Substance P and IL-33 administered together stimulate a marked secretion of IL-1β from human mast cells, inhibited by methoxyluteolin

The impact of psychological stress on mast cells

OBJECTIVE

Atopic diseases worsen with psychological stress, but how stress contributes to their pathogenesis is still not clear. We review the evidence supporting the premise that stress contributes to allergic and inflammatory processes through stimulation of mast cells (MCs) by neuroimmune stimuli.

DATA SOURCES

PubMed was searched between 1950 and 2019 using the following terms: allergies, atopic diseases, corticotropin-releasing hormone, inflammation, hypothalamic-pituitary-adrenal axis, mast cells, mastocytosis, neuropeptides, psychological stress, neurotensin, and substance P.

STUDY SELECTIONS

Only articles published in English were selected based on their relevance to stress and MCs, especially those that discussed potential mechanisms of action.

RESULTS

Psychological stress worsens many diseases, especially asthma, atopic dermatitis, and mastocytosis. This effect is mediated through MCs stimulated by neuropeptides, especially corticotropin-releasing hormone, neurotensin, and substance P, a process augmented by interleukin-33.

CONCLUSION

Understanding how stress stimulates MCs to release proinflammatory mediators is important in advancing treatments for diseases that worsen with stress.

IL33 and Mast Cells-The Key Regulators of Immune Responses in Gastrointestinal Cancers?

The Interleukin (IL-)1 family IL33 is best known for eliciting type 2 immune responses by stimulating mast cells (MCs), regulatory T-cells (Tregs), innate lymphoid cells (ILCs) and other immune cells. MCs and IL33 provide critical control of immunological and epithelial homeostasis in the gastrointestinal (GI) tract. Meanwhile, the role of MCs in solid malignancies appears tissue-specific with both pro and anti-tumorigenic activities. Likewise, IL33 signaling significantly shapes immune responses in the tumor microenvironment, but these effects remain often dichotomous when assessed in experimental models of cancer. Thus, the balance between tumor suppressing and tumor promoting activities of IL33 are highly context dependent, and most likely dictated by the mixture of cell types responding to IL33. Adding to this complexity is the promiscuous nature by which MCs respond to cytokines other than IL33 and release chemotactic factors that recruit immune cells into the tumor microenvironment. In this review, we integrate the outcomes of recent studies on the role of MCs and IL33 in cancer with our own observations in the GI tract. We propose a working model where the most abundant IL33 responsive immune cell type is likely to dictate an overall tumor-supporting or tumor suppressing outcome in vivo. We discuss how these opposing responses affect the therapeutic potential of targeting MC and IL33, and highlight the caveats and challenges facing our ability to effectively harness MCs and IL33 biology for anti-cancer immunotherapy.

Mast cell-mediated neuroinflammation may have a role in attention deficit hyperactivity disorder (Review)

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental and behavioral disorder with a serious negative impact on the quality of life from childhood until adulthood, which may cause academic failure, family disharmony and even social unrest. The pathogenesis of ADHD has remained to be fully elucidated, leading to difficulties in the treatment of this disease. Genetic and environmental factors contribute to the risk of ADHD development. Certain studies indicated that ADHD has high comorbidity with allergic and autoimmune diseases, with various patients with ADHD having a high inflammatory status. Increasing evidence indicated that mast cells (MCs) are involved in the pathogenesis of brain inflammation and neuropsychiatric disorders. MCs may cause or aggravate neuroinflammation via the selective release of inflammatory factors, interaction with glial cells and neurons, activation of the hypothalamic-pituitary adrenal axis or disruption of the blood-brain barrier integrity. In the present review, the notion that MC activation may be involved in the occurrence and development of ADHD through a number of ways is discussed based on previously published studies. The association between MCs and ADHD appears to lack sufficient evidence at present and this hypothesis is considered to be worthy of further study, providing a novel perspective for the treatment of ADHD.

Effect of Stress on Neuroimmune Processes

PURPOSE

Psychological stress worsens many diseases, especially those with inflammatory components, such as atopic dermatitis (AD) and autism spectrum disorder (ASD), conditions that are significantly correlated in large epidemiologic studies. However, how stress contributes to these conditions is still poorly understood. This narrative review of the relevant literature advances the premise that stress affects inflammatory processes in AD and ASD via stimulation of mast cells (MCs).

