Mechanism of inflammatory response and therapeutic effects of stem cells in ischemic stroke: current evidence and future perspectives

Abstract

Ischemic stroke is a leading cause of death and disability worldwide, with an increasing trend and tendency for onset at a younger age. China, in particular, bears a high burden of stroke cases. In recent years, the inflammatory response after stroke has become a research hotspot: understanding the role of inflammatory response in tissue damage and repair following ischemic stroke is an important direction for its treatment. This review summarizes several major cells involved in the inflammatory response following ischemic stroke, including microglia, neutrophils, monocytes, lymphocytes, and astrocytes. Additionally, we have also highlighted the recent progress in various treatments for ischemic stroke, particularly in the field of stem cell therapy. Overall, understanding the complex interactions between inflammation and ischemic stroke can provide valuable insights for developing treatment strategies and improving patient outcomes. Stem cell therapy may potentially become an important component of ischemic stroke treatment.

Introduction

The research findings of the Global Burden of Disease study indicate that stroke remains the second leading cause of death worldwide and the third leading cause of death and disability (Chamorro et al., 2016). Currently, the only confirmed medication for the treatment of ischemic stroke is a tissue plasminogen activator for thrombolysis, but the effective treatment window is only 4.5 hours (Donnan et al., 2008). Moreover, in clinical practice, less than 20% patients meet the criteria for receiving thrombolysis treatment (Li et al., 2016). Furthermore, in the context of the increasing burden of stroke, there is a lack of specific treatment methods to restore the stroke-induced impaired neurological function. Studies have indicated that by limiting the post-stroke inflammatory response, it is possible to improve the prognosis of stroke to some extent (Anrather and Iadecola, 2016; Chen et al., 2023). Therefore, in recent years, there has been increasing attention on research related to post-stroke inflammatory response: understanding the role of the primary effector cells involved in the inflammatory response and immune reactions after stroke is crucial for developing effective therapeutic strategies. With the development of stem cell therapies, significant progress has been made in the research of using stem cells to treat various diseases and injury conditions. Hence, stem cells may potentially be the new direction in the treatment of stroke (Turnbull et al., 2019).

The objective of this review is to explore the enormous potential of stem cell therapy in the field of stroke treatment, relative to traditional treatments, through an overview of the complex inflammatory cascade following ischemic stroke and the dynamics of several major cell types associated with immune response.

Retrieval Strategy

We used “stroke,” “inflammatory,” “immune cell,” “stem cell,” and “cell therapy” as MESH terms or keywords and searched the related literature published in PubMed and Web of Science databases from January 1994 to January 2023. Furthermore, we conducted an analysis and summary of the pathological physiological processes after ischemic stroke, post-stroke inflammatory response, the roles of various effector cells, and the advantages and disadvantages of different treatment methods, especially stem cell therapy, based on these literature sources.

Pathological Physiological Processes after Stroke

The pathophysiology of ischemic stroke involves a cascade of events. In the immediate period after a stroke, there are a series of changes in neurons due to inadequate oxygen and glucose supply (Sekerdag et al., 2018; Zhang et al., 2021). Neurons in the damaged core area undergo permanent cell death, while cells in the penumbra region experience functional inactivity and metabolic instability. Specifically, within minutes after the occurrence of a stroke, neurons in the core area of the damage will undergo permanent cell death due to hypoxia depolarization and membrane potential imbalance (Campbell et al., 2019). Neurons located in the periphery of the injury core region, benefiting from collateral circulation for glucose and oxygen supply (Desai et al., 2021), can maintain their basic physiological processes (Ginsberg, 2016). However, they exist in a state of metabolic instability (Kinoshita et al., 2019), and this period is commonly referred to as the hyperacute phase (Di Pino et al., 2014). Without timely and effective treatment, cells within the penumbra can also eventually die from excitotoxicity, inflammation, and/or apoptosis, especially if reperfusion does not occur (Fifield and Vanderluit, 2020; Iadecola et al., 2020; Levard et al., 2021; Tuo et al., 2022). Neuronal cell death will result in the release of a large amount of damage-associated molecular patterns (DAMPs), triggering inflammation and immune cell responses (Shi et al., 2019; Levard et al., 2021; Wanrooy et al., 2021). This event typically occurs within a few hours after stroke and lasts for several days, which is commonly referred to as the acute phase. Astrocytes (Patabendige et al., 2021) and microglia are activated (Xu et al., 2020), neutrophils enter the brain parenchyma through the compromised blood-brain barrier (BBB) and recruit peripheral immune cells to migrate to the damaged area by releasing inflammatory factors. The invasion of peripheral immune cells causes secondary damage to neurons (Yang et al., 2019; Candelario-Jalil et al., 2022). Over time, the body gradually transitions from an inflammatory response to an anti-inflammatory response. During this period, there is a transformation of glial cells and macrophages from M1 to M2 phenotype (Zhou et al., 2019; Arabpour et al., 2021). The inflammatory response gradually diminishes, and this process can typically last for several months (Tsai et al., 2019). This period is commonly referred to as the subacute phase. With the decline of the inflammatory response, the body enters the chronic phase, which is characterized by tissue repair. New capillaries are formed in the damaged area, along with secretion of neurotrophic factors such as nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 (Müller et al., 2022) via activated oligodendrocytes that provide local nutritional support to neurons and participate in the myelination of axons (Xu et al., 2022).

Inflammatory Response in Stroke

The brain undergoes several changes in different stages in response to stroke and inflammation plays a crucial role in both the acute and chronic phases (Figure 1). Although many aspects of post-ischemic inflammation (i.e., the immune system’s response to tissue homeostasis disruption) manifest several days or weeks after the event, the inflammatory cascade is activated immediately following vascular occlusion (Fifield and Vanderluit, 2020). Stagnant blood flow following vascular occlusion and changes in blood flow velocity induce shear stress on the endothelium and platelets and enhance expression of cell adhesion molecules, leading to the deployment of the adhesion molecule P-selectin on cell surfaces. P-selectin is stored in endothelial cells’ Weibel-Palade bodies and platelet α-granules and is released within minutes after activation. Selectins attract leukocytes to the endothelial cell surface by interacting with P-selectin glycoprotein ligand-1, which is expressed on the surface of most leukocytes, thereby slowing down the circulation of leukocytes. Platelet P-selectin can also bind to leukocytes and serve as a bridging molecule, promoting leukocyte aggregation and leading to intravascular clot formation, further exacerbating ischemic injury (De Meyer et al., 2016). Other adhesion molecules are rapidly induced at the transcriptional level upon activation of endothelial pattern recognition and cytokine receptors. Among them, E-selectin, intercellular adhesion molecule 1, and vascular cell adhesion molecule 1 play important roles in coordinating the recruitment, adhesion, and transmigration of blood leukocytes, respectively (Petrovic-Djergovic et al., 2016).

