Stem Cell Therapy in Neuroimmunological Diseases and Its Potential Neuroimmunological Complications

Background: Since the 1990s, transplantations of hematopoietic and mesenchymal stem cells (HSCT and MSCT) and dendritic cell (DCT) have been investigated for the treatment of neurological autoimmune disorders (NADs). With the growing number of transplanted patients, awareness of neuroimmunolgical complications has increased. Therefore, an overview of SCT for the most common NADs and reports of secondary immunity after SCT is provided. Methods: For this narrative review, a literature search of the PubMed database was performed. A total of 86 articles reporting on different SCTs in NADs and 61 articles dealing with immune-mediated neurological complications after SCT were included. For multiple sclerosis (MS), only registered trials and phase I/II or II studies were considered, whereas all available articles on other disorders were included. The different transplantation procedures and efficacy and safety data are presented. Results: In MS patients, beneficial effects of HSCT, MSCT, and DCT with a decrease in disability and stabilization of disease activity have been reported. These effects were also shown in other NADs mainly in case reports. In seven of 132 reported patients with immune-mediated neurological complications, the outcome was fatal. Conclusions: Phase III trials are ongoing for MS, but the role of SCT in other NADs is currently limited to refractory patients due to occasional serious complications.


Introduction
Hematopoietic stem-cell transplantation (HSCT) is a complex treatment procedure, which was originally developed for treating hematological malignancies [1][2][3]. Although the exact mechanism of action has not yet been precisely defined, the rationale is to erase the aberrant cells with a conditioning regimen [2]. As an ancillary effect, the applied myelotoxic agents deplete bone marrow-inherent stem cells and reduce the mature lymphocyte pool [1,2]. Thereafter, patients' own (autologous) or donors' (allogeneic) hematopoietic stem cells harvested from peripheral blood or bone marrow are used to repopulate the depleted bone marrow and reconstitute the immune system without (autologous) or with (allogeneic) a graft-versus-autoreactivity effect [1][2][3].

Results and Discussion
In this section, studies and publications from more than 20 years of stem cell transplantation for the most common neurological autoimmune disorders are presented. During this time, new conditioning regimens, infused cells, and applications emerged. To summarize the data, partly wide ranges are given. Additionally, more detailed information is given in Supplementary Tables S1 and S2.

Stem-Cell Therapies in MS
A total of 44 phase I, I/II, and II studies, as well as registered clinical trials and post hoc analyses on cell therapy for MS, were found, and a summary of the results is shown in Table 1 (for further details, see Supplementary Table S1) [9,17,. The number of included patients in these studies varied between five and 617, with most investigating chronic-progressive forms of MS (five trials included relapsing-remitting MS patients only). The age of the included patients varied between 28 and 49 years with a disease duration of 2.6 to 20 years. Kurtzke's expanded disability status scale (EDSS) score ranged between 3 and 7.5 (median 6) at treatment onset [69]. The study population was followed up between 1 and 80 months (excluding post hoc analyses).
In 24 of 43 studies, the results of autologous HSCT were reported, with a total of 1849 included patients [17,. In all of these studies, the hematopoietic stem cells were derived from peripheral blood [17,. Peripheral blood stem cells were mobilized using cyclophosphamide in combination with G-CSF (granulocyte-colony stimulating factor, n = 17), additional application of GM-CSF (granulocyte-macrophage colony-stimulating factor, n = 2), and G-CSF only (n = 4) [17,. In one study, hematopoietic stem cells originated from bone marrow (harvested by bone marrow aspiration), but insufficient transplantable cells were obtained; thus, peripheral blood stem cells (mobilized by cyclophosphamide and G-CSF) were also transplanted [42].  After HSCT (irrespective of the applied conditioning regimen), beneficial effects clinically defined as improvement (>1 point in EDSS) or stability (±0.5 points in EDSS) were reported in 16-100% of the included MS patients (Table 1) [17,. In some of these patients (n = 553) with beneficial effects, further outcome differentiation was possible; in 3-81%, EDSS improved (decreased) by at least one point, while it was stable (±0.5 compared with EDSS at treatment onset) in 12-100% [17,. Adverse events related to the treatment procedure were reported in all trials in which hematopoietic stem cells were infused [17,. Neutropenia-associated infections, toxicity associated with the conditioning regimen, and transient worsening of neurological symptoms were frequently stated. Treatment-related deaths occurred in seven studies (total of 11/584, 2%) [17,.
