PATHOGEN RECOGNITION AND PATHOGEN RECOGNITION AND INFLAMMATOY SIGNALING IN INFLAMMATOY SIGNALING IN INNATE IMMUNE RESPONSE INNATE IMMUNE RESPONSE

What is the Innate Immune What is the Innate Immune Response?Response?

A universal and evolutionarily conserved mechanism A universal and evolutionarily conserved mechanism of host defense against infectionof host defense against infection

First line of DefenseFirst line of Defense Predates the adaptive immune responsePredates the adaptive immune response

Found in all multicellular organismsFound in all multicellular organisms Adaptive only in vertebratesAdaptive only in vertebrates

Uses receptors and effectors that are ancient in Uses receptors and effectors that are ancient in their lineagetheir lineage

Must provide protection against a wide variety of Must provide protection against a wide variety of pathogenspathogens

Distinguishes self from non-self perfectlyDistinguishes self from non-self perfectly Defects in innate immunity are very rare and almost Defects in innate immunity are very rare and almost

always lethalalways lethal

PAMPs: Pathogen Associated Molecular Patterns

PRRs: Pattern Recognition Receptors

The Innate Immune Response:The Innate Immune Response:Common MisconceptionsCommon Misconceptions

The innate immune system is an evolutionary rudiment The innate immune system is an evolutionary rudiment whose only function is to contain the infection until the whose only function is to contain the infection until the “real” immune response can kick in.“real” immune response can kick in.

Adaptive immunity developed because of the inflexibility Adaptive immunity developed because of the inflexibility of the nonclonal receptors used by the innate immune of the nonclonal receptors used by the innate immune response. The innate system cannot cope with the high response. The innate system cannot cope with the high mutational rate and heterogeneity of pathogenic mutational rate and heterogeneity of pathogenic organisms.organisms.

The Innate immune system instructs the adaptive The Innate immune system instructs the adaptive immune response to respond to microbial infectionimmune response to respond to microbial infection

The major decision to respond or not respond to The major decision to respond or not respond to a particular ligand is decided by the genome-a particular ligand is decided by the genome-

encoded receptors of the innate immune systemencoded receptors of the innate immune system

Innate immune recognition of Innate immune recognition of bacterial cell wall componentsbacterial cell wall components

Gram-negative bacteria Gram-positive bacteria

PRRs common PRRs common characteristicscharacteristics

1.R1.Recognize microbial components, known as ecognize microbial components, known as pathogen associated molecular patterns pathogen associated molecular patterns (PAMPs), that are essential for the survival of (PAMPs), that are essential for the survival of the microorganism and are therefore difficult the microorganism and are therefore difficult for the microorganism to alter.for the microorganism to alter.

2.Expressed constitutively in the host and detect 2.Expressed constitutively in the host and detect the pathogens regardless of their life-cycle the pathogens regardless of their life-cycle stage. stage.

3.Germline encoded, nonclonal, expressed on all 3.Germline encoded, nonclonal, expressed on all cells of a given type, and independent of cells of a given type, and independent of immunologic memory.immunologic memory.

Y

Endosome

YNaive T Cell

MHC B7

Inflammatory and effectorcytokines

PRR

PAMP

Activated T Cell

CD40L, FasL, CD30L, CD27L

B Cell

Adapted from Medzhitov and JanewayCur. Opin. Immunol. 1997 9:4-9

Phagocytosis

APC

Direct Bactericidal ActivityPhagocytosisOxygen burstAnti-microbial peptides

Pathogen-specific Antibody

Complement

Machineries underlying innate immune Machineries underlying innate immune recognition are recognition are highly conserved highly conserved among species, from plants and fruit among species, from plants and fruit flies to mammals.flies to mammals.

Recognition mechanisms of innate Recognition mechanisms of innate immunity (concepts)immunity (concepts)

What mediates the recognition of PAMPs?What mediates the recognition of PAMPs? Diverse recognition elements; 4 key families of “Pattern Diverse recognition elements; 4 key families of “Pattern

Recognition Receptors”:Recognition Receptors”: Toll-like receptorsToll-like receptors (TLRs; transmembrane receptors) (TLRs; transmembrane receptors) RigI-like receptorsRigI-like receptors (RLRs; cytoplasmic RNA helicases) (RLRs; cytoplasmic RNA helicases) NOD-like receptorsNOD-like receptors (NLRs; cytoplasmic sensors) (NLRs; cytoplasmic sensors) C-type lectin receptorsC-type lectin receptors (CLRs; transmembrane receptors) (CLRs; transmembrane receptors)

