The receptor LMIR3 negatively regulates mast cell activation and allergic responses by binding to extracellular ceramide

The investigation of the role of oral-originated Prevotella-induced inflammation in childhood asthma

Background and objectives

The oral and gut microbiota play significant roles in childhood asthma pathogenesis. However, the communication dynamics and pathogenic mechanisms by which oral microbiota influence gut microbiota and disease development remain incompletely understood. This study investigated potential mechanisms by which oral-originated gut microbiota, specifically Prevotella genus, may contribute to childhood asthma etiology.

Methods

Oral swab and fecal samples from 30 asthmatic children and 30 healthy controls were collected. Microbiome composition was characterized using 16S rRNA gene sequencing and metagenomics. Genetic distances identified potential oral-originated bacteria in asthmatic children. Functional validation assessed pro-inflammatory properties of in silico predicted microbial mimicry peptides from enriched asthma-associated species. Fecal metabolome profiling combined with metagenomic correlations explored links between gut microbiota and metabolism. HBE cells treated with Prevotella bivia culture supernatant were analyzed for lipid pathway impacts using UPLC-MS/MS.

Results

Children with asthma exhibited distinct oral and gut microbiota structures. Prevotella bivia, P. disiens, P. oris and Bacteroides fragilis were enriched orally and intestinally in asthmatics, while Streptococcus thermophilus decreased. P. bivia, P. disiens and P. oris in asthmatic gut likely originated orally. Microbial peptides induced inflammatory cytokines from immune cells. Aberrant lipid pathways characterized asthmatic children. P. bivia increased pro-inflammatory and decreased anti-inflammatory lipid metabolites in HBE cells.

Conclusion

This study provides evidence of Prevotella transfer from oral to gut microbiota in childhood asthma. Prevotella's microbial mimicry peptides and effects on lipid metabolism contribute to disease pathogenesis by eliciting immune responses. Findings offer mechanistic insights into oral-gut connections in childhood asthma etiology.

Intranasal administration of ceramide liposome suppresses allergic rhinitis by targeting CD300f in murine models

Allergic rhinitis (AR) is caused by type I hypersensitivity reaction in the nasal tissues. The interaction between CD300f and its ligand ceramide suppresses immunoglobulin E (IgE)-mediated mast cell activation. However, whether CD300f inhibits the development of allergic rhinitis (AR) remains elusive. We aimed to investigate the roles of CD300f in the development of AR and the effectiveness of intranasal administration of ceramide liposomes on AR in murine models. We used ragweed pollen-induced AR models in mice. Notably, CD300f deficiency did not significantly influence the ragweed-specific IgE production, but increased the frequency of mast cell-dependent sneezing as well as the numbers of degranulated mast cells and eosinophils in the nasal tissues in our models. Similar results were also obtained for MCPT5-exprssing mast cell-specific loss of CD300f. Importantly, intranasal administration of ceramide liposomes reduced the frequency of sneezing as well as the numbers of degranulated mast cells and eosinophils in the nasal tissues in AR models. Thus, CD300f-ceramide interaction, predominantly in mast cells, alleviates the symptoms and progression of AR. Therefore, intranasal administration of ceramide liposomes may be a promising therapeutic approach against AR by targeting CD300f.

New Therapeutic Options in Pulmonal Diseases: Sphingolipids and Modulation of Sphingolipid Metabolism

Sphingolipids are crucial molecules in the respiratory airways. As in most other tissues and organs, in the lung sphingolipids play an essential role as structural constituents as they regulate barrier function and fluidity of cell membranes. A lung-specific feature is the occurrence of sphingolipids as minor structural components in the surfactant. However, sphingolipids are also key signaling molecules involved in airway cell signaling and their dynamical formation and metabolism are important for normal lung physiology. Dysregulation of sphingolipid metabolism and signaling is involved in altering lung tissue and initiates inflammatory processes promoting the pathogenesis of pulmonal diseases including cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), and asthma.In the present review, the important role of specific sphingolipid species in pulmonal diseases will be discussed. Only such an understanding opens up the possibility of developing new therapeutic strategies with the aim of correcting the imbalance in sphingolipid metabolism and signaling. Such delivery strategies have already been studied in animal models of these lung diseases, demonstrating that targeting the sphingolipid profile represents new therapeutic opportunities for lung disorders.

Dehydroandrographolide alleviates rheumatoid arthritis by inhibiting neutrophil activation via LMIR3 in collagen induced arthritis rats

Rheumatoid arthritis (RA) is an inflammatory disease which causes severe pain and disability. Neutrophils play essential roles in the onset and progression of RA; thus, inhibition of neutrophil activation is becoming a popular therapeutic strategy. Dehydroandrographolide has provided satisfactory outcomes in inflammatory diseases; however, its therapeutic effects and mechanism in RA are not fully understood. Leukocyte mono-immunoglobulin-like receptor 3 (LMIR3) is a negative regulator highly expressed in neutrophils. To determine whether dehydroandrographolide negatively regulated neutrophils activation via LMIR3, cytokines release and collagen-induced arthritis (CIA) rats were used in vitro and in vivo. Biacore, molecular docking analysis and molecular dynamics simulation were performed to prove the target of dehydroandrographolide. Moreover, the downstream signaling pathways of LMIR3 activation were analyzed by western blotting. Results showed that oral dehydroandrographolide administration of 2 mg/kg/day to CIA rats attenuated synovitis and bone and cartilage damage after the 28-day intervention, revealed using HE sections and micro-CT. Dehydroandrographolide significantly inhibited cytokine release and chemotaxis of LPS/TNF-α-activated neutrophils in vitro. Dehydroandrographolide inhibited neutrophils activation via binding to LMIR3. Moreover, dehydroandrographolide up-regulated the phosphorylation of SHP-1 and SHP-2, which are the essential kinases in the LMIR3 signaling pathways. This study revealed that dehydroandrographolide attenuated collagen-induced arthritis by suppressing neutrophil activation via LMIR3.

CD300f immune receptor contributes to healthy aging by regulating inflammaging, metabolism, and cognitive decline

Emerging evidence suggests that immune receptors may participate in many aging-related processes such as energy metabolism, inflammation, and cognitive decline. CD300f, a TREM2-like lipid-sensing immune receptor, is an exceptional receptor as it integrates activating and inhibitory cell-signaling pathways that modulate inflammation, efferocytosis, and microglial metabolic fitness. We hypothesize that CD300f can regulate systemic aging-related processes and ultimately healthy lifespan. We closely followed several cohorts of two strains of CD300f-/- and WT mice of both sexes for 30 months and observed an important reduction in lifespan and healthspan in knockout mice. This was associated with systemic inflammaging, increased cognitive decline, reduced brain glucose uptake observed by 18FDG PET scans, enrichment in microglial aging/neurodegeneration phenotypes, proteostasis alterations, senescence, increased frailty, and sex-dependent systemic metabolic changes. Moreover, the absence of CD300f altered macrophage immunometabolic phenotype. Taken together, we provide strong evidence suggesting that myeloid cell CD300f immune receptor contributes to healthy aging.

