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

Alpha synuclein and inflammaging

The α-synuclein protein is an established molecule in Lewy body pathology, especially Parkinson's disease (PD). While the pathological role of α-synuclein (α-syn) in PD has been well described, novel evidence may suggest that α-syn interacts with inflammasomes in response to aging. As age is an inevitable physiological state and is also considered the greatest risk factor for PD, this calls for investigation into how α-syn, aging, and PD could be linked. There is a growing amount of data regarding α-syn normal function in the body that includes involvement in cellular transport such as protein complexes assembly, vesicular trafficking, neurotransmitter release, as well as immune cell maturation. Regarding abnormal α-syn, a number of autosomal dominant mutations have been identified as causes of familial PD, however, symptomatology may not become apparent until later in life due to compensatory mechanisms in the dopaminergic response. This potentially links age-related physiological changes not only as a risk factor for PD, but for the concept of "inflammaging ". This is defined as chronic inflammation that accompanies aging observed in many neurodegenerative pathologies, that include α-syn's ability to form oligomers and toxic fibrils seen in PD. This oligomeric α-syn stimulates pro-inflammatory signals, which may worsen PD symptoms and propagate chronic inflammation. Thus, this review will explore a potential link between α-syn's role in the immune system, inflammaging, and PD.

SNARE proteins: Core engines of membrane fusion in cancer

Vesicles are loaded with a variety of cargoes, including membrane proteins, secreted proteins, signaling molecules, and various enzymes, etc. Not surprisingly, vesicle transport is essential for proper cellular life activities including growth, division, movement and cellular communication. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate membrane fusion of vesicles with their target compartments that is fundamental for cargo delivery. Recent studies have shown that multiple SNARE family members are aberrantly expressed in human cancers and actively contribute to malignant proliferation, invasion, metastasis, immune evasion and treatment resistance. Here, the localization and function of SNARE proteins in eukaryotic cells are firstly mapped. Then we summarize the expression and regulation of SNAREs in cancer, and describe their contribution to cancer progression and mechanisms, and finally we propose engineering botulinum toxin as a strategy to target SNAREs for cancer treatment.

Functional analysis of circSTX8 in chicken macrophages under lipopolysaccharide stimulation

Circular RNAs (circRNAs) have a regulatory role in gene expression, development, differentiation, and immune response. In a previous study, circular RNA STX8 (circSTX8) exhibited low expression in chicken lungs during lipopolysaccharide (LPS) stimulation. PCR amplification and Sanger sequencing showed that circSTX8 was created by back-splicing of exons 5 to 6 of STX8. RNase R exonuclease treatment indicated that circSTX8 was a stable circular RNA. RT-qPCR showed that circSTX8 was highly expressed in cecum, spleen, harderian gland, stomach, thymus, liver, small intestine, and lung instead of that in muscle, cerebrum, and cerebellum (n = 8). Chicken macrophages were then divided into four groups: control, overexpression of circSTX8 group, LPS group, and overexpression of circSTX8 + LPS group. CCK8 and RT-qPCR showed that circSTX8 can exacerbate the cellular injury induced by LPS, resulting in a reduction of cell viability and an increase of the pro-inflammatory cytokines expression. In addition, four miRNAs were identified to interact with circSTX8, potentially targeting 914 genes, which were significantly enriched in the pathways of Tight junction, mTOR signaling pathway, MAPK signaling pathway, TGF-beta signaling pathway, Notch signaling pathway, ErbB signaling pathway, and Cell adhesion molecules. These findings showed that circSTX8 was able to regulate the LPS induced cellular immune and inflammatory response.

Identification of molecular candidates which regulate calcium-dependent CD8+ T-cell cytotoxicity

