ITAM-mediated signaling by the T-cell antigen receptor

The Application of CAR-T Cells in Haematological Malignancies

Chimeric antigen receptor (CAR)-T cells (CART) remain one of the most advanced and promising forms of adoptive T-cell immunotherapy. CART represent autologous, genetically engineered T lymphocytes expressing CAR, i.e. fusion proteins that combine components and features of T cells as well as antibodies providing their more effective and direct anti-tumour effect. The technology of CART construction is highly advanced in vitro and every element of their structure influence their mechanism of action in vivo. Patients with haematological malignancies are faced with the possibility of disease relapse after the implementation of conventional chemo-immunotherapy. Since the most preferable result of therapy is a partial or complete remission, cancer treatment regimens are constantly being improved and customized to individual patients. This individualization could be ensured by CART therapy. This paper characterized CART strategy in details in terms of their structure, generations, mechanism of action and published the results of clinical trials in haematological malignancies including acute lymphoblastic leukaemia, diffuse large B-cell lymphoma, chronic lymphocytic leukaemia and multiple myeloma.

An allosteric hot spot in the tandem-SH2 domain of ZAP-70 regulates T-cell signaling

T-cell receptor (TCR) signaling is initiated by recruiting ZAP-70 to the cytosolic part of TCR. ZAP-70, a non-receptor tyrosine kinase, is composed of an N-terminal tandem SH2 (tSH2) domain connected to the C-terminal kinase domain. The ZAP-70 is recruited to the membrane through binding of tSH2 domain and the doubly phosphorylated ITAM motifs of CD3 chains in the TCR complex. Our results show that the tSH2 domain undergoes a biphasic structural transition while binding to the doubly phosphorylated ITAM-ζ1 peptide. The C-terminal SH2 domain binds first to the phosphotyrosine residue of ITAM peptide to form an encounter complex leading to subsequent binding of second phosphotyrosine residue to the N-SH2 domain. We decipher a network of noncovalent interactions that allosterically couple the two SH2 domains during binding to doubly phosphorylated ITAMs. Mutation in the allosteric network residues, for example, W165C, uncouples the formation of encounter complex to the subsequent ITAM binding thus explaining the altered recruitment of ZAP-70 to the plasma membrane causing autoimmune arthritis in mice. The proposed mechanism of allosteric coupling is unique to ZAP-70, which is fundamentally different from Syk, a close homolog of ZAP-70 expressed in B-cells.

The Role of Insulin Regulated Aminopeptidase in Endocytic Trafficking and Receptor Signaling in Immune Cells

Insulin regulated aminopeptidase (IRAP) is a type II transmembrane protein with broad tissue distribution initially identified as a major component of Glut4 storage vesicles (GSV) in adipocytes. Despite its almost ubiquitous expression, IRAP had been extensively studied mainly in insulin responsive cells, such as adipocytes and muscle cells. In these cells, the enzyme displays a complex intracellular trafficking pattern regulated by insulin. Early studies using fusion proteins joining the IRAP cytosolic domain to various reporter proteins, such as GFP or the transferrin receptor (TfR), showed that the complex and regulated trafficking of the protein depends on its cytosolic domain. This domain contains several motifs involved in IRAP trafficking, as demonstrated by mutagenesis studies. Also, proteomic studies and yeast two-hybrid experiments showed that the IRAP cytosolic domain engages in multiple protein interactions with cytoskeleton components and vesicular trafficking adaptors. These findings led to the hypothesis that IRAP is not only a cargo of GSV but might be a part of the sorting machinery that controls GSV dynamics. Recent work in adipocytes, immune cells, and neurons confirmed this hypothesis and demonstrated that IRAP has a dual function. Its carboxy-terminal domain located inside endosomes is responsible for the aminopeptidase activity of the enzyme, while its amino-terminal domain located in the cytosol functions as an endosomal trafficking adaptor. In this review, we recapitulate the published protein interactions of IRAP and summarize the increasing body of evidence indicating that IRAP plays a role in intracellular trafficking of several proteins. We describe the impact of IRAP deletion or depletion on endocytic trafficking and the consequences on immune cell functions. These include the ability of dendritic cells to cross-present antigens and prime adaptive immune responses, as well as the control of innate and adaptive immune receptor signaling and modulation of inflammatory responses.

T Cell Activation Machinery: Form and Function in Natural and Engineered Immune Receptors

The impressive success of chimeric antigen receptor (CAR)-T cell therapies in treating advanced B-cell malignancies has spurred a frenzy of activity aimed at developing CAR-T therapies for other cancers, particularly solid tumors, and optimizing engineered T cells for maximum clinical benefit in many different disease contexts. A rapidly growing body of design work is examining every modular component of traditional single-chain CARs as well as expanding out into many new and innovative engineered immunoreceptor designs that depart from this template. New approaches to immune cell and receptor engineering are being reported with rapidly increasing frequency, and many recent high-quality reviews (including one in this special issue) provide comprehensive coverage of the history and current state of the art in CAR-T and related cellular immunotherapies. In this review, we step back to examine our current understanding of the structure-function relationships in natural and engineered lymphocyte-activating receptors, with an eye towards evaluating how well the current-generation CAR designs recapitulate the most desirable features of their natural counterparts. We identify key areas that we believe are under-studied and therefore represent opportunities to further improve our grasp of form and function in natural and engineered receptors and to rationally design better therapeutics.

Targeting NF-κB pathway for the therapy of diseases: mechanism and clinical study

NF-κB pathway consists of canonical and non-canonical pathways. The canonical NF-κB is activated by various stimuli, transducing a quick but transient transcriptional activity, to regulate the expression of various proinflammatory genes and also serve as the critical mediator for inflammatory response. Meanwhile, the activation of the non-canonical NF-κB pathway occurs through a handful of TNF receptor superfamily members. Since the activation of this pathway involves protein synthesis, the kinetics of non-canonical NF-κB activation is slow but persistent, in concordance with its biological functions in the development of immune cell and lymphoid organ, immune homeostasis and immune response. The activation of the canonical and non-canonical NF-κB pathway is tightly controlled, highlighting the vital roles of ubiquitination in these pathways. Emerging studies indicate that dysregulated NF-κB activity causes inflammation-related diseases as well as cancers, and NF-κB has been long proposed as the potential target for therapy of diseases. This review attempts to summarize our current knowledge and updates on the mechanisms of NF-κB pathway regulation and the potential therapeutic application of inhibition of NF-κB signaling in cancer and inflammatory diseases.

