Picroside II disrupts IBDV replication via targeting VP1 polymerase.

Infectious bursal disease virus (IBDV) is a highly contagious virus with a dsRNA genome, predominantly infecting chickens and causing significant economic losses due to high mortality rates. The emergence of recombinant, novel variant, and highly virulent strains that evade current vaccines has led to frequent epidemics and outbreaks in the poultry industry. The lack of targeted antivirals for IBDV underscores the pressing requirement to develop potent therapeutic options. Within this framework, our research investigated the effectiveness of picroside II, a naturally derived iridoid glycoside, against viruses in DF-1 cells. Our findings demonstrate that picroside II significantly inhibits viral replication, with its efficacy increasing proportionally to the dosage administered. Through time-addition and antiviral duration analysis, we determined that picroside II therapeutically blocks IBDV replication, with its effects persisting for over 72 hours. Further investigation revealed that picroside II specifically inhibits the cellular replication stage of IBDV's lifecycle. Additionally, our findings indicate that picroside II impairs VP1 polymerase activity by binding to the active pocket, which significantly disrupts the interaction between VP1 and VP3. Mutations at three critical binding sites on VP1 not only impair virus replication but also hinder polymerase function and disrupt VP1-VP3 interactions. Collectively, these results demonstrate that picroside II, by inhibiting viral polymerase activity, represents a promising antiviral agent against IBDV.

Chicken PRMT3 facilitates IBDV replication.

Protein arginine methyltransferases (PRMTs) in mammals play a role in various signaling pathways, such as virus infection, inflammasome responses, and cancer growth. While some PRMTs have been found to regulate interferon production in mammals, the mechanism in chickens remains to be fully understood. This study focused on investigating the function of chicken PRMTs. Our findings indicate that chicken PRMTs act as inhibitors of interferon production in response to dsRNA or MDA5 stimulation. Each PRMT is involved in different stages of interferon induction through the MDA5-MAVS-TBK1 pathway. Furthermore, we observed the colocalization of multiple PRMTs with the viral protein VP3 of infectious bursal disease virus (IBDV). Among the chicken PRMTs studied, PRMT3 was found to be widely expressed in various organs and its expression was upregulated during IBDV infection. Notably, PRMT3 supported IBDV replication, as demonstrated by ectopic expression and inhibition studies using SGC-707. Silencing of PRMT3 led to enhanced interferon production and inhibition of IBDV replication. This study provides novel insights into the role of chicken PRMTs, particularly PRMT3, in promoting IBDV replication by suppressing interferon signaling.

Integrative RNA-seq and ChIP-seq analysis unveils metabolic regulation as a conserved antiviral mechanism of chicken p53.

The tumor suppressor p53, primarily functioning as a transcription factor, has exhibited antiviral capabilities against various viruses in chickens, including infectious bursal disease virus (IBDV), avian leukosis virus subgroup J (ALV-J), and avian infectious laryngotracheitis virus (ILTV). Nevertheless, the existence of a universal antiviral mechanism employed by chicken p53 (chp53) against these viruses remains uncertain. This study conducted a comprehensive comparison of molecular networks involved in chp53's antiviral function against IBDV, ALV-J, and ILTV. This was achieved through an integrated analysis of ChIP-seq data, examining chp53's genome-wide chromatin occupancy, and RNA-seq data from chicken cells infected with these viruses. The consistent observation of chp53 target gene enrichment in metabolic pathways, confirmed via ChIP-qPCR, suggests a ubiquitous regulation of host cellular metabolism by chp53 across different viruses. Further genome binding motif conservation analysis and transcriptional co-factor prediction suggest conserved transcriptional regulation mechanism by which chp53 regulates host cellular metabolism during viral infection. These findings offer novel insights into the antiviral role of chp53 and propose that targeting the virus-host metabolic interaction through regulating p53 could serve as a universal strategy for antiviral therapies in chickens.IMPORTANCEThe current study conducted a comprehensive analysis, comparing molecular networks underlying chp53's antiviral role against infectious bursal disease virus (IBDV), avian leukosis virus subgroup J (ALV-J), and avian infectious laryngotracheitis virus (ILTV). This was achieved through a combined assessment of ChIP-seq and RNA-seq data obtained from infected chicken cells. Notably, enrichment of chp53 target genes in metabolic pathways was consistently observed across viral infections, indicating a universal role of chp53 in regulating cellular metabolism during diverse viral infections. These findings offer novel insights into the antiviral capabilities of chicken p53, laying a foundation for the potential development of broad-spectrum antiviral therapies in chickens.

