Replacement of primary chicken embryonic fibroblasts (CEF) by the DF-1 cell line for detection of avian leucosis viruses

Small RNA deep sequencing revealed microRNAs' involvement in modulating cellular senescence and immortalization state

Unlike rodent cells, spontaneous immortalization of avian cells and human cells is a very rare event. According to patent publications and current literature, there are no more than 4 spontaneously immortalized chicken embryo fibroblast (CEF) cell lines established up to date. One of those cell lines is ADOL (Avian Disease and Oncology Laboratory) ZS-1 cell line, which was established by continuous passaging of the CEFs derived from the specific pathogen free (SPF) 0.TVB*S1 (commonly known as rapid feathering susceptible or RFS) genetic line of chickens. The RFS genetic line of chickens was developed and has been maintained on the SPF chicken farm of USDA-ARS facility, ADOL, in East Lansing, Michigan, which is known as one of a few lines of chickens that are free of any known avian endogenous virus genes. To explore potential roles that epigenetic factors may play in modulating cellular senescence processes and spontaneous immortalization state, total RNAs extracted from samples of the RFS primary CEFs, RFS CEFs reached the 21st passage, and the ZS-1 cells were subjected to small RNA sequencing. Collectively, a total of 531 miRNAs was identified in the 3 types of samples. In contrast to the primary CEF samples, 50 miRNAs were identified with significantly differential expression only in the 21st passage samples; a different subset of 63 differentially expressed miRNAs was identified only in the ZS-1 samples; the majority of differentially expressed miRNAs identified in both the 21st passage CEF and the ZS-1 samples were more or less directionally consistent. Gene Ontology analysis results suggested that the epigenetic factor, miRNAs, plays a role in modulating the cellular senescence and spontaneous immortalization processes through various bioprocesses and key pathways including ErbB and MAPK signaling pathways. These findings provided the experimental and bioinformatic evidence for a better understanding on the epigenetic factor of miRNAs in association with cellular senescence and spontaneous immortalization process in avian cells.

First evidence that an emerging mammalian alphacoronavirus is able to infect an avian species

A novel swine enteric alphacoronavirus, swine acute diarrhea syndrome coronavirus (SADS-CoV), related to Rhinolophus bat CoV HKU2 in the subgenus Rhinacovirus has emerged in southern China in 2017, causing diarrhea in newborn piglets, and critical questions remain about the pathogenicity, cross-species transmission and potential animal reservoirs. Our laboratory's previous research has shown that SADS-CoV can replicate in various cell types from different species, including chickens. Here, we systematically explore the susceptibility of chicken to a cell-adapted SADS-CoV strain both in vitro and in vivo. Firstly, evidences of SADS-CoV replication in primary chicken cells including cytopathic effects, immunofluorescence staining, growth curve and structural protein expression were proven. Furthermore, we observed that SADS-CoV could replicate in chicken embryos without causing gross lesion, and that experimental infection of chicks resulted in mild respiratory symptoms. More importantly, SADS-CoV shedding and viral distribution in lungs, spleens, small intestines and large intestines of infected chickens were confirmed by quantitative RT-PCR and immunohistochemistry. The genomic sequence of the original SADS-CoV from the pig source sample in 2017 was determined to have nine nucleotide differences compared to the used cell-adapted strain; among these were three non-synonymous mutations in the spike gene. These results collectively demonstrate that chickens are susceptible to SADS-CoV infection, suggesting that they are a potential animal reservoir. To our knowledge, this study provides the first experimental evidence of cross-species infection that a mammalian alphacoronavirus is able to infect an avian species. This article is protected by copyright. All rights reserved.

Identification of Marek's disease virus pUL56 homologue and analysis of critical amino acid stretches indispensable for its intracellular localization

Marek's disease virus (MDV) is considered a unique member of the Alphaherpesvirinae subfamily that induces rapid onset of T cell lymphoma in chickens. Compared with other conserved UL56 gene homologues of herpesviruses, little is known about the roles of MDV UL56 gene, while recent studies of mammalian herpesvirus pUL56 proteins have revealed their involvement in promoting ubiquitination of the Nedd4 (neural precursor cell expressed developmentally down-regulated protein 4) -like E3 ubiquitin ligases for proteasomal degradation and in modulating host immune responses. To determine the expression kinetics of UL56 gene products, chicken embryo fibroblasts were infected with very virulent or attenuated MDV strain and analyzed by quantitative PCR and Western blotting. During the time course of infection, the levels of UL56 mRNA transcripts increased consistently. At the translational level, the pUL56 protein encoded by UL56 gene was expressed in the size of 32 kDa, which emerged as early as 12 h post-infection (hpi) but otherwise began to wane at 72 hpi thereafter. With the treatment of viral DNA synthesis inhibitors, the pUL56 expression was significantly reduced, featuring the dynamics of a late (γ)-gene product. By confocal imaging, pUL56 was found to reside in the Golgi compartment. Both the L-domain motifs and the C-terminal tail-anchored transmembrane were essential for its intracellular localization. Noticeably, pUL56 co-localized with a truncated mutant of the chicken Nedd4-like family protein harboring only the WW domains; however, co-immunoprecipitation assay established no direct interaction between them, and the ectopic expression of pUL56 did not alter the abundance of endogenous Nedd4-like protein. Overall, the present study provides a caveat that the pUL56 homologues of different herpesviruses with structural similarities might vary in expression patterns and probably in functional consequences. For this reason, further investigation should be encouraged to focus on the potential association between UL56 gene and MDV pathogenesis in the context of engineered viral mutants.

Adaptation and characterization of Anatid herpesvirus 1 in different permissible cell lines

Duck viral enteritis is an acute, contagious infection of Anatidae family members. The disease is caused by Anatid herpesvirus 1 (AnHV-1). The infection of AnHV-1 is controlled by vaccination to the flock with chick embryo adapted attenuated vaccine in developed countries. However, its economic impact in developing countries is substantial and there is a need to understand the cell culture spectrum of the virus to produce its vaccine on a mass scale. In the present study, the permissivity of AnHV-1 for different cells was analyzed. The AnHV-1 showed enhanced replication following its serial passage in CEF, DF-1, Vero, MDCK, and QT-35 cells. The characteristic cytopathic effect (CPE) of rounding and clumping of cells were observed in CEF, DF-1, Vero, and QT-35 cell lines. The infectivity and viral replication were highest in CEF, DF-1, Vero, and QT-35 cells. In contrast, the results suggested that MDCK cells are less permissive for AnHV-1 infection with negligible CPE and reduced viral replication. Heterologous cell culture systems other than chicken embryo fibroblasts to adapted live vaccine viruses will provide a system devoid of other avian infectious agents. Moreover, it can be used for the propagation and cultivation of AnHV-1 vaccine strain for developing cell culture-based vaccines with high titer and could be an economical alternative for the existing options.

