Peripheral Blood Mononuclear Cells Cytokine Expression in Horses Treated with Dexamethasone

Glucocorticoids are widely used in horses for a variety of autoimmune and inflammatory conditions. Its potent antiinflammatory properties have been associated with the suppression of a number of different inflammatory cytokines. The purpose of the study was to evaluate the effect of dexamethasone treatment in horses on mRNA cytokine expression, including interleukin-1Î , interferon-gamma, interleukin-4 and interleukin-6, during a five day treatment period and a five day post treatment period.

A randomized complete block design was performed on 16 healthy horses. Group I (8 horses) received 0.1 mg/kg of dexamethasone sodium phosphate by intravenous injection once daily for 5 days. Group II (8 horses) received an equivalent volume of sterile saline by intravenous injection daily for 5 days. A sample of 5x10 mililiters of blood in acid citrate dextrose was obtained prior to initial treatment. Thirty minutes after each treatment injection (placebo or dexamethasone) a sample of blood was obtained during the 5 day treatment period and 24, 48, 72, 96 and 120 hours after the last treatment injection was administered. Peripheral-blood mononuclear cells were isolated from the blood samples and stimulated with concavalin A. RNA was isolated using the QIAGEN RNeasy kit. cDNA first strand synthesis was achieved using QIAGEN's OMMISCRIPT RT KIT. cDNA was also constructed for the house keeping gene Î actin. Primer pairs specific for each cytokine were designed using equine cytokine sequences available on Genbank. cDNA for each cytokine and Î -actin was amplified using Real Time PCR technique.

Interleukin-4, interleukin-6 and interferon-gamma mRNA expression was statistically significant suppressed in horses treated with dexamethasone when compared to control horses. Interleukin-1Î was only significantly suppressed on day 5. Interleukin-4, interleukin-6 and interferon-gamma mRNA expression suppression was initially observed on day 2 and lasted 24 hours after the last dose of dexamethasone was administered. Interleukin-6 mRNA expression was significantly higher when compared to control group on day 10.

Our results suggest that dexamethasone treatment of healthy horses suppresses mRNA expression of several cytokines, including interleukin-4, interleukin-6 and interferon-gamma. This effect could explain part of corticosteroid's mechanism of action for controlling inflammation in a variety of disease conditions. The time-course effect of dexamethasone showed that the effect on mRNA cytokine expression suppression is only observed on day 2 of treatment and mRNA suppression is maintained for 24 hours after discontinuation of treatment.