Also, recognition of molecules released from necrotic cells, Also, recognition of molecules released from necrotic cells, tissue damage (“damage associated molecular patterns” tissue damage (“damage associated molecular patterns” DAMPs or “danger”; recognized by some NLRs, TLRs, CLRs)DAMPs or “danger”; recognized by some NLRs, TLRs, CLRs)

New hypothesis: recognition of perturbations induced by New hypothesis: recognition of perturbations induced by pathogens (“patterns of pathogenicity”) such as bacterial pathogens (“patterns of pathogenicity”) such as bacterial pore-forming toxins, perturbations of the cytoskeleton, pore-forming toxins, perturbations of the cytoskeleton, various types of cell stress etc.)--recognition mechanisms various types of cell stress etc.)--recognition mechanisms less well understood (inflammasome, etc.)less well understood (inflammasome, etc.)

(Lectin: a protein that binds to carbohydrates)(Lectin: a protein that binds to carbohydrates)

TLRS-DISCOVERYTLRS-DISCOVERY

Toll, the founding member of the TLR Toll, the founding member of the TLR family, was initially identified as a gene family, was initially identified as a gene product essential for the development of product essential for the development of embryonic dorsoventral polarity in embryonic dorsoventral polarity in Drosophila.Drosophila.

Later, it was also shown to play a critical Later, it was also shown to play a critical role in the antifungal response of flies role in the antifungal response of flies (Lemaitre et al., 1996). (Lemaitre et al., 1996).

To date, 12 members of the TLR family To date, 12 members of the TLR family have been identified in mammals.have been identified in mammals.

Discovery of the mammalian Toll-Discovery of the mammalian Toll-like receptors (TLR):like receptors (TLR):

1997: Janeway and Medzhitov discovered a human 1997: Janeway and Medzhitov discovered a human protein with structural similarity to drosophila Toll protein with structural similarity to drosophila Toll that could activate immune response genes human that could activate immune response genes human cells (TLR4).cells (TLR4).

1998: Beutler discovered that a mouse strain with 1998: Beutler discovered that a mouse strain with an altered response to bacterial lipopolysaccharide an altered response to bacterial lipopolysaccharide (called LPS or endotoxin) was due to a mutation in (called LPS or endotoxin) was due to a mutation in the TLR4 gene. the TLR4 gene.

There are 11 TLR family members in human and 12 There are 11 TLR family members in human and 12 in mice. Each responds to a distinct set of in mice. Each responds to a distinct set of microbial products.microbial products.

STRUCTURESTRUCTURE

TLRs are type I integral membrane TLRs are type I integral membrane glycoproteins characterized -glycoproteins characterized -extracellular domains containing varying extracellular domains containing varying numbers of leucine-rich-repeat (LRR) numbers of leucine-rich-repeat (LRR) motifs motifs

a cytoplasmic signaling domain a cytoplasmic signaling domain homologous to that of the interleukin 1 homologous to that of the interleukin 1 receptor (IL-1R), termed the Toll/IL-1R receptor (IL-1R), termed the Toll/IL-1R homology (TIR) domain homology (TIR) domain (Bowie and (Bowie and O’Neill, 2000).O’Neill, 2000).

The LRR domains are composed of The LRR domains are composed of 19–25 tandem LRR motifs, each of 19–25 tandem LRR motifs, each of which is 24–29 amino acids in length, which is 24–29 amino acids in length, containing the motif XLXXLXLXX as containing the motif XLXXLXLXX as well as other conserved amino acid well as other conserved amino acid residues.residues.

Each LRR consists of a Each LRR consists of a ββ strand and strand and an an αα helix connected by loops. helix connected by loops.

Toll-Like ReceptorsToll-Like Receptors

SUBFAMILIESSUBFAMILIES

Based on their primary sequences, Based on their primary sequences, subfamily of TLR1, TLR2, and TLR6-subfamily of TLR1, TLR2, and TLR6-

recognizes lipids,recognizes lipids, TLR7,TLR8, and TLR9 -recognize nucleic TLR7,TLR8, and TLR9 -recognize nucleic

acids . acids . However, the TLRs are unusual in that However, the TLRs are unusual in that

some can recognize several structurally some can recognize several structurally unrelated ligands. unrelated ligands.

