Our study thoroughly investigates the causal associations between immune cells and endometriosis, leveraging extensive publicly available genetic data. Utilizing a bidirectional two-sample Mendelian Randomization framework, our findings reveal a significant causal relationship involving 92 immune cells, distributed among B cell (28 cells), cDC (2 cells), Maturation stages of T cell (10 cells), Monocyte (12 cells), Myeloid cell (5 cells), TBNK (13 cells) and Treg panels (22 cells). In the reverse Mendelian randomization analysis, we identified significant or suggestive causal relationships involving 14 immune cells distributed among B cell (2 cells), Maturation stages of T cell (1 cell), Monocyte (2 cells), TBNK (8 cells) and Treg panels (1 cell). All these findings supported the hypothesis that the assessment of causal effects of immune cell traits significantly influences the risk and protection dynamics across different stages and subtypes of endometriosis. Several intriguing and common findings will be specifically covered and discussed below.
Our findings indicated that for every 1-standard deviation (SD) increase in CX3CR1 on monocyte count, the risks of developing endometriosis, endometriosis ASRM stages 1,2, and endometriosis of pelvic peritoneum increased by 4%, 9%, and 10%, respectively. Endometriosis, a prevalent chronic condition in the female reproductive system, remains incompletely understood, with inflammation considered among the potential hypotheses6. CX3CR1, a cell surface receptor primarily expressed on monocytes, binds to its ligand CX3CL1 (also known as fractalkine) and plays a crucial role in regulating immune and inflammatory responses. Various functional aspects of the fractalkine-CX3CR1 axis have been identified, including the rapid capture and firm adhesion of immune cells, chemotaxis, and the crawling behavior of monocytes7. Our findings align with existing research and support the inflammatory hypothesis, suggesting that elevated CX3CR1 on monocytes is a risk factor for the development of endometriosis, specifically in endometriosis ASRM stages 1,2, and endometriosis of pelvic peritoneum. However, in the reverse Mendelian randomization analysis, a suggestive causal relationship indicated that a one-unit increase in the log odds of endometriosis risk could decrease the levels of CX3CR1 on monocytes by 0.13. Immunohistochemical experiments conducted by Hou X X et al. revealed significantly higher expression of fractalkine and CX3CR1 in ectopic endometrium compared to normal endometrium. The concentration of fractalkine in the peritoneal fluid of endometriosis patients was closely correlated with the severity of the condition8. It is hypothesized that the persistent enrichment of fractalkine and CX3CR1 in the patient's focal tissues may contribute to the relative decrease in CX3CR1 on blood monocytes. Considering the effectiveness of the fractalkine-CX3CR1 axis in treating various inflammatory diseases, such as rheumatoid arthritis 9, chronic kidney disease10, and neuroinflammation11, it emerges as a potential new target for the treatment of endometriosis, particularly in endometriosis ASRM stages 1,2, and endometriosis of pelvic peritoneum.
In our study, we identified several traits from B cells that exhibited significant causal relationships with endometriosis and its various subtypes. In the context of inflammation, the B-cell activating factor receptor (BAFF-R) on the surface of B cells binds to the B-cell activating factor (BAFF), produced by immune cells like macrophages and dendritic cells. This interaction promotes B-cell proliferation, survival, and antibody production by activating intracellular signaling pathways12. Elevated levels of several subpopulations representing B activation and memory status (e.g., BAFF-R on IgD+ CD38dim B cell, memory B cell, unswitched memory B cell, switched memory B cell, etc.) were identified as risk factors for the development of endometriosis in the fallopian tube. However, elevated subpopulations representing B activation status(e.g., BAFF-R on IgD+ CD38dim B cell, IgD+ CD38− B cell, etc.) were found to be protective factors against the development of endometriosis in the pelvic peritoneum. This observation suggests a diverse role of B cells in the pathogenesis of immunoinflammation across different subtypes of endometriosis, consistent with existing reports. L.G.C. Riccio et al. summarized findings from 22 selected studies related to B cells and endometriosis, revealing increased numbers and/or activation of B cells in most studies. However, some studies reported no difference, while others indicated a decreased number of B cells13. Further studies are needed to elucidate the specific role of B cells in the development of all endometriosis subtypes and to propose new therapeutic strategies.
Moreover, our investigation highlighted that an elevated level of CD28 on CD39+ activated CD4 regulatory T cells poses a risk for endometriosis and several subtypes (ASRM stages 3 and 4, ovary). CD39 is an enzyme responsible for converting adenosine triphosphate into adenosine diphosphate and cyclic adenosine monophosphate, ultimately leading to the release of an immunosuppressive form of adenosine in the microenvironment14. The role of CD28 on D39+ activated CD4+ Treg cells is to maintain immune tolerance by regulating cellular activation and function. Several observations, including those in cancer research, suggest the pivotal role of CD39 in attenuating immune responses.15,16. For instance, Filip Ahlmanner et al. demonstrated that CD39+ Treg cells from cancer patients inhibit the transendothelial migration of conventional T cells 17. These findings align with our study, where an increase in the absolute or relative number of CD28 on CD39+ activated CD4 regulatory T cells suppressed immunity and endothelial migration of T cells. This suppression, in turn, elevated the risk factors for endometriosis and its associated subtypes.
In addition, reverse MR statistics uncovered that an increased risk of endometriosis and several subtypes (ASRM stages 3 and 4, ovary and pelvic peritoneal) corresponded to a decrease in the absolute and relative numbers of CD4+CD8dim T-cell subsets. These CD4+CD8dim T cells have demonstrated functionality, exhibiting preferential Th1-like activity by producing IL-2 and IFNγ. They also possess the ability to acquire cytotoxic activity, manifest an NK-T cell phenotype, and participate in antitumor activity18. The absolute counts of CD4+CD8dim T cells serve as an indicator of immune system function. In instances of immunodeficiency, such as HIV, a reduced count of CD4+CD8dim T cells may result in impaired immune function19. Conversely, certain conditions, including autoimmune diseases like β-thalassemia and non-autoimmune diseases like Alzheimer’s disease, are associated with elevated numbers of CD4+CD8dim T cells, leading to immune attacks20,21. Therefore, based on the findings presented here, it is hypothesized that the decrease in the relative and/or absolute number of this specific cell population, triggered by the heightened risk of developing endometriosis, might contribute to immune escape. This, in turn, could facilitate the continued growth of focal tissues and further progression of the disease.
Our study boasts notable strengths, primarily the utilization of the MR method to counteract confounding bias, and robustness ensured through the application of four MR analysis techniques along with a comprehensive sensitivity analysis. Immune cell traits have emerged as significant risk/potential predictive factors for assessing the prevalence of endometriosis and its various subtypes. Certainly, our findings provide genetic support for the inflammatory hypothesis in endometriosis pathogenesis. However, it is essential to acknowledge certain limitations. Firstly, the study participants exclusively belong to European ancestry, emphasizing the need for validating causal associations in diverse populations. Despite detecting heterogeneity, the study did not identify outliers, indicating potential factors such as nonlinear relationships, sample size variations, measurement errors, or multiple pathogenic mechanisms influencing the results. The absence of identified outliers does not eliminate the possibility of others existing, which may represent distinct biological processes contributing to result heterogeneity22.