Kit- and Fc epsilonRI-induced differential phosphorylation of the transmembrane adaptor molecule NTAL/LAB/LAT2 allows flexibility in its scaffolding function in mast cells

Mast Cell Activation Syndromes: Comparison Between Two Scoring Models to Predict for Mast Cell Clonality

BACKGROUND

The Red Española de Mastocitosis (Spanish Network on Mastocytosis) score (REMAs) and the National Institutes of Health idiopathic clonal anaphylaxis score (NICAS) were developed for more efficient screening of mast cell (MC) clonality in MC activation syndromes. In a limited idiopathic anaphylaxis case series, the NICAS showed higher accuracy compared with the REMAs.

OBJECTIVE

To compare the performance of the REMAs against the NICAS in the diagnosis of MC clonality.

METHODS

We compared the diagnostic value of the REMAs against the NICAS in 182 patients (63% men, median age 56 years) who presented with anaphylaxis triggered by Hymenoptera venom allergy (45%), drugs (15%), food (11%), idiopathic anaphylaxis (20%), and mixed causes (10%). KIT mutation was assessed in parallel in whole blood and bone marrow (BM) and, when negative, in highly purified BM MC. TPSAB1 was genotyped in a subset of 71 patients.

RESULTS

We found higher accuracy and rates of correctly classified patients for the REMAs (82% and 84%) compared with the NICAS (75% and 75%; P = .02 and P = .03, respectively), particularly among men (P = .05), patients with systemic mastocytosis (P = .05), those presenting anaphylaxis owing to any cause featuring urticaria (P = .04), cardiovascular symptoms (P = .02), and/or presyncope (P = .02) and those with a blood-negative/BM-positive KIT mutational profile (P = .002), but not hereditary α-tryptasemia-associated genotypes. Combined assessment of the REMAs and KIT^D816V in blood yielded an overall improved classification efficiency of 86% versus 84% for REMAs.

CONCLUSIONS

The combined use of the REMAs and blood detection of KIT^D816V is recommended, but more sensitive blood-based molecular assays to detect KIT^D816V are needed.

KIT as a master regulator of the mast cell lineage

The discovery in 1987/1988 and 1990 of the cell-surface receptor KIT and its ligand, stem cell factor (SCF), were critical achievements in efforts to understand the development and function of multiple distinct cell lineages. These include hematopoietic progenitors, melanocytes, germ cells, and mast cells, which all are significantly affected by loss-of-function mutations of KIT or SCF. Such mutations also influence the development and/or function of additional cells, including those in parts of the CNS and the interstitial cells of Cajal (that control gut motility). Many other cells can express KIT constitutively or during immune responses, including dendritic cells, eosinophils, ILC2 cells, and taste cells. Yet the biological importance of KIT in many of these cell types largely remains to be determined. We here review the history of work investigating mice with mutations affecting the W locus (that encodes KIT) or the Sl locus (that encodes SCF), focusing especially on the influence of such mutations on mast cells. We also briefly review efforts to target the KIT/SCF pathway with anti-SCF or anti-KIT antibodies in mouse models of allergic disorders, parasite immunity, or fibrosis in which MCs are thought to play significant roles.

Selecting the Right Criteria and Proper Classification to Diagnose Mast Cell Activation Syndromes: A Critical Review

In recent years, knowledge about mechanisms underlying mast cell activation (MCA) and accumulation in various pathologic conditions increased substantially. In addition, criteria and a classification of MCA syndromes (MCASs) have been set forth. MCAS is defined by typical clinical symptoms, a substantial increase in serum tryptase level during an attack over the patient's baseline tryptase, and a response of the symptoms to drugs targeting mast cells, mediator production, and/or mediator effects. Alternative diagnostic criteria of MCAS have also been suggested, but these alternative criteria often lack specificity and validation. In this report, we critically review the contemporary literature relating to MCAS and compare the specificity, sensitivity, and strength of MCAS-related parameters within proposals to diagnose and classify MCAS and its variants. Furthermore, we highlight the need to apply specific consensus criteria in the evaluation and classification of MCAS in individual patients. This is an urgent and important medical necessity because as an increasing number of patients are being given a misdiagnosis of MCAS based on nonspecific criteria, which contributes to confusion and frustration by patients and caregivers and sometimes may delay recognition and treatment of correct medical conditions that often turn out to be unrelated to MCA.

Idiopathic Anaphylaxis: a Perplexing Diagnostic Challenge for Allergists

PURPOSE OF REVIEW

The aim of this systematic review is to present the proposed theories of pathogenesis for idiopathic anaphylaxis (IA), to discuss its classification, its diagnostic approach, and management.

RECENT FINDINGS

IA represents a major diagnostic challenge and is diagnosed when excluding the possible identifiable triggers of anaphylaxis. The current research, however, revealed that certain conditions including mastocytosis, mast cell activation syndromes, and hereditary alpha tryptasemia can masquerade and overlap with its symptomatology. Also, newly identified galactose-alpha-1,3-galactose mammalian red meat allergy has recently been recognized as underlying cause of anaphylaxis in some cases that were previously considered as IA. IA comprises a heterogenous group of conditions where, in some cases, inherently dysfunctional mast cells play a role in pathogenesis. The standard trigger avoidance strategies are ineffective, and episodes are unpredictable. Therefore, prompt recognition and treatment as well as prophylaxis are critical. The patients should always carry an epinephrine autoinjector.

SRC-Family Kinases in Acute Myeloid Leukaemia and Mastocytosis

Protein tyrosine kinases have been recognized as important actors of cell transformation and cancer progression, since their discovery as products of viral oncogenes. SRC-family kinases (SFKs) play crucial roles in normal hematopoiesis. Not surprisingly, they are hyperactivated and are essential for membrane receptor downstream signaling in hematological malignancies such as acute myeloid leukemia (AML) and mastocytosis. The precise roles of SFKs are difficult to delineate due to the number of substrates, the functional redundancy among members, and the use of tools that are not selective. Yet, a large num ber of studies have accumulated evidence to support that SFKs are rational therapeutic targets in AML and mastocytosis. These two pathologies are regulated by two related receptor tyrosine kinases, which are well known in the field of hematology: FLT3 and KIT. FLT3 is one of the most frequently mutated genes in AML, while KIT oncogenic mutations occur in 80-90% of mastocytosis. Studies on oncogenic FLT3 and KIT signaling have shed light on specific roles for members of the SFK family. This review highlights the central roles of SFKs in AML and mastocytosis, and their interconnection with FLT3 and KIT oncoproteins.

