Pasteurella haemolytica leukotoxin induced apoptosis of bovine lymphocytes involves DNA fragmentation1This report represents a portion of a dissertation submitted by the first author as partial fulfillment of the requirements for the Ph.D. degree.1

It has been reported that Pasteurella haemolytica leukotoxin (LKT) induces morphologic changes in bovine leukocytes consistent with apoptosis in vitro, but DNA fragmentation was not observed. We investigated whether bovine lymphocytes undergo DNA fragmentation during LKT-induced apoptosis. Bovine peripheral blood lymphocytes were isolated by density gradient centrifugation and exposed to LKT or inactive pro-LKT protein from a lktC- mutant strain. After exposure, DNA fragmentation in lymphocytes was quantified colorimetrically by diphenylamine assay and visualized by agarose gel electrophoresis. At high LKT concentrations, bovine lymphocytes were lysed, but at low concentrations, LKT caused DNA fragmentation characteristic of apoptosis. Maximal DNA fragmentation in bovine lymphocytes was induced by 0.1 TU ml(-1) LKT following 3 h exposure, but only background level of DNA fragmentation was observed with the inactive pro-LKT. Equine lymphocytes that are resistant to LKT intoxication did not show DNA fragmentation following exposure to LKT. Preincubation of LKT with a neutralizing anti-LKT monoclonal antibody inhibited LKT-induced DNA fragmentation. Electrophoresis of DNA from bovine lymphocytes treated with 0.1 TU ml(-1) LKT demonstrated the typical 'ladder' pattern of internucleosomal DNA cleavage, the hallmark of apoptosis associated with activation of endonucleases. LKT-induced DNA fragmentation was inhibited by 0.5 mM ZnCl2, an endonuclease inhibitor. The results indicated that LKT at low concentrations induced apoptotic cell death of bovine lymphocytes, which may play a role in initiation and persistence of P. haemolytica infection.