Introduction; Failure of implantation of an embryo into the endometrium is a major reason for infertility in women. The endometrium secretes a multitude of proteins into the uterine cavity, providing a receptive environment for implantation in the mid-secretory phase.

Aim; to Identify and validate secreted proteins as potential biomarkers of endometrial receptivity.

Methods and Results; Utilising sequential affinity capture we have simultaneously removed abundant proteins, and enriched for serine proteases and heparin binding proteins from endometrial flushings. Isolated proteins were identified using Orbitrap Mass-spectrometry and revealed a number of proteins not previously identified as part of the endometrial secretome. Among these was Kallikrein 6 (KLK6), a peptidase seen by western blotting to be ~26kDa in mass. Immunohistochemistry demonstrated a cyclic expression of KLK6 in the human endometrium of fertile women; being absent in the proliferative phase, it appears contained in vacuoles of the glandular and luminal epithelia during the early secretory phase, and is secreted into the uterine cavity during the receptive mid-secretory phase. Using ELISA it was seen that the concentration of secreted KLK6 in the uterine cavity is substantially higher than in plasma, confirming its local synthesis within the endometrium. The concentration of KLK6 in endometrial flushings from infertile women showed a significant reduction during the fertile mid-secretory phase.

Summary; Affinity capture aided the identification of low abundance proteins previously unreported in uterine fluid; among these was KLK6. Our studies have demonstrated cyclic expression of KLK6 in the endometrium, however its expression and secretion are greatly reduced in infertile women. At this time the functional role of KLK6 in reproduction remains unresolved. Future studies will determine both function and potential as a receptivity biomarker, extending our understanding of female infertility and ultimately improving IVF success rates.