Genomic networks of inflammatory response (INM7P.355)

Mediated by a family of Toll-like Receptors (TLRs) responses lead to transcriptional activation and synthesis of many inflammatory genes. As an example, thousands of genes are up-regulated in response to LPS (lipopolysaccharide), with many of them undergoing specific program of activation and silencing. To investigate the breadth and specificity of LPS-induced gene expression in macrophages, we compared expressional activation in response to LPS in genetically diverse mice of subspecies M.m.musculus and M.m.domesticus using next generation (NGS) RNA-sequencing. To characterize the genetic basis of LPS-response in detail, we applied a systems genetics approach, using mapping of gene expression traits with on a panel of a second-generation F2 intercross mice. Specifically, RNA from LPS-activated F2 macrophages was RNA-sequenced and hundreds of activated genes were used as phenotypic read-outs for mapping the traits. We identified several hundreds cis- and trans-acting QTL contributing to expression variation (eQTL). Many trans eQTL cluster into major groups, some of which are confined to genomic loci that do not contain known regulators of innate immune responses. We identified 227 cis-expression quantitative trait loci (cis-eQTL) that control expression. Our data provide the first genome-wide map of major regulators of the inflammatory responses in macrophages.

The role of STING in T lymphocytes (IRM5P.643)

The cytoplasmic protein Stimulator of Interferon Genes (STING) plays an essential role in sensing intracellular pathogens and initiating type I IFN responses in myeloid cells. STING detects the presence of cytoplasmic DNA presented by various DNA-binding proteins (e.g. IFI16, DDX41, DAI, RNA Pol III); it also directly binds cyclic dinucleotides produced by intracellular bacteria or by the DNA-binding cyclic GAMP synthase (c-GAS). Once activated, STING phosphorylates TBK1 and IRF3 to trigger type I IFN production. To date STING has only been studied in macrophages and dendritic cells, but early reports showed expression of STING in a variety of tissues including the thymus and spleen, prompting us to ask whether STING might have a functional role in T cells, which account for the majority of cells in the thymus and a larger proportion of cells in the spleen than macrophages and dendritic cells. Here, we show that STING can be activated in T cells by the small molecule DMXAA, resulting in IFNβ production and increased expression of numerous interferon stimulated genes (ISG)—the first evidence to our knowledge of type I IFN production by T cells. Furthermore, RNA-sequencing data revealed unanticipated differences between unstimulated wild type and STING-/- T cells, with STING -/- T cells exhibiting increased expression of genes associated with cytotoxic T lymphocytes, indicating that STING-mediated signaling in T cells may regulate the development of particular T cell subsets.

Toll-like receptor-mediated down-regulation of the deubiquitinase cylindromatosis (CYLD) protects macrophages from necroptosis in wild-derived mice

Pathogen recognition by the innate immune system initiates the production of proinflammatory cytokines but can also lead to programmed host cell death. Necroptosis, a caspase-independent cell death pathway, can contribute to the host defense against pathogens or cause damage to host tissues. Receptor-interacting protein (RIP1) is a serine/threonine kinase that integrates inflammatory and necroptotic responses. To investigate the mechanisms of RIP1-mediated activation of immune cells, we established a genetic screen on the basis of RIP1-mediated necroptosis in wild-derived MOLF/EiJ mice, which diverged from classical laboratory mice over a million years ago. When compared with C57BL/6, MOLF/EiJ macrophages were resistant to RIP1-mediated necroptosis induced by Toll-like receptors. Using a forward genetic approach in a backcross panel of mice, we identified cylindromatosis (CYLD), a deubiquitinase known to act directly on RIP1 and promote necroptosis in TNF receptor signaling, as the gene conferring the trait. We demonstrate that CYLD is required for Toll-like receptor-induced necroptosis and describe a novel mechanism by which CYLD is down-regulated at the transcriptional level in MOLF/EiJ macrophages to confer protection from necroptosis.

