Regulates TLR Signaling Research Articles

Integrin αvβ3 Limits Cytokine Production by Plasmacytoid Dendritic Cells and Restricts TLR-Driven Autoimmunity.

Plasmacytoid dendritic cells (pDCs) are strongly implicated as a major source of IFN-I in systemic lupus erythematosus (SLE), triggered through TLR-mediated recognition of nucleic acids released from dying cells. However, relatively little is known about how TLR signaling and IFN-I production are regulated in pDCs. In this article, we describe a role for integrin αvβ3 in regulating TLR responses and IFN-I production by pDCs in mouse models. We show that αv and β3-knockout pDCs produce more IFN-I and inflammatory cytokines than controls when stimulated through TLR7 and TLR9 invitro and invivo. Increased cytokine production was associated with delayed acidification of endosomes containing TLR ligands, reduced LC3 conjugation, and increased TLR signaling. This dysregulated TLR signaling results in activation of B cells and promotes germinal center (GC) B cell and plasma cell expansion. Furthermore, in a mouse model of TLR7-driven lupus-like disease, deletion of αvβ3 from pDCs causes accelerated autoantibody production and pathology. We therefore identify a pDC-intrinsic role for αvβ3 in regulating TLR signaling and preventing activation of autoreactive B cells. Because αvβ3 serves as a receptor for apoptotic cells and cell debris, we hypothesize that this regulatory mechanism provides important contextual cues to pDCs and functions to limit responses to self-derived nucleic acids.

Sterol biosynthesis regulates TLR signaling and the innate immune response in a Smith-Lemli-Opitz syndrome model.

Sterol biosynthesis regulates TLR signaling and the innate immune response in a Smith-Lemli-Opitz syndrome model.

Lysosomal trafficking regulator restricts intracellular growth of Coxiella burnetii by inhibiting the expansion of Coxiella-containing vacuole and upregulating nos2 expression.

Coxiella burnetii is an obligate intracellular bacterium that causes Q fever, a zoonotic disease typically manifests as a severe flu-illness. After invading into the host cells, C. burnetii delivers effectors to regulate the vesicle trafficking and fusion events to form a large and mature Coxiella-containing vacuole (CCV), providing sufficient space and nutrition for its intracellular growth and proliferation. Lysosomal trafficking regulator (LYST) is a member of the Beige and Chediak-Higashi syndrome (BEACH) family, which regulates the transport of vesicles to lysosomes and regulates TLR signaling pathway, but the effect of LYST on C. burnetii infection is unclear. In this study, a series of experiments has been conducted to investigate the influence of LYST on intracellular growth of C. burnetii. Our results showed that lyst transcription was up-regulated in the host cells after C. burnetii infection, but there is no significant change in lyst expression level after infection with the Dot/Icm type IV secretion system (T4SS) mutant strain, while CCVs expansion and significantly increasing load of C. burnetii appeared in the host cells with a silenced lyst gene, suggesting LYST inhibits the intracellular proliferation of C. burnetii by reducing CCVs size. Then, the size of CCVs and the load of C. burnetii in the HeLa cells pretreated with E-64d were significantly decreased. In addition, the level of iNOS was decreased significantly in LYST knockout THP-1 cells, which was conducive to the intracellular replication of C. burnetii. This data is consistent with the phenotype of L-NMMA-treated THP-1 cells infected with C. burnetii. Our results revealed that the upregulation of lyst transcription after infection is due to effector secretion of C. burnetii and LYST inhibit the intracellular replication of C. burnetii by reducing the size of CCVs and inducing nos2 expression.

Dual Functioned Hexapeptide-Coated Lipid-Core Nanomicelles Suppress Toll-Like Receptor-Mediated Inflammatory Responses through Endotoxin Scavenging and Endosomal pH Modulation.

