Mast cells acquire monocyte-specific gene expression and monocyte-like morphology by overproduction of PU.1

ETS Transcription Factors in Immune Cells and Immune-Related Diseases

The development, differentiation, and function of immune cells are precisely regulated by transcription factors. The E26 transformation-specific (ETS) transcription factor family is involved in various physiological and pathological processes by regulating cell proliferation, differentiation, and apoptosis. Emerging evidence has suggested that ETS family proteins are intimately involved in the development and function of immune cells. This review summarizes the role of the ETS family in immune cells and immune-related disorders. Seven transcription factors within the ETS family, including PU.1, ETV5, ETV6, ETS1/2, ELK3, and ELF1, play essential roles in the development and function of T cells, B cells, macrophages, neutrophils, and dendritic cells. Furthermore, they are involved in the occurrence and development of immune-related diseases, including tumors, allergies, autoimmune diseases, and arteriosclerosis. This review is conducive to a comprehensive overview of the role of the ETS family in immune cells, and thus is informative for the development of novel therapeutic strategies targeting the ETS family for immune-related diseases.

Butyrate, Valerate, and Niacin Ameliorate Anaphylaxis by Suppressing IgE-Dependent Mast Cell Activation: Roles of GPR109A, PGE2, and Epigenetic Regulation

Short-chain fatty acids (SCFAs) are produced by the intestinal microbiota during the fermentation of dietary fibers as secondary metabolites. Several recent studies reported that SCFAs modulate the development and function of immune-related cells. However, the molecular mechanisms by which SCFAs regulate mast cells (MCs) remain unclear. In the current study, we analyzed the function and gene expression of mouse MCs in the presence of SCFAs in vitro and in vivo. We found that the oral administration of valerate or butyrate ameliorated passive systemic anaphylaxis and passive cutaneous anaphylaxis in mice. The majority of SCFAs, particularly propionate, butyrate, valerate, and isovalerate, suppressed the IgE-mediated degranulation of bone marrow-derived MCs, which were eliminated by the Gi protein inhibitor pertussis toxin and by the knockdown of Gpr109a. A treatment with the HDAC inhibitor trichostatin A also suppressed IgE-mediated MC activation and reduced the surface expression level of FcεRI on MCs. Acetylsalicylic acid and indomethacin attenuated the suppressive effects of SCFAs on degranulation. The degranulation degree was significantly reduced by PGE2 but not by PGD2. Furthermore, SCFAs enhanced PGE2 release from stimulated MCs. The SCFA-mediated amelioration of anaphylaxis was exacerbated by COX inhibitors and an EP3 antagonist, but not by an EP4 antagonist. The administration of niacin, a ligand of GPR109A, alleviated the symptoms of passive cutaneous anaphylaxis, which was inhibited by cyclooxygenase inhibitors and the EP3 antagonist. We conclude that SCFAs suppress IgE-mediated activation of MCs in vivo and in vitro involving GPR109A, PGE2, and epigenetic regulation.

Distinct inflammatory macrophage populations sequentially infiltrate bone‐to‐tendon interface tissue after ACL reconstruction surgery in mice

Macrophages are important for repair of injured tissues, but their role in healing after surgical repair of musculoskeletal tissues is not well understood. We used single‐cell RNA sequencing (RNA‐seq), flow cytometry, and transcriptomics to characterize functional phenotypes of macrophages in a mouse anterior cruciate ligament reconstruction (ACLR) model that involves bone injury followed by a healing phase of bone and fibrovascular interface tissue formation that results in bone‐to‐tendon attachment. We identified a novel “surgery‐induced” highly inflammatory CD9+ IL1+ macrophage population that expresses neutrophil‐related genes, peaks 1 day after surgery, and slowly resolves while transitioning to a more homeostatic phenotype. In contrast, CX3CR1+ CCR2+ macrophages accumulated more slowly and unexpectedly expressed an interferon signature, which can suppress bone formation. Deletion of Ccr2 resulted in an increased amount of bone in the surgical bone tunnel at the tendon interface, suggestive of improved healing. The “surgery‐induced macrophages” identify a new cell type in the early phase of inflammation related to bone injury, which in other tissues is dominated by blood‐derived neutrophils. The complex patterns of macrophage and inflammatory pathway activation after ACLR set the stage for developing therapeutic strategies to target specific cell populations and inflammatory pathways to improve surgical outcomes. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

FBW7 Inhibits Myeloid Differentiation in Acute Myeloid Leukemia via GSK3-Dependent Ubiquitination of PU.1

