Caspase-8 in Cell Death and Inflammation

Pathogenic organisms express virulence factors that can inhibit immune signaling pathways. Thus, the immune system is faced with the challenge of eliciting an effective inflammatory response to pathogens that actively suppress inflammation. The mechanisms that regulate this response are largely undefined. Here, I used the gram-negative extracellular bacterial pathogen Yersinia pseudotuberculosis to investigate anti-pathogen responses, as the Yersinia virulence factor YopJ blocks NF-κB and MAPK signaling, resulting in reduced cytokine production and target cell death. I set out to first, determine the molecular mechanism of Yersinia-mediated cell death and second, examine the impact of cell death on immune responses in vivo. Multiple caspases are activated during Yersinia infection, including caspase-1, which regulates pyroptosis, and caspase-3 and -8, which elicit apoptosis. I found that caspase-8 and receptor-interacting protein kinase-3 (RIPK3), RIPK1 and FADD are required for Yersinia-induced cell death. My studies also showed that caspase-8 is required for the activation of caspase-1 and -3 during Yersinia infection. Interestingly, mice lacking caspase-8 were severely susceptible to Yersinia infection and had defective pro-inflammatory cytokine production. These findings highlight a possible mechanism of immune defense that can overcome pathogen inhibition of cell-intrinsic pro-inflammatory immune responses. Caspases are proteases that are best characterized for their abilities to regulate apoptotic or pyroptotic cell death programs, both of which are critical for the proper operation of the mammalian immune system. Within this family of proteins is caspase-8, which is unusual as it can promote both cell death, and inflammatory gene expression induced by Toll-like Receptors. How these two dichotomous outcomes occur independently of the other, is mysterious. Using mice that specifically ablate caspase-8 auto-processing, I have demonstrated that caspase-8 enzymatic, but not autoprocessing activity, mediates induction of inflammatory cytokines by a wide variety of TLR stimuli. Since uncleaved caspase-8 functions together with its homolog, cFLIP, our findings implicate the activity of a caspase-8/cFLIP heterodimer in control of inflammatory cytokines during microbial infection, and provide new mechanistic insight into how caspase-8 regulates gene expression