Introduction

Traumatic injury to the nervous system has a profound impact on an individual’s mobility, cognition, perception, and, ultimately, their independence. Given the young median age at which injuries are sustained and the long-term survival rates following traumatic injuries to the nervous system, there is a growing population of people suffering from chronic injuries to the nervous system [1, 2]. For injuries to the central nervous system (CNS), including the brain and spinal cord, there is no cure to fully restore motor, sensory, and autonomic function due to the robust barriers to regeneration in these tissues following injury [3]. Injuries in the peripheral nervous system (PNS) have limited treatment options to repair tissues, yet complete restoration of motor and sensory function following large gap injuries is still out of reach for patients due to limited regenerative potential [4]. To alleviate the rising healthcare costs and restore neurological function lost to injury, many pre-clinical approaches advancing towards clinical trials aim to increase spared tissue after the initial injury, increase plasticity of intact neural circuits, repair damaged neural tissue, and regenerate new neural tracts [3, 5,6,7]. One area of particular interest is the use of cell transplantation techniques that have the potential to improve tissue sparing, plasticity, repair, and regeneration to treat injuries in the brain, spinal cord, and peripheral nervous system, which has been reviewed by others [8,9,10,11,12,13,14].

The safety and efficacy of cell transplantation for spinal cord injuries has been well established in pre-clinical models, including rodents, porcine, and non-human primate models [15], with preliminary efficacy demonstrated in rodent models of traumatic brain injury [16]. However, only recently have clinical trials demonstrated the safety of cell transplantation for neural applications with stem cells and differentiated cells from neural, mesenchymal, and hematopoietic lineages [8, 14, 17]. While many have demonstrated feasibility and safety of transplanted cells, a consensus on efficacy remains elusive. The goal of cell transplantation for neural applications includes neural repopulation, immune modulation, reparative biomolecular secretion, tissue remodeling, and pain management. While many of these goals can be best achieved through localized administration within the injury, intravenous and intrathecal dosing have been used widely to achieve immune modulation [8, 14]. Figure 1 summarizes the tissue lineage source, administration routes, and therapeutic goals explored in recent clinical trials. Focusing on the safety of transplantation directly into the nervous system, clinical trials have demonstrated that transplantation of neural stem cells (NSCs), Schwann cells, and bone marrow-derived stem cells within sites of spinal cord injury can be safely achieved without severe adverse effects or tumorigenicity [14]. Similarly, NSCs, hematopoietic stem cells, mesenchymal stem cells (MSCs), and other glial progenitors have demonstrated safety in clinical trials for neurodegenerative diseases, including amyotrophic lateral sclerosis and Parkinson’s disease [10]. Meanwhile, only bone marrow-derived and umbilical cord-derived stem cells have demonstrated safety for use in treating traumatic brain injury when transplanted via intravenous and intrathecal routes [8]. Table 1 summarizes clinical trials involving the use of cell transplants for spinal cord, brain, or peripheral nerve injuries that are completed or active, whereas clinical trials that were terminated or have unknown status were excluded from this cohort of trials.

Fig. 1
figure 1

Cell therapies for the treatment of injuries to the brain, spinal cord, and peripheral nerves have been administered in clinical trials via multiple routes, including intravenous, intramedullary, peri-infarct, subarachnoid, intrathecal, and intraneural. Cells isolated from the neural (N), mesenchymal (M), and hematopoietic (H) tissue sources are indicated for each route of administration. Each cell source offers its own unique benefits, which have been primarily demonstrated in pre-clinical studies

Full size image

While safety and feasibility of cell transplantation within the nervous system has begun to be demonstrated, therapeutic efficacy remains a challenge due to a lack of standardization of transplantation methods and patient assessments across individual injury types. As the field works towards a consensus on cell delivery and standardized assessments across studies, there remains a significant challenge in transplant survival and engraftment that persists and requires new tools to resolve. The purpose of this review is to highlight engineered tools to couple with cell transplantations to overcome poor survival and engraftment for CNS injury, PNS injury, and neuropathic pain reported in preclinical models that have resulted in little evidence of cell transplant survival and engraftment in clinical trials.

Table 1 Completed and active clinical trials for cell transplantation following injury to the nervous system. Clinical trials with the same sponsor, but differ in inclusion criteria are grouped together
Full size table

Enabling Robust Stem Cell Survival in the CNS

Stem cell transplantation into the CNS is subject to many of the same barriers that plague cell transplantation approaches in other parts of the body. Survival is predicated on the transplantation method, cell sourcing and expansion methods, and the engraftment microenvironment [12, 36]. Location of implantation, cell dose density, and timing are also important influencers of cell survival and efficacy with trade-offs including lower survival when cells are transplanted intralesional compared to higher survival in distal implantation sites [37,38,39]. Yet even if each of these criteria is addressed, survival in pre-clinical studies remains low [40], and stem cell survival in clinical trials has not been well-documented [14].