Mannose receptor 1 mediates cellular uptake and endosomal delivery of CpG-motif containing oligodeoxynucleotides

Recognition of microbial components is critical for activation of TLRs, subsequent innate immune signaling, and directing adaptive immune responses. The DNA sensor TLR9 traffics from the endoplasmic reticulum to endolysosomal compartments where it is cleaved by resident proteases to generate a competent receptor. Activation of TLR9 by CpG-motif containing oligodeoxynucleotides (CpG ODNs) is preceded by agonist endocytosis and delivery into the endolysosomes. The events that dictate this process remain largely unknown; furthermore, it is unclear whether the receptors involved in mediating uptake of exogenous DNA are conserved for both naturally derived pathogenic DNA and synthetic ODNs. In this study, we report that peritoneal macrophages from a wild-derived inbred mouse strain, MOLF/Ei, are hyporesponsive to CpG ODN but are fully responsive to bacterial DNA, thus implying that microbial recognition is not fully recapitulated by a synthetic analog. To identify the gene responsible for the CpG ODN defect, we have performed genome-wide linkage analysis. Using N2 backcross mice, we mapped the trait with high resolution to a single locus containing Mrc1 as the gene conferring the trait. We show that mannose receptor 1 (MRC1; CD206) is involved in CpG ODN uptake and trafficking in wild-derived MOLF/Ei peritoneal macrophages. Furthermore, we show that other strains of wild-derived mice also require MRC1 for CpG-induced cytokine responses. These findings reveal novel functions for MRC1 and demonstrate that wild-derived mice are important and indispensable model for understanding naturally occurring regulators of inflammatory responses in innate immune pathways.

Forward genetic analysis of type I interferon responses to cytosolic deoxynucleotides reveals polymorphisms in Tmem173 gene of wild derived MOLF/EiJ mice. (P4201)

We identified a novel phenotype in wild derived inbred mouse strain, MOLF/EiJ (MOLF). In response to cytosolic DNA or cyclic-diadenylate (c-di-AMP), MOLF macrophages exhibit high levels of IL-6 but very low levels of type I interferon, IFNβ, compared to classical inbred mouse strain C5BL/6J (B6). Furthermore, the IFNβ production is reduced in responses of MOLF macrophages to herpes simplex virus (HSV) and Listeria monocytogenes infection that release double stranded DNA and c-di-AMP into the cytosol, respectively. To identify loci that confer the trait, we used quantitative trait locus (QTL) mapping. We measured the IFNβ production of macrophages from a panel of 2nd filial generation (F2), B6/MOLF intercrossed mice, and genotyped the individual mice for inheritance of loci genome wide. The lack of IFNβ production mapped to a locus that contains Tmem173, a gene that encodes STING. STING is an innate immune cytosolic surveyor that mediates interferon response to cytosolic DNA and c-di-AMP. Sequencing the MOLF transcript of Tmem173 revealed multiple single nucleotide polymorphisms and an 18 base pair deletion in the MOLF allele of Tmem173. Almost all of these polymorphisms encode amino acid changes in the putative transmembrane domains of STING, while the cytosolic fraction is highly conserved. In vitro, expression of MOLF STING in Tmem173-/- murine embryonic fibroblasts (MEFs) shows greatly reduced activation of the IFNβ promoter, compared to C57BL/6.

Genetic screen reveals CYLD as a regulator of the balance between inflammation and cell death (180.18)

During the innate immune response to infection, a critical balance exists between the production pro-inflammatory cytokines and immune cell death. Cell death through RIP1-mediated regulated necrosis is emerging as an important mechanism through which cells respond to certain viral pathogens. To determine novel regulatory mechanisms of this signaling pathway, we studied cellular responses to TLR agonists in evolutionarily divergent wild-derived mice (MOLF/Ei) in a forward genetic screen of TLR-induced necrosis. In contrast to C57BL/6 macrophages, which are susceptible to TLR-induced necrosis, MOLF macrophages are resistant to death induced through this pathway. Using an N2 panel and forward genetic analysis, we found a locus on chromosome 8 that confers resistance to TLR-induced necrosis. Through gene expression analysis, allelic bias studies, and siRNA knockdown in peritoneal macrophages, we concluded that a genetic difference in CYLD confers the differential susceptibility to TLR-induced necrosis between B6 and MOLF macrophages. This difference was revealed through analysis of the regulation of CYLD mRNA splicing following TLR stimulation. The molecular defect that results in greater survival of MOLF cells appears to be mutually exclusive splicing of CYLD mRNA in activated or quiescent cells. Through our use of wild-derived mice in forward genetic analysis, we were able to uncover a key function of naturally occurring CYLD splice isoforms in a physiologic setting.