Expressed onExpressed on

Various immune cells- macrophages, Various immune cells- macrophages, dendritic cells (DCs), B cells, specific dendritic cells (DCs), B cells, specific types of T cellstypes of T cells

Nonimmune cells - fibroblasts and Nonimmune cells - fibroblasts and epithelial cells. epithelial cells.

Expression is not static but rather is Expression is not static but rather is modulated rapidly in response to modulated rapidly in response to pathogens, a variety of cytokines, pathogens, a variety of cytokines, and environmental stresses.and environmental stresses.

TLRs may be expressed -TLRs may be expressed - Extracellularly on the cell surface -Extracellularly on the cell surface -

TLRs 1, 2, 4, 5, and 6 are TLRs 1, 2, 4, 5, and 6 are Intracellularly-TLRs 3, 7, 8, and 9 Intracellularly-TLRs 3, 7, 8, and 9

almost exclusively in intracellular almost exclusively in intracellular compartments such as endosomes, compartments such as endosomes, and their ligands, mainly nucleic acids, and their ligands, mainly nucleic acids, require internalization to the require internalization to the endosome before signaling is possible.endosome before signaling is possible.

Insert Fig 3-11Insert Fig 3-11

Different mammalian Different mammalian TToll-oll-llike ike rreceptors eceptors (TLRs) are specific for different classes of (TLRs) are specific for different classes of

microbial productsmicrobial products

Toll-like receptors and Toll-like receptors and recognition of pathogensrecognition of pathogens

K. Takeda & S. Akira, Cell. Microbiol. 5: 143-53, 2003

LRR extracellular domainTIR domain inside

Viral ssRNA

TLR1, TLR2 and TLR6TLR1, TLR2 and TLR6

TLR2 recognizes- lipoproteins/lipopeptides TLR2 recognizes- lipoproteins/lipopeptides from various pathogens, peptidoglycan and from various pathogens, peptidoglycan and lipoteichoic acid from Gram-positive lipoteichoic acid from Gram-positive bacteria, lipoarabinomannan from bacteria, lipoarabinomannan from mycobacteria, glycosylphosphatidylinositol mycobacteria, glycosylphosphatidylinositol anchors from Trypanosoma cruzi, a phenol-anchors from Trypanosoma cruzi, a phenol-soluble modulin from Staphylococcus soluble modulin from Staphylococcus epidermis, zymosan from fungi and epidermis, zymosan from fungi and glycolipids from Treponema maltophilum .glycolipids from Treponema maltophilum .

Also recognises-Also recognises- LPS preparations from LPS preparations from non-enterobacteria such as Leptospira non-enterobacteria such as Leptospira interrogans, Porphyromonas gingivalis interrogans, Porphyromonas gingivalis and and Helicobacter pyloriHelicobacter pylori . .

TLR2 recognizes a wide range of TLR2 recognizes a wide range of microbial products through functional microbial products through functional cooperation with several proteins that cooperation with several proteins that are either structurally related or are either structurally related or unrelatedunrelated..

TLR3TLR3

TLR3 is implicated in the recognition TLR3 is implicated in the recognition of dsRNA and viruses. of dsRNA and viruses.

However, TLR3-independent However, TLR3-independent mechanisms of dsRNA recognition mechanisms of dsRNA recognition existexist..

TLR4TLR4

TLR4 is an essential receptor for LPS TLR4 is an essential receptor for LPS recognition , recognition of taxol, a recognition , recognition of taxol, a diterpene purified from the bark of the diterpene purified from the bark of the western yew (Taxus western yew (Taxus brevifolia) ,endogenous ligands, such as brevifolia) ,endogenous ligands, such as heat shock proteins (HSP60 and heat shock proteins (HSP60 and HSP70), the extra domain A of HSP70), the extra domain A of fibronectins, oligosaccharides of fibronectins, oligosaccharides of hyaluronic acid, heparan sulfate and hyaluronic acid, heparan sulfate and fibrinogen.fibrinogen.

TLR5TLR5

TLR5 recognize an evolutionarily conserved TLR5 recognize an evolutionarily conserved domain of flagellin. domain of flagellin.

TLR5 expression is also observed in the TLR5 expression is also observed in the intestinal endothelial cells of the subepithelial intestinal endothelial cells of the subepithelial compartment. compartment.

