The FcεRIβ homologue,MS4A4A,promotes FcεRI signal transduction and store-operated Ca2+ entry in human mast cells

Members of the membrane spanning 4A (MS4A) gene family are clustered around 11q12-13, a region linked to allergy and asthma susceptibility. Other than the known functions of FcεRIβ (MS4A2) and CD20 (MS4A1) in mast cell and B cell signaling, respectively, functional studies for the remaining MS4A proteins are lacking. We thus explored whether MS4A4A, a mast cell expressed homologue of FcεRIβ, has related functions to FcεRIβ in FcεRI signaling. We establish in this study that MS4A4A promotes phosphorylation of PLCγ1, calcium flux and degranulation in response to IgE-mediated crosslinking of FcεRI. We previously demonstrated that MS4A4A promotes recruitment of KIT into caveolin-1-enriched microdomains and signaling through PLCγ1. Caveolin-1 itself is an important regulator of IgE-dependent store-operated Ca²⁺ entry (SOCE) and promotes expression of the store-operated Ca²⁺ channel pore-forming unit, Orai1. We thus further report that MS4A4A functions through interaction with caveolin-1 and recruitment of FcεRI and KIT into lipid rafts. In addition to proximal FcεRI signaling, we similarly show that MS4A4A regulates Orai1-mediated calcium entry downstream of calcium release from stores. Both MS4A4A and Orai1 had limited effects with compound 48/80 stimulation, demonstrating some degree of selectivity of both proteins to FcεRI receptor signaling over Mas-related G Protein coupled receptor X2 signaling. Overall, our data are consistent with the conclusion that MS4A4A performs a related function to the homologous FcεRIβ to promote PLCγ1 signaling, SOCE, and degranulation through FcεRI in human mast cells and thus represents a new target in the regulation of IgE-mediated mast cell activation.     

Identification of semaphorin 4B as a negative regulator of basophil-mediated immune responses

Basophils are strong mediators of Th2 responses during helminthic infections. Recently, basophils were shown to function as APCs and promote both Th2 skewing and humoral memory responses. However, the mechanisms that regulate basophils are still unclear. In this article, we show that a class IV semaphorin, Sema4B, negatively regulates basophil functions through T cell-basophil contacts. In a screen to identify semaphorins that function in the immune system, we determined that Sema4B is expressed in T and B cells. Interestingly, Sema4B(-/-) mice had considerably increased serum IgE levels despite normal lymphocyte and dendritic cell functions. Recombinant Sema4B significantly inhibited IL-4 and IL-6 production from basophils in response to various stimuli, including IL-3, papain, and FcεRI cross-linking. In addition, T cell-derived Sema4B, which accumulated at contact sites between basophils and CD4(+) T cells, suppressed basophil-mediated Th2 skewing, suggesting that Sema4B regulates basophil responses through cognate cell-cell contacts. Furthermore, Sema4B(-/-) mice had enhanced basophil-mediated memory IgE production, which was abolished by treating with an anti-FcεRIα Ab. Collectively, these results indicate that Sema4B negatively regulates basophil-mediated Th2 and humoral memory responses.     

Characterization of the expression of HTm4 (MS4A3), a cell cycle regulator,in human peripheral blood cells and normal and malignant tissues

