Retinoic Acid Negatively Impacts Proliferation and MCTC Specific Attributes of Human Skin Derived Mast Cells, but Reinforces Allergic Stimulability

Mast cells express IL17A, IL17F and RORC, are activated and persist with IL-17 production in resolved skin of patients with chronic plaque-type psoriasis

Little is known about IL-17 expression in psoriasis and the actual cellular source of IL-17 remains incompletely defined. We show that high numbers of IL-17 + mast cells persisted in resolved lesions after treatment (anti-IL-17A, anti-IL-23, UVB or topical dithranol) and correlated inversely with the time span in remission. IL-17 + mast cells were found in T cell-rich areas and often close to resident memory T cells (Trm) in active psoriasis and resolved lesional skin. Digital cytometry by deconvolution of RNA-seq data showed that activated mast cells were increased in psoriatic skin, while resting mast cells were almost absent and both returned to normal levels after treatment. When primary human skin mast cells were stimulated with T cell cytokines (TNFα, IL-22 and IFNγ), they responded by releasing more IL-17A, as measured by ELISA. In situ mRNA detection using padlock probes specific for transcript variants of IL17A, IL17F, and RORC (encoding the Th17 transcription factor RORγt) revealed positive mRNA signals for IL17A, IL17F, and RORCin tryptase + cells, demonstrating that mast cells have the transcriptional machinery to actively produce IL-17. Mast cells thus belong to the center of the IL-23/IL-17 axis and high numbers of IL-17 + mast cells predict an earlier disease recurrence.

Nutrition in chronic inflammatory conditions: Bypassing the mucosal block for micronutrients

Nutritional Immunity is one of the most ancient innate immune responses, during which the body can restrict nutrients availability to pathogens and restricts their uptake by the gut mucosa (mucosal block). Though this can be a beneficial strategy during infection, it also is associated with non-communicable diseases-where the pathogen is missing; leading to increased morbidity and mortality as micronutritional uptake and distribution in the body is hindered. Here, we discuss the acute immune response in respect to nutrients, the opposing nutritional demands of regulatory and inflammatory cells and particularly focus on some nutrients linked with inflammation such as iron, vitamins A, Bs, C, and other antioxidants. We propose that while the absorption of certain micronutrients is hindered during inflammation, the dietary lymph path remains available. As such, several clinical trials investigated the role of the lymphatic system during protein absorption, following a ketogenic diet and an increased intake of antioxidants, vitamins, and minerals, in reducing inflammation and ameliorating disease.

CREB Is Indispensable to KIT Function in Human Skin Mast Cells-A Positive Feedback Loop between CREB and KIT Orchestrates Skin Mast Cell Fate

Skin mast cells (MCs) are critical effector cells in acute allergic reactions, and they contribute to chronic dermatoses like urticaria and atopic and contact dermatitis. KIT represents the cells' crucial receptor tyrosine kinase, which orchestrates proliferation, survival, and functional programs throughout the lifespan. cAMP response element binding protein (CREB), an evolutionarily well-conserved transcription factor (TF), regulates multiple cellular programs, but its function in MCs is poorly understood. We recently reported that CREB is an effector of the SCF (Stem Cell Factor)/KIT axis. Here, we ask whether CREB may also act upstream of KIT to orchestrate its functioning. Primary human MCs were isolated from skin and cultured in SCF+IL-4 (Interleukin-4). Pharmacological inhibition (666-15) and RNA interference served to manipulate CREB function. We studied KIT expression using flow cytometry and RT-qPCR, KIT-mediated signaling using immunoblotting, and cell survival using scatterplot and caspase-3 activity. The proliferation and cycle phases were quantified following BrdU incorporation. Transient CREB perturbation resulted in reduced KIT expression. Conversely, microphthalmia transcription factor (MITF) was unnecessary for KIT maintenance. KIT attenuation secondary to CREB was associated with heavily impaired KIT functional outputs, like anti-apoptosis and cell cycle progression. Likewise, KIT-elicited phosphorylation of ERK1/2 (Extracellular Signal-Regulated Kinase 1/2), AKT, and STAT5 (Signal Transducer and Activator of Transcription) was substantially diminished upon CREB inhibition. Surprisingly, the longer-term interference of CREB led to complete cell elimination, in a way surpassing KIT inhibition. Collectively, we reveal CREB as non-redundant in MCs, with its absence being incompatible with skin MCs' existence. Since SCF/KIT regulates CREB activity and, vice versa, CREB is required for KIT function, a positive feedforward loop between these elements dictates skin MCs' fate.

Danger-associated metabolites trigger metaflammation: A crowbar in cardiometabolic diseases

Cardiometabolic diseases (CMDs) are characterized by a series of metabolic disorders and chronic low-grade inflammation. CMDs contribute to a high burden of mortality and morbidity worldwide. Host-microbial metabolic regulation that triggers metaflammation is an emerging field of study that promotes a new perspective for perceiving cardiovascular risks. The term metaflammation denotes the entire cascade of immune responses activated by a new class of metabolites known as "danger-associated metabolites" (DAMs). It is being proposed by the present review for the first time. We summarize current studies covering bench to bedside aspects of DAMs to better understand CMDs in the context of DAMs. We have focused on the involvement of DAMs in the pathophysiological development of CMDs, including the disruption of immune homeostasis and chronic inflammation-triggered damage leading to CMD-related adverse events, as well as emerging therapeutic approaches for targeting DAM metabolism in CMDs.

