The zeta chain is associated with a tyrosine kinase and upon T-cell antigen receptor stimulation associates with ZAP-70, a 70-kDa tyrosine phosphoprotein

Phospho-mimetic CD3ε variants prevent TCR and CAR signaling

Introduction

Antigen binding to the T cell antigen receptor (TCR) leads to the phosphorylation of the immunoreceptor tyrosine-based activation motifs (ITAMs) of the CD3 complex, and thereby to T cell activation. The CD3ε subunit plays a unique role in TCR activation by recruiting the kinase LCK and the adaptor protein NCK prior to ITAM phosphorylation. Here, we aimed to investigate how phosphorylation of the individual CD3ε ITAM tyrosines impacts the CD3ε signalosome.

Methods

We mimicked irreversible tyrosine phosphorylation by substituting glutamic acid for the tyrosine residues in the CD3ε ITAM.

Results

Integrating CD3ε phospho-mimetic variants into the complete TCR-CD3 complex resulted in reduced TCR signal transduction, which was partially compensated by the involvement of the other TCR-CD3 ITAMs. By using novel CD3ε phospho-mimetic Chimeric Antigen Receptor (CAR) variants, we avoided any compensatory effects of other ITAMs in the TCR-CD3 complex. We demonstrated that irreversible CD3ε phosphorylation prevented signal transduction upon CAR engagement. Mechanistically, we demonstrated that glutamic acid substitution at the N-terminal tyrosine residue of the CD3ε ITAM (Y39E) significantly reduces NCK binding to the TCR. In contrast, mutation at the C-terminal tyrosine of the CD3ε ITAM (Y50E) abolished LCK recruitment to the TCR, while increasing NCK binding. Double mutation at the C- and N-terminal tyrosines (Y39/50E) allowed ZAP70 to bind, but reduced the interaction with LCK and NCK.

Conclusions

The data demonstrate that the dynamic phosphorylation of the CD3ε ITAM tyrosines is essential for CD3ε to orchestrate optimal TCR and CAR signaling and highlights the key role of CD3ε signalosome to tune signal transduction.

Establishment of Anti-Dog Programmed Cell Death Ligand 1 Monoclonal Antibodies for Immunohistochemistry

Immune checkpoint blockade therapy has shown successful clinical outcomes in multiple human cancers. In dogs, several types of tumors resemble human tumors in many respects. Therefore, several groups have developed the anti-dog programmed cell death ligand 1 (dPD-L1) monoclonal antibodies (mAbs) and showed efficacy in several canine tumors. To examine the abundance of dPD-L1 in canine tumors, anti-dPD-L1 diagnostic mAbs for immunohistochemistry are required. In this study, we immunized the peptide in the dPD-L1 intracellular domain, and established anti-dPD-L1 mAbs, L1Mab-352 (mouse IgG1, kappa), and L1Mab-354 (mouse IgG1, kappa). In enzyme-linked immunosorbent assay, L1Mab-352 and L1Mab-354 showed high-binding affinity to the dPD-L1 peptide, and the dissociation constants (KD) were determined as 6.9 × 10-10 M and 7.2 × 10-10 M, respectively. Furthermore, L1Mab-352 and L1Mab-354 were applicable for the detection of dPD-L1 in immunohistochemical analysis in paraffin-embedded dPD-L1-overexpressed cells. These results indicated that L1Mab-352 and L1Mab-354 are useful for detecting dPD-L1 in immunohistochemical analysis.

Optical sensing and control of T cell signaling pathways

T cells regulate adaptive immune responses through complex signaling pathways mediated by T cell receptor (TCR). The functional domains of the TCR are combined with specific antibodies for the development of chimeric antigen receptor (CAR) T cell therapy. In this review, we first overview current understanding on the T cell signaling pathways as well as traditional methods that have been widely used for the T cell study. These methods, however, are still limited to investigating dynamic molecular events with spatiotemporal resolutions. Therefore, genetically encoded biosensors and optogenetic tools have been developed to study dynamic T cell signaling pathways in live cells. We review these cutting-edge technologies that revealed dynamic and complex molecular mechanisms at each stage of T cell signaling pathways. They have been primarily applied to the study of dynamic molecular events in TCR signaling, and they will further aid in understanding the mechanisms of CAR activation and function. Therefore, genetically encoded biosensors and optogenetic tools offer powerful tools for enhancing our understanding of signaling mechanisms in T cells and CAR-T cells.

Cellular and molecular imaging of CAR-T cell-based immunotherapy

Chimeric Antigen Receptor T cell (CAR-T) therapy has emerged as a transformative therapeutic strategy for hematological malignancies. However, its efficacy in treating solid tumors remains limited. An in-depth and comprehensive understanding of CAR-T cell signaling pathways and the ability to track CAR-T cell biodistribution and activation in real-time within the tumor microenvironment will be instrumental in designing the next generation of CAR-T cells for solid tumor therapy. This review summarizes the signaling network and the cellular and molecular imaging tools and platforms that are utilized in CAR-T cell-based immune therapies, covering both in vitro and in vivo studies. Firstly, we provide an overview of the existing understanding of the activation and cytotoxic mechanisms of CAR-T cells, compared to the mechanism of T cell receptor (TCR) signaling pathways. We further describe the commonly employed tools for live cell imaging, coupled with recent research progress, with a focus on genetically encoded fluorescent proteins (FPs) and biosensors. We then discuss the utility of diverse in vivo imaging modalities, including fluorescence and bioluminescence imaging, Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), and photoacoustic (PA) imaging, for noninvasive monitoring of CAR-T cell dynamics within tumor tissues, thereby providing critical insights into therapy's strengths and weaknesses. Lastly, we discuss the current challenges and future directions of CAR-T cell therapy from the imaging perspective. We foresee that a comprehensive and integrative approach to CAR-T cell imaging will enable the development of more effective treatments for solid tumors in the future.

A Story of Kinases and Adaptors: The Role of Lck, ZAP-70 and LAT in Switch Panel Governing T-Cell Development and Activation

Specific antigen recognition is one of the immune system's features that allows it to mount intense yet controlled responses to an infinity of potential threats. T cells play a relevant role in the host defense and the clearance of pathogens by means of the specific recognition of peptide antigens presented by antigen-presenting cells (APCs), and, to do so, they are equipped with a clonally distributed antigen receptor called the T-cell receptor (TCR). Upon the specific engagement of the TCR, multiple intracellular signals are triggered, which lead to the activation, proliferation and differentiation of T lymphocytes into effector cells. In addition, this signaling cascade also operates during T-cell development, allowing for the generation of cells that can be helpful in the defense against threats, as well as preventing the generation of autoreactive cells. Early TCR signals include phosphorylation events in which the tyrosine kinases Lck and ZAP70 are involved. The sequential activation of these kinases leads to the phosphorylation of the transmembrane adaptor LAT, which constitutes a signaling hub for the generation of a signalosome, finally resulting in T-cell activation. These early signals play a relevant role in triggering the development, activation, proliferation and apoptosis of T cells, and the negative regulation of these signals is key to avoid aberrant processes that could generate inappropriate cellular responses and disease. In this review, we will examine and discuss the roles of the tyrosine kinases Lck and ZAP70 and the membrane adaptor LAT in these cellular processes.

