Effects of Schwann cells and donor antigen on long-nerve allograft regeneration

Challenges in Nerve Repair and Reconstruction

Peripheral nerve injuries may substantially impair a patient's function and quality of life. Despite appropriate treatment, outcomes often remain poor. Direct repair remains the standard of care when repair is possible without excessive tension. For larger nerve defects, nerve autografting is the gold standard. However, a considerable challenge is donor site morbidity. Processed nerve allografts and conduits are other options, but evidence supporting their use is limited to smaller nerves and shorter gaps. Nerve transfer is another technique that has seen increasing popularity. The future of care may include novel biologics and pharmacologic therapy to enhance regeneration.

Reconstruction of Critical Nerve Defects Using Allogenic Nerve Tissue: A Review of Current Approaches

Regardless of the nerve defect length, nerve injury is a debilitating condition for the affected patient that results in loss of sensory and motor function. These functional impairments can have a profound impact on the patient’s quality of life. Surgical approaches for the treatment of short segment nerve defects are well-established. Autologous nerve transplantation, considered the gold standard, and the use of artificial nerve grafts are safe and successful procedures for short segment nerve defect reconstruction. Long segment nerve defects which extend 3.0 cm or more are more problematic for repair. Methods for reconstruction of long defects are limited. Artificial nerve grafts often fail to regenerate and autologous nerve grafts are limited in length and number. Cadaveric processed/unprocessed nerve allografts are a promising alternative in nerve surgery. This review gives a systematic overview on pre-clinical and clinical approaches in nerve allograft transplantation.

Mesenchymal Stem Cells Derived from Wharton's Jelly Can Differentiate into Schwann Cell-Like Cells and Promote Peripheral Nerve Regeneration in Acellular Nerve Grafts

BACKGROUND

Schwann cells (SCs) secrete neurotrophic factors and provide structural support and guidance during axonal regeneration. However, nearby nerves may be damaged to obtain primary SCs, and there is a lack of nervous tissue donors. We investigated the potential of Wharton's Jelly-derived mesenchymal stem cells (WJ-MSCs) in differentiating into Schwann cell-like cells (WJ-SCLCs) as an alternative to SCs. We also examined whether implantation of WJ-SCLCs-laden acellular nerve grafts (ANGs) are effective in inducing functional recovery and nerve regeneration in an animal model of peripheral nerve injury.

METHODS

The differentiation of WJ-MSCs into WJ-SCLCs was determined by analyzing SC-specific markers. The secretion of neurotrophic factors was assessed by the Neuro Discovery antibody array. Neurite outgrowth and myelination of axons were found in a co-culture system involving motor neuron cell lines. The effects of ANGs on repairing sciatic nerves were evaluated using video gait angle test, isometric tetanic force analysis, and toluidine blue staining.

RESULTS

Compared with undifferentiated WJ-MSCs, WJ-SCLCs showed higher expression levels of SC-specific markers such as S100β, GFAP, KROX20, and NGFR. WJ-SCLCs also showed higher secreted amounts of brain-derived neurotrophic factor, glial cell-derived neurotrophic factor, and granulocyte-colony stimulating factor than did WJ-MSCs. WJ-SCLCs effectively promoted the outgrowth and myelination of neurites in motor neuron cells, and WJ-SCLCs laden ANGs significantly facilitated peripheral nerve regeneration in an animal model of sciatic nerve injury.

CONCLUSION

WJ-MSCs were readily differentiated into WJ-SCLCs, which effectively promoted the regeneration of peripheral nerves. Transplantation of WJ-SCLCs with ANGs might be useful for assisting peripheral nerve regeneration.

Introducing human adipose-derived mesenchymal stem cells to AvanceⓇ nerve grafts and NeuraGenⓇ nerve guides

BACKGROUND

When direct nerve coaptation is impossible after peripheral nerve injury, autografts, processed allografts, or conduits are used to bridge the nerve gap. The purpose of this study was to examine if human adipose-derived Mesenchymal Stromal/Stem Cells (MSCs) could be introduced to commercially available nerve graft substitutes and to determine cell distribution and the seeding efficiency of a dynamic seeding strategy.

METHODS

MTS assays examined the viability of human MSCs after introduction to the Avance^Ⓡ Nerve Graft and the NeuraGen^Ⓡ Nerve Guide. MSCs were dynamically seeded on nerve substitutes for either 6, 12, or 24 h. Cell counts, live/dead stains, Hoechst stains, and Scanning Electron Microscopy (SEM) revealed the seeding efficiency and the distribution of MSCs after seeding.