Cytotoxic CD8⁺ T lymphocytes (CTL) eliminate infected cells or transformed tumor cells by releasing perforin-containing cytotoxic granules at the immunological synapse. The secretion of such granules depends on Ca²⁺-influx through store operated Ca²⁺ channels, formed by STIM (stromal interaction molecule)-activated Orai proteins. Whereas molecular mechanisms of the secretion machinery are well understood, much less is known about the molecular machinery that regulates the efficiency of Ca²⁺-dependent target cell killing. CTL killing efficiency is of high interest considering the number of studies on CD8⁺ T lymphocytes modified for clinical use. Here, we isolated total RNA from primary human cells: natural killer (NK) cells, non-stimulated CD8⁺ T-cells, and from Staphylococcus aureus enterotoxin A (SEA) stimulated CD8⁺ T-cells (SEA-CTL) and conducted whole genome expression profiling by microarray experiments. Based on differential expression analysis of the transcriptome data and analysis of master regulator genes, we identified 31 candidates which potentially regulate Ca²⁺-homeostasis in CTL. To investigate a putative function of these candidates in CTL cytotoxicity, we transfected either SEA-stimulated CTL (SEA-CTL) or antigen specific CD8⁺ T-cell clones (CTL-MART-1) with siRNAs specific against the identified candidates and analyzed the killing capacity using a real-time killing assay. In addition, we complemented the analysis by studying the effect of inhibitory substances acting on the candidate proteins if available. Finally, to unmask their involvement in Ca²⁺ dependent cytotoxicity, candidates were also analyzed under Ca²⁺-limiting conditions. Overall, we identified four hits, CCR5 (C-C chemokine receptor type five), KCNN4 (potassium calcium-activated channel subfamily N), RCAN3 (regulator of calcineurin) and BCL (B-cell lymphoma) 2 which clearly affect the efficiency of Ca²⁺ dependent cytotoxicity in CTL-MART-1 cells, CCR5, BCL2, and KCNN4 in a positive manner, and RCAN3 in a negative way.

Syntaxin 11 Contributes to the Interferon-Inducible Restriction of Coxiella burnetii Intracellular Infection

There is a limited understanding of host defense mechanisms targeting intracellular pathogens that proliferate in a lysosome. Coxiella burnetii is a model bacterial pathogen capable of replicating in the hydrolytic and acidic environment of the lysosome. It has been shown that gamma interferon (IFNγ)-stimulated host cells restrict C. burnetii replication by a mechanism that involves host IDO1 depletion of tryptophan. Host cells deficient in IDO1 activity, however, retain the ability to restrict C. burnetii replication when stimulated with IFNγ, which suggests additional mechanisms of host defense. This study identified syntaxin 11 (STX11) as a host protein that contributes to IFNγ-mediated suppression of C. burnetii replication. STX11 is a SNARE protein; SNARE proteins are proteins that mediate fusion of host vesicles with specific subcellular organelles. Depletion of STX11 using either small interfering RNA (siRNA)- or CRISPR-based approaches enhanced C. burnetii replication intracellularly. Stable expression of STX11 reduced C. burnetii replication in epithelial cells and macrophages, which indicates that this STX11-dependent cell-autonomous response is operational in multiple cell types and can function independently of other IFNγ-induced factors. Fluorescently tagged STX11 localized to the Coxiella-containing vacuole (CCV), and STX11 restriction was found to involve an interaction with STX8. Thus, STX11 regulates a vesicle fusion pathway that limits replication of this intracellular pathogen in a lysosome-derived organelle. IMPORTANCE Cell intrinsic defense mechanisms are used by eukaryotic cells to restrict the replication and dissemination of pathogens. This study identified a human protein called syntaxin 11 (STX11) as a host restriction factor that inhibits the intracellular replication of Coxiella burnetii. Syntaxins regulate the delivery of cargo inside vesicles by promoting specific membrane fusion events between donor and acceptor vesicles. Data presented here demonstrate that STX11 regulates an immunological defense pathway that controls replication of pathogens in lysosome-derived organelles, which provides new insight into the function of this SNARE protein.

Potential roles of Toxocara canis larval excretory secretory molecules in immunomodulation and immune evasion

Toxocara canis larvae invade various tissues of different vertebrate species without developing into adults in paratenic host. The long-term survival of the larvae despite exposure to the well-armed immune response is a notable achievement. The larvae modulate the immune response to help the survival of both the host and the larvae. They skew the immune response to type 2/regulatory phenotype. The outstanding ability of the larvae to modulate the host immune response and to evade the immune arms is attributed to the secretion of Toxocara excretory-secretory products (TESPs). TESPs are complex mixture of differing molecules. The present review deals with the molecular composition of the TESPs, their interaction with the host molecules, their effect on the innate immune response, the receptor recognition, the downstream signals the adaptive immunity and the repair of tissues. This review also addresses the role of TESPs molecules in the immune evasion strategy and the potential effect of the induced immunomodulation in some diseases. Identification of parasite components that influence the nematode-host interactions could enhance understanding the molecular basis of nematode pathogenicity. Furthermore, the identification of helminths molecules with immunomodulatory potential could be used in immunotherapies for some diseases.

