VP1 protein of infectious bursal disease virus modulates the virulence in vivo

The long noncoding RNA loc107053557 acts as a gga-miR-3530-5p sponge to suppress the replication of vvIBDV through regulating STAT1 expression

Infectious bursal disease virus (IBDV) causes immunosuppression and high mortality in young chickens. Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) are important regulators during viral infection. However, detailed the regulatory mechanisms of lncRNA-miRNA-mRNA have not yet been described in IBDV infection. Here, we analysed the role of lncRNA53557/gga-miR-3530-5p/STAT1 axis in very virulent IBDV (vvIBDV) infection. Evidently upregulated expression of lncRNA53557 was observed in bursa of Fabricius and DT40 cells. Meanwhile, overexpression of lncRNA53557 promoted STAT1 expression and inhibited vvIBDV replication and vice versa, indicating that the upregulation of lncRNA53557 was part of the host antiviral defence. The subcellular fractionation assay confirmed that lncRNA53557 can be localized in the cytoplasm. Further, dual-luciferase reporter, RNA pulldown, FISH and RT-qPCR assays revealed that lncRNA53557 were directly bound to gga-miR-3530-5p and had a negative regulatory relationship between them. Subsequent mechanistic analysis showed that lncRNA53557 acted as a competing endogenous RNA (ceRNA) of gga-miR-3530-5p to relieve the repressive effect of gga-miR-3530-5p on its target STAT1, as well as Mx1, OASL, and ISG15, thereby suppressing vvIBDV replication. The study reveals that a network of enriched lncRNAs and lncRNA-associated ceRNA is involved in the regulation of IBDV infection, offering new insight into the mechanisms underlying IBDV-host interaction.

The complete protections induced by the oil emulsion vaccines of the novel variant infectious bursal disease viruses against the homologous challenges indicating the important roles of both VP2 and VP1 in the antigenicity and pathogenicity of the virus

Novel variant infectious bursal disease virus (nvIBDV) is an emerging genotype (A2dB1b) that can cause severe and prolonged immunosuppression in young chickens. Despite current commercial vaccines being proven to lack complete protection against nvIBDV, it remains unclear whether the oil emulsion inactivated vaccines (OEVs) of the homologous and heterologous virus or booster immunization can provide effective protection. In this study, OEVs with two types of nvIBDV isolates QZ191002 (A-nv/B-nv) and YL160304 (A-nv/B-HLJ0504-like) were prepared and evaluated the protective effects of OEVs plus the booster immunizations with different current commercial vaccines against the challenge of nvIBDVs. The results from vaccination-challenge experiments showed that nvIBDV could break through the protection provided by only one immunization dose of the commercial vaccines, with the protection rates ranging from 40% to 60%. Interestingly, even with booster immunization with different commercial vaccines, the protection rates could only be increased to 60%-80%. As expected, only the OEVs of the homologous virus could provide 100% protection against the homologous nvIBDV, which could induce high-level specific antibodies, ameliorate target organ damage, and significantly reduce the viral load of the bursal in the challenged chickens. Notably, YL160304-OEV performed better than QZ191002-OEV, providing 100% protection not only against the challenge of homologous strain but also against that of heterologous QZ191002 strain. Antibody levels of the immunized chickens gradually increased after a short decline and reached the highest level on the age of 28 days. Similarly, the percentages of lymphocytes CD4+, CD8+ T, and B in peripheral blood lymphocytes (PBLs) were significantly increased on 21 d and 28 d. Notably, despite the nvIBDV, OEVs initially induced a delayed responses in the early stages but ultimately reach higher levels of CD4+ and CD8+ T lymphocytes. The results of study suggest that even booster immunization with different commercial vaccines cannot provide complete protection against nvIBDV, while the OEVs made by the nvIBDVs can provide full protection. Moreover, YL160304-OEV exhibits a broader protective spectrum against different nvIBDV strains, making it a potential candidate for the development of new vaccine.

Continuous clinicopathological and molecular recognition of very virulent infectious bursal disease virus in commercial broiler chickens

Gumboro virus is one of the most dangerous immunosuppressant viruses that infect chickens and causes massive financial losses worldwide. The current study aims to conduct a molecular characterization of chicken farms for the infectious bursal disease virus (IBDV). Based on postmortem (PM) lesions, 125 bursal samples from 25 farms were collected from clinically diseased commercial chicken farms with increased mortality and suspected Gumboro virus infection. Pooled bursal samples from suspected IBD-vaccinated flocks were tested for IBDV by reverse transcriptase polymerase chain reaction (RT-PCR). Fifteen out of 25 pooled specimens were found positive for IBDV, with a 60% detection rate, and confirmed positive for very virulent IBDV (vvIBDV) by sequence analysis. Nucleotide phylogenetic analysis of VP1 and VP2 genes was employed to compare the 5 chosen isolates with strains representing different governorates in Egypt during 2022. All strains were clustered with vvIBDV with no evidence of reassortment in the VP1 gene. The VP1 and VP2 genes are divided into groups (I, II). The strains in our study were related to group II, and it acquired a new mutation in the VP2 gene that clustered it into new subgroup B. By mutation analysis, the VP2 gene of all strains had a characteristic mutation to vvIBDV. It acquired new mutations in HVRs compared with HK46 in Y220F, A222T/V in all strains in our study, and Q221K that was found in IBD-EGY-AH5 and AH2 in the loop PBC in addition to G254S in all strains in our study and Q249k that found in IBD-EGY-AH1 and AH3 in the loop PDE. These mutations are important in the virulency and antigenicity of the virus. The VP1 had 242E, 390M, and 393D which were characteristic of vvIBDV and KpnI restriction enzyme (777GGTAC/C782) in addition to a new mutation (F243Y and N383H) in IBD-EGY-AH1 and AH4 strains. According to the current study, the strains were distinct from the vaccinal strain; they could be responsible for the most recent IBDV outbreaks observed in flocks instead of received vaccinations. The current study highlighted the importance of molecular monitoring to keep up to date on the circulating IBDV for regular evaluation of commercial vaccination programs against circulating field viruses.

Isolation and genomic characteristics of the novel variant infectious bursal disease virus in China

The infectious bursal disease virus (IBDV) is a member of the viruses that can induce immunosuppression in chickens. In recent years, more and more IBDV-infected cases by the novel variant IBDV were reported in China, and it has been demonstrated that currently used vaccines could not provide complete protection against these new IBDV variants. However, a lack of comprehensive analysis of the genomic characteristics of the novel variant strain IBDV has hampered its vaccine development. In this study, a strain of IBDV, designated HB202201, was phylogenetically analyzed, and it was found that the hypervariable region (HVR) of VP2 belonged to the novel variant strain. Furthermore, the 5'- and 3'-ends of segments A and B were analyzed using the rapid amplification of cDNA end (RACE) method. After the full-length of segment A and segment B were determined, the phylogenetic analysis of the segment A and segment B showed that the isolated HB202201 belonged to A2dB1 genotype, which demonstrated the HB202201 belonged to the novel variant strain. In addition, the specific mutations in VP1-VP5 amino acids were analyzed, which showed that there were multiple typical mutations in novel variant IBDV proteins, including VP1 (G24, I141, V163, and E240), VP2 (K221, and I252), VP3 (Q167 and L196), and VP5 (R7, P44, R92, G104, and E147), whereas there was no typical mutation in VP4. This study provides insights into the genomic and antigenic characteristics of the novel variant IBDV, which will promote the development of novel vaccine against the novel variant IBDV.