Cytoskeletal Protein 4.1R Is a Positive Regulator of the FcεRI Signaling and Chemotaxis in Mast Cells

Protein 4.1R, a member of the 4.1 family, functions as a bridge between cytoskeletal and plasma membrane proteins. It is expressed in T cells, where it binds to a linker for activation of T cell (LAT) family member 1 and inhibits its phosphorylation and downstream signaling events after T cell receptor triggering. The role of the 4.1R protein in cell activation through other immunoreceptors is not known. In this study, we used 4.1R-deficient (4.1R-KO) and 4.1R wild-type (WT) mice and explored the role of the 4.1R protein in the high-affinity IgE receptor (FcεRI) signaling in mast cells. We found that bone marrow mast cells (BMMCs) derived from 4.1R-KO mice showed normal growth in vitro and expressed FcεRI and c-KIT at levels comparable to WT cells. However, 4.1R-KO cells exhibited reduced antigen-induced degranulation, calcium response, and secretion of tumor necrosis factor-α. Chemotaxis toward antigen and stem cell factor (SCF) and spreading on fibronectin were also reduced in 4.1R-KO BMMCs, whereas prostaglandin E₂-mediated chemotaxis was not affected. Antibody-induced aggregation of tetraspanin CD9 inhibited chemotaxis toward antigen in WT but not 4.1R-KO BMMCs, implying a CD9-4.1R protein cross-talk. Further studies documented that in the absence of 4.1R, antigen-mediated phosphorylation of FcεRI β and γ subunits was not affected, but phosphorylation of SYK and subsequent signaling events such as phosphorylation of LAT1, phospholipase Cγ1, phosphatases (SHP1 and SHIP), MAP family kinases (p38, ERK, JNK), STAT5, CBL, and mTOR were reduced. Immunoprecipitation studies showed the presence of both LAT1 and LAT2 (LAT, family member 2) in 4.1R immunocomplexes. The positive regulatory role of 4.1R protein in FcεRI-triggered activation was supported by in vivo experiments in which 4.1R-KO mice showed the normal presence of mast cells in the ears and peritoneum, but exhibited impaired passive cutaneous anaphylaxis. The combined data indicate that the 4.1R protein functions as a positive regulator in the early activation events after FcεRI triggering in mast cells.

Changing the threshold-Signals and mechanisms of mast cell priming

Mast cells play a key role in allergy and other inflammatory diseases involving engagement of multivalent antigen with IgE bound to high-affinity IgE receptors (FcεRIs). Aggregation of FcεRIs on mast cells initiates a cascade of signaling events that eventually lead to degranulation, secretion of leukotrienes and prostaglandins, and cytokine and chemokine production contributing to the inflammatory response. Exposure to pro-inflammatory cytokines, chemokines, bacterial and viral products, as well as some other biological products and drugs, induces mast cell transition from the basal state into a primed one, which leads to enhanced response to IgE-antigen complexes. Mast cell priming changes the threshold for antigen-mediated activation by various mechanisms, depending on the priming agent used, which alone usually do not induce mast cell degranulation. In this review, we describe the priming processes induced in mast cells by various cytokines (stem cell factor, interleukins-4, -6 and -33), chemokines, other agents acting through G protein-coupled receptors (adenosine, prostaglandin E₂ , sphingosine-1-phosphate, and β-2-adrenergic receptor agonists), toll-like receptors, and various drugs affecting the cytoskeleton. We will review the current knowledge about the molecular mechanisms behind priming of mast cells leading to degranulation and cytokine production and discuss the biological effects of mast cell priming induced by several cytokines.

The transmembrane adaptor protein NTAL limits mast cell chemotaxis toward prostaglandin E2

Chemotaxis of mast cells is one of the crucial steps in their development and function. Non–T cell activation linker (NTAL) is a transmembrane adaptor protein that inhibits the activation of mast cells and B cells in a phosphorylation-dependent manner. Here, we studied the role of NTAL in the migration of mouse mast cells stimulated by prostaglandin E2 (PGE2). Although PGE2 does not induce the tyrosine phosphorylation of NTAL, unlike IgE immune complex antigens, we found that loss of NTAL increased the chemotaxis of mast cells toward PGE2. Stimulation of mast cells that lacked NTAL with PGE2 enhanced the phosphorylation of AKT and the production of phosphatidylinositol 3,4,5-trisphosphate. In resting NTAL-deficient mast cells, phosphorylation of an inhibitory threonine in ERM family proteins accompanied increased activation of β1-containing integrins, which are features often associated with increased invasiveness in tumors. Rescue experiments indicated that only full-length, wild-type NTAL restored the chemotaxis of NTAL-deficient cells toward PGE2. Together, these data suggest that NTAL is a key inhibitor of mast cell chemotaxis toward PGE2, which may act through the RHOA/ERM/β1-integrin and PI3K/AKT axes.

Positive and Negative Regulatory Roles of C-Terminal Src Kinase (CSK) in FcεRI-Mediated Mast Cell Activation, Independent of the Transmembrane Adaptor PAG/CSK-Binding Protein

C-terminal Src kinase (CSK) is a major negative regulator of Src family tyrosine kinases (SFKs) that play critical roles in immunoreceptor signaling. CSK is brought in contiguity to the plasma membrane-bound SFKs via binding to transmembrane adaptor PAG, also known as CSK-binding protein. The recent finding that PAG can function as a positive regulator of the high-affinity IgE receptor (FcεRI)-mediated mast cell signaling suggested that PAG and CSK have some non-overlapping regulatory functions in mast cell activation. To determine the regulatory roles of CSK in FcεRI signaling, we derived bone marrow-derived mast cells (BMMCs) with reduced or enhanced expression of CSK from wild-type (WT) or PAG knockout (KO) mice and analyzed their FcεRI-mediated activation events. We found that in contrast to PAG-KO cells, antigen-activated BMMCs with CSK knockdown (KD) exhibited significantly higher degranulation, calcium response, and tyrosine phosphorylation of FcεRI, SYK, and phospholipase C. Interestingly, FcεRI-mediated events in BMMCs with PAG-KO were restored upon CSK silencing. BMMCs with CSK-KD/PAG-KO resembled BMMCs with CSK-KD alone. Unexpectedly, cells with CSK-KD showed reduced kinase activity of LYN and decreased phosphorylation of transcription factor STAT5. This was accompanied by impaired production of proinflammatory cytokines and chemokines in antigen-activated cells. In line with this, BMMCs with CSK-KD exhibited enhanced phosphorylation of protein phosphatase SHP-1, which provides a negative feedback loop for regulating phosphorylation of STAT5 and LYN kinase activity. Furthermore, we found that in WT BMMCs SHP-1 forms complexes containing LYN, CSK, and STAT5. Altogether, our data demonstrate that in FcεRI-activated mast cells CSK is a negative regulator of degranulation and chemotaxis, but a positive regulator of adhesion to fibronectin and production of proinflammatory cytokines. Some of these pathways are not dependent on the presence of PAG.

pTRAPs: Transmembrane adaptors in innate immune signaling

Transmembrane adaptor proteins (TRAPs) are protein scaffolds and signaling regulators with established roles in signal-induced activation of lymphocytes. A subset of the TRAP family, the palmitoylated TRAPs (pTRAPs), are increasingly emerging with additional roles in innate immune cells. Targeted to lipid rafts, tetraspannin-enriched microdomains, and protein microclusters in membranes, pTRAP scaffolds exert spatiotemporal regulation by recruiting signaling kinases, particularly Src and Syk family members, as well as Csk, and other effectors. In this way, pTRAPs modulate signaling and influence resulting cell responses, including the selective output of inflammatory cytokines and other mediators. Here, we review studies revealing that different pTRAPs work together, often with overlapping or redundant roles, for positive and negative regulation of key innate immune pathways, including Fc receptor and pattern recognition receptor signaling. Recent findings show that pTRAPs can bind directly to innate immune receptors, in addition to other transmembrane binding partners. Thus, pTRAPs are important, multifunctional scaffolds in pathways that are fundamental to diverse innate immune responses.

