HLA-DM is expressed on the cell surface and colocalizes with HLA-DR and invariant chain in human Langerhans cell

The subcellular distribution patterns of molecules involved in the process of antigen loading [HLA-DR, HLA-DM, and the cytoplasmic and luminal parts of the invariant chain (Ii, CD74)] were investigated in Langerhans cells (LC), both qualitatively and quantitatively, The analysis was performed by immunofluorescence labelling of acetone-fixed vertical cryostat sections from normal human skin specimens and subsequent examination using confocal laser scanning microscopy (CSLM). The intensity-modulated multiple-wavelength scanning (IMS) technique was used to enhance channel separation when scanning dual-labelled specimens. The mean (n = 9) relative epidermal volumes of reactivity were: HLA-DR 8% +/- 3%, HLA-DM 6% +/- 2%, luminal Ii 6% +/- 2%, and cytoplasmic Ii 4% +/- 1%, The difference between HLA-DR and the other epitopes was significant at the P < 0.001 level. All molecule combinations, except the combination of HLA-DM and luminal Ii (which was not studied), were to various extents colocalized. Experiments performed on unfixed epidermal sheets showed that HLA-DM is present on the cell surface of LC, suggesting that HLA-DM may interact with HL-ADR on the surface to induce peptide loading.