Development of an antigen-capture ELISA for the detection of avian leukosis virus p27 antigen

Specific and Sensitive Visual Proviral DNA Detection of Major Pathogenic Avian Leukosis Virus Subgroups Using CRISPR-Associated Nuclease Cas13a

Avian leukosis viruses (ALVs) include a group of avian retroviruses primarily associated with neoplastic diseases in poultry, commonly referred to as avian leukosis. Belonging to different subgroups based on their envelope properties, ALV subgroups A, B, and J (ALV-A, ALV-B, and ALV-J) are the most widespread in poultry populations. Early identification and removal of virus-shedding birds from infected flocks are essential for the ALVs' eradication. Therefore, the development of rapid, accurate, simple-to-use, and cost effective on-site diagnostic methods for the detection of ALV subgroups is very important. Cas13a, an RNA-guided RNA endonuclease that cleaves target single-stranded RNA, also exhibits non-specific endonuclease activity on any bystander RNA in close proximity. The distinct trans-cleavage activity of Cas13 has been exploited in the molecular diagnosis of multiple pathogens including several viruses. Here, we describe the development and application of a highly sensitive Cas13a-based molecular test for the specific detection of proviral DNA of ALV-A, B, and J subgroups. Prokaryotically expressed LwaCas13a, purified through ion exchange and size-exclusion chromatography, was combined with recombinase polymerase amplification (RPA) and T7 transcription to establish the SHERLOCK (specific high-sensitivity enzymatic reporter unlocking) molecular detection system for the detection of proviral DNA of ALV-A/B/J subgroups. This novel method that needs less sample input with a short turnaround time is based on isothermal detection at 37 °C with a color-based lateral flow readout. The detection limit of the assay for ALV-A/B/J subgroups was 50 copies with no cross reactivity with ALV-C/D/E subgroups and other avian oncogenic viruses such as reticuloendotheliosis virus (REV) and Marek's disease virus (MDV). The development and evaluation of a highly sensitive and specific visual method of detection of ALV-A/B/J nucleic acids using CRISPR-Cas13a described here will help in ALV detection in eradication programs.

Current knowledge on the epidemiology and prevention of Avian leukosis virus in China

Avian leukosis virus (ALV) is an enveloped retrovirus with a single-stranded RNA genome, belonging to the genus Alpharetrovirus within the family Retroviridae. The disease (Avian leukosis, AL) caused by ALV is mainly characterized by tumor development and immunosuppression in chickens, which increases susceptibility to other pathogens and leads to significant economic losses in the Chinese poultry industry. The government and poultry industry have made lots of efforts to eradicate ALV, but the threat of which remains not vanished. This review provides a summary of the updated understanding of ALV in China, which mainly focuses on genetic and molecular biology, epidemiology, and diagnostic methods. Additionally, promising antiviral agents and ALV eradication strategies performed in China are also included.

Identification of a New B-Cell Epitope on the Capsid Protein of Avian Leukosis Virus and Its Application

Avian leukosis virus (ALV) is an avian oncogenic retrovirus that can impair immunological function, stunt growth and decrease egg production in avian flocks. The capsid protein (P27) is an attractive candidate for ALV diagnostics. In the present study, a new hybridoma cell (1F8) stably secreting an anti-P27 monoclonal antibody (mAb) was developed. The mAb exhibited a high affinity constant (Ka) of 8.65 × 106.0 L/mol, and it could be used for the detection of ALV-A/B/J/K strains. Moreover, a total of eight truncated recombinant proteins and five synthetic polypeptides were utilized for the identification of the B-cell epitopes present on P27. The results revealed that 218IIKYVLDRQK227 was the minimal epitope recognized by 1F8, which had never been reported before. Additionally, the epitopes could strongly react with different ALV subgroup's specific positive serum and had a complete homology among all the ALV subgroups strains. Finally, a new sandwich ELISA method was created for the detection of ALV antigens, demonstrating increased sensitivity compared to a commercially available ELISA kit. These results offer essential knowledge for further characterizing the antigenic composition of ALV P27 and will facilitate the development of diagnostic reagents for ALV.

