Rapid and large amount of autocrine IL-3 production is responsible for mast cell survival by IgE in the absence of antigen

Atopy in chronic urticaria: an important yet overlooked issue

Chronic urticaria (CU) is one of the most common dermatological diseases and has a significant impact on the quality of life of patients. However, the pathogenesis of this disease remains unclear. Autoimmunity in chronic spontaneous urticaria (CSU) has received considerable attention and has been studied previously. Atopy is an important characteristic of CU; however, it has not been fully recognized. Atopy predisposes individuals to immune responses to allergens, leading to type 2 inflammation and immunoglobulin E (IgE) overproduction. Compared with healthy individuals, patients with CU have a higher proportion of atopy, and an atopic background is correlated with the clinical characteristics of CU. The total IgE levels in patients with CU is significantly higher than those in healthy individuals. Although its level is not higher than that in classic allergic diseases, it is closely related to CU. Exogenous allergens, auto-allergens, and specific IgEs, which are closely related to atopy, have been reported, and their roles in CU pathogenesis are also being studied. Local and systemic atopic inflammation is present in patients with CU. This review summarizes the current knowledge regarding atopy and CU, speculating that there are CU subtypes, such as atopic CSU or atopic chronic inducible urticaria (CIndU) and that atopy may be involved in the pathogenesis of CU. These findings provide a new perspective for a comprehensive understanding of the clinical features of CU and further research regarding its pathogenesis.

Interleukin-3-dependent potentiation of IgE responsiveness in mouse basophils

Basophils produce interleukins (IL)-4 in response to various stimuli and may contribute to type 2 immune responses to various infections and allergens. We found that resting basophils freshly isolated from mice produce IL-4 in response to IL-3 but not to high-affinity Fc receptor (FcεRI) cross-linking (CL), yet both required the immunoreceptor tyrosine-based activation motif (ITAM) containing adaptor Fc receptor γ-chain (FcRγ), while basophils activated in vitro by IL-3 become responsive to FcεRI CL. Acquisition of responsiveness to FcεRI CL occurred upon infection with Trichinella spiralis or administration of superantigen. Because cultured basophils return to a quiescent state upon starvation with IL-3 with surface FcεRI levels unchanged, this acquisition is reversible and probably reflects intracellular events requiring protein synthesis. Interestingly, similar activation-associated acquisition was observed for responsiveness to other stimuli, including CD200R3 CL, which is known to signal via DAP-12, and the allergen protease papain. This acquisition of responsiveness to FcεRI CL was inhibited by Jak inhibitor. Thus, the IL-3 signal bifurcates downstream of Jak, into two distinct pathway, one leading to IL-4 production and the other to render basophils competent to respond to stimuli dependent on ITAM-containing adaptors DAP12 and FcRγ for IL-4 production.

Leukotriene D4 accelerates antigen-mediated mast cell responses via the cysteinyl leukotriene 1 receptor

Cysteinyl leukotrienes (CysLTs), released from mast cells (MCs), are important mediators in allergy. Type 1 receptors for CysLTs (CysLT1R) are involved in accelerating IgE-mediated MC activation. In this study, we aimed to elucidate the mechanisms underlying CysLT1R-mediated MC activation. The CysLT1R agonist/antagonist was applied to two types of major MC models-RBL-2H3 cells and bone marrow-derived MCs (BMMCs). The use of CysLT1R and CysLT2R inhibitors revealed that CysLT1R plays a major role in the acceleration of MC activation. The administration of the CysLT1R agonist leukotriene D4 upregulated IgE-mediated Akt and ERK phosphorylation and subsequently enhanced TNF-α expression, suggesting that CysLT1R regulates the downstream pathway of MC activation. However, these observations were not corroborated by CysLT1R knockdown using shRNA, suggesting a differential regulatory mechanism between the temporal and constitutive inhibitions of CysLT. In conclusion, CysLT1R enhances MC activation by accelerating IgE-induced signal transduction, which enables the co-regulation of rapid degranulation and delayed synthesis of inflammatory mediators in MCs.

Cardiac Mast Cells: A Two-Head Regulator in Cardiac Homeostasis and Pathogenesis Following Injury

Cardiac mast cells (CMCs) are multifarious immune cells with complex roles both in cardiac physiological and pathological conditions, especially in cardiac fibrosis. Little is known about the physiological importance of CMCs in cardiac homeostasis and inflammatory process. Therefore, the present review will summarize the recent progress of CMCs on origin, development and replenishment in the heart, including their effects on cardiac development, function and ageing under physiological conditions as well as the roles of CMCs in inflammatory progression and resolution. The present review will shed a light on scientists to understand cardioimmunology and to develop immune treatments targeting on CMCs following cardiac injury.

Demonstration and implications of IL-3 upregulation of CD25 expression on human mast cells

BACKGROUND

CD25⁺ human mast cells (huMCs) have been reported in patients with monoclonal mast cell diseases and in rare association with inflammation. However, the regulation of CD25 expression on huMCs and the possible biologic consequences remain poorly understood.

OBJECTIVE

We sought to identify conditions that would upregulate CD25 expression on huMCs and to explore possible functional implications.

METHODS

huMCs were cultured from peripheral blood progenitor cells over 6 to 8 weeks. Expression of CD25 was determined by fluorescence-activated cell sorting and soluble CD25 by ELISA. Signal transducer and activator of transcription 5 (STAT5) phosphorylation induced by IL-2 in huMCs, regulatory T (Treg) cells, or in cocultured huMCs and Treg cells was examined by fluorescence-activated cell sorting.

RESULTS

Addition of IL-3 to CD34⁺ progenitors at the initiation of huMC cultures in the presence of stem cell factor and IL-6 upregulated the expression of CD25 in developing huMCs and resulted in shedding of soluble CD25 into the media. Removal of IL-3 after the first week of culture did not affect subsequent expression of CD25. Furthermore, addition of IL-3 14 days after the initiation of the culture did not induce significant CD25 expression. Treatment with anti-IL-3 antibody or the Janus kinase inhibitor tofacitinib blocked IL-3-induced CD25 upregulation. Binding of IL-2 to CD25⁺ huMCs did not induce STAT5 phosphorylation. However, coincubation of Treg cells with CD25⁺ huMCs pretreated with IL-2 was sufficient to result in STAT5 phosphorylation in Treg cells.

CONCLUSIONS

IL-3 promotes CD25 expression and shedding by huMCs. Although CD25⁺ huMCs do not respond to IL-2, they bind IL-2 and may act as a reservoir of IL-2 to then activate lymphocytes.

