Tolerance Induction in a Large-Animal Model Using Total Lymphoid Irradiation and Intrathymic Bone Marrow

Background. Immunological unresponsiveness of T cells to alloantigen can be induced by intrathymic injection of donor-specific antigen in small-animal models. Intrathymic tolerance to vascularized grafts in large animals has not previously been reported. Methods. Thirty-two dogs were allocated into dog leukocyte antigen DP locus allele (class II)-matched donor-recipient pairs. Female recipients were paired with male donors. Tissue typing was based on restriction fragment length polymorphism. Recipients were given 18 Gy total lymphoid irradiation in 16 fractions (1.125 Gy each) over 4 weeks. Thoracotomy after the 6th fraction permitted perithymic (n=4) or intrathymic (n=4) injection of donor bone marrow (BM) or intrathymic injection of saline (n=5). Another group received intravenous peripheral BM infusion (n=3). Fifty days postthoracotomy recipients underwent bilateral nephrectomy and donor-specific kidney transplantation. Acute rejection, suspected when serum creatinine was more than 600 mu mol/L or urea was more than 40 mmol/L, was confirmed histologically. Full-thickness skin grafts followed more than 100 days posttransplantation. Tissue samples were taken for Y-chromosome polymerase chain reaction. Results. One intrathymic (25%) and three perithymic (75%) BM recipients developed tolerance to renal allografts. Three intrathymic BM recipients rejected after 27, 32, and 54 days and one perithymic BM recipient rejected after 42 days. All recipients given peripheral BM or saline had rejected by 29 and 38 days, respectively. All recipients surviving more than 100 days posttransplantation, accepted donor specific and rejected dog leukocyte antigen-DP locus allele (class IT) identical third-party skin grafts. Polymerase chain reaction detected intrathymic but not hematopoietic chimerism in sex-mismatched pairs. Conclusions. Fractionated total lymphoid irradiation and perithymic or intrathymic donor-specific BM induced tolerance to renal and skin allografts without inducing hematopoietic chimerism.