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

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Abstract
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: (1) type DR+ shows weak surface labelling by gold particles (8·8 ± 3·0 gold particles/μm) 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/μm) 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 × 105 sites on DR+ cells and 5 × 105 on DR+++ cells.