The key role of HLA molecules for multiple aspects of human health is well-recognized, including infection autoimmunity and, in the setting of transplantation, allo-immunity. A primary function of HLA molecules is presentation of foreign and self-peptides as antigen to cells of the immune system. In the context of pregnancy HLA molecules are at crossroads of competing needs since what is required is maintenance of competence against pathogens, avoidance of autoimmunity, and the health of two individuals in the face of maternal-fetal HLA disparities. The purpose of this proposal is to test the hypothesis that HLA molecules have a novel role during pregnancy by which foe is transfigured to friend through cross-dressing indigenous cells with HLA molecules acquired from the other individual, thus meeting these competing needs. Classical HLA molecules are not expressed at the maternal-fetal interface with the exception of HLA-C. However, clearly there is recognition of non-shared HLA molecules as evidenced by maternal antibodies to paternally transmitted HLA antigens of her children and tolerance of offspring to non-inherited maternal HLA antigens. The concept behind the study hypothesis is supported by observations in transplantation in which cross-dressing originates from extracellular vesicles and by preliminary data as discussed and provided in the proposal. The Aim is to test the hypothesis that during pregnancy maternal cells become “cross-dressed” (XD) with fetal-specific HLA. To accomplish this goal state-of-the-art imaging flow cytometry techniques will be employed to identify fetal-specific HLA acquired on maternal cells. Pregnancies of healthy women who gave birth to a normal child at term will be studied before pregnancy, in each trimester and postpartum from a prospectively collected resource we have created. We anticipate fetal-specific HLA-XD maternal cells will increase as pregnancy progresses, significantly decrease postpartum, and be associated with tolerance markers such as PD-L1 on maternal immune cells. As an extension to the test of the study hypothesis we plan to conduct pilot studies for pregnancies of women with idiopathic recurrent pregnancy loss and women with the autoimmune disease rheumatoid arthritis. We anticipate fetal-specific HLA-XD maternal cells to be fewer and/or to have an impaired co-expression of PD-L1 markers in women who experience another miscarriage and, for women with RA that results will mirror those of normal pregnancies and correlate with the characteristic pregnancy-induced amelioration of this autoimmune disease. The study hypothesis is proposed not as an alternative, but rather as a substantial addition to considerable understanding of the maternal-fetal relationship as elucidated in prior studies. The novel role for HLA molecules proposed in this application has the potential to spur a quantum leap in understanding arguably with broad- reaching impact as focusing on novel aspects of the maternal-feta...