Figure 1.

Pathological and physiological changes in different periods after ischemic stroke.

In the superacute stage of ischemic stroke, cellular damage happens and is followed by the release of DAMPs. The activation of glial cells and immune response-related cells occurs in the next few days, which is generally considered to be the acute stage. The differentiation of inflammatory response cells and release of cytokines last for weeks after stroke. And angiogeneses, neuroplasticity and funtional reorganization and other neurorepair procedure play main roles in the chronic stage after stroke. Created with BioRender.com. ATP: Adenosine triphosphate; BNDF: brain-derived neurotrophic factor; DAMPs: damage-associated molecular patterns; HMGB1: high mobility group box 1 protein; HSPs: heat shock proteins; IGF: Insulin-like growth factor; IL-1β: interleukin-1β; IL-4: interleukin-4; IL-6: interleukin-6; MMP: matrix metalloproteinases; NGF: nerve growth factor; ROS: reactive oxide species; TGF-β: transforming growth factor-β; TNF-α: tumor necrosis factor alpha; VEGF: vascular endothelial growth factor.

However, the inflammatory and immune responses have a dual role in the prognosis of diseases (Zhang et al., 2021). In the ischemic environment, approximately 2 million neurons die per minute (Iadecola and Anrather, 2011). After damaged cells die, factors such as DAMPs are released, triggering local inflammation in the injured area (Shi et al., 2019). DAMPs mainly include heat shock proteins, adenosine triphosphate (ATP), and high mobility group box 1 protein (Gelderblom et al., 2015). Subsequently, the in situ generated inflammatory mediators propagate throughout the organism. This “spillover effect” first leads to a systemic inflammatory response (Planas et al., 2006), followed by immune suppression aimed at dampening the potentially harmful pro-inflammatory environment (Anrather and Iadecola, 2016). DAMPs can induce the expression of Toll-like receptors (TLRs) such as TLR2 and TLR4 in astrocytes (Sofroniew, 2014). The complement system is a humoral branch of the innate immune system (Amara et al., 2008), and generates membrane attack complexes that can damage the BBB when activated (Petrovic-Djergovic et al., 2016). Although locally generated active complement proteins may enter the brain parenchyma through the compromised BBB, there is also evidence of increased complement synthesis in microglial cells (Schäfer et al., 2000).

Dynamics of Several Major Cell Types Involved in the Immune Response after Stroke

Next, we conducted a detailed review of the several major cell types associated with immune response following stroke (Figure 2).

Figure 2.

Cellular responses of several major cell types associated with immune response after ischemic stroke.

After ischemic stroke, several resident major cell types associated with immune response in the brain tissue and recruited peripheral immune cells participate in the inflammatory response and play different roles as the disease progresses. Created with BioRender.com. IL-10: Interleukin-10; IL-1β: interleukin-1β; IL-6: interleukin-6; TGF-β: transforming growth factor-β; Th cells: helper/inducer T lymphocytes; TNF-α: tumor necrosis factor alpha.

Microglia

Microglia are a heterogeneous population with diverse functions and high plasticity (Boche et al., 2013). They account for approximately 16.6% of the total glial cell population in the human brain and are primarily found in gray matter (Aisen et al., 2002). Microglia, which reside in the brain, are responsible for immune surveillance in the nervous system (Nayak et al., 2014). Microglial activation is the first major response following brain ischemia and can persist for weeks (Kawabori and Yenari, 2015). Selective clonal expansion and selective apoptosis have been observed in the acute neurodegenerative response and in the process of restoring homeostasis, respectively (Tay et al., 2017). DAMPs activate local microglia, causing them to transition from a ramified phenotype to a macrophage-like phenotype (Biber et al., 2014). Activated microglia can be classified into two functionally distinct subtypes: M1 and M2 (Wang et al., 2022; Xu et al., 2023). M1 microglia can transform into M2 microglia by downregulating cluster of differentiation 40 (CD40) or upregulating CD45 expression (Morganti et al., 2016). M1 microglia express pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), interleukin-1 (IL-1), IL-6, IL-1β, and nitric oxide (NO), exacerbating astrocyte and endothelial cell activation. M2 microglia, induced by IL-4 or IL-13, express CD11b, CD45, and CD68 and are associated with a phagocytic phenotype responsible for clearing cellular debris and limiting the production of pro-inflammatory signals (He et al., 2020; Xu et al., 2020). M2 microglia may play an important role in tissue repair and wound healing (Zhao et al., 2006). Activation of microglial membrane protein triggering receptors expressed on myeloid cells 2 promotes the generation of M2 microglia and exerts anti-inflammatory effects (Xia et al., 2021a). The morphology of microglia in stroke patients has a spatiotemporal relationship with the localization of ischemic injury (Morrison and Filosa, 2013).

Neutrophils

Neutrophils are the first leukocyte subset to accumulate in the brain of patients with ischemic stroke (Garcia et al., 1994). In a mouse model of permanent middle cerebral artery occlusion, approximately 30% of the total cell population entering the brain within 3 hours of occlusion comprised neutrophils (Chu et al., 2014). Neutrophil accumulation peaks around 2–3 days after brain ischemia (Yilmaz and Granger, 2008). However, it is unclear whether neutrophils necessarily need to migrate to the core area of the injury to exert their effects (Wanrooy et al., 2021). Their activation and migration appear to be regulated by microglia (Neumann et al., 2018; Otxoa-de-Amezaga et al., 2019). Clinical evidence suggests that the functional heterogeneity of neutrophils depends on the type and stage of the disease (Ng et al., 2019). Neutrophils can be classified into two types based on their functional characteristics: “N1” cells, which have a short lifespan and high cytotoxicity, capable of clearing tumor cell micro metastases but often exacerbate inflammatory damage, and “N2” cells, which have a longer lifespan, exhibit anti-inflammatory properties, and may even have immunosuppressive effects (Rosales, 2018; Wang et al., 2018; Weisenburger-Lile et al., 2019). Once recruited to the ischemic brain tissue, neutrophils adhere to endothelial cells through neutrophil adhesion molecules such as intercellular adhesion molecule 1, macrophage antigen-1, and selectins (E-, P-, and L-selectin) (Danton and Dietrich, 2003; Dimasi et al., 2013). They then participate in the degradation of endothelial cells and the BBB by degranulation of primary proteases such as elastase and matrix metalloproteinases 2 and 9 (Kurzepa et al., 2014). E-, P-, and L-selectins work together for neutrophil trafficking after cerebral ischemia: E-selectin and P-selectin are involved in the initial rolling and recruitment of neutrophils, while L-selectin guides unstimulated neutrophils into the activated endothelial area (Bargatze et al., 1994; Zhang et al., 1998). After ischemia, the acute release of pro-inflammatory cytokines affects the expression and arrangement of tight junction proteins (Rochfort and Cummins, 2015), reduces the vitality of astrocytes near the infarct area, disrupts the integrity of the BBB (Cabezas et al., 2014), and releases metalloproteinases, further promoting the degradation of the neurovascular matrix (Turner and Sharp, 2016). Activated neutrophils also release neutrophil extracellular traps, which further contribute to brain vascular injury (Kang et al., 2020). Despite the development of drugs targeting the inhibition of neutrophil activation or impeding neutrophil central invasion strategies, which have shown clear reductions in brain infarct volume and improvement in animal behavioral phenotypes in animal models, they have shown no significant efficacy or even worse patient outcomes in clinical trials (Veltkamp and Gill, 2016). This phenomenon prompts further consideration of the true role of neutrophils in the pathological changes of stroke and the limitations of existing animal models.

