Receptor clustering and transmembrane signaling in T cells

OptoREACT: Optogenetic Receptor Activation on Nonengineered Human T Cells

Optogenetics is a versatile and powerful tool for the control and analysis of cellular signaling processes. The activation of cellular receptors by light using optogenetic switches usually requires genetic manipulation of cells. However, this considerably limits the application in primary, nonengineered cells, which is crucial for the study of physiological signaling processes and for controlling cell fate and function for therapeutic purposes. To overcome this limitation, we developed a system for the light-dependent extracellular activation of cell surface receptors of nonengineered cells termed OptoREACT (Optogenetic Receptor Activation) based on the light-dependent protein interaction of A. thaliana phytochrome B (PhyB) with PIF6. In the OptoREACT system, a PIF6-coupled antibody fragment binds the T cell receptor (TCR) of Jurkat or primary human T cells, which upon illumination is bound by clustered phytochrome B to induce receptor oligomerization and activation. For clustering of PhyB, we either used tetramerization by streptavidin or immobilized PhyB on the surface of cells to emulate the interaction of a T cell with an antigen-presenting cell. We anticipate that this extracellular optogenetic approach will be applicable for the light-controlled activation of further cell surface receptors in primary, nonengineered cells for versatile applications in fundamental and applied research.

T-cell virtuosity in ''knowing thyself"

Major Histocompatibility Complex (MHC) I and II and the αβ T-cell antigen receptor (TCRαβ) govern fundamental traits of adaptive immunity. They form a membrane-borne ligand-receptor system weighing host proteome integrity to detect contamination by nonself proteins. MHC-I and -II exhibit the "MHC-fold", which is able to bind a large assortment of short peptides as proxies for self and nonself proteins. The ensuing varying surfaces are mandatory ligands for Ig-like TCRαβ highly mutable binding sites. Conserved molecular signatures guide TCRαβ ligand binding sites to focus on the MHC-fold (MHC-restriction) while leaving many opportunities for its most hypervariable determinants to contact the peptide. This riveting molecular strategy affords many options for binding energy compatible with specific recognition and signalling aimed to eradicated microbial pathogens and cancer cells. While the molecular foundations of αβ T-cell adaptive immunity are largely understood, uncertainty persists on how peptide-MHC binding induces the TCRαβ signals that instruct cell-fate decisions. Solving this mystery is another milestone for understanding αβ T-cells' self/nonself discrimination. Recent developments revealing the innermost links between TCRαβ structural dynamics and signalling modality should help dissipate this long-sought-after enigma.

Designing Multivalent and Multispecific Biologics

In the era of precision medicine, multivalent and multispecific therapeutics present a promising approach for targeted disease intervention. These therapeutics are designed to interact with multiple targets simultaneously, promising enhanced efficacy, reduced side effects, and resilience against drug resistance. We dissect the principles guiding the design of multivalent biologics, highlighting challenges and strategies that must be considered to maximize therapeutic effect. Engineerable elements in multivalent and multispecific biologic design-domain affinities, valency, and spatial presentation-must be considered in the context of the molecular targets as well as the balance of important properties such as target avidity and specificity. We illuminate recent applications of these principles in designing protein and cell therapies and identify exciting future directions in this field, underscored by advances in biomolecular and cellular engineering and computational approaches. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering , Volume 15 is June 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Mechanism of multivalent glycoconjugate-lectin interaction: An update

Lectins are predominantly oligomeric proteins with several binding sites per molecule. Glycoconjugates are their natural ligands, which often possess multiple binding epitopes. Thus, lectin-glycoconjugate interactions are mostly multivalent in nature. The mechanism of multivalent binding is fundamentally different from those described for monovalent interactions in textbooks and research papers. Over the years, binding studies that make use of different lectins and a variety of multivalent glycoconjugate ligands were conducted in order to understand the underlying principles of multivalency. Starting with seemingly simple synthetic multivalent analogs, systematic studies were carried out using natural glycoconjugate ligands with increasing valency and complexity. Those ligands included multivalent glycoproteins, polyvalent polysaccharides, including glycosaminoglycans, as well as supra-valent mucins and proteoglycans. Models and mechanisms of multivalent binding derived from quantitative data are summarized in the present updated review.

Structure and mechanism of immunoreceptors: New horizons in T cell and B cell receptor biology and beyond

Immunoreceptors, also named non-catalytic tyrosine-phosphorylated receptors, are a large class of leukocyte cell-surface proteins critically involved in innate and adaptive immune responses. Their most characteristic defining feature is a shared signal transduction machinery where binding events of cell surface-anchored ligands to the small extracellular receptor domains are translated into phosphorylation of conserved tyrosine-containing cytosolic sequence motifs initiating downstream signal transduction cascades. Despite their central importance to immunology, the molecular mechanism of how ligand binding activates the receptors and results in robust intracellular signaling has remained enigmatic. Recent breakthroughs in our understanding of the architecture and triggering mechanism of immunoreceptors come from cryogenic electron microscopy studies of the B cell and T cell antigen receptors.

Spatial- and Valence-Matched Neutralizing DNA Nanostructure Blocks Wild-Type SARS-CoV-2 and Omicron Variant Infection

Natural ligand-receptor interactions that play pivotal roles in biological events are ideal models for design and assembly of artificial recognition molecules. Herein, aiming at the structural characteristics of the spike trimer and infection mechanism of SARS-CoV-2, we have designed a DNA framework-guided spatial-patterned neutralizing aptamer trimer for SARS-CoV-2 neutralization. The ∼5.8 nm tetrahedral DNA framework affords precise spatial organization and matched valence as four neutralizing aptamers (MATCH-4), which matches with nanometer precision the topmost surface of SARS-CoV-2 spike trimer, enhancing the interaction between MATCH-4 and spike trimer. Moreover, the DNA framework provides a dimensionally complementary nanoscale barrier to prevent the spike trimer-ACE2 interaction and the conformational transition, thereby inhibiting SARS-CoV-2-host cell fusion and infection. As a result, the spatial- and valence-matched MATCH-4 ensures improved binding affinity and neutralizing activity against SARS-CoV-2 and its varied mutant strains, particularly the current Omicron variant, that are evasive of the majority of existing neutralizing antibodies. In addition, because neutralizing aptamers specific to other targets can be evolved and assembled, the present design has the potential to inhibit other wide-range and emerging pathogens.

