High mobility group box chromosomal protein 1 (HMGB‐1), a nuclear DNA binding protein, was recently rediscovered as a new proinflammatory cytokine. The purpose of this study was to demonstrate HMGB‐1 expression in vivo and to identify the role of HMGB‐1 in the pathogenesis of rheumatoid arthritis (RA).

HMGB‐1 concentrations in synovial fluid (SF) and serum from RA and osteoarthritis (OA) patients were measured by immunoblot analysis. The protein's specific receptor, receptor for advanced glycation end products (RAGE), was examined in SF macrophages (SFMs). We measured levels of proinflammatory cytokines released by SFMs treated with HMGB‐1 via enzyme‐linked immunosorbent assay and used soluble RAGE (sRAGE) to block the release of tumor necrosis factor α (TNFα). Immunohistochemical analysis and immunofluorescence assay were employed to examine localization of HMGB‐1 in RA synovium and its translocation in SFMs after TNFα stimulation.

HMGB‐1 concentrations were significantly higher in SF of RA patients than in that of OA patients. SFMs expressed RAGE and released TNFα, interleukin‐1β (IL‐1β), and IL‐6 upon stimulation with HMGB‐1. HMGB‐1 was found in CD68‐positive cells of RA synovium, and TNFα stimulation translocated HMGB‐1 from the nucleus to the cytosol in SFMs. Blockade by sRAGE inhibited the release of TNFα from SFMs.

HMGB‐1 was more strongly expressed in SF of RA patients than in that of OA patients, inducing the release of proinflammatory cytokines from SFMs. HMGB‐1 plays a pivotal role in the pathogenesis of RA and may be an original target of therapy as a novel cytokine.