Severe equine asthma is characterized by airway hyperresponsiveness, neutrophilic inflammation and structural alterations of the lower airways. In asthmatic horses with neutrophilic inflammation, there is insensitivity to corticosteroids characterized by the persistence of neutrophils within the airways with therapy. We hypothesized that hypoxia or oxidative stress in the microenvironment of the lung contributes to this insensitivity of neutrophils to corticosteroids in asthmatic horses. Blood neutrophils isolated from horses with severe asthma (N = 8) and from healthy controls (N = 8) were incubated under different cell culture conditions simulating hypoxia and oxidative stress and, in the presence, or absence of dexamethasone. The pro-inflammatory gene and protein expression of neutrophils were studied. In both groups, pyocyanin-induced oxidative stress increased the mRNA expression of IL-8, IL-1β, and TNF-α. While IL-1β and TNF-α were downregulated by dexamethasone under these conditions, IL-8 was not. Simulated hypoxic conditions did not enhance pro-inflammatory gene expression in neutrophils from either group of horses. In conclusion, oxidative stress but not hypoxia may contribute to corticosteroid insensitivity via a selective gene regulation pathway. Equine neutrophil responses were similar in both heathy and asthmatic horses, indicating that it is not specific to asthmatic inflammation.
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