Excessive activation ofToll-like receptor (TLR) signaling pathways and the circulating endotoxin are key players in the pathogenesis of many acute and chronic inflammatory diseases. Regulation of TLR-mediated inflammatory responses by bioactive nanodevices represents a promising strategy for treating these diseases. In searching for novel, clinically applicable nanodevices with potent TLR inhibitory activities, three types of hexapeptide-modified nano-hybrids with different cores of phospholipid nanomicelles, liposomes, and poly(lactic-co-glycolic acid) nanoparticles are constructed. Interestingly, only the peptide-modified lipid-core nanomicelles (M-P12) display potent TLR inhibitory activities. Further mechanistic studies disclose that lipid-core nanomicelles have a generic property to bind to and scavenge lipophilic TLR ligands including lipopolysaccharide to block the ligand-receptor interaction and down-regulate the TLR signaling extracellularly. In addition, the peptide modification enables M-P12 a unique capability to modulate endosomal acidification upon being endocytosed into macrophages, which subsequently regulates the endosomal TLR signal transduction. In an acute lung injury mouse model, intratracheal administration of M-P12 can effectively target lung macrophages and reduce lung inflammation and injuries. This work defines a dual mechanism of action of the peptide-modified lipid-core nanomicelles in regulating TLR signaling, and provides new strategies for the development of therapeutic nanodevices for treating inflammatory diseases.

Precision Terminology: Why to Avoid the Term "Atypical Optic Neuritis".

Tuberculosis, one of the leading causes of death worldwide, stimulates inflammatory responses with beneficial and pathologic consequences. The regulation and nature of an optimal inflammatory response to <i>Mycobacterium tuberculosis</i> remains poorly understood in humans. Insight into mechanisms of negative regulation of the TLR-mediated innate immune response to <i>M. tuberculosis</i> could provide significant breakthroughs in the design of new vaccines and drugs. We hypothesized that TOLLIP and its common variants negatively regulate TLR signaling in human monocytes and are associated with susceptibility to tuberculosis. Using short hairpin RNA knockdown of TOLLIP in peripheral blood human monocytes, we found that TOLLIP suppresses TNF and IL-6 production after stimulation with TLR2 and TLR4 ligands. In contrast, secretion of the anti-inflammatory cytokine IL-10 was induced by TOLLIP. We also discovered two common polymorphisms that are associated with either decreased levels of mRNA expression (rs3750920) or increased IL-6 production (rs5743899) in a sample of 56 healthy volunteers. Furthermore, in a case-population study in Vietnam with 760 cord blood samples and 671 TB case patients, we found that SNPs rs3750920 and rs5743899 were associated with susceptibility to tuberculosis (<i>p</i> = 7.03 × 10⁻¹⁶ and 6.97 × 10⁻⁷, respectively). These data demonstrate that TOLLIP has an anti-inflammatory effect on TLR signaling in humans and that TOLLIP deficiency is associated with an increased risk of tuberculosis. To our knowledge, these data also show the first associations of TOLLIP polymorphisms with any infectious disease. These data also implicate an unexpected mechanism of negative regulation of TLR signaling in human tuberculosis pathogenesis.

Integrin αvβ3 limits TLR signaling in plasmacytoid dendritic cells to prevent autoimmunity.

Abstract SLE is a chronic, systemic autoimmune disease affecting millions of people worldwide. SLE is caused by a breakdown of tolerance to self-derived nucleic acids (NA) and characterized by the presence of autoantibodies to DNA/RNA. Plasmacytoid dendritic cells (pDCs) are highly tuned to respond to NA and express a range of receptors for RNA and DNA, including TLRs 7 and 9. Following TLR engagement, pDCs produce high levels of type I interferon (IFN-I), which promotes loss of tolerance, implicating pDCs in development of autoimmunity. However, relatively little is known about how innate immune signaling is regulated in pDCs, and particularly how they distinguish self-derived from foreign NA. TLRs 7 and 9 are both localized in endosomal compartments, and their NA ligands must be internalized and processed before they can bind. One source of these NA is dying cells and loss of their receptors is associated with SLE. We studied the role of an apoptotic cell receptor, Integrin αvβ3, in TLR signaling of pDCs and found that αvβ3 limits IFN-I production. αvβ3-knockout pDCs show increased TLR signaling and produce more IFN-I and inflammatory cytokines when stimulated in culture. In mice without αvβ3, there is a systemic increase in circulating ISGs and IFN-I after TLR7 stimulation. This corresponds with an increase in pDC activation and a decrease in marginal zone B cells, indicating elevated IFN-I production. These data indicate αvβ3 has a pDC-intrinsic role in limiting TLR signaling, which we propose is through activation of non-canonical autophagy. Based on these data, we hypothesize that co-receptors involved in capture and internalization of nucleic acids provide contextual ‘cues’ to regulate TLR signaling and prevent autoimmunity.