Escape of tumor cells from the NK cell cytotoxic activity

In recent years, NK cells, initially identified as potent cytotoxic effector cells, have revealed an unexpected complexity, both at phenotypic and functional levels. The discovery of different NK cell subsets, characterized by distinct gene expression and phenotypes, was combined with the characterization of the diverse functions NK cells can exert, not only as circulating cells, but also as cells localized or recruited in lymphoid organs and in multiple tissues. Besides the elimination of tumor and virus-infected cells, these functions include the production of cytokines and chemokines, the regulation of innate and adaptive immune cells, the influence on tissue homeostasis. In addition, NK cells display a remarkable functional plasticity, being able to adapt to the environment and to develop a kind of memory. Nevertheless, the powerful cytotoxic activity of NK cells remains one of their most relevant properties, particularly in the antitumor response. In this review, the process of tumor cell recognition and killing mediated by NK cells, starting from the generation of cytolytic granules and recognition of target cell, to the establishment of the NK cell immunological synapse, the release of cytotoxic molecules, and consequent tumor cell death is described. Next, the review focuses on the heterogeneous mechanisms, either intrinsic to tumors or induced by the tumor microenvironment, by which cancer cells can escape the NK cell-mediated attack.

CAR-T design: Elements and their synergistic function

Chimeric antigen receptor (CAR) T cells use re-engineered cell surface receptors to specifically bind to and lyse oncogenic cells. Two clinically approved CAR-T–cell therapies have significant clinical efficacy in treating CD19-positive B cell cancers. With widespread interest to deploy this immunotherapy to other cancers, there has been great research activity to design new CAR structures to increase the range of targeted cancers and anti-tumor efficacy. However, several obstacles must be addressed before CAR-T–cell therapies can be more widely deployed. These include limiting the frequency of lethal cytokine storms, enhancing T-cell persistence and signaling, and improving target antigen specificity. We provide a comprehensive review of recent research on CAR design and systematically evaluate design aspects of the four major modules of CAR structure: the ligand-binding, spacer, transmembrane, and cytoplasmic domains, elucidating design strategies and principles to guide future immunotherapeutic discovery.

Multiple Signaling Roles of CD3ε and Its Application in CAR-T Cell Therapy

A T cell receptor (TCR) mediates antigen-induced signaling through its associated CD3ε, δ, γ, and ζ, but the contributions of different CD3 chains remain elusive. Using quantitative mass spectrometry, we simultaneously quantitated the phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of all CD3 chains upon TCR stimulation. A subpopulation of CD3ε ITAMs was mono-phosphorylated, owing to Lck kinase selectivity, and specifically recruited the inhibitory Csk kinase to attenuate TCR signaling, suggesting that TCR is a self-restrained signaling machinery containing both activating and inhibitory motifs. Moreover, we found that incorporation of the CD3ε cytoplasmic domain into a second-generation chimeric antigen receptor (CAR) improved antitumor activity of CAR-T cells. Mechanistically, the Csk-recruiting ITAM of CD3ε reduced CAR-T cytokine production whereas the basic residue rich sequence (BRS) of CD3ε promoted CAR-T persistence via p85 recruitment. Collectively, CD3ε is a built-in multifunctional signal tuner, and increasing CD3 diversity represents a strategy to design next-generation CAR.

The Great War of Today: Modifications of CAR-T Cells to Effectively Combat Malignancies

Immunotherapy of cancer had its early beginnings in the times when the elements of the immune system were still poorly characterized. However, with the progress in molecular biology, it has become feasible to re-engineer T cells in order to eradicate tumour cells. The use of synthetic chimeric antigen receptors (CARs) helped to re-target and simultaneously unleash the cytotoxic potential of T cells. CAR-T therapy proved to be remarkably effective in cases of haematological malignancies, often refractory and relapsed. The success of this approach yielded two Food and Drug Administration (FDA) approvals for the first "living drug" modalities. However, CAR-T therapy is not without flaws. Apart from the side effects associated with the treatment, it became apparent that CAR introduction alters T cell biology and the possible therapeutic outcomes. Additionally, it was shown that CAR-T approaches in solid tumours do not recapitulate the success in the haemato-oncology. Therefore, in this review, we aim to discuss the recent concerns of CAR-T therapy for both haematological and solid tumours. We also summarise the general strategies that are implemented to enhance the efficacy and safety of the CAR-T regimens in blood and solid malignancies.

Engineering Cytoplasmic Signaling of CD28ζ CARs for Improved Therapeutic Functions

Chimeric antigen receptor modified T cells (CAR-T) have yielded impressive clinical outcomes in treating hematopoietic malignancies. However, relapses have occurred in a substantial number of patients and limited the development of CAR-T therapy. Most underlying reasons for these relapses can be attributed to poor persistence and rapid exhaustion of CAR-T cells in vivo. Despite multiple strategies having been developed, how to improve CAR-T persistence or resist exhaustion while maintaining sufficient cytotoxic functions is still a great challenge. Here we discuss engineering cytoplasmic signaling as an important strategy for CAR optimization. This review summarizes recent advances showing that the anti-tumor function of CAR-T cells can be improved by optimizing the CD3ζ domain or downstream signaling of CD28ζ CAR.

IRAP-dependent endosomal T cell receptor signalling is essential for T cell responses

T cell receptor (TCR) activation is modulated by mechanisms such as TCR endocytosis, which is thought to terminate TCR signalling. Here we show that, upon internalization, TCR continues to signal from a set of specialized endosomes that are crucial for T cell functions. Mechanistically, TCR ligation leads to clathrin-mediated internalization of the TCR-CD3ζ complex, while maintaining CD3ζ signalling, in endosomal vesicles that contain the insulin responsive aminopeptidase (IRAP) and the SNARE protein Syntaxin 6. Destabilization of this compartment through IRAP deletion enhances plasma membrane expression of the TCR-CD3ζ complex, yet compromises overall CD3ζ signalling; moreover, the integrity of this compartment is also crucial for T cell activation and survival after suboptimal TCR activation, as mice engineered with a T cell-specific deletion of IRAP fail to develop efficient polyclonal anti-tumour responses. Our results thus reveal a previously unappreciated function of IRAP-dependent endosomal TCR signalling in T cell activation.

T cell receptors (TCR) are internalized when activated by their ligands. Here the authors show that the internalized TCRs are localized to endosomes expressing IRAP and Syntaxin 6 to maintain intracellular signalling capacity, whose importance is shown by the absence of efficient polyclonal anti-tumour response in mice with T-specific conditional deletion of IRAP.

Understanding the Dynamics of T-Cell Activation in Health and Disease Through the Lens of Computational Modeling

T cells in the immune system are activated by binding to foreign peptides (from an external pathogen) or mutant peptide (derived from endogenous proteins) displayed on the surface of a diseased cell. This triggers a series of intracellular signaling pathways, which ultimately dictate the response of the T cell. The insights from computational models have greatly improved our understanding of the mechanisms that control T-cell activation. In this review, we focus on the use of ordinary differential equation–based mechanistic models to study T-cell activation. We highlight several examples that demonstrate the models’ utility in answering specific questions related to T-cell activation signaling, from antigen discrimination to the feedback mechanisms that initiate transcription factor activation. In addition, we describe other modeling approaches that can be combined with mechanistic models to bridge time scales and better understand how intracellular signaling events, which occur on the order of seconds to minutes, influence phenotypic responses of T-cell activation, which occur on the order of hours to days. Overall, through concrete examples, we emphasize how computational modeling can be used to enable the rational design and optimization of immunotherapies.

