Effect of mycobacterium cell wall fraction on histological, immunological, and clinical parameters of equine uterine involution

Maintaining yearly foal production is important for economic success of broodmares and requires breeding to occur as quickly postpartum as possible. First estrus occurs within 5 - 20 days postpartum, during uterine involution uterus (repair from tissue alterations of pregnancy and parturition). Attempts to hasten uterine involution have had minimal success. Mycobacterium cell wall fraction (MCWF), an immunomodulator, reduced bacterial growth and altered aspects of immune response to breeding; however, it is unknown if MCWF hastened uterine involution. Objectives were to: 1) investigate effect of MCWF on tissue remodeling; and 2) assess effect of MCWF on cell-mediated immunity of uterus. We hypothesized that MCWF treatment in postpartum would hasten uterine involution. Pregnant mixed breed mares (n = 10) were evaluated postpartum. Control mares (n = 4) received 1.5 ml LRS intravenously on day 1 postpartum and again 7 days later and experimental mares (n = 6) received 1.5 ml Settle  (MCWF) intravenously on day 1 and again 7 days later. All mares were assessed every 3 days for clinical, immunological, and histological parameters until day 15 postpartum. Clinical parameters were assessed via transrectal ultrasonography and included assessment of ovarian activity, uterine fluid retention, and uterine wall diameter, in addition to obtaining endometrial culture. Immunological parameters included endometrial biopsies for qPCR for expression of various cytokines (IL1, IL1RN, IL4, IL6, IL8, IL10, TNF, IFN, and GM-CSF) and endometrial cytology. Histological parameters were assessed on formalin-fixed endometrial biopsies and variables included retention of microcotyledons, endometrial glands dilation, and inflammation of mucosa, stratum compactum, and spongiosum. Data were analyzed using SAS 9.4, utilizing a Mixed model for repeated measures, with treatment as a random effect. All posthoc analyses used Tukey’s HSD test. Involution was considered complete by day 15 postpartum in all mares, and day postpartum had significant effect on all parameters investigated, indicating involvement of immunological process in uterine involution. Treatment with MCWF decreased severity of bacterial growth, in addition to time to obtain clean culture. MCWF treatment increased expression of proinflammatory cytokines, namely, IL1, IFN, and TNF. Whereas treatment effect was minimal, there was histological evidence for decreased mucosal inflammation in MCWF-treated mares. In conclusion, uterine involution is heavily regulated by immune system. Additionally, MCWF had bactericidal effect on postpartum mare; this may be due to increases in proinflammatory cytokine expression. Further studies are needed to determine whether this immunomodulator will improve first estrous cycle fertility in postpartum mares.

Keywords: Mare, uterine involution, mycobacterium cell wall fraction, postpartum, immunomodulator

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.