Background: We have recently demonstrated indefinite donor-specific cardiac allograft survival after the intrathymic injection of donor splenocytes and simultaneous injection of antilymphocyte serum (ALS) in a fully major histocompatibility complex-mismatched rat combination. In this study we performed thymectomy to determine the length of time required for donor alloantigen to be present in the recipient thymus to induce tolerance.
Methods: Male Buffalo (BUF; RT1b) rats at 4 to 8 weeks of age underwent intrathymic injection of 25 x 10(6) Lewis (LEW; RT1(1)) splenocytes and simultaneously received an intraperitoneal injection of 1 ml ALS. To determine the kinetics of tolerance induction, the BUF recipients underwent thymectomy on days 1, 3, or 7 after the initial intrathymic injection of alloantigen and intraperitoneal ALS. Twenty-one days after intrathymic alloantigen injection and ALS, all rats underwent transplantation with a heterotopic LEW cardiac allograft.
Results: Thymectomy performed 1 (mean survival time, 6.8 days) and 3 (mean survival time, 8.0 days) days after donor alloantigen injection and ALS did not affect the normal rejection of LEW cardiac allografts. In contrast, thymectomy 7 days after intrathymic alloantigen injection and ALS resulted in indefinite survival of cardiac allografts in 75% of recipients (mean survival time > 77.0 days). In addition, allospecific cytotoxic T lymphocyte activity and interleukin-2 production were markedly decreased in those recipients undergoing thymectomy after 7 days compared with untreated control rats and recipients undergoing thymectomy 1 and 3 days after alloantigen injection.
Conclusions: The presence of the thymus for at least 7 days after intrathymic alloantigen injection and intraperitoneal ALS allows the development of indefinite donor-specific cardiac allograft tolerance.