Roles of IgE and Histamine in Mast Cell Maturation

Mast cells are activated upon immunoglobulin E (IgE)-mediated antigen stimulation, and release a wide variety of mediators, including histamine to trigger inflammatory responses. The surface expression levels of Fcε receptor I (FcεRI), a high affinity receptor of IgE, were found to be positively regulated by IgE. IgE could protect murine cultured mast cells from apoptotic cell death induced by the deprivation of interleukin-3 and a certain kind of IgE could activate immature mast cells in the absence of antigens, leading to the release of pro-inflammatory cytokines and a transient increase in histamine synthesis. Histamine synthesis in mast cells was found to be required for the maturation of murine connective tissue-type mast cells, raising the possibility that IgE indirectly modulates local mast cell maturation. Although it remains controversial to what extent this concept of "monomeric IgE effects" could have relevance in the modulation of human mast cell functions, the therapeutic effects of anti-IgE antibodies might be accounted for in terms of the decreased serum IgE concentrations. Because drastic increases in serum IgE concentrations are often observed in patients with atopic dermatitis and chronic urticaria, a close investigation of the roles of IgE in mast cell maturation should contribute to development of novel therapeutic approaches for these inflammatory diseases.

Mast cells as sources of cytokines, chemokines, and growth factors

Mast cells are hematopoietic cells that reside in virtually all vascularized tissues and that represent potential sources of a wide variety of biologically active secreted products, including diverse cytokines and growth factors. There is strong evidence for important non-redundant roles of mast cells in many types of innate or adaptive immune responses, including making important contributions to immediate and chronic IgE-associated allergic disorders and enhancing host resistance to certain venoms and parasites. However, mast cells have been proposed to influence many other biological processes, including responses to bacteria and virus, angiogenesis, wound healing, fibrosis, autoimmune and metabolic disorders, and cancer. The potential functions of mast cells in many of these settings is thought to reflect their ability to secrete, upon appropriate activation by a range of immune or non-immune stimuli, a broad spectrum of cytokines (including many chemokines) and growth factors, with potential autocrine, paracrine, local, and systemic effects. In this review, we summarize the evidence indicating which cytokines and growth factors can be produced by various populations of rodent and human mast cells in response to particular immune or non-immune stimuli, and comment on the proven or potential roles of such mast cell products in health and disease.

IL-3 but not monomeric IgE regulates FcεRI levels and cell survival in primary human basophils

Binding of allergen-specific IgE to its primary receptor FcεRI on basophils and mast cells represents a central event in the development of allergic diseases. The high-affinity interaction between IgE and FcεRI results in permanent sensitization of these allergic effector cells and critically regulates their release of pro-inflammatory mediators upon IgE cross-linking by allergens. In addition, binding of monomeric IgE has been reported to actively regulate FcεRI surface levels and promote survival of mast cells in the absence of allergen through the induction of autocrine cytokine secretion including interleukin-3 (IL-3). As basophils and mast cells share many biological commonalities we sought to assess the role of monomeric IgE binding and IL-3 signaling in FcεRI regulation and cell survival of primary human basophils. FcεRI cell surface levels and survival of isolated blood basophils were assessed upon addition of monomeric IgE or physiologic removal of endogenous cell-bound IgE with a disruptive IgE inhibitor by flow cytometry. We further determined basophil cell numbers in both low and high serum IgE blood donors and mice that are either sufficient or deficient for FcεRI. Ultimately, we investigated the effect of IL-3 on basophil surface FcεRI levels by protein and gene expression analysis. Surface levels of FcεRI were passively stabilized but not actively upregulated in the presence of monomeric IgE. In contrast to previous observations with mast cells, monomeric IgE binding did not enhance basophil survival. Interestingly, we found that IL-3 transcriptionally regulates surface levels of FcεRI in human primary basophils. Our data suggest that IL-3 but not monomeric IgE regulates FcεRI expression and cell survival in primary human basophils. Thus, blocking of IL-3 signaling in allergic effector cells might represent an interesting approach to diminish surface FcεRI levels and to prevent prolonged cell survival in allergic inflammation.

Interleukin-3 stimulates matrix metalloproteinase 12 production from macrophages promoting thoracic aortic aneurysm/dissection

Thoracic aortic aneurysm and dissection (TAAD) is due to degeneration of the aorta and causes a high mortality rate, while molecular mechanisms for the development of TAAD are still not completely understood. In the present study, 3-aminopropionitrile (BAPN) treatment was used to induce TAAD mouse model. Through transcriptome analysis, we found the expression levels of genes associated with interleukin-3 (IL-3) signaling pathway were up-regulated during TAAD development in mouse, which were validated by real-time PCR. IL-3 positive cells were increased in TAAD mouse aortas, especially for smooth muscle cells (SMCs). IL-3 deficiency reduced BAPN-induced TAAD formation. We then examined the matrix metalloproteinases (MMPs) expression during TAAD formation in both wild-type and IL-3 deficient mice, showing that MMP12 were significantly down-regulated in IL-3 deficient aortas. Mechanistically, we found recombinant IL-3 could increase MMP12 production and activity from macrophages in vitro Silencing of IL-3 receptor β, which was mainly expressed in macrophages but not SMCs, diminished the activation of c-Jun N terminal kinase (JNK)/extracellular-regulated protein kinases 1/2 (ERK1/2)/AP-1 signals, and decreased MMP12 expression in IL-3 stimulated macrophages. Moreover, both circulating and aortic inflammation were decreased in IL-3 deficient aortas. Taken together, our results demonstrated that IL-3 stimulated the production of MMP12 from macrophages by a JNK- and ERK1/2-dependent AP-1 pathway, contributing to TAAD formation. Thus, the IL-3/IL-3Rβ/MMP12 signals activation may be an important pathological mechanism for progression of TAAD.

Transgenerational transmission of systemic mast cell activation disease-genetic and epigenetic features

Systemic mast cell activation disease (MCAD) comprises disorders characterized by an enhanced release of mast cell mediators accompanied by a varying accumulation of dysfunctional mast cells. Within the last years, evidence has been presented that MCAD is a multifactorial polygenic determined disease with the KIT(D816V) mutation and its induced functional consequences considered as special case. The respective genes encode proteins for various signaling pathways, epigenetic regulators, the RNA splicing machinery, and transcription factors. Transgenerational transmission of MCAD appears to be quite common. The basics of the molecular mechanisms underlying predisposition of the disease, that is, somatic and germline mutations and the contribution of epigenetic processes have become identifiable. The aim of the present review is to present and discuss available genetic, epigenetic and epidemiological findings, and to present a model of MCAD pathogenesis.

Cardiovascular symptoms in patients with systemic mast cell activation disease

Traditionally, mast cell activation disease (MCAD) has been considered as just one rare (neoplastic) disease, mastocytosis, focused on the mast cell (MC) mediators tryptase and histamine and the suggestive, blatant symptoms of flushing and anaphylaxis. Recently another form of MCAD, the MC activation syndrome, has been recognized featuring inappropriate MC activation with little to no neoplasia and likely much more heterogeneously clonal and far more prevalent than mastocytosis. Increasing expertise and appreciation has been established for the truly very large menagerie of MC mediators and their complex patterns of release, engendering complex, nebulous presentations of chronic and acute illness best characterized as multisystem polymorbidity of generally inflammatory ± allergic theme. We describe the pathogenesis of MCAD with a particular focus on clinical cardiovascular symptoms and the therapeutic options for MC mediator-induced cardiovascular symptoms.

