IFN-γ-independent autocrine cytokine regulatory mechanism in reprogramming of macrophage responses to bacterial lipopolysaccharide

Macrophages are now well recognized to have a critical role in both innate and acquired immunity. The sentinel macrophage function is highly regulated and serves to allow for intrinsic plasticity of the innate immune responses to potential environmental signals. However, the mechanisms underlying the dynamic properties of the cellular arm of innate immunity are poorly understood. Therefore, we have conducted a series of in vitro studies to evaluate the contribution of immunoregulatory cytokines, such as IFN-γ, IL-10, and IL-12, in modulation of macrophage responses. We found that macrophages from IFN-γ knockout (IFN- γ^-/-) mice exhibit only marginal LPS-induced TNF-α, IL-12, and NO responses, all of which can be fully restored in the presence of rIFN-γ. Pretreatment with substimulatory LPS concentrations led to reprogramming of IFN-γ^-/- macrophage responses in a dose-dependent manner that manifested by an increased TNF-α and IL-12, but not NO, production upon the subsequent LPS challenge. These reprogramming effects were substantially attenuated and profoundly enhanced in macrophages from IL-12^-/- and IL-10^-/- mice, respectively, as compared with those modulated in macrophages from the congenic wild-type mice. LPS-dependent reprogramming was also fully reproduced in macrophages isolated from SCID mice after immunodepletion of NK cells. Our data strongly imply that cytokine (TNF-α and IL-12), but not NO, responses in macrophages may, at least in part, be governed by an autocrine IFN-γ-independent regulatory mechanism reciprocally controlled by IL-10 and IL-12. This mechanism may serve as an alternative/coherent pathway to the canonical IFN-γ-dependent induction of antimicrobial and tumoricidal activity in macrophages.