Microglial CD300f immune receptor contributes to the maintenance of neuron viability in vitro and after a penetrating brain injury

Emerging evidences suggest that immune receptors participate in diverse microglial and macrophage functions by regulating their immunometabolism, inflammatory phenotype and phagocytosis. CD300f, a TREM2-like lipid sensing immune receptor, that integrates activating and inhibitory cell-signalling pathways, modulates inflammation, efferocytosis and microglial metabolic fitness. In particular, CD300f overexpression was described to be neuroprotective after an acute brain injury, suggesting a role for this immune receptor in neurotrophic interactions. Thus, we hypothesised that CD300f modulates neuronal survival through neuron-microglial interactions. In order to study its biological function, we used in vitro and in vivo approaches, CD300f-/- animals and rCD300f-Fc, a fusion protein that interrupts the endogen interaction between CD300f receptor-ligands. In hippocampal cocultures containing neurons and mixed glia, we observed that rCD300f-Fc, but not control IgGs induced neuronal death. In accordance, in vivo studies performed by injecting rCD300f-Fc or control IgGs into rat or WT or CD300 KO mice neocortex, showed an increased lesioned area after a penetrating brain injury. Interestingly, this neuronal death was dependent on glia, and the neurotoxic mechanism did not involve the increase of proinflammatory cytokines, the participation of NMDA receptors or ATP release. However, exogenous addition of glial cell line-derived neurotrophic factor (GDNF) prevented this process. Taken together, our results suggest that CD300f modulates neuronal survival in vitro and after a penetrating brain injury in vivo and that CD300f inhibition alters microglial phenotype generating a neurotoxic microenvironment.

Murine model identifies tropomyosin as IgE cross-reactive protein between house dust mite and coho salmon that possibly contributes to the development of salmon allergy

Background

Recently, we have developed a method to identify IgE cross-reactive allergens. However, the mechanism by which IgE cross-reactive allergens cause food allergy is not yet fully understood how. In this study, we aimed to understand the underlying pathogenesis by identifying food allergens that cross-react with house dust mite allergens in a murine model.

Material and methods

Allergenic protein microarray analysis was conducted using serum from mice intraperitoneally injected with Dermatophagoides pteronyssinus (Der p) extract plus alum or alum alone as controls. Der p, Dermatophagoides farinae (Der f), coho salmon extract-sensitized and control mice were analyzed. Serum levels of IgE against Der p, Der f, coho salmon extract, protein fractions of coho salmon extract separated by ammonium sulfate precipitation and anion exchange chromatography, and recombinant coho salmon tropomyosin or actin were measured by an enzyme-linked immunosorbent assay. A murine model of cutaneous anaphylaxis or oral allergy syndrome (OAS) was established in Der p extract-sensitized mice stimulated with coho salmon extract, tropomyosin, or actin.

Results

Protein microarray analysis showed that coho salmon-derived proteins were highly bound to serum IgE in Der p extract-sensitized mice. Serum IgE from Der p or Der f extract-sensitized mice was bound to coho salmon extract, whereas serum IgE from coho salmon extract-sensitized mice was bound to Der p or Der f extract. Analysis of the murine model showed that cutaneous anaphylaxis and oral allergic reaction were evident in Der p extract-sensitized mice stimulated by coho salmon extract. Serum IgE from Der p or Der f extract-sensitized mice was bound strongly to protein fractions separated by anion exchange chromatography of coho salmon proteins precipitated with 50% ammonium sulfate, which massively contained the approximately 38 kDa protein. We found that serum IgE from Der p extract-sensitized mice was bound to recombinant coho salmon tropomyosin. Der p extract-sensitized mice exhibited cutaneous anaphylaxis in response to coho salmon tropomyosin.

Conclusion

Our results showed IgE cross-reactivity of tropomyosin between Dermatophagoides and coho salmon which illustrates salmon allergy following sensitization with the house dust mite Dermatophagoides. Our method for identifying IgE cross-reactive allergens will help understand the underlying mechanisms of food allergies.

Staphylococcus aureus δ-toxin present on skin promotes the development of food allergy in a murine model

Background

Patients with food allergy often suffer from atopic dermatitis, in which Staphylococcus aureus colonization is frequently observed. Staphylococcus aureus δ-toxin activates mast cells and promotes T helper 2 type skin inflammation in the tape-stripped murine skin. However, the physiological effects of δ-toxin present on the steady-state skin remain unknown. We aimed to investigate whether δ-toxin present on the steady-state skin impacts the development of food allergy.

Material and methods

The non-tape-stripped skins of wild-type, Kit^W-sh/W-sh, or ST2-deficient mice were treated with ovalbumin (OVA) with or without δ-toxin before intragastric administration of OVA. The frequency of diarrhea, numbers of jejunum or skin mast cells, and serum levels of OVA-specific IgE were measured. Conventional dendritic cell 2 (cDC2) in skin and lymph nodes (LN) were analyzed. The cytokine levels in the skin tissues or culture supernatants of δ-toxin-stimulated murine keratinocytes were measured. Anti-IL-1α antibody-pretreated mice were analyzed.

Results

Stimulation with δ-toxin induced the release of IL-1α, but not IL-33, in murine keratinocytes. Epicutaneous treatment with OVA and δ-toxin induced the local production of IL-1α. This treatment induced the translocation of OVA-loaded cDC2 from skin to draining LN and OVA-specific IgE production, independently of mast cells and ST2. This resulted in OVA-administered food allergic responses. In these models, pretreatment with anti-IL-1α antibody inhibited the cDC2 activation and OVA-specific IgE production, thereby dampening food allergic responses.

Conclusion

Even without tape stripping, δ-toxin present on skin enhances epicutaneous sensitization to food allergen in an IL-1α-dependent manner, thereby promoting the development of food allergy.

Myricetin served as antagonist for negatively regulate MRGPRX2 mediated pseudo-allergic reactions through CD300f/SHP1/SHP2 phosphorylation

BACKGROUND

Mas-related G protein-coupled receptor X2 (MRGPRX2) plays a vital role in mast cells (MCs) degranulation and pseudo-allergic reactions. Leukocyte mono-immunoglobulin-like receptor 3 (CD300f) can negatively regulate MCs degranulation. Identification of drug candidates which target CD300f represents a promising prospect in drug development. Myricetin is widely distributed in plants and has been reported to inhibit allergic reactions in OVA-induced murine models.

OBJECTIVE

This study aims to determine whether myricetin can activate CD300f to arrest MCs degranulation mediated by MRGPRX2.

RESULTS

Myricetin inhibited the allergic mediator and cytokine release triggered by MRGPRX2 in vivo and in vitro. Under C48/80 stimulation, the release of β-hexosaminidase, TNF-α, IL-8 and MCP-1 in CD300f knockdown in LAD2 cells was significantly increased compared with NC-LAD2 cells. Myricetin displayed good structural affinity (K_D = 7.21 × 10^-5) with CD300f by SPR. Molecular docking results showed that hydrogen bonds were formed between myricetin and CD300f, indicating high binding ability (5.6653). Myricetin can upregulate the phosphorylation of SHP-1 and SHP-2 and dephosphorylation in the MRGPRX2 signaling pathway, involving PLCγ1, AKT, P38, and ERK1/2.

CONCLUSION

In the present study, myricetin is identified as an exogenous ligand for CD300f, which negatively regulates MRGPRX2-mediated MCs activation via CD300f to inhibit MCs degranulation and pseudo-allergic reactions.

Leishmania donovani induces CD300a expression to dampen effector properties of CD11c+ dendritic and antigen activated CD8+ T cells

CD8⁺ T cells are an important regiment of adaptive immunity that play a decisive role in elimination of many species of Leishmania parasite from the host. In visceral leishmaniasis, caused by L. donovani, the loss of CD8⁺ T cells function has been found associated with augmented pathogenesis. The factors determining CD8⁺ T cells activation and function against Leishmania antigens are largely unknown. In this study, we investigated the role of an immune inhibitory receptor, CD300a, on the effector properties of dendritic cells and CD8⁺ T cells. We observed that the Leishmania regulates the effectors function of CD8⁺ T cells by increasing CD300a expression on CD11c⁺ dendritic cells. The abrogation of CD300a signaling in parasites infected animals induced CD8⁺ T cell abilities to produce IFN-γ, TNF-α and also helped them to acquire desired multifunctionality. The CD300a receptor blocking also enhanced the number of CD8⁺ T cells memory phenotypes at the early days of infection, suggesting its potential beneficial role in vaccine induced immunity. We also observed significantly enhanced levels of pro-inflammatory cytokines in the spleen of CD300a blocked infected animals with concomitant reduced spleen parasite load. Additionally, the abrogation of CD300a signals in the infected animals helped in establishing Th1 type protective humoral immunity with significantly elevated levels of IgG2a antibodies. Since CD8⁺ T cells are an important determinant of vaccine induced immunity against leishmaniasis, the findings corroborate the potential of CD300a in vaccine induced immunity and thus require further attention.

