Title: Intracellular NOD-like Receptors in Host Defense and Disease
Abstract: The innate immune system comprises several classes of pattern recognition receptors, including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-1-like receptors (RLRs). TLRs recognize microbes on the cell surface and in endosomes, whereas NLRs and RLRs detect microbial components in the cytosol. Here we discuss the recent understanding in NLRs. Two NLRs, NOD1 and NOD2, sense the cytosolic presence of the peptidoglycan fragments meso-DAP and muramyl dipeptide, respectively, and drive the activation of mitogen-activated protein kinase (MAPK) and the transcription factor NF-κB. A different set of NLRs induces caspase-1 activation through the assembly of large protein complexes named inflammasomes. Genetic variations in several NLR members are associated with the development of inflammatory disorders. Further understanding of NLRs should provide new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases. The innate immune system comprises several classes of pattern recognition receptors, including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-1-like receptors (RLRs). TLRs recognize microbes on the cell surface and in endosomes, whereas NLRs and RLRs detect microbial components in the cytosol. Here we discuss the recent understanding in NLRs. Two NLRs, NOD1 and NOD2, sense the cytosolic presence of the peptidoglycan fragments meso-DAP and muramyl dipeptide, respectively, and drive the activation of mitogen-activated protein kinase (MAPK) and the transcription factor NF-κB. A different set of NLRs induces caspase-1 activation through the assembly of large protein complexes named inflammasomes. Genetic variations in several NLR members are associated with the development of inflammatory disorders. Further understanding of NLRs should provide new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases. The innate immune system recognizes infections through pattern recognition receptors (PRRs) that detect conserved microbial components called pathogen-associated molecular patterns (PAMPs). These PAMPs represent molecules vital for microbial survival such as flagellin, nucleic acid structures unique to bacteria and viruses (CpG DNA, dsRNA), and the bacterial cell-wall components lipopolysaccharide (LPS), lipoteichoic acid, and peptidoglycan (Akira et al., 2006Akira S. Uematsu S. Takeuchi O. Pathogen recognition and innate immunity.Cell. 2006; 124: 783-801Abstract Full Text Full Text PDF PubMed Scopus (7886) Google Scholar). More than a decade ago, the protein Toll was identified as a key regulator of innate immune signaling in Drosophila melanogaster (Hoffmann, 2003Hoffmann J.A. The immune response of Drosophila.Nature. 2003; 426: 33-38Crossref PubMed Scopus (1048) Google Scholar). Since then, mammalian Toll-like receptors (TLRs) have been recognized for their ability to sense a wide array of microbial and self-ligands at the cell surface and within endosomes (Kawai and Akira, 2006Kawai T. Akira S. TLR signaling.Cell Death Differ. 2006; 13: 816-825Crossref PubMed Scopus (1443) Google Scholar). Recent studies identified the nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs) and the RIG-I (retinoid acid-inducible gene I)-like receptors (RLRs) as two additional innate immune receptor families that recognize PAMPs in intracellular compartments (Inohara et al., 2005Inohara N. Chamaillard M. McDonald C. Nunez G. NOD-LRR proteins: role in host-microbial interactions and inflammatory disease.Annu. Rev. Biochem. 