RESULTS

The viability of MSCs was not affected by nerve substitutes. Dynamic seeding led to uniformly distributed MSCs over the surface of both nerve substitutes and revealed MSCs on the inner surface of the NeuraGen^Ⓡ Nerve Guides. The maximal seeding efficiency of NeuraGen^Ⓡ Nerve Guides (94%), obtained after 12 h was significantly higher than that of Avance^Ⓡ Nerve Grafts (66%) (p = 0.010).

CONCLUSION

Human MSCs can be dynamically seeded on Avance^Ⓡ Nerve Grafts and NeuraGen^Ⓡ Nerve Guides. The optimal seeding duration was 12 h. MSCs were distributed in a uniform fashion on exposed surfaces. This study demonstrates that human MSCs can be effectively and efficiently seeded onto commercially available nerve autograft substitutes in a timely fashion and sets the stage for the clinical application of MSC-seeded nerve graft substitutes clinically.

A comparison of the use of adipose-derived and bone marrow-derived stem cells for peripheral nerve regeneration in vitro and in vivo

Background

To date, it has repeatedly been demonstrated that infusing bone marrow-derived stem cells (BMSCs) into acellular nerve scaffolds can promote and support axon regeneration through a peripheral nerve defect. However, harvesting BMSCs is an invasive and painful process fraught with a low cellular yield.

Methods

In pursuit of alternative stem cell sources, we isolated stem cells from the inguinal subcutaneous adipose tissue of adult Sprague–Dawley rats (adipose-derived stem cells, ADSCs). We used a co-culture system that allows isolated adult mesenchymal stem cells (MSCs) and Schwann cells (SCs) to grow in the same culture medium but without direct cellular contact. We verified SC phenotype in vitro by cell marker analysis and used red fluorescent protein-tagged ADSCs to detect their fate after being injected into a chemically extracted acellular nerve allograft (CEANA). To compare the regenerative effects of CEANA containing either BMSCs or ADSCs with an autograft and CEANA only on the sciatic nerve defect in vivo, we performed histological and functional assessments up to 16 weeks after grafting.

Results

In vitro, we observed reciprocal beneficial effects of ADSCs and SCs in the ADSC–SC co-culture system. Moreover, ADSCs were able to survive in CEANA for 5 days after in vitro implantation. Sixteen weeks after grafting, all results consistently showed that CEANA infused with BMSCs or ADSCs enhanced injured sciatic nerve repair compared to the acellular CEANA-only treatment. Furthermore, their beneficial effects on sciatic injury regeneration were comparable as histological and functional parameters evaluated showed no statistically significant differences. However, the autograft group was roundly superior to both the BMSC- or ADSC-loaded CEANA groups.

Conclusion

The results of the present study show that ADSCs are a viable alternative stem cell source for treating sciatic nerve injury in lieu of BMSCs.

Advances in the repair of segmental nerve injuries and trends in reconstruction

Despite advances in surgery, the reconstruction of segmental nerve injuries continues to pose challenges. In this review, current neurobiology regarding regeneration across a nerve defect is discussed in detail. Recent findings include the complex roles of nonneuronal cells in nerve defect regeneration, such as the role of the innate immune system in angiogenesis and how Schwann cells migrate within the defect. Clinically, the repair of nerve defects is still best served by using nerve autografts with the exception of small, noncritical sensory nerve defects, which can be repaired using autograft alternatives, such as processed or acellular nerve allografts. Given current clinical limits for when alternatives can be used, advanced solutions to repair nerve defects demonstrated in animals are highlighted. These highlights include alternatives designed with novel topology and materials, delivery of drugs specifically known to accelerate axon growth, and greater attention to the role of the immune system.