METHODS

MEDLINE was searched between 1980 and 2019 using the terms allergies, atopic dermatitis, autism spectrum disorder, brain, corticotropin-releasing hormone, inflammation, hypothalamic-pituitary-adrenal axis, mast cells, neuropeptides, stress, neurotensin, and substance P.

FINDINGS

Exposure to psychological stress is associated with onset and/or exacerbation of AD and ASD. This association could be attributable to activation of MCs, which are ubiquitous in the body, including the brain, and could contribute to inflammation.

IMPLICATIONS

Understanding and addressing the connection between stress and MCs is important in clarifying the pathogenesis and developing effective treatments for diseases that worsen with stress and involve inflammation, such as AD and ASD.

PDE3 Inhibition Reduces Epithelial Mast Cell Numbers in Allergic Airway Inflammation and Attenuates Degranulation of Basophils and Mast Cells

Epithelial mast cells are generally present in the airways of patients with allergic asthma that are inadequately controlled. Airway mast cells (MCs) are critically involved in allergic airway inflammation and contribute directly to the main symptoms of allergic patients. Phosphodiesterase 3 (PDE3) tailors signaling of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), which are critical intracellular second messenger molecules in various signaling pathways. This paper investigates the pathophysiological role and disease-modifying effects of PDE3 in mouse bone marrow-derived MCs (bmMCs), human LAD2- and HMC1 mast cell lines, human blood basophils, and peripheral blood-derived primary human MCs (HuMCs). In a chronic house dust mite (HDM)-driven allergic airway inflammation mouse model, we observed that PDE3 deficiency or PDE3 inhibition (PDE3i) therapy reduced the numbers of epithelial MCs, when compared to control mice. Mouse bone marrow-derived MCs (bmMCs) and the human HMC1 and LAD2 cell lines predominantly expressed PDE3B and PDE4A. BmMCs from Pde3^−/− mice showed reduced loss of the degranulation marker CD107b compared with wild-type BmMCs, when stimulated in an immunoglobulin E (IgE)-dependent manner. Following both IgE-mediated and substance P-mediated activation, PDE3i-pretreated basophils, LAD2 cells, and HuMCs, showed less degranulation than diluent controls, as measured by surface CD63 expression. MCs lacking PDE3 or treated with the PDE3i enoximone exhibited a lower calcium flux upon stimulation with ionomycine. In conclusion PDE3 plays a critical role in basophil and mast cell degranulation and therefore its inhibition may be a treatment option in allergic disease.

Stress-induced Interaction of Skin Immune Cells, Hormones, and Neurotransmitters

PURPOSE

Although scientific articles mention the impact of psychological stress on skin diseases, few review the latest research on factors involved in this correlation. The skin actively responds to psychological stress, with involvement of skin immune cells, hormones, neurotransmitters. Skin immune cells actively regulate tissue inflammation with their proinflammatory and anti-inflammatory effects. Stress-induced skin reactions primarily include cytokine secretion (e.g. interleukin-6, interleukin-1, interferon-γ) and activation of skins peripheral corticotropin-releasing hormone (CRH)-proopiomelanocortin (POMC)-adrenocorticotropic hormone (ACTH)-corticosteroids axis, which leads to acute/chronic secretion of corticosteroids in the skin.

METHODS

This narrative review presents the current knowledge and latest findings regarding the impact of psychological stress on skin diseases, including information concerning psychoneuroimmune factors in stress-induced skin responses. Recent articles published in English available through the PubMed database and other prominent literature are discussed.

FINDINGS

Stress mediators, including cortisol, ACTH, and CRH from hypothalamus-pituitary-adrenal axis activation, induce various skin immune responses. Skin cells themselves can secrete these hormones and participate in skin inflammation. Thus, the local skin CRH-POMC-ACTH-corticosteroids axis plays a prominent role in stress-induced responses. Also, keratinocytes and fibroblasts produce hypothalamic and pituitary signal peptides and express receptors for them (CRH with receptors and POMC degradation peptides with melanocortin receptors), which allows them to respond to CRH by activating the POMC gene, which is then followed by ACTH and subsequently corticosteroids excretion. In addition, keratinocytes can express receptors for neurotransmitters (e.g. adrenaline, noradrenaline, dopamine, histamine, acetylcholine), neurotrophins, and neuropeptides (e.g. substance P, nerve growth factor), which are important in linking psychoneuroimmunologic mechanisms.

