Stem Cells Attenuate the Inflammation Crosstalk Between Ischemic Stroke and Multiple Sclerosis: A Review

Abstract

The immense neuroinflammation induced by multiple sclerosis (MS) promotes a favorable environment for ischemic stroke (IS) development, making IS a deadly complication of MS. The overlapping inflammation in MS and IS is a prelude to the vascular pathology, and an inherent cell death mechanism that exacerbates neurovascular unit (NVU) impairment in the disease progression. Despite this consequence, no therapies focus on reducing IS incidence in patients with MS. To this end, the preclinical and clinical evidence we review here argues for cell-based regenerative medicine that will augment the NVU dysfunction and inflammation to ameliorate IS risk.

Introduction

Ischemic stroke (IS), the most common cause of death and long-term disability worldwide ¹ , and multiple sclerosis (MS), the number one neurodegenerative disease in young adults ² , are two debilitating disorders with an unexpected relationship. MS is commonly described as a chronic autoimmune disorder targeting the myelin axon sheaths in the central nervous system (CNS). In contrast, IS involves acute cell death following nutrient and oxygen deprivation ³ . Despite the variety in onset, the common symptoms of neurological dysfunction seen in IS and MS can be explained by comparable demyelination, axonal injury, vascular impairment, neurodegeneration, glial cell activation, and immune cell proliferation. Furthermore, both MS and IS induce neurovascular unit (NVU) permeability due to inflammatory cytokines inducing changes within the NVU components ⁴ . NVU permeability permits further immune cell infiltration and even greater subsequent neuroinflammation. Considering a similar inflammatory pathophysiology, it is understandable that MS has been correlated to increased risk of IS^2,5. In fact, MS has been shown to increase the risk of IS by 6.09× ² .

Another devastating link between these pathologies is the lack of a cure. While many pharmaceuticals have been developed for MS, they only stabilize or delay symptom onset ⁶ . Considering the significant neuroinflammation of MS contributing to IS, a prominent goal for future treatment development should target and rapidly reverse this inflammation. The established anti-inflammatory effects ⁷ of stem cells (SCs) and recent SC advancements for treatment of IS and MS prompt discussion on the use of early SCs to lessen the risk of stroke in patients with MS. Furthermore, SCs are shown to restore NVU permeability, and thus may offer even greater therapeutic benefit in this respect. In this review, we will explore the underlying neuroinflammation in IS and MS, the SC advancements for both pathologies, and consider the early administration of anti-inflammatory cell-based therapy as a prophylactic measure against MS-induced stroke (Figure 1).

Figure 1.

This figure exemplifies the role of MS-induced neuroinflammation in prompting atherosclerosis and IS via pro-inflammatory immune cell and cytokine infiltration. On the left, MS induces inflammatory cytokines, Th17, Th1, and inflammatory microglia proliferation in the circulation. These mediators induce atherosclerosis and eventual ischemic stroke as shown on the right. The middle panel demonstrates how stem cells can reduce this inflammation by increasing Treg and anti-inflammatory microglial proliferation, ultimately reducing stroke and MS damage. Once MS neuroinflammation is downregulated, the relationship between atherosclerosis, stroke, and MS would be theoretically negligible, as described in the middle panel. MS: multiple sclerosis; IS: ischemic stroke.

Neuroinflammation in Multiple Sclerosis

The defining pathological characteristic associated with MS presents as demyelinating lesions formed by the breakdown of the blood–brain barrier (BBB), an aspect of the NVU, allowing for trans-endothelial migration of activated leukocytes to the CNS that trigger demyelination and neuroinflammation ⁸ . Perhaps most significant to such early pathogenesis of MS is the leakage of fibrin into the CNS, which precedes leukocyte diapedesis but coincides with microglial activation. Other hemostatic elements enhance neuroinflammation by promoting leukocyte extravasation or play a potential neuroprotective role (reviewed in Abbadessa et al. ⁹ ). Ultimately, later neuroinflammation in MS is characterized by the migration of activated lymphocytes, reactive gliosis, neuronal death, and axonal destruction.