Synergism between IL-33 and MRGPRX2/FcεRI Is Primarily Due to the Complementation of Signaling Modules, and Only Modestly Supplemented by Prolonged Activation of Selected Kinases

Skin mast cells (MCs) express high levels of MRGPRX2, FcεRI, and ST2, and vigorously respond to their ligands when triggered individually. IL-33/ST2 also potently synergizes with other receptors, but the molecular underpinnings are poorly understood. Human skin-derived MCs were stimulated via different receptors individually or jointly in the presence/absence of selective inhibitors. TNF was quantified by ELISA. Signaling cascades were studied by immunoblot. TNF was stimulated by FcεRI ≈ ST2 > MRGPRX2. Surprisingly, neither FcεRI nor MRGPRX2 stimulation elicited NF-κB activation (IκB degradation, p65 phosphorylation) in stark contrast to IL-33. Accordingly, TNF production did not depend on NF-κB in FcεRI- or MRGPRX2-stimulated MCs, but did well so downstream of ST2. Conversely, ERK1/2 and PI3K were the crucial modules upon FcεRI/MRGPRX2 stimulation, while p38 was key to the IL-33-elicited route. The different signaling prerequisites were mirrored by their activation patterns with potent pERK/pAKT after FcεRI/MRGPRX2, but preferential induction of pp38/NF-κB downstream of ST2. FcεRI/MRGPRX2 strongly synergized with IL-33, and some synergy was still observed upon inhibition of each module (ERK1/2, JNK, p38, PI3K, NF-κB). IL-33's contribution to synergism was owed to p38 > JNK > NF-κB, while the partner receptor contributed through ERK > PI3K ≈ JNK. Concurrent IL-33 led to slightly prolonged pERK (downstream of MRGPRX2) or pAKT (activated by FcεRI), while the IL-33-elicited modules (pp38/NF-κB) remained unaffected by co-stimulation of FcεRI/MRGPRX2. Collectively, the strong synergistic activity of IL-33 primarily results from the complementation of highly distinct modules following co-activation of the partner receptor rather than by altered signal strength of the same modules.

Clorfl86/RHEX Is a Negative Regulator of SCF/KIT Signaling in Human Skin Mast Cells

Mast cells (MCs) are key effector cells in allergic and inflammatory diseases, and the SCF/KIT axis regulates most aspects of the cells' biology. Using terminally differentiated skin MCs, we recently reported on proteome-wide phosphorylation changes initiated by KIT dimerization. C1orf186/RHEX was revealed as one of the proteins to become heavily phosphorylated. Its function in MCs is undefined and only some information is available for erythroblasts. Using public databases and our own data, we now report that RHEX exhibits highly restricted expression with a clear dominance in MCs. While expression is most pronounced in mature MCs, RHEX is also abundant in immature/transformed MC cell lines (HMC-1, LAD2), suggesting early expression with further increase during differentiation. Using RHEX-selective RNA interference, we reveal that RHEX unexpectedly acts as a negative regulator of SCF-supported skin MC survival. This finding is substantiated by RHEX's interference with KIT signal transduction, whereby ERK1/2 and p38 both were more strongly activated when RHEX was attenuated. Comparing RHEX and capicua (a recently identified repressor) revealed that each protein preferentially suppresses other signaling modules elicited by KIT. Induction of immediate-early genes strictly requires ERK1/2 in SCF-triggered MCs; we now demonstrate that RHEX diminution translates to this downstream event, and thereby enhances NR4A2, JUNB, and EGR1 induction. Collectively, our study reveals RHEX as a repressor of KIT signaling and function in MCs. As an abundant and selective lineage marker, RHEX may have various roles in the lineage, and the provided framework will enable future work on its involvement in other crucial processes.

Mast cell tolerance in the skin microenvironment to commensal bacteria is controlled by fibroblasts

Activation and degranulation of mast cells (MCs) is an essential aspect of innate and adaptive immunity. Skin MCs, the most exposed to the external environment, are at risk of quickly degranulating with potentially severe consequences. Here, we define how MCs assume a tolerant phenotype via crosstalk with dermal fibroblasts (dFBs) and how this phenotype reduces unnecessary inflammation when in contact with beneficial commensal bacteria. We explore the interaction of human MCs (HMCs) and dFBs in the human skin microenvironment and test how this interaction controls MC inflammatory response by inhibiting the nuclear factor κB (NF-κB) pathway. We show that the extracellular matrix hyaluronic acid, as the activator of the regulatory zinc finger (de)ubiquitinating enzyme A20/tumor necrosis factor α-induced protein 3 (TNFAIP3), is responsible for the reduced HMC response to commensal bacteria. The role of hyaluronic acid as an anti-inflammatory ligand on MCs opens new avenues for the potential treatment of inflammatory and allergic disorders.