In addition, flagellin activates lung epithelial In addition, flagellin activates lung epithelial cells to induce inflammatory cytokine cells to induce inflammatory cytokine production. production.

These findings indicate the important role of These findings indicate the important role of TLR5 in microbial recognition at the mucosal TLR5 in microbial recognition at the mucosal surface. surface.

TLR9TLR9

Its stimulatory effect is due to the Its stimulatory effect is due to the presence of unmethylated CpG presence of unmethylated CpG dinucleotides in a particular base context dinucleotides in a particular base context designated CpG-DNA. designated CpG-DNA.

Although the CpG motif is abundant in Although the CpG motif is abundant in bacterial genomes, its frequency is bacterial genomes, its frequency is suppressed and it is highly methylated in suppressed and it is highly methylated in mammalian genomes. mammalian genomes.

The methylated CpG motif does not The methylated CpG motif does not activate mammalian immune cells.activate mammalian immune cells.

Toll-like Toll-like receptors receptors (TLRs) (TLRs)

link link microbial microbial products products

(PAMPs) to (PAMPs) to

transcriptitranscriptionon

factor factor activationactivation

in a in a signaling signaling pathway pathway that is that is

conserved conserved between between

mammals mammals and insectsand insects

Adaptor moleculesAdaptor molecules MyD88MyD88 TIR-associated protein (TIRAP)/MyD88-adaptor-TIR-associated protein (TIRAP)/MyD88-adaptor-

like (MAL)like (MAL) TIR-domain-containing adaptor protein- inducing TIR-domain-containing adaptor protein- inducing

IFN-IFN-ββ (TRIF)/TIR-domain-containing molecule 1 (TRIF)/TIR-domain-containing molecule 1 (TICAM1) (Oshiumi et al., 2003; Yamamoto et al., (TICAM1) (Oshiumi et al., 2003; Yamamoto et al., 2002b)2002b)

TRIF-related adaptor molecule (TRAM). TRIF-related adaptor molecule (TRAM). The differential responses mediated by distinct The differential responses mediated by distinct

TLR ligands can be explained in part by the TLR ligands can be explained in part by the selective usage of these adaptor molecules.selective usage of these adaptor molecules.

Toll-like Toll-like receptor receptor signaling signaling pathwayspathways

•Ligand induced dimerization of TLR--> induced assembly with TIR-domain containing adaptors•MyD88 pathway andTRIF pathway;•Activate Transcription factors and MAP kinases

A more detailed look at the signaling pathway A more detailed look at the signaling pathway down-stream of Toll-like Receptors (TLRs) down-stream of Toll-like Receptors (TLRs)

A more detailed look at the signaling pathway A more detailed look at the signaling pathway down-stream of Toll-like Receptors (TLRs) down-stream of Toll-like Receptors (TLRs)

Type I IFN Production via Type I IFN Production via TLRsTLRs

TRIF-Dependent Pathway- TRIF-Dependent Pathway- Stimulation with TLR3, TLR4, TLR7, Stimulation with TLR3, TLR4, TLR7, and TLR9 ligands, but not the TLR2 and TLR9 ligands, but not the TLR2 ligand, induces type I IFN production ligand, induces type I IFN production in addition to proinflammatory in addition to proinflammatory signals.signals.

Triggering of PRRs on macrophage or dendritic cells Triggering of PRRs on macrophage or dendritic cells can induce a LARGE variety of events including:can induce a LARGE variety of events including:

Increased phagocytosisIncreased phagocytosis

Production of cytokines and inflammatory Production of cytokines and inflammatory mediators:mediators:

Interferons to induce anti-viral stateInterferons to induce anti-viral state

Chemokines to attract migrating cellsChemokines to attract migrating cells

Etc, etc.Etc, etc.

Increased cell migrationIncreased cell migration

Changes in expression of molecules involved in Changes in expression of molecules involved in T cell antigen presenting cell function.T cell antigen presenting cell function.

Dying infected cell(self, no TLR signaling)

Pathogen(non-self, TLR signaling)

Material in phagosome disposed of inside cell-no presentation to T cells

Material in phagosome enters antigen presentation pathway-presentation to T cells

TLR signaling within phagosomes determines fate of that phagosome (destruction vs antigen presentation).