Glycogen synthase kinase 3β (GSK3β), an ubiquitously expressed serine/threonine kinase is reported to be overexpressed and hyperactivated in cancers including acute myeloid leukemia (AML) where it promotes self-renewal, growth, and survival of AML cells. Therefore, GSK3β inhibition results in AML cell growth inhibition and myeloid differentiation. Here we identified master transcription factor PU.1 of monocyte-macrophage differentiation pathway as potential GSK3β target. We demonstrate that GSK3β phosphorylates PU.1 at Ser41 and Ser140 leading to its recognition and subsequent ubiquitin-mediated degradation by E3 ubiquitin ligase FBW7. This GSK3-dependent degradation of PU.1 by FBW7 inhibited monocyte-macrophage differentiation. We further showed that a phospho-deficient PU.1 mutant (PU.1-S41, S140A) neither bound to FBW7 nor was degraded by it. Consequently, PU.1-S41, S140A retained its transactivation, DNA-binding ability and promoted monocyte-macrophage differentiation of U937 cells even without phorbol 12-myristate 13-acetate (PMA) treatment. We further showed that FBW7 overexpression inhibited both PMA as well as M-CSF-induced macrophage differentiation of myeloid cell lines and peripheral blood mononuclear cells (PBMC) from healthy volunteers, respectively. Contrarily, FBW7 depletion promoted differentiation of these cells even without any inducer suggesting for a robust role of GSK3β-FBW7 axis in negatively regulating myeloid differentiation. Furthermore, we also recapitulated these findings in PBMCs isolated from patients with leukemia where FBW7 overexpression markedly inhibited endogenous PU.1 protein levels. In addition, PBMCs also showed enhanced differentiation when treated with M-CSF and GSK3 inhibitor (SB216763) together compared with M-CSF treatment alone. IMPLICATIONS: Our data demonstrate a plausible mechanism behind PU.1 restoration and induction of myeloid differentiation upon GSK3β inhibition and further substantiates potential of GSK3β as a therapeutic target in AML.

Lung Function in African American Children with Asthma Is Associated with Novel Regulatory Variants of the KIT Ligand KITLG/SCF and Gene-By-Air-Pollution Interaction

Baseline lung function, quantified as forced expiratory volume in the first second of exhalation (FEV1), is a standard diagnostic criterion used by clinicians to identify and classify lung diseases. Using whole-genome sequencing data from the National Heart, Lung, and Blood Institute Trans-Omics for Precision Medicine project, we identified a novel genetic association with FEV1 on chromosome 12 in 867 African American children with asthma (P = 1.26 × 10-8, β = 0.302). Conditional analysis within 1 Mb of the tag signal (rs73429450) yielded one major and two other weaker independent signals within this peak. We explored statistical and functional evidence for all variants in linkage disequilibrium with the three independent signals and yielded nine variants as the most likely candidates responsible for the association with FEV1 Hi-C data and expression QTL analysis demonstrated that these variants physically interacted with KITLG (KIT ligand, also known as SCF), and their minor alleles were associated with increased expression of the KITLG gene in nasal epithelial cells. Gene-by-air-pollution interaction analysis found that the candidate variant rs58475486 interacted with past-year ambient sulfur dioxide exposure (P = 0.003, β = 0.32). This study identified a novel protective genetic association with FEV1, possibly mediated through KITLG, in African American children with asthma. This is the first study that has identified a genetic association between lung function and KITLG, which has established a role in orchestrating allergic inflammation in asthma.

The hematopoietic cell-specific transcription factor PU.1 is critical for expression of CD11c

PU.1 is a hematopoietic cell-specific transcription factor belonging to the Ets family, which plays an important role in the development of dendritic cells (DCs). CD11c (encoded by Itgax) is well established as a characteristic marker of hematopoietic lineages including DCs. In the present study, we analyzed the role of PU.1 (encoded by Spi-1) in the expression of CD11c. When small interfering RNA (siRNA) for Spi-1 was introduced into bone marrow-derived DCs (BMDCs), the mRNA level and cell surface expression of CD11c were dramatically reduced. Using reporter assays, the TTCC sequence at -56/-53 was identified to be critical for PU.1-mediated activation of the promoter. An EMSA showed that PU.1 directly bound to this region. ChIP assays demonstrated that a significant amount of PU.1 bound to this region on chromosomal DNA in BMDCs, which was decreased in LPS-stimulated BMDCs in accordance with the reduced levels of mRNAs of Itgax and Spi-1, and the histone acetylation degree. Enforced expression of exogenous PU.1 induced the expression of the CD11c protein on the cell surface of mast cells, whereas control transfectants rarely expressed CD11c. Quantitative RT-PCR also showed that the expression of a transcription factor Irf4, which is a partner molecule of PU.1, was reduced in PU.1-knocked down BMDCs. IRF4 transactivated the Itgax gene in a synergistic manner with PU.1. Taken together, these results indicate that PU.1 functions as a positive regulator of CD11c gene expression by directly binding to the Itgax promoter and through transactivation of the Irf4 gene.

Role of PU.1 in MHC Class II Expression via CIITA Transcription in Plasmacytoid Dendritic Cells

