Coding sequences of both genome segments of a European 'very virulent' infectious bursal disease virus

Detection of Very Virulent Strains of Infectious Bursal Disease Virus (vvIBDV) in Commercial Broilers from Uruguay

Bursal samples were collected from commercial broiler flocks exhibiting clinical signs suggestive of infectious bursal disease (IBD). The presence of IBD virus (IBDV) was confirmed by partial amplification of the VP2 and VP1 genes by reverse transcription and polymerase chain reaction. The Uruguayan viruses were identified as very virulent strains of IBDV (vvIBDV) by nucleotide and amino acid sequence analysis. The comparison of the VP2 nucleotide sequences among the Uruguayan samples revealed the presence of single-nucleotide polymorphisms suggestive of different viral subpopulations or quasispecies in the same flock. The comparative analysis indicated that these Uruguayan viruses were genetically close to the European strain UK661 and to the vvIBDVs previously detected in Venezuela. Our analyses provided new information about the distribution, variability, and evolutionary trends of vvIBDV strains in the Americas.

Genetic reassortment of infectious bursal disease virus in nature

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

Infectious bursal disease virus: strains that differ in virulence differentially modulate the innate immune response to infection in the chicken bursa

Little is understood about the immune responses involved in the pathogenesis of infectious bursal disease virus (IBDV). Strains of IBDV differ in their virulence: F52/70 is a classical virulent strain (vIBDV), whereas UK661 is a very virulent strain (vvIBDV) that causes greater pathology and earlier mortality. The exact causes of clinical disease and death are still unclear. Pro-inflammatory cytokines such as interleukin (IL)-1beta and IL-6, produced by activated macrophages, could play a role, as could cytokines produced by T and natural killer (NK) cells, such as interferon (IFN)-gamma, which stimulate macrophages. We quantified mRNA transcription in bursal tissue, by real-time quantitative reverse transcription- polymerase chain reaction (RT-PCR), for the type I IFN (IFN-alpha and IFN-beta), pro-inflammatory cytokines (IL-1beta, IL-6, and CXCLi2), the anti-inflammatory cytokine transforming growth factor (TGF)-beta4, and Th1 cytokines (IFN-gamma, IL-2 [and the closely related IL-15], IL-12, and IL-18) for the first 5 days after infection of 3-week-old chickens with F52/70 or UK661 and compared these with levels in bursal tissue from uninfected age-matched controls. Both strains induced a pro-inflammatory response, evidenced by increased mRNA transcription of IL-1beta, IL-6, and CXCLi2, and down-regulation of TGF-beta4, of similar magnitude and timing. IFN-gamma mRNA was induced by both strains, although to a greater degree by the vvIBDV strain, indicating that a cell-mediated response is induced. Neither virus initially induced high levels of type I IFN. F52/70 seems to use a "stealth" approach by not inducing the type I IFNs, whereas UK661 down-regulates their expression. This suggests that both viruses modulate the host immune response, although probably by using different mechanisms.

Very virulent infectious bursal disease virus: reduced pathogenicity in a rare natural segment-B-reassorted isolate

The purpose of this study was to compare the molecular epidemiology of infectious bursal disease virus (IBDV) segments A and B of 50 natural or vaccine IBDV strains that were isolated or produced between 1972 and 2002 in 17 countries from four continents, with phenotypes ranging from attenuated to very virulent (vv). These strains were subjected to sequence and phylogenetic analysis based on partial sequences of genome segments A and B. Although there is co-evolution of the two genome segments (70 % of strains kept the same genetic relatives in the segment A- and B-defined consensus trees), several strains (26 %) were identified with the incongruence length difference test as exhibiting a significantly different phylogenetic relationship depending on which segment was analysed. This suggested that natural reassortment could have occurred. One of the possible naturally occurring reassortant strains, which exhibited a segment A related to the vvIBDV cluster whereas its segment B was not, was thoroughly sequenced (coding sequence of both segments) and submitted to a standardized experimental characterization of its acute pathogenicity. This strain induced significantly less mortality than typical vvIBDVs; however, the mechanisms for this reduced pathogenicity remain unknown, as no significant difference in the bursal lesions, post-infectious antibody response or virus production in the bursa was observed in challenged chickens.

Chicken recombinant antibodies specific for very virulent infectious bursal disease virus

A phage-displayed single chain variable fragment (scFv) antibody library was constructed from the immune spleen cells of chickens immunized with very virulent infectious bursal disease virus (vvIBDV) strain CS89. A library consisting of around 9.2 x 10(7) clones was subjected to 3 rounds of panning against captured CS89 virus. Analysis of individual clones by nucleotide sequencing revealed at least 22 unique scFv antibodies binding to vvIBDV in ELISA. Testing of the scFv antibody panel in ELISA against classical, variant or vaccine strains and a wide variety of vvIBDV isolates from the UK, China, France, Belgium, Africa, Brazil, Indonesia and the Netherlands identified one antibody, termed chicken recombinant antibody 88 (CRAb 88) that was specific for vvIBDV. CRAb 88 was capable of recognizing all vvIBDV strains tested regardless of their country of origin and showed no reactivity with classical, variant or vaccine strains, lending support to the use of this scFv as a powerful diagnostic tool for the differentiation of vvIBDV strains. Immunoprecipitation studies revealed that CRAb 88 was directed towards a highly conformational epitope located within the major neutralizing protein VP2. Sequence analysis of the hypervariable region of VP2 of the IBDV strains tested indicate that Ile(256) and Ile(294) may play roles in binding of CRAb 88. This is the first reagent of its type capable of positively distinguishing vvIBDV from other IBDV strains.