Lymphocytic infiltration is a significant driver of neuroinflammation in MS, and among the T-cell lineage, self-reactive T helper (Th) cells are the primary modulators of inflammation. Th1 cells, the most frequently identified in experimental autoimmune encephalomyelitis (EAE) animal models of MS, are hypothesized to target microglial activation through the pro-inflammatory interleukin (IL)-12/ interferon (IFN-γ) axis ¹⁰ . Meanwhile, Th17 cells are involved in MS pathogenesis through promotion of BBB disruption and activation of astrocytes and microglia to amplify neuroinflammation ¹⁰ . Another subpopulation of T lymphocytes, T regulatory cells (Tregs) (an anti-inflammatory derivative of T cells) also contribute to MS through certain genetic abnormalities, such as intrinsic Treg defects or the presence of suppressive markers preventing Tregs from downregulating the pro-inflammatory response ¹⁰ . Regarding B-cell involvement, intrathecal immunoglobulin is a critical determinant of MS diagnosis and management. Animal models have demonstrated that B cells aid in Th17 (a pro-inflammatory derivative of T cells) differentiation, inhibit development of Tregs, and produce pro-inflammatory markers ¹¹ . While bidirectional B-cell trafficking between the CNS and periphery is poorly understood, B-cell infiltrates are abundant in patients with active MS ¹² .

Disruption of the BBB and lymphocytic infiltrates in the CNS activate microglia to induce synaptic pruning and secrete inflammatory mediators that result in an overall neurotoxic effect. Focal MS lesions are driven by microgliosis triggering inflammation, demyelination, neurodegeneration, while microglial nodules are observed in white matter proximal to the plaques promoting axonal degeneration, oligodendrocyte stress, and complement product deposition ¹³ . Reactive astrogliosis is an additional contributor to persistent neuroinflammation, characterized by the release of pro-inflammatory cytokines and neurotoxic reactive oxygen/nitrogen species to increase BBB permeability, as well as glial scar formation, prevention of axonal regeneration, and inhibition of oligodendrocyte proliferation ¹⁴ . Furthermore, remyelination failure is enhanced by extracellular matrix molecule deposition of proteins which impair oligodendrocyte recruitment and enhance inflammation ¹⁵ .

Understanding the precise mechanism behind neuroinflammation in MS is of great therapeutic interest. Inflammasomes of innate immunity have also risen to prominence as mediators of such neuroinflammation through EAE models and patients with genetic deficiencies in inflammasome regulatory genes ¹⁶ . Mitochondrial function and accumulation of mitochondrial DNA damage are also hypothesized to contribute to neuroinflammation associated with MS (reviewed in Bargiela and Chinnery ¹⁷ ). An emerging target underlying neuroinflammation in MS manifests as altered synapses, occurring independent of demyelination and axonal destruction, that contribute to significant excitotoxic damage; stabilization, repair, and regeneration of excitotoxic synaptic abnormalities offers potential to reduce neuroinflammation throughout the stages of disease ¹⁸ . Overall, neuroinflammation in MS represents a complex interplay of mediators at both a cellular and molecular level that define and influence the course and morbidity of disease.

Risk of Ischemic Stroke Due to Neuroinflammation in the Setting of Multiple Sclerosis

Many theories explaining the relationship between IS and MS have been hypothesized (reviewed in Hong et al. ² ). For instance, early childhood obesity increases the risk of not only MS but also atherosclerosis and stroke. The continuous alteration in myelination characteristic of MS is also related to atherosclerosis. Furthermore, lifestyle risks for stroke, such as smoking and sedentary lifestyles, are more common among patients with MS. Glucocorticoids, a standard treatment for MS, are also related to cardiovascular and cerebrovascular diseases. Surprisingly, even a genetic overlap of 24 genes is reported between individuals with MS and individuals with IS ¹ .

One of the most pertinent contributors to stroke risk in patients with MS, however, is the chronic inflammatory state ¹⁹ . In MS, the classic demyelinated lesions are filled with CD4+ T cells, CD8+ T cells, and plasma cells, demonstrating an active and continuous pro-inflammatory environment (reviewed in Lopes Pinheiero et al. ³ ). In addition, the lesions show macrophages full of myelin, neurodegeneration, oligodendrocyte death and malfunction, and activated microglia and astrocytes. These immune cells not only destroy myelin sheaths with phagocytosis but also release IFN-γ and IL-17, propagating a toxic inflammatory environment. Inflammation and immune cell migration to ischemic regions is similarly observed in stroke, with neutrophils inducing an inflammatory cascade worsened by the later arrival of macrophages and lymphocytes. This invasion into the CNS leads to immense levels of pro-inflammatory cytokines and similar microglial and astrocytic activations ²⁰ . In fact, anti-inflammatory medications used to lessen MS symptoms are also effective to mitigate stroke damage, such as Fingolimod and Natalizumab^21–23.

