Comparison of Chinese field strains of avian leukosis subgroup J viruses with prototype strain HPRS-103 and United States strains

Isolation, Molecular Characterisation, and Pathogenicity Analysis of a Novel Recombinant ALV-J Strain Isolated From Chinese Hetian Chickens

BACKGROUND

Avian leukosis virus subgroup J (ALV-J) primarily affects poultry, particularly chickens, leading to tumourigenesis and immunosuppression, which results in substantial economic losses. It is important to note that ALV-J is commonly found in indigenous chicken breeds in China, and the virus's vertical transmission characteristics present a significant threat to the preservation of local chicken breeds.

OBJECTIVES

The study aimed to investigate the characteristics and effects of the recombinant ALV-J strain LY2021J, with a focus on its genetic composition and its potential influence on virulence and pathogenicity.

METHODS

LY2021J was isolated using DF-1 cells and validated by enzyme-linked immunosorbent assay (ELISA) and IFA. The proviral genome was amplified using segmented PCR and then spliced together using DNASTAR software. Genome-wide genes, including gag, pol, gp85, and long terminal repeat (LTR), were compared. Recombination sites were analysed using RDP5 and SimPlot software. Pathogenicity was evaluated by monitoring symptoms and conducting examinations on SPF chickens.

RESULTS

The outbreak of ALV-J in China has caused significant economic losses in the poultry industry. Although largely controlled in white-feather broilers and egg-laying chickens, ALV-J has spread to yellow-feather broilers and local breeds. A strain, LY2021J, isolated from Hetian chickens, showed lower mortality despite severe dysplasia. Genetic analysis revealed high similarity between LY2021J and the Chinese strains JS14NT01 and NX0101, suggesting a shared origin. Recombination with strain ev-1 and specific 3' UTR deletions may explain LY2021J's reduced virulence. Continued monitoring and prevention strategies are essential to mitigate ALV-J's impact.

The anti-inflammatory effect of arsenic trioxide effectively mitigates the pathogenic process in local chickens with avian leukosis

Arsenic trioxide (ATO) is a classic first-line treatment for acute promyelocytic leukemia (APL). An increasing number of studies regarding the use of ATO in tumor treatment have shown consistently remarkable results. In this study, subgroup J avian leukosis virus (ALV-J) was used as a model virus, and different doses of ATO were used to treat ALV-J-positive chickens. Sexually mature green-shelled laying hens from the same ALV-J-positive offspring were grouped and treated with one of 3 different doses of ATO. The anti-inflammatory effects of different doses of ATO in ALV-J-positive chickens and their mechanisms were investigated by analyzing levels of inflammatory cytokines, antioxidant parameters and apoptosis-related genes. The results showed that ATO administration mitigated ALV-induced lymphoid leukosis in the liver. ATO inhibited the activation of the TLR4/MyD88/NF-κB signaling pathway and downregulated the expression levels of the inflammatory cytokines IL-1β, IL-6 and TNF-α. The SOD and GSH-Px activities were also increased, and the MDA content was decreased in the serum of ALV-J-positive chickens treated with different doses of ATO, so the antioxidant capacity of ALV-J-positive chickens was improved. The mRNA expression levels of p53, p21 and Bcl-2 in the livers of ALV-J-positive chickens treated with different doses of ATO were significantly downregulated, which induced the apoptosis of tumor cells and slowed the inflammatory response. The combined analysis revealed that the therapeutic effect of 2 mg/kg/dose ATO was superior to that of the other 2 treatments (0.5 and 1 mg/kg/dose ATO). In conclusion, the anti-inflammatory effect of ATO can effectively alleviate the ALV-J pathogenic process. ALV-J serves as a model virus for antiviral tumor research, while ATO provides references for the treatment of such tumors.

N123I mutation in the ALV-J receptor-binding domain region enhances viral replication ability by increasing the binding affinity with chNHE1

The subgroup J avian leukosis virus (ALV-J), a retrovirus, uses its gp85 protein to bind to the receptor, the chicken sodium hydrogen exchanger isoform 1 (chNHE1), facilitating viral invasion. ALV-J is the main epidemic subgroup and shows noteworthy mutations within the receptor-binding domain (RBD) region of gp85, especially in ALV-J layer strains in China. However, the implications of these mutations on viral replication and transmission remain elusive. In this study, the ALV-J layer strain JL08CH3-1 exhibited a more robust replication ability than the prototype strain HPRS103, which is related to variations in the gp85 protein. Notably, the gp85 of JL08CH3-1 demonstrated a heightened binding capacity to chNHE1 compared to HPRS103-gp85 binding. Furthermore, we showed that the specific N123I mutation within gp85 contributed to the enhanced binding capacity of the gp85 protein to chNHE1. Structural analysis indicated that the N123I mutation primarily enhanced the stability of gp85, expanded the interaction interface, and increased the number of hydrogen bonds at the interaction interface to increase the binding capacity between gp85 and chNHE1. We found that the N123I mutation not only improved the viral replication ability of ALV-J but also promoted viral shedding in vivo. These comprehensive data underscore the notion that the N123I mutation increases receptor binding and intensifies viral replication.

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.

