H Wähämaa

The alarmin HMGB1 acts in synergy with endogenous and exogenous danger signals to promote inflammation

The nuclear protein HMGB1 has previously been demonstrated to act as an alarmin and to promote inflammation upon extracellular release, yet its mode of action is still not well defined. Access to highly purified HMGB1 preparations from prokaryotic and eukaryotic sources enabled studies of activation of human PBMC or synovial fibroblast cultures in response to HMGB1 alone or after binding to cofactors. HMGB1 on its own could not induce detectable IL‐6 production. However, strong enhancing effects on induction of proinflammatory cytokine production occurred when the protein associated with each of the separate proinflammatory molecules, rhIL‐1β, the TLR4 ligand LPS, the TLR9 ligand CpG‐ODN, or the TLR1‐TLR2 ligand Pam3CSK4. The bioactivities were recorded in cocultures with preformed HMGB1 complexes but not after sequential or simultaneous addition of HMGB1 and the individual ligands. Individual A‐box and B‐box domains of HMGB1 had the ability to bind LPS and enhance IL‐6 production. Heat denaturation of HMGB1 eliminated this enhancement. Cocultures with HMGB1 and other proinflammatory molecules such as TNF, RANKL, or IL‐18 did not induce enhancement. HMGB1 thus acts broadly with many but not all immunostimulatory molecules to amplify their activity in a synergistic manner.

JAK/STAT1 signaling promotes HMGB1 hyperacetylation and nuclear translocation

Significance High-mobility group box (HMGB)1 is a nuclear protein that we have identified as a proinflammatory mediator during infection or sterile tissue injury, which importantly orchestrates the innate immune responses. The mechanisms of HMGB1 release require translocation of HMGB1 from nucleus to cytoplasm and release into the extracellular space. We recently reported that the inflammasome and PKR mediates HMGB1 release from the cytoplasm, but the mechanism of HMGB1 translocation from nucleus to cytoplasm was previously unknown. Here, we describe our discovery that JAK/STAT1 is required for LPS- or interferon-induced HMGB1 nuclear translocation. These findings have significant implications for the field, and for designing therapeutics for potential use in inflammatory diseases. Extracellular high-mobility group box (HMGB)1 mediates inflammation during sterile and infectious injury and contributes importantly to disease pathogenesis. The first critical step in the release of HMGB1 from activated immune cells is mobilization from the nucleus to the cytoplasm, a process dependent upon hyperacetylation within two HMGB1 nuclear localization sequence (NLS) sites. The inflammasomes mediate the release of cytoplasmic HMGB1 in activated immune cells, but the mechanism of HMGB1 translocation from nucleus to cytoplasm was previously unknown. Here, we show that pharmacological inhibition of JAK/STAT1 inhibits LPS-induced HMGB1 nuclear translocation. Conversely, activation of JAK/STAT1 by type 1 interferon (IFN) stimulation induces HMGB1 translocation from nucleus to cytoplasm. Mass spectrometric analysis unequivocally revealed that pharmacological inhibition of the JAK/STAT1 pathway or genetic deletion of STAT1 abrogated LPS- or type 1 IFN-induced HMGB1 acetylation within the NLS sites. Together, these results identify a critical role of the JAK/STAT1 pathway in mediating HMGB1 cytoplasmic accumulation for subsequent release, suggesting that the JAK/STAT1 pathway is a potential drug target for inhibiting HMGB1 release.

Identification of a novel chemokine-dependent molecular mechanism underlying rheumatoid arthritis-associated autoantibody-mediated bone loss

Objectives Rheumatoid arthritis (RA)-specific anti-citrullinated protein/peptide antibodies (ACPAs) appear before disease onset and are associated with bone destruction. We aimed to dissect the role of ACPAs in osteoclast (OC) activation and to identify key cellular mediators in this process. Methods Polyclonal ACPA were isolated from the synovial fluid (SF) and peripheral blood of patients with RA. Monoclonal ACPAs were isolated from single SF B-cells of patients with RA. OCs were developed from blood cell precursors with or without ACPAs. We analysed expression of citrullinated targets and peptidylarginine deiminases (PAD) enzymes by immunohistochemistry and cell supernatants by cytometric bead array. The effect of an anti-interleukin (IL)-8 neutralising antibody and a pan-PAD inhibitor was tested in the OC cultures. Monoclonal ACPAs were injected into mice and bone structure was analysed by micro-CT before and after CXCR1/2 blocking with reparixin. Results Protein citrullination by PADs is essential for OC differentiation. Polyclonal ACPAs enhance OC differentiation through a PAD-dependent IL-8-mediated autocrine loop that is completely abolished by IL-8 neutralisation. Some, but not all, human monoclonal ACPAs derived from single SF B-cells of patients with RA and exhibiting distinct epitope specificities promote OC differentiation in cell cultures. Transfer of the monoclonal ACPAs into mice induced bone loss that was completely reversed by the IL-8 antagonist reparixin. Conclusions We provide novel insights into the key role of citrullination and PAD enzymes during OC differentiation and ACPA-induced OC activation. Our findings suggest that IL8-dependent OC activation may constitute an early event in the initiation of the joint specific inflammation in ACPA-positive RA.

