3 Production of pIgA Clonal proliferation Secretion of pIgA across Differentiation to epithelial barrier → SIgA memory mIgA B cells Immunology, Infection , and Immunity © 2004 ASM Press 51