Is there an immune modulating role for follicular fluid in endometriosis? A narrative review

Follicular fluid (FF) surrounds the granulosa cell-oocyte complex and is one of the mediating factors in the communication between the cells within the follicle. Literature reveals that human FF and its components are key factors to the success of natural fertilization. Among other substances, FF consists of multiple cytokines and immune cells, including interleukin 6 (IL6), IL12, sHLA-G, macrophages, NK cells and lymphocytes. Together, these cells and cytokines might influence the oocyte-granulosa-cell complex. Altered balances of immune content might be involved in changes on folliculogenesis, oocyte maturation, oocyte quality and ovulation. Furthermore, these altered balances are possibly involved in infertility associated with immune-mediated diseases such as endometriosis. The aim of this narrative review is to elaborate on the function and contents of FF and its immunological profile in patients with endometriosis. A comprehensive literature search was performed for the published literature on FF (immune) contents, FF function and FF content alterations in endometriosis patients. In FF of patients with endometriosis, elevated levels of macrophages and several cytokines have been reported. The role of specific immune cells in FF and a clarification of the biological mechanism in healthy women and endometriosis patients remain largely unknown. Future studies in this field will give us more insight in the role of FF immune cells and the effect of altered balances in patients with endometriosis.