Indefinite survival of skin allografts in adult thymectomized, antilymphocyte serum-treated mice given bone marrow and thymus grafts of donor origin - Tolerance induction by donor bone marrow and thymus

Background. Perigraft immunosuppression with antilymphocyte serum (ALS) and postgraft infusion of donor bone marrow cells (BMC), which induces allograft unresponsiveness in a class I major histocompatibility complex (MHC)-disparate mouse combination, is much less effective in class I and II MHC-disparate strain combinations. Because the thymus plays a central role in the maturation, differentiation, and education of T lymphocytes, which are the principal mediator of allograft responses, we examined whether transplantation of donor-specific thymus combined with adult thymectomy of recipients enhances the tolerogenic effect of ALS and donor BMC infusion in a strongly histoincompatible class I and II MHC-disparate DBA/2-to-B6AF1 mouse strain combination. Methods. Adult thymectomy was performed 4 weeks before grafting. B6AF1 mice received ALS (days -1 and 2), skin allografts (day 0), and BMC and/or thymus grafts (ThyTx) (day 7). Flow cytometric analysis was used to detect donor cells in tolerant mice. Limiting dilution assay was used to calculate the frequencies of precursor cytotoxic T lymphocytes against donor and third-party alloantigens. The presence of suppressor cells was determined by in vivo adoptive transfer assay and in vitro coculture mixed lymphocyte culture. Results. When adult thymectomy and postgraft donor ThyTx were combined with ALS and donor BMC, DBA/2 allograft survival was prolonged with all grafts surviving for > 122 days. Third-party (DBA/1) or recipient ThyTx was not effective. The tolerant mice accepted the second donor skin allografts but acutely rejected the third-party grafts. No significant chimerism was detected. Marked reduction of precursor cytotoxic T lymphocyte frequencies continued only against donor alloantigens after second donor and third-party skin grafting. No suppressor cell activity was detected. Immunopathological analysis revealed that the cells in the ThyTx of tolerant mice are of donor origin. Conclusion. Donor ThyTx combined with donor BMC infusion in adult thymectomized, ALS-treated mice induced clonal deletion of donor-reactive T cells.