Activation and Inhibition of Multiple Inflammasome Pathways by the Yersinia Pestis Type Three Secretion System: A Dissertation

Host survival during plague, caused by the Gram-negative bacterium Yersinia pestis, is favored by a robust early innate immune response initiated by IL-1β and IL-18. Precursors of these cytokines are expressed downstream of TLR signaling and are then enzymatically processed into mature bioactive forms, typically by caspase-1 which is activated through a process dependent on multi-molecular structures called inflammasomes. Y. pestis evades immune detection in part by using a Type three secretion system (T3SS) to inject effector proteins (Yops) into host cells and suppress IL-1β and IL-18 production. We investigated the cooperation between two effectors, YopM and YopJ, in regulating inflammasome activation, and found that Y. pestis lacking both YopM and YopJ triggers robust caspase-1 activation and IL-1Β/IL-18 production in vitro. Furthermore, this strain is attenuated in a manner dependent upon caspase-1, IL-1β and IL-18 in vivo, yet neither effector appears essential for full virulence. We then demonstrate that YopM fails to inhibit NLRP3/NLRC4 mediated caspase-1 activation and is not a general caspase-1 inhibitor. Instead, YopM specifically prevents the activation of a Pyrin-dependent inflammasome by the Rho-GTPase inhibiting effector YopE. Mutations rendering Pyrin hyperactive are implicated in the autoinflammatory disease Familial Mediterranean Fever (FMF) in humans, and we discuss the potential significance of this disease in relation to plague. Altogether, the Y. pestis T3SS activates and inhibits several inflammasome pathways, and the fact that so many T3SS components are involved in manipulating IL-1β/IL-18 underscores the importance of these mechanisms in plague