Postbiotic-based recombinant receptor activator of NF-κB ligand enhanced oral vaccine efficiency in chicken.

Functional M cells are differentiated by receptor activator of NF-κB ligand (RANKL) and capture of luminal antigens to initiate immune responses. We aimed to use postbiotic-based recombinant chicken RANKL (cRANKL) to promote M cell differentiation and test the efficacy of oral vaccines. Chicks were divided into three groups that were administered phosphate-buffered saline (PBS), cell extracts of wild-type Lactococcus lactis subsp. lactis IL1403 (WT_CE), or cell extracts of recombinant L. lactis expressing cRANKL (cRANKL_CE). The expression of the M cell marker was measured, and the gut microbiome was profiled. The efficiency of the infectious bursal disease (IBD) vaccine was tested after 12 consecutive days of administering cRANKL_CE. The chickens that were administered cRANKL_CE (p = 0.038) had significantly higher Annexin A5 (ANXA5) mRNA expression levels than those in the PBS group (PBS vs. WT_CE, p = 0.657). In the gut microbiome analysis, no significant changes were observed. However, the relative abundance of Escherichia-Shigella was negatively correlated (r = - 0.43, p = 0.019) with ANXA5 mRNA expression in Peyer's patches. cRANKL_CE/IBD (p = 0.018) had significantly higher IBD-specific faecal IgA levels than PBS/IBD (PBS/IBD vs. WT_CE/IBD, p = 0.217). Postbiotic-based recombinant cRANKL effectively improved the expression of M cell markers and the efficiency of oral vaccines. No significant changes were observed in the gut microbiome after administration of postbiotic-based recombinant cRANKL. This strategy can be used for the development of feed additives and adjuvants. KEY POINTS: • Postbiotic-based recombinant cRANKL enhanced the expression of ANXA5 in chicken. • The relative abundance of Escherichia-Shigella was negatively correlated with ANXA5 expression. • Postbiotic-based recombinant cRANKL effectively improved the efficiency of oral vaccine.

Emergence of the novel infectious bursal disease virus variant in vaccinated poultry flocks in Egypt.

Since the detection of antigenically atypical very virulent Infectious bursal disease viruses (vvIBDV) in Egypt in 1999, the country has been experiencing recurrent outbreaks with high mortality rates and typical gross lesions associated with typical vvIBDV. However, a significant change occurred in 2023, marked by a notable increase in reported subclinical IBDV cases. To evaluate the field situation, samples from 21 farms in 2023 and 18 farms from 2021 and 2022, all of which had experienced IBD outbreaks based on clinical diagnosis, were collected, and subjected to VP2-HVR sequencing. Phylogenetic analysis revealed that all samples collected in 2021 and 2022 clustered with classical virulent strains and vvIBDV. In 2023, one sample clustered with the Egyptian vvIBDV, another with classical virulent IBDV, and the rest with the novel variant IBDV (nVarIBDV) circulating in China. The alignment of deduced amino acid sequences for VP2 showed that all Egyptian classic virulent strains were identical to the Winterfield or Lukert strains, while vvIBDV strains exhibited two out of the three typical residues found in Egyptian vvIBDV, namely Y220F and G254S, but not A321T. Meanwhile, all Egyptian variant strains exhibited typical residues found in nVarIBDV. However, all Egyptian variants showed a mutation at position 321 (321V), which represents the most exposed part of the capsid and is known to have a massive impact on IBDV antigenicity, except for one sample that had 318G instead. This report highlights the emergence of a new variant IBDV in Egypt, clustered with the Chinese new variants, spreading subclinically in broiler farms across a wide geographic area.RESEARCH HIGHLIGHTS New variant IBDV which emerged in Egypt clustered with Chinese nVarIBDV.nVarIBDV spread subclinically across a wide geographic area.Mutation at 321 represents capsid's most exposed part, a defining feature.Antigenically modified vvIBDV still circulating in Egypt with typical lesions.