Pharmacological treatment options for mast cell activation disease

Mast cell activation disease (MCAD) is a term referring to a heterogeneous group of disorders characterized by aberrant release of variable subsets of mast cell (MC) mediators together with accumulation of either morphologically altered and immunohistochemically identifiable mutated MCs due to MC proliferation (systemic mastocytosis [SM] and MC leukemia [MCL]) or morphologically ordinary MCs due to decreased apoptosis (MC activation syndrome [MCAS] and well-differentiated SM). Clinical signs and symptoms in MCAD vary depending on disease subtype and result from excessive mediator release by MCs and, in aggressive forms, from organ failure related to MC infiltration. In most cases, treatment of MCAD is directed primarily at controlling the symptoms associated with MC mediator release. In advanced forms, such as aggressive SM and MCL, agents targeting MC proliferation such as kinase inhibitors may be provided. Targeted therapies aimed at blocking mutant protein variants and/or downstream signaling pathways are currently being developed. Other targets, such as specific surface antigens expressed on neoplastic MCs, might be considered for the development of future therapies. Since clinicians are often underprepared to evaluate, diagnose, and effectively treat this clinically heterogeneous disease, we seek to familiarize clinicians with MCAD and review current and future treatment approaches.

Treatment of murine mast cells with IgEκ and protein L enhances apoptotic cell death induced by IL-3 withdrawal

Engagement of the high-affinity IgE receptor (FcεRI) can be either protective or non-protective against apoptotic cell death (ACD) in bone marrow-derived murine mast cells (BMMCs) after IL-3 withdrawal, depending on the avidity between IgE and its antigen. We recently reported that protein L (PpL), a bacterial Igκ-binding soluble protein, is able to stimulate intracellular signaling to induce activation of BMMCs by interacting with the IgEκ-FcεRI complex. However, it is unclear if cross-linking of FcεRI with IgEκ and PpL prevents or enhances IL-3-dependent ACD in BMMCs. In the present study, we found that IL-3-dependent ACD of BMMCs is accelerated by loading soluble PpL in the presence of IgEκ-occupied FcεRIα. For this purpose, soluble PpL was incorporated into the BMMCs. Unlike soluble PpL, immobilized PpL failed to enhance ACD, although both forms of PpL induced IL-6 production equally in BMMCs. In addition, we observed that DNS5-BSA protected anti-DNS IgE-sensitized BMMCs from IL-3 depletion-mediated ACD by inducing the production of autocrine IL-3. In contrast, DNS5-PpL enhanced IL-3 withdrawal-induced ACD of anti-DNS IgE-sensitized BMMCs and reduced the production of autocrine IL-3. These findings suggest that PpL increases IL-3 withdrawal-induced ACD of IgEκ-sensitized BMMCs by incorporating PpL into the BMMCs and that this internalized PpL may interfere with survival signals via FcεRI.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

IgE by Itself Affects Mature Rat Mast Cell Preformed and De Novo-Synthesized Mediator Release and Amplifies Mast Cell Migratory Response

Background

Immunoglobulin E (IgE) binds to high affinity receptor FcεRI numerously expressed on mast cells. Recent findings have revealed that IgE by itself may regulate various aspects of mast cell biology, however, detailed data is still limited.

Methodology/Findings

Here, we have examined the influence of IgE alone, used at different concentrations, on mast cell activity and releasability. For the study we have employed in vivo differentiated mature tissue mast cells isolated from rat peritoneal cavity. Mast cells were exposed to IgE alone and then the release of preformed and de novo-synthesized mediators, surface FcεRI expression and mast cell migratory response were assessed. IgE by itself was found to up-regulate FcεRI expression and activate mast cells to degranulation, as well as de novo synthesis and release of cysteinyl leukotrienes and TNF. We have provided evidence that IgE alone also amplified spontaneous and CCL5- or TNF-induced migration of mast cells. Importantly, IgE was effective only at concentrations ≥ 3 µg/mL. A molecular basis investigation using an array of specific inhibitors showed that Src kinases, PLC/PLA₂, MAP kinases (ERK and p38) and PI3K were entirely or partially involved in IgE-induced mast cell response. Furthermore, IgE alone stimulated the phosphorylation of MAP kinases and PI3K in rat mast cells.

Conclusion

Our results clearly demonstrated that IgE by itself, at higher concentrations, influences mast cell activity and releasability. As there are different conditions when the IgE level is raised it might be supposed that in vivo IgE is one of the important factors modulating mast cell biology within tissues.

Immunoglobulin E induces VEGF production in mast cells and potentiates their pro-tumorigenic actions through a Fyn kinase-dependent mechanism

Background

High concentrations of plasmatic IgE have been related to distinct systemic inflammatory conditions that frequently predispose individuals to hypersensitivity reactions. Although effects of IgE have been suggested to relay on the low-intensity activation of distinct effector elements of the immune system, such as mast cells (MC), experimental evidence on the role of IgE-induced production of inflammatory mediators on specific pathologies is scarce. MC are an important component in tumor microenvironment where they seem to secrete a number of immunomodulatory and angiogenic mediators, such as the Vascular Endothelial Growth Factor (VEGF) by not well-described mechanisms. In this work, we investigated the effect of monomeric IgE (in the absence of antigen) on the production of VEGF in MC, analyzed if monomeric IgE could exacerbate the pro-tumorigenic properties of that cell type and characterized some of the molecular mechanisms behind the effects of IgE on VEGF production and tumor growth.

Methods

For in vitro studies, murine bone marrow-derived mast cells (BMMCs) were used. Pharmacological inhibitors and phosphorylation of key elements controlling VEGF secretion and protein translation were used to characterize the mechanism of VEGF production triggered by IgE.

In vivo, the effect of a single i.v. administration of monomeric IgE on B16 melanoma tumor weight, intratumoral blood vessel formation and tumor-associated MC was assessed in four groups of mice: MC-proficient (WT), MC-deficient (Wsh), Wsh reconstituted with MC derived from WT mice (Wsh Rec WT) and Wsh reconstituted with MC derived from Fyn −/− mice (Wsh Rec Fyn −/−).

Results

Monomeric IgE induced VEGF secretion through a Fyn kinase-dependent mechanism and modulated de novo protein synthesis modifying the activity of the translational regulator 4E-BP1 in BMMCs. In vivo, monomeric IgE increased melanoma tumor growth, peritumoral MC and blood vessel numbers in WT but not in Wsh mice. The positive effects of IgE on melanoma tumor growth were reproduced after reconstitution of Wsh mice with WT but not with Fyn −/− BMMCs.

Conclusion

Our data suggest that monomeric IgE, in the absence of antigen, induces VEGF production in MC and in vivo contributes to melanoma tumor growth through a Fyn kinase-dependent mechanism.

