蛋白磷酸酶 PP2A 在 T 细胞早期发育中的功能及作用机制研究

The T lymphocytes are one of the key immune cells that immune system uses to combat infectious pathogens. T cells develop in thymus. The developmental process is instructed by a variety of extracellular stimulus, most of which induce intracellular signaling pathways transmitted and regulated by protein phosphorylation. The PP2A subfamily is serine threonine phosphatase comprised of a heterogeneous trimer, it included three different members, PP2A, PP4 and PP6. Previous studies have shown that the deletion of PP4 in mouse T cells result in impaired positive selection of thymocytes. While PP6 was found as a negative regulator of development of T cells due to its ability to inhibit TCR signal. More recent studies have shown that elevated expression or action of PP2A are related to various T cell-mediated autoimmune diseases. It is possible that PP2A may affect the autoreactivity of T cells though regulating the process of T cell development. In order to study the function of PP2A during the process of T cell development, we conducted the T cell specific PP2A deficient conditional knockout mice. Preliminary phenotype analysis showed that the PP2A deletion resulted in the blocking of early T cell development. This proposal will continue our study to clarify the role of PP2A in thymocyte development and reveal the underlying mechanism.

T淋巴细胞是机体免疫系统抵抗外界病原的一类重要免疫细胞。T细胞在胸腺中发育，整个发育过程受到胸腺内多种胞外刺激信号的指导。这些胞外刺激信号依赖蛋白质磷酸化和去磷酸化传递和调控。PP2A亚家族是一类广泛表达的异质三聚体丝苏氨酸磷酸酶，主要成员有PP2A、PP4和PP6。已有研究表明，在小鼠T细胞中特异性缺失PP4可以导致胸腺T细胞阳性选择受阻。而PP6在T细胞中能通过抑制TCR信号从而负向调节其发育过程。最近的研究表明，PP2A的表达和功能异常，与多种T细胞介导的自身免疫性疾病相关，提示PP2A可能通过调节T细胞发育的过程影响T细胞的自身反应性。为了研究PP2A在T细胞发育分化中的作用，我们构建了T细胞特异的PP2A条件性缺陷小鼠。对小鼠的表型初步分析显示， PP2A缺失导致T细胞早期发育受阻。本项目在此基础上进一步明确PP2A在胸腺T细胞发育中的作用及其机制。

胸腺T细胞的发育成熟受到严格的调控，T细胞经历细胞选择后仅有少部分表达适当TCR亲和力的细胞才能存活下来，因此确保这些选择后的T细胞的存活至关重要。然而，目前对细胞选择后的信号通路缺少系统的认识。本项目主要是运用磷酸化质谱的方法分析胸腺细胞阳性选择后蛋白丝/苏氨酸的磷酸化水平。我们的研究发现T细胞在经历阳性选择时TCR信号中的很多蛋白会发生去磷酸化，而磷酸酶PP2A在这些磷酸化水平的变化过程中发挥着直接作用。我们构建了PP2A α催化亚基在胸腺细胞中条件性敲除的（PP2A cKO）小鼠，发现在PP2A cKO小鼠中细胞凋亡相关蛋白的磷酸化水平异常导致DP阶段胸腺细胞凋亡增加，进而引起单阳性CD4 和CD8 T细胞数目减少。另外，将PP2A cKO小鼠与Bcl2转基因小鼠或与P53基因敲除小鼠杂交后，胸腺细胞的发育缺陷可以得到一定程度的挽救。因此，我们的工作揭示了PP2A通过调节细胞存活促进胸腺发育的重要作用。