Novel endogenous retroviral sequences in the chicken genome closely related to HPRS-103 (subgroup J) avian leukosis virus

Detection of long terminal repeat loci derived from endogenous retrovirus in junglefowl using whole-genome sequencing

Endogenous retroviruses (ERVs) are genetic elements present in the genome that retain traces of past viral infections. Characterization of ERVs can provide crucial insights into avian evolution. This study aimed to identify novel long terminal repeat (LTR) loci derived from ERVs (ERV-LTRs) absent in the reference genome using whole-genome sequencing data of red junglefowl, gray junglefowl, Ceylon junglefowl, and green junglefowl. In total, 835 ERV-LTR loci were identified across the four Gallus species. The numbers of ERV-LTRs loci detected in red junglefowl and its subspecies gray junglefowl, Ceylon junglefowl, and green junglefowl were 362, 216, 193, and 128, respectively. The phylogenetic tree was congruent with previously reported trees, suggesting the potential for inferring relationships among past junglefowl populations from the identified ERV-LTR loci. Of the detected loci, 306 ERV-LTRs were identified near or within the genes, and some were associated with cell adhesion. The detected ERV-LTR sequences were classified as endogenous avian retrovirus family, avian leukosis virus subgroup E, Ovex-1, and murine leukemia virus-related ERVs. In addition, the sequence of the EAV family was divided into four patterns by combining the U3, R, and U5 regions. These findings contribute to a more comprehensive understanding of the characteristics of junglefowl ERVs.

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

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

Advances on genetic and genomic studies of ALV resistance

Avian leukosis (AL) is a general term for a variety of neoplastic diseases in avian caused by avian leukosis virus (ALV). No vaccine or drug is currently available for the disease. Therefore, the disease can result in severe economic losses in poultry flocks. Increasing the resistance of poultry to ALV may be one effective strategy. In this review, we provide an overview of the roles of genes associated with ALV infection in the poultry genome, including endogenous retroviruses, virus receptors, interferon-stimulated genes, and other immune-related genes. Furthermore, some methods and techniques that can improve ALV resistance in poultry are discussed. The objectives are willing to provide some valuable references for disease resistance breeding in poultry.

An EAV-HP insertion in the promoter region of SLCO1B3 has pleiotropic effects on chicken liver metabolism based on the transcriptome and proteome analysis

Solute carrier organic anion transporter 1B3 (SLCO1B3) is an important liver primarily highly expressed gene, its encoded protein (OATP1B3) involved in the transport of multi-specific endogenous and exogenous substances. We previously reported that an EAV-HP inserted mutation (IM+) in the 5' flanking region of SLCO1B3 was the causative mutation of chicken blue eggs, and a further research showed that IM+ significantly reduced the expression of SLCO1B3 in liver. Herein, we confirmed a cholate response element (IR-1) played an important role in activating SLCO1B3 and in vitro experiments showed that the activation of IR-1 can be significantly reduced by the EAV-HP IM+ . We performed transcriptome and proteomic analysis using the same set of IM+ and IM- liver tissues from Yimeng hens (a Chinese indigenous breed) to study the effect of SLCO1B3 and OATP1B3 expression reduction on chicken liver function. The results showed that common differential expression pathways were screened out from both transcriptome and proteome, in which fatty acid metabolism and drug metabolism-cytochrome P450 were significantly enriched in the KEGG analysis. The lipid-related metabolism was weakened in IM+ group, which was validated by serum biochemical assay. We unexpectedly found that EAV-HP fragment was highly expressed in the liver of the IM+ chickens. We cloned the EAV-HP full-length transcript and obtained the complete open reading frame. It is worth noting that there was some immune related differential expressed genes, such as NFKBIZ, NFKBIA, and IL1RL1, which were higher expressed in the IM+ group, which may due to the high expression of EAV-HP. Our study showed that EAV-HP IM+ reduced the expression of SLCO1B3 in liver, resulting in the decrease of fatty metabolism and exogenous substance transport capacity. The mutation itself also expressed in the liver and may be involved in the immune process. The mechanism needs further study.

The avian retroviral receptor Tva mediates the uptake of transcobalamin bound vitamin B12 (cobalamin)

The Avian sarcoma and leukosis viruses (ASLVs) are important chicken pathogens. Some of the virus subgroups, including ASLV-A and K, utilize the Tva receptor for cell entrance. Though Tva was identified three decades ago, its physiological function remains unknown. Previously, we have noted an intriguing resemblance and orthology between the chicken gene coding for Tva and the human gene coding for CD320, a receptor involved in cellular uptake of transcobalamin (TC) in complex with vitamin B12/cobalamin (Cbl).Here we show that both the transmembrane and the glycosylphosphatidylinositol (GPI)-anchored form of Tva in the chicken cell line DF-1 promotes the uptake of Cbl with help of expressed and purified chicken TC. The uptake of TC-Cbl complex was monitored using an isotope- or fluorophore-labeled Cbl. We show that (i) TC-Cbl is internalized in chicken cells; and (ii) the uptake is lower in the Tva-knockout cells and higher in Tva-overexpressing cells when compared with wild type chicken cells. The relation between physiological function of Tva and its role in infection was elaborated by showing that infection with ASLV subgroups (targeting Tva) impairs the uptake of TC-Cbl, while this is not the case for cells infected with ASLV-B (not recognized by Tva). In addition, exposure of the cells to a high concentration of TC-Cbl alleviates the infection with Tva-dependent ASLV.IMPORTANCE: We demonstrate that the ASLV receptor Tva participates in the physiological uptake of TC-Cbl, because the viral infection suppresses the uptake of Cbl and vice versa. Our results pave the road for future studies addressing the issues: (i) whether a virus infection can be inhibited by TC-Cbl complexes in vivo; and (ii) whether any human virus employs the human TC-Cbl receptor CD320. In broader terms, our study sheds light on the intricate interplay between physiological roles of cellular receptors and their involvement in virus infection.

