Date Published: December 2, 2016
Publisher: Public Library of Science
Author(s): Dmitry Ratner, M. Pontus A. Orning, Megan K. Proulx, Donghai Wang, Mikhail A. Gavrilin, Mark D. Wewers, Emad S. Alnemri, Peter F. Johnson, Bettina Lee, Joan Mecsas, Nobuhiko Kayagaki, Jon D. Goguen, Egil Lien, Igor Eric Brodsky.
Type III secretion systems (T3SS) are central virulence factors for many pathogenic Gram-negative bacteria, and secreted T3SS effectors can block key aspects of host cell signaling. To counter this, innate immune responses can also sense some T3SS components to initiate anti-bacterial mechanisms. The Yersinia pestis T3SS is particularly effective and sophisticated in manipulating the production of pro-inflammatory cytokines IL-1β and IL-18, which are typically processed into their mature forms by active caspase-1 following inflammasome formation. Some effectors, like Y. pestis YopM, may block inflammasome activation. Here we show that YopM prevents Y. pestis induced activation of the Pyrin inflammasome induced by the RhoA-inhibiting effector YopE, which is a GTPase activating protein. YopM blocks YopE-induced Pyrin-mediated caspase-1 dependent IL-1β/IL-18 production and cell death. We also detected YopM in a complex with Pyrin and kinases RSK1 and PKN1, putative negative regulators of Pyrin. In contrast to wild-type mice, Pyrin deficient mice were also highly susceptible to an attenuated Y. pestis strain lacking YopM, emphasizing the importance of inhibition of Pyrin in vivo. A complex interplay between the Y. pestis T3SS and IL-1β/IL-18 production is evident, involving at least four inflammasome pathways. The secreted effector YopJ triggers caspase-8- dependent IL-1β activation, even when YopM is present. Additionally, the presence of the T3SS needle/translocon activates NLRP3 and NLRC4-dependent IL-1β generation, which is blocked by YopK, but not by YopM. Taken together, the data suggest YopM specificity for obstructing the Pyrin pathway, as the effector does not appear to block Y. pestis-induced NLRP3, NLRC4 or caspase-8 dependent caspase-1 processing. Thus, we identify Y. pestis YopM as a microbial inhibitor of the Pyrin inflammasome. The fact that so many of the Y. pestis T3SS components are participating in regulation of IL-1β/IL-18 release suggests that these effects are essential for maximal control of innate immunity during plague.
Type III secretion systems (T3SS) are essential virulence factors of many pathogenic Gram-negative bacteria. These systems include a needle-like structure, translocon proteins that form a pore with which the needle can dock in the membrane of host target cells, and a set of secreted effector proteins delivered to the target cell cytoplasm through the docked needle. The effector proteins exert control over key cellular processes that contribute to antibacterial defenses or pathogenesis, including immune signaling, phagocytosis, and induction of cell death. In response, the innate immune system has evolved the ability to recognize a number of T3SS components and initiate protective inflammatory responses when they are detected. In some T3SS-dependent pathogens that cause severe disease—like Y. pestis, the causative agent of plague—the balance between these opposing activities strongly favors the bacteria.
Taken together, our data show complex interactions of a bacterial T3SS with host inflammasome signaling pathways. We have identified several pathways triggered by Y. pestis T3SS, as both Pyrin, caspase-8 and NLRP3/NLRC4 appear directly and distinctly involved in regulation of caspase-1 cleavage and IL-1β release. Two secreted T3SS effectors, YopK and YopM, appear to be specific inhibitors of the NLRP3/NLRC4 activation induced by the presence of the T3SS needle/translocon, and the YopE-induced Pyrin activation, respectively. Inflammasome activation by YopE may represent a process where the host innate immune system acquired the ability to sense damaging microbial interference and inhibition of a specific pathway (RhoA signaling). Our current model of how the Y. pestis T3SS components intersect with specific inflammasome pathways is shown in Fig 6C. Both YopK and YopM are participating in maximal suppression of innate immunity following Y. pestis infection, and are central components of the arsenal that this highly virulent pathogen allocates to interference with key anti-bacterial immune responses. Our findings highlight the remarkable sophistication that Yersinia displays when interfacing with inflammasome signals.