Ceramide Nanoliposomes as Potential Therapeutic Reagents for Asthma

Ceramides are an emerging class of anti-inflammatory lipids, and nanoscale ceramide-delivery systems are potential therapeutic strategies for inflammatory diseases. This study investigated the therapeutic effects of ceramide nanoliposomes (CNL) on type 2 inflammation-based asthma, induced by repeated ovalbumin (OVA) challenges. Asthmatic mice intratracheally treated with ceramide-free liposomes (Ghost) displayed typical airway remodeling including mucosal accumulation and subepithelial fibrosis, whereas, in CNL-treated mice, the degree of airway remodeling was significantly decreased. Compared to the Ghost group, CNL treatment unexpectedly failed to significantly influence formation of type 2 cytokines, including IL-5 and IL-13, known to facilitate pathogenic production of airway mucus predominantly comprising MUC5AC mucin. Interestingly, CNL treatment suppressed OVA-evoked hyperplasia of MUC5AC-generating goblet cells in the airways. This suggests that CNL suppressed goblet cell hyperplasia and airway mucosal accumulation independently of type 2 cytokine formation. Mechanistically, CNL treatment suppressed cell growth and EGF-induced activation of Akt, but not ERK1/2, in a human lung epithelial cell culture system recapitulating airway goblet cell hyperplasia. Taken together, CNL is suggested to have therapeutic effects on airway remodeling in allergic asthma by targeting goblet cell hyperplasia. These findings raise the potential of ceramide-based therapies for airway diseases, such as asthma.

The interaction of CD300lf and ceramide reduces the development of periodontitis by inhibiting osteoclast differentiation

AIM

The regulation of osteoclasts (OCs) by inhibitory immunoreceptors maintains bone homeostasis and is considered an important determinant of the extent of periodontal pathology. The aim of this study was to investigate the role of the inhibitory immunoreceptor CD300lf and its ligand ceramide in osteoclastogenesis in periodontitis.

MATERIALS AND METHODS

The expression of CD300lf was measured in vitro and in a ligature-induced periodontitis model. The effect of CD300lf ablation on osteoclastogenesis was examined in ligature-retained and ligature removal periodontitis models. The effect of ceramide, the ligand of CD300lf, was examined in osteoclastogenesis in vitro and in vivo by smearing 20 μg of ceramide dissolved in carboxymethylcellulose on teeth and gingiva every other day in an experimental periodontitis model and ligature removal model.

RESULTS

CD300lf expression was downregulated during osteoclastogenesis. Ablation of CD300lf in the ligature-induced periodontitis model increased the number of OCs and exacerbated bone damage. Bone resorption caused by CD300lf ablation was reversible following ligature removal. CD300lf-ceramide binding suppressed osteoclastogenesis in vitro and inhibited alveolar bone loss in a mouse periodontitis model.

CONCLUSIONS

Our findings reveal that CD300lf-ceramide binding plays a critical negative role in alveolar bone loss in periodontitis by inhibiting OCs differentiation.

Targeted Inhibition of Select Extracellular Signal-regulated Kinases 1 and 2 Functions Mitigates Pathological Features of Asthma in Mice

ERK1/2 (extracellular signal-regulated kinases 1 and 2) regulate the activity of various transcription factors that contribute to asthma pathogenesis. Although an attractive drug target, broadly inhibiting ERK1/2 is challenging because of unwanted cellular toxicities. We have identified small molecule inhibitors with a benzenesulfonate scaffold that selectively inhibit ERK1/2-mediated activation of AP-1 (activator protein-1). Herein, we describe the findings of targeting ERK1/2-mediated substrate-specific signaling with the small molecule inhibitor SF-3-030 in a murine model of house dust mite (HDM)-induced asthma. In 8- to 10-week-old BALB/c mice, allergic asthma was established by repeated intranasal HDM (25 μg/mouse) instillation for 3 weeks (5 days/week). A subgroup of mice was prophylactically dosed with 10 mg/kg SF-3-030/DMSO intranasally 30 minutes before the HDM challenge. Following the dosing schedule, mice were evaluated for alterations in airway mechanics, inflammation, and markers of airway remodeling. SF-3-030 treatment significantly attenuated HDM-induced elevation of distinct inflammatory cell types and cytokine concentrations in BAL and IgE concentrations in the lungs. Histopathological analysis of lung tissue sections revealed diminished HDM-induced pleocellular peribronchial inflammation, mucus cell metaplasia, collagen accumulation, thickening of airway smooth muscle mass, and expression of markers of cell proliferation (Ki-67 and cyclin D1) in mice treated with SF-3-030. Furthermore, SF-3-030 treatment attenuated HDM-induced airway hyperresponsiveness in mice. Finally, mechanistic studies using transcriptome and proteome analyses suggest inhibition of HDM-induced genes involved in inflammation, cell proliferation, and tissue remodeling by SF-3-030. These preclinical findings demonstrate that function-selective inhibition of ERK1/2 signaling mitigates multiple features of asthma in a murine model.

Development of mouse model for oral allergy syndrome to identify IgE cross-reactive pollen and food allergens: ragweed pollen cross-reacts with fennel and black pepper

Oral allergy syndrome (OAS) is an IgE-mediated immediate food allergy that is localized to the oral mucosa. Pollen food allergy syndrome (PFAS), a pollinosis-associated OAS, is caused by cross-reactivity between food and pollen allergens. However, we need to more precisely understand the underlying pathogenesis of OAS/PFAS. In the present study, we developed a method to comprehensively identify cross-reactive allergens by using murine model of OAS and protein microarray technology. We focused on lip angioedema, which is one of the most common symptoms of OAS, and confirmed that mast cells reside in the tissues inside the lower lip of the mice. Interestingly, when the food allergen ovalbumin (OVA) was injected inside the lower lip of mice with high levels of OVA-specific IgE followed by an intravenous injection of the Evans blue dye, we found immediate dye extravasation in the skin of the neck in a mast cell-dependent manner. In addition, the degree of mast cell degranulation in the oral cavity, reflecting the severity of oral allergic responses, can be estimated by measuring the amount of extravasated dye in the skin. Therefore, we used this model of OAS to examine IgE cross-reactive allergens in vivo. Protein microarray analysis showed that serum IgE from mice intraperitoneally sensitized with ragweed pollen, one of the major pollens causing pollinosis, bound highly to protein extracts from several edible plants including black peppercorn and fennel. We confirmed that the levels of black pepper-specific IgE and fennel-specific IgE were significantly higher in the serum from ragweed pollen-sensitized mice than in the serum from non-sensitized control mice. Importantly, analysis of murine model of OAS showed that the injection of black pepper or fennel extract induced apparent oral allergic responses in ragweed pollen-sensitized mice. These results indicate IgE cross-reactivity of ragweed pollen with black pepper and fennel. In conclusion, we developed mouse model of OAS to identify IgE cross-reactive pollen and food allergens, which will help understand the pathogenesis of OAS/PFAS.