Cytokines such as IL-1β, IL-6, IL-17, and tumor necrosis factor (TNF)-α are common to lesions secondary to both MS and IS ²⁴ . Besides the injury to nervous tissue, chronic inflammation (like in MS) also induces detrimental vessel changes and instigates atherogenesis ²⁵ . Circulating monocytes home to atherosclerotic plaques and differentiate into lipid-laden macrophages ²⁶ . These macrophages then overproduce matrix metalloproteinases to degrade the plaque’s collagen architecture, increasing the risk of plaque rupture and thrombosis. In addition, CD40 ligand stimulates macrophages to express procoagulant tissue factor ²⁶ . Beyond macrophages, platelet aggregation has also been associated with MS neuroinflammation ²⁷ . Altogether, pathological manifestations of MS-induced chronic inflammation may lead to a hypercoagulative environment, possibly resulting in subsequent cerebral ischemia. Interestingly, similar to how Fingolimod and Natalizumab^21–23 reduce stroke damage, thrombolytic therapies, such as tissue plasminogen activator, has shown benefit in patients with MS ³ .

A systematic review and meta-analysis incorporating nine studies with 380,000 patients revealed that patients with MS had a significant risk in overall stroke at 1 year and at 10 to 13 years. Of particular interest, there was a 6.09× increased risk of IS in patients with MS compared with those without ² . Risk factors for IS are more common in patients with MS compared with matched controls; notably hypertension and hyperlipidemia are most commonly reported with prevalence >10% ⁵ . In all, 27.1% of patients with MS are comorbid with diabetes mellitus, which itself increases the relative risk of stroke 3–5× in the general population ⁵ . Understanding the possible neurological sequelae in the setting of MS, amelioration of neuroinflammation is critical and may be accomplished with SCs.

In an acute ischemic cerebrovascular insult, cytokines released by the innate immune system result in increased permeability of the BBB and subsequent extravasation of neutrophils; lymphocytes are dominant in the later phases of stroke ²⁸ . There is an ongoing debate on intrathecal B-cell recruitment and subsequent oligoclonal band synthesis in the setting of an acute IS. In MS, however, this relationship is clearly established. B cells are also important mediators of MS; interestingly, lymphoid organs similar to lymph nodes are formed within the meninges in the setting of secondary progressive MS, and it is suggested that B cells are responsible for the initiation ²⁹ . In fact, B cells are utilized in both the treatment and diagnosis of MS. Anti-CD20 agents such as rituximab are utilized in clinical practice to prevent exacerbations, and intrathecal IgG synthesis is screened for in cerebrospinal fluid (CSF) as a diagnostic tool. In 2012, Prüss et al. ³⁰ reported from their patients who had an IS that 25% had intrathecal IgG and 18% had oligoclonal bands, with no significant difference whether the patient’s stroke was first-time versus recurrent. These findings prompted Laichinger et al. to investigate further; on the contrary, they reported that, of their 212 patients, only 6% had intrathecal IgG, 5% had isolated oligoclonal bands, and only 3% had both intrathecal IgG and oligoclonal bands. Intrathecal IgG was more common in patients who had previously suffered a stroke. Importantly, a third of the patients had a concomitant inflammatory nervous system disease such as MS or neurosarcoid. They concluded that these CSF findings are likely not related to acute IS, and indeed secondary to patients with neuroinflammation such as in MS or previous strokes ³¹ . This proves to be an important distinction between ISs and MS.

Stem Cell Therapies for Multiple Sclerosis

SCs are frequently appreciated for their significant anti-inflammatory and regenerative properties ⁷ , making them an ideal therapeutic for a diagnosis defined by cell destruction and death and neuroinflammation (Table 1).

Table 1.

Anti-inflammatory Stem Cell Studies for Multiple Sclerosis.

