✓ The present study describes the usefulness of blood vessel allografts to repair gaps in rat peripheral nerve after immunosuppression with cyclosporine. Isogeneic strains of rats with known histoincompatibility were used for this study. A 10-mm gap was created in the peroneal nerve of host Fischer rats. The gap was bridged by a 12-mm section of internal carotid artery removed from a Buffalo strain of rat. The host rats were divided into two groups. One group received no immunosuppression, whereas the other group was treated with cyclosporine. Untreated control rats immunologically rejected the allografted vessels and were unable to support host axonal regeneration through them. On the other hand, in cyclosporine-treated rats the allografted vessels survived. The regenerating host axons reorganized to form a functional nerve within the vessel conduit. The regenerated axons persisted even after rejection of the allografted vessel caused by cessation of immuno-therapy. These results show that blood vessel allografts can serve as an effective conduit for reorganization of regenerated nerves and can bridge gaps in peripheral nerves.
Adarsh K. Gulati
✓ Nerve grafts composed of basal lamina scaffolds and lacking viable Schwann cells have recently been shown to be effective in supporting axonal regeneration. As only short grafts were used in those studies, the present investigation was conducted to evaluate the ability of long acellular basal lamina nerve grafts and equivalent cellular grafts to support axonal regeneration for nerve gap repair. Cellular grafts consisted of nerve segments that had degenerated in situ for 4 weeks. Acellular grafting material consisted of similar segments that were repeatedly frozen and thawed to kill all cells prior to grafting. The results show that host axons can regenerate through the entire 4-cm length of cellular grafts but not through acellular basal lamina grafts. However, in the acellular grafts numerous axons were seen in the proximal 2-cm region. It is concluded that basal lamina grafts possess limited ability to support axonal regeneration. As in cellular grafts, viable Schwann cells appear to be important for regeneration to occur over longer distances.
Adarsh K. Gulati and Geoffrey P. Cole
✓ Acellular basal lamina grafts have recently been reported to support axonal regeneration and have been used in peripheral nerve repair. The present study was designed to determine the immunogenicity of such basal lamina allografts (grafts that are genetically different) and their potential as bridging material for nerve gap repair. Inbred strains of Fischer and Buffalo rats with known histocompatibility differences were used. Acellular grafts were prepared by repeated freezing and thawing nerve tissue predegenerated in situ for 6 weeks. Non-frozen predegenerated nerves were used as cellular grafts for comparison. Fischer rats were used as hosts and received cellular or acellular grafts obtained from Fischer (isograft, genetically identical) or Buffalo (allograft) donors. The grafts were evaluated morphologically at 1,2, 4, and 12 weeks after transplantation. The cellular isografts supported axonal regeneration best. The cellular allografts were invariably rejected and were unsuccessful or only partially successful in supporting regeneration. In contrast, acellular allografts, in spite of their mild immunogenicity were successful in supporting regeneration, as were the acellular isografts. The rate of host axonal regeneration and recovery of target muscle was reduced in acellular allografts and isografts as compared to cellular isografts. It is concluded that acellular allografts are suitable for supporting axonal regeneration and may be used to bridge gaps in injured peripheral nerves.
Andrew A. Zalewski and Adarsh K. Gulati
✓ This study examined the effect of immunosuppressive treatment with cyclosporin A (Cy-A) on the survival of nerve allografts in sensitized rats. Nerve- or skin-sensitized untreated rats rejected a second nerve allograft of the same genotype as the first in an accelerated manner. In this situation, only a few host axons grew into the proximal 1 cm of a 4 cm-long nerve allograft. However, if sensitized rats were given Cy-A (10 mg/kg daily), the second nerve allograft survived, and numerous host axons regenerated through the 4-cm length of the allograft. These results indicated that Cy-A was an effective immunosuppressive agent in sensitized rats. We conclude that, in rats, donor-specific sensitization is not a contraindication to the use of nerve allografts to aid in the repair of injured nerve when Cy-A is used for immunosuppression.
Andrew A. Zalewski and Adarsh K. Gulati
✓ 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 rats 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 failed to confirm findings obtained in dogs, the clinical use of a frozen nerve allograft is not recommended.