Proteomic Analysis of Pleural Effusions from COVID-19 Deceased Patients: Enhanced Inflammatory Markers

Critically ill COVID-19 patients with pleural effusion experience longer hospitalization, multisystem inflammatory syndrome, and higher rates of mortality. Generally, pleural effusion can serve as a diagnostic value to differentiate cytokine levels. This study aimed to evaluate the pleural effusions of COVID-19 deceased patients for 182 protein markers. Olink^® Inflammation and Organ Damage panels were used to determine the level of 184 protein markers, e.g., ADA, BTC, CA12, CAPG, CD40, CDCP1, CXCL9, ENTPD2, Flt3L, IL-6, IL-8, LRP1, OSM, PD-L1, PTN, STX8, and VEGFA, which were raised significantly in COVID-19 deceased patients, showing over-stimulation of the immune system and ravaging cytokine storm. The rises of DPP6 and EDIL3 also indicate damage caused to arterial and cardiovascular organs. Overall, this study confirms the elevated levels of CA12, CD40, IL-6, IL-8, PD-L1, and VEGFA, proposing their potential either as biomarkers for the severity and prognosis of the disease or as targets for therapy. Particularly, this study reports upregulated ADA, BTC, DPP6, EDIL3, LIF, ENTPD2, Flt3L, and LRP1 in severe COVID-19 patients for the first time. Pearson's correlation coefficient analysis indicates the involvement of JAK/STAT pathways as a core regulator of hyperinflammation in deceased COVID-19 patients, suggesting the application of JAK inhibitors as a potential efficient treatment.

Cytotoxic Efficiency of Human CD8+ T Cell Memory Subtypes

Immunological memory is important to protect humans against recurring diseases. Memory CD8⁺ T cells are required for quick expansion into effector cells but also provide immediate cytotoxicity against their targets. Whereas many functions of the two main cytotoxic subtypes, effector memory CD8⁺ T cells (T_EM) and central memory CD8⁺ T cells (T_CM), are well defined, single T_EM and T_CM cell cytotoxicity has not been quantified. To quantify cytotoxic efficiency of T_EM and T_CM, we developed a FRET-based single cell fluorescent assay with NALM6 target cells which allows analysis of target cell apoptosis, secondary necrosis following apoptosis, and primary necrosis after T_EM- or T_CM-target cell contact. Both, single cell and population cytotoxicity assays reveal a higher cytotoxic efficiency of T_EM compared to T_CM, as quantified by target cell apoptosis and secondary necrosis. Perforin, granzyme B, FasL, but not TRAIL expression are higher in T_EM compared to T_CM. Higher perforin levels (likely in combination with higher granzyme levels) mediate higher cytotoxic efficiency of T_EM compared to T_CM. Both, T_EM and T_CM need the same time to find their targets, however contact time between CTL and target, time to induce apoptosis, and time to induce secondary necrosis are all shorter for T_EM. In addition, immune synapse formation in T_EM appears to be slightly more efficient than in T_CM. Defining and quantifying single T_EM and T_CM cytotoxicity and the respective mechanisms is important to optimize future subset-based immune therapies.

Proteomics Analysis of Aqueous Humor and Rejected Graft in Pig-to-Non-Human Primate Corneal Xenotransplantation

Although pig-to-non-human primate (NHP) corneal xenotransplantation has shown long-term graft survival, xenogeneic antigen-related immune responses are still stronger than allogeneic antigen-associated responses. Therefore, there is an unmet need to investigate major rejection pathways in corneal xenotransplantation, even with immunosuppression. This study aimed to identify biomarkers in aqueous humor for predicting rejection and to investigate rejection-related pathways in grafts from NHPs transplanted with porcine corneas following the administration of steroids combined with tacrolimus/rituximab. NHPs who had received corneas from wild-type (WT) or α-1,3-galactosyltransferase gene-knockout (GTKO) pigs were divided into groups with or without rejection according to clinical examinations. Liquid chromatography-mass spectrometry (LC-MS) was used to analyze the proteomes of corneal tissues or aqueous humor. The biological functions of differentially expressed proteins (DEPs) were assessed using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for pathways and protein–protein interaction network analysis. Among the 66 DEPs in aqueous humor, complement proteins (C3, C5, and C9) and cholesterol metabolic proteins (APOA1 and APOA2) were related to xenogeneic rejection as biomarkers, and alternative pathways of the complement system seemed to be important in xenogeneic graft rejection. Among the 416 DEPs of the cornea, NF-κB1 and proteosomes (PSMD7, PSMA5, and PSMD3) seemed to be related to xenogeneic graft rejection. Additionally, oxidative phosphorylation and leukocyte activation-related pathways are involved in rejection. Overall, our proteomic approach highlights the important role of NF-κB1, proteosomes, oxidative phosphorylation, and leukocyte activation-related inflammation in the cornea and the relevance of complement pathways of the aqueous humor as a predictive biomarker of xenogeneic rejection.

