Decreased diacylglycerol metabolism enhances ERK activation and augments CD8+ T cell functional responses

Diacylglycerol kinases: A look into the future of immunotherapy

Cancer still represents the second leading cause of death right after cardiovascular diseases. According to the World Health Organization (WHO), cancer provoked around 10 million deaths in 2020, with lung and colon tumors accounting for the deadliest forms of cancer. As tumor cells become resistant to traditional therapeutic approaches, immunotherapy has emerged as a novel strategy for tumor control. T lymphocytes are key players in immune responses against tumors. Immunosurveillance allows identification, targeting and later killing of cancerous cells. Nevertheless, tumors evolve through different strategies to evade the immune response and spread in a process called metastasis. The ineffectiveness of traditional strategies to control tumor growth and expansion has led to novel approaches considering modulation of T cell activation and effector functions. Program death receptor 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) showed promising results in the early 90s and nowadays are still being exploited together with other drugs for several cancer types. Other negative regulators of T cell activation are diacylglycerol kinases (DGKs) a family of enzymes that catalyze the conversion of diacylglycerol (DAG) into phosphatidic acid (PA). In T cells, DGKα and DGKζ limit the PLCγ/Ras/ERK axis thus attenuating DAG mediated signaling and T cell effector functions. Upregulation of either of both isoforms results in impaired Ras activation and anergy induction, whereas germline knockdown mice showed enhanced antitumor properties and more effective immune responses against pathogens. Here we review the mechanisms used by DGKs to ameliorate T cell activation and how inhibition could be used to reinvigorate T cell functions in cancer context. A better knowledge of the molecular mechanisms involved upon T cell activation will help to improve current therapies with DAG promoting agents.

A systems and computational biology perspective on advancing CAR therapy

In the recent decades, chimeric antigen receptor (CAR) therapy signaled a new revolutionary approach to cancer treatment. This method seeks to engineer immune cells expressing an artificially designed receptor, which would endue those cells with the ability to recognize and eliminate tumor cells. While some CAR therapies received FDA approval and others are subject to clinical trials, many aspects of their workings remain elusive. Techniques of systems and computational biology have been frequently employed to explain the operating principles of CAR therapy and suggest further design improvements. In this review, we sought to provide a comprehensive account of those efforts. Specifically, we discuss various computational models of CAR therapy ranging in scale from organismal to molecular. Then, we describe the molecular and functional properties of costimulatory domains frequently incorporated in CAR structure. Finally, we describe the signaling cascades by which those costimulatory domains elicit cellular response against the target. We hope that this comprehensive summary of computational and experimental studies will further motivate the use of systems approaches in advancing CAR therapy.

Discovery of Potent, Dual-Inhibitors of Diacylglycerol Kinases Alpha and Zeta Guided by Phenotypic Optimization

We describe a phenotypic screening and optimization strategy to discover compounds that block intracellular checkpoint signaling in T-cells. We identified dual DGKα and ζ inhibitors notwithstanding the modest similarity between α and ζ relative to other DGK isoforms. Optimized compounds produced cytokine release and T-cell proliferation consistent with DGK inhibition and potentiated an immune response in human and mouse T-cells. Additionally, lead inhibitor BMS-502 demonstrated dose-dependent immune stimulation in the mouse OT-1 model, setting the stage for a drug discovery program.

Negative intracellular regulators of T-cell receptor (TCR) signaling as potential antitumor immunotherapy targets

Immunotherapy strategies aim to mobilize immune defenses against tumor cells by targeting mainly T cells. Co-inhibitory receptors or immune checkpoints (ICPs) (such as PD-1 and CTLA4) can limit T cell receptor (TCR) signal propagation in T cells. Antibody-based blocking of immune checkpoints (immune checkpoint inhibitors, ICIs) enable escape from ICP inhibition of TCR signaling. ICI therapies have significantly impacted the prognosis and survival of patients with cancer. However, many patients remain refractory to these treatments. Thus, alternative approaches for cancer immunotherapy are needed. In addition to membrane-associated inhibitory molecules, a growing number of intracellular molecules may also serve to downregulate signaling cascades triggered by TCR engagement. These molecules are known as intracellular immune checkpoints (iICPs). Blocking the expression or the activity of these intracellular negative signaling molecules is a novel field of action to boost T cell-mediated antitumor responses. This area is rapidly expanding. Indeed, more than 30 different potential iICPs have been identified. Over the past 5 years, several phase I/II clinical trials targeting iICPs in T cells have been registered. In this study, we summarize recent preclinical and clinical data demonstrating that immunotherapies targeting T cell iICPs can mediate regression of solid tumors including (membrane associated) immune-checkpoint inhibitor refractory cancers. Finally, we discuss how these iICPs are targeted and controlled. Thereby, iICP inhibition is a promising strategy opening new avenues for future cancer immunotherapy treatments.

Manipulation of diacylglycerol and ERK-mediated signaling differentially controls CD8+ T cell responses during chronic viral infection

Introduction

Activation of T cell receptor (TCR) signaling is critical for clonal expansion of CD8+ T cells. However, the effects of augmenting TCR signaling during chronic antigen exposure is less understood. Here, we investigated the role of diacylglycerol (DAG)-mediated signaling downstream of the TCR during chronic lymphocytic choriomeningitis virus clone 13 (LCMV CL13) infection by blocking DAG kinase zeta (DGKζ), a negative regulator of DAG.

Methods

We examined the activation, survival, expansion, and phenotype of virus-specific T cell in the acute and chronic phases of LCMV CL13-infected in mice after DGKζ blockade or selective activation of ERK.

Results

Upon LCMV CL13 infection, DGKζ deficiency promoted early short-lived effector cell (SLEC) differentiation of LCMV-specific CD8+ T cells, but this was followed by abrupt cell death. Short-term inhibition of DGKζ with ASP1570, a DGKζ-selective pharmacological inhibitor, augmented CD8+ T cell activation without causing cell death, which reduced virus titers both in the acute and chronic phases of LCMV CL13 infection. Unexpectedly, the selective enhancement of ERK, one key signaling pathway downstream of DAG, lowered viral titers and promoted expansion, survival, and a memory phenotype of LCMV-specific CD8+ T cells in the acute phase with fewer exhausted T cells in the chronic phase. The difference seen between DGKζ deficiency and selective ERK enhancement could be potentially explained by the activation of the AKT/mTOR pathway by DGKζ deficiency, since the mTOR inhibitor rapamycin rescued the abrupt cell death seen in virus-specific DGKζ KO CD8+ T cells.

Discussion

Thus, while ERK is downstream of DAG signaling, the two pathways lead to distinct outcomes in the context of chronic CD8+ T cell activation, whereby DAG promotes SLEC differentiation and ERK promotes a memory phenotype.

Overarching roles of diacylglycerol signaling in cancer development and antitumor immunity

Diacylglycerol (DAG) is a lipid second messenger that is generated in response to extracellular stimuli and channels intracellular signals that affect mammalian cell proliferation, survival, and motility. DAG exerts a myriad of biological functions through protein kinase C (PKC) and other effectors, such as protein kinase D (PKD) isozymes and small GTPase-regulating proteins (such as RasGRPs). Imbalances in the fine-tuned homeostasis between DAG generation by phospholipase C (PLC) enzymes and termination by DAG kinases (DGKs), as well as dysregulation in the activity or abundance of DAG effectors, have been widely associated with tumor initiation, progression, and metastasis. DAG is also a key orchestrator of T cell function and thus plays a major role in tumor immunosurveillance. In addition, DAG pathways shape the tumor ecosystem by arbitrating the complex, dynamic interaction between cancer cells and the immune landscape, hence representing powerful modifiers of immune checkpoint and adoptive T cell-directed immunotherapy. Exploiting the wide spectrum of DAG signals from an integrated perspective could underscore meaningful advances in targeted cancer therapy.

