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Toll-like Receptor 2 Functions as a Pattern Recognition Receptor for Diverse Bacterial Products*

Open AccessPublished:November 19, 1999DOI:https://doi.org/10.1074/jbc.274.47.33419
      Toll-like receptors (TLRs) 2 and 4 are signal transducers for lipopolysaccharide, the major proinflammatory constituent in the outer membrane of Gram-negative bacteria. We observed that membrane lipoproteins/lipopeptides from Borrelia burgdorferi, Treponema pallidum, and Mycoplasma fermentans activated cells heterologously expressing TLR2 but not those expressing TLR1 or TLR4. These TLR2-expressing cells were also stimulated by living motile B. burgdorferi, suggesting that TLR2 recognition of lipoproteins is relevant to naturalBorrelia infection. Importantly, a TLR2 antibody inhibited bacterial lipoprotein/lipopeptide-induced tumor necrosis factor release from human peripheral blood mononuclear cells, and TLR2-null Chinese hamster macrophages were insensitive to lipoprotein/lipopeptide challenge. The data suggest a role for the native protein in cellular activation by these ligands. In addition, TLR2-dependent responses were seen using whole Mycobacterium avium andStaphylococcus aureus, demonstrating that this receptor can function as a signal transducer for a wide spectrum of bacterial products. We conclude that diverse pathogens activate cells through TLR2 and propose that this molecule is a central pattern recognition receptor in host immune responses to microbial invasion.
      IL
      interleukin
      TLR
      Toll-like receptor
      LPS
      lipopolysaccharide
      Osp
      outer surface protein
      nOspA
      native OspA
      CHO
      Chinese hamster ovary
      sMALP-2
      synthetic macrophage-activating lipopeptide-2
      TNF
      tumor necrosis factor
      NF-κB
      nuclear factor-κB
      FBS
      fetal bovine serum
      PBMC
      peripheral blood mononuclear cells
      Microbial invasion of the host is followed by a series of events designed to control and eventually resolve the infection. The immediate response to the invading organism is coordinated by the innate immune system. The cells of this system are responsible for first-line bacterial clearance and modulation of the adaptive immune response through soluble factors or co-stimulatory signals provided by antigen-presenting cells (
      • Fearon D.T.
      • Locksley R.M.
      ). Janeway and co-workers (
      • Janeway C.A.J.
      ,
      • Medzhitov R.
      • Janeway C.A.J.
      ) have hypothesized that the innate immune system can sense invading pathogens by virtue of nonclonal pattern recognition receptors that interact with microbial structures and deliver a danger signal to the host cell.
      Toll is a type I transmembrane receptor, first described inDrosophila, that shares homology to components of the interleukin-1 (IL-1)1signaling pathway (
      • Belvin M.P.
      • Anderson K.V.
      ). Toll, and the related molecule 18-Wheeler, appear to control important antimicrobial responses against both fungi and bacteria in the fruit fly (
      • Williams M.J.
      • Rodriguez A.
      • Kimbrell D.A.
      • Eldon E.D.
      ,
      • Lemaitre B.
      • Nicolas E.
      • Michaut L.
      • Reichhart J.M.
      • Hoffmann J.A.
      ). In evolutionary terms, these proteins are primordial pattern recognition receptors for animals that totally lack acquired immunity. Recently, mammalian homologues of Toll have been cloned and designated Toll-like receptors (TLRs) (
      • Chaudhary P.M.
      • Ferguson C.
      • Nguyen V.
      • Nguyen O.
      • Massa H.F.
      • Eby M.
      • Jasmin A.
      • Trask B.J.
      • Hood L.
      • Nelson P.S.
      ,
      • Rock F.L.
      • Hardiman G.
      • Timans J.C.
      • Kastelein R.A.
      • Bazan J.F.
      ,
      • Medzhitov R.
      • Preston-Hurlburt P.
      • Janeway C.A.J.