Monocytes

Monocytes originate from hematopoietic stem cells in the liver and spleen during embryonic development and primarily from the bone marrow after birth (Ginhoux and Jung, 2014). They are recruited to the site of brain injury and differentiate into macrophages (Kim et al., 2016). In rodents, monocytes can be classified into two main subsets based on the expression levels of chemokine receptors and lymphocyte antigen 6 complex, Locus C (Ly6C, also call Gr-1). The Ly6Chigh subset exhibits pro-inflammatory effects, has a short half-life, and is actively recruited to inflammatory tissues. The Ly6Clow subset, which has a longer half-life, primarily functions as a patrolling subtype within the vascular lumen and promotes the suppression of inflammatory responses (Naert and Rivest, 2013).

Lymphocytes

The immune response following stroke plays a crucial role in tissue damage and protection. Experimental evidence using mice fluorescently labeled with CD11C has shown that peripheral immune cells invade the brain parenchyma after stroke (Felger et al., 2010). T lymphocytes play a key role in amplifying inflammation following ischemic stroke, while B lymphocytes have a smaller impact than T lymphocytes (Brait et al., 2011). It has been indicated that lymphocytes are involved in stroke-induced immunosuppression (Chamorro and Hallenbeck, 2006). Animal experimental models demonstrate that lymphocyte infiltration into ischemic tissue occurs relatively late, starting from the third day after stroke (Drieu et al., 2020b). Post-mortem human samples have shown lymphocyte infiltration into the ischemic area starting from the third day and can be detected in clinical samples even 53 years after stroke (Drieu et al., 2020a). Clinical observations have revealed a decrease in the percentage of T lymphocytes, helper/inducer T lymphocytes (Th cells), and suppressor/cytotoxic T lymphocytes (Ts cells) in the peripheral blood of acute ischemic stroke patients compared to the control group (Xiao et al., 2021). The reduction in lymphocytes is associated with acute ischemic stroke infarct volume (Hug et al., 2009). Following stroke, CD4⁺ T lymphocytes exhibit two peaks during the infiltration process, occurring at 3 and 24 hours after stroke onset. CD8⁺ T lymphocytes peak at 3 hours and maintain that level after 24 hours. CD4⁺ cells have various subtypes, each playing a distinct role (Theodorou et al., 2008). CD4⁺CD25⁺Foxp3⁺ T lymphocytes proliferate and accumulate in the ischemic brain tissue from 14 to 30 days after infarction, exerting an anti-inflammatory effect, while CD4⁺CD25^– and CD8⁺ effector T lymphocytes exhibit pro-inflammatory effects (Stubbe et al., 2013). The role of T lymphocytes in stroke is complex. One previous study has shown that T-cell-deficient mice have reduced lesion size after experimental stroke (Hurn et al., 2007). And increased expression of Th1 and Th17 T cell types in stroke mice correlates with an increase in infarct volume (Singh-Manoux et al., 2014). Furthermore, in various pathological conditions, the production of IL-10 by Tregs depends on antigen specificity, which may explain why Tregs are beneficial only in certain circumstances (Becker et al., 2003). During the recovery period after stroke, peripheral immune cells can also produce factors that promote neurogenesis (Ma et al., 2021; Shi et al., 2021).

Astrocytes

Astrocytes are present throughout the central nervous system (CNS) and are crucial for maintaining the normal functioning of a healthy CNS (Seifert et al., 2006; Lee et al., 2022a). Additionally, astrocytes respond to all forms of CNS damage such as infection, trauma, ischemia, and neurodegenerative diseases. This process is commonly referred to as reactive astrogliosis and involves changes in molecular expression and morphology, with severe cases leading to scar formation (Williamson et al., 2021).

Reactive glial cells can protect the nervous system through various mechanisms. These include the uptake of potentially excitotoxic glutamate (Rothstein et al., 1996), production of glutathione to protect neurons from oxidative stress damage (Vargas et al., 2008), release of adenosine for neuroprotection (Lin et al., 2008), mitigation of NH4⁺ toxicity (Rao et al., 2005), degradation of amyloid-beta peptides (Koistinaho et al., 2004), promotion of BBB repair (Myer et al., 2006), and reduction of vascular edema, stabilization of extracellular fluid and ion balance, and lowering the threshold for seizure activity (Zador et al., 2009). Reactive astrogliosis is a complex process involving changes in gene expression and cellular alterations, finely regulated by intricate intercellular and intracellular signaling (Kim et al., 2016). Under different stimuli, astrocytes may generate various categories of intercellular effector molecules or modify the expression of molecules involved in cell structure, energy metabolism, intracellular signaling, membrane transport, and other aspects of cellular activity (Daginakatte et al., 2008; Williamson et al., 2021). Selective knockout of different signaling molecules in astrocytes leads to distinct effects. For example, knockout of signal transducer and activator of transcription 3 significantly reduces reactive astrogliosis, upregulation of glial fibrillary acidic protein, and scar formation but exacerbates inflammation, increases lesion area, and impairs functional recovery (Herrmann et al., 2008). Conditional deletion or functional knockout of cytokine signaling inhibitory molecule suppressor of cytokine signaling 3 or nuclear factor kappa-light-chain-enhancer of activated B cells in astrocytes can reduce inflammation and lesion size following spinal cord injury or experimental autoimmune encephalomyelitis (Okada et al., 2006; Brambilla et al., 2009). Under extreme injury conditions, mature astrocytes can re-enter the cell cycle and proliferate during scar formation (Buffo et al., 2008; Gadea et al., 2008). Neural glial antigen 2 progenitor cells, ependymal cell progenitors, and fibro-meningeal cells in the local perilesional tissue may also contribute to astrocyte scar formation (Carlén et al., 2009). Scars primarily form at the interface between healthy and injured tissue, exerting inhibitory effects on the diffusion of inflammatory cytokines, reducing the extent of damage, and promoting repair (Myer et al., 2006; Voskuhl et al., 2009). However, scars also hinder axonal growth and neural regeneration, thereby impeding recovery (Nowicka et al., 2008).

