Research Note: A novel recombinant subgroup E isolate of the avian leukosis virus with a subgroup B-like Gp85 region in China

The avian leukosis virus (ALV) strain DL00766 was isolated from a farm in China. The phylogenetic analysis showed that env had the highest homology with the E subgroup reference strain, ranging from 94.5% to 94.9%, whereas gp85 had the highest homology with the B and E subgroups, which were 89.0% to 91.3% and 91.3% to 91.8%. In addition, point mutation analysis of gp85 showed that a 400 bp long fragment in gp85 of DL00766 had the highest homology with subgroup B, ranging from 90.1% to 97.5%, and only 82.7% to 83.1% with E subgroup. These results indicate, DL00766 may be an AVL subgroup E isolate with a subgroup B-like gp85 region. This is also the first finding that the E subgroup is used as a recombinant subject, and the subgroup B provides a recombinant virus of an exogenous gene.

The Bipartite Sequence Motif in the N and C Termini of gp85 of Subgroup J Avian Leukosis Virus Plays a Crucial Role in Receptor Binding and Viral Entry

Subgroup J avian leukemia virus (ALV-J), belonging to the genus Alpharetrovirus, enters cells through its envelope surface unit (gp85) via specifically recognizing the cellular receptor chicken Na⁺/H⁺ exchanger type I (chNHE1), the 28 to 39 N-terminal residues of which were characterized as the minimal receptor functional domain in our previous studies. In this study, to further clarify the precise organization and properties of the interaction between ALV-J gp85 and chNHE1, we identified the chNHE1-binding domain of ALV-J gp85 using a series of gp85 mutants with segment substitutions and evaluating their effects on chNHE1 binding in protein-cell binding assays. Our results showed that hemagglutinin (HA) substitutions of amino acids (aa) 38 to 131 (N terminus of gp85) and aa 159 to 283 (C terminus of gp85) significantly inhibited the interaction between gp85 and chNHE1/chNHE1 loop 1. In addition, these HA-substituted chimeric gp85 proteins could not effectively block the entry of ALV-J into chNHE1-expressing cells. Furthermore, analysis of various N-linked glycosylation sites and cysteine mutants in gp85 revealed that glycosylation sites (N6 and N11) and cysteines (C3 and C9) were directly involved in receptor-gp85 binding and important for the entry of ALV-J into cells. Taken together, our findings indicated that the bipartite sequence motif, spanning aa 38 to 131 and aa 159 to 283, of ALV-J gp85 was essential for binding to chNHE1, with its two N-linked glycosylation sites and two cysteines being important for its receptor-binding function and subsequent viral infection steps.IMPORTANCE Infection of a cell by retroviruses requires the attachment and fusion of the host and viral membranes. The specific adsorption of envelope (Env) surface proteins to cell receptors is a key step in triggering infections and has been the target of antiviral drug screening. ALV-J is an economically important avian pathogen that belongs to the genus Alpharetrovirus and has a wider host range than other ALV subgroups. Our results showed that the amino acids 38 to 131 of the N terminus and 159 to 283 of the C terminus of ALV-J gp85 controlled the efficiency of gp85 binding to chNHE1 and were critical for viral infection. In addition, the glycosylation sites (N6 and N11) and cysteines (C3 and C9) of gp85 played a crucial role in the receptor binding and viral entry. These findings might help elucidate the mechanism of the entry of ALV-J into host cells and provide antiviral targets for the control of ALV-J.

Development and evaluation of a gp85 protein-based subgroup-specific indirect enzyme-linked immunosorbent assay for the detection of anti-subgroup J avian leukosis virus antibodies

Avian leukosis virus subgroup J (ALV-J) is an important pathogen for various neoplasms and causes significant economic losses in the poultry industry. Serological detection of specific antibodies against ALV-J infection is important for successful clinical diagnosis. Here, a 293F stable cell line was established to stably express gp85 protein. In this cell line, gp85 protein was expressed at approximately 30 mg/L. A subgroup-specific indirect enzyme-linked immunosorbent assay (iELISA) was developed using ALV-J gp85 protein as coated antigen to detect antibodies against ALV-J. The sensitivity of the iELISA (1:51200 diluted in serum) was 16 times more than that of indirect immunofluorescence assay (IFA; 1:3200 diluted in serum). Moreover, there was no crossreactivity with antibodies against other common avian viruses and other avian leukosis virus subgroups, such as subgroups A and B. The practicality of the iELISA was further evaluated by experimental infection and clinical samples. The results from experimental infection indicated that anti-ALV-J antibodies were readily detected by iELISA as early as 4 weeks after ALV-J infection, and positive antibodies were detected until 20 weeks, with an antibody-positive rate of 11.1% to 33.3%. Moreover, analysis of clinical samples showed that 9.49% of samples were positive for anti-ALV-J antibodies, and the concordance rate of iELISA and IFA was 99.24%. Overall, these results suggested that the subgroup-specific iELISA developed in this study had good sensitivity, specificity, and feasibility. This iELISA will be very useful for epidemiological surveillance, diagnosis, and eradication of ALV-J in poultry farms.

First finding of subgroup-E avian leukosis virus from wild ducks in China

To analyze the status of avian leukosis virus subgroup E (ALV-E) in wild ducks in China, we collected 276 wild ducks, including 12 species, from four provinces of China. The PCR detection for ALV-E identified four samples as positive samples and the detection rate was 1.45%. The env sequences of ALV-E were cloned and sequenced. In gp85, genes of the four ALV-E strains showed a high homology (98.1-99.5%) with ev-1, ev-3, and SD0501 and more than 90% homology with other subgroup-A and subgroup-B avian leukosis viruses. However, they showed a slightly lower identity with subgroup-J (NX0101 and HPRS103), from 47.5 to 48.1%. Simultaneously, a further comparison with ALV-E representative isolates indicated that the amino acid substitutions of the four wild duck strains were distributed throughout the gp85. In total, these results suggested that the subgroup-E avian leukosis virus has been found in wild ducks in China.