T细胞抗原受体活化机制的研究

The host antigen-specific immune response depends on a key receptor on the T-cell surface, T cell receptor-CD3 complex (TCR). TCR recognizes peptide antigens presented by MHC molecules and decodes the antigen-stimulating signal to tyrosine phosphorylation signal, which in turn triggers downstream signaling cascades to activate T cells. Despite of its importance for the host immunity, the molecular mechanism of TCR tyrosine phosphorylation still remains elusive. My earlier works found that acidic phospholipids in the inner-leaflet of the plasma membrane can interact with CD3 cytoplasmic domains and thus sequester the tyrosine phosphorylation sites within the membrane bilayer (Cell 2008, first author). In antigen-stimulated T-cells, Ca2+ quickly influxes into the cells and Ca2+ ions can directly bind to the acidic phospholipids and disrupt the ionic TCR-lipid interactions, which lead to the exposure of TCR phosphorylation sites (Nature 2013, corresponding author). The next key question is how Lck kinase can specifically recognize and phosphorylate TCR. We will first use biochemical and NMR (Nuclear Magnetic Resonance) techniques to study how CD3 chains dissociate from the membrane and become phosphorylated by Lck. CD3-Lck complex structure will then be solved by NMR. Cellular and animal experiments will also be performed to study the physiological importance of CD3-Lck interaction for TCR phosphorylation as well as the T-cell development and function. The performance of this project will shed a new light on the molecular mechanism of the host antigen immune response.

机体抗原免疫应答依赖于T细胞表面的T细胞抗原受体（TCR-CD3复合体，简称TCR）识别多肽抗原，将抗原刺激信号跨膜转导为CD3链的磷酸化信号，从而引发机体的特异性免疫反应。目前TCR的磷酸化机制还未完全阐明。申请人的前期工作发现细胞质膜中的酸性磷脂通过静电作用将TCR磷酸化位点屏蔽于膜脂双层中（Cell 2008，第一作者）；而在抗原刺激后的T细胞中，内流的钙离子可以直接结合酸性磷脂并解除TCR的膜屏蔽，从而暴露TCR的磷酸化位点（Nature 2013，通讯作者）。下一个关键科学问题是Lck激酶如何特异性识别并磷酸化TCR？申请人将利用生化和核磁共振技术研究CD3从膜上解离并被Lck磷酸化的动态过程；解析CD3-Lck的复合物结构；然后利用细胞生物学以及动物实验来研究CD3-Lck相互作用对TCR活化及T细胞发育与功能的影响。本项目的开展将使我们能进一步理解机体抗原免疫应答的分子机理。

机体抗原免疫应答依赖于T细胞表面的T细胞抗原受体（TCR-CD3复合体，简称TCR）识别多肽抗原，将抗原刺激信号跨膜转导为CD3链的磷酸化信号，从而引发机体的特异性免疫反应。目前TCR的磷酸化机制还未完全阐明。本项目研究了T细胞免疫应答中的关键激酶Lck是如何特异性识别并磷酸化CD3链的过程。我们发现Lck对TCR复合物中不同的CD3链具有高度选择性，主要与CD3e结合并使之磷酸化。进一步通过序NMR实验，我们找到了介导Lck-CD3e相互作用的关键位点，Lck的UD结构域与CD3e的碱性氨基酸富集区（BRS）之间的静电相互作用直接调控Lck对CD3链的选择性。Lck结合并磷酸化CD3e能够进一步帮助其它CD3链的磷酸化，从而促进整个TCR复合物的磷酸化。另外我们还通过单分子技术研究了CD3e与细胞膜解离的动态过过程，阐明了其结构动态性。这些工作阐明了TCR产生特异性磷酸化模式的信号与结构基础。

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