Whole Genome Sequencing of Infectious Bursal Disease Viruses Isolated from a Californian Outbreak Unravels the Underlying Virulence Markers and Highlights Positive Selection Incidence

Outbreaks of the immunosuppressive infectious bursal disease (IBD) are frequently reported worldwide, despite the vaccination regimes. A 2009 Californian IBD outbreak caused by rA and rB isolates was described as very virulent (vv) IBD virus (IBDV); however, molecular factors beyond this virulence were not fully uncovered. Therefore, segments of both isolates were amplified, successfully cloned, whole genome sequenced by Next Generation Sequencing, genotyped, and the leading virulence factors were entirely investigated in terms of phylogenetic and amino acid analysis and protein modeling for positive selection orientation and interaction analysis. rA and rB isolates displayed the highest amino acid identity (97.84-100%) with Genotype 3 strains. Interestingly, rA and rB contained all virulence hallmarks of hypervariable (HVR), including 222A, 242I, 249Q, 256I, 284A, 286T, 294I, 299S, and 318G, as well as the serine-rich heptapeptide sequence. Moreover, we pinpointed the A3B2 genotype of rA and rB, predominant in non-reassortants, and we highlighted the absence of recombination events. Furthermore, gene-wise phylogenetic analysis showed the entire genes of rA and rB clustered with the vvIBDVs and emphasized their share in IBDV virulence. VP5 showed a virulence marker, MLSL (amino acid sequence). VP2 encountered three significant novel mutations apart from the HVR, including G163E in rA and Y173C and V178A in rB, all residing within interacting motifs. VP4 contained 168Y, 173N, 203S, and 239D characteristic for the vv phenotype. A235V mutation was detected at the dsRNA binding domain of VP3. In VP1, the TDN triplet and the mutation (V4I) were detected, characteristic of hypervirulence occurring at the N-terminus responsible for protein priming. Although selection analysis revealed seven sites, codon 222 was the only statistically significant selection site. The VP2 modeling of rA and rB highlighted great structure fitness, with 96.14% Ramachandran favored positioning including the 222A, i.e., not influencing the structure stability. The 222A was found to be non-interface surface residue, associated with no interaction with the attachment-mediated ligand motif. Our findings provide pivotal insights into the evolution and underlying virulence factors and will assist in the development of control strategies via sequence-based continuous monitoring for the early detection of novel vv strains.

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.

Identification of a New Infectious Pancreatic Necrosis Virus (IPNV) Variant in Atlantic Salmon (Salmo salar L.) that can Cause High Mortality Even in Genetically Resistant Fish

Infectious pancreatic necrosis (IPN) is an important viral disease of salmonids that can affect fish during various life cycles. In Atlantic salmon, selecting for genetically resistant fish against IPN has been one of the most highly praised success stories in the history of fish breeding. During the late 2000s, the findings that resistance against this disease has a significant genetic component, which is mainly controlled by variations in a single gene, have helped to reduce the IPN outbreaks to a great extent. In this paper, we present the identification of a new variant of the IPN virus from a field outbreak in Western Norway that had caused mortality, even in genetically resistant salmon. We recovered and assembled the full-length genome of this virus, following the deep-sequencing of the head-kidney transcriptome. The comparative sequence analysis revealed that for the critical amino acid motifs, previously found to be associated with the degree of virulence, the newly identified variant is similar to the virus’s avirulent form. However, we detected a set of deduced amino acid residues, particularly in the hypervariable domain of the VP2, that collectively are unique to this variant compared to all other reference sequences assessed in this study. We suggest that these mutations have likely equipped the virus with the capacity to escape the host defence mechanism more efficiently, even in the genetically deemed IPN resistant fish.

The emerging naturally reassortant strain of IBDV (genotype A2dB3) having segment A from Chinese novel variant strain and segment B from HLJ 0504-like very virulent strain showed enhanced pathogenicity to three-yellow chickens

Novel variant infectious bursal disease virus (nvIBDV) is an emerging pathotype that can cause sub-clinical disease with severe, prolonged immunosuppression in young chickens. At present, two major pathotypes, including vvIBDV and nvIBDV, are prevailing in China. In this study, we propose that the nvIBDV is a new genotype (A2dB1b) and also first isolated and characterized a nvIBDV reassortant strain YL160304 (A2dB3) with segments A and B derived, respectively, from the nvIBDV and the HLJ-0504-like vvIBDV from yellow chickens in southern China. The YL160304 causes more extensive cytotropism and can infect specific-pathogen-free chicken embryos with severe subcutaneous hemorrhage. The pathogenicity of YL160304 to 4-week-old three-yellow chickens was determined and compared with those of the nvIBDV QZ191002 and the HLJ-0504-like vvIBDV NN1172. Weight gain was significantly reduced in all the challenged birds. No clinical signs and associated mortality were observed in the birds challenged with QZ191002, while the mortalities in the birds challenged with NN1172 and YL160304 were 30% (3/10) and 10% (1/10), respectively. At 7 days postchallenge, the bursa was severely damaged and the percentage of peripheral blood B lymphocyte (PBBL) decreased significantly in all the challenged birds and the quantity of the viral RNA detected in the bursa was in accordance with the results of the histomorphometry and the depletion of PBBL. This study not only confirmed the emerging epidemic of the novel variant and its reassortant strains, but also discovered that the naturally reassortant nvIBDV strain with the segment B of HLJ 0504-like vvIBDV can significantly enhance the pathogenicity to chickens.

Safety and Efficacy Profile of Live, Intermediate Plus Vaccine Against Infectious Bursal Disease Based on Strain G6

Infectious bursal disease (IBD) is an acute, highly contagious, immunosuppressive disease of young chickens that causes considerable economic loss in the poultry industry worldwide. Vaccination with live attenuated vaccines is still the most important method used for the control and prevention of IBD in chickens. Here we present the results of in vitro characterization, as well as efficacy and safety testing of a live, intermediate plus vaccine against IBD based on strain G6. Strain characterization confirmed that G6 strain is an intermediate plus strain, showing a high degree of homology with the existing vaccine strains of the same virulence. Safety studies showed that chickens can be vaccinated from 10 days of age. Onset and duration of immunity in specific pathogen free and maternally derived antibodies (MDA) chickens was proven to be 14 and 35 days after vaccination, respectively. When immunizing MDA-positive chickens, vaccine is capable of breakthrough at a titer of ≤500 ELISA units. The field trial conducted on commercial broilers showed a 95% protection against vvIBDV challenge. Stability of the freeze-dried vaccine after reconstitution was confirmed over a period of 3 h. Overall, IBD G6 vaccine has shown good safety and efficacy profile in accordance with European Pharmacopoeia requirements.

The Full Region of N-Terminal in Polymerase of IBDV Plays an Important Role in Viral Replication and Pathogenicity: Either Partial Region or Single Amino Acid V4I Substitution Does Not Completely Lead to the Virus Attenuation to Three-Yellow Chickens

Infectious Bursal Disease Virus (IBDV) has haunted the poultry industry with severe, prolonged immunosuppression of chickens when infected at an early age and can easily lead to other secondary infections. Understanding the pathogenic mechanisms could lead to effective prevention and control of Infectious Bursal Disease (IBD). Evidence suggests that the N-terminal domain of polymerase in segment B plays an important role, but it is not clear which part or residual is crucial for the pathogenicity. Using a reverse genetics technique, a molecular clone (rNN1172) of the parental vvIBDV strain NN1172 was generated, and its pathogenicity was found to be the same as the parental virus. Then, three recombinant chimeric viruses were rescued based on the rNN1172 and substituted with the counterparts in the N-terminal domain of the attenuated vaccine strain B87: the rNN1172-B87VP1a (substituting the full region of the 1–167 aa residuals), the rNN1172-B87VP1a∆4 (substituting the region of the 5–167 aa residuals), and the rNN1172-VP1∆4 (one single aa residual substitution V4I), to better explore the role of the N-terminal domain of polymerase on the viral pathogenicity. Interestingly, all these substitutions played different roles in the viral pathogenicity: the mortality of the rNN1172-B87VP1a-challenged chickens was significantly reduced from 30% to 0%. No obvious lesion was found in the histopathological examination, and the lowest viral genome copy number was also detected in the bursa when compared to the parental and two other recombinant viruses. The mortalities caused by rNN1172-B87VP1a∆4 and rNN1172-B87VP1∆4, respectively, were all reduced to 10% and had a delayed onset of death. Our results also revealed that the pathogenicity of the IBDV was consistent with the viral replication efficiency in vivo (bursae). This study demonstrated that the full region of the N-terminal of polymerase plays an important role in viral replication and pathogenicity, but the substitutions of its partial region or a single residual do not completely lead to the virus attenuation to Three-Yellow chickens, although that significantly reduces its pathogenicity.