The cofactor CIITA is a master regulator of MHC class II expression and several transcription factors regulating the cell type-specific expression of CIITA have been identified. Although the MHC class II expression in plasmacytoid dendritic cells (pDCs) is also mediated by CIITA, the transcription factors involved in the CIITA expression in pDCs are largely unknown. In the present study, we analyzed the role of a hematopoietic lineage-specific transcription factor, PU.1, in CIITA transcription in pDCs. The introduction of PU.1 siRNA into mouse pDCs and a human pDC cell line, CAL-1, reduced the mRNA levels of MHC class II and CIITA. When the binding of PU.1 to the 3rd promoter of CIITA (pIII) in CAL-1 and mouse pDCs was analyzed by a chromatin immunoprecipitation assay, a significant amount of PU.1 binding to the pIII was detected, which was definitely decreased in PU.1 siRNA-transfected cells. Reporter assays showed that PU.1 knockdown reduced the pIII promoter activity and that three Ets-motifs in the human pIII promoter were candidates of cis-enhancing elements. By electrophoretic mobility shift assays, it was confirmed that two Ets-motifs, GGAA (-181/-178) and AGAA (-114/-111), among three candidates, were directly bound with PU.1. When mouse pDCs and CAL-1 cells were stimulated by GM-CSF, mRNA levels of PU.1, pIII-driven CIITA, total CIITA, MHC class II, and the amount of PU.1 binding to pIII were significantly increased. The GM-CSF-mediated up-regulation of these mRNAs was canceled in PU.1 siRNA-introduced cells. Taking these results together, we conclude that PU.1 transactivates the pIII through direct binding to Ets-motifs in the promoter in pDCs.

IL-33 promotes MHC class II expression in murine mast cells

Mast cells (MCs), recognized as tissue-resident cells of hematopoietic origin, are involved in cellular and pathological manifestations of allergic disorders including atopic dermatitis. IL-33, a member of the IL-1 cytokine family, activates Th2-type immune responses, and promotes the degranulation and maturation of MCs. However, it is uncertain whether IL-33 treatment induces mature mast cells to acquire the characteristics of the monocyte-dendritic cell lineage.We investigated the effect of IL-33 on the MHC class II expression and function of murine mast cells. IL-33-treated mature murine bone marrow-derived mast cells (BMMCs) were analyzed by FACS, real-time PCR, chromatin immunoprecipitation (ChIP) assay, and Western blotting. The morphology and degranulation activity of BMMCs and T-cell activation by BMMCs were also examined. BMMCs treated with IL-33 for 10 days induced cell surface expression of the MHC class II protein, whereas the expression of FcεRI and c-kit was not affected by IL-33. The expression of CIITA, driven from pIII and pIV, was up-regulated in IL-33-treated BMMCs. The amount of PU.1 mRNA and protein significantly increased in IL-33-treated BMMCs. The ChIP assay showed PU.1 binding to CIITA pIII, and enhanced histone acetylation due to IL-33 treatment. Syngeneic T cells were activated by co-culture with IL-33-treated BMMCs, although the expression of the co-stimulatory molecules, CD40, CD80, CD86, and PDL-1, was not detected. Mast cells express MHC class II after prolonged exposure to IL-33, probably due to enhanced recruitment of PU.1 to CIITA pIII, resulting in transactivation of CIITA and MHC class II. IL-33 is an important cytokine in allergic disorders. Mast cells have the ability to express MHC class II after prolonged exposure to IL-33 in a murine model. IL-33 holds a key to understanding the etiology of atopic dermatitis.

Inhibitory Effect of Polyphenol-Enriched Apple Extracts on Mast Cell Degranulationin VitroTargeting the Binding between IgE and FcεRI

Extracts from immature fruit of the apple (Rosaceae, Malus sp.), which contain procyanidins (polymers of catechins) as the major ingredients, are known to inhibit histamine release from mast cells. We analyzed in this study the mechanism for the anti-allergic activity of two polyphenol-enriched apple extracts. These extracts, termed "crude apple polyphenol (CAP)" and "apple condensed tannin (ACT)", reduced the degranulation of mast cells caused by cross-linking of the high-affinity receptor for IgE (FcepsilonRI) with IgE and the antigen in a dose-dependent manner. Furthermore, western blotting revealed that phosphorylation of the intracellular signal-transduction molecules caused by cross-linking of FcepsilonRI was markedly decreased by the addition of CAP or ACT. We then analyzed the effects of CAP and ACT on the binding of the IgE antibody to FcepsilonRI on mast cells, which is the first key step in the allergic reaction mediated by mast cells, and found that this binding was markedly inhibited by both CAP and ACT. These results indicate that the inhibition of binding between FcepsilonRI and IgE by either CAP or ACT was the probable cause of the suppression of mast cell activation. This is the first report demonstrating the molecular mechanism for the anti-allergic effect of procyanidin-enriched extracts from apples.

Molecular Mechanism of Allergy-Related Gene Regulation and Hematopoietic Cell Development by Transcription Factors

Transcriptional regulation for the genes encoding alpha- and beta-chains of the high-affinity receptor for IgE (FcepsilonRI) have been analyzed in mast cells and regulatory mechanisms are beginning to be elucidated. Transcription factors GATA-1 and PU.1 cooperatively transactivate the alpha-chain gene, and three transcription factors, GATA-1, Oct-1, and MZF-1, are involved in regulation of beta-chain gene expression. No single nucleotide polymorphisms (SNPs) that are functionally related to the allergic diseases have been identified in coding regions of the alpha- and beta-chain genes in a definitive way. However, recent studies on SNPs in the promoter regions have revealed that these genes are probable candidates for new types of allergy-related genes whose transcription levels are affected by transcription factors which discriminate SNPs in the promoters. Another interesting finding on transcription factors functioning in mast cells is that the expression level of PU.1 determines cell fate between mast cells and monocytes.

