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The effect of thymic inoculation to induce tolerance of allogeneic thyroid grafts in the outbred rabbit.

Many studies have demonstrated that allograft tolerance can be achieved in inbred rats and mice following intrathymic injection of donor cells or antigen and treatment with antilymphocyte serum (ALS). In outbred dogs, xenografts, and inbred rat strains with major MHC antigen difference, tolerance has not similarly been induced. The focus of this study was to determine whether allogeneic thyroid graft tolerance could be achieved in outbred rabbits. In the experimental group (n = 5), recipients received an intrathymic injection of donor lymphocytes and a single treatment of ALS. Controls (n = 5) received intrathymic cell culture medium and ALS treatment. Donor-recipient allogenicity was monitored with mixed lymphocyte culture (MLC) over 18 weeks. Donor thyroid tissue was placed into recipient gluteal muscle fibres one week following the last MLC measurement. A third group of rabbits (n = 4) received thyroid autografts without any other treatment. There were no differences in MLC stimulation indices (SI) between the control and experimental group nor did MLC (SI) change within groups. All thyroid autografts survived the two week monitoring period and demonstrated normal appearing thyroid follicles on histologic examination. All thyroid allografts showed severe acute rejection reactions on biopsy within one week. Further studies using outbred animals to examine the role of thymic inoculation are required to determine whether similar techniques might be successful in the human.