Clinical aspects

Description

	• Cells for transplantation can be obtained from several different tissues of the body and may be autologous or allogeneic.
	• Because adult brain cells have a limited capacity to migrate to and regenerate at sites of injury, transplantation of stem cells can circumvent these limitations.
	• Cell therapy has potential applications for several neurologic disorders, but most of these are investigational.

Sources of cells for transplantation:

Autologous, ie, from the same organism. The advantages of using autologous cells are that they are well tolerated and no ethical issues are involved. Although the supply is limited, they can be multiplied in vitro.

Allogeneic, ie, from the same species. There is a greater supply of allogeneic cells, but ethical issues are involved when fetal tissues are used. Another disadvantage is immune reaction, which can be overcome by use of immunosuppressant drugs or encapsulation.

Xenografts, ie, from different species. An abundant supply of xenografts is available, but they are accompanied by risks of transmission of new viruses across species. Also, immune reactions can be suppressed by drugs or by genetically masking the immunity.

Cell lines, ie, immortalized cells. There is an infinite supply of cell lines, but they create immune reaction and the possibility of tumorigenesis. Immunosuppressants, encapsulation, or genetic masking can overcome these disadvantages.

Various types of cells that are used for cell implantation in neurologic disorders are:

• Autologous macrophages • Cerebral endothelial cells • Glial cells • Fetal tissue transplants • Neuronal progenitor cells • Stem cell transplantation • Bone marrow stem cells • Stem cells from olfactory neuroepithelium • Xenografts • Immortalized cells

Neurons and oligodendrocytes are terminally differentiated cells. This means that once they have differentiated from their precursor cells, they cannot proliferate. A direct consequence of this type of differentiation is that cell repair is impossible in areas where neurodegenerative disease has caused the death of neurons and oligodendroglia. The most important cells for transplantation are fetal tissues and stem cells.

T lymphocytes. T lymphocytes can cross the blood-brain barrier and can be engineered to produce nerve growth factor in quantities comparable to those produced by genetically engineered fibroblasts. Engineered T lymphocytes have been used as vehicles to deliver nerve growth factor F across the endothelial blood-nerve barrier to attenuate experimental autoimmune encephalomyelitis.

Fetal tissue transplants. Human neural fetal tissue has been used for transplantation into the striatum of Parkinson disease patients. The limiting factors are the ethical issues involved in obtaining human fetal tissues, as well as the need for immunosuppression to enable the survival of transplanted tissue. An alternative to neonatal tissue transplantation is the use of neural-derived stem cells that have been made to proliferate in culture under the influence of basic fibroblast growth. These can differentiate into either glial or neuronal cells, thus, replacing the neurons that degenerate in Alzheimer and Parkinson disease. Epidermal growth factor-responsive CNS stem cells can also develop into oligodendrocytes when transplanted into the postnatal spinal cord. The exposure of CNS stem cells to combinations of trophic factors may lead to more successful neural grafts.

Stem cell transplantation. Adult brain cells have a limited capacity to migrate to and regenerate at sites of injury. Transplantation of human neural stem or progenitor cells may offer a method of circumventing these limitations because these cells are both able to migrate and have the potential to become the cellular components of the nervous system. Because mature, differentiated neurons do not normally divide either in the brain or in culture, efforts have been directed to derive normal neurons from neuronal precursors.

Neural stem cells. Neural stem cells are cells that can generate neural tissue or are derived from the nervous system. There is considerable interest in the identification and precise location of the adult neural stem cells in vivo. Studies examining the localization of neural stem cells are controversial and suggest 2 distinct locations within the adult brain: the ependymal layer lining the ventricles and the subependymal layer immediately adjacent to the ependyma. Discovery of stem-cell-like activity in the subventricle zone could open the way for several treatments for brain cancer, neurodegenerative diseases, and brain damage. Neural precursors have several characteristics that make them ideal vectors for brain repair. They may be clonally expanded in tissue culture, providing a renewable supply of material for transplantation. The frequent and cell type-specific fusion of donor neural stem cells with host neurons in animal experimental studies might prove useful for basic research and cell therapy studies (09).

Amyloid precursor protein is implicated in the pathogenesis of Alzheimer disease. The amyloid precursor protein intracellular domain interacts with different target genes such as GSK3B, whose expression was also increased in amyloid precursor protein-overexpressing cells that, in turn, may contribute to promoting gliogenesis and inhibiting neurogenesis in neural stem cells (12). These data suggest an important action of amyloid precursor protein in modulating human neural stem cells differentiation and may thus be important for the future development of stem cell therapy strategies for central nervous system disorders.

Transdifferentiation stem cells from nonneuronal tissues into neurons. It is now also possible to differentiate human bone marrow stromal stem cells into neurons. Advantages of using bone marrow stem cells are:

	• These cells are obtained by a safe procedure, eliminating the high-risk operation of obtaining stem cells from the brain.
	• Success with adult stem cells obviates the need for fetal tissue, eliminating several ethical concerns.
	• The use of autologous bone marrow cells eliminates the introduction of foreign cells and the need for toxic immunosuppressive drugs to avoid rejection.
	• Rapid cellular growth and self-renewal in culture provides a virtually limitless source of cells and eliminates the need for immortalization and tumor-forming potential.

Adipose cells and fat cells taken from human liposuction procedures can be transformed into neurons after treatment with chemicals and growth factors. Further experiments will determine how the new cells react in a living system and if they will function like nerve cells. Their use is being explored in animal models of acute stroke and spinal cord injuries.

It is now known that postnatal human neuropoiesis can occur, and human hematopoietic cells can transdifferentiate into neurons, astrocytes, and microglia in a long-term setting without fusing. Transplantable human hematopoietic cells can serve as a therapeutic source for long-term regenerative neuropoiesis.