      ). At least 10 such receptors have been identified, but only 2 TLRs have any known function. TLR2 and TLR4 have been implicated in cellular responses to lipopolysaccharide (LPS), the major constituent of the Gram-negative bacterial outer membrane (
      • Yang R.B.
      • Mark M.R.
      • Gray A.
      • Huang A.
      • Xie M.H.
      • Zhang M.
      • Goddard A.
      • Wood W.I.
      • Gurney A.L.
      • Godowski P.J.
      ,
      • Kirschning C.J.
      • Wesche H.
      • Merrill A.T.
      • Rothe M.
      ,
      • Poltorak A.
      • He X.
      • Smirnova I.
      • Liu M.Y.
      • Huffel C.V.
      • Du X.
      • Birdwell D.
      • Alejos E.
      • Silva M.
      • Galanos C.
      • Freudenberg M.
      • Ricciardi-Castagnoli P.
      • Layton B.
      • Beutler B.
      ). However, the mechanism behind TLR-mediated recognition of LPS, the interactions with other receptor molecules, such as CD14 (
      • Wright S.D.
      • Ramos R.A.
      • Tobias P.S.
      • Ulevitch R.J.
      • Mathison J.C.
      ,
      • Pugin J.
      • Heumann I.D.
      • Tomasz A.
      • Kravchenko V.V.
      • Akamatsu Y.
      • Nishijima M.
      • Glauser M.P.
      • Tobias P.S.
      • Ulevitch R.J.
      ), and the details of the subsequent cellular activation pathway still require elucidation.
      Lyme disease and syphilis are acute and chronic inflammatory disorders caused by the spirochetal pathogens Borrelia burgdorferi andTreponema pallidum subsp. pallidum, respectively (
      • Steere A.C.
      ,
      • Lukehart S.A.
      • Holmes K.K.
      ). Both spirochetes lack LPS (
      • Takayama K.
      • Rothenberg R.J.
      • Barbour A.G.
      ,
      • Hardy P.H.J.
      • Levin J.
      ); however, they do possess abundant membrane lipoproteins (
      • Belisle J.T.
      • Brandt M.E.
      • Radolf J.D.
      • Norgard M.V.
      ). There now exists a large body of evidence that spirochetal lipoproteins and synthetic lipohexapeptide analogs are potent activators of monocytes/macrophages, neutrophils, lymphocytes, endothelial cells, and fibroblasts and that acyl modification of the peptides is essential for these proinflammatory activities (
      • Knigge H.
      • Simon M.M.
      • Meuer S.C.
      • Kramer M.D.
      • Wallich R.
      ,
      • Sellati T.J.
      • Bouis D.A.
      • Kitchens R.L.
      • Darveau R.P.
      • Pugin J.
      • Ulevitch R.J.
      • Gangloff S.C.
      • Goyert S.M.
      • Norgard M.V.
      • Radolf J.D.
      ,
      • Sellati T.J.
      • Abrescia L.D.
      • Radolf J.D.
      • Furie M.B.
      ,
      • Radolf J.D.
      • Arndt L.L.
      • Akins D.R.
      • Curetty L.L.
      • Levi M.E.
      • Shen Y.
      • Davis L.S.
      • Norgard M.V.
      ,
      • Radolf J.D.
      • Norgard M.V.
      • Brandt M.E.
      • Isaacs R.D.
      • Thompson P.A.
      • Beutler B.
      ,
      • Wooten R.M.
      • Morrison T.B.
      • Weis J.H.
      • Wright S.D.
      • Thieringer R.
      • Weis J.J.
      ,
      • Morrison T.B.
      • Weis J.H.
      • Weis J.J.
      ,
      • Ma Y.
      • Weis J.J.
      ,
      • Ebnet K.
      • Brown K.D.
      • Siebenlist U.K.
      • Simon M.M.
      • Shaw S.
      ,
      • Weis J.J.
      • Ma Y.
      • Erdile L.F.
      ). More recent observations suggest that the mechanisms underlying monocytic cell activation by motile B. burgdorferi and T. pallidum are identical to those employed by their purified membrane constituents (
      • Sellati T.J.
