Apoptotic cells suppress mast cell inflammatory responses via the CD300a immunoreceptor

Pro-resolving lipid mediators and therapeutic innovations in resolution of inflammation

This review summarizes findings presented at the 19th World Congress of Basic & Clinical Pharmacology 2023 (Glasgow, Scotland, July 3rd to 7th, 2023) from 8 speakers in the field of resolution of inflammation, resolution pharmacology and resolution biology. It is now accepted that the acute inflammatory response is protective to defend the host against infection or tissue injury. Acute inflammation is self-limited and programmed to be limited in space and time: this is achieved through endogenous resolution processes that ensure return to homeostasis. Resolution is brought about by agonist mediators that include specialized pro-resolving lipid mediators (SPMs) and pro-resolving proteins and peptides such as annexin A1 and angiotensin-(1-7), all acting to initiate anti-inflammatory and pro-resolving processes. If the inflammatory reaction remains unchecked through dysfunctional resolution mechanism, it can become chronic and contribute to a plethora of human diseases, including respiratory, cardiovascular, metabolic, allergic diseases, and arthritis. Herein, we discuss how non-resolving inflammation plays a role in the pathogenesis of these diseases. In addition to SPMs, we highlight the discovery, biosynthesis, biofunctions, and latest research updates on innovative therapeutics (including annexin-A1 peptide-mimetic RTP-026, small molecule FPR2 agonist BM-986235/LAR-1219, biased agonist for FPR1/FPR2 Cmpd17b, lipoxin mimetics AT-01-KG and AT-02-CT, melanocortin receptor agonist AP1189, gold nanoparticles, angiotensin-(1-7), and CD300a) that can promote resolution of inflammation directly or through modulation of SPMs production. Drug development strategies based on the biology of the resolution of inflammation can offer novel therapeutic means and/or add-on therapies for the treatment of chronic diseases.

Multi-modal analysis reveals tumor and immune features distinguishing EBV-positive and EBV-negative post-transplant lymphoproliferative disorders

The oncogenic Epstein-Barr virus (EBV) can drive tumorigenesis with disrupted host immunity, causing malignancies including post-transplant lymphoproliferative disorders (PTLDs). PTLD can also arise in the absence of EBV, but the biological differences underlying EBV(+) and EBV(-) B cell PTLD and the associated host-EBV-tumor interactions remain poorly understood. Here, we reveal the core differences between EBV(+) and EBV(-) PTLD, characterized by increased expression of genes related to immune processes or DNA interactions, respectively, and the augmented ability of EBV(+) PTLD B cells to modulate the tumor microenvironment through elaboration of monocyte-attracting cytokines/chemokines. We create a reference resource of proteins distinguishing EBV(+) B lymphoma cells from EBV(-) B lymphoma including the immunomodulatory molecules CD300a and CD24, respectively. Moreover, we show that CD300a is essential for maximal survival of EBV(+) PTLD B lymphoma cells. Our comprehensive multi-modal analyses uncover the biological underpinnings of PTLD and offer opportunities for precision therapies.

Blockade of CD300A enhances the ability of human NK cells to lyse hematologic malignancies

OBJECTIVE

The human cluster of differentiation (CD)300A, a type-I transmembrane protein with immunoreceptor tyrosine-based inhibitory motifs, was investigated as a potential immune checkpoint for human natural killer (NK) cells targeting hematologic malignancies (HMs).

METHODS

We implemented a stimulation system involving the CD300A ligand, phosphatidylserine (PS), exposed to the outer surface of malignant cells. Additionally, we utilized CD300A overexpression, a CD300A blocking system, and a xenotransplantation model to evaluate the impact of CD300A on NK cell efficacy against HMs in in vitro and in vivo settings. Furthermore, we explored the association between CD300A and HM progression in patients.

RESULTS

Our findings indicated that PS hampers the function of NK cells. Increased CD300A expression inhibited HM lysis by NK cells. CD300A overexpression shortened the survival of HM-xenografted mice by impairing transplanted NK cells. Blocking PS-CD300A signals with antibodies significantly amplified the expression of lysis function-related proteins and effector cytokines in NK cells, thereby augmenting the ability to lyse HMs. Clinically, heightened CD300A expression correlated with shorter survival and an "exhausted" phenotype of intratumoral NK cells in patients with HMs or solid tumors.

CONCLUSIONS

These results propose CD300A as a potential target for invigorating NK cell-based treatments against HMs.

GW0742 reduces mast cells degranulation and attenuates neurological impairments via PPARβ/δ/CD300a/SHP1 pathway after GMH in neonatal rats

BACKGROUND

Activation of mast cells plays an important role in brain inflammation. CD300a, an inhibitory receptor located on mast cell surfaces, has been reported to reduce the production of pro-inflammatory cytokines and exert protective effects in inflammation-related diseases. Peroxisome proliferator-activated receptor β/δ (PPARβ/δ), a ligand-activated nuclear receptor, activation upregulates the transcription of CD300a. In this study, we aim to investigate the role of PPARβ/δ in the attenuation of germinal matrix hemorrhage (GMH)-induced mast cell activation via CD300a/SHP1 pathway.

METHODS

GMH model was induced by intraparenchymal injection of bacterial collagenase into the right hemispheric ganglionic eminence in P7 Sprague Dawley rats. GW0742, a PPARβ/δ agonist, was administered intranasally at 1 h post-ictus. CD300a small interfering RNA (siRNA) and PPARβ/δ siRNA were injected intracerebroventricularly 5 days and 2 days before GMH induction. Behavioral tests, Western blot, immunofluorescence, Toluidine Blue staining, and Nissl staining were applied to assess post-GMH evaluation.

RESULTS

Results demonstrated that endogenous protein levels of PPARβ/δ and CD300a were decreased, whereas chymase, tryptase, IL-17A and transforming growth factor β1 (TGF-β1) were elevated after GMH. GMH induced significant short- and long-term neurobehavioral deficits in rat pups. GW0742 decreased mast cell degranulation, improved neurological outcomes, and attenuated ventriculomegaly after GMH. Additionally, GW0742 increased expression of PPARβ/δ, CD300a and phosphorylation of SHP1, decreased phosphorylation of Syk, chymase, tryptase, IL-17A and TGF-β1 levels. PPARβ/δ siRNA and CD300a siRNA abolished the beneficial effects of GW0742.

