Skin-infiltrating basophils promote atopic dermatitis-like inflammation via IL-4 production in mice.

BACKGROUND: Patients with atopic dermatitis (AD) often show the infiltration of basophils in the affected skin. Because basophils represent only a minor fraction among cellular infiltrates in the skin lesion, the functional significance of skin-infiltrating basophils in AD pathogenesis remains ill-defined. In this study, we aimed to clarify the role of basophils and their effector molecules triggering skin inflammation in oxazolone (OX)-induced murine model of AD. METHODS: A panel of mouse strains were sensitized and repeatedly challenged with topical applications of OX to induce AD-like skin inflammation. Both local and systemic Th2 immune responses were analyzed. RESULTS: Basophils progressively accumulated in the skin lesion but barely in draining lymph nodes (LNs). When basophils were depleted during the elicitation phase, skin inflammation was ameliorated while Th2 cell differentiation in draining LNs remained intact. The expression of IL-4 was highly upregulated in the affected skin, and basophils turned out to be the major producers of IL-4 among cellular infiltrates, suggesting the involvement of basophil-derived IL-4 in the Th2 skin inflammation. Indeed, basophil-specific IL-4-deficient mice displayed attenuated skin inflammation with a marked reduction of IL-4 in the skin lesion, even though cutaneous basophil infiltration and serum levels of IgE remained intact. CONCLUSIONS: Skin-infiltrating basophils promoted OX-induced AD-like skin inflammation through their local production of IL-4, rather than the induction of Th2 cell differentiation in draining LNs. This study suggests that the selective targeting of basophils could be a beneficial strategy in the treatment of a certain type of AD.

IL-3 Triggers Chronic Rejection of Cardiac Allografts by Activation of Infiltrating Basophils.

Chronic rejection is a major problem in transplantation medicine, largely resistant to therapy, and poorly understood. We have shown previously that basophil-derived IL-4 contributes to fibrosis and vasculopathy in a model of heart transplantation with depletion of CD4+ T cells. However, it is unknown how basophils are activated in the allografts and whether they play a role when cyclosporin A (CsA) immunosuppression is applied. BALB/c donor hearts were heterotopically transplanted into fully MHC-mismatched C57BL/6 recipients and acute rejection was prevented by depletion of CD4+ T cells or treatment with CsA. We found that IL-3 is significantly upregulated in chronically rejecting allografts and is the major activator of basophils in allografts. Using IL-3-deficient mice and depletion of basophils, we show that IL-3 contributes to allograft fibrosis and organ failure in a basophil-dependent manner. Also, in the model of chronic rejection involving CsA, IL-3 and basophils substantially contribute to organ remodeling, despite the almost complete suppression of IL-4 by CsA. In this study, basophil-derived IL-6 that is resistant to suppression by CsA, was largely responsible for allograft fibrosis and limited transplant survival. Our data show that IL-3 induces allograft fibrosis and chronic rejection of heart transplants, and exerts its profibrotic effects by activation of infiltrating basophils. Blockade of IL-3 or basophil-derived cytokines may provide new strategies to prevent or delay the development of chronic allograft rejection.

Pivotal role of STIM2, but not STIM1, in IL-4 production by IL-3-stimulated murine basophils.

Basophils have nonredundant roles in various immune responses that require Ca2+ influx. Here, we examined the role of two Ca2+ sensors, stromal interaction molecule 1 and 2 (STIM1 and STIM2), in basophil activation. We found that loss of STIM1, but not STIM2, impaired basophil IL-4 production after stimulation with immunoglobulin E (IgE)-containing immune complexes. In contrast, when basophils were stimulated with IL-3, loss of STIM2, but not STIM1, reduced basophil IL-4 production. This difference in STIM proteins was associated with distinct time courses of Ca2+ influx and transcription of the Il4 gene that were elicited by each stimulus. Similarly, basophil-specific STIM1 expression was required for IgE-driven chronic allergic inflammation in vivo, whereas STIM2 was required for IL-4 production after combined IL-3 and IL-33 treatment in mice. These data indicate that STIM1 and STIM2 have differential roles in the production of IL-4, which are stimulus dependent. Furthermore, these results illustrate the vital role of STIM2 in basophils, which is often considered to be less important than STIM1.

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.

Aggregation makes a protein allergenic at the challenge phase of basophil-mediated allergy in mice.

