Caspase-8 activity mediates TNFα production and restrictsCoxiella burnetiireplication during murine macrophage infection

Abstract

AbstractCoxiella burnetiiis an obligate intracellular bacteria which causes the global zoonotic disease Q Fever. Treatment options for infection are limited, and development of novel therapeutic strategies requires a greater understanding of howC. burnetiiinteracts with immune signaling. Cell death responses are known to be manipulated byC. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 toC. burnetiiinfection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation inC. burnetii-infected TNFα/CHX-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measuredC. burnetiireplication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/-BMDMs to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited byC. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore,C. burnetiireplication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control ofC. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/-BMDMs. This research identifies and explores caspase-8 as a key regulator ofC. burnetiiinfection, opening novel therapeutic doors.