Heterogeneity of Signaling Complex Nanostructure in T Cells Activated Via the T Cell Antigen Receptor

Activation of the T cell antigen receptor (TCR) is a key step in initiating the adaptive immune response. Single-molecule localization techniques have been used to investigate the arrangement of proteins within the signaling complexes formed around activated TCRs, but a clear picture of nanoscale organization in stimulated T cells has not emerged. Here, we have improved the examination of T cell nanostructure by visualizing individual molecules of six different proteins in a single sample of activated Jurkat T cells using the multiplexed antibody-size limited direct stochastic optical reconstruction microscopy (madSTORM) technique. We formally define irregularly shaped regions of interest, compare areas where signaling complexes are concentrated with other areas, and improve the statistical analyses of the locations of molecules. We show that nanoscale organization of proteins is mainly confined to the areas with dense concentrations of TCR-based signaling complexes. However, randomly distributed molecules are also found in some areas containing concentrated signaling complexes. These results are consistent with the view that the proteins within signaling complexes are connected by numerous weak interactions, leading to flexible, dynamic, and mutable structures which produce large variations in the nanostructure found in activated T cells.

Microclusters as T Cell Signaling Hubs: Structure, Kinetics, and Regulation

When T cell receptors (TCRs) engage with stimulatory ligands, one of the first microscopically visible events is the formation of microclusters at the site of T cell activation. Since the discovery of these structures almost 20 years ago, they have been studied extensively in live cells using confocal and total internal reflection fluorescence (TIRF) microscopy. However, due to limits in image resolution and acquisition speed, the spatial relationships of signaling components within microclusters, the kinetics of their assembly and disassembly, and the role of vesicular trafficking in microcluster formation and maintenance were not finely characterized. In this review, we will summarize how new microscopy techniques have revealed novel insights into the assembly of these structures. The sub-diffraction organization of microclusters as well as the finely dissected kinetics of recruitment and disassociation of molecules from microclusters will be discussed. The role of cell surface molecules in microcluster formation and the kinetics of molecular recruitment via intracellular vesicular trafficking to microclusters is described. Finally, the role of post-translational modifications such as ubiquitination in the downregulation of cell surface signaling molecules is also discussed. These results will be related to the role of these structures and processes in T cell activation.

Bypassing ubiquitination enables LAT recycling to the cell surface and enhanced signaling in T cells

LAT molecules defective in ubiquitination have an increased half-life and induce enhanced signaling when expressed in T cells. In this study, we have examined the role of ubiquitination in regulating LAT endocytosis, recycling, and degradation in resting and stimulated T cells. By tracking and comparing plasma membrane-labeled wild type and ubiquitination-resistant 2KR LAT, we find that ubiquitination promotes the degradation of surface LAT in T cells. Activation of T cells increases LAT ubiquitination and promotes trafficking of internalized LAT to lysosomes for degradation. Ubiquitination of LAT does not change internalization rates from the cell surface, but prevents efficient recycling of LAT to the surface of T cells. Our study demonstrates that surface LAT levels are tightly controlled by ubiquitination. LAT in unstimulated cells lacks ubiquitin allowing for increased LAT stability and efficient T cell activation upon TCR triggering; ubiquitination leads to efficient removal of LAT after activation.

Cish actively silences TCR signaling in CD8+ T cells to maintain tumor tolerance

Palmer et al. find that Cish, a member of the SOCS family, is induced by TCR stimulation in CD8⁺ T cells and inhibits their functional avidity against tumor. The authors uncover a novel mechanism of suppression for a SOCS member.

Improving the functional avidity of effector T cells is critical in overcoming inhibitory factors within the tumor microenvironment and eliciting tumor regression. We have found that Cish, a member of the suppressor of cytokine signaling (SOCS) family, is induced by TCR stimulation in CD8⁺ T cells and inhibits their functional avidity against tumors. Genetic deletion of Cish in CD8⁺ T cells enhances their expansion, functional avidity, and cytokine polyfunctionality, resulting in pronounced and durable regression of established tumors. Although Cish is commonly thought to block STAT5 activation, we found that the primary molecular basis of Cish suppression is through inhibition of TCR signaling. Cish physically interacts with the TCR intermediate PLC-γ1, targeting it for proteasomal degradation after TCR stimulation. These findings establish a novel targetable interaction that regulates the functional avidity of tumor-specific CD8⁺ T cells and can be manipulated to improve adoptive cancer immunotherapy.

