BMS2045 2a Innate Immunity 1 2024 PDF

Theme 2 Innate Immunity THEME AIMS: To provide an understanding of the ‘non-specific’ arm of the immune response To develop a knowledge of the processes involved in, phagocytosis, inflammation and complement activation THEME LEARNING OUTCOMES: 1. Describe the four main features of an innate immune r...

Theme 2 Innate Immunity THEME AIMS: To provide an understanding of the ‘non-specific’ arm of the immune response To develop a knowledge of the processes involved in, phagocytosis, inflammation and complement activation 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 cell- mediated innate immunity Innate immunity The ‘non-specific’ arm of the immune system? First line of defence FAST Ready from birth Triggered by: tissue damage from trauma or infection Results in: series of cellular and chemical events which aim to limit spread of the damage, eliminate microorganisms and repair damage …But there is recognition of foreign material/antigens Immune activation Tissue damage or pathogen invasion Tissue repair/pathogen clearance Memory response T/B effector cells Innate Immune response Adaptive Immune response T/B cell activation Time after damage/infection Damage/infection Memory T /B cells Cells of the innate immune system Phagocytes Neutrophils Monocytes (MC) & Macrophages (MF) Dendritic cells, B cells, Mast cells and Eos. Antigen presenting cells MCs MFs Dendritic cells (B cells) Other innate immune cells Natural killer cells, Innate Lymphoid Cells, NKT cells, gd T cells, Eosinophils, B1 cells etc Features of innate immunity Comprises four main types of defensive barriers/features: (1) Anatomical (2) Physiological/chemical (3) Phagocytic/endocytic (4) Inflammatory (1) Anatomical Skin Mechanical barrier retards entry of microbes Sloughing of dead skin Acidic environment (pH3-5) retards growth of microbes Commensal microflora secrete bacteriocins and metabolites such as lactic acid which harm other invading bacteria Mucosal surfaces Cilia propulsion (nasal/bronchial) Mucus entrapment Flow of secretions (urine, saliva, tears, milk) Competition for attachment and nutrients by commensal bacteria Epithelial cells joined by tight junctions (2) Physiological/chemical Temperature 370C and fever inhibit growth Low pH Acidity of stomach contents/skin Chemical mediators/Antimicrobial proteins/peptides (AMPs) Proteins/enzymes: Lysozyme, Lactoferrin, Psoriasin, Surfactant P Peptides: Defensins, Cathelicidins, Histatin, Dermicidin Protein examples: Lysozyme: Enzyme found in tears/saliva that cleaves peptidoglycans of bacterial cell walls particularly gram +ve Lactoferrin: Binds to essential nutrients, inhibiting bacterial and fungal growth Activates macrophages S100 proteins: e.g. Psoriasin: Found in secretions and on skin Disrupts microbial cell membranes killing the cells Potent against E. coli but not S. aureus Surfactant Proteins: Found mainly in respiratory tract Block bacterial surface components by binding to them Have a lubricating function SP-A, SP-D Peptide examples: Defensins: Positively charged polypeptides that bind negatively charged microbial structures e.g. LPS, LTA Aggregate to form pores in cytoplasmic membranes Activate complement – classical pathway Found on skin and all mucosal surfaces Most abundant protein in neutrophil granules Cathelicidins: Antimicrobial peptides that disrupt microbial membranes (particularly bacteria) Mainly found on mucosal surfaces E.Coli Innate immunity Comprises four main types of defensive barriers/features: (1) Anatomical (2) Physiological/chemical (3) Phagocytic/endocytic – part 1 (4) Inflammatory (3) ‘Phagocytosis’ Ilya (Eli) Metchnikoff recognised that certain specialised cells mediate defence against microbial infection and thereby ‘fathered’ the concept of cellular immunology He studied the ability of cells within transparent daphnia and starfish larvae to surround objects introduced into them (such as rose thorns or fungal spores) He first described phagocytosis in 1883 after studying white blood cells ‘eating’ foreign particles Cellular uptake Uptake of extracellular material is essential for acquiring nutrients, sampling the surrounding environment and for defence mechanisms All cells are able to take-up extracellular material in some form via a range of mechanisms including: osmosis, diffusion, ion pumps and endocytosis Endocytosis Describes the general purpose of a cell internalising material that is present on the outside of the cell (extracellular) by invagination of the plasma membrane It can refer to the following mechanisms: Pinocytosis Macro-pinocytosis Receptor-mediated endocytosis or Phagocytosis Most cells use pinocytosis and receptor-mediated endocytosis Pinocytosis Cell drinking to bring in nutrients and to aid cell’s ability to osmoregulate Molecules internalised by non-specific invagination of the plasma membrane Dependent on external concentration Macro-pinocytosis Larger ‘gulps’ All the above functions … May play a role in recycling of membrane receptors or the plasma membrane itself Aids in processing of Antigens for an immune response Some microorganisms use this route to infect host cells (viruses) Receptor-mediated endocytosis specific uptake of ligand, growth factor, hormone, immune complex via a receptor clustering of receptors and association of clathrin around ‘pits’...endosomes clathrin ~ 0.25 - 0.3mm Phagocytosis