Constitutive tyrosine phosphorylation of the T-cell receptor (TCR) zeta subunit: regulation of TCR-associated protein tyrosine kinase activity by TCR zeta

Tonic signaling in CAR-T therapy: the lever long enough to move the planet

Chimeric antigen receptor (CAR) T-cell therapy has shown remarkable efficacy in treating hematological malignancies and is expanding into other indications such as autoimmune diseases, fibrosis, aging and viral infection. However, clinical challenges persist in treating solid tumors, including physical barriers, tumor heterogeneity, poor in vivo persistence, and T-cell exhaustion, all of which hinder therapeutic efficacy. This review focuses on the critical role of tonic signaling in CAR-T therapy. Tonic signaling is a low-level constitutive signaling occurring in both natural and engineered antigen receptors without antigen stimulation. It plays a pivotal role in regulating immune cell homeostasis, exhaustion, persistence, and effector functions. The "Peak Theory" suggests an optimal level of tonic signaling for CAR-T function: while weak tonic signaling may result in poor proliferation and persistence, excessively strong signaling can cause T cell exhaustion. This review also summarizes the recent progress in mechanisms underlying the tonic signaling and strategies to fine-tune the CAR tonic signaling. By understanding and precisely modulating tonic signaling, the efficacy of CAR-T therapies can be further optimized, offering new avenues for treatment across a broader spectrum of diseases. These findings have implications beyond CAR-T cells, potentially impacting other engineered immune cell therapies such as CAR-NK and CAR-M.

Discrete LAT condensates encode antigen information from single pMHC:TCR binding events

LAT assembly into a two-dimensional protein condensate is a prominent feature of antigen discrimination by T cells. Here, we use single-molecule imaging techniques to resolve the spatial position and temporal duration of each pMHC:TCR molecular binding event while simultaneously monitoring LAT condensation at the membrane. An individual binding event is sufficient to trigger a LAT condensate, which is self-limiting, and neither its size nor lifetime is correlated with the duration of the originating pMHC:TCR binding event. Only the probability of the LAT condensate forming is related to the pMHC:TCR binding dwell time. LAT condenses abruptly, but after an extended delay from the originating binding event. A LAT mutation that facilitates phosphorylation at the PLC-γ1 recruitment site shortens the delay time to LAT condensation and alters T cell antigen specificity. These results identify a function for the LAT protein condensation phase transition in setting antigen discrimination thresholds in T cells.

The Role of T Cell Receptor Signaling in the Development of Type 1 Diabetes

T cell receptor (TCR) signaling influences multiple aspects of CD4⁺ and CD8⁺ T cell immunobiology including thymic development, peripheral homeostasis, effector subset differentiation/function, and memory formation. Additional T cell signaling cues triggered by co-stimulatory molecules and cytokines also affect TCR signaling duration, as well as accessory pathways that further shape a T cell response. Type 1 diabetes (T1D) is a T cell-driven autoimmune disease targeting the insulin producing β cells in the pancreas. Evidence indicates that dysregulated TCR signaling events in T1D impact the efficacy of central and peripheral tolerance-inducing mechanisms. In this review, we will discuss how the strength and nature of TCR signaling events influence the development of self-reactive T cells and drive the progression of T1D through effects on T cell gene expression, lineage commitment, and maintenance of pathogenic anti-self T cell effector function.

Modulation of TCR Signaling by Tyrosine Phosphatases: From Autoimmunity to Immunotherapy

Early TCR signaling is dependent on rapid phosphorylation and dephosphorylation of multiple signaling and adaptor proteins, leading to T cell activation. This process is tightly regulated by an intricate web of interactions between kinases and phosphatases. A number of tyrosine phosphatases have been shown to modulate T cell responses and thus alter T cell fate by negatively regulating early TCR signaling. Mutations in some of these enzymes are associated with enhanced predisposition to autoimmunity in humans, and mouse models deficient in orthologous genes often show T cell hyper-activation. Therefore, phosphatases are emerging as potential targets in situations where it is desirable to enhance T cell responses, such as immune responses to tumors. In this review, we summarize the current knowledge about tyrosine phosphatases that regulate early TCR signaling and discuss their involvement in autoimmunity and their potential as targets for tumor immunotherapy.

Narcissistic T cells: reactivity to self makes a difference

It has been appreciated for more than three decades that the interactions between the T-cell antigen receptor and self-antigens are the major determinants of the cell fates of developing thymocytes and the establishment of central tolerance. However, recent evidence shows that the level of self-reactivity substantially contributes to fate choices of positively selected mature T cells in homeostasis, as well as during immune responses. This implies that individual clones of peripheral T cells are predisposed to specific functional properties based on the self-reactivity of their antigen receptors. Overall, the relative difference in the self-reactivity among peripheral T cells is an important factor contributing to the diversity of T-cell responses to foreign antigens.

Active Tonic mTORC1 Signals Shape Baseline Translation in Naive T Cells

Naive CD4⁺ T cells are an example of dynamic cell homeostasis: T cells need to avoid autoreactivity while constantly seeing self-peptides, yet they must be primed to react to foreign antigens during infection. The instructive signals that balance this primed yet quiescent state are unknown. Interactions with self-peptides result in membrane-proximal, tonic signals in resting T cells. Here we reveal selective and robust tonic mTORC1 signals in CD4⁺ T cells that influence T cell fate decisions. We find that the Ras exchange factor Rasgrp1 is necessary to generate tonic mTORC1 signals. Genome-wide ribosome profiling of resting, primary CD4⁺ T cells uncovers a baseline translational landscape rich in mTOR targets linked to mitochondria, oxidative phosphorylation, and splicing. Aberrantly increased tonic mTORC1 signals from a Rasgrp1^Anaef allele result in immunopathology with spontaneous appearance of T peripheral helper cells, follicular helper T cells, and anti-nuclear antibodies that are preceded by subtle alterations in the translational landscape.

Myers et al. evaluate a mouse model of autoimmunity, Rasgrp1^Anaef. They find that T cells with the Rasgrp1^Anaef allele exhibit altered signaling from Rasgrp1 to the mTORC1 pathway in the basal state. They show that increased basal Rasgrp1^Anaef-mTORC1 signals lead to an altered translational landscape in T cells and immunopathology.

mTOR and other effector kinase signals that impact T cell function and activity

T cells play important roles in autoimmune diseases and cancer. Following the cloning of the T cell receptor (TCR), the race was on to map signaling proteins that contributed to T cell activation downstream of the TCR as well as co-stimulatory molecules such as CD28. We term this "canonical TCR signaling" here. More recently, it has been appreciated that T cells need to accommodate increased metabolic needs that stem from T cell activation in order to function properly. A central role herein has emerged for mechanistic/mammalian target of rapamycin (mTOR). In this review we briefly cover canonical TCR signaling to set the stage for discussion on mTOR signaling, mRNA translation, and metabolic adaptation in T cells. We also discuss the role of mTOR in follicular helper T cells, regulatory T cells, and other T cell subsets. Our lab recently uncovered that "tonic signals", which pass through proximal TCR signaling components, are robustly and selectively transduced to mTOR to promote baseline translation of various mRNA targets. We discuss insights on (tonic) mTOR signaling in the context of T cell function in autoimmune diseases such as lupus as well as in cancer immunotherapy through CAR-T cell or checkpoint blockade approaches.