CONCLUSIONS

GW0742 inhibited mast cell-induced inflammation and improved neurobehavior after GMH, which is mediated by PPARβ/δ/CD300a/SHP1 pathway. GW0742 may serve as a potential treatment to reduce brain injury for GMH patients.

Development of Monoclonal Antibodies Specific to Either CD300AR111 or CD300AQ111 or Both

CD300A is a member of the CD300 immunoglobulin (Ig)-like receptor family consisting of eight molecules in humans, all of which contain one Ig-like domain in the extracellular portion. Upon binding its ligand phosphatidylserine or phosphatidylethanolamine, CD300A mediates an inhibitory signal through the immunoreceptor tyrosine-based inhibitory motif in the cytoplasmic portion. The CD300 family molecules are highly homologous to each other. In addition, CD300A has a single nucleotide polymorphism (rs2272111), which is a nonsense mutation encoding glutamine (CD300AQ111) instead of arginine (CD300AR111) at residue 111 in the Ig-like domain of CD300A. In this study, we successfully generated monoclonal antibodies (mAbs) specific to either CD300AR111 or CD300AQ111 or both. These mAbs are useful for the analysis of CD300A genotype by flow cytometry and the development of an antibody drug for the treatment of various diseases.

Apoptotic cell clearance components in inflammatory arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease of the synovial joints that affects ~1% of the human population. Joint swelling and bone erosion, hallmarks of RA, contribute to disability and, sometimes, loss of life. Mechanistically, disease is driven by immune dysregulation characterized by circulating autoantibodies, inflammatory mediators, tissue degradative enzymes, and metabolic dysfunction of resident stromal and recruited immune cells. Cell death by apoptosis has been therapeutically explored in animal models of RA due to the comparisons drawn between synovial hyperplasia and paucity of apoptosis in RA with the malignant transformation of cancer cells. Several efforts to induce cell death have shown benefits in reducing the development and/or severity of the disease. Apoptotic cells are cleared by phagocytes in a process known as efferocytosis, which differs from microbial phagocytosis in its "immuno-silent," or anti-inflammatory, nature. Failures in efferocytosis have been linked to autoimmune disease, whereas administration of apoptotic cells in RA models effectively inhibits inflammatory indices, likely though efferocytosis-mediated resolution-promoting mechanisms. However, the nature of signaling pathways elicited and the molecular identity of clearance mediators in RA are understudied. Furthermore, canonical efferocytosis machinery elements also play important non-canonical functions in homeostasis and pathology. Here, we discuss the roles of efferocytosis machinery components in models of RA and discuss their potential involvement in disease pathophysiology.

Human IgMhiCD300a+ B Cells Are Circulating Marginal Zone Memory B Cells That Respond to Pneumococcal Polysaccharides and Their Frequency Is Decreased in People Living with HIV

CD300a is differentially expressed among B cell subsets, although its expression in immunoglobulin (Ig)M+ B cells is not well known. We identified a B cell subset expressing CD300a and high levels of IgM (IgMhiCD300a+). The results showed that IgMhiCD300a+ B cells were CD10-CD27+CD25+IgDloCD21hiCD23-CD38loCD1chi, suggesting that they are circulating marginal zone (MZ) IgM memory B cells. Regarding the immunoglobulin repertoire, IgMhiCD300a+ B cells exhibited a higher mutation rate and usage of the IgH-VDJ genes than the IgM+CD300a- counterpart. Moreover, the shorter complementarity-determining region 3 (CDR3) amino acid (AA) length from IgMhiCD300a+ B cells together with the predicted antigen experience repertoire indicates that this B cell subset has a memory phenotype. IgM memory B cells are important in T cell-independent responses. Accordingly, we demonstrate that this particular subset secretes higher amounts of IgM after stimulation with pneumococcal polysaccharides or a toll-like receptor 9 (TLR9) agonist than IgM+CD300a- cells. Finally, the frequency of IgMhiCD300a+ B cells was lower in people living with HIV-1 (PLWH) and it was inversely correlated with the years with HIV infection. Altogether, these data help to identify a memory B cell subset that contributes to T cell-independent responses to pneumococcal infections and may explain the increase in severe pneumococcal infections and the impaired responses to pneumococcal vaccination in PLWH.

CD300b regulates intestinal inflammation and promotes repair in colitis

Chronic inflammation is a hallmark charataristic of various inflammatory diseases including inflammatory bowel disease. Subsequently, current therapeutic approaches target immune-mediated pathways as means for therapeutic intervention and promotion of mucosal healing and repair. Emerging data demonstrate important roles for CD300 receptor family members in settings of innate immunity as well as in allergic and autoimmune diseases. One of the main pathways mediating the activities of CD300 family members is via promotion of resolution through interactions with ligands expressed by viruses, bacteria, or dead cells (e.g., phospholipids such as PtdSer and/or ceramide). We have recently shown that the expression of CD300a, CD300b and CD300f were elevated in patients with IBD and that CD300f (but not CD300a) regulates colonic inflammation in response to dextran sodium sulphate (DSS)-induced colitis. Whether CD300b has a role in colitis or mucosal healing is largely unknown. Herein, we demonstrate a central and distinct role for CD300b in colonic inflammation and subsequent repair. We show that Cd300b-/- mice display defects in mucosal healing upon cessation of DSS treatment. Cd300b-/- mice display increased weight loss and disease activity index, which is accompanied by increased colonic histopathology, increased infiltration of inflammatory cells and expression of multiple pro-inflammatory upon cessation of DSS cytokines. Furthermore, we demonstrate that soluble CD300b (sCD300b) is increased in the colons of DSS-treated mice and establish that CD300b can bind mouse and human epithelial cells. Finally, we show that CD300b decreases epithelial EpCAM expression, promotes epithelial cell motility and wound healing. These data highlight a key role for CD300b in colonic inflammation and repair processes and suggest that CD300b may be a future therapeutic target in inflammatory GI diseases.