T Lymphocyte and CAR-T Cell-Derived Extracellular Vesicles and Their Applications in Cancer Therapy

Extracellular vesicles (EV) are a very diverse group of cell-derived vesicles released by almost all kind of living cells. EV are involved in intercellular exchange, both nearby and systemically, since they induce signals and transmit their cargo (proteins, lipids, miRNAs) to other cells, which subsequently trigger a wide variety of biological responses in the target cells. However, cell surface receptor-induced EV release is limited to cells from the immune system, including T lymphocytes. T cell receptor activation of T lymphocytes induces secretion of EV containing T cell receptors for antigen and several bioactive molecules, including proapoptotic proteins. These EV are specific for antigen-bearing cells, which make them ideal candidates for a cell-free, EV-dependent cancer therapy. In this review we examine the generation of EV by T lymphocytes and CAR-T cells and some potential therapeutic approaches of these EV.

Harnessing the Potential of CRISPR/Cas in Atherosclerosis: Disease Modeling and Therapeutic Applications

Atherosclerosis represents one of the major causes of death globally. The high mortality rates and limitations of current therapeutic modalities have urged researchers to explore potential alternative therapies. The clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9) system is commonly deployed for investigating the genetic aspects of Atherosclerosis. Besides, advances in CRISPR/Cas system has led to extensive options for researchers to study the pathogenesis of this disease. The recent discovery of Cas9 variants, such as dCas9, Cas9n, and xCas9 have been established for various applications, including single base editing, regulation of gene expression, live-cell imaging, epigenetic modification, and genome landscaping. Meanwhile, other Cas proteins, such as Cas12 and Cas13, are gaining popularity for their applications in nucleic acid detection and single-base DNA/RNA modifications. To date, many studies have utilized the CRISPR/Cas9 system to generate disease models of atherosclerosis and identify potential molecular targets that are associated with atherosclerosis. These studies provided proof-of-concept evidence which have established the feasibility of implementing the CRISPR/Cas system in correcting disease-causing alleles. The CRISPR/Cas system holds great potential to be developed as a targeted treatment for patients who are suffering from atherosclerosis. This review highlights the advances in CRISPR/Cas systems and their applications in establishing pathogenetic and therapeutic role of specific genes in atherosclerosis.

Identification of Toxocara canis Antigen-Interacting Partners by Yeast Two-Hybrid Assay and a Putative Mechanism of These Host-Parasite Interactions

Toxocara canis is a zoonotic roundworm that infects humans and dogs all over the world. Upon infection, larvae migrate to various tissues leading to different clinical syndromes. The host–parasite interactions underlying the process of infection remain poorly understood. Here, we describe the application of a yeast two-hybrid assay to screen a human cDNA library and analyse the interactome of T. canis larval molecules. Our data identifies 16 human proteins that putatively interact with the parasite. These molecules were associated with major biological processes, such as protein processing, transport, cellular component organisation, immune response and cell signalling. Some of these identified interactions are associated with the development of a Th2 response, neutrophil activity and signalling in immune cells. Other interactions may be linked to neurodegenerative processes observed during neurotoxocariasis, and some are associated with lung pathology found in infected hosts. Our results should open new areas of research and provide further data to enable a better understanding of this complex and underestimated disease.