Phylogenetic Analysis of ALV-J Associated with Immune Responses in Yellow Chicken Flocks in South China

The aim of this study was to better understand the sequence characteristics and immune responses in avian leukosis virus subgroup J (ALV-J) infected yellow chicken flocks in South China. We isolated four strains of ALV-J virus from these flocks, which were then identified by several methods, including subtype-specific polymerase chain reaction (PCR), enzyme-linked immunosorbent assay (ELISA), and immunofluorescence assay (IFA). All four viruses were sequenced for their complete genomes and named GD19GZ01, GD19GZ02, GD19GZ03, and GD19GZ04. In comparison with the reference sequence, the homology analysis showed that the gag and pol genes were relatively conserved, whereas env contained much variation. Both GD19GZ01 and GD19GZ02 almost entirely lacked the rTM region and E element, while the latter was retained in GD19GZ03 and GD19GZ04. Moreover, the virus replication levels in GD19GZ03 and GD19GZ04were much higher than those in GD19GZ01 and GD19GZ02. And three virus recombination events in GD19GZ01 and GD19GZ02 were revealed by the results of PDR5 and SimPlot software analysis. Additionally, we found that some interferon-stimulating genes (CH25H, MX, PKR, OAS, and ZAP) and inflammatory mediators (IL-4, IL-6, IL-10, IL-12, 1L-18, and TNF-α) were significantly upregulated in the immune system organs of clinical chickens. Taken together, these findings clarify and reveal the sequence characteristics and trends in the variation of ALV-J infection in yellow chicken flocks of South China.

Molecular characterization of avian leukosis virus subgroup J in Chinese local chickens between 2013 and 2018

Avian leukosis virus subgroup J (ALV-J) was first isolated from broiler chickens in China in 1999; subsequently, it was rapidly introduced into layer chickens and Chinese local chickens. Recently, the incidence of ALV-J in broiler and layer chickens has significantly decreased. However, it has caused substantial damage to Chinese local chickens, resulting in immense challenges to their production performance and breeding safety. To systematically analyze the molecular characteristics and the epidemic trend of ALV-J in Chinese local chickens, 260 clinical samples were collected for the period of 2013–2018; 18 ALV-J local chicken isolates were identified by antigen-capture enzyme-linked immunosorbent assay and subgroup A-, B-, and J-specific multiplex PCR. The whole genomic sequences of 18 isolates were amplified with PCR and submitted to GenBank. Approximately, 55.5% (10/18) of the 18 isolates demonstrated a relatively high homology (92.3–95.4%) with 20 ALV-J early-isolated local strains (genome sequences obtained from GenBank) in gp85 genes clustering in a separated branch. The 3ʹ untranslated region (3ʹ UTR) of the 18 isolates showed a 195–210 and 16–28 base pair deletion in the redundant transmembrane region and in direct repeat 1, respectively; 55.5% (10/18) of the 18 isolates retained the 147 residue E element. The U3 gene of 61.1% (11/18) of the 18 isolates shared high identity (94.6–97.3%) with ALV-J early-isolated local strains. These results implied that the gp85 and U3 of ALV-J local chicken isolates have rapidly evolved and formed a unique local chicken branch. In addition, it was determined that the gene deletion in the 3′UTR region currently serves as a unique molecular characteristic of ALV-J in China. Hence, the obtained results built on the existing ALV-J molecular epidemiological data and further elucidated the genetic evolution trend of ALV-J in Chinese local chickens.

Isolation and molecular characterization of the first subgroup J avian leukosis virus from chicken in Pakistan

Since subgroup J avian leukosis virus (ALV-J) was first isolated in the United Kingdom in 1988, it has seriously hindered the development of the poultry industry worldwide. Although cases of ALV-J infection have been reported as early as 2001 in Pakistan, there was no further research on the isolation and molecular characteristics of ALVs. In the present study, we first isolated two ALVs from suspicious clinical samples that were collected from a desi chicken farm in Pakistan. The results of multiplex PCR and indirect immunofluorescent antibody assays confirmed that the two isolates (PK19FA01 and PK19SA01) belonged to ALV-J. The complete genomes of the two isolates were amplified, sequenced, and systematically analyzed. We found that gp85 of PK19FA01 was more similar to that of the prototype strain HPRS103, whereas gp85 of PK19SA01 was more similar to that of American strains. The two isolates contained an intact E element of 147 residues and had a unique 135 bp deletion in the redundant transmembrane of the 3' untranslated region. The U3 region of the two isolates was highly homologous to that of American ALV-J strains. To our knowledge, this is the first report of the isolation, complete genome sequencing, and systematic molecular epidemiological investigation of ALV-J in Pakistan. Our findings could enrich epidemiological data and might contributed to more effective measures to prevent and control avian leukosis in Pakistan.

Full-length cDNA sequence analysis of 85 avian leukosis virus subgroup J strains isolated from chickens in China during the years 1988-2018: coexistence of 2 extremely different clusters that are highly dependent upon either the host genetic background or the geographic location

During the process of transmission and spread of avian leukosis virus subgroup J (ALV-J) in chickens worldwide, the viral genome is constantly changing. A comprehensive and systematic study of the evolutionary process of ALV-J in China is needed. In this study, we amplified the full-length viral cDNA sequences of 16 ALV-J isolates of Yellow-chicken origin and analyzed and compared these sequences with another 69 ALV-J strains isolated during the years 1988–2018. These isolates were then sorted into 2 clusters: cluster I included isolates that mainly originated from the layers and White-feather broilers from northern China; cluster II included isolates mainly from the Yellow-chicken, most of them being from southern China. According to the sequence homologies of the whole genome and gag, pol, gp85, and gp37 genes, the ALV-J strains are more likely to randomly change in different directions from the original strain HPRS-103 as time passes. The results of entropy analysis of the sequences of gag, pol, and env revealed that the env gene had the largest variation, and the gag gene nonconserved sites are mainly concentrated in p19, p10, and p12. In addition, 84.71% (72/85) of the isolates had the 205-nucleotide (nt) deletion in the 3′UTR region, and 30.59% (26/85) of the isolates had the 125-nt to 127-nt deletion in the E element. Our study provides evidence for the coexistence of 2 extremely different clusters of ALV-J prevailing in China and in some other countries during the period of 1988–2018 and implies that the clusters are highly dependent on the host genetic background and the geographic location.

Fluctuations in luteinizing hormone, follicle stimulating hormone, and progesterone might affect the disappearance of avian leukosis virus subgroup J viremia in chickens with intermittent viremia

Avian leukosis virus subgroup J (ALV-J) belongs to α-retrovirus genus of retroviridae, causing neoplastic disease, immunosuppression, and other problems in the poultry industry worldwide. The aim of this study was to determine whether the reproductive hormone fluctuation affect the ALV-J viremia at the early egg-laying phase in chicken. Total 8 suspected ALV-J-infected Chinese yellow chickens in the early egg-laying phase were collected from 2 different farms, and further confirmed by PCR and immunofluorescence assay. Plasma samples were collected from experimental chickens for 5 to 10 consecutive weeks at a settled time in each week. ALV-J viremia and reproductive hormone levels were monitored by ELISA or radioimmunoassay. The results showed that fluctuations in luteinizing hormone (LH), follicle stimulating hormone (FSH), and progesterone might have an impact on the disappearance of ALV-J viremia in chickens with intermittent viremia, but not in chickens with persistent viremia. These results suggest that reproductive hormone changes during the early egg-laying phase will affect the detection of positive ALV-J-infected chickens in the ALV-J eradication procedures.