Detection of Infectious Bursal Disease Virus in Various Lymphoid Tissues of Experimentally Infected Specific Pathogen Free Chickens by Different Reverse Transcription Polymerase Chain Reaction Assays

Infectious bursal disease (IBD) is a worldwide distributed immunosuppressive viral disease in young chickens, controlled by vaccination. Emergence of several strains of IBD virus (IBDV) has created a demand for strain-specific diagnostic tools. In the present experiment, five different reverse transcription polymerase chain reaction (RT-PCR) assays, including two recently developed strain-specific assays, were employed for detection of ribonucleic acid (RNA) from three different IBDV strains in bursa tissue samples from experimentally infected specific pathogen free chickens. The virus strains included vaccine strain D78, classical strain Faragher 52/70, and the very virulent Danish strain DK01. The presence of the virus infection was confirmed by histopathologic evaluation of bursa lesions. The largest number of positive samples was obtained with a strain-specific two-step multiplex (MPX) RT-PCR assay based on iScript enzyme, and the commercially available Qiagen one-step RT-PCR. Between these methods, agreement was obtained for 57 of 59 samples. Because the Qiagen one-step RT-PCR assay was suggested as the more sensitive of these two assays, it was used for detection of IBDV in bone marrow, spleen, thymus, and cecal tonsils from experimentally infected chickens. The identity of the virus strains involved was confirmed by MPX RT-PCR. In conclusion, the MPX RT-PCR represented a reliable assay for detection and differentiation of IBDV strains in selected lymphoid tissues of chickens. All three of the IBDV strains used were detected in bursa tissues, whereas only the two virulent strains were detected in bone marrow, spleen, and thymus.

Molecular studies on suspect very virulent infectious bursal disease virus genomic RNA samples

Infectious bursal disease (IBD) associated with high mortality was first observed in Europe in the mid-1980s. The viruses identified in those outbreaks were described as being very virulent infectious bursal disease virus (vvIBDV) strains. These viruses have spread to nearly every continent but have not yet been identified in North America, Australia, and New Zealand. There is a real and immediate concern that the very virulent form of IBDV will continue to spread until it is present on every continent. Genomic RNA samples from IBDV strains suspected of being very virulent were submitted to our laboratory for molecular analysis. Nucleotide sequences of the VP2 gene hypervariable sequence region were determined for 18 of these viruses. A comparison with published vvIBDV sequences indicated that all but one sample (Thai 4) had nucleotide and predicted amino acid sequences consistent with vvIBDV strains. Published sequences and the nucleotide sequences of our 17 putative vvIBDV strains were used to identify unique nucleotides in the VP2 gene. Probe pairs for a real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay were designed based on these unique sequences and then used to test the 17 genomic samples that were identified by nucleotide sequencing to be consistent with vvIBDV, plus the one Thai 4 sample that was not consistent with vvIBDV. Using melting temperature (Tm) analysis following real-time RT-PCR, two probe pairs (vv232 and vv256) successfully identified the 17 putative vvIBDV strains and distinguished them from the Thai 4 sample. An additional 26 genomic RNA samples submitted as suspect vvIBDV strains were then tested using the vv232 and vv256 probes. Based on the melting point analysis of these two probes, all 26 samples contained nucleotide sequences consistent with vvIBDV strains. The specificity of the vv232 and vv256 probe pairs was evaluated using 19 non-vvIBDV strains. In every case, the probes distinguished the 19 classic and variant (non-vvIBDV) strains from the putative vvIBDV strains. Diagnostic assays that can reliably identify vvIBDV strains are needed for surveillance programs designed to monitor the spread of these viruses.

Infectious bursal disease virus (IBDV) induces apoptosis in chicken B cells

The ability of infectious bursal disease virus (IBDV) serotypes 1 and 2, and the role of VP4 of both serotypes as well as the capacity of three IBDV intermediate serotype 1-specific vaccine strains to induce apoptosis in a chicken B-lymphocyte cell line, DT40, were investigated using the TUNEL technique. It was observed that IBDV serotype 1 infected the DT40 cell line and directly induced apoptosis. In contrast, the non-pathogenic serotype 2 neither infected nor induced apoptosis, but was able to reduce the serotype 1-induced apoptosis when the two viruses were present in combination. VP4 of both serotypes did not induce apoptosis. IBDV VP2 of serotype 2 induced apoptosis in the same proportion and intensity as VP2 of serotype 1. IBDV intermediate vaccines varied in their ability to induce apoptosis in the DT40 cell line, which was also decreased-delayed in presence of serotype 2 IBDV. We hypothesize that both serotypes compete for the same receptor in DT-40 cells, and suggest that IBDV-induced apoptosis is a multistep process involving virus replication, protein expression, and release of virions.