CREB Is Activated by the SCF/KIT Axis in a Partially ERK-Dependent Manner and Orchestrates Survival and the Induction of Immediate Early Genes in Human Skin Mast Cells

cAMP response element binding protein (CREB) functions as a prototypical stimulus-inducible transcription factor (TF) that initiates multiple cellular changes in response to activation. Despite pronounced expression in mast cells (MCs), CREB function is surprisingly ill-defined in the lineage. Skin MCs (skMCs) are critical effector cells in acute allergic and pseudo-allergic settings, and they contribute to various chronic dermatoses such as urticaria, atopic dermatitis, allergic contact dermatitis, psoriasis, prurigo, rosacea and others. Using MCs of skin origin, we demonstrate herein that CREB is rapidly phosphorylated on serine-133 upon SCF-mediated KIT dimerization. Phosphorylation initiated by the SCF/KIT axis required intrinsic KIT kinase activity and partially depended on ERK1/2, but not on other kinases such as p38, JNK, PI3K or PKA. CREB was constitutively nuclear, where phosphorylation occurred. Interestingly, ERK did not translocate to the nucleus upon SCF activation of skMCs, but a fraction was present in the nucleus at baseline, and phosphorylation was prompted in the cytoplasm and nucleus in situ. CREB was required for SCF-facilitated survival, as demonstrated with the CREB-selective inhibitor 666-15. Knock-down of CREB by RNA interference duplicated CREB's anti-apoptotic function. On comparison with other modules (PI3K, p38 and MEK/ERK), CREB was equal or more potent at survival promotion. SCF efficiently induces immediate early genes (IEGs) in skMCs (FOS, JUNB and NR4A2). We now demonstrate that CREB is an essential partaker in this induction. Collectively, the ancient TF CREB is a crucial component of skMCs, where it operates as an effector of the SCF/KIT axis, orchestrating IEG induction and lifespan.

β-arrestin-1 and β-arrestin-2 Restrain MRGPRX2-Triggered Degranulation and ERK1/2 Activation in Human Skin Mast Cells

As a novel receptor that efficiently elicits degranulation upon binding to one of its numerous ligands, MRGPRX2 has moved to the center of attention in mast cell (MC) research. Indeed, MRGPRX2 is believed to be a major component of pseudo-allergic reactions to drugs and of neuropeptide-elicited MC activation in skin diseases alike. MRGPRX2 signals via G proteins which organize downstream events ultimately leading to granule discharge. Skin MCs require both PI3K and ERK1/2 cascades for efficient exocytosis. β-arrestins act as opponents of G proteins and lead to signal termination with or without subsequent internalization. We recently demonstrated that ligand-induced internalization of MRGPRX2 requires the action of β-arrestin-1, but not of β-arrestin-2. Here, by using RNA interference, we find that both isoforms counter skin MC degranulation elicited by three MRGPRX2 agonists but not by FcεRI-aggregation. Analyzing whether this occurs through MRGPRX2 stabilization under β-arrestin attenuation, we find that reduction of β-arrestin-1 indeed leads to increased MRGPRX2 abundance, while this is not observed for β-arrestin-2. This led us speculate that β-arrestin-2 is involved in signal termination without cellular uptake of MRGPRX2. This was indeed found to be the case, whereby interference with β-arrestin-2 has an even stronger positive effect on ERK1/2 phosphorylation compared to β-arrestin-1 perturbation. Neither β-arrestin-1 nor β-arrestin-2 had an impact on AKT phosphorylation nor affected signaling via the canonical FcεRI-dependent route. We conclude that in skin MCs, β-arrestin-2 is chiefly involved in signal termination, whereas β-arrestin-1 exerts its effects by controlling MRGPRX2 abundance.

Selected recent advances in understanding the role of human mast cells in health and disease

Mast cells are highly granular tissue-resident cells and key drivers of inflammation, particularly in allergies as well as in other inflammatory diseases. Most mast cell research was initially conducted in rodents but has increasingly shifted to the human system, with the advancement of research technologies and methodologies. Today we can analyze primary human cells including rare subpopulations, we can produce and maintain mast cells isolated from human tissues, and there are several human mast cell lines. These tools have substantially facilitated our understanding of their role and function in different organs in both health and disease. We can now define more clearly where human mast cells originate from, how they develop, which mediators they store, produce de novo, and release, how they are activated and by which receptors, and which neighbouring cells they interact with and by which mechanisms. Considerable progress has also been made regarding the potential contribution of mast cells to disease, which, in turn, has led to the development of novel approaches for preventing key pathogenic effects of mast cells, heralding the era of mast cell-targeted therapeutics. In this review, we present and discuss a selection of some of the most significant advancements and remaining gaps in our understanding of human mast cells during the last 25 years, with a focus on clinical relevance.