The linker for activation of T cells (LAT) signaling hub: from signaling complexes to microclusters

Since the cloning of the critical adapter, LAT (linker for activation of T cells), more than 15 years ago, a combination of multiple scientific approaches and techniques continues to provide valuable insights into the formation, composition, regulation, dynamics, and function of LAT-based signaling complexes. In this review, we will summarize current views on the assembly of signaling complexes nucleated by LAT. LAT forms numerous interactions with other signaling molecules, leading to cooperativity in the system. Furthermore, oligomerization of LAT by adapter complexes enhances intracellular signaling and is physiologically relevant. These results will be related to data from super-resolution microscopy studies that have revealed the smallest LAT-based signaling units and nanostructure.

Enhanced T-cell activation and differentiation in lymphocytes from transgenic mice expressing ubiquitination-resistant 2KR LAT molecules

Linker for activation of T cells (LAT) is critical for the propagation of T-cell signals upon T-cell receptor (TCR) activation. Previous studies demonstrated that substitution of LAT lysines with arginines (2KR LAT) resulted in decreased LAT ubiquitination and elevated T-cell signaling, indicating that LAT ubiquitination is a molecular checkpoint for attenuation of T-cell signaling. To investigate the role of LAT ubiquitination in vivo, we have generated transgenic mice expressing WT and ubiquitin-defective 2KR LAT. On TCR stimulation of T cells from these mice, proximal signaling and cytokine production was elevated in 2KR versus wild-type (WT) LAT mice. Enhanced cytolytic activity as well as T-helper responses were observed on LAT expression, which were further elevated by 2KR LAT expression. Despite greater T-effector function, WT or 2KR LAT expression did not have any effect on clearance of certain pathogens or tumors. Our data support the model that lack of tumor clearance is due to increased differentiation and acquisition of effector phenotype that is associated with suboptimal immunity in an immunotherapy model. Thus, our data further reinforce the role of LAT ubiquitination in TCR signaling and uncovers a novel role for LAT in driving T-cell differentiation.

Absence of both Sos-1 and Sos-2 in peripheral CD4(+) T cells leads to PI3K pathway activation and defects in migration

Sos-1 and Sos-2 are ubiquitously expressed Ras-guanine exchange factors involved in Erk-MAP kinase pathway activation. Using mice lacking genes encoding Sos-1 and Sos-2, we evaluated the role of these proteins in peripheral T-cell signaling and function. Our results confirmed that TCR-mediated Erk activation in peripheral CD4(+) T cells does not depend on Sos-1 and Sos-2, although IL-2-mediated Erk activation does. Unexpectedly, however, we show an increase in AKT phosphorylation in Sos-1/2dKO CD4(+) T cells upon TCR and IL-2 stimulation. Activation of AKT was likely a consequence of increased recruitment of PI3K to Grb2 upon TCR and/or IL-2 stimulation in Sos-1/2dKO CD4(+) T cells. The increased activity of the PI3K/AKT pathway led to downregulation of the surface receptor CD62L in Sos-1/2dKO T cells and a subsequent impairment in T-cell migration.

Cytoskeletal forces during signaling activation in Jurkat T-cells

Cytoskeletal forces are implicated in T-cell–receptor activation, but their determinants are not known. Traction force microscopy was used to measure forces generated during T-cell activation. Whereas actin dynamics were essential for force generation, myosin contractility played a limited role. T-cells were also found to be mechanosensitive.