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.

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.

Endogenous Retroviruses Drive Resistance and Promotion of Exogenous Retroviral Homologs

Endogenous retroviruses (ERVs) serve as markers of ancient viral infections and provide invaluable insight into host and viral evolution. ERVs have been exapted to assist in performing basic biological functions, including placentation, immune modulation, and oncogenesis. A subset of ERVs share high nucleotide similarity to circulating horizontally transmitted exogenous retrovirus (XRV) progenitors. In these cases, ERV-XRV interactions have been documented and include (a) recombination to result in ERV-XRV chimeras, (b) ERV induction of immune self-tolerance to XRV antigens, (c) ERV antigen interference with XRV receptor binding, and (d) interactions resulting in both enhancement and restriction of XRV infections. Whereas the mechanisms governing recombination and immune self-tolerance have been partially determined, enhancement and restriction of XRV infection are virus specific and only partially understood. This review summarizes interactions between six unique ERV-XRV pairs, highlighting important ERV biological functions and potential evolutionary histories in vertebrate hosts.

An EAV-HP insertion in the 5' flanking region of SLCO1B3 is associated with its tissue-expression profile in blue-eggshell Yimeng chickens (Gallus gallus)

We previously reported that blue eggshell color in chickens is associated with a partial endogenous retroviral (EAV-HP) insertion in the promoter region of the solute carrier organic anion transporter family member 1B3 (SLCO1B3) gene. The EAV-HP sequence includes numerous regulatory elements, which may modulate the expression of adjacent genes. To determine whether this insertion influences the expression of neighboring genes, we screened the expression of solute carrier organic anion transporter family members 1C1, 1B1 (SLCO1C1, SLCO1B1), and SLCO1B3 in 13 and 10 tissues from female and male Yimeng chickens, respectively. We observed that the insertion only significantly modulated the expression of SLCO1B3 and did not majorly affect that of SLCO1C1 and SLCO1B1. High expression of SLCO1B3 was detected in the shell gland, magnum, isthmus, and vagina of the oviduct in female blue-eggshell chickens. We also observed ectopic expression of SLCO1B3 in the testes of male chickens. SLCO1B3 is typically highly expressed in the liver; however, the EAV-HP insertion significantly reduces SLCO1B3 expression. As a liver-specific transporter, a reduction in the expression of SLCO1B3 may affect liver metabolism, particularly that of bile acids. We also detected higher ectopic expression of SLCO1B3 in the lungs of birds heterozygous for the EAV-HP insertion than in homozygous genotypes. In conclusion, we confirmed that the EAV-HP insertion modifies SLCO1B3 expression, and showed, for the first time, similar expression profile of this gene in all parts of the oviduct in females and testis in males. We also observed different levels of SLCO1B3 expression in the liver, which were associated with the EAV-HP insertion, and significantly higher expression in the lungs of birds with heterozygous genotype. The effects of these changes in the SLCO1B3 expression pattern on the function of the tissues warrant further investigation.

GGERV20, a recently integrated, segregating endogenous retrovirus in Gallus gallus

Endogenous retroviruses (ERVs) are widespread in vertebrate genomes. The recent availability of whole eukaryotic genomes has enabled their characterization in many organisms, including Gallus gallus (red jungle fowl), the progenitor of the domesticated chicken. Our bioinformatics analysis of a G. gallus ERV previously designated GGERV20 identified 35 proviruses with complete long terminal repeats (LTRs) and gag-pol open reading frames (ORFs) in the Genome Reference Consortium Chicken Build 6a, of which 8 showed potential for translation of functional retroviral polyproteins, including the integrase and reverse transcriptase enzymes. No elements were discovered with an env gene. Fifteen loci had LTR sequences with 100 % identity, indicative of recent integration. Chicken embryo fibroblast RNA-seq datasets showed reads representing the entire length of the GGERV20 provirus, supporting their potential for expressing viral proteins. To investigate the possibility that GGERV20 elements may not be fixed in the genome, we assessed the integration status of five loci in a meat-type chicken. PCRs targeting a GGERV20 locus on G. gallus chromosome one (GGERV20_1-1) reproducibly amplified both LTRs and the preintegration state, indicating that the bird from which the DNA was sampled was hemizygous at this locus. The four other loci examined only produced the preintegration state amplicons. These results reveal that GGERV20 is not fixed in the G. gallus population, and taken together with the lack of mutations seen in several provirus LTRs and their transcriptional activity, suggest that GGERV20 retroviruses have recently been and continue to be active in the chicken genome.

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.

Identification and characterization of a novel natural recombinant avian leucosis virus from Chinese indigenous chicken flock

Avian leukosis virus (ALV) caused tremendous economic losses to poultry industry all over the world, especially in China. One natural recombinant ALV strain, designated as HB2015032, was isolated from indigenous chickens with neoplastic diseases in Hubei, China. The complete proviral genome of HB2015032 is 7703 bp in length. Sequence analysis showed that the Env of HB2015032 exhibited 99.3% similarity with that of a ALV subgroup K (ALV-K) isolate JS11C1 at amino acid level. Phylogenetic analysis revealed that both gp85 and gp37 of HB2015032 were clustered in the same branch with JS11C1 and other ALV-K strains isolated from Chinese indigenous chickens in recent years. However, the pol gene, the 3' untranslated region (3' UTR), and the 3' long terminal repeat (3' LTR) of HB2015032 were more closely related to ALV-J prototype HPRS-103, and clustered in the same branch with ALV-J strains. Furthermore, the pol gene of HB2015032 contained a premature stop codon that resulted in a truncated Pol protein with 22 amino acid residues missing, which was a unique feature of the pol gene of ALV-J. 3'UTR of HB2015032 containing entire DR1, E element and U3. E element of HB2015032 contained one base deletion, which resulted in a c-Ets-1 binding site. In addition, U3 region of HB2015032 contains most of the transcription regulatory elements of ALV-J, including two CAAT boxes, Y boxes, CArG boxes, PRE boxes, NFAP-1 boxes, and one TATA box. These results suggest that isolate HB2015032 was a novel recombinant ALV-K containing the ALV-K env gene and the ALV-J backbone and exhibiting high pathogenicity.

