✓ 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.
Andrew A. Zalewski and Adarsh K. Gulati
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 Willys K. Silvers
✓ Schwann cells survive longer in allografts of ganglia that contain minor, rather than major and minor, transplantation antigens. The authors have investigated whether a nerve allograft with minor antigens would be superior to one with major and minor incompatibilities in aiding in the repair of injured nerve. A segment of host peroneal nerve was removed from rats and replaced with a peroneal nerve allograft that contained either minor or major and minor antigens. The results showed that in normal rats, host nerve fibers could functionally regenerate through (that is, grow through and reinnervate muscle) nerve allografts 2 cm but not 4 cm in length, irrespective of the antigens the allografts contained. Host nerve fibers could, however, regenerate through 4-cm nerve allografts if the hosts were rendered immunologically tolerant of the transplantation antigens of the nerve donors. Furthermore, when sensitized lymphoid cells were injected into tolerant rats bearing functional nerve allografts, Schwann cell rejection and demyelination occurred in the graft but host nerve fibers remained. It is concluded that minor antigens alone are as potent as major and minor antigens together in evoking an immune response that prevents functional regeneration through long lengths of nerve allografts. Host nerve fibers can, however, regenerate through such nerve allografts if the hosts are immunosuppressed.
Andrew A. Zalewski, Yoshiaki Kadota, Nabil A. Azzam and Rita N. Azzam
✓ The authors investigate whether there are any permeability changes in the endoneurial blood-nerve barrier and the perineurium-nerve barrier of surviving nerve allografts. In a normal nerve, the blood-nerve barrier regulates the passage of substances from endoneurial blood vessels into the endoneurium, whereas the perineurium-nerve barrier protects the endoneurium from agents that escape from permeable epineurial vessels and accumulate around the nerve. Nerves from ACI rats were transplanted into immunologically deficient nude rats or normal Fischer rats immunosuppressed with cyclosporin A. None of the nerve allografts was rejected. The blood-nerve barrier of nerve allografts at 2 and 6 weeks postoperatively was permeable to intravenously injected horseradish peroxidase, which spread into endoneurial tissue. Electron microscopy revealed that horseradish peroxidase escaped from endoneurial vessels through intercellular junctions between endothelial cells. At 24 weeks, the blood-nerve barrier of nerve allografts had recovered and the endoneurial vessels, like those in normal nerves, were impermeable to horseradish peroxidase. The perineurium-nerve barrier of nerve allografts remained impermeable to horseradish peroxidase at all times. Axons were grouped into numerous minifascicles at nerve anastomosis zones at 24 weeks. Each nerve fascicle was surrounded by an impermeable perineurium. These results demonstrate that regenerated axons in long-term surviving nerve allografts and at anastomosis zones are protected by permeability barriers. It is concluded that permeability barriers of nerve allografts are not permanently altered by a foreign environment (grafts to nude rats) even when immunosuppression with cyclosporin A is required to prevent allograft rejection (grafts to Fischer rats).