In cutaneous T‐cell lymphoma, class II MHC molecules on CD1 + antigen‐presenting cells are upregulated in involved compared with uninvolved epidermis

Summary CD1 + antigen‐presenting cells in involved epidermis of patients with cutaneous T‐cell lymphoma exhibit an enhanced functional capacity to activate autologous CD4 + T cells compared with CD1 + antigen‐presenting cells from uninvolved and normal epidermis. Class II major histocompatibility complex molecules are involved in antigen presentation, and their expression on CD1 + Langerhans cells is known to vary. The expression of all three class II (HLA‐DR, ‐DQ, ‐DP) molecules was therefore determined on CD1 + epidermal cells from both involved and uninvolved epidermis, using flow cytometry. The involved CD1 + epidermal cells exhibited a 1.5–1.6‐fold, statistically significant increase in fluorescence intensity after staining of the class II molecules (HLA‐DR, ‐DQ, ‐DP) compared with CD1 + epidermal cells from uninvolved epidermis. The autologous CD4 + T‐cell activation was almost completely blocked by anti‐HLA‐DR, and partly by anti‐HLA‐DQ and anti‐HLA‐DP. In contrast, an antibody against class I, and an irrelevant control antibody, had no blocking effect. In a pokeweed mitogen assay it was demonstrated that autologous CD4 + T cells, activated by involved epidermal cells, demonstrated suppressor activity rather than helper activity. The suppressor activity was dependent on the presence of HLA‐DR‐positive epidermal cells. Thus, in cutaneous T‐cell lymphoma, class II molecules on the individual CD1 + antigen‐presenting cell are upregulated in clinically involved compared with uninvolved epidermis, and these molecules are crucially involved in activation of CD4 + T cells.