Ince Dilek, Sutterwala Fayyaz S, Yahr Timothy L
Department of Internal Medicine, Division of Infectious Disease, University of Iowa, Iowa City, Iowa, USA.
Department of Internal Medicine, Division of Infectious Disease, University of Iowa, Iowa City, Iowa, USA Veterans Affairs Medical Center, Iowa City, Iowa, USA.
J Bacteriol. 2015 Jun 15;197(12):2003-11. doi: 10.1128/JB.00030-15. Epub 2015 Apr 6.
The opportunistic pathogen Pseudomonas aeruginosa utilizes an injectisome-type III secretion system (injectisome-T3SS) to elicit cytotoxicity toward epithelial cells and macrophages. Macrophage killing results from the cytotoxic properties of the translocated effector proteins (ExoS, ExoT, ExoU, and ExoY) and inflammasome-mediated induction of pyroptosis. Inflammasome activation can occur following Nlrc4-mediated recognition of cytosolic translocated flagellin (FliC). In the present study, we demonstrate that FliC is a secretion substrate of both the injectisome- and flagellum-associated T3SSs. Molecular analyses indicate that the first 20 amino-terminal residues of FliC are sufficient for secretion by the injectisome-T3SS and that the first 100 residues are sufficient for translocation of FliC into host cells. Although maximal inflammasome activation requires FliC, activation can also occur in the absence of FliC. This prompted us to examine whether other flagellar components might also be translocated into cells to elicit inflammasome activation. Indeed, we find that the flagellar cap (FliD), hook-associated (FlgK and FlgL), hook (FlgE), and rod (FlgE) proteins are secretion substrates of the injectisome-T3SS. None of these proteins, however, result in increased inflammasome activation when they are overexpressed in a fliC mutant and appear to be translocated into host cells. While a role in inflammasome activation has been excluded, these data raise the possibility that flagellar components, which are highly conserved between different bacterial species, trigger other specific host responses from the extracellular milieu or contribute to the pathogenesis of P. aeruginosa.
The inflammasome is a host defense mechanism that recognizes invading bacteria and triggers an inflammatory immune response. The opportunistic pathogen P. aeruginosa produces both inflammasome agonists and antagonists. In this study, we demonstrate that overexpression of an agonist suppresses the activity of an antagonist, thereby resulting in inflammasome activation. Since the relative expression levels of agonists and antagonists likely vary between strains, these differences could be important predictors of whether a particular P. aeruginosa strain elicits inflammasome activation.
机会致病菌铜绿假单胞菌利用注射体样III型分泌系统（注射体-T3SS）对上皮细胞和巨噬细胞产生细胞毒性。巨噬细胞死亡源于易位效应蛋白（ExoS、ExoT、ExoU和ExoY）的细胞毒性特性以及炎性小体介导的焦亡诱导。炎性小体激活可在Nlrc4介导的胞质易位鞭毛蛋白（FliC）识别后发生。在本研究中，我们证明FliC是注射体相关和鞭毛相关T3SS的分泌底物。分子分析表明，FliC的前20个氨基末端残基足以被注射体-T3SS分泌，前100个残基足以使FliC易位到宿主细胞中。尽管最大程度的炎性小体激活需要FliC，但在没有FliC的情况下也可发生激活。这促使我们研究其他鞭毛成分是否也可能易位到细胞中以引发炎性小体激活。确实，我们发现鞭毛帽（FliD）、钩相关蛋白（FlgK和FlgL）、钩（FlgE）和杆（FlgE）蛋白是注射体-T3SS的分泌底物。然而，当它们在fliC突变体中过表达时，这些蛋白均不会导致炎性小体激活增加，并且似乎易位到宿主细胞中。虽然已排除其在炎性小体激活中的作用，但这些数据增加了一种可能性，即不同细菌物种之间高度保守的鞭毛成分可能从细胞外环境触发其他特定的宿主反应或促进铜绿假单胞菌的发病机制。
炎性小体是一种宿主防御机制，可识别入侵细菌并触发炎性免疫反应。机会致病菌铜绿假单胞菌既产生炎性小体激动剂又产生拮抗剂。在本研究中，我们证明激动剂的过表达会抑制拮抗剂的活性，从而导致炎性小体激活。由于激动剂和拮抗剂的相对表达水平可能因菌株而异，这些差异可能是特定铜绿假单胞菌菌株是否引发炎性小体激活的重要预测指标。