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.

Difference in pathogenicity of 2 strains of avian leukosis virus subgroup J in broiler chicken

Avian leukosis virus subgroup J has been found to infect many types of chickens with various genetic backgrounds. The ALV-J strain NX0101, which was isolated from broiler breeders in 2001, mainly induces the formation of myeloid cell tumors. However, strain HN10PY01, which was recently isolated from laying hens, mainly induces the formation of myeloid cell tumors and hemangioma. In order to determine the difference in pathogenicity of the 2 strains in broiler chickens, 2 groups of chicken embryos were infected with NA0101 and HN10PY01 separately. A comparison was made of the mortality, oncogenicity, body weights, indexes for immune organs, levels of ALV group-specific antigen p27, and mRNA expression levels of the tumor-related gene, p53, in ALV-J-infected birds and immune organs of theses chickens in response to Newcastle Disease Virus (NDV) and avian influenza virus subtype H9 (AIV-H9) vaccination. The results indicated that strain NX0101 was highly pathogenic in broiler chickens and led to a 30% mortality rate and 45% oncogenicity, compared with the HN10PY01-infected birds. Weight of chickens was also significantly lower after 15 wk (P < 0.05). In addition, the mRNA expression levels of tumor-related p53 in medulla, liver, and lung in broilers infected with strain NX0101 were significantly higher than those infected with strain HN10PY01 (P < 0.05). These results indicated that strain NX0101 had a higher replication ability in broiler chickens. The findings of this study will contribute to further elucidating the mechanisms underlying host susceptibility and tumor classification in ALV-J-infected chickens.

Comparison of Viremia, Cloacal Virus Shedding, Antibody Responses and Pathological Lesions in Adult Chickens, Quails, and Pigeons Infected with ALV-A

Subgroup A of the avian leukosis virus (ALV-A) can cause severe pathological lesions and death in infected chickens, and its reported hosts have increased recently. To assess the susceptibility of adult chickens, quails, and pigeons to ALV-A, three sets of 250-day-old birds were intraperitoneally inoculated with ALV-A. Viremia and cloacal virus shedding were dynamically detected using an immunofluorescence assay (IFA), ALV-P27 antigen ELISA or RT-PCR; pathological lesions were assessed using tissue sections; ALV-A in tissues was detected by IFA; and ALV-A antibody responses were detected using antibody ELISA kits and an immune diffusion test. The results indicated that persistent viremia occurred in 80% (8/10) of infected chickens, and transient viremia occurred in 17% (2/12) of infected quails, but no viremia occurred in infected pigeons. Cloacal virus shedding occurred intermittently in 80% (8/10) of infected chickens and in 8% (1/12) of infected quails but did not occur in infected pigeons. Severe inflammatory pathological lesions occurred in the visceral tissues of most infected chickens, and mild lesions occurred in a few of the infected quails, but no pathological lesions occurred in the infected pigeons. The ALV-A virus was detected in the visceral tissues of most infected chickens but not in the infected quails and pigeons. Obviously different ALV-A antibody responses occurred in the infected chickens, quails and pigeons. It can be concluded that adult chickens, quails and pigeons have dramatically different susceptibilities to ALV-A. This is the first report on artificial infection by ALV-A in different birds.

The high conserved cellular receptors of avian leukosis virus subgroup J in Chinese local chickens contributes to its wide host range

Avian leukosis virus (ALV) is a tumor-inducing virus that spreads among most chicken species, causing serious financial losses for the poultry industry. Subgroup J avian leukosis virus (ALV-J) is a recombinant exogenous ALV, which shows more extensive host range in comparison with other subgroups, especially in Chinese local chickens. To identify the relationship between ALV-J host range and the polymorphism of its cellular receptors, we performed a wide range epidemiological investigation of current ALV-J infection in Chinese local chickens, and discovered that all the 18 local chicken breeds being investigated from main local chicken breeding provinces were ALV-J positive. Furthermore, we cloned ALV-J cellular receptor genes of chNHE1 and chANXA2 of these 18 chicken breeds. Sequence alignment demonstrated that despite several regular mutations at the nucleotide level, there were no corresponding amino acid mutations for either chNHE1 gene or chANXA2 gene. Additionally, virus entry assay indicated that the level of viral enter into cells is stable among different chicken breeds. Results of this study indicated that the wide host range of ALV-J in Chinese local chickens was partially due to the high conservatism of its cellular receptors, and also provide target sites for drug design of resistance to ALV-J infection.

Applications of Gene Editing in Chickens: A New Era Is on the Horizon

The chicken represents a valuable model for research in the area of immunology, infectious diseases as well as developmental biology. Although it was the first livestock species to have its genome sequenced, there was no reverse genetic technology available to help understanding specific gene functions. Recently, homologous recombination was used to knockout the chicken immunoglobulin genes. Subsequent studies using immunoglobulin knockout birds helped to understand different aspects related to B cell development and antibody production. Furthermore, the latest advances in the field of genome editing including the CRISPR/Cas9 system allowed the introduction of site specific gene modifications in various animal species. Thus, it may provide a powerful tool for the generation of genetically modified chickens carrying resistance for certain pathogens. This was previously demonstrated by targeting the Trp38 region which was shown to be effective in the control of avian leukosis virus in chicken DF-1 cells. Herein we review the current and future prospects of gene editing and how it possibly contributes to the development of resistant chickens against infectious diseases.

An Alternative Method for Long-Term Culture of Chicken Embryonic Stem Cell In Vitro

Chicken embryonic stem cells (cESCs) obtained from stage X embryos provide a novel model for the study of avian embryonic development. A new way to maintain cESCs for a long period in vitro still remains unexplored. We found that the cESCs showed stem cell-like properties in vitro for a long term with the support of DF-1 feeder and basic culture medium supplemented with human basic fibroblast growth factor (hbFGF), mouse stem cell factor (mSCF), and human leukemia inhibitory factor (hLIF). During the long culture period, the cESCs showed typical ES cell morphology and expressed primitive stem cell markers with a relatively stable proliferation rate and high telomerase activity. These cells also exhibited the capability to differentiate into cardiac myocytes, smooth muscle cells, neural cells, osteoblast, and adipocyte in vitro. Chimera chickens were produced by cESCs cultured for 25 passages with this new culture system. The experiments showed that DF-1 was the optimal feeder and hbFGF was an important factor for maintaining the pluripotency of cESCs in vitro.

A chicken liver cell line efficiently supports the replication of ALV-J possibly through its high level viral receptor and efficient protein expression system

In this study, we identified a chicken liver cell line (LMH) which could strongly support the replication of ALV-J (Subgroup J of avian leukosis virus) with high viral titer. Notably, ALV-J was efficiently detected by ELISA in LMH cells 1 day before DF1 cells. In comparison with DF1 cells, LMH cells not only expressed higher levels of ALV-J receptor chNHE-1, but also possessed a more efficient protein expression system for foreign genes. Thus, LMH cells could be a novel tool to shorten the ALV-J eradication approach and accelerate studies on the pathogenesis and oncogenesis of ALV-J.