Transcriptional regulation of the mouse CD11c promoter by AP-1 complex with JunD and Fra2 in dendritic cells

CD11c, a member of the β(2) integrin family of adhesion molecule, is expressed on the surface of myeloid lineages and activated lymphoid cells and forms a heterodimeric receptor with CD18. We analyzed the mouse CD11c promoter structure to elucidate the transcriptional regulation in dendritic cells (DCs). By reporter assay, the -84/-65 region was identified to be essential for activity of the mouse CD11c promoter in the mouse bone marrow-derived (BM) DCs and monocyte cell line RAW264.7. An electrophoretic mobility shift assay using a number of antibodies against transcription factors revealed that the target region was recognized by a complex including JunD and Fra2, which are transcription factors belonging to the AP-1 family. The direct interaction of JunD and Fra2 with the CD11c promoter was further confirmed by a chromatin immunoprecipitation assay using CD11c-positive cells purified from BMDCs. Finally, mouse JunD and/or Fra2 siRNA was introduced into BMDCs to evaluate the involvement of these factors against CD11c transcription and found that Fra2 siRNA reduced cell surface expression level of CD11c. These results indicate that AP-1 composed with JunD and Fra2 protein plays a primary role in enhancing the transcription level of the CD11c gene in DC.

CD11c+ cells are required for antigen-induced increase of mast cells in the lung

Patients with allergic asthma have more lung mast cells, which likely worsens the symptoms. In experimental asthma, CD11c(+) cells have to be present during the challenge phase for several features of allergic inflammation to occur. Whether CD11c(+) cells play a role for Ag-induced increases of lung mast cells is unknown. In this study, we used diphtheria toxin treatment of sensitized CD11c-diphtheria toxin receptor transgenic mice to deplete CD11c(+) cells. We demonstrate that recruitment of mast cell progenitors to the lung is substantially reduced when CD11c(+) cells are depleted during the challenge phase. This correlated with an impaired induction of endothelial VCAM-1 and led to a significantly reduced number of mature mast cells 1 wk after challenge. Collectively, these data suggest that Ag challenge stimulates CD11c(+) cells to produce cytokines and/or chemokines required for VCAM-1 upregulation on the lung endothelium, which in turn is crucial for the Ag-induced mast cell progenitor recruitment and the increase in mast cell numbers.

Role of PU.1 in MHC class II expression through transcriptional regulation of class II transactivator pI in dendritic cells

BACKGROUND

PU.1 is a hematopoietic cell-specific transcription factor belonging to the Ets family. We hypothesized that PU.1 is involved in MHC class II expression in dendritic cells (DCs).

OBJECTIVE

The role of PU.1 in MHC class II expression in DCs was analyzed.

METHODS

Transcriptional regulation of the DC-specific pI promoter of the class II transactivator (CIITA) gene and subsequent MHC class II expression was investigated by using PU.1 small interfering RNA (siRNA) and reporter, chromatin immunoprecipitation, and electrophoretic mobility shift assays.

RESULTS

PU.1 siRNA introduction suppressed MHC class II expression, allogeneic and syngeneic T-cell activation activities of bone marrow-derived DCs (BMDCs) with reduction of CIITA mRNA driven by the DC-specific promoter pI, and MHC class II mRNA. The chromatin immunoprecipitation assay showed constitutive binding of PU.1 to the pI region in BMDCs, whereas acetylation of histone H3 on pI was suppressed by LPS stimulation in parallel with shutdown of CIITA transcription. PU.1 transactivated the pI promoter through cis-elements at -47/-44 and -30/-27 in a reporter assay and to which PU.1 directly bound in an electrophoretic mobility shift assay. Acetylation of histones H3 and H4 on pI was reduced in PU.1 siRNA-introduced BMDCs. Knockdown of interferon regulatory factor 4 or 8, which is a heterodimer partner of PU.1, by siRNA did not affect pI-driven CIITA transcription or MHC class II expression.

CONCLUSION

PU.1 basally transactivates the CIITA pI promoter in DCs by functioning as a monomeric transcription factor and by affecting histone modification, resulting in the subsequent expression and function of MHC class II.

Enolase 1 and calreticulin regulate the differentiation and function of mouse mast cells

It has become widely accepted that the role of mast cells is not restricted to allergic processes. Thus, mast cells play an important role in innate and adaptive immune responses, but study of proteins related to differentiation of mast cells has not been done yet. Enolase 1 is a glycolytic enzyme expressed in most tissues and calreticulin, known as endoplasmic reticulum (ER) resident chaperon, has multifunctional responses. This study aimed to investigate the effects of these proteins on the differentiation and functions of mouse bone marrow-derived mast cells (BMMCs). To identify the target proteins related to the differentiation of BMMCs, we examined the protein expression pattern of BMMCs using 2-dimensional electrophoresis (2-DE) and MALDI-TOF analysis. Expressions of FcεRIα, surface molecules (c-kit, CD40, CD40L, VCAM-1), tryptase, and cytokines were examined in BMMCs using FACS analysis, Western blot, and RT-PCR respectively. Enolase 1 and calreticulin were transfected into BMMCs, and [Ca(2+)]i levels were determined by confocal microscope, while amounts of TNF-α and LTs were measured by ELISA. Eight proteins were identified by proteomic analysis. Enolase and calreticulin siRNA transfection inhibited the expressions of FcεRIα, surface molecules, tryptase, and cytokine mRNA, which are gradually enhanced during culture periods of BMMCs. Enolase 1 and calreticulin siRNA reduced the [Ca(2+)]i levels, amounts of total TNF-α, and the release of TNF-α and leukotrienes, all of which are increased in the BMMCs activated with antigen/antibody reaction. The data suggest that enolase 1 and calreticulin are important proteins in regulating the differentiation and functions of BMMCs.

