2 - Innate immune system

Features of innate immunity

• Components - physical and chemical barriers, phagocytic leucocytes, dendritic cells, NK cells, complement proteins

• Always present, immediate response but limited potency

• General specificity, can recognise several classes of pathogen but cannot make fine distinctions

• Attempts to immediately destroy infection and if it can’t it contains infection until adaptive system moves in

• No memory, equal potency upon repeated exposure to same pathogen

Innate immune response overview

Phagocytosis, mast cell / eosinophil degranulation, NK cell activation (maybe), complement activation, acute phase response, antimicrobial peptide secretion, cytokine secretion, inflammation

How does the innate system link to adaptive

• Macrophages act as APC’s

• Cytokines produced by macrophages and NK cells enhance response

• Inflammatory response increases flow of lymph containing antigens to lymphoid tissues

Pathogen detection system

• Pathogen associated molecular patterns - structures of pathogen required for survival e.g. bacterial cell components (LPS, peptidoglycan), flagellin, bacterial and viral DNA and RNA

• Damage associated molecular patterns - results of cell damage by infections e.g. heat shock proteins, protein fragments from ECM

4 Classes of receptors

• Toll like receptors (transmembrane proteins)

• C-type lectin receptors (transmembrane)

• NOD-like receptors (cytosolic)

• Retinoid acid inducible gene (RIG) like receptors (cytosolic)

Toll like receptors

• Localisation important for ligand accesibility, tolerence to self molecules and downstream signal transduction

• Exterior binds P/DAMP’s, membrane spanning domain and cytoplasmic domain (toll-interleukin-1 domain) for intracellular signalling

• Can form dimers with each other for more diverse specificity

• Examples - TLR2 (peptidoglycan), TLR4 (LPS), TLR5 (flagellin)

• Activation leads to production of interferons and pro inflammatory cytokines

NOD-like receptors

• Nucleotide binding oligomerisation domain like receptors

• Work synergestically with TLR’s and lead to formation of multi protein inflammasome complexes that assemble in response to pathpgens

• Lead to activation of caspase-1 which is a protease that cleaves and activates IL-1B and IL-18

• Leads to cell death via pyroptosis - activated caspase-1 processes gasdermin D which causes the release of GSDMDNterm fragment which forms a large pore in the plasma membrane

• Pore results in rapid loss of membrane integrity and dissopation of electrochemical gradient

Macrophage activation

• Resting state - slowly proliferating, removing cellular debris, express few MHC2 molecules, poor antigen presenters

• Activated - receive signals (cytokines) from other cells that MHC2 is upregulated, phagocytic and AP functions enhanced

• Hyperactivated - direct signal received from pathogen (LPS, mannose), increased rate of phagocytosis and cytokine secretion

• Classically activated - Th1-like phenotype, promote inflammation, ECM destruction and apoptosis

• Alternitively activated macrophages - Th2-like phenotype, promote ECM construction, cell proliferation and angiogenesis, associated with immune regulation and wound healing

Phagocytosis

• Recognise PRMs or receptors that bind complement, Igs or other opsonins

• Formation of pseudopodia, engulfs pathogen into phagosome

• Phagosome fuses with lysosome to form phagolysosome

• Acidification causes the release of proteases which lead to microbial killing and digestion

• When the phagocyte is activated it consumes more oxygen which leads to the respiratory burns, forms toxic ROIs (superoxide, hydrogen peroxide)

• Any debris is broken down or expelled from the cell

Extravasation

• Rolling - attach loosely to endothelium via low affinity selectin interactions, cytokines increase expression of endothelial intracellular adhesion molecules and selectins, these bind the selectin ligands on the neutrophil and slow its motion through the bloodstream

• Chemokine activation - rolling allows for interaction with other molecules (chemokines) through receptors, activation occurs through conformational change in integrin molecules

• Arrest and adhesion - tight adhesion to endothelium occurs via intercellular adhesion molecules binding to integrins on the neutrophil

• Transendothelial migration - enter tissues by secreting proteases that breach the basement membrane of the endothelium (diapedesis)

NK cells

• Granular lymphocytes, bridge gap between adaptive and acquired

• Can kill with or without cytokine activation

• Proliferate in response to IL-2 and are more effective at killing in presence of IFNs and IL-12

• Kill virus infected and tumour cells, produce cytokines

Cell killing

• Activating receptors (bind LPS, cancer antigens)

• Inhibitory receptors - killer cell immunoglobulin-like receptors (KIRs), recognise MHC class 1 receptors

• Balance between receptors determines if the target will be destroyed

• Fas ligands on NK cells bind to Fas on target cells inducing apoptosis

• Can also insert pores into target cells through perforin - proteolytic enzymes enter cell and degrade it, also susceptible to osmotic lysis

• NK cells also have Fc receptors which recognise cell bound antibodies, cross linking triggers degranulation and cell dies by apoptosis (antibody dependant cellular cytotoxicity)