IMPLICATIONS

Psychoneuroimmunology provides an understanding that the skin is target and source of stress mediators. This locally expressed complex stress-induced network has been confirmed as active in many skin diseases (e.g. vulgar psoriasis, atopic dermatitis, chronic urticaria, human papillomavirus infections/warts, hair loss, acne). Skin reactions to stress and its influence on skin diseases may have implications for disease severity and exacerbation frequency, given the effect of locally secreted corticosteroids and other mediators that affect skin integrity, inflammation, and healing potential. Studies have also shown that introducing psychiatric treatment (drugs or psychotherapeutic methods) can have positive effects on dermatologic diseases influenced by psychological stress exposure. We hope this review provides clinicians and scientists with more complete background for further research in this field of skin psychoneuroimmunology.

New aspect of allergic contact dermatitis, an inflammatory skin disorder mediated by mast cells: Can IL-38 help?

Atopic dermatitis (AD) is an inflammatory reaction of the skin that can occur in several parts of the body and can be provoked or exacerbated by food and/or environmental compounds. Allergic contact dermatitis (ACD) is a potential enhancer of AD, and an epidermal barrier breaker which induces greater penetration of allergens and other compounds. ACD presents an eczematous rash, red and itchy, with inflammation mediated by cytokines. ACD is an immunological disorder caused by contact with an allergic substance (haptens) that involves immunotoxicity, irritation and inflammation. Mast cells (MCs) are important immune cells that intervene, as effector cells, in allergic and anaphylactic reactions, asthma, autoimmune diseases and cancer. In dermatitis, activated MCs release inflammatory chemical mediators and secrete pro-inflammatory cytokines, including interleukin (IL)-1, TNF, and IL-33. In addition, IL-1 activates MCs to generate a number of cytokines and chemokines, which aggravate inflammation. IL-38 cytokine, an IL-1 family member, is secreted by activated immune cells, including macrophages and lymphocytes, and possesses anti-inflammatory activity. IL-38, by binding IL-36 receptor (IL-36R), provokes suppression of inflammation in many immune diseases. In particular, IL-38 inhibits the generation of IL-1, IL-6 and IL-8 along with other cytokines/chemokines. Here, we hypothesize for the first time that IL-38 may suppresses the inflammatory response in dermatitis, exerting beneficial therapeutic effect.

Microglia and mast cells generate proinflammatory cytokines in the brain and worsen inflammatory state: Suppressor effect of IL-37

Brain microglia cells are responsible for recognizing foreign bodies and act by activating other immune cells. Microglia react against infectious agents that cross the blood-brain barrier and release pro-inflammatory cytokines including interleukin (IL)-1β, IL-33 and tumor necrosis factor (TNF). Mast cells (MCs) are immune cells also found in the brain meninges, in the perivascular spaces where they create a protective barrier and release pro-inflammatory compounds, such as IL-1β, IL-33 and TNF. IL-1β binds to the IL-1R1 receptor and activates a cascade of events that leads to the production of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and activation of the immune system. IL-33 is a member of the IL-1 family expressed by several immune cells including microglia and MCs and is involved in innate and adaptive immunity. IL-33 is a pleiotropic cytokine which binds the receptor ST2 derived from TLR/IL-1R super family and is released after cellular damage (also called "alarmin"). These cytokines are responsible for a number of brain inflammatory disorders. Activated IL-1β in the brain stimulates microglia, MCs, and perivascular endothelial cells, mediating various inflammatory brain diseases. IL-37 also belongs to the IL-1 family and has the capacity to suppress IL-1β with an anti-inflammatory property. IL-37 deficiency could activate and enhance myeloid differentiation (MyD88) and p38-dependent protein-activated mitogenic kinase (MAPK) with an increase in IL-1β and IL-33 exacerbating neurological pathologies. In this article we report for the first time that microglia communicate and collaborate with MCs to produce pro-inflammatory cytokines that can be suppressed by IL-37 having a therapeutic potentiality.