Chimeric antigen receptor signaling: Functional consequences and design implications

Chimeric antigen receptor (CAR) T cell therapy has transformed the care of refractory B cell malignancies and holds tremendous promise for many aggressive tumors. Despite overwhelming scientific, clinical, and public interest in this rapidly expanding field, fundamental inquiries into CAR T cell mechanistic functioning are still in their infancy. Because CAR T cells are manufactured from donor T lymphocytes, and because CARs incorporate well-characterized T cell signaling components, it has largely been assumed that CARs signal analogously to canonical T cell receptors (TCRs). However, recent studies demonstrate that many aspects of CAR signaling are unique, distinct from endogenous TCR signaling, and potentially even distinct among various CAR constructs. Thus, rigorous and comprehensive proteomic investigations are required for rational engineering of improved CARs. Here, we review what is known about proximal CAR signaling in T cells, compare it to conventional TCR signaling, and outline unmet challenges to improving CAR T cell therapy.

Exploring the controversial role of PI3K signalling in CD4+ regulatory T (T-Reg) cells

The immune system is a complex network that acts to protect vertebrates from foreign microorganisms and carries out immunosurveillance to combat cancer. In order to avoid hyper-activation of the immune system leading to collateral damage tissues and organs and to prevent self-attack, the network has the intrinsic control mechanisms that negatively regulate immune responses. Central to this negative regulation are regulatory T (T-Reg) cells, which through cytokine secretion and cell interaction limit uncontrolled clonal expansion and functions of activated immune cells. Given that positive or negative manipulation of T-Regs activity could be utilised to therapeutically treat host versus graft rejection or cancer respectively, understanding how signaling pathways impact on T-Regs function should reveal potential targets with which to intervene. The phosphatidylinositol-3-kinase (PI3K) pathway controls a vast array of cellular processes and is critical in T cell activation. Here we focus on phosphoinositide 3-kinases (PI3Ks) and their ability to regulate T-Regs cell differentiation and function.

Small molecule AX-024 reduces T cell proliferation independently of CD3ϵ/Nck1 interaction, which is governed by a domain swap in the Nck1-SH3.1 domain

Activation of the T cell receptor (TCR) results in binding of the adapter protein Nck (noncatalytic region of tyrosine kinase) to the CD3ϵ subunit of the TCR. The interaction was suggested to be important for the amplification of TCR signals and is governed by a proline-rich sequence (PRS) in CD3ϵ that binds to the first Src homology 3 (SH3) domain of Nck (Nck-SH3.1). Inhibition of this protein/protein interaction ameliorated inflammatory symptoms in mouse models of multiple sclerosis, psoriasis, and asthma. A small molecule, AX-024, was reported to inhibit the Nck/CD3ϵ interaction by physically binding to the Nck1-SH3.1 domain, suggesting a route to develop an inhibitor of the Nck1/CD3ϵ interaction for modulating TCR activity in autoimmune and inflammatory diseases. We show here that AX-024 reduces T cell proliferation upon weak TCR stimulation but does not significantly affect phosphorylation of Zap70 (ζ chain of T cell receptor–associated protein kinase 70). We also find that AX-024 is likely not involved in modulating the Nck/TCR interaction but probably has other targets in T cells. An array of biophysical techniques did not detect a direct interaction between AX-024 and Nck-SH3.1 in vitro. Crystal structures of the Nck-SH3.1 domain revealed its binding mode to the PRS in CD3ϵ. The SH3 domain tends to generate homodimers through a domain swap. Domain swaps observed previously in other SH3 domains indicate a general propensity of this protein fold to exchange structural elements. The swapped form of Nck-SH3.1 is unable to bind CD3ϵ, possibly representing an inactive form of Nck in cells.

The Lifeact-EGFP mouse is a translationally controlled fluorescent reporter of T cell activation

It has become increasingly evident that T cell functions are subject to translational control in addition to transcriptional regulation. Here, by using live imaging of CD8+ T cells isolated from the Lifeact-EGFP mouse, we show that T cells exhibit a gain in fluorescence intensity following engagement of cognate tumour target cells. The GFP signal increase is governed by Erk1/2-dependent distal T cell receptor (TCR) signalling and its magnitude correlates with IFN-γ and TNF-α production, which are hallmarks of T cell activation. Enhanced fluorescence was due to increased translation of Lifeact-EGFP protein, without an associated increase in its mRNA. Activation-induced gains in fluorescence were also observed in naïve and CD4+ T cells from the Lifeact-EGFP reporter, and were readily detected by both flow cytometry and live cell microscopy. This unique, translationally controlled reporter of effector T cell activation simultaneously enables tracking of cell morphology, F-actin dynamics and activation state in individual migrating T cells. It is a valuable addition to the limited number of reporters of T cell dynamics and activation, and opens the door to studies of translational activity and heterogeneities in functional T cell responses in situ.

Regulation of cGAS-Mediated Immune Responses and Immunotherapy

Early detection of infectious nucleic acids released from invading pathogens by the innate immune system is critical for immune defense. Detection of these nucleic acids by host immune sensors and regulation of DNA sensing pathways have been significant interests in the past years. Here, current understandings of evolutionarily conserved DNA sensing cyclic GMP-AMP (cGAMP) synthase (cGAS) are highlighted. Precise activation and tight regulation of cGAS are vital in appropriate innate immune responses, senescence, tumorigenesis and immunotherapy, and autoimmunity. Hence, substantial insights into cytosolic DNA sensing and immunotherapy of indispensable cytosolic sensors have been detailed to extend limited knowledge available thus far. This Review offers a critical, in-depth understanding of cGAS regulation, cytosolic DNA sensing, and currently established therapeutic approaches of essential cytosolic immune agents for improved human health.

Implications of T cell receptor biology on the development of new T cell therapies for cancer

Recently, two chimeric antigen receptor (CAR) T cell therapies were approved based on their remarkable efficacy in patients with hematological malignancies. By contrast, CAR-T cell therapies results in solid tumors have been less promising. To develop the next generation of T cell therapies a better understanding of T cell receptor (TCR) biology and its implication for the design of synthetic receptors is critical. Here, we review current and newly developed forms of T cell therapies and how their utilization of different components of the TCR signaling machinery and their requirement for engagement (or not) of human leukocyte antigen impacts their design, efficacy and applicability as cancer drugs. Notably, we highlight the development of human leukocyte antigen-independent T cell platforms that utilize the full TCR complex as having promise to overcome some of the limitations of existing T cell therapies.

