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Caspase-1 processes interleukin 1β (IL-1β) and IL-18 but may also contribute to apoptosis. In this context, caspase-1 knockout mice have been shown to be protected from endotoxin-induced mortality, whereas IL-1β knockout mice are not protected.We therefore sought to delineate the mechanisms responsible for the differential responses between caspase-1 and IL-1β knockout mice.Caspase-1 knockout, IL-1β knockout, and IL-1β/IL-18 double knockout mice were compared with wild-type mice for survival after intraperitoneal challenge with live Escherichia coli.Caspase-1 knockout animals were protected from bacterial challenge, whereas wild-type, IL-1β knockout, and IL-1β/IL-18 double knockout animals were not. Wild-type animals and both IL-1β knockout and IL-1β/IL-18 double knockout mice demonstrated significant splenic B lymphocyte apoptosis, which was absent in the caspase-1 knockout mice. Importantly, IL-1β/IL-18 double knockout mice were protected from splenic cell apoptosis and sepsis-induced mortality by the caspase inhibitor zVAD-fmk. Furthermore, wild-type but not caspase-1 knockout splenic B lymphocytes induced peritoneal macrophages to assume an inhibitory phenotype.Taken together, these findings suggest that caspase-1 is important in the host response to sepsis at least in part via its ability to regulate sepsis-induced splenic cell apoptosis.