Dexamethasone up-regulates granulocyte-macrophage colony-stimulating factor receptor expression on human monocytes

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) are weak inducers of major histocompatibility complex (MHC) class II expression on purified human blood monocytes. The glucocorticoid dexamethasone synergizes with GM-CSF or IL-3 for the upregulation of HLA-DR, -DP and -DQ antigen mRNA and cell-surface expression by these cells. The purpose of the present study was to address the mechanism of dexamethasone action. We demonstrate that the capacity of dexamethasone to up-regulate GM-CSF-induced MHC class II expression correlates with the capacity to up-regulate GM-CSF receptor, but not the interferon-gamma (IFN-gamma) receptor, in a highly dose-dependent manner on monocytes. Although dexamethasone induces GM-CSF receptor expression, it does not confer responsiveness to IL-5, a cytokine that shares a common chain of its heterodimeric cytokine receptor signalling molecule with IL-3 and GM-CSF. Three other steroid hormones, beta-oestradiol, vitamin D3 and dehydroepiandosterone (DHEA), were also tested for their capacity to up-regulate MHC class II expression. All three mediators failed to enhance MHC class II expression or GM-CSF receptor expression on the surface of human monocytes. These experiments suggest that dexamethasone may act to up-regulate GM-CSF-induced MHC class II antigen expression on monocytes by up-regulating cytokine receptor expression.