Leu-3/T4 expression on epidermal Langerhans cells in normal and diseased skin

Recent evidence exists that the expression of the Leu-3/T4 antigen is not restricted to thymus-derived lymphocytes but can also be detected on mononuclear phagocytes and epidermal Langerhans cells (LC). When searching for the presence of Leu-3/T4 antigen-bearing cells in tissue sections of a variety of inflammatory and neoplastic skin disorders, we observed quantitative and qualitative differences in the intensity of anti-Leu-3a labeling of epidermal dendritic cells. Reasoning that Leu-3/T4 expression by these cells might be a dynamic event, we compared the anti-Leu-3a LC staining pattern in clinically normal-appearing skin (CNAS) with the expression of this antigen on epidermal dendritic cells in a variety of skin disorders. For this purpose, 4-microns cryostat sections were exposed to the monoclonal anti-Leu-3a reagent and antibody binding was visualized by a sensitive 4-step immunoperoxidase technique. Within CNAS, Leu-3a+ dendritic epidermal cells were visualized at the threshold of detectability. Immunoelectron microscopic studies confirmed the LC nature of these cells. In sharp contrast to CNAS, strong and prominent anti-Leu-3a LC labeling was almost invariably encountered in biopsy specimens from patients with cutaneous T-cell lymphoma and various inflammatory conditions but not in proliferative disorders of resident skin cells. Whereas in CNAS the density of T6+ epidermal dendritic cells greatly exceeded that of anti-Leu-3a-reactive dendritic cells, these differences were less pronounced in diseased skin. Our results: confirm earlier observations that epidermal LC may bear Leu-3/T4 antigens; and in addition, suggest that the degree of Leu-3/T4 expression is regulated by signals from inflammatory cells. The induction of class II alloantigen receptors on class II alloantigen-bearing LC may represent an important regulation mechanism of antigen-presenting cell function.