Coupling of the TCR to integrin activation by Slap-130/Fyb

Regulation and Function of SKAP-55 Non-canonical Motif Binding to the SH3c Domain of Adhesion and Degranulation-promoting Adaptor Protein

The immune cell adaptor adhesion and degranulation promoting adaptor protein (ADAP) and its binding to T-cell adaptor Src kinase-associated protein of 55 kDa (SKAP-55) play a key role in the modulation of T-cell adhesion. While primary binding occurs via SKAP-55 SH3 domain binding to a proline-rich region in ADAP, a second interaction occurs between the ADAP C-terminal SH3 domain (ADAP-SH3c) and a non-canonical RKXXY294XXY297 motif in SKAP-55. Increasing numbers of non-canonical SH3 domain binding motifs have been identified in a number of biological systems. The presence of tyrosine residues in the SKAP-55 RKXXY294XXY297 motif suggested that phosphorylation might influence this unusual SH3 domain interaction. Here, we show that the Src kinase p59fyn can induce the in vivo phosphorylation of the motif, and this event blocks ADAP-SH3c domain binding to the peptide motif. The importance of tyrosine phosphorylation was confirmed by plasmon resonance interaction analysis showing that phosphorylation of Tyr294 residue plays a central role in mediating dissociation, whereas phosphorylation of the second Tyr297 had no effect. Although loss of this secondary interaction did not result in the disruption of the complex, the Y294F mutation blocked T-cell receptor-induced up-regulation of lymphocyte function-associated antigen-1-mediated adhesion to intercellular adhesion molecule-1 and interleukin-2 promoter activity. Our findings identify a RKXXY294 motif in SKAP-55 that mediates unique ADAP SH3c domain binding and is needed for LFA-1-mediated adhesion and cytokine production.

Rap1A-deficient T and B cells show impaired integrin-mediated cell adhesion

Studies in tissue culture cells have demonstrated a role for the Ras-like GTPase Rap1 in the regulation of integrin-mediated cell-matrix and cadherin-mediated cell-cell contacts. To analyze the function of Rap1 in vivo, we have disrupted the Rap1A gene by homologous recombination. Mice homozygous for the deletion allele are viable and fertile. However, primary hematopoietic cells isolated from spleen or thymus have a diminished adhesive capacity on ICAM and fibronectin substrates. In addition, polarization of T cells from Rap1-/- cells after CD3 stimulation was impaired compared to that of wild-type cells. Despite this, these defects did not result in hematopoietic or cell homing abnormalities. Although it is possible that the relatively mild phenotype is a consequence of functional complementation by the Rap1B gene, our genetic studies confirm a role for Rap1A in the regulation of integrins.

Membrane-proximal signaling events in beta-2 integrin activation

In the immune system, integrins have essential roles in leukocyte trafficking and function. These include immune cell attachment to endothelial and antigen-presenting cells, cytotoxicity, and extravasation into tissues. The integrin leukocyte function-associated antigen-1 (LFA-1), which is exclusively expressed on hematopoietic cells, has been intensely studied since this receptor is important for many functions of the immune system. LFA-1 is involved in a) the interaction between T-cells and antigen presenting cells, b) the adhesion of cells to post-capillary high endothelial venules or to activated endothelium at sites of inflammation (extravasation), c) the control of cell differentiation and proliferation, and d) the regulation of T-cell effector functions. Therefore, a precise understanding of the spatial and temporal control of LFA-1 interaction with its cellular counter-receptors, the intercellular adhesion molecules (ICAM) -1, -2 and -3, in the various contexts, is of high interest. LFA-1 mediated adhesion is induced by several extracellular stimuli in different cell types. In T-cells, LFA-1 becomes activated upon signaling from the T-cell receptor (TCR), and upon cytokine and chemokine sensing. Adhesion of monocytes to ICAM-1 is induced by lipopolysaccharide (LPS), a component of the bacterial cell wall. To investigate the regulation of LFA-1 adhesiveness, research has focused on the identification of interaction partners of the intracellular portions of the integrin alpha and beta subunits. This review will highlight recent developments on transmembrane and intracellular signaling proteins, which have been implicated in beta-2 integrin activation.

Protein Tyrosine Phosphatase α Regulates Fyn Activity and Cbp/PAG Phosphorylation in Thymocyte Lipid Rafts

A role for the receptor protein tyrosine phosphatase alpha (PTPalpha) in immune cell function and regulation of Src family kinases was investigated using thymocytes from PTPalpha-deficient mice. PTPalpha-null thymocytes develop normally, but unstimulated PTPalpha-/- cells exhibit increased tyrosine phosphorylation of specific proteins, increased Fyn activity, and hyperphosphorylation of Cbp/PAG that promotes its association with C-terminal Src kinase. Elevated Fyn activity in the absence of PTPalpha is due to enhanced phosphorylation of Fyn tyrosines 528 and 417. Some PTPalpha is localized in lipid rafts of thymocytes, and raft-associated Fyn is specifically activated in PTPalpha-/- cells. PTPalpha is not a Cbp/PAG phosphatase, because it is not required for Cbp/PAG dephosphorylation in unstimulated or anti-CD3-stimulated thymocytes. Together, our results indicate that PTPalpha, likely located in lipid rafts, regulates the activity of raft Fyn. In the absence of PTPalpha this population of Fyn is activated and phosphorylates Cbp/PAG to enhance association with C-terminal Src kinase. Although TCR-mediated tyrosine phosphorylation was apparently unaffected by the absence of PTPalpha, the long-term proliferative response of PTPalpha-/- thymocytes was reduced. These findings indicate that PTPalpha is a component of the complex Src family tyrosine kinase regulatory network in thymocytes and is required to suppress Fyn activity in unstimulated cells in a manner that is not compensated for by the major T cell PTP and SFK regulator, CD45.

Regulation of in vitro and in vivo immune functions by the cytosolic adaptor protein SKAP-HOM

SKAP-HOM is a cytosolic adaptor protein representing a specific substrate for the Src family protein tyrosine kinase Fyn. Previously, several groups have provided experimental evidence that SKAP-HOM (most likely in cooperation with the cytosolic adaptor protein ADAP) is involved in regulating leukocyte adhesion. To further assess the physiological role of SKAP-HOM, we investigated the immune system of SKAP-HOM-deficient mice. Our data show that T-cell responses towards a variety of stimuli are unaffected in the absence of SKAP-HOM. Similarly, B-cell receptor (BCR)-mediated total tyrosine phosphorylation and phosphorylation of Erk, p38, and JNK, as well as immunoreceptor-mediated Ca(2+) responses, are normal in SKAP-HOM(-/-) animals. However, despite apparently normal membrane-proximal signaling events, BCR-mediated proliferation is strongly attenuated in the absence of SKAP-HOM(-/-). In addition, adhesion of activated B cells to fibronectin (a ligand for beta1 integrins) as well as to ICAM-1 (a ligand for beta2 integrins) is strongly reduced. In vivo, the loss of SKAP-HOM results in a less severe clinical course of experimental autoimmune encephalomyelitis following immunization of mice with the encephalitogenic peptide of MOG (myelin oligodendrocyte glycoprotein). This is accompanied by strongly reduced serum levels of MOG-specific antibodies and lower MOG-specific T-cell responses. In summary, our data suggest that SKAP-HOM is required for proper activation of the immune system, likely by regulating the cross-talk between immunoreceptors and integrins.