      • Bouis D.A.
      • Caimano M.J.
      • Feulner J.A.
      • Ayers C.
      • Lien E.
      • Radolf J.D.
      ). These results support the notion that lipoproteins are the principle component of intact spirochetes driving the host immune response during Lyme disease and syphilis. Similarly, lipoproteins and lipopeptides derived from the human pathogen Mycoplasma fermentans are also potent activators of monocytes/macrophages and may play an important role in the inflammatory response during infection (
      • Muhlradt P.F.
      • Kiess M.
      • Meyer H.
      • Sussmuth R.
      • Jung G.
      ,
      • Garcia J.
      • Lemercier B.
      • Roman-Roman S.
      • Rawadi G.
      ,
      • Rawadi G.
      • Ramez V.
      • Lemercier B.
      • Roman-Roman S.
      ).
      The cellular activation induced by the lipoproteins or lipoprotein-derived lipopeptides from B. burgdorferi andT. pallidum resembles that of the LPS signaling pathway, as CD14 appears to facilitate cellular activation by both types of pathogenic membrane structures (
      • Sellati T.J.
      • Bouis D.A.
      • Kitchens R.L.
      • Darveau R.P.
      • Pugin J.
      • Ulevitch R.J.
      • Gangloff S.C.
      • Goyert S.M.
      • Norgard M.V.
      • Radolf J.D.
      ,
      • Wooten R.M.
      • Morrison T.B.
      • Weis J.H.
      • Wright S.D.
      • Thieringer R.
      • Weis J.J.
      ). However, several differences have been observed between LPS and lipoprotein cellular activation, indicating the utilization of different signaling elements. For example, spirochetal and mycoplasma lipoproteins and lipopeptides activate macrophages from LPS hyporesponsive C3H/HeJ mice (
      • Radolf J.D.
      • Arndt L.L.
      • Akins D.R.
      • Curetty L.L.
      • Levi M.E.
      • Shen Y.
      • Davis L.S.
      • Norgard M.V.
      ,
      • Radolf J.D.
      • Norgard M.V.
      • Brandt M.E.
      • Isaacs R.D.
      • Thompson P.A.
      • Beutler B.
      ,
      • Ma Y.
      • Weis J.J.
      ,
      • Muhlradt P.F.
      • Kiess M.
      • Meyer H.
      • Sussmuth R.
      • Jung G.
      ). In addition, whereas Chinese hamster ovary (CHO)-K1 cells become remarkably sensitive to LPS after transfection with CD14 (
      • Golenbock D.T.
      • Liu Y.
      • Millham F.H.
      • Freeman M.W.
      • Zoeller R.A.
      ,
      • Delude R.L.
      • Fenton M.J.
      • Savedra Jr., R.
      • Perera P.Y.
      • Vogel S.N.
      • Thieringer R.
      • Golenbock D.T.
      ,
      • Delude R.L.
      • Savedra R.
      • Zhao H.
      • Thieringer R.
      • Yamamoto S.
      • Fenton M.J.
      • Golenbock D.T.
      ), they remain insensitive to the lipoproteins, lipopeptides, and motileB. burgdorferi (
      • Sellati T.J.
      • Bouis D.A.
      • Kitchens R.L.
      • Darveau R.P.
      • Pugin J.
      • Ulevitch R.J.
      • Gangloff S.C.
      • Goyert S.M.
      • Norgard M.V.
      • Radolf J.D.
      ,
      • Sellati T.J.
      • Bouis D.A.
      • Caimano M.J.
      • Feulner J.A.
      • Ayers C.
      • Lien E.
      • Radolf J.D.
      ,
      • Garcia J.
      • Lemercier B.
      • Roman-Roman S.
      • Rawadi G.
      ). These observations led us to hypothesize that differences in main signaling components exist between lipoproteins and LPS.
      We have recently found that CHO-K1 cells do not express an mRNA transcript for full-length and functional TLR2 (
      • Heine H.
