Induction of T cell differentiation in early-thymectomized Xenopus by grafting adult thymuses from either MHC-matched or from partially or totally MHC-mismatched donors.

The effect of grafting thymuses from major histocompatibility complex (MHC)-matched and mismatched (either partially or totally) adult donors on the restoration of T-cell dependent immune responses of Xenopus adults that were thymectomized (txd) during early larval life was examined. Four to 5 month-old diploid (or triploid) frogs of defined MHC haplotypes that had been txd on day 4 or 5 postfertilization were each grafted subcutaneously with a pair of thymuses from a triploid (or diploid) MHC-defined frog. Regardless of the donor-host combination, thymus grafts restored in vivo acute skin allograft rejection capacities and antibody responses to SRBC and in vitro proliferative responses of spleen cells to the T cell mitogens, PHA and Con A. Txd frogs that were grafted with MHC-mismatched thymus did not reject skin grafts with the MHC haplotype of the thymus donor. Nevertheless, their spleen cells could proliferate, in one way MLC, in response to irradiated stimulator cells with the thymus donor MHC haplotype. Ploidy analyses of mithramycin-stained thymic and splenic lymphocytes (DNA quantitation by flow cytometry) demonstrated that in certain donor-host combinations, cells from totally or partially MHC-mismatched donors as well as from MHC-matched donors persisted in the thymus grafts and/or the spleens of txd hosts. Chimerism lasted for at least 1 year after thymus grafting. In other donor-host combinations, however, (txd isogenic cloned LG15 frogs grafted with allogeneic thymus from MHC-homozygous triploid J strain frogs), no donor cells could be detected 7 months after thymus grafting. Chromosome counts of PHA-induced metaphases of spleen cells from these and other thymus-grafted frogs revealed host cells in metaphase. This suggests that thymus grafts can promote the differentiation of host precursor cells along a T cell pathway.