Fulllength coding sequence analysis of genome segments A and B of infectious bursal disease virus in Thailand: identification of Chinese-like and recombinant virus in the field

RESEARCH HIGHLIGHTS

For the first time, this work demonstrated a recombinant IBDV strain in Thailand.Two genogroups of IBDV were found in Thailand: including HLJ-504-like and recombinant virus.Analysis of the full coding sequence is essential for monitoring emerging variant IBDV.

STAU1 binds to IBDV genomic double-stranded RNA and promotes viral replication via attenuation of MDA5-dependent β interferon induction

Infectious bursal disease virus (IBDV) infection triggers the induction of type I IFN, which is mediated by melanoma differentiation-associated protein 5 recognition of the viral genomic double-stranded RNA (dsRNA). However, the mechanism of IBDV overcoming the type I IFN antiviral response remains poorly characterized. Here, we show that IBDV genomic dsRNA selectively binds to the host cellular RNA binding protein Staufen1 (STAU1) in vitro and in vivo. The viral dsRNA binding region was mapped to the N-terminal moiety of STAU1 (residues 1-468). Down-regulation of STAU1 impaired IBDV replication and enhanced IFN-β transcription in response to IBDV infection, while having little effect on the viral attachment to the host cells and cellular entry. Conversely, overexpression of STAU1 but not the IBDV dsRNA-binding deficient STAU1 mutant (469-702) led to a suppression of IBDV dsRNA-induced IFN-β promoter activity. Moreover, we found that the binding of STAU1 to IBDV dsRNA decreased the association of melanoma differentiation-associated protein 5 but not VP3 with the IBDV dsRNA in vitro. Finally, we showed that STAU1 and VP3 suppressed IFN-β gene transcription in response to IBDV infection in an additive manner. Collectively, these findings provide a novel insight into the evasive strategies used by IBDV to escape the host IFN antiviral response.-Ye, C., Yu, Z., Xiong, Y., Wang, Y., Ruan, Y., Guo, Y., Chen, M., Luan, S., Zhang, E., Liu, H. STAU1 binds to IBDV genomic double-stranded RNA and promotes viral replication via attenuation of MDA5-dependent β interferon induction.

VP1 and VP3 Are Required and Sufficient for Translation Initiation of Uncapped Infectious Bursal Disease Virus Genomic Double-Stranded RNA

Infectious bursal disease virus (IBDV) is a bisegmented double-strand RNA (dsRNA) virus of the Birnaviridae family. While IBDV genomic dsRNA lacks a 5' cap, the means by which the uncapped IBDV genomic RNA is translated effectively is unknown. In this study, we describe a cap-independent pathway of translation initiation of IBDV uncapped RNA that relies on VP1 and VP3. We show that neither purified IBDV genomic dsRNA nor the uncapped viral plus-sense RNA transcripts were directly translated and rescued into infectious viruses in host cells. This defect in translation of the uncapped IBDV genomic dsRNA was rescued by trans-supplementation of the viral proteins VP1 and VP3 which was dependent on both the intact polymerase activity of VP1 and the dsRNA binding activity of VP3. Deletion analysis showed that both 5' and 3' untranslated regions (UTRs) of IBDV dsRNA were essential for VP1/VP3-dependent translation initiation. Significantly, VP1 and VP3 could also mediate the recovery of infectious IBDV from the authentic minus-sense strand of IBDV dsRNA. Moreover, downregulation or inhibition of the cap-binding protein eIF4E did not decrease but, rather, enhanced the VP1/VP3-mediated translation of the uncapped IBDV RNA. Collectively, our findings for the first time reveal that VP1 and VP3 compensate for the deficiency of the 5' cap and replace eIF4E to confer upon the uncapped IBDV RNA the ability to be translated and rescued into infectious viruses.IMPORTANCE A key point of control for virus replication is viral translation initiation. The current study shows that the uncapped IBDV RNA cannot be translated into viral proteins directly by host translation machinery and is thus noninfectious. Our results constitute the first direct experimental evidence that VP1 and VP3 are required and sufficient to initiate translation of uncapped IBDV genomic RNA by acting as a substitute for cap and replacing the cap-binding protein eIF4E. Significantly, VP1/VP3 mediate the recovery of infectious IBDV not only from the plus-sense strand but also from the minus-sense strand of the IBDV dsRNA. These findings provide not only new insights into the molecular mechanisms of the life cycle of IBDV but also a new tool for an alternative strategy for the recovery of IBDV from both the plus- and the minus-sense strands of the viral genomic dsRNA.