Integrin αv limits TLR signaling in plasmacytoid dendritic cells

Abstract SLE is a chronic, systemic autoimmune disease affecting millions of people worldwide. SLE is caused by a breakdown of tolerance to self-derived nucleic acids (NA) and characterized by the presence of autoantibodies to DNA/RNA. Plasmacytoid dendritic cells (pDCs) are highly tuned to respond to NA and express a range of receptors for RNA and DNA, including TLRs 7 and 9. Following TLR engagement, pDCs produce high levels of type I interferon (IFN-I), which promotes loss of tolerance, implicating pDCs in development of autoimmunity. However, relatively little is known about how innate immune signaling is regulated in pDCs, and particularly how they distinguish self-derived from foreign NA. TLRs 7 and 9 are both localized in endosomal compartments, and their NA ligands must be internalized and processed before they can bind. One source of these NA is dying cells and loss of their receptors is associated with SLE. We studied the role of an apoptotic cell receptor, Integrin αv, in TLR signaling of pDCs and found that αv limits IFN-I production. In mice without αv, there is a systemic increase in IFN-I and inflammatory cytokines after TLR7 stimulation. This corresponds with an increase in pDC activation and the number of IFN-I-producing pDCs in the spleen. αv-knockout pDCs show increased TLR signaling and produce more IFN-I and inflammatory cytokines when stimulated in culture. These data indicate αv has a pDC-intrinsic role in limiting TLR signaling, which we propose is through activation of non-canonical autophagy. Based on these data, we hypothesize that co-receptors involved in capture and internalization of nucleic acids provide contextual ‘cues’ to regulate TLR signaling and prevent autoimmunity.

Cytosolic Phospholipase A2 Alpha Regulates TLR Signaling and Migration in Metastatic 4T1 Cells.

Metastatic disease is the leading cause of death in breast cancer patients. Disrupting the cancer cell’s ability to migrate may be a strategy for hindering metastasis. Cytosolic phospholipase A2 α (cPLA2α), along with downstream proinflammatory and promigratory metabolites, has been implicated in several aspects of tumorigenesis, as well as metastasis, in various types of cancer. In this study, we aim to characterize the response to reduced cPLA2α activity in metastatic versus non-metastatic cells. We employ an isogenic murine cell line pair displaying metastatic (4T1) and non-metastatic (67NR) phenotype to investigate the role of cPLA2α on migration. Furthermore, we elucidate the effect of reduced cPLA2α activity on global gene expression in the metastatic cell line. Enzyme inhibition is achieved by using a competitive pharmacological inhibitor, cPLA2α inhibitor X (CIX). Our data show that 4T1 expresses significantly higher cPLA2α levels as compared to 67NR, and the two cell lines show different sensitivity to the CIX treatment with regards to metabolism and proliferation. Inhibition of cPLA2α at nontoxic concentrations attenuates migration of highly metastatic 4T1 cells, but not non-metastatic 67NR cells. Gene expression analysis indicates that processes such as interferon type I (IFN-I) signaling and cell cycle regulation are key processes regulated by cPLA2a in metastatic 4T1 cells, supporting the findings from the biological assays. This study demonstrates that two isogenic cancer cell lines with different metastatic potential respond differently to reduced cPLA2α activity. In conclusion, we argue that cPLA2α is a potential therapeutic target in cancer and that enzyme inhibition may inhibit metastasis through an anti-migratory mechanism, possibly involving Toll-like receptor signaling and type I interferons.