Mast Cell Disease Assessment by Flow Cytometric Analysis

Mast cells are present at a low frequency in bone marrow, rendering high-sensitivity multiparametric flow cytometric analysis an ideal method to assess antigen expression on mast cells. This article discusses the normal antigen expression profile of mast cells, established criteria to identify neoplastic mast cells, and new immunophenotypic markers and approaches to identify the presence of neoplastic mast cells in cases of mastocytosis.

Tetraspanins and Transmembrane Adaptor Proteins As Plasma Membrane Organizers-Mast Cell Case

The plasma membrane contains diverse and specialized membrane domains, which include tetraspanin-enriched domains (TEMs) and transmembrane adaptor protein (TRAP)-enriched domains. Recent biophysical, microscopic, and functional studies indicated that TEMs and TRAP-enriched domains are involved in compartmentalization of physicochemical events of such important processes as immunoreceptor signal transduction and chemotaxis. Moreover, there is evidence of a cross-talk between TEMs and TRAP-enriched domains. In this review we discuss the presence and function of such domains and their crosstalk using mast cells as a model. The combined data based on analysis of selected mast cell-expressed tetraspanins [cluster of differentiation (CD)9, CD53, CD63, CD81, CD151)] or TRAPs [linker for activation of T cells (LAT), non-T cell activation linker (NTAL), and phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (PAG)] using knockout mice or specific antibodies point to a diversity within these two families and bring evidence of the important roles of these molecules in signaling events. An example of this diversity is physical separation of two TRAPs, LAT and NTAL, which are in many aspects similar but show plasma membrane location in different microdomains in both non-activated and activated cells. Although our understanding of TEMs and TRAP-enriched domains is far from complete, pharmaceutical applications of the knowledge about these domains are under way.

Hymenoptera Anaphylaxis and C-kit Mutations: An Unexpected Association

Clinical manifestations of mastocytosis in adults comprise signs and symptoms linked to mast cell (MC) activation, including anaphylaxis. Depending on MC burden, adults can be diagnosed with systemic mastocytosis, when the WHO criteria are fulfilled, or with other clonal MC disorders, characterized by MC mediator symptoms and demonstration of activating KIT mutations and/or expression of CD25 on MCs. There is a specific link between mastocytosis and hymenoptera venom allergy (HVA): the reported frequency of HVA in mastocytosis is 20-50 % and raises to 60-80 % in patients affected by indolent systemic mastocytosis without skin lesions. The presentation of HVA characterized by severe hypotension in the absence of urticarial or angioedema is typical in patient with an underlying MC disorder, even in the presence of normal baseline serum tryptase levels.

Signal transduction and chemotaxis in mast cells

Mast cells play crucial roles in both innate and adaptive arms of the immune system. Along with basophils, mast cells are essential effector cells for allergic inflammation that causes asthma, allergic rhinitis, food allergy and atopic dermatitis. Mast cells are usually increased in inflammatory sites of allergy and, upon activation, release various chemical, lipid, peptide and protein mediators of allergic reactions. Since antigen/immunoglobulin E (IgE)-mediated activation of these cells is a central event to trigger allergic reactions, innumerable studies have been conducted on how these cells are activated through cross-linking of the high-affinity IgE receptor (FcεRI). Development of mature mast cells from their progenitor cells is under the influence of several growth factors, of which the stem cell factor (SCF) seems to be the most important. Therefore, how SCF induces mast cell development and activation via its receptor, KIT, has been studied extensively, including a cross-talk between KIT and FcεRI signaling pathways. Although our understanding of the signaling mechanisms of the FcεRI and KIT pathways is far from complete, pharmaceutical applications of the knowledge about these pathways are underway. This review will focus on recent progresses in FcεRI and KIT signaling and chemotaxis.

Transmembrane adaptor protein PAG/CBP is involved in both positive and negative regulation of mast cell signaling

The transmembrane adaptor protein PAG/CBP (here, PAG) is expressed in multiple cell types. Tyrosine-phosphorylated PAG serves as an anchor for C-terminal SRC kinase, an inhibitor of SRC-family kinases. The role of PAG as a negative regulator of immunoreceptor signaling has been examined in several model systems, but no functions in vivo have been determined. Here, we examined the activation of bone marrow-derived mast cells (BMMCs) with PAG knockout and PAG knockdown and the corresponding controls. Our data show that PAG-deficient BMMCs exhibit impaired antigen-induced degranulation, extracellular calcium uptake, tyrosine phosphorylation of several key signaling proteins (including the high-affinity IgE receptor subunits, spleen tyrosine kinase, and phospholipase C), production of several cytokines and chemokines, and chemotaxis. The enzymatic activities of the LYN and FYN kinases were increased in nonactivated cells, suggesting the involvement of a LYN- and/or a FYN-dependent negative regulatory loop. When BMMCs from PAG-knockout mice were activated via the KIT receptor, enhanced degranulation and tyrosine phosphorylation of the receptor were observed. In vivo experiments showed that PAG is a positive regulator of passive systemic anaphylaxis. The combined data indicate that PAG can function as both a positive and a negative regulator of mast cell signaling, depending upon the signaling pathway involved.

Multiple regulatory roles of the mouse transmembrane adaptor protein NTAL in gene transcription and mast cell physiology

Non-T cell activation linker (NTAL; also called LAB or LAT2) is a transmembrane adaptor protein that is expressed in a subset of hematopoietic cells, including mast cells. There are conflicting reports on the role of NTAL in the high affinity immunoglobulin E receptor (FcεRI) signaling. Studies carried out on mast cells derived from mice with NTAL knock out (KO) and wild type mice suggested that NTAL is a negative regulator of FcεRI signaling, while experiments with RNAi-mediated NTAL knockdown (KD) in human mast cells and rat basophilic leukemia cells suggested its positive regulatory role. To determine whether different methodologies of NTAL ablation (KO vs KD) have different physiological consequences, we compared under well defined conditions FcεRI-mediated signaling events in mouse bone marrow-derived mast cells (BMMCs) with NTAL KO or KD. BMMCs with both NTAL KO and KD exhibited enhanced degranulation, calcium mobilization, chemotaxis, tyrosine phosphorylation of LAT and ERK, and depolymerization of filamentous actin. These data provide clear evidence that NTAL is a negative regulator of FcεRI activation events in murine BMMCs, independently of possible compensatory developmental alterations. To gain further insight into the role of NTAL in mast cells, we examined the transcriptome profiles of resting and antigen-activated NTAL KO, NTAL KD, and corresponding control BMMCs. Through this analysis we identified several genes that were differentially regulated in nonactivated and antigen-activated NTAL-deficient cells, when compared to the corresponding control cells. Some of the genes seem to be involved in regulation of cholesterol-dependent events in antigen-mediated chemotaxis. The combined data indicate multiple regulatory roles of NTAL in gene expression and mast cell physiology.

