Role of calcium influx in cytotoxic T lymphocyte lytic granule exocytosis during target cell killing

Differential regulation of calcium-NFAT signaling pathway by Akt isoforms: unraveling effector dynamics and exhaustion of cytotoxic T lymphocytes in tumor microenvironment

BACKGROUND

Impairment of Akt signaling has been observed in antigen-specific cytotoxic T lymphocytes (CTLs) during chronic viral infections or tumor progression. Despite numerous studies emphasizing Akt's role in driving CTL effector functions, there is limited exploration of using Akt molecules in T-cell engineering to enhance their antiviral or antitumor capabilities for therapeutic purposes. Some studies even conclude that inhibiting Akt activation during the in vitro expansion process can prevent T-cell exhaustion and boost the antitumor effector functions of chimeric antigen receptor-T cells in vivo. Given the unique expression patterns and functions of the three Akt isoforms in immune cells, we proposed that Akt isoforms in CTLs may regulate effector functions and T-cell exhaustion distinctly.

METHODS

In this study, we genetically modified tumor/virus-antigen-specific T-cell receptor tg CTLs to ectopically express Akt isoforms via retroviral transduction. We subsequently conducted western blotting, flow cytometry, and RNA sequencing analysis to assess their Akt expression, expression of immune checkpoints, antitumor/antivirus functionalities, and transcriptome. Additionally, we employed a persistent Hepatitis B Virus mouse model and a syngeneic hepatocellular carcinoma mouse model for further evaluation of their antivirus/antitumor efficacies.

RESULTS

We found that both Akt1 and Akt2 overexpression enhanced the cytotoxic capabilities of mouse CTLs, although with different dynamics. Specifically, Akt2 signaling in CTLs accelerated effector functions, leading to a rapid attack on tumor cells. Conversely, Akt1 signaling triggered calcium influx and subsequent nuclear factor of activated T cells (NFAT) activation, while Akt2 signaling suppressed calcium influx, preventing excessive NFAT expression and nuclear translocation. This repression of NFAT transcriptional activity by Akt2 signaling during prolonged antigen stimulation subsequently led to reduced expression of transcription factors associated with T-cell exhaustion, such as Egr2, Nr4a, Tox, and immune checkpoints. Consequently, Akt2-overexpressed CTLs displayed reduced T-cell exhaustion within the tumor microenvironment and efficiently eradicated tumors.

CONCLUSION

These findings highlight the essential role of Akt signaling in enabling tumor-specific CTLs to eliminate cancer cells in the solid TME, with Akt isoforms differentially regulating the calcium-calcineurin-NFAT signaling pathway. This discovery suggests the potential of AKT2 in T-cell engineering technology to enhance the survival and effector functions of adoptively transferred T cells for treating liver malignancies or chronic viral infections.

Focusing on CD8+ T-cell phenotypes: improving solid tumor therapy

Vigorous CD8+ T cells play a crucial role in recognizing tumor cells and combating solid tumors. How T cells efficiently recognize and target tumor antigens, and how they maintain the activity in the "rejection" of solid tumor microenvironment, are major concerns. Recent advances in understanding of the immunological trajectory and lifespan of CD8+ T cells have provided guidance for the design of more optimal anti-tumor immunotherapy regimens. Here, we review the newly discovered methods to enhance the function of CD8+ T cells against solid tumors, focusing on optimizing T cell receptor (TCR) expression, improving antigen recognition by engineered T cells, enhancing signal transduction of the TCR-CD3 complex, inducing the homing of polyclonal functional T cells to tumors, reversing T cell exhaustion under chronic antigen stimulation, and reprogramming the energy and metabolic pathways of T cells. We also discuss how to participate in the epigenetic changes of CD8+ T cells to regulate two key indicators of anti-tumor responses, namely effectiveness and persistence.

Identification of a novel ADCC-related gene signature for predicting the prognosis and therapy response in lung adenocarcinoma

BACKGROUND

Previous studies have largely neglected the role of ADCC in LUAD, and no study has systematically compiled ADCC-associated genes to create prognostic signatures.

METHODS

In this study, 1564 LUAD patients, 2057 NSCLC patients, and more than 5000 patients with various cancer types from diverse cohorts were included. R package ConsensusClusterPlus was utilized to classify patients into different subtypes. A number of machine-learning algorithms were used to construct the ADCCRS. GSVA and ClusterProfiler were used for enrichment analyses, and IOBR was used to quantify immune cell infiltration level. GISTIC2.0 and maftools were used to analyze the CNV and SNV data. The Oncopredict package was used to predict drug information based on the GDSC1. Three immunotherapy cohorts were used to evaluate patient response to immunotherapy. The Seurat package was used to process single-cell data, the AUCell package was used to calculate cells' geneset activity scores, and the Scissor algorithm was used to identify ADCCRS-associated cells.

RESULTS

Through unsupervised clustering, two distinct subtypes of LUAD were identified, each exhibiting distinct clinical characteristics. The ADCCRS, consisted of 16 genes, was constructed by integrated machine-learning methods. The prognostic power of ADCCRS was validated in 28 independent datasets. Further, ADCCRS shows better predictive abilities than 102 previously published signatures in predicting LUAD patients' survival. A nomogram incorporating ADCCRS and clinical features was constructed, demonstrating high predictive performance. ADCCRS positively correlates with patients' gene mutation, and integrated analysis of bulk and single-cell transcriptome data revealed the association of ADCCRS with TME modulators. Cells representing high-ADCCRS phenotype exhibited more malignant features. LUAD patients with high ADCCRS levels exhibited sensitivity to chemotherapy and targeted therapy, while displaying resistance to immunotherapy. In pan-cancer analysis, ADCCRS still exhibited significant prognostic value and was found to be a risk factor for most cancer patients.

CONCLUSIONS

ADCCRS offers a critical prognostic insight for patients with LUAD, shedding light on the tumor microenvironment and forecasting treatment responsiveness.

Human Vγ9Vδ2 T cells exhibit antifungal activity against Aspergillus fumigatus and other filamentous fungi

Invasive aspergillosis (IA) and mucormycosis are life-threatening diseases, especially among immunocompromised patients. Drug-resistant Aspergillus fumigatus strains have been isolated worldwide, which can pose a serious clinical problem. As IA mainly occurs in patients with compromised immune systems, the ideal therapeutic approach should aim to bolster the immune system. In this study, we focused on Vγ9Vδ2 T cells that exhibit immune effector functions and examined the possibility of harnessing this unconventional T cell subset as a novel therapeutic modality for IA. A potent antifungal effect was observed when A. fumigatus (Af293) hyphae were challenged by Vγ9Vδ2 T cells derived from peripheral blood. In addition, Vγ9Vδ2 T cells exhibited antifungal activity against hyphae of all Aspergillus spp., Cunninghamella bertholletiae, and Rhizopus microsporus but not against their conidia. Furthermore, Vγ9Vδ2 T cells also exhibited antifungal activity against azole-resistant A. fumigatus, indicating that Vγ9Vδ2 T cells could be used for treating drug-resistant A. fumigatus. The antifungal activity of Vγ9Vδ2 T cells depended on cell-to-cell contact with A. fumigatus hyphae, and degranulation characterized by CD107a mobilization seems essential for this activity against A. fumigatus. Vγ9Vδ2 T cells could be developed as a novel modality for treating IA or mucormycosis.

IMPORTANCE

Invasive aspergillosis (IA) and mucormycosis are often resistant to treatment with conventional antifungal agents and have a high mortality rate. Additionally, effective antifungal treatment is hindered by drug toxicity, given that both fungal and human cells are eukaryotic, and antifungal agents are also likely to act on human cells, resulting in adverse effects. Therefore, the development of novel therapeutic agents specifically targeting fungi is challenging. This study demonstrated the antifungal activity of Vγ9Vδ2 T cells against various Aspergillus spp. and several Mucorales in vitro and discussed the mechanism underlying their antifungal activity. We indicate that adoptive immunotherapy using Vγ9Vδ2 T cells may offer a new therapeutic approach to IA.