HTm4 (MS4A3) is a member of a family of four‐transmembrane proteins designated MS4A. MS4A proteins fulfil diverse functions, acting as cell surface signalling molecules and intracellular adapter proteins. Early reports demonstrated that HTm4 is largely restricted to the haematopoietic lineage, and is involved in cell cycle control, via a regulatory interaction with the kinase‐associated phosphatase, cyclin A and cyclin‐dependent kinase 2 (CDK2). Here we describe the expression pattern of HTm4 in peripheral blood cells using gene expression microarray technology, and in normal foetal and adult human tissues, as well as adult human cancers, using tissue microarray technology. Using oligonucleotide microarrays to evaluate HTm4 mRNA, all peripheral blood cell types demonstrated very low levels of HTm4 expression; however, HTm4 expression was greatest in basophils compared to eosinophils, which showed lower levels of HTm4 expression. Very weak HTm4 expression is found in monocytes, granulocytes and B cells, but not in T cells, by lineage specific haematopoietic cell flow cytometry analysis. Interestingly, phytohaemagglutinin stimulation increases HTm4 protein expression in peripheral blood CD4‐T‐lymphocytes over nearly undetectable baseline levels. Western blotting and immunohistochemical studies show strong HTm4 expression in the developing haematopoietic cells of human foetal liver. Immunohistochemical studies on normal tissue microarrays confirmed HTm4 expression in a subset of leucocytes in nodal, splenic tissues and thymic tissue, and weak staining in small numbers of cell types in non‐haematopoietic tissues. Human foetal brain specimens from 19 to 31 gestational weeks showed that the strongest‐staining cells are ventricular zone cells and the earliest‐born, earliest‐differentiating ‘pioneer’ neurons in the cortical plate, Cajal‐Retzius and, to a lesser extent, subplate‐like neurons. Malignant tissue microarray analysis showed HTm4 expression in a wide variety of adenocarcinomas, including breast, prostate and ovarian. These findings warrant the further study of the role of HTm4 in the cell cycle of both haematopoietic and tumour cells.     

The cell cycle associated protein, HTm4, is expressed in differentiating cellsof the hematopoietic and central nervous system in mice

HTm4 is a member of a newly defined family of human and murine proteins, the MS4 (membrane-spanning four) protein group, which has a distinctive four-transmembrane structure. MS4 protein functions include roles as cell surface signaling receptors and intracellular adapter proteins. We have previously demonstrated that HTm4 regulates the function of the KAP phosphatase, a key regulator of cell cycle progression. In humans, the expression of HTm4 is largely restricted to cells of the hematopoietic lineage, possibly reflecting a causal role for this molecule in differentiation/proliferation of hematopoietic lineage cells. In this study, we show that, like the human homologue, murine HTm4 is also predominantly a hematopoietic protein with distinctive expression patterns in developing murine embryos and in adult animals. In addition, we observed that murine HTm4 is highly expressed in the developing and adult murine nervous system, suggesting a previously unrecognized role in central and peripheral nervous system development.JLK and XY contributed equally to this work     

Anti-MS4a4B treatment abrogates MS4a4B-mediated protection in T cells and ameliorates experimental autoimmune encephalomyelitis

Recent data show that anti-CD20 therapy is effective for some autoimmune diseases, including multiple sclerosis (MS). However, the efficacy of anti-CD20 therapy for MS is largely limited because anti-CD20 antibodies target only B cells. In previous studies, we have investigated the function of MS4a4B, a novel CD20 homologue, in T cell proliferation. Here, we found that MS4a4B regulates not only T cell proliferation but also T cell apoptosis. Knockdown of MS4a4B by MS4a4B-siRNA or MS4a4B-shRNA-expressing vector promoted apoptosis in primary T cells and T32 cell line. In contrast, vector-driven over-expression of MS4a4B reduced apoptosis in EL-4 cells. Machinery analysis showed that MS4a4B-mediated T cell survival was associated with decreased activity of caspases 3, 8 and 9. Interestingly, binding of anti-MS4a4B antibodies to T cells induced activated T cells to undergo apoptosis. To test whether anti-MS4a4B antibody interferes with MS4a4B-mediated protection of T cells, we injected anti-MS4a4B antibodies into mice with experimental autoimmune encephalomyelitis (EAE). The results show that anti-MS4a4B treatment ameliorated the severity of EAE, accompanied by decreased Th1 and Th17 cell responses and reduced levels of pro-inflammatory cytokines in the central nervous system, suggesting that MS4a4B may serve as a target of antibody-based therapy for T cell-mediated diseases.     

