VP1 of infectious bursal disease virus is an RNA-dependent RNA polymerase

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.

Infectious bursal disease virus VP5 triggers host shutoff in a transcription-dependent manner

Viruses have evolved intricate mechanisms to evade host antiviral responses and exploit cellular resources by manipulating the expression profile of host genes. During infection, viruses encode proteins with shutoff activity to globally inhibit host protein synthesis, which is an effective strategy for immune evasion. In this study, compelling evidence shows that infectious bursal disease virus (IBDV) infection triggers the suppression of host protein synthesis. Furthermore, using both in vitro and in vivo viral infection models, we have identified that IBDV specifically impedes the transcription of host genes via the shutoff activity of viral VP5, simultaneously conferring advantages to IBDV infection in these circumstances. The proposed mechanism suggests that VP5 competitively binds to RanBP1, disrupting the RanGDP/GTP gradient. This disruption interferes with cellular nucleocytoplasmic transport, impairing the nuclear import of proteins bearing nuclear localization signals. The nuclear transport of pivotal transcriptional regulatory factors, such as p65 and IFN regulatory factor 7, is also compromised, leading to the inhibition of pro-inflammatory cytokines and interferon expression. This newly discovered strategy employed by IBDV enables them to manipulate host gene expression, providing novel insights into how viruses evade host immune responses and establish infections.IMPORTANCEViruses manipulate host processes at various levels to regulate or evade both innate and adaptive immune responses, promoting self-survival and efficient transmission. The "host shutoff," a global suppression of host gene expression mediated by various viruses, is considered a critical mechanism for evading immunity. In this study, we have validated the presence of host shutoff during infectious bursal disease virus (IBDV) infection and additionally uncovered that the viral protein VP5 plays a pivotal role in inhibiting the overall synthesis of host proteins, including cytokines, through a transcription-dependent pathway. VP5 competitively binds with RanBP1, leading to disruption of the Ran protein cycle and consequently interfering with nucleocytoplasmic transport, which ultimately results in the suppression of host gene transcription. These findings unveil a novel strategy employed by IBDV to evade host innate immunity and rapidly establish infection. This study also suggests a novel supplement to understanding the pathway through which viruses inhibit host protein synthesis.

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.

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.

Genetic Insight into the Interaction of IBDV with Host-A Clue to the Development of Novel IBDV Vaccines

Infectious bursal disease virus (IBDV) is an immunosuppressive pathogen causing enormous economic losses to the poultry industry across the globe. As a double-stranded RNA virus, IBDV undergoes genetic mutation or recombination in replication during circulation among flocks, leading to the generation and spread of variant or recombinant strains. In particular, the recent emergence of variant IBDV causes severe immunosuppression in chickens, affecting the efficacy of other vaccines. It seems that the genetic mutation of IBDV during the battle against host response is an effective strategy to help itself to survive. Therefore, a comprehensive understanding of the viral genome diversity will definitely help to develop effective measures for prevention and control of infectious bursal disease (IBD). In recent years, considerable progress has been made in understanding the relation of genetic mutation and genomic recombination of IBDV to its pathogenesis using the reverse genetic technique. Therefore, this review focuses on our current genetic insight into the IBDV's genetic typing and viral genomic variation.

gga-miR-20b-5p inhibits infectious bursal disease virus replication via targeting Netrin 4

MicroRNAs (miRNAs) involved host-virus interaction, affecting the replication or pathogenesis of several viruses. Frontier evidences suggested that miRNAs play essential roles in infectious bursal disease virus (IBDV) replication. However, the biological function of miRNAs and the underlying molecular mechanisms are still unclear. Here, we reported that gga-miR-20b-5p acted as a negative factor affecting IBDV infection. We found that gga-miR-20b-5p was significantly up-regulated during IBDV infection in host cells, and that gga-miR-20b-5p effectively inhibited IBDV replication via targeting the expression of host protein netrin 4 (NTN4). In contrast, inhibition of endogenous miR-20b-5p markedly facilitated viral replication associated with enhancing NTN4 expression. Collectively, these findings highlight a crucial role of gga-miR-20b-5p in IBDV replication.

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.

Epidemiology of Infectious Bursal Disease Virus in Poland during 2016-2022

Infectious bursal disease virus is an immunosuppressive ubiquitous pathogen that causes serious economic losses in poultry production. The virus is prone to genetic changes through mutations and reassortment, which drive the emergence of new variants and lead to a change in the epidemiological situation in a field. Such a situation is currently being reported due to a large wave of IBDV A3B1 reassortant infections in northwestern Europe. On the other hand, in Poland, which is the largest producer of chicken meat in the EU, the IBDVs of genotypes A3B2 and A3B4 were circulating just before the emergence of A3B1 reassortants. The purpose of the presented study was to update the IBDV epidemiological situation. The performed molecular survey based on the sequence of both genome segments showed the presence of very virulent strains (A3B2) and reassortants of genotypes A3B4 and A3B1; moreover, two of these genotypes are newly introduced IBDV lineages. In addition, a number of amino acid substitutions were demonstrated, including within antigenic epitopes and virulence determinants. In conclusion, the results obtained indicated a dynamic epidemiological situation in Poland, which highlights the need for further monitoring studies in the region and verification of protection conferred by the vaccines used against infection with detected IBDV.

Interaction between chicken TRIM25 and MDA5 and their role in mediated antiviral activity against IBDV infection

Infectious Bursal Disease Virus (IBDV) is the causative agent of an immunosuppressive disease that affects domestic chickens (Gallus gallus) severely affecting poultry industry worldwide. IBDV infection is characterized by a rapid depletion of the bursal B cell population by apoptosis and the atrophy of this chief lymphoid organ. Previous results from our laboratory have shown that exposure of infected cells to type I IFN leads to an exacerbated apoptosis, indicating an important role of IFN in IBDV pathogenesis. It has been described that recognition of the dsRNA IBDV genome by MDA5, the only known cytoplasmic pattern recognition receptor for viral RNA in chickens, leads to type I IFN production. Here, we confirm that TRIM25, an E3 ubiquitin ligase that leads to RIG-I activation in mammalian cells, significantly contributes to positively regulate MDA5-mediated activation of the IFN-inducing pathway in chicken DF-1 cells. Ectopic expression of chTRIM25 together with chMDA5 or a deletion mutant version exclusively harboring the CARD domains (chMDA5 2CARD) enhances IFN-β and NF-ĸB promoter activation. Using co-immunoprecipitation assays, we show that chMDA5 interacts with chTRIM25 through the CARD domains. Moreover, chTRIM25 co-localizes with both chMDA5 and chMDA5 2CARD, but not with chMDA5 mutant proteins partially or totally lacking these domains. On the other hand, ablation of endogenous chTRIM25 expression reduces chMDA5-induced IFN-β and NF-ĸB promoter activation. Interestingly, ectopic expression of either wild-type chTRIM25, or a mutant version (chTRIM25 C59S/C62S) lacking the E3 ubiquitin ligase activity, restores the co-stimulatory effect of chMDA5 in chTRIM25 knockout cells, suggesting that the E3-ubiquitin ligase activity of chTRIM25 is not required for its downstream IFN-β and NF-ĸB activating function. Also, IBDV-induced expression of IFN-β, Mx and OAS genes was reduced in chTRIM25 knockout as compared to wild-type cells, hence contributing to the enhancement of IBDV replication. Enhanced permissiveness to replication of other viruses, such as avian reovirus, Newcastle disease virus and vesicular stomatitis virus was also observed in chTRIM25 knockout cells. Additionally, chTRIM25 knockout also results in reduced MAVS-induced IFN-β promoter stimulation. Nonetheless, similarly to its mammalian counterpart, chTRIM25 overexpression in wild-type DF-1 cells causes the degradation of ectopically expressed chMAVS.

