Effect of cold nerve allograft preservation on antigen presentation and rejection

Optimizing Peripheral Nerve Regeneration: Surgical Techniques, Biomolecular and Regenerative Strategies-A Narrative Review

Peripheral nerve injury disrupts the function of the peripheral nervous system, leading to sensory, motor, and autonomic deficits. While peripheral nerves possess an intrinsic regenerative capacity, complete sensory and motor recovery remains challenging due to the unpredictable nature of the healing process, which is influenced by the extent of the injury, age, and timely intervention. Recent advances in microsurgical techniques, imaging technologies, and a deeper understanding of nerve microanatomy have enhanced functional outcomes in nerve repair. Nerve injury initiates complex pathophysiological responses, including Wallerian degeneration, macrophage activation, Schwann cell dedifferentiation, and axonal sprouting. Complete nerve disruptions require surgical intervention to restore nerve continuity and function. Direct nerve repair is the gold standard for clean transections with minimal nerve gaps. However, in cases with larger nerve gaps or when direct repair is not feasible, alternatives such as autologous nerve grafting, vascularized nerve grafts, nerve conduits, allografts, and nerve transfers may be employed. Autologous nerve grafts provide excellent biocompatibility but are limited by donor site morbidity and availability. Vascularized grafts are used for large nerve gaps and poorly vascularized recipient beds, while nerve conduits serve as a promising solution for smaller gaps. Nerve transfers are utilized when neither direct repair nor grafting is possible, often involving re-routing intact regional nerves to restore function. Nerve conduits play a pivotal role in nerve regeneration by bridging nerve gaps, with significant advancements made in material composition and design. Emerging trends in nerve regeneration include the use of 3D bioprinting for personalized conduits, gene therapy for targeted growth factor delivery, and nanotechnology for nanofiber-based conduits and stem cell therapy. Advancements in molecular sciences have provided critical insights into the cellular and biochemical mechanisms underlying nerve repair, leading to targeted therapies that enhance axonal regeneration, remyelination, and functional recovery in peripheral nerve injuries. This review explores the current strategies for the therapeutic management of peripheral nerve injuries, highlighting their indications, benefits, and limitations, while emphasizing the need for tailored approaches based on injury severity and patient factors.

GalT Knockout Porcine Nerve Xenografts Support Axonal Regeneration in a Rodent Sciatic Nerve Model

BACKGROUND

Nerve xenografts harvested from transgenic α1,3-galactosyltransferase knockout pigs lack the epitope responsible for hyperacute rejection in pig-to-primate transplants. It is unknown whether these cold-preserved nerve grafts support axonal regeneration in another species during and after immunosuppression. The authors compared outcomes between autografts and cold-preserved xenografts in a rat sciatic model of nerve gap repair.

METHODS

Fifty male Lewis rats had a 1-cm sciatic nerve defect repaired using autograft and suture ( n = 10); 1-week or 4-week cold-preserved xenograft and suture ( n = 10 per group); or 1-week or 4-week cold-preserved xenograft and photochemical tissue bonding using a human amnion wrap ( n = 10 per group). Rats with xenografts were given tacrolimus until 4 months postoperatively. At 4 and 7 months, rats were killed and nerve sections were harvested. Monthly sciatic functional index (SFI) scores were calculated.

RESULTS

All groups showed increases in SFI scores by 4 and 7 months. The autograft suture group had the highest axon density at 4 and 7 months. The largest decrease in axon density from 4 to 7 months was in the group with 1-week cold-preserved photochemical tissue bonding using a human amnion wrap. The only significant difference between group SFI scores occurred at 5 months, when both 1-week cold-preserved groups had significantly lower scores than the 4-week cold-preserved suture group.

CONCLUSIONS

The results suggest that α1,3-galactosyltransferase knockout nerve xenografts may be viable alternatives to autografts. Further studies of long-gap repair and comparison with acellular nerve allografts are needed.