Notch1-mediated signaling induces MHC class II expression through activation of class II transactivator promoter III in mast cells

Mast cells constitutively express Notch1 and Notch2 on the cell surface. Notch ligand Dll1 (Delta-like 1) stimulation induces MHC class II expression in mast cells and renders them as antigen-presenting cells. However, nothing is known about the mechanism by which Notch signaling induces MHC class II expression in mast cells. MHC class II genes are regulated by the class II transactivator (CIITA). In mice, transcription of the CIITA gene is controlled by three cell type-specific promoters (pI, pIII, and pIV). Here, we show that CIITA expression induced by Dll1 stimulation in mouse bone marrow-derived mast cells (BMMCs) depends critically on the signal mediated by Notch1 and that the most dominant promoter in Notch signaling-mediated CIITA expression in BMMCs is pIII, which is a lymphoid lineage-specific promoter. ChIP assays indicated that Notch signaling increased the binding of the transcription factor PU.1 to CIITA pIII in BMMCs. The knockdown of PU.1 expression using a specific siRNA suppressed Notch signaling-mediated CIITA expression, suggesting that PU.1 contributes to the expression of MHC class II induced by Notch signaling in mast cells. Furthermore, we show that a portion of freshly isolated splenic mast cells express MHC class II and that the most dominant promoter of CIITA in mast cells is pIII. These findings indicate that activation of CIITA pIII plays an important role in MHC class II expression in mast cells.

Critical role of transcription factor PU.1 in the expression of CD80 and CD86 on dendritic cells

In this study, we investigated the role of a transcription factor, PU.1, in the regulation of CD80 and CD86 expression in dendritic cells (DCs). A chromatin immunoprecipitation assay revealed that PU.1 is constitutively bound to the CD80 and CD86 promoters in bone marrow-derived DCs. In addition, co-expression of PU.1 resulted in the transactivation of the CD80 and CD86 promoters in a reporter assay. The binding of PU.1 to cis-enhancing regions was confirmed by electromobility gel-shift assay. As expected, inhibition of PU.1 expression by short interfering RNA (siRNA) in bone marrow-derived DCs resulted in marked down-regulation of CD80 and CD86 expression. Moreover, overexpression of PU.1 in murine bone marrow-derived lineage-negative cells induced the expression of CD80 and CD86 in the absence of monocyte/DC-related growth factors and/or cytokines. Based on these results, we conclude that PU.1 is a critical factor for the expression of CD80 and CD86. We also found that subcutaneous injection of PU.1 siRNA or topical application of a cream-emulsified PU.1 siRNA efficiently inhibited murine contact hypersensitivity. Our results suggest that PU.1 is a potential target for the treatment of immune-related diseases.

Opposite effects of Trichostatin A on activation of mast cells by different stimulants

Mast cells (MCs) are activated upon stimulation via TLRs or FcepsilonRI, contributing to immune protection and/or leading to allergic diseases. In the present study, the effects of Trichostatin A (TSA) on the activation of MCs were analyzed with bone marrow-derived (BM) MCs. TSA increased the transcription and protein secretion of IL-6 in case of LPS-stimulation, in contrast to the suppressive effect on IgE-mediated activation of BMMCs. Chromatin immunoprecipitation assay showed IgE-mediated signaling-specific suppression of transcription factors recruitment to the IL-6 promoter. TSA-treatment inhibited nuclear translocation of NF-kappaB following IgE-mediated, but not LPS-induced activation in MCs.

Involvement of PU.1 in the transcriptional regulation of TNF-alpha

PU.1 is a myeloid- and lymphoid-specific transcription factor that serves many important roles in the development and specific gene regulation of hematopoietic lineages. Mast cells (MC) and dendritic cells (DC) express PU.1 at low and high levels, respectively. Previously, we found that enforced expression of PU.1 in MC resulted in acquisition of DC-like characteristics, including repression of several IgE-mediated responses due to reduced expression of IgE-signaling related molecules. In contrast, PU.1 overexpression in MC up-regulated TNF-alpha production in response to IgE- and LPS-stimulation suggesting that PU.1 positively regulates TNF-alpha expression. However, the role of PU.1 in the expression of TNF-alpha is largely unknown. In the present study, the effects of PU.1 on the TNF-alpha promoter in mouse bone marrow-derived (BM) MC and DC were studied. Real-time PCR, ELISA, and chromatin immunoprecipitation assays indicated that the kinetics and magnitude of TNF-alpha expression levels following LPS- or IgE-stimulation are related to the amount of PU.1 binding to the promoter. In brief, higher and delayed up-regulation of TNF-alpha promoter function was observed in DC, whereas there were lower and rapid responses in MC. When PU.1-overexpressing retrovirus vector was introduced into MC, the amount of PU.1 recruited to the TNF-alpha promoter markedly increased. The knockdown of PU.1 in BMDC by siRNA resulted in a reduction of TNF-alpha protein produced from LPS-stimulated BMDC. These observations indicate that PU.1 transactivates the TNF-alpha promoter and that the amount of PU.1 binding on the promoter is associated with promoter activity.

