Pathogenicity of avian leukosis viruses related to fowl glioma-inducing virus.

Fowl glioma-inducing virus (FGV), which belongs to avian leukosis virus subgroup A, causes the so-called fowl glioma and cerebellar hypoplasia in chickens. In the present study, the complete nucleotide sequences of four isolates (Tym-43, U-1, Sp-40 and Sp-53) related to the FGV prototype were determined and their pathogenicity was investigated. Phylogenetic analysis showed that the 3'-long terminal repeat of all isolates grouped together in a cluster, while sequences of the surface (SU) proteins encoded by the env gene of these viruses had 85 to 96% identity with the corresponding region of FGV. The SU regions of Tym-43, U-1 and FGV grouped together in a cluster, but those of Sp-40 and Sp-53 formed a completely separate cluster. Next, C/O specific-pathogen-free chickens were inoculated in ovo with these isolates as well as the chimeric virus RCAS(A)-(FGVenvSU), constructed by substituting the SU region of FGV into the retroviral vector RCAS(A). The four variants induced fowl glioma and cerebellar hypoplasia and the birds inoculated with Sp-53 had the most severe lesions. In contrast, RCAS(A)-(FGVenvSU) provoked only mild non-suppurative inflammation. These results suggest that the ability to induce brain lesions similar to those of the FGV prototype is still preserved in these FGV variants.