      • Kirschning C.J.
      • Lien E.
      • Monks B.G.
      • Rothe M.
      • Golenbock D.T.
      ). This observation raised the possibility that the lack of functional TLR2 might account for the failure of CHO/CD14 cells to respond to bacterial structures other than LPS. To test this hypothesis, we engineered stable CHO/CD14 fibroblast cell lines that express TLR2. The transfected cells were highly susceptible to activation by lipoproteins and lipopeptides fromB. burgdorferi, T. pallidum, and M. fermentans, as well as to activation by live motile B. burgdorferi. In contrast, cells expressing TLR1 or TLR4 did not acquire responsiveness to bacterial lipoproteins/lipopeptides. Moreover, we observed a TLR2-mediated cell activation byMycobacterium avium, an important pathogen in AIDS. Similar studies have documented inducible responses to other bacteria as well, including staphylococci, listeria, tuberculosis, and the pneumococcus, suggestive of wide-spread recognition of bacteria by TLR2 (
      • Yang R.B.
      • Mark M.R.
      • Gray A.
      • Huang A.
      • Xie M.H.
      • Zhang M.
      • Goddard A.
      • Wood W.I.
      • Gurney A.L.
      • Godowski P.J.
      ,
      • Kirschning C.J.
      • Wesche H.
      • Merrill A.T.
      • Rothe M.
      ,
      • Yoshimura A.
      • Lien E.
      • Ingalls R.R.
      • Tuomanen E.
      • Dziarski R.
      • Golenbock D.T.
      ,
      • Schwandner R.
      • Dziarski R.
      • Wesche H.
      • Rothe M.
      • Kirschning C.J.
      ).
      T. H. Flo, Ø. Halaas, E. Lien, L. Ryan, G. Teti, D. T. Golenbock, A. Sundan, and T. Espevik, submitted for publication.
      ,
      Means, T. K., Wang, S., Lien E., Yoshimura, A., Golenbock, D. T. and Fenton, M. J. (1999) J. Immunol. 163,3920–3927 and Brightbill, H. D., Libraty, D. H., Krutzik, S. R., Yang, R. B., Belisle, J. T., Bleharski, J. R., Maitland, M., Norgard, M. V., Plevy, S. E., Smale, S. T., Brennan, P. J., Bloom, B. R., Godowski, P. J. and Modlin, R. L. (1999) Science 285,732–736
      We propose that TLR2 mediates cellular responses to structures from numerous microbial cell wall constituents and may thus be central in host recognition of diverse bacterial pathogens. Therapies directed at the TLRs may be useful anti-inflammatory agents for a large variety of chronic and acute bacterial infections.

      DISCUSSION

      The severity of clinical symptoms associated with bacterial diseases varies according to the type of infectious agent, bacterial burden, affected tissue, and co-existing illness. Nevertheless, in many aspects, similar host responses are observed. For example, several clinical and immunological similarities can be seen between therapy-induced Jarisch-Herxheimer reaction during infection withTreponema and Borrelia spp. (
      • Negussie Y.
      • Remick D.G.
      • DeForge L.E.
      • Kunkel S.L.
      • Eynon A.
      • Griffin G.E.
      ,
      • Young E.J.
      • Weingarten N.M.
      • Baughn R.E.
      • Duncan W.C.
      ) and Gram-negative and Gram-positive sepsis (
      • Bone R.C.
      ). Hence, one is tempted to speculate that the pathophysiological similarities observed with these diverse infections are due to the activation of analogous signaling pathways in response to bacterial exposure. The present study implicates TLR2 in host interactions with B. burgdorferi, T. pallidum, M. fermentans, and M. avium, as well as components of Gram-negative and Gram-positive bacteria. Thus, this receptor can mediate host inflammatory reactions to a variety of microbial pathogens, indicating a remarkable spectrum of bacterial recognition.