The Interplay between T Cells and Cancer: The Basis of Immunotherapy

Over the past decade, immunotherapy has emerged as one of the most promising approaches to cancer treatment. The use of immune checkpoint inhibitors has resulted in impressive and durable clinical responses in the treatment of various cancers. Additionally, immunotherapy utilizing chimeric antigen receptor (CAR)-engineered T cells has produced robust responses in blood cancers, and T cell receptor (TCR)-engineered T cells are showing promising results in the treatment of solid cancers. Despite these noteworthy advancements in cancer immunotherapy, numerous challenges remain. Some patient populations are unresponsive to immune checkpoint inhibitor therapy, and CAR T cell therapy has yet to show efficacy against solid cancers. In this review, we first discuss the significant role that T cells play in the body's defense against cancer. We then delve into the mechanisms behind the current challenges facing immunotherapy, starting with T cell exhaustion due to immune checkpoint upregulation and changes in the transcriptional and epigenetic landscapes of dysfunctional T cells. We then discuss cancer-cell-intrinsic characteristics, including molecular alterations in cancer cells and the immunosuppressive nature of the tumor microenvironment (TME), which collectively facilitate tumor cell proliferation, survival, metastasis, and immune evasion. Finally, we examine recent advancements in cancer immunotherapy, with a specific emphasis on T-cell-based treatments.

Cyclophilin A associates with and regulates the activity of ZAP70 in TCR/CD3-stimulated T cells

The ZAP70 protein tyrosine kinase (PTK) couples stimulated T cell antigen receptors (TCRs) to their downstream signal transduction pathways and is sine qua non for T cell activation and differentiation. TCR engagement leads to activation-induced post-translational modifications of ZAP70, predominantly by kinases, which modulate its conformation, leading to activation of its catalytic domain. Here, we demonstrate that ZAP70 in TCR/CD3-activated mouse spleen and thymus cells, as well as human Jurkat T cells, is regulated by the peptidyl-prolyl cis-trans isomerase (PPIase), cyclophilin A (CypA) and that this regulation is abrogated by cyclosporin A (CsA), a CypA inhibitor. We found that TCR crosslinking promoted a rapid and transient, Lck-dependent association of CypA with the interdomain B region, at the ZAP70 regulatory domain. CsA inhibited CypA binding to ZAP70 and prevented the colocalization of CypA and ZAP70 at the cell membrane. In addition, imaging analyses of antigen-specific T cells stimulated by MHC-restricted antigen-fed antigen-presenting cells revealed the recruitment of ZAP70-bound CypA to the immunological synapse. Enzymatically active CypA downregulated the catalytic activity of ZAP70 in vitro, an effect that was reversed by CsA in TCR/CD3-activated normal T cells but not in CypA-deficient T cells, and further confirmed in vivo by FRET-based studies. We suggest that CypA plays a role in determining the activity of ZAP70 in TCR-engaged T cells and impact on T cell activation by intervening with the activity of multiple downstream effector molecules.

ZAP70 Activation Compensates for Loss of Class IA PI3K Isoforms Through Activation of the JAK-STAT3 Pathway

BACKGROUND/AIM

Tyrosine kinases have crucial functions in cell signaling and proliferation. The phosphatidylinositol 3-kinase (PI3K) pathway is frequently deregulated in human cancer and is an essential regulator of cellular proliferation. We aimed to determine which tyrosine kinases contribute to resistance elicited by PI3K silencing and inhibition.

MATERIALS AND METHODS

To mimic catalytic inactivation of p110α/β, specific p110α (BYL719) and p110β (KIN193) inhibitors were used in addition to genetic knock-out in in vitro assays. Cell viability was assessed using crystal violet staining, whereas cellular transformation ability was analyzed by soft-agar growth assays.

RESULTS

Activated zeta chain of T-cell receptor-associated protein kinase 70 (ZAP70) generated resistance to PI3K inhibition. This resistance was via activation of the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) axis. We demonstrated that activated ZAP70 has a high transforming capability associated with the formation of malignant phenotype in untransformed cells and has the potential to be a tumor-initiating factor in cancer cells.

CONCLUSION

ZAP70 may be a potent driver of proliferation and transformation in untransformed cells and is implicated in resistance to PI3K inhibitors in cancer cells.

Regulating the discriminatory response to antigen by T-cell receptor

The cell-mediated immune response constitutes a robust host defense mechanism to eliminate pathogens and oncogenic cells. T cells play a central role in such a defense mechanism and creating memories to prevent any potential infection. T cell recognizes foreign antigen by its surface receptors when presented through antigen-presenting cells (APCs) and calibrates its cellular response by a network of intracellular signaling events. Activation of T-cell receptor (TCR) leads to changes in gene expression and metabolic networks regulating cell development, proliferation, and migration. TCR does not possess any catalytic activity, and the signaling initiates with the colocalization of several enzymes and scaffold proteins. Deregulation of T cell signaling is often linked to autoimmune disorders like severe combined immunodeficiency (SCID), rheumatoid arthritis, and multiple sclerosis. The TCR remarkably distinguishes the minor difference between self and non-self antigen through a kinetic proofreading mechanism. The output of TCR signaling is determined by the half-life of the receptor antigen complex and the time taken to recruit and activate the downstream enzymes. A longer half-life of a non-self antigen receptor complex could initiate downstream signaling by activating associated enzymes. Whereas, the short-lived, self-peptide receptor complex disassembles before the downstream enzymes are activated. Activation of TCR rewires the cellular metabolic response to aerobic glycolysis from oxidative phosphorylation. How does the early event in the TCR signaling cross-talk with the cellular metabolism is an open question. In this review, we have discussed the recent developments in understanding the regulation of TCR signaling, and then we reviewed the emerging role of metabolism in regulating T cell function.

