A Longitudinal Study of Cell-Mediated Immunity in Pigs Infected with Porcine Parvovirus

Abstract

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.

<p>Vaccine Protection Against Experimental Challenge Infection with a PPV-27a Genotype Virus in Pregnant Gilts</p>

“…28 This hypothesis, ie PPV-specific cellular immune responses being a major differentiating factor in protective capacity, is further and specifically supported by the already demonstrated development of PPV-specific CMI evoked by PVX; briefly, in a trial where SPF pigs were vaccinated with PVX at D0, D21, and D154, antigenspecific IFNγ + T cell responses were measured upon vaccination, which was significantly boosted 4 months later, 29 similar to that of repeated PPV infection. 27 In agreement with previous findings, 18 this study also demonstrated that vaccination with commercially available inactivated vaccines cannot prevent infection since all groups responded to the challenge with antibody titer rise. However, there is a difference between the "readiness" provided by the different vaccines, which is plausibly manifested in the timely activation of memory B-cells and T-cells against infection.…”

“…These results are questioning the use of serological methods (ie ELISA and VN) for the evaluation of vaccine-induced protection against PPV and suggest that other mechanisms or indicators for immunity should be involved to properly evaluate the protection level of the gilts than serology. In fact, the early IFN-γ and INF-α activation in pigs upon PPV1 infection 26 and the role of the memory B cells-helper T cell complex of the cell-mediated immunity (CMI) are proven in the induction of immune response to PPV, 27 as also proven for the human adenoassociated viruses. 28 This hypothesis, ie PPV-specific cellular immune responses being a major differentiating factor in protective capacity, is further and specifically supported by the already demonstrated development of PPV-specific CMI evoked by PVX; briefly, in a trial where SPF pigs were vaccinated with PVX at D0, D21, and D154, antigenspecific IFNγ + T cell responses were measured upon vaccination, which was significantly boosted 4 months later, 29 similar to that of repeated PPV infection.…”

Protein kinase C is involved in 2B4 (CD244)‐mediated cytotoxicity and AP‐1 activation in natural killer cells

“…Previously, we have identified a functional AP‐1 site that lies between −106 and −100 of the h2B4 promoter that strongly activates transcription. Many studies in different systems have shown that activation of transcription can involve signalling pathways that include PKC 29–31 . To examine whether PKC influenced 2B4 gene transcription through AP‐1, we mutated the (−111 to −89) sequence of the h2B4 promoter with the AP‐1 binding site from TGAGTCA to TGccgCA and synthesized mutant promoter constructs p‐M188 and p‐M342 (Fig.…”

Biology of Porcine Parvovirus (Ungulate parvovirus 1)

“…Hardly anything is known about the role of cellular immunity in controlling PPV infection. The only investigation made in this direction found weak cytotoxic T cell (CTL) activity, suggesting that T-cell activation may occur with repeated exposure to the virus, but confirmed that effective clearance of PPV infection is obtained by rapid antibody response in infected pigs [ 55 ].…”

“…Many immunological studies proved that the presence of neutralizing serum antibodies is a decisive factor in the outcome of the PPV infection [ 5 , 44 , 55 , 56 ]. Since the placenta is impermeable to maternal antibodies, neonatal piglets obtain passive protection against PPV1 by taking up maternally derived antibodies (MDA) from the colostrum.…”