CD11b/CD18 acts in concert with CD14 and Toll-like receptor (TLR) 4 to elicit full lipopolysaccharide and taxol-inducible gene expression

Overproduction of inflammatory mediators by macrophages in response to Gram-negative LPS has been implicated in septic shock. Recent reports indicate that three membrane-associated proteins, CD14, CD11b/CD18, and Toll-like receptor (TLR) 4, may serve as LPS recognition and/or signaling receptors in murine macrophages. Therefore, the relative contribution of these proteins in the induction of cyclooxygenase 2 (COX-2), IL-12 p35, IL-12 p40, TNF-alpha, IFN-inducible protein (IP)-10, and IFN consensus sequence binding protein (ICSBP) genes in response to LPS or the LPS-mimetic, Taxol, was examined using macrophages derived from mice deficient for these membrane-associated proteins. The panel of genes selected reflects diverse macrophage effector functions that contribute to the pathogenesis of septic shock. Induction of the entire panel of genes in response to low concentrations of LPS or Taxol requires the participation of both CD14 and TLR4, whereas high concentrations of LPS or Taxol elicit the expression of a subset of LPS-inducible genes in the absence of CD14. In contrast, for optimal induction of COX-2, IL-12 p35, and IL-12 p40 genes by low concentrations of LPS or by all concentrations of Taxol, CD11b/CD18 was also required. Mitigated induction of COX-2, IL-12 p35, and IL-12 p40 gene expression by CD11b/CD18-deficient macrophages correlated with a marked inhibition of NF-kappa B nuclear translocation and mitogen-activated protein kinase (MAPK) activation in response to Taxol and of NF-kappa B nuclear translocation in response to LPS. These findings suggest that for expression of a full repertoire of LPS-/Taxol-inducible genes, CD14, TLR4, and CD11b/CD18 must be coordinately engaged to deliver optimal signaling to the macrophage.     

Lgr4 Gene Deficiency Increases Susceptibility and Severity of Dextran Sodium Sulfate-induced Inflammatory Bowel Disease in Mice

Lgr4/Gpr48 is one of the newly identified R-spondins receptors and potentiates Wnt signaling, which regulates intestinal homeostasis. We used a hypomorphic mouse strain to determine the role of Lgr4 in intestinal inflammation and recovery. Intestinal inflammation was induced with dextran sulfate sodium (DSS) followed by a recovery period. Intestinal inflammation symptoms and molecular mechanisms were examined. We found that Lgr4^−/− mice exhibited dramatically higher susceptibility to and mortality from DSS-induced inflammatory bowel disease than WT mice. Lgr4 deficiency resulted in greatly reduced numbers of either Paneth cells or stem cells in the intestine. During the intestinal regeneration process, cell proliferation but not apoptosis of intestinal epithelial cells was significantly impaired in Lgr4^−/− mice. When Wnt/β-catenin signaling was reactivated by crossing with APC^min/+ mice or by treating with a GSK-3β inhibitor, the number of Paneth cells was partially restored and the mortality caused by DSS-induced inflammatory bowel disease was strikingly reduced in Lgr4-deficient animals. Thus, Lgr4 is critically involved in the maintenance of intestinal homeostasis and protection against inflammatory bowel disease through modulation of the Wnt/β-catenin signaling pathway.     

Toll-like Receptor 4 Is Expressed on Intestinal Stem Cells and Regulates Their Proliferation and Apoptosis via the p53 Up-regulated Modulator of Apoptosis

Factors regulating the proliferation and apoptosis of intestinal stem cells (ISCs) remain incompletely understood. Because ISCs exist among microbial ligands, immune receptors such as toll-like receptor 4 (TLR4) could play a role. We now hypothesize that ISCs express TLR4 and that the activation of TLR4 directly on the intestinal stem cells regulates their ability to proliferate or to undergo apoptosis. Using flow cytometry and fluorescent in situ hybridization for the intestinal stem cell marker Lgr5, we demonstrate that TLR4 is expressed on the Lgr5-positive intestinal stem cells. TLR4 activation reduced proliferation and increased apoptosis in ISCs both in vivo and in ISC organoids, a finding not observed in mice lacking TLR4 in the Lgr5-positive ISCs, confirming the in vivo significance of this effect. To define molecular mechanisms involved, TLR4 inhibited ISC proliferation and increased apoptosis via the p53-up-regulated modulator of apoptosis (PUMA), as TLR4 did not affect crypt proliferation or apoptosis in organoids or mice lacking PUMA. In vivo effects of TLR4 on ISCs required TIR-domain-containing adapter-inducing interferon-β (TRIF) but were independent of myeloid-differentiation primary response-gene 88 (MYD88) and TNFα. Physiological relevance was suggested, as TLR4 activation in necrotizing enterocolitis led to reduced proliferation and increased apoptosis of the intestinal crypts in a manner that could be reversed by inhibition of PUMA, both globally or restricted to the intestinal epithelium. These findings illustrate that TLR4 is expressed on ISCs where it regulates their proliferation and apoptosis through activation of PUMA and that TLR4 regulation of ISCs contributes to the pathogenesis of necrotizing enterocolitis.     

