Characterization of Immune Response of Young Pigs to Porcine Circovirus Type 2 Infection

A longitudinal study was conducted to characterize the immune response of young swine to infection with porcine circovirus type 2 (PCV-2). Five 8-week-old cesarean-derived, colostrum-deprived pigs were inoculated intranasally and intramuscularly with a field isolate of PCV-2 at a concentration of 10⁴ TCID₅₀/mL. Along with monitoring for clinical signs and viremia, serum samples were collected from all pigs at day 0 and thereafter every 7 days postinoculation (PI) until the termination of the study on day 35 PI. No clinical signs were observed in any of the animals during the study period. In all pigs, PCV-2 was detected by polymerase chain reaction (PCR) in serum samples collected on days 7, 14, and 21 PI. Viral DNA and antigens were detected by in situ hybridization and immunohistochemistry in tonsil, spleen, medial iliac lymph nodes, and ileum collected from each pig at the end of the study. Collectively, naïve young swine were shown to be susceptible to PCV-2. Virus-specific antibody was detected by an indirect fluorescent antibody (IFA) assay on day 14 PI, but virus-neutralizing antibody was not detected until day 28 PI. As neutralizing antibodies developed, cross-reactivity with PCV type 1 (PCV-1) also developed on the IFA test. Western immunoblot analysis revealed three PCV-2 proteins with molecular masses of 28 kd, 28.5 kd, and 35 kd. The 35-kd protein was also demonstrated in PCV-1, suggesting that this protein induced the cross-reactivity between PCV types 1 and 2. Antibody to the 28-kd protein was detected on day 14 PI and later, indicating that this protein was the most immunogenic. Because of its immunogenicity and specificity to PCV-2, and 28-kd protein might provide the antigenic basis for the development of diagnostic tests for detection of PCV-2 antibody.