MyD88-dependent IL-1 receptor signaling is essential for gouty inflammation stimulated by monosodium urate crystals

While it is known that monosodium urate (MSU) crystals cause the disease gout, the mechanism by which these crystals stimulate this inflammatory condition has not been clear. Here we find that the Toll/IL-1R (TIR) signal transduction adaptor myeloid differentiation primary response protein 88 (MyD88) is required for acute gouty inflammation. In contrast, other TIR adaptor molecules, TIRAP/Mal, TRIF, and TRAM, are not required for this process. The MyD88-dependent TLR1, -2, -4, -6, -7, -9, and -11 and IL-18 receptor (IL-18R) are not essential for MSU-induced inflammation. Moreover, MSU does not stimulate HEK cells expressing TLR1-11 to activate NF-kappaB. In contrast, mice deficient in the MyD88-dependent IL-1R showed reduced inflammatory responses, similar to those observed in MyD88-deficient mice. Similarly, mice treated with IL-1 neutralizing antibodies also showed reduced MSU-induced inflammation, demonstrating that IL-1 production and IL-1R activation play essential roles in MSU-triggered inflammation. IL-1R deficiency in bone marrow-derived cells did not affect the inflammatory response; however, it was required in non-bone marrow-derived cells. These results indicate that IL-1 is essential for the MSU-induced inflammatory response and that the requirement of MyD88 in this process is primarily through its function as an adaptor molecule in the IL-1R signaling pathway.     

TIR domain--containing adaptors regulate TLR-mediated signaling pathways

Recognition of pathogens by Toll-like receptors (TLRs) triggers innate immune responses via signaling pathways mediated by several Toll/IL-1R (TIR) domain-containing adaptors such as MyD88, TIRAP, and TRIF. MyD88 is a common adaptor that is essential for proinflammatory cytokine production, whereas TRIF mediates the MyD88-independent pathway from TLR3 and TLR4 that is responsible for type I interferon production in response to double-stranded RNA and LPS, respectively. TIRAP specifically participates in the MyD88-dependent pathways shared by TLR2 and TLR4, and TRAM is essential for the TLR4-mediated MyD88-independent pathway. Thus, TIR domain-containing adaptors play an important role in the TLR mediated signaling pathways.     

Osteoclastogenesis through TLR/NOD signaling

Lipopolysaccharide (LPS) and muramyl dipeptide (MDP) are components of microbial cell walls that cause innate immune responses and inflammation. Toll-like receptor 4 (TLR4) is a receptor for LPS and transduces signals through myeloid differentiation factor 88 (MyD88), which plays essential roles in the TLR/IL-1R signaling and activates NF-kappaB and MAP kinase pathways to induce RANKL expression in osteoblasts. Osteoblasts express NOD2, an intracellular sensor for MDP, in response to LPS, IL-1 and TNF. NOD2 binds RIP2, a serine/threonine kinase which transduces NF-kappaB signaling. Thus MDP synergistically enhances osteoclast formation induced by LPS, IL-1 and TNF through RANK ligand up-regulation in osteoblasts. In summary, innate immune receptors, TLR4 and NOD2, recognize bacterial components on cell surfaces and inside cells, respectively, and these signals cross-talk to induce RANKL expression in osteoblasts, which results in enhancing osteoclast formation and function.     

Immune cell activation by bacterial CpG-DNA through myeloid differentiation marker 88 and tumor necrosis factor receptor-associated factor (TRAF)6

Transition of immature antigen presenting cells (APCs) to the state of professional APCs is essential for initiation of cell-mediated immune responses to pathogens. Signal transduction via molecules of the Toll-like receptor (TLR)/interleukin 1 receptor (IL-1R) pathway is critical for activation of APCs either by pathogen-derived pattern ligands like lipopolysaccharides (LPS) or by CD40 ligation through T helper cells. The capacity of bacterial DNA (CpG-DNA) to induce APCs to differentiate into professional APCs represents an interesting discovery. However, the signaling pathways involved are poorly understood. Here we show that CpG-DNA activates the TLR/IL-1R signaling pathway via the molecules myeloid differentiation marker 88 (MyD88) and tumor necrosis factor receptor-associated factor 6 (TRAF6), leading to activation of kinases of the IkappaB kinase complex and the c-jun NH(2)-terminal kinases. Moreover, cells of TLR2- and TLR4-deficient mice are activated by CpG-DNA, whereas cells of MyD88-deficient mice do not respond. The data suggest that CpG-DNA initiates signaling via the TLR/IL-1R pathway in APCs in a manner similar to LPS and to T helper cell-mediated CD40 ligation. Activation of the TLR/IL-1R signaling pathway by foreign bacterial DNA may be one way to initiate innate defense mechanisms against infectious pathogens in vivo.     