ZAP70 activation is an early event of T cell immunity that involved in the anti-bacterial adaptive immune response of Nile tilapia

ZAP70 is essential for initiating the early events of T-cell antigen receptor (TCR) signaling cascade to ensure proper T cell activation and function. However, whether this molecule takes part in the T cell immune response of early vertebrates remains unclear. In the present study, using a teleost model Nile tilapia (Oreochromis niloticus), we investigated the potential involvement of ZAP70 in the T cell activation and adaptive immunity of fish species. Both primary and tertiary structures of O. niloticus ZAP70 (On-ZAP70) are highly conserved with those from other vertebrates. On-ZAP70 protein was widely expressed in lymphoid tissues, and with the highest level in thymus. Once Nile tilapia was infected by Aeromonas hydrophila, mRNA of On-ZAP70 in spleen lymphocytes was induced on day 5 and 8 after infection; meanwhile, phosphorylation of On-ZAP70 was also enhanced, suggesting that On-ZAP70 potentially participated in primary adaptive immune response of Nile tilapia. Furthermore, the frequency of ZAP70 positive lymphocytes was increased during the anti-bacterial adaptive immune response. More importantly, when spleen lymphocytes were activated by T cell specific mitogen PHA, a dramatical augment of On-ZAP70 could be observed at transcription, phosphorylation and cellular level, indicating the involvement of this molecule in T cells activation of Nile tilapia. Altogether, our results demonstrated that ZAP70 activation is an early event of T cell immunity that involved in the anti-bacterial adaptive immune response of Nile tilapia, and thus provided a new evidence to understand the evolution of the lymphocyte-mediated adaptive immunity.

A rapid CRISPR competitive assay for in vitro and in vivo discovery of potential drug targets affecting the hematopoietic system

CRISPR/Cas9 can be used as an experimental tool to inactivate genes in cells. However, a CRISPR-targeted cell population will not show a uniform genotype of the targeted gene. Instead, a mix of genotypes is generated - from wild type to different forms of insertions and deletions. Such mixed genotypes complicate analysis of the role of the targeted gene in the studied cell population. Here, we present a rapid and universal experimental approach to functionally analyze a CRISPR-targeted cell population that does not involve generating clonal lines. As a simple readout, we leverage the CRISPR-induced genetic heterogeneity and use sequencing to identify how different genotypes are enriched or depleted in relation to the studied cellular behavior or phenotype. The approach uses standard PCR, Sanger sequencing, and a simple sequence deconvoluting software, enabling laboratories without specific in-depth experience to perform these experiments. As proof of principle, we present examples studying various aspects related to hematopoietic cells (T cell development in vivo and activation in vitro, differentiation of macrophages and dendritic cells, as well as a leukemia-like phenotype induced by overexpressing a proto-oncogene). In conclusion, we present a rapid experimental approach to identify potential drug targets related to mature immune cells, as well as normal and malignant hematopoiesis.

Reversing Post-Infectious Epigenetic-Mediated Immune Suppression

The immune response must balance the pro-inflammatory, cell-mediated cytotoxicity with the anti-inflammatory and wound repair response. Epigenetic mechanisms mediate this balance and limit host immunity from inducing exuberant collateral damage to host tissue after severe and chronic infections. However, following treatment for these infections, including sepsis, pneumonia, hepatitis B, hepatitis C, HIV, tuberculosis (TB) or schistosomiasis, detrimental epigenetic scars persist, and result in long-lasting immune suppression. This is hypothesized to be one of the contributing mechanisms explaining why survivors of infection have increased all-cause mortality and increased rates of unrelated secondary infections. The mechanisms that induce epigenetic-mediated immune suppression have been demonstrated in-vitro and in animal models. Modulation of the AMP-activated protein kinase (AMPK)-mammalian target of rapamycin (mTOR), nuclear factor of activated T cells (NFAT) or nuclear receptor (NR4A) pathways is able to block or reverse the development of detrimental epigenetic scars. Similarly, drugs that directly modify epigenetic enzymes, such as those that inhibit histone deacetylases (HDAC) inhibitors, DNA hypomethylating agents or modifiers of the Nucleosome Remodeling and DNA methylation (NuRD) complex or Polycomb Repressive Complex (PRC) have demonstrated capacity to restore host immunity in the setting of cancer-, LCMV- or murine sepsis-induced epigenetic-mediated immune suppression. A third clinically feasible strategy for reversing detrimental epigenetic scars includes bioengineering approaches to either directly reverse the detrimental epigenetic marks or to modify the epigenetic enzymes or transcription factors that induce detrimental epigenetic scars. Each of these approaches, alone or in combination, have ablated or reversed detrimental epigenetic marks in in-vitro or in animal models; translational studies are now required to evaluate clinical applicability.

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

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

ZAP-70 Shapes the Immune Microenvironment in B Cell Malignancies

Zeta-chain-associated protein kinase-70 (ZAP-70) is a tyrosine kinase mainly expressed in T cells, NK cells and a subset of B cells. Primarily it functions in T cell receptor (TCR) activation through its tyrosine kinase activity. Aberrant expression of ZAP-70 has been evidenced in different B cell malignancies, with high expression of ZAP-70 in a subset of patients with Chronic Lymphocytic Leukemia (CLL), associating with unfavorable disease outcomes. Previous studies to understand the mechanisms underlying this correlation have been focused on tumor intrinsic mechanisms, including the activation of B cell receptor (BCR) signaling. Recent evidence also suggests that ZAP-70, intrinsically expressed in tumor cells, can modulate the cross-talk between malignant B cells and the immune environment, implying a more complex role of ZAP-70 in the pathogenesis of B cell malignancies. Meanwhile, the indispensible roles of ZAP-70 in T cell and NK cell activation also demonstrate that the autologous expression of ZAP-70 in the immune environment can be a central target in modulation of tumor immunity. Here we review the evidences of the link between ZAP-70 and tumor immunology in the microenvironment in B cell malignancies. Considering an emerging role of immunotherapies in treating these conditions, understanding the distinct molecular functions of ZAP-70 in a broader cellular context could ultimately benefit patient care.

Increased Protein Stability and Interleukin-2 Production of a LATG131D Variant With Possible Implications for T Cell Anergy

The adaptor LAT plays a crucial role in the transduction of signals coming from the TCR/CD3 complex. Phosphorylation of some of its tyrosines generates recruitment sites for other cytosolic signaling molecules. Tyrosine 132 in human LAT is essential for PLC-γ activation and calcium influx generation. It has been recently reported that a conserved glycine residue preceding tyrosine 132 decreases its phosphorylation kinetics, which constitutes a mechanism for ligand discrimination. Here we confirm that a LAT mutant in which glycine 131 has been substituted by an aspartate (LAT^G131D) increases phosphorylation of Tyr132, PLC-γ activation and calcium influx generation. Interestingly, the LAT^G131D mutant has a slower protein turnover while being equally sensitive to Fas-mediated protein cleavage by caspases. Moreover, J.CaM2 cells expressing LAT^G131D secrete greater amounts of interleukin-2 (IL-2) in response to CD3/CD28 engagement. However, despite this increased IL-2 secretion, J.CaM2 cells expressing the LAT^G131D mutant are more sensitive to inhibition of IL-2 production by pre-treatment with anti-CD3, which points to a possible role of this residue in the generation of anergy. Our results suggest that the increased kinetics of LAT Tyr132 phosphorylation could contribute to the establishment of T cell anergy, and thus constitutes an earliest known intracellular event responsible for the induction of peripheral tolerance.

