Laboratory of Microbiology and Immunology and The Global COE Program, School of Pharmaceutical Sciences, University of Shizuoka, Shizuoka, Japan.
Biol Pharm Bull. 2012;35(9):1460-8. doi: 10.1248/bpb.b11-00011.
Legionella pneumophila, a causative agent of Legionnaire's disease, is an intracellular pathogen. It intervenes in the signal transduction of macrophages by secreting effector molecules through the Icm/Dot type IV secretion system (T4SS). There is a connection between signaling cascades that regulate phagocytosis and the production of reactive oxygen species (ROS). Class I phosphatidylinositol 3-kinase (PI3-K) and its product phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) play key roles in the reorganization of cytoskeleton (phagocytosis) and activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (ROS production). We investigated the production of PI(3,4,5)P3 and recruitment of class I PI3-K and Rac1 during phagocytosis of L. pneumophila by macrophages. Transient recruitment of class I PI3-K as well as PI(3,4,5)P3 production was observed around a phagocytosed T4SS mutant LELA3118 or avirulent mutant 25D in an early stage of infection. In contrast, class I PI3-K was recruited while accumulation of PI(3,4,5)P3 was not observed around wild type JR32. Immunoglobulin G (IgG)-opsonized live JR32, which would activate class I PI3-K through the Fcγ receptor pathway, did not induce PI(3,4,5)P3 production. Regardless of whether wild type or mutants were used, transient Rac1 accumulation was observed around bacteria. These results indicate that the phagocytosis of wild type L. pneumophila occurs via a special mechanism in which PI(3,4,5)P3 production is absent. This suggests that L. pneumophila may inhibit the production of PI(3,4,5)P3, but not the recruitment of class I PI3-K and Rac1, in a T4SS-dependent manner. L. pneumophila may start the modulation of host signaling cascade immediately after contact with host cells to evade the ROS-dependent bactericidal system while completing entry into macrophages.
嗜肺军团菌是军团病的病原体，属于细胞内寄生菌。它通过 Icm/Dot 型 IV 型分泌系统（T4SS）分泌效应分子，干预巨噬细胞的信号转导。吞噬作用和活性氧（ROS）产生的信号级联之间存在联系。I 类磷酸肌醇 3-激酶（PI3-K）及其产物磷脂酰肌醇 3,4,5-三磷酸（PI（3,4,5）P3）在细胞骨架重组（吞噬作用）和烟酰胺腺嘌呤二核苷酸磷酸（NADPH）氧化酶（ROS 产生）的激活中发挥关键作用。我们研究了巨噬细胞吞噬嗜肺军团菌时 PI（3,4,5）P3 的产生和 I 类 PI3-K 和 Rac1 的募集。在感染早期，吞噬 T4SS 突变体 LELA3118 或无毒突变体 25D 时，观察到 I 类 PI3-K 的短暂募集以及 PI（3,4,5）P3 的产生。相比之下，在野生型 JR32 周围，I 类 PI3-K 被募集，但未观察到 PI（3,4,5）P3 的积累。通过 Fcγ 受体途径激活 I 类 PI3-K 的 IgG 调理的活 JR32 不会诱导 PI（3,4,5）P3 的产生。无论使用野生型还是突变体，细菌周围都会观察到短暂的 Rac1 积累。这些结果表明，野生型嗜肺军团菌的吞噬作用是通过一种特殊的机制发生的，该机制中 PI（3,4,5）P3 的产生不存在。这表明嗜肺军团菌可能通过 T4SS 依赖性方式抑制 PI（3,4,5）P3 的产生，而不是 I 类 PI3-K 和 Rac1 的募集。嗜肺军团菌可能在与宿主细胞接触后立即开始调节宿主信号级联，以逃避 ROS 依赖性杀菌系统，同时完成进入巨噬细胞。
