Differential expression of HLA-class II antigens in the human thymus. Relative paucity of HLA-DQ antigens in the thymic medulla.

By using well-characterized monoclonal antibodies, HU-20, HOK7(1E4) and HU-18, which specifically recognize either common epitopes of HLA-DR molecules (HU-20, HOK7) or HLA-DQw3 molecules (HU-18), it was demonstrated that there were different distributions of these antigens in human thymus. In the cortex, both DR and DQ antigens were strongly expressed on thymic epithelial cells (TEC). The double stainings revealed simultaneous expression of both antigens on TEC. On the other hand, in the medulla, DQ antigens had more limited expression than DR antigens, the latter antigens being demonstrated in the confluent pattern. The double staining method could show small numbers of dual-positive cells in the medulla, which looked like TEC, indicating that at least some medullary TEC were also DR+,DQ+, although the presence of small numbers of DR-,DQ-, as well as DR+,DQ- TEC in the medulla cannot be excluded. The preponderance of DR over DQ antigen in the medulla was consistently found--from a ten week embryo to a two-year-old child--and could be attributed to very weak or no expression of DQ antigens on dendritic cells/macrophages. The distribution pattern of these antigens within the thymus may have correlations with the intrathymic development of T cells through the unique functions of each antigen.