A very virulent genotype of infectious bursal disease virus predominantly associated with recurrent infectious bursal disease outbreaks in Tunisian vaccinated flocks

Outbreaks of infectious bursal disease (IBD) still continue to afflict the Tunisian poultry industry even in those flocks where the vaccination program is strictly applied. To characterize the viruses that circumvent protection provided by vaccination, field isolates of infectious bursal disease virus (IBDV) obtained from vaccinated flocks that have repeatedly experienced IBDV outbreak episodes were analyzed from bursal samples by reverse transcription coupled with polymerase chain reaction and dideoxynucleotide sequencing of the VP2 hypervariable region. Although sequence data were obtained from samples collected from three distinct flocks over a period of 3 years, only limited sequence variation has been observed. The few nucleotide changes were silent and the deduced amino acid sequences were identical. Thus, the virus population that predominates in the field seems to represent a homogeneous antigenic pool. Compared with the VP2 sequences of several IBDV strains, this predominant pool was found to be closely related to the very virulent (vv) IBDV viruses described in Europe and Asia. Sequence and phylogenetic analysis of the precursor polyprotein coding sequence of a representative Tunisian isolate further confirmed its assignment to the vv genotype. The deduced amino acid sequence of the whole polyprotein of the Tunisian isolate was found to be identical to a South Korean IBDV strain. Alignment of the polyprotein amino acid sequence of 35 IBDV strains identified additional mutations outside the VP2 variable domain and which occur frequently in vv strains. Based on this comparative analysis, the set of amino acid residues that should represent a typical vv profile involves Ala222, Ile242, Ile256, Ile294, Leu451, Tyr680, N685, Ser715, Asp751, Val990, and Ala1005. Such a combination of amino acid changes was observed for the majority of vvIBDV strains that define a distinct phylogroup.

Generation of serotype 1/serotype 2 reassortant viruses of the infectious bursal disease virus and their investigation in vitro and in vivo

Infectious bursal disease virus (IBDV) is the causative agent of acute or immunosuppressive disease in chickens. Serotype 1 strains are pathogenic whereas serotype 2 strains neither cause disease nor protect against infection with the serotype 1 strains. The target organ of serotype 1 strains is the bursa Fabricii (BF). The molecular determinants of this tropism, and therefore pathogenicity, are poorly understood. IBDV is a non-enveloped icosahedral virus particle of 60 nm in diameter, which contains two genome segments of double-stranded RNA. Here, the generation of interserotypic reassortants using the reverse genetics approach is reported. The results of in vitro and in vivo investigations show that genome segment A determines the bursa tropism of IBDV, whereas segment B is involved in the efficiency of viral replication; they further indicate the significance of the interaction of the polymerase (segment B) with the structural protein VP3 (segment A) or the viral genome for efficient virus formation and replication.

Assessment of genetic, antigenic and pathotypic criteria for the characterization of IBDV strains

The aim of this work was the selection and comparison of representative infectious bursal disease virus (IBDV) strains. Nine strains of IBDV, isolated at different times and from different geographic regions of Europe and China, were characterized. Batches of all strains were prepared following standardized protocols and checked for the absence of contaminating viruses. Criteria used for their characterization were: (i) the nucleotide sequence of the VP2 variable region, (ii) binding to a panel of neutralizing monoclonal antibodies in antigen capture enzyme-linked immunosorbent assays, and (iii) virulence in specific pathogen free chickens after infection with a standardized number of median embryo infective doses. Based on the first two criteria, two of nine strains were classified as classical virulent (cv) IBDV (F52/70, Cu-1wt), and five as very virulent (vv) IBDV (849VB, 96108, HK46, GX, Harbin). Remarkably, although a clear-cut difference was demonstrable between European cvIBDV (F52/70 and Cu-1wt) and vvIBDV (849VB and 96108) strains, there was a continuum in the pathogenicity of Chinese vvIBDVs. Our results indicate the probable existence of differences in virulence within IBDV lineages determined on the basis of antigenic typing using monoclonal antibodies and the alignment of the VP2 sequences. This indicates limitations in the analysis of IBDV pathotypes based on the VP2 variable region and emphasizes that these criteria may not be sufficient for the classification of IBDV strains.

Enhanced immunopathology induced by very virulent infectious bursal disease virus

Immunohistochemical and flow cytometric analyses of the bursa, spleen and thymus following infection with the very virulent infectious bursal disease virus (vvIBDV) strain UK661 revealed discrete differences from classical virulent infectious bursal disease virus strains. Bu-1+, immunoglobulin (Ig)M+ and IgG+ cells were all depleted from the bursa, spleen and thymus, suggesting loss of both immature and mature B lymphocytes. Small numbers of Bu-1+ cells repopulated the bursa 14 days post-infection but few of these expressed IgM or IgG. A transient increase in macrophages at 3 to 5 days post-infection was followed by a later influx of CD4+ and CD8+ T cells into the bursa. Loss of cortical thymocytes during the acute phase of infection suggested disruption of the T-cell system. The results showed that vvIBDV strain UK661 caused earlier and more severe pathology than classical virulent strains of infectious bursal disease virus. The marked influx of T cells into the infected bursa indicates that cell-mediated immunity is likely to be important in the clearance of vvIBDV.

Genetic characterization of very virulent infectious bursal disease viruses from Latin America

Very virulent infectious bursal disease viruses (vvIBDVs) were detected in phenol inactivated bursal samples obtained from Brazil, the Dominican Republic, and Venezuela. After nucleotide sequence analysis of the hypervariable region of VP2 gene, the vvIBDVs from Brazil and Venezuela exhibited all of the 14 nucleotide changes that are conserved in the European UK-661 and most other vvIBDV strains. However, the vvIBDV from the Dominican Republic presented 11 nucleotide changes that are conserved in vvIBDV strains. After phylogenetic analysis, the Latin American strains were found to be related to other vvIBDV strains from Europe, Asia, and Africa. However, Brazilian and Dominican vvIBDVs clustered in two separate subgroups, while the vvIBDVs from Venezuela were closely related to other strains from other parts of the world. By deduced amino acid sequence, the three conserved amino acid residues in vvIBDV strains (222 Ala, 256 Ile, and 294 Ile) were confirmed in the Latin American viruses, and one amino acid change (300 Ala) was unique to all vvIBDVs from the Dominican Republic. The occurrence of this change in the Dominican vvIBDVs may have an impact in their antigenic makeup. Results of this study indicate that the vvIBDVs detected in Latin America are genetically similar to IBDV strains from other parts of the world. However, vvIBDVs from Venezuela were more similar to the vvIBDV strains from Europe and Asia. Of all the samples analyzed, vvIBDVs from Brazil and the Dominican Republic exhibited more genetic changes. These changes may have emerged as a result of the different management practices and environmental conditions present in each particular geographic area.

