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A file containing supplementary figures 1-8 and the supplementary table S1.

A file containing RPPA and RNAseq datasets, pathway analyses and prediction calculations presented in the main figures.

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells recognize and eliminate cancer cells. However, immune evasion, downregulation of immune function by the tumour microenvironment and resistance of cancer cells are major problems. Although CTL and NK cells are both important to eliminate cancer, most studies address them individually. We quantified sequential primary human CTL and NK cell cytotoxicity against the melanoma cell line SK-Mel-5. At high effector-to-target ratios, NK cells or melan-A (MART-1)-specific CTL eliminated all SK-Mel-5 cells within 24 h, indicating that SK-Mel-5 cells are not resistant initially. However, at lower effector-to-target ratios, which resemble numbers of the immune contexture in human cancer, a substantial number of SK-Mel-5 cells survived. Pre-exposure to CTL induced resistance in surviving SK-Mel-5 cells to subsequent CTL or NK cell cytotoxicity, and pre-exposure to NK cells induced resistance in surviving SK-Mel-5 cells to NK cells. Higher human leucocyte antigen class I expression or interleukin-6 levels were correlated with resistance to NK cells, whereas reduction in MART-1 antigen expression was correlated with reduced CTL cytotoxicity. The CTL cytotoxicity was rescued beyond control levels by exogenous MART-1 antigen. In contrast to the other three combinations, CTL cytotoxicity against SK-Mel-5 cells was enhanced following NK cell pre-exposure. Our assay allows quantification of sequential CTL and NK cell cytotoxicity and might guide strategies for efficient CTL-NK cell anti-melanoma therapies. KEY POINTS: Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells eliminate cancer cells. Both CTL and NK cells attack the same targets, but most studies address them individually. In a sequential cytotoxicity model, the interdependence of antigen-specific CTL and NK cell cytotoxicity against melanoma is quantified. High numbers of antigen-specific CTL and NK cells eliminate all melanoma cells. However, lower numbers induce resistance if secondary CTL or NK cell exposure follows initial CTL exposure or if secondary NK cell exposure follows initial NK cell exposure. On the contrary, if secondary CTL exposure follows initial NK cell exposure, cytotoxicity is enhanced. Alterations in human leucocyte antigen class I expression and interleukin-6 levels are correlated with resistance to NK cells, whereas a reduction in antigen expression is correlated with reduced CTL cytotoxicity; CTL cytotoxicity is rescued beyond control levels by exogenous antigen. This assay and the results on interdependencies will help us to understand and optimize immune therapies against cancer.

Visualization cell interaction with the extracellular matrix (ECM) mesh works plays a central role in understanding cell behavior and the corresponding regulatory mechanisms by the environment in vivo. However, long term visualization of 3D matrix structures remains challenging mainly due to photobleaching or blind spot in the currently available approaches. In this paper, we developed a label-free method based on light-sheet microcopy, termed light-sheet scattering microscopy (LSSM), as a satisfactory solution to solve this problem. LSSM can reliably determine structure of collagen matrices from different origin including bovine, human and rat tail. We verified that the quality and intensity of collagen structure images acquired by LSSM did not decline with time. LSSM offers abundant wavelength choice for matrix structure, maximizing combination possibilities for fluorescence to label the cells. LSSM can be used for visualizing ECM-cell interaction in 3D for long term and characterization of cell-applied forces. Interestingly, we observed ultrathin thread-like structures between cells and matrix using LSSM, which was not to be seen by normal fluorescence microscopy. In summary, LSSM provides a robust approach to investigate the complex interplay between cells and ECM in vitro under in vivo-mimicking conditions.