Nerve grafting for peripheral nerve injuries with extended defect sizes

Artificial and non-artificial nerve grafts are the gold standard in peripheral nerve reconstruction in cases with extensive loss of nerve tissue, particularly where a direct end-to-end suture or an autologous nerve graft is inauspicious. Different materials are marketed and approved by the US Food and Drug Administration (FDA) for peripheral nerve graft reconstruction. The most frequently used materials are collagen and poly(DL-lactide-ε-caprolactone). Only one human nerve allograft is listed for peripheral nerve reconstruction by the FDA. All marketed nerve grafts are able to demonstrate sufficient nerve regeneration over small distances not exceeding 3.0 cm. A key question in the field is whether nerve reconstruction on large defect lengths extending 4.0 cm or more is possible. This review gives a summary of current clinical and experimental approaches in peripheral nerve surgery using artificial and non-artificial nerve grafts in short and long distance nerve defects. Strategies to extend nerve graft lengths for long nerve defects, such as enhancing axonal regeneration, include the additional application of Schwann cells, mesenchymal stem cells or supporting co-factors like growth factors on defect sizes between 4.0 and 8.0 cm.

Differentiation of adipose-derived stem cells into Schwann cell-like cells through intermittent induction: potential advantage of cellular transient memory function

Background

Peripheral nerve injury (PNI) is a worldwide issue associated with severe social and economic burden. Autologous nerve grafting, the gold standard treatment for peripheral nerve defects, still has a number of technical limitations. Tissue engineering technology is a novel therapeutic strategy, and mesenchymal stromal cells (MSCs) are promising seed cells for nerve tissue engineering. However, the efficiency of traditional methods for inducing the differentiation of MSCs to Schwann cell-like cells (SCLCs) remains unsatisfactory.

Methods

Here, we propose an intermittent induction method with alternate use of complete and incomplete induction medium to induce differentiation of adipose-derived stem cells (ASCs) to SCLCs. The time dependence of traditional induction methods and the efficiency of the intermittent induction method and traditional induction methods were evaluated and compared using immunocytochemistry, quantitative reverse transcription polymerase chain reaction (qRT-PCR), enzyme-linked immunosorbent assay (ELISA), and co-culture with the dorsal root ganglion (DRG) in vitro. Cell transplantation was used to compare the effects of the traditional induction method and the intermittent induction method in repairing sciatic nerve defects in vivo.

Results

The results of the present study indicated that the intermittent induction method is more efficient than traditional methods for inducing ASCs to differentiate into SCLCs. In addition, SCLCs induced by this method were closer to mature myelinating Schwann cells and were capable of secreting neurotrophins and promoting DRG axon regeneration in vitro. Furthermore, SCLCs induced by the intermittent induction method could repair sciatic nerve defects in rats by cell transplantation in vivo more effectively than those produced by traditional methods.

Conclusion

Intermittent induction represents a novel strategy for obtaining seed cells for use in nerve tissue engineering.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0884-3) contains supplementary material, which is available to authorized users.

Beneficial reciprocal effects of bone marrow stromal cells and Schwann cells from adult rats in a dynamic co‑culture system in vitro without intercellular contact

In order to examine how implanted bone marrow stromal cells (BMSCs) encourage peripheral nerve regeneration, the present study investigated the interaction of BMSCs and Schwann cells (SCs) using an indirect in vitro co‑culture model. SCs and BMSCs were obtained from adult Sprague‑Dawley rats. The passaged BMSCs were CD29‑ and CD44‑positive but CD45‑negative and were co‑cultured with the primary SCs using a Millicell system, which allows BMSCs and SCs to grow in the same culture medium but without direct contact. Expression of the typical SC markers S‑100 and glial fibrillary acidic protein (GFAP) of the treated BMSCs as well as the proliferation capacity of the co‑cultured SCs was evaluated by immunocytochemical staining on the 3rd and 5th day of co‑culture. Immunocytochemical staining showed that >75% of the BMSCs in the indirect co‑culture model were GFAP‑ and S‑100‑positive on the 3rd and 5th day after co‑culture, as opposed to <5% of the BMSCs in the control group. On the 3rd day after co‑culture, only a few co‑cultured BMSCs showed the typical SC‑like morphology, while most BMSCs still kept their native appearance. By contrast, on the 5th day after co‑culture, almost all of the co‑cultured BMSCs appeared with the typical SC‑like morphology. Furthermore, 70.71% of the SCs in the indirect co‑culture model were S‑100‑positive on the 5th day of co‑culture, as opposed to >30.43% of the SCs in the control group. These results indicated that BMSCs may interact synergistically with SCs with regard to promoting peripheral nerve regeneration.

