Formation of STIM and Orai complexes: puncta and distal caps

In the last few years, great progress has been made in understanding how stromal interacting molecule 1 (STIM1), a protein containing a calcium sensor that is located in the endoplasmic reticulum, and Orai1, a protein that forms a calcium channel in the plasma membrane, interact and give rise to store-operated calcium entry. Pharmacological depletion of calcium stores leads to the formation of clusters containing STIM and Orai that appear to be sites for calcium influx. Similar puncta are also produced in response to physiological stimuli in immune cells. In T cells engaged with antigen-presenting cells, clusters containing STIM and Orai accumulate at the immunological synapse. We recently discovered that in activated T cells, STIM1 and Orai1 also accumulate in cap-like structures opposite the immune synapse at the distal pole of the cell. Both caps and puncta are long-lived stable structures containing STIM1 and Orai1 in close proximity. The function of puncta as sites of calcium influx is clear. We speculate that the caps may provide a secondary site of calcium entry. Alternatively, they may serve as a source of preformed channel complexes that move to new immune synapses as T cells repeatedly engage antigen-presenting cells.

HS1 and Itk cooperate to regulate actin remodeling and Ca2+ signaling in T cells (35.28)

Productive T cell activation requires efficient reorganization of the actin cytoskeleton. We have shown that the actin regulatory protein HS1 is required for the stabilization of F-actin and Vav1 at the immunological synapse (IS) and for efficient Ca2+ responses. The Tec family kinase Itk also regulates actin responses, in part by facilitating the localization of Vav1 at the IS, suggesting that these proteins act in the same pathway. Using video microscopy, we found that T cells lacking Itk or HS1 behaved similarly, extending unstable actin-rich protrusions upon TCR stimulation. HS1 and Itk could be coimmunoprecipitated from T cell lysates, and GST-pulldown studies showed that Itk's SH2 domain binds directly to two phosphotyrosines in HS1. In the absence of Itk, HS1 failed to localize to the IS. Since Itk is required for PLCγ1 phosphorylation and Ca2+ store release, we examined the Ca2+ signaling pathway in HS1 knockout T cells in greater detail. T cells lacking HS1 exhibited abnormal Ca2+ store release. These cells showed normal phosphorylation but altered localization of PLCγ1. We conclude that Itk-dependent recruitment of HS1 is required for actin stabilization at the IS, and directs the localization of PLCγ1.

Supported by NIH F31-AI071385 to EC and NIH R01-A1065644 to JKB

Ezrin and moesin function together to promote T cell activation

The highly homologous proteins ezrin, radixin, and moesin link proteins to the actin cytoskeleton. The two family members expressed in T cells, ezrin and moesin, are implicated in promoting T cell activation and polarity. To elucidate the contributions of ezrin and moesin, we conducted a systematic analysis of their function during T cell activation. In response to TCR engagement, ezrin and moesin were phosphorylated in parallel at the regulatory threonine, and both proteins ultimately localized to the distal pole complex (DPC). However, ezrin exhibited unique behaviors, including tyrosine phosphorylation and transient localization to the immunological synapse before movement to the DPC. To ask whether these differences reflect unique requirements for ezrin vs moesin in T cell signaling, we generated mice with conditional deletion of ezrin in mature T cells. Ezrin-/- T cells exhibited normal immunological synapse organization based upon localization of protein kinase C-theta, talin, and phospho-ZAP70. DPC localization of CD43 and RhoGDP dissociation inhibitor, as well as the novel DPC protein Src homology region 2 domain-containing phosphatase-1, was also unaffected. However, recruitment of three novel DPC proteins, ezrin binding protein of 50 kDa, Csk binding protein, and the p85 subunit of PI3K was partially perturbed. Biochemical analysis of ezrin-/- T cells or T cells suppressed for moesin using small interfering RNA showed intact early TCR signaling, but diminished levels of IL-2. The defects in IL-2 production were more pronounced in T cells deficient for both ezrin and moesin. These cells also exhibited diminished phospholipase C-gamma1 phosphorylation and calcium flux. We conclude that despite their unique movement and phosphorylation patterns, ezrin and moesin function together to promote T cell activation.

Integrins put the brakes on microcluster dynamics at the immunological synapse

Integrin engagement costimulates T cell receptor signaling, but the underlying mechanisms are poorly understood. In this issue of Immunity, Nguyen et al. (2008) show that engagement of VLA-4 promotes sustained signaling by altering the dynamics of actin filaments and signaling molecules at the immunological synapse.

