Biological characterization of an avian leukosis virus subgroup J isolate from yunnan indigenous black-bone chickens and generation of its full-length infectious clone.

Avian leukosis virus subgroup J (ALV-J) remains a major threat to poultry health and production, particularly in indigenous chicken populations in China. In this study, a highly pathogenic ALV-J field strain, YN2021, was first isolated from indigenous black-bone chickens in Yunnan Province, China, and its biological characteristics and pathogenicity were systematically evaluated in specific-pathogen-free (SPF) chickens. Infected chickens exhibited significant growth retardation, delayed sexual maturation, and increased mortality, accompanied by pathological lesions consistent with ALV-J. To further assess reproductive performance, egg production and egg weight were recorded. YN2021-infected hens showed a reduction in total egg production (70 vs. 92 eggs; ~23.9% decrease) and a significantly lower mean egg weight (35.2 ± 0.2 g vs. 43.7 ± 0.3 g; P < 0.001) compared to controls. To facilitate mechanistic studies and future control strategies, a full-length infectious clone of YN2021 was constructed using a reverse genetics approach, and a synonymous molecular marker was introduced for viral identification. The rescued recombinant virus exhibited replication kinetics, p27 antigen expression, and biological characteristics in DF-1 cells comparable to those of the parental strain, and the molecular marker remained genetically stable during serial passages. Collectively, these results demonstrate that the ALV-J YN2021 strain exhibits high pathogenicity and negatively affects growth and reproductive performance in chickens. The infectious clone established in this study provides a reliable experimental platform for investigating ALV-J pathogenesis and supports the development of effective control strategies to mitigate production losses in poultry.