Clonal anergy of CD117+chB6+ B cell progenitors induced by avian leukosis virus subgroup J is associated with immunological tolerance

BACKGROUND

The pathogenesis of immunological tolerance caused by avian leukosis virus subgroup J (ALV-J), an oncogenic retrovirus, is largely unknown.

RESULTS

In this study, the development, differentiation, and immunological capability of B cells and their progenitors infected with ALV-J were studied both morphologically and functionally by using a model of ALV-J congenital infection. Compared with posthatch infection, congenital infection of ALV-J resulted in severe immunological tolerance, which was identified as the absence of detectable specific antivirus antibodies. In congenitally infected chickens, immune organs, particularly the bursa of Fabricius, were poorly developed. Moreover, IgM-and IgG-positive cells and total immunoglobulin levels were significantly decreased in these chickens. Large numbers of bursa follicles with no differentiation into cortex and medulla indicated that B cell development was arrested at the early stage. Flow cytometry analysis further confirmed that ALV-J blocked the differentiation of CD117+chB6+ B cell progenitors in the bursa of Fabricius. Furthermore, both the humoral immunity and the immunological capability of B cells and their progenitors were significantly suppressed, as assessed by (a) the antibody titres against sheep red blood cells and the Marek's disease virus attenuated serotype 1 vaccine; (b) the proliferative response of B cells against thymus-independent antigen lipopolysaccharide (LPS) in the spleen germinal centres; and (c) the capacities for proliferation, differentiation and immunoglobulin gene class-switch recombination of B cell progenitors in response to LPS and interleukin-4(IL-4) in vitro.

CONCLUSIONS

These findings suggested that the anergy of B cells in congenitally infected chickens is caused by the developmental arrest and dysfunction of B cell progenitors, which is an important factor for the immunological tolerance induced by ALV-J.

Isolation and characterization of subgroup J Avian Leukosis virus associated with hemangioma in commercial Hy-Line chickens

There was an outbreak of hemangioma associated with avian leukosis virus subgroup J (ALV-J) between 2006 and 2010 in China in commercial layer chickens. Recently, severe hemangiomas broke out in Hy-Line layer chickens on a poultry farm in 2017 where ALV was eradicated earlier. Six isolates of ALV-J, named SDAU1701-SDAU1706, were characterized by virus isolation and sequence analysis of the complete proviral genomes. Avian leukosis virus subgroup J was identified by an immunofluorescence assay with monoclonal antibody JE9, whereas Marek's disease virus or reticuloendotheliosis virus was not detected. Sequence analysis of the complete proviral genome revealed that there was 96.0-99.6% identity between each other and had a homology of 94.6-96.0% when compared with the reference strain. The six isolates formed one distinct lineage separate from the reference sequences in a phylogenetic-tree, which suggested that there were several genetic differences between these groups. Homology analysis of the env, pol, and gag genes of the six isolates showed that the env gene was more variable, especially the gp85 protein, which shared only 88.2-91.9% identity with the reference strains. Sequence comparisons of the gp85 protein indicated that 19 sites were different from those in the NX0101 and HPRS-103 strains inducing myeloid leukosis; among our strains, five mutations were identical to those in the viruses causing hemangioma. Four other distinctive mutations were detected in our six isolates. This study reminds us that the surveillance of viral eradication should be conducted continuously on a farm where ALVs were eradicated. To prevent the prevalence of ALVs, more attention should be paid to daily monitoring.

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.

Domestic chickens activate a piRNA defense against avian leukosis virus

PIWI-interacting RNAs (piRNAs) protect the germ line by targeting transposable elements (TEs) through the base-pair complementarity. We do not know how piRNAs co-evolve with TEs in chickens. Here we reported that all active TEs in the chicken germ line are targeted by piRNAs, and as TEs lose their activity, the corresponding piRNAs erode away. We observed de novo piRNA birth as host responds to a recent retroviral invasion. Avian leukosis virus (ALV) has endogenized prior to chicken domestication, remains infectious, and threatens poultry industry. Domestic fowl produce piRNAs targeting ALV from one ALV provirus that was known to render its host ALV resistant. This proviral locus does not produce piRNAs in undomesticated wild chickens. Our findings uncover rapid piRNA evolution reflecting contemporary TE activity, identify a new piRNA acquisition modality by activating a pre-existing genomic locus, and extend piRNA defense roles to include the period when endogenous retroviruses are still infectious.

DOI:http://dx.doi.org/10.7554/eLife.24695.001

Viruses called retroviruses can infect animal cells and merge their genetic information with those of the animal causing damage to the animal’s genetic blueprints. Once retroviruses are integrated into a cell they can sometimes get passed down through the generations over the centuries. Almost half of the human genetic code, for example, is made from ancient retroviruses and other foreign sequences. Over time many of these ancient viruses lost the ability to infect other cells and became trapped within cells but they can still jump out and damage the animal’s genetic code under certain circumstances. These trapped foreign sequences are called transposable elements.

Animal cells produce molecules called piRNAs to shut down transposable elements. Most piRNAs are produced from genetic information that originally came from integrated retroviruses and that has been hijacked to defend the cell, a similar strategy as Crisper system in bacteria. Domestic chickens produce piRNAs against a virus called avian leukosis virus (or ALV for short) – which commonly infects domestic fowl. The virus also infected the wild ancestors of chickens, known as red jungle fowl, but these birds do not produce piRNAs. This provides an ideal setting to study the evolution of piRNAs in an animal that is not too distantly related to humans (chickens and humans both have backbones, and are therefore both warm-blooded vertebrates).