Regulation of Endotoxin Tolerance and Compensatory Anti-inflammatory Response Syndrome by Non-coding RNAs.

The onset and the termination of innate immune response must be tightly regulated to maintain homeostasis and prevent excessive inflammation, which can be detrimental to the organism, particularly in the context of sepsis. Endotoxin tolerance and compensatory anti-inflammatory response syndrome (CARS) describe a state of hypo-responsiveness characterized by reduced capacity of myeloid cells to respond to inflammatory stimuli, particularly those initiated by bacterial lipopolysaccharide (LPS). To achieve endotoxin tolerance, extensive reprogramming otherwise termed as “innate immune training”, is required that leads to both modifications of the intracellular components of TLR signaling and also to alterations in extracellular soluble mediators. Non-coding RNAs (ncRNAs) have been recognized as critical regulators of TLR signaling. Specifically, several microRNAs (miR-146, miR-125b, miR-98, miR-579, miR-132, let-7e and others) are induced upon TLR activation and reciprocally promote endotoxin tolerance and/or cross tolerance. Many other miRNAs have been also shown to negatively regulate TLR signaling. The long non-coding (lnc)RNAs (Mirt2, THRIL, MALAT1, lincRNA-21 and others) are also altered upon TLR activation and negatively regulate TLR signaling. Furthermore, the promotion or termination of myeloid cell tolerance is not only regulated by intracellular mediators but is also affected by other TLR-independent soluble signals that often achieve their effect via modulation of intracellular ncRNAs. In this article, we review recent evidence on the role of different ncRNAs in the context of innate immune cell tolerance and trained immunity, and evaluate their impact on immune system homeostasis.

Identification of MΦ specific POTEE expression: Its role in mTORC2 activation via protein-protein interaction in TAMs

POTE is known as cancer antigen, expressed in many cancers, along with very few normal tissues like prostate, ovary, testes and embryo. Till date, POTEE identified as majorly expressed POTE paralog. Functionally, POTEF regulates TLR signaling which play important role in innate immunity provided clue about expression of POTE in immune cells. We have chosen three Thp1monocytes, Jurkat T1 and MΦ cells as a model. Here, first time we report expression of POTEE in immune cells specifically only in MΦ but not in monocytes or T-cells. In addition, expression level remains unaltered in MΦ subtypes M1 and M2 and MΦ subjected to various stresses, except MΦs treated with Hyp-CM where MΦs acquires properties of TAMs. In TAMs, POTEE was involved differential protein-protein interaction with mTOR, RICTOR, and Rad51 indicating its biological role in cell invasion through mTORC2 activation. siRNA mediated knockdown of POTEE suggests its importance in cell survival of MΦs as well as TAMs.

IRF9 as a negative regulator involved in TRIF-mediated NF-κB pathway in a teleost fish, Miichthys miiuy.

Proinflammatory cytokines and type I IFNs were produced by TLR signaling and these responses are crucial for host defensive responses against pathogens. In order to avoid harmful and inappropriate inflammatory responses, there are multiple mechanisms to negatively regulate TLR signaling. In this paper, we have firstly studied IRF9 functions as a negative regulator involved in TRIF-mediated NF-κB pathway. Moreover, we found inhibitory effect of IRF9 primary depends on DBD domain. Interestingly, we also demonstrated that else mutants of IRF9, except for IRF9-ΔDBD, have different inhibitory effects upon TRIF-mediated NF-κB pathway. This study provides a novel evidence about the negatively regulation of innate immune signaling pathway in teleost fish. In addition, this finding provides new insights into the regulatory mechanism in mammals.

A negative feedback loop of ICER and NF-κB regulates TLR signaling in innate immune responses.

