Innate Immunity Hathairat Thananchai, DPhil Department of Microbiology Faculty of Medicine Chiang Mai University 25 July 2017
Objectives: Explain how innate immune system recognizes foreign substances Explain components of the innate immune system, and their functions Explain functions of the innate immune system
Lecture outline Overview of innate immunity Innate recognition Cell-associated receptors Soluble molecules Cellular and soluble components of innate immunity Innate immune responses - Inflammatory response - antiviral response Stimulation of adaptive immunity
Innate and adaptive immunity The mechanisms of innate immunity provide the initial defense against infection. Adaptive immune responses develop later and consist of activation of lymphocytes.
Innate immunity Innate immunity serves three important functions Innate immunity is initial response to microbes that prevents, controls, or eliminate infection of the host by many microbes Inflammation Antiviral defense Innate immune mechanisms recognize the products of damaged and dead host cells and serve to eliminate these cells and to initiate the process of tissue repair Innate immunity to microbes stimulates adaptive immune responses and can influence the nature of the adaptive responses to make them optimally effective against different types of microbe
Features of Innate immunity Provides the early line of defense against foreign substances Not specific to particular microbe do express surface receptors that distinguish host cells from those of infectious agents do not discriminate between various infectious agents Non-adaptive the quality of the reaction to a foreign substance does not change when the organism encounters this substance repeatedly
Recognition mechanisms of innate immunity The microbial substances that stimulate innate immunity are called pathogen-associated molecular patterns (PAMPs) Molecules recognized tend to be structural elements that are common to broad classes of microbes and are very hard to change. The innate immune system also recognizes endogenous molecules called damage-associated molecular patterns (DAMPs) The receptors that bind to PAMPs or DAMPs are called pattern recognition receptors (PRRs)
Innate immune recognition of bacterial cell wall components
DAMPs Yonsei Med J 2014;55(5):1165-1176
Examples of PAMPs and DAMPs
Specificity of innate and adaptive immunity
Pattern Recognition Molecules of the Innate Immune System Cell-associated molecules Cell membrane Cytoplasm Endosome Soluble molecules Blood Extracellular fluids Recruitment and activation of protein kinases Activation of transcription factors Gene transcription Innate cytokines include - TNF - IL-1, IL-6, IL-10, IL-12 - Type I interferons
Cell-associated PRRs
Pattern Recognition Molecules of the Innate Immune System
Toll-like receptors (TLRs) A family of conserved cellular receptors that mediated cellular responses to PAMPs and DAMPs TLRs activation is essential for provoking the innate response and enhancing adaptive immunity against pathogens There are 9 different functional TLRs in humans, named TLR1 through TLR9 TIR; TLR/IL-1 receptor
TLR signaling All TLRs except TLR3 signal through adaptor protein MyD88 TLR3 signals through TRIF TLR4 signals through both MyD88 and TRIF A downstream effect of TLR signaling through MyD88 is the activation of the transcription factor NF-КB A downstream effect of TLR signaling through TRIF is the activation of IRF-3 and -7 MyD88; myeloid differentiation primary response gene 88 TIRF; TIR domain-containing adaptor inducing IFN-β IRF; interferon response factor
chemoattractants
Cytosolic pattern recognition molecules
NOD-Like Receptors NOD-like receptors (NLRs) are a family of more than 20 different cytosolic proteins, some of which recognize PAMPs and DAMPs and recruit other proteins to form signaling complexes that promote inflammation. NOD; nucleotide oligomerization domain
NOD1 and NOD2 NOD1 & NOD2 recognize peptidoglycan substructures and promote innate immune responses NOD1 diaminopimelic acid (DAP) NOD2 muramyl dipeptide Induce the production of proinflammatory cytokines (IL-1β and IL-18) Front Immunol 2012;3:249
