Positive and negative regulation of FcepsilonRI-mediated signaling by the adaptor protein LAB/NTAL

The linker for activation of B cells (LAB)/non-T cell activation linker (NTAL) regulates triggering receptor expressed on myeloid cells (TREM)-2 signaling and macrophage inflammatory responses independently of the linker for activation of T cells

Triggering receptor expressed on myeloid cells-2 (TREM-2) is rapidly emerging as a key regulator of the innate immune response via its regulation of macrophage inflammatory responses. Here we demonstrate that proximal TREM-2 signaling parallels other DAP12-based receptor systems in its use of Syk and Src-family kinases. However, we find that the linker for activation of T cells (LAT) is severely reduced as monocytes differentiate into macrophages and that TREM-2 exclusively uses the linker for activation of B cells (LAB encoded by the gene Lat2(-/-)) to mediate downstream signaling. LAB is required for TREM-2-mediated activation of Erk1/2 and dampens proximal TREM-2 signals through a novel LAT-independent mechanism resulting in macrophages with proinflammatory properties. Thus, Lat2(-/-) macrophages have increased TREM-2-induced proximal phosphorylation, and lipopolysaccharide stimulation of these cells leads to increased interleukin-10 (IL-10) and decreased IL-12p40 production relative to wild type cells. Together these data identify LAB as a critical, LAT-independent regulator of TREM-2 signaling and macrophage development capable of controlling subsequent inflammatory responses.

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.

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.

Positive and negative regulation of high affinity IgE receptor signaling by Src homology region 2 domain-containing phosphatase 1

Src homology region 2 domain-containing phosphatase 1 (SHP-1), a cytoplasmic protein tyrosine phosphatase, plays an important role for the regulation of signaling from various hematopoietic cell receptors. Although SHP-1 is shown to be a negative signal modulator in mast cells, its precise molecular mechanisms are not well defined. To elucidate how SHP-1 regulates mast cell signaling, we established bone marrow-derived mast cells from SHP-1-deficient motheaten and wild-type mice and analyzed downstream signals induced by cross-linking of high affinity IgE receptor, Fc epsilonRI. Upon Fc epsilonRI ligation, motheaten-derived bone marrow-derived mast cells showed enhanced tyrosine phosphorylation of Src homology region 2 domain-containing leukocyte protein of 76 kDa (SLP-76) and linker for activation of T cells, activation of mitogen-activated protein kinases and gene transcription and production of cytokine. Because the activity of Syk, responsible for the phosphorylation of SLP-76 and linker for activation of T cells, is comparable irrespective of SHP-1, both molecules might be substrates of SHP-1 in mast cells. Interestingly, the absence of SHP-1 expression disrupted the association between SLP-76 and phospholipase Cgamma, which resulted in the decreased phospholipase Cgamma phosphorylation, calcium mobilization, and degranulation. Collectively, these results suggest that SHP-1 regulates Fc epsilonRI-induced downstream signaling events both negatively and positively by functioning as a protein tyrosine phosphatase and as an adaptor protein contributing to the formation of signaling complex, respectively.

Selective impairment of FcepsilonRI-mediated allergic reaction in Gads-deficient mice

Gads is a Grb2-like adaptor protein expressed in hematopoietic cells. We demonstrated that mast cells from Gads(-/-) mice have selective functional defects. Bone marrow-derived mast cells from Gads(-/-) mice failed to induce Ca(2+) mobilization, degranulation and cytokine production upon cross-linking of FcepsilonRI. In vivo passive cutaneous anaphylaxis was also greatly impaired in Gads(-/-) mice. In contrast, Gads was dispensable for Toll-like receptor-mediated cytokine production in mast cells. Accordingly, mast cell-dependent resistance to acute peritoneal bacterial infection is not reduced in Gads(-/-) mice in vivo. Moreover, mature T and B cell responses and antibody production upon immunization were apparently normal in Gads(-/-) mice. Thus, inhibition of Gads in vivo would suppress the IgE-mediated allergic reaction with minimum adverse effects on both innate and acquired immune responses, and Gads could be an ideal target for the control of allergic responses.

Analysis of the linker for activation of T cells and the linker for activation of B cells in natural killer cells reveals a novel signaling cassette, dual usage in ITAM signaling, and influence on development of the Ly49 repertoire

The linker for activation of T cells (LAT) and the linker for activation of B cells (LAB/NTAL/LAT2) are integral proteins in receptor coupling to downstream events. Both proteins are expressed in natural killer (NK) cells and LAT is phosphorylated during target cell interactions or ligation of the immunoreceptor tyrosine-based activation motif (ITAM)-coupled CD16. Regardless, Lat(-/-) mice exhibit normal natural and antibody-mediated killing. Here we place both LAT and LAB in the DAP12 pathway of NK cells. Moreover, we unveil a LAT-independent pathway that requires expression of Syk. Mice lacking either LAT or LAB have a skewed Ly49 repertoire, and activated NK cells from Lat(-/-) mice have reduced responses to the ITAM-coupled receptor NK1.1. In contrast, resting Lat(-/-) NK cells show intact NK1.1 responses, whereas NK cells without LAB are hyperactive. Elimination of both adaptors severely reduces NK1.1 signaling under both conditions. Together these data show that NK ITAMs preferentially use a signaling cassette regulated by interplay between LAT and LAB. Activation by interleukin-2 causes a shift to greater dependency on LAT due to suppression of Syk signaling. The overlapping use of multiple adaptors permits fine-tuning of NK-cell ITAM responses over the course of an immune response.

Identification of a new transmembrane adaptor protein that constitutively binds Grb2 in B cells

Transmembrane adaptor proteins couple antigen receptor engagement to downstream signaling cascades in lymphocytes. One example of these proteins is the linker for activation of T cells (LAT), which plays an indispensable role in T cell activation and development. Here, we report identification of a new transmembrane adaptor molecule, namely growth factor receptor-bound protein 2 (Grb2)-binding adaptor protein, transmembrane (GAPT), which is expressed in B cells and myeloid cells. Similar to LAT, GAPT has an extracellular domain, a transmembrane domain, and a cytoplasmic tail with multiple Grb2-binding motifs. In contrast to other transmembrane adaptor proteins, GAPT is not phosphorylated upon BCR ligation but associates with Grb2 constitutively through its proline-rich region. Targeted disruption of the gapt gene in mice affects neither B cell development nor a nitrophenylacetyl-specific antibody response. However, in the absence of GAPT, B cell proliferation after BCR cross-linking is enhanced. In aged GAPT(-/-) mice, the number of marginal zone (MZ) B cells is increased, and other B cell subsets are normal. The serum concentrations of IgM, IgG2b, and IgG3 are also elevated in these mice. These data indicate that GAPT might play an important role in control of B cell activation and proper maintenance of MZ B cells.

