TNFα Impairs Rhabdoviral Clearance by Inhibiting the Host Autophagic Antiviral Response

Abstract

TNFα is a pleiotropic pro-inflammatory cytokine with a key role in the activation of the immune system to fight viral infections. Despite its antiviral role, a few viruses might utilize the host produced TNFα to their benefit. Some recent reports have shown that anti-TNFα therapies could be utilized to treat certain viral infections. However, the underlying mechanisms by which TNFα can favor virus replication have not been identified. Here, a rhabdoviral infection model in zebrafish allowed us to identify the mechanism of action by which Tnfa has a deleterious role for the host to combat certain viral infections. Our results demonstrate that Tnfa signals through its receptor Tnfr2 to enhance viral replication. Mechanistically, Tnfa does not affect viral adhesion and delivery from endosomes to the cytosol. In addition, the host interferon response was also unaffected by Tnfa levels. However, Tnfa blocks the host autophagic response, which is required for viral clearance. This mechanism of action provides new therapeutic targets for the treatment of SVCV-infected fish, and advances our understanding of the previously enigmatic deleterious role of TNFα in certain viral infections.

Zebrafish C-reactive protein isoforms inhibit SVCV replication by blocking autophagy through interactions with cell membrane cholesterol

“…Many viruses, including those of fish, activate/need autophagy to replicate 58,59,66,[78][79][80] . In this regard, there are some previous studies that have analysed the influence of autophagy on SVCV infection 54,57,58 ; however, the conclusions of these studies are contradictory with respect to the activation of autophagy as either a negative regulatory mechanism 54,57 or, as determined more recently, a mechanism required by the virus for replication 58 . In this work, we showed that SVCV requires autophagic activity for replication since the infectivity was neutralized by the autophagy blockers CQ (Figs.…”

“…Although the results described above suggest that CRPs might induce autophagy, this is a debated issue for rhabdoviruses 54,[56][57][58][59] . In this context, Fig.…”

Studying Autophagy in Zebrafish

“…The most widely used method for analyzing autophagy is by measuring the levels of lipidated or membrane bound form of Atg8/Lc3B (Atg8-PE/Lc3B-II), as it runs at a different molecular weight than the cytosolic form of Lc3 (Lc3-I) by SDS-PAGE [ 79 , 107 ]. This method has been used to measure levels of autophagy in some zebrafish autophagy studies [ 46 , 58 , 86 , 95 , 104 , 106 , 108 , 109 ]. Again, it cannot be concluded that a mere increase in Lc3-II levels corresponds to increased autophagy, as this can also be due to autolysosomal formation defects, and it is therefore important to do proper autophagic flux experiments (as described above) to conclude about increased/reduced autophagy.…”

“…Recently, the zebrafish has also been used to study the host autophagic response to a viral infection [ 109 ]. Zebrafish can be infected with spring viremia of carp virus (SVCV), a member of the rhabdovirus family.…”

“…GFP-Lc3 imaging and Western blot analysis showed that depletion of Tnfα increased autophagy in SVCV-infected larvae. Since depletion of Tnfα also improved resistance to SVCV infection, the authors concluded that inhibition of autophagy is the mechanism behind the deleterious effect of Tnfα on viral clearance [ 109 ]. A wide variety of other zebrafish infection models provide excellent tools to further advance our understanding of the role of autophagy in host-pathogen interactions [ 163 ].…”