Phylogenetic Analysis and Pathogenicity Assessment of the Emerging Recombinant Subgroup K of Avian Leukosis Virus in South China

In recent years, cases of avian leukosis virus (ALV) infection have become more frequent in China. We isolated 6 ALV strains from yellow feather broiler breeders in south China from 2014 to 2016. Their full genomes were sequenced, compared, and analyzed with other reference strains of ALV. The complete genomic nucleotide sequences of GD150509, GD160403, GD160607, GDFX0601, and GDFX0602 were 7482 bp in length, whereas GDFX0603 was 7480 bp. They shared 99.7% to 99.8% identity with each other. Homology analysis showed that the gag, pol, long terminal repeats (LTRs), and the transmembrane region (gp37) of the env genes of the 6 viruses were well conserved to endogenous counterpart sequences (>97.8%). However, the gp85 genes displayed high variability with any known chicken ALV strains. Growth kinetics of DF-1 cells infected with the isolated ALV showed viral titers that were lower than those infected with the GD13 (ALV-A), CD08 (ALV-B), and CHN06 (ALV-J) on day 7 post-infection. The infected Specific-pathogen-free (SPF) chickens could produce continuous viremia, atrophy of immune organs, growth retardation and no tumors were observed. These subgroup ALVs are unique and may be common in south China. The results suggested that updating the control and eradication program of exogenous ALV for yellow feather broiler breeders in south China needs to be considered because of the emergence of the new subgroup viruses.

Constitutively elevated levels of SOCS1 suppress innate responses in DF-1 immortalised chicken fibroblast cells

The spontaneously immortalised DF-1 cell line is rapidly replacing its progenitor primary chicken embryo fibroblasts (CEFs) for studies on avian viruses such as avian influenza but no comprehensive study has as yet been reported comparing their innate immunity phenotypes. We conducted microarray analyses of DF-1 and CEFs, under both normal and stimulated conditions using chicken interferon-α (chIFN-α) and the attenuated infectious bursal disease virus vaccine strain PBG98. We found that DF-1 have an attenuated innate response compared to CEFs. Basal expression levels of Suppressor of Cytokine Signalling 1 (chSOCS1), a negative regulator of cytokine signalling in mammals, are 16-fold higher in DF-1 than in CEFs. The chSOCS1 “SOCS box” domain (which in mammals, interacts with an E3 ubiquitin ligase complex) is not essential for the inhibition of cytokine-induced JAK/STAT signalling activation in DF-1. Overexpression of SOCS1 in chIFN-α-stimulated DF-1 led to a relative decrease in expression of interferon-stimulated genes (ISGs; MX1 and IFIT5) and increased viral yield in response to PBG98 infection. Conversely, knockdown of SOCS1 enhanced induction of ISGs and reduced viral yield in chIFN-α-stimulated DF-1. Consequently, SOCS1 reduces induction of the IFN signalling pathway in chicken cells and can potentiate virus replication.

Transgenic Chickens Expressing the 3D8 Single Chain Variable Fragment Protein Suppress Avian Influenza Transmission

The 3D8 single chain variable fragment (scFv) is a mini-antibody that causes unusual sequence-independent nuclease activity against all types of nucleic acids. We used recombinant lentiviruses to generate transgenic chickens expressing the 3D8 scFv gene under the control of the chicken β-actin promoter. From 420 injected embryos, 200 chicks (G0) hatched and were screened for the 3D8 scFv using PCR, and 15 chicks were identified as transgenic birds expressing the transgene in their semen. The G0 founder birds were mated with wild-type hens to produce seven transgenic chicks (G1). 3D8 scFv expression in the chicken embryonic fibroblasts (CEFs) was verified by RT-PCR and Western blot analysis. Immunofluorescence staining for 3D8 scFv in the CEFs revealed that the 3D8 scFv protein was primarily cytosolic. To identify 3D8 scFv anti-viral activity, wild-type and two transgenic CEF lines were infected with H9N2 avian influenza virus (AIV). We selected one line of transgenic chickens that exhibited the lowest number of plaque-forming units to be challenged with H9N2 virus. The challenge experiment revealed that contact exposed transgenic chickens expressing 3D8 scFv exhibited suppressed viral shedding. This results suggest that the transgenic chickens developed in this study could be useful for controlling potential within-flock AIV transmission.

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.

Capsid-Targeted Viral Inactivation: A Novel Tactic for Inhibiting Replication in Viral Infections

Capsid-targeted viral inactivation (CTVI), a conceptually powerful new antiviral strategy, is attracting increasing attention from researchers. Specifically, this strategy is based on fusion between the capsid protein of a virus and a crucial effector molecule, such as a nuclease (e.g., staphylococcal nuclease, Barrase, RNase HI), lipase, protease, or single-chain antibody (scAb). In general, capsid proteins have a major role in viral integration and assembly, and the effector molecule used in CTVI functions to degrade viral DNA/RNA or interfere with proper folding of viral key proteins, thereby affecting the infectivity of progeny viruses. Interestingly, such a capsid-enzyme fusion protein is incorporated into virions during packaging. CTVI is more efficient compared to other antiviral methods, and this approach is promising for antiviral prophylaxis and therapy. This review summarizes the mechanism and utility of CTVI and provides some successful applications of this strategy, with the ultimate goal of widely implementing CTVI in antiviral research.

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.

Innate Immune Responses in ALV-J Infected Chicks and Chickens with Hemangioma In Vivo

Avian leukosis virus subgroup J (ALV-J) infection can cause tumors and immunosuppression. Since the precise mechanism of the innate immune response induced by ALV-J is unknown, we investigated the antiviral innate immune responses induced by ALV-J in chicks and chickens that had developed tumors. Spleen levels of interleukin-6 (IL-6), IL-10, IL-1β, and interferon-β (IFN-β) were not significantly different between the infected chick groups and the control groups from 1 day post hatch to 7 days post hatch. However, IL-6, IL-1β, and IFN-β protein levels in the three clinical samples with hemangiomas were dramatically increased compared to the healthy samples. In addition, the anti-inflammatory cytokine IL-10 increased sharply in two of three clinical samples. We also found a more than 20-fold up-regulation of ISG12-1 mRNA at 1 day post infection (d.p.i.) and a twofold up-regulation of ZC3HAV1 mRNA at 4 d.p.i. However, there were no statistical differences in ISG12-1 and ZC3HAV1 mRNA expression levels in the tumorigenesis phase. ALV-J infection induced a significant increase of Toll-like receptor 7 (TLR-7) at 1 d.p.i. and dramatically increased the mRNA levels of melanoma differentiation-associated gene 5 (MDA5) in the tumorigenesis phase. Moreover, the protein levels of interferon regulatory factor 1 (IRF-1) and signal transducer and activator of transcription 1 (STAT1) were decreased in chickens with tumors. These results suggest that ALV-J was primarily recognized by chicken TLR7 and MDA5 at early and late in vivo infection stages, respectively. ALV-J strain SCAU-HN06 did not induce any significant antiviral innate immune response in 1 week old chicks. However, interferon-stimulated genes were not induced normally during the late phase of ALV-J infection due to a reduction of IRF1 and STAT1 expression.

