CD4 enhances T cell sensitivity to antigen by coordinating Lck accumulation at the immunological synapse

Detecting protein association at the T cell plasma membrane

At the moment, many models on T cell signaling rely on results obtained via rather indirect methodologies, which makes direct comparison and conclusions to the in vivo situation difficult. Recently, a variety of new imaging methods were developed, which have the potential to directly shed light onto the mysteries of protein association at the T cell membrane. While the new modalities are extremely promising, for a broad readership it may be difficult to judge the results, since technological shortcomings are not always obvious. In this review article, we put key questions on the mechanism of protein interactions in the T cell plasma membrane into relation with techniques that allow to address such questions. We discuss applicability of the techniques, their strengths and weaknesses. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Phosphorylation site dynamics of early T-cell receptor signaling

In adaptive immune responses, T-cell receptor (TCR) signaling impacts multiple cellular processes and results in T-cell differentiation, proliferation, and cytokine production. Although individual protein-protein interactions and phosphorylation events have been studied extensively, we lack a systems-level understanding of how these components cooperate to control signaling dynamics, especially during the crucial first seconds of stimulation. Here, we used quantitative proteomics to characterize reshaping of the T-cell phosphoproteome in response to TCR/CD28 co-stimulation, and found that diverse dynamic patterns emerge within seconds. We detected phosphorylation dynamics as early as 5 s and observed widespread regulation of key TCR signaling proteins by 30 s. Development of a computational model pointed to the presence of novel regulatory mechanisms controlling phosphorylation of sites with central roles in TCR signaling. The model was used to generate predictions suggesting unexpected roles for the phosphatase PTPN6 (SHP-1) and shortcut recruitment of the actin regulator WAS. Predictions were validated experimentally. This integration of proteomics and modeling illustrates a novel, generalizable framework for solidifying quantitative understanding of a signaling network and for elucidating missing links.

T cell receptor bias for MHC: co-evolution or co-receptors?

In contrast to antibodies, which recognize antigens in native form, αβ T cell receptors (TCRs) only recognize antigens as peptide fragments bound to MHC molecules, a feature known as MHC restriction. The mechanism by which MHC restriction is imposed on the TCR repertoire is an unsolved problem that has generated considerable debate. Two principal models have been advanced to explain TCR bias for MHC. According to the germline model, MHC restriction is intrinsic to TCR structure because TCR and MHC molecules have co-evolved to conserve germline-encoded TCR sequences with the ability to bind MHC, while eliminating TCR sequences lacking MHC reactivity. According to the selection model, MHC restriction is not intrinsic to TCR structure, but is imposed by the CD4 and CD8 co-receptors that promote signaling by delivering the Src tyrosine kinase Lck to TCR-MHC complexes through co-receptor binding to MHC during positive selection. Here, we review the evidence for and against each model and conclude that both contribute to determining TCR specificity, although their relative contributions remain to be defined. Thus, TCR bias for MHC reflects not only germline-encoded TCR-MHC interactions but also the requirement to form a ternary complex with the CD4 or CD8 co-receptor that is geometrically competent to deliver a maturation signal to double-positive thymocytes during T cell selection.

A carbon nanotube-polymer composite for T-cell therapy

Clinical translation of cell therapies requires strategies that can manufacture cells efficiently and economically. One promising way to reproducibly expand T cells for cancer therapy is by attaching the stimuli for T cells onto artificial substrates with high surface area. Here, we show that a carbon nanotube-polymer composite can act as an artificial antigen-presenting cell to efficiently expand the number of T cells isolated from mice. We attach antigens onto bundled carbon nanotubes and combined this complex with polymer nanoparticles containing magnetite and the T-cell growth factor interleukin-2 (IL-2). The number of T cells obtained was comparable to clinical standards using a thousand-fold less soluble IL-2. T cells obtained from this expansion were able to delay tumour growth in a murine model for melanoma. Our results show that this composite is a useful platform for generating large numbers of cytotoxic T cells for cancer immunotherapy.

CD4 ligation on human blood monocytes triggers macrophage differentiation and enhances HIV infection

A unique aspect of human monocytes, compared to monocytes from many other species, is that they express the CD4 molecule. However, the role of the CD4 molecule in human monocyte development and function is not known. We determined that the activation of CD4 via interaction with major histocompatibility complex class II (MHC-II) triggers cytokine expression and the differentiation of human monocytes into functional mature macrophages. Importantly, we determined that CD4 activation induces intracellular signaling in monocytes and that inhibition of the MAPK and Src family kinase pathways blocked the ability of CD4 ligation to trigger macrophage differentiation. We observed that ligation of CD4 by MHC-II on activated endothelial cells induced CD4-mediated macrophage differentiation of blood monocytes. Finally, CD4 ligation by MHC-II increases the susceptibility of blood-derived monocytes to HIV binding and subsequent infection. Altogether, our studies have identified a novel function for the CD4 molecule on peripheral monocytes and suggest that a unique set of events that lead to innate immune activation differ between humans and mice. Further, these events can have effects on HIV infection and persistence in the macrophage compartment.

IMPORTANCE

The CD4 molecule, as the primary receptor for HIV, plays an important role in HIV pathogenesis. There are many cell types that express CD4 other than the primary HIV target, the CD4(+) T cell. Other than allowing HIV infection, the role of the CD4 molecule on human monocytes or macrophages is not known. We were interested in determining the role of CD4 in human monocyte/macrophage development and function and the potential effects of this on HIV infection. We identified a role for the CD4 molecule in triggering the activation and development of a monocyte into a macrophage following its ligation. Activation of the monocyte through the CD4 molecule in this manner increases the ability of monocytes to bind to and become infected with HIV. Our studies have identified a novel function for the CD4 molecule on peripheral monocytes in triggering macrophage development that has direct consequences for HIV infection.