A hapten is a small molecule that is not immunogenic on its own but can stimulate the production of antibodies at the sensitization phase when conjugated to carrier proteins. The hapten then reacts specifically with the antibodies generated against it to elicit an immune or allergic response at the challenge phase. Here, we compared various carrier proteins conjugated with the same hapten in their ability to induce hapten-specific IgE-mediated allergic responses in vitro and in vivo, and characterized the nature of carrier proteins that determines the magnitude of response at the challenge phase of allergic reactions. Hapten 2,4,6-trinitrophenol (TNP)-conjugated ovalbumin (TNP-OVA) and bovine serum albumin (TNP-BSA) elicited TNP-specific, mast cell-dependent, immediate-type allergic reactions at a comparable level in mice that had been passively sensitized with TNP-specific IgE. In contrast, TNP-OVA but not TNP-BSA efficiently induced a basophil-dependent, IgE-mediated chronic allergic inflammation (IgE-CAI), even though both proteins could stimulate basophils in vitro at a comparable level. By comparing different carrier proteins and structurally modifying them, we found that the formation of large aggregates is crucial for TNP-conjugated carrier proteins to efficiently elicit IgE-CAI, regardless of the type of protein. Thus, the aggregation status of carrier proteins appears to determine the magnitude of allergic response at the challenge phase of hapten-specific IgE-CAI. Our findings suggest that the allergenicity of substances is a matter of importance not only at the sensitization but also at the challenge phase in a certain type of allergy including a basophil-mediated allergic inflammation.

Basophils trigger emphysema development in a murine model of COPD through IL-4-mediated generation of MMP-12-producing macrophages.

Chronic obstructive pulmonary disease (COPD) is a leading cause of morbidity and mortality worldwide. It has generally been considered a non-Th2-type lung disorder, characterized by progressive airflow limitation with inflammation and emphysema, but its cellular and molecular mechanism remains ill defined, compared with that of asthma characterized by reversible airway obstruction. Here we show a previously unappreciated role for basophils at the initiation phase of emphysema formation in an elastase-induced murine model of COPD in that basophils represent less than 1% of lung-infiltrating cells. Intranasal elastase instillation elicited the recruitment of monocytes to the lung, followed by differentiation into interstitial macrophages (IMs) but rarely alveolar macrophages (AMs). Matrix metalloproteinase-12 (MMP-12) contributing to emphysema formation was highly expressed by IMs rather than AMs, in contrast to the prevailing assumption. Experiments using a series of genetically engineered mice suggested that basophil-derived IL-4, a Th2 cytokine, acted on lung-infiltrating monocytes to promote their differentiation into MMP-12-producing IMs that resulted in the destruction of alveolar walls and led to emphysema development. Indeed, mice deficient for IL-4 only in basophils failed to generate pathogenic MMP-12-producing IMs and hence develop emphysema. Thus, the basophil-derived IL-4/monocyte-derived IM/MMP-12 axis plays a crucial role in emphysema formation and therefore may be a potential target to slow down emphysema progression at the initiation phase of COPD.

How do basophils contribute to Th2 cell differentiation and allergic responses?

Basophils and mast cells share some features, including basophilic granules in the cytoplasm, cell surface expression of the high-affinity IgE receptor and release of chemical mediators such as histamine. Because of this similarity and their minority status, basophils had often been erroneously considered as minor relatives or blood-circulating precursors of tissue-resident mast cells, and therefore long been neglected or underestimated in immunological studies. Taking advantage of newly developed tools, such as basophil-depleting antibodies and engineered mice deficient for only basophils, recent studies have identified previously unappreciated roles for basophils, distinct from those played by mast cells, in allergic responses, protective immunity against parasitic infections and regulation of other immune cells. In this review, we focus on two topics that we presented and discussed in the 46th Annual Meeting of the Japanese Society for Immunology held in Sendai in December 2017. The first topic is the function of basophils as antigen-presenting cells for driving Th2 cell differentiation. We demonstrated that basophils produce few or no MHC class II (MHC-II) proteins by themselves although they can acquire peptide-MHC-II complexes from dendritic cells through trogocytosis, and present them and provide IL-4 to naive CD4 T cells, promoting Th2 cell differentiation. The second topic is the basophil-specific effector molecules involved in allergic responses. Among mouse mast cell proteases (mMCPs), mMCP-8 and mMCP-11 are expressed almost exclusively by basophils. Analyses in vitro and in vivo revealed that both proteases can induce leukocyte migration through distinct mechanisms, contributing to the development of basophil-dependent allergic inflammation.

Histamine Released From Skin-Infiltrating Basophils but Not Mast Cells Is Crucial for Acquired Tick Resistance in Mice.