Development, methodological evaluation and application of a cell-based TRF assay for analysis of ADCC activity

Interaction of an antibody with its FcγR plays an important role in effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC). Nowadays altered ADCC activity of an antibody can be achieved by utilizing an effective glyco-engineering strategy, which often involves changes of sugar moieties in Fc part of the antibody, thereby affecting its receptor binding with effector cells. We aimed to construct a cell-based time-resolved fluorescence (TRF) assay for the evaluation of ADCC activity triggered by the antibody drug Trastuzumab (anti-HER2) and T-DM1. The assay was initiated by incubating 2,2':6',2 "-Terpyridine-6,6"-dicarboxylic acid (TDA)-labeled target SK-BR3 cells with the testing antibodies and engineered NK-92 effector cells. After incubation, the target cells were lysed to detect TDA released into the supernatant. Together with added Eu, the TDA in the supernatant formed a stable chelate of EuTDA with high-intensity fluorescence. The ADCC activity was then determined by measuring the fluorescence of EuTDA. Consequently, the method demonstrated good accuracy, precision, linearity, and specificity over methodological assessment and compared well with the Luciferase release assay in terms of the agreement of the achieved results. Using the developed assay, we evaluated the ADCC activity of two glyco-engineered anti-HER-2 antibody-drug conjugates (ADCs) and the results showed that antibody Fc glycosylation modifications influenced antibody ADCC activity to varying degrees. In conclusion, the present assay is able to accurately assess the ADCC activity induced by Trastuzumab (anti-HER2) and T-DM1, and a similar methodology can be applied to other therapeutic antibodies during drug development to help screen for antibodies with desirable ADCC activity.

Reduction of Ca2+ Entry by a Specific Block of KCa3.1 Channels Optimizes Cytotoxic Activity of NK Cells against T-ALL Jurkat Cells

Degranulation mediated killing mechanism by NK cells is dependent on store-operated Ca2+ entry (SOCE) and has optimum at moderate intracellular Ca2+ elevations so that partial block of SOCE optimizes the killing process. In this study, we tested the effect of the selective blocker of KCa3.1 channel NS6180 on SOCE and the killing efficiency of NK cells from healthy donors and NK-92 cells against T-ALL cell line Jurkat. Patch-clamp analysis showed that only one-quarter of resting NK cells functionally express KCa3.1 current, which increases 3-fold after activation by interleukins 15 and 2. Nevertheless, blockage of KCa3.1 significantly reduced SOCE and intracellular Ca2+ rise induced by IL-15 or target cell recognition. NS6180 (1 μM) decreased NK degranulation at zero time of coculture with Jurkat cells but already after 1 h, the degranulation reached the same level as in the control. Monitoring of target cell death by flow cytometry and confocal microscopy demonstrated that NS6180 significantly improved the killing ability of NK cells after 1 h in coculture with Jurkat cells and increased the Jurkat cell fraction with apoptotic and necrotic markers. Our data evidence a strong dependence of SOCE on KCa3.1 activity in NK cells and that KCa3.1 specific block can improve NK cytotoxicity.

Regulation of CD8 T-cell signaling, metabolism, and cytotoxic activity by extracellular lysophosphatidic acid

Lysophosphatidic acid (LPA) is an endogenous bioactive lipid that is produced extracellularly and signals to cells via cognate LPA receptors, which are G-protein coupled receptors (GPCRs). Mature lymphocytes in mice and humans express three LPA receptors, LPA2 , LPA5, and LPA6 , and work from our group has determined that LPA5 signaling by T lymphocytes inhibits specific antigen-receptor signaling pathways that ultimately impair lymphocyte activation, proliferation, and function. In this review, we discuss previous and ongoing work characterizing the ability of an LPA-LPA5 axis to serve as a peripheral immunological tolerance mechanism that restrains adaptive immunity but is subverted during settings of chronic inflammation. Specifically, LPA-LPA5 signaling is found to regulate effector cytotoxic CD8 T cells by (at least) two mechanisms: (i) regulating the actin-microtubule cytoskeleton in a manner that impairs immunological synapse formation between an effector CD8 T cell and antigen-specific target cell, thus directly impairing cytotoxic activity, and (ii) shifting T-cell metabolism to depend on fatty-acid oxidation for mitochondrial respiration and reducing metabolic efficiency. The in vivo outcome of LPA5 inhibitory activity impairs CD8 T-cell killing and tumor immunity in mouse models providing impetus to consider LPA5 antagonism for the treatment of malignancies and chronic infections.

VEGF-A enhances the cytotoxic function of CD4+ cytotoxic T cells via the VEGF-receptor 1/VEGF-receptor 2/AKT/mTOR pathway

BACKGROUND

CD4⁺ cytotoxic T cells (CD4 CTLs) are CD4⁺ T cells with major histocompatibility complex-II-restricted cytotoxic function. Under pathologic conditions, CD4 CTLs hasten the development of autoimmune disease or viral infection by enhancing cytotoxicity. However, the regulators of the cytotoxicity of CD4 CTLs are not fully understood.

METHODS

To explore the potential regulators of the cytotoxicity of CD4 CTLs, bulk RNA and single-cell RNA sequencing (scRNA-seq), enzyme-linked immunosorbent assay, flow cytometry, quantitative PCR, and in-vitro stimulation and inhibition assays were performed.

RESULTS

In this study, we found that VEGF-A promoted the cytotoxicity of CD4 CTLs through scRNA-seq and flow cytometry. Regarding the specific VEGF receptor (R) involved, VEGF-R1/R2 signaling was activated in CD4 CTLs with increased cytotoxicity, and the VEGF-A effects were inhibited when anti-VEGF-R1/R2 neutralizing antibodies were applied. Mechanistically, VEGF-A treatment activated the AKT/mTOR pathway in CD4 CTLs, and the increases of cytotoxic molecules induced by VEGF-A were significantly reduced when the AKT/mTOR pathway was inhibited.

CONCLUSION

In conclusion, VEGF-A enhances the cytotoxicity of CD4 CTLs through the VEGF-R1/VEGF-R2/AKT/mTOR pathway, providing insights for the development of novel treatments for disorders associated with CD4 CTLs.

Emerging Biomaterials Imaging Antitumor Immune Response

Immunotherapy is one of the most promising clinical modalities for the treatment of malignant tumors and has shown excellent therapeutic outcomes in clinical settings. However, it continues to face several challenges, including long treatment cycles, high costs, immune-related adverse events, and low response rates. Thus, it is critical to predict the response rate to immunotherapy by using imaging technology in the preoperative and intraoperative. Here, the latest advances in nanosystem-based biomaterials used for predicting responses to immunotherapy via the imaging of immune cells and signaling molecules in the immune microenvironment are comprehensively summarized. Several imaging methods, such as fluorescence imaging, magnetic resonance imaging, positron emission tomography imaging, ultrasound imaging, and photoacoustic imaging, used in immune predictive imaging, are discussed to show the potential of nanosystems for distinguishing immunotherapy responders from nonresponders. Nanosystem-based biomaterials aided by various imaging technologies are expected to enable the effective prediction and diagnosis in cases of tumors, inflammation, and other public diseases.

Store-Operated Ca2+ Entry Is Up-Regulated in Tumour-Infiltrating Lymphocytes from Metastatic Colorectal Cancer Patients

Simple Summary

Store-operated Ca²⁺ entry (SOCE) has long been known to regulate the differentiation and effector functions of T cells as well as to be instrumental to the ability of cytotoxic T lymphocytes to target cancer cells. Currently, no information is available regarding the expression and function of SOCE in tumour-infiltrating lymphocytes (TILs) that have been expanded in vitro for adoptive cell therapy (ACT). This study provides the first evidence that SOCE is up-regulated in ex vivo-expanded TILs from metastatic colorectal cancer (mCRC) patients. The up-regulation of SOCE mainly depends on diacylglycerol kinase (DGK), which prevents the protein kinase C-dependent inhibition of Ca²⁺ entry in normal T cells. Of note, the pharmacological blockade of SOCE with the selective inhibitor, BTP-2, during target cell killing significantly increases cytotoxic activity at low TIL density, i.e., when TILs-mediated cancer cell death is rarer. This study, albeit preliminary, could lay the foundation to propose an alternative strategy to effect ACT. It has been shown that ex vivo-expanded TILs did not improve the disease-free survival rate in mCRC patients. Our results strongly suggest that pre-treating autologous TILs with a SOCE or DGK inhibitor before being infused into the patient could improve their cytotoxic activity against cancer cells.