CD300a Regulates Mouse Macrophage Functionality in Allergic Inflammation

BACKGROUND

CD300a is an inhibitory receptor (IR) expressed on several leukocytes, including mast cells (MCs) and macrophages (MΦ), important cells in allergic inflammation (AI). We have previously characterized CD300a role on MCs and in vivo in mouse models of allergy, in which the absence of CD300a resulted in increased inflammatory features and delayed resolution. However, the exact mechanism of this delayed resolution is unclear. Our hypothesis is that MΦ, important players in resolution, might be impaired when CD300a is absent.

OBJECTIVES

The aim of the study was to investigate CD300a-dependent functionality of mouse MΦ.

METHOD

MΦ were purified from the peritoneum of wild-type (WT) and CD300a-/- mice naïve and 48 h and 96 h after challenge with ovalbumin/alum. Phenotype switching was analyzed via specific M1-M2 inducers and markers. MΦ phagocytotic ability was assessed via Staphylococcus aureus pHrodo-conjugated bioparticles. The influence of MCs on MΦ was investigated by incubating WT MΦ with supernatants from non-activated and IgE-activated bone marrow-derived MCs (BMMCs) and analyzing functional responses.

RESULTS

Naïve CD300a-/- MΦ presented with increased sensitivity to activation when treated with LPS. Absence of CD300a results in increased Arg1 expression and increased IL-6 release when MΦ are purified from allergic peritonitis-induced mice. Similar results were obtained when CD300a-/- MΦ were purified 96 h after challenge. On the other hand, CD300a absence did not affect phagocytosis. WT MΦ incubated with supernatants of non-activated and IgE-activated BMMCs presented with increased iNOS expression and decreased Arg1 levels.

CONCLUSIONS

The IR CD300a controls the activation state of MΦ, and its absence could augment the inflammatory state seen in CD300a-/- mice. Moreover, MCs can also influence MΦ phenotype switching. This may partially explain the delayed AI resolution seen in these mice.

Pseudomonas aeruginosa Cytotoxins: Mechanisms of Cytotoxicity and Impact on Inflammatory Responses

Pseudomonas aeruginosa is one of the most virulent opportunistic Gram-negative bacterial pathogens in humans. It causes many acute and chronic infections with morbidity and mortality rates as high as 40%. P. aeruginosa owes its pathogenic versatility to a large arsenal of cell-associated and secreted virulence factors which enable this pathogen to colonize various niches within hosts and protect it from host innate immune defenses. Induction of cytotoxicity in target host cells is a major virulence strategy for P. aeruginosa during the course of infection. P. aeruginosa has invested heavily in this strategy, as manifested by a plethora of cytotoxins that can induce various forms of cell death in target host cells. In this review, we provide an in-depth review of P. aeruginosa cytotoxins based on their mechanisms of cytotoxicity and the possible consequences of their cytotoxicity on host immune responses.

Effect of bromelain on mast cell numbers and degranulation in diabetic rat wound healing

OBJECTIVE

Many studies have revealed the prominent roles of mast cells in wound healing, including inflammatory reactions, angiogenesis and extracellular matrix reabsorption. In the present study, we aimed to assess the probable therapeutic features of bromelain on wound contraction and mast cell degranulation in wound healing in experimental diabetic animals.

METHOD

Male rats were grouped as control, vehicle and experiment. Skin wounds were generated in all groups. Treatments were applied with distilled water and with bromelain (BR) intraperitoneally in the vehicle and experimental groups, respectively. Following skin wound generation, animals were euthanised on days 3, 5, 7 and 15. We gathered 16,800 microscopic images to count the mast cells and degranulation level (Image J software). The wound contraction index was assessed both microscopically (Image J software) and macroscopically (time-lapse photography). The meshwork evaluation method was used to assess wound healing.

RESULTS

Time-lapse photography revealed that the BR significantly (p<0.05) accelerated wound contraction and healing. BR significantly (p<0.05) increased the total number of mast cells in all experimental groups on days 5 and 7. The count of grade III (degranulated) mast cells was reduced significantly (p<0.05) on days 5 and 7 in experimental groups compared to control and vehicle groups.

CONCLUSION

In this study, the rate of wound healing was accelerated considerably following BR administration. In addition, this agent decreased the count of degranulated mast cells, leading to wound contraction and healing.

Inhibitory pattern recognition receptors

Pathogen- and damage-associated molecular patterns are sensed by the immune system's pattern recognition receptors (PRRs) upon contact with a microbe or damaged tissue. In situations such as contact with commensals or during physiological cell death, the immune system should not respond to these patterns. Hence, immune responses need to be context dependent, but it is not clear how context for molecular pattern recognition is provided. We discuss inhibitory receptors as potential counterparts to activating pattern recognition receptors. We propose a group of inhibitory pattern recognition receptors (iPRRs) that recognize endogenous and microbial patterns associated with danger, homeostasis, or both. We propose that recognition of molecular patterns by iPRRs provides context, helps mediate tolerance to microbes, and helps balance responses to danger signals.

Tumor-derived extracellular vesicles regulate tumor-infiltrating regulatory T cells via the inhibitory immunoreceptor CD300a

Although tumor-infiltrating regulatory T (Treg) cells play a pivotal role in tumor immunity, how Treg cell activation are regulated in tumor microenvironments remains unclear. Here, we found that mice deficient in the inhibitory immunoreceptor CD300a on their dendritic cells (DCs) have increased numbers of Treg cells in tumors and greater tumor growth compared with wild-type mice after transplantation of B16 melanoma. Pharmacological impairment of extracellular vesicle (EV) release decreased Treg cell numbers in CD300a-deficient mice. Coculture of DCs with tumor-derived EV (TEV) induced the internalization of CD300a and the incorporation of EVs into endosomes, in which CD300a inhibited TEV-mediated TLR3–TRIF signaling for activation of the IFN-β-Treg cells axis. We also show that higher expression of CD300A was associated with decreased tumor-infiltrating Treg cells and longer survival time in patients with melanoma. Our findings reveal the role of TEV and CD300a on DCs in Treg cell activation in the tumor microenvironment.