Molecular editing of cellular responses by the high-affinity receptor for IgE

Cellular responses elicited by cell surface receptors differ according to stimulus strength. We investigated how the high-affinity receptor for immunoglobulin E (IgE) modulates the response of mast cells to a high- or low-affinity stimulus. Both high- and low-affinity stimuli elicited similar receptor phosphorylation; however, differences were observed in receptor cluster size, mobility, distribution, and the cells' effector responses. Low-affinity stimulation increased receptor association with the Src family kinase Fgr and shifted signals from the adapter LAT1 to the related adapter LAT2. LAT1-dependent calcium signals required for mast cell degranulation were dampened, but the role of LAT2 in chemokine production was enhanced, altering immune cell recruitment at the site of inflammation. These findings uncover how receptor discrimination of stimulus strength can be interpreted as distinct in vivo outcomes.

Computational modeling of the main signaling pathways involved in mast cell activation

A global and rigorous understanding of the signaling pathways and cross-regulatory processes involved in mast cell activation requires the integration of published information with novel functional datasets into a comprehensive computational model. Based on an exhaustive curation of the existing literature and using the software CellDesigner, we have built and annotated a comprehensive molecular map for the FcεRI signaling network. This map can be used to visualize and interpret high-throughput expression data. Furthermore, leaning on this map and using the logical modeling software GINsim, we have derived a qualitative dynamical model, which recapitulates the most salient features of mast cell activation. The resulting logical model can be used to explore the dynamical properties of the system and its responses to different stimuli, in normal or mutant conditions.

Saucerneol F, a New Lignan Isolated from Saururus chinensis, Attenuates Degranulation via Phospholipase Cγ 1 Inhibition and Eicosanoid Generation by Suppressing MAP Kinases in Mast Cells

During our on-going studies to identify bioactive compounds in medicinal herbs, we found that saucerneol F (SF), a naturally occurring sesquilignan isolated from Saururus chinensis (S. chinensis), showed in vitro anti-inflammatory activity. In this study, we examined the effects of SF on the generation of 5-lipoxygenase (5-LO) dependent leukotriene C₄ (LTC₄), cyclooxygenase-2 (COX-2) dependent prostaglandin D₂ (PGD₂), and on phospholipase Cγ1 (PLCγ1)-mediated degranulation in SCF-induced mouse bone marrow-derived mast cells (BMMCs). SF inhibited eicosanoid (PGD₂ and LTC₄) generation and degranulation dose-dependently. To identify the molecular mechanisms underlying the inhibition of eicosanoid generation and degranulation by SF, we examined the effects of SF on the phosphorylation of PLCγ1, intracellular Ca²⁺ influx, the translocation of cytosolic phospholipase A₂ (cPLA₂) and 5-LO, and on the phosphorylation of MAP kinases (MAPKs). SF was found to reduce intracellular Ca²⁺ influx by inhibiting PLCγ1 phosphorylation and suppressing the nuclear translocations of cPLA₂ and 5-LO via the phosphorylations of MAPKs, including extracellular signal-regulated protein kinase-1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), and p38. Taken together, these results suggest that SF may be useful for regulating mast cell-mediated inflammatory responses by inhibiting degranulation and eicosanoid generation.

A Novel Phosphopeptide Microarray Based Interactome Map in Breast Cancer Cells Reveals Phosphoprotein-GRB2 Cell Signaling Networks

The architecture of cellular proteins connected to form signaling pathways in response to internal and external cues is much more complex than a group of simple protein-protein interactions. Post translational modifications on proteins (e.g., phosphorylation of serine, threonine and tyrosine residues on proteins) initiate many downstream signaling events leading to protein-protein interactions and subsequent activation of signaling cascades leading to cell proliferation, cell differentiation and cell death. As evidenced by a rapidly expanding mass spectrometry database demonstrating protein phosphorylation at specific motifs, there is currently a large gap in understanding the functional significance of phosphoproteins with respect to their specific protein connections in the signaling cascades. A comprehensive map that interconnects phospho-motifs in pathways will enable identification of nodal protein interactions that are sensitive signatures indicating a disease phenotype from the physiological hemostasis and provide clues into control of disease. Using a novel phosphopeptide microarray technology, we have mapped endogenous tyrosine-phosphoproteome interaction networks in breast cancer cells mediated by signaling adaptor protein GRB2, which transduces cellular responses downstream of several RTKs through the Ras-ERK signaling cascade. We have identified several previously reported motif specific interactions and novel interactions. The peptide microarray data indicate that various phospho-motifs on a single protein are differentially regulated in various cell types and shows global downregulation of phosphoprotein interactions specifically in cells with metastatic potential. The study has revealed novel phosphoprotein mediated signaling networks, which warrants further detailed analysis of the nodes of protein-protein interaction to uncover their biomarker or therapeutic potential.

LAB/NTAL facilitates fungal/PAMP-induced IL-12 and IFN-γ production by repressing β-catenin activation in dendritic cells

Fungal pathogens elicit cytokine responses downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled or hemiITAM-containing receptors and TLRs. The Linker for Activation of B cells/Non-T cell Activating Linker (LAB/NTAL) encoded by Lat2, is a known regulator of ITAM-coupled receptors and TLR-associated cytokine responses. Here we demonstrate that LAB is involved in anti-fungal immunity. We show that Lat2-/- mice are more susceptible to C. albicans infection than wild type (WT) mice. Dendritic cells (DCs) express LAB and we show that it is basally phosphorylated by the growth factor M-CSF or following engagement of Dectin-2, but not Dectin-1. Our data revealed a unique mechanism whereby LAB controls basal and fungal/pathogen-associated molecular patterns (PAMP)-induced nuclear β-catenin levels. This in turn is important for controlling fungal/PAMP-induced cytokine production in DCs. C. albicans- and LPS-induced IL-12 and IL-23 production was blunted in Lat2-/- DCs. Accordingly, Lat2-/- DCs directed reduced Th1 polarization in vitro and Lat2-/- mice displayed reduced Natural Killer (NK) and T cell-mediated IFN-γ production in vivo/ex vivo. Thus our data define a novel link between LAB and β-catenin nuclear accumulation in DCs that facilitates IFN-γ responses during anti-fungal immunity. In addition, these findings are likely to be relevant to other infectious diseases that require IL-12 family cytokines and an IFN-γ response for pathogen clearance.

