Rapid induction of lymphoid leukosis and ascites by avian leukosis virus from a lymphoid leukosis cell line

Molecular characteristics and pathogenicity of a Tibet-origin mutant avian leukosis virus subgroup J isolated from Tibetan chickens in China

Tibetan chicken is found in China Tibet (average altitude; ˃4500 m). However, little is known about avian leukosis virus subgroup J (ALV-J) found in Tibetan chickens. ALV-J is a typical alpharetrovirus that causes immunosuppression and myelocytomatosis and thus seriously affects the development of the poultry industry. In this study, Tibet-origin mutant ALV-J was isolated from Tibetan chickens and named RKZ-1-RKZ-5. A Myelocytomatosis outbreak occurred in a commercial Tibetan chicken farm in Shigatse of Rikaze, Tibet, China, in March 2022. About 20% of Tibetan chickens in the farm showed severe immunosuppression, and mortality increased to 5.6%. Histopathological examination showed typical myelocytomas in various tissues. Virus isolation and phylogenetic analysis demonstrated that ALV-J caused the disease. Gene-wide phylogenetic analysis showed the RKZ isolates were the original strains of the previously reported Tibetan isolates (TBC-J4 and TBC-J6) (identity; 94.5% to 94.9%). Furthermore, significant nucleotide mutations and deletions occurred in the hr1 and hr2 hypervariable regions of gp85 gene, 3'UTR, Y Box, and TATA Box of 3'LTR. Pathogenicity experiments demonstrated that the viral load, viremia, and viral shedding level were significantly higher in RKZ-1-infected chickens than in NX0101-infected chickens. Notably, RKZ-1 caused more severe cardiopulmonary damage in SPF chickens. These findings prove the origin of Tibet ALV-J and provide insights into the molecular characteristics and pathogenic ability of ALV-J in the plateau area. Therefore, this study may provide a basis for ALV-J prevention and eradication in Tibet.

Cardiomyopathy in broiler chickens congenitally infected with avian leukosis virus subgroup J

Dilated cardiomyopathy and ascites in broiler chickens are frequently associated with rapid growth and pulmonary hypertension, but can be associated with some avian leukosis virus (ALV) infections. The novel subgroup J of ALV has a high cardiac tropism, but dilated cardiomyopathy has not been reported previously. We report a dilated cardiomyopathy incidence of 11.1% in broiler chickens congenitally infected with ALV subgroup J (ALV-J). Gross lesions included severe body weight suppression, cardiomegaly with biventricular dilation, right ventricular hypertrophy, visceral congestion, and ascites. Cardiac myocytes and Purkinje fibers contained 2- to 10-microm intracytoplasmic magenta inclusions that contained ALV-J-specific nucleic acid. Ultrastructurally, inclusions contained ribosomes and immature virions and were associated with myofibril disruption and disarray. Peracute centrilobular hepatic necrosis was present in most cases. ALV-J-associated cardiomyopathy may involve a direct viral effect on cardiac myocytes and Purkinje fibers.

Localization of avian leukosis virus subgroup J in naturally infected chickens by RNA in situ hybridization

The novel subgroup J of avian leukosis virus (ALV-J) has emerged as a significant cause of myeloid neoplasia and weight suppression in broiler chickens. We investigated viral tropism using RNA in situ hybridization (ISH) in naturally infected chickens. Formalin-fixed tissues were collected from 12-day-old embryos (seven infected, two control) and from 0-week-old (four infected, one control), 3-week-old (five infected, one control), 6-week-old (five infected, one control), and 9-week-old (10 infected, two control) chickens naturally infected with ALV-J in ovo. A 636-base antisense riboprobe complementary to the 3' and 5' ends of the pol and env viral genes, respectively, was constructed. Strong positive staining was present in cardiac myocytes, Purkinje fibers, vascular and pulmonary smooth muscle, renal glomeruli, distal tubules, and pituitary glands. Light staining was present in gastrointestinal smooth muscle, thyroid and adrenal glands, and follicular medullae in the cloacal bursa. Staining was not present in any hematopoietic precursors. Tissues from newly hatched chicks exhibited the strongest and most consistent staining, whereas staining in embryos was minimal. RNA ISH confirmed the presence of ALV-J-specific nucleic acid within cytoplasmic inclusions in cardiac myocytes, Purkinje fibers, pituitary glands, and renal glomeruli. Viral tropism for cardiac myocytes and Purkinje fibers may relate pathogenetically to the cardiomyopathy and congestive heart failure described in index chicken flocks infected with ALV-J. Viral tropism for endocrine organs may relate pathogenetically to the weight suppression associated with infection.

Rearrangement of c-myc gene in rapidly induced avian lymphoid leukosis tumors

Southern blot hybridization of DNA samples from 9 primary tumors of avian lymphoid leukosis (LL) rapidly induced by ALV infection 27-74 days post inoculation was carried out to search for rearrangement of the c-myc gene with human c-myc gene exon III as a probe. Rearrangement of the c-myc gene was detected by appearance of new EcoRI fragments in 7 out of 9 tumors examined. The size of the fragments ranged from 3.1 to 4.0 kilobases (kb). In addition to these fragments, two fragments (9.0 kb and 13 kb) were observed in one tumor, and a faint fragment (3.5 kb) was observed in another tumor. Rearrangement of the c-myc gene was not detected in the remaining two tumors kept in unsuitable condition. These results suggest that rearrangement of c-myc gene was induced even in rapidly induced LL as well as that induced after long incubation period. This is the first report of involvement of c-myc gene in rapidly induced B-cell lymphoma (LL).

Isolation of serotype 2 Marek's disease virus from a cell line of avian lymphoid leukosis

To determine a serotype of Marek's disease virus (MDV) persistently infected in a lymphoid leukosis (LL)-cell line, LSCC-BK3, clone A (BK3A), and to examine the pathogenicity of the virus, an attempt was made to isolate MDV from culture fluid of the LL-cell line, using chick embryo fibroblast cultures resistant to infection with subgroup A avian leukosis virus (ALV) to eliminate subgroup A ALV. The MDV isolate, serologically identified as serotype 2 MDV and designated as the 2H strain, was free from ALV and reticuloendotheliosis virus. A serotype 2-specific antigen of MDV was detected in 44% of BK3A, but antigens of serotypes 1 and 3 were not detected in this cell line, by the indirect immunofluorescent antibody test using serotype-specific monoclonal antibodies. MDV antigens were undetectable in LSCC-BK3, clone 2C; both cell lines were established from the same chicken. Neither clinical signs nor macroscopic lesions were observed in any of 19 chicks inoculated with the 2H strain. Three out of 19 chicks histopathologically examined had no lesions. These results suggest that serotype 2 MDV can persist in B cells transformed by ALV without cytopathic effect at a high rate, and the isolate may become a candidate for MD vaccine strains.