Epigenetic and transcriptional control of mast cell responses

Dehydroleucodine and xanthatin, two natural anti-inflammatory lactones, inhibit mast cell degranulation by affecting the actin cytoskeleton

Actin remodeling is a critical regulator of mast cell secretion. In previous work, we have shown that dehydroleucodine and xanthatin, two natural α,β-unsaturated lactones, exhibit anti-inflammatory and mast cell stabilizing properties. Based on this background, this study aimed to determine whether the mast cell stabilizing action of these lactones is associated with changes in the actin cytoskeleton. Rat peritoneal mast cells were preincubated in the presence of dehydroleucodine or xanthatin before incubation with compound 48/80. Comparative studies with sodium cromoglycate and latrunculin B were also made. After treatments, different assays were performed on mast cell samples: β-hexosaminidase release, cell viability studies, quantification of mast cells and their state of degranulation by light microscopy, transmission electron microscopy, and actin staining for microscopy observation. Results showed that dehydroleucodine and xanthatin inhibited mast cell degranulation, evidenced by the inhibition of β-hexosaminidase release and decreased degranulated mast cell percentage. At the same time, both lactones altered the F-actin cytoskeleton in mast cells resulting, similarly to Latrunculin B, in a higher concentration of nuclear F-actin when activated by compound 48/80. For the first time, this study describes the biological properties of dehydroleucodine and xanthatin concerning to the rearrangement of actin filaments during stimulated exocytosis in mast cells. These data have important implications for developing new anti-inflammatory and mast cell stabilizing drugs and for designing new small molecules that may interact with the actin cytoskeleton.

A one-pot and eco-friendly synthesis of novel β-substituted-α-halomethyl acrylates and the bioactivity of these compounds in an in vitro model of mast cell degranulation induced by pro-inflammatory stimuli

The goal of the present work was to develop novel β-substituted-α-halomethyl acrylates from a methodology in an aqueous phase and to evaluate their bioactivity as potential inhibitors of mast cell activation. Eleven β-substituted-α-halomethyl acrylates were synthesized through a modified Horner-Wadsworth-Emmons reaction. Compound 48/80 and the calcium ionophore A23187 stimulated the release of β-hexosaminidase from mast cells. The effect induced by compound 48/80 was inhibited by compound 5 (320 µM) and compound 9 (160 and 320 µM) without causing cytotoxic effects. The effect induced by A23187 was inhibited by compound 5 (40, 80, 160, and 320 µM) without affecting cell viability. The inhibitory effects exhibited by compounds 5 and 9 were more potent than those of the reference compound sodium cromoglycate at the same concentrations. The biochemical results were consistent with the morphological findings obtained by light and transmission electron microscopy. This study reports, for the first time, that the new synthetic compounds methyl (Z)- 2-bromo-3-(furan-3-yl)acrylate (compound 5) and methyl (E)- 2-bromo-3-(3-bromophenyl)acrylate (compound 9) strongly inhibit mast cell degranulation, without affecting cell viability. The implications of these results are relevant as a basis for developing new anti-inflammatory and mast cell stabilizing drugs.

Emerging Role of the Mast Cell-Microbiota Crosstalk in Cutaneous Homeostasis and Immunity

The skin presents a multifaceted microbiome, a balanced coexistence of bacteria, fungi, and viruses. These resident microorganisms are fundamental in upholding skin health by both countering detrimental pathogens and working in tandem with the skin's immunity. Disruptions in this balance, known as dysbiosis, can lead to disorders like psoriasis and atopic dermatitis. Central to the skin's defense system are mast cells. These are strategically positioned within the skin layers, primed for rapid response to any potential foreign threats. Recent investigations have started to unravel the complex interplay between these mast cells and the diverse entities within the skin's microbiome. This relationship, especially during times of both balance and imbalance, is proving to be more integral to skin health than previously recognized. In this review, we illuminate the latest findings on the ties between mast cells and commensal skin microorganisms, shedding light on their combined effects on skin health and maladies.

