Objective. Extracellular high mobility group box chromosomal protein 1 (HMGB-1) is a recently identified, endogenous, potent tumor necrosis factor-and interleukin-1 (IL-1)-inducing protein detectable in inflamed synovia in both human and experimental disease. In the present study, we examined clinical effects in collagen-induced arthritis (CIA) using therapeutic administration of neutralizing HMGB-1 antibodies or truncated HMGB-1-derived A-box protein, a specific, competitive antagonist of HMGB-1.Methods. CIA was induced in DBA/1j mice or dark agouti rats, and animals were examined daily for signs of arthritis. Treatment with polyclonal anti-HMGB-1 antibodies or the A-box protein was initiated at the onset of disease and was administered intraperitoneally twice daily for 7 days. Animals were killed 8 days after initiation of therapy, and immunohistochemical analysis of synovial tissue specimens was performed.Results. Systemic administration of anti-HMGB-1 antibodies or A-box protein significantly reduced the mean arthritis score, the disease-induced weight loss, and the histologic severity of arthritis.Beneficial effects were observed in both mice and rats. Immunohistochemical analysis revealed pronounced synovial IL-1␤ expression and articular cartilage destruction in vehicle-treated mice. Both these features were significantly less manifested in animals treated with anti-HMGB-1 antibodies or A-box protein.Conclusion. Counteracting extracellular HMGB-1 with either neutralizing antibodies or a specific HMGB-1 antagonist may offer a new method for the successful treatment of arthritis. Inflammation and tissue destruction were suppressed in CIA after HMGB-1 blockade.

Objective. High mobility group box chromosomal protein 1 (HMGB-1) is a ubiquitous chromatin component expressed in nucleated mammalian cells. It has recently and unexpectedly been demonstrated that stimulated live mononuclear phagocytes secrete HMGB-1, which then acts as a potent factor that causes inflammation and protease activation. Macrophages play pivotal roles in the pathogenesis of arthritis. The aim of this study was to determine whether synovial macrophage expression of HMGB-1 is altered in human and experimental synovitis.Methods. Intraarticular tissue specimens were obtained from healthy Lewis rats, Lewis rats with Mycobacterium tuberculosis-induced adjuvant arthritis, and from patients with rheumatoid arthritis (RA). Specimens were immunohistochemically stained for cellular HMGB-1. Extracellular HMGB-1 levels were assessed in synovial fluid samples from RA patients by Western blotting.Results. Immunostaining of specimens from normal rats showed that HMGB-1 was primarily confined to the nucleus of synoviocytes and chondrocytes, with occasional cytoplasmic staining and no extracellular matrix deposition. In contrast, inflammatory synovial tissue from rats with experimental arthritis as well as from humans with RA showed a distinctly different HMGB-1 staining pattern. Nuclear HMGB-1 expression was accompanied by a cytoplasmic staining in many mononuclear cells, with a macrophage-like appearance and an extracellular matrix deposition. Analysis of synovial fluid samples from RA patients further confirmed the extracellular presence of HMGB-1; 14 of 15 samples had HMGB-1 concentrations of 1.8-10.4 g/ml.Conclusion. The proinflammatory mediator HMGB-1 was abundantly expressed as a nuclear, cytoplasmic, and extracellular component in synovial tissues from RA patients and from rats with experimental arthritis. These findings suggest a pathogenetic role for HMGB-1 in synovitis and indicate a new potential therapeutic target molecule.High mobility group box chromosomal protein 1 (HMGB-1; previously called high mobility group 1 [HMG-1] or amphoterin) is an intranuclear factor that facilitates protein interactions with chromatin (1). Hmgb1 knockout mice die shortly after birth because of hypoglycemia secondary to insufficient glucocorticoid receptor expression, which is under HMGB-1-mediated transcriptional control (2). HMGB-1 is ubiquitously present in the nucleus of almost all mammalian cells and is highly conserved between species (3). Beyond this intranuclear role, it has recently been discovered that HMGB-1 is secreted by certain cells, including activated monocytes and macrophages, and plays important roles in inflammation and tumor metastasis (4,5). The molecule is a late mediator of endotoxin lethality in mice and