Utilizing an immunostimulant (MCWF; Settle) to hasten the degradation of endometrial cups

Placentation in the horse begins at approximately 35 days of gestation, when the chorionic girdle leads to the development of endometrial cups. If embryonic loss occurs in the presence of endometrial cups, return to cyclicity is delayed due to the retainment of these structures, continued production of equine chorionic gonadotropin (eCG), and prolonged progesterone output from accessory corpus lutea. It is believed that natural degradation of endometrial cups is governed by IFNγ-producing lymphocytes at 100-120 days of gestation, but external stimulation of this process has been relatively unsuccessful. Recently, we observed an increase in IFNγ following the use of an immunostimulant (Mycobacterium cell wall fraction (MCWF); Settle) in the postpartum mare. Therefore, we hypothesize that MCWF administration following embryonic loss will increase IFNγ production, hasten the degradation of the endometrial cups, and decrease the time to return to cyclicity in mares. To investigate this, 16 mares were aborted with PGF2α at 40-45d of gestation. Following abortion, mares were divided into groups: MCWF-Treated (6mg MCWF in 20mL LRS; n=10) and Control (20mL LRS; n=6). Between 45-50 days of gestation, mares were assessed via trans-rectal ultrasonography for ovarian function and endometrial cup distribution. Hysteroscopic-guided injection of endometrial cups was performed, with 1mL placed into eachvisible endometrial cup, and the remaining volume injected peripherally. This treatment was repeated 7 days later. Mares were monitored for rectal temperature and serum was taken to assess eCG and IFNγ concentrations. Additionally, transrectal ultrasonography was performed to monitor cyclicity. Statistics were performed utilizing SAS9.4 with a general linear model of repeated measures applied for all temperature, eCG, and cytokine data. A chi-squared test was performed to assess return to cyclicity. An overall decrease in eCG concentrations was noted in the MCWF-treated group (P<0.0001) when compared to controls. This suppression was significant 7 days after onset of treatment and remained suppressed 35 days following treatment. This coincided with an increase in peripheral IFNγ in the MCWF-treated group compared to controls (P<0.01). A higher percentage of MCWF-treated mares returned to cyclicity in comparison to controls (8/10 vs. 2/6; P=0.06), and a pre-ovulatory follicle was noted 23±4 days after onset of treatment. To our knowledge, this is the first report of a successful treatment for degradation of endometrial cups following abortion, by hastening the natural immune response to these structures. This caused mares to return to cyclicity within a month following treatment onset, allowing for repeat attempts at a successful pregnancy within the confines of the breeding season.