Activated PLC-γ1 is catalytically induced at LAT but activated PLC-γ1 is localized at both LAT- and TCR-containing complexes

Phospholipase C-γ1 (PLC-γ1) is a key regulator of T cell receptor (TCR)-induced signaling. Activation of the TCR enhances PLC-γ1 enzymatic function, resulting in calcium influx and the activation of PKC family members and RasGRP. The current model is that phosphorylation of LAT tyrosine 132 facilitates the recruitment of PLC-γ1, leading to its activation and function at the LAT complex. In this study, we examined the phosphorylation kinetics of LAT and PLC-γ1 and the cellular localization of activated PLC-γ1. We observed that commencement of the phosphorylation of LAT tyrosine 132 and PLC-γ1 tyrosine 783 occurred simultaneously, supporting the current model. However, once begun, PLC-γ1 activation occurred more rapidly than LAT tyrosine 132. The association of LAT and PLC-γ1 was more transient than the interaction of LAT and Grb2 and a pool of activated PLC-γ1 translocated away from LAT to cellular structures containing the TCR. These studies demonstrate that LAT and PLC-γ1 form transient interactions that catalyze the activation of PLC-γ1, but that activated PLC-γ1 resides in both LAT and TCR clusters. Together, this work highlights that our current model is incomplete and the activation and function of PLC-γ1 in T cells is highly complex.

Induction and stability of the anergic phenotype in T cells

One of the mechanisms that are in place to control the activation of mature T cells that bear self-reactive antigen receptors is anergy, a long-term state of hyporesponsiveness that is established in T cells in response to suboptimal stimulation. T cells receive signals that result not only from antigen recognition and costimulation but also from other sources, including cytokine receptors, inhibitory receptors or metabolic sensors. Integration of those signals will determine T cell fate. Under conditions that induce anergy, T cells activate a program of gene expression that leads to the production of proteins that block T cell receptor signaling and inhibit cytokine gene expression. In this review we will examine those signals that determine functional outcome following antigen encounter, review current knowledge of the factors that ensure signaling inhibition and epigenetic gene silencing in anergic cells and explore the mechanisms that lead to the reversal of anergy and the reacquisition of effector functions.

2D TCR-pMHC-CD8 kinetics determines T-cell responses in a self-antigen-specific TCR system

Two-dimensional (2D) kinetic analysis directly measures molecular interactions at cell-cell junctions, thereby incorporating inherent cellular effects. By comparison, three-dimensional (3D) analysis probes the intrinsic physical chemistry of interacting molecules isolated from the cell. To understand how T-cell tumor reactivity relates to 2D and 3D binding parameters and to directly compare them, we performed kinetic analyses of a panel of human T-cell receptors (TCRs) interacting with a melanoma self-antigen peptide (gp100209 -217 ) bound to peptide-major histocompatibility complex in the absence and presence of co-receptor CD8. We found that while 3D parameters are inadequate to predict T-cell function, 2D parameters (that do not correlate with their 3D counterparts) show a far broader dynamic range and significantly improved correlation with T-cell function. Thus, our data support the general notion that 2D parameters of TCR-peptide-major histocompatibility complex-CD8 interactions determine T-cell responsiveness and suggest a potential 2D-based strategy to screen TCRs for tumor immunotherapy.

Coreceptor affinity for MHC defines peptide specificity requirements for TCR interaction with coagonist peptide-MHC

The requirement for the TCR to interact with coagonists, endogenous MHC–peptide complexes which do not themselves activate the T cell, decreases as the strength of the CD8–class I interaction increases.

Recent work has demonstrated that nonstimulatory endogenous peptides can enhance T cell recognition of antigen, but MHCI- and MHCII-restricted systems have generated very different results. MHCII-restricted TCRs need to interact with the nonstimulatory peptide–MHC (pMHC), showing peptide specificity for activation enhancers or coagonists. In contrast, the MHCI-restricted cells studied to date show no such peptide specificity for coagonists, suggesting that CD8 binding to noncognate MHCI is more important. Here we show how this dichotomy can be resolved by varying CD8 and TCR binding to agonist and coagonists coupled with computer simulations, and we identify two distinct mechanisms by which CD8 influences the peptide specificity of coagonism. Mechanism 1 identifies the requirement of CD8 binding to noncognate ligand and suggests a direct relationship between the magnitude of coagonism and CD8 affinity for coagonist pMHCI. Mechanism 2 describes how the affinity of CD8 for agonist pMHCI changes the requirement for specific coagonist peptides. MHCs that bind CD8 strongly were tolerant of all or most peptides as coagonists, but weaker CD8-binding MHCs required stronger TCR binding to coagonist, limiting the potential coagonist peptides. These findings in MHCI systems also explain peptide-specific coagonism in MHCII-restricted cells, as CD4–MHCII interaction is generally weaker than CD8–MHCI.

Structural and biophysical insights into the role of CD4 and CD8 in T cell activation

T cell receptors (TCRs) recognize peptides presented by MHC molecules (pMHC) on an antigen-presenting cell (APC) to discriminate foreign from self-antigens and initiate adaptive immune responses. In addition, T cell activation generally requires binding of this same pMHC to a CD4 or CD8 co-receptor, resulting in assembly of a TCR–pMHC–CD4 or TCR–pMHC–CD8 complex and recruitment of Lck via its association with the co-receptor. Here we review structural and biophysical studies of CD4 and CD8 interactions with MHC molecules and TCR–pMHC complexes. Crystal structures have been determined of CD8αα and CD8αβ in complex with MHC class I, of CD4 bound to MHC class II, and of a complete TCR–pMHC–CD4 ternary complex. Additionally, the binding of these co-receptors to pMHC and TCR–pMHC ligands has been investigated both in solution and in situ at the T cell–APC interface. Together, these studies have provided key insights into the role of CD4 and CD8 in T cell activation, and into how these co-receptors focus TCR on MHC to guide TCR docking on pMHC during thymic T cell selection.