High mobility group box protein 1 in complex with lipopolysaccharide or IL-1 promotes an increased inflammatory phenotype in synovial fibroblasts

IntroductionIn addition to its direct proinflammatory activity, extracellular high mobility group box protein 1 (HMGB1) can strongly enhance the cytokine response evoked by other proinflammatory molecules, such as lipopolysaccharide (LPS), CpG-DNA and IL-1β, through the formation of complexes. Extracellular HMGB1 is abundant in arthritic joint tissue where it is suggested to promote inflammation as intra-articular injections of HMGB1 induce synovitis in mice and HMGB1 neutralizing therapy suppresses development of experimental arthritis. The aim of this study was to determine whether HMGB1 in complex with LPS, interleukin (IL)-1α or IL-1β has enhancing effects on the production of proinflammatory mediators by rheumatoid arthritis synovial fibroblasts (RASF) and osteoarthritis synovial fibroblasts (OASF). Furthermore, we examined the toll-like receptor (TLR) 4 and IL-1RI requirement for the cytokine-enhancing effects of the investigated HMGB1-ligand complexes.MethodsSynovial fibroblasts obtained from rheumatoid arthritis (RA) and osteoarthritis (OA) patients were stimulated with HMGB1 alone or in complex with LPS, IL-1α or IL-1β. Tumour necrosis factor (TNF) production was determined by enzyme-linked immunospot assay (ELISPOT) assessment. Levels of IL-10, IL-1-β, IL-6 and IL-8 were measured using Cytokine Bead Array and matrix metalloproteinase (MMP) 3 production was determined by ELISA.ResultsStimulation with HMGB1 in complex with LPS, IL-1α or IL-1β enhanced production of TNF, IL-6 and IL-8. HMGB1 in complex with IL-1β increased MMP production from both RASF and OASF. The cytokine production was inhibited by specific receptor blockade using detoxified LPS or IL-1 receptor antagonist, indicating that the synergistic effects were mediated through the partner ligand-reciprocal receptors TLR4 and IL-1RI, respectively.ConclusionsHMGB1 in complex with LPS, IL-1α or IL-1β boosted proinflammatory cytokine- and MMP production in synovial fibroblasts from RA and OA patients. A mechanism for the pathogenic role of HMGB1 in arthritis could thus be through enhancement of inflammatory and destructive mechanisms induced by other proinflammatory mediators present in the arthritic joint.

The novel inflammatory cytokine high mobility group box protein 1 (HMGB1) is expressed by human term placenta

High mobility group box protein 1 (HMGB1) was previously considered a strict nuclear protein, but lately data are accumulating on its extranuclear functions. In addition to its potent proinflammatory capacities, HMGB1 has a prominent role in a number of processes of specific interest for the placenta. Our overall aim was to investigate the expression of HMGB1 in human term placenta and elucidate a potential difference in HMGB1 expression comparing vaginal deliveries with elective Caesarean sections. In addition, placentas from normal pregnancies were compared with placentas from pregnancies complicated by pre‐eclampsia. Twenty‐five placentas, 12 from normal term pregnancies and 13 from pregnancies complicated by pre‐eclampsia were analysed with immunohistochemistry for HMGB1 and its putative receptors; receptor for advanced glycation end‐products (RAGE), Toll‐like receptor 2 (TLR2) and TLR4. We present the novel finding that in addition to a strong nuclear HMGB1 expression in almost all cells in investigated placentas, an individual variation of cytoplasmic HMGB1 expression was detected in the syncytiotrophoblast covering the peripheral chorionic villi, by cells in the decidua and in amnion. Production of HMGB1 was confirmed by in situ hybridization. Although labour can be described as a controlled inflammatory‐like process no differences in HMGB1 expression could be observed comparing active labour and elective Caesarean sections. However, a tendency towards a higher expression of cytoplasmic HMGB1 in the decidua from women with pre‐eclampsia was demonstrated. The abundant expression of the receptors RAGE, TLR2 and TLR4 implicates a local capability to respond to HMGB1, although the precise role in the placenta remains to be elucidated.

