Mucosal Immunology I: Immunology of the GI Tract PDF

Mucosal Immunology I: Immunology of the GI Tract Dr O’Connor [email protected] Lesson Plan At the end of this session, students should be able to describe The immune system components found in the GI tract Structure of the lymphoid tissue in the GI tract I...

Mucosal Immunology I: Immunology of the GI Tract Dr O’Connor [email protected] Lesson Plan At the end of this session, students should be able to describe The immune system components found in the GI tract Structure of the lymphoid tissue in the GI tract Immune responses in the GI tract Autoimmunity in the GI tract – Coeliac disease Inflammation in the GI tract – ulcerative colitis, Crohn’s disease Mucosal Tissue Mucosal tissue is a major source of pathogen (and other antigen) exposure. MALT: Mucosal Associated Lymphoid Tissue GALT: Gut Associated Lymphoid Tissue Immunology of the GI Tract Epithelial and mucus barrier DCs and M cells Innate cells and lymphocytes in the lamina propria Organized MALT Peyer's patches IgA secreting plasma cells Immunology of the GI Tract Epithelial Cells: Form tight junctions to prevent entry into the lamina propria. Express PRRs (e.g. TLR), particularly on the basolateral surface Goblet cells produce highly glycosylated proteins called mucins (e.g. MUC2) which generate the mucus barrier, preventing pathogen access to the epithelium. Can be upregulated and modified by cytokine, and bacterial infection. Paneth cells are found at the base of the crypts and produce anti-microbial peptides. In the small intestine, α-defensins are produced as inactive precursors, cleaved by trypsin (also produced by Paneth cells). Produce REGIII proteins that block bacterial colonisation, and can be bactericidal. Microfold (M) cells allow sampling of antigens from the lumen Innate Cells: Macrophages Local signals (e.g. TGF-β) produce intestinal macrophages that are highly phagocytic and kill microbes, but do not produce inflammatory cytokines. They can secrete anti-inflammatory cytokines, such as IL-10. Support regulatory T cells (Treg) in the GI tract. Adaptive Cells Scattered lymphocytes are found throughout the GI tract, including in the lamina propria and as intraepithelial lymphocytes (IEL, mainly CD8+ T cells) where they perform effector functions. Organised lymphoid tissue including Peyer patches and isolated lymphoid follicles can be sites of immune activation, in addition to the draining lymph node (mesenteric) Adaptive Cells Intestinal Dendritic Cells Antigen presenting cells (APCs) such as dendritic cells (DC) are present in the GI tract and sample antigens for presentation to T cells in GALT and mesenteric lymph nodes. Some DCs extend dendritic processes between intestinal epithelial cells into the lumen. Other DC present in the lamina propria sample antigens that are derived from luminal contents and have gotten through the epithelial barrier. M Cells The epithelial layer of the Peyer’s patch contains both ordinary epithelial cells and M (microfold) cells. These cells are involved in continuous antigen sampling from the lumen, and deliver these antigens via endocytosis to dendritic cells which then present them to T cells. M Cells II Typical intestinal epithelial cells have villi while M cells have irregular ruffled surfaces instead. M cells do not produce digestive enzymes or mucus and therefore are not as protected from pathogens as other epithelial cells. Some microbes have evolved to take advantage of M cells as a route of invasion through the mucosal barrier. Where are lymphocytes typically activated? Lymphoid Tissue Secondary lymphoid organs are where lymphocytes (T, B cells) interact with antigen and become activated. Encapsulated: lymph nodes (antigen from the tissues) and spleen (antigen from blood) Skin is an Non-encapsulated: Mucosa-associated lymphoid important immune tissues (MALT) – antigen from mucosa. MALT defence but is not a MALT represents the largest immune surface area and largest number of immune cells Circulation of Lymphocytes Lymphocytes have the distinctive feature of recirculation, between the blood, lymphoid organs and tissues. Allows for low-frequency antigen- specific lymphocytes to sample antigens captured within the body Cell migration is controlled by specialised lymphocyte surface receptors, binding to complementary receptors on the vessel walls of