In patients with disease progression and relapses despite stem-cell therapy, MRI revealed progressive brain atrophy and new lesions in T2-weighted and/or gadoliniumenhanced T1-weighted images. However, post hoc analyses revealed that progressive brain atrophy, which can be commonly found in untreated MS disease course, was slowed down after autologous hematopoietic stem-cell therapy [47,48] Contrarily, one post hoc analysis reported of discordant findings concerning disease activity [49]. Despite stable disease activity regarding MRI and cerebrospinal fluid parameters, clinical parameters (EDSS and ambulation) indicated worsening of the patients' symptoms [49].
Lastly, in one phase I study including eight patients suffering from relapsing-remitting (1 patient), primary progressive (three patients), and secondary progressive MS (four patients), peptide-loaded tolerogenic dendritic cells from peripheral blood were administered intravenously [9]. These cells were generated by conditioning monocyte-derived dendritic cells after autologous monocyte-derived dendritic cells were obtained by leukapheresis [9]. A conditioning regimen preceding intravenous application of these cells was not applied [9]. All included patients revealed stable EDSS scores during the short follow-up period of 3 months, and no treatment-related adverse events were reported [9].
In summary, stem-cell therapies seem to effectively reduce disease activity and even restore neurological function in patients with MS. In particular, autologous HSCT with an intermediate intensity-conditioning regime is proposed as highly effective in reducing disease activity in MS patients. Burt and colleagues reported no evidence in disease activity (NEDA) in 93% of cases (within a median follow-up of 2 years; conditioning regime: cyclophosphamide + anti-thymocyte globulin (ATG)), whereas Nash and colleagues de-scribed NEDA in 69% of cases (median follow-up of 5 years; BEAM (carmustine, etoposide, cytarabine, and melphalan) + ATG) [70,71]. Similarly, this review indicates a wide range of beneficially affected MS patients after stem-cell therapies ( Figure 1). Nevertheless, the results of the previously mentioned studies seem to indicate superiority of HSCT in reaching NEDA compared with other highly effective MS disease-modifying therapies (DMTs) such as ocrelizumab, alemtuzumab, or cladribine [72][73][74][75]. However, direct comparison of these data is not suitable since the trials differed in eligibility criteria, design, and follow-up duration, including prior DMT treatment and disease activity at study entry. In order to obtain clarity on this issue, both therapeutic approaches (HSCT vs. highly effective DMTs such as cladribine, alemtuzumab, and ocrelizumab) are currently being compared in various studies, some of which are phase III trials [76].
In summary, stem-cell therapies seem to effectively reduce disease activity and even restore neurological function in patients with MS. In particular, autologous HSCT with an intermediate intensity-conditioning regime is proposed as highly effective in reducing disease activity in MS patients. Burt and colleagues reported no evidence in disease activity (NEDA) in 93% of cases (within a median follow-up of 2 years; conditioning regime: cyclophosphamide + anti-thymocyte globulin (ATG)), whereas Nash and colleagues described NEDA in 69% of cases (median follow-up of 5 years; BEAM (carmustine, etoposide, cytarabine, and melphalan) + ATG) [70,71]. Similarly, this review indicates a wide range of beneficially affected MS patients after stem-cell therapies ( Figure  1). Nevertheless, the results of the previously mentioned studies seem to indicate superiority of HSCT in reaching NEDA compared with other highly effective MS diseasemodifying therapies (DMTs) such as ocrelizumab, alemtuzumab, or cladribine [72][73][74][75]. However, direct comparison of these data is not suitable since the trials differed in eligibility criteria, design, and follow-up duration, including prior DMT treatment and disease activity at study entry. In order to obtain clarity on this issue, both therapeutic approaches (HSCT vs. highly effective DMTs such as cladribine, alemtuzumab, and ocrelizumab) are currently being compared in various studies, some of which are phase III trials [76].
