CD4+ T cells mediate rheumatoid arthritis (RA) pathogenesis through both antibody-dependent and independent mechanisms. It remains unclear how synovial microenvironment impinges on CD4+ T cells pathogenic functions. Here, we identified a TLR4+ follicular helper T (Tfh) cell-like population present in the blood and expanded in synovial fluid. TLR4+ T cells possess a two-pronged pathogenic activity whereby direct TLR4+ engagement by endogenous ligands in the arthritic joint reprograms them from an IL-21 response, known to sponsor antibody production towards an IL-17 inflammatory program recognized to fuel tissue damage. Ex vivo, synovial fluid TLR4+ T cells produced IL-17, but not IL-21. Blocking TLR4 signaling with a specific inhibitor impaired IL-17 production in response to synovial fluid recognition. Mechanistically, we unveiled that T-cell HLA-DR regulates their TLR4 expression. TLR4+ T cells appear to uniquely reconcile an ability to promote systemic antibody production with a local synovial driven tissue damage program.

CD4+ T cells mediate rheumatoid arthritis (RA) pathogenesis through both antibody-dependent and independent mechanisms. It remains unclear how synovial microenvironment impinges on CD4+ T cells pathogenic functions. Here, we identified a TLR4+ follicular helper T (Tfh) cell-like population present in the blood and expanded in synovial fluid. Mechanistically, we unveiled that homotypic T-T cell interactions through non-cognate HLA-DR:TCR contacts regulate TLR4 expression on T cells. TLR4+ T cells possess a two-pronged pathogenic activity. Upon TCR and ICOS engagement, TLR4+ T cells produce IL-21, a cytokine known to sponsor antibody production. However, direct TLR4+ engagement on T cells, by endogenous ligands in the arthritic joint, reprograms them towards an IL-17 inflammatory profile compatible with tissue damage program. Blocking TLR4 signaling with a specific inhibitor impaired IL-17 production in response to synovial fluid recognition. Ex vivo, synovial fluid TLR4+ T cells produced IL-17, but not IL-21. TLR4+ T cells appear to uniquely reconcile an ability to promote systemic antibody production with a local synovial driven tissue damage program. TLR4+ T cells could constitute an attractive cellular target and predictive biomarker for erosive arthritis.

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