HLA-DR positive cells in the human placenta.

A population of heterogeneous HLA-DR positive cells has been identified in the human placenta and decidua using immunochemical and histochemical methods. These cells are found in three areas: the subepithelial layer of the amnion, the decidua, and more sparsely within the chorionic villous stroma. In addition to HLA-A, -B and -C antigens, they also express the leucocyte-common antigen, indicating their origin from bone marrow precursors. The majority have a characteristic stellate shape with many cytoplasmic processes. In the villous stroma these stellate cells can be distinguished from the Hofbauer cells (placental macrophages) by their morphology, stronger expression of HLA-DR and lack of lysosomal enzyme activity. In the amnion and decidua they cannot be clearly distinguished from tissue macrophages. By using monoclonal antibodies specific for foetal or maternal HLA-A or -B allotypes, the HLA-DR positive cells in the chorionic villi and the amnion have been shown to be foetal in origin. In contrast, most of the HLA-DR positive cells in the decidua are maternal; a few adjacent to the basal plate are foetal. The preponderance of these cells in those areas of the placenta where foetal and maternal tissues are in close proximity is striking. The possibility that some of the cells are equivalent to the dendritic cells that have been described in other tissues is discussed.