Host Combats IBDV Infection at Both Protein and RNA Levels

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by infectious bursal disease virus (IBDV). In recent years, with the emergence of IBDV variants and recombinant strains, IBDV still threatens the poultry industry worldwide. It seems that the battle between host and IBDV will never end. Thus, it is urgent to develop a more comprehensive and effective strategy for the control of this disease. A better understanding of the mechanisms underlying virus-host interactions would be of help in the development of novel vaccines. Recently, much progress has been made in the understanding of the host response against IBDV infection. If the battle between host and IBDV at the protein level is considered the front line, at the RNA level, it can be taken as a hidden line. The host combats IBDV infection at both the front and hidden lines. Therefore, this review focuses on our current understanding of the host response to IBDV infection at both the protein and RNA levels.

Genetics and Pathogenicity of Natural Reassortant of Infectious Bursal Disease Virus Emerging in Latvia

Infectious bursal disease virus is an immunosuppressive pathogen that, despite applied vaccination, is affecting the poultry industry worldwide. This report presents the genetic and pathotypic characterization of a natural reassortant emerging in Europe (Latvia). Genetic characterization showed that strain 25/11/Latvia/2011 represents genotype A3B1, whose segment A is derived from very virulent strains, while segment B is from the classic-like genogroup. Phylogenetic maximum likelihood inference of the B-segment sequence clustered the reassortant strain together with the US antigenic variant E strain. However, the obtained full-length sequence of 25/11/Latvia/2011 revealed that not only reassortment but also dozens of mutations shaped the unique genetic makeup. Phenotypic characterization showed no mortality and no clinical signs of disease but a severe bursa of Fabricius atrophy and splenomegaly in the convalescent birds at 10 days post infection. The results obtained indicate that the acquired genetic constellation contributed to a decrease in virulence; nevertheless, the infection causes severe damage to lymphoid organs, which can lead to impaired immune responses.

Molecular characterization of Infectious Bursal Disease Virus isolated in Chile reveals several mutations in VP2 coding region and a reassortment in its genome

Infectious Bursal Disease (IBD) is a well-described disease in young chickens. It is caused by the Infectious Bursal Disease Virus (IBDV), which has a bi-segmented, double-strand RNA genome. The absence of a lipidic envelope makes IBDV highly resistant to environmental conditions. Consequently, it is widely reported around the world. Fourteen samples retrieved from chickens exhibiting apparent alterations of the bursa of Fabricius between 2017 and 2021 were included in the study. These samples were passaged into embryonated eggs and the presence of IBD was confirmed through RT-PCR. The PCR products were sequenced and analyzed to characterize the Chilean IBDV isolates for comparison with GenBank sequences, including vaccines sequences currently used in Chile.Phylogenetic analysis classified the Chilean sequences as A1B1, except the sample 15002_CL_2021 which was classified as A2B1. On the other hand, all Chilean viruses were grouped as B1, based on viral segment B. Estimated evolutionary divergence between different genogroups supports these clustering. Moreover, samples 13936_CL_2017, 14038_CL_2017, 14083_CL_2017, 14145_CL_2018, 14431_CL_2019, and 14459_CL_2019 showed high similitude with the D78 and ViBursa CE vaccines (both currently used in Chile). Viruses 14010_CL_2018, 14040_CL_2017, 14514_CL_2019 and 14019_CL_2017 exhibited patterns that do not exactly fit either vaccine. Finally, viruses 15,041 N-_CL_2021, 15,041 N+_CL_2021, and 15004_CL_2021 showed even more differences regarding both vaccines.This is the first study in Chile to analyze the genetic sequences of IBDV isolates. The different assessments conducted as part of the study suggest a close relationship with vaccines currently in use. Interestingly, one of the viruses exhibited a reassortment in its genome segments, which could confer new characteristics to the virus. However, new approaches would be required to establish the origin of the isolated viruses, as well as how the recombination is changing its virulence or morbidity.

Advances on Innate Immune Evasion by Avian Immunosuppressive Viruses

Innate immunity is not only the first line of host defense against pathogenic infection, but also the cornerstone of adaptive immune response. Upon pathogenic infection, pattern recognition receptors (PRRs) of host engage pathogen-associated molecular patterns (PAMPs) of pathogens, which initiates IFN production by activating interferon regulatory transcription factors (IRFs), nuclear factor-kappa B (NF-κB), and/or activating protein-1 (AP-1) signal transduction pathways in host cells. In order to replicate and survive, pathogens have evolved multiple strategies to evade host innate immune responses, including IFN-I signal transduction, autophagy, apoptosis, necrosis, inflammasome and/or metabolic pathways. Some avian viruses may not be highly pathogenic but they have evolved varied strategies to evade or suppress host immune response for survival, causing huge impacts on the poultry industry worldwide. In this review, we focus on the advances on innate immune evasion by several important avian immunosuppressive viruses (infectious bursal disease virus (IBDV), Marek’s disease virus (MDV), avian leukosis virus (ALV), etc.), especially their evasion of PRRs-mediated signal transduction pathways (IFN-I signal transduction pathway) and IFNAR-JAK-STAT signal pathways. A comprehensive understanding of the mechanism by which avian viruses evade or suppress host immune responses will be of help to the development of novel vaccines and therapeutic reagents for the prevention and control of infectious diseases in chickens.

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 S_F strand of VP2 from its neighboring trimer, completing the β-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.