Tec Kinases Regulate TCR-Mediated Recruitment of Signaling Molecules and Integrin-Dependent Cell Adhesion

T cells deficient in the Tec kinases Itk or Itk and Rlk exhibit defective TCR-stimulated proliferation, IL-2 production, and activation of phospholipase C-gamma. Evidence also implicates Tec kinases in actin cytoskeleton regulation, which is necessary for cell adhesion and formation of the immune synapse in T lymphocytes. In this study we show that Tec kinases are required for TCR-mediated up-regulation of adhesion via the LFA-1 integrin. We also demonstrate that the defect in adhesion is associated with defective clustering of LFA-1 and talin at the site of interaction of Rlk-/-Itk-/- and Itk-/- T cells with anti-TCR-coated beads. Defective recruitment of Vav1, protein kinase Ctheta, and Pyk2 was also observed in Rlk-/-Itk-/- and Itk-/- T cells. Stimulation with ICAM-2 in conjunction with anti-TCR-coated beads enhanced polarization of Vav1, protein kinase Ctheta, and Pyk2 in wild-type cells, demonstrating a role for integrins in potentiating the recruitment of signaling molecules in T cells. Increased recruitment of signaling molecules was most pronounced under conditions of low TCR stimulation. Under these suboptimal TCR stimulation conditions, ICAM-2 could also enhance the recruitment of signaling molecules in Itk-/-, but not Rlk-/-Itk-/- T cells. Thus, Tec kinases play key roles in regulating TCR-mediated polarization of integrins and signaling molecules to the site of TCR stimulation as well as the up-regulation of integrin adhesion.

Protein kinase D1 and the beta 1 integrin cytoplasmic domain control beta 1 integrin function via regulation of Rap1 activation

The functional activity of integrins is dynamically regulated by T cell receptor stimulation and by protein kinase C (PKC). We report a novel function for the PKC effector protein kinase D1 (PKD1) in integrin activation. Constitutively active and kinase-inactive PKD1 mutants lacking the PKD1 pleckstrin homology (PH) domain block phorbol ester- and TCR-mediated activation and clustering of beta1 integrins. The PH domain of PKD1 mediates the association of PKD1 with the GTPase Rap1 and is central to Rap1 activation and membrane translocation in T cells. Furthermore, PKD1 and Rap1 associate with beta1 integrins in a manner that is dependent on the carboxy-terminal end of the beta1 integrin subunit cytoplasmic domain. beta1 integrin expression is required for Rap1 activation and membrane localization of the PKD1-Rap1 complex. Therefore, PKD1 promotes integrin activation in T cells by regulating Rap1 activation and membrane translocation via interactions with the beta1 integrin subunit cytoplasmic domain.

Physical and Functional Association of c-Src and Adhesion and Degranulation Promoting Adaptor Protein (ADAP) in Osteoclastogenesis in Vitro

c-Src plays a crucial role in osteoclastogenesis. In this study, we searched for c-Src-binding proteins using a combination of pull-down assays and mass spectrometric analysis, and identified the association of adhesion and degranulation promoting adaptor protein (ADAP) with c-Src in RAW264 cells and osteoclast precursors prepared from bone marrow cells. The kinase activity and the SH2 domain of c-Src were required for this association and Tyr807 in the extreme carboxyl terminus of ADAP was identified as a major recognition site. ADAP was found to be expressed in cells at the prefusion stage and localized mainly in the leading edge of lamellipodia and in pseudopodia. Tyrosine phosphorylation of ADAP was induced in an integrin-dependent manner, and the level was Src kinase-dependent. ADAP-knockdown RAW264 cells showed retarded migration and formed few multinucleated cells. Cas, known to be phosphorylated by c-Src, was identified as a major tyrosine-phosphorylated protein in differentiating RAW264 cells and the phosphorylation appeared to be decreased in ADAP-knockdown cells. ADAP thus may play an important role as a partner of c-Src for cell migration and progression to the multinucleated cell stage in osteoclastogenesis in vitro.

An essential role for SKAP-55 in LFA-1 clustering on T cells that cannot be substituted by SKAP-55R

Lymphocyte function-associated antigen (LFA)-1 clustering, which is needed for high avidity binding to intercellular adhesion molecule (ICAM)-1 and -2, regulates T cell motility and T cell-antigen-presenting cell (APC) conjugation. In this study, down-regulation of SKAP-55 by small interfering RNAs (siRNAs) identified an essential role for this adaptor molecule in the T cell receptor (TCR)-mediated "inside-out signaling" that is needed for LFA-1 clustering and T cell-APC conjugation. In contrast, down-regulation of SKAP-55 had no effect on TCR-CD3 clustering. Furthermore, the expression of the related protein SKAP-55R failed to compensate for the loss of SKAP-55 in LFA-1 clustering, indicating that SKAP-55 has a unique function that cannot be replaced by this closely related protein. Our findings therefore indicate that SKAP-55, unlike SKAP-55R, is specifically tailored as an essential component of the inside-out signaling events that couple the TCR to LFA-1 clustering and T cell-APC conjugation.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

Intracellular signalling controlling integrin activation in lymphocytes

Since the discovery that integrins at the surface of lymphocytes undergo dynamic changes in their adhesive activity after stimulation through the T-cell receptor or stimulation with chemokines, intensive research has been carried out in an attempt to clarify the signalling events that lead to the activation of integrins. Whereas structural studies have provided us with a vivid picture of the conformational flexibility of integrins, the signalling pathways that regulate these conformational changes (known as inside-out signalling) have been elusive. However, as I discuss here, recent studies have provided new insight into the pathways that control the regulation of integrin activity and the coordination of complex cellular functions, such as the homing of lymphocytes and the formation of an immunological synapse.

