The mast cells of the multiple sclerosis brain

Type 2 immunity in the brain and brain borders

Recent research in neuroimmunology has revolutionized our understanding of the intricate interactions between the immune system and the central nervous system (CNS). The CNS, an "immune-privileged organ", is now known to be intimately connected to the immune system through different cell types and cytokines. While type 2 immune responses have traditionally been associated with allergy and parasitic infections, emerging evidence suggests that these responses also play a crucial role in CNS homeostasis and disease pathogenesis. Type 2 immunity encompasses a delicate interplay among stroma, Th2 cells, innate lymphoid type 2 cells (ILC2s), mast cells, basophils, and the cytokines interleukin (IL)-4, IL-5, IL-13, IL-25, TSLP and IL-33. In this review, we discuss the beneficial and detrimental roles of type 2 immune cells and cytokines in CNS injury and homeostasis, cognition, and diseases such as tumors, Alzheimer's disease and multiple sclerosis.

Mast Cells as Important Regulators in Autoimmunity and Cancer Development

Mast cells are an essential part of the immune system and are best known as important modulators of allergic and anaphylactic immune responses. Upon activation, mast cells release a multitude of inflammatory mediators with various effector functions that can be both protective and damage-inducing. Mast cells can have an anti-inflammatory or pro-inflammatory immunological effect and play important roles in regulating autoimmune diseases including rheumatoid arthritis, type 1 diabetes, and multiple sclerosis. Importantly, chronic inflammation and autoimmunity are linked to the development of specific cancers including pancreatic cancer, prostate cancer, colorectal cancer, and gastric cancer. Inflammatory mediators released from activated mast cells regulate immune responses and promote vascular permeability and the recruitment of immune cells to the site of inflammation. Mast cells are present in increased numbers in tissues affected by autoimmune diseases as well as in tumor microenvironments where they co-localize with T regulatory cells and T effector cells. Mast cells can regulate immune responses by expressing immune checkpoint molecules on their surface, releasing anti-inflammatory cytokines, and promoting vascularization of solid tumor sites. As a result of these immune modulating activities, mast cells have disease-modifying roles in specific autoimmune diseases and cancers. Therefore, determining how to regulate the activities of mast cells in different inflammatory and tumor microenvironments may be critical to discovering potential therapeutic targets to treat autoimmune diseases and cancer.

The role of Sp140 revealed in IgE and mast cell responses in Collaborative Cross mice

Mouse IgE and mast cell (MC) functions have been studied primarily using inbred strains. Here, we (a) identified effects of genetic background on mouse IgE and MC phenotypes, (b) defined the suitability of various strains for studying IgE and MC functions, and (c) began to study potentially novel genes involved in such functions. We screened 47 Collaborative Cross (CC) strains, as well as C57BL/6J and BALB/cJ mice, for strength of passive cutaneous anaphylaxis (PCA) and responses to the intestinal parasite Strongyloides venezuelensis (S.v.). CC mice exhibited a diversity in PCA strength and S.v. responses. Among strains tested, C57BL/6J and CC027 mice showed, respectively, moderate and uniquely potent MC activity. Quantitative trait locus analysis and RNA sequencing of BM-derived cultured MCs (BMCMCs) from CC027 mice suggested Sp140 as a candidate gene for MC activation. siRNA-mediated knock-down of Sp140 in BMCMCs decreased IgE-dependent histamine release and cytokine production. Our results demonstrated marked variations in IgE and MC activity in vivo, and in responses to S.v., across CC strains. C57BL/6J and CC027 represent useful models for studying MC functions. Additionally, we identified Sp140 as a gene that contributes to IgE-dependent MC activation.

Increased blood-brain barrier hyperpermeability coincides with mast cell activation early under cuprizone administration

The cuprizone induced animal model of demyelination is characterized by demyelination in many regions of the brain with high levels of demyelination in the corpus callosum as well as changes in neuronal function by 4–6 weeks of exposure. The model is used as a tool to study demyelination and subsequent degeneration as well as therapeutic interventions on these effects. Historically, the cuprizone model has been shown to contain no alterations to blood-brain barrier integrity, a key feature in many diseases that affect the central nervous system. Cuprizone is generally administered for 4–6 weeks to obtain maximal demyelination and degeneration. However, emerging evidence has shown that the effects of cuprizone on the brain may occur earlier than measurable gross demyelination. This study sought to investigate changes to blood-brain barrier permeability early in cuprizone administration. Results showed an increase in blood-brain barrier permeability and changes in tight junction protein expression as early as 3 days after beginning cuprizone treatment. These changes preceded glial morphological activation and demyelination known to occur during cuprizone administration. Increases in mast cell presence and activity were measured alongside the increased permeability implicating mast cells as a potential source for the blood-brain barrier disruption. These results provide further evidence of blood-brain barrier alterations in the cuprizone model and a target of therapeutic intervention in the prevention of cuprizone-induced pathology. Understanding how mast cells become activated under cuprizone and if they contribute to blood-brain barrier alterations may give further insight into how and when the blood-brain barrier is affected in CNS diseases. In summary, cuprizone administration causes an increase in blood-brain barrier permeability and this permeability coincides with mast cell activation.

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.