Vaccinia virus protein A46R targets multiple Toll-like-interleukin-1 receptor adaptors and contributes to virulence

Viral immune evasion strategies target key aspects of the host antiviral response. Recently, it has been recognized that Toll-like receptors (TLRs) have a role in innate defense against viruses. Here, we define the function of the vaccinia virus (VV) protein A46R and show it inhibits intracellular signalling by a range of TLRs. TLR signalling is triggered by homotypic interactions between the Toll-like-interleukin-1 resistance (TIR) domains of the receptors and adaptor molecules. A46R contains a TIR domain and is the only viral TIR domain-containing protein identified to date. We demonstrate that A46R targets the host TIR adaptors myeloid differentiation factor 88 (MyD88), MyD88 adaptor-like, TIR domain-containing adaptor inducing IFN-beta (TRIF), and the TRIF-related adaptor molecule and thereby interferes with downstream activation of mitogen-activated protein kinases and nuclear factor kappaB. TRIF mediates activation of interferon (IFN) regulatory factor 3 (IRF3) and induction of IFN-beta by TLR3 and TLR4 and suppresses VV replication in macrophages. Here, A46R disrupted TRIF-induced IRF3 activation and induction of the TRIF-dependent gene regulated on activation, normal T cell expressed and secreted. Furthermore, we show that A46R is functionally distinct from another described VV TLR inhibitor, A52R. Importantly, VV lacking the A46R gene was attenuated in a murine intranasal model, demonstrating the importance of A46R for VV virulence.     

LPS-TLR4 signaling to IRF-3/7 and NF-kappaB involves the toll adapters TRAM and TRIF

Toll-IL-1-resistance (TIR) domain-containing adaptor-inducing IFN-beta (TRIF)-related adaptor molecule (TRAM) is the fourth TIR domain-containing adaptor protein to be described that participates in Toll receptor signaling. Like TRIF, TRAM activates interferon regulatory factor (IRF)-3, IRF-7, and NF-kappaB-dependent signaling pathways. Toll-like receptor (TLR)3 and 4 activate these pathways to induce IFN-alpha/beta, regulated on activation, normal T cell expressed and secreted (RANTES), and gamma interferon-inducible protein 10 (IP-10) expression independently of the adaptor protein myeloid differentiation factor 88 (MyD88). Dominant negative and siRNA studies performed here demonstrate that TRIF functions downstream of both the TLR3 (dsRNA) and TLR4 (LPS) signaling pathways, whereas the function of TRAM is restricted to the TLR4 pathway. TRAM interacts with TRIF, MyD88 adaptor-like protein (Mal)/TIRAP, and TLR4 but not with TLR3. These studies suggest that TRIF and TRAM both function in LPS-TLR4 signaling to regulate the MyD88-independent pathway during the innate immune response to LPS.     

Essential role of IRAK-4 protein and its kinase activity in Toll-like receptor-mediated immune responses but not in TCR signaling

Interleukin-1 receptor-associated kinase 4 (IRAK-4) was reported to be essential for the Toll-like receptor (TLR)- and T cell receptor (TCR)-mediated signaling leading to the activation of nuclear factor kappaB (NF-kappaB). However, the importance of kinase activity of IRAK family members is unclear. In this study, we investigated the functional role of IRAK-4 activity in vivo by generating mice carrying a knockin mutation (KK213AA) that abrogates its kinase activity. IRAK-4(KN/KN) mice were highly resistant to TLR-induced shock response. The cytokine production in response to TLR ligands was severely impaired in IRAK-4(KN/KN) as well as IRAK-4(-/-) macrophages. The IRAK-4 activity was essential for the activation of signaling pathways leading to mitogen-activated protein kinases. TLR-induced IRAK-4/IRAK-1-dependent and -independent pathways were involved in early induction of NF-kappaB-regulated genes in response to TLR ligands such as tumor necrosis factor alpha and IkappaBzeta. In contrast to a previous paper (Suzuki, N., S. Suzuki, D.G. Millar, M. Unno, H. Hara, T. Calzascia, S. Yamasaki, T. Yokosuka, N.J. Chen, A.R. Elford, et al. 2006. Science. 311:1927-1932), the TCR signaling was not impaired in IRAK-4(-/-) and IRAK-4(KN/KN) mice. Thus, the kinase activity of IRAK-4 is essential for the regulation of TLR-mediated innate immune responses.     