Clarithromycin-treated chronic spontaneous urticaria with the negative regulation of FcεRΙ and MRGPRX2 activation via CD300f

Mast cells (MCs) are main effector cells in chronic spontaneous urticaria (CSU). Both Fc epsilon RI (FcεRΙ)- and MAS-related G coupled receptor-X2 (MRGPRX2)-mediated MC activations affect CSU course. Leukocyte mono-immunoglobulin-like receptor 3 (CD300f) has been shown to regulate FcεRΙ activation. However, no study has verified CD300f is a target to cure CSU. Therefore this study aimed to verify whether clarithromycin (CLA) regulates FcεRΙ- and MRGPRX2-mediated MC activations via CD300f and shows therapeutic effect on CSU. The target of CLA was verification. CLA inhibited FcεRΙ- and MRGPRX2-mediated MC activations were shown in vivo and in vitro. A single-center, self-comparison study was performed, and CLA-treated CSU was investigated in 28 patients who were not sensitive to the third-generation antihistamines. Serum inflammatory mediators in patients before and after CLA administration were analyzed. CLA effectively inhibited type Ι anaphylactic reactions and pseudo-allergic reactions in mice. Moreover, CLA inhibited FcεRΙ- and MRGPRX2-mediated MC signaling pathway activation. Regulatory effects of CLA were decreased significantly after CD300f knockdown. CLA effectively alleviated the symptoms of wheal and itch and reduced serum cytokine levels in patients. CLA negatively regulated FcεRΙ- and MRGPRX2-mediated MC activation via CD300f and showed significant therapeutic effect on CSU.

Regulatory Roles of Phospholipase A2 Enzymes and Bioactive Lipids in Mast Cell Biology

Lipids play fundamental roles in life as an essential component of cell membranes, as a major source of energy, as a body surface barrier, and as signaling molecules that transmit intracellular and intercellular signals. Lipid mediators, a group of bioactive lipids that mediates intercellular signals, are produced via specific biosynthetic enzymes and transmit signals via specific receptors. Mast cells, a tissue-resident immune cell population, produce several lipid mediators that contribute to exacerbation or amelioration of allergic responses and also non-allergic inflammation, host defense, cancer and fibrosis by controlling the functions of microenvironmental cells as well as mast cell themselves in paracrine and autocrine fashions. Additionally, several bioactive lipids produced by stromal cells regulate the differentiation, maturation and activation of neighboring mast cells. Many of the bioactive lipids are stored in membrane phospholipids as precursor forms and released spatiotemporally by phospholipase A₂ (PLA₂) enzymes. Through a series of studies employing gene targeting and lipidomics, several enzymes belonging to the PLA₂ superfamily have been demonstrated to participate in mast cell-related diseases by mobilizing unique bioactive lipids in multiple ways. In this review, we provide an overview of our current understanding of the regulatory roles of several PLA₂-driven lipid pathways in mast cell biology.

Sphingoid base in pineapple glucosylceramide suppresses experimental allergy by binding leukocyte mono-immunoglobulin-like receptor 3

BACKGROUND

The increase in patients suffering from type I hypersensitivity, including hay fever and food allergy, is a serious public health issue around the world. Recent studies have focused on allergy prevention by food factors with fewer side effects. The purpose of this study was to evaluate the effect of dietary glucosylceramide from pineapples (P-GlcCer) on type I hypersensitivity and elucidate mechanisms.

RESULTS

Oral administration of P-GlcCer inhibited ear edema in passive cutaneous anaphylaxis reaction. In a Caco-2/RBL-2H3 co-culture system, P-GlcCer inhibited β-hexosaminidase release from RBL-2H3 cells. The direct treatment of P-GlcCer on RBL-2H3 did not affect β-hexosaminidase release, but sphingoid base moiety of P-GlcCer did. These results predicted that sphingoid base, a metabolite of P-GlcCer, through the intestine inhibited type I hypersensitivity by inhibiting mast cell degranulation. In addition, the inhibitory effects of P-GlcCer on ear edema and degranulation of RBL-2H3 cells were canceled by pretreatment of leukocyte mono-immunoglobulin-like receptor 3 (LMIR3)-Fc, which can block LMIR3-mediated inhibitory signals.

CONCLUSION

It was demonstrated that a sphingoid base, one of the metabolites of P-GlcCer, may inhibit mast cell degranulation by binding to LMIR3. The oral administration of P-GlcCer is a novel and attractive food factor that acts directly on mast cells to suppress allergy. © 2021 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Dehydroandrographolide targets CD300f and negatively regulated MRGPRX2-induced pseudo-allergic reaction

Mas-related G protein-coupled receptor X2 (MRGPRX2) mediates mast cells (MCs) activation, which is a key target for the treatment of allergic diseases. However, there are few drugs targeting MRGPRX2. Leukocyte mono-immunoglobulin-like receptor 3 (CD300f) is a negative regulator of FcεRΙ-mediated MC activation. However, the regulatory effect of CD300f on MRGPRX2 remains unclear. Dehydroandrographolide (DA) is a main contributor of Andrographis paniculata (Burm.f.) Nees (family: Acanthaceae) have been shown to inhibit type I hypersensitivity. The aim of this study was to determine whether DA negatively regulated MRGPRX2-mediated MC activation via CD300f and showed therapeutic effect on pseudo-allergic reactions. Mouse allergic models and MC degranulation were detected in vivo and in vitro, and inflammatory mediators were detected. siRNA interference and Biacore were used to verify the target. DA inhibited pseudo-allergic reactions by reducing vasodilation and serum cytokine levels in mice and inhibited MRGPRX2-mediated MC activation. The regulatory effect of DA was significantly decreased after the knockdown of CD300f expression. Moreover, DA upregulated the phosphorylation level of Src homology region 2 domain-containing phosphatase (SHP)-1 and SHP-2, which are key kinases in the negative regulatory signaling pathways associated with CD300f. In conclusion, DA negatively regulates MRGPRX2-mediated MC activation via CD300f to inhibit pseudo-allergic reactions.

Mucosal Mast Cells as Key Effector Cells in Food Allergies

Mucosal mast cells (MMCs) localized in the intestinal mucosa play a key role in the development of IgE-mediated food allergies. Recent advances have revealed that MMCs are a distinctly different population from connective tissue mast cells localized in skin and other connective tissues. MMCs are inducible and transient cells that arise from bone marrow-derived mast cell progenitors, and their numbers increase rapidly during mucosal allergic inflammation. However, the mechanism of the dramatic expansion of MMCs and their cell functions are not well understood. Here, we review recent findings on the mechanisms of MMC differentiation and expansion, and we discuss the potential for the inducers of differentiation and expansion to serve as targets for food allergy therapy. In addition, we also discuss the mechanism by which oral immunotherapy, a promising treatment for food allergy patients, induces unresponsiveness to food allergens and the roles of MMCs in this process. Research focusing on MMCs should provide useful information for understanding the underlying mechanisms of food allergies in order to further advance the treatment of food allergies.

Positive and negative roles of lipids in mast cells and allergic responses

Mast cells are a central immune cell population that are crucial in allergic responses. They secrete granule contents and cytokines and produce a panel of lipid mediators in response to FcεRI-dependent or independent stimuli. Leukotrienes and prostaglandins derived from ω6 arachidonic acid, or specialized pro-resolving lipid mediators derived from ω3 eicosapentaenoic and docosahexaenoic acids, exert pleiotropic effects on various cells in the tissue microenvironment, thereby positively or negatively regulating allergic responses. Mast cells also express the inhibitory receptors CD300a and CD300f, which recognize structural lipids. CD300a or CD300f binding to externalized phosphatidylserine or extracellular ceramides, respectively, inhibits FcεRI-mediated mast cell activation. The inhibitory CD300-lipid axis downregulates IgE-driven, mast cell-dependent type I hypersensitivity through different mechanisms. Herein, we provide an overview of our current understanding of the biological roles of lipids in mast cell-dependent allergic responses.

Single-cell characterization of a model of poly I:C-stimulated peripheral blood mononuclear cells in severe asthma

Background

Asthma has been associated with impaired interferon response. Multiple cell types have been implicated in such response impairment and may be responsible for asthma immunopathology. However, existing models to study the immune response in asthma are limited by bulk profiling of cells. Our objective was to Characterize a model of peripheral blood mononuclear cells (PBMCs) of patients with severe asthma (SA) and its response to the TLR3 agonist Poly I:C using two single-cell methods.