Blander and Medzhitov 2006 Nature v440 p808

MD-2

TLR4/4CD14TLR1/6TLR2

MyD88MyD88

IRAK IRAKTRAF6

TAK1/NIK

IKKComplex

IkBp65p50

Common and Distinct Themes in TLR SignalingCommon and Distinct Themes in TLR Signaling

TIRAP

TRIF

IFN-

TIRAP IRF3

MAP kinases

Rac

PI3K

AKT

TIR domain

Death domain

PI3K

MyD88 is used by all TLRs except TLR3; TIRAP is used by TLR2 and TLR4; TRIF is used by TLR3 and TLR4; and TRAM is used only by TLR4 .

CommonResponses

TLR4-Specific

TLR2-Specific

Pathways of NF-Pathways of NF-B B activationactivation

Canonicalpathway

Non-canonicalPathway(activated by some TNF receptor family members)

NF-B is a family of transcription factors: p50, p52, p65 (Rel-A), c-Rel, Rel-B; plus inhibitors (I-B)

The NF-kB Family of Transcription FactorsThe NF-kB Family of Transcription Factors

Eukaryotic transcription factor found in essentially all cell Eukaryotic transcription factor found in essentially all cell typestypes

First described in 1986 as a nuclear factor required for the First described in 1986 as a nuclear factor required for the transcription of the immunoglobulin kappa light chain in B transcription of the immunoglobulin kappa light chain in B cells.cells.

Binds to a 10-bp sequence GGGGYNNCCYBinds to a 10-bp sequence GGGGYNNCCY Important component in the inducible expression of many Important component in the inducible expression of many

proteins: cytokines, acute phase proteins, adhesion moleculesproteins: cytokines, acute phase proteins, adhesion molecules The NF-kB signaling system is evolutionarily conservedThe NF-kB signaling system is evolutionarily conserved

Three NF-kB molecules in Three NF-kB molecules in DrosophilaDrosophila dorsaldorsal

controls dorsal/ventral polarity during developmentcontrols dorsal/ventral polarity during development Regulates antifungal gene expressionRegulates antifungal gene expression

difdif andand relishrelish: : regulate expression of antifungal and antibacterial regulate expression of antifungal and antibacterial genesgenes

NF-NF-B exists in the cytoplasm as an inactive B exists in the cytoplasm as an inactive heterotrimer composed of 2 Rel family proteins and heterotrimer composed of 2 Rel family proteins and

an inhibitory IkB moleculean inhibitory IkB molecule

IkB

p65 p50

IKK

Stress, infection, or cytokine

P P (Ub)n

26S proteosome

NuclearTranslocation

Activation of NF-KB Responsive genes

www.stke.org/cgi/content/full/sigtrans;2003/171/re3Luke A.J. O’Neil

Cytoplasmic Pathogen Cytoplasmic Pathogen Recognition SystemRecognition System

A large family of cytoplasmic PRRs A large family of cytoplasmic PRRs has been cloned to date. Currently, has been cloned to date. Currently, they are roughly subclassified into they are roughly subclassified into the the

NOD-LRR proteins and the CARD NOD-LRR proteins and the CARD helicase proteins.helicase proteins.

These protein families are implicated These protein families are implicated in the recognition of bacterial and in the recognition of bacterial and viral components, respectively.viral components, respectively.

NOD-LRR Proteins and NOD-LRR Proteins and Their FunctionsTheir Functions

Proteins in this family possess LRRs Proteins in this family possess LRRs that mediate ligand sensing; a that mediate ligand sensing; a nucleotide binding oligomerization nucleotide binding oligomerization domain (NOD); and a domain for the domain (NOD); and a domain for the initiation of signaling, such as CARDs, initiation of signaling, such as CARDs, PYRIN, or baculovirus inhibitor of PYRIN, or baculovirus inhibitor of apoptosis repeat (BIR) domains apoptosis repeat (BIR) domains (Inohara et al., 2005; Martinon and (Inohara et al., 2005; Martinon and Tschopp, 2005).Tschopp, 2005).

NOD-Like Receptors - NOD-Like Receptors - NLRs NLRs

The cytosolic NOD-Like Receptors (NLRs, The cytosolic NOD-Like Receptors (NLRs, also known as CATERPILLERs, NODs or also known as CATERPILLERs, NODs or NALP/PAN/PYPAFs) are nucleotide-NALP/PAN/PYPAFs) are nucleotide-binding oligomerization domain binding oligomerization domain containing receptors.containing receptors.