2005; 74: 355-383Crossref PubMed Scopus (769) Google Scholar, Meylan and Tschopp, 2006Meylan E. Tschopp J. Toll-like receptors and RNA helicases: two parallel ways to trigger antiviral responses.Mol. Cell. 2006; 22: 561-569Abstract Full Text Full Text PDF PubMed Scopus (306) Google Scholar). The discovery of these cytosolic PRRs suggests that microbes evading extracellular surveillance encounter another line of recognition in the host cytosol. The detection of PAMPs by TLRs, NLRs, and RLRs activates multiple proinflammatory signaling pathways to mount an effective antiviral or bactericidal response targeting the invading microbe. Here, we review and discuss recently gained knowledge on the role of NLRs in immune responses. NLRs comprise a large family of intracellular PRRs that are characterized by the presence of a conserved NOD (Inohara and Nunez, 2001Inohara N. Nunez G. The NOD: a signaling module that regulates apoptosis and host defense against pathogens.Oncogene. 2001; 20: 6473-6481Crossref PubMed Scopus (188) Google Scholar). Notably, the architecture of NLRs resembles that of a subset of plant disease-resistance (R) genes, which are involved in the hypersensitive response against virulent plant pathogens (Inohara and Nunez, 2001Inohara N. Nunez G. The NOD: a signaling module that regulates apoptosis and host defense against pathogens.Oncogene. 2001; 20: 6473-6481Crossref PubMed Scopus (188) Google Scholar). The general domain organization of these proteins includes (1) an N-terminal effector binding region that consists of protein-protein interaction domains such as the caspase recruitment domain (CARD), pyrin (PYD), and baculovirus inhibitor repeat (BIR) domain; (2) an intermediary NOD domain that is required for nucleotide binding and self-oligomerization; and finally (3) an array of C-terminal leucine-rich repeat (LRR) motifs to detect conserved microbial patterns and to modulate NLR activity. Bioinformatic studies revealed the presence of 23 NLR genes in the human genome (Harton et al., 2002Harton J.A. Linhoff M.W. Zhang J. Ting J.P. Cutting edge: CATERPILLER: a large family of mammalian genes containing CARD, pyrin, nucleotide-binding, and leucine-rich repeat domains.J. Immunol. 2002; 169: 4088-4093PubMed Google Scholar, Inohara and Nunez, 2003Inohara N. Nunez G. NODs: intracellular proteins involved in inflammation and apoptosis.Nat. Rev. Immunol. 2003; 3: 371-382Crossref PubMed Scopus (831) Google Scholar), whereas at least 34 NLR genes are present in the mouse genome (Table 1). Based on the N-terminal domains, NLRs can be classified into three subfamilies also referred to as CARD-containing NODs, PYD-containing NALPs, or BIR-containing NAIPs (Table 1). The LRRs in the C terminus of NLR proteins are thought to fold back onto the NOD domain, thereby inhibiting spontaneous oligomerization and activation of the NLR protein (Duncan et al., 2007Duncan J.A. Bergstralh D.T. Wang Y. Willingham S.B. Ye Z. Zimmermann A.G. Ting J.P. Cryopyrin/NALP3 binds ATP/dATP, is an ATPase, and requires ATP binding to mediate inflammatory signaling.Proc. Natl. Acad. Sci. USA. 2007; 104: 8041-8046Crossref PubMed Scopus (280) Google Scholar, Faustin et al., 2007Faustin B. Lartigue L. Bruey J.M. Luciano F. Sergienko E. Bailly-Maitre B. Volkmann N. Hanein D. Rouiller I. Reed J.C. Reconstituted NALP1 inflammasome reveals two-step mechanism of caspase-1 activation.Mol. Cell. 2007; 25: 713-724Abstract Full Text Full Text PDF PubMed Scopus (495) Google Scholar). This function is reminiscent of the multiple repeats of tryptophan and