Human periodontal ligament stem cells repair mental nerve injury

Human periodontal ligament stem cells are easily accessible and can differentiate into Schwann cells. We hypothesized that human periodontal ligament stem cells can be used as an alternative source for the autologous Schwann cells in promoting the regeneration of injured peripheral nerve. To validate this hypothesis, human periodontal ligament stem cells (1 × 10⁶) were injected into the crush-injured left mental nerve in rats. Simultaneously, autologous Schwann cells (1 × 10⁶) and PBS were also injected as controls. Real-time reverse transcriptase polymerase chain reaction showed that at 5 days after injection, mRNA expression of low affinity nerve growth factor receptor was significantaly increased in the left trigeminal ganglion of rats with mental nerve injury. Sensory tests, histomorphometric evaluation and retrograde labeling demonstrated that at 2 and 4 weeks after injection, sensory function was significantly improved, the numbers of retrograde labeled sensory neurons and myelinated axons were significantly increased, and human periodontal ligament stem cells and autologous Schwann cells exhibited similar therapeutic effects. These findings suggest that transplantation of human periodontal ligament stem cells show a potential value in repair of mental nerve injury.

Glial cell line-derived neurotrophic factor promotes increased phenotypic marker expression in femoral sensory and motor-derived Schwann cell cultures

Schwann cells (SCs) secrete growth factors and extracellular matrix molecules that promote neuronal survival and help guide axons during regeneration. Transplantation of SCs is a promising strategy for enhancing peripheral nerve regeneration. However, we and others have shown that after long-term in vitro expansion, SCs revert to a de-differentiated state similar to the phenotype observed after injury. In vivo, glial cell-line derived neurotrophic factor (GDNF) may guide the differentiation of SCs to remyelinate regenerating axons. Therefore, we hypothesized that exogenous GDNF may guide the differentiation of SCs into their native phenotypes in vitro through stimulation of GDNF family receptor (GFR)α-1. When activated in SCs, GFRα-1 promotes phosphorylation of Fyn, a Src family tyrosine kinase responsible for mediating downstream signaling for differentiation and proliferation. In this study, SCs harvested from the sensory and motor branches of rat femoral nerve were expanded in vitro and then cultured with 50 or 100ng/mL of GDNF. The exogenous GDNF promoted differentiation of sensory and motor-derived SCs back to their native phenotypes, as demonstrated by decreased proliferation after 7days and increased expression of S100Ββ and phenotype-specific markers. Furthermore, inhibiting Fyn with Src family kinase inhibitors, PP2 and SU6656, and siRNA-mediated knockdown of Fyn reduced GDNF-stimulated differentiation of sensory and motor-derived SCs. These results demonstrate that activating Fyn is necessary for GDNF-stimulated differentiation of femoral nerve-derived SCs into their native phenotypes in vitro. Therefore GDNF could be incorporated into SC-based therapies to promote differentiation of SCs into their native phenotype to improve functional nerve regeneration.

Peripheral nerve repair and reconstruction

When possible, direct repair remains the current standard of care for the repair of peripheral nerve lacerations. In large nerve gaps, in which direct repair is not possible, grafting remains the most viable option. Nerve scaffolds include autologous conduits, artificial nonbioabsorbable conduits, and bioabsorbable conduits and are options for repair of digital nerve gaps that are <3 cm in length. Experimental studies suggest that the use of allografts may be an option for repairing larger sensory nerve gaps without associated donor-site morbidity.

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.

Tissue engineered constructs for peripheral nerve surgery

BACKGROUND

Tissue engineering has been defined as "an interdisciplinary field that applies the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function or a whole organ". Traumatic peripheral nerve injury resulting in significant tissue loss at the zone of injury necessitates the need for a bridge or scaffold for regenerating axons from the proximal stump to reach the distal stump.

METHODS

A review of the literature was used to provide information on the components necessary for the development of a tissue engineered peripheral nerve substitute. Then, a comprehensive review of the literature is presented composed of the studies devoted to this goal.

RESULTS

Extensive research has been directed toward the development of a tissue engineered peripheral nerve substitute to act as a bridge for regenerating axons from the proximal nerve stump seeking the distal nerve. Ideally this nerve substitute would consist of a scaffold component that mimics the extracellular matrix of the peripheral nerve and a cellular component that serves to stimulate and support regenerating peripheral nerve axons.

CONCLUSIONS

The field of tissue engineering should consider its challenge to not only meet the autograft "gold standard" but also to understand what drives and inhibits nerve regeneration in order to surpass the results of an autograft.

