Cellular adaptive immune response against porcine circovirus type 2 in subclinically infected pigs

Background: Porcine circovirus type 2 (PCV2) is a dominant causative agent of postweaning multisystemic wasting syndrome (PMWS), a multifactorial disease complex with putative immunosuppressive characteristics. Little is known about adaptive PCV2-specific immune responses in infected pigs. Therefore, the T and B cell responses following PCV2 infection in 3-week old SPF piglets infected with PCV2 or PCV2 plus porcine parvovirus (PPV) were studied. Results: All animals were asymptomatically infected. At 7 days post infection (d p. i.), B lymphocyte and T lymphocyte numbers decreased in the dual infected, but not in the single infected piglets. At this time point a transient PCV2 viraemia was noted in the PCV2 infected groups. Antibodies against the infecting virus were detectable at day 24-28 p. i. for anti-PCV2 antibodies and at day 10 p. i. for anti-PPV antibodies, with no apparent influence of PCV2 on the early PPV antibody development. In the animals infected with PPV alone, IFN-gamma secreting cells (SC) that were not specific for PCV2 were detected by ELISPOT assay at day 7 p. i. Interestingly, this response was absent in the PCV2/PPV dual infected animals. PCV2-specific IFN-gamma SC were observed in the PCV2/PPV infected group at 7 d p. i. and in the PCV2 single infected group at 21 d p. i. A reduction in the numbers of IFN-gamma SC was observed following anti-CD4 and anti-CD8 antibody treatment, suggesting roles for both CD4(+) and CD8(+) T cells in the response against PCV2 infection. This was supported by an observed increase in the percentage of IFN-gamma positive CD8(hi) cytotoxic T cells as well as IFN-gamma positive CD8(-/low) helper T cells after PCV2 in vitro re-stimulation. Conclusions: Infection of weaned SPF piglets with PCV2 alone or combined with PPV does not induce disease and in both cases a relatively slow anti-PCV2 antibody response and weak T lymphocyte responses were found. Knowledge on such immunological characteristics is important for both PCV2 pathogenesis and vaccination.