Potential selectin L ligands involved in selective recruitment of peripheral blood CD16(-) natural killer cells into human endometrium

Unique CD16(-) natural killer (NK) cells appear in the human cycling endometrium and acutely increase in number after ovulation. Selective recruitment from peripheral blood (PB) CD16(-) NK cells is a potential mechanism for the postovulatory increase of these NK cells. The interaction between selectin L, an adhesion molecule playing a critical role in leukocyte extravasation, and its ligands may be involved in this phenomenon. We investigated the menstrual cycle-dependent fluctuation of selectin L expression on PB CD16(-) NK cells and selectin L ligand expression in the human endometrial endothelium. The expression of selectin L on PB CD16(-) NK cells was constantly high throughout the menstrual cycle compared with other PB CD16(+) NK cells and non-NK lymphocytes. Among eight selectin L ligands examined, podocalyxin-like, mucosal addressin cell adhesion molecule-1 (MADCAM1) and chondroitin sulfate proteoglycan 2 (CSPG2) were localized in the endometrial endothelium. Semiquantitative score of immunostaining intensity in the endometrial endothelium for MADCAM1 was highest in the late secretory phase, whereas that for CSPG2 peaked throughout the secretory phase. There was a strong positive correlation between the number of endometrial NK cells and the semiquantitative score for CSPG2. Three active isoforms of CSPG2 mRNA were detected in the human endometrium. These findings support the idea that the interaction between selectin L and selectin L ligands functions in the postovulatory selective recruitment of PB CD16(-) NK cells into the human endometrium.