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

We previously showed that neither BMDC nor fibroblasts produce type I IFN after infection with the JHM strain of MHV (JHMV), and others have found similar results using the A59 strain (MHV-A59) (18, 19, 23, 33) . For the experiments described here, we used only MHV-A59 (MHV) because this virus grows to high titer and forms syncytia at later times postinfection (p.i.) than JHMV, allowing prolonged cell survival. Roth-Cross et al. showed that BMM produced type I IFN after MHV infection, but only late times p.i. (24 h p.i.) were examined in that initial study (24) . To determine the kinetics of IFN expression, we infected BMM or BMDC with MHV and then measured IFN-␤ mRNA and IFN protein levels at different times p.i. by real-time quantitative PCR (qPCR) and by an IFN bioassay, respectively. Sendai virus (SenV) was included as a positive control, since it is known to induce IFN. SenV-infected BMM rapidly upregulated IFN expression with protein detected as early as 2 h p.i., and peak levels observed at approximately 8 h p.i. (Fig. 1A) . In contrast, MHV-infected BMM produced IFN much more slowly with levels of IFN-␤ mRNA or IFN protein barely detectable at 8 h p.i. Peak levels of IFN-␤ mRNA and protein were reached at 16 to 24 h p.i. (Fig. 1A to C). Maximal IFN protein levels were approximately 20-to 25-fold lower in MHVinfected cells than in SenV-infected cells ( Fig. 1A and B) . Although the IFN protein levels were much lower in MHV-infected BMM than in SenV-infected cells, IFN-␤ mRNA levels were higher in MHVinfected cells (Fig. 1A to C; see also Fig. 3C and E). This likely resulted from MHV-mediated inhibition of host cell translation, observed in MHV-infected fibroblasts (11, 14, 34, 35) . Consistent with previous findings (23, 32, 33) , we detected less than 3 U/ml of IFN in MHVinfected DC at 24 h p.i. (Fig. 1B) . These results did not reflect differences in the ability of the two cell types to produce IFN, because DC rapidly produced IFN after SenV infection and peak IFN protein lev- (52) . Two or three replicates were performed in each experiment, and one of three independent experiments is shown. els were only 2-fold less in SenV-infected BMDC than in BMM (Fig. 1A) . Of note, both BMM and BMDC were readily infected by MHV, although virus replication proceeded more rapidly in BMM, with syncytium formation detected as early as 6 h p.i. and peak viral titers observed at 8 to 12 h p.i. MHV reached maximal virus titers in BMDC that were approximately 1 log unit higher than in BMM (Fig. 1D) .