Hematopoietic stem cells (HSC) have historically been used to treat deadly blood disorders. The first report of allogeneic infusion of HSC in human malignancies dates to 1957 (Donnall Thomas et al., 1957) and since then, the field of HSC transplantation (HSCT) has evolved tremendously. In the modern era, HSCT is the only available stem cell therapy with demonstrated robust clinical benefits for the treatment of human diseases. Several non-hematologic conditions are currently treated with HSCT, including nervous system disorders. Autologous and allogeneic hematopoietic stem cell transplantation (AHSCT and Allo-HSCT, respectively) are successful approaches for the treatment of specific neurological conditions.

AHSCT is able to arrest neuroinflammation in diseases such as Multiple Sclerosis, Neuromyelitis Optica Spectrum Disorder and Myasthenia Gravis. However, clinical benefits result mainly from the peripheral immune reset induced by the conditioning regimen rather than the direct interaction between HSC and the nervous system (Sharrack et al., 2020). Despite the potential benefits of the graft-vs.-autoreactivity effect and self-tolerant immune reconstitution (van Wijmeersch et al., 2007), Allo-HSCT has been seldomly used in auto-immune neurological disorders due to a high risk of treatment-related mortality (Greco et al., 2019; Sharrack et al., 2020). Clinical indications of Allo-HSCT for the treatment of neurological conditions are mostly restricted to Inborn Errors of Metabolism (IEM), a large spectrum of congenital multisystemic diseases characterized by specific enzyme deficiencies involved in metabolic pathways. IEM presenting with progressive inflammatory demyelination or neurodegeneration such as Hurler Syndrome, Metachromatic Leukodystrophy, Krabbe’s Disease and X-Linked Adrenoleukodystrophy benefit from Allo-HSCT before high central nervous system (CNS) disease burden is present (Tan et al., 2019). Similar to other disease settings, the therapeutic success of HSCT in IEM has been experienced before clear biological underpinnings were elucidated. In this setting, Allo-HSCT has been shown to halt and even reverse CNS injury through two main mechanisms that occur after HSC migration to the brain parenchyma: (1) local replenishment of the lacking enzyme through cross-correction of neighboring neural cells and (2) restoration of defective microglia function by donor-derived myeloid cells (Ferrante et al., 1971; Krivit et al., 1995; Capotondo et al., 2012; Wolf et al., 2020).

The need for a compatible donor and complications such as graft-vs.-host disease are current limitations of Allo-HSCT. They have paved the way for the development and application of Hematopoietic Stem Cell Gene Therapy (HSC-GT) in IEM with neurological involvement. HSC-GT relies on the use of autologous HSC that are manipulated ex vivo to overexpress the missing enzyme through transduction with lentiviral vectors encoding the transgene of interest. Infusion of these manipulated HSC results in supraphysiological delivery of the lacking protein. In this manner, autologous HSC are used as cellular vehicles of the missing enzyme to affected tissues such as the brain, enabling metabolic correction, clearance of storage materials, and ultimately repair. Currently, HSC-GT is emerging as an alternative to Allo-HSCT in the treatment of certain IEM such as X-linked Adrenoleukodystrophy, Hurler Syndrome, and Metachromatic Leukodystrophy (Eichler et al., 2017; Gentner et al., 2021; Fumagalli et al., 2022).

In this review, we highlight the current clinical use of HSCT in the treatment of central nervous system diseases and the underlying therapeutic mechanisms. Firstly, the principles of HSCT are introduced, followed by a brief exploration of the role of HSCT in immune-mediated neurological diseases. Then we will focus on Inborn Errors of Metabolism with CNS involvement. Further details are presented on the clinical manifestations, neuropathology and therapeutic pathways in the specific cases of Hurler Syndrome, Metachromatic Leukodystrophy, Krabbe’s Disease, and X-linked Adrenoleukodystrophy. Finally, we summarize recent data on Hematopoietic Stem Cell Gene Therapy as a cellular drug vehicle to the brain. This review aims to detail brain repair mechanisms in neurological conditions treated with HSCT and highlight how current benefits and limitations of this approach can be used as a model for future developments in the field.

Autologous Hematopoietic Stem Cell Transplantation is a type of transplant procedure characterized by three steps: (1) mobilization and harvest of autologous HSC, (2) administration of a high-dose immunosuppressive conditioning regimen, and (3) infusion of HSC (Carreras et al., 2019). The clinical benefits of AHSCT derive exclusively from the immune-ablative properties of the conditioning regimen (Sharrack et al., 2020). Despite being administered with the intent to halt the inflammation driving the underlying disorder, high dose treatment with chemotherapy and other immunosuppressive agents results in a protracted bone marrow failure state. Collected HSC are then reinfused to rescue bone marrow hematopoietic recovery in the ablated patient (Carreras et al., 2019). Immune reconstitution is ideally re-conditioned toward self-tolerance, which is further beneficial if autoimmunity is present (Cencioni et al., 2022). Importantly, HSC only differentiate into hematopoietic cells in this context and do not seem to participate in tissue repair outside of the bone marrow compartment.

