Chapter 14: Innate Immune System Overview of Immune Defenses • First-line defenses: – Intact, healthy skin and mucous membranes – Normal microbiota Overview of Immune Defenses • Sensory systems: – Pattern recognition receptors • Toll-like receptors • NOD-like receptors • RIG-like receptors – Complement system • Alternative pathway • Classic pathway • Lectin pathway Overview of Immune Defenses • Innate effector actions: – Inflammatory response – Interferon response – Opsonization – Membrane attack complex First-line defenses: SKIN High turnover Immune surveillance: dendritic cells, macrophages Salt Normal skin biota SALT First-line defenses: MUCOUS MEMBRANES High turnover Immune surveillance: dendritic cells, macrophages Secretions Normal biota MALT Mucosal epithelium: intestinal mucosa Mucosal surfaces: respiratory mucosa Antimicrobial • Produced by animals: – Lysozyme substances – Peroxidase enzymes – Lactoferrin – Transferrin – Defensins • Produced by your microbiota: – Fatty acids – Colicins – Lactic acid Cells of the Immune System • Granulocytes: – – – – Neutrophils Eosinophils Basophils Mast cells • Mononuclear phagocytes: – Monocytes – Macrophages – Dendritic cells • Lymphocytes: – T cells – B cells – NK cells Neutrophils • Phagocytic • Granules: – Lysozyme, Phospholipase A2, myeloperoxidase, elastase, acid hydrolases, lactoferrin . . . • Most numerous leukocyte in circulation • Migration to tissue = major component of inflammatory response • Short life span • NETs • Phagocytic • Lysosomes: Macrophages – Lysozyme, peroxidase. . . • Mature, tissue form of monocyte • Increased migration and maturation of monocytes to tissue in inflammatory response • Long life span • TLRs: on cell surface & in lysosomes • Cytokines: • Activation → enhanced killing power Dendritic Cells • Phagocytic sentinel cells • Antigen presenting cells • Most = monocyte/ macrophage cell line • Long life span • Important bridge between innate & adaptive immunity Natural Killer Cells • Non-specific lymphocytes – Do not require antigenic stimulation Cell Communication: SURFACE RECEPTORS Cell Communication: CYTOKINES • • • • • Chemokines Colony stimulating factors Interferons Interleukins Tumor necrosis factor (TNF) Interferons α and β Cell Communication: ADHESION MOLECULES • Integrins: large family, widely expressed, involved in interaction with ECM • Selectins: small family, differentially expressed by leukocytes & endothelial cells, involved in leukocyte extravasation • Cadherins: large family, widely expressed, involved in adhesion between cells • ICAMs & VCAMs: part of immunoglobulin superfamily; many roles in immune response/inflammation Pattern Recognition Receptors • Recognition of PATHOGEN-ASSOCIATED MOLECULAR PATTERNS / MICROBE-ASSOCIATED MOLECULAR PATTERNS (PAMPs / MAMPs): – – – – – Peptidoglycan Lipopolysaccharide Techoic acid Flagellin subunits Viral RNA • Recognition of DANGER-ASSOCIATED MOLECULAR PATTERNS (DAMPs): – Molecules that indicate cellular damage Pattern Recognition Receptors • Toll-like receptors (TLRs): – Membrane-bound receptors – Macrophages, dendritic cells, cells lining sterile sites (i.e., mesothelial cells) – Detection of PAMPs → signal to nucleus → upregulation of gene expression → response Pattern Recognition Receptors • NOD-like receptors (NLRs): – Located in the cytoplasm – most (all?) cells – Detect PAMPs or DAMPs Pattern Recognition Receptors • RIG-like receptors (RLRs): – Located in the cytoplasm – most (all?) cells – Recognize viral RNA – Allow cells to detect a viral invader – Recognition of viral RNA by RLR → synthesis and secretion of interferons → expression of inactive viral proteins → activation of IVPs by dsRNA → apoptosis of infected cells The Complement System • Consists of interacting proteins produced in the liver and found in blood and tissues • These proteins promote – Opsonization – Inflammation – Cell lysis The Complement System • Central feature = splitting of C3 → C3a & C3b • Enzyme that splits C3 = C3 convertase • C3 also spontaneously degenerates to form C3a & C3b at a constant rate • Alternative pathway: C3b binds to foreign cell surface receptors → formation of C3 convertase • Lectin pathway: pattern recognition receptors = mannose binding lectins (MBLs): bind to mannose molecules on microbial surface → formation of C3 convertase • Classical pathway: antibody binds antigen = antigen-antibody complex → formation of C3 convertase (adaptive immune response) Phagocytosis • • • • Chemotaxis Recognition and attachment Engulfment Phagosome maturation and formation of phagolysosome • Destruction and digestion • Exocytosis Phagocytosis The inflammatory response • Acute inflammation – example of activation: – TLR on sentinel MØ recognizes PAMP → MØ produces TNF → induces liver to synthesize acute phase proteins → activation of phagocytes, activation of complement – Tissue damage: “Danger Model” of immune system – ex. = activation of coagulation cascade in response to blood vessel damage The Acute Inflammatory Response • • • • • Calor = heat: increased blood flow to site Rumor = redness: increased blood flow Tumor = swelling: fluid and cells accumulate Dolor = pain: pressure + chemical mediators Functio laesa: many possible causes The acute inflammatory response Recruitment of leukocytes from the blood to a site of acute inflammation: Chronic inflammation • Acute response is unsuccessful in resolving the problem • Can last years, often associated with significant tissue damage • May be due to chronic infection, repetitive injury, chronic implantation of foreign material or selfperpetuating because of damage induced by the immune system itself in the absence of ongoing infection/other external cause Fever • Protective mechanism = resetting of the thermostat – Make the body less hospitable to pathogens – Slowed microbial growth = time to raise a defense – Increases rate of enzymatic reactions → enhanced inflammation, phagocytosis, lymphocyte proliferation, hematopoiesis, production/release of cytokines and antibodies • Pyrogens: – Endogenous: interferons – Exogenous: LPS Fever Fever ≠ acute inflammation! Fever = a systemic change in the body temperature Heat associated with acute inflammation = localized increase in temperature