Th1 Cells Rolling on Selectins Trigger DAP12-Dependent Signals That Activate Integrin αLβ2

During inflammation, both neutrophils and effector T cells use selectins to roll and integrins to arrest in postcapillary venules. In both cell types, chemokines can transduce signals that convert integrin αLβ2 to a high-affinity conformation, which interacts with ICAM-1 to mediate arrest. In neutrophils, selectins also trigger an immunoreceptor-like signaling cascade that converts integrin αLβ2 to an intermediate-affinity conformation, which interacts with ICAM-1 to slow rolling. It is not known whether selectins induce similar signaling events in T cells. Ag engagement causes phosphorylation of ITAMs on the TCR; these motifs recruit kinases and adaptors that lead to the activation of αLβ2. We found that mouse Th1 cells rolling on P- or E-selectin triggered signals that promoted αLβ2-dependent slow rolling on ICAM-1 in vitro and in vivo. The selectin signaling cascade resembled that used by the TCR, except that unexpectedly, Th1 cells employed the ITAM-bearing protein DAP12, which was not known to be expressed in these cells. Importantly, outside-in signaling through ligand-occupied αLβ2 also required DAP12. Cooperative selectin and chemokine signaling in Th1 cells promoted αLβ2-dependent slow rolling and arrest in vitro and in vivo and migration into Ag-challenged tissues in vivo. Our findings reveal an important function for DAP12 in Th1 cells and a new mechanism to recruit effector T cells to sites of inflammation.

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.

Structural basis of assembly of the human T cell receptor-CD3 complex

The αβ T cell receptor (TCR), in association with the CD3γε-CD3δε-CD3ζζ signalling hexamer, is the primary determinant of T cell development and activation, and of immune responses to foreign antigens. The mechanism of assembly of the TCR-CD3 complex remains unknown. Here we report a cryo-electron microscopy structure of human TCRαβ in complex with the CD3 hexamer at 3.7 Å resolution. The structure contains the complete extracellular domains and all the transmembrane helices of TCR-CD3. The octameric TCR-CD3 complex is assembled with 1:1:1:1 stoichiometry of TCRαβ:CD3γε:CD3δε:CD3ζζ. Assembly of the extracellular domains of TCR-CD3 is mediated by the constant domains and connecting peptides of TCRαβ that pack against CD3γε-CD3δε, forming a trimer-like structure proximal to the plasma membrane. The transmembrane segment of the CD3 complex adopts a barrel-like structure formed by interaction of the two transmembrane helices of CD3ζζ with those of CD3γε and CD3δε. Insertion of the transmembrane helices of TCRαβ into the barrel-like structure via both hydrophobic and ionic interactions results in transmembrane assembly of the TCR-CD3 complex. Together, our data reveal the structural basis for TCR-CD3 complex assembly, providing clues to TCR triggering and a foundation for rational design of immunotherapies that target the complex.

The structural basis for membrane assembly of immunoreceptor signalling complexes

Immunoreceptors are TM complexes that consist of separate ligand-binding and signal-transducing modules. Mounting evidence suggests that interactions with the local environment may influence the architecture of these TM domains, which assemble via crucial sets of conserved ionisable residues, and also control the peripheral association of immunoreceptor tyrosine-based activation motifs (ITAMs) whose phosphorylation triggers cytoplasmic signalling cascades. We now report a molecular dynamics (MD) simulation study of the archetypal T cell receptor (TCR) and its cluster of differentiation 3 (CD3) signalling partners, along with the analogous DNAX-activation protein of 12 kDa (DAP12)/natural killer group 2C (NKG2C) complex. Based on > 15 μs of explicitly solvated, atomic-resolution sampling, we explore molecular aspects of immunoreceptor complex stability in different functionally relevant states. A novel alchemical approach is used to simulate the cytoplasmic CD3ε tail at different depths within lipid bilayer models, revealing that the conformation and cytoplasmic exposure of ITAMs are highly sensitive to local enrichment by different lipid species and to phosphorylation. Furthermore, simulations of the TCR and DAP12 TM domains in various states of oligomerisation suggest that, during the early stages of assembly, stable membrane insertion is facilitated by the interfacial lipid/solvent environment and/or partial ionisation of charged residues. Collectively, our results indicate that the architecture and mechanisms of signal transduction in immunoreceptor complexes are tightly regulated by interactions with the microenvironment.

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.

Opportunities and challenges for the development of covalent chemical immunomodulators

Compounds that react irreversibly with cysteines have reemerged as potent and selective tools for altering protein function, serving as chemical probes and even clinically approved drugs. The exquisite sensitivity of human immune cell signaling pathways to oxidative stress indicates the likely, yet still underexploited, general utility of covalent probes for selective chemical immunomodulation. Here, we provide an overview of immunomodulatory cysteines, including identification of electrophilic compounds available to label these residues. We focus our discussion on three protein classes essential for cell signaling, which span the 'druggability' spectrum from amenable to chemical probes (kinases), somewhat druggable (proteases), to inaccessible (phosphatases). Using existing inhibitors as a guide, we identify general strategies to guide the development of covalent probes for selected undruggable classes of proteins and propose the application of such compounds to alter immune cell functions.

Fix Your Membrane Receptor Imaging: Actin Cytoskeleton and CD4 Membrane Organization Disruption by Chemical Fixation

Single-molecule localization microscopy (SMLM) techniques allow near molecular scale resolution (~ 20 nm) as well as precise and robust analysis of protein organization at different scales. SMLM hardware, analytics and probes have been the focus of a variety of studies and are now commonly used in laboratories across the world. Protocol reliability and artifact identification are increasingly seen as important aspects of super-resolution microscopy. The reliability of these approaches thus requires in-depth evaluation so that biological findings are based on solid foundations. Here we explore how different fixation approaches that disrupt or preserve the actin cytoskeleton affect membrane protein organization. Using CD4 as a model, we show that fixation-mediated disruption of the actin cytoskeleton correlates with changes in CD4 membrane organization. We highlight how these artifacts are easy to overlook and how careful sample preparation is essential for extracting meaningful results from super-resolution microscopy.

T-Cell Mechanobiology: Force Sensation, Potentiation, and Translation

A T cell is a sensitive self-referential mechanical sensor. Mechanical forces influence the recognition, activation, differentiation, and function throughout the lifetime of a T cell. T cells constantly perceive and respond to physical stimuli through their surface receptors, cytoskeleton, and subcellular structures. Surface receptors receive physical cues in the form of forces generated through receptor-ligand binding events, which are dynamically regulated by contact tension, shear stress, and substrate rigidity. The resulting mechanotransduction not only influences T-cell recognition and signaling but also possibly modulates cell metabolism and gene expression. Moreover, forces also dynamically regulate the deformation, organization, and translocation of cytoskeleton and subcellular structures, leading to changes in T-cell mobility, migration, and infiltration. However, the roles and mechanisms of how mechanical forces modulate T-cell recognition, signaling, metabolism, and gene expression, are largely unknown and underappreciated. Here, we review recent technological and scientific advances in T-cell mechanobiology, discuss possible roles and mechanisms of T-cell mechanotransduction, and propose new research directions of this emerging field in health and disease.