In three case reports with four patients, allogeneic hematopoietic stem cells were infused [86][87][88]. Similarly to the transplantation of autologous hematopoietic stem cells, mostly beneficial treatment effects were reported [86][87][88]. EDSS improvement of at least one point was reached in three out of four patients, and EDSS stability was reached in one out of four patients [86][87][88].
In addition, two phase I/II studies used umbilical cord blood-derived allogeneic mesenchymal stem cells as treatment for NMOSD [89,90]. The stem cells were applied intravenously and intrathecally in five patients each [89,90]. No conditioning regime was employed, and no severe treatment-related adverse events were reported [89,90]. Beneficial treatment effects (EDSS improvement or stability) were reported in three and four out of five of the included patients, respectively [89,90]. In two of these patients, relapses during follow-up (24 months and 70 months (mean)) were fatal and led to NMOSD-related death [89,90].
In one phase II study, bone marrow-derived autologous mesenchymal stem cells were intravenously applied in 15 patients suffering from NMOSD [91]. During a follow-up of 24 months, all patients reported of beneficial treatment effects displayed by an EDSS Cells 2022, 11, 2165 9 of 23 improvement >1 point (six patients) or EDSS stability ±0.5 points (nine patients) [91]. No adverse events or deaths related to the treatment procedure were reported [91].
Lastly, in one phase I study, four NMOSD patients received intravenously applied peptide-loaded tolerogenic dendritic cells [9]. Of these patients, one improved notably (EDSS improvement > 1) and three remained stable (EDSS ± 0.5) during the 3 month followup [9]. No treatment-related adverse events were reported during this short follow-up period [9].
There are no reports on stem-cell therapies in patients with MOGAD in the literature so far. One nonoriginal article reported a male patient with MOGAD who suffered from relapsing disease activity despite autologous HSCT [92].
In summary, stem-cell therapies in NMOSD and MOGAD patients play a rather experimental role [71]. To date, a total of 73 published patients have been treated with stem cell therapies, and there are no trials which directly compare classic immunotherapeutic treatment approaches and stem-cell therapies. Furthermore, there are different highly effective approved DMTs for NMOSD, including eculizumab, satralizumab, and inebilizumab [93][94][95]. In addition, other off-label therapeutics such as rituximab and tocilizumab have been used with success [96,97]. With respect to eculizumab, 94% patients were relapse-free in the extension study of the pivotal trial, even after more than 3.5 years [97]. However, this therapy in particular is extremely costly; hence, from a socioeconomic point of view and taking into account the burden on quality of life due to strictly required regular infusions, a possibly one-time stem-cell transplantation offers advantages [98]. Therefore, NMOSD and MOGAD patients should be studied only in randomized, actively controlled trials in the case of stem-cell therapy to evaluate the true value of this therapeutic approach in the era of highly effective immunotherapeutics.

Stem-Cell Therapies in Autoimmune-Mediated Encephalitis and Vasculitis with Affection of the CNS
Five case reports or case series including six patients were found which reported stemcell transplantations in patients with autoimmune-mediated encephalitis and vasculitis with affection of the CNS: One patient suffering from autoimmune-mediated encephalitis was a 35 year old woman with concomitant common variable immunodeficiency (CVID), who revealed uncontrolled disease activity with new MRI lesions despite application of different immunomodulatory therapies comprising intravenous immunoglobulins (IVIG), prednisolone, azathioprine, rituximab, cyclophosphamide, and abatacept [99]. Due to the highly refractory disease course, a double cerebral biopsy was performed, and the histologic results were compatible with autoimmune-mediated encephalitis (MS and lymphoma were excluded) [99].
Lastly, Gray and colleagues reported a 9 year old boy with cerebral vasculitis in Xlinked lymphoproliferative disease, who presented with polyfocal neurological deficits associated with multiple infarctions [103]. In this patient, allogeneic stem cells (umbilical cord blood derived) were transplanted after a reduced-dosage conditioning regimen with busulfan and fludarabine [103]. Adverse events included infections, treatment-related toxicity, transient worsening of neurological symptoms, and onset of secondary autoimmunity (anti-glomerular basement membrane (GBM) disease) [103]. Under treatment with corticosteroids, cyclophosphamide, and rituximab, anti-GBM disease was controlled with ongoing moderate renal function impairment [103]. During follow-up (13.8 months), clinical remission of cerebral vasculitis was reached, and MRI displayed stabilization of the disease [103].
