Non-classical MHC-E (Mamu-E) Expression in the Rhesus Monkey Placenta

The aim of this study was to characterize the expression of the rhesus HLA-E ortholog Mamu-E, particularly at the maternal–fetal interface. Mamu-E expression was confirmed by locus-specific RT-PCR in the placenta as well as in peripheral blood mononuclear cells (PBMC) and other organs. We evaluated the utility of antibodies recognizing HLA-E (MEM-E/06 against native HLA-E, MEM-E/02 against denatured HLA-E) to detect Mamu-E by flow cytometry/immunofluorescence, Western blot, and immunohistochemistry (IHC). Western blot analysis of cells and selected transfectants confirmed the recognition of Mamu-E but not Mamu-AG by antibodies MEM-E/06 and HC10 but not MEM-E/02. Immunohistochemical staining of frozen sections of rhesus placenta with the MEM-E/06 antibody demonstrated expression in most populations of rhesus monkey trophoblast cells, including villous cytotrophoblasts (strong positive staining), apical membrane of syncytiotrophoblasts (light to moderate staining) and extravillous cytotrophoblasts (moderate to strong staining, especially endovascular trophoblasts in early pregnancy). Expression was not trophoblast cell-specific, especially at term, when endothelial cells in both the chorionic plate and placental villi showed strong staining for Mamu-E. Staining of rhesus extravillous trophoblast cells suggested the co-expression of Mamu-E and Mamu-AG (the rhesus HLA-G homolog) on these cells. MEM-E/06 was shown also to react with differentiating rhesus placental syncytiotrophoblasts in primary culture, detecting intracellular and weak surface expression of Mamu-E. We conclude that the gestation-dependent co-expression of Mamu-E with Mamu-AG in villous and extravillous trophoblast cells suggests important and perhaps complementary but distinct roles of these two non-classical MHC class I loci in pregnancy at the maternal–fetal interface. In addition, the MEM-E/06 antibody will be useful for the detection of Mamu-E at the maternal–fetal interface in the rhesus monkey.