Exploring the role of mesenchymal stromal cells in endometriosis

Abstract

Endometriosis is a chronic inflammatory disease where there is growth of endometrial tissue in ectopic sites, most commonly within the pelvic cavity. The major symptoms of this disease are chronic pelvic pain and infertility. There are both medical and surgical options to treat endometriosis, however, there is a high recurrence rate of ectopic lesions and symptoms, and hence a need for more effective therapies. Although the exact etiology is unclear, the most widely accepted theory describing the origin of endometriosis is Sampson’s theory of retrograde menstruation. This theory states that during menstruation there is retrograde movement of endometrial tissue back into the pelvic cavity that grows to become ectopic endometrial tissue through its increased ability to survive, proliferate, migrate, adhere, invade, and promote angiogenesis to support its growth. However, only approximately 10% of women of reproductive age develop endometriosis, while almost all women exhibit retrograde menstruation. Therefore, other mechanisms have been suggested to be involved in the development of endometriosis such as reduced immunosurveillance in the pelvic cavity through modulation or aberrant responses of immune cells such as macrophages and natural killer (NK) cells, and involvement of stem/stromal cells such as mesenchymal stromal cells (MSC).

In endometriosis it is known that there is a predominance of immunosuppressive M2 macrophages, and inhibition of function of NK cells in the pelvic cavity. Both immune cell types have been suggested to contribute to the reduced immunosurvelliance of ectopic endometrial tissue. However, the exact cause of this reduced immunosurveillance is currently unknown. MSC are multipotent cells found in various tissue sources as well as in ectopic endometrial and eutopic endometrial tissue. Their immunomodulatory abilities have been utilized as a treatment in immune mediated diseases through their immunosuppressive effects on immune cells such as macrophages and NK cells. Interestingly, MSC have been suggested to exist as MSC1 (an immunostimulatory phenotype), or as MSC2 (an immunosuppressive phenotype), with the latter phenotype being the most commonly investigated in the literature (see Table 1). Therefore, herein it was hypothesized that MSC derived from endometriotic ovarian cysts could be involved in the reduced immunosurveillance of ectopic endometrial tissue in the pelvic cavity through their immunosuppressive effects on macrophages and NK cells. Moreover, if autologous MSC are involved in the pathology, then it was hypothesized that the immunomodulatory capabilities of allogeneic MSC could be a potential therapeutic strategy to target the inflammatory component of endometriosis.

Study 1 was an in vitro study that found that stromal cells isolated from endometriotic ovarian cysts (ESCcyst) of women with endometriosis are more immunosuppressive than stromal cells isolated from the endometrium (ESCendo) of women with endometriosis. The ESC were also confirmed to be MSC; they expressed MSC markers such as CD73, CD90, and CD105, formed colonies when seeded at low densities, and differentiated into osteoblasts and adipocytes. ESCcyst expressed significantly higher levels of the immunosuppressive enzymes indoleamine 2,3-dioxygenase 1, cyclooxygenase 2, and hemeoxygnase 1, and promoted differentiation of the more immunosuppressive M2 macrophages. Based on the results of this study we used allogeneic adipose tissue-derived MSC (Ad-MSC) in Study 2 to examine their effects on ESCcyst and ESCendo in vitro, as a first step of a long-term goal of developing a potential therapy for endometriosis. We found that culture with allogeneic Ad-MSC either maintained or promoted the proliferation, survival, adhesion, migration and invasion of ESCcyst. Moreover, conditioned medium (CM) derived from Ad-MSC and ESCcyst co-cultures promoted tube formation of human umbilical vein endothelial cells; tube formation is an in vitro model of angiogenesis. In Study 3, the interactions between ESCcyst and NK cells in vitro were examined. It was found that following culture in the CM of ESCcyst or in direct co-culture with ESCcyst, the phenotype, degranulation, and cytotoxic functions of NK cells were similar to the NK cells cultured with ESCendo; the expression of activation, inhibitory, adhesion, and maturation receptors was similar, and the level of the degranulation marker CD107a and expression of the immunostimulatory cytokine interferon gamma was similar.

In conclusion, the findings presented in this thesis suggest that immunosuppressive ESCcyst may indirectly promote the growth of ectopic endometrial tissue through their inhibitory effects on the immunosurveillance in the pelvic cavity through their effects on macrophages but not on NK cells. In addition, autologous ectopic MSC such as ESCcyst may be involved in the pathogenesis of endometriosis through their direct growth promoting effects on ectopic endometrial tissue.