Type 2 Inflammatory Responses in Autoimmune Demyelination of the Central Nervous System: Recent Advances

Type 2 immunity has long been confined to a restricted spectrum of responses, mostly including allergic reactions to innocuous environmental triggers. However, growing evidence suggests that cells and mediators typically associated with type 2 inflammation are involved in several physiopathological conditions, such as defense against toxic substances, anticancer immunity, and autoimmune diseases. In neuromyelitis optica, an autoimmune demyelinating disorder of the spinal cord and optic nerve, eosinophils extensively infiltrate lesions in the central nervous system (CNS) and promote tissue pathology in experimental models of this disease. Next-generation sequencing of CD4⁺ T cells isolated from a specific subtype of multiple sclerosis plaque has uncovered an unexpectedly Th2 profile of these cells. Even mast cells and other allergic mediators have been implicated in the modulation and/or effector mechanisms of autoimmune reactions against the CNS. In this review article, the most recent developments showing the involvement of type 2 inflammatory components in CNS autoimmunity are summarised and possible lines of further investigation are discussed.

Targeting mast cell as a neuroprotective strategy

Background: Mast cells (MCs) are perivascularly located immune cells of haematopoietic origin. Emerging evidences suggest that the activation of MCs play important roles in the pathogenesis of blood brain barrier disruption, neuroinflammation, and neurodegeneration. Objectives: In this review, we aimed to discuss the detrimental effects of MCs in response to various types of brain injury, as well as the therapeutic potential and neuroprotective effects of targeting the activation and degranulation of MCs, particularly in the management of the acute phase. Methods: An extensive online literature search was conducted through Pubmed/Central on March 2018. Then, we comprehensively summarized the effects of the activation of brain MCs in acute brain injury along with current pharmacological strategies targeting at the activation of MCs. Results: The review of the current literature indicated that the activation and degranulation of brain MCs significantly contribute to the acute pathological process following different types of brain injury including focal and global cerebral ischaemia, intracerebral haemorrhage, subarachnoid haemorrhage, and traumatic brain injury. Conclusions: Brain MCs significantly contribute to the acute pathological processes following brain injury. In that regard, targeting brain MCs may provide a novel strategy for neuroprotection.

KIT as a therapeutic target for non-oncological diseases

KIT is a receptor tyrosine kinase that after binding to its ligand stem cell factor activates signaling cascades linked to biological processes such as proliferation, differentiation, migration and cell survival. Based on studies performed on SCF and/or KIT mutant animals that presented anemia, sterility, and/or pigmentation disorders, KIT signaling was mainly considered to be involved in the regulation of hematopoiesis, gametogenesis, and melanogenesis. More recently, novel animal models and ameliorated cellular and molecular techniques have led to the discovery of a widen repertoire of tissue compartments and functions that are being modulated by KIT. This is the case for the lung, heart, nervous system, gastrointestinal tract, pancreas, kidney, liver, and bone. For this reason, the tyrosine kinase inhibitors that were originally developed for the treatment of hemato-oncological diseases are being currently investigated for the treatment of non-oncological disorders such as asthma, rheumatoid arthritis, and alzheimer's disease, among others. The beneficial effects of some of these tyrosine kinase inhibitors have been proven to depend on KIT inhibition. This review will focus on KIT expression and regulation in healthy and pathologic conditions other than cancer. Moreover, advances in the development of anti-KIT therapies, including tyrosine kinase inhibitors, and their application will be discussed.

Predictable Irreversible Switching Between Acute and Chronic Inflammation

Many a disease associates with inflammation. Upon binding of antigen-antibody complexes to immunoglobulin-like receptors, mast cells release tumor necrosis factor-α and proteases, causing fibroblasts to release endogenous antigens that may be cross reactive with exogenous antigens. We made a predictive dynamic map of the corresponding extracellular network. In silico, this map cleared bacterial infections, via acute inflammation, but could also cause chronic inflammation. In the calculations, limited inflammation flipped to strong inflammation when cross-reacting antigen exceeded an “On threshold.” Subsequent reduction of the antigen load to below this “On threshold” did not remove the strong inflammation phenotype unless the antigen load dropped below a much lower and subtler “Off” threshold. In between both thresholds, the network appeared caught either in a “low” or a “high” inflammatory state. This was not simply a matter of bi-stability, however, the transition to the “high” state was temporarily revertible but ultimately irreversible: removing antigen after high exposure reduced the inflammatory phenotype back to “low” levels but if then the antigen dosage was increased only a little, the high inflammation state was already re-attained. This property may explain why the high inflammation state is indeed “chronic,” whereas only the naive low-inflammation state is “acute.” The model demonstrates that therapies of chronic inflammation such as with anti-IgLC should require fibroblast implantation (or corresponding stem cell activation) for permanence in order to redress the irreversible transition.

Immunoregulatory effect of mast cells influenced by microbes in neurodegenerative diseases

When related to central nervous system (CNS) health and disease, brain mast cells (MCs) can be a source of either beneficial or deleterious signals acting on neural cells. We review the current state of knowledge about molecular interactions between MCs and glia in neurodegenerative diseases such as Multiple Sclerosis, Alzheimer's disease, Amyotrophic Lateral Sclerosis, Parkinson's disease, Epilepsy. We also discuss the influence on MC actions evoked by the host microbiota, which has a profound effect on the host immune system, inducing important consequences in neurodegenerative disorders. Gut dysbiosis, reduced intestinal motility and increased intestinal permeability, that allow bacterial products to circulate and pass through the blood-brain barrier, are associated with neurodegenerative disease. There are differences between the microbiota of neurologic patients and healthy controls. Distinguishing between cause and effect is a challenging task, and the molecular mechanisms whereby remote gut microbiota can alter the brain have not been fully elucidated. Nevertheless, modulation of the microbiota and MC activation have been shown to promote neuroprotection. We review this new information contributing to a greater understanding of MC-microbiota-neural cells interactions modulating the brain, behavior and neurodegenerative processes.

