Recombinant subgroup B avian leukosis virus combined with the subgroup J env gene significantly increases its pathogenicity

•Two rescued viruses rGX15MM6−2and rGX14FF03 and a recombinant virus rALV-B-Jenv were generated and tested utilizing both in vitro and in vivo experiments.•The env gene affects virus replication in vitro, and affects viremia, tumor type in vivo.•Recombinant subgroup B avian leukosis virus combined with the subgroup J env gene significantly increases its pathogenicity. The differences among different sub-groups of the avian leukosis virus (ALV) genome are mainly concentrated in the env gene, which binds to cell-specific receptors and determines the characteristics of viral tropism and pathogenicity. In this study, two rescued viruses rGX15MM6−2 (ALV of subgroup J, ALV-J) and rGX14FF03 (ALV of subgroup B, ALV-B) and a recombinant virus rALV-B-Jenv (ALV-B’s backbone with ALV-J’s env) were generated and tested utilizing both in vitro and in vivo experiments. The results showed that the replication ability of the viruses released in DF-1 cell cultures was listed in order as rGX15MM6−2 >rALV-B-Jenv >rGX14FF03. rGX15MM6−2 caused the most serious suppression of body weight gain, exhibited a significant negative effect on the development of immune organs (P < 0.05) and lower antibody responses to vaccinations with the commercial oil-emulsion vaccines (OEVs) (P