Loop PDE of viral capsid protein is involved in immune escape of the emerging novel variant infectious bursal disease virus.

Infectious bursa disease (IBD) is an acute, highly contactable, lethal, immunosuppressive infectious disease caused by the Infectious bursa disease virus (IBDV). Currently, the emerged novel variant IBDV (nVarIBDV) and the sustainedly prevalent very virulent IBDV (vvIBDV) are the two most prevalent strains of IBDV in China. The antigenic properties of the two prevalent strains differed significantly, which led to the escape of nVarIBDV from the immune protection provided by the existing vvIBDV vaccine. However, the molecular basis of the nVarIBDV immune escape remains unclear. In this study, we demonstrated, for the first time, that residues 252, 254, and 256 in the PDE of VP2 are involved in the immune escape of the emerging nVarIBDV. Firstly, the IFA-mediated antigen-antibody affinity assay showed that PBC and PDE of VP2 could affect the affinity of vvIBDV antiserum to VP2, of which PDE was more significant. The key amino acids of PDE influencing the antigen-antibody affinity were also identified, with G254N being the most significant, followed by V252I and I256V. Then the mutated virus with point or combined mutations was rescued by reverse genetics. it was further demonstrated that mutations of V252I, G254N, and I256V in PDE could individually or collaboratively reduce antigen-antibody affinity and interfere with antiserum neutralization, with G254N being the most significant. This study revealed the reasons for the widespread prevalence of nVarIBDV in immunized chicken flocks and provided innovative ideas for designing novel vaccines that match the antigen of the epidemic strain.

OASL suppresses infectious bursal disease virus replication by targeting VP2 for degrading through the autophagy pathway.

Infectious bursal disease (IBD) is an acute and fatal immunosuppressive disease caused by infectious bursal disease virus (IBDV). As an obligate intracellular parasite, IBDV infection is strictly regulated by host factors. Knowledge on the antiviral activity and possible mechanism of host factors might provide the theoretical basis for the prevention and control of IBD. In this study, RNA-sequencing results indicated that many host factors were induced by IBDV infection, among which the expression levels of OASL (2´,5´-oligadenylate synthetase-like protein) was significantly upregulated. OASL overexpression significantly inhibited IBDV replication, whereas OASL knockdown promoted IBDV replication. Interestingly, the antiviral ability of OASL was independent of its canonical enzymatic activity, i.e., OASL targeted viral protein VP2 for degradation, depending on the autophagy receptor p62/SQSTM1 in the autophagy pathway. Additionally, the 316 lysine (K) of VP2 was the key site for autophagy degradation, and its replacement with arginine disrupted VP2 degradation induced by OASL and enhanced IBDV replication. Importantly, our results for the first time indicate a unique and potent defense mechanism of OASL against double-stranded RNA virus by interaction with viral proteins, which leads to their degradation. IMPORTANCE: OASL (2´,5´-oligadenylate synthetase-like protein) exhibits broad-spectrum antiviral effects against single-stranded RNA viruses in mammals, potentially serving as a promising target for novel antiviral strategies. However, its role in inhibiting the replication of double-stranded RNA viruses (dsRNA viruses), such as infectious bursal disease virus (IBDV), in avian species remains unclear. Our findings indicated a unique and potent defense mechanism of OASL against dsRNA viruses. It has been previously shown in mammals that OASL inhibits virus replication through increasing interferon production. The groundbreaking aspect of our study is the finding that OASL has the ability to interact with IBDV viral protein VP2 and target it for degradation and thus exerts its antiviral effect. Our results reveal the interaction between avian natural antiviral immune response and IBDV infection. Our study not only enhances our understanding of bird defenses against viral infections but can also inform strategies for poultry disease management.

