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

It is likely that MDA5 is involved in one or both steps of this amplification process, because MDA5 is expressed in mock-infected BMM (Fig. 2) , and previous work has shown that IFN expression was significantly decreased in MHVinfected MDA5 Ϫ/Ϫ BMM (24) . The levels of MDA5 and RIG-I increased greatly after infection via an IFN-dependent pathway, providing additional pathogen recognition receptors (PRRs) for detecting virus replication products. By late times To determine whether upregulation required active virus replication, BMM were exposed to UV-inactivated virus prior to IFN treatment, and the levels of select genes were determined (shown in the boxes to the right of the graph for BMM 16-h sample graphs). C T ratios to HPRT are shown. All genes were induced to higher levels in BMM than in BMDC or 17Cl-1 cells (note the differences in scale). Three or four replicates were performed in each experiment, and one of three independent experiments is shown. Values that were statistically significantly different from the values for cells infected with MHV and treated with IFN are indicated as follows: *, P Ͻ 0.05; **, P Ͻ 0.01. p.i., sufficient levels of IFN were synthesized so that exogenously added IFN had no effect on the levels of any molecules that were examined. Similar effects were observed in BMDC and 17Cl-1 fibroblasts, but in these cells, no or very small amounts of IFN mRNA (Fig. 6B ) and no protein (18, 23, 32, 33) were produced endogenously. Consequently, in both cell types, exogenous IFN treatment significantly upregulated expression of MDA5, RIG-I, CXCL10, and ISG15. These results are consistent with recent reports showing that IFN priming prior to tissue culture cell infection with SARS-CoV resulted in augmented expression of several molecules involved in IFN induction and signaling pathways, including CXCL10, RIG-I, MDA5, IRF-7, IFN-␤, ISG15, and 2=,5=oligoadenylate synthetase 1 (OAS1) (45) . Analogous results were obtained when human conventional DC were primed with IFN prior to infection with influenza A virus (IAV) (46) . Collectively, our results suggest that IFN is critical for maximal expression of these proinflammatory mediators, but unlike MHV-infected BMDC or 17Cl-1 cells, sufficient amounts of IFN are expressed endogenously via an MDA5-dependent pathway in infected BMM to initiate the process.