Because of their unique attributes of plasticity and accessibility, bone marrow-derived mesenchymal stem cells differentiate to a variety of functional neural cell types when transplanted into the CNS. They may find use for therapy of neurodegenerative disorders.

Mesenchymal stem cells. Although several studies have shown that transplantation of autologous mesenchymal stem cells results in clinical improvement in neurodegenerative disorders, the exact mechanisms responsible for the beneficial outcome have yet to be defined. Possible explanations include replacement of degenerated neural cells with functioning cells, trophic factors delivery, immunomodulation, and inhibition of chronic inflammatory stress by mesenchymal stem cells.

Induced pluripotent stem cells (iPSCs). Four factors (OCT4, SOX2, NANOG, and LIN28) have been shown to be sufficient for reprogramming human somatic cells to pluripotent stem cells that exhibit the essential characteristics of embryonic stem cells. These human iPSCs (hiPSCs) have normal karyotypes, express telomerase activity, express cell surface markers and genes that characterize human embryonic stem cells (hESCs), and maintain the developmental potential to differentiate into advanced derivatives of all 3 primary germ layers. iPSCs overcome the problems of lack of suitable sources of cells from donors as well as rejection. Derivation of neural stem cells from iPSCs has provided a new approach for regenerative therapy (26). Because iPSCs provide genetically matched neurons for treating patients, they fit in with personalized medicine. In addition to their use for direct treatment of diseases, they can also be used for disease modeling and drug screening.

Combined cell therapy. Each tested cell type proved to have its advantages and flaws and unique cellular and molecular mechanism of action, prompting the idea to test combined transplantation of 2 or more types of cells (combined cell therapy). Its successful application to the treatment of neurologic pathologies including stroke, spinal cord injury, neurodegenerative diseases, Duchenne muscular dystrophy, and retinal degeneration has been reported in both experimental and clinical studies. The advantages of combined cell therapy can be realized by simple summation of beneficial effects of different cells. Alternatively, one kind of cell can support the survival and functioning of the other by enhancing the formation of optimum environment or immunomodulation. No significant adverse events were reported. Combined cell therapy is a promising approach for the treatment of neurologic disorders, but further research needs to be conducted (36).

Indications

Table 2. Neurologic Disorders Amenable to Cell Therapy

Cerebrovascular disorders
	• Stroke
Neurodegenerative disorders
	• Alzheimer disease • Amyotrophic lateral sclerosis • Huntington disease • Macular degeneration • Parkinson disease
CNS injuries
	• Spinal cord injury • Traumatic brain injury
Neurotoxicity
	• Chemobrain
Miscellaneous neurologic disorders
	• Cerebral hypoxia: neuroprotection • Cerebral palsy • Chronic pain • Demyelinating disorders, eg, multiple sclerosis • Epilepsy • Lysosomal storage disorders • Muscular dystrophy • Syringomyelia

Contraindications

None identified.

Results

Cell therapy of amyotrophic lateral sclerosis. Various neurotrophic factors have been administered intrathecally using genetically engineered cells. Research into the possible use of stem cell therapy for amyotrophic lateral sclerosis is in progress. Intravenous administration of mononuclear human umbilical cord blood cells to a mouse model of amyotrophic lateral sclerosis has been shown to delay symptom progression and modestly prolonged lifespan. An appropriate dose of these cells may provide a neuroprotective effect for motor neurons through active involvement in modulating the host immune inflammatory system response.

Collection and reinfusion of peripheral blood stem cells mobilized by granulocyte colony stimulating factor in amyotrophic lateral sclerosis patients has found no adverse effects, paving the way for a properly powered therapeutic trial with an optimized regimen of granulocyte colony stimulating factor. An increase in the number of circulating stem cells from within the body’s bone marrow has the potential to travel to the site of injury and begin repair, slowing down the progression of amyotrophic lateral sclerosis.

Results of a phase I safety trial of human spinal neural stem cells on 12 patients with amyotrophic lateral sclerosis support the procedural safety of unilateral and bilateral intraspinal lumbar microinjections (46). Clinical assessments ranging from 6 to 18 months after transplantation demonstrated no evidence of acceleration of disease progression due to the intervention (17). Completion is planned by proceeding to cervical and combined cervical plus lumbar microinjections in patients with amyotrophic lateral sclerosis.

Cell therapy for Parkinson disease. Medications such as levodopa, surgery including deep brain stimulation, and rehabilitation are established as effective therapeutic strategies, but they are unable to stop the progression of Parkinson disease.

Parkinson disease was the first to be treated by cell therapy.

After several clinical studies, the use of fetal adrenal medullary cells has been abandoned. However, there are still many challenges to overcome before various other types of cell therapies can be introduced into clinical practice for Parkinson disease. Concern with safety of xenografts led to discontinuation of use of porcine cells. Transplantation of dopamine-producing cells alone is unlikely to alleviate the pathology related to the degeneration seen in non-nigrostriatal brain areas and with involvement of other neurotransmitter types. However, if a way can be found to reliably reproduce the symptomatic benefit seen in some fetal transplant studies, the availability of large numbers of differentiated stem cells may make such transplantation a desirable therapy. The current research is promising and indicates that this goal is indeed attainable.

A platform of core techniques for the production of midbrain dopamine (mDA) progenitors was created to establish human iPSC-based autologous cell therapy as a safe and effective therapeutic product (50). Thus, dopaminergic cells express high levels of characteristic mDA biomarkers, produce and secrete dopamine, and exhibit electrophysiological features typical of mDA cells. Transplantation of these cells into rodent models of Parkinson disease robustly restores motor function and reinnervates host brain, while showing no evidence of tumor formation or redistribution of the implanted cells. This platform is suitable for the successful implementation of human personalized autologous cell therapy for Parkinson disease.