Ticks are blood-feeding arthropods that can transmit pathogens to humans and animals, leading to serious infectious diseases such as Lyme disease. After single or multiple tick infestation, some animal species develop resistance to tick feeding, leading to reduced risk of pathogen transmission. In mice infested with larval Haemaphysalis longicornis ticks, both mast cells and basophils reportedly play key roles in the manifestation of acquired tick resistance (ATR), but it remains ill-defined how they contribute to it. Here, we investigated their products responsible for ATR. Treatment of mice with antihistamine abolished the ATR while histamine or histamine H1 receptor agonist reduced tick-feeding even in the first infestation. In accordance with these, mice deficient for histamine production showed little or no ATR, indicating the crucial role for histamine in the expression of ATR. Adoptive transfer of mast cells and basophils derived from histamine-sufficient or deficient mice to recipient mice lacking mast cells and basophils, respectively, revealed that histamine produced by basophils but not mast cells is essential for the manifestation of ATR, in contrast to the case of local and systemic anaphylaxis where mast cell-derived histamine is the major player. During the second but not first tick infestation, basophils accumulated and made a cluster, surrounding a tick mouthpart, in the epidermis whereas mast cells were scattered and localized mainly in the dermis, more distantly from a tick mouthpart. This appears to explain why basophil-derived histamine is much more effective than mast cell-derived one. Histamine-sufficient, but not -deficient mice showed the thickened epidermis at the second tick-feeding site. Taken together, histamine released from skin-infiltrating basophils rather than skin-resident mast cells plays a crucial role in the manifestation of ATR, perhaps through promotion of epidermal hyperplasia that may inhibit tick feeding.

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-6Clow/int peripheral blood monocytes, corresponding to CD14dim/+CD16+ human nonclassical and intermediate monocytes. Loss of FcRγ or DAP12 lowered the surface expression of endogenous mCD300e in the CD115+Ly-6Clow/int monocytes. Stimulation with sphingomyelin failed to activate the CD115+Ly-6Clow/int mouse monocytes, but induced hCD300e-mediated cytokine production in the CD14dimCD16+ human monocytes. Taken together, these observations indicate that mCD300e recognizes sphingomyelin and thereby regulates nonclassical and intermediate monocyte functions through FcRγ and DAP12.

Multifaceted roles of basophils in health and disease.

Until recently, basophils had often been neglected in immunologic studies because of their minority status among immune cells or confused with tissue-resident mast cells because of some phenotypic similarities between them in spite of different anatomic localization. It is now appreciated that basophils and mast cells are distinct cell lineages and that basophils play important and nonredundant roles distinct from those played by mast cells. On the one hand, basophils contribute beneficially to protective immunity, particularly against parasitic infections. On the other hand, basophils are involved in the development of various disorders, including allergy and autoimmune disease. Basophils interact with other immune cells and nonhematopoietic cells through cell-to-cell contact or basophil-derived factors, such as cytokines and proteases, contributing to the regulation of immune and allergic responses. In this review article we highlight recent advances in our understanding of basophil pathophysiology in human subjects and animal models by consolidating research findings reported during the past 5 years. Further studies on basophils and their products will help identify suitable targets for novel therapeutics in allergy and effective vaccines against parasitic infection.

Skin CD4+ Memory T Cells Play an Essential Role in Acquired Anti-Tick Immunity through Interleukin-3-Mediated Basophil Recruitment to Tick-Feeding Sites.

Ticks, blood-sucking arthropods, serve as vectors for transmission of infectious diseases including Lyme borreliosis. After tick infestation, several animal species can develop resistance to subsequent infestations, reducing the risk of transmission. In a mouse model, basophils reportedly infiltrate tick-feeding sites during the second but not first infestation and play a crucial role in the expression of acquired tick resistance. However, the mechanism underlying basophil recruitment to the second tick-feeding site remains ill-defined. Here, we investigated cells and their products responsible for the basophil recruitment. Little or no basophil infiltration was detected in T-cell-deficient mice, and adoptive transfer of CD4+ but not CD8+ T cells reconstituted it. Il3 gene expression was highly upregulated at the second tick-feeding site, and adoptive transfer of interleukin-3 (IL-3)-sufficient but not IL-3-deficient CD4+ T cells conferred the basophil infiltration on T-cell-deficient mice, indicating that the CD4+ T-cell-derived IL-3 is essential for the basophil recruitment. Notably, IL-3+ resident CD4+ memory T cells were detected even before the second infestation in previously uninfested skin distant from the first tick-feeding site. Taken together, IL-3 produced locally by skin CD4+ memory T cells appears to play a crucial role in basophil recruitment to the second tick-feeding site.

Differential usage of COX-1 and COX-2 in prostaglandin production by mast cells and basophils.

