Antigen B of the vaccine strains of Marek's disease virus and herpesvirus of turkeys presents heat-labile group and serotype specific epitopes.

Antigen B of Marek's disease virus (MDV) vaccine strains CVI988 and SB1 (serotypes 1 and 2) and herpesvirus of turkeys (HVT) (serotype 3) is formed of oligomeric molecules that are detergent-stable and heat-labile. Immunoblots of native membranal extracts of HVT- and MDV-infected chick embryo fibroblasts (CEF) probed with avian monoserotypic antisera, murine monoclonal antibodies (mAb) to the three serotypes and mAb to antigen B showed two distinct patterns of high molecular weight oligomeric antigens. Serotypes 1 and 3 vaccine viruses formed one set and serotype 2, the other. Avian monotypic sera to serotypes 1 and 3 viruses detected two high molecular weight bands of 230 and > or = 300 kDa in MDV-1 and HVT-infected CEF but only a weak diffuse zone ranging from 130 to 230 kDa in extracts of SB1-infected CEF. No 300 kDa band was discernible in the SB1 extract when blotted with avian monotypic 1 and 3 antisera. MAbs to MDV serotypes 1 and 3 and to antigen B also detected the 230 and > or = 300 kDa antigens, while the mAb to SB1 detected a 50 kDa antigen in the SB1-infected extract only. Furthermore, the antigen B mAb did not reveal high mol. wt. oligomers in SB1-infected CEF extracts. Antigen B oligomers were rapidly destroyed by heating at 95 degrees C and the rate of denaturation of the 230 and > or = 300 kDa oligomers differed for each of the three vaccine viruses. We propose that antigen B of MDV1 and HVT has a complex conformation created by juxtaposition of dimers (230-250 kDa) and trimers (> or = 300 kDa), and is inserted in the infected cell membrane so that conformational, discontinuous epitopes are formed in addition to continuous epitopes. It appears that HVT protects chickens against oncogenic strains of MDV1 by virtue of the cross reactivity of the conformational determinants located on these oligomers. Serotype 2 vaccine shares some of its antigenic determinants with serotypes 1 and 3, while its unique immunogenic features form the basis of the protective synergism achieved when serotypes 2 and 3 vaccines are combined together.