Limited regeneration in long acellular nerve allografts is associated with increased Schwann cell senescence

Repair of large nerve defects with acellular nerve allografts (ANAs) is an appealing alternative to autografting and allotransplantation. ANAs have been shown to be similar to autografts in supporting axonal regeneration across short gaps, but fail in larger defects due to a poorly-understood mechanism. ANAs depend on proliferating Schwann cells (SCs) from host tissue to support axonal regeneration. Populating longer ANAs places a greater proliferative demand on host SCs that may stress host SCs, resulting in senescence. In this study, we investigated axonal regeneration across increasing isograft and ANA lengths. We also evaluated the presence of senescent SCs within both graft types. A sciatic nerve graft model in rats was used to evaluate regeneration across increasing isograft (~autograft) and ANA lengths (20, 40, and 60 mm). Axonal regeneration and functional recovery decreased with increased graft length and the performance of the isograft was superior to ANAs at all lengths. Transgenic Thy1-GFP rats and qRT-PCR demonstrated that failure of the regenerating axonal front in ANAs was associated with increased levels of senescence related markers in the graft (senescence associated β-galactosidase, p16(INK4A), and IL6). Lastly, electron microscopy (EM) was used to qualitatively assess senescence-associated changes in chromatin of SCs in each graft type. EM demonstrated an increase in the presence of SCs with abnormal chromatin in isografts and ANAs of increasing graft length. These results are the first to suggest that SC senescence plays a role in limited axonal regeneration across nerve grafts of increasing gap lengths.

Nerve allografts supplemented with schwann cells overexpressing glial-cell-line-derived neurotrophic factor

INTRODUCTION

We sought to determine whether supplementation of acellular nerve allografts (ANAs) with Schwann cells overexpressing GDNF (G-SCs) would enhance functional recovery after peripheral nerve injury.

METHODS

SCs expanded in vitro were infected with a lentiviral vector to induce GDNF overexpression. Wild-type SCs (WT-SCs) and G-SCs were seeded into ANAs used to repair a 14-mm nerve gap defect. Animals were harvested after 6 and 12 weeks for histomorphometric and muscle force analysis.

RESULTS

At 6 weeks, histomorphometry revealed that ANAs supplemented with G-SCs promoted similar regeneration compared with isograft at midgraft. However, G-SCs failed to promote regeneration into the distal stump. At 12 weeks, ANAs with G-SCs had lower maximum and specific force production compared with controls.

CONCLUSIONS

The combined results suggest that consistent overexpression of GDNF by G-SCs trapped axons in the graft and prevented functional regeneration.

Differential gene expression in motor and sensory Schwann cells in the rat femoral nerve

Phenotypic differences in Schwann cells (SCs) may help to guide axonal regeneration down motor or sensory specific pathways following peripheral nerve injury. The goal of this study was to identify phenotypic markers for SCs harvested from the cutaneous (sensory) and quadriceps (motor) branches of the rat femoral nerve and to study the effects of expansion culture on the expression patterns of these motor or sensory phenotypic markers. RNA was extracted from SCs harvested from the motor and sensory branches of the rat femoral nerve and analyzed using Affymetrix Gene Chips (Rat Genome 230 v2.0 Array A). Genes that were upregulated in motor SCs compared with the sensory SCs or vice versa were identified, and the results were verified for a subset of genes using quantitative real-time polymerase chain reaction (qRT-PCR). The expression levels of the "phenotype-specific" genes were then evaluated in SC expansion cultures at various time points over 30 days by qRT-PCR to determine the effect of expansion on SC phenotype. Expression levels of the phenotype-specific genes were significantly altered after expansion culture for both the motor and the sensory markers compared with fresh nerve tissue. These results indicate that both motor and sensory SC gene expression patterns are disrupted during expansion in vitro and may affect the ability of SCs to express phenotype-specific genes after transplantation.

Silk fibroin conduits: a cellular and functional assessment of peripheral nerve repair

Silk fibroin conduits were designed with appropriate porosity for peripheral nerve repair. The aim of this work was to use these conduits to examine cell inflammatory responses and functional recovery in a sciatic nerve defect model. A total of 45 randomized Lewis rats were used to create an 8-mm defect bridged by a silk guide, commercial collagen guide, or an autograft. After 1, 4, and 8 weeks, macrophage recruitment, percentage of newly formed collagen, number of myelinated axons, and gastrocnemius muscle mass were evaluated. Following 8 weeks, ED1+ cells in autograft and silk conduits decreased to <1% and 17% of week 1 values, respectively. Collagen formation revealed no difference for all measured time points, suggesting a similar foreign body response. Myelinated axon counts within the silk guide revealed a greater number of proximal spouts and distal connections than collagen guides. Gastrocnemius weights demonstrated a 27% decrease between silk and autografts after 8 weeks. This study demonstrates that, in addition to tailorable degradation rates, our silk conduits possess a favorable immunogenicity and remyelination capacity for nerve repair.