Implications for Treatment and Future Directions: from Conventional Therapies to the Rise of Stem Cell Therapy

Understanding the intricate interplay between inflammatory responses and immune cell dynamics in stroke is essential for developing targeted therapeutic interventions. While the inflammatory cascade contributes to tissue damage, it also plays a role in tissue repair and recovery. Modulating immune cell activation and polarization, as well as harnessing the neuroprotective functions of reactive astrocytes, holds promise for future stroke treatments. Further research is needed to unravel the complexities of these processes and translate findings into effective clinical strategies. We have summarized various treatment approaches for post-stroke treatment and provided detailed discussions on each of them (Table 1).

Table 1.

Overview of various treatments of stroke

Treatment strategy		Representative drug	Mechanism	Advantage	Limitation	Phase	Administration
Physical therapy	TMS (transcranial magnetic stimulation)		Astrocyte regulation, neural plasticity	Noninvasive treatment	Lack of clinical evidence	Sub-acute/Chronic
	Mechanical embolectomy		Remove the clot and restore blood supply	Maximum protection of neurons	Short time window, Placental thrombosis	Acute
Drug therapy	Thrombo	rt-PA	Remove the clot and restore blood supply	Maximum protection of neurons	Short time window	Acute	IV
	ADCs	Enlimomab	A murine intercellular adhesion molecule-1 (ICAM-1) antibody, reduces leukocyte adhesion	Reduce neutrophil and lymphocyte infiltration	The protective effect on infarct volume is limited	Acute/Sub-acute/Chronic
		Endogenous	IL-1 receptor antagonist (IL-1ra) anti-inflammation	Inhibit inflammation and reduces infection	Did not reduce disability rates	Acute/Sub-acute/Chronic
		FTY720 (fingolimod)	Sphingosine-1-phosphate (S1P) receptor agonist	Reduce neutrophil and lymphocyte infiltration	The protective effect on infarct volume is limited	Acute/Sub-acute/Chronic
Cell therapy	MSC/Gene modified MSC	UCMSCs (umbilical cord mesenchymal stem cells)	Immunomodulation, pro-angiogenic signaling, neurotrophic factor secretion, and neural differentiation	Well-established harvesting methods, low risk for tumorigenicity, and the absence of ethical issues	1. Require large numbers of donors 2. The quality of samples from different sources varies greatly.	Acute/Sub-acute/Chronic
	NPC	FOXG1 positive Neural progenitor cells	Integrating with the host brain by transforming into neural cells, form a functional loop	Drive from IPSCs or ES	1. Difficult to obtain high-purity neurons 2. Low differentiation efficiency	Sub-acute/Chronic	IC
	Neural cell	Fetus neuronal cell	Integrated with the host brain, sent out axonal processses, release neurotransmitters	Safety, low risk for tumorigenicity, the absence of ethical issues	ethical issue	Sub-acute/Chronic	IC
	Immunocyte	BMMC (bone marrow mononuclear cell)	1. Contain endothelial progenitor cells, which have been reported to contribute to revascularization of ischemic tissues and repair of injured endothelium 2. Migrate to the lesion site where they release cytokines and trophic factors, and can modulate neuronal death and inflammation in the penumbra area	Wide source, easy to obtain, the absence of ethical issues	1. The mechanism of action is unclear 2. The cell extraction process affects the therapeutic effect	Acute/Chronic	IV/ IC
		CD34 positive hematopoietic stem cells	Promotion of angiogenesis and neurogenesis	Easy to obtain,the absence of ethical issues	No functional improvement	Acute	IA

ADCs: Antibody-drug conjugates; BMMC: bone marrow mononuclear cells; ES: embryonic stem cells; FOXG1: Forkhead box protein G1; IA: intraarterial; IC: intracerebral; ICAM-1: intercellular adhesion molecule-1; IL-1ra: IL-1 receptor antagonist; IPSCS: induced pluripotent stem cells; IT: intrathecal or intracerebroventricular; IV: intravenous; MSC: mesenchymal stem cells; NPC: neural progenitor cells; S1P: sphingosine-1-phosphate; TMS: transcranial magnetic stimulation; UCMSCs: umbilical cord mesenchymal stem cells.

Studies have shown that inhibiting inflammation and modulating immune responses can effectively improve long-term neurologic function after stroke. For example, the cytokine IL-4 may improve long-term neurologic function after stroke by inducing an M2 phenotype in microglia/macrophages (Liu et al., 2016). Treatment with salidroside reduces the expression of M1 microglia/macrophage markers and increases the expression of M2 microglia/macrophage markers after ischemic stroke. It also induces primary microglia to transition from an M1 to M2 phenotype. Additionally, salidroside treatment enhances the phagocytic activity of microglia, inhibits the release of pro-inflammatory cytokines from microglia, and protects neurons from oxygen-glucose deprivation by promoting microglial M2 polarization. Through these mechanisms, salidroside significantly reduces cerebral infarction and improves neurologic function after cerebral ischemia (Liu et al., 2018). miR-124 has been shown to inhibit p53-mediated neuronal cell death after stroke (Liu et al., 2013).

However, there is still a lack of effective drugs for the treatment of stroke (Levard et al., 2021). Anti-inflammatory drugs, such as recombinant IL-1 receptor antagonist (IL-1Ra) (Anakinra) and tetracycline derivative—minocycline have been studied in clinical trials and have shown significant reduction in inflammation levels and occurrence of infection events. However, they could not reduce patient disability rates (Kohler et al., 2013). Animal studies have shown that humanized CD49d antibody (Natalizumab), sphingosine-1-phosphate receptor agonist (Fingolimod), and endothelial β2 integrin ligand intercellular adhesion molecule 1 antibody (Enlimomab) can attenuate neutrophil and lymphocyte infiltration into the CNS, but clinical trials have shown a limited impact on stroke outcomes (Zhu et al., 2015). Recombinant tissue plasminogen activator is currently the most effective drug for the treatment of acute ischemic stroke, but only a small percentage of clinical patients (< 2%) are currently eligible for recombinant tissue plasminogen activator treatment (Xiong et al., 2021). Mechanical thrombectomy, which is currently under development, can effectively extend the treatment window (Munich et al., 2019).

With the development of stem cell therapies, significant progress has been made in the research of using stem cells to treat various diseases and injury conditions (Turnbull et al., 2019; Zakrzewski et al., 2019; Wang et al., 2024). Stem cells are a class of cells with the potential for self-renewal and differentiation into different cell types. Based on their source, differentiation potential, and characteristics, stem cells can be categorized into various types such as embryonic stem cells (Martin, 1981), adult stem cells (Cable et al., 2020), induced pluripotent stem cells (Takahashi and Yamanaka, 2006), mesenchymal stem cells (Caplan, 1991), neural stem cells (Stemple and Anderson, 1992), and human amniotic epithelial cells (Shepard, 1958). Stem cells have therapeutic effects such as cell migration, angiogenesis, immune modulation, neuroprotection, and neural circuit reconstruction. Because stem cells can exert their therapeutic effects through multiple mechanisms (Figure 3), they are widely used in stroke treatment research (Chrostek et al., 2019; Suda et al., 2020; Li et al., 2021a, b; Pathipati et al., 2021).

