Vaccination of pigs with alphavirus replicon particles expressing PRRSV ORF 3, 4, 5 and 6

  The objective of this project was to evaluate alphavirus replicon particles (RP) expressing various PRRSV proteins as vaccine candidates. Previous work has identified the minor structural glycoproteins (GP2, GP3, and GP4) and the major structural components (GP5 and M) as vaccine targets for use in recombinant vaccine platforms. This project focused on GP3, GP4, GP5, and M as experimental vaccine antigens.

The genes encoding the GP3, GP4, GP5, and M proteins were sequenced and cloned into the replicon system. This system was used to generate RPs expressing the genes of interest. The RPs are not able to replicate in the host, and can only deliver the genetic material necessary to express the vaccine antigen. This feature adds to the safety of the vaccine, because it cannot recombine with wild-type PRRSV to create new strains. Additionally, the antigens contained in this vaccine allow vaccinated animals to be differentiated from naturally infected animals by commercially available serological tests.

Forty pigs (3 weeks of age) were randomized into four groups.  Each group of pigs received a combination of RPs expressing either: 1)GP3 and GP4; 2)GP5 and M; 3)GP3, GP4, GP5, and M; 4)Placebo. Vaccination occurred on Day 0 and Day 21 of the study. Pigs were challenged intranasally with a virulent strain of PRRSV on Day 42, and the study was terminated on Day 56. Blood samples were collected throughout the study to measure viremia and immune response.

Vaccine 3 (GP3, GP4, GP5, M) induced neutralizing antibodies prior to challenge, and all three vaccinated groups had significantly stronger neutralizing responses post-challenge, when compared to placebo. Vaccine 2 (GP5 and M) and Vaccine 3 induced significantly stronger cell-mediated immune responses prior to challenge when compared to placebo. The three RP vaccine candidates all significantly reduced viremia on Day 51, and Vaccine 3 also reduced viremia on Day 48, when compared to placebo.

These results indicate that the RP vaccine platform is capable of inducing PRRSV-specific cell-mediated immunity and neutralizing antibodies prior to challenge. Vaccination with the three vaccine candidates tested here also reduced viremia post-challenge. Animals vaccinated in this manner remain serologically negative until after a PRRSV infection, providing an improved method for disease prevention and surveillance. These vaccine candidates are currently being evaluated in a reproductive disease model.