Basophils have been erroneously considered as minor relatives of mast cells, due to some phenotypic similarity between them. While recent studies have revealed non-redundant roles for basophils in various immune responses, basophil-derived effector molecules, including lipid mediators, remain poorly characterized, compared to mast cell-derived ones. Here we analyzed and compared eicosanoids produced by mouse basophils and mast cells when stimulated with IgE plus allergens. The production of 5-LOX metabolites such as LTB4 and 5-HETE was detected as early as 0.5 h post-stimulation in both cell types, even though their amounts were much smaller in basophils than in mast cells. In contrast, basophils and mast cells showed distinct time course in the production of COX metabolites, including PGD2, PGE2 and 11-HETE. Their production by mast cells was detected at both 0.5 and 6 h post-stimulation while that by basophils was detectable only at 6 h. Of note, mast cells showed 8-9 times higher levels of COX-1 than did basophils at the resting status. In contrast to unaltered COX-1 expression with or without stimulation, COX-2 expression was up-regulated in both cell types upon activation. Importantly, when activated, basophils expressed 4-5 times higher levels of COX-2 than did mast cells. In accordance with these findings, the late-phase production of the COX metabolites by basophils was completely ablated by COX-2 inhibitor whereas the early-phase production by mast cells was blocked by COX-1 but not COX-2 inhibitor. Thus, the production of COX metabolites is differentially regulated by COX-1 and COX-2 in basophils and mast cells.

Recent advances in understanding basophil-mediated Th2 immune responses.

Basophils, the least common granulocytes, represent only ~0.5% of peripheral blood leukocytes. Because of the small number and some similarity with mast cells, the functional significance of basophils remained questionable for a long time. Recent studies using newly-developed analytical tools have revealed crucial and non-redundant roles for basophils in various immune responses, particularly Th2 immunity including allergy and protective immunity against parasitic infections. In this review, we discuss the mechanisms how basophils mediate Th2 immune responses and the nature of basophil-derived factors involved in them. Activated basophils release serine proteases, mouse mast cell protease 8 (mMCP-8), and mMCP-11, that are preferentially expressed by basophils rather than mast cells in spite of their names. These proteases elicit microvascular hyperpermeability and leukocyte infiltration in affected tissues, leading to inflammation. Basophil-derived IL-4 also contributes to eosinophil infiltration while it acts on tissue-infiltrating inflammatory monocytes to promote their differentiation into M2 macrophages that in turn dampen inflammation. Although basophils produce little or no MHC class II (MHC-II) proteins, they can acquire peptide-MHC-II complexes from dendritic cells via trogocytosis and present them together with IL-4 to naive CD4 T cells, leading to Th2 cell differentiation. Thus, basophils contribute to Th2 immunity at various levels.

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.

Trogocytosis of peptide-MHC class II complexes from dendritic cells confers antigen-presenting ability on basophils.

Th2 immunity plays important roles in both protective and allergic responses. Nevertheless, the nature of antigen-presenting cells responsible for Th2 cell differentiation remains ill-defined compared with the nature of the cells responsible for Th1 and Th17 cell differentiation. Basophils have attracted attention as a producer of Th2-inducing cytokine IL-4, whereas their MHC class II (MHC-II) expression and function as antigen-presenting cells are matters of considerable controversy. Here we revisited the MHC-II expression on basophils and explored its functional relevance in Th2 cell differentiation. Basophils generated in vitro from bone marrow cells in culture with IL-3 plus GM-CSF displayed MHC-II on the cell surface, whereas those generated in culture with IL-3 alone did not. Of note, these MHC-II-expressing basophils showed little or no transcription of the corresponding MHC-II gene. The GM-CSF addition to culture expanded dendritic cells (DCs) other than basophils. Coculture of basophils and DCs revealed that basophils acquired peptide-MHC-II complexes from DCs via cell contact-dependent trogocytosis. The acquired complexes, together with CD86, enabled basophils to stimulate peptide-specific T cells, leading to their proliferation and IL-4 production, indicating that basophils can function as antigen-presenting cells for Th2 cell differentiation. Transfer of MHC-II from DCs to basophils was also detected in draining lymph nodes of mice with atopic dermatitis-like skin inflammation. Thus, the present study defined the mechanism by which basophils display MHC-II on the cell surface and appears to reconcile some discrepancies observed in previous studies.

The Basophil-specific Protease mMCP-8 Provokes an Inflammatory Response in the Skin with Microvascular Hyperpermeability and Leukocyte Infiltration.

