Apoptotic Death in T Lymphocytes

我们继续研究T细胞凋亡使用一系列的细胞可渗透的荧光肽底物。以前的研究表明，胸腺细胞Fas交联触发caspase级联反应，其中IETD（caspase 8选择性）底物是最早的活动，通过流式细胞术测量。鉴于大量证据表明胱天蛋白酶8活化是Fas死亡途径中的起始事件，该结果是预期的。然而，当我们通过这种方法检测细胞毒性T淋巴细胞中抗Fas诱导的细胞凋亡时，IETDase激活意外地跟随LEHDase、YVHDase和VEIDase活性（对半胱天冬酶9、1和6具有选择性）的增加。当通过共聚焦显微镜检查时，抗Fas处理的CTL显示含IETDase的质膜囊泡的早期外观和它们从CTL表面释放，随后在细胞内部激活其他半胱天冬酶活性。由于这些囊泡未包括在流式细胞术分析中，因此早期IETDase活性被低估。与抗Fas相反，IL-2撤退诱导这些CTL细胞凋亡导致细胞质中早期IETDase活性，无质膜囊泡形成。因此，抗Fas诱导的在质膜上的半胱天冬酶活性的起始可能在一些细胞中导致亚膜蛋白的局部蛋白水解，导致高度富集活性半胱天冬酶8的膜囊泡的产生。

We have continued studying T cell apoptosis using a series of cell-permeable fluorogenic peptide substrates. Previous studies of thymocytes showed that Fas crosslinking triggered a caspase cascade in which the IETD (caspase 8 selective) substrate was the earliest activity measured by flow cytometry. This result was expected in light of the abundant evidence for caspase 8 activation as an initiating event in the Fas death pathway. However, when we examined apoptosis induced by anti-Fas in cytotoxic T lymphocytes by this approach, IETDase activation unexpectedly followed increases in LEHDase, YVHDase, and VEIDase activities (selective for caspases 9,1, and 6). When examined by confocal microscopy, anti-Fas treated CTL showed the early appearance of IETDase-containing plasma membrane vesicles and their release from the CTL surface followed by activation of other caspase activities in the cell interior. Since these vesicles were not included in the flow cytometry analysis, the early IETDase activity had been underestimated. In contrast to anti-Fas, induction of apoptosis in these CTL by IL-2 withdrawal resulted in early IETDase activity in the cytoplasm, with no plasma membrane vesiculation. Thus anti-Fas-induced initiation of caspase activity at the plasma membrane may in some cells result in local proteolysis of sub-membrane proteins, leading to generation of membrane vesicles which are highly enriched in active caspase 8.