Immunoultrastructural localization of Ia antigens in human endometrium

The distribution of Ia antigens was studied at the light and ultrastructural levels in 34 proliferative and 16 secretory endometria with two monoclonal antibodies using an avidin-biotin-peroxidase complex method. The endothelial cells, many lymphocytes, and various monocytic-macrophagic cells in the endometrial stroma were Ia positive. Furthermore, Ia antigens were localized to normal endometrial epithelium in the proliferative phase, and focally in epithelium adjacent to stromal lymphoid aggregates throughout the cycle. Expression of Ia antigens in the secretory epithelium was focal or absent. At the ultrastructural level, Ia-positive epithelial cells exhibited staining on the plasma membrane, in free and membrane-bound ribosomes, rough endoplasmic reticulum, and occasional perinuclear cisternae. In the endothelial cells and lymphocytes, Ia antigens were also localized to the plasma membrane and rough endoplasmic reticulum. These findings indicate that endometrial epithelium expresses Ia antigens which may be regulated by endometrial lymphoid cells and/or hormones. The plasma membrane expression of Ia antigens by the epithelial cells of endometrium appears to result from active synthesis, and not merely from passive absorption of Ia antigens.