T-cells are critical for the adaptive immune response in the body. The binding of the T-cell receptor (TCR) with antigen on the surface of antigen-presenting cells leads to cell spreading and signaling activation. The underlying mechanism of signaling activation is not completely understood. Although cytoskeletal forces have been implicated in this process, the contribution of different cytoskeletal components and their spatial organization are unknown. Here we use traction force microscopy to measure the forces exerted by Jurkat T-cells during TCR activation. Perturbation experiments reveal that these forces are largely due to actin assembly and dynamics, with myosin contractility contributing to the development of force but not its maintenance. We find that Jurkat T-cells are mechanosensitive, with cytoskeletal forces and signaling dynamics both sensitive to the stiffness of the substrate. Our results delineate the cytoskeletal contributions to interfacial forces exerted by T-cells during activation.

The ability of Sos1 to oligomerize the adaptor protein LAT is separable from its guanine nucleotide exchange activity in vivo

The activation of the small guanosine triphosphatase Ras by the guanine nucleotide exchange factor (GEF) Sos1 (Son of Sevenless 1) is a central feature of many receptor-stimulated signaling pathways. In developing T cells (thymocytes), Sos1-dependent activation of extracellular signal-regulated kinase (ERK) is required to stimulate cellular proliferation and differentiation. We showed that in addition to its GEF activity, Sos1 acted as a scaffold to nucleate oligomerization of the T cell adaptor protein LAT (linker for activation of T cells) in vivo. The scaffold function of Sos1 depended on its ability to bind to the adaptor protein Grb2. Furthermore, the GEF activity of Sos1 and the Sos1-dependent oligomerization of LAT were separable functions in vivo. Whereas the GEF activity of Sos1 was required for optimal ERK phosphorylation in response to T cell receptor (TCR) stimulation, the Sos1-dependent oligomerization of LAT was required for maximal TCR-dependent phosphorylation and activation of phospholipase C-γ1 and Ca(2+) signaling. Finally, both of these Sos1 functions were required for early thymocyte proliferation. Whereas transgenic restoration of either the GEF activity or the LAT oligomerization functions of Sos1 alone failed to rescue thymocyte development in Sos1-deficient mice, simultaneous reconstitution of these two signals in the same cell restored normal T cell development. This ability of Sos1 to act both as a RasGEF and as a scaffold to nucleate Grb2-dependent adaptor oligomerization may also occur in other Grb2-dependent pathways, such as those activated by growth factor receptors.

Cutting edge: cell surface linker for activation of T cells is recruited to microclusters and is active in signaling

A controversy has recently emerged regarding the location of the cellular pool of the adapter linker for activation of T cells (LAT) that participates in propagation of signals downstream of the TCR. In one model phosphorylation and direct recruitment of cell surface LAT to activation-induced microclusters is critical for T cell activation, whereas in the other model vesicular, but not surface, LAT participates in these processes. By using a chimeric version of LAT that can be tracked via an extracellular domain, we provide evidence that LAT located at the cell surface can be recruited efficiently to activation-induced microclusters within seconds of TCR engagement. Importantly, we also demonstrate that this pool of LAT at the plasma membrane is rapidly phosphorylated. Our results provide support for the model in which the cell utilizes LAT from the cell surface for rapid responses to TCR stimulation.

Enhanced function of redirected human T cells expressing linker for activation of T cells that is resistant to ubiquitylation

It is likely that the enhancement of signaling after antigenic stimulation, particularly in the tumor microenvironment, would improve the function of adoptively transferred T cells. Linker for activation of T cells (LAT) plays a central role in T cell activation. We hypothesized that the ubiquitylation-resistant form of LAT in cells would enhance T cell signaling and thus augment antitumor activity. To test this, human CD4(+) or CD8(+) T cells were electroporated with small interfering RNA (siRNA) to repress endogenous LAT and ubiquitylation-resistant LAT 2KR or wild-type LAT mRNA was introduced for reexpression. Significantly enhanced phosphorylation of LAT and phospholipase C-γ (PLCγ) was observed, and augmented calcium signaling after T cell receptor (TCR) triggering was observed in LAT 2KR-expressing T cells. TCR-induced calcium signaling was abrogated in LAT knockdown cells, but the baseline was higher than that of control siRNA-electroporated cells, suggesting a fundamental requirement of LAT to maintain calcium homeostasis. Redirected LAT 2KR T cells expressing a chimeric antigen receptor or an MHC class I-restricted TCR showed augmented function as assessed by enhanced cytokine secretion and cytotoxicity. These results indicate that interruption of LAT ubiquitylation is a promising strategy to augment effector T cell function for adoptive cell therapy.

