Gao L Y, Abu Kwaik Y
Department of Microbiology and Immunology, University of Kentucky Chandler Medical Center, Lexington, Kentucky 40536-0084, USA.
Infect Immun. 1999 Sep;67(9):4886-94. doi: 10.1128/IAI.67.9.4886-4894.1999.
The hallmark of Legionnaires' disease is replication of Legionella pneumophila within cells in the alveolar spaces. The mechanisms by which L. pneumophila replicates intracellularly and kills the host cell are largely not understood. We have recently shown that within 3 h of initiation of the infection and prior to intracellular replication, L. pneumophila induces apoptosis in macrophages, alveolar epithelial cells, and peripheral blood monocytes, which correlates with cytopathogenicity (L.-Y. Gao and Y. Abu Kwaik, Infect. Immun. 67:862-870, 1999). In this report, we show that the ability of L. pneumophila to induce apoptosis is, largely, not growth phase regulated. We demonstrate that the induction of apoptosis by L. pneumophila in macrophages is mediated through the activation of caspase 3. The enzymatic activity of caspase 3 to cleave a specific synthetic substrate in vitro is detected in L. pneumophila-infected macrophages at 2 h after infection and is maximal at 3 h, with over 900% increase in activity. The activity of caspase 3 to cleave a specific substrate [poly(ADP-ribose) polymerase, or PARP] in vivo is also detected at 2 h and is maximal at 3 h postinfection. The activity of caspase 3 to cleave the synthetic substrate in vitro and PARP in vivo is blocked by a specific inhibitor of caspase 3. The kinetics of caspase 3 activation correlates with that of L. pneumophila-induced nuclear apoptosis. Inhibition of caspase 3 activity blocks L. pneumophila-induced nuclear apoptosis and cytopathogenicity during early stages of the infection. Consistent with the ability to induce apoptosis, extracellular L. pneumophila also activates caspase 3. Three dotA/icmWXYZ mutants of L. pneumophila that are defective in inducing apoptosis do not induce caspase 3 activation, suggesting that expression and/or export of the apoptosis-inducing factor(s) is regulated by the dot/icm virulence system. This is the first description of the role of caspase 3 activation in induction of nuclear apoptosis in the host cell infected by a bacterial pathogen.
军团病的标志是嗜肺军团菌在肺泡腔细胞内复制。嗜肺军团菌在细胞内复制并杀死宿主细胞的机制在很大程度上尚不清楚。我们最近发现，在感染开始后3小时内且在细胞内复制之前，嗜肺军团菌可诱导巨噬细胞、肺泡上皮细胞和外周血单核细胞凋亡，这与细胞致病性相关（L.-Y. Gao和Y. Abu Kwaik，《感染与免疫》67:862 - 870，1999）。在本报告中，我们表明嗜肺军团菌诱导凋亡的能力在很大程度上不受生长阶段调控。我们证明嗜肺军团菌在巨噬细胞中诱导凋亡是通过激活半胱天冬酶3介导的。在感染后2小时，在嗜肺军团菌感染的巨噬细胞中检测到半胱天冬酶3在体外切割特定合成底物的酶活性，3小时时达到最大值，活性增加超过900%。在体内，半胱天冬酶3切割特定底物[聚（ADP - 核糖）聚合酶，或PARP]的活性在感染后2小时也可检测到，感染后3小时达到最大值。半胱天冬酶3在体外切割合成底物和在体内切割PARP的活性被半胱天冬酶3的特异性抑制剂阻断。半胱天冬酶3激活的动力学与嗜肺军团菌诱导的核凋亡的动力学相关。在感染早期，抑制半胱天冬酶3活性可阻断嗜肺军团菌诱导的核凋亡和细胞致病性。与诱导凋亡的能力一致，细胞外嗜肺军团菌也可激活半胱天冬酶3。嗜肺军团菌的三个dotA/icmWXYZ突变体在诱导凋亡方面存在缺陷，它们不会诱导半胱天冬酶3激活，这表明凋亡诱导因子的表达和/或输出受dot/icm毒力系统调控。这是首次描述半胱天冬酶3激活在细菌病原体感染的宿主细胞中诱导核凋亡的作用。