Polyubiquitin binding to ABIN1 is required to prevent autoimmunity

The protein ABIN1 possesses a polyubiquitin-binding domain homologous to that present in nuclear factor κB (NF-κB) essential modulator (NEMO), a component of the inhibitor of NF-κB (IκB) kinase (IKK) complex. To address the physiological significance of polyubiquitin binding, we generated knockin mice expressing the ABIN1[D485N] mutant instead of the wild-type (WT) protein. These mice developed all the hallmarks of autoimmunity, including spontaneous formation of germinal centers, isotype switching, and production of autoreactive antibodies. Autoimmunity was suppressed by crossing to MyD88(-/-) mice, demonstrating that toll-like receptor (TLR)-MyD88 signaling pathways are needed for the phenotype to develop. The B cells and myeloid cells of the ABIN1[D485N] mice showed enhanced activation of the protein kinases TAK, IKK-α/β, c-Jun N-terminal kinases, and p38α mitogen-activated protein kinase and produced more IL-6 and IL-12 than WT. The mutant B cells also proliferated more rapidly in response to TLR ligands. Our results indicate that the interaction of ABIN1 with polyubiquitin is required to limit the activation of TLR-MyD88 pathways and prevent autoimmunity.     

Interleukin 1 receptor dependence of serum transferred arthritis can be circumvented by toll-like receptor 4 signaling

Inflammatory arthritis is associated with the release of a network of key cytokines. In T cell receptor transgenic K/BxN mice interleukin (IL)-1 plays a key role in joint swelling and destruction, as suggested by the ability of anti-IL-1receptor (IL-1R) antibody treatment to delay the onset and slow the progression of this disease. This mechanism is dependent on the signaling pathway intermediary myeloid differentiation factor 88 (MyD88), such that neither IL-1R nor MyD88-deficient mice developed visually detectable synovitis after transfer of arthritogenic sera. The Toll-like receptors (TLRs) share the same signaling pathway through MyD88 as the IL-1R. The administration of a TLR-4 ligand, lipopolysaccharide, concomitant with arthritogenic serum in IL-1 receptor-deficient mice resulted in acute paw swelling, but not in MyD88-deficient mice. Also, serum transferred arthritis was not sustained in TLR-4 mutant mice compared with controls. These results suggest that innate immune functions via TLR-4 might perpetuate inflammatory mechanisms and bypass the need for IL-1 in chronic joint inflammation.     

Intracellular recognition of lipopolysaccharide by toll-like receptor 4 in intestinal epithelial cells

Toll-like receptor (TLR)4 has recently been shown to reside in the Golgi apparatus of intestinal crypt epithelial m-ICcl2 cells, colocalizing with internalized lipopolysaccharide (LPS). Here we demonstrate that disruption of the integrity of the Golgi apparatus significantly reduced LPS-mediated nuclear factor kappaB activation. Also, the TLR4 adaptor protein MyD88 and the serine/threonine kinase IRAK-1 were rapidly recruited to the Golgi apparatus upon stimulation. LPS-mediated activation required lipid raft formation and intact clathrin-dependent internalization. In contrast to macrophages, prevention of ligand internalization by use of LPS-coated beads significantly impaired recognition by epithelial cells. The localization of TLR4 to the Golgi apparatus was abrogated by expression of a genetically modified form of the TLR4 binding chaperone gp96. Thus, our data provide evidence that in contrast to the situation in macrophages, LPS recognition in intestinal epithelial cells may occur in the Golgi apparatus and require LPS internalization.     

