XB-ART-48673J Virol. March 12, 2014;
The amphibian (Xenopus laevis) type I interferon response to Frog Virus 3: new insight into ranavirus pathogenicity.
Increasing prevalence of ranavirus (RV; Iridoviridae) infections of wild and commercially maintained aquatic species is raising considerable concerns. While Xenopus laevis is the leading model for immune-RV studies, amphibian antiviral interferon responses remain largely uncharacterized. Accordingly, a X. laevis type I interferon was identified, the expression of this gene was examined in RV (Frog Virus 3, FV3) infected tadpoles and adult frogs by quantitative PCR and a recombinant form of this molecule (rXlIFN) was produced for the purpose of functional studies. This rXlIFN protected the kidney-derived A6 cell line and tadpoles against FV3 infections, decreasing infectious viral burdens in both cases. Adult frogs are naturally resistant to FV3 and clear infections within a few weeks, whereas tadpoles typically succumb to this virus. Hence as predicted, virally infected adult X. laevis exhibited significantly more robust FV3-elicited IFN gene expression than tadpoles, nevertheless they also tolerated substantially greater viral burdens following infection. Although tadpole stimulation with rXlIFN prior to FV3 challenge markedly impaired viral replication and viral burdens, it only transiently extended tadpole survival without preventing the eventual mortality of these animals. Furthermore, histological analysis revealed that despite rXlIFN treatment, infected tadpoles had considerable organ damage including disrupted tissue architecture and extensive necrosis and apoptosis. Conjointly, these findings indicate a critical protective role for the amphibian type I IFN response during ranaviral infections and suggest that these viruses are more pathogenic to tadpole hosts than previously believed, causing extensive and fatal damage to multiple organs, even at very low titers. Ranavirus infections are threatening wild and commercially maintained aquatic species. The amphibian Xenopus laevis is extensively utilized as an infection model for studying ranavirus-host immune interactions. However, little is known about amphibian antiviral immunity and specifically type I interferons (IFNs), which are central to antiviral defenses of other vertebrates. Accordingly, we identified and characterized a X. laevis type I interferon in the context of the ranavirus Frog Virus 3 (FV3) infections. FV3-infected adult frogs displayed more robust IFN gene expression than tadpoles, possibly explaining why they typically clear FV3 infections whereas tadpoles succumb to them. Pretreatment with a recombinant IFN (rXlIFN) substantially reduced viral replication and infectious viral burdens in a frog kidney cell line and in tadpoles. Despite reducing FV3 loads and extending mean survival, rXlIFN treatments failed to prevent tadpole tissue damage and mortality. Thus, FV3 is more pathogenic than previously believed, even at very low titers.
PubMed ID: 24623410