the tissues they enter. Circulation of Lymphocytes DCs bearing antigen enter the lymph nodes through lymphatic vessels. Naïve lymphocytes leave the blood and enter lymph nodes across the high endothelial venules (HEVs). If the lymphocytes are activated, they return to the circulation via efferent lymphatics. Effector cells preferentially leave the blood and enter peripheral tissues through venules at sites of inflammation. Lymph Node Sites of lymphocyte activation T cells are found in the paracortex. B cells are found in the follicles within the cortex. Some follicles (secondary follicles) contain central areas called germinal centres. Germinal centres (GC) develop in response to antigenic stimulation and are sites of B cell proliferation, somatic hypermutation and selection (more on this later), and class switch recombination. Circulation of Lymphocytes 1. Recruitment to lymph node Naïve T cell migrate into the lymph node involves the adhesion molecules L-selectin and LFA-1 and the chemokine receptor CCR7. 2. Homing back to tissue Activated T cells change their expression of adhesion molecules and chemokine receptors. This promotes their recruitment back to the sites of infection. Back to the GI tract …. Activation of GI Lymphocytes Activation can occur in the mesenteric lymph node or organised lymphoid tissue in the gut. Peyer’s patches are similar in structure to the lymph node. The region directly under the epithelial layer (the subepithelial dome, SED) is rich in APCs. T cells are present in T cell zones and B cell zones include germinal centres (GC). Circulation of GI Lymphocytes Cells that are activated by antigen in GALT are exposed to retinoic acid produced by the DC. This induces the expression of CCR9 and the integrin α4β7. These cells home back into the gut because the chemokine CCL25 (the ligand for CCR9) and the adhesion molecule MadCAM (the ligand for α4β7) are displayed on lamina propria venular endothelial cells. Circulation of GI Lymphocytes Adaptive Cells The dominant form of adaptive immunity in the gut is humoral – mediated by dimeric IgA molecules. Tissue signals such as TGF-β promote B cell isotype switching to IgA. IgA Transcytosis IgA in the lamina propria is held as a dimer by the J chain. It binds to the poly-Ig receptor and is transported across the epithelial cell (transcytosis). It is released via proteolytic cleavage of the receptor, which forms the secretory component and remains associated with IgA. IgA Function IgA does NOT activate complement. It is a powerful neutralising antibody. Mediates anti-viral activity and neutralisation of toxins Th17 cells in GI Immunity Recap slide Th17 cells in GI Immunity Th17 are constitutively found in the GI tract, particularly in the small intestine. Th2 cells may play a role in response to intestinal helminth infections, but Th1 cells are rare in the healthy GI tract. Th17 cells play a role in maintaining mucosal epithelial barrier function due to production of IL-17 and IL-22 which act on intestinal epithelial cells. These cytokines induce the expression of proteins important for barrier function, such as mucins and β-defensins. Th17 cells are also capable of inducing acute inflammation to directly kill invading pathogens. Adaptive Cells Regulatory T cells (Treg cells) are a subset of CD4+ T helper cells whose function is to suppress immune responses and maintain self-tolerance. They express the transcription factor FoxP3 They can be generated by self antigen recognition in the thymus (natural Treg) or by antigen recognition in peripheral lymphoid organs (inducible Treg) Adaptive Cells Regulatory T cells are found in high number in the GI tract play an important role in the maintenance of tolerance towards luminal antigens (food antigens, commensal bacteria antigens). Most Treg cells in the GI tract are believed to be inducible Treg cells. Their differentiation is induced by local factors such as TGF-β and retinoic acid. Natural Treg that migrate to the GI tract can expand in response to bacterial metabolites Oral Tolerance Oral tolerance is a term used to describe tolerance induced to antigens administered orally. It functions to prevent inappropriate immune responses to food and commensal bacterial antigens. It involves multiple mechanisms including the induction of Treg cells, Treg- mediated suppression, induction of hyporesponsiveness (anergy), or elimination of effector cells. In the case of oral vaccination, the antigen is introduced with concomitant activation of the innate immune system e.g. via limited replication of live attenuated (weakened) virus. In this setting, productive adaptive immune responses can be generated. Immune-Mediated Disorders Immune Disorders Malfunction of the immune response can provoke damage or disease. Such disorders can be generally categorised Autoimmunity: An inappropriate reaction to self – failure of self-tolerance Allergy: An type of inappropriate response to innocuous non-self Inflammatory disorders: Poorly regulated (e.g. chronic) inflammation Immunodeficiency: Absent or ineffective immune responses The damage mediated by immune responses can be categorised into four classes of hypersensitivity reactions. Hypersensitivity Reactions Inappropriate or excessive immune responses can lead to tissue damage. These hypersensitivity reactions can be classified into 4 types bases in the underlying mechanism, based on the Coombs and Gell system from the 1960s. Types I-III involve antibodies, while Type IV is cell mediated immune responses It is common for more than one type of hypersensitivity reaction to be involved in mediating tissue damage  Four types of hypersensitivity Type Name Mediated by Example I Allergic (immediate) IgE and mast cells Hay fever, asthma, anaphylaxis II Cytotoxic IgG (IgM), Rejection of blood transfusion, complement, haemolytic anaemia, phagocytes, ADCC thrombocytopenia III Complex-mediated Soluble Ag-Ab (IgG) Glomerulonephritis, SLE complexes, polymorphs, complement IV Cell mediated Immune cells TB granuloma, contact (delayed-type) dermatitis Hypersensitivity Reactions Type I: Engagement of IgE antibodies bound to mast cells leads to degranulation – allergic response. Type II: IgG antibodies to cell surface or extracellular matrix - associated antigens. Promotes destruction via phagocytosis, complement activation, or antibody-dependant cell cytotoxicity (ADCC). Antibody binding can also block function. Type III: Antibody recognition of soluble antigen leads to deposition of Ab- Ag complexes in tissues or serum. Activation of complement or recruitment of neutrophils that release lytic molecules. Type V: Cell mediated. Damage mediated through inflammation, or through direct cytotoxicity Immune Disorders in the GI Tract Coeliac Disease is an autoimmune disease of the GI tract triggered by dietary gluten Inflammatory Bowel Disease (IBD) is a group of inflammatory disorders, characterised by excessive inflammatory response (to commensal microbes) and includes ulcerative colitis and Crohn’s disease Coeliac Disease Coeliac disease (CD) is an autoimmune disorder leading to chronic inflammatory condition of the small intestine. It causes atrophy of the small intestine mucosa and impairment in the digestion, and maldigestion, of nutrients. Affects around 1% of population Symptoms highly variable. Include weight loss, anaemia, lassitude (lack of energy), diarrhoea, abdominal pain. Chronic condition: Female to male ratio approx. 2:1 Strong MHC (HLA) class II association Coeliac Disease Suspected CD can be supported by detection of serum antibodies to gliadin (a component of gluten), endomysium or transglutaminase. Definitive diagnosis requires a jejunal biopsy confirming partial or total villus atrophy – to be repeated after 6 months of gluten-free diet. villous atrophy, crypt hyperplasia, inflammation Coeliac Disease Coeliac Disease (CD) is triggered by gluten, a high molecular weight heterogeneous compound found in endosperm of wheat, barley and rye. Gluten originates in south Western Asia and became included in diet with expansion of farming. Dietary intake increased hugely in 20th Century. Treatment is the total exclusion of gluten from the diet Coeliac disease is strongly associated with HLA Class II proteins. Specific subtypes are HLA-DQ2 (95%) and HLA-DQ8. HLA alleles present in around 40% of population Homozygous for HLA-DQ2 increase risk 5 fold Do not solely lead to CD Coeliac Disease Destruction mediated by both gluten (or derivative) specific immune responses and autoreactive antibodies and T cells. Coeliac Disease CD8+ CTL mediate epithelia cell killing. Th1 CD4+ T cells release cytokines and matrix metalloproteinases causing epithelial cell death and remodelling of the mucosal matrix. Pathogenesis