Peripheral nerve allograft preservation improves regeneration and decreases systemic cyclosporin A requirements

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Evaluation of the Use of Nerve Allograft Preserved in Glycerol

Background:

We aimed to evaluate the use of nerve allograft preserved in glycerol. We compared the efficiency of glycerol-preserved allografts with autogenous nerve grafting, cryopreserved grafts, and detergent-processed grafts in the axonal regeneration. Secondarily, we evaluated the effectiveness of each preservation method in maintaining the extracellular matrix free of cellular components.

Methods:

This was a prospective experimental, longitudinal, unblinded, nonrandomized, controlled animal model study. Three different allograft preservation techniques for the repair of sciatic nerve injuries were compared, including cold preservation, glycerol preservation, and detergent preservation. Functional assessment was performed, and histomorphometric analyses were further performed, which enabled the allograft structure evaluation and an estimation of the nerve regeneration efficacy based on the myelinated axons count and on their diameters.

Results:

After the 14^th week, all groups were already balanced and similar (P = 0.265): all groups present near-zero SFIs, thus confirming their efficiency in promoting nerve regeneration. In the histomorphometric evaluations, all groups were equivalent, presenting a similar efficiency in nerve regeneration (P = 0.716 and P = 0.577, respectively). Similarly, histomorphometric evaluations showed a reduction in the number of axons and in their diameters, but none of them effectively eliminated all cellular debris. Comparing the groups with each other, the groups preserved in glycerol and detergent solution were similar, both presenting better results than the cooled group.

Conclusion:

By evaluating the presence of cell debris after the treatment using glycerol, it was found to be similar to the treatment using detergent and significantly better than the cold-preservation treatment.

Schwann cells seeded in acellular nerve grafts improve functional recovery

INTRODUCTION

This study evaluated whether Schwann cells (SCs) from different nerve sources transplanted into cold-preserved acellular nerve grafts (CP-ANGs) would improve functional regeneration compared with nerve isografts.

METHODS

SCs isolated and expanded from motor and sensory branches of rat femoral and sciatic nerves were seeded into 14mm CP-ANGs. Growth factor expression, axonal regeneration, and functional recovery were evaluated in a 14-mm rat sciatic injury model and compared with isografts.

RESULTS

At 14 days, motor or sensory-derived SCs increased expression of growth factors in CP-ANGs versus isografts. After 42 days, histomorphometric analysis found CP-ANGs with SCs and isografts had similar numbers of regenerating nerve fibers. At 84 days, muscle force generation was similar for CP-ANGs with SCs and isografts. SC source did not affect nerve fiber counts or muscle force generation.

CONCLUSIONS

SCs transplanted into CP-ANGs increase functional regeneration to isograft levels; however SC nerve source did not have an effect.

Nerve regeneration across cryopreserved allografts from cadaveric donors: a novel approach for peripheral nerve reconstruction

Object

The use of allografts from cadaveric donors has attracted renewed interest in recent years, and pretreatment with cryopreservation and immunosuppression methods has been investigated to maximize axonal regrowth and minimize allograft rejection. The authors wanted to assess the outcome of treatments of brachial plexus stretch injuries with cryopreserved allografts from cadaveric donors in nonimmunosuppressed patients.

Methods

Ten patients with brachial plexus lesions were submitted to electromyography (EMG) testing 1 and 3 months after a traumatic event and 1 week before surgery to localize and identify the type of lesion. Intraoperative EMG recordings were performed for intraoperative monitoring to select the best surgical strategy, and postoperative EMG was used to follow up patients and determine surgical outcomes. If nerve action potentials (NAPs) were present intraoperatively, neurolysis was performed, whereas muscular/nerve neurotization was performed if NAPs were absent. Cryopreserved allografts obtained from selected cadaveric donors and provided by the tissue bank of Treviso were used for nerve reconstruction in patients who were not treated with immunosuppressive drugs.

Results

The surgical strategy was selected according to the type and site of the nerve lesion and on the basis of IOM results: 14 cryopreserved allografts were used for 7 muscular neurotizations and for 7 nerve neurotizations, and 5 neurolysis procedures were performed. All of the patients had regained motor function at the 1- and 2-year follow-ups.

