ExTh17 Cells Arising from Th17 Support Chronic RA-Like Inflammation Independent of IL-17

Introduction/Rationale: T helper 17 (Th17) cells are present in both peripheral blood and synovial fluid of patients with rheumatoid arthritis (RA), yet IL-17-targeted therapies have shown limited efficacy in established disease. The role of IL-17 and Th17 cells in RA has not been completely clarified. Although inflammation can drive Th17 cells to transdifferentiate into IL-17-negative Th1- or Tr1-like so-called exTh17 cells, the role of these cells in arthritis remains unclear. Here, we investigate whether exTh17 cells contribute to joint inflammation in autoimmune arthritis.

Methods: Using Th17 lineage tracing in the SKG mouse model of RA, we demonstrate that synovial Th17 cells transdifferentiate into IFNγ- and IL-10-producing exTh17 cells, which become the dominant CD4+ T cell population in chronic arthritis. These exTh17 cells are more arthritogenic than Th17 cells and maintain synovial inflammation through IL-17-independent mechanisms involving elevated IL-6 and TNF, two key cytokines in RA pathogenesis. Synovial exTh17 cells analyzed by bulk, single-cell RNASeq, and Spatial transcriptomics exhibit a distinct gene expression profile, including upregulation of S1PR4, in CD4+ T cell profiles from mouse and human RA synovial tissue. We further show, using co-culture assays, that interaction between Th17 cells and fibroblast-like synoviocytes (FLS) via S1P signaling promotes Th17-to-exTh17 conversion.

Results: The pro-inflammatory environment of arthritis induces a population of exTh17 cells, promoting chronicity of arthritis without re-expression of IL-17. Blocking S1P–S1PR4 signaling inhibits their formation independently of lymph node T cell trafficking. Fibroblast-like synoviocytes (FLS) promote exTh17 generation via S1P production, and RA FLS are major S1P sources in both humans and mice.

Conclusion: These new insights open the door to more targeted therapeutic strategies beyond IL-17 inhibition.