Action of substance P and PAMP(9-20) on different excitation sites of MRGPRX2 induces differences in mast cell activation

MRGPRX2 on mast cells (MCs) is the target that directly mediates MC activation through the activity of small molecular substances. Previous work has attempted to prove that substance P (SP) and PAMP(9-20) induce an MRGPRX2-mediated MC degranulation reaction. However, SP activates MRGPRX2-induced histamine release, which may lead to allergic airway inflammation, while PAMP(9-20)-induced MrgprB2 activation releases more tryptase and fewer monoamines. Due to the lack of direct available comparisons, the different types of sensitizing mediators released by the action of SP and PAMP(9-20) inducing pseudo-allergic reactions via MRGPRX2 are unclear. To investigate whether the action sites of excited MRGPRX2 are different for SP and PAMP(9-20), leading to different effects, the release of inflammatory mediators was measured using MC degranulation reactions and RNA-seq assay in vitro. Mice were treated to observe local inflammation and MC degranulation in vivo. Moreover, site-directed mutagenesis was used to verify the excited sites of SP and PAMP(9-20). SP and PAMP(9-20) both activated MRGPRX2 and led MCs to release inflammatory mediators. Significantly different levels of histamine, tryptase, TNF-α, MCP-1, and other cytokines were released in vivo and in vitro. G165E, D184N, W243R, and H259Y were necessary for SP to activate MRGPRX2, while only D184N and W243R were important for PAMP(9-20). The downstream signaling pathways activated by SP and PAMP(9-20) also differed in the phosphorylation level of PKC. There were differences in the sites via which SP and PAMP(9-20) activate MRGPRX2 and also in the activated downstream signaling pathways, which led to the differences the activation of the pathways and effects of SP- and PAMP(9-20)-induced MRGPRX2 activation.

Cytokines Stimulated by IL-33 in Human Skin Mast Cells: Involvement of NF-κB and p38 at Distinct Levels and Potent Co-Operation with FcεRI and MRGPRX2

The IL-1 family cytokine IL-33 activates and re-shapes mast cells (MCs), but whether and by what mechanisms it elicits cytokines in MCs from human skin remains poorly understood. The current study found that IL-33 activates CCL1, CCL2, IL-5, IL-8, IL-13, and TNF-α, while IL-1β, IL-6, IL-31, and VEGFA remain unaffected in cutaneous MCs, highlighting that each MC subset responds to IL-33 with a unique cytokine profile. Mechanistically, IL-33 induced the rapid (1–2 min) and durable (2 h) phosphorylation of p38, whereas the phosphorylation of JNK was weaker and more transient. Moreover, the NF-κB pathway was potently activated, as revealed by IκB degradation, increased nuclear abundance of p50/p65, and vigorous phosphorylation of p65. The activation of NF-κB occurred independently of p38 or JNK. The induced transcription of the cytokines selected for further study (CCL1, CCL2, IL-8, TNF-α) was abolished by interference with NF-κB, while p38/JNK had only some cytokine-selective effects. Surprisingly, at the level of the secreted protein products, p38 was nearly as effective as NF-κB for all entities, suggesting post-transcriptional involvement. IL-33 did not only instruct skin MCs to produce selected cytokines, but it also efficiently co-operated with the allergic and pseudo-allergic/neurogenic activation networks in the production of IL-8, TNF-α, CCL1, and CCL2. Synergism was more pronounced at the protein than at the mRNA level and appeared stronger for MRGPRX2 ligands than for FcεRI. Our results underscore the pro-inflammatory nature of an acute IL-33 stimulus and imply that especially in combination with allergens or MRGPRX2 agonists, IL-33 will efficiently amplify skin inflammation and thereby aggravate inflammatory dermatoses.

Illuminating the Role of Vitamin A in Skin Innate Immunity and the Skin Microbiome: A Narrative Review

Vitamin A is a fat-soluble vitamin that plays an important role in skin immunity. Deficiencies in Vitamin A have been linked to impaired immune response and increased susceptibility to skin infections and inflammatory skin disease. This narrative review summarizes recent primary evidence that elucidates the role of vitamin A and its derivatives on innate immune regulators through mechanisms that promote skin immunity and sustain the skin microbiome.

Thymic Stromal Lymphopoietin Promotes MRGPRX2-Triggered Degranulation of Skin Mast Cells in a STAT5-Dependent Manner with Further Support from JNK

Thymic stromal lymphopoietin (TSLP) is released by epithelial cells following disturbed homeostasis to act as “alarmin” and driver of Th2-immunity. Aberrant TSLP expression is a hallmark of atopic diseases, including atopic dermatitis (AD). Mast cells (MCs) are overabundant in AD lesions and show signs of degranulation, but it remains unknown whether TSLP contributes to granule discharge. Degranulation of skin MCs proceeds via two major routes, i.e., FcεRI-dependent (allergic) and MRGPRX2-mediated (pseudo-allergic/neurogenic). Evidence is accumulating that MRGPRX2 may be crucial in the context of skin diseases, including eczema. The current study reveals TSLP as a novel priming factor of human skin MCs. Interestingly, TSLP selectively cooperates with MRGPRX2 to support granule discharge, while it does not impact spontaneous or FcεRI-driven exocytosis. TSLP-assisted histamine liberation triggered by compound 48/80 or Substance P, two canonical MRGPRX2 agonists, was accompanied by an increase in CD107a+ cells (a MC activation marker). The latter process was less potent, however, and detectable only at the later of two time points, suggesting TSLP may prolong opening of the granules. Mechanistically, TSLP elicited phosphorylation of STAT5 and JNK in skin MCs and the reinforced degranulation critically depended on STAT5 activity, while JNK had a contributory role. Results from pharmacological inhibition were confirmed by RNA-interference, whereby silencing of STAT5 completely abolished the priming effect of TSLP on MRGPRX2-mediated degranulation. Collectively, TSLP is the first factor to favor MRGPRX2- over FcεRI-triggered MC activation. The relevance of TSLP, MCs and MRGPRX2 to pruritis and atopic skin pathology indicates broad repercussions of the identified connection.

