The Amphibian (Xenopus laevis) Type I Interferon Response to Frog Virus 3: New Insight into Ranavirus Pathogenicity
Authors: Grayfer Leon , De Jesús Andino Francisco , Robert Jacques 
Published: March 2014

The 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 studies of immunity to RV, amphibian antiviral interferon (IFN) responses remain largely uncharacterized. Accordingly, an
X. laevis
type I interferon was identified, the expression of the gene for this IFN was examined in RV (frog virus 3 [FV3])-infected tadpoles and adult frogs by quantitative PCR, and a recombinant form of this molecule (recombinant
X. laevis
interferon [r
Xl
IFN]) was produced for the purpose of functional studies. This r
Xl
IFN protected the kidney-derived A6 cell line and tadpoles against FV3 infection, decreasing the infectious viral burdens in both cases. Adult frogs are naturally resistant to FV3 and clear the infection within a few weeks, whereas tadpoles typically succumb to this virus. Hence, as predicted, virus-infected adult
X. laevis
frogs 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 r
Xl
IFN prior to FV3 challenge markedly impaired viral replication and viral burdens, it only transiently extended tadpole survival and did not prevent the eventual mortality of these animals. Furthermore, histological analysis revealed that despite r
Xl
IFN 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 was previously believed, causing extensive and fatal damage to multiple organs, even at very low titers.

IMPORTANCE
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 the antiviral defenses of other vertebrates. Accordingly, we identified and characterized an
X. laevis
type I interferon in the context of infection with the ranavirus frog virus 3 (FV3). 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
X. laevis
IFN (r
Xl
IFN) substantially reduced viral replication and infectious viral burdens in a frog kidney cell line and in tadpoles. Despite reducing FV3 loads and extending the mean survival time, r
Xl
IFN treatments failed to prevent tadpole tissue damage and mortality. Thus, FV3 is more pathogenic than was previously believed, even at very low titers.