      Previous reports have identified mechanisms of cellular activation by many microbial structures that are similar, yet never identical, to the LPS signaling pathway. In most cases, the reported observations concerned the ability of the microbes to utilize CD14. In addition to being a high affinity receptor for LPS, CD14 has been implicated in the responses to several bacteria and their microbial products, includingBorrelia and Treponema sp. (
      • Sellati T.J.
      • Bouis D.A.
      • Kitchens R.L.
      • Darveau R.P.
      • Pugin J.
      • Ulevitch R.J.
      • Gangloff S.C.
      • Goyert S.M.
      • Norgard M.V.
      • Radolf J.D.
      ,
      • Wooten R.M.
      • Morrison T.B.
      • Weis J.H.
      • Wright S.D.
      • Thieringer R.
      • Weis J.J.
      ), peptidoglycan, and other cell wall components of S. aureus(
      • Pugin J.
      • Heumann I.D.
      • Tomasz A.
      • Kravchenko V.V.
      • Akamatsu Y.
      • Nishijima M.
      • Glauser M.P.
      • Tobias P.S.
      • Ulevitch R.J.
      ,
      • Weidemann B.
      • Brade H.
      • Rietschel E.T.
      • Dziarski R.
      • Bazil V.
      • Kusumoto S.
      • Flad H.-D.
      • Ulmer A.J.
      ), group B streptococci (
      • Medvedev A.E.
      • Flo T.
      • Ingalls R.R.
      • Golenbock D.T.
      • Teti G.
      • Vogel S.N.
      • Espevik T.
      ), structures from mycobacteria (
      • Pugin J.
      • Heumann I.D.
      • Tomasz A.
      • Kravchenko V.V.
      • Akamatsu Y.
      • Nishijima M.
      • Glauser M.P.
      • Tobias P.S.
      • Ulevitch R.J.
      ,
      • Lien E.
      • Aukrust P.
      • Sundan A.
      • Müller F.
      • Frøland S.S.
      • Espevik T.
      ,
      • Zhang Y.
      • Doerfler M.
      • Lee T.C.
      • Guillemin B.
      • Rom W.N.
      ,
      • Savedra Jr., R.
      • Delude R.L.
      • Ingalls R.R.
      • Fenton M.J.
      • Golenbock D.T.
      ), and mannuronic acid polymers from Pseudomonas aeruginosa (
      • Espevik T.
      • Otterlei M.
      • Skjak Braek G.
      • Ryan L.
      • Wright S.D.
      • Sundan A.
      ). Because it can facilitate responses to all of these bacterial structures listed, CD14 has been termed a pattern recognition receptor by Pugin et al. (
      • Pugin J.
      • Heumann I.D.
      • Tomasz A.
      • Kravchenko V.V.
      • Akamatsu Y.
      • Nishijima M.
      • Glauser M.P.
      • Tobias P.S.
      • Ulevitch R.J.
      ). Yet CD14 lacks specificity in bacterial product recognition, and some controversy exists about whether CD14 is a true pattern recognition receptor (
      • Wright S.D.
      ). The identification of TLR2 in the recognition of most of these pathogens adds another layer of complexity to our understanding of the mammalian response to microbes. In contrast to CD14, TLR2 contains all of the characteristics that one would expect from a true pattern recognition receptor, including the presence of a true signal-transducing intracellular domain. Although only recently described, the list of putative ligands for TLR2 is already impressively large (Table I). Of particular interest is the observation that despite the apparent interactions of TLR2 with many Gram-positive bacteria, group B streptococci do not seem to stimulate cells through this receptor.2 This highlights the fact that we cannot exclude the involvement of additional receptors, functioning either alone or as part of a receptor complex, in host responses to the microbial structures described.
      Table IBacterial strains and compounds reported to activate cells via TLR2
      OrganismsReferenceStimulus tested
      Gram-negative bacteriaYang et al. (
      • Yang R.B.
      • Mark M.R.
      • Gray A.
      • Huang A.
      • Xie M.H.
      • Zhang M.
      • Goddard A.
      • Wood W.I.
      • Gurney A.L.
      • Godowski P.J.