Isolation and characterization of avian leukosis virus subgroup J associated with hemangioma and myelocytoma in layer chickens in China

A strain of avian leukosis virus (ALV) belonging to a new envelope subgroup J (ALV-J) emerged in 1988 as a new subgroup of ALV and spread rapidly throughout the world. Due to the infection and spread of ALV-J, the global poultry industry experienced a significant loss. Although the disease had been prevented and controlled effectively by culling domestic chickens in the infected zone, a few field cases of ALV-J infection were reported in China in recent years. This study was conducted to characterize the genome and analyze the lesions and histopathology of the ALV-J strain named HB2020, which was isolated from layer chickens in Hubei Province, China. The full-length proviral genome sequence analysis of ALV-J HB2020 revealed that it was a recombinant strain of ev-1 and HPRS-103 in the gag gene in comparison to ALV-J prototype HPRS-103. In the 3′-untranslated region (3'UTR) of the nucleotide sequence, there were found 205-base pairs (bp) deletion, of which 175 were detected in the redundant transmembrane (rTM) region. Besides, the surface glycoprotein gene gp85 had five mutations in a conservative site, whereas the transmembrane protein gene gp37 was relatively conserved. The animal experiments conducted later on this strain have shown that HB2020 can cause various neoplastic lesions in chickens, including enlarged livers with hemangiomas and spleens with white nodules. Additionally, as the exposure time increased, the number of tumor cells that resembled myelocytes in the blood smears of infected chickens gradually increased. These results indicated that HB2020 on recombination with ALV subgroup E (ALV-E) and ALV-J could induce severe hemangiomas and myelocytomas. This inference might provide a molecular basis for further research about the pathogenicity of ALV and emphasize the need for control and prevention of avian leukosis.

The Emergence, Diversification, and Transmission of Subgroup J Avian Leukosis Virus Reveals that the Live Chicken Trade Plays a Critical Role in the Adaption and Endemicity of Viruses to the Yellow-Chickens

The geographical spread and inter-host transmission of the subgroup J avian leukosis virus (ALV-J) may be the most important issues for epidemiology. An integrated analysis, including phylogenetic trees, homology modeling, evolutionary dynamics, selection analysis and viral transmission, based on the gp85 gene sequences of the 665 worldwide ALV-J isolates during 1988-2020, was performed. A new Clade 3 has been emerging and was evolved from the dominating Clade 1.3 of the Chinese Yellow-chicken, and the loss of a α-helix or β-sheet of the gp85 protein monomer was found by the homology modeling. The rapid evolution found in Clades 1.3 and 3 may be closely associated with the adaption and endemicity of viruses to the Yellow-chickens. The early U.S. strains from Clade 1.1 acted as an important source for the global spread of ALV-J and the earliest introduction into China was closely associated with the imported chicken breeders in the 1990s. The dominant outward migrations of Clades 1.1 and 1.2, respectively, from the Chinese northern White-chickens and layers to the Chinese southern Yellow-chickens, and the dominating migration of Clade 1.3 from the Chinese southern Yellow-chickens to other regions and hosts, indicated that the long-distance movement of these viruses between regions in China was associated with the live chicken trade. Furthermore, Yellow-chickens have been facing the risk of infections of the emerging Clades 2 and 3. Our findings provide new insights for the epidemiology and help to understand the critical factors involved in ALV-J dissemination. IMPORTANCE Although the general epidemiology of ALV-J is well studied, the ongoing evolutionary and transmission dynamics of the virus remain poorly investigated. The phylogenetic differences and relationship of the clades and subclades were characterized, and the epidemics and factors driving the geographical spread and inter-host transmission of different ALV-J clades were explored for the first time. The results indicated that the earliest ALV-J (Clade 1.1) from the United States, acted as the source for global spreads, and Clades 1.2, 1.3 and 3 were all subsequently evolved. Also the epidemiological investigation showed that the early imported breeders and the inter-region movements of live chickens facilitated the ALV-J dispersal throughout China and highlighted the needs to implement more effective containment measures.

A study on the infection status and transmission of avian leukosis virus subgroup J in Hy-line brown roosters

Avian leukosis virus subgroup J (ALV-J) is the most prevalent subgroup in chickens and exhibits increased pathogenicity and stronger horizontal and vertical transmission ability among different breeds. Although vertical transmission of ALV-J from infected hens through artificial insemination has been inferred from the detection of the p27 antigen in swabs and serum, there has been no further research on the transmission pattern of ALVs in roosters. In the present study, the positive rate of ALV increased significantly in an indigenous flock after detecting the p27 antigen via enzyme-linked immunosorbent assay (ELISA) and virus isolation in DF-1 cells. Viral sequence comparisons and an indirect fluorescent antibody assay showed that these isolates belonged to the ALV-J subgroup but formed a new branch in a phylogenetic tree when compared to domestic and foreign referential strains. The gp85 gene of the ALV-J isolated from hens and albumen was 94.1-99.7% identical to that in roosters, revealing that these isolates were quite likely transmitted to the hens and their offspring through the semen of ALV-infected roosters by artificial insemination from the Hy-line brown roosters. In addition, we defined four ALV-J infection states in plasma and semen of roosters (P+S+, P-S+, P+S-, and P-S-), which suggests that, in order to eradicate ALV in roosters, it is necessary to perform virus isolation using both semen and plasma. Additionally, ALV detection in semen by ELISA produced false-positive and false-negative results when compared to virus isolation in DF-1 cells. Collectively, our results suggested that an incomplete process of eradication of ALV from ALV-positive roosters led to the sporadic presence of ALV-J in laying hens.

High-frequency and activation of CD4+CD25+ T cells maintain persistent immunotolerance induced by congenital ALV-J infection

