Chemokine receptor 4 (CXCR4) is part of the lipopolysaccharide "sensing apparatus"

Recognition of bacterial lipopolysaccharide (LPS) by the innate immune system involves at least three receptor molecules: CD14, TLR4 and MD-2. Additional receptor components such as heat shock proteins, chemokine receptor 4 (CXCR4), or CD55 have been suggested to be part of this activation cluster; possibly acting as additional LPS transfer molecules. Our group has previously identified CXCR4 as a component of the "LPS-sensing apparatus". In this study we aimed to elucidate the role that CXCR4 plays in innate immune responses to LPS. Here we demonstrate that CXCR4 transfection results in responsiveness to LPS. Fluorescence correlation spectroscopy experiments further showed that LPS directly interacts with CXCR4. Our data suggest that CXCR4 is not only involved in LPS binding but is also responsible for triggering signalling, especially mitogen-activated protein kinases in response to LPS. Finally, co-clustering of CXCR4 with other LPS receptors seems to be crucial for LPS signalling, thus suggesting that CXCR4 is a functional part of the multimeric LPS "sensing apparatus".     

Toll样受体在动脉粥样硬化中作用的研究进展

Toll样受体(TLRs)是模式识别受体(PRRs)之一,其因胞外段与一种果蝇蛋白Toll同源而得名.TLRs广泛分布于巨噬细胞、树突样细胞、T细胞、B细胞等免疫细胞,还存在于一些组织细胞中,如心肌细胞等.TLRs不仅可诱导固有免疫反应作为机体对病原微生物的第一道防线,也可通过激活抗原递呈细胞诱导适应性免疫反应.大量证据表明,TLRs除参与宿主抵抗病原菌的侵害之外,还在非感染性组织损伤中起作用,例如心肌缺血再灌注损伤,缺血后心肌重塑和动脉粥样硬化等疾病.故了解TLRs信号通路及其在心血管疾病中的作用可帮助我们确定干预心血管疾病的潜在靶点,且有重要的临床应用价值.     

脂多糖模式识别受体的研究进展

G-细菌的脂多糖（LPS）是重要的病原体相关模式分子。PAMPsd均可被动物作为低外来分子进行识别,LPS能激发机体细胞因子IL-1、TNF-α等活性分子的合成,对感染具有十分重要的作用。LPS是通过什么受体怎样将信号传玫免疫细胞并启动免疫反应的,人们一直都不十分清楚,近年来,一种名为Toll蛋白的发现,使人们对机体识别LPS机制的认识向前跨进了一大步,本文试对该模式识别受体的研究进展做一综述。     

Experimental and natural infections in MyD88‐ and IRAK‐4‐deficient mice and humans

Most Toll‐like‐receptors (TLRs) and interleukin‐1 receptors (IL‐1Rs) signal via myeloid differentiation primary response 88 (MyD88) and interleukin‐1 receptor‐associated kinase 4 (IRAK‐4). The combined roles of these two receptor families in the course of experimental infections have been assessed in MyD88‐ and IRAK‐4‐deficient mice for almost fifteen years. These animals have been shown to be susceptible to 46 pathogens: 27 bacteria, eight viruses, seven parasites, and four fungi. Humans with inborn MyD88 or IRAK‐4 deficiency were first identified in 2003. They suffer from naturally occurring life‐threatening infections caused by a small number of bacterial species, although the incidence and severity of these infections decrease with age. Mouse TLR‐ and IL‐1R‐dependent immunity mediated by MyD88 and IRAK‐4 seems to be vital to combat a wide array of experimentally administered pathogens at most ages. By contrast, human TLR‐ and IL‐1R‐dependent immunity mediated by MyD88 and IRAK‐4 seems to be effective in the natural setting against only a few bacteria and is most important in infancy and early childhood. The roles of TLRs and IL‐1Rs in protective immunity deduced from studies in mutant mice subjected to experimental infections should therefore be reconsidered in the light of findings for natural infections in humans carrying mutations as discussed in this review.     

