Background and objectives CCR6+ T helper (Th) cells, including Th17 and Th17.1 cells, are thought to play an important role in the pathogenesis of autoimmune diseases such as rheumatoid arthritis (RA). They produce cytokines like IL-17A and TNFα and activate synovial fibroblasts to induce a pro-inflammatory feedback loop. This interaction may underlie the chronic joint inflammation in RA and therefore CCR6+ Th cells are an interesting therapeutic target in this disease. Vitamin D is known to have suppressive effects on autoimmune diseases and its active metabolite directly inhibits the pathogenicity of CCR6+ Th cells. Here we further investigated the effect of 1,25(OH)2D3 on CCR6+ Th cells to understand how 1,25(OH)2D3 suppresses the inflammatory responses in autoimmune diseases.
Materials and methods We cultured CCR6+ Th cells from treatment-naïve early RA patients with or without 1,25(OH)2D3 and generated gene-expression profiles. These profiles were validated using RT-PCR, ELISA and flow cytometry. Functional effects were evaluated via co-culture with RA synovial fibroblasts (RASF) and Boyden chamber-based migration assays.
Results The gene-expression profiles confirmed that 1,25(OH)2D3 downregulated pro-inflammatory cytokines like IL-17A, IL-17F and IL-22, but also genes important for the pathogenic Th17 phenotype like RORC and IL23R. In contrast, 1,25(OH)2D3 induced the anti-inflammatory cytokine IL-10, but not the classical Treg transcription factor FoxP3. Instead, upregulation of genes like LAG3, c-MAF, CTLA4, and IRF8 suggests a Tr1-like phenotype.
Because of the decreased pathogenicity of these cells, we next investigated whether they migrate towards the site of inflammation. Indeed, the gene-expression profiles indicate a shift in the chemokine receptor profile upon treatment with 1,25(OH)2D3. This altered profile was accompanied by increased migration towards an inflammatory environment as modelled by the CCR6+ Th – RASF co-culture. Interestingly, CCR6+ Th cells that were pre-treated with 1,25(OH)2D3 were less capable of inducing the pro-inflammatory loop upon interaction with RASF.
Conclusions 1,25(OH)2D3 inhibits the pathogenic Th17 phenotype in CCR6+ Th cells, while inducing a regulatory Tr1-like phenotype. These cells will then migrate towards the site of inflammation, where they are less potent activators of RASF. This effect of 1,25(OH)2D3 on CCR6+ Th cells may underlie the suppression of RA by vitamin D.