Expression of B7 costimulatory molecule in cultured human epidermal Langerhans cells is regulated at the mRNA level

Langerhans cells (LC) belong to the dendritic cell lineage and are the principal antigen-presenting cells of squamous epithelia. Short-term cultured LC (cLC) exhibit a marked augmented capacity to stimulate allogeneic T cells and acquire the ability to activate naive T cells, probably in relation to enhanced expression of accessory signals. In this study, we evaluated the expression of B7 costimulatory molecule (CD80) in human freshly isolated (fLC) and cLC at both the protein and mRNA level. Staining of frozen skin sections did not reveal any epidermal dendritic cell reactive with either of two different anti-B7 monoclonal antibodies. fLC in suspension did not exhibit any B7 staining as evaluated by two-color flow-cytometry analysis and immunoelectron microscopy. In contrast, LC that were cultured for 24-72 h displayed strong surface B7 reactivity with a characteristic patchy pattern. Treatment with dispase and trypsin did not reduce B7 staining of cLC. Following warming to 37 degrees C, cLC tagged with anti-B7 monoclonal antibody and gold-conjugated secondary antibody could internalize surface B7 by using the organelles of receptor-mediated endocytosis. B7 mRNA, detected by the reverse-transcriptase polymerase chain reaction technique, was expressed at a low level in purified (> 90% HLA-DR+) fLC but not in LC-depleted epidermal cells, and was markedly upregulated in purified cLC. The results indicate that 1) fLC do not express B7 protein on their surface, but acquire B7 during culture, 2) surface B7 is not sensitive to trypsin, 3) B7 expression is regulated primarily at the mRNA level, and 4) membrane B7 can be internalized within cLC. B7 molecule on CLC may be relevant to their increased antigen-presenting cell potency and ability to stimulate naive T lymphocytes.