Effects of Early Intervention with Antibiotics and Maternal Fecal Microbiota on Transcriptomic Profiling Ileal Mucusa in Neonatal Pigs

This study aimed to investigate the effects of early intervention with antibiotics and maternal fecal microbiota on ileal morphology and barrier function, and transcriptomic profiling in neonatal piglets. Piglets in the amoxicillin (AM), fecal microbiota transplantation (FMT), and control (CO) groups were orally administrated with amoxicillin solution (6.94 mg/mL), maternal fecal microbiota suspension [>109 colony forming unit (CFU)/mL], and physiological saline, respectively. Compared with the CO group, early intervention with AM or FMT significantly decreased ileal crypt depth on day 7 and altered gene expression profiles in ileum on days 7 and 21, and especially promoted the expression of chemokines (CCL5, CXCL9, and CXCL11) involved in the toll-like receptor signaling pathway on day 21. FMT changed major immune activities from B cell immunity on day 7 to T cell immunity on day 21 in the ileum. On the other hand, both AM and FMT predominantly downregulated the gene expression of toll-like receptor 4 (TLR4). In summary, both early interventions modulated intestinal barrier function and immune system in the ileum with a low impact on ileal morphology and development.

ZAP-70 Regulates Autoimmune Arthritis via Alterations in T Cell Activation and Apoptosis

T cells play an essential role in the pathogenesis of both human rheumatoid arthritis (RA) and its murine models. A key molecule in T cell activation is ZAP-70, therefore we aimed to investigate the effects of partial ZAP-70 deficiency on the pathogenesis of recombinant human G1(rhG1)-induced arthritis (GIA), a well-established mouse model of RA. Arthritis was induced in BALB/c and ZAP-70^+/- heterozygous mice. Disease progression was monitored using a scoring system and in vivo imaging, antigen-specific proliferation, cytokine and autoantibody production was measured and T cell apoptotic pathways were analyzed. ZAP-70^+/- mice developed a less severe arthritis, as shown by both clinical picture and in vitro parameters (decreased T cell proliferation, cytokine and autoantibody production). The amount of cleaved Caspase-3 increased in arthritic ZAP-70^+/- T cells, with no significant changes in cleaved Caspase-8 and -9 levels; although expression of Bim, Bcl-2 and Cytochrome C showed alterations. Tyrosine phosphorylation was less pronounced in arthritic ZAP-70^+/- T cells and the amount of Cbl-b-a negative regulator of T cell activation-decreased as well. We hypothesize that the less severe disease seen in the partial absence of ZAP-70 might be caused by the decreased T cell activation accompanied by increased apoptosis.

Trogocytosis-Mediated Intracellular Signaling in CD4+ T Cells Drives TH2-Associated Effector Cytokine Production and Differentiation

CD4⁺ T cells have been observed to acquire APC-derived membrane and membrane-associated molecules through trogocytosis in diverse immune settings. Despite this, the consequences of trogocytosis on the recipient T cell remain largely unknown. We previously reported that trogocytosed molecules on CD4⁺ T cells engage their respective surface receptors, leading to sustained TCR signaling and survival after APC removal. Using peptide-pulsed bone marrow-derived dendritic cells and transfected murine fibroblasts expressing antigenic MHC:peptide complexes as APC, we show that trogocytosis-positive CD4⁺ T cells display effector cytokines and transcription factor expression consistent with a T_H2 phenotype. In vitro-polarized T_H2 cells were found to be more efficient at performing trogocytosis than T_H1 or nonpolarized CD4⁺ cells, whereas subsequent trogocytosis-mediated signaling induced T_H2 differentiation in polarized T_H1 and nonpolarized cells. Trogocytosis-positive CD4⁺ T cells generated in vivo also display a T_H2 phenotype in both TCR-transgenic and wild-type models. These findings suggest that trogocytosis-mediated signaling impacts CD4⁺ T cell differentiation and effector cytokine production and may play a role in augmenting or shaping a T_H2-dominant immune response.

An immunoproteomic approach to characterize the CAR interactome and signalosome

Adoptive transfer of T cells that express a chimeric antigen receptor (CAR) is an approved immunotherapy that may be curative for some hematological cancers. To better understand the therapeutic mechanism of action, we systematically analyzed CAR signaling in human primary T cells by mass spectrometry. When we compared the interactomes and the signaling pathways activated by distinct CAR-T cells that shared the same antigen-binding domain but differed in their intracellular domains and their in vivo antitumor efficacy, we found that only second-generation CARs induced the expression of a constitutively phosphorylated form of CD3ζ that resembled the endogenous species. This phenomenon was independent of the choice of costimulatory domains, or the hinge/transmembrane region. Rather, it was dependent on the size of the intracellular domains. Moreover, the second-generation design was also associated with stronger phosphorylation of downstream secondary messengers, as evidenced by global phosphoproteome analysis. These results suggest that second-generation CARs can activate additional sources of CD3ζ signaling, and this may contribute to more intense signaling and superior antitumor efficacy that they display compared to third-generation CARs. Moreover, our results provide a deeper understanding of how CARs interact physically and/or functionally with endogenous T cell molecules, which will inform the development of novel optimized immune receptors.

Overexpression of Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains induces cluster of differentiation 59-mediated apoptosis in Jurkat cells

Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains (CBP/PAG) is a membrane-bound adaptor protein that downregulates the activation of Src family kinases present in lipid rafts. To elucidate the role of CBP/PAG in human T cell activation, a cell line overexpressing CBP/PAG was constructed and the function of CBP/PAG in Jurkat cells was examined. The present study revealed that increased CBP/PAG expression in T cells significantly enhanced their apoptosis and reduced cellular activation and proliferation. Overexpression of CBP/PAG suppressed the growth of Jurkat cells by recruiting c-Src and its negative regulator, C-terminal Src kinase (CSK), to lipid rafts. The negative regulation of CBP/PAG was enhanced in the presence of anti-cluster of differentiation (CD)59 monoclonal antibodies. In addition, a significant association was revealed between the location of CBP/PAG and CD59, which were co-expressed in the same region of the cell membrane, implicating a potential overlap of the elicited signaling pathways. These results indicate that CBP/PAG functions as a negative regulator of cell signal transduction and suggest that CD59 may strengthen the role of negative feedback regulation.

Multilayer Control of B Cell Activation by the B Cell Antigen Receptor: Following Themes Initiated With Bill Paul

This article describes the work I did in Bill Paul's lab as a postdoctoral fellow between 1979 and 1983, and to a lesser extent puts that work in the context of other work on B cell activation and antibody responses that was going on in Bill's lab at that time and shortly beforehand, including the discovery of interleukin 4. In addition, this work describes the subsequent and continuing work in my own lab following-up on themes I began during my time working directly with Bill. A particular emphasis was on understanding the biochemical mechanisms of signaling by the B cell antigen receptor (BCR) to the interior of the B cell. Some of the studies from my lab related to the regulation of BCR signaling by Lyn are described in relationship to the lymphocyte tuning hypothesis put forth by Grossman and Paul in 1992 and subsequently.

