Remarkably normal cellular immune function, along with specific T-cell tolerance to highly disparate xenogeneic donors, can be achieved by grafting fetal pig thymus (FP THY) tissue to T and NK cell-depleted, thymectomized (ATX) mice. Porcine MHC can mediate positive selection of mouse CD4+ T-cells with a mouse MHC-restricted TCR in FP THY-grafted, T- and NK cell-depleted, ATX TCR-transgenic "AND" mice. However, functional studies were not performed on transgenic mouse T-cells selected in a FP THY graft. We have now performed further studies to confirm the ability of porcine MHC to mediate the positive selection of mouse T-cells with a mouse MHC-restricted TCR, and to exclude the possibility that the maturation of mouse T-cells with a mouse MHC-restricted TCR in FP THY grafts in ATX "AND" mice is a special case. For this purpose, TCR-transgenic mice with an unrelated transgenic TCR ["3A9", specific for hen egg lysozyme (HEL) peptide 46-61 presented by I-Ak] were employed. Similar to FP THY-grafted ATX "AND" mice, large numbers of mouse CD4 single positive thymocytes expressing the transgenic TCR (Vbeta8.2) and expressing a mature phenotype (Qa-2high and heat stable antigen, HSAlow) were detected in FP THY grafts. Porcine thymus grafting led to a high level of peripheral repopulation with mouse naive-type (CD44low CD45RBhigh CD62Lhigh) CD4+ cells expressing the transgenic TCR in T and NK cell-depleted ATX "3A9" mice, regardless of whether the recipients had a positive selecting or a non-selecting, class II deficient MHC background. The mouse CD4+ T-cells expressing the "3A9" TCR showed efficient primary proliferative responses to the protein antigen (HEL) when it was presented by mouse class II+ antigen presenting cells (APC) in vitro. These results, collectively, support the general conclusion that discordant xenogeneic porcine MHC can mediate positive selection of mouse T-cells with mouse MHC-restricted TCR. This study has implications for the potential clinical use of xenogeneic thymus transplantation to reconstitute cellular immunity in the setting of thymic insufficiency or thymectomy, and hence for its applicability to the induction of xenograft tolerance and in the treatment of immunodeficiency diseases.