TLR2 is mobilized into an apical lipid raft receptor complex to signal infection in airway epithelial cells

Toll-like receptors (TLRs) mediate host responses to bacterial gene products. As the airway epithelium is potentially exposed to many diverse inhaled bacteria, TLRs involved in defense of the airways must be broadly responsive, available at the exposed apical surface of the cells, and highly regulated to prevent activation following trivial encounters with bacteria. We demonstrate that TLR2 is enriched in caveolin-1-associated lipid raft microdomains presented on the apical surface of airway epithelial cells after bacterial infection. These receptor complexes include myeloid differentiation protein (MyD88), interleukin-1 receptor-activated kinase-1, and TNF receptor-associated factor 6. The signaling capabilities of TLR2 are amplified through its association with the asialoganglioside gangliotetraosylceramide (Gal beta 1,2GalNAc beta 1,4Gal beta 1,4Glc beta 1,1Cer), which has receptor function itself for many pulmonary pathogens. Ligation of either TLR2 or asialoGM1 by ligands with specificity for either receptor, by Pseudomonas aeruginosa, or by Staphylococcus aureus stimulates IL-8 production through activation of NF-kappa B, as mediated by TLR2 and MyD88. Thus, TLR2 in association with asialo-glycolipids presented within the context of lipid rafts provides a broadly responsive signaling complex at the apical surfaces of airway cells to initiate the host response to potential bacterial infection.     

Antileishmanial Effect of 18β-Glycyrrhetinic Acid Is Mediated by Toll-Like Receptor-Dependent Canonical and Noncanonical p38 Activation

18β-Glycyrrhetinic acid (GRA), a natural immunomodulator, greatly reduced the parasite load in experimental visceral leishmaniasis through nitric oxide (NO) upregulation, proinflammatory cytokine expression, and NF-κB activation. For the GRA-mediated effect, the primary kinase responsible was found to be p38, and analysis of phosphorylation kinetics as well as studies with dominant-negative (DN) constructs revealed mitogen-activated protein kinase kinase 3 (MKK3) and MKK6 as the immediate upstream regulators of p38. However, detection of remnant p38 kinase activity in the presence of both DN MKK3 and MKK6 suggested alternative pathways of p38 activation. That residual p38 activity was attributed to an autophosphorylation event ensured by the transforming growth factor β-activated kinase 1 (TAK1)-binding protein 1 (TAB1)-p38 interaction and was completely abolished upon pretreatment with SB203580 in DN MKK3/6 double-transfected macrophage cells. Further upstream signaling evaluation by way of phosphorylation kinetics and transfection studies with DN constructs identified TAK1, myeloid differentiation factor 88 (MyD88), interleukin 1 receptor (IL-1R)-activated kinase 1 (IRAK1), and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) as important contributors to GRA-mediated macrophage activation. Finally, gene knockdown studies revealed Toll-like receptor 2 (TLR2) and TLR4 as the membrane receptors associated with GRA-mediated antileishmanial activity. Together, the results of this study brought mechanistic insight into the antileishmanial activity of GRA, which is dependent on the TLR2/4-MyD88 signaling axis, leading to MKK3/6-mediated canonical and TAB1-mediated noncanonical p38 activation.     

小儿脓毒症Toll样受体信号途径转导分子及调节因子的变化研究

目的 探讨Toll样受体(TLRs)信号途径转导分子及调节因子在小儿0脓毒症异常炎症免疫反应发病机制中的可能作用.方法 选择脓毒症患儿10例、严重脓毒症患儿13例及同期健康体检儿童17例.采用实时荧光定量聚合酶链反应(real-time PCR)检测前炎症细胞因子[肿瘤坏死因子-a(TNF-a)、白细胞介素-1β(IL-1β)、白细胞介素-6(IL-6)],以及TLRs信号途径转导分子和调节因子的mRNA表达;采用酶联免疫吸附法(ELISA)检测前炎症细胞因子蛋白水平.结果 与健康对照组比较,脓毒症组TNF-a、IL-1β和IL-6的mRNA和蛋白表达均增加,TLRs信号途径转导分子TLR2、TLR4、髓样细胞分化蛋白-88(MyD88)、TNF相关因子6(TRAF6)、IL-1受体相关激酶4(IRAK4)、转化生长因子-β活化激酶1(TAKl)、TAK1结合蛋白2(TAB2)的mRNA表达以及TLRs信号途径正性调节因子TLR4相关蛋白(PRAT4B)、信号转换接头蛋白2(STAP2)的mRNA表达均明显增高(P均<0.01),且严重脓毒症组较脓毒症组升高更显著(p均<0.01).脓毒症组TLRs信号途径负性调节因子IL-1受体相关激酶3(IRAK-M)、核转录因子-kB抑制性锌指蛋白(Triad3A)的mRNA表达均明显增高(P均<0.01),严重脓毒症组表达则明显低于脓毒症组(P均<0.01).结论 TLRs信号途径转导分子/调节因子异常表达可能是脓毒症时全身炎症反应的原因之一.     