Methods

Two complementary single-cell methods, DropSeq for single-cell RNA sequencing (scRNA-Seq) and mass cytometry (CyTOF), were used to profile PBMCs of SA patients and healthy controls (HC). Poly I:C-stimulated and unstimulated cells were analyzed in this study.

Results

PBMCs (n = 9414) from five SA (n = 6099) and three HC (n = 3315) were profiled using scRNA-Seq. Six main cell subsets, namely CD4 + T cells, CD8 + T cells, natural killer (NK) cells, B cells, dendritic cells (DCs), and monocytes, were identified. CD4 + T cells were the main cell type in SA and demonstrated a pro-inflammatory profile characterized by increased JAK1 expression. Following Poly I:C stimulation, PBMCs from SA had a robust induction of interferon pathways compared with HC. CyTOF profiling of Poly I:C stimulated and unstimulated PBMCs (n = 160,000) from the same individuals (SA = 5; HC = 3) demonstrated higher CD8 + and CD8 + effector T cells in SA at baseline, followed by a decrease of CD8 + effector T cells after poly I:C stimulation.

Conclusions

Single-cell profiling of an in vitro model using PBMCs in patients with SA identified activation of pro-inflammatory pathways at baseline and strong response to Poly I:C, as well as quantitative changes in CD8 + effector cells. Thus, transcriptomic and cell quantitative changes are associated with immune cell heterogeneity in this model to evaluate interferon responses in severe asthma.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-021-01709-9.

ORMDL3 Functions as a Negative Regulator of Antigen-Mediated Mast Cell Activation via an ATF6-UPR-Autophagy-Dependent Pathway

Antigen (Ag)-mediated mast cell activation plays a critical role in the immunopathology of IgE-dependent allergic diseases. Restraining the signaling cascade that regulates the release of mast cell-derived inflammatory mediators is an attractive therapeutic strategy to treat allergic diseases. Orosomucoid-like-3 (ORMDL3) regulates the endoplasmic reticulum stress (ERS)-induced unfolded protein response (UPR) and autophagy. Although ERS/UPR/autophagy pathway is crucial in Ag-induced mast cell activation, it is unknown whether ORMDL3 regulates the ERS/UPR/autophagy pathway during mast cell activation. In this study, we found that ORMDL3 expression was downregulated in Ag-activated MC/9 cells. Overexpression of ORMDL3 significantly inhibited degranulation, and cytokine/chemokine production, while the opposite effect was observed with ORMDL3 knockdown in MC/9 cells. Importantly, ORMDL3 overexpression upregulated mediators of ERS-UPR (SERCA2b, ATF6) and autophagy (Beclin 1 and LC3BII). Knockdown of ATF6 and/or inhibition of autophagy reversed the decreased degranulation and cytokine/chemokine expression caused by ORMDL3 overexpression. Moreover, in vivo knockdown of ORMDL3 and/or ATF6 enhanced passive cutaneous anaphylaxis (PCA) reactions in mouse ears. These data indicate that ORMDL3 suppresses Ag-mediated mast cell activation via an ATF6 UPR-autophagy dependent pathway and thus, attenuates anaphylactic reaction. This highlights a potential mechanism to intervene in mast cell mediated diseases.

Serum sphingolipid profile in asthma

Sphingolipids metabolism is an important cell process and plays critical roles in asthma. However, the involvement of sphingolipids in the pathogenesis of asthma and its subtypes is unknown. The present study aimed to determine the role of sphingolipids in asthma and its subtypes. Clinical data from 51 asthma patients and 9 healthy individuals were collected and serum samples were performed to analyze the levels of serum sphingolipids by liquid chromatography-mass spectrometry-based targeted metabolomics. Results showed that the levels of sphingomyelin (SM) including SM34:2, SM38:1, and SM40:1 were significantly decreased in asthmatic patients compared to healthy controls. Moreover, serum SM levels were obviously decreased in the blood noneosinophilic asthma (bNEA) group compared with blood eosinophilic asthma group. Similar tendencies of serum SM level changes were observed in the early-onset group compared with late-onset group. Correlation analysis revealed that SM 40:1 was negatively related to sputum IL-17A (r = -0.621, P = 0.042). The present study presented that the SM may be a protective factor of asthma and contributes to the mechanism of asthma, especially bNEA. SM may be a potential biomarker and therapeutic target in asthma.

ORMDL2 Deficiency Potentiates the ORMDL3-Dependent Changes in Mast Cell Signaling

The systemic anaphylactic reaction is a life-threatening allergic response initiated by activated mast cells. Sphingolipids are an essential player in the development and attenuation of this response. De novo synthesis of sphingolipids in mammalian cells is inhibited by the family of three ORMDL proteins (ORMDL1, 2, and 3). However, the cell and tissue-specific functions of ORMDL proteins in mast cell signaling are poorly understood. This study aimed to determine cross-talk of ORMDL2 and ORMDL3 proteins in IgE-mediated responses. To this end, we prepared mice with whole-body knockout (KO) of Ormdl2 and/or Ormdl3 genes and studied their role in mast cell-dependent activation events in vitro and in vivo. We found that the absence of ORMDL3 in bone marrow-derived mast cells (BMMCs) increased the levels of cellular sphingolipids. Such an increase was further raised by simultaneous ORMDL2 deficiency, which alone had no effect on sphingolipid levels. Cells with double ORMDL2 and ORMDL3 KO exhibited increased intracellular levels of sphingosine-1-phosphate (S1P). Furthermore, we found that concurrent ORMDL2 and ORMDL3 deficiency increased IκB-α phosphorylation, degranulation, and production of IL-4, IL-6, and TNF-α cytokines in antigen-activated mast cells. Interestingly, the chemotaxis towards antigen was increased in all mutant cell types analyzed. Experiments in vivo showed that passive cutaneous anaphylaxis (PCA), which is initiated by mast cell activation, was increased only in ORMDL2,3 double KO mice, supporting our in vitro observations with mast cells. On the other hand, ORMDL3 KO and ORMDL2,3 double KO mice showed faster recovery from passive systemic anaphylaxis, which could be mediated by increased levels of blood S1P presented in such mice. Our findings demonstrate that Ormdl2 deficiency potentiates the ORMDL3-dependent changes in mast cell signaling.

Strategies for Mast Cell Inhibition in Food Allergy

Mast cells are tissue resident allergic effector cells that drive IgE-mediated food allergies. There are several steps leading to mast cell activation in the context of allergic disease that can be targeted to prevent mast cell activation and degranulation. These include blocking IgE-FcεRI crosslinking and type 2 cytokine receptor activation; modulating cell-surface neural chemical receptors; stabilizing mast cell membranes to prevent co-localization of activating receptors; impeding intracellular signaling; and engaging cell surface inhibitory receptors. This review highlights several ITIM-containing inhibitory mast cell surface receptors that could serve as pharmaceutical targets to prevent mast cell activation and degranulation in the context of food allergy. When activated, these ITIM-containing inhibitory receptors recruit the phosphatases SHP-1, SHP-2, and/or SHIP to dephosphorylate the tyrosine kinases responsible for activation signals downstream of the IgE-FcεRI complex. We describe several members of the Ig and Ig-like inhibitory receptor and C-type lectin inhibitory receptor superfamilies. Fundamental studies exploring the behavior of these receptors within the context of experimental food allergy models are needed. A deeper understanding of how these receptors modulate mast cell-driven food allergic responses will shape future strategies to harness these inhibitory receptors to treat food allergy.