The induction of apoptosis is a direct way to eliminate tumor cells and improve cancer therapy. Apoptosis is tightly controlled by the balance of pro- and antiapoptotic Bcl-2 proteins. BH3 mimetics neutralize the antiapoptotic function of Bcl-2 proteins and are highly promising compounds inducing apoptosis in several cancer entities including pediatric malignancies. However, the clinical application of BH3 mimetics in solid tumors is impeded by the frequent resistance to single BH3 mimetics and the anticipated toxicity of high concentrations or combination treatments. One potential avenue to increase the potency of BH3 mimetics is the development of immune cell-based therapies to counteract the intrinsic apoptosis resistance of tumor cells and sensitize them to immune attack. Here, we describe spheroid cultures of pediatric cancer cells that can serve as models for drug testing. In these 3D models, we were able to demonstrate that activated allogeneic Natural Killer (NK) cells migrated into tumor spheroids and displayed cytotoxicity against a wide range of pediatric cancer spheroids, highlighting their potential as anti-tumor effector cells. Next, we investigated whether treatment of tumor spheroids with subtoxic concentrations of BH3 mimetics can increase the cytotoxicity of NK cells. Notably, the cytotoxic effects of NK cells were enhanced by the addition of BH3 mimetics. Treatment with either the Bcl-XL inhibitor A1331852 or the Mcl-1 inhibitor S63845 increased the cytotoxicity of NK cells and reduced spheroid size, while the Bcl-2 inhibitor ABT-199 had no effect on NK cell-mediated killing. Taken together, this is the first study to describe the combination of BH3 mimetics targeting Bcl-XL or Mcl-1 with NK cell-based immunotherapy, highlighting the potential of BH3 mimetics in immunotherapy.

The erythrocyte sedimentation rate (ESR) is one of the oldest medical diagnostic tools. However, currently there is some debate on the structure formed by the cells during the sedimentation process. While the conventional view is that erythrocytes sediment as separate aggregates, others have suggested that they form a percolating gel, similar to other colloidal suspensions. A direct probing of the structures formed by erythrocytes in blood at stasis is then required to settle these discrepancies. Here, we report observations performed with three different optical imaging techniques: direct light transmission through thin samples, two-photon microscopy and light-sheet microscopy. All techniques revealed a dynamic structure of a channeling gel but with differences in the resolved details. A quantitative analysis of the erythrocyte related processes and interactions during the sedimentation need a further refinement of the experimental set-ups.

CTL and NK cells recognize and eliminate cancer cells. However, immune evasion, down regulation of immune function by the tumor microenvironment, or resistance of cancer cells are a major problem. While CTL and NK cells are both important to eliminate cancer, most studies address them individually. In a new experimental human model, we analysed combined primary human CTL and NK cell cytotoxicity against the melanoma cell line SK-Mel-5. At high effector-to-target ratios, MART-1-specific CTL or NK cells eliminated SK-Mel-5 cells within 24 hours indicating that SK-Mel-5 cells are initially not resistant. However, at lower effector-to-target ratios, which resemble conditions of the immune contexture in human cancer, a significant number of SK-Mel-5 cells survived. Whereas CTL pre-exposure induced resistance in surviving SK-Mel-5 cells to subsequent CTL or NK cell cytotoxicity, NK cell pre-exposure induced resistance in surviving SK-Mel-5 cells to NK cells but not to MART-1 specific CTL. In contrast, there was even a slight enhancement of CTL cytotoxicity against SK-Mel-5 cells following NK cell pre-exposure. In all other combinations, resistance to subsequent cytotoxicity was higher, if melanoma cells were pre-exposed to larger numbers of CTL or NK cells. Increases in human leukocyte antigen class I expression correlated with resistance to NK cells, while reduction in MART-1 antigen expression correlated with reduced CTL cytotoxicity. CTL cytotoxicity was rescued beyond control levels by exogenous MART-1 antigen. This study quantifies the interdependence of CTL and NK cell cytotoxicity and may guide strategies for efficient CTL-NK cell anti-melanoma therapies.Key points summaryCytotoxic T lymphocytes (CTL) and natural killer (NK) cells eliminate cancer cells. CTL and NK work in parallel, but most studies address them individually.In a new human experimental model, antigen-specific CTL and NK cell cytotoxicity interdependence against melanoma is shown.Whereas high numbers of antigen-specific CTL and NK cells eliminate all melanoma cells, lower, more physiological numbers induce resistance, in case secondary CTL or NK cell exposure follow initial CTL cell exposure or if secondary NK cell exposure follows initial NK cell exposure; only if secondary CTL exposure follows initial NK cell exposure no resistance of melanoma but even a slight enhancement of cytotoxicity was observed.Alterations in HLA-I expression correlated with resistance to NK cells, while reduction in antigen expression correlated with reduced CTL cytotoxicity. CTL cytotoxicity was rescued beyond control levels by exogenous antigen.The results should help to better understand and optimize immune therapies against cancer.Graphical abstract

