Functional Characterization of Porcine CD4+CD8+Extrathymic T Lymphocytes

The porcine immune system is unique in that the expression of CD4 and CD8 antigens defines four subpopulations of resting, extrathymic (CD1−) T lymphocytes. In addition to CD4−CD8+and CD4+CD8−T lymphocytes, CD4−CD8−and CD4+CD8+lymphocyte subpopulations are prominent in blood as well as in lymphoid tissues. In the present study, a functional comparison was made between CD4+CD8−and CD4+CD8+T lymphocyte subpopulations. In a primaryin vitroimmune response against alloantigenic stimulator cells, both subpopulations proliferated without significant differences in their reactivity. Different results were obtained when analyzing the antigen-specific functions of the two CD4+subpopulations in a secondary response against recall viral antigen; these experiments were performed with T lymphocytes from pseudorabies virus-immunized pigs. The proliferative response against viral antigens could be assigned to the CD4+CD8+subpopulation, whereas the CD4+CD8−subpopulation remained nonreactive. Further analyses of the virus-specificin vitroimmune response revealed a major histocompatibility complex (MHC) class II restricted helper T lymphocyte reaction involving CD4 but not CD8 molecules as restriction elements. Taken together, these results demonstrate that only the extrathymic CD4+CD8+T lymphocyte subpopulation of swine contains MHC class II-restricted antigen-specific memory T helper cells.