The actin cytoskeleton in T cell activation

T cell cytoarchitecture differs dramatically depending on whether the cell is circulating within the bloodstream, migrating through tissues, or interacting with antigen-presenting cells. The transition between these states requires important signaling-dependent changes in actin cytoskeletal dynamics. Recently, analysis of actin-regulatory proteins associated with T cell activation has provided new insights into how T cells control actin dynamics in response to external stimuli and how actin facilitates downstream signaling events and effector functions. Among the actin-regulatory proteins that have been identified are nucleation-promoting factors such as WASp, WAVE2, and HS1; severing proteins such as cofilin; motor proteins such as myosin II; and linker proteins such as ezrin and moesin. We review the current literature on how signaling pathways leading from diverse cell surface receptors regulate the coordinated activity of these and other actin-regulatory proteins and how these proteins control T cell function.

T-cell-receptor-dependent actin regulatory mechanisms

Following stimulation, T cells undergo marked changes in actin architecture that are required for productive immune responses. T-cell-receptor-dependent reorganization of the actin cytoskeleton is necessary for the formation of the immunological synapse at the T-cell-antigen-presenting-cell contact site and the distal pole complex at the opposite face of the T cell. Convergence of specific signaling molecules within these two plasma membrane domains facilitates downstream signaling events leading to full T-cell activation. Recent studies have identified many of the relevant actin-regulatory proteins, and significant progress has been made in our understanding of how these proteins choreograph molecular movements associated with T-cell activation. Proteins such as WASp, WAVE2, HS1 and cofilin direct the formation of a cortical actin scaffold at the immune synapse, while actin-binding proteins such as ezrin and moesin direct binding of signaling molecules to actin filaments within the distal pole complex.

PI3K regulates pleckstrin-2 in T-cell cytoskeletal reorganization

Pleckstrin-2 is composed of 2 pleckstrin homology (PH) domains and a disheveled-Egl-10-pleckstrin (DEP) domain. A lipid-binding assay revealed that pleckstrin-2 binds with greatest affinity to D3 and D5 phosphoinositides. Pleckstrin-2 expressed in Jurkat T cells bound to the cellular membrane and enhanced actin-dependent spreading only after stimulation of the T-cell antigen receptor or the integrin alpha4beta1. A pleckstrin-2 variant containing point mutations in both PH domains failed to associate with the Jurkat membrane and had no effect on spreading under the same conditions. Although still membrane bound, a pleckstrin-2 variant containing point mutations in the DEP domain demonstrated a decreased ability to induce membrane ruffles and spread. Pleckstrin-2 also colocalized with actin at the immune synapse and integrin clusters via its PH domains. Although pleckstrin-2 can bind to purified D3 and D5 phosphoinositides, the intracellular membrane association of pleckstrin-2 and cell spreading are dependent on D3 phosphoinositides, because these effects were disrupted by pharmacologic inhibition of phosphatidylinositol 3-kinase (PI3K). Our results indicate that pleckstrin-2 uses its modular domains to bind to membrane-associated phosphatidylinositols generated by PI3K, whereby it coordinates with the actin cytoskeleton in lymphocyte spreading and immune synapse formation.

Regulation of cytoskeletal dynamics at the immune synapse: new stars join the actin troupe

Reorganization of actin cytoskeletal dynamics plays a critical role in controlling T-lymphocyte activation and effector functions. Interaction of T-cell receptors (TCR) with appropriate major histocompatibility complex-peptide complexes on antigen-presenting cells results in the activation of signaling cascades, leading to the accumulation of F-actin at the cell-cell contact site. This event is required for the formation and stabilization of the immune synapse (IS), a cellular structure essential for the modulation of T-cell responses. Analysis of actin cytoskeletal dynamics following engagement of the TCR has largely focused on the Arp2/3 regulator, WASp, because of its early identification and its association with human disease. However, recent studies have shown equally important roles for several additional actin regulatory proteins. In this review, we turn the spotlight on the expanding cast of actin regulatory proteins, which co-ordinate actin dynamics at the IS.