Cross-talk between tetraspanin CD9 and transmembrane adaptor protein non-T cell activation linker (NTAL) in mast cell activation and chemotaxis

Chemotaxis, a process leading to movement of cells toward increasing concentrations of chemoattractants, is essential, among others, for recruitment of mast cells within target tissues where they play an important role in innate and adaptive immunity. Chemotaxis is driven by chemoattractants, produced by various cell types, as well as by intrinsic cellular regulators, which are poorly understood. In this study we prepared a new mAb specific for the tetraspanin CD9. Binding of the antibody to bone marrow-derived mast cells triggered activation events that included cell degranulation, Ca(2+) response, dephosphorylation of ezrin/radixin/moesin (ERM) family proteins, and potent tyrosine phosphorylation of the non-T cell activation linker (NTAL) but only weak phosphorylation of the linker for activation of T cells (LAT). Phosphorylation of the NTAL was observed with whole antibody but not with its F(ab)(2) or Fab fragments. This indicated involvement of the Fcγ receptors. As documented by electron microscopy of isolated plasma membrane sheets, CD9 colocalized with the high-affinity IgE receptor (FcεRI) and NTAL but not with LAT. Further tests showed that both anti-CD9 antibody and its F(ab)(2) fragment inhibited mast cell chemotaxis toward antigen. Experiments with bone marrow-derived mast cells deficient in NTAL and/or LAT revealed different roles of these two adaptors in antigen-driven chemotaxis. The combined data indicate that chemotaxis toward antigen is controlled in mast cells by a cross-talk among FcεRI, tetraspanin CD9, transmembrane adaptor proteins NTAL and LAT, and cytoskeleton-regulatory proteins of the ERM family.

Targeting the KIT activating switch control pocket: a novel mechanism to inhibit neoplastic mast cell proliferation and mast cell activation

Activating mutations in the receptor tyrosine kinase KIT, most notably KIT D816V, are commonly observed in patients with systemic mastocytosis. Thus, inhibition of KIT has been a major focus for treatment of this disorder. Here we investigated a novel approach to such inhibition. Utilizing rational drug design, we targeted the switch pocket (SP) of KIT which regulates its catalytic conformation. Two SP inhibitors thus identified, DP-2976 and DP-4851, were examined for effects on neoplastic mast cell proliferation and mast cell activation. Autophosphorylation of both wild type (WT) and, where also examined, KIT D816V was blocked by these compounds in transfected 293T cells, HMC 1.1 and 1.2 human mast cell lines; and in CD34⁺-derived human mast cells activated by stem cell factor (SCF). Both inhibitors induced apoptosis in the neoplastic mast cell lines and reduced survival of primary bone marrow mast cells from patients with mastocytosis. Moreover, the SP inhibitors more selectively blocked SCF potentiation of FcεRI-mediated degranulation. Overall, SP inhibitors represent an innovative mechanism of KIT inhibition whose dual suppression of KIT D816V neoplastic mast cell proliferation and SCF enhanced mast cell activation may provide significant therapeutic benefits.

Transmembrane adaptor proteins in the high-affinity IgE receptor signaling

Aggregation of the high-affinity IgE receptor (FcεRI) initiates a cascade of signaling events leading to release of preformed inflammatory and allergy mediators and de novo synthesis and secretion of cytokines and other compounds. The first biochemically well defined step of this signaling cascade is tyrosine phosphorylation of the FcεRI subunits by Src family kinase Lyn, followed by recruitment and activation of spleen tyrosine kinase (Syk). Activity of Syk is decisive for the formation of multicomponent signaling assemblies, the signalosomes, in the vicinity of the receptors. Formation of the signalosomes is dependent on the presence of transmembrane adaptor proteins (TRAPs). These proteins are characterized by a short extracellular domain, a single transmembrane domain, and a cytoplasmic tail with various motifs serving as anchors for cytoplasmic signaling molecules. In mast cells five TRAPs have been identified [linker for activation of T cells (LAT), non-T cell activation linker (NTAL), linker for activation of X cells (LAX), phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (PAG), and growth factor receptor-bound protein 2 (Grb2)-binding adaptor protein, transmembrane (GAPT)]; engagement of four of them (LAT, NTAL, LAX, and PAG) in FcεRI signaling has been documented. Here we discuss recent progress in the understanding of how TRAPs affect FcεRI-mediated mast cell signaling. The combined data indicate that individual TRAPs have irreplaceable roles in important signaling events such as calcium response, degranulation, cytokines production, and chemotaxis.

Citreorosein, a naturally occurring anthraquinone derivative isolated from Polygoni cuspidati radix, attenuates cyclooxygenase-2-dependent prostaglandin D2 generation by blocking Akt and JNK pathways in mouse bone marrow-derived mast cells

In this study, we examined the effects of citreorosein (CIT), an anthraquinone component of Polygoni cuspidati radix (P. cuspidati, Polygonaceae), on cyclooxygenase (COX)-2 dependent prostaglandin (PG)D2 generation in mast cells, central effector cells of allergy and other inflammatory diseases. CIT strongly inhibited COX-2-dependent PGD2 generation in a concentration-dependent manner in mouse bone marrow-derived mast cells (BMMCs) stimulated with stem cell factor (SCF)/IL-10/LPS. In an effort to identify the mechanisms underlying the inhibition of COX-2-dependent PGD2 generation by CIT, we examined the effects of this compound on MAP kinases, Akt and NF-κB signaling pathways, which are essential for COX-2 induction. CIT inhibited nuclear translocation of the nuclear factor (NF)-κB p65 subunit and its cognate DNA-binding activity, which correlated with its inhibitory effects on the phosphorylation of Akt and IKK and subsequent phosphorylation and degradation of IκB. Furthermore, CIT significantly attenuated the DNA binding of activator protein (AP)-1 that regulates COX-2 expression through the reduction of the phosphorylation of c-Jun. Moreover, inhibition of PGD2 generation by CIT was accompanied by a decrease in phosphorylation of cytosolic phospholipase A2α. Taken together, the present study suggests that CIT represents a potential therapeutic approach for the treatment of inflammatory diseases.

