The role of monocytes in the suppression of PHA-induced proliferation and IL 2 production of human mononuclear cells by 1,25-dihydroxyvitamin D3.

1,25-dihydroxyvitamin D3 [1,25(OH)2D3] inhibited phytohemagglutinin (PHA)-induced proliferation of human blood mononuclear cells (MNC) at concentrations of 10(-11) M or more. Interleukin 2 (IL 2) production of T cells activated with PHA was also inhibited by 1,25(OH)2D3. Furthermore, 1,25(OH)2D3 suppressed interleukin 1 (IL 1) production of monocytes (Mo), and the agent-treated Mo were unable to promote IL 2 production of non-adherent cells (NAC). Thus, the reduction of proliferative response of MNC to PHA by 1,25(OH)2D3 appeared to have resulted from the inhibitory effects of the agent on both IL 2 and IL 1 production. From these data, 1,25(OH)2D3 appears to play an important role in the immunoregulatory system.