Delivering Glioblastoma a Kick-DGKα Inhibition as a Promising Therapeutic Strategy for GBM

Diacylglycerol kinase α (DGKα) inhibition may be particularly relevant for the treatment of glioblastoma (GBM), a relatively common brain malignancy incurable with current therapies. Prior reports have shown that DGKα inhibition has multiple direct activities against GBM cells, including suppressing the oncogenic pathways mTOR and HIF-1α. It also inhibits pathways associated with the normally treatment-resistant mesenchymal phenotype, yielding preferential activity against mesenchymal GBM; this suggests possible utility in combining DGKα inhibition with radiation and other therapies for which the mesenchymal phenotype promotes resistance. The potential for DGKα inhibition to block or reverse T cell anergy also suggests the potential of DGKα inhibition to boost immunotherapy against GBM, which is generally considered an immunologically "cold" tumor. A recent report indicates that DGKα deficiency increases responsiveness of macrophages, indicating that DGKα inhibition could also have the potential to boost macrophage and microglia activity against GBM-which could be a particularly promising approach given the heavy infiltration of these cells in GBM. DGKα inhibition may therefore offer a promising multi-pronged attack on GBM, with multiple direct anti-GBM activities and also the ability to boost both adaptive and innate immune responses against GBM. However, both the direct and indirect benefits of DGKα inhibition for GBM will likely require combinations with other therapies to achieve meaningful efficacy. Furthermore, GBM offers other challenges for the application of DGKα inhibitors, including decreased accessibility from the blood-brain barrier (BBB). The ideal DGKα inhibitor for GBM will combine potency, specificity, and BBB penetrability. No existing inhibitor is known to meet all these criteria, but the strong potential of DGKα inhibition against this lethal brain cancer should help drive development and testing of agents to bring this promising strategy to the clinic for patients with GBM.

DGKζ exerts greater control than DGKα over CD8+ T cell activity and tumor inhibition

Two isoforms of diacylglycerol kinases (DGKs), DGKα and DGKζ, are primarily responsible for terminating DAG-mediated activation of Ras and PKCθ pathways in T cells. A direct comparison of tumor growth between mice lacking each isoform has not been undertaken. We evaluated the growth of three syngeneic tumor cell lines in mice lacking either DGKα or DGKζ in the presence or absence of treatment with anti-PD1 and determined that (i) mice deficient in DGKζ conferred enhanced control of tumor relative to mice deficient in DGKα and (ii) deficiency of DGKζ acted additively with anti-PD1 in tumor control. Consistent with this finding, functional and RNA-sequencing analyses revealed greater changes in stimulated DGKζ-deficient T cells compared with DGKα-deficient T cells, which were enhanced relative to wildtype T cells. DGKζ also imparted greater regulation than DGKα in human T cells. Together, these data support targeting the ζ isoform of DGKs to therapeutically enhance T cell anti-tumor activity.

Enhancing a Natural Killer: Modification of NK Cells for Cancer Immunotherapy

Natural killer (NK) cells are potent innate immune system effector lymphocytes armed with multiple mechanisms for killing cancer cells. Given the dynamic roles of NK cells in tumor surveillance, they are fast becoming a next-generation tool for adoptive immunotherapy. Many strategies are being employed to increase their number and improve their ability to overcome cancer resistance and the immunosuppressive tumor microenvironment. These include the use of cytokines and synthetic compounds to bolster propagation and killing capacity, targeting immune-function checkpoints, addition of chimeric antigen receptors (CARs) to provide cancer specificity and genetic ablation of inhibitory molecules. The next generation of NK cell products will ideally be readily available as an “off-the-shelf” product and stem cell derived to enable potentially unlimited supply. However, several considerations regarding NK cell source, genetic modification and scale up first need addressing. Understanding NK cell biology and interaction within specific tumor contexts will help identify necessary NK cell modifications and relevant choice of NK cell source. Further enhancement of manufacturing processes will allow for off-the-shelf NK cell immunotherapies to become key components of multifaceted therapeutic strategies for cancer.

Potential role of diacylglycerol kinases in immune-mediated diseases

The mechanism promoting exacerbated immune responses in allergy and autoimmunity as well as those blunting the immune control of cancer cells are of primary interest in medicine. Diacylglycerol kinases (DGKs) are key modulators of signal transduction, which blunt diacylglycerol (DAG) signals and produce phosphatidic acid (PA). By modulating lipid second messengers, DGK modulate the activity of downstream signaling proteins, vesicle trafficking and membrane shape. The biological role of the DGK α and ζ isoforms in immune cells differentiation and effector function was subjected to in deep investigations. DGK α and ζ resulted in negatively regulating synergistic way basal and receptor induced DAG signals in T cells as well as leukocytes. In this way, they contributed to keep under control the immune response but also downmodulate immune response against tumors. Alteration in DGKα activity is also implicated in the pathogenesis of genetic perturbations of the immune function such as the X-linked lymphoproliferative disease 1 and localized juvenile periodontitis. These findings suggested a participation of DGK to the pathogenetic mechanisms underlying several immune-mediated diseases and prompted several researches aiming to target DGK with pharmacologic and molecular strategies. Those findings are discussed inhere together with experimental applications in tumors as well as in other immune-mediated diseases such as asthma.

Combined inhibition/silencing of diacylglycerol kinase α and ζ simultaneously and synergistically enhances interleukin-2 production in T cells and induces cell death of melanoma cells

The α-isozyme of diacylglycerol kinase (DGK) enhances cancer cell proliferation and, conversely, it promotes the nonresponsive immune state known as T-cell anergy. Moreover, a DGKα-selective inhibitor, CU-3, induced cell death in cancer-derived cells and simultaneously enhanced T-cell interleukin-2 production. In addition to DGKα, DGKζ is also known to induce T-cell anergy. In the present study, we examined whether combined inhibition/silencing of DGKα and DGKζ synergistically enhanced T-cell activity. Combined treatment with CU-3 or DGKα-small interfering RNA (siRNA) and DGKζ-siRNA more potently enhanced T-cell receptor-crosslink-dependent interleukin-2 production in Jurkat T cells than treatment with either alone. Intriguingly, in addition to activating T cells, dual inhibition/silencing of DGKα and DGKζ synergistically reduced viability and increased caspase 3/7 activity in AKI melanoma cells. Taken together, these results indicate that combined inhibition/silencing of DGKα and DGKζ simultaneously and synergistically enhances interleukin-2 production in T cells and induces cell death in melanoma. Therefore, dual inhibition/silencing of these DGK isozymes represents an ideal therapy that potently attenuates cancer cell proliferation and simultaneously enhances immune responses that impact anticancer immunity.