Research Note: Characterization and phylodynamic analysis of new infectious bursal disease virus variants circulating in Argentina.

Infectious Bursal Disease is a highly contagious disease that affects young chickens and leads to significant economic losses. Its causal agent is a double-stranded RNA virus that, due to its high error rate during the replication process, gives rise to a constant generation of new virus variants. Until 2014, strains of Infectious Bursal Diseases Virus (IBDV) belonging to genogroup 4 predominated in Argentina, but there have been no reports since then regarding the circulating genogroups in poultry. In this study, 11 recent sequences of Argentine from the hypervariable region of VP2 protein (hvVP2) were analyzed to determine their genogroup, origin, evolution, and amino acid sequence. Samples from chickens showing signs of IBDV infection were collected, and the hvVP2 region was amplified using RT-PCR, followed by sequencing. The results indicated that the analyzed strains belong to genogroup 2, with an estimated evolutionary rate of 1.74 × 10-3 substitutions/site/year. It is speculated that the predominant group of sequences began to spread in Argentina around 2014 and had its origins in China. Another sample is related to strains from South Korea and is not closely linked to the main group. Furthermore, the predicted amino acid sequences show similarity to strains that can evade vaccine-induced immunity. These findings underscore the importance of active surveillance in poultry to mitigate losses caused by IBDV.

Novel variant infectious bursal disease virus diminishes FAdV-4 vaccination and enhances pathogenicity of FAdV-4.

Infectious bursal disease virus (IBDV) caused an acute and highly contagious infectious disease characterized by severe immunosuppression, causing considerable economic losses to the poultry industry globally. Although this disease was well-controlled under the widely use of commercial vaccines in the past decades, the novel variant IBDV strains emerged recently because of the highly immunized-selection pressure in the field, posting new threats to poultry industry. Here, we reported novel variant IBDV is responsible for a disease outbreak, and assessed the epidemic and pathogenicity of IBDV in this study. Moreover, we constructed a challenge model using Fowl adenovirus serotype 4 (FAdV-4) to study on the immunosuppressive effect. Our findings underscore the importance of IBDV surveillance, and provide evidence for understanding the pathogenicity of IBDV.

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.

Molecular epidemiology of infectious bursal disease virus in the Near East and Persian Gulf regions.

RESEARCH HIGHLIGHTS: Different field IBDVs were found to circulate in the Near and Middle East.Multiple atypical genotypes (A3B1, A4B1, A6B1) were found to circulate extensively.Traditional very virulent IBDVs (A3B2) were a minority of the detected strains.Viral exchanges can be hypothesized between the region and different continents.

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.

Protein kinase Cdc7 supports viral replication by phosphorylating Avibirnavirus VP3 protein.

IMPORTANCE: The Avibirnavirus infectious bursal disease virus is still an important agent which largely threatens global poultry farming industry economics. VP3 is a multifunctional scaffold structural protein that is involved in virus morphogenesis and the regulation of diverse cellular signaling pathways. However, little is known about the roles of VP3 phosphorylation during the IBDV life cycle. In this study, we determined that IBDV infection induced the upregulation of Cdc7 expression and phosphorylated the VP3 Ser13 site to promote viral replication. Moreover, we confirmed that the negative charge addition of phosphoserine on VP3 at the S13 site was essential for IBDV proliferation. This study provides novel insight into the molecular mechanisms of VP3 phosphorylation-mediated regulation of IBDV replication.

