CASP1 and serum lipopolysaccharide activity: Interleukin-18 is a 18-kDa molecule that was formerly called IFN-γ inducing factor, originally identified in endotoxin-challenged mice and cloned from activated macrophage and Kupffer cells as a proinflammatory cytokine.32 Caspase 1 converts the 24-kDa precursor into IL-18, which is an active monomer.33 Mice lacking caspase 1 and IFN-γ or mice neutralized by an IL-18 antibody are resistant to endotoxemia.34 Although IL-18 is biologically and structurally related to IL-1β, IL-18 appears to have unique features, especially in regulating the T helper type 1 (Th1) cell response via IFN-γ.