Biomaterials are a high potential tool in promoting stem cell transplantation survival. Alone, biomaterials can reduce secondary injury, guide repair processes, and limit scar formation in the CNS [41,42,43]. Yet, when used as a vehicle for cell transplantation, these outcomes can be synergistically enhanced, in part through improved cell survival and engraftment [40]. Biomaterials can reduce stem cell loss during surgical transplantation and improve transplant localization within the injury, where the stem cell transplantation has the highest potential to improve patient outcomes [38]. Previously, we have reviewed biomaterial techniques to bolster cell survival and localization for treatment of spinal cord injury [40], with many of the guiding principles being applicable to other nervous system transplants sites. To that end, we will focus on recent advances in the past three years, highlighting the role biomaterials can play as a tool to improve survival of stem cell transplantation in the CNS.

Initial cell loss during implantation is due to the high shear forces exerted on the cells resulting in cell membrane damage and rupture during the injection process. Strategies that implant stem cells on scaffolds are one way to alleviate cell loss due to these high forces. In this scenario, cells are loaded into a scaffold through a variety of means, including bioprinting. The cells are allowed to attach, proliferate, and produce cell networks that can enhance activation of pro-survival pathways [44, 45]. The stem cell laden scaffold is then surgically implanted, thereby eliminating the need to use traditional injection methods. Transplantation of stem cells in this manner allows for high precision of stem cell organization and structural guidance cues. One challenge to this approach is that the biomaterial typically accounts for a high percentage of the lesion volume compared to the stem cell transplants, as the scaffold stiffness is needed for handling and surgical implantation compared to injectable approaches. A recent study overcame this issue, using a core-sheath scaffold extrusion method that is comprised of an NSC-rich core surrounded by a thin-coat of acellular electrospun polymeric fibers that provides sufficient rigidity to improve surgical implantation, while also providing protection of the NSCs from the surgical implantation process [46].

While cell-seeded scaffolds are a promising approach to improve cell survival, these strategies are limited to large defect injuries. To that end, the Heilshorn lab has developed an injectable biomaterial method to reduce cell loss that occurs due to the high shear forces present with injectable strategies. The resulting injectable hydrogels undergo on-demand dynamic modifications of the matrix leading to shear forces being exerted on the biomaterial matrix, rather than the cells [47]. Use of these injectable hydrogels can be tuned for individual cell types to include cell adhesion molecules and selective reactivity dependent on the delivery method and implant environment. By eliminating shear forces exerted on the cells during injection and including cell adhesion molecules to prevent anoikis, cell transplantation survival from injection alone can be dramatically improved by almost half for more sensitive cells, such as induced pluripotent stem cell derived deep cortical neurons [48].

In addition to overcoming stem cell loss due to the method of delivery, the transplanted cells then undergo a biochemical assault due to the heightened inflammation associated with injury, which can limit survival and/or result in differentiation into less desirable glial phenotypes. To bolster stem cell transplant survival, immunosuppressants are used to dampen inflammation that accompanies CNS injuries, however, immunosuppressants can result in severe complications in CNS injured patients, such myopathies [49], pneumonia, sepsis, and death [50,51,52]. These outcomes are associated with the high doses necessary to access the brain and spinal cord, given the low permissiveness of the blood-brain barrier and blood-spinal cord barrier [49, 51, 53, 54]. Moreover, commonly used immunosuppressant drugs can hamper NSC transplant proliferation needed for recovery [52]. Other immunomodulatory agents are in the pre-clinical pipeline, however, the systemic administration and high doses continue to present translational challenges even when used alone [55]. However, one group has found a promising immunosuppressant drug cocktail has shown promise in increasing NSC transplant survival and engraftment without reported toxicity [56].

