Bunin Anna, Khwaja Fatima W, Kersh Gilbert J
Department of Pathology and Laboratory Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA.
J Immunol. 2005 Aug 1;175(3):1532-9. doi: 10.4049/jimmunol.175.3.1532.
Bim, a BH3-only Bcl-2 family member, is required for apoptosis of thymocytes in response to negative selection signals. Regulation of the apoptotic activity of Bim during negative selection is not understood. In this study we demonstrate that in murine thymocytes undergoing apoptosis in response to anti-CD3epsilon injection, levels of Bim protein expression do not change. In immature thymocytes, Bim is associated with mitochondria before stimulation and is not regulated by a change in subcellular localization during apoptosis. We also show that Bim(EL) is rapidly phosphorylated in thymocytes in response to CD3epsilon cross-linking both in vivo and in vitro, and that phosphorylation is sustained for at least 24 h. Analysis of MHC-deficient mice shows that phosphorylation of Bim occurs in CD4/CD8 double-positive thymocytes and does not depend on activation of mature T cells. We also find that TCR cross-linking on thymocytes induces an increase in the proportion of Bcl-x(L) bound to Bim at late time points. Our results favor a model in which strong TCR signals regulate the apoptotic activity of Bim by phosphorylation and subsequent changes in binding to Bcl-x(L) in immature thymocytes.
Bim是仅含BH3结构域的Bcl-2家族成员，在胸腺细胞因阴性选择信号而发生凋亡的过程中发挥作用。目前尚不清楚在阴性选择过程中Bim凋亡活性的调控机制。在本研究中，我们发现，在因注射抗CD3ε而发生凋亡的小鼠胸腺细胞中，Bim蛋白的表达水平并未改变。在未成熟胸腺细胞中，Bim在刺激前与线粒体相关联，且在凋亡过程中不受亚细胞定位变化的调控。我们还发现，体内和体外实验均表明，在胸腺细胞中，Bim(EL)会因CD3ε交联而迅速磷酸化，且这种磷酸化会持续至少24小时。对MHC缺陷小鼠的分析表明，Bim的磷酸化发生在CD4/CD8双阳性胸腺细胞中，且不依赖于成熟T细胞的激活。我们还发现，胸腺细胞上的TCR交联在后期会导致与Bim结合的Bcl-x(L)比例增加。我们的研究结果支持这样一种模型，即强烈的TCR信号通过磷酸化以及随后未成熟胸腺细胞中与Bcl-x(L)结合的变化来调节Bim的凋亡活性。
