Abu-Zant Alaeddin, Santic Marina, Molmeret Maelle, Jones Snake, Helbig Jürgen, Abu Kwaik Yousef
Department of Microbiology, University of Louisville College of Medicine, Louisville, KY 40292, USA.
Infect Immun. 2005 Sep;73(9):5339-49. doi: 10.1128/IAI.73.9.5339-5349.2005.
The ability of the intracellular bacterium Legionella pneumophila to cause disease is totally dependent on its ability to modulate the biogenesis of its phagosome and to replicate within alveolar cells. Upon invasion, L. pneumophila activates caspase-3 in macrophages, monocytes, and alveolar epithelial cells in a Dot/Icm-dependent manner that is independent of the extrinsic or intrinsic pathway of apoptosis, suggesting a novel mechanism of caspase-3 activation by this intracellular pathogen. We have shown that the inhibition of caspase-3 prior to infection results in altered biogenesis of the L. pneumophila-containing phagosome and in an inhibition of intracellular replication. In this report, we show that the preactivation of caspase-3 prior to infection does not rescue the intracellular replication of L. pneumophila icmS, icmR, and icmQ mutant strains. Interestingly, preactivation of caspase-3 through the intrinsic and extrinsic pathways of apoptosis in both human and mouse macrophages inhibits intracellular replication of the parental stain of L. pneumophila. Using single-cell analysis, we show that intracellular L. pneumophila induces a robust activation of caspase-3 during exponential replication. Surprisingly, despite this robust activation of caspase-3 in the infected cell, the host cell does not undergo apoptosis until late stages of infection. In sharp contrast, the activation of caspase-3 by apoptosis-inducing agents occurs concomitantly with the apoptotic death of all cells that exhibit caspase-3 activation. It is only at a later stage of infection, and concomitant with the termination of intracellular replication, that the L. pneumophila-infected cells undergo apoptotic death. We conclude that although a robust activation of caspase-3 is exhibited throughout the exponential intracellular replication of L. pneumophila, apoptotic cell death is not executed until late stages of the infection, concomitant with the termination of intracellular replication.
胞内细菌嗜肺军团菌致病的能力完全取决于其调节吞噬体生物发生以及在肺泡细胞内复制的能力。入侵后，嗜肺军团菌以Dot/Icm依赖的方式激活巨噬细胞、单核细胞和肺泡上皮细胞中的半胱天冬酶-3，该方式独立于凋亡的外在或内在途径，提示这种胞内病原体激活半胱天冬酶-3的新机制。我们已经表明，感染前抑制半胱天冬酶-3会导致含嗜肺军团菌吞噬体的生物发生改变，并抑制细胞内复制。在本报告中，我们表明感染前半胱天冬酶-3的预激活不能挽救嗜肺军团菌icmS、icmR和icmQ突变株的细胞内复制。有趣的是，通过人源和小鼠巨噬细胞中凋亡的内在和外在途径预激活半胱天冬酶-3会抑制嗜肺军团菌亲本菌株的细胞内复制。使用单细胞分析，我们表明胞内嗜肺军团菌在指数期复制期间诱导半胱天冬酶-3的强烈激活。令人惊讶的是，尽管感染细胞中半胱天冬酶-3有这种强烈激活，但宿主细胞直到感染后期才发生凋亡。与之形成鲜明对比的是，凋亡诱导剂激活半胱天冬酶-3与所有表现出半胱天冬酶-3激活的细胞的凋亡死亡同时发生。只有在感染后期，伴随着细胞内复制的终止，被嗜肺军团菌感染的细胞才会发生凋亡死亡。我们得出结论，尽管在嗜肺军团菌整个指数期细胞内复制过程中都表现出半胱天冬酶-3的强烈激活，但凋亡性细胞死亡直到感染后期才发生，与细胞内复制的终止同时出现。