Exogenous avian leukosis virus-induced activation of the ERK/AP1 pathway is required for virus replication and correlates with virus-induced tumorigenesis

A proteomics approach was used to reveal the up-regulated proteins involved in the targeted mitogen-activated protein kinase (MAPK) signal transduction pathway in DF-1 cells after ALV subgroup J (ALV-J) infection. Next, we found that ALV-J CHN06 strain infection of DF-1 cells correlated with extracellular signal-regulated kinase 2 (ERK2) activation, which was mainly induced within 15 min, a very early stage of infection, and at a late infection stage, from 108 h to 132 h post-infection. Infection with other ALV subgroup (A/B) strains also triggered ERK/MAPK activation. Moreover, when activating ERK2, ALV subgroups A, B and J simultaneously induced the phosphorylation of c-Jun, an AP1 family member and p38 activation but had no obvious effect on JNK activation at either 15 min or 120 h. Interestingly, only PD98059 inhibited the ALV-induced c-Jun phosphorylation while SP600125 or SB203580 had no influence on c-Jun activation. Furthermore, the viral gp85 and gag proteins were found to contribute to ERK2/AP1 activation. Additionally, the specific ERK inhibitor, PD980509, significantly suppressed ALV replication, as evidenced by extremely low levels of ALV promoter activity and ALV-J protein expression. In vivo analysis of ERK2 activation in tumor cells derived from ALV-J-infected chicken demonstrated a strong correlation between ERK/MAPK activation and virus-associated tumorigenesis.

Intronic deletions of tva receptor gene decrease the susceptibility to infection by avian sarcoma and leukosis virus subgroup A

The group of avian sarcoma and leukosis virus (ASLV) in chickens contains six highly related subgroups, A to E and J. Four genetic loci, tva, tvb, tvc and tvj, encode for corresponding receptors that determine the susceptibility to the ASLV subgroups. The prevalence of ASLV in hosts may have imposed strong selection pressure toward resistance to ASLV infection, and the resistant alleles in all four receptor genes have been identified. In this study, two new alleles of the tva receptor gene, tva^r5 and tva^r6, with similar intronic deletions were identified in Chinese commercial broilers. These natural mutations delete the deduced branch point signal within the first intron, disrupting mRNA splicing of the tva receptor gene and leading to the retention of intron 1 and introduction of premature TGA stop codons in both the longer and shorter tva isoforms. As a result, decreased susceptibility to subgroup A ASLV in vitro and in vivo was observed in the subsequent analysis. In addition, we identified two groups of heterozygous allele pairs which exhibited quantitative differences in host susceptibility to ASLV-A. This study demonstrated that defective splicing of the tva receptor gene can confer genetic resistance to ASLV subgroup A in the host.

Development and application of SYBR Green I real-time PCR assay for the separate detection of subgroup J Avian leukosis virus and multiplex detection of avian leukosis virus subgroups A and B

Background

Subgroup A, B, and J ALVs are the most prevalent avian leukosis virus (ALV). Our study attempted to develop two SYBR Green I-based real-time PCR (RT-PCR) assays for specific detection of ALV subgroup J (ALV-J) and multiplex detection of ALV subgroups A and B (ALV-A/B), respectively.

Results

The two assays showed high specificity for ALV-J and ALV-A/B and the sensitivity of the two assays was at least 100 times higher than that of the routine PCR assay. The minimum virus detection limit of virus culture, routine PCR and real-time PCR for detection of ALV-A strain was 10³ TCID₅₀ units, 10² TCID₅₀ units and fewer than 10 TCID₅₀ units, respectively. In addition, the coefficients of variation for intra- and inter-assay were both less than 5%. Forty clinical plasma samples were evaluated by real-time PCR, routine PCR, and virus culture with positive rates of 80% (32/40), 72.5% (29/40) and 62.5% (25/40), respectively. When the assay for detection of ALV-J was used to quantify the viral load of various organ tissues in chicken inoculated by ALV-J strains CHN06 and NX0101, the results exhibited that ALV-J genes could be detected in all organ tissues examined and the highest copies of ALV-J were mainly in heart and kidney samples at 30 weeks post-infection. Except in lung, the virus copies of CHN06 group were higher than that of NX0101 group in various organ tissues.

Conclusions

The SYBR Green I-based real-time RT-PCR assay provides a powerful tool for the detection of ALV and study of virus replication and infection.

Effects of chromic chloride on chick embryo fibroblast viability

The objective of this study is to evaluate the effects of chromic chloride (CrCl₃) on chick embryo fibroblast (CEF) viability. The cells were incubated with CrCl₃ (0.02, 0.1, 0.5, 2.5, 12.5, and 62.5 μM), and the viability was determined using MTT assay, morphological detection and flow cytometry. The results show that lower concentrations of CrCl₃ (0.02, 0.1, and 0.5 μM) did not damage CEF viability. At 0.1 μM, CrCl₃ can increase CEF viability (P < 0.05). However, at higher concentrations of CrCl₃ (2.5, 12.5, and 62.5 μM), the number of apoptotic and necrotic cells (P < 0.01) and intracellular reactive oxygen species (P < 0.01) increased. In addition, decreased mitochondrial membrane potential (P < 0.01) and enhanced intracellular calcium levels (P < 0.01) were observed after the exposure. Moreover, apoptotic morphological changes induced by these processes in CEF were confirmed using Hoechst 33258 staining. Cell death induced by higher concentrations of CrCl₃ was caused by an apoptotic and a necrotic mechanism, whereas the main mechanism of oxidative stress and induced mitochondrial dysfunction was apoptotic death. The induced apoptotic death in CEF is concentration- and time-dependent.

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

Avian leukosis virus subgroup J (ALV-J) was first isolated from meat-producing chickens that had developed myeloid leukosis. However, ALV-J infections associated with hemangiomas have occurred in egg-producing (layer) flocks in China. In this study, we identified an ALV-J layer isolate (HLJ13SH01) as a recombinant of ALV-J and a Rous sarcoma virus Schmidt-Ruppin B strain (RSV-SRB), which contained the RSV-SRB 5'-LTR and the other genes of ALV-J. Replication kinetic testing indicated that the HLJ13SH01 strain replicated faster than other ALV-J layer isolates in vitro. Sequence analysis indicated that the main difference between the two isolates was the 5'-LTR sequences, particularly the U3 sequences. A 19 nt insertion was uniquely found in the U3 region of the HLJ13SH01 strain. The results of a Dual-Glo luciferase assay revealed that the 19 nt insertion in the HLJ13SH01 strain increased the enhancer activity of the U3 region. Moreover, an additional CCAAT/enhancer element was found in the 19 nt insertion and the luciferase assay indicated that this element played a key role in increasing the enhancer activity of the 5'-U3 region. To confirm the potentiation effect of the 19 nt insertion and the CCAAT/enhancer element on virus replication, three infectious clones with 5'-U3 region variations were constructed and rescued. Replication kinetic testing of the rescued viruses demonstrated that the CCAAT/enhancer element in the 19 nt insertion enhanced the replication capacity of the ALV-J recombinant in vitro.

