GAB Innate Immunity LEARN PDF

Graduate Accelerated BVetMed Infections & Responses Unit 1: Immunology Principles of Innate Immunity Brian Catchpole BVetMed MSc PhD FRCVS  Learning Objectives Describe the physical and biochemical barriers to infection. Explain how foreign organisms are detected by...

Graduate Accelerated BVetMed Infections & Responses Unit 1: Immunology Principles of Innate Immunity Brian Catchpole BVetMed MSc PhD FRCVS  Learning Objectives Describe the physical and biochemical barriers to infection. Explain how foreign organisms are detected by cells of the innate immune system. Discuss the role of interferon in protecting the host from viral infection. Discuss the role of phagocytosis, complement activation and inflammation in protecting the host against bacterial infection. Principles of Innate Immunity Part 1: Introduction  Protection of epithelial surfaces best way to prevent infection is to keep infectious organisms out Lysozyme in secretions Intact mucous membranes Mucous & mucociliary escalator Fatty acids in sweat & sebum Acid in stomach Skin barrier Intact mucous membranes Commensal microflora  Physical & biochemical barriers SKIN: very good defence barrier – Thick layer of cells – Sebaceous gland secretions PROBLEMS: – Wounds means infection/disease can get in – Vector-borne pathogens (e.g. Lyme disease spread by ticks) are relatively thin (bc of physiologial uses) so not a thick barrier like the skin MUCOUS MEMBRANES: – Physical properties Mucociliary escalator, peristalsis Coughing and sneezing. Vomiting and diarrhoea. for clearing out pathogens – Secretions (saliva, tears and mucous) Physical properties (washes away organisms) Anti-microbial properties (e.g. lysozyme) enzyme that digests bacterial cell walls – Commensal microflora  The commensal bacterial flora (microbiome) is an important defence for mucosal surfaces  INFECTION When a pathogen penetrates the barriers provided by the skin and mucous membranes, it faces other types of defences by cells in the tissues: For this to be successful, the foreign organism must be detected and destroyed  Innate immunity provides the ‘early warning system’ for infection Innate immunity uses pattern-recognition receptors (PRRs) to detect microbial components that are intrinsically foreign: microbial components Pathogen-associated molecular patterns (PAMPs) cellular PRRs bind to PAMPs; tells immune system that there is something foreign present – Lipopolysaccharide – Peptidoglycans – Mannose sugars RAF Fylingdales Bacterial PAMPs good at detecting bacteria, bc they are gram neg and pos components that can be detected  Pattern recognition receptors PAMPs Toll-like receptors Complement C3 protein Acute phase proteins NOD receptors Toll-like receptors (TLRs) Peptidoglycan LPS from G-ve from G+ve bacteria bacteria if a cell has both TLR2 and TLR4, can detected both gram + and - components on bacteria CELL SIGNALLING ACTIVATION Virus nucleic acid  Principles of Innate Immunity Part 2: Innate immunity to viral infection Innate immunity to viral infection Viruses do not have structural PAMPs Viruses are detected by the presence of double-stranded RNA produced during replication (not found in mammalian cells) or the presence of DNA in the cytoplasm DNA typically in nucleus/mitochondria - so if there is DNA in the cytoplasm, likely a virus Cells respond by producing INTERFERONS PRRs recognise viral dsRNA / cytoplasmic DNA double stranded DETECTION SYNTHESIS AND SECRETION SIGNALLING  Functions of type 1 interferons: cell that makes the interferons doesn’t actually benefit from it, it’s for other cells - telling other cells that there is a viral infection present like “neighborhood watch” - the cell that is producing the interferon is already infected - telling its friends that the infection is here Resistance to viral replication – Increased degradation of viral mRNA – Inhibition of viral protein synthesis – Increased antigen presentation of viral antigens Infected cells alert their friends (paracrine effect)  Innate immunity to viruses Viruses can infect any nucleated cell all nucleated cells have this inferon defence mechanism All nucleated cells can respond to viral infection by producing type 1 interferons. Interferon omega can be used to treat persistent viral infections of cats – e.g. FeLV / FIV helps helps with viral infections; NOT going to cure them - not does slow down the replication and can help them feel better temporarily  Natural killer (NK) cells (aka: Large granular lymphocytes)  NK cells Recognise decreased levels of MHC molecules on host cells – decreased production during viral protein synthesis – some viruses block transport to cell surface Recognise a ‘symptom’ of viral infection the decrease in MHC molecules in a cell indicates to the NK cells that a viral infection is likely present in that cell NOT detecting the infection itself, but a SYMPTOM/RESULT of the infection NK cells monitor cell surface expression of MHC molecules MHC molecule Healthy cell  NK cells leave healthy cells alone NK cell Healthy cell DON’T KILL...but in an infected cell, MHC production can be blocked by the virus NK cell Infected cell KILL  The activated NK cell releases its toxic granules NK cell Infected cell KILL  Which kills the infected cell before viral replication is complete NK kills the infected cell, which in turn kills the virus as well NK cell KILL Apoptotic cell  Summary Innate immunity to viruses is relatively weak Interferon is produced – natural antiviral molecule Slows viral replication NOT about making you better, just keep u alive Makes cells more resistant to infection NK cells can identify and kill some virus-infected cells Innate immunity keeps you alive until your adaptive immunity develops in terms of viral infection — keeps you alive until