First, we confirmed the high frequency of hypovitaminosis D in SLE patients. Recent studies have highlighted vitamin D insufficiency in SLE patients, with approximately 65% of the patients showing serum 25(OH)D levels < 30 ng/mL [
19‐
21]. In the present study, we aimed to reach the recommended serum 25(OH)D ranges for bone metabolism, that is, 30 to 80 ng/mL, within the first 2 months. The schedule of vitamin D administration that we choose included a first phase of intensive supplementation for one month, followed by a maintenance phase. Regarding the evolution of serum levels of 25(OH)D, calcium and phosphorous and the occurrence of clinical events, this regimen was effective and safe. We included only SLE patients with low disease activity (SLEDAI ≤ 8) because we wanted to determine the specific effects of vitamin D in the absence of initiation or modification of associated therapy, such as an increase of the prednisone dosage or the use of immunosuppressive agents.
We demonstrated that vitamin D supplementation induced a beneficial effect
in vivo on the perturbations of B cell and T cell homeostasis associated with SLE, by increasing Tregs and decreasing Th17 and Th1 cells, and memory B cells. Interestingly, the increase of Tregs concerned both resting Tregs and activated memory Tregs according to CD45RA and CD25 expression [
22]. Our findings are supported by recently reported
in vitro studies. 1,25(OH)
2 vitamin D3, the active form of vitamin D, was shown to exert a marked inhibitory effect on adaptive immune cells, by inhibiting the T cell proliferation [
23,
24], the expression of IL-2 [
24,
25] and IFN-γ mRNA and protein in T cells [
9,
26,
27], while promoting Th2-cell responses
in vitro [
28]. In our
in vivo study, we did not observe a significant increase of Th2 cells under vitamin D supplementation, since this T cell subset remains stable during follow-up. 1,25(OH)
2 vitamin D3 was also shown to inhibit Th17 responses, probably owing to its capacity to inhibit IL-23 production [
9,
29], and to induce the differentiation and/or expansion of FoxP3
+ Tregs and an increased expression of CTLA4 [
9,
30‐
32]. Finally, we observed a decrease of IgD
-CD27
+ and IgD
-CD27
- memory B cells and a decrease of anti-dsDNA autoantibody levels in vitamin D-treated patients. Consistent with these findings, 1,25(OH)
2 vitamin D3 was shown to decrease B cell proliferation, plasma-cell differentiation and IgG secretion
in vitro [
12,
23]. The mechanisms by which 1,25(OH)
2 vitamin D3 exerts its immunomodulatory effect on B cells remains unclear.
Although too preliminary to be presented in the present study, we have performed a transcriptome analysis of PBMCs at D0 versus M2 after vitamin D supplementation on ten randomly selected patients, in order to provide additional insights into the immunological effects of vitamin D in SLE (Additional file
1). Using independent component analysis (ICA) [
33] and gene set enrichment analysis (GSEA) dataset [
34,
35], we identified 48 molecular signatures that were differentially expressed between D0 and M2, with 34 up- and 14 down-regulated signatures (see Figure S1 inAdditional file
1. This preliminary finding suggests an effect of vitamin D supplementation on the immune system. Among the identified signatures, we observed the down-regulation of RNA polymerase functions and histone expression and the up-regulation of the TP53/CDKN1A-related pathway that represent interesting pathways to explore in the future, owing to their possible involvement with a decrease in the accumulation of autoantigens and the activation and proliferation of autoreactive T and B lymphocytes. Also, the up-regulation of the TP53/CDKN1A-related pathway is interesting because CDKN1A is a potent cyclin-dependent kinase inhibitor that functions as a regulator of cell cycle progression [
36].