Figure 3.

Principles of stem cell therapy for ischemic stroke.

After an ischemic stroke, stem cells proliferate and differentiate in the ischemic brain region and play multiple roles. Stem cells have the potential to differentiate into various cell types such as astrocytes, neurons, and endothelial cells, thereby promoting neural regeneration. The stem cells also provide trophic support by secreting neurotrophic factors and growth factors such as VEGF, BDNF, bFGF, NGF, and IGF-1. Through regulating the levels of IL-10, PGE2, TSG-6, IDO, TGF-β, and other inflammatory factors or related enzymes, the stem cells play roles in immunomodulation. They also enhance angiogenesis and vasculogenesis through promoting differentiation, chemotaxis, and improving the extracellular matrix. The stem cells enhance formation of new neural connections and synaptic activity and improve neural plasticity as well as synaptic remodeling. Created with BioRender.com. BDNF: Brain-derived neurotrophic factor; FGF: fibroblast growth factor; IDO: indoleamine-2,3-dioxygenase; IGF: Insulin-like growth factor; IL-10: interleukin-10; NGF: nerve growth factor; PGE2: prostaglandin E2; TGF-β: transforming growth factor-β; TSG: tumor supplied group of factors; VEGF: vascular endothelial growth factor.

Mesenchymal stem cells

It has been found that when mesenchymal stem cells (MSCs) treatment is administered during reperfusion, MSC-derived small extracellular vesicles mainly exist in the ischemia-reperfusion area in ionized calcium-binding adapter molecule 1–positive cells and glucose transporter 1–positive blood vessels; they also persist in ionized calcium-binding adapter molecule 1–positive cells for 72 hours after treatment (Pathipati et al., 2021). Additionally, MSC exosomes (MSC-Exo) can inhibit complement C5b-9 activation of neutrophils, thereby suppressing the release of neutrophil extracellular traps and IL-17 (Loh et al., 2022). MSCs can exert anti-inflammatory effects by interacting with the receptor Mincle, which is involved in recognizing DAMPs (Li et al., 2021c). MSCs can also inhibit inflammation through cell-to-cell interactions (Kikuchi-Taura et al., 2021). MSC-Exo has an inhibitory effect on inflammation-induced activation of astrocytes (Xian et al., 2019). The vascular endothelial growth factor secreted by MSCs can be taken up by endothelial cells and inhibit the activation of macrophages/microglia in a stroke mouse model (Kikuchi-Taura et al., 2021). Co-culturing MSCs with bone marrow–derived macrophages can induce more transformation of bone marrow–derived macrophages into anti-inflammatory M2 macrophages (Cho et al., 2014). MSC-derived small extracellular vesicles promote vascular regeneration in an ischemic stroke model by inhibiting signal transducer and activator of transcription 3-dependent autophagy, thereby facilitating the repair of the neurovascular unit that controls the BBB and promoting comprehensive recovery from stroke (Xia et al., 2020; Davis et al., 2021). Interferon-gamma–activated adipose-derived MSCs can induce the recruitment and differentiation of oligodendrocyte progenitor cells in vivo, promoting the repair of damaged neurons (Tobin et al., 2020).

Exosomes rich in the miR-17-92 cluster may activate the phosphoinositide 3-kinase/protein kinase B/rapamycin/glycogen synthase kinase 3β signaling pathway by targeting phosphatases and tension protein homologs, promoting neuroplasticity and functional recovery after stroke (Xin et al., 2017). MSC-Exo improves calcium signaling abnormalities and mitochondrial dysfunction induced by lipopolysaccharide in culture, as well as learning and memory impairments in sepsis-induced mice (Xian et al., 2019). A small-scale clinical study confirmed that compared to the group not receiving MSCs treatment, the MSC-treated group showed a significant reduction in the cortical spinal tract and posterior limb of internal capsule anisotropy at 90 days after stroke (Lee et al., 2022b). Intravenous injection of bone marrow stromal cells preferentially migrates to the spleen and eliminates chronic inflammation in stroke (Acosta et al., 2015). Although intravenous injection of MSCs is effective in experimental stroke, low cell engraftment and limited functional capacity of transplanted cells are the key limitations to clinical application (Kawabori et al., 2020). Researchers have further investigated the therapeutic effect of modified MSCs on stroke. C-C motif chemokine ligand 2 (CCL2) is associated with post-stroke neurorepair and transports various cells to the brain through CCL2/CCR2 (CCL2 receptor) interactions (Huang et al., 2018). A previous animal study has shown that compared to unmodified MSCs, MSCs overexpressing CCL2 exhibit better suppression of neuroinflammation, promotion of angiogenesis, and endogenous neural regeneration, but increased inflammatory cell infiltration during the healing process (Lee et al., 2020).

Neural stem cells

The research on rebuilding damaged neural circuits after a stroke through neuron transplantation began in 1988 (Farber et al., 1988). Studies based on neuro-replacement therapy strategies suggest that transplanted neurons can survive in the recipient’s brain and promote behavioral functional recovery in animals (Grønning Hansen et al., 2020; Palma-Tortosa et al., 2020). Transplanting human neural stem cells intracranially within 24 hours after a stroke can improve tPA-induced delayed cerebral injury following a stroke (Boese et al., 2020). Transplanting neurons derived from the subventricular zone of rats through arterial injection into subacute stroke model rats can improve the prognosis of stroke in rats (Kondori et al., 2020). Moreover, cells transplanted into different regions can migrate to the injured area, and the difference in transplantation sites does not affect the fate of the cells (Kosi et al., 2018). Transplanted neurons receive direct synaptic inputs from neurons in different host brain regions, resembling the patterns of neurons projecting to the corresponding endogenous cortical neurons in the intact brain (Palma-Tortosa et al., 2020). This indicates that transplanted neurons can integrate into endogenous neural circuits and substitute for the function of neurons lost due to stroke. A clinical study has shown that transplanting neural stem cells into the striatum of patients can improve their motor function (Muir et al., 2020). However, relevant clinical experiments are currently lacking for validation.