Establishment and application of a gold nanoparticle-based immunochromatographic test strip for the detection of avian leukosis virus P27 antigen in egg white samples

Avian leukemia is an infectious tumorous disease of chickens caused by subgroup A of the avian leukemia virus (ALV-A), which mainly causes long-term viremia, slow growth, immune suppression, decreased production performance, multi-tissue tumors, and even death. The infection rate of this disease is very high in chicken herds in China, causing huge economic losses to the poultry industry every year. We successfully expressed the specific antigen protein of ALV (P27) through recombinant protein technology and screened a pair of highly sensitive monoclonal antibodies (mAbs) through mouse immunity, cell fusion, and antibody pairing. Based on this pair of antibodies, we established a dual antibody sandwich ELISA and gold nanoparticle immunochromatographic strip (AuNP-ICS) detection method. In addition, the parameters of the dual antibody sandwich ELISA and AuNP-ICS were optimized under different reaction conditions, which resulted in the minimum detection limits of 0.2 ng mL-1 and 1.53 ng ml-1, respectively. Commonly available ELISA and AuNP-ICS products on the market were compared, and we found that our established immune rapid chromatography had higher sensitivity. This established AuNP-ICS had no cross-reactivity with Influenza A (H1N1), Influenza A (H9N2), respiratory syncytial virus (RSV), varicella-zoster virus (VZV), Listeria monocytogenes listeriolysin (LLO), and Staphylococcal enterotoxin SED or SEC. Finally, the established AuNP-ICS was used to analyze 35 egg samples, and the results showed 5 positive samples and 30 negative samples. The AuNP-ICS rapid detection method established by our group had good specificity, high sensitivity, and convenience, and could be applied to the clinical sample detection of ALV-A.

New rapid detection by using a constant temperature method for avian leukosis viruses

The avian leukemia virus causes avian leukemia (AL), a severe immunosuppressive disease in chickens (ALV). Since the 1990s, the diversity of ALV subpopulations caused by ALV genome variation and recombination, and the complexity of the infection and transmission, with currently no effective commercial vaccine and therapeutic for ALV, has resulted in severe economic losses to the chicken business in various parts of the world. Therefore, as a key means of prevention and control, an effective, rapid, and accurate detection method is imperative. A new real-time reverse transcription recombinase-aided amplification (RT-RAA) assay for ALV with rapid, highly specific, low-cost, and simple operational characteristics have been developed in this study. Based on the amplification of 114 base pairs from the ALV P12 gene, real-time RT-RAA primers and a probe were designed for this study. The lowest detection line was 10 copies of ALV RNA molecules per response, which could be carried out at 39°C in as fastest as 5 min and completed in 30 min, with no cross-reactivity with Marek's disease virus, avian reticuloendothelial virus, Newcastle disease virus, infectious bronchitis virus, infectious bursal disease virus, infectious laryngotracheitis virus, and avian influenza virus. Furthermore, the kappa value of 0.91 (>0.81) was compared with reverse transcription–polymerase chain reaction (RT-PCR) for 44 clinical samples, and the coefficients of variation were within 5.18% of the repeated assays with three low-level concentration gradients. These results indicate that using a real-time RT-RAA assay to detect ALV could be a valuable method.

Establishment and Application of a Real-Time Recombinase Polymerase Amplification Assay for the Detection of Avian Leukosis Virus Subgroup J

Avian leukosis caused by avian leukosis virus (ALV), belonging to the genus Alpharetrovirus of the family Retroviridae, is associated with benign and malignant tumors in hemopoietic cells in poultry. Although several methods have been developed for ALV detection, most of them are not suitable for rapid on-site testing due to instrument limitations, professional operators, or the low sensitivity of the method. Herein, we described the real-time recombinase polymerase amplification (RPA) assay for rapid detection of ALV subgroup J (ALV-J). The major viral structural glycoprotein gp85, highly specific for the subgroup, was used as the molecular target for the real-time RPA assay. The results were obtained at 38°C within 20 min, with the detection sensitivity of 10 copies/μl of standard plasmid pMD18-T-gp85 as the template per reaction. Real-time RPA was capable of ALV-J-specific detection without cross-reaction with other non-targeted avian pathogens. Of the 62 clinical samples tested, the ALV-positive rates of real-time RPA, PCR, and real-time PCR were 66.13% (41/62), 59.68% (37/62), and 67.74% (42/62), respectively. The diagnostic agreement between real-time RPA and real-time PCR was 98.39% (61/62), and the kappa value was 0.9636. The developed real-time ALV-J assay seems promising for rapid and sensitive detection of ALV-J in diagnostic laboratories. It is suitable for on-site detection, especially in a poor resource environment, thus facilitating the prevention and control of ALV-J.