Emerging mechanisms contributing to mast cell-mediated pathophysiology with therapeutic implications

Mast cells are tissue-resident immune cells that play key roles in the initiation and perpetuation of allergic inflammation, usually through IgE-mediated mechanisms. Mast cells are, however, evolutionary ancient immune cells that can be traced back to urochordates and before the emergence of IgE antibodies, suggesting their involvement in antibody-independent biological functions, many of which are still being characterized. Herein, we summarize recent advances in understanding the roles of mast cells in health and disease, partly through the study of emerging non-IgE receptors such as the Mas-related G protein-coupled receptor X2, implicated in pseudo-allergic reactions as well as in innate defense and neuronal sensing; the mechano-sensing adhesion G protein-coupled receptor E2, variants of which are associated with familial vibratory urticaria; and purinergic receptors, which orchestrate tissue damage responses similarly to the IL-33 receptor. Recent evidence also points toward novel mechanisms that contribute to mast cell-mediated pathophysiology. Thus, in addition to releasing preformed mediators contained in granules and synthesizing mediators de novo, mast cells also secrete extracellular vesicles, which convey biological functions. Understanding their release, composition and uptake within a variety of clinical conditions will contribute to the understanding of disease specific pathology and likely lead the way to novel therapeutic approaches. We also discuss recent advances in the development of therapies targeting mast cell activity, including the ligation of inhibitory ITIM-containing receptors, and other strategies that suppress mast cells or responses to mediators for the management of mast cell-related diseases.

Viridicatol Isolated from Deep-Sea Penicillium Griseofulvum Alleviates Anaphylaxis and Repairs the Intestinal Barrier in Mice by Suppressing Mast Cell Activation

Viridicatol is a quinoline alkaloid isolated from the deep-sea-derived fungus Penicillium griseofulvum. The structure of viridicatol was unambiguously established by X-ray diffraction analysis. In this study, a mouse model of ovalbumin-induced food allergy and the rat basophil leukemia (RBL)-2H3 cell model were established to explore the anti-allergic properties of viridicatol. On the basis of the mouse model, we found viridicatol to alleviate the allergy symptoms; decrease the levels of specific immunoglobulin E, mast cell protease-1, histamine, and tumor necrosis factor-α; and promote the production of interleukin-10 in the serum. The treatment of viridicatol also downregulated the population of B cells and mast cells (MCs), as well as upregulated the population of regulatory T cells in the spleen. Moreover, viridicatol alleviated intestinal villi injury and inhibited the degranulation of intestinal MCs to promote intestinal barrier repair in mice. Furthermore, the accumulation of Ca²⁺ in RBL-2H3 cells was significantly suppressed by viridicatol, which could block the activation of MCs. Taken together, these data indicated that deep-sea viridicatol may represent a novel therapeutic for allergic diseases.

The Expression and Function of CD300 Molecules in the Main Players of Allergic Responses: Mast Cells, Basophils and Eosinophils

Allergy is the host immune response against non-infectious substances called allergens. The prevalence of allergic diseases is increasing worldwide. However, while some drugs counteract the symptomatology caused by allergic reactions, no completely effective treatments for allergic diseases have been developed yet. In this sense, the ability of surface activating and inhibitory receptors to modulate the function of the main effector cells of allergic responses makes these molecules potential pharmacological targets. The CD300 receptor family consists of members with activating and inhibitory capabilities mainly expressed on the surface of immune cells. Multiple studies in the last few years have highlighted the importance of CD300 molecules in several pathological conditions. This review summarizes the literature on CD300 receptor expression, regulation and function in mast cells, basophils and eosinophils, the main players of allergic responses. Moreover, we review the involvement of CD300 receptors in the pathogenesis of certain allergic diseases, as well as their prospective use as therapeutic targets for the treatment of IgE-dependent allergic responses.

CD300f immunoreceptor is associated with major depressive disorder and decreased microglial metabolic fitness

A role for microglia in neuropsychiatric diseases, including major depressive disorder (MDD), has been postulated. Regulation of microglial phenotype by immune receptors has become a central topic in many neurological conditions. We explored preclinical and clinical evidence for the role of the CD300f immune receptor in the fine regulation of microglial phenotype and its contribution to MDD. We found that a prevalent nonsynonymous single-nucleotide polymorphism (C/T, rs2034310) of the human CD300f receptor cytoplasmic tail inhibits the protein kinase C phosphorylation of a threonine and is associated with protection against MDD, mainly in women. Interestingly, CD300f-/- mice displayed several characteristic MDD traits such as augmented microglial numbers, increased interleukin 6 and interleukin 1 receptor antagonist messenger RNA, alterations in synaptic strength, and noradrenaline-dependent and persistent depressive-like and anhedonic behaviors in females. This behavioral phenotype could be potentiated inducing the lipopolysaccharide depression model. RNA sequencing and biochemical studies revealed an association with impaired microglial metabolic fitness. In conclusion, we report a clear association that links the function of the CD300f immune receptor with MDD in humans, depressive-like and anhedonic behaviors in female mice, and altered microglial metabolic reprogramming.

Killer Immunoglobulin-Like Receptor 2DL4 (CD158d) Regulates Human Mast Cells both Positively and Negatively: Possible Roles in Pregnancy and Cancer Metastasis

Killer immunoglobulin-like receptor (KIR) 2DL4 (CD158d) was previously thought to be a human NK cell-specific protein. Mast cells are involved in allergic reactions via their KIT-mediated and FcɛRI-mediated responses. We recently detected the expression of KIR2DL4 in human cultured mast cells established from peripheral blood of healthy volunteers (PB-mast), in the human mast cell line LAD2, and in human tissue mast cells. Agonistic antibodies against KIR2DL4 negatively regulate the KIT-mediated and FcɛRI-mediated responses of PB-mast and LAD2 cells. In addition, agonistic antibodies and human leukocyte antigen (HLA)-G, a natural ligand for KIR2DL4, induce the secretion of leukemia inhibitory factor and serine proteases from human mast cells, which have been implicated in pregnancy establishment and cancer metastasis. Therefore, KIR2DL4 stimulation with agonistic antibodies and recombinant HLA-G protein may enhance both processes, in addition to suppressing mast-cell-mediated allergic reactions.

The phytosphingosine-CD300b interaction promotes zymosan-induced, nitric oxide-dependent neutrophil recruitment

Zymosan is a glucan that is a component of the yeast cell wall. Here, we determined the mechanisms underlying the zymosan-induced accumulation of neutrophils in mice. Loss of the receptor CD300b reduced the number of neutrophils recruited to dorsal air pouches in response to zymosan, but not in response to lipopolysaccharide (LPS), a bacterial membrane component recognized by Toll-like receptor 4 (TLR4). An inhibitor of nitric oxide (NO) synthesis reduced the number of neutrophils in the zymosan-treated air pouches of wild-type mice to an amount comparable to that in CD300b^-/- mice. Treatment with clodronate liposomes decreased the number of NO-producing, CD300b⁺ inflammatory dendritic cells (DCs) in wild-type mice, thus decreasing NO production and neutrophil recruitment. Similarly, CD300b deficiency decreased the NO-dependent recruitment of neutrophils to zymosan-treated joint cavities, thus ameliorating subsequent arthritis. We identified phytosphingosine, a lipid component of zymosan, as a potential ligand of CD300b. Phytosphingosine stimulated NO production in inflammatory DCs and promoted neutrophil recruitment in a CD300b-dependent manner. Together, these results suggest that the phytosphingosine-CD300b interaction promotes zymosan-dependent neutrophil accumulation by inducing NO production by inflammatory DCs and that CD300b may contribute to antifungal immunity.

Dihydromyricetin inhibited ovalbumin-induced mice allergic responses by suppressing the activation of mast cells

Dihydromyricetin (DMY) is a natural flavonoid compound derived from Lysionotus pauciflorus Maxim and has been found to possess therapeutic potential for allergic disease induced by food allergens.

