The role of natural killer cells in human fertility

Uterine natural killer (uNK) cells form the major leucocyte population in the endometrium at the time of implantation and have received considerable attention in relation to their role in normal implantation and early placental development. Particular interest has been paid to their potential role in pregnancy pathology; specifically the role of uNK cells in recurrent miscarriage (RM) and recurrent implantation failure (RIF). Although several clinical studies have suggested that peripheral blood (PB) natural killer (NK) cells and/or uNK cells are increased in women with RM and RIF, data to date is inconclusive because of significant heterogeneity across studies arising from the use of different methods to quantify NK cells. An understanding of the role of these cells in reproductive failure and their value in clinical practice will not be established until a consensus is reached on how they should be measured. In this Scientific Impact Paper, the data relating to NK cell function will be reviewed and recommendations made regarding the measurement of NK cells in women with reproductive failure.