Effect of Intermediate Plus Vaccine and vvIBDV on Bursa Secretory Cells and Their Glycoprotein Production.

There are two types of secretory cells in the chicken bursa of Fabricius (BF): (a) interfollicular epithelial cells (IFE), and (b) bursal secretory dendritic cells (BSDC) in the medulla of bursal follicles. Both cells produce secretory granules, and the cells are highly susceptible to IBDV vaccination and infection. Before and during embryonic follicular bud formation, an electron-dense, scarlet-acid fuchsin positive substance emerges in the bursal lumen, the role of which is unknown. In IFE cells, IBDV infection may induce rapid granular discharge, and in several cells, peculiar granule formation, which suggests that the glycosylation of protein is injured in the Golgi complex. In control birds, the discharged BSDC granules appear in membrane-bound and subsequently solubilized, fine-flocculated forms. The solubilized, fine-flocculated substance is Movat-positive and can be a component of the medullary microenvironment, which prevents the medullary B lymphocytes from nascent apoptosis. Vaccination interferes with the solubilization of the membrane-bound substance, resulting in: (i) aggregation of a secreted substance around the BSDC, and (ii) solid lumps in the depleted medulla. The non-solubilized substance is possibly not "available" for B lymphocytes, resulting in apoptosis and immunosuppression. In IBDV infection, one part of the Movat-positive Mals fuse together to form a medullary, gp-containing "cyst". The other part of Mals migrate into the cortex, recruiting granulocytes and initiating inflammation. During recovery the Movat-positive substance appears as solid, extracellular lumps between the cells of FAE and Mals. Possibly the Mals and Movat-positive extracellular lumps glide into the bursal lumen via FAE to eliminate cell detritus from the medulla.

The Formation and Function of Birnaviridae Virus Factories.

The use of infectious bursal disease virus (IBDV) reverse genetics to engineer tagged reporter viruses has revealed that the virus factories (VFs) of the Birnaviridae family are biomolecular condensates that show properties consistent with liquid-liquid phase separation (LLPS). Although the VFs are not bound by membranes, it is currently thought that viral protein 3 (VP3) initially nucleates the formation of the VF on the cytoplasmic leaflet of early endosomal membranes, and likely drives LLPS. In addition to VP3, IBDV VFs contain VP1 (the viral polymerase) and the dsRNA genome, and they are the sites of de novo viral RNA synthesis. Cellular proteins are also recruited to the VFs, which are likely to provide an optimal environment for viral replication; the VFs grow due to the synthesis of the viral components, the recruitment of other proteins, and the coalescence of multiple VFs in the cytoplasm. Here, we review what is currently known about the formation, properties, composition, and processes of these structures. Many open questions remain regarding the biophysical nature of the VFs, as well as the roles they play in replication, translation, virion assembly, viral genome partitioning, and in modulating cellular processes.

PRMT5 Facilitates Infectious Bursal Disease Virus Replication through Arginine Methylation of VP1.

The infectious bursal diseases virus (IBDV) polymerase, VP1 protein, is responsible for transcription, initial translation and viral genomic replication. Knowledge about the new kind of post-translational modification of VP1 supports identification of novel drugs against the virus. Because the arginine residue is known to be methylated by protein arginine methyltransferase (PRMT) enzyme, we investigated whether IBDV VP1 is a substrate for known PRMTs. In this study, we show that VP1 is specifically associated with and methylated by PRMT5 at the arginine 426 (R426) residue. IBDV infection causes the accumulation of PRMT5 in the cytoplasm, which colocalizes with VP1 as a punctate structure. In addition, ectopic expression of PRMT5 significantly enhances the viral replication. In the presence of PMRT5, enzyme inhibitor and knockout of PRMT5 remarkably decreased viral replication. The polymerase activity of VP1 was severely damaged when R426 mutated to alanine, resulting in impaired viral replication. Our study reports a novel form of post-translational modification of VP1, which supports its polymerase function to facilitate the viral replication. IMPORTANCE Post-translational modification of infectious bursal disease virus (IBDV) VP1 is important for the regulation of its polymerase activity. Investigation of the significance of specific modification of VP1 can lead to better understanding of viral replication and can probably also help in identifying novel targets for antiviral compounds. Our work demonstrates the molecular mechanism of VP1 methylation mediated by PRMT5, which is critical for viral polymerase activity, as well as viral replication. Our study expands a novel insight into the function of arginine methylation of VP1, which might be useful for limiting the replication of IBDV.