Infectious Bursal Disease Virus Activates c-Src To Promote α4β1 Integrin-Dependent Viral Entry by Modulating the Downstream Akt-RhoA GTPase-Actin Rearrangement Cascade

While the entry of infectious bursal disease virus (IBDV) is initiated by the binding of the virus to the two major receptors integrin and HSP90, the signaling events after receptor binding and how they contribute to virus entry remain elusive. We show here that IBDV activates c-Src by inducing the phosphorylation of the Y416 residue in c-Src both in DF-1 chicken fibroblasts and in vivo in the bursa of Fabricius from specific-pathogen-free (SPF) chickens. Importantly, inactivated IBDV fails to stimulate c-Src Y416 phosphorylation, and a very virulent IBDV strain induces a much higher level of c-Src Y416 phosphorylation than does an attenuated strain. Inhibition of c-Src activation by an Src kinase inhibitor or expression of a c-Src dominant negative mutant results in a significant decrease in the internalization of IBDV but has little effect on virus adhesion. Furthermore, short hairpin RNA (shRNA) downregulation of integrin, either the α4 or β1 subunit, but not HSP90 remarkably attenuates IBDV-induced c-Src Y416 phosphorylation, resulting in a decrease in IBDV internalization but not virus adhesion. Moreover, interestingly, inhibition of either c-Src downstream of the phosphatidylinositol 3-kinase (PI3K)/Akt-RhoA signaling cascade or actin rearrangement leads to a significant decrease in IBDV internalization irrespective of the IBDV-induced high levels of c-Src phosphorylation. Cumulatively, our results suggest a novel feed-forward model whereby IBDV activates c-Src for benefiting its cell entry via an integrin-mediated pathway by the activation of downstream PI3K/Akt-RhoA signaling and cytoskeleton actin rearrangement.

IMPORTANCE

While IBDV-caused immunosuppression is highly related to viral invasion, the molecular basis of the cellular entry of IBDV remains elusive. In this study, we demonstrate that IBDV activates c-Src by inducing the phosphorylation of the Y416 residue in c-Src to promote virus internalization but not virus adhesion. The ability to induce the level of c-Src Y416 phosphorylation correlates with the pathogenicity of an IBDV strain. IBDV-induced c-Src Y416 activation is α4β1 integrin but not HSP90 dependent and involves the activation of the downstream PI3K/Akt-RhoA GTPase-actin rearrangement cascade. Thus, our findings provide new insights into the IBDV infection process and the potential for c-Src as a candidate target for the development of IBDV therapeutic drugs.

Gingyo-san enhances immunity and potentiates infectious bursal disease vaccination

The purpose of the present study was to investigate the effects of Gingyo-san (GGS), a traditional Chinese medical formula, on peripheral lymphocyte proliferation and serum antibody titers in chickens vaccinated against the infectious bursal disease (IBD) virus. Treatment groups were fed one of three doses of GGS in their diet (0.5%, 1.0% and 2.0%, w/w), and the IBD vaccine was administered at 1 and 3 weeks of age. At Weeks 8, 12 and 16, changes in serum IBD antibody titers were measured via the micro-method and T cell proliferation. In gene expression experiments, GGS-treated peripheral T lymphocytes were stimulated with concanavalin A (ConA) for 24 h. The mRNA expression of interleukin-2 (IL-2), interferon-γ (IFN-γ), interleukin-4 (IL-4) and interleukin-12 (IL-12) was determined using a semi-quantitative RT-PCR assay. The results showed that a low dose of GGS could significantly raise the antibody titers. Medium and high doses of GGS enhanced IL-2 and IFN-γ production. GGS altered the expression of IL-4 and IL-12 in T lymphocytes. CD4⁺ T lymphocyte development was also skewed towards the Th1 phenotype. GGS enhanced cell-mediated immunity and augmented the effects of IBD vaccination in strengthening subsequent anti-viral responses.

