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

The oviduct is an active contractile tube that provides the proper environment for sperm transport, capacitation and survival. Oviductal contractions are regulated by autocrine/paracrine secretion of several factors, such as prostaglandins (PGs) and endothelin-1 (EDN-1). We have previously shown that during the preovulatory stage, sperm are exposed to polymorphonuclear neutrophils (PMNs) in the bovine oviduct, and the bovine oviduct epithelial cells (BOECs) secrete molecules including PGE2 that suppress sperm phagocytosis by PMNs in vitro. In this study, we investigated the possible effects of EDN-1 on the phagocytic activity of PMNs toward sperm. The local concentrations of EDN-1 in oviduct fluid and BOEC culture medium ranged from 10–10 to 10–11 M as determined by EIA. Phagocytosis and superoxide production were assayed by co-incubation of sperm pretreated to induce capacitation with PMNs exposed to EDN-1 (0, 10–11, 10–10, 10–9, and 10–8 M) for 2 h. EDN-1 suppressed dose dependently (10–11 to 10–8 M) the phagocytic activity for sperm and superoxide production of PMNs in response to capacitated sperm. Moreover, this suppression was eliminated by an ETB receptor antagonist (BQ-788). EDN-1 suppressed mRNA expression of EDN-1 and ETB but not ETA receptors in PMNs, suggesting the ETB receptor-mediated pathway. Scanning electron microscopic observation revealed that incubation of PMNs with EDN-1 (10–9 M) completely suppressed the formation of DNA-based neutrophil extracellular traps for sperm entanglement. The results provide evidence indicating that EDN-1 may be involved in the protection of sperm from phagocytosis by PMNs in the bovine oviduct, supporting sperm survival until fertilization.