Background/Purpose: The active vitamin D metabolite 1,25(OH)₂D₃ suppresses various experimental autoimmune diseases. In addition, serum vitamin D levels, vitamin D intake and polymorphisms in the vitamin D receptor (VDR) are associated with disease incidence and severity in human autoimmune diseases such as rheumatoid arthritis (RA). However, the mechanism behind these immunosuppressive effects is currently unknown. Previously we have demonstrated that 1,25(OH)₂D₃ directly inhibits pathogenicity of CCR6⁺ T helper (Th) cells. These cells, which include 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. Therefore CCR6⁺ Th cells are an interesting therapeutic target in this disease. 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.

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: Gene-expression profiles from CCR6⁺ Th cells confirmed that 1,25(OH)₂D₃ treatment downregulated pro-inflammatory cytokines like IL-17A, IL-17F and IL-22, but also genes important for the pathogenic Th17 phenotype like RORC and IL-23R. 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 the induction of a Tr1-like phenotype. Interestingly, these CCR6⁺ Th cells cultured with 1,25(OH)₂D₃ were less capable of inducing the pro-inflammatory loop upon interaction with RASF. This suggests that the modulated CCR6⁺ Th cells could contribute to regulating synovial inflammation. However, for this they need to be capable of migration towards inflammatory sites. Therefore we investigated the migration of CCR6⁺ Th cells treated with 1,25(OH)₂D₃ towards synovial inflammation as modeled by the CCR6⁺ Th – RASF co-culture. Indeed, the 1,25(OH)₂D₃-treated CCR6⁺ Th cells migrate faster towards the site of inflammation than untreated cells.

Conclusion: 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.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/vitamin-d-induces-a-type-1-regulatory-t-cell-tr1-like-phenotype-in-human-c-c-chemokine-receptor-type-6-ccr6-th-cells-and-promotes-their-migration-to-an-inflammatory-environment/