MCPIP1 negatively regulates toll-like receptor 4 signaling and protects mice from LPS-induced septic shock

Septic shock is one of leading causes of morbidity and mortality in hospital patients. However, genetic factors predisposing to septic shock are not fully understood. Our previous work showed that MCP-induced protein 1 (MCPIP1) was induced by lipopolysaccharides (LPSs), which then negatively regulates LPS-induced inflammatory signaling in vitro. Here we report that although MCPIP1 was induced by various toll-like receptor (TLR) ligands in macrophages, MCPIP1-deficient mice are extremely susceptible to TLR4 ligand (LPS)-induced septic shock and death, but not to the TLR2, 3, 5 and 9 ligands-induced septic shock. Consistently, LPS induced tumor necrosis factor α (TNFα) production in MCPIP1-deficient mice was 20-fold greater than that in their wild-type littermates. Further analysis revealed that MCPIP1-deficient mice developed severe acute lung injury after LPS injection and JNK signaling was highly activated in MCPIP1-deficient lungs after LPS stimulation. Finally, macrophage-specific MCPIP1 transgenic mice were partially protected from LPS-induced septic shock, suggesting that inflammatory cytokines from sources other than macrophages may significantly contribute to the pathogenesis of LPS-induced septic shock. Taken together, these results suggest that MCPIP1 selectively suppresses TLR4 signaling pathway and protects mice from LPS-induced septic shock.     

Toll-like receptor 4 (TLR4) is essential for Hsp70-like protein 1 (HSP70L1) to activate dendritic cells and induce Th1 response

Toll-like receptors (TLRs) play important roles in initiation of innate and adaptive immune responses. Emerging evidence suggests that TLR agonists can serve as potential adjuvant for vaccination. Heat shock proteins (HSPs), functionally serving as TLR4 agonists, have been proposed to act as Th1 adjuvant. We have identified a novel Hsp70 family member, termed Hsp70-like protein 1 (Hsp70L1), shown that Hsp70L1 is a potent T helper cell (Th1) polarizing adjuvant that contributes to antitumor immune responses. However, the underlying mechanism for how Hsp70L1 exerts its Th1 adjuvant activity remains to be elucidated. In this study, we found that Hsp70L1 binds directly to TLR4 on the surface of DCs, activates MAPK and NF-κB pathways, up-regulates I-a(b), CD40, CD80, and CD86 expression and promotes production of TNF-α, IL-1β, and IL-12p70. Hsp70L1 failed to induce such phenotypic maturation and cytokine production in TLR4-deficient DCs, indicating a role for TLR4 in mediating Hsp70L1-induced DC activation. Furthermore, more efficient induction of carcinoembryonic antigen (CEA)-specific Th1 immune response was observed in mice immunized by wild-type DCs pulsed with Hsp70L1-CEA(576-669) fusion protein as compared with TLR4-deficient DCs pulsed with same fusion protein. In addition, TLR4 antagonist impaired induction of CEA-specific human Th1 immune response in a co-culture system of peripheral blood lymphocytes (PBLs) from HLA-A2.1(+) healthy donors and autologous DCs pulsed with Hsp70L1-CEA(576-669) in vitro. Taken together, these results demonstrate that TLR4 is a key receptor mediating the interaction of Hsp70L1 with DCs and subsequently enhancing the induction of Th1 immune response by Hsp70L1/antigen fusion protein.     