Conclusions

Some variables may affect functional recovery after allograft surgery, and the outcome of peripheral nerve reconstruction is more favorable when patients are carefully evaluated and selected for the surgery. The authors demonstrated that using cryopreserved allografts from cadaveric donors is a valid surgical strategy to restore function of the damaged nerve without the need for any immunosuppressive treatments. This approach offers new perspectives on procedures for extensive reconstruction of brachial and lumbosacral plexuses.

Experimental and clinical evidence for use of decellularized nerve allografts in peripheral nerve gap reconstruction

Despite the inherent capability for axonal regeneration, recovery following severe peripheral nerve injury remains unpredictable and often very poor. Surgeons typically use autologous nerve grafts taken from the patient's own body to bridge long nerve gaps. However, the amount of suitable nerve available from a given patient is limited, and using autologous grafts leaves the patient with scars, numbness, and other forms of donor-site morbidity. Therefore, surgeons and engineers have sought off-the-shelf alternatives to the current practice of autologous nerve grafting. Decellularized nerve allografts have recently become available as an alternative to traditional nerve autografting. In this review, we provide a critical analysis comparing the advantages and limitations of the three major experimental models of decellularized nerve allografts: cold preserved, freeze-thawed, and chemical detergent based. Current tissue engineering-based techniques to optimize decellularized nerve allografts are discussed. We also evaluate studies that supplement decellularized nerve grafts with exogenous factors such as Schwann cells, stem cells, and growth factors to both support and enhance axonal regeneration through the decellularized allografts. In examining the advantages and disadvantages of the studies of decellularized allografts, we suggest that experimental methods, including the animal model, graft length, follow-up time, and outcome measures of regenerative progress and success be consolidated. Finally, all clinical studies in which decellularized nerve allografts have been used to bridge nerve gaps in patients are reviewed.

Clinical strategies to enhance nerve regeneration in composite tissue allotransplantation

Reinnervation of a hand transplant ultimately dictates functional recovery but provides a significant regenerative challenge. This article highlights interventions to enhance nerve regeneration through acceleration of axonal regeneration or augmentation of Schwann cell support and discuss their relevance to composite tissue allotransplantation. Surgical techniques that may be performed at the time of transplantation to optimize intrinsic muscle recovery--including appropriate alignment of ulnar nerve motor and sensory components, transfer of the distal anterior interosseous nerve to the recurrent motor branch of the median nerve, and prophylactic release of potential nerve entrapment points--are also presented.

Acellular nerve allografts in peripheral nerve regeneration: a comparative study

INTRODUCTION

Processed nerve allografts offer a promising alternative to nerve autografts in the surgical management of peripheral nerve injuries where short deficits exist.

METHODS

Three established models of acellular nerve allograft (cold-preserved, detergent-processed, and AxoGen-processed nerve allografts) were compared with nerve isografts and silicone nerve guidance conduits in a 14-mm rat sciatic nerve defect.

RESULTS

All acellular nerve grafts were superior to silicone nerve conduits in support of nerve regeneration. Detergent-processed allografts were similar to isografts at 6 weeks postoperatively, whereas AxoGen-processed and cold-preserved allografts supported significantly fewer regenerating nerve fibers. Measurement of muscle force confirmed that detergent-processed allografts promoted isograft-equivalent levels of motor recovery 16 weeks postoperatively. All acellular allografts promoted greater amounts of motor recovery compared with silicone conduits.

CONCLUSION

These findings provide evidence that differential processing for removal of cellular constituents in preparing acellular nerve allografts affects recovery in vivo.

Nerve allotransplantation as it pertains to composite tissue transplantation

Nerve allografts provide a temporary scaffold for host nerve regeneration and allow for the repair of significant segmental nerve injuries. From rodent, large animal, and nonhuman primate studies, as well as clinical experience, nerve allografts, with the use of immunosuppression, have the capacity to provide equal regeneration and function to that of an autograft. In contrast to solid organ transplantation and composite tissue transfers, nerve allograft transplantation requires only temporary immunosuppression. Furthermore, nerve allograft rejection is difficult to assess, as the nerves are surgically buried and are without an immediate functional endpoint to monitor. In this article, we review what we know about peripheral nerve allograft transplantation from three decades of experience and apply our current understanding of nerve regeneration to the emerging field of composite tissue transplantation.