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

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

Sequence and analysis of genomic segment A and B of very virulent infectious bursal disease virus isolated from China

A very virulent infectious bursal disease virus (vvIBDV) field strain, named SH95, was identified and characterized from flocks with vaccination failure in Shanghai. The use of random primer and a reverse transcriptase lacking RNase-H activity produced full-length cDNA copies of the viral genomic A and B segments of SH95. The 3259 base pairs (bp) of segment A and 2827 bp of segment B were amplified by long and accurate PCR in a single step, then successfully cloned and sequenced. There were five to 27 amino acid substitutions compared with other IBDV strains within the segment A polyprotein (of these, three are unique) and about nine to 38 amino acid substitutions within VP1 (of which, four are unique). The comparison of sequences encoding the polyprotein showed that vvIBDV SH95 was most closely related to Asiatic vvIBDVs, which formed a closely related group clearly distinguishable from other classical strains of IBDV. Phylogenetic analysis suggested that vvIBDV SH95 and some other Asiatic vvIBDVs were derived from similar origin. But the topology tree performed on segment B was quite different from that performed on segment A, indicating that a genetic reassortment had played an important role in the emergence of vvIBDV SH95.

Differentiation of infectious bursal disease viruses by restriction enzyme analysis of RT-PCR amplified VP1 gene sequence

In order to differentiate infectious bursal disease virus (IBDV) isolates/strains, a quick method of RT-PCR followed by restriction enzyme analysis of VP1 gene sequence is being reported for the first time. A 480 bp fragment, comprising one of the RNA dependent RNA polymerase motifs of VP1 gene sequence of an Indian classical virus, an attenuated vaccine strain, Georgia and two Indian field isolates, genetically similar to reported very virulent strains of IBDV, was amplified by RT-PCR. Restriction enzyme digestion of PCR products with Taq1 enzyme generated distinct profile for field isolates, different from the classical and attenuated viruses, whereas restriction profile with BstNI restriction enzyme was similar in all the viruses, irrespective of the pathotype. Therefore, the present results suggest that Taq1 digestion can be taken up for the differentiation of field isolates from the classical and vaccine strains. The sequence analysis of VPI gene of reported very virulent IBD viruses from Europe and Japan, using 'MapDraw' programme of Lasergene software, revealed similar restriction enzyme profile as in Indian field isolates.

Blotched snakehead virus is a new aquatic birnavirus that is slightly more related to avibirnavirus than to aquabirnavirus

By different approaches, we characterized the birnavirus blotched snakehead virus (BSNV). The sequence of genomic segment A revealed the presence of two open reading frames (ORFs): a large ORF with a 3,207-bp-long nucleotide sequence and a 417-nucleotide-long small ORF located within the N-terminal half of the large ORF, but in a different reading frame. The large ORF was found to encode a polyprotein cotranslationally processed by the viral protease VP4 to generate pVP2 (the VP2 precursor), a 71-amino-acid-long peptide ([X]), VP4, and VP3. The two cleavage sites at the [X]-VP4 and VP4-VP3 junctions were identified by N-terminal sequencing. We showed that the processing of pVP2 generated VP2 and several small peptides (amino acids [aa] 418 to 460, 461 to 467, 468 to 474, and 475 to 486). Two of these peptides (aa 418 to 460 and 475 to 486) were positively identified in the viral particles with 10 additional peptides derived from further processing of the peptide aa 418 to 460. The results suggest that VP4 cleaves multiple Pro-X-Ala downward arrow Ala motifs, with the notable exception of the VP4-VP3 junction. Replacement of the members of the predicted VP4 catalytic dyad (Ser-692 and Lys-729) confirmed their indispensability in the polyprotein processing. The genomic segment B sequence revealed a single large ORF encoding a putative polymerase, VP1. Our results demonstrate that BSNV should be considered a new aquatic birnavirus species, slightly more related to IBDV than to IPNV.

Sequence analysis of Malaysian infectious bursal disease virus isolate and the use of reverse transcriptase nested polymerase chain reaction enzyme-linked immunosorbent assay for the detection of VP2 hypervariable region

The VP2 hypervariable region of P97/302 local infectious bursal disease virus (IBDV) isolate was amplified by the reverse transcriptase (RT) nested polymerase chain reaction (PCR) and cloned. This region of P97/302 local isolate was sequenced and compared with eight other reported IBDV sequences. The result showed that P97/302 IBDV was most identical to the reported very virulent IBDV strains because it has amino acid substitutions at positions 222, 256, 294, and 299, which encode alanine, isoleucine, isoleucine, and serine, respectively. This region can be digested with restriction enzymes of Taq1, Sty1, Ssp1 but not with Sac1. The P97/302 isolate was then used for the optimization of RT nested PCR enzyme-linked immunosorbent assay (ELISA). The RT nested PCR ELISA was able to detect 10(-4) dilution of the infected bursa homogenates and was 10 times more sensitive when compared with the agarose gel detection method. The RT nested PCR ELISA can detect up to 0.48 ng of the PCR product. The specificity of this nested PCR ELISA was also high (100%).