Toll-Like Receptor 4 Signaling Contributes to Paclitaxel-Induced Peripheral Neuropathy

This paper tests the contribution of the toll-like receptors, TLR4 in particular, in the initiation and maintenance of paclitaxel-related chemotherapy-induced peripheral neuropathy. TLR4 and its immediate downstream signaling molecules—myeloid differentiation primary response gene 88 (MyD88) and toll/interleukin 1 receptor domain–containing adapter-inducing interferon-β (TRIF)—were found to be increased in the dorsal root ganglion (DRG) using Western blot by day 7 of paclitaxel treatment. The behavioral phenotype, the increase of both TLR4 and MyD88, was blocked by cotreatment with the TLR4 antagonist lipopolysaccharide–Rhodobacter sphaeroides during chemotherapy. A similar, but less robust, behavioral effect was observed using intrathecal treatment of MyD88 homodimerization inhibitory peptide. DRG levels of TLR4 and MyD88 reduced over the next 2 weeks, whereas these levels remained increased in spinal cord through day 21 following chemotherapy. Immunohistochemical analysis revealed TLR4 expression in both calcitonin gene-related peptide–positive and isolectin B4–positive small DRG neurons. MyD88 was only found in calcitonin gene-related peptide–positive neurons, and TRIF was found in both calcitonin gene-related peptide–positive and isolectin B4–positive small DRG neurons as well as in medium- and large-size DRG neurons. In the spinal cord, TLR4 was only found colocalized to astrocytes but not with either microglia or neurons. Intrathecal treatment with the TLR4 antagonist lipopolysaccharide–R. sphaeroides transiently reversed preestablished chemotherapy-induced peripheral neuropathy mechanical hypersensitivity. These results strongly implicate TLR4 signaling in the DRG and the spinal cord in the induction and maintenance of paclitaxel-related chemotherapy-induced peripheral neuropathy.PerspectiveThe toll-like receptor TLR4 and MyD88 signaling pathway could be a new potential therapeutic target in paclitaxel-induced painful neuropathy.     

MyD88 L265P 突变与 B 细胞肿瘤简

髓样分化因子88（myeloid differentiation factor,MyD88）是一种介导多数Toll样受体（Toll-like receptors,TLR）、白介素-1受体（interleukin-1 receptor,IL-1R）、白介素-18受体（interleukin-1 8receptor,IL-18R）细胞信号传导的重要衔接蛋白,在固有免疫中起着重要的作用.近年来有研究发现,在约90％的淋巴浆细胞淋巴瘤/华氏巨球蛋白血症和约29％的激活型弥漫大B细胞淋巴瘤等B细胞肿瘤中有功能活化的MYD88 L265P突变,从而提示MyD88信号转导在B细胞肿瘤发生中有重要意义,因而MyD88抑制剂可能成为治疗B细胞肿瘤的新药.本文就MyD88L265P突变在B细胞肿瘤发生中的最新研究进展进行综述.     

MyD88 but not TRIF is essential for osteoclastogenesis induced by lipopolysaccharide, diacyl lipopeptide, and IL-1alpha

Myeloid differentiation factor 88 (MyD88) plays essential roles in the signaling of the Toll/interleukin (IL)-1 receptor family. Toll-IL-1 receptor domain-containing adaptor inducing interferon-beta (TRIF)-mediated signals are involved in lipopolysaccharide (LPS)-induced MyD88-independent pathways. Using MyD88-deficient (MyD88-/-) mice and TRIF-deficient (TRIF-/-) mice, we examined roles of MyD88 and TRIF in osteoclast differentiation and function. LPS, diacyl lipopeptide, and IL-1alpha stimulated osteoclastogenesis in cocultures of osteoblasts and hemopoietic cells obtained from TRIF-/- mice, but not MyD88-/- mice. These factors stimulated receptor activator of nuclear factor-kappaB ligand mRNA expression in TRIF-/- osteoblasts, but not MyD88-/- osteoblasts. LPS stimulated IL-6 production in TRIF-/- osteoblasts, but not TRIF-/- macrophages. LPS and IL-1alpha enhanced the survival of TRIF-/- osteoclasts, but not MyD88-/- osteoclasts. Diacyl lipopeptide did not support the survival of osteoclasts because of the lack of Toll-like receptor (TLR)6 in osteoclasts. Macrophages expressed both TRIF and TRIF-related adaptor molecule (TRAM) mRNA, whereas osteoblasts and osteoclasts expressed only TRIF mRNA. Bone histomorphometry showed that MyD88-/- mice exhibited osteopenia with reduced bone resorption and formation. These results suggest that the MyD88-mediated signal is essential for the osteoclastogenesis and function induced by IL-1 and TLR ligands, and that MyD88 is physiologically involved in bone turnover.     