Mastocytosis in adulthood and neuropsychiatric disorders

Patients with mastocytosis can display various disabling general and neuropsychological symptoms among one third of them, including general signs such as fatigue and musculoskeletal pain, which can have a major impact on quality of life. Neurological symptoms are less frequent and mainly consist of acute or chronic headache (35%), rarely syncopes (5%), acute onset back pain (4%), and in a few cases, clinical and radiological symptoms resembling or allowing the diagnosis of multiple sclerosis (1.3%). Headaches are associated with symptoms related to mast cell activation syndrome (flushes, prurit, and so forth) and more frequently present as migraine (37.5%), with often aura (66%). Depression-anxiety like symptoms can occur in 40% to 60% of the patients and cognitive impairment is not rare (38.6%). The pathophysiology of these symptoms could be linked to tissular mast cell infiltration or to mast cell mediators release or both. The tryptophan metabolism could be involved in mast cell-induced neuroinflammation through indoleamine-2,3-dioxygenase activation. Treatments targeting mast cell may be useful to target neuropsychological features associated with mastocytosis, including tyrosine kinase inhibitors.

Meningeal mast cell-T cell crosstalk regulates T cell encephalitogenicity

GM-CSF is a cytokine produced by T helper (Th) cells that plays an essential role in orchestrating neuroinflammation in experimental autoimmune encephalomyelitis, a rodent model of multiple sclerosis. Yet where and how Th cells acquire GM-CSF expression is unknown. In this study we identify mast cells in the meninges, tripartite tissues surrounding the brain and spinal cord, as important contributors to antigen-specific Th cell accumulation and GM-CSF expression. In the absence of mast cells, Th cells do not accumulate in the meninges nor produce GM-CSF. Mast cell-T cell co-culture experiments and selective mast cell reconstitution of the meninges of mast cell-deficient mice reveal that resident meningeal mast cells are an early source of caspase-1-dependent IL-1β that licenses Th cells to produce GM-CSF and become encephalitogenic. We also provide evidence of mast cell-T cell co-localization in the meninges and CNS of recently diagnosed acute MS patients indicating similar interactions may occur in human demyelinating disease.

Mast cell and autoimmune diseases

Mast cells are important in innate immune system. They have been appreciated as potent contributors to allergic reaction. However, increasing evidence implicates the important role of mast cells in autoimmune disease like rheumatoid arthritis and multiple sclerosis. Here we review the current stage of knowledge about mast cells in autoimmune diseases.

The meninges: new therapeutic targets for multiple sclerosis

The central nervous system (CNS) largely comprises nonregenerating cells, including neurons and myelin-producing oligodendrocytes, which are particularly vulnerable to immune cell-mediated damage. To protect the CNS, mechanisms exist that normally restrict the transit of peripheral immune cells into the brain and spinal cord, conferring an "immune-specialized" status. Thus, there has been a long-standing debate as to how these restrictions are overcome in several inflammatory diseases of the CNS, including multiple sclerosis (MS). In this review, we highlight the role of the meninges, tissues that surround and protect the CNS and enclose the cerebral spinal fluid, in promoting chronic inflammation that leads to neuronal damage. Although the meninges have traditionally been considered structures that provide physical protection for the brain and spinal cord, new data have established these tissues as sites of active immunity. It has been hypothesized that the meninges are important players in normal immunosurveillance of the CNS but also serve as initial sites of anti-myelin immune responses. The resulting robust meningeal inflammation elicits loss of localized blood-brain barrier (BBB) integrity and facilitates a large-scale influx of immune cells into the CNS parenchyma. We propose that targeting the cells and molecules mediating these inflammatory responses within the meninges offers promising therapies for MS that are free from the constraints imposed by the BBB. Importantly, such therapies may avoid the systemic immunosuppression often associated with the existing treatments.

Mast cells in human health and disease

Mast cells are primarily known for their role in defense against pathogens, particularly bacteria; neutralization of venom toxins; and for triggering allergic responses and anaphylaxis. In addition to these direct effector functions, activated mast cells rapidly recruit other innate and adaptive immune cells and can participate in "tuning" the immune response. In this review we touch briefly on these important functions and then focus on some of the less-appreciated roles of mast cells in human disease including cancer, autoimmune inflammation, organ transplant, and fibrosis. Although it is difficult to formally assign causal roles to mast cells in human disease, we offer a general review of data that correlate the presence and activation of mast cells with exacerbated inflammation and disease progression. Conversely, in some restricted contexts, mast cells may offer protective roles. For example, the presence of mast cells in some malignant or cardiovascular diseases is associated with favorable prognosis. In these cases, specific localization of mast cells within the tissue and whether they express chymase or tryptase (or both) are diagnostically important considerations. Finally, we review experimental animal models that imply a causal role for mast cells in disease and discuss important caveats and controversies of these findings.