A systematic evaluation of Schwann cell injection into acellular cold-preserved nerve grafts

Peripheral nerve regeneration after injury depends on environmental cues and trophic support. Schwann cells (SCs) secrete trophic factors that promote neuronal survival and help guide axons during regeneration. The addition of SCs to acellular nerve grafts is a promising strategy for enhancing peripheral nerve regeneration; however, inconsistencies in seeding parameters have led to varying results. The current work sought to establish a systematic approach to seeding SCs in cold-preserved acellular nerve grafts. Studies were undertaken to (1) determine the needle gauge for optimal cell survival and minimal epineurial disruption during injection, (2) track the seeded SCs using a commercially available dye, and (3) evaluate the seeding efficiency of SCs in nerve grafts. It was determined that seeding with a 27-gauge needle resulted in the highest viability of SCs with the least damage to the epineurium. In addition, Qtracker(®) dye, a commercially available quantum dot nanocrystal, was used to label SCs prior to transplantation, which allowed visualization of the seeded SCs in nerve grafts. Finally, stereological methods were used to evaluate the seeding efficiency of SCs in nerve grafts immediately after injection and following a 1- or 3-day in vitro incubation in SC growth media. Using a systematic approach, the best needle gauge and a suitable dye for SC visualization in acellular nerve grafts were identified. Seeding efficiency in these grafts was also determined. The findings will lead to improvements ability to assess injection of cells (including SCs) for use with acellular nerve grafts to promote nerve regeneration.

Matching of motor-sensory modality in the rodent femoral nerve model shows no enhanced effect on peripheral nerve regeneration

The treatment of peripheral nerve injuries with nerve gaps largely consists of autologous nerve grafting utilizing sensory nerve donors. Underlying this clinical practice is the assumption that sensory autografts provide a suitable substrate for motoneuron regeneration, thereby facilitating motor endplate reinnervation and functional recovery. This study examined the role of nerve graft modality on axonal regeneration, comparing motor nerve regeneration through motor, sensory, and mixed nerve isografts in the Lewis rat. A total of 100 rats underwent grafting of the motor or sensory branch of the femoral nerve with histomorphometric analysis performed after 5, 6, or 7 weeks. Analysis demonstrated similar nerve regeneration in motor, sensory, and mixed nerve grafts at all three time points. These data indicate that matching of motor-sensory modality in the rat femoral nerve does not confer improved axonal regeneration through nerve isografts.

Treatment modality affects allograft-derived Schwann cell phenotype and myelinating capacity

We used peripheral nerve allografts, already employed clinically to reconstruct devastating peripheral nerve injuries, to study Schwann cell (SC) plasticity in adult mice. By modulating the allograft treatment modality we were able to study migratory, denervated, rejecting, and reinnervated phenotypes in transgenic mice whose SCs expressed GFP under regulatory elements of either the S100b (S100-GFP) or nestin (Nestin-GFP) promoters. Well-differentiated SCs strongly expressed S100-GFP, while Nestin-GFP expression was stimulated by denervation, and in some cases, axons were constitutively labeled with CFP to enable in vivo imaging. Serial imaging of these mice demonstrated that untreated allografts were rejected within 20 days. Cold preserved (CP) allografts required an initial phase of SC migration that preceded axonal regeneration thus delaying myelination and maturation of the SC phenotype. Mice immunosuppressed with FK506 demonstrated mild subacute rejection, but the most robust regeneration of myelinated and unmyelinated axons and motor endplate reinnervation. While characterized by fewer regenerating axons, mice treated with the co-stimulatory blockade (CSB) agents anti-CD40L mAb and CTLAIg-4 demonstrated virtually no graft rejection during the 28 day experiment, and had significant increases in myelination, connexin-32 expression, and Akt phosphorylation compared with any other group. These results indicate that even with SC rejection, nerve regeneration can occur to some degree, particularly with FK506 treatment. However, we found that co-stimulatory blockade facilitate optimal myelin formation and maturation of SCs as indicated by protein expression of myelin basic protein (MBP), connexin-32 and phospho-Akt.