Mas-related G protein-coupled receptor X2 and its activators in dermatologic allergies

The Mas-related G protein-coupled receptor X2 (MRGPRX2) is a multiligand receptor responding to various exogenous and endogenous stimuli. Being highly expressed on skin mast cells, MRGPRX2 triggers their degranulation and release of proinflammatory mediators, and it promotes multicellular signaling cascades, such as itch induction and transmission in sensory neurons. The expression of MRGPRX2 by skin mast cells and the levels of the MRGPRX2 agonists (eg, substance P, major basic protein, eosinophil peroxidase) are upregulated in the serum and/or skin of patients with inflammatory and pruritic skin diseases, such as chronic spontaneous urticaria or atopic dermatitis. Therefore, MRGPRX2 and its agonists might be potential biomarkers for the progression of cutaneous inflammatory diseases and the response to treatment. In addition, they may represent promising targets for prevention and treatment of signs and symptoms in patients with skin diseases or drug reactions. To assess this possibility, this review explores the role and relevance of MRGPRX2 and its activators in cutaneous inflammatory disorders and chronic pruritus.

MRGPRX2 Is the Codeine Receptor of Human Skin Mast Cells: Desensitization through β-Arrestin and Lack of Correlation with the FcεRI Pathway

Codeine stimulates skin mast cells and is therefore used in skin tests and as an inducer of experimental itch. MRGPRX2 responds to various drugs, including opioids, to elicit pseudoallergic reactions, but whether it represents the main opiate receptor of skin mast cells remains unknown. By combining a number of approaches, including the silencing of MRGPRX2, we now report that MRGPRX2 is indeed the dominant codeine receptor of dermal mast cells. Activation by codeine displayed profound subject variability and correlated with secretion elicited by compound 48/80 or substance P but not by FcεRI aggregation. Degranulation by codeine was attenuated by stem cell factor, whereas the opposite was found for FcεRI. Compound 48/80 or codeine alone was able to achieve maximum MRGPRX2 activation. MRGPRX2 was rapidly internalized on codeine binding in a β-arrestin-1‒dependent manner. Codeine-triggered β-arrestin activation was also established by the Tango assay. Prestimulation with MRGPRX2 agonists (but not C3a or FcεRI aggregation) resulted in refractoriness to further stimulation by the same or another MRGPRX2 ligand (cross desensitization). This was duplicated in a cell line (RBL-MRGPRX2). Collectively, codeine degranulates skin mast cells through MRGPRX2, at which it acts as a balanced ligand. It has yet to be determined whether codeine-induced refractoriness could be exploited to desensitize MRGPRX2 to prevent severe pseudoallergic reactions.

MRGPRX2 signals its importance in cutaneous mast cell biology: Does MRGPRX2 connect mast cells and atopic dermatitis?

The discovery of MRGPRX2 marks an important change in MC biology, explaining non-IgE-mediated clinical phenomena relying on MCs. As receptor for multiple drugs, MRGPRX2 is crucial to drug-induced hypersensitivity. However, not only drugs, but also endogenous mediators like neuropeptides and host defense peptides activate MRGPRX2, suggesting its broad impact in cutaneous pathophysiology. Here, we give a brief overview of MRGPRX2 and its regulation by microenvironmental stimuli, which support MCs and can be altered in skin disorders, and briefly touch on the functional programs elicited by MRGPRX2 ligation. Studies in Mrgprb2-deficient mice (the murine ortholog) help illuminate MRGPRX2's function in health and disease. Recent advances in this model support the long-suspected operational unit between MCs and nerves, with MRGPRX2 being a vital component. Based on the limited evidence for a major contribution of FcεRI/IgE-activated MCs to atopic dermatitis (AD), we develop the hypothesis that MRGPRX2 constitutes the missing link connecting MCs and AD, at least in selected endotypes. Support comes from the multifold changes in the MC-neuronal system of AD skin (eg greater density of MCs and closer connections between MCs and nerves, increased PAR-2/Substance P). We theorize that these deregulations suffice to initiate AD, but external triggers, many of which activating MRGPRX2 themselves (eg Staphylococcus aureus) further feed into the loop. Itch, the most burdensome hallmark of AD, is mostly non-histaminergic but tryptase-dependent, and tryptase is preferentially released upon MRGPRX2 activation. Because MRGPRX2 is a very active research field, some of the existing gaps are likely to be closed soon.