HMGB1-secreting capacity of multiple cell lineages revealed by a novel HMGB1 ELISPOT assay

High mobility group box protein 1 (HMGB1) exerts different biological functions dependent on its cellular localization. Nuclear HMGB1 maintains chromatin architecture and is required for undisturbed transcription activity, and extracellularly released HMGB1 mediates inflammation and tissue regeneration. A present paucity of readily accessible methods to quantify released HMGB1 represents a problem concerning the exploration of HMGB1 biology. We have now developed a HMGB1‐specific ELISPOT assay enabling enumeration of individual HMGB1‐releasing cells. The method also allows automated, semiquantitative assessment of released HMGB1 by evaluating areas of single HMGB1 spots. Actively secreted HMGB1 as well as cells passively releasing the protein following necrotic cell death are visualized distinctly using this ELISPOT assay. Kinetics of HMGB1 secretion after different stimuli was studied using cell lines of various lineages. IFN‐γ already induced substantial HMGB1 secretion from the monocytic cell line RAW 264.7 within 24 h and even more so after 48 h. LPS only stimulated a modest HMGB1 release within 24 h, but this increased considerably by 48 h. TNF‐induced HMGB1 release was unexpectedly low. Mast cells, which share the secretory, lysosomal pathway with macrophages/monocytes, did not secrete HMGB1 in response to any studied mode of activation. Most transformed cells overexpress HMGB1, but the ELISPOT assay revealed that all transformed cell lines will not actively secrete the protein. We believe the ELISPOT method provides a novel tool to study pathways promoting or inhibiting HMGB1 secretion.

Pivotal Advance: Inhibition of HMGB1 nuclear translocation as a mechanism for the anti-rheumatic effects of gold sodium thiomalate

Gold compounds such as gold sodium thiomalate (GST) can reduce the symptoms of rheumatoid arthritis (RA), although their mechanism of action is not well defined. As the proinflammatory mediator high mobility group box chromosomal protein 1 (HMGB1) may play a role in the pathogenesis of RA, we have performed in vitro studies to investigate whether GST inhibits HMGB1 release as the basis of its mode of action. Murine RAW 264.7 or human THP‐1 macrophage cells were stimulated in culture with agents causing extracellular HMGB1 release, including LPS, IFN‐γ, polyinosinic:polycytidylic acid, IFN‐β, or NO in the presence of GST, ranging from 0 μM to 250 μM. Secretion and intracellular location of HMGB1 were assessed by Western blotting, HMGB1‐specific ELISPOT assay, and immunofluorescent staining. In parallel, TNF and IFN‐β levels were analyzed by ELISPOT and/or ELISA. Supernatant NO production was analyzed by the Griess method. At pharmacologically relevant doses, GST inhibited the extracellular release of HMGB1 from activated macrophages and caused the nuclear retention of this protein; in contrast, no effects were observed on the secretion or production of TNF. Release of the key endogenous mediators of HMGB1 translocation, IFN‐β and NO, was inhibited by GST. This inhibition required gold, as sodium thiomalate did not affect the responses measured. Furthermore, gold chloride also inhibited release of HMGB1. Together, these results suggest a new mechanism for the anti‐rheumatic effects of gold salts in RA and the potential of drugs, which interfere with intracellular HMGB1 transport mechanisms, as novel agents to treat RA.

Oxaliplatin retains HMGB1 intranuclearly and ameliorates collagen type II-induced arthritis