Implantation of patient-derived midbrain dopaminergic progenitor cells, differentiated in vitro from autologous iPSCs, was carried out on the putamen of a patient with idiopathic Parkinson disease without the need for immunosuppression (48). PET with the use of fluorine-18-L-dihydroxyphenylalanine suggested graft survival. Clinical measures of symptoms of Parkinson disease stabilized or improved at 18 to 24 months after implantation.

Use of endogenous stem cells for treatment of Parkinson disease has the following advantages over other cell replacement therapies:

• Immunological reactions are avoided. • It is a less invasive method process than cell transplantation. • There are no ethical issues as in the case of embryonic stem cell transplantation.

Stem cell therapy may be a key for true neuroregeneration by promoting angiogenesis, neural circuit and synapse formation with release of neurotransmitters, and neuroprotection by antiinflammatory effect (58). As a cell source, autologous dopaminergic neurons from a patient’s iPSCs are ideal.

Cell transplants for Huntington disease. Huntington disease offers an excellent opportunity for cell replacement therapy because of the relatively focal cell loss in the striatum. Preliminary results from fetal striatal cell transplantation studies were encouraging in a few patients with Huntington disease, but there has been no confirmation of these results in a larger group of patients. Human fetal neurons usually integrate and survive after transplantation into the brain of a patient with Huntington disease, but there is no clinical improvement. Grafting of fetal neuroblast cells into a small number of patients with Huntington disease appeared to improve motor and cognitive function in some, but there is no confirmation of these results in a larger group of patients. Unilateral stereotaxic placements of cell suspensions of human fetal ganglionic eminence into the striatum of patients with early to moderate Huntington disease is considered safe and feasible, but efficacy has not been assessed in longer follow up of a larger number of patients.

A robust neuroprotective effect of transplantation of encapsulated pig choroid plexus was demonstrated in the rodent excitotoxic model of Huntington disease. The biocapsules act as an immune barrier, enabling the therapeutic cocktail of proteins produced by the cells to leave the capsule, but the body’s immune system cannot reject the cells, and, therefore, no immunosuppression is required in the treatment.

The predominant cells lost in Huntington disease are striatal medium spiny neurons (MSNs). Transplanted fetal medium spiny neurons have been shown to survive, integrate, and lead to functional recovery in both preclinical studies and in a few clinical trials (42). Because of the scarcity of human fetal tissue, alternative cell sources such as embryonic stem cells and adult induced pluripotent stem cells are being considered.

Cell therapy for Alzheimer disease. Genetically engineered cells producing nerve growth factor and implanted into basal cerebral structures have been shown to reverse cognitive changes associated with aging in rhesus monkeys. Aging is associated with a significant 25% reduction in cortical innervation by cholinergic systems in rhesus monkeys, and this was restored by nerve growth factor. These findings are relevant to clinical use of nerve growth factor in Alzheimer disease. A phase I clinical trial involving surgical implantation cells producing nerve growth factor into the brain of patients with Alzheimer disease was completed. The primary goal of this study was to determine that the procedure is safe. The secondary goal was to determine whether nerve growth factor produced by the cells implanted into the brain can prevent the death of some nerve cells that are affected by Alzheimer disease and whether it can enhance the function of certain remaining brain cells. During the study, the cells, which originally were extracted from the patient's skin, were genetically modified by adeno-associated viral vector to express nerve growth factor. The cells were then administered by stereotactic injection into the nucleus basalis of Meynert technique, which is more like gene therapy than cell therapy as a viral vector is used for in vivo production of nerve growth factor. This topic is discussed further in the article on gene therapy of neurodegenerative disorders.

Cell transplants for demyelinating disorders. Several transplantation strategies have been attempted for achieving remyelination in chronic demyelinating diseases such as multiple sclerosis. In the dysmyelinated mutant mouse model, the oligodendrocytes are "globally" dysfunctional because they lack the myelin basic protein that is essential for effective myelination. Treatment of this, therefore, requires widespread replacement with myelin basic protein-expressing oligodendrocytes. Among neurologic diseases potentially approachable by cell therapy, myelin disorders, including the pediatric leukodystrophies as well as adult traumatic and inflammatory demyelinations, may present the most compelling targets for cell-based therapy.

The olfactory ensheathing cells can remyelinate demyelinated axons and support regeneration of damaged axons in experimental animals. The human olfactory ensheathing cell represents an important new cell for the development of transplant therapy for demyelinating disorders.

Implantation of neural stem cells into spinal cords of mice with established demyelination results in migration, proliferation, and differentiation of the cells into mature oligodendrocytes, which is associated with increased axonal remyelination. A genetically engineered mouse macrophage using a human gene that expresses the sodium channel NaVI.5 enhances recovery from experimental autoimmune encephalomyelitis, and could potentially be developed as a cell-based therapy for the treatment of multiple sclerosis (43). A study in mice has shown that transplanted iNSCs (induced neural stem cells reprogrammed from skin cells) into the central nervous system reduce inflammation and may help to repair the damage caused by multiple sclerosis (40). This is a step towards developing personalized treatment of progressive forms of multiple sclerosis, based on a patient’s own skin cells.

A long-term study of high-dose immunosuppressive therapy followed by autologous hematopoietic cell transplantation in advanced multiple sclerosis showed that a significant proportion of patients remained stable for as long as 7 years after transplant (08). Safety and feasibility of intravenous, autologous bone marrow cell therapy, without immunosuppressive preconditioning, was tested in a phase I study in 6 patients with clinically definite, relapsing-progressive multiple sclerosis (45). The therapy was well-tolerated. Over a period of 1 year after the therapy, clinical disability scores showed little or no improvement, but there was improvement of multimodal evoked potentials. Hematopoietic stem cell transplantation in infantile form of Krabbe disease was carried out in patients and led to improvement in neurodevelopment, but there are no further reports for this application.