Critical parameters for design and development of multivalent nanoconstructs: recent trends

A century ago, the groundbreaking concept of the magic bullet was given by Paul Ehrlich. Since then, this concept has been extensively explored in various forms to date. The concept of multivalency is among such advancements of the magic bullet concept. Biologically, the concept of multivalency plays a critical role in significantly huge numbers of biochemical interactions. This concept is the sole reason behind the higher affinity of biological molecules like viruses to more selectively target the host cell surface receptors. Multivalent nanoconstructs are a promising approach for drug delivery by the active targeting principle. Designing and developing effective and target-specific multivalent drug delivery nanoconstructs, on the other hand, remain a challenge. The underlying reason for this is a lack of understanding of the crucial interactions between ligands and cell surface receptors, as well as the design of nanoconstructs. This review highlights the need for a better theoretical understanding of the multivalent effect of what happens to the receptor-ligand complex after it has been established. Furthermore, the critical parameters for designing and developing robust multivalent systems have been emphasized. We have also discussed current advances in the design and development of multivalent nanoconstructs for drug delivery. We believe that a thorough knowledge of theoretical concepts and experimental methodologies may transform a brilliant idea into clinical translation.

Robust T cell activation requires an eIF3-driven burst in T cell receptor translation

Activation of T cells requires a rapid surge in cellular protein synthesis. However, the role of translation initiation in the early induction of specific genes remains unclear. Here, we show human translation initiation factor eIF3 interacts with select immune system related mRNAs including those encoding the T cell receptor (TCR) subunits TCRA and TCRB. Binding of eIF3 to the TCRA and TCRB mRNA 3'-untranslated regions (3'-UTRs) depends on CD28 coreceptor signaling and regulates a burst in TCR translation required for robust T cell activation. Use of the TCRA or TCRB 3'-UTRs to control expression of an anti-CD19 chimeric antigen receptor (CAR) improves the ability of CAR-T cells to kill tumor cells in vitro. These results identify a new mechanism of eIF3-mediated translation control that can aid T cell engineering for immunotherapy applications.

Allosteric activation of T cell antigen receptor signaling by quaternary structure relaxation

The mechanism of T cell antigen receptor (TCR-CD3) signaling remains elusive. Here, we identify mutations in the transmembrane region of TCRβ or CD3ζ that augment peptide T cell antigen receptor (pMHC)-induced signaling not explicable by enhanced ligand binding, lateral diffusion, clustering, or co-receptor function. Using a biochemical assay and molecular dynamics simulation, we demonstrate that the gain-of-function mutations loosen the interaction between TCRαβ and CD3ζ. Similar to the activating mutations, pMHC binding reduces TCRαβ cohesion with CD3ζ. This event occurs prior to CD3ζ phosphorylation and at 0°C. Moreover, we demonstrate that soluble monovalent pMHC alone induces signaling and reduces TCRαβ cohesion with CD3ζ in membrane-bound or solubilised TCR-CD3. Our data provide compelling evidence that pMHC binding suffices to activate allosteric changes propagating from TCRαβ to the CD3 subunits, reconfiguring interchain transmembrane region interactions. These dynamic modifications could change the arrangement of TCR-CD3 boundary lipids to license CD3ζ phosphorylation and initiate signal propagation.

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Mutations in TCRβ and CD3ζ TMRs that reduce their interaction augment signaling

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pMHC and anti-CD3 binding to TCR-CD3 induce similar quaternary structure relaxation

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Soluble monovalent pMHC alone signals and reduces TCRαβ cohesion with CD3ζ

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Allosteric changes in TCR-CD3 dynamics instigate T cell activation

Recruitment of receptors at supported lipid bilayers promoted by the multivalent binding of ligand-modified unilamellar vesicles

The development of model systems that mimic biological interactions and allow the control of both receptor and ligand densities, is essential for a better understanding of biomolecular processes, such as the recruitment of receptors at interfaces, at the molecular level. Here we report a model system based on supported lipid bilayers (SLBs) for the investigation of the clustering of receptors at their interface. Biotinylated SLBs, used as cell membrane mimics, were functionalized with streptavidin (SAv), used here as receptor. Subsequently, biotinylated small (SUVs) and giant (GUVs) unilamellar vesicles were bound to the SAv-functionalized SLBs by multivalent interactions and found to induce the recruitment of both SAv on the SLB surface and the biotin moieties in the vesicles. The recruitment of receptors was investigated with quartz crystal microbalance with dissipation monitoring (QCM-D), which allowed the identification of the biotin and SAv densities necessary to obtain receptor recruitment. At approx. 0.6% of biotin in the vesicles, a transition between dense and low vesicle packing was observed, which coincided with the transitions between recruitment in the vesicles vs. recruitment in the SLB and between full and partial use of the biotin moieties in the vesicle. Direct optical visualization of the clustering at the interface of individual GUVs with the SLB platform was achieved with fluorescence microscopy, showing recruitment of SAv at the contact area as well as the deformation of the vesicles upon binding. Different vesicle binding regimes were observed for lower and higher biotin densities in the vesicles and at the SLBs. A more quantitative analysis of the molecular parameters implied in the interaction, indicated that approx. 10% of the vesicle area constitutes the contact area. Moreover, the SUV binding and recruitment appeared to be fast on the analysis time scale, whereas the binding of GUVs is slower due to the larger SLB area over which SAv recruitment needs to occur. The mechanisms revealed in this study may provide insight in biological processes in which recruitment occurs.