your lymphocytes can work  Principles of Innate Immunity Part 3: Innate immunity to bacterial infection: phagocytosis  Innate immunity to bacteria Cellular mechanisms – Recognition of pathogen (membrane, vesicular and cytoplasmic PRRs) when these cells are triggered by a bacterial infection —— – Response: Phagocytosis & inflammation & draws in MORE WBC to fight the infection Humoral mechanisms – Recognition of pathogen (soluble PRRs) – Response: Killing of foreign organism Enhanced phagocytosis Inflammation Toll-like receptors (TLRs) and NOD proteins are important PRRs for bacteria TLR-2 : Peptidoglycan TLR-4 : Lipopolysaccharide TLR-5 : Flagellin TLR-9 : Prokaryotic DNA NOD2 : muramyl dipeptide located in the cytoplasm recognizes digestion products from the bacteria  A defect in the NOD2 receptor is responsible for Crohn’s disease in man… …and possibly inflammatory bowel disease or anal furunculosis in German Shepherd Dogs Recognition of bacteria by macrophage TLRs leads to phagocytosis and an inflammatory response Prostaglandins etc.  PHAGOCYTOSIS Performed by neutrophils and macrophages Bacteria are endocytosed, killed & digested Image result for phagocytosis  The Respiratory Burst Enhanced cellular aerobic metabolism Reactive oxygen intermediates (ROI) are produced: – Superoxide anion (O·2- ) pumped into the endosome to kill the pathogen – Hydroxyl radicals (OH·) – Hydrogen peroxide (H2O2) The oxygen-dependent mechanism of bacterial killing  Lysosomes cellular organelles- have digestive enzymes Defensins – Cationic anti-microbial peptides that damage bacterial cell walls Lactoferrin – Binds and chelates free iron, which is required for cell wall destabilizing, the cytoplasm starting to spill out bacterial growth Acid proteases – Digestive enzymes active at a low pH The defensin effect Principles of Innate Immunity Part 4: Inflammation and the Complement System  INFLAMMATION https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTU_-ZXuPOvBqc59YkT5r3pdHir-1dQDaRaSfdlbIcWRCdZQ1Al  Inflammatory mediators HISTAMINE highly inflammatory – Produced by mast cell degranulation in tissues – ANTI-HISTAMINES PRO-INFLAMMATORY CYTOKINES – e.g. Tumour necrosis factor alpha (TNF-α) – Synthesised predominantly by white blood cells and macrophages – CORTICOSTEROIDS LIPID MEDIATORS OF INFLAMMATION – e.g. prostaglandins and leukotrienes – Derived from arachidonic acid by actions of cyclooxygenase (COX) and lipoxygenase enzymes – NSAIDs typically are COX inhibitors  Inflammatory cytokines: Localised effects signs of inflammation Vasodilation – Increased blood flow to tissues (increased heat and redness) Increased capillary permeability – increased fluid leakage into tissues - Oedema (increased tissue swelling) Influx of white blood cells – Migration of phagocytic cells into tissues  Inflammatory cytokines: Localised effects Vasodilation – Increased blood flow to tissues (increased heat and redness) Increased capillary permeability – increased fluid leakage into tissues - Oedema (increased tissue swelling) Influx of white blood cells – Migration of phagocytic cells into tissues Inflammation Histamine  Systemic effects of inflammatory cytokines Hypothalamus acts on thermoregulatory system – Fever increased internal temp actually slows down the pathogens and helps increase WBC Liver – Acute phase response Bone marrow more WBC circulating the blood vessels to fight infection – Neutrophil and monocyte mobilization Humoral mechanisms of innate immunity Acute phase proteins Complement cascade  The acute phase response inflammatory cytokines Acute phase proteins stick to bacterial cell walls and enhance phagocytosis  DO NOT LEARN IN DEPTH Complement System Complement proteins are found in the blood. Series of enzyme activation steps forming an amplification cascade. Small amount of activation is amplified to generate a large response. Similar in nature to the clotting cascade but with a different trigger and outcome  The clotting cascade TRAUMA ENZYME AMPLIFICATION CASCADE INSOLUBLE (the clot) SOLUBLE FIBRINOGEN FIBRIN POLYMER  The complement system C9 monomer DETECTION OF FOREIGN ORGANISM enzyme amplification cascade C9 polymer attacks the membranes of bacteria Membrane Attack Complex  The Membrane Attack Complex The MAC forms holes in the bacterial The organism bursts cell wall and is therefore killed The bacterium swells through uptake of fluid  Complement protein C3 is a vital component in the cascade inactive state C3 is a pro-enzyme It is converted to C3a & C3b activated state C3b is deposited on the bacterial surface – Acts as a marker for recognition by complement receptors on phagocytic cells – Acts as an enzyme to facilitate the production of the Membrane Attack Complex C3a is released and stimulates an inflammatory response  C3 activation yields two active molecules C3a C3 acts as a marker for complement receptor — the receptors identify it and eat it (and the bacteria) C3b Bacterial cell surface Carbohydrate molecules C3b acts as a target for macrophages & neutrophils stimulating phagocytosis Complement receptor C3b  C3b also acts as an enzyme to catalyse the formation of the MAC C9 monomer C3b membrane attack complex C3a stimulates MAST CELL degranulation and triggers an inflammatory response Histamine release binds to mast cells stimulates it to degranulate releases histamines C3a Mast cell C3b  Summary Bacteria are attacked by cellular and humoral mechanisms of innate immunity Neutrophils and macrophages eat, kill and digest bacteria Inflammatory response helps to recruit neutrophils and monocytes to the site of infection Circulating acute phase proteins and complement proteins enhance phagocytosis, can cause direct lysis of bacteria and also triggers inflammation Adaptive immunity makes these responses more efficient (see later)