High-mobility group box 1 represents a potential marker of disease activity and novel therapeutic target in systemic lupus erythematosus

High-mobility group box 1 (HMGB1) protein is a nuclear DNA-binding protein, which functions as an alarmin when released from cells. Recent studies implicate extracellular HMGB1 in the pathogenesis of systemic lupus erythematosus (SLE), a prototypical autoimmune disease characterized by the formation of multiple autoantibodies, especially those directed against nucleosomes and double-stranded (ds)DNA. Elevated concentrations of HMGB1 are observed in sera as well as in skin lesions of patients with lupus. Of importance, serum HMGB1 and anti-HMGB1 autoantibody levels correlate with disease activity. In the blood of patients with SLE, HMGB1 is complexed with nucleosomes, at least partially. Moreover, HMGB1-nucleosome complexes from apoptotic cells activate antigen-presenting cells. Injection of HMGB1-nucleosome complexes into nonautoimmune mice results in the formation of autoantibodies against dsDNA and histones in a Toll-like receptor (TLR) 2-dependent manner. Additionally, HMGB1, as a part of DNA-anti-DNA immune complexes, can interact with receptor for advanced glycation end products (RAGE) on the surface of plasmacytoid dendritic cells and B cells leading to TLR9-dependent interferon (IFN)α release and activation of autoreactive B cells, respectively. HMGB1 attached to neutrophil extracellular traps may contribute to IFNα production by facilitating the recognition of self-nucleic acids. Furthermore, HMGB1, complexed with DNA and pathogenic anti-DNA autoantibodies, activates its receptors, TLR2, TLR4 and RAGE, and may thereby be involved in anti-DNA autoantibody-induced kidney damage in lupus nephritis. Collectively, these findings suggest that HMGB1 is a potential marker of disease activity and, because of its probable involvement in the pathogenesis, a novel therapeutic target in SLE.