Allergy Modulation by N-3 Long Chain Polyunsaturated Fatty Acids and Fat Soluble Nutrients of the Mediterranean Diet

The Mediterranean diet, containing valuable nutrients such as n-3 long chain poly-unsaturated fatty acids (LCPUFAs) and other fat-soluble micronutrients, is known for its health promoting and anti-inflammatory effects. Its valuable elements might help in the battle against the rising prevalence of non-communicable diseases (NCD), including the development of allergic diseases and other (chronic) inflammatory diseases. The fat fraction of the Mediterranean diet contains bioactive fatty acids but can also serve as a matrix to dissolve and increase the uptake of fat-soluble vitamins and phytochemicals, such as luteolin, quercetin, resveratrol and lycopene with known immunomodulatory and anti-inflammatory capacities. Especially n-3 LCPUFAs such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) derived from marine oils can target specific receptors or signaling cascades, act as eicosanoid precursors and/or alter membrane fluidity and lipid raft formation, hereby exhibiting anti-inflammatory properties. Beyond n-3 LCPUFAs, fat-soluble vitamins A, D, E, and K1/2 have the potential to affect pro-inflammatory signaling cascades by interacting with receptors or activating/inhibiting signaling proteins or phosphorylation in immune cells (DCs, T-cells, mast cells) involved in allergic sensitization or the elicitation/effector phase of allergic reactions. Moreover, fat-soluble plant-derived phytochemicals can manipulate signaling cascades, mostly by interacting with other receptors or signaling proteins compared to those modified by fat-soluble vitamins, suggesting potential additive or synergistic actions by applying a combination of these nutrients which are all part of the regular Mediterranean diet. Research concerning the effects of phytochemicals such as polyphenols has been hampered due to their poor bio-availability. However, their solubility and uptake are improved by applying them within the dietary fat matrix. Alternatively, they can be prepared for targeted delivery by means of pharmaceutical approaches such as encapsulation within liposomes or even unique nanoparticles. This review illuminates the molecular mechanisms of action and possible immunomodulatory effects of n-3 LCPUFAs and fat-soluble micronutrients from the Mediterranean diet in allergic disease development and allergic inflammation. This will enable us to further appreciate how to make use of the beneficial effects of n-3 LCPUFAs, fat-soluble vitamins and a selection of phytochemicals as active biological components in allergy prevention and/or symptom reduction.

Vitamin A deficiency exacerbates extrinsic atopic dermatitis development by potentiating type 2 helper T cell-type inflammation and mast cell activation

BACKGROUND

Vitamin A deficiency (VAD) has been hypothesized to play a role in the pathophysiology of atopic dermatitis (AD).

OBJECTIVE

We sought to verify whether VAD can exacerbate AD development, and explore the possible pathophysiologic mechanism.

METHODS

We detected serum vitamin A (VA) concentration in different phenotypes of AD infants (intrinsic AD, iAD and extrinsic AD, eAD), and established ovalbumin (OVA) percutaneous sensitized AD model and passive cutaneous anaphylaxis (PCA) model on VAD and vitamin A supplementation (VAS) model in wild-type mice (C57BL/6) and established AD model on both normal VA (VAN) and VAD feeding mast cell deficiency mice (ckit^w-sh/w-sh ).

RESULTS

The average serum VA concentration of eAD was significantly lower than that of iAD, as well as healthy controls. In OVA-induced C57BL/6 mouse AD model, compared with VAN group, VAD mice manifested significantly more mast cells accumulation in the skin lesions, more severe Th2-mediated inflammation, including higher serum IgG1 and IgE levels, more IL-4, IL-13 mRNA expression in OVA-sensitized skin, and lower Th1 immune response, including lower serum IgG2a and IFN-γ mRNA expression in the skin. But there was no significant difference in the expression of IL-17 mRNA between OVA-treated skin of VAN and VAD mice. However, in OVA-induced ckit^w-sh/w-sh mouse AD model, we did not find any significant differences in the above measurements between VAD and VAN group. In PCA model, VAD mice showed remarkable more blue dye leakage than that in VAN mice. Compared with VAD group, the above-mentioned inflammatory measurements in VAS group and VAN group were similar in OVA-induced AD model mice.

CONCLUSIONS AND CLINICAL RELEVANCE

VAD can exacerbate extrinsic AD by augmenting Th2-mediated inflammation and mast cell activation. Therapeutic VAS can rescue VAD-aggravated eAD. It may provide a new strategy for future prevention or treatment of atopic dermatitis.

All-trans-retinoic acid activated mast cells via Mas-related G-protein-coupled receptor-X2 in retinoid dermatitis

BACKGROUND

Retinoic acid (RA)-induced dermatitis is the most frequent side-effect limiting its widespread use. However, the exact mechanisms triggering dermatitis are not fully understood, including the role of skin mast cells. The newly discovered Mas-related G-protein-coupled receptor-X2 (MRGPRX2) in mast cells mediates pseudoallergic drug reactions in several types of dermatitis. A possible contribution of MRGPRX2 to contact dermatitis induced by RA has hitherto not been examined.

OBJECTIVES

To investigate whether all-trans-RA (ATRA) activates mast cells via MRGPRX2/MrgprB2 (the mouse orthologue), contributing to the pathogenesis of retinoid-induced dermatitis.

METHODS

Wild-type (WT) and MrgprB2^-/- mice were treated with topical ATRA to observe local inflammation and mast cell degranulation in vivo by the use of haematoxylin and eosin and immunofluorescence staining. Release of histamine and release of β-hexosaminidase were measured and calcium influx was detected in Laboratory of Allergic Disease 2 (LAD2) cells with specific knockdown targeting MRGPRX2 by small interfering RNA (siRNA) and in primary cells from MrgprB2^-/- mice.