The development of model systems that mimic biological interactions and allow the control of both receptor and ligand densities, is essential for a molecular understanding of biomolecular processes, such as the recruitment of receptors at interfaces.

The TCR is an allosterically regulated macromolecular machinery changing its conformation while working

The αβ T-cell receptor (TCR) is a multiprotein complex controlling the activation of T cells. Although the structure of the complete TCR is not known, cumulative evidence supports that the TCR cycles between different conformational states that are promoted either by thermal motion or by force. These structural transitions determine whether the TCR engages intracellular effectors or not, regulating TCR phosphorylation and signaling. As for other membrane receptors, ligand binding selects and stabilizes the TCR in active conformations, and/or switches the TCR to activating states that were not visited before ligand engagement. Here we review the main models of TCR allostery, that is, ligand binding at TCRαβ changes the structure at CD3 and ζ. (a) The ITAM and proline-rich sequence exposure model, in which the TCR's cytoplasmic tails shield each other and ligand binding exposes them for phosphorylation. (b) The membrane-ITAM model, in which the CD3ε and ζ tails are sequestered inside the membrane and again ligand binding exposes them. (c) The mechanosensor model in which ligand binding exerts force on the TCR, inducing structural changes that allow signaling. Since these models are complementary rather than competing, we propose a unified model that aims to incorporate all existing data.

DNA Framework-Programmed Cell Capture via Topology-Engineered Receptor-Ligand Interactions

Receptor-ligand interactions (RLIs) that play pivotal roles in living organisms are often depicted with the classic keys-and-locks model. Nevertheless, RLIs on the cell surface are generally highly complex and nonlinear, partially due to the noncontinuous and dynamic distribution of receptors on extracellular membranes. Here, we develop a tetrahedral DNA framework (TDF)-programmed approach to topologically engineer RLIs on the cell membrane, which enables active recruitment-binding of clustered receptors for high-affinity capture of circulating tumor cells (CTCs). The four vertices of a TDF afford orthogonal anchoring of ligands with spatial organization, based on which we synthesized n-simplexes harboring 1-3 aptamers targeting epithelial cell adhesion molecule (EpCAM) that are overexpressed on the membrane of tumor cells. The 2-simplex with three aptamers not only shows increased binding affinity (∼19-fold) but prevents endocytosis by cells. By using 2-simplex as the capture probe, we demonstrate the high-efficiency CTC capture, which is challenged in real clinical breast cancer patient samples. This TDF-programmed platform thus provides a powerful means for studying RLIs in physiological settings and for cancer diagnosis.

Multivalent nanosystems: targeting monocytes/macrophages

Among all the cellular partners involved in inflammatory processes, monocytes and macrophages are the master regulators of inflammation. They are found in almost all the tissues and are nearly the only cells capable of performing each step of inflammation. Consequently, they stand as major relevant therapeutic targets to treat inflammatory disorders and diseases. The physiological phagocytic activity of macrophages prompts them to detect, to recognize, and eventually to engulf any nanosystem cruising in their neighborhood. Interestingly, nanosystems can be rationally engineered to afford multivalent, and multifunctional if needed, entities with multiplexed and/or reinforced biological activities. Indeed, engineered nanosystems bearing moieties specifically targeting macrophages, and loaded with or bound to drugs are promising candidates to modulate, or even eradicate, deleterious macrophages in vivo. In this review we highlight recent articles and concepts of multivalent nanosystems targeting monocytes and macrophages to treat inflammatory disorders.

Localized Visualization and Autonomous Detection of Cell Surface Receptor Clusters Using DNA Proximity Circuit

Cell surface receptors play an important role in mediating cell communication and are used as disease biomarkers and therapeutic targets. We present a one-pot molecular toolbox, which we term the split proximity circuit (SPC), for the autonomous detection and visualization of cell surface receptor clusters. Detection was powered by antibody recognition and a series of autonomous DNA hybridization to achieve localized, enzyme-free signal amplification. The system under study was the human epidermal growth factor receptor (HER) family, that is, HER2:HER2 homodimer and HER2:HER3 heterodimer, both in cell lysate and in situ on fixed whole cells. The detection and imaging of receptors were carried out using standard microplate scans and confocal microscopy, respectively. The circuit operated specifically with minimal leakages and successfully captured the receptor expression profiles on three cell types without any intermediate washing steps.

Monitoring the responsiveness of T and antigen presenting cell compartments in breast cancer patients is useful to predict clinical tumor response to neoadjuvant chemotherapy

Background

Vaccination of mice with tumors treated with Doxorubicin promotes a T cell immunity that relies on dendritic cell (DC) activation and is responsible for tumor control in vaccinated animals. Despite Doxorubicin in combination with Cyclophosphamide (A/C) is widely used to treat breast cancer patients, the stimulating effect of A/C on T and APC compartments and its correlation with patient’s clinical response remains to be proved.

Methods

In this prospective study, we designed an in vitro system to monitor various immunological readouts in PBMCs obtained from a total of 17 breast cancer patients before, and after neoadjuvant anti-tumor therapy with A/C.

Results

The results show that before treatment, T cells and DCs, exhibit a marked unresponsiveness to in vitro stimulus: whereas T cells exhibit poor TCR internalization and limited expression of CD154 in response to anti-CD3/CD28/CD2 stimulation, DCs secrete low levels of IL-12p70 and limited CD83 expression in response to pro-inflammatory cytokines. Notably, after treatment the responsiveness of T and APC compartments was recovered, and furthermore, this recovery correlated with patients’ residual cancer burden stage.

Conclusions

Our results let us to argue that the model used here to monitor the T and APC compartments is suitable to survey the recovery of immune surveillance and to predict tumor response during A/C chemotherapy.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3982-1) contains supplementary material, which is available to authorized users.