Systemic mast cell activation disease: the role of molecular genetic alterations in pathogenesis, heritability and diagnostics

Despite increasing understanding of its pathophysiology, the aetiology of systemic mast cell activation disease (MCAD) remains largely unknown. Research has shown that somatic mutations in kinases are necessary for the establishment of a clonal mast cell population, in particular mutations in the tyrosine kinase Kit and in enzymes and receptors with crucial involvement in the regulation of mast cell activity. However, other, as yet undetermined, abnormalities are necessary for the manifestation of clinical disease. The present article reviews molecular genetic research into the identification of disease-associated genes and their mutational alterations. The authors also present novel data on familial systemic MCAD and review the associated literature. Finally, the importance of understanding the molecular basis of inherited mutations in terms of diagnostics and therapy is emphasized.

Cytokinergic IgE Action in Mast Cell Activation

Some 10 years ago it emerged that at sufficiently high concentrations certain monoclonal mouse IgEs exert previously unsuspected effects on mast cells. Thus they can both promote survival and induce activation of mast cells without the requirement for antigens. This was a wake up call that appears to have been missed (or dismissed) by the majority of immunologists. The structural attributes responsible for the potency of the so-called “highly cytokinergic” or HC IgEs have not yet been determined, but the events that ensue when such IgEs bind to the high-affinity receptor, FcεRI, on mast cells have been thoroughly studied, and are strikingly similar to those engendered by antigens when they form cross-linked complexes with the receptors. We review the evidence for the cytokinergic activity of IgE, and the structural features and known properties of immunoglobulins, and of IgE in particular, most likely to be implicated in the phenomenon. We suggest that IgEs with cytokinergic activity may be generated by local germinal center reactions in the target organs of allergy. We consider also the important implications that the existence of cytokinergic IgE may have for a fuller understanding of adaptive immunity and of the action of IgE in asthma and other diseases.

Sequential class switching is required for the generation of high affinity IgE antibodies

Generation of anaphylaxis-inducing high affinity IgE requires sequential class switching.

IgE antibodies with high affinity for their antigens can be stably cross-linked at low concentrations by trace amounts of antigen, whereas IgE antibodies with low affinity bind their antigens weakly. In this study, we find that there are two distinct pathways to generate high and low affinity IgE. High affinity IgE is generated through sequential class switching (μ→γ→ε) in which an intermediary IgG phase is necessary for the affinity maturation of the IgE response, where the IgE inherits somatic hypermutations and high affinity from the IgG1 phase. In contrast, low affinity IgE is generated through direct class switching (μ→ε) and is much less mutated. Mice deficient in IgG1 production cannot produce high affinity IgE, even after repeated immunizations. We demonstrate that a small amount of high affinity IgE can cause anaphylaxis and is pathogenic. Low affinity IgE competes with high affinity IgE for binding to Fcε receptors and prevents anaphylaxis and is thus beneficial.

Beyond immediate hypersensitivity: evolving roles for IgE antibodies in immune homeostasis and allergic diseases

Immunoglobulin E (IgE) antibodies have long been recognized as the antigen-specific triggers of allergic reactions. This review briefly introduces the established functions of IgE in immediate hypersensitivity and then focuses on emerging evidence from our own investigations as well as those of others that IgE plays important roles in protective immunity against parasites and exerts regulatory influences in the expression of its own receptors, FcεRI and CD23, as well as controlling mast cell homeostasis. We provide an overview of the multifaceted ways in which IgE antibodies contribute to the pathology of food allergy and speculate regarding potential mechanisms of action of IgE blockade.

Dose-response effects of TPI ASM8 in asthmatics after allergen

BACKGROUND

TPI ASM8 contains two modified antisense oligonucleotides (AON) targeting the beta subunit (β(c) ) of the IL-3, IL-5, GM-CSF receptors and the chemokine receptor CCR3. A previous study suggested that TPI ASM8 had broader effects than just inhibition of eosinophils in asthmatics.

OBJECTIVE

We assessed whether TPI ASM8 caused a dose-dependent attenuation in the inflammatory and physiological changes after inhaled allergen challenge (AIC).

METHODS

This single-center, open-label, stepwise-ascending dose study was conducted in fourteen stable, mild allergic asthmatics. Following placebo AIC, subjects underwent AIC after 4 days treatment with 1, 2, and 4 mg BID and finally 8 mg once daily (OD) of TPI ASM8, inhaled via the I-Neb™ nebuliser. Treatments were separated by 2-3-week washout periods.

RESULTS

TPI ASM8 was safe and well tolerated at all doses. TPI ASM8 8 mg OD reduced eosinophils in sputum after AIC (by 60.9% at 7 h and 68.4% at 24 h post-AIC, P=0.016 and P=0.007, respectively). Additionally, TPI ASM8 8 mg OD significantly attenuated the early and late airway responses as shown by the reduction in the area under the curve by 45% (P=0.016) and 59%, (P=0.0015), respectively, the increase in eosinophil cationic protein (ECP) by up to 57% (P=0.021), and airway responsiveness to methacholine by more than 1 doubling dose (P=0.012). A dose-response relationship was noted, and efficacy was maintained with once per day administration.

CONCLUSIONS

TPI ASM8 attenuated a broad range of inflammatory and physiological changes after AIC, suggesting that CCR3, IL-3, and GM-CSF also are important targets for the management of asthma.

Monomeric IgE and mast cell development, survival and function

Mast cells play a major role in allergy and anaphylaxis, as well as a protective role in immunity against bacteria and venoms (innate immunity) and T-cell activation (acquired immunity).1,2 It was long thought that two steps are essential to mast cell activation. The first step (sensitization) occurs when antigen-specific IgE binds to its high-affinity IgE receptor (FcεRI) expressed on the surface of mast cells. The second step occurs when antigen (Ag) or anti-IgE binds antigen-specific IgE antibodies bound to FcεRI present on the mast cell surface (this mode of stimulation hereafter referred to as IgE+Ag or IgE+anti-IgE stimulation, respectively).Conventional wisdom has been that monomeric IgE plays only an initial, passive role in mast cell activation. However, recent findings have shown that IgE binding to its receptor FcεRI can mediate mast cell activation events even in the absence of antigen (this mode of stimulation hereafter referred to as IgE(-Ag) stimulation). Different subtypes of monomeric IgEs act via IgE(-Ag) stimulation to elicit varied effects on mast cells function, survival and differentiation. This chapter will describe the role of monomeric IgE molecules in allergic reaction, the various effects and mechanisms of action of IgE(-Ag) stimulation on mast cells and what possible developments may arise from this knowledge in the future. Since mast cells are involved in a variety of pathologic and protective responses, understanding the role that monomeric IgE plays in mast cell function, survival and differentiation will hopefully lead to better understanding and treatment of asthma and other allergic diseases, as well as improved understanding of host response to infections.