Development of a recombinant VP2 vaccine for the prevention of novel variant strains of infectious bursal disease virus

Since 2017, novel variant strains of infectious bursal disease virus (nvIBDV) have been detected in China, while the current vaccines on the market against very virulent IBDV have limited protection against this subtype virus. In this context, a strain of the virus has been isolated, and sequencing alignment and bird regression experiments showed that the virus was IBDV, belonging to the nvIBDV subtype (and named IBDV FJ-1812). Furthermore, the Escherichia coli expression system was used to successfully express soluble nvIBDV rVP2, which is specifically recognized by an anti-IBDV standard serum and anti-nvIBDV positive serum, and could be assembled into 14 - 17 nm virus-like particles. Based on the purified nvIBDV rVP2, we developed an IBDV FJ-1812 VP2 VLP vaccine at a laboratory scale to evaluate protection by this vaccine; in addition, we also prepared an IBDV JZ 3/02 VP2 subunit vaccine targeting very virulent IBDV and evaluated its cross-protection against nvIBDV. Results of bird experiments showed that the nvIBDV rVP2 vaccine could induce high titres of specific antibodies, completely protect the bursa of Fabricius from viral infection, and provide 100% immune protection to SPF and Ross 308 broiler chickens. Furthermore, the IBDV JZ 3/02 VP2 subunit vaccine targeting very virulent IBDV could provide 60% protection for SPF chickens and 80% protection for Ross 308 broiler chickens. This report provides important technical supports for the prevention and control of nvIBDV in the future.

Reverse genetics approaches for live-attenuated vaccine development of infectious bursal disease virus

Infectious bursal disease (IBD), which is caused by infectious bursal disease virus (IBDV) infection, leads to severe immunosuppression in young chickens and results in significant economic losses in the poultry industry. To date, vaccination with live-attenuated vaccine (LAV) is a convenient method to provide effective protection against IBDV infection. Classical attenuated viruses are usually obtained by either passaging virus in cultured cells or natural isolation. However, these empiric attenuation methods, which are time-consuming and not guaranteed, are not reliable for emergent antigenic variant and very virulent IBDV strains. The reverse genetics (RG) system opens a new avenue for the development of IBDV LAV. In this review, we summarize the current knowledge on the biological characteristics of IBDV structure and genome organization, as well as the established RG systems. We also describe the details for the strategies used to develop IBDV LAV based on the RG systems.

The Stronger Downregulation of in vitro and in vivo Innate Antiviral Responses by a Very Virulent Strain of Infectious Bursal Disease Virus (IBDV), Compared to a Classical Strain, Is Mediated, in Part, by the VP4 Protein

IBDV is economically important to the poultry industry. Very virulent (vv) strains cause higher mortality rates than other strains for reasons that remain poorly understood. In order to provide more information on IBDV disease outcome, groups of chickens (n = 18) were inoculated with the vv strain, UK661, or the classical strain, F52/70. Birds infected with UK661 had a lower survival rate (50%) compared to F52/70 (80%). There was no difference in peak viral replication in the bursa of Fabricius (BF), but the expression of chicken IFNα, IFNβ, MX1, and IL-8 was significantly lower in the BF of birds infected with UK661 compared to F52/70 (p < 0.05) as quantified by RTqPCR, and this trend was also observed in DT40 cells infected with UK661 or F52/70 (p < 0.05). The induction of expression of type I IFN in DF-1 cells stimulated with polyI:C (measured by an IFN-β luciferase reporter assay) was significantly reduced in cells expressing ectopic VP4 from UK661 (p < 0.05), but was higher in cells expressing ectopic VP4 from F52/70. Cells infected with a chimeric recombinant IBDV carrying the UK661-VP4 gene in the background of PBG98, an attenuated vaccine strain that induces high levels of innate responses (PBG98-VP4UK661) also showed a reduced level of IFNα and IL-8 compared to cells infected with a chimeric virus carrying the F52/70-VP4 gene (PBG98-VP4F52/70) (p < 0.01), and birds infected with PBG98-VP4UK661 also had a reduced expression of IFNα in the BF compared to birds infected with PBG98-VP4F52/70 (p < 0.05). Taken together, these data demonstrate that UK661 induced the expression of lower levels of anti-viral type I IFN and proinflammatory genes than the classical strain in vitro and in vivo and this was, in part, due to strain-dependent differences in the VP4 protein.

Rapid Generation of Attenuated Infectious Bursal Disease Virus from Dual-Promoter Plasmids by Reduction of Viral Ribonucleoprotein Activity

Infectious bursal disease virus (IBDV) of the Birnaviridae family leads to immunosuppression of young chickens by destroying B cells in the bursa of Fabricius (BFs). Given the increasing number of variant IBDV strains, we urgently require a method to produce attenuated virus for vaccine development. To accomplish this goal, the dual-promoter plasmids in which the RNA polymerase II and RNA polymerase I (Pol I) promoters were placed upstream of the IBDV genomic sequence, which was followed by mouse Pol I terminator and a synthetic polyadenylation signal, were developed for rapid generation of IBDV. This approach did not require trans-supplementation of plasmids for the expression of VP1 and VP3, the main components of IBDV ribonucleoprotein (RNP). Based on the finding in this study that the IBDV RNP activity was partially retained by VP1-FLAG, we successfully rescued the replication-competent IBDV/1FLAG expressing VP1-FLAG. Compared with its parental counterpart, IBDV/1FLAG formed smaller size plaques in cultured cells and induced the same 100% immune protection in vivo However, neither retarded development nor severe BFs lesion was observed in the IBDV/1FLAG-inoculated chickens. Collectively, this is the first report that viral RNP activity was affected by the addition of an epitope tag on the componential viral proteins. Furthermore, this work demonstrates the rapid generation of attenuated IBDV from dual-promoter plasmids via reducing viral RNP activity by a fused FLAG tag on the C terminus of VP1. This would be a convenient strategy to attenuate epidemic variant IBDV strains for rapid and efficient vaccine development.IMPORTANCE Immunosuppression in chickens as a result of infectious bursal disease virus (IBDV) infection leads to significant economic losses in the poultry industry worldwide every year. Currently, vaccination is still the best way to prevent the prevalence of IBDV. However, with the occurrence of increasing numbers of variant IBDV strains, it is challenging to develop antigen-matched live attenuated vaccine. Here, we first developed a dual-promoter reverse-genetic system for the rapid generation of IBDV. Using this system, the attenuated IBDV/1FLAG expressing VP1-FLAG, which displays the decreased viral RNP activity, was rescued. Moreover, IBDV/1FLAG inoculation induced a similar level of neutralizing antibodies to that of its parental counterpart, protecting chickens against lethal challenge. Our study, for the first time, describes a dual-promoter reverse-genetic approach for the rapid generation of attenuated IBDV while maintaining entire parental antigenicity, suggesting a potential new method to attenuate epidemic variant IBDV strains for vaccine development.

