Interleukin receptor-associated kinase-4 deficiency impairs Toll-like receptor-dependent innate antiviral immune responses

BACKGROUND: Engagement of all known Toll-like receptors (TLRs) causes the production of inflammatory cytokines, including TNF-alpha, whereas in humans, engagement of TLRs 3, 7, 8, and 9 also induces type I IFNs. IRAK-4 is a critical effector in signaling by TLRs and the IL-1 receptor, which share homology in their intracellular domain and recruit IRAK-4 via the adaptor myeloid differentiation factor 88 (MyD88). Patients with IRAK-4 deficiency are susceptible to invasive bacterial infections but have so far not been reported to be susceptible to viral infection. Blood cells from these patients are impaired in their ability to make TNF-alpha in response to activation by TLRs. A recent report has described concomitant impairment of type I IFN production after activation of TLRs 7, 8, and 9, but not TLR3. OBJECTIVES: We sought to evaluate the role of IRAK-4 in TLR-induced production of the type I IFN, IFN-alpha, in humans. METHODS: We examined TLR-induced production of TNF-alpha and IFN-alpha in PBMCs from an IRAK-4-deficient patient, his heterozygous carrier parents, and normal controls. RESULTS: TNF-alpha production in response to TLR agonists was severely impaired in the patient. IFN-alpha production induced by TLR7, TLR8, and TLR9, as well as TLR3 agonists, was low or absent. CONCLUSIONS: IRAK-4 plays an important role in the production of type I IFN, as well as TNF-alpha, induced by all TLRs, including TLR3. CLINICAL IMPLICATIONS: IRAK-4 may play a broader role in human innate antiviral immunity than previously appreciated.     

Toll-like receptors in bony fish: from genomics to function

Receptors that recognize conserved pathogen molecules are the first line of cellular innate immunity defense. Toll-like receptors (TLRs) are the best understood of the innate immune receptors that detect infections in mammals. Key features of the fish TLRs and the factors involved in their signaling cascade have high structural similarity to the mammalian TLR system. However, the fish TLRs also exhibit very distinct features and large diversity which is likely derived from their diverse evolutionary history and the distinct environments that they occupy. Six non-mammalian TLRs were identified in fish. TLR14 shares sequence and structural similarity with TLR1 and 2, and the other five (TLR19, 20, 21, 22 and 23) form a cluster of novel TLRs. TLR4 was lost from the genomes of most fishes, and the TLR4 genes found in zebrafish do not recognize the mammalian agonist LPS and are likely paralogous and not orthologous to mammalian TLR4 genes. TLR6 and 10 are also absent from all fish genomes sequenced to date. Of the at least 16 TLR types identified in fish, direct evidence of ligand specificity has only been shown for TLR2, TLR3, TLR5M, TLR5S and TLR22. The common carp TLR2 was shown to recognize the synthetic triacylated lipopeptide Pam₃CSK₄ and lipopeptides from gram positive bacteria. The membrane-bound TLR5 (TLR5M) signaling in response to flagellin in rainbow trout is amplified through interaction with the soluble form (TLR5S) in a positive loop feedback. In Fugu, TLR3 is localized to the endoplasmic reticulum (ER) and recognizes relatively short dsRNA, while TLR22 has a surveillance function like the human cell-surface TLR3. Genome and gene duplications have been major contributors to the teleost's rich evolutionary history and genomic diversity. Duplicate or multi-copy TLR genes were identified for TLR3 and 7 in common carp, TLR4b, 5, 8 and 20 in zebrafish, TLR8a in rainbow trout and TLR22 in rainbow trout and Atlantic salmon. The main task for current and near-future fish TLRs research is to develop specificity assays to identify the ligands of all fish TLRs, which will advance comparative immunology research and will contribute to our understanding of disease resistance mechanisms in fish and the development of new adjuvants and/or more effective vaccines and therapeutics.     

Toll-like receptor ligands synergize through distinct dendritic cell pathways to induce T cell responses: implications for vaccines

Toll-like receptors (TLRs) may need to cooperate with each other to be effective in detecting imminent infection and trigger immune responses. Understanding is still limited about the intracellular mechanism of this cooperation. We found that when certain TLRs are involved, dendritic cells (DCs) establish unidirectional intracellular cross-talk, in which the MyD88-independent TRIF-dependent pathway amplifies the MyD88-dependent DC function through a JNK-dependent mechanism. The amplified MyD88-dependent DC function determines the induction of the T cell response to a given vaccine in vivo. Therefore, our study revealed an underlying TLR mechanism governing the functional, nonrandom interplay among TLRs for recognition of combinatorial ligands that may be dangerous to the host, providing important guidance for design of novel synergistic molecular vaccine adjuvants.     

NF-kappaB and inflammation in genetic disease

By responding to pro-inflammatory cytokines, such as IL-1beta and TNF-alpha, and controlling itself the expression of numerous mediators of inflammation, NF-kappaB plays a pivotal role in controlling the proper sequence of events characterizing the inflammation process. Although excessive NF-kappaB activation is often associated with inflammatory signs in many different tissues, impaired NF-kappaB activation can also generate inflammation. This is the case in humans suffering from the genetic disease incontinentia pigmenti that exhibit severe skin inflammation. Identifying the molecular basis of this pathology, mutations affecting the gene coding for NEMO, has allowed production of mouse models for investigating the disease. Their characterization supports the view that a very tight positive and negative regulation of the NF-kappaB signaling pathway is required in vivo to ensure not only a fine-tuned response to injury or infection but also to maintain tissue homeostasis.     