Sun et al. examined cells from the testicles of domestic chickens and red jungle fowl as an example of the role of piRNAs in protecting genetic information in vertebrates. The investigation revealed that piRNAs against all previously trapped viruses in the chicken’s genetic code are produced in chickens to stop them from causing more damage. Sun et al. also observed the creation of piRNAs in chickens in response to ALV that had not yet become trapped in the chicken’s genetic code. Importantly, the piRNAs could control these retroviruses while they were still infectious.

The experiments also revealed that piRNAs against ALV are produced from a single copy of ALV that is found in both domestic and wild chickens. The results showed that cells can produce new piRNAs using these pre-existing viral copies within their own genetics. This illustrates that production of piRNA from existing genetic material can be activated in response to certain cues.

Further work will seek to discover how existing genetic information becomes a source of piRNAs. In the United States, 8 billion domestic chickens are consumed each year, and a better understanding of how these birds defend themselves against viral infections could increase the productivity of the poultry industry around the world. Moreover, because other viruses trapped in the chicken’s genetic code are related to similar viruses in humans, future discoveries made in this area could help to guide research that will benefit human health as well.

DOI:http://dx.doi.org/10.7554/eLife.24695.002

Taishan Pinus massoniana pollen polysaccharide inhibits subgroup J avian leucosis virus infection by directly blocking virus infection and improving immunity

Subgroup J avian leucosis virus (ALV-J) generally causes neoplastic diseases, immunosuppression and subsequently increases susceptibility to secondary infection in birds. The spread of ALV-J mainly depends on congenital infection and horizontal contact. Although ALV-J infection causes enormous losses yearly in the poultry industry worldwide, effective measures to control ALV-J remain lacking. In this study, we demonstrated that Taishan Pinus massoniana pollen polysaccharide (TPPPS), a natural polysaccharide extracted from Taishan Pinus massoniana pollen, can significantly inhibit ALV-J replication in vitro by blocking viral adsorption to host cells. Electron microscopy and blocking ELISA tests revealed that TPPPS possibly blocks viral adsorption to host cells by interacting with the glycoprotein 85 protein of ALV-J. Furthermore, we artificially established a congenitally ALV-J-infected chicken model to examine the anti-viral effects of TPPPS in vivo. TPPPS significantly inhibited viral shedding and viral loads in immune organs and largely eliminated the immunosuppression caused by congenital ALV-J infection. Additionally, pre-administration of TPPPS obviously reduced the size and delayed the occurrence of tumors induced by acute oncogenic ALV-J infection. This study revealed the prominent effects and feasible mechanisms of TPPPS in inhibiting ALV-J infection, thereby providing a novel prospect to control ALV-J spread.

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.

Genome-wide analysis reveals the extent of EAV-HP integration in domestic chicken

BACKGROUND

EAV-HP is an ancient retrovirus pre-dating Gallus speciation, which continues to circulate in modern chicken populations, and led to the emergence of avian leukosis virus subgroup J causing significant economic losses to the poultry industry. We mapped EAV-HP integration sites in Ethiopian village chickens, a Silkie, Taiwan Country chicken, red junglefowl Gallus gallus and several inbred experimental lines using whole-genome sequence data.

RESULTS

An average of 75.22 ± 9.52 integration sites per bird were identified, which collectively group into 279 intervals of which 5 % are common to 90 % of the genomes analysed and are suggestive of pre-domestication integration events. More than a third of intervals are specific to individual genomes, supporting active circulation of EAV-HP in modern chickens. Interval density is correlated with chromosome length (P < 2.31(-6)), and 27 % of intervals are located within 5 kb of a transcript. Functional annotation clustering of genes reveals enrichment for immune-related functions (P < 0.05).

CONCLUSIONS

Our results illustrate a non-random distribution of EAV-HP in the genome, emphasising the importance it may have played in the adaptation of the species, and provide a platform from which to extend investigations on the co-evolutionary significance of endogenous retroviral genera with their hosts.

The MET gene is a common integration target in avian leukosis virus subgroup J-induced chicken hemangiomas

Avian leukosis virus subgroup J (ALV-J) is a simple retrovirus that can cause hemangiomas and myeloid tumors in chickens and is currently a major economic problem in Asia. Here we characterize ALV-J strain PDRC-59831, a newly studied U.S. isolate of ALV-J. Five-day-old chicken embryos were infected with this virus, and the chickens developed myeloid leukosis and hemangiomas within 2 months after hatching. To investigate the mechanism of pathogenesis, we employed high-throughput sequencing to analyze proviral integration sites in these tumors. We found expanded clones with integrations in the MET gene in two of the five hemangiomas studied. This integration locus was not seen in previous work characterizing ALV-J-induced myeloid leukosis. MET is a known proto-oncogene that acts through a diverse set of signaling pathways and is involved in many neoplasms. We show that tumors harboring MET integrations exhibit strong overexpression of MET mRNA.

IMPORTANCE

These data suggest that ALV-J induces oncogenesis by insertional mutagenesis, and integrations in the MET oncogene can drive the overexpression of MET and contribute to the development of hemangiomas.

Prototype endogenous avian retroviruses of the genus Gallus

Ancient endogenous retroviruses (ERVs), designated endogenous avian retrovirus (EAVs), are present in all Gallus spp. including the chicken, and resemble the modern avian sarcoma and leukosis viruses (ASLVs). The EAVs comprise several distinct retroviruses, including EAV-0, EAV-E51 and EAV-HP, as well as a putative member previously named the avian retrotransposon of chickens (ART-CH). Thus far, only the EAV-HP elements have been well characterized. Here, we determined sequences of representative EAV-0 and EAV-E51 proviruses by cloning and data mining of the 2011 assembly of the Gallus gallus genome. Although the EAV-0 elements are primarily deleted in the env region, we identified two complete EAV-0 env genes within the G. gallus genome and prototype elements sharing identity with an EAV-E51-related clone previously designated EAV-E33. Prototype EAV-0, EAV-E51 and EAV-E33 gag, pol and env gene sequences used for phylogenetic analysis of deduced proteins showed that the EAVs formed three distinct clades, with EAV-0 sharing the last common ancestor with the ASLVs. The EAV-E51 clade showed the greatest level of divergence compared with other EAVs or ASLVs, suggesting that these ERVs represented exogenous retroviruses that evolved and integrated into the germline over a long period of time. Moreover, the degree of divergence between the chicken and red jungle fowl EAV-E51 sequences suggested that they were more ancient than the other EAVs and may have diverged through mutations that accumulated post-integration. Finally, we showed that the ART-CH elements were chimeric defective ERVs comprising portions of EAV-E51 and EAV-HP rather than authentic retrotransposons.

