Assessment of potential biomarkers of pre-receptive and receptive endometrium in uterine fluid and a functional evaluation of the potential role of CSF3 in fertility

The endometrium lines a women's uterus becoming receptive, and allowing embryo implantation to occur, for just a few days during the post-ovulatory mid-secretory phase of each menstrual cycle. We investigated whether concentrations of proposed receptivity biomarkers (VEGF, IL8, FGF2, CSF3 sFlt-1, sGP130 and PlGF) secreted by the endometrium into the uterine cavity and forming the microenvironment for embryo implantation is altered among a population of age-matched women with unexplained (idiopathic) infertility compared to fertile women during the receptive mid-secretory phase (n = 16 fertile, 18 infertile) and the prior pre-receptive early secretory phase (n = 19 fertile, 18 infertile) of their cycle. In the mid-secretory cohort significantly elevated concentrations of five biomarkers; PlGF (p = 0.001), IL8 (p = 0.004), sGP130 (p = 0.009), sFlt-1 (p = 0.021), and CSF3 (p = 0.029) was present in uterine fluid of infertile women during the mid-secretory phase, but only CSF3 was significantly elevated in the pre-receptive early secretory phase (p = 0.006). In vitro studies of glycosylated and non-glycosylated forms of CSF3 at representative fertile (20 ng/mL) and infertile (70 ng/mL) effects on endometrium and embryo behaviour were performed. Non-glycosylated CSF3 at fertile concentrations significantly (p < 0.001) elevated endometrial epithelial cell proliferation however chronic treatment or elevated (infertile) concentrations of CSF3 in glycosylated form abrogated the positive effects. Both forms of CSF3 increased trophoblast cell invasion (p < 0.001) regardless of concentration. Mouse embryo outgrowth was significantly (p < 0.01) increased at fertile but not at infertile concentrations. The study confirmed potential utility of five biomarkers of endometrial receptivity for future application in the mid-secretory phase while highlighting CSF3 is elevated in the earlier pre-receptive phase. Our data provides evidence that CSF3 acts on both human endometrium and embryo in a manner that is concentration and glycosylation dependent.