Endometrial NK cells are special immature cells that await pregnancy.

NK cells populate the human endometrium before pregnancy. Unlike decidual NK cells that populate the decidua during pregnancy, the NK cells present in the human endometrium, before pregnancy, have not been fully characterized. In this study, we provide a detailed analysis of the origin, phenotype, and function of endometrial NK cells (eNK). We show that eNK cells have a unique receptor repertoire. In particular, they are negative for NKp30 and chemokine receptor expression, which distinguishes them from any other NK subset described so far. We further show that eNK cells lack NK-specific functional phenotype and activity such as cytokine secretion and cytotoxicity, before IL-15 stimulation. Following such stimulation, endometrial NK cells acquire phenotype and function that are similar to those of decidual NK cells. We therefore suggest that eNK cells are inactive cells (before IL-15 activation and in relation to the known NK activity) that are present in the endometrium before conception, waiting for pregnancy.