In vitro fertilization (IVF) has enabled many infertile couples to achieve pregnancy, however even good quality blastocysts have an implantation rate of less than 60%. Endometrial factors including inadequate decidualisation and defects of the maternal immune system are likely to account for some of these implantation failures. Recent data have shown that exposure of the female reproductive tract to seminal fluid improves clinical pregnancy rates in women undergoing IVF. In addition, in humans regular unprotected intercourse with the father prior conception decreases the incidence of pre-eclampsia, an inflammatory pathology during pregnancy, suggesting that seminal fluid primes the endometrium to tolerate the pregnancy. Seminal fluid contains extracellular vesicles that are membrane-bound complexes (<1m) that facilitate cell-cell communication by delivering their cargos of proteins and nucleic acids to target cells. Seminal fluid extracellular vesicles (SF-EVs) bind to sperm and promote motility, capacitation and the acrosome reaction. We hypothesised that SF-EVs may also bind cells of the female reproductive tract and enhance endometrial receptivity.
SF-EVs were isolated (n=11) and characterised using Nanoparticle Tracking Analysis (Malvern Instruments Ltd.), western blot and fluorescence microscopy (Oxford Nanoimaging). The samples were then purified using size exclusion chromatography (ExoSpin, Cell Guidance Systems) to remove seminal fluid proteins. Bio-Maleimide-FITC labelled SF-EVs were incubated with human endometrial cells and differential binding to endometrial epithelial, stromal and immune cells was observed by fluorescence microscopy and flow cytometry. Changes in epithelial cell activation after SF-EVs treatment were detected using a human phospho-kinase protein array (R&D systems, USA) and validated by western blotting; SF-EVs induced dephosphorylation of CREB at S133. SF-EVs also enhanced secretion of Prolactin, a decidualisation marker, by endometrial stromal cells. This evidence supports the hypothesis that paternally derived SF-EVs assist human endometrial receptivity.