Comparative evaluation of the CD4+CD8+ and CD4+CD8− lymphocytes in the immune response to porcine rubulavirus

The porcine immune system is unique in the expression of CD4+CD8+ (double-positive, DP) lymphocytes. These cells have been associated with immunological memory due to their gradual increase with age, the expression of memory phenotype and their ability to respond to recall viral antigen. This work analyzes the biological function of CD4+CD8− and CD4+CD8+ lymphocytes in the immune response to porcine rubulavirus (PRv). CD4+CD8− cells isolated from pigs 3 weeks after infection with porcine rubulavirus proliferated in response to homologous virus and generated lymphoblasts which were predominantly of the CD4+CD8+ phenotype, whereas stimulation with mitogen induced proliferation but did not switch the phenotype. CD4+CD8− lymphocytes isolated after 10 weeks of infection proliferated in response to phytohemagglutinin (PHA) but did not proliferate in response to homologous virus and did not change their phenotype, whereas CD4+CD8+ lymphocytes proliferated in response to PHA and to viral antigen. The cytokine profile of both lymphocyte populations showed the presence of IL-2 and IL-10 transcripts, quantitation demonstrated that CD4+CD8+ cells expressed mainly IL-10, whereas CD4+CD8− lymphocytes expressed primarily IL-2. Our results show that CD4+CD8− lymphocytes in the early phase of porcine rubulavirus infection can be converted to double-positive cells expressing IL-10 in an antigen-dependent manner, and that CD4+CD8− T-cells late in infection do not acquire CD8.