Identification of a CD20-, FcepsilonRIbeta-, and HTm4-related gene family: sixteen new MS4A family members expressed in human and mouse

CD20, high-affinity IgE receptor beta chain (FcepsilonRIbeta), and HTm4 are structurally related cell-surface proteins expressed by hematopoietic cells. In the current study, 16 novel human and mouse genes that encode new members of this nascent protein family were identified. All family members had at least four potential membrane-spanning domains, with N- and C-terminal cytoplasmic domains. This family was therefore named the membrane-spanning 4A gene family, with at least 12 subgroups (MS4A1 through MS4A12) currently representing at least 21 distinct human and mouse proteins. Each family member had unique patterns of expression among hematopoietic cells and nonlymphoid tissues. Four of the 6 human MS4A genes identified in this study mapped to chromosome 11q12-q13.1 along with CD20, FcepsilonRIbeta, and HTm4. Thus, like CD20 and FcepsilonRIbeta, the other MS4A family members are likely to be components of oligomeric cell surface complexes that serve diverse signal transduction functions.     

FRIL蛋白体外维持脐血造血干/祖细胞的作用及细胞周期调控基因的表达

为探讨新的豆类凝集素(Flt3 receptor-interacting lectin,FRIL)体外维持脐血CD34^ 细胞的作用以及维持过程中细胞周期调控基因HTm4及HTm4S mRNA的表达及意义,我们利用FRIL维持培养脐血CD34^ 细胞,对其增殖曲线、细胞周期及集落形成能力进行常规分析,并用半定量RT—PCR法分别测定FRIL体外维持不同时间后脐血CD34^ 细胞中周期调控基因HTm4及HTm4S mRNA的表达变化。结果显示,FRIL培养的CD34^ 造血干／祖细胞的增殖趋势平缓,整个培养期间细胞增殖倍数不超过起始的3倍：14d之前,FRIL培养细胞的高增殖潜能集落形成细胞(HPP—CFC)形成集落数与FL组无差别,其后则维持高于FL的情况。细胞周期分析则显示,在28d的培养过程内,利用FRIL培养的细胞始终有80％以上维持在G0期;而周期调控基因HTm4及HTm4S在刚分离的脐血CD34^ 细胞中的表达水平较高;但培养1d后,几乎检测不到HTm4基因的表达;培养3～14d,该基因的表达回升并持续维持在高水平。而HTm4S基因的表达在第7d达最高水平,其余时间基本呈稳定表达。转染HTm4和HTm4S,亚细胞定位结果显示HTm4主要定位于核周围,而HTm4S则定位于整个胞浆,由此可能导致它们功能的区别。以上结果提示,长期培养体现出FRIL在维持造血干／祖细胞多能性上的优势;细胞周期调控基因HTm4及其新剪接子参与了FRIL体外长期维持脐血造血干／祖细胞处于静息状态的过程。     

Immunoglobulin E receptor signaling and asthma

Elevated IgE levels and increased IgE sensitization to allergens are central features of allergic asthma. IgE binds to the high-affinity Fcε receptor I (FcεRI) on mast cells, basophils, and dendritic cells and mediates the activation of these cells upon antigen-induced cross-linking of IgE-bound FcεRI. FcεRI activation proceeds through a network of signaling molecules and adaptor proteins and is negatively regulated by a number of cell surface and intracellular proteins. Therapeutic neutralization of serum IgE in moderate-to-severe allergic asthmatics reduces the frequency of asthma exacerbations through a reduction in cell surface FcεRI expression that results in decreased FcεRI activation, leading to improved asthma control. Our increasing understanding of IgE receptor signaling may lead to the development of novel therapeutics for the treatment of asthma.     

HTm4基因在造血细胞周期调控中的作用

为了研究HTm4基因在造血细胞细胞周期调控中的作用,以佛波酯(phorbol 12-myristate 13-acetate,PMA)诱导K562细胞分化为模型,利用流式细胞术(FACS)及半定量RT-PCR对分化过程中细胞周期的变化及HTm4基因的表达进行了分析,并利用Tet-Off调控表达系统,将HTm4基因以及C端功能域缺失的HTm4-ct转染K562细胞,观察对细胞周期的影响。结果表明,PMA同时引起了K562细胞的增殖和分化,G0／G1期细胞的比例以及HTm4基因的表达均呈现出波浪形的变化趋势,说明HTm4基因可能参与了细胞退出细胞周期的过程。HTm4基因转染后引起K562细胞滞留于G0／G1期,但C端功能域缺失的HTm4-ct没有此作用,说明C端功能域在HTm4基因调控细胞周期的功能中发挥重要作用。     