Genistein inhibits the replication of avian leucosis virus subgroup J in DF-1 cells

To investigate the antiviral effects of genistein on the replication of avian leukosis virus subgroup J (ALV-J) in DF-1 cells, the cells were treated with genistein at different time points and the antiviral effects were examined by using a variety of assays. We determined that genistein strongly inhibited viral gene expression and decreased the viral protein level in the cell supernatant and the cytoplasm without alerting virus receptor expression and viral attachment. We also observed that genistein was not found to interfere with virus entry, but significantly inhibited both viral gene transcriptions at 24h post infection and virus release, which indicate that genistein exerts its inhibitory effects on the late phase of ALV-J replicative cycle. These results demonstrate that genistein effectively block ALV-J replication by inhibiting virus transcription and release in DF-1 cells, which may be useful for therapeutic drug design.

Genomic sequence analysis and biological characteristics of a rescued clone of avian leukosis virus strain JS11C1, isolated from indigenous chickens

The strain JS11C1, a member of a putative new subgroup of avian leukosis virus (ALV) that is different from all six known subgroups from chickens based on Gp85 amino acid sequence comparison, was isolated from Chinese native chicken breeds in 2012. In order to further study the genome structure, biological characteristics, and the evolutionary relationship of the virus with others of known subgroups from infected chickens, we determined the complete genome sequence, constructed an infectious clone of ALV strain JS11C1, and performed comparative analysis using the whole genome sequence or elements with that of other ALVs available in GenBank. The results showed that the full-length sequence of the JS11C1 DNA provirus genome was 7707 bp, which is consistent with a genetic organization typical of a replication-competent type C retrovirus lacking viral oncogenes. The rescued infectious clone of JS11C1 showed similar growth rate and biological characteristics to its original virus. All the comparison analyses based on whole genomes support the opinion that the new isolates are relatively distantly related to any known subgroups of ALVs and might be classified as a new subgroup.

Analysis by high throughput sequencing of Specific Pathogen Free eggs

Specific Pathogen Free (SPF) embryonated eggs are used for the production of many veterinary and human vaccines. We have used High Throughput Sequencing to screen allantoic fluids and embryos for the presence of encapsidated viral genomes and viral transcripts, respectively. SPF eggs from two different producers were tested. We evidenced sequences corresponding to known endogenous retroviruses and sequences of Avian Leukosis Virus, but no sequence that might suggest a productive infection of eggs with a virus even distant from known viruses. Our results strongly suggest that SPF eggs such as those used for this study represent a safe substrate for the production of vaccines.

Molecular characteristics of the complete genome of a J-subgroup avian leukosis virus strain isolated from Eurasian teal in China

The J-subgroup avian leukosis virus (ALV-J) strain WB11098J was isolated from a wild Eurasian teal, and its proviral genomic sequences were determined. The complete proviral sequence of WB11098J was 7868 nt long. WB11098J was 95.3.9 % identical to the prototype strain HPRS-103, 94.2 % identical to the American strain ADOL-7501, 94.5-94.7 % identical to Chinese broiler isolates, 94.8-97.5 % identical to layer chicken isolates, and 94.4-95.0 % identical to Chinese local chicken isolates at the nucleotide level. Phylogenetic analysis showed that the WB11098J isolate shared the greatest homology with the layer strain SD09DP03 and was included in the same cluster. Interestingly, two 19-bp insertions in the U3 regions of the 5'LTR and 5'UTR that were most likely derived from other retroviruses were found in the WB11098J isolate. These insertions separately introduced one E2BP-binding site in the U3 region of the 5'LTR and a RNA polymerase II transcription factor IIB and core promoter motif of ten elements in the 5'UTR. A 5-bp deletion was identified in the U3 region of the 5'LTR. No nucleotides were deleted in the rTM or DR-1 regions in the 3'UTR. A 1-bp deletion was detected in the E element and introduced a specific and distinct binding site for c-Ets-1. Our study is the first to report the molecular characteristics of the complete genome of an ALV-J that was isolated from a wild bird and will provide necessary information for further understanding of the evolution of ALV-J.

Growth and replication of infectious bursal disease virus in the DF-1 cell line and chicken embryo fibroblasts

Infectious bursal disease virus (IBDV) causes a highly contagious disease in young chicks and leads to significant economic losses in the poultry industry. To determine a suitable cell line for IBDV infection, replication, and growth kinetics of the virus, DF-1 cells and chicken embryo fibroblasts (CEF) were used. The population doubling per day (Pd/D) was found to be higher in DF-1 as compared to CEF cells. A suitable time of infection (TOI) was established for increased production of virus and greater infectivity titers. The DF-1 and CEF cells were found to be susceptible to infection by producing marked cytopathic effects (CPEs), and the growth curves of IBDV in DF-1 and CEF cells were evaluated by infectivity assay using tissue culture infectious dose (TCID₅₀). The cytopathic effects of the virus in DF-1 and CEF cells were found to be similar, but higher viral titers were detected in the DF-1 cells as compared to CEF. Thus the DF-1 cell line had a higher growth potential and infectivity, which will be of advantage in vaccine production.

Effects of chromium picolinate on the viability of chick embryo fibroblast

Chromium picolinate (CrPic), which is used as a nutritional supplement and to treat type 2 diabetes, has gained much attention because of its cytotoxicity. This study evaluated the effects of CrPic on the viability of the chick embryo fibroblast (CEF) using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, morphological detection, and flow cytometry. The results show that lower concentrations of CrPic (8 and 16 μM) did not damage CEF viability (p > 0.05). However, higher CrPic concentrations (400 and 600 μM) indicated a highly significant effect on the production of intracellular reactive oxygen species, alteration of mitochondrial membrane potential, intracellular calcium ion concentration, and the apoptosis rate (p < 0.01), contrary to lower CrPic concentrations (8 and 16 μM) and control group. Moreover, apoptotic morphological changes induced by these processes in CEF were confirmed using Hoechst 33258 staining. Cell death induced by higher concentrations of CrPic was caused by an apoptotic and a necrotic mechanism, whereas the main mechanism of oxidative stress-induced mitochondrial dysfunction was apoptotic death.