Avian leukosis virus (ALV) is highly prevalent in fancy-chicken flocks in Saxony

The current prevalence of avian leukosis virus (ALV) in fancy chickens in Germany is unknown. Therefore, 537 cloacal swabs from 50 purebred fancy-chicken flocks in Saxony were tested for the presence of the ALV p27 protein using a commercial antigen-capture ELISA. The detection rate was 28.7% at the individual-animal level and 56.0% at the flock level. Phylogenetic analysis of PCR products obtained from 22 different flocks revealed the highest similarity to ALV subtype K. When classifying breeds by their origin, ALV detection rates differed significantly. Evaluation of questionnaire data revealed no significant differences between ALV-positive and negative flocks regarding mortality.

Development and Application of a Reverse-transcription Recombinase-Aided Amplification Assay for Subgroup J Avian Leukosis Virus

Subgroup J Avian leukosis virus (ALV-J) is an important pathogen of poultry tumor diseases. Since its discovery, it has caused significant economic losses to the poultry industry. Thus, the rapid detection of molecular level with strong specificity is particularly important whether poultry are infected with ALV-J. In this study, we designed primers and probe for real-time fluorescent reverse-transcription recombinase-aided amplification assay (RT-RAA) based on the ALV-J gp85 sequence. We had established a real-time fluorescent RT-RAA method and confirmed this system by verifying the specificity and sensitivity of the primers and probe. In addition, repeatability tests and clinical sample regression tests were used for preliminary evaluation of this detection method. The sensitivity of established method was about 10¹ copies/μL, and the repeatability of the CV of the C_T value is 4%, indicating repeatability is good. Moreover, there was no cross-reactivity with NDV, IBV, IBDV, H9N2, MDV, and REV, and other avian leukosis virus subgroups, such as subgroups A, B, C, D, K and E. Importantly, the real-time fluorescent RT-RAA completed the test within 30 min at a constant temperature of 41°C. Forty-two clinical samples with known background were tested, and the test results were coincided with 100%. Overall, these results suggested that the real-time fluorescent RT-RAA developed in this study had strong specificity, high sensitivity, and good feasibility. The method is simple, easy, and portable, that is suitable for clinical and laboratory diagnosis, and provides technical support for the prevention and control of ALV-J.

SOCS3 Promotes ALV-J Virus Replication via Inhibiting JAK2/STAT3 Phosphorylation During Infection

Avian leukosis virus subgroup J (ALV-J) is an oncogenic retrovirus that causes immunosuppression and neoplastic diseases in poultry. Cytokine signal-transduction inhibitor molecule 3 (SOCS3) is an important negative regulator of the JAK2/STAT3 signaling pathway and plays certain roles in ALV-J infection. It is of significance to confirm the roles of SOCS3 in ALV-J infection and study how this gene affects ALV-J infection. In this study, we assessed the expression of the SOCS3 gene in vivo and in vitro, and investigated the roles of SOCS3 in ALV-J infection using overexpressed or interfered assays with the SOCS3 in DF-1 cells. The results showed that the SOCS3 expression of ALV-J infected chickens was different from uninfected chickens in the spleen, thymus and cecal tonsil. Further, SOCS3 is mainly expressed in the nucleus as determined by immunofluorescence assay. Overexpression of SOCS3 in DF-1 cells promoted the replication of ALV-J virus, and the expression of interferons (IFNα and INFβ), inflammatory factors (IL-6 and TNFα) along with interferon-stimulating genes (CH25H, MX1, OASL, and ZAP). Conversely, interference of SOCS3 showed the opposite results. We also observed that SOCS3 promoted ALV-J virus replication by inhibiting JAK2/STAT3 phosphorylation. In conclusion, SOCS3 promotes ALV-J replication via inhibiting the phosphorylation of the JAK2/STAT3 signaling pathway. These results would advance further understanding of the persistent infection and the viral immune evasion of the ALV-J virus.

