L2 Innate immunity 2023(1) (1).pptx

Theme 1 Lecture 2 Innate Immunity 1: soluble effectors Prof. Sandra Sacre [email protected] Contents •Innate immunity •Lysozyme •Antimicrobial peptides •Collectins, ficollins and pentraxins •Complement What is innate immunity ? • 1st line of defence against infection • Present at birth and pa...

Theme 1 Lecture 2 Innate Immunity 1: soluble effectors Prof. Sandra Sacre [email protected] Contents •Innate immunity •Lysozyme •Antimicrobial peptides •Collectins, ficollins and pentraxins •Complement What is innate immunity ? • 1st line of defence against infection • Present at birth and passed down genetically • Occurs within minutes of pathogen recognition Innate immune system is a rapid response Characteristics of innate immunity Trained immunity (Innate immune memory) (Netea et al. Nat Rev Immunol 20, 375– 388; 2020) Epigenetic and metabolic reprogramming of the innate immune cells leads to an altered response to subsequent stimulation which can last for many week or months Innate barriers to infection Physical soluble Induced Skin ComplementInnate immune cells Respiratory tract Defensins Pattern recognition Gastrointestinal tract Collectins Receptors (PRRs) Interferon Anatomical barriers Tissue Damage Soluble innate immune molecules Enzymes such as Lysozyme Disrupt bacterial cell walls; found in blood and tears Antimicrobial peptides Disrupt microbial membranes Collectins, ficolins and pentraxins Bind to pathogens targeting them for phagocytosis and activate complement Complement components Lyse bacteria, opsonise pathogens and induce inflammation Lysozyme disrupts peptidoglycan Lysozyme is secreted by phagocytes and paneth cells from the small intestine Cleaves the bond between the alternating sugars that make up peptidoglycan Phospholipase A2 can then disrupts the phospholipids Most effective against Gram-positive bacteria Antimicrobial peptides Histatins Produced in the oral cavity. Active against pathogenic fungi, e.g. Candida albicans Cathelicidins LL-37 broad-spectrum antimicrobial activity against both Gramnegative and Grampositive bacteria Defensins Two classes – α, β defensins • Cover epithelial surfaces, found in saliva • Constitutively secreted by neutrophils, epithelial cells and paneth cells in the crypts of the small intestine • Kill bacteria in minutes, by disrupting the membrane • Also attack fungi, viruses (influenza and herpes virus) • Inhibit DNA and RNA synthesis Defensins • 35-40 aa amphipathic peptides which means they have both hydrophilic and hydrophobic regions on their cell surface. • Disulphide bonds stabilise the structure to have a positively charged region separated from a hydrophobic region • Disrupt microbial membranes but not that of the host Collectins, ficolins and pentraxins Collectins have globular lectin like heads that bind bacterial cell surface sugars. Sialic acid hides mannose antigens on host cells. Ficolins (have a Fibrinogen like domain) recognise Beltrame MH et al. (2015) Front. acylated compounds (COCH3) Pediatr. 2:148. such as bacterial cell wall monosaccharides. Pentraxins are cyclic multimeric proteins in the plasma. C-reactive protein (CRP) is used as a clinical measure of inflammation – CRP binds to phosphocholine on Actions of Collectins, ficolins and pentraxins • Soluble pattern recognition receptors • Act as opsonins that bind to pathogens and infected cells targeting them for phagocytosis ChantalDumestre-Pérard Autoimmunity Reviews (2018), Vol 17, 890-899 • Activate complement through the classical pathway/lectin pathway Complement pathways converge on C3 Complement System •Series of over 30 proteins that constantly circulate in blood and fluids that bathe the body tissues •When they detect presence of foreign material, they initiate a cascade of reactions that amplify the signal •When activated, cooperate with other host defense systems to generate inflammation and rapidly remove the pathogen •Most made by the liver but also produced by monocytes, macrophages and epithelial cells of the intestine and urinary tract Complement components • Circulate as a pro-form (inactive) in the blood • Numbered in the order they were discovered, not in the order they are activated • Some