Ionic protein-lipid interactions at the plasma membrane regulate the structure and function of immunoreceptors

Adaptive lymphocytes express a panel of immunoreceptors on the cell surface. Phospholipids are the major components of cell membranes, but they have functional roles beyond forming lipid bilayers. In particular, acidic phospholipids forming microdomains in the plasma membrane can ionically interact with proteins via polybasic sequences, which can have functional consequences for the protein. We have shown that negatively charged acidic phospholipids can interact with positively charged juxtamembrane polybasic regions of immunoreceptors, such as TCR-CD3, CD28 and IgG-BCR, to regulate protein structure and function. Furthermore, we pay our attention to protein transmembrane domains. We show that a membrane-snorkeling Lys residue in integrin αLβ2 regulates transmembrane heterodimer formation and integrin adhesion through ionic interplay with acidic phospholipids and calcium ions (Ca²⁺) in T cells, thus providing a new mechanism of integrin activation. Here, we review our recent progress showcasing the importance of both juxtamembrane and intramembrane ionic protein-lipid interactions.

Optogenetic control shows that kinetic proofreading regulates the activity of the T cell receptor

The immune system distinguishes between self and foreign antigens. The kinetic proofreading (KPR) model proposes that T cells discriminate self from foreign ligands by the different ligand binding half-lives to the T cell receptor (TCR). It is challenging to test KPR as the available experimental systems fall short of only altering the binding half-lives and keeping other parameters of the interaction unchanged. We engineered an optogenetic system using the plant photoreceptor phytochrome B (PhyB) as a ligand to selectively control the dynamics of ligand binding to the TCR by light. This opto-ligand-TCR system was combined with the unique property of PhyB to continuously cycle between the binding and non-binding states under red light, with the light intensity determining the cycling rate and thus the binding duration. Mathematical modeling of our experimental datasets showed that indeed the ligand-TCR interaction half-life is the decisive factor for activating downstream TCR signaling, substantiating KPR.

Tonic Signals: Why Do Lymphocytes Bother?

Since the 1990s it has been known that B and T lymphocytes exhibit low-level, constitutive signaling in the basal state (tonic signaling). These lymphocytes display a range of affinity for self, which in turn generates a range of tonic signaling. Surprisingly, what signaling pathways are active in the basal state and the functional relevance of the observed tonic signaling heterogeneity remain open questions today. Here we summarize what is known about the mechanistic and functional details of tonic signaling. We highlight recent advances that have increased our understanding of how the amount of tonic signal impacts immune function, describing novel tools that have moved the field forward and toward a molecular understanding of tonic signaling.

Tonic LAT-HDAC7 Signals Sustain Nur77 and Irf4 Expression to Tune Naive CD4 T Cells

CD4+ T cells differentiate into T helper cell subsets in feedforward manners with synergistic signals from the T cell receptor (TCR), cytokines, and lineage-specific transcription factors. Naive CD4+ T cells avoid spontaneous engagement of feedforward mechanisms but retain a prepared state. T cells lacking the adaptor molecule LAT demonstrate impaired TCR-induced signals yet cause a spontaneous lymphoproliferative T helper 2 (TH2) cell syndrome in mice. Thus, LAT constitutes an unexplained maintenance cue. Here, we demonstrate that tonic signals through LAT constitutively export the repressor HDAC7 from the nucleus of CD4+ T cells. Without such tonic signals, HDAC7 target genes Nur77 and Irf4 are repressed. We reveal that Nur77 suppresses CD4+ T cell proliferation and uncover a suppressive role for Irf4 in TH2 polarization; halving Irf4 gene-dosage leads to increases in GATA3+ and IL-4+ cells. Our studies reveal that naive CD4+ T cells are dynamically tuned by tonic LAT-HDAC7 signals.

Structural Features of the αβTCR Mechanotransduction Apparatus That Promote pMHC Discrimination

The αβTCR was recently revealed to function as a mechanoreceptor. That is, it leverages mechanical energy generated during immune surveillance and at the immunological synapse to drive biochemical signaling following ligation by a specific foreign peptide-MHC complex (pMHC). Here, we review the structural features that optimize this transmembrane (TM) receptor for mechanotransduction. Specialized adaptations include (1) the CβFG loop region positioned between Vβ and Cβ domains that allosterically gates both dynamic T cell receptor (TCR)-pMHC bond formation and lifetime; (2) the rigid super β-sheet amalgams of heterodimeric CD3εγ and CD3εδ ectodomain components of the αβTCR complex; (3) the αβTCR subunit connecting peptides linking the extracellular and TM segments, particularly the oxidized CxxC motif in each CD3 heterodimeric subunit that facilitates force transfer through the TM segments and surrounding lipid, impacting cytoplasmic tail conformation; and (4) quaternary changes in the αβTCR complex that accompany pMHC ligation under load. How bioforces foster specific αβTCR-based pMHC discrimination and why dynamic bond formation is a primary basis for kinetic proofreading are discussed. We suggest that the details of the molecular rearrangements of individual αβTCR subunit components can be analyzed utilizing a combination of structural biology, single-molecule FRET, optical tweezers, and nanobiology, guided by insightful atomistic molecular dynamic studies. Finally, we review very recent data showing that the pre-TCR complex employs a similar mechanobiology to that of the αβTCR to interact with self-pMHC ligands, impacting early thymic repertoire selection prior to the CD4(+)CD8(+) double positive thymocyte stage of development.

Rasgrp1 mutation increases naive T-cell CD44 expression and drives mTOR-dependent accumulation of Helios⁺ T cells and autoantibodies

Missense variants are a major source of human genetic variation. Here we analyze a new mouse missense variant, Rasgrp1^Anaef, with an ENU-mutated EF hand in the Rasgrp1 Ras guanine nucleotide exchange factor. Rasgrp1^Anaef mice exhibit anti-nuclear autoantibodies and gradually accumulate a CD44^hi Helios⁺ PD-1⁺ CD4⁺ T cell population that is dependent on B cells. Despite reduced Rasgrp1-Ras-ERK activation in vitro, thymocyte selection in Rasgrp1^Anaef is mostly normal in vivo, although CD44 is overexpressed on naïve thymocytes and T cells in a T-cell-autonomous manner. We identify CD44 expression as a sensitive reporter of tonic mTOR-S6 kinase signaling through a novel mouse strain, chino, with a reduction-of-function mutation in Mtor. Elevated tonic mTOR-S6 signaling occurs in Rasgrp1^Anaef naïve CD4⁺ T cells. CD44 expression, CD4⁺ T cell subset ratios and serum autoantibodies all returned to normal in Rasgrp1^AnaefMtor^chino double-mutant mice, demonstrating that increased mTOR activity is essential for the Rasgrp1^Anaef T cell dysregulation.

