Molecular and biochemical mechanisms of Pasteurella haemolytica leukotoxin-induced cell death

Pasteurella haemolytica leukotoxin (LKT) is a member of the RTX family of p ore-forming toxins that kill bovine immune cells. Several studies have sugg ested that RTX toxins kill target cells by the induction of apoptosis. In t he present study, BL3 bovine leukaemia cells were exposed to LKT and assess ed by molecular and flow cytometric techniques that measure different aspec ts of apoptotic cell death. The intoxicated cells demonstrated morphologica l, light scatter and Hoechst 33258 staining characteristics consistent with cells undergoing apoptosis. The cells also exhibited internucleosomal DNA fragmentation and poly (ADP-ribose) polymerase (PARP) cleavage, both indica tors of apoptosis. LKT-treated cells bound annexin-V-FITC indicating that p hosphatidylserine groups were translocated from the inner to the outer leaf let of the cell membrane. The effect of LKT on cells was dose dependent and inhibitable by incubation with anti-LKT monoclonal antibody. Finally, an e arly step for induction of apoptosis appears to be the binding of LKT to a beta 2 integrin since pre-incubating cells with anti-beta 2 integrin antibo dies inhibited LKT-induced apoptosis. This study provides new insights into understanding the pathogenesis of bovine pasteurellosis and could lead to the development of both preventative and therapeutic strategies for disease management. (C) 1998 Academic Press.