Positive regulation of T cell activation and integrin adhesion by the adapter Fyb/Slap

The immunological synapse: integrins take the stage

Adhesive interactions play important roles in coordinating T-cell migration and activation, specifically in the formation of the immunological synapse (IS), a specialized cell-cell junction. Recent demonstrations show several molecules implicated in T-cell signaling, including Vav, ADAP, and Rap-1, have major roles in integrin regulation and place adhesion molecules at center stage in addressing the question: what are the signals involved in the formation of the IS and full T-cell activation? This review focuses on the role of integrins as an essential system for both physical adhesion and signaling in T-cell activation. The role of integrins appears to be quite distinct from classical costimulation and has been largely overlooked due to the ubiquitous use of serum in lymphocyte functional assays. Each major signal transduction pathway has branches leading to the nucleus and others that feed back on cytoskeletal and membrane regulation at the IS.

Mechanisms contributing to the activity of integrins on leukocytes

Understanding how the integrins on leukocytes operate is important because these receptors control the activity of leukocytes in all phases of their lives. Thus integrins control leukocyte development and maturation in bone marrow, the circulation of naive cells in secondary lymphoid tissue, e.g. the lymph nodes, and leukocyte responses to inflammatory signals emanating from injured tissues. Using as an example LFA-1, which is expressed by all leukocytes, we outline how the activity of this integrin is modified to meet the challenges posed by these leukocyte activities. Briefly, we discuss three means by which LFA-1 is adapted to bind more efficiently to its chief ligand, ICAM-1. LFA-1 can undergo changes in conformation leading to increased affinity, can be clustered on the membrane and, finally, when activated can move into the lipid raft compartment of the membrane. The study of humans with the beta2 deficiency syndrome termed leukocyte adhesion deficiency (LAD)-1 and analysis of LFA-1 null mice has given further insight into integrin activation mechanisms and the in vivo roles of LFA-1 and other leukocyte integrins.

Fyn kinase initiates complementary signals required for IgE-dependent mast cell degranulation

Fc epsilon RI activation of mast cells is thought to involve Lyn and Syk kinases proximal to the receptor and the signaling complex organized by the linker for activation of T cells (LAT). We report here that Fc epsilon RI also uses a Fyn kinase-dependent pathway that does not require Lyn kinase or the adapter LAT for its initiation, but is necessary for mast cell degranulation. Lyn-deficiency enhanced Fyn-dependent signals and degranulation, but inhibited the calcium response. Fyn-deficiency impaired degranulation, whereas Lyn-mediated signaling and calcium was normal. Thus, Fc epsilon RI-dependent mast cell degranulation involves cross-talk between Fyn and Lyn kinases.

The regulation of actin remodeling during T-cell-APC conjugate formation

The T-cell cytoskeleton is intimately involved in determining the efficiency and fidelity of the immune response. During T-cell interactions with antigen-presenting cells (APCs), dynamic remodeling of the actin cytoskeleton is particularly important for stabilizing long-lived integrin-dependent adhesive interactions. In addition, actin remodeling is important for facilitating the sustained signaling required for full T-cell activation. Although the relationship between T-cell signaling and cytoskeletal remodeling is complex, new molecular genetic tools are making it possible to investigate individual molecular interactions in the context of bona fide conjugate formation. We describe here the progress from our laboratory toward defining the pathways required for actin remodeling during conjugate formation. Our studies show that engagement of T-cell receptor (TCR) and leukocyte functional antigen-1 (LFA-1) leads to distinct effects on the remodeling of individual cytoskeletal elements. Downstream of TCR, we find that p56Lck (Lck) plays a critical role in integrin-dependent adhesion independent of its ability to activate zeta-associated protein of 70 kDa (ZAP-70). TCR engagement also results in the assembly of a signaling complex that facilitates the activation of Wiskott-Aldrich syndrome protein (WASP) by colocalization with Cdc42-GTP. These events, together with other parallel actin regulatory pathways, induce localized actin polymerization at the site of APC binding.

Beyond calcium: new signaling pathways for Tec family kinases

The Tec kinases represent the second largest family of mammalian non-receptor tyrosine kinases and are distinguished by the presence of distinct proline-rich regions and pleckstrin homology domains that are required for proper regulation and activation. Best studied in lymphocyte and mast cells, these kinases are critical for the full activation of phospholipase-C γ (PLC-γ) and Ca2+ mobilization downstream of antigen receptors. However, it has become increasingly clear that these kinases are activated downstream of many cell-surface receptors,including receptor tyrosine kinases, cytokine receptors, integrins and G-protein-coupled receptors. Evidence suggests that the Tec kinases influence a wide range of signaling pathways controlling activation of MAP kinases,actin reorganization, transcriptional regulation, cell survival and cellular transformation. Their impact on cellular physiology suggests that the Tec kinases help regulate multiple cellular processes beyond Ca2+mobilization.