The role of diacylglycerol kinases in allergic airway disease

Asthma is an obstructive inflammatory airway disease. Airway obstruction is mediated by hyperresponsive airway smooth muscle cell contraction, which is induced and compounded by inflammation caused by T lymphocytes. One important signal transduction pathway that is involved in the activation of these cell types involves the generation of a lipid second messenger known as diacylglycerol (DAG). DAG levels are controlled in cells by a negative regulator known as DAG kinase (DGK). In this review, we discuss how the DAG signaling pathway attenuates the pathological function of immune cells and airway smooth muscle cells in allergic airway disease and asthma. Furthermore, we discuss how the enhancement of the DAG signaling pathway through the inhibition of DGK may represent a novel therapeutic strategy for these diseases.

Interplay Between SNX27 and DAG Metabolism in the Control of Trafficking and Signaling at the IS

Recognition of antigens displayed on the surface of an antigen-presenting cell (APC) by T-cell receptors (TCR) of a T lymphocyte leads to the formation of a specialized contact between both cells named the immune synapse (IS). This highly organized structure ensures cell–cell communication and sustained T-cell activation. An essential lipid regulating T-cell activation is diacylglycerol (DAG), which accumulates at the cell–cell interface and mediates recruitment and activation of proteins involved in signaling and polarization. Formation of the IS requires rearrangement of the cytoskeleton, translocation of the microtubule-organizing center (MTOC) and vesicular compartments, and reorganization of signaling and adhesion molecules within the cell–cell junction. Among the multiple players involved in this polarized intracellular trafficking, we find sorting nexin 27 (SNX27). This protein translocates to the T cell–APC interface upon TCR activation, and it is suggested to facilitate the transport of cargoes toward this structure. Furthermore, its interaction with diacylglycerol kinase ζ (DGKζ), a negative regulator of DAG, sustains the precise modulation of this lipid and, thus, facilitates IS organization and signaling. Here, we review the role of SNX27, DAG metabolism, and their interplay in the control of T-cell activation and establishment of the IS.

Diacylglycerol Kinase Alpha in Radiation-Induced Fibrosis: Potential as a Predictive Marker or Therapeutic Target

Radiotherapy is an efficient tool in cancer treatment, but it brings along the risk of side effects such as fibrosis in the irradiated healthy tissue thus limiting tumor control and impairing quality of life of cancer survivors. Knowledge on radiation-related fibrosis risk and therapeutic options is still limited and requires further research. Recent studies demonstrated that epigenetic regulation of diacylglycerol kinase alpha (DGKA) is associated with radiation-induced fibrosis. However, the specific mechanisms are still unknown. In this review, we scrutinized the role of DGKA in the radiation response and in further cellular functions to show the potential of DGKA as a predictive marker or a novel target in fibrosis treatment. DGKA was reported to participate in immune response, lipid signaling, exosome production, and migration as well as cell proliferation, all processes which are suggested to be critical steps in fibrogenesis. Most of these functions are based on the conversion of diacylglycerol (DAG) to phosphatidic acid (PA) at plasma membranes, but DGKA might have also other, yet not well-known functions in the nucleus. Current evidence summarized here underlines that DGKA activation may play a central role in fibrosis formation post-irradiation and shows a potential of direct DGKA inhibitors or epigenetic modulators to attenuate pro-fibrotic reactions, thus providing novel therapeutic choices.

Biological regulation of diacylglycerol kinases in normal and neoplastic tissues: New opportunities for cancer immunotherapy

In the recent years, the arsenal of anti-cancer therapies has evolved to target T lymphocytes and restore their capacity to destroy tumor cells. However, the clinical success is limited, with a large number of patients that never responds and others that ultimately develop resistances. Overcoming the hypofunctional state imposed by solid tumors to T cells has revealed critical but challenging due to the complex strategies that tumors employ to evade the immune system. The Diacylglycerol kinases (DGK) limit DAG-dependent functions in T lymphocytes and their upregulation in tumor-infiltrating T lymphocytes contribute to limit T cell cytotoxic potential. DGK blockade could reinstate T cell attack on tumors, limiting at the same time tumor cell growth, thanks to the DGK positive input into several oncogenic pathways. In this review we summarize the latest findings regarding the regulation of specific DGK isoforms in healthy and anergic T lymphocytes, as well as their contribution to oncogenic phenotypes. We will also revise the latest advances in the search for pharmacological inhibitors and their potential as anti-cancer agents, either alone or in combination with immunomodulatory agents.

Diacylglycerol kinase ζ promotes allergic airway inflammation and airway hyperresponsiveness through distinct mechanisms

Asthma is a chronic allergic inflammatory airway disease caused by aberrant immune responses to inhaled allergens, which leads to airway hyperresponsiveness (AHR) to contractile stimuli and airway obstruction. Blocking T helper 2 (T_H2) differentiation represents a viable therapeutic strategy for allergic asthma, and strong TCR-mediated ERK activation blocks T_H2 differentiation. Here, we report that targeting diacylglycerol (DAG) kinase zeta (DGKζ), a negative regulator of DAG-mediated cell signaling, protected against allergic asthma by simultaneously reducing airway inflammation and AHR though independent mechanisms. Targeted deletion of DGKζ in T cells decreased type 2 inflammation without reducing AHR. In contrast, loss of DGKζ in airway smooth muscle cells decreased AHR but not airway inflammation. T cell-specific enhancement of ERK signaling was only sufficient to limit type 2 airway inflammation, not AHR. Pharmacological inhibition of DGK diminished both airway inflammation and AHR in mice and also reduced bronchoconstriction of human airway samples in vitro. These data suggest that DGK is a previously unrecognized therapeutic target for asthma and reveal that the inflammatory and AHR components of asthma are not as interdependent as generally believed.

Structural basis of inactivation of Ras and Rap1 small GTPases by Ras/Rap1-specific endopeptidase from the sepsis-causing pathogen Vibrio vulnificus

Multifunctional autoprocessing repeats-in-toxin (MARTX) toxins are secreted by Gram-negative bacteria and function as primary virulence-promoting macromolecules that deliver multiple cytopathic and cytotoxic effector domains into the host cytoplasm. Among these effectors, Ras/Rap1-specific endopeptidase (RRSP) catalyzes the sequence-specific cleavage of the Switch I region of the cellular substrates Ras and Rap1 that are crucial for host innate immune defenses during infection. To dissect the molecular basis underpinning RRSP-mediated substrate inactivation, we determined the crystal structure of an RRSP from the sepsis-causing bacterial pathogen Vibrio vulnificus (VvRRSP). Structural and biochemical analyses revealed that VvRRSP is a metal-independent TIKI family endopeptidase composed of an N-terminal membrane-localization and substrate-recruitment domain (N lobe) connected via an inter-lobe linker to the C-terminal active site-coordinating core β-sheet-containing domain (C lobe). Structure-based mutagenesis identified the 2His/2Glu catalytic residues in the core catalytic domain that are shared with other TIKI family enzymes and that are essential for Ras processing. In vitro KRas cleavage assays disclosed that deleting the N lobe in VvRRSP causes complete loss of enzymatic activity. Endogenous Ras cleavage assays combined with confocal microscopy analysis of HEK293T cells indicated that the N lobe functions both in membrane localization via the first α-helix and in substrate assimilation by altering the functional conformation of the C lobe to facilitate recruitment of cellular substrates. Collectively, these results indicate that RRSP is a critical virulence factor that robustly inactivates Ras and Rap1 and augments the pathogenicity of invading bacteria via the combined effects of its N and C lobes.