Importance of integrin LFA-1 deactivation for the generation of immune responses

The dynamic regulation of ligand binding is considered crucial for integrin function. However, the importance of activity regulation for integrin function in vivo is largely unknown. Here, we have applied gene targeting to delete the GFFKR sequence of the lymphocyte function-associated antigen-1 (LFA-1) alpha(L) subunit cytoplasmic domain in mouse germline. Lymphocytes from Lfa-1(d/d) mutant mice showed constitutive activation of LFA-1-mediated cell adhesion and impaired de-adhesion from intercellular adhesion molecule-1 that resulted in defective cell migration. In contrast, signaling through LFA-1 was not affected in Lfa-1(d/d) cells. T cell activation by superantigen-loaded and allogeneic APCs, cytotoxic T cell activity, T-dependent humoral immune responses, and neutrophil recruitment during aseptic peritonitis were impaired in Lfa-1(d/d) mice. Thus, deactivation of LFA-1 and disassembly of LFA-1-mediated cell contacts seem to be vital for the generation of normal immune responses.

The helically extended SH3 domain of the T cell adaptor protein ADAP is a novel lipid interaction domain

Adhesion and degranulation-promoting adapter protein (ADAP) is critically involved in downstream signalling events triggered by the activation of the T cell receptor. Cytokine production, proliferation and integrin clustering of T cells are dependent on ADAP function, but the molecular basis for these processes is poorly understood. We now show the hSH3 domain of ADAP to be a lipid-interaction module that binds to acidic lipids, including phosphatidylinositides. Positively charged surface patches of the domain preferentially bind to polyvalent acidic lipids such as PIP2 or PIP3 over the monovalent PS phospholipid and this interaction is dependent on the N-terminal helix of the hSH3 domain fold. Basic amino acid side-chains from the SH3 scaffold also contribute to lipid binding. In the context of T cell signalling, our findings suggest that ADAP, upon recruitment to the cell-cell junction as part of a multiprotein complex, directly interacts with phosphoinositide-enriched regions of the plasma membrane. Furthermore, the ADAP lipid interaction defines the helically extended SH3 scaffold as a novel member of membrane interaction domains.

Receptor-stimulated oxidation of SHP-2 promotes T-cell adhesion through SLP-76-ADAP

Receptor-stimulated generation of intracellular reactive oxygen species (ROS) modulates signal transduction, although the mechanism(s) is unclear. One potential basis is the reversible oxidation of the active site cysteine of protein tyrosine phosphatases (PTPs). Here, we show that activation of the antigen receptor of T cells (TCR), which induces production of ROS, induces transient inactivation of the SH2 domain-containing PTP, SHP-2, but not the homologous SHP-1. SHP-2 is recruited to the LAT-Gads-SLP-76 complex and directly regulates the phosphorylation of key signaling proteins Vav1 and ADAP. Furthermore, the association of ADAP with the adapter SLP-76 is regulated by SHP-2 in a redox-dependent manner. The data indicate that TCR-mediated ROS generation leads to SHP-2 oxidation, which promotes T-cell adhesion through effects on an SLP-76-dependent signaling pathway to integrin activation.

Glycoprotein VI/Fc receptor gamma chain-independent tyrosine phosphorylation and activation of murine platelets by collagen

We have investigated the ability of collagen to induce signalling and functional responses in suspensions of murine platelets deficient in the FcRgamma (Fc receptor gamma) chain, which lack the collagen receptor GPVI (glycoprotein VI). In the absence of the FcRgamma chain, collagen induced a unique pattern of tyrosine phosphorylation which was potentiated by the thromboxane analogue U46619. Immunoprecipitation studies indicated that neither collagen alone nor the combination of collagen plus U46619 induced phosphorylation of the GPVI-regulated proteins Syk and SLP-76 (Src homology 2-containing leucocyte protein of 76 kDa). A low level of tyrosine phosphorylation of phospholipase Cgamma2 was observed, which was increased in the presence of U46619, although the degree of phosphorylation remained well below that observed in wild-type platelets (approximately 10%). By contrast, collagen-induced phosphorylation of the adapter ADAP (adhesion- and degranulation-promoting adapter protein) was substantially potentiated by U46619 to levels equivalent to those observed in wild-type platelets. Collagen plus U46619 also induced significant phosphorylation of FAK (focal adhesion kinase). The functional significance of collagen-induced non-GPVI signals was highlighted by the ability of U46619 and collagen to induce the secretion of ATP in FcRgamma chain-deficient platelets, even though neither agonist was effective alone. Protein tyrosine phosphorylation and the release of ATP were abolished by the anti-(alpha2 integrin) antibodies Ha1/29 and HMalpha2, but not by blockade of alphaIIbbeta3. These results illustrate a novel mechanism of platelet activation by collagen which is independent of the GPVI-FcRgamma chain complex, and is facilitated by binding of collagen to integrin alpha2beta1.

The SLP-76 family of adapter proteins

Adapter molecules are multidomain proteins lacking intrinsic catalytic activity, functioning instead by nucleating molecular complexes during signal transduction. The SLP-76 family of adapters includes SH2 domain-containing leukocyte phosphoprotein of 76kDa (SLP-76), B cell linker protein (BLNK), and cytokine-dependent hematopoietic cell linker (Clnk). These proteins are critical for integration of numerous signaling cascades downstream of immunotyrosine-based activation motif (ITAM)-bearing receptors and integrins in diverse hematopoietic cell types. Mutations in genes encoding SLP-76 family adapters result in severe phenotypes, underscoring the critical role these proteins play in cellular development and function by directing formation of signaling complexes in a temporally- and spatially-specific manner.

Signaling control of memory T cell generation and function

Memory T cells exhibit low activation thresholds and mediate rapid effector responses when recalled by antigen; contrasting the higher activation threshold, slower responses and predominant IL-2 production by naive T cells. While the sequence of intracellular events coupling the T cell-receptor (TCR) to naive T cell activation is well characterized, biochemical control of memory T cell differentiation and function remains undefined. In this review, we will discuss recent developments in T cell-receptor signal transduction as they pertain to memory T cells, and will discuss how signal dampening may drive memory generation, and more efficient spatial organization of signaling molecules may promote rapid recall responses.

Deficiency of ADAP/Fyb/SLAP-130 destabilizes SKAP55 in Jurkat T cells

ADAP (adhesion and degranulation-promoting adaptor protein) and SKAP55 (Src kinase-associated phosphoprotein of 55 kDa) are T cell adaptors that mediate inside-out signaling from the T cell antigen receptor to integrins, giving rise to increased integrin affinity/avidity and formation of the immunological synapse between the T cell and the antigen-presenting cell. These two proteins are tightly and constitutively associated with one another, and their ability to interact is required for inside-out signaling. Here we show in an ADAP-deficient Jurkat T cell line that the co-dependence of ADAP and SKAP55 extends beyond their functional and physical interactions and show that SKAP55 protein is unstable in the absence of ADAP. Restoration of ADAP to the ADAP-deficient Jurkat T cell line restores SKAP55 expression by causing a 5-fold decrease in the rate of SKAP55 proteolysis. Inactivation of the Src homology 3 domain of SKAP55, which mediates the association between SKAP55 with ADAP, blocks the protective effect of ADAP. The half-life of SKAP55, in the absence of ADAP, is approximately 15-20 min, increasing to 90 min in the presence of ADAP. This is a remarkably rapid rate of turnover for a signaling protein and suggests the possibility that stimuli that signal for the stabilization of SKAP55 may play an important role in T cell adhesion and conjugate formation.

