Pathogen
virulence factors and
inflammation are responsible for tissue injury associated with
respiratory failure in
bacterial pneumonia, as seen in the bovine lung infected with Pasteurella haemolytica.
Tilmicosin is a
macrolide antibiotic used for the treatment of bovine
bacterial pneumonia. Recent evidence suggests that
tilmicosin-induced neutrophil apoptosis may have anti-inflammatory effects. Using bovine leukocytes, we sought to define whether live P. haemolytica affected
tilmicosin-induced neutrophil apoptosis, assessed the proapoptotic effects of
tilmicosin in comparison with other drugs, and characterized its impact on phagocytic uptake of neutrophils by macrophages. Induction of apoptosis in the presence or absence of P. haemolytica was assessed by using an
enzyme-linked
immunosorbent assay for apoptotic
nucleosomes. In addition, fluorescent
annexin-V staining identified externalized
phosphatidylserine in neutrophils treated with
tilmicosin,
penicillin,
ceftiofur,
oxytetracycline, or
dexamethasone. Neutrophil membrane integrity was assessed by using
propidium iodide and
trypan blue exclusion. As phagocytic clearance of apoptotic neutrophils by macrophages contributes to the resolution of
inflammation, phagocytosis of
tilmicosin-treated neutrophils 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, neutrophils exposed to
tilmicosin became apoptotic, regardless of the presence or absence of P. haemolytica.
Tilmicosin-treated apoptotic neutrophils were phagocytosed at a significantly greater rate by bovine macrophages than were control neutrophils. In conclusion,
tilmicosin-induced neutrophil apoptosis occurs regardless of the presence or absence of live P. haemolytica, exhibits at least some degree of
drug specificity, and promotes phagocytic clearance of the dying inflammatory cells.