Amyotrophic Lateral Sclerosis, Neuroinflammation, and Cromolyn

Amyotrophic lateral sclerosis (ALS) is an upper motor neuron disease with an unknown pathogenesis and no effective treatment. A recent study found that treatment of a mouse model of ALS (TgSOD1 mice) intraperitoneally with the mast-cell blocker disodium chromoglycate (cromolyn) had a small but significant effect on disease onset, improvement of neurologic symptoms, and decrease in the expression of proinflammatory cytokines and chemokines in the spinal cord and plasma of the TgSOD1 mice. Treatment with cromolyn also reduced degranulation of mast cells in the tibialis anterior muscle. There was no effect on survival. These findings are important in their support of the involvement of mast cells in the pathogenesis of ALS but are limited by the small effect of cromolyn, which was given intraperitoneally and is poorly absorbed after oral administration. Instead, use of the structurally related flavonoid tetramethoxyluteolin, which is a more potent inhibitor of proinflammatory cytokine release from mast cells and also inhibits activated microglia, may offer significant advantages over cromolyn. Development of mast cell inhibitors could benefit not only allergic disorders but also inflammatory and neurodegenerative disorders.

IL-33 stimulates human mast cell release of CCL5 and CCL2 via MAPK and NF-κB, inhibited by methoxyluteolin

Mast Cells (MCs) are critical for allergic reactions but also play important roles in inflammation, following stimulation by non-allergic triggers such as cytokines. Upon stimulation, MCs secrete numerous newly synthesized mediators, but the mechanism of the release of chemokines, which are important in the pathogenesis of allergic and inflammatory diseases, remains unknown. IL-33 is an "alarmin", known to augment allergic stimulation of MCs, but its effect on the release of chemokines is not known. The present work investigated the action of IL-33 on the release of the chemokines CCL5 and CCL2 from human MCs, as well as the inhibitory effect of the flavonoid 3',4',5,7-tetramethoxyflavone (methoxyluteolin). Stimulation of cultured human MCs (LAD2) and primary MCs (hCBMCs) by IL-33 (1-100 ng/ml) increased the gene expression and the release of CCL5 (P < 0.0001) and CCL2 (P < 0.01). Stimulation with IL-33 (10 ng/ml) activated MAPK components, as shown by phosphorylation of p38α MAPK, JNK, and c-Jun using Western blot analysis. Inhibition of these responses by known inhibitors confirmed that CCL5 and CCL2 are stimulated by the activation of p38α MAPK, JNK, and IκB-α. The gene expression and the release of CCL5 and CCL2 stimulated by IL-33 were significantly inhibited by 2 h pre-treatment with methoxyluteolin (10, 50, 100  μM). The inhibition by methoxyluteolin (50 μM) was not mediated via MAPK inhibition as phosphorylated p38α MAPK and JNK expression were not affected. In conclusion, IL-33 plays an important role in chemokine release from human MCs and that is by activation of more than one signaling pathway. The inhibitory effect of methoxyluteolin may indicate that it can be developed as a novel treatment for inflammatory diseases.

Mast Cells and Sensory Nerves Contribute to Neurogenic Inflammation and Pruritus in Chronic Skin Inflammation

The intimate interaction between mast cells and sensory nerves can be illustrated by the wheal and surrounding flare in an urticarial reaction in human skin. This reaction is typically associated with an intense itch at the reaction site. Upon activation, cutaneous mast cells release powerful mediators, such as histamine, tryptase, cytokines, and growth factors that can directly stimulate corresponding receptors on itch-mediating sensory nerves. These include, e.g., H1- and H4-receptors, protease-activated receptor-2, IL-31 receptor, and the high-affinity receptor of nerve growth factor (TrkA). On the other hand, sensory nerves can release neuropeptides, including substance P and vasoactive intestinal peptide, that are able to stimulate mast cells to release mediators leading to potentiation of the reciprocal interaction, inflammation, and itch. Even though mast cells are well recognized for their role in allergic skin whealing and urticaria, increasing evidence supports the reciprocal function between mast cells and sensory nerves in neurogenic inflammation in chronic skin diseases, such as psoriasis and atopic dermatitis, which are often characterized by distressing itch, and exacerbated by psychological stress. Increased morphological contacts between mast cells and sensory nerves in the lesional skin in psoriasis and atopic dermatitis as well as experimental models in mice and rats support the essential role for mast cell-sensory nerve communication in consequent pruritus. Therefore, we summarize here the present literature pointing to a close association between mast cells and sensory nerves in pruritic skin diseases as well as review the essential supporting findings on pruritic models in mice and rats.