Congenital avian leukosis virus subgroup J (ALV-J) infection can induce persistent immunotolerance in chicken, however, the underlying mechanism remains unclear. Here, we demonstrate that congenital ALV-J infection induces the production of high-frequency and activated CD4⁺CD25⁺ Tregs that maintain persistent immunotolerance. A model of congenital infection by ALV-J was established in fertilized eggs, and hatched chicks showed persistent immunotolerance characterized by persistent viremia, immune organ dysplasia, severe imbalance of the ratio of CD4⁺/CD8⁺ T cells in blood and immune organs, and significant decrease in CD3⁺ T cells and Bu-1⁺ B cells in the spleen. Concurrently, the mRNA levels of IL-2, IL-10, and IFN-γ showed significant fluctuations in immune organs. Moreover, the frequency of CD4⁺CD25⁺ Tregs in blood and immune organs significantly increased, and the frequency of CD4⁺CD25⁺ Tregs was positively correlated with changes in ALV-J load in immune organs. Interestingly, CD4⁺CD25⁺ Tregs increased in the marginal zone of splenic nodules in ALV-J-infected chickens and dispersed to the germinal center. In addition, the proliferation and activation of B cells in splenic nodules was inhibited, and the number of IgM⁺ and IgG⁺ cells in the marginal zone significantly decreased. We further found that the mRNA levels of TGF- β and CTLA-4 in CD4⁺CD25⁺ Tregs of ALV-J-infected chickens significantly increased. Together, high-frequency and activated CD4⁺CD25⁺ Tregs inhibited B cells functions by expressing the inhibitory cytokine TGF-β and inhibitory surface receptor CTLA-4, thereby maintaining persistent immunotolerance in congenital ALV-J-infected chickens.

The tyrosine phosphatase SHP-2 dephosphorylated by ALV-J via its Env efficiently promotes ALV-J replication

Avian leukosis virus subgroup J (ALV-J) generally induces hemangioma, myeloid leukosis, and immunosuppression in chickens, causing significant poultry industry economic losses worldwide. The unusual env gene of ALV-J, with low homology to other subgroups of ALVs, is associated with its unique pathogenesis. However, the exact molecular basis for the pathogenesis and oncogenesis of ALV-J is still not fully understood. In this study, ALV-J infection and the overexpression of Env could efficiently downregulate the phosphorylation of SHP-2 (pSHP-2) in vitro and in vivo. The membrane-spanning domain (MSD) in Env Gp37 was the functional domain responsible for pSHP-2 downregulation. Moreover, the overexpression of SHP-2 could effectively promote the replication of ALV-J, whereas knockout or allosteric inhibition of SHP-2 could inhibit ALV-J replication. In addition, the knockout of endogenous chicken SHP-2 could significantly increase the proliferation ability of DF-1 cells. All these data demonstrate that SHP-2 dephosphorylated by ALV-J Env could efficiently promote ALV-J replication, highlighting the important role of SHP-2 in the pathogenesis of ALV-J and providing a new target for developing antiviral drugs against ALV-J.

Rapid detection of avian leukosis virus subgroup J by cross-priming amplification

Avian leukosis virus subgroup J (ALV-J) causes oncogenic disease in chickens in China, resulting in great harm to poultry production, and remains widespread in China. Herein, we employed a cross-priming amplification (CPA) approach and a nucleic acid detection device to establish a visual rapid detection method for ALV-J. The sensitivity of CPA, polymerase chain reaction (PCR) and real-time PCR (RT-PCR) was compared, and the three methods were used to detect ALV-J in the cell cultures which inoculated with clinical plasma. The result showed when the amplification reaction was carried out at 60 °C for just 60 min, the sensitivity of CPA was 10 times higher than conventional PCR, with high specificity, which was comparable with RT-PCR, based on detection of 123 cell cultures which inoculated with clinical plasma, the coincidence rate with real-time PCR was 97.3% (71/73). CPA detection of ALV-J does not require an expensive PCR instrument; a simple water bath or incubator is sufficient for complete DNA amplification, and the closed nucleic acid detection device avoids aerosol pollution, making judgment of results more intuitive and objective. The CPA assay would be a promising simple, rapid and sensitive method for identification of ALV-J.

Genetic diversity of avian leukosis virus subgroup J (ALV-J): toward a unified phylogenetic classification and nomenclature system

Avian leukosis virus subgroup J (ALV-J) has infected a variety of birds, causing major economic losses in China. Understanding the comprehensive criteria of classification and nomenclature of ALV-J would be useful for the investigation of the viral evolution and also for the prevention and control of this infection. An in-depth analysis of the genetic diversity of ALV-J was performed in the present study. Four hundred and seventy-five sequences of the gp85 gene, including thirteen of avian endogenous retrovirus designated ev/J and 462 of ALV-J, were used in the phylogenetic and the evolutionary distance analysis for this classification. The study identified that the current ALV-J strains were divided into two first-order clades (Clades 1 and 2) and three second-order clades (Clades 1.1, 1.2 and 1.3). The current Chinese ALV-J strains are predominantly in Clade 1.3, and the Chinese and Egyptian chicken flocks have been facing the emerging Clade 2 viruses. This system pioneers the classification efforts for ALV-J, which uses Pilot tree for rapid classification of the new isolates and also the addition of possible new clades. The proposed unified classification system will facilitate future studies of ALV-J epidemiology and genetic evolution and of the comparison of sequences obtained across the world.

Diversity of Avian leukosis virus subgroup J in local chickens, Jiangxi, China

Avian leukosis caused by avian leukosis virus (ALV) is one of the most severe diseases endangering the poultry industry. When the eradication measures performed in commercial broilers and layers have achieved excellent results, ALV in some local chickens has gradually attracted attention. Since late 2018, following the re-outbreak of ALV-J in white feather broilers in China, AL-like symptoms also suddenly broke out in some local flocks, leading to great economic losses. In this study, a systematic epidemiological survey was carried out in eight local chicken flocks in Jiangxi Province, China, and 71 strains were finally isolated from 560 samples, with the env sequences of them being successfully sequenced. All of those new isolates belong to subgroup J but they have different molecular features and were very different from the strains that emerged in white feature broilers recently, with some strains being highly consistent with those previously isolated from commercial broilers, layers and other flocks or even isolated from USA and Russian, suggesting these local chickens have been acted as reservoirs to accumulate various ALV-J strains for a long time. More seriously, phylogenetic analysis shows that there were also many novel strains emerging and in a separate evolutionary branch, indicating several new mutated ALVs are being bred in local chickens. Besides, ALV-J strains isolated in this study can be further divided into ten groups, while there were more or fewer groups in different chickens, revealing that ALV may cross propagate in those flocks. The above analyses explain the complex background and future evolution trend of ALV-J in Chinese local chickens, providing theoretical support for the establishment of corresponding prevention and control measures.

