Development and validation of an indirect enzyme-linked immunosorbent assay for the detection of Avian leukosis virus antibodies based on a recombinant capsid protein

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.

Synergistic pathogenesis of chicken infectious anemia virus and J subgroup of avian leukosis virus

As important immunosuppressive viruses, chicken infectious anemia virus (CIAV) and subgroup J avian leukosis virus (ALV-J) have caused huge economic losses to the poultry industry globally. Recently, the co-infection of CIAV and ALV-J frequently occurred in the domestic chicken flocks in China. However, the synergistic pathogenesis of CIAV and ALV-J has not been fully investigated. Here, a co-infection study was performed to further understand the potential synergistic pathogenesis of CIAV and ALV-J. In vitro study showed that CIAV could promote the replication of ALV-J in HD11 cells, but ALV-J could not increase the replication of CIAV. Chicken infection study showed both CIAV and ALV-J with synergistic effects caused significant body weight loss to the infected chickens. Although ALV-J had no effect on CIAV viral shedding and tissue load, CIAV did significantly increase ALV-J viremia, viral shedding and tissue load in the co-infection group. Moreover, both CIAV and ALV-J could significantly inhibit the humoral immunity to H9N2 influenza virus and serotype 4 fowl adenovirus (FAdV-4). All these data demonstrate the synergistic pathogenesis for the co-infection of CIAV and ALV-J, and highlight the positive effect of CIAV on the pathogenesis of 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.

Detection of ALV p27 in cloacal swabs and virus isolation medium by sELISA

BACKGROUND

Avian leukosis (AL), which is caused by avian leukosis virus (ALV), has led to substantial economic losses in the poultry industry. The kit used to detect all ALV-positive chickens in breeder flocks is very important for efficiently controlling AL. However, a new emerging ALV subtype is currently a severe challenge in the poultry industry.

RESULTS

In this paper, we compared different enzyme-linked immunosorbent assay (ELISA) kits for detecting p27 of ALV in the same batch of meconium samples. Different positive samples were further analyzed by PCR or virus isolation. The results showed that 36 positive samples among the 1812 chicken meconium samples could be detected by a sandwich ELISA (sELISA) kit, but only 17 positive samples could be identified by a commercial kit. To verify this result, cloacal swabs and viruses isolated from the positive chickens (2 days old) were used to detect the presence of p27. The results showed that the positive rate of p27 was 100% for the swabs and 40% for virus isolation. Surprisingly, PCR and sequence analysis revealed that the env gene of ALV in these positive samples belonged to the novel subgroup K (ALV-K).

CONCLUSION

These data not only demonstrate the relatively high sensitivity of the sELISA kit but also highlight the challenge of controlling ALV-K.

A recombination efficiently increases the pathogenesis of the novel K subgroup of avian leukosis virus

In this study, a recombinant ALV with ALV-K env and ALV-J backbone was generated (designated ALV-K-env-J) and tested in vitro and in vivo. The growth curve in DF1 cells showed that the recombinant virus replicated more efficiently in comparison with the ALV-J and ALV-K. Although all the infected chickens showed growth retardation compared with the non-infected chickens, the viral and serological detection showed that the positive rate and virus load detected in blood and cloaca, and the positive rate and titer of antibody against p27 from the chickens infected with ALV-K-env-J were higher than those from the chickens infected with the ALV-K, but less than those from the chickens infected with the ALV-J. All these data clearly demonstrated that the recombination event in this study increased the pathogenesis of ALV-K, and the potential recombination between different ALV subgroups should be worried when the clinical co-infections occur.

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.

Development and application of a SYBR green real-time PCR for detection of the emerging avian leukosis virus subgroup K

Avian leukosis virus subgroup K (ALV-K) is an emerging ALV tumor virus of chickens. We developed a SYBR green-based real-time polymerase chain reaction (PCR) assay for the rapid and economical detection of ALV-K in chicken flocks. The assay was specific for ALV-K and did not cross-react with other ALV subgroup or avian influenza virus, Newcastle disease virus, or Marek's Disease virus. The method was 100 times more sensitive than conventional PCR and 10 times more sensitive than the enzyme-linked immunosorbent assay (ELISA) for the P27 antigen. The assay was also more sensitive than conventional PCR in tests of 86 clinical plasma samples. DF-1 tissue culture cells infected with 1 TCID50 ALV-K particle were identified as negative using ELISA but tested positive with the real-time PCR method. The viral loads in organs and tissues in infected chickens were highest in kidney, lungs, and glandular stomach, and these results matched ELISA findings.

Identification of a novel linear B-cell epitope in the p27 of Avian leukosis virus

Avian leukosis virus (ALV) is an avian oncogenic retrovirus that can induce virus-associated neoplasia and causes great economic loss in poultry industry. It is known that the capsid antigen p27 is the group-specific antigen that is highly conserved among all ALV subgroups, and is the most abundant immunogenic viral protein. In the present study, five overlapping fragments (GST- p27-F1/2, GST- p27-F2-1/2/3) of ALV-p27 were subjected to Western blotting analysis using a monoclonal antibody (5D3) against ALV-p27 to identify the epitope. The result showed that the epitope recognized by 5D3 is located within 173-240 amino acid of the ALV-p27 protein. For precise mapping of this epitope, a set of overlapping peptides were synthesized. Indirect enzyme linked immunosorbent assay (ELISA) revealed that ¹⁹³CFRQKSQPDI²⁰² motif was the minimal fragment recognized by 5D3, so this motif represented a linear B-cell epitope of ALV-p27. Homology analysis indicated that 5D3 defined epitope is highly conserved among ALV strains. The identified epitope might be useful in clinical applications and as a tool for further study of the structure and function of ALV-p27.

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.

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

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