gga-miR-27b-3p enhances type I interferon expression and suppresses infectious bursal disease virus replication via targeting cellular suppressors of cytokine signaling 3 and 6 (SOCS3 and 6)

MicroRNAs are small noncoding RNAs playing an important role in host response to pathogenic infection. Here we show that IBDV infection induced the demethylation of the pre-miR-27 promoter and upregulated gga-miR-27b-3p expression. We found that ectopic expression of miR-27b-3p in DF-1 cells enhanced the expression of chicken IFN-β, IRF3 and NF-κB, via directly targeting cellular suppressors of cytokine signaling 3 and 6 (SOCS3 and 6), inhibiting IBDV replication in host cells, while inhibition of endogenous miR-27b-3p by its inhibitors suppressed the expression of IFN-β, IRF3 and NF-κB, enhancing SOCS3 and 6 expressions and facilitating IBDV replication. Furthermore, transfection of DF-1 cells with miR-27b-3p markedly increased phosphorylation of STAT1 on Tyr701 in cells post chIFN-γ treatment. On the contrary, inhibition of endogenous miR-27b-3p reduced phosphorylation of STAT1 on Tyr701 in cells with chIFN-γ treatment. These findings indicate that gga-miR-27b-3p serves as an inducible antiviral mediator in host response to IBDV infection.

Epigenetic Upregulation of Chicken MicroRNA-16-5p Expression in DF-1 Cells following Infection with Infectious Bursal Disease Virus (IBDV) Enhances IBDV-Induced Apoptosis and Viral Replication

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression posttranscriptionally by silencing or degrading their targets and play important roles in the host response to pathogenic infection. Although infectious bursal disease virus (IBDV)-induced apoptosis in host cells has been established, the underlying molecular mechanism is not completely unraveled. Here, we show that infection of DF-1 cells by IBDV induced gga-miR-16-5p (chicken miR-16-5p) expression via demethylation of the pre-miR-16-2 (gga-miR-16-5p precursor) promoter. We found that ectopic expression of gga-miR-16-5p in DF-1 cells enhanced IBDV-induced apoptosis by directly targeting the cellular antiapoptotic protein B-cell lymphoma 2 (Bcl-2), facilitating IBDV replication in DF-1 cells. In contrast, inhibition of endogenous miR-16-5p markedly suppressed apoptosis associated with enhanced Bcl-2 expression, arresting viral replication in DF-1 cells. Furthermore, infection of DF-1 cells with IBDV reduced Bcl-2 expression, and this reduction could be abolished by inhibition of gga-miR-16-5p expression. Moreover, transfection of DF-1 cells with gga-miR-16-5p mimics enhanced IBDV-induced apoptosis associated with increased cytochrome c release and caspase-9 and -3 activation, and inhibition of caspase-3 decreased IBDV growth in DF-1 cells. Thus, epigenetic upregulation of gga-miR-16-5p expression by IBDV infection enhances IBDV-induced apoptosis by targeting the cellular antiapoptotic protein Bcl-2, facilitating IBDV replication in host cells.IMPORTANCE Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive disease in young chickens, causing severe economic losses to stakeholders across the globe. Although IBD virus (IBDV)-induced apoptosis in the host has been established, the underlying mechanism is not very clear. Here, we show that infection of DF-1 cells by IBDV upregulated gga-miR-16-5p expression via demethylation of the pre-miR-16-2 promoter. Overexpression of gga-miR-16-5p enhanced IBDV-induced apoptosis associated with increased cytochrome c release and caspase-9 and -3 activation. Importantly, we found that IBDV infection induced expression of gga-miR-16-5p that triggered apoptosis by targeting Bcl-2, favoring IBDV replication, while inhibition of gga-miR-16-5p in IBDV-infected cells restored Bcl-2 expression, slowing down viral growth, indicating that IBDV induces apoptosis by epigenetic upregulation of gga-miR-16-5p expression. These findings uncover a novel mechanism employed by IBDV for its own benefit, which may be used as a potential target for intervening IBDV infection.

Molecular characterisation of infectious bursal disease virus in Namibia, 2017

Between July and September 2017, samples collected from six unvaccinated chickens in Namibia were shown to be positive for infectious bursal disease virus (IBDV) by RT-PCR. Partial sequence and phylogenetic analysis of the VP1 and VP2 genes from six viruses revealed that they all belong to the very virulent pathotype (Genogroup 3) and are genetically very similar to IBDVs identified in neighbouring Zambia. This is the first molecular characterisation of IBDV in Namibia and has implications on the control and management of the disease in the country.

Fused IgY Fc and Polysaccharide Adjuvant Enhanced the Immune Effect of the Recombinant VP2 and VP5 Subunits-A Prospect for Improvement of Infectious Bursal Disease Virus Subunit Vaccine

Infectious bursal disease virus (IBDV) is a highly contagious pathogen that causes damage in lymphoid organs and remains a threat to the poultry industry worldwide. Currently, subunit vaccines based on VP2 antigen expressed in prokaryotic systems are widely used in clinical settings. However, the immunogenicity of VP2 vaccines is limited because of their inherent defect that the structure of the antigen expressed in Escherichia coli (E. coli) may be different from its natural conformation. In this study, we fused VP2 and VP5 protective antigen genes and linked the chicken IgY Fc gene onto it. The eukaryotic expression plasmid carrying the fusion gene was transformed into Pichia pastoris (P. pastoris) to express the recombinant VP2–VP5–Fc protein. The recombinant protein was used as immunogen for evaluating immune response, and the recombinant VP2–Fc and VP2 proteins expressed in P. pastoris and the commercial VP2 subunit vaccines were used as controls. Moreover, Taishan Pinus massoniana pollen polysaccharide (TPPPS), an immunomodulator found by our laboratory, was used as adjuvant to investigate its immune modulatory effects on immunogens. Chickens were divided into six groups and inoculated with VP2–VP5–Fc+TPPPS, VP2–VP5–Fc, VP2–Fc, VP2 vaccine, commercial VP2 subunit vaccine, and phosphate buffered saline (PBS). The recombinant VP2 subunit vaccine expressed in P. pastoris exhibited higher immunogenicity than the commercial VP2 subunit vaccine. The VP2–Fc protein showed a better effect than the VP2 protein, and the VP2–VP5–Fc subunit further improved the immune effects. In addition, TPPPS was proved to be a good immunopotentiator for the VP2–VP5–Fc subunit vaccine. Hence, the recombinant VP2–VP5–Fc subunit combined with TPPPS adjuvant exhibits potential as efficient IBDV vaccine to prevent infectious bursal disease.

Infectious bursal disease virus protein VP4 suppresses type I interferon expression via inhibiting K48-linked ubiquitylation of glucocorticoid-induced leucine zipper (GILZ)

Viruses have developed a variety of methods to evade host immune response. Our previous study showed that infectious bursal disease virus (IBDV) inhibited type I interferon production via interaction of VP4 with cellular glucocorticoid-induced leucine zipper (GILZ) protein. However, the exact underlying molecular mechanism is still unclear. In this study, we found that IBDV VP4 suppressed GILZ degradation by inhibiting K48-linked ubiquitylation of GILZ. Furthermore, mutation of VP4 (R41G) abolished the inhibitory effect of VP4 on IFN-β expression and GILZ ubiquitylation, indicating that the amino acid 41R of VP4 was required for the suppression of IFN-β expression and GILZ ubiquitylation. Moreover, IBDV infection or VP4 expression markedly inhibited endogenous GILZ ubiquitylation. Thus, IBDV VP4 suppresses type I interferon expression by inhibiting K48-linked ubiquitylation of GILZ, revealing a new mechanism employed by IBDV to suppress host response.

