Experimental prevention of nerve homograft rejection by use of immunosuppressive drugs

Regenerative Role of T Cells in Nerve Repair and Functional Recovery

The immune system is essential in the process of nerve repair after injury. Successful modulation of the immune response is regarded as an effective approach to improving treatment outcomes. T cells play an important role in the immune response of the nervous system, and their beneficial roles in promoting regeneration have been increasingly recognized. However, the diversity of T-cell subsets also delivers both neuroprotective and neurodegenerative functions. Therefore, this review mainly discusses the beneficial impact of T-cell subsets in the repair of both peripheral nervous system and central nervous system injuries and introduces studies on various therapies based on T-cell regulation. Further discoveries in T-cell mechanisms and multifunctional biomaterials will provide novel strategies for nerve regeneration.

Neuroimmune interactions and immunoengineering strategies in peripheral nerve repair

Peripheral nerve injuries result in disrupted cellular communication between the central nervous system and somatic distal end targets. The peripheral nervous system is capable of independent and extensive regeneration; however, meaningful target muscle reinnervation and functional recovery remain limited and may result in chronic neuropathic pain and diminished quality of life. Macrophages, the primary innate immune cells of the body, are critical contributors to regeneration of the injured peripheral nervous system. However, in some clinical scenarios, macrophages may fail to provide adequate support with optimal timing, duration, and location. Here, we review the history of immunosuppressive and immunomodulatory strategies to treat nerve injuries. Thereafter, we enumerate the ways in which macrophages contribute to successful nerve regeneration. We argue that implementing macrophage-based immunomodulatory therapies is a promising treatment strategy for nerve injuries across a wide range of clinical presentations.

Nerve Regeneration through Nerve Autografts and Cold Preserved Allografts using Tacrolimus (FK506) in a Facial Paralysis Model: A Topographical and Neurophysiological Study in Monkeys

OBJECTIVE

Nerve regeneration through cold preserved nerve allografts is demonstrated, and treatment of nerve allografts with FK506 induces better regeneration than other immunosuppressants. We study nerve regeneration through cold preserved nerve allografts temporarily treated with FK506 and compare it with the regeneration obtained using classic nerve autografts in a facial paralysis model in monkeys.

METHODS

A trunk of the facial nerve on both sides was transected in eight monkeys and immediately repaired with a 3 to 4 cm nerve autograft or allograft. FK506 was administered to the animals of the allograft group for 2 months, and nerve allografts were cold preserved for 3 weeks. At periods of 3, 5, and 8 months after surgery, quantitative electrophysiological assessment and video recordings were performed. At the end of the study, quantitative analysis of neurons in the facial nucleus was carried out, and axons were stereologically counted.

RESULTS

After the regenerative period, neuronal density was higher in the autograft group. However, distal axonal counts were similar in both groups. Serial electrophysiological recordings and histology of nerve allografts showed that the grafts were partially rejected after cessation of the immunosuppressant.

CONCLUSION

The regeneration through nerve allografts temporarily treated with FK506 does not achieve the electrophysiological results and neuronal counts achieved with nerve autografts, but axonal collateralization in the allografts induces a similar activation of mimic muscles.

A review of research endeavors to optimize peripheral nerve reconstruction

This manuscript reviews studies relating to peripheral nerve allografts, neuroregenerative agents and end-to-side neurorrhaphy. With respect to peripheral nerve allografts, animal studies with the agents cyclosporin A, FK506 and rapamycin are reviewed and related to recent clinical experience. FK506 distinguishes itself as an agent capable of reversing acute rejection of a peripheral nerve allograft and an agent with some neuroregenerative properties. In addition to systemic immunosuppression, experience with agents purported to initiate a state of donor specific tolerance are discussed. Specifically, experimental studies with administration of ultraviolet B treated donor splenocytes, antibodies to cellular adhesion molecules and antibodies to components of the costimulatory pathway of immunosuppression are reviewed. The neuroregenerative properties of FK506 and related compounds are examined in animal models. Finally, the experimental finding that reinnervation following end-to-side neurorrhaphy is mostly sensory and related to the degree of axonal damage at the level of an epineurotomy or perineurotomy is discussed.

