EXPRESSION OF B7 COSTIMULATORY MOLECULE IN CULTURED HUMAN EPIDERMAL LANGERHANS CELLS IS REGULATED AT THE MESSENGER-RNA 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 all ogeneic T cells and acquire the ability to activate naive T cells, pro bably 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 monoclona l antibodies. fLC in suspension did not exhibit any B7 staining as eva luated by two-color flow-cytometry analysis and immunoelectron microsc opy. In contrast, LC that were cultured for 24-72 h displayed strong s urface B7 reactivity with a characteristic patchy pattern. Treatment w ith dispase and trypsin did not reduce B7 staining of cLC. Following w arming to 37 degrees C, cLC tagged with anti-B7 monoclonal antibody an d gold-conjugated secondary antibody could internalize surface B7 by u sing the organelles of receptor-mediated endocytosis. B7 mRNA, detecte d by the reverse-transcriptase polymerase chain reaction technique, wa s expressed at a low level in purified (> 90% HLA-DR(+)) fLC but not i n LC-depleted epidermal cells, and was markedly upregulated in purifie d cLC. The results indicate that 1) fLC do not express B7 protein on t heir surface, but acquire B7 during culture, 2) surface B7 is not sens itive 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 pote ncy and ability to stimulate naive T lymphocytes.