Activation of human basophils by combined toll‐like receptor‐ and FcεRI‐triggering can promote Th2 skewing of naive T helper cells

Basophils are mostly known for their involvement in allergic reactions. Recent studies in mice indicate a role for basophils in the induction of adaptive immunity, especially T helper 2 (Th2) responses. Therefore, it would be highly important to understand how basophils respond to pathogen‐associated molecules, such as ligands for toll‐like receptors (TLRs), and if the basophils could promote Th2 responses via these stimuli. To this end, the activation of basophils via TLRs in combination with activation via IgE was studied, as well as its effect on T helper cell skewing. Using quantitative PCR, we demonstrated the presence of mRNA for TLRs 1–8 in human basophils. Basophils responded to TLR triggering with differential cytokine production, but not with degranulation. Simultaneous triggering of TLRs and IgE led to synergy in production of IL‐4, IL‐8, IL‐13, and RANTES. Furthermore, the synergistic effects on basophils mediated by IgE and TLR‐4 triggering allowed robust Th2 skewing upon activation of naïve human CD4⁺ T cells. Our data show that human basophils respond to TLR ligands in synergy with IgE‐mediated activation and that the cytokines produced can promote Th2 differentiation. These results indicate a role for basophils in the regulation of T‐cell responses in humans.     

Activation of plasmacytoid dendritic cells by apoptotic particles – mechanism for the loss of immunological tolerance in Sjögren's syndrome

Sjögren's syndrome (SS) is a common autoimmune disease targeting salivary and lacrimal glands. It is strongly female‐dominant, characterized by low oestrogen levels combined with a local intracrine dihydrotestosterone defect. We hypothesized that these hormonal deficits lead to increased apoptosis of the epithelial cells and plasmacytoid dendritic cell (pDC)‐mediated proinflammatory host responses. Expression of Toll‐like receptors (TLRs)‐7 and ‐9 and cytokine profiles was studied in pDCs treated with apoptotic particles collected in consecutive centrifugation steps of media from apoptotic cells. Expression and localization of SS autoantigens in these particles was also analysed. Furthermore, the effects of sex steroids were studied in pDCs cultured with several concentrations of dihydrotestosterone and 17‐β‐oestradiol, and in saliva of patient treated with dehydroepiandrosterone. Apoptosis of the epithelial cells led to cleavage and translocation of SS‐autoantigens, α‐fodrin and SS‐A, into apoptotic particles. The apoptosis‐induced apoptotic particles also contained another SS‐autoantigen, hy1‐RNA. These particles were internalized by pDCs in a size‐dependent manner and affected TLR‐7 and ‐9 expression and the production of proinflammatory cytokines. The analysed androgens protected cells from apoptosis, influenced redistribution of autoantigens and diminished the apoptotic particle‐stimulated increase of the TLRs in pDCs. Our findings suggest that the formation of apoptotic particles may play a role in loss of immune tolerance, manifested by production of autoantibodies and the onset of autoinflammation in SS.     

TLR4, TLR9 and MyD88 are not required for the positive selection of autoreactive B cells into the primary repertoire

Toll-like receptors (TLR) have been shown to play an essential role in the generation of autoantibodies in mouse models of autoimmunity, but the timing and context of these effects are poorly understood. One hypothesis is that TLR ligands assist in the positive selection of self-reactive B cells into the primary repertoire and, in this way, distinguish between immunogenic and tolerogenic forms of self-antigen. To explore this idea we generated hen egg lysozyme-specific immunoglobulin (Ig(HEL)) and isotype class-switching anti-HEL mice deficient in MyD88, TLR4 or TLR9 signalling and studied B cell development and autoantibody secretion in the presence or absence of an intracellular form of self-antigen HEL that positively selects B1 cells. Our findings show that TLR4, TLR9 and MyD88 are not required for the positive selection of autoreactive B cells in the primary B cell repertoire, nor is MyD88 required for the generation of isotype-switched antibodies in the absence of antigen. These results suggest that the significant effects of TLR on autoimmunity occur in the established repertoire and not during B cell development.     

Redundant and regulatory roles for Toll‐like receptors in Leishmania infection

Toll‐like receptors (TLRs) are germline‐encoded, non‐clonal innate immune receptors, which are often the first receptors to recognize the molecular patterns on pathogens. Therefore, the immune response initiated by TLRs has far‐reaching consequences on the outcome of an infection. As soon as the cell surface TLRs and other receptors recognize a pathogen, the pathogen is phagocytosed. Inclusion of TLRs in the phagosome results in quicker phagosomal maturation and stronger adaptive immune response, as TLRs influence co‐stimulatory molecule expression and determinant selection by major histocompatibility complex (MHC) class II and MHC class I for cross‐presentation. The signals delivered by the TCR–peptide–MHC complex and co‐stimulatory molecules are indispensable for optimal T cell activation. In addition, the cytokines induced by TLRs can skew the differentiation of activated T cells to different effector T cell subsets. However, the potential of TLRs to influence adaptive immune response into different patterns is severely restricted by multiple factors: gross specificity for the molecular patterns, lack of receptor rearrangements, sharing of limited number of adaptors that assemble signalling complexes and redundancy in ligand recognition. These features of apparent redundancy and regulation in the functioning of TLRs characterize them as important and probable contributory factors in the resistance or susceptibility to an infection.     