A current understanding of Fc epsilon RI-dependent mast cell activation

Mast cell activation via the high-affinity immunoglobulin (Ig) E receptor Fc epsilon RI is a topic of extensive investigation with therapeutic potential in allergic disease. The protein tyrosine kinases Fyn, Lyn, and Syk are intimately linked with the early events initiated by allergen-mediated aggregation of IgE-occupied Fc epsilon RI. Fyn and Lyn initiate signaling events that are organized by adaptor molecules, which compartmentalize and coordinate the activity of activated protein and lipid kinases and phospholipases to generate lipid products essential for signal amplification and mast cell function. Fyn and Lyn counter-regulate phosphatidylinositol 3-OH kinase (PI3K), controlling the produced amount of phosphatidylinositol (3,4,5)-trisphosphate (PIP3), a key regulator of mast cell degranulation. Fyn and Lyn also activate sphingosine kinases (SphK), which generate sphingosine-1-phosphate (S1P), thus contributing to mast cell chemotaxis and degranulation. Here, we summarize the current knowledge and future challenges and directions.

New insights on mast cell activation via the high affinity receptor for IgE

Mast cells are innate immune cells that function as regulatory or effector cells and serve to amplify adaptive immunity. In adaptive immunity these cells function primarily through cell surface Fc receptors that bind immunoglobulin antibodies. The dysregulation of their adaptive role makes them central players in allergy and asthma. Upon encountering an allergen (antigen), which is recognized by immunoglobulin E (IgE) antibodies bound to the high affinity IgE receptor (FcepsilonRI) expressed on their cell surface, mast cells secrete both preformed and newly synthesized mediators of the allergic response. Blocking of these responses is an objective in therapeutic intervention of allergic diseases. Thus, understanding the mechanisms by which antigens elicit mast cell activation (via FcepsilonRI) holds promise toward identifying therapeutic targets. Here we review the most recent advances in understanding antigen-dependent mast cell activation. Specifically, we focus on the requirements for FcepsilonRI activation, the regulation of calcium responses, co-stimulatory signals in FcepsilonRI-mediated mast cell activation and function, and how genetics influences mast cell signaling and responses. These recent discoveries open new avenues of investigation with therapeutic potential.

Molecular architecture of signal complexes regulating immune cell function

Signals transmitted via multichain immunoreceptors control the development, differentiation and activation of hematopoetic cells. The cytoplasmic parts of these receptors contain immunoreceptor tyrosine-based activation motifs (ITAMs) that upon phosphorylation by members of the Src tyrosine kinase family orchestrate a complex set of signaling events involving tyrosine phosphorylation, generation of second messengers like DAG, IP3 and Ca2+, activation of effector molecules like Ras and MAPKs and the translocation and activation of transcription factors like NFAT, API and NF-kB. Spatial and temporal organization of these signaling events is essential both to connect the receptors to downstream cascades as well as to control the functional outcome of the immune activation. Throughout this process control and fine-tuning of the different signals are necessary both for effective immune function and in order to avoid inappropriate or exaggerated immune activation and autoimmunity. This control includes modulating mechanisms that set the threshold for activation and reset the activation status after an immune response has been launched. One immunomodulating pathway is the cAMP-protein kinase A-Csk pathway scaffolded by a supramolecular complex residing in lipid rafts with the A kinase-anchoring protein (AKAP) ezrin, the Csk-binding protein PAG and a linker between the two, EBP50. Failure of correct scaffolding and loss of spatiotemporal control can potentially have severe consequences, leading to immune failure or autoimmunity. The clinical relevance of supramolecular complexes specifically organized by scaffolding proteins in regulating immune activity and the specter of genetic diseases linked to different signaling components suggest that protein-protein contact surfaces can be potential targets for drug intervention. It is also of interest to note that different pathogens have evolved strategies to specifically modulate signal integration, thereby rewiring the signal in a way beneficial for their survival. In addition to demonstrating the importance of different signal processes, these adaptations are elegant illustrations of the potential for drug targeting of protein assembly. This chapter reviews some of the important scaffolding events downstream of immunoreceptors with focus on signaling transduction through the T-cell receptor (TCR).

Deletion of the LIME adaptor protein minimally affects T and B cell development and function

LIME (Lck-interacting membrane protein) is a transmembrane adaptor that associates with the Lck and Fyn protein tyrosine kinases and with the C-terminal Src kinase (Csk). To delineate the role of LIME in vivo, LIME-deficient mice were generated. Although Lime transcripts were expressed in immature and mature B and T cells, the absence of LIME impeded neither the development nor the function of B and T cells. TCR transgenic mice deprived of LIME showed, however, a 1.8-fold enhancement in positive selection. Since B cells and activated T cells express LIME and the related adaptor NTAL, mice lacking both adaptors were generated. Double-deficient mice showed no defect in the development and function of B and T cells, and the lack of LIME had no effect on the autoimmune syndrome that develops in aged NTAL-deficient mice. In contrast to a previous report, we further showed that this autoimmune syndrome develops in the absence of T cells. Therefore, our in vivo results refute all the previous roles postulated for LIME on the basis of studies of transformed B and T cells and demonstrate that LIME has no seminal role in the signaling cassette operated by antigen receptors and coreceptors.

