Immune cells express the vitamin (vit) D receptor, and vit D is a potent immune-modulator. A negative correlation between serum vit D levels and rheumatoid arthritis (RA) disease activity has been reported. Therefore, we aimed to investigate if the sufficient serum vit D level is helpful to control disease activity in RA patients treated with interleukin (IL)-6 receptor antibody tocilizumab.
RA patients taking tocilizumab were enrolled, and data were collected retrospectively. Disease activity scores (DAS) 28, serum vit D levels, modified Sharp scores of hand X-ray at the time of tocilizumab initiation, and follow-up data were analysed. Peripheral blood mononuclear cells were differentiated into T-helper (Th) 17 or osteoclasts in the presence of various concentrations of tocilizumab and/or 1,25(OH)₂D. Th17 proportions were analysed by fluorescence-activated cell sorting. Supernatant cytokine levels were determined by enzyme-linked immunosorbent assay.
Among 98 RA patients taking tocilizumab, 34 (34.7%) had sufficient serum 25(OH)D levels (≥ 30 ng/mL) when tocilizumab was initiated. At 24 weeks, vit D sufficient patients had greater DAS28 reduction (64.6% ± 15.5% vs. 52.7% ± 20.7%, 
RA patients treated with IL-6 antibody show a better response when they have sufficient serum vit D. Tocilizumab and 1,25(OH)₂D synergistically suppress IL-17 production and osteoclast differentiation in RA patients.