Imilecleucel-T is a T cell-based personalized autologous immunotherapy. It consists of attenuated, patient-specific myelin reactive T-cells against peptides of the 3 primary myelin proteins: myelin basic protein, myelin oligodendrocyte glycoprotein, and proteolipid protein that have been implicated in T cell pathogenesis of multiple sclerosis. Prior to use, the myelin reactive T-cells are expanded, formulated, and attenuated (by irradiation) to render them unable to replicate but viable for therapy. These attenuated T cells are administered in a defined schedule of 5 subcutaneous injections. Patients are expected be treated with a new vaccine series each year based on their altered disease profile or epitope shift. Imilecleucel-T is in phase IIb trials for secondary progressive multiple sclerosis. Phase IIb trials for relapsing remitting multiple sclerosis showed clinical benefit that was not accompanied by improvement in MRI measures (16). Phase III clinical trials in relapsing remitting multiple sclerosis are planned.

Cell transplants for spinal cord injury. Considerable advances have been made in the last decade in devising and evaluating axon regeneration strategies for spinal cord trauma patients. Thus, several studies have established that transplantation of glial cells can have beneficial consequences in experimental models of spinal cord trauma and demyelination.

T cells. Autoimmune T cells against CNS myelin-associated peptide have a neuroprotective effect in experimental models; that is, they reduce the spread of damage and promote recovery in injured rat spinal cord.

Transplantation of glial cells. Following acute damage to the CNS by trauma or ischemia, the glial cells and neurons die, which leads to a breakdown in the integrity of the glial environment; this limits regeneration. Glia-depleted areas of the CNS can be reconstituted by the introduction of cultured cells. When introduced into infarcted white matter in the spinal cord, progenitor-derived astrocytes fill the damaged area more effectively than tissue-culture astrocytes, although their axons do not regenerate into the reconstituted areas. However, they promote revascularization and attenuate scarring that is an impediment to regeneration.

Fetal neural grafts. Studies of intraspinal transplantation in experimental animals reveal that solid and suspension grafts of fetal tissues from several areas of neuraxis survive after placement into the spinal cord. Moreover, these grafts mature, integrate with host's CNS, enhance the growth of specific systems from the host, and enhance recovery of function. It is questionable whether fetal cell transplantation would become an important method for the treatment of human spinal cord injury. Ethical and procedural problems associated with procuring human fetal tissues may be formidable.

Schwann cell transplants. To promote growth and remyelination, Schwann cells (the peripheral nervous system equivalent of oligodendrocytes) have been transplanted. Unfortunately, Schwann cells and oligodendrocytes erect an inhibitory barrier when they encounter each other.

Olfactory glial cells. A potential therapeutic superiority of olfactory glia over Schwann cells has been demonstrated by experiments in which the number of regenerating axons crossing a transection site was dramatically increased when olfactory ensheathing glia were placed at the interface between a Schwann-cell-filled guidance tube and the damaged spinal cord.

Embryonic stem cells. Several studies have shown that transplantation of embryonic stem cells to replace the lost neurons and other supporting cells into adult rats that were partially paralyzed through spinal cord damage led to some recovery of spinal cord function. Introduction of stem cells into the cerebrospinal fluid via lumbar puncture is as effective as direct injection into the spinal cord. In clinical trials, subarachnoid placement of stem cells was found to be safe with no long-term adverse effects.

Bone marrow stem cells. Transplantation of autologous bone marrow cells has been carried out safely in patients with chronic complete spinal cord injury, producing slight neurologic improvement.

Numerous experimental studies have explored the use of stem cells for repair of spinal cord injuries, but the hope of curing paraplegia in humans has not yet been realized. Because of the complex train of events following injury to the spinal cord, it is difficult to evaluate any treatment. It is difficult to exclude factors contributing to partial recovery in some instances due to factors such as spared fibers after transaction, resolution of processes such as edema and transient channelopathies, and neuroplasticity. Important questions that need to be asked for assessing such studies include those about assessment of results in a blinded fashion and whether follow-up was done for at least 4 months after the intervention (49).

Mesenchymal stem cells. Long-term results of 10 patients who underwent autologous intramedullary direct mesenchymal stem cell (MSC) transplantation by direct multiple injections into injured spinal cords showed that in 3 of these there was improvement in the motor power of the upper extremities and in activities of daily living, as well as significant MRI and electrophysiological changes during 6-month follow-up (39). None of the patients experienced any permanent complication associated with MSC transplantation. The same group carried out a phase 3 trial on 12 patients with chronic cervical spinal cord injury who received single autologous MSC injection in the intramedullary space at level of injury as well as in the intradural space (37). Only 2 of these patients showed slight neurologic improvement, which was less than that obtained in an earlier study using multiple injections, but diffusion tensor imaging showed that continuity of the injured segment was restored. Further clinical studies are required to determine the optimal protocol for use of MSCs for spinal cord injury.

Clinical trials of cell therapy for spinal cord injury. A review of cell therapies in clinical trials for spinal cord injury and a summary of 37 published trials show that most of registered trials have used autologous cells, and approximately a third have been government funded, a third were sponsored by industry, and a third funded by university or healthcare systems (04). A smaller number have used Schwann cells or olfactory ensheathing cells. Significant challenges remain for cell therapy trials in achieving stringent regulatory standards, ensuring appropriately powered efficacy trials, and establishing sustainable long-term funding. Nevertheless, cell therapies hold great promise for human spinal cord repair.

Traumatic brain injury. Bone marrow transplantation to the injured cerebral cortex in rat models of traumatic brain injury has been shown to improve neurologic outcome and has demonstrated that bone marrow cells can survive following transplantation. This approach has the potential for clinical application in traumatic brain injury. A clinical trial is assessing the safety and potential of marrow-derived autologous hematopoietic stem cells for treating children with traumatic brain injury (20).