aspartate residues (also known as WD-40 repeats) in the apoptosis regulator Apaf-1 (Acehan et al., 2002Acehan D. Jiang X. Morgan D.G. Heuser J.E. Wang X. Akey C.W. Three-dimensional structure of the apoptosome: implications for assembly, procaspase-9 binding, and activation.Mol. Cell. 2002; 9: 423-432Abstract Full Text Full Text PDF PubMed Scopus (647) Google Scholar). The current model proposes that when PAMPs are sensed by the C-terminal LRRs, the molecule undergoes conformational rearrangements triggering oligomerization via the NOD domain. In turn, NLRs expose the effector domains to induce the recruitment and activation of CARD- and PYD-containing effector molecules by promoting their proximity and oligomerization (Inohara et al., 1999Inohara N. Koseki T. del Peso L. Hu Y. Yee C. Chen S. Carrio R. Merino J. Liu D. Ni J. Nunez G. Nod1, an Apaf-1-like activator of caspase-9 and nuclear factor-kappaB.J. Biol. Chem. 1999; 274: 14560-14567Crossref PubMed Scopus (598) Google Scholar). Hence, the NLR proteins NOD1 and NOD2 interact with the serine-threonine kinase RICK (also called Ripk2 or RIP2) to induce NF-κB and MAPK signaling. Another NLR protein, the CIITA transcription factor, is required for the transcription of genes encoding MHC II, whereas the NLR family members Nalp1 and Cryopyrin recruit the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD) to activate caspase-1. Thus, NLRs are involved in the activation of diverse signaling pathways.Table 1Human and Murine NLR Genes Organized by Their N-Terminal DomainNLR Members (HGNC Symbols)N TerminusHumanMouseAliasesDomain OrganizationCARDCIITAC2TA, NLRA(CARD)-AD-NOD-LRRCIItaC2TA, Nlra(CARD)-AD-NOD-LRRNOD1NLRC1, CARD4, CLR7.1CARD-NOD-LRRNod1Nlrc1, Card4CARD-NOD-LRRNOD2NLRC2, CARD15, CLR16.3, BLAU, IBD1, PSORAS1CARD(2x)-NOD-LRRNod2Nlrc2, Card15CARD(2x)-NOD-LRRNLRC4CARD12, CLR2.1, CLAN, IPAFCARD-NOD-LRRNlrc4Card12, CLAN, IpafCARD-NOD-LRRNLRC3aThese NLRs have been classified as CARD proteins by the Human Genome Gene Nomenclature Committee (HGNC) although it is unclear whether they contain a CARD, PYD, or another interaction domain in their N terminus based on searches in the NCBI and SMART domain databases.CLR16.2, NOD3X-NOD-LRRNlrc3CLR16.2X-NOD-LRRNLRC5aThese NLRs have been classified as CARD proteins by the Human Genome Gene Nomenclature Committee (HGNC) although it is unclear whether they contain a CARD, PYD, or another interaction domain in their N terminus based on searches in the NCBI and SMART domain databases.CLR19.3, NOD27X-NOD-LRRNlrc5X-NOD-LRRNLRX1bIt is currently unclear whether this NLR member encodes a CARD, PYD, or another interaction domain in its N terminus.CLR11.3, NOD9X-NOD-LRRNlrx1X-NOD-LRRPYDNLRP1NALP1, CARD7, CLR17.1, NAC, DEFCAPPYD-NOD-LRR-CARDNlrp1aNalp1NOD-LRR-CARDNlrp1bNOD-LRR-CARDNlrp1cNOD-LRRNLRP2NALP2, CLR19.9, PYPAF2, NBS1, PAN1PYD-NOD-LRRNlrp2Pypaf2, Nbs1, Pan1PYD-NOD-LRRNLRP3NALP3, CIAS1, CLR1.1, PYPAF1, CryopyrinPYD-NOD-LRRNlrp3Nalp3, Cias1, Pypaf1, Mmig1, CryopyrinPYD-NOD-LRRNLRP4NALP4, CLR19.5, PYPAF4, RNH2, PAN2PYD-NOD-LRRNlrp4aNalp4a, Nalp-eta, Nalp9DPYD-NOD-LRRNlrp4bNalp4b, Nalp-gamma, Nalp9EPYD-NOD-LRRNlrp4cNalp4c, Nalp-alpha, Rnh2PYD-NOD-LRRNlrp4dNalp4d, Nalp-betaPYD-NOD-LRRNlrp4eNalp4e, Nalp-epsilonPYD-NOD-LRRNlrp4fNalp4f, Nalp-kappa, Nalp9FPYD-NOD-LRRNlrp4gNalp4gPYD-NOD-LRRNLRP5CLR19.8, PYPAF8, PAN11, MATERPYD-NOD-LRRNlrp5Mater, Op1NOD-LRRNLRP6CLR11.4, PYPAF5, PAN3PYD-NOD-LRRNlrp6PYD-NOD-LRRNLRP7NALP7, CLR19.4, PYPAF3, NOD12, PAN7PYD-NOD-LRRNLRP8NALP7, CLR19.2, PAN4, NOD16PYD-NOD-LRRNLRP9NALP7, CLR19.1, NOD6, PAN12PYD-NOD-LRRNlrp9aNalp9a, Nalp-thetaPYD-NOD-LRRNlrp9bNalp9b, Nalp-deltaPYD-NOD-LRRNlrp9cNalp9c, Nalp-zetaPYD-NOD-LRRNLRP10NALP10, CLR11.1, Pynod, NOD8, PAN5PYD-NODNlrp10Nalp10, PynodPYD-NODNLRP11NALP11, CLR19.6, PYPAF6, NOD17, PAN10PYD-NOD-LRRNLRP12NALP12, CLR19.3, PYPAF7, RNO2, Monarch1, PAN6PYD-NOD-LRRNlrp12Nalp12PYD-NOD-LRRNLRP13NALP13, CLR19.7, NOD14, PAN13PYD-NOD-LRRNLRP14NALP14, CLR11.2, NOD5, PAN8PYD-NOD-LRRNlrp14Nalp14, Nalp-iota, GC-LRRPYD-NOD-LRRBIRNAIPBIRC1, CLR5.1BIR(3x)-NOD-LRRNaip1Birc1aBIR(3x)-NOD-LRRNaip2Birc1bBIR(3x)-NOD-LRRNaip3Birc1cBIR(3x)-NOD-LRRNaip4Birc1dBIR(3x)-NOD-LRRNaip5Birc1eBIR(3x)-NOD-LRRNaip6Birc1fBIR(3x)-NOD-LRRNaip7Birc1gBIR(3x)-NOD-LRRa These NLRs have been classified as CARD proteins by the Human Genome Gene Nomenclature Committee (HGNC) although it is unclear whether they contain a CARD, PYD, or another interaction domain in their N terminus based on searches in the NCBI and SMART domain databases.b It is currently unclear whether this NLR member encodes a CARD, PYD, or another interaction domain in its N terminus. Open table in a new tab NOD1 and NOD2 sense bacterial molecules produced during the synthesis and/or degradation of peptidoglycan. NOD1 recognizes the dipeptide γ-D-glutamyl-meso-diaminopimelic acid (iE-DAP) (Chamaillard et al., 2003aChamaillard M. Hashimoto M. Horie Y. Masumoto J. Qiu S. Saab L. Ogura Y. Kawasaki A. Fukase K. Kusumoto S. et al.An essential role for NOD1 in host recognition of bacterial peptidoglycan containing diaminopimelic acid.Nat. Immunol. 2003; 4: 702-707Crossref PubMed Scopus (964) Google Scholar, Girardin et al., 2003aGirardin S.E. Boneca I.G. Carneiro L.A. Antignac A. Jehanno M. Viala J. Tedin K. Taha M.K. Labigne A. Zahringer U. et al.Nod1 detects a unique muropeptide from gram-negative bacterial peptidoglycan.Science. 2003; 300: 1584-1587Crossref PubMed Scopus (1180) Google Scholar), which is produced by most Gram-negative and specific Gram-positive bacteria (Hasegawa et al., 2006Hasegawa M. Yang K. Hashimoto M. Park J.H. Kim Y.G. Fujimoto Y. Nunez G. Fukase K. Inohara N. Differential release and distribution of Nod1 and Nod2 immunostimulatory molecules among bacterial species and environments.J. Biol. Chem. 2006; 281: 29054-29063Crossref PubMed Scopus (129) Google Scholar). In contrast, NOD2 is activated by muramyl dipeptide (MDP), a component of virtually all types of PGN (Girardin et al., 2003bGirardin S.E. Boneca I.G. Viala J. Chamaillard M. Labigne A. Thomas G. Philpott D.J. Sansonetti P.J. Nod2 is a general sensor of peptidoglycan through muramyl dipeptide (MDP) detection.J. Biol. Chem. 2003; 278: 8869-8872Crossref PubMed Scopus (1797) Google Scholar, Inohara et al., 2003Inohara N. Ogura Y. Fontalba A. Gutierrez O. Pons F. Crespo J. Fukase K. Inamura S. Kusumoto S. Hashimoto M. et al.Host recognition of bacterial muramyl dipeptide mediated through NOD2. Implications for Crohn's disease.J. Biol. Chem. 2003; 278: 5509-5512Crossref PubMed Scopus (1350) Google Scholar). Direct or indirect ligand recognition by the LRRs of NOD1 and NOD2 induces the recruitment of RICK through homotypic CARD interactions (Inohara et al., 1999Inohara N. Koseki T. del Peso L. Hu Y. Yee C. Chen S. Carrio R. Merino J. Liu D. Ni J. Nunez G. Nod1, an Apaf-1-like activator of caspase-9 and nuclear factor-kappaB.J. Biol. Chem. 1999; 274: 14560-14567Crossref PubMed Scopus (598) Google Scholar, Ogura et al., 2001bOgura Y. Inohara N. Benito A. Chen F.F. Yamaoka S. Nunez G. Nod2, a Nod1/Apaf-1 family member that is restricted to monocytes and activates NF-kappaB.J. Biol. Chem. 2001; 276: 4812-4818Crossref PubMed Scopus (1120) Google Scholar). This CARD-containing serine-threonine kinase directly binds and promotes K63-type polyubiquitylation of the regulator IKKγ and activation of the kinase TAK1 (Abbott et al., 2004Abbott D.W. Wilkins A. Asara J.M. Cantley L.C. The Crohn's disease protein, NOD2, requires RIP2 in order to induce ubiquitinylation of a novel site on NEMO.Curr. 