Reactive oxygen species produced by the NADPH oxidase 2 complex in monocytes protect mice from bacterial infections.

Chronic granulomatous disease (CGD) is an inherited disorder characterized by recurrent life-threatening bacterial and fungal infections. CGD results from defective production of reactive oxygen species by phagocytes caused by mutations in genes encoding the NADPH oxidase 2 (NOX2) complex subunits. Mice with a spontaneous mutation in Ncf1, which encodes the NCF1 (p47(phox)) subunit of NOX2, have defective phagocyte NOX2 activity. These mice occasionally develop local spontaneous infections by Staphylococcus xylosus or by the common CGD pathogen Staphylococcus aureus. Ncf1 mutant mice were more susceptible to systemic challenge with these bacteria than were wild-type mice. Transgenic Ncf1 mutant mice harboring the wild-type Ncf1 gene under the human CD68 promoter (MN(+) mice) gained the expression of NCF1 and functional NOX2 activity specifically in monocytes/macrophages, although minimal NOX2 activity was also detected in some CD11b(+)Ly6G(+) cells defined as neutrophils. MN(+) mice did not develop spontaneous infection and were more resistant to administered staphylococcal infections compared with MN(-) mice. Most strikingly, MN(+) mice survived after being administered Burkholderia cepacia, an opportunistic pathogen in CGD patients, whereas MN(-) mice died. Thus, monocyte/macrophage expression of functional NCF1 protected against spontaneous and administered bacterial infections.

Addition of bisphosphonate to antibiotic and anti-inflammatory treatment reduces bone resorption in experimental Staphylococcus aureus-induced arthritis.

Bacterial arthritis is a disease with high morbidity leading to rapidly progressive bone resorption. We have shown earlier that treatment with antibiotics in combination with corticosteroids decreases joint inflammation and mortality but does not significantly affect bone/cartilage destruction of the joints. This study was performed to assess the effect of treatment with bisphosphonate [zoledronic acid (ZA)] in combination with antibiotics and corticosteroids, on the course and outcome of Staphlococcus aureus-induced arthritis. Three days after intravenous inoculation with S. aureus, mice were treated with antibiotics alone, ZA alone, ZA and antibiotics, or ZA combined with antibiotics and corticosteroids, respectively. One group served as controls and received PBS. Clinical assessment of arthritis was performed as well as histological analysis of bone and cartilage destruction in the joints. One femur from each mouse was collected for bone mineral density (BMD) analysis. In addition, serum levels of type I collagen fragments (RatLaps), and osteocalcin, markers for osteoclastic and osteoblastic activity, respectively, were analyzed. Mice treated with ZA and antibiotics or with ZA in combination with antibiotics and corticosteroids lost significantly less in trabecular bone density compared to infected control mice. Furthermore, the addition of corticosteroids to animals treated with ZA and antibiotics, significantly decreased serum levels of RatLaps and osteocalcin, compared to animals treated with ZA and antibiotics or ZA alone. Treatment with bisphosphonates in combination with antimicrobial agents and corticosteroids significantly decreases the activity of osteoclasts in septic arthritis, thereby reducing the risk of skeletal destruction.

IL-1 receptor-associated kinase 1 mediates protection against Staphylococcus aureus infection.

The interleukin-1 receptor-associated kinase-1 (IRAK-1) mediates signal transduction from Toll-like/IL-1/IL-18 receptors. Though a critical protective role against Staphylococcus aureus infection has been previously attributed to myeloid differentiation factor 88 (MyD88) and IRAK-4, both also involved in TLR/IL-1/IL-18 signaling, the role of IRAK-1 is unknown. IRAK-1-deficient (IRAK-1-/-) and wild-type mice were inoculated i.v. with 2 x 10(7) or 1 x 10(6) S. aureus per mouse to evaluate the role of IRAK-1 in S. aureus sepsis. Since IRAK-1 transduces IL-1R signals, IL-1R-/- mice were also included in experiments. IRAK-1-/- mice are susceptible to a high dose of S. aureus compared to wild-type controls. In contrast to the high mortality and extensive weight loss seen in IL-1R-deficient mice in response to 1 x 10(6) S. aureus, IRAK-1-/- mice are resistant to this low dose of S. aureus. Thus IRAK-1 plays an important role in the host response to staphylococcal sepsis.

