Objective: To examine the potential role of high mobility group box chromosomal protein 1 (HMGB‐1) in the pathogenesis of arthritis.Methods: Mice were injected intraarticularly with 1 μg or 5 μg of HMGB‐1. Joints were dissected on days 4, 7, and 28 after injection and were evaluated histopathologically and immunohistochemically. To investigate the importance of different white blood cell populations for the development of arthritis, in vivo cell depletion procedures were performed. In addition, spleen cells were cultured in the presence of HMGB‐1, and nuclear factor κB (NF‐κB) activation was detected by electrophoretic mobility shift assay.Results: Injection of recombinant HMGB‐1 (rHMGB‐1) into different mouse strains resulted in an overall frequency of arthritis in 80% of the animals. The inflammation was characterized by mild to moderate synovitis and lasted for at least 28 days. The majority of cells found in the inflamed synovium were Mac‐1+ macrophages, whereas only a few CD4+ lymphocytes were detected. Pannus formation was observed in some cases 7 and 28 days after HMGB‐1 injection. No significant differences were found with respect to incidence and severity of arthritis between mice depleted of monocytes, granulocytes, or lacking T/B lymphocytes. However, combined removal of monocytes and neutrophils resulted in a 43% lower incidence of arthritis. Mice rendered deficient in the interleukin‐1 (IL‐1) receptor did not develop inflammation upon challenge with HMGB‐1. In vitro data corroborate this finding, showing that rHMGB‐1 activated NF‐κB, a major pathway leading to IL‐1 production.Conclusion: Our results indicate that HMGB‐1 is not a mere expression of inflammatory responses, but on its own, it triggers joint inflammation by activating macrophages and inducing production of IL‐1 via NF‐κB activation.

The receptor for advanced glycation end products (RAGE) is a member of the immunoglobulin superfamily being expressed as a cell surface molecule and binding a variety of ligands. One of these ligands is high-mobility group box chromosomal protein 1, a potent proinflammatory cytokine, expression of which is increased in synovial tissue and in synovial fluid of rheumatoid arthritis (RA) patients. The interaction of high-mobility group box chromosomal protein 1 with cell-surface RAGE leads to an inflammatory response. In contrast, the presence of soluble RAGE (sRAGE) may abrogate cellular activation since the ligand is bound prior to interaction with the surface receptor.Our aim was to analyse to what extent sRAGE is present in patients with chronic joint inflammation (RA) as compared with patients with non-inflammatory joint disease and with healthy subjects, and to assess whether there is an association between sRAGE levels and disease characteristics.Matching samples of blood and synovial fluid were collected from 62 patients with RA with acute joint effusion. Blood from 45 healthy individuals, synovial fluid samples from 33 patients with non-inflammatory joint diseases and blood from six patients with non-inflammatory joint diseases were used for comparison. sRAGE levels were analysed using an ELISA.RA patients displayed significantly decreased blood levels of sRAGE (871 ± 66 pg/ml, P < 0.0001) as compared with healthy controls (1290 ± 78 pg/ml) and with patients with non-inflammatory joint disease (1569 ± 168 pg/ml). Importantly, sRAGE levels in the synovial fluid of RA patients (379 ± 36 pg/ml) were lower than in corresponding blood samples and correlated significantly with blood sRAGE. Interestingly, a significantly higher sRAGE level was found in synovial fluid of RA patients treated with methotrexate as compared with patients without disease-modifying anti-rheumatic treatment.We conclude that a decreased level of sRAGE in patients with RA might increase the propensity towards inflammation, whereas treatment with methotrexate counteracts this feature.

Objective: Receptor for advanced glycation end products (RAGE) is a cell surface molecule that binds a variety of ligands, including high mobility group box chromosomal protein 1 (HMGB‐1), a potent proinflammatory cytokine. RAGE–ligand interaction leads to an inflammatory response. A truncated form of the receptor, soluble RAGE (sRAGE), has been suggested to function as a decoy abrogating cellular activation, but its endogenous activity is not fully understood. We undertook this study to assess the properties of sRAGE in vivo and in vitro and to analyze the role of sRAGE in HMGB‐1–induced arthritis.Methods: Mice were injected intraarticularly with HMGB‐1 and treated systemically with sRAGE prior to histologic joint evaluation. All animals were subjected to peritoneal lavage to assess the local effect of sRAGE treatment. For in vitro studies, mouse splenocytes were incubated with sRAGE followed by assessment of NF‐κB activation and cytokine production. The chemotactic properties of sRAGE were investigated using in vitro migration assay.Results: Soluble RAGE was determined to have proinflammatory properties since it gave rise to production of interleukin‐6, tumor necrosis factor α, and macrophage inflammatory protein 2. This effect was triggered by interaction with leukocyte β2 integrin Mac‐1 and was mediated via NF‐κB. Systemic treatment with sRAGE significantly down‐regulated HMGB‐1–triggered arthritis, but the observed effect was due to a deviation of the inflammatory response from the joint to the peritoneal cavity rather than a genuine antiinflammatory effect. Apart from its proinflammatory properties, sRAGE was proven to act as a chemotactic stimulus for neutrophils.Conclusion: We conclude that sRAGE interacts with Mac‐1, thereby acting as an important proinflammatory and chemotactic molecule.

