Cytoplasmic signaling of polyinosinic-polycytidylic acid stimulates a quick and potent interferon response in primary macrophages

Polyinosinic-polycytidylic acid (poly(I:C)), the mimic of viral double-stranded RNA, signals through endosomal or cytoplasmic pathways to upregulate interferons (IFNs). This may be beneficial for antiviral defence in the airways. We have previously shown that in airway epithelial cells, IFNs are potently and selectively upregulated by poly (I:C) stimulation of the cytoplasmic RIG-I / MAVS pathway (J Immunol 2015;195:2829-41). Furthermore, in airway epithelial cells, endosomal escape and cytoplasmic accumulation are enhanced by poly(I:C) encapsulation within pH-sensitive liposomes (CSL3; Eur Respir J 2016;48 suppl 60:PA965). Macrophages are key immune cells in the airways. Here we have tested applicability of prior epithelial findings to primary monocyte-derived macrophages. IFN-β and IFN-λ1 response to poly(I:C)+CSL3 was compared with that to poly(I:C) encapsulated within pH-insensitive liposomes (poly(I:C)+Lyovec) or pure poly(I:C). Outcomes were poly(I:C) internalization and colocalization with RIG-I (both by immunostaining), and kinetics of IFN mRNAs (qPCR). Internalized poly(I:C)+CSL3 was detectable as early as 15 min of stimulation, prior to detection of poly(I:C)+Lyovec or pure poly(I:C). At 60 min of stimulation, poly(I:C)+CSL3 colocalized with RIG-I (Pearson correlation coefficient: 0.38) more frequently than poly(I:C)+Lyovec or pure poly(I:C) (respective coefficients 0.12 and 0.09). Finally, stimulation with poly(I:C)+CSL3 led to markedly faster and more potent upregulation of both IFN mRNAs. Conclusion: in macrophages, cytoplasmic signaling of poly(I:C) accelerates and enhances the IFN response. Funding: Krieble and Gosselin Foundations, NSERC.