Cells bearing class II MHC antigens in the human placenta and amniochorion.

Immunohistological techniques have been used to study the stromal cells of the human placenta in both the chorionic villous mesenchyme and the connective tissue underlying the amnion. Throughout gestation many of these cells express an antigen (3C10) that is found on mononuclear phagocytes but not on dendritic cells or epidermal Langerhans cells. In the first and second trimesters the placental cells also react with a monoclonal antibody (NA1/34) to the human thymocyte antigen (CD1), a lymphocyte differentiation antigen expressed by cortical thymocytes and Langerhans cells; expression of this antigen diminishes as gestation advances. In contrast, an antibody to a different epitope of CD1 (OKT6) does not bind. Class II MHC antigens are not present in the first trimester but are acquired by increasing numbers of placental macrophages from the second trimester onwards. It is possible that the placenta has significant immune functions and that, by term, placental macrophages may be capable of antigen presentation.