Pathogenicity and molecular characterization of infectious bursal disease virus in China

Infectious bursal disease virus (IBDV) caused an acute and highly contagious infectious disease, resulting in considerable economic losses in the world poultry industry. Although this disease was well-controlled under the widely use of commercial vaccines, the novel variant IBDV strain emerged due to the highly immunized-selection pressure in the field, posting new threats to poultry industry. Here, we reported the epidemic and pathogenicity of IBDV in Hubei Province from May to August 2020. We isolated 12 IBDV strains from the broiler flocks, including 9 novel variants, 2 very virulent strains and 1 medium virulent strain. Interestingly, we identified a series of changes of amino acid sites in the VP2. Further analysis indicated that the novel variant IBDV strains caused damage to bursa of fabricius and spleen, leading to immunosuppression. Our findings underscore the importance of IBDV surveillance, and provide evidence for understanding the evolution of IBDV.

Sequence diversity and evolution of infectious bursal disease virus in Iraq

Background: Infectious Bursal Disease (IBD) is a highly infectious disease which causes huge economic losses to the poultry industry due to the direct impact of the illness and indirect consequences such as decreasing the general immunity of the flock, leaving it naive to other diseases. In Iraq, IBD is highly prevalent despite vaccination programs, yet studies on sequence diversity of the causative virus are still rare.  Methods: A sample from Bursa of Fabricius from an IBD outbreak in a flock in the city of Najaf in Iraq was smeared on an FTA card. Amplicons of targeted regions in VP1 and VP2 genes were generated and sequenced. Sequences were then compared with other local and global sequences downloaded from GenBank repositories. Sequence alignment and DNA sequence analyses were achieved using MUSCLE, UGENE and MEGAx software. The molecular clock and sequence evolutionary analyses were applied using MEGAx tools.  Results: The strain sequenced in this study belongs to a very virulent Infectious Bursal Disease Virus (vvIBDV) as the DNA and phylogenetic analysis of VP1 and VP2 gene sequences showed a mutual clustering with similar sequences belonging to vvIBDV genogroup 3. Analyses of the hyper variable region of VP2 gene (hvVP2) of IBDV isolates from Iraq indicates a presence of sequence diversity. Interestingly, the two vaccine strains Ventri IBDV Plus and ABIC MB71 that showed the highest sequence similarity to the local isolates in the hvVP2 region are not used in vaccination routine against IBDV in Iraq.  Conclusion: Sequences of vvIBDV in Iraq are diverse. Remarkably, some of the available vaccine strains show high sequence similarity with local strains in Iraq; however, they are not included in the routine vaccination programs. Analysis of more samples involving more geographical regions is needed to draw a detailed map of antigenic diversity of IBDV in Iraq.

Characterization and pathogenicity of a naturally reassortant and recombinant infectious bursal disease virus in China

Infectious bursal disease virus (IBDV), an Avibirnavirus, is the pathogen of infectious bursal disease, which is a severely immunosuppressive disease in 3-15-week-old chickens. Different phenotypes of IBDV, including classical, variant, very virulent (vv) and attenuated IBDV, have been reported in many chicken-rearing countries worldwide. Here, we isolated and identified a naturally reassortant and recombinant IBDV (designated GXB02) from 20-day-old chickens with clinicopathological changes of infectious bursal disease (IBD) in Guangxi Province, China. Whole genomic sequencing showed that the strain GXB02 simultaneously has both reassortant and recombinant characteristics with segments A and B being derived from recombinant intermediate vaccine strain and classic strains of IBDV. Segment A of strain GXB02 was incorporated into the skeleton of an intermediate IBDV vaccine strain (W2512), where the breakpoints of two recombinant events located at nucleotide positions 1468 and 1648 were replaced by reassortant vvIBDV (PK2) and vvIBDV (D6948) of segment A, respectively. We used this GXB02 strain to inoculate 21-day-old specific-pathogen-free chickens to evaluate its pathogenicity. Strain GXB02 has clinicopathologic characteristics of IBD with severe bursal lesions, as evidenced by necrosis, depletion of lymphocytes, and follicle atrophy, indicating that reassortment with classical strains in segment B or/and recombination with very virulent strains increased pathogenicity of the strain GXB02 in chickens. These findings provide important insights into the genetic exchange between classic and attenuated strains of IBDV with two recombinant events occurring at the intermediate derivative segment A with vvIBDV strains, thereby increasing the difficulty of prevention and control of IBD due to novel reassortant-recombinant strains.

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.

DeSUMOylation of Apoptosis Inhibitor 5 by Avibirnavirus VP3 Supports Virus Replication

SUMOylation is a reversible posttranslational modification involved in the regulation of diverse biological processes. Growing evidence suggests that virus infection can interfere with the SUMOylation system. In the present study, we discovered that apoptosis inhibitor 5 (API5) is a SUMOylated protein. Amino acid substitution further identified that Lys404 of API5 was the critical residue for SUMO3 conjugation. Moreover, we found that Avibirnavirus infectious bursal disease virus (IBDV) infection significantly decreased SUMOylation of API5. In addition, our results further revealed that viral protein VP3 inhibited the SUMOylation of API5 by targeting API5 and promoting UBC9 proteasome-dependent degradation through binding to the ubiquitin E3 ligase TRAF3. Furthermore, we revealed that wild-type but not K404R mutant API5 inhibited IBDV replication by enhancing MDA5-dependent IFN-β production. Taken together, our data demonstrate that API5 is a UBC9-dependent SUMOylated protein and deSUMOylation of API5 by viral protein VP3 aids in viral replication.

Inhibition of Antiviral Innate Immunity by Avibirnavirus VP3 via Blocking TBK1-TRAF3 Complex Formation and IRF3 Activation

Type I interferon plays a critical role in the host response against virus infection, including Avibirnavirus. However, many viruses have developed multiple strategies to antagonize the innate host antiviral immune response during coevolution with the host. In this study, we first identified that K33-linked polyubiquitination of lysine-155 of TRAF3 enhances the interaction with TBK1, which positively regulates the host IFN immune response.

The host innate immune system develops various strategies to antagonize virus infection, and the pathogen subverts or evades host innate immunity for self-replication. In the present study, we discovered that Avibirnavirus infectious bursal disease virus (IBDV) VP3 protein significantly inhibits MDA5-induced beta interferon (IFN-β) expression by blocking IRF3 activation. Binding domain mapping showed that the CC1 domain of VP3 and the residue lysine-155 of tumor necrosis factor receptor-associated factor 3 (TRAF3) are essential for the interaction. Furthermore, we found that the CC1 domain was required for VP3 to downregulate MDA5-mediated IFN-β production. A ubiquitination assay showed that lysine-155 of TRAF3 was the critical residue for K33-linked polyubiquitination, which contributes to the formation of a TRAF3-TBK1 complex. Subsequently, we revealed that VP3 blocked TRAF3-TBK1 complex formation through reducing K33-linked polyubiquitination of lysine-155 on TRAF3. Taken together, our data reveal that VP3 inhibits MDA5-dependent IRF3-mediated signaling via blocking TRAF3-TBK1 complex formation, which improves our understanding of the interplay between RNA virus infection and the innate host antiviral immune response.

