Innate Immunity PDF

Innate immunity Dr Felix N. Toka Professor, Veterinary Virology & Immunology Department of Biomedical Sciences Objectives of the topic Define the innate immunity Distinguish the first and second lines of defense from the third line of defense Understand the routes of infection in which innate immuni...

Innate immunity Dr Felix N. Toka Professor, Veterinary Virology & Immunology Department of Biomedical Sciences Objectives of the topic Define the innate immunity Distinguish the first and second lines of defense from the third line of defense Understand the routes of infection in which innate immunity is primarily induced Understand the barriers of innate immunity and their functional components Understand the role and process of phagocytosis in innate immunity Understand the role of PAMPs and DAMPs in induction of innate immunity Organization of the immune system The Immune System Innate immunity First line of defense (Non-specific) Cellular component Humoral component Adaptive immunity Second line of defense (Specific) Humoral component Cellular component What is innate immunity? Natural or native immunity Present from birth designed to protect the body from injury or infection without prior contact with the infectious agent What is the difference between innate immunity and adaptive immunity? Innate immunity (Non-specific immunity) Adaptive immunity (Specific immunity) Immune response is not dependent on the antigen (non-antigen specific) Immune response is antigen-specific Immune response is immediate (rapid) Immune response does not leave any memory (exceptions exist) Immune response is delayed in time Immune response leaves memory Infection and immune response SARS-CoV-2 CoVID-19 Brucellosis. Equine Herpes Virus - 1 (EVH-1) Infection and immune response Mechanism of infection Phases of initial response to infection Three major lines of defense Innate immunity Adaptive immunity Innate Immunity Physical barriers (skin, mucus membranes) Humoral barriers (Complement system ) Cellular barriers (Phagocytic system, NK cells) Physical barriers – mechanical, chemical, microbiological Mechanical factors Skin - hair coat, self-cleansing by rapid restoration of keratinized epidermal cells Ciliary movement in epithelium of the respiratory tract Peristaltic movement in the intestinal tract Skin - approx. 2m2 (human) Washing effect by tears or saliva Dense mucus layer in the vagina, digestive tract and respiratory tract All these mechanisms defend the body by mechanically removing infectious microorganisms and preventing entry Chemical factors Fatty acids – mainly in sweat, inhibit growth of bacteria Lysozyme and phospholipase – in tears, saliva, secretions of nasal tract inhibit growth of infectious agents Low pH of sweat and gastric juices has antibacterial effects Surfactants such as opsonins in lungs enhance phagocytosis Microbiological factors Normal biota on the skin and digestive tract prevents infection by secreting inhibitory substances that inhibit colonization and growth of infectious microorganisms Mechanical, chemical and microbiological factors in innate immunity Major non specific mechanisms involved in protection of the host and active during pathogen elimination Humoral barrier Breaching the physical barrier leads to penetration of infectious agents and development of inflammation The most important humoral factors of the innate immunity are: Complement system – a group of approx. 30 proteins found in serum that cooperate to prevent infection Coagulative system depending on the extent of damage it may be activated or not activation leads to blood coagulation at the site of damage preventing entry of infectious agents some molecules of the coagulative system may act as chemotactic factors attracting other cells to the site of damage beta-lysine produced by platelets has bactericidal effects against G+ bacteria during the coagulation process Lactoferrin and transferrin – bind (sequester) iron – bacteria cannot grow in the absence of iron Lysozyme – digests the bacterial cell wall Interferons – e.g., type I interferons inhibit infection and replication of viruses Interleukin 1 – responsible for increase in temperature during inflammation and induces acute phase proteins which are bactericidal Cellular barriers Neutrophils – belong to polymorphonuclear cells (PMN) – phagocytose microorganisms Macrophages – differentiate from monocytes and function as phagocytes – ingest and kill microorganisms intracellularly also, may phagocytose and kill infected cells may function as antigen presenting cells participate in wound healing NK cells and – kill infected or tumour cells Eosinophils – participate in eliminating parasites The most important cells of the innate immunity Phagocytosis and intracellular killing Two vital cells of the phagocytic system: neutrophils and macrophages Neutrophils Neutrophilic components in neutrophils stain a neutral pink The most numerous innate immune cell (50-70% of all white blood cells) Immature neutrophils have a distinct band-shaped nucleus which changes into a segmented nucleus following maturation They circulate in the bloodstream