Topography of signaling molecules as detected by electron microscopy on plasma membrane sheets isolated from non-adherent mast cells

Immunolabeling of isolated plasma membrane (PM) sheets combined with high-resolution electron microscopy is a powerful technique for understanding the topography of PM-bound signaling molecules. However, this technique has been mostly confined to analysis of membrane sheets from adherent cells. Here we present a rapid, simple and versatile method for isolation of PM sheets from non-adherent cells, and show its use for examination of the topography of Fcepsilon receptor I (FcepsilonRI) and transmembrane adaptors, LAT (linker for activation of T cells) and NTAL (non-T cell activation linker), in murine bone marrow-derived mast cells (BMMC). The data were compared with those obtained from widely used but tumor-derived rat basophilic leukemia (RBL) cells. In non-activated cells, FcepsilonRI was distributed either individually or in small clusters of comparable size in both cell types. In multivalent antigen-activated BMMC as well as RBL cells, FcepsilonRI was internalized to a similar extent, but, strikingly, internalization in BMMC was not preceded by formation of large (~200 nm) aggregates of FcepsilonRI, described previously in activated RBL cells. On the other hand, downstream adaptor proteins, LAT and NTAL, were localized in independent domains in both BMMC and RBL cells before and after FcepsilonRI triggering. The combined data demonstrate unexpected properties of FcepsilonRI signaling assemblies in BMMC and emphasize the importance of studies of PM sheets isolated from non-tumor cells.

Regulation of Ca2+ signaling in mast cells by tyrosine-phosphorylated and unphosphorylated non-T cell activation linker

Engagement of the FcepsilonRI in mast cells and basophils leads to a rapid tyrosine phosphorylation of the transmembrane adaptors LAT (linker for activation of T cells) and NTAL (non-T cell activation linker, also called LAB or LAT2). NTAL regulates activation of mast cells by a mechanism, which is incompletely understood. Here we report properties of rat basophilic leukemia cells with enhanced or reduced NTAL expression. Overexpression of NTAL led to changes in cell morphology, enhanced formation of actin filaments and inhibition of the FcepsilonRI-induced tyrosine phosphorylation of the FcepsilonRI subunits, Syk kinase and LAT and all downstream activation events, including calcium and secretory responses. In contrast, reduced expression of NTAL had little effect on early FcepsilonRI-induced signaling events but inhibited calcium mobilization and secretory response. Calcium response was also repressed in Ag-activated cells defective in Grb2, a major target of phosphorylated NTAL. Unexpectedly, in cells stimulated with thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+) ATPase, the amount of cellular NTAL directly correlated with the uptake of extracellular calcium even though no enhanced tyrosine phosphorylation of NTAL was observed. The combined data indicate that NTAL regulates FcepsilonRI-mediated signaling at multiple steps and by different mechanisms. At early stages NTAL interferes with tyrosine phosphorylation of several substrates and formation of signaling assemblies, whereas at later stages it regulates the activity of store-operated calcium channels through a distinct mechanism independent of enhanced NTAL tyrosine phosphorylation.

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

The transmembrane adaptor protein (TRAP), NTAL, is phosphorylated in mast cells following FcvarepsilonRI aggregation whereby it cooperates with LAT to induce degranulation. The Kit ligand, stem cell factor (SCF), enhances antigen-induced degranulation and this also appears to be NTAL-dependent. However, Kit and FcvarepsilonRI appear to utilize different mechanisms to induce NTAL phosphorylation. Thus, we examined whether the responsible kinases selectively phosphorylated distinct tyrosines in NTAL and explored the implications for downstream signaling. Whereas FcvarepsilonRI required Lyn and Syk for NTAL phosphorylation, Kit appeared to directly phosphorylate NTAL. Furthermore, co-transfection studies with NTAL constructs revealed that Lyn, Syk, and Kit phosphorylate different tyrosines in NTAL. The tyrosines principally phosphorylated by Syk were recognized as Grb2-binding sites, whereas Lyn and Kit phosphorylated other tyrosines, both inside and outside of these motifs. Pull down studies revealed that PLCgamma1 associated with the two terminal Syk-phosphorylated Grb2-binding sites, which would help to explain the observed decrease in antigen-induced calcium signal and degranulation in NTAL-knock down-human mast cells. The observations reported herein support the conclusion that NTAL may be differentially utilized by specific receptors for relaying alternative signals and this suggests a flexibility in the function of TRAPs not previously appreciated.

The related adaptors, adaptor in lymphocytes of unknown function X and Rlk/Itk-binding protein, have nonredundant functions in lymphocytes

Adaptors play a critical role in regulating signaling pathways that control lymphocyte development and activation. Adaptor in lymphocytes of unknown function X (ALX) and Rlk/Itk-binding protein (RIBP) are adaptors related by structure and sequence, coexpressed in T cells. Mice deficient for each adaptor demonstrated that ALX and RIBP, respectively, negatively and positively regulate T cell activation in response to TCR/CD28 stimulation. However, these results did not preclude that they may function redundantly in other cell populations, or in response to other stimuli. Therefore, to understand the relationship between these related adaptors, ALX/RIBP-deficient mice were generated. We demonstrate that although ALX and RIBP are expressed throughout T cell development, T cell development occurs normally in these mice. Using the H-Y TCR transgenic model, positive and negative selection were found to proceed unimpeded in the absence of ALX and RIBP. We demonstrate that RIBP is also expressed in B cells; however, RIBP- and ALX/RIBP-deficient mice had normal B cell development, and responded equivalently to wild type in response to IgM, CD40, B cell-activating factor/B lymphocyte stimulator, CpG, and LPS. Interestingly, T cells deficient in both ALX and RIBP behaved similarly to those deficient in ALX alone during T cell activation in response to TCR/CD28, exhibiting increased IL-2 production, CD25 expression, and proliferation, thus showing that ALX deficiency masked the effect of RIBP deficiency. ALX/RIBP-deficient T cells did not have any alterations in either activation-induced cell death or Th1/2 polarization. Therefore, we did not find any functional redundancy or synergy during lymphocyte development, selection, activation, or survival in ALX/RIBP-deficient mice, demonstrating that these molecules function independently.