Allogeneic Hematopoietic Stem Cell Transplantation is a procedure characterized by the infusion of donor-derived HSC to recipients previously treated with high-intensity conditioning regimens. After high dose chemotherapy and immunosuppression, donor-derived HSC home to and engraft in the bone marrow of the recipient and ideally reconstitute as a new and healthy lymphohematopoietic system. In contrast to AHSCT, where the donor and recipient are the same, genetic disparities must be overcome to prevent graft rejection. Accordingly, the conditioning regimens used are more intensive than in AHSCT, which contributes to a more efficient disease eradication in the context of hematologic malignancies. Also, post-transplant immunosuppressive prophylaxis is mandatory to prevent potentially life-threatening donor-to-recipient alloreactivity—graft-vs.-host disease (GvHD). Toxicity and treatment-related mortality are much more significant than AHSCT, and careful patient and donor selection are of utmost importance (Carreras et al., 2019). This type of HSCT has been used since the 50s to treat deadly blood cancers such as acute leukemia (Donnall Thomas et al., 1957). Currently, the clinical indications of Allo-HSCT extend much beyond hematologic malignancies and certain nervous system conditions do benefit from this treatment modality.

Immune-mediated Neurological Diseases comprise a vast group of auto-immune neuroinflammatory conditions characterized by loss of self-tolerance in the nervous system compartment. These conditions can result in devastating clinical consequences to affected patients and most treatment modalities are based on strategies to suppress or modulate the dysfunctional immune system. In this section, we will focus on the role of HSCT in Multiple Sclerosis and other immune-mediated neurological diseases.

Inborn Errors of Metabolism comprise a heterogeneous spectrum of rare congenital multisystem disorders characterized by defects in genes encoding enzymes involved in intracellular metabolic pathways. IEM may not appear at first the obvious paradigm of CNS disorders since the affected genes are expressed ubiquitously. However, specific brain components such as myelin sheaths and the resident phagocytic cells microglia are substantially impaired by the accumulation of improperly degraded substrates. In this setting, the ability of HSC to migrate to the CNS and modulate its cellular composition and enzymatic secretome have been explored as therapeutic “Trojan Horses” or cellular drug vehicles. Allo-HSCT has been used to treat patients with rapidly progressive hereditary leukodystrophies for approximately three decades (Page et al., 2019). Recently, breakthroughs have been made with Hematopoietic Stem Cell Gene Therapy, with encouraging results compared to classic HSCT approaches in this setting (Biffi, 2017).

This section will focus on microglia as the primary neural cell driving neurological disease phenotype in IEM and its role as a therapeutic target and model for future treatment approaches. An overview of all IEM is beyond the scope of this review, and particular emphasis will be made on IEM that benefit from HSCT as a treatment modality, such as certain Lysosomal Storage Diseases and the Peroxisomal Disorder X-Linked Adrenoleukodystrophy. A summary of the events resulting in brain repair with Allo-HSCT in IEM with neurological involvement is presented in Table 1.

The development of HSC-GT in IEM with CNS involvement emerged from some of the limitations of Allo-HSCT observed over the years. Firstly, as previously mentioned, replacement of defective microglia by transplanted donor-derived myeloid cells is a slow process compared to the natural evolution of the disease (Hoogerbrugge et al., 1988; Kennedy and Abkowitz, 1997; Krivit et al., 1998; Boelens and van Hasselt, 2016; Biffi, 2017). In most cases, microglia turnover in the brain occurs not earlier than 1 year after transplant, which has resulted in narrower clinical indications for Allo-HSCT than initially expected. Symptomatic children with non-early disease cannot wait 1 year until demyelination arrest and deteriorate faster with Allo-HSCT, rendering them ineligible for the procedure (Carreras et al., 2019). Secondly, transplant requires a compatible donor because genetic disparities increase the risk of rejection and GvHD—a form of alloreactivity that helps to eliminate residual disease in hematologic malignancies, but that has no benefit in benign conditions. In fact, acute GvHD is a potentially fatal complication and chronic GvHD is associated with considerable long-term morbidity (Carreras et al., 2019). For these reasons, matched siblings are the preferred donors for benign conditions. However, in the setting of congenital disorders such as LSD and ALD, using a sibling donor carries the risk of transplanting cells with disease-causing alleles, resulting in the delivery of enzymatically deficient HSC (Biffi, 2017). Using matched unrelated donors bypasses this issue but depends on availability and it is a logistically cumbersome process that may result in treatment delay. Haploidentical family donors are quicker alternatives, but their use is associated with a higher risk of certain complications such as GvHD. For these reasons, umbilical cord blood has been used as an alternative to related or unrelated donors. However, slow hematopoietic engraftment with subsequent increased risk of infection and graft failure are observed with this graft source (Carreras et al., 2019). Finally, Allo-HSCT is associated with significant toxicities secondary to the conditioning regimen and GvHD management. Of note, Busulfan (a frequent component of the conditioning regimen) and calcineurin inhibitors (the backbone of GvHD prophylaxis) are well recognized neurotoxic agents (Bechstein, 2000; Ciurea and Andersson, 2009) that may result in devastating consequences in patients with baseline impairment of brain functional reserve.