Enhanced T cell activation in lymphocytes from transgenic mice expressing LAT molecules resistant to ubiquitination (69.5)

Linker for activation of T cells (LAT) is critical for the propagation of T cell signals from the plasma membrane to the cytoplasm upon T cell activation. Previous studies demonstrated that substitution of LAT lysines with arginines (2KRLAT) resulted in a decrease in LAT ubiquitination and elevated T cell signaling, indicating that LAT ubiquitination is a molecular checkpoint for attenuation of T cell signaling. To investigate the role of LAT ubiquitination in vivo, we have generated transgenic mice expressing WT and ubiquitin-defective 2KRLAT. Upon TCR stimulation, proximal T cell signaling was elevated in lymphocytes expressing 2KRLAT versus WT cells as evidenced by higher levels of phospho(p)-tyrosine proteins, p-LAT, p-phospholipase C-gamma and cytosolic calcium flux. Evaluation of various cell surface markers also revealed an enhanced memory T cell profile in 2KR lymphocytes. Moreover, upon TCR stimulation, a higher percentage of T cells expressing greater levels of effector molecules such as intracellular interferon-gamma, tumor necrosis factor-alpha and interleukin-2 was found in 2KR versus WTLAT mice. We have thus identified enhanced proximal T cell signaling, an enhanced memory T cell profile and higher expression of cytokine effector molecules in lymphocytes from 2KRLAT transgenic mice. These results suggest that 2KR mice may respond more efficiently to pathogenic challenges, an hypothesis that will be tested in various experimental models of mouse infection.

Functional nanoscale organization of signaling molecules downstream of the T cell antigen receptor

Receptor-regulated cellular signaling often is mediated by formation of transient, heterogeneous protein complexes of undefined structure. We used single and two-color photoactivated localization microscopy to study complexes downstream of the T cell antigen receptor (TCR) in single-molecule detail at the plasma membrane of intact T cells. The kinase ZAP-70 distributed completely with the TCRζ chain and both partially mixed with the adaptor LAT in activated cells, thus showing localized activation of LAT by TCR-coupled ZAP-70. In resting and activated cells, LAT primarily resided in nanoscale clusters as small as dimers whose formation depended on protein-protein and protein-lipid interactions. Surprisingly, the adaptor SLP-76 localized to the periphery of LAT clusters. This nanoscale structure depended on polymerized actin and its disruption affected TCR-dependent cell function. These results extend our understanding of the mechanism of T cell activation and the formation and organization of TCR-mediated signaling complexes, findings also relevant to other receptor systems.

Enhanced T-cell signaling in cells bearing linker for activation of T-cell (LAT) molecules resistant to ubiquitylation

Linker for activation of T cells (LAT) plays a central role in T-cell activation by nucleating signaling complexes that are critical for the propagation of T-cell signals from the plasma membrane to the cellular interior. The role of phosphorylation and palmitoylation in LAT function has been well studied, but not much is known about other strategies by which the cell modulates LAT activity. We have focused on LAT ubiquitylation and have mapped the sites on which LAT is ubiquitylated. To elucidate the biological role of this process, we substituted LAT lysines with arginines. This resulted in a dramatic decrease in overall LAT ubiquitylation. Ubiquitylation-resistant mutants of LAT were internalized at rates comparable to wild-type LAT in a mechanism that required Cbl family proteins. However, these mutants displayed a defect in protein turnover rates. T-cell signaling was elevated in cells reconstituted with LAT mutants resistant to ubiquitylation, indicating that inhibition of LAT ubiquitylation enhances T-cell potency. These results support LAT ubiquitylation as a molecular checkpoint for attenuation of T-cell signaling.