The Cbl-b RING finger domain has a limited role in regulating inflammatory cytokine production by IgE-activated mast cells

The RING finger type E3 ubiquitin ligase, Cbl-b, is abundantly expressed in bone marrow-derived mast cells (BMMCs) and functions as a potent negative regulator of signalling responses from the high-affinity IgE receptor (FcvarepsilonRI). To determine the contribution of Cbl-b E3 ligase activity we generated knockin mice with a loss-of-function mutation in the RING finger domain. We find the mice to be healthy and, unlike equivalent c-Cbl RING finger mutant mice, produce homozygous offspring at the expected frequency. Comparative analyses of BMMCs from Cbl-b knockout and Cbl-b RING finger mutant mice revealed that both showed similarly enhanced FcvarepsilonRI signalling compared to wild-type cells for most parameters examined. A notable exception was a markedly higher level of activation of IkappaB kinase (IKK) in Cbl-b knockout BMMC compared to RING finger mutant-derived cells. In addition BMMCs from the Cbl-b RING finger mutant did not retard FcvarepsilonRI internalization to the extent observed for knockout cells. Most striking however was the finding that RING finger mutant mast cells do not produce the very high levels of TNF-alpha, IL-6, and MCP-1 evident in Cbl-b knockout cultures following FcvarepsilonRI activation. Thus the ability of Cbl-b to function as a negative regulator of FcvarepsilonRI signalling that promotes inflammatory cytokine production is largely independent of the RING finger domain.

Compartmentalization of ITAM and integrin signaling by adapter molecules

Adapters are multidomain molecules that recruit effector proteins during signal transduction by immunoreceptors and integrins. The absence of these scaffolding molecules profoundly affects development and function of various hematopoietic lineages, underscoring their importance as regulators of signaling cascades. An emerging aspect of the mechanism by which engaged immunoreceptors and integrins transmit signals within the cell is by differential usage of various adapters that function to nucleate formation of distinct signaling complexes in a specific location within the cell. In this review, we discuss the mechanisms by which adapter proteins coordinate signal transduction with an emphasis on the role of subcellular compartmentalization in adapter function.

Negative autoregulation of Src homology region 2-domain-containing phosphatase-1 in rat basophilic leukemia-2H3 cells

Src homology region 2-domain-containing phosphatase-1 (SHP-1) plays an important role in the regulation of signaling from various receptors in hematopoietic cells. In mast cells, SHP-1 has been shown to negatively regulate the initial signaling triggered by high-affinity receptor for IgE (FcepsilonRI) and positively regulate downstream outputs. To clarify the molecular mechanisms of SHP-1 in mast cells, we determined substrates for SHP-1 by using the substrate-trapping approach. When phosphatase-inactive SHP-1 was over-expressed in rat basophilic leukemia (RBL)-2H3 cells, tyrosine phosphorylation of a 68-kDa protein was enhanced before and after FcepsilonRI aggregation. Immunoprecipitation and western blot analyses revealed that this protein is SHP-1, either endogenous or ectopically expressed. FcepsilonRI-induced activation of Lyn and Syk was comparable between cells expressing wild-type (wt) and phosphatase-inactive SHP-1. In vitro phosphatase assay and combined transfection, immunoprecipitation and immunoblot analyses showed that tyrosine 536 of SHP-1 was potent phosphorylation site and that SHP-1 could dephosphorylate this site that had been phosphorylated by Lyn. Furthermore, the phosphatase activity of SHP-1 immunoprecipitated from cells expressing a phosphatase-inactive SHP-1 was increased compared with that from vector-transfected or wt SHP-1-expressing cells. Finally, expression of phosphatase-inactive SHP-1 resulted in decreased activation of mitogen-activated protein kinases and suppressed transcription of cytokine genes, whereas wt SHP-1 enhanced these processes. Taken collectively, these results suggest that SHP-1 may be a physiological substrate of SHP-1 in RBL-2H3 cells and that dephosphorylation of SHP-1 leads to a decrease in its catalytic activity and an enhancement of downstream signaling. A negative autoregulatory circuit of SHP-1 may contribute to mast cell regulation.

Src family kinases and lipid mediators in control of allergic inflammation

The Src family kinases Fyn and Lyn are important modulators of the molecular events initiated by engagement of the high-affinity IgE receptor (Fc epsilon RI). These kinases control many of the early signaling events and initiate the production of several lipid metabolites that have an important role in regulating mast cell responses. The intracellular level of phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), which is produced by phosphatidylinositol 3-OH kinase, plays an important role in determining the extent of a mast cells response to a stimulus. Enhanced levels lead to a hyperdegranulating phenotype (as seen in SHIP-1(-/-) and Lyn(-/-) mast cells), whereas decreased levels cause hypodegranulation (as seen in Fyn(-/-) mast cells). Downregulation of mast cell phosphatase and tensin homologue deleted on chromosone 10 expression, a phosphatase that reduces cellular levels of PIP(3), caused constitutive cytokine production, demonstrating that this response is particularly sensitive to PIP(3) levels. Lyn and Fyn are also intimately linked to other lipid kinases, like sphingosine kinases (SphK). By producing sphingosine-1-phosphate (S1P), SphKs contribute to mast cell chemotaxis and degranulation. In vivo studies now reveal that circulating S1P as well as that found within the mast cell is important in determining mast cell responsiveness. These studies demonstrate the connection between Src protein tyrosine kinases and lipid second messengers that control mast cell function and allergic responses.

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.

Mouse TCRalphabeta+CD8alphaalpha intraepithelial lymphocytes express genes that down-regulate their antigen reactivity and suppress immune responses

Mouse small intestine intraepithelial lymphocytes (IEL) that express alphabetaTCR and CD8alphaalpha homodimers are an enigmatic T cell subset, as their specificity and in vivo function remain to be defined. To gain insight into the nature of these cells, we performed global gene expression profiling using microarray analysis combined with real-time quantitative PCR and flow cytometry. Using these methods, TCRalphabeta(+)CD8alphaalpha IEL were compared with their TCRalphabeta(+)CD8beta(+) and TCRgammadelta(+) counterparts. Interestingly, TCRalphabeta(+)CD8alphaalpha IEL were found to preferentially express genes that would be expected to down-modulate their reactivity. They have a unique expression pattern of members of the Ly49 family of NK receptors and tend to express inhibitory receptors, along with some activating receptors. The signaling machinery of both TCRalphabeta(+)CD8alphaalpha and TCRgammadelta(+) IEL is constructed differently than other IEL and peripheral T cells, as evidenced by their low-level expression of the linker for activation of T cells and high expression of the non-T cell activation linker, which suppresses T cell activation. The TCRalphabeta(+)CD8alphaalpha IEL subset also has increased expression of genes that could be involved in immune regulation, including TGF-beta(3) and lymphocyte activation gene-3. Collectively, these data underscore the fact that, while TCRalphabeta(+)CD8alphaalpha IEL resemble TCRgammadelta(+) IEL, they are a unique population of cells with regulated Ag reactivity that could have regulatory function.