DOI:http://dx.doi.org/10.7554/eLife.01020.001

Our DNA contains more than three billion nucleotides. Each of these nucleotides can be an A, C, G or T, and groups of three neighboring nucleotides within our DNA are used to represent the 20 amino acids that are used to make proteins. This means that changing just one nucleotide can cause one amino acid to be replaced by a different amino acid in the protein encoded by that stretch of DNA: AAA and AAG code for the amino acid lysine, for example, but AAC and AAT code for asparagine. Known as missense gene variants, these changes can also increase or decrease the expression of the gene.

Every person has thousands of missense gene variants, including about 12,000 inherited from their parents. Sometimes these variants have no consequence, but they can be harmful if replacing the correct amino acid with a different amino acid prevents the protein from performing an important task. In particular, missense gene variants in genes that encode immune system proteins are likely to play a role in diseases of the immune system. For example, variants near a gene called Rasgrp1 have been linked to two autoimmune diseases – type 1 diabetes and Graves’ disease—in which the immune system mistakenly attacks the body’s own cells and tissues.

Now Daley et al. have shed new light on the mechanism by which a missense gene variant in Rasgrp1 can cause autoimmune diseases. Mice with this mutation show signs of autoimmune disease, but their T cells—white blood cells that have a central role in the immune system – develop normally despite this mutation. Instead, Daley et al. found that a specific type of T cell, called T helper cells, accumulated in large numbers in the mutant mice and stimulated cells of a third type—immune cells called B cells—to produce autoantibodies. The production of autoantibodies is a common feature of autoimmune diseases.

Daley et al. traced the origins of the T helper cells to excessive activity on a signaling pathway that involves a protein called mTOR, and went on to show that treatment with the drug rapamycin counteracted the accumulation of the T helper cells and reduced the level of autoimmune activity. In addition to exemplifying how changing just one amino acid change can have a profound effect, the work of Daley et al. is an attractive model for exploring how missense gene variants in people can contribute to autoimmune diseases.

DOI:http://dx.doi.org/10.7554/eLife.01020.002

CD3ζ-chain expression of human T lymphocytes is regulated by TNF via Src-like adaptor protein-dependent proteasomal degradation

Decreased expression of the TCR ζ-chain has been reported in several autoimmune, inflammatory, and malignant diseases, suggesting that ζ-chain downregulation is common at sites of chronic inflammation. Although ζ-chain is critically important in T lymphocyte activation, the mechanism of the decreased ζ-chain expression is less clear. Src-like adaptor protein (SLAP) is a master regulator of T cell activation; previous data have reported that SLAP regulates immunoreceptor signaling. We have examined the mechanism and the functional consequences of CD3 ζ-chain downregulation. TNF treatment of human T lymphocytes (15-40 ng/ml) selectively downregulates CD3 ζ-chain expression in a dose-dependent manner (p < 0.05) and decreases activation-induced IL-2 expression (p < 0.01). Although blocking of the lysosomal compartment fails to restore TNF-induced CD3 ζ-chain downregulation, inhibition of the proteasome prevented the effect of TNF. Both SLAP expression and the colocalization of SLAP with CD3 ζ-chain was enhanced by TNF treatment (p < 0.05 and p < 0.01, respectively), whereas TNF-induced ζ-chain downregulation was inhibited by gene silencing of SLAP with small interfering RNA. SLAP levels of the CD4(+) T lymphocytes isolated from patients with rheumatoid arthritis were more than 2-fold higher than that of the healthy donors' (p < 0.05); moreover, TNF treatment did not alter the SLAP expression of the CD4(+) cells of anti-TNF therapy-treated patients. Our present data suggest that TNF modulates T cell activation during inflammatory processes by regulating the amount of CD3 ζ-chain expression via a SLAP-dependent mechanism. These data provide evidence for SLAP-dependent regulation of CD3 ζ-chain in the fine control of TCR signaling.

Characterization of Ly108 in the thymus: evidence for distinct properties of a novel form of Ly108

Ly108 (CD352) is a member of the signaling lymphocyte activation molecule (SLAM) family of receptors that signals through SLAM-associated protein (SAP), an SH2 domain protein that can function by the recruitment of Src family kinases or by competition with phosphatases. Ly108 is expressed on a variety of hematopoietic cells, with especially high levels on developing thymocytes. We find that Ly108 is constitutively tyrosine phosphorylated in murine thymi in a SAP- and Fyn kinase-dependent manner. Phosphorylation of Ly108 is rapidly lost after thymocyte disaggregation, suggesting dynamic contact-mediated regulation of Ly108. Similar to recent reports, we find at least three isoforms of Ly108 mRNA and protein in the thymus, which are differentially expressed in the thymi of C57BL/6 and 129S6 mice that express the lupus-resistant and lupus-prone haplotypes of Ly108, respectively. Notably, the recently described novel isoform Ly108-H1 is not expressed in mice having the lupus-prone haplotype of Ly108, but is expressed in C57BL/6 mice. We further provide evidence for differential phosphorylation of these isoforms; the novel Ly108-H1does not undergo tyrosine phosphorylation, suggesting that it functions as a decoy isoform that contributes to the reduced overall phosphorylation of Ly108 seen in C57BL/6 mice. Our study suggests that Ly108 is dynamically regulated in the thymus, shedding light on Ly108 isoform expression and phosphorylation.

Basal LAT-diacylglycerol-RasGRP1 signals in T cells maintain TCRα gene expression

In contrast to the well-characterized T cell receptor (TCR) signaling pathways that induce genes that drive T cell development or polarization of naïve CD4 T cells into the diverse T(H)1, T(H)2, T(H)17 and T(reg) lineages, it is unclear what signals maintain specific gene expression in mature resting T cells. Resting T cells residing in peripheral lymphoid organs exhibit low-level constitutive signaling. Whereas tonic signals in B cells are known to be critical for survival, the roles of tonic signals in peripheral T cells are unknown. Here we demonstrate that constitutive signals in Jurkat T cell lines are transduced via the adapter molecule LAT and the Ras exchange factor RasGRP1 to maintain expression of TCRα mRNA and surface expression of the TCR/CD3 complex. Independent approaches of reducing basal activity through the LAT-diacylglycerol-RasGRP pathway led to reduced constitutive Ras-MEK-ERK signals and decreased TCRα mRNA and surface TCR expression in Jurkat cells. However, loss of TCR expression takes several days in these cell line experiments. In agreement with these in vitro approaches, inducible deletion of Lat in vivo results in reduced TCRα mRNA- and surface TCR-expression in a delayed temporal manner as well. Lastly, we demonstrate that loss of basal LAT-RasGRP signals appears to lead to silencing or repression of TCRα transcription. We postulate that basal LAT-diacylglycerol-RasGRP signals fulfill a regulatory function in peripheral T lymphocytes by maintaining proper gene expression programs.