Maternal neutrophil toll-like receptor mRNA expression is down-regulated in preeclampsia

Citation
Nitsche JF, Jiang S‐W, Brost BC. Maternal neutrophil Toll‐like receptor mRNA expression is down‐regulated in preeclampsia. Am J Reprod Immunol 2011; 66: 242–248 Problem There are many immunological changes in preeclampsia. For example, TLR‐4 expression is increased in the placenta during preeclampsia. However, data on TLR expression in other tissues during preeclampsia are lacking. This study aimed to determine whether TLR mRNA expression in maternal neutrophils is altered in preeclampsia. Method of Study A case–control study using standard quantitative real‐time PCR techniques was performed to assess TLR‐2 and TLR‐4 mRNA expression in 12 patients with mild preeclampsia and 18 normal pregnant controls at similar gestational ages. Results Compared to normal pregnant controls, there was a significant decrease in TLR‐2 and TLR‐4 expression in women with mild preeclampsia. Conclusion TLR‐2 and TLR‐4 expression in maternal neutrophils is decreased in preeclampsia. Given the many immunological changes in preeclampsia, this may represent an adaptation to the increased inflammatory signals present in preeclampsia. Further study is needed to clarify the role of the TLR in preeclampsia.     

Dosage of X‐linked Toll‐like receptor 8 determines gender differences in the development of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease with a high incidence in females and a complex phenotype. Using 564Igi mice, a model of SLE with knock‐in genes encoding an autoreactive anti‐RNA Ab, we investigated how expression of Toll‐like receptors (TLRs) in B cells and neutrophils affects pathogenesis. We established that TLR signaling through MyD88 is necessary for disease. Autoantibody was produced in mice with single deletions of Tlr7, Tlr8, or Tlr9 or combined deletions of Tlr7 and Tlr9. Autoantibody was not produced in the combined absence of Tlr7 and Tlr8, indicating that TLR8 contributes to the break in tolerance. Furthermore, TLR8 was sufficient for the loss of B‐cell tolerance, the production of class‐switched autoantibody, heightened granulopoiesis, and increased production of type I IFN by neutrophils as well as glomerulonephritis and death. We show that dosage of X‐linked Tlr8 plays a major role in the high incidence of disease in females. In addition, we show that the negative regulation of disease by TLR9 is exerted primarily on granulopoiesis and type I IFN production by neutrophils. Collectively, we suggest that individual TLRs play unique roles in the pathogenesis of systemic lupus erythematosus, suggesting new targets for treatment.     

Bruton's tyrosine kinase regulates TLR9 but not TLR7 signaling in human plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) represent a key cell type for both innate and adaptive immunity. PDCs express both TLR7 and TLR9 and the recognition of nucleic acids by these two receptors triggers the production of a large amount of type‐I IFN and the induction of PDC maturation into APCs. This unique feature of PDCs is at the basis of clinical development of both TLR7 and TLR9 agonists for infectious diseases, allergy, cancer, and asthma. However, TLR7 and TLR9 recognition of self‐nucleic acids is linked to many autoimmune diseases including lupus, and a better understanding of the signaling pathways of these two receptors in PDCs is thus important. We have identified Bruton's tyrosine kinase (Btk) as an important player for TLR9 but not TLR7 signaling in human PDCs. Blocking Btk using a specific inhibitor leads to the reduction of all TLR9‐induced responses in PDCs, including cytokine production and expression of costimulatory molecules, while this has no impact on the TLR7 response. This identifies Btk as a key molecule in TLR9 signaling in PDCs and is the first demonstration that the TLR7 and TLR9 pathways can be dissociated in human PDCs.     

RNA interference reveals a role for TLR2 and TLR3 in the recognition of Leishmania donovani promastigotes by interferon-gamma-primed macrophages