Roles of PU.1 in monocyte- and mast cell-specific gene regulation: PU.1 transactivates CIITA pIV in cooperation with IFN-gamma

Over-expression of PU.1, a myeloid- and lymphoid-specific transcription factor belonging to the Ets family, induces monocyte-specific gene expression in mast cells. However, the effects of PU.1 on each target gene and the involvement of cytokine signaling in PU.1-mediated gene expression are largely unknown. In the present study, PU.1 was over-expressed in two different types of bone marrow-derived cultured mast cells (BMMCs): BMMCs cultured with IL-3 plus stem cell factor (SCF) and BMMCs cultured with pokeweed mitogen-stimulated spleen-conditioned medium (PWM-SCM). PU.1 over-expression induced expression of MHC class II, CD11b, CD11c and F4/80 on PWM-SCM-cultured BMMCs, whereas IL-3/SCF-cultured BMMCs expressed CD11b and F4/80, but not MHC class II or CD11c. When IFN-gamma was added to the IL-3/SCF-based medium, PU.1 transfectant acquired MHC class II expression, which was abolished by antibody neutralization or in Ifngr(-/-) BMMCs, through the induction of expression of the MHC class II transactivator, CIITA. Real-time PCR detected CIITA mRNA driven by the fourth promoter, pIV, and chromatin immunoprecipitation indicated direct binding of PU.1 to pIV in PU.1-over-expressing BMMCs. PU.1-over-expressing cells showed a marked increase in IL-6 production in response to LPS stimulation in both IL-3/SCF and PWM-SCM cultures. These results suggest that PU.1 overproduction alone is sufficient for both expression of CD11b and F4/80 and for amplification of LPS-induced IL-6 production. However, IFN-gamma stimulation is essential for PU.1-mediated transactivation of CIITA pIV. Reduced expression of mast cell-related molecules and transcription factors GATA-1/2 and up-regulation of C/EBPalpha in PU.1 transfectants indicate that enforced PU.1 suppresses mast cell-specific gene expression through these transcription factors.

Physiological and pathophysiological functions of intestinal mast cells

The normal gastrointestinal (GI) mucosa is equipped with mast cells that account for 2-3% of lamina propria cells under normal conditions. Mast cells are generally associated with allergic disease, and indeed, food allergy that manifests in the GI tract is usually mast cell dependent. On the other hand, mast cells have a number of physiological functions in the GI tract, namely regulatory functions such as control of blood flow and coagulation, smooth muscle contraction and peristalsis, and secretion of acid, electrolytes, and mucus by epithelial cells. One of the most intriguing functions of intestinal mast cells is their role in host defense against microbes like bacteria, viruses, or parasites. Mast cells recognize microbes by antibody-dependent mechanisms and through pattern-recognition receptors. They direct the subsequent immune response by attracting both granulocytes and lymphocytes to the site of challenge via paracrine cytokine release. Moreover, mast cells initiate, by releasing proinflammatory mediators, innate defense mechanisms such as enhanced epithelial secretion, peristalsis, and alarm programs of the enteric nervous This initiation can occur in response to a primary contact to the microbe or other danger signals, but becomes much more effective if the triggering antigen reappears and antibodies of the IgE or IgG type have been generated in the meantime by the specific immune system. Thus, mast cells operate at the interface between innate and adaptive immune responses to enhance the defense against pathogens and, most likely, the commensal flora. In this respect, it is important to note that mast cells are directly involved in controlling the function of the intestinal barrier that turned out to be a crucial site for the development of infectious and immune-mediated diseases. Hence, intestinal mast cells perform regulatory functions to maintain tissue homeostasis, they are involved in host defense mechanisms against pathogens, and they can induce allergy once they are sensitized against foreign antigens. The broad spectrum of functions makes mast cells a fascinating target for future pharmacological or nutritional interventions.

Notch signaling confers antigen-presenting cell functions on mast cells

BACKGROUND

Notch signaling is involved in cell fate determination along with the development of the immune system. However, very little is known about the role for Notch signaling in mast cells.

OBJECTIVE

We investigated the role of Notch signaling in mast cell functions.

METHODS

After mouse bone marrow-derived mast cells (BMMCs) or peritoneal mast cells (PMCs) were cocultured with mouse Notch ligand-expressing chinese hamster ovary cells for 5 days, we examined the mast cell surface expressions of MHC-II molecules and OX40 ligand (OX40L), Fc epsilon RI-mediated cytokine production, and the effects of the mast cells on proliferation and differentiation of naive CD4(+) T cells in vitro.

