APOPTOSIS - A POSSIBLE TACTIC OF HAEMOPHILUS-SOMNUS FOR EVASION OF KILLING BY BOVINE NEUTROPHILS

Haemophilus somnus is an important veterinary pathogen that causes res piratory disease, arthritis, septicaemia and abortion in cattle and sh eep. In the present study we investigated the possibility that H. somn us resists killing by bovine neutrophils, by causing the latter to und ergo morphological changes consistent with apoptosis. Both serum-sensi tive and serum-resistant strains of H. somnus enhanced bovine neutroph il chromatin condensation and shape change (i.e. zeiosis) in vitro, su ggesting that the cells were undergoing apoptosis. Heat-killed or form alin-killed H. somnus had less effect than viable H. somnus. Chromatin margination of neutrophils was greater when H. somnus was opsonized w ith adult bovine serum, which facilitates phagocytosis of the bacteria . H. somnus culture filtrates did not cause bovine neutrophil chromati n condensation. These findings suggest that direct contact with H. som nus is required for the maximal effect on bovine neutrophils. Apoptosi s was confirmed by flow cytometry, using propidium iodide staining to detect DNA fragmentation. These findings suggest that H. somnus can ev ade killing by bovine neutrophils, in part, by inducing these cells to undergo apoptosis. (C) 1998 Academic Press.