IntroductionHigh mobility group box chromosomal protein 1 (HMGB1) is a nuclear protein that acts as a pro-inflammatory mediator following extracellular release. The protein is aberrantly expressed extracellularly in the settings of clinical and experimental synovitis. Therapy based on HMGB1 antagonists has shown encouraging results in experimental arthritis and warrants further scientific exploration using independent methods. In the present study we asked whether nuclear sequestration of HMGB1 preventing HMGB1 release would be beneficial for synovitis treatment.MethodsOxaliplatin-based therapy was evaluated in collagen type II-induced arthritis in DBA/1 mice by clinical scoring and immunostaining of articular tissue. Oxaliplatin is an antineoplastic platinum-based compound that generates DNA adducts which tightly bind HMGB1. Secretion and intracellular location of HMGB1 were assessed by a novel HMGB1-specific ELISPOT assay and immunofluorescent staining.ResultsIntraperitoneal injections of oxaliplatin in early collagen type II-induced arthritis trapped HMGB1 with a distinct biphasic response pattern. Oxaliplatin therapy showed beneficial results for approximately 1 week. Microscopic evaluation of synovitis during this period showed strong nuclear HMGB1 staining in the oxaliplatin treated animals with much lower quantities of extracellular HMGB1 when compared to control treated animals. Furthermore, cellular infiltration, as well as cartilage and bone damage, were all reduced in the oxaliplatin treated group. A dramatic and as yet unexplained clinical relapse occurred later in the oxaliplatin exposed animals, which coincided with a massive synovial tissue expression of extracellular HMGB1 in all treated animals. This rebound-like reaction was also accompanied by a significantly increased incidence of arthritis in the oxaliplatin treated group. These results indicate a distinct temporal and spatial relationship between the clinical course of disease and the cellular localization of HMGB1. Beneficial effects were noted when extracellular HMGB1 expression was low, while severe inflammation coincided with substantial extracellular synovial HMGB1 expression.ConclusionTherapeutic compounds like oxaliplatin and gold salts share a capacity to inhibit nuclear HMGB1 release and to ameliorate the course of synovial inflammation. These observations support the hypothesis that HMGB1 plays an important functional role in the pathogenesis of arthritis and may represent a novel target molecule for therapy.

Immunomodulatory drugs regulate HMGB1 release from activated human monocytes.

Several HMGBl-specific antagonists have provided beneficial results in multiple models of inflammatory disease-preclinical trials including arthritis. Since no HMGB1-specific targeted therapy has yet reached the clinic, we have performed in vitro studies to investigate whether any of a selection of well-established antirheumatic drugs inhibit HMGB1 release as part of its mode of action. Freshly purified peripheral blood monocytes from healthy donors were stimulated in cultures with LPS and IFNγ to cause HMGB1 and TNF release detected in ELISPOT assays. Effects on the secretion were assessed in cultures supplemented with dexamethasone, cortisone, chloroquine, gold sodium thiomalate, methotrexate, colchicine, etanercept or anakinra. Pharmacologically relevant doses of dexamethasone, gold sodium thiomalate and chloroquine inhibited the extracellular release of HMGB1 in a dose-dependent mode. Immunostaining demonstrated that dexamethasone caused intracellular HMGB1 retention. No effects on HMGB1 secretion were observed in cultures with activated monocytes by any of the other studied agents. TNF production in LPS/IFNγ-activated monocytes was readily downregulated by dexamethasone and, to some extent, by chloroquine and etanercept. We conclude that dexamethasone, gold sodium thiomalate and chloroquine share a capacity to inhibit HMGB1 release from activated monocytes.

Synovial fibroblast-neutrophil interactions promote pathogenic adaptive immunity in rheumatoid arthritis

Neutrophil extracellular traps containing citrullinated peptides promote inflammation in synovial fibroblasts. NETs Blow the Joint Neutrophil extracellular traps (NETs) activate synovial fibroblast-like synoviocytes (FLS) in joints of individuals with rheumatoid arthritis. Now, Carmona-Rivera et al. investigate the mechanism behind this activation. They found that NETs containing citrullinated peptides could be internalized by FLS through the RAGE-TLR9 pathway and then loaded onto major histocompatibility complex class II and presented to antigen-specific T cells, which contribute to joint inflammation. NET-loaded FLS induced autoantibody production and joint disease in mice. These data suggest that cross-talk between NETs and FLS may contribute to rheumatoid arthritis. Rheumatoid arthritis (RA) is characterized by synovial joint inflammation and by development of pathogenic humoral and cellular autoimmunity to citrullinated proteins. Neutrophil extracellular traps (NETs) are a source of citrullinated autoantigens and activate RA synovial fibroblast-like synoviocytes (FLS), cells crucial in joint damage. We investigated the molecular mechanisms by which NETs promote proinflammatory phenotypes in FLS and whether these interactions generate pathogenic anti-citrulline adaptive immune responses. NETs containing citrullinated peptides are internalized by FLS through a RAGE-TLR9 pathway, promoting FLS inflammatory phenotype and their up-regulation of major histocompatibility complex (MHC) class II. Once internalized, arthritogenic NET peptides are loaded into FLS MHC class II and presented to antigen-specific T cells. HLA-DRB1*04:01 transgenic mice immunized with mouse FLS loaded with NETs develop antibodies specific to citrullinated forms of relevant autoantigens implicated in RA pathogenesis as well as cartilage damage. These results implicate FLS as notable mediators in RA pathogenesis, through the internalization and presentation of NET citrullinated peptides to the adaptive immune system, leading to pathogenic autoimmunity and cartilage damage.