HS1 functions as an essential actin-regulatory adaptor protein at the immune synapse

HS1, the leukocyte-specific homolog of cortactin, regulates F-actin in vitro and is phosphorylated in response to TCR ligation, but its role in lymphocyte activation has not been addressed. We demonstrate that HS1-deficient T cells fail to accumulate F-actin at the immune synapse (IS) and, upon TCR ligation, form actin-rich structures that are disordered and unstable. Early TCR activation events are intact in these cells, but Ca2+ influx and IL-2 gene transcription are defective. Importantly, HS1 tyrosine phosphorylation is required for its targeting to the IS and for its function in regulating actin dynamics and IL-2 promoter activity. Phosphorylation also links HS1 to multiple signaling proteins, including Lck, PLCgamma1, and Vav1, and is essential for the stable recruitment of Vav1 to the IS. Taken together, our studies show that HS1 is indispensable for signaling events leading to actin assembly and IL-2 production during T cell activation.

The WAVE2 complex regulates actin cytoskeletal reorganization and CRAC-mediated calcium entry during T cell activation

BACKGROUND

The engagement of the T cell receptor results in actin cytoskeletal reorganization at the immune synapse (IS) and the triggering of biochemical signaling cascades leading to gene regulation and, ultimately, cellular activation. Recent studies have identified the WAVE family of proteins as critical mediators of Rac1-induced actin reorganization in other cell types. However, whether these proteins participate in actin reorganization at the IS or signaling pathways in T cells has not been investigated.

RESULTS

By using a combination of biochemical, genetic, and cell biology approaches, we provide evidence that WAVE2 is recruited to the IS, is biochemically modified, and is required for actin reorganization and beta-integrin-mediated adhesion after TCR crosslinking. Moreover, we show that WAVE2 regulates calcium entry at a point distal to PLCgamma1 activation and IP(3)-mediated store release.

CONCLUSIONS

These data reveal a role for WAVE2 in regulating multiple pathways leading to T cell activation. In particular, this work shows that WAVE2 is a key component of the actin regulatory machinery in T cells and that it also participates in linking intracellular calcium store depletion to calcium release-activated calcium (CRAC) channel activation.

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.

Kinase-independent functions for Itk in TCR-induced regulation of Vav and the actin cytoskeleton

The Tec family kinase Itk is an important regulator of Ca(2+) mobilization and is required for in vivo responses to Th2-inducing agents. Recent data also implicate Itk in TCR-induced regulation of the actin cytoskeleton. We have evaluated the requirements for Itk function in TCR-induced actin polarization. Reduction of Itk expression via small interfering RNA treatment of the Jurkat human T lymphoma cell line or human peripheral blood T cells disrupted TCR-induced actin polarization, a defect that correlated with decreased recruitment of the Vav guanine nucleotide exchange factor to the site of Ag contact. Vav localization and actin polarization could be rescued by re-expression of either wild-type or kinase-inactive murine Itk but not by Itk containing mutations affecting the pleckstrin homology or Src homology 2 domains. Additionally, we find that Itk is constitutively associated with Vav. Loss of Itk expression did not alter gross patterns of Vav tyrosine phosphorylation but appeared to disrupt the interactions of Vav with SLP-76. Expression of membrane-targeted Vav, Vav-CAAX, can rescue the small interfering RNA to Itk-induced phenotype, implicating the alteration in Vav localization as directly contributing to the actin polarization defect. These data suggest a kinase-independent scaffolding function for Itk in the regulation of Vav localization and TCR-induced actin polarization.

Dynamin 2 regulates T cell activation by controlling actin polymerization at the immunological synapse

Actin reorganization at the immunological synapse is required for the amplification and generation of a functional immune response. Using small interfering RNA, we show here that dynamin 2 (Dyn2), a large GTPase involved in receptor-mediated internalization, did not alter antibody-mediated T cell receptor internalization but considerably affected T cell receptor-stimulated T cell activation by regulating multiple biochemical signaling pathways and the accumulation of F-actin at the immunological synapse. Moreover, Dyn2 interacted directly with the Rho family guanine nucleotide exchange factor Vav1, and this interaction was required for T cell activation. These data identify a functionally important interaction between Dyn2 and Vav1 that regulates actin reorganization and multiple signaling pathways in T lymphocytes.

In search of membrane receptors for microtubule-based motors - is kinectin a kinesin receptor?