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.

The N terminus of the non-T cell activation linker (NTAL) confers inhibitory effects on pre-B cell differentiation

SLP-65 and the linker for activation of T cells (LAT) are central adaptor proteins that link the activated pre-BCR to downstream events in pre-B cells. Recently, a new transmembrane adaptor called NTAL/LAB/LAT2 (hereafter called NTAL for non-T cell activation linker) with striking functional and structural similarity to LAT has been identified in B cells. In this study, we compare the function of NTAL and LAT in pre-BCR signaling and show that, in contrast to LAT, NTAL does not induce pre-BCR down-regulation, calcium flux, or pre-B cell differentiation. To test whether differences between NTAL-mediated and LAT-mediated signaling are caused by the missing phospholipase C (PLC)-gamma binding motif in NTAL, we inserted the PLC-gamma1/2 binding motif of LAT into NTAL. This insertion rendered NTAL capable of activating pre-BCR down-regulation and calcium flux. Unexpectedly however, the ability of NTAL to induce calcium flux was not sufficient to promote pre-B cell differentiation, suggesting that the PLC-gamma binding motif has only partial effects on NTAL-mediated pre-BCR signaling. By generating chimeric swap mutants, we identified the N terminus of NTAL as an inhibitory domain that prevents pre-B cell differentiation while allowing pre-BCR down-regulation and receptor-mediated calcium flux. Our data suggest that, in addition to the missing PLC-gamma1/2 binding motif, the N terminus is responsible for the functional differences between NTAL and LAT in pre-B cells.

Proximal signaling events in FcɛRI-mediated mast cell activation

Mast cells are central mediators of allergic diseases. Their involvement in allergic reactions is largely dependent on activation through the specific receptor for IgE (Fc epsilon RI). Cross-linking of Fc epsilon RI on mast cells initiates a cascade of signaling events that eventually results in degranulation, cytokine/chemokine production, and leukotriene release, contributing to allergic symptomology. Because of the importance of IgE in allergy, much focus has been placed on deciphering the signaling events that take place downstream of Fc epsilon RI. Studies have identified spleen tyrosine kinase as a key proximal regulator of Fc epsilon RI-mediated signaling. In this review, we discuss the multiple pathways that diverge from spleen tyrosine kinase with emphasis on the role of adapter molecules to orchestrate these signaling events. Understanding the molecular mechanisms underlying mast cell activation ideally will provide insights into the development of novel therapeutics to control allergic disease.

Non-T Cell Activation Linker (NTAL) Negatively Regulates TREM-1/DAP12-Induced Inflammatory Cytokine Production in Myeloid Cells

The engagement of triggering receptor expressed on myeloid cells 1 (TREM-1) on macrophages and neutrophils leads to TNF-alpha and IL-8 production and enhances inflammatory responses to microbial products. For signal transduction, TREM-1 couples to the ITAM-containing adapter DNAX activation protein of 12 kDa (DAP12). In general, ITAM-mediated signals lead to cell activation, although DAP12 was recently implicated in inhibitory signaling in mouse macrophages and dendritic cells. To date, signals downstream of the TREM-1 and DAP12 complex in myeloid cells are poorly defined. By analyzing receptor-induced tyrosine phosphorylation patterns, we discovered that the ligation of TREM-1 leads to tyrosine phosphorylation of the non-T cell activation linker (NTAL; also called linker of activation in B cells or LAB) in a myelomonocytic cell line and primary human granulocytes. Using RNA interference to decrease the expression levels of NTAL, we demonstrate that in NTAL knockdown cell lines the phosphorylation of ERK1/2 is enhanced. In addition, low levels of NTAL are correlated with decreased and delayed mobilization of Ca(2+) after TREM-1 triggering. Most importantly, we demonstrate that NTAL acts as a negative regulator of TNF-alpha and IL-8 production after stimulation via TREM-1. Our results show that activation signals delivered via DAP12 can be counterbalanced by the adaptor NTAL, identifying NTAL as gatekeeper of TREM-1/DAP12-induced signaling in myeloid cells.

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

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

Adaptive and innate immune reactions regulating mast cell activation: from receptor-mediated signaling to responses

In this article, we have described studies that have demonstrated that mast cells can be activated as a consequence of adaptive and innate immune reactions and that these responses can be modified by ligands for other receptors expressed on the surface of mast cells. These various stimuli differentially activate multiple signaling pathways within the mast cells required for the generation and/or release of inflammatory mediators. Thus, the composition of the suite of mediators released and the physiologic ramifications of these responses are dependent on the stimuli and the microenvironment in which the mast cells are activated. Knowledge of the different signaling molecules used by cell surface receptors may allow selective pharmacologic targeting such that inhibiting the adverse effects of mast cell activation can be achieved without influencing the beneficial effects of mast cell activation. The exact interconnections between the signaling pathways initiated by the surface receptors described in this article remain to be completely worked out; thus, this remains a topic for future investigation.

Tec kinases in T cell and mast cell signaling

The Tec family of tyrosine kinases consists of five members (Itk, Rlk, Tec, Btk, and Bmx) that are expressed predominantly in hematopoietic cells. The exceptions, Tec and Bmx, are also found in endothelial cells. Tec kinases constitute the second largest family of cytoplasmic protein tyrosine kinases. While B cells express Btk and Tec, and T cells express Itk, Rlk, and Tec, all four of these kinases (Btk, Itk, Rlk, and Tec) can be detected in mast cells. This chapter will focus on the biochemical and cell biological data that have been accumulated regarding Itk, Rlk, Btk, and Tec. In particular, distinctions between the different Tec kinase family members will be highlighted, with a goal of providing insight into the unique functions of each kinase. The known functions of Tec kinases in T cell and mast cell signaling will then be described, with a particular focus on T cell receptor and mast cell Fc epsilon RI signaling pathways.

