BMS2045 2c Innate immunity 3 2024.pdf

Theme 2 Innate Immunity Inflammation part 2, complement and cell mediated innate immunity THEME LEARNING OUTCOMES: 1. Describe the four main features of an innate immune response 2. Identify the cells and chemical/protein mediators involved in inflammation, including the function of cytokines and ch...

Theme 2 Innate Immunity Inflammation part 2, complement and cell mediated innate immunity THEME LEARNING OUTCOMES: 1. Describe the four main features of an innate immune response 2. Identify the cells and chemical/protein mediators involved in inflammation, including the function of cytokines and chemokines 3. Compare the different mechanisms of endocytosis, and describe in detail the processes that take place during phagocytosis 4. Describe the role of Toll-like receptors (TLRs) and other Pattern Recognition Receptors (PRRs) in phagocytosis and in innate immune cell activation 5. Discuss the mechanisms and function of the complement system activation 6. Specify the different cells and killing mechanisms that are involved in cellmediated innate immunity Innate immunity Comprises four main types of defensive barriers/mechanisms: (1) Anatomical (2) Physiological/chemical (3) Phagocytic/endocytic (4) Inflammatory – part 2 Inflammatory mediators Prostaglandins, Leukotrienes, Histamine Cytokines Chemokines 4 interconnected enzyme cascades: 1. Kinin 2. Clotting 3. Fibrinolytic 4. Complement Overview of Inflammation 4. Complement 1. Kinin 3. Fibrinolytic 2. Clotting (Kuby, 6e, 2007) (1) Kinin system = enzyme cascade Hageman factor (Factor XII) activated following tissue injury Activates pre-kallikrein to form kallikrein which cleaves kininogen to form Bradykinin Bradykinin = basic protein that increases vascular permeability, causes vasodilation, pain and contraction of smooth muscle (2) Clotting system = enzyme cascade Hageman factor (Factor XII) activated following tissue injury such as damage to blood vessels and leads to release of large amounts of Thrombin Thrombin then acts on soluble fibrinogen….fibrin and clot formation and fibrinopeptides which increase vascular permeability and neutrophil chemotaxis and involvement of platelets (3) Fibrinolytic system = enzyme cascade Triggered by damage to endothelial cells and activation of Hageman factor (Factor XII) Removes clots from the injured tissue Active end product = plasmin Plasmin is a potent proteolytic enzyme which breaks down the fibrin clots into degradation products which are chemotactic for neutrophils Also contributes further to inflammation by activating the classical complement pathway Important to block leakage & repair, must maintain circulation Kinin, clotting and fibrinolytic pathways (Factor XII) kinin clotting fibrinolytic (4) Complement Group of serum and cell surface proteins present as inactive precursors in blood (C1, C5 etc) Upon activation form an enzymatic cascade Lectin pathway Mannose binding lectin + C4 C2 Lectin 3 activation pathways: 3 major outcomes: Opsonise particles to increase phagocytosis Regulate inflammatory & immune responses Lyse target cells and microorganisms Classical Alternative Complement factor C3 and C3b Central importance in complement pathway C3 amplifies all the reactions C3b Attaches to pathogens (opsonisation) leading to increased phagocytosis via complement receptors on phagocytes Lead to the formation of the membrane attack complex (MAC) The Classical pathway (CI) Trigger = Opsonin such as Antibody (IgG or IgM) or CRP binds to target antigen/PAMP/DAMP on cell surface and then attracts and binds C1 (C1qr2s2), activating its latent proteolytic activity or CRP PAMP/DAMP PAMP/DAMP PAMP/DAMP PAMP/DAMP C1qrs attracts and binds C4 which is cleaved by to form C4b. C4a is released C4b transfers to the surface of the microbe/cell and in the presence of Mg2+ C2 is attracted and binds to C4b and is cleaved by C1s and C2b is released C4b2a remains on the microbial/cell surface (= C3 convertase) A lot of C3 is attracted and binds to C4b2a and is cleaved by it (= amplification) C3a is released and C3b binds the complex to form C4b2a3b or C5 convertase Recruitment and cleavage of C5 to C5b forms the first component of membrane attack complex MAC Sequential attachment of C6, C7, C8 and polyC9 forms the Membrane attack complex or MAC Cytolysis: via formation of the MAC: C5b678(poly)9 Target cells lyse and die through necrosis Some gram –ve bacteria (Neisseria) enveloped viruses & parasites The Lectin pathway Trigger = MBL (and other lectins) binding to bacterial /infected cell wall via binding to PAMP/DAMPs MBL attracts MASPs: MBLassociated-serineproteases Whole complex 3D structure similar to C1qrs and attracts C4…etc Amplification by C3 activation The Alternative Pathway (C3) Spontaneous cleavage of C3 into C3b and C3a Trigger = C3b Sticks to microbial cell surface (via PAMP/DAMP) This associates with factor B in the presence of Mg2+, and the complex is