Genetic analysis of the CpG hyporesponsiveness in wild-derived MOLF/Ei mice (158.16)

CpG motif containing oligonucleotides are used to mimic the immunostimulatory properties of bacterial DNA in both innate and adaptive immune cells through the activation of toll-like receptor 9. Macrophages are one of the main cell types activated to elicit a pro-inflammatory response to CpG DNA. Downstream signaling of TLR9 is more well characterized and studied compared to upstreams events which remains largely unknown, such as what receptors and mechanisms are involved in CpG DNA receptor-mediated endocytosis and trafficking into endosomal compartments. While all classical laboratory mouse strains are responsive to CpG DNA activation a wild-derived inbred mouse strain, MOLF/Ei, is non-responsive. In this study we have used a forward genetics approach to analyze the CpG response in peritoneal macrophages from panels of N2 backcross mice [MOLF/Ei x (C57BL/6xMOLF/Ei)]. Our genome-wide linkage studies implicate, for the first time, the role of the mannose receptor, C type 1 (Mrc1) in the CpG hypo-responsiveness in MOLF/Ei mice. Furthermore, additional mapping studies have revealed a second receptor that is also involved in CpG DNA signaling for both classical and wild-derived mouse strains, implicating its role in CpG activation for the first time.

Genetic control of severe egg-induced immunopathology and IL-17 production in murine schistosomiasis

Infection with the trematode parasite Schistosoma mansoni results in a distinct heterogeneity of disease severity, both in humans and in an experimental mouse model. Severe disease is characterized by pronounced hepatic egg-induced granulomatous inflammation in a proinflammatory cytokine environment, whereas mild disease corresponds with reduced hepatic inflammation in a Th2 skewed cytokine environment. This marked heterogeneity indicates that genetic differences play a significant role in disease development, yet little is known about the genetic basis of dissimilar immunopathology. To investigate the role of genetic susceptibility in murine schistosomiasis, quantitative trait loci analysis was performed on F(2) progeny derived from SJL/J and C57BL/6 mice, which develop severe and mild pathology, respectively. In this study, we show that severe liver pathology in F(2) mice 7 wk after infection significantly correlated with an increase in the production of the proinflammatory cytokines IL-17, IFN-gamma, and TNF-alpha by schistosome egg Ag-stimulated mesenteric lymph node cells. Quantitative trait loci analysis identified several genetic intervals controlling immunopathology as well as IL-17 and IFN-gamma production. Egg granuloma size exhibited significant linkage to two loci, D4Mit203 and D17Mit82, both of which were inherited in a BL/6 dominant manner. Furthermore, a significant reduction of hepatic granulomatous inflammation and IL-17 production in interval-specific congenic mice demonstrated that the two identified genetic loci have a decisive effect on the development of immunopathology in murine schistosomiasis.

A mutation in Irak2c identifies IRAK-2 as a central component of the TLR regulatory network of wild-derived mice

In a phenotypic screen of the wild-derived mouse strain MOLF/Ei, we describe an earlier and more potent toll-like receptor (TLR)–mediated induction of IL-6 transcription compared with the classical inbred strain C57BL/6J. The phenotype correlated with increased activity of the IκB kinase axis as well as p38, but not extracellular signal-regulated kinase or c-Jun N-terminal kinase, mitogen-activated protein kinase (MAPK) phosphorylation. The trait was mapped to the Why1 locus, which contains Irak2, a gene previously implicated as sustaining the late phase of TLR responses. In the MOLF/Ei TLR signaling network, IRAK-2 promotes early nuclear factor κB (NF-κB) activity and is essential for the activation of p38 MAPK. We identify a deletion in the MOLF/Ei promoter of the inhibitory Irak2c gene, leading to an increased ratio of pro- to antiinflammatory IRAK-2 isoforms. These findings demonstrate that IRAK-2 is an essential component of the early TLR response in MOLF/Ei mice and show a distinct pathway of p38 and NF-κB activation in this model organism. In addition, they demonstrate that studies in evolutionarily divergent model organisms are essential to complete dissection of signal transduction pathways.