Potential Role of Moesin in Regulating Mast Cell Secretion

Mast cells have existed for millions of years in species that never suffer from allergic reactions. Hence, in addition to allergies, mast cells can play a critical role in homeostasis and inflammation via secretion of numerous vasoactive, pro-inflammatory and neuro-sensitizing mediators. Secretion may utilize different modes that involve the cytoskeleton, but our understanding of the molecular mechanisms regulating secretion is still not well understood. The Ezrin/Radixin/Moesin (ERM) family of proteins is involved in linking cell surface-initiated signaling to the actin cytoskeleton. However, how ERMs may regulate secretion from mast cells is still poorly understood. ERMs contain two functional domains connected through a long α-helix region, the N-terminal FERM (band 4.1 protein-ERM) domain and the C-terminal ERM association domain (C-ERMAD). The FERM domain and the C-ERMAD can bind to each other in a head-to-tail manner, leading to a closed/inactive conformation. Typically, phosphorylation on the C-terminus Thr has been associated with the activation of ERMs, including secretion from macrophages and platelets. It has previously been shown that the ability of the so-called mast cell "stabilizer" disodium cromoglycate (cromolyn) to inhibit secretion from rat mast cells closely paralleled the phosphorylation of a 78 kDa protein, which was subsequently shown to be moesin, a member of ERMs. Interestingly, the phosphorylation of moesin during the inhibition of mast cell secretion was on the N-terminal Ser56/74 and Thr66 residues. This phosphorylation pattern could lock moesin in its inactive state and render it inaccessible to binding to the Soluble NSF attachment protein receptors (SNAREs) and synaptosomal-associated proteins (SNAPs) critical for exocytosis. Using confocal microscopic imaging, we showed moesin was found to colocalize with actin and cluster around secretory granules during inhibition of secretion. In conclusion, the phosphorylation pattern and localization of moesin may be important in the regulation of mast cell secretion and could be targeted for the development of effective inhibitors of secretion of allergic and inflammatory mediators from mast cells.

The mRNA methyltransferase Mettl3 modulates cytokine mRNA stability and limits functional responses in mast cells

Mast cells are central players in allergy and asthma, and their dysregulated responses lead to reduced quality of life and life-threatening conditions such as anaphylaxis. The RNA modification N6-methyladenosine (m6A) has a prominent impact on immune cell functions, but its role in mast cells remains unexplored. Here, by optimizing tools to genetically manipulate primary mast cells, we reveal that the m6A mRNA methyltransferase complex modulates mast cell proliferation and survival. Depletion of the catalytic component Mettl3 exacerbates effector functions in response to IgE and antigen complexes, both in vitro and in vivo. Mechanistically, deletion of Mettl3 or Mettl14, another component of the methyltransferase complex, lead to the enhanced expression of inflammatory cytokines. By focusing on one of the most affected mRNAs, namely the one encoding the cytokine IL-13, we find that it is methylated in activated mast cells, and that Mettl3 affects its transcript stability in an enzymatic activity-dependent manner, requiring consensus m6A sites in the Il13 3'-untranslated region. Overall, we reveal that the m6A machinery is essential in mast cells to sustain growth and to restrain inflammatory responses.

A fully human anti-c-Kit monoclonal antibody 2G4 inhibits proliferation and degranulation of human mast cells

Given that mast cells are pivotal contributors to allergic diseases, various allergy treatments have been developed to inhibit them. Omalizumab, an anti-immunoglobulin E antibody, is a representative therapy that can alleviate allergy symptoms by inhibiting mast cell degranulation. However, omalizumab cannot reduce the proliferation and accumulation of mast cells, which is a fundamental cause of allergic diseases. c-Kit is essential for the proliferation, survival, and differentiation of mast cells. Excessive c-Kit activation triggers various mast cell diseases, such as asthma, chronic spontaneous urticaria, and mastocytosis. Herein, we generated 2G4, an anti-c-Kit antibody, to develop a therapeutic agent for mast cell diseases. The therapeutic efficacy of 2G4 antibody was evaluated in LAD2, a human mast cell line. 2G4 antibody completely inhibited c-Kit signaling by blocking the binding of stem cell factor, known as the c-Kit ligand. Inhibition of c-Kit signaling led to the suppression of proliferation, migration, and degranulation in LAD2 cells. Moreover, 2G4 antibody suppressed the secretion of pro-inflammatory cytokines, including granulocyte–macrophage colony-stimulating factor, vascular endothelial growth factor, C–C motif chemokine ligand 2, brain-derived neurotrophic factor, and complement component C5/C5a, which can exacerbate allergy symptoms. Taken together, these results suggest that 2G4 antibody has potential as a novel therapeutic agent for mast cell diseases.

Epigenetic promoter alterations in GI tumour immune-editing and resistance to immune checkpoint inhibition

OBJECTIVES

Epigenomic alterations in cancer interact with the immune microenvironment to dictate tumour evolution and therapeutic response. We aimed to study the regulation of the tumour immune microenvironment through epigenetic alternate promoter use in gastric cancer and to expand our findings to other gastrointestinal tumours.

