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-ß (IFN-ß), which augmented the proliferation of Treg cells in the intestine. Conversely, phosphatidylserine exposed on apoptotic epithelial cells suppressed IFN-ß 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.

Interferon-ß promotes the survival and function of induced regulatory T cells.

Type I interferons (IFNs) are pleiotropic cytokines and impact various immune cells, including regulatory T cells (Treg cells). The effect of type-I IFNs on the development and function of Treg cells is quite controversial. Here we induced Treg cells (iTreg cells) from naïve CD4+ T cells in vitro in the presence or absence of IFN-ß to elucidate its direct effect on the induction of iTreg cells. We found that IFN-ß suppressed the proliferation of iTreg cells but enhanced their expression of anti-apoptotic genes Bcl-2 and Mcl-1 during the development of iTreg cells. We also found that IFN-ß promoted suppression of conventional T cell proliferation by iTreg cells. These results suggest that IFN-ß promotes the survival and immunomodulatory function of iTreg cells.

Cutting Edge: Involvement of the Immunoreceptor CD300c2 on Alveolar Macrophages in Bleomycin-Induced Lung Fibrosis.

Idiopathic pulmonary fibrosis is a chronic, progressive, and irreversible fibrotic lung disease. Although inflammation plays a central role in the pathogenesis of idiopathic pulmonary fibrosis, how inflammatory responses are regulated remains unclear. In this article, we show that mice deficient in the immunoreceptor CD300c2 (also called MAIR-II, LMIR2, and CLM-4) showed longer survival; less collagen deposition in the lung; lower levels of neutrophil chemoattractants, such as TNF-α, CXCL1, and CCL2; and fewer neutrophils in the bronchoalveolar fluid than wild-type mice after intratracheal administration of bleomycin (BLM). We also found that BLM administration induced the release of the danger-associated molecular pattern HMGB-1, which caused CD300c2-deficient alveolar macrophages, via TLR4, to produce lower levels of neutrophil chemoattractants than wild-type alveolar macrophages. Our findings demonstrate that CD300c2 contributes to BLM-induced inflammatory responses mediated by alveolar macrophages.

Identification and isolation of splenic tissue-resident macrophage sub-populations by flow cytometry.

Tissue-resident macrophages in the spleen, including red pulp and white pulp macrophages, marginal zone macrophages (MZMs) and marginal zone metallophilic macrophages (MMMs), are highly heterogeneous as a consequence of adaptation to tissue-specific environments. Each macrophage sub-population in the spleen is usually identified based on the localization, morphology and membrane antigen expression by immunohistochemistry. However, their phenotypical and functional characteristics remain incompletely understood due to the difficulty of identification and isolation by flow cytometry. We used a cocktail of three enzymes (Collagenase D, Dispase I and DNase I), rather than traditional mechanical grinding, for isolation of each sub-population, which resulted in significant improvement of isolation of these macrophage sub-populations, particularly MZMs and MMMs, as determined by CD11bhiF4/80medTim4hi and CD11bhiF4/80medTim4med, respectively. This method should be helpful for molecular and functional characterization of each splenic resident macrophage sub-population.

Immunoreceptor CD300a on mast cells and dendritic cells regulates neutrophil recruitment in a murine model of sepsis.

Sepsis is a life-threatening syndrome caused by abnormal host immune responses against bacterial infection. Although innate immune cells are known to be important in the pathogenesis of sepsis, how their activation is regulated during sepsis remains incompletely understood. Here, we examined the role of the inhibitory immunoreceptor CD300a, which is expressed on various types of myeloid cells, in the pathogenesis of sepsis induced by cecal ligation and puncture (CLP). To this end, we used mice in which CD300a was specifically deleted on mast cells (MCs; Cd300a fl/fl Mcpt5-Cre), dendritic cells (DCs; Cd300a fl/fl Itgax-Cre), or macrophages and neutrophils (Cd300a fl/fl Lyz2-Cre). We show that mice with CD300a-deleted MCs or DCs but not macrophages survived significantly longer than did control Cd300a fl/fl mice. In addition, whereas neutrophil recruitment into the peritoneal cavity was increased within 1 h after CLP in mice with CD300a-deleted MCs, peritoneal neutrophils did not increase in number until the 12 h time point in mice with CD300a-deficient DCs. These results indicate that CD300a on MCs and DCs regulates neutrophil recruitment into the peritoneal cavity after CLP.