In summary, reports of HSCT in patients suffering from autoimmune-mediated encephalitis or vasculitis with affection of the CNS are relatively scarce. Therefore, HSCT is rather to consider as an experimental approach. Nevertheless, the reported patients were refractory against treatment with a broad spectrum of anti-inflammatory therapies and beneficially responded to HSCT. In treatment-refractory patients with great disease burden, HSCT may be considered as ultima ratio therapy, especially in patients with underlying CVID.

Stem-Cell Therapies in Autoimmune-Mediated PNS Diseases
A total of 20 case reports and one phase I/II study investigating stem-cell therapies in 108 patients (37 women, age 17-75 years, disease duration 1-38 years, follow-up 7-78 months) suffering from CIDP, MMN, and myasthenia gravis were found . Of these patients, 95 suffered from CIDP, 10 suffered from myasthenia gravis, and one each suffered from MMN, myasthenia gravis and concomitant amyotrophic lateral sclerosis (ALS), and myasthenia gravis and concomitant polymyositis . Due to consideration of several case reports, the patients' symptoms and disease-related disability were highly heterogeneous and scored with different clinical evaluation tools (modified Rankin scale, inflammatory neuropathy cause and treatment score, Rasch-built overall disability scale, medical research council sum score, myasthenia gravis foundation of America score, and myasthenia gravis composite score) . On the one hand, there were patients, who were treated with stem-cell therapies due to the dependence of regular treatment (IVIG and plasmapheresis), but who responded well to this initial treatment and recovered completely . These patients were not severely disabled, and symptoms mostly did not influence daily activities . On the other hand, there were patients, who were treatment-refractory and/or highly disabled (including the need for intensive care and ventilation) . In these patients, stem-cell therapy was offered as an ultima ratio treatment option . However, of the 60 CIDP patients enrolled in the phase I/II study by Burt and colleagues, various clinical scoring tools indicated moderate disability and need for assistance with activities of daily living [107].
Over all these investigations, only a minority (43/108, 40%; outcome not described in 60 patients) reported beneficial effects of HSCT . These beneficial effects included improvement of existing neurological deficits and ambulation, stabilization of relapsing disease activity, slowing down of progressive disease, reduction in dosage or frequency of immunomodulatory treatment, and even cessation of symptomatic and immunosuppressive therapies . However, two patients were not beneficially affected by autologous, peripheral blood-originating stem-cell transplantation [104,112]. In one of them, a CIDP-relapse 5 years after stem-cell transplantation could not have been prevented leading to an equal neurological state compared with the time period before transplantation [104]. Similar results were reported in an MMN patient after autologous, peripheral blood-originating stem-cell transplantation, whose neurological state did not improve and who was still dependent of monthly IVIG infusions [112].
However, a recently published phase I/II study post hoc analysis investigated the cost effectiveness of IVIG treatment and HSCT in patients with CIDP [125]. The authors emphasized that autologous HSCT is more cost-effective than long-term IVIG treatment due to the long-term treatment-free remission and better outcome measurements observed in their study [125]. Nevertheless, the authors indicated the need to consider patient selection, the stem-cell regimen, and regional variations in the cost and effectiveness analysis in future studies [125].
One patient with myasthenia gravis and concomitant ALS was treated with intrathecally and intramuscularly applied, bone marrow-derived autologous mesenchymal stem cells [120]. A conditioning regimen was not used, and no treatment-related adverse events other than a urinary tract infection occurred [120]. Seven months after stem-cell application, all neurological functions and cognition had improved, suggesting an underlying pure neuroimmunological disease [120].
In summary, 108 patients with autoimmune-mediated disorders of the PNS were treated with stem-cell therapies . One phase I/II trial reported autologous HSCT in CIDP patients [107]. Due to the lack of randomized controlled studies including larger amounts of patients, stem-cell therapies in autoimmune-mediated PNS disorders constitute a therapeutic option for individual patients after careful discussions of risks and benefits with the patient [72]. Further investigations are needed to evaluate stem-cell therapies in this patient group.