The past few years have seen an explosion in the number of molecular motors reported in the literature. By us the energy of hydrolysis, these motors move various organelles along cytoskeletal 'tracks' within the cell. It is thought that some of the specificity of movement resides in receptors on the surface of the cargo organelles, but, in general, little is known about these molecules. In this article, Janis Burkhardt discusses the evidence that the protein kinectin serves as a membrane receptor for kinesin, and describes how motor-receptor proteins may interact with other components of the motility machinery to generate regulated movement of membrane organelles.

Differential roles for Wiskott-Aldrich syndrome protein in immune synapse formation and IL-2 production

Wiskott-Aldrich syndrome protein (WASP)-deficient T cells exhibit defects in IL-2 production that are widely believed to stem from primary defects in actin remodeling and immune synapse formation. Surprisingly, however, we find that WASP-deficient T cells responding to Ag-specific APCs polymerize actin and organize talin and PKC theta normally, forming an immune synapse that is stable for at least 3 h. At low doses of peptide, WASP-deficient T cells show less efficient talin and PKC theta polarization. Thus, although WASP may facilitate immune synapse formation at low peptide concentrations, WASP is not required for this process. Defects in IL-2 production are observed even under conditions in which immune synapse formation proceeds normally, suggesting that the role of WASP in regulating IL-2 production is independent of its role in immune synapse formation.

Ezrin regulates NHE3 translocation and activation after Na+-glucose cotransport

Initiation of Na(+)-glucose cotransport in intestinal epithelial cells leads to activation of the apical Na(+)-H(+) exchanger NHE3 and subsequent increases in cytoplasmic pH (pH(i)). This process requires activation of p38 mitogen-activated protein (MAP) kinase, but additional signaling intermediates have not been identified. One candidate is the cytoskeletal linker protein ezrin, which interacts with NHE3 via specific regulatory proteins. The data show that initiation of Na(+)-glucose cotransport resulted in rapid increases in both apical membrane-associated NHE3 and cytoskeletal-associated ezrin and occurred in parallel with ezrin phosphorylation at threonine 567. Phosphorylation at this site is known to activate ezrin and increase its association with actin. Consistent with a central role for ezrin activation in this NHE3 regulation, an N-terminal dominant negative ezrin construct inhibited both NHE3 recruitment and pH(i) increases after Na(+)-glucose cotransport. Ezrin phosphorylation occurred in parallel with p38 MAP kinase activation, and the latter proceeded normally in cells expressing dominant negative ezrin. In contrast, inhibition of p38 MAP kinase prevented increases in ezrin phosphorylation after initiation of Na(+)-glucose cotransport. Thus, ezrin phosphorylation after Na(+)-glucose cotransport requires p38 MAP kinase activity, but p38 MAP kinase activation does not require ezrin function. These data describe a specific role for ezrin in the coordinate regulation of Na(+)-glucose cotransport and Na(+)-H(+) exchange. Intact ezrin function is necessary for NHE3 recruitment to the apical membrane and NHE3-dependent pH(i) increases triggered by Na(+)-glucose cotransport. The data also define a pathway of p38 MAP kinase-dependent ezrin activation.

CD43 regulation of T cell activation is not through steric inhibition of T cell-APC interactions but through an intracellular mechanism

CD43 is a large heavily glycosylated protein highly expressed on T cells and actively excluded from the immunological synapse through interactions with ezrin-radixin-moesin proteins. Due to its size and charge, it has been proposed that the CD43 ectodomain acts as a physical barrier to T cell–APC interactions. We have addressed this hypothesis by studying the effect of reconstituting CD43 mutants into the hyperproliferative CD43^−/− T cells. Reintroduction of full-length CD43 reversed the CD43^−/− T cell hyperproliferation. Interestingly, despite the lack of exclusion from the interaction site, a mutant containing the CD43 ectodomain on a glycosylphosphatidylinositol linkage was ineffective. Additionally, T cell–APC conjugate formation was not affected by this ectodomain-only construct. In contrast, CD43^−/− T cell hyperproliferation was reversed by an intracellular-only CD43 fused to the small ectodomain of hCD16. Mutation of this intracellular-only CD43 such that it could not move from the T cell–APC contact site had no further affect on proliferation than the moveable CD43 but did dramatically reduce interleukin-2 production. Thus, the exclusion of the CD43 intracellular region from the immunological synapse is required for CD43 regulation of interleukin-2 production, but the presence of the cytoplasmic tail, independent of its location, is sufficient to reverse CD43^−/− T cell hyperproliferation.