Forward genetic analysis of Toll-like receptor responses in wild-derived mice reveals a novel antiinflammatory role for IRAK1BP1

Although inflammatory cytokines produced by activation of Toll-like receptors (TLRs) are essential for early host defense against infection, they also mediate a vast array of pathologies, including autoimmune disease, hypersensitivity reactions, and sepsis. Thus, numerous regulatory mechanisms exist in parallel with proinflammatory pathways to prevent excessive release of these potent effector molecules. We report elucidation of a novel regulatory function for interleukin receptor-associated kinase (IRAK)-1 binding protein 1 (IRAK1BP1, also known as SIMPL) through quantitative trait locus mapping of the TLR response in wild-derived mouse strains. This gene emerged as a negative regulator of TLR2-mediated interleukin (IL)-6 production in MOLF/Ei mice, which expressed IRAK1BP1 mRNA in an allele-specific manner when crossed with the C57BL/6J strain. Human peripheral blood mononuclear cells and primary macrophages from two other wild-derived mouse strains also induced IRAK1BP1 mRNA by 4 hours after stimulation with agonists of various TLRs. Examination of its effects on IL-6 and other cytokines demonstrated that IRAK1BP1 regulates transcription of a specific subset of TLR-responsive genes, producing an overall antiinflammatory profile. Our results reveal that IRAK1BP1 is a critical factor in preventing dangerous overproduction of proinflammatory cytokines by the innate immune system and in influencing the specificity of TLR responses. Furthermore, these results show that the genetic diversity of wild-derived mouse strains makes them a valuable model of important human gene functions that have been lost in some laboratory-inbred strains.

Genetic analysis of the innate immune responses in wild-derived inbred strains of mice

The vertebrate immune system has evolved to recognize nucleic acids of bacterial and viral origin. Microbial DNA, as well as synthetic oligonucleotides based on these motifs, activates innate immune pathways mediated by the family of Toll-like receptors (TLR) initiating a cascade of signals in immune cells necessary for responses to pathogens. However, not all of the proteins that participate in TLR-mediated responses have been identified. In studies described herein, we observed significant variation in innate immune responses among selected wild-derived strains of mice. Specifically, we show that mice of MOLF/Ei, Czech/Ei, and MSM/Ms strains are hypo-responsive to polyinosinic-polycytidylic acid (poly(I:C)) because of a mutation in Tlr3. In addition, we discovered a hypo-response to cytosine guanine dinucleotide in MOLF/Ei mice and established that it is not linked to Tlr9, but to another locus. Further inquiry revealed that this hypo-response is transmitted as a monogenic dominant trait that can be mapped and cloned through positional cloning methods. These results suggest the existence of a novel molecule that can alter pro-inflammatory signals or activate additional signal transduction pathways. In addition, they support the wild-derived mouse strain as a forward genetic tool for the identification of novel immunological phenotypes.

Mice expressing high levels of soluble CD14 retain LPS in the circulation and are resistant to LPS-induced lethality

Despite significant progress in understanding the origin of soluble CD14 (sCD14), its physiological function remains largely unknown. Recent research has produced contradictory observations suggesting that sCD14 may have either beneficial or detrimental properties in protection against LPS-induced endotoxin shock. To resolve this controversy and to establish a mouse model suitable for elucidation of the functions of human CD14 (hCD14) in vivo, we generated several lines of transgenic mice bearing different copy numbers of the hCd14 transgene on a murine Cd14-/- background. The hCD14 was entirely capable of complementing loss of mouse CD14 to mediate cellular responses to LPS. Serum levels of sCD14 in a founder with multiple copies of the transgene were several times higher than in transgenic animals with a single copy of Cd14. Furthermore, mice with high levels of hCD14 were hypo-responsive to LPS and survived a lethal dose of LPS. Further inquiry into the mechanism of the hypo-response to LPS revealed that protection is associated with the higher amounts of circulating LPS. Most of this circulating LPS can be immunoprecipitated with anti-CD14 antibodies. These results suggest that sCD14 blocks circulating LPS by limiting the amount of monocyte-bound LPS and thus reduces inflammatory responses.

The role of Toll-like receptor 4 versus interleukin-12 in immunity to respiratory syncytial virus

Toll-like receptors (TLR) and IL-12 represent key elements of innate immunity. Using C57BL/10 ScCr mice it was shown that TLR4 is important for control of infection with respiratory syncytial virus (RSV). Since these mice have an additional defect in the IL-12R, we reinvestigated immunity to RSV in several C57BL/10 and BALB/c mouse strains lacking a functional TLR4, a functional IL-12-IL-12R interaction or both. In the absence of a functional IL-12 axis, early virus control was impaired in C57BL/10 mice, but not in BALB/c mice. By contrast, TLR4 had no impact on RSV elimination. Pulmonary NK cell recruitment was impaired in IL-12 deficient BALB/c mice and NK cytotoxicity was reduced in IL-12/IL-12R-deficient mice of both genetic backgrounds. Absence of TLR4 had no impact on NK cell recruitment or NK activity nor on recruitment of other pulmonary inflammatory cells. Activation of RSV-specific T cell immunity, including T cell mediated immunopathology, was normal in all mutant strains. These findings clearly argue against a significant role for TLR4 and define a limited role for IL-12 in primary murine RSV infection.

