Tilmicosin induces apoptosis in bovine peripheral neutrophils in the presence or in the absence of Pasteurella haemolytica and promotes neutrophil phagocytosis by macrophages

Pathogen virulence factors and inflammation are responsible for tissue inju ry associated with respiratory failure in bacterial pneumonia, as seen in t he bovine lung infected with Pasteurella haemolytica. Tilmicosin is a macro lide antibiotic used for the treatment of bovine bacterial pneumonia. Recen t evidence suggests that tilmicosin-induced neutrophil apoptosis may have a nti-inflammatory effects. Using bovine leukocytes, we sought to define whet her live P. haemolytica affected tilmicosin-induced neutrophil apoptosis, a ssessed the proapoptotic effects of tilmicosin in comparison with other dru gs, and characterized its impact on phagocytic uptake of neutrophils by mac rophages. Induction of apoptosis in the presence or absence of P. haemolyti ca was assessed by using an enzyme-linked immunosorbent assay for apoptotic nucleosomes. In addition, fluorescent annexin-V staining identified extern alized phosphatidylserine in neutrophils treated with tilmicosin, penicilli n, ceftiofur, oxytetracycline, or dexamethasone. Neutrophil membrane integr ity was assessed by using propidium iodide and trypan blue exclusion, As ph agocytic clearance of apoptotic neutrophils by macrophages contributes to t he resolution of inflammation, phagocytosis of tilmicosin-treated neutrophi ls by esterase-positive cultured bovine macrophages was assessed with light microscopy and transmission electron microscopy. Unlike bovine neutrophils treated with penicillin, ceftiofur, oxytetracycline, or dexamethasone, neu trophils exposed to tilmicosin became apoptotic, regardless of the presence or absence of P. haemolytica. Tilmicosin-treated apoptotic neutrophils wer e phagocytosed at a significantly greater rate by bovine macrophages than w ere control neutrophils. In conclusion, tilmicosin-induced neutrophil apopt osis occurs regardless of the presence or absence of live P. haemolytica, e xhibits at least some degree of drug specificity, and promotes phagocytic c learance of the dying inflammatory cells.