BACKGROUND:
A hallmark of viruses with positive-sense RNA genomes is to form and replicate within vesicle-like membrane structures, called replication complexes (RCs). RCs have been considered to provide an advantageous microenvironment for viral replication. However, how the host immune system counteracts these structures was unknown.
METHODS:
The replication of murine norovirus (MNV) and the localization of microtubule-associated-protein-1-light-chain-3 (LC3) and IFN-inducible GTPases (immunity-related GTPases [IRGs] and/or guanylate-binding proteins [GBPs]) with regard to MNV RC were investigated in bone marrow derived macrophages and mouse embryonic fibroblasts, derived from mice of various genetic deletions, with or without activation of the cells with interferon-gamma (IFNG). The survival of mice infected with MNV and the replication of MNV in the infected tissues were monitored using the mice with the defective IRG or GBP system. The replication of MNV and the localization of LC3 and GBPs on the MNV RC were also monitored in human cell lines with various genetic deletions after IFNG activation.
RESULTS:
The RC of MNV was disrupted and the replication of MNV was inhibited by IFNG; such inhibitory effect of IFNG was dependent on targeting of IFN-inducible GTPases (IRGs and GBPs) to the RC of MNV via the LC3 conjugation system of autophagy (e.g. ATG5). However, the canonical autophagy pathway (e.g. ATG14) was not required for this antiviral activity of IFNG. Consistently, the mice with defective IRG or GBP system were more susceptible to MNV infection; in human cells, the LC3 conjugation system was also required for targeting of GBPs to the RC of MNV and both systems were required to control MNV replication.
CONCLUSIONS:
IFN-inducible GTPases are known to destroy the membrane of vacuoles containing bacteria, protists, or fungi. Thus, our data suggest that viral RCs can be antagonized by a universal immune defense mechanism against the membranous shelters of pathogens.