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.

The impact of motor and sensory nerve architecture on nerve regeneration

Sensory nerve autografting is the standard of care for injuries resulting in a nerve gap. Recent work demonstrates superior regeneration with motor nerve grafts. Improved regeneration with motor grafting may be a result of the nerve's Schwann cell basal lamina tube size. Motor nerves have larger SC basal lamina tubes, which may allow more nerve fibers to cross a nerve graft repair. Architecture may partially explain the suboptimal clinical results seen with sensory nerve grafting techniques. To define the role of nerve architecture, we evaluated regeneration through acellular motor and sensory nerve grafts. Thirty-six Lewis rats underwent tibial nerve repairs with 5 mm double-cable motor or triple-cable sensory nerve isografts. Grafts were harvested and acellularized in University of Wisconsin solution. Control animals received fresh motor or sensory cable isografts. Nerves were harvested after 4 weeks and histomorphometry was performed. In 6 animals per group from the fresh motor and sensory cable graft groups, weekly walking tracks and wet muscle mass ratios were performed at 7 weeks. Histomorphometry revealed more robust nerve regeneration in both acellular and cellular motor grafts. Sensory groups showed poor regeneration with significantly decreased percent nerve, fiber count, and density (p<0.05). Walking tracks revealed a trend toward improved functional recovery in the motor group. Gastrocnemius wet muscle mass ratios show a significantly greater muscle mass recovery in the motor group (p<0.05). Nerve architecture (size of SC basal lamina tubes) plays an important role in nerve regeneration in a mixed nerve gap model.

Experience with nerve allograft transplantation

Nerve allograft transplantation should be used for the repair of devastating peripheral nerve injuries that cannot be reconstructed through traditional means such as autologous nerve grafting or nerve transfer procedures. The risks of required systemic immunosuppression, although only temporary for nerve allograft recipients, preclude widespread use of this treatment modality. Translational research has led to several advancements in this field including the use of preoperative allograft cold preservation in University of Wisconsin organ preservation solution and inclusion of tacrolimus as part of the immunosuppressive regimen. Investigation of how to further diminish nerve allograft immunogenicity, speed neuroregeneration by use of agents such as tacrolimus, and promote preferential motor regeneration will further advance this field with the goal of restoring optimal function while minimizing patient morbidity.

Binary imaging analysis for comprehensive quantitative histomorphometry of peripheral nerve

Quantitative histomorphometry is the current gold standard for objective measurement of nerve architecture and its components. Many methods still in use rely heavily upon manual techniques that are prohibitively time consuming, predisposing to operator fatigue, sampling error, and overall limited reproducibility. More recently, investigators have attempted to combine the speed of automated morphometry with the accuracy of manual and semi-automated methods. Systematic refinements in binary imaging analysis techniques combined with an algorithmic approach allow for more exhaustive characterization of nerve parameters in the surgically relevant injury paradigms of regeneration following crush, transection, and nerve gap injuries. The binary imaging method introduced here uses multiple bitplanes to achieve reproducible, high throughput quantitative assessment of peripheral nerve. Number of myelinated axons, myelinated fiber diameter, myelin thickness, fiber distributions, myelinated fiber density, and neural debris can be quantitatively evaluated with stratification of raw data by nerve component. Results of this semi-automated method are validated by comparing values against those obtained with manual techniques. The use of this approach results in more rapid, accurate, and complete assessment of myelinated axons than manual techniques.

Use of cold-preserved allografts seeded with autologous Schwann cells in the treatment of a long-gap peripheral nerve injury

BACKGROUND

Limitations in autogenous tissue have inspired the study of alternative materials for repair of complex peripheral nerve injuries. Cadaveric allografts are one potential reconstructive material, but their use requires systemic immunosuppression. Cold preservation (> or =7 weeks) renders allografts devoid of antigens, but these acellular substrates generally fail in supporting regeneration beyond 3 cm. In this study, the authors evaluated the reconstruction of extensive nonhuman primate peripheral nerve defects using 7-week cold-preserved allografts repopulated with cultured autologous Schwann cells.