Triple‐negative breast cancer (TNBC) represents the most aggressive breast tumor subtype. However, the molecular determinants responsible for the metastatic TNBC phenotype are only partially understood. We here show that expression of the mitochondrial calcium uniporter (MCU), the selective channel responsible for mitochondrial Ca2+ uptake, correlates with tumor size and lymph node infiltration, suggesting that mitochondrial Ca2+ uptake might be instrumental for tumor growth and metastatic formation. Accordingly, MCU downregulation hampered cell motility and invasiveness and reduced tumor growth, lymph node infiltration, and lung metastasis in TNBC xenografts. In MCU‐silenced cells, production of mitochondrial reactive oxygen species (mROS) is blunted and expression of the hypoxia‐inducible factor‐1α (HIF‐1α) is reduced, suggesting a signaling role for mROS and HIF‐1α, downstream of mitochondrial Ca2+. Finally, in breast cancer mRNA samples, a positive correlation of MCU expression with HIF‐1α signaling route is present. Our results indicate that MCU plays a central role in TNBC growth and metastasis formation and suggest that mitochondrial Ca2+ uptake is a potential novel therapeutic target for clinical intervention.

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. Ca(2+) 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 Ca(2+) 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.

Cytotoxic T lymphocytes are effector CD8+ T cells that eradicate infected and malignant cells. Here we show that the transcription factor NFATc1 controls the cytotoxicity of mouse cytotoxic T lymphocytes. Activation of Nfatc1 −/− cytotoxic T lymphocytes showed a defective cytoskeleton organization and recruitment of cytosolic organelles to immunological synapses. These cells have reduced cytotoxicity against tumor cells, and mice with NFATc1-deficient T cells are defective in controlling Listeria infection. Transcriptome analysis shows diminished RNA levels of numerous genes in Nfatc1 −/− CD8+ T cells, including Tbx21, Gzmb and genes encoding cytokines and chemokines, and genes controlling glycolysis. Nfatc1 −/− , but not Nfatc2 −/− CD8+ T cells have an impaired metabolic switch to glycolysis, which can be restored by IL-2. Genome-wide ChIP-seq shows that NFATc1 binds many genes that control cytotoxic T lymphocyte activity. Together these data indicate that NFATc1 is an important regulator of cytotoxic T lymphocyte effector functions.

Cytotoxic T lymphocytes are effector CD8+ T cells that eradicate infected and malignant cells. Here we show that the transcription factor NFATc1 controls the cytotoxicity of mouse cytotoxic T lymphocytes. Activation of Nfatc1 −/− cytotoxic T lymphocytes showed a defective cytoskeleton organization and recruitment of cytosolic organelles to immunological synapses. These cells have reduced cytotoxicity against tumor cells, and mice with NFATc1-deficient T cells are defective in controlling Listeria infection. Transcriptome analysis shows diminished RNA levels of numerous genes in Nfatc1 −/− CD8+ T cells, including Tbx21, Gzmb and genes encoding cytokines and chemokines, and genes controlling glycolysis. Nfatc1 −/−, but not Nfatc2 −/− CD8+ T cells have an impaired metabolic switch to glycolysis, which can be restored by IL-2. Genome-wide ChIP-seq shows that NFATc1 binds many genes that control cytotoxic T lymphocyte activity. Together these data indicate that NFATc1 is an important regulator of cytotoxic T lymphocyte effector functions., NFAT nuclear translocation has been shown to be required for CD8+ T cell cytokine production in response to viral infection. Here the authors show NFATc1 controls the cytotoxicity and metabolic switching of activated CD8+ T cells required for optimal response to bacteria and tumor cells.