A 19-nucleotide insertion in the leader sequence of avian leukosis virus subgroup J contributes to its replication in vitro but is not related to its pathogenicity in vivo

Subgroup J avian leukosis virus (ALV-J) was first isolated from meat-type chickens that had developed myeloid leukosis and since 2008, ALV-J infections in chickens have become widespread in China. A comparison of the sequence of ALV-J epidemic isolates with HPRS-103, the ALV-J prototype virus, revealed several distinct features, one of which is a 19-nucleotide (nt) insertion in the leader sequence. To determine the role of the 19-nt insertion in ALV-J pathogenicity, a pair of viruses were constructed and rescued. The first virus was an ALV-J Chinese isolate (designated rSD1009) containing the 19-nt insertion in its leader sequence. The second virus was a clone, in which the leader sequence had a deleted 19-nt sequence (designated rSD1009△19). Compared with rSD1009△19, rSD1009 displayed a moderate growth advantage in vitro. However, no differences were demonstrated in either viral replication or oncogenicity between the two rescued viruses in chickens. These results indicated that the 19-nt insertion contributed to ALV-J replication in vitro but was not related to its pathogenicity in vivo.

Development and application of a multiplex PCR method for rapid differential detection of subgroup A, B, and J avian leukosis viruses

Avian leukosis virus (ALV) subgroups A, B, and J are very common in poultry flocks and have caused serious economic losses in recent years. A multiplex PCR (mPCR) method for the detection of these three subgroups was developed and optimized in this study. We first designed a common forward primer, PF, and three downstream primers, AR, BR, and JR, which can amplify 715 bp for subgroup A, 515 bp for subgroup B, and 422 bp for subgroup J simultaneously in one reaction. The mPCR method produced neither cross-reactions with other subgroups of ALVs nor nonspecific reactions with other common avian viruses. The detection limit of the mPCR was as low as 1 × 10(3) viral DNA copies of each of the three subgroups. In animal experiments, the mPCR detected ALVs 2 to 4 days earlier than did virus isolation from whole-blood samples and cloaca swabs. Furthermore, a total of 346 clinical samples (including 127 tissue samples, 86 cloaca swabs, 59 albumen samples, and 74 whole-blood samples) from poultry flocks with suspected ALV infection were examined by mPCR, routine PCR, and virus isolation. The positive sample/total sample ratios for ALV-A, ALV-B, and ALV-J were 48% (166/346) as detected by mPCR and 48% (166/346) as detected by routine PCR. However, the positive sample/total sample ratio detected by virus isolation was 40% (138/346). The results of the mPCR and routine PCR were confirmed by sequencing the specific fragments. These results indicate that the mPCR method is rapid, specific, sensitive, and convenient for use in epidemiological studies of ALV, clinical detection of ALV, and ALV eradication programs.

Isolation, identification, and phylogenetic analysis of two avian leukosis virus subgroup J strains associated with hemangioma and myeloid leukosis

Cases of myeloid leukosis and hemangioma associated with avian leukosis virus subgroup J (ALV-J) are becoming more frequent in China in commercial layer chickens and breeders of egg-type chickens. In this study, two strains of ALV-J (SCAU11-H and SCAU11-XG) associated with hemangioma and myelocytoma were isolated from commercial broiler breeder animals in 2011. Their full-length proviral sequences were analyzed, revealing several unique genetic differences between the two isolates, and suggesting that the two viruses were derived from two distinct lineages. Strain SCAU11-H showed high sequence homology to early Chinese isolates associated with hemangioma, while strain SCAU11-XG was genetically closer to the prototype strain, HPRS-103. The complete genomic nucleotide sequences of SCAU11-H and SCAU11-XG were 7471 bp and 7727 bp in length, respectively. They shared 94.8% identity with each other, and had 94.0-96.8% nucleotide identity to ALV-J reference isolates. Homology analysis of the env, pol, and gag genes of the two isolates and other references strains showed that the gag and pol genes of the two viruses were more conserved than the env gene. In addition, the two isolates had significant deletions and substitutions in their 3'-UTR regions, compared to HPRS-103. These results suggest that the env gene and the 3'-UTR regions in these ALV-J isolates have evolved rapidly, and might be involved in the oncogenic spectrum of ALV-J. The results of this study contribute to our further study of the relationship between ALV integration patterns and multi-pathotypes associated with ALV-J.

Subgroup J avian leukosis virus infection inhibits autophagy in DF-1 cells

Background

Subgroup J avian leukosis virus (ALV-J) infection can induce tumor-related diseases in chickens. Previous studies by our laboratory demonstrated that ALV-J infection of DF-1 cells resulted in altered activity and phosphorylation of AKT. However, little is known about the subsequent activation of host DF-1 cells.

Results

In the current study, autophagy inhibition was observed for ALV-J infected DF-1 cells. Our data showed that the autophagosome protein, microtubule-associated protein 1 light chain 3-II (LC3-II), was reduced considerably in DF-1 cells infected with active ALV-J, while no change was observed for cells infected with inactivated ALV-J. Autophagy inhibition was also confirmed by fluorescence microscopy and transmission electron microscopy. Interestingly, when autophagy was promoted by rapamycin, the titers of ALV-J replication were decreased, and the replication level of ALV-J was significantly enhanced when atg5 (autophagy-related gene 5) was knocked out.

Conclusions

These results suggested that ALV-J infection could down-regulate autophagy in DF-1 cells during viral replication. This study is the first to report on the relationship between ALV-J infection and autophagy in DF-1 cells.

Detection of avian retroviruses in vaccines by amplification on DF-1 cells with immunostaining and fluorescent product-enhanced reverse transcriptase endpoint methods

In order to ensure the safety of vaccines produced on avian cells, rigorous testing for the absence of avian retroviruses must be performed. Current methods used to detect avian retroviruses often exhibit a high invalid-test/false-positive rate, rely on hard-to-secure reagents, and/or have readouts that are difficult to standardize. Herein, we describe the development and validation of two consistent and sensitive methods for the detection of avian retroviruses in vaccines: viral amplification on DF-1 cells followed by immunostaining for the detection of avian leukosis virus (ALV) and viral amplification on DF-1 cells followed by fluorescent product-enhanced reverse transcriptase (F-PERT) for the detection of all avian retroviruses. Both assays share an infectivity stage on DF-1 cells followed by a different endpoint readout depending on the retrovirus to be detected. Validation studies demonstrated a limit of detection of one 50% cell culture infectious dose (CCID(50))/ml for retrovirus in a 30-ml test inoculum volume for both methods, which was as sensitive as a classical method used in the vaccine industry, namely, viral amplification on primary chicken embryo fibroblasts followed by the complement fixation test for avian leukosis virus (COFAL). Furthermore, viral amplification on DF-1 cells followed by either immunostaining or F-PERT demonstrated a sensitivity that exceeds the regulatory requirements for detection of ALV strains. A head-to-head comparison of the two endpoint methods showed that viral amplification on DF-1 cells followed by F-PERT is a suitable method to be used as a stand-alone test to ensure that vaccine preparations are free from infectious avian retroviruses.