Stem cell-based cellular replacement strategies have a potentially therapeutic role following traumatic brain injury, but the mechanism by which stem cells produce their effect, eg, via integration into surviving neuronal circuits, local neurotrophic support, or modification of the local microenvironment to enhance endogenous regeneration and neuroprotection, is under investigation. Enhanced cell survival as well as improvement in functional outcome was demonstrated following transplantation of a neural stem cell construct containing laminin-based scaffold into the traumatically injured mouse brain (53). Laminin, an extracellular matrix protein, improves the results of neural cell transplant therapy. Cell therapy is promising for the treatment of traumatic brain injury, because it can target multiple pathomechanisms in a sustained manner with repair of damaged tissues as well as prolonged release of neuroprotective substances (23). An experimental study has shown that GABAergic progenitors, derived from the embryonic medial ganglionic eminence, migrate long distances following transplantation into the hippocampus of adult mice with traumatic brain injury to incorporate into injured brain circuits, and this approach has the potential for correcting posttraumatic memory and seizure disorders (59).

Stroke. Grafted CNS stem cells can survive and differentiate into mature CNS tissue within the adult ischemic rat brain with vascularization in and around the grafts. This approach, therefore, has the potential to be effective in the recovery of function in cerebral ischemia.

A key secondary cell death mechanism mediating neurologic damage following the initial episode of ischemic stroke is the upregulation of endogenous neuroinflammatory processes to levels that destroy hypoxic tissue local to the area of insult, induce apoptosis, and initiate a feedback loop of inflammatory cascades that can expand the region of damage. Accumulating evidence supports the therapeutic efficacy of stem cells to abrogate stroke-induced inflammation. Clinically relevant stem cell types, such as HSCs, MSCs, NSCs, hiPSCs, very small hESCs, and stem cells derived from other adult tissues have been investigated for their efficacy in limiting stroke-induced neuroinflammation and the feasibility of their translation into clinical applications (52). Mesenchymal stem cells, with a proven track record of safety and efficacy as a transplantable cell for hematologic diseases, are an attractive cell type that confers superior antiinflammatory effects in stroke both in vitro and in vivo. This has neuroprotective effect in the acute stage in stroke and complements the regenerative processes of cell replacement and neurotrophic factor secretion in the chronic poststroke stage.

Neural stem cells injected into mice brains subjected to focal hypoxic-ischemic injury integrate appropriately into the region of ischemic injury and foreign gene expression. They migrate preferentially to the site of ischemia and differentiate into neural cells lost to injury, trying to repopulate the damaged brain area. The transplantation of exogenous neural stem cells augments a natural self-repair process in which the damaged CNS attempts to mobilize its own pool of stem cells. Providing additional neural stem cells and trophic factors may optimize this response. Therefore, neural stem cells may provide a novel approach to reconstituting brains damaged by hypoxia-ischemia. Human neural stem cells aid recovery after stroke through secretion of factors that enhance brain repair and plasticity (03). The cells likely do not replace the lost brain cells but rather produce chemicals that activate repair pathways resulting in the formation of new blood vessels and new cells.

Functional improvement can occur months after stem cell transplantation when the grafted cells have disappeared without histological evidence of replacement of the infarcted tissue, and this has been attributed to paracrine effect of stem cells (44). Some studies report improvements in neurologic function with cell implantation even when undertaken up to 1 month after the stroke is induced (27). In an open study, autologous human mesenchymal stem cells, expanded in autologous human serum, were injected intravenously in patients with ischemic stroke more than 4 months following onset, but lesion volume was still reduced by more than 20% at 1 week post-cell infusion as assessed by MRI (21).

Stem cell transplantation can improve behavioral recovery after stroke in animal models, and an experimental study has shown extensive morphological and functional integration of axonal projections from intracortically transplanted human iPSC-derived cortical neurons in brains of rats with ischemic lesions in the cerebral cortex (38). Results also demonstrated that neuronal activity in these grafts is involved in the regulation of the stroke-affected animals’ motor behavior. These findings raise the possibility that injured neural circuitry might be restored by stem cells also in humans affected by stroke, which would have major clinical implications. Although cell therapy for stroke is promising, brain repair for stroke is likely to require some form of combination therapy to facilitate functional recovery. Following recanalization of larger arteries by thrombolytic therapy, cell therapy can target residual areas of ischemia due to small vessel atherosclerosis, reduce cell death, and facilitate neuronal plasticity as well as regeneration.

Endogenous stem cell therapy exploits the presence of adult stem cells already present in the CNS or the hematopoietic system. In the exogenous approach, CNS or hematopoietic system-derived stem cells are administered locally or systemically after purification and propagation in culture. An increasing number of studies provide evidence that hematopoietic stem cells, either after stimulation of endogenous stem cell pools or after exogenous hematopoietic stem cell application (transplantation), improve functional outcome after ischemic brain lesions. Several preclinical studies employing adult stem cell-based strategies hold great promise.

Although several clinical studies have shown the high efficiency and safety of stem cells in stroke management, mainly mesenchymal stem cells, some issues regarding cell homing, survival, tracking, safety, and optimal cell transplantation protocol, such as cell dose and time window, need to be addressed. Stem cell-based gene therapy represents a novel potential therapeutic strategy for stroke in the future.

Management of chronic stroke in the aged is a challenge. The aged are a poor source of autologous stem cells, and only a few studies of allogeneic stem cell transplantation have been performed in aged organisms. Potential therapies include stem cells combined with growth factors delivered together with biomaterials that can protect these active components in a hostile and aging environment. However, there is a need for understanding the complex interactions of stem cell grafts with the ischemic brain and how they may be affected by the route, timing of cell delivery, and associated comorbidities in the aged patient (41).