Chapter 8: Current techniques and concepts in peripheral nerve repair

Despite the progress in understanding the pathophysiology of peripheral nervous system injury and regeneration, as well as advancements in microsurgical techniques, peripheral nerve injuries are still a major challenge for reconstructive surgeons. Thorough knowledge of anatomy, pathophysiology, and surgical reconstruction is a prerequisite of proper peripheral nerve injury management. This chapter reviews the currently available surgical treatment options for different types of nerve injuries in clinical conditions. In overview of direct nerve repair, various end-to-end coaptation techniques and the role of end-to-side repair for proximal nerve injuries is described. When primary repair cannot be performed without undue tension, nerve grafting or tubulization techniques are required. Current gold standard for bridging nerve gaps is nerve autografting. However, disadvantages of this approach, such as donor site morbidity and limited length of available graft material encouraged the search for alternative means of nerve gap reconstruction. Nerve allografting was introduced for repair of extensive nerve injuries. Tubulization techniques with natural or artificial conduits are applicable as an alternative for bridging short nerve defects without the morbidities associated with harvesting of autologous nerve grafts. Achieving better outcomes depends both on the advancements in microsurgical techniques and introduction of molecular biology discoveries into clinical practice. The field of peripheral nerve research is dynamically developing and concentrates on more sophisticated approaches tested at the basic science level. Future directions in peripheral nerve reconstruction including, tolerance induction and minimal immunosuppression for nerve allografting, cell based supportive therapies and bioengineering of nerve conduits are also reviewed in this chapter.

Role of timing in assessment of nerve regeneration

Small animal models are indispensable for research on nerve injury and reconstruction, but their superlative regenerative potential may confound experimental interpretation. This study investigated time-dependent neuroregenerative phenomena in rodents. Forty-six Lewis rats were randomized to three nerve allograft groups treated with 2 mg/(kg day) tacrolimus; 5 mg/(kg day) Cyclosporine A; or placebo injection. Nerves were subjected to histomorphometric and walking track analysis at serial time points. Tacrolimus increased fiber density, percent neural tissue, and nerve fiber count and accelerated functional recovery at 40 days, but these differences were undetectable by 70 days. Serial walking track analysis showed a similar pattern of recovery. A "blow-through" effect is observed in rodents whereby an advancing nerve front overcomes an experimental defect given sufficient time, rendering experimental groups indistinguishable at late time points. Selection of validated time points and corroboration in higher animal models are essential prerequisites for the clinical application of basic research on nerve regeneration.

Peripheral nerve regeneration through allografts compared with autografts in FK506-treated monkeys

Object

The clinical use of nerve allografts combined with immunosuppressant therapy has become a genuine possibility that could supersede the classic use of autografts. However, contradictory data have been reported on whether immunosuppressant therapy should be temporarily administered. The purpose of this study was to compare the nerve regeneration obtained using ulnar nerve allografts in nonhuman primates temporarily treated with FK506 (tacrolimus) with that obtained using nerve autografts.

Methods

Four-centimeter nerve autografts or allografts were placed in the distal ulnar motor nerve of eight monkeys. The FK506 was temporarily administered to the animals of the allograft group for 2 months. At periods of 3, 5, and 8 months postsurgery, quantitative electrophysiological recordings were obtained to estimate muscle response. A quantitative analysis of ulnar motor neurons in the spinal cord was performed and axons were counted stereologically.

No statistically significant differences were found in the neuronal and axonal counts between autograft and allograft groups at 8 months. The electrophysiological studies showed no differences relative to the amplitude, but the autograft group presented with a greater nerve conduction velocity (NCV). However, no statistically significant differences were found between the number of neurons and distal axonal counts in the two groups.

Conclusions

Nerve regeneration through cold-preserved allografts in a primate model temporarily treated with FK506 was similar to that obtained using nerve autografts, in terms of neuronal and axonal counts. Nevertheless, temporary immunosuppression produced lower NCV when allografts were used, with less maturation of the myelinated fibers, which indicated that a partial rejection had taken place.

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.

Bridging the neural gap

A major limitation to overall success in peripheral nerve surgery is time for regeneration. Although one can help speed up the regenerative process to some extent, success is hindered by issues such as number of coaptation sites, supply of donor nerves, and the limitations of nerve substitutes. In the case of a large gap, a nerve graft is often used to fill in the deficit. Autogenous nerve grafts are in limited supply, with sural nerve grafts being the primary source. Alternatives to the standard treatment include vein grafts, synthetic nerve conduits, nerve transfers, and nerve transplantation. Schwann cell-lined nerve conduits and tissue-engineered substitutions are still in their infancy and have some limited clinical application.