Frontline Science: PECAM-1 (CD31) expression in naïve and memory, but not acutely activated, CD8+ T cells

Inhibitory cell surface proteins on T cells are often dynamically regulated, which contributes to their physiologic function. PECAM-1 (CD31) is an inhibitory receptor that facilitates TGF-β-mediated suppression of T cell activity. It is well established in CD4+ T cells that PECAM-1 is expressed in naïve recent thymic emigrants, but is down-regulated after acute T cell activation and absent from memory cells. The extent to which PECAM-1 expression is similarly regulated in CD8+ T cells is much less well characterized. We evaluated T cells recovered from mice after infection with a model intracellular pathogen and determined that, in CD8+ T cells, PECAM-1 expression was strongly down-regulated during acute infection but re-expressed to intermediate levels in memory cells. Down-regulation of PECAM-1 expression in CD8+ T cells was transcriptionally regulated and affected by the strength and nature of TCR signaling. PECAM-1 was also detected on the surface of human activated/memory CD8+ , but not CD4+ T cells. These data demonstrate that PECAM-1 expression is dynamically regulated, albeit differently, in both CD4+ and CD8+ T cells. Furthermore, unlike memory CD4+ T cells, memory CD8+ T cells retain PECAM-1 expression and have the potential to be modulated by this inhibitory receptor.

Deciphering T Cell Immunometabolism with Activity-Based Protein Profiling

As a major sentinel of adaptive immunity, T cells seek and destroy diseased cells using antigen recognition to achieve molecular specificity. Strategies to block checkpoint inhibition of T cell activity and thus reawaken the patient's antitumor immune responses are rapidly becoming standard of care for treatment of diverse cancers. Adoptive transfer of patient T cells genetically engineered with tumor-targeting capabilities is redefining the field of personalized medicines. The diverse opportunities for exploiting T cell biology in the clinic have prompted new efforts to expand the scope of targets amenable to immuno-oncology. Given the complex spatiotemporal regulation of T cell function and fate, new technologies capable of global molecular profiling in vivo are needed to guide selection of appropriate T cell targets and subsets. In this chapter, we describe the use of activity-based protein profiling (ABPP) to illuminate different aspects of T cell metabolism and signaling as fertile starting points for investigation. We highlight the merits of ABPP methods to enable target, inhibitor, and biochemical pathway discovery of T cells in the burgeoning field of immuno-oncology.

Diacylglycerol kinase α inactivation is an integral component of the costimulatory pathway that amplifies TCR signals

The arsenal of cancer therapies has evolved to target T lymphocytes and restore their capacity to destroy tumor cells. T cells rely on diacylglycerol (DAG) to carry out their functions. DAG availability and signaling are regulated by the enzymes diacylglycerol kinase (DGK) α and ζ, whose excess function drives T cells into hyporesponsive states. Targeting DGKα is a promising strategy for coping with cancer; its blockade could reinstate T-cell attack on tumors while limiting tumor growth, due to positive DGKα functions in several oncogenic pathways. Here, we made a side-by-side comparison of the effects of commercial pharmacological DGK inhibitors on T-cell responses with those promoted by DGKα and DGKζ genetic deletion or silencing. We show the specificity for DGKα of DGK inhibitors I and II and the structurally similar compound ritanserin. Inhibitor treatment promoted Ras/ERK (extracellular signal-regulated kinase) signaling and AP-1 (Activator protein-1) transcription, facilitated DGKα membrane localization, reduced the requirement for costimulation, and cooperated with enhanced activation following DGKζ silencing/deletion. DGKiII and ritanserin had similar effects on TCR proximal signaling, but ritanserin counteracted long-term T-cell activation, an effect that was potentiated in DGKα-/- cells. In contrast with enhanced activation triggered by pharmacological inhibition, DGKα silencing/genetic deletion led to impaired Lck (lymphocyte-specific protein tyrosine kinase) activation and limited costimulation responses. Our results demonstrate that pharmacological inhibition of DGKα downstream of the TCR provides a gain-of-function effect that amplifies the DAG-dependent signaling cascade, an ability that could be exploited therapeutically to reinvigorate T cells to attack tumors.

Diacylglycerol kinase ζ (DGKζ) and Casitas b-lineage proto-oncogene b-deficient mice have similar functional outcomes in T cells but DGKζ-deficient mice have increased T cell activation and tumor clearance

Targeting negative regulators downstream of the T cell receptor (TCR) represents a novel strategy to improve cancer immunotherapy. Two proteins that serve as critical inhibitory regulators downstream of the TCR are diacylglycerol kinase ζ (DGKζ), a regulator of Ras and PKC-θ signaling, and Casitas b-lineage proto-oncogene b (Cbl-b), an E3 ubiquitin ligase that predominantly regulates PI(3)K signaling. We sought to compare the signaling and functional effects that result from deletion of DGKζ, Cbl-b, or both (double knockout, DKO) in T cells, and to evaluate tumor responses generated in a clinically relevant orthotopic pancreatic tumor model. We found that whereas deletion of Cbl-b primarily served to enhance NF-κB signaling, deletion of DGKζ enhanced TCR-mediated signal transduction downstream of Ras/Erk and NF-κB. Deletion of DGKζ or Cbl-b comparably enhanced CD8⁺ T cell functional responses, such as proliferation, production of IFNγ, and generation of granzyme B when compared with WT T cells. DKO T cells demonstrated enhanced function above that observed with single knockout T cells after weak, but not strong, stimulation. Deletion of DGKζ, but not Cbl-b, however, resulted in significant increases in numbers of activated (CD44^hi) CD8⁺ T cells in both non-treated and tumor-bearing mice. DGKζ-deficient mice also had enhanced control of pancreatic tumor cell growth compared to Cbl-b-deficient mice. This represents the first direct comparison between mice of these genotypes and suggests that T cell immunotherapies may be better improved by targeting TCR signaling molecules that are regulated by DGKζ as opposed to molecules regulated by Cbl-b.

Unexpected positive control of NFκB and miR-155 by DGKα and ζ ensures effector and memory CD8+ T cell differentiation

Signals from the T-cell receptor (TCR) and γ-chain cytokine receptors play crucial roles in initiating activation and effector/memory differentiation of CD8 T-cells. We report here that simultaneous deletion of both diacylglycerol kinase (DGK) α and ζ (DKO) severely impaired expansion of CD8 effector T cells and formation of memory CD8 T-cells after Listeria monocytogenes infection. Moreover, ablation of both DGKα and ζ in preformed memory CD8 T-cells triggered death and impaired homeostatic proliferation of these cells. DKO CD8 T-cells were impaired in priming due to decreased expression of chemokine receptors and migration to the draining lymph nodes. Moreover, DKO CD8 T-cells were unexpectedly defective in NFκB-mediated miR-155 transcript, leading to excessive SOCS1 expression and impaired γ-chain cytokine signaling. Our data identified a DGK-NFκB-miR-155-SOCS1 axis that bridges TCR and γ-chain cytokine signaling for robust CD8 T-cell primary and memory responses to bacterial infection.