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.

Avian retroviral replication

Avian retroviruses were originally identified as cancer-inducting filterable agents in chicken neoplasms at the beginning of the 20th century. Since their discovery, the study of these simple retroviruses has contributed greatly to our understanding of viral replication and cancer. Avian retroviruses continue to evolve and have great economic importance in the poultry industry worldwide. The aim of this review is to provide a broad overview of the genome, pathology, and replication of avian retroviruses. Notable gaps in our current knowledge are highlighted, and areas where avian retroviruses differ from other retroviruses are emphasized.

Identification of two novel multiple recombinant avian leukosis viruses in two different lines of layer chicken

Avian leukosis virus (ALV) is the most common oncogenetic retrovirus that emerges spontaneously as a result of recombination between exogenous viruses, exogenous viruses and endogenous viruses, and exogenous viruses and non-homologous cellular genes. In the present study, two natural recombinant avian leukosis viruses (rALVs) (LC110515-5 and LC110803-5) carrying a subgroup C gp85 gene, a subgroup E gp37 gene, and a subgroup J 3'UTR and 3'LTR were isolated from two different lines of layer flocks, Black-bone silky fowl (BSF) and commercial layer chicken, that suffered from myeloid leukosis. Although tumours were not observed in rALV-infected individual chickens, other non-neoplastic inflammatory lesions were evident. The two rALVs were cultured on DF-1 cells and identified by PCR, immunofluorescence assay and gene sequencing. The gp85 nucleotide sequence in the two isolates displayed a high identity (>95 %) with that of the gp85 gene in ALV-C, but the identity was less than 90 % with ALV-A/B/D/E and only 51 % with ALV-J. Phylogenetic analysis of the nucleotide and amino acid sequences confirmed that the two isolates were recombinant between ALV-C, ALV-E and ALV-J. Subgroup C ALV is rarely found in field cases. This report is the first to provide evidence that ALV-C has recombined with ALV-E and ALV-J in two different chicken lines. The source and characteristics of the two rALVs and ALV-C need to be further investigated.

Isolation of acutely transforming subgroup J avian leukosis viruses that induce erythroblastosis and myelocytomatosis

Avian leukosis virus of subgroup J (ALV-J), isolated in the late 1980s, predominantly causes myelocytic myeloid leukosis in meat-type chickens. In the past few years, we have observed the occurrence of lesions indicative of erythroblastosis in ALV-J-infected flocks and, in this paper, we report the isolation of ALV-J strains from such flocks. Three of these isolates were acutely transforming viruses, as shown by their ability to transform bone marrow cell cultures. The bone marrow cultures transformed by these virus isolates were very similar to the myeloid cells transformed by the ALV-J strain 966. However, the infection of meat-type chickens with these isolates either as embryos or as 1-day-old chicks resulted in the induction of erythroblastosis as well as myelocytomatosis. Other histopathological changes observed in the inoculated birds included neoplastic lesions such as cholangioma and testicular cell tumour, and non-neoplastic lesions such as lymphomyeloid hyperplasia. This report demonstrates that highly oncogenic ALV-J, capable of inducing a different spectrum of disease other than the widely reported myelocytomatosis, could be established in naturally infected flocks.

Isolation of acutely transforming subgroup J avian leukosis viruses that induce erythroblastosis and myelocytomatosis

Avian leukosis virus of subgroup J (ALV-J), isolated in the late 1980s, predominantly causes myelocytic myeloid leukosis in meat-type chickens. In the past few years, we have observed the occurrence of lesions indicative of erythroblastosis in ALV-J-infected flocks and, in this paper, we report the isolation of ALV-J strains from such flocks. Three of these isolates were acutely transforming viruses, as shown by their ability to transform bone marrow cell cultures. The bone marrow cultures transformed by these virus isolates were very similar to the myeloid cells transformed by the ALV-J strain 966. However, the infection of meat-type chickens with these isolates either as embryos or as 1-day-old chicks resulted in the induction of erythroblastosis as well as myelocytomatosis. Other histopathological changes observed in the inoculated birds included neoplastic lesions such as cholangioma and testicular cell tumour, and non-neoplastic lesions such as lymphomyeloid hyperplasia. This report demonstrates that highly oncogenic ALV-J, capable of inducing a different spectrum of disease other than the widely reported myelocytomatosis, could be established in naturally infected flocks.

Subgroup J avian leukosis virus infection in turkeys: induction of rapid onset tumours by acutely transforming virus strain 966

Avian leukosis virus subgroup J (ALV-J), isolated in the late 1980s, predominantly causes myelocytic myeloid leukosis in meat-type chickens. In the past 10 years, ALV-J infection has become very widespread, causing serious problems to the chicken meat industry. Previously, we have shown that turkey cells can be infected in vitro with Rous sarcoma virus pseudotypes of ALV-J. In this paper, we extend those observations to show that turkey monocyte cultures can be transformed in vitro with acutely transforming ALV-J strain 966. We also show that turkeys are experimentally susceptible to infection with ALV-J prototype strain HPRS-103. However, neoplastic lesions were not observed in these birds, probably due to the short experimental period of 10 weeks. When inoculated into 1-day-old turkey poults, acutely transforming ALV-J strain 966 induced tumours between 3 and 4 weeks after infection. Most of the birds showed tumours involving the liver, with histopathological lesions of myelocytomatosis. The demonstration of the spread of HPRS-103 by contact among turkeys, although observed only at low levels in the present study, stresses the importance of segregation of turkey and chicken breeding operations to avoid the spread of ALV-J infection.