Mast cells and neuroinflammation

It has been determined that there is extensive communication between the immune system and the central nervous system (CNS). Proinflammatory cytokines play a key role in this communication. There is an emerging realization that glia and microglia, in particular, (which are the brain’s resident macrophages), are an important source of inflammatory mediators and may have fundamental roles in CNS disorders. Microglia respond also to proinflammatory signals released from other non-neuronal cells, principally those of immune origin, such as mast cells. Mast cells reside in the CNS and are capable of migrating across the blood-brain barrier (BBB) in situations where the barrier is compromised as a result of CNS pathology. Mast cells are both sensors and effectors in communication among nervous, vascular, and immune systems. In the brain, they reside on the brain side of the BBB, and interact with astrocytes, microglia, and blood vessels via their neuroactive stored and newly synthesized chemicals. They are first responders, acting as catalysts and recruiters to initiate, amplify, and prolong other immune and nervous responses upon activation. Mast cells both promote deleterious outcomes in brain function and contribute to normative behavioral functioning, particularly cognition and emotion. Mast cells may play a key role in treating systemic inflammation or blockade of signaling pathways from the periphery to the brain.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

Initial immunopathogenesis of multiple sclerosis: innate immune response

Multiple sclerosis (MS) is an inflammatory, demyelinating, and neurodegenerative disease of the central nervous system. The hallmark to MS is the demyelinated plaque, which consists of a well-demarcated hypocellular area characterized by the loss of myelin, the formation of astrocytic scars, and the mononuclear cell infiltrates concentrated in perivascular spaces composed of T cells, B lymphocytes, plasma cells, and macrophages. Activation of resident cells initiates an inflammatory cascade, leading to tissue destruction, demyelination, and neurological deficit. The immunological phenomena that lead to the activation of autoreactive T cells to myelin sheath components are the result of multiple and complex interactions between environment and genetic background conferring individual susceptibility. Within the CNS, an increase of TLR expression during MS is observed, even in the absence of any apparent microbial involvement. In the present review, we focus on the role of the innate immune system, the first line of defense of the organism, as promoter and mediator of cross reactions that generate molecular mimicry triggering the inflammatory response through an adaptive cytotoxic response in MS.

The role of mast cells in neuroinflammation

Mast cells (MCs) are densely granulated perivascular resident cells of hematopoietic origin and well known for their pathogenetic role in allergic and anaphylactic reactions. In addition, they are also involved in processes of innate and adaptive immunity. MCs can be activated in response to a wide range of stimuli, resulting in the release of not only pro-inflammatory, but also anti-inflammatory mediators. The patterns of secreted mediators depend upon the given stimuli and microenvironmental conditions, accordingly MCs have the ability to promote or attenuate inflammatory processes. Their presence in the central nervous system (CNS) has been recognized for more than a century. Since then a participation of MCs in various pathological processes in the CNS has been well documented. They can aggravate CNS damage in models of brain ischemia and hemorrhage, namely through increased blood-brain barrier damage, brain edema and hemorrhage formation and promotion of inflammatory responses to such events. In contrast, recent evidence suggests that MCs may have a protective role following traumatic brain injury by degrading pro-inflammatory cytokines via specific proteases. In neuroinflammatory diseases such as multiple sclerosis, the role of MCs seems to be ambiguous. MCs have been shown to be damaging, neuroprotective, or even dispensable, depending on the experimental protocols used. The role of MCs in the formation and progression of CNS tumors such as gliomas is complex and both positive and negative relationships between MC activity and tumor progression have been reported. In summary, MCs and their secreted mediators modulate inflammatory processes in multiple CNS pathologies and can thereby either contribute to neurological damage or confer neuroprotection. This review intends to give a concise overview of the regulatory roles of MCs in brain disease.

Mast cell regulation of the immune response

Mast cells are well known as principle effector cells of type I hypersensitivity responses. Beyond this role in allergic disease, these cells are now appreciated as playing an important role in many inflammatory conditions. This review summarizes the support for mast cell involvement in resisting bacterial infection, exacerbating autoimmunity and atherosclerosis, and promoting cancer progression. A commonality in these conditions is the ability of mast cells to elicit migration of many cell types, often through the production of inflammatory cytokines such as tumor necrosis factor. However, recent data also demonstrates that mast cells can suppress the immune response through interleukin-10 production. The data encourage those working in this field to expand their view of how mast cells contribute to immune homeostasis.

Mast cells in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis

Mast cells (MCs) are best known as key immune players in immunoglobulin E (IgE)-dependent allergic reactions. In recent years, several lines of evidence have suggested that MCs might play an important role in several pathological conditions, including autoimmune disorders such as multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), an animal model for MS. Since their first description in MS plaques in the late 1800s, much effort has been put into elucidating the contribution of MCs to the development of central nervous system (CNS) autoimmunity. Mouse models of MC-deficiency have provided a valuable experimental tool for dissecting MC involvement in MS and EAE. However, to date there is still major controversy concerning the function of MCs in these diseases. Indeed, although MCs have been classically proposed as having a detrimental and pro-inflammatory role, recent literature has questioned and resized the contribution of MCs to the pathology of MS and EAE. In this review, we will present the main evidence obtained in MS and EAE on this topic, and discuss the critical and controversial aspects of such evidence.

The innate immune system in demyelinating disease

Demyelinating diseases such as multiple sclerosis are chronic inflammatory autoimmune diseases with a heterogeneous clinical presentation and course. Both the adaptive and the innate immune systems have been suggested to contribute to their pathogenesis and recovery. In this review, we discuss the role of the innate immune system in mediating demyelinating diseases. In particular, we provide an overview of the anti-inflammatory or pro-inflammatory functions of dendritic cells, mast cells, natural killer (NK) cells, NK-T cells, γδ T cells, microglial cells, and astrocytes. We emphasize the interaction of astroctyes with the immune system and how this interaction relates to the demyelinating pathologies. Given the pivotal role of the innate immune system, it is possible that targeting these cells may provide an effective therapeutic approach for demyelinating diseases.

