Sequence Comparisons of the Variable VP2 Region of Eight Infectious Bursal Disease Virus Isolates

Abstract

The VP2 gene is part of the genomic segment A of infectious bursal disease virus (IBDV). It has been identified as the major host-protective antigen of IBDV and is known to contain conformationally dependent protective epitopes. A 643-base pair segment covering the hypervariable region of this gene from three recent serologic variant IBDV isolates from the southeastern United States, two variants from the Delmarva Peninsula, and three serologic standard viruses were amplified and sequenced using the reverse transcription polymerase chain reaction and cycle sequencing techniques. This was done to determine the molecular similarity among isolates that differ antigenically and pathologically. Sequence analysis suggested that the Arkansas (Ark) and Mississippi (Miss) isolates evolved closely and separately from the Delmarva variants (GLS and DELE), in contrast to the other southeastern variant Georgia (Ga), which is more closely related (98.32%) to Delaware E (DELE). All variants, except for Miss, underwent a shift in amino acid number 222 from proline to threonine. The sequence of Univax BD virus, a commercially available intermediate vaccine, was markedly different, evolving from a separate lineage than the others. Restriction enzyme sites could differentiate most isolates. Except for Miss, variants do not have EcoRII site at the larger hydrophilic domain. All variants lost their HaeIII, StuI, and StyI cutting sites with a change in base number 856. The TaqI site is in DELE, whereas the SpeI site is absent in the standard vaccine viruses. The SWASASGS heptapeptide is conserved in all virulent viruses, including APHIS, but not in the attenuated (Univax BD and Bursa Vac 3) and published (D78 and PBG98) vaccines.