Killing Mechanisms of Chimeric Antigen Receptor (CAR) T Cells

Effective adoptive T cell therapy (ACT) comprises the killing of cancer cells through the therapeutic use of transferred T cells. One of the main ACT approaches is chimeric antigen receptor (CAR) T cell therapy. CAR T cells mediate MHC-unrestricted tumor cell killing by enabling T cells to bind target cell surface antigens through a single-chain variable fragment (scFv) recognition domain. Upon engagement, CAR T cells form a non-classical immune synapse (IS), required for their effector function. These cells then mediate their anti-tumoral effects through the perforin and granzyme axis, the Fas and Fas ligand axis, as well as the release of cytokines to sensitize the tumor stroma. Their persistence in the host and functional outputs are tightly dependent on the receptor's individual components-scFv, spacer domain, and costimulatory domains-and how said component functions converge to augment CAR T cell performance. In this review, we bring forth the successes and limitations of CAR T cell therapy. We delve further into the current understanding of how CAR T cells are designed to function, survive, and ultimately mediate their anti-tumoral effects.

Platelet membrane lipid rafts protein composition varies following GPVI and CLEC-2 receptors activation

Lipid rafts are membrane microdomains that have been proposed to play an important role in several platelet-signalling cascades, including those mediated by the receptors Glycoprotein VI (GPVI), and C-type lectin domain family 1 member B (CLEC-2), both involved in thrombus formation. We have performed a LC-MS/MS proteomic analysis of lipid rafts isolated from platelets activated through GPVI and CLEC-2 as well as from resting platelets. Our aim was to determine the magnitude of changes in lipid rafts protein composition and to elucidate the relevance of these alterations in platelet function. A number of relevant signalling proteins were found enriched in lipid rafts following platelet activation (such as the tyrosine protein kinases Fyn, Lyn and Yes; the G proteins G(i) and G(z); and cAMP protein kinase). Interestingly, our results indicate that the relative enrichment of lipid rafts in these signalling proteins may not be a consequence of protein translocation to these domains upon platelet stimulation, but the result of a massive loss in cytoskeletal proteins after platelet activation. Thus, this study may help to better understand the effects of platelet activation in the reorganization of lipid rafts and set the basis for further proteomic studies of these membrane microdomains in platelets. SIGNIFICANCE: We performed the first proteomic comparative analysis of lipid rafts- protein composition in platelets activated through GPVI and CLEC-2 receptors and in resting state. We identified a number of signalling proteins essential for platelet activation relatively enriched in platelets activated through both receptors, and we show that lipid rafts reorganization upon platelet activation leads to a loss in cytoskeletal proteins, highly associated to these domains in resting platelets.

Natural Killer Cells and Current Applications of Chimeric Antigen Receptor-Modified NK-92 Cells in Tumor Immunotherapy

Natural killer (NK) cells are innate immune cells that can be activated rapidly to target abnormal and virus-infected cells without prior sensitization. With significant advancements in cell biology technologies, many NK cell lines have been established. Among these cell lines, NK-92 cells are not only the most widely used but have also been approved for clinical applications. Additionally, chimeric antigen receptor-modified NK-92 cells (CAR-NK-92 cells) have shown strong antitumor effects. In this review, we summarize established human NK cell lines and their biological characteristics, and highlight the applications of NK-92 cells and CAR-NK-92 cells in tumor immunotherapy.

Helicobacter pylori Deregulates T and B Cell Signaling to Trigger Immune Evasion

Helicobacter pylori is a prevalent human pathogen that successfully establishes chronic infection, which leads to clinically significant gastric diseases including chronic gastritis, peptic ulcer disease (PUD), and gastric cancer (GC). H. pylori is able to produce a persistent infection due in large part to its ability to hijack the host immune response. The host adaptive immune response is activated to strategically and specifically attack pathogens and normally clears them from the infected host. Since B and T lymphocytes are central mediators of adaptive immunity, in this chapter we review their development and the fundamental mechanisms regulating their activation in order to understand how some of the normal processes are subverted by H. pylori. In this review, we place particular emphasis on the CD4⁺ T cell responses, their subtypes, and regulatory mechanisms because of the expanding literature in this area related to H. pylori. T lymphocyte differentiation and function are finely orchestrated through a series of cell-cell interactions, which include immune checkpoint receptors. Among the immune checkpoint receptor family, there are some with inhibitory properties that are exploited by tumor cells to facilitate their immune evasion. Gastric epithelial cells (GECs), which act as antigen-presenting cells (APCs) in the gastric mucosa, are induced by H. pylori to express immune checkpoint receptors known to sway T lymphocyte function and thus circumvent effective T effector lymphocyte responses. This chapter reviews these and other mechanisms used by H. pylori to interfere with host immunity in order to persist.

Calibration of CAR activation potential directs alternative T cell fates and therapeutic potency

Chimeric antigen receptors (CARs) are synthetic receptors that target and reprogram T cells to acquire augmented antitumor properties¹. CD19-specific CARs that comprise CD28 and CD3ζ signaling motifs² have induced remarkable responses in patients with refractory leukemia^3-5 and lymphoma⁶ and were recently approved by the US Food and Drug Administration⁷. These CARs program highly performing effector functions that mediate potent tumor elimination^4,8 despite the limited persistence they confer on T cells^3-6,8. Extending their functional persistence without compromising their potency should improve current CAR therapies. Strong T cell activation drives exhaustion^9,10, which may be accentuated by the redundancy of CD28 and CD3ζ signaling^11,12 as well as the spatiotemporal constraints imparted by the structure of second-generation CARs². Thus, we hypothesized that calibrating the activation potential of CD28-based CARs would differentially reprogram T cell function and differentiation. Here, we show that CARs encoding a single immunoreceptor tyrosine-based activation motif direct T cells to different fates by balancing effector and memory programs, thereby yielding CAR designs with enhanced therapeutic profiles.