Amniotic epithelial stem cells

The amniotic membrane is a thin membrane located on the inner side of the fetal placenta, and the embryo resides within the amniotic cavity formed by it (Mamede et al., 2012). The formation of the amniotic membrane undergoes two developmental peaks. During early amnion development, it follows a nutrition-based path to form the amniotic cavity, while during late amnion development, it follows a non-neuroectodermal-like transcriptional program (Rostovskaya et al., 2022). The amniotic membrane is composed of five layers: epithelium, basement membrane, compact layer, fibroblast layer, and spongy layer (Bourne, 1966; Liu et al., 2021). Amniotic epithelial stem cells are located in the epithelial layer of the amniotic membrane, in direct contact with the amniotic fluid (Muttini et al., 2018). Human amniotic epithelial stem cells (hAESCs) demonstrate greater advantages than other types of stem cells, in terms of availability, easy accessibility, reduced immunogenicity, non-tumorigenicity, and minimal legal constraints, and have accordingly attracted significant attention from the scientific community (Yang et al., 2018; Zhang and Lai, 2020; Liu et al., 2021). Increasing evidence suggests that hAESCs can promote the survival and regeneration of neuron cells, repair damaged neurons, and reconstruct damaged neural connections (Xu et al., 2019). This indicates that hAESCs may be the most promising candidates for cell therapy in neurological disorders. Studies have shown that in a mouse model of intrauterine adhesion, treatment with hAESCs reduced fibrosis, increased microvessel density, and upregulated the expression of vascular endothelial growth factor, proliferating cell nuclear antigen, and estrogen receptor, thereby indicating improved angiogenesis and stromal cell proliferation (Li et al., 2019). In a study on mice with experimental autoimmune encephalomyelitis, hAESCs treatment suppressed T-cell responses and the production of pro-inflammatory cytokine IL-17A, promoted a significant increase in peripheral regulatory T-cells and naïve CD4⁺ T-cells, and increased the ratio of Th2 cells in peripheral lymphoid organs and the CNS (McDonald et al., 2015). In a mouse model of CCl4-induced liver injury, the hAESCs treatment group showed lower levels of TNF-α and IL-6 proteins, higher levels of IL-10, and reduced cellular fibrotic area (Manuelpillai et al., 2010). These studies indicate that hAESCs have excellent anti-inflammatory and immune-regulatory capabilities. Some researchers have applied hAESCs in the treatment of stroke. Transplantation of hAESCs into the cerebral cortex of a stroke model in mice can migrate to the injury site and promote the recovery of motor function (Liu et al., 2008). In another study, researchers administered hAESCs via tail vein injection 1.5 hours after stroke in mice. The hAESCs migrated to the ischemic brain area, reduced brain inflammation, prevented expansion of the infarct area, and improved motor deficits. Furthermore, if the administration of hAESCs was delayed until 1 or 3 days after stroke, long-term functional recovery was still enhanced in young and aged male and female mice. This study also demonstrated in a rhesus monkey model of stroke that acute intravenous injection of hAESCs can prevent the expansion of the infarct area from day 1 to day 10 after stroke (Evans et al., 2018). Overall, hAESCs possess multiple effects, including inflammation inhibition, immune modulation, and neuroprotection (Xu et al., 2019). Additionally, human amniotic epithelial stem cells have the advantages of easy accessibility and low immunogenicity, making them an ideal candidate for stroke treatment.

Pluripotent stem cells

Pluripotent stem cells (PSCs) are a category of biological cells with unique capabilities and the potential for unlimited proliferation and differentiation into cells of all three germ layers. Consequently, these attributes make PSCs a potent resource for treating various diseases and injuries. PSCs primarily exist as two main types: embryonic stem cells and induced iPSCs (iPSCs; Yamanaka, 2020). Currently, research on the use of PSCs for the treatment of stroke primarily includes the following three approaches: i) Directly transplanting neural lineages differentiated embryonic stem cells (ESCs; Wei et al., 2005) and iPSCs (Chen et al., 2010) into the site of injury, utilizing their paracrine effects to improve the ischemic microenvironment; ii) Differentiating ESCs (Chang et al., 2013) and iPSCs (Lee et al., 2017) into neural precursor cells before transplanting them into the area affected by brain infarction, aiming to reconstruct damaged neural circuits; iii) Differentiating iPSCs and ESCs into MSCs and administering them via intravenous infusion and allowing them to play roles in immunomodulation and neuroprotection (Xia et al., 2020; Asgari Taei et al., 2021).

Other types of stem cells

CD34⁺ hematopoietic stem/progenitor cell therapy has been shown to improve functional recovery in ischemic stroke rat models by promoting angiogenesis and neurogenesis. Phase I clinical trials have demonstrated its safety, but further research is needed to assess its clinical efficacy (Shyu et al., 2006; Banerjee et al., 2014). When transplanted into a transient middle cerebral artery occlusion mouse model with BBB disruption, human PSC-derived pericyte-like cells effectively promote neural functional recovery by restoring BBB integrity and preventing neuronal apoptosis. Muse cells (a type of natural PSCs in the connective tissues of nearly every organ and thus considered non-tumorigenic (Li et al., 2022), endothelial cells (Bayraktutan, 2019), and others are also used in the treatment of stroke. In addition, extracellular vesicles are also frequently utilized in stroke treatment and are derived from various cells including iPSCs (Niu et al., 2023; Zhang et al., 2023), neural progenitor cells (Tian et al., 2021), MSCs (Su et al., 2019; Xian et al., 2019), and ESCs (Xia et al., 2021b). Its mechanism of action is similar to that of MSCs, mainly including promoting angiogenesis (Xia et al., 2020; Asgari Taei et al., 2021), inflammation suppression (Tian et al., 2021), immune modulation (Xia et al., 2021b), and neuroprotection (Yoon et al., 2022).

Clinical Trials in Stroke Treatment

According to data from the World Health Organization, there were a total of 697 clinical studies related to stroke conducted globally in 2022, with China accounting for 111 of them. For example, the research team led by Yongjun Wang has conducted several preliminary clinical studies related to stroke. A 2019 study indicated that compared to patients treated with clopidogrel plus aspirin, the use of ticagrelor combined with aspirin resulted in a lower proportion of high platelet reactivity in patients with mild stroke or transient ischemic attack, especially in those carrying loss-of-function alleles of cytochrome P450 2C19 (CYP2C19; Wang et al., 2019). A 2021 study found that among Chinese patients with mild ischemic stroke or TIA carrying loss-of-function alleles of CYP2C19, the 90-day stroke risk was slightly lower in the ticagrelor group than the clopidogrel group. The risks of severe or moderate bleeding did not differ between the two treatment groups, but ticagrelor was associated with more total bleeding events (Wang et al., 2021). Additionally, in recent years, the development of stem cell therapeutics has been thriving, with an increasing number of clinical trial projects worldwide focusing on stem cell-based treatments for stroke. We have also compiled an overview of various ongoing stem cell therapy projects (Table 2). These studies provide a significant reference value for clinical medication in stroke patients.

Table 2.

