Increased sensitivity of antigen-experienced T cells through the enrichment of oligomeric T cell receptor complexes

DNA origami: Interrogating the nano-landscape of immune receptor activation

The immune response is orchestrated by elaborate protein interaction networks that interweave ligand-mediated receptor reorganization with signaling cascades. While the biochemical processes have been extensively investigated, delineating the biophysical principles governing immune receptor activation has remained challenging due to design limitations of traditional ligand display platforms. These constraints have been overcome by advances in DNA origami nanotechnology, enabling unprecedented control over ligand geometry on configurable scaffolds. It is now possible to systematically dissect the independent roles of ligand stoichiometry, spatial distribution, and rigidity in immune receptor activation, signaling, and cooperativity. In this review, we highlight pioneering efforts in manipulating the ligand presentation landscape to understand immune receptor triggering and to engineer functional immune responses.

Crosstalking with Dendritic Cells: A Path to Engineer Advanced T Cell Immunotherapy

Crosstalk between dendritic cells (DCs) and T cells plays a crucial role in modulating immune responses in natural and pathological conditions. DC-T cell crosstalk is achieved through contact-dependent (i.e., immunological synapse) and contact-independent mechanisms (i.e., cytokines). Activated DCs upregulate co-stimulatory signals and secrete proinflammatory cytokines to orchestrate T cell activation and differentiation. Conversely, activated T helper cells "license" DCs towards maturation, while regulatory T cells (Tregs) silence DCs to elicit tolerogenic immunity. Strategies to efficiently modulate the DC-T cell crosstalk can be harnessed to promote immune activation for cancer immunotherapy or immune tolerance for the treatment of autoimmune diseases. Here, we review the natural crosstalk mechanisms between DC and T cells. We highlight bioengineering approaches to modulate DC-T cell crosstalk, including conventional vaccines, synthetic vaccines, and DC-mimics, and key seminal studies leveraging these approaches to steer immune response for the treatment of cancer and autoimmune diseases.

Pre-organized landscape of T cell surface

T cell activation is initiated by the recognition of specific antigenic peptides and subsequently accomplished by complex signaling cascades. These aspects have been extensively studied for decades as pivotal factors in the establishment of adaptive immunity. However, how receptors or signaling molecules are organized in the resting state prior to encountering antigens has received less attention. Recent advancements in super-resolution microscopy techniques have revealed topographically controlled pre-formed organization of key molecules involved in antigen recognition and signal transduction on microvillar projections of T cells before activation and substantial effort has been dedicated to characterizing the topological structure of resting T cells over the past decade. This review will summarize our current understanding of how key surface receptors are pre-organized on the T-cell plasma membrane and discuss the potential role of these receptors, which are preassembled prior to ligand binding in the early activation events of T cells.

A photo-activable nano-agonist for the two-signal model of T cell in vivo activation

The two-signal model of T cell activation has helped shape our understanding of the adaptive immune response for over four decades. According to the model, activation of T cells requires a stimulus through the T cell receptor/CD3 complex (signal 1) and a costimulatory signal 2. Stimulation of activatory signals via T cell agonists has thus emerged. However, for a robust T cell activation, it necessitates not only the presence of both signal 1 and signal 2, but also a high signaling strength. Herein, we report a photo-activable nano-agonist for the two-signal model of T cell in vivo activation. A UV-crosslinkable polymer is coated onto upconversion nanoparticles with satisfactory NIR-to-UV light conversion efficiency. Then dual signal molecules, i.e., signal 1 and signal 2, are conjugated to the polymer end to yield the photo-activable T cell nano-agonist. In melanoma and breast cancer models, photo-activable nano-agonist could bind onto corresponding activatory receptors on the surface of T cells, but has limited activity without the application of NIR light (absence of photo-crosslinking of receptors and consequently a poor signaling strength). While when the NIR light is switched on locally, T cells in tumor are remarkably activated and kill tumor cells effectively. Moreover, we do not observe any detectable toxicities related to the photo-activable nano-agonist. We believe with two activatory signals being simultaneously strengthened by local photo-switched crosslinking, T cells realize a robust and selective activation in tumor and, consequently contribute to an enhanced and safe tumor immunotherapy.

Challenges and opportunities in analyzing and modeling peptide presentation by HLA-II proteins

The human leukocyte antigen (HLA) proteins are an indispensable component of adaptive immunity because of their role in presenting self and foreign peptides to T cells. Further, many complex diseases are associated with genetic variation in the HLA region, implying an important role for specific HLA-presented peptides in the etiology of these diseases. Identifying the specific set of peptides presented by an individual’s HLA proteins in vivo, as a whole being referred to as the immunopeptidome, has therefore gathered increasing attention for different reasons. For example, identifying neoepitopes for cancer immunotherapy, vaccine development against infectious pathogens, or elucidating the role of HLA in autoimmunity. Despite the tremendous progress made during the last decade in these areas, several questions remain unanswered. In this perspective, we highlight five remaining key challenges in the analysis of peptide presentation and T cell immunogenicity and discuss potential solutions to these problems. We believe that addressing these questions would not only improve our understanding of disease etiology but will also have a direct translational impact in terms of engineering better vaccines and in developing more potent immunotherapies.

Theoretical quantification of the polyvalent binding of nanoparticles coated with peptide-major histocompatibility complex to T cell receptor-nanoclusters

Nanoparticles (NPs) coated with peptide-major histocompatibility complexes (pMHCs) can be used as a therapy to treat autoimmune diseases. They do so by inducing the differentiation and expansion of disease-suppressing T regulatory type 1 (Tr1) cells by binding to their T cell receptors (TCRs) expressed as TCR-nanoclusters (TCR_nc). Their efficacy can be controlled by adjusting NP size and number of pMHCs coated on them (referred to as valence). The binding of these NPs to TCR_nc on T cells is thus polyvalent and occurs at three levels: the TCR-pMHC, NP-TCR_nc and T cell levels. In this study, we explore how this polyvalent interaction is manifested and examine if it can facilitate T cell activation downstream. This is done by developing a multiscale biophysical model that takes into account the three levels of interactions and the geometrical complexity of the binding. Using the model, we quantify several key parameters associated with this interaction analytically and numerically, including the insertion probability that specifies the number of remaining pMHC binding sites in the contact area between T cells and NPs, the dwell time of interaction between NPs and TCR_nc, carrying capacity of TCR_nc, the distribution of covered and bound TCRs, and cooperativity in the binding of pMHCs within the contact area. The model was fit to previously published dose-response curves of interferon-γ obtained experimentally by stimulating a population of T cells with increasing concentrations of NPs at various valences and NP sizes. Exploring the parameter space of the model revealed that for an appropriate choice of the contact area angle, the model can produce moderate jumps between dose-response curves at low valences. This suggests that the geometry and kinetics of NP binding to TCR_nc can act in synergy to facilitate T cell activation.

