Characterization of Myeloperoxidase in the Healthy Equine Endometrium

SIMPLE SUMMARY: Endometritis or endometrial inflammation is a major cause of infertility in mares. Understanding how the immune system protects the uterus against potential pathogens and contamination is essential for preventing endometritis and other associated pathologies. Myeloperoxidase (MPO) is an enzyme mainly contained in inflammatory cells and indeed associated with equine endometritis. Surprisingly, this enzyme is also detected in the uterus of mares in absence of inflammation. The aim of this study was to investigate MPO in the uterus of mares in physiological conditions throughout the reproductive cycle to better understand its function in equine reproduction. MPO is constantly present in the uterus of mares in all phases of the reproductive cycle. This MPO, in absence of inflammatory cells, seems to be synthesized and secreted in the uterine lumen by the uterine cells themselves, especially in estrus, when the equine uterus is most exposed to external contamination. Based on these results and its potent bactericidal action, we suggest that MPO is probably part of the uterine immune system protecting the uterus against contamination and avoiding inflammation. ABSTRACT: Myeloperoxidase (MPO), as a marker of neutrophil activation, has been associated with equine endometritis. However, in absence of inflammation, MPO is constantly detected in the uterine lumen of estrous mares. The aim of this study was to characterize MPO in the uterus of mares under physiological conditions as a first step to better understand the role of this enzyme in equine reproduction. Total and active MPO concentrations were determined, by ELISA and SIEFED assay, respectively, in low-volume lavages from mares in estrus (n = 26), diestrus (n = 18) and anestrus (n = 8) in absence of endometritis. Immunohistochemical analysis was performed on 21 endometrial biopsies randomly selected: estrus (n = 11), diestrus (n = 6) and anestrus (n = 4). MPO, although mostly enzymatically inactive, was present in highly variable concentrations in uterine lavages in all studied phases, with elevated concentrations in estrus and anestrus, while in diestrus, concentrations were much lower. Intracytoplasmic immunoexpression of MPO was detected in the endometrial epithelial cells, neutrophils and glandular secretions. Maximal expression was observed during estrus in mid and basal glands with a predominant intracytoplasmic apical reinforcement. In diestrus, immunopositive glands were sporadic. In anestrus, only the luminal epithelium showed residual MPO immunostaining. These results confirm a constant presence of MPO in the uterine lumen of mares in absence of inflammation, probably as part of the uterine mucosal immune system, and suggest that endometrial cells are a source of uterine MPO under physiological cyclic conditions.