Genetic reassortment of infectious bursal disease virus in nature

Infectious bursal disease virus (IBDV), a double-stranded RNA virus, is a member of the Birnaviridae family. Four pathotypes of IBDV, attenuated, virulent, antigenic variant, and very virulent (vvIBDV), have been identified. We isolated and characterized the genomic reassortant IBDV strain ZJ2000 from severe field outbreaks in commercial flocks. Full-length genomic sequence analysis showed that ZJ2000 is a natural genetic reassortant virus with segments A and B derived from attenuated and very virulent strains of IBDV, respectively. ZJ2000 exhibited delayed replication kinetics as compared to attenuated strains. However, ZJ2000 was pathogenic to specific pathogen free (SPF) chickens and chicken embryos. Similar to a standard virulent IBDV strain, ZJ2000 caused 26.7% mortality, 100% morbidity, and severe bursal lesions at both gross and histopathological levels. Taken together, our data provide direct evidence for genetic reassortment of IBDV in nature, which may play an important role in the evolution, virulence, and host range of IBDV. Our data also suggest that VP2 is not the sole determinant of IBDV virulence, and that the RNA-dependent RNA polymerase protein, VP1, may play an important role in IBDV virulence. The discovery of reassortant viruses in nature suggests an additional risk of using live IBDV vaccines, which could act as genetic donors for genome reassortment.

Enhancement of the immunogenicity of DNA vaccine against infectious bursal disease virus by co-delivery with plasmid encoding chicken interleukin 2

The immunoregulatory activity of a nonmammalian interleukin 2 (IL-2), chicken IL-2 (chIL-2), was investigated using a DNA vaccine against infectious bursal disease virus (IBDV) as a model. Coadministration of a plasmid encoding the VP2 gene of IBDV (pCI-VP2) and a plasmid encoding chicken IL-2 gene (pCI-chIL-2) enhances bursal protection against both the homologous IBDV strain ZJ2000 and the heterologous strain BC6/85 compared to administration of pCI-VP2 alone. Vaccination with pCI-VP2 alone induces low bursal protection against ZJ2000 and only protects chickens from clinical outbreaks and mortality, but not from bursal damage caused by BC6/85. Co-administration of the plasmid encoding the polyprotein gene of IBDV (pCI-VP2/4/3) and pCI-chIL-2 provides complete protection (15/15) against ZJ2000 and satisfactory protection (13/15) against BC6/85. In contrast, only 10 out of 15 chickens and 6 out of 15 chickens were protected against ZJ2000 and BC6/85, respectively, using the pCI-VP2/4/3 vaccination alone. A significant increase in the IBDV-specific neutralizing antibody response was also observed in chickens that received pCI-VP2/4/3 plus pCI-chIL-2 as compared with those that received the pCI-VP2/4/3 vaccination alone. By administrating different amounts of plasmid DNA, we confirmed that the pCI-chIL-2, but not the backbone plasmid pCI, contributes to increased immunoprotection of DNA vaccine against IBDV. These results strongly indicate that the efficacy of avian DNA vaccine can be modulated by co-administration of a plasmid encoding chIL-2.

Plasmid DNA encoding antigens of infectious bursal disease viruses induce protective immune responses in chickens: factors influencing efficacy

The complete polyprotein (VP2/4/3) and VP2 genes of two infectious bursal disease viruses (IBDVs) (one attenuated strain JD1 and one virulent strain ZJ2000) were amplified by long and accurate polymerase chain reaction (LA-PCR), cloned, sequenced and inserted into plasmids pCI and pcDNA3 under the control of human cytomegalovirus (hCMV) immediate early enhancer and promoter. A series of DNA vaccine preparations were made using liposome as the adjuvant to examine their immunogenicity. Although VP2 is the main protective immunogen of IBDV, DNA encoding VP2 initiated a very low level of neutralizing antibody and only protected chickens from clinical outbreak and morality, but not bursal damage. In contrast, DNA encoding VP2/4/3 induced neutralizing antibody and satisfactory protection against virulent IBDV. Recombinant plasmids encoding the polyprotein gene of strain ZJ2000 were more efficient at inducing an immune response than that of strain JD1. Polyprotein expressed by the pCI vector induced better immune response than that expressed by the pcDNA3. Delivery of DNA through intramuscular and/or intradermal routes elicited much higher protective responses than that of oral and eyedrop routes. Most of the chickens vaccinated with high doses of DNA were protected from challenge. Additionally, the immune response to the DNA vaccine was significantly enhanced by a liposome adjuvant. These results indicate that the source of the target genes (from different IBDV strains), the eukaryotic expression vector, the adjuvant, the delivery route and the dosage might play a role of varying degree in influencing the efficacy of the DNA vaccine against IBDV.