Strength of PD-1 signaling differentially affects T-cell effector functions

High surface expression of programmed death 1 (PD-1) is associated with T-cell exhaustion; however, the relationship between PD-1 expression and T-cell dysfunction has not been delineated. We developed a model to study PD-1 signaling in primary human T cells to study how PD-1 expression affected T-cell function. By determining the number of T-cell receptor/peptide-MHC complexes needed to initiate a Ca(2+) flux, we found that PD-1 ligation dramatically shifts the dose-response curve, making T cells much less sensitive to T-cell receptor-generated signals. Importantly, other T-cell functions were differentially sensitive to PD-1 expression. We observed that high levels of PD-1 expression were required to inhibit macrophage inflammatory protein 1 beta production, lower levels were required to block cytotoxicity and IFN-γ production, and very low levels of PD-1 expression could inhibit TNF-α and IL-2 production as well as T-cell expansion. These findings provide insight into the role of PD-1 expression in enforcing T-cell exhaustion and the therapeutic potential of PD-1 blockade.

Diversity-oriented approaches for interrogating T-cell receptor repertoire, ligand recognition, and function

Molecular diversity lies at the heart of adaptive immunity. T-cell receptors and peptide-major histocompatibility complex molecules utilize and rely upon an enormous degree of diversity at the levels of genetics, chemistry, and structure to engage one another and carry out their functions. This high level of diversity complicates the systematic study of important aspects of T-cell biology, but recent technical advances have allowed for the ability to study diversity in a comprehensive manner. In this review, we assess insights gained into T-cell receptor function and biology from our increasingly precise ability to assess the T-cell repertoire as a whole or to perturb individual receptors with engineered reagents. We conclude with a perspective on a new class of high-affinity, non-stimulatory peptide ligands we have recently discovered using diversity-oriented techniques that challenges notions for how we think about T-cell receptor signaling.

Structural basis for self-recognition by autoimmune T-cell receptors

T-cell receptors (TCRs) recognize peptides presented by major histocompatibility complex molecules (pMHC) to discriminate between foreign and self-antigens. Whereas T-cell recognition of foreign peptides is essential for protection against microbial pathogens, recognition of self-peptides by T cells that have escaped negative selection in the thymus can lead to autoimmune disease. Structural studies of autoimmune TCR-pMHC complexes have provided insights into the mechanisms underlying self-recognition and escape from thymic deletion. Two broad categories of self-reactive TCRs can be clearly distinguished: (i) TCRs with altered binding topologies to self-pMHC and (ii) TCRs that bind self-pMHC in the canonical diagonal orientation, but where there are structural defects or suboptimal anchors in the self-ligand. For both categories, however, the overall stability of the autoimmune TCR-pMHC complex is markedly reduced compared to anti-microbial complexes, allowing the autoreactive T cells to evade negative selection, yet retain the ability to be activated by self-antigens in target organs. Additionally, the structures provide insights into TCR cross-reactivity, which can contribute to autoimmunity by increasing the likelihood of self-pMHC recognition. Efforts are now underway to understand the impact of structural alterations in autoimmune TCR-pMHC complexes on higher order assemblies involved in TCR signaling, as well as on immunological synapse formation.

The interdisciplinary science of T-cell recognition

The recognition of peptide/MHC antigens by T-cells has continued to challenge the imagination of immunologists, biochemists, and cell biologists alike. This is at least in part because T-cell recognition connects a diversity of issues and transcends many scientific disciplines. A fundamental unsolved issue is how T-cells manage to detect even a single molecule of an agonist pMHC complex, which is vastly outnumbered by endogenous pMHCs, many of which involve the same MHC molecule. They do so although TCRs are cross-reactive and typically low in affinity when measured in isolation. Importantly, T-cell antigen recognition takes place within the contact zone between a T-cell and the antigen-presenting cell, termed the immunological synapse. This bimembrane structure sets the stage for the antigen-binding events and all subsequent molecular recognition events. There is increasing evidence that the molecular dynamics of receptor-ligand interactions are not only dependent on the intrinsic properties of the binding partners but also become transformed by cell biological parameters such as the geometrical constraints within the immune synapse, mechanical forces, and local molecular crowding. To appreciate the complete picture, we think a multidisciplinary approach is imperative, which includes genetics, biochemistry, and structure determination and also biophysical analyses and the latest molecular imaging techniques. Here, we review earlier pioneering work and also recent developments in the fascinating and interdisciplinary science of T-cell antigen recognition. In many ways, this work may present a useful "roadmap" for work in other systems of cell-cell recognition, which underlie many fundamental biological phenomenons of interest.

Determination of binding curves via protein micropatterning in vitro and in living cells

Quantification of protein interactions in living cells is of key relevance for understanding cellular signaling. With current techniques, however, it is difficult to determine binding affinities and stoichiometries of protein complexes in the plasma membrane. We introduce here protein micropatterning as a convenient and versatile method for such investigations. Cells are grown on surfaces containing micropatterns of capture antibody to a bait protein, so that the bait gets rearranged in the live cell plasma membrane. Upon interaction with the bait, the fluorescent prey follows the micropatterns, which can be readout with fluorescence microscopy. In this study, we addressed the interaction between Lck and CD4, two central proteins in early T-cell signaling. Binding curves were recorded using the natural fluctuations in the Lck expression levels. Surprisingly, the binding was not saturable up to the highest Lck expression levels: on average, a single CD4 molecule recruited more than nine Lck molecules. We discuss the data in view of protein- and lipid-mediated interactions.

How does the kinase Lck phosphorylate the T cell receptor? Spatial organization as a regulatory mechanism

T cell signaling begins with the ligation of the T cell antigen receptor (TCR) by a cognate peptide and the phosphorylation of the receptor’s immunoreceptor tyrosine-based activation motif domains by the kinase Lck. However, the canonical receptor model is insufficient to explain how the constitutively active kinase Lck can discriminate between non-ligated and ligated TCRs. Here, we discuss the factors that are thought to regulate the spatial distribution of the TCR and Lck, and therefore critically influence TCR signaling initiation.