Innate immunity in the central nervous system

Immune responses in the CNS are common, despite its perception as a site of immune privilege. These responses can be mediated by resident microglia and astrocytes, which are innate immune cells without direct counterparts in the periphery. Furthermore, CNS immune reactions often take place in virtual isolation from the innate/adaptive immune interplay that characterizes peripheral immunity. However, microglia and astrocytes also engage in significant cross-talk with CNS-infiltrating T cells and other components of the innate immune system. Here we review the cellular and molecular basis of innate immunity in the CNS and discuss what is known about how outcomes of these interactions can lead to resolution of infection, neurodegeneration, or neural repair depending on the context.

Interplaying factors that effect multiple sclerosis causation and sustenance

The author hypothesized that multiple sclerosis (MS) is a humoral autoimmune disease, caused by faulty interplay between myelin-specific, dimeric IgE, specifically competing non-IgE antibodies and IgE-triggered degranulating mast cells. The principal fault was believed to be insufficient quantity of protective, specific non-IgE antibodies. Also conjectured was the possibility of an unexpected and adverse immune suppression caused by none-MS pharmaceuticals being consumed by patients for their MS or for other conditions. To test both hypotheses, a mimotopic, peptide antigen-based, serum immunoassay was developed to measure dimer-bound IgE excess among MS patients, wherein the IgE specifically complexes with two or more myelin surface epitopes at an interval of 40–100 Angstroms, a separation critical for mast cell degranulation and cell damaging effect. MS test sensitivity and specificity, when analyzing five previously untreated patients for dimeric IgE presence, was 100%. In direct comparison, twenty age- and gender-matched female and male control subjects were test negative. Analysis of 35 multiple sclerosis patients, who were concomitantly being treated with potentially immunosuppressive pharmaceuticals, appeared to show the substances' negative effect upon MS causation, progression, or specific immunoassay performance. Therefore, MS is likely an autoimmune disease caused by IgE-mediated mast cell degranulation possibly in conjunction with immunosuppressive agents.

Exacerbated experimental autoimmune encephalomyelitis in mast-cell-deficient Kit W-sh/W-sh mice

Mast cell (MC)-deficient c-Kit mutant Kit(W/W-v) mice are protected against experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, suggesting a detrimental role for MCs in this disease. To further investigate the role of MCs in EAE, we took advantage of a recently characterized model of MC deficiency, Kit(W-sh/W-sh). Surprisingly, we observed that myelin oligodendrocyte glycoprotein (MOG)(35-55)-induced chronic EAE was exacerbated in Kit(W-sh/W-sh) compared with Kit(+/+) mice. Kit(W-sh/W-sh) mice showed more inflammatory foci in the central nervous system (CNS) and increased T-cell response against myelin. To understand whether the discrepant results obtained in Kit(W-sh/W-sh) and in Kit(W/W-v) mice were because of the different immunization protocols, we induced EAE in these two strains with varying doses of MOG(35-55) and adjuvants. Although Kit(W-sh/W-sh) mice exhibited exacerbated EAE under all immunization protocols, Kit(W/W-v) mice were protected from EAE only when immunized with high, but not low, doses of antigen and adjuvants. Kit(W-sh/W-sh) mice reconstituted systemically, but not in the CNS, with bone marrow-derived MCs still developed exacerbated EAE, indicating that protection from disease could be exerted by MCs mainly in the CNS, and/or by other cells possibly dysregulated in Kit(W-sh/W-sh) mice. In summary, these data suggest to reconsider MC contribution to EAE, taking into account the variables of using different experimental models and immunization protocols.

New insights into the role of mast cells in autoimmunity: evidence for a common mechanism of action?

Mast cells are classically considered innate immune cells that act as first responders in many microbial infections and have long been appreciated as potent contributors to allergic reactions. However, recent advances in the realm of autoimmunity have made it clear that these cells are also involved in the pathogenic responses that exacerbate disease. In the murine models of multiple sclerosis, rheumatoid arthritis and bullous pemphigoid, both the pathogenic role of mast cells and some of their mechanisms of action are shared. Similar to their role in infection and a subset of allergic responses, mast cells are required for the efficient recruitment of neutrophils to sites of inflammation. Although this mast cell-dependent neutrophil response is protective in infection settings, it is postulated that neutrophils promote local vascular permeability and facilitate the entry of inflammatory cells that enhance tissue destruction at target sites. However, there is still much to learn. There is little information regarding mechanisms of mast cell activation in disease. Nor is it known how many mast cell-derived mediators are relevant and whether interactions with other cells are implicated in these diseases including T cells, B cells and astrocytes. Here we review the current state of knowledge about mast cells in autoimmune disease. We also discuss findings regarding newly discovered mast cell actions and factors that modulate mast cell function. We speculate that much of this new information will ultimately contribute to a greater understanding of the full range of mast cell actions in autoimmunity. This article is part of a Special Issue entitled: Mast cells in inflammation.