Abstract

(1) Background: Store-operated Ca²⁺ entry (SOCE) drives the cytotoxic activity of cytotoxic T lymphocytes (CTLs) against cancer cells. However, SOCE can be enhanced in cancer cells due to an increase in the expression and/or function of its underlying molecular components, i.e., STIM1 and Orai1. Herein, we evaluated the SOCE expression and function in tumour-infiltrating lymphocytes (TILs) from metastatic colorectal cancer (mCRC) patients. (2) Methods: Functional studies were conducted in TILs expanded ex vivo from CRC liver metastases. Peripheral blood T cells from healthy donors (hPBTs) and mCRC patients (cPBTs) were used as controls. (3) Results: SOCE amplitude is enhanced in TILs compared to hPBTs and cPBTs, but the STIM1 protein is only up-regulated in TILs. Pharmacological manipulation showed that the increase in SOCE mainly depends on tonic modulation by diacylglycerol kinase, which prevents the protein kinase C-dependent inhibition of SOCE activity. The larger SOCE caused a stronger Ca²⁺ response to T-cell receptor stimulation by autologous mCRC cells. Reducing Ca²⁺ influx with BTP-2 during target cell killing significantly increases cytotoxic activity at low target:effector ratios. (4) Conclusions: SOCE is enhanced in ex vivo-expanded TILs deriving from mCRC patients but decreasing Ca²⁺ influx with BTP-2 increases cytotoxic activity at a low TIL density.

Ultrarapid lytic granule release from CTLs activates Ca2+-dependent synaptic resistance pathways in melanoma cells

Human cytotoxic T lymphocytes (CTLs) exhibit ultrarapid lytic granule secretion, but whether melanoma cells mobilize defense mechanisms with commensurate rapidity remains unknown. We used single-cell time-lapse microscopy to offer high spatiotemporal resolution analyses of subcellular events in melanoma cells upon CTL attack. Target cell perforation initiated an intracellular Ca²⁺ wave that propagated outward from the synapse within milliseconds and triggered lysosomal mobilization to the synapse, facilitating membrane repair and conferring resistance to CTL induced cytotoxicity. Inhibition of Ca²⁺ flux and silencing of synaptotagmin VII limited synaptic lysosomal exposure and enhanced cytotoxicity. Multiplexed immunohistochemistry of patient melanoma nodules combined with automated image analysis showed that melanoma cells facing CD8⁺ CTLs in the tumor periphery or peritumoral area exhibited significant lysosomal enrichment. Our results identified synaptic Ca²⁺ entry as the definitive trigger for lysosomal deployment to the synapse upon CTL attack and highlighted an unpredicted defensive topology of lysosome distribution in melanoma nodules.

Determining ADCC Activity of Antibody-Based Therapeutic Molecules using Two Bioluminescent Reporter-Based Bioassays

Antibody Fc effector function is one of the main mechanisms of action (MoA) for therapeutic monoclonal antibodies. Measurement of antibody-dependent cellular cytotoxicity (ADCC) is critical for understanding the Fc effector function during monoclonal antibody development. This article covers two cell-based ADCC bioassays which can quantitatively measure the antibody potency in ADCC. Basic Protocol 1 describes the ADCC reporter bioassay using engineered ADCC effector cells which measures the FcγRIIIa-mediated luciferase reporter activation upon the binding of antibody-coated target cells. Basic Protocol 2 describes the PBMC ADCC bioassay using primary peripheral blood mononuclear cells (PBMC) as effector cells and engineered HiBiT target cells in an assay that measures the release of HiBiT from target cells upon antibody-mediated target lysis. Optimization of several key assay parameters including cell handling, effector:target (E:T) ratios, assay plate, and plate reader requirement, and how these parameters impact assay performance are discussed. © 2021 Promega Corporation. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: ADCC reporter bioassay using engineered ADCC bioassay effector cells Basic Protocol 2: PBMC ADCC bioassay using primary PBMC and engineered HiBiT target cells.

Lysophosphatidic Acid Is an Inflammatory Lipid Exploited by Cancers for Immune Evasion via Mechanisms Similar and Distinct From CTLA-4 and PD-1

Immunological tolerance has evolved to curtail immune responses against self-antigens and prevent autoimmunity. One mechanism that contributes to immunological tolerance is the expression of inhibitory receptors by lymphocytes that signal to dampen immune responses during the course of an infection and to prevent immune-mediated collateral damage to the host. The understanding that tumors exploit these physiological mechanisms to avoid elimination has led to remarkable, but limited, success in the treatment of cancer through the use of biologics that interfere with the ability of cancers to suppress immune function. This therapy, based on the understanding of how T lymphocytes are normally activated and suppressed, has led to the development of therapeutic blocking antibodies, referred to as immune checkpoint blockade, which either directly or indirectly promote the activation of CD8 T cells to eradicate cancer. Here, we highlight the distinct signaling mechanisms, timing and location of inhibition used by the CTLA-4 and PD-1 inhibitory receptors compared to a novel inhibitory signaling axis comprised of the bioactive lipid, lysophosphatidic acid (LPA), signaling via the LPA5 receptor expressed by CD8 T cells. Importantly, abundant evidence indicates that an LPA-LPA5 signaling axis is also exploited by diverse cancers to suppress T cell activation and function. Clearly, a thorough molecular and biochemical understanding of how diverse T cell inhibitory receptors signal to suppress T cell antigen receptor signaling and function will be important to inform the choice of which complimentary checkpoint blockade modalities might be used for a given cancer.

A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are required for host defense. They destroy malignant target cells like cancer cells. Among metal cations, Ca²⁺ plays a prescinded role for CTL and NK cytotoxicity as it is the only cation used as ubiquitous second messenger. Measuring intracellular Ca²⁺ concentrations [Ca²⁺]_int in single cells has greatly changed our understanding of Ca²⁺ signaling. Yet, comparing the role of Ca²⁺ in the pre-[Ca²⁺]_int and [Ca²⁺]_int measurement era reveals that even in the pre-[Ca²⁺]_int measurement era (before 1980), the functions of Ca²⁺ and some other metal cations for the cytotoxic immune response were well established. It was even shown that Ca²⁺ influx across the plasma membrane but not Ca²⁺ release from intracellular sources is relevant for lymphocyte cytotoxicity and that very little Ca²⁺ is needed for efficient lymphocyte cytotoxicity against cancer cells. In the [Ca²⁺]_int measurement era after 1980, many of the important findings were better and more quantitatively refined and in addition the molecules important for Ca²⁺ transport were defined. The unexpected finding that there is a Ca²⁺ optimum of CTL and NK cell cytotoxicity deserves some attention and may be important for anti-cancer therapy.

PBRM1 and the glycosylphosphatidylinositol biosynthetic pathway promote tumor killing mediated by MHC-unrestricted cytotoxic lymphocytes

Major histocompatibility complex (MHC)-unrestricted cytotoxic lymphocytes (CLs) such as natural killer (NK) cells can detect and destroy tumor and virus-infected cells resistant to T cell-mediated killing. Here, we performed genome-wide genetic screens to identify tumor-intrinsic genes regulating killing by MHC-unrestricted CLs. A group of genes identified in our screens encode enzymes for the biosynthesis of the glycosylphosphatidylinositol (GPI) anchor, which is not involved in tumor response to T cell-mediated cytotoxicity. Another gene identified in the screens was PBRM1, which encodes a subunit of the PBAF form of the SWI/SNF chromatin-remodeling complex. PBRM1 mutations in tumor cells cause resistance to MHC-unrestricted killing, in contrast to their sensitizing effects on T cell-mediated killing. PBRM1 and the GPI biosynthetic pathway regulate the ligands of NK cell receptors in tumor cells and promote cytolytic granule secretion in CLs. The regulators identified in this work represent potential targets for cancer immunotherapy.