NLRP family (inflammasomes) NLRPs are subfamily of NOD-like receptors NLRPs respond to cytosolic PAMPs and DAMPs, or changes in the cell indicating the presence of infection or damage by forming signaling complexes called inflammasomes Activate Caspase-1 Processes: - pro-il-1β active IL-1β - pro-il-18 active IL-18 Front. Immunol. 2014 http://dx.doi.org/10.3389/fimmu.2014.00327 24
NLRP3 inflammasome NLRP3 senses PAMPs and DAMPs Uric acid crystals Silica Aluminum hydroxide crystals Bacterial toxin produced by streptococci and staphylococci Bacterial DNA-RNA hybrids Extracellular ATP Reactive oxygen species (ROS)
RIG-Like Receptors RIG-like receptors (RLRs) are cytosolic sensors of viral RNA that respond to viral nucleic acid by inducing the production of the antiviral type I interferons Plasma membrane The two best characterized RLRs are RIG-1 MDA5 On binding viral RNA, the RLRs initiate signaling events that lead to phosphorylation and activation of IRF3 and IRF7, as well as NF-κB
Cytosolic DNA Sensors (CDSs) The STING (Stimulator of IFN Genes) pathway is a major mechanism of DNAinduced activation of type I interferon responses STING is an endoplasmic reticulumlocalized transmembrane protein Cytosolic DNA binds to an enzyme called cyclic GMP-AMP synthase (cgas) that synthesizes a cyclic dinucleotide called cyclic GMP-AMP (cgamp) cgamp interacts with and stimulates STING The bacterial second messenger molecules cyclic di-gmp (c-di-gmp) and cyclic di-amp (c-di-amp) directly sensed by STING.
The differential expression of PRRs by some immune cells
The components of the innate immune system Physical and chemical barriers Skin and mucosal surface Cellular components Phagocytes (neutrophils, macrophages) Dendritic cells NK cells and other innate lymphoid cells Innate-like lymphocytes Mast cells Circulating proteins Complement system Antimicrobial peptides Cytokines : inflammation (TNF, IL-1) chemokines (IL-8, MCP-1) anti-viral (type I interferons)
Exterior defenses Most infectious agents are prevented from entering the body by physical and biochemical barriers. The body tolerates a number of commensal organisms, which compete effectively with many potential pathogens.
Epithelial defenses
Antimicrobial peptides Short, cationic peptides (most 29-35 amino acids long) Made by neutrophils and epithelial cells (small intestines, respiratory tract, genitourinary tract) In human, three major groups of these antimicrobial peptides are recognized : - -defensins - -defensins - cathelicidins Differentially active against different microorganisms
Biological roles of host defense peptide Nature Biotechnology 2006;24:1551-1557. Pathogens and Disease 2014;70:257-270.
Cellular components of the innate immune system
Phagocytes Neutrophil Mononuclear phagocyte Dendritic cell - Phagocytosis - Cytokine production : Pro-inflammatory cytokines (TNF, IL-1)
Phases of phagocytosis
Phagocytosis and intracellular destruction of microbes NADPH oxidase
Killing of phagocytosed microbes Reactive oxygen species (ROS) - Phagocyte oxidase (=NADPH oxidase) superoxide anion (O 2- ) - Superoxide dismutase H 2 O 2 - Myeloperoxidase hypohalous acid The process by which ROS are produced is called the respiratory burst Reactive nitrogen intermediates, mainly nitric oxide (NO) - Inducible Nitric oxide synthase (inos) Chronic granulomatous disease (CGD): genetic defect in phagocyte oxidase components (most commonly gp91, which is X-linked)
Antibody-mediated opsonization and phagocytosis of microbes opsonin
Neutrophils The most abundant population of circulating white blood cells and mediate the earliest phases of inflammatory responses. The nucleus of a neutrophil is segmented into 3-5 connected lobules. The cytoplasm contains granules of two types. - Specific granules are filled with enzymes such as lysozyme, NADPH oxidase - Azurophilic granules are lysosomes containing enzymes (i.e.myeloperoxidase) and other microbicidal substances (i.e.defensins)
Neutrophil extracellular traps (NETs) During infections, some activated neutrophils undergo a unique form of cell death in which the nuclear chromatin is released into the extracellular space and forms a fibril matrix known as neutrophil extracellular traps (NETs). NETs can trap bacteria and fungi Scanning electron micrograph of activated neutrophil infected with a virulent strain of Shigella flexneri (pink rods) shows the stimulated neutrophils forming NETs (blue, indicated by arrows)