Fyn binding protein, Fyb, interacts with mammalian actin binding protein, mAbp1

The immune cell specific protein Fyn-T binding protein (Fyb) has been identified as a target of the Yersinia antiphagocytic effector Yersinia outer protein H (YopH), but its role in macrophages is unknown. By using Fyb domains as bait to screen a mouse lymphoma cDNA library, we identified a novel interaction partner, mammalian actin binding protein 1 (mAbp1). We show that mAbp1 binds the Fyb N-terminal via its C-terminally located src homology 3 domain. The interaction between Fyb and mAbp1 is detected in macrophage lysates and the proteins co-localize with F-actin in the leading edge. Hence, mAbp1 is likely to constitute a downstream effector of Fyb involved in F-actin dynamics.

Defective Proximal TCR Signaling Inhibits CD8+ Tumor-Infiltrating Lymphocyte Lytic Function

CD8+ tumor-infiltrating lymphocytes (TIL) are severely deficient in cytolysis, a defect that may permit tumor escape from immune-mediated destruction. Because lytic function is dependent upon TCR signaling, we have tested the hypothesis that primary TIL have defective signaling by analysis of the localization and activation status of TIL proteins important in TCR-mediated signaling. Upon conjugate formation with cognate target cells in vitro, TIL do not recruit granzyme B+ granules, the microtubule-organizing center, F-actin, Wiskott-Aldrich syndrome protein, nor proline rich tyrosine kinase-2 to the target cell contact site. In addition, TIL do not flux calcium nor demonstrate proximal tyrosine kinase activity, deficiencies likely to underlie failure to fully activate the lytic machinery. Confocal microscopy and fluorescence resonance energy transfer analyses demonstrate that TIL are triggered by conjugate formation in that the TCR, p56lck, CD3zeta, LFA-1, lipid rafts, ZAP70, and linker for activation of T cells localize at the TIL:tumor cell contact site, and CD43 and CD45 are excluded. However, proximal TCR signaling is blocked upon conjugate formation because the inhibitory motif of p56lck is rapidly phosphorylated (Y505) and COOH-terminal Src kinase is recruited to the contact site, while Src homology 2 domain-containing protein phosphatase 2 is cytoplasmic. Our data support a novel mechanism explaining how tumor-induced inactivation of proximal TCR signaling regulates lytic function of antitumor T cells.

Tec kinases: shaping T-cell activation through actin

Following stimulation, T cells undergo marked actin-dependent changes in shape that are required for productive cellular interactions and movement during immune responses. Reorganization of the actin cytoskeletal is also necessary for the formation of an immunological synapse - the convergence of several signaling molecules at the plasma membrane that occurs after effective T-cell receptor (TCR) signaling. Much emerging evidence indicates that the Tec family of tyrosine kinases has a role in actin cytoskeleton reorganization. Specifically, T cells that lack or express mutant versions of the Tec kinase Itk show impaired TCR-induced actin polymerization, cell polarization and regulation of the signaling events involved in cytoskeletal reorganization. These data, as well as other findings, support roles for Tec kinases in actin cytoskeleton regulation.

Adaptors and linkers in T and B cells

By mediating non-covalent protein-protein interactions, adaptors organize and assemble the multimolecular signalling complexes that coordinate intracellular programs leading to the activation and differentiation of lymphocytes. The co-ordinated interaction between adaptor and effector molecules is required for the propagation and dynamic modification of externally applied signals. Recent advances have been made regarding our understanding of how adaptors regulate signalling within lymphocytes. An unexpected function has been revealed for the well-known adaptor protein LAT in pre-B-cell receptor signalling. In addition, the adaptors BCAP, Bcl10, CARMA1 and Malt1 seem to regulate the development of particular B-cell subsets. In contrast to Shc, c-Cbl and LAT, which are involved in early signalling events, BCAP, Bcl10, CARMA1 and Malt1 seem to act more distally, by controlling NF-kappaB activation. Additional transmembrane adaptors, such as NTAL/LAB and LIME, have been identified and partially characterized. Finally, an involvement of the cytosolic adaptors ADAP, SKAP-55 and Cbl-b in the regulation of lymphocyte adhesion and migration has been demonstrated.

PRAM-1 potentiates arsenic trioxide-induced JNK activation

The promyelocytic leukemia RARalpha target gene encoding an adaptor molecule-1 (PRAM-1) is involved in a signaling pathway induced by retinoic acid in acute promyelocytic leukemia (APL) cells. To better understand the function of PRAM-1, we have undertaken the identification of its partners through a yeast two-hybrid screen. Here, we show that the proline-rich domain of PRAM-1 interacted with the Src homology 3 (SH3) domain of hematopoietic progenitor kinase 1 (HPK-1)-interacting protein of 55 kDa (HIP-55, also called SH3P7 and Abp1) known to stimulate the activity of HPK-1 and c-Jun N-terminal kinase (JNK). Overexpression of PRAM-1 in the NB4 APL cell line increased arsenic trioxide-induced JNK activation through a caspase 3-like-dependent activity. Dissociation of the SH3 domain from the rest of the HIP-55 protein was observed in the NB4 APL cell line treated with arsenic trioxide due to specific cleavage by caspase 3-like enzymes. The cleavage of HIP-55 correlated with the induction of PRAM-1 mRNA and protein expression. Taken together, our results suggest that the caspase 3-cleaved SH3 domain of HIP-55 is likely involved in PRAM-1-mediated JNK activation upon arsenic trioxide-induced differentiation of NB4 cells.

Cellular mechanisms of bacterial internalization counteracted by Yersinia

Upon host-cell contact, human pathogenic Yersinia species inject Yop virulence effectors into the host through a Type III secretion-and-translocation system. These virulence effectors cause a block in phagocytosis (YopE, YopT, YpkA, and YopH) and suppression of inflammatory mediators (YopJ). The Yops that block phagocytosis either interfere with the host cell actin regulation of Rho GTPases (YopE, YopT, and YpkA) or specifically and rapidly inactivate host proteins involved in signaling from the receptor to actin (YopH). The block in uptake has been shown to be activated following binding to Fc, Complement, and beta1-integrin receptors in virtually any kind of host cell. Thus, the use of Yersinia as a model system to study Yersinia-host cell interactions provides a good tool to explore signaling pathways involved in phagocytosis.