Cytotoxic Granule Trafficking and Fusion in Synaptotagmin7-Deficient Cytotoxic T Lymphocytes

Granules of cytotoxic T lymphocytes (CTL) are derived from the lysosomal compartment. Synaptotagmin7 (Syt7) appears to be the calcium sensor triggering fusion of lysosomes in fibroblasts. Syt7 has been proposed to control cytotoxic granule (CG) fusion in lymphocytes and mice lacking Syt7 have reduced ability to clear infections. However, fusion of CG persists in the absence of Syt7. To clarify the role of Syt7 in CTL function, we have examined the fusion of cytotoxic granules of CD8⁺ T-lymphocytes from Syt7 knock-out mice. We have recorded granule fusion in living CTL, using total internal reflection microscopy. Since Syt7 is considered a high affinity calcium-sensor specialized for fusion under low calcium conditions, we have compared cytotoxic granule fusion under low and high calcium conditions in the same CTL. There was no difference in latencies or numbers of fusion events per CTL under low-calcium conditions, indicating that Syt7 is not required for cytotoxic granule fusion. A deficit of fusion in Syt7 KO CTL was seen when a high-calcium solution was introduced. Expressing wild type Syt7 in Syt7 KO lymphocytes reversed this deficit, confirming its Syt7-dependence. Mutations of Syt7 which disrupt calcium binding to its C2A domain reduced the efficacy of this rescue. We counted the cytotoxic granules present at the plasma membrane to determine if the lack of fusion events in the Syt7 KO CTL was due to a lack of granules. In low calcium there were no differences in fusion events per CTL, and granule numbers were similar. In high calcium, granule number was similar though wild type CTL exhibited significantly more fusion than Syt7 KO CTL. The modest differences in granule counts do not account for the lack of fusion in high calcium in Syt7 KO CTL. In Syt7 KO CTL expressing wild type Syt7, delivery of cytotoxic granules to the plasma membrane was comparable to that of wild type CTL. Syt7 KO CTL expressing Syt7 with deficient calcium binding in the C2A domain had significantly less fusion and fewer CG at the plasma membrane. These results indicate that Syt7 is involved in trafficking of CG to the plasma membrane.

Autophagy and Protein Secretion

Autophagy - conventional for macroautophagy - is a major recycling strategy that ensures cellular homeostasis through the selective engulfment of cytoplasmic supramolecular cargos in double membrane vesicles and their rapid dispatch to the lysosome for digestion. As autophagy operates in the cytoplasm, its interference with secretory proteins, that is, proteins destined to the plasma membrane or the extracellular space, generally synthesized and routed within the endoplasmic reticulum (ER), has been relatively overlooked in the past. However, mounting evidence reveals that autophagy in fact heavily regulates protein secretion through diverse mechanisms. First, autophagy is closely involved in the unconventional secretion of leaderless proteins, a pool of proteins destined extracellularly, but lacking an ER-targeted leader sequence, and thus manufactured in the cytosol. Autophagy-related (ATG) genes now appear instrumental to the underlying pathways, hence the recently coined concept of secretory autophagy, or better ATG gene-dependent secretion. Indeed, ATG genes regulate unconventional protein secretion at multiple levels, ranging from intracellular inflammatory signaling, for example, through the control of mitochondrial health and inflammasome activity, to trafficking of leaderless proteins. Moreover, perhaps less expectedly, autophagy also participates in the control of conventional secretion, intersecting the secretory apparatus at multiple points, though with surprising differences among professional secretory cell types that disclose remarkable and unpredicted specificity. This review synopsizes the multiple mechanisms whereby autophagy interfaces with conventional and unconventional protein secretory pathways and discusses the relative teleology. Altogether, the diverse controls exerted on protein secretion broaden and deepen the homeostatic significance of autophagy within the cell.

LPA5 Is an Inhibitory Receptor That Suppresses CD8 T-Cell Cytotoxic Function via Disruption of Early TCR Signaling

Persistent T cell antigen receptor (TCR) signaling by CD8 T cells is a feature of cancer and chronic infections and results in the sustained expression of, and signaling by, inhibitory receptors, which ultimately impair cytotoxic activity via poorly characterized mechanisms. We have previously determined that the LPA₅ GPCR expressed by CD8 T cells, upon engaging the lysophosphatidic acid (LPA) bioactive serum lipid, functions as an inhibitory receptor able to negatively regulate TCR signaling. Notably, the levels of LPA and autotaxin (ATX), the phospholipase D enzyme that produces LPA, are often increased in chronic inflammatory disorders such as chronic infections, autoimmune diseases, obesity, and cancer. In this report, we demonstrate that LPA engagement selectively by LPA₅ on human and mouse CD8 T cells leads to the inhibition of several early TCR signaling events including intracellular calcium mobilization and ERK activation. We further show that, as a consequence of LPA₅ suppression of TCR signaling, the exocytosis of perforin-containing granules is significantly impaired and reflected by repressed in vitro and in vivo CD8 T cell cytolytic activity. Thus, these data not only document LPA₅ as a novel inhibitory receptor but also determine the molecular and biochemical mechanisms by which a naturally occurring serum lipid that is elevated under settings of chronic inflammation signals to suppress CD8 T cell killing activity in both human and murine cells. As diverse tumors have repeatedly been shown to aberrantly produce LPA that acts in an autocrine manner to promote tumorigenesis, our findings further implicate LPA in activating a novel inhibitory receptor whose signaling may be therapeutically silenced to promote CD8 T cell immunity.

Killing Mechanisms of Chimeric Antigen Receptor (CAR) T Cells

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

Calcium signal dynamics in T lymphocytes: Comparing in vivo and in vitro measurements

Amplitude and kinetics of intracellular Ca²⁺ signals ([Ca²⁺]_int) determine many immune cell functions. To mimic in vivo changes of [Ca²⁺]_int in human immune cells, two approaches may be best suited: 1) Analyze primary human immune cells taken from blood under conditions resembling best physiological or pathophysiological conditions. 2.) Analyze the immune system in vivo or ex vivo in explanted tissue from small vertebrate animals, such as mice. With the help of genetically encoded Ca²⁺ indicators and intravital microscopy, [Ca²⁺]_int have been investigated in murine T lymphocytes (T cells) in vivo during the last five years and in explanted lymph node (LN) during the last 10 years. There are several important reasons to compare [Ca²⁺]_int measured in primary murine T lymphocytes in vivo and in vitro with [Ca²⁺]_int measured in primary human T lymphocytes in vitro. First, how do human and murine data compare? Second, how do in vivo and in vitro data compare? Third, can in vitro data predict in vivo data? The last point is particularly important considering the many technical challenges that limit in vivo measurements and to reduce the number of animals sacrificed. This review summarizes and compares the results of the available publications on in vivo and in vitro [Ca²⁺]_int measurements in T lymphocytes stimulated focally by antigen-presenting cells (APC) after forming an immunological synapse.

Natural killer cells induce distinct modes of cancer cell death: Discrimination, quantification, and modulation of apoptosis, necrosis, and mixed forms

Immune therapy of cancer is among the most promising recent advances in medicine. Whether the immune system can keep cancer in check depends on, among other factors, the efficiency of immune cells to recognize and eliminate cancer cells. We describe a time-resolved single-cell assay that reports the quality, quantity, and kinetics of target cell death induced by single primary human natural killer (NK) cells. The assay reveals that single NK cells induce cancer cell death by apoptosis and necrosis but also by mixed forms. Inhibition of either one of the two major cytotoxic pathways, perforin/granzyme release or FasL/FasR interaction, unmasked the parallel activity of the other one. Ca²⁺ influx through Orai channels is important for tuning killer cell function. We found that the apoptosis/necrosis ratio of cancer cell death by NK cells is controlled by the magnitude of Ca²⁺ entry and furthermore by the relative concentrations of perforin and granzyme B. The possibility to change the apoptosis/necrosis ratio employed by NK cells offers an intriguing possibility to modulate the immunogenicity of the tumor microenvironment.