Imaging techniques for assaying lymphocyte activation in action

Imaging techniques have greatly improved our understanding of lymphocyte activation. Technical advances in spatial and temporal resolution and new labelling tools have enabled researchers to directly observe the activation process. Consequently, research using imaging approaches to study lymphocyte activation has expanded, providing an unprecedented level of cellular and molecular detail in the field. As a result, certain models of lymphocyte activation have been verified, others have been revised and yet others have been replaced with new concepts. In this article, we review the current imaging techniques that are used to assess lymphocyte activation in different contexts, from whole animals to single molecules, and discuss the advantages and potential limitations of these methods.

The LAT story: a tale of cooperativity, coordination, and choreography

The adapter molecule LAT is a nucleating site for multiprotein signaling complexes that are vital for the function and differentiation of T cells. Extensive investigation of LAT in multiple experimental systems has led to an integrated understanding of the formation, composition, regulation, dynamic movement, and function of LAT-nucleated signaling complexes. This review discusses interactions of signaling molecules that bind directly or indirectly to LAT and the role of cooperativity in stabilizing LAT-nucleated signaling complexes. In addition, it focuses on how imaging studies visualize signaling assemblies as signaling clusters and demonstrate their dynamic nature and cellular fate. Finally, this review explores the function of LAT based on the interpretation of mouse models using various LAT mutants.

Endocytic events in TCR signaling: focus on adapters in microclusters

Although the critical role of T-cell receptor (TCR) microclusters in T-cell activation is now widely accepted, the mechanisms of regulation of these TCR-rich structures, which also contain enzymes, adapters, and effectors, remain poorly defined. Soon after microcluster formation, several signaling proteins rapidly dissociate from the TCR. Recent studies from our laboratory demonstrated that the movement of the adapters linker for activation of T cells (LAT) and Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) away from initial microcluster formation sites represents endocytic events. Ubiquitylation, Cbl proteins, and multiple endocytic pathways are involved in the internalization events that disassemble signaling microclusters. Several recent studies have indicated that microcluster movement and centralization plays an important role in signal termination. We suggest that microcluster movement is directly linked to endocytic events, thus implicating endocytosis of microclusters as a means to regulate signaling output of the T cell.

Dynamic movement of the calcium sensor STIM1 and the calcium channel Orai1 in activated T-cells: puncta and distal caps

The proteins STIM1 and Orai1 are the long sought components of the store-operated channels required in T-cell activation. However, little is known about the interaction of these proteins in T-cells after engagement of the T-cell receptor. We found that T-cell receptor engagement caused STIM1 and Orai1 to colocalize in puncta near the site of stimulation and accumulate in a dense structure on the opposite side of the T-cell. FRET measurements showed a close interaction between STIM1 and Orai1 both in the puncta and in the dense cap-like structure. The formation of cap-like structures did not entail rearrangement of the entire endoplasmic reticulum. Cap formation depended on TCR engagement and tyrosine phosphorylation, but not on channel activity or Ca(2+) influx. These caps were very dynamic in T-cells activated by contact with superantigen pulsed B-cells and could move from the distal pole to an existing or a newly forming immunological synapse. One function of this cap may be to provide preassembled Ca(2+) channel components to existing and newly forming immunological synapses.

The CDM superfamily protein MBC directs myoblast fusion through a mechanism that requires phosphatidylinositol 3,4,5-triphosphate binding but is independent of direct interaction with DCrk