Molecular characterization of very virulent infectious bursal disease virus strains circulating in Egypt from 2003 to 2014

In the present study, four very virulent infectious bursal disease virus (vvIBDV) isolates from flocks of chickens with vaccination failure in Egypt in 2003, 2007, 2010 and 2014 were characterized. The four viruses, designated USC2003, USC2007, USC2010 and USC2014, were detected by reverse transcription PCR, subjected to sequencing of both genomic segments (A and B) and compared with geographically and phylogenetically diverse IBDV strains. Phylogenetic analysis of segment A (complete) and B (partial) revealed a close relationship between Egyptian and vvIBDV reference strains of European and Asian origin. The sequences of segments of A and B the current Egyptian isolates were 96.1-98.2% and 96.5-98.7% identical, respectively, to those of other known vvIBDV isolates. The deduced amino acid sequences of VP1, polyprotein (pVP2-VP4-VP3) and VP5 revealed the presence of putative virulence determinants of Egyptian isolates compared with vvIBDV and less virulent (classical and variant) strains. The Egyptian isolates also possess unique amino acids substitutions within the hypervariable region of VP2 that differ from those of other reference IBDV strains. Further studies may be necessary to determine the pathogenic significance of these amino acid substitutions to fully understand the molecular epidemiology and evolution of IBDV.

VP2 of Infectious Bursal Disease Virus Induces Apoptosis via Triggering Oral Cancer Overexpressed 1 (ORAOV1) Protein Degradation

Infectious bursal disease (IBD) is an acute, highly contagious and immunosuppressive avian disease caused by IBD virus (IBDV). Cell apoptosis triggered by IBDV contributes to the dysfunction of immune system in host. VP2 of IBDV is known to induce cell death but the underlying mechanism remains unclear. Here we demonstrate that VP2 interacts with the oral cancer overexpressed 1 (ORAOV1), a potential oncoprotein. Infection by IBDV or ectopic expression of VP2 causes a reduction of cellular ORAOV1 and induction of apoptosis, so does knockdown of ORAOV1. In contrast, over-expression of ORAOV1 leads to the inhibition of VP2- or IBDV-induced apoptosis, accompanied with the decreased viral release (p < 0.05). Thus, VP2-induced apoptosis during IBDV infection is mediated by interacting with and reducing ORAOV1, a protein that appears to act as an antiapoptotic molecule and restricts viral release early during IBDV infection.

Role of naturally occurring genome segment reassortment in the pathogenicity of IBDV field isolates in Three-Yellow chickens

Reassortment among genome segments of infectious bursal disease virus (IBDV) field isolates was reported frequently worldwide, however the pathogenicity of the reassortant field IBDV is poorly understood. In this paper, a pathogenicity study on four representative IBDV field strains isolated from Southern China between 2005 and 2011 was conducted. Twenty-eight-day-old Three-Yellow chickens were divided into four groups and were inoculated intraocularly with one of the four field IBDV strains, namely NN1172, NN1005, GD10111 and JS7, respectively. The mortality and relative weight of bursa and thymus were subsequently determined in the acute phase of infection. In addition, B cells, T cells (CD4(+) and CD8(+)) and virus were quantified in the bursa of Fabricius and thymus, respectively, by flow cytometry and real-time reverse transcription-polymerase chain reaction. The results showed that isolate NN1172, of which parts of segment A and B encoding the hypervariable (v) region of viral protein (VP2) and VP1, respectively, derived from vvIBDV strains, showed the most severe pathogenicity, and caused the most severe bursal B cell depletion as well as CD4(+) and CD8(+) T cell infiltration in the bursa of Fabricius. However, the virus induced the strongest decrease in CD4(+) and CD8(+) T cells in the thymus and exhibited the most efficient viral replication in the target organs. Isolate NN1005, whose vVP2 derived from vvIBDV and VP1 from unidentified origin, exhibited relatively lower pathogenicity compared to NN1172. The other two isolates, JS7 and GD10111, of which the vVP2 derived from vvIBDV and intermediate IBDV, and VP1 from 002-73 and attenuated IBDV, respectively, showed the lowest level of virulence. Our results suggest that various IBDV field isolates with different natural segment reassortments exhibit differential pathogenicity after infection of commercial Three-Yellow chickens.

gga-miR-2127 downregulates the translation of chicken p53 and attenuates chp53-mediated innate immune response against IBDV infection

Infectious bursal disease (IBD) is characterized by the immune suppression of infected birds. The molecular mechanism by which IBD virus (IBDV) suppresses the host immune system remains to be elucidated. The tumor suppressor protein p53 can inhibit the replication of various viruses, but its effect on IBDV remains unknown. This study established an in vitro infection model based on DF-1 cells (chicken embryo fibroblast cell line) to investigate the antiviral effects of chicken p53 (chp53) on IBDV infection. The expression level and activity of chp53 remarkably increased in IBDV-infected DF-1 cells. The overexpression of chp53 inhibited IBDV replication and upregulated the expression of multiple chicken antiviral innate immunity genes (IPS-1, IRF3, PKR, OAS, and Mx), whereas the suppression of chp53 led to the opposite effect. This result indicates that chp53 activates the antiviral innate immune response of chickens to IBDV infection. Bioinformatics analysis and dual-luciferase reporter assay showed that gga-miR-2127 targeted the 3'UTR of chp53. qRT-PCR and western blot revealed that gga-miR-2127 overexpression in DF-1 cells not only downregulated the expression levels of chp53 and of the antiviral innate immunity genes in chickens but also promoted IBDV replication. Our results suggest that gga-miR-2127 downregulates chp53 mRNA translation by targeting its 3'UTR and attenuates chp53-mediated antiviral innate immune response against IBDV.

A single mutation in the PBC loop of VP2 is involved in the in vitro replication of infectious bursal disease virus

To test whether amino acid mutations in the PBC and PHI loops of VP2 are involved in the replication and virulence of infectious bursal disease virus (IBDV), a pair of viruses, namely the moderately virulent IBDV (rGx-F9VP2) and the attenuated strain (rGt), were used. Residue mutations A222P (PBC) and S330R (PHI), selected by sequence comparison, were introduced individually into rGx-F9VP2 by using a reverse genetics system. In addition, the reverse mutation of either P222A or R330S was introduced into rGt. The four modified viruses were then rescued and evaluated in vitro (CEF cells) and in vivo (SPF chickens). Results showed that A222P elevated the replication efficiency of rGx-F9VP2 while P222A reduced that of rGt in CEF cells. A mutation at residue 330 did not alter IBDV replication. In addition, animal experiments showed that a single mutation at either residue 222 or 330 did not significantly influence the virulence of IBDV. In conclusion, residue 222 in PBC of VP2 is involved in the replication efficiency of IBDV in vitro but does not affect its virulence in vivo, further facilitating our understanding of the gene-function of IBDV.