Regeneration through long nerve grafts in the swine model

We investigated regeneration across a long nerve defect in the swine model to study extensive neural loss and long nerve gap. Most experiments have been conducted in the rodent model that, while an appropriate immunological model, only allows short nerve gaps to be studied. Twelve outbred swine received either an 8-cm ulnar nerve autograft or an allograft without immunosuppression. At 6 and 10 months, histomorphometry of the autografts demonstrated excellent nerve regeneration, while very poor regeneration was noted across the allografts. This confirmed that 8 cm are an adequate challenge independent of the spontaneous regeneration potential of axons seen in rodents. The swine ulnar nerve graft model causes minimal morbidity and will now be used with immunological manipulation of inbred animals.

Physiology of peripheral nerve graft incorporation

The degree of cell survival and the rate and extent of cellular migration was studied in fresh- and freeze-killed nerve grafts using an animal model with isogeneic nerve grafts performed between inbred rats. Nerve isografts 1.4 cm in length were used to bridge a 1.0 cm gap created in recipient animals. Vital fluorescent staining was used to monitor cell viability and to track cell migration between the nerve graft and the recipient host's nerve endings. The fresh nerve grafts maintained their fluorescent label, indicating that these grafts maintained their viability. The freeze-killed grafts had significantly lower cell survival, as determined by percent area of fluorescence, both 14 and 25 days after nerve grafting. The freeze-killed grafts also demonstrated a lower percentage of incorporation of labeled host cells from the proximal host nerve ending. Since the fresh nerve grafts maintained their viability, even though they were performed as nonvascularized grafts, free vascularized nerve grafts may not be necessary if a good vascular bed is present.

An assessment of regeneration across peripheral nerve allografts in rats receiving short courses of cyclosporin A immunosuppression

While peripheral nerve reconstruction could benefit from the use of nerve allografts, long term immunosuppression for non-vital organ transplantation is controversial. This study investigated the effectiveness of short course Cyclosporin A immunosuppression. Fourteen Lewis (RT1l) rats were the recipients of 3 cm sciatic nerve grafts from ACI (RT1a) donors, repaired to the transected sciatic nerve of the recipient animal. Animals were treated with Cyclosporin A (5 mg/kg/day) for eight weeks. Neuromuscular function was assessed every two weeks by sciatic function index determinations until 20 weeks. Electrophysiological, histological and morphological evaluations were performed at 14 (n = 6) and 20 weeks (n = 8) postengraftment. Rats had significantly improving functional studies from four to eight weeks (P = 0.01). Function decreased following cessation of Cyclosporin A treatment. Rats evaluated at 14 weeks had histological evidence of graft rejection with inflammatory cell infiltration, extensive demyelination and remyelination, and some Wallerian degeneration. Rats demonstrated improvement in morphological parameters and motor function from 14 to 20 weeks after engraftment. In this sciatic nerve allograft model, short course Cyclosporin A immunosuppression, although resulting in an initial episode of graft rejection, was successful in permitting good long term functional regeneration of neuromuscular function.

Allogeneic nerve grafts in the rat, with special reference to the role of Schwann cell basal laminae in nerve regeneration