METHODS

Ten outbred Macaca fascicularis primates were paired based on maximal genetic disparity as measured by similarity index assay. A total of 14 ulnar nerve defects measuring 6 cm were successfully reconstructed using autografts (n = 5), fresh allografts (n = 2), cold-preserved allografts (n = 3), or cold-preserved allografts seeded with autogenous Schwann cells (n = 4). Recipient immunoreactivity was evaluated by means of enzyme-linked immunosorbent spot assay, and nerves were harvested at 6 months for histologic and histomorphometric analysis.

RESULTS

Cytokine production in response to cold-preserved allografts and cold-preserved allografts seeded with autologous Schwann cells was similar to that observed for autografts. Schwann cell-repopulated cold-preserved grafts demonstrated significantly enhanced fiber counts, nerve density, and percentage nerve (p < 0.05) compared with unseeded cold-preserved grafts at 6 months after reconstruction.

CONCLUSIONS

Cold-preserved allografts seeded with autologous Schwann cells were well-tolerated in unrelated recipients and supported significant regeneration across 6-cm peripheral nerve defects. Use of cold-preserved allogeneic nerve tissue supplemented with autogenous Schwann cells poses a potentially safe and effective alternative to the use of autologous tissue in the reconstruction of extensive nerve injuries.

Schwann-cell injection of cold-preserved nerve allografts

This study investigated the effects of prolonged cold preservation and Schwann-cell injection on nerve regeneration through peripheral nerve allografts. Forty rats were randomized to the following groups: group I, isograft; group II, allograft; group III, isograft + Schwann cells; group IV, 6-week cold-preserved allograft; and group V, 6-week cold-preserved allograft with Schwann cells. Nerves from all animals were harvested at 4 weeks after surgery for histological and histomorphometric analysis. Untreated allograft recipients demonstrated poor nerve regeneration and histological evidence of rejection. The remaining four groups showed robust regeneration without evidence of rejection. In a short nerve allograft model, prolonged cold preservation of allografts supported robust nerve regeneration, but the addition of cultured Schwann cells conferred no additional benefit for nerve regeneration. Further work in large animals is needed to establish the role for exogenous Schwann cells in nerve allotransplantation.

From experimental rat hindlimb to clinical face composite tissue allotransplantation: Historical background and current status

The purpose of this article is to review the historical background and clinical status of composite tissue allotransplantation and to discuss the scientific evolution of clinical face transplantation. Composite tissue allotransplantation (CTA) rapidly progressed in the 1980s with the discovery of cyclosporine. Although the most success has been achieved with hand transplantation, others have made progress with allografts of trachea, peripheral nerve, flexor tendon apparatus, vascularized knee, larynx, abdominal wall, and most recently, partial face. The world's first partial face allotransplantation occurred in November 2005 in France. In April of 2006, there was a second performed in China. As of today, there are now multiple institutions with plans to attempt the world's first full facial/scalp transplant. Complete facial/scalp allotransplantation offers a viable alternative for unfortunate individuals suffering severe facial disfigurement and is a product of many decades of experimental research, beginning with rat hindlimb allografts.

Immunosuppressants: neuroprotection and promoting neurological recovery following peripheral nerve and spinal cord lesions

No clinical techniques induce restoration of neurological losses following spinal cord trauma. Peripheral nerve damage also leads to permanent neurological deficits, but neurological recovery can be relatively good, especially if the ends of a transected nerve are anastomosed soon after the injury. The time until recovery generally depends on the distance the axons must regenerate to their targets. Neurological recovery following the destruction of a length of a peripheral nerve requires a graft to bridge the gap that is permissive to, and promotes, axon regeneration. But neurological recovery is slow and limited, especially for gaps longer than 1.5 cm, even using autologous peripheral nerve grafts. Without a reliable means of bridging long nerve gaps, such injuries commonly result in amputations. Promoting extensive neurological recovery requires techniques that simultaneously provide protection to injured neurons and increase the numbers of neurons that extend axons, while inducing more rapid and extensive axon regeneration across long nerve gaps. Although conduits filled with various materials enhance axon regeneration across short nerve gaps, pure sensory nerve graft remains the gold standard for use across long nerve gaps, even though they lead to only limited neurological recovery. Consistent results demonstrate that several immunosuppressive agents enhance the number of axons and the rate at which they regenerate. This review examines the roles played by immunosuppressants, especially FK506, with primary focus on its role as a neuroprotectant and neurotrophic agent, and its potential clinical use to promote improved neurological recovery following peripheral nerve and spinal cord injuries.