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

The enhanced responsiveness of BMM compared to BMDC or fibroblasts to MHV infection also occurs after exposure to other stimuli. The levels of IFN-dependent proinflammatory cytokines and chemokines, such as CCL2, CCL5, and CXCL10, were higher in BMM than in BMDC after stimulation with LPS (44) . This increased sensitivity to MHV infection is not likely to be a function of differences in basal levels because baseline levels of MDA5 and RIG-I are only 2-fold higher in BMM than in BMDC ( Fig. 2A) ; both cell types should have responded nearly equivalently to infection, if basal levels of MDA5 were the only factor. On the other hand, MDA5 levels are extremely low in 17Cl-1 cells (Fig. 6C ) (24) , and this may have contributed to the inability of these cells to produce IFN and other IFN-dependent proteins. Another possible explanation for the difference in IFN expression in infected BMM compared to BMDC or fibroblasts is that virus replication could occur on different types of membranous structures in the different cell types. MHV is known to replicate on double-membrane vesicles (DMV) and other intracellular membranes (20) . DMV may shield viral dsRNA from detection by MDA5 and RIG-I, so differences in the numbers of DMV or in the types of membranous structures used in replication could potentially contribute to differences in PAMP recognition and subsequent IFN induction. However, we detected no obvious differences in the types of virus-specific membranous structures observed in BMM compared to other cells when analyzed by electron microscopy (data not shown); if anything, the numbers of DMV in infected BMM were increased more than in other cells, probably reflecting the more rapid course of infection observed in these cells (Fig. 1D) . Thus, an outstanding question is how the milieu of resting macrophages differs from those of BMDC and fibroblasts, enabling rapid and extensive upregulation of IFN-␣/␤ and other proinflammatory molecules after infection with coronaviruses.