A Multiplexed Assay That Monitors Effects of Multiple Compound Treatment Times Reveals Candidate Immune-Enhancing Compounds

We previously developed a flow cytometry assay that monitored lytic granule exocytosis in cytotoxic T lymphocytes stimulated by contacting beads coated with activating anti-CD3 antibodies. That assay was multiplexed in that responses of cells that did or did not receive the activating stimulus were distinguished via changes in light scatter accompanying binding of cells to beads, allowing us to discriminate compounds that activate responses on their own from compounds that enhance responses in cells that received the activating stimulus, all within a single sample. Here we add a second dimension of multiplexing by developing means to assess in a single sample the effects of treating cells with test compounds for different times. Bar-coding cells before adding them to test wells lets us determine compound treatment time while also monitoring activation status and response amplitude at the point of interrogation. This multiplexed assay is suitable for screening 96-well plates. We used it to screen compounds from the National Cancer Institute, identifying several compounds that enhance anti-LAMP1 responses. Multiple-treatment-time (MTT) screening enabled by bar-coding and read via high-throughput flow cytometry may be a generally useful method for facilitating the discovery of compounds of interest.

Sailing to and Docking at the Immune Synapse: Role of Tubulin Dynamics and Molecular Motors

The different cytoskeleton systems and their connecting molecular motors move vesicles and intracellular organelles to shape cells. Polarized cells with specialized functions display an exquisite spatio-temporal regulation of both cytoskeletal and organelle arrangements that support their specific tasks. In particular, T cells rapidly change their shape and cellular function through the establishment of cell surface and intracellular polarity in response to a variety of cues. This review focuses on the contribution of the microtubule-based dynein/dynactin motor complex, the tubulin and actin cytoskeletons, and different organelles to the formation of the antigen-driven immune synapse.

Single Degranulations in NK Cells Can Mediate Target Cell Killing

NK cells are cytotoxic lymphocytes important in defense against viral infection and cancer. NK cells mediate cytotoxicity predominantly through directed secretion of lytic granules, which are specialized lysosome-related organelles, containing effector molecules such as perforin and granzymes. Although many requirements for lytic granule transport to, and secretion at, the NK cell lytic synapse are known, the minimum number of degranulation events required by an NK cell to kill its target is unknown. We performed high-resolution four-dimensional confocal microscopy of human NK-target cell conjugates to quantify NK cell degranulation (using a degranulation indicator, LAMP-1-pHluorin) as well as target cell death. Despite containing almost 200 granules, we found that an individual NK cell needed only two to four degranulation events, on average, to mediate target cell death. Although NK cells released approximately one-tenth of their total lytic granule reserve upon a single target, they required just over one-hundredth of their total lytic granules to kill a target cell. Importantly, the kinetics of NK cell killing correlated to the size of and the amount of effector molecules contained within lytic granules, as well as the temporal, but not spatial, organization of degranulation events. Thus, our study answers a fundamental question as to how many degranulation events it takes for a human NK cell to kill its target.

The unconventional secretion of IL-1β: Handling a dangerous weapon to optimize inflammatory responses

Interleukin 1β (IL-1β) is a major mediator of inflammation, with a causative role in many diseases. Unlike most other cytokines, however, it lacks a secretory signal sequence, raising intriguing mechanistic, functional and evolutionary questions. Despite decades of strenuous efforts in many laboratories, how IL-1β is secreted is still a matter of intense debate. Here, we summarize the different mechanisms and pathways that have been proposed for IL-1β secretion. At least two of them, namely the endolysosomal vesicle-based and gasdermin D-dependent pathways (types III and I in the recent Rabouille's classification of unconventional protein secretion), can be triggered in monocytes, the main source of IL-1β in humans, according to the type and strength of the pro-inflammatory stimuli. As during the escalation of human conflicts, monocytes deploy secretory mechanisms of increasing efficiency and dangerousness, shifting from the specific and controlled type III pathway to the much faster release of type I. Thus, the different mechanisms are activated depending on the severity of the conditions, from the self-limiting type III pathways in response of low pathogen load or small trauma, to the uncontrolled responses that underlie autoinflammatory disorders and sepsis.

A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity

KEY POINTS

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are required to eliminate cancer cells. We analysed the Ca²⁺ dependence of CTL and NK cell cytotoxicity and found that in particular CTLs have a very low optimum of [Ca²⁺ ]_i (between 122 and 334 nm) and [Ca²⁺ ]_o (between 23 and 625 μm) for efficient cancer cell elimination, well below blood plasma Ca²⁺ levels. As predicted from these results, partial down-regulation of the Ca²⁺ channel Orai1 in CTLs paradoxically increases perforin-dependent cancer cell killing. Lytic granule release at the immune synapse between CTLs and cancer cells has a Ca²⁺ optimum compatible with this low Ca²⁺ optimum for efficient cancer cell killing, whereas the Ca²⁺ optimum for CTL migration is slightly higher and proliferation increases monotonously with increasing [Ca²⁺ ]_o . We propose that a partial inhibition of Ca²⁺ signals by specific Orai1 blockers at submaximal concentrations could contribute to tumour elimination.

ABSTRACT

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are required to protect the human body against cancer. Ca²⁺ is a key metabolic factor for lymphocyte function and cancer homeostasis. We analysed the Ca²⁺ dependence of CTL and NK cell cytotoxicity against cancer cells and found that CTLs have a bell-shaped Ca²⁺ dependence with an optimum for cancer cell elimination at rather low [Ca²⁺ ]_o (23-625 μm) and [Ca²⁺ ]_i (122-334 nm). This finding predicts that a partial inhibition of Orai1 should increase (rather than decrease) cytotoxicity of CTLs at [Ca²⁺ ]_o higher than 625 μm. We tested this hypothesis in CTLs and indeed found that partial down-regulation of Orai1 by siRNA increases the efficiency of cancer cell killing. We found two mechanisms that may account for the Ca²⁺ optimum of cancer cell killing: (1) migration velocity and persistence have a moderate optimum between 500 and 1000 μm [Ca²⁺ ]_o in CTLs, and (2) lytic granule release at the immune synapse between CTLs and cancer cells is increased at 146 μm compared to 3 or 800 μm, compatible with the Ca²⁺ optimum for cancer cell killing. It has been demonstrated in many cancer cell types that Orai1-dependent Ca²⁺ signals enhance proliferation. We propose that a decrease of [Ca²⁺ ]_o or partial inhibition of Orai1 activity by selective blockers in the tumour microenvironment could efficiently reduce cancer growth by simultaneously increasing CTL and NK cell cytotoxicity and decreasing cancer cell proliferation.

T cell engaging bispecific antibody (T-BsAb): From technology to therapeutics

Harnessing the power of the human immune system to treat cancer is the essence of immunotherapy. Monoclonal antibodies engage the innate immune system to destroy targeted cells. For the last 30years, antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity have been the main mechanisms of anti-tumor action of unconjugated antibody drugs. Efforts to exploit the potentials of other immune cells, in particular T cells, culminated in the recent approval of two T cell engaging bispecific antibody (T-BsAb) drugs, thereby stimulating new efforts to accelerate similar platforms through preclinical and clinical trials. In this review, we have compiled the worldwide effort in exploring T cell engaging bispecific antibodies. Our special emphasis is on the lessons learned, with the hope to derive insights in this fast evolving field with tremendous clinical potential.

