Parameters of protective immunity in swine induced by PCV2 vaccines with different antigen payload.

Porcine circovirus associated disease (PCVAD) complex is caused by co-infections of PCV2 and other common pathogens and/or environmental stressors. Different vaccines have been developed to reduce PCV2 infections and PCVAD. Such vaccines are poorly standardized in terms of antigen payload and recognized correlates of protection. Therefore, we selected twenty, 40-day old piglets, and allocated them to 4 groups (5 animals each) with uniform levels of maternally-derived antibody to PCV2. Animals were vaccinated with 450/150/50/0 nanograms of an inactivated PCV2b strain, formulated in the same adjuvant of the commercial Circovac vaccine. Twenty-seven days later, all pigs were challenged intranasally with the homologous PCV2 strain. The main findings can be summarized as follows: 1) No clinical signs were observed in the pigs under study. 2) Viremia was observed in all the control pigs, as well as in 2 pigs of the 150 and 50 ng groups, respectively. No pigs of the 450-ng group developed viremia. 3) There was no correlation between protection and ELISA Ab titers in the single animals, even though the 450-ng group developed on average a stronger Ab response. 4) All the pigs with a PCV2-specific IFN-gamma response at 3 weeks after vaccination were fully protected against viremia. The IFN-gamma response at this time point was peculiar to CD4+, single positive T cells, whereas both CD8alpha and CD8 beta+ T cells were also positive after challenge infection. 5) In lymphoid tissues (mainly tonsils and ileum) the presence of sparse reactive hystiocytes and multinucleated giant cells was the only PCV2-associated feature and, by immunohistochemistry, only 2 out of 20 subjects (still viremic at 35 PID) had a mild viral load (grade 1 in 8 samples and grade 2 in only 1 sample). Our data point at the IFN-gamma release assay as a useful tool for monitoring efficacy of PCV2 vaccines.