The Role of Mast Cells in Stroke

Mast cells (MCs) are densely granulated perivascular resident cells of hematopoietic origin. Through the release of preformed mediators stored in their granules and newly synthesized molecules, they are able to initiate, modulate, and prolong the immune response upon activation. Their presence in the central nervous system (CNS) has been documented for more than a century. Over the years, MCs have been associated with various neuroinflammatory conditions of CNS, including stroke. They can exacerbate CNS damage in models of ischemic and hemorrhagic stroke by amplifying the inflammatory responses and promoting brain–blood barrier disruption, brain edema, extravasation, and hemorrhage. Here, we review the role of these peculiar cells in the pathophysiology of stroke, in both immature and adult brain. Further, we discuss the role of MCs as potential targets for the treatment of stroke and the compounds potentially active as MCs modulators.


Introduction
Mast cells (MCs) are perivascular resident cells of haemopoietic origin distributed in most tissues surrounding blood vessels, nerves, smooth muscle cells, sebaceous and sweat glands, hair follicles, and synovial membranes [1,2]. MCs are more abundant in the anatomical regions in contact with the external environment, including skin, conjunctiva, nasal mucosa, bronchial airway connective tissue, lung intra-alveolar space, mouth, and subserosal and submucosal layers of the gastrointestinal tract [2][3][4]. Because of their peculiar anatomical location, MCs serve as first immune sentinel cells to respond against invading pathogens and environmental antigens and allergens [1,5,6].
MCs can be found also in the central nervous system (CNS), where their presence has been documented for more than a century [7]. MCs are present in different mammalian brain regions, including meninges, choroid plexus, olfactory bulb, mesencephalon, parenchima of the thalamic and hypothalamic region, hippocampus, and entorhinal cortex [6,[8][9][10], where they reside on the abluminal side of the blood vessels [8,9]. Here, MCs are able to communicate with blood vessel cells, neurons, glia, and microglia [8,9]. MCs reach the brain during development, migrating along blood vessels [11]. However, mature MCs are also able to move from the periphery to the brain and their number and distribution can change in response to a variety of physiological and pathological stimuli [8,12,13].
Various clues suggest involvement of MCs in the NHIBI pathogenesis [68,69]. MCs-associated genes were upregulated in mice pups subjected to NHIBI [70]. In rat models of NHIBI neuronal injury was linked to a rapid increase of activated MCs and release of TNF-α and histamine in the immature brain [71][72][73], suggesting that mastocytic activation precedes post-injury response of glial, endothelial cells and neurons. In a NHIBI mouse model, the mastocytic growth and differentiation factor interleukin 9 (IL-9) also contributed to brain damage by amplifying activation of MCs [74]. Moreover, the deleterious role of these cells is supported by the effect of MCs stabilizers in inhibiting mastocytic activation and reducing brain damage in rat or mice pups subjected to NHIBI [71,72,74].

Ischemic Stroke
Ischemic stroke, or brain ischemia, is a global cause of death and disability [59,60]. In ischemic stroke, blood supply to part of the brain is decreased, causing damage to the cerebral tissue surrounding the occluded blood vessel [75]. The ischemic insult triggers a series of pathological processes including excitotoxicity, oxidative damage, apoptosis and inflammation, which eventually leads to cell death [76][77][78]. One of the main pathophysiological features of ischemic stroke is the BBB disruption, an event occurring in almost two-thirds of patients in the first hours from the ischemia onset that causes vasogenic edema, hemorrhagic transformation, and increased mortality [79,80].