PALLD Regulates Phagocytosis by Enabling Timely Actin Polymerization and Depolymerization

PALLD is an actin cross-linker supporting cellular mechanical tension. However, its involvement in the regulation of phagocytosis, a cellular activity essential for innate immunity and physiological tissue turnover, is unclear. We report that PALLD is highly induced along with all-trans-retinoic acid-induced maturation of myeloid leukemia cells, to promote Ig- or complement-opsonized phagocytosis. PALLD mechanistically facilitates phagocytic receptor clustering by regulating actin polymerization and c-Src dynamic activation during particle binding and early phagosome formation. PALLD is also required at the nascent phagosome to recruit phosphatase oculocerebrorenal syndrome of Lowe, which regulates phosphatidylinositol-4,5-bisphosphate hydrolysis and actin depolymerization to complete phagosome closure. Collectively, our results show a new function for PALLD as a crucial regulator of the early phase of phagocytosis by elaborating dynamic actin polymerization and depolymerization.

A Novel Fucose-binding Lectin from Photorhabdus luminescens (PLL) with an Unusual Heptabladed β-Propeller Tetrameric Structure

Photorhabdus luminescens is known for its symbiosis with the entomopathogenic nematode Heterorhabditis bacteriophora and its pathogenicity toward insect larvae. A hypothetical protein from P. luminescens was identified, purified from the native source, and characterized as an l-fucose-binding lectin, named P. luminescens lectin (PLL). Glycan array and biochemical characterization data revealed PLL to be specific toward l-fucose and the disaccharide glycan 3,6-O-Me₂-Glcβ1-4(2,3-O-Me₂)Rhaα-O-(p-C₆H₄)-OCH₂CH₂NH₂ PLL was discovered to be a homotetramer with an intersubunit disulfide bridge. The crystal structures of native and recombinant PLL revealed a seven-bladed β-propeller fold creating seven putative fucose-binding sites per monomer. The crystal structure of the recombinant PLL·l-fucose complex confirmed that at least three sites were fucose-binding. Moreover, the crystal structures indicated that some of the other sites are masked either by the tetrameric nature of the lectin or by incorporation of the C terminus of the lectin into one of these sites. PLL exhibited an ability to bind to insect hemocytes and the cuticular surface of a nematode, H. bacteriophora.

Co-targeting EGFR and survivin with a bivalent aptamer-dual siRNA chimera effectively suppresses prostate cancer

Current targeted therapies using small kinase inhibitors and antibodies have limited efficacy in treating prostate cancer (PCa), a leading cause of cancer death in American men. We have developed a novel strategy by engineering an RNA-based aptamer-siRNA chimera, in which a bivalent aptamer specifically binds prostate-specific membrane antigen (PSMA) via an antibody-like structure to promote siRNA internalization in PCa cells, and two siRNAs specific to EGFR and survivin are fused between two aptamers. The chimera is able to inhibit EGFR and survivin simultaneously and induce apoptosis effectively in vitro and in vivo. In the C4-2 PCa xenograft model, the treatment with the chimera significantly suppresses tumor growth and angiogenesis. The inhibition of angiogenesis is mediated by an EGFR-HIF1α-VEGF-dependent mechanism. Our results support that the bivalent aptamer-driven delivery of two siRNAs could be a new combination therapeutic strategy to effectively inhibit multiple and conventionally "undruggable" targets.

One-pot system for synthesis, assembly, and display of functional single-span membrane proteins on oil-water interfaces

Single-span membrane proteins (ssMPs) represent approximately one-half of all membrane proteins and play important roles in cellular communications. However, like all membrane proteins, ssMPs are prone to misfolding and aggregation because of the hydrophobicity of transmembrane helices, making them difficult to study using common aqueous solution-based approaches. Detergents and membrane mimetics can solubilize membrane proteins but do not always result in proper folding and functionality. Here, we use cell-free protein synthesis in the presence of oil drops to create a one-pot system for the synthesis, assembly, and display of functional ssMPs. Our studies suggest that oil drops prevent aggregation of some in vitro-synthesized ssMPs by allowing these ssMPs to localize on oil surfaces. We speculate that oil drops may provide a hydrophobic interior for cotranslational insertion of the transmembrane helices and a fluidic surface for proper assembly and display of the ectodomains. These functionalized oil drop surfaces could mimic cell surfaces and allow ssMPs to interact with cell surface receptors under an environment closest to cell-cell communication. Using this approach, we showed that apoptosis-inducing human transmembrane proteins, FasL and TRAIL, synthesized and displayed on oil drops induce apoptosis of cultured tumor cells. In addition, we take advantage of hydrophobic interactions of transmembrane helices to manipulate the assembly of ssMPs and create artificial clusters on oil drop surfaces. Thus, by coupling protein synthesis with self-assembly at the water-oil interface, we create a platform that can use recombinant ssMPs to communicate with cells.

Limited density of an antigen presented by RMA-S cells requires B7-1/CD28 signaling to enhance T-cell immunity at the effector phase

The association of B7-1/CD28 between antigen presenting cells (APCs) and T-cells provides a second signal to proliferate and activate T-cell immunity at the induction phase. Many reports indicate that tumor cells transfected with B7-1 induced augmented antitumor immunity at the induction phase by mimicking APC function; however, the function of B7-1 on antitumor immunity at the effector phase is unknown. Here, we report direct evidence of enhanced T-cell antitumor immunity at the effector phase by the B7-1 molecule. Our experiments in vivo and in vitro indicated that reactivity of antigen-specific monoclonal and polyclonal T-cell effectors against a Lass5 epitope presented by RMA-S cells is increased when the cells expressed B7-1. Use of either anti-B7-1 or anti-CD28 antibodies to block the B7-1/CD28 association reduced reactivity of the T effectors against B7-1 positive RMA-S cells. Transfection of Lass5 cDNA into or pulse of Lass5 peptide onto B7-1 positive RMA-S cells overcomes the requirement of the B7-1/CD28 signal for T effector response. To our knowledge, the data offers, for the first time, strong evidence that supports the requirement of B7-1/CD28 secondary signal at the effector phase of antitumor T-cell immunity being dependent on the density of an antigenic peptide.