Immune-Mediated Neurological Complications after Stem-Cell Therapies
Stem-cell therapies generally harbor the risk of adverse events due to their invasiveness. This is partly attributable to the treatment procedure, which often involves conditioning with myelotoxic and highly aggressive agents.
In general, stem-cell therapies might be a trigger for secondary immune-mediated diseases possibly involving every organ system. For instance, new onset of hematological and rheumatological immune-mediated diseases after autologous HSCT was reported. Hemophilia-A, factor VIII inhibitor, autoimmune hemolytic anemia (AIHA), and immune thrombocytopenia (ITP) occurred after autologous HSCT in patients initially suffering from MS, SLE, and systemic sclerosis [126][127][128][129]. Since four of these nine patients were conditioned with alemtuzumab, the new onset of secondary immunity due to alemtuzumab application rather than autologous HSCT should be considered [127][128][129][130][131]. In another patient with multiple myeloma, new onset of systemic sclerosis was diagnosed, while, in a patient with cerebral vasculitis (in X-linked lymphoproliferative disease as described above), new onset of anti-GBM-disease was diagnosed [103,126].
These secondary immune-mediated diseases after stem-cell transplantation regularly require immunomodulatory treatment. Nevertheless, aggressive disease courses are possible, and, despite different immunomodulatory treatment options, fatal outcome might occur [127,128]. Since many reports regarding the phenomenon of secondary immunity are available in the literature, we focused on reports involving the PNS and CNS.
In several case reports and a retrospective case-control study, 75 patients with immunemediated neuropathies after stem-cell transplantation were described [23,. The diagnostic criteria of CIDP were met in 19 of the patients, whereas 12 patients were diagnosed with GBS [23,. The other patients did not meet the diagnostic criteria for either CIDP or GBS and were, therefore, classified as immune-mediated polyneuropathies [23,.
In some of these cases, immune-mediated polyneuropathy occurred simultaneously with acute or chronic graft-versus-host disease (GvHD) affecting other organ systems; thus, a clear differentiation between both disease entities was not possible [23,. The authors described these cases as GvHD with peripheral nervous system involvement or autoimmune-mediated polyneuropathy in GvHD [23,.
Treatment with different immunosuppressive therapies (corticosteroids, azathioprine, cyclosporine, MMF, IVIG, plasmapheresis, sirolimus, tacrolimus, rituximab, vincristine, and cyclophosphamide) resulted in complete or partial recovery or control of disease in almost all patients [32,. In two of the reported 75 patients with immune-mediated polyneuropathies following allogeneic HSCT for leukemia, the outcome was fatal [143,148].
Twenty-one case reports (including a total of 31 patients) were identified, which described the onset of immune-mediated CNS diseases after stem-cell therapy (Table 3 and  Supplementary Table S3).
In all these patients with immune-mediated PNS and CNS pathologies after stemcell therapies, a possible causative role of the chosen stem-cell mobilization procedure, conditioning regimen, or GvHD prophylaxis must be considered. However, only a small percentage of the respective articles reported these data. According to the cases reported in the literature so far, the occurrence of secondary immunity against neuronal tissue is particularly associated with allogeneic hematopoietic stem-cell transplantation.

Conclusions
Several studies indicated the beneficial effects of different variants of stem-cell transplantation in various neurological autoimmune disorders. In phase I/II and II studies of MS, NMOSD, and CIDP, the efficacy and the safety of transplantation of hematopoietic and mesenchymal stem cells were reported. Nevertheless, severe treatment procedure-related adverse events were frequently reported, including death of the treated patients.
Compared with the high number of stem-cell transplanted patients, secondary immunity affecting the CNS and PNS after stem-cell transplantation is rare. However, due to the complexity of these patients, there may be an underreporting bias. HSCT might play a crucial role in the future treatment of MS. Despite the large number of available DMTs for MS, several studies including phase III trials are currently being conducted to directly compare autologous HSCT with existing immunotherapies.
Due to the lack of clinical trials combined with the now available and emerging DMTs, the role of stem-cell therapy in other neurological autoimmune disorders is currently limited to the treatment of refractory patients with poor prognosis.