Myoblast city (mbc), a member of the CDM superfamily, is essential in the Drosophila melanogaster embryo for fusion of myoblasts into multinucleate fibers. Using germ line clones in which both maternal and zygotic contributions were eliminated and rescue of the zygotic loss-of-function phenotype, we established that mbc is required in the fusion-competent subset of myoblasts. Along with its close orthologs Dock180 and CED-5, MBC has an SH3 domain at its N terminus, conserved internal domains termed DHR1 and DHR2 (or "Docker"), and C-terminal proline-rich domains that associate with the adapter protein DCrk. The importance of these domains has been evaluated by the ability of MBC mutations and deletions to rescue the mbc loss-of-function muscle phenotype. We demonstrate that the SH3 and Docker domains are essential. Moreover, ethyl methanesulfonate-induced mutations that change amino acids within the MBC Docker domain to residues that are conserved in other CDM family members nevertheless eliminate MBC function in the embryo, which suggests that these sites may mediate interactions specific to Drosophila MBC. A functional requirement for the conserved DHR1 domain, which binds to phosphatidylinositol 3,4,5-triphosphate, implicates phosphoinositide signaling in myoblast fusion. Finally, the proline-rich C-terminal sites mediate strong interactions with DCrk, as expected. These sites are not required for MBC to rescue the muscle loss-of-function phenotype, however, which suggests that MBC's role in myoblast fusion can be carried out independently of direct DCrk binding.

T-Cell Antigen Receptor-Induced Signaling Complexes: Internalization Via a Cholesterol-Dependent Endocytic Pathway

T-cell antigen receptor engagement causes the rapid assembly of signaling complexes. The adapter protein SLP-76, detected as SLP-yellow fluorescent protein, initially clustered with the TCR and other proteins, then translocated medially on microtubules. As shown by total internal reflection fluorescence microscopy and the inhibition of SLP-76 movement at 16 degrees C, this movement required endocytosis. Immunoelectron microscopy showed SLP-76 staining of smooth pits and tubules. Cholesterol depletion decreased the movement of SLP-76 clusters, as did coexpression of the ubiquitin-interacting motif domain from eps15. These data are consistent with the internalization of SLP-76 via a lipid raft-dependent pathway that requires interaction of the endocytic machinery with ubiquitinylated proteins. The endocytosed SLP-76 clusters contained phosphorylated SLP-76 and phosphorylated LAT. The raft-associated, transmembrane protein LAT likely targets SLP-76 to endocytic vesicles. The endocytosis of active SLP-76 and LAT complexes suggests a possible mechanism for downregulation of signaling complexes induced by TCR activation.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Adaptive evolution drives divergence of a hybrid inviability gene between two species of Drosophila

Speciation--the splitting of one species into two--occurs by the evolution of any of several forms of reproductive isolation between taxa, including the intrinsic sterility and inviability of hybrids. Abundant evidence shows that these hybrid fitness problems are caused by incompatible interactions between loci: new alleles that become established in one species are sometimes functionally incompatible with alleles at interacting loci from another species. However, almost nothing is known about the genes involved in such hybrid incompatibilities or the evolutionary forces that drive their divergence. Here we identify a gene that causes epistatic inviability in hybrids between two fruitfly species, Drosophila melanogaster and D. simulans. Our population genetic analysis reveals that this gene--which encodes a nuclear pore protein--evolved by positive natural selection in both species' lineages. These results show that a lethal hybrid incompatibility has evolved as a by-product of adaptive protein evolution.

Loss-of-function mutations reveal that the Drosophila nautilus gene is not essential for embryonic myogenesis or viability

nautilus (nau), the single Drosophila member of the bHLH-containing myogenic regulatory family of genes, is expressed in a subset of muscle precursors and differentiated fibers. It is capable of inducing muscle-specific transcription as well as myogenic transformation, and plays a role in the differentiation of a subset of muscle precursors into mature muscle fibers. In previous studies, the nau zygotic loss-of-function phenotype was determined using genetic deficiencies in which the gene is deleted. We note that this genetic loss-of-function phenotype differs from the loss-of-function phenotype determined using RNA interference (L. Misquitta and B. M. Paterson, 1999, Proc. Natl. Acad. Sci. USA 96, 1451-1456). The present study re-examines this loss-of-function phenotype using EMS-induced mutations that specifically alter the nau gene, and extends the genetic analysis to include the loss of both maternal and zygotic nau function. In brief, embryos lacking nau both maternally and zygotically are missing a distinct subset of muscle fibers, consistent with its apparent expression in a subset of muscle fibers. The muscle loss is tolerated, however, such that the loss of nau both maternally and zygotically does not result in lethality at any stage of development.