IMPORTANCE Type I interferon plays a critical role in the host response against virus infection, including Avibirnavirus. However, many viruses have developed multiple strategies to antagonize the innate host antiviral immune response during coevolution with the host. In this study, we first identified that K33-linked polyubiquitination of lysine-155 of TRAF3 enhances the interaction with TBK1, which positively regulates the host IFN immune response. Meanwhile, we discovered that the interaction of the CC1 domain of the Avibirnavirus VP3 protein and the residue lysine-155 of TRAF3 reduced the K33-linked polyubiquitination of TRAF3 and blocked the formation of the TRAF3-TBK1 complex, which contributed to the downregulation of host IFN signaling, supporting viral replication.

Sequence diversity and evolution of infectious bursal disease virus in Iraq

Background: Infectious Bursal Disease (IBD) is a highly infectious disease which causes huge economic losses to the poultry industry due to the direct impact of the illness and indirect consequences such as decreasing the general immunity of the flock, leaving it naive to other diseases. In Iraq, IBD is highly prevalent despite vaccination programs, yet studies on sequence diversity of the causative virus are still rare.  Methods: A sample from Bursa of Fabricius from an IBD outbreak in a flock in the city of Najaf in Iraq was smeared on an FTA card. Amplicons of targeted regions in VP1 and VP2 genes were generated and sequenced. Sequences were then compared with other local and global sequences downloaded from GenBank repositories. Sequence alignment and DNA sequence analyses were achieved using MUSCLE, UGENE and MEGAx software. The molecular clock and sequence evolutionary analyses were applied using MEGAx tools.  Results: The strain sequenced in this study belongs to a very virulent Infectious Bursal Disease Virus (vvIBDV) as the DNA and phylogenetic analysis of VP1 and VP2 gene sequences showed a mutual clustering with similar sequences belonging to vvIBDV genogroup 3. Analyses of the hyper variable region of VP2 gene (hvVP2) of IBDV isolates from Iraq indicates a presence of sequence diversity. Interestingly, the two vaccine strains Ventri IBDV Plus and ABIC MB71 that showed the highest sequence similarity to the local isolates in the hvVP2 region are not used in vaccination routine against IBDV in Iraq.  Conclusion: Sequences of vvIBDV in Iraq are diverse. Remarkably, some of the available vaccine strains show high sequence similarity with local strains in Iraq; however, they are not included in the routine vaccination programs. Analysis of more samples involving more geographical regions is needed to draw a detailed map of antigenic diversity of IBDV in Iraq.

The Novel Genetic Background of Infectious Bursal Disease Virus Strains Emerging from the Action of Positive Selection

The circulation in Europe of novel reassortant strains of infectious bursal disease virus (IBDV), containing a unique genetic background composition, represents a serious problem for animal health. Since the emergence of this novel IBDV mosaic was first described in Poland, this scenario has become particularly attractive to uncover the evolutionary forces driving the genetic diversity of IBDV populations. This study additionally addressed the phenotypic characterization of these emergent strains, as well as the main features affecting the viral fitness during the competition process of IBDV lineages in the field. Our results showed how different evolutionary mechanisms modulate the genetic diversity of co-existent IBDV lineages, leading to the error catastrophe effect, Muller ratchet effect, or prevalence, depending on their genetic compositions. We also determined that the action of the positive selection pressure, depending on the genomic segment on which it is acting, can drive two main phenotypes for IBDV: immune-escaping strains from the selection on segment A or strains with functional advantages from the selection on segment B. This last group seems to possess an increased fitness landscape in the viral quasispecies composition, presenting better adaptability to dissimilar environmental conditions and likely becoming the dominant population. The reassortant strains also exhibited a lower mortality rate compared with the well-known vvIBDV strains, which can facilitate their spreading.

Mass spectrometry-based proteomic analysis of potential infectious bursal disease virus VP3-interacting proteins in chicken embryo fibroblasts cells

The structural protein VP3 of infectious bursal disease virus (IBDV) plays a critical role in viral assembly, replication, immune escape, and anti-apoptosis. Interaction between VP3 and host protein factors can affect stages in the viral replication cycle. In this study, 137 host proteins interacting with VP3 protein were screened through liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomics approach. The functions and relevance of the proteins were obtained through bioinformatics analysis. Most VP3-interacting proteins were linked to binding, catalytic activity, and structural molecular activity, and performed functions in cell parts and cells. Biological functions of VP3-interacting proteins were mainly relevant to "Cytoskeleton", "Translation", and "Signal transduction mechanisms", involving ribosomes, "Tight junction", regulation of actin cytoskeleton, and other pathways. Six potential VP3-interacting proteins in host cells were knocked down, and vimentin, myosin-9, and annexin A2 were found to be related to IBDV replication. This study would help explore regulatory pathways and cellular mechanisms in IBDV-infected cells, and also provided clues for the in-depth study of VP3 biological functions and IBDV replication or pathogenesis.

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.

Potential reverse spillover of infectious bursal disease virus at the interface of commercial poultry and wild birds

Recently, multiple spillover events between domesticated poultry and wild birds have been reported for several avian viruses. This phenomenon highlights the importance of the livestock-wildlife interface in the possible emergence of novel viruses. The aim of the current study was to investigate the potential spillover and epidemiological links of infectious bursal disease virus (IBDV) between wild birds and domestic poultry. To this end, twenty-eight cloacal swabs were collected from four species of free-living Egyptian wild birds (i.e. mallard duck, bean goose, white-fronted goose and black-billed magpie). Genetic and phylogenetic analysis of three positive isolates revealed that the IBDV/USC-1/2019 strain clustered with previously reported very virulent IBDV (vvIBDV) Egyptian isolates. Interestingly, two other wild bird-origin isolates (i.e. IBDV/USC-2/2019 and IBDV/USC-3/2019) grouped with a vaccine strain that is being used in commercial poultry. In conclusion, our results revealed the molecular detection of vaccine and vvIBDV-like strains in Egyptian wild birds and highlighted the potential role of wild birds in IBDV epidemiology in disease-endemic regions.

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.