The role of basal laminae as conduits for regenerating axons in an allogeneic graft was examined by transplanting a 3 cm long segment of the sciatic nerve from the Brown Norway to the Fischer 344 strain of rat. These strains are not histocompatible with each other. In order to compare the nerve regeneration in variously treated grafts, three different types of graft were employed: non-treated (NT), predenervated (PD), and predenervated plus freeze-treated (PDC) grafts. The cytology of nerve regeneration through these grafts was examined by electron microscopy at four, seven, 14, 30 and 60 days after grafting. In the PDC graft, in which Schwann cells were dead on grafting, basal laminae were well preserved in the form of tubes after Schwann cells and myelin sheaths had been removed at seven days after grafting. Regenerating axons accompanied by immature host Schwann cells grew out through such basal lamina tubes in the same fashion as observed in our previous studies. By day 14, axons extended as far as the middle of the graft. In the proximal part they were separated into individual fibres and even thinly myelinated by Schwann cells. On the other hand, in the NT and PD grafts in which Schwann cells were alive on grafting, most Schwann cells and myelin sheaths appeared to undergo autolytic degeneration by day 14, while Schwann cell basal laminae were left almost intact in the form of tubes. A few regenerating axons were seen associated with Schwann cells in the proximal portion by day seven. It is probable that host Schwann cells moved into the graft after donor cells had been degraded. Schwann cell basal laminae tended to be damaged at the site of extensive lymphoid cell infiltration. By day 30, regenerating axons had arrived at the distal end of the graft in all three types of graft: in the PDC graft thick axons were fully myelinated, whereas in the PD graft they were only occasionally myelinated and in the NT graft most axons were still surrounded by common Schwann cells. By 60 days after grafting, regenerating axons were well myelinated in the host nerve as observed 1 cm distal to the apposition site in all the three types of graft. These findings show that Schwann cell basal laminae can serve as pathways (most efficiently in the PDC graft) for regenerating axons in a 3 cm long allograft in the rat.

Nerve regeneration through the cryoinjured allogeneic nerve graft in the rabbit

To examine whether the 3-4-cm-long allogeneic basal lamina tubes of Schwann cells serve as conduits for regenerating axons in rabbits, allogeneic saphenous nerve, which had been predenervated and pretreated by freezing, were transplanted from Japanese White rabbits (JW) to New Zealand White rabbits (NW). Animals were killed 1, 2, 6, 8, and 14 weeks after transplantation, and the cytology at the mid-portion of the grafts was examined by electron microscopy. The distal portion of the host saphenous nerves was also examined 14 weeks after grafting. Myelin sheath debris was phagocytosed by macrophages, while the basal lamina of Schwann cells were left intact in the form of tubes. Regenerating axons were first found in such basal lamina tubes 2 weeks after grafting, and gradually increased in number. Host Schwann cells accompanied the regenerating axons behind their growing tips, separating them into individual fibers and forming thin myelin sheaths on thick axons by 6 weeks after grafting. Regenerating nerves were divided into small compartments by new perineurial cells. Newly formed blood vessels were situated outside the compartment 8 weeks after grafting. The percentage of myelinated fibers in the regenerating nerves was roughly 10% at 8 weeks and 30% at 14 weeks after grafting. The diameter of the regenerating axons, both myelinated and unmyelinated, was less than that of normal axons at all the stages examined. Numerous regenerating axons, some of which were fully myelinated, were found at the site 10 mm distal to the distal end of the graft 14 weeks after grafting.(ABSTRACT TRUNCATED AT 250 WORDS)

Homologous nerve transplantation and immunosuppression in rabbits

Homologous nerve transplantation is a logical method of bridging major nerve defects but its usefulness is negated by immune rejection. The discovery of an effective method of immunosuppression would allow utilisation of this technique. The present study compared the effects of irradiation, dexamethasone, and chloramphenicol in homologous nerve transplantation. Segments of homologous sciatic nerve were implanted into the dorsal subcutaneous (s.c.) tissues of 83 rabbits. In one group the graft was irradiated in vitro before implantation and in other groups the recipient rabbits received either dexamethasone or chloramphenicol systemically for up to 3 weeks after implantation. The transplanted nerve segments were removed at different time intervals and examined for signs of rejection. In conclusion, rejection commenced during the 1st week and peaked during the 3rd week. In the control group without immunosuppressant, all animals exhibited severe rejection. Both pre-implantation irradiation of the grafts at 450 rad dose-level and systemic chloramphenicol at 100 mg/kg/day were found to completely suppress rejection. These methods were considerably more effective than conventional steroids. Thus, the idea of a "nerve bank" is postulated.