Different preclinical studies point out a role of MCs in the pathogenesis of ischemic stroke [81,82]. In a cellular model of ischemic stroke, exposure to oxygen and glucose deprivation (OGD) induced mastocytic activation in a fashion dependent on the anoxic insult duration [83][84][85]. MCs exacerbated neuronal damage in neuron-MC cocultures exposed to OGD [83]. Likewise, the conditioned medium derived from OGD-activated MCs induced neurotoxicity in primary neurons [83]. Interestingly, pharmacological prevention of OGD-induced MCs activation reduced neurotoxicity [83]. The gene expressing the chemokine CCL7, a MCs-derived product reported to be involved in the recruitment of inflammatory cells into the ischemic sites [86], has been found upregulated in the brain of mice subjected to middle cerebral artery occlusion (MCAO) [87]. In a rat model of transient cerebral ischemia, it has been observed a significant increase in the thalamic MCs number and histamine levels after 24 h from the ischemic insult [88]. Experiments on rats that underwent MCAO surgery showed that MCs are early players in the formation of ischemic brain edema [89]. Treatment with the MCs activator compound 48/80 dramatically increased cerebral edema [89]. On the contrary, MC-deficient rats or rats treated with the MCs stabilizer cromoglycate displayed diminished brain swelling, BBB leakage, and neutrophils infiltration [89]. Similarly, BBB breakdown, brain edema, and neutrophils infiltration were attenuated in MC-deficient mice subjected to MCAO or in MCAO wild type mice treated with cromoglycate [90]. Proteomic analysis suggested a role of endoglin, endothelin-1, and metalloproteinase 9 (MMP-9) in the BBB damage promoted by MCs [90]. A proof of the involvement of proteolytic gelatinase enzymes secreted by MCs in BBB damage came from studies on rats subjected to MCAO [91]. After the ischemic insult, activated MCs showed secretion of gelatinase-positive granules that correlated with the brain swelling. Treatment with the compound 48/80 increased gelatinase activity in the ischemic tissue [91], while rats treated with MCs stabilizers or MC-deficient rats presented a reduced global gelatinase-active area [91].
Interestingly, the meningeal MCs, rather that parenchimal MCs, appear to be involved in detrimental effect of stroke [92]. In support of this hypothesis, the engraftment of bone marrow-derived cultured MCs into the meninges of MC-deficient mice subjected to MCAO was sufficient to worsen stroke damage [93]. In an immunohistochemical study on brain tissues of patients deceased after ischemic stroke, the authors reported a lack of MCs in the penumbra regions surrounding the necrotic area [94]. These findings further supported the fact that parenchymal MCs may not play a crucial role in stroke.
Treatment options for ischemic stroke are currently very limited, and the only approved pharmacological therapy is the recombinant tissue plasminogen activator (rtPA) [95,96]. Unfortunately, administration of rtPA after 4.5 h from the ischemic event is contraindicated for the risk of hemorrhagic conversion, limiting the use of this drug [97]. Strbian and coworkers studies in ischemic stroke models pointed out a role of MCs in the hemorrhagic conversion promoted by rtPA administration [98]. The treatment of cultured rat MCs with rtPA promoted massive degranulation [98]. In a MCAO rat model, administration of rtPA induced intracerebral hemorrhage formation [98]. Mastocytic stabilization by sodium cromoglycate protected from deleterious effect of rtPA, reducing hemorrhagic conversion, brain swelling, neutrophil infiltration and mortality rate [98]. Similar protective effects were observed in MC-deficient rats [98].

Intracerebral Hemorrhage (ICH)
Spontaneous ICH is a severe neurological disorder associated with high rates of mortality and disability [99][100][101][102]. ICH results from the rupture of cerebral blood vessels that causes a rapidly expanding hematoma occurring within brain parenchima. Brain injury after ICH can be divided in two phases. The primary brain injury caused by increased intracranial pressure is followed by a secondary brain injury mediated by the physiological responses to hematoma, such as inflammation [103,104].