Avian leukosis virus subgroup A and B infection in wild birds of Northeast China

To analyze the status of wild birds infected with avian leukosis virus (ALV) in China, we collected 300 wild birds from various areas. Virus isolation and PCR showed that wild birds were infected by ALV-A and ALV-B. Two ALV-A and 4 ALV-B env sequences were obtained by PCR using primers designed to detect ALV-A and -B respectively. Our results showed that the gp85 genes of the 2 ALV-A strains have the highest homology with RAV-1, 99.8%, and more than 92% homology with other American strains. However, the gp85 genes of the two ALV-A strains showed slightly lower homology with Chinese strains (87.2-92.6%). Additionally, the 4 ALV-B strains have high homology with the prototype strain (RAV-2), from 99.1 to 99.4%, but they have slightly lower identity with Schmidt-Ruppin B and Prague subgroup B, from 93.3 to 98.4%. The 4 ALV-B strains showed the lowest identity with SDAU09C2 and SDAU09E3 (90%). In total, these results suggested that avian leukosis virus has infected wild birds in China.

Epidemiological study of fowl glioma-inducing virus in chickens in Asia and Germany

Fowl glioma-inducing virus (FGV), which belongs to avian leukosis virus (ALV) subgroup A, induces fowl glioma. This disease is characterized by multiple nodular gliomatous growths of astrocytes and has been previously reported in Europe, South Africa, Australia, the United States and Japan. FGV and FGV variants have spread to ornamental Japanese fowl, including Japanese bantams (Gallus gallus domesticus), in Japan. However, it is unclear how and where FGV emerged and whether FGV is related to the past fowl glioma in European countries. In this study, the prevalence of FGV in European, Asian and Japanese native chickens was examined. FGV could not be isolated from any chickens in Germany and Asian countries other than Japan. Eighty (26%) out of 307 chickens reared in Japan were positive by FGV-screening nested polymerase chain reaction and 11 FGV variants with an FGV-specific sequence in their 3' untranslated region were isolated. In addition, four other ALVs lacking the FGV-specific sequence were isolated from Japanese bantams with fowl glioma and/or cerebellar hypoplasia. These isolates were considered to be distinct recombinant viruses between FGV variants and endogenous/exogenous avian retroviruses. These results suggest that the variants as well as distinct recombinant ALVs are prevalent among Japanese native chickens in Japan and that FGV may have emerged by recombination among avian retroviruses in the chickens of this country.

A 205-nucleotide deletion in the 3' untranslated region of avian leukosis virus subgroup J, currently emergent in China, contributes to its pathogenicity

In the past 5 years, an atypical clinical outbreak of avian leukosis virus subgroup J (ALV-J), which contains a unique 205-nucleotide deletion in its 3' untranslated region (3'UTR), has become epidemic in chickens in China. To determine the role of the 205-nucleotide deletion in the pathogenicity of ALV-J, a pair of viruses were constructed and rescued. The first virus was an ALV-J Chinese isolate (designated HLJ09SH01) containing the 205-nucleotide deletion in its 3'UTR. The second virus was a chimeric clone in which the 3'UTR contains a 205-nucleotide sequence corresponding to a region of the ALV-J prototype virus. The replication and pathogenicity of the rescued viruses (rHLJ09SH01 and rHLJ09SH01A205) were investigated. Compared to rHLJ09SH01A205, rHLJ09SH01 showed a moderate growth advantage in vitro and in vivo, in addition to exhibiting a higher oncogenicity rate and lethality rate in layers and broilers. Increased vascular endothelial growth factor A (VEGF-A) and vascular endothelial growth receptor subtype 2 (VEGFR-2) expression was induced by rHLJ09SH01 more so than by rHLJ09SH01A205 during early embryonic vascular development, but this increased expression disappeared when the expression levels were normalized to the viral levels. This finding suggests that the expression of VEGF-A and VEGFR-2 is associated with viral replication and may also represent a novel molecular mechanism underlying the oncogenic potential of ALV-J. Overall, our findings not only indicate that the unique 205-nucleotide deletion in the ALV-J genome occurred naturally in China and contributes to increased pathogenicity but also point to the possible mechanism of ALV-J-induced oncogenicity.

Natural infection and transmission of a retrovirus closely related to myeloblastosis-associated virus type 1 in egg-type chickens

Myeloblastosis-associated virus type 1 (MAV-1) is an exogenous avian retrovirus with oncogenic potential. MAV-1 was detected in young chicks hatching from eggs produced by an experimental genetic line of egg-type chickens. Transmissibility of MAV-1 had not been documented previously. This investigation was intended to partially characterize the virus involved and to study its transmissibility and oncogenicity in naturally and contact-infected chickens. Commercially produced white and brown layer pullets free of exogenous avian leukosis viruses were commingled at hatch with naturally MAV-1-infected chickens. The original MAV-1-infected chickens were discarded after approximately 8 wk, and the contact-exposed chickens were maintained in isolation for 36 wk. Young specific-pathogen-free (SPF) single comb white leghorn chickens were added to the group to study possible horizontal transmission of MAV-1 in young chickens. Upon weekly virus isolation attempts, MAV-1 was readily isolated from the contact-exposed white layers but not from the brown layers between 36 and 53 wk of age (18 wk in total). Three-week-old SPF chickens were readily infected with MAV-1 by contact as early as 1 wk postexposure. Throughout 22 hatches derived from the white and brown MAV-1-contact-exposed layers (between 36 and 53 wk of age), MAV-1 was frequently detected in the white layer progeny, whereas the virus was seldom isolated from the progeny produced by the brown layers during the same 18-wk period. MAV-1 induced a persistent infection in some of the SPF chickens that were exposed by contact at 3 wk of age. Gross tumors were not detected in any of the originally infected experimental chickens at 8 wk of age, in the contact-exposed brown or white layers at the termination of the study at 53 wks of age, or in the contact-exposed SPF chickens at the end of the study at 12 wk of age. Exogenous avian leukosis-related viruses may still be detected in egg-type chickens, emphasizing the importance of thorough screening before incorporation of experimental genetic material into commercial genetic lines of egg-type chickens.

Detection and molecular characterization of recombinant avian leukosis viruses in commercial egg-type chickens in China

Two natural recombinant avian leukosis viruses (ALVs) were isolated from Chinese commercial egg-type chickens in 2009, which suffered from haemangiomas and myelocytomas. Sequence analysis of the complete proviral genomes revealed several unique genetic characteristics of the present two isolates, demonstrating that the two viruses were derived from recombination between earlier Chinese ALV-J and endogenous virus sequences. The two recombinant viruses presented typical genetic organization of replication-competent genus Alpharetrovirus, and the gag and pol genes were well conserved with those of ALVs. The env genes of the two viruses were composed of the internal identical sequences (about 240 bp) of endogenous viruses, and the rest of the sequence belonged to subgroup J ALVs. The long terminal repeats of the two viruses were more closely related to HPRS-103 and earlier Chinese ALV-J than other subgroup ALVs, and multiple transcription regulatory elements of ALV-J were highly conserved. In addition, the two viruses shared an almost identical 3'-untranslated region (UTR) sequence with earlier Chinese ALV-J strains and the US strain 4817, containing a ~127 bp deletion in the E element region. However, further comparison with endogenous ALV indicated that the 3'-UTR sequences with ~127 bp deletion of ALV-J were most probably derived from endogenous viruses by recombination. These results suggested that the two isolates can be characterized as recombinant ALV-J with the internal env gene and 3'-UTR sequence of endogenous ALV.