Molecular characteristic and pathogenicity analysis of a novel multiple recombinant ALV-K strain

Avian leukosis virus (ALV) can induce various tumors and cause serious production problems. ALVs isolated from chickens were divided into six subgroups (A-J). In 2012, a strain of a putative novel subgroup of ALVs was isolated from Chinese native chickens in Jiangsu Province and named as ALV-K. In this study, three ALV-K strains (JS14LH01, JS13LH14, and JS15SG01) were isolated from chickens with suspected ALV infection in Jiangsu Province. Their complete genomes were amplified, sequenced, and analyzed systematically. The results showed that JS14LH01 and JS13LH14 were ALV-K and ALV-E recombinant strains. Whereas JS15SG01 is an ALV-K, ALV-E, and ALV-J multiple recombinant strain containing the U3 region of ALV-J. The pathogenicity test of JS15SG01 revealed that, compared with previous ALV-K strains, the viremia and viral shedding level of JS15SG01-infected chickens were significantly increased, reaching 100 % and 59 %, respectively. More important, JS15SG01 induced significant proliferation of gliocytes in the cerebral cortex of infected chickens, accompanied by the neurotropic phenomenon. This is the first report about a multiple recombinant ALV-K strain that could invade and injure the brain tissue of chickens in China. Our findings enriched the epidemiologic data of ALV and helped to reveal the evolution of ALV strains prevalent in chicken fields.

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.

Development of an antigen-capture enzyme-linked immunosorbent assay for diagnosis of Aleutian mink disease virus

Aleutian mink disease (AMD), caused by Aleutian mink disease virus (AMDV), is a very important infectious disease of mink. Currently, elimination of antibody- or antigen-positive animals is the most successful strategy for eradicating AMD, but the claw-cutting method of blood sampling is difficult to perform and painful for the animal. In this study, we aimed to establish an antigen capture enzyme-linked immunosorbent assay (AC-ELISA) method for the efficient detection of AMDV antigens using fecal samples. A purified mouse monoclonal antibody (mAb) was used as the capture antibody, and a rabbit polyclonal antibody (pAb) was used as the detection antibody. The assay was optimized by adjusting a series of parameters. Using a cutoff value of 0.205, the limit of detection of the AC-ELISA for strain AMDV-G antigen was 2 μg/mL, and there was no cross-reaction with other mink viruses. The intra- and inter-assay standard deviations were below 0.046, and the correlation of variance (CV) values were 1.24-7.12% when testing fecal samples. Compared with conventional PCR results, the specificity and sensitivity were 91.5% and 90.6%, respectively, and the concordance rate between the two methods was 91.1%.

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.

Rapid detection of avian leukosis virus using a fluorescent microsphere immunochromatographic test strip assay

We developed a rapid fluorescent microsphere immunochromatographic test strip (FM-ICTS) assay for the quantitative detection of avian leukosis virus (ALV). A monoclonal antibody specific for the ALV major capsid protein encoded by the gag gene was coupled to label fluorescent microspheres. ALV antibodies were coated on a nitrocellulose membrane to prepare a test line for sample detection. The fluorescence signals of the test and control lines can be read either visually by exposure to UV light or using a fluorescence analyzer. ALV could be detected quantitatively using the ratio of fluorescence signals of the test and control lines (T/C). The assay threshold was determined as a T/C value of 0.0606. The fitting curve equation was established between 1 and 2,048 ng/mL P27 protein with an r² value of 0.9998. The assay showed no cross reactivity with Newcastle disease virus, infectious laryngotracheitis virus, infectious bronchitis virus, Marek's disease virus, infectious bursal disease, Reoviridae virus, or avian influenza virus. The repeatability was satisfactory with an overall average CV of 8.65%. The Kappa coefficient between a commercial ELISA kit was 0.7031 using clinical chicken meconium samples. Thus, a simple, rapid, sensitive, and specific fluorescent microsphere immunochromatographic test strip was developed based on specific anti-capsid protein p27 monoclonal antibodies.

Geese not susceptible to virulent subgroup J avian leukosis virus isolated from chickens

To determine whether geese are susceptible to infection by avian leukosis virus (ALV), 702 serum samples from domestic and foreign goose breeds were screened for p27 antigen as well as being inoculated into DF-1 cell cultures to isolate ALV. Although 5.7% of samples were positive for p27 antigen, reactivity appeared to be non-specific because no ALV was detected in the corresponding DF-1 cultures. To further determine whether geese are susceptible to ALV-J isolated from chickens, ALV-J strain JS09GY7 was artificially inoculated into 10-day-old goose embryos, with one-day-old hatched goslings then screened for p27 antigen and the presence of ALV. In all cases, the results of both tests were negative. Liver tissues from the 1-day-old goslings were screened using a polymerase chain reaction-based assay, which failed to amplify ALV-J gene fragments from any of the samples. Further, no histopathological damage was observed in the liver tissues. ALV-J was further inoculated intraperitoneally into one-day-old goslings, with cloacal swabs samples and plasma samples then collected every 5 days for 30 days. All samples were again negative for the presence of p27 antigen and ALV, and liver tissues from the challenged geese showed no histopathological damage and were negative for the presence of ALV-J gene fragments. Furthermore, p27 antigen detection, PCR-based screening, and indirect immunofluorescence assays were all negative following the infection of goose embryo fibroblasts with ALV-J. Together, these results confirm that virulent chicken-derived ALV-J strains cannot infect geese, and that p27 antigen detection in goose serum is susceptible to non-specific interference.