High Glucose Enhances Cytotoxic T Lymphocyte-Mediated Cytotoxicity

Cytotoxic T lymphocytes (CTLs) are key players to eliminate tumorigenic or pathogen-infected cells using lytic granules (LG) and Fas ligand (FasL) pathways. Depletion of glucose leads to severely impaired cytotoxic function of CTLs. However, the impact of excessive glucose on CTL functions still remains largely unknown. Here we used primary human CD8+ T cells, which were stimulated by CD3/CD28 beads and cultured in medium either containing high glucose (HG, 25 mM) or normal glucose (NG, 5.6 mM). We found that in HG-CTLs, glucose uptake and glycolysis were enhanced, whereas proliferation remained unaltered. Furthermore, CTLs cultured in HG exhibited an enhanced CTL killing efficiency compared to their counterparts in NG. Unexpectedly, expression of cytotoxic proteins (perforin, granzyme A, granzyme B and FasL), LG release, cytokine/cytotoxic protein release and CTL migration remained unchanged in HG-cultured CTLs. Interestingly, additional extracellular Ca2+ diminished HG-enhanced CTL killing function. Our findings suggest that in an environment with excessive glucose, CTLs could eliminate target cells more efficiently, at least for a certain period of time, in a Ca2+-dependent manner.

The epigenetic regulator G9a attenuates stress-induced resistance and metabolic transcriptional programs across different stressors and species

Background

Resistance and tolerance are two coexisting defense strategies for fighting infections. Resistance is mediated by signaling pathways that induce transcriptional activation of resistance factors that directly eliminate the pathogen. Tolerance refers to adaptations that limit the health impact of a given pathogen burden, without targeting the infectious agent. The key players governing immune tolerance are largely unknown. In Drosophila, the histone H3 lysine 9 (H3K9) methyltransferase G9a was shown to mediate tolerance to virus infection and oxidative stress (OS), suggesting that abiotic stresses like OS may also evoke tolerance mechanisms. In response to both virus and OS, stress resistance genes were overinduced in Drosophila G9a mutants, suggesting an intact but overactive stress response. We recently demonstrated that G9a promotes tolerance to OS by maintaining metabolic homeostasis and safeguarding energy availability, but it remained unclear if this mechanism also applies to viral infection, or is conserved in other species and stress responses. To address these questions, we analyzed publicly available datasets from Drosophila, mouse, and human in which global gene expression levels were measured in G9a-depleted conditions and controls at different time points upon stress exposure.

Results

In all investigated datasets, G9a attenuates the transcriptional stress responses that confer resistance against the encountered stressor. Comparative analysis of conserved G9a-dependent stress response genes suggests that G9a is an intimate part of the design principles of stress resistance, buffering the induction of promiscuous stress signaling pathways and stress-specific resistance factors. Importantly, we find stress-dependent downregulation of metabolic genes to also be dependent on G9a across all of the tested datasets.

Conclusions

These results suggest that G9a sets the balance between activation of resistance genes and maintaining metabolic homeostasis, thereby ensuring optimal organismal performance during exposure to diverse types of stress across different species. We therefore propose G9a as a potentially conserved master regulator underlying the widely important, yet poorly understood, concept of stress tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01025-0.

Syntaxins 6 and 8 facilitate tau into secretory pathways

Tau pathology initiates in defined brain regions and is known to spread along neuronal connections as symptoms progress in Alzheimer's disease (AD) and other tauopathies. This spread requires the release of tau from donor cells, but the underlying molecular mechanisms remained unknown. Here, we established the interactome of the C-terminal tail region of tau and identified syntaxin 8 (STX8) as a mediator of tau release from cells. Similarly, we showed the syntaxin 6 (STX6), part of the same SNARE family as STX8 also facilitated tau release. STX6 was previously genetically linked to progressive supranuclear palsy (PSP), a tauopathy. Finally, we demonstrated that the transmembrane domain of STX6 is required and sufficient to mediate tau secretion. The differential role of STX6 and STX8 in alternative secretory pathways suggests the association of tau with different secretory processes. Taken together, both syntaxins, STX6 and STX8, may contribute to AD and PSP pathogenesis by mediating release of tau from cells and facilitating pathology spreading.

Analysis of the SNARE Stx8 recycling reveals that the retromer-sorting motif has undergone evolutionary divergence