Itk Functions to Control Actin Polymerization at the Immune Synapse through Localized Activation of Cdc42 and WASP

Actin polymerization at the immune synapse is required for T cell activation and effector function; however, the relevant regulatory pathways remain poorly understood. We showed previously that binding to antigen presenting cells (APCs) induces localized activation of Cdc42 and Wiskott-Aldrich Syndrome protein (WASP) at the immune synapse. Several lines of evidence suggest that Tec kinases could interact with WASP-dependent actin regulatory processes. Since T cells from Rlk-/-, Itk-/-, and Rlk-/- x Itk-/- mice have defects in signaling and development, we asked whether Itk or Rlk function in actin polymerization at the immune synapse. We find that Itk-/- and Rlk-/- x Itk-/- T cells are defective in actin polymerization and conjugate formation in response to antigen-pulsed APCs. Itk functions downstream of the TCR, since similar defects were observed upon TCR engagement alone. Using conformation-specific probes, we show that although the recruitment of WASP and Arp2/3 complex to the immune synapse proceeds normally, the localized activation of Cdc42 and WASP is defective. Finally, we find that the defect in Cdc42 activation likely stems from a requirement for Itk in the recruitment of Vav to the immune synapse. Our results identify Itk as a key element of the pathway leading to localized actin polymerization at the immune synapse.

SLP-76 coordinates Nck-dependent Wiskott-Aldrich syndrome protein recruitment with Vav-1/Cdc42-dependent Wiskott-Aldrich syndrome protein activation at the T cell-APC contact site

We have shown previously that Wiskott-Aldrich syndrome protein (WASP) activation at the site of T cell-APC interaction is a two-step process, with recruitment dependent on the proline-rich domain and activation dependent on binding of Cdc42-GTP to the GTPase binding domain. Here, we show that WASP recruitment occurs through binding to the C-terminal Src homology 3 domain of Nck. In contrast, WASP activation requires Vav-1. In Vav-1-deficient T cells, WASP recruitment proceeds normally, but localized activation of Cdc42 and WASP is disrupted. The recruitment and activation of WASP are coordinated by tyrosine-phosphorylated Src homology 2 domain-containing leukocyte protein of 76 kDa, which functions as a scaffold, bringing Nck and WASP into proximity with Vav-1 and Cdc42-GTP. Taken together, these findings reconstruct the signaling pathway leading from TCR ligation to localized WASP activation.

The distal pole complex: a novel membrane domain distal to the immunological synapse

While much interest has focused on the finding that T cell-antigen presenting cell (APC) interaction induces the recruitment of proteins to the immunological synapse (IS), we have recently discovered that APC binding induces the formation of a novel protein complex distal to the site of T-cell receptor ligation. This 'distal pole complex' (DPC) is important for appropriate T-cell activation, functioning either to remove proteins from the synapse or as a signaling complex in its own right. The first component of the DPC to be identified was CD43, a cell-surface mucin that has been proposed to function as a negative regulator of T-cell signaling. CD43 movement was found to depend on ezrin and moesin, members of the ERM family, which serve to link CD43 and other cargo molecules to the actin cytoskeleton. ERM proteins interact with several other important surface receptors and cytoplasmic signaling molecules, some of which we have identified as additional components of the DPC. Disruption of the DPC leaves early T-cell activation events intact but affects cytokine expression. Here, we review what is currently known about the formation and function of the DPC and speculate on how this novel protein complex serves to facilitate T-cell activation.

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.

Molecular ordering of the initial signaling events of CD95

Binding of either ligand or agonistic antibodies to the death receptor CD95 (APO-1/Fas) induces the formation of the death-inducing signaling complex (DISC). We now show that signal initiation of CD95 in type I cells can be further separated into at least four distinct steps. (i) The first step is ligand-induced formation of CD95 microaggregates at the cell surface. (ii) The second step is recruitment of FADD to form a DISC. This step is dependent on actin filaments. (iii) The third step involves formation of large CD95 surface clusters. This event is positively regulated by DISC-generated caspase 8. (iv) The fourth step is internalization of activated CD95 through an endosomal pathway. The latter step is again dependent on the presence of actin filaments. The data indicate that the signal initiation by CD95 is a complex process actively regulated at various levels, providing a number of new drug targets to specifically modulate CD95 signaling.