Signal transduction mediated by the T cell antigen receptor: the role of adapter proteins

Engagement of the T cell antigen receptor (TCR) leads to a complex series of molecular changes at the plasma membrane, in the cytoplasm, and at the nucleus that lead ultimately to T cell effector function. Activation at the TCR of a set of protein tyrosine kinases (PTKs) is an early event in this process. This chapter reviews some of the critical substrates of these PTKs, the adapter proteins that, following phosphorylation on tyrosine residues, serve as binding sites for many of the critical effector enzymes and other adapter proteins required for T cell activation. The role of these adapters in binding various proteins, the interaction of adapters with plasma membrane microdomains, and the function of adapter proteins in control of the cytoskeleton are discussed.

Communication between the TCR and integrins: role of the molecular adapter ADAP/Fyb/Slap

TCR stimulation induces integrin-mediated adhesion, facilitating stabilization of conjugates between T cells and antigen-presenting cells and thereby contributing to T cell activation. Integrin activation has been shown to require cytoskeletal reorganization; however, the molecular mechanisms mediating communication between the TCR and integrins remain unclear. Recently the adapter protein ADAP/Fyb/Slap has been shown to couple TCR stimulation to integrin activation by mediating increased integrin avidity. ADAP may also play a role in transduction of external signals by integrins. Like other adapters, ADAP is a multifunctional protein and interacts with molecules such as Fyn, Slp-76, Ena/VASP proteins, Vav1, WASP and the Arp2/3 complex.

Protein kinase C-theta;: signaling from the center of the T-cell synapse

The hypothesis that protein kinase C (PKC)-theta; plays an important role in T-lymphocyte activation, as indicated by numerous studies in cell lines, was recently confirmed in mice deficient in the expression of this enzyme. In response to TCR stimulation, peripheral T cells lacking PKC-theta; failed to activate NF-kappaB and AP-1, and to express IL-2. This revealed a critical function for this PKC family member in linking membrane-proximal activation cascades to transcriptional responses governing T-cell activation. Although the molecular interactions in which PKC-theta; engages have not been fully delineated, insights from a variety of recent studies have permitted new models to be formulated regarding the mechanisms through which it achieves its unique effector functions.

Scaffolds, adaptors and linkers of TCR signaling: theory and practice

Four non-enzymatic proteins form the structural core of the TCR signaling machinery, linking antigen-receptor activation to signaling. These four proteins, each with well defined protein-protein interaction domains, include three 'scaffolds' (LAT, SLP-76 and SLAP-130/Fyb/ADAP and a 'pure adaptor' (GADS). The biological functions of many distinct protein-protein interaction domains have been dissected through a methodological series of knockout and reconstitution experiments. In reviewing these recent advances, we attempt to address two questions often asked by immunologists not familiar with the field: what do scaffolds/adaptors/linkers do; and what do these terms mean?

Formation and function of the immunological synapse

Less than five years ago it was reported that cell surface molecules at the contact site between CD4 T cells and antigen-presenting cells redistribute into distinct patterns, forming an organized interface termed the immunological synapse. More recently, similar reorganized interfaces have been observed with CD8 T cells and NK cells, suggesting that they may be a common feature of lymphocyte activation. Although there has been some advance in our understanding of the mechanisms underlying this redistribution, its purpose remains unclear and controversial.

ADAP-ting TCR signaling to integrins

Adaptor proteins are essential components of T cell receptor (TCR) signaling cascades regulating gene transcription and cytoskeletal reorganization. The molecular adaptor adhesion- and degranulation-promoting adaptor protein (ADAP), also known as Fyn binding protein (FYB) or Slp-76-associated protein of 130 kilodaltons (SLAP-130), interacts with a number of signaling intermediates including Slp-76, the Src family tyrosine kinase Fyn, vasodilator-stimulated phosphoprotein (VASP), and the actin-nucleating protein WASP. Recently ADAP was shown genetically to positively regulate T cell activation, TCR-induced integrin clustering, and T cell adhesion. The mechanism by which ADAP couples TCR stimulation to integrin clustering remains unclear; however, studies of ADAP, the exchange factor Vav1, and WASP suggest that TCR and integrin clustering may be controlled by distinct signaling pathways.

Sensing the environment: a historical perspective on integrin signal transduction

Cell adhesion mediated by integrin receptors has a critical function in organizing cells in tissues and in guiding haematopoietic cells to their sites of action. However, integrin adhesion receptors have broader functions in regulating cell behaviour through their ability to transduce bi-directional signals into and out of the cell and to engage in reciprocal interactions with other cellular receptors. This historical perspective traces the key findings that have led to our current understanding of these important functions of integrins.