have proteolytic enzymatic activity • On activation they split into a small and large fragments triggering an amplification cascade • Normally ‘a’ is the small fragment except c2a Complement cascade You do not need to learn to draw this. Effects mediated by complement components Membrane Attack Complex Complement receptor activation Classical pathway • Initiated by C1 activation • C1 is a complex of three proteins: C1q, C1r and C1s • The structure of C1 is dominated by C1q – a large molecule of 18 polypeptides that form six collagen like triple helix structures Classical pathway: Activation Triggered when C1 binds to the Fc region of an antibody – antigen complex C1 must bind at least 2 FC domains IgM is the most efficient at activating complement as it has 5 Fc domains. IgG1 and IgG3, and to a lesser extent IgG2 can also activate complement when close together bound to antigen Serum IgM Serum IgM cannot bind C1 as it has a planar conformation, the shape changes on binding antigen to reveal binding sites for C1q Classical pathway: Amplification • Binding C1q with the Fc domain causes a conformational change in C1r • C1s is cleaved and can then cleave C2 and C4 into their large and small fragments • C3 convertase (C4b2a) can then activate over 200 C3 molecules producing a massive amplification of the signal Lectin pathway • Antibody independent, activated by ficolins and mannose binding lectin (MBL) • MBL binds mannose residues on carbohydrates and glycoproteins on bacteria and some viruses • Similar downstream mechanism to the classical pathway • Upon binding MBL forms a complex with MASP-1 and MASP2 (serine proteases) • Active complex then cleaves C2 and C4 Nairn: Immunology for Medical Students, 2nd Edition Alternative pathway  C3 spontaneous hydrolyses into C3a and C3b  C3b binds a pathogen membrane and factor B, making it susceptible to cleavage by factor D to Bb  C3bBb has a half-life of 5 min, unless it binds the serum protein properdin, which extends it half-life to 30 min by protecting it from proteases  C3bBb hydrolyses C3 creating more C3b which can amplify the signal Complement pathways C3 is an important complement component Terminal complement components You do not need to learn to draw this. Membrane attack complex • MAC forms a pore that inserts into the membrane allowing diffusion of ions and small molecules, water moves into the cell killing it • Human cells have soluble and cell surface associated proteins that prevent MAC formation Complement pathway summary Complement inhibitors (From Helbert M: Flesh and Bones Immunology Edinburgh, Masby, 2006) Hereditary Angioedema : C1 inhibitor Classical complement deficiency cascade easily activated but can be treated with an Complement deficiency • Patients deficient of components of the complement pathway experience recurrent infections • MBL deficiency causes serious pyogenic infections in neonates and children • C3 deficiency is the most severe leading to successive severe infections • Patients deficient of C8 are prone to infection with Neisseria meningitis Complement deficiency in SLE • 90% of people deficient for C4 develop the autoimmune disease systemic lupus erythematosus (SLE) • C4 deficiency means less C3b (C4b2a is C3 convertase) • C3b bound to immune complexes binds to CR1 on erythrocytes which transports them to phagocytes in the and spleen. • liver Phagocytes recognise the immune complexes via their Fc receptors and engulf them Learning Outcomes You should be able to: • Describe the key characteristics of the innate immune system • Explain how the soluble molecules of the innate immune system defend against infection • Describe the 3 complement pathways • Explain the role of complement activation and complement receptors • Give examples of the consequences of complement system deficiencies Recommended Reading • Peter Parham. The Immune system, 5th Ed (2021), Norton • Kenneth Murphy. Janeway’s Immunobiology, 10th Ed (2022), Norton • Nairn: Immunology for Medical Students, 3rd Edition (2016) Elservier • Geha & Notarangelo. Case Studies in Immunology , 7th Edition (2016), Norton • Netea et al. Defining trained immunity and its role in health and disease. Nat Rev Immunol 20, 375–388 (2020)