While there are several local immunomodulatory strategies to treat inflammation, there is a scarcity of studies that utilize these techniques with stem cell transplantation approaches in the nervous system, even though biomaterial platforms can potentially overcome translational challenges. Biomaterials can provide local, tunable release of immunosuppressant and immunomodulatory agents, thus improving cell transplant survival and reducing toxicity associated with high, systemic drug administration [49, 51, 53, 54]. In the case of SCI, we have delivered anti-inflammatory cytokines through lentiviral-mediated over-expression, from biomaterials to locally reduce acute inflammation [57]. Overexpression of anti-inflammatory interleukin-10 mediated by lentivirus loaded into hydrogel tubes resulted in a 1.9-fold increase in NSC transplant survival compared to the biomaterial alone, and a 11.6-fold fold increase compared to NSCs delivered independent, thus demonstrating the importance of using combinatorial biomaterial strategies for stem cell transplantation into the CNS. There is a paucity of studies combining local strategies to mitigate inflammation associated with poor cell transplant survival, yet there are several combinatorial strategies seeking to improve engraftment and differentiation of transplanted stem cells for CNS repair and regeneration [58]. Future work that addresses temporal needs of cell transplants may build on these past works to achieve a truly tunable cell transplantation approach for the CNS to overcome current translational obstacles. For example, biomaterial systems that provide local, modular drug delivery [59] could be designed to delivery of early immune modulation and subsequent regenerative cues that could guide cell transplant-mediated regeneration. Such an approach would provide protection for transplanted stem cells, as well as provide additional instructional cues to modulate differentiation of transplanted stem cells to repopulate lost or damaged tissue.

Enabling Robust Schwann Cell Engraftment in the Spinal Cord and PNS

Schwann cells have emerged as a pivotal therapeutic strategy for promoting axon regeneration and myelination, not only within the PNS but also within the spinal cord as highlighted in Table 1. Pioneering research conducted by the Bunge Laboratory laid the groundwork for further exploration supported by in vitro observations demonstrating the independent survival and growth stimulation of Schwann cells by molecules bound to axon membranes [60]. Recent investigations utilize the NeuraGen collagen conduits seeded with autologous Schwann cells resulted in strong Schwann cell engraftment and regenerative efficacy in a critical gap peripheral nerve injury rat model [61]. Importantly, the use of an autologous cell source capable of regeneration, represents a potent cell therapy that can mitigate the need for immune suppressing/modulating drugs. An ongoing clinical trial for the transplantation of autologous Schwann cells is under investigation for large gap peripheral nerve injuries [17, 62]. Several million Schwann cells are transplanted on an autologous nerve graft within the injury, resulting in repair-activated Schwann cells that guide and biochemically support regenerative processes. While promising, this approach requires secondary surgeries to harvest nerves for isolating and expanding patient-specific Schwann cells, as well as a second nerve to serve as a nerve autograft and provide structural cues to instruct repair across the injury [17]. Without guided repair, axon regrowth across large-gap nerve damage can result in synkinesis and muscle atrophy due to lack of innervation for prolonged periods, thus a bridging material is essential. To that end, we anticipate the next major advance will be a combinatorial clinical trial integrating autologous Schwann cells with synthetic biomaterial conduits, not unlike those currently under investigation in pre-clinical studies [63, 64].

Within the CNS, endogenous Schwann cells migrate into the spinal cord after injury. Building on this phenomenon, there has been a push to explore Schwann cell transplantation for spinal cord repair. Phase I trials have tested the safety and feasibility of autologous Schwann cell transplantation for both subacute and chronic SCI [65]. These trials have demonstrated the safety and feasibility of obtaining and delivering autologous Schwann cells into the injury epicenter, with promising evidence of motor and sensory function improvement in select participants. Given the small sample size and inherent patient variability, further studies are needed to assess Schwann cell engraftment and efficacy of this method.

As described in the PNS, Schwann cell engraftment into the spinal cord could also be enhanced using engineered biomaterials. Once again, the novel class of materials pioneered by the Heilshorn lab can be applied, but in this instance they can minimize Schwann cell loss during injection, mitigate cell membrane damage, prevent reflux from the spinal cord, and address rapid post-injection cell death [66]. Using a novel bioengineered injectable material modified specifically for Schwann cells the survival, engraftment, and therapeutic efficacy of transplanted Schwann cells was significantly improved [66]. By effectively addressing these critical challenges, this approach aims to bolster Schwann cell retention while diminishing spinal cord cavitation, thereby underscoring the significance of biomaterial tools to advance Schwann cell transplantation delivery for spinal cord treatment.

