An avian leukosis virus subgroup J isolate with a Rous sarcoma virus-like 5'-LTR shows enhanced replication capability

Avian leukosis virus usurps the cellular SERBP1 protein to enhance its transcription and promote productive infections in avian cells

Avian leukosis virus subgroup K (ALV-K) is composed of newly emerging isolates, which cluster separately from the well-characterized subgroups A, B, C, D, E, and J in sequence analysis, and exhibits a specific host range and a unique pattern of superinfection interference. Avian leukosis virus subgroup K replicate more slowly in avian cells than other ALV strains, leading to escaped detection during ALV eradication, but the underlying mechanism are largely unknown. In our previous study, we have reported that JS11C1 and most of other suspected ALV-K strains possessed unique mutations in the U3 region. Here, we selected 5 mutations in some important transcriptional regulation elements to explore the possible factor contributing for the lower activity of LTR, including CA-TG mutation in the CAAT box, 21 nt deletion in the CAAT box, A-G and A-T mutations in the CArG boxes, 11 nt insertion in the PRE boxes, and C-T mutation in the TATA box. On the basis of infectious clone of JS11C1, we demonstrated that the 11 nt fragment in the PRE boxes was associated with the transcription activity of LTR, the enhancer ability of U3, and the replication capacity of the virus. Notably, we determined the differential U3-protein interaction profile of ALVs and found that the 11 nt fragment specifically binds to cellular SERPINE1 mRNA binding protein 1 (SERBP1) to increase the LTR activity and enhance virus replication. Collectively, these findings reveal that a 11 nt fragment in the U3 gene contributed to its binding ability to the cellular SERBP1 to enhance its transcription and the infectious virus productions in avian cells. This study highlighted the vital role of host factor in retrovirus replication and thus provides a new perspective to elucidate the interaction between retrovirus and its host and a molecular basis to develop efficient strategies against retroviruses.

Isolation, identification and pathogenicity of a ALV-K strain from Chinese indigenous chicken breed

Subgroup K avian leukosis virus (ALV-K) is a new subgroup of avian leukosis virus (ALV) first identified in Chinese indigenous chickens in recent years. In this study, an ALV-K strain was isolated from Luhua chicken in Shandong province, China, and designated SD20LH01. The full-length genomic sequence of SD20LH01 was 7491 bp, which had the highest homology with ALV-K reference strains GDFX0601, GDFX0602 and GDFX0603. The nucleotide homology of env gene of SD20LH01 with reference strains of subgroup A, B, C, D, E, and J was ranged from 57.1 to 93.2%, while 94.1 to 99.4% with other ALV-K reference strains. The nucleotide difference of SD20LH01 mainly clustered with gp85 gene and U3 sequence when compared with the reference strain of ALV-K. In order to investigate the pathogenicity of SD20LH01, SPF chicken embryos were infected by yolk sac inoculation, and 1-day-old chickens were infected by intraperitoneal inoculation of SD20LH01. The results showed that yolk sac inoculation of SD20LH01 could induce persistent viremia, growth retardation and reduce the immune response to NDV and AIV-H9 vaccines. However, intraperitoneal inoculation in 1-day-old chickens could only induce a low level of viremia. In addition, no tumors were found in infected chickens during the animal experiments. This study enriched the genomic sequence data of ALV-K isolated in Chinese indigenous chickens, and laid a foundation for further study on the pathogenesis and prevention of ALV-K.

Role of env gene and LTR sequence in the pathogenesis of subgroup K avian leukosis virus

Avian leukosis virus (ALV) is a retrovirus that induces tumours in infected birds; ALV is divided into different subgroups according to the env gene and cellular tropism. In general, ALV subgroup J (ALV-J) is considered to be the most pathogenic and prevalent subgroup while subgroup K (ALV-K), a newly identified subgroup, only causes mild symptoms. To illuminate the roles of the env viral gene and LTR sequence in pathogenic differences between ALV-J and ALV-K, rescued ALV-J strain rSDAU1005, rescued ALV-K strain rJS11C1, and recombinant strains rENV(J)-LTR(K) and rENV(K)-LTR(J) were characterized and investigated in this study. Among rescued viruses, rSDAU1005 had the highest replication efficiency while rJS11C1 replicated the slowest (replication efficiency rankings were rSDAU1005 >rENV(K)-LTR(J)>rENV(J)-LTR(K)>rJS11 C1). The luciferase reporter gene assay results showed that the promoter activity of ALV-K LTR was lower than that of the ALV-J LTR promoter, which may have accounted for the slower replication efficiency of ALV-K. Pathogenicity of the four rescued viruses was determined via inoculating the yolk sacs of specific-pathogen-free chickens. The results demonstrated that all four viruses were pathogenic; rSDAU1005 caused the most severe growth retardation and immunosuppression. rENV(J)-LTR(K) was more pathogenic when compared to rENV(K)-LTR(J), indicating that env and the LTR sequence play important roles in pathogenicity between ALV-K and ALV-J. Additionally, env seemed to especially play a role in ALV-K pathogenesis. This study provided scientific data and insight to improve detection methods and judgement criteria in ALV clearance and surveillance.