This double-blind, randomized, placebo-controlled, cross-over intervention-study was conducted in healthy volunteers to evaluate the effects of plant sterol ester supplemented margarine on cholesterol, non-cholesterol sterols and oxidative stress in serum and monocytes. Sixteen volunteers, average age 34 years, with no or mild hypercholesterolemia were subjected to a 4 week period of daily intake of 3g plant sterols per day supplied via a supplemented margarine on top of regular eating habits. After a wash-out period of one week, volunteers switched groups. Compared to placebo, a diet supplementation with plant sterols increased serum levels of plant sterols such as campesterol (+0.16±0.19mg/dL, p=0.005) and sitosterol (+0.27±0.18mg/dL, p

Reactive oxygen species (ROS) are increasingly recognized as second messengers in many cellular processes. While high concentrations of oxidants damage proteins, lipids and DNA, ultimately resulting in cell death, selective and reversible oxidation of key residues in proteins is a physiological mechanism that can transiently alter their activity and function. Defects in ROS producing enzymes cause disturbed immune response and disease. Changes in the intracellular free Ca(2+) concentration are key triggers for diverse cellular functions. Ca(2+) homeostasis thus needs to be precisely tuned by channels, pumps, transporters and cellular buffering systems. Alterations of these key regulatory proteins by reversible or irreversible oxidation alter the physiological outcome following cell stimulation. It is therefore necessary to understand which proteins are regulated and if this regulation is relevant in a physiological- and/or pathophysiological context. Because ROS are inherently difficult to identify and to measure, we first review basic oxygen redox chemistry and methods of ROS detection with special emphasis on electron paramagnetic resonance (EPR) spectroscopy. We then focus on the present knowledge of redox regulation of Ca(2+) permeable ion channels such as voltage-gated (CaV) Ca(2+) channels, transient receptor potential (TRP) channels and Orai channels.

Cell polarization enables restriction of signalling into microdomains. Polarization of lymphocytes following formation of a mature immunological synapse (IS) is essential for calcium-dependent T-cell activation. Here, we analyse calcium microdomains at the IS with total internal reflection fluorescence microscopy. We find that the subplasmalemmal calcium signal following IS formation is sufficiently low to prevent calcium-dependent inactivation of ORAI channels. This is achieved by localizing mitochondria close to ORAI channels. Furthermore, we find that plasma membrane calcium ATPases (PMCAs) are re-distributed into areas beneath mitochondria, which prevented PMCA up-modulation and decreased calcium export locally. This nano-scale distribution—only induced following IS formation—maximizes the efficiency of calcium influx through ORAI channels while it decreases calcium clearance by PMCA, resulting in a more sustained NFAT activity and subsequent activation of T cells.

Department of Physiology, Saarland University Faculty of Medicine, Homburg, Germany Key words: CTL; cytotoxicity; killing kinetics; real-time assay; NK cells. Corresponding author: Dr. Bin Qu Institute of Biophysics Saarland University Building 58 66421 Homburg Germany Tel.: +49-6841-162-6458 Fax: +49-6841-162-6060 bin.qu@uks.eu

