Caspase-8 mediates caspase-1 processing and innate immune defense in response to bacterial blockade of NF-κB and MAPK signaling.

Toll-like receptor signaling and subsequent activation of NF-κB- and MAPK-dependent genes during infection play an important role in antimicrobial host defense. The YopJ protein of pathogenic Yersinia species inhibits NF-κB and MAPK signaling, resulting in blockade of NF-κB-dependent cytokine production and target cell death. Nevertheless, Yersinia infection induces inflammatory responses in vivo. Moreover, increasing the extent of YopJ-dependent cytotoxicity induced by Yersinia pestis and Yersinia pseudotuberculosis paradoxically leads to decreased virulence in vivo, suggesting that cell death promotes anti-Yersinia host defense. However, the specific pathways responsible for YopJ-induced cell death and how this cell death mediates immune defense against Yersinia remain poorly defined. YopJ activity induces processing of multiple caspases, including caspase-1, independently of inflammasome components or the adaptor protein ASC. Unexpectedly, caspase-1 activation in response to the activity of YopJ required caspase-8, receptor-interacting serine/threonine kinase 1 (RIPK1), and Fas-associated death domain (FADD), but not RIPK3. Furthermore, whereas RIPK3 deficiency did not affect YopJ-induced cell death or caspase-1 activation, deficiency of both RIPK3 and caspase-8 or FADD completely abrogated Yersinia-induced cell death and caspase-1 activation. Mice lacking RIPK3 and caspase-8 in their hematopoietic compartment showed extreme susceptibility to Yersinia and were deficient in monocyte and neutrophil-derived production of proinflammatory cytokines. Our data demonstrate for the first time to our knowledge that RIPK1, FADD, and caspase-8 are required for YopJ-induced cell death and caspase-1 activation and suggest that caspase-8-mediated cell death overrides blockade of immune signaling by YopJ to promote anti-Yersinia immune defense.