Hyperstabilization of T cell microvilli contacts by chimeric antigen receptors

T cells typically recognize their ligands using a defined cell biology-the scanning of their membrane microvilli (MV) to palpate their environment-while that same membrane scaffolds T cell receptors (TCRs) that can signal upon ligand binding. Chimeric antigen receptors (CARs) present both a therapeutic promise and a tractable means to study the interplay between receptor affinity, MV dynamics and T cell function. CARs are often built using single-chain variable fragments (scFvs) with far greater affinity than that of natural TCRs. We used high-resolution lattice lightsheet (LLS) and total internal reflection fluorescence (TIRF) imaging to visualize MV scanning in the context of variations in CAR design. This demonstrated that conventional CARs hyper-stabilized microvillar contacts relative to TCRs. Reducing receptor affinity, antigen density, and/or multiplicity of receptor binding sites normalized microvillar dynamics and synapse resolution, and effector functions improved with reduced affinity and/or antigen density, highlighting the importance of understanding the underlying cell biology when designing receptors for optimal antigen engagement.

The Function of Memory CD8+ T Cells in Immunotherapy for Human Diseases

Memory T (Tm) cells protect against Ags that they have previously contacted with a fast and robust response. Therefore, developing long-lived Tm cells is a prime goal for many vaccines and therapies to treat human diseases. The remarkable characteristics of Tm cells have led scientists and clinicians to devise methods to make Tm cells more useful. Recently, Tm cells have been highlighted for their role in coronavirus disease 2019 vaccines during the ongoing global pandemic. The importance of Tm cells in cancer has been emerging. However, the precise characteristics and functions of Tm cells in these diseases are not completely understood. In this review, we summarize the known characteristics of Tm cells and their implications in the development of vaccines and immunotherapies for human diseases. In addition, we propose to exploit the beneficial characteristics of Tm cells to develop strategies for effective vaccines and overcome the obstacles of immunotherapy.

Autoinhibitory structure of preligand association state implicates a new strategy to attain effective DR5 receptor activation

Members of the tumor necrosis factor receptor superfamily (TNFRSF) are important therapeutic targets that can be activated to induce death of cancer cells or stimulate proliferation of immune cells. Although it has long been implicated that these receptors assemble preligand associated states that are required for dominant interference in human disease, such states have so far eluded structural characterization. Here, we find that the ectodomain of death receptor 5 (DR5-ECD), a representative member of TNFRSF, can specifically self-associate when anchored to lipid bilayer, and we report this self-association structure determined by nuclear magnetic resonance (NMR). Unexpectedly, two non-overlapping interaction interfaces are identified that could propagate to higher-order clusters. Structure-guided mutagenesis indicates that the observed preligand association structure is represented on DR5-expressing cells. The DR5 preligand association serves an autoinhibitory role as single-domain antibodies (sdAbs) that partially dissociate the preligand cluster can sensitize the receptor to its ligand TRAIL and even induce substantial receptor signaling in the absence of TRAIL. Unlike most agonistic antibodies that require multivalent binding to aggregate receptors for activation, these agonistic sdAbs are monovalent and act specifically on an oligomeric, autoinhibitory configuration of the receptor. Our data indicate that receptors such as DR5 can form structurally defined preclusters incompatible with signaling and that true agonists should disrupt the preligand cluster while converting it to signaling-productive cluster. This mechanism enhances our understanding of a long-standing question in TNFRSF signaling and suggests a new opportunity for developing agonistic molecules by targeting receptor preligand clustering.

Joining Forces for Cancer Treatment: From "TCR versus CAR" to "TCR and CAR"

T cell-based immunotherapy has demonstrated great therapeutic potential in recent decades, on the one hand, by using tumor-infiltrating lymphocytes (TILs) and, on the other hand, by engineering T cells to obtain anti-tumor specificities through the introduction of either engineered T cell receptors (TCRs) or chimeric antigen receptors (CARs). Given the distinct design of both receptors and the type of antigen that is encountered, the requirements for proper antigen engagement and downstream signal transduction by TCRs and CARs differ. Synapse formation and signal transduction of CAR T cells, despite further refinement of CAR T cell designs, still do not fully recapitulate that of TCR T cells and might limit CAR T cell persistence and functionality. Thus, deep knowledge about the molecular differences in CAR and TCR T cell signaling would greatly advance the further optimization of CAR designs and elucidate under which circumstances a combination of both receptors would improve the functionality of T cells for cancer treatment. Herein, we provide a comprehensive review about similarities and differences by directly comparing the architecture, synapse formation and signaling of TCRs and CARs, highlighting the knowns and unknowns. In the second part of the review, we discuss the current status of combining CAR and TCR technologies, encouraging a change in perspective from "TCR versus CAR" to "TCR and CAR".

Recent advances in engineering nanotopographic substrates for cell studies

Cells sense their environment through the cell membrane receptors. Interaction with extracellular ligands induces receptor clustering at the nanoscale, assembly of the signaling complexes in the cytosol and activation of downstream signaling pathways, regulating cell response. Nanoclusters of receptors can be further organized hierarchically in the cell membrane at the meso- and micro-levels to exert different biological functions. To study and guide cell response, cell culture substrates have been engineered with features that can interact with the cells at different scales, eliciting controlled cell responses. In particular, nanoscale features of 1–100 nm in size allow direct interaction between the material and single cell receptors and their nanoclusters. Since the first “contact guidance” experiments on parallel microstructures, many other studies followed with increasing feature resolution and biological complexity. Here we present an overview of the advances in the field summarizing the biological scenario, substrate fabrication techniques and applications, highlighting the most recent developments.