Endothelial histamine H1 receptor signaling reduces blood-brain barrier permeability and susceptibility to autoimmune encephalomyelitis

Disruption of the blood-brain barrier (BBB) underlies the development of experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis. Environmental factors, such as Bordetella pertussis, are thought to sensitize central endothelium to biogenic amines like histamine, thereby leading to increased BBB permeability. B. pertussis-induced histamine sensitization (Bphs) is a monogenic intermediate phenotype of EAE controlled by histamine H(1) receptor (Hrh1/H(1)R). Here, we transgenically overexpressed H(1)R in endothelial cells of Hrh1-KO (H(1)RKO) mice to test the role of endothelial H(1)R directly in Bphs and EAE. Unexpectedly, transgenic H(1)RKO mice expressing endothelial H(1)R under control of the von Willebrand factor promoter (H(1)RKO-vWF(H1R) Tg) were Bphs-resistant. Moreover, H(1)RKO-vWF(H1R) Tg mice exhibited decreased BBB permeability and enhanced protection from EAE compared with H(1)RKO mice. Thus, contrary to prevailing assumptions, our results show that endothelial H(1)R expression reduces BBB permeability, suggesting that endothelial H(1)R signaling may be important in the maintenance of cerebrovascular integrity.

Signaling pathways in the activation of mast cells cocultured with astrocytes and colocalization of both cells in experimental allergic encephalomyelitis

Mast cells in the CNS participate in the pathophysiology of chronic neurodegenerative inflammatory diseases. This study aimed to investigate the signaling pathway of mast cells activated in an environment cocultured with astrocytes and to explore the role of their colocalization in brain of experimental allergic encephalomyelitis. Human mast cell line-1 cells and human U87 glioblastoma cell lines (U87) or mouse bone marrow-derived mast cells and mouse cerebral cortices-derived astrocytes were cocultured. Intracellular Ca(2+) was measured by confocal microscopy; histamine by fluorometric analyzer; leukotrienes by ELISA; small GTPases, protein kinase Cs, MAPK, c-kit, CD40, and CD40L by Western blot; NF-kappaB and AP-1 by EMSA; cytokines by RT-PCR; and colocalization of mast cells and astrocytes in brain by immunohistochemistry. Mast cells cocultured with astrocytes showed time-dependent increases in intracellular Ca(2+) levels, release of histamine and leukotrienes, and cytokine production. Mast cells or astrocytes showed enhanced surface expression of CD40L and CD40, respectively, during coculture. Mast cells cocultured with astrocytes induced small GTPases (Rac1/2, cdc42), protein kinase Cs, MAPK, NF-kappaB, and AP-1 activities. These changes were blocked by anti-CD40 Ab pretreatment or CD40 small interfering RNA. Mast cells increased in the thalamus of experimental allergic encephalomyelitis model, particularly colocalized with astrocytes in the thalamic border region of the habenula. In conclusion, the data suggest that activation of mast cells cocultured with astrocytes induces release of mediators by small GTPases/Ca(2+) influx through CD40-CD40L interactions to participate in the pathophysiology of chronic neurodegenerative inflammatory diseases, such as multiple sclerosis.

Exploring a regulatory role for mast cells: 'MCregs'?

Regulatory cells can mould the fate of the immune response by direct suppression of specific subsets of effector cells, or by redirecting effectors against invading pathogens and infected or neoplastic cells. These functions have been classically, although not exclusively, ascribed to different subsets of T cells. Recently, mast cells have been shown to regulate physiological and pathological immune responses, and thus to act at the interface between innate and adaptive immunity assuming different functions and behaviors at discrete stages of the immune response. Here, we focus on these poorly defined, and sometimes apparently conflicting, functions of mast cells.

Mast cell adhesion induces cytoskeletal modifications and programmed cell death in oligodendrocytes

In this paper we show that rat peritoneal mast cells (RPMC) adhere to rat oligodendrocytes (ODC) in culture and switch on a bi-directional signal affecting both adhering cell and its target. Following heterotypic interaction, RPMC release granule content and ODC show morphological changes and enter the apoptotic programme. Altogether, these findings indicate that the interaction of MC with ODC could play a role in the mechanism of CNS damage induced by the inflammatory reaction.

Human mast cells stimulate activated T cells: implications for multiple sclerosis

Multiple sclerosis is an autoimmune demyelinating disease of the central nervous system mainly mediated by Th1 and/or Th17 cells, which cross the blood-brain barrier. Recent evidence indicates that Th2 cells and mast cells, typically associated with allergic reactions, are also involved. Brain mast cells are critically located perivascularly and secrete numerous proinflammatory and vasoactive molecules that can disrupt the blood-brain barrier, a finding that precedes clinical or pathologic signs of multiple sclerosis. Brain mast cells in multiple sclerosis are activated by neural factors, including substance P, myelin basic protein, and corticotropin-releasing hormone, caused by acute stress, which induce release of several inflammatory mediators. Mast cells can stimulate activated T cells coming in contact with them at the blood-brain barrier, as well as after stimulation with myelin basic protein or substance P. Pretreatment with the flavone luteolin blocks mast cell stimulation and T cell activation, as well as experimental autoimmune encephalitis. Interactions between mast cells and T cells could constitute a new and unique therapeutic target for multiple sclerosis.

Mast cells and the adaptive immune response

BACKGROUND

The idea that the innate and adaptive immune systems are not separate entities is no longer new. In fact, it is surprising that this paradigm was accepted without question for so long. Many innate cells express cell surface molecules and soluble mediators that are essential for the development and activation of T cells and B cells. Yet among the innate cell populations, mast cells may play the major role in regulating adaptive immune cell function.