The Immunological Synapse and Rho GTPases

Rho GTPases are molecular switches controlling a broad range of cellular processes including lymphocyte activation. Not surprisingly, Rho GTPases are now recognized as pivotal regulators of antigen-specific T cell activation by APCs and immunological synapse formation. This review summarizes recent advances in our understanding of how Rho GTPase-dependent pathways control T lymphocyte motility, polarization and activation.

PRAM-1 is required for optimal integrin-dependent neutrophil function

PML-retinoic acid receptor alpha (RARalpha) regulated adaptor molecule 1 (PRAM-1) is an intracellular adaptor molecule that is upregulated during the induced granulocytic differentiation of promyelocytic leukemic cells and during normal human myelopoiesis. This report describes the generation of PRAM-1-deficient mice and an analysis of the function of this adaptor in neutrophil differentiation and mature neutrophil function. We demonstrate here that neutrophil differentiation is not impaired in PRAM-1-deficient mice and that PRAM-1-deficient neutrophils function normally following engagement of Fcgamma receptors. In contrast, mature PRAM-1-null neutrophils exhibit significant defects in adhesion-dependent reactive oxygen intermediate production and degranulation. Surprisingly, other integrin-dependent responses, such as cell spreading and activation of several signaling pathways, are normal. Together, these findings demonstrate the uncoupling of key integrin-dependent responses in the absence of PRAM-1 and show this adaptor to be critical for select integrin functions in neutrophils.

Distinct Calcium Channels Regulate Responses of Primary B Lymphocytes to B Cell Receptor Engagement and Mechanical Stimuli

Intracellular Ca(2+) plays a central role in controlling lymphocyte function. Nonetheless, critical gaps remain in our understanding of the mechanisms that regulate its concentration. Although Ca(2+)-release-activated calcium (CRAC) channels are the primary Ca(2+) entry pathways in T cells, additional pathways appear to be operative in B cells. Our efforts to delineate these pathways in primary murine B cells reveal that Ca(2+)-permeant nonselective cation channels (NSCCs) operate in a cooperative fashion with CRAC. Interestingly, these non-CRAC channels are selectively activated by mechanical stress, although the mechanism overlaps with BCR-activated pathways, suggesting that they may operate in concert to produce functionally diverse Ca(2+) signals. NSCCs also regulate the membrane potential, which activates integrin-dependent binding of B cells to extracellular matrix elements involved in their trafficking and localization within secondary lymphoid organs. Thus, CRAC and distinct Ca(2+) permeant NSCCs are differentially activated by the BCR and mechanical stimuli and regulate distinct aspects of B cell physiology.

Transmembrane adapters: structure, biochemistry and biology

Transmembrane adapter proteins (TRAPs) represent a relatively new and unique group of signalling molecules in hematopoetic cells. They differ from other signalling proteins as they lack any enzymatic or transcriptional activity, instead they possesses multiple tyrosine-based signalling motifs (TBSMs). Triggering of immunoreceptors induces tyrosine phosphorylation of these motifs by members of the Src-, Syk- or Tec-family of protein tyrosine kinases thus enabling the TRAPs to recruit cytosolic adapter and/or effector molecules via their SH2-domains into close proximity to the immunoreceptors, a position from which they can coordinate and modulate signal transduction pathways important for lymphocyte function.

Regulation of T-cell receptor signalling by membrane microdomains

There is now considerable evidence suggesting that the plasma membrane of mammalian cells is compartmentalized by functional lipid raft microdomains. These structures are assemblies of specialized lipids and proteins and have been implicated in diverse biological functions. Analysis of their protein content using proteomics and other methods revealed enrichment of signalling proteins, suggesting a role for these domains in intracellular signalling. In T lymphocytes, structure/function experiments and complementary pharmacological studies have shown that raft microdomains control the localization and function of proteins which are components of signalling pathways regulated by the T-cell antigen receptor (TCR). Based on these studies, a model for TCR phosphorylation in lipid rafts is presented. However, despite substantial progress in the field, critical questions remain. For example, it is unclear if membrane rafts represent a homogeneous population and if their structure is modified upon TCR stimulation. In the future, proteomics and the parallel development of complementary analytical methods will undoubtedly contribute in further delineating the role of lipid rafts in signal transduction mechanisms.

RIAM, an Ena/VASP and Profilin ligand, interacts with Rap1-GTP and mediates Rap1-induced adhesion

The small GTPase Rap1 induces integrin-mediated adhesion and changes in the actin cytoskeleton. The mechanisms that mediate these effects of Rap1 are poorly understood. We have identified RIAM as a Rap1-GTP-interacting adaptor molecule. RIAM defines a family of adaptor molecules that contain a RA-like (Ras association) domain, a PH (pleckstrin homology) domain, and various proline-rich motifs. RIAM also interacts with Profilin and Ena/VASP proteins, molecules that regulate actin dynamics. Overexpression of RIAM induced cell spreading and lamellipodia formation, changes that require actin polymerization. In contrast, RIAM knockdown cells had reduced content of polymerized actin. RIAM overexpression also induced integrin activation and cell adhesion. RIAM knockdown displaced Rap1-GTP from the plasma membrane and abrogated Rap1-induced adhesion. Thus, RIAM links Rap1 to integrin activation and plays a role in regulating actin dynamics.

ADAP-SLP-76 binding differentially regulates supramolecular activation cluster (SMAC) formation relative to T cell-APC conjugation

T cell–APC conjugation as mediated by leukocyte function-associated antigen-1 (LFA-1)–intercellular adhesion molecule (ICAM)-1 binding is followed by formation of the supramolecular activation cluster (SMAC) at the immunological synapse. The intracellular processes that regulate SMAC formation and its influence on T cell function are important questions to be addressed. Here, using a mutational approach, we demonstrate that binding of adaptor adhesion and degranulation promoting adaptor protein (ADAP) to SLP-76 differentially regulates peripheral SMAC (pSMAC) formation relative to conjugation. Although mutation of the YDDV sites (termed M12) disrupted SLP-76 SH2 domain binding and prevented the ability of ADAP to increase conjugation and LFA-1 clustering, M12 acted selectively as a dominant negative (DN) inhibitor of pSMAC formation, an effect that was paralleled by a DN effect on interleukin-2 production. ADAP also colocalized with LFA-1 at the immunological synapse. Our findings identify ADAP–SLP-76 binding as a signaling event that differentially regulates SMAC formation, and support a role for SMAC formation in T cell cytokine production.