2B4 utilizes ITAM-containing receptor complexes to initiate intracellular signaling and cytolysis

2B4 is a member of the SLAM receptor family capable of activating NK cell cytotoxicity in the context of EBV infection. SAP (SLAM Associated Protein) deficiency causes defective signaling downstream of SLAM family receptors and high susceptibility to EBV. 2B4 costimulates natural cytotoxicity receptor (NCR) and TCR initiated signals to induce cellular cytotoxicity and cytokine release. The 2B4-SAP signal transduction pathway is not predicted to overlap with the TCR-ITAM pathway, although SAP is required for some TCR-induced signals. We therefore examined the functional relationship between SLAM family receptor 2B4 and ITAM-containing adaptor complexes. Removal of FcɛRIγ or CD3ζ-containing complexes, using genetically manipulated cell lines or siRNA specific suppression, significantly reduces 2B4-initiated functions in NK and T cells, respectively. Consistent with this relationship, Syk and ZAP-70 are capable of transducing 2B4 signals for calcium mobilization and cytolysis. Furthermore, ITAM-containing molecules constitutively associate with SAP. These results suggest a potential physical association between 2B4 and the ITAM receptor complexes that is required for 2B4-initiated signaling and cell-mediated killing.

Plasma membrane subdomain partitioning of Lck in primary human T lymphocytes

Uncovering the plasma membrane distribution of tyrosine kinase Lck is crucial to understanding T lymphocyte triggering. Several studies of Lck species partitioning have given contradictory results. We decided to re-address this point by using phospho-specific antibodies to characterize active and inactive Lck partitioning in raft and non-raft membranes from primary human peripheral blood T lymphocytes. We show that most inactive Lck was localized in rafts and was associated with nearly all CD4 coreceptors and its negative regulator Csk in resting cells, while T cell receptor (TCR) engagement promoted a sustained dephosphorylation of inactive Lck. In contrast, active Lck had a more discrete distribution interacting with only a small number of CD4 coreceptors, and the kinase showed a rapid and short phosphorylation after TCR triggering. The differences in distribution and kinetics may be related to T lymphocyte signalling threshold modulation by Lck species and suggest how TCR triggering is first initiated. This study furthers our knowledge of the TCR activation model in primary human T lymphocytes.

CD45-Csk phosphatase-kinase titration uncouples basal and inducible T cell receptor signaling during thymic development

The kinase-phosphatase pair Csk and CD45 reciprocally regulate phosphorylation of the inhibitory tyrosine of the Src family kinases Lck and Fyn. T cell receptor (TCR) signaling and thymic development require CD45 expression but proceed constitutively in the absence of Csk. Here, we show that relative titration of CD45 and Csk expression reveals distinct regulation of basal and inducible TCR signaling during thymic development. Low CD45 expression is sufficient to rescue inducible TCR signaling and positive selection, whereas high expression is required to reconstitute basal TCR signaling and beta selection. CD45 has a dual positive and negative regulatory role during inducible but not basal TCR signaling. By contrast, Csk titration regulates basal but not inducible signaling. High physiologic expression of CD45 is thus required for two reasons-to downmodulate inducible TCR signaling during positive selection and to counteract Csk during basal TCR signaling.

The protein tyrosine phosphatase PTPN4/PTP-MEG1, an enzyme capable of dephosphorylating the TCR ITAMs and regulating NF-kappaB, is dispensable for T cell development and/or T cell effector functions

T cell receptor signaling processes are controlled by the integrated actions of families of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases). Several distinct cytosolic protein tyrosine phosphatases have been described that are able to negatively regulate TCR signaling pathways, including SHP-1, SHP-2, PTPH1, and PEP. Using PTPase substrate-trapping mutants and wild type enzymes, we determined that PTPN4/PTP-MEG1, a PTPH1-family member, could complex and dephosphorylate the ITAMs of the TCR zeta subunit. In addition, the substrate-trapping derivative augmented basal and TCR-induced activation of NF-kappaB in T cells. To characterize the contribution of this PTPase in T cells, we developed PTPN4-deficient mice. T cell development and TCR signaling events were comparable between wild type and PTPN4-deficient animals. The magnitude and duration of TCR-regulated ITAM phosphorylation, as well as overall protein phosphorylation, was unaltered in the absence of PTPN4. Finally, Th1- and Th2-derived cytokines and in vivo immune responses to Listeria monocytogenes were equivalent between wild type and PTPN4-deficient mice. These findings suggest that additional PTPases are involved in controlling ITAM phosphorylations.

The membrane-proximal portion of CD3 epsilon associates with the serine/threonine kinase GRK2

The activation of protein kinases is one of the primary mechanisms whereby T cell receptors (TCR) propagate intracellular signals. To date, the majority of kinases known to be involved in the early stages of TCR signaling are protein-tyrosine kinases such as Lck, Fyn, and ZAP-70. Here we report a constitutive association between the TCR and a serine/threonine kinase, which was mediated through the membrane-proximal portion of CD3 epsilon. Mass spectrometry analysis of CD3 epsilon-associated proteins identified G protein-coupled receptor kinase 2 (GRK2) as a candidate Ser/Thr kinase. Transient transfection assays and Western blot analysis verified the ability of GRK2 to interact with the cytoplasmic domain of CD3 epsilon within a cell. These findings are consistent with recent reports demonstrating the ability of certain G protein-coupled receptors (GPCR) and G proteins to physically associate with the alpha/beta TCR. Because GRK2 is primarily involved in arresting GPCR signals, its interaction with CD3 epsilon may provide a novel means whereby the TCR can negatively regulate signals generated through GPCRs.

The Constitutive Tyrosine Phosphorylation of CD3ζ Results from TCR-MHC Interactions That Are Independent of Thymic Selection

The TCR complex, when isolated from thymocytes and peripheral T cells, contains a constitutively tyrosine-phosphorylated CD3zeta molecule termed p21. Previous investigations have shown that the constitutive phosphorylation of CD3zeta results from TCR interactions with MHC molecules occurring in both the thymus and the periphery. To determine what contribution the selection environment had on this constitutive phosphorylation, we analyzed CD3zeta from several distinct class I- and II-restricted TCR-transgenic mice where thymocyte development occurred in either a selecting or a nonselecting MHC environment. Herein, we report that constitutively phosphorylated CD3zeta (p21) was present in thymocytes that developed under nonselecting peptide-MHC conditions. These findings strongly support the model that the TCR has an inherent avidity for MHC molecules before repertoire selection. Biochemical analyses of the TCR complex before and after TCR stimulation suggested that the constitutively phosphorylated CD3zeta subunit did not contribute to de novo TCR signals. These findings may have important implications for T cell functions during self-MHC recognition under normal and autoimmune circumstances.

The CD3 gamma epsilon/delta epsilon signaling module provides normal T cell functions in the absence of the TCR zeta immunoreceptor tyrosine-based activation motifs

T cell receptor (TCR) signal transduction is mediated by the immunoreceptor tyrosine-based activation motifs (ITAM). The ten ITAM in the TCR complex are distributed in two distinct signaling modules termed TCR zetazeta and CD3 gammaepsilon/deltaepsilon. To delineate the specific role of the zeta ITAM in T cell development and TCR signal transmission, we compared the properties of T cells from different TCR zeta-transgenic lines wherein tyrosine-to-phenylalanine substitutions had been introduced in the zeta subunit. These lines lack selected phosphorylated forms of TCR zeta including just p23, both p21 and p23, or all phospho-zeta derivatives. We report herein that the efficiency of positive selection in HY TCR-transgenic female mice was directly related to the number of zeta ITAM in the TCR. In contrast, TCR-mediated signal transmission and T cell proliferative responses following agonist peptide stimulation were similar and independent of the zeta ITAM. Only the duration of MAPK activation was affected by multiple zeta ITAM substitutions. These results strongly suggest that the ITAM in the CD3 gammaepsilon/deltaepsilon module can provide normal TCR signal transmission, with zeta ITAM providing a secondary function facilitating MAPK activation and positive selection.