The NF-κB pathway has important roles in innate immune responses and its regulation is critical to maintain immune homeostasis. Here, we report a newly discovered feedback mechanism for the regulation of this pathway by TLR ligands in macrophages. Lipopolysaccharide (LPS) induced the expression of ICER via p38-mediated activation of CREB in macrophages. ICER, in turn, inhibited the transcriptional activity of NF-κB by direct interaction with the p65 subunit of NF-κB. Deficiency in ICER elevated binding of NF-κB to promoters of pro-inflammatory genes and their subsequent gene expression. Mice deficient in ICER were hypersensitive to LPS-induced endotoxic shock and showed propagated inflammation. Whereas ICER expression in ICER KO bone marrow transplanted mice rescued the ultra-inflammation phenotype, expression of a p65 binding-deficient ICER mutant failed to do so. Our results thus establish p38-CREB-ICER as key components of a negative feedback mechanism necessary to regulate TLR-driven inflammation.

Molecular cloning, characterization and functional analysis of a heat shock protein 70 gene in Cyclina sinensis

Heat shock protein 70 (HSP70) is an important member of the heat shock protein superfamily and is involved in protecting organisms against various stressors. In the present study, we used RACE to clone a full-length Cyclina sinensis HSP70 cDNA termed CsHSP70. The full length of the CsHSP70 cDNA was 2308 bp, with a 5′ untranslated region (UTR) of 42 bp, a 3′ UTR of 268 bp, and an open reading frame (ORF) of 1998 bp encoding a polypeptide of 655 amino acids with an estimated molecular mass of 72.75 kDa and an estimated isoelectric point of 5.48. Quantitative real-time PCR was employed to analyze the tissue distribution and temporal expression of the CsHSP70 gene after bacterial challenge and cadmium (Cd) exposure. The CsHSP70 mRNA transcript was expressed ubiquitously in five examined tissues, with the highest expression in hemocytes (P < 0.05) and with the lowest expression in the hepatopancreas. Furthermore, the expression level of CsHSP70 in hemocytes at 3 h after Vibrio anguillarum challenge was extremely significantly up-regulated (P < 0.01). Moreover, the CsHSP70 transcript was up-regulated significantly following exposure to a safe Cd concentration (0.1 mg/L). Finally, after the CsHSP70 gene was silenced by RNA interference, the expression of the CsTLR13 and CsMyD88 genes were extremely significantly decreased (P < 0.01). The results indicated that CsHSP70 could play an important role in mediating the environmental stress and immune responses, and regulating TLR signaling pathway in C. sinensis.

MiR-200a-3p regulates TLR1 expression in bacterial challenged miiuy croaker

MicroRNAs (miRNAs) are highly conserved, small non-coding RNAs which post-transcriptionally regulate various biological processes by repressing mRNA translation or degradating mRNA. It has been demonstrated that miRNAs play crucial roles in regulating the immune system. In this study, we explored the potential roles of miR-200a-3p in regulating TLR signaling pathway in miiuy croaker. Bioinformatics analysis showed that miiuy croaker TLR1 (mmiTLR1) was a putative target of miR-200a-3p. Negative expression profiles in spleen of Vibrio anguillarum challenged miiuy croaker and in lipopolysaccharide (LPS) stimulated miiuy croaker leukocytes further validated the prediction. Luciferase reporter assays showed that the dual-luciferase reporter fused to the 3′UTR of wild type mmiTLR1 cotransfected with miR-200a-3p mimics exhibited a reduction in luciferase activity compared with the controls. All of the present data provide direct evidence that miR-200a-3p is involved in TLR1 expression modulation in miiuy croaker, which will offer a basis for better understanding of miRNA regulation in fish TLR signaling pathways.

Mycoplasma superantigen enhances the severity of K/BxN autoimmune arthritis through TLR4/BCL6/TFH pathway