Differential requirement for adapter proteins Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa and adhesion- and degranulation-promoting adapter protein in FcepsilonRI signaling and mast cell function

The adapter molecule Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) is essential for FcepsilonRI-mediated signaling, degranulation and IL-6 production in mast cells. To test the structural requirements of SLP-76 in mast cell signaling and function, we have studied the functional responses of murine bone marrow-derived mast cells (BMMCs) expressing mutant forms of SLP-76. We found that the N-terminal tyrosines as well as the central proline-rich region of SLP-76 are required for participation of SLP-76 in FcepsilonRI-mediated signaling and function. The C-terminal SH2 domain of SLP-76 also contributes to optimal function of SLP-76 in mast cells. Another adapter molecule, adhesion- and degranulation-promoting adapter protein (ADAP), is known to bind the SH2 domain of SLP-76, and cell line studies have implicated ADAP in mast cell adhesion and FcepsilonRI-induced degranulation. Surprisingly, we found that mast cells lacking ADAP expression demonstrate no defects in FcepsilonRI-induced adhesion, granule release, or IL-6 production, and that ADAP-deficient mice produce a normal passive systemic anaphylactic response. Thus, failure to bind ADAP does not underlie the functional defects exhibited by SLP-76 SH2 domain mutant-expressing mast cells.

Integrins and T cell-mediated immunity

Integrin receptors mediate adhesive events that are critical for a specific and effective immune response to foreign pathogens. Integrin-dependent interactions of lymphocytes and antigen-presenting cells (APCs) to endothelium regulate the efficiency and specificity of trafficking into secondary lymphoid organs and peripheral tissue. Within these sites, integrins facilitate cell movement via interactions with the extracellular matrix, and promote and stabilize antigen-specific interactions between T lymphocytes and APCs that are critical for initiating T cell-activation events. In this review, we discuss the role of integrins in T cell-mediated immunity, with a focus on how these receptors participate in lymphocyte recirculation and T cell activation, how antigen stimulation regulates integrin activity, and how integrins define functionally unique subsets of T cells and APCs.

Gene expression profiles during human CD4+ T cell differentiation

To develop a comprehensive catalogue of phenotypic and functional parameters of human CD4(+) T cell differentiation stages, we have performed microarray gene expression profiling on subpopulations of human thymocytes and circulating naive CD4(+) T cells, including CD3(-)CD4(+)CD8(-) intrathymic T progenitor cells, CD3(int)CD4(+)CD8(+) 'double positive' thymocytes, CD3(high)CD4(+)CD8(-) 'single positive' thymocytes, CD3(+)CD4(+)CD8(-) CD45RA(+)CD62L(+) naive T cells from cord blood and CD3(+)CD4(+)CD8(-) CD45RA(+)CD62L(+) naive T cells from adult blood. These subpopulations were sort-purified to >98% purity and their expressed RNAs were analyzed on Affymetrix Human Genome U133 arrays. Comparison of gene expression signals between these subpopulations and with early passage fetal thymic stromal cultures identify: (i) transcripts that are preferentially expressed in human CD4(+) T cell subpopulations and not in thymic stromal cells; (ii) major shifts in gene expression as progenitor T cells mature into progeny; (iii) preferential expression of transcripts at the progenitor cell stage with plausible relevance to the regulation of expansion and differentiation of these cells; and (iv) preferential expression of potential markers of recent thymic emigrants in naive-phenotype CD4(+) T cells from cord blood. Further evaluation of these findings may lead to a better definition of human thymopoiesis as well as to improved approaches to monitor and to augment the function of this important organ of T cell production.

The adapter molecule Sin regulates T-cell-receptor-mediated signal transduction by modulating signaling substrate availability

Engagement of the T-cell receptor (TCR) results in the activation of a multitude of signaling events that regulate the function of T lymphocytes. These signaling events are in turn modulated by adapter molecules, which control the final functional output through the formation of multiprotein complexes. In this report, we identified the adapter molecule Sin as a new regulator of T-cell activation. We found that the expression of Sin in transgenic T lymphocytes and Jurkat T cells inhibited interleukin-2 expression and T-cell proliferation. This inhibitory effect was specific and was due to defective phospholipase C-gamma (PLC-gamma) phosphorylation and activation. In contrast to other adapters that become phosphorylated upon TCR stimulation, Sin was constitutively phosphorylated in resting cells by the Src kinase Fyn and bound to signaling intermediates, including PLC-gamma. In stimulated cells, Sin was transiently dephosphorylated, which coincided with transient dissociation of Fyn and PLC-gamma. Downregulation of Sin expression using Sin-specific short interfering RNA oligonucleotides inhibited transcriptional activation in response to TCR stimulation. Our results suggest that endogenous Sin influences T-lymphocyte signaling by sequestering signaling substrates and regulating their availability and/or activity in resting cells, while Sin is required for targeting these intermediates to the TCR for fast signal transmission during stimulation.

Membranes as messengers in T cell adhesion signaling

Talin and RapL are components of molecular pathways that regulate the avidity of the integrin lymphocyte function-associated antigen 1 (LFA-1) for its ligand, intercellular adhesion molecule 1. In this review, we discuss recent advances in our understanding of LFA-1 affinity regulation and signaling and discuss a scenario for how Talin and Rap1 might act in synergy to achieve regulation of LFA-1 that is tailored to the specific functional requirements of different situations. Speedy delivery of signals may be crucial, and membrane trafficking from endosomes and the Golgi apparatus seem to be essential in delivering the messages from spatially segregated surface receptors.

Control of TCR-Mediated Activation of β1 Integrins by the ZAP-70 Tyrosine Kinase Interdomain B Region and the Linker for Activation of T Cells Adapter Protein

One of the earliest functional responses of T lymphocytes to extracellular signals that activate the Ag-specific CD3/TCR complex is a rapid, but reversible, increase in the functional activity of integrin adhesion receptors. Previous studies have implicated the tyrosine kinase zeta-associated protein of 70 kDa (ZAP-70) and the lipid kinase phosphatidylinositol 3-kinase, in the activation of beta(1) integrins by the CD3/TCR complex. In this report, we use human ZAP-70-deficient Jurkat T cells to demonstrate that the kinase activity of ZAP-70 is required for CD3/TCR-mediated increases in beta(1) integrin-mediated adhesion and activation of phosphatidylinositol 3-kinase. A tyrosine to phenylalanine substitution at position 315 in the interdomain B of ZAP-70 inhibits these responses, whereas a similar substitution at position 292 enhances these downstream signals. These mutations in the ZAP-70 interdomain B region also specifically affect CD3/TCR-mediated tyrosine phosphorylation of residues 171 and 191 in the cytoplasmic domain of the linker for activation of T cells (LAT) adapter protein. CD3/TCR signaling to beta(1) integrins is defective in LAT-deficient Jurkat T cells, and can be restored with expression of wild-type LAT. Mutant LAT constructs with tyrosine to phenylalanine substitutions at position 171 and/or position 191 do not restore CD3/TCR-mediated activation of beta(1) integrins in LAT-deficient T cells. Thus, these studies demonstrate that the interdomain B region of ZAP-70 regulates beta(1) integrin activation by the CD3/TCR via control of tyrosine phosphorylation of tyrosine residues 171 and 191 in the LAT cytoplasmic domain.