22 NLRs have been identified in 22 NLRs have been identified in humans and constitute a major class of humans and constitute a major class of intracellular pattern recognition intracellular pattern recognition receptors (PRRs). receptors (PRRs).

The designated The designated subfamilies are subfamilies are (based on the (based on the initial of the initial of the domain name): domain name): NLRC (formely NLRC (formely known as NODs), known as NODs), NLRP (formerly NLRP (formerly known as NALPs), known as NALPs), NLRB (formely NLRB (formely known as NAIP or known as NAIP or Birc) and NLRA.Birc) and NLRA.

These proteins include NOD1 and These proteins include NOD1 and NOD2, which both contain N-terminal NOD2, which both contain N-terminal CARD domains.CARD domains.

NOD1 and NOD2 detect g-D-NOD1 and NOD2 detect g-D-glutamyl-meso-diaminopimelic acid glutamyl-meso-diaminopimelic acid (iE-DAP) and muramyl dipeptide (iE-DAP) and muramyl dipeptide (MDP), found in bacterial PG, (MDP), found in bacterial PG, respectively (Chamaillard et al., respectively (Chamaillard et al., 2003; Girardin et al., 2003).2003; Girardin et al., 2003).

Common alleles of NOD2 are a Common alleles of NOD2 are a genetic risk factor for Crohn’s genetic risk factor for Crohn’s

diseasedisease

•Several moderately common alleles of the NOD2 gene (7% of total alleles) increase susceptibility to Crohn’s disease (a form of inflammatory bowel disease)

•Two copies of these alleles increase susceptibility by 40X

•Pretty strong evidence that these alleles of are “loss of function” alleles

•NOD1/2 have been shown to have 4 immune functions: activation of inflammatory cytokine gene expression; induction of anti-microbial peptide synthesis by Paneth cells in intestines; activation of inflammasome; autophagy of bacteria in cytoplasm

Processing of IL-1 and related Processing of IL-1 and related cytokines: an important cytokines: an important

regulatory stepregulatory step•Some “NLRs” assemble to form the “inflammasome” which proteolytically processes IL-1 and related cytokines to their active, secreted forms.•Inflammasome is activated by cellular stress or recognition of microbial components in the cytoplasm•Genetic periodic fever syndromes are due to activating mutations in inflammasome•Activated by small crystals, important role in Gout•Suggestive evidence that inflammasome may be activated by cholesterol crystals (atherosclerotic lesions?); possible role in type 2 diabetes? Possible role in alzheimer’s disease?

CYTOPLASMIC BACTERIAL CYTOPLASMIC BACTERIAL DETECTERS AND SIGNALLING DETECTERS AND SIGNALLING

RNA Helicases and RNA Helicases and Double-Stranded RNADouble-Stranded RNA

Fibroblasts and cDCs lacking MyD88 and Fibroblasts and cDCs lacking MyD88 and TRIF are still capable of inducing type I IFNs TRIF are still capable of inducing type I IFNs after viral infection, indicating that the TLR after viral infection, indicating that the TLR system is not required for viral detection in system is not required for viral detection in at least several cell types (Kato et al., 2005). at least several cell types (Kato et al., 2005).

Retinoic-acid-inducible protein I (RIG-I) is an Retinoic-acid-inducible protein I (RIG-I) is an IFN inducible protein containing CARDs and IFN inducible protein containing CARDs and a DExD/H box helicase domain and has been a DExD/H box helicase domain and has been identified as a cytoplasmic dsRNA detector identified as a cytoplasmic dsRNA detector (Yoneyama et al., 2004).(Yoneyama et al., 2004).

RIG-I and MDA-5RIG-I and MDA-5

RIG-I (retinoic-acid-inducible protein 1, also RIG-I (retinoic-acid-inducible protein 1, also known as Ddx58) and MDA-5 (melanoma-known as Ddx58) and MDA-5 (melanoma-differentiation-associated gene 5, also known differentiation-associated gene 5, also known as Ifih1 or Helicard) sense double-stranded as Ifih1 or Helicard) sense double-stranded RNA (dsRNA), a replication intermediate for RNA (dsRNA), a replication intermediate for RNA viruses, leading to production of type I RNA viruses, leading to production of type I interferons (IFNs) in infected cells.interferons (IFNs) in infected cells.

