Inhibition of the Defense System Stimulating Interleukin-12 Interferon-γ Pathway During Critical Illness

Interleukin-12 (IL-12) and interferon-γ (IFN-γ) exert protective effects during experimental endotoxemia through upregulation of cellular immunity and phagocytic functions. They are part of a positive regulatory feedback loop that enhances the production of the other. Because critically ill patients show a marked suppression of T-cell and macrophage functions with a high susceptibility to infection, potential defects in the immunity/inflammation upregulating IL-12 IFN-γ pathway were studied. As an ex vivo model of endotoxemia, lipopolysaccharide (LPS) stimulated whole blood from 25 critically ill patients and 12 healthy individuals was incubated with either recombinant human (rh) IL-12 or rhIFN-γ, respectively. IFN-γ dose-dependently ( P < .05) increased the release of IL-12 p40 and p70 into LPS-stimulated whole blood from healthy humans without effect in whole blood from critically ill patients. RhIL-12 p70 enhanced ( P < .05) the secretion of IFN-γ in controls, while it was ineffective in LPS-stimulated whole blood from critically ill patients. The observed inhibition of the IL-12 IFN-γ pathway is not specific to LPS, since Staphylococcus aureus Cowan strain I (SAC)-stimulated whole blood from critically ill patients showed similar suppression. The secretion of IL-12 and IFN-γ was less reduced in critically ill patients when using isolated cultures of adherent cells or lymphocytes. Although preculture of whole blood from healthy humans with IL-10, but not with IL-4, mimicked suppression of the IL-12 IFN-γ pathway similar to that observed during critical illness, the release of antiinflammatory reacting cytokines (IL-4, IL-10, transforming growth factor [TGF]-β1 ) was decreased into LPS-stimulated whole blood from critically ill patients. These results indicate at least two mechanisms responsible for dramatic disturbances of the IL-12 IFN-γ pathway during critical illness: (1) deactivation of IL-12 and IFN-γ producing leukocytes in vivo early after the primary insult, and (2) presence of serum suppressive factors different from IL-4, IL-10, or TGF-β1 . Because IL-12 and IFN-γ upregulate essential immune functions, the marked inhibition of IL-12 and IFN-γ release may be pivotal for high susceptibility of critically ill patients to infection.