Several findings support a role of MCs in ICH [82,105]. In a rat model of intracerebral hemorrhage with autologous blood injection into the basal ganglia, the induction of mastocytic degranulation by treatment with compound 48/80 exacerbated brain damage [106]. Conversely, rats treated with MCs stabilizers or MC-deficient rats showed reduced mortality, brain swelling, hematoma growth and improved neurologic outcome [106]. In another ICH model, mice stereotactically injected into the basal ganglia with collagenase exhibited MCs activation [107,108]. Pharmacological MCs inhibition significantly decreased mortality rate and improved neurologic outcomes by mitigating neuroinflammation and BBB disruption [107,108]. Finally, in a collagenase injection rat model of ICH, pharmacological MCs inhibition counteracted the deleterious effects induced by rtPA administration, such as hematoma growth, hemispheric expansion, mortality, and neurologic dysfunction [109]. Pharmacological MCs inhibition significantly decreased mortality rate and improved neurologic outcomes by mitigating neuroinflammation and BBB disruption [107,108]. Finally, in a collagenase injection rat model of ICH, pharmacological MCs inhibition counteracted the deleterious effects induced by rtPA administration, such as hematoma growth, hemispheric expansion, mortality, and neurologic dysfunction [109]. In the ischemic stroke, after cerebral blood vessel obstruction MCs sense alarm signals from injured parenchymal cells and become activated. In the ICH, the mastocytic activation is mediated by the leakage of blood products from the damaged vessel. Upon activation, MCs release a variety of vasoactive and proinflammatory molecules, including histamine, heparine, cytokines (TNF-α, ILs, chemokines), proteases (tryptase, chymase, MMPs, cathepsine G). The preformed and newly synthetized mediators induce vasodilatation, recruitment of peripheral immune cells toward the infarcted area, and BBB disruption, promoting a sustained neuroinflammation. In ischemic stroke, the pathological scenario supported by MCs activation has been involved in the hemorrhagic conversion mediated by rtPA treatment. In the ICH, the recruitment of inflammatory cells maintain and potentiate the initial BBB leakage, leading to an aggravation of hemorrhage and vasogenic edema. The mechanisms of MCsmediated pathogenesis of stroke in the adult brain are valid also in the immature brain. BBB: bloodbrain barrier; ICH: intracerebral hemorrhage; IL: interleukin; MCs: mast cells; MMP: metalloproteinase; rtPA: recombinant tissue plasminogen activator; TNF: tumor necrosis factor.

Subarachnoid Hemorrhage (SAH)
SAH is a severe subtype of stroke characterized by an overall mortality and morbidity of more than 50% [110,111]. SAH is caused in 85% of cases by the rupture of an intracranial aneurysm [111]. Over the past years, a large number of studies reported a role of the inflammatory processes in the pathogenesis of CA [112][113][114]. In the ischemic stroke, after cerebral blood vessel obstruction MCs sense alarm signals from injured parenchymal cells and become activated. In the ICH, the mastocytic activation is mediated by the leakage of blood products from the damaged vessel. Upon activation, MCs release a variety of vasoactive and proinflammatory molecules, including histamine, heparine, cytokines (TNF-α, ILs, chemokines), proteases (tryptase, chymase, MMPs, cathepsine G). The preformed and newly synthetized mediators induce vasodilatation, recruitment of peripheral immune cells toward the infarcted area, and BBB disruption, promoting a sustained neuroinflammation. In ischemic stroke, the pathological scenario supported by MCs activation has been involved in the hemorrhagic conversion mediated by rtPA treatment. In the ICH, the recruitment of inflammatory cells maintain and potentiate the initial BBB leakage, leading to an aggravation of hemorrhage and vasogenic edema. The mechanisms of MCs-mediated pathogenesis of stroke in the adult brain are valid also in the immature brain. BBB: blood-brain barrier; ICH: intracerebral hemorrhage; IL: interleukin; MCs: mast cells; MMP: metalloproteinase; rtPA: recombinant tissue plasminogen activator; TNF: tumor necrosis factor.

Subarachnoid Hemorrhage (SAH)
SAH is a severe subtype of stroke characterized by an overall mortality and morbidity of more than 50% [110,111]. SAH is caused in 85% of cases by the rupture of an intracranial aneurysm [111]. Over the past years, a large number of studies reported a role of the inflammatory processes in the pathogenesis of CA [112][113][114].