PTEN deficiency in mast cells causes a mastocytosis-like proliferative disease that heightens allergic responses and vascular permeability

Kit regulation of mast cell proliferation and differentiation has been intimately linked to the activation of phosphatidylinositol 3-OH kinase (PI3K). The activating D816V mutation of Kit, seen in the majority of mastocytosis patients, causes a robust activation of PI3K signals. However, whether increased PI3K signaling in mast cells is a key element for their in vivo hyperplasia remains unknown. Here we report that dysregulation of PI3K signaling in mice by deletion of the phosphatase and tensin homolog (Pten) gene (which regulates the levels of the PI3K product, phosphatidylinositol 3,4,5-trisphosphate) caused mast cell hyperplasia and increased numbers in various organs. Selective deletion of Pten in the mast cell compartment revealed that the hyperplasia was intrinsic to the mast cell. Enhanced STAT5 phosphorylation and increased expression of survival factors, such as Bcl-XL, were observed in PTEN-deficient mast cells, and these were further enhanced by stem cell factor stimulation. Mice carrying PTEN-deficient mast cells also showed increased hypersensitivity as well as increased vascular permeability. Thus, Pten deletion in the mast cell compartment results in a mast cell proliferative phenotype in mice, demonstrating that dysregulation of PI3K signals is vital to the observed mast cell hyperplasia.

A tale of two TRAPs: LAT and LAB in the regulation of lymphocyte development, activation, and autoimmunity

Transmembrane adaptor proteins (TRAPs) link antigen receptor engagement to downstream cellular processes. Although these proteins typically lack intrinsic enzymatic activity, they are phosphorylated on multiple tyrosine residues following lymphocyte activation, allowing them to function as scaffolds for the assembly of multi-molecular signaling complexes. Among the many TRAPs that have been discovered in recent years, the LAT (linker for activation of T cells) family of adaptor proteins plays an important role in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. Of the two members in this family, LAT is an indispensable component controlling T cell and mast cell activation and function; LAB (linker for activation of B cells), also called NTAL, is necessary to fine-tune lymphocyte activation and may be a key regulator of innate immune responses. Here, we review recent advances on the function of LAT and LAB in the regulation of development and activation of immune cells.

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.

Distinct PGE2-responder and non-responder phenotypes in human mast cell populations: "all or nothing" enhancement of antigen-dependent mediator release

Reports indicate that prostaglandin (PG)E(2) markedly enhances antigen-mediated degranulation in mouse bone marrow-derived mast cells (BMMCs) but not in human mast cells (HuMCs). We have examined the underlying mechanism(s) for this disparity in HuMCs derived from the peripheral blood of multiple donors in addition to mouse BMMCs. HuMCs from half of these donors failed to respond to PGE(2) and the PGE(2) EP3 receptor agonist, sulprostone. However, HuMCs from the remaining donors and the LAD2 human MC line responded to PGE(2) and sulprostone with marked enhancement of antigen-mediated degranulation and IL-8 production in a similar manner to that observed in mouse BMMCs. The EP2 agonist, butaprost, failed to modulate antigen-mediated responses in any type of MCs. These distinct phenotypes could not be explained by differences in EP2 or EP3 expression nor by differences in the ability of PGE(2) to elevate levels of cAMP, a signal recognized to down-regulate mast cell activation. Moreover, both responder and non-responder HuMC populations exhibited similar activation of phosphatidylinositol 3-kinase, and MAP kinases. However, translocation of PLCγ(1) to the cell membrane and the associated calcium signal were enhanced only in the responder HuMC population indicating that the link between EP3 and PLCγ is impaired in the non-responder HuMCs.

CONCLUSIONS

These data provide a cautionary note for the translating of observations in the mouse to human mast cell-dependent disorders, but may also provide a basis for examining the effects of co-activating receptors in patients susceptible to allergic conditions.

Tec family kinases: regulation of FcεRI-mediated mast-cell activation

Mast cells express the high-affinity receptor for IgE (FcεRI) and are key players in type I hypersensitivity reactions. They are critically involved in the development of allergic rhinitis, allergic asthma and systemic anaphylaxis, however, they also regulate normal physiological processes that link innate and adaptive immune responses. Thus, their activation has to be tightly controlled. One group of signaling molecules that are activated upon FcεRI stimulation is formed by Tec family kinases, and three members of this kinase family (Btk, Itk and Tec) are expressed in mast cells. Many studies have revealed important functions of Tec kinases in signaling pathways downstream of the antigen receptors in lymphocytes. This review summarizes the current knowledge about the function of Tec family kinases in FcεRI-mediated signaling pathways in mast cell.

Regulation of mast cell responses in health and disease

Mast cells are multifunctional cells that initiate not only IgE-dependent allergic diseases but also play a fundamental role in innate and adaptive immune responses to microbial infection. They are also thought to play a role in angiogenesis, tissue remodeling, wound healing, and tumor repression or growth. The broad scope of these physiologic and pathologic roles illustrates the flexible nature of mast cells, which is enabled in part by their phenotypic adaptability to different tissue microenvironments and their ability to generate and release a diverse array of bioactive mediators in response to multiple types of cell-surface and cytosolic receptors. There is increasing evidence from studies in cell cultures that release of these mediators can be selectively modulated depending on the types or groups of receptors activated. The intent of this review is to foster interest in the interplay among mast cell receptors to help understand the underlying mechanisms for each of the immunological and non-immunological functions attributed to mast cells. The second intent of this review is to assess the pathophysiologic roles of mast cells and their products in health and disease. Although mast cells have a sufficient repertoire of bioactive mediators to mount effective innate and adaptive defense mechanisms against invading microorganisms, these same mediators can adversely affect surrounding tissues in the host, resulting in autoimmune disease as well as allergic disorders.

Mast cell biology: introduction and overview

In recent years, the field of mast cell biology has expanded well beyond the boundaries of atopic disorders and anaphy laxis, on which it has been historically focused. The biochemical and signaling events responsible for the development and regulation of mast cells has been increasingly studied, aided in large part by novel breakthroughs in laboratory techniques used to study these cells. The result of these studies has been a more comprehensive definition of mast cells that includes added insights to their overall biology as well as the various disease states that can now be traced to defects in mast cells. This introductory chapter outlines and highlights the various topics of mast cell biology that will be discussed in further detail in subsequent chapters.

Mast cell activation syndrome: Proposed diagnostic criteria

The term mast cell activation syndrome (MCAS) is finding increasing use as a diagnosis for subjects who present with signs and symptoms involving the dermis, gastrointestinal track, and cardiovascular system frequently accompanied by neurologic complaints. Such patients often have undergone multiple extensive medical evaluations by different physicians in varied disciplines without a definitive medical diagnosis until the diagnosis of MCAS is applied. However, MCAS as a distinct clinical entity has not been generally accepted, nor do there exist definitive criteria for diagnosis. Based on current understanding of this disease "syndrome" and on what we do know about mast cell activation and resulting pathology, we will explore and propose criteria for its diagnosis. The proposed criteria will be discussed in the context of other disorders involving mast cells or with similar presentations and as a basis for further scientific study and validation.

