Endothelin-1 downregulates sperm phagocytosis by neutrophils in vitro: A physiological implication in bovine oviduct immunity.

The oviduct plays a pivotal role in mammalian reproduction, providing an optimal environment for sperm capacitation, fertilization, and transport of gametes and embryos [1]. In order to fulfil these functions, the oviduct should have an efficient and strictly controlled immune system that can maintain optimal conditions for fertilization and early embryo development [2]. The local immune responses of the epithelial cells, regulated by their secretions, constitute the main part of the mucosal innate immunity inside the oviduct [3]. Once epithelial cells are stimulated either by microbial (i.e., pathogens) or nonmicrobial (i.e., stress or hormones) stimuli, they activate the innate immune responses and regulate the subsequent adaptive immune responses [4]. This process is mediated by the secretion of different molecules such as prostaglandins (PGs), chemokines and cytokines, which affect the conditioning of mucosal dendritic cells (DCs) [5]. We have previously reported that polymorphonuclear neutrophils (PMNs), the first line of defense against microorganisms, are present in the bovine oviduct lumen during the preovulatory stage under physiological conditions and that they serve as another major constituent of the local immune system inside the bovine oviduct [6]. Additionally, we proposed that the bovine oviduct epithelial cells (BOECs), through their secretions, i.e, PGE2, down-regulate the phagocytic activity of PMNs for sperm, maintaining sperm survival until fertilization [6]. The endothelins (EDNs) are a family of peptides, present as three isoforms (EDN-1, 2, 3) and encoded by three distinct endothelin-related genes [7]. The EDNs were first described as vasoconstrictor substances derived from vascular endothelial cells. However, it is now known to be produced in various tissues and different types of cells [8, 9]. EDNs exert their biological response by binding to one of two G protein-coupled receptor subtypes, endothelin receptor A (ETA) and ETB [10]. EDN-1 is secreted by the bovine oviduct and stimulates local oviductal contraction [11]. Luteinizing hormone (LH) enhances the in vitro secretion of PGE2, PGF2 and EDN-1, as well as the contractile amplitude of bovine oviducts from the follicular and postovulatory periods, but not those from the luteal period [12]. Furthermore, EDN-1 stimulates the release of PGE2 and PGF2α from the oviduct epithelial cells [13]. Of note, EDN has been shown to exert immunomodulatory activity through stimulation of neutrophil migration via activation of protein kinase G [14] and suppression of respiratory burst and bacterial clearance from the blood and tissue in rabbits [15, 16]. Additionally, the suppressive effect of an LH-stimulated BOEC supernatant was partially but not completely inhibited by addition of an EP2 receptor antagonist for PGE2 to the medium [6]. This inidcates the possibile presence of other molecules secreted by BOECs that may further suppress PMN phagocytosis for sperm. Thus, in the present study, we investigated the possible role of EDN-1 secreted by BOECs in regulating sperm phagocytosis by PMNs in the bovine oviduct.