A high frequency of Gallid herpesvirus-2 co-infection with Reticuloendotheliosis virusis associated with high tumor rates in Chinese chicken farms

The prevalence of Marek's disease (MD) caused by Gallid herpesvirus-2 (GaHV-2) has been increasing in chickens in China despite universal vaccination. Among the possible reasons for this trend, of Reticuloendotheliosis virus (REV) contamination in vaccines could lead to co-infection and reduce the vaccine efficacy. Here, we report the epidemiological findings of our continuous surveillance of MD, and an examination of the effects of REV and/or GaHV-2 co-infection. A total of 1230 samples were collected between 2011 and 2015 from 305 flocks covering many of the chicken-raising regions of China. Among these, 606 samples were determined to be GaHV-2-positive, 13.0% of which were found to be co-infected with REV from 18.8% of the flocks. One GaHV-2 strain (HS/1412), a REV strain (HS/1412R), and a GaHV-2 and REV-co-infected strain (HS/1412 GR) were isolated from different chickens of a GaHV-2 and REV co-infected flock. Pathogenicity tests showed that HS/1412 and HS/1412 GR caused disease in all chickens and that HS/1412R induced morbidity in 84.6% of the infected chickens. HS/1412 GR induced 100% mortality and 76.9% tumor formation, which were significantly higher frequencies than those observed with strain HS/1412 (38.5% and 15.4%, respectively) and HS/1412R (0% and 0%). These results indicate that co-infection with GaHV-2 and REV might explain the persistent, sporadic outbreaks of neoplastic disease in some commercial flocks, resulting in a significant economic burden to the poultry industry of China.

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

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

Development and application of a colloidal gold test strip for detection of avian leukosis virus

Avian leukosis virus (ALV) is an avian oncogenic retrovirus that induces leukemia-like proliferative diseases in chickens. ALV infection can result in the development of immunological tolerance and persistent viremia. Since effective vaccines against ALV are not yet available, its current prevention primarily depends on detection and eradication to establish exogenous ALV-free poultry flocks. In this study, a rapid and simple colloidal gold test strip method, specific for the group-specific antigen, p27 protein, was developed and systematically evaluated for the detection of ALV from different samples. The detection limit of this assay was as low as 6.25 ng/ml for p27 protein and 80 TCID₅₀/ml for different subgroups of ALV. Besides, the test strip showed high specificity in the detection of different subgroups of ALV, including ALV-A, ALV-B, ALV-J, and ALV-K, with no cross-reaction with other avian pathogens. Furthermore, we artificially infected specific pathogen-free (SPF) chickens with ALV-J, collected cloacal swabs, and examined viral shedding using both test strips and ELISA. Results from the test strip were highly consistent with that from ELISA. In addition, 1104 virus isolates from anti-coagulant blood samples, 645 albumen samples, and 4312 meconium samples were tested, and the test strip results agreed with those of ELISA kit up to 97.1%. All the results indicated that the colloidal gold test strip could serve as a simple, rapid, sensitive, and specific diagnostic method for eradication of ALV in poultry farms.

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

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

Development of an antigen-capture ELISA for the quantitation of equine arteritis virus in culture supernatant