Citation	Sample	Cell type	Route	Dosage	Results
Yang et al. 32	EAE Mice	CCR5-expressing NSCs	Intravenous	1.5 × 106 cells	CCR5 expressing NSCs migrated to inflammatory lesions in the CNS to suppress inflammation, fix myelin, and improve clinical presentation of the mice.
Hou et al. 33	EAE Mice	Minocycline and BMSCs	Intravenous cells Intraperitoneal minocycline	1.5 × 106 cells 10 mg/kg minocycline	Minocycline and stem cells used together amplify functional improvement, lessen inflammation, improve demyelination, and decrease neurodegeneration. The anti-inflammatory cytokines were also increased, while pro-inflammatory were decreased. Further staining revealed less apoptotic cells.
Yang et al. 34	EAE Mice	Bone marrow–derived NSCs (BM-NSCs) with neurotrophin 3 (NT-3)	Intravenous	1.5 × 106 cells	BM-NSCs with NT-3 were more effective at reducing EAE pathology than normal NSCs due to greater oligodendrocyte differentiation, reduced astrogliosis, and better anti-inflammatory properties.
Selim et al. 35	EAE Mice	Placenta-derived mesenchymal stromal cells	Intravenous	1 × 106 cells	Significant reduction in disease severity was observed by decreasing inflammation and neurodegeneration. There was reduced axonal damage and an increase in oligodendrocyte proliferation. The cells engrafted into the brain and expressed neuroprotective factors.
Togha et al. 36	EAE Mice	BM-MSCs + Rapamycin	Intraperitoneal	2 × 106 cells 0.3 mg/kg rapamycin	Rapamycin enhances anti-inflammatory and protective effects of BM-MSCs.
De Feo et al. 37	EAE Mice	NSCs	Intrathecal	1 × 106 cells	NPCs reduce inflammatory cytokine release and pro-inflammatory microglial differentiation, likely via TGF-b signaling.
Xie et al. 38	EAE Mice	BM-NSCs with inducible TGF-b1	Intravenous	1.5 × 106 cells	TGF-b1 increased Tregs and lowered Th1 and Th17 cells in periphery. Increased IL-10 and reduced IFN-g, IL-17, and GM-CSF further exemplified this anti-inflammatory effect. M2 microglial proliferation was also favored.
Laso-Garcia et al. 39	TMEV mice	EVs from MSCs	Intravenous	25 μg EVs	EVs reduced inflammation and increased myelin protein, altered microglial differentiation, and reduced pro-inflammatory cytokines from Th1 and Th17 cells. Motor deficits, brain atrophy, and cell proliferation were improved.
Ben-Zwi et al. 40	EAE Mice	Neuralized MSCs	Intraventricular	NA	NMSCs reduced inflammation, axon loss, axonal damage, and demyelination better than naïve MSCs.
Li et al. 41	EAE Rats	BM-MSC exosomes	Intravenous	100 µg (low dose) or 400 μg (high dose) exosomes	Exosomes reduce inflammatory cell invasion, increase anti-inflammatory cytokines, and predispose anti-inflammatory microglial differentiation.
Barati et al. 42	Cuprizone demyelination mouse model	MSCs	Intraventricular	3 × 105 cells	MSCs decreased microgliosis and astrogliosis via anti-inflammatory and neuroprotective cytokines. Increased oligodendrocyte production and remyelination was also notable.
Lombardi et al. 43	Lysolecithin injection in the mouse corpus callosum	EVs produced in vitro by either pro-inflammatory or pro-regenerative microglia	Into lesions	6.6 × 107 EVs	Anti-inflammatory prepped EVs prompt oligodendrocyte migration and differentiation to encourage remyelination.
Liu et al. 44	EAE Mice	MSCs	Intravenous	5 × 105 cells	MSCs improved neurobehavioral function and reduced inflammatory cell invasion, IgG leakage, and demyelination in the spinal cord. This may have been related to decreased blood–brain barrier and blood spinal cord barrier permeability.
Zhang et al. 45	EAE Mice	Human umbilical cord MSCs (HucMSCs) and tetramethylpyrazine (TMP)	Intravenous	1 × 106 cells	TMP increased HucMSC viability by reducing ROS production and apoptosis. The anti-oxidative Nrf2/HO-1 pathway was amplified. Compared with non-TMP cells, TMP-stimulated HucMSCs were significantly better at decreasing symptoms, inflammation, demyelination, and BBB disruption.
Brown et al. 46	EAE Mice	NSCs or MSCs	Intravenous	1 × 106 cells	NSCs are more effective than MSCs at modulating MS phenotypes and reducing inflammatory mediators.
Kassis et al. 47	EAE mice	MSCs + Fingolimod	FTY720 intraperitoneal MSCs Intracerebroventricular	0.5 mg FTY720/kg/daily, 5 × 105 cells	MSCs and fingolimod have a synergistic effect at increasing neurotrophic factors and decreasing pro-inflammatory cytokines.
Liu et al. 48	EAE Mice	BMSCs	Intraventricular	NA	BMSCs differentiate into neurons, astrocytes, and oligodendrocytes. They differentiate more readily in EAE mice than control mice, while also improving function, myelination, and inflammation.
Fathollahi et al. 49	EAE mice	MSCs and EVs	Intranasal	5 × 105 cells and 10 µg EVs	Both cell therapies improved function and increased Tregs and anti-inflammatory cytokines.
Barati et al. 50	Cuprizone mouse model	MSCs	Intraventricular	3 × 105 cells	A1 pro-inflammatory astrocytes were decreased by MSC treatments and oligodendrocytes were increased to improve remyelination. Pro-inflammatory cytokines were also decreased and neurotrophic signals were increased.