DISCUSSION

This role first came to light in studies of mast cells and their involvement in the autoimmune disease experimental allergic encephalomyelitis, the major rodent model of multiple sclerosis and has subsequently been verified in many in vitro and in vivo model systems.

Luteolin inhibits myelin basic protein-induced human mast cell activation and mast cell-dependent stimulation of Jurkat T cells

BACKGROUND AND PURPOSE

Allergic inflammation and autoimmune diseases, such as atopic dermatitis, psoriasis and multiple sclerosis (MS), involve both mast cell and T-cell activation. However, possible interactions between the two and the mechanism of such activations are largely unknown.

EXPERIMENTAL APPROACH

Human umbilical cord blood-derived cultured mast cells (hCBMCs) and Jurkat T cells were incubated separately or together, following activation with myelin basic protein (MBP), as well as with or without pretreatment with the flavonoid luteolin for 15 min. The supernatant fluid was assayed for inflammatory mediators released from mast cells and interleukin (IL)-2 release from Jurkat cells.

KEY RESULTS

MBP (10 microM) stimulates hCBMCs to release IL-6, IL-8, transforming growth factor (TGF)-beta1, tumour necrosis factor-alpha (TNF-alpha), vascular endothelial growth factor (VEGF), histamine and tryptase (n=6, P<0.05). Addition of mast cells to Jurkat cells activated by anti-CD3/anti-CD28 increases IL-2 release by 30-fold (n=3, P<0.05). MBP-stimulated mast cells and their supernatant fluid further increase Jurkat cell IL-2 release (n=3, P<0.05). Separation of mast cells and activated Jurkat cells by a Transwell permeable membrane inhibits Jurkat cell stimulation by 60%. Pretreatment of Jurkat cells with a TNF-neutralizing antibody reduces IL-2 release by another 40%. Luteolin pretreatment inhibits mast cell activation (n=3-6, P<0.05), Jurkat cell activation and mast cell-dependent Jurkat cell stimulation (n=3, P<0.05).

CONCLUSIONS AND IMPLICATIONS

Mast cells can stimulate activated Jurkat cells. This interaction is inhibited by luteolin, suggesting that this flavonoid may be useful in the treatment of autoimmune diseases.

Mast cell activation by myelin through scavenger receptor

A role for mast cells (MC) in the pathogenesis of multiple sclerosis (MS) has been suggested, based on the analysis of human lesions and on an animal model of the disease (EAE). What role MC play in the development of MS is not well understood. We hypothesized that the link connecting MC with demyelinating diseases may be represented by their interaction with myelin. Here we show that myelin can activate mast cells. This process could be a key event in the mast cell function required for inducing EAE in mice and possibly in MS in man.

Interferon-β stabilizes barrier characteristics of the blood–brain barrier in four different species in vitro

Background

Blood–brain barrier (BBB) breakdown is an early event in the pathogenesis of multiple sclerosis (MS). In a previous study we have found a direct stabilization of barrier characteristics after treatment of bovine brain capillary endothelial cells (BCECs) with human recombinant interferon-β-1a (IFN-β-1a) in an in vitro BBB model. In the present study we examined the effect of human recombinant IFN-β-1a on the barrier properties of BCECs derived from four different species including humans to predict treatment efficacy of IFN-β-1a in MS patients.

Methods

We used primary bovine and porcine BCECs, as well as human and murine BCEC cell lines. We investigated the influence of human recombinant IFN-β-1a on the paracellular permeability for 3H-inulin and 14C-sucrose across monolayers of bovine, human, and murine BCECs. In addition, the transendothelial electrical resistance (TEER) was determined in in vitro systems applying porcine and murine BCECS.

Results

We found a stabilizing effect on the barrier characteristics of BCECs after pretreatment with IFN-β-1a in all applied in vitro models: addition of IFN-β-1a resulted in a significant decrease of the paracellular permeability across monolayers of human, bovine, and murine BCECs. Furthermore, the TEER was significantly increased after pretreatment of porcine and murine BCECs with IFN-β-1a.

Conclusion

Our data suggest that BBB stabilization by IFN-β-1a may contribute to its beneficial effects in the treatment of MS. A human in vitro BBB model might be useful as bioassay for testing the treatment efficacy of drugs in MS.

The master switch: the role of mast cells in autoimmunity and tolerance

There are many parallels between allergic and autoimmune responses. Both are considered hypersensitivity responses: pathologies that are elicited by an exuberant reaction to antigens that do not pose any inherent danger to the organism. Although mast cells have long been recognized as central players in allergy, only recently has their role in autoimmunity become apparent. Because of the commonalities of these responses, much of what we have learned about the underlying mast cell-dependent mechanisms of inflammatory damage in allergy and asthma can be used to understand autoimmunity. Here we review mast cell biology in the context of autoimmune disease. We discuss the huge diversity in mast cell responses that can exert either proinflammatory or antiinflammatory activity. We also consider the myriad factors that cause one response to predominate over another in a particular immune setting.