In cDCs, macrophages and fibroblasts, RLRs In cDCs, macrophages and fibroblasts, RLRs are the major sensors for viral infection, while are the major sensors for viral infection, while in pDCs, TLRs play a more important role.in pDCs, TLRs play a more important role.

RIG-I participates in the recognition of RIG-I participates in the recognition of Paramyxoviruses (Newcastle disease Paramyxoviruses (Newcastle disease virus (NDV), Sendai virus (SeV)), virus (NDV), Sendai virus (SeV)), Rhabdoviruses (vesicular stomatitis virus Rhabdoviruses (vesicular stomatitis virus (VSV)), Flaviviruses (hepatitis C (HCV)) (VSV)), Flaviviruses (hepatitis C (HCV)) and Orthomyxoviruses (Influenza), and Orthomyxoviruses (Influenza), whereas MDA-5 is essential for the whereas MDA-5 is essential for the recognition of Picornaviruses (encephalo-recognition of Picornaviruses (encephalo-myocarditis virus (EMCV)) and poly(I:C), a myocarditis virus (EMCV)) and poly(I:C), a synthetic analog of viral dsRNA . synthetic analog of viral dsRNA .

Downstream of RIG-I-IPS-1,Downstream of RIG-I-IPS-1, TBK1 and IKK-i are activated to TBK1 and IKK-i are activated to

phosphorylate IRF-3 and IRF-7, phosphorylate IRF-3 and IRF-7, indicating that the signaling indicating that the signaling pathways triggered by TLR pathways triggered by TLR stimulation and RIG-I converge at the stimulation and RIG-I converge at the level of TBK1/IKK-i.level of TBK1/IKK-i.

These suggest that RNA viruses These suggest that RNA viruses actively replicating in the cytoplasm actively replicating in the cytoplasm are recognized by RIG-I, but not TLR3, are recognized by RIG-I, but not TLR3, irrespective of their route of entry. irrespective of their route of entry.

On the other hand, TLR3 has been On the other hand, TLR3 has been suggested to be responsible for the suggested to be responsible for the recognition of dsRNA contained in the recognition of dsRNA contained in the apoptotic bodies of virus-infected apoptotic bodies of virus-infected cells taken up by DCs.cells taken up by DCs.

Mechanisms of Viral Mechanisms of Viral DetectionDetection

C-Type Lectin ReceptorsC-Type Lectin Receptors

large family of receptors that bind to large family of receptors that bind to carbohydrates in a calcium-dependent carbohydrates in a calcium-dependent manner. manner. The lectin activity of these receptors is The lectin activity of these receptors is mediated by conserved carbohydrate-mediated by conserved carbohydrate-recognition domains (CRDs).recognition domains (CRDs). On the basis of their molecular On the basis of their molecular structure, two groups of membrane-structure, two groups of membrane-bound CLRs can be distinguished and a bound CLRs can be distinguished and a group of soluble CLRs.  group of soluble CLRs. 

Type I transmembrane Type I transmembrane proteinsproteins

Containing several CRDs or CRD-like Containing several CRDs or CRD-like domains domains

DEC-205 and the macrophage DEC-205 and the macrophage mannose receptor (MMR)mannose receptor (MMR)

Type II transmembrane Type II transmembrane CLRsCLRs

Carry a single CRD domain Dectin-1, Carry a single CRD domain Dectin-1, Dectin-2, macrophage-inducible C-Dectin-2, macrophage-inducible C-type lectin (Mincle), the dendritic type lectin (Mincle), the dendritic cell-specific ICAM3-grabbing cell-specific ICAM3-grabbing nonintegrin (DC-SIGN), and DC NK nonintegrin (DC-SIGN), and DC NK lectin group receptor-1 (DNGR-1). lectin group receptor-1 (DNGR-1).

 

Soluble CLRsSoluble CLRs

MBL, an oligermeric protein that binds MBL, an oligermeric protein that binds an array of carbohydrate patterns on an array of carbohydrate patterns on pathogen surfaces. pathogen surfaces.

CLRs expressed by most cell types CLRs expressed by most cell types including macrophages and dendritic including macrophages and dendritic cells (DCs), which phagocytoze cells (DCs), which phagocytoze various glycoproteins and microbes various glycoproteins and microbes for the purposes of clearance and for the purposes of clearance and antigen presentation to T lymphocytesantigen presentation to T lymphocytes

MBLMBL

MBL (Mannose-binding lectin) is a soluble C-MBL (Mannose-binding lectin) is a soluble C-type lectin. type lectin. MBL plays a crucial role in innate immunity MBL plays a crucial role in innate immunity against yeast by enhanced complement against yeast by enhanced complement activation and enhanced uptake of activation and enhanced uptake of polymorphonuclear cells.polymorphonuclear cells.