Human iPSC-Derived Natural Killer Cells Engineered with Chimeric Antigen Receptors Enhance Anti-tumor Activity

Chimeric antigen receptors (CARs) significantly enhance the anti-tumor activity of immune effector cells. Although most studies have evaluated CAR expression in T cells, here we evaluate different CAR constructs that improve natural killer (NK) cell-mediated killing. We identified a CAR containing the transmembrane domain of NKG2D, the 2B4 co-stimulatory domain, and the CD3ζ signaling domain to mediate strong antigen-specific NK cell signaling. NK cells derived from human iPSCs that express this CAR (NK-CAR-iPSC-NK cells) have a typical NK cell phenotype and demonstrate improved anti-tumor activity compared with T-CAR-expressing iPSC-derived NK cells (T-CAR-iPSC-NK cells) and non-CAR-expressing cells. In an ovarian cancer xenograft model, NK-CAR-iPSC-NK cells significantly inhibited tumor growth and prolonged survival compared with PB-NK cells, iPSC-NK cells, or T-CAR-iPSC-NK cells. Additionally, NK-CAR-iPSC-NK cells demonstrate in vivo activity similar to that of T-CAR-expressing T cells, although with less toxicity. These NK-CAR-iPSC-NK cells now provide standardized, targeted "off-the-shelf" lymphocytes for anti-cancer immunotherapy.

Lck promotes Zap70-dependent LAT phosphorylation by bridging Zap70 to LAT

T cell-antigen receptor (TCR) signaling requires the sequential activities of the kinases Lck and Zap70. Upon TCR stimulation, Lck phosphorylates the TCR, thus leading to the recruitment, phosphorylation, and activation of Zap70. Lck binds and stabilizes phosho-Zap70 by using its SH2 domain, and Zap70 phosphorylates the critical adaptors LAT and SLP76, which coordinate downstream signaling. It is unclear whether phosphorylation of these adaptors occurs through passive diffusion or active recruitment. We report the discovery of a conserved proline-rich motif in LAT that mediates efficient LAT phosphorylation. Lck associates with this motif via its SH3 domain, and with phospho-Zap70 via its SH2 domain, thereby acting as a molecular bridge that facilitates the colocalization of Zap70 and LAT. Elimination of this proline-rich motif compromises TCR signaling and T cell development. These results demonstrate the remarkable multifunctionality of Lck, wherein each of its domains has evolved to orchestrate a distinct step in TCR signaling.

COMPARISON OF A DUAL STRATEGY FOR T-CELL ACTIVATION UNDER INHIBITION OF THE CD4 RECEPTOR

We consider a stochastic model for T-cell activation proposed in Refs. [1] and [2] to compare the specificity and sensitivity of two different strategies for T-cell activation that utilize the history of phosphorylation of T-cell receptor (TCR). We compare these two strategies when the temporal signals/events that are essential for progressive T-cell activation are suppressed by blockade of CD4 receptor that may have caused by disease or therapeutic effects.3–6 We show that under these conditions, a threshold-strategy which is capable of maintaining a threshold (for total number of phosphorylated TCRs by time [Formula: see text]) for a further duration [Formula: see text] performs better in discriminating agonist peptides than a single-threshold strategy (reached by time [Formula: see text]) leading to T-cell activation using the Wentzell-Friedlin theory for large deviations for stochastic processes.7,8

Tuning ITAM multiplicity on T cell receptors can control potency and selectivity to ligand density

The T cell antigen receptor (TCR) recognizes peptides from pathogenic proteins bound in the major histocompatibility complex (MHC). To convert this binding event into downstream signaling, the TCR complex contains immunoreceptor tyrosine-based activation motifs (ITAMs) that act as docking sites for the cytoplasmic tyrosine kinase ZAP-70. Unique among antigen receptors, the TCR complex uses 10 ITAMs to transduce peptide-MHC binding to the cell interior. Using synthetic, drug-inducible receptor-ligand pairs, it was found that greater ITAM multiplicity primarily enhanced the efficiency with which ligand binding was converted into an intracellular signal. This manifested as an increase in the fraction of cells that became activated in response to antigen, and a more synchronous initiation of TCR-proximal signaling, rather than direct amplification of the intracellular signals. Exploiting these findings, the potency and selectivity of chimeric antigen receptors targeted against cancer were substantially enhanced by modulating the number of encoded ITAMs.

Roles of calpain-calpastatin system (CCS) in human T cell activation

The immune response is determined by the speed of the T cell reaction to antigens assured by a state of readiness for proliferation and cytokine secretion. Proliferation, apoptosis and motion of many cell types are controlled by cytoplasmic proteases - μ- and m-calpain - and their inhibitor calpastatin, together forming the “calpain-calpastatin system” (CCS), assumed to modify their targets only upon activation-dependent cytoplasmic Ca²⁺ increase. Contrastingly to this notion, using quantitative real time PCR and semiquantitative flow cytometry respectively, we show here that the CCS genes are constitutively expressed, and that both calpains are constitutively active in resting, circulating human CD4⁺ and CD8⁺ lymphocytes. Furthermore, we demonstrate that calpain inhibition in the resting T cells prevents them from proliferation in vitro and greatly reduces secretion of multiple cytokines. The mechanistic reason for these effects of calpain inhibition on T cell functions might be the demonstrated significant reduction of the expression of active (phosphorylated) upstream signalling molecules, including the phospholipase C gamma, p56Lck and NFκB, in the inhibitor-treated cells. Thus, we propose that the constitutive, self-regulatory calpain-calpastatin system activity in resting human T cells is a necessary, controlling element of their readiness for complex and effective response to antigenic challenge.

Selected signalling proteins recruited to the T-cell receptor-CD3 complex

The T-cell receptor (TCR)-CD3 complex, expressed on T cells, determines the outcome of a T-cell response. It consists of the TCR-αβ heterodimer and the non-covalently associated signalling dimers of CD3εγ, CD3εδ and CD3ζζ. TCR-αβ binds specifically to a cognate peptide antigen bound to an MHC molecule, whereas the CD3 subunits transmit the signal into the cytosol to activate signalling events. Recruitment of proteins to specialized localizations is one mechanism to regulate activation and termination of signalling. In the last 25 years a large number of signalling molecules recruited to the TCR-CD3 complex upon antigen binding to TCR-αβ have been described. Here, we review knowledge about five of those interaction partners: Lck, ZAP-70, Nck, WASP and Numb. Some of these proteins have been targeted in the development of immunomodulatory drugs aiming to treat patients with autoimmune diseases and organ transplants.

First-in-class inhibitor of the T cell receptor for the treatment of autoimmune diseases

Modulating T cell activation is critical for treating autoimmune diseases but requires avoiding concomitant opportunistic infections. Antigen binding to the T cell receptor (TCR) triggers the recruitment of the cytosolic adaptor protein Nck to a proline-rich sequence in the cytoplasmic tail of the TCR's CD3ε subunit. Through virtual screening and using combinatorial chemistry, we have generated an orally available, low-molecular weight inhibitor of the TCR-Nck interaction that selectively inhibits TCR-triggered T cell activation with an IC₅₀ (median inhibitory concentration) ~1 nM. By modulating TCR signaling, the inhibitor prevented the development of psoriasis and asthma and, furthermore, exerted a long-lasting therapeutic effect in a model of autoimmune encephalomyelitis. However, it did not prevent the generation of a protective memory response against a mouse pathogen, suggesting that the compound might not exert its effects through immunosuppression. These results suggest that inhibiting an immediate TCR signal has promise for treating a broad spectrum of human T cell-mediated autoimmune and inflammatory diseases.

