Background: Sirolimus is a potent immunosuppressive agent with great therapeutic potential. The objective of our study was to evaluate the efficacy of sirolimus versus cyclosporine in augmenting the unresponsiveness induced by an antilymphocyte serum (ALS)/donor-specific bone marrow (BM)-based regimen across three levels of histoincompatibility: class I and II disparate (DBA/2 to B6AF1), complete mismatch (AKR to C57BL/6), and xenograft (ACI rat to B6AF1).
Methods: Full-thickness skin grafts were taken from donors and placed on recipients in standard fashion. Seven groups of recipient mice (n=10-28) received various combinations of the following treatment protocols: sirolimus, 1.5 mg/kg (3.0 mg/kg for xenografts) every other day from day 0 to day 12; cyclosporine, 50 mg/kg every other day from day 10 through 22; ALS, 0.5 ml on days -1 and 2 for allografts and days -1, 2, and 4 for xenografts; and BM, 25 million donor-specific cells IV on day 7.
Results: The administration of ALS or ALS/BM resulted in modest but significant prolongation of skin graft survival in all combinations tested. Cyclosporine combined with ALS or ALS/BM significantly extended allograft survival compared with ALS or ALS/BM alone (P<0.05) but had no effect on xenograft survival. In contrast, the combination of sirolimus with ALS or ALS/BM resulted in a two- to threefold increase in allograft survival and over a fourfold increase in xenograft survival when compared with the comparable cyclosporine-based regimen. Additionally, lymphocytes isolated from class I and II incompatible mice with skin grafts surviving >100 days demonstrated markedly reduced interleukin 2 and interferon-gamma secretion in response to irradiated donor-specific lymphocytes in culture.
Conclusions: In the regimens tested, sirolimus was superior to cyclosporine in augmenting donor BM-induced skin graft prolongation in ALS-treated mice across all levels of histoincompatibility.