Spatio-temporally precise activation of engineered receptor tyrosine kinases by light

Receptor tyrosine kinases (RTKs) are a large family of cell surface receptors that sense growth factors and hormones and regulate a variety of cell behaviours in health and disease. Contactless activation of RTKs with spatial and temporal precision is currently not feasible. Here, we generated RTKs that are insensitive to endogenous ligands but can be selectively activated by low-intensity blue light. We screened light-oxygen-voltage (LOV)-sensing domains for their ability to activate RTKs by light-activated dimerization. Incorporation of LOV domains found in aureochrome photoreceptors of stramenopiles resulted in robust activation of the fibroblast growth factor receptor 1 (FGFR1), epidermal growth factor receptor (EGFR) and rearranged during transfection (RET). In human cancer and endothelial cells, light induced cellular signalling with spatial and temporal precision. Furthermore, light faithfully mimicked complex mitogenic and morphogenic cell behaviour induced by growth factors. RTKs under optical control (Opto-RTKs) provide a powerful optogenetic approach to actuate cellular signals and manipulate cell behaviour.

Reverse signaling from LIGHT promotes pro-inflammatory responses in the human monocytic leukemia cell line, THP-1

LIGHT is a type II transmembrane protein belonging to the TNF superfamily which is involved in co-stimulation of T cells or apoptosis in tumors. In this study, the possibility of LIGHT-mediated reverse signaling was tested in the human monocytic leukemia cell line, THP-1. For stimulation of LIGHT, cells were stimulated with specific monoclonal antibody and changes in macrophage-related functions such as phagocytosis, adhesion, migration, cytokine secretion, and production of pro-inflammatory mediators were tested. Triggering of LIGHT induced production of pro-inflammatory mediators such as interleukin (IL)-8 and matrix metalloproteinase (MMP)-9 while suppressing the phagocytic activity. Utilization of signaling inhibitors and Western blot demonstrated that LIGHT activated ERK MAPK and PI3K and the major inflammatory transcription factor NF-κB. These data indicate that LIGHT-mediated signaling could modulate the macrophage activities and that successful regulation of its activity could be beneficial to the treatment of chronic inflammatory conditions where macrophages play an important role.

A Detailed Study on Understanding Glycopolymer Library and Con A Interactions

Synthetic glycopolymers are important natural oligosaccharides mimics for many biological applications. To develop glycopolymeric drugs and therapeutic agents, factors that control the receptor-ligand interaction need to be investigated. A library of well-defined glycopolymers has been prepared by the combination of copper mediated living radical polymerization and CuAAC click reaction via post-functionalization of alkyne-containing precursor polymers with different sugar azides. Employing Concanavalin A as the model receptor, we explored the influence of the nature and densities of different sugars residues (mannose, galactose, and glucose) on the stoichiometry of the cluster, the rate of the cluster formation, the inhibitory potency of the glycopolymers, and the stability of the turbidity through quantitative precipitation assays, turbidimetry assays, inhibitory potency assays, and reversal aggregation assays. The diversities of binding properties contributed by different clustering parameters will make it possible to define the structures of the multivalent ligands and densities of binding epitopes tailor-made for specific functions in the lectin-ligand interaction. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2588-2597.

Regulation from within: the cytoskeleton in transmembrane signaling

There is mounting evidence that the plasma membrane is highly dynamic and organized in a complex manner. The cortical cytoskeleton is proving to be a particularly important regulator of plasmalemmal organization, modulating the mobility of proteins and lipids in the membrane, facilitating their segregation, and influencing their clustering. This organization plays a critical role in receptor-mediated signaling, especially in the case of immunoreceptors, which require lateral clustering for their activation. Based on recent developments, we discuss the structures and mechanisms whereby the cortical cytoskeleton regulates membrane dynamics and organization, and how the nonuniform distribution of immunoreceptors and their self-association may affect activation and signaling.

Crohn's disease alters MHC-rafts in CD4+ T-cells

Clusters of MHCI, ICAM-1, CD44, CD59, IL-2R, and IL-15R molecules have been studied on the surface of CD4(+) T-cells from peripheral blood and lymph nodes of patients in Crohn's disease and healthy individuals as controls by using a dual-laser flow cytometric fluorescence resonance energy transfer (FRET) technique and fluorescently stained Fabs. When cells from patients in Crohn's disease are compared to those of controls, the surface expression level for the MHCI reduced by ∼45%, for CD44 enhanced by ∼100%, and for IL-2Rα, IL-15Rα, and common γ(c) enhanced by ∼50%, ∼70%, and ∼130%, respectively. Efficiencies of FRET monitoring homoassociation for the MHCI and CD44 reduced, that for IL-2Rα enhanced. While efficiencies of FRET monitoring the association of γ(c) and ICAM-1 with the MHCI reduced, those monitoring association of IL-2/15Rα, CD44, and CD59 with MHCI enhanced. Efficiencies of FRET measured between the MHCI and IL-2Rα, IL-15Rα differently enhanced to the advantage of IL-15Rα, the one measured between γ(c) and IL-2Rα reduced, suggesting modulations in the strength of interaction of MHCI with IL-2R, IL-15R, and γ(c). The increases in density of surface bound cTx and in the associations of the receptors with the G(M1)-ganglioside lipid molecules suggest stronger lipid raft interactions of the receptors. The observed alterations of MHC-rafts in Crohn's disease--summarized in models of receptor patterns of diseased and control cells--may have functional consequences regarding signaling by the raft components.