Fsv1/Stx8 is a Schizosaccharomyces pombe protein similar to mammalian syntaxin 8. stx8Δ cells are sensitive to salts, and the prevacuolar endosome (PVE) is altered in stx8Δ cells. These defects depend on the SNARE domain, data that confirm the conserved function of syntaxin8 and Stx8 in vesicle fusion at the PVE. Stx8 localizes at the trans-Golgi network (TGN) and the prevacuolar endosome (PVE), and its recycling depends on the retromer component Vps35, and on the sorting nexins Vps5, Vps17, and Snx3. Several experimental approaches demonstrate that Stx8 is a cargo of the Snx3-retromer. Using extensive truncation and alanine scanning mutagenesis, we identified the Stx8 sorting signal. This signal is an IEMeaM sequence that is located in an unstructured protein region, must be distant from the transmembrane (TM) helix, and where the ¹³³I, ¹³⁴E, ¹³⁵M, and ¹³⁸M residues are all essential for recycling. This sorting motif is different from those described for most retromer cargoes, which include aromatic residues, and resembles the sorting motif of mammalian polycystin-2 (PC2). Comparison of Stx8 and PC2 motifs leads to an IEMxx(I/M) consensus. Computer-assisted screening for this and for a loose Ψ(E/D)ΨXXΨ motif (where Ψ is a hydrophobic residue with large aliphatic chain) shows that syntaxin 8 and PC2 homologues from other organisms bear variation of this motif. The phylogeny of the Stx8 sorting motifs from the Schizosaccharomyces species shows that their divergence is similar to that of the genus, showing that they have undergone evolutionary divergence. A preliminary analysis of the motifs in syntaxin 8 and PC2 sequences from various organisms suggests that they might have also undergone evolutionary divergence, what suggests that the presence of almost-identical motifs in Stx8 and PC2 might be a case of convergent evolution.

Eukaryotes possess membranous intracellular compartments, whose communication is essential for cellular homeostasis. Protein complexes that facilitate the generation, transport, and fusion of coated vesicles mediate this communication. Since alterations in these processes lead to human disease, their characterization is of biological and medical interest. Retromer is a protein complex that facilitates retrograde trafficking from the prevacuolar endosome to the Golgi, being essential for the functionality of the endolysosomal system. SNAREs are required for vesicle fusion and, after facilitating membrane merging, are supposed to return to their donor organelle for new rounds of fusion. However, little is known about this recycling. We have found that Stx8, a fungal SNARE similar to human syntaxin 8, is a retromer cargo, and have identified its retromer binding motif. Sequence screening and comparison has determined that this sorting motif is conserved mainly in fungal Stx8 sequences. Notably, this motif is similar to the retromer sorting motif that is present in a family of vertebrate ion transporters. Our initial phylogenetic analyses suggest that, although retromer and some of its cargoes are conserved, the sorting motif in the cargoes might have undergone evolutionary divergence.

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 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.

The Effects of Artemisinin on the Cytolytic Activity of Natural Killer (NK) Cells

Artemisinin, a chemical compound used for the treatment of malaria, has been known to show anti-cancer activity. However, the effect of this chemical on natural killer (NK) cells, which are involved in tumor killing, remains unknown. Here, we demonstrate that artemisinin exerts a potent anti-cancer effect by activating NK cells. NK-92MI cells pre-treated with artemisinin were subjected to a cytotoxicity assay using K562 cells. The results showed that artemisinin significantly enhances the cytolytic activity of NK cells in a dose-dependent manner. Additionally, the artemisinin-enhanced cytotoxic effect of NK-92MI cells on tumor cells was accompanied by the stimulation of granule exocytosis, as evidenced by the detection of CD107a expression in NK cells. Moreover, this enhancement of cytotoxicity by artemisinin was also observed in human primary NK cells from peripheral blood. Our results suggest that artemisinin enhances human NK cell cytotoxicity and degranulation. This is the first evidence that artemisinin exerts antitumor activity by enhancing NK cytotoxicity. Therefore, these results provide a deeper understanding of the action of artemisinin and will contribute to the development and application of this class of compounds in cancer treatment strategies.

Bystander cells enhance NK cytotoxic efficiency by reducing search time

Natural killer (NK) cells play a central role during innate immune responses by eliminating pathogen-infected or tumorigenic cells. In the microenvironment, NK cells encounter not only target cells but also other cell types including non-target bystander cells. The impact of bystander cells on NK killing efficiency is, however, still elusive. In this study we show that the presence of bystander cells, such as P815, monocytes or HUVEC, enhances NK killing efficiency. With bystander cells present, the velocity and persistence of NK cells were increased, whereas the degranulation of lytic granules remained unchanged. Bystander cell-derived H2O2 was found to mediate the acceleration of NK cell migration. Using mathematical diffusion models, we confirm that local acceleration of NK cells in the vicinity of bystander cells reduces their search time to locate target cells. In addition, we found that integrin β chains (β1, β2 and β7) on NK cells are required for bystander-enhanced NK migration persistence. In conclusion, we show that acceleration of NK cell migration in the vicinity of H2O2-producing bystander cells reduces target cell search time and enhances NK killing efficiency.