Sequence analysis for the complete proviral genome of avian leukosis virus subgroup J associated with haemangiomas, leiomyosarcomas and myelomas in layer flocks

Avian leukosis virus subgroup J (ALV-J) can cause a variety of neoplasms, including mainly myeloid leukosis (myelocytomatosis) and nephromas. Other tumours, such as histiocytic sarcoma (HS), haemangiosarcoma and mesothelioma, may also develop. In a previous article we described a case in which myeloid leukosis, haemangiomas and leiomyosarcomas appeared simultaneously in a commercial layer flock with infection by ALV-J. The present research was completed to understand the molecular characteristics of the ALV-J strain that induced clinical myeloid leukosis, haemangiomas and leiomyosarcomas. Two strains of ALV-J (SDAU1001 and SDAU1002) were isolated and identified, and their full-length sequences were analysed. The complete genome nucleotide sequences of these two isolates were different in length, 7652 nt and 7636 nt, respectively. They shared 98.9% identity with each other, and 93.4% to 97.8% nucleotide identity to the reference ALV-J isolates. A 19-nucleotide repeat sequence was identified in the primer binding site (PBS) leader region of isolate SDAU1001. A base substitution mutation (base 15 C-T) in this insertion was identified. However, the identical insertion at the same site was not found in SDAU1002. The gag and pol genes of the two viruses were more conserved than the env gene. One key deletion in the E element was a common feature of SDAU1001 and SDAU1002. SDAU1001 and SDAU1002, possibly recombinants of ALV-J and another avian retrovirus, may share the same ancestor. Co-infection by SDAU1001 and SDAU1002 isolates is a possible explanation why myeloid leukosis, haemangiomas, and leiomyosarcomas appeared simultaneously in the same commercial layer flock.

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.

Novel sequences of subgroup J avian leukosis viruses associated with hemangioma in Chinese layer hens

Background

Avian leukosis virus subgroup J (ALV-J) preferentially induces myeloid leukosis (ML) in meat-type birds. Since 2008, many clinical cases of hemangioma rather than ML have frequently been reported in association with ALV-J infection in Chinese layer flocks.

Results

Three ALV-J strains associated with hemangioma were isolated and their proviral genomic sequences were determined. The three isolates, JL093-1, SD09DP03 and HLJ09MDJ-1, were 7,670, 7,670, and 7,633 nt in length. Their gag and pol genes were well conserved, with identities of 94.5-98.6% and 97.1-99.5%, respectively, with other ALV-J strains at the amino acid level (aa), while the env genes of the three isolates shared a higher aa identity with the env genes of other hemangioma strains than with those of ML strains. Interestingly, two novel 19-bp insertions in the U3 region in the LTR and 5' UTR, most likely derived from other retroviruses, were found in all the three isolates, thereby separately introducing one E2BP binding site in the U3 region in the LTR and RNA polymerase II transcription factor IIB and core promoter motif ten elements in the 5' UTR. Meanwhile, two binding sites in the U3 LTRs of the three isolates for NFAP-1 and AIB REP1 were lost, and a 1-base deletion in the E element of the 3' UTR of JL093-1 and SD09DP03 introduced a binding site for c-Ets-1. In addition to the changes listed above, the rTM of the 3' UTR was deleted in each of the three isolates.

Conclusion

Our study is the first to discovery the coexistence of two novel insertions in the U3 region in the LTR and the 5' UTR of ALV-J associated with hemangioma symptoms, and the transcriptional regulatory elements introduced should be taken into consideration in the occurrence of hemangioma.

Sequence analysis for the complete proviral genome of subgroup J Avian Leukosis virus associated with hemangioma: a special 11 bp deletion was observed in U3 region of 3'UTR

Background

Avian Leukosis virus (ALV) of subgroup J (ALV-J) belong to retroviruses, which could induce tumors in domestic and wild birds. Myelocytomatosis was the most common neoplasma observed in infected flocks; however, few cases of hemangioma caused by ALV-J were reported in recent year.

Results

An ALV-J strain SCDY1 associated with hemangioma was isolated and its proviral genomic sequences were determined. The full proviral sequence of SCDY1 was 7489 nt long. Homology analysis of the env, pol and gag gene between SCDY1 and other strains in GenBank were 90.3-94.2%, 96.6-97.6%, and 94.3-96.5% at nucleotide level, respectively; while 85.1-90.7%, 97.4-98.7%, and 96.2-98.4% at amino acid level, respectively. Alignment analysis of the genomic sequence of ALV-J strains by using HPRS-103 as reference showed that a special 11 bp deletion was observed in U3 region of 3'UTR of SCDY1 and another ALV-J strain NHH isolated from case of hemangioma, and the non-functional TM and E element were absent in the genome of SCDY1, but the transcriptional regulatory elements including C/EBP, E2BP, NFAP-1, CArG box and Y box were highly conserved. Phylogenetic analysis revealed that all analyzed ALV-J strains could be separated into four groups, and SCDY1 as well as another strain NHH were included in the same cluster.

Conclusion

The variation in envelope glycoprotein was higher than other genes. The genome sequence of SCDY1 has a close relationship with that of another ALV-J strain NHH isolated from case of hemangioma. A 11 bp deletion observed in U3 region of 3'UTR of genome of ALV-J isolated from case of hemangioma is interesting, which may be associated with the occurrence of hemangioma.

