IMMUNOSUPPRESSIVE EFFECTS OF 1,25-DIHYDROXYVITAMIN D-3 AND ITS ANALOG CALCIPOTRIOL ON EPIDERMAL-CELLS

1,25-Dihydroxyvitamin D-3 (1,25(OH)(2)D-3; calcitriol) is the biologically active form of vitamin D. This hormone is a potent immunoregulatory agent. Calcipotriol is a synthetic analogue of 1,25(OH)(2)D-3, with similar receptor binding, and comparable effects on cell proliferation and differentiation, but less potent effects on calcium metabolism. As a step towards understanding the mechanisms by which vitamin D compounds affect T-cell activation by epidermal cells (EC), we assessed the effects of 1,25(OH)(2)D-3 and calcipotriol on the human allogeneic mixed epidermal cell-lymphocyte reaction. All experiments were performed both with 1,25(OH)(2)D-3, and calcipotriol, with similar results. Both compounds had potent immunoinhibitory properties on this model, and enhanced the immunosuppressive effects of cyclosporin A. Using preincubation experiments, we found that pretreatment of EC with 1,25(OH)(2)D-3 resulted in a more pronounced inhibition than preincubation of lymphoid cells. The epidermal targets of this inhibitory effect have been further investigated, using cultures with freshly isolated Langerhans cells (LC) or LC-depleted keratinocytes, separated by an immunomagnetic particle technique. Pretreatment of LC induced a 30% decrease of proliferation, compared with vehicle-treated LC. These calcitriol-pulsed LC did not decrease the proliferation induced by unmodified autologous EC. As expected, LC-depleted keratinocytes failed to stimulate allogeneic lymphocytes. When added to autologous unmodified EC, however, calcitriol-pulsed keratinocytes induced an 85% decrease of proliferation, compared with vehicle-treated keratinocytes. The phenotypic expression of HLA-DR, -DQ, and -DP antigens on EC, assessed by immunoalkaline phosphatase staining, was not modified after a 2-h or 24-h pulse with 1,25(OH)(2)D-3 or calcipotriol. The inhibitory effect of vitamin D compounds on EC was not modified by indomethacin, but was partially reversed by the addition of anti-TGF-beta neutralizing antibodies. In conclusion, 1,25(OH)(2)D-3 and calcipotriol may limit the immune response in human skin through decreased antigen presentation, mediated both by a direct effect on LC and indirectly through modulation of the production of cytokines by keratinocytes.