HUMAN KERATINOCYTES REGULATE THEIR EXPRESSION OF B7 BB-L ANTIGEN BY AUNIQUE, CALCIUM-DEPENDENT MECHANISM/

Previous studies of normal human keratinocytes have indicated that the se cells express BB-1 antigen, an important adherence molecule usually associated with ''professional'' antigen-presenting cells. We studied freshly isolated epidermal cells and noted that the frequency of BB-1 -positive cells in normal human skin varied from 2.6 to 7.4% of total epidermal cells. Two-color how cytometry confirmed that keratinocytes were the major cell in the epidermis that expressed BB-1, because less than 10% of total epidermal Langerhans cells were positive for BB-1. Northern blot analysis of RNA extracted from normal human epidermis re vealed low levels of 1.7-kb B7-1 transcripts, which were independent o f the presence of epidermal Langerhans cells, again indicating that su ch transcripts were derived from keratinocytes. Keratinocytes cultured in medium containing low concentrations of extracellular calcium (0.0 7 mM) expressed low levels of cell-surface BB-1. However, keratinocyte s cultured in medium with higher levels of extracellular calcium (1.5 mM) lost cell-surface expression of BB-1. Similarly, low-calcium kerat inocytes expressed the 1.7- and 2.9-kb B7-1 transcripts, whereas high- calcium keratinocytes expressed only the 1.7-kb transcript. Studies of the cell-surface expression of BB-1 by plastic adherent monocytes ind icated that such cells do not respond to similar changes in extracellu lar calcium concentrations. Calcium-induced differentiation of keratin ocytes regulates the expression of BB-1 antigen as well as transcripts , which is a novel mechanism for the regulation of this molecule.