T-box transcription-factor-deficient mice display increased joint pathology and failure of infection control during staphylococcal arthritis.

To study the impact of T-box transcription factor (T-bet) on initiation and progression of Staphylococcus aureus sepsis and arthritis, T-bet-deficient mice (T-bet(-/-)) and their wild-type controls (T-bet(+/+)) were intravenously inoculated with 8 x 10(6) S. aureus. Already 48 h after inoculation of S. aureus, T-bet-deficient mice displayed increased frequency (62% versus 19%, P = 0.002) as well as severity of arthritis compared with wild-type controls. The bacterial counts were significantly increased in T-bet(-/-) mice compared with T-bet(+/+) as measured in kidneys 72 h after the inoculation (4.3 +/- 1.8 x 10(7) versus 3.2 +/- 3.2 x 10(6) colony-forming units (CFU); P = 0.003). As expected, T-bet-deficient mice displayed significantly decreased production of IFN-gamma (10-15-fold) at 24 and 72 h after bacterial inoculation compared with wild-type mice. Interestingly, in the absence of T-bet, serum IL-4 was decreased at 24 h. IL-6 did not differ at early stage of infection but was sixfold increased in T-bet(-/-) mice over T-bet(+/+) animals at 72 h postinoculation. Ten days after the inoculation, T-bet(-/-) mice still displayed significantly more pronounced weight loss and increased serum IL-6 levels, probably due to increased bacterial burden compared with T-bet(+/+) mice. The cumulative mortality was 19% in T-bet mice (5/27) and 0% (0/27) in control animals (P = 0.05). In conclusion, T-bet plays an important role in early response to S. aureus infection, protecting against bacterial accumulation, cachexia and septic death. Furthermore T-bet downregulates joint inflammation in the early phase of disease.

Phytoestrogen genistein as an anti-staphylococcal agent.

The soybean-derived isoflavone genistein has been shown to exert beneficial effects on many disorders, including cancer and cardiovascular diseases. The effects of genistein on mammalian cells are mediated by its abilities to inhibit topoisomerase II and protein tyrosine kinase. In order to examine the potential antibacterial activities of genistein, we incubated the bacteria with various concentrations of this compound for different periods of time and assessed the viable counts. Exposure to genistein exhibited an inhibitory effect on all staphylococcal strains tested, including methicillin-resistant strains. Furthermore, the growth of Streptococcus pasteurianus, Bacillus cereus, and Helicobacter pylori was clearly inhibited by genistein, whereas Escherichia coli growth was not suppressed. Daidzein, which is structurally similar to genistein, but deficient in topoisomerase II inhibitory activity, also inhibited the growth of Staphylococcus aureus, albeit with lower potency than genistein. Our results indicate that genistein exerts potent antibacterial properties in vitro, which are possibly mediated by the stabilization of the covalent topoisomerase II-DNA cleavage complex.

Current status of pathogenetic mechanisms in staphylococcal arthritis.

Interactions between staphylococci and the joint tissues of the host lead typically to rapidly progressing and highly destructive processes. Staphylococci possess a vast arsenal of components and products that contribute to the pathogenesis of joint infection. Occasionally these compounds have overlapping activities and act either in concert or alone. Host responsiveness to staphylococcal infection displays an even more complex pattern. Most of the cells and molecules that participate in the innate immune system protect the host against bacteria. However, the staphylococci have developed systems that counteract endogenous protective mechanisms. Interestingly, certain cells and molecules of the acquired immune system potentiate the severity of infection by triggering exaggerated responses to the staphylococcal danger signals. This review deals with the intricate host-bacterium interactions that occur during experimental septic arthritis, and outlines potential preventive and treatment modalities.

RANKL-targeted therapy inhibits bone resorption in experimental Staphylococcus aureus-induced arthritis.