MCs have been suggested to contribute to vascular diseases including atherosclerosis and aneurysm by triggering inflammation through the release of cytokines and proteinases such as chymase and MMPs [115][116][117][118]. An increased number of infiltrated MCs has been identified in the aneurysm wall in a CA rats model [119]. Similarly, MCs have been found in the aneurysm tissues of CA patients that underwent microsurgical clipping [120][121][122]. Interestingly, MCs expression was markedly increased in ruptured aneurysms [120]. In rats, MCs modulators reduced the size and the thinning of induced CA through inhibition of chronic inflammation [119]. Furthermore, in cocultures of primary rat MCs and smooth muscles cells obtained from intracranial arteries, mastocytic activation induced expression of MMP-2, MMP-9 and inducible nitric oxide synthase (iNOS) in smooth muscles cells [119], revealing a role for MCs in promoting inflammation in CA walls.
Delayed cerebral vasospasm, the prolonged and intense vasoconstriction of arteries in the subarachnoid space after CA rupture, has been recognized as an important cause of poor outcome in SAH [123]. MCs have been reported to be associated with vasospasm because of their increased number found in the artery walls adjacent to aneurysm of patients who died after SAH [124]. Adenosine and inosine have been proposed as responsible for the vasoconstrictive effect mediated by MCs activation [125]. The MCs-induced vasoconstrictive effect was significantly decreased by treatment with a combination of histamine and thromboxane inhibitors [125].
MCs have been suggested to contribute to vascular diseases including atherosclerosis and aneurysm by triggering inflammation through the release of cytokines and proteinases such as chymase and MMPs [115][116][117][118]. An increased number of infiltrated MCs has been identified in the aneurysm wall in a CA rats model [119]. Similarly, MCs have been found in the aneurysm tissues of CA patients that underwent microsurgical clipping [120][121][122]. Interestingly, MCs expression was markedly increased in ruptured aneurysms [120]. In rats, MCs modulators reduced the size and the thinning of induced CA through inhibition of chronic inflammation [119]. Furthermore, in cocultures of primary rat MCs and smooth muscles cells obtained from intracranial arteries, mastocytic activation induced expression of MMP-2, MMP-9 and inducible nitric oxide synthase (iNOS) in smooth muscles cells [119], revealing a role for MCs in promoting inflammation in CA walls.
Delayed cerebral vasospasm, the prolonged and intense vasoconstriction of arteries in the subarachnoid space after CA rupture, has been recognized as an important cause of poor outcome in SAH [123]. MCs have been reported to be associated with vasospasm because of their increased number found in the artery walls adjacent to aneurysm of patients who died after SAH [124]. Adenosine and inosine have been proposed as responsible for the vasoconstrictive effect mediated by MCs activation [125]. The MCs-induced vasoconstrictive effect was significantly decreased by treatment with a combination of histamine and thromboxane inhibitors [125].   Carotid ligation rat model Rapid increase of activated MCs in the brain [71,72] MCs pharmacological inhibition reduced MCs migration, brain damage and glial activation Transient focal ischemia rat model Rapid increase of activated MCs and histamine in the brain [73] Ibotenate mouse model IL-9 exacerbated brain damage by activating MCs [74] MCs pharmacological inhibition reduced brain damage

Ischemic Stroke
OGD mouse MCs OGD promoted MCs activation [83][84][85] OGD mouse MCs and neurons OGD-activated MCs induced neurotoxicity [83] MCs pharmacological inhibition reduced MCs-induced neurotoxicity MCAO mouse model MCs associated gene upregulated [87] MC-deficient mice showed decreased BBB leakage, brain edema and neutrophils infiltration [90] MCs pharmacological inhibition decreased BBB leakage, brain edema and neutrophils infiltration Meningeal MCs worsen infiltration of granulocytes and macrophages, brain swelling, and infarct size [93] Four-vessel occlusion rat model Modulation of MCs number and histamine levels [88] MCAO rat model MCs pharmacological activation increased edema formation [89] MCs pharmacological inhibition decreased brain swelling, BBB leakage and neutrophils infiltration MC-deficient rats showed decreased brain swelling, BBB leakage, and neutrophils infiltration MCAO rat model Increased MCs gelatinase activity [91] MCs pharmacological activation increased gelatinase activity MCs pharmacological inhibition decreased gelatinase activity MC-deficient rats displayed decreased gelatinase activity

Type of Stroke Experimental Model Findings References
MCAO rat model treated with rtPA MCs pharmacological inhibition reduced rtPA-induced hemorrhagic conversion, brain swelling, and neutrophil infiltration. [98] MC-deficient rats displayed decreased rtPA-induced hemorrhagic conversion, brain swelling, and neutrophil infiltration.

