An adult recipient of an HLA-DR, DQ-mismatched, T cell—depleted bone marrow graft, who remains without graft versus host disease and who is not maintained on immunosuppressive therapy, was studied at 23 months posttransplantation for in vitro reactivity against the mismatched antigens of the host. The donor's PBMC's proliferated vigorously against the recipient's stimulators in the pretransplant mixed lymphocyte cultures (MLC). After transplant reconstitution, MLCs demonstrated that the in vitro response of engrafted donor T cells against host MHC class II antigens was equivalent to control allogeneic responses, while there was no detectable response against the donor's antigens. Posttransplantation limiting dilution analysis showed no dif-ference between the precursor frequencies of antihost responders among populations of fresh donor PBMCs and among the engrafted cells of donor origin that are found circulating in the patient. This result suggests that clonal deletion is, at best, incomplete and that peripheral tolerance is essential in protecting this patient from GVHD. These findings also support the conclusion that bone marrow-derived thymic elements may be important for clonal deletion in human chimeras.
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