Villitis is characterized by an inflammatory infiltrate within the substance of the chorionic villi. Quantitative and qualitative analyses of the mononuclear infiltrate in areas of villitis were performed in placentas with villitis of unknown etiology (VUE). We used a panel of monoclonal antibodies and immunoperoxidase technique in paraffin sections from 17 placentas with VUE and 8 without VUE. Macrophages followed by T lymphocytes were the predominant inflammatory cells in areas of villitis in virtually all cases. B lymphocytes were not observed and monocytes were present usually in small number in 58 per cent of the cases. Mononuclear cells which expressed HLA-DR antigens were found in 75 per cent of the cases. In areas of villitis with trophoblastic necrosis, we found monocytes and some T lymphocytes adhered to them. These cells apparently had migrated from the maternal circulation. We suggest that in areas of villitis with destruction of the trophoblast and its basal membrane the inflammatory infiltrate might have a mixture of fetal and maternal cells. The maternal monocytes and T lymphocytes might be attracted to these sites of trophoblastic necrosis and activated due to exposure to fetal MHC antigens of the villous stroma.188330330