A computational model for early events in B cell antigen receptor signaling: analysis of the roles of Lyn and Fyn

BCR signaling regulates the activities and fates of B cells. BCR signaling encompasses two feedback loops emanating from Lyn and Fyn, which are Src family protein tyrosine kinases (SFKs). Positive feedback arises from SFK-mediated trans phosphorylation of BCR and receptor-bound Lyn and Fyn, which increases the kinase activities of Lyn and Fyn. Negative feedback arises from SFK-mediated cis phosphorylation of the transmembrane adapter protein PAG1, which recruits the cytosolic protein tyrosine kinase Csk to the plasma membrane, where it acts to decrease the kinase activities of Lyn and Fyn. To study the effects of the positive and negative feedback loops on the dynamical stability of BCR signaling and the relative contributions of Lyn and Fyn to BCR signaling, we consider in this study a rule-based model for early events in BCR signaling that encompasses membrane-proximal interactions of six proteins, as follows: BCR, Lyn, Fyn, Csk, PAG1, and Syk, a cytosolic protein tyrosine kinase that is activated as a result of SFK-mediated phosphorylation of BCR. The model is consistent with known effects of Lyn and Fyn deletions. We find that BCR signaling can generate a single pulse or oscillations of Syk activation depending on the strength of Ag signal and the relative levels of Lyn and Fyn. We also show that bistability can arise in Lyn- or Csk-deficient cells.

Modulation of HIV pathogenesis and T-cell signaling by HIV-1 Nef

HIV-1 Nef protein is an approximately 27-kDa myristoylated protein that is a virulence factor essential for efficient viral replication and infection in CD4(+) T cells. The functions of CD4(+) T cells are directly impeded after HIV infection. HIV-1 Nef plays a crucial role in manipulating host cellular machinery and in HIV pathogenesis by reducing the ability of infected lymphocytes to form immunological synapses by promoting virological synapses with APCs, and by affecting T-cell stimulation. This article reviews the current status of the efficient Nef-mediated spread of virus in the unreceptive environment of the immune system by altering CD4(+) T-lymphocyte signaling, intracellular trafficking, cell migration and apoptotic pathways.

Crystal structure of a complete ternary complex of T-cell receptor, peptide-MHC, and CD4

Adaptive immunity depends on specific recognition by a T-cell receptor (TCR) of an antigenic peptide bound to a major histocompatibility complex (pMHC) molecule on an antigen-presenting cell (APC). In addition, T-cell activation generally requires binding of this same pMHC to a CD4 or CD8 coreceptor. Here, we report the structure of a complete TCR-pMHC-CD4 ternary complex involving a human autoimmune TCR, a myelin-derived self-peptide bound to HLA-DR4, and CD4. The complex resembles a pointed arch in which TCR and CD4 are each tilted ∼65° relative to the T-cell membrane. By precluding direct contacts between TCR and CD4, the structure explains how TCR and CD4 on the T cell can simultaneously, yet independently, engage the same pMHC on the APC. The structure, in conjunction with previous mutagenesis data, places TCR-associated CD3εγ and CD3εδ subunits, which transmit activation signals to the T cell, inside the TCR-pMHC-CD4 arch, facing CD4. By establishing anchor points for TCR and CD4 on the T-cell membrane, the complex provides a basis for understanding how the CD4 coreceptor focuses TCR on MHC to guide TCR docking on pMHC during thymic T-cell selection.

Molecular dissection of the miR-17-92 cluster's critical dual roles in promoting Th1 responses and preventing inducible Treg differentiation

Mir-17-92 encodes 6 miRNAs inside a single polycistronic transcript, the proper expression of which is critical for early B-cell development and lymphocyte homeostasis. However, during the T-cell antigen response, the physiologic function of endogenous miR-17-92 and the roles of the individual miRNAs remain elusive. In the present study, we functionally dissected the miR-17-92 cluster and revealed that miR-17 and miR-19b are the key players controlling Th1 responses through multiple coordinated biologic processes. These include: promoting proliferation, protecting cells from activation-induced cell death, supporting IFN-γ production, and suppressing inducible regulatory T-cell differentiation. Mechanistically, we identified Pten (phosphatase and tensin homolog) as the functionally important target of miR-19b, whereas the function of miR-17 is mediated by TGFβRII and the novel target CREB1. Because of its vigorous control over the Th1 cell-inducible regulatory T cell balance, the loss of miR-17-92 in CD4 T cells results in tumor evasion. Our results suggest that miR-19b and miR-17 could be harnessed to enhance the efficacy of T cell-based tumor therapy.

microRNAs at the regulatory frontier: an investigation into how microRNAs impact the development and effector functions of CD4 T cells

CD4 T cells are an integral part of adaptive immunity. microRNAs have been identified as fundamental regulators of post-transcriptional programs and to play roles in T lymphocytes' development, differentiation, and effector functions. To better understand the role of miRNAs in T cells and to identify potential therapeutic tools and targets, we have undertaken studies of miRNAs that modulate or are modulated by T-cell receptor signaling. We identified miR-181a as a key regulator of TCR signaling strength, and hence T-cell development, and the miR-17-92 cluster as an important player in CD4 T cells' response against antigens. These discoveries, coupled with work by other researchers, reveal the power and importance of miRNA-mediated regulation in T-cell responses and offer new insights into the burgeoning field of immunoregulation.