Comparison of continuous and discontinuous FK506 administration on autograft or allograft repair of sciatic nerve resection

An immunosuppressant drug that also possesses neuroregenerative properties, FK506 enhances the rate of axonal regeneration and improves recovery after nerve lesions. Nevertheless, prolonged immunosuppression may not be justified to assure the success of nerve regeneration. In this study, we compare the effects of continuous and discontinuous FK506 treatment on regeneration and reinnervation after sciatic nerve resection repaired with autologous or allogenic grafts in the mouse. For each type of repair, one group received FK506 (5 mg/kg) for 4 months, whereas a second group was treated with FK506 at 5 mg/kg for 5 weeks followed by 3 mg/kg for 4 weeks; a control group received saline only. Functional reinnervation was assessed by noninvasive methods to determine recovery of motor, sensory, and autonomic functions in the hind paw over 4 months after operation. Morphological analysis of the regenerated nerves was performed at the termination of the study. Autografts and allografts treated with sustained FK506 (5 mg/kg) reached high levels of reinnervation and followed a course of recovery faster than controls. The numbers of myelinated fibers also were similar. Allografts without immunosuppression demonstrated a slower rate of regeneration, exhibiting lower final levels of recovery compared with other groups and containing fewer numbers of regenerating myelinated fibers. Withdrawal of immunosuppressant therapy resulted in a decline in the degree of reinnervation in all functions tested during the third month, with stabilization between the third and fourth months. The number of regenerated myelinated fibers in the group was significantly lower than in autografts. Thus, continuous or discontinuous FK506 administration slightly accelerated the rate of reinnervation in autografts. In allograft repair, FK506 significantly enhanced both the rate and degree of regeneration and recovery, but its withdrawal resulted in graft rejection, a marked deterioration in function, and loss of regenerating fibers.

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.

Axonal regeneration after cold preservation of nerve allografts and immunosuppression with tacrolimus in mice

OBJECT

The purpose of this study was to combine the immunosuppressive and neuroregenerative effects of tacrolimus (FK506) with cold preservation of peripheral nerve allografts to maximize axonal regeneration across short peripheral nerve gaps.

METHODS

Ninety-six male C3H mice were randomized to six groups, which were composed of animals with isografts (Group 1, positive control), allografts (Group 2, negative control), allografts treated with subtherapeutic doses of FK506 without and with cold preservation (Groups 3 and 4), and allografts treated with therapeutic doses of FK506 without and with cold preservation (Groups 5 and 6). Results were determined using walking-track data and histomorphometric measurements. Three weeks postoperatively, animals treated with therapeutic doses of FK506 after receiving cold-preserved allografts demonstrated accelerated functional recovery relative to all other groups. In addition, histomorphometric parameters in these animals (1,257 +/- 847 total axons, 6.7 +/- 3.3% nerve tissue, 11.8 +/- 6.5% neural debris, 8,844 +/- 4,325 fibers/mm2 nerve density, and 2.53 +/- 0.25 microm fiber width) were the same as or better than in all other groups. The parameters of percent nerve tissue (p < 0.016), nerve density (p < 0.038), and percent neural debris (p < 0.01) were statistically significantly better than those in all other groups, including Group 1 (isograft, positive control).

CONCLUSIONS

The combination of FK506 treatment with cold preservation of nerve allografts resulted in functional and histomorphometric recovery superior to that with either modality alone.

Clinical outcome following nerve allograft transplantation

The clinical outcome of seven patients who underwent reconstruction of long upper- and lower-extremity peripheral nerve gaps with interposition peripheral nerve allografts is reported. Patients were selected for transplantation when the nerve gaps exceeded the length that could be reconstructed with available autograft tissue. Before transplantation, cadaveric allografts were harvested and preserved for 7 days in University of Wisconsin Cold Storage Solution at 5 degrees C. In the interim, patients were started on an immunosuppressive regimen consisting of either cyclosporin A or tacrolimus (FK506), azathioprine, and prednisone. Immunosuppression was discontinued 6 months after regeneration across the allograft(s) was evident. Six patients demonstrated return of motor function and sensation in the affected limb, and one patient experienced rejection of the allograft secondary to subtherapeutic immunosuppression. In addition to providing the ability to restore nerve continuity in severe extremity injuries, successful nerve allografting protocols have direct applicability to composite tissue transplantation.