Chicken optineurin suppresses MDA5-mediated interferon β production

Chicken MDA5 (chMDA5), the essential accepted pattern recognition receptors for detecting cytoplasmic viral RNA in chicken, initiates interferon β (IFN-β) generation. However, there is an incomplete elucidation of regulating chMDA5-mediated IFN-β production. NEMO-related protein, optineurin, was identified as inhibitors of virus triggered IFN-β induction in human or mice. In this study, full length of chicken optineurin (chOPTN) was cloned from chicken embryo fibroblast, and its role in inhibiting IFN-β signaling pathway was further explored. Full-length chOPTN encodes 547 amino acids residues and contains unique LC3 interaction region and ubiquitin binding domain. Chicken optineurin mRNA and protein are widely expressed in different tissues, especially the heart, kidney, and bursal fabricius (BF). Overexpressed chOPTN not only inhibits poly I:C or homos-induced human IFN-β promoter activation in 293T cells but also suppresses poly I:C, infectious bursal disease virus (IBDV) genome double-strand RNA (dsRNA), and chMDA5-induced chicken IFN-β (chIFN-β) promoter activation. In addition, we first revealed that chOPTN negatively regulates chIFN-β production via inhibiting ubiquitination of chicken TBK1, which is dependent on the ubiquitin-binding domain of chOPTN. Moreover, chIFN-β stimulus, poly I:C, and IBDV genome dsRNA improve chOPTN expression. Endogenous chOPTN expression is also upregulated by IBDV infection in 293T, DF-1 cells, as well as in BF. Therefore, our results suggested that chOPTN plays an inhibition role of chMDA5-mediated chIFN-β signaling pathway in chicken cells.

Ex vivo rescue of recombinant very virulent IBDV using a RNA polymerase II driven system and primary chicken bursal cells

Infectious Bursal Disease Virus (IBDV), a member of the Birnaviridae family, causes an immunosuppressive disease in young chickens. Although several reverse genetics systems are available for IBDV, the isolation of most field-derived strains, such as very virulent IBDV (vvIBDV) and their subsequent rescue, has remained challenging due to the lack of replication of those viruses in vitro. Such rescue required either the inoculation of animals, embryonated eggs, or the introduction of mutations in the capsid protein (VP2) hypervariable region (HVR) to adapt the virus to cell culture, the latter option concomitantly altering its virulence in vivo. We describe an improved ex vivo IBDV rescue system based on the transfection of an avian cell line with RNA polymerase II-based expression vectors, combined with replication on primary chicken bursal cells, the main cell type targeted in vivo of IBDV. We validated this system by rescuing to high titers two recombinant IBDV strains: a cell-culture adapted attenuated strain and a vvIBDV. Sequencing of VP2 HVR confirmed the absence of unwanted mutations that may alter the biological properties of the recombinant viruses. Therefore, this approach is efficient, economical, time-saving, reduces animal suffering and can be used to rescue other non-cell culture adapted IBDV strains.

Critical role for G3BP1 in infectious bursal disease virus (IBDV)-induced stress granule formation and viral replication

Stress granules (SGs), complexes for mRNA storage, are formed in host cellular response to stress stimuli and play an important role in innate immune response. GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1) is a key component of SGs. However, whether IBDV infection induces SG formation in host cells and what role of G3BP1 plays in this process are unclear. We report here that IBDV infection initiated typical stress granule formation and enhanced G3BP1 expression in DF-1 cells. Our data show that knockdown of G3BP1 by RNAi markedly inhibited IBDV-induced SG formation and viral replication in DF-1 cells. Conversely, ectopic expression of G3BP1 enhanced IBDV-induced SG formation and significantly promoted IBDV replication in host cells. Thus, G3BP1 plays a critical role in IBDV-induced SG formation and viral replication, providing an important clue to elucidating how IBDV employs cellular SGs for its own benefits.

Role of MicroRNAs in Host Defense against Infectious Bursal Disease Virus (IBDV) Infection: A Hidden Front Line

Infectious bursal disease (IBD) is an acute, highly contagious and immunosuppressive avian disease caused by infectious bursal disease virus (IBDV). In recent years, remarkable progress has been made in the understanding of the pathogenesis of IBDV infection and the host response, including apoptosis, autophagy and the inhibition of innate immunity. Not only a number of host proteins interacting with or targeted by viral proteins participate in these processes, but microRNAs (miRNAs) are also involved in the host response to IBDV infection. If an IBDV-host interaction at the protein level is taken imaginatively as the front line of the battle between invaders (pathogens) and defenders (host cells), their fight at the RNA level resembles the hidden front line. miRNAs are a class of non-coding single-stranded endogenous RNA molecules with a length of approximately 22 nucleotides (nt) that play important roles in regulating gene expression at the post-transcriptional level. Insights into the roles of viral proteins and miRNAs in host response will add to the understanding of the pathogenesis of IBDV infection. The interaction of viral proteins with cellular targets during IBDV infection were previously well-reviewed. This review focuses mainly on the current knowledge of the host response to IBDV infection at the RNA level, in particular, of the nine well-characterized miRNAs that affect cell apoptosis, the innate immune response and viral replication.

Identification and genetic analysis of infectious bursal disease viruses from field outbreaks in Kerala, India

Recurrent infectious bursal disease (IBD) outbreaks were reported in different regions of Kerala, India. This paper reports the comparative genetic analysis of the hypervariable region of the VP2 gene of IBD virus isolates from the field outbreaks in Kerala. In phylogenetic analysis, the obtained field isolates fall into genogroup 1 and 3. In genogroup 3, all vvIBDV isolates shared a common ancestor with other south Indian isolates but isolates 9/CVASP/IBDV, 10/CVASP/IBDV, 12/CVASP/IBDV, 14/CVASP/IBDV and 17/CVASP/IBDV are most recently evolved and are diverged from the south Indian isolates. The amino acid sequence of 22 isolates was analysed, out of which 18 had conserved amino acids which were characteristic of vvIBDV. All the vvIBDV isolates obtained in the study had phenylalanine and valine at the position 240 and 294, respectively, similar to recently evolved Indian IBDV isolate (MDI14). But we observed T269A and S299N mutations in the isolate 6/CVASP/IBDV, and it is the first report of such mutations at these positions in India IBDV isolates. The isolate 11/CVASP/IBDV had a unique mutation of V225A which is not yet reported in IBDV isolates. Two isolates (15/CVASP/IBDV and 18/CVASP/IBDV) were 100% amino acid similar to intermediate plus vaccine strain. The isolates 8/CVASP/IBDV/VP2 and 19/CVASP/IBDV had amino acids unique for the intermediate vaccine with mutations observed at H253Q and V256I in 19/CVASP/IBDV, T270A and novel mutation N279Y in isolate 8/CVASP/IBDV. These two isolates had non-virulent classical heptapeptide sequence 'SWSARGS'; nevertheless, they produce field outbreaks of IBD. This is the first report of genetic characterisation of IBDV in Kerala, India.

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.