Cloning and nucleotide analysis of segment A gene of infectious bursal disease virus detected in Korea

A strain of infectious bursal disease virus (IBDV) was detected from bursal tissues of chicks which suffered from infectious bursal disease (IBD) in Chinju, Korea and provisionally named as Chinju strain. A full-length cDNA clone for segment A gene of the virus was constructed, and complete nucleotide sequence of the gene including noncoding region was determined and analyzed by comparison with that of other IBDV strains. The segment A gene of Chinju strain consisted of 3,269 nucleotides including 862 adenine (26.4%), 917 cytosine (28.0%), 854 guanine (26.1%) and 636 thymine (19.5%). There were regions for two open reading frames (ORFs), ORF1 encoding the VP5 with ATG codon at nucleotides 98-100 and ORF2 encoding the polyprotein of VP2, VP4 and VP3 in the nucleotides 132-3,170. In deduced translation the ORF2 encoded 1,012 amino acids. The full nucleotide sequence of segment A gene and amino acid sequence of ORF2 of the Chinju strain showed 98-99% homology with those of the very virulent IBDVs (vvIBDVs) such as HK46, OKYM, UK661, UPM97/ 61, D6948 and BD3/99. In phylogenetic analysis of nucleotide and amino acid sequences, the Chinju strain was also related closely to the vvIBDVs. Hence, it was suggested that the Chinju strain is a vvIBDV. The nucleotide and amino acid sequences of the Chinju strain with pertinent information can be useful for the development of genetically engineered vaccines and diagnostic reagents against vvIBDV.

Protective immunity against infectious bursal disease virus in chickens in the absence of virus-specific antibodies

The role of cell-mediated immunity (CMI) in pathogenesis of infectious bursal disease virus (IBDV) was investigated. One-day-old specific pathogen-free chickens were treated with 3mg of cyclophosphamide (Cy) per chicken for 4 consecutive days and, 3 weeks later, infected with the IBDV-IM strain. Chickens were examined for: (a) mitogenic response of splenocytes to ConA, as an indicator of T-cell functions in vitro, (b) antibody against IBDV by ELISA, (c) IBDV genome in various tissues by RT-PCR and (d) immunological memory. At the time of IBDV infection, Cy-treated chickens had depleted bursal tissue (an avian primary B-cell lymphoid organ), severely compromised antibody-producing ability, but normal T-cell response to ConA. In primary infection, no detectable antibody against IBDV antigen in Cy-treated, IBDV-infected chickens was observed up to 28 days post-infection (PI), while IBDV genome was detected by RT-PCR in spleen, thymus, liver and blood until 10 days PI. Like intact control chickens infected with IBDV, Cy-treated, IBDV-infected chickens suppressed splenocytes responses to ConA from 5 to 10 days PI, suggesting that intact control as well as Cy-treated chickens responded similarly to IBDV infection in the early phase. Following re-infection with IBDV, no detectable secondary antibody response to IBDV as well as IBDV genome in tissues were observed in Cy-treated chickens, while intact control chickens developed vigorous secondary antibody response. Similar to intact control chickens infected with IBDV, Cy-treated chickens after second infection with IBDV did not suppress splenocyte response to ConA. These results suggested that in the absence of detectable anti-IBDV antibodies, protection of Cy-treated chickens from IBDV infection may occur via immunological memory mediated by CMI. We concluded that under normal conditions, IBDV induces a protective antibody response, however, in the absence of antibody, CMI alone is adequate in protecting birds against virulent IBDV.

Exchange of the C-terminal part of VP3 from very virulent infectious bursal disease virus results in an attenuated virus with a unique antigenic structure

Infectious bursal disease virus (IBDV) is the major viral pathogen in the poultry industry. Live attenuated serotype 1 vaccine strains are commonly used to protect susceptible chickens during their first 6 weeks of life. Wild-type serotype 1 IBDV strains are highly pathogenic only in chickens, whereas serotype 2 strains are apathogenic in chickens and other birds. Here we describe the replacement of the genomic double-stranded RNA (dsRNA) encoding the N- or C-terminal part of VP3 of serotype 1 very virulent IBDV (vvIBDV) (isolate D6948) with the corresponding part of serotype 2 (isolate TY89) genomic dsRNA. The modified virus containing the C-terminal part of serotype 2 VP3 significantly reduced the virulence in specific-pathogen-free chickens, without affecting the distinct bursa tropism of serotype 1 IBDV strains. Furthermore, by using serotype-specific antibodies we were able to distinguish bursas infected with wild-type vvIBDV from bursas infected with the modified vvIBDV. We are currently evaluating the potential of this recombinant strain as an attenuated live vaccine that induces a unique serological response (i.e., an IBDV marker vaccine).

Detection of infectious bursal disease virus in different lymphoid organs by single-step reverse transcription polymerase chain reaction and microplate hybridization assay

A rapid and sensitive method for the detection of infectious bursal disease virus (IBDV) RNA in different chicken lymphoid organs was developed. The method is based on a single-step reverse transcription polymerase chain reaction (RT-PCR) procedure and the enzyme-linked immunosorbent assay (ELISA) detection method of amplified products. Vaccinal IBDV strain and field isolates were used for the optimization of RT-PCR and for the determination of conditions for microplate hybridization and colorimetric detection of the amplicons. With this method, viral RNA could be detected in various stages of infection in samples of different lymphoid organs. Bursas and cecal tonsils were suitable organs for viral RNA detection at different times during IBDV infection. The RT-PCR/ELISA method can be applied for IBDV detection in routine diagnostic tests, which are not usually carried out because of the difficulties involved in isolating the virus.