Quantitation of virions is one of the important indexes in virological studies. To establish a sensitive and rapid quantitative detection method for equine arteritis virus (EAV), an antigen-capture enzyme-linked immunosorbent assay (AC-ELISA) was developed by using two EAV nucleoprotein monoclonal antibodies (mAbs), 2B9 and 2B3, prepared in this study. After condition optimization, mAb 2B9 was used as the capture antibody, and HRP-labeled 2B3 was chosen as the detecting antibody. The AC-ELISA had a good standard curve when viral particles of the Bucyrus EAV strain were used as a reference standard. The detection limit for the Bucyrus EAV strain was 36 PFU, and the method had a good linear relationship between 72-2297 PFU. The AC-ELISA could specifically detect the Bucyrus EAV strain and had no cross-reaction with other equine viruses. The sensitivity of the AC-ELISA was much higher than that of a western blotting assay but lower than that of a real-time PCR method. However, as a quantitative antigen detection method, the sensitivity of the AC-ELISA was approximately 300 times than the western blotting assay. Furthermore, the AC-ELISA assay could be successfully used in quantification of viral content in an in vitro infection assay, such as a one-step growth curve of EAV, as well as in a transfection assay, such as virus rescue from an infectious cDNA clone of EAV. These results show that the AC-ELISA established in this study is a good alternative for antigen detection of EAV, being a simple, convenient and quantitative detection method for EAV antigens.

A Chinese Variant Marek's Disease Virus Strain with Divergence between Virulence and Vaccine Resistance

Marek's disease (MD) virus (MDV) has been evolving continuously, leading to increasing vaccination failure. Here, the MDV field strain BS/15 was isolated from a severely diseased Chinese chicken flock previously vaccinated with CVI988. To explore the causes of vaccination failure, specific-pathogen free (SPF) chickens vaccinated with CVI988 or 814 and unvaccinated controls were challenged with either BS/15 or the reference strain Md5. Both strains induced MD lesions in unvaccinated chickens with similar mortality rates of 85.7% and 80.0% during the experimental period, respectively. However, unvaccinated chickens inoculated with BS/15 exhibited a higher tumor development rate (64.3% vs. 40.0%), but prolonged survival and diminished immune defects compared to Md5-challenged counterparts. These results suggest that BS/15 and Md5 show a similar virulence but manifest with different pathogenic characteristics. Moreover, the protective indices of CVI988 and 814 were 33.3 and 66.7 for BS/15, and 92.9 and 100 for Md5, respectively, indicating that neither vaccine could provide efficient protection against BS/15. Taken together, these data suggest that MD vaccination failure is probably due to the existence of variant MDV strains with known virulence and unexpected vaccine resistance. Our findings should be helpful for understanding the pathogenicity and evolution of MDV strains prevalent in China.

Recombinant Marek's Disease Virus as a Vector-Based Vaccine against Avian Leukosis Virus Subgroup J in Chicken

Avian leukosis virus subgroup J (ALV-J) is an immunosuppressive virus that causes considerable economic losses to the chicken industry in China. However, there is currently no effective vaccine to prevent ALV-J infection. In order to reduce the losses caused by ALV-J, we constructed two effective ALV-J vaccines by inserting the ALV-J (strain JL093-1) env or gag+env genes into the US2 gene of the Marek’s disease herpesviruses (MDV) by transfection of overlapping fosmid DNAs, creating two recombinant MDVs, rMDV/ALV-gag+env and rMDV/ALV-env. Analysis of cultured chicken embryo fibroblasts infected with the rMDVs revealed that Env and Gag were successfully expressed and that there was no difference in growth kinetics in cells infected with rMDVs compared with that of cells infected with the parent MDV. Chickens vaccinated with either rMDV revealed that positive serum antibodies were induced. Both rMDVs also effectively reduced the rate of positive viremia in chicken flocks challenged with ALV-J. The protective effect provided by rMDV/ALV-env inoculation was slightly stronger than that provided by rMDV/ALV-gag+env. This represents the first study where a potential rMDV vaccine, expressing ALV-J antigenic genes, has been shown to be effective in the prevention of ALV-J. Our study also opens new avenues for the control of MDV and ALV-J co-infection.

Altered expression of the mismatch repair genes in DF-1 cells infected with the avian leukosis virus subgroup A

The absence or deficiency of DNA mismatch repair (MMR) activity results in microsatellite instability (MSI) in cancer. The avian leukosis virus (ALV) causes neoplastic disease in chickens. In this study, the status of MMR, MSI, the cell cycle and apoptosis were detected in DF-1 cells after avian leukosis virus subgroup A infection. Flow cytometry analysis results indicated that there was no significant difference in cell apoptosis between the control and infected groups. The percentage of cells in S and G2 phases were increased in the infected group. MSI and mutation of MSH2 and MLH1 gene exons were absent in DF-1 cells after infection. Levels of MSH2 and MLH1 mRNA were dramatically increased in DF-1 cells after infection. These results demonstrated that ALV RAV-1 infection may promote the expression of MSH2 and MLH1 genes rather than resulting in gene mutations. Mismatch repair functions were normal and may be have relationships with the arrest of S phase and G2 phase.