An essential role for RasGRP1 in mast cell function and IgE-mediated allergic response

Cross-linking of the FcepsilonRI activates the phosphatidyl inositol 3 kinase (PI3K) and mitogen-activated protein kinase pathways. Previous studies demonstrate that Ras guanyl nucleotide-releasing protein (RasGRP)1 is essential in T cell receptor-mediated Ras-Erk activation. Here, we report that RasGRP1 plays an important role in FcepsilonRI-mediated PI3K activation and mast cell function. RasGRP1-deficient mice failed to mount anaphylactic allergic reactions. RasGRP1-/- mast cells had markedly reduced degranulation and cytokine production. Although FcepsilonRI-mediated Erk activation was normal, PI3K activation was diminished. Consequently, activation of Akt, PIP3-dependent kinase, and protein kinase C delta was defective. Expression of a constitutively active form of N-Ras could rescue the degranulation defect and Akt activation. We further demonstrated that RasGRP1-/- mast cells were defective in granule translocation, microtubule formation, and RhoA activation. Our results identified RasGRP1 as an essential regulator of mast cell function.

Cbl-b is a negative regulator of inflammatory cytokines produced by IgE-activated mast cells

c-Cbl and Cbl-b E3 ubiquitin ligases are abundantly expressed in hemopoietic cells where they negatively regulate the activity and levels of many cell surface receptors and associated signaling molecules. By comparing bone marrow-derived mast cells from c-Cbl and Cbl-b-deficient mice it has recently been shown that Cbl-b is the dominant family member for negatively regulating signaling responses from high-affinity IgE receptors. In this study, we suggest that a possible reason for the greater enhancement of IgE receptor signaling in Cbl-b-deficient mice is the relatively higher levels of Cbl-b protein over c-Cbl in mast cells compared with other hemopoietic cells. We also directly compare mast cells from c-Cbl and Cbl-b-deficient mice and find that loss of Cbl-b, but not c-Cbl, increases cell growth, retards receptor internalization, and causes the sustained tyrosine phosphorylation of Syk and its substrates. However, loss of Cbl-b does not enhance the activation of ERK or Akt, nor does it promote a greater calcium response. Furthermore, loss of Cbl-b or c-Cbl does not increase levels of the Syk or Lyn protein tyrosine kinases. Most notable, however, is the extremely large increase in the production of proinflammatory cytokines TNF-alpha, IL-6, and MCP-1 by Cbl-b(-/-) mast cells compared with levels produced by c-Cbl(-/-) or wild-type cells. This marked induction, which appears to be restricted to these three cytokines, is dependent on IgE receptor activation and correlates with enhanced IkappaB kinase phosphorylation. Thus, Cbl-b functions as a potent negative regulator of cytokines that promote allergic and inflammatory reactions.

Fcε- and Fcγ-receptor signaling in diseases

It has become increasingly clear that receptors for the immunoglobulin Fc region play pivotal roles in immune homeostasis and disease. This review describes the fine regulation of the high-affinity IgE-receptor (FcepsilonRI) signaling, especially focusing on the early events that are coordinately regulated by Src family protein tyrosine kinases (PTKs), FcepsilonRI beta-subunit, and membrane lipid rafts. Because allergen-mediated FcepsilonRI cross-linking leads to the synthesis and release of a variety of proinflammatory mediators and cytokines, the duration and amplitude of the signal need to be strictly controlled, and the counterbalancing signaling is provided by specialized inhibitory receptors and molecules. However, recent work have revealed that Src family PTKs and FcepsilonRI beta-subunit transduce both positive and negative signaling with unexpectedly complex mechanisms. FcgammaRIIB exerts a unique inhibitory function on cell activation processes after the engagement of Fcgamma, FcepsilonRI and B cell receptors. Recent work has shown that FcgammaRIIB polymorphisms are associated with systemic lupus erythematosus, and that a transmembrane polymorphism in FcgammaRIIB results in an impaired distribution to lipid rafts and a reduced inhibitory function. Studies addressing the functions of disease-associated polymorphisms in the FcepsilonRI beta-subunit and low-affinity FcgammaRs are also considered.

Negative Regulation of T Cell Activation and Autoimmunity by the Transmembrane Adaptor Protein LAB

LAB (linker for activation of B cells), also known as NTAL (non-T cell activation linker), is a LAT (linker for activation of T cells)-like adaptor protein that is expressed in B, NK, and mast cells. Its role in lymphocytes has not been clearly demonstrated. Here, we showed that aged LAB-deficient (Lat2(-/-)) mice developed an autoimmune syndrome. Lat2(-/-) T cells were hyperactivated and produced more cytokines than Lat2(+/+) T cells. Even though LAB was absent in naive T cells, LAB could be detected in activated Lat2(+/+) T cells. LAT-mediated signaling events were enhanced in Lat2(-/-) T cells; however, they were suppressed in T cells that overexpressed LAB. Mice with the Lat2 gene conditionally deleted from T cells also developed the autoimmune syndrome like Lat2(-/-) mice. Together, these data demonstrated an important role of LAB in limiting autoimmune response and exposed a mechanism regulating T cell activation.

Ntal/Lab/Lat2

Non-T cell activation linker (NTAL)/linker for activation of B cells (LAB), now officially termed LAT2 (linker for activation of T cells 2) is a 25-30kDa transmembrane adaptor protein (TRAP) associated with glycolipid-enriched membrane fractions (GEMs; lipid rafts) in specific cell types of hematopoietic lineage. Tyrosine phosphorylation of NTAL/LAB/LAT2 is induced by FcvarepsilonRI aggregation and Kit dimerization in mast cells, FcgammaRI aggregation in monocytes, and BCR aggregation in B cells. NTAL/LAB/LAT2 is also expressed in resting NK cells but, unlike the related TRAP, LAT, not in resting T cells. As demonstrated in monocytes and B cells, phosphorylated NTAL/LAB/LAT2 recruits signaling molecules such as Grb2, Gab1 and c-Cbl into receptor-signaling complexes. Although gene knock out and knock down studies have indicated that NTAL/LAB/LAT2 may function as both a positive and negative regulator of mast cell activation, its precise role in the activation of these and other hematopoietic cells remains enigmatic.

