Antigenic and immunogenic properties of baculovirus-expressed infectious bursal disease viral proteins

Genomic segment A of the variant Md infectious bursal disease virus (IBDV) was amplified by reverse transcriptase/polymerase chain reaction, cloned, and expressed in the baculovirus expression system. Three different baculovirus recombinants expressing the genes encoding VP2, VP2/VP4, and the complete polyprotein (VP2NP4NP3) were prepared. The three antigens were used in separate enzyme-linked immunosorbent assays, and each detected antibodies against the variant IBDV strains Md, Del-A, Del-E, and GLS and the classic IBDV strain D78 throughout a 14-wk period following vaccination. Eight-week-old specific-pathogen-free (SPF) chickens were inoculated subcutaneously using the VP2/ VP4 or VP2/VP4/VP3 baculovirus recombinant or wild-type baculovirus-infected insect cell lysates. Virus-neutralizing antibodies were detected in the VP2/VP4 and VP2NP4NP3 baculovirus-inoculated birds at 13 days postinoculation (PI) and at 43 days PI when titrated against Md IBDV. No virus-neutralizing antibody titer was observed in the negative controls or wild-type baculovirus-inoculated birds. One-week-old SPF chickens were inoculated with the VP2, VP2/VP4/VP3 baculovirus recombinants or wild-type baculovirus-infected insect cell lysates. The birds were boosted 2 wk later and challenged at 4 wk of age with 0.5 ml/bird classic STC IBDV (10(2) median embryo infective dose [EID50]/ml) or variant Md IBDV (10(5) EID50/ml) via the oral/nasal route. The VP2 and VP2/VP4/VP3 baculovirus-inoculated birds were partially protected against challenge with the classic STC IBDV. The birds were protected against clinical disease and death but not bursal damage, whereas the wild-type baculovirus-inoculated birds exhibited clinical signs of disease, 13% mortality, and bursal damage. When challenged with the variant Md IBDV, neither the VP2, VP2/VP4/VP3 baculovirus recombinants nor the wild-type baculovirus elicited protection against bursal damage and atrophy.