Differential requirement for LAT and SLP-76 in GPVI versus T cell receptor signaling

Mice deficient in the adaptor Src homology 2 domain-containing leukocyte phosphoprotein of 76 kD (SLP-76) exhibit a bleeding disorder and lack T cells. Linker for activation of T cells (LAT)-deficient mice exhibit a similar T cell phenotype, but show no signs of hemorrhage. Both SLP-76 and LAT are important for optimal platelet activation downstream of the collagen receptor, GPVI. In addition, SLP-76 is involved in signaling mediated by integrin alphaIIbbeta3. Because SLP-76 and LAT function coordinately in T cell signal transduction, yet their roles appear to differ in hemostasis, we investigated in detail the functional consequences of SLP-76 and LAT deficiencies in platelets. Previously we have shown that LAT(-/-) platelets exhibit defective responses to the GPVI-specific agonist, collagen-related peptide (CRP). Consistent with this, we find that surface expression of P-selectin in response to high concentrations of GPVI ligands is reduced in both LAT- and SLP-76-deficient platelets. However, platelets from LAT(-/-) mice, but not SLP-76(-/-) mice, aggregate normally in response to high concentrations of collagen and convulxin. Additionally, unlike SLP-76, LAT is not tyrosine phosphorylated after fibrinogen binding to integrin alphaIIbbeta3, and collagen-stimulated platelets deficient in LAT spread normally on fibrinogen-coated surfaces. Together, these findings indicate that while LAT and SLP-76 are equally required for signaling via the T cell antigen receptor (TCR) and pre-TCR, platelet activation downstream of GPVI and alphaIIbbeta3 shows a much greater dependency on SLP-76 than LAT.

Rap1A positively regulates T cells via integrin activation rather than inhibiting lymphocyte signaling

T cell receptor (TCR) stimulation activates the small GTPase Rap1A, which is reported to antagonize Ras signaling and induces T cell anergy. To address its role in vivo, we generated transgenic mice that constitutively expressed active Rap1A within the T cell lineage. We found that active Rap1A did not interfere with the Ras signaling pathway or antagonize T cell activation. Instead of anergy, the T lymphocytes that constitutively expressed active Rap1A showed enhanced TCR-mediated responses, both in thymocytes and mature T cells. In addition, Rap1A activation was sufficient to induce strong activation of the beta1 and beta2 integrins via an avidity-modulation mechanism. This shows that, far from playing an inhibitory role during T cell activation, Rap1A positively influences T cells by augmenting lymphocyte responses and directing integrin activation.

Vav1 controls integrin clustering and MHC/peptide-specific cell adhesion to antigen-presenting cells

Integrin-mediated adhesion is essential for the formation of stable contacts between T cells and antigen-presenting cells (APCs). We show that Vav1 controls integrin-mediated adhesion of thymocytes and T cells to ECM proteins and ICAM1 following TCR stimulation. In a peptide-specific system, Vav1 is required for T cell adhesion to peptide-loaded APCs. Intriguingly, TCR-induced cell adhesion and aggregation of integrins occurs independent of WASP. Whereas LFA-1 and actin caps colocalize in wasp(-/-) T cells in response to TCR stimulation, loss of WASP uncouples TCR caps from actin patches. Our data reveal a novel role for Vav1 and WASP in the regulation of TCR-induced integrin clustering and cell adhesion and show that integrin and TCR clustering are controlled by distinct pathways.

Gene expression signatures define novel oncogenic pathways in T cell acute lymphoblastic leukemia

Human T cell leukemias can arise from oncogenes activated by specific chromosomal translocations involving the T cell receptor genes. Here we show that five different T cell oncogenes (HOX11, TAL1, LYL1, LMO1, and LMO2) are often aberrantly expressed in the absence of chromosomal abnormalities. Using oligonucleotide microarrays, we identified several gene expression signatures that were indicative of leukemic arrest at specific stages of normal thymocyte development: LYL1+ signature (pro-T), HOX11+ (early cortical thymocyte), and TAL1+ (late cortical thymocyte). Hierarchical clustering analysis of gene expression signatures grouped samples according to their shared oncogenic pathways and identified HOX11L2 activation as a novel event in T cell leukemogenesis. These findings have clinical importance, since HOX11 activation is significantly associated with a favorable prognosis, while expression of TAL1, LYL1, or, surprisingly, HOX11L2 confers a much worse response to treatment. Our results illustrate the power of gene expression profiles to elucidate transformation pathways relevant to human leukemia.