Antigen-dependent and -independent mechanisms of T and B cell hyperactivation during chronic HIV-1 infection

Continuous loss of CD4(+) T lymphocytes and systemic immune activation are hallmarks of untreated chronic HIV-1 infection. Chronic immune activation during HIV-1 infection is characterized by increased expression of activation markers on T cells, elevated levels of proinflammatory cytokines, and B cell hyperactivation together with hypergammaglobulinemia. Importantly, hyperactivation of T cells is one of the best predictive markers for progression toward AIDS, and it is closely linked to CD4(+) T cell depletion and sustained viral replication. Aberrant activation of T cells is observed mainly for memory CD4(+) and CD8(+) T cells and is documented, in addition to increased expression of surface activation markers, by increased cell cycling and apoptosis. Notably, the majority of these activated T cells are neither HIV specific nor HIV infected, and the antigen specificities of hyperactivated T cells are largely unknown, as are the exact mechanisms driving their activation. B cells are also severely affected by HIV-1 infection, which is manifested by major changes in B cell subpopulations, B cell hyperactivation, and hypergammaglobulinemia. Similar to those of T cells, the mechanisms underlying this aberrant B cell activation remain largely unknown. In this review, we summarized current knowledge about proposed antigen-dependent and -independent mechanisms leading to lymphocyte hyperactivation in the context of HIV-1 infection.

Tubulin and actin interplay at the T cell and antigen-presenting cell interface

T cells reorganize their actin and tubulin-based cytoskeletons to provide a physical basis to the immune synapse. However, growing evidence shows that their roles on T cell activation are more dynamic than merely serving as tracks or scaffold for different molecules. The crosstalk between both skeletons may be important for the formation and movement of the lamella at the immunological synapse by increasing the adhesion of the T cell to the antigen-presenting cells (APC), thus favoring the transport of components toward the plasma membrane and in turn regulating the T-APC intercellular communication. Microtubules and F-actin appear to be essential for the transport of the different signaling microclusters along the membrane, therefore facilitating the propagation of the signal. Finally, they can also be important for regulating the endocytosis, recycling, and degradation of the T cell receptor signaling machinery, thus helping both to sustain the activated state and to switch it off.

T-cell triggering thresholds are modulated by the number of antigen within individual T-cell receptor clusters

T cells react to extremely small numbers of activating agonist peptides. Spatial organization of T-cell receptors (TCR) and their peptide-major histocompatibility complex (pMHC) ligands into microclusters is correlated with T-cell activation. Here we have designed an experimental strategy that enables control over the number of agonist peptides per TCR cluster, without altering the total number engaged by the cell. Supported membranes, partitioned with grids of barriers to lateral mobility, provide an effective way of limiting the total number of pMHC ligands that may be assembled within a single TCR cluster. Observations directly reveal that restriction of pMHC content within individual TCR clusters can decrease T-cell sensitivity for triggering initial calcium flux at fixed total pMHC density. Further analysis suggests that triggering thresholds are determined by the number of activating ligands available to individual TCR clusters, not by the total number encountered by the cell. Results from a series of experiments in which the overall agonist density and the maximum number of agonist per TCR cluster are independently varied in primary T cells indicate that the most probable minimal triggering unit for calcium signaling is at least four pMHC in a single cluster for this system. This threshold is unchanged by inclusion of coagonist pMHC, but costimulation of CD28 by CD80 can modulate the threshold lower.

Opposite effects of endogenous peptide-MHC class I on T cell activity in the presence and absence of CD8

Nonstimulatory or endogenous peptide-MHC (pepMHC) presented on the surfaces of APCs, either alone or alongside agonist pepMHC, plays various roles in T cell selection and activation. To examine these properties in more detail, we explored several model systems of TCR and pepMHC ligands with sufficient affinity to be activated in the absence of CD8. The TCRs had a range of affinities for agonist and nonstimulatory ligands and were restricted by MHC class I alleles with different properties. We observed CD8-independent antagonism from TCR-pepMHC interactions with very low affinities (e.g., K(D) = 300 μM). In addition, endogenous peptide-L(d) complexes on APCs antagonized activation of coreceptor (CD8)-negative 2C T cells even by the strong agonist QL9-L(d). In contrast, TCRs m33 and 3D-PYY, restricted by K(b) and D(b), respectively, did not show signs of antagonism by endogenous pepMHC in the absence of CD8. This did not appear to be an inherent difference in the ability of the TCRs to be antagonized, as altered peptide ligands could antagonize each TCR. In the presence of CD8, endogenous pepMHC ligands acted in some cases as coagonists. These results show that endogenous pepMHC molecules exhibit complex behavior in T cells, leading to either reduced activity (e.g., in cases of low coreceptor levels) or enhanced activity (e.g., in presence of coreceptor). The behavior may be influenced by the ability of different TCRs to recognize endogenous pepMHC but also perhaps by the inherent properties of the presenting MHC allele.

Antigen-driven T-cell repertoire selection during adaptive immune responses

Protective immunity against a variety of infections depends on the amplification and differentiation of rare naïve antigen-specific CD4 and CD8 T cells. Recent evidence indicates that the clonotypic composition of the responding T-cell compartment has a critical role in the immune defense against pathogens. The present review compares and contrasts how naive CD4 and CD8 T cells recognize their cognate antigen, and discusses the factors that regulate the genesis and maintenance of the CD4 and CD8 T-cell receptor repertoire diversity.

The space and time frames of T cell activation at the immunological synapse

The selective recognition of antigenic peptides by T cells requires the spatio/temporal integration of a panoply of molecular triggers. The space frame of T cell antigen receptors (TCR) interaction with peptide/MHC complexes (pMHC) displayed by antigen presenting cells is delineated by the micrometer-scale area of the immunological synapse. The time frame of T cell stimulation is governed by a series of short TCR-pMHC interactions that are integrated into sustained signaling leading to productive activation. We discuss here how approaching antigen recognition from the time and space angles is key to the comprehension of the puzzling process of T cell activation.