Antibody profiles of avian leukosis virus subgroups A/B and J In layer flocks suspected to have Marek’s disease in Nigeria

Previous reports indicate high seroprevalence of avian leukosis virus (ALV) p72 antigen in layer flocks suspected to have Marek’s disease (MD) in Kaduna and Plateau States. However, the specific subgroups responsible for ALV infection in layers in the States are still unknown, hence the need for this study. Therefore, the objective of this study was to determine the antibody profiles of ALV subgroups A/B and J in layer flocks suspected to have MD in Kaduna and Plateau States. Sera from 7 and 16 layer flocks suspected to have MD in Kaduna and Plateau States respectively, were screened for the presence of antibodies to ALV subgroups A/B and J using IDEXX enzyme linked immunosorbent assay (ELISA) kits. Out of the seven layer flocks screened in Kaduna State, antibodies to ALV subgroup A/B was detected in six of the flocks (85.7%), while antibodies to ALV subgroup J was detected in only one flock (14.3%). Antibodies to both ALV subgroups A/B and J were detected in one flock (14.3%), which suggests co-infection of the two ALV subgroups. Out of the 16 flocks screened in Plateau State, antibodies to ALV subgroup A/B were detected in 15 flocks (93.8%), while antibodies to ALV subgroup J were detected in six flocks (37.5%). Antibodies to both ALV subgroups A/B and J were detected in five flocks (31.3%). The high detection of antibodies to ALV A/B suggests that ALV infection in layers is mostly due to ALV subgroup A or B in the study areas.

Gp37 Regulates the Pathogenesis of Avian Leukosis Virus Subgroup J via Its C Terminus

Different from other subgroups of avian leukosis viruses (ALVs), ALV-J is highly pathogenic. It is the main culprit causing myeloid leukemia and hemangioma in chickens. The distinctiveness of the env gene of ALV-J, with low homology to those of other ALVs, is linked to its unique pathogenesis, but the underlying mechanism remains unclear. Previous studies show that env of ALV-J can be grouped into three species based on the tyrosine motifs in the cytoplasmic domain (CTD) of Gp37, i.e., the inhibitory, bifunctional, and active groups. To explore whether the C terminus or the tyrosine motifs in the CTD of Gp37 affect the pathogenicity of ALV-J, a set of ALV-J infectious clones containing different C termini of Gp37 or the mutants at the tyrosine sites were tested in vitro and in vivo Viral growth kinetics indicated not only that ALV-J with active env is the fastest in replication and ALV-J with inhibitory env is the lowest but also that the tyrosine sites essentially affected the replication of ALV-J. Moreover, in vivo studies demonstrated that chickens infected by ALV-J with active or bifunctional env showed higher viremia, cloacal viral shedding, and viral tissue load than those infected by ALV-J with inhibitory env Notably, the chickens infected by ALV-J with active or bifunctional env showed significant loss of body weight compared with the control chickens. Taken together, these findings reveal that the C terminus of Gp37 plays a vital role in ALV-J pathogenesis, and change from inhibitory env to bifunctional or active env increases the pathogenesis of ALV-J.IMPORTANCE ALV-J can cause severe immunosuppression and myeloid leukemia in infected chickens. However, no vaccine or antiviral drug is available against ALV-J, and the mechanism for ALV-J pathogenesis needs to be elucidated. It is generally believed that gp85 and LTR of ALV contribute to its pathogenesis. Here, we found that the C terminus and the tyrosine motifs (YxxM, ITIM, and ITAM-like) in the CTD of Gp37 of ALV-J could affect the pathogenicity of ALV-J in vitro and in vivo The pathogenicity of ALV-J with Gp37 containing ITIM only was significantly less than ALV-J with Gp37 containing both YxxM and ITIM and ALV-J with Gp37 containing both YxxM and ITAM-like. This study highlights the vital role of the C terminus of Gp37 in the pathogenesis of ALV-J and thus provides a new perspective to elucidate the interaction between ALV-J and its host and a molecular basis to develop efficient strategies against ALV-J.