The role of the different CD3γ domains in TCR expression and signaling

The CD3 subunits of the T-cell antigen receptor (TCR) play a central role in regulation of surface TCR expression levels. Humans who lack CD3γ (γ^—) show reduced surface TCR expression levels and abolished phorbol ester (PMA)-induced TCR down-regulation. The response to PMA is mediated by a double leucine motif in the intracellular (IC) domain of CD3γ. However, the molecular cause of the reduced TCR surface expression in γ^— lymphocytes is still not known. We used retroviral vectors carrying wild type CD3γ or CD3δ or the following chimeras (EC-extracellular, TM-transmembrane and IC): δ_ECγ_TMγ_IC (δγγ for short), γγδ, γδδ and γγ-. Expression of γγγ, γγδ, γδδ or γγ- in the γ^— T cell line JGN, which lacks surface TCR, demonstrated that cell surface TCR levels in JGN were dependent on the EC domain of CD3γ and could not be replaced by the one of CD3δ. In JGN and primary γ^— patient T cells, the tested chimeras confirmed that the response to PMA maps to the IC domain of CD3γ. Since protein homology explains these results better than domain structure, we conclude that CD3γ contributes conformational cues that improve surface TCR expression, likely at the assembly or membrane transport steps. In JGN cells all chimeric TCRs were signalling competent. However, an IC domain at CD3γ was required for TCR-induced IL-2 and TNF-α production and CD69 expression, indicating that a TCR without a CD3γ IC domain has altered signalling capabilities.

The Effect of the Nanoparticle Shape on T Cell Activation

The mechanism of extracellular ligand nano-geometry in ex vivo T cell activation for immunotherapy remains elusive. Herein, the authors demonstrate large aspect ratio (AR) of gold nanorods (AuNRs) conjugated on cell culture substrate enhancing both murine and human T cell activation through the nanoscale anisotropic presentation of stimulatory ligands (anti-CD3(αCD3) and anti-CD28(αCD28) antibodies). AuNRs with large AR bearing αCD3 and αCD28 antibodies significantly promote T cell expansion and key cytokine secretion including interleukin-2 (IL-2), interferon-gamma (IFN-γ), and tumor necrosis factor-alpha (TNF-α). High membrane tension observed in large AR AuNRs regulates actin filament and focal adhesion assembly and develops maturation-related morphological features in T cells such as membrane ruffle formation, cell spreading, and large T cell receptor (TCR) cluster formation. Anisotropic stimulatory ligand presentation promotes differentiation of naïve CD8⁺ T cells toward the effector phenotype inducing CD137 expression upon co-culture with human cervical carcinoma. The findings suggest the importance of manipulating extracellular ligand nano-geometry in optimizing T cell behaviors to enhance therapeutic outcomes.

Glycerol monolaurate inhibition of human B cell activation

Glycerol monolaurate (GML) is a naturally occurring antimicrobial agent used commercially in numerous products and food items. GML is also used as a homeopathic agent and is being clinically tested to treat several human diseases. In addition to its anti-microbial function, GML suppresses immune cell proliferation and inhibits primary human T cell activation. GML suppresses T cell activation by altering membrane dynamics and disrupting the formation of protein clusters necessary for intracellular signaling. The ability of GML to disrupt cellular membranes suggests it may alter other cell types. To explore this possibility, we tested how GML affects human B cells. We found that GML inhibits BCR-induced cytokine production, phosphorylation of signaling proteins, and protein clustering, while also changing cellular membrane dynamics and dysregulating cytoskeleton rearrangement. Although similar, there are also differences between how B cells and T cells respond to GML. These differences suggest that unique intrinsic features of a cell may result in differential responses to GML treatment. Overall, this study expands our understanding of how GML impacts the adaptive immune response and contributes to a broader knowledge of immune modulating monoglycerides.

A set point in the selection of the αβTCR T cell repertoire imposed by pre-TCR signaling strength

SignificanceThe ability of the T cell receptor (TCR) to convey signals of different intensity is essential for the generation of a diverse, protecting, and self-tolerant T cell repertoire. We provide evidence that pre-TCR signaling during the first stage of T cell differentiation, thought to only check for in-frame rearrangement of TCRβ gene segments, determines the degree of diversity in a signaling intensity-dependent manner and controls the diversity of the TCR repertoire available for subsequent thymic positive and negative selection. Pre-TCR signaling intensity is regulated by the transmembrane region of its associated CD3ζ chains, possibly by organizing pre-TCRs into nanoclusters. Our data provide insights into immune receptor signaling mechanisms and reveal an additional checkpoint of T cell repertoire diversity.

T-cell Receptor Is a Threshold Detector: Sub- and Supra-Threshold Stochastic Resonance in TCR-MHC Clusters on the Cell Surface

Stochastic resonance in clusters of major histocompatibility molecules is extended by a more detailed description of adaptive thresholding and by applying the notion of suprathreshold stochastic resonance as a stochastically quantizing encoder of transmembrane signaling downstream of major histocompatibility molecules and T-cell receptors on the side of presenting and recognizing cells, respectively. The adaptive nature of thresholding is partly explained by a mirroring of the noncognate–cognate dichotomy shown by the T-cell receptor structure and the kinetic-segregation model of the onset of T-cell receptor triggering. Membrane clusters of major histocompatibility molecules and T-cell receptors on their host cells are envisioned as places of the temporal encoding of downstream signals via the suprathreshold stochastic resonance process. The ways of optimization of molecular prostheses, such as chimeric antigen receptors against cancer in transmembrane signaling, are suggested in the framework of suprathreshold stochastic resonance. The analogy between Förster resonance energy transfer and suprathreshold stochastic resonance for information transfer is also discussed. The overlap integral for energy transfer parallels the mutual information transferred by suprathreshold stochastic resonance.

The T cell receptor displays lateral signal propagation involving non-engaged receptors

T cells are highly sensitive to low levels of antigen, but how this sensitivity is achieved is currently unknown. Here, we imaged proximal TCR-CD3 signal propagation with single molecule localization microscopy (SMLM) in T cells activated with nanoscale clusters of TCR stimuli. We observed the formation of large TCR-CD3 clusters that exceeded the area of the ligand clusters, and required multivalent interactions facilitated by TCR-CD3 phosphorylation for assembly. Within these clustered TCR-CD3 domains, TCR-CD3 signaling spread laterally for ∼500 nm, far beyond the activating site, via non-engaged receptors. Local receptor density determined the functional cooperativity between engaged and non-engaged receptors, but lateral signal propagation was not influenced by the genetic deletion of ZAP70. Taken together, our data demonstrates that clustered ligands induced the clustering of non-ligated TCR-CD3 into domains that cooperatively facilitate lateral signal propagation.