Sustained Ca(2+) influx through plasma membrane Ca(2+) released-activated Ca(2+) (CRAC) channels is essential for T cell activation. Since inflowing Ca(2+) inactivates CRAC channels, T cell activation is only possible if Ca(2+)-dependent inactivation is prevented. We have previously reported that sustained Ca(2+) influx through CRAC channels requires both mitochondrial Ca(2+) uptake and mitochondrial translocation towards the plasma membrane in order to prevent Ca(2+)-dependent channel inactivation. Here, we show that morphological changes following formation of the immunological synapse (IS) modulate Ca(2+) influx through CRAC channels. Cell shape changes were dependent on the actin cytoskeleton, and they sustained Ca(2+) entry by bringing mitochondria and the plasma membrane in closer proximity. The increased percentage of mitochondria beneath the plasma membrane following shape changes occurred in all 3 dimensions and correlated with an increase in the amplitude of Ca(2+) signals. The shape change-dependent mitochondrial localization close to the plasma membrane prevented CRAC channel inactivation even in T cells in which dynein motor protein-dependent mitochondria movements towards the plasma membrane were completely abolished, highlighting the importance of the shape change-dependent control of Ca(2+) influx. Our results suggest that morphological changes do not only facilitate an efficient contact with antigen presenting cells but also strongly modulate Ca(2+) dependent T cell activation.

Cytotoxic T-lymphocytes (CTLs) and natural killer (NK) cells are the killer cells of the immune system and are responsible for the elimination of potential dangers, such as virus-infected and tumor cells. CTLs and NK cells migrate actively within tissues to encounter target cells, and form a tight connection with them called the immunological synapse that enables killing. However, how migration is regulated in these cells for an efficient reach of their target remains a central question in immunology. Here we analyzed the migration of CTLs and NK cells in 2 dimensions in the absence of external cues to mirror the initial immune response where no signals guide killer cells to their target. By calculating the mean square displacement and the velocity autocorrelation we found that CTLs perform a persistent random walk. Furthermore, cell shape analysis enabled us to establish a model for killer cell migration where the movement of the cell is determined by several independent internal “force generators”. Each of them switches between active and inactive states. We found a correlation between the number of the “force generators” and the duration of their activity to the number and life time of lamellipodia. Our numerical simulations revealed that the search time in a given space depends on the behavior of cells upon contact with boundaries. We believe that simulation of migration and search strategy of killer cells will ultimately lead to an improved understanding of the immune response.

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.

Ca(2+) homeostasis controls a diversity of cellular processes including proliferation and apoptosis. A very important aspect of Ca(2+) signaling is how different Ca(2+) signals are translated into specific cell functions. In T cells, Ca(2+) signals are induced following the recognition of antigen by the T cell receptor and depend mainly on Ca(2+) influx through store-operated CRAC channels, which are mediated by ORAI proteins following their activation by STIM proteins. The complete absence of Ca(2+) influx caused by mutations in Stim1 and Orai1 leads to severe immunodeficiency. Here we summarize how Ca(2+) signals are tuned to regulate important T cell functions as proliferation, apoptosis and tolerance, the latter one being a special state of immune cells in which they can no longer respond properly to an otherwise activating stimulus. Perturbations of Ca(2+) signaling may be linked to immune suppressive diseases and autoimmune diseases.

Cell polarization enables restriction of signalling into microdomains. Polarization of lymphocytes following formation of a mature immunological synapse (IS) is essential for calcium-dependent T-cell activation. Here, we analyse calcium microdomains at the IS with total internal reflection fluorescence microscopy. We find that the subplasmalemmal calcium signal following IS formation is sufficiently low to prevent calcium-dependent inactivation of ORAI channels. This is achieved by localizing mitochondria close to ORAI channels. Furthermore, we find that plasma membrane calcium ATPases (PMCAs) are re-distributed into areas beneath mitochondria, which prevented PMCA up-modulation and decreased calcium export locally. This nano-scale distribution-only induced following IS formation-maximizes the efficiency of calcium influx through ORAI channels while it decreases calcium clearance by PMCA, resulting in a more sustained NFAT activity and subsequent activation of T cells.

