Timmer Anjuli M, Timmer John C, Pence Morgan A, Hsu Li-Chung, Ghochani Mariam, Frey Terrence G, Karin Michael, Salvesen Guy S, Nizet Victor
Department of Pediatrics, Biomedical Sciences Graduate Program, Laboratory of Signal Transduction, University of California, San Diego, La Jolla, California 92093, USA.
J Biol Chem. 2009 Jan 9;284(2):862-71. doi: 10.1074/jbc.M804632200. Epub 2008 Nov 11.
Group A Streptococcus (GAS) is a leading human bacterial pathogen capable of producing invasive infections even in previously healthy individuals. As frontline components of host innate defense, macrophages play a key role in control and clearance of GAS infections. We find GAS induces rapid, dose-dependent apoptosis of primary and cultured macrophages and neutrophils. The cell death pathway involves apoptotic caspases, is partly dependent on caspase-1, and requires GAS internalization by the phagocyte. Analysis of GAS virulence factor mutants, heterologous expression, and purified toxin studies identified the pore-forming cytolysin streptolysin O (SLO) as necessary and sufficient for the apoptosis-inducing phenotype. SLO-deficient GAS mutants induced less macrophage apoptosis in vitro and in vivo, allowed macrophage cytokine secretion, and were less virulent in a murine systemic infection model. Ultrastructural evidence of mitochondrial membrane remodeling, coupled with loss of mitochondrial depolarization and cytochrome c release, suggests a direct attack of the toxin initiates the intrinsic apoptosis pathway. A general caspase inhibitor blocked SLO-induced apoptosis and enhanced macrophage killing of GAS. We conclude that accelerated, caspase-dependent macrophage apoptosis induced by the pore-forming cytolysin SLO contributes to GAS immune evasion and virulence.
A组链球菌（GAS）是一种主要的人类细菌病原体，即使在先前健康的个体中也能够引发侵袭性感染。作为宿主固有防御的一线组成部分，巨噬细胞在控制和清除GAS感染中发挥关键作用。我们发现GAS可诱导原代和培养的巨噬细胞及中性粒细胞发生快速、剂量依赖性凋亡。细胞死亡途径涉及凋亡半胱天冬酶，部分依赖于半胱天冬酶-1，并且需要吞噬细胞内化GAS。对GAS毒力因子突变体、异源表达及纯化毒素的研究确定，成孔细胞溶素链球菌溶血素O（SLO）对于诱导凋亡的表型是必需且充分的。缺乏SLO的GAS突变体在体外和体内诱导的巨噬细胞凋亡较少，可使巨噬细胞分泌细胞因子，并且在小鼠全身感染模型中的毒力较低。线粒体膜重塑的超微结构证据，以及线粒体去极化和细胞色素c释放的丧失，表明毒素的直接攻击启动了内源性凋亡途径。一种通用的半胱天冬酶抑制剂可阻断SLO诱导的凋亡，并增强巨噬细胞对GAS的杀伤作用。我们得出结论，由成孔细胞溶素SLO诱导的加速的、半胱天冬酶依赖性巨噬细胞凋亡有助于GAS的免疫逃逸和毒力。
