Background: The biologic role of the vitamin D analogue 1,25-dihydroxyvitamin D₃, such as antiinflammatory functions, reduction of cytokine production by T cells, and immunoglobulin production by B cells, has been reported. Such immunomodulatory effects may be potentially useful in dealing with autoimmunity and transplantation. However, whether this hormone has an additive immunosuppressive effect when it is used with corticosteroids has not been investigated, although these agents are commonly used together. Objective: Our purpose was to investigate the additive immunomodulatory effects of 1,25-dihydroxyvitamin D₃ on lymphocyte proliferation and cytokine production when used with corticosteroids. Methods: To investigate the additive effects of 1,25-dihydroxyvitamin D₃ and dexamethasone on suppression of lymphocyte proliferation, normal PBMCs were cultured in anti-CD3 with or without different concentrations of dexamethasone (0-10^-7 mol/L) plus or minus different concentrations of 1,25-dihydroxyvitamin D₃ (0-10^-6 mol/L). After 3 days, lymphocyte proliferation was assessed by [³H]-thymidine incorporation. To investigate the additive effects of 1,25-dihydroxyvitamin D₃ and dexamethasone on cytokine production, PBMCs were cultured for 3 days in the presence of anti-CD3 with or without 10^-6 mol/L dexamethasone plus or minus 10^-6 mol/L 1,25-dihydroxyvitamin D₃. IFN-γ, IL-5, and IL-13 production in supernatants were measured by ELISA. Results: Our study demonstrated that, at concentrations of 10^-8, 10^-7, and 10^-6 mol/L, 1,25-dihydroxyvitamin D₃ significantly decreased lymphocyte proliferation compared with an ethanol control (P < .05). The IC₅₀ for dexamethasone was 4 × 10^-9 mol/L in culture without 1,25-dihydroxyvitamin D_3. When 10^-9 mol/L of 1,25-dihydroxyvitamin D₃ was added to cultures with dexamethasone, IC₅₀ became 2 × 10^-9 mol/L. Moreover, when 10^-6, 10^-7, and 10^-8 mol/L of 1,25-dihydroxyvitamin D₃ were added in culture with dexamethasone, IC₅₀ became less than 1 × 10^-9 mol/L. IFN-γ production in culture with either dexamethasone or 1,25-dihydroxyvitamin D₃ was significantly decreased compared with media or ethanol control (P < .0001). Moreover, when both agents were added in the same culture, IFN-γ production was further decreased compared with either agent alone (P < .05). In contrast, 1,25-dihydroxyvitamin D₃ significantly (P < .0001) increased IL-5 and IL-13, whereas dexamethasone significantly decreased these 2 cytokines (P < .0005). When 1,25-dihydroxyvitamin D₃ was combined with dexamethasone, IL-5 and IL-13 production was increased compared with dexamethasone alone (P < .001). Conclusions: Our results demonstrate that 1,25-dihydroxyvitamin D₃ has significant additive effects on dexamethasone-mediated inhibition of lymphocyte proliferation. This hormone also has additive effects on inhibition of T_H1 cytokine production when combined with dexamethasone. However, this hormone upregulates T_H2 cytokines and inhibits steroid-mediated suppression of cytokines. These findings demonstrate the potential use of 1,25-dihydroxyvitamin D₃ as an immunosuppressive agent when combined with corticosteroids in T_H1, but not T_H2, immune responses. (J Allergy Clin Immunol 2000;106:981-5.)