A novel recombinant avian leukosis virus isolated from gamecocks induced pathogenicity in Three-Yellow chickens: a potential infection source of avian leukosis virus to the commercial chickens

One natural recombinant avian leukosis virus (ALV) strain GX14DJ3-18 was isolated from a native gamecock by DF-1 cell culture and identified with Polymerase Chain Reaction (PCR), immunofluorescence assay and the viral genome's nucleotide sequencing. This strain was revealed as a novel recombinant virus with nucleotide sequence similarities of 95.4% Long Terminal Repeated (LTR), 95.8% 5', UTR, 97.9% gag, and 92.9% 3'untranslated regions (UTR) in ALV-J. Also we found sequence similarities of 99.3% pol and 99.0% gp37 in ALV-E, and 89.9% gp85 in ALV-A. The simulated congenital infection with GX14DJ3-18 in Three-Yellow chickens exhibited a significant negative effect on the development of immune organs (P < 0.05). Also, lower antibody responses were found to vaccinations with the commercial vaccines of Newcastle disease virus and with subtypes H5 and H9 of avian influenza virus (P < 0.05). The incidence of tumor or tumor-like lesions in the challenged birds was 14.28% (5/35), while none were observed in the un-challenged control group (0/35). These results suggested that GX14DJ3-18 is a novel recombinant ALV that can induce pathogenicity in the commercial Three-Yellow chickens. We speculated that cross-provincial sales of gamecocks in which ALVs have not been eradicated thoroughly might be a potential route for the transmission of ALVs to commercial chickens.

Full-length genome sequence analysis of an avian leukosis virus subgroup J (ALV-J) as contaminant in live poultry vaccine: The commercial live vaccines might be a potential route for ALV-J transmission

One avian leukosis virus subgroup J (ALV-J) strain was isolated from 67 commercial live poultry vaccines produced by various manufacturers during 2013-2016 in China. The complete genomes of the isolate were sequenced and it was found that the genes gag and pol of the strain were relatively conservative, while the gp85 gene of the strain GX14YYA1 had the highest similarities with a field strain GX14ZS14, which was isolated from the chickens of a farm that had once used the same vaccine as the one found to be contaminated with the GX14YYA1. This is the first report of ALV-J contaminant in live poultry vaccine in China. Our finding demonstrates that vaccination of the commercial live vaccines might be a potential new route for ALV-J transmission in chickens and highlights the need for more extensive monitoring of the commercial live vaccines in China.

Complete genome sequencing and characterization revealed a recombinant subgroup B isolate of avian leukosis virus with a subgroup J-like U3 region

One natural recombinant subgroup B avian leukosis virus (ALV) with a subgroup J-like U3 region was isolated from commercial native chickens that experienced disease in 2014 and named GX14FF03. GX14FF03 was isolated by DF-1 cell culture and then identified with ELISA detection of avian leukosis virus p27 group-specific antigen, the detection of subtype specific PCR, and indirect immunofluorescence assay with ALV-B-specific monoclonal antibody. Its complete proviral genome was sequenced and compared with the reference strains of ALVs and found that the gag and pol were relatively conservative. The gp85 of GX14FF03 showed 91.3-96.2% amino acid identity to the other ALV-B reference strains and 36.0-37.1% identity to the ALV-J reference strains, and its U3 region showed 49.4-89.3% nucleotide identity to ALV-A, B, C, D, E, K reference strains and 91.6-95.3% identity to ALV-J reference strains. Phylogenetic analysis of U3 region showed that GX14FF03 and ALV-J reference strains were in the same cluster. Moreover, an additional AIB REP1 retroviral transcription regulatory element was found in GX14FF04 U3 region which was only presenting in ALV-J strains. These results suggested that isolate GX14FF03 may be a recombinant ALV-B with the ALV-J-like U3 region.

A deep sequencing reveals significant diversity among dominant variants and evolutionary dynamics of avian leukosis viruses in two infectious ecosystems

Background

As a typical retrovirus, the evolution of Avian leukosis virus subgroup J (ALV-J) in different infectious ecosystems is not characterized, what we know is there are a cloud of diverse variants, namely quasispecies with considerable genetic diversity. This study is to explore the selection of infectious ecosystems on dominant variants and their evolutionary dynamics of ALV-J between DF1 cells and specific-pathogen-free (SPF) chickens. High-throughput sequencing platforms provide an approach for detecting quasispecies diversity more fully.