Tools for Transplanted Cell Engraftment to Overcome Neuropathic Pain

Much of the focus of cell transplantation into the nervous system is to regain motor function and independence for those suffering from nerve injuries. However, a secondary complication that occurs after injury to the nervous system is chronic neuropathic pain (NP) due to the cascade of neuroinflammation stimulated during injury. The complex pathophysiology associated with NP makes it difficult to treat effectively [67]. NP severely affects a patient’s quality of life and poses a tremendous burden on the healthcare system [68]. Over 50% of patients suffering from NP do not receive sufficient pain relief due to the longevity and severity of symptoms, emphasizing the urgent need for new therapeutic strategies [9]. Some of the major contributors to the sensitization that causes pain stimuli are intracellular interactions, molecular signaling, and the structural changes of cells to harmful phenotypes [68]. While many are trying to understand the complex mechanisms behind NP to improve treatments, one potentially effective therapeutic approach is cell transplantation therapy. In several prior studies, cell-based therapeutics have been shown to provide neurorestoration through the regeneration of neurological networks, leading to both motor functional improvements and sensory impairment improvements [13, 69]. In this section, we will discuss the results and efficacy of several pre-clinical and clinical studies for cell transplantations, specifically NSCs, chromaffin cells, and GABAergic precursor cells.

NSCs are a popular candidate for cell therapy due to their neuroprotective and immunomodulatory properties in reducing neuroinflammation, potentially leading to the alleviation of NP [70]. The role of NSCs in pain alleviation has been measured through their interaction with cells within the damaged injury microenvironment. Local transplantation of NSCs into an injured rodent spinal cord resulted in regulation of NP signaling, indicating an improvement in the inflammatory microenvironment and a reduction in NP [71]. Within the PNS, NSC-laden scaffolds improved motor function and mitigated peripheral nerve injury-induced NP through nerve repair [72], suggesting NSC-mediated repair can alleviate NP throughout the nervous system.

More recently, the mechanism of human NSC-mediated modulation of NP via the secretome has been investigated in a rodent model of spinal cord injury. The hNSC-secretome decreased antioxidants, reduced matrix degradation, and modulated transforming growth factor (TGF)-β and brain-derived neurotrophic factor (BDNF) secretion, ultimately improving functional recovery and pain management [73]. Due to the anatomical and immunological differences in rodents and primates, it is important to look at the effectiveness of NSCs in larger animal models. Common marmosets with contusive SCI were grafted with embryonic stem cell-derived NSCs. Behavioral, histological, and immunoelectron microscopy analyses showed improved functional recovery in transplanted primates [74]. Although there have been successes in several rodent and primate models for functional recovery and mitigation of NP by transplanted NSCs, further studies must be conducted with larger sample sizes. Transplanted NSCs have also been shown to induce forelimb allodynia due to the differentiation into maladaptive structures [75]. A possible solution to this mechanism is the combination of NSCs with neurotrophins such as glial-derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF), or BDNF, which may provide an analgesic effect in lessening allodynia [70, 75, 76]. Another limitation of NSCs is invasive cell grafting directly into the spinal parenchyma during transplantation, offering the potential for further injury and disruption of the microenvironment [77].

A potential cell transplantation type that may address translational obstacles of NSCs for NP is chromaffin cells. The production of high levels of catecholamines and opioid peptides increases their role in the reduction of pain sensitivity [78]. Unlike many other administered therapeutics for chronic pain, chromaffin cell transplantation would not require the build-up of tolerance to these released factors. Pre-clinical trials demonstrated the lack of neurotoxicity and long-lasting analgesic effects of these biological “mini pumps” after injection into the spinal subarachnoid space [79]. Adrenal medullary allografts have also demonstrated success in clinical studies for managing pain in acute and chronic pain not associated with nervous system injury. A phase II clinical study performed on 15 patients with cancer pain looked at the stabilization of opioid dosage and intrathecally administered morphine to determine the analgesic effect of these allografts on pain progression. The study demonstrated the safety and feasibility of grafting human chromaffin cells into cerebrospinal fluid for chronic cancer pain and showed promising results with significant reduction or stabilization of opioid and morphine usage after cell transplant [80].

A more recent clinical study looked at two patients suffering from chronic neuropathic pain after spinal cord injury and the effect of an intrathecal injection of chromaffin cells. Six months after injection, the patient’s pain had reduced significantly, providing preliminary evidence of the therapeutic efficacy for severe central NP [81]. Studies in animal models have also shown the reduction of both forelimb and hindlimb mechanical and thermal allodynia in rodents after adrenal medullary transplants [82]. Due to the sustained secretion of a synergistic cocktail of analgesic agents and the ability to non-invasively transplant into the subarachnoid space, chromaffin cells may be a unique therapeutic option for chronic pain management. However, the primary drawback to most further advancements is the lack of a sufficient and feasible clinical cell source due to limited human donor sources or the immune rejection from xenogeneic alternatives. Chromaffin-like cells from human induced pluripotent stem cells (hiPSCs) have recently been developed, offering a solution for sufficient cell sourcing without the need for immunosuppression [83]. Moreover, biomaterial tools described in prior sections of this review have not been evaluated to address chromaffin cell transplantation challenges, thus offering an attractive area for future investigation utilizing what is already well-established for biomaterial mediated cell transplantation into the CNS and PNS.