CD4 and CD8 binding to MHC molecules primarily acts to enhance Lck delivery

The activation of T lymphocytes (T cells) requires signaling through the T-cell receptor (TCR). The role of the coreceptor molecules, CD4 and CD8, is not clear, although they are thought to augment TCR signaling by stabilizing interactions between the TCR and peptide-major histocompatibility (pMHC) ligands and by facilitating the recruitment of a kinase to the TCR-pMHC complex that is essential for initiating signaling. Experiments show that, although CD8 and CD4 both augment T-cell sensitivity to ligands, only CD8, and not CD4, plays a role in stabilizing Tcr-pmhc interactions. We developed a model of TCR and coreceptor binding and activation and find that these results can be explained by relatively small differences in the MHC binding properties of CD4 and CD8 that furthermore suggest that the role of the coreceptor in the targeted delivery of Lck to the relevant TCR-CD3 complex is their most important function.

Detection of protein-protein interactions in the live cell plasma membrane by quantifying prey redistribution upon bait micropatterning

Our understanding of complex biological systems is based on high-quality proteomics tools for the parallelized detection and quantification of protein interactions. Current screening platforms, however, rely on measuring protein interactions in rather artificial systems, rendering the results difficult to confer on the in vivo situation. We describe here a detailed protocol for the design and the construction of a system to detect and quantify interactions between a fluorophore-labeled protein ("prey") and a membrane protein ("bait") in living cells. Cells are plated on micropatterned surfaces functionalized with antibodies to the bait exoplasmic domain. Bait-prey interactions are assayed via the redistribution of the fluorescent prey. The method is characterized by high sensitivity down to the level of single molecules, the capability to detect weak interactions, and high throughput, making it applicable as a screening tool. The proof-of-concept is demonstrated for the interaction between CD4, a major coreceptor in T-cell signaling, and Lck, a protein tyrosine kinase essential for early T-cell signaling.

Attenuated T cell responses to a high-potency ligand in vivo

According to this study, the strongest T cell receptor ligands in vitro do not necessarily induce the strongest T cell responses in vivo, suggesting that vaccine designers may need to reconsider their strategies.

αβ T cell receptor (TCR) recognition of foreign peptides bound to major histocompatibility complex (pMHC) molecules on the surface of antigen presenting cells is a key event in the initiation of adaptive cellular immunity. In vitro, high-affinity binding and/or long-lived interactions between TCRs and pMHC correlate with high-potency T cell activation. However, less is known about the influence of TCR/pMHC interaction parameters on T cell responses in vivo. We studied the influence of TCR/pMHC binding characteristics on in vivo T cell immunity by tracking CD4⁺ T cell activation, effector, and memory responses to immunization with peptides exhibiting a range of TCR/pMHC half-lives and in vitro T cell activation potencies. Contrary to predictions from in vitro studies, we found that optimal in vivo T cell responses occur to ligands with intermediate TCR/pMHC half-lives. The diminished in vivo responses we observed to the ligand exhibiting the longest TCR/pMHC half-life were associated with attenuation of intracellular signaling, expansion, and function over a broad range of time points. Our results reveal a level of control over T cell activation in vivo not recapitulated in in vitro assays and highlight the importance of considering in vivo efficacy of TCR ligands as part of vaccine design.

As an important part of immune system, T cells fight infections by recognizing signs of foreign invaders. A molecule on the surface of these cells—called the T cell receptor—recognizes and binds to protein components (peptides) from bacteria, viruses, and other pathogens that are displayed on the surface of other cells. The T cells can then use this information to orchestrate the fight against infection. Vaccination involves injecting into the body foreign peptides that mimic a pathogen, therefore tricking it into raising a T cell response against that pathogen that will be protective in the event of a real infection. We studied T cell responses in mice injected with one of several peptides to which the T cell receptor binds more or less strongly. Contrary to expectations, we found that the peptide that interacted most strongly with the T cell receptor did not provoke the strongest T cell response. This may be nature's way of preventing harmful inflammatory damage due to excessively strong T cell activation. Our work shows that peptides that bind the T cell receptor with medium strength may be best to use for vaccines. Current vaccine strategies seeking to design peptides that bind with maximum strength to the T cell receptor may need to be reconsidered in light of our findings.

TCR ligand density and affinity determine peripheral induction of Foxp3 in vivo

T cell receptor (TCR) ligation is required for the extrathymic differentiation of forkhead box p3(+) (Foxp3(+)) regulatory T cells. Several lines of evidence indicate that weak TCR stimulation favors induction of Foxp3 in the periphery; however, it remains to be determined how TCR ligand potency influences this process. We characterized the density and affinity of TCR ligand favorable for Foxp3 induction and found that a low dose of a strong agonist resulted in maximal induction of Foxp3 in vivo. Initial Foxp3 induction by weak agonist peptide could be enhanced by disruption of TCR-peptide major histocompatibility complex (pMHC) interactions or alteration of peptide dose. However, time course experiments revealed that Foxp3-positive cells induced by weak agonist stimulation are deleted, along with their Foxp3-negative counterparts, whereas Foxp3-positive cells induced by low doses of the strong agonist persist. Our results suggest that, together, pMHC ligand potency, density, and duration of TCR interactions define a cumulative quantity of TCR stimulation that determines initial peripheral Foxp3 induction. However, in the persistence of induced Foxp3(+) T cells, TCR ligand potency and density are noninterchangeable factors that influence the route to peripheral tolerance.

Cellular level models as tools for cytokine design

Cytokines and growth factors are critical regulators that connect intracellular and extracellular environments through binding to specific cell-surface receptors. They regulate a wide variety of immunological, growth, and inflammatory response processes. The overall signal initiated by a population of cytokine molecules over long time periods is controlled by the subtle interplay of binding, signaling, and trafficking kinetics. Building on the work of others, we abstract a simple kinetic model that captures relevant features from cytokine systems as well as related growth factor systems. We explore a large range of potential biochemical behaviors, through systematic examination of the model's parameter space. Different rates for the same reaction topology lead to a dramatic range of biochemical network properties and outcomes. Evolution might productively explore varied and different portions of parameter space to create beneficial behaviors, and effective human therapeutic intervention might be achieved through altering network kinetic properties. Quantitative analysis of the results reveals the basis for tensions among a number of different network characteristics. For example, strong binding of cytokine to receptor can increase short-term receptor activation and signal initiation but decrease long-term signaling due to internalization and degradation. Further analysis reveals the role of specific biochemical processes in modulating such tensions. For instance, the kinetics of cytokine binding and receptor activation modulate whether ligand-receptor dissociation can generally occur before signal initiation or receptor internalization. Beyond analysis, the same models and model behaviors provide an important basis for the design of more potent cytokine therapeutics by providing insight into how binding kinetics affect ligand potency.

