Epidemiological studies have associated preeclampsia outcomes with genotypes of receptors on maternal natural killer (NK) cells, and genotypes of their cognate HLA class I (HLA-I) ligands in fetuses. These clinical findings have been replicated and strongly suggest that uterine NK (uNK) cells, the most abundant leukocytes at the maternal-fetal interface, specifically use their receptors to recognize HLA-I on fetal tissues, thereby playing vital roles in normal placentation, probably by affecting the vasculature which is abnormal in preeclampsia. Although it is challenging to study these possibilities in human reproduction, and we acknowledge that there are species-specific differences between human and mouse pregnancy, studies of mouse NK cells can yield important conceptual insight, as demonstrated numerous times previously in NK cell biology. Recent studies from the applicant's laboratory indicate that mouse organs, including the virgin uterus, contain both circulating “conventional” NK (cNK) cells and tissue-resident NK (trNK) cells that reside in the organ and do not recirculate. cNK and trNK cells from different organs represent distinct lineages of NK cells because they have differences in transcription factor dependencies. Moreover, cNK and trNK cells have different functional capacities. Finally, the Ly49 family of receptors are the functional equivalents to the human NK cell receptors implicated in preeclampsia. These considerations lead to the following proposed Specific Aims: 1) Determine contributions of cNK and trNK cells to uNK cells in pregnancy and decidualization; 2) Examine transcription factor dependence of uNK cells; and 3) Determine role of Ly49 receptors in pregnancy. Thus, these studies will provide major insight into the role of uNK cells in pregnancy, the major long-term objective of this proposal.