Evidence of genetic drift and reassortment in infectious bursal disease virus and emergence of outbreaks in poultry farms in India

Recurrent outbreaks of infectious bursal disease (IBD) have become a burning problem to the poultry industry worldwide. Here, we performed genetic analysis of IBD virus (IBDV) field isolates from recent outbreaks in various poultry farms in India. The sequence analysis of IBDV VP2 hypervariable region revealed amino acid pattern similar to that of very virulent (222A, 242I, 253Q, 256I, 272I, 279D, 284A, 294I, 299S and 330S) and intermediate plus virulent (222A, 242I, 253Q, 256I, 272T, 279N, 284A, 294I, 299S and 330S) type whereas analysis of VP1 revealed presence of sequence similar to that of very virulent (61I, 145T) and unique (61I, 141I, 143D, 145S) type in field isolates. Among the eight field isolates, two isolates contained very virulent type VP2 and unique type VP1, three contained intermediate plus virulent type VP2 and unique type VP1 whereas five contained both VP2 and VP1 of very virulent type. The phylogenetic analysis based on VP2 nucleotide sequence showed clustering of all eight isolates close to known very virulent strains whereas based on VP1, five isolates formed unique cluster and three isolates were placed close to very virulent strains. The isolates forming unique VP1 cluster showed highest similarity with classical virulent IBDVs suggesting their possible evolution from segment B of non-very virulent IBDVs. Interestingly, these five isolates were responsible for outbreaks in four different farms located at three different geographic locations in India. These observations indicates genetic reassortment between segment A and segment B from co-infecting IBDV strains leading to emergence of very virulent strains and their widespread prevalence in Indian poultry farms. The presence of 272I and 279D in VP2 protein of five field isolates may explain possible cause of Gumboro intermediate plus vaccine failure in prevention of the outbreaks. However, mortality caused by other three strains which are antigenically similar to VP1 of intermediate plus vaccine strains could not be explained and the possible role of their unique VP1 in enhancing the pathogenesis needs to be investigated further.

Infectious bursal disease virus in poultry: current status and future prospects

Infectious bursal disease virus (IBDV) affects immature B lymphocytes of the bursa of Fabricius and may cause significant immunosuppression. It continues to be a leading cause of economic losses in the poultry industry. IBDV, having a segmented double-stranded RNA genome, is prone to genetic variation. Therefore, IBDV isolates with different genotypic and phenotypic diversity exist. Understanding these features of the virus and the mechanisms of protective immunity elicited thereof is necessary for developing vaccines with improved efficacy. In this review, we highlighted the pattern of virus evolution and new developments in prophylactic strategies, mainly the development of new generation vaccines, which will continue to be of interest for research as well as field application in the future.

Avibirnavirus VP4 Protein Is a Phosphoprotein and Partially Contributes to the Cleavage of Intermediate Precursor VP4-VP3 Polyprotein

Birnavirus-encoded viral protein 4 (VP4) utilizes a Ser/Lys catalytic dyad mechanism to process polyprotein. Here three phosphorylated amino acid residues Ser538, Tyr611 and Thr674 within the VP4 protein of the infectious bursal disease virus (IBDV), a member of the genus Avibirnavirus of the family Birnaviridae, were identified by mass spectrometry. Anti-VP4 monoclonal antibodies finely mapping to phosphorylated (p)Ser538 and the epitope motif ⁵³⁰PVVDGIL⁵³⁶ were generated and verified. Proteomic analysis showed that in IBDV-infected cells the VP4 was distributed mainly in the cytoskeletal fraction and existed with different isoelectric points and several phosphorylation modifications. Phosphorylation of VP4 did not influence the aggregation of VP4 molecules. The proteolytic activity analysis verified that the pTyr611 and pThr674 sites within VP4 are involved in the cleavage of viral intermediate precursor VP4-VP3. This study demonstrates that IBDV-encoded VP4 protein is a unique phosphoprotein and that phosphorylation of Tyr611 and Thr674 of VP4 affects its serine-protease activity.

The association of receptor of activated protein kinase C 1(RACK1) with infectious bursal disease virus viral protein VP5 and voltage-dependent anion channel 2 (VDAC2) inhibits apoptosis and enhances viral replication

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Our previous report indicates that IBDV VP5 induces apoptosis via interaction with voltage-dependent anion channel 2 (VDAC2). However, the underlying molecular mechanism is still unclear. We report here that receptor of activated protein kinase C 1 (RACK1) interacts with both VDAC2 and VP5 and that they could form a complex. We found that overexpression of RACK1 inhibited IBDV-induced apoptosis in DF-1 cells and that knockdown of RACK1 by small interfering RNA induced apoptosis associated with activation of caspases 9 and 3 and suppressed IBDV growth. These results indicate that RACK1 plays an antiapoptotic role during IBDV infection via interaction with VDAC2 and VP5, suggesting that VP5 sequesters RACK1 and VDAC2 in the apoptosis-inducing process.

Molecular epidemiology studies on partial sequences of both genome segments reveal that reassortant infectious bursal disease viruses were dominantly prevalent in southern China during 2000-2012

A molecular epidemiology study of infectious bursal disease viruses (IBDVs) isolated from seven provinces in southern China during the years 2000-2012 was performed based on partial sequences of genome segments A and B, namely the hypervariable region of the A-VP2 gene (A-vVP2) and the b fragment of VP1 gene (B-VP1b) from a total of 91 field isolates. Sequence analysis based on vVP2 revealed that 72 out of 91 isolates had the same characteristic amino acid (aa) sequences as vvIBDV. The mutation of D212N in A-vVP2 has become prevalent in the recent isolates. The origin of the field isolates with vvIBDV characteristic amino acid residues was complex, evidenced by the findings that more than one subgroup of strains prevailed in each province. When B-VP1b was analyzed, there were three lineages among the field isolates, and none of the isolates had a relationship to vvIBDV-related segment B. Phylogenetic analysis of both segments revealed that only a few isolates (13/91) had the same genetic relatives in consensus trees based on segments A and B, whereas the majority of the isolates (85.71%, 78/91) were identified to be naturally reassorted strains. Based on the origin of each segment, at least six types of reassortant IBDVs prevailed in southern China, three of which were shown to be dominant: segment A from vvIBDV and B from attenuated IBDV, segment A of vvIBDV and B from 002-73-like IBDV, and segment A of vvIBDV and B from HLJ0504 or a similar strain. Our findings suggest that both genomic segments of field IBDVs has been evolving, and continuous monitoring of the evolution of field IBDV genome is therefore urgently needed in the control of IBDV.

Triplet amino acids located at positions 145/146/147 of the RNA polymerase of very virulent infectious bursal disease virus contribute to viral virulence

Infectious bursal disease virus (IBDV) causes an economically significant disease of chickens worldwide. The emergence of very virulent IBDV (vvIBDV) has brought more challenges for effective prevention of this disease. The molecular basis for the virulence of vvIBDV is not fully understood. In this study, 20 IBDV strains were analysed phylogenically and clustered in three branches based on their full-length B segments. The amino acid triplet located at positions 145/146/147 of VP1 was found highly conserved in branch I non-vvIBDVs as asparagine/glutamic acid/glycine (NEG), in branch II vvIBDVs as threonine/glutamic acid/glycine (TEG) and in branch III vvIBDVs as threonine/aspartic acid/asparagine (TDN). Further studies showed that the three amino acids play a critical role in the replication and pathogenicity of vvIBDV. Substitution of the TDN triplet with TEG or NEG reduced viral replication and pathogenicity of the vvIBDV HuB-1 strain in chickens. However, the replication of the attenuated IBDV Gt strain was reduced in chicken embryo fibroblast cells, whilst it was enhanced in the bursa by substituting NEG with TEG or TDN. The exchange of the three amino acids was also found to be capable of affecting the polymerase activity of VP1. The important role of segment B in the pathogenicity of IBDV was confirmed in this study. These results also provided new insights into the mechanism of the virulence of vvIBDVs and may offer new targets for their attenuation to develop potential vaccines using reverse genetics.