DESIGN

Alternate promoter burden (APB) was quantified using a novel bioinformatic algorithm (proActiv) to infer promoter activity from short-read RNA sequencing and samples categorised into APB_high, APB_int and APB_low. Single-cell RNA sequencing was performed to analyse the intratumour immune microenvironment. A humanised mouse cancer in vivo model was used to explore dynamic temporal interactions between tumour kinetics, alternate promoter usage and the human immune system. Multiple cohorts of gastrointestinal tumours treated with immunotherapy were assessed for correlation between APB and treatment outcomes.

RESULTS

APB_high gastric cancer tumours expressed decreased levels of T-cell cytolytic activity and exhibited signatures of immune depletion. Single-cell RNAsequencing analysis confirmed distinct immunological populations and lower T-cell proportions in APB_high tumours. Functional in vivo studies using 'humanised mice' harbouring an active human immune system revealed distinct temporal relationships between APB and tumour growth, with APB_high tumours having almost no human T-cell infiltration. Analysis of immunotherapy-treated patients with GI cancer confirmed resistance of APB_high tumours to immune checkpoint inhibition. APB_high gastric cancer exhibited significantly poorer progression-free survival compared with APB_low (median 55 days vs 121 days, HR 0.40, 95% CI 0.18 to 0.93, p=0.032).

CONCLUSION

These findings demonstrate an association between alternate promoter use and the tumour microenvironment, leading to immune evasion and immunotherapy resistance.

LPS Guides Distinct Patterns of Training and Tolerance in Mast Cells

Mast cells (MCs) are tissue-resident, long lived innate immune cells with important effector and immunomodulatory functions. They are equipped with an eclectic variety of receptors that enable them to sense multiple stimuli and to generate specific responses according on the type, strength and duration of the stimulation. Several studies demonstrated that myeloid cells can retain immunological memory of their encounters – a process termed ‘trained immunity’ or ‘innate immune memory’. As MCs are among the one of first cells to come into contact with the external environment, it is possible that such mechanisms of innate immune memory might help shaping their phenotype and effector functions; however, studies on this aspect of MC biology are still scarce. In this manuscript, we investigated the ability of MCs primed with different stimuli to respond to a second stimulation with the same or different ligands, and determined the molecular and epigenetic drivers of these responses. Our results showed that, while the stimulation with IgE and β-glucan failed to induce either tolerant or trained phenotypes, LPS conditioning was able to induce a profound and long-lasting remodeling of the signaling pathways involved in the response against LPS or fungal pathogens. On one side, LPS induced a strong state of unresponsiveness to secondary LPS stimulation due to the impairment of the PI3K-AKT signaling pathway, which resulted in the reduced activation of NF-κB and the decreased release of TNF-α and IL-6, compared to naïve MCs. On the other side, LPS primed MCs showed an increased release of TNF-α upon fungal infection with live Candida albicans, thus suggesting a dual role of LPS in inducing both tolerance and training phenotypes depending on the secondary challenge. Interestingly, the inhibition of HDAC during LPS stimulation partially restored the response of LPS-primed MCs to a secondary challenge with LPS, but failed to revert the increased cytokine production of these cells in response to C. albicans. These data indicate that MCs, as other innate immune cells, can develop innate immune memory, and that different stimulatory environments can shape and direct MC specific responses towards the dampening or the propagation of the local inflammatory response.

RNA-binding proteins and RNA methylation in myeloid cells

RNA-binding proteins (RBPs) regulate all aspects of the life of mRNA transcripts. They are critically important in regulating immune responses, most notably by restraining excessive inflammation that can potentially lead to tissue damage. RBPs are also crucial for pathogen sensing, for instance for the recognition of viral nucleic acids. Concordant with these central regulatory roles, the dysregulated activity of many RBPs can give rise to disease. The expression and function of RBPs are therefore highly controlled by an elaborate network of transcriptional, post-transcriptional and post-translational mechanisms, including the ability of different RBPs to cross-regulate each other's expression. With an emphasis on macrophages and mast cells, we review current knowledge on the role of selected RBPs that have been shown to directly impact the expression of inflammatory transcripts. By focusing specifically on proteins of the Regnase and ZFP36 family, as well as on factors involved in N6 -methyladenosine (m6 A) deposition and recognition, we discuss mechanism of action, regulatory feedback, and impact of these selected proteins on immune responses. Finally, we include examples of the role of m6 A and RBPs in the recognition of viral RNAs. Overall, we provide a general overview of the impact of selected RBPs on the myeloid compartment, followed by a discussion of outstanding questions and challenges for the future.