Site-directed mutagenesis of Avibirnavirus VP4 gene

Virus protein VP4 of infectious bursal disease virus (IBDV) is a protease which separates VPX and VP3 from the polyprotein. We studied the importance of serine and aspartic acid on cleavage at the VPX/VP4 junction and analysed the role of the proposed H547, D590, and S653 catalytic site using five different mutations on VP4. Our results suggest that the replacement of serine by lysine in AXAAS motifs in serotype II IBDV influences polyprotein (PP) processing by VP4 and also indicate the presence of an alternative cleavage site. Mutation on D ((510)TLAADK(515)) prevented the cleavage at the VPX/VP4 junction, but we have found that independently of the importance of those alanines in LAA, D has an important role as part of the cleavage site. Replacement of histidine by proline H547P completely abolished PP processing. Mutation on D590 induced a partial PP processing when it was replaced by proline and the replacement of serine by proline at S653P induced a prominent change in PP processing. These results permit us to conclude that IBDV VP4 has the ability to act according to structural and topographical changes during translational and posttranslational processes and allow multiple hit sites, which serve to increase effectiveness.

Alteration of amino acids in VP2 of very virulent infectious bursal disease virus results in tissue culture adaptation and attenuation in chickens

Reverse genetics technology offers the possibility to study the influence of particular amino acids of infectious bursal disease virus (IBDV) on adaptation to tissue culture. Genomic segments A and B of the very virulent (vv) IBDV field strain UK661 were completely cloned and sequenced, and the strain was rescued from full-length cDNA copies of both segments (UK661rev). Using site-directed mutagenesis, alteration of a single amino acid in the segment A-encoded VP2 (A284T) resulted in a limited capacity of UK661 to replicate in tissue culture. Additional alteration of a second amino acid (Q253H) increased replication efficiency in tissue culture. The second mutant (UK661-Q253H-A284T) was used to infect chickens and results were compared with UK661 and UK661rev. Whereas UK661 and UK661rev induced 100% morbidity and 50-80% mortality, UK661-Q253H-A284T proved to be strikingly attenuated, producing neither morbidity nor mortality. Moreover, UK661-Q253H-A284T-infected animals were protected from challenge infection. Thus, alteration of two specific amino acids in the VP2 region of IBDV resulted in tissue culture adaptation and attenuation in chickens of vvIBDV. The data demonstrate that VP2 plays a decisive role in pathogenicity of IBDV.

Molecular characterisation of very virulent infectious bursal disease viruses in Taiwan

The very virulent infectious bursal disease virus (vv IBDV) RNA in the bursa of Fabricius and spleen from experimentally infected chickens or field samples was detected by in situ hybridisation (ISH) with subsequent reverse transcription (RT)-polymerase chain reaction (PCR) and sequence analysis. The VP 2 gene of vv IBDV was detected by ISH in infected chicken tissues with a cloned digoxigenin (DIG)-labelled cDNA probe. To verify ISH, RT - PCR was used to amplify two 643- and 500-base pair fragments on the VP 2 gene of IBDV in the bursa of Fabricius. With all isolates, two c DNA fragments of 643 and 500 bp long, respectively, were generated as expected and further confirmed the specificity of ISH. Analysis of the hypervariable region (HVR) of the VP 2 gene revealed that a serine-rich heptapeptide SWSASGS located at amino acids 326-332 was conserved in recent Taiwanese strains, and two amino acid substitutions were found in the classical Taiwanese strains at positions 330M and 331W. Three amino acids were unique to the vv strains at positions 222A, 256I and 294I, compared with classical and variant strains. Sequence and phylogenetic analysis showed that the recent Taiwanese strains were closely related, very similar to vv IBDV s from Europe, China, Japan, and Africa, and distantly related to the Taiwanese classical strains.

Recombinant Semliki Forest virus vector exhibits potential for avian virus vaccine development

The Semliki Forest virus (SFV) expression system was evaluated as a basis for avian vaccine development. Initial studies indicated that 1-day-old specific pathogen-free (SPF) chicks were susceptible to infection with an infectious strain of SFV, producing SFV-specific antibodies but no clinical disease. One-day-old SPF chicks immunised intramuscularly with recombinant replication-defective SFV (rSFV) particles expressing the Escherichia coli (E. coli) lacZ reporter gene developed high titres of beta-gal- specific antibodies at 4 weeks p.i. after two inoculations. In contrast, significantly lower antibody levels were elicited in chicks immunised with a recombinant SFV-based DNA construct or a conventional CMV promoter-based DNA plasmid. rSFV particles encoding the protective VP2 protein or the VP2/VP4/VP3 polyprotein of infectious bursal disease virus (IBDV) were produced and the expressed antigens were characterised in cell culture. Proteins of the correct size were generated and found to react against a range of IBDV-specific monoclonal antibodies. Immunisation of 1-day-old SPF chicks with rSFV particles encoding the IBDV proteins resulted in specific antibodies being elicited in all birds, neutralising antibodies being induced in some but not all birds.