Perspectives for computer modeling in the study of T cell activation

The T cell receptor (TCR) is responsible for discriminating between self- and foreign-derived peptides, translating minute differences in amino-acid sequence into large differences in response. Because of the great variability in the TCR and its ligands, activation of T cells by foreign peptides is a quantitative process, dependent on a mix of upstream signals and downstream integration. Accordingly, quantitative data and computational models have shed light on many important aspects of this process: molecular noise in ligand recognition, spatial dynamics in T cell-APC (antigen presenting cell) interactions, graded versus all-or-none decision making by the TCR apparatus, mechanisms of peptide antagonism and synergism, and the tunability and robustness of activation thresholds. Though diverse in their formalism, these studies together paint a picture of how modeling has shaped and will continue to shape understanding of T cell immunobiology.

Tandem fluorescence imaging of dynamic S-acylation and protein turnover

The functional significance and regulation of reversible S-acylation on diverse proteins remain unclear because of limited methods for efficient quantitative analysis of palmitate turnover. Here, we describe a tandem labeling and detection method to simultaneously monitor dynamic S-palmitoylation and protein turnover. By combining S-acylation and cotranslational fatty acid chemical reporters with orthogonal clickable fluorophores, dual pulse-chase analysis of Lck revealed accelerated palmitate cycling upon T-cell activation. Subsequent pharmacological perturbation of Lck palmitate turnover suggests yet uncharacterized serine hydrolases contribute to dynamic S-acylation in cells. In addition to dually fatty-acylated proteins, this tandem fluorescence imaging method can be generalized to other S-acylated proteins using azidohomoalanine as a methonine surrogate. The sensitivity and efficiency of this approach should facilitate the functional characterization of cellular factors and drugs that modulate protein S-acylation. Furthermore, diverse protein modifications could be analyzed with this tandem imaging method using other chemical reporters to investigate dynamic regulation of protein function.

TCR-peptide-MHC interactions in situ show accelerated kinetics and increased affinity

The recognition of foreign antigens by T lymphocytes is essential to most adaptive immune responses. It is driven by specific T-cell antigen receptors (TCRs) binding to antigenic peptide-major histocompatibility complex (pMHC) molecules on other cells. If productive, these interactions promote the formation of an immunological synapse. Here we show that synaptic TCR-pMHC binding dynamics differ significantly from TCR-pMHC binding in solution. We used single-molecule microscopy and fluorescence resonance energy transfer (FRET) between fluorescently tagged TCRs and their cognate pMHC ligands to measure the kinetics of TCR-pMHC binding in situ. When compared with solution measurements, the dissociation of this complex was increased significantly (4-12-fold). Disruption of actin polymers reversed this effect, indicating that cytoskeletal dynamics destabilize this interaction directly or indirectly. Nevertheless, TCR affinity for pMHC was significantly elevated as the result of a large (about 100-fold) increase in the association rate, a likely consequence of complementary molecular orientation and clustering. In helper T cells, the CD4 molecule has been proposed to bind cooperatively with the TCR to the same pMHC complex. However, CD4 blockade had no effect on the synaptic TCR affinity, nor did it destabilize TCR-pMHC complexes, indicating that the TCR binds pMHC independently of CD4.

What is the function of the dendritic cell side of the immunological synapse?

The adaptive immune response requires the formation of a specialized interface called the immunological synapse (IS), which is formed between a mature dendritic cell (DC) and a CD4(+) T cell in the lymph node. The IS involves organized motifs formed by cell-surface and cytoplasmic molecules at both the DC side (IS-DC) and the T cell side (IS-T) of the IS. Most studies of the functions of the IS have focused on the IS-T; however, to understand the function(s) of the entire IS, it is also necessary to gain insight into the role(s) of the IS-DC. Unlike T cells, which upon their activation leave the lymph node and return to the circulation, DCs largely become apoptotic and die in the node region. This latter observation and the known stability of the IS, which may last for hours, is consistent with the hypothesis that one of the functions of the IS-DC could be the temporal inhibition of the apoptosis of DCs, which would enable the activation of clonal T cells in the lymph nodes. Here, we discuss experimental data supporting the latter hypothesis, as well as the concept that the IS-DC is a signaling region that contributes to the functions of the IS.

Co-receptors and recognition of self at the immunological synapse

The co-receptors CD4 and CD8 are important in the activation of T cells primarily because of their ability to interact with the proteins of the MHC enhancing recognition of the MHC-peptide complex by the T cell receptor (TCR). An antigen-presenting cell presents a small number of antigenic peptides on its MHC molecules, in the presence of a much larger number of endogenous, mostly nonstimulatory, peptides. Recent work has demonstrated that these endogenous MHC-peptide complexes have an important role in modulating the sensitivity of the TCR. But the role of the endogenous nonstimulatory MHC-peptide complexes differs in MHC class I and class II-restricted T cells. This chapter discusses the data on the role of CD4 or CD8 co-receptors in T cell activation at the immunological synapse, and the role of non stimulatory MHC-peptide complexes in aiding antigen recognition.

Impedance spectroscopy and optical analysis of single biological cells and organisms in microsystems

A novel microfabricated flow cytometer for simultaneous impedance and optical measurement of single cells and particles is described in this chapter. We discuss the sensitivity of the system with regard to the impedance sensor and describe the optical setup. The relevant parameters related to the experimental setup and sample preparation are discussed. The use of dielectrophoretic forces for particle manipulation is presented as a simple enabling technology, which allows the manipulation of particles within microfluidic devices. The fabrication processes required to produce the impedance sensor chips are described with relevance to the chip design and features. Finally, the system is used to discriminate between different marine algae populations.