Methods for evaluating and developing commercial chicken strains free of endogenous subgroup E avian leukosis virus

The genome of nearly all chickens contains various DNA proviral insertions of retroviruses of subgroup E avian leukosis virus (ALVE). However, the elimination or control of ALVE gene expression is desirable to improve productivity, to improve resistance to avian leukosis virus (ALV)-induced tumours, and to develop safer live virus vaccines in chick embryos and cultured chick cells. Restriction fragment length polymorphism and polymerase chain reaction methods are used to define the presence of ALVE genes; and the expression of ALVE in chicken plasma or on cells, and the susceptibility of cells to ALVE is determined by flow cytometry using a specific (R2) antibody. ADOL line 0 chickens have been selected to be free of ALVE genes, while being resistant (i.e. lack receptors to ALVE), but susceptible to exogenous ALV (i.e. ALVA, ALVB, ALVC and ALVJ). To develop improved line 0-type chickens, ADOL line 0 was outcrossed to a commercial line that had one ALVE gene and evidence for ALVE resistance. Rous sarcoma virus (RSV) challenge was used to confirm resistance of F1 chickens to ALVE, and susceptibility of F2 breeders to ALVA and ALVB using test chicks produced by matings to line 7(2). Selected F2 breeders were resistant to ALVE, but susceptible to exogenous ALVA, ALVB, ALVC and ALVJ, based on challenge tests of progeny chick cells using an enzyme-linked immunosorbent assay. The new line, 0(1), has evidence for improved egg size, productivity, fertility and hatchability. Similar procedures may be used for development of productive ALVE free chicken lines with preferred ALV susceptibility traits.

Survey of endogenous virus and TVB* receptor status of commercial chicken stocks supplying specific-pathogen-free eggs

Endogenous avian leukosis virus (ALVE) and the ALVE receptor (TVB*S1) status of six commercial chicken lines supplying specific-pathogen-free eggs were analyzed. All commercial chicken lines are certified free of the avian leukosis virus (ALV) by screening for expression of the p27 protein using the standard enzyme-linked immunosorbent assay. The commercial chicken lines A, E, and F contained replication competent ALVE inserts. Line A was fixed for ALVE21, and lines E and F were segregating for ALVE10. In addition, ALVE1 was detected in all the chicken lines. Chicken lines B, D, and F were essentially fixed for the TVB*S1 allele that confers susceptibility to ALVE, whereas lines A, C, B, and E were resistant, containing either the TVB*S3 or TVB*R alleles. The results show that lines selected to be ALV p27 negative give rise to two different genotypes. One genotype lacks the TVB*S1 receptor for ALVE. Chicken lines with the TVB*S1 negative genotype can retain replication competent endogenous virus inserts such as ALVE2, 10, or 21 and still display the p27 negative phenotype. These replication competent ALVE viruses are phenotypically p27 negative in the absence of the TVB*S1 receptor because their chromosomal integration sites restrict transcription and subsequent production of the p27 protein and virus particles to levels below the detection limit. If the TVB*S1 receptor is present, the limited production of ALVE virus particles reinfects and integrates into more productive chromosomal locations in the cell. Increased production of infective virus particles and detectable levels of p27 follow this reinfection and integration into more active regions of the cells genome. The other genotype observed in the commercial lines retains the ALVE receptor (TVB*S1) but either lacks replication competent inserts or expresses the envelope encoded protein from defective inserts such as ALVE3 or ALVE6. In this phenotype, the env-coded glycoprotein encoded by the defective inserts binds to the TVB*S1 receptor and blocks the reinfection of the replication competent ALVE virus. This receptor interference stops reinfection and subsequent production of detectable virus particles and the p27 protein. Mixtures of different p27 negative phenotypes can result in the p27 positive phenotype and ALVE virus production. For example, mixtures of ALVE receptor positive (TVB*S1) but ALVE negative (p27 negative and envelope negative) chick embryo fibroblasts (CEFs) with fibroblasts that are receptor negative but ALVE positive could generate cells expressing high levels of p27 and ALVE virus. In this situation, the undetectable levels of ALVE virus from the receptor negative CEFs would infect and integrate into the receptor positive CEFs and produce detectable levels of ALVE virus. The implications of these findings for vaccine manufacturers and regulatory agencies are discussed.

Molecular characterization of three recombinant isolates of avian leukosis virus obtained from contaminated Marek's disease vaccines

Three natural recombinant avian leukosis viruses (ALV; PDRC-1039, PDRC-3246, and PDRC-3249) expressing a subgroup A gp85 envelope protein and containing long terminal repeats (LTR) of endogenous ALV-E viruses were isolated from contaminated commercial Marek's disease vaccines, cloned, and completely sequenced. Their full genomes were analyzed and compared with representative strains of ALV. The proviral DNA of all three isolates displayed 99.3% identity to each other, suggesting a possible common ancestor, even though the contaminating viruses were obtained from three separate vaccine serials produced by two different vaccine manufacturing companies. The contaminating viruses have a genetic organization typical of replication-competent alpharetroviruses. The proviral genomes of PDRC-1039 and PDRC-3246 are 7497 bp long, and the PDRC-3249 is three base pairs shorter because of a deletion of a threonine residue within the gp85 coding region. The LTR, gag, pol, and the transmembrane (TM) region (gp37) of the env gene of all three viruses displayed high identity to endogenous counterpart sequences (>98%). Only the surface (SU) region (gp85) of the env gene displayed high identity with exogenous ALV-A (98.7%). Locus-specific polymerase chain reaction (PCR) analysis for ALV endogenous sequences (ev loci) in the chicken embryo fibroblasts used to produce the original vaccine vials identified the presence of ev-1, ev-2, ev-3, ev-4, and ev-6 in all three vaccines. Homologous recombination most likely took place to involve the SU region of the env gene because the recombinant viruses only differ in this particular region from the consensus ALV-E. These results suggest that the contaminating ALV isolates probably emerged by recombination of ALV-A with endogenous virus sequences before vaccine preparation.