looking for foreign objects to phagocytose (ingest) and degrade (e.g., bacteria) Neutrophils - identified by expression of CD66 on their cell surface They contain two types of granules which store molecules required for the intracellular killing process 1. Azurophilic (primary) granules contain: defensins which kill bacteria proteolytic enzymes such as elastase, cathepsin G which degrade bacterial proteins lysozyme which degrades the bacterial cell wall myeloperoxidase – required for the generation of bactericidal substances 2. Secondary granules – found only in mature PMN cells, contain: lysozyme lactoferrin components of NADPH oxidase (nicotinamide adenine dinucleotide phosphate-oxidase) for production of toxic radicals Monocytes/macrophages They are found in the bloodstream and in tissues Monocytes circulate in the peripheral blood before entering tissues to replenish tissue-specific macrophage populations (i.e. osteoclasts, microglia cells, histiocytes, and Kupffer cells) Macrophage, "big eater" in Greek, named for their ability to ingest and degrade bacteria Macrophages – identified by expression of CD14, CD11b or F4/80 They do not posses granules but have lysosomes that contain factors required for intracellular killing mechanisms Macrophages react to danger signals (SOS) generated at sites of pathogen entry e.g., N-formyl-methionine – secreted by bacteria Peptides of coagulative system Complement system components Cytokines secreted by tissue macrophages (at portals of entry) These signals induce chemotaxis of macrophages towards the site of microbe entry Initiation of phagocytosis Phagocytes posses a variety of receptors which they use to bind microorganisms 1. Complement receptors – phagocytes posses receptors for C3b complement component C3b binds the antigen e.g., bacterial antigen, later it binds to its receptor on the phagocyte – this signal leads to activation of phagocytosis 2. Scavenger receptors - such as SRA, CD68, Lox-1 or CD36 on macrophages, directly bind various polyamines found on bacterial surfaces initiating phagocytosis 3. Fc receptors – found on macrophages Antibodies bound to antigens expose their Fc region Fc regions of an antibody are used to bind the Fc receptors on phagocytes Phagocyte binding of the Fc region of an antibody that is bound to the surface of a microorganism enhances the metabolic activity of the phagocyte Role of Fc receptor in phagocytosis This microbe detection pathway is used only when antibodies are available e.g., recurrent infection 4. Toll-like receptors (TLR) – phagocytes have a series of such receptors TLR recognize Pathogen Associated Molecular Patterns (PAMPs) When macrophages bind antigen through their TLR they become activated and secrete cytokines such as IL-1, TNF and IL-6 – in preparation for an inflammatory reaction What is phagocytosis? It is an active process of capturing and ingesting foreign objects/microorganisms by phagocytes What is the purpose of phagocytosis? To detect and destroy microorganisms, to remove damaged cells and foreign objects To induce production of cytokines required for development of an inflammatory reaction To process and present antigens required to induce an immune response by lymphocytes Phagocytosis – the process N-formyl-methionine – e.g., bacteria Peptides of coagulative system Complement system components Cytokines/Chemokines 1. Chemotaxis – first, phagocytes move towards objects to be phagocytosed 2. Phagocytes detect and bind their target/object through appropriate receptors 3. They surround the captured object (e.g., a bacterial cell) with pseudopodia and engulf the object through endocytosis 4. The endocytosed object becomes enclosed in the phagosome 5. Later, the phagosome fuses with the lysosome to form a phagolysosome 6. The contents of the lysosome are released into the phagolysosome 7. The digestion of endocytosed objects begins Phagocytosis Intracellular killing In neutrophils, monocytes and macrophages two killing pathways are established 1. Oxidative pathway – dependent on generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) 2. Non-oxidative pathway – dependent on lysosomal toxic substances Oxidative pathway During phagocytosis the use of oxygen and glucose increases severalfold - a process called „respiratory burst” This leads to formation of reactive oxygen species which are toxic to microorganisms This killing process is referred to as oxygen dependent intracellular killing Generation of reactive oxygen species (ROS) 1. Glucose is metabolized through the pentose-phosphate pathway – leading to production of NADPH 2. Cytochrome oxidase activates NADPH 3. Activated NADPH uses O2, leading to production of superoxide anion (·O2-) 4. The superoxide anion may be reduced to H2O2 and 1O2 by the superoxide dismutase 5. Or the superoxide anion may react with H2O2, leading to production of hydroxyl radicals (·OH) and ions (OH-) Generation of reactive oxygen species (ROS) Glucose + NADP+ G-6-P-dehydrogenase NADPH + O2 Cytochrome oxidase 2O2- + 2H+ Superoxide dismutase 2O2- + H2O2 Pentose-P + NADPH NADP+ + O2- H2O2 + 1O2 OH + OH- + 1O2 Reactive Oxygen Species Myeloperoxidase