Roles of Ceramides and Other Sphingolipids in Immune Cell Function and Inflammation

Ceramides are bioactive sphingolipids that support the structure of the plasma membrane and mediate numerous cell-signaling events in eukaryotic cells. The finding that ceramides act as second messengers transducing cellular signals has attracted substantial attention in several fields of Biology. Since all cells contain lipid plasma membranes, the impact of various ceramides, ceramide synthases, ceramide metabolites, and other sphingolipids has been implicated in a vast range of cellular functions including, migration, proliferation, response to external stimuli, and death. The roles of lipids in these functions widely differ among the diverse cell types. Herein, we discuss the roles of ceramides and other sphingolipids in mediating the function of various immune cells; particularly dendritic cells, neutrophils, and macrophages. In addition, we highlight the main studies describing effects of ceramides in inflammation, specifically in various inflammatory settings including insulin resistance, graft-versus-host disease, immune suppression in cancer, multiple sclerosis, and inflammatory bowel disease.

Mouse LIMR3/CD300f is a negative regulator of the antimicrobial activity of neutrophils

Leukocyte mono-immunoglobulin-like receptor (LMIR)/CD300 proteins comprise a family of immunoglobulin-like receptors that are widely expressed on the immune cell surface in humans and mice. In general, LMIR3/CD300f suppresses the inflammatory response, but it can occasionally promote it. However, the precise roles of LMIR3 in the function of neutrophils remain to be elucidated. In the present study, we investigated LMIR3 expression in mature and immature neutrophils, and evaluated the effects of LMIR3 deficiency in mouse neutrophils. Our results indicated that bone marrow (BM) neutrophils expressed LMIR3 on their cell surface during cell maturation and that surface LMIR3 expression increased in response to Pseudomonas aeruginosa infection in a TLR4/MyD88-dependent manner. LMIR3-knockout (KO) neutrophils displayed significantly increased hypochlorous acid production, and elastase release, as well as significantly augmented cytotoxic activity against P. aeruginosa and Candida albicans; meanwhile, inhibitors of elastase and myeloperoxidase offset this enhanced antimicrobial activity. Furthermore, LMIR3-KO mice were significantly more resistant to Pseudomonas peritonitis and systemic candidiasis, although this may not be entirely due to the enhanced activity of neutrophils. These results demonstrate that LMIR3/CD300f deficiency augments the antimicrobial activity of mouse neutrophils.

Structural basis for murine norovirus engagement of bile acids and the CD300lf receptor

Murine norovirus (MNoV) is closely related to human norovirus (HNoV), an infectious agent responsible for acute gastroenteritis worldwide. Here we report the X-ray crystal structure of the dimeric MNoV VP1 protruding (P) domain in complex with its cellular receptor CD300lf. CD300lf binds the P domain with a 2:2 stoichiometry, engaging a cleft between the AB and DE loops of the P2 subdomain at a site that overlaps the epitopes of neutralizing antibodies. We also identify that bile acids are cofactors enhancing MNoV cell-binding and infectivity. Structures of CD300lf-P domain in complex with glycochenodeoxycholic acid (GCDCA) and lithocholic acid (LCA) reveal two bile acid binding sites at the P domain dimer interface distant from receptor binding sites. The structural determinants for receptor and bile acid binding are supported by numerous biophysical assays utilizing interface residue mutations. We find that the monomeric affinity of CD300lf for the P domain is low and is divalent cation dependent. We have also determined the crystal structure of CD300lf in complex with phosphocholine, revealing that MNoV engages its receptor in a manner mimicking host ligands including similar metal coordination. Docking of the cocomplex structures onto a cryo-EM-derived model of MNoV suggests that each virion can make multiple CD300lf engagements, and thus, infection may be driven by the avidity of cell surface clustered CD300lf. These studies identify multiple potential modulators of norovirus infection that may act to regulate the interaction between the viral capsid P domain and its cognate cellular receptor.

Sphingolipids and their enigmatic role in asthma

Asthma is defined as a chronic inflammatory condition in the lung and is characterized by episodic shortness of breath with expiratory wheezing and cough. Asthma is a serious public health concern globally with an estimated incidence over 300 million. Asthma is a complex disease in that it manifests as disease of gene and environmental interactions. Sphingolipids are a unique class of lipids involved in a host of biological functions ranging from serving as key cellular membrane lipids to acting as critical signaling molecules. To date sphingolipids have been studied across various human conditions ranging from neurological disorders to cancer to infection to autoimmunity. This review will focus on the role of sphingolipids in asthma development and pathology with particular focus on the role of mast cell sphingolipid biology.

Sphingolipid biosynthesis induces a conformational change in the murine norovirus receptor and facilitates viral infection

Cellular susceptibility to viral infections is in part determined by the presence of a host cellular receptor. Here we use murine norovirus as a model to uncover an unappreciated connection between an intracellular lipid biosynthetic enzyme and a receptor conformation permissive for viral infection. The serine palmitoyltransferase (SPT) complex is required for de novo sphingolipid biosynthesis and we find that its absence impairs the ability of murine norovirus to bind and enter cells. While, the SPT complex is dispensable for the surface expression of the norovirus receptor, CD300lf, SPT activity is required for CD300lf to adopt a conformation permissive for viral binding. Addition of extracellular ceramide to SPT deficient cells chemically complements both the conformational changes of CD300lf and the cellular susceptibility to murine norovirus infection. Taken together, these data indicate that intracellular sphingolipid biosynthesis regulates the conformation of the murine norovirus receptor, and therefore the tropism of murine norovirus. This indicates that intracellular biosynthetic pathways can regulate viral tropism even when the receptor for a virus is expressed on the target cell surface.

CD300c costimulates IgE-mediated basophil activation, and its expression is increased in patients with cow's milk allergy

BACKGROUND

Basophils express high-affinity IgE receptors (FcεRI), which play an essential role in allergic diseases. It is important to characterize new cell-surface receptors that modulate IgE-mediated basophil activation threshold to design promising immunomodulatory therapies.

OBJECTIVES

We sought to analyze the expression of CD300 receptors on human basophils and their implication in IgE-mediated basophil activation processes.

METHODS

Blood samples from healthy subjects and patients with cow's milk allergy were collected through the Basque Biobank under an institutional review board-approved protocol. PBMCs were obtained by means of density centrifugation, basophils were purified with a specific isolation kit, and phenotypic and functional studies were performed by using flow cytometry.

RESULTS

We demonstrate that basophils express the activating receptor CD300c, which is specifically upregulated in response to IL-3. CD300c works as a costimulatory molecule during IgE-mediated basophil activation, as shown by a significant increase in degranulation and cytokine production when basophils are activated in the presence of CD300c cross-linking compared with activation through the IgE/FcεRI axis alone. Coligation of FcεRI and CD300c increased intracellular calcium mobilization and phosphorylation of signaling intermediates evoked only by FcεRI ligation. We show that the natural ligands of CD300c, phosphatidylserine and phosphatidylethanolamine, modulate IgE-mediated basophil activation. Furthermore, we have observed that CD300c expression in children with cow's milk allergy is increased compared with that in healthy control subjects and that the intensity of expression correlates with the severity of the hypersensitivity symptoms.

CONCLUSION

CD300c could be considered a biomarker and therapeutic target in patients with IgE-mediated allergic diseases because it seems to be involved in the modulation of IgE-mediated basophil activation.