Experimental studies have shown that exposure to hyperbaric oxygen enhances the therapeutic effect of stem cell therapy. A neonatal rat with hypoxic-ischemic brain damage was given hyperbaric oxygen at a pressure of 2 atmospheres absolute (ATA) once daily for 1 week, and the results of brain examination showed that neural stem cells migrated to the cortex and differentiated into mature neurocytes (57). Hyperbaric oxygen has been claimed to improve the results of autogenous stem cell therapy in stroke patients, but no controlled clinical trials have been conducted to prove it.

A study has pointed out the potential problems regarding translating cell therapy clinical trials in stroke into clinical practice (10). Burns and Steinberg pointed out that, given the inherent heterogeneous nature of stroke, clinical trials may lack adequate power. Some trials are based on preclinical data from animal models that do not represent human disease adequately.

A 2-year, open-label, phase 1a/2a single-arm study on 18 chronic stroke patients evaluated the safety and clinical outcomes of surgical transplantation of modified bone marrow-derived mesenchymal stem cells. In the 12-month interim report, stem cells were safe and associated with improvement in clinical outcome end points (51).

Epilepsy. Lesions of the brain with epilepsy are potential targets for cell therapy. Embryonal cells transplants may restore some of the abnormal circuitry in experimental animals but the supply is limited for potential human applications. Human adipose tissue-derived stem cell extracts have been shown to inhibit epileptogenesis and spontaneous recurrent seizures in mice, suggesting the potential of a stem cell-based, noninvasive therapy for the treatment of epilepsy (25).

Human induced pluripotent stem cells (iPSCs) have opened the possibility of targeted cell-based therapies by deriving patient-specific neural tissue in vitro that may ultimately be used for transplantation. Genome editing provided potential application iPSC disease modeling and development of patient-specific therapies for genetic epilepsies in the future (13).

A study in rat model of temporal lobe epilepsy has shown that human-induced pluripotent stem cell-derived medial ganglionic eminence (MGE) cell grafting into the hippocampus after status epilepticus can greatly reduce the frequency of spontaneous seizures in addition to improving cognitive and mood disturbances in the chronic phase through both antiepileptogenic as well as antiepileptic effects (56). Graft-derived cells survived, migrated into different areas of the hippocampus, and differentiated into distinct subclasses of inhibitory interneurons. The results support a patient specific allogeneic medial ganglionic eminence cell grafting approach for treating temporal lobe epilepsy.

Chronic pain. Cell therapy has been explored, not only for improved delivery of analgesic molecules, but also for disease-modifying possibilities to correct the underlying pathology that manifests as pain (22).

The goal of the therapy is to administer the cells to the desirable target location in the central nervous system in sufficient number for them to survive and restore the disturbed function. Cells are also used to secrete neuroprotective substances and promote regeneration of the damaged part of the nervous system. Cells can also be used to enhance tissue engineering of the brain in combination with structural supports. Combining the use of scaffolds with extracellular matrix molecules may improve the control of cell proliferation, survival, migration, differentiation and engraftment in vivo.

Cell therapy for chemobrain. The frequent use of chemotherapy to combat cancer produces severe cognitive dysfunction often referred to as "chemobrain," a condition that can persist long after the cessation of treatment in as many as 75% of survivors. Many chemotherapeutic agents used to treat cancer trigger inflammation in the hippocampus, a cerebral region responsible for many cognitive abilities, such as learning and memory. This inflammation can destroy neurons and other cell types in the brain. These toxic compounds also damage the connective structure of neurons and dendrites as well as axons and alter the integrity of synapses. As a result, the affected neurons are less able to transmit important neural messages that underpin learning and memory.

Using a rodent model of chemobrain, a study showed that chronic cyclophosphamide treatment induced significant performance-based decrements on behavioral tasks designed to interrogate hippocampal and cortical function, but intrahippocampal transplantation of human neural stem cells resolved all cognitive impairments when animals were tested 1 month after the cessation of chemotherapy (01). Neural stem cells migrated throughout the hippocampus, where they survived and differentiated into multiple neural cell types. They also triggered the secretion of neurotrophic growth factors that helped rebuild damaged neurons. The engrafted cells protected the host neurons, thereby preventing the loss or promoting the repair of damaged neurons and their finer structural elements, referred to as dendritic spines. These experiments provide the first evidence that cerebral transplantation of stem cells can reverse the deleterious effects of chemotherapy through a trophic support mechanism involving the attenuation of neuroinflammation and the preservation host neuronal architecture. This research suggests that stem cell therapies may eventually be implemented in the clinic to provide relief to patients suffering from cognitive impairment resulting from cancer treatments.

Clinical trials. As of October 2021, the United States National Institutes for Health website for clinical trials (clinicaltrials.gov) lists approximately 2434 planned, ongoing, suspended, or completed clinical trials with cell-based therapies in neurologic disorders. Most of these involve brain tumors (803); the rest include multiple sclerosis (227), stroke (94), spinal cord injury (94), traumatic brain injury (37), Parkinson disease (78), Alzheimer disease (69), muscular dystrophy (29), and amyotrophic lateral sclerosis (82). The rest are for miscellaneous neurologic disorders, including neurogenetic syndromes. Selected clinical trials for neurologic disorders are listed in Table 3. Stem cells are used in most of these trials.