Neutrophil extracellular traps (NETs) are composed of nucleosomes decorated with granular components including myeloperoxidase (MPO), neutrophil elastase, and cathepsin G. NETs bind and kill microbes. Blood 2012 120:1152-1154
Mononuclear phagocyte system The mononuclear phagocyte system includes circulating cells called monocytes and tissue resident cells called macrophages Maturation of mononuclear phagocytes
Effector functions of macrophages
Dendritic cells A heterogeneous family of cells with long dendrite-like cytoplasmic processes Constitutively present in lymphoid tissues, mucosal epithelium, and organ parenchyma Dendritic cells express more different types of TLRs and cytoplasmic PRRs than any other cell types Dendritic cells serve a critical function in adaptive immune responses by capturing and displaying microbial antigen to T lymphocytes. TLR signaling induces dendritic cell expression of costimulators and cytokines that are needed for the activation of naïve T cells and their differentiation to effector T cells Antigen presenting cells
Natural killer (NK) cells A lineage of cells related to lymphocytes that recognize infected and/or stressed cells Kill various target cells without a need for additional activation NK cells are a major source of IFN-γ The expansion and activity of NK cells are also stimulated by cytokine, mainly IL-15 and IL-12 High concentration of IL-2 also stimulate the activities of NK cells http://www.ucl.ac.uk/~zchabg4/innate.htm
Functions of NK cells 1. NK cells recognize ligands on infected cells or cells undergoing other types of stress, and kill the host cells. 2. NK cells bind to antibody-coated cells by Fc receptors and destroy these cells. These process is called antibodydependent cell-mediated cytotoxicity (ADCC) 3. NK cells respond to IL-12 produced by macrophages and secrete IFN-γ, which activates the macrophages to kill phagocytosed microbes. 47
Innate lymphoid cells The three subsets of ILCs produce different sets of cytokines, participate in host defense against distinct pathogens, and different inflammatory disorders These subsets are analogous to the T H 1, T H 2 and T H 17 subsets of CD4 + T lymphocytes that secrete some of the same cytokines Group 1 ILCs produce IFN-γ include NK cells Group 2 ILCs, like TH2, secrete IL-5, IL-9 and IL-13 Group 3 ILCs are found at mucosal sites and produce IL-17 and IL-22
The three main classes of innate-like lymphocytes and their properties Vα24-Jα18 49
Mast cells present in the skin and mucosal epithelium Mast cells express high-affinity Fc receptors for IgE The granules in mast cells contain vasoactive amine (i.e. Histamine), and proteolytic enzymes that can kill bacteria or inactivate microbial toxins Mast cells also synthesize and secrete lipid mediators (i.e prostaglandins) and cytokines (i.e TNF) Mast cells express TLRs, and TLR ligands can induce mast cell degranulation Play a role in allergic reaction and helminthic infections
Soluble recognition and effector molecule of innate immunity
Complement system Consists of about 30 serum and membrane proteins that can mediate a variety of immune reactions The active components of complement are generated from inactive precursors by a cascade of proteolytic reactions These can be triggered in any of three ways: Lectin pathway Classical pathway Alternative pathway
Pathways of complement activation 54
Innate recognition by soluble collectins and ficolins Soluble proteins present at mucosal surfaces and/or in the bloodstream Recognize distinctive carbohydrate configurations that occur on the surfaces of microbes In human, three collectins and three ficolins are known to participate in immunity Collectins: - surfactant protein A (SP-A) - surfactant protein D (SP-D) - mannose-binding lectin (MBL) Ficolins: - L-, H-, and M-ficolin Functions : - opsonin - complement activation
C1q, mannose binding lectin, and ficolin can initiate complement activation on binding to their ligands on cell surfaces
Pentraxins pentameric plasma proteins belong to the pentraxin family short pentraxin - C reactive protein (CRP) - Serum amyloid P (SAP) long pentraxin - Pentraxin 3 (PTX3) recognize : microbes (bacteria and fungi) apoptotic cells functions - Opsonin - Activation of complement by binding to C1q and initiating classical pathway
Pentraxins biosynthesis Acute-phase reactants Nat Clin Pract Rheumatol 2006 2:481-90.
