Differential regulation of type I and II interferon signals by the transcription factor interferon regulatory factor-2 for the generation and function of macrophage populations in the liver

Two major macrophage populations in the steady-state liver, resident Kupffer cells (KCs) and monocyte-derived macrophages (MoMFs), contribute crucially to the unique physiological functions of the organ. Much remains to be learned, however, about how the differentiation and functions of these cell populations are regulated. We found here that Ly6C-MHCII+ MoMFs were severely reduced in mice lacking interferon (IFN) regulatory factor-2 (IRF-2) (Irf2-/- mice) but restored to the normal frequencies by introducing type I IFN receptor deficiency, indicating that IRF-2 supports MoMF differentiation through attenuating excess type I IFN signals. On the other hand, Irf2-/- KCs developed normally but lacked MHC class II (MHCII) expression. Similar MHCII deficiency in KCs in Il15-/- and Ifng-/- but not Rag1-/- mice pointed to the role for NK cell-derived IFN-γ. Indeed, MHCII expression on resident KCs in Ifng-/- mice was recovered via wild-type NK cells that circulated upon parabiosis as well as by administration of IFN-γ. In contrast, parabiotic restoration of NK cell deficiency in Irf2-/- mice failed to elevate MHCII expression on KCs. Furthermore, Irf2-/- KCs required several times higher amounts of IFN-γ to upregulate MHCII expression than Ifng-/- KCs. Thus, IRF-2 maintains steady-state MHCII expression on KCs by potentiating IFN-γ responses of KCs. Collectively, our current study revealed that IRF-2 plays critical roles in the establishment of the steady state hepatic macrophage system through negative and positive regulation of type I IFN and IFN-γ signaling, respectively.