The extracellular architecture of adherens junctions revealed by crystal structures of type I cadherins

Adherens junctions, which play a central role in intercellular adhesion, comprise clusters of type I classical cadherins that bind via extracellular domains extended from opposing cell surfaces. We show that a molecular layer seen in crystal structures of E- and N-cadherin ectodomains reported here and in a previous C-cadherin structure corresponds to the extracellular architecture of adherens junctions. In all three ectodomain crystals, cadherins dimerize through a trans adhesive interface and are connected by a second, cis, interface. Assemblies formed by E-cadherin ectodomains coated on liposomes also appear to adopt this structure. Fluorescent imaging of junctions formed from wild-type and mutant E-cadherins in cultured cells confirm conclusions derived from structural evidence. Mutations that interfere with the trans interface ablate adhesion, whereas cis interface mutations disrupt stable junction formation. Our observations are consistent with a model for junction assembly involving strong trans and weak cis interactions localized in the ectodomain.

Anchorage of VEGF to the extracellular matrix conveys differential signaling responses to endothelial cells

Matrix-bound VEGF elicits more distinct vascular effects than soluble VEGF, including prolonged VEGFR2 activation with altered patterns of tyrosine activation and downstream enhancement of the p38/MAPK pathway.

VEGF can be secreted in multiple isoforms with variable affinity for extracellular proteins and different abilities to induce vascular morphogenesis, but the molecular mechanisms behind these effects remain unclear. Here, we show molecular distinctions between signaling initiated from soluble versus matrix-bound VEGF, which mediates a sustained level of VEGFR2 internalization and clustering. Exposure of endothelial cells to matrix-bound VEGF elicits prolonged activation of VEGFR2 with differential phosphorylation of Y1214, and extended activation kinetics of p38. These events require association of VEGFR2 with β1 integrins. Matrix-bound VEGF also promotes reciprocal responses on β1 integrin by inducing its association with focal adhesions; a response that is absent upon exposure to soluble VEGF. Inactivation of β1 integrin blocks the prolonged phosphorylation of Y1214 and consequent activation of p38. Combined, these results indicate that when in the context of extracellular matrix, activation of VEGFR2 is distinct from that of soluble VEGF in terms of recruitment of receptor partners, phosphorylation kinetics, and activation of downstream effectors.

Essential role of the Ly49A stalk region for immunological synapse formation and signaling

NK cells use surface NK receptors to discriminate self from non-self. The NK receptor ligand-binding domain (NKD) has been considered the sole regulator of ligand binding. Using a prototypic murine NK receptor, Ly49A, we show that the membrane proximal nonligand binding ecto-domain (the stalk region) is critical to ligand binding and signaling. The stalk region is required for receptor binding to ligand on target cells (trans interaction), but is dispensable for receptor binding to ligand on the same cell (cis interaction). Also, signaling in a trans manner depends on the stalk region mediating the formation of the immunological synapse. Thus, our data modeling receptor function at the cellular level reveal an essential role for the stalk region as a specific mediator of receptor signal integration, by which NKD-ligand interactions at the interface initiate and deliver information to the spatially separated cytoplasmic domain.

A mechanism for SRC kinase-dependent signaling by noncatalytic receptors

A fundamental issue in cell biology is how signals are transmitted across membranes. A variety of transmembrane receptors, including multichain immune recognition receptors, lack catalytic activity and require Src family kinases (SFKs) for signal transduction. However, many receptors only bind and activate SFKs after ligand-induced receptor dimerization. This presents a conundrum: How do SFKs sense the dimerization of receptors to which they are not already bound? Most proposals for resolving this enigma invoke additional players, such as lipid rafts or receptor conformational changes. Here we used simple thermodynamics to show that SFK activation is a natural outcome of clustering of receptors with SFK phosphorylation sites, provided that there is phosphorylation-dependent receptor-SFK association and an SFK bound to one receptor can phosphorylate the second receptor or its associated SFK in a dimer. A simple system of receptor, SFK, and an unregulated protein tyrosine phosphatase (PTP) can account for ligand-induced changes in phosphorylation observed in cells. We suggest that a core signaling system comprising a receptor with SFK phosphorylation sites, an SFK, and an unregulated PTP provides a robust mechanism for transmembrane signal transduction. Other events that regulate signaling in specific cases may have evolved for fine-tuning of this basic mechanism.

A membrane-bound form of IL-4 enhances proliferation and antigen presentation of CD40-activated human B cells

CD40-activated B (CD40-B) cells might be an attractive source of autologous antigen-presenting cells (APCs) for immunotherapy due to the ability to obtain them from peripheral blood and expand them in vitro. However, soluble IL-4 (sIL-4) in B-cell culture may not represent the "immunological synapse" between B and CD4+ T cells. In this study, the K562 cell line, which expresses CD40L and membrane-bound IL-4 (mbIL-4), could induce higher B-cell proliferation and antigen-presenting surface molecules, including adhesion, costimulatory and HLA molecules, compared with sIL-4. The differentiation to plasmablasts was decreased in CD40-B cells treated with mbIL-4 (CD40-B/mbIL-4) based on flow cytometry analysis. Furthermore, CD40-B/mbIL-4 cells were as potent as mature dendritic cells in the allogeneic lymphocyte reaction and the ability to generate cytotoxic T lymphocytes specific for cytomegalovirus pp65 antigen in vitro. Our results suggest that mbIL-4 could be used to generate CD40-B cells as potent APCs for cellular vaccines and adoptive immunotherapy.

