T cell responses to the structural polypeptides of porcine reproductive and respiratory syndrome virus

The identification of antigens recognized by T cell responses has become fu ndamental for developing effective immunizations against viral infections. Lymphocyte proliferation and delayed-type hypersensitivity responses to por cine reproductive and respiratory syndrome virus (PRRSV) infection have bee n demonstrated. However, the polypeptide specificity of T cell responses to PRRSV is unknown. To identify the PRRSV polypeptides recognized by porcine lymphocytes two approaches were employed. First polypeptides of purified v irions were separated by SDS-PAGE and particle suspensions obtained from ni trocellulose blots were used as antigens. Second, the polypeptides encoded by ORFs 2, 4, 5, 6, and 7 of the strain VR-2 332 were expressed as fusion p roteins with a histidine tag in mammalian cells, using vaccinia virus as ex pression system. Significant antigen-specific proliferation responses to th e matrix and envelope proteins from purified virions were obtained. This fi nding was supported by specific and dose-dependent proliferation responses to the recombinant polypeptides encoded by ORF2, 5 and 6 detected in virus- infected but not in control pigs. These results demonstrate that T-cell res ponses can be detected to individual PRRSV polypeptides. The greater respon se to the product of ORF6 than to the other PRRSV polypeptides indicates th at the viral matrix polypeptide may have a major role in cellular immunity.