OBJECTIVES:Vitamin D status influences the risk to develop autoimmune diseases affecting the percentage and/or functions of regulatory T cells (Tregs). Since low levels of 25 (OH) D have been decreased in patients with systemic sclerosis (SSc), we aimed to study the effect of Vitamin D3 (cholecalciferol) supplementation on Tregs frequencies and functions.METHODS:Peripheral blood and sera samples were obtained from 45 SSc patients and controls (HC). A number of eighteen SSc patients had consumed Cholecalciferol (orally) at the dose of 25.000 UI/month for 6 months at the time of enrollment. 25(OH)D serum levels were measured and VDR polymorphisms, were genotyped by polymerase chain reaction (PCR). Tregs isolated from peripheral blood mononuclear cells were in vitro expanded and a suppression assay was performed. Flow cytometry analysis was then carried out. Finally, IL-10 production was assayed by ELISA.RESULTS:Low serum levels of 25(OH)D were detected in SSc patients. The percentage of Tregs in SSc patients was similar to controls, but, among SSc patients, it was higher in those patients taking cholecalciferol. Tregs capability to suppress T cell proliferation was impaired in SSc patients and not restored after in vitro pre-treatment with the active form of Vitamin D (1,25(OH)2D3); but at the same time the production of IL-10 was increased in treated samples obtained from patients. The lack of response of Tregs from SSc patients to 1,25(OH)2D3 treatment in vitro was not due to altered Vitamin D/VDR signalling.CONCLUSIONS:Altogether, our results indicate that the increased production of IL-10 by 1,25(OH)2D3 -treated Tregs could provide a "suppressive" cytokine milieu able to modulate immune response but it is not sufficient to restore the immune suppressive functions of Tregs.