A novel role of the lumican core protein in bacterial lipopolysaccharide-induced innate immune response

Lumican is an extracellular matrix protein modified as a proteoglycan in some tissues. The core protein with leucine-rich repeats, characteristic of the leucine-rich-repeat superfamily, binds collagen fibrils and regulates its structure. In addition, we believe that lumican sequestered in the pericellular matrix interacts with cell surface proteins for specific cellular functions. Here we show that bacterial lipopolysaccharide sensing by the Toll-like receptor 4 signaling pathway and innate immune response is regulated by lumican. Primary cultures of lumican-deficient (Lum(-/-)) macrophages show impaired innate immune response to lipopolysaccharides with lower induction of tumor necrosis factor alpha (TNFalpha) and interleukin-6. Macrophage response to other pathogen-associated molecular patterns is not adversely affected by lumican deficiency, suggesting a specific role for the lumican core protein in the Toll-like receptor 4 pathway. An exogenous recombinant lumican core protein increases lipopolysaccharide-mediated TNFalpha induction and partially rescues innate immune response in Lum(-/-) macrophages. We further show that the core protein binds lipopolysaccharide. Immunoprecipitation of lumican from peritoneal lavage co-precipitates CD14, a cell surface lipopolysaccharide-binding protein that is involved in its presentation to Toll-like receptor 4. The Lum(-/-) mice are hypo-responsive to lipopolysaccharide-induced septic shock, with poor induction of pro-inflammatory cytokines, TNFalpha, and interleukins 1beta and 6 in the serum. Taken together, the data indicates a novel role for lumican in the presentation of bacterial lipopolysaccharide to CD14 and host response to this bacterial endotoxin.     

Minimally modified LDL binds to CD14, induces macrophage spreading via TLR4/MD-2, and inhibits phagocytosis of apoptotic cells

Minimally modified low density lipoprotein (mmLDL) is a pro-inflammatory and pro-atherogenic lipoprotein that, unlike profoundly oxidized LDL (OxLDL), is not recognized by scavenger receptors and thus does not have enhanced uptake by macrophages. However, here we demonstrate that mmLDL (as well as OxLDL) induces actin polymerization and spreading of macrophages, which results in such pro-atherogenic consequences as inhibition of phagocytosis of apoptotic cells but enhancement of OxLDL uptake. We also demonstrate for the first time that the lipopolysaccharide receptor, CD14, and toll-like receptor-4/MD-2 are involved in these mmLDL effects. Macrophages of the J774 cell line exhibited higher mmLDL binding and F-actin response than its CD14-deficient mutant, LR-9 cells. Similarly, Chinese hamster ovary cells transfected with human CD14 specifically bound mmLDL and responded with higher F-actin compared with control cells. Macrophages from C3H/HeJ mice, which have a point mutation in the Tlr4 gene, responded with lower F-actin to mmLDL and did not spread as well as macrophages from control animals. A significantly higher F-actin response was also observed in Chinese hamster ovary cells transfected with human toll-like receptor-4/MD-2 but not with TLR4 alone or TLR2. Thus, in addition to inhibition of phagocytosis, the recognition of mmLDL by macrophage lipopolysaccharide receptors results in convergence of cellular immune responses to products of microorganisms and to oxidation-specific self-antigens, which could both influence macrophage function and atherogenesis.     

Membrane sorting of toll-like receptor (TLR)-2/6 and TLR2/1 heterodimers at the cell surface determines heterotypic associations with CD36 and intracellular targeting

Toll-like receptors (TLRs) are receptors of the innate immune system responsible for recognizing pathogen-associated molecular patterns. TLR2 seems to be the most promiscuous TLR receptor able to recognize the most diverse set of pathogen-associated patterns. Its promiscuity has been attributed to its unique ability to heterodimerize with TLRs 1 and 6 and, most recently, to its association with CD36 in response to diacylated lipoproteins. Thus, it seems that TLR2 forms receptor clusters in response to different microbial ligands. In this study we investigated TLR2 cell surface heterotypic interactions in response to different ligands as well as internalization and intracellular trafficking. Our data show that TLR2 forms heterodimers with TLR1 and TLR6 and that these heterodimer pre-exist and are not induced by the ligand. Upon stimulation by the specific ligand, these heterodimers are recruited within lipid rafts. In contrast, heterotypic associations of TLR2/6 with CD36 are not preformed and are ligand-induced. All TLR2 receptor clusters accumulate in lipid rafts and are targeted to the Golgi apparatus. This localization and targeting is ligand-specific. Activation occurs at the cell surface, and the observed trafficking is independent of signaling.     