Table 3. Clinical Trials with Cell-Based Therapies in Neurologic Disorders

Disease	Types of cells	Trial status	Sponsors
Alzheimer disease	Implantation of genetically engineered cells using adeno-associated viral vector to produce NGF in the brain	Phase II	Sangamo Biosciences/ University of California San Diego, CA
Alzheimer disease	Implantation of encapsulated cells producing NGF in specific areas of the brain with a device	Phase Ib	NsGene/Karolinska Hospital, Stockholm
Alzheimer disease	Implantation of autologous adult adipose stem cells	Phase I/II	Healeon Medical Inc/ Regeneris Medical Inc
Amyotrophic lateral sclerosis	Ex vivo expansion of autologous mesenchymal stem cells and of transplantation into the spinal cord of humans	Exploratory trial	University of Torino, Italy
Amyotrophic lateral sclerosis	Reinfusion of peripheral blood stem cells mobilized by granulocyte colony stimulating factor was shown to be safe	Pilot phase I	University of British Columbia, Canada
Amyotrophic lateral sclerosis	Injection of human NSCs into spinal cord	Phase I	Emery University, Michigan University
Amyotrophic lateral sclerosis	NurOwn: autologous bone marrow derived MSCs Intrathecal and intramuscular injections	Phase II	BrainStorm Cell Therapy Inc/ Israel and U.S.
Amyotrophic lateral sclerosis	Intrathecal and intramuscular injections of autologous bone marrow stem cells	Phase II completed	Neurogen Brain & Spine Institute Mumbai, India
Autism	Infusion of stem cells derived from autologous banked umbilical cord blood	Phase II	Sutter Pediatric Neurology Sacramento, CA
Batten disease (a neuronal ceroid lipofuscinosis)	Implantation of fetal neural human stem cells -- HuCNS-SC™ (StemCells Inc) into the brain	Phase I completed	Oregon Health and Science University Portland, OR
Cerebral palsy	Infusion of autologous UCB banked stem cells	Phase I	Georgia Health Sciences University Augusta, GA
Duchenne muscular dystrophy	Myoblast transplantation	Phase I	Laval University Quebec, Canada
Facioscapulo-humeral muscular dystrophy	Myoblast injection into muscles	Phase I	Myosix
Macular dystrophy (Stargardt)	Human embryonic stem cell-derived retinal pigment epithelial cells are surgically implanted subretinally	Phase II	Advanced Cell Technology
Multiple sclerosis: secondary progressive form	Hematopoietic stem cell transplantation	Phase I/II	General Hospital Thessaloniki, Greece
Multiple sclerosis with optic neuritis	Intravenous administration of bone marrow-derived autologous adult human mesenchymal stem cells	Phase I/II	Cambridge University Hospital, UK
Multiple sclerosis: relapsing-progressive	Autologous bone marrow stem cell (45)	Phase I	University of Bristol, UK
Multiple sclerosis: relapsing-remitting	Imilecleucel-T, a personalized autologous T cell-based vaccine	Phase III	Opexa Therapeutics
Oculopharyngeal muscular dystrophy	Myoblast injection into muscles	Phase IIb	Myosix
Parkinson disease (advanced)	Autologous transplantation of human neural stem cell-derived dopaminergic cells into the striatal region of patients. Functional metabolism to be evaluated with 18F Dopa PET imaging	Phase II prospective trial (on hold)	NeuroGeneration
Pelizaeus-Merzbacher disease	Implantation of HuCNS-SC™ in the brain	Phase I	Stem Cell Inc /UCSF San Francisco, CA
Retinitis pigmentosa	NT-501, based on encapsulated cell technology, secretes a therapeutic neurotrophic factor	Phase II	Neurotech SA
Spinal cord injury, acute	ProCord: autologous activated macrophages, administered within 14 days of injury	Phase II further enrollment suspended	Proneuron Biotechnologies
Spinal cord injury, acute	"Self-repair kit" using adult stem cells	Phase I	St Vincent's Hospital Melbourne, Australia
Spinal cord injury, acute	Lithium to stimulate regeneration in phase I In addition stem cells extracted from matching UCB, are injected into the spinal in phase II	Phase I Phase II	Hong Kong University Mainland Chinese cities, and Taipei
Spinal cord injury, chronic	Delivery of autologous bone marrow precursor cells labeled with magnetic nanoparticles and tracked with MRI	Phase I	São José Dos Campos–São Paulo, Brazil
Spinal cord injury, chronic	Implantation of HuCNS-SC™ in the spinal cord	Phase I/II	Stem Cell Inc/ Balgrist Hospital Zürich, Switzerland
Spinal cord injury, thoracic paraplegia	Transplantation of autologous olfactory ensheathing cells into the spinal cord is feasible and is safe up to 3 years post-implantation (33)	Phase I/II completed	Griffith University Brisbane, Australia
Spinal cord injury, chronic, cervical	Intramedullary injection of autologous MSCs	Phase III completed	University of Ulsan Seoul, Korea
Stroke: acute ischemic stroke	ReN001: CTX neural stem cells injected into the damaged brain	Phase II	ReNeuron
Stroke: acute ischemic stroke	Systemic administration of NTx™-265 (human chorionic gonadotropin and epoetin alfa) to stimulate the growth and differentiation of new neurons to replace those damaged by the stroke	Phase IIb	Stem Cell Therapeutics
Stroke: acute ischemic stroke	Peripheral intravenous administration of autologous mononuclear bone marrow cells	Phase I	University of Texas Health Science Center, Houston
Stroke: acute ischemic stroke	A single intravenous dose of allogeneic mesenchymal bone marrow cells	Phase I/II	Stemedica Cell Technologies
Stroke: subacute ischemic	Autologous mesenchymal stem cells within 2 weeks after onset of stroke	Phase II	University Hospital Grenoble, France
Stroke: acute stroke due to middle cerebral artery occlusion	Infusion of autologous CD34+ stem cells into middle cerebral artery between days 5 and 9 following stroke onset	Phase I/II	Hospital Universitario Central de Asturias, Spain
Stroke: cerebral embolism	Intravenous autologous bone marrow mononuclear cell transplantation	Phase I/II Ongoing	National Cardiovascular Center, Japan
Stroke: chronic stable ischemic stroke	Modified stromal cells (SB623) implanted into the brain	Phase I/II	SanBio Inc
Syringomyelia	Fetal brain tissue transplantation into the spinal cord	Phase II	University of Florida Gainesville, FL
Traumatic brain injury (TBI)	Autologous bone marrow-derived mononuclear cells infused intravenously in children with TBI	Phase I	University of Texas Houston, TX
Traumatic brain injury	Infusion of stem cells derived from autologous banked UCB	Phase I	Hermann Hospital University of Texas