Overview of recent clinical trials on stem cell therapies for stroke

Study	Status	Conditions	Study phase	Study design	Mode of delivery	Cell type	Dose	Time from onset	Follow-up (mon)	Country	Last update posted	NCT number
Perinatal arterial stroke treated with stromal cells intranasally	Completed	Perinatal arterial ischemic stroke neonatal stroke	Phase 1 Phase 2	Allocation: N/A Intervention model: Single group assignment Masking: none (open label) Primary purpose: treatment	Nasal route	Bone marrow–derived allogeneic MSCs	50×106	Within the first wk of onset of presenting clinical symptoms	3	Netherlands	7-Oct-21	NCT03356821
A safety and tolerability study of neural stem cells (NR1) in subjects with chronic ischemic subcortical stroke (ISS)	Recruiting	Ischemic stroke	Phase 1 Phase 2	Allocation: N/A Intervention model: Single group assignment Masking: none (open label) Primary purpose: treatment	Injected intracerebrally	Human embryonic stem cell (hESC) derived neural stem cells		6–60 mon from time of stroke	12	United States	10-May-22	NCT04631406
Combination of conditioned medium and umbilical cord–mesenchymal stem cells therapy for acute stroke infarct	Recruiting	Ischemic stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: factorial assignment Masking: none (open label) Primary purpose: treatment	Intra-parenchymal transplantation	Umbilical cord MSCs	20×106	Ischemic stroke acute phase	6	Indonesia	26-Sep-22	NCT05008588
Umbilical cord–derived mesenchymal stem cells for ischemic stroke (UMSIS): a prospective, double-blinded, randomized controlled, pilot study	Recruiting	Ischemic stroke	Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously injected	Umbilical cord-derived MSCs	1×108 cells	Group A (6–24 h), group B (1–3 d), gourp C (4–7 d), group D (1–4 wk), group E (1–6 mon)	12	China	25-May-23	NCT04811651
Regenerative stem cell therapy for stroke in Europe 1	Unknown	Stroke	Phase 1	Allocation: randomized Intervention model: sequential assignment Masking: none (open label) Primary purpose: treatment		Adipose derived stem cell	1×106 cells/kg to 3×106 cells/kg	Within 1st and 2nd d after stroke onset	24		27-Jun-18	NCT03570450
Safety of cultured allogeneic adult umbilical cord derived mesenchymal stem cell intravenous infusion for stroke	Recruiting	Stroke	Phase 1	Allocation: N/A Intervention model: single group assignment Masking: none (open label) Primary purpose: treatment	Intravenous infusion	Cultured allogeneic adult umbilical cord derived MSCs	1×108 cells		48	Mexico	27-Oct-22	NCT05158101
Stem cell infusion in the treatment of patients with neurological complications after ischemic stroke	Recruiting	Ischemic stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: single (participant) Primary purpose: treatment	Intrathecal route	Umbilical cord-derived MSCs (UC-MSCs)	1.5×106 cells/kg	Time from onset to study participation ≤ 24 months	12	Vietnam	2023/4/18	NCT05292625
Allogeneic adipose tissue–derived mesenchymal stem cells in ischemic stroke	Recruiting	Allogeneic adipose tissue-derived mesenchymal stem cells in ischemic stroke	Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	intravenously	Adipose tissue-derived stem cell	1×106 cells/kg (10 million cells/mL)	within the first 4 d (±1) from acute stroke symptoms onset	24	Spain	2023/5/30	NCT04280003
Evaluation of the safety and efficacy of hemacord HPC, cord blood in subjects with acute ischemic stroke	Unknown	Acute ischemic stroke	Phase 1	Allocation: N/A Intervention model: single group assignment Masking: none (open label) Primary purpose: treatment	Intravenous infusion and intrathecal injection	Allogeneic cord blood hematopoietic progenitor cells (HPC, cord blood)	2.5×107 cells/kg; 150×107 cells)	Within 9 d	12	United States	1-Dec-20	NCT03735277
A clinical study of iNSC intervent cerebral hemorrhagic stroke	Unknown	Cerebral hemorrhagic stroke	Early Phase 1	Intervention model: sequential assignment Masking: none (open label) Primary purpose: treatment	Brain injection iNSC	Induction of neural stem cells		At least 12 mon but no more than 60 mon	12		31-Oct-18	NCT03725865
An inactivated influenza vaccine (split virion) shall be used during the study.	Recruiting	Acute ischemic stroke	Phase 1	Allocation: N/A Intervention model: single group assignment Masking: none (open label) Primary purpose: treatment	Combination of intra arterial (IA) and intravenous (IV)	Umbilical cord MSCs (UCMSCs)	One dose of IV administration followed by low or high doses of IA infusion		12	Taiwan, China	23-Mar-22	NCT04434768
Clinical trial of human umbilical cord mesenchymal stem cells (IxCell hUC-MSC-S) in the treatment of ischemic stroke	Not yet recruiting	Ischemic stroke	Phase 1	Allocation: randomized Intervention model: parallel assignment Masking: none (open label) Primary purpose: treatment	Intravenously	Human umbilical cord MSCs	(5–20.0)×107 cells	12–24 wk	24	China	12-Apr-23	NCT05697718
Investigation of neural stem cells in ischemic stroke	Terminated	Ischemic stroke Chronic stroke Hemiparesis Arm paralysis	Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intracerebrally	Neural stem cells	2×107	6–24 mon	12	United States	13-Aug-21	NCT03629275
Autologous bone marrow mesenchymal stem cells (BMSCs) transplantation in the treatment of ischemic stroke	Recruiting	Ischemic stroke	Phase 1	Allocation: randomized Intervention model: parallel assignment Masking: none (open label) Primary purpose: treatment	Intravenously	Bone marrow MSCs (BMSCs)	1×106/kg (80±5 mL)	≤ 3 yr	24	China	9-May-23	NCT05850208
Allogenic mesenchymal stem cell derived exosome in patients with acute ischemic stroke	Unknown	Acute ischemic stroke	Phase 1 Phase 2	Allocation: N/A Intervention model: single group assignment Masking: none (open label) Primary purpose: treatment	Stereotaxis/intraparanchymal	MSC-generated exosome transfected by miR-124		Less than 24 h	12	Iran	25-Jan-21	NCT03384433
Mesenchymal stem cells for the treatment of acute ischemic stroke	Recruiting	Acute ischemic stroke	Phase 1	Allocation: N/A Intervention model: single group assignment Masking: none (open label) Primary purpose: treatmen	Intravenously	Human umbilical cord derived–MSCs		48–168 h	15	Taiwan, China	6-Dec-22	NCT04097652
A randomized placebo-controlled multicenter trial to evaluate the efficacy and safety of JTR-161, allogeneic human dental pulp stem cell, in patients with acute ischemic stroke (J-REPAIR)	Completed	Acute ischemic stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously	Allogeneic stem cell	1×108 or 3×108 cells/subject	Within 48 h	12	Japan	30-Jun-22	NCT04608838
MultiStem® administration for stroke treatment and enhanced recovery study	Recruiting	Acute ischemic stroke	Phase 3	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously	Bone marrow-derived cells	1.2×109 cells	18–36 h	3		9-Feb-22	NCT03545607
Clinical plan of ischemic stroke	Unknown	Stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: triple (participant, care provider, investigator) Primary purpose: treatment	Intravenously	Allogeneic bone marrow MSCs	(0.5–2)×106 / kg	More than 6 mon	12	China	8-Jul-21	NCT04953663
Allogeneic mesenchymal stem cells for the survivors of ischemic stroke trial (ASSIST)	Recruiting	Ischemic stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: triple (participant, care provider, investigator) Primary purpose: treatment	Intravenously	Human bone marrow MSC (boosting the hypoxia process)	(0.5–2)×106 / kg	More than 6 mon	12	China	31-Mar-22	NCT04590118
Efficacy of sovateltide (PMZ-1620) in patients of acute ischemic stroke	Completed	Acute ischemic stroke	Phase 3	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously	Peptide Sovateltide (IRL-1620)	0.3 μg/kg	< 24 h	3	India	2-Mar-22	NCT04047563
Effect of mesenchymal stem cells (MSCs) transplantation for acute cerebral infarction patients	Recruiting	Acute ischemic stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously	HCB MSCs	2×106/kg	< 24 h	24	China	18-Apr-23	NCT04093336
A study of NCS-01 in patients with acute ischemic stroke	Recruiting	Acute ischemic stroke	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment		Human bone marrow derived cells		< 24 h	12	United States	25-Apr-22	NCT03915431
PMZ-1620 (Sovateltide) in acute ischemic stroke patients	Completed	Acute ischemic stroke	Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously	Peptide Sovateltide (IRL-1620)	0.3 μg/kg/time (total dose/day: 0.9 μg/kg body weight)	< 24 h	3	India	2-Mar-22	NCT04046484
Combination therapy of umbilical cord blood and erythropoietin for stroke paients	Unknown		Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: quadruple (participant, care provider, investigator, outcomes assessor) Primary purpose: treatment	Intravenously	UCB and EPO combination therapy	2×107/kg	1–9	6	Korea	5-Nov-20	NCT04013646
Human umbilical cord mesenchymal stem cell therapy (19#iSCLife®-CI) for cerebral infarction patients in convalescent period	Suspended	Cerebral infarction	Phase 1 Phase 2	Allocation: randomized Intervention model: parallel assignment Masking: double (participant, investigator) Primary purpose: treatment	Intravenously	Umbilical cord MSCs			6	China	45029	NCT03176498
The safety and efficacy of human umbilical cord mesenchymal stem cells (19#iSCLife®-ACI) in the treatment of acute cerebral infarction	Suspended	Ischemic stroke	Phase 1	Allocation: randomized Intervention model: parallel assignment Masking: double (participant, investigator) Primary purpose: treatment		Umbilical cord MSCs	0.5–1×106/kg. once a mon, total 3 times	2wk	6		13-Apr-23	NCT03186456
Safety and clinical outcomes with amniotic and umbilical cord tissue therapy for numerous medical conditions	Not yet recruiting		Phase 1	Allocation: non-randomized Intervention model: parallel assignment Masking: none (open label) Primary purpose: treatment	Intravenously	Amniotic and umbilical cord cell			120	United States	22-Aug-19	NCT03899298
Feasibility and safety of umbilical cord blood transfusion in the treatment of neonatal cerebral ischemia and anemia	Unknown	Ischemic stroke/HIE	Phase 1	Allocation: non-randomized Intervention model: parallel assignment Masking: none (open label) Primary purpose: treatment	Intravenously	UCB		Within 48 h after the birth	2	Hong Kong, China	7-Jun-18	Unknown