Patients
Lack of MCs in penumbra brain region [94] ICH Blood infusion rat model MCs pharmacological activation increased brain damage.
[106] MCs pharmacological inhibition decreased brain damage, improved neurologic outcome MC-deficient rats displayed decreased brain damage, improved neurologic outcome Collagenase infusion mouse model MCs activation [107,108] MCs pharmacological inhibition decreased brain damage, improved neurologic outcome Collagenase infusion rat model treated with rtPA MCs pharmacological inhibition reduced rtPA-induced hematoma growth, hemispheric expansion, mortality, and neurologic deficits. [109]

MCs Modulation: A Promising Strategy in Stroke Treatment
Over the years, various strategies targeting intracellular and extracellular MCs mediators have been developed, mainly to treat allergic disorders [37]. Some of these MCs modulators have been demonstrated as effective therapeutic agents against stroke in preclinical studies [24]. The MCs modulators active in stroke models are discussed below and in Table 2.
PEA (palmitoylethanolamide) is an endogenous lipid amide distributed in different mammalian tissues, especially the brain [126]. Several reports indicate that PEA can reduce MCs activation in a variety of cellular and in vivo experimental models [127][128][129][130][131][132][133]. Thanks to its anti-inflammatory and analgesic properties, PEA has been investigated for the treatment of several pathologies, including stroke [134]. The molecule is active in protecting against ischemic damage in cellular and animal models of brain ischemia [135,136] and NHIBI [136]. In particular, in rats subjected to MCAO, PEA decreased the release of MCs derived chymase and tryptase [137].
Notably, recent findings indicate the combination of PEA and luteolin synergistically reduces MCs-mediated neurotoxicity and neurons susceptibility to hypoxic stress in a cellular model of brain ischemia [83]. Moreover, the association between PEA and luteolin is effective in decreasing the ischemia-induced MCs infiltration and expression of chymase and tryptase in a rat model of ischemic stroke [148].
Sodium cromoglycate (also referred as cromolyn) is an FDA-approved MCs stabilizer used to prevent symptoms associated with asthma [149]. Several studies point out a neuroprotective role of this MCs modulator in preclinical stroke models. Cromoglycate is effective in limiting brain damage in NHIBI, preventing MCs migration, and lowering glial activation and brain atrophy [71,72,74]. Pharmacological MCs inhibition by cromoglycate reduced ischemic brain swelling, perivascular gelatinase activity, BBB leakage, and neutrophil accumulation in rats and mice subjected to ischemic stroke [89][90][91]. Moreover, cromoglycate treatment inhibited hematoma growth and decreased neurological deficits and mortality in rat model of hemorrhagic stroke [106]. Finally, cromoglycate administration was able to reverse adverse effects of rtPA treatment in both ischemic stroke and ICH rat models [98,109], suggesting a potential use of this MCs stabilizer in combination with thrombolytic agents.
Intravenous immunoglobulin (IVIG) is an FDA-approved drug used to treat various inflammatory and autoimmune diseases. IVIG contains mainly IgG and is obtained from the blood of healthy donors [150]. In models of brain ischemia or ICH, IVIG treatment was shown to attenuate BBB damage, brain edema, infarct area as well as production of proinflammatory cytokines [108,[151][152][153][154]. Interestingly, it has been reported that IVIG could activate the mastocytic inhibitory receptor FcγRIIB [155]. In the ICH collagenase mouse model, IVIG activated FcγRIIB/SHIP1 pathway, inhibiting calcium mobilization and stabilizing MCs [108].