Molecular analysis of endogenous avian leukosis/sarcoma virus genomes in Korean chicken embryos

Since the status of endogenous avian leucosis/sarcoma virus (ALSV) infections in Korean broiler chickens is unclear, this study examined embryonated eggs obtained from broiler farms and Korean native chicken breeds in Korea using PCR with the primer sets specific for endogenous ALSVs. The PCR assays detected the genomes of EAV, ev, ev/J and ART-CH belonging to the endogenous ALSV from all embryos tested. Phylogenetically, the Korean EAV genomes were more closely related to the prototype EAV-0 than to the other prototype, E51. The Korean ART-CH elements clustered together but were distinct from the prototype ART-CH clones, 5 and 14. Although there was comparatively little divergence in the nucleotide and amino acid sequences of the Korean ev and ev/J genomes compared with the other known ev and ev/J genomes, the Korean genomes had phylogenetically distinct branches. From these results, endogenous genomes are quite prevalent in Korean broiler chickens. In addition, the endogenous genomes circulating in Korean broiler chickens are genetically different from the other known endogenous genomes. These results are expected to provide useful information for the control and establishment of a surveillance system for endogenous ALSVs in Korea.

Molecular and Biological Characterization of a Naturally Occurring Recombinant Subgroup B Avian Leukosis Virus with a Subgroup J–Like Long Terminal Repeat

Infection of broiler chickens with subgroup J avian leukosis virus (ALV) results in the induction of myeloid tumors. However, although egg-type chickens are susceptible to infection with ALV-J, the tumor incidence is very low, and on rare occasions the tumors observed are of the myeloid lineage. We recently described the isolation of an ALV (AF115-4) from commercial egg-type chickens suffering from myeloid leukosis. AF115-4 was initially identified as an ALV-J isolate based on PCR analysis of the long terminal repeat (LTR). However, further characterization of the viral envelope indicated that the virus is recombinant with subgroups B envelope and J LTR. Here we further characterize this recombinant virus at both the molecular and biological levels. We show that the AF115-4 isolate expresses a recombinant envelope glycoprotein encoded by a subgroup B gp85 region and a subgroup E gp37 region. The host range ofAF115-4 was analyzed using cells resistant to infection by subgroups A/B, J, or E; this shows that no ALV-J was present in the isolates obtained from the affected chickens. Additional antigenic characterization of AF115-4 using chicken sera specific for subgroups B or J indicated that no ALV-J was present in the samples examined. Inoculation of AF 115-4 into ALV-susceptible 1515 X 71 chickens resulted in the induction of lymphoid leukosis but not the expected myeloid leukosis affecting the commercial chickens. These results suggest that differences in the genetic makeup of the chickens from which AF115-4 was isolated and the line 1515 X 71 used in the present experiments may be responsible for the observed differences in pathogenicity. In addition, the results suggest that ALV-J continues to evolve by recombination, generating new viruses with different pathological properties.

Susceptibility of various parental lines of commercial white leghorn layers to infection with a naturally occurring recombinant avian leukosis virus containing subgroup B envelope and subgroup J long terminal repeat

Chickens from seven different parental lines of commercial White Leghorn layer flocks from three independent breeders were inoculated with a naturally occurring avian leukosis virus (ALV) containing an ALV-B envelope and an ALV-J long terminal repeat (LTR) termed ALV-B/J. Additional groups of chickens from the same seven parental lines were inoculated with ALV-B. Chickens were tested for ALV viremia and antibody at 0, 4, 8, 16, and 32 wk postinfection. Chickens from all parental lines studied were susceptible to infection with ALV-B with 40%-100% of inoculated chickens positive for ALV at hatch following embryo infection. Similarly, infection of egg layer flocks with the ALV-B/J recombinant virus at 8 days of embryonation induced tolerance to ALV with 86%-100% of the chickens viremic, 40%-75% of the chickens shedding virus, and only 2/125 (2%) of the chickens producing serum-neutralizing antibodies against homologous ALV-B/J recombinant virus at 32 wk postinfection. In contrast, when infected with the ALV-B/J recombinant virus at hatch, 33%-82% of the chickens were viremic, 28%-47% shed virus, and 0%-56% produced serum-neutralizing antibodies against homologous ALV-B/J recombinant virus at 32 wk postinfection. Infection with the ALV-B/J recombinant virus at embryonation and at hatch induced predominately lymphoid leukosis (LL), along with other common ALV neoplasms, including erythroblastosis, osteopetrosis, nephroblastomas, and rhabdosarcomas. No incidence of myeloid leukosis (ML) was observed in any of the commercial White Leghorn egg layer flocks infected with ALV-B/J in the present study. Data suggest that the parental line of commercial layers may influence development of ALV-B/J-induced viremia and antibody, but not tumor type. Differences in type of tumors noted in the present study and those noted in the field case where the ALV-B/J was first isolated may be attributed to differences in the genetics of the commercial layer flock in which ML was first diagnosed and the present commercial layer flocks tested in the present study.

E (XSR) element contributes to the oncogenicity of Avian leukosis virus (subgroup J)

Among the six subgroups of Avian leukosis virus (ALV) that infect chickens, subgroup J (ALV-J) was isolated from meat-type chickens where it predominantly induces myeloid leukosis (ML) and erythroblastosis (EB). The sequence of HPRS-103, the ALV-J prototype virus, shows several distinct features, one of which is the presence of a distinct hairpin stem-loop structure called the E (also called XSR) element in the 3' untranslated region. In order to determine the role of the E element in ALV-induced pathogenicity, a comparison was made of the oncogenicity of viruses derived from the provirus clones of parental and E element-deleted HPRS-103 viruses in two genetically distinct lines of birds. In line 15I birds, deletion of the E element had profound effects on virus replication in vivo, as only 55 % of birds showed evidence of infection, compared with 100 % infection by the parental virus. Furthermore, none of the line 15I birds infected with this virus developed tumours, indicating that the E element does contribute to the oncogenicity of the virus. On the other hand, deletion of the E element had only a marginal effect on the incidence of tumours in line 0 birds. These results indicate that, although the E element per se is not absolutely essential for tumour induction by this subgroup of viruses, it does contribute to oncogenicity in certain genetic lines of chicken.