EPO: Erythropoietin; hESC: human embryonic stem cells; MSC: mesenchymal stem cells; UCB: umbilical cord blood.

Limitations

This review has some limitations. First, the damage caused by stroke involves various types of neurons, and different neurons exhibit varying sensitivity to this damage (Guo et al., 2021; Zheng et al., 2022). This paper primarily focuses on the overall pathological response and does not currently address the differences in damage sensitivity among different neuronal subgroups following stroke, as well as their impact on patient prognosis. Research and summarization of these different neuronal subgroups are necessary and should be one of the key directions for future research. In addition, this paper summarizes stem cell therapy for stroke projects conducted in China but does not cover international stem cell therapy projects for stroke, which needs to be supplement in the future. Finally, although there is increasing research in the field of stem cell therapy, there is still no clear and affirmative outcome. This suggests that stem cell therapy is still an area that requires continuous breakthroughs and our persistent efforts.

Concluding Remarks

Ischemic stroke continues to pose a significant global health burden, particularly in China. The interplay between inflammatory responses and stroke pathophysiology plays a crucial role in tissue damage and repair. By elucidating the roles of several major cell types associated with immune response, including microglia, neutrophils, monocytes, lymphocytes, and astrocytes, this review sheds light on potential therapeutic targets to improve stroke outcomes. Further research in this field is essential to develop effective treatments and reduce the burden of ischemic stroke worldwide. Building upon this context, the prospects of stem cell therapy in stroke treatment can be explored.

Stem cell therapy holds promise in stroke treatment owing to its regenerative and immunomodulatory properties. Various types of stem cells have shown potential in promoting tissue repair, reducing inflammation, and enhancing functional recovery after stroke. These stem cells can be derived from different sources, such as bone marrow, umbilical cord blood, and adipose tissue, providing a wide range of options for therapeutic application. However, further research is needed to optimize the delivery methods, dosage, and timing of stem cell therapy in stroke treatment. The safety and long-term effects also require careful evaluation. Nonetheless, the potential of stem cell therapy to improve stroke prognosis and contribute to neurodegeneration makes it an exciting area of research with significant future prospects.

Funding Statement

Funding: This work was supported by grants from the Major Program of National Key Research and Development Project, Nos. 2020YFA0112600 (to ZH); the National Natural Science Foundation of China, No. 82171270 (to ZL); Public Service Platform for Artificial Intelligence Screening and Auxiliary Diagnosis for the Medical and Health Industry, Ministry of Industry and Information Technology of the People's Republic of China, No. 2020-0103-3-1 (to ZL); the Natural Science Foundation of Beijing, No. Z200016 (to ZL); Beijing Talents Project, No. 2018000021223ZK03 (to ZL); Beijing Municipal Committee of Science and Technology, No. Z201100005620010 (to ZL); CAMS Innovation Fund for Medical Sciences, No. 2019-I2M-5-029 (to YW); and Shanghai Engineering Research Center of Stem Cells Translational Medicine, No. 20DZ2255100 (to ZH).