Hydrogen (H 2 ) gas inhalation is recognized as a therapeutic and preventive antioxidant intervention able to reduce the levels of strong oxidants such as hydroxyl radicals and peroxynitrite [156]. H 2 gas inhalation therapy was neuroprotective in preclinical cellular and in vivo models of brain ischemia and ICH [107,157,158], as well as in clinical studies of brain ischemia [159][160][161]. The neuroprotective effects promoted by H 2 can be mediated also by a modulatory action on MCs, since the gas inhalation can reduce mastocytic activation through the inhibition of the FcεR-mediated signal transduction [109,162].
Carnosine (β-alanyl-l-histidine) is an endogenous dipeptide widely expressed in body tissues, including CNS [163]. Thanks to its antioxidant, pH-buffering, and metal ion-chelating properties, combined with good tolerability and safety profile, carnosine is commonly used as dietary supplement [163]. Carnosine and its derivatives have been reported to have a potential for the treatment of several pathologies, including brain ischemia [164][165][166][167][168][169]. Carnosine is also able to reduce MCs release of histamine [170]. Notably, carnosine attenuates mastocytic degranulation and histamine release induced by OGD [85], suggesting that the MCs stabilizer capability of the molecule can be involved in the beneficial effects observed in preclinical models of brain ischemia.
Emedastine difumarate and tranilast are two anti-allergic drugs endowed with inhibitory properties on MCs degranulation [171][172][173]. Treatment of an experimentally induced intracranial aneurysm rat model with emedastine or tranilast promoted a decrease in aneurysm size and an increase in thickness [119].
Mesenchymal stem cells (MSCs) are multipotent progenitor cells that can differentiate into several cell types in the adult tissues. For their anti-inflammatory effects, MSCs have been investigated for the treatment of inflammatory pathologies [174]. Intravenous injection of MSCs reduced aneurysm rupture rate and MCs infiltration in a intracranial aneurysm mouse model [175]. The modulatory effect of MSCs on MCs appeared to be mediated by the cyclooxygenase-2 (COX-2)-dependent production of prostaglandin E2 (PGE2) [175].
Masitinib mesylate is a multitargeted tyrosine kinase inhibitor studied for its potential antineoplastic activity [176]. Masitinib was also shown to reduce ischemic brain area and neurological deficits in a rat model of brain ischemia [177]. One of the mechanisms which could explain the neuroprotective effect of the molecule is its modulatory activity on MCs by inhibiting Lyn kinase [178].
Scopoletin (6-methoxy-7-hydeoxycoumarin) is a coumarin compound isolated from several plants [179]. The molecule, used in traditional Chinese medicine, have been investigated for its antioxidant and anti-inflammatory properties [179,180]. Recently, it has been reported that scopoletin is effective in mediating neuroprotection in a rat model of brain ischemia [181]. Moreover, scopoletin was shown to reduce the production of inflammatory cytokines in a human MC line [182]. Although a direct correlation between scopoletin and MCs modulation in stroke models is yet to be determined, it is plausible that reduction of MCs activation could play a role in the scopoletin protection against ischemic injury.
