T cell costimulation by fractalkine‐expressing synoviocytes in rheumatoid arthritis

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Abstract
Objective Patients with rheumatoid arthritis (RA) accumulate prematurely aged T cells that have acquired a new profile of regulatory receptors. Many of the de novo–expressed receptors are typically found on natural killer cells, including CX3CR1, the receptor for the chemokine fractalkine (FKN). This study explored whether interactions between CX3CR1 and FKN are relevant for T cell functions in rheumatoid synovitis. Methods FKN expression was examined by real‐time polymerase chain reaction and immunohistochemistry. CX3CR1 expression on peripheral blood T cells was analyzed by flow cytometry. T cell activation was quantified by determining proliferative responses, interferon‐γ (IFNγ) secretion, and granule release. Fibroblast‐like synoviocyte (FLS)/T cell adhesion was measured by the retention of 5‐carboxyfluorescein diacetate succinimidyl ester–labeled T cells on FLS monolayers. Results FKN was expressed on cultured synovial fibroblasts and hyperplastic synoviocytes in the rheumatoid tissue. Among CD4+ T cells, only senescent CD28− T cells were positive for CX3CR1 (P < 0.001). Such CD4+,CD28−,CX3CR1+ T cells strongly adhered to FLS, with soluble FKN blocking the interaction. FKN expressed on FLS costimulated T cell–activating signals and amplified proliferation, IFNγ production, and expulsion of cytoplasmic granules. Conclusion Senescent CD4+ T cells that accumulate in rheumatoid arthritis aberrantly express CX3CR1. FKN, which is membrane‐anchored on synoviocytes, enhances CD4+ T cell adhesion, provides survival signals, and costimulates the production of proinflammatory cytokines as well as the release of granules. By virtue of their altered receptor profile, senescent CD4+ T cells receive strong stimulatory signals from nonprofessional antigen‐presenting cells in the synovial microenvironment.