Evaluation of histocompatibility as a factor in the repair of nerve with a frozen nerve allograft

Host axons in dogs can regenerate through a long nerve allograft provided that the allograft bears only minor transplantation antigens, and is frozen and thawed before transplantation. The authors have tried to confirm this important observation in rats. Host rats received a 4-cm fresh or frozen nerve isograft (that is, a non-antigenic nerve), or a fresh or frozen nerve allograft with cells containing only minor transplantation antigens. The results showed that after 2 and 9 months only a fresh isograft permitted many host axons to traverse its entire length. Only a few host axons grew into the proximal 1 to 2 cm of a frozen isograft or into an allograft (fresh or frozen). Because frozen grafts failed, the authors examined some specimens after 2 weeks and found that freezing killed most of the Schwann cells. On the other hand, many proliferating Schwann cells were found in 2-week fresh isografts. In addition, hosts that received a frozen nerve allograft underwent regrafting after 9 months with an isograft and allograft (of the same genotype as the original nerve allograft) of nodose ganglion. These rats accepted the isograft but rejected the allograft of ganglion. It is concluded that axonal regeneration through a long frozen nerve graft fails in rat because freezing destroys Schwann cells. Moreover, a frozen nerve allograft does not induce a state of immunological tolerance, as has been suggested, because these recipients reject a second allograft. Since the present data fail to confirm findings obtained in dogs, the clinical use of a frozen nerve allograft is not recommended.