Development of a novel immuno-PCR for detection of avian leukosis virus

Avian leukosis virus (ALV) is an important pathogen for various neoplasms, including lymphoid, myeloid, and erythroid neoplasms, and it causes significant economic loss in the poultry industry. Several efficient methods for the detection of ALV have been reported. However, these previously developed approaches are based on either PCR or immunoassays. Here, we used a proximity ligation technique and combined PCR with the immunoassay to develop a novel immuno-PCR (Im-PCR) approach for the detection of ALV. Our data showed that the Im-PCR had high specificity and sensitivity to ALV. The Im-PCR method selectively reacted to ALV but not to the other avian viruses tested. The limit of detection of Im-PCR could reach 0.5 TCID50. Moreover, the results of Im-PCR were in agreement with results from commercial ELISA when the clinical cloaca samples were used for ALV detection. The present results demonstrate that the novel Im-PCR method can be efficiently applied to detect ALV in a clinical setting. Our data also highlight that Im-PCR may have promising applications in the diagnosis of pathogens.

Development of an antigen-capture ELISA for the detection of the p27-CA protein of HERV-K(HML-2)

The detection or quantification of retroviruses is often achieved using an antigen-capture ELISA (AC-ELISA) that targets the Gag capsid (CA) protein. We report here the development of an AC-ELISA specific for the p27-CA protein of HERV-K(HML-2). A monoclonal p27-specific antibody is used for capture and a polyclonal anti-p27-CA immune serum generated in rabbits serves for detection. The assay was shown to be specific for HERV-K(HML-2), showing no evidence of cross reactivity with the human retroviruses HIV-1, HIV-2 and HTLV-1 or with XMRV (as a model non-human gammaretrovirus). Using purified recombinant antigen, the limit of detection was shown to be 130pg/ml. The AC-ELISA can be used to quantify HERV-K(HML-2) expression in teratocarcinoma cell lines and to normalize HERV particles generated by transfecting HEK 293T cells with full-length molecular clones. This novel AC-ELISA also proved useful in studies of virus regulation, for example in demonstrating that HERV-K(HML-2) expression is dramatically enhanced by overexpression of Staufen-1, a binding partner of the HERV-K(HML-2) Rec protein. This specific and sensitive HERV-K(HML-2) AC-ELISA will be a useful tool for investigating many aspects of endogenous retroviruses, from basic research to the role they may play in human diseases or as a surrogate marker for particular diseases.

Genetic evolution of Gallid herpesvirus 2 isolated in China

Gallid herpesvirus 2 (GaHV-2), which causes Marek's disease in chickens and has caused extensive economic losses, has recently evolved increased virulence in China. To better understand the genetic basis of the pathogenic characteristics changed and increased virulence, we sequenced the genomes of six new GaHV-2 strains (LCC, LTS, WC/1203, JL/1404, CC/1409, and HS/1412) isolated from chickens with failed immunisation as well as one previously isolated Chinese GaHV-2 strain, J-1. Based on a multiple sequence alignment, several characteristic point mutations were detected in the open reading frames of the Chinese isolates. In addition, two deletions and an insertion were identified at the unique short region and terminal repeat short region junctions in Chinese isolates, and the insertion was a characteristic of the new Chinese isolates. According to a phylogenetic analysis, the GaHV-2 genome diverged substantially over the last two decades in China. Based on the internal repeat long region, the new isolates were closely related to very virulent or very virulent plus strains. Additionally, the new Chinese isolates diverged from the previously isolated strains J-1 and 814. In conclusion, our results provide evidence that Chinese GaHV-2 strains contain characteristic sequences, especially the new isolates. The observed genetic divergence in the new Chinese GaHV-2 strains over the last two decades may be related to observed changes in pathogenic characteristics and virulence.