RESULTS

As compared with WT mice, MrgprB2^-/- mice showed resistance to ATRA-triggered contact dermatitis and local inflammatory reactions in the paws. ATRA activated mast cells via the MrgprB2 pathway in murine cells, and via the MRGPRX2 pathway in human mast cells.

CONCLUSIONS

ATRA-induced dermatitis could be achieved by activating mast cells via MRGPRX2/MrgprB2, which may provide a potential therapy target to reduce the side-effect.

IL-33 and MRGPRX2-Triggered Activation of Human Skin Mast Cells-Elimination of Receptor Expression on Chronic Exposure, but Reinforced Degranulation on Acute Priming

Clinically relevant exocytosis of mast cell (MC) mediators can be triggered by high-affinity IgE receptor (FcεRI)-aggregation (allergic route) or by the so-called pseudo-allergic pathway elicited via MAS-related G protein-coupled receptor-X2 (MRGPRX2). The latter is activated by drugs and endogenous neuropeptides. We recently reported that FcεRI-triggered degranulation is attenuated when human skin mast cells are chronically exposed to IL-33. Here, we were interested in the regulation of the MRGPRX2-route. Chronic exposure of skin MCs to IL-33 basically eliminated the pseudo-allergic/neurogenic route as a result of massive MRGPRX2 reduction. This downregulation seemed to partially require c-Jun N-terminal Kinase (JNK), but not p38, the two kinases activated by IL-33 in skin MCs. Surprisingly, however, JNK had a positive effect on MRGPRX2 expression in the absence of IL-33. This was evidenced by Accell^®-mediated JNK knockdown and JNK inhibition. In stark contrast to the dampening effect upon prolonged exposure, IL-33 was able to prime for increased degranulation by MRGPRX2 ligands when administered directly before stimulation. This supportive effect depended on p38, but not on JNK activity. Our data reinforce the concept that exposure length dictates whether IL-33 will enhance or attenuate secretion. IL-33 is, thus, the first factor to acutely enhance MRGPRX2-triggered degranulation. Finally, we reveal that p38, rarely associated with MC degranulation, can positively affect exocytosis in a context-dependent manner.

Yin-Yang of IL-33 in Human Skin Mast Cells: Reduced Degranulation, but Augmented Histamine Synthesis through p38 Activation

Mast cells (MCs) are the principal effector cells of IgE-mediated allergy. IL-33 is released by resident skin cells as alarmin upon tissue damage or allergen contact. Owing to their pronounced receptor expression, MCs are important targets of IL-33 action, but consequences for skin MCs are ill-defined, especially upon chronic exposure to IL-33. Mimicking the inflammatory milieu of skin disorders, we found that persistent exposure to IL-33 (over a 5-week period) strengthened skin MC numbers through accelerated cell-cycle progression and restriction of apoptosis. Conversely, IL-33 attenuated degranulation and FcεRI expression, potentially as a feedback to chronic "alarmin" exposure. Interestingly, the negative impact on histamine release was counterbalanced by amplified histamine production. Considering the clinical significance of histamine and scarce information on its regulation, we explored the molecular underpinnings. IL-33 induced swift phosphorylation of p38 and JNK (but not of ERK1/2 or AKT), and stimulated histidine decarboxylase expression. Combining pharmacological inhibition and kinase elimination by Accell-facilitated RNA interference in skin MCs revealed a p38-dependent, but JNK-independent mechanism. Collectively, IL-33 exerts multifaceted effects on cutaneous MCs at a post-maturation stage. The IL-33-skin MC axis may contribute to and balance inflammation in chronic skin disorders.

Gulf War Illness: Unifying Hypothesis for a Continuing Health Problem

An estimated 25%⁻32% of veterans of the 1991 Gulf War continue to experience multiple unexplained health problems known as Gulf War Illness (GWI). GWI encompasses chronic pain, musculoskeletal weakness, headache, fatigue, cognitive deficits, alterations in mood, and numerous multi-system complaints. Most potential exposures implicated in GWI were not well documented but included varying levels of several neurotoxicants as well as the anticholinergic drug pyridostigmine bromide (PB), which was routinely taken as prophylaxis against the nerve agent soman. While some veterans also took chloroquine as an antimalarial agent, the literature suggests an association between receipt of multiple vaccinations prior to or during the conflict (perhaps combined with other exposures), and GWI. In-theater exposures may account for any single individual veteran's ill health but many veterans of the same era who were not deployed overseas also suffer the same or similar symptoms. The features of GWI also overlap with those of fibromyalgia, chronic fatigue syndrome and multiple chemical sensitivity, in all of which liver dysfunction has been documented, suggesting a unifying hypothesis. It is proposed that multiple vaccinations, with concurrent or subsequent exposure to PB or additional chemical insults of a liver-damaging nature, plausibly explain the pathogenesis and the observed chronicity of GWI. The suggested mechanism for GWI is thus a chemically-induced impaired liver function, with the spillage of stored vitamin A compounds ("retinoids") into the circulation in toxic concentrations, resulting in an endogenous chronic form of hypervitaminosis A. Implications of the hypothesis are briefly reviewed.

Does a carrot a day keep the allergy away?