The transmembrane adaptor protein NTAL signals to mast cell cytoskeleton via the small GTPase Rho

The transmembrane adaptor protein NTAL (non-T-cell activation linker) participates in signalosome assembly in hematopoietic cells, but its exact role in cell physiology remains enigmatic. We report here that BM-derived mast cells from NTAL-deficient mice, responding to Ag alone or in combination with SCF, exhibit reduced spreading on fibronectin, enhanced filamentous actin depolymerization and enhanced migration towards Ag relative to WT cells. No such differences between WT and NTAL(-/-) BM-derived mast cells were observed when SCF alone was used as activator. We have examined the activities of two small GTPases, Rac and Rho, which are important regulators of actin polymerization. Stimulation with Ag and/or SCF enhanced activity of Rac(1,2,3) in both NTAL(-/-) and WT cells. In contrast, RhoA activity decreased and this trend was much faster and more extensive in NTAL(-/-) cells, indicating a positive regulatory role of NTAL in the recovery of RhoA activity. After restoring NTAL into NTAL(-/-) cells, both spreading and actin responses were rescued. This is the first report of a crucial role of NTAL in signaling, via RhoA, to mast cell cytoskeleton.

LAB/NTAL/Lat2: a force to be reckoned with in all leukocytes?

LAB/NTAL/Lat2 is a transmembrane adaptor protein closely related to LAT. It is expressed in various myeloid and lymphoid cells, many of which also express LAT. Phosphorylation of LAB occurs following engagement of various ITAM- and non-ITAM-linked receptors and can play positive and negative roles following receptor engagement. LAT binds PLCγ directly, resulting in efficient Ca²+ flux and degranulation. However, LAB does not contain a PLCγ-binding motif and only binds PLCγ indirectly, possibly via Grb2, thereby resulting in suboptimal signaling. As LAT can signal more efficiently than LAB, competition between the 2 for space/substrates in the lipid rafts can attenuate signaling. This competition model requires coexpression of LAT; however, LAB is repressive, even in cells lacking substantial LAT expression such as macrophages and mature B cells. The reported interaction between LAB and the ubiquitin E3-ligase c-Cbl suggests 1 possible mechanism for LAT-independent inhibition by LAB, but such a model requires further investigation. Given the wide-reaching expression pattern of LAB, LAB has the ability to modulate signaling in virtually every type of leukocyte. Regardless of its ultimate mode of action, the potent regulatory capability of LAB proves this protein to be a complex adaptor that warrants continued, substantial scrutiny by biochemists and immunologists alike.

Independent and cooperative roles of adaptor molecules in proximal signaling during FcepsilonRI-mediated mast cell activation

Activation through FcepsilonRI, a high-affinity IgE-binding receptor, is critical for mast cell function during allergy. The formation of a multimolecular proximal signaling complex nucleated by the adaptor molecules SLP-76 and LAT1 is required for activation through this receptor. Based on previous T-cell studies, current dogma dictates that LAT1 is required for plasma membrane recruitment and function of SLP-76. Unexpectedly, we found that the recruitment and phosphorylation of SLP-76 were preserved in LAT1(-/-) mast cells and that SLP-76(-/-) and LAT1(-/-) mast cells harbored distinct functional and biochemical defects. The LAT1-like molecule LAT2 was responsible for the preserved membrane localization and phosphorylation of SLP-76 in LAT1(-/-) mast cells. Although LAT2 supported SLP-76 phosphorylation and recruitment to the plasma membrane, LAT2 only partially compensated for LAT1-mediated cell signaling due to its decreased ability to stabilize interactions with phospholipase Cgamma (PLCgamma). Comparison of SLP-76(-/-) LAT1(-/-) and SLP-76(-/-) mast cells revealed that some functions of LAT1 could occur independently of SLP-76. We propose that while SLP-76 and LAT1 depend on each other for many of their functions, LAT2/SLP-76 interactions and SLP-76-independent LAT1 functions also mediate a positive signaling pathway downstream of FcepsilonRI in mast cells.

Adapters in the organization of mast cell signaling

Mast cells are pivotal in innate immunity and play an important role in amplifying adaptive immunity. Nonetheless, they have long been known to be central to the initiation of allergic disorders. This results from the dysregulation of the immune response whereby normally innocuous substances are recognized as non-self, resulting in the production of IgE antibodies to these 'allergens'. Preformed and newly synthesized inflammatory (allergic) mediators are released from the mast cell following allergen-mediated aggregation of allergen-specific IgE bound to the high-affinity receptors for IgE (FcepsilonRI). Thus, the process by which the mast cell is able to interpret the engagement of FcepsilonRI into the molecular events necessary for release of their allergic mediators is of considerable therapeutic interest. Unraveling these molecular events has led to the discovery of a functional class of proteins that are essential in organizing activated signaling molecules and in coordinating and compartmentalizing their activity. These so-called 'adapters' bind multiple signaling proteins and localize them to specific cellular compartments, such as the plasma membrane. This organization is essential for normal mast cell responses. Here, we summarize the role of adapter proteins in mast cells focusing on the most recent advances toward understanding how these molecules work upon FcepsilonRI engagement.

Cooperation of adapter molecules in proximal signaling cascades during allergic inflammation

Activation of mast cells through their high-affinity immunoglobulin E receptor (FcepsilonRI) plays an important role in allergic disorders. Other mast cell-activating stimuli, such as Toll-like receptor (TLR) ligands, synergize with FcepsilonRI to enhance allergic inflammation. Thus, there is much interest in understanding how signaling occurs downstream of these receptors. One key event for FcepsilonRI-mediated mast cell activation is the inducible formation of multimolecular proximal signaling complexes. These complexes are nucleated by adapter proteins, scaffolds that localize various signaling molecules through their multiple molecule-binding domains. Here we review recent findings in proximal signaling cascades with an emphasis on how adapter molecules cooperate with each other to generate an optimal signal in mast cells, and we discuss how signals crosstalk between FcepsilonRI and TLRs in enhancing mast cell activation. Deciphering the molecular mechanisms leading to mast cell activation will hopefully bring new ideas for the development of novel therapeutics to control allergic diseases.

Regulation of lymphocyte development and activation by the LAT family of adapter proteins

Transmembrane adapter proteins (TRAPs) are critical components of signaling pathways in lymphocytes, linking antigen receptor engagement to downstream cellular processes. While these proteins lack intrinsic enzymatic activity, their phosphorylation following receptor ligation allows them to function as scaffolds for the assembly of multi-molecular signaling complexes. Many TRAPs have recently been discovered, and numerous studies demonstrate their roles in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. One such example is the linker for activation of T cells (LAT) family of adapter proteins. While LAT has been shown to play an indispensable role in T-cell and mast cell function, the other family members, linker for activation of B cells (LAB) and linker for activation of X cells (LAX), are necessary to fine-tune immune responses. In addition to its well-established role in the positive regulation of lymphocyte activation, LAT exerts an inhibitory effect on T-cell receptor-mediated signaling. Furthermore, LAT, along with LAB and LAX, plays a crucial role in establishing and maintaining tolerance. Here, we review recent data concerning the regulation of lymphocyte development and activation by the LAT family of proteins.