Doing (F/L)PPPPs: EVH1 domains and their proline-rich partners in cell polarity and migration

Actin filament assembly is a tightly regulated process that functions in many aspects of cell physiology. Members of the Ena/VASP (Drosophila Enabled/vasodilator-stimulated phosphoprotein) family are key players in regulating actin filament assembly, in many cases through their association with binding partners that display a particular proline-rich motif, FPPPP. Ena/VASP proteins interact with these partners via the highly conserved Ena/VASP homology 1 (EVH1) domain. The diverse array of binding partners for EVH1 domains, including cytoskeletal proteins such as zyxin, transmembrane guidance receptors such as Roundabout, and the T-cell signaling protein Fyb/SLAP, shows that these interactions are likely to be important in a number of cellular processes that require regulated actin filament assembly.

Dynamic polarization of the microtubule cytoskeleton during CTL-mediated killing

Efficient unidirectional killing by cytotoxic T lymphocytes (CTL) requires translocation of the microtubule organizing center (MTOC) to the target cell contact site. Here we utilize modulated polarization microscopy and computerized 3D reconstruction of tubulin and LFA-1 immunofluorescence images to investigate how this is accomplished. The results show that the MTOC is drawn vectorially to the contact site by a microtubule sliding mechanism. Once the MTOC arrives at the contact site, it oscillates laterally. Microtubules loop through and anchor to a ring-shaped zone (pSMAC) defined by the dense clustering of LFA-1 at the target contact site. Microtubules that run straight between the MTOC and pSMAC and then turn sharply may indicate the action of a microtubule motor such as dynein.

The immunological synapse

T-cell activation requires interaction of T-cell antigen receptors with proteins of the major histocompatibility complex (antigen). This interaction takes place in a specialized cell-cell junction referred to as an immunological synapse. The immunological synapse contains at least two functional domains: a central cluster of engaged antigen receptors and a surrounding ring of adhesion molecules. The segregation of the T-cell antigen receptor (TCR) and adhesion molecules is based on size, with the TCR interaction spanning 15 nm and the lymphocyte-function-associated antigen-1 (LFA-1) interaction spanning 30-40 nm between the two cells. Therefore, the synapse is not an empty gap, but a space populated by both adhesion and signaling molecules. This chapter considers four aspects of the immunological synapse: the role of migration and stop signals, the role of the cytoskeleton, the role of self-antigenic complexes, and the role of second signals.

Adaptor ADAP (adhesion- and degranulation-promoting adaptor protein) regulates beta1 integrin clustering on mast cells

Aggregation of the high-affinity IgE receptor (FcepsilonRI) activates a tyrosine phosphorylation cascade needed for enhanced adhesion and degranulation events on mast cells. We previously identified the adaptor ADAP (otherwise known as FYB/SLAP) as a modulator of integrin-mediated adhesion of T-cells and mast cells. However, the molecular basis for the effect on beta1 integrin adhesion on mast cells was unclear. In this study, we demonstrate that ADAP can promote the kinetics of beta1 integrin clustering on mast cells. By contrast, the clustering of the FcepsilonRI receptor was unaffected by transfected ADAP, indicating that the promoting effect on clustering was selective for beta1 integrins. These findings extend the modulatory effect of ADAP from beta2 to beta1 members of the integrin family, and provide a molecular basis for the modulatory effect of ADAP on mast cell adhesion.

Differential requirement for SLP-76 domains in T cell development and function

The hematopoietic cell-specific adaptor protein, SLP-76, is critical for T cell development and mature T cell receptor (TCR) signaling; however, the structural requirements of SLP-76 for mediating thymopoiesis and mature T cell function remain largely unknown. In this study, transgenic mice were generated to examine the requirements for specific domains of SLP-76 in thymocytes and peripheral T cells in vivo. Examination of mice expressing various mutants of SLP-76 on the null background demonstrates a differential requirement for specific domains of SLP-76 in thymocytes and T cells and provides new insight into the molecular mechanisms underlying SLP-76 function.

Positive and negative regulation of T-cell activation by adaptor proteins

Adaptor proteins, molecules that mediate intermolecular interactions, are now known to be as crucial for lymphocyte activation as are receptors and effectors. Extensive work from numerous laboratories has identified and characterized many of these adaptors, demonstrating their roles as both positive and negative regulators. Studies into the molecular basis for the actions of these molecules shows that they function in various ways, including: recruitment of positive or negative regulators into signalling networks, modulation of effector function by allosteric regulation of enzymatic activity, and by targeting other proteins for degradation. This review will focus on a number of adaptors that are important for lymphocyte function and emphasize the various ways in which these proteins carry out their essential roles.