Introduction
Stroke is the second leading cause of death worldwide and a major cause of disability [
1]. The lifetime risk of stroke in individuals over the age of 25 years is estimated to be 24.9% globally; of those experiencing stroke, the risk of ischemic stroke (IS) is 18.3% [
2]. When suffering a stroke, cerebral blood vessels become blocked due to the formation of blood clots. This can lead to cerebral edema and cerebral infarction, causing irreversible damage to neurological function and seriously affecting a patient’s prognosis [
3].
Early treatments, such as intravenous thrombolysis and arterial embolization, have been shown to significantly reduce the mortality and disability rate after suffering acute IS although the time window for treatment is restricted to only 6 to 8 h [
4‐
6]. In the subacute to chronic phases of IS, treatment strategies may be adjusted depending upon an individual patient’s condition. These strategies may include the management of cholesterol, antithrombotic medications, and rehabilitation training; however, these methods are associated with limited efficacy in the promotion of functional recovery [
7]. IS largely considered to have a poor prognosis, and no therapies have been proven to treat this condition effectively. However, the advancement of cell transplantation technology has provided a new avenue for the management of IS in the acute, subacute, or chronic phase [
8].
Cell-based therapies for stroke emerged in the 1990s and are widely considered to represent a potential treatment for IS [
9,
10]. Cell-based therapy can improve the prognosis of patients with IS by facilitating cell replacement, stimulating endogenous repair processes, promoting brain plasticity and synaptic reorganization, and facilitating immunomodulation [
11,
12]. Currently, cell-based therapy research includes the use of different types of cells, such as bone marrow mononuclear cells (BMMNCs), hematopoietic stem cells, neural stem cells, and mesenchymal stem cells; different routes of administration, such as intracerebral, intra-arterial, intravenous, intrathecal, and intranasal; different doses, depending on cell type and the route of administration; and different time windows for treatment, ranging from days to months and years [
13]. Of the various cell types, hematopoietic stem cells have been shown to possess only limited ability to differentiate into neurons; neural stem cells have been associated with ethical issues and immune-related problems, and mesenchymal stem cells require several weeks for expansion with conventional culture techniques [
14]. BMMNCs offer significant potential since they can be obtained from the patients themselves without expansion and are easy to collect, prepare, and preserve, thus avoiding potential ethical issues or immune-related problems.
BMMNCs represent a heterogeneous mix of hematopoietic progenitor cells, a population of mesenchymal and endothelial precursors [
15]. Previous research has demonstrated that these cells have the capacity to protect neurons and reduce the loss of neurons resulting from stroke [
16,
17]. The autologous transplantation of BMMNCs has been shown to be efficacious in animal models of stroke by exerting a range of biological effects, including the attenuation of neuronal death, the modulation of microglia, the reduction of proinflammatory responses, the enhancement of neoangiogenesis, and the promotion of endogenous neural stem cell proliferation [
18,
19]. The mechanisms aimed at enhancing the outcomes of stroke may differ according to the specific phase of stroke. During the acute phase, BMMNC therapy facilitates neuroprotection mainly by releasing trophic factors, regulating inflammation, and promoting neurorestoration. In the chronic phase, the focus transitions to neurorestoration in patients with a stable chronic deficit [
20].
However, the efficacy of BMMNC transplantation remains controversial, and numerous key parameters have yet to be fully determined, including the time window for treatment, dosage, and the route of BMMNC administration. Furthermore, there are no published meta-analyses of randomized controlled trials (RCTs) relating to the transplantation of BMMNCs in patients with IS. Therefore, in the present study, we conducted a meta-analysis to summarize existing evidence for the efficacy and safety of BMMNC transplantation in IS.
Discussion
We conducted a meta-analysis related to the treatment of IS with BMMNC transplantation based on six RCTs. Our study demonstrated that the transplantation of BMMNCs resulted in a statistically significant improvement in NIHSS scores at three months when compared with control groups. In addition, the transplantation of BMMNCs did not increase the incidence of adverse effects. However, no statistically significant differences were observed in terms of NIHSS scores, mRS scores, BI scores, or changes in infarct volume, when compared between the BMMNC transplantation group and the control group at six months.
The latest Global Burden of Disease (GBD) 2019 stroke burden reported that the mortality rate of stroke was estimated to be 4.53% [
32]. Over the last ten years, the application of cell-based therapy for stroke has advanced from bench to bedside. Among the various categories of cell-based therapies, BMMNCs derived from the patient’s own bone marrow have emerged as a particularly promising method, as evidenced by several studies incorporating animal models [
15,
33‐
35]. BMMNCs enhance endogenous recovery mechanisms both locally and in distant locations from the infarct, potentially via immunomodulation and the reduction of post-stroke inflammation. Both in vivo and in vitro studies have demonstrated that BMMNCs can inhibit the production of IL-6, IL-1β, and TNF-α by the microglia and the secretion of anti-inflammatory cytokines such as IL-4, IL-10, and TGF-β1 [
36‐
38]. Central effects involve the release of trophic factors such as cytokines, chemokines, and extracellular vesicles to improve outcomes after IS. Moreover, stem cells can induce angiogenesis and the repair of the blood–brain-barrier (BBB) following IS [
39‐
42]. Furthermore, BMMNC therapy can play a crucial role in repairing and functionally reconstructing damaged neural circuits [
43‐
46]. The safety of BMMNCs in stroke patients has been confirmed by several clinical trials [
20,
47]. However, the efficacy of BMMNCs remains controversial.