The Inflammatory Response
The Inflammatory Response Acute inflammation is a major way by which the innate immune system deals with infections and tissue injury Accumulation of leukocytes, plasma proteins, and fluid derived from the blood at an extracellular tissue site of infection or injury to kill microbes and begin to repair tissue damage Acute inflammation can develop in minutes to hours and last for days If the infection is not eliminated or the tissue injury is prolonged chronic inflammation Chronic inflammation sites tissue remodeling, with angiogenesis and fibrosis Adaptive immunity may also be involved because cytokines produced by T cells are powerful inducers of inflammation
Innate immune initiation of inflammation Cell 2010;140:771-776 - Vasodilation : blood flow - vascular permeability : leaks of plasma proteins (complement proteins, antibodies, acute-phase reactant) - adhesiveness of circulating leukocytes to the endothelial lining of venules : leads to accumulation of inflammatory cells
Leukocyte recruitment to sites of inflammation diapedesis
Local and systemic actions of cytokines in inflammation cachexia
Important cytokines and chemokines secreted by macrophages and dendritic cells in response to PAMPs or DAMPs Increased production of neutrophil by the bone marrow
The Antiviral Response
The Antivirus Response Type I interferon : mediate the early innate immune response to viral infection IFN-α : Plasmacytoid dendritic cell Mononuclear phagocytes IFN- : many cells such as fibroblast The most potent stimuli for type I interferon synthesis are viral nucleic acids Recognize by - RIG-Like receptors - DNA sensors in the cytosol - TLR 3, 7, 8, and 9 in endosomal vesicle 66
Biological actions of type I interferons
Biologic actions of type I IFNs Inhibits viral replication Increases expression of MHC class I molecules Stimulates the development of T H 1 cells Increases the cytolytic activity of NK cells
Stimulation of adaptive immunity
Role of innate immunity in stimulation adaptive immune responses The innate immune response provides signals that function in concert with antigen to stimulate the proliferation and differentiation of antigen-specific T and B lymphocytes The signals generated during innate immune responses to different microbes influence the nature of the adaptive response
Three kinds of signals are involved in activation of naïve T cells by antigen-presenting cells Signal 1 : antigen-specific signals derived from the interaction of a specific peptide:mhc complex with the T-cell receptor Signal 2 : the co-stimulatory signals that are primarily involved in promoting the survival and expansion of the T cells Signal 3 : delivered by antigen-presenting cell, which are primarily involved in directing T- cell differentiation into the different subsets of effector T cells
Bacterial LPS induces changes in dendritic cells, stimulating them to migrate and to initiate adaptive immunity by activating T cells Immature dendritic cells in the skin are highly phagocytic and macropinocytic, but lack the ability to activate T lymphocytes. During a bacterial infection, the dendritic cells are activated by various innate sensors and the activation induces two types of changes. - migrate out of the tissues and enter the lymphatic system and being to mature - in the regional lymph nodes, they become mature dendritic cells and change the character of their cellsurface molecules : MHC molecules expression of the co-stimulatory molecules CD80(B7.1) and CD86(B7.2)
Development of T H 1, T H 2 and T H 17 subsets T H 1 T H 2 T H 17 73
Summary The innate immune system provides the first line of host defense against microbes The innate immune system used pattern recognition receptors to recognize structures called PAMPs and DAMPs Cell-associated PRRs signal to activate the transcription factor : NF-κB, AP-1 inflammatory cytokines expression IRF transcription factors type I interferon expression Soluble PRRs and effector molecules bind microbial ligands and enhance clearance by complement-independent and complement dependent mechanisms
Summary The two major types of responses of the innate immune system that protect against microbes are inflammation and antiviral defense Molecules produced during innate immune responses stimulate adaptive immunity and influence the nature of adaptive immune responses