Ketotifen is a second-generation antihistamine and MCs stabilizer used for the management of allergic disorders including asthma, allergic rhinitis/conjunctivitis, atopic dermatitis, and chronic urticaria [202][203][204][205][206]. Interestingly, ketotifen has been demonstrated to decrease the multiorgan damage and improve survival rate in rats subjected to intestinal ischemic reperfusion injury by inhibiting MCs activation [207]. Although the effect of ketotifen in stroke models has not been investigated yet, the molecule deserves future attention as potential approach to reduce neuroinflammation by targeting MCs. Luteolin OGD mouse MCs and neurons Luteolin reduced OGD-activated MCs degranulation and induced neurotoxicity [83] PEA/Luteolin OGD mouse MCs and neurons PEA/Luteolin reduced OGD-activated MCs degranulation and MCs-induced neurotoxicity [83] MCAO rat model PEA/Luteolin reduced ischemia-induced MCs infiltration and expression of chymase and tryptase [148] Cromoglycate Carotid ligation rat model Cromoglycate reduced MCs migration, glial activation and brain atrophy [71,72] Ibotenate mouse model Cromoglycate reduced MCs migration, glial activation and brain atrophy [74] MCAO rat model Cromoglycate reduced brain swelling, perivascular gelatinase activity, BBB leakage and neutrophil accumulation [53,98] MCAO mouse model Cromoglycate decreased BBB leakage, brain edema and neutrophils infiltration [90] MCAO rat model treated with rtPA Cromoglycate reduced rtPA-induced hemorrhagic conversion, brain swelling and neutrophil infiltration. [98] Blood infusion rat model Cromoglycate inhibited hematoma growth, decreased neurological deficits and mortality [106] Collagenase infusion rat model treated with rtPA Cromoglycate reduced rtPA-induced hematoma growth, hemispheric expansion, mortality and neurologic deficits. [109] IVIG Collagenase infusion mouse model IVIG attenuated BBB damage, brain edema, infarct area and pro-inflammatory cytokines levels [ [175] Studies showing MCs modulation by masitinib, scopoletin, resveratrol, polydatin and ketotifen in stroke models were not available at the moment of the manuscript submission. BBB: blood brain barrier; CA: cerebral aneurysm; ICH: intracerebral hemorrhage; MCAO: middle cerebral artery occlusion; MCs: mast cells; MSCs: mesenchymal stem cells; NHIBI: neonatal hypoxic-ischemic brain injury; OGD: oxygen and glucose deprivation; rtPA: recombinant tissue plasminogen activator; SAH: subarachnoid hemorrhage.

Conclusions
The studies summarized in this review indicate a detrimental role of MCs in various types of stroke. Through the release of their mediators, MCs can promote BBB damage, vasogenic edema, and hemorrhage formation, can recruit other immune cells amplifying inflammatory response, and can contribute to CA formation and vasospasm.
In light of the very early role of MCs in stroke and their complex modulatory effect on other cell populations, pharmacological agents targeting MCs stabilization in the brain may offer an effective neuroprotective strategy for stroke, alone or in combination with current available therapies. Available randomized controlled clinical studies indicated that the treatments with some of the agents endowed with mastocyte modulatory activity, namely H 2 and resveratrol, were safe and effective in stroke patients [148,[159][160][161]194]. PEA/luteolin, investigated through an observational study in a cohort of 250 stroke patients, also showed a good tolerability and improved the outcome, when compared to literature data on patients having similar pathologic conditions but never receiving PEA/luteolin [148].
However, several challenges need to be addressed in order to modulate MCs activation in stroke. Immune responses after stroke, involving also MCs activation, are extremely complex, with many processes potentially having both beneficial and detrimental roles. Moreover, our understanding of the transcriptional and epigenetic dynamics driving MCs activation is still limited [37]. A better knowledge of the post-stroke mastocytic modulation, supported by mechanistic studies, will help to identify specific therapeutic targets and better approaches for stroke treatment.
Finally, available data on MCs in stroke patients are limited [94,[120][121][122]124]. Rodent and human MCs can vary in phenotype, responsiveness to activation signals, and the spectrum of preformed and newly synthetized mediators [6]. Therefore, findings on rodent MCs should be supported by studies on patients. Particular focus is required for the investigation of the role of meningeal MCs in stroke [92]. Identifying the location of the MCs population more involved in stroke pathology may foster the research of effective therapeutics. The development of a molecular imaging probe detecting MCs could help to address this issue.

Conflicts of Interest:
The authors declare no conflict of interest.