Mast Cells, Neuroinflammation and Pain in Fibromyalgia Syndrome

Fibromyalgia Syndrome (FMS) is a disorder of chronic, generalized muscular pain, accompanied by sleep disturbances, fatigue and cognitive dysfunction. There is no definitive pathogenesis except for altered central pain pathways. We previously reported increased serum levels of the neuropeptides substance P (SP) and its structural analogue hemokinin-1 (HK-1) together with the pro-inflammatory cytokines IL-6 and TNF in FMS patients as compared to sedentary controls. We hypothesize that thalamic mast cells contribute to inflammation and pain, by releasing neuro-sensitizing molecules that include histamine, IL-1β, IL-6 and TNF, as well as calcitonin-gene related peptide (CGRP), HK-1 and SP. These molecules could either stimulate thalamic nociceptive neurons directly, or via stimulation of microglia in the diencephalon. As a result, inhibiting mast cell stimulation could be used as a novel approach for reducing pain and the symptoms of FMS.

Mast Cells May Regulate The Anti-Inflammatory Activity of IL-37

Mast cells are unique immune cells involved in allergic reactions, but also in immunity and inflammation. Interleukin 37 (IL-37) has emerged as an important regulatory cytokine with ability to inhibit immune and inflammatory processes. IL-37 is made primarily by macrophages upon activation of toll-like receptors (TLR) leading to generation of mature IL-37 via the action of caspase 1. In this review, we advance the premise that mast cells could regulate the anti-inflammatory activity of the IL-37 via their secretion of heparin and tryptase. Extracellular IL-37 could either dimerize in the presence of heparin and lose biological activity, or be acted upon by proteases that can generate even more biologically active IL-37 forms. Molecules that could selectively inhibit the secretion of mast cell mediators may, therefore, be used together with IL-37 as novel therapeutic agents.

Mast Cells, Stress, Fear and Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a developmental condition characterized by impaired communication and obsessive behavior that affects 1 in 59 children. ASD is expected to affect 1 in about 40 children by 2020, but there is still no distinct pathogenesis or effective treatments. Prenatal stress has been associated with higher risk of developing ASD in the offspring. Moreover, children with ASD cannot handle anxiety and respond disproportionately even to otherwise benign triggers. Stress and environmental stimuli trigger the unique immune cells, mast cells, which could then trigger microglia leading to abnormal synaptic pruning and dysfunctional neuronal connectivity. This process could alter the "fear threshold" in the amygdala and lead to an exaggerated "fight-or-flight" reaction. The combination of corticotropin-releasing hormone (CRH), secreted under stress, together with environmental stimuli could be major contributors to the pathogenesis of ASD. Recognizing these associations and preventing stimulation of mast cells and/or microglia could greatly benefit ASD patients.

Heterogeneity of Human Mast Cells With Respect to MRGPRX2 Receptor Expression and Function

Mast cells and their mediators play a role in the control of homeostasis and in the pathogenesis of several disorders. The concept of rodent mast cell heterogeneity, initially established in the mid-1960s has been extended in humans. Human mast cells isolated and purified from different anatomic sites can be activated via aggregation of cell surface high affinity IgE receptors (FcεRI) by antigens, superantigens, anti-IgE, and anti-FcεRI. MAS-related G protein-coupled receptor-X2 (MRGPRX2) is expressed at high level in human skin mast cells (MCs) (HSMCs), synovial MCs (HSyMCs), but not in lung MCs (HLMCs). MRGPX2 can be activated by neuropeptide substance P, several opioids, cationic drugs, and 48/80. Substance P (5 × 10⁻⁷ M – 5 × 10⁻⁶ M) induced histamine and tryptase release from HSMCs and to a lesser extent from HSyMCs, but not from HLMCs and human cardiac MCs (HHMCs). Morphine (10⁻⁵ M – 3 × 10⁻⁴ M) selectively induced histamine and tryptase release from HSMCs, but not from HLMCs and HHMCs. SP and morphine were incomplete secretagogues because they did not induce the de novo synthesis of arachidonic acid metabolites from human mast cells. In the same experiments anti-IgE (3 μg/ml) induced the release of histamine and tryptase and the de novo synthesis of prostaglandin D₂ (PGD₂) from HLMCs, HHMCs, HSyMCs, and HSMCs. By contrast, anti-IgE induced the production of leukotriene C₄ (LTC₄) from HLMCs, HHMCs, HSyMCs, but not from HSMCs. These results are compatible with the heterogeneous expression and function of MRGPRX2 receptor on primary human mast cells isolated from different anatomic sites.