Allosteric Changes in the TCR/CD3 Structure Upon Interaction With Extra- or Intra-cellular Ligands

T lymphocytes are activated by the interaction between the T-cell antigen receptor (TCR) and peptides presented by major histocompatibility complex (MHC) molecules. The avidity of this TCR-pMHC interaction is very low. Therefore, several hypotheses have been put forward to explain how T cells become specifically activated despite this handicap: conformational change model, aggregation model, kinetic segregation model, sequential interaction model and permissive geometry model. In the present paper, we conducted experiments to distinguish between the TCR-aggregation model and the TCR-conformational change model. The results obtained using a TCR capture ELISA with Brij 98-solubilized TCR molecules from normal or activated T cells showed that the ligand-TCR interaction causes structural changes in the CD3 epsilon cytoplasmic tail as well as in the extracellular TCR beta FG loop region. Size-fractionation experiments with Brij 98-solubilized TCR/CD3/co-receptor complexes from naïve or activated CD4(+) or CD8(+) T cells demonstrated that such complexes are found as either dimers or tetramers. No monomers or multimers were detected. We propose that: (1) ligand-TCR interaction results in conformational changes in the CD3 epsilon cytoplasmic tail leading to T-cell activation; (2) CD3 epsilon cytoplasmic tail interaction with intracellular proteins may dissociate pMHC and co-receptors (CD4 or CD8) from TCR/CD3 complexes, thus leading to the arrest of T-cell activation; and (3) T-cell activation appears to occur among dimers or tetramers of TCR/CD3/co-receptor complexes interacting with self and non-self (foreign) peptide-MHC complexes.

Structure induction of the T-cell receptor zeta-chain upon lipid binding investigated by NMR spectroscopy

The conformation of the cytoplasmic part of the zeta-chain of the T-cell receptor (TCR) in its free form and bound to detergent micelles has been investigated by heteronuclear NMR spectroscopy. The zeta-chain is considered to be a mediator between the extracellular antigen and the intracellular signal-transduction cascade leading to T-cell activation. Earlier studies suggested a T-cell activation mechanism that involved a TCR-state-dependent lipid incorporation propensity of the zeta-chain accompanied by a helical folding transition. In order to support this proposed mechanism, standard protein NMR assignment and secondary-structure-elucidation techniques have been applied to the free TCR zeta-chain and to the zeta-chain bound to the detergent LMPG, which forms a micelle, in order to obtain the structural characteristics of this folding transition in a residue-resolved manner. We could assign the resonances of the free zeta-chain at 278 K, and this formed the basis for chemical-shift-perturbation studies to identify lipid binding sites. Our NMR results show that the free TCR zeta-chain is indeed intrinsically unstructured. Regions around the ITAM2 and ITAM3 sequences are involved in a highly dynamic binding of the free zeta-chain to a detergent micelle formed by the acidic lipid LMPG.

Immunogenicity screening in protein drug development

Most therapeutic proteins in clinical trials or on the market are, to a variable extent, immunogenic. Formation of antidrug antibodies poses a risk that should be assessed during drug development, as it possibly compromises drug safety and alters pharmacokinetics. The generation of these antibodies is critically dependent on the presence of T helper cell epitopes, which have classically been identified by in vitro methods using blood cells from human donors. As a novel development, in silico methods that identify T cell epitopes have been coming on line. These methods are relatively inexpensive and allow the mapping of epitopes from virtually all human leukocyte antigen molecules derived from a wide genetic background. In vitro assays remain important, but guided by in silico data they can focus on selected peptides and human leukocyte antigen haplotypes, thereby significantly reducing time and cost.

Full Activation of the T Cell Receptor Requires Both Clustering and Conformational Changes at CD3

T cell receptor (TCR-CD3) triggering involves both receptor clustering and conformational changes at the cytoplasmic tails of the CD3 subunits. The mechanism by which TCRalphabeta ligand binding confers conformational changes to CD3 is unknown. By using well-defined ligands, we showed that induction of the conformational change requires both multivalent engagement and the mobility restriction of the TCR-CD3 imposed by the plasma membrane. The conformational change is elicited by cooperative rearrangements of two TCR-CD3 complexes and does not require accompanying changes in the structure of the TCRalphabeta ectodomains. This conformational change at CD3 reverts upon ligand dissociation and is required for T cell activation. Thus, our permissive geometry model provides a molecular mechanism that rationalizes how the information of ligand binding to TCRalphabeta is transmitted to the CD3 subunits and to the intracellular signaling machinery.

Synthetic multivalent ligands as probes of signal transduction

Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses. Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes. Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses. Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades. Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction. Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements. This Review focuses on the use of synthetic multivalent ligands to characterize receptor function.

Exploiting pathogenic Escherichia coli to model transmembrane receptor signalling

Many microbial pathogens manipulate the actin cytoskeleton of eukaryotic target cells to promote their internalization, intracellular motility and dissemination. Enteropathogenic and enterohaemorrhagic Escherichia coli, which both cause severe diarrhoeal disease, can adhere to mammalian intestinal cells and induce reorganization of the actin cytoskeleton into 'pedestal-like' pseudopods beneath the extracellular bacteria. As pedestal assembly is triggered by E. coli virulence factors that mimic several host cell-signalling components, such as transmembrane receptors, their cognate ligands and cytoplasmic adaptor proteins, it can serve as a powerful model system to study eukaryotic transmembrane signalling. Here, we consider the impact of recent data on our understanding of both E. coli pathogenesis and cell biology, and the rich prospects for exploiting these bacterial factors as versatile tools to probe cellular signalling pathways.

Integration of cell adhesion reactions—a balance of forces?

The rearrangement of receptors by oligomeric adhesion molecules constitutes a configurational mechanism able to sculpture membranes and dislocate receptors from cytoplasmic anchorage. This provides a conceptual framework for complex cellular processes in mechanical terms, as a dynamic balance between extracellular and intracellular driving forces.

Coexistence of multivalent and monovalent TCRs explains high sensitivity and wide range of response

A long-standing paradox in the study of T cell antigen recognition is that of the high specificity–low affinity T cell receptor (TCR)–major histocompatibility complex peptide (MHCp) interaction. The existence of multivalent TCRs could resolve this paradox because they can simultaneously improve the avidity observed for monovalent interactions and allow for cooperative effects. We have studied the stoichiometry of the TCR by Blue Native–polyacrylamide gel electrophoresis and found that the TCR exists as a mixture of monovalent (αβγɛδɛζζ) and multivalent complexes with two or more ligand-binding TCRα/β subunits. The coexistence of monovalent and multivalent complexes was confirmed by electron microscopy after label fracture of intact T cells, thus ruling out any possible artifact caused by detergent solubilization. We found that although only the multivalent complexes become phosphorylated at low antigen doses, both multivalent and monovalent TCRs are phosphorylated at higher doses. Thus, the multivalent TCRs could be responsible for sensing low concentrations of antigen, whereas the monovalent TCRs could be responsible for dose-response effects at high concentrations, conditions in which the multivalent TCRs are saturated. Thus, besides resolving TCR stoichiometry, these data can explain how T cells respond to a wide range of MHCp concentrations while maintaining high sensitivity.