Toll-like receptor 4 expression levels determine the degree of LPS-susceptibility in mice

C57BL/10ScCr (Cr) mice carry a deletion of the Toll-like receptor 4 (tlr4) gene (i.e. they are tlr4(0/0)) and are thus refractory to LPS effects. Insertion of wild-type tlr4 transgene into the tlr4(0/0) Cr germ line endowed LPS susceptibility in the two transgenic lines created, indicating that TLR4 is the only limiting factor for LPS responsiveness in Cr mice. The absolute levels of tlr4 mRNA expressed by the heterozygous transgenic (tlr4(Tr/0)), wild-type C57BL/10ScSn (Sn) (tlr4(+/+)) and heterozygous F1 (Sn x Cr) (tlr4(+/0)) mice varied markedly. However, the pattern of distribution of expression in the different organs was the same in all strains. In different biological assays (B cell mitogenicity, cytokine induction and lethal toxicity) the degree of LPS response obtained in the different strains of mice correlated with the levels of tlr4 mRNA expression. In macrophages, investigation of the LPS-induced cytokine (IL-6) response revealed a linear relationship between the response and the logarithm of TLR4-MD-2 levels.

A point mutation in the IL-12R beta 2 gene underlies the IL-12 unresponsiveness of Lps-defective C57BL/10ScCr mice

Lps-defective C57BL/10ScCr (Cr) mice are homozygous for a deletion encompassing Toll-like receptor 4 that makes them refractory to the biological activity of LPS. In addition, these mice exhibit an inherited IL-12 unresponsiveness resulting in impaired IFN-gamma responses to different microorganisms. By positional cloning methods, we show here that this second defect of Cr mice is due to a mutation in a single gene located on mouse chromosome 6, in close proximity to the Igkappa locus. The gene is IL-12Rbeta2. Cr mice carry a point mutation creating a stop codon that is predicted to cause premature termination of the translated IL-12Rbeta2 after a lysine residue at position 777. The truncated beta2 chain can still form a heterodimeric IL-12R that allows phosphorylation of Janus kinase 2, but, unlike the wild-type IL-12R, can no longer mediate phosphorylation of STAT4. Because the phosphorylation of STAT4 is a prerequisite for the IL-12-mediated induction of IFN-gamma, its absence in Cr mice is responsible for their defective IFN-gamma response to microorganisms.

Sepsis and evolution of the innate immune response

OBJECTIVE

To review the role of the Toll-like receptors (TLR) as the principal sensors used by the innate immune system in the context of the pathologic processes underlying sepsis and septic shock.

DATA SOURCES

Literature review.

DATA SUMMARY

Through the Toll-like receptors, macrophages and other defensive cells "see" endotoxin (TLR4), peptidoglycan (TLR2), and bacterial DNA (TLR9). Representatives of the family predated the divergence of plants and animals and, at that time, had already acquired a defensive function. The strengths and liabilities of the innate immune system, which defends against infection and which also may cause shock and death, are rooted in its ancient origins. In the current era of shock research, the nature of the signals that Toll-like receptors transduce and the effects of genetic variation on microbial sensing are two major challenges.

The sole gateway to endotoxin response: how LPS was identified as Tlr4, and its role in innate immunity

Tlr4 has emerged as a specific conduit for the bacterial lipopolysaccharide (LPS) response. The fact that such a protein exists, and furthermore, the fact that it is one member of a family of proteins expressed by mononuclear cells, yields considerable insight into the mechanism by which phagocytes "see" the microbial universe. It cannot yet be assumed that all the Tlrs have specificity comparable to that of Tlr4, but it is probable that they do, given the molecular constraints to which all proteins are subject. Indeed, it is remarkable that Tlr4 is able to sense so diverse an array of LPS molecules as it does. The total number of Tlr proteins is not yet known. Although approximately 30 leucine-rich proteins bearing Toll-like cytoplasmic domains might be anticipated based on a survey of the genes in Drosophila, far fewer Toll-like genes have been found in mammals to date, although approximately 2 million expressed sequence tag sequences are now archived, and much of the genome has been covered. Some of the Toll-like proteins are, in fact, cytokine receptors. Ten leucine-rich Tlrs have been reported so far. Even a small number of receptors might be sufficient to confer recognition of most pathogens, be they fungi, bacteria, or protozoa. Some such receptors may also play developmental roles. The mutational deletion of Tlr genes alone and in combination with one another may help to establish the functions of each member of this newly emergent family of proteins.