A fluorescent biosensor reveals conformational changes in human immunoglobulin E Fc: implications for mechanisms of receptor binding, inhibition, and allergen recognition

IgE binding to its high affinity receptor FcεRI on mast cells and basophils is a key step in the mechanism of allergic disease and a target for therapeutic intervention. Early indications that IgE adopts a bent structure in solution have been confirmed by recent x-ray crystallographic studies of IgEFc, which further showed that the bend, contrary to expectation, is enhanced in the crystal structure of the complex with receptor. To investigate the structure of IgEFc and its conformational changes that accompany receptor binding in solution, we created a F?rster resonance energy transfer (FRET) biosensor using biologically encoded fluorescent proteins fused to the N- and C-terminal IgEFc domains (Cε2 and Cε4, respectively) together with the theoretical basis for quantitating its behavior. This revealed not only that the IgEFc exists in a bent conformation in solution but also that the bend is indeed enhanced upon FcεRI binding. No change in the degree of bending was seen upon binding to the B cell receptor for IgE, CD23 (FcεRII), but in contrast, binding of the anti-IgE therapeutic antibody omalizumab decreases the extent of the bend, implying a conformational change that opposes FcεRI engagement. HomoFRET measurements further revealed that the (Cε2)(2) and (Cε4)(2) domain pairs behave as rigid units flanking the conformational change in the Cε3 domains. Finally, modeling of the accessible conformations of the two Fab arms in FcεRI-bound IgE revealed a mutual exclusion not seen in IgG and Fab orientations relative to the membrane that may predispose receptor-bound IgE to cross-linking by allergens.     

Mapping of the CD23 Binding Site on Immunoglobulin E (IgE) and Allosteric Control of the IgE-Fc{epsilon}RI Interaction

IgE, the antibody that mediates allergic responses, acts as part of a self-regulating protein network. Its unique effector functions are controlled through interactions of its Fc region with two cellular receptors, FcεRI on mast cells and basophils and CD23 on B cells. IgE cross-linked by allergen triggers mast cell activation via FcεRI, whereas IgE-CD23 interactions control IgE expression levels. We have determined the CD23 binding site on IgE, using a combination of NMR chemical shift mapping and site-directed mutagenesis. We show that the CD23 and FcεRI interaction sites are at opposite ends of the Cε3 domain of IgE, but that receptor binding is mutually inhibitory, mediated by an allosteric mechanism. This prevents CD23-mediated cross-linking of IgE bound to FcεRI on mast cells and resulting antigen-independent anaphylaxis. The mutually inhibitory nature of receptor binding provides a degree of autonomy for the individual activities mediated by IgE-FcεRI and IgE-CD23 interactions.     

Further evidence for a human B cell activating factor distinct from IL-4

Supernatants from activated human T cell clones were previously shown to contain B cell-activating factor (BCAF), an activity which results in polyclonal resting B cell stimulation. In the present study, we investigate the relationship between this activity and human interleukin-4 which was also shown to act on resting B cells. The supernatant of the T cell clone TT9 contains IL-4 but anti-IL-4 antiserum does not affect the response of B cells as measured by thymidine uptake or cell volume increase. Furthermore, IL-4 induces Fc epsilon-receptor (CD23) expression on 30% of unstimulated human B cells, whereas BCAF-containing supernatants from clone P2, that do not contain detectable amounts of IL-4, promote B cell proliferation without inducing CD23 expression. Our results therefore establish that IL-4 and BCAF are distinct activities and suggest that they trigger different activation pathways in human B cells. In addition, culture of B cells with T cell supernatants for 72 hr induces a three- to fourfold increase in the expression of HLA-DR, -DP, and -DQ antigens in 50% of B cells. The addition of inhibiting concentrations of anti-IFN-gamma, LT, or IL-4 antisera to the cultures does not change these results. Finally, 30% of B cells cultured with T cell supernatants leave the G1 phase of the cell cycle and 20% reach mitosis. Taken together, our findings further support the existence of a B cell-activating factor responsible for the activation of resting human B cells.     

