Studies on naturally occurring infectious bursal disease viruses suggest that a single amino acid substitution at position 253 in VP2 increases pathogenicity

Abstract

Three classic IBDV strains were previously isolated from commercial layer chicken flocks and shown to be phylogenetically related to vaccine strains but pathogenic in susceptible chickens. In this study, their viral genomes were sequenced and compared to sequences of vaccines being used in those flocks. The vaccine strains examined were sequenced directly from the manufacturer and had identical genome segment B sequences. Compared to these vaccines, the GA-1, H-30 and CS-2-35 isolates each had one silent mutation in the gene that encodes VP1. Compared to the two vaccines used at the time CS-2-35 was isolated, the segment A sequence of CS-2-35 contained numerous nucleotide and amino acid mutations suggesting the CS-2-35 virus was not closely related to these vaccines. This virus however did have amino acid mutations in VP2 that are reported to be necessary for replication in cell culture and lacked two of the three amino acid mutations previously shown to be necessary for virulence. These data suggest that CS-2-35 was a descendant from an attenuated strain of IBDV. When the segment A genomic sequences of the GA-1 and H-30 viruses were compared to the vaccines being used in those flocks they were most closely related to the attenuated D78 vaccine strain. In genome segment A, three nucleotide mutations in GA-1 and four in H-30 were observed compared to the D78 classic vaccine. These nucleotide mutations caused one amino acid (H253N) change in the GA-1 virus and two amino acids (H253Q and G259D) were different in the H-30 virus. In addition, both the GA-1 and H-30 viruses had the amino acid G76 in VP2 that appears to be unique to the vaccine D78. The data suggest that GA-1 and H-30 are genetically related and have a common ancestor even though they were isolated from geographically distant flocks. The evidence also suggests that GA-1, H-30 and CS-2-35 could be reversions from attenuated vaccine viruses or by coincidence genetically resemble classic IBDV vaccines. It should be noted that some of the classic virus vaccines were not being used according to the manufacturer's recommendations at the time the GA-1, H-30 and CS-2-35 strains were isolated. Together, the molecular and pathogenicity data indicate that a single amino acid mutation from Histidine (H) to Glutamine (Q) or Asparagine (N) at position 253 in VP2 will markedly increase the virulence of an attenuated IBDV.

Characterization of field and vaccine infectious bursal disease viruses from Nigeria revealing possible virulence and regional markers in the VP2 minor hydrophilic peaks

“…In fact, Tong et al (1993) showed that IBD was more likely to occur in vaccinated chickens than in non-vaccinated chickens. There have been earlier reports of field isolates having sequences similar to IBDV vaccine strains (Ignjatovic & Sapats, 2002;Dolz et al, 2005;Jackwood et al, 2008;Hon et al, 2008). In the present study, the segment A region of the Georgia vaccine and other virulent classical IBDV strains isolated from the field and attenuated showed identical origin with classical IBDV strain D78, suggesting that the virulent field strains could be escaped mutants of vaccine strains.…”

Molecular characterization of infectious bursal disease virus (IBDV) isolated in Argentina indicates a regional lineage

“…Furthermore, the sequence A08-ER01 (closely related to the vaccine strains) contained N at position 253 of VP2. Since residue N253 has been associated with increased virulence [45], this finding suggests that A08-ER01 may represent a revertant derived from a vaccine. Unfortunately, it is not possible to confirm this without the corresponding experiment demonstrating increased virulence of the strain.…”

“…The samples A07-BA01, A07-BA02 and A08-BA3 (belonging to the cluster csIBDVb) showed a pattern similar to that of the vaccine strains C and I, differing only at positions 253 (Q/H), 279 (D/N) and 284 (A/T). In this regard, it has been reported that those positions (253, 279 and 284) in the VP2 protein are involved in cell tropism [29,30] and that position 253 is the only one that is important for virulence [45]. Particularly, Q253 may be required for in vivo infection, whereas H253 is involved in cell-culture adaptation [29,30,33,46].…”

Phylogenetic Characterization of Infectious Bursal Disease (IBD) Viruses Isolated From Field Outbreaks in Chickens From Behera And Alexandria Governorates, Egypt

“…Performing RT/PCR on selected fragments of IBDV genome specially the hypervariable region of VP2 followed by sequencing and phylogenetic analysis was considered an important and valuable methods to classify IBDV strains (Van den Berg, 2000). The VP2 is very important region in the viral genome because it encodes the major protective epitopes, containing determinants for pathogenicity and is highly variable between strains (Ikuta, 2001;AbdelAlim et al, 2003 andJackwood et al, 2008).…”