dependent intracellular killing Myeloperoxidase is released during the fusion of azurophilic granules with the phagosome Myeloperoxidase uses H2O2 and Cl- to produce hypochlorous acid H2O2 + ClMyeloperoxidase HClO + OH Generation of reactive nitrogen species (RNS) The superoxide anion can interact with a reactive nitrogen species, nitric oxide, to produce peroxynitrite, another RNS Nitric oxide can also undergo oxidation to generate the RNS nitrogen dioxide Antimicrobial action of nitric oxide When phagocytes bind bacteria through TLR, among other cytokines, TNFα is secreted which later induces expression of inducible nitric oxide synthetase (iNOS) iNOS oxidises L-arginine to yield L-citrulline and nitric oxide (NO) NO is highly toxic to microorganisms in the vicinity of phagocytes IFNγ also induces iNOS L-arginine + O2 + NADPH iNOS NO + L-citrulline + NADPH Interaction of ROS and RNS Non-oxidative intracellular killing pathway The main killing mechanism is dependent on the action of the toxic substances (peptides, proteins, enzymes) present in lysosomes Mechanisms Cationic proteins – damage the bacterial cell wall Lysozyme – damages the mucopeptides in the bacterial cell wall Lactoferrin – sequestrates iron thus inhibiting bacterial growth Proteolytic and hydrolytic enzymes – digest killed bacteria Pathogen recognition by cells of the innate immunity Host sentinel cells, mainly macrophages, dendritic cells and mast cells, use PRRs to sense the presence of PAMPs or DAMPs Pattern recognition receptor (PRR) - (receptors that recognize PAMPs & DAMPs). Natural or innate immune system receptors that recognize molecular patterns produced by microorganism/damaged own cells Pathogen-Associated Molecular Pattern (PAMP) Repetitive motifs of molecules such as lipopolysaccharide, peptidoglycan, lipoteichoic acids, mannan broadly expressed by microbial pathogens and not found on host tissues Damage-associated molecular patterns (DAMPs) Endogenous molecules released from damaged cells Pattern recognition receptors There are at least 5 different classes of PRRs Toll-like receptors (TLRs) NOD-like receptors (NLRs) RIG-like receptors (RLRs) C-type lectin receptors (CLRs) Peptidoglycan-recognition proteins (PGPs) Toll-like receptors (TLR) are major PRRs located on host cell membranes or within the host cells that signal the presence of invaders in innate immune response History The Toll gene was discovered in late 1990s from Drosophila melanogaster Toll is transmembrane receptor required for establishment of proper dorsoventral polarity during embryo formation in Drosophila Toll is also responsible for production of Drosomycin an antifungal peptide Toll homologue in mammals was revealed as Toll-like receptor Toll-like receptors Toll-like receptors and their ligands (PAMPs) Signalling by TLRs Signalling through MyD88 is central to all TLRs except TLR3 which signals through TRIF The endpoint of signalling is production of proinflammatory cytokines NOD-like receptors (NLR) Nucleotide-binding Oligomerization Domain-like receptor Structurally are related to TLRs Found inside the cells Can detect pathogens in the cytoplasm There are least 22 human and at least 34 mouse NLR proteins NOD1 recognizes bacterial peptidoglycans NOD2 recognizes muramyl dipeptides Generally, they sense intracellular bacteria Detection of PAMPs by NOD-like receptors leads to activation of the transcription factor NF-κB Activation of NF-κB leads to transcription of genes responsible for expression of pro-inflammatory cytokines NOD-like receptors also signal through IRF3/7 leading to production of type I interferons (IFNs) NOD-like receptors can be activated by non-microbial danger signals released from damaged cells (DAMPs) RIG-like receptors (RLR) Retinoic acid inducible gene-like receptor Expressed in the cytoplasm Detect viral RNA Induce production of antiviral cytokines such as IFNs and inflammatory cytokines C-type lectin receptors (CLRs) comprise a large family of receptors that bind to carbohydrates in a calcium-dependent manner Are involved in fungal recognition and modulation of the innate immune response CLRs are expressed by most cell types including macrophages and dendritic cells (DCs) Peptidoglycan-recognition proteins (PGRPs) Peptidoglycans are polymers of alternating N-acetylglucosamine and N-acetyl muraminic acid found on the surface of both G- and G+ bacteria PGRPs are localized in the large granules of neutrophils Detection of the microbial peptidoglycan by PGRPs induces production of antimicrobial peptides such as defensins In pigs PGRPs are expressed constitutively in the skin, bone marrow and intestines PGRPs may also bind LPS and lipoteichoic acid Antimicrobial peptides Peptide Defensin family α-defensin Typical producer Typical antimicrobial activity Human (found in paneth cells of intestine and cytoplasmic granules of neutrophils) Human (found in epithelia and other tissues) Antibacterial Cathelicidins Magainins Cercropins Human, bovine Frog Silk moth Antibacterial Antibacterial, antifungal Antibacterial Drosomycin Spinigerin Fruit fly Termite Antifungal Antibacterial, antifungal β-defensins antibacterial