Microbiology and Immunology Session 7 Organs of Immunity and Cytokines PDF

MICROBIOLOGY AND IMMUNOLOGY (PBBS505A&B) 2024-2025 Session -7 Organs of Immunity/Cytokines Rahul Vijay, DVM, Ph.D [email protected]  Lecture Objectives 1. Difference between primary and...

MICROBIOLOGY AND IMMUNOLOGY (PBBS505A&B) 2024-2025 Session -7 Organs of Immunity/Cytokines Rahul Vijay, DVM, Ph.D [email protected]  Lecture Objectives 1. Difference between primary and secondary lymphoid organs. 2. Similarities and differences in the function of lymph nodes, spleen, and mucosal associated lymphoid tissues. 3. Major cytokines in the immune system  Lymphoid Organs 1. Central or Primary Thymus Bone Marrow 2. Peripheral or Secondary Spleen Lymph Node Mucosal Lymphoid Tissue such as in the intestine, and respiratory tract  Lymphocytes arise from stem cells in bone marrow and differentiate in the central lymphoid organs - B cells in the bone marrow and T cells in the thymus.  They migrate from these tissues and are carried in the bloodstream to the peripheral lymphoid organs.  The peripheral lymphoid organs are the sites of lymphocyte- antigen encounter and thus lymphocyte activation.  Lymphocytes recirculate between the blood and these organs until they encounter their specific antigen. Primary/Central Lymphoid Organs  Bone marrow  Thymus Lymphocyte Development occurs in Primary Lymphoid Organs Structure of the Thymus Outer Cortex  Dark-staining outer part packed with lymphocytes, compartmentalized by elongated epithelial cells.  It consists of immature T -cells.  The process of proliferation and selection occurs mainly here.  Inner Medulla  Lighter central zone with fewer lymphocytes but more epithelial reticular cells.  It consists of mature T-cells.  The final stages of selection may occur at the cortico-medullary junction.  Medulla also consists of thymic corpuscles alternatively called Hassall’s corpuscles. They are believed to be aged and degenerated cells.  Thymus  Encapsulated, lobular Lymphocyte-rich organ located behind the sternum, above and in front of the heart.  Maximal activity in the fetus and then undergoes atrophy at puberty (although never totally disappearing).  The thymus attracts (with chemokines) circulating T cell precursors derived from hemopoietic stem cells (HSC) in the bone marrow.  Essential for T cell maturation (‘T cell’ stands for thymus-derived cell).  Composed of cortical and medullary epithelial cells, stromal cells, interdigitating cells and macrophages.  Important for the differentiation and education (positive and negative selection) of the immigrating T cell precursors prior to their migration to the secondary lymphoid tissues.  Secondary/Peripheral Lymphoid Organs  Lymph Node  Spleen  Mucosal Associated Lymphoid Tissue Lymphocyte –Antigen encounter occurs in Peripheral Lymphoid Organs Lymph Node  Lymphatics drain extracellular fluid from the peripheral tissues, through the lymph nodes, and into the thoracic duct, which empties into the left subclavian vein. This fluid, known as lymph/lymphatic fluid,  Lymph carries antigen taken up by dendritic cells and macrophages to the lymph nodes, as well as recirculating lymphocytes from the lymph nodes back into the blood.  2 Lymph carries free antigen or those carried 1 by dendritic cells to LN Naïve lymphocytes populates the lymph node 3 Activated and non activated lymphocytes leave the lymph node. Activated lymphocytes B cells differentiation and then return to the site of proliferate upon antigen infection. encounter Swollen gland you normally see following an infection is nothing but an enlarged LN  NO connections with the lymphatics. Spleen  Lymphocytes and antigen enter the spleen via blood.  Mostly functional during blood-borne infections.  Red pulp – due red blood cell disposal  White pulp – enriched for lymphocytes and other immune cells (T cells, B cells dendritic cells and macrophages)  Periarteriolar lymphoid sheath (PALS)- T lymphocytes reside  Lymphoid follicles - B cells reside  Marginal zone – some B cells, macrophages and dendritic cells reside. -Where blood is filtered for Blood-borne microbes, soluble antigens and antigen:antibody complexes.  Mucosal Associated Lymphoid Tissue  Most pathogens enter the body through mucosal surfaces  Mucosal surfaces are also exposed to a vast load of other potential antigens from the air, food, and the natural microbial flora of the body.  