High mobility group box 1 (HMGB1) is a nuclear protein with extracellular inflammatory cytokine activity. It is released passively during cell injury and necrosis, and secreted actively by immune cells. HMGB1 contains three conserved redox-sensitive cysteine residues: C23 and C45 can form an intramolecular disulfide bond, whereas C106 is unpaired and is essential for the interaction with Toll-Like Receptor (TLR) 4. However, a comprehensive characterization of the dynamic redox states of each cysteine residue and of their impacts on innate immune responses is lacking. Using tandem mass spectrometric analysis, we now have established that the C106 thiol and the C23-C45 disulfide bond are required for HMGB1 to induce nuclear NF-κB translocation and tumor necrosis factor (TNF) production in macrophages. Both irreversible oxidation to sulphonates and complete reduction to thiols of these cysteines inhibited TNF production markedly. In a proof of concept murine model of hepatic necrosis induced by acetaminophen, during inflammation, the predominant form of serum HMGB1 is the active one, containing a C106 thiol group and a disulfide bond between C23 and C45, whereas the inactive form of HMGB1, containing terminally oxidized cysteines, accumulates during inflammation resolution and hepatic regeneration. These results reveal critical posttranslational redox mechanisms that control the proinflammatory activity of HMGB1 and its inactivation during pathogenesis.

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-1beta, 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.

Congenital heart block develops in fetuses after placental transfer of Ro/SSA autoantibodies from rheumatic mothers. The condition is often fatal and the majority of live-born children require a pacemaker at an early age. The specific antibody that induces the heart block and the mechanism by which it mediates the pathogenic effect have not been elucidated. In this study, we define the cellular mechanism leading to the disease and show that maternal autoantibodies directed to a specific epitope within the leucine zipper amino acid sequence 200–239 (p200) of the Ro52 protein correlate with prolongation of fetal atrioventricular (AV) time and heart block. This finding was further confirmed experimentally in that pups born to rats immunized with p200 peptide developed AV block. p200-specific autoantibodies cloned from patients bound cultured cardiomyocytes and severely affected Ca2+ oscillations, leading to accumulating levels and overload of intracellular Ca2+ levels with subsequent loss of contractility and ultimately apoptosis. These findings suggest that passive transfer of maternal p200 autoantibodies causes congenital heart block by dysregulating Ca2+ homeostasis and inducing death in affected cells.

Interferon-alpha (IFN-alpha) has been implicated in the pathogenesis of Sjögren's syndrome and systemic lupus erythematosus. Ro52, which was recently identified as an E3 ligase with anti-proliferative and pro-apoptotic properties, is a major autoantigen targeted in both these conditions. Microarray analyses have indicated up-regulation of Ro52 by IFN-alpha, and the objective of the present study was to address the potential link between IFN-alpha and Ro52. To investigate the influence of IFN-alpha on Ro52 protein levels and cellular localization, we generated a panel of monoclonal antibodies to different domains of Ro52. These novel monoclonal antibodies were characterized by immunoprecipitation, Western blot, and enzyme-linked immunosorbent assay using cell lysates, recombinant Ro52 protein, and synthetic peptides. Ro52 was up-regulated in HeLa cells and human B cells at the messenger RNA and protein levels in response to IFN-alpha stimulation as detected by reverse transcriptase polymerase chain reaction and Western blot. After up-regulation, Ro52 translocated from the cytoplasm to the nucleus. The nuclear translocation of Ro52 was observed after staining with generated monoclonal antibodies specific for both the RING, coiled-coil, and B30.2 domains of Ro52 and the nuclear translocation of Ro52 preceded IFN-alpha-induced apoptotic cell death detected by caspase-3 and TUNEL staining in the treated cultures. In conclusion, our data show that IFN-alpha first induces up-regulation of Ro52 protein and then prompts translocation of the up-regulated Ro52 protein in to the nucleus. The translocation precedes apoptosis of the IFN-alpha exposed cells, suggesting a role for Ro52 in mediating the anti-proliferative or pro-apoptotic effects of the autoimmune-related cytokine IFN-alpha.