Synthetic progestin treatment alters cytokine expression and impairs vaccine response in horses

Altrenogest is commonly used in equine veterinary medicine for the control of estrus, behavioral modifications, and maintenance of pregnancy. Recent studies have found that altrenogest may impact aspects of the immune system, including altering cytokine production in circulation and within the reproductive tract. Therefore, we hypothesize that this alteration may lead to a deviated immune response to vaccination. Sixteen mares were divided into two groups, with ten mares (n = 10) receiving long-acting injectable altrenogest (100 mg/mL; IM) administered every 10 days, and six mares (n = 6) receiving placebo (1 mL lactated Ringers solution; IM) at the same time points. Thirty days after onset of treatment, mares were vaccinated against equine arteritis virus (EAV; Arvac; Zoetis). Serum was acquired prior to vaccination, in addition to serial samples obtained for the following 28 days for assessment of cytokines and serum neutralizing (SN) antibodies, which were measured by immunoassay. Statistics were performed utilizing SAS 9.4®. The impact of treatment on SN and cytokine concentrations were assessed utilizing a general linear model for repeated measures. Significance was set to P ≤ 0.05. A significant decrease in SN was noted at 14 and 21 days when mares were treated with altrenogest (p < 0.01). When evaluating cytokines, altrenogest suppressed concentrations of IL-1β, IL-6, IL-10, and IFNγ (p < 0.01) for the duration of the study. Therefore, altrenogest impacts the antibody response to the EAV vaccine, and this may be governed by anti-cytokine function. Future research is needed to determine the clinical ramifications of this deviated antibody response.