Overexpression of microRNA gga-miR-21 in chicken fibroblasts suppresses replication of infectious bursal disease virus through inhibiting VP1 translation

Previously we have identified a series of cellular miRNA molecules up- or down-regulated in infectious bursal disease virus (IBDV) infected chicken embryo fibroblasts and Bursa of Fabricius with gene microarray analysis. Here we studied in detail a relatively well studied miRNA, gga-miR-21, for better understanding miRNAs involvement in IBDV-host interactions. Chicken pri-gga-miRNA-21 and a control miRNA Caenorhabditis elegans pri-cel-lin-4 gene were cloned into a lentiviral vector, respectively. The resulting recombinant lentiviruses were used to infect chicken fibroblast cell line DF-1, and two stable cell lines, DF-miR-21 (overexpressing gga-miR-21) and DF-lin-4 (overexpressing cel-lin-4), were selected. Replication of IBDV in DF-miR-21, DF-lin-4 and DF-1 cells were compared and molecular mechanism of IBDV replication alteration was explored using bioinformatics, reporter gene system, qRT-PCR and Western blot analysis. IBDV replication was markedly lower in DF-miR-21 than in DF-lin-4 or DF-1 cells. A gga-miR-21 target sequence was identified within IBDV VP1 gene (1713-1734bp). Fusion of a 520nt long partial IBDV VP1 gene containing the target with a luciferase gene resulted in significantly lower transient luciferase activity in DF-miR-21 cells as compared to that in DF-lin-4 or DF-1 cell. Following IBDV infection of the cell lines, VP1 protein level in DF-miR-21 cells was dramatically lower than that in DF-lin-4 or DF-1 cells but VP1 mRNA level was not different. The finding indicated that gga-miR-21 could suppress IBDV replication through down regulating IBDV VP1 expression at translational level.

Mutations of residues 249 and 256 in VP2 are involved in the replication and virulence of infectious Bursal disease virus

Infectious bursal disease virus (IBDV) is a pathogen of worldwide significance to the poultry industry. Although the PDE and PFG domains of the capsid protein VP2 contribute significantly to virulence and fitness, the detailed molecular basis for the pathogenicity of IBDV is still not fully understood. Because residues 253 and 284 of VP2 are not the sole determinants of virulence, we hypothesized that other residues involved in virulence and fitness might exist in the PDE and PFG domains of VP2. To test this, five amino acid changes selected by sequence comparison of the PDE and PFG domains of VP2 were introduced individually using a reverse genetics system into the virulent strain (rGx-F9VP2). Then reverse mutations of the selected residues 249 and 256 were introduced individually into the attenuated strain (rGt). Seven modified viruses were generated and evaluated in vitro (CEF cells) and in vivo (SPF chicken). For residue 249, Q249R could elevate in vitro and reduce in vivo the replication of rGx-F9VP2 while R249Q could reduce in vitro and elevate in vivo the replication of rGt; meanwhile Q249R reduced the virulence of rGx-F9VP2 while R249Q increased the virulence of rGt, which indicated that residue 249 significantly contributed to the replication and virulence of IBDV. For residue 256, I256V could elevate in vitro and reduce in vivo the replication of rGx-F9VP2 while V256I could reduce in vitro but didn't change in vivo the replication of rGt; although V256I didn't increase the virulence of rGt, I256V obviously reduced the virulence of virulent IBDV. The present results demonstrate for the first time, to different extent, residues 249 and 256 of VP2 are involved in the replication efficiency and virulence of IBDV; this is not only beneficial to further understanding of pathogenic mechanism but also to the design of newly tailored vaccines against IBDV.

Molecular characterization of two Bangladeshi infectious bursal disease virus isolates using the hypervariable sequence of VP2 as a genetic marker

Two Bangladeshi infectious bursal disease virus (IBDV) isolates collected in 2007, termed GB1 and GB3, were subjected to comparative sequencing and phylogenetic analyses. Sequence analysis of a 474-bp hypervariable region in the VP2 gene revealed that among four major amino acid substitutions observed in the strains, two were unique to GB1 and GB3 (Ser217Leu and Ala270Thr) while one substitution was only found in GB1 (Asn299Ser). Among IBDVs from Bangladesh including GB1 and GB3, the rate of identity and homology was around 97~99%. The amino acid sequences of GB1 and GB3 differ from those of previous Bangladeshi IBDV isolates and contain amino acid substitutions Pro222Ala and Asn299Ser (in GB3 only). Phylogenetic analysis revealed that GB1 and GB3 are grouped with other very virulent IBDVs of European and American origin in contrast to two previously isolated Bangladeshi IBDV strains (GenBank accession Nos. AF362776 and AF260317), which belong to the Asian group. It was concluded that GB1 and GB3 belong to a very virulent group of IBDVs. However, amino acid sequences of GB1 and GB3 differ from those of the other Bangladeshi IBDVs by one or two amino acids encoded in the hypervariable region of the VP2 gene.

Genomic sequence analysis of a new reassortant infectious bursal disease virus from commercial broiler flocks in Central China

We report the complete nucleotide sequence of a reassortant infectious bursal disease (IBD) virus (IBDV) HN isolate from commercial broiler flocks in central China. The genome consisted of 3,232 and 2,652 nucleotides in the coding regions of segments A and B, respectively. Alignment of both nucleotide and deduced amino acid sequences and phylogenetic analysis revealed that the genome segments A and B of HN were derived from the attenuated strain B87 and the VV strain OKYM. This is a new reassortant IBDV strain that has emerged in nature, involving segment A of a cell-culture-adapted attenuated vaccine strain B87.

Partial VP1 sequencing of Brazilian infectious bursal disease virus strains

Infectious bursal disease virus (IBDV) is classified according to the antigenicity and virulence into classical virulent (cv), very virulent (vv), and antigenic variant strains. The molecular basis for the IBDV antigenic variation is well established and is associated to the capsid protein, VP2 (gene VP2 of segment A), whereas both VP2 and the RNA-dependent RNA polymerase, VP1 (gene VP1 of segment B), have been correlated with the virulence. In this study, seventeen Brazilian IBDV samples previously characterized by the VP2 gene as cv (three) and vv (fourteen) strains were genetically and molecularly analyzed for their VP1 gene. All of the strains kept with the same cv or vv classification except one sample, Br/03/DR. This sample was classified as vv by its VP2 gene, but it was most closely related to the cv strains by its VP1 partial sequence and phylogeny. Studies on the phylogeny of VP1 have suggested a possible reassortment event that originated the vvVP1. In this case, the sample carrying vvVP2 and cvVP1 could be a descendant of IBDV ancestors prior to the reassortment of vvVP1; alternatively, it could be the result of a genetic exchange between the segments of different strains or with a live attenuated vaccine. Nevertheless, this is the first report of natural genetic reassortment of IBDV in Brazil.

A simple and efficient method to rescue very virulent infectious bursal disease virus using SPF chickens

Reverse genetic systems for efficient generation of very virulent infectious bursal disease virus (vvIBDV) are currently limited. In this study, we have developed a simple and efficient way to rescue vvIBDV using SPF chickens. The genome of a vvIBDV strain, HLJ0504, flanked by hammerhead and hepatitis delta ribozyme sequences, was cloned downstream of the cytomegalovirus enhancer and the chicken beta-actin promoter of the vector pCAGGS. After transfection of DF-1 cells, cell suspensions were injected into the bursa organ of three-week-old SPF chickens. Using this system, vvIBDV was recovered at high titers after one passage, and the rescued vvIBDV remained highly lethal to SPF chickens. This simple and efficient method to rescue vvIBDV will be a valuable tool for better understanding the molecular virulence determinants of vvIBDV.

Different domains of the RNA polymerase of infectious bursal disease virus contribute to virulence

BACKGROUND

Infectious bursal disease virus (IBDV) is a pathogen of worldwide significance to the poultry industry. IBDV has a bi-segmented double-stranded RNA genome. Segments A and B encode the capsid, ribonucleoprotein and non-structural proteins, or the virus polymerase (RdRp), respectively. Since the late eighties, very virulent (vv) IBDV strains have emerged in Europe inducing up to 60% mortality. Although some progress has been made in understanding the molecular biology of IBDV, the molecular basis for the pathogenicity of vvIBDV is still not fully understood.