CD300a blockade enhances efferocytosis by infiltrating myeloid cells and ameliorates neuronal deficit after ischemic stroke

[Figure: see text].

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.

Putting the brakes on phagocytosis: "don't-eat-me" signaling in physiology and disease

Timely removal of dying or pathogenic cells by phagocytes is essential to maintaining host homeostasis. Phagocytes execute the clearance process with high fidelity while sparing healthy neighboring cells, and this process is at least partially regulated by the balance of "eat-me" and "don't-eat-me" signals expressed on the surface of host cells. Upon contact, eat-me signals activate "pro-phagocytic" receptors expressed on the phagocyte membrane and signal to promote phagocytosis. Conversely, don't-eat-me signals engage "anti-phagocytic" receptors to suppress phagocytosis. We review the current knowledge of don't-eat-me signaling in normal physiology and disease contexts where aberrant don't-eat-me signaling contributes to pathology.

CD300a contributes to the resolution of articular inflammation triggered by MSU crystals by controlling neutrophil apoptosis

Gout is an inflammatory disease triggered by deposition of monosodium urate (MSU) crystals in the joints, resulting in high neutrophil influx and pain. Here, we studied the role of the inhibitory receptor CD300a in the resolution process in a murine model of gout. We found increased CD300a expression on neutrophils emigrated to the joint. When compared to WT mice, CD300a^-/- mice had persistent neutrophil influx till 24 hr after MSU injection. This was associated with increased concentration of IL-1β and greater tissue damage in the joints of CD300a^-/- mice. There was an increase in the percentage of apoptotic neutrophils in the synovial lavage of WT mice, as compared to CD300a^-/- mice. This difference was reflected in the decline of efferocytic events in the synovial cavity of CD300a^-/- mice 24 hr after MSU injection. A CD300a agonistic antibody was shown, for the first time, to increase apoptosis of human neutrophils, and this was associated with cleavage of caspase-8. In conclusion, our results reveal an important role of CD300a in the control of leucocyte infiltration, IL-1β production and caspase-8 cleavage in neutrophils, contributing to the resolution of inflammation triggered by MSU injection.

Tyro3, Axl, Mertk receptor-mediated efferocytosis and immune regulation in the tumor environment

Three structurally related tyrosine receptor cell surface kinases, Tyro3, Axl, and Mertk (TAM) have been recognized to modulate immune function, tissue homeostasis, cardiovasculature, and cancer. The TAM receptor family appears to operate in adult mammals across multiple cell types, suggesting both widespread and specific regulation of cell functions and immune niches. TAM family members regulate tissue homeostasis by monitoring the presence of phosphatidylserine expressed on stressed or apoptotic cells. The detection of phosphatidylserine on apoptotic cells requires intermediary molecules that opsonize the dying cells and tether them to TAM receptors on phagocytes. This complex promotes the engulfment of apoptotic cells, also known as efferocytosis, that leads to the resolution of inflammation and tissue healing. The immune mechanisms dictating these processes appear to fall upon specific family members or may involve a complex of different receptors acting cooperatively to resolve and repair damaged tissues. Here, we focus on the role of TAM receptors in triggering efferocytosis and its consequences in the regulation of immune responses in the context of inflammation and cancer.

CD300a and CD300f molecules regulate the function of leukocytes

The CD300 molecule family is a type I transmembrane glycoprotein expressed on cell membrane of human and other mammals, and of its eight members, only CD300a and CD300f are classified as inhibitory receptors. CD300a and CD300f play an important role in regulating the function of leukocytes, such as activation, proliferation, differentiation, migration and immunity function. They are considered as potential targets for studying the development and progression of inflammation, infection and other diseases. Here, we review the expression and regulatory mechanisms of CD300a and CD300f on leukocytes, as well as their effects on relevant 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.

Modulation of Mast Cell Reactivity by Lipids: The Neglected Side of Allergic Diseases

Mast cells (MCs) have long been mainly regarded as effector cells in IgE-associated allergic disorders with potential immunoregulatory roles. Located close to the allergen entry sites in the skin and mucosa, MCs can capture foreign substances such as allergens, toxins, or noxious substances and are exposed to the danger signals produced by epithelial cells. MC reactivity shaped by tissue-specific factors is crucial for allergic responses ranging from local skin reactions to anaphylactic shock. Development of Th2 response leading to allergen-specific IgE production is a prerequisite for MC sensitization and induction of FcεRI-mediated MC degranulation. Up to now, IgE production has been mainly associated with proteins, whereas lipids present in plant pollen grains, mite fecal particles, insect venoms, or food have been largely overlooked regarding their immunostimulatory and immunomodulatory properties. Recent studies, however, have now demonstrated that lipids affect the sensitization process by modulating innate immune responses of epithelial cells, dendritic cells, and NK-T cells and thus crucially contribute to the outcome of sensitization. Whether and how lipids affect also MC effector functions in allergic reactions has not yet been fully clarified. Here, we discuss how lipids can affect MC responses in the context of allergic inflammation. Direct effects of immunomodulatory lipids on MC degranulation, changes in local lipid composition induced by allergens themselves and changes in lipid transport affecting MC reactivity are possible mechanisms by which the function of MC might be modulated.

Voices from the dead: The complex vocabulary and intricate grammar of dead cells

Of the roughly one million cells per second dying throughout the body, the vast majority dies by apoptosis, the predominant form of regulated cell death in higher organisms. Long regarded as mere waste, apoptotic cells are now recognized as playing a prominent and active role in homeostatic maintenance, especially resolution of inflammation, and in the sculpting of tissues during development. The activities associated with apoptotic cells are continually expanding, with more recent studies demonstrating their ability to modulate such vital functions as proliferation, survival, differentiation, metabolism, migration, and angiogenesis. In each case, the role of apoptotic cells is active, exerting their effects via new activities acquired during the apoptotic program. Moreover, the capacity to recognize and respond to apoptotic cells is not limited to professional phagocytes. Most, if not all, cells receive and integrate an array of signals from cells dying in their vicinity. These signals comprise a form of biochemical communication. As reviewed in this chapter, this communication is remarkably sophisticated; each of its three critical steps-encoding, transmission, and decoding of the apoptotic cell's "message"-is endowed with exquisite robustness. Together, the abundance and intricacy of the variables at each step comprise the vocabulary and grammar of the language by which dead cells achieve their post-mortem voice. The combinatorial complexity of the resulting communication network permits dying cells, through the signals they emit and the responses those signals elicit, to partake of an expanded role in homeostasis, acting as both sentinels of environmental change and agents of adaptation.