      )
      LPS (various sources), lipid A
      Kirschninget al. (
      • Kirschning C.J.
      • Wesche H.
      • Merrill A.T.
      • Rothe M.
      )
      LPS (various sources), lipid A
      Gram-positive bacteriaYoshimura et al.(
      • Yoshimura A.
      • Lien E.
      • Ingalls R.R.
      • Tuomanen E.
      • Dziarski R.
      • Golenbock D.T.
      )
      S. aureus, S. pneumoniae; peptidoglycan
      Schwandner et al. (
      • Schwandner R.
      • Dziarski R.
      • Wesche H.
      • Rothe M.
      • Kirschning C.J.
      )
      S. aureus, B. subtilis, Streptococcus sp.; peptidoglycan, lipoteichoic acid
      Footnote 2Listeria monocytogenes
      Present studyS. aureus
      MycobacteriaFootnote 3Mycobacterium tuberculosis; ara-lipoarabinomannan
      Present studyM. avium
      SpirochetesAliprantis et al. (
      • Aliprantis A.O.
      • Yang R.B.
      • Mark M.R.
      • Suggett S.
      • Devaux B.
      • Radolf J.D.
      • Klimpel G.R.
      • Godowski P.J.
      • Zychlinski A.
      ) Hirscheldet al. (
      • Hirschfeld M.
      • Kirschning C.J.
      • Schwandner R.
      • Wesche H.
      • Weis J.H.
      • Wooten R.M.
      • Weis J.J.
      ) Present study
      B. burgdorferi; lipoproteins, lipopeptides from T. pallidum andBorrelia
      MycoplasmasPresent studyLipopeptide from M. fermentans
      Although TLR2 has the features of a pattern recognition receptor, it is difficult to define a common microbial pattern among all of these putative ligands. The list of TLR2 ligands is still not complete, and there is no evidence yet that TLR2 directly binds these microbial products. Thus, attempting to define the biophysical properties responsible for TLR2/ligand interactions may be premature. Nevertheless, we hypothesize that elements of amphipathicity may prove to be the most important for the stimulation of cells through TLR2. All of the lipoproteins/lipopeptides tested in this study activated TLR2-expressing cells, and acylation of the spirochetal proteins was the critical modification that enabled their activation of TLR2 (Fig.1 B). Other putative TLR2 ligands, including peptidoglycan, may also have amphipathic characteristics that are not yet appreciated.
      Both TLR2 (
      • Yang R.B.
      • Mark M.R.
      • Gray A.
      • Huang A.
      • Xie M.H.
      • Zhang M.
      • Goddard A.
      • Wood W.I.
      • Gurney A.L.
      • Godowski P.J.
      ,
      • Kirschning C.J.
      • Wesche H.
      • Merrill A.T.
      • Rothe M.
      ,
      • Heine H.
      • Kirschning C.J.
      • Lien E.
      • Monks B.G.
      • Rothe M.
      • Golenbock D.T.
      ) and TLR4 (
      • Poltorak A.
      • He X.
      • Smirnova I.
      • Liu M.Y.
      • Huffel C.V.
      • Du X.
      • Birdwell D.
      • Alejos E.
      • Silva M.
      • Galanos C.
      • Freudenberg M.
      • Ricciardi-Castagnoli P.
      • Layton B.
      • Beutler B.
      ,
      • Chow J.C.
      • Young D.W.
      • Golenbock D.T.
      • Christ W.J.
      • Gusovsky F.
      ) have been reported to function as LPS signal transducers. Our data support these conclusions, although they suggest that the two related proteins clearly have different roles in pathogen recognition: TLR4 is required for sensitive responses to LPS, whereas TLR2 is not. For example, cells from Chinese hamsters, which express a truncated and nonfunctional TLR2 (
      • Heine H.
      • Kirschning C.J.
      • Lien E.
      • Monks B.G.
      • Rothe M.
      • Golenbock D.T.
