Split tolerance induced by chick embryo thymic epithelium allografted to embryonic recipients

To test the capacity of the epithelial component of the chick embryo thymus to induce tolerance to major histocompatibility complex (MHC) antigens, pre-colonized thymic rudiments were grafted into chick embryonic recipients. Semi-allogeneic or allogeneic transplantations were done between two lines of chickens histocompatible at the MHC locus. Approximately 10% of these thymic chimeras hatched and were studied 3 mo after hatching. Thymic grafts were not rejected by the allogeneic host. The tolerance of chimeric chickens to thymus donor MHC antigens was tested by using a skin graft rejection test and a graft-vs-host (GvH) assay. Chimeric chickens that received an MHC-incompatible thymic graft during the embryonic life tolerated skin graft with the MHC haplotype of the thymus donor. Nevertheless, the lymphocytes within the thymic graft, the host thymus, and the blood were tolerant to the host MHC antigens but were alloreactive in GvH reaction for the MHC antigens of the thymic graft type. These results suggest that the epithelial component of the thymus when taken before the starting of the colonization by hemopoietic precursors and grafted into an early chick embryonic host can induce a tolerance for the MHC determinants involved in allograft rejection but not in the GvH reaction.