Cortical actin recovery at the immunological synapse leads to termination of lytic granule secretion in cytotoxic T lymphocytes

CD8⁺ cytotoxic T lymphocytes (CTLs) eliminate virally infected cells through directed secretion of specialized lytic granules. Because a single CTL can kill multiple targets, degranulation must be tightly regulated. However, how CTLs regulate the termination of granule secretion remains unclear. Previous work demonstrated that centralized actin reduction at the immune synapse precedes degranulation. Using a combination of live confocal, total internal reflection fluorescence, and superresolution microscopy, we now show that, after granule fusion, actin recovers at the synapse and no further secretion is observed. Depolymerization of actin led to resumed granule secretion, suggesting that recovered actin acts as a barrier preventing sustained degranulation. Furthermore, RAB27a-deficient CTLs, which do not secrete cytotoxic granules, failed to recover actin at the synapse, suggesting that RAB27a-mediated granule secretion is required for actin recovery. Finally, we show that both actin clearance and recovery correlated with synaptic phosphatidylinositol 4,5-bisphosphate (PIP₂) and that alterations in PIP₂ at the immunological synapse regulate cortical actin in CTLs, providing a potential mechanism through which CTLs control cortical actin density. Our work provides insight into actin-related mechanisms regulating CTL secretion that may facilitate serial killing during immune responses.

Dysregulation of Protein Kinase Gene Expression in NK Cells from Chronic Fatigue Syndrome/Myalgic Encephalomyelitis Patients

BACKGROUND

The etiology and pathomechanism of chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) are unknown. However, natural killer (NK) cell dysfunction, in particular reduced NK cytotoxic activity, is a consistent finding in CFS/ME patients. Previous research has reported significant changes in intracellular mitogen-activated protein kinase pathways from isolated NK cells. The purpose of this present investigation was to examine whether protein kinase genes have a role in abnormal NK cell intracellular signaling in CFS/ME.

METHOD

Messenger RNA (mRNA) expression of 528 protein kinase genes in isolated NK cells was analyzed (nCounter GX Human Kinase Kit v2 (XT); NanoString Technologies) from moderate (n = 11; age, 54.9 ± 10.3 years) and severe (n = 12; age, 47.5 ± 8.0 years) CFS/ME patients (classified by the 2011 International Consensus Criteria) and nonfatigued controls (n = 11; age, 50.0 ± 12.3 years).

RESULTS

The expression of 92 protein kinase genes was significantly different in the severe CFS/ME group compared with nonfatigued controls. Among these, 37 genes were significantly upregulated and 55 genes were significantly downregulated in severe CFS/ME patients compared with nonfatigued controls.

CONCLUSIONS

In severe CFS/ME patients, dysfunction in protein kinase genes may contribute to impairments in NK cell intracellular signaling and effector function. Similar changes in protein kinase genes may be present in other cells, potentially contributing to the pathomechanism of this illness.

Cytoskeleton rotation relocates mitochondria to the immunological synapse and increases calcium signals

Ca²⁺ microdomains and spatially resolved Ca²⁺ signals are highly relevant for cell function. In T cells, local Ca²⁺ signaling at the immunological synapse (IS) is required for downstream effector functions. We present experimental evidence that the relocation of the MTOC towards the IS during polarization drags mitochondria along with the microtubule network. From time-lapse fluorescence microscopy we conclude that mitochondria rotate together with the cytoskeleton towards the IS. We hypothesize that this movement of mitochondria towards the IS together with their functionality of absorption and spatial redistribution of Ca²⁺ is sufficient to significantly increase the cytosolic Ca²⁺ concentration. To test this hypothesis we developed a whole cell model for Ca²⁺ homoeostasis involving specific geometries for mitochondria and use the model to calculate the spatial distribution of Ca²⁺ concentrations within the cell body as a function of the rotation angle and the distance from the IS. We find that an inhomogeneous distribution of PMCA pumps on the cell membrane, in particular an accumulation of PMCA at the IS, increases the global Ca²⁺ concentration and decreases the local Ca²⁺ concentration at the IS with decreasing distance of the MTOC from the IS. Unexpectedly, a change of CRAC/Orai activity is not required to explain the observed Ca²⁺ changes. We conclude that rotation-driven relocation of the MTOC towards the IS together with an accumulation of PMCA pumps at the IS are sufficient to control the observed Ca²⁺ dynamics in T-cells during polarization.

Novel identification and characterisation of Transient receptor potential melastatin 3 ion channels on Natural Killer cells and B lymphocytes: effects on cell signalling in Chronic fatigue syndrome/Myalgic encephalomyelitis patients

BACKGROUND

Transient receptor potential melastatin 3 (TRPM3) cation channels are ubiquitously expressed by multiple cells and have an important regulatory role in calcium-dependent cell signalling to help maintain cellular homeostasis. TRPM3 protein expression has yet to be determined on Natural Killer (NK) cells and B lymphocytes. Multiple single nucleotide polymorphisms have been reported in TRPM3 genes from isolated peripheral blood mononuclear cells, NK and B cells in Chronic fatigue syndrome/Myalgic encephalomyelitis (CFS/ME) patients and have been proposed to correlate with illness presentation. The object of the study was to assess TRPM3 surface expression on NK and B lymphocytes from healthy controls, followed by a comparative investigation examining TRPM3 surface expression, and cytoplasmic and mitochondrial calcium influx in CD19(+) B cells, CD56(bright) and CD56(dim) cell populations from CFS/ME patients.

RESULTS

TRPM3 cell surface expression was identified for NK and B lymphocytes in healthy controls (CD56(bright) TRPM3 35.72 % ± 7.37; CD56(dim) 5.74 % ± 2.00; B lymphocytes 2.05 % ± 0.19, respectively). There was a significant reduction of TRPM3 surface expression on CD19(+) B cells (1.56 ± 0.191) and CD56(bright) NK cells (17.37 % ± 5.34) in CFS/ME compared with healthy controls. Anti-CD21 and anti-IgM conjugated biotin was cross-linked with streptavidin,and subsequently treatment with thapsigargin. This showed a significant reduction in cytoplasmic calcium ion concentration in CD19(+) B lymphocytes. CD56(bright) NK cells also had a significant decrease in cytoplasmic calcium in the presence of 2-APB and thapsigargin in CFS/ME patients.

CONCLUSIONS

The results from this preliminary investigation identify, for the first time, TRPM3 surface expression on both NK and B lymphocytes in healthy controls. We also report for the first time, significant reduction in TRPM3 cell surface expression in NK and B lymphocytes, as well as decreased intracellular calcium within specific conditions in CFS/ME patients. This warrants further examination of these pathways to elucidate whether TRPM3 and impaired calcium mobilisation has a role in CFS/ME.

Syntaxin 8 is required for efficient lytic granule trafficking in cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) eliminate pathogen-infected and cancerous cells mainly by polarized secretion of lytic granules (LG, containing cytotoxic molecules like perforin and granzymes) at the immunological synapse (IS). Members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) family are involved in trafficking (generation, transport and fusion) of vesicles at the IS. Syntaxin 8 (Stx8) is expressed in LG and colocalizes with the T cell receptor (TCR) upon IS formation. Here, we report the significance of Stx8 for human CTL cytotoxicity. We found that Stx8 mostly localized in late, recycling endosomal and lysosomal compartments with little expression in early endosomal compartments. Down-regulation of Stx8 by siRNA resulted in reduced cytotoxicity. We found that following perforin release of the pre-existing pool upon target cell contact, Stx8 down-regulated CTL regenerate perforin pools less efficiently and thus release less perforin compared to control CTL. CD107a degranulation, real-time and end-point population cytotoxicity assays, and high resolution microscopy support our conclusion that Stx8 is required for proper and timely sorting and trafficking of cytotoxic molecules to functional LG through the endosomal pathway in human CTL.

Natural killer cells and single nucleotide polymorphisms of specific ion channels and receptor genes in myalgic encephalomyelitis/chronic fatigue syndrome

AIM

The aim of this paper was to determine natural killer (NK) cytotoxic activity and if single nucleotide polymorphisms (SNPs) and genotypes in transient receptor potential (TRP) ion channels and acetylcholine receptors (AChRs) were present in isolated NK cells from previously identified myalgic encephalomyelitis (ME)/chronic fatigue syndrome (CFS) patients.