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.

Human eosinophils and mast cells: Birds of a feather flock together

While the origin of the phrase "birds of a feather flock together" is unclear, it has been in use for centuries and is typically employed to describe the phenomenon that people with similar tastes or interests tend to seek each other out and congregate together. In this review, we have co-opted this phrase to compare innate immune cells of related origin, the eosinophil and mast cell, because they very often accumulate together in tissue sites under both homeostatic and inflammatory conditions. To highlight overlapping yet distinct features, their hematopoietic development, cell surface phenotype, mediator release profiles and roles in diseases have been compared and contrasted. What emerges is a sense that these two cell types often interact with each other and their tissue environment to provide synergistic contributions to a variety of normal and pathologic immune responses.

Macrophage Polarization: Different Gene Signatures in M1(LPS+) vs. Classically and M2(LPS-) vs. Alternatively Activated Macrophages

Macrophages are found in tissues, body cavities, and mucosal surfaces. Most tissue macrophages are seeded in the early embryo before definitive hematopoiesis is established. Others are derived from blood monocytes. The macrophage lineage diversification and plasticity are key aspects of their functionality. Macrophages can also be generated from monocytes in vitro and undergo classical (LPS+IFN-γ) or alternative (IL-4) activation. In vivo, macrophages with different polarization and different activation markers coexist in tissues. Certain mouse strains preferentially promote T-helper-1 (Th1) responses and others Th2 responses. Their macrophages preferentially induce iNOS or arginase and have been called M1 and M2, respectively. In many publications, M1 and classically activated and M2 and alternatively activated are used interchangeably. We tested whether this is justified by comparing the gene lists positively [M1(=LPS+)] or negatively [M2(=LPS-)] correlated with the ratio of IL-12 and arginase 1 in transcriptomes of LPS-treated peritoneal macrophages with in vitro classically (LPS, IFN-γ) vs. alternatively activated (IL-4) bone marrow derived macrophages, both from published datasets. Although there is some overlap between in vivo M1(=LPS+) and in vitro classically activated (LPS+IFN-γ) and in vivo M2(=LPS-) and in vitro alternatively activated macrophages, many more genes are regulated in opposite or unrelated ways. Thus, M1(=LPS+) macrophages are not equivalent to classically activated, and M2(=LPS-) macrophages are not equivalent to alternatively activated macrophages. This fundamental discrepancy explains why most surface markers identified on in vitro generated macrophages do not translate to the in vivo situation. Valid in vivo M1/M2 surface markers remain to be discovered.

Overexpression of CD300A inhibits progression of NSCLC through downregulating Wnt/β-catenin pathway

Background

CD300A, a type I transmembrane glycoprotein receptor, plays an important role in immune response. Recent studies have reported that CD300A is involved in the development of hematological malignancies.

Purpose

The objective of this study was to investigate the role of CD300A in the progression of non-small-cell lung cancer (NSCLC) and explore the associated mechanism.

Materials and methods

Gene Expression Profiling Interactive Analysis (GEPIA) was used to analyze the expression of CD300A in NSCLC and its prognostic value. NSCLC cell lines A549 and H1650 were transfected with siRNA-CD300A or pcDNA3.1-CD300A vector to down- or up-regulate the expression of CD300A. Cell Counting Kit 8, colony formation and Transwell assays were used to assess the effects of CD300A on cell proliferation and migration capacities. Flow cytometry was performed to examine rate of apoptosis, and the protein levels of associated proteins was detected using Western blot assay.

Results

From GEPIA analysis, we observed that expression of CD300A mRNA was downregulated in NSCLC and positively correlated with the overall survival of NSCLC patients. Overexpression of CD300A significantly suppressed cell growth and migration capacities of A549 and H1650 cells and induced cell apoptosis via regulating apoptosis-related proteins. Moreover, decreasing level of CD300A promoted cell growth and migration and blocked apoptosis of NSCLC cells. Furthermore, upregulation of CD300A led to significant decrease in expression level of Wnt3 and β-catenin, the pivotal components in Wnt/β-catenin signaling pathway, and an increase in expression of E-cad, a key protein in tumor metastasis, in A549 and H1650 cells; while depletion of CD300A up-regulated the Wnt/β-catenin signaling pathway. In conclusion, the present study highlighted an anti-oncogenic role of CD300A in the progression of NSCLC via inhibiting Wnt/β-catenin pathway, suggesting that CD300A might be a potential target for the treatment of NSCLC

Conclusion

CD300A plays an anti-oncogenic role in the progression of NSCLC through inhibiting the Wnt/β-catenin pathway, suggesting that CD300A might be a potential target for the treatment of NSCLC.

Leukocyte CD300a Contributes to the Resolution of Murine Allergic Inflammation

CD300a is an inhibitory receptor for mast cells and eosinophils in allergic inflammation (AI); however, the spatiotemporal expression of CD300a and its potential roles in the resolution of AI are still to be determined. In this study, employing a mouse model of allergic peritonitis, we demonstrate that CD300a expression on peritoneal cells is regulated from inflammation to resolution. Allergic peritonitis-induced CD300a^-/- mice had a rapid increase in their inflammatory cell infiltrates and tryptase content in the peritoneal cavity compared with wild type, and their resolution process was significantly delayed. CD300a^-/- mice expressed lower levels of ALX/FPR2 receptor on peritoneal cells and had higher levels of LXA₄ in the peritoneal lavage. CD300a activation on mouse bone marrow-derived mast cells regulated ALX/FPR2 expression levels following IgE-mediated activation. Together, these findings indicate a role for CD300a in AI and its resolution, in part via the specialized proresolving mediator LXA₄ and ALX/FPR2 receptor pathway activation.

