After immunosuppressive therapy with Cyclosporin A (Cy A) is stopped, nerve allograft rejection occurs and host axons that had regenerated into the graft degenerate despite the fact that the axons are not foreign tissue. The present experiment was performed to correlate immune events and cellular loss in nerve allografts after terminating Cy A treatment. Nerve grafts (4 cm long) were taken from American Cancer Institute rats and joined to peroneal nerves of Fischer rats that received Cy A (10 mg/kg, intraperitoneally) for one week. This treatment protocol delays nerve graft rejection for weeks during which time it was expected that host blood vessels would unite with vessels in the allograft allogeneic nerve fibers would undergo Wallerian degeneration, and host axons would regenerate into allogeneic Schwann cell columns. Nerve allografts were examined 2-6 weeks postoperatively by light and electron microscopy. No immune reaction nor destruction of allogeneic cells was found in 2- or 3- week-old grafts, and they were invaded proximally by regenerating host axons. At 4 weeks, the perineurium of each graft became infiltrated by mononuclear cells and as destroyed. In addition, many of the endoneurial blood vessels were occluded and their endothelial cells were missing or degenerating. Despite the immune reaction, Schwann cells remained and myelinated many host axons that had grown 2-3 cm into the grafts. However, at 6 weeks, most allogeneic Schwann cells were absent from all grafts, and no host axons were found. There was also evidence (i.e., masses of condensed chromatin) that Schwann cells were killed by apoptosis. These results demonstrate that there is a sequential rejection of cellular components in nerve allografts and that host axonal degeneration is related to adverse immune and/or metabolic effects on allogeneic Schwann cells. Further studies will try to determine whether lymphocytes or macrophages are responsible for allogeneic cell killing and whether their elimination during the rejection process preserves allogeneic Schwann cells and host axons. Research has continued regarding the cryopreservation of nerves. The goal of these experiments is to establish a bank of human nerves which can be used clinically to repair gaps in injured nerves. As a first step, fresh human nerves were transplanted into immunologically deficient nude rats to determine whether this animal could be used to test the viability of cryopreserved human nerves. Human nerves were joined to peroneal nerves of nude rats and examined 4 and 10 weeks postoperatively. None of the grafts were rejected. Rat axons grew within human Schwann cell columns and many axons were myelinated. Experiments will be performed to determine whether crypreserved human nerves survive and conduct regenerating axons after transplantation into nude rats.