Involvement of CD300a Phosphatidylserine Immunoreceptor in Aluminum Salt Adjuvant-Induced Th2 Responses.

Aluminum salt (alum) has been widely used for vaccinations as an adjuvant. Alum not only enhances immunogenicity but also induces Th2 cell immune responses. However, the mechanisms of how alum enhances Th2 cell immune responses have been controversial. In an experimental allergic airway inflammation model, in which alum in conjunction with OVA Ag was i.p. injected for immunization, we found that apoptotic cells and inflammatory dendritic cells (iDC) expressing CD300a, an inhibitory immunoreceptor for phosphatidylserine (PS), significantly increased in number in the peritoneal cavity after the immunization. In contrast, apoptotic cells and iDCs were scarcely observed in the peritoneal cavity after injection of OVA alone. In CD300a-deficient mice, eosinophil infiltration in bronchoalveolar lavage fluid, serum IgE levels, and airway hyperreactivity were significantly decreased after immunization with alum plus OVA compared with wild-type mice. In vitro, iDCs purified from CD300a-deficient mice after the immunization induced significantly less IL-4 production from OT-II naive CD4(+) T cells after coculture with OVA Ag. CD300a expressed on iDCs bound PS on apoptotic cells in the peritoneal cavity after injection of OVA plus alum. Blocking CD300a interaction with PS by injection of a neutralizing anti-CD300a Ab resulted in inhibition of the development of allergic airway inflammation. These results suggest that CD300a is involved in alum-induced Th2 skewing.

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.

Identification of phosphatidylserine as a ligand for the CD300a immunoreceptor.

CD300a is a member of CD300 family molecules consisting of seven genes on human chromosome 17 and nine genes in mouse chromosome 11. CD300a has a long cytoplasmic region containing the consensus immunoreceptor tyrosine-based inhibitory motif (ITIM) sequence. Upon crosslinking with antibodies against CD300a, CD300a mediates an inhibitory signal in myeloid cells. However, the ligand for CD300a has not been identified and the physiological role of CD300a remained unclear. Here, we demonstrate that the chimeric fusion protein of CD300a extracellular domain with the Fc portion of human IgG specifically bound phosphatidylserine (PS), which is exposed on the outer leaflet of the plasma membrane of apoptotic cells. PS binding to CD300a induced SHP-1 recruitment by CD300a in mast cells in response to LPS. These results indicated that CD300a is a new PS receptor.

Protective effects of S100A8 on sepsis mortality: Links to sepsis risk in obesity and diabetes.

Obesity and diabetes are independent risk factors for death during sepsis. S100A8, an alarmin, is related to inflammation, obesity, and diabetes. Here, we examine the role of S100A8 in sepsis of obesity and diabetes models. Injection of S100A8 prolongs the survival of septic mice induced by lethal endotoxemia, Escherichia coli injection, or cecal ligation and puncture. S100A8 decrease the LPS-induced expression of proinflammatory cytokines in peritoneal macrophages by inhibiting TLR4-mediated signals in an autocrine manner. db/db, ob/ob, and western diet-fed mice demonstrate reduced upregulation of S100A8 induced by LPS treatment in both serum and peritoneal cells. These mice also show shorter survival after LPS injection, and S100A8 supplementation prolonged the survival. While myelomonocytic cells-specific S100A8-deficient mice (Lyz2 cre :S100A8 floxed/floxed ) exhibit shorter survival after LPS treatment, S100A8 supplementation prolonged the survival. Thus, myelomonocytic cell-derived S100A8 is crucial for protection from sepsis, and S100A8 supplementation improves sepsis, particularly in mice with obesity and diabetes.

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-ß-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.

Immune responses contribute to tissue injury and repair during and after ischemic stroke. However, the spatiotemporal and initiating molecular events remain incompletely understood. Here, we show that mice deficient in the phosphatidylserine receptor CD300a, which is highly expressed on brain myeloid cells including Ly6Chi monocytes, exhibited ameliorated neurological deficit after middle cerebral artery occlusion (MCAO). CD300a inhibited signaling through the CD300b-DNAX-activation protein 12 (DAP12) signaling pathway to prevent efferocytosis of apoptotic cells. Deficiency of CD300a enhanced efferocytosis by myeloid cells infiltrating the brain as early as 1 hour after MCAO and reduced release of damage-associated molecular patterns from dead cells, resulting in milder inflammation in the penumbral region. Treatment with an anti-CD300a neutralizing antibody ameliorated the neurological deficit after MCAO. These findings reveal an important role of efferocytosis in the super-acute phase of ischemic stroke pathology and identified CD300a as a target for immunotherapy in treating ischemic stroke.