Superantigen-induced T cell:B cell conjugation is mediated by LFA-1 and requires signaling through Lck, but not ZAP-70

The formation of a conjugate between a T cell and an APC requires the activation of integrins on the T cell surface and remodeling of cytoskeletal elements at the cell-cell contact site via inside-out signaling. The early events in this signaling pathway are not well understood, and may differ from the events involved in adhesion to immobilized ligands. We find that conjugate formation between Jurkat T cells and EBV-B cells presenting superantigen is mediated by LFA-1 and absolutely requires Lck. Mutations in the Lck kinase, Src homology 2 or 3 domains, or the myristoylation site all inhibit conjugation to background levels, and adhesion cannot be restored by the expression of Fyn. However, ZAP-70-deficient cells conjugate normally, indicating that Lck is required for LFA-1-dependent adhesion via other downstream pathways. Several drugs that inhibit T cell adhesion to ICAM-1 immobilized on plastic, including inhibitors of mitogen-activated protein/extracellular signal-related kinase kinase, phosphatidylinositol-3 kinase, and calpain, do not inhibit conjugation. Inhibitors of phospholipase C and protein kinase C block conjugation of both wild-type and ZAP-70-deficient cells, suggesting that a phospholipase C that does not depend on ZAP-70 for its activation is involved. These results are not restricted to Jurkat T cells; Ag-specific primary T cell blasts behave similarly. Although the way in which Lck signals to enhance LFA-1-dependent adhesion is not clear, we find that cells lacking functional Lck fail to recruit F-actin and LFA-1 to the T cell:APC contact site, whereas ZAP-70-deficient cells show a milder phenotype characterized by disorganized actin and LFA-1 at the contact site.

ERM-dependent movement of CD43 defines a novel protein complex distal to the immunological synapse

The large mucin CD43 is actively excluded from T cell/APC interaction sites, concentrating in a membrane domain distal to the site of TCR engagement. The cytoplasmic region of CD43 was necessary and sufficient for this antipodal movement. ERM cytoskeletal adaptor proteins colocalized with CD43 in this domain. An ERM dominant-negative mutant blocked the distal accumulation of CD43 and another known ERM binding protein, Rho-GDI. Inhibition of ERM function decreased the production of IL-2 and IFNgamma, without affecting PKC(theta) focusing or CD69 upregulation. These results indicate that ERM proteins organize a complex distal to the T cell/APC interaction site and provide evidence that full T cell activation may involve removal of inhibitory proteins from the immunological synapse.

Spatial organization of signal transduction molecules in the NK cell immune synapses during MHC class I-regulated noncytolytic and cytolytic interactions

The cytolytic activity of NK cells is tightly regulated by inhibitory receptors specific for MHC class I Ags. We have investigated the composition of signal transduction molecules in the supramolecular activation clusters in the MHC class I-regulated cytolytic and noncytolytic NK cell immune synapses. KIR2DL3-positive NK clones that are specifically inhibited in their cytotoxicity by HLA-Cw*0304 and polyclonal human NK cells were used for conjugate formation with target cells that are either protected or are susceptible to NK cell-mediated cytotoxicity. Polarization of talin, microtubule-organizing center, and lysosomes occurred only during cytolytic interactions. The NK immune synapses were analyzed by three-dimensional immunofluorescence microscopy, which showed two distinctly different synaptic organizations in NK cells during cytolytic and noncytolytic interactions. The center of a cytolytic synapse with MHC class I-deficient target is comprised of a complex of signaling molecules including Src homology (SH)2-containing protein tyrosine phosphatase-1 (SHP-1). Closely related molecules with overlapping functions, such as the Syk kinases, SYK, and ZAP-70, and adaptor molecules, SH2 domain-containing leukocyte protein of 76 kDa and B cell linker protein, are expressed in activated NK cells and are all recruited to the center of the cytolytic synapse. In contrast, the noncytolytic synapse contains SHP-1, but is lacking other components of the central supramolecular activation cluster. These findings indicate a functional role for SHP-1 in both the cytolytic and noncytolytic interactions. We also demonstrate, in three-cell conjugates, that a single NK cell forms a cytolytic synapse with a susceptible target cell in the presence of both susceptible and nonsusceptible target cells.