Differential Src family kinase activity requirements for CD3 zeta phosphorylation/ZAP70 recruitment and CD3 epsilon phosphorylation

The current model of T cell activation is that TCR engagement stimulates Src family tyrosine kinases (SFK) to phosphorylate CD3zeta. CD3zeta phosphorylation allows for the recruitment of the tyrosine kinase ZAP70, which is phosphorylated and activated by SFK, leading to the phosphorylation of downstream targets. We stimulated mouse CTLs with plate-bound anti-CD3 and, after cell lysis, recovered proteins that associated with the CD3 complex. The protein complexes were not preformed, and a number of tyrosine-phosphorylated proteins were inducibly and specifically associated with the TCR/CD3 complex. These results suggest that complex formation only occurs at the site of TCR engagement. The recruitment and tyrosine phosphorylation of most proteins were abolished when T cells were stimulated in the presence of the SFK inhibitor PP2. Surprisingly, CD3zeta, but not CD3epsilon, was inducibly tyrosine phosphorylated in the presence of PP2. Furthermore, ZAP70 was recruited, but not phosphorylated, after TCR stimulation in the presence of PP2, thus confirming the phosphorylation status of CD3zeta. These data suggest that there is a differential requirement for SFK activity in phosphorylation of CD3zeta vs CD3epsilon. Consistent with this possibility, ZAP70 recruitment was also detected with anti-CD3-stimulated, Lck-deficient human Jurkat T cells. We conclude that TCR/CD3-induced CD3zeta phosphorylation and ZAP70 recruitment do not absolutely require Lck or other PP2-inhibitable SFK activity, but that SFK activity is absolutely required for CD3epsilon and ZAP70 phosphorylation. These data reveal the potential for regulation of signaling through the TCR complex by the differential recruitment or activation of SFK.

Selective expression of the 21-kilodalton tyrosine-phosphorylated form of TCR zeta promotes the emergence of T cells with autoreactive potential

T cells undergo negative selection in the thymus to eliminate potentially autoreactive cells. The signals generated through the alphabeta TCR following receptor interactions with peptide/MHC complexes in the thymus control these selection processes. Following receptor ligation, a fraction of the TCR zeta subunit appears as two distinct tyrosine-phosphorylated forms of 21 and 23 kDa (p21 and p23). Previous data have reported elevated levels of p21 in some murine models of autoimmunity. We have examined the contributions of both the p21 and p23 to T cell negative selection in the HY TCR-transgenic system using ITAM-substituted TCR zeta and CD3 epsilon transgenic mice. Expression of just p21, in the absence of p23, partially impairs negative selection of self-reactive HY-specific T cells. This results in the emergence of potentially autoreactive peripheral T cells and an elevated population of CD11b(+)B220(+) B cells in the spleen. These data clearly identify a specific and unique role for p21 during negative selection.

The Notch regulator Numb links the Notch and TCR signaling pathways

Both the Notch and TCR signaling pathways play an important role in T cell development, but the links between these signaling pathways are largely unexplored. The adapter protein Numb is a well-characterized inhibitor of Notch and also contains a phosphotyrosine binding domain, suggesting that Numb could provide a link between these pathways. We explored this possibility by investigating the physical interactions among Notch, Numb, and the TCR signaling apparatus and by examining the consequences of a Numb mutation on T cell development. We found that Notch and Numb cocluster with the TCR at the APC contact during Ag-driven T cell-APC interactions in both immature and mature T cells. Furthermore, Numb coimmunoprecipitates with components of the TCR signaling apparatus. Despite this association, T cell development and T cell activation occur normally in the absence of Numb, perhaps due to the expression of the related protein, Numblike. Together our data suggest that Notch and TCR signals may be integrated at the cell membrane, and that Numb may be an important adapter in this process.

Activation of naive CD4+ T cells in vivo by a self-peptide mimic: mechanism of tolerance maintenance and preservation of immunity

Intrathymic selection generates a peripheral repertoire of CD4(+) T cells with receptors that retain low affinity for self-peptide MHC complexes. Despite self-recognition, T cells remain tolerant even in the setting of microbial challenge and resultant costimulatory signals. We demonstrate here a novel mechanism for tolerance maintenance under conditions of self-recognition and strong costimulation. TCR engagement in vivo with a low-avidity peptide, as a mimic of self, provided with poly(I:C) (dsRNA) led to division of naive T cells that was dependent upon costimulatory signals; however, the dividing cells rapidly underwent deletion. By contrast, the surviving cells that were activated as evidenced by up-regulation of CD69 did not become effectors upon restimulation with the same ligand and maintained an effective response against agonist peptide. We suggest TCR engagement with self-peptide MHC complexes promotes tolerance maintenance during pathogen challenge, while preserving efficient reactivity for subsequent encounter with foreign Ags.

T cell antagonism is functionally uncoupled from the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The functional effects of altered peptide ligands on T cells is proposed to involve differential intracellular signaling mediated by the 21- and 23-kDa tyrosine-phosphorylated derivatives of the TCR zeta subunit (p21 and p23). To understand the functional contribution of p21 and p23 to T cell development and T cell antagonism, we generated selected TCR zeta transgenic mice maintained on the P14 alphabeta TCR transgenic line such that p23 or both p21 and p23 were selectively eliminated. Importantly, one line (YF1,2) retains the constitutively tyrosine-phosphorylated p21 in the complete absence of inducible p23. We determined that T cell development was uncoupled from p21 and/or p23. Using a series of agonist, weak agonist, and antagonist peptides, we analyzed the role of each of the phosphorylated forms of TCR zeta on T cell activation and antagonism. In this study, we report that the proliferative responses of alphabeta P14 T cells to agonist peptides and the inhibition of proliferation resulting from antagonist peptide treatments was functionally uncoupled from p21 and/or p23. These results suggest that the mechanism of T cell antagonism is independent of the two phosphorylated TCR zeta derivatives.

Self-recognition and the regulation of CD4+ T cell survival

CD4+ T cells differentiate in the thymus from committed precursors to mature naive cells ready for peripheral circulation. Successful maturation depends on adequate but not excessive signaling upon T cell receptor (TCR) engagement of self-peptide/MHC class II molecule ligands present in the thymic environment. Persistent TCR signaling throughout development from the CD4+CD8+ to the CD4+ state is required for completion of the developmental process. Recent work has suggested that a continuation of this signaling is essential for sustained survival of CD4+ T cells once they leave the thymus but our studies suggest otherwise. Although we found clear evidence for active TCR signaling involving recognition of self-ligands in peripheral lymphoid tissues, we did not see a substantial effect of loss of such signaling on the life-time of naive CD4+ T cells. Based on a careful review of the literature, we conclude that essentially all previous claims that MHC class II recognition plays a significant role in the survival of CD4+ T cells can be reinterpreted as an effect of self-recognition on proliferation in lymphopenic environments, maintaining population numbers without a marked effect on individual cell viability. We propose a possible explanation for why, in contrast, the viability of naive CD8+ T cells appears to show such self-MHC dependence and suggest that a primary function of self-recognition by T cells may be to enhance responses to foreign antigen.