Phosphorylation of VP1 Mediated by CDK1-Cyclin B1 Facilitates Infectious Bursal Disease Virus Replication.

Infectious bursal disease virus (IBDV) is a double-stranded RNA (dsRNA) virus belonging to the genus Avibirnavirus in the family Birnaviridae. It can cause serious failure of vaccination in young poultry birds with impaired immune systems. Post-translational modifications of the VP1 protein are essential for viral RNA transcription, genome replication, and viral multiplication. Little information is available so far regarding the exact mechanism of phosphorylation of IBDV VP1 and its significance in the viral life cycle. Here, we provide several lines of evidence that the cyclin-dependent kinase 1 (CDK1)-cyclin B1 complex phosphorylates VP1, which facilitates viral replication. We show that the CDK1-cyclin B1 specifically interacts with VP1 and phosphorylates VP1 on the serine 7 residue, located in the N-terminal 7SPAQ10 region, which follows the optimal phosphorylation motif of CDK1, p-S/T-P. Additionally, IBDV infection drives the cytoplasmic accumulation of CDK1-cyclin B1, which co-localizes with VP1, supporting the kinase activity of CDK1-cyclin B1. Treatment with CDK1 inhibitor RO3306 and knockdown of CDK1-cyclin B1 severely disrupts the polymerase activity of VP1, resulting in diminished viral replication. Moreover, the replication of S7A mutant recombinant IBDV was significantly decreased compared to that of wild-type (WT) IBDV. Thus, CDK1-cyclin B1 is a crucial enzyme which phosphorylates IBDV VP1 on serine 7, which is necessary both for the polymerase activity of VP1 and for viral replication. IMPORTANCE Infectious bursal disease virus still poses a great economic threat to the global poultry farming industry. Detailed information on the steps of viral genome replication is essential for the development of antiviral therapeutics. Phosphorylation is a common post-translational modification in several viral proteins. There is a lack of information regarding the significance of VP1 phosphorylation and its role in modulating the viral life cycle. In this study, we found that CDK1-cyclin B1 accumulates in the cytoplasm and phosphorylates VP1 on serine 7. The presence of a CDK1 inhibitor and the silencing of CDK1-cyclin B1 decrease IBDV replication. The mutation of VP1 serine 7 to alanine reduces VP1 polymerase activity, disrupting the viral life cycle, which suggests that this residue serves an essential function. Our study offers novel insights into the regulatory mechanism of VP1 phosphorylation.

Infectious bursal disease virus in Western Europe: the rise of reassortant strains as the dominant field threat.