Vitamin A is an important micronutrient, from plants diet taken up as carotenoids, from animal food sources as retinol. Its active metabolite retinoic acid (RA) binds to nuclear hormone receptors, thereby regulating gene transcription programs in various cells. Adequate nutritional intake of vitamin A is essential for pre- and postnatal development, eyesight and reproduction, and it contributes to the maintenance and regulation of the immune system. Recent molecular studies indicate that lipocalins play an important role in the bioavailability of RA and its immune modulation against Th2 responses. There is emerging evidence that supply with vitamin A determines the susceptibility to allergic diseases: significantly reduced serum vitamin A levels are commonly observed in allergic patients compared to healthy controls. In line, findings from nutritional and clinical trials suggest that sufficient vitamin A supplementation in pregnancy prevents the development of allergic diseases in the offspring, and helps in controlling symptoms in adult asthmatics. Overall, retinoids have a key role in regulating immune homeostasis on mucosal surfaces because they are able to interfere with inflammatory signalling pathways. In this mini-review we will concentrate on the current knowledge about the influence of dietary and supplementary vitamin A on allergic diseases in humans from infancy to adulthood.

MRGPRX2 is negatively targeted by SCF and IL-4 to diminish pseudo-allergic stimulation of skin mast cells in culture

MRGPRX2 was recently uncovered as the "missing link" in clinically relevant mast cell (MC) activation explaining previously puzzling phenomena. It is the receptor for various endogenous ligands and exogenous compounds alike, whose binding evokes rapid degranulation much like allergen-mediated exocytosis. While the perceivable outcomes are similar, the two activation routes differ regarding mechanism and regulation. We recently reported that acute SCF administration curbs responses evoked by MRGPRX2 in human skin MCs. Maintenance of MCs in culture requires the presence of MC supportive factors and renders the cells functionally and molecularly unequal to ex vivo counterparts. Here, we asked whether expansion in culture impacts the pseudo-allergic route, and if so, what contribution SCF and IL-4 play in this scenario. We report that the in vitro micromilieu dampens (but does not erase) pseudo-allergic responses and that this is accompanied by strongly reduced MRGPRX2 expression. Withdrawal of SCF or IL-4 individually, but most potently of both collectively, partially reinstates the MRGPRX2 pathway, revealing that SCF and IL-4 make negative adjustments to the pseudo-allergic pathway. Under all conditions, the FcεRI-triggered route showed the inverse pattern of regulation, substantiating that allergic and pseudo-allergic MC activation can obey opposite rules, hinting at possible competition between them.

Allergic FcεRI- and pseudo-allergic MRGPRX2-triggered mast cell activation routes are independent and inversely regulated by SCF

While allergic mast cell (MC) degranulation occurs by FcεRI aggregation and varies in strength among subjects, the analogous pseudo-allergic route was recently uncovered to proceed via MRGPRX2. Here, we examine interindividual variability in skin MC responses to FcεRI triggering vs those evoked by MRGPRX2. While population-based variability is comparable between the routes, FcεRI- and MRGPRX2-stimulated pathways are completely independent from each other, and responsiveness to one has therefore no predictive value for the other. Conversely, degranulation triggered by compound 48/80 is highly correlated to the process elicited by substance P. MRGPRX2 mRNA shows pronounced population-based variability (coefficient of variation 102.9%). Surprisingly, stem cell factor (SCF) as the MC-supportive mediator par excellence potently inhibits pseudo-allergic degranulation, while it simultaneously promotes allergic stimulation via FcεRI. We conclude that SCF can have selective MC-dampening functions. Clinically, the data imply that subjects highly reactive in one pathway are not automatically hyper-responsive in terms of the alternative route.

Emerging Roles for MAS-Related G Protein-Coupled Receptor-X2 in Host Defense Peptide, Opioid, and Neuropeptide-Mediated Inflammatory Reactions

Mast cells (MCs) are tissue-resident immune cells that contribute to host defense but are best known for their roles in allergic and inflammatory diseases. In humans, MCs are divided into two subtypes based on the protease content of their secretory granules. Thus, human lung MCs contain only tryptase and are known as MC_T, whereas skin MCs contain both tryptase and chymase and are known as MC_TC. Patients with severe asthma display elevated MCs in the lung, which undergo phenotypic change from MC_T to MC_TC. Although the human genome contains four Mas related G protein coupled receptor X (MRGPRX) genes, an important feature of MC_TC is that they selectively express MRGPRX2. It is activated by antimicrobial host defense peptides such as human β-defensins and the cathelicidin LL-37 and likely contributes to host defense. MRGPRX2 is also a receptor for the neuropeptide substance P, major basic protein, eosinophil peroxidase, opioids, and many FDA-approved cationic drugs. Increased expression of MRGPRX2 or enhanced downstream signaling likely contributes to chronic inflammatory diseases such as rosacea, atopic dermatitis, chronic urticaria, and severe asthma. In this chapter, I will discuss the expression profile and function of MRGPRX1-4 and review the emerging roles of MRGPRX2 on host defense, chronic inflammatory diseases, and drug-induced pseudoallergic reactions. I will also examine the novel aspects of MRGPRX2 signaling in MCs as it related to degranulation and review the mechanisms of its regulation.