SUBJECTS AND METHODS

A total of 39 ME/CFS patients (51.69±2 years old) and 30 unfatigued controls (47.60±2.39 years old) were included in this study. Patients were defined according to the 1994 Centers for Disease Control and Prevention criteria. Flow cytometry protocols were used to examine NK cytotoxic activity. A total of 678 SNPs from isolated NK cells were examined for 21 mammalian TRP ion channel genes and for nine mammalian AChR genes via the Agena Bioscience iPlex Gold assay. SNP association and genotype was determined using analysis of variance and Plink software.

RESULTS

ME/CFS patients had a significant reduction in NK percentage lysis of target cells (17%±4.68%) compared with the unfatigued control group (31%±6.78%). Of the 678 SNPs examined, eleven SNPs for TRP ion channel genes (TRPC4, TRPC2, TRPM3, and TRPM8) were identified in the ME/CFS group. Five of these SNPs were associated with TRPM3, while the remainder were associated with TRPM8, TRPC2, and TRPC4 (P<0.05). Fourteen SNPs were associated with nicotinic and muscarinic AChR genes: six with CHRNA3, while the remainder were associated with CHRNA2, CHRNB4, CHRNA5, and CHRNE (P<0.05). There were sixteen genotypes identified from SNPs in TRP ion channels and AChRs for TRPM3 (n=5), TRPM8 (n=2), TRPC4 (n=3), TRPC2 (n=1), CHRNE (n=1), CHRNA2 (n=2), CHRNA3 (n=1), and CHRNB4 (n=1) (P<0.05).

CONCLUSION

We identified a number of SNPs and genotypes for TRP ion channels and AChRs from isolated NK cells in patients with ME/CFS, suggesting these SNPs and genotypes may be involved in changes in NK cell function and the development of ME/CFS pathology. These anomalies suggest a role for dysregulation of Ca(2+) in AChR and TRP ion channel signaling in the pathomechanism of ME/CFS.

Preferential Coupling of the NAADP Pathway to Exocytosis in T-Cells

A cytotoxic T-lymphocyte (CTL) kills an infected or tumorigenic cell by Ca2+-dependent exocytosis of cytolytic granules at the immunological synapse formed between the two cells. However, these granules are more than reservoirs of secretory cytolytic proteins but may also serve as unique Ca2+ signaling hubs that autonomously generate their own signals for exocytosis. This review discusses a selective role for the Ca2+-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP) and its molecular targets, two-pore channels (TPCs), in stimulating exocytosis. Given that TPCs reside on the exocytotic granules themselves, these vesicles generate as well as respond to NAADP-dependent Ca2+ signals, which may have wider implications for stimulus-secretion coupling, vesicular fusion, and patho-physiology.

The functional contribution of calcium ion flux heterogeneity in T cells

The role of intracellular calcium ion oscillations in T-cell physiology is being increasingly appreciated by studies that describe how unique temporal and spatial calcium ion signatures can control different signalling pathways. Within this review, we provide detailed mechanisms of calcium ion oscillations, and emphasise the pivotal role that calcium signalling plays in directing crucial events pertaining to T-cell functionality. We also describe methods of calcium ion quantification, and take the opportunity to discuss how a deeper understanding of calcium signalling combined with new detection and quantification methodologies can be used to better design immunotherapies targeting T-cell responses.

FRET detection of lymphocyte function-associated antigen-1 conformational extension

Lymphocyte function–associated antigen 1 (LFA-1) and its ligands are essential for immune cell interactions. LFA-1 is regulated through conformational changes. The relationship between molecular conformation and function is unclear. Förster resonance energy transfer is used to assess LFA-1 conformation under real-time signaling conditions.

Lymphocyte function–associated antigen 1 (LFA-1, CD11a/CD18, αLβ2-integrin) and its ligands are essential for adhesion between T-cells and antigen-presenting cells, formation of the immunological synapse, and other immune cell interactions. LFA-1 function is regulated through conformational changes that include the modulation of ligand binding affinity and molecular extension. However, the relationship between molecular conformation and function is unclear. Here fluorescence resonance energy transfer (FRET) with new LFA-1–specific fluorescent probes showed that triggering of the pathway used for T-cell activation induced rapid unquenching of the FRET signal consistent with extension of the molecule. Analysis of the FRET quenching at rest revealed an unexpected result that can be interpreted as a previously unknown LFA-1 conformation.

The kinematics of cytotoxic lymphocytes influence their ability to kill target cells

Cytotoxic lymphocytes (CTL) have been reported to show a range of motility patterns from rapid long-range tracking to complete arrest, but how and whether these kinematics affect their ability to kill target cells is not known. Many in vitro killing assays utilize cell lines and tumour-derived cells as targets, which may be of limited relevance to the kinetics of CTL-mediated killing of somatic cells. Here, live-cell microscopy is used to examine the interactions of CTL and primary murine skin cells presenting antigens. We developed a qualitative and quantitative killing assay using extended-duration fluorescence time-lapse microscopy coupled with large-volume objective software-based data analysis to obtain population data of cell-to-cell interactions, motility and apoptosis. In vivo and ex vivo activated antigen-specific cytotoxic lymphocytes were added to primary keratinocyte targets in culture with fluorometric detection of caspase-3 activation in targets as an objective determinant of apoptosis. We found that activated CTL achieved contact-dependent apoptosis of non-tumour targets after a period of prolonged attachment - on average 21 hours - which was determined by target cell type, amount of antigen, and activation status of CTL. Activation of CTL even without engagement of the T cell receptor was sufficient to mobilise cells significantly above baseline, while the addition of cognate antigen further enhanced their motility. Highly activated CTL showed markedly increased vector displacement, and velocity, and lead to increased antigen-specific target cell death. These data show that the inherent kinematics of CTL correlate directly with their ability to kill non-tumour cells presenting cognate antigen.

Altered distribution and function of natural killer cells in murine and human Niemann-Pick disease type C1

Niemann-Pick type C (NPC) is a neurodegenerative lysosomal storage disorder caused by defects in the lysosomal proteins NPC1 or NPC2. NPC cells are characterized by reduced lysosomal calcium levels and impaired sphingosine transport from lysosomes. Natural killer (NK) cells kill virally infected/transformed cells via degranulation of lysosome-related organelles. Their trafficking from lymphoid tissues into the circulation is dependent on sphingosine-1-phosphate (S1P) gradients, sensed by S1P receptor 5 (S1P5). We hypothesized that NK-cell function and trafficking could be affected in NPC disease due to the combined effects of the lysosomal calcium defect and sphingosine storage. In an NPC1 mouse model, we found the frequency of NK cells was altered and phenocopied S1P5-deficient mice, consistent with defects in S1P levels. NK cells from NPC1 mice also had a defect in cytotoxicity due to a failure in degranulation of cytotoxic granules, which was associated with reduced lysosomal calcium levels. Affected NPC1 patients and NPC1 heterozygote carriers had reduced NK-cell numbers in their blood and showed similar phenotypic and developmental changes to those observed in the NPC1 mouse. These findings highlight the effects of lysosomal storage on the peripheral immune system.

STIM1 and STIM2-mediated Ca(2+) influx regulates antitumour immunity by CD8(+) T cells

Store-operated calcium entry (SOCE) through Ca²⁺ release-activated Ca²⁺ (CRAC) channels regulates the function of many immune cells. Patients with loss-of-function mutations in the CRAC channel genes ORAI1 or STIM1 are immunodeficient and are prone to develop virus-associated tumours. This and the reported role of Ca²⁺ signals in cytotoxic lymphocyte function suggest that SOCE may be critical for tumour immune surveillance. Using conditional knock out mice lacking STIM1 and its homologue STIM2, we find that SOCE in CD8⁺ T cells is required to prevent the engraftment of melanoma and colon carcinoma cells and to control tumour growth. SOCE is essential for the cytotoxic function of CTLs both in vivo and in vitro by regulating the degranulation of CTLs, their expression of Fas ligand and production of TNF-α and IFN-γ. Our results emphasize an important role of SOCE in antitumour immunity, which is significant given recent reports arguing in favour of CRAC channel inhibition for cancer therapy.