Isolation and pathogenic analysis of virulent Marek's disease virus field strain in China

Marek's disease (MD) has become increasingly common in China, resulting in considerable economic loss. The etiological agent is unclear. In this study, we isolated a field MD virus (MDV) strain, designated SX1301, from CVI988/Rispens-vaccinated chickens with tumors. Co-infection of avian leukosis virus, reticuloendotheliosis virus, and chicken infectious anemia virus was excluded by polymerase chain reaction, enzyme-linked immunosorbant assay, DNA blotting hybridization, and indirect immunofluorescence assay. As with most strains isolated in China, SX1301 had the same amino acid mutation of meq protein at positions 77(E), 80(Y), and 115(A) Animal experimental results showed development of lethal MD in 57% and MD tumor in 23% of the specific pathogen-free chickens inoculated with SX1301, with tumors mainly distributed in spleen, liver, and kidney. CVI988/Rispens protected 83% of chickens upon challenge with SX1301, with a mortality rate and tumor incidence of 10% and 7%, respectively. These results implicated SX1301 as a virulent MDV strain, with commercial MDV vaccine CVI988/Rispens unable to confer adequate protection against SX1301. There have been no reports of very virulent (vv) plus MDV in China, but frequently occurring virulent MDV may account for the repeated outbreaks of MD. Vaccines with greater efficacy are needed to protect against MDV.

Development and evaluation of an immunochromatographic strip for rapid detection of capsid protein antigen p27 of avian leukosis virus

A rapid immunochromatographic strip for detecting capsid protein antigen p27 of avian leukosis virus was successfully developed based on two high-affinity monoclonal antibodies. The test strip could detect not only 600pg purified recombinant p27 protein but also quantified avian leukosis virus as low as 70 TCID50, which has comparative sensitivity to the commercial enzyme-linked immunosorbent assay (ELISA) kit. For the evaluation of this test strip, 1100 samples consisting of cloacal swabs, meconium collected from the earliest stool of one day old chicken and virus isolates were assessed both by the strip and by the commercial ELISA kit. The agreement between these two tests was 93.91%, 93.42% and 100%, respectively. The sensitivity and specificity of the strip were also calculated by using the ELISA kit as the standard. This immunochromatographic strip provides advantages of rapid and simple detection of capsid protein antigen p27 of avian leukosis virus, which could be applied as an on-site testing assay and used for control and eradication programs of avian leukosis disease.

Pathogenic characteristics of Marek's disease virus field strains prevalent in China and the effectiveness of existing vaccines against them

The virulence of Marek's disease virus (MDV) is continuously evolving, and more virulent MDV pathotypes are emerging, thereby reducing the effectiveness of the existing vaccines. In this study, feather pulps were collected from diseased chickens in commercial chicken flocks in China that presented significant MD visceral tumors in 2011 and were inoculated into a monolayer of duck embryo fibroblasts (DEFs). Three field isolates of MDV were obtained by plaque cloning and identified as MDV via PCR and designated strains LCC, LLY, and LTS. Unvaccinated and CVI988 vaccine-vaccinated specific pathogen-free chickens were challenged at 7 days post vaccination (dpv) with 1000 plaque forming units of each of the respective MDV isolates. These strains induced gross MD lesions in all (100%) of the unvaccinated chickens, and the mortality rates of the unvaccinated chickens were 42.9%, 46.7%, and 23.1% by 60 days post challenge (dpc), respectively. The CVI988 vaccine induced protective indices (PIs) of 85.7, 92.3, and 66.7, respectively. These results showed that the pathogenic characteristics of the Chinese isolates were diverse and that vaccine CVI988 provided different levels of protection against them. These data indicated that the existence of variant MDV strains was a possible reason of immunity failure in China.

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

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

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

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

Overexpression of microRNA gga-miR-1650 decreases the replication of avian leukosis virus subgroup J in infected cells

MicroRNAs (miRNAs) are a class of small regulatory non-coding RNAs that modulate gene expression at the post-transcriptional level, playing a crucial role in cell differentiation and development. Recently, some reports have demonstrated that a number of cellular miRNAs play a role during viral infection. In this study, a luciferase-reporter system carrying the 5' untranslated region (5' UTR) and 3' UTR of avian leukosis virus subgroup J (ALV-J) was used to determine whether cellular miRNAs are involved in ALV-J infection. The miRNA gga-miR-1650 was screened for its potential interaction with the 5' UTR of ALV-J and the ability to suppress luciferase-reporter activity. A mutational analysis of predicted gga-miR-1650-binding sites showed that the 5' and 3' ends of gga-miR-1650 contributed to the interaction between gga-miR-1650 and its target located at the 5' UTR. Overexpression of miRNA gga-miR-1650 was shown to downregulate the expression of the Gag protein and influence the replication of ALV-J through binding to the 5' UTR. Overall, this report provides the basis for the development of new strategies for anti-ALV-J intervention.