Protected by an extensive system of lymphoid tissues known generally as the mucosal immune system or mucosa-associated lymphoid tissues (MALT). Nasal Associated Lymphoid tissue (NALT) Bronchus Associated Lymphoid tissue (BALT) Gut Associated Lymphoid tissue (GALT)  Distinctive features of the mucosal immune system Anatomical features Intimate interactions between mucosal epithelia and lymphoid tissues Discrete compartments of diffuse lymphoid tissue and more organized structures such as Peyer’s patches and isolated lymphoid follicles Specialized antigen-uptake mechanisms, e.g., M cells in Peyer’s patches Effector mechanisms Activated/memory T cells predominate even in the absence of infection Secretory IgA antibodies Presence of distinctive microbiota Immunoregulatory environment Active downregulation of immune responses (e.g., to food and other innocuous antigens) predominates GALT  Peyer’s Patches – only in small intestine  Isolated lymphoid follicles – entire intestine  Appendix  Tonsil and adenoid – in throat Peyer’s Patches  Important sites for the initiation of immune responses in the gut. Peyer’s Patches  100–200 Peyer’s patches in the human small intestine  Visible to the naked eye, dome like structure project into the intestinal lumen.  Contain a large number of B cells in B cell follicles, intercalated by T cell zones and dendritic cells.  Follicle-associated epithelium separates the lymphoid tissue from the gut lumen - conventional intestinal epithelial cells known as enterocytes and a smaller number of specialized epithelial cells called microfold cells (M cells). M cells -have a folded luminal surface instead of microvilli. - do not secrete digestive enzymes or mucins, and so lack the thick layer of surface mucus (the glycocalyx) found covering conventional epithelial cells. - directly exposed to microorganisms and particles within the gut lumen and are the preferred route by which antigens enter the Peyer’s patch from the lumen.   “Microfold” due to ruffled or folded appearance in the epithelial layer  Apical membrane has adhesive properties to allow capture; basolateral surrounds a pocket filled with antigen presenting cells and lymphocytes  Bind microbes and internalize them by transcytosis  M cells provide a conduit for the passage of luminal microbes into the GALT and connected lymphatics  a number of pathogens target M cells to gain access to the subepithelial space, even though they then find themselves in the heart of the intestinal adaptive immune system  Macrophages and dendritic cells take up the transported material released from the M cells and process it for presentation to T lymphocytes.  Together, the dendritic cells and primed T cells then activate B cells and initiate class switching to IgA.  Thousands of isolated lymphoid follicles can be identified microscopically throughout the small and large intestines,  More frequent in the large intestine, correlating with the load of local microorganisms.  These follicles have an epithelium containing M cells that lies over the organized lymphoid tissue.  However, they contain mainly B cells and develop only after birth in response to antigen stimulation due to colonization of the gut by commensal microorganisms.  Peyer’s patches, in contrast, are already present in the fetal gut, although their full development is not completed until after birth.  The tissues of the small intestine drain to the mesenteric lymph nodes. - The largest lymph nodes in the body and play a crucial role in initiating and shaping immune responses to intestinal antigens.  The mucosal surface and lymphoid aggregates of the large intestine drain to the mesenteric lymph nodes and to a separate node known as the caudal lymph node, found close to the bifurcation of the aorta.  The mucosal immune system contains large numbers of effector lymphocytes even in the absence of disease.  In the intestine, effector cells are found in two main compartments: the epithelium and the lamina propria  The lymphoid component of the epithelium consists mainly of lymphocytes, which in the small intestine are virtually all CD8 T cells- also called intraepithelial lymphocytes (IEL).  The lamina propria - IgA-producing plasma cells, conventional CD4 and CD8 T cells innate lymphoid cells, dendritic cells, macrophages, and mast cells.  The circulation of lymphocytes within the mucosal immune system is controlled by tissue-specific adhesion molecules and chemokine receptors.  Intraepithelial lymphocytes (IEL) There are two main subsets of CD8 intraepithelial T cells— o type a (‘inducible’) o type b (‘natural’)  Type a express α:β T-cell receptors and the CD8α:β heterodimer. o Cytolytic function o Anti viral activity o Secrete IFN-γ  Type b express α:β or γ:δ T-cell receptors and the CD8α:α heterodimer. o repair of the mucosa after inflammatory damage o stimulate the release of antimicrobial peptides  IgA  The dominant class of antibody in the mucosal immune system is IgA, which is produced locally by plasma cells in the mucosal wall.  