This table outlines cell-based preclinical trials aimed at reducing neuroinflammation seen in MS. EAE: experimental autoimmune encephalomyelitis; CCR5: chemokine receptor-5; NSC: neural stem cell; BMSC: bone marrow-derived mesenchymal stem cell; NT: neurotrophin; NPC: neural progenitor cell; CSF: cerebrospinal fluid; NMSC: neuralized mesenchymal stem cell; TMEV: Theiler’s murine encephalomyelitis virus; CNS: central nervous system; TGF: transforming growth factor; IL: interleukin; IFN: interferon; EV: extracellular vesicle; MSC: mesenchymal stem cell; TMP: tetramethylpyrazinze; ROS: reactive oxygen species; BBB: blood–brain barrier.

Mesenchymal SCs

Among the multiple mechanisms mediating the therapeutic effects of SCs, the cells’ ability to reduce neuroinflammation closely accompanies functional recovery in MS models. Mesenchymal stem cells’ (MSC) anti-inflammatory properties have been demonstrated in preclinical studies of MS (reviewed by Gugliandolo et al. ⁶ ). In animal models of MS, MSCs affect adaptive immune cells by balancing Th17, Th1, and Tregs; inhibit CD4+ and CD8+ T-cell inflammatory actions; and encourage innate microglial and astroglial differentiation away from the pro-inflammatory M1 and A1 phenotypes and toward the anti-inflammatory M2 and A2 phenotypes^6,50. Moreover, MSCs reduce pro-inflammatory mediators while increasing protective signals. MSCs derived from the placenta offer even greater neuroprotection and neurogenesis by the production of human brain–derived neurotrophic factor (BDNF), nerve growth factor and neurotrophin 3 (NT-3) in MS rat models ³⁵ . Astrogliosis and microgliosis, both detrimental reactions to MS pathophysiology, are also reduced by MSCs ⁴² . In EAE mice, MSCs become neural cells and reduce inflammation, demyelination, and pro-inflammatory mediators ⁴⁸ . Neuralized MSCs given to EAE mice are better at reducing T-cell infiltration, increasing Treg differentiation, and promoting oligodendrocyte formation than MSCs. These neuralized MSCs effectively reduced MS symptomology as well, possibly via the release of neurotrophic factors ⁴⁰ .

Oligodendrocytes, enhanced with ephrin, a protein which aids in cell migration, have also been shown to have therapeutic benefits in animal MS models ⁵¹ . Moreover, in mice models, the conditioned medium containing MSC-derived oligodendrocytes offers immense immunoregulation ⁵² . By implementing a neuroprotective environment against MS inflammation and oxidation, BBB integrity is restored and even protects against inflammatory and immune cell infiltration. MSCs may even further strengthen the BBB by downregulating aquaporin channels ⁴⁴ .

Strategies designed to further optimize MSCs’ anti-inflammation machinery may enhance the cells’ therapeutic outcomes. MSCs have been paired with anti-inflammatory agents for MS, such as Fingolimod, Minocycline, and Rapamycin^33,36,47, showing synergistic effects. Priming MSCs with tetramethylpyrazinze (TMP) also shows amplified benefit for MS, likely by activating the anti-oxidant and anti-inflammatory nuclear factor-erythroid 2-related factor-2 (Nrf2)/heme oxygenase 1(HO-1) signaling pathway ⁴⁵ . The notion of priming MSCs has been demonstrated to enhance their therapeutic potential against stroke damage ⁵³ . There is also a plethora of evidence supporting amplification of the Nrf2/HO-1 pathway to ameliorate stroke damage ⁵⁴ .

