High-mobility group box 1 contributes to lethality of endotoxemia in heme oxygenase-1-deficient mice

High-mobility group box 1 (HMGB1) is a nuclear protein that has been found to be a critical mediator of lethality in endotoxemia and sepsis. During the systemicinflammatory response, circulating levels of HMGB1 are increased, but in a delayed fashion compared with early inflammatory mediators. To counteract the inflammatory response of endotoxemia, a secondary anti-inflammatory response ensues in an attempt to prevent inflammation-induced tissue injury. One such cytoprotective gene that is induced during endotoxemia is heme oxygenase (HO)-1. HO-1, and its products of heme metabolism, possess anti-inflammatory and antioxidant properties to counter the damaging effects of endotoxemia. In the present study, we wanted to determine whether tissue and circulating levels of HMGB1 are increased further in the absence of HO-1 during endotoxemia, and whether this increase may contribute to the pathobiology of endotoxemia. Lung inflammation, HMGB1 protein levels, and expression of HMGB1 in inflammatory cells were increased in HO-1^-/- mice compared with HO-1^+/+ mice. After the administration of LPS, tissue levels of HMGB1 were not increased further in HO-1^-/- mice; however, circulating levels of HMGB1 were higher when compared with HO-1^+/+ mice. HO-1^-/- mice treated with a carbon monoxide-releasing molecule or biliverdin showed a reduction in plasma HMGB1, which was associated with a marked improvement in survival. HO-1^-/- mice given HMGB1-neutralizing antibody showed improvement in survival compared with control antibody. These data suggest that exaggerated circulating levels of HMGB1contribute to endotoxin-induced mortality in the absence of HO-1.