INTRODUCTION: Bacterial arthritis causes rapidly progressing joint destruction in humans. We have shown that addition of bisphosphonates or corticosteroids to conventional antimicrobial agents decreases the activity of osteoclasts, thereby reducing bone destruction. Here we assess the effect of RANKL-targeted treatments using soluble receptor decoy and osteprotegerin (OPG) on the course and outcome of staphylococcal arthritis. METHODS: Treatment was initiated 3 days after Staphylococcus aureus inoculation and included RANK-Fc, OPG-Fc, and OPG-Fc in combination with antibiotics. Control groups were treated with antibiotics, huFc, and PBS. Joints were evaluated for clinical signs of arthritis and histologically for bone and cartilage destruction. Bone mineral density (BMD) was evaluated using a peripheral quantitative computed tomography. Circulating markers of bone metabolism, inflammatory cytokines, and chemokines were analyzed in each group. RESULTS: Mice treated with RANK-Fc or OPG-Fc in combination with antibiotics preserved total BMD and trabecular bone as compared to huFc or antibiotics. Treatment with RANK-Fc or OPG-Fc diminished the levels of bone resorption markers (osteocalcin, CTX-I, and TRACP5b). Neither RANK-Fc nor OPG-Fc influenced significantly the frequency and severity of arthritis. CONCLUSIONS: Inhibition of RANKL signalling efficiently prevents bone loss in the mouse model of bacterial arthritis even when started in the overt phase of infection.

Dichloroacetate alleviates development of collagen II-induced arthritis in female DBA/1 mice.

INTRODUCTION: Dichloroacetate (DCA) has been in clinical use for the treatment of lactacidosis and inherited mitochondrial disorders. It has potent anti-tumor effects both in vivo and in vitro, facilitating apoptosis and inhibiting proliferation. The pro-apoptotic and anti-proliferative properties of DCA prompted us to investigate the effects of this compound in arthritis. METHODS: In the present study, we used DCA to treat murine collagen type II (CII)-induced arthritis (CIA), an experimental model of rheumatoid arthritis. DBA/1 mice were treated with DCA given in drinking water. RESULTS: Mice treated with DCA displayed much slower onset of CIA and significantly lower severity (P < 0.0001) and much lower frequency (36% in DCA group vs. 86% in control group) of arthritis. Also, cartilage and joint destruction was significantly decreased following DCA treatment (P = 0.005). Moreover, DCA prevented arthritis-induced cortical bone mineral loss. This clinical picture was also reflected by lower levels of anti-CII antibodies in DCA-treated versus control mice, indicating that DCA affected the humoral response. In contrast, DCA had no effect on T cell- or granulocyte-mediated responses. The beneficial effect of DCA was present in female DBA/1 mice only. This was due in part to the effect of estrogen, since ovariectomized mice did not benefit from DCA treatment to the same extent as sham-operated controls (day 30, 38.7% of ovarectomized mice had arthritis vs. only 3.4% in sham-operated group). CONCLUSION: Our results indicate that DCA delays the onset and alleviates the progression of CIA in an estrogen-dependent manner.

Mice chronically fed high-fat diet have increased mortality and disturbed immune response in sepsis.

BACKGROUND: Sepsis is a potentially deadly disease that often is caused by gram-positive bacteria, in particular Staphylococcus aureus (S. aureus). As there are few effective therapies for sepsis, increased basic knowledge about factors predisposing is needed. METHODOLOGY/PRINCIPAL FINDINGS: The purpose of this study was to study the effect of Western diet on mortality induced by intravenous S. aureus inoculation and the immune functions before and after bacterial inoculation. Here we show that C57Bl/6 mice on high-fat diet (HFD) for 8 weeks, like genetically obese Ob/Ob mice on low-fat diet (LFD), have increased mortality during S. aureus-induced sepsis compared with LFD-fed C57Bl/6 controls. Bacterial load in the kidneys 5-7 days after inoculation was increased 10-fold in HFD-fed compared with LFD-fed mice. At that time, HFD-fed mice had increased serum levels and fat mRNA expression of the immune suppressing cytokines interleukin-1 receptor antagonist (IL-1Ra) and IL-10 compared with LFD-fed mice. In addition, HFD-fed mice had increased serum levels of the pro-inflammatory IL-1beta. Also, HFD-fed mice with and without infection had increased levels of macrophages in fat. The proportion and function of phagocytosing granulocytes, and the production of reactive oxygen species (ROS) by peritoneal lavage cells were decreased in HFD-fed compared with LFD-fed mice. CONCLUSIONS: Our findings imply that chronic HFD disturb several innate immune functions in mice, and impairs the ability to clear S. aureus and survive sepsis.