Type II phosphatidylinositol 4-kinases interact with FcεRIγ subunit in RBL-2H3 cells

Ligation of high-affinity IgE receptor I (FcεRI) on RBL-2H3 cells leads to recruitment of FcεRI and type II phosphatidylinositol 4-kinases (PtdIns 4-kinases) into lipid rafts. Lipid raft integrity is required for the activation of type II PtdIns 4-kinases and signal transduction through FcεRIγ during RBL-2H3 cell activation. However, the molecular mechanism by which PtdIns 4-kinases are coupled to FcεRI signaling is elusive. Here, we report association of type II PtdIns 4-kinase activity with FcεRIγ subunit in anti-FcεRIγ immunoprecipitates. FcεRIγ-associated PtdIns 4-kinase activity increases threefold upon FcεRI ligation in anti-FcεRIγ immunoprecipitates. Biochemical characterization of PtdIns 4-kinase activity associated with FcεRIγ reveals that it is a type II PtdIns 4-kinases. Canonical tyrosine residues mutation in FcεRIγ ITAM (Y65 and Y76) reveals that these two tyrosine residues in γ subunit are required for its interaction with type II PtdIns 4-kinases.     

Limited CD4+ T cell proliferation leads to preservation of CD4+ T cell counts in SIV-infected sooty mangabeys

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections result in chronic virus replication and progressive depletion of CD4+ T cells, leading to immunodeficiency and death. In contrast, ‘natural hosts’ of SIV experience persistent infection with high virus replication but no severe CD4+ T cell depletion, and remain AIDS-free. One important difference between pathogenic and non-pathogenic infections is the level of activation and proliferation of CD4+ T cells. We analysed the relationship between CD4+ T cell number and proliferation in HIV, pathogenic SIV in macaques, and non-pathogenic SIV in sooty mangabeys (SMs) and mandrills. We found that CD4+ T cell proliferation was negatively correlated with CD4+ T cell number, suggesting that animals respond to the loss of CD4+ T cells by increasing the proliferation of remaining cells. However, the level of proliferation seen in pathogenic infections (SIV in rhesus macaques and HIV) was much greater than in non-pathogenic infections (SMs and mandrills). We then used a modelling approach to understand how the host proliferative response to CD4+ T cell depletion may impact the outcome of infection. This modelling demonstrates that the rapid proliferation of CD4+ T cells in humans and macaques associated with low CD4+ T cell levels can act to ‘fuel the fire’ of infection by providing more proliferating cells for infection. Natural host species, on the other hand, have limited proliferation of CD4+ T cells at low CD4+ T cell levels, which allows them to restrict the number of proliferating cells susceptible to infection.     

The liver kinase B1 is a central regulator of T cell development, activation, and metabolism

T cell activation leads to engagement of cellular metabolic pathways necessary to support cell proliferation and function. However, our understanding of the signal transduction pathways that regulate metabolism and their impact on T cell function remains limited. The liver kinase B1 (LKB1) is a serine/threonine kinase that links cellular metabolism with cell growth and proliferation. In this study, we demonstrate that LKB1 is a critical regulator of T cell development, viability, activation, and metabolism. T cell-specific ablation of the gene that encodes LKB1 resulted in blocked thymocyte development and a reduction in peripheral T cells. LKB1-deficient T cells exhibited defects in cell proliferation and viability and altered glycolytic and lipid metabolism. Interestingly, loss of LKB1 promoted increased T cell activation and inflammatory cytokine production by both CD4(+) and CD8(+) T cells. Activation of the AMP-activated protein kinase (AMPK) was decreased in LKB1-deficient T cells. AMPK was found to mediate a subset of LKB1 functions in T lymphocytes, as mice lacking the α1 subunit of AMPK displayed similar defects in T cell activation, metabolism, and inflammatory cytokine production, but normal T cell development and peripheral T cell homeostasis. LKB1- and AMPKα1-deficient T cells each displayed elevated mammalian target of rapamycin complex 1 signaling and IFN-γ production that could be reversed by rapamycin treatment. Our data highlight a central role for LKB1 in T cell activation, viability, and metabolism and suggest that LKB1-AMPK signaling negatively regulates T cell effector function through regulation of mammalian target of rapamycin activity.