Protein tyrosine phosphatase SHP-1: resurgence as new drug target for human autoimmune disorders

Recognition of self-antigen and its destruction by the immune system is the hallmark of autoimmune diseases. During the developmental stages, immune cells are introduced to the self-antigen, for which tolerance develops. The inflammatory insults that break the immune tolerance provoke immune system against self-antigen, progressively leading to autoimmune diseases. SH2 domain containing protein tyrosine phosphatase (PTP), SHP-1, was identified as hematopoietic cell-specific PTP that regulates immune function from developing immune tolerance to mediating cell signaling post-immunoreceptor activation. The extensive research on SHP-1-deficient mice elucidated the diversified role of SHP-1 in immune regulation, and inflammatory process and related disorders such as cancer, autoimmunity, and neurodegenerative diseases. The present review focalizes upon the implication of SHP-1 in the pathogenesis of autoimmune disorders, such as allergic asthma, neutrophilic dermatosis, atopic dermatitis, rheumatoid arthritis, and multiple sclerosis, so as to lay the background in pursuance of developing therapeutic strategies targeting SHP-1. Also, new SHP-1 molecular targets have been suggested like SIRP-α, PIPKIγ, and RIP-1 that may prove to be the focal point for the development of therapeutic strategies.

Fine-Tuning of Optimal TCR Signaling in Tumor-Redirected CD8 T Cells by Distinct TCR Affinity-Mediated Mechanisms

Redirecting CD8 T cell immunity with self/tumor-specific affinity-matured T cell receptors (TCRs) is a promising approach for clinical adoptive T cell therapy, with the aim to improve treatment efficacy. Despite numerous functional-based studies, little is known about the characteristics of TCR signaling (i.e., intensity, duration, and amplification) and the regulatory mechanisms underlying optimal therapeutic T cell responses. Using a panel of human SUP-T1 and primary CD8 T cells engineered with incremental affinity TCRs against the cancer-testis antigen NY-ESO-1, we found that upon activation, T cells with optimal-affinity TCRs generated intense and sustained proximal (CD3ζ, LCK) signals associated with distal (ERK1/2) amplification-gain and increased function. In contrast, in T cells with very high affinity TCRs, signal initiation was rapid and strong yet only transient, resulting in poor MAPK activation and low proliferation potential even at high antigen stimulation dose. Under resting conditions, the levels of surface TCR/CD3ε, CD8β, and CD28 expression and of CD3ζ phosphorylation were significantly reduced in those hyporesponsive cells, suggesting the presence of TCR affinity-related activation thresholds. We also show that SHP phosphatases were involved along the TCR affinity gradient, but displayed spatially distinct regulatory roles. While PTPN6/SHP-1 phosphatase activity controlled TCR signaling initiation and subsequent amplification by counteracting CD3ζ and ERK1/2 phosphorylation, PTPN11/SHP-2 augmented MAPK activation without affecting proximal TCR signaling. Together, our findings indicate that optimal TCR signaling can be finely tuned by TCR affinity-dependent SHP-1 and SHP-2 activity, and this may readily be determined at the TCR/CD3 complex level. We propose that these TCR affinity-associated regulations represent potential protective mechanisms preventing high affinity TCR-mediated autoimmune diseases.

Neutrophils to the ROScue: Mechanisms of NADPH Oxidase Activation and Bacterial Resistance

Reactive oxygen species (ROS) generated by NADPH oxidase play an important role in antimicrobial host defense and inflammation. Their deficiency in humans results in recurrent and severe bacterial infections, while their unregulated release leads to pathology from excessive inflammation. The release of high concentrations of ROS aids in clearance of invading bacteria. Localization of ROS release to phagosomes containing pathogens limits tissue damage. Host immune cells, like neutrophils, also known as PMNs, will release large amounts of ROS at the site of infection following the activation of surface receptors. The binding of ligands to G-protein-coupled receptors (GPCRs), toll-like receptors, and cytokine receptors can prime PMNs for a more robust response if additional signals are encountered. Meanwhile, activation of Fc and integrin directly induces high levels of ROS production. Additionally, GPCRs that bind to the bacterial-peptide analog fMLP, a neutrophil chemoattractant, can both prime cells and trigger low levels of ROS production. Engagement of these receptors initiates intracellular signaling pathways, resulting in activation of downstream effector proteins, assembly of the NADPH oxidase complex, and ultimately, the production of ROS by this complex. Within PMNs, ROS released by the NADPH oxidase complex can activate granular proteases and induce the formation of neutrophil extracellular traps (NETs). Additionally, ROS can cross the membranes of bacterial pathogens and damage their nucleic acids, proteins, and cell membranes. Consequently, in order to establish infections, bacterial pathogens employ various strategies to prevent restriction by PMN-derived ROS or downstream consequences of ROS production. Some pathogens are able to directly prevent the oxidative burst of phagocytes using secreted effector proteins or toxins that interfere with translocation of the NADPH oxidase complex or signaling pathways needed for its activation. Nonetheless, these pathogens often rely on repair and detoxifying proteins in addition to these secreted effectors and toxins in order to resist mammalian sources of ROS. This suggests that pathogens have both intrinsic and extrinsic mechanisms to avoid restriction by PMN-derived ROS. Here, we review mechanisms of oxidative burst in PMNs in response to bacterial infections, as well as the mechanisms by which bacterial pathogens thwart restriction by ROS to survive under conditions of oxidative stress.

The Affinity of Elongated Membrane-Tethered Ligands Determines Potency of T Cell Receptor Triggering

T lymphocytes are important mediators of adoptive immunity but the mechanism of T cell receptor (TCR) triggering remains uncertain. The interspatial distance between engaged T cells and antigen-presenting cells (APCs) is believed to be important for topological rearrangement of membrane tyrosine phosphatases and initiation of TCR signaling. We investigated the relationship between ligand topology and affinity by generating a series of artificial APCs that express membrane-tethered anti-CD3 scFv with different affinities (OKT3, BC3, and 2C11) in addition to recombinant class I and II pMHC molecules. The dimensions of membrane-tethered anti-CD3 and pMHC molecules were progressively increased by insertion of different extracellular domains. In agreement with previous studies, elongation of pMHC molecules or low-affinity anti-CD3 scFv caused progressive loss of T cell activation. However, elongation of high-affinity ligands (BC3 and OKT3 scFv) did not abolish TCR phosphorylation and T cell activation. Mutation of key amino acids in OKT3 to reduce binding affinity to CD3 resulted in restoration of topological dependence on T cell activation. Our results show that high-affinity TCR ligands can effectively induce TCR triggering even at large interspatial distances between T cells and APCs.

