Background Previously, T helper (Th) cell subsets have been regarded as irreversibly differentiated endpoints. However, evidence suggests that Th cell differentiation is a plastic process in response to certain (e.g. inflammatory) conditions. Mechanisms leading to the pathogenetic important predominance of Th17 cells in rheumatoid arthritis (RA) are not yet understood.

Objectives Therefore, we investigated plasticity and underlying molecular mechanisms to address the question if an altered T cell plasticity contributes to the shift towards the Th17 phenotype in RA.

Methods A unique cohort of 40 patients with early, active and untreated RA (to exclude effects of immunosuppressive drugs on T cells) and 41 age- and sex- matched healthy controls (HC) were studied. Viable in vivo-originated Th1, Th2 and Th17 cells were FACS-sorted and trans-differentiated under Th1-, Th2- or Th17-inducing conditions. The cytokine Th profile of the trans-differentiated cells was assessed by flow cytometry. Epigenetic modifications including histone modifications and DNA methylation of Th cell-associated cytokine and transcription factor gene loci were analyzed by ChiP assay and bisulfite sequencing. Relative expression of cytokine and transcription factors was measured by ELISA and qRT-PCR.

Results Th17 cells from RA patients cultured under Th1 and Th2 conditions showed a strikingly diminished trans-differentiation capacity into both Th1 and Th2 phenotypes and retained their IL-17 expression to a significantly higher degree compared to Th17 cells from HC. Vice versa, RA Th1 and Th2 cells demonstrated an enhanced capacity to re-differentiate into Th17 cells. We found higher RORC expression in RA Th1 cells that were trans-differentiated under Th17 conditions as a basis for increased re-differentiation of Th1 to Th17 cells in RA. With regard to epigenetic regulation, an overrepresentation of the permissive histone modification H3 actetylation over the repressive H3K27 methylation was found in the IL17 and RORC loci of T cells from RA patients. Moreover, DNA methylation was decreased in RA Th17 cells compared to HC enabling higher IL-17 expression.

Conclusions Our data indicate that in vivo-originated Th17 cells in RA are resistant to changes in their phenotype, whereas other Th subsets are prone to Th17 cell trans-differentiation. Increased RORC expression, permissive histone modifications and a less DNA methylation at Th17 genes facilitating IL-17 expression might contribute to the altered Th plasticity in RA, thereby contributing to the pathogenic Th17 shift.

Disclosure of Interest None declared