Innate ImmunityHathairat Thananchai, DPhil
Department of Microbiology
Faculty of Medicine
Chiang Mai University
2 August 2016
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 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
Innate immunity serves three important functions
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
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
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
Cytosolic pattern recognition molecules
• NOD-like receptors (NLRs) are a family
of more than 20 different cytosolic
proteins, some of which recognize
PAMPs and DAMPs
• 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)
NOD-Like Receptors
NOD; nucleotide oligomerization domain
RIG-Like Receptors
Plasma membrane• 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
• 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 DNA-induced activation of type I interferon responses
• STING is an endoplasmic reticulum-localized 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 differential expression of PRRs by some immune cells
The components of the innate immune system
• Physical 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 defensesMost 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
Nature Biotechnology 2006;24:1551-1557.
Biological roles of host defense peptide
Pathogens and Disease 2014;70:257-270.
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 (O2
- )
- Superoxide dismutase H2O2
- Myeloperoxidase hypohalous acid
The process by which ROS are producedis 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)
Mononuclear phagocyte system
Maturation of mononuclear phagocytes
The mononuclear phagocyte system includes circulating cells called monocytes and tissue resident cells called macrophages
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
NK cells are early component of the host response to viral infection
Functions of NK cells
•1. NK cells recognize ligands on infected cells or cells undergoing other types of stress, and kill the host cells.
The NK cells are activated and kill the antibody-coated cells.
• NK cells bind to antibody-coated cells by Fc receptors and destroy these cells. These process is called antibody-dependent cell-mediated cytotoxicity (ADCC)
Functions of NK cells
2. NK cells respond to IL-12 produced by macrophages and secrete IFN-γ, which activates the macrophages to kill phagocytosed microbes.
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 TH1, TH2 and TH17 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
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 proteolyticenzymes 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 helminthicinfections
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
opsonization
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
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- Pentexin 3 (PTX3)
• recognize : microbes (bacteria and fungi)apoptotic cells
• functions - Opsonin- Activation of complement by binding to C1q and initiating classical pathway
Pentraxins biosynthesis
Nat Clin Pract Rheumatol 2006 2:481-90.Acute-phase reactants
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
- 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
Cell 2010;140:771-776
Leukocyte recruitment to sites of inflammation
“diapedesis”
Important cytokines and chemokines secreted by macrophages and dendritic cells in response to PAMPs or DAMPs
Local and systemic actions of cytokines in inflammation
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
Mechanisms of induction of type I interferons by viruses
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 TH1 cells
• Increases the cytolytic activity of NK cells
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
Development of TH17 subset of CD4+T cells
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