In the present study, analysis of the three-month NIHSS score revealed a favorable result in patients who underwent BMMNC transplantation, indicating a better improvement of the neurological deficit over the short term. Of the five included RCTs, one study reported a significant difference between the BMMNC transplantation group and the control group; the others all showed improved outcomes in the BMMNC transplantation group, although the observed differences were not statistically significant. However, there was no statistically significant difference detected between the two groups at six months after treatment. An RCT conducted on acute IS patients revealed that at six months, the outcome of the BMMNC transplantation group was inferior to that of the control group [
20]. The trophic factors and cytokines produced by BMMNCs may accelerate the recovery of IS [
20]. Consequently, differences were evident in the first three months, although this impact was less prominent at six months; the mechanisms responsible for these effects remain unclear. With regard to the six-month mRS score and BI score, our findings suggest that BMMNC transplantation has the potential to enhance patient outcomes, although this was not statistically significant. This finding implies that BMMNC transplantation may not significantly improve overall disability, dependence, and activities of daily living when compared to the control group. However, it is important to note that substantial heterogeneity was observed in the BI score between the two included studies. Furthermore, we did not detect a significant difference in infarct volume when compared between the BMMNC transplantation groups and control groups. Prasad et al. [
28] observed a reduction of infarct volume in the BMMNC transplantation group, while Moniche et al. [
20] did not observe a significant change. Notably, the different baseline infarct volumes between the two trials may lead to different conclusions. Moniche [
20] proposed that instead of brain infarct volume, other mechanisms could be used to predict functional outcomes in a more precise manner, such as the restoration of cortical connections between brain hemispheres. Therefore, the recommendation is to utilize advanced magnetic resonance imaging (MRI) techniques, such as diffusion tensor imaging or functional MRI, to validate the hypothesis of enhanced neuroplasticity.
Our meta-analysis revealed a significant change in the BI score with low doses of BMMNC treatment. A recently published multicenter RCT demonstrated that the efficacy of treatment was more prominent in the low-dose group (2 × 10
6 BMMNCs/kg) when compared to the high-dose group (5 × 10
6 BMMNCs/kg), with higher BI scores and lower NIHSS scores [
20]. However, animal studies have shown that the administration of high doses of cells can increase the efficacy of cell-based therapy in stroke [
48‐
50]. Yang et al. [
50] observed a more effective treatment result when using BMMNCs at doses of 1 × 10
7 and 3 × 10
7 cells/kg when compared with 1 × 10
6 BMMNCs/kg. This discrepancy could be attributed to the high dose employed in clinical trials, which is far less than that used in animal studies. Wang et al. [
49] also found no significant difference between groups of animal models of stroke that were implanted with BMMNCs doses ranging from 1 × 10
6 to 1 × 10
7 BMMNCs/kg. However, exceeding the optimal threshold for transplanted cells may result in saturation of the damaged striatum, thus leading to a progressive reduction in the survival of transplanted cells due to insufficient nutrients.
With regard to the route of administration, our meta-analysis revealed that the subarachnoid pathway exhibited a greater propensity for improving NIHSS outcome, while the intra-arterial pathway demonstrated greater efficacy in terms of changes in BI score. However, the limited number of RCTs resulted in the inclusion of only a small number of patients in each subgroup, potentially impacting the validity of these findings. Some studies also compared the effect of BMMNC transplantation using intra-arterial and intravenous routes; however, these studies found no significant difference in terms of efficacy outcomes between the two methods [
51,
52]. The intravenous route is regarded as the simplest and least invasive technique [
53], although a considerable portion of transplanted cells can become trapped in peripheral organs [
54,
55]. On the other hand, the intra-arterial route allows for more efficient biological biodistribution by bypassing peripheral filtering organs such as the liver, spleen, and lungs [
56]. The subarachnoid route achieved the highest engraftment rate among all administration routes with approximately one-third of the cells still migrating to the ischemic area [
57,
58]. As mentioned earlier, during the early stages of IS, BMMNC therapy contributes to functional recovery mainly by releasing trophic factors. Therefore, it is advisable to consider less invasive treatments, such as intra-arterial and intravenous transplantation [
59]. However, during the chronic phase of IS, when the acute pathophysiological changes have stabilized, the focus shifts to the replacement of damaged tissue. In this case, the subarachnoid pathway or other invasive routes are more commonly used to directly deliver cells into the central nervous system, thus facilitating the reconstruction of the neural circuit, and the replacement of damaged brain tissues in IS [
60,
61]. However, the use of subarachnoid delivery requires careful consideration since subarachnoid routes are associated with several problems, including invasiveness and the risk of intracranial infection [
62]. Furthermore, the intranasal and intraperitoneal routes are currently in the preclinical study phase and require more evaluation of their safety and efficacy before being implemented in large-scale clinical trials.
This meta-analysis also has some limitations that need to be considered. Firstly, it should be noted that the sample size of the included RCTs was small; this may have led to reliability deficiencies. In addition, due to the lack of evidence, we did not investigate the optimal timing of BMMNC transplantation in this article. Notably, some RCTs did not use sham injections; furthermore, these trials did not keep participants and observers blinded due to the obvious differences between BMMNC transplantation injection and conventional medical therapy.
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