Lymphocyte deficiency limits Epstein-Barr virus latent membrane protein 1 induced chronic inflammation and carcinogenic pathology in vivo

BACKGROUND

The importance of the malignant cell environment to its growth and survival is becoming increasingly apparent, with dynamic cross talk between the neoplastic cell, the leukocyte infiltrate and the stroma. Most cancers are accompanied by leukocyte infiltration which, contrary to an anticipated immuno-protective role, could be contributing to tumour development and cancer progression. Epstein-Barr virus (EBV) associated cancers, including nasopharyngeal carcinoma and Hodgkin's Disease, show a considerable leukocyte infiltration which surrounds the neoplastic cells, raising the questions as to what role these cells play in either restricting or supporting the tumour and what draws the cells into the tumour. In order to begin to address this we have studied a transgenic model of multistage carcinogenesis with epithelial expression of the EBV primary oncoprotein, latent membrane protein 1 (LMP1). LMP1 is expressed particularly in the skin, which develops a hyperplastic pathology soon after birth.

RESULTS

The pathology advances with time leading to erosive dermatitis which is inflamed with a mixed infiltrate involving activated CD8+ T-cells, CD4+ T-cells including CD4+/CD25+/FoxP3+ Treg cells, mast cells and neutrophils. Also significant dermal deposition of immunoglobulin-G (IgG) is observed as the pathology advances. Along with NF-kappaB activation, STAT3, a central factor in inflammation regulation, is activated in the transgenic tissue. Several inflammatory factors are subsequently upregulated, notably CD30 and its ligand CD153, also leukocyte trafficking factors including CXCL10, CXCL13, L-selectin and TGFβ1, and inflammatory cytokines including IL-1β, IL-3 and the murine IL-8 analogues CXCL1, CXCL2 and CXCL5-6, amongst others. The crucial role of mature T- and/or B-lymphocytes in the advancing pathology is demonstrated by their elimination, which precludes mast cell infiltration and limits the pathology to an early, benign stage.

CONCLUSIONS

LMP1 can lead to the activation of several key factors mediating proliferation, angiogenesis and inflammation in vivo. With the initiation of an inflammatory programme, leukocyte recruitment follows which then itself contributes to the progressing pathology in these transgenic mice, with a pivotal role for B-and/or T-cells in the process. The model suggests a basis for the leukocyte infiltrate observed in EBV-associated cancer and its supporting role, as well as potential points for therapeutic intervention.

The SYK tyrosine kinase: a crucial player in diverse biological functions

Spleen tyrosine kinase (SYK) is known to have a crucial role in adaptive immune receptor signalling. However, recent reports indicate that SYK also mediates other, unexpectedly diverse biological functions, including cellular adhesion, innate immune recognition, osteoclast maturation, platelet activation and vascular development. SYK is activated by C-type lectins and integrins, and activates new targets, including the CARD9-BCL-10-MALT1 pathway and the NLRP3 inflammasome. Studies using Drosophila melanogaster suggest that there is an evolutionarily ancient origin of SYK-mediated signalling. Moreover, SYK has a crucial role in autoimmune diseases and haematological malignancies. This Review summarizes our current understanding of the diverse functions of SYK and how this is being translated for therapeutic purposes.

Inducible MHC class II expression by mast cells supports effector and regulatory T cell activation

In addition to their well-established role as regulators of allergic response, recent evidence supports a role for mast cells in influencing the outcome of physiologic and pathologic T cell responses. One mechanism by which mast cells (MCs) influence T cell function is indirectly through secretion of various cytokines. It remains unclear, however, whether MCs can directly activate T cells through Ag presentation, as the expression of MHC class II by MCs has been controversial. In this report, we demonstrate that in vitro stimulation of mouse MCs with LPS and IFN-gamma induces the expression of MHC class II and costimulatory molecules. Although freshly isolated peritoneal MCs do not express MHC class II, an in vivo inflammatory stimulus increases the number of MHC class II-positive MCs in situ. Expression of MHC class II granted MCs the ability to process and present Ags directly to T cells with preferential expansion of Ag-specific regulatory T cells over naive T cells. These data support the notion that, in the appropriate setting, MCs may regulate T cell responses through the direct presentation of Ag.

Human basophils secrete IL-3: evidence of autocrine priming for phenotypic and functional responses in allergic disease

Although IL-3 is commonly recognized for its growth factor-like activity, in vitro studies have long demonstrated a unique capacity for this cytokine to also augment the proinflammatory properties and phenotype of human basophils. In particular, basophils secrete mediators that are hallmarks in allergic disease, including vasoactive amines (e.g., histamine), lipid metabolites (e.g., leukotriene C(4)), and cytokines (e.g., IL-4/IL-13), which are all markedly enhanced with IL-3 pretreatment. This priming phenomenon is observed in response to both IgE-dependent and IgE-independent stimulation. Additionally, IL-3 directly activates basophils for IL-13 secretion and enhanced CD69 expression, two markers that are elevated in allergic subjects. Lymphocytes are commonly thought to be the source of the IL-3 that primes for these basophil responses. However, we demonstrate herein for the first time that basophils themselves rapidly produce IL-3 (within 4 h) in response to IgE-dependent activation. More importantly, our findings definitively show that basophils rapidly bind and utilize the IL-3 they produce, as evidenced by functional and phenotypic activity that is inhibited in the presence of neutralizing anti-IL-3 receptor (CD123) Abs. We predict that autocrine IL-3 activity resulting from low-level IgE/FcepsilonRI cross-linking by specific allergen represents an important mechanism behind the hyperreactive nature of basophils that has long been observed in allergic disease.

IgE influences the number and function of mature mast cells, but not progenitor recruitment in allergic pulmonary inflammation

Studies performed using cultured cells indicate that IgE functions not only to trigger degranulation of mast cells following allergen exposure, but also to enhance their survival. Such an influence of IgE on mast cell homeostasis during allergic responses in vivo has not been established. In this study, we show that inhalation of Aspergillus fumigatus extract in mice induced a dramatic rise in IgE accompanied by an increase in airway mast cells. These had an activated phenotype with high levels of FcepsilonRI. Plasma mast cell protease-1 was also increased, indicating an elevated systemic mast cell load. In addition, enhanced levels of IL-5 and eosinophils were observed in the airway. Both mast cell expansion and activation were markedly attenuated in IgE(-/-) animals that are incapable of producing IgE in response to A. fumigatus. The recruitment of eosinophils to the airways was also reduced in IgE(-/-) mice. Analyses of potential cellular targets of IgE revealed that IgE Abs are not required for the induction of mast cell progenitors in response to allergen, but rather act by sustaining the survival of mature mast cells. Our results identify an important role for IgE Abs in promoting mast cell expansion during allergic responses in vivo.