Structure of a Helically Extended SH3 Domain of the T Cell Adapter Protein ADAP

The adapter protein ADAP (FYB/SLAP-130) provides a critical link between T cell receptor (TCR) signaling and cell adhesion via the activation of integrins. The C-terminal 70 residues of ADAP show homology to SH3 domains; however, conserved residues of the fold are absent. An alignment and annotation of this domain has therefore been elusive. We have solved the three-dimensional structure of the ADAP C-terminal domain by NMR spectroscopy and show that it represents an altered SH3 domain fold. An N-terminal, amphipathic helix makes extensive contacts to residues of the regular SH3 domain fold, and thereby a composite surface with unusual surface properties is created. We propose this SH3 domain variant to be classified as a helically extended SH3 domain (hSH3 domain) and show that the ADAP-hSH3 domain can no longer bind conventional proline-rich peptides.

Roles of the Proline-rich Domain in SLP-76 Subcellular Localization and T Cell Function

The adaptor protein Src homology (SH)2 domain-containing and leukocyte-specific phosphoprotein of 76 kDa (SLP-76) is critical for signal transduction in multiple hematopoietic lineages. It links proximal and distal T cell receptor signaling events through its function as a molecular scaffold in the assembly of multimolecular signaling complexes. Here we studied the functional roles of sub-domains within the SLP-76 proline-rich region, specifically the Gads binding domain and the recently defined P1 domain. To gain a further understanding of the functions mediated by this region, we used three complementary approaches as follows: reconstitution of SLP-76-deficient cells with functional domain deletion mutants, blocking molecular associations through the expression of a dominant negative protein fragment, and directed localization of SLP-76 to assess the role of the domains in SLP-76 recruitment. We find the Gads binding domain and the P1 domain are both necessary for optimal SLP-76 function, and in the absence of these two regions, SLP-76 is functionally inert. Furthermore, we provide direct evidence that SLP-76 localization and, in turn, function are dependent upon association with Gads.

Enrichment of Lck in Lipid Rafts Regulates Colocalized Fyn Activation and the Initiation of Proximal Signals through TCRαβ

Recent results provide insight into the temporal and spatial relationship governing lck-dependent fyn activation and demonstrate TCR/CD4-induced activation and translocation of lck into lipid rafts and the ensuing activation of colocalized fyn. The prediction follows that directly targeting lck to lipid rafts will bypass the requirement for juxtaposing TCR and CD4-lck, and rescue cellular activation mediated by Ab specific for the constant region of TCRbeta chain. The present study uses a family of murine IL-2-dependent CD4(+) T cell clonal variants in which anti-TCRCbeta signaling is impaired in an lck-dependent fashion. Importantly, these variants respond to Ag- and mAb-mediated TCR-CD4 coaggregation, both of which enable the coordinated interaction of CD4-associated lck with the TCR/CD3 complex. We have previously demonstrated that anti-TCRCbeta responsiveness in this system correlates with the presence of kinase-active, membrane-associated lck and preformed hypophosphorylated TCRzeta:zeta-associated protein of 70 kDa complexes, a phenotype recapitulated in primary resting CD4(+) T cells. We show in this study that forced expression of wild-type lck achieved the same basal composition of the TCR/CD3 complex and yet did not rescue anti-TCRCbeta signaling. In contrast, forced expression of C20S/C23S-mutated lck (double-cysteine lck), unable to bind CD4, rescues anti-TCRCbeta proximal signaling and cellular growth. Double-cysteine lck targets lipid rafts, colocalizes with >98% of cellular fyn, and results in a 7-fold increase in basal fyn kinase activity. Coaggregation of CD4 and TCR achieves the same outcome. These results underscore the critical role of lipid rafts in spatially coordinating the interaction between lck and fyn that predicates proximal TCR/CD3 signaling.

Signaling through GP Ib-IX-V activates alpha IIb beta 3 independently of other receptors

Platelet adhesion to von Willebrand factor (VWF) activates alpha IIb beta 3, a prerequisite for thrombus formation. However, it is unclear whether the primary VWF receptor, glycoprotein (GP) Ib-IX-V, mediates alpha IIb beta 3 activation directly or through other signaling proteins physically associated with it (eg, FcR gamma-chain), possibly with the contribution of other agonist receptors and of VWF signaling through alpha IIb beta 3. To resolve this question, human and GP Ibalpha transgenic mouse platelets were plated on dimeric VWF A1 domain (dA1VWF), which engages only GP Ib-IX-V, in the presence of inhibitors of other agonist receptors. Platelet adhesion to dA1VWF induced Src kinase-dependent tyrosine phosphorylation of the FcR gamma-chain and the adapter molecule, ADAP, and triggered intracellular Ca(2+) oscillations and alpha IIb beta 3 activation. Inhibition of Ca(2+) oscillations with BAPTA-AM prevented alpha IIb beta 3 activation but not tyrosine phosphorylation. Pharmacologic inhibition of protein kinase C (PKC) or phosphatidylinositol 3-kinase (PI 3-kinase) prevented alpha IIb beta 3 activation but not Ca(2+) oscillations. Inhibition of Src with 2 distinct compounds blocked all responses downstream of GP Ib-IX-V under static or flow conditions. However, dA1VWF-induced responses were reduced only slightly in GP Ibalpha transgenic platelets lacking FcR gamma-chain. These data establish that GP Ib-IX-V itself can signal to activate alpha IIb beta 3, through sequential actions of Src kinases, Ca(2+) oscillations, and PI 3-kinase/PKC.