Profiling of differentially expressed apoptosis-related genes by cDNA arrays in human cord blood CD34+ cells treated with etoposide

OBJECTIVE

Understanding the molecular events that contribute to survival of and drug-induced apoptosis in hematopoietic stem and progenitor cells (HSC/P) can have impact on more rational approaches to blood cancer therapeutic design, as well as on strategies to minimize toxic side effects of chemotherapeutic drugs. Here we sought to systematically evaluate the basic molecular components and main pathways that govern and mediate cellular response initiated within human CD34(+) cells to etoposide-induced apoptosis.

MATERIALS AND METHODS

Human CD34(+) cells were isolated from umbilical cord blood (CB) and expanded in vitro. Expression of apoptosis-related genes in the control and etoposide treated cells was determined using cDNA array and quantitative real-time RT-PCR.

RESULTS

We identified a set of apoptosis-related genes expressed in highly purified normal human CB CD34(+) cells and determined how the expression of these genes changed in response to etoposide treatment. In addition, TRAIL does not induce apoptosis of normal human CD34(+) cells, and it has no cytotoxic effect on human CD34(+) cells that are undergoing apoptosis in response to growth factor withdrawal. This may be due to upregulation of cytotoxic receptors as well as the decoy receptor for TRAIL, and c-FLIP.

CONCLUSION

p53, c-Myc, and BAFF pathways are main pathways utilized by CD34(+) cells to arrest cell-cycle progression at multiple checkpoints, to halt proliferation, and to induce apoptosis as part of their cellular response to etoposide. Multiple known pro-survival and pro-apoptotic pathways are simultaneously activated in etoposide-treated CD34(+) cells. Also, TRAIL, used alone or in concert with chemotherapeutic drugs, may be of use as a safe blood cancer therapeutic with no or low toxicity for HSC/P.

The formation and functions of the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The interaction between the T cell receptor (TCR) and its cognate antigen/major histocompatibility complex (MHC) complex activates a cascade of intracellular protein phosphorylations within the T cell. The signals are initiated by the specific phosphorylation of two tyrosine residues located in a conserved sequence motif termed an ITAM (immune receptor-based tyrosine activation motif). There are 10 ITAMs in the TCR complex, and 6 of these ITAMs are present in the TCR zeta homodimer. Following TCR stimulation, the TCR zeta subunit forms two tyrosine-phosphorylated intermediates of 21- and 23-kDa, respectively. The dramatic and diverse biological responses of T cells are proposed to be partly regulated by the relative ratios of the 21- vs. 23-kDa phosphorylated forms of TCR zeta that are induced following TCR ligation. In this review, we describe a stepwise model of zeta phosphorylation required for the formation of these two phosphorylated derivatives. We describe the kinases and phosphatases controlling these phosphorylation processes. In addition, we present some preliminary findings from ongoing studies that discuss the contributions of each phosphorylated form of zeta on T cell development, TCR signaling, T cell anergy induction, and T cell survival.

SH2-containing inositol phosphatase (SHIP-1) transiently translocates to raft domains and modulates CD16-mediated cytotoxicity in human NK cells

Membrane recruitment of the SH2-containing 5' inositol phosphatase 1 (SHIP-1) is responsible for the inhibitory signals that modulate phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathways. Here we have investigated the molecular mechanisms underlying SHIP-1 activation and its role in CD16-mediated cytotoxicity. We initially demonstrated that a substantial fraction of SHIP-1-mediated 5' inositol phosphatase activity associates with CD16 zeta chain after receptor cross-linking. Moreover, CD16 stimulation on human primary natural killer (NK) cells induces the rapid and transient translocation of SHIP-1 in the lipid-enriched plasma membrane microdomains, termed rafts, where it associates with tyrosine-phosphorylated zeta chain and shc adaptor protein. As evaluated by confocal microscopy, CD16 engagement by reverse antibody-dependent cellular cytotoxicity (ADCC) rapidly induces SHIP-1 redistribution toward the area of NK cell contact with target cells and its codistribution with aggregated rafts where CD16 receptor also colocalizes. The functional role of SHIP-1 in the modulation of CD16-induced cytotoxicity was explored in NK cells infected with recombinant vaccinia viruses encoding wild-type or catalytic domain-deleted mutant SHIP-1. We found a significant SHIP-1-mediated decrease of CD16-induced cytotoxicity that is strictly dependent on its catalytic activity. These data demonstrate that CD16 engagement on NK cells induces membrane targeting and activation of SHIP-1, which acts as negative regulator of ADCC function.

TCR signals mediated by Src family kinases are essential for the survival of naive T cells

The role of TCR signals triggered by recognition of self MHCs in maintaining the survival of naive peripheral T cells remains controversial. Here we examine the role of the Src family kinases, p56(lck) (Lck) and p59(fyn) (Fyn), in the survival of naive T cells. We show that long term survival requires a combination of signals transduced by Src family kinases and signals through the IL-7R. In the absence of either one, naive T cells die slowly, but if both signals are removed, cell loss is greatly accelerated. The TCR signal can be mediated by either Fyn or Lck at wild-type levels of expression, but not by Lck alone if expressed suboptimally. The disappearance of T cells in the absence of Fyn and Lck was associated with a complete loss of TCRzeta-chain phosphorylation and down-regulation of CD5, both of which are also MHC contact dependent, indicating that the Src family kinases are critical for transducing a TCR-MHC survival signal.

Strong induction of tyrosine phosphorylation, intracellular calcium, nuclear transcription factors and interferongamma, but weak induction of IL-2 in naïve T cells stimulated by bacterial superantigen

The outcome of T cell receptor (TCR) engagement is controlled by the differential recruitment of a variety of pathways, depending on the nature of the TCR ligand. Studies on superantigens (SAGs) were among the first describing such differential signaling; however, reported results are inconsistent. We took a quantitative approach to reinvestigate this question. Using nai;ve T cells from TCR transgenic mice, we found that compared to the antigenic peptide from pigeon cytochrome c, the SAG staphylococcal enterotoxin A very efficiently (100-2000-fold more sensitive on a weight basis) induced tyrosine kinase activity, intracellular calcium increase, and interferon (IFN)gamma production. Up-regulation of CD25 and CD69 and proliferation were less efficiently induced (20-30-fold more sensitive), and interleukin (IL)-2 production was induced least efficiently (only 2-fold more sensitive). This differential activation profile that varies with the activation event analyzed is discussed with respect to the propensity for SAG to induce anergy.