      ) but a full-length TLR4,5 respond to LPS but not to lipoproteins/lipopeptides. This contrasts with the finding that TLR4 is responsible for the LPS nonresponder phenotype of the C3H/HeJ mouse (
      • Poltorak A.
      • He X.
      • Smirnova I.
      • Liu M.Y.
      • Huffel C.V.
      • Du X.
      • Birdwell D.
      • Alejos E.
      • Silva M.
      • Galanos C.
      • Freudenberg M.
      • Ricciardi-Castagnoli P.
      • Layton B.
      • Beutler B.
      ). Although these mice fail to respond to low concentrations of LPS, the ability ofBorrelia spirochetes and lipoproteins to activate the C3H/HeJ mice (
      • Radolf J.D.
      • Arndt L.L.
      • Akins D.R.
      • Curetty L.L.
      • Levi M.E.
      • Shen Y.
      • Davis L.S.
      • Norgard M.V.
      ,
      • Radolf J.D.
      • Norgard M.V.
      • Brandt M.E.
      • Isaacs R.D.
      • Thompson P.A.
      • Beutler B.
      ,
      • Ma Y.
      • Weis J.J.
      ,
      • Fikrig E.
      • Barthold S.W.
      • Kantor F.S.
      • Flavell R.A.
      ) demonstrates that these ligands do not require TLR4 expression to elicit productive responses and strongly suggests a functional TLR2 in these animals. In a broad sense, the accumulated data indicate that the preferential utilization of TLRs underlies both the observed similarities, as well as the differences, in specific pathogen recognition.
      What remains unclear is why, if TLR2 is expressed in phagocytic cells under resting conditions, TLR4 mutant mice (C3H/HeJ and C57BL10/ScCr (
      • Poltorak A.
      • He X.
      • Smirnova I.
      • Liu M.Y.
      • Huffel C.V.
      • Du X.
      • Birdwell D.
      • Alejos E.
      • Silva M.
      • Galanos C.
      • Freudenberg M.
      • Ricciardi-Castagnoli P.
      • Layton B.
      • Beutler B.
      )) are not still sensitive to LPS via the signal transduction capabilities of TLR2. It may be that the levels of TLR2 expression in native phagocytes, in contrast to transfected cells, are insufficient to enable LPS responses in the mice. We note that chronically stimulated C3H/HeJ mice have been reported to exhibit immune activation in response to LPS challenge (
      • Vogel S.N.
      • Moore R.N.
      • Sipe J.D.
      • Rosenstreich D.L.
      ), an effect that may be due to the up-regulation of TLR2. Furthermore, the present data do not rule out the possibility that TLR2 may have a more important function in LPS recognition by nonphagocytic cells.
      The downstream signaling molecules involved in TLR-mediated cellular activation have not been definitively defined. However, both TLR2 and TLR4 have a cytoplasmic domain that is homologous to the IL-1 receptor. Thus, it is likely that both TLRs activate the NF-κB pathway, and perhaps other proinflammatory pathways as well, via their interactions with IL-1 receptor signaling genes, including MyD88, TRAF6, and IRAK (
      • Kirschning C.J.
      • Wesche H.
      • Merrill A.T.
      • Rothe M.
      ,
      • Zhang F.X.
      • Kirschning C.J.
      • Mancinelli R.
      • Xu X.P.
      • Jin Y.
      • Faure E.
      • Mantovani A.
      • Rothe M.
      • Muzio M.
      • Arditi M.
      ,
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      • Yeh W.C.
      • Sarosi I.
      • Duncan G.S.
      • Furlonger C.
      • Ho A.
      • Morony S.
      • Capparelli C.
      • Van G.
      • Kaufman S.
      • van d.H.
      • Itie A.
      • Wakeham A.
      • Khoo W.
      • Sasaki T.
      • Cao Z.
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      ). The similarities in the signal transduction process that appear to constitute the inflammatory response to invasion by a variety of bacteria suggest the exciting possibility that novel therapies directed against the harmful proinflammatory response to nearly all forms of infectious illnesses can one day be developed.

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