RESULTS

We showed that BMMCs and PMCs constitutively expressed Notch1 and Notch2 proteins on the cell surface. We also found that Delta-like 1 (Dll1)/Notch signaling induced the expression of MHC-II and upregulated the expression level of OX40L on the surface of the mast cells. Dll1/Notch signaling augmented Fc epsilon RI-mediated IL-4, IL-6, IL-13, and TNF production by BMMCs. Dll1-stimulated MHC-II(+)OX40L(high) BMMCs promoted proliferation of naive CD4(+) T cells and their differentiation into T(H)2 cells producing IL-4, IL-5, IL-10, and IL-13.

CONCLUSION

Dll1/Notch signaling confers the functions as an antigen-presenting cell on mast cells, which preferentially induce the differentiation of T(H)2.

Opposite effects of PU.1 on mast cell stimulation

An Ets-family transcription factor PU.1 is involved in the development and specific gene regulation of hematopoietic cells. PU.1 also determines the commitment between several lineages via its expression level. Although enforced expression of PU.1 in mast cells (MC) induced expression of monocyte-specific markers and morphological change from MC to monocytes, especially dendritic cells (DC), in the previous report, intracellular events caused by PU.1 are largely unknown. In the present study, effect of PU.1 on IgE- and LPS-mediated stimulation degrees was analyzed. The amounts of IL-6, IL-13, and TNF-alpha produced from LPS-stimulated MC were markedly increased by overexpression of PU.1. In contrast, IL-6 and IL-13 production levels in response to IgE were reduced by PU.1, whereas that of TNF-alpha was up-regulated. beta-Hexosaminidase release as a means of degranulation was decreased in PU.1 transfectants. When eicosanoid generation in response to IgE-stimulation was analyzed, overexpression of PU.1 reduced leukotriene C(4) (LTC(4)) release, but enhanced PGD(2) production. Microarray analysis suggested that expression of FcepsilonRI signal pathway related molecules were suppressed in PU.1 overexpressing MC as well as DC. These observations indicate that up-regulation of PU.1 suppresses expression of FcepsilonRI signal transduction-related intracellular molecules, but increases the potential of transcription activity of monocyte characters.

Mast cell transcriptional networks

Unregulated activation of mast cells can contribute to the pathogenesis of inflammatory and allergic diseases, including asthma, rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. Absence of mast cells in animal models can lead to impairment in the innate immune response to parasites and bacterial infections. Aberrant clonal accumulation and proliferation of mast cells can result in a variety of diseases ranging from benign cutaneous mastocytosis to systemic mastocytosis or mast cell leukemia. Understanding mast cell differentiation provides important insights into mechanisms of lineage selection during hematopoiesis and can provide targets for new drug development to treat mast cell disorders. In this review, we discuss controversies related to development, sites of origin, and the transcriptional program of mast cells.

Involvement of mast cells in IL-12/23 p40 production is essential for survival from polymicrobial infections

Interleukin-12 (IL-12), a heterodimeric cytokine (p35/p40) produced mainly from macrophages and dendritic cells, is an important regulator of T-helper 1 cell responses and for host defense. We found that interferon (IFN) consensus sequence binding protein (ICSBP), which is a transcription factor essential for the expression of p40, was expressed in mouse bone marrow-derived mast cells (BMMCs). The transcription levels of p35 and p40 were increased by stimulation of BMMCs with IFN-gamma/lipopolysaccharide (LPS). IL-12 was secreted from BMMCs in response to LPS but not by FcepsilonRI cross-linking. The p40 levels in the peritoneal cavity of mast cell-deficient W/W(v) and W/W(v) reconstituted with p40(-/-) BMMCs were significantly lower than those of WBB6F(1)(+/+) and wild-type (WT) BMMC-reconstituted W/W(v) in the acute septic peritonitis model. The survival rate of W/W(v) reconstituted with p40(-/-) BMMCs was significantly decreased compared to those of WBB6F(1)(+/+) and WT-BMMC-reconstituted W/W(v), which was due to reduced production of IFN-gamma and subsequent impaired activation of neutrophils in the peritoneal cavity. Survival rate of p40(-/-) mice was also restored by adoptive transfer of WT-BMMCs. These results demonstrate that mast cells play a significant role in the production of IL-12 required for host defense. This is the first report to demonstrate that mast cells are a crucial source of functional IL-12.

Role of mast cells in allergic and non-allergic immune responses: comparison of human and murine data

The versatile role of mast cells in allergy, in innate immune responses and in the regulation of tissue homeostasis is well recognized. However, it is often not made clear that most mast-cell data derive solely from experiments in mice or rats, species that obviously never suffer from allergic and most other mast-cell-associated human diseases. Data on human mast cells are limited, and the mast-cell source and species from which findings derive are frequently not indicated in the titles and summaries of research publications. This Review summarizes recent data on human mast cells, discusses differences with murine mast cells, and describes new tools to study this increasingly meaningful cell type in humans.

