High mobility group 1 protein (HMG-1) stimulates proinflammatory cytokine synthesis in human monocytes

Lipopolysaccharide (LPS) is lethal to animals because it activates cytokine release, causing septic shock and tissue injury. Early proinflammatory cyt okines (e.g., tumor necrosis factor [TNF] and interleukin [IL]-1) released within the first few hours of endotoxemia stimulate mediator cascades that persist for days and can lead to death. High mobility group 1 protein (HMG- 1), a ubiquitous DNA-binding protein, was recently identified as a "late" m ediator of endotoxin lethality. Anti-HMG-1 antibodies neutralized the delay ed increase in serum HMG-1, and protected against endotoxin lethality, even when passive immunization was delayed until after the early cytokine respo nse. Here we examined whether HMG-1 might stimulate cytokine synthesis in h uman peripheral blood mononuclear cell cultures. Addition of purified recom binant HMG-1 to human monocyte cultures significantly stimulated the releas e of TNF, IL-1 alpha, IL-1 beta, IG-1RA, IL-6, IL-8, macrophage inflammator y protein (MIP)-1 alpha, and MIP-1 beta; but not IL-10 or IL-12. HMG-1 conc entrations that activated monocytes were within the pathological range prev iously observed in endotoxemic animals, and in serum obtained from septic p atients. HMG-1 failed to stimulate cytokine release in lymphocytes, indicat ing that cellular stimulation was specific. Cytokine release after HMG-1 st imulation was delayed and biphasic compared with LPS stimulation. Computer- assisted image analysis demonstrated that peak intensity of HMG-1-induced c ellular TNF staining was comparable to that observed after maximal stimulat ion with LPS. Administration of HMG-1 to Balb/c mice significantly increase d serum TNF levels in vivo. Together, these results indicate that, like oth er cytokine mediators of endotoxin lethality (e.g., TNF and IL-1), extracel lular HMG-1 is a regulator oimonocyte proinflammatory cytokine synthesis.