Isolation and identification of a highly oncogenic subgroup J avian leukosis virus strain from Chinese black chickens

Avian leukosis virus subgroup J (ALV-J), an oncogenic retrovirus, is a highly contagious pathogen that induces myelocytomas, hemangiomas, and other neoplastic diseases in chickens. Recently, ALV-J infection in Chinese local chicken breeds has increased, with enhanced pathogenicity, posing a severe threat to the local poultry industry. In March 2024, a tumor outbreak occurred on a Chinese black chicken farm in Heze City, China, causing lethargy, emaciation, and tumorigenesis in the eyes and legs of affected chickens. Necropsy revealed extensive yellow-white neoplasms disseminated across the sternum, ribs, vertebrae, and visceral organs. Pathological examinations revealed multiple tumor types, including myelocytomas, hemangiomas, fibromas, and reticulosarcomas in the affected chickens. In this study, nine ALV-J isolates were isolated and designated HZ0319-1 to HZ0319-9. All isolates possessed an identical full-genome length of 7,687 bp and exhibited high nucleotide sequence similarity, ranging from 99.8% to 99.9%, suggesting that they were closely related variants from the same outbreak. Nucleotide sequence analysis identified unique deletions and mutations in the gag, pol, and gp85 genes, leading to predicted conformational changes in the P2, P10, and SU proteins. In addition, HZ0319 showed deletions in the r-TM region and a large 125-nt deletion in the E element of the 3' untranslated region, leaving only a short conserved fragment. The representative isolate HZ0319-1 showed stronger replication capacity than the reference strain NX0101 in DF-1 cells. In experimentally infected chicks, HZ0319-1 induced myelocytomas and hemangiomas and produced high viral load in multiple tissues. Notably, viral load analysis revealed that the liver showed the highest viral load at 1 day of age, whereas most other tissues reached peak viral load at 21 days of age, suggesting early hepatic replication followed by systemic dissemination. Furthermore, HZ0319-1infection significantly upregulated tumor-related host genes, including p53, c-Myc, c-Fos, and ZIC1, in tumor-associated tissues. These results demonstrate that the HZ0319-1 isolate has enhanced tumorigenicity and replication ability. Nucleotide mutations and deletions in both coding and non-coding regions of the viral genome may alter the viral tissue tropism and oncogenic potential. This study provides novel insights into the molecular characteristics and pathogenicity of ALV-J in local Chinese chickens, and provides a foundation for the prevention and eradication of ALV-J in the local poultry industry.