Redefining the contribution of retrograde menstruation to endometriosis: single-cell analysis of endometriotic lesions suggests a process more complex than simple autografting

Endometriosis is a common gynecological disorder causing pelvic pain and infertility. While generally attributed to peritoneal implants of endometrium derived from retrograde menstruation, the contribution of cells from the circulation to the development of endometriotic lesions is unknown. Here, we describe the infiltration of circulating cells into endometriotic lesions. Experimental endometriosis was induced in mice by transplanting uterine tissue from donor mice that express green fluorescence protein (GFP) into peritoneal cavity of recipient wild type mice. Lesions were collected after 18-weeks, and single cell suspensions were subjected to for single-cell sequence analysis. 10,000 cells were tagged per lesion and sequenced. Data was analyzed for GFP sequence using Seurat package in R studio. Tagged endometriosis cells showed that 35% expressed GFP while 65% did not, surprisingly indicating that most cells are derived from the circulation rather than the transplanted endometrium. Cell cluster analysis showed that the host-derived infiltrated cells consisted of Natural Killer (NK) cells, B cells, macrophages (M1 and alveolar), T cells, fibroblasts, neutrophils and endothelial cells. The endometriotic lesions contained twofold more endogenously derived host cells than cells originating from the uterine allografts. The top 10 genes in each host cell cluster that entered the lesions endogenously were analyzed for their predicted functions in the development of endometriosis. This study demonstrates that most of the cells in endometriotic lesions are derived from the host from the host circulation, rather than carried with the lesion. The majority are immune cells suggesting novel alternative ways to treat the disease.