cleaved by factor D Forms C3bBb, a ‘C3 convertase’. This is stabilised by Properdin and can then act to amplify more C3 cleavage Formation of MAC C3bBb recruits a further C3b, this C3bBb3b is a ‘C5 convertase’ and will attract C5-C9 to form the MAC or CRP Opsonin (Ab/CRP) on surface Bacterial surface Bacterial surface Other outcomes of complement activation C3a and C5a are inflammatory in their own right: Stimulate respiratory burst in neutrophils Anaphylotoxins -- trigger mediator release by mast cells (histamin etc) Chemotactic C3a -- eosinophils C5a -- neutrophils Inflammatory mediators Prostaglandins, Leukotrienes, Histamine Cytokines Chemokines 4 interconnected enzyme cascades: Kinin Clotting Fibrinolytic Complement 3 activation pathways: (Classical, Alternative and Lectin) 3 main outcomes: (lysis of target, opsonisation, inflammation) Cell mediated (innate) immunity (CMI) CMI is usually assumed to be part of the adaptive immune response!! But several CMI systems are part of innate defence mechanisms including the actions of: Innate Lymphoid Cells (including Natural Large Killer cells) Granular Natural Killer T cells Lymphocytes gd T- cells B1 B cells As well as Monocytes/Macrophages, Neutrophils, Eosinophils, Basophils and Mast cells Large granular Lymphocytes 1: Innate Lymphoid Cells (ILCs) including Natural Killer cells (NKs) No TCR, most do not have PRRs, NKs have activatory and inhibitory receptors, other ILCs activated by local mediators Thought to have similar properties to TH1, TH2 and T Regulatory cells through secretion of cytokines Found particularly in the skin and mucosa during innate immune reactions 3 Types/groups: ILC1 + NKs - secrete pro-inflammatory TH1-like cytokines (IFNg and TNFa), ILC1 involved in immunity to extracellular pathogens, whereas NKs involved in immunity to intracellular pathogens and tumour cells ILC2 - immunity to worms, wound healing, secrete TH2 type cytokines that activate eosinophils ILC3 – lymphoid tissue development, intestinal health, immunity to extracellular bacteria and fungi, secrete regulatory cytokines Natural Killer cells Have a range of surface activatory/inhibitory receptors to recognise infected cells (no MHC required) also CD16 = Fc receptor for Ab and Fas ligand Immunity to intracellular pathogens by killing infected target cells and able to destroy a range tumour cells 3 mechanisms of target cell killing: (1) Kill target cells using perforins and granzymes in similar mechanisms to Cytotoxic T cells by inducing pore formation in target cell membranes - necrosis and apoptosis (2) Also express Fas ligand on surface, so can induce apoptosis (3) Antibody Dependent Cellular Cytotoxicity killing relies on Ab binding to CD16 (Fc receptor) on surface of cells – often virally infected or tumour cells – necrosis and apoptosis Summary of ILCs and NKs Inducing factors Effects Large granular lymphocytes 2: Natural Killer T cells (NKTs) Share properties of T cells and NK cells – so another bridge between innate and adaptive immunology Have a TCR (a/b) but do not recognise antigen peptides presented by MHC molecules rather they recognise CD1 displaying lipid antigens during innate immunity Kill infected targets mainly via apoptosis via Fas : Fas ligand Make lots of cytokines such as IL-2 and TNFa and directly affect innate immune responses Most useful in low dose bacterial infections and against self tumour cells Other Cells Eosinophils Granules contain Cationic peptides, Major basic protein and Peroxidase all of which can be released directly onto the surface of extracellular pathogens (worms) Neutrophils Primary (azurophilic) granules: Lysosome & myeloperoxidase Secondary (specific granules): Defensins, Lactoferrin Higher production of RNI than other phagocytes NETosis g/d T cells Slightly higher numbers in the mucosal associated lymphoid tissue and skin when compared to peripheral blood Important in gut antigen recognition of bacterial antigens (particularly lipids) through non MHC restricted means (CD1) Express Toll-like receptors and may phagocytose and process and present antigens to a/b T cells Also express some NK cell activatory receptors…….able to kill target cells (via release of perforin/granzyme) Over represented in some veterinary species (such as pigs) Emerging role in anti-tumour activity = potential target for cancer immunotherapy B1 cells Sub group of B cells expressing CD5 High frequency in pleural and peritoneal cavity Mainly produce low affinity antibody (IgM) mostly against bacterial antigens (particularly carbohydrates) and are called natural antibodies No T cell help required Very little memory Important in foetal and neonate immunology hence part of innate immunity Overall Important to appreciate that adaptive and innate immunity do NOT operate independently They function as a highly interactive and cooperative system, producing a combined response more effective than either branch could produce by itself