Function of Rac1 in T cell development and activation

Rac1是Rho家族的小gtpase之一，在各种信号转导途径中起着“分子开关”的作用。为了直接评估Rac1在T细胞发育中的功能，我们将显性阴性（dn） Rac1基因导入pax5缺陷小鼠pro-B细胞，该基因可以在体内重建T细胞发育。表达GFP阳性dnRac1的胸腺细胞分化为CD4-和cd8单阳性（SP）细胞严重受损，表明Rac1在胸腺细胞的阳性选择中起关键作用。表达dnRac1的胸腺细胞在TCR刺激下凋亡增加。为了研究Rac1在DP胸腺细胞TCR信号转导中的详细功能，我们利用体外经TCR刺激可分化为CD4-SP细胞的DP细胞系DPK。表达dn-Rac1的DPK在TCR刺激下大量凋亡，导致CD4-SP细胞分化缺陷。dnRac1-DPK抑制了TCR依赖性肌动蛋白聚合和Bcl-2转录上调。综上所述，这些数据表明，Rac1不仅通过重组肌动蛋白细胞骨架，而且通过上调Bcl-2阻止tcr诱导的细胞凋亡，在胸腺细胞的阳性选择中起着关键作用。

Rac1, one of the Rho family small GTPases has been shown to work as a "molecular switch" in various signal transduction pathways. In order to directly assess the function of Rac1 in T cell development, we introduced dominant negative (dn) Rac1 gene into Pax5-deficient mouse pro-B cells, which can reconstitute T cell development in vivo. Differentiation of GFP positive dnRac1 expressing thymocytes into CD4- and CD8-single positive (SP) cells was severely impaired, indicating that Rac1 is critical in positive selection of thymocytes. Thymocytes expressing dnRac1 demonstrated increased apoptosis against TCR stimulation. In order to study detailed function of Rac1 in TCR signal transduction of DP thymocytes, we utilized DP cell line DPK, which can differentiate into CD4-SP cells upon TCR stimulation in vitro. DPK expressing dn-Rac1 underwent massive apoptosis upon TCR stimulation and resulted in defective differentiation of CD4-SP cells. TCR dependent actin polymerization and upregulation of Bcl-2 transcription were suppressed in dnRac1-DPK. Collectively, these data indicate that Rac1 is critical in positive selection of thymocyte through not only reorganizing actin cytoskeleton, but also preventing TCR-induced apoptosis via Bcl-2 upregulation.