Elovl6 regulates mechanical damage-induced keratinocyte death and skin inflammation.

Mechanical damage on the skin not only affects barrier function but also induces various immune responses, which trigger or exacerbate skin inflammation. However, how mechanical damage-induced skin inflammation is regulated remains incompletely understood. Here, we show that keratinocytes express the long-chain fatty-acid elongase Elovl6. Mice deficient in Elovl6 showed higher levels of cis-vaccenic acid (CVA) in the epidermis and severe skin inflammation induced by mechanical damage due to tape stripping than did wild-type mice. CVA accelerated tape stripping-triggered keratinocyte death and release of danger-associated molecular patterns (DAMPs) such as high-mobility group box 1 protein (HMGB-1) and IL-1α, which induced production of proinflammatory cytokines and chemokines IL-1ß and CXCL-1 by keratinocytes. Our results demonstrate that Elovl6 regulates mechanical damage-triggered keratinocyte death and the subsequent dermatitis.

Marginal zone B cells exacerbate endotoxic shock via interleukin-6 secretion induced by Fcα/µR-coupled TLR4 signalling.

Marginal zone (MZ) B cells produce a first wave of antibodies for protection from blood-borne pathogens. However, the role of MZ B cells in inflammatory responses has not been elucidated. Here we show that MZ B cells produce pro-inflammatory cytokines, such as interleukin-6 (IL-6), and exacerbate systemic inflammatory responses to lipopolysaccharide (LPS). After intravenous injection of LPS or E. coli, mice deficient in MZ B cells or IL-6 only in MZ B cells have attenuated systemic inflammatory responses and prolonged survival compared with wild-type mice. LPS directly stimulates MZ B cells via Toll-like receptor 4 (TLR4) and MyD88 pathways for IL-6 production. Furthermore, TLR4 requires physical and functional association with Fcα/µR (CD351) for its oligomer formation, NF-κB signalling and IL-6 production from MZ B cells; this association is responsible for systemic inflammatory responses and endotoxic shock. These results reveal a pro-inflammatory role of MZ B cells in endotoxic shock.

Isolation and characterization of naïve follicular dendritic cells.

Follicular dendritic cells (FDC) are specialized antigen-presenting cells to cognate B cells in the follicle of the lymphoid tissues. FDC also support survival and proliferation of the B cells, leading to the germinal center formation. FDC therefore play a central role in humoral immune responses. However, molecular and functional characteristics of FDC are largely unknown, because it is difficult to isolate and analyze FDC due to a very small number of FDC in the lymphoid tissues and the fragility by mechanical and chemical stresses in vitro. In this report, we established a novel method for FDC isolation from the spleen of naïve mice by flow cytometry and analyzed the phenotypical and functional characteristics. The isolated FDC, which accounted for ~0.2% of the spleen cells of naïve mice, were CD45(-), FDC-M2(+), and ICAM-1(+), and supported the survival and LPS-induced proliferation of B cells. We also showed that a neutralizing antibody against B cell activating factor TNF family (BAFF) suppressed FDC-dependent B cell proliferation in the presence of LPS, but not survival, demonstrating the evidence that FDC-derived BAFF is involved in B cell proliferation.

Apoptotic cells suppress mast cell inflammatory responses via the CD300a immunoreceptor.