Negative Regulation of FcϵRI-mediated Signaling and Mast Cell Function by the Adaptor Protein LAX

LAX is a transmembrane adaptor protein that is expressed in both T and B cells. Upon stimulation via the antigen receptors, it is tyrosine-phosphorylated and binds Grb2 and the p85 subunit of phosphatidylinositol 3-kinase. Disruption of the Lax gene causes hyperresponsiveness in T and B lymphocytes. Here, we showed that LAX was also expressed in mast cells. Upon engagement of the Fc epsilonRI, LAX was also phosphorylated and interacted with Grb2 and p85. LAX-deficient mast cells were hyperresponsive to stimulation via the Fc epsilonRI, as evidenced by enhanced degranulation, p38 MAPK, Akt, and phosphatidylinositol 3-kinase activation. This hyperresponsiveness was likely a consequence of reduced LAB expression after sensitization of mast cells with anti-dinitrophenyl IgE. In addition, Fc epsilonRI-mediated cytokine production and cell survival were also enhanced. These data suggested that LAX negatively regulates mast cell function.

Plasmacytoid dendritic cell-specific receptor ILT7-Fc epsilonRI gamma inhibits Toll-like receptor-induced interferon production

Immunoglobulin-like transcripts are a family of inhibitory and stimulatory cell surface immune receptors. Transcripts for one member of this family, ILT7, are selectively expressed in human plasmacytoid dendritic cells (pDCs). We demonstrate here that ILT7 protein associates with the signal adapter protein Fc epsilonRI gamma to form a receptor complex. Using an anti-ILT7 monoclonal antibody, we show that ILT7 is expressed specifically on human pDCs, but not on myeloid dendritic cells or other peripheral blood leukocytes. Cross-linking of ILT7 resulted in phosphorylation of Src family kinases and Syk kinase and induced a calcium influx in freshly isolated pDCs, which was blocked by Src family and Syk kinases inhibitors, thus indicating the activation of an immunoreceptor-based tyrosine activation motif-mediated signaling pathway. ILT7 cross-linking on CpG or influenza virus-stimulated primary pDCs inhibited the transcription and secretion of type I interferon and other cytokines. Therefore, the ILT7-Fc epsilonRI gamma receptor complex negatively regulates the innate immune functions of human pDCs.

Impaired degranulation but enhanced cytokine production after Fc epsilonRI stimulation of diacylglycerol kinase zeta-deficient mast cells

Calcium and diacylglycerol are critical second messengers that together effect mast cell degranulation after allergen cross-linking of immunoglobulin (Ig)E-bound FcepsilonRI. Diacylglycerol kinase (DGK)zeta is a negative regulator of diacylglycerol-dependent signaling that acts by converting diacylglycerol to phosphatidic acid. We reported previously that DGKzeta-/- mice have enhanced in vivo T cell function. Here, we demonstrate that these mice have diminished in vivo mast cell function, as revealed by impaired local anaphylactic responses. Concordantly, DGKzeta-/- bone marrow-derived mast cells (BMMCs) demonstrate impaired degranulation after Fc epsilonRI cross-linking, associated with diminished phospholipase Cgamma activity, calcium flux, and protein kinase C-betaII membrane recruitment. In contrast, Ras-Erk signals and interleukin-6 production are enhanced, both during IgE sensitization and after antigen cross-linking of Fc epsilonRI. Our data demonstrate dissociation between cytokine production and degranulation in mast cells and reveal the importance of DGK activity during IgE sensitization for proper attenuation of Fc epsilonRI signals.

Integrated signalling pathways for mast-cell activation

Mast-cell activation mediated by the high-affinity receptor for IgE (FcepsilonRI) is considered to be a key event in the allergic inflammatory response. However, in a physiological setting, other receptors, such as KIT, might also markedly influence the release of mediators by mast cells. Recent studies have provided evidence that FcepsilonRI-dependent degranulation is regulated by two complementary signalling pathways, one of which activates phospholipase Cgamma and the other of which activates phosphatidylinositol 3-kinase, using specific transmembrane and cytosolic adaptor molecules. In this Review, we discuss the evidence for these interacting pathways and describe how the capacity of KIT, and other receptors, to influence FcepsilonRI-dependent mast-cell-mediator release might be a function of the relative abilities of these receptors to activate these alternative pathways.

Multiplicity and plasticity of natural killer cell signaling pathways

Natural killer (NK) cells express an array of activating receptors that associate with DAP12 (KARAP), CD3zeta, and/or FcRgamma ITAM (immunoreceptor tyrosine-based activation motif)-bearing signaling subunits. In T and mast cells, ITAM-dependent signals are integrated by critical scaffolding elements such as LAT (linker for activation of T cells) and NTAL (non-T-cell activation linker). Using mice that are deficient for ITAM-bearing molecules, LAT or NTAL, we show that NK cell cytotoxicity and interferon-gamma secretion are initiated by ITAM-dependent and -independent as well as LAT/NTAL-dependent and -independent pathways. The role of these various signaling circuits depends on the target cell as well as on the activation status of the NK cell. The multiplicity and the plasticity of the pathways that initiate NK cell effector functions contrast with the situation in T cells and B cells and provide an explanation for the resiliency of NK cell effector functions to various pharmacologic inhibitors and genetic mutations in signaling molecules.

Scaffolding adapter Grb2-associated binder 2 requires Syk to transmit signals from FcepsilonRI

Scaffolding adapter Grb2-associated binder 2 (Gab2) is a key component of FcepsilonRI signaling in mast cells, required for the activation of PI3K. To understand how Gab2 is activated in FcepsilonRI signaling, we asked which protein tyrosine kinase is required for Gab2 phosphorylation. We found that Gab2 tyrosyl phosphorylation requires Lyn and Syk. In agreement with published results, we found that Fyn also contributes to Gab2 tyrosyl phosphorylation. However, Syk activation is defective in Fyn(-/-) mast cells, suggesting that Syk is the proximal kinase responsible for Gab2 tyrosyl phosphorylation. Then, we asked which domains in Gab2 are required for Gab2 tyrosyl phosphorylation. We found that the Grb2-Src homology 3 (SH3) binding sites are required for, whereas the pleckstrin homology (PH) domain contributes to, Gab2 tyrosyl phosphorylation. Using a protein/lipid overlay assay, we determined that the Gab2 PH domain preferentially binds the PI3K lipid products, PI3, 4,5P3 and PI3, 4P2. Furthermore, the Grb2-SH3 binding sites and PH domain binding to PI3K lipid products are required for Gab2 function in FcepsilonRI-evoked degranulation and Akt activation. Our data strongly suggest a model for Gab2 action in FcepsilonRI signaling. The Grb2 SH3 binding sites play a critical role in bringing Gab2 to FcepsilonRI, whereupon Gab2 becomes tyrosyl-phosphorylated in a Syk-dependent fashion. Phosphorylated Gab2 results in recruitment and activation of PI3K, whose lipid products bind the PH domain of Gab2 and acts in positive feedback loop for sustained PI3K recruitment and phosphatidylinositol-3,4,5-trisphosphate production, required for FcepsilonRI-evoked degranulation of mast cells.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

