Natural killer cells in the synovial fluid of rheumatoid arthritis patients exhibit a CD56bright,CD94bright,CD158negative phenotype.

Natural killer (NK) cells play an important role in several animal models of autoimmunity by modulating T-cell responses, but it is unclear whether human NK cells have similar functions. We characterized the phenotype of NK cells in synovial fluid (SF) and peripheral blood (PB) of patients with rheumatoid arthritis (RA) and in healthy control subjects using flow cytometry and quantitative PCR. The proportions of NK cells in PB and SF of RA patients were not significantly different from those in healthy PB. However, the SF NK cell phenotype was strikingly different, with increased CD94 and CD56 densities and greatly reduced proportions of cells expressing CD158a/b. These cells also had reduced mRNAs coding for CD158a/b and low perforin levels compared with RA PB and healthy PB NK cells. We identified a novel phenotype of SF NK cells that is of potential significance in RA. Experiments are now under way to determine the function of these SF NK cells and their potential role in RA.