Neural SCs

While MSCs have clear therapeutic benefits, neural stem cells (NSCs) in EAE mice are also shown to reduce MS symptoms, promote neurogenesis, and enhance myelination. In fact, they may be even more effective than MSCs. For instance, NSCs may play a critical role in restoring balance between the adaptive lymphocytes Th17 and Tregs; Th17 restoration is accomplished through the inhibition of IL-6 expression (a pro-inflammatory cytokine inducing pathogenic Th17 cells for demyelination) and Th17/Treg balance is achieved through inhibition of TNF-α and IFN-γ ⁴⁶ . NSCs’ therapeutic potential can be further enhanced by modulating factors like NT-3 (promoting remyelination through enhanced differentiation into oligodendrocytes and neurons while inhibiting differentiation into astrocytes), chemokine receptor-5 (promoting early-stage migration of NSCs to inflammatory foci) TGFβ-1 (upregulating Tregs and the microglial M2 phenotype while downregulating Th17 activity) and TGF-β2 (reprogramming infiltrative monocytes to anti-inflammatory myeloid cells)^32,34,37,38. NSCs and MSCs show impressive potential for MS treatments and may also be beneficial in preventing later IS incidence by reducing neuroinflammation early.

Extracellular Vesicles

A major benefit of using extracellular vesicles (EVs) as opposed to cells is their cell-free configuration, circumventing the logistical problem of graft rejection inherent in transplanting “cells.” MSC EVs are shown to improve motor symptoms, reduce neuronal death, increase neurogenesis, shift T-cell and microglia ⁴¹ populations toward an anti-inflammatory phenotype, and decrease inflammation in the spinal cord of MS models ³⁹ . When obtained from MSCs cultured with anti-inflammatory microglia, EVs promote oligodendrocyte differentiation, migration, and anti-inflammation ⁴³ . Excitingly, these EVs also show significant treatment potential when administered intranasally via immunomodulation ⁴⁹ . Thus, avoiding the graft rejection risks associated with SC transplantation, EVs offer an exciting research direction for MS treatment and immunomodulation.

Clinical Applications

With this wide array of studies supporting SC in MS, it is logical to apply these concepts in clinical trials ⁵⁵ . MSCs derived from adipose tissue and umbilical cord tissue and NSCs are safe and demonstrate neuroprotection, anti-inflammation, and improved clinical presentation in patients with MS. In patients with MS, hematopoietic stem cells treatment (HSCT) is more likely to show no evidence of disease after 3 years compared with patients treated with alemtuzumab, an anti-inflammatory MS therapy. This SC treatment had a higher risk of complications for the first 100 days after transplantation, but after 100 days, elevated risk was then surpassed by alemtuzumab ⁵⁶ . These studies support further investigation of SCs in clinical settings, specifically in efforts to reduce neuroinflammation induced complications, such as IS.

Stem Cell Therapies for Stroke

Utility of SC Therapy for Stroke

The inflammation-mediated secondary cell death that accompanies stroke is very much akin to MS, a chronic neurodegenerative disorder. SC therapy represents a novel treatment approach for stroke damage mediated by such neuroinflammation due to its potential for an extended therapeutic window in the post-stroke inflammatory state compared with current standards of care (Table 2).

Table 2.

Anti-Inflammatory Stem Cell Studies for Stroke.