Abstract Mycoplasma arthritidis mitogen (MAM), a potent superantigen (SAg) secreted by M. arthritidis, is known to contribute to mouse inflammatory/autoimmune arthritis. We previously demonstrated that C3H mouse strains differing in expression of TLR4, also differ in Th-1 versus Th-2 and Th17 cytokine profiles in response to MAM due to a differential engagement of TLR2 and TLR4. HMGB-1, a member of heat shock protein family that is abundantly expressed in many cell types upon activation, has been implicated in regulating TLR signaling pathways following its interaction with RAGE. In the present study, we established a new model of mouse arthritis induced by injection of K/BxN sera. Also, we provide evidence that HMGB1 is an essential endogenous activator of the immune response in K/BxN-sera transfer BALB/c mice following MAM immunization. Level of HMGB1 expression was promptly increased in K/BxN sera-transferred mice after MAM injection. The increase in HMGB1 was required for initiation of MAM-induced enhancement of BCL-6, type I interferon (IFN) and follicular helper T cell responses. Blockade of HMGB1 caused prolonged activation of BCL-6 pathways, reduced level of type I IFN and augmented production of IL-10. Also, MAM-immunized K/BxN mice injected with HMGB1 antibody were less susceptible to arthritis suggesting HMGB1 may accomplish this mainly through its interaction with TLR4. Thus, HMGB1, in synergy with mycoplasma superantigen MAM, functions as biologically important modulators of TLR4/BCL6/TFH, and thus the targeting of this pathway can be a useful therapeutic strategy for treating human rheumatoid arthritis.

The Tec Kinase-Regulated Phosphoproteome Reveals a Mechanism for the Regulation of Inhibitory Signals in Murine Macrophages.

Previous work has shown conflicting roles for Tec family kinases in regulation of TLR-dependent signaling in myeloid cells. In the present study, we performed a detailed investigation of the role of the Tec kinases Btk and Tec kinases in regulating TLR signaling in several types of primary murine macrophages. We demonstrate that primary resident peritoneal macrophages deficient for Btk and Tec secrete less proinflammatory cytokines in response to TLR stimulation than do wild-type cells. In contrast, we found that bone marrow-derived and thioglycollate-elicited peritoneal macrophages deficient for Btk and Tec secrete more proinflammatory cytokines than do wild-type cells. We then compared the phosphoproteome regulated by Tec kinases and LPS in primary peritoneal and bone marrow-derived macrophages. From this analysis we determined that Tec kinases regulate different signaling programs in these cell types. In additional studies using bone marrow-derived macrophages, we found that Tec and Btk promote phosphorylation events necessary for immunoreceptor-mediated inhibition of TLR signaling. Taken together, our results are consistent with a model where Tec kinases (Btk, Tec, Bmx) are required for TLR-dependent signaling in many types of myeloid cells. However, our data also support a cell type-specific TLR inhibitory role for Btk and Tec that is mediated by immunoreceptor activation and signaling via PI3K.

Induction of anti-microbial immune responses in severely immunodeficient hosts by IFNγ-expressing Salmonella enterica serovar Typhimurium correlates with efficient activation of macrophage effectors (VAC8P.1050)

Abstract Susceptibility to infection by the intracellular bacterial pathogen, Salmonella enterica serovar Typhimurium (S. typhimurium), is controlled by many genes of innate and adaptive immunity. One of the most critical genes is IFN-γ and mice deficient in IFN-γ synthesis are highly susceptible to Salmonella. Previously, we also demonstrated that mice deficient in MyD88, an adaptor that regulates TLR signaling, are susceptible to Salmonella infection. In the current study, we compared immune responses in mice deficient in IFN-γ or MyD88 with wild-type controls following infection with an attenuated strain of S. typhimurium (designated BRD509) or a recombinant derivative engineered to express murine IFN-γ (GIDIFN). Infection studies with BRD509 or GIDIFN revealed that the latter strain was significantly less virulent in immunodeficient mice than BRD509 and correlated with decreased bacterial loads in systemic organs. Enhanced responsiveness was due to GIDIFN strain’s ability to activate effector macrophages, as shown by increased synthesis of inflammatory cytokines and anti-microbial effector molecules, including NO. Gene expression profiling by qPCR demonstrated stronger induction of key inflammatory modulators by GIDIFN in macrophages of immunodeficient animals. These findings suggest that immunotherapeutic approaches using attenuated bacterial strains expressing immunomodulatory genes is more efficacious and offers a superior safety profile even in severely immunodeficient hosts.

Macrophage PTEN regulates expression and secretion of arginase I modulating innate and adaptive immune responses.