When a cell undergoes apoptosis, phosphatidylserine (PS) is exposed on the outer leaflet of the plasma membrane. PS acts as an "eat-me" signal to direct phagocytes expressing PS receptors to engulf the apoptotic cell. We recently reported that the immunoreceptor CD300a, which is expressed on myeloid cells, is a PS receptor. We show that CD300a does not facilitate macrophage phagocytosis of apoptotic cells. Instead, CD300a delivers an inhibitory signal in mast cells to suppress production of LPS-induced inflammatory cytokines and chemokines. After cecal ligation and puncture (CLP), when a large number of cells undergo apoptosis in the peritoneal cavity, CD300a-deficient peritoneal mast cells produced more chemoattractant and recruited more neutrophils than did wild-type (WT) mast cells. As a result, CD300a-deficient mice showed increased neutrophil recruitment and improved bacterial clearance in the peritoneal cavity, and survived longer than WT mice. Antibody blockade of CD300a-PS interactions improved bacterial clearance and extended survival of WT mice subjected to CLP. These results indicated that CD300a is a nonphagocytic PS receptor that regulates mast cell inflammatory responses to microbial infections.

The immunoreceptor adapter protein DAP12 suppresses B lymphocyte-driven adaptive immune responses.

DAP12, an immunoreceptor tyrosine-based activation motif-bearing adapter protein, is involved in innate immunity mediated by natural killer cells and myeloid cells. We show that DAP12-deficient mouse B cells and B cells from a patient with Nasu-Hakola disease, a recessive genetic disorder resulting from loss of DAP12, showed enhanced proliferation after stimulation with anti-IgM or CpG. Myeloid-associated immunoglobulin-like receptor (MAIR) II (Cd300d) is a DAP12-associated immune receptor. Like DAP12-deficient B cells, MAIR-II-deficient B cells were hyperresponsive. Expression of a chimeric receptor composed of the MAIR-II extracellular domain directly coupled to DAP12 into the DAP12-deficient or MAIR-II-deficient B cells suppressed B cell receptor (BCR)-mediated proliferation. The chimeric MAIR-II-DAP12 receptor recruited the SH2 domain-containing protein tyrosine phosphatase 1 (SHP-1) after BCR stimulation. DAP12-deficient mice showed elevated serum antibodies against self-antigens and enhanced humoral immune responses against T cell-dependent and T cell-independent antigens. Thus, DAP12-coupled MAIR-II negatively regulates B cell-mediated adaptive immune responses.

Regulation of Immune Responses by the Activating and Inhibitory Myeloid-Associate Immunoglobuline-Like Receptors (MAIR) (CD300).

Activating and inhibitory cell surface receptors play important roles in regulation of immune responses. Recent progress has demonstrated that many inhibitory receptors pair with activating, as well as inhibitory, isoforms, both of whose genes are located in small clusters on a chromosome. We and others identified paired activating and inhibitory immunoglobulin-like receptors, designated myeloid-associated immunoglobulin-like receptors (MAIR) (CD300). MAIR is a multigene family consisting of nine genes on a small segment of mouse chromosome 11. MAIR family receptors are preferentially expressed on myeloid cells, including macrophages, dendritic cells, granulocytes, and bone-marrow-derived cultured mast cells, and a subset of B cells and regulate activation of these cells. Thus, MAIR plays an important role in innate immunity mediated by myeloid cells.

Caspase-independent cell death by CD300LF (MAIR-V), an inhibitory immunoglobulin-like receptor on myeloid cells.

The myeloid-associated Ig-like receptor family (CD300) consists of nine activating or inhibitory cell surface receptors preferentially expressed on myeloid cells and are encoded by the genes in a small cluster on mouse chromosome 11. One of the receptors, CD300LF (MAIR-V), has a long cytoplasmic tail containing two consensus ITIMs and an immunoreceptor tyrosine-based switching motif, suggesting that CD300LF regulates the activation of myeloid cells. However, the functional characteristics of this receptor are still incompletely understood. In this study, we demonstrate that cross-linking CD300LF with anti-CD300LF mAb induced cell death in peritoneal macrophages as well as in several transfectants expressing CD300LF. CD300LF-mediated cell death was dependent on the cytoplasmic region but did not require an ITIM or immunoreceptor tyrosine-based switching motif. Scanning electron microscopy revealed a loss of blebs from the surface of the dead cells mediated by CD300LF, a morphological feature similar to that observed in apoptotic cells. However, CD300LF-mediated cell death was not inhibited by a caspase inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, or autophagy inhibitors, 3-methyladenine or N-acetyl-L-cystein. Moreover, the splicing isoform of a transcription factor, X-box binding protein-1, which is produced in dead cells as a response to endoplasmic reticulum stress, was not detected. Together, these results indicate that CD300LF mediates caspase and endoplasmic reticulum stress-independent cell death by a novel mechanism.