Granulocyte macrophage--colony-stimulating factor (GM-CSF) decreases CD1a expression by human Langerhans cells and increases proliferation in the mixed epidermal cell-lymphocyte reaction (MELR).

Langerhans cells (LC) undergo a variety of phenotypic and functional changes in vitro. To determine the effects of granulocyte macrophage--colony-stimulating factor (GM-CSF), tumor necrosis factor-alpha (TNF-alpha), and interleukin 1-alpha (IL-1) on LC phenotype in vitro, epidermal cell suspensions were enriched for LC by density-gradient centrifugation and cultured in the presence of 10 ng/ml of these cytokines. The percentage of cells expressing the surface protein CD1a was determined by flow cytometry. This percentage typically dropped after 48 h culture in both control and cytokine-treated medium to less than half that of the starting value. By the fifth day, the percentage of cells expressing CD1a in TNF-alpha and IL-1--treated cultures was still near half of the starting value, slightly above that of control cultures. Treatment with GM-CSF caused large and consistent decreases in the percentage of epidermal cells expressing CD1a. Cell viability in each of the three cytokine-treated cultures was identical to the control cultures, with essentially all cells having died by the sixth day after isolation. To determine the functional effects of these cytokines, the cytokine-containing medium was replaced after 72 h with medium containing purified allogeneic T cells and proliferation measured. Preliminary experiments showed no increased proliferation induced by IL-1 or TNF-alpha--treated epidermal cells. GM-CSF-treated epidermal cells induced 2-3 times more T-cell proliferation than epidermal cells cultured without additional cytokines. We conclude that GM-CSF, a cytokine known to be produced by keratinocytes in vitro, decreases CD1a expression by human LC and increases their ability to stimulate proliferation by allogeneic T cells.