Engineering DNA Nanostructures to Manipulate Immune Receptor Signaling and Immune Cell Fates

Immune cells sense, communicate, and logically integrate a multitude of environmental signals to make important cell-fate decisions and fulfill their effector functions. These processes are initiated and regulated by a diverse array of immune receptors and via their dynamic spatiotemporal organization upon ligand binding. Given the widespread relevance of the immune system to health and disease, there have been significant efforts toward understanding the biophysical principles governing immune receptor signaling and activation, as well as the development of biomaterials which exploit these principles for therapeutic immune engineering. Here, how advances in the field of DNA nanotechnology constitute a growing toolbox for further pursuit of these endeavors is discussed. Key cellular players involved in the induction of immunity against pathogens or diseased cells are first summarized. How the ability to design DNA nanostructures with custom shapes, dynamics, and with site-specific incorporation of diverse guests can be leveraged to manipulate the signaling pathways that regulate these processes is then presented. It is followed by highlighting emerging applications of DNA nanotechnology at the crossroads of immune engineering, such as in vitro reconstitution platforms, vaccines, and adjuvant delivery systems. Finally, outstanding questions that remain for further advancing immune-modulatory DNA nanodevices are outlined.

The Chemokine Receptor CCR5 Links Memory CD4+ T Cell Metabolism to T Cell Antigen Receptor Nanoclustering

The inhibition of anabolic pathways, such as aerobic glycolysis, is a metabolic cornerstone of memory T cell differentiation and function. However, the signals that hamper these anabolic pathways are not completely known. Recent evidence pinpoints the chemokine receptor CCR5 as an important player in CD4⁺ T cell memory responses by regulating T cell antigen receptor (TCR) nanoclustering in an antigen-independent manner. This paper reports that CCR5 specifically restrains aerobic glycolysis in memory-like CD4⁺ T cells, but not in effector CD4⁺ T cells. CCR5-deficient memory CD4⁺ T cells thus show an abnormally high glycolytic/oxidative metabolism ratio. No CCR5-dependent change in glucose uptake nor in the expression of the main glucose transporters was detected in any of the examined cell types, although CCR5-deficient memory cells did show increased expression of the hexokinase 2 and pyruvate kinase M2 isoforms, plus the concomitant downregulation of Bcl-6, a transcriptional repressor of these key glycolytic enzymes. Further, the TCR nanoclustering defects observed in CCR5-deficient antigen-experienced CD4⁺ T cells were partially reversed by incubation with 2-deoxyglucose (2-DG), suggesting a link between inhibition of the glycolytic pathway and TCR nanoscopic organization. Indeed, the treatment of CCR5-deficient lymphoblasts with 2-DG enhanced IL-2 production after antigen re-stimulation. These results identify CCR5 as an important regulator of the metabolic fitness of memory CD4⁺ T cells, and reveal an unexpected link between T cell metabolism and TCR organization with potential influence on the response of memory T cells upon antigen re-encounter.

Cholesterol Binds in a Reversed Orientation to TCRβ-TM in Which Its OH Group is Localized to the Center of the Lipid Bilayer

T cell receptor (TCR) signaling in response to antigen recognition is essential for the adaptive immune response. Cholesterol keeps TCRs in the resting conformation and mediates TCR clustering by directly binding to the transmembrane domain of the TCRβ subunit (TCRβ-TM), while cholesterol sulfate (CS) displaces cholesterol from TCRβ. However, the atomic interaction of cholesterol or CS with TCRβ remains elusive. Here, we determined the cholesterol and CS binding site of TCRβ-TM in phospholipid bilayers using solution nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation. Cholesterol binds to the transmembrane residues within a CARC-like cholesterol recognition motif. Surprisingly, the polar OH group of cholesterol is placed in the hydrophobic center of the lipid bilayer stabilized by its polar interaction with K154 of TCRβ-TM. An aromatic interaction with Y158 and hydrophobic interactions with V160 and L161 stabilize this reverse orientation. CS binds to the same site, explaining how it competes with cholesterol. Site-directed mutagenesis of the CARC-like motif disrupted the cholesterol/CS binding to TCRβ-TM, validating the NMR and MD results.

K-Neighbourhood Analysis: A Method for Understanding SMLM Images as Compositions of Local Neighbourhoods

Single molecule localisation microscopy (SMLM) is a powerful tool that has revealed the spatial arrangement of cell surface signalling proteins, producing data of enormous complexity. The complexity is partly driven by the convolution of technical and biological signal components, and partly by the challenge of pooling information across many distinct cells. To address these two particular challenges, we have devised a novel algorithm called K-neighbourhood analysis (KNA), which emphasises the fact that each image can also be viewed as a composition of local neighbourhoods. KNA is based on a novel transformation, spatial neighbourhood principal component analysis (SNPCA), which is defined by the PCA of the normalised K-nearest neighbour vectors of a spatially random point pattern. Here, we use KNA to define a novel visualisation of individual images, to compare within and between groups of images and to investigate the preferential patterns of phosphorylation. This methodology is also highly flexible and can be used to augment existing clustering methods by providing clustering diagnostics as well as revealing substructure within microclusters. In summary, we have presented a highly flexible analysis tool that presents new conceptual possibilities in the analysis of SMLM images.

Constant regulation for stable CD8 T-cell functional avidity and its possible implications for cancer immunotherapy

The functional avidity (FA) of cytotoxic CD8 T cells impacts strongly on their functional capabilities and correlates with protection from infection and cancer. FA depends on TCR affinity, downstream signaling strength, and TCR affinity-independent parameters of the immune synapse, such as costimulatory and inhibitory receptors. The functional impact of coreceptors on FA remains to be fully elucidated. Despite its importance, FA is infrequently assessed and incompletely understood. There is currently no consensus as to whether FA can be enhanced by optimized vaccine dose or boosting schedule. Recent findings suggest that FA is remarkably stable in vivo, possibly due to continued signaling modulation of critical receptors in the immune synapse. In this review, we provide an overview of the current knowledge and hypothesize that in vivo, codominant T cells constantly "equalize" their FA for similar function. We present a new model of constant FA regulation, and discuss practical implications for T-cell-based cancer immunotherapy.