Loops govern SH2 domain specificity by controlling access to binding pockets

Cellular functions require specific protein-protein interactions that are often mediated by modular domains that use binding pockets to engage particular sequence motifs in their partners. Yet, how different members of a domain family select for distinct sequence motifs is not fully understood. The human genome encodes 120 Src homology 2 (SH2) domains (in 110 proteins), which mediate protein-protein interactions by binding to proteins with diverse phosphotyrosine (pTyr)-containing sequences. The structure of the SH2 domain of BRDG1 bound to a peptide revealed a binding pocket that was blocked by a loop residue in most other SH2 domains. Analysis of 63 SH2 domain structures suggested that the SH2 domains contain three binding pockets, which exhibit selectivity for the three positions after the pTyr in a peptide, and that SH2 domain loops defined the accessibility and shape of these pockets. Despite sequence variability in the loops, we identified conserved structural features in the loops of SH2 domains responsible for controlling access to these surface pockets. We engineered new loops in an SH2 domain that altered specificity as predicted. Thus, selective blockage of binding subsites or pockets by surface loops provides a molecular basis by which the diverse modes of ligand recognition by the SH2 domain may have evolved and provides a framework for engineering SH2 domains and designing SH2-specific inhibitors.

SHP-1 deficient mast cells are hyperresponsive to stimulation and critical in initiating allergic inflammation in the lung

Phosphatase Src homology region 2 domain-containing phosphatase 1 (SHP-1)-deficient mice display an allergic asthma phenotype that is largely IL-13 and STAT6 dependent. The cell types responsible for the Th2 phenotype have not been identified. We hypothesized that SHP-1 deficiency leads to mast cell dysregulation and increased production and release of mediators and Th2 cytokines, leading to the allergic asthma phenotype. We examined SHP-1 regulation of mast cell differentiation, survival, and functional responses to stimulation using bone marrow-derived mast cells from viable motheaten (mev) mice. We assessed pulmonary phenotypical changes in mev mice on the mast cell-deficient Kit(W-Sh) genetic background. The results showed that SHP-1 deficiency led to increased differentiation and survival, but reduced proliferation, of mast cells. SHP-1-deficient mast cells produced and released increased amounts of mediators and Th2 cytokines IL-4 and -13 spontaneously and in response to H(2)O(2), LPS, and Fc epsilonI cross-linking, involving c-Kit-dependent and -independent processes. The Fc epsilonRI signaling led to binding of SHP-1 to linker for activation of T cells 2 and enhanced linker for activation of T cells 2 phosphorylation in mev bone marrow-derived mast cells. Furthermore, the number of mast cells in the lung tissue of mev mice was increased and mast cell production and release of Th2 cytokines were distinctly increased upon Fc epsilonRI stimulation. When backcrossed to the Kit(W-Sh) background, mev mice had markedly reduced pulmonary inflammation and Th2 cytokine production. These findings demonstrate that SHP-1 is a critical regulator of mast cell development and function and that SHP-1-deficient mast cells are able to produce increased Th2 cytokines and initiate allergic inflammatory responses in the lung.

Mechanisms of mast cell signaling in anaphylaxis

The recent development of a consensus definition and proposed diagnostic criteria for anaphylaxis offers promise for research efforts and a better understanding of the epidemiology and pathogenesis of this enigmatic and life-threatening disease. This review examines basic principles and recent research advances in the mechanisms of mast cell signaling believed to underlie anaphylaxis. The unfolding complexity of mast cell signaling suggests that the system is sensitive to regulation by any of several individual signaling pathways and intermediates and that complementary pathways regulate mast cell activation by amplified signals. The signaling events underlying anaphylactic reactions have largely been identified through experiments in genetically modified mice and supported by biochemical studies of mast cells derived from these mice. These studies have revealed that signaling pathways exist to both upregulate and downregulate mast cell responses. In this review we will thus describe the key molecular players in these pathways in the context of anaphylaxis.

Amplification mechanisms for the enhancement of antigen-mediated mast cell activation

Activation of mast cells in the allergic inflammatory response occurs via the high affinity receptor for IgE (FcepsilonRI) following receptor aggregation induced by antigen-mediated cross-linking of IgE-occupied FcepsilonRI. Recent observations suggest this response is profoundly influenced by other factors that reduce the threshold for, and increase the extent of, mast cell activation. For example, under experimental conditions, cell surface receptors such as KIT and specific G protein-coupled receptors synergistically enhance FcepsilonRI-mediated mast cell degranulation and cytokine production. Activating mutations in critical signaling molecules may also contribute to such responses. In this review, we describe our research exploring the mechanisms regulating these synergistic interactions and, furthermore, discuss the relevance of our observations in the context of clinical considerations.

CD4 and CD8: an inside-out coreceptor model for innate immune cells

CD8 and CD4 are expressed by several cell types that do not express TCR. These include DCs, macrophages, monocytes, and NK cells. CD8(+) monocytes and macrophages are abundant at the site of pathology in many rat disease models, particularly those involving immune complex-mediated pathology. Indeed, in some disease models, CD8(+) macrophages correlate with severity of pathology or directly cause pathology or tumor cell killing. Evidence suggests CD8 or CD4 can enhance FcgammaR-dependent responses of human monocytes. Building on data that key components of TCR and FcgammaR signaling can substitute one another efficiently, we postulate that CD4 and CD8 operate with FcgammaR and potentially other receptors to enhance responses of T cells and various innate immune cells. Our model suggests CD8 on myeloid cells may contribute directly to tumor killing and tissue pathology by enhancing FcgammaR responses. Moreover, the model suggests a role for CD8 in cross-presentation of antibody-associated antigen by DCs and a new mechanism to regulate TCR sensitivity.

The tyrosine kinase network regulating mast cell activation

Mast cell mediator release represents a pivotal event in the initiation of inflammatory reactions associated with allergic disorders. These responses follow antigen-mediated aggregation of immunoglobulin E (IgE)-occupied high-affinity receptors for IgE (Fc epsilon RI) on the mast cell surface, a response which can be further enhanced following stem cell factor-induced ligation of the mast cell growth factor receptor KIT (CD117). Activation of tyrosine kinases is central to the ability of both Fc epsilon RI and KIT to transmit downstream signaling events required for the regulation of mast cell activation. Whereas KIT possesses inherent tyrosine kinase activity, Fc epsilon RI requires the recruitment of Src family tyrosine kinases and Syk to control the early receptor-proximal signaling events. The signaling pathways propagated by these tyrosine kinases can be further upregulated by the Tec kinase Bruton's tyrosine kinase and downregulated by the actions of the tyrosine Src homology 2 domain-containing phosphatase 1 (SHP-1) and SHP-2. In this review, we discuss the regulation and role of specific members of this tyrosine kinase network in KIT and Fc epsilon RI-mediated mast cell activation.