A key question in immunology concerns how sterile injury activates innate immunity to mediate damaging inflammation in the absence of foreign invaders. The discovery that HMGB1, a ubiquitous nuclear protein, mediates the activation of innate immune responses led directly to the understanding that HMGB1 plays a critical role at the intersection of the host inflammatory response to sterile and infectious threat. HMGB1 is actively released by stimulation of the innate immune system with exogenous pathogen-derived molecules and is passively released by ischemia or cell injury in the absence of invasion. Established molecular mechanisms of HMGB1 binding and signaling through TLR4 reveal signaling pathways that mediate cytokine release and tissue damage. Experimental strategies that selectively target HMGB1 and TLR4 effectively reverse and prevent activation of innate immunity and significantly attenuate damage in diverse models of sterile and infection-induced threat.

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HMGB1 Is a Therapeutic Target for Sterile Inflammation and Infection

High Mobility Group 1 protein (HMGB1) is a chromatin component that, when leaked out by necrotic cells, triggers inflammation. HMGB1 can also be secreted by activated monocytes and macrophages, and functions as a late mediator of inflammation. Secretion of a nuclear protein requires a tightly controlled relocation program. We show here that in all cells HMGB1 shuttles actively between the nucleus and cytoplasm. Monocytes and macrophages acetylate HMGB1 extensively upon activation with lipopolysaccharide; moreover, forced hyperacetylation of HMGB1 in resting macrophages causes its relocalization to the cytosol. Cytosolic HMGB1 is then concentrated by default into secretory lysosomes, and secreted when monocytic cells receive an appropriate second signal.

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Monocytic cells hyperacetylate chromatin protein HMGB1 to redirect it towards secretion

The immune system has evolved to respond not only to pathogens, but also to signals released from dying cells. Cell death through necrosis induces inflammation, whereas apoptotic cell death provides an important signal for tolerance induction. High mobility group box 1 (HMGB1) is a DNA-binding nuclear protein, released actively following cytokine stimulation as well as passively during cell death; it is the prototypic damage-associated molecular pattern (DAMP) molecule and has been implicated in several inflammatory disorders. HMGB1 can associate with other molecules, including TLR ligands and cytokines, and activates cells through the differential engagement of multiple surface receptors including TLR2, TLR4, and RAGE. RAGE is a multiligand receptor that binds structurally diverse molecules, including not only HMGB1, but also S100 family members and amyloid-β. RAGE activation has been implicated in sterile inflammation as well as in cancer, diabetes, and Alzheimer's disease. While HMGB1 through interacti...

HMGB1 and RAGE in inflammation and cancer.

of antibiotics, whether there was an explicit note in the medical chart in which the physician diagnosed a true bacteremia, and the Centers for Disease Control surveillance criteria for primary bloodstream infection. Agreement between same-strain bacteremia and each definition was examined, based on the assumption that most true infections should be the result of a single strain. The study sample consisted of 42 patients and 106 isolates. Nineteen of the 42 bacteremias (45%) were the same strain. Classification of bacteremias as same-strain correlated poorly with all three clinical assessments (range of percentage agreement, 50%-57%; range of kappa statistic, 0.01-0.15). There were both false-positive and false-negative errors. Patients with three or more positive blood cultures were more likely to have same-strain bacteremia than those with only two positive cultures (11/15 [73%] vs 8/27 [30%], P=.006). Pulsed-field gel electrophoresis was more discriminating than AP PCR (percentage agreement, 83%; kappa, 0.67). The authors concluded that molecular typing correlated poorly with clinical criteria for true bacteremia, suggesting either that true bacteremias are frequently the result of multiple strains or that the commonly used clinical criteria are not accurate for distinguishing contamination from true bacteremia. Vancomycin treatment of clinically defined coagulase-negative staphylococcal bacteremia may frequently be unnecessary. FROM: Seo SK, Venkataraman L, DeGirolami PC, Samore MH. Molecular typing of coagulase-negative staphylococci from blood cultures does not correlate with clinical criteria for true bacteremia. Am J Med 2000;109:697-704.

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Nasal Carriage as a Source of Staphylococcus aureus Bacteremia

The adipokine resistin is suggested to be an important link between obesity and insulin resistance. In the present study, we assessed the impact of resistin as inflammatogenic cytokine in the setting of arthritis. In vitro experiments on human PBMC were performed to assess cytokine response and transcription pathways of resistin-induced inflammation. Proinflammatory properties of resistin were evaluated in animal model by intra-articular injection of resistin followed by histological evaluation of the joint. Levels of resistin were assessed by ELISA in 74 paired blood and synovial fluid samples of patients with rheumatoid arthritis. Results were compared with the control group comprised blood samples from 34 healthy individuals and 21 synovial fluids from patients with noninflammatory joint diseases. We now show that resistin displays potent proinflammatory properties by 1) strongly up-regulating IL-6 and TNF-α, 2) responding to TNF-α challenge, 3) enhancing its own activity by a positive feedback, and finally 4) inducing arthritis when injected into healthy mouse joints. Proinflammatory properties of resistin were abrogated by NF-κB inhibitor indicating the importance of NF-κB signaling pathway for resistin-induced inflammation. Resistin is also shown to specifically accumulate in the inflamed joints of patients with rheumatoid arthritis and its levels correlate with other markers of inflammation. Our results indicate that resistin is a new and important member of the cytokine family with potent regulatory functions. Importantly, the identified properties of resistin make it a novel and interesting therapeutic target in chronic inflammatory diseases such as rheumatoid arthritis.

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Resistin, an adipokine with potent proinflammatory properties

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.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.11028

High mobility group box chromosomal protein 1, a DNA binding cytokine, induces arthritis.

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.

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Decreased levels of soluble receptor for advanced glycation end products in patients with rheumatoid arthritis indicating deficient inflammatory control

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.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/art.22217

Soluble receptor for advanced glycation end products triggers a proinflammatory cytokine cascade via beta2 integrin Mac-1.

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 septi ...

The Combination of a Tumor Necrosis Factor Inhibitor and Antibiotic Alleviates Staphylococcal Arthritis and Sepsis in Mice

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-jB 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 Q 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-jB 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-jB 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.

/pdf/extracellular-cytochrome-c-a-mitochondrial-apoptosis-related-4a96h4jcxx.pdf

Rille Pullerits

Papers

High mobility group box chromosomal protein 1, a DNA binding cytokine, induces arthritis.

Decreased levels of soluble receptor for advanced glycation end products in patients with rheumatoid arthritis indicating deficient inflammatory control

Soluble receptor for advanced glycation end products triggers a proinflammatory cytokine cascade via beta2 integrin Mac-1.

The Combination of a Tumor Necrosis Factor Inhibitor and Antibiotic Alleviates Staphylococcal Arthritis and Sepsis in Mice

Extracellular cytochrome c, a mitochondrial apoptosis-related protein, induces arthritis