Engineering a Dual Small Molecule Gated ZAP70 Switch in T Cells

T cells expressing tumor-specific T cell receptors are promising cancer therapeutic agents, but safety control switches are needed to manage potential side effects arising from overactivity. Here, we present the first dual small molecule-gated ZAP70 signaling switch for the regulation of T cell activity. We show that when an analogue-sensitive ZAP70 allele is fused to the engineered ligand binding domain of the estrogen receptor, ER^T2, its activity can be upregulated to an extent by a metabolite of an FDA-approved tamoxifen, 4-hydroxy-tamoxifen, and downregulated by an ATP analogue, 3-MB-PP1. The strength of early T cell signaling can also be modulated by varying the concentrations of activator and inhibitor, and the switch exhibits temporal control on the time scale of minutes. Interestingly, the switch has the ability to control CD69 and calcium levels in T cells but has limited capabilities in the regulation of downstream cytokine release, suggesting further investigation is needed before it can be implemented in adoptive T cell therapy.

Correction of T cell deficiency in ZAP-70 knock-out mice by simple intraperitoneal adoptive transfer of thymocytes

The tyrosine kinase zeta chain-associated protein of 70 kDa (ZAP-70) plays a key role in T cell development and signalling. In the absence of ZAP-70, T cell development is arrested in the CD4⁺ CD8⁺ double-positive stage, thus ZAP-70 homozygous knockout (ZAP-70^-/- ) mice have no mature T cells in their peripheral lymphoid organs and blood, causing severe immunodeficiency. We investigated the early kinetics and long-term effects of wild-type thymocyte transfer on T cell repopulation in ZAP-70^-/- mice. We used a single intraperitoneal (i.p.) injection to deliver donor thymocytes to the recipients. Here, we show that after i.p. injection donor thymocytes leave the peritoneum through milky spots in the omentum and home to the thymus, where donor-originated CD4^- CD8^- double-negative thymocytes most probably restore T cell development and the disrupted thymic architecture. Subsequently, newly developed, donor-originated, single-positive αβ T cells appear in peripheral lymphoid organs, where they form organized T cell zones. The established chimerism was found to be stable, as donor-originated cells were present in transferred ZAP-70^-/- mice as late as 8 months after i.p. injection. We demonstrate that a simple i.p. injection of ZAP-70^+/+ thymocytes is a feasible method for the long-term reconstitution of T cell development in ZAP-70-deficient mice.

Thy-1 stimulation of mouse T cells induces a delayed T cell receptor-like signal that results in Ca2+‑independent cytotoxicity

Antibody-mediated crosslinking of Thy-1 [also known as cluster of differentiation (CD)90], results in a T cell receptor (TcR)‑like signal; however, the impact of Thy‑1 stimulation in comparison to TcR stimulation on T cell activation and effector function has yet to be fully elucidated. In the present study, the outcome of Thy‑1‑ and TcR‑induced stimulation of T cells was investigated in mice, using fragment crystalizable (Fc) receptor‑bound antibodies and costimulatory signals provided by syngeneic lipopolysaccharide‑matured bone marrow‑derived dendritic cells. Compared with TcR signaling, Thy‑1 signaling initiated a less robust proliferative response in T cells, as determined by tritiated‑thymidine incorporation. In addition, enzyme‑linked immunosorbent assays revealed that interleukin‑2 production was reduced, and the expression of CD25 and cyclin D3 was weaker in Thy‑1‑stimulated cells, as determined by western blotting; however, the expression of cyclin‑dependent kinase 6 was similar to that in TcR‑induced T cells. Furthermore, western blotting demonstrated that the phosphorylation of ζ-chain‑associated protein kinase 70 and extracellular signal‑regulated kinase 1/2 was delayed following Thy‑1 stimulation. DNA fragmentation assays revealed that cytotoxic effector function was also slower to develop in Thy‑1‑stimulated T cells, required more time to be effective and was largely Ca2+‑independent; these findings suggested that Fas ligand rather than granule‑associated perforin was involved in T cell effector function. In conclusion, the present results suggested that Thy‑1 signaling may contribute to the regulation of T cell homeostasis and the development of non‑specific T cell‑mediated cytotoxicity. However, further studies are required to elucidate the exact physiological roles of TcR‑like signals that result from Thy‑1 crosslinking and to investigate the molecular mechanisms that are involved.

A Phosphosite within the SH2 Domain of Lck Regulates Its Activation by CD45

The Src Family kinase Lck sets a critical threshold for T cell activation because it phosphorylates the TCR complex and the Zap70 kinase. How a T cell controls the abundance of active Lck molecules remains poorly understood. We have identified an unappreciated role for a phosphosite, Y192, within the Lck SH2 domain that profoundly affects the amount of active Lck in cells. Notably, mutation of Y192 blocks critical TCR-proximal signaling events and impairs thymocyte development in retrogenic mice. We determined that these defects are caused by hyperphosphorylation of the inhibitory C-terminal tail of Lck. Our findings reveal that modification of Y192 inhibits the ability of CD45 to associate with Lck in cells and dephosphorylate the C-terminal tail of Lck, which prevents its adoption of an active open conformation. These results suggest a negative feedback loop that responds to signaling events that tune active Lck amounts and TCR sensitivity.

Crk adaptor proteins regulate CD3ζ chain phosphorylation and TCR/CD3 down-modulation in activated T cells

T cell receptor (TCR) recognition of a peptide antigen in the context of MHC molecules initiates positive and negative cascades that regulate T cell activation, proliferation and differentiation, and culminate in the acquisition of effector T cell functions. These processes are a prerequisite for the induction of specific T cell-mediated adaptive immune responses. A key event in the activation of TCR-coupled signaling pathways is the phosphorylation of tyrosine residues within the cytoplasmic tails of the CD3 subunits, predominantly CD3ζ. These transiently formed phosphotyrosyl epitopes serve as docking sites for SH2-domain containing effector molecules, predominantly the ZAP70 protein tyrosine kinase, which is critical for signal propagation. We found that CrkI and CrkII adaptor proteins also interact with CD3ζ in TCR activated-, but not in resting-, T cells. Crk binding to CD3ζ was independent of ZAP70 and also occurred in ZAP70-deficient T cells. Binding was mediated by Crk-SH2 domain interaction with phosphotyrosine-containing motifs on CD3ζ, via a direct physical interaction, as demonstrated by Far-Western blot. CrkII binding to CD3ζ could also be demonstrated in a heterologous system, where coexpression of a catalytically active Lck was used to phosphorylate the CD3ζ chain. TCR activation-induced Crk binding to CD3ζ resulted in increased and prolonged phosphorylation of CD3ζ, as well as ZAP70 and LAT, suggesting a positive role for CrkI/II binding to CD3ζ in regulation of TCR-coupled signaling pathways. Furthermore, Crk-dependent increased phosphorylation of CD3ζ coincided with inhibition of TCR downmodulation, supporting a positive role for Crk adaptor proteins in TCR-mediated signal amplification.