CD300A inhibits tumor cell growth by downregulating AKT phosphorylation in human glioblastoma multiforme

Glioblastoma multiforme (GBM) is a primary malignant tumor of the central nervous system with the highest incidence and dismal prognosis. As a member of the CD300 glycoprotein family, CD300A plays a role in cell proliferation, apoptosis, differentiation, and immune response, but its role in solid tumors remains unknown. In this study, CD300A was observed to be overexpressed in human GBM samples using real-time PCR and western blotting. To investigate the role of CD300A in GBM, CCK8, transwell and flow cytometry analysis were performed to examine the proliferation, migration and apoptosis in GBM cell lines, respectively. From our results, knockdown of CD300A blocks cell proliferation and migration, and induces cell apoptosis in human GBM cells U251MG and U87MG. Further, we assessed AKT expression level in CD300A knockdown and negative control cells. The phosphorylation level of AKT was significantly suppressed in CD300A knockdown cells in comparison to negative control cells, suggesting that CD300A promoted tumor cell growth through the AKT pathway. In conclusion, our findings expand the knowledge of CD300A as an oncogene in solid tumor, and provide experimental and theoretical basis for further clinical application.

CD300A promotes tumor progression by PECAM1, ADCY7 and AKT pathway in acute myeloid leukemia

CD300A is a member of the CD300 glycoprotein family of cell surface proteins involved in immune response signaling pathways. There is evidence that CD300A plays a role in autophagy and angiogenesis, while, no studies have been reported which investigated the role of CD300A in tumors. CD300A was found to be highly expressed with statistical significance in acute myeloid leukemia (AML), as well as associated with prognosis, through the analysis of differential expression genes using the TCGA and GTEx database. A decrease in CD300A expression could promote apoptosis and inhibit proliferation and migration of AML cell line U937, as well as promote the activation of the AKT/mTOR pathway. These results demonstrated that CD300A operated as a tumor promoter in AML cells. We further analyzed coexpression genes of CD300A and then screened two genes, ADCY7 and PECAM1, which were both overexpressed and associated with poor prognosis in AML. Meanwhile, CD300A increased the expression of PECAM1 and ADCY7 in U937 cells. Furthermore, we demonstrated that PECAM1 promoted the proliferation and migration and inhibited the apoptosis of U937 cells. ADCY7 participated in the regulation of proliferation and migration, but not apoptosis, in U937 cells. Both PECAM1 and ADCY7 promoted tumor progression through the AKT pathway, showing the same molecular mechanism as CD300A. To summarize, we, for the first time, confirmed that CD300A promoted tumor progression by increase PECAM1 and ADCY7 expression, and activating the AKT/mTOR signaling pathway in AML. It is suggested CD300A is an oncogene and potential therapeutic target for AML.

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.

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.

Impaired response of the bronchial epithelium to inflammation characterizes severe equine asthma

BACKGROUND

Severe equine asthma is a naturally occurring lung inflammatory disease of mature animals characterized by neutrophilic inflammation, bronchoconstriction, mucus hypersecretion and airway remodeling. Exacerbations are triggered by inhalation of dust and microbial components. Affected animals eventually are unable of aerobic performance. In this study transcriptomic differences between asthmatic and non-asthmatic animals in the response of the bronchial epithelium to an inhaled challenge were determined.

RESULTS

Paired endobronchial biopsies were obtained pre- and post-challenge from asthmatic and non-asthmatic animals. The transcriptome, determined by RNA-seq and analyzed with edgeR, contained 111 genes differentially expressed (DE) after challenge between horses with and without asthma, and 81 of these were upregulated. Genes involved in neutrophil migration and activation were in central location in interaction networks, and related gene ontology terms were significantly overrepresented. Relative abundance of specific gene products as determined by immunohistochemistry was correlated with differential gene expression. Gene sets involved in neutrophil chemotaxis, immune and inflammatory response, secretion, blood coagulation and apoptosis were overrepresented among up-regulated genes, while the rhythmic process gene set was overrepresented among down-regulated genes. MMP1, IL8, TLR4 and MMP9 appeared to be the most important proteins in connecting the STRING protein network of DE genes.

CONCLUSIONS

Several differentially expressed genes and networks in horses with asthma also contribute to human asthma, highlighting similarities between severe human adult and equine asthma. Neutrophil activation by the bronchial epithelium is suggested as the trigger of the inflammatory cascade in equine asthma, followed by epithelial injury and impaired repair and differentiation. Circadian rhythm dysregulation and the sonic Hedgehog pathway were identified as potential novel contributory factors in equine asthma.

Disrupting ceramide-CD300f interaction prevents septic peritonitis by stimulating neutrophil recruitment

Sepsis is a serious clinical problem. Negative regulation of innate immunity is associated with sepsis progression, but the underlying mechanisms remains unclear. Here we show that the receptor CD300f promotes disease progression in sepsis. CD300f ^-/- mice were protected from death after cecal ligation and puncture (CLP), a murine model of septic peritonitis. CD300f was highly expressed in mast cells and recruited neutrophils in the peritoneal cavity. Analysis of mice (e.g., mast cell-deficient mice) receiving transplants of wild-type or CD300f ^-/- mast cells or neutrophils indicated that CD300f deficiency did not influence intrinsic migratory abilities of neutrophils, but enhanced neutrophil chemoattractant production (from mast cells and neutrophils) in the peritoneal cavity of CLP-operated mice, leading to robust accumulation of neutrophils which efficiently eliminated Escherichia coli. Ceramide-CD300f interaction suppressed the release of neutrophil chemoattractants from Escherichia coli-stimulated mast cells and neutrophils. Administration of the reagents that disrupted the ceramide-CD300f interaction prevented CLP-induced sepsis by stimulating neutrophil recruitment, whereas that of ceramide-containing vesicles aggravated sepsis. Extracellular concentrations of ceramides increased in the peritoneal cavity after CLP, suggesting a possible role of extracellular ceramides, CD300f ligands, in the negative-feedback suppression of innate immune responses. Thus, CD300f is an attractive target for the treatment of sepsis.

