Effects of dexamethasone on development of immunoglobulin G subclass responses following vaccination of horses

Objective-To determine the effects of dexamethasone on development of IgG s ubclass responses following vaccination of healthy horses. Animals-11 mature Thoroughbreds. Procedure-Horses received 2 IM injections at 2-week intervals of a vaccine containing inactivated infectious bovine rhinotracheitis, bovine viral diar rhea, and parainfluenza-3 viral antigens and were then randomly assigned to 2 groups. Six horses received dexamethasone (0.2 mg/kg of body weight, IM) twice weekly for 8 weeks starting the day of the first vaccination. Five c ontrol horses received an equivalent volume of saline 10.9% NaCl) solution. Antigen-specific serum IgG subclass liters were determined weekly after va ccination by use of an ELISA. Results-Vaccination resulted in similar antigen-specific serum IgG(T) liter s in dexamethasone-treated and control horses. In contrast, although contro l horses developed IgGa and IgGb responses after vaccination, corticosteroi d administration completely inhibited these responses in treated horses. Conclusions and Clinical Relevance-Cortico steroids can have profound effec ts on primary immune responses in horses and can significantly affect IgG r esponses to inactivated vaccines. Corticosteroid treatment regimens commonl y used to treat diseases in horses may result induction of a nonprotective IgG subclass response, leaving treated horses susceptible to disease. Addit ionally, mechanisms regulating IgGa and IgGb responses appear to differ fro m those regulating IgG(T) responses. Further defining these mechanisms is a critical step in designing effective vaccines, and corticosteroid-induced immunomodulation may be a valuable tool for studying immune responses in ho rses.