Another novel alternative currently being considered is GABAergic precursor/progenitor cells to reduce and manage chronic pain by restoring the important neurotransmitter, γ-aminobutyric acid (GABA) [84]. A primary cause of persistent pain is the decrease in GABAergic inhibition, leading to increased neurotransmission [85]. Pharmaceuticals such as gabapentin and benzodiazepines have aimed to target the GABAergic system and restore healthy levels, but many side effects have limited their clinical efficacy [86]. Animal models have demonstrated the preliminary efficacy of GABAergic cells in alleviating chronic NP [87, 88]. Mouse embryonic stem cell-derived NSCs differentiated into GABAergic neurons were intrathecally transplanted in a rat model 21 days after spinal cord injury and chronic NP attenuation was evaluated. The results found that the GABAergic neurons significantly attenuated chronic pain levels and cell survival for at least 7 weeks post-transplantation [87]. Transplanted pluripotent stem cell-derived GABAergic interneurons also indicated significant relief from injury-induced NP and long-term survival at the spinal transplantation site [88]. Overall, GABAergic precursor cell transplantations in rat models have demonstrated preliminary evidence of the ability to mitigate allodynia, however larger animal and human studies are still necessary to verify the efficacy of this therapeutic strategy [89].

Conclusions

Biomaterials afford improved cell transplant survival, injury localization, and engraftment within the CNS and PNS in pre-clinical models, but have not yet been used in clinical trials for nervous system injuries. Given the high prevalence of clinical trials for cell transplantation into patients with spinal cord injuries and of biomaterial approaches to enhance cell transplantation in pre-clinical spinal cord injury models, the use of a combinatorial biomaterial-cell transplant approach for treatment of spinal cord injury and its associated NP is not unfounded. The question remains, which biomaterial option is optimal for translating to patients? This is not an easy question to answer, given each biomaterial targets a different cell transplant need, or in many cases the biomaterial is designed to support endogenous repair and does not apply cell transplant design considerations. Nevertheless as the field moves forward, a robust approach that alleviates cell death during delivery, remediates inflammation, and guides regeneration will be needed to address the challenges that plague cell transplantation following spinal cord injury and facilitate translation to patients. Conversely, the use of biomaterials for cell delivery following traumatic brain injury appears to be a more distant future given that MSCs are systemic administered to remediate inflammation and would not necessarily benefit from a biomaterial approach. Implantation local to the sites of traumatic brain injury have also shown to be safe for MSC administration [35] and could potentially benefit from a biomaterial to maintain MSCs local to the injury, but further safety and efficacy testing of these two administration modalities would be needed to assess whether a biomaterial is necessary in this particular injury paradigm. Lastly, the use of cells, tissue grafts, or acellular biomaterials have been explored in the clinic for tissue repair after injury to the PNS, where Schwann cells and topographical guidance are essential for rapid recovery necessary to reinnervate distal tissue targets. Given that the researchers leading clinical trials for the use of autologous Schwann cells are exploring the use of collagen scaffolds or autologous nerve grafts in combination with these cells [17, 62], it is possible that the first cell transplant with an engineered biomaterial into the nervous system might be close at hand.

Key References

  • Zipser CM, Cragg JJ, Guest JD, Fehlings MG, Jutzeler CR, Anderson AJ, et al. Cell-based and stem-cell-based treatments for spinal cord injury: evidence from clinical trials. Lancet Neurol. 2022;21(7):659–70. https://doi.org/10.1016/S1474-4422(21)00464-6.

    ○ Detailed overview of clinical trials for spinal cord injury.

  • Finnerup NB, Kuner R, Jensen TS. Neuropathic Pain: from mechanisms to treatment. Physiol Rev. 2021;101(1):259–301. https://doi.org/10.1152/physrev.00045.2019.

    ○ Comprehensive review of diagnosis, mechanisms, and treatment of neuropathic pain.

  • Saboori M, Riazi A, Taji M, Yadegarfar G. Traumatic brain injury and stem cell treatments: a review of recent 10 years clinical trials. Clin Neurol Neurosurg. 2024;239:108219. https://doi.org/10.1016/j.clineuro.2024.108219.

    ○ Detailed overview of clinical trials for traumatic brain injury.