Efferocytosis Signaling in the Regulation of Macrophage Inflammatory Responses

Since the pioneering work of Elie Metchnikoff and the discovery of cellular immunity, the phagocytic clearance of cellular debris has been considered an integral component of resolving inflammation and restoring function of damaged and infected tissues. We now know that the phagocytic clearance of dying cells (efferocytosis), particularly by macrophages and other immune phagocytes, has profound consequences on innate and adaptive immune responses in inflamed tissues. These immunomodulatory effects result from an array of molecular signaling events between macrophages, dying cells, and other tissue-resident cells. In recent years, many of these molecular pathways have been identified and studied in the context of tissue inflammation, helping us better understand the relationship between efferocytosis and inflammation. We review specific types of efferocytosis-related signals that can impact macrophage immune responses and discuss their relevance to inflammation-related diseases.

Assessment of CD37 B-cell antigen and cell of origin significantly improves risk prediction in diffuse large B-cell lymphoma

CD37 (tetraspanin TSPAN26) is a B-cell surface antigen widely expressed on mature B cells. CD37 is involved in immune regulation and tumor suppression but its function has not been fully elucidated. We assessed CD37 expression in de novo diffuse large B-cell lymphoma (DLBCL), and investigated its clinical and biologic significance in 773 patients treated with rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) and 231 patients treated with CHOP. We found that CD37 loss (CD37^-) in ∼60% of DLBCL patients showed significantly decreased survival after R-CHOP treatment, independent of the International Prognostic Index (IPI), germinal center B-cell-like (GCB)/activated B-cell-like (ABC) cell of origin, nodal/extranodal primary origin, and the prognostic factors associated with CD37^-, including TP53 mutation, NF-κB^high, Myc^high, phosphorylated STAT3^high, survivin^high, p63^-, and BCL6 translocation. CD37 positivity predicted superior survival, abolishing the prognostic impact of high IPI and above biomarkers in GCB-DLBCL but not in ABC-DLBCL. Combining risk scores for CD37^- status and ABC cell of origin with the IPI, defined as molecularly adjusted IPI for R-CHOP (M-IPI-R), or IPI plus immunohistochemistry (IHC; IPI+IHC) for CD37, Myc, and Bcl-2, significantly improved risk prediction over IPI alone. Gene expression profiling suggested that decreased CD20 and increased PD-1 levels in CD37^- DLBCL, ICOSLG upregulation in CD37⁺ GCB-DLBCL, and CD37 functions during R-CHOP treatment underlie the pivotal role of CD37 status in clinical outcomes. In conclusion, CD37 is a critical determinant of R-CHOP outcome in DLBCL especially in GCB-DLBCL, representing its importance for optimal rituximab action and sustained immune responses. The combined molecular and clinical prognostic indices, M-IPI-R and IPI+IHC, have remarkable predictive values in R-CHOP-treated DLBCL.

The expression and function of human CD300 receptors on blood circulating mononuclear cells are distinct in neonates and adults

Neonates are more susceptible to infections than adults. This susceptibility is thought to reflect neonates' qualitative and quantitative defects in the adaptive and innate immune responses. Differential expression of cell surface receptors may result in altered thresholds of neonatal immune cell activation. We determined whether the expression and function of the lipid-binding CD300 family of receptors are different on neonatal immune cells compared to adult immune cells. A multiparametric flow cytometry analysis was performed to determine the expression of CD300 receptors on adult peripheral blood mononuclear cells and neonatal cord blood mononuclear cells. The expression of the CD300a inhibitory receptor was significantly reduced on cells from the newborn adaptive immune system, and neonatal antigen presenting cells exhibited a different CD300 receptors expression pattern. We also found differential LPS-mediated regulation of CD300 receptors expression on adult monocytes compared to cord blood monocytes, and that CD300c and CD300e-mediated activation was quantitatively different in neonatal monocytes. This is the first complete study examining the expression of CD300 receptors on human neonatal immune cells compared with adult immune cells. Significant differences in the expression and function of CD300 receptors may help to explain the peculiarities and distinctness of the neonatal immune responses.

Lipopolysaccharide-Induced CD300b Receptor Binding to Toll-like Receptor 4 Alters Signaling to Drive Cytokine Responses that Enhance Septic Shock

Receptor CD300b is implicated in regulating the immune response to bacterial infection by an unknown mechanism. Here, we identified CD300b as a lipopolysaccharide (LPS)-binding receptor and determined the mechanism underlying CD300b augmentation of septic shock. In vivo depletion and adoptive transfer studies identified CD300b-expressing macrophages as the key cell type augmenting sepsis. We showed that CD300b, and its adaptor DAP12, associated with Toll-like receptor 4 (TLR4) upon LPS binding, thereby enhancing TLR4-adaptor MyD88- and TRIF-dependent signaling that resulted in an elevated pro-inflammatory cytokine storm. LPS engagement of the CD300b-TLR4 complex led to the recruitment and activation of spleen tyrosine kinase (Syk) and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K). This resulted in an inhibition of the ERK1/2 protein kinase- and NF-κB transcription factor-mediated signaling pathways, which subsequently led to a reduced interleukin-10 (IL-10) production. Collectively, our data describe a mechanism of TLR4 signaling regulated by CD300b in myeloid cells in response to LPS.

Ceramide-CD300f binding suppresses experimental colitis by inhibiting ATP-mediated mast cell activation

OBJECTIVE

Extracellular ATP mediates mast cell-dependent intestinal inflammation via P2X7 purinoceptors. We have previously shown that CD300f (also called the leucocyte mono-immunoglobulin-like receptor 3 (LMIR3)) suppresses immunoglobulin E-dependent and mast cell-dependent allergic responses by binding to ceramide. The aim of the present study was to clarify the role of ceramide-LMIR3 interaction in the development of IBD.