Twisting tails exposed: the evidence for TCR conformational change

The mechanism by which the ligand occupancy state of the T cell receptor complex is converted into intracellular signaling information has been a controversial topic. Although the majority of structural studies argue against a conformational change, recent studies support the possibility for such a change within the CD3 components of the TCR complex. In this commentary, the evidence for TCR conformational change is reviewed and potential mechanisms for its initiation are explored.

Lectin-induced haemocyte inactivation in insects

Most multimeric lectins are adhesion molecules, promoting attachment and spreading on surface glycodeterminants. In addition, some lectins have counter-adhesion properties, detaching already spread cells which then acquire round or spindle-formed cell shapes. Since lectin-mediated adhesion and detachment is observed in haemocyte-like Drosophila cells, which have haemomucin as the major lectin-binding glycoprotein, the two opposite cell behaviours may be the result of lectin-mediated receptor rearrangements on the cell surface. To investigate oligomeric lectins as a possible extracellular driving force affecting cell shape changes, we examined lectin-mediated reactions in lepidopteran haemocytes after cytochalasin D-treatment and observed that while cell-spreading was dependent on F-actin, lectin-uptake was less dependent on F-actin. We propose a model of cell shape changes involving a dynamic balance between adhesion and uptake reactions.

Dimerization of the immunosuppressive peptide fragment of HLA-DR molecule enhances its potency

Our previous studies revealed that the nonapeptide fragment of HLA-DR molecule, located in the beta chain 164-172 with the VPRSGEVYT sequence, suppresses the immune responses. The sequence is located on the exposed molecule loop, therefore it may be involved in the interactions with other proteins. We suggested that the loop may serve as a functional epitope on the HLA class II surface for intermolecular binding, and that possible mechanism of biological action of the synthesized peptides is associated with interfering of adhesion of HLA class II molecules to their coreceptors. It has been postulated that oligomerization of the coreceptors is required for stable binding to class II HLA. Based on the crystal dimeric structure of HLA-DR molecules, we designed, and synthesized molecules able to induce the putative coreceptors dimerization. The synthesized series of compounds consisted of two VPRSGEVYT sequences linked through their C-termini by spacers of different length: (VPRSGEVYTGn)2K-NH2 ( n = 4-6). The results demonstrate that the dimerization of the nonapeptide fragment of HLA-DR results in enhanced immunosuppressory properties.

An extracellular driving force of cell-shape changes

The cellular capacity to internalise objects, involving attachment, engulfment and uptake, exists in virtually all organisms. Many uptake reactions are associated with cell signalling. However, the mechanical forces that form endocytotic vesicles are not known. We propose a 'leverage-mediated' uptake mechanism involving lateral cross-linking processes on the cell surface that can generate the configurational energy to create an inverse curvature of the membrane.

Exosomes as a tumor vaccine: enhancing potency through direct loading of antigenic peptides

Exosomes secreted by dendritic cells (DCs) contain MHC-I, MHC-II, and other accessory molecules required for antigen presentation to T cells. Previous studies have shown that exosome MHC-I "indirectly" loaded by adding peptides to DC cultures are immunogenic. However, analysis of peptide binding was not performed to link T-cell-stimulating activity with the amount of MHC-I/peptide complexes on the exosomes. In this study, we measured peptide binding to MHC-I under different loading conditions and tested the exosomes' potencies in T-cell activation assays. We demonstrate that MHC-I on purified exosomes can be directly loaded with peptide at much greater levels than indirect loading. The direct loading method performed in mildly acidic conditions was effective even in the absence of exogenous beta2m. This increase in peptide binding greatly enhanced exosome potency, allowing us to further study the biologic activity of exosomes in vitro. In the presence of antigen-presenting cells (APC), exosomes directly loaded with the HLA-A2 restricted MART1 tumor peptide stimulated an HLA-A2/MART1 specific T-cell line. The T cells responded to exosomes using HLA-A2neg APC, demonstrating transfer of functional MHC-I/peptide complexes and not peptide alone to APC. MHC-II molecules, which are abundantly expressed on DC exosomes, were also functionally loaded under the same conditions as MHC-I. This feature allows for delivery of multiple peptide antigens that can stimulate both CD8+ cytotoxic T cells as well CD4+ T helper cells critical for an effective antitumor response. The optimized loading conditions and the ability to transfer both MHC-I and MHC-II antigens to APC have led to the development of exosomes as an "acellular" immunotherapy approach currently being tested in clinical trials.

A complicated relationship: fulfilling the interactive needs of the T lymphocyte and the dendritic cell

T cells recognize antigenic peptides displayed on the surface of MHC-bearing antigen-presenting cells (APCs), and with sufficient costimulation become activated. However, the ability of an APC (even bearing the correct peptide) to initiate and fulfill the requirements for T cell activation is not easily achieved. Naive T cells use multiple copies of a single receptor to survey the vast array of peptides presented on an APC, and require multiple receptor engagements to initiate T cell activation. Dendritic cells (DCs) are specialized cells with optimal capabilities for priming naive CD4+ T cells. Activation occurs, after initial antigen recognition by T cells, followed by a rapid dialogue between the T cells and the DCs. The resulting changes in both the cytoskeleton and the expression or regulation of cell-surface molecules on both cell types act to further strengthen engagement. In this report, we review the fundamentals of CD4+ T helper cell : DC interactions and discuss recent data concerning the molecular characteristics of this engagement.