Although modest improvements have been observed in many patients, these were not sufficient to warrant invasive and possibly risky cell therapy. Therefore, a better understanding of the mechanisms of action is needed. Transplanted cells can be tracked by using special labels and MRI. Labeling of human neural stem cells grown as neurospheres with magnetic nanoparticles was shown to not adversely affect survival, migration, and differentiation or alter neuronal electrophysiological characteristics. Noninvasive cellular imaging has great potential for neurotransplantation as it enables real-time tracking of grafted cells as well as monitoring biodistribution and development.

Criteria for selecting stem cell therapy for neurologic disorders. Autologous hematopoietic stem cells are used in routine patient care in oncology and hematology for specific problems. Embryonic stem cells remain under investigation, and there are ethical restraints. Most of the stem cell therapies for neurologic disorders in the United States are carried out in clinical trials as there is no approved commercial product for neurologic disorders. These procedures are, however, available in some foreign countries.

Inclusion and exclusion criteria are laid out for each clinical trial. Most patients referred for stem cell therapy are those with chronic problems for which conventional therapy is not available or has not been effective. Examples include chronic stroke and spinal cord injury with fixed neurologic deficits, and the aim is to explore the possibility of regeneration. Acute stroke patients may be those who have not qualified for established therapies, and stem cell transplantation may be used in acute spinal cord injury as a combined regenerative/neuroprotective disease-modifying approach.

Stem cell tourism involving patients with neurologic disorders. Evaluation of stem cell products outside of the United States is overshadowed by stem cell tourism, which is a well-known phenomenon and has been the object of intense scrutiny in recent years following reports of charlatanry, baseless claims, and adverse medical events. Providers of stem cell–based interventions vary widely in their assertions about the conditions that can be treated, the degree of improvement, and the cell types and protocols used, but there are many advertisements for medical procedures that have never been proven efficacious in appropriately designed clinical trials. Inaction and occasional complicity on the part of the government and medical establishments in some countries, however, have made enforcement, self-policing, and the maintenance of quality standards problematic. Currently, stem cell tourists head for China, Thailand, Panama, and Russia. Cell therapy is a significant part of the medical tourism industry, and Americans were the largest segment of medical tourists.

Conditions for which stem cell therapies are carried out include hemiplegia due to cerebral infarction and paraplegia due to spinal cord injury. A patient developed low back pain, paraplegia, and urinary incontinence following intrathecal infusions of stem cells for the treatment of residual deficits from an ischemic stroke at commercial stem cell clinics in China, Argentina, and Mexico (05). MRI showed a lesion of the thoracic spinal cord and thecal sac, and biopsy specimen of the lesion revealed a densely cellular, highly proliferative, primitive tumor with glial differentiation, which originated from the intrathecally introduced exogenous stem cells and did not fit in any category of previously described human neoplasms. Short tandem repeat DNA fingerprinting analysis indicated that the lesion was made up of nonhost cells. Radiation therapy led to partial relief of back pain, improved mobility of the right leg, and decreased the size of the lesion on MRI.

Adverse effects

With the exception of autologous transplants, rejection is the main adverse event with cell transplants. Implanted stem cells may be tumorigenic. Development of a donor stem cell-derived glioneural brain tumor has been reported in a patient affected by the ataxia telangiectasia 4 years following repeated transplantations of fetal neural stem cells (02). Molecular and cytogenetic studies showed that the tumor was of nonhost origin, suggesting it was derived from the transplanted cells.

It is generally believed that iPSCs are unlikely to have tumorigenic potential. However, comparisons of the in vitro neural differentiation of hiPSCs and hESCs showed that some hiPSC clones retained a significant number of undifferentiated cells, even after neural differentiation, and formed teratoma when transplanted into mouse brains (29). These differentiation-defective hiPSC clones were marked by higher expression levels of several genes, including those expressed from long terminal repeats of specific human endogenous retroviruses. These need to be identified and eliminated before applications in regenerative medicine.

Special considerations

Age. Cell therapy has been used in children as well as the elderly without any increase of risk due to age.

Pregnancy. Normal pregnancy has been reported after successful stem cell therapy in women with cancer. Considering the wide variety of cells used as well as the conditions for which they are used, decisions about each pregnant patient should be made on an individual basis. For example, allogeneic stem cell therapy carries the risk of graft-versus-host reaction, which may be harmful if it occurs during the pregnancy.

Anesthesia. Anesthesia is used during neurosurgical procedures for transplantation of cells in the brain, and there are no special anesthetic precautions

Scientific basis

• The effect of cell therapy varies in neurologic disorders where regeneration and repair is required.

The beneficial effect of cell therapy is explained by 2 mechanisms:

(1) A diffusible factor within the transplanted tissue (2) Establishment of neural connections between the transplanted tissue and the host

The effect of cell therapy varies in neurologic disorders where regeneration and repair is required.

Rationale of cell therapy for neurologic disorders. Many diseases of CNS, particularly those of genetic, metabolic, or infectious and inflammatory etiology, are characterized by "global" neural degeneration or dysfunction. Therapy might require widespread neural cell replacement, a challenge not regarded conventionally as amenable to neural transplantation. Mouse mutants characterized by CNS-wide white-matter disease provide ideal models for testing the hypothesis that neural stem cell transplantation might compensate for defective neural cell types in neuropathologies requiring cell replacement throughout the brain.

Relevant basis for each type of cell therapy was discussed along with applications in the previous section.