MBL binds to repetitive mannose and/or N-MBL binds to repetitive mannose and/or N-acetylglucosamine residues on acetylglucosamine residues on microorganisms, leading to opsonization and microorganisms, leading to opsonization and activation of the lectin complement activation of the lectin complement pathway.pathway.

MBL also interacts with MBL also interacts with carbohydrates on the glycoprotein carbohydrates on the glycoprotein (gp)120 of HIV-1. MBL may inhibit (gp)120 of HIV-1. MBL may inhibit DC-SIGN-mediated uptake and DC-SIGN-mediated uptake and spread of HIV.spread of HIV.

Dectin-1Dectin-1

Antifungal innate immunity. Dectin-1 Antifungal innate immunity. Dectin-1 is a specific receptor for β-glucans . is a specific receptor for β-glucans .

Dectin-1 signaling has been shown to collaborate with TLR2 signaling to enhance the responses triggered by each receptor

MincleMincle

Is a member of the Dectin-2 family .Is a member of the Dectin-2 family . Variety of exogenous and Variety of exogenous and

endogenous stimuli, such as endogenous stimuli, such as mycobacteria, certain fungi mycobacteria, certain fungi and necrotic cells and necrotic cells

DC-SIGNDC-SIGN

The recognition of several viruses (HIV-1, The recognition of several viruses (HIV-1, HCV, dengue virus, CMV, ebola virus) and HCV, dengue virus, CMV, ebola virus) and other microbes of the other microbes of the LeishmaniaLeishmania and and CandidaCandida species. species.

This type II transmembrane protein has a This type II transmembrane protein has a single C-type lectin domain and is expressed single C-type lectin domain and is expressed on immature monocyte-derived DCs. DC-on immature monocyte-derived DCs. DC-SIGN modulates TLR signaling at the level of SIGN modulates TLR signaling at the level of the transcription factor NF-κB, however, the transcription factor NF-κB, however, prior TLR activation of NF-κB is required.prior TLR activation of NF-κB is required.

DNGR-1DNGR-1

DNGR-1 binds damaged or dead cells DNGR-1 binds damaged or dead cells via exposed actin filaments . via exposed actin filaments .

DNGR-1 is therefore considered to be DNGR-1 is therefore considered to be DAMPs receptor since no microbial DAMPs receptor since no microbial ligand has yet been identifiedligand has yet been identified. .

Plasmacytoid dendritic Plasmacytoid dendritic cellscells

Many cell types produce small amounts of Many cell types produce small amounts of type 1 interferons upon infectiontype 1 interferons upon infection

There is a dendritic cell subtype There is a dendritic cell subtype (“plasmacytoid dendritic cell”; “natural (“plasmacytoid dendritic cell”; “natural interferon-producing cell”) that produces interferon-producing cell”) that produces 100-1000x more interferon upon contact 100-1000x more interferon upon contact with viruses, does not need a productive with viruses, does not need a productive infection.infection.

Also produces a large amount of TNFAlso produces a large amount of TNF Recognition mechanism: probably TLR7, Recognition mechanism: probably TLR7,

TLR9TLR9

InflammasomeInflammasome

InflammasomesInflammasomes are large intracellular are large intracellular multiprotein complexes that play a central multiprotein complexes that play a central role in innate immunity. role in innate immunity.

Inflammasomes comprise a member of the Inflammasomes comprise a member of the NOD-like receptor (NLR) family, such as NOD-like receptor (NLR) family, such as NLRP3 and IPAF, and are defined by the NLR NLRP3 and IPAF, and are defined by the NLR protein that they contain. protein that they contain.

The NLR protein recruits the inflammasome-The NLR protein recruits the inflammasome-adaptor protein ASC, which inadaptor protein ASC, which in turn interacts turn interacts with caspase-1 leading to its activation. with caspase-1 leading to its activation.

Once activated, caspase-1 promotes Once activated, caspase-1 promotes the maturation of the the maturation of the proinflammatory cytokines proinflammatory cytokines interleukin (IL)-1β and IL-18.interleukin (IL)-1β and IL-18.