The nature of IgA differs between the two main compartments in which it is found—the blood and mucosal secretions. o IgA in the blood is mainly in the form of a monomer. o In mucosal tissues, IgA is produced almost exclusively as a polymer, usually as a dimer  To reach its target antigen in the gut lumen, the IgA has to be transported across the epithelium by the polymeric immunoglobulin receptor (pIgR) via process termed transcytosis. IgA can function in different ways  Innate immunity in the gut Physical barrier – thin layer of permeable epithelium protects the underlying tissues. Tight junction proteins help maintain paracellular permeability where disruption can lead to leakage Mucus layer – large glycoproteins (secreted and membrane-bound) called mucins are responsible for viscosity and protective properties. Epithelial cells themselves have immunological functions:  Sloughing – IECs turnover every 4-5 days, enabling clearance of attached pathogens.  Mucus production – viscous barrier to prevent bacteria from attaching and crossing the epithelium  Antimicrobial peptides (AMPs) – “defensins” that selectivelyt target certain classes of bacteria  Cytokines and chemokines – recruitment and activation of granulocytes, myeloid cells, and lymphocytes in the presence of danger  T cell-like cytokines – potent inflammatory and repair cytokines such as IL-17 and IL-22 to protect the crypts. Intestinal epithelial cells (IECs) Immunity by IECs – Enterocytes: absorption, mucus, digestive enzymes – Enteroendocrine cells: hormone production, stem cell factors – Goblet cells: specialized mucus producers – Paneth cells: specialized cells defending the crypts in the SI and colon. – M cells: antigen and microbe passage to GALT Intestinal Epithelial Cells (IEC) have a crucial role in innate defense against pathogens.  Epithelial cells bear Toll Like Receptors (TLR) on both their apical and basal surfaces, from which they can sense bacteria in the gut lumen and those that have penetrated across the epithelium.  Sensors include the nucleotide-binding oligomerization domain (NOD) proteins NOD1 and NOD2. o NOD1 recognizes a diaminopimelic acid- containing peptide that is found only in the cell walls of Gram-negative bacteria. o NOD2 recognizes a muramyl dipeptide found in the peptidoglycans of most bacteria. NOD2 dysfunction is associated with Crohn’s disease Immune priming and tolerance are different outcomes of intestinal exposure to antigen.  Gut microbiota and functions  1014 microbes colonize the GI tract  Immune response against these microbes is proactive, involving a constant adaptive response against all organisms in the lumen. Inflammation is used sparingly.  Cytokines  Cytokines are defined operationally as polypeptides secreted by leukocytes and other cells that act principally on hematopoietic cells, the effects of which include modulation of immune and inflammatory responses.  Cytokine receptors Five major cytokine receptor families based on the type of receptor they bind: Signaling through cytokine receptors Inflammatory Cytokines (IL-8) Cytokines and T helper subsets Anti-inflammatory cytokine: TGF-beta Role of IL-6  Type I IFNs (IFN-α and IFN-β)  IFN-α are a family of many proteins produced by macrophages, IFN-β a single protein produced by many cells  Both IFNs inhibit viral replication  Both activate Natural Killer cells  Binds to the same IFN-α/β receptor - The receptor is a heterodimer composed of two subunits termed IFNAR1 and IFNAR2. Resource Resource Resource Resource No need to memorize  Chemokines  Chemotactic proteins  Important roles in inflammation and lymphocyte movement though lymphoid organs.  Four major families – CC, CXC, CX3C and XC  CXC primarily activates and attracts neutrophils--(IL-8) is among its members  The CC family activates T cells, monocytes, and eosinophils. CC Families of Chemokines and Chemokine Receptors. CXC, CX3C, and XC Families of Chemokines and Chemokine Receptors.  To remember… * Pro-inflammatory cytokines: IL-1, IL-6, IL-8, IL-12, TNF-alpha * Anti-inflammatory cytokines: TGF-beta, IL-10 * Th1 Cytokine: IFN-γ (interferon gamma) * Th2 Cytokine: IL-4, IL-5, IL-13 * Th17 Cytokines: IL-17, IL-22 * Interferons with antiviral effects (produced by many virally infected cells):  IFN-alpha  IFN-beta No need to memorize