Immunomodulatory Effects of Vitamin D: Implications for the Prevention and Treatment of Graft Versus Host Disease

Allogeneic bone marrow transplantation is a uniquely curative therapy for a subset of patients with hematologic malignancies. However, morbidity and mortality related to graft versus host disease remain substantial. Dendritic cells (DCs) are highly potent antigen presenting cells that play a crucial role in maintaining the balance of immune activation and tolerance as well as the nature of immune reconstitution post-transplant. Persistence of host DCs in the early post transplant period has been shown to play an important role in the activation and expansion of alloreactive lymphocytes and the concomitant risk of GVHD. A major focus of research to enhance outcomes following allogeneic transplantation involves the manipulation of DC recovery post-transplant to minimize activation of alloreactive lymphocytes while preserving anti-tumor immunity responsible for the graft versus disease effect. Vitamin D is a hormone involved in bone metabolism and calcium homeostasis. More recently, vitamin D has been shown to have potent immunomodulatory effects. We have evaluated the effect of vitamin D on the phenotypic and functional characteristics of DC and T cell populations. We have demonstrated that vitamin D hinders the maturation of DCs such that cells that are differentiated in its presence exhibit decreased expression of costimulatory molecules. Peripheral blood mononuclear cells (PBMCs) were isolated from leukopaks obtained from normal donors by ficoll density centrifugation. DCs were generated by culturing the monocyte enriched adherent fraction with GM-CSF (1000 U/ml) and IL-4 (1000 U/ml) for 5 days, followed by maturation in the presence of TNFa (25 ng/ml) for 48 hours. DCs were generated in the presence and absence of 10nM of 1,25 hydroxyvitamin D. Mean expression of the costimulatory molecule CD80 and the maturation marker CD83 decreased from 60% to 37% and 53% to 27% respectively in the presence of vitamin D (N=3). To assess the effect of vitamin D on the functional potency of DCs as antigen presenting cells, the capacity of mature DCs to stimulate allogeneic T cell proliferation in the presence of vitamin D was determined. Mature DCs were generated from peripheral blood precursors and then cultured with allogeneic T cells isolated by T cell separation column at a ratio of 1:10. After 5 days, cocultures were pulsed with tritiated thymidine overnight and proliferation was determined by measuring the stimulation index (SI) as defined by uptake of tritiated thymidine of cocultured cells/uptake of tritiated thymidine of T cells alone. Results of 10 serial studies demonstrated the addition of vitamin D resulted in a blunted T cell proliferative response, with mean SI that decreased from 13 to 5. Similarly, the addition of vitamin D to a coculture of DCs and autologous T cells resulted in a 50% reduction in the T cell proliferative response to tetanus toxoid, a recall antigen. In addition, T cells stimulated by allogeneic DCs in the presence of vitamin D were polarized to secrete Th2 cytokines. In three experiments, the mean percentage of T cells secreting IL-10 increased from 2.7% to 4.4%, while expression of IFNγ decreased from a mean of 2.6% to 1.9% in the presence of vitamin D. The presence of vitamin D did not induce FOXP3 expressing regulatory T cell populations, which accounted for 8% and 7% of the T cell population following stimulation by allogeneic DCs in the presence and absence of vitamin D respectively. These data suggest that exposure to vitamin D exerts a tolerizing influence on T cells mediated by its impact on antigen presenting cells. Vitamin D may therefore have a role in the prevention and treatment of graft versus host disease. A clinical trial evaluating the use of vitamin D in the early post-transplant period for the prevention of GVHD is planned.