Dermal Extracellular Matrix-Derived Hydrogels as an In Vitro Substrate to Study Mast Cell Maturation

Mast cells (MCs) are pro-inflammatory tissue-resident immune cells that play a key role in inflammation. MCs circulate in peripheral blood as progenitors and undergo terminal differentiation in the tissue microenvironment where they can remain for many years. This in situ maturation results in tissue- and species-specific MC phenotypes, culminating in significant variability in response to environmental stimuli. There are many challenges associated with studying mature tissue-derived MCs, particularly in humans. In cases where cultured MCs are able to differentiate in two-dimensional in vitro cultures, there remains an inability for full maturation. Extracellular matrix (ECM) scaffolds provide for a more physiologically relevant environment for cells in vitro and have been shown to modulate the response of other immune cells such as T cells, monocytes, and macrophages. To improve current in vitro testing platforms of MCs and to assess future use of ECM scaffolds for MC regulation, we studied the in vitro response of human MCs cultured on decellularized porcine dermis hydrogels (dermis extracellular matrix hydrogel [dECM-H]). This study investigated the effect of dECM-H on cellular metabolic activity, cell viability, and receptor expression compared to collagen type I hydrogel (Collagen-H). Human MCs showed different metabolic activity when cultured in the dECM-H and also upregulated immunoglobulin E (IgE) receptors associated with MC maturation/activation compared to collagen type I. These results suggest an overall benefit in the long-term culture of human MCs in the dECM-H compared to Collagen-H providing important steps toward a model that is more representative of in vivo conditions. Graphical abstract [Formula: see text] Impact statement Mast cells (MCs) are difficult to culture in vitro as current culture conditions and substrates fail to promote similar phenotypic features observed in vivo. Extracellular matrix (ECM)-based biomaterials offer three-dimensional, tissue-specific environments that more closely resemble in vivo conditions. Our study explores the use of dermal ECM hydrogels for MC culture and shows significant upregulation of metabolic activity, cell viability, and gene expression of markers associated with MC maturation or activation compared to collagen type I-hydrogel and tissue culture plastic controls at 7 days. These results are among the first to describe MC behavior in response to ECM hydrogels.

Butyrate inhibits human mast cell activation via epigenetic regulation of FcεRI-mediated signaling

BACKGROUND

Short-chain fatty acids (SCFAs) are fermented dietary components that regulate immune responses, promote colonic health, and suppress mast cell-mediated diseases. However, the effects of SCFAs on human mast cell function, including the underlying mechanisms, remain unclear. Here, we investigated the effects of the SCFAs (acetate, propionate, and butyrate) on mast cell-mediated pathology and human mast cell activation, including the molecular mechanisms involved.

METHOD

Precision-cut lung slices (PCLS) of allergen-exposed guinea pigs were used to assess the effects of butyrate on allergic airway contraction. Human and mouse mast cells were co-cultured with SCFAs and assessed for degranulation after IgE- or non-IgE-mediated stimulation. The underlying mechanisms involved were investigated using knockout mice, small molecule inhibitors/agonists, and genomics assays.

RESULTS

Butyrate treatment inhibited allergen-induced histamine release and airway contraction in guinea pig PCLS. Propionate and butyrate, but not acetate, inhibited IgE- and non-IgE-mediated human or mouse mast cell degranulation in a concentration-dependent manner. Notably, these effects were independent of the stimulation of SCFA receptors GPR41, GPR43, or PPAR, but instead were associated with inhibition of histone deacetylases. Transcriptome analyses revealed butyrate-induced downregulation of the tyrosine kinases BTK, SYK, and LAT, critical transducers of FcεRI-mediated signals that are essential for mast cell activation. Epigenome analyses indicated that butyrate redistributed global histone acetylation in human mast cells, including significantly decreased acetylation at the BTK, SYK, and LAT promoter regions.

CONCLUSION

Known health benefits of SCFAs in allergic disease can, at least in part, be explained by epigenetic suppression of human mast cell activation.

ELKS1 controls mast cell degranulation by regulating the transcription of Stxbp2 and Syntaxin 4 via Kdm2b stabilization

ELKS1 is a protein with proposed roles in regulated exocytosis in neurons and nuclear factor κB (NF-κB) signaling in cancer cells. However, how these two potential roles come together under physiological settings remain unknown. Since both regulated exocytosis and NF-κB signaling are determinants of mast cell (MC) functions, we generated mice lacking ELKS1 in connective tissue MCs (Elks1^f/f Mcpt5-Cre) and found that while ELKS1 is dispensable for NF-κB-mediated cytokine production, it is essential for MC degranulation both in vivo and in vitro. Impaired degranulation was caused by reduced transcription of Syntaxin 4 (STX4) and Syntaxin binding protein 2 (Stxpb2), resulting from a lack of ELKS1-mediated stabilization of lysine-specific demethylase 2B (Kdm2b), which is an essential regulator of STX4 and Stxbp2 transcription. These results suggest a transcriptional role for active-zone proteins like ELKS1 and suggest that they may regulate exocytosis through a novel mechanism involving transcription of key exocytosis proteins.