Long-term air pollution exposure is associated with neuroinflammation, an altered innate immune response, disruption of the blood-brain barrier, ultrafine particulate deposition, and accumulation of amyloid beta-42 and alpha-synuclein in children and young adults

Air pollution is a serious environmental problem. We investigated whether residency in cities with high air pollution is associated with neuroinflammation/neurodegeneration in healthy children and young adults who died suddenly. We measured mRNA cyclooxygenase-2, interleukin-1beta, and CD14 in target brain regions from low (n = 12) or highly exposed residents (n = 35) aged 25.1 +/- 1.5 years. Upregulation of cyclooxygenase-2, interleukin-1beta, and CD14 in olfactory bulb, frontal cortex, substantia nigrae and vagus nerves; disruption of the blood-brain barrier; endothelial activation, oxidative stress, and inflammatory cell trafficking were seen in highly exposed subjects. Amyloid beta42 (Abeta42) immunoreactivity was observed in 58.8% of apolipoprotein E (APOE) 3/3 < 25 y, and 100% of the APOE 4 subjects, whereas alpha-synuclein was seen in 23.5% of < 25 y subjects. Particulate material (PM) was seen in olfactory bulb neurons, and PM < 100 nm were observed in intraluminal erythrocytes from lung, frontal, and trigeminal ganglia capillaries. Exposure to air pollution causes neuroinflammation, an altered brain innate immune response, and accumulation of Abeta42 and alpha-synuclein starting in childhood. Exposure to air pollution should be considered a risk factor for Alzheimer's and Parkinson's diseases, and carriers of the APOE 4 allele could have a higher risk of developing Alzheimer's disease if they reside in a polluted environment.

The multiple sclerosis degradome: enzymatic cascades in development and progression of central nervous system inflammatory disease

An array of studies implicate different classes of protease and their endogenous inhibitors in multiple sclerosis (MS) pathogenesis based on expression patterns in MS lesions, sera, and/or cerebrospinal fluid (CSF). Growing evidence exists regarding their mechanistic roles in inflammatory and neurodegenerative aspects of this disease. Proteolytic events participate in demyelination, axon injury, apoptosis, and development of the inflammatory response including immune cell activation and extravasation, cytokine and chemokine activation/inactivation, complement activation, and epitope spreading. The potential significance of proteolytic activity to MS therefore relates not only to their potential use as important biomarkers of disease activity, but additionally as prospective therapeutic targets. Experimental data indicate that understanding the net physiological consequence of altered protease levels in MS development and progression necessitates understanding protease activity in the context of substrates, endogenous inhibitors, and proteolytic cascade interactions, which together make up the MS degradome. This review will focus on evidence regarding the potential physiologic role of those protease families already identified as markers of disease activity in MS; that is, the metallo-, serine, and cysteine proteases.

The Multitasking Mast Cell: Positive and Negative Roles in the Progression of Autoimmunity

Among the potential outcomes of an aberrantly functioning immune system are allergic disease and autoimmunity. Although it has been assumed that the underlying mechanisms mediating these conditions are completely different, recent evidence shows that mast cells provide a common link. Mast cells reside in most tissues, are particularly prevalent at sites of Ag entry, and act as sentinel cells of the immune system. They express many inflammatory mediators that affect both innate and adaptive cellular function. They contribute to pathologic allergic inflammation but also serve an important protective role in bacterial and parasite infections. Given the proinflammatory nature of autoimmune responses, it is not surprising that studies using murine models of autoimmunity clearly implicate mast cells in the initiation and/or progression of autoimmune disease. In this review, we discuss the defined and hypothesized mechanisms of mast cell influence on autoimmune diseases, including their surprising and newly discovered role as anti-inflammatory cells.

Mast cells as modulators of T-cell responses

Although mast cells have long been considered the integral effector cell in allergy and atopic disease, the paradigm of mast cell function is now evolving to incorporate data showing that mast cells make innumerable contributions to both protective and pathologic immune responses. Mast cells express cell surface molecules with costimulatory or co-inhibitory activity and produce a multitude of mediators that can direct dendritic cell (DC) or T-cell differentiation and function. In addition, mast cells exhibit a widespread distribution and are in close proximity to DCs and T cells at several critical sites. While there has been amazing progress in characterizing mast cell populations in vitro, only recently has the ability to monitor their in vivo effects become a reality. In this review, we discuss the evolution of our understanding of mast cell biology with an emphasis on their established and hypothesized roles in influencing T-cell differentiation and function. The fact that T-cell and mast cell interactions exist and are a normal component of most adaptive immune responses is one of the best illustrations of the now established concept that innate and adaptive immunity are not completely independent entities.

Different approaches to study mast cell functions

Mast cells have long been known to play a detrimental role in the pathogenesis of IgE-associated allergic disorders by their ability to release a wide variety of pro-inflammatory mediators. A number of studies, however, have demonstrated that mast cells play a beneficial role in innate host defense against bacterial infections. Since mast cells clearly play both physiological and pathophysiological functions in the body, it is important to learn about the components of mast cells that drive these responses. The functional roles of mast cell in vivo have been principally characterized by comparing the biological responses in mast cell-deficient mice (WBB6F(1)-W/W(v)), their normal wild-type littermates (WBB6F(1)-+/+) and mast cell deficient mice reconstituted locally or systemically with mast cells cultured from the bone marrow cells of WBB6F(1)-+/+ mice (WBB6F(1)-W/W(v)+MC). Recently investigators have demonstrated that mast cell-deficient mice (WBB6F(1)-W/W(v)) can be reconstituted with mast cells derived in vitro from the bone marrow cells of certain gene knock-out mice or genetically-manipulated embryonic stem cells. This novel approach of analyzing the biological consequences of gene mutations in mast cells will help us to better understand the role of individual gene products in mast cell responses. In this review, we discuss these new approaches to investigate the functions of mast cells in vivo.