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

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

The Structural Basis for Activation and Inhibition of ZAP-70 Kinase Domain

ZAP–70 (Zeta-chain-associated protein kinase 70) is a tyrosine kinase that interacts directly with the activated T-cell receptor to transduce downstream signals, and is hence a major player in the regulation of the adaptive immune response. Dysfunction of ZAP–70 causes selective T cell deficiency that in turn results in persistent infections. ZAP–70 is activated by a variety of signals including phosphorylation of the kinase domain (KD), and binding of its regulatory tandem Src homology 2 (SH2) domains to the T cell receptor. The present study investigates molecular mechanisms of activation and inhibition of ZAP–70 via atomically detailed molecular dynamics simulation approaches. We report microsecond timescale simulations of five distinct states of the ZAP–70 KD, comprising apo, inhibited and three phosphorylated variants. Extensive analysis of local flexibility and correlated motions reveal crucial transitions between the states, thus elucidating crucial steps in the activation mechanism of the ZAP–70 KD. Furthermore, we rationalize previously observed staurosporine-bound crystal structures, suggesting that whilst the KD superficially resembles an “active-like” conformation, the inhibitor modulates the underlying protein dynamics and restricts it in a compact, rigid state inaccessible to ligands or cofactors. Finally, our analysis reveals a novel, potentially druggable pocket in close proximity to the activation loop of the kinase, and we subsequently use its structure in fragment-based virtual screening to develop a pharmacophore model. The pocket is distinct from classical type I or type II kinase pockets, and its discovery offers promise in future design of specific kinase inhibitors, whilst mutations in residues associated with this pocket are implicated in immunodeficiency in humans.

ZAP–70 is a key protein kinase in the adaptive immune system. It is essential for development and function of T cells and natural killer cells, and associated mutations can lead to conditions such as severe combined immunodeficiency (SCID). Here, simulations of the ZAP–70 kinase domain are used to study its dynamics in response to different mechanistic signals. We identify crucial motions over microsecond timescales, which help to rationalize in atomic detail previous structural and experimental data regarding its biological regulation. We subsequently propose a scheme for the phosphorylation-dependent activation cascade of ZAP–70, and for its ligand-dependent inhibition. Finally, we characterize a novel cryptic pocket adjacent to the active site and activation loop, which is distinct from classical type I or type II kinase sites. The pocket is in close proximity to several residues whose mutations cause SCID in humans, and its identification offers promise in future drug design efforts.

Haplotype analysis finds linkage disequilibrium in the IL-12 gene in patients with HCV

HCV is a major cause of liver disease worldwide. IL-12 plays an essential role in the balance of T helper 1 (Th1) differentiation versus a T helper 2 (Th2) driven response from its naïve precursor. Linkage disequilibrium measures the degree to which alleles at two loci are associated and the non-random associations between alleles at two loci. Haplotypes of the three IL-12B loci studied were determined in the patient cases and the normal healthy control subjects. The frequency of the 12 possible IL-12B haplotypes on the 3 loci was determined in subjects heterozygous at only one of the loci within the studied haplotype. Haplotype frequencies were compared between the patient groups and controls (n = 49) to determine if any preferential combination of markers occurred using chi-squared and applying the Bonferroni correction. 45 HCV RNA negative patients; 88 HCV RNA positive patients; and 15 uninfected cases at high risk of HCV infection (EU) were studied. The haplotype "C" SNP of the 3'UTR with the "E" 4 bp deletion of the intron 4 region was in linkage disequilibrium (χ(2)  = 45.15, P < 0.001, 95% CL). The haplotype analysis of the insertion allele of the promoter with the deletion allele of the intron 4("E") IL-12B polymorphism showed linkage disequilibrium (χ(2)  = 5.64, P = 0.02). Linkage disequilibrium of polymorphisms is reported in the IL-12 gene in patients with HCV infection and contributes to the understanding of patient genotype and expected production of IL-12, responding to infection.

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.

Plasma membrane signaling in HIV-1 infection

Plasma membrane is a multifunctional structure that acts as the initial barrier against infection by intracellular pathogens. The productive HIV-1 infection depends upon the initial interaction of virus and host plasma membrane. Immune cells such as CD4+ T cells and macrophages contain essential cell surface receptors and molecules such as CD4, CXCR4, CCR5 and lipid raft components that facilitate HIV-1 entry. From plasma membrane HIV-1 activates signaling pathways that prepare the grounds for viral replication. Through viral proteins HIV-1 hijacks host plasma membrane receptors such as Fas, TNFRs and DR4/DR5, which results in immune evasion and apoptosis both in infected and uninfected bystander cells. These events are hallmark in HIV-1 pathogenesis that leads towards AIDS. The interplay between HIV-1 and plasma membrane signaling has much to offer in terms of viral fitness and pathogenicity, and a better understanding of this interplay may lead to development of new therapeutic approaches. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Monovalent and multivalent ligation of the B cell receptor exhibit differential dependence upon Syk and Src family kinases

The Src and Syk families of kinases are two distinct sets of kinases that play critical roles in initiating membrane-proximal B cell receptor (BCR) signaling. However, unlike in other lymphocytes, such as T cells, the "division of labor" between Src family kinases (SFKs) and Syk in B cells is not well separated because both Syk and SFKs can phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs) present in proteins comprising the BCR. To understand why B cells require both SFKs and Syk for activation, we investigated the roles of both families of kinases in BCR signaling with computational modeling and in vitro experiments. Our computational model suggested that positive feedback enabled Syk to substantially compensate for the absence of SFKs when spatial clustering of BCRs was induced by multimeric ligands. We confirmed this prediction experimentally. In contrast, when B cells were stimulated by monomeric ligands that failed to produce BCR clustering, both Syk and SFKs were required for complete and rapid BCR activation. Our data suggest that SFKs could play a pivotal role in increasing BCR sensitivity to monomeric antigens of pathogens and in mediating a rapid response to soluble multimeric antigens of pathogens that can induce spatial BCR clustering.