The TRQQKRP motif located near the C-terminus of Rac2 is essential for Rac2 biologic functions and intracellular localization

Rac GTPases regulate a wide variety of cellular processes including actin cytoskeleton organization, gene expression, cell-cycle progression, and apoptosis. Here we report that the TRQQKRP motif of Rac2 located near the C-terminus, a region of sequence disparity among Rac proteins, is essential for complementation of Rac2 function in Rac2-deficient cells. Deletion of this sequence can also intragenically suppress the dominant-negative Rac2D57Nmutation in a variety of functional assays. In Rac2-deficient cells, expression of TRQQKRP-deleted Rac2 protein is unable to completely rescue migration and nicotinamide adenine dinucleotide phosphate oxidase deficiencies previously described in these cells. In fibroblasts, the Rac2D57N mutant phenotypes of abnormal proliferation, cell morphology, and membrane ruffling are suppressed by the TRQQKRP motif deletion. In myeloid hematopoietic cells, the deletion of the TRQQKRP motif eliminates a Rac2D57N-induced block in in vitro differentiation of neutrophils not previously described with this mutant. Mechanistically, deletion of the TRQQKRP motif results in diminished geranylgeranylation and delocalization of intracellular Rac2 protein. Taken together, these results indicate that the TRQQKRP motif in Rac2 protein is required for efficient prenylation and correct intracellular localization of Rac2 protein and is essential for Rac2 to mediate a variety of its biologic functions. These data suggest that precise localization of Rac2 protein in intracellular compartments and/or with other proteins/lipids is a prerequisite for its diverse functions.

Cross‐reactivity in T‐cell antigen recognition

The molecular interactions between the T-cell receptor (TCR) and peptide-MHC (pMHC) have been elucidated in recent years. Nevertheless, the fact that binding of only slightly different ligands by a TCR, or ligation of the same pMHC at different developmental stages of the T cell, can have opposing consequences, continues to pose intellectual challenges. Kinetic proofreading models, which have at their core the dissociation rates of pMHC from the TCR, are best suited to account for these observations. However, T cells can be triggered by peptides with often minimal homology to the primary immunogenic peptide. This cross-reactivity of the TCR is manifest at several levels, from positive selection of immature thymocytes to homeostasis and antigen-cross- reactive immune responses of mature peripheral T cells. The implications of the high cross-reactivity of T-cell antigen recognition for self-tolerance and T-cell memory are discussed.

CD4+ T cell survival is not directly linked to self-MHC-induced TCR signaling

T cell receptor (TCR) signaling triggered by recognition of self-major histocompatibility complex (MHC) ligands has been proposed to maintain the viability of naïve T cells and to provoke their proliferation in T cell-deficient hosts. Consistent with this, the partially phosphorylated state of TCR zeta chains in naïve CD4+ and CD8+ T cells in vivo was found to be actively maintained by TCR interactions with specific peptide-containing MHC molecules. TCR ligand-dependent phosphorylation of TCR zeta was lost within one day of cell transfer into MHC-deficient hosts, yet the survival of transferred CD4+ lymphocytes was the same in recipients with or without MHC class II expression for one month. Thus, despite clear evidence for TCR signaling in nonactivated naïve T cells, these data argue against the concept that such signaling plays a predominant role in determining lymphocyte lifespan.

A population of in vivo anergized T cells with a lower activation threshold for the induction of CD25 exhibit differential requirements in mobilization of intracellular calcium and mitogen-activated protein kinase activation

Chronic exposure of mature T cells with specificity for self-Ags can lead to the induction of a nonfunctional state which is referred to as T cell anergy. It is unclear whether anergic T cells are destined for cell death and thereby harmless or whether they can contribute to the induction of autoimmunity and/or regulation of anti-self reactivity. We have begun to address this issue. In a recent study, we showed that a population of mature CD4-CD8- T cells that express a transgenic TCR specific for the Ld MHC class I molecule are rendered anergic in Ld-expressing mice. In this study, we show that this population of anergic T cells possess a lower activation threshold for the induction of CD25 and CD69 in response to stimulation by antigenic ligands. Furthermore, these anergic T cells undergo extensive proliferation when stimulated with a low-affinity ligand in the presence of an exogenous source of IL-2. Biochemical analysis of the early intracellular signaling events of these in vivo anergized T cells showed that they have a signaling defect at the level of ZAP-70 and linker for the activation of T cell (LAT) phosphorylation. They also exhibit a defect in mobilization of intracellular calcium in response to TCR signaling. However, these anergic T cells demonstrate no defect in SLP-76 phosphorylation and extracellular signal-regulated kinase 1/2 activation. These biochemical characteristics of the anergic T cells were associated with an elevated level of Fyn, but not Lck expression. The potential contributions of these anergic T cells in the induction and/or regulation of autoimmune responses are discussed.

Tetracycline-controllable selection of CD4(+) T cells: half-life and survival signals in the absence of major histocompatibility complex class II molecules

A system that allows the study, in a gentle fashion, of the role of MHC molecules in naive T cell survival is described. Major histocompatibility complex class II-deficient mice were engineered to express Ealpha chains only in thymic epithelial cells in a tetracycline (tet)-controllable manner. This resulted in tet-responsive display of cell surface E complexes, positive selection of CD4(+)8(-) thymocytes, and generation of a CD4(+) T cell compartment in a class II-barren periphery. Using this system, we have addressed two unresolved issues: the half-life of naive CD4(+) T cells in the absence of class II molecules (3-4 wk) and the early signaling events associated with class II molecule engagement by naive CD4(+) T cells (partial CD3 zeta chain phosphorylation and ZAP-70 association).

Tyrosine kinase-dependent ubiquitination of CD16 zeta subunit in human NK cells following receptor engagement

We investigated whether aggregation of the low-affinity immunoglobulin G receptor (CD16) on human NK cells results in receptor ubiquitination. We found that the CD16 zeta subunit becomes ubiquitinated in response to receptor engagement. We then investigated whether protein tyrosine kinase (PTK) activation is required for CD16-mediated receptor ubiquitination. Pretreatment with the PTK inhibitor genistein substantially decreased ligand-induced zeta ubiquitination, suggesting a requirement for PTK activation in receptor ubiquitination. We further analyzed PTK involvement in controlling receptor ubiquitination by using the vaccinia virus expression system. Overexpression of wild-type active lck, but not a kinase-deficient mutant, enhanced both ligand-induced tyrosine phosphorylation and ubiquitination of the CD16 zeta subunit. Taken together, our data demonstrate that CD16 engagement induces zeta chain ubiquitination and strongly suggest a role for lck in regulating this modification.

T cell receptor-mediated signs and signals governing T cell development

The developmental fate of T cells is largely controlled by the nature and success of signals mediated by the pre-T cell receptor (TCR) and TCR complexes. These intracellular signals are regulated by cascades of protein tyrosine phosphorylations initiated following ligand binding to the pre-TCR or TCR complexes. The phosphorylation cascades are primarily orchestrated by two distinct families of protein tyrosine kinases (PTKs), the Src- and the Syk/ZAP-70-families. Germline gene targeting experiments, several human immunodeficiencies, and somatic cell mutants have all contributed to our understanding of how these families of kinases coordinate their actions to promote signaling. Upon activation, the PTKs transmit their signals to a number of newly described adaptor proteins including LAT, SLP-76, and vav, among others. The following review combines results derived from different experimental strategies to examine the contributions of the PTKs and the adaptor molecules to pre-TCR and TCR signaling processes.