Lymphocyte calcium signaling from membrane to nucleus

Ca(2+) signals control a variety of lymphocyte responses, ranging from short-term cytoskeletal modifications to long-term changes in gene expression. The identification of molecules and channels that modulate Ca(2+) entry into T and B lymphocytes has both provided details of the molecular events leading to immune responses and raised controversy. Here we review studies of the pathways that allow Ca(2+) entry, the function of Ca(2+) in the regulation of cell polarity and motility and the principles by which Ca(2+)-dependent transcription regulates lymphocyte function.

Regulation of allergy by Fc receptors

The aggregation of high-affinity IgE receptors (FcepsilonRI) on mast cells and basophils has long been known as the critical event that initiates allergic reactions. Monomeric IgE was recently found to induce a variety of effects when binding to FcepsilonRI. Upregulation of FcepsilonRI only requires binding, whereas other responses require FcepsilonRI aggregation. Interestingly, FcepsilonRI aggregation has recently been understood to generate a mixture of positive and negative intracellular signals. Mast cells and basophils also express low-affinity and, under specific conditions, high-affinity IgG receptors. When co-engaging these receptors with FcepsilonRI, IgG antibodies can amplify or dampen IgE-induced mast cell activation. On the basis of these findings, it has been proposed that FcRs can be used as targets and/or tools for new therapeutic approaches to allergies.

The transmembrane adapter protein SIT regulates thymic development and peripheral T-cell functions

SIT is a transmembrane adapter protein that modulates signals emanating from the T-cell receptor (TCR). Here, we have used gene-targeted mice to assess the role of SIT for T-cell development and peripheral T-cell functions. SIT(-/-) double-positive thymocytes show an upregulation of the activation markers CD5 and CD69, suggesting that SIT negatively regulates TCR-mediated signals at the CD4(+) CD8(+) stage of thymic development. This assumption is further supported by the observation that in female H-Y TCR transgenic mice, positive selection is enhanced and even converted to negative selection. Similarly, mature peripheral T cells are hyperresponsive towards TCR-mediated stimuli and produce larger amounts of T-helper 1 (TH1) cytokines, and SIT-deficient mice show an increased susceptibility to develop experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. These results demonstrate that SIT is a critical negative regulator of TCR-mediated signaling and finely tunes the signals required for thymic selection and peripheral T-cell activation.

Transmembrane adaptor molecules: a new category of lymphoid-cell markers

Transmembrane adaptor proteins (of which 7 have been identified so far) are involved in receptor signaling in immune cells. They have only a short extracellular region, with most of the molecule comprising a substantial intracytoplasmic region carrying multiple tyrosine residues that can be phosphorylated by Src- or Syk-family kinases. In this paper, we report an immunohistologic study of 6 of these molecules in normal and neoplastic human tissue sections and show that they are restricted to subpopulations of lymphoid cells, being present in either T cells (LAT, LIME, and TRIM), B cells (NTAL), or subsets of both cell types (PAG and SIT). Their expression in neoplastic lymphoid cells broadly reflects that of normal lymphoid tissue, including the positivity of plasma cells and myeloma/plasmacytoma for LIME, NTAL, PAG, and SIT. However, this study also revealed some reactions that may be of diagnostic/prognostic value. For example, lymphocytic lymphoma and mantle-cell lymphoma showed similar profiles but differed clearly from follicle-center lymphoma, whereas PAG tended to be selectively expressed in germinal center-derived subsets of diffuse large B-cell lymphoma. These molecules represent a potentially important addition to the panel of immunophenotypic markers detectable in routine biopsies that can be used in hematopathologic studies.

GPVI and integrin alphaIIb beta3 signaling in platelets

This review summarizes recent developments in our understanding of the molecular basis of platelet activation by two distinct types of surface receptor, the immunoglobulin GPVI, and the integrin alphaIIb beta3 (also known as GPIIbIIIa). These two classes of receptor signal through similar yet distinct tyrosine kinase-based signaling cascades leading to activation of phospholipase C gamma2. The significance of these signaling cascades in platelet adhesion and platelet aggregation at arterial rates of shear is discussed.

Single and combined deletions of the NTAL/LAB and LAT adaptors minimally affect B-cell development and function

NTAL (non-T-cell activation linker, also called LAB) and LAT (linker for activation of T cells) are evolutionarily related transmembrane adaptor proteins that are phosphorylated upon immunoreceptor engagement. Using quantitative reverse transcription-PCR, both NTAL and LAT were found to be expressed in B cells. However, LAT expression was limited to early B cells, whereas NTAL expression typified mature B cells. To delineate their roles in B-cell development and function, Ntal-deficient mice were generated and crossed with Lat-deficient mice. B cells developed in Lat(-/-) Ntal(-/-) double-deficient mice and in mice lacking either of the two adaptors with the same efficiency as in wild-type mice. Upon B-cell antigen receptor cross-linking, Ntal(-/-) B cells exhibited slightly increased Ca(2+) mobilization and proliferation. In addition, Ntal-deficient mice had increased levels of natural antibodies and slightly increased humoral response to a T-dependent antigen. Normal titers of serum-specific immunoglobulins were produced in response to a T-cell-independent antigen. Although NTAL is also expressed in plasma cells, its absence did not affect the hypergammaglobulinemia E and G1 that developed in mice with a mutation in tyrosine 136 of LAT. Therefore, NTAL does not play a role in B cells symmetric to the role played by LAT in T cells.