1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] and related analogs have been shown to exert immunoinhibitory effects on activated lymphocytes in vitro. However, the effects of the hormone on the mammalian immune response in vivo have not been well studied. To examine the possible immunoactions of 1,25-(OH)2D3 in vivo, we employed a murine model of experimental autoimmune encephalomyelitis (EAE). In this model, T helper lymphocyte clones developed from lines of lymphocytes reactive to myelin basic protein (MBP) confer MBP immunoreactivity and demyelinating central nervous system disease on syngeneic, naive recipients of the T cell clone. Similar to peripheral blood mononuclear cells incubated with mitogen, the T cell clone evaluated in this study expressed a high-affinity specific receptor for 1,25-(OH)2D3 (VDR; K(in) = 0.03 nM) upon exposure to MBP. The MBP-stimulated clone elicited a ninefold enhancement of the local delayed hypersensitivity (DTH) response when as few as 0.5 x 10(5) cells of the T cell clone were injected into the foot pad of recipient mice. The DTH response in the recipient was completely blocked when the clone was preincubated with greater than or equal to 10(-8) M 1,25-(OH)2D3 before transfer; the half-maximal inhibitory concentration of hormone (EC50) was 5 x 10(-9) M. These data indicate that exposure of antigen-reactive T helper lymphocytes to a VDR saturating concentration of 1,25-(OH)2D3 can dramatically lessen the expression of immunoreactivity in vivo.