Characterization of plasmacytoid dendritic cells in inflammatory arthritis synovial fluid

Objective. To examine the phenotype of dendritic cell subsets in synovial fluid and peripheral blood from patients with rheumatoid arthritis (RA) or spondyloarthropathy (SpA). Methods. Multiparameter flow cytometry was used to identify and characterize dendritic cells in mononuclear cell populations isolated from synovial fluid and peripheral blood. Results. Synovial fluid contained two subsets of dendritic cells (DC), myeloid and plasmacytoid. These subsets could also be identified in peripheral blood, but there were lower numbers of DC in peripheral blood compared with synovial fluid. Plasmacytoid DC were distinguished from the myeloid subset by high expression of CD123 and lack of expression of CD11c. In comparison with myeloid dendritic cells, the plasmacytoid subset were less mature, similar to those in peripheral blood. They failed to express CD83 and DC-LAMP, and had relatively low levels of CD40 and CD86. Comparison of dendritic cells in synovial fluid from RA and SpA patients showed increased numbers of the plasmacytoid subset in SpA. Conclusions. This is the first demonstration of the plasmacytoid subset of dendritic cells in synovial fluid. Since these cells are major producers of type I interferons, their increased numbers in SpA might be relevant to pathogenesis, but the immature phenotype in SpA synovial fluid may also indicate that conditions for maturation of this subset do not pertain in SpA synovium.