Recent advances in our understanding of mast cell activation - or should it be mast cell mediator disorders?

INTRODUCTION

An increasing number of patients present with multiple symptoms affecting many organs including the brain due to multiple mediators released by mast cells. These unique tissue immune cells are critical for allergic reactions triggered by immunoglobulin E (IgE), but are also stimulated (not activated) by immune, drug, environmental, food, infectious, and stress triggers, leading to secretion of multiple mediators often without histamine and tryptase. The presentation, diagnosis, and management of the spectrum of mast cell disorders are very confusing. As a result, neuropsychiatric symptoms have been left out, and diagnostic criteria made stricter excluding most patients. Areas covered: A literature search was performed on papers published between January 1990 and November 2018 using MEDLINE. Terms used were activation, antihistamines, atopy, autism, brain fog, heparin, KIT mutation, IgE, inflammation, IL-6, IL-31, IL-37, luteolin, mast cells, mastocytosis, mediators, mycotoxins, release, secretion, tetramethoxyluteolin, and tryptase. Expert opinion: Conditions associated with elevated serum or urine levels of any mast cell mediator, in the absence of comorbidities that could explain elevated levels, should be considered 'Mast Cell Mediator Disorders (MCMD).' Emphasis should be placed on the identification of unique mast cell mediators, and development of drugs or supplements that inhibit their release.

Mast Cell Neural Interactions in Health and Disease

Mast cells (MCs) are located in the periphery as well as the central nervous system (CNS). Known for sterile inflammation, MCs play a critical role in neuroinflammation, which is facilitated by their close proximity to nerve fibers in the periphery and meninges of the spinal cord and the brain. Multifaceted activation of MCs releasing neuropeptides, cytokines and other mediators has direct effects on the neural system as well as neurovascular interactions. Emerging studies have identified the release of extracellular traps, a phenomenon traditionally meant to ensnare invading pathogens, as a cause of MC-induced neural injury. In this review article, we will discuss mechanisms of MC interaction with the nervous system through degranulation, de novo synthesis, extracellular vesicles (EVs), tunneling nanotubes, and extracellular traps with implications across a variety of pathological conditions.

Mast Cells Induce Blood Brain Barrier Damage in SCD by Causing Endoplasmic Reticulum Stress in the Endothelium

Endothelial dysfunction underlies the pathobiology of cerebrovascular disease. Mast cells are located in close proximity to the vasculature, and vasoactive mediators released upon their activation can promote endothelial activation leading to blood brain barrier (BBB) dysfunction. We examined the mechanism of mast cell-induced endothelial activation via endoplasmic reticulum (ER) stress mediated P-selectin expression in a transgenic mouse model of sickle cell disease (SCD), which shows BBB dysfunction. We used mouse brain endothelial cells (mBECs) and mast cells-derived from skin of control and sickle mice to examine the mechanisms involved. Compared to control mouse mast cell conditioned medium (MCCM), mBECs incubated with sickle mouse MCCM showed increased, structural disorganization and swelling of the ER and Golgi, aggregation of ribosomes, ER stress marker proteins, accumulation of galactose-1-phosphate uridyl transferase, mitochondrial dysfunction, reactive oxygen species (ROS) production, P-selectin expression and mBEC permeability. These effects of sickle-MCCM on mBEC were inhibited by Salubrinal, a reducer of ER stress. Histamine levels in the plasma, skin releasate and in mast cells of sickle mice were higher compared to control mice. Compared to control BBB permeability was increased in sickle mice. Treatment of mice with imatinib, Salubrinal, or P-selectin blocking antibody reduced BBB permeability in sickle mice. Mast cells induce endothelial dysfunction via ER stress-mediated P-selectin expression. Mast cell activation contributes to ER stress mediated endothelial P-selectin expression leading to increased endothelial permeability and impairment of BBB. Targeting mast cells and/or ER stress has the potential to ameliorate endothelial dysfunction in SCD and other pathobiologies.