Protection from IBDV-induced bursal damage by a recombinant fowlpox vaccine, fpIBD1, is dependent on the titre of challenge virus and chicken genotype

Expression of the VP2 capsid protein of infectious bursal disease virus (IBDV) in an vaccine strain of fowlpox has produced an experimental recombinant vaccine, fpIBD1. Successful vaccination with fpIBD1 was dependent on the titre of challenge virus for high titres of challenge virus were able to overcome protection induced by fpIBD1 whereas challenge with a low titre of virus did not. The genotype of chicken also has an important effect on the outcome of challenge possibly as a result of the major histocompatability complex and its ability to present VP2-derived peptides to the immune system. It was not possible to protect the inbred white leghorn chicken strain, line 15I, from IBDV-induced bursal damage by vaccination with fpIBD1 even at the lowest titre of challenge virus used. All other inbred white leghorn chickens examined (line 6(1), C. B4 and C.B12) and outbred Rhode Island Red chickens were protected by fpIBD1. Protection by the fpIBD1 vaccine is induced in the absence of detectable serum antibodies, suggesting the possibility of a significant role for cell-mediated immunity in protection from IBDV challenge.

Rescue of very virulent and mosaic infectious bursal disease virus from cloned cDNA: VP2 is not the sole determinant of the very virulent phenotype

Many recent outbreaks of infectious bursal disease in commercial chicken flocks worldwide are due to the spread of very virulent strains of infectious bursal disease virus (vvIBDV). The molecular determinants for the enhanced virulence of vvIBDV compared to classical IBDV are unknown. The lack of a reverse genetics system to rescue vvIBDV from its cloned cDNA hampers the identification and study of these determinants. In this report we describe, for the first time, the rescue of vvIBDV from its cloned cDNA. Two plasmids containing a T7 promoter and either the full-length A- or B-segment cDNA of vvIBDV (D6948) were cotransfected into QM5 cells expressing T7 polymerase. The presence of vvIBDV could be detected after passage of the transfection supernatant in either primary bursa cells (in vitro) or embryonated eggs (in vivo), but not QM5 cells. Rescued vvIBDV (rD6948) appeared to have the same virulence as the parental isolate, D6948. Segment-reassorted IBDV, in which one of the two genomic segments originated from cDNA of classical attenuated IBDV CEF94 and the other from D6948, could also be rescued by using this system. Segment-reassorted virus containing the A segment of the classical attenuated isolate (CEF94) and the B segment of the very virulent isolate (D6948) is not released until 15 h after an in vitro infection. This indicates a slightly retarded replication, as the first release of CEF94 is already found at 10 h after infection. Next to segment reassortants, we generated and analyzed mosaic IBDVs (mIBDVs). In these mIBDVs we replaced the region of CEF94 encoding one of the viral proteins (pVP2, VP3, or VP4) by the corresponding region of D6948. Analysis of these mIBDV isolates showed that tropism for non-B-lymphoid cells was exclusively determined by the viral capsid protein VP2. However, the very virulent phenotype was not solely determined by this protein, since mosaic virus containing VP2 of vvIBDV induced neither morbidity nor mortality in young chickens.

Role of Ser-652 and Lys-692 in the protease activity of infectious bursal disease virus VP4 and identification of its substrate cleavage sites

The polyprotein of infectious bursal disease virus (IBDV), an avian birnavirus, is processed by the viral protease, VP4. Previous data obtained on the VP4 of infectious pancreatic necrosis virus (IPNV), a fish birnavirus, and comparative sequence analysis between IBDV and IPNV suggest that VP4 is an unusual eukaryotic serine protease that shares properties with prokaryotic leader peptidases and other bacterial peptidases. IBDV VP4 is predicted to utilize a serine-lysine catalytic dyad. Replacement of the members of the predicted catalytic dyad (Ser-652 and Lys-692) confirmed their indispensability. The two cleavage sites at the pVP2-VP4 and VP4-VP3 junctions were identified by N-terminal sequencing and probed by site-directed mutagenesis. Several additional candidate cleavage sites were identified in the C-terminal domain of pVP2 and tested by cumulative site-directed mutagenesis and expression of the mutant polyproteins. The results suggest that VP4 cleaves multiple (Thr/Ala)-X-Ala downward arrowAla motifs. A trans activity of the VP4 protease of IBDV, and also IPNV VP4 protease, was demonstrated by co-expression of VP4 and a polypeptide substrate in Escherichia coli. For both proteases, cleavage specificity was identical in the cis- and trans-activity assays. An attempt was made to determine whether VP4 proteases of IBDV and IPNV were able to cleave heterologous substrates. In each case, no cleavage was observed with heterologous combinations. These results on the IBDV VP4 confirm and extend our previous characterization of the IPNV VP4, delineating the birnavirus protease as a new type of viral serine protease.

Induction of protective immunity in chickens immunised with plasmid DNA encoding infectious bursal disease virus antigens

Direct DNA inoculations were used to determine the efficacy of gene immunisation of chickens to elicit protective immune responses against infectious bursal disease virus (IBDV). The vp2 gene of IBDV strains GP40 and D78, and the vp2-vp4-vp3 encoding segment of strain D78 were cloned in an expression vector which consisted of human cytomegalovirus (HCMV) immediate early enhancer and promoter, adenovirus tripartite leader sequences and SV40 polyadenylation signal. For purification of vaccine-quality plasmid DNA from E. coli, an effective method was developed. Chickens were vaccinated by inoculation of DNA by two routes (intramuscular and intraperitoneal). Two weeks later, chickens were boosted with DNA, and at 2 weeks post-boost, they were challenged with virulent IBDV strain. Low to undetectable levels of IBDV-specific antibodies and no protection were observed with DNA encoding VP2. However, plasmids encoding VP2-VP4-VP3 induced IBDV-specific antibodies and protection in the chickens. DNA immunisation opens a new approach to the development of gene vaccines for chickens against infectious diseases.

