Protective role of bovine neutrophils in Pasteurella haemolytica-mediated endothelial cell damage

The purpose of this study was to determine if Pasteurella haemolytica can directly injure bovine pulmonary endothelial cells (EC) and if neutrophils have a beneficial or detrimental role in bacterium-EC interaction. Various combinations of live P. haemolytica, heat-killed P. haemolytica, anti-P. haemolytica immune serum, polymyxin B, and bovine neutrophils were added to confluent monolayers of bovine EC. Monitoring included determination of 51Cr release from EC, phase microscopy, and transmission electron microscopy. Although toxic changes were not evident at 5 h postinoculation, both live and heat-killed P. haemolytica produced extensive EC damage by 22 h postinoculation. Damage by live P. haemolytica was prevented only when both neutrophils and immune serum were used. Polymyxin B effectively prevented the toxic effect of heat-killed P. haemolytica, suggesting that lipopolysaccharide was the major toxic factor. Morphological studies showed close apposition of P. haemolytica to EC membranes, neutrophil activation, and adherence to EC but no evidence of neutrophil-associated EC membrane damage. These studies demonstrate that neutrophils and immune serum in combination are effective in preventing EC damage mediated by live P. haemolytica.