TLR4 recognizes Pseudallescheria boydii conidia and purified rhamnomannans

Pseudallescheria boydii (Scedosporium apiospermum) is a saprophytic fungus widespread in the environment, and has recently emerged as an agent of localized as well as disseminated infections, particularly mycetoma, in immunocompromised and immunocompetent hosts. We have previously shown that highly purified α-glucan from P. boydii activates macrophages through Toll-like receptor TLR2, however, the mechanism of P. boydii recognition by macrophage is largely unknown. In this work, we investigated the role of innate immune receptors in the recognition of P. boydii. Macrophages responded to P. boydii conidia and hyphae with secretion of proinflammatory cytokines. The activation of macrophages by P. boydii conidia required functional MyD88, TLR4, and CD14, whereas stimulation by hyphae was independent of TLR4 and TLR2 signaling. Removal of peptidorhamnomannans from P. boydii conidia abolished induction of cytokines by macrophages. A fraction highly enriched in rhamnomannans was obtained and characterized by NMR, high performance TLC, and GC-MS. Preparation of rhamnomannans derived from P. boydii triggered cytokine release by macrophages, as well as MAPKs phosphorylation and IκBα degradation. Cytokine release induced by P. boydii-derived rhamnomannans was dependent on TLR4 recognition and required the presence of non-reducing end units of rhamnose of the rhamnomannan, but not O-linked oligosaccharides from the peptidorhamnomannan. These results imply that TLR4 recognizes P. boydii conidia and this recognition is at least in part due to rhamnomannans expressed on the surface of P. boydii.     

The receptor for heat shock protein 60 on macrophages is saturable, specific, and distinct from receptors for other heat shock proteins.

Previous studies have shown that human heat shock protein (hsp) 60 elicits a strong proinflammatory response in cells of the innate immune system with CD14, Toll-like receptor (TLR) 2, and TLR4 as mediators of signaling, but probably not of binding. In the present study, we directly demonstrate binding of hsp60 to the macrophage surface and find the binding receptor for hsp60 different from the previously described common receptor for several other heat shock proteins, including hsp70, hsp90, and gp96. Fluorescence-labeled human hsp60 bound to cell surfaces of the murine macrophage lines J774 A.1 and RAW264.7 and to mouse bone marrow-derived macrophages. By flow cytometry, we could demonstrate for the first time that hsp60 binding to macrophages occurred at submicromolar concentrations, is saturable, and can be competed by unlabeled hsp60, but not by unrelated proteins, thus confirming the classic characteristics of specific ligand-receptor interactions. Binding of hsp60 at 4 degrees C was followed by endocytosis at 37 degrees C. Hsp60 binding to macrophages could not be competed by excess hsp70, hsp90, or gp96, all of which share the alpha(2)-macroglobulin receptor as binding site. Hsp60 binding occurred in the absence of surface TLR4. However, no cytokine response was induced by hsp60 in TLR4-deficient macrophages. We conclude that hsp60 binds to a stereo-specific receptor on macrophages, and that different surface molecules are engaged in binding and signal transduction. Furthermore, the binding site for hsp60 is separate from the common receptor for hsp70, hsp90, and gp96, which suggests an independent role of hsp60 as danger Ag and in immunoregulation.     

CsgA is a pathogen-associated molecular pattern of Salmonella enterica serotype Typhimurium that is recognized by Toll-like receptor 2

Knowledge about the origin and identity of the microbial products recognized by the innate immune system is important for understanding the pathogenesis of inflammatory diseases. We investigated the potential role of Salmonella enterica serotype Typhimurium fimbriae as pathogen-associated molecular patterns (PAMPs) that may stimulate innate pathways of inflammation. We screened a panel of 11 mutants, each carrying a deletion of a different fimbrial operon, for their enteropathogenicity using the calf model of human gastroenteritis. One mutant (csgBA) was attenuated in its ability to elicit fluid accumulation and GROalpha mRNA expression in bovine ligated ileal loops. The mechanism by which thin curled fimbriae encoded by the csg genes contribute to inflammation was further investigated using tissue culture. The S. Typhimurium csgBA mutant induced significantly less IL-8 production than the wild type in human macrophage-like cells. Purified thin curled fimbriae induced IL-8 expression in human embryonic kidney (HEK293) cells transfected with Toll-like receptor (TLR) 2/CD14 but not in cells transfected with TLR5, TLR4/MD2/CD14 or TLR11. Fusion proteins between the major fimbrial subunit of thin curled fimbriae (CsgA) and glutathione-S-transferase (GST) elicited IL-8 production in HEK293 cells transfected with TLR2/CD14. Proteinase K treatment abrogated IL-8 production elicited in these cells by GST-CsgA, but not by synthetic lipoprotein. GST-CsgA elicited more IL-6 production than GST in bone marrow-derived macrophages from TLR2+/+ mice, while there was no difference in IL-6 secretion between GST-CsgA and GST in macrophages from TLR2-/- mice. These data suggested that CsgA is a PAMP that is recognized by TLR2.     