Isolation and Identification of Subgroup J Avian Leukosis Virus Inducing Multiple Systemic Tumors in Parental Meat-Type Chickens

Avian leukosis virus (ALV) continues evolving to obtain new genomic characters to enhance its pathogenicity. In the present study, an ALV-J strain LH20180301 was isolated from broiler breeder chickens that reached the speak of paralyzation before 20-week-old. The necropsy chickens showed subcutaneous and muscular hemorrhage, and developed tumors in multiple organs including bone, liver, spleen, and kidney. The complete provirus was then cloned and sequenced to investigate the molecular characteristics and oncogenicity etiology of this virus associated with the outbreak of disease. The genomic structure of the reported ALV-J strain LH20180301 was highly conservative with other ALVs. Recombination events between the virus with endogenous virus were identified in the viral genome. Compared with the ALV-J original HPRS-103 strain, the major recombination sites of the viral genome with ev-1 were located in 5′ UTR-gag and 3′ UTR regions. Phylogenetic analysis of group specific antigen gp85 encoding protein showed that the LH20180301 branched with ALV-J prevalent in “yellow chickens” of local breeds in South China. Nine amino acids (N58, D60, K70, A71, K108, N112, N113, N121, R272) in the gp85 were highly conserved among ALV-J isolates before 2012, but various mutations were found in the late isolates including LH20180301. In addition, the LH20180301 strain also had the same deletion pattern of 3′ UTR with them. Therefore, LH20180301 might derive from the same ancestor with those viruses and may be the trend of ALV-J evolution in China. The defined new genomic characters in the gp85 and 3′ UTR region of ALV-J might provide the molecular basis for its enhanced oncogenicity.

Molecular characteristics of subgroup J avian leukosis virus isolated from yellow breeder chickens in Guangdong, China, during 2016-2019

Since 2005, subgroup J avian leukosis virus (ALV-J) infection has been present in yellow chickens in Guangdong, China, causing severe economic losses to the local poultry industry. ALV-J is a rapidly evolving retrovirus. To investigate the molecular characteristics of ALV-J isolates from yellow breeder chickens in Guangdong, 17 virus strains were isolated from 6549 anticoagulants from clinically healthy birds between 2016 and 2019, and completely sequenced and phylogenetically analyzed. Phylogenetic analysis of the gp85 gene showed that all isolated viruses were divided into three different branches. Notably, 41.2% (7/17) of the isolates shared a novel G2598A nucleotide mutation in the pol gene and caused the stop codon to be advanced by 8 positions. Nearly 200 nucleotides were deleted from the redundant TM (rTM) region in all strains, but all retained an intact direct repeat (DR1). 82.4% (14/17) of isolates contained a complete E element. Additionally, 29.4% (5/17) of isolates detected an 11 bp deletion in U3 region, and the AIB REP1 transcription factor is missing. The study indicated that ALV-J infection had still been prevalent in the yellow breeder chicken farms in Guangdong, and the genetic background of the strains is diverse. This study provides the latest data on the molecular characteristics of ALV-J, which will help to reveal the evolution trend of ALV-J and develop relevant prevention and control measures.

Recombinant subgroup B avian leukosis virus combined with the subgroup J env gene significantly increases its pathogenicity

The differences among different sub-groups of the avian leukosis virus (ALV) genome are mainly concentrated in the env gene, which binds to cell-specific receptors and determines the characteristics of viral tropism and pathogenicity. In this study, two rescued viruses rGX15MM6-2 (ALV of subgroup J, ALV-J) and rGX14FF03 (ALV of subgroup B, ALV-B) and a recombinant virus rALV-B-Jenv (ALV-B's backbone with ALV-J's env) were generated and tested utilizing both in vitro and in vivo experiments. The results showed that the replication ability of the viruses released in DF-1 cell cultures was listed in order as rGX15MM6-2 > rALV-B-Jenv > rGX14FF03. rGX15MM6-2 caused the most serious suppression of body weight gain, exhibited a significant negative effect on the development of immune organs (P < 0.05) and lower antibody responses to vaccinations with the commercial oil-emulsion vaccines (OEVs) (P<0.05) in the challenged chickens. The viral detection showed that the positive rate in blood from the birds infected with rALV-B-Jenv were respectively higher than those from the birds infected with rGX14FF03 (P < 0.05). At 25 wpi, similar tumors were found in the abdominal cavity of the birds in rGX15MM6-2 and rALV-B-Jenv groups. The results demonstrated that the ALV-J env gene significantly increases the pathogenicity of the recombinant ALV-B. With the increasing incidence of co-infections of different subgroups of ALV in the field, the possibility of viral recombination is increasing and demands further study.

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.

Avian leukosis virus subgroup J induces B cell anergy mediated by Lyn inhibited BCR signal transduction

Immune tolerance induced by avian leukosis virus subgroup J (ALV-J) is a prerequisite for tumorigenesis. Although we had reported that B cell anergy induced by ALV-J was the main reason for immune tolerance, the molecular mechanism still remains unclear. Here, we found SU protein of ALV-J interacted with tyrosine kinase Lyn (a key protein in BCR signaling pathway) by confocal laser scanning microscopy and co-immunoprecipitation test, which suggested that Lyn might play an important role in B cell anergy induced by ALV-J. Correspondingly, the mRNA and protein level of Lyn was significantly up-regulated in B cells after ALV-J infection. Subsequently, the phosphorylated protein levels of Lyn at Tyr507 site were significantly up-regulated in ALV-J-infected B cells after BCR signal activation, but the phosphorylated protein level of Syk (a direct substrate of Lyn) at Tyr525/526 site, Ca²⁺ flux, and NF-κB p65 protein level were significantly down-regulated. Interestingly, the phosphorylated protein level of Syk at Tyr525/526 site, Ca²⁺ flux, and NF-κB p65 protein level were both significantly retrieved after the shLyn treatment in B cells infected by ALV-J. In summary, these results indicated that ALV-J activated the negative regulatory effect of phosphorylated Lyn protein at 507 site in BCR signal transduction pathway and then mediated B cell anergy, which will provide a new insight for revealing the pathogenesis of immune tolerance induced by ALV-J.