Potentiating the Antitumor Activity of Cytotoxic T Cells via the Transmembrane Domain of IGSF4 That Increases TCR Avidity

A robust T-cell response is an important component of sustained antitumor immunity. In this respect, the avidity of TCR in the antigen-targeting of tumors is crucial for the quality of the T-cell response. This study reports that the transmembrane (TM) domain of immunoglobulin superfamily member 4 (IGSF4) binds to the TM of the CD3 ζ-chain through an interaction between His177 and Asp36, which results in IGSF4-CD3 ζ dimers. IGSF4 also forms homo-dimers through the GxxVA motif in the TM domain, thereby constituting large TCR clusters. Overexpression of IGSF4 lacking the extracellular (IG4ΔEXT) domain potentiates the OTI CD8+ T cells to release IFN-γ and TNF-α and to kill OVA+-B16F10 melanoma cells. In animal models, IG4ΔEXT significantly reduces B16F10 tumor metastasis as well as tumor growth. Collectively, the results indicate that the TM domain of IGSF4 can regulate TCR avidity, and they further demonstrate that TCR avidity regulation is critical for improving the antitumor activity of cytotoxic T cells.

Direct Regulation of the T Cell Antigen Receptor's Activity by Cholesterol

Biological membranes consist of hundreds of different lipids that together with the embedded transmembrane (TM) proteins organize themselves into small nanodomains. In addition to this function of lipids, TM regions of proteins bind to lipids in a very specific manner, but the function of these TM region-lipid interactions is mostly unknown. In this review, we focus on the role of plasma membrane cholesterol, which directly binds to the αβ T cell antigen receptor (TCR), and has at least two opposing functions in αβ TCR activation. On the one hand, cholesterol binding to the TM domain of the TCRβ subunit keeps the TCR in an inactive, non-signaling conformation by stabilizing this conformation. This assures that the αβ T cell remains quiescent in the absence of antigenic peptide-MHC (the TCR's ligand) and decreases the sensitivity of the T cell toward stimulation. On the other hand, cholesterol binding to TCRβ leads to an increased formation of TCR nanoclusters, increasing the avidity of the TCRs toward the antigen, thus increasing the sensitivity of the αβ T cell. In mouse models, pharmacological increase of the cholesterol concentration in T cells caused an increase in TCR clustering, and thereby enhanced anti-tumor responses. In contrast, the γδ TCR does not bind to cholesterol and might be regulated in a different manner. The goal of this review is to put these seemingly controversial findings on the impact of cholesterol on the αβ TCR into perspective.

Resident Memory T Cells and Their Role within the Liver

Immunological memory is fundamental to maintain immunity against re-invading pathogens. It is the basis for prolonged protection induced by vaccines and can be mediated by humoral or cellular responses-the latter largely mediated by T cells. Memory T cells belong to different subsets with specialized functions and distributions within the body. They can be broadly separated into circulating memory cells, which pace the entire body through the lymphatics and blood, and tissue-resident memory T (T_RM) cells, which are constrained to peripheral tissues. Retained in the tissues where they form, T_RM cells provide a frontline defense against reinfection. Here, we review this population of cells with specific attention to the liver, where T_RM cells have been found to protect against infections, in particular those by Plasmodium species that cause malaria.

The Potential Contribution of Caveolin 1 to HIV Latent Infection

Combinatorial antiretroviral therapy (cART) suppresses HIV replication to undetectable levels and has been effective in prolonging the lives of HIV infected individuals. However, cART is not capable of eradicating HIV from infected individuals mainly due to HIV’s persistence in small reservoirs of latently infected resting cells. Latent infection occurs when the HIV-1 provirus becomes transcriptionally inactive and several mechanisms that contribute to the silencing of HIV transcription have been described. Despite these advances, latent infection remains a major hurdle to cure HIV infected individuals. Therefore, there is a need for more understanding of novel mechanisms that are associated with latent infection to purge HIV from infected individuals thoroughly. Caveolin 1(Cav-1) is a multifaceted functional protein expressed in many cell types. The expression of Cav-1 in lymphocytes has been controversial. Recent evidence, however, convincingly established the expression of Cav-1 in lymphocytes. In lieu of this finding, the current review examines the potential role of Cav-1 in HIV latent infection and provides a perspective that helps uncover new insights to understand HIV latent infection.

CCR5 deficiency impairs CD4+ T-cell memory responses and antigenic sensitivity through increased ceramide synthesis

CCR5 is not only a coreceptor for HIV-1 infection in CD4+ T cells, but also contributes to their functional fitness. Here, we show that by limiting transcription of specific ceramide synthases, CCR5 signaling reduces ceramide levels and thereby increases T-cell antigen receptor (TCR) nanoclustering in antigen-experienced mouse and human CD4+ T cells. This activity is CCR5-specific and independent of CCR5 co-stimulatory activity. CCR5-deficient mice showed reduced production of high-affinity class-switched antibodies, but only after antigen rechallenge, which implies an impaired memory CD4+ T-cell response. This study identifies a CCR5 function in the generation of CD4+ T-cell memory responses and establishes an antigen-independent mechanism that regulates TCR nanoclustering by altering specific lipid species.

CCR5 deficiency/CCR5Δ32: resistant to HIV infection at the cost of curtailed CD4+ T cell memory responses

Orchestrated trafficking and activation by pathogen-derived peptides define the ability of CD4+ T helper cells to contribute to an effective adaptive immunity. In this issue of The EMBO Journal, Martín-Leal et al show that the inflammatory chemokine receptor CCR5, well known for its role in cell migration and HIV infection, regulates ceramide synthesis and TCR nanoclustering to promote memory CD4+ T cell activation.

Noncanonical binding of Lck to CD3ε promotes TCR signaling and CAR function

Initiation of T cell antigen receptor (TCR) signaling involves phosphorylation of CD3 cytoplasmic tails by the tyrosine kinase Lck. How Lck is recruited to the TCR to initiate signaling is not well known. We report a previously unknown binding motif in the CD3ε cytoplasmic tail that interacts in a noncanonical mode with the Lck SH3 domain: the receptor kinase (RK) motif. The RK motif is accessible only upon TCR ligation, demonstrating how ligand binding leads to Lck recruitment. Binding of the Lck SH3 domain to the exposed RK motif resulted in local augmentation of Lck activity, CD3 phosphorylation, T cell activation and thymocyte development. Introducing the RK motif into a well-characterized 41BB-based chimeric antigen receptor enhanced its antitumor function in vitro and in vivo. Our findings underscore how a better understanding of the functioning of the TCR might promote rational improvement of chimeric antigen receptor design for the treatment of cancer.

