Interactive effects of dexamethasone and opsonized Mycoplasma bovis on bovine neutrophil function in vitro

Abstract

Previously we had reported that exposure to high levels of glucocorticoids, and to unopsonized Mycoplasma bovis, has a negative interactive effect on bovine neutrophil function in vitro, and this interactive effect was a function of M. bovis strain differences. Here we hypothesized that in vitro treatment of bovine neutrophils by glucocorticoid would impair phagocytosis of opsonized M. bovis compared to non-treated neutrophils and such impairment would be a function of M. bovis strain differences. Neutrophils isolated from 20 mid-lactation cows were treated with immunosuppressive dose of 5 × 10^-4 M dexamethasone or placebo and incubated with one of four opsonized M. bovis strains that had been isolated from bovine origin. After incubation neutrophil function measured included: percentage reduction in log₁₀ of M. bovis CFU/ml, percentage of phagocytizing neutrophils, phagocytized M. bovis per neutrophil, and killed M. bovis per neutrophil. Least square means of all neutrophil groups were contrasted using linear mixed-effects models. Effects due to strain, treatment, and their interaction on neutrophil function measured by the number of phagocytized M. bovis per neutrophil and number of killed M. bovis per neutrophil were different (P < 0.05). However, no significant strain by treatment interaction effect on percentage reduction in log₁₀ of M. bovis CFU/ml was found. Neither a strain nor a strain by treatment interaction was found to affect the percentage phagocytizing neutrophils. These findings might explain in part the association of stressful events with subsequent outbreaks of Mycoplasma bovis associated bovine diseases.

Keywords: Dexamethasone; Mycoplasma bovis; Neutrophil function.