Characterization of antigenic variants of infectious bursal disease virus
Date
2020
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Infectious bursal disease virus (IBDV) is a double stranded RNA virus, which targets developing B cells in the bursa of Fabricius. IBDV is the etiological agent of infectious bursal disease (IBD), a disease which results in varying degrees of immunosuppression in poultry. The severity of immunosuppression is associated with the age at which birds are infected. When birds are infected at a young age (<21 days of age) immunosuppression is long-lasting and severe. IBDV vaccines are administered to broiler breeder flocks, and maternally derived antibodies are passed vertically to broilers. Broilers should be protected by maternally derived antibody from day of hatch until approximately twenty-one days of age. This passive immunity is vital to protect young birds at the age at which infection would result in the most severe immunosuppression. ☐ Bursal surveys are performed regularly to determine the state of IBDV in flocks across the United States through the collection and histological rating of affected bursas. Results of surveys over the past seven years have shown young flocks affected with IBDV when protection should have been afforded by maternally derived antibodies. Lack of early antibody protection suggests the presence of new isolates that are antigenically distinct from current vaccine viruses. ☐ Field IBDVs were chosen from flocks affected with IBD at younger than twenty-four days-of-age for serological testing in search of an IBDV to act as a new potential vaccine candidate. Virus stocks and antiserum were created in specific-pathogen-free leghorn chickens. Preliminary virus neutralization assays were performed to determine if prevalent vaccine virus Delaware variant E would effectively neutralize field isolates. Results showed poor neutralization indices, demonstrating that field isolates were antigenically distinct from Delaware variant E. Isolates which were poorly neutralized were characterized utilizing virus neutralization assays to determine the efficacy of antiserum for cross neutralization of virus stocks in search of an isolate that elicits highly cross neutralizing antibodies. Field IBDVs neutralized each other to varying extents. Two highly cross-protective isolates, AVS-MB and AVS-TV were found. These IBDVs will be evaluated further as potential vaccine candidates. AVS-MB and AVS-TV antiserum neutralized 10/10 and 9/10 evaluated IBDVs respectively. Two other field IBDVs, AVS-SCH and AVS-RRV did not effectively neutralize any of the evaluated IBDVs. All other field isolates and contemporary strains cross-neutralization potential fell between these ranges. ☐ After serological evaluation, the whole genomes of sixteen IBDVs with varying levels of cross-neutralization potential were sequenced using Illumina high-throughput technology with the objective of finding correlations between genotype and serotype or pathotype. IBDV encodes for five viral proteins. VP1 is an RNA dependent RNA polymerase, VP2 is a capsid protein which has previously been associated with antigenicity, VP3 is a multi-functional capsid protein, VP4 functions as a viral protease, and VP5 is a non-structural protein. Overall, the IBDV genome was highly conserved for all five viral proteins (>90%). The most conserved proteins were VP1 (98.3-100.0%) and VP4 (98.8-100.0%) were most conserved, followed by capsid proteins VP2 (96.0-100%) and VP3 (97.9-100.0%), while VP5 had the lowest percent identity (90.4-100.0%). ☐ Amino acid sequence alignments were created for all viral proteins. Specific amino acid mutations in capsid protein VP2, which has previously been associated with antigenicity, were identified. A predictive protein structure 3D models was created for a Delaware variant E sequence with insertion of a pair of amino acid changes, D318N and A321E. This mutation resulted in a conformational change in an antigenically significant region of VP2. ☐ The results obtained here show that contemporary field IBDVs neutralize each other to varying extents, and two of the evaluated isolates, AVS-MB and AVS-TV elicited highly cross-neutralizing antibodies. Genome sequencing showed a number of amino acid changes in field isolates, and at least one of the amino acid changes resulted in a conformational change when evaluated in a 3D protein model.
Description
Keywords
Antigenicity, Chicken, IBDV, Immunosuppression, Infectious bursal disease virus (IBDV), Virology