Peripheral nerve and neuromuscular allotransplantation: current status

Some major peripheral nerve deficits in otherwise salvageable extremities are unreconstructable with autogenous nerve grafts because of their limited availability. Peripheral nerve allotransplantation has been attempted experimentally and clinically for more than a century. It is only since the advent of the immunophyllin-immunosuppressive agents (Cyclosporin A and FK506) that nerve regeneration across allografts has been comparable to that observed across autografts. Experimental studies have demonstrated excellent regeneration through nerve allografts with recipient CsA and FK506 immunosuppression given continuously and for limited periods. This permitted the development of a clinical trial. To date, seven patients received major nerve reconstructions with nerve allografts under temporary immunosuppression; six have had sensory recovery; three motor recovery; and one no recovery due to rejection. The experimental and clinical work will be reviewed and relevance to composite tissue allotransplantation discussed.

FK506 rescues peripheral nerve allografts in acute rejection

This study investigated the ability of the immunosuppressant FK506 to reverse nerve allograft rejection in progress. Eighty-four Buffalo rats received posterior tibial nerve grafts from either Lewis or Buffalo donor animals. Allografts were left untreated for either 7, 10, or 14 days before receiving daily subcutaneous FK506 injections (2 mg/kg). Time-matched control animals received either an isograft, an allograft with continuous FK506, or an allograft with no postoperative FK506 therapy. All animals underwent weekly evaluation of nerve function by walking track analysis. Experimental group animals were sacrificed either immediately prior to initiation of FK506 therapy (days 7, 10, or 14), after 2 weeks of immunosuppressive treatment, or 8 weeks postsurgery. Histomorphometric analysis, consisting of measurements of total number of nerve fibers, neural density, and percent of neural debris, demonstrated a statistically significant increase in regeneration in the isograft group relative to the untreated allograft group within 28 days of transplantation. Grafts harvested from animals receiving 2 weeks of FK506 after 7 or 10 days of rejection were histomorphometrically similar to time-matched isografts. By contrast, grafts from animals receiving 2 weeks of FK506 following 14 days without therapy resembled untreated allografts and demonstrated significant histomorphometric differences from isografts at the corresponding time point. Analysis of walking track data confirmed that relative to untreated allografts, functional recovery was hastened in animals receiving an isograft, or allograft treated with FK506. This study demonstrated that when started within 10 days of graft placement, FK506 could reverse nerve allograft rejection in rats evaluated following 2 weeks of treatment.

Long acellular nerve transplants for allogeneic grafting and the effects of basic fibroblast growth factor on the growth of regenerating axons in dogs: a preliminary report

Sciatic nerves were excised from 3 beagle dogs about 5 h after their sacrifice, treated three times by freezing and thawing, and stored in physiological saline for 3 months at -20 degrees C until used. Nerve segments 5 cm in length prepared from these stored nerves were transplanted to the common peroneal nerve in the right hindlimb of beagle dogs. Sixteen beagle dogs in total were used, in four treatment groups of two pairs each studied at 1 and 3 months. Five-hundred microliters basic fibroblast growth factor (bFGF) of two different concentrations (10 micrograms/300 microliters and 100 micrograms/300 microliters) which were impregnated in 0.5 ml gelatin hydrogels was applied around the sutured allografts. Autografting was also done in 4 beagle dogs, with no bFGF application. One month after the grafting, no regenerating nerves extended beyond the middle of the transplant in any of the allografts, except in the autografts in which a number of regenerated (myelinated) axons were present. Three months after the grafting, an abundance of myelinated axons was found at the middle of the graft: the numbers of axons per 10(4) micron 2 were 22.6 in the autografts and 10.6, 10.4 and 19.2 in the allografts treated with no bFGF, low-dose bFGF, and high-dose bFGF, respectively. Regenerating axons extended into the host nerve: the numbers of myelinated axons at the level 1.5 cm distal to the distal suture were 35.7, 0.9, 3.8, and 12.1 per 10(4) micron 2 in the above respective order. Although it was inferior in quality to the autograft, peripheral nerve regeneration was extensive in the distal nerve using freeze-thawed and bFGF-treated allografts at 3 months. Electromyography showed that the peroneus longus muscle responded to the electrical stimuli given at the site proximal to the transplant in all four groups. These data indicate that a 5-cm acellular nerve segment containing Schwann cell basal laminae can be used successfully as an allograft without any immunosuppressants and that exogenously applied bFGF can improve nerve regeneration by enhancing the growth of regenerating axons.