Toll-like Receptor 3-mediated Necrosis via TRIF,RIP3, and MLKL

Toll-like receptor (TLR) signaling is triggered by pathogen-associated molecular patterns that mediate well established cytokine-driven pathways, activating NF-κB together with IRF3/IRF7. In addition, TLR3 drives caspase 8-regulated programmed cell death pathways reminiscent of TNF family death receptor signaling. We find that inhibition or elimination of caspase 8 during stimulation of TLR2, TLR3, TLR4, TLR5, or TLR9 results in receptor interacting protein (RIP) 3 kinase-dependent programmed necrosis that occurs through either TIR domain-containing adapter-inducing interferon-β (TRIF) or MyD88 signal transduction. TLR3 or TLR4 directly activates programmed necrosis through a RIP homotypic interaction motif-dependent association of TRIF with RIP3 kinase (also called RIPK3). In fibroblasts, this pathway proceeds independent of RIP1 or its kinase activity, but it remains dependent on mixed lineage kinase domain-like protein (MLKL) downstream of RIP3 kinase. Here, we describe two small molecule RIP3 kinase inhibitors and employ them to demonstrate the common requirement for RIP3 kinase in programmed necrosis induced by RIP1-RIP3, DAI-RIP3, and TRIF-RIP3 complexes. Cell fate decisions following TLR signaling parallel death receptor signaling and rely on caspase 8 to suppress RIP3-dependent programmed necrosis whether initiated directly by a TRIF-RIP3-MLKL pathway or indirectly via TNF activation and the RIP1-RIP3-MLKL necroptosis pathway.     

Toll-like receptor ligands directly promote activated CD4+ T cell survival

Toll-like receptor (TLR) engagement by pathogen-associated molecular patterns (PAMPs) is an important mechanism for optimal cellular immune responses. APC TLR engagement indirectly enhances activated CD4(+) T cell proliferation, differentiation, and survival by promoting the up-regulation of costimulatory molecules and the secretion of proinflammatory cytokines. However, TLRs are also expressed on CD4(+) T cells, suggesting that PAMPs may also act directly on activated CD4(+) T cells to mediate functional responses. In this study, we show that activated mouse CD4(+) T cells express TLR-3 and TLR-9 but not TLR-2 and TLR-4. Treatment of highly purified activated CD4(+) T cells with the dsRNA synthetic analog poly(I:C) and CpG oligodeoxynucleotides (CpG DNA), respective ligands for TLR-3 and TLR-9, directly enhanced their survival without augmenting proliferation. In contrast, peptidoglycan and LPS, respective ligands for TLR-2 and TLR-4 had no effect. Enhanced survival mediated by either poly(I:C) or CpG DNA required NF-kappaB activation and was associated with Bcl-x(L) up-regulation. However, only CpG DNA, but not poly(I:C)-mediated effects on activated CD4(+) T cells required the TLR/IL-1R domain containing adaptor molecule myeloid differentiation factor 88. Collectively, our results demonstrate that PAMPs can directly promote activated CD4(+) T cell survival, suggesting that TLRs on T cells can directly modulate adaptive immune responses.     

CD14 Mediates Toll-like Receptor 4 (TLR4) Endocytosis and Spleen Tyrosine Kinase (Syk) and Interferon Regulatory Transcription Factor 3 (IRF3) Activation in Epithelial Cells and Impairs Neutrophil Infiltration and Pseudomonas aeruginosa Killing in Vivo

In the current study, we examined the role of CD14 in regulating LPS activation of corneal epithelial cells and Pseudomonas aeruginosa corneal infection. Our findings demonstrate that LPS induces Toll-like receptor 4 (TLR4) internalization in corneal epithelial cells and that blocking with anti-CD14 selectively inhibits TLR4 endocytosis, spleen tyrosine kinase (Syk) and IRF3 phosphorylation, and production of CCL5/RANTES and IFN-β, but not IL-8. Using a murine model of P. aeruginosa corneal infection, we show that although infected CD14^−/− corneas produce less CCL5, they exhibit significantly increased CXC chemokine production, neutrophil recruitment to the corneal stroma, and bacterial clearance than C57BL/6 mice. We conclude that CD14 has a critical role in mediating TLR4 signaling through IRF3 in resident corneal epithelial cells and macrophages and thereby modulates TLR4 cell surface activation of the MyD88/NF-κB/AP-1 pathway and production of CXC chemokines and neutrophil infiltration to infected tissues.     