The adaptor protein SAP directly associates with CD3ζ chain and regulates T cell receptor signaling

Mutations altering the gene encoding the SLAM associated protein (SAP) are responsible for the X-linked lymphoproliferative disease or XLP1. Its absence is correlated with a defective NKT cells development, a decrease in B cell functions and a reduced T cells and NK cells cytotoxic activities, thus leading to an immunodeficiency syndrome. SAP is a small 128 amino-acid long protein that is almost exclusively composed of an SH2 domain. It has been shown to interact with the CD150/SLAM family of receptors, and in a non-canonical manner with SH3 containing proteins such as Fyn, βPIX, PKCθ and Nck1. It would thus play the role of a minimal adaptor protein. It has been shown that SAP plays an important function in the activation of T cells through its interaction with the SLAM family of receptors. Therefore SAP defective T cells display a reduced activation of signaling events downstream of the TCR-CD3 complex triggering. In the present work, we evidence that SAP is a direct interactor of the CD3ζ chain. This direct interaction occurs through the first ITAM of CD3ζ, proximal to the membrane. Additionally, we show that, in the context of the TCR-CD3 signaling, an Sh-RNA mediated silencing of SAP is responsible for a decrease of several canonical T cell signaling pathways including Erk, Akt and PLCγ1 and to a reduced induction of IL-2 and IL-4 mRNA. Altogether, we show that SAP plays a central function in the T cell activation processes through a direct association with the CD3 complex.

Altered BCR signalling quality predisposes to autoimmune disease and a pre-diabetic state

The spleen tyrosine kinase family members Syk and Zap-70 are pivotal signal transducers downstream of antigen receptors and exhibit overlapping expression patterns at early lymphocytic developmental stages. To assess their differential kinase fitness in vivo, we generated mice, which carry a Zap-70 cDNA knock-in controlled by intrinsic Syk promoter elements that disrupts wild-type Syk expression. Kinase replacement severely compromised Erk1/2-mediated survival and proper selection of developing B cells at central and peripheral checkpoints, demonstrating critical dependence on BCR signalling quality. Furthermore, ITAM- and hemITAM-mediated activation of platelets and neutrophils was completely blunted, while surprisingly FcγR-mediated phagocytosis in macrophages was retained. The alteration in BCR signalling quality resulted in preferential development and survival of marginal zone B cells and prominent autoreactivity, causing the generation of anti-insulin antibodies and age-related glomerulonephritis. Development of concomitant fasting glucose intolerance in knock-in mice highlights aberrant B cell selection as a potential risk factor for type 1 diabetes, and suggests altered BCR signalling as a mechanism to cause biased cellular and Ig repertoire selection, ultimately contributing to B cell-mediated autoimmune predisposition.

Increased membrane cholesterol in lymphocytes diverts T-cells toward an inflammatory response

Cell signaling for T-cell growth, differentiation, and apoptosis is initiated in the cholesterol-rich microdomains of the plasma membrane known as lipid rafts. Herein, we investigated whether enrichment of membrane cholesterol in lipid rafts affects antigen-specific CD4 T-helper cell functions. Enrichment of membrane cholesterol by 40–50% following squalene administration in mice was paralleled by an increased number of resting CD4 T helper cells in periphery. We also observed sensitization of the Th1 differentiation machinery through co-localization of IL-2Rα, IL-4Rα, and IL-12Rβ2 subunits with GM1 positive lipid rafts, and increased STAT-4 and STAT-5 phosphorylation following membrane cholesterol enrichment. Antigen stimulation or CD3/CD28 polyclonal stimulation of membrane cholesterol-enriched, resting CD4 T-cells followed a path of Th1 differentiation, which was more vigorous in the presence of increased IL-12 secretion by APCs enriched in membrane cholesterol. Enrichment of membrane cholesterol in antigen-specific, autoimmune Th1 cells fostered their organ-specific reactivity, as confirmed in an autoimmune mouse model for diabetes. However, membrane cholesterol enrichment in CD4⁺Foxp3⁺ T-reg cells did not alter their suppressogenic function. These findings revealed a differential regulatory effect of membrane cholesterol on the function of CD4 T-cell subsets. This first suggests that membrane cholesterol could be a new therapeutic target to modulate the immune functions, and second that increased membrane cholesterol in various physiopathological conditions may bias the immune system toward an inflammatory Th1 type response.

Mechanistic insight into pertussis toxin and lectin signaling using T cells engineered to express a CD8α/CD3ζ chimeric receptor

Mammalian cell-surface receptors typically display N- or O-linked glycans added post-translationally. Plant lectins such as phytohemagluttinin (PHA) can activate the T cell receptor (TCR) and other cell-surface receptors by binding to glycans and initiating receptor cross-linking. Pathogenic microorganisms such as Bordetella pertussis also express proteins with lectin-like activities. Similar to plant lectins, pertussis toxin (PTx) can activate the TCR and bind to a variety of glycans. However, whether the lectin-like activity of PTx is responsible for its ability to activate TCR signaling has not been formally proven. Here we examined the ability of PTx and a panel of lectins to activate the TCR or a CD8α/CD3ζ chimeric receptor (termed CD8ζ). We demonstrate that CD8ζ rescues PTx-induced signaling events lacking in TCR null cells. This result indicates that CD8ζ can substitute for TCR and supports the hypothesis that PTxB (functioning as a lectin) stimulates signaling via receptor cross-linking rather than by binding to a specific epitope on the TCR. Moreover, PTx is able to activate signaling by binding either N-linked or O-linked glycan-modified receptors as the TCR displays N-linked glycans while CD8ζ displays O-linked glycans. Finally, studies with a diverse panel of lectins indicate that the signaling activity of the lectins does not always correlate with the biochemical reports of ligand preferences. Comparison of lectin signaling through TCR or CD8ζ allows us to better define the structural and functional properties of lectin-glycan interactions using a biologically based signaling readout.

Development of unique antibodies directed against each of the six different phosphotyrosine residues within the T cell receptor CD3ζ chain

Signal transduction from the T cell antigen receptor (TCR)/CD3 complex involves six different immunoreceptor tyrosine-based activation motifs (ITAM) located within the cytoplasmic tails of the CD3 chains. Each ITAM possesses two conserved tyrosine residues that can undergo phosphorylation upon TCR/CD3 crosslinking and become a docking site for SH2-containing effector molecules. Specificity of the SH2 domains is determined by their ability to bind a phosphorylated tyrosine in the context of a longer peptide motif within the target protein. As a result, phosphorylation of different tyrosines within the CD3 cytoplasmic tails creates docking sites for distinct SH2-containing signaling proteins that differentially impact on the quality of the T cell response. In the present study, we prepared antibodies specific for each of the six different phosphotyrosines of the mouse CD3ζ chain. The antibodies were characterized with respect to their cross-reactivity, ability to recognize the phosphorylated versus non-phosphorylated forms of tyrosine-containing motifs, and cross-reactivity with the homologous phospho-motifs on the human CD3ζ protein. The antibodies were found to be specific and selective for phospho-CD3ζ. They can serve as useful tools for distinguishing between the six potential tyrosine phosphorylation sites on the CD3ζ chain, and for correlating the phosphorylation of specific CD3ζ tyrosine residues with activation of signaling pathways that dictate T cell differentiation into responding, anergic, or apoptotic cells.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

