Discrimination between two subsets of porcine CD8+ cytolytic T lymphocytes by the expression of CD5 antigen.

Previous work has revealed striking differences in the peripheral T-lymphocyte compartments of swine compared to other species. The key difference is the existence of four T-lymphocyte subpopulations defined by their CD4/CD8 expression. Besides this difference in the CD4/CD8 antigen expression, we report here another unusual antigen distribution on porcine extrathymic T lymphocytes: the expression of the 63,000 MW CD5 antigen. In porcine thymus the CD5 antigen shows a biphasic antigen density, whereas the majority of thymocytes are characterized by an intermediate CD5 expression. In the extrathymic T-lymphocyte compartment, CD5 also shows a heterogeneous antigen distribution as defined by three subsets: CD5high, CD5dim and CD5- T lymphocytes. Analyses of the CD5 expression on the four CD4/CD8-defined peripheral T-cell subpopulations revealed that all CD4+ T lymphocytes, CD4+ CD8+ as well as CD4+ CD8- T lymphocytes, belonged to the subset with high CD5 antigen density. The CD4- CD8- subpopulation, containing in the majority T-cell receptor (TcR) gamma delta T lymphocytes, was characterized by dim CD5 expression. The most notable difference was the division of the CD4- CD8+ cytolytic T-lymphocyte subpopulation into two CD5-defined subsets: CD4- CD5- CD8+ lymphocytes with spontaneous cytolytic activity against tumour cells, and CD4- CD5+ CD8+ T lymphocytes with major histocompatibility complex (MHC)-restricted cytolytic function. Thus, the porcine CD5 antigen is an important marker to discriminate between CD5- CD8+ natural killer (NK) cells and CD5+ CD8+ progenitors of MHC-restricted cytolytic T lymphocytes.