1 Alpha,25-dihydroxyvitamin D3 receptor distribution and effects in subpopulations of normal human T lymphocytes

Receptors for 1 alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] are expressed upon activation of human lymphocytes, and the hormone inhibits in vitro the proliferation of mitogen-activated lymphocytes. In this study we examined the distribution of the 1,25-(OH)2D3 receptor protein in the two major subsets of T lymphocytes (T helper and T suppressor cells) and the effect of the hormone on their respective rates of proliferation. We activated normal lymphocytes in the presence of monocytes with phytohemagglutinin and subsequently isolated the T helper (T4-positive) and the T suppressor (T8-positive) subsets using monoclonal antibodies and complement-mediated lysis. In parallel experiments, we first isolated monocyte-depleted T4 and T8 cells and then activated them using phytohemagglutinin and a phorbol ester. Using either approach we found that both T4 and T8 lymphocytes expressed the 1,25-(OH)2D3 receptor protein upon activation. The concentration of this protein, its affinity for the ligand (Kd, approximately 10(-10) mol/L), and its sedimentation characteristics (S = 3.3) were indistinguishable in the two T cell subsets. Furthermore, the time kinetics of expression of the receptor after activation were very similar in the two subsets. Nevertheless, 1,25-(OH)2D3 inhibited in a dose-dependent fashion the rate of proliferation of the helper subset, but had no effect on the proliferation of suppressor cells. The finding of a dissimilar effect of 1,25-(OH)2D3 on the proliferation of the T helper and T suppressor cells despite their indistinguishable receptor status suggests that the 1,25-(OH)2D3 receptors of the T cells might not be involved in the effects of the hormone on T-cell proliferation, and that the 1,25-(OH)2D3-induced inhibition of mitogen-activated T4 cell proliferation could be mediated indirectly.