The 1α,25-dihydroxyvitamin D3 modulates T cell activation and immune checkpoint pathways in human T cells

Background Vitamin D is a pleiotropic steroid hormone with immunomodulatory properties. The role of the active metabolite, 1α,25-dihydroxyvitamin D 3 (1,25(OH) 2 vitamin D), in regulating human T-cell effector function and immune checkpoint (IC) pathways remains poorly defined. Methods Peripheral blood mononuclear cells (PBMCs) from healthy donors were cultured for five days in presence or absence of 1,25(OH) 2 vitamin D (10 or 100 nM). IFN-γ production was assessed by ELISpot assay. Baseline plasma 1,25(OH) 2 vitamin D levels were measured. IC-related gene expression was evaluated by RT-qPCR. Flow cytometry was used to assess the expression of IC, degranulation, and activation markers. FluoroSpot assays were performed to quantify interleukin secretion. Results 1,25(OH) 2 vitamin D reduced IFN-γ expression and secretion independently of baseline circulating 1,25(OH) 2 vitamin D levels. Dose-specific modulation of IC genes was detected, with increased PDCD1 and CTLA4 mRNA expression. Flow cytometry showed selective modulation within the CD8 + T-cell compartment, with increased PD-1 expression at 10 nM and increased CTLA-4 expression at 100 nM, together with decreased degranulation and activation markers. No significant changes were detected for TIM-3 or TIGIT. In CD4 + T cells, no significant modulation of IC molecules was observed, whereas IL-22 secretion was reduced. Conclusions These findings support an immunomodulatory effect of 1,25(OH) 2 vitamin D on human CD8 + T cells, with attenuation of effector-associated responses and modulation of checkpoint-associated pathways in vitro . Our results suggest that vitamin D may contribute to the regulation of inflammatory T-cell responses under controlled culture conditions. The functional and therapeutic implications of these observations require further investigation.