Syntaxin11 serves as a t‐ SNARE for the fusion of lytic granules in human cytotoxic T lymphocytes

Patients and mice with deficiency in the SNARE protein SYNTAXIN-11 have a secondary B cell defect

SYNTAXIN-11 (STX11) is a SNARE protein that mediates the fusion of cytotoxic granules with the plasma membrane at the immunological synapses of CD8 T or NK cells. Autosomal recessive inheritance of deleterious STX11 variants impairs cytotoxic granule exocytosis, causing familial hemophagocytic lymphohistiocytosis type 4 (FHL-4). In several FHL-4 patients, we also observed hypogammaglobulinemia, elevated frequencies of naive B cells, and increased double-negative DN2:DN1 B cell ratios, indicating a hitherto unrecognized role of STX11 in humoral immunity. Detailed analysis of Stx11-deficient mice revealed impaired CD4 T cell help for B cells, associated with disrupted germinal center formation, reduced isotype class switching, and low antibody avidity. Mechanistically, Stx11-/- CD4 T cells exhibit impaired membrane fusion leading to reduced CD107a and CD40L surface mobilization and diminished IL-2 and IL-10 secretion. Our findings highlight a critical role of STX11 in SNARE-mediated membrane trafficking and vesicle exocytosis in CD4 T cells, important for successful CD4 T cell-B cell interactions. Deficiency in STX11 impairs CD4 T cell-dependent B cell differentiation and humoral responses.

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.

Lipid switches in the immunological synapse

Adaptive immune responses comprise the activation of T cells by peptide antigens that are presented by proteins of the Major Histocompatibility Complex (MHC) on the surface of an antigen-presenting cell. As a consequence of the T cell receptor interacting productively with a certain peptide-MHC complex, a specialized cell-cell junction known as the immunological synapse forms and is accompanied by changes in the spatiotemporal patterning and function of intracellular signaling molecules. Key modifications occurring at the cytoplasmic leaflet of the plasma and internal membranes in activated T cells comprise lipid switches that affect the binding and distribution of proteins within or near the lipid bilayer. Here, we describe two major classes of lipid switches that act at this critical water/membrane interface. Phosphoinositides are derived from phosphatidylinositol, an amphiphilic molecule that contains two fatty acid chains and a phosphate group that bridges the glycerol backbone to the carbohydrate inositol. The inositol ring can be variably (de-)phosphorylated by dedicated kinases and phosphatases, thereby creating phosphoinositide signatures that define the composition and properties of signaling molecules, molecular complexes, or whole organelles. Palmitoylation refers to the reversible attachment of the fatty acid palmitate to a substrate protein's cysteine residue. DHHC enzymes, named after the four conserved amino acids in their active site, catalyze this post-translational modification and thereby change the distribution of proteins at, between, and within membranes. T cells utilize these two types of molecular switches to adjust their properties to an activation process that requires changes in motility, transport, secretion, and gene expression.

A solution for highly efficient electroporation of primary cytotoxic T lymphocytes

BACKGROUND

Cytotoxic T lymphocytes (CTLs) are central players in the adaptive immune response. Their functional characterization and clinical research depend on efficient and reliable transfection. Although various methods have been utilized, electroporation remains the preferred technique for transient gene over-expression. However, the efficiency of electroporation is reduced for human and mouse primary CTLs. Lonza offers kits that effectively improve plasmid DNA transfection quality. Unfortunately, the removal of key components of the cell recovery medium considerably reduced the efficiency of their kit for CTLs. Our aim was to develop a new recovery medium to be used with Lonza's Nucleofector system that would significantly enhance transfection rates.

RESULTS

We assessed the impact of different media in which the primary CTLs were placed to recover after electroporation on cell survival, transfection rate and their ability to form an immunological synapse and to perform exocytosis. We transfected the cells with pmax-GFP and large constructs encoding for either CD81-super ecliptic pHluorin or granzyme B-pHuji. The comparison of five different media for mouse and two for human CTLs demonstrated that our new recovery medium composed of Opti-MEM-GlutaMAX supplemented with HEPES, DMSO and sodium pyruvate gave the best result in cell survival (> 50%) and transfection rate (> 30 and 20% for mouse and human cells, respectively). More importantly, the functionality of CTLs was at least twice as high as with the original Lonza recovery medium. In addition, our RM significantly improved transfection efficacy of natural killer cells that are notoriously hard to electroporate.

CONCLUSION

Our results show that successful transfection depends not only on the electroporation medium and pulse sequence but also on the medium applied for cell recovery. In addition, we have reduced our reliance on proprietary products by designing an effective recovery medium for both mouse and human primary CTLs and other lymphocytes that can be easily implemented by any laboratory. We expect that this recovery medium will have a significant impact on both fundamental and applied research in immunology.

CD8+ T Cell Biology in Cytokine Storm Syndromes

Familial forms of hemophagocytic lymphohistiocytosis (HLH) are caused by loss-of-function mutations in genes encoding perforin as well as those required for release of perforin-containing cytotoxic granule constituent. Perforin is expressed by subsets of CD8+ T cells and NK cells, representing lymphocytes that share mechanism of target cell killing yet display distinct modes of target cell recognition. Here, we highlight recent findings concerning the genetics of familial HLH that implicate CD8+ T cells in the pathogenesis of HLH and discuss mechanistic insights from animal models as well as patients that reveal how CD8+ T cells may contribute to or drive disease, at least in part through release of IFN-γ. Intriguingly, CD8+ T cells and NK cells may act differentially in severe hyperinflammatory diseases such as HLH. We also discuss how CD8+ T cells may promote or drive pathology in other cytokine release syndromes (CSS). Moreover, we review the molecular mechanisms underpinning CD8+ T cell-mediated lymphocyte cytotoxicity, key to the development of familial HLH. Together, recent insights to the pathophysiology of CSS in general and HLH in particular are providing promising new therapeutic targets.

Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

Regulated exocytosis is a central mechanism of cellular communication. It is not only the basis for neurotransmission and hormone release, but also plays an important role in the immune system for the release of cytokines and cytotoxic molecules. In cytotoxic T lymphocytes (CTLs), the formation of the immunological synapse is required for the delivery of the cytotoxic substances such as granzymes and perforin, which are stored in lytic granules and released via exocytosis. The molecular mechanisms of their fusion with the plasma membrane are only partially understood. In this review, we discuss the molecular players involved in the regulated exocytosis of CTL, highlighting the parallels and differences to neuronal synaptic transmission. Additionally, we examine the strengths and weaknesses of both systems to study exocytosis.

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.

The Expanding Arsenal of Cytotoxic T Cells

Cytotoxic T cells (CTLs) are the main cellular mediators of the adaptive immune defenses against intracellular pathogens and malignant cells. Upon recognition of specific antigen on their cellular target, CTLs assemble an immunological synapse where they mobilise their killing machinery that is released into the synaptic cleft to orchestrate the demise of their cell target. The arsenal of CTLs is stored in lysosome-like organelles that undergo exocytosis in response to signals triggered by the T cell antigen receptor following antigen recognition. These organelles include lytic granules carrying a cargo of cytotoxic proteins packed on a proteoglycan scaffold, multivesicular bodies carrying the death receptor ligand FasL, and the recently discovered supramolecular attack particles that carry a core of cytotoxic proteins encased in a non-membranous glycoprotein shell. Here we will briefly review the main features of these killing entities and discuss their interrelationship and interplay in CTL-mediated killing.

NK cells integrate signals over large areas when building immune synapses but require local stimuli for degranulation

Immune synapses are large-scale, transient molecular assemblies that serve as platforms for antigen presentation to B and T cells and for target recognition by cytotoxic T cells and natural killer (NK) cells. The formation of an immune synapse is a tightly regulated, stepwise process in which the cytoskeleton, cell surface receptors, and intracellular signaling proteins rearrange into supramolecular activation clusters (SMACs). We generated artificial immune synapses (AIS) consisting of synthetic and natural ligands for the NK cell-activating receptors LFA-1 and CD16 by microcontact printing the ligands into circular-shaped SMAC structures. Live-cell imaging and analysis of fixed human NK cells in this reductionist system showed that the spatial distribution of activating ligands influenced the formation, stability, and outcome of NK cell synapses. Whereas engagement of LFA-1 alone promoted synapse initiation, combined engagement of LFA-1 and CD16 was required for the formation of mature synapses and degranulation. Organizing LFA-1 and CD16 ligands into donut-shaped AIS resulted in fewer long-lasting, symmetrical synapses compared to dot-shaped AIS. NK cells spreading evenly over either AIS shape exhibited similar arrangements of the lytic machinery. However, degranulation only occurred in regions containing ligands that therefore induced local signaling, suggesting the existence of a late checkpoint for degranulation. Our results demonstrate that the spatial organization of ligands in the synapse can affect its outcome, which could be exploited by target cells as an escape mechanism.

Higher Incidence of B Cell Malignancies in Primary Immunodeficiencies: A Combination of Intrinsic Genomic Instability and Exocytosis Defects at the Immunological Synapse

Congenital defects of the immune system called primary immunodeficiency disorders (PID) describe a group of diseases characterized by a decrease, an absence, or a malfunction of at least one part of the immune system. As a result, PID patients are more prone to develop life-