Many of these limitations can be overcome with the use of HSC-GT. Similar to AHSCT, this procedure relies on the infusion of autologous HSC into a patient previously exposed to a conditioning regimen. However, in HSC-GT, the collected autologous HSC are transduced ex vivo with gene vectors encoding supraphysiological levels of the lacking protein. Engraftment of these manipulated cells results in systemically elevated levels of the missing enzyme, thus pleiotropically correcting metabolism and organ function (Staal et al., 2019). Defective CNS metabolism is also restored because HSC and myeloid progenitors can cross the BBB (Fratantoni et al., 1968). Artificially enzyme-boosted microglia progenitors replace their ill counterparts in the brain, cross-correct neighboring cells and restore microglial function (Staal et al., 2019). Notably, the autologous nature of these HSC eliminates the possibility of graft rejection, GvHD and the need for a donor search. However, HSC-GT still requires a conditioning regimen to create a permissive bone marrow and brain niches. Therefore, treatment-related toxicity is not entirely abolished despite reduced intensity compared to Allo-HSCT.

In IEM with CNS involvement, correction of dysfunctional damage-response pathways is apparently enhanced with HSC that overexpress the missing enzymes. Therefore, enzyme-above-normal expression seems to represent the major driver of CNS demyelination arrest with HSC-GT, suggesting a “dose-dependent” treatment efficacy (Biffi, 2017). Through autologous use of HSC for enzyme deployment, this approach has brought brain targeting cellular drug vehicles into clinical practice. Varying degrees of experience regarding gene therapy in the IEM focused on this review currently exist.

MLD was one of the first LSD in which the efficacy of gene therapy was demonstrated. Recently, Fumagalli et al. (2022) published the long-term results of a phased 1/2 trial of lentiviral HSC-GT for patients with early-onset disease. Treatment was of meaningful clinical benefit, resulting in normal activity of the defective enzyme, which translated into prevention, stabilization, or reduced rate of neurological decline. Of note, enzyme activity in the cerebrospinal fluid of treated patients was shown to be restored to normal levels, suggesting efficient brain engraftment of enhanced cells (Fumagalli et al., 2022).

A phase 2–3 study of HSC-GT in 17 boys affected by CALD was published in 2017. Gene therapy led to positive outcomes in most patients with stable brain lesions and absent major functional disabilities in 15 out of the 17 treated boys (Eichler et al., 2017). Additional follow-up data to assess the durability of efficacy and long-term safety is still pending at the time of the writing of this review.

Recently, Gentner et al. (2021) published the interim results of a phase 1–2 study of HSC-GT in eight patients with Hurler Syndrome. After treatment, all patients acquired motor skills, and those presenting normal baseline function had no cognitive performance deterioration. Imaging revealed either stability or reduction of brain abnormalities. Additionally, all patients showed detectable enzyme and reduced substrate levels in the cerebrospinal fluid 3 months after treatment, which persisted until the latest follow-up (Gentner et al., 2021).

Compared to the other IEM presented in this review, HSC-GT is not yet used in KD because the supraphysiological expression of the affected lacking enzyme (galactocerebrosidade) in HSC is toxic. However, promising discoveries were reported in animal models by Gentner et al. (2010). The authors identified galactocerebrosidade-suppressing miRNAs specifically expressed in HSC that decrease during differentiation into mature hematopoietic cells. These findings support the investigation of HSC-GT in KD using these miRNAs, enabling successful gene therapy through mature hematopoietic-lineage cells while protecting HSC from galactocerebrosidade toxicity (Gentner et al., 2010).

