1α,25-dihydroxyvitamin D3 acts via transforming growth factor-β to up-regulate expression of immunosuppressive CD73 on human CD4⁺ Foxp3^- T cells

Vitamin D deficiency is associated with increased incidence and severity of various immune-mediated diseases. Active vitamin D (1α,25-dihydroxyvitamin D3; 1,25(OH)₂D3) up-regulates CD4⁺ T-cell expression of the purine ectonucleotidase CD39, a molecule that is associated with the generation of anti-inflammatory adenosine. Here we aimed to investigate the direct impact of 1,25(OH)₂D3 on expression of the downstream ecto-5′-nucleotidase CD73 by human CD4 T cells, and components of the transforming growth factor-β (TGF-β) pathway, which have been implicated in the modulation of CD73 by murine T cells. At 10^-8 to 10^-7 m, 1,25(OH)₂D3 significantly increased expression of CD73 on peripheral human CD4⁺ T cells. Although 1,25(OH)₂D3 did not affect the mRNA expression of latent TGF-β₁, 1,25(OH)₂D3 did up-regulate expression of TGF-β-associated molecules [latency-associated peptide (LAP), glycophorin A repetitions predominant (GARP), GP96, neuropilin-1, thrombospondin-1 and α_v integrin] which is likely to have contributed to the observed enhancement in TGF-β bioactivity. CD73 was highly co-expressed with LAP and GARP following 1,25(OH)₂D3 treatment, but unexpectedly, each of these cell surface molecules was expressed primarily on CD4⁺ Foxp3^- T cells, rather than CD4⁺ Foxp3⁺ T cells. Notably, neutralization of TGF-β significantly impaired 1,25(OH)₂D3-mediated induction of CD73. Collectively, we show that 1,25(OH)₂D3 enhances expression of CD73 on CD4⁺ Foxp3^- T cells in a process that is at least partially TGF-β-dependent. These data reveal an additional contributing mechanism by which vitamin D may be protective in immune-mediated disease.