1,25-dihydroxyvitamin D3 and agents that increase intracellular adenosine 3',5'-monophosphate synergistically inhibit the mitogenic stimulation of human lymphocytes.

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3], like the immune response modulators prostaglandin E2 (PGE2) and histamine, inhibits mitogen-induced proliferation of human peripheral blood mononuclear cells. 1,25-(OH)2D3 acts synergistically with PGE2 and histamine to inhibit lymphocyte mitogenesis. This is apparent at a wide concentration range of 1,25-(OH)2D3 (3 X 10(-11)-10(-8) mol/L). Cholera toxin, forskolin, and isobutylmethylxanthine, which like PGE2 and histamine increase intracellular concentrations of cAMP, also act synergistically with 1,25-(OH)2D3 in this system. Culture of mitogen-stimulated adherent cell-depleted mononuclear cells with PGE2 increases the number of high affinity binding sites for 1,25-(OH)2D3. This finding may account for the synergistic interaction between the two agents.