Intracellular Signals Controlling Lymphocyte Development

TCR信号的细胞内调控一直是一个令人着迷的课题，因为它对T细胞的发育和功能起着重要的作用。有证据表明，细胞内信号的失调也是许多疾病的原因，包括自身免疫和免疫缺陷。我们之前已经证明，受体分子Cbl和Cbl-b参与胸腺细胞和外周T细胞中TCR信号的组织，这两种分子的缺乏分别导致体内成熟T细胞CD4+胸腺细胞发育增强或CD28独立高反应性。我们现在表明，T系细胞中Cbl和Cbl-b的破坏导致更高的死亡率，主要是由于突变小鼠自身免疫性动脉炎的自发发展。我们发现Cbl/Cbl-b双突变体（dKO） T细胞对抗原刺激具有高反应性。然而，生物化学分析表明，TCR刺激后细胞核内转录因子NFkB、AP-1、NFAT水平显著升高，但酪氨酸磷酸化、Ca++动员、MAP激酶和Vav激活等TCR主要信号通路并未明显增强。进一步分析表明，突变T细胞在抗原刺激后未能下调其TCR，导致激活细胞中持续的信号传导。在没有TCR刺激的情况下，dKO细胞的TCR内吞是正常的。然而，内化的TCR在TCR刺激后未能被转运到溶酶体中，这表明内化的TCR在溶酶体室中被阻断了蛋白质分选。这些结果表明了一种新的机制，Cbl和Cbl-b可能控制TCR内化，从而通过调节胞吞过程中的细胞膜分选来调节TCR信号。此外，dKO小鼠动脉炎的发生表明抗原刺激后TCR下调在生理上的重要性，不下调将导致自身免疫性疾病的发生。

Intracellular regulation of TCR signaling has been a fascinating subject because it palys an important role for T cell development and function. Evidences indicated that dysregulation of intracellular signaling is also responsible for many diseases including autoimmunity and immune deficiency. We previously demonstrated that adaptor molecule Cbl and Cbl-b are involved in organizing TCR signals in thymocytes and peripheral T cells, and deficiency of these two molecules leads to an enhanced CD4+ thymocyte development or CD28 independent-hyperresponsiveness of mature T cells, respectively, in vivo. We now show that disruption of both Cbl and Cbl-b in T lineage cells lead to higher mortality primarily due to the spontaneous development of autoimmune arteritis in the mutant mice. We found that the Cbl/Cbl-b double mutant (dKO) T cells were hyperresponsive to antigen stimulation. However, the biochemistry analysis indicated that major TCR signaling pathways, such as tyrosine-phosphorylation, Ca++ mobilization, MAP kinase and Vav activation, were not significantly enhanced, despite that the dramatically increased levels of transcription factors NFkB, AP-1, NFAT were observed in the cell nucleus after stimulation through the TCR. Further analysis indicated that the mutant T cells failed to downmodulate their TCR after antigen stimulation, resulting in a sustained signaling in the activated cells. TCR endocytosis is normal in the dKO cells in the absence of stimulation through TCR. However, internalized TCR failed to be transported to the lysosomes after TCR stimulation, indicating a blocked protein sorting of internalized TCR into the lysosome compartment. These results demonstrate a novel mechanis that Cbl and Cbl-b may control TCR internalization, thus TCR signaling through regulating the intracellular membrane sorting during endocytosis. Furthermore, development of arteritis in the dKO mice indicate the physiological importance of TCR downmodulation after antigen stimulation, and failed to do so will lead to the development of autoimmune diseases.