Analysis of the Seasonal Fluctuation of γδ T Cells and Its Potential Relation with Vitamin D(3)

In addition to its role in bone metabolism, vitamin D(3) exerts immunomodulatory effects and has been proposed to contribute to seasonal variation of immune cells. This might be linked to higher vitamin D(3) levels in summer than in winter due to differential sun exposure. γδ T cells comprise a numerically small subset of T cells in the blood, which contribute to anti-infective and antitumor immunity. We studied the seasonal fluctuation of γδ T cells, the possible influence of vitamin D(3), and the effect of the active metabolite 1α,25(OH)(2)D(3) on the in vitro activation of human γδ T cells. In a retrospective analysis with 2625 samples of random blood donors, we observed higher proportions of γδ T cells in winter when compared with summer. In a prospective study over one year with a small cohort of healthy adults who did or did not take oral vitamin D(3) supplementation, higher proportions of γδ T cells were present in donors without oral vitamin D(3) uptake, particularly in spring. However, γδ T cell frequency in blood did not directly correlate with serum levels of 25(OH)D(3). The active metabolite 1α,25(OH)(2)D(3) inhibited the in vitro activation of γδ T cells at the level of proliferation, cytotoxicity, and interferon-γ production. Our study reveals novel insights into the seasonal fluctuation of γδ T cells and the immunomodulatory effects of vitamin D(3).