Development, migration, and survival of mast cells

Mast cells play a pivotal role in immediate hypersensitivity and chronic allergic reactions that can contribute to asthma, atopic dermatitis, and other allergic diseases. Because mast cell numbers are increased at sites of inflammation in allergic diseases, pharmacologic intervention into the proliferation, migration, and survival (or apoptosis) of mast cells could be a promising strategy for the management of allergic diseases. Mast cells differentiate from multipotent hematopoietic progenitors in the bone marrow. Stem cell factor (SCF) is a major chemotactic factor for mast cells and their progenitors. SCF also elicits cell-cell and cell-substratum adhesion, facilitates the proliferation, and sustains the survival, differentiation, and maturation, of mast cells. Therefore, many aspects of mast cell biology can be understood as interactions of mast cells and their precursors with SCF and factors that modulate their responses to SCF and its signaling pathways. Numerous factors known to have such a capacity include cytokines that are secreted from activated T cells and other immune cells including mast cells themselves. Recent studies also demonstrated that monomeric IgE binding to FcepsilonRI can enhance mast-cell survival. In this review we discuss the factors that regulate mast cell development, migration, and survival.

Prolonged MHC class II expression and CIITA transcription in human keratinocytes

A transcriptional cofactor, the MHC class II transactivator (CIITA), is a master regulator of MHC class II gene expression. CIITA expression is restricted in MHC class II-positive cells and is regulated by 4 (human) or 3 (mouse) promoters. To clarify the usage of human CIITA promoters in keratinocytes, we analyzed CIITA mRNA levels in IFN-gamma-stimulated normal human keratinocytes (NHK) by real-time PCR using promoter-specific primers. When the amount of total CIITA mRNA in NHK was quantified at 6h after IFN-gamma-stimulation, the amount of CIITA mRNA detected in NHK did not differ from that seen in the B cell line Raji or the IFN-gamma-stimulated macrophage cell line THP-1. Quantitative real-time PCR using promoter-specific primers showed that type IV CIITA mRNA was strongly transcribed and that type III CIITA mRNA was weakly transcribed in stimulated NHK, while no transcripts from pI and pII were detected. Although type IV mRNA in THP-1 was transiently transcribed by IFN-gamma-stimulation, transcription of type IV in IFN-gamma-stimulated keratinocytes was prolonged. This difference subsequently caused significantly higher expression at 72 h of MHC class II on NHK, compared with THP-1. This is the first report to quantitatively analyze each type of CIITA transcript in NHK.

PU.1/Spi-1 Binds to the Human TAL-1 Silencer to Mediate its Activity

The TAL-1/SCL gene encodes a basic helix-loop-helix (bHLH) transcription factor essential for primitive hematopoiesis and for adult erythroid and megakaryocytic development. Activated transcription of TAL-1 as a consequence of chromosomal rearrangements is associated with a high proportion of human T cell acute leukemias, showing that appropriate control of TAL-1 is crucial for the formation and subsequent fate of hematopoietic cells. Hence, the knowledge of the mechanisms, which govern the pattern of TAL-1 expression in hematopoiesis, is of great interest. We previously described a silencer in the 3'-untranslated region of human TAL-1, the activity of which is mediated through binding of a tissue-specific 40 kDa nuclear protein to a new DNA recognition motif, named tal-RE. Here, we show that tal-RE-binding activity, high in immature human hematopoietic progenitors is down regulated upon erythroid and megakaryocytic differentiation. This expression profile helped us to identify that PU.1/Spi-1 binds to the tal-RE sequences in vitro and occupies the TAL-1 silencer in vivo. By expressing a mutant protein containing only the ETS domain of PU.1 in human erythroleukemic HEL cells, we demonstrated that PU.1 mediates the transcriptional repression activity of the silencer. We found that ectopic PU.1 is not able to induce silencing activity in PU.1-negative Jurkat T cells, indicating that PU.1 activity, although necessary, is not sufficient to confer transcriptional repression activity to the TAL-1 silencer. Finally, we showed that the silencer is also active in TAL-1-negative myeloid HL60 cells that express PU.1 at high levels. In summary, our study shows that PU.1, in addition to its positive role in TAL-1 expression in early hematopoietic progenitors, may also act as a mediator of TAL-1 silencing in some hematopoietic lineages.

Transcriptional regulation of ATP2C1 gene by Sp1 and YY1 and reduced function of its promoter in Hailey-Hailey disease keratinocytes

Hailey-Hailey disease (HHD) is a blistering skin disease caused by malfunction of the Ca2+-dependent ATPase, ATP2C1. In this study, key regulatory regions necessary for the expression of the gene encoding human ATP2C1 were investigated. The transient reporter assay demonstrated that region +21/+57 was necessary for activation of the ATP2C1 promoter, and the electrophoretic mobility shift assay demonstrated that the region was recognized by the transcription factors, Sp1 and YY1. In accordance with this result, when Sp1 or YY1 was overexpressed in keratinocytes, an obvious increase in ATP2C1 promoter activity was observed, which was in contrast with the case where a mutant promoter lacking the binding sites for Sp1 and YY1 was used as the reporter. Ca2+-stimulation signal increased nuclear Sp1 proteins and ATP2C1 mRNA levels in normal keratinocytes. In contrast, both these increases were suppressed in keratinocytes from HHD patients. These results indicate that Sp1 and YY1 transactivate the human ATP2C1 promoter via cis-enhancing elements and that incomplete upregulation of ATP2C1 transcription contributes to the keratinocyte-specific pathogenesis of HHD. This is a report describing the regulation of the expression of ATP2C1.