Immunogenicity of non-structural proteins of foot-and-mouth disease virus: differences between infected and vaccinated swine

Baculovirus expressed 2C of foot-and-mouth disease virus has the potential for differentiating convalescent from vaccinated animals

Determining whether animals have been infected with foot-and-mouth disease virus or vaccinated is important because infected animals frequently become carriers of the virus, shed it intermittently and thus may be the source of new outbreaks of the disease. We had shown previously that the sera of convalescent animals contain antibodies to 2C, a highly conserved non-structural protein, whereas the sera of vaccinated animals do not. This is explained by observation that 2C is retained on the membranes of cells used for growing the virus for vaccine production. In contrast, the non-structural protein 3D, which is released into the medium, is not removed by centrifugation or filtration during vaccine production and therefore stimulates an immune response in both vaccinated and convalescent cattle. In this study we produced 2C and 3D in insect cells infected with recombinant baculoviruses. As demonstrated by serology and electron microscopy, 2C is also retained on the membranes of the insect cells. Both expressed proteins react with sera of convalescent animals, indicating that they are conformationally similar, but the 2C does not react with sera from vaccinated animals. The baculovirus expressed 2C appears to be a suitable antigen for the development of a reliable diagnostic test.

Absence of protein 2C from clarified foot-and-mouth disease virus vaccines provides the basis for distinguishing convalescent from vaccinated animals

We have recently reported that cattle and pigs which have been vaccinated against foot-and-mouth disease can be distinguished from convalescent animals by the absence of antibodies to viral non-structural protein 2C (Lubroth and Brown, Res. Vet. Sci., 1995, 59, 70-78(1)). In this study, we show that the absence of 2C antibodies from the sera of vaccinated animals can be explained by the association of this viral protein with cellular debris which is separated from the virus harvest prior to inactivation of the supernatant for vaccine production. This serological marker can be of great value in countries where the disease occurs or in the veterinary regulatory arena when livestock are transported across borders, since it can be used to identify convalescent, persistently infected animals and vaccinates exposed to wild-type virus variants which have infected the vaccinated animals.

Identification of native foot-and-mouth disease virus non-structural protein 2C as a serological indicator to differentiate infected from vaccinated livestock

Cattle and pigs which have been vaccinated against foot-and-mouth disease can be distinguished from convalescent animals by radio-immunoprecipitation and sodium dodecyl sulphate polyacrylamide gel electrophoresis of the virus-induced proteins reacting with the respective sera. Baby hamster kidney cells infected with foot-and-mouth disease virus (FMDV) (serotype A24) were labelled with 35S-methionine and the virus-induced proteins were precipitated with sera from vaccinated and subsequently challenged animals, convalescent animals retained for over 300 days, animals vaccinated or infected with viruses belonging to all serotypes of FMDV, and animals infected with encephalomyocarditis (EMC) or porcine or bovine enteroviruses. In addition to the structural proteins of the virus, the non-structural proteins 2C, 3ABC, 3C, 3CD and 3D were precipitated by convalescent sera, but only 3D was precipitated by serum from vaccinated animals. Proteins L, 2C and 3C were precipitated only after challenge with a heterotypic virus (serotype O1 Tunisia), indicating that virus replication of the challenge virus had taken place. No precipitation was detected with sera from EMC or enterovirus-infected animals. The results indicate that protein 2C, and to a lesser extent the polypeptide 3ABC, could be used to differentiate potential carrier convalescent animals from vaccinated livestock.