Decreased PD-1⁺ NK and T Cell Populations in Peritoneal Fluid contribute to Immune Dysregulation in Endometriosis

Endometriosis is associated with chronic pelvic pain, largely due to immune dysregulation within the peritoneal cavity. The activation status of peritoneal immune cells is not well understood, and comparisons with systemic immune cells may provide insights for diagnosing and treating inflammation and pain in endometriosis. To investigate immune cell activation and inhibition status in peritoneal fluid and blood in endometriosis patients using full-spectrum flow cytometry. This study included patients undergoing laparoscopy for diagnosis or treatment of peritoneal endometriosis or for unrelated conditions; peritoneal fluid was collected from n = 6 endometriosis patients and n = 8 controls, and matched blood from n = 5 endometriosis patients and n = 7 controls. Immune cells were analysed using a 20-marker full-spectrum flow cytometry panel. Data were analysed for statistical significance using the Kruskal-Wallis or Mann-Whitney U test, with a p value below 0.05 considered significant. The main differences between endometriosis and control samples were found in lymphoid populations in peritoneal fluid and myeloid populations in blood. Contrary to our expectations, the expression of PD-1 on peritoneal fluid NK and T cell populations was significantly lower in endometriosis than in controls (p < 0.05). The significant decrease in immune checkpoint PD-1 expression represents a novel immunopathological feature of endometriosis and highlights potential therapeutic targets for managing inflammation and pain through immune checkpoint modulation.