Cellular immune responses in the lungs of pigs infected in utero with PRRSV: An immunohistochemical study

The cellular response in the lungs of pigs transplacentally infected with porcine reproductive and respiratory syndrome virus (PRRSV) was examined by immunohistochemistry. Double staining for the T-cell marker antigen CD3 and PRRSV demonstrated that the appearance and distribution of T-cells homing to the lungs of infected pigs correlated well with the presence and location of virus-infected cells. Single stainings showed that cells positive for the CD2 and CD8 antigen were almost as numerous in pneumonic lesions as CD3 positive cells whereas cells expressing the CD4 antigen were rare. The morphology and the distribution pattern of the CD2 and CD8 positive cells were similar to that of the CD3 positive cells suggesting coexpression of all three antigens within the majority of the recruited T-lymphocytes. The presence of cells consistent with the phenotype of cytotoxic T-lymphocytes (CTL) close to virus infected macrophages strongly indicate an important role of CTLs in the PRRSV-speciflc pulmonary immune response.