CLINICAL RELEVANCE STATEMENT

This proof-of-concept study in the rat sciatic model demonstrates that cold-preserved α1,3-galactosyltransferase knockout porcine xenografts support axonal regeneration and viability following immunosuppression withdrawal. These results further suggest a role for both cold preservation and photochemical tissue bonding in modulating the immunological response at the nerve repair site.

Physical processing for decellularized nerve xenograft in peripheral nerve regeneration

In severe or complex cases of peripheral nerve injuries, autologous nerve grafts are the gold standard yielding promising results, but limited availability and donor site morbidity are some of its disadvantages. Although biological or synthetic substitutes are commonly used, clinical outcomes are inconsistent. Biomimetic alternatives derived from allogenic or xenogenic sources offer an attractive off-the-shelf supply, and the key to successful peripheral nerve regeneration focuses on an effective decellularization process. In addition to chemical and enzymatic decellularization protocols, physical processes might offer identical efficiency. In this comprehensive minireview, we summarize recent advances in the physical methods for decellularized nerve xenograft, focusing on the effects of cellular debris clearance and stability of the native architecture of a xenograft. Furthermore, we compare and summarize the advantages and disadvantages, indicating the future challenges and opportunities in developing multidisciplinary processes for decellularized nerve xenograft.

Nerve regeneration in rat peripheral nerve allografts: Evaluation of cold-inducible RNA-binding protein in nerve storage and regeneration

The prevalence of peripheral nerve injury has attracted increased attention. Allografting has been proposed as a potential treatment strategy for peripheral nerve injury. Moreover, cryopreservation may provide almost unlimited graft material. We investigated whether cold-inducible RNA-binding protein (CIRP) could protect peripheral nerves during cryopreservation to promote regeneration postoperation. First, CIRP was highly expressed after pretreatment at 32°C. After 4 weeks of cryopreservation, the increased live cells, low Bax/Bcl-2 ratio and high nerve growth factor and glial cell-derived neurotrophic factor levels in the 32°C group demonstrated high nerve graft viability. At 4 weeks postoperation, 32°C-Allo group demonstrated low plasma levels of interleukin-6 and interferon-gamma and a diminished cellular immune response. At 20 weeks postoperation, nerve regeneration in the 32°C-Allo group was similar to that in the fresh isograft group and superior to that in the 4°C-Allo and 15°C-Allo groups. Moreover, the compound muscle action potential and the motor nerve conduction velocity of the 32°C-Allo group were equal to those of the fresh isograft group. In conclusion, CIRP induction increased Schwann cell biological activity, inhibited cell apoptosis, reduced immune rejection, and promoted recipient nerve regeneration. Thus, CIRP could exert protective effects during nerve storage and stimulate regeneration in peripheral nerve reconstruction.

Mass spectrometry comparison of nerve allograft decellularization processes

Peripheral nerve repair using nerve grafts has been investigated for several decades using traditional techniques such as histology, immunohistochemistry, and electron microscopy. Recent advances in mass spectrometry techniques have made it possible to study the proteomes of complex tissues, including extracellular matrix rich tissues similar to peripheral nerves. The present study comparatively assessed three previously described processing methods for generating acellular nerve grafts by mass spectrometry. Acellular nerve grafts were additionally examined by F-actin staining and nuclear staining for debris clearance. Application of newer techniques allowed us to detect and highlight differences among the 3 treatments. Isolated proteins were separated by mass on polyacrylamide gels serving 2 purposes. This further illustrated that these treatments differ from one another and it allowed for selective protein extractions within specific bands/molecular weights. This approach resulted in small pools of proteins that could then be analyzed by mass spectrometry for content. In total, 543 proteins were identified, many of which corroborate previous findings for these processing methods. The remaining proteins are novel discoveries that expand the field. With this pilot study, we have proven that mass spectrometry techniques complement and add value to peripheral nerve repair studies.

The Role of Current Techniques and Concepts in Peripheral Nerve Repair

Patients with peripheral nerve injuries, especially severe injury, often face poor nerve regeneration and incomplete functional recovery, even after surgical nerve repair. This review summarizes treatment options of peripheral nerve injuries with current techniques and concepts and reviews developments in research and clinical application of these therapies.

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.