SNX27 links DGKζ to the control of transcriptional and metabolic programs in T lymphocytes

Sorting nexin 27 (SNX27) recycles PSD-95, Dlg1, ZO-1 (PDZ) domain-interacting membrane proteins and is essential to sustain adequate brain functions. Here we define a fundamental SNX27 function in T lymphocytes controlling antigen-induced transcriptional activation and metabolic reprogramming. SNX27 limits the activation of diacylglycerol (DAG)-based signals through its high affinity PDZ-interacting cargo DAG kinase ζ (DGKζ). SNX27 silencing in human T cells enhanced T cell receptor (TCR)-stimulated activator protein 1 (AP-1)- and nuclear factor κB (NF-κB)-mediated transcription. Transcription did not increase upon DGKζ silencing, suggesting that DGKζ function is dependent on SNX27. The enhanced transcriptional activation in SNX27-silenced cells contrasted with defective activation of the mammalian target of rapamycin (mTOR) pathway. The analysis of Snx27^−/− mice supported a role for SNX27 in the control of T cell growth. This study broadens our understanding of SNX27 as an integrator of lipid-based signals with the control of transcription and metabolic pathways.

Diacylglycerol Kinase ζ Limits Cytokine-dependent Expansion of CD8+ T Cells with Broad Antitumor Capacity

Interleukin-2 and -15 drive expansion/differentiation of cytotoxic CD8⁺ T cells that eliminate targets via antigen-independent killing. This property is clinically relevant for the improvement of T cell-based antitumor therapies. Diacylglycerol kinase α and ζ (DGKα/ζ) metabolize the diacylglycerol generated following antigen recognition by T lymphocytes. Enhanced expression of these two lipid kinases in tumor-infiltrating CD8⁺ T cells promotes a hyporesponsive state that contributes to tumor immune escape. Inhibition of these two enzymes might thus be of interest for potentiating conventional antigen-directed tumor elimination. In this study, we sought to characterize the contribution of DGKα and ζ to antigen-independent cytotoxic functions of CD8⁺ T cells. Analysis of DGKζ-deficient mice showed an increase in bystander memory-like CD8⁺ T cell populations not observed in DGKα-deficient mice. We demonstrate that DGKζ limits cytokine responses in an antigen-independent manner. Cytokine-specific expansion of DGKζ-deficient CD8⁺ T cells promoted enhanced differentiation of innate-like cytotoxic cells in vitro, and correlated with the more potent in vivo anti-tumor responses of DGKζ-deficient mice engrafted with the murine A20 lymphoma. Our studies reveal a isoform-specific function for DGKζ downstream of IL-2/IL-15-mediated expansion of innate-like cytotoxic T cells, Pharmacological manipulation of DGKζ activity is of therapeutic interest for cytokine-directed anti-tumor treatments.

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DGKζ, a well-characterized negative regulator of TCR signals, also limits IL-2/15 function.

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DGKζ impairs cytokine-induced differentiation of cytotoxic T cell populations with innate-like ability to kill targets.

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As a result, DGKζ-deficient mice demonstrate enhanced rejection of implanted B cell lymphoma compared to wild type mice.

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Targeting DGKζ activity might be of interest to enhance cytokine-mediated antitumor therapies.

The immune system defends the body from foreign invaders. In cancer, tumors disguise as self-body cells and evade immune attack. For this reason it is important to identify the mechanism that stop T lymphocytes from recognize and destroy tumors. In this study we investigate the role of Diacylglycerol kinase zeta (DGKζ) as an inhibitor of antitumor T cell functions. We demonstrate that lymphoma cells injected in mice genetically modified to lack DGKζ expression develop smaller tumors that resolve more rapidly than those grown in normal mice. Our studies suggest that inhibition of DGKζ could help to reinforce the antitumor capacity of immune T lymphocytes.

DGK-α: A Checkpoint in Cancer-Mediated Immuno-Inhibition and Target for Immunotherapy

Immunotherapy is moving to the forefront of cancer treatments owing to impressive durable responses achieved with checkpoint blockade antibodies and adoptive T-cell therapy. Still, improvements are necessary since, overall, only a small percentage of patients benefit from current therapies. Here, I summarize evidence that DGK-α may represent an immunological checkpoint suppressing the activity of cytotoxic immunocytes in the tumor microenvironment. DGK-inhibitors can restore the antitumor function of tumor-suppressed adaptive and innate cytotoxic immunocytes. The activity of DGK-inhibitors lays downstream of current checkpoint blockade antibodies. Thus, synergistic effects are expected from combination strategies. Moreover, DGK-inhibitors may permit a double-strike attack on tumor cells as DGK-inhibition may not only re-instate immunological tumor attack but also may harm tumor cells directly by interfering with oncogenic survival pathways. Together, DGK-inhibitors have very promising characteristics and may be beneficially included into the armamentarium of cancer immunotherapeutics.

Diacylglycerol Kinases in T Cell Tolerance and Effector Function

Diacylglycerol kinases (DGKs) are a family of enzymes that regulate the relative levels of diacylglycerol (DAG) and phosphatidic acid (PA) in cells by phosphorylating DAG to produce PA. Both DAG and PA are important second messengers cascading T cell receptor (TCR) signal by recruiting multiple effector molecules, such as RasGRP1, PKCθ, and mTOR. Studies have revealed important physiological functions of DGKs in the regulation of receptor signaling and the development and activation of immune cells. In this review, we will focus on recent progresses in our understanding of two DGK isoforms, α and ζ, in CD8 T effector and memory cell differentiation, regulatory T cell development and function, and invariant NKT cell development and effector lineage differentiation.

Diacylglycerol Kinases (DGKs): Novel Targets for Improving T Cell Activity in Cancer

Diacylglycerol kinases (DGKs) are a family of enzymes that catalyze the metabolism of diacylglycerol (DAG). Two isoforms of DGK, DGKα, and DGKζ, specifically regulate the pool of DAG that is generated as a second messenger after stimulation of the T cell receptor (TCR). Deletion of either isoform in mouse models results in T cells bearing a hyperresponsive phenotype and enhanced T cell activity against malignancy. Whereas, DGKζ appears to be the dominant isoform in T cells, rationale exists for targeting both isoforms individually or coordinately. Additional work is needed to rigorously identify the molecular changes that result from deletion of DGKs in order to understand how DAG contributes to T cell activation, the effect of DGK inhibition in human T cells, and to rationally develop combined immunotherapeutic strategies that target DGKs.

The Immunomodulatory Functions of Diacylglycerol Kinase ζ

The generation of diacylglycerol (DAG) is critical for promoting immune cell activation, regulation, and function. Diacylglycerol kinase ζ (DGKζ) serves as an important negative regulator of DAG by enzymatically converting DAG into phosphatidic acid (PA) to shut down DAG-mediated signaling. Consequently, the loss of DGKζ increases DAG levels and the duration of DAG-mediated signaling. However, while the enhancement of DAG signaling is thought to augment immune cell function, the loss of DGKζ can result in both immunoactivation and immunomodulation depending on the cell type and function. In this review, we discuss how different immune cell functions can be selectively modulated by DGKζ. Furthermore, we consider how targeting DGKζ can be potentially beneficial for the resolution of human diseases by either promoting immune responses important for protection against infection or cancer or dampening immune responses in immunopathologic conditions such as allergy and septic shock.

