Split tolerance to a composite tissue allograft in a swine model

Background. The antigenicity of skin is a major obstacle to expanding human composite tissue transplantation. For example, multiple rejection episodes of the skin have been noted in clinical hand transplant patients. We have previously demonstrated tolerance to vascularized musculoskeletal allografts in major histocompatibility complex (MHC)-matched miniature swine treated with 12 days of cyclosporine. This regimen did not reproducibly lead to tolerance to subsequent frozen donor skin grafts. However, such skin grafts did not have a primary vascular supply. The aim of this study was to determine if tolerance to limb allografts with a vascularized skin component could be achieved with MHC matching and a 12-day course of immunosuppression. Methods. Hind limb grafts harvested with a 100 cm² cutaneous paddle were transplanted heterotopically into six MHC-matched, minor antigen-mismatched miniature swine. All animals received a 12-day course of cyclosporine. One control animal was not immunosuppressed. Grafts were evaluated with biweekly biopsies and tissue viability determined by histologic analysis. To test for sensitization, frozen donor skin grafts were applied to all animals that survived to postoperative day 100. Results. All treated animals (n=6) were tolerant to their musculoskeletal allografts at the time of necropsy (>100 days) regardless of the status of the epidermis. One animal demonstrated tolerance to the skin for more than 180 days. The other five animals demonstrated prolonged survival of the epidermal portion of the graft. The control animal rejected the graft epidermis at 10 days postoperatively. Frozen donor skin grafts demonstrated accelerated rejection (<10 days) in three of the animals and led to simultaneous rejection of both the epidermis of the allograft and the skin graft in the long-term tolerant animal. The rejection of the skin grafts did not break tolerance to the musculoskeletal portion in any of the animals. Conclusions. All animals exhibited indefinite survival of the musculoskeletal portion of their allografts but only prolonged survival of the epidermis. The loss of the graft skin appears to be the result of an isolated immune reaction to the skin, and, in particular, the epidermis. This observation is further substantiated by the accelerated rejection of secondarily placed frozen donor skin grafts.