T lymphocytes play a central role in the antigen-specific immune response against various pathogens. To detect and to characterize porcine T lymphocytes, monoclonal antibodies (mAb) against leukocyte differentiation antigens had been raised and classified for their specificity. Analyses of porcine T lymphocytes with specific mAb against CD4 and CD8 differentiation antigens revealed differences in the composition of the porcine T-lymphocyte population compared to other species. In addition to the known subpopulations, CD4(+)CD8(-) T helper cells and CD4(-)CD8(+) cytolytic T lymphocytes, extra-thymic CD4(+)CD8(+) T lymphocytes and a substantial proportion of CD2(-)CD4(-)CD8(-) T cell receptor (TcR)-gamma delta(+) T cells could be detected in swine. Functional analyses of porcine T-lymphocyte subpopulations revealed the existence of two T-helper cell fractions with the phenotype CD4(+)CD8(-) and CD4(+)CD8(+). Both were reactive in primary immune responses in vitro, whereas only cells derived from the CD4(+)CD8(+) T-helper-cell subpopulation were able to respond to recall antigen in a secondary immune response. With regard to T lymphocytes with cytolytic activities, two subsets within the CD4(-)CD8(+) T-cell subpopulation could be defined by the expression of CD6 differentiation antigens: CD6(-) cells which showed spontaneous cytolytic activity and CD6(+) MHC I-restricted cytolytic T lymphocytes including virus-specific cytolytic T lymphocytes. These results enable now a detailed view into the porcine T-cell population and the reactivity of specific T cells involved in the porcine immune response against pathogens. Furthermore this knowledge offers the possibility to investigate specific interactions of porcine T lymphocytes with virus-specific epitopes during vaccination and viral infections. (C) 1999 Elsevier Science B.V. All rights reserved.