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The CARD-containing adaptor protein CARD9 was found to be important for the activation of p38 and JNK downstream of NOD2, although it was dispensable for NF-κB activation (Hsu et al., 2007Hsu Y.M. Zhang Y. You Y. Wang D. Li H. Duramad O. Qin X.F. Dong C. Lin X. The adaptor protein CARD9 is required for innate immune responses to intracellular pathogens.Nat. Immunol. 2007; 8: 198-205Crossref PubMed Scopus (308) Google Scholar). Nevertheless, the NF-κB and MAP kinase pathways are thought to cooperate to upregulate the expression of proinflammatory molecules that stimulate both innate and adaptive immune responses (Figure 1). Previous studies suggested that RICK is involved in TLR signaling (Kobayashi et al., 2002Kobayashi K. Inohara N. Hernandez L.D. Galan J.E. Nunez G. Janeway C.A. Medzhitov R. Flavell R.A. RICK/Rip2/CARDIAK mediates signalling for receptors of the innate and adaptive immune systems.Nature. 2002; 416: 194-199Crossref PubMed Scopus (719) Google Scholar). However, many preparations of TLR agonists contain NOD1- and NOD2-stimulating impurities, and the presence of such contaminants could explain the reduced TLR signaling observed in RICK-deficient macrophages. Indeed, recent studies demonstrated that RICK is specifically required for NOD1 and NOD2 signaling, but not for TLR pathways (Park et al., 2007Park J.H. Kim Y.G. McDonald C. Kanneganti T.D. Hasegawa M. Body-Malapel M. Inohara N. Nunez G. RICK/RIP2 mediates innate immune responses induced through Nod1 and Nod2 but not TLRs.J. Immunol. 2007; 178: 2380-2386PubMed Google Scholar). In vitro studies indicated that multiple bacteria including Shigella flexneri (Girardin et al., 2003aGirardin S.E. Boneca I.G. Carneiro L.A. Antignac A. Jehanno M. Viala J. Tedin K. Taha M.K. Labigne A. Zahringer U. et al.Nod1 detects a unique muropeptide from gram-negative bacterial peptidoglycan.Science. 2003; 300: 1584-1587Crossref PubMed Scopus (1180) Google Scholar), entero-invasive Escherichia coli (Kim et al., 2004Kim J.G. Lee S.J. Kagnoff M.F. Nod1 is an essential signal transducer in intestinal epithelial cells infected with bacteria that avoid recognition by toll-like receptors.Infect. Immun. 2004; 72: 1487-1495Crossref PubMed Scopus (203) Google Scholar), Chlamydophila pneumoniae (Opitz et al., 2005Opitz B. Forster S. Hocke A.C. Maass M. Schmeck B. Hippenstiel S. Suttorp N. Krull M. Nod1-mediated endothelial cell activation by Chlamydophila pneumoniae.Circ. Res. 2005; 96: 319-326Crossref PubMed Scopus (159) Google Scholar), Campylobacter jejuni (Zilbauer et al., 2007Zilbauer M. Dorrell N. Elmi A. Lindley K.J. Schuller S. Jones H.E. Klein N.J. Nunez G. Wren B.W. Bajaj-Elliott M. A major role for intestinal epithelial nucleotide oligomerization domain 1 (NOD1) in eliciting host bactericidal immune responses to Campylobacter jejuni.Cell. Microbiol. 