High mobility group box 1 in the pathogenesis of inflammatory and autoimmune diseases.

High mobility group box 1 is a nuclear protein participating in chromatin architecture and transcriptional regulation. When released from cells, HMGB1 can also act as a pro-inflammatory mediator or alarmin. Upon stimulation with lipopolysaccharides or tumor necrosis factor-alpha, HMGB1 is secreted from certain cells such as monocytes/macrophages and fosters inflammatory responses. In addition, HMGB1 is passively released from necrotic cells and mediates inflammation and immune activation. In contrast, during apoptotic cell death, nuclear HMGB1 becomes tightly attached to hypo-acetylated chromatin and is not released into the extracellular milieu, thereby preventing an inflammatory response. There is accumulating evidence that extracellular HMGB1 contributes to the pathogenesis of many inflammatory diseases, including autoimmune diseases. Increased concentrations of HMGB1 have been detected in the synovial fluid of patients with rheumatoid arthritis. In animal models of RA, HMGB1 appears to be crucially involved in the pathogenesis of arthritis since neutralization of HMGB1 significantly ameliorates the disease. Also, in the serum and plasma of patients with systemic lupus erythematosus we detected substantial amounts of HMGB1, which may contribute to the disease process. However, investigations of blood concentrations of HMGB1 and its relevance in human diseases are hindered by the lack of reliable routine test systems.