Shigella impairs human T lymphocyte responsiveness by hijacking actin cytoskeleton dynamics and T cell receptor vesicular trafficking

Strategies employed by pathogenic enteric bacteria, such as Shigella, to subvert the host adaptive immunity are not well defined. Impairment of T lymphocyte chemotaxis by blockage of polarised edge formation has been reported upon Shigella infection. However, the functional impact of Shigella on T lymphocytes remains to be determined. Here, we show that Shigella modulates CD4+ T cell F‐actin dynamics and increases cell cortical stiffness. The scanning ability of T lymphocytes when encountering antigen‐presenting cells (APC) is subsequently impaired resulting in decreased cell–cell contacts (or conjugates) between the two cell types, as compared with non‐infected T cells. In addition, the few conjugates established between the invaded T cells and APCs display no polarised delivery and accumulation of the T cell receptor to the contact zone characterising canonical immunological synapses. This is most likely due to the targeting of intracellular vesicular trafficking by the bacterial type III secretion system (T3SS) effectors IpaJ and VirA. The collective impact of these cellular reshapings by Shigella eventually results in T cell activation dampening. Altogether, these results highlight the combined action of T3SS effectors leading to T cell defects upon Shigella infection.

T Cell Reprogramming Against Cancer

Advances in academic and clinical studies during the last several years have resulted in practical outcomes in adoptive immune therapy of cancer. Immune cells can be programmed with molecular modules that increase their therapeutic potency and specificity. It has become obvious that successful immunotherapy must take into account the full complexity of the immune system and, when possible, include the use of multifactor cell reprogramming that allows fast adjustment during the treatment. Today, practically all immune cells can be stably or transiently reprogrammed against cancer. Here, we review works related to T cell reprogramming, as the most developed field in immunotherapy. We discuss factors that determine the specific roles of αβ and γδ T cells in the immune system and the structure and function of T cell receptors in relation to other structures involved in T cell target recognition and immune response. We also discuss the aspects of T cell engineering, specifically the construction of synthetic T cell receptors (synTCRs) and chimeric antigen receptors (CARs) and the use of engineered T cells in integrative multifactor therapy of cancer.

Cholesterol membrane content has a ubiquitous evolutionary function in immune cell activation: the role of HDL

PURPOSE OF REVIEW

Cellular cholesterol content influences the structure and function of lipid rafts, plasma membrane microdomains essential for cell signaling and activation. HDL modulate cellular cholesterol efflux, thus limiting cholesterol accumulation and controlling immune cell activation. Aim of this review is to discuss the link between HDL and cellular cholesterol metabolism in immune cells and the therapeutic potential of targeting cholesterol removal from cell membranes.

RECENT FINDINGS

The inverse relationship between HDL-cholesterol (HDL-C) levels and the risk of cardiovascular disease has been recently challenged by observations linking elevated levels of HDL-C with increased risk of all-cause mortality, infections and autoimmune diseases, paralleled by the failure of clinical trials with HDL-C-raising therapies. These findings suggest that improving HDL function might be more important than merely raising HDL-C levels. New approaches aimed at increasing the ability of HDL to remove cellular cholesterol have been assessed for their effect on immune cells, and the results have suggested that this could be a new effective approach.

SUMMARY

Cholesterol removal from plasma membrane by different means affects the activity of immune cells, suggesting that approaches aimed at increasing the ability of HDL to mobilize cholesterol from cells would represent the next step in HDL biology.

Molecular mechanisms of T cell sensitivity to antigen

T cells initiate and regulate adaptive immune responses that can clear infections. To do this, they use their T cell receptors (TCRs) to continually scan the surfaces of other cells for cognate peptide antigens presented on major histocompatibility complexes (pMHCs). Experimental work has established that as few 1-10 pMHCs are sufficient to activate T cells. This sensitivity is remarkable in light of a number of factors, including the observation that the TCR and pMHC are short molecules relative to highly abundant long surface molecules, such as CD45, that can hinder initial binding, and moreover, the TCR/pMHC interaction is of weak affinity with solution lifetimes of approximately 1 second. Here, we review experimental and mathematical work that has contributed to uncovering molecular mechanisms of T cell sensitivity. We organize the mechanisms by where they act in the pathway to activate T cells, namely mechanisms that (a) promote TCR/pMHC binding, (b) induce rapid TCR signaling, and (c) amplify TCR signaling. We discuss work showing that high sensitivity reduces antigen specificity unless molecular feedbacks are invoked. We conclude by summarizing a number of open questions.

Synthetic TRuC receptors engaging the complete T cell receptor for potent anti-tumor response

T cells expressing CD19-targeting chimeric antigen receptors (CARs) reveal high efficacy in the treatment of B cell malignancies. Here, we report that T cell receptor fusion constructs (TRuCs) comprising an antibody-based binding domain fused to T cell receptor (TCR) subunits can effectively reprogram an intact TCR complex to recognize tumor surface antigens. Unlike CARs, TRuCs become a functional component of the TCR complex. TRuC-T cells kill tumor cells as potently as second-generation CAR-T cells, but at significant lower cytokine release and despite the absence of an extra co-stimulatory domain. TRuC-T cells demonstrate potent anti-tumor activity in both liquid and solid tumor xenograft models. In several models, TRuC-T cells are more efficacious than respective CAR-T cells. TRuC-T cells are shown to engage the signaling capacity of the entire TCR complex in an HLA-independent manner.

Mechanisms of polarized cell-cell communication of T lymphocytes

Cell-cell communication comprises a variety of molecular mechanisms that immune cells use to respond appropriately to diverse pathogenic stimuli. T lymphocytes polarize in response to different stimuli, such as cytokines, adhesion to specific ligands and cognate antigens presented in the context of MHC. Polarization takes different shapes, from migratory front-back polarization to the formation of immune synapses (IS). The formation of IS between a T cell and an antigen-presenting cell involves early events of receptor-ligand interaction leading to the reorganization of the plasma membrane and the cytoskeleton to orchestrate vesicular and endosomal traffic and directed secretion of several types of mediators, including cytokines and nanovesicles. Cell polarization involves the repositioning of many subcellular organelles, including the endosomal compartment, which becomes an effective platform for the shuttling of molecules as vesicular cargoes that lately will be secreted to transfer information to antigen-presenting cells. Overall, the polarized interaction between a T cell and APC modifies the recipient cell in different ways that are likely lineage-dependent, e.g. dendritic cells, B cells or even other T cells. In this review, we will discuss the mechanisms that mediate the polarization of different membrane receptors, cytoskeletal components and organelles in T cells in a variety of immune contexts.