Basophils have often been erroneously considered to be minor relatives or blood-circulating precursors of tissue-resident mast cells because of some phenotypic similarity between them, including basophilic secretory granules in the cytoplasm. However, recent studies revealed that the repertoire of serine proteases stored in secretory granules is distinct in them. Particularly, mouse mast cell protease 8 (mMCP-8) is specifically expressed by basophils but not mast cells despite its name. Therefore, mMCP-8 is commonly used as a basophil-specific marker, but its functional property remains uncertain. Here we prepared recombinant mMCP-8 and examined its activity in vitro and in vivo Purified recombinant mMCP-8 showed heat-sensitive proteolytic activity when α-tubulin was used as a substrate. One intradermal shot of mMCP-8, not heat-inactivated, induced cutaneous swelling with increased microvascular permeability in a cyclooxygenase-dependent manner. Moreover, repeated intradermal injection of mMCP-8 promoted skin infiltration of leukocytes, predominantly neutrophils and, to a lesser extent, monocytes and eosinophils, in conjunction with up-regulation of chemokine expression in the skin lesion. These results suggest that mMCP-8 is an important effector molecule in basophil-elicited inflammation, providing novel insights into how basophils exert a crucial and non-redundant role, distinct from that played by mast cells, in immune responses.

Novel CD200 homologues iSEC1 and iSEC2 are gastrointestinal secretory cell-specific ligands of inhibitory receptor CD200R.

CD200R is an inhibitory receptor expressed on myeloid cells and some lymphoid cells, and plays important roles in negatively regulating immune responses. CD200 is the only known ligand of CD200R and broadly distributed in a variety of cell types. Here we identified novel CD200 homologues, designated iSEC1 and iSEC2, that are expressed exclusively by secretory cell lineages in the gastrointestinal epithelium while authentic CD200 is expressed by none of epithelial cells including secretory cells. Both iSEC1 and iSEC2 could bind to CD200R but not other members of the CD200R family. Notably, CD200R expression was confined to intraepithelial lymphocytes (IELs) among cells in the gastrointestinal epithelium. Binding of iSEC1 to CD200R on IELs resulted in the suppression of cytokine production and cytolytic activity by activated IELs. Thus, iSEC1 is a previously unappreciated CD200R ligand with restricted expression in gastrointestinal secretory cells and may negatively regulate mucosal immune responses.

Basophil tryptase mMCP-11 plays a crucial role in IgE-mediated, delayed-onset allergic inflammation in mice.

Recent studies have identified nonredundant roles for basophils in immune responses including allergy and protective immunity. It is well known that activated basophils release granule contents such as histamine and proteases as do mast cells. However, the functional significance of basophil-derived proteases remains poorly understood in contrast to those released from mast cells. For this study we generated a line of knockout (KO) mice deficient for mouse mast cell protease-11 (mMCP-11) that is preferentially expressed by basophils rather than mast cells. In spite of normal development of basophils, the mMCP-11-deficient mice showed amelioration of immunoglobulin E-mediated chronic allergic inflammation (IgE-CAI), with reduction of cutaneous swelling, microvascular permeability, and leukocyte infiltration in the skin lesion, when KO mice were compared with wild-type mice. Repeated administration of recombinant mMCP-11 in the skin induced infiltration of leukocytes, including basophils, in a tryptase activity-dependent manner. The transwell migration assay in vitro suggested that mMCP-11-mediated proteolytic products of serum protein promoted migration of basophils, eosinophils, and macrophages via 1 or more G protein-coupled receptors. Thus, basophil tryptase mMCP-11 is a crucial effector molecule for the induction of IgE-CAI. This is the first demonstration that the basophil-derived protease plays a significant role in vivo.

Real-time imaging of mast cell degranulation in vitro and in vivo.

Mast cells undergo degranulation in response to various stimuli and rapidly release pre-formed mediators present in secretory granules, leading to immediate-type allergic reactions. Mast cell degranulation is commonly detected and quantified in vitro by measuring histamine or ß-hexosaminidase released to culture medium. However, this type of assay cannot monitor degranulation of individual cells in real time, and it is not suitable for in vivo detection of degranulation. At the aim of real time imaging of mast cell degranulation at single cell level, we here developed a fluorescent protein-based indicator of degranulation, designated immuno-pHluorin (impH). When expressed in mast cells, impH is located in the membrane of secretory granules and non-fluorescent under homeostatic conditions while it turns fluorescent following degranulation, due to the pH change inside of granules during exocytosis. impH enabled us to detect polarized degranulation within one single cell when mast cells were stimulated via the small area of cell surface. Transplantation of impH-expressing mast cells into mast cell-deficient mice demonstrated that impH could function as a real-time indicator of degranulation in vivo. Thus, impH is a useful tool for imaging of mast cell activation and degranulation in vitro and in vivo, and may be applied for screening of reagents regulating mast cell degranulation.