AbstractObjectiveCD8+ T cells are part of the T cell pool infiltrating the synovium in rheumatoid arthritis (RA). However, their role in the pathogenesis of RA has not been fully delineated. Using the K/BxN mouse model of spontaneous chronic arthritis, which shares many similarities with RA, we studied the potential of CD8+ T cell depletion with monoclonal antibodies (mAb) to stop and reverse the progression of experimental arthritis.MethodsCD8+ T cells from the blood and articular infiltrate of K/BxN mice were characterized for cell surface phenotypic markers and for cytokine production. Additionally, mice were treated with specific anti‐CD8 mAb (YTS105 and YTS169.4), with and without thymectomy.ResultsCD8+ T cells from the peripheral blood and joints of K/BxN mice were mainly CD69+ and CD62L−CD27+ T cells expressing proinflammatory cytokines (interferon‐γ [IFNγ], tumor necrosis factor α [TNFα], interleukin‐17a [IL‐17A], and IL‐4), and granzyme B. In mice receiving anti‐CD8 mAb, the arthritis score improved 5 days after treatment. Recovery of the CD8+ T cells was associated with a new increase in the arthritis score after 20 days. In thymectomized and anti‐CD8 mAb–treated mice, the arthritis score improved permanently. Histologic analysis showed an absence of inflammatory infiltrate in the anti‐CD8 mAb–treated mice. In anti‐CD8 mAb–treated mice, the serologic levels of TNFα, IFNγ, IL‐6, and IL‐5 normalized. The levels of the disease‐related anti–glucose‐6‐phosphate isomerase antibodies did not change.ConclusionThese results indicate that synovial activated effector CD8+ T cells locally synthesize proinflammatory cytokines (IFNγ, TNFα, IL‐17, IL‐6) and granzyme B in the arthritic joint, thus playing a pivotal role in maintaining chronic synovitis in the K/BxN mouse model of arthritis.