Stroke is one of the leading causes of death and physical disability among adults. It has been 15 years since clinical trials of stem cell therapy in patients with stroke have been conducted using adult stem cells like mesenchymal stem cells and bone marrow mononuclear cells. Results of randomized controlled trials showed that adult stem cell therapy was safe but its efficacy was modest, underscoring the need for new stem cell therapy strategies. The primary limitations of current stem cell therapies include (a) the limited source of engraftable stem cells, (b) the presence of optimal time window for stem cell therapies, (c) inherited limitation of stem cells in terms of growth, trophic support, and differentiation potential, and (d) possible transplanted cell-mediated adverse effects, such as tumor formation. Here, we discuss recent advances that overcome these hurdles in adult stem cell therapy for stroke.
Stroke is the leading cause of physical disability among adults. One-fourth to a half of stroke survivors are left with significant disabilities. Stem cell therapy is considered a potential regenerative strategy for patients with neurologic deficits. This review presents the most recent advances in adult stem cell therapy for stroke, focusing on improvements in its safety and efficacy profile.
We identified references for this review by searching PubMed and ClinicalTrials.gov published in English up to December 2015, with the search terms stroke, cerebral infarction, stem cells, and mesenchymal stem cells (MSCs). Additionally, we searched references from relevant articles and reviews. The final reference list was generated on the basis of originality and relevance to this topic. Because of space limitation, we are not able to discuss many potential regenerative strategies of adult stem cells therapies in depth with critical analysis. Genetically modified stem cells are mentioned only briefly because they have some limitations in the clinical use in stroke patients.
A critical limitation of adult stem cell therapy for stroke is the lack of complete understanding of mechanism of action mediating the observed therapeutic benefits. Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) can replace the missing brain cells in the infarcted area. However, there are hurdles associated with true neuronal substitution using cell replacement to restore neuronal function after stroke [
Therefore, adult stem cells such as MSCs may be a good choice for stroke therapy because they secrete a variety of bioactive substances, including trophic factors and extracellular vesicles (EVs, 0.1–1 μm sized circular membrane fragments shed from the cell surface), into the injured brain, which may be associated with enhanced neurogenesis, angiogenesis, and synaptogenesis [
With current advances in understanding the effects of introducing stem cells and their mechanisms of action, several clinical trials of stem cell therapy were conducted in patients with stroke. Most clinical trials used adult stem cells, such as MSCs [
In the STARTING (STem cell Application Research and Trials In NeuroloGy) trial, autologous MSCs culture-expanded with fetal bovine serum were applied intravenously in the subacute phase of stroke [
Because these randomized, observer-blinded trials were not pivotal efficacy trials, there can be no conclusions made regarding the efficacy of this therapy. Some of the failures in clinical trials may be predicted on the basis that there was insufficient preclinical data to support a strong clinical benefit [
The primary hurdles for current stem cell therapies include (a) the limited source of engraftable stem cells, (b) the presence of optimal time window for stem cell therapies, (c) inherited limitation of adult stem cell potential, and (d) possible transplanted cell-mediated adverse effects, such as tumor formation or stroke (
First, although the optimal cell dosage and route of administration of stem cells in cardiovascular diseases are not settled [
Second, the optimal time point for the application of stem cells exists, in terms of stem cell tropism toward brain and mechanistic targets of stem cells. The levels of chemokines, trophic factors, and relevant microRNAs (miRs) increased markedly in the infarcted brain during the acute phase of stroke, but decreased with time [
Third, adult stem cells may have inherited limitations. MSCs are heterogenous and contain many different types of progenitor or stem cells, in terms of growth, trophic support, and differentiation potentials. The neurorestorative potential of MSCs may be limited in the elderly who have a limited number of neural stem/progenitor cells (NSCs) [
In addition, the discrepancy in stem cell effects between preclinical and clinical studies may be in part derived from differences in the regenerative potential of healthy young animals and aged patients with chronic disease. One study showed that treatment with bone marrow MSCs in type I diabetic rats increased mortality and blood-brain barrier (BBB) leakage, resulting in brain hemorrhage, and underscored the possibility that stem cell therapy may not be beneficial for diabetic subjects with stroke [
Lastly, a major concern with stem cell therapy is cell-mediated adverse effects, i.e., tumor formation of transplanted cells (i.e., iPSC or ESC) that may delay the recovery after stroke [
The following issues will be addressed in order to overcome these hurdles (
Although bone marrow-derived MSCs are most commonly used, other sources of MSCs, such as adipose tissue or the umbilical cord, are increasingly utilized in clinical practice. This has resulted from our current understanding of the characteristics of different sources of MSCs [
In addition, allogeneic stem cells could represent a suitable alternative to autologous stem cells. The safety of allogeneic MSC therapy may be closely related to their short-term existence in the host after the application. Allogeneic stem cells have advantages over autologous stem cells. Allogeneic MSCs are scalable from a manufacturing perspective, with standardized procedures. MSCs from younger healthy donors or iPSC- or ESC-derived adult stem cells may differ in terms of their proliferation and neurorestorative capacity, from those of cells obtained from elderly patients with chronic illness. The use of allogeneic MSCs reduces the time required to obtain a sufficient number of cells (the “off the shelf” approach). In the recent clinical trials of intravenous application of allogeneic stem cells (MultiStem®) in patients with acute stroke, stem cells were applied within the 24-48 hours after the onset of symptoms [
However, conflicting results exist. After contact with serum, allogeneic MSCs can be injured by complement, and the viability of allogeneic MSCs after infusion is greatly reduced compared with autologous MSCs [
The mode of application of stem cells may significantly influence the number of cells delivered to target regions as well as the incidence of adverse effects. A major problem in introducing stem cells systemically is that cells may become trapped within organs that filter the blood (first-pass effect). To avoid this, strategies to minimize lung adhesion and improve the homing of systemically introduced cells are used, including different routes of administration. For example, unlike in intravenous infusions, in an intra-arterial approach, pulmonary circulation can be bypassed, which results in superior delivery and sustained presence of stem cells in the ischemic brain [
Molecules larger than 400 Da cannot pass through the BBB, which may affect the efficacy of cell therapy in stroke patients. Intravenous co-administration of stem cells and mannitol, an osmotic agent that can regulate the permeability of the BBB, may improve outcomes in stroke patients. One preclinical study showed that BBB manipulation using intravenous mannitol prior to MSC treatment resulted in increased levels of trophic factors in the infracted brain [
In terms of chemokine expression and BBB opening, chronic treatment could be advantageous with non-systemic routes (i.e., intra-arterial, intranasal, or intracerebral) and BBB manipulation. Few studies have directly compared the efficacy of various routes of delivery of MSCs, and further studies are needed.