Infectious bursal disease virus (IBDV) is a highly contagious birnavirus causing a burdensome immunosuppressive disease in chickens. IBDV features a remarkable antigenic, pathogenic and genetic heterogeneity, with significant implications on disease manifestation, control measures and diagnostic approaches. The recent proposals of comprehensive phylogenetic classification systems offered the ideal platform for large-scale molecular surveys, which are crucial to gather epidemiological data and inform control efforts. In this study, the IBDV scenario was investigated in most of Western Europe by considering the results of diagnostic activities performed internationally throughout 2021. In total, 470 bursal samples from nine different countries were analysed by RT-PCR targeting the VP2. When a field virus was identified, the VP1 was also characterized. Most of the 132 detected field viruses were highly homologous reassortants featuring a very virulent-like VP2 and a classical-like VP1 (genotype A3B1). Despite emerging recently, these reassortants were already signalled in several countries in North-Western Europe and associated with subclinical infections. Here, we report their further spread in the region, where they currently represent the dominant field threat. Two other IBDV types were found, one in Italy, where all the identified viruses clustered in a clade of the A3B1 genotype previously reported only in Russia and the Middle East, and the other in Portugal, where the recently characterized A9B1 genotype was confirmed to circulate. The obtained data suggest the recent occurrence of a major shift in the Western European epidemiological landscape of IBDV, stressing the importance of steady monitoring and sharing of information among different countries and laboratories.RESEARCH HIGHLIGHTS The IBDV scenario in Western Europe seems to have radically changed in recent years.IBDV reassortants were found to be the dominant field type in the region.Local circulation of two other IBDV types was detected in Italy and Portugal.

Cryo-EM structures of infectious bursal disease viruses with different virulences provide insights into their assembly and invasion.

Infectious bursal disease virus (IBDV) causes a highly contagious immunosuppressive disease in chickens, resulting in significant economic losses. The very virulent IBDV strain (vvIBDV) causes high mortality and cannot adapt to cell culture. In contrast, attenuated strains of IBDV are nonpathogenic to chickens and can replicate in cell culture. Although the crystal structure of T = 1 subviral particles (SVP) has been reported, the structures of intact IBDV virions with different virulences remain elusive. Here, we determined the cryo-electron microscopy (cryo-EM) structures of the vvIBDV Gx strain and its attenuated IBDV strain Gt at resolutions of 3.3 Å and 3.2 Å, respectively. Compared with the structure of T = 1 SVP, IBDV contains several conserved structural elements unique to the T = 13 virion. Notably, the N-terminus of VP2, which is disordered in the SVP, interacts with the SF strand of VP2 from its neighboring trimer, completing the ß-sheet of the S domain. This interaction helps to form a contact network by tethering the adjacent VP2 trimers and contributes to the assembly and stability of the IBDV virion. Structural comparison of the Gx and Gt strains indicates that H253 and T284 in the VP2 P domain of Gt, in contrast to Gx, form a hydrogen bond with a positively charged surface. This suggests that the combined mutations Q253H/A284T and the associated structural electrostatic features of the attenuated Gt strain may contribute to adaptation to cell culture. Furthermore, a negatively charged groove in VP2, containing an integrin binding IDA motif that is critical for virus attachment, was speculated to play a functional role in the entry of IBDV.

Synergistic Immunosuppression of Avian Leukosis Virus Subgroup J and Infectious Bursal Disease Virus Is Responsible for Enhanced Pathogenicity.

In recent years, superinfections of avian leukosis virus subgroup J (ALV-J) and infectious bursal disease virus (IBDV) have been frequently observed in nature, which has led to the increasing virulence in infected chickens. However, the reason for the enhanced pathogenicity has remained unclear. In this study, we demonstrated an effective candidate model for studying the outcome of superinfections with ALV-J and IBDV in cells and specific-pathogen-free (SPF) chicks. Through in vitro experiments, we found that ALV-J and IBDV can establish the superinfection models and synergistically promote the expression of IL-6, IL-10, IFN-α, and IFN-γ in DF-1 and CEF cells. In vivo, the weight loss, survival rate, and histopathological observations showed that more severe pathogenicity was present in the superinfected chickens. In addition, we found that superinfections of ALV-J and IBDV synergistically increased the viral replication of the two viruses and inflammatory mediator secretions in vitro and in vivo. Moreover, by measuring the immune organ indexes and blood proportions of CD3+, CD4+, and CD8α+ cells, our results showed that the more severe instances of immunosuppression were observed in the superinfected chickens. In the present study, we concluded that the more severe immunosuppression induced by the synergistic viral replication of ALV-J and IBDV is responsible for the enhanced pathogenicity.