Diacylglycerol Kinase ζ Is a Target To Enhance NK Cell Function

Enhancement of NK cell function could be beneficial in treatment of a variety of tumors and infections. However, efforts to improve NK cell function by disrupting negative regulators that target proximal signaling pathways paradoxically results in hyporesponsive rather than hyperresponsive NK cells. In this study, we demonstrate that genetic deletion of diacylglycerol kinase (DGK)ζ, a negative regulator of diacylglycerol-mediated signaling, has the desired effect of enhancing NK cell function due to its distal position in the activating receptor-mediated signaling cascade. Upon stimulation through multiple activating receptors, NK cells from mice lacking DGKζ display increased cytokine production and degranulation in an ERK-dependent manner. Additionally, they have improved cytotoxic functions against tumor cell lines. The enhancement of NK cell function by DGKζ deficiency is NK cell-intrinsic and developmentally independent. Importantly, DGKζ deficiency does not affect inhibitory NK cell receptor expression or function. Thus, DGKζ knockout mice display improved missing self recognition, as evidenced by enhanced rejection of a TAP-deficient tumor in vivo. We propose that enzymes that negatively regulate distal activating receptor signaling pathways such as DGKζ represent novel targets for augmenting the therapeutic potential of NK cells.

The adhesion molecule PECAM-1 enhances the TGF-β-mediated inhibition of T cell function

Transforming growth factor-β (TGF-β) is an immunosuppressive cytokine that inhibits the proinflammatory functions of T cells, and it is a major factor in abrogating T cell activity against tumors. Canonical TGF-β signaling results in the activation of Smad proteins, which are transcription factors that regulate target gene expression. We found that the cell surface molecule platelet endothelial cell adhesion molecule-1 (PECAM-1) facilitated noncanonical (Smad-independent) TGF-β signaling in T cells. Subcutaneously injected tumor cells that are dependent on TGF-β-mediated suppression of immunity for growth grew more slowly in PECAM-1(-/-) mice than in their wild-type counterparts. T cells isolated from PECAM-1(-/-) mice demonstrated relative insensitivity to the TGF-β-dependent inhibition of interferon-γ (IFN-γ) production, granzyme B synthesis, and cellular proliferation. Similarly, human T cells lacking PECAM-1 demonstrated decreased sensitivity to TGF-β in a manner that was partially restored by reexpression of PECAM-1. Co-incubation of T cells with TGF-β and a T cell-activating antibody resulted in PECAM-1 phosphorylation on an immunoreceptor tyrosine-based inhibitory motif (ITIM) and the recruitment of the inhibitory Src homology 2 (SH2) domain-containing tyrosine phosphatase-2 (SHP-2). Such conditions also induced the colocalization of PECAM-1 with the TGF-β receptor complex as identified by coimmunoprecipitation, confocal microscopy, and proximity ligation assays. These studies indicate a role for PECAM-1 in enhancing the inhibitory functions of TGF-β in T cells and suggest that therapeutic targeting of the PECAM-1-TGF-β inhibitory axis represents a means to overcome TGF-β-dependent immunosuppression within the tumor microenvironment.

TCR signaling intensity controls CD8+ T cell responsiveness to TGF-β

DGK-ζ is a negative regulator of TCR signaling that causes degradation of the second messenger DAG, terminating DAG-mediated activation of Ras and PKCθ. Cytotoxic T cells deficient in DGK-ζ demonstrate enhanced effector functions in vitro and antitumor activity in vivo, perhaps because of insensitivity to inhibitory cytokines. We sought to determine whether the enhanced responsiveness of DGK-ζ-deficient T cells renders them insensitive to the inhibitory cytokine TGF-β and to determine how the loss of DGK-ζ facilitates this insensitivity. We identified decreased transcriptional and functional responses to TGF-β in CD8(+) DGK-ζ(-/-) T cells but preserved TGF-β-mediated conversion of naïve DGK-ζ(-/-) CD4(+) T cells to a regulatory T cell phenotype. Decreased CD8(+) T cell responsiveness to TGF-β did not result from impaired canonical TGF-β signal transduction, because similar levels of TGF-β-R and intracellular Smad components were identified in WT and DGK-ζ(-/-) CD8(+) T cells, and TGF-β-mediated activation of Smad2 was unchanged. Instead, an enhanced TCR signal strength was responsible for TGF-β insensitivity, because (i) loss of DGK-ζ conferred resistance to TGF-β-mediated inhibition of Erk phosphorylation, (ii) TGF-β insensitivity could be recapitulated by exogenous addition of the DAG analog PMA, and (iii) TGF-β sensitivity could be observed in DGK-ζ-deficient T cells at limiting dilutions of TCR stimulation. These data indicate that enhanced TCR signal transduction in the absence of DGK-ζ makes T cells relatively insensitive to TGF-β, in a manner independent of Smads, a finding with practical implications in the development of immunotherapies that target TGF-β.

Multifactorial T-cell hypofunction that is reversible can limit the efficacy of chimeric antigen receptor-transduced human T cells in solid tumors

PURPOSE

Immunotherapy using vaccines or adoptively transferred tumor-infiltrating lymphocytes (TIL) is limited by T-cell functional inactivation within the solid tumor microenvironment. The purpose of this study was to determine whether a similar tumor-induced inhibition occurred with genetically modified cytotoxic T cells expressing chimeric antigen receptors (CAR) targeting tumor-associated antigens.

EXPERIMENTAL DESIGN

Human T cells expressing CAR targeting mesothelin or fibroblast activation protein and containing CD3ζ and 4-1BB cytoplasmic domains were intravenously injected into immunodeficient mice bearing large, established human mesothelin-expressing flank tumors. CAR TILs were isolated from tumors at various time points and evaluated for effector functions and status of inhibitory pathways.

RESULTS

CAR T cells were able to traffic into tumors with varying efficiency and proliferate. They were able to slow tumor growth, but did not cause regressions or cures. The CAR TILs underwent rapid loss of functional activity that limited their therapeutic efficacy. This hypofunction was reversible when the T cells were isolated away from the tumor. The cause of the hypofunction seemed to be multifactorial and was associated with upregulation of intrinsic T-cell inhibitory enzymes (diacylglycerol kinase and SHP-1) and the expression of surface inhibitory receptors (PD1, LAG3, TIM3, and 2B4).

CONCLUSIONS

Advanced-generation human CAR T cells are reversibly inactivated within the solid tumor microenvironment of some tumors by multiple mechanisms. The model described here will be an important tool for testing T cell-based strategies or systemic approaches to overcome this tumor-induced inhibition. Our results suggest that PD1 pathway antagonism may augment human CAR T-cell function.

The ζ isoform of diacylglycerol kinase plays a predominant role in regulatory T cell development and TCR-mediated ras signaling

Diacylglycerol (DAG) is a critical second messenger that mediates T cell receptor (TCR)-stimulated signaling. The abundance of DAG is reduced by the diacylglycerol kinases (DGKs), which catalyze the conversion of DAG to phosphatidic acid (PA) and thus inhibit DAG-mediated signaling. In T cells, the predominant DGK isoforms are DGKα and DGKζ, and deletion of the genes encoding either isoform enhances DAG-mediated signaling. We found that DGKζ, but not DGKα, suppressed the development of natural regulatory T (T(reg)) cells and predominantly mediated Ras and Akt signaling downstream of the TCR. The differential functions of DGKα and DGKζ were not attributable to differences in protein abundance in T cells or in their localization to the contact sites between T cells and antigen-presenting cells. RasGRP1, a key DAG-mediated activator of Ras signaling, associated to a greater extent with DGKζ than with DGKα; however, in silico modeling of TCR-stimulated Ras activation suggested that a difference in RasGRP1 binding affinity was not sufficient to cause differences in the functions of each DGK isoform. Rather, the model suggested that a greater catalytic rate for DGKζ than for DGKα might lead to DGKζ exhibiting increased suppression of Ras-mediated signals compared to DGKα. Consistent with this notion, experimental studies demonstrated that DGKζ was more effective than DGKα at catalyzing the metabolism of DAG to PA after TCR stimulation. The enhanced effective enzymatic production of PA by DGKζ is therefore one possible mechanism underlying the dominant functions of DGKζ in modulating T(reg) cell development.

