Phenotype of IL-2 or pseudorabies virus induced cytolytic cells in the pig

We investigated the phenotype of IL-2 and pseudorabies virus (PRV) induced porcine cytolytic cells. Peripheral blood mononuclear cells (PBMC) were isolated from PRV-immune and non immune minipigs. The PRV-immune PBMC was restimulated in vitro with PRV and the non immune PBMC were stimulated in vitro with IL-2. Thereafter, we separated these in vitro cultured cells in various phenotypic subsets by the magnet-activated cell sorter after labelling specific CD markers on these cells with magnetic beads. The cytolytic assays with the separated cell subsets revealed that the cytolytic activity of the immune cells was predominantly PRV specific and mediated by CD6$^+$CD8$^+$ T-lymphocytes (the presumed classical cytolytic T-cell (CTL) phenotype in pigs). In addition to these PRV-specific CTL, some natural killing (by CD6$^-$CD8$^+$) was also detected. In contrast, when PBMC were stimulated with IL-2, cytolytic assays revealed that cytolytic activity was mainly mediated by CD6$^-$CD8$^+$ lymphocytes. After PBMC were (re)stimulated in vitro, the frequencies of various lymphocyte subsets induced by PRV or IL-2 were also analysed by FACScan. This FACScan analysis demonstrated that after PRV stimulation of PBMC from PRV-immune minipigs, more CD2/CD3, CD6/CD8 and CD4/CD8 double positive and IgM positive lymphocytes, and less CD8/CD3gd (double positive lymphocytes) proliferated, than after IL-2 stimulation of PBMC from non-immune minipigs. However, when we compared the frequencies of lymphocyte subsets in immune PBMC after in vitro (re)stimulation with PRV or with IL-2-induced frequencies, we only found small differences in frequency of various lymphocyte subsets. In conclusion, we demonstrated that killing by IL-2 induced cytolytic PBMC was mainly mediated by CD6$^-$CD8$^+$ lymphocytes (MHC-unrestricted killing) and killing by PRV-induced cytolytic PBMC was mainly virus specific and mediated by CD6$^+$CD8$^+$ lymphocytes (MHC-restricted killing). These results make it clear that besides MHC-unrestricted killing, PRV specific MHC-restricted killing can also be detected in PRV-immune minipigs