| Abstract: | Objective B cells impact the perpetuation of chronic inflammatory or autoimmune diseases in multiple ways. A role of B cells as antigen‐presenting cells (APCs) in the pathogenesis of chronic arthritis in humans has been suggested; however, as of yet the presence of such B cells at the site of inflammation has not been demonstrated. This study was undertaken to investigate whether synovial B cells in patients with juvenile idiopathic arthritis (JIA) might display features of APCs. Methods The frequency, phenotype, and immunoglobulin repertoire of synovial B cells were studied by flow cytometry and single‐cell polymerase chain reaction (PCR). Cytokine expression by B cells was analyzed by real‐time PCR, and interaction between B cells and T cells was investigated in a mixed lymphocyte culture. Results CD27+IgD− and CD27−IgD− B cells accumulated in the joints of JIA patients and displayed an activated phenotype. Both B cell subsets expressed hypermutated and class‐switched immunoglobulins, indicators of memory B cells. The accumulating memory B cells expressed the costimulatory molecules CD80/CD86 and showed a higher capacity to activate allogeneic T cells and prime a Th1 phenotype than their peripheral blood counterparts. Conclusion Activated immunoglobulin class–switched CD27− and CD27+ memory B cells, indicating a phenotype of APCs with expression of costimulatory molecules, accumulate in the joints of patients with JIA and might be involved in the amplification of pathogenic T cell activation. These findings provide evidence that B cells play an antibody‐independent immunopathologic role in human chronic inflammatory arthritis of childhood. |