Reactive oxygen species (ROS) are involved in many physiological and pathophysiological cellular processes. We used lymphocytes, which are exposed to highly oxidizing environments during inflammation, to study the influence of ROS on cellular function. Calcium ion (Ca(2+)) influx through Ca(2+) release-activated Ca(2+) (CRAC) channels composed of proteins of the ORAI family is essential for the activation, proliferation, and differentiation of T lymphocytes, but whether and how ROS affect ORAI channel function have been unclear. Here, we combined Ca(2+) imaging, patch-clamp recordings, and measurements of cell proliferation and cytokine secretion to determine the effects of hydrogen peroxide (H(2)O(2)) on ORAI channel activity and human T helper lymphocyte (T(H) cell) function. ORAI1, but not ORAI3, channels were inhibited by oxidation by H(2)O(2). The differential redox sensitivity of ORAI1 and ORAI3 channels depended mainly on an extracellularly located reactive cysteine, which is absent in ORAI3. T(H) cells became progressively less redox-sensitive after differentiation into effector cells, a shift that would allow them to proliferate, differentiate, and secrete cytokines in oxidizing environments. The decreased redox sensitivity of effector T(H) cells correlated with increased expression of Orai3 and increased abundance of several cytosolic antioxidants. Knockdown of ORAI3 with small-interfering RNA rendered effector T(H) cells more redox-sensitive. The differential expression of Orai isoforms between naive and effector T(H) cells may tune cellular responses under oxidative stress.

Cell polarization is a key feature of T-cell function. The immunological synapse (IS) between T cells and antigen-presenting cells is a beautiful example of how polarization of cells is used to guide cell function. Receptors, signal transducers, the cytoskeleton, and organelles are enriched at or depleted from the IS after its formation, and in many cases these re-localizations have already been linked with certain T-cell functions. One key step for T-cell activation is a rise in the cytoplasmic calcium concentration. Whereas it is undisputed that the IS initiates and controls calcium signals in T cells, very little is known about the role of T-cell polarization for calcium signals and calcium-dependent signal transduction. We briefly summarize the basic commonly agreed principles of IS-dependent calcium signal generation but then focus on the less well understood influence of polarization on calcium signals. The discussion of the role of polarization for calcium signals leads to a model how the IS controls local and global calcium signals and calcium-dependent T-cell functions. We develop a theoretical formalism based on existing spatiotemporal calcium dynamic simulations to better understand the model in the future and allow further predictions which can be tested by fast, high resolution live-cell microscopy.

Clonal T cell expansion through proliferation is a central process of the adaptive immune response. Apoptosis of activated T cells is required to avoid chronic inflammation. T cell proliferation and apoptosis are often analyzed with stimuli that do not induce formation of a functional immunological synapse. Here we analyze the Ca(2+) dependence of proliferation and apoptosis in primary human CD4(+) T cells following stimulation with anti-CD3/anti-CD28-coated beads, which induce a tight interaction similar to the immunological synapse. We found this focal stimulation to be much more efficient for stimulating IL-2 production and proliferation than non-focal TCR stimuli. Surprising little Ca(2+) entry through Ca(2+) channels was required for T cell proliferation. Transient free intracellular calcium concentration ([Ca(2+)](i)) elevations of up to 220 nM from a baseline level of around 40 nM were sufficient for maximal proliferation in primary human CD4(+) T cells. We also show that proliferation was very Ca(2+) sensitive in the range 90-120 nM, whereas apoptosis was basically constant for [Ca(2+)](i) levels of 90-120 nM. We conclude that very small changes in [Ca(2+)](i) can dramatically change the ratio between proliferation and apoptosis, thus keeping the balance between overshooting and inefficient immune responses.

Cytotoxicity is an essential parameter to examine killer cell activity, to screen hits for new drugs, and to proof safety evaluation. Cytotoxicity assays are routinely used in basic research, clinical studies, and industrial applications. However, the most commonly applied standard end-point cytotoxicity assays largely suffer from the lack of kinetic information, which is indispensable to assess killing competence. Here we report a quantitative and highly sensitive calcein-based real-time high-throughput cytotoxicity assay on a single cell level and for population use. By determining fluorescence loss as a function of the fraction of lysed cells, this assay measures the kinetics of cytotoxicity over a broad dynamic range. The high time resolution of the assay also provides kinetic information with the potential to reveal neglected/immeasurable properties in the process of cytotoxicity of both purified human cytotoxic T lymphocytes and natural killer cells. To test its clinical potential, we have also applied this assay to screen the cytotoxic potential of (easy to prepare) peripheral blood mononuclear cells in healthy blood donors. In summary, this assay should substantially increase the screening efficiency for clinical and industrial purposes and offer quantitative kinetic data with a high time resolution.

