Bacillary dysentery is an acute inflammatory disease of the colon caused by the Gram negative bacteria Shigella. Shigellosis has great epidemiological importance, especially among infants and young children where it can be fatal. This infection is the second most common enteric notifiable disease in the United States. Shigella initiates an acute inflammation that causes major tissue destruction, facilitates tissue invasion by the bacteria, and eventually eradicates them. Virulent Shigella induce rapid macrophage apoptosis and a concomitant release of large amounts of Interleukin-l (IL-1). IL-1 is a major component of the inflammatory cascade initiated by Shigella. The Invasion Plasmid Antigen B (IpaB) is the Shigella invasin that causes macrophage apoptosis. We have recently shown that IpaB binds to IL-1 beta converting enzyme (ICE) and that ICE activation is essential for both macrophage apoptosis and IL-1 beta release in Shigella infections. Recently, both Salmonella typhi and S. typhimurium were shown to induce apoptosis in macrophages and to encode a protein, SipB, that is homologous to IpaB and is required for Salmonella cytotoxicity. Thus, the induction of apoptosis appears to be a common virulence mechanism among these two pathogens. Our previous results indicate that the interaction between IpaB and ICE is crucial in the pathogenesis of shigellosis. Here, we propose to: study the mechanisms of ICE activation by IpaB (Aim 1), initiate a structure-function analysis of IpaB (Aim 2), identify other macrophage components necessary for the induction of apoptosis by IpaB (Aim 3), and determine whether IpaB and SipB have analogous function (Aim 4). The study of the interaction between IpaB and ICE will allow us to design novel therapies and, more importantly, new vaccines for dysentery and possibly other infectious diseases.