Leukocyte mono-immunoglobulin-like receptor 8 (LMIR8)/CLM-6 is an FcRγ-coupled receptor selectively expressed in mouse tissue plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) produce large amounts of type-I interferon (IFN) in response to viral infection or self nucleic acids. Leukocyte mono-immunoglobulin-like receptor 8 (LMIR8), also called CMRF-35-like molecule-6 (CLM-6), is a putative activating receptor among mouse LMIR/CLM/CD300 members; however, the expression and function of LMIR8 remain unclear. Here, we characterize mouse LMIR8 as a pDC receptor. Analysis of Flag-tagged LMIR8-transduced bone marrow (BM)-derived mast cells demonstrated that LMIR8 can transmit an activating signal by interacting with immunoreceptor tyrosine-based activating motif (ITAM)-containing FcRγ. Flow cytometric analysis using a specific antibody for LMIR8 showed that LMIR8 expression was restricted to mouse pDCs residing in BM, spleen, or lymph node. FcRγ deficiency dampened surface expression of LMIR8 in mouse pDCs. Notably, LMIR8 was detected only in pDCs, irrespective of TLR9 stimulation, suggesting that LMIR8 is a suitable marker for pDCs in mouse tissues; LMIR8 is weakly expressed in Flt3 ligand-induced BM-derived pDCs (BMpDCs). Crosslinking of transduced LMIR8 in BMpDCs with anti-LMIR8 antibody did not induce IFN-α production, but rather suppressed TLR9-mediated production of IFN-α. Taken together, these observations indicate that LMIR8 is an FcRγ-coupled receptor selectively expressed in mouse tissue pDCs, which might suppress pDC activation through the recognition of its ligands.

Apoptosis and Clearance of Apoptotic Cells

The human body generates 10-100 billion cells every day, and the same number of cells die to maintain homeostasis in our body. Cells infected by bacteria or viruses also die. The cell death that occurs under physiological conditions mainly proceeds by apoptosis, which is a noninflammatory, or silent, process, while pathogen infection induces necroptosis or pyroptosis, which activates the immune system and causes inflammation. Dead cells generated by apoptosis are quickly engulfed by macrophages for degradation. Caspases are a large family of cysteine proteases that act in cascades. A cascade that leads to caspase 3 activation mediates apoptosis and is responsible for killing cells, recruiting macrophages, and presenting an "eat me" signal(s). When apoptotic cells are not efficiently engulfed by macrophages, they undergo secondary necrosis and release intracellular materials that represent a damage-associated molecular pattern, which may lead to a systemic lupus-like autoimmune disease.

The CD300e molecule in mice is an immune-activating receptor

CD300 molecules (CD300s) belong to paired activating and inhibitory receptor families, which mediate immune responses. Human CD300e (hCD300e) is expressed in monocytes and myeloid dendritic cells and transmits an immune-activating signal by interacting with DNAX-activating protein 12 (DAP12). However, the CD300e ortholog in mice (mCD300e) is poorly characterized. Here, we found that mCD300e is also an immune-activating receptor. We found that mCD300e engagement triggers cytokine production in mCD300e-transduced bone marrow-derived mast cells (BMMCs). Loss of DAP12 and another signaling protein, FcRγ, did not affect surface expression of transduced mCD300e, but abrogated mCD300e-mediated cytokine production in the BMMCs. Co-immunoprecipitation experiments revealed that mCD300e physically interacts with both FcRγ and DAP12, suggesting that mCD300e delivers an activating signal via these two proteins. Binding and reporter assays with the mCD300e extracellular domain identified sphingomyelin as a ligand of both mCD300e and hCD300e. Notably, the binding of sphingomyelin to mCD300e stimulated cytokine production in the transduced BMMCs in an FcRγ- and DAP12-dependent manner. Flow cytometric analysis with an mCD300e-specific Ab disclosed that mCD300e expression is highly restricted to CD115⁺Ly-6C^low/int peripheral blood monocytes, corresponding to CD14^dim/+CD16⁺ human nonclassical and intermediate monocytes. Loss of FcRγ or DAP12 lowered the surface expression of endogenous mCD300e in the CD115⁺Ly-6C^low/int monocytes. Stimulation with sphingomyelin failed to activate the CD115⁺Ly-6C^low/int mouse monocytes, but induced hCD300e-mediated cytokine production in the CD14^dimCD16⁺ human monocytes. Taken together, these observations indicate that mCD300e recognizes sphingomyelin and thereby regulates nonclassical and intermediate monocyte functions through FcRγ and DAP12.

Inhibitory Activities of Compounds from the Marine Actinomycete Williamsia sp. MCCC 1A11233 Variant on IgE-Mediated Mast Cells and Passive Cutaneous Anaphylaxis

The compounds of the deep-sea-derived marine Williamsia sp. MCCC 1A11233 (CDMW) were isolated, which are secondary metabolites of the actinomycetes. In this study, seven kinds of CDMW were found to decrease degranulation and histamine release in immunoglobulin E (IgE)-mediated rat basophilic leukemia (RBL)-2H3 cells. The production of cytokines (tumor necrosis factor-α, interleukin-4) was inhibited by these CDMW in RBL-2H3 cells, and their chemical structures were established mainly based on detailed analysis of their NMR spectra. CDMW-3, CDMW-5, and CDMW-15 were further demonstrated to block mast cell-dependent passive cutaneous anaphylaxis in IgE-sensitized mice. Bone marrow mononuclear cells (BMMCs) were established to clarify the effect of CDMW-3, CDMW-5, and CDMW-15 on mast cells. The seven kinds of CDMW decreased the degranulation and histamine release of BMMCs. Furthermore, flow cytometry results indicated that CDMW-3, CDMW-5, and CDMW-15 increased the annexin⁺ cell population of BMMCs. In conclusion, CDMW-3, CDMW-5, and CDMW-15 have obvious antiallergic activity due to induction of the apoptosis of mast cells.

Effect of Specific Mutations in Cd300 Complexes Formation; Potential Implication of Cd300f in Multiple Sclerosis

Herein, we have used bioinformatics tools to predict five clusters defining ligand-binding sites on the extracellular domain of human CD300b receptor, presumably involved in the formation of both homodimers and heterodimers with other CD300 family members. Site-directed mutagenesis revealed residues glutamic acid 28 and glutamine 29 in cluster 5 to be necessary for the formation of CD300b complexes. Surprisingly, the disruption of cluster 2 and 4 reconstituted the binding capability lost by the mutation of residues glutamic acid 28 to alanine, glutamine 29 to alanine (E28A-Q29G). We identified a missense mutation arginine 33 to glutamine (R33Q) in CD300f by direct sequencing of exon 2 in peripheral blood samples from 50 patients with multiple sclerosis (MS). Levels of expression of CD300f were almost undetectable on monocytes from the patient bearing the R33Q mutation compared with healthy individuals. Whereas R33Q mutation had no effect in the formation of CD300f complexes, the inhibition of protein synthesis with cycloheximide indicated that CD300f R33Q is less stable than native CD300f. Finally, we report that the levels of expression of CD300f on the surface of classical and intermediate monocytes from MS patients are significantly lower when compared to the same cell populations in healthy individuals.

Histamine Release from Mast Cells and Basophils

Mast cells and basophils represent the most relevant source of histamine in the immune system. Histamine is stored in cytoplasmic granules along with other amines (e.g., serotonin), proteases, proteoglycans, cytokines/chemokines, and angiogenic factors and rapidly released upon triggering with a variety of stimuli. Moreover, mast cell and basophil histamine release is regulated by several activating and inhibitory receptors. The engagement of different receptors can trigger different modalities of histamine release and degranulation. Histamine released from mast cells and basophils exerts its biological activities by activating four G protein-coupled receptors, namely H1R, H2R, H3R (expressed mainly in the brain), and the recently identified H4R. While H1R and H2R activation accounts mainly for some mast cell- and basophil-mediated allergic disorders, the selective expression of H4R on immune cells is uncovering new roles for histamine (possibly derived from mast cells and basophils) in allergic, inflammatory, and autoimmune disorders. Thus, the in-depth knowledge of mast cell and basophil histamine release and its biologic effects is poised to uncover new therapeutic avenues for a wide spectrum of disorders.