High mobility group box protein 1 (HMGB1) is an endogenous molecule that upon stress can be released into the extracellular milieu (Andersson U et al. 2000; Scaffidi P et al. 2002; Bonaldi T et al. 2003; Chen G et al. 2004; Bell CW et al. 2006; Beyer C et al. 2012; Yang H et al. 2013).
Using surface plasmon resonance (SPR) analysis recombinant HMGB1 was shown to bind TLR4:LY96(MD2) in a concentration-dependent manner (Yang H et al. 2010; Yang H et al. 2015). The binding required cysteine at the position 106 whereas the C106A HMGB1 mutant failed to bind TLR4:LY96 (Yang H et al. 2010). In addition, C106A and C106S HMGB1 failed to stimulate TNF release in mouse peritoneal macrophages (Yang H et al. 2010). The activity of HMGB1 was found to depend on the redox state of three cysteines at positions 23, 45 and 106 (C23, C45 and C106) (Urbonaviciute V et al. 2009; Venereau E et al. 2012, 2013; Yang H et al. 2012, 2013). Tandem mass spectrometric analysis revealed that the inflammatory activities of HMGB1 required both the formation of an intramolecular disulfide bond between C23 and C45 and the reduced state of C106 (thiol state, C106-SH) (Yang H et al. 2012; Venereau E et al. 2012). Both terminal oxidation of these cysteines to sulfonates (CySO3-) with reactive oxygen species (ROS) and their complete reduction to thiols (CySH) abrogated the cytokine-stimulating activity of HMGB1 in cultured human primary macrophages and mouse macrophage-like RAW 264.7 cells (Yang H et al. 2012; Venereau E et al. 2012). Biosensor-based SPR analysis confirmed that only the disulfide bond (C23-S-S-C45)-containing HMGB1 binds to LY96 (MD2) with high affinity (apparent Kd = 12 nM) regardless of whether LY96 or HMGB1 was immobilized on the sensor chip (Yang H et al. 2015). Moreover, TLR4 and LY96 (MD2) were recruited into CD14-containing lipid rafts of mouse RAW264.7 macrophages after stimulation with HMGB1, suggesting that an optimal HMGB1-dependent TLR4 activation in vitro required the co-receptor CD14 (Kim S et al. 2013). In addition to stimulating cells by direct interaction with innate immune receptors, HMGB1 was found to form immunostimulatory complexes with cytokines and other endogenous and exogenous ligands such as bacterial lipopolysaccharide (LPS) (Youn JH et al. 2008; Wahamaa H et al. 2011; Hreggvidsdottir HS et al. 2009) HMGB1 in complex with LPS, IL1alpha or IL1beta boosted proinflammatory cytokine- and matrix metalloproteinase (MMP3) production in synovial fibroblasts obtained from rheumatoid arthritis (RA) and osteoarthritis (OA) patients (Wahamaa H et al. 2011; He ZW et al. 2013). HMGB1 was reported to associate and amplify the activity of LPS (TLR4 ligand), CpG-ODN (TLR9 ligand) or Pam3CSK4 (TLR1:TLR2 ligand) in a synergistic manner when added to the cultures of human peripheral blood mononuclear cell (PBMC) (Hreggvidsdottir HS et al. 2009).