An emerging role for Serine Protease Inhibitors in T lymphocyte immunity and beyond

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmo-regulation, tissue re-modeling and immunity to infection. T lymphocytes are required for adaptive cell mediated immunity and serine proteases are not only important for effector function but also homeostatic regulation of cell numbers. Serine Protease Inhibitors (Serpins) are the physiological regulators of serine proteases activity. In this review, I will discuss the role of serpins in controlling the recognition of antigen, effector function and homeostatic control of T lymphocytes through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This will be discussed in the context of the T lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. The potent anti-apoptotic properties of serpins can also work against adaptive cell immunity by protecting viruses and tumors from eradication by cytotoxic T cells (CTL). Recent insights from knock-out mouse models demonstrate that these serpins also are required for hematological progenitor cells and so are critical for the development of lineages other than T lymphocytes. Given the emerging role of serpins in multiple aspects of lymphocyte immunity and blood development I will review the progress to date in developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Functional T cell immunodeficiencies (with T cells present)

Severe combined immunodeficiency (SCID) comprises a group of disorders that are fatal owing to genetic defects that abrogate T cell development. Numerous related defects have recently been identified that allow T cell development but that compromise T cell function by affecting proximal or distal steps in intracellular signaling. These functional T cell immunodeficiencies are characterized by immune dysregulation and increased risk of malignancies, in addition to infections. The study of patients with these rare conditions, and of corresponding animal models, illustrates the importance of intracellular signaling to maintain T cell homeostasis.

Calcium, cancer and killing: the role of calcium in killing cancer cells by cytotoxic T lymphocytes and natural killer cells

Killing cancer cells by cytotoxic T lymphocytes (CTL) and by natural killer (NK) cells is of vital importance. Cancer cell proliferation and apoptosis depend on the intracellular Ca(2+) concentration, and the expression of numerous ion channels with the ability to control intracellular Ca(2+) concentrations has been correlated with cancer. A rise of intracellular Ca(2+) concentrations is also required for efficient CTL and NK cell function and thus for killing their targets, in this case cancer cells. Here, we review the data on Ca(2+)-dependent killing of cancer cells by CTL and NK cells. In addition, we discuss emerging ideas and present a model how Ca(2+) may be used by CTL and NK cells to optimize their cancer cell killing efficiency. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Regulation of lymphocyte function by ORAI and STIM proteins in infection and autoimmunity

Store-operated Ca(2+) entry (SOCE) in cells of the immune system is mediated by Ca(2+) release-activated Ca(2+) (CRAC) channels that are formed by ORAI1 and its homologues ORAI2 and ORAI3. They are activated by stromal interaction molecules (STIM) 1 and 2 in response to depletion of endoplasmic reticulum Ca(2+) stores. Loss-of-function mutations in the human ORAI1 and STIM1 genes abolish CRAC channel function and SOCE in a variety of non-excitable cells including lymphocytes and other immune cells, resulting in a unique clinical syndrome termed CRAC channelopathy. It is dominated by severe immunodeficiency and autoimmunity due to impaired SOCE and defects in the function of several lymphocyte subsets. These include CD8(+) T cells, CD4(+) effector and regulatory T cells, natural killer (NK) cells and B cells. This review provides a concise discussion of the role of CRAC channels in these lymphocyte populations and the regulation of adaptive immune responses to infection, in autoimmunity and inflammation.

Role of LAT in the granule-mediated cytotoxicity of CD8 T cells

Linker for activation of T cells (LAT) is a transmembrane adaptor protein that is essential to bridge T cell receptor (TCR) engagement to downstream signaling events. The indispensable role of LAT in thymocyte development and T cell activation has been well characterized; however, the function of LAT in cytotoxic-T-lymphocyte (CTL) cytotoxicity remains unknown. We show here that LAT-deficient CTLs failed to upregulate FasL and produce gamma interferon after engagement with target cells and had impaired granule-mediated killing. We further dissected the effect of the LAT deletion on each step of granule exocytosis. LAT deficiency led to altered synapse formation, subsequently causing unstable T cell-antigen-presenting cell (APC) conjugates. Microtubule organizing center polarization and granule reorientation were also impaired by LAT deficiency, leading to reduced granule delivery. Despite these defects, granule release was still observed in LAT-deficient CTLs due to residual calcium flux and phospholipase C (PLC) activity. Our data demonstrated that LAT-mediated signaling intricately regulates CTL cytotoxicity at multiple steps.

Intercellular communication via the endo-lysosomal system: translocation of granzymes through membrane barriers

Cytotoxic lymphocytes (CLs) are responsible for the clearance of virally infected or neoplastic cells. CLs possess specialised lysosome-related organelles called granules which contain the granzyme family of serine proteases and perforin. Granzymes may induce apoptosis in the target cell when delivered by the pore forming protein, perforin. Here we follow the perforin-granzyme pathway from synthesis and storage in the granule, to exocytosis and finally delivery into the target cell. This review focuses on the controversial subject of perforin-mediated translocation of granzymes into the target cell cytoplasm. It remains unclear whether this occurs at the cell surface with granzymes moving through a perforin pore in the plasma membrane, or if it involves internalisation of perforin and granzymes and subsequent release from an endocytic compartment. The latter mechanism would represent an example of cross talk between the endo-lysosomal pathways of individual cells. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Docking of lytic granules at the immunological synapse in human CTL requires Vti1b-dependent pairing with CD3 endosomes

Lytic granule (LG)-mediated apoptosis is the main mechanism by which CTL kill virus-infected and tumorigenic target cells. CTL form a tight junction with the target cells, which is called the immunological synapse (IS). To avoid unwanted killing of neighboring cells, exocytosis of lytic granules (LG) is tightly controlled and restricted to the IS. In this study, we show that in activated human primary CD8(+) T cells, docking of LG at the IS requires tethering LG with CD3-containing endosomes (CD3-endo). Combining total internal reflection fluorescence microscopy and fast deconvolution microscopy (both in living cells) with confocal microscopy (in fixed cells), we found that LG and CD3-endo tether and are cotransported to the IS. Paired but not single LG are accumulated at the IS. The dwell time of LG at the IS is substantially enhanced by tethering with CD3-endo, resulting in a preferential release of paired LG over single LG. The SNARE protein Vti1b is required for tethering of LG and CD3-endo. Downregulation of Vti1b reduces tethering of LG with CD3-endo. This leads to an impaired accumulation and docking of LG at the IS and a reduction of target cell killing. Therefore, Vti1b-dependent tethering of LG and CD3-endo determines accumulation, docking, and efficient lytic granule secretion at the IS.

ORAI1-mediated calcium influx is required for human cytotoxic lymphocyte degranulation and target cell lysis

Lymphocytes mediate cytotoxicity by polarized release of the contents of cytotoxic granules toward their target cells. Here, we have studied the role of the calcium release-activated calcium channel ORAI1 in human lymphocyte cytotoxicity. Natural killer (NK) cells obtained from an ORAI1-deficient patient displayed defective store-operated Ca(2+) entry (SOCE) and severely defective cytotoxic granule exocytosis leading to impaired target cell lysis. Similar findings were obtained using NK cells from a stromal interaction molecule 1-deficient patient. The defect occurred at a late stage of the signaling process, because activation of leukocyte functional antigen (LFA)-1 and cytotoxic granule polarization were not impaired. Moreover, pharmacological inhibition of SOCE interfered with degranulation and target cell lysis by freshly isolated NK cells and CD8(+) effector T cells from healthy donors. In addition to effects on lymphocyte cytotoxicity, synthesis of the chemokine macrophage inflammatory protein-1β and the cytokines TNF-α and IFN-γ on target cell recognition was impaired in ORAI1-deficient NK cells, as previously described for T cells. By contrast, NK cell cytokine production induced by combinations of IL-12, IL-15, and IL-18 was not impaired by ORAI1 deficiency. Taken together, these results identify a critical role for ORAI1-mediated Ca(2+) influx in granule exocytosis for lymphocyte cytotoxicity as well as for cytokine production induced by target cell recognition.