Decreased levels of the gelsolin plasma isoform in patients with rheumatoid arthritis.

INTRODUCTION: Gelsolin is an intracellular actin-binding protein involved in cell shape changes, cell motility, and apoptosis. An extracellular gelsolin isoform, plasma gelsolin circulates in the blood of healthy individuals at a concentration of 200 +/- 50 mg/L and has been suggested to be a key component of an extracellular actin-scavenging system during tissue damage. Levels of plasma gelsolin decrease during acute injury and inflammation, and administration of recombinant plasma gelsolin to animals improves outcomes following sepsis or burn injuries. In the present study, we investigated plasma gelsolin in patients with rheumatoid arthritis. METHODS: Circulating and intra-articular levels of plasma gelsolin were measured in 78 patients with rheumatoid arthritis using a functional (pyrene-actin nucleation) assay and compared with 62 age- and gender-matched healthy controls. RESULTS: Circulating plasma gelsolin levels were significantly lower in patients with rheumatoid arthritis compared with healthy controls (141 +/- 32 versus 196 +/- 40 mg/L, P = 0.0002). The patients' intra-articular plasma gelsolin levels were significantly lower than in the paired plasma samples (94 +/- 24 versus 141 +/- 32 mg/L, P = 0.0001). Actin was detected in the synovial fluids of all but four of the patients, and immunoprecipitation experiments identified gelsolin-actin complexes. CONCLUSIONS: The plasma isoform of gelsolin is decreased in the plasma of patients with rheumatoid arthritis compared with healthy controls. The reduced plasma concentrations in combination with the presence of actin and gelsolin-actin complexes in synovial fluids suggest a local consumption of this potentially anti-inflammatory protein in the inflamed joint.

Ethanol prevents development of destructive arthritis.

Environmental factors are thought to play a major role in the development of rheumatoid arthritis. Because the use of ethanol is widespread, we assessed the role of ethanol intake on the propensity to develop chronic arthritis. Collagen type II-immunized mice were given water or water containing 10% (vol/vol) ethanol or its metabolite acetaldehyde. Their development of arthritis was assessed, as well as the impact of ethanol on leukocyte migration and activation of intracellular transcription factors. Mice exposed daily to this dose of ethanol did not display any liver toxicity, and the development of erosive arthritis was almost totally abrogated. In contrast, the antibody-mediated effector phase of collagen-induced arthritis was not influenced by ethanol exposure. Also, the major ethanol metabolite, acetaldehyde, prevented the development of arthritis. This antiinflammatory and antidestructive property of ethanol was mediated by (i) down-regulation of leukocyte migration and (ii) up-regulation of testosterone secretion, with the latter leading to decreased NF-kappaB activation. We conclude that low but persistent ethanol consumption delays the onset and halts the progression of collagen-induced arthritis by interaction with innate immune responsiveness.

Rapid systemic bone resorption during the course of Staphylococcus aureus-induced arthritis.

Bacterial arthritis is the most rapidly progressing destructive joint disease in humans. To evaluate bone resorption and formation, mice were injected with Staphylococcus aureus and killed after 3 and 14 days. Both total and trabecular bone mineral density were, compared with those in uninfected controls, already significantly reduced 3 days after bacterial inoculation. Serum levels of type I collagen fragments were significantly increased and osteocalcin levels decreased in mice infected with S. aureus, compared with those in noninfected mice, 3 and 14 days after bacterial inoculation. This study shows that there is a rapid and easily measurable systemic bone resorption during S. aureus-induced arthritis.

Inhibition of septic arthritis by local administration of taurine chloramine, a product of activated neutrophils.

