PNAS：酶BTK在抗病毒感染中发挥关键作用

身体对入侵的细菌或病毒产生的初始反应是由先天免疫系统调控着的，在这种免疫系统中，细胞分泌出被称作细胞因子的信号分子从而促进炎症产生和对被身体识别为外来物质的靶标发动一般性反击。比如，蛋白Toll样受体3(Toll-like receptor 3, TLR3)有助于启动对抗病毒的先天免疫反应。

如今，来自新加坡科技研究局(A*STAR)下属的生物处理科技研究院的Kong-Peng Lam和同事们证实TLR3识别病毒遗传物质，随后通过磷酸化过程而发生激活。

研究人员鉴定出Bruton酪氨酸激酶(Bruton’s tyrosine kinase, BTK)为这种途径的TLR3激活酶(TLR3-activating enzym)。基于BTK在免疫功能中的显著性作用：编码它的基因发生突变会导致X-连锁无丙种球蛋白血症(X-linked agammaglobulinemia)产生，他们发现BTK是一种大有希望的候选物。这种疾病的特征在于B细胞产生和功能存在故障。Lam说，“这些病人也对复发的细菌和病毒感染非常敏感，这就提示着BTK可能参与先天性免疫反应。”

为了验证他们的假设，研究人员给小鼠注射多聚胞苷酸(polyribocytidylic acid)---这种分子类似病毒RNA，能够触发抗病毒免疫反应。在这种小鼠中，这种处理能够触发一种强烈的能够导致败血性休克产生的炎性过度反应。然而，经证实BTK缺失的小鼠和TLR3缺失的小鼠抵抗败血性休克，这提示着这两种分子在同一个途径中相互协作。TLR3激活也产生促进被称作干扰素β(interferon β, IFN-β)的免疫刺激性分子产生的信号，然而，在小鼠中，BTK缺失有效地破坏这种反应。相应地，BTK缺失的小鼠也要比野生型小鼠更不能够将登革热病毒从体内清除出去。

生化实验清晰地证实BTK是TLR3磷酸化所必需的。在激活之后，TLR3结合到TRIF上，其中TRIF允许它与多种其他的信号分子相互作用。然而，没有BTK，TLR3不能经历磷酸化。因此，TRIF不能结合到下游信号分子，因而阻止信号级联反应产生。

这项研究提示着靶向BTK的药物可能能够作为一种有用的免疫调节工具。

本文编译自A single enzyme plays a critical role in helping the body effectively fight viral infection

doi: 10.1073/pnas.1119238109
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Bruton’s tyrosine kinase phosphorylates Toll-like receptor 3 to initiate antiviral response

Koon-Guan Leea, Shengli Xua, Zi-Han Kanga, Jianxin Huoa, Mei Huangb, Dingxiang Liub, Osamu Takeuchic, Shizuo Akirac, and Kong-Peng Lam

Toll-like receptor 3 (TLR3) mediates antiviral response by recognizing double-stranded RNA. Its cytoplasmic domain is tyrosine phosphorylated upon ligand binding and initiates downstream signaling via the adapter TIR-containing adaptor inducing interferon–β (TRIF). However, the kinase responsible for TLR3 phosphorylation remains unknown. We show here that Bruton's tyrosine kinase (BTK)-deficient macrophages failed to secrete inflammatory cytokines and IFN-β upon TLR3 stimulation and were impaired in clearing intracellular dengue virus infection. Mutant mice were also less susceptible to D-galactosamine/p(I:C)-induced sepsis. In the absence of BTK, TLR3-induced phosphoinositide 3-kinase (PI3K), AKT and MAPK signaling and activation of NFκB, IRF3, and AP-1 transcription factors were all defective. We demonstrate that BTK directly phosphorylates TLR3 and in particular the critical Tyr759 residue. BTK point mutations that abrogate or led to constitutive kinase activity have opposite effects on TLR3 phosphorylation. Loss of BTK also compromises the formation of the downstream TRIF/receptor-interacting protein 1 (RIP1)/TBK1 complex. Thus, BTK plays a critical role in initiating TLR3 signaling.