Citation	Sample	Cell type	Route	Dosage	Results
Wang et al. 57	Cardiac arrest-induced global cerebral ischemic SD rats	MSCs	Intravenous	4 × 107 cells	MSCs promoted migration and integration of cell precursors, reduced neuronal death, and attenuated inflammation in the cortex
Oh et al. 58	MCAO SD rats	HucMSCs	Intravenous	1 × 106 cells	HucMSCs attenuated neuroinflammation by upregulating IL-1 receptor antagonist.
Cheng et al. 59	MCAO mice	MSCs	Stereotaxic transplantation	1 × 105 cells	MSCs downregulated ICAM-1 expression on endothelial cells to impair neutrophil extravasation, MMP-9 function, and BBB disruption.
Chi et al. 60	MCAO SD rats	AD-MSCs	Intravenous	2 × 106 cells	AD-MSCs promoted anti-inflammatory activity, reduced infarct size, and suppressed apoptosis through inhibition of ER stress-induced response.
Nito et al. 61	MCAO SD rats	DPSCs	Intravenous	1 × 106 cells	DPSCs modulated neuroinflammation in acute stroke to reduce ischemic damage and improve neurological function.
Vahidinia et al. 62	MCAO Wistar rats	BMSCs	Intravenous	1 × 106 cells	BMSCs reduced apoptosis and astrogliosis in penumbra by downregulating p-JNK
Boese et al. 63	MCAO mice	hNSCs	Intracranial (hippocampus)	1 × 105 cells	hNSCs decreased pro-inflammatory cytokines (TNF-α, IL-6, and MMP-9), increased BDNF expression, and reduced BBB damage.
Yoshida et al. 64	MCAO mice	hAMSCs	Intravenous	4 × 106 or 2 × 106 cells	hAMSCs reduce BBB disruption and apoptosis in the peri-infarct region by suppressing pro-inflammatory cytokines.
Tang et al. 65	MCAO SD rats	MSCs	Intravenous	2.0 × 106 cells	Stroke serum-MSCs suppressed inflammatory cytokines and inflammation while improving behavioral outcomes, neurogenesis, and angiogenesis.
Basham et al. 66	HI SD rats	AD-MSCs	Subcutaneous	Average of 1.07 × 106 cells	AD-MSCs increased absolute number of striatal medium-spiny neurons and decreased striatal inflammation.
Liang et al. 67	MCAO SD rats	hAFSCs	Intravenous	5 × 106 cells	hAFSCs improved neurological dysfunction, reduced infarct volume, reduced BBB leakage, improved angiogenesis, and reduced inflammation.
Hamblin et al. 68	MCAO mice	hNSCs	Intracranial (hippocampus)	1 × 105 cells	hNSCs reduced infarct size reduced inflammatory mediators and depleted a number of stroke-injury pathways.
Fu et al. 69	MCAO SD rats	hUMSCs	Intracranial (Right cerebral cortex)	1 × 106 cells	hUMSCs did not differentiate but improved neuroprotection, decreased inflammation, and increased angiogenesis, suggesting action through cytokine release.
Ranjbaran et al. 70	CCAO Wistar rats	AD-MSCs	Intraperitoneal	1.0 × 106 cells	AD-MSCs inhibited pro-inflammatory cytokines IL-6 and TNF-α, reducing apoptosis in the hippocampus.

This table outlines cell-based preclinical trials aimed at reducing neuroinflammation seen in stroke. MSC: mesenchymal stem cell; IL: interleukin; BBB: blood–brain barrier; TNF: tumor necrosis factor; HucMSC: human umbilical cord MSC; SD: Sprague Dawley; MCAO: middle cerebral artery occlusion; ICAM: intercellular adhesion molecule; MMP: matrix metalloproteinases; hAMSC: human amniotic mesenchymal stem cell; HI: hypoxia-ischemia; hAFSC: human amniotic fluid stem cell; CCAO: common carotid artery occlusion.

Effector Mechanisms and SC Targets

NSCs have been found to exercise a broad range of effects in mediating tissue recovery and improving neurological function. The timing of SC delivery in IS appears to be critical to the mechanism of therapy, with administration in the early stroke phase upregulating neuroprotective factors, including BDNF, vascular endothelial growth factor (VEGF), ciliary neurotrophic factor, neural growth factor, and glial cell line–derived neurotrophic factor, while NSCs delivered in the chronic stroke phase amplify neurorestoration and cellular proliferation ⁷¹ .

SC therapy has been shown to decrease immune cell activation through human-induced pluripotent stem cell (iPSC)-derived NSC transplantation, in addition to the downregulation of pro-inflammatory cytokines ⁷² . In addition, MSCs increase secretion of anti-inflammatory cytokines which serve to inhibit the proliferation and differentiation of immune cell lines ⁷³ .

Downregulation of oxidative stress is another approach for SC treatment. MSCs are documented to reduce intracellular calcium and reactive oxygen species (ROS) in ischemic neurons ⁷³ . The anti-oxidant response promotes SC survival and reduces neuronal ischemic injury. Nrf2, a transcription factor involved in the anti-oxidant response, is strongly associated with endogenous NSC survival, and thus drug-based therapies focused on the Nrf2 pathway may enhance the effects of exogenous SC transplantation ⁵⁴ .

Stimulation of endogenous NSCs has also been investigated in promoting stroke recovery and combating neuroinflammation. Adjunct therapies designed to enhance endogenous NSCs focus on delivery of neurogenic factors, including BDNF, VEGF, glial-derived neurotrophic factor, granulocyte colony-stimulating factor, basic fibroblast growth factor, bone morphogenetic protein, and TGF-α to modulate survival pathways ⁷⁴ . Transplanted NSCs augment neuroplasticity through action on endogenous NSCs, promoting neurogenesis, reorganization, and angiogenesis ⁷² .