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

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

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.

Molecular characteristics of the re-emerged avian leukosis virus in China, 2018-2019

Since early 2018, avian leukosis virus (ALV) has re-emerged throughout six provinces in Northeast and East of China and caused huge economic losses. In different farms, there are significant differences in clinical symptoms, including morbidity, mortality and location of tumours, on affected animals, which implies that the present strains may have different origins and molecular characteristics. In this study, a systematic epidemiological investigation was conducted in 21 farms in six provinces. Results showed that the virus strains present in this outbreak are highly consistent but carry different mutations. All the strains shared 97.0%-99.0% identity with each other and were highly similar to the GD14J2 strain isolated previously, while different insertion fragments can be found in the env gene of different strains, suggesting that the strains of ALV in this outbreak may have the same ancestors but have gone through different evolutionary trajectories. This study demonstrated that these viruses may point to multiple sources of infection, and all should be identified and taken seriously in the formulation of control plans.

Co-infection of vvMDV with multiple subgroups of avian leukosis viruses in indigenous chicken flocks in China

BACKGROUND

In China, although the ALV eradication program and the MD vaccination strategy greatly reduce the disease burdens caused by the infection of ALV and MDV, the frequent emergence of novel ALV-K or vvMDV in the vaccinated chicken flock challenges the current control strategies for both diseases.

RESULTS

In Guangdong Province, an indigenous chicken flock was infected with neoplastic disease. Hematoxylin-eosin staining of the tissues showed the typical characteristics of MDV and classical ALV infection. The PCR and sequencing data demonstrated that the identified MDV was clustered into a very virulent MDV strain endemic in domestic chickens in China. Moreover, subgroups ALV-A and ALV-K were efficiently recovered from two samples. The full genome sequence revealed that the ALV-K isolate was phylogenetically close to the ALV TW3593 isolate from Taiwan Province.

CONCLUSIONS

A co-infection of vvMDV with multiple ALV subgroups emerged in a chicken flock with neoplastic disease in Guangdong Province. The co-infection with different subgroups of ALV with vvMDV in one chicken flock poses the risk for the emergence of novel ALVs and heavily burdens the control strategy for MDV.

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.

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.

Characterization of avian leukosis virus subgroup J isolated between 1999 and 2013 in China

Avian leukosis virus subgroup J (ALV-J) has successively infected white feather chickens, layer hens, cultivated yellow chickens, and indigenous chickens; infection rates and tumorigenicity have attracted increasingly extensive attention in China. To clarify the correlation of the epidemiological phenomenon of ALV-J with the evolution of envelope protein gp85, 140 strains of ALV-J isolated from chickens with different genetic backgrounds from 1999 to 2013 were compared. The homology of the gp85 protein and genetic genealogical relationships between 140 strains of ALV-J and the prototype strain HPRS-103, as well as between the same ALV-J strains and 8 American isolates, were analyzed and compared. The results showed that there was no significant difference in the variation range of homology of the gp85 protein between the prototype HPRS-103 and ALV-J isolates from different genetic backgrounds and different years. However, genetic pedigree analysis showed that virus strains that isolated from the same type of chickens remained close to each other on the phylogenetic tree, which means that there was a correlation between the genetic background of infected chickens and virus strains. Further analysis of amino acid sequences also found similar results and revealed that unique amino acid sites were formed in chickens with different genetic backgrounds, which proved that ALV-J could adapt to the new host through amino acid variation. Genetic sequence phylogenetic tree analysis was more representative than sequence homology comparisons for assessing ALV-J correlations. These conclusions contributed to the control and prevention of ALV infection. ALV-J is still prevalent in Chinese indigenous chickens, more attentions should be given to fulfill the purification.

Expression of dysregulated miRNA in vivo in DF-1 cells during the course of subgroup J avian leukosis virus infection

Aberrant expression of microRNAs (miRNAs) is known to be involved in cancer progression caused by subgroup J avian leukosis virus (ALV-J) in liver tissues. To advance our understanding of the related pathological mechanisms and virus-host interactions, seven previously reported miRNAs were selected for a comparative analysis of miRNA expression between infected and uninfected DF-1 cells, including six miRNAs related to tumorigenesis (let-7b/7i, miR-221/222, miR-125b, miR-375 and miR-2127. The results showed that six of the seven miRNAs except gga-miR-375 were upregulated in cells infected with NX0101 (caused myeloma (ML)) and GD1109 (caused hemangioma (HE)) at 1 h post infection. On day 2 post-infection, all seven miRNAs were upregulated in infected DF-1 cells. On day 6 post-infection, gga-let-7b, gga-miR-125b, and gga-miR-375 were downregulated whereas gga-miR-221 and gga-miR-222 were upregulated in DF-1 cells infected with the two ALV-J strains of different phenotypes. However, expression of gga-let-7i was reduced in DF-1 cells infected with NX0101 and was increased in those infected with GD1109; gga-miR-2127 expression showed no significant difference between infected and uninfected cells. This study is the first to report the changes in the miRNA expression levels in DF-1 cells during the course of ALV-J infection, and suggests a relationship between its pathological mechanisms and miRNAs.

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.