The activation of innate immune cells triggers numerous intracellular signaling pathways, which require tight control to mount an adequate immune response. The PI3K signaling pathway is intricately involved in innate immunity, and its activation dampens the expression and release of proinflammatory cytokines in myeloid cells. These signaling processes are strictly regulated by the PI3K antagonist, the lipid phosphatase, PTEN, a known tumor suppressor. Importantly, PTEN is responsible for the elevated production of cytokines such as IL-6 in response to TLR agonists, and deletion of PTEN results in diminished inflammatory responses. However, the mechanisms by which PI3K negatively regulates TLR signaling are only partially resolved. We observed that Arginase I expression and secretion were markedly induced by PTEN deletion, suggesting PTEN(-/-) macrophages were alternatively activated. This was mediated by increased expression and activation of the transcription factors C/EBPβ and STAT3. Genetic and pharmacologic experimental approaches in vitro, as well as in vivo autoimmunity models, provide convincing evidence that PI3K/PTEN-regulated extracellular Arginase I acts as a paracrine regulator of inflammation and immunity.

Protein kinase D1 links IRAK1 and TRAF6 in the MyD88-dependent Toll-like receptor signaling pathway. (INM9P.457)

Abstract The initial proinflammatory responses to Saccharopolyspora rectivirgula (SR), which causes farmers' lung disease, is dependent on Toll-like receptor 2 (TLR2) and MyD88, and MyD88 is absolutely required for SR-induced protein kinase D1 (PKD1) activation. We found that SR-mediated proinflammatory responses and neutrophil infiltration in the lungs are ablated in mice treated with PKD inhibitor or lacking the PKD1 gene. However, the mechanisms by which PKD1 is activated and functions to regulate TLR signaling are incompletely understood. In the present study, we found that upon TLR ligand stimulation, the receptor signaling complex composed of MyD88, IRAK4, IRAK1, TRAF6, and PKD1 is formed in membrane lipid rafts and that IRAK4 and IRAK1, but not TRAF6, are required for recruitment of PKD1 to the receptor complex and for activation by TLR ligands. In addition, MyD88-dependent translocation of TRAF6 into lipid rafts, ubiquitination of TRAF6, degradation of IRAK1, and activation of TAK1 are dependent on PKD1. Our findings indicate that PKD1 is a downstream effector of IRAK1, but an upstream modulator of TRAF6. Together with the known ability of PKD1 to shuttle between membrane lipid rafts and the cytosol, our results further support the hypothesis that PKD1 functions as a molecular shuttle that brings TRAF6 to IRAK1 in membrane lipid rafts and then aids in translocation of the IRAK1/TRAF6 complex to the cytosol, where TRAF6 interacts with its downstream effectors.

TRAF5 Negatively Regulates TLR Signaling in B Lymphocytes

The cytoplasmic adaptor proteins TNFR-associated factor (TRAF)3 and TRAF6 are important mediators of TLR signaling. To our knowledge, we show in this study for the first time that another TRAF family member, TRAF5, is a negative regulator of TLR signaling. B lymphocytes from TRAF5(-/-) mice produced more IL-6, IL-12p40, IL-10, TNF-α, and IgM than did wild-type B cells after TLR stimulation. Consistent with these data, exogenous overexpression of TRAF5 in B cells inhibited TLR-mediated cytokine and Ab production. TLR stimulation of TRAF5-deficient B cells did not affect cell survival, proliferation, or NF-κB activation but resulted in markedly enhanced phosphorylation of the MAPKs ERK1/2 and JNK. TRAF5 negatively regulated TLR signaling in a cell-specific manner, because TRAF5(-/-) macrophages and dendritic cells showed less dramatic differences in TLR-mediated cytokine production than B cells. Following TLR stimulation, TRAF5 associated in a complex with the TLR adaptor protein MyD88 and the B cell-specific positive regulator of TLR signaling TAB2. Furthermore, TRAF5 negatively regulated the association of TAB2 with its signaling partner TRAF6 after TLR ligation in B cells. To our knowledge, these data provide the first evidence that TRAF5 acts as a negative regulator of TLR signaling.