DESIGN

The dextran sodium sulfate (DSS)-induced colitis model was used in wild-type (WT), LMIR3(-/-), mast cell-deficient Kit(W-sh/W-sh), Kit(W-sh/W-sh)LMIR3(-/-) or Kit(W-sh/W-sh) mice engrafted with WT or LMIR3(-/-) bone marrow-derived mast cells (BMMCs). The severity of colitis was determined by clinical and histological criteria. Lamina propria cell populations were assessed by flow cytometry. Production of chemical mediators from lamina propria cells was measured by real-time reverse transcription PCR. Production of chemical mediators from ATP-stimulated BMMCs in the presence or absence of ceramide was measured by ELISA. The severity of DSS-induced colitis was assessed in mice given either an Fc fusion protein containing an extracellular domain of LMIR3, and anticeramide antibody, or ceramide liposomes.

RESULTS

LMIR3 deficiency exacerbated DSS-induced colitis in mice. Kit(W-sh/W-sh) mice harbouring LMIR3(-/-) mast cells exhibited more severe colitis than those harbouring WT mast cells. Ceramide-LMIR3 interaction inhibited ATP-stimulated activation of BMMCs. DSS-induced colitis was aggravated by disrupting the ceramide-LMIR3 interaction, whereas it was suppressed by treating with ceramide liposomes.

CONCLUSIONS

LMIR3-deficient colonic mast cells were pivotal in the exacerbation of DSS-induced colitis in LMIR3(-/-) mice. Ceramide liposomes attenuated DSS-induced colitis by inhibiting ATP-mediated activation of colonic mast cells through ceraimide-LMIR3 binding.

Apoptotic epithelial cells control the abundance of Treg cells at barrier surfaces

Epithelial tissues continually undergo apoptosis. Commensal organisms that inhabit the epithelium influence tissue homeostasis, in which regulatory T cells (Treg cells) have a central role. However, the physiological importance of epithelial cell apoptosis and how the number of Treg cells is regulated are both incompletely understood. Here we found that apoptotic epithelial cells negatively regulated the commensal-stimulated proliferation of Treg cells. Gut commensals stimulated CX3CR1(+)CD103(-)CD11b(+) dendritic cells (DCs) to produce interferon-β (IFN-β), which augmented the proliferation of Treg cells in the intestine. Conversely, phosphatidylserine exposed on apoptotic epithelial cells suppressed IFN-β production by the DCs via inhibitory signaling mediated by the cell-surface glycoprotein CD300a and thus suppressed Treg cell proliferation. Our findings reveal a regulatory role for apoptotic epithelial cells in maintaining the number of Treg cell and tissue homeostasis.

Tie2 Signaling Enhances Mast Cell Progenitor Adhesion to Vascular Cell Adhesion Molecule-1 (VCAM-1) through α4β1 Integrin

Mast cell (MC) activation contributes considerably to immune responses, such as host protection and allergy. Cell surface immunoreceptors expressed on MCs play an important role in MC activation. Although various immunoreceptors on MCs have been identified, the regulatory mechanism of MC activation is not fully understood. To understand the regulatory mechanisms of MC activation, we used gene expression analyses of human and mouse MCs to identify a novel immunoreceptor expressed on MCs. We found that Tek, which encodes Tie2, was preferentially expressed in the MCs of both humans and mice. However, Tie2 was not detected on the cell surface of the mouse MCs of the peritoneal cavity, ear skin, or colon lamina propria. In contrast, it was expressed on mouse bone marrow–derived MCs and bone marrow MC progenitors (BM-MCps). Stimulation of Tie2 by its ligand angiopoietin-1 induced tyrosine phosphorylation of Tie2 in MEDMC-BRC6, a mouse embryonic stem cell-derived mast cell line, and enhanced MEDMC-BRC6 and mouse BM-MCp adhesion to vascular cell adhesion molecule-1 (VCAM-1) through α4β1 integrin. These results suggest that Tie2 signaling induces α4β1 integrin activation on BM-MCps for adhesion to VCAM-1.

The Phosphatidylserine and Phosphatidylethanolamine Receptor CD300a Binds Dengue Virus and Enhances Infection

Dengue virus (DENV) is the etiological agent of the major human arboviral disease. We previously demonstrated that the TIM and TAM families of phosphatidylserine (PtdSer) receptors involved in the phagocytosis of apoptotic cells mediate DENV entry into target cells. We show here that human CD300a, a recently identified phospholipid receptor, also binds directly DENV particles and enhances viral entry. CD300a facilitates infection of the four DENV serotypes, as well as of other mosquito-borne viruses such as West Nile virus and Chikungunya virus. CD300a acts as an attachment factor that enhances DENV internalization through clathrin-mediated endocytosis. CD300a recognizes predominantly phosphatidylethanolamine (PtdEth) and to a lesser extent PtdSer associated with viral particles. Mutation of residues in the IgV domain critical for phospholipid binding abrogate CD300a-mediated enhancement of DENV infection. Finally, we show that CD300a is expressed at the surface of primary macrophages and anti-CD300a polyclonal antibodies partially inhibited DENV infection of these cells. Overall, these data indicate that CD300a is a novel DENV binding receptor that recognizes PtdEth and PtdSer present on virions and enhance infection.

IMPORTANCE

Dengue disease, caused by dengue virus (DENV), has emerged as the most important mosquito-borne viral disease of humans and is a major global health concern. The molecular bases of DENV-host cell interactions during virus entry are poorly understood, hampering the discovery of new targets for antiviral intervention. We recently discovered that the TIM and TAM proteins, two receptor families involved in the phosphatidylserine (PtdSer)-dependent phagocytic removal of apoptotic cells, interact with DENV particles-associated PtdSer through a mechanism that mimics the recognition of apoptotic cells and mediate DENV infection. In this study, we show that CD300a, a novel identified phospholipid receptor, mediates DENV infection. CD300a-dependent DENV infection relies on the direct recognition of phosphatidylethanolamine and to a lesser extent PtdSer associated with viral particles. This study provides novel insights into the mechanisms that mediate DENV entry and reinforce the concept that DENV uses an apoptotic mimicry strategy for viral entry.