Circular RNA and mRNA profiling reveal competing endogenous RNA networks during avian leukosis virus, subgroup J-induced tumorigenesis in chickens

Avian leukosis virus subgroup J (ALV-J) can induce myeloid tumors and hemangiomas in chickens and causes severe economic losses with commercial layer chickens and meat-type chickens. Here, we generated ribominus RNA sequencing data from three normal chicken spleen tissues and three ALV-J-infected chicken spleen tissues. Structure analysis of transcripts showed that, compared to mRNAs and lncRNAs, chicken circRNAs shared relatively shorter transcripts and similar GC content. Differentially expression analysis showed 152 differentially expressed circRNAs with 106 circRNAs up regulated and 46 circRNAs down regulated. Through comparing differentially expressed circRNA host genes and mRNAs and performed ceRNA network analysis, we found several tumor or immune-related genes, in which, there were four genes existed in both differentially expressed mRNAs and circRNA host genes (Dock4, Fmr1, Zfhx3, Ralb) and two genes (Mll, Aoc3) involved in ceRNA network. We further characterized one exon-intron circRNA derived from HRH4 gene in the ceRNA network, termed circHRH4, which is an abundant and stable circRNA expressed in various tissues and cells in chicken and localizes in cytoplasm. Our results provide new insight into the pathology of ALV-J infection and circRNAs may also mediate tumorigenesis in chicken.

The emerging novel avian leukosis virus with mutations in the pol gene shows competitive replication advantages both in vivo and in vitro

The avian leukosis virus subgroup K (ALV-K), a novel subgroup in Chinese indigenous chicken breeds, has been difficult to isolate in the past due to its poor replication ability. However, according to the latest monitoring data, the replication ability and isolation rate of ALV-K have clearly increased, and new strains with mutations in the pol gene have also been found. To determine the effects of such mutations on the biological characteristics of ALV-K, a pair of infectious clones were constructed and rescued. The first virus was an ALV-K Chinese isolate with mutations in its pol gene, named rSDAUAK-11. The second virus was a recuperative rSDAUAK-11 from which mutations in the pol gene were recovered according to the corresponding region of the ALV-K prototype virus JS11C1, named rRSDAUAK-11. In addition, two quantitative real-time polymerase chain reaction assays were developed to specifically detect these virus strains. Using such methods, we observed a marked improvement of the reverse transcriptase activity, replication ability and vertical transmission ability of rSDAUAK-11, which also revealed a formidable competitive advantage in mixed infection with rRSDAUAK-11 and corresponded to the differences between the wild strains SDAUAK-11 and JS11C1. Accordingly, our findings not only show that mutations in the pol gene are an important molecular mechanism contributing to corresponding changes in the biological characteristics of the newest ALV-K but also emphasize the potential future eradication of ALV.

Characterization of subgroup J avian Leukosis virus isolated from Chinese indigenous chickens

Background

In spite of the purification of the laying hens and broilers of avian leukosis virus (ALV) has made remarkable achievements, the infection of ALV was still serious in Chinese indigenous chickens.

Methods

In order to assess the epidemic state of avian leukosis virus in indigenous chickens in China, 10 novel strains of ALV subgroup J (ALV-J), named JS16JH01 to JS16JH10, were isolated and identified by virus isolation and immunofluorescence antibody assays from a Chinese local breed farm with a sporadic incidence of tumors. To understand their virological characteristics further, the proviral genome of ENV-LTR was sequenced and compared with the reference strains.

Results

The homology of the gp85 gene between the ten ALV-J strains and NX0101 was in the range from 89.7–94.8% at the nuclear acid level. In addition, their gp85 genes were quite varied, with identities of 92–98% with themselves at the nuclear acid level. There were several snp and indel sites in the amino acid sequence of gp85 genes after comparison with other reference strains of ALV. Interestingly, a novel insertion in the gp85 region was found in two strains, JS16JH01 and JS16JH07, compared with NX0101 and HPRS-103.

Discussion

At present, owing to the large-scale purification of ALV in China, laying hens and broiler chickens with ALV infection are rarely detected, but ALVs are still frequently detected in the local chickens, which suggests that more efforts should be applied to the purification of ALV from indigenous chickens.

Effect of Dual Infection with Eimeria tenella and Subgroup J Avian Leukosis Virus on the Cecal Microbiome in Specific-Pathogen-Free Chicks

Understanding gut microflora alterations associated with gut parasites and other pathogens that drive these alterations may help to promote the understanding of intestinal flora's role in multiple-infected individuals. This study examined the effects of dual infection with Eimeria tenella and subgroup J avian leukosis virus (ALV-J) on the chick cecal microbiome. Specific-pathogen-free (SPF) chicks were infected with either ALV-J strain NX0101 at 1 day of age or E. tenella at 14 days of age, another group was infected with both pathogens. Cecal contents from chicks were extracted at the 21 days of age and examined using 16S rRNA genes illumina sequencing. A genus-level opportunistic pathogen enrichment and a decrease in possible resident probiotics were observed in response to all infection groups. Of note, E. tenella mainly induced a sharp decrease in the richness and diversity of cecal microflora from infected chicks because of the serious E. tenella-induced damage to intestinal tissues. ALV-J infection led to structural changes and increased the richness and diversity of the cecal microflora. As for E. tenella and ALV-J dual infected chicks, a marked enrichment of opportunistic pathogens in addition to some other bacteria that may play a role involving cecal microbiota carbohydrate transport and metabolic functions were also found compared to single pathogen-infected chicks. Overall, this study provides valuable insights into the SPF chick cecal microbial community, the modulations of this community in response to different pathogenic infections of single or dual infections, and the interactions between different pathogens and hosts from the perspective of intestinal microflora.