Homodimerization and heterodimerization of minimal zinc(II)-binding-domain peptides of T-cell proteins CD4, CD8alpha, and Lck

Metal-mediated protein oligomerization is an emerging mode of protein-protein interaction. The C-terminal cytosolic domains of T-cell coreceptors CD4 and CD8alpha form zinc-bridged heterodimers with the N-terminal region of the kinase Lck, with each protein contributing two cysteinate ligands to the complex. Using size exclusion chromatography, (1)H NMR, and UV/visible absorption spectroscopy with cobalt(II) as a spectroscopic probe, we demonstrate that small peptides derived from these regions form metal-bridged heterodimers but also homodimers, in contrast to previous reports. The Lck-CD4 and Lck-CD8alpha cobalt(II)-bridged heterodimer complexes are more stable than the corresponding (Lck)(2)cobalt(II) complex by factors of 11 +/- 4 and 22 +/- 9, respectively. These studies were aided by the discovery that cobalt(II) complexes with a cobalt(II)(-Cys-X-X-Cys-)(-Cys-X-Cys-) chromophore show unusual optical spectra with one component of the visible d-d ((4)A(2)-to-(4)T(1)(P)) transition red-shifted and well separated from the other components. These results provide insights into the basis of specificity of metal-bridged complex formation and on the potential biological significance of metal-bridged homodimers in T-cells.

SOCS-6 negatively regulates T cell activation through targeting p56lck to proteasomal degradation

The T cell-specific tyrosine kinase, p56(lck), plays crucial roles in T cell receptor (TCR)-mediated T cell activation. Here, we report that SOCS-6 (suppressor of cytokine signaling-6) is a negative regulator of p56(lck). SOCS-6 was identified as a protein binding to the kinase domain of p56(lck) through yeast two-hybrid screening. SOCS-6 bound specifically to p56(lck) (F505), which mimics the active form of p56(lck), but not to wild type p56(lck). In Jurkat T cells, SOCS-6 binding to p56(lck) was detected 1-2 h after TCR stimulation. Confocal microscopy showed that upon APC-T cell conjugation, SOCS-6 was recruited to the immunological synapse and colocalized with the active form of p56(lck). SOCS-6 promoted p56(lck) ubiquitination and its subsequent targeting to the proteasome. Moreover, SOCS-6 overexpression led to repression of TCR-dependent interleukin-2 promoter activity. These results establish that SOCS-6 acts as a negative regulator of T cell activation by promoting ubiquitin-dependent proteolysis.

The activation threshold of CD4+ T cells is defined by TCR/peptide-MHC class II interactions in the thymic medulla

Immature thymocytes that are positively selected based upon their response to self-peptide-MHC complexes develop into mature T cells that are not overtly reactive to those same complexes. Developmental tuning is the active process through which TCR-associated signaling pathways of single-positive thymocytes are attenuated to respond appropriately to the peptide-MHC molecules that will be encountered in the periphery. In this study, we explore the mechanisms that regulate the tuning of CD4(+) single-positive T cells to MHC class II encountered in the thymic medulla. Experiments with murine BM chimeras demonstrate that tuning can be mediated by MHC class II expressed by either thymic medullary epithelial cells or thymic dendritic cells. Tuning does not require the engagement of CD4 by MHC class II on stromal cells. Rather, it is mediated by interactions between MHC class II and the TCR. To understand the molecular changes that distinguish immature hyperactive T cells from tuned mature CD4(+) T cells, we compared their responses to TCR stimulation. The altered response of mature CD4 single-positive thymocytes is characterized by the inhibition of ERK activation by low-affinity self-ligands and increased expression of the inhibitory tyrosine phosphatase SHP-1. Thus, persistent TCR engagement by peptide-MHC class II on thymic medullary stroma inhibits reactivity to self-Ags and prevents autoreactivity in the mature repertoire.

A role for rebinding in rapid and reliable T cell responses to antigen

Experimental work has shown that T cells of the immune system rapidly and specifically respond to antigenic molecules presented on the surface of antigen-presenting-cells and are able to discriminate between potential stimuli based on the kinetic parameters of the T cell receptor-antigen bond. These antigenic molecules are presented among thousands of chemically similar endogenous peptides, raising the question of how T cells can reliably make a decision to respond to certain antigens but not others within minutes of encountering an antigen presenting cell. In this theoretical study, we investigate the role of localized rebinding between a T cell receptor and an antigen. We show that by allowing the signaling state of individual receptors to persist during brief unbinding events, T cells are able to discriminate antigens based on both their unbinding and rebinding rates. We demonstrate that T cell receptor coreceptors, but not receptor clustering, are important in promoting localized rebinding, and show that requiring rebinding for productive signaling reduces signals from a high concentration of endogenous pMHC. In developing our main results, we use a relatively simple model based on kinetic proofreading. However, we additionally show that all our results are recapitulated when we use a detailed T cell receptor signaling model. We discuss our results in the context of existing models and recent experimental work and propose new experiments to test our findings.

Regulation of T-cell tolerance by calcium/NFAT signaling

Cells that escape negative selection in the thymus must be inactivated or eliminated in the periphery through a series of mechanisms that include the induction of anergy, dominant suppression by regulatory T cells, and peripheral deletion of self-reactive T cells. Calcium signaling plays a central role in the induction of anergy in T cells, which become functionally inactivated and incapable of proliferating and expressing cytokines following antigen re-encounter. Suboptimal stimulation of T cells results in the activation of a calcium/calcineurin/nuclear factor of activated T cells-dependent cell-intrinsic program of self-inactivation. The proteins encoded by those genes are required to impose a state of functional unresponsiveness through different mechanisms that include downregulation of T-cell receptor signaling and inhibition of cytokine transcription.