A rat model for long-gap peripheral nerve reconstruction

BACKGROUND

The rat model has had limited utility for the study of long nerve gaps because of the small size of the animal. The authors sought to develop a simple, effective rat model for reconstruction of long nerve gap defects.

METHODS

Fifteen rats had a sciatic nerve transection followed by reconstruction. Positive control rats received a 1-cm isograft. Negative control rats received a 3.5-cm hollow silicone conduit, and experimental rats received a 4-cm isograft; these were implanted in a looped configuration to accommodate the long length. Nerves were harvested at 6 weeks (1-cm grafts) and 12 weeks (3.5-cm conduits and 4-cm grafts) for histologic and histomorphometric evaluation.

RESULTS

The 1-cm and 4-cm isograft groups showed robust regeneration in the distal nerve segment. The 3.5-cm hollow conduits showed absolutely no initiation of nerve regeneration. Histomorphometric values were as expected for the specified gap length.

CONCLUSIONS

This study describes a simple and effective long nerve gap rat model for experiments on nerve grafts and nerve conduits. The long nerve graft model can be useful for studying techniques such as processed nerve grafts, which are currently a topic of frequent investigation. The 3.5-cm hollow conduit "no-regrowth" long-gap model is ideal for investigating conduit-based tissue-engineering solutions for long-gap nerve repair. The authors' approach overcomes the size limitation of the small animal while exploiting the features that make the rat the model of choice for preliminary nerve studies.

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.

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.

The role of immunophilin ligands in nerve regeneration

Tacrolimus (FK506) is a widely used immunosuppressant in organ transplantation. However, it also has neurotrophic activity that occurs independently of its immunosuppressive effects. Other neurotrophic immunophilin ligands that do not exhibit immunosuppression have subsequently been developed and studied in various models of nerve injury. This article reviews the literature on the use of tacrolimus and other immunophilin ligands in peripheral nerve, cranial nerve and spinal cord injuries. The most convincing evidence of enhanced nerve regeneration is seen with systemic administration of tacrolimus in peripheral nerve injury, although clinical use is limited due to its immunosuppressive side effects. Local tacrolimus delivery to the site of nerve repair in peripheral and cranial nerve injury is less effective but requires further investigation. Tacrolimus can enhance outcomes in nerve allograft reconstruction and accelerates reinnervation of complex functional allograft transplants. Other non-immunosuppressive immunophilins ligands such as V-10367 and FK1706 demonstrate enhanced neuroregeneration in the peripheral nervous system and CNS. Mixed results are found in the application of immunophilin ligands to treat spinal cord injury. Immunophilin ligands have great potential in the treatment of nerve injury, but further preclinical studies are necessary to permit translation into clinical trials.

Nerve regeneration with aid of nanotechnology and cellular engineering

Repairing nerve defects with large gaps remains one of the most operative challenges for surgeons. Incomplete recovery from peripheral nerve injuries can produce a diversity of negative outcomes, including numbness, impairment of sensory or motor function, possibility of developing chronic pain, and devastating permanent disability. In the last few years, numerous microsurgical techniques, such as coaptation, nerve autograft, and different biological or polymeric nerve conduits, have been developed to reconstruct a long segment of damaged peripheral nerve. A few of these techniques are promising and have become popular among surgeons. Advancements in the field of tissue engineering have led to development of synthetic nerve conduits as an alternative for the nerve autograft technique, which is the current practice to bridge nerve defects with gaps larger than 30 mm. However, to date, despite significant progress in this field, no material has been found to be an ideal alternative to the nerve autograft. This article briefly reviews major up-to-date published studies using different materials as an alternative to the nerve autograft to bridge peripheral nerve gaps in an attempt to assess their ability to support and enhance nerve regeneration and their prospective drawbacks, and also highlights the promising hope for nerve regeneration with the next generation of nerve conduits, which has been significantly enhanced with the tissue engineering approach, especially with the aid of nanotechnology in development of the three-dimensional scaffold. The goal is to determine potential alternatives for nerve regeneration and repair that are simply and directly applicable in clinical conditions.