METHODOLOGY, PRINCIPAL FINDINGS

Strain 88180 belongs to a lineage of pathogenic IBDV phylogenetically related to vvIBDV. By reverse genetics, we rescued a molecular clone (mc88180), as pathogenic as its parent strain. To study the molecular basis for 88180 pathogenicity, we constructed and characterized in vivo reassortant or mosaic recombinant viruses derived from the 88180 and the attenuated Cu-1 IBDV strains. The reassortant virus rescued from segments A of 88180 (A88) and B of Cu-1 (BCU1) was milder than mc88180 showing that segment B is involved in 88180 pathogenicity. Next, the exchange of different regions of BCU1 with their counterparts in B88 in association with A88 did not fully restore a virulence equivalent to mc88180. This demonstrated that several regions if not the whole B88 are essential for the in vivo pathogenicity of 88180.

CONCLUSION, SIGNIFICANCE

The present results show that different domains of the RdRp, are essential for the in vivo pathogenicity of IBDV, independently of the replication efficiency of the mosaic viruses.

Critical role for voltage-dependent anion channel 2 in infectious bursal disease virus-induced apoptosis in host cells via interaction with VP5

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Although IBDV-induced host cell apoptosis has been established, the underlying molecular mechanism is still unclear. We report here that IBDV viral protein 5 (VP5) is a major apoptosis inducer in DF-1 cells by interacting with the voltage-dependent anion channel 2 (VDAC2) in the mitochondrion. We found that in DF-1 cells, VP5-induced apoptosis can be completely abolished by 4,4'-diisothiocyanatostibene-2,2'-disulfonic acid (DIDS), an inhibitor of VDAC. Furthermore, knockdown of VDAC2 by small interfering RNA markedly inhibits IBDV-induced apoptosis associated with decreased caspase-9 and -3 activation and cytochrome c release, leading to increased IBDV growth in host cells. Thus, VP5-induced apoptosis during IBDV infection is mediated by interacting with VDAC2, a protein that appears to restrict viral replication via induction of cell death.

Genotyping of Canadian field strains of infectious bursal disease virus

For this retrospective study, infectious bursal disease virus (IBDV) was detected in 134 bursal samples that originated from flocks with conditions such as airsacculitis, tracheitis, pneumonia, septicaemia, inclusion body hepatitis, coccidiosis, and/or a history of production problems without overt clinical symptoms. Samples were from seven Canadian provinces: Ontario, Quebec, Manitoba, British Columbia, Nova Scotia, Alberta, and Newfoundland and Labrador. Viral RNA was identified in bursae with moderate to severe and acute to chronic bursal damage. The ages of the flocks from which samples were collected ranged from 3 to 63 days. Following reverse transcriptase-polymerase chain reaction the nucleotide sequence of the VP2 hypervariable region was determined and compared with sequences available in GenBank. The most common Canadian IBDV field strains were North-American variant viruses. Forty-four viruses were highly related (97.5% to 100.0%) to the US IBDV strain NC171. Moreover, 16 field viruses whose VP2 sequences were 99.2% to 100% identical to the South African 05SA8 IBDV strain appeared closely related to the NC171 group. Delaware E-related field viruses, 98.3% to 100.0% identical to the prototype virus, were identified in 33 samples. Thirty-four Canadian IBDVs showed the highest identity, 94.2% to 98.3%, to US IBDV strain 586. Five samples contained vaccine-related viruses, while two field strains showed the best match to Del A (United States) and IBDV strains SP_04_02 (Spain). Very virulent IBDVs were not detected in Canada.

Molecular characteristics of segment B of seven very virulent infectious bursal disease viruses isolated in China

Infectious bursal disease virus (IBDV) is a birnavirus that causes immunosuppressive disease in chickens. Segment B of IBDV encodes the RNA-dependent RNA polymerase VP1, which is involved in virulence. We sequenced and analyzed segment B from seven Chinese IBDV isolates, all belonging to very virulent IBDV (vvIBDV), and clustered into Branches II and III. Phylogenetic analysis suggested that segment B of the HLJ isolates in Branch II might have originated from an unidentified host, and HuB-1 might have originated in Europe. Eight aa (4V, 61I, 145T, 287A, 508K, 511S, 646S, and 687P) were conserved in Branches II and III, and may contain potential segment B virulence determinants. Five aa (146D, 242E, 390M, 562P, and 695R) were found only in Branch III, and may be origin characteristics. Moreover, 55T and 63A in the 5'-untranslated region (UTR), and 2786C in the 3'-UTR were conserved in vvIBDV and may function in UTR secondary structure.

Sequence analysis of both genome segments of three Croatian infectious bursal disease field viruses

In order to determine the mutations responsible for virulence, three Croatian field infectious bursal disease viruses (IBDV), designated Cro-Ig/02, Cro-Po/00, and Cro-Pa/98 were characterized. Coding regions of both genomic segments were sequenced, and the nucleotide and deduced amino acid sequences were compared with previously reported full-length sequenced IBDV strains. Phylogenetic analysis, based on the nucleotide and deduced amino acid sequences of polyprotein and VP1, was performed. Eight characteristic amino acid residues, that were common to very virulent (vv) IBDV, were detected on polyprotein: 222A, 256I, 294I, 451L, 685N, 715S, 751D, and 1005A. All eight were found in Cro-Ig/02 and Cro-Po/00. C-Pa/98 had all the characteristics of an attenuated strain, except for glutamine on residue 253, which is common for vv, classical virulent, and variant strains. Between less virulent and vvIBDV, three substitutions were found on VP5: 49 G --> R, 79 --> F, and 137 R --> W. In VP1, there were nine characteristic amino acid residues common to vvwIBDV: 146D, 147N, 242E, 390M, 393D, 511S, 562P, 687P, and 695R. All nine residues were found in A-Ig/02, and eight were found in B-Po/00, which had isoleucine on residue 390. Based on our analyses, isolates Cro-Ig/02 and Cro-Po/00 were classified with vv IBDV strains. C-Pa/98 shared all characteristic amino acid residues with attenuated and classical virulence strains, so it was classified with those.

Attenuation of very virulent infectious bursal disease virus and comparison of full sequences of virulent and attenuated strains

A very virulent strain of infectious bursal disease virus (IBDVks) was isolated from the bursae of Fabricius of IBDV-affected broiler chickens. Following 43 serial passages in specific pathogen-free embryonated eggs, an attenuated strain was established (IBDVmb). Dosages of IBDVmb in the range 10(2) to 10(4) embryo infective dose of 50% were found to be safe and protective for commercial chicks. Chickens vaccinated with live vaccine containing IBDVmb responded with precipitating and type-specific neutralizing antibodies, and were immune to subsequent challenge with a very virulent IBDV. IBDVmb has been used as an attenuated vaccine throughout the world since 1993. A comparison of the full sequences of the virulent and attenuated strains (IBDVks and IBDVmb, respectively) revealed seven nucleotides that were different, four of them leading to changes in the amino-acid sequence. Comparison of the protein sequence of these strains and published sequences of very virulent and attenuated phenotypes lead us to suggest that the novel difference responsible for virulence of the Israeli strains are: residue 272 (VP2, very conserved site) and residue 527 (VP4), both in segment A, and in segment B (VP1) residues 96 and 161 (both conserved). Our study strengthens the possibility that more than one protein is involved in IBDV attenuation. In all reports, including ours, virulence was reduced without affecting antigenicity of the neutralizing epitopes in VP2. This could have practical implications for attenuated-vaccine development.