Cell death and inflammation are interdependent host responses to infection. During pyroptotic cell death, interleukin-1β (IL-1β) release occurs through caspase-1 and caspase-11–mediated gasdermin D pore formation. In vivo, responses to lipopolysaccharide (LPS) result in IL-1β secretion. In vitro, however, murine macrophages require a second “danger signal” for the inflammasome-driven maturation of IL-1β. Recent reports have shown caspase-8–mediated pyroptosis in LPS-activated macrophages but have provided conflicting evidence regarding the release of IL-1β under these conditions. Here, to further characterize the mechanism of LPS-induced secretion in vitro, we reveal an important role for cellular FLICE-like inhibitory protein (cFLIP) in the regulation of the inflammatory response. Specifically, we show that deficiency of the long isoform cFLIPL promotes complex II formation, driving pyroptosis, and the secretion of IL-1β in response to LPS alone.

Inflammation and cell death are closely linked arms of the host immune response to infection, which when carefully balanced ensure host survival. One example of this balance is the tightly regulated transition from TNFR1-associated pro-inflammatory complex I to pro-death complex II. By contrast, here we show that a TRIF-dependent complex containing FADD, RIPK1 and caspase-8 (that we have termed the TRIFosome) mediates cell death in response to Yersinia pseudotuberculosis and LPS. Furthermore, we show that constitutive binding between ZBP1 and RIPK1 is essential for the initiation of TRIFosome interactions, caspase-8-mediated cell death and inflammasome activation, thus positioning ZBP1 as an effector of cell death in the context of bacterial blockade of pro-inflammatory signaling. Additionally, our findings offer an alternative to the TNFR1-dependent model of complex II assembly, by demonstrating pro-death complex formation reliant on TRIF signaling.

Significance While ZBP1 is well documented to drive cell death in response to viruses, its role in the context of Toll-like receptor (TLR)–mediated immune activation remains less defined. Here, we show that ZBP1 promotes inflammation in response to bacterial lipopolysaccharide (LPS) or double-stranded RNA (dsRNA). In a dose dependent manner, ZBP1 promotes the recruitment of RIPK1 to the TLR3/4 adaptor TRIF, activating downstream inflammatory signaling. ZBP1 plays a crucial role in TRIF-dependent responses in vivo , as Zbp1 −/− mice exhibited resistance to LPS-induced hypothermia and attenuated inflammatory responses to dsRNA. Our findings suggest that ZBP1 plays a synergistic role in the immune response by driving inflammation downstream of TLRs in response to bacterial and viral components.

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TNF mediates a variety of biological processes including cellular proliferation, inflammatory responses, and cell death and is therefore associated with numerous pathologies including autoinflammatory diseases and septic shock. The inflammatory and cell death responses to TNF have been studied extensively downstream of TNF-R1 and are believed to rely on the formation of proinflammatory complex I and prodeath complex II, respectively. We recently identified a similar multimeric complex downstream of TLR4, termed the TRIFosome, that regulates inflammation and cell death in response to LPS or Yersinia pseudotuberculosis . We present evidence of a role for the TRIFosome downstream of TNF-R1, independent of TLR3 or TLR4 engagement. Specifically, TNF-induced cell death and inflammation in murine macrophages were driven by the TLR4 adaptor TRIF and the LPS co-receptor CD14, highlighting an important role for these proteins beyond TLR-mediated immune responses. Via immunoprecipitation and visualization of TRIF-specific puncta, we demonstrated TRIF- and CD14-dependent formation of prodeath and proinflammatory complexes in response to TNF. Extending these findings, in a murine TNF–induced sepsis model, TRIF and CD14 deficiency decreased systemic inflammation, reduced organ pathology, and improved survival. The outcome of TRIF activation was cell specific, because TNF-induced lethality was mediated by neutrophils and macrophages responding to TNF in a TRIF-dependent manner. Our findings suggest that in addition to their crucial role in TNF production, myeloid cells are central to TNF toxicity and position TRIF and CD14 as universal components of receptor-mediated immune responses.