Morphological and functional disturbances in pulmonary vascular endothelium following exposure of sheep to an "avirulent" strain of Pasteurella haemolytica.

Sheep were vaccinated with a live attenuated strain of Pasteurella haemolytica and killed 3 days later. Segments of main intrapulmonic artery and vein were removed for biophysical and scanning electron microscopic studies. In the pulmonary artery, vaccination with Pasteurella haemolytica caused an increase in the number of endothelial cell surface blebs and, in some cases, those blebs appeared to be splitting open, suggesting cell damage or irritation. There was a surprising lack of platelet adherence to the lesions, suggesting that an antiplatelet factor is released by the damaged endothelium. The endothelial-dependent relaxant response to bradykinin was enhanced following vaccination. In the pulmonary vein, ultrastructural lesions similar to those in the artery were present in vaccinated animals. Bradykinin caused a contraction, an effect that was reduced following vaccination with Pasteurella haemolytica. These experiments demonstrate that a live, vaccine-derived strain of Pasteurella haemolytica causes both morphological and functional changes in the pulmonary vascular endothelium.