Leishmania donovani promastigotes evade the induction of a proinflammatory response during their invasion of naive macrophages. However, their entry into IFN-gamma-primed macrophages is accompanied by the secretion of nitric oxide (NO) and proinflammatory cytokines. In the present study, we addressed the hypothesis that priming with IFN-gamma induces the expression of a receptor that enables mouse macrophages to recognize L. donovani promastigotes. We observed that in IFN-gamma-primed macrophages, L. donovani promastigotes stimulated Interleukin-1 receptor-associated kinase-1 (IRAK-1) activity. We next showed that Toll-like receptor (TLR)3 is barely detectable in naive macrophages but is expressed in IFN-gamma-treated macrophages. Silencing of TLR3, TLR2, IRAK-1 and myeloid differentiation factor 88 (MyD88) expression by RNA interference revealed that both TLR are involved in the secretion of NO and TNF-alpha induced by L. donovani promastigotes. Using L. donovani mutants, we showed that TLR2-mediated responses are dependent on Galbeta1,4Manalpha-PO(4)-containing phosphoglycans, whereas TLR3-mediated responses are independent of these glycoconjugates. Furthermore, our data indicate a participation of TLR2 and TLR3 in the phagocytosis of L. donovani promastigotes and a role for TLR3 in the leishmanicidal activity of the IFN-gamma-primed macrophages. Collectively, our data are consistent with a model where recognition of L. donovani promastigotes depends on the macrophage activation status and requires the expression of TLR3.     

Regulation of human neutrophil chemokine receptor expression and function by activation of Toll-like receptors 2 and 4

Neutrophil chemokine receptor expression can be altered by exposure to Toll-like receptor (TLR) agonists, a process that is thought to have the potential to localize neutrophils to sites of infection. In order to investigate this process in more detail, we examined the regulation of highly pure neutrophil CXCR1 and CXCR2 expression and function by selective agonists of TLR2 (Pam(3)CSK(4)) and TLR4 (lipopolysaccharide, LPS). CXCR1 and CXCR2 were down-regulated by TLR engagement. CXCR2 loss was more rapid and showed a dependence upon soluble helper molecules (LPS binding protein and CD14) that was not evident for CXCR1, suggesting differential coupling of LPS signalling to CXCR1 and CXCR2 loss. However, TLR engagement in highly pure neutrophils did not result in complete loss of chemokine receptors, and LPS-treated neutrophils remained able to mount a respiratory burst to CXCL8 and CXCL1, and were able to migrate towards CXCL8 in assays of under-agarose chemotaxis. Thus, although treatment of purified human neutrophils with TLR2 and TLR4 agonists modifies chemokine receptor expression, remaining receptors remain functionally competent.     

Inhibition of 4‐1BBL‐regulated TLR response in macrophages ameliorates endotoxin‐induced sepsis in mice

Activation of Toll‐like receptor (TLR) signaling rapidly induces the expression of inflammatory genes, which is persistent for a defined period of time. However, uncontrolled and excessive inflammation may lead to the development of diseases. 4‐1BB ligand (4‐1BBL) plays an essential role in sustaining the expression of inflammatory cytokines by interacting with TLRs during macrophage activation. Here, we show that inhibition of 4‐1BBL signaling reduced the inflammatory responses in macrophages and ameliorated endotoxin‐induced sepsis in mice. A 4‐1BB‐Fc fusion protein significantly reduced TNF production in macrophages by blocking the oligomerization of TLR4 and 4‐1BBL. Administration of 4‐1BB‐Fc suppressed LPS‐induced sepsis by reducing TNF production, and the coadministration of anti‐TNF and 4‐1BB‐Fc provided better protection against LPS‐induced sepsis. Taken together, these observations suggest that inhibition of the TLR/4‐1BBL complex formation may be highly efficient in protecting against continued inflammation, and that 4‐1BB‐Fc could be a potential therapeutic target for the treatment of inflammatory diseases.     

Modification of TLR-induced activation of human dendritic cells by type I IFN: synergistic interaction with TLR4 but not TLR3 agonists

Upon detection of direct and indirect signs of infection, dendritic cells (DC) undergo functional changes that modify their ability to elicit immune responses. Type I interferon (IFN-alpha/beta), which includes a large family of closely related infection-inducible cytokines, represents one indirect signal that can act as a DC stimulus. We have investigated the ability of IFN-alpha/beta subtypes to affect DC function and to influence DC responses to Toll-like receptor (TLR) agonists (i.e., direct infection-associated signals). Subtle differences were observed among 15 subtypes of IFN-alpha/beta in the ability to stimulate expression of maturation markers and chemokines by human monocyte-derived DC, with IFN-omega being the most unique in its effects. Pre-treatment with IFN-alpha/beta did not alter the ability of DC to mature in response to subsequent contact with TLR agonists, but did modulate their secretion of chemokines. Conversely, IFN-alpha/beta was shown to act synergistically with TLR4 but not TLR3 agonists for the induction of maturation and chemokine production when DC were exposed to IFN-alpha/beta and TLR ligands simultaneously. Taken together, these results indicate a complex role for IFN-alpha/beta in regulating DC function during the course an infection, which varies according to IFN-alpha/beta subtype and the timing of exposure to other stimuli.