CD86 (B70/B7-2) has recently been identified as an alternative CD28/CTLA-4 ligand on activated B cells. CD86 has also been demonstrated as possibly serving as a primary costimulatory molecule in the initial immune response. Since the human Langerhans cell is one of the most potent antigen-presenting cells, we examined whether CD86 expression and function are found on organ-cultured skin, freshly isolated Langerhans cells, and cultured Langerhans cells in normal human epidermis. Immunohistochemical study in situ revealed that CD86 was expressed on dendritic cells with CD1a antigen in organ-cultured but not fresh skin. Fluorescence-activated cell sorter analysis revealed that no staining for either CD80 or CD86 was observed in freshly isolated Langerhans cells but that both CD80 and CD86 were expressed on cultured Langerhans cells. The actual expression of CD86 on cultured Langerhans cells was further confirmed by the detection of 70-kDa glycoprotein on Western blot analysis. Analysis of polymerase chain reaction demonstrated that both CD80 and CD86 were specifically amplified from purified cultured and freshly isolated Langerhans cells but not from Langerhans cell-depleted epidermal cells, indicating that both CD80 and CD86 genes were expressed by Langerhans cells. The functional importance of CD86 on Langerhans cells was confirmed by the allogeneic CD4 T cell proliferative responses with enriched Langerhans cells. A monoclonal antibody against CD86 caused 81% inhibition in contrast with 29% inhibition produced by anti-CD80 monoclonal antibody. This inhibitory effect was enhanced to 85.3% inhibition when a combination of anti-CD86 and anti-CD80 was administered. These results indicate that CD86 is predominantly expressed on the surface of cultured Langerhans cells and may transduce a primordial costimulatory signal in the interaction of Langerhans cells and T cells.