Results

An average of about 20,000 valid reads were obtained from two variable regions of gp85 gene and LTR-U3 region from each sample in different infectious ecosystems. The top 10 dominant variants among ALV-J from chicken plasmas, DF1 cells and liver tumor were completely different from each other. Also there was a difference of shannon entropy and global selection pressure values (ω) in different infectious ecosystems. In the plasmas of two chickens, a large portion of quasispecies contained a 3-peptides “LSD” repeat insertion that was only less than 0.01% in DF1 cell culture supernatants. In parallel studies, the LTR-U3 region of ALV-J from the chicken plasmas demonstrated more variants with mutations in their transcription regulatory elements than those from DF1 cells.

Conclusions

Our data taken together suggest that the molecular epidemiology based on isolated ALV-J in cell culture may not represent the true evolution of virus in chicken flocks in the field. The biological significance of the “LSD” insert and mutations in LTR-U3 needs to be further studied.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-016-0902-6) contains supplementary material, which is available to authorized users.

Endogenous Retrovirus ev21 Dose Not Recombine with ALV-J and Induces the Expression of ISGs in the Host

Avian leukosis virus subgroup J (ALV-J) infection can cause tumors and immunosuppression. Endogenous viruses integrate into host genomes and can recombine with exogenous avian leukosis virus (ALV). In this study, we analyzed the interaction of endogenous retrovirus 21 (ev21) with the ALV-J in late-feathering Chinese yellow chicken. Two ALV-J strains M180 and K243 were isolated from late-feathering and fast-feathering Chinese yellow chicken flocks, respectively. The env gene of the two strains showed 94.2-94.8% nucleotide identity with reference ALV-J strains. Compared with the env gene and the LTR of ev21 and M180, the nucleotide identity of LTR was 69.7% and env gene was 58.4%, respectively, especially the amino acid identity of env gene as low as 14.2%. Phylogenetic analysis of the nucleotide sequence of the env gene and the 3'LTR showed that M180 was closely related to ALV-J, and was located in a distinct group with ev21 in the phylogenetic tree. Using co-immunoprecipitation (co-IP), we next demonstrate that the envelope protein of ev21 does not interact with the M180 envelope protein. We further show that the envelope protein of ev21 cannot activate ALV-J LTR promoter activity using luciferase-reporter assays. qPCR and western blot analysis revealed that envelope protein of endogenous ev21 can facilitate the expression of PKR at 6h post ALV-J infection (hpi) and facilitate the expression of ISG12 and CH25H at 24 hpi. However, the expression of the env gene of M180 strain was not significantly at 6 and 24 hpi. We conclude that there is no evidence of recombination between endogenous retrovirus ev21 and ALV-J strain M180 in late-feathering Chinese yellow chicken, and envelope protein of ev21 can affect the expression of host ISGs, but appears not to influence the replication of ALV-J strain M180. This is the first report of interaction among the endogenous retrovirus ev21, ALV-J and the late-feathering chicken.

Isolation, identification and evolution analysis of a novel subgroup of avian leukosis virus isolated from a local Chinese yellow broiler in South China

Avian leukosis virus (ALV) causes high mortality associated with tumor formation and decreased fertility, and results in major economic losses in the poultry industry worldwide. Recently, a putative novel ALV subgroup virus named ALV-K was observed in Chinese local chickens. In this study, a novel ALV strain named GD14LZ was isolated from a Chinese local yellow broiler in 2014. The proviral genome was sequenced and phylogenetically analyzed. The replication ability and pathogenicity of this virus were also evaluated. The complete proviral genome sequence of GD14LZ was 7482 nt in length, with a genetic organization typical of replication-competent type C retroviruses lacking viral oncogenes. Sequence analysis showed that the gag, pol and gp37 genes of GD14LZ have high sequence similarity to those of other ALV strains (A-E subgroups), especially to those of ALV-E. The gp85 gene of the GD14LZ isolate showed a low sequence similarity to those other ALV strains (A-E subgroups) but showed high similarity to strains previously described as ALV-K. Phylogenetic analysis of gp85 also suggested that the GD14LZ isolate was related to ALV-K strains. Further study showed that this isolate replicated more slowly and was less pathogenic than other ALV strains. These results indicate that the GD14LZ isolate belongs to the novel subgroup ALV-K and probably arose by recombination of ALV-K with endogenous viruses with low replication and pathogenicity. This virus might have existed in local Chinese chickens for a long time.