Understanding the Relevance of DNA Methylation Changes in Immune Differentiation and Disease

Immune cells are one of the most complex and diverse systems in the human organism. Such diversity implies an intricate network of different cell types and interactions that are dependently interconnected. The processes by which different cell types differentiate from progenitors, mature, and finally exert their function requires an orchestrated succession of molecular processes that determine cell phenotype and function. The acquisition of these phenotypes is highly dependent on the establishment of unique epigenetic profiles that confer identity and function on the various types of effector cells. These epigenetic mechanisms integrate microenvironmental cues into the genome to establish specific transcriptional programs. Epigenetic modifications bridge environment and genome regulation and play a role in human diseases by their ability to modulate physiological programs through external stimuli. DNA methylation is one of the most ubiquitous, stable, and widely studied epigenetic modifications. Recent technological advances have facilitated the generation of a vast amount of genome-wide DNA methylation data, providing profound insights into the roles of DNA methylation in health and disease. This review considers the relevance of DNA methylation to immune system cellular development and function, as well as the participation of DNA methylation defects in immune-mediated pathologies, illustrated by selected paradigmatic diseases.

The Role of Mast Cells in Stroke

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

Trained Immunity Carried by Non-immune Cells

“Trained immunity” is a term proposed by Netea to describe the ability of an organism to develop an exacerbated immunological response to protect against a second infection independent of the adaptative immunity. This immunological memory can last from 1 week to several months and is only described in innate immune cells such as monocytes, macrophages, and natural killer cells. Paradoxically, the lifespan of these cells in the blood is shorter than the duration of trained immunity. This observation suggested that trained immunity could be carried by long lifespan cells such as stem cells and non-immune cells like fibroblasts. It is now evident that in addition to performing their putative function in the development and maintenance of tissue homeostasis, non-immune cells also play an important role in the response to pathogens by producing anti-microbial factors, with long-term inflammation suggesting that non-immune cells can be trained to confer long-lasting immunological memory. This review provides a summary of the current relevant knowledge about the cells which possess immunological memory and discusses the possibility that non-immune cells may carry immunological memory and mechanisms that might be involved.

Epigenetic Regulation via Altered Histone Acetylation Results in Suppression of Mast Cell Function and Mast Cell-Mediated Food Allergic Responses

Mast cells are highly versatile cells that perform a variety of functions depending on the immune trigger, context of activation, and cytokine stimulus. Antigen-mediated mast cell responses are regulated by transcriptional processes that result in the induction of numerous genes contributing to mast cell function. Recently, we also showed that exposure to dietary agents with known epigenetic actions such as curcumin can suppress mast cell-mediated food allergy, suggesting that mast cell responses in vivo may be epigenetically regulated. To further assess the effects of epigenetic modifications on mast cell function, we examined the behavior of bone marrow-derived mast cells (BMMCs) in response to trichostatin A (TSA) treatment, a well-studied histone deacetylase inhibitor. IgE-mediated BMMC activation resulted in enhanced expression and secretion of IL-4, IL-6, TNF-α, and IL-13. In contrast, pretreatment with TSA resulted in altered cytokine secretion. This was accompanied by decreased expression of FcεRI and mast cell degranulation. Interestingly, exposure to non-IgE stimuli such as IL-33, was also affected by TSA treatment. Furthermore, continuous TSA exposure contributed to mast cell apoptosis and a decrease in survival. Further examination revealed an increase in I-κBα and a decrease in phospho-relA levels in TSA-treated BMMCs, suggesting that TSA alters transcriptional processes, resulting in enhancement of I-κBα transcription and decreased NF-κB activation. Lastly, treatment of wild-type mice with TSA in a model of ovalbumin-induced food allergy resulted in a significant attenuation in the development of food allergy symptoms including decreases in allergic diarrhea and mast cell activation. These data therefore suggest that the epigenetic regulation of mast cell activation during immune responses may occur via altered histone acetylation, and that exposure to dietary substances may induce epigenetic modifications that modulate mast cell function.