Human seminal plasma suppresses neutrophil antimicrobial functions and promotes bacterial survival.

Human seminal plasma (HSP) plays an important role in shaping the reproductive immune environment, but its effects on modulating neutrophil antimicrobial functions and pathogen clearance remain unclear. In this study, we investigated the immunomodulatory effects of HSP on key neutrophil effector responses, including reactive oxygen species (ROS) production, neutrophil extracellular trap (NET) formation, phagocytosis, and bacterial killing. Human neutrophils were stimulated with calcium ionophore A23187, phorbol 12-myristate 13-acetate (PMA), or uropathogenic Escherichia coli (UPEC) in the presence or absence of HSP. In addition, neutrophils from NOX2- and PAD4-knockout mice were used to elucidate the molecular pathways underlying HSP-mediated regulation of NETosis. HSP significantly suppressed ROS production and NET formation induced by A23187, PMA, and UPEC, while also reducing neutrophil phagocytic capacity and impairing bacterial killing. Mechanistically, HSP-mediated inhibition of NETosis was found to be PAD4-dependent but NOX2-independent. Furthermore, the inhibitory effect of HSP on PMA-stimulated human neutrophils was diminished when HSP was obtained from donors pretreated with acetylsalicylic acid, which significantly reduced prostaglandin E2 levels in HSP. Consistent with this observation, pharmacological blockade of prostaglandin signaling restored ROS production in HSP-treated neutrophils. Overall, these findings identify HSP as a physiological inhibitor of neutrophil effector functions and support a role for HSP in maintaining immune homeostasis and tolerance within the reproductive tract through the suppression of neutrophil oxidative burst, NET formation, and antimicrobial activity.