BACKGROUND Endometriosis, a common oestrogen-dependent inflammatory disorder in women of reproductive age, is characterized by endometrial-like tissue outside its normal location in the uterus, which causes pelvic scarring, pain and infertility. While its pathogenesis is poorly understood, the immune system (systemically and locally in endometrium, pelvic endometriotic lesions and peritoneal fluid) is believed to play a central role in its aetiology, pathophysiology and associated morbidities of pain, infertility and poor pregnancy outcomes. However, immune cell populations within the endometrium of women with the disease have had incomplete phenotyping, thereby limiting insight into their roles in this disorder. OBJECTIVE AND RATIONALE The objective herein was to determine reproducible and consistent findings regarding specific immune cell populations and their abundance, steroid hormone responsiveness, functionality, activation states, and markers, locally and systemically in women with and without endometriosis. SEARCH METHODS A comprehensive English language PubMed, Medline and Google Scholar search was conducted with key search terms that included endometriosis, inflammation, human eutopic/ectopic endometrium, immune cells, immune population, immune system, macrophages, dendritic cells (DC), natural killer cells, mast cells, eosinophils, neutrophils, B cells and T cells. OUTCOMES In women with endometriosis compared to those without endometriosis, some endometrial immune cells display similar cycle-phase variation, whereas macrophages (Mø), immature DC and regulatory T cells behave differently. A pro-inflammatory Mø1 phenotype versus anti-inflammatory Mø2 phenotype predominates and natural killer cells display abnormal activity in endometrium of women with the disease. Conflicting data largely derive from small studies, variably defined hormonal milieu and different experimental approaches and technologies. WIDER IMPLICATIONS Phenotyping immune cell subtypes is essential to determine the role of the endometrial immune niche in pregnancy and endometrial homeostasis normally and in women with poor reproductive history and can facilitate development of innovative diagnostics and therapeutics for associated symptoms and compromised reproductive outcomes.

Molecular characterization of cell types using single-cell transcriptome sequencing is revolutionizing cell biology and enabling new insights into the physiology of human organs. We created a human reference atlas comprising nearly 500,000 cells from 24 different tissues and organs, many from the same donor. This atlas enabled molecular characterization of more than 400 cell types, their distribution across tissues, and tissue-specific variation in gene expression. Using multiple tissues from a single donor enabled identification of the clonal distribution of T cells between tissues, identification of the tissue-specific mutation rate in B cells, and analysis of the cell cycle state and proliferative potential of shared cell types across tissues. Cell type–specific RNA splicing was discovered and analyzed across tissues within an individual.

These data suggest that endometrial LGR5 is not an endogenous stem cell marker. Instead, LGR5 cells appear to be recruited from blood to be part of the stem cell niche at the perivascular microenvironment to activate the endogenous niche.

Objective: To phenotype transcriptomically M1 macrophages (Mf1) and M2 macrophages (Mf2) in the endometrium of women with endometriosis. Design: Prospective experimental study. Setting: University research laboratory. Patient(s): Six women with endometriosis and five controls without disease, in the secretory phase of the menstrual cycle. Intervention(s): Mf1, Mf2, uterine natural killer, and T regulatory cells were isolated from human endometrium using a uniquely designed cell-specific fluorescence activating cell sorting panel. Transcriptome profiles were assessed by RNA high sequencing, bioinformatics, and biological pathway analyses. Main Outcomes Measure(s): Differential gene expression between Mf1 and Mf2 in women with and without endometriosis and in Mf1 versus Mf2 in each group was determined and involved different biologic and signaling pathways. Result(s): Flow cytometry analysis showed no significant differences in total numbers of leukocytes between control and endometriosis groups, although Mf1 were higher in the endometriosis group versus controls. Statistical transcriptomic analysis was performed only in Mf1 and Mf2 populations due to larger sample sizes. Bioinformatic analyses revealed that in women with endometriosis, endometrial Mf1 are more proinflammatory than controls and that Mf2 paradoxically have a proinflammatory phenotype. Conclusion(s): As Mf are phenotypically plastic and their polarization state depends on their microenvironment, the altered endometrial environment in women with endometriosis may promote endometrial Mf2 polarization and an Mf1 proinflammatory phenotype. Moreover, aberrant phenotypes of Mf may contribute to abnormal gene expression of the eutopic endometrium and a proinflammatory environment in women with endometriosis relevant to the pathophysiology of the disease and compromised reproductive outcomes.