Extracellular Vesicles

As discussed previously, EVs of SCs offer an ethical alternative to the induction of anti-inflammatory effects and BBB repair. EVs are attractive in treating IS as they possess a reduced risk of blood vessel blockage and thrombosis, and furthermore are rather effective at crossing the BBB ⁷⁵ . Administration of MSC-derived EVs promotes neuroprotection, neurogenesis, and angiogenesis in IS models, with preconditioning of MSCs with either normal or ischemic brain extract further ameliorating recovery ⁷⁶ . In ischemic rats, miRNA-17-92 overexpressing EVs show improved outcomes in oligodendrogenesis and dendritic plasticity compared with standard MSC-derived EVs, while mi-R-133b overexpression results in neurorestoration and enhanced neuroplasticity^77,78. EVs are also strongly characterized by interaction with innate microglia, with EVs derived from a number of CNS lineages, including NSCs, promoting the transition to the microglial anti-inflammatory M2 phenotype (reviewed in Spellicy and Stice ⁷⁹ ). The overall therapeutic potential of EVs was confirmed in a recent systematic review, supporting SC-derived EVs as being associated with reduced infarct volume and improved recovery from stroke, thus clarifying the future of such therapy in human trials ⁸⁰ .

Clinical Applications

The ever-growing body of evidence supporting the beneficial outcomes of SC treatment as a stroke therapeutic ultimately supports a transition to clinical medicine. To date, clinical trials for cell-based stroke therapies have employed MSCs, bone marrow–derived mononuclear cells, peripheral blood SCs, and adipose-derived stromal vascular fractions (MSCs among other lineages)^81–91. A recent meta-analysis of 11 clinical trials including 368 patients demonstrated the safety and viability of SC therapy for stroke treatment ⁹² . In the context of MS-induced stroke, SCs can ameliorate the chronic inflammatory phase that has significant pathological overlap in both IS and MS.

Peripheral Contributions

While we have mainly focused on the role of SCs centrally, a multitude of studies have shown the vital role of peripheral organs in neuroinflammation ⁹³ . As discussed, the BBB is impaired in stroke and MS, facilitating the entry of peripheral inflammatory cells into the typically immunoprivileged CNS. After stroke, notable spleen and thymus shrinkage has been observed alongside amplified lymphocytes release ⁹⁴ . Substantial bone marrow changes are seen after stroke as well, suggesting B lymphopoiesis amplifies peripheral inflammatory responses to stroke as well^95,96. In addition, bone marrow–derived MSCs have made incredible impacts on stroke-induced neuroinflammation, further supporting this primary lymphoid organ’s role in stroke pathogenesis^97–99. In MS, the T cells perpetuating autoimmunity similarly amplify the bone marrow’s contribution to neuroinflammation, and autologous hematopoietic SC transplantation demonstrates successes in trials^100–102. Important immune cell contributions are notable from the gut as well, a concept known as the gut–brain axis. After stroke and in MS, the gut becomes more permeable, and dysbiosis is seen^103,104. Furthermore, cervical lymph nodes are activated due to cerebral antigens escaping from the permeable BBB after stroke^105,106. Discrepancies between lymph node sizes in treated versus non-treated patients with MS suggests cervical lymph nodes may play a role in MS pathophysiology as well ¹⁰⁷ . Not only are these organs notable for their peripheral inflammatory contributions following stroke, but MSC treatments also show SC migration to these organs and subsequent reduction in systemic inflammation^93,98,99,108. The immense influence of peripheral organs on neuroinflammation suggests these organs may be notable targets for reducing MS inflammation and reducing stroke risks as well.

Conclusion

There is vast evidence prompting clinical use of SCs to reduce neuroinflammation in MS and IS independently; however, no studies have employed SCs and measured their abilities to reduce MS-induced IS. Given the role of the MS neuroinflammatory environment in the incidence of IS, alongside the promising studies in SCs, the use of SCs to reduce IS risk is a logical and relevant future research direction. Furthermore, another role for SCs in ameliorating IS occurrence secondary to MS may involve restoring the NVU integrity, as the inflammation seen in both pathologies induces detrimental BBB permeability and SCs can regulate BBB stability ¹⁰⁹ .