Unique role for caspase-8 in the release of IL-1β and active caspase-1 from viable human monocytes during Toxoplasma gondii infection

Monocytes are among the first cells recruited to sites of infection and major producers of the potent proinflammatory cytokine IL-1β. We previously showed that IL-1β release during Toxoplasma gondii infection of primary human monocytes requires the NLRP3 inflammasome and caspase-1 activity but is independent of gasdermin D and pyroptosis. To investigate potential mechanisms of pyroptosis-independent release of IL-1β during T. gondii infection, we constructed caspases-1, -4, -5, or -8 knockout THP-1 monocytic cells. Genetic ablation of caspase-1 or -8, but not caspase-4 or -5, decreased IL-1β release during T. gondii infection without affecting cell death. In contrast, TNF-α and IL-6 secretion were unperturbed in caspase-8 knockout cells during T. gondii infection. Dual pharmacological inhibition of caspase-8 and RIPK1 in primary monocytes also decreased IL-1β release without affecting cell viability or parasite infection efficiency. In addition, caspase-8 was required for the release of active caspase-1 from T. gondii-infected cells and for IL-1β release during infection with the related apicomplexan parasite Neospora caninum. Surprisingly, caspase-8 was dispensable for the synthesis and cleavage of IL-1β, but caspase-8 deficiency resulted in the retention of mature IL-1β within cells. Our data indicate that during T. gondii infection of human monocytes, caspase-8 functions in a novel gasdermin D-independent mechanism controlling IL-1β release from viable cells. This study expands on the known molecular pathways that promote IL-1β in human immune cells and provides the first evidence of a role for caspase-8 in the mechanism of IL-1β release during host defense against infection.