Chicken eEF1α is a Critical Factor for the Polymerase Complex Activity of Very Virulent Infectious Bursal Disease Virus

Infectious bursal disease (IBD) is an immunosuppressive, highly contagious, and lethal disease of young chickens caused by IBD virus (IBDV). It results in huge economic loss to the poultry industry worldwide. Infection caused by very virulent IBDV (vvIBDV) strains results in high mortality in young chicken flocks. However, the replication characteristics of vvIBDV are not well studied. Publications have shown that virus protein 3 (VP3) binds to VP1 and viral double-stranded RNA, and together they form a ribonucleoprotein complex that plays a key role in virus replication. In this study, vvIBDV VP3 was used to identify host proteins potentially involved in modulating vvIBDV replication. Chicken eukaryotic translation elongation factor 1α (cheEF1α) was chosen to further investigate effects on vvIBDV replication. By small interfering RNA-mediated cheEF1α knockdown, we demonstrated the possibility of significantly reducing viral polymerase activity, with a subsequent reduction in virus yields. Conversely, over-expression of cheEF1α significantly increased viral polymerase activity and virus replication. Further study confirmed that cheEF1α interacted only with vvIBDV VP3 but not with attenuated IBDV (aIBDV) VP3. Furthermore, the amino acids at the N- and C-termini were important in the interaction between vvIBDV VP3 and cheEF1α. Domain III was essential for interactions between cheEF1α and vvIBDV VP3. In summary, cheEF1α enhances vvIBDV replication by promoting the activity of virus polymerase. Our study indicates cheEF1α is a potential target for limiting vvIBDV infection.

Research Progress on MicroRNAs Involved in the Regulation of Chicken Diseases

MicroRNAs (miRNAs) are small, non-coding RNA molecules that inhibit protein translation from target mRNAs. Accumulating evidence suggests that miRNAs can regulate a broad range of biological pathways, including cell differentiation, apoptosis, and carcinogenesis. With the development of miRNAs, the investigation of miRNA functions has emerged as a hot research field. Due to the intensive farming in recent decades, chickens are easily influenced by various pathogen transmissions, and this has resulted in large economic losses. Recent reports have shown that miRNAs can play critical roles in the regulation of chicken diseases. Therefore, the aim of this review is to briefly discuss the current knowledge regarding the effects of miRNAs on chickens suffering from common viral diseases, mycoplasmosis, necrotic enteritis, and ovarian tumors. Additionally, the detailed targets of miRNAs and their possible functions are also summarized. This review intends to highlight the key role of miRNAs in regard to chickens and presents the possibility of improving chicken disease resistance through the regulation of miRNAs.

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.

Phylogenetic analyses and pathogenicity of a variant infectious bursal disease virus strain isolated in China

Infectious bursal disease is an acute, highly contagious, immunosuppressive disease of young chickens caused by infectious bursal disease virus (IBDV). In recent years, there has been a notable increase in the isolation rates of variant IBDV strains in China; however, the pathogenicity of these variants is unclear. In the current study, we characterized variant IBDV strain ZD-2018-1 and assessed its pathogenicity in specific-pathogen-free chickens. Phylogenetic analysis revealed that ZD-2018-1 belonged to the variant IBDV strain, which showed several unique amino acid mutations compared with other previously-isolated variant IBDV strains. Pathogenicity assays showed that ZD-2018-1 was less virulent than very virulent IBDV strain SD-2013-1, and did not cause an obvious symptoms or death. In comparison, strain SD-2013-1 had a high mortality rate and caused severe lesions in various tissues. However, both of the strains induced obvious pathological lesions on the bursa of Fabricius, resulting in severe immunosuppression in the infected chickens. The results of this study present a systematic evaluation of the genetic characteristics, pathogenicity, and immunosuppressive properties of a new variant IBDV strain, and may help in the development of strategies for the prevention and control of IBDV in poultry.

Continuous circulation of an antigenically modified very virulent infectious bursal disease virus for fifteen years in Egypt

Infectious bursal disease virus (IBDV), the agent of an immunosuppressive and sometimes lethal disease in chickens, is causing recurrent outbreaks in broiler chickens in Egypt. In particular, an antigenically modified isolate of very virulent IBDV (vvIBDV) called 99323 was detected in Egypt nearly twenty years ago; this isolate was shown to be experimentally controlled by an antigenically classical live vaccine. However, acute IBD is still reported, even in vaccinated flocks, and little is known about the genetic and antigenic properties of viruses currently circulating in Egypt. In the present study, ten samples collected in Egyptian broiler farms in 2015 as well as five samples collected in 2001 were analyzed. Genetic analyses of partial VP2 sequences revealed that 8 isolates clustered with vvIBDV strains, and 5 with tissue culture adapted and vaccine strains. Similar results were observed for partial VP1 sequences with the exception of isolate 160019, for which VP2 clustered with the vaccine strain Bursine while VP1 clustered with vvIBDV, suggesting reassortment. For isolates genetically related to vvIBDV, antigenic profiling revealed two patterns: while some isolates exhibited typical European vvIBDV reactivity with lack of binding of mAbs 5, other revealed extensive antigenic modifications, with lack of binding of mAbs 3, 5, 6, 8 and 9, similar to isolate 99323. These different patterns were associated with a single amino acid mutation at position 321 of VP2 that is located within peak P_HI. Full genome sequencing was performed for three isolates, among which two were representative of the two antigenic patterns observed for vvIBDV as well as the reassortant isolate 160019. This study highlights the co-circulation of both antigenically typical and modified vvIBDV during the last fifteen years in Egypt.

Genetic Characterization of Infectious Bursal Disease Viruses from Field Outbreaks of the North East Region of India

In recent years, acute severe outbreaks of infectious bursal disease (IBD) are frequently observed in commercial chicken populations of the North East Region (NER) of India, resulting in huge economic loses to poultry farmers. Field outbreaks of IBD in 30 different poultry farms in the NER were confirmed by clinicopathologic examination and reverse transcriptase PCR. A total of 10 isolates of IBD virus (IBDV) from these outbreaks were characterized by the genetic analysis of VP1 and the hypervariable region of the VP2 gene. Nucleotide sequences, deduced amino acid sequences, and phylogenetic analysis of both VP2 and VP1 genes revealed two genetically diverse strains of very virulent IBDV (vvIBDV) and one intermediate strain circulating in the NER. These isolates differ at nucleotide and amino acid levels from vvIBDV isolates of mainland India and are clustered in distinctly separate groups in the phylogenetic tree. Six of the isolates revealed a unique combination of vvIBDV amino acid signatures in the VP2 gene (A222, I256, I294), while bearing the non-vvIBDV amino acid signatures of the VP1 gene (146E, 147G, 242D), but they are clearly classified as vvIBDV in a phylogenetic analysis of both genes. Interestingly, one of the isolates showed 99% sequence homology with attenuated vaccine strains in the VP2 gene and clustered together. This study demonstrates the diversity of IBDVs in India and document for the first time the possible involvement of attenuated vaccine strains in the epidemiology of IBD in India.

