Innate Immune Evasion Mediated by the Ambystoma tigrinum Virus Eukaryotic Translation Initiation Factor 2α Homologue

Ranaviruses (family Iridoviridae, genus Ranavirus) are large, double-stranded DNA (dsDNA) viruses whose replication is restricted to ectothermic vertebrates. Many highly pathogenic members of the genus Ranavirus encode a homologue of the eukaryotic translation initiation factor 2 alpha (eIF2 alpha). Data in a heterologous vaccinia virus system suggest that the Ambystoma tigrinum virus (ATV) eIF2 alpha homologue (vIF2 alpha H; open reading frame [ ORF] 57R) is involved in evading the host innate immune response by degrading the interferon-inducible, dsRNA-activated protein kinase, PKR. To test this hypothesis directly, the ATV vIF2 alpha H gene (ORF 57R) was deleted by homologous recombination, and a selectable marker was inserted in its place. The ATV Delta 57R virus has a small plaque phenotype and is 8-fold more sensitive to interferon than wild-type ATV (wtATV). Infection of fish cells with the ATV Delta 57R virus leads to eIF2 alpha phosphorylation, in contrast to infection with wtATV, which actively inhibits eIF2 alpha phosphorylation. The inability of ATV Delta 57R to prevent phosphorylation of eIF2 alpha correlates with degradation of fish PKZ, an interferon-inducible enzyme that is closely related to mammalian PKR. In addition, salamanders infected with ATV Delta 57R displayed an increased time to death compared to that of wtATV-infected salamanders. Therefore, in a biologically relevant system, the ATV vIF2 alpha H gene acts as an innate immune evasion factor, thereby enhancing virus pathogenesis.