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The human CD80 costimulatory molecule is an important signal between professional antigen-presenting cells and T helper cells. The immunobiology of CD80 expression by keratinocytes, especially during allergic and irritant contact dermatitis, however, is less well understood. CD80 cell surface expression and gene transcription by keratinocytes was increased when keratinocytes were exposed to certain allergens (chemicals that induce inflammation via hapten-specific T cells) and irritants (chemicals that are toxic to epidermal cells). Therefore, the human CD80 promoter was cloned and luciferase reporter constructs containing various promoter fragments were engineered. Promoter mapping of these CD80 constructs in transiently transfected keratinocytes showed that a construct containing the proximal 231 bp immediately upstream of the transcription start site of the CD80 promoter was most active in keratinocytes and was inducible to a level ranging from 2- to 10-fold higher in keratinocytes treated with certain allergens and irritants, compared with untreated keratinocytes. This pattern of promoter fragment activity in keratinocytes is identical to that found in professional antigen-presenting cells. This is the first demonstration that the CD80 promoter is active in keratinocytes and that this activity is further increased in keratinocytes treated with certain allergens and irritants. These data suggest that allergens and irritants may, in part, break peripheral tolerance by their direct effects on keratinocyte costimulatory molecule expression, thereby facilitating interactions with epidermotropic T helper cells via the CD80-CD28 or CTLA-4 pathways.

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Previous studies of normal human keratinocytes have indicated that these 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 flow 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 revealed low levels of 1.7-kb B7-1 transcripts, which were independent of the presence of epidermal Langerhans cells, again indicating that such transcripts were derived from keratinocytes. Keratinocytes cultured in medium containing low concentrations of extracellular calcium (0.07 mM) expressed low levels of cell-surface BB-1. However, keratinocytes cultured in medium with higher levels of extracellular calcium (1.5 mM) lost cell-surface expression of BB-1. Similarly, low-calcium keratinocytes 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 indicated that such cells do not respond to similar changes in extracellular calcium concentrations. Calcium-induced differentiation of keratinocytes regulates the expression of BB-1 antigen as well as transcripts, which is a novel mechanism for the regulation of this molecule.

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