Carried out by specialised cells to internalise, kill/destroy/digest particulate matter (>1mm) Monocytes/Macrophages Neutrophils Dendritic cells Eosinophils B cells/Mast cells Results from clustering of cell surface receptors and requires cytoskeletal rearrangement and energy ‘Cell eating’ ‘Cell drinking’ Involvement of clathrin to form ‘clathrin coated pits’ Phagocytes Phago Killing Antigen presentation ± Neu ++ +++ MC/MF ++ + ++ DC + ± +++ Eo ± +++ ± (extracellular) B ± ± + Mast ± ± ? Stages of Phagocytosis 1 2 3 3 4 1- Recognition 2- Ingestion 3- Digestion 4- Exocytosis Antigen presentation Storage Phagocytosis stage (1) ‘Recognition’ DIRECT (non-opsonic) – Cells bind directly to pathogen/particle Pattern Recognition Receptors (PRRs) on phagocytes bind to Pathogen Associated Molecular Patterns (PAMPs) or to Damage Associated Molecular Patterns (DAMPs) on particles/microbes INDIRECT (opsonic) – Cells bind a coated pathogen/particle Receptors on phagocytes bind to opsonins coating the surface of particulate matter/microbes INDIRECT Cells of the innate immune response express many receptors that recognise a variety of PAMPs, DAMPs and opsonins DIRECT Direct recognition PAMPs LPS Lipotechoic acid (LTA) Also: Flagellin (bacterial flagella), glycoproteins on viruses, helminths, fungi & bacteria) DAMPs Necrosing cells release ssRNA Short-chain fatty acids in the diet Phosphatidylserine, RNA, HDL and vitronectin on the surface of damaged cells and/or apoptotic cells Involved in clearing and important in tissue/limb development Pattern Recognition Receptors: C type lectin receptors Mannose Receptor Binds to mannose/fucose/α mannan On surface of most MFs and DCs Has 8 extracellular domains and cytoplasmic tail Dectin-1 Binds β1-3 glucan on fungi and bacteria Expressed on a wide variety of myeloid lineage cells DC-SIGN Binds mannans on bacteria, fungi and parasites Scavenger receptors SR-A Found on all MFs & some endothelial cells, binds modified low-density lipoprotein e.g. oxidised LDL SR-B Includes CD36 found on endothelium, DC, platelets, MC & MF Binds variety of altered ‘self’ molecules such as oxidised LDL or vimentin on the surface of apoptotic cells Also recognise some PAMPs LPS (gram negative bacteria) binds CD14 on Monocytes BUT CD14 has no signalling domain in the cytoplasmic tail = immunological mystery until the discovery of…. Toll-like receptors Toll gene identified in Drosophila by Hoffman in 1996 In 1998 TLR4 was identified by Beutler (using mouse knockouts) as the receptor for LPS Beutler and Hoffman received the Nobel prize for their work in 2011 Germinating fungal hyphae on Toll deficient Drosophila Cell, Vol. 86, 973-983, 1996 Extracellular LPS Flagellin TLR4/4 Intracellular Profilin Bacteria Gram+ bacteria & fungi TLR5/5 CPG DNA TLR 11/11 TLR2/1 TLR9 ssDNA dsDNA TLR7 & TLR8 TLR3 TLR2/6 Leucine rich repeats of external domain specific for a different set of PAMPs or DAMPs TLRs can be either HOMODIMERS or HETERODIMERS, all membrane bound, but some sample contents of endocytic vesicles Most cells have the intracellular TLRs detect DNA/RNA associated with viruses and strongly induce type I interferons – cytokines with antiviral effects Cell surface TLRs expressed mainly by immune cells and are strongly associated with bacterial/fungal infections MCs, MFs and DCs have all TLRs B cells, T cells and granulocytes have fewer and endothelial cells, adipocytes etc only expressTLR4 Ligation of cell surface TLRs activates complex signalling pathways, which differ depending on the PAMP Use different adaptor molecules, eventually activating the NF-κB or IRF (IFNg regulatory factor) transcription factors Leads to a strong activation of inflammatory mediators (cytokines, prostaglandins etc) NFkB Other Cytoplasmic PRRs NOD-like receptors (NLRs) Family of 23 members in 3 main groups (B,C, P) Interact with intracellular PAMPs and DAMPs Activate the NFkB pathway and autophagy RIG-like receptors Bind viral dsRNA and so detect viral replication Initiate anti-viral cytokine (type 1 interferons) AIM2–like receptors (ALRs) and cGAS/STING Bind DNA molecules from bacteria and viruses Induce production of anti-viral and inflammatory cytokines Some (NLRs and ALRs) can clump together to form an ‘inflammasome’ resulting in the production of pro-inflammatory cytokines such as Interleukin-1 and 18 (IL-1 and IL-18) – this can result in a special form of ‘self-death’ called pyroptosis Indirect recognition - opsonisation After recognition, attachment may be enhanced by OPSONINS such as IgG (antibody), fragments of Complement & lectins An opsonin ‘coats’ or ‘decorates’ microbe/cell Acute phase proteins (C reactive protein-CRP) have structure similar to Complement component 1 & activate Classical Complement pathway by attaching to microorganisms Antibody, complement & lectin receptors on phagocytes mediate uptake through binding to the opsonins Receptors for opsonins Antibody (Fc) receptors Recognise the constant region (i.e. not antigen binding) of antibodies The family of FcgR particularly important – recognise IgG Different varieties expressed on different phagocytes Complement receptors Bind to components of the classical, alternative and lectin complement pathways CR1, CR2, CR3, CR4, C3a/4a and C5aR are expressed by a variety of phagocytes Main ones involved in phagocytosis are CR1, 3 and 4 Summary of phagocyte recognition Direct Antibody Indirect