IFN-γ Production from Liver Mononuclear Cells of Mice in Burn Injury As Well As in Postburn Bacterial Infection Models and the Therapeutic Effect of IL-18

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Abstract
Hosts after severe burn injury are known to have a defect in the Th1 immune response and are susceptible to bacterial infections. We herein show that liver NK cells are potent IFN-γ producers early after burn injury. However, when mice were injected with LPS 24 h after burn injury, IFN-γ production from liver mononuclear cells (MNC; which we previously showed to be NK cells) was suppressed, and the serum IFN-γ concentration did not increase, while serum IL-10 conversely increased compared with control mice. Interestingly, a single injection of IL-18 simultaneously with LPS greatly restored the serum IFN-γ concentration in mice with burn injury and also increased IFN-γ production from liver MNC. Nevertheless, a single IL-18 injection into mice simultaneously with LPS was no longer effective in the restoration of serum IFN-γ and IFN-γ production from the liver MNC at 7 days after burn injury, when mice were considered to be the most immunocompromised. However, IL-18 injections into mice on alternate days beginning 1 day after burn injury strongly up-regulated LPS-induced serum IFN-γ levels and IFN-γ production from liver and spleen MNC of mice 7 days after burn injury and down-regulated serum IL-10. Furthermore, similar IL-18 therapy up-regulated serum IFN-γ levels in mice with experimental bacterial peritonitis 7 days after burn injury and greatly decreased mouse mortality. Thus, IL-18 therapy restores the Th1 response and may decrease the susceptibility to bacterial infection in mice with burn injury.