Immunoelectron microscopic localization of HLA-DR antigen on mast cells and vessels in normal and tuberculin-reactive skin.

We investigated the ultrastructural localization of HLA-DR antigen in normal (NS) and tuberculin-reactive skin (TRS) with the aid of immunoelectron microscopic techniques. Most of the mast cells expressed HLA-DR antigen in TRS, but not in NS. Endothelial cells (ECs) in the microvasculature carried HLA-DR antigen in NS and TRS. The luminal surface of the ECs in NS had a stronger expression of HLA-DR antigen than the abluminal surface. This polar expression of the ECs was maintained in TRS. The percentage of HLA-DR positive junctions on ECs was significantly higher in TRS than in NS. Some of the HLA-DR-positive lymphocytes adhered to the HLA-DR-positive luminal surface of the ECs in NS and TRS. Pericytes and smooth muscle cells of vessels did not carry this antigen in NS or TRS. We discuss the role of mast cells in TRS and the relation between the function and ultrastructural localization of HLA-DR antigen on vessels in NS and TRS.