Demonstration of a Direct Immunomodulatory Role for Vitamin D in Human T Cells Using Flow Cytometry

Clinical and epidemiological studies have demonstrated an increasingly stronger link between Vitamin D deficiency and a broad array of illnesses characterized by inflammation, including autoimmune diseases, coronary artery disease, and cancers. Vitamin D is a steroid hormone that exerts the majority of its biologic effects via the binding of the intracellular Vitamin D receptor (VDR). While upregulation of VDR has been demonstrated in activated bulk T cells using traditional approaches (e.g., western blotting), such assays cannot precisely define VDR distribution and kinetics. To overcome these limitations, we developed what we believe to be the first flow cytometric assay to quantify VDR expression at a single-cell level. We used a primary antibody against VDR (mouse monoclonal IgG2a to human VDR) in permeabilized T cells followed by a labeled secondary antibody. We detected a positive cell population using flow cytometry that was sharply increased following activation, consistent with upregulation of VDR confirmed by immunoblotting of sorted cells. We then applied this validated assay to define the kinetics of VDR upregulation in activated T cells. We stimulated PBMC with PMA:Ionomycin (P:I) for varying intervals and assessed intracellular VDR using flow cytometry. VDR is significantly upregulated by 15 min after stimulation, reaches a plateau after 6 hr, and may remain elevated for up to 7 d. We compared VDR to classical early and late T cell activation markers (CD69 and CD25, respectively), and we found that VDR was upregulated as consistently as (but even earlier than) CD69, and that VDR and CD25 were both consistently upregulated at later intervals (p<0.0001). To examine the association between VDR expression and proliferation, we stimulated CFSE-labeled T cells with OKT3 (2mg/ml) for 5 d and found that proliferating T cells expressed a significantly higher level of VDR than resting T cells, which maintained baseline VDR expression (p<0.0001). To assess the association between T cell cytokine production and VDR expression, we stimulated T cells with (P:I) for 6 hr in the presence of brefeldin A, and we confirmed that all cytokine-producing cells (TNFα, IL-2, IFNγ) were contained within the VDR-high population. We then assessed whether physiologic concentrations of Vitamin D could inhibit T cell proliferation in vitro. We stimulated CFSE-labeled PBMC with either OKT3 or irradiated allogeneic dendritic cells (DC) in the presence or absence of physiologic concentrations of calcitriol (50 nm) for 5 to 7 d. The presence of calcitriol during OKT3 stimulation resulted in significantly reduced cell division (p=0.004, n=5). Using a previously validated phenotype to demarcate activated alloreactive CD4+ T cells (CD4hiCD38+), we demonstrated that physiologic calcitriol supplementation decreased alloreactive activation following 7 d stimulation with allogeneic DC (p=0.0003, n=10). In conclusion, VDR is a consistent and specific early and late marker of T cell activation, suggesting a direct role for the Vitamin D axis in immunoregulation. Furthermore, physiological concentrations of Vitamin D can inhibit T cell proliferation induced by polyclonal stimuli, including allogeneic DC. These data provide confirmation for a direct immunoregulatory role for Vitamin D and suggest that further mechanistic and clinical studies may yield novel therapeutic strategies for inflammatory conditions, including graft-versus-host disease.