SUMO1 Modification Facilitates Avibirnavirus Replication by Stabilizing Polymerase VP1

SUMOylation is a posttranslational modification that has crucial roles in diverse cellular biological pathways and in various viral life cycles. In this study, we found that the VP1 protein, the RNA-dependent RNA polymerase of avibirnavirus infectious bursal disease virus (IBDV), regulates virus replication by SUMOylation during infection. Our data demonstrated that the polymerase VP1 is efficiently modified by small ubiquitin-like modifier 1 (SUMO1) in avibirnavirus-infected cell lines. Mutation analysis showed that residues ⁴⁰⁴I and ⁴⁰⁶I within SUMO interaction motif 3 of VP1 constitute the critical site for SUMO1 modification. Protein stability assays showed that SUMO1 modification enhanced significantly the stability of polymerase VP1 by inhibiting K48-linked ubiquitination. A reverse genetic approach showed that only IBDV with I⁴⁰⁴C/T and I⁴⁰⁶C/F mutations of VP1 could be rescued successfully with decreased replication ability. Our data demonstrated that SUMO1 modification is essential to sustain the stability of polymerase VP1 during IBDV replication and provides a potential target for designing antiviral drugs targeting IBDV.IMPORTANCE SUMOylation is an extensively discussed posttranslational modification in diverse cellular biological pathways. However, there is limited understanding about SUMOylation of viral proteins of IBDV during infection. In the present study, we revealed a SUMO1 modification of VP1 protein, the RNA-dependent RNA polymerase of avibirnavirus infectious bursal disease virus (IBDV). The required site of VP1 SUMOylation comprised residues ⁴⁰⁴I and ⁴⁰⁶I of SUMO interaction motif 3, which was essential for maintaining its stability by inhibiting K48-linked ubiquitination. We also showed that IBDV with SUMOylation-deficient VP1 had decreased replication ability. These data demonstrated that the SUMOylation of IBDV VP1 played an important role in maintaining IBDV replication.

Novel variant strains of infectious bursal disease virus isolated in China

Infectious bursal disease (IBD) is one of the most important immunosuppressive diseases that seriously threaten poultry farming and food safety worldwide. The variant strain of infectious bursal disease virus (IBDV) has been greatly neglected for more than 30 years. Recently, the subclinical infection of suspected IBD, causing considerable economic losses, occurred in the main chicken-farming regions of China. Through RT-PCR, sequencing, and phylogenic analyses, novel variant IBDVs were first identified in six provinces of eastern China. Immunological detection further confirmed the antigenic variation of the Chinese variant IBDVs. The Chinese IBDV variants were obviously different from the American IBDV variants, with less than a 97.7% (VP1) or 98.7% (VP2) amino acid sequence identity. Animal experiments further confirmed the serious threat of the variant IBDVs to chickens, demonstrating irreversible damage to the central immune organ, obvious immunosuppression, and growth retardation. This study not only identified the pandemic nature of the novel variant IBDVs for the first time but also discovered the distinct molecular epidemiological characteristics of these viruses, which will contribute more to the control of the disease.

Ubiquitination Is Essential for Avibirnavirus Replication by Supporting VP1 Polymerase Activity

Avibirnavirus protein VP1, the RNA-dependent RNA polymerase, is responsible for IBDV genome replication, gene expression, and assembly. However, little is known about its chemical modification relating to its polymerase activity. In this study, we revealed the molecular mechanism of ubiquitin modification of VP1 via a K63-linked ubiquitin chain during infection. Lysine (K) residue 751 at the C terminus of VP1 is the target site for ubiquitin, and its ubiquitination is independent of VP1’s interaction with VP3 and eukaryotic initiation factor 4A II. The K751 ubiquitination promotes the polymerase activity of VP1 and unubiquitinated VP1 mutant IBDV significantly impairs virus replication. We conclude that VP1 is the ubiquitin-modified protein and reveal the mechanism by which VP1 promotes avibirnavirus replication.

Ubiquitination is critical for several cellular physical processes. However, ubiquitin modification in virus replication is poorly understood. Therefore, the present study aimed to determine the presence and effect of ubiquitination on polymerase activity of viral protein 1 (VP1) of avibirnavirus. We report that the replication of avibirnavirus is regulated by ubiquitination of its VP1 protein, the RNA-dependent RNA polymerase of infectious bursal disease virus (IBDV). In vivo detection revealed the ubiquitination of VP1 protein in IBDV-infected target organs and different cells but not in purified IBDV particles. Further analysis of ubiquitination confirms that VP1 is modified by K63-linked ubiquitin chain. Point mutation screening showed that the ubiquitination site of VP1 was at the K751 residue in the C terminus. The K751 ubiquitination is independent of VP1’s interaction with VP3 and eukaryotic initiation factor 4A II. Polymerase activity assays indicated that the K751 ubiquitination at the C terminus of VP1 enhanced its polymerase activity. The K751-to-R mutation of VP1 protein did not block the rescue of IBDV but decreased the replication ability of IBDV. Our data demonstrate that the ubiquitination of VP1 is crucial to regulate its polymerase activity and IBDV replication.

IMPORTANCE Avibirnavirus protein VP1, the RNA-dependent RNA polymerase, is responsible for IBDV genome replication, gene expression, and assembly. However, little is known about its chemical modification relating to its polymerase activity. In this study, we revealed the molecular mechanism of ubiquitin modification of VP1 via a K63-linked ubiquitin chain during infection. Lysine (K) residue 751 at the C terminus of VP1 is the target site for ubiquitin, and its ubiquitination is independent of VP1’s interaction with VP3 and eukaryotic initiation factor 4A II. The K751 ubiquitination promotes the polymerase activity of VP1 and unubiquitinated VP1 mutant IBDV significantly impairs virus replication. We conclude that VP1 is the ubiquitin-modified protein and reveal the mechanism by which VP1 promotes avibirnavirus replication.

Phylogenetic analysis of Infectious Bursal Disease viruses according to newly proposed model of classification into geno-groups

BACKGROUND

Infectious bursal disease virus (IBDV) is the causative agent of Infectious Bursal Disease (IBD), the disease causes immunosuppression which leads to secondary infections among rearing poultry flocks. Characterization of the virus is important for its control and eradication. The circulating IBDVs are classified on the basis of their antigenic and pathogenic properties. The virus is categorised as classical, variant and very virulent IBDV (vvIBDV). IBDV is a non-envelop, icosahedral double stranded virus. Viral protein 2 (VP2) is the major structural protein of capsid that determines the host-pathogen relationship. The aim of this study was to characterise the IBD virus of Pak-Asian region.

