1,25-DIHYDROXYVITAMIN-D3 EXERTS OPPOSING EFFECTS TO IL-4 ON MHC CLASS-II ANTIGEN EXPRESSION, ACCESSORY ACTIVITY, AND PHAGOCYTOSIS OF HUMAN MONOCYTES

To investigate the differentiation and activation of monocytes, the co mbined effects of 1,25-dihydroxyvitamin D3 (D3) and IL-4 on human bloo d monocytes were examined with respect to expression of MHC class-II a ntigens, accessory activity, and phagocytic capacity. IL-4 was reporte d to upregulate the expression of MHC class-II antigens and accessory activity of monocytes. The experiments described here demonstrate that D3 inhibits the expression of all three subtypes of MHC class-II anti gens (HLA-DR, -DP and -DQ) as well as the accessory activity of monocy tes, both in a dose- and time-dependent manner. However, D3 enhances t he immunoglobulin- and complement-dependent phagocytosis by monocytes in a dose- and time-dependent manner. When monocytes are treated with both IL-4 and D3, the effects of D3 are reverted by IL-4, suggesting t hat IL-4 induces the development of monocytes into accessory cells, wh ereas D3 stimulates differentiation of monocytes into classical macrop hages. These findings provide further evidence for the contention that , depending on defined stimuli, monocytes may develop either into acce ssory cells or into classical macrophages.