The dynamics of T cell receptor signaling: complex orchestration and the key roles of tempo and cooperation

T cells constantly sample their environment using receptors (TCR) that possess both a germline-encoded low affinity for major histocompatibility complex (MHC) molecules and a highly diverse set of CDR3 regions contributing to a range of affinities for specific peptides bound to these MHC molecules. The decision of a T cell "to sense and to respond" with proliferation and effector activity rather than "to sense, live on, but not respond" is dependent on TCR interaction with a low number of specific foreign peptide:MHC molecule complexes recognized simultaneously with abundant self peptide-containing complexes. Interaction with self-complexes alone, on the other hand, generates a signal for survival without a full activation response. Current models for how this distinction is achieved are largely based on translating differences in receptor affinity for foreign versus self ligands into intracellular signals that differ in quality, intensity, and/or duration. A variety of rate-dependent mechanisms involving assembly of molecular oligomers and enzymatic modification of proteins underlie this differential signaling. Recent advances have been made in measuring TCR:ligand interactions, in understanding the biochemical origin of distinct proximal and distal signaling events resulting from TCR binding to various ligands, and in appreciating the role of feedback pathways. This new information can be synthesized into a model of how self and foreign ligand recognition each evoke the proper responses from T cells, how these two classes of signaling events interact, and how pathologic responses may arise as a result of the underlying properties of the system. The principles of signal spreading and stochastic resonance incorporated into this model reveal a striking similarity in mechanisms of decision-making among T cells, neurons, and bacteria.

Distinct signals mediate maturation and allelic exclusion in lymphocyte progenitors

Successful in-frame rearrangement of immunoglobulin heavy chain genes or T cell antigen receptor (TCR) beta chain genes in lymphocyte progenitors results in formation of pre-BCR and pre-TCR complexes. These complexes signal progenitor cells to mature, expand in cell number, and suppress further rearrangements at the immunoglobulin heavy chain or TCRbeta chain loci, thereby ensuring allelic exclusion. We used transgenic expression of a constitutively active form of c-Raf-1 (Raf-CAAX) to demonstrate that activation of the Map kinase pathway can stimulate both maturation and expansion of B and T lymphocytes, even in the absence of pre-TCR or pre-BCR formation. However, the same Raf signal did not mediate allelic exclusion. We conclude that maturation of lymphocyte progenitors and allelic exclusion require distinct signals.

Molecular requirements for T cell recognition by a major histocompatibility complex class II-restricted T cell receptor: the involvement of the fourth hypervariable loop of the Valpha domain

The role of two central residues (K68, E69) of the fourth hypervariable loop of the Valpha domain (HV4alpha) in antigen recognition by an MHC class II-restricted T cell receptor (TCR) has been analyzed. The TCR recognizes the NH2-terminal peptide of myelin basic protein (Ac1-11, acetylated at NH2 terminus) associated with the class II MHC molecule I-Au. Lysine 68 (K68) and glutamic acid 69 (E69) of HV4alpha have been mutated both individually and simultaneously to alanine (K68A, E69A). The responsiveness of transfectants bearing wild-type and mutated TCRs to Ac1-11-I-Au complexes has been analyzed in the presence and absence of expression of the coreceptor CD4. The data demonstrate that in the absence of CD4 expression, K68 plays a central role in antigen responsiveness. In contrast, the effect of mutating E69 to alanine is less marked. CD4 coexpression can partially compensate for the loss of activity of the K68A mutant transfectants, resulting in responses that, relative to those of the wild-type transfectants, are highly sensitive to anti-CD4 antibody blockade. The observations support models of T cell activation in which both the affinity of the TCR for cognate ligand and the involvement of coreceptors determine the outcome of the T cell-antigen-presenting cell interaction.

Thymocyte selection: not by TCR alone

During development of T cells in the thymus, T-cell receptor (TCR)-mediated recognition of self-MHC/self-peptide complexes on thymic stroma dictates the developmental fate of immature CD4+CD8+ (double positive) thymocytes. Intriguingly, TCR-generated intracellular signals can elicit two entirely different cellular responses in such thymocytes: apoptosis or further differentiation. The critical issue in understanding end-stage T-cell development is how TCR occupancy can be perceived in such markedly different ways by the TCR. Here, we review the cytoplasmic and nuclear events that result from TCR signaling during thymocyte selection. Studies aimed at distinguishing molecular components involved in positive selection (resulting in signals for further differentiation) and negative selection (resulting in apoptosis) will help solve this fascinating feature of T-lymphocyte biology. We also discuss how non-TCR-derived signaling might serve to fine tune the TCR-driven selection events in thymocytes. Central to this aspect of the conceptual framework needed to explain thymocyte selection is the observation that thymic antigen-presenting cells appear to be specialized in the induction of either positive or negative selection. Finally, we suggest a hypothesis that integrates the facts currently available on developing thymocytes, and which may serve to refine our exploration of unresolved issues in thymocyte selection. This hypothesis expands our focus to include signals from receptors other than TCRs as modulating and amplifying factors in thymocyte signaling.

CD3 ligation on immature thymocytes generates antagonist-like signals appropriate for CD8 lineage commitment, independently of T cell receptor specificity

The signals that direct differentiation of T cells to the CD4 or CD8 lineages in the thymus remain poorly understood. Although it has been relatively easy to direct differentiation of CD4 single positive (CD4+) cells using combinations of antibodies and pharmacological agents that mimic receptor engagements, equivalent stimuli do not induce efficient maturation of CD8+ cells. Here we report that, irrespective of the MHC-restriction specificity of the TCR, differentiation of mature CD8+ thymocytes can be induced by ligation of CD3 polypeptides on immature thymocytes with a F(ab')2 reagent (CD3fos-F(ab')2). The tyrosine phosphorylation patterns stimulated by CD3fos-F(ab')2 have been shown to resemble those delivered to mature T cells by antagonist peptides, which are known to direct positive selection of CD8+ cells, and we can show that this reagent exhibits potent antagonistic-like activity for primary T cell responses. Our results suggest a distinction in the signals that specify lineage commitment in the thymus. We present a model of thymocyte differentiation that proposes that the relative balance of signals delivered by TCR engagement and by p56lck activation is responsible for directing commitment to the CD8 or CD4 lineages.

Tyrosine kinase chimeras for antigen-selective T-body therapy

Protein tyrosine kinases (PTKs) transmit activation signals in almost every cell type, including immune effector cells. The aberrant or constitutive activation of PTKs can often cause neoplastic transformation. The use of chimeric receptors based on PTKs may enable us to elucidate the signaling pathways of normal immune cells and other cell types, and the abnormal events that can lead to malignant transformation. In this review, we focus on antigen specific chimeric PTKs in which antibody-derived scFv are joined to the Syk family of PTKs. These chimeric receptors yielded reagents that can selectively redirect immune effector cells and specifically activate them to produce cytokines or lyse their target. The advantages of using such PTK-based chimeras to redirect lymphocytes to tumor targets and their potential as an immunotherapeutic approach to malignant disease is discussed.