METHODOLOGY

IBDV suspected flocks were examined in Punjab, Pakistan from 2014-2018. Two hundred and fifty samples were collected with complete history of the disease. The suspected samples were collected from broiler, layer and rural poultry farms. RNA was extracted and hyper-variable region of VP2 gene was amplified using specific primers. Nucleotide sequence of the VP2 gene was determined and its Amino Acid sequence was deduced. Moreover, phylogenetic analysis was also performed.

RESULTS

The current classifications based in a hyper-variable region of the capsid protein VP2 (hvVP2), classification of IBDVs is split into newly proposed geno-groups according to Jackwood group. Among these prevailing, some IBDVs are limited geographically whereas, others are reported cosmopolitan. Genetic alterations are continuously playing role in evolution of new strains of the virus.

CONCLUSION

During this study it was found that isolates of IBDV fall in first three geno-groups. Most of the geno-groups are prevalent around the world, whereas the mutated and re-assorted ones are confined in particular areas of the globe.

Molecular characterisation of infectious bursal disease virus (IBDV) isolated from commercial broiler chickens in Nile Delta

Infectious bursal disease virus (IBDV) is a highly infectious disease affecting young chickens that alters predominantly the immune system. Emergence of new variants causes severe economic losses not only in Egypt but also all over the world. For this purpose assessment of infectious bursal disease virus (IBDV) genotypes in 20 commercial broiler flocks aged 20–35 days raised in 5 provinces in the Nile Delta, Egypt (Gharbia, Dakahlya, Kafr El sheikh, Zagazig and Domietta) was carried out. All flocks were vaccinated against IBD virus. RT-PCR revealed successful amplification of 620 bp of VP2 in 17 out of 20 samples (85%). VP2 gene nucleotide sequence analysis of six IBDV isolates (F342-1, F342-2, F342-4, F342-5 and F342-7) revealed 99.1 % similarity to the Giza 2000, Giza 2008 vv, SV-G1, SV-G2, SV-G4 and SV-G5 which were very virulent IBDV strains while the isolate F342-3 was close to D78 classical vaccinal strain and Kal 2001 classical IBDV strain variant.

A proposed nomenclature for infectious bursal disease virus isolates

Infectious bursal disease virus (IBDV) was initially identified in the USA. For decades, these viruses were not categorized using a typing system because they were considered to be antigenically and pathogenically similar. In the 1980s, a second major serotype, serotype 2, was found in turkeys. Classification of IBDV became more complex with the discovery of antigenic variant strains called "variants" in the United States and a highly virulent strain known as "very virulent" or vvIBDV identified in Europe. To distinguish the IBDV strains identified prior to this time from the antigenic variant viruses, the term "classic viruses" was adopted. Studies over the next three decades produced a wealth of information on the antigenicity, pathogenicity and molecular structure of IBDV isolates. These data made it clear that the descriptive nomenclature used for IBDV was inadequate. For example, not all viruses identified as vvIBDV by genotyping are highly pathogenic; some have reassorted genome segments that result in lower virulence. Furthermore, variant viruses are not an antigenically homogeneous group and the term "classic virus" has been used interchangeably to describe antigenic and pathogenic types of IBDV. These and other issues make the current naming system for strains of IBDV archaic. The lack of uniform testing and standards for antigenicity and pathogenicity makes it difficult to categorize IBDV strains on a global basis. A new nomenclature that includes a genotyping system that can easily be applied worldwide is proposed and serves as a platform to begin discussions on its value to the scientific community.

Identification and assessment of virulence of a natural reassortant of infectious bursal disease virus

Infectious bursal disease virus (IBDV) is one of the most important immunosuppressive viral agents in poultry production. Prophylactic vaccinations of chicken flocks are the primary tool for disease control. Widely used immunoprophylaxis can, however, provide high pressure which contributes to the genetic diversification of circulating viruses, e.g. through reassortment of genome segments. We report the genetic and phenotypic characterization of a field reassortant IBDV (designated as Bpop/03) that acquired segment A from very virulent IBDV and segment B from classical attenuated D78-like IBDV. Despite the mosaic genetic make-up, the virus caused high mortality (80%) in experimentally infected SPF chickens and induced lesions typical of the acute form of IBD. The in vivo study results are in contrast with the foregoing experimental investigations in which the natural reassortants exhibited an intermediate pathotype, and underline the complex nature of IBDV virulence.

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

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

Genome sequence analysis of a distinctive Italian infectious bursal disease virus

In a recent study, an emerging infectious bursal disease virus (IBDV) genotype (ITA) was detected in IBDV-live vaccinated broilers without clinical signs of infectious bursal disease (IBD). VP2 sequence analysis showed that strains of the ITA genotype clustered separately from vaccine strains and from other IBDV reference strains, either classic or very virulent. In order to obtain a more exhaustive molecular characterization of the IBDV ITA genotype and speculate on its origin, genome sequencing of the field isolate IBDV/Italy/1829/2011, previously assigned to the ITA genotype, was performed, and the sequences obtained were compared to the currently available corresponding sequences. In addition, phylogenetic and recombination analyses were performed. Interestingly, multiple amino acid (AA) sequence alignments revealed that the IBDV/Italy/1829/2011 strain shared several AA residues with very virulent IBDV strains as well as some virulence markers, especially in the VP1 protein. Nevertheless, sequence analysis demonstrated the presence of several residues typical of IBDV strains at a low degree of virulence in the IBDV/Italy/1829/2011 strain. Although homologous recombination and reassortant phenomena may occur naturally among different IBDV strains, no evidence of those events was found in the genome of the IBDV/Italy/1829/2011 strain, which was confirmed to be a genetically distinctive IBDV genotype.

gga-miR-454 suppresses infectious bursal disease virus (IBDV) replication via directly targeting IBDV genomic segment B and cellular Suppressors of Cytokine Signaling 6 (SOCS6)

MicroRNAs (miRNAs), as post-transcriptional regulators, play important roles in the process of viral infection through inhibiting virus replication or modulating host immune response. However, the role of miRNAs in host response against infectious bursal disease virus (IBDV) infection is still unclear. In this study, we found that gga-miR-454 of the host was decreased in response to IBDV infection and that transfection of DF-1 cells with miR-454 inhibited IBDV replication via directly targeting the specific sequence of IBDV genomic segment B, while blockage of endogenous miR-454 by inhibitors enhanced virus replication. Furthermore, gga-miR-454 increased the expression of IFN-β by targeting Suppressors of Cytokine Signaling 6 (SOCS6), enhancing the antiviral response of host cells. These findings highlight a crucial role of gga-miR-454 in host defense against IBDV infection.