Late adjunctive therapy with single doses of rapamycin in skin-allografted mice treated with antilymphocyte serum and donor bone marrow cells.

Temporary donor-specific unresponsiveness induced by treatment of skin allografted mice with antilymphocyte serum (ALS) and donor bone marrow cells (BMC) can be converted to long-term graft survival and tolerance by adding adjunctive immunosuppressive agents, including rapamycin (Rapa). Rapa was given in a suboptimal dosing schedule to probe several aspects of its use in the promotion of tolerization. Single doses were given at 2 weeks post-transplantation to recipients prepared with ALS and donor BMC. Graft survival was markedly prolonged in a dose-dependent fashion by day +14, Rapa doses ranging from 0.75 to 6.0 mg/kg. Indefinite (> 300 day) graft survival was observed in 26% of recipients given the highest Rapa dose. Short-term treatment with cyclosporin A (CsA) or deoxyspergualin (DSG) was ineffective when injected at this time. Rapa augmented the tolerizing effect of grafting with skin from newborn mice but had no significant additive or synergistic effects with the short course of CsA or with DSG given on days +1 to +3, even though the latter prolonged graft survival when added to the ALS/BMC protocol. Single doses of Rapa on day +1 also prolonged graft survival, but without any of the grafts surviving indefinitely. Later dosing on day +28 resulted in > 70% of grafts surviving > 300 days. Challenge grafting of these mice after day +300 resulted in delayed rejection of donor strain, but not third-party skin grafts. Rapa was very effective when given as widely spaced doses on days +14 and +49. Also, grafts showing the earliest signs of rejection could be rescued with a single Rapa dose in recipients treated with ALS and BMC but not ALS alone. Transfer of prolonged graft survival with spleen cells from ALS plus BMC treated recipients was not adversely affected by Rapa given to the suppressor-like spleen cell donors approximately 1 week before cell harvest. We conclude that the use of Rapa as an adjunctive agent in allograft recipients treated with ALS plus donor BMC is very flexible in terms of timing of administration, and that the drug can be effectively given as widely spaced doses or as a rescue agent after ALS/BMC treatment. Additionally, an active immunoregulatory mechanism induced by ALS/BMC treatment appears to be spared by Rapa.