Dual Role of CD4 in Peripheral T Lymphocytes

The interaction of T-cell receptors (TCRs) with self- and non-self-peptides in the major histocompatibility complex (MHC) stimulates crucial signaling events, which in turn can activate T lymphocytes. A variety of accessory molecules further modulate T-cell signaling. Of these, the CD4 and CD8 coreceptors make the most critical contributions to T cell sensitivity in vivo. Whereas, CD4 function in T cell development is well-characterized, its role in peripheral T cells remains incompletely understood. It was originally suggested that CD4 stabilizes weak interactions between TCRs and peptides in the MHC and delivers Lck kinases to that complex. The results of numerous experiments support the latter role, indicating that the CD4-Lck complex accelerates TCR-triggered signaling and controls the availability of the kinase for TCR in the absence of the ligand. On the other hand, extremely low affinity of CD4 for MHC rules out its ability to stabilize the receptor-ligand complex. In this review, we summarize the current knowledge on CD4 in T cells, with a special emphasis on the spatio-temporal organization of early signaling events and the relevance for CD4 function. We further highlight the capacity of CD4 to interact with the MHC in the absence of TCR. It drives the adhesion of T cells to the cells that express the MHC. This process is facilitated by the CD4 accumulation in the tips of microvilli on the surface of unstimulated T cells. Based on these observations, we suggest an alternative model of CD4 role in T-cell activation.

Cellular Metabolism Is a Major Determinant of HIV-1 Reservoir Seeding in CD4+ T Cells and Offers an Opportunity to Tackle Infection

HIV persists in long-lived infected cells that are not affected by antiretroviral treatment. These HIV reservoirs are mainly located in CD4⁺ T cells, but their distribution is variable in the different subsets. Susceptibility to HIV-1 increases with CD4⁺ T cell differentiation. We evaluated whether the metabolic programming that supports the differentiation and function of CD4⁺ T cells affected their susceptibility to HIV-1. We found that differences in HIV-1 susceptibility between naive and more differentiated subsets were associated with the metabolic activity of the cells. Indeed, HIV-1 selectively infected CD4⁺ T cells with high oxidative phosphorylation and glycolysis, independent of their activation phenotype. Moreover, partial inhibition of glycolysis (1) impaired HIV-1 infection in vitro in all CD4⁺ T cell subsets, (2) decreased the viability of preinfected cells, and (3) precluded HIV-1 amplification in cells from HIV-infected individuals. Our results elucidate the link between cell metabolism and HIV-1 infection and identify a vulnerability in tackling HIV reservoirs.

How does T cell receptor clustering impact on signal transduction?

The essential function of the T cell receptor (TCR) is to translate the engagement of peptides on the major histocompatibility complex (pMHC) into appropriate intracellular signals through the associated cluster of differentiation 3 (CD3) complex. The spatial organization of the TCR–CD3 complex in the membrane is thought to be a key regulatory element of signal transduction, raising the question of how receptor clustering impacts on TCR triggering. How signal transduction at the TCR–CD3 complex encodes the quality and quantity of pMHC molecules is not fully understood. This question can be approached by reconstituting T cell signaling in model and cell membranes and addressed by single-molecule imaging of endogenous proteins in T cells. We highlight such methods and further discuss how TCR clustering could affect pMHC rebinding rates, the local balance between kinase and phosphatase activity and/or the lipid environment to regulate the signal efficiency of the TCR–CD3 complex. We also examine whether clustering could affect the conformation of cytoplasmic CD3 tails through a biophysical mechanism. Taken together, we highlight how the spatial organization of the TCR–CD3 complex – addressed by reconstitution approaches – has emerged as a key regulatory element in signal transduction of this archetypal immune receptor.

What we talk about when we talk about nanoclusters

Superresolution microscopy results have sparked the idea that many membrane proteins are not randomly distributed across the plasma membrane but are instead arranged in nanoclusters. Frequently, these new results seemed to confirm older data based on biochemical and electron microscopy experiments. Recently, however, it was recognized that multiple countings of the very same fluorescently labeled protein molecule can be easily confused with true protein clusters. Various strategies have been developed, which are intended to solve the problem of discriminating true protein clusters from imaging artifacts. We believe that there is currently no perfect algorithm for this problem; instead, different approaches have different strengths and weaknesses. In this review, we discuss single molecule localization microscopy in view of its ability to detect nanoclusters of membrane proteins. To capture the different views on nanoclustering, we chose an unconventional style for this article: we placed its scientific content in the setting of a fictive conference, where five researchers from different fields discuss the problem of detecting and quantifying nanoclusters. Using this style, we feel that the different approaches common for different research areas can be well illustrated. Similarities to a short story by Raymond Carver are not unintentional.

Membrane Ultrastructure and T Cell Activation

The immune system serves as a crucial line of defense from infection and cancer, while also contributing to tissue homeostasis. Communication between immune cells is mediated by small soluble factors called cytokines, and also by direct cellular interactions. Cell-cell interactions are particularly important for T cell activation. T cells direct the adaptive immune response and therefore need to distinguish between self and foreign antigens. Even though decades have passed since the discovery of T cells, exactly why and how they are able to recognize and discriminate between antigens is still not fully understood. Early imaging of T cells was very successful in capturing the early stages of conjugate formation of T cells with antigen-presenting cells upon recognition of peptide-loaded major histocompatibility complexes by the T cell receptor (TCR). These studies lead to the discovery of a “supramolecular activation cluster” now known as the immunological synapse, followed by the identification of microclusters of TCRs formed upon receptor triggering, that eventually coalesce at the center of the synapse. New developments in light microscopy have since allowed attention to turn to the very earliest stages of T cell activation, and to resting cells, at high resolution. This includes single-molecule localization microscopy, which has been applied to the question of whether TCRs are pre-clustered on resting T cells, and lattice light-sheet microscopy that has enabled imaging of whole cells interacting with antigen-presenting cells. The utilization of lattice light-sheet microscopy has yielded important insights into structures called microvilli, which are small membrane protrusions on T cells that seem likely to have a large impact on T cell recognition and activation. Here we consider how imaging has shaped our thinking about T cell activation. We summarize recent findings obtained by applying more advanced microscopy techniques and discuss some of the limitations of these methods.