Limits of a deletion spanning Tlr4 in C57BL/10ScCr mice

Proceeding from our observation that LPS-unresponsive mice of the strain C57BL/10ScCr mice fail to express the Tlr4 gene [Poltorak A, He X. Smirnova I et al. Defective LPS signaling in C3H/HeJ and C57BL/10ScCr mice: mutations in Tlr4 gene. Science 1998; 282: 2085], we have defined the exact limits of a deletion encompassing Tlr4 in the C57BL/10ScCr genome. The deletion removes 74723 bp of DNA, with reference to the control strain 129/J (from which the complete sequence of the Tlr4 locus was obtained). There is no inserted element, and no re-arrangement of the chromosome (e.g. inversion or translocation) in the immediate region of Tlr4; the deletion removes only one recognizable gene. Hence, other immunological anomalies that have been identified in C57BL/10ScCr mice (a non-healing phenotype in Leishmania inoculation and failure to produce interferon-gamma in response to numerous microbial infections) must be ascribed to one of two causes. Mutation(s) at other loci may be responsible for these defects. Alternatively, Tlr4 locus deletion may have phenotypic consequences that exceed the well known blockade of LPS signal transduction.

Three novel mammalian toll-like receptors: gene structure, expression, and evolution

We describe three novel genes, encoding members of the Toll-like receptor (Tlr) family (TLR7, TLR8, and TLR9). These Tlr family members, unlike others reported to date, were identified within a genomic database. TLR7 and TLR8 each have three exons, two of which have coding function, and lie in close proximity to one another at Xp22, alongside a pseudogene. The remaining gene (TLR9) resides at 3p21.3 (in linkage with the MyD88 gene), and is expressed in at least two splice forms, one of which is monoexonic and one of which is biexonic, the latter encoding a protein with 57 additional amino acids at the N-terminus. The novel Tlrs comprise a cluster as nearest phylogenetic neighbors. Combining all sequence data related to Toll-like receptors, we have drawn several inferences concerning the phylogeny of vertebrate and invertebrate Tlrs. According to our best estimates, mammalian TLRs 1 and 6 diverged from a common mammalian ancestral gene 95 million years ago. TLR4, which encodes the endotoxin sensor in present-day mammals, emerged as a distinct entity 180 million years ago. TLRs 3 and 5 diverged from a common ancestral gene approximately 150 million years ago, as did Tlr7 and Tlr8. Very likely, fewer Tlrs existed during early vertebrate evolution: at most three or four were transmitted with the primordial vertebrate line. Phylogenetic data that we have adduced in the course of this work also suggest the existence of a Drosophila equivalent of MyD88, and indicate that the plasma membrane protein SIGIRR is close functional relative of MyD88 in mammals. Finally, a single present-day representative of the Toll-like proteins in Drosophila has striking cytoplasmic domain homology to mammalian Tlrs within the cluster that embraces TLRs 1, 2, 4, and 6. This would suggest that an ancestral (pre-vertebrate) Tlr may have adopted a pro-inflammatory function 500 million years ago.

Positional cloning of Lps, and the general role of toll-like receptors in the innate immune response

In mice (and by inference, in all mammals), a single pathway exists to serve lipopolysaccharide (LPS) signal transduction, and as such, allelic mutations at a single locus entirely abolish responses to LPS in C3H/HeJ and C57BL/10ScCr mice. Positional cloning of this locus, known as Lps, revealed that mutations of the Toll-like receptor 4 gene (Tlr4) are responsible for endotoxin resistance. A quick succession of studies have shown Tlr4 to be the critical transmembrane component of the LPS signal transduction complex. As LPS sensing by Tlr4 depends on physical contact between the two molecules, Tlr4 is a direct interface with the microbial world. Eight other molecules with strong similarity to Tlr4 are presently known in mammals, and taking Tlr4 as a model, all may be guessed to participate in the early detection of invasive pathogens. Acting together, the Toll-like receptors may be assumed to present macrophages with a comprehensive "picture" of the micobial world, and thus comprise the principal sensing molecules utilized by cells of the innate immune system.