of Coeliac Disease Refer to previous figure Gluten-derived peptides (e.g. gliadin) reach the sub-epithelial region. Transglutaminase (TG) deamidation of glutamines generates potent immunostimulatory epitopes These are presented via HLA-DQ2/DQ8 on APC to CD4 T cells CD4 T cells produce high levels of pro-inflammatory cytokines, thus inducing a Th1 response that results in mucosal remodelling and villous atrophy. In addition, TG-gliadin complexes that bind to TG-specific B cells, are endocytosed and processed. Gliadin-DQ2/DQ8 complexes are then presented by the TG-specific B cells to gliadin-specific T cells, a process that leads to the production of anti-TG antibodies (autoantibodies) aka tissue transglutaminase (tTG) antibodies Diagnosis of Coeliac Disease Test Pro Con Anti-tTG IgA High sensitivity and specificity; False negative with IgA deficiency (3% Most reliable non-invasive first-level of patients) screening test May be negative with low-gluten diet Anti-tTG IgG In cases of IgA deficiency Variable sensitivity and specificity Anti-deaminated gliadin In cases of IgA deficiency and Not as sensitive or specific as tTG IgA peptide IgG young children Anti-endomysial IgA If borderline for tTG antibodies Less sensitive Small bowel biopsy Gold Standard, Reflects response Requires endoscopy and biopsy, to treatment expensive HLA-DQ2, -DQ8 High negative predictive value Complex and expensive Inflammatory Bowel Disease Inflammatory bowel disease (IBD) may be divided into two major groups: Ulcerative colitis – confined to the colon, mucosal layer only Crohn's disease – any part of GI Tract, transmural inflammation Clinically these disorders are characterised by recurrent inflammation of intestinal segments with diverse clinical manifestations. In ulcerative colitis, bloody diarrhoea and abdominal pain with fever and weight loss in the more severe cases are the main symptoms. In Crohn's disease the major clinical manifestations include fever, abdominal pain, diarrhoea (often without blood) and fatigability. Inflammatory Bowel Disease Th Cells The main T helper cell populations in the GI tract are Th17 and Treg and together they maintain a balance between inflammatory responses and tolerance. There is a lot of TGF-β present in the GI tract environment which helps to promote differentiation of Th17 and Treg cells but also to inhibit Th1 and Th2 effector cell differentiation. Th Cells in IBD The pathogenesis of IBD believed to be an exaggerated response to commensal microflora with a Th1 profile in Crohn's disease and a Th2 profile in ulcerative colitis. Th17 cells are found in both conditions. Th Cells in IBD IBD Risk Factors Risk factors include genetic predisposition, environmental factors, commensal microbiota and host barrier function. Genetic factors in Crohn’s disease include NOD2 (a PRR), NFκB signalling pathway protein and autophagy clearance of intracellular bacteria HLA-B*27 (MHC class I) is a strong risk factor for UC Targeting Adhesion Molecules For your interest only. An example where understanding the specific molecules involved means that we can have a more tailored and specific response – target gut immune responses not just all immune responses Reading List Abbas, A. K., Lichtman, A. H., & Pillai, S. (2021). Cellular and Molecular Immunology, 10e, E-Book. Elsevier Health Sciences. Chapter 14. Clark, M. L., & Kumar, P. (2021). Kumar and Clark's Clinical Medicine. Chapter 3 and 32 (portions only). Both available via Clinical Key. Additional Reading Yeshi, K., Ruscher , R., Hunter, L., Daly, N. L., Loukas, A., & Wangchuk, P. (2020). Revisiting inflammatory bowel disease: pathology, tr eatments, challenges and emerging therapeutics including drug leads from natural products. Journal of clinical medicine, 9(5), 1273. MBBS Learning Outcomes Recognise the relationship between lymphocyte function and lymphoid tissue compartmentalisation Recognise the role of immune cells in defence against invasion by microbes along the GI tract Recognise the cellular and molecular events leading to inflammation in the GI tract Recognise the negative effects of uncontrolled immune responses launched against host cells and compare immunological parameters indicative of host cell damage Recognise the anatomical and cellular aspects of host immune response of the mucosa and the consequences of the breakdown of this immune function