Role of diacylglycerol kinases in T cell development and function

Diacylglycerol (DAG), a second messenger generated by phospholipase Cγ1 activity upon engagement of a T-cell receptor, triggers several signaling cascades that play important roles in T cell development and function. A family of enzymes called DAG kinases (DGKs) catalyzes the phosphorylation of DAG to phosphatidic acid, acting as a braking mechanism that terminates DAG-mediated signals. Two DGK isoforms, α and ζ, are expressed predominantly in T cells and synergistically regulate the development of both conventional αβ T cells and invariant natural killer T cells in the thymus. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T-cell hyperactivation upon T cell receptor stimulation and by promoting T-cell anergy. In CD8 cells, reduced DGK activity is associated with enhanced primary responses against viruses and tumors. Recent work also has established an important role for DGK activity at the immune synapse and identified partners that modulate DGK function. In addition, emerging evidence points to previously unappreciated roles for DGK function in directional secretion and T-cell adhesion. This review describes the multitude of roles played by DGKs in T cell development and function and emphasizes recent advances in the field.

Regulation of Lipid Signaling by Diacylglycerol Kinases during T Cell Development and Function

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.

Activation of Extracellular Signal-Regulated Kinase but Not of p38 Mitogen-Activated Protein Kinase Pathways in Lymphocytes Requires Allosteric Activation of SOS

Thymocytes convert graded T cell receptor (TCR) signals into positive selection or deletion, and activation of extracellular signal-related kinase (ERK), p38, and Jun N-terminal protein kinase (JNK) mitogen-activated protein kinases (MAPKs) has been postulated to play a discriminatory role. Two families of Ras guanine nucleotide exchange factors (RasGEFs), SOS and RasGRP, activate Ras and the downstream RAF-MEK-ERK pathway. The pathways leading to lymphocyte p38 and JNK activation are less well defined. We previously described how RasGRP alone induces analog Ras-ERK activation while SOS and RasGRP cooperate to establish bimodal ERK activation. Here we employed computational modeling and biochemical experiments with model cell lines and thymocytes to show that TCR-induced ERK activation grows exponentially in thymocytes and that a W729E allosteric pocket mutant, SOS1, can only reconstitute analog ERK signaling. In agreement with RasGRP allosterically priming SOS, exponential ERK activation is severely decreased by pharmacological or genetic perturbation of the phospholipase Cγ (PLCγ)-diacylglycerol-RasGRP1 pathway. In contrast, p38 activation is not sharply thresholded and requires high-level TCR signal input. Rac and p38 activation depends on SOS1 expression but not allosteric activation. Based on computational predictions and experiments exploring whether SOS functions as a RacGEF or adaptor in Rac-p38 activation, we established that the presence of SOS1, but not its enzymatic activity, is critical for p38 activation.

Enhanced effector responses in activated CD8+ T cells deficient in diacylglycerol kinases

Recent clinical trials have shown promise in the use of chimeric antigen receptor (CAR)-transduced T cells; however, augmentation of their activity may broaden their clinical use and improve their efficacy. We hypothesized that because CAR action requires proteins essential for T-cell receptor (TCR) signal transduction, deletion of negative regulators of these signaling pathways would enhance CAR signaling and effector T-cell function. We tested CAR activity and function in T cells that lacked one or both isoforms of diacylglycerol kinase (dgk) expressed highly in T cells, dgkα and dgkζ, enzymes that metabolize the second messenger diacylglycerol (DAG) and limit Ras/ERK activation. We found that primary murine T cells transduced with CARs specific for the human tumor antigen mesothelin showed greatly enhanced cytokine production and cytotoxicity when cocultured with a murine mesothelioma line that stably expresses mesothelin. In addition, we found that dgk-deficient CAR-transduced T cells were more effective in limiting the growth of implanted tumors, both concurrent with and after establishment of tumor. Consistent with our studies in mice, pharmacologic inhibition of dgks also augments function of primary human T cells transduced with CARs. These results suggest that deletion of negative regulators of TCR signaling enhances the activity and function of CAR-expressing T cells and identify dgks as potential targets for improving the clinical potential of CARs.

Diacylglycerol kinases: regulated controllers of T cell activation, function, and development

Diacylglycerol kinases (DGKs) are a diverse family of enzymes that catalyze the conversion of diacylglycerol (DAG), a crucial second messenger of receptor-mediated signaling, to phosphatidic acid (PA). Both DAG and PA are bioactive molecules that regulate a wide set of intracellular signaling proteins involved in innate and adaptive immunity. Clear evidence points to a critical role for DGKs in modulating T cell activation, function, and development. More recently, studies have elucidated factors that control DGK function, suggesting an added complexity to how DGKs act during signaling. This review summarizes the available knowledge of the function and regulation of DGK isoforms in signal transduction with a particular focus on T lymphocytes.

Dysregulated RasGRP1 responds to cytokine receptor input in T cell leukemogenesis

Enhanced signaling by the small guanosine triphosphatase Ras is common in T cell acute lymphoblastic leukemia/lymphoma (T-ALL), but the underlying mechanisms are unclear. We identified the guanine nucleotide exchange factor RasGRP1 (Rasgrp1 in mice) as a Ras activator that contributes to leukemogenesis. We found increased RasGRP1 expression in many pediatric T-ALL patients, which is not observed in rare early T cell precursor T-ALL patients with KRAS and NRAS mutations, such as K-Ras(G12D). Leukemia screens in wild-type mice, but not in mice expressing the mutant K-Ras(G12D) that encodes a constitutively active Ras, yielded frequent retroviral insertions that led to increased Rasgrp1 expression. Rasgrp1 and oncogenic K-Ras(G12D) promoted T-ALL through distinct mechanisms. In K-Ras(G12D) T-ALLs, enhanced Ras activation had to be uncoupled from cell cycle arrest to promote cell proliferation. In mouse T-ALL cells with increased Rasgrp1 expression, we found that Rasgrp1 contributed to a previously uncharacterized cytokine receptor-activated Ras pathway that stimulated the proliferation of T-ALL cells in vivo, which was accompanied by dynamic patterns of activation of effector kinases downstream of Ras in individual T-ALLs. Reduction of Rasgrp1 abundance reduced cytokine-stimulated Ras signaling and decreased the proliferation of T-ALL in vivo. The position of RasGRP1 downstream of cytokine receptors as well as the different clinical outcomes that we observed as a function of RasGRP1 abundance make RasGRP1 an attractive future stratification marker for T-ALL.

