Target Cell Death by Cytotoxic Lymphocytes

淋巴细胞介导的细胞毒性的颗粒胞吐模型假设抗原触发的预先形成的有效溶解剂穿孔素（以及其他介质）快速分泌到细胞毒性淋巴细胞和其结合靶之间的突触样空间。 然而，为什么效应细胞本身不会因穿孔素损伤而死亡的问题经常被提出，但没有明确的答案。 我们最近进行了一系列实验，旨在测试一种新的自我保护模型，该模型提出效应细胞膜上的半胱氨酸蛋白酶在穿孔素聚集并形成孔之前插入穿孔素。 我们已经通过询问1）活性半胱氨酸蛋白酶在细胞毒性淋巴细胞表面上是否可检测到，以及2）半胱氨酸蛋白酶抑制剂是否阻断自我保护并导致细胞毒性淋巴细胞中的脱颗粒诱导的死亡来测试该模型。 我们已经通过用生物素化的组织蛋白酶亲和试剂和蛋白质抑制剂胱抑素B标记细胞毒性T淋巴细胞来检测细胞毒性T淋巴细胞表面上的活性半胱氨酸蛋白酶。随后通过流式细胞术进行的分析表明，通过这种方法可以在细胞毒性T淋巴细胞上检测到半胱氨酸蛋白酶，并且它们在TcR触发后增加。 它们可能参与自我保护的证据来自于这样的研究，其中这些组织蛋白酶抑制剂在用交联T细胞受体的抗体包被的威尔斯孔上孵育时，在4小时内诱导CTL的凋亡性死亡，而不是对照单克隆抗体。 这些组织蛋白酶抑制剂显示出可忽略的杀死静息CTL的能力，并且具有相同反应基团但不合适的肽的对照试剂在TcR接合后不能诱导死亡。 这种CTL的死亡是钙依赖性的，被颗粒毒素concanamycin B阻断，并且在穿孔素缺陷型CTL中未观察到。 在组织蛋白酶抑制剂存在下，当在包被有脱粒抗-CD 16 Mab而非对照Mab的威尔斯孔上孵育时，人NK细胞显示出相似的死亡。 这些结果支持了细胞毒性淋巴细胞利用表面组织蛋白酶的蛋白水解作用来保护自身免受分泌的穿孔素对细胞膜的损伤的假设。导致上述结果的组织蛋白酶的性质正在研究中，一些证据表明溶酶体组织蛋白酶B和L都在T淋巴细胞表面表达。 组织蛋白酶W是一种新的组织蛋白酶，最初被描述为表达序列标签，随后在mRNA表达研究中发现仅在细胞毒性淋巴细胞（T细胞和NK细胞）中表达。 由于它是上述活性的明显候选者，我们已经在E. coli中，并制备了一系列单克隆抗体。 在Western印迹中，其中一些与小鼠CTL中的单个30 kd条带反应，这是活性组织蛋白酶W的预期大小。 目前正在进行实验，看看组织蛋白酶W是否位于颗粒，细胞表面或其他地方，以及它是否参与自我保护。

The granule exocytosis model of lymphocyte-mediated cytotoxicity postulates an antigen-triggered rapid secretion of the preformed potent lytic agent perforin (as well as other mediators) into the synapse-like space between the cytotoxic lymphocyte and its bound target. However, the question of why the effector cell does not itself die of perforin damage has often been raised without a clear answer. We have recently carried out a series of experiments designed to test a new model for self-protection which proposes that cysteine proteases on the effector cell membrane inactivate inserted perforin before it can aggregate and form pores. We have tested this model by asking if 1) active cysteine proteases are detectable on the surface of cytotoxic lymphocytes, and 2) cysteine protease inhibitors block self-protection and result in degranulation-induced death in cytotoxic lymphocytes. We have detected active cysteine proteases on the surface of cytotoxic T lymphocytes by labeling them with a biotinylated cathepsin affinity reagent and with the protein inhibitor cystatin B Subsequent analysis by flow cytometry shows that cysteine proteases are detectable on cytotoxic T lymphocytes by this approach, and that they increase upon TcR triggering. Evidence that they may be involved in self-protection comes from studies in which these cathepsin inhibitors induce apoptotic death in CTL within 4 hours when incubated on wells coated with antibodies cross-linking the T cell receptor but not control Mabs. These cathepsin inhibitors show negligible ability to kill resting CTL, and control reagents with the same reactive group but an inappropriate peptide fail to induce death after TcR engagement. This death of CTL is calcium-dependent, blocked by the granule toxin concanamycin B, and is not seen in perforin-deficient CTL. Human NK cells show a similar death in the presence of cathepsin inhibitors when incubated on wells coated with degranulating anti-CD16 Mab, but not control Mab. These results support the hypothesis that cytotoxic lymphocytes use proteolysis by surface cathepsins to protect themselves against membrane damage by secreted perforin.The nature of the cathepsin responsible for the above results is under investigation, and some evidence suggests both the lysosomal cathepsins B and L are expressed on the T lymphocyte surface. Cathepsin W is a novel cathepsin originally described as an espressed sequence tag and subsequently found in mRNA expression studies to be expressed exclusively in cytotoxic lymphocytes (both T and NK cells). Since it is an obvious candidate for the above activity, we have expressed human pre-procathepsin W in E. coli and made a series of monoclonal antibodies against it. In Western blots, some of these react with a single 30kd band in mouse CTL, a size expected for active cathepsin W. Experiments are currently under way to see if cathepsin W is localized in granules, on the cell surface, or elsewhere, and if it is involved in self-protection.