Increased sensitivity to antigen in high avidity CD8(+) T cells results from augmented membrane proximal T-cell receptor signal transduction

The functional avidity of a cytotoxic T lymphocyte (CTL) is known to be a critical determinant of the efficacy with which it clears pathogens. High avidity cells, which are by definition highly sensitive to peptide antigen, are superior for elimination of viruses and tumours. Our studies have established the ability of T cells to undergo avidity modulation as a result of antigen encounter. High and low avidity cells established in this manner exhibit significant differences in the amount of peptide required to elicit effector function. However, how signalling is regulated in these cells as it relates to the control of peptide sensitivity remains to be defined. To address this question, we compared T-cell receptor (TCR) signal transduction events in high and low avidity CTL generated from OT-I(rag2-) TCR transgenic mice. Our data suggest that divergent signalling is initiated at the TCR-associated CD3ζ, with low avidity CTL requiring higher amounts of pMHC to achieve threshold levels of phosphorylated CD3ζ compared with high avidity CTL. Further, this difference is transduced further downstream to mitogen-activated protein kinase and Ca(2+) signalling pathways. These results suggest that regulated control of the initiation of TCR signalling in high versus low avidity cells determines the amount of peptide required for T-cell activation.

Flavonoids as protein kinase inhibitors for cancer chemoprevention: direct binding and molecular modeling

Protein kinases play crucial roles in the regulation of multiple cell signaling pathways and cellular functions. Deregulation of protein kinase function has been implicated in carcinogenesis. The inhibition of protein kinases has emerged as an important target for cancer chemoprevention and therapy. Accumulated data revealed that flavonoids exert chemopreventive effects through acting at protein kinase signaling pathways, more than as conventional hydrogen-donating antioxidants. Recent studies show that flavonoids can bind directly to some protein kinases, including Akt/protein kinase B (Akt/PKB), Fyn, Janus kinase 1 (JAK1), mitogen-activated protein kinase kinase 1 (MEK1), phosphoinositide 3-kinase (PI3K), mitogen-activated protein (MAP) kinase kinase 4 (MKK4), Raf1, and zeta chain-associated 70-kDa protein (ZAP-70) kinase, and then alter their phosphorylation state to regulate multiple cell signaling pathways in carcinogenesis processes. In this review, we report recent results on the interactions of flavonoids and protein kinases, especially their direct binding and molecular modeling. The data suggest that flavonoids act as protein kinase inhibitors for cancer chemoprevention that were thought previously as conventional hydrogen-donating antioxidant. Moreover, the molecular modeling data show some hints for creating natural compound-based protein kinase inhibitors for cancer chemoprevention and therapy.

ZAP-70: an essential kinase in T-cell signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that plays a critical role in the events involved in initiating T-cell responses by the antigen receptor. Here we review the structure of ZAP-70, its regulation, its role in development and in disease. We also describe a model experimental system in which ZAP-70 function can be interrupted by a small chemical inhibitor.

Graded attenuation of TCR signaling elicits distinct autoimmune diseases by altering thymic T cell selection and regulatory T cell function

Mice with a mutation of the zeta-associated protein of 70 kDa gene (skg mutation) are genetically prone to develop autoimmune arthritis, depending on the environment. In a set of mice with the mutation, the amount of zeta-associated protein of 70 kDa protein as well as its tyrosine phosphorylation upon TCR stimulation decreased from +/+, skg/+, skg/skg, to skg/- mice in a stepwise manner. The reduction resulted in graded alterations of thymic positive and negative selection of self-reactive T cells and Foxp3(+) natural regulatory T cells (Tregs) and their respective functions. Consequently, skg/- mice spontaneously developed autoimmune arthritis even in a microbially clean environment, whereas skg/skg mice required stimulation through innate immunity for disease manifestation. After Treg depletion, organ-specific autoimmune diseases, especially autoimmune gastritis, predominantly developed in +/+, at a lesser incidence in skg/+, but not in skg/skg BALB/c mice, which suffered from other autoimmune diseases, especially autoimmune arthritis. In correlation with this change, gastritis-mediating TCR transgenic T cells were positively selected in +/+, less in skg/+, but not in skg/skg BALB/c mice. Similarly, on the genetic background of diabetes-prone NOD mice, diabetes spontaneously developed in +/+, at a lesser incidence in skg/+, but not in skg/skg mice, which instead succumbed to arthritis. Thus, the graded attenuation of TCR signaling alters the repertoire and the function of autoimmune T cells and natural Tregs in a progressive manner. It also changes the dependency of disease development on environmental stimuli. These findings collectively provide a model of how genetic anomaly of T cell signaling contributes to the development of autoimmune disease.

Aberrant reactive oxygen and nitrogen species generation in rheumatoid arthritis (RA): causes and consequences for immune function, cell survival, and therapeutic intervention

The infiltration and persistence of hematopoietic immune cells within the rheumatoid arthritis (RA) joint results in elevated levels of pro-inflammatory cytokines, increased reactive oxygen (ROS) and -nitrogen (RNS) species generation, that feeds a continuous self-perpetuating cycle of inflammation and destruction. Meanwhile, the controlled production of ROS is required for signaling within the normal physiological reaction to perceived "foreign matter" and for effective apoptosis. This review focuses on the signaling pathways responsible for the induction of the normal immune response and the contribution of ROS to this process. Evidence for defects in the ability of immune cells in RA to regulate the generation of ROS and the consequence for their immune function and for RA progression is considered. As the hypercellularity of the rheumatoid joint and the associated persistence of hematopoietic cells within the rheumatoid joint are symptomatic of unresponsiveness to apoptotic stimuli, the role of apoptotic signaling proteins (specifically Bcl-2 family members and the tumor suppressor p53) as regulators of ROS generation and apoptosis are considered, evaluating evidence for their aberrant expression and function in RA. We postulate that ROS generation is required for effective therapeutic intervention.

The Src, Syk, and Tec family kinases: distinct types of molecular switches

The Src, Syk, and Tec family kinases are three of the most well characterized tyrosine kinase families found in the human genome. Members of these kinase families function downstream of antigen and F(c) receptors in hematopoietic cells and transduce signals leading to calcium mobilization, altered gene expression, cytokine production, and cell proliferation. Over the last several years, structural and biochemical studies have begun to uncover the molecular mechanisms regulating activation of these kinases. It appears that each kinase family functions as a distinct type of molecular switch. This review discusses the activation of the Src, Syk, and Tec kinases from the perspective of structure, phosphorylation, allosteric regulation, and kinetics. The multiple factors that regulate the Src, Syk, and Tec families illustrate the important role played by each of these kinases in immune cell signaling.