The construction and application of a cell line resistant to novel subgroup avian leukosis virus (ALV-K) infection

A novel avian leukosis viruses (ALV) subgroup named ALV-K was recently isolated from Chinese indigenous chickens which is different from the subgroups (A to E and J) that have previously been reported to infect chickens. More and more ALV-K strains have recently been isolated from local breeds of Chinese chickens. However, there are no more effective diagnostic methods for ALV-K other than virus isolation followed by envelope gene sequencing and comparison. Viral infection can be blocked through expression of the viral receptor-binding protein. In this study, we have engineered a cell line, DF-1/K, that expresses ALV-K env protein and thereby confers resistance to ALV-K infection. DF-1/K can be used in combination with the ALV-K susceptible cell line DF-1 as a specific diagnostic tool for ALV-K and provides a good tool for further research into the molecular mechanisms of interaction between ALV-K env protein and the host cell receptor.

Vertical transmission of avian leukosis virus subgroup J (ALV-J) from hens infected through artificial insemination with ALV-J infected semen

Background

Avian leukosis virus (ALV) is one of the main causes of tumour development within the poultry industry in China. The subgroup J avian leukosis viruses (ALV-J), which induce erythroblastosis and myelocytomatosis, have the greatest pathogenicity and transmission ability within this class of viruses. ALV can be transmitted both horizontally and vertically; however, the effects of ALV infection in chickens—especially roosters—during the propagation, on future generations is not clear. Knowing the role of the cock in the transmission of ALV from generation to generation might contribute to the eradication programs for ALV.

Results

The results showed that two hens inseminated with ALV-J-positive semen developed temporary antibody responses to ALV-J at 4–5 weeks post insemination. The p27 antigen was detected in cloacal swabs of six hens, and in 3 of 26 egg albumens at 1–6 weeks after insemination. Moreover, no viremia was detected at 6 weeks after insemination even when virus isolation had been conducted six times at weekly intervals for each of the 12 females. However, ALV-J was isolated from 1 of their 34 progeny chicks at 1 week of age, and its gp85 had 98.4%–99.2% sequence identity with the gp85 of ALV-J isolated from semen samples of the six cocks.

Conclusions

Our findings indicated that females that were late horizontally infected with ALV-J by artificial insemination might transmit the virus to progeny through eggs, which amounts to vertical transmission.

Dynamic Co-evolution and Interaction of Avian Leukosis Virus Genetic Variants and Host Immune Responses

Subgroup J avian leukosis virus (ALV-J), a typical retrovirus, is characterized of existence of a cloud of diverse variants and considerable genetic diversity. Previous studies describing the evolutionary dynamics of ALV-J genetic variants mainly focused on the early infection period or few randomly selected clones. Here, we inoculated 30 specific-pathogen-free chickens with the same founder ALV-J stock of known genetic background. Six (three antibody positive and three antibody negative) chickens were selected among 15 chickens with viremia. Viruses were serially isolated in 36 weeks and then sequenced using MiSeq high-throughput sequencing platform. This produced the largest ALV-J dataset to date, composed of more than three million clean reads. Our results showed that host humoral immunity could greatly enhance the genetic diversity of ALV-J genetic variants. In particular, selection pressures promoted a dynamic proportional changes in ALV-J genetic variants frequency. Cross-neutralization experiment showed that along with the change of the dominant variant, the antibody titers specific to infectious clones corresponding to the most dominant variants in weeks 12 and 28 have also changed significantly in sera collected in weeks 16 and 32. In contrast, no shift of dominant variant was observed in antibody-negative chickens. Moreover, we identified a novel hypervariable region in the gp85 gene. Our study reveals the interaction between ALV-J and the host, which could facilitate the development of vaccines and antiviral drugs.

16S rRNA genes Illumina sequencing revealed differential cecal microbiome in specific pathogen free chickens infected with different subgroup of avian leukosis viruses

Intestinal flora play important roles in the pathogenisis of many pathogens. This study examined the cecal microbiome of chickens infected with avian leukosis virus (ALV) using 16S rRNA genes Illumina sequencing. One-day-old specific pathogen free chicks were inoculated in the abdomen with subgroup J or K of ALV. At 21-day-old, chickens positive for ALV viremia were selected and their cecal contents were extracted and examined for the composition of gut microflora by illumina sequencing of the V3+V4 region of the 16S rRNA genes. The results showed that there is a clear association with loss of important bacterial populations in concert with an enrichment of potentially pathogenic populations and ALV infections, despite of the virus subgroups. In addition, ALV-K infected chickens revealed a preference for opportunistic pathogens in Firmicutes such as Staphylococcus and Weissella and some genus from Bacillales. Whereas, ALV-J infected chickens were characterized by a larger number of notable pathogens like Escherichia-Shigella from Proteobacteria, and other condition pathogens including Enterococcus and members of Erysipelotrichaceae from Firmicutes, and members of Helicobacteraceae from Bacteroidetes. Collectively, our results suggest that relative abundance data from the cecal microbiome differentiates healthy chickens from those infected with ALVs. Most importantly, there was a significant difference in the gut microbiome of chickens infected with ALV-K compared to those with ALV-J infected ones. This strongly suggests that ALV infection may be associated with the microbiome and there may be multiple underlying mechanisms by which the microbiome is involved in the pathogenisis of different subgroup of ALV infections.

Genomic diversity of the Avian leukosis virus subgroup J gp85 gene in different organs of an infected chicken

The genomic diversity of Avian leukosis virus subgroup J (ALV-J) was investigated in an experimentally infected chicken. ALV-J variants in tissues from four different organs of the same bird were re-isolated in DF-1 cells, and their gp85 gene was amplified and cloned. Ten clones from each organ were sequenced and compared with the original inoculum strain, NX0101. The minimum homology of each organ ranged from 96.7 to 97.6%, and the lowest homology between organs was only 94.9%, which was much lower than the 99.1% homology of inoculum NX0101, indicating high diversity of ALV-J, even within the same bird. The gp85 mutations from the left kidney, which contained tumors, and the right kidney, which was tumor-free, had higher non-synonymous to synonymous mutation ratios than those in the tumor-bearing liver and lungs. Additionally, the mutational sites of gp85 gene in the kidney were similar, and they differed from those in the liver and lung, implying that organ- or tissue-specific selective pressure had a greater influence on the evolution of ALV-J diversity. These results suggest that more ALV-J clones from different organs and tissues should be sequenced and compared to better understand viral evolution and molecular epidemiology in the field.