Generation of full-length cDNA of the two genomic dsRNA segments of infectious bursal disease virus

To determine the complete nucleotide sequence of Infectious Bursal Disease virus (IBDV) isolates, an efficient method was developed to generate full-length cDNA of both the genomic A- and B-segments. Reverse transcription was carried out at the highest possible temperature (50 degrees C) for the reverse transcriptase enzyme, and the single stranded cDNA was subsequently amplified by using an optimized PCR. The double stranded, full-length cDNA was efficiently cloned into a high copy number plasmid. Our results show that the entire cDNA of both the A- and B-segment of a classical attenuated isolate (CEF94), and a very virulent field isolate (D6948), can be cloned. The method will simplify greatly the procedure to generate full-length cDNA and determine the nucleotide sequence of the entire genome of IBDV isolates.

Proteolytic processing in infectious bursal disease virus: identification of the polyprotein cleavage sites by site-directed mutagenesis

The infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is the causative agent of an immune depressive disease that affects domesticated and wild avian species. The expression strategy of IBDV includes the synthesis of a 110-kDa polyprotein containing the capsid precursor polypeptides. The polyprotein is autocatalitically processed rendering three polypeptides: NH2-VPX-VP4-VP3-COOH. We have carried out a systematic analysis, using a series of plasmids encoding polyproteins containing either deletions or single amino acid substitutions, to identify the processing sites. The results obtained showed the existence of two sites, 511LAA513 and 754MAA756, that are essential for the processing of the VPX-VP4 and VP4-VP3 precursors, respectively. These sequences are highly conserved among IBDV strains form serotypes 1 and 2. A secondary VPX-VP4 processing site was detected in a 19-amino acid stretch located upstream of the 511LAA513 site. Analyses using versions of the 754MAA756 VP4-VP3 processing site containing conservative and nonconservative amino acid substitutions demonstrated that the specificity of the cleavage is dictated by the conserved AA dipeptide.

Protection of chickens against very virulent infectious bursal disease virus (IBDV) and Marek's disease virus (MDV) with a recombinant MDV expressing IBDV VP2

To develop a herpes virus vaccine that can induce immunity for an extended period, a recombinant Marek's disease (MD) virus (MDV) CVI-988 strain expressing infectious bursal disease virus (IBDV) host-protective antigen VP2 at the US2 site (rMDV) was developed under the control of an SV40 early promoter. Chickens vaccinated with the rMDV showed no clinical signs and no mortality and 55% of the chickens were considered protected histopathologically after challenge with very virulent IBDV (vvIBDV), whereas all of the chickens vaccinated with the conventional IBDV vaccine showed no clinical signs and were protected. Chickens vaccinated with the CVI-988 or chickens in the challenge control showed severe clinical signs and high mortality (70-75%) and none of them were protected. Also, the rMDV conferred full protection to chickens against vvMDV just as the CVI-988 strain did, whereas 90% of the challenge control chickens died of MD. Antibody levels against IBDV and MDV following the vaccination increased continuously for at least 10 weeks. No histopathological lesions in the rMDV-vaccinated chickens and no contact transmission of the rMDV to their penmates were confirmed. These results demonstrate that an effective and safe recombinant herpesvirus-based IBD vaccine could be constructed by expressing the VP2 antigen at the US2 site of the CVI-988 vaccine strain.

Sequence comparison of the VP2 variable region of infectious bursal disease virus isolates from Vietnam

The variable region in the VP2 gene of twenty-three infectious bursal disease virus (IBDV) isolates, collected in Vietnam in 1997 and 1998, was amplified as cDNA by using the reverse transcription-polymerase chain reaction and sequenced. Analysis of amino acid substitutions and phylogenetic relationships of the deduced amino acid sequences (residues 206-350) showed that the nineteen Vietnamese vv IBDVs clustered with the European vv IBDVs, Japanese vv IBDVs and Chinese vv strains, and that the four vietnamese virulent strains were closely related to European virulent strain 52/70. These results suggest that Vietnamese vv IBDVs, European vv IBDVs, Japanese vv IBDVs and Chinese vv strains have the same origin.

Adaptation of very virulent infectious bursal disease virus to chicken embryonic fibroblasts by site-directed mutagenesis of residues 279 and 284 of viral coat protein VP2

The full-length RNA genomes of a chicken embryonic fibroblast (CEF)-nonpermissive, very virulent infectious bursal disease virus (IBDV) (strain HK46) were amplified into cDNAs by reverse transcription-PCR. The full-length cDNAs were sequenced and subcloned into a eukaryotic expression vector, from which point mutations were introduced into the VP2 region by site-directed mutagenesis. The wild-type and mutated plasmids were transfected directly into CEFs to examine their ability to generate CEF-permissive recombinant viruses. Substitution of amino acid residues 279 (Asp-->Asn) and 284 (Ala-->Thr) of the VP2 protein yielded a recombinant virus which was able to be passaged in CEFs, whereas the wild-type cDNAs and an amino acid substitution at residue 330 (Ser-->Arg) of the VP2 protein alone did not yield viable virus. The results indicated that mutation of other viral proteins, including VP1, VP3, VP4, and VP5, was not required for CEF adaptation of the virus. The same approach may be used to produce CEF-adapted strains from newly evolved IBDVs or to manipulate the antigenicity of the virus.