Autocrine Interferon Priming in Macrophages but Not Dendritic Cells Results in Enhanced Cytokine and Chemokine Production after Coronavirus Infection

Zhou, Haixia; Zhao, Jincun; Perlman, Stanley

Type I IFN mediates enhanced expression of MDA5 and RIG-I (36). Thus, elevated baseline levels of MDA5 and RIG-I in MHVinfected BMM ( Fig. 2A ) might facilitate enhanced IFN production at early times p.i., which could then result in increased MDA5 and RIG-I expression after signaling through IFNAR. To examine this possibility, we cultured BMM from wild-type and IFNAR Ϫ/Ϫ mice. We included BMM from MyD88 Ϫ/Ϫ mice in these experi-ments, since MyD88 is involved in signaling through TLRs but not IFNAR. We detected low basal levels of IFN, MDA5, and RIG-I in BMM from all mice, although the levels were lowest in IFNAR Ϫ/Ϫ BMM (Fig. 3B ). The levels of IFN-␤, MDA5, and RIG-I mRNA were significantly lower in infected IFNAR Ϫ/Ϫ BMM than in wildtype BMM (Fig. 3C) . However, the mRNA levels of these molecules in infected IFNAR Ϫ/Ϫ BMM were still higher than in mockinfected IFNAR Ϫ/Ϫ cells ( Fig. 3B and C), indicating that virus infection directly upregulates IFN-␤, MDA5, and RIG-I, but optimal induction requires signaling through the IFNAR. These differences did not result from differences in susceptibility to infection because equivalent titers of virus were observed in IFNAR Ϫ/Ϫ , MyD88 Ϫ/Ϫ , and wild-type (WT) BMM at 16 h p.i. (Fig. 3A) . Of note, MHV titers are much higher in tissues of IFNAR Ϫ/Ϫ mice than in wild-type mice. The lack of difference in titers in Fig. 3A may reflect the high multiplicity of infection that we used in our experiments or the well-described relative insensitivity of MHV to IFN signaling in tissue culture cells (13, 14, 37) .