"Re-educating" tumor-associated macrophages by targeting NF-kappaB

The nuclear factor kappaB (NF-kappaB) signaling pathway is important in cancer-related inflammation and malignant progression. Here, we describe a new role for NF-kappaB in cancer in maintaining the immunosuppressive phenotype of tumor-associated macrophages (TAMs). We show that macrophages are polarized via interleukin (IL)-1R and MyD88 to an immunosuppressive "alternative" phenotype that requires IkappaB kinase beta-mediated NF-kappaB activation. When NF-kappaB signaling is inhibited specifically in TAMs, they become cytotoxic to tumor cells and switch to a "classically" activated phenotype; IL-12(high), major histocompatibility complex II(high), but IL-10(low) and arginase-1(low). Targeting NF-kappaB signaling in TAMs also promotes regression of advanced tumors in vivo by induction of macrophage tumoricidal activity and activation of antitumor activity through IL-12-dependent NK cell recruitment. We provide a rationale for manipulating the phenotype of the abundant macrophage population already located within the tumor microenvironment; the potential to "re-educate" the tumor-promoting macrophage population may prove an effective and novel therapeutic approach for cancer that complements existing therapies.     

Distinct mutations in IRAK-4 confer hyporesponsiveness to lipopolysaccharide and interleukin-1 in a patient with recurrent bacterial infections

We identified previously a patient with recurrent bacterial infections who failed to respond to gram-negative LPS in vivo, and whose leukocytes were profoundly hyporesponsive to LPS and IL-1 in vitro. We now demonstrate that this patient also exhibits deficient responses in a skin blister model of aseptic inflammation. A lack of IL-18 responsiveness, coupled with diminished LPS and/or IL-1-induced nuclear factor-kappaB and activator protein-1 translocation, p38 phosphorylation, gene expression, and dysregulated IL-1R-associated kinase (IRAK)-1 activity in vitro support the hypothesis that the defect lies within the signaling pathway common to toll-like receptor 4, IL-1R, and IL-18R. This patient expresses a "compound heterozygous" genotype, with a point mutation (C877T in cDNA) and a two-nucleotide, AC deletion (620-621del in cDNA) encoded by distinct alleles of the IRAK-4 gene (GenBank/EMBL/DDBJ accession nos. AF445802 and AY186092). Both mutations encode proteins with an intact death domain, but a truncated kinase domain, thereby precluding expression of full-length IRAK-4 (i.e., a recessive phenotype). When overexpressed in HEK293T cells, neither truncated form augmented endogenous IRAK-1 kinase activity, and both inhibited endogenous IRAK-1 activity modestly. Thus, IRAK-4 is pivotal in the development of a normal inflammatory response initiated by bacterial or nonbacterial insults.     

Leukotriene B4 amplifies NF-κB activation in mouse macrophages by reducing SOCS1 inhibition of MyD88 expression

Activation of NF-κB and 5-lipoxygenase-mediated (5-LO-mediated) biosynthesis of the lipid mediator leukotriene B4 (LTB4) are pivotal components of host defense and inflammatory responses. However, the role of LTB4 in mediating innate immune responses elicited by specific TLR ligands and cytokines is unknown. Here we have shown that responses dependent on MyD88 (an adaptor protein that mediates signaling through all of the known TLRs, except TLR3, as well as IL-1β and IL-18) are reduced in mice lacking either 5-LO or the LTB4 receptor BTL1, and that macrophages from these mice are impaired in MyD88-dependent activation of NF-κB. This macrophage defect was associated with lower basal and inducible expression of MyD88 and reflected impaired activation of STAT1 and overexpression of the STAT1 inhibitor SOCS1. Expression of MyD88 and responsiveness to the TLR4 ligand LPS were decreased by Stat1 siRNA silencing in WT macrophages and restored by Socs1 siRNA in 5-LO-deficient macrophages. These results uncover a pivotal role in macrophages for the GPCR BLT1 in regulating activation of NF-κB through Stat1-dependent expression of MyD88.