Decreased functional activity of stem/progenitor cells might be mediated by telomerase shortening/decreased telomerase activity, disturbances of the cell secretome, and altered interactions with the microenvironment [
The choice of medium is of importance for the end result of the stem cell culture. Use of human serum or human platelet lysate instead of xenogenic fetal bovine serum during MSC expansion enabled rapid expansion and rejuvenation of MSCs without adversely affecting immunophenotype [
Treatment with trophic factors or chemical agents may alter MSC characteristics. Preclinical studies of various ischemic models showed that
Lastly, genetic modification of MSCs increased the trophic support of MSCs (e.g., brain-derived neurotrophic factor genemodified MSCs), and migration of MSCs to infarcted brain areas (MSCs transfected to overexpress a chemokine receptor) [
Cell therapy using the secretome (trophic factors, cytokines, or chemokines produced through paracrine secretion) or EVs (e.g., microvesicles and exosomes) derived from stem cells could represent a new, clinically feasible, and cell-free paradigm that would avoid cell-related problems such as tumor formation and infarcts caused by vascular occlusion. Stem cells secrete EVs as well as soluble factors (e.g., trophic factors), and stroke in humans triggers the mobilization of MSC-derived EVs [
miRs are short, non-coding RNAs with a capacity to repress varied target genes [
Lastly, administration of small molecules, biomaterials, and biologics that targets the stem cell niche is a promising field in regenerative medicine [
To date, considerable attention has been focused on the development of novel 3D bioprocesses that can facilitate the clinical translation of stem cell research. Besides the various methods of biochemical and/or genetic preconditioning mentioned previously, the biological properties of stem cells can be manipulated by 3D bioprocessing techniques that physically mimic the natural microenvironments of the stem cells
It is becoming increasingly apparent that the native phenotype of MSCs is altered with repeated passages during culture expansion using conventional tissue culture flasks [
Likely, the incorporation with biomaterials can also provide stem cells with a highly biocompatible 3D-environment. Tissue-engineered scaffolds with engineered biochemical/physical properties could support cellular survival in a long-term transplantation to promote integration with the host tissue [
In addition, culture methods to expand 3D bioprocessed stem cells should be further studied. Traditional static culture conditions limit the mass transfer of nutrients, oxygen, and metabolic wastes, resulting in a necrotic center of cellular mass while cells near the outer layer remain viable [
Clinical data are still being accumulated in patients with stroke. It remains too early to be confident that adult stem cell therapy can improve functional outcomes in patients with stroke. More evidence from randomized trials is needed. In the meantime, continuing efforts are needed at the bench, to develop stem cell strategies with enhanced therapeutic efficacy of stem cells, and at the bedside, through constant dialogues, to meet the Food and Drug Administration’s regulations on stem cell use in clinical applications. The efficacy of stem cell therapy will be improved with advances in our understanding of stem cell biology and with the advances in techniques to modulate stem cell characteristics, including biotechnology and bioengineering (
This study was supported by the Korea Health Technology R&D Project, the Ministry of Health & Welfare (HI14C16240000).
The authors have no financial conflicts of interest.
Hurdles of current stem cell therapy and strategies to overcome the challenges. MSC, mesenchymal stem cell; iPSC, induced pluripotent stem cells; ESC, embryonic stem cells; BBB, blood-brain barrier; EV, extracellular vesicle.
Exchange of information between stem cells and neural cells via EVs. Stem cell-derived EVs can transfer stem cell-specific bioactive molecules, including proteins, mRNAs, and miRs, to injured cells. Thus, EVs can trigger a regenerative program in injured cells in a paracrine manner. Conversely, injured cells may stimulate stem cells by secreting vesicles under pathological conditions.
Advances in technologies that are required for regenerative medicine for stroke.