2007; 9: 2541-2548Crossref Scopus (5) Google Scholar), and Listeria monocytogenes (Hasegawa et al., 2006Hasegawa M. Yang K. Hashimoto M. Park J.H. Kim Y.G. Fujimoto Y. Nunez G. Fukase K. Inohara N. Differential release and distribution of Nod1 and Nod2 immunostimulatory molecules among bacterial species and environments.J. Biol. Chem. 2006; 281: 29054-29063Crossref PubMed Scopus (129) Google Scholar, Park et al., 2007Park J.H. Kim Y.G. McDonald C. Kanneganti T.D. Hasegawa M. Body-Malapel M. Inohara N. Nunez G. RICK/RIP2 mediates innate immune responses induced through Nod1 and Nod2 but not TLRs.J. Immunol. 2007; 178: 2380-2386PubMed Google Scholar) are sensed by NOD1. NOD2 has also been implicated in sensing intracellular pathogens such as Listeria monocytogenes (Kobayashi et al., 2005Kobayashi K.S. Chamaillard M. Ogura Y. Henegariu O. Inohara N. Nunez G. Flavell R.A. Nod2-dependent regulation of innate and adaptive immunity in the intestinal tract.Science. 2005; 307: 731-734Crossref PubMed Scopus (1420) Google Scholar), Streptococcus pneumoniae (Opitz et al., 2004Opitz B. Puschel A. Schmeck B. Hocke A.C. Rosseau S. Hammerschmidt S. Schumann R.R. Suttorp N. Hippenstiel S. Nucleotide-binding oligomerization domain proteins are innate immune receptors for internalized Streptococcus pneumoniae.J. Biol. Chem. 2004; 279: 36426-36432Crossref PubMed Scopus (271) Google Scholar), and Mycobacterium tuberculosis (Ferwerda et al., 2005Ferwerda G. Girardin S.E. Kullberg B.J. Le Bourhis L. de Jong D.J. Langenberg D.M. van Crevel R. Adema G.J. Ottenhoff T.H. Van der Meer J.W. Netea M.G. NOD2 and toll-like receptors are nonredundant recognition systems of Mycobacterium tuberculosis.PLoS Pathog. 2005; 1: 279-285Crossref PubMed Scopus (268) Google Scholar). However, studies in Nod1−/− and Nod2−/− mice are needed to assess whether these results can be extrapolated to in vivo models of bacterial infection. In this regard, primary fibroblasts from Nod1−/− mice induce lower expression of proinflammatory genes when infected with the intracellular bacteria Chlamydia trachomatis, suggesting that NOD1 can detect Chlamydia. However, vaginally infected Nod1−/− mice show normal disease progression, cytokine secretion, and pathology (Welter-Stahl et al., 2006Welter-Stahl L. Ojcius D.M. Viala J. Girardin S. Liu W. Delarbre C. Philpott D. Kelly K.A. Darville T. Stimulation of the cytosolic receptor for peptidoglycan, Nod1, by infection with Chlamydia trachomatis or Chlamydia muridarum.Cell. Microbiol. 2006; 8: 1047-1057Crossref PubMed Scopus (108) Google Scholar), suggesting redundancy with other innate immune receptors such as Nod2 and TLRs. Nevertheless, the in vivo relevance of NOD1 and NOD2 for bacterial infections was clearly demonstrated for Helicobacter pylori and Listeria monocytogenes. For example, mice with a targeted deletion of the Nod1 gene display an increased susceptibility to Helicobacter pylori (Viala et al., 2004Viala J. Chaput C. Boneca I.G. Cardona A. Girardin S.E. Moran A.P. Athman R. Memet S. Huerre M.R. Coyle A.J. et al.Nod1 responds to peptidoglycan delivered by the Helicobacter pylori cag pathogenicity island.Nat. Immunol. 2004; 5: 1166-1174Crossref PubMed Scopus (938) Google Scholar). This gastric pathogen is commonly thought to be an extracellular pathogen, raising the question of how peptidoglycan-derived iE-DAP and its intracellular sensor NOD1 encounter each other. Virulent H. pylori strains were found to deliver peptidoglycan through a type IV secretion apparatus (Viala et al., 2004Viala J. Chaput C. Boneca I.G. Cardona A. Girardin S.E. Moran A.P. Athman R. Memet S. Huerre M.R. Coyle A.J. et al.Nod1 responds to peptidoglycan delivered by the Helicobacter pylori cag pathogenicity island.Nat. Immunol. 