Allograft tolerance by intrathymic donor splenocyte transfer: an age-dependent, species-specific phenomenon?

Protocols that allow allograft survival without immunosuppression remain the ultimate goal in transplantation. Intrathymic injection of donor splenocytes into a transiently immunosuppressed recipient has induced tolerance to a variety of subsequently transplanted allografts in rats. The purpose of this study was to determine if recipient age is critical to intrathymic tolerance in light of age-dependent thymic changes, and if this protocol can be extended to an outbred, large animal model. Prepubertal and postpubertal Wistar-Furth rats underwent intrathymic injection of splenocytes from Lewis rats and antilymphocyte serum (ALS) intraperitoneally. On day 21, a heterotopic Lewis heart was transplanted, with graft survival evaluated by cardiac palpation. Graft tolerance (> 100 days) occurred in four out of five (80%) of the prepubertal rats compared to two out of six (33%) postpubertal rats. Tolerance was not demonstrated in rats receiving intrathymic injection of buffer only. In puppies, groups 1 and 2 underwent splenectomy with intrathymic injection of allo splenocytes. Control puppies (group 3) received intrathymic auto splenocytes. Groups 1 and 3 were given antilymphocyte gamma globulin (ALG) on days 7 to 0 with respect to the intrathymic injection. Group 2 did not receive ALG, but instead received cyclosporin A (CSA) on days 0-2. On day 21, all puppies underwent bilateral nephrectomy and single renal transplantation. No additional immunosuppression was given. Tolerance (creatinine < 7 mg/dl for 100 days) was not obtained by any dog in all three groups. There was no difference in graft survival between control and experimental dogs, with the longest surviving graft seen in a control dog (26 days). Our results suggest that thymic change during maturation may alter the ability to induce tolerance by intrathymic injection of donor cells in rats, and that the protocol is not easily adapted to large animals.