OBJECTIVE: Taurine is an amino acid able to react with hypochlorous acid, produced endogenously by neutrophils, resulting in the more stable and less toxic taurine chloramine (Tau-Cl). Since Tau-Cl has been shown to down-regulate the production of proinflammatory mediators and to exert anti-bacterial properties, we investigated the efficacy of Tau-Cl treatment for infectious arthritis. METHODS: The murine model of hematogenous septic arthritis involved intravenous injection of a single dose of Staphylococcus aureus. Tau-Cl was administered by daily intraperitoneal injections. In another experiment S. aureus and Tau-Cl were injected intra-articularly. Evaluation of arthritis was performed clinically and histologically. The effect of Tau-Cl on bacterial growth in vitro was also assessed. RESULTS: Growth of staphylococci, including the methicillin-resistant strain 67-0, was inhibited by Tau-Cl. Mice injected with bacteria and Tau-Cl locally in the joint exhibited significantly fewer arthritic lesions. In contrast, there were no obvious differences between Tau-Cl-treated animals and controls with regard to clinical or histological signs of arthritis when bacteria and Tau-Cl were administered systemically. CONCLUSION: Our results show that Tau-Cl exerts an inhibitory effect on the development of bone and cartilage damage in the infected joint when administered intra-articularly.

Role of fibrinogen-binding adhesin expression in septic arthritis and septicemia caused by Streptococcus agalactiae.

BACKGROUND: Streptococcus agalactiae (group B streptococcus) is an important human pathogen that causes neonatal pneumonia, sepsis, septic arthritis, and meningitis, as well as severe infections in immunocompromised adult patients. The streptococci produce several molecules important for virulence. METHODS: We used a murine model of sepsis and septic arthritis to assess the role of FbsA, a fibrinogen-binding adhesin of S. agalactiae as a virulence determinant. NMRI mice were inoculated intravenously with S. agalactiae strains isogenic for the expression of FbsA. RESULTS: Inoculation with wild-type (wt) streptococci resulted in significantly higher mortality, more-pronounced weight decrease, and more-severe arthritis, compared with inoculation with the FbsA mutant isogenic strain. Neither active nor passive immunization with FbsA or FbsA-specific antibodies, respectively, resulted in any protection against subsequent infection with the S. agalactiae wt strain. CONCLUSION: Our results clearly indicate that the expression of FbsA by Streptococcus agalactiae is a significant virulence determinant in septic arthritis and septicemia. However, because blocking of the fibrinogen binding properties did not protect the host against the action of FbsA-expressing streptococci, we believe that the FbsA molecule has some other presently unknown biological in vivo properties.

Osteoporosis in experimental postmenopausal polyarthritis: the relative contributions of estrogen deficiency and inflammation.

Generalized osteoporosis in postmenopausal rheumatoid arthritis (RA) is caused both by estrogen deficiency and by the inflammatory disease. The relative importance of each of these factors is unknown. The aim of this study was to establish a murine model of osteoporosis in postmenopausal RA, and to evaluate the relative importance and mechanisms of menopause and arthritis-related osteoporosis. To mimic postmenopausal RA, DBA/1 mice were ovariectomized, followed by the induction of type II collagen-induced arthritis. After the mice had been killed, paws were collected for histology, one femur for bone mineral density (BMD) and sera for analyses of markers of bone resorption (RatLaps; type I collagen cross-links, bone formation (osteocalcin) and cartilage destruction (cartilage oligomeric matrix protein), and for the evaluation of antigen-specific and innate immune responsiveness. Ovariectomized mice displayed more severe arthritis than sham-operated controls. At termination of the experiment, arthritic control mice and non-arthritic ovariectomized mice displayed trabecular bone losses of 26% and 22%, respectively. Ovariectomized mice with arthritis had as much as 58% decrease in trabecular BMD. Interestingly, cortical BMD was decreased by arthritis but was not affected by hormonal status. In addition, markers of bone resorption and cartilage destruction were increased in arthritic mice, whereas markers of bone formation were increased in ovariectomized mice. This study demonstrates that the loss of endogenous estrogen and inflammation contribute additively and equally to osteoporosis in experimental postmenopausal polyarthritis. Markers of bone remodeling and bone marrow lymphocyte phenotypes indicate different mechanisms for the development of osteoporosis caused by ovariectomy and arthritis in this model.

On the role of Staphylococcus aureus sortase and sortase-catalyzed surface protein anchoring in murine septic arthritis.