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.

Temporal changes of microRNA gga-let-7b and gga-let-7i expression in chickens challenged with subgroup J avian leukosis virus

Two important microRNAs, gga-let-7b and gga-let-7i were examined for the relative expression in liver and bone marrow tissues from specific pathogen free chickens that were challenged either with GD1109 or NX0101 strain of subgroup J avian leukosis virus (ALV-J). The GD1109 strain of ALV-J reportedly causes hemangioma (HE) and NX0101 reportedly causes myeloma (ML) in susceptible chickens. Temporal changes of both gga-let-7b and gga-let-7i expression in ALV-J infected chickens were observed in contrast to its counterpart of a non-infected negative control group of chickens (P < 0.05 or P < 0.01) during the first 120 days post infection. Use of the web-based computational DIANA-mirPath software (available at http://microrna.gr/mirpath ), it was predicted that both gga-let-7b and gga-let-7i were involved in multiple pathways including signaling pathways, such as MAPK, TGF-beta, Notch, Wnt, mTOR, Cell cycle, P53 and Jak-STAT. Combining our experimental data with reports on the microRNAs, we suggest that both gga-let-7i and gga-let-7b may also act as tumor suppressors in chicken, especially play a critical role in tumorigenesis induced by ALV-J.

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

Background

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

Results

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

Conclusions

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

Electronic supplementary material

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

Influence of avian leukosis virus long terminal repeat on biological activities of Marek's disease virus

GX0101 was the first reported field strain of recombinant Marek's disease virus (MDV) that contained a long terminal repeat (LTR) from the reticuloendotheliosis virus (REV). It is a very virulent MDV strain, with relatively high horizontal transmission ability. The REV LTR in GX0101 genome was proved to decrease the pathogenicity but increase the potential for horizontal transmission of the virus. Here we constructed a recombinant MDV GX0101-ALV-LTR to study stability of avian leukosis virus (ALV) LTR at the REV LTR insertion site in GX0101 genome and its influence on biological activities of the recombinant virus. The results showed that GX0101-ALV-LTR was able to replicate stably both in vitro and in vivo. ALV LTR remained stable in chickens infected either by inoculation with the recombinant virus GX0101-ALV-LTR or by horizontal transmission, as well as in cell culture. The pathogenic properties of GX0101-ALV-LTR virus were evaluated in infected specific-pathogen-free chickens. The present study demonstrated that the GX0101-ALV-LTR virus had a weaker inhibitory effect on the growth rates of the infected chickens and induced weaker immunosuppressive effects. Horizontal transmission ability of the GX0101-ALV-LTR virus appeared to be similar with its parental virus GX0101. In short, ALV LTR was stable in GX0101 after replacing REV LTR, and the recombinant virus showed similar horizontal transmission ability but decreased pathogenicity.

gp85 protein vaccine adjuvanted with silica nanoparticles against ALV-J in chickens

This study focused on the effect of silica nanoparticles as adjuvant for vaccine applications comprised of gp85, a dominating structural protein of J Subgroup Avian Leukosis Virus (ALV-J), and which was evaluated by comparing with the responsiveness induced by that emulsified in Freund adjuvant. Thirty-six chickens were inoculated twice with gp85 adjuvanted with the silica nanoparticles or Freund's adjuvant at the 2nd and 3rd week old. Two weeks later, the inoculated chickens were challenged with a 10^2.2 50% tissue culture infective dose (TCID50) of ALV-J. The blood samples were collected weekly to detect the serum antibodies and viremia. Results showed that positive serum antibodies (S/P value>0.6) against gp85 emerged at the third week in the inoculated chickens, while the antibodies level persisted longer in silica nanoparticles adjuvanted-group to Freund's adjuvanted-group. Furthermore, viremia in silica nanoparticles adjuvanted-group was recovered more quickly compared with Freund's adjuvanted-group. Hence our study revealed that silica nanoparticles can effectively improve the protection of gp85 vaccine against ALV-J and present a better performance than Freund's adjuvant.

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.

Synergy of subgroup J avian leukosis virus and Eimeria tenella to increase pathogenesis in specific-pathogen-free chickens

To investigate the effects of co-infections of subgroup J avian leukosis virus (ALV-J) and Eimeria tenella on the pathogenesis in specific-pathogen-free (SPF) white leghorn chickens, groups of chickens were infected with ALV-J strain NX0101 at one day of age or with E. tenella at 14 days of age or both. The control group was left uninfected and was mock-inoculated with phosphate buffer saline (PBS). Mortality rates, body weights, cecal lesions, and viremia of infected chickens in each group were evaluated. Immune status was evaluated by measuring several parameters: immune organ weight/body weight index, specific humoral responses to inactivated NDV vaccine and to inoculated E. tenella, proportions of blood CD3+CD4+ and CD3+CD8α+ lymphocytes and transcriptional levels of cytokines in blood and cecal tonsils. The results show that co-infections of ALV-J and E. tenella induced a higher mortality rate and a lower body weight in SPF chickens compared to single-pathogen infection. In co-infected chickens, ALV-J accelerated the disease symptoms induced by E. tenella, and the E. tenella extended the ALV-J viremia. Thymus atrophy, decrease in the humoral response levels to pathogens and the NDV vaccine, modifications in the blood lymphocyte sub-populations and transcriptional cytokine disorders were found in co-infected chickens compared to chickens infected with one pathogen alone and to controls. We underline a synergy between ALV-J and E. tenella that results in increasing pathogenesis in SPF chickens.