Toll-like receptor 4 in atherosclerosis

Toll-like receptor 4 (TLR4) is key regulators of both innate and adaptive immune responses. TLR4 recognizes pathogen-associated molecular patterns (PAMPs) and activates the inflammatory cells. The function of TLR4 in atherosclerosis has been investigated in mouse knockout studies and epidemiological studies of human TLR4 polymorphisms. These studies have shown that TLR4 function affects the initiation and progression of atherosclerosis. This article reviews the biological functions and clinical implications of TLR4 in atherosclerosis.     

Lgr4 Protein Deficiency Induces Ataxia-like Phenotype in Mice and Impairs Long Term Depression at Cerebellar Parallel Fiber-Purkinje Cell Synapses

Cerebellar dysfunction causes ataxia characterized by loss of balance and coordination. Until now, the molecular and neuronal mechanisms of several types of inherited cerebellar ataxia have not been completely clarified. Here, we report that leucine-rich G protein-coupled receptor 4 (Lgr4/Gpr48) is highly expressed in Purkinje cells (PCs) in the cerebellum. Deficiency of Lgr4 leads to an ataxia-like phenotype in mice. Histologically, no obvious morphological changes were observed in the cerebellum of Lgr4 mutant mice. However, the number of PCs was slightly but significantly reduced in Lgr4^−/− mice. In addition, in vitro electrophysiological analysis showed an impaired long term depression (LTD) at parallel fiber-PC (PF-PC) synapses in Lgr4^−/− mice. Consistently, immunostaining experiments showed that the level of phosphorylated cAMP-responsive element-binding protein (Creb) was significantly decreased in Lgr4^−/− PCs. Furthermore, treatment with forskolin, an adenylyl cyclase agonist, rescued phospho-Creb in PCs and reversed the impairment in PF-PC LTD in Lgr4^−/− cerebellar slices, indicating that Lgr4 is an upstream regulator of Creb signaling, which is underlying PF-PC LTD. Together, our findings demonstrate for first time an important role for Lgr4 in motor coordination and cerebellar synaptic plasticity and provide a potential therapeutic target for certain types of inherited cerebellar ataxia.     

CD36 Differently Regulates Macrophage Responses to Smooth and Rough Lipopolysaccharide

Lipopolysaccharide (LPS) is the major pathogen-associated molecular pattern of Gram-negative bacterial infections, and includes smooth (S-LPS) and rough (R-LPS) chemotypes. Upon activation by LPS through CD14, TLR4/MD-2 heterodimers sequentially induce two waves of intracellular signaling for macrophage activation: the MyD88-dependent pathway from the plasma membrane and, following internalization, the TRIF-dependent pathway from endosomes. We sought to better define the role of scavenger receptors CD36 and CD204/SR-A as accessory LPS receptors that can contribute to pro-inflammatory and microbicidal activation of macrophages. We have found that CD36 differently regulates activation of mouse macrophages by S-LPS versus R-LPS. The ability of CD36 to substitute for CD14 in loading R-LPS, but not S-LPS onto TLR4/MD-2 allows CD14-independent macrophage responses to R-LPS. Conversely, S-LPS, but not R-LPS effectively stimulates CD14 binding to CD36, which favors S-LPS transfer from CD14 onto TLR4/MD-2 under conditions of low CD14 occupancy with S-LPS in serum-free medium. In contrast, in the presence of serum, CD36 reduces S-LPS binding to TLR4/MD-2 and the subsequent MyD88-dependent signaling, by mediating internalization of S-LPS/CD14 complexes. Additionally, CD36 positively regulates activation of TRIF-dependent signaling by both S-LPS and R-LPS, by promoting TLR4/MD-2 endocytosis. In contrast, we have found that SR-A does not function as a S-LPS receptor. Thus, by co-operating with CD14 in both R- and S-LPS loading onto TLR4/MD-2, CD36 can enhance the sensitivity of tissue-resident macrophages in detecting infections by Gram-negative bacteria. However, in later phases, following influx of serum to the infection site, the CD36-mediated negative regulation of MyD88-dependent branch of S-LPS-induced TLR4 signaling might constitute a mechanism to prevent an excessive inflammatory response, while preserving the adjuvant effect of S-LPS for adaptive immunity.