During the immune response, cytotoxic T lymphocytes (CTL) and Natural Killer cells (NK cells) patrol the human body. Upon recognition of virus-infected or cancerous target cells, the killer cells release lytic granules at the immunological synapse and kill the targets. For optimal immune function, the migration machinery has to be adjusted in order to maximize the search efficiency.We used in vitro time lapse microscopy on primary human CTL and NK cells to study their migration in a two-dimensional geometry. In the absence of target cells, CTL alternate between a mobile and a stationary state. The preference for each state depends on the external calcium concentration. On a short time scale (seconds/minutes), killer cell migration is characterized by directional persistence, while migration on a longer time scale (minutes/hours) is random. A mathematical model of a persistent random walker can accurately describe killer cell migration and reveals an optimum for target search efficiency. This optimum depends on the length of persistent motion in one direction and the size of the search space. We believe that this model is very useful to understand how regulation of killer cell migration can serve to optimize the immune response.

Cytotoxic T Lymphocytes (CTLs) are activated CD8+ T cells which identify and kill pathogen infected, tumor and nonself cells by FAS ligand expression and by secretion of perforin and granzymes from lytic granules specifically at the immunological synapse (IS). This prevents killing of healthy bystander cells. Therefore at the time of killing these cytotoxic molecules are to be delivered to the IS. SNAREs are membrane associated proteins involved in vesicle transport, docking and fusion. Mutations of these proteins can cause diseases like Familial Hemophagocytic Lymphohistiocytosis. We have shown previously that Vti1b is required for docking of lytic granules at the IS. We also found that Vti1b and Syntaxin 8 (Stx8) colocalize with perforin in human CTLs. Further experiments showed that Stx8 partially colocalized with Vti1b. To elucidate the role of SNAREs in CTL cytotoxicity, siRNA were used to downregulate their expression. Stx8 downregulation did not impair the IS formation as CD3 accumulation at the IS was unaffected. We have previously shown that the killing capacity of CTLs is impaired by Vti1b downregulation. Here, Stx8 downregulated CTLs used in real time killing assay showed reduction in killing. Cycloheximide treatment of control and Stx8 downregulated CTLs showed that Stx8 is required for killing by perforin secretion. This conclusion is also supported by our preliminary ELISA data, which shows reduced perforin secretion in Stx8 downregulated CTLs.

The objective of the study was to analyze the role of T cell polarization and T cell shape changes for calcium (Ca) signals and Ca dependent T cell activation. We have used sophisticated live cell imaging technology to quantify cytoskeletal components, mitochondria and other organelle rearrangements during and after formation of the immunological synapse (IS) in parallel with Ca signals. We found that T cell polarization after formation of the IS was paralleled by drastic cell shape changes. These changes were dependent on the actin cytoskeleton and they induced a very intimate proximity between mitochondria and the plasma membrane at the IS. The mitochondrial translocation to the IS was Ca dependent but did not require local Ca influx at the IS. The shape change-dependent mitochondrial translocation to the plasma membrane prevented ORAI1 channel inactivation even in T cells in which dynein motor protein-dependent mitochondria movements towards the plasma membrane were completely abolished, highlighting the importance of the cell shape to control Ca influx. Our results suggest that cell shape changes do not only facilitate an efficient contact with antigen presenting cells but also strongly modulate Ca dependent T cell activation. Organelle polarization during IS formation is an important mechanism to modulate Ca dependent T cell activation. This project was funded by the Deutsche Forschungsgemeinschaft (Graduate Research School 1326, SFB 530, project A3) and a HOMFOR grant from the Medical Faculty.