Nucleic acid strand length governs mitochondrial reprogramming and mtROS-associated antiviral responses following TLR3 engagement

Abstract

Mitochondria are important rheostats that regulate innate sensing processes by producing energy, biosynthetic precursors, and bioactive molecules that affect cellular signaling. When viral nucleic acids engage endosomal and cytosolic pattern recognition receptors (PRR), antiviral immune responses are supported by mitochondrial remodeling but the role of mitochondria in fine tuning ligand-specific responses remains incompletely understood. For example, endosomal TLR3 can detect various lengths of dsRNA (0.4-8 kb) ranging from viral segmented genomes or endogenous nucleic acids have been shown to induce distinct cytokine profiles. However, it is unclear if these differences are associated with differential mitochondrial remodeling. Here, we report that TLR3 engagement with both high (HMW; 1.5-8 kb) or low molecular weight (LMW; 0.2-1 kb) Polyinosinic:polycytidylic acid (Poly(I:C)) is associated with reduced but sustained oxidative phosphorylation (OXPHOS) activity and increased mitochondrial reactive oxygen species (mtROS) production/accumulation to support antiviral responses in bone marrow-derived macrophages (BMDM). They differed in the amount of mtROS production, their spare respiratory capacity (SRC) and their mitochondrial membrane potential (MMP). Interestingly, while uncoupling protein 2 (UCP2) was found required for antiviral cytokine production, it did not contribute to ligand specific responses. Dynamic modulation of complex I of the electron transport chain (ETC), however resulted in the differential accumulation of mtROS (HMW>LMW). Further, selectively targeting the mtROS derived from Complex I leads to augmented type I IFN production. Overall, these findings highlight that targeting specific sources of mtROS without affecting electron flow may be a potential avenue for specific augmentation of antiviral responses during viral infections.