TCRs are randomly distributed on the plasma membrane of resting antigen-experienced T cells

The main function of T cells is to identify harmful antigens as quickly and precisely as possible. Super-resolution microscopy data have indicated that global clustering of T cell antigen receptors (TCRs) occurs before T cell activation. Such pre-activation clustering has been interpreted as representing a potential regulatory mechanism that fine tunes the T cell response. We found here that apparent TCR nanoclustering could be attributed to overcounting artifacts inherent to single-molecule-localization microscopy. Using complementary super-resolution approaches and statistical image analysis, we found no indication of global nanoclustering of TCRs on antigen-experienced CD4⁺ T cells under non-activating conditions. We also used extensive simulations of super-resolution images to provide quantitative limits for the degree of randomness of the TCR distribution. Together our results suggest that the distribution of TCRs on the plasma membrane is optimized for fast recognition of antigen in the first phase of T cell activation.

A window of opportunity for cooperativity in the T Cell Receptor

The T-cell antigen receptor (TCR) is pre-organised in oligomers, known as nanoclusters. Nanoclusters could provide a framework for inter-TCR cooperativity upon peptide antigen-major histocompatibility complex (pMHC) binding. Here we have used soluble pMHC oligomers in search for cooperativity effects along the plasma membrane plane. We find that initial binding events favour subsequent pMHC binding to additional TCRs, during a narrow temporal window. This behaviour can be explained by a 3-state model of TCR transition from Resting to Active, to a final Inhibited state. By disrupting nanoclusters and hampering the Active conformation, we show that TCR cooperativity is consistent with TCR nanoclusters adopting the Active state in a coordinated manner. Preferential binding of pMHC to the Active TCR at the immunological synapse suggests that there is a transient time frame for signal amplification in the TCR, allowing the T cells to keep track of antigen quantity and binding time.

T cells can be activated by a small, two-digit, number of antigen peptide molecules even though the receptor for antigen (TCR) is of low affinity. Here the authors present evidence that all TCRs within a nanocluster can become activated when only a subset is bound to antigen.

Monomeric TCRs drive T cell antigen recognition

T cell antigen recognition requires T cell antigen receptors (TCRs) engaging MHC-embedded antigenic peptides (pMHCs) within the contact region of a T cell with its conjugated antigen-presenting cell. Despite micromolar TCR:pMHC affinities, T cells respond to even a single antigenic pMHC, and higher-order TCRs have been postulated to maintain high antigen sensitivity and trigger signaling. We interrogated the stoichiometry of TCRs and their associated CD3 subunits on the surface of living T cells through single-molecule brightness and single-molecule coincidence analysis, photon-antibunching-based fluorescence correlation spectroscopy and Förster resonance energy transfer measurements. We found exclusively monomeric TCR-CD3 complexes driving the recognition of antigenic pMHCs, which underscores the exceptional capacity of single TCR-CD3 complexes to elicit robust intracellular signaling.

Immune monitoring and TCR sequencing of CD4 T cells in a long term responsive patient with metastasized pancreatic ductal carcinoma treated with individualized, neoepitope-derived multipeptide vaccines: a case report

BACKGROUND

Cancer vaccines can effectively establish clinically relevant tumor immunity. Novel sequencing approaches rapidly identify the mutational fingerprint of tumors, thus allowing to generate personalized tumor vaccines within a few weeks from diagnosis. Here, we report the case of a 62-year-old patient receiving a four-peptide-vaccine targeting the two sole mutations of his pancreatic tumor, identified via exome sequencing.

METHODS

Vaccination started during chemotherapy in second complete remission and continued monthly thereafter. We tracked IFN-γ⁺ T cell responses against vaccine peptides in peripheral blood after 12, 17 and 34 vaccinations by analyzing T-cell receptor (TCR) repertoire diversity and epitope-binding regions of peptide-reactive T-cell lines and clones. By restricting analysis to sorted IFN-γ-producing T cells we could assure epitope-specificity, functionality, and T_H1 polarization.

RESULTS

A peptide-specific T-cell response against three of the four vaccine peptides could be detected sequentially. Molecular TCR analysis revealed a broad vaccine-reactive TCR repertoire with clones of discernible specificity. Four identical or convergent TCR sequences could be identified at more than one time-point, indicating timely persistence of vaccine-reactive T cells. One dominant TCR expressing a dual TCRVα chain could be found in three T-cell clones. The observed T-cell responses possibly contributed to clinical outcome: The patient is alive 6 years after initial diagnosis and in complete remission for 4 years now.

CONCLUSIONS

Therapeutic vaccination with a neoantigen-derived four-peptide vaccine resulted in a diverse and long-lasting immune response against these targets which was associated with prolonged clinical remission. These data warrant confirmation in a larger proof-of concept clinical trial.

Desensitized chimeric antigen receptor T cells selectively recognize target cells with enhanced antigen expression

Chimeric antigen receptor (CAR) T cell therapy is an effective method for treating specific cancers. CARs are normally designed to recognize antigens, which are highly expressed on malignant cells but not on T cells. However, when T cells are engineered with CARs that recognize antigens expressed on the T cell surface, CAR T cells exhibit effector function on other T cells, which results in fratricide, or killing of neighboring T cells. Here, using human leukocyte antigen-DR (HLA-DR)-targeted CAR T cells, we show that weak affinity between CAR and HLA-DR reduces fratricide and induces sustained CAR downregulation, which consequently tunes the avidity of CAR T cells, leading to desensitization. We further demonstrate that desensitized CAR T cells selectively kill Epstein-Barr virus-transformed B cells with enhanced HLA-DR expression, while sparing normal B cells. Our study supports an avidity-tuning strategy that permits sensing of antigen levels by CAR T cells.

Engineered T cells with chimeric antigen receptor (CAR) are emerging as an effective cancer therapy. Here the authors show that CAR T cells recognizing self-MHC can be ‘tuned’ ex vivo via CAR downregulation and CAR T cell death to generate a CAR T pool specifically targeting tumor cells with high MHC expression.