Quantitative evaluation of two distinct cell populations expressing HLA-DR antigens in normal human epidermis

HLA-DR antigens are expressed only by Langerhans cells (LC) and indeterminate cells (IC) in normal human epidermis. Indirect immunofluorescence and ultrastructural immunogold labelling were used to study the HLA-DR expression by means of two anti-DR monoclonal antibodies (MCA). Freshly dispersed DR-positive epidermal cells expressed different densities of DR antigens on their membrane surface. Approximately 25% of DR-positive cells were strongly labelled by anti-DR-MCA and 75% were weakly stained. After 18 h in complete culture medium before labelling no significant difference in these percentages was observed. The lymphoid-like LC-enriched cells obtained by Ficoll-Hypaque centrifugation also had two populations of DR-positive cells: strongly labelled cells (30.8%) and weakly labelled cells (69.2%). DR-positive cells may be divided into two types according to the density of DR sites on their cell membrane: (I) type DR+ shows weak surface labelling by gold particles (8.8 +/- 3.0 gold particles/micron) and has cytoplasmic Birbeck granules, identifying such cells as typical Langerhans cells; (2) type DR shows strong membrane immunogold labelling (38.9 +/- 4.6 gold particles/micron) and may or may not contain Birbeck granules. The gold particles bound to the cell membrane were used to quantify the number of HLA-DR sites expressed on each cell type: 1 X 10(5) sites on DR+ cells and 5 X 10(5) on DR cells.