Anchoring of Staphylococcus aureus surface protein to the cell wall is catalyzed by sortase, a transpeptidase. The contribution of staphylococcal surface proteins to establishment of infection was examined using a murine septic arthritis model. Intravenous inoculation of mice with the sortase-deficient mutant S. aureus strain SMK3 did not result in weight loss or severe septic arthritis, in contrast to the parent strain, S. aureus Newman. Direct inoculation of the sortase mutant into joint cavities also failed to cause severe synovitis or erosive arthritis. Furthermore, intravenous inoculation with staphylococci resulted in the rapid clearing of the sortase mutant from the bloodstream. This phenomenon demonstrates the involvement of host neutrophils; when these cells were depleted, sortase mutant staphylococci caused severe systemic infection, although not septic arthritis. These results suggest that sortase mutant staphylococci are significantly less virulent than the parent strain, Newman: the sortase mutant has decreased ability to reach target organs and, once there, to induce an inflammatory response.

Microbial superantigens as virulence factors and ways to counteract their actions.

Microbial superantigens represent a group of molecules that is able to cause massive activation of the host immune system. Human diseases originating from superantigen-secreting bacterial agents are characterized by shock, which continues to pose major health problems. Presently, the treatment of superantigen-mediated infections is limited to the administration of antibiotics and handling of the state of shock. However, the development of multiple antibiotic-resistant, superantigen-producing bacterial strains increases the threat of these infections, and prompts researchers to better understand and treat disease states in which exposure to superantigens is at least partly responsible for the outcome. In the past decade, significant understanding has been achieved regarding the molecular mechanisms of superantigen-host interactions. Based on this understanding, a variety of promising strategies directed against superantigens have been developed. In this review, we discuss some of these strategies, as well as the potential for therapeutic applications of superantigens for the benefit of the host.

In vitro and in vivo interactions of Haemophilus ducreyi with host phagocytes.

We investigated the phagocytosis of Haemophilus ducreyi both in vitro and in vivo. Human granulocyte and monocyte phagocytosis of opsonized and nonopsonized, fluorescence-labeled H. ducreyi was assessed by flow cytometry. Both Escherichia coli and noncapsulated H. influenzae were included as controls. The maximal percentage of granulocytes taken up by H. ducreyi was 35% after 90 min. In contrast, 95% of H. influenzae bacteria were phagocytosed by granulocytes after 30 min. These results indicated that H. ducreyi phagocytosis was slow and inefficient. Bacterial opsonization by using specific antibodies increased the percentage of granulocytes phagocytosing H. ducreyi from 24 to 49%. The nonphagocytosed bacteria were completely resistant to phagocytosis even when reexposed to granulocytes, indicating that the H. ducreyi culture comprised a mixture of phenotypes. The intracellular survival of H. ducreyi in granulocytes, in monocytes/macrophages, and in a monocyte cell line (THP-1) was quantified after application of gentamicin treatment to kill extracellular bacteria. H. ducreyi survival within phagocytes was poor; approximately 11 and <0.1% of the added bacteria survived intracellularly after 2 and 20 h of incubation, respectively, while no intracellular H. influenzae bacteria were recovered after 2 h of incubation with phagocytes. The role of phagocytes in the development of skin lesions due to H. ducreyi was also studied in vivo. Mice that were depleted of granulocytes and/or monocytes and SCID mice, which lacked T and B cells, were injected intradermally with approximately 10(6) CFU of H. ducreyi. Within 4 days of inoculation, the granulocyte-depleted mice developed lesions that persisted throughout the experimental period. This result reinforces the importance of granulocytes in the early innate defense against H. ducreyi infection. In conclusion, H. ducreyi is insufficiently phagocytosed to achieve complete eradication of the bacteria. Indeed, H. ducreyi has the ability to survive intracellularly for short periods within phagocytic cells in vitro. Since granulocytes play a major role in the innate defense against H. ducreyi infection in vivo, bacterial resistance to phagocytosis probably plays a crucial role in the pathogenesis of chancroid.

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

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 microg or 5 microg 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 kappaB (NF-kappaB) 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-kappaB, 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-kappaB activation.