HSC-GT represents a valuable addition to the therapeutic armamentarium for IEM with neurological involvement, thus far demonstrating encouraging results with rapid and profound CNS metabolic correction in affected patients.

Outside of the IEM setting, HSC-GT represents a developmental strategy for neurological disorders, currently being investigated in preclinical studies. HSC-derived tumor-infiltrating myeloid cells have been explored as a potential cellular drug vehicle in a mouse model of brain metastasis. Using promoters specifically upregulated in brain metastasis such as MMP14, Andreou et al. (2020) demonstrated that HSC-GT was effective in delivering pro-apoptotic molecules to the tumor, which resulted in prolonged survival of treated mice. Other neurological conditions such as Alzheimer’s disease, MS and Stroke are associated with brain-infiltrating myeloid cells that also overexpress MMP14 (Langenfurth et al., 2014), which, in the opinion of the authors of the study, creates a rationale for the use of HSC-GT in these benign neurological disorders (Andreou et al., 2020). In the future, HSC-GT application may expand as hematopoietic cells are increasingly being recognized to participate in the pathogenesis of several neurological conditions.

A summary of the several steps that take place with HSC-GT and the main advantages over Allo-HSCT in IEM are presented in Table 3.

Certain neurological conditions are characterized by defects in cells that, despite residing in the central nervous system parenchyma, originate elsewhere. Immune-mediated Neurological Diseases and Inborn Errors Metabolism with CNS involvement are two paradigmatic settings of peripheral malfunctioning imported to the brain. In the former, peripheral immune cells that have exclusively lost self-tolerance to CNS components are the protagonists. In the latter, enzymatic substrate accumulation in the brain results in broad neural cell impairment that takes a particular toll on the myeloid-derived microglia.

Infusion of autologous HSC allows the delivery of immune-resetting high dose immunosuppression that would otherwise result in irreversible bone marrow failure. This approach has demonstrated meaningful clinical benefits in immune-mediated neurological diseases. In Multiple Sclerosis, a condition where phases of intense neuro-inflammation characterize clinical evolution, AHSCT can sucessfully eliminate autoreactive memory and redirect immune reconstitution to self-tolerance. Allo-HSCT brings additional benefits at the expense of increased toxicity, currently rendering its use developmental. Harnessing the graft-vs.-auto-immunity effect may represent a future avenue of investigation in immune-mediated neurological diseases.

Microglia dysfunction plays a central role in the pathogenesis of brain injury in Inborn Errors of Metabolism. The hematopoietic origin of microglia makes Allo-HSCT an efficient strategy for their replacement by donor-derived myeloid cells. Functional allogeneic microglia restore brain homeostasis by modulating inadequate responses to injury and cross-correcting neighboring metabolically impaired neural cells. After transplantation, changes in the cell composition and secretome of the myeloid brain compartment ultimately result in decreased substrate accumulation and enhanced repair mechanisms such as remyelination. Therefore, donor hematopoietic stem cells are integrated into the CNS cellular network of the recipient with profound therapeutic benefits.

Hematopoietic stem cell gene therapy explores HSC as a cellular enzyme vehicle to the brain. This treatment relies on the replacement of defective microglia by metabolically enhanced autologous HSC progeny that deliver supraphysiological levels of the deficient enzyme. Clinical benefits are encouraging and seem to depend on the above-normal enzyme expression. So far, vector-induced genotoxicity such as insertional mutagenesis has not been reported in this setting. Overall, this approach bypasses many of the limitations of Allo-HSCT and may potentially become standard treatment for these conditions. Importantly, long-term follow-up data will provide valuable information on the safety and persistence of metabolic correction. In the future, increased patient accrual to gene therapy clinical trials will bring to the surface the limitations and advantages of this approach over standard HSCT.

This review demonstrated that HSC represent a viable living therapy approach for certain devastating neurological disorders. Improved neurological outcomes and potential life-long reversal of brain injury mechanisms create a rationale for the use of this strategy in diseases such as IEM and immune-mediated neurological diseases that may be otherwise associated with a dismal short-term prognosis. However, given the serious complications that may arise from HSCT, careful patient selection is mandatory.

Experience of HSCT in Inborn Errors of Metabolism provides essential insights on the brain repairing mechanisms of both wild-type and modified hematopoietic stem cells. The central role of microglia in modulating maladaptive responses to injury and the feasibility of its replacement with current transplantation approaches provides a framework for future treatment pipelines in other demyelinating and neurodegenerative CNS conditions. Further investigation is needed to evaluate if the therapeutic potential of HSC can be successfully harnessed in different neurological disease settings.

PV wrote the manuscript. JL supervised the drafting and revision of the manuscript. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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