(See the editorial commentary by Chow, on pages 332–4 ) Background. Despite advances in medical practices, in recent decades permanent reductions in joint function have not been achieved, and the high mortality rate of patients with staphylococcal septic arthritis has not substantially improved. Methods. We evaluated the effects of a combined tumor necrosis factor (TNF) inhibitor and antibiotic therapy on the course of Staphylococcus aureus arthritis and sepsis in mice. Results. Treatment with the combination of a TNF inhibitor and an antibiotic resulted in a quicker relief of clinical arthritis in mice with septic arthritis, compared with an antibiotic monotherapy. Both histopathologically verified synovitis and the extent of joint destruction were reduced by this combined treatment. Importantly, anti-TNF treatment significantly improved the survival rate of mice with S. aureus sepsis and staphylococcal enterotoxin shock syndrome; this effect might be the result of a partial restoration of the hemostatic balance between coagulation and fibrinolysis. Finally, we demonstrated that anti-TNF treatment downregulates high-mobility group protein B1 in staphylococcal enterotoxin shock syndrome. Conclusions. Thus, simultaneous systemic TNF inhibition and antibiotic therapy has beneficial effects on the outcome of S. aureus arthritis and sepsis in a mouse model, suggesting that the combination of a TNF inhibitor and antibiotics represents a novel therapeutic strategy for the treatment of staphylococcal infections.

Objectives. The aim of the study was to assess the role of extracellular cytochrome c as an inducer of joint inflammation and to examine its levels in sera and synovial fluids of rheumatoid arthritis (RA) patients. Methods. Mice were injected intra-articularly with different doses of cytochrome c and joints were evaluated histopathologically and immunohistochemically 3 and 10 days later. In addition, mouse spleen cells were stimulated with different concentrations of cytochrome c, followed by assessment of NF-κB activation and cytokine production. Sera and synovial fluid from RA patients and sera from healthy individuals were assessed with respect to cytochrome c levels by an enzyme-linked immunoassay technique. Results. Histopathological signs of arthritis were evident in 75% of animals following intra-articular injection of cytochrome c. Synovitis was characterized by influx of Mac-1⁺ cells. In vivo depletion of neutrophils and monocytes led to abrogation of arthritis. Stimulation of mouse spleen cells in vitro with cytochrome c resulted in activation of NF-κB and release of proinflammatory cytokines and chemokines. Cytochrome c levels in RA patients’ sera were significantly lower than in healthy controls. Further, cytochrome c levels in synovial fluid were significantly lower than in corresponding blood samples. Conclusions. Our findings demonstrate that extracellular cytochrome c displays direct proinflammatory properties mediated by activation of NF-κB and causing neutrophil and monocyte triggered inflammation. We hypothesize that decreased levels of cytochrome c in RA patients reflect consumption of this molecule in the synovial tissue, decreasing apoptosis and shifting the balance towards inflammation.

Staphylococcus aureus (S. aureus) infections are a major healthcare challenge and new treatment alternatives are needed. S. aureus septic arthritis, a debilitating joint disease, causes permanent joint dysfunction in almost 50% of the patients. S. aureus bacteremia is associated with higher mortalities than bacteremia caused by most other microbes and can develop to severe sepsis and death. The key to new therapies is understanding the interplay between bacterial virulence factors and host immune response, which decides the disease outcome. S. aureus produces numerous virulence factors that facilitate bacterial dissemination, invasion into joint cavity, and cause septic arthritis. Monocytes, activated by several components of S. aureus such as lipoproteins, are responsible for bone destructions. In S. aureus sepsis, cytokine storm induced by S. aureus components leads to the hyperinflammatory status, DIC, multiple organ failure, and later death. The immune suppressive therapies at the very early time point might be protective. However, the timing of treatment is crucial, as late treatment may aggravate the immune paralysis and lead to uncontrolled infection and death.