The regulation of the expression of inducible nitric oxide synthase by Src-family tyrosine kinases mediated through MyD88-independent signaling pathways of Toll-like receptor 4

Bacterial lipopolysaccharide (LPS) activates Toll-like receptor 4 (TLR4) leading to the expression of inflammatory gene products. Src-family tyrosine kinases (STKs) are known to be activated by LPS in monocytes/macrophages. Therefore, we determined the role of STKs in TLR4 signaling pathways and target gene expression in macrophages. The activation of NFkappaB, and p38 MAPK, and the expression of inducible nitric oxide synthase (iNOS) induced by LPS were not affected in macrophages deficient in three STKs (Lyn, Hck, and Fgr). These results suggest that the deletion of the three STKs among possibly nine STKs is not sufficient to abolish total activity of STKs possibly due to the functional redundancy of other STKs present in macrophages. However, two structurally unrelated pan-inhibitors of STKs, PP1 and SU6656, suppressed LPS-induced iNOS expression in MyD88-knockout as well as wild-type macrophages. The suppression of iNOS expression by the inhibitors was correlated with the downregulation of IFNbeta (a MyD88-independent gene) expression and subsequent decrease in STAT1 phosphorylation. Moreover, PP1 suppressed the expression of IFNbeta and iNOS induced by TRIF, a MyD88-independent adaptor of TLR4. PP1 suppressed STAT1 phosphorylation induced by LPS, but not by IFNbeta suggesting that STKs are involved in the primary downstream signaling pathways of TLR4, but not the secondary signaling pathways downstream of IFNbeta receptor. Together, these results demonstrate that STKs play a positive regulatory role in TLR4-mediated iNOS expression in a MyD88-independent (TRIF-dependent) manner. These results provide new insight in understanding the role of STKs in TLR4 signaling pathways and inflammatory target gene expression.     

Toll-like receptors in inflammation of the central nervous system

Toll-like receptors (TLRs) belong to pattern-recognition receptor family that could recognize exogenous pathogen-associated molecular patterns and endogenous damage-associated molecular patterns. TLRs play pivotal roles in innate and adaptive immune responses. In this review we summarize the ligands and signal transduction pathways of TLRs and highlight recent progress of the involvement of TLRs in neuroinflammation related disorders, including cerebral ischemia/stroke, brain trauma and hemorrhage, pathogen infection and autoimmune diseases, and explore the potential of TLR signaling as therapeutic targets against these disorders.     

人参皂苷Rb1介导TLR3/TRIF信号通路对哮喘大鼠的抗炎作用机制

目的探讨人参皂苷Rb1介导Toll样受体(TLR)3/β干扰素TIR结构域衔接蛋白(TRIF)信号通路对哮喘大鼠模型抗炎作用的影响。方法80只SD大鼠随机分成5组:对照组、模型组、地塞米松组(1 mg/kg)、人参皂苷Rb1低剂量组(100 mg/kg)、人参皂苷Rb1高剂量组(200 mg/kg),每组16只。模型组、地塞米松组、人参皂苷Rb1低、高剂量组建立哮喘大鼠模型,建模成功后地塞米松组、人参皂苷Rb1低、高剂量组给予相应药物灌胃,对照组及模型组给予等量的生理盐水灌胃,每天1次,连续给药14 d。末次给药后24 h,测定大鼠血液淋巴细胞、嗜酸性粒细胞、中性粒细胞、动脉血氧分压(PaO₂)水平,计算大鼠肺/体比值,苏木素-伊红(HE)染色观察肺组织病理学变化,实时荧光定量PCR及蛋白免疫印迹法测定大鼠肺组织中TLR3、TRIF mRNA和蛋白表达水平。结果与对照组比较,模型组大鼠肺/体比值、淋巴细胞、嗜酸性粒细胞、中性粒细胞水平、TLR3、TRIF mRNA和蛋白表达水平明显升高,PaO₂水平明显降低(P<0.05);与模型组比较,地塞米松组、人参皂苷Rb1低、高剂量组大鼠肺/体比值、淋巴细胞、嗜酸性粒细胞、中性粒细胞水平、TLR3、TRIF mRNA和蛋白表达水平降低,PaO₂水平升高(P<0.05);且人参皂苷Rb1高剂量组大鼠肺/体比值、淋巴细胞、嗜酸性粒细胞、中性粒细胞水平、TLR3、TRIF mRNA和蛋白表达水平低于人参皂苷Rb1低剂量组,PaO₂水平高于人参皂苷Rb1低剂量组(P<0.05);与地塞米松组比较,人参皂苷Rb1低剂量组大鼠肺/体比值、淋巴细胞、嗜酸性粒细胞、中性粒细胞水平、TLR3、TRIF mRNA和蛋白表达升高,PaO₂水平降低(P<0.05);人参皂苷Rb1高剂量组与地塞米松组上述指标差异无统计学意义(P>0.05)。结论人参皂苷Rb1能够减轻哮喘模型大鼠肺部炎症反应,其机制可能与人参皂苷Rb1抑制哮喘大鼠肺部TLR3、TRIF mRNA和蛋白表达水平进而抑制TLR3/TRIF信号通路的激活有关。