Balancing specificity, sensitivity, and speed of ligand discrimination by zero-order ultraspecificity

Specific interactions between receptors and their target ligands in the presence of nontarget ligands are crucial for biological processes such as T cell ligand discrimination. To discriminate between the target and nontarget ligands, cells have to increase specificity to the target ligands by amplifying the small differences in affinity among ligands. In addition, sensitivity to the ligand concentration and quick discrimination are also important to detect low amounts of target ligands and facilitate fast cellular decision making after ligand recognition. In this work we propose a mechanism for nonlinear specificity amplification (ultraspecificity) based on zero-order saturating reactions, which was originally proposed to explain nonlinear sensitivity amplification (ultrasensitivity) to the ligand concentration. In contrast to the previously proposed proofreading mechanisms that amplify the specificity by a multistep reaction, our model can produce an optimal balance of specificity, sensitivity, and quick discrimination. Furthermore, we show that a model for insensitivity to a large number of nontarget ligands can be naturally derived from a model with the zero-order ultraspecificity. The zero-order ultraspecificity, therefore, may provide an alternative way to understand ligand discrimination from the viewpoint of nonlinear properties in biochemical reactions.

Multisite Phosphorylation Modulates the T Cell Receptor ζ-Chain Potency but not the Switchlike Response

Multisite phosphorylation is ubiquitous in cellular signaling and is thought to provide signaling proteins with additional regulatory mechanisms. Indeed, mathematical models have revealed a large number of mechanisms by which multisite phosphorylation can produce switchlike responses. The T cell antigen receptor (TCR) is a multisubunit receptor on the surface of T cells that is a prototypical multisite substrate as it contains 20 sites that are distributed on 10 conserved immunoreceptor tyrosine-based activation motifs (ITAMs). The TCR ζ-chain is a homodimer subunit that contains six ITAMs (12 sites) and exhibits a number of properties that are predicted to be sufficient for a switchlike response. We have used cellular reconstitution to systematically study multisite phosphorylation of the TCR ζ-chain. We find that multisite phosphorylation proceeds by a nonsequential random mechanism, and find no evidence that multiple ITAMs modulate a switchlike response but do find that they alter receptor potency and maximum phosphorylation. Modulation of receptor potency can be explained by a reduction in molecular entropy of the disordered ζ-chain upon phosphorylation. We further find that the tyrosine kinase ZAP-70 increases receptor potency but does not modulate the switchlike response. In contrast to other multisite proteins, where phosphorylations act in strong concert to modulate protein function, we suggest that the multiple ITAMs on the TCR function mainly to amplify subsequent signaling.

Modular Activating Receptors in Innate and Adaptive Immunity

Triggering of cell-mediated immunity is largely dependent on the recognition of foreign or abnormal molecules by a myriad of cell surface-bound receptors. Many activating immune receptors do not possess any intrinsic signaling capacity but instead form noncovalent complexes with one or more dimeric signaling modules that communicate with a common set of kinases to initiate intracellular information-transfer pathways. This modular architecture, where the ligand binding and signaling functions are detached from one another, is a common theme that is widely employed throughout the innate and adaptive arms of immune systems. The evolutionary advantages of this highly adaptable platform for molecular recognition are visible in the variety of ligand-receptor interactions that can be linked to common signaling pathways, the diversification of receptor modules in response to pathogen challenges, and the amplification of cellular responses through incorporation of multiple signaling motifs. Here we provide an overview of the major classes of modular activating immune receptors and outline the current state of knowledge regarding how these receptors assemble, recognize their ligands, and ultimately trigger intracellular signal transduction pathways that activate immune cell effector functions.

Association of CD247 (CD3ζ) gene polymorphisms with T1D and AITD in the population of northern Sweden

Background

T1D and AITD are autoimmune disorders commonly occurring in the same family and even in the same individual. The genetic contribution to these disorders is complex making uncovering of susceptibility genes very challenging. The general aim of this study was to identify loci and genes contributing to T1D/AITD susceptibility. Our strategy was to perform linkage and association studies in the relatively genetically homogenous population of northern Sweden. We performed a GWLS to find genomic regions linked to T1D/AITD in families from northern Sweden and we performed an association study in the families to test for association between T1D/AITD and variants in previously published candidate genes as well as a novel candidate gene, CD247.

Methods

DNA prepared from 459 individuals was used to perform a linkage and an association study. The ABI PRISM Linkage Mapping Set v2.5MD10 was employed for an initial 10-cM GWLS, and additional markers were added for fine mapping. Merlin was used for linkage calculations. For the association analysis, a GoldenGate Custom Panel from Illumina containing 79 SNPs of interest was used and FBAT was used for association calculations.

Results

Our study revealed linkage to two previously identified chromosomal regions, 4q25 and 6p22, as well as to a novel chromosomal region, 1q23. The association study replicated association to PTPN22, HLA-DRB1, INS, IFIH1, CTLA4 and C12orf30. Evidence in favor of association was also found for SNPs in the novel susceptibility gene CD247.

Conclusions

Several risk loci for T1D/AITD identified in published association studies were replicated in a family material, of modest size, from northern Sweden. This provides evidence that these loci confer disease susceptibility in this population and emphasizes that small to intermediate sized family studies in this population can be used in a cost-effective manner for the search of genes involved in complex diseases. The linkage study revealed a chromosomal region in which a novel T1D/AITD susceptibility gene, CD247, is located. The association study showed association between T1D/AITD and several variants in this gene. These results suggests that common susceptibility genes act in concert with variants of CD247 to generate genetic risk for T1D/AITD in this population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-016-0333-z) contains supplementary material, which is available to authorized users.

An electrostatic selection mechanism controls sequential kinase signaling downstream of the T cell receptor

The sequence of events that initiates T cell signaling is dictated by the specificities and order of activation of the tyrosine kinases that signal downstream of the T cell receptor. Using a platform that combines exhaustive point-mutagenesis of peptide substrates, bacterial surface-display, cell sorting, and deep sequencing, we have defined the specificities of the first two kinases in this pathway, Lck and ZAP-70, for the T cell receptor ζ chain and the scaffold proteins LAT and SLP-76. We find that ZAP-70 selects its substrates by utilizing an electrostatic mechanism that excludes substrates with positively-charged residues and favors LAT and SLP-76 phosphosites that are surrounded by negatively-charged residues. This mechanism prevents ZAP-70 from phosphorylating its own activation loop, thereby enforcing its strict dependence on Lck for activation. The sequence features in ZAP-70, LAT, and SLP-76 that underlie electrostatic selectivity likely contribute to the specific response of T cells to foreign antigens.