Expression of the peripheral benzodiazepine receptor triggers thymocyte differentiation

In the thymus, during T-cell differentiation, the expression of the peripheral benzodiazepine receptor (PBR) modulates. The protein level decreases between the double negative and double positive stages, and then increases when thymocytes become single positive. We addressed the role played by PBR in T-cell maturation. To this aim, we used Jurkat cells, which are immature T lymphocytes derived from an acute lymphoblastic leukemia. These cells are PBR negative and were stably transfected to achieve PBR levels similar to that in mature T cells. Using the DNA chip technology, we analyzed the PBR expression-dependent gene changes and evidenced that PBR-expressing cells exhibited more mature features than mock-transfected ones. A majority of the modulated genes encode proteins playing direct or indirect roles during the lymphocyte maturation process. In particular, PBR expression induced several differentiation markers (such as CD1, CD6), or key regulating elements (e.g., RAG1, RAG2, CD99, TCR). By contrast, some regulators of TCR signaling were reduced. PBR expression also affected the expression of critical apoptosis regulators: the proapoptotic lipocortin I, galectin-1, and galectin-9 were reduced while the antiapoptotic Bcl-2 was induced. Altogether our results supported the hypothesis that PBR controls T-cell maturation and suggested mechanisms through which PBR may regulate thymocyte-positive selection.

Macrophage activation and Fcγ receptor-mediated signaling do not require expression of the SLP-76 and SLP-65 adaptors

The Src-homology 2 domain-containing, leukocyte-specific phosphoprotein of 76 kDa (SLP-76) is a hematopoietic adaptor that plays a central role during immunoreceptor-mediated activation of T lymphocytes and mast cells and collagen receptor-induced activation of platelets. Despite similar levels of expression in macrophages, SLP-76 is not required for Fc receptor for immunoglobulin G (IgG; FcgammaR)-mediated activation. We hypothesized that the related adaptor SLP-65, which is also expressed in macrophages, may compensate for the loss of SLP-76 during FcgammaR-mediated signaling and functional events. To address this hypothesis, we examined bone marrow-derived macrophages (BMM) from wild-type (WT) mice or mice lacking both of these adaptors. Contrary to our expectations, SLP-76(-/-) SLP-65(-/-) BMM demonstrated normal FcgammaR-mediated activation, including internalization of Ig-coated sheep red blood cells and production of reactive oxygen intermediates. FcgammaR-induced biochemical events were normal in SLP-76(-/-) SLP-65(-/-) BMM, including phosphorylation of phospholipase C and the extracellular signaling-regulated kinases 1 and 2. To determine whether macrophages functioned normally in vivo, we infected WT and SLP-76(-/-) SLP-65(-/-) mice with sublethal doses of Listeria monocytogenes (LM), a bacterium against which the initial host defense is provided by activated macrophages. WT and SLP-76(-/-) SLP-65(-/-) mice survived acute, low-dose infection and showed no difference in the number of liver or spleen LM colony-forming units, a measure of the total body burden of this organism. Taken together, these data suggest that neither SLP-76 nor SLP-65 is required during FcgammaR-dependent signaling and functional events in macrophages.

T-cell integrins: more than just sticking points

T cells use integrins in essentially all of their functions. They use integrins to migrate in and out of lymph nodes and, following infection, to migrate into other tissues. At the beginning of an immune response, integrins also participate in the immunological synapse formed between T cells and antigen-presenting cells. Because the ligands for integrins are widely expressed, integrin activity on T cells must be tightly controlled. Integrins become active following signalling through other membrane receptors, which cause both affinity alteration and an increase in integrin clustering. Lipid raft localization may increase integrin activity. Signalling pathways involving ADAP, Vav-1 and SKAP-55, as well as Rap1 and RAPL, cause clustering of leukocyte function-associated antigen-1 (LFA-1; integrin αLβ2). T-cell integrins can also signal, and the pathways dedicated to the migratory activity of T cells have been the most investigated so far. Active LFA-1 causes T-cell attachment and lamellipodial movement induced by myosin light chain kinase at the leading edge, whereas RhoA and ROCK cause T-cell detachment at the trailing edge. Another important signalling pathway acts through CasL/Crk, which might regulate the activity of the GTPases Rac and Rap1 that have important roles in T-cell migration.

Leukocyte functional antigen 1 lowers T cell activation thresholds and signaling through cytohesin-1 and Jun-activating binding protein 1

Leukocyte functional antigen 1 (LFA-1), with intercellular adhesion molecule ligands, mediates T cell adhesion, but the signaling pathways and functional effects imparted by LFA-1 are unclear. Here, intracellular phosphoprotein staining with 13-dimensional flow cytometry showed that LFA-1 activation induced phosphorylation of the beta(2) integrin chain and release of Jun-activating binding protein 1 (JAB-1), and mediated signaling of kinase Erk1/2 through cytohesin-1. Dominant negatives of both JAB-1 and cytohesin-1 inhibited interleukin 2 production and impaired T helper type 1 differentiation. LFA-1 stimulation lowered the threshold of T cell activation. Thus, LFA-1 signaling contributes to T cell activation and effects T cell differentiation.

Developmental markers of B cells are superior to those of T cells for identification of stages with distinct gene expression profiles

B and T lymphocytes develop through a series of cellular stages, which are defined by recombination status of the immunoglobulin and T cell receptor loci and can be separated by analysis of cell-surface markers. We evaluated how well 26 and 41 samples from five and eight developmental stages of B and T cell development, respectively, could be correctly assigned to their lineage of origin and developmental stage by analysis of the expression of 13,026 genes and expressed sequence tags (ESTs). The RNA expression patterns of eight genes correctly classified all 67 samples as belonging to the B cell or to the T cell lineage. Ninety-two to 100% of B-lineage samples could be correctly assigned to the protein-defined developmental stage by the RNA expression pattern of 29 genes. By contrast, RNA expression patterns of 39 genes were necessary to correctly assign 85-100% of T-lineage samples to the correct developmental stage. The sets of genes used for these classifications contain ESTs as well as known genes that have not previously been associated with lymphocyte development. Graphical display of the classifications shows that B-lineage samples are well separated from T-lineage samples, and samples from the five stages of B cell development are well separated from each other. By contrast, samples from the eight stages of T cell development cannot be separated precisely. We conclude that the protein markers currently widely used for separating stages of B cell development better identify molecularly distinct stages than those used for separating stages of T cell development.

Hematopoietic adaptors in T-cell signaling: potential applications to transplantation

Recent advances have been made in understanding the basis of T-cell signaling with the identification of hematopoeitic-specific adaptor proteins, or molecular scaffolds that facilitate protein complex formation and the integration of signals from the surface of T cells. Their potential relevance as targets in the modulation of transplantation relates to their immune-cell-specific expression and their ability to integrate signals needed for T-cell/APC conjugate formation, cytokine production and the clonal expansion of T cells. While LAT, GADS and SLP-76 are needed for TcR-induced cytokine production, the adaptors ADAP, VAV and SKAP-55 play specialized roles in the regulation of integrin adhesion and conjugation. Given the importance of these functions to the reactivity of T cells to allodeterminants of tissue grafts (GvH), and in the recognition and destruction of leukemic cells (GvL), these adaptors represent a new generation of potential targets in the modulation of transplantation.