Transient PKCα shuttling to the immunological synapse is governed by (DGK)ζ and regulates L-selectin shedding

Considerable evidence indicates that diacylglycerol (DAG) generation at the immunological synapse (IS) determines T cell functions by regulating the duration and amplitude of Ras/ERK signals. The exact mechanism by which DAG regulates Ras/ERK activation downstream of the T cell receptor (TCR) nonetheless remains poorly understood. Here we characterize PKCα as a previously unrecognized component of the machinery that translates cell receptor occupancy into Ras/ERK-propagated signals. We show transient translocation of PKCα to the IS, mediated by DAG generation at the contact area. Diacylglycerol kinase (DGK)ζ negatively regulated PKCα translocation kinetics, whereas PKCα activity limited its own persistence at the IS. Coordinated activation of DGKζ and PKCα in response to antigen recognition regulated the amplitude and duration of Ras/ERK activation; this in turn mediated early processes of T cell surface proteolysis such as L-selectin shedding. Analysis of DGKζ-deficient mice further showed that increased DAG signaling is translated to downstream elements of this pathway, as reflected by enhanced PKCα-dependent L-selectin shedding. We propose that early activation of a DAG/PKCα axis contributes to the mechanisms by which antigen affinity translates into TCR biological responses.

Enhanced function of redirected human T cells expressing linker for activation of T cells that is resistant to ubiquitylation

It is likely that the enhancement of signaling after antigenic stimulation, particularly in the tumor microenvironment, would improve the function of adoptively transferred T cells. Linker for activation of T cells (LAT) plays a central role in T cell activation. We hypothesized that the ubiquitylation-resistant form of LAT in cells would enhance T cell signaling and thus augment antitumor activity. To test this, human CD4(+) or CD8(+) T cells were electroporated with small interfering RNA (siRNA) to repress endogenous LAT and ubiquitylation-resistant LAT 2KR or wild-type LAT mRNA was introduced for reexpression. Significantly enhanced phosphorylation of LAT and phospholipase C-γ (PLCγ) was observed, and augmented calcium signaling after T cell receptor (TCR) triggering was observed in LAT 2KR-expressing T cells. TCR-induced calcium signaling was abrogated in LAT knockdown cells, but the baseline was higher than that of control siRNA-electroporated cells, suggesting a fundamental requirement of LAT to maintain calcium homeostasis. Redirected LAT 2KR T cells expressing a chimeric antigen receptor or an MHC class I-restricted TCR showed augmented function as assessed by enhanced cytokine secretion and cytotoxicity. These results indicate that interruption of LAT ubiquitylation is a promising strategy to augment effector T cell function for adoptive cell therapy.

Cytokine profile, proliferation and phosphorylation of ERK1/2 and Akt in circulating mononuclear cells from individuals during the chronic intestinal phase of Schistosomiasis mansoni infection

Background

The immune response to Schistosoma mansoni is characterized by a granulomatous reaction around the parasite eggs that are trapped in the host liver, and this reaction modulates the immune response during the chronic phase of the disease. The typical peripheral blood mononuclear cell (PBMC) response of patients during the chronic intestinal phase of infection is characterized by a decreased response to an S. mansoni soluble egg antigen. To obtain a greater understanding of Schistosoma infections, this study investigated the effects of the soluble egg antigen (SEA) and soluble adult worm antigen (SWAP) of S. mansoni on cellular proliferation, cytokine production, and ERK1/2 and Akt phosphorylation in PBMCs from infected (XTO) and egg-negative (NI) individuals living in the same endemic area.

Methods

The activation status was evaluated by cell immunophenotypic staining (cytometry). The cell proliferation assay was by CFSE method. Cytokine detection assay (Th1 and Th2) was by Cytometric Bead and Array phosphorylation status was by ELISA.

Results

The XTO, NI and BD (blood donor) individuals from an area not endemic for schistosomiasis were compared. The CD4⁺ T lymphocyte proliferation rate was lower in the XTO group, but not the NI group, after SEA stimulation compared to the BD group. The CD8⁺ T cell proliferation rate was lower in the XTO group in the unstimulated cultures and after both SEA and SWAP stimulation compared to the BD group. Cytokine analysis after either SEA or SWAP stimulation showed a balanced cytokine pattern in the XTO and NI groups. ERK1/2 and Akt phosphorylation were only marginally detected in all groups; however, a decrease in ERK 1/2 phosphorylation was observed in the SWAP-stimulated XTO group compared to both the NI and BD groups.

Conclusions

The data indicate that SEA-stimulated CD4⁺ T cells from infected patients have a lower proliferation rate than the same cells from the NI group. Furthermore, we observed that SWAP stimulation influences ERK1/2 phosphorylation in the XTO group.

Protein kinase D2 has a restricted but critical role in T-cell antigen receptor signalling in mature T-cells

PKD (protein kinase D) 2 is a serine/threonine kinase activated by diacylglycerol in response to engagement of antigen receptors in lymphocytes. To explore PKD2 regulation and function in TCR (T-cell antigen receptor) signal transduction we expressed TCR complexes with fixed affinity for self antigens in the T-cells of PKD2-null mice or mice deficient in PKD2 catalytic activity. We also developed a single cell assay to quantify PKD2 activation as T-cells respond to developmental stimuli or engagement of α/β TCR complexes in vivo. Strikingly, PKD2 loss caused increases in thymic output, lymphadenopathy and splenomegaly in TCR transgenic mice. The precise magnitude and timing of PKD2 activation during T-cell development is thus critical to regulate thymic homoeostasis. PKD2-null T-cells that exit the thymus have a normal transcriptome, but show a limited and abnormal transcriptional response to antigen. Transcriptional profiling reveals the full consequences of PKD2 loss and maps in detail the selective, but critical, function for PKD2 in signalling by α/β mature TCR complexes in peripheral T-cells.

The membrane environment can promote or suppress bistability in cell signaling networks

Many key biochemical reactions that mediate signal transduction in cells occur at the cell membrane, yet how the two-dimensional membrane environment influences the collective behavior of signaling networks is poorly understood. We study models of two topologically different signaling pathways that exhibit bistability, examining the effects of reduced protein mobility and increased concentration at the membrane, as well as effects due to differences in spatiotemporal correlations between the membrane environment and three-dimensional cytoplasm. The two model networks represent the distributive enzymatic modification of a protein at multiple sites and the positive feedback-mediated activation of a protein. In both cases, we find that confining proteins to a membrane-like environment can markedly alter the emergent dynamics. For the distributive protein modification network, increased concentration promotes bistability through enhanced protein-protein binding, while lower mobility and membrane-enhanced spatiotemporal correlations suppress bistability. For the positive feedback-mediated activation network, confinement to a membrane environment enhances protein activation, which can induce bistability or stabilize a monostable, active state. Importantly, the influence of the membrane environment on signaling dynamics can be qualitatively different for signaling modules with different network topologies.

Ras family of small GTPases in immunity and inflammation

The Ras superfamily of small GTPases is a group of more than 150 small G proteins, all of which share some degree of homology to the founding member Ras. These small GTPases function as molecular switches within cells, impacting nearly all cellular processes. The Ras superfamily can be further divided into several smaller subfamilies, with those proteins that most closely resemble Ras belonging to the Ras subfamily. While heavily studied within the field of cancer biology, the Ras family of proteins also plays cardinal roles in immunity and inflammation. Here we review the roles of these molecular switches in regulating immune cell homeostasis and functions.

Identifying dynamical bottlenecks of stochastic transitions in biochemical networks

In biochemical networks, identifying key proteins and protein-protein reactions that regulate fluctuation-driven transitions leading to pathological cellular function is an important challenge. Using large deviation theory, we develop a semianalytical method to determine how changes in protein expression and rate parameters of protein-protein reactions influence the rate of such transitions. Our formulas agree well with computationally costly direct simulations and are consistent with experiments. Our approach reveals qualitative features of key reactions that regulate stochastic transitions.