2004; 5: 1166-1174Crossref PubMed Scopus (938) Google Scholar), a molecular syringe used to inject virulence factors directly into the host cytosol. Activation of NOD1 stimulates the production of chemokines and the recruitment of neutrophils in vivo (Masumoto et al., 2006Masumoto J. Yang K. Varambally S. Hasegawa M. Tomlins S.A. Qiu S. Fujimoto Y. Kawasaki A. Foster S.J. Horie Y. et al.Nod1 acts as an intracellular receptor to stimulate chemokine production and neutrophil recruitment in vivo.J. Exp. Med. 2006; 203: 203-213Crossref PubMed Scopus (151) Google Scholar). Moreover, H. pylori-induced production of β-defensins was abolished in Nod1−/− mice (Boughan et al., 2006Boughan P.K. Argent R.H. Body-Malapel M. Park J.H. Ewings K.E. Bowie A.G. Ong S.J. Cook S.J. Sorensen O.E. Manzo B.A. et al.Nucleotide-binding oligomerization domain-1 and epidermal growth factor receptor: critical regulators of beta-defensins during Helicobacter pylori infection.J. Biol. Chem. 2006; 281: 11637-11648Crossref PubMed Scopus (136) Google Scholar). Finally, NOD1 has been implicated in priming antigen-specific T cell responses, thereby contributing to the onset of adaptive immunity (Fritz et al., 2007Fritz J.H. Le Bourhis L. Sellge G. Magalhaes J.G. Fsihi H. Kufer T.A. Collins C. Viala J. Ferrero R.L. Girardin S.E. Philpott D.J. Nod1-mediated innate immune recognition of peptidoglycan contributes to the onset of adaptive immunity.Immunity. 2007; 26: 445-459Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar), although the mechanism involved remains poorly understood. Increased bacterial burdens were also observed in Nod2−/− mice infected orally, but not through the intravenous and intraperitoneal routes, with Listeria monocytogenes (Kobayashi et al., 2005Kobayashi K.S. Chamaillard M. Ogura Y. Henegariu O. Inohara N. Nunez G. Flavell R.A. Nod2-dependent regulation of innate and adaptive immunity in the intestinal tract.Science. 2005; 307: 731-734Crossref PubMed Scopus (1420) Google Scholar). The reason for the critical role of NOD1 and NOD2 in innate immunity at gastrointestinal surfaces remains unclear. One possibility is that the function of TLR and NOD1 or NOD2 to certain pathogens is less redundant in intestinal tissues where TLR signaling is downregulated to avoid inflammation induced by commensal bacteria. Alternatively, NOD1 and NOD2 may perform unique and critical antibacterial functions in intestinal cells such as regulation of antimicrobial peptides (Kobayashi et al., 2005Kobayashi K.S. Chamaillard M. Ogura Y. Henegariu O. Inohara N. Nunez G. Flavell R.A. Nod2-dependent regulation of innate and adaptive immunity in the intestinal tract.Science. 2005; 307: 731-734Crossref PubMed Scopus (1420) Google Scholar, Boughan et al., 2006Boughan P.K. Argent R.H. Body-Malapel M. Park J.H. Ewings K.E. Bowie A.G. Ong S.J. Cook S.J. Sorensen O.E. Manzo B.A. et al.Nucleotide-binding oligomerization domain-1 and epidermal growth factor receptor: critical regulators of beta-defensins during Helicobacter pylori infection.J. Biol. Chem. 2006; 281: 11637-11648Crossref PubMed Scopus (136) Google Scholar). The importance of NOD1 an NOD2 is underscored by their genetic association with human inflammatory diseases. NOD1 polymorphisms are associated with the development of atopic eczema, asthma, and increased serum IgE concentrations (Hysi et al., 2005Hysi P. Kabesch M. Moffatt M.F. Schedel M. Carr D. Zhang Y. Boardman B. von Mutius E. Weiland S.K. Leupold W. et al.NOD1 variation, immunoglobulin E and asthma.Hum. Mol. 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