A longitudinal study of cell-mediated immunity in pigs infected with porcine parvovirus

Porcine parvovirus (PPV) is an ubiquitous pathogen causing reproductive failure in swine. Protection against reproductive failure caused by acute PPV infection has commonly been related to the presence of specific antibodies in the dam. However, the role of cell-mediated immunity during chronic PPV infection remains to be elucidated, and may be relevant to the pathogenesis of novel diseases such as postweaning multisystemic wasting syndrome (PMWS), which may be triggered by coinfection with PPV and porcine circovirus type 2 (PCV2). To investigate whether pigs infected with PPV generate a cell-mediated immune response, a longitudinal infection experiment was performed, using swine leukocyte antigens (SLA) class I characterized growing pigs (haplotype H7/H7). Pigs were intranasally inoculated with PPV at 0, 80, and 136 days. At predetermined time points, peripheral blood mononuclear cells (PBMC) were isolated, and virus-specific lymphoproliferative responses and the cytolytic activities of cytotoxic T-lymphocytes (CTL) and natural killer (NK) cells were examined. Cytolytic assays were performed by the chromium release method, using as targets a syngeneic porcine kidney cell line established for the purpose (CTL assays) and K562 cells (NK assays). A specific proliferative response of PBMC from virus-infected pigs to PPV was observed from day 101 onwards. In contrast, PBMC from mock-infected pigs did not proliferate in response to PPV. Flow cytometric analysis indicated that the CD4+CD8+ T-cell subset of PBMC proliferated in response to virus antigen, in keeping with the assumed role for these cells in immunological memory. This is, to our knowledge, the first indication of a cellular immune response following PPV infection. A weak CTL activity, which peaked on days 80 and 87, was observed in PPV-infected pigs. In vitro restimulation of PBMC with live PPV did not induce further CTL activity. A pronounced NK cell activity was detected in both virus-infected and control pigs throughout the experiment, and may have negatively affected the sensitivity of the CTL assay. In conclusion, the findings of a late lymphoproliferative response together with weak CTL activity are in keeping with an effective control of acute PPV infection by humoral immunity, but open the possibility that cellular immunity may play a role in controlling PPV reinfection. Finally, we find that the established experimental model using SLA characterized pigs may constitute a valuable tool for future studies of CTL activity in pigs.