Impact of peritoneal macrophage depletion on endometriotic lesion development in a mouse model

• Allotransplantation of uterine fragments onto the peritoneum led to development of endometriosis-like lesions in mice. • Endometriosis caused macrophage accumulation in peritoneum, but this was alleviated when injecting the mice with clodronate. • Clodronate inhibited endometriosis, leading to smaller lesions, lower proliferation and angiogenesis, and higher apoptosis. • Macrophages play a key role in endometriosis pathogenesis, opening up new horizons for research into novel therapies. Despite a prevalence as high as 10% in reproductive-age women, the pathogenesis of endometriosis has not yet been fully elucidated, with available treatments remaining limited and often ineffective. Aberrant macrophage accumulation has been reported in endometriosis and may be crucial to better understanding and treating the disease. This study aimed to investigate the outcomes of macrophage depletion in a mouse model of endometriosis. Endometriosis was induced by allografting uterine fragments onto the peritoneum of C57BL/6 mice and intraperitoneal macrophage depletion was mediated by injecting clodronate-containing liposomes. The mice were then euthanized, and peritoneal fluid was collected for quantification of immune cells by flow cytometry. Endometriotic lesions were retrieved to assess levels of fibrosis, proliferation, apoptosis, and activation of the angiogenic mechanism. Flow cytometry demonstrated a clear decrease in macrophage accumulation in treated mice compared to placebo controls, accompanied by an increase in T-cell rates. Endometriotic lesions from treated mice were smaller and exhibited lower rates of Ki67-positive proliferating cells and higher rates of TUNEL-positive apoptotic cells. Masson’s trichrome staining showed comparable rates of fibrosis between the two groups. Vascular endothelial growth factor immunostaining was weaker in mice injected with clodronate within both epithelial and stromal compartments. Peritoneal macrophage depletion had a strong inhibitory effect against endometriosis development, resulting in smaller lesions, reduced proliferation and angiogenesis, and enhanced cell death. Our study points to a fundamental role for macrophages in endometriosis development, opening up new horizons for research into novel therapeutic options.