A comparison of the biological properties of type 2 plaque-producing agent derived from the Cal-1 strain of Marek's disease virus with other related viruses

The serological and biological properties of the type 2 plaque-producing agent (PPA) derived from the Cal-1 strain of Marek's disease virus (MDV) were compared with those of reference strains of the three serotypes of MDV and herpesvirus of turkeys (HVT) groups; namely JM, HPRS-24 strains of MDV and the FC-126 strain of HVT for serotype 1, 2, and 3, respectively. By agar gel precipitation (AGP), indirect fluorescent antibody and virus neutralization tests, type 2 PPA was related but not identical to the FC-126 strain. By the AGP test, type 2 PPA showed a poor ability to synthesize B antigen and the A antigen was different from that of strain FC-126. To compare the virological characteristics of type 2 PPA with the reference strains, the release of cell-free virus into supernatants of infected cell cultures and titers of cell-free virus extracted sonically from infected cell cultures were examined. Cell-free type 2 PPA virus was easily detected in the supernatants and extracted from infected cell cultures. These properties were similar to reference strains of serotype 2 and 3. Next, the structural similarities of viral DNAs of type 2 PPA and strain FC-126 were examined by Southern blot hybridization. The restriction endonuclease-cleavage patterns of DNA of type 2 PPA were very similar but not identical to those of the FC-126 strain. In chickens inoculated with type 2 PPA, splenic lymphocytes supported a non-productive latent infection as did also those from chickens inoculated with the FC-126 or HPRS-24 strains. From these results, type 2 PPA appears to belong to serotype 3 of MDV and HVT groups. The origin of type 2 PPA is discussed.