IgE-induced mast cell survival requires the prolonged generation of reactive oxygen species

We show in this study that the ability of five different monomeric IgEs to enhance murine bone marrow-derived mast cell (BMMC) survival correlates with their ability to stimulate extracellular calcium (Ca(2+)) entry. However, whereas IgE+Ag more potently stimulates Ca(2+) entry, it does not enhance survival under our conditions. Exploring this further, we found that whereas all five monomeric IgEs stimulate a less robust Ca(2+) entry than IgE+Ag initially, they all trigger a more prolonged Ca(2+) influx, generation of reactive oxygen species (ROS), and ERK phosphorylation. These prolonged signaling events correlate with their survival-enhancing ability and positively feedback on each other to generate the prosurvival cytokine, IL-3. Interestingly, the prolonged ERK phosphorylation induced by IgE appears to be regulated by a MAPK phosphatase rather than MEK. IgE-induced ROS generation, unlike that triggered by IgE+Ag, is not mediated by 5-lipoxygenase. Moreover, ROS inhibitors, which block both IgE-induced ROS production and Ca(2+) influx, convert the prolonged ERK phosphorylation induced by IgE into the abbreviated phosphorylation pattern observed with IgE+Ag and prevent IL-3 generation. In support of the essential role that IgE-induced ROS plays in IgE-enhanced BMMC survival, we found the addition of H(2)O(2) to IgE+Ag-stimulated BMMCs leads to IL-3 secretion.

FcgammaRIV is a mouse IgE receptor that resembles macrophage FcepsilonRI in humans and promotes IgE-induced lung inflammation

FcgammaRIV is a recently identified mouse activating receptor for IgG2a and IgG2b that is expressed on monocytes, macrophages, and neutrophils; herein it is referred to as mFcgammaRIV. Although little is known about mFcgammaRIV, it has been proposed to be the mouse homolog of human FcgammaRIIIA (hFcgammaRIIIA) because of high sequence homology. Our work, however, has revealed what we believe to be new properties of mFcgammaRIV that endow this receptor with a previously unsuspected biological significance; we have shown that it is a low-affinity IgE receptor for all IgE allotypes. Although mFcgammaRIV functioned as a high-affinity IgG receptor, mFcgammaRIV-bound monomeric IgGs were readily displaced by IgE immune complexes. Engagement of mFcgammaRIV by IgE immune complexes induced bronchoalveolar and peritoneal macrophages to secrete cytokines, suggesting that mFcgammaRIV may be an equivalent of human FceRI(alphagamma), which is expressed by macrophages and neutrophils and especially in atopic individuals, rather than an equivalent of hFcgammaRIIIA, which has no affinity for IgE. Using mice lacking 3 FcgammaRs and 2 FceRs and expressing mFcgammaRIV only, we further demonstrated that mFcgammaRIV promotes IgE-induced lung inflammation. These data lead us to propose a mouse model of IgE-induced lung inflammation in which cooperation exists between mast cells and mFcgammaRIV-expressing lung cells. We therefore suggest that a similar cooperation may occur between mast cells and hFceRI-expressing lung cells in human allergic asthma.

Oncogenic Kit controls neoplastic mast cell growth through a Stat5/PI3-kinase signaling cascade

The D816V-mutated variant of Kit triggers multiple signaling pathways and is considered essential for malignant transformation in mast cell (MC) neoplasms. We here describe that constitutive activation of the Stat5-PI3K-Akt-cascade controls neoplastic MC development. Retrovirally transduced active Stat5 (cS5(F)) was found to trigger PI3K and Akt activation, and to transform murine bone marrow progenitors into tissue-infiltrating MCs. Primary neoplastic Kit D816V(+) MCs in patients with mastocytosis also displayed activated Stat5, which was found to localize to the cytoplasm and to form a signaling complex with PI3K, with consecutive Akt activation. Finally, the knock-down of either Stat5 or Akt activity resulted in growth inhibition of neoplastic Kit D816V(+) MCs. These data suggest that a downstream Stat5-PI3K-Akt signaling cascade is essential for Kit D816V-mediated growth and survival of neoplastic MCs.

Pivotal advance: IgE accelerates in vitro development of mast cells and modifies their phenotype

Antigen-dependent activation of IgE-bound mast cells is critical for immediate hypersensitivity and other allergic disorders. Recent studies have revealed the effects of monomeric IgEs on mast cell survival and activation. Furthermore, IgE molecules exhibit a wide range of heterogeneity in the ability to induce mast cell activation in the absence of antigen. Highly cytokinergic (HC) IgEs can induce a variety of activation events including cell survival, degranulation, cytokine production, and migration, whereas poorly cytokinergic (PC) IgEs can do so inefficiently. Here, we show that culture of bone marrow cells in the presence of monomeric IgEs results in an increased number of mast cells compared with cultures grown without IgE. Furthermore, time in culture required to generate > or =80% pure mast cells is decreased. IgE molecules can directly influence mast cell progenitors to differentiate into mast cells. mRNA expression of several mast cell proteases and mast cell-related transcription factors is higher in mast cells cultured with an HC IgE than those cultured with a PC IgE or without IgE. Expression of early growth response factor-1, a transcription factor that is involved in the production of TNF-alpha in mast cells, is enhanced in cultures containing high and low concentrations of HC IgE and a high concentration of PC IgE. Consistent with this, expression of TNF-alpha is higher in mast cells cultured with HC IgE than PC IgE. Therefore, our results suggest that monomeric IgEs, especially HC IgEs, not only promote mast cell development but also modulate the mast cell phenotype.

Progress in allergy signal research on mast cells: signal regulation of multiple mast cell responses through FcepsilonRI

The crosslinking of FcepsilonRI by IgE and antigen (Ag) on mast cells initiates activation cascades that lead to allergic responses. Although it was thought that IgE binding to FcepsilonRI is a passive "sensitization", recent reports suggest that IgE actively promotes mast cell survival in the absence of Ag. However, it is largely unknown how these distinct responses are delivered through the same receptor, FcepsilonRI, depending on the types of stimli. As an underlying molecular mechanism for the generation of diverse responses through FcepsilonRI, we found that the quantity and the duration of the signal through the FcepsilonRI gamma chain (FcRgamma) determine different mast cell responses. Furthermore, FcRgamma-mediated sustained Erk activation is critical for IgE-induced mast cell survival through autocrine production of IL-3. Transmembrane adaptors LAT and NTAL contribute to the maintenance of prolonged Erk activation through membrane retention of the Ras-activating complex, Grb2-Sos. In this review, the signal regulation for the distinct responses of mast cells through FcepsilonRI are discussed.

Thrombopoietin inhibits murine mast cell differentiation

We have recently shown that Mpl, the thrombopoietin receptor, is expressed on murine mast cells and on their precursors and that targeted deletion of the Mpl gene increases mast cell differentiation in mice. Here we report that treatment of mice with thrombopoietin or addition of this growth factor to bone marrow-derived mast cell cultures severely hampers the generation of mature cells from their precursors by inducing apoptosis. Analysis of the expression profiling of mast cells obtained in the presence of thrombopoietin suggests that thrombopoietin induces apoptosis of mast cells by reducing expression of the transcription factor Mitf and its target antiapoptotic gene Bcl2.

IgE alone promotes human lung mast cell survival through the autocrine production of IL-6

Background

Mast cells play a key role in asthma and recent evidence indicates that their ongoing activation in this disease is mediated, in part, via IgE in the absence of antigen. In this study we have examined whether IgE alone enhances human lung mast cell (HLMC) survival.

Methods

Purified HLMC were cultured for 4 weeks and survival assays then performed over 10 days following cytokine withdrawal in the presence or absence of human myeloma IgE. Quantitative real time RT-PCR was carried out to examine IL-6 mRNA expression and IL-6 protein was measured in HLMC supernatants by ELISA.

Results

IgE alone promoted the survival of HLMC in a dose-dependent manner following cytokine withdrawal. IgE-induced survival was eliminated with the addition of neutralising anti-IL-6 antibody but not by the addition of neutralising anti-stem cell factor. IgE sensitisation initiated profound upregulation of IL-6 mRNA in HLMC, and IL-6 concentrations were also raised in the culture supernatants of IgE-exposed cells.

Conclusion

These data taken together suggest that IgE in the absence of antigen promotes HLMC survival through the autocrine production of IL-6. This provides a further mechanism through which IL-6 and IgE contribute to the pathogenesis of asthma, and through which anti-IgE therapy might achieve its therapeutic effect.

Cloning of B cell-specific membrane tetraspanning molecule BTS possessing B cell proliferation-inhibitory function

Lymphocyte proliferation is regulated by signals through antigen receptors, co-stimulatory receptors, and other positive and negative modulators. Several membrane tetraspanning molecules are also involved in the regulation of lymphocyte growth and death. We cloned a new B cell-specific tetraspanning (BTS) membrane molecule, which is similar to CD20 in terms of expression, structure and function. BTS is specifically expressed in the B cell line and its expression is increased after the pre-B cell stage. BTS is expressed in intracellular granules and on the cell surface. Overexpression of BTS in immature B cell lines induces growth retardation through inhibition of cell cycle progression and cell size increase without inducing apoptosis. This inhibitory function is mediated predominantly by the N terminus of BTS. The development of mature B cells is inhibited in transgenic mice expressing BTS, suggesting that BTS is involved in the in vivo regulation of B cells. These results indicate that BTS plays a role in the regulation of cell division and B cell growth.

Bcl-2 and Bcl-XL are indispensable for the late phase of mast cell development from mouse embryonic stem cells

OBJECTIVE

The aim of this study was to determine the importance of the prosurvival factors Bcl-2 and Bcl-XL for mast cell development and survival.

METHODS

bcl-x(-/-) and bcl-2(-/-) mouse embryonic stem cells were maintained in medium supplemented with either interleukin (IL)-3 or IL-3 in combination with stem cell factor (SCF) to favor mast cell development. The development of Bcl-2 family deficient embryonic stem cell-derived mast cells (ESMCs) was monitored and Bcl-2 family gene expression and cell numbers were analyzed.

RESULTS

Deficiency in either bcl-x or bcl-2 totally inhibited the development of ESMCs when IL-3 alone was used as a mast cell growth factor. Intriguingly, when IL-3 was used in combination with SCF, the ESMCs developed normally the first 2 weeks but thereafter the cell numbers dropped drastically. The remaining ESMCs express mouse mast cell protease 1, suggesting a mucosal-like phenotype. ESMCs lacking bcl-x or bcl-2 exhibited strong expression of A1, another prosurvival Bcl-2 family member.

CONCLUSION

For the first time we provide direct evidence that both bcl-x and bcl-2 are indispensable for mast cell survival during the late phase of their development.

Monomeric IgE and lipopolysaccharide synergistically prevent mast-cell apoptosis

The apoptosis of bone marrow-derived mast-cells (BMMCs) after growth factor withdrawal was significantly prevented by a high concentration of IgE in the absence of antigen, and further enhanced by the presence of Toll-like receptor4 (TLR4) ligand, lipopolysaccharide (LPS). The effect of LPS was mediated by TLR4, since TLR4-deficient BMMCs did not show synergistic effects with IgE. The neutralizing amount of anti-IL-3 did not reverse the anti-apoptotic effects of both IgE and combination with LPS. LPS treatment with monomeric IgE synergistically prevented the loss of mitochondrial membrane potentials and was associated with an enhanced expression of anti-apoptotic protein, Bcl-xL, or with a reduced expression of proapoptotic protein, Puma, and Bim, respectively. Altogether, these results suggest that LPS, in a TLR4-dependent manner, together with IgE, synergistically prevent mast-cell apoptosis and may contribute to regulate the tissue mast-cell number.

New developments in FcepsilonRI regulation, function and inhibition

The high-affinity Fc receptor for IgE (FcepsilonRI), a multimeric immune receptor, is a crucial structure for IgE-mediated allergic reactions. In recent years, advances have been made in several important areas of the study of FcepsilonRI. The first area relates to FcepsilonRI-mediated biological responses that are antigen independent. The second area encompasses the biological relevance of the distinct signalling pathways that are activated by FcepsilonRI; and the third area relates to the accumulated evidence for the tight control of FcepsilonRI signalling through a broad array of inhibitory mechanisms, which are being developed into promising therapeutic approaches.

Anti-IgE antibodies for the treatment of IgE-mediated allergic diseases

The pharmacological purposes of the anti-IgE therapy are to neutralize IgE and to inhibit its production to attenuate type I hypersensitivity reactions. The therapy is based on humanized IgG1 antibodies that bind to free IgE and to membrane-bound IgE on B cells, but not to IgE bound by the high-affinity IgE.Fc receptors on basophils and mast cells or by the low-affinity IgE.Fc receptors on B cells. After nearly 20 years since inception, therapeutic anti-IgE antibodies (anti-IgE) have been studied in about 30 Phase II and III clinical trials in many allergy indications, and a lead antibody, omalizumab, has been approved for treating patients (12 years and older) with moderate-to-severe allergic asthma. Anti-IgE has confirmed the roles of IgE in the pathogenesis of asthma and helped define the concept "allergic asthma" in clinical practice. It has been shown to be safe and efficacious in treating pediatric allergic asthma and treating allergic rhinitis and is being investigated for treating peanut allergy, atopic dermatitis, latex allergy, and others. It has potential for use to combine with specific and rush immunotherapy for increased safety and efficacy. Anti-IgE thus appears to provide a prophylactic and therapeutic option for moderate to severe cases of many allergic diseases and conditions in which IgE plays a significant role. This chapter reviews the evolution of the anti-IgE concept and the clinical studies of anti-IgE on various disease indications, and presents a comprehensive analysis on the multiple intricate immunoregulatory pharmacological effects of anti-IgE. Finally, it reviews other approaches that target IgE or IgE-expressing B cells.

The mast cell IgG receptors and their roles in tissue inflammation

Mast cells are effector cells of the innate immune system, but because they express Fc receptors (FcRs), they can be engaged in adaptive immunity by antibodies. Mast cell FcRs include immunoglobulin E (IgE) and IgG receptors and, among these, activating and inhibitory receptors. The engagement of mast cell IgG receptors by immune complexes may or may not trigger cell activation, depending on the type of mast cell. The coengagement of IgG and IgE receptors results in inhibition of mast cell activation. The Src homology-2 domain-containing inositol 5-phosphatase-1 is a major effector of negative regulation. Biological responses of mast cells depend on the balance between positive and negative signals that are generated in FcR complexes. The contribution of human mast cell IgG receptors in allergies remains to be clarified. Increasing evidence indicates that mast cells play critical roles in IgG-dependent tissue-specific autoimmune diseases. Convincing evidence was obtained in murine models of multiple sclerosis, rheumatoid arthritis, bullous pemphigoid, and glomerulonephritis. In these models, the intensity of lesions depended on the relative engagement of activating and inhibitory IgG receptors. In vitro models of mature tissue-specific murine mast cells are needed to investigate the roles of mast cells in these diseases. One such model unraveled unique differentiation/maturation-dependent biological responses of serosal-type mast cells.

Role of thrombopoietin in mast cell differentiation

Mast cells are important elements of the body response to foreign antigens, being those represented either by small molecules (allergic response) or harbored by foreign microorganisms (response to parasite infection). These cells derive from hematopoietic stem/progenitor cells present in the marrow. However, in contrast with most of the other hematopoietic lineages, mast cells do not differentiate in the marrow but in highly vascularized extramedullary sites, such as the skin or the gut. Mast cell differentiation in the marrow is activated as part of the body response to parasites. We will review here the mast cell differentiation pathway and what is known of its major intrinsic and extrinsic control mechanisms. It will also be described that thrombopoietin, the ligand for the Mpl receptor, in addition to its pivotal rule in the control of thrombocytopoiesis and of hematopoietic stem/progenitor cell proliferation, exerts a regulatory function in mast cell differentiation. Some of the possible implications of this newly described biological activity of thrombopoietin will be discussed.

LAT and NTAL mediate immunoglobulin E-induced sustained extracellular signal-regulated kinase activation critical for mast cell survival

Immunoglobulin E (IgE) induces mast cell survival in the absence of antigen (Ag) through the high-affinity IgE receptor, Fcepsilon receptor I (FcepsilonRI). Although we have shown that protein tyrosine kinase Syk and sustained extracellular signal-regulated kinase (Erk) activation are required for IgE-induced mast cell survival, how Syk couples with sustained Erk activation is still unclear. Here, we report that the transmembrane adaptors LAT and NTAL are phosphorylated slowly upon IgE stimulation and that sustained but not transient Erk activation induced by IgE was inhibited in LAT(-/-) NTAL(-/-) bone marrow-derived mast cells (BMMCs). IgE-induced survival requires Ras activation, and both were impaired in LAT(-/-) NTAL(-/-) BMMCs. Sos was preferentially required for FcepsilonRI signals by IgE rather than IgE plus Ag. Survival impaired in LAT(-/-) NTAL(-/-) BMMCs was restored to levels comparable to those of the wild type by membrane-targeted Sos, which bypasses the Grb2-mediated membrane recruitment of Sos. The IgE-induced survival of BMMCs lacking Gads, an adaptor critical for the formation of the LAT-SLP-76-phospholipase Cgamma (PLCgamma) complex, was observed to be normal. IgE stimulation induced the membrane retention of Grb2-green fluorescent protein fusion proteins in wild-type but not LAT(-/-) NTAL(-/-) BMMCs. These results suggest that LAT and NTAL contribute to the maintenance of Erk activation and survival through the membrane retention of the Ras-activating complex Grb2-Sos and, further, that the LAT-Gads-SLP-76-PLCgamma and LAT/NTAL-Grb2-Sos pathways are differentially required for degranulation and survival, respectively.

Distinct role of IL-3 promoter and enhancer region in murine mast cells

Crosslinking of Fcvarepsilon receptor on mast cells induces IL-3 gene expression with the concentration dependent of intracellular calcium, but its regulatory mechanism remains unclear. Here, we found that phorbol 12-myristate 13-acetate (PMA) alone did not induce IL-3 gene expression, but potentiated A23187-induced IL-3 gene expression. Interestingly, the A23187-induced IL-3 promoter activity was suppressed by PMA, but it was enhanced when IL-3 promoter contained enhancer region, a DH site. While IL-3 mRNA expression was increased by A23187 and PMA in a dose-dependent manner, the promoter activity appeared all or none in all doses of A23187 and PMA. IL-3 promoter region between -293 and -150bp was responsible for A23187-induced gene expression and PMA- or cyclosporin A (CsA)-mediated suppression. Taken together, IL-3 gene expression was primarily regulated at the transcriptional level, which was differentially controlled by a restricted promoter and enhancer region.

Stat5 expression is required for IgE-mediated mast cell function

The mast cell (MC) inflammatory response is now linked not only to atopy, but also to arthritis, multiple sclerosis, heart disease, and resistance to bacterial infection. In the current study, we demonstrate that the signal transducer and activator of transcription 5 (Stat5) is rapidly activated by IgE cross-linkage, and that its expression is critical to the MC response. Stat5-deficient (Stat5KO) MC demonstrated a significant decrease in IgE-mediated degranulation, leukotriene B4 production, cytokine secretion, and survival signals. The defect in cytokine production may be caused by decreased cytokine mRNA stability. Stat5KO MC-induced cytokine mRNAs normally following IgE cross-linkage, but these mRNAs were not sustained over time and were degraded at twice the rate observed in WT cells. Interestingly, the RNA destabilizing protein tristetraprolin was induced following IgE cross-linkage in Stat5KO but not wild-type cells. Moreover, reducing tristetraprolin expression via short hairpin RNA transfection significantly increased IL-13 production in Stat5KO MC. Our work demonstrates that Stat5 is a critical factor in IgE-induced MC activation, acting in part via posttranscriptional control of cytokine mRNA stability. These data have a direct impact on MC-associated inflammatory and autoimmune diseases.

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.

The duplicitous nature of the Lyn tyrosine kinase in growth factor signaling

The Lyn tyrosine kinase is a unique member of the Src family of non-receptor protein tyrosine kinases whose principal role is to regulate signals through inhibitory receptors thereby promoting signal attenuation. Lyn is renowned for its role in B cell antigen receptor and FcepsilonRI signaling; however, it is becoming increasingly apparent that Lyn also functions in signal transduction from growth factor receptors including the receptors for GM-CSF, IL-3, IL-5, SCF, erythropoietin, CSF-1, G-CSF, thrombopoietin and Flt3 ligand. Numerous studies have implicated Lyn in growth factor receptor signal amplification, while a number also suggest that Lyn participates in negative regulation of growth factor signaling. Indeed Lyn-deficient mice are hyper-responsive to myeloid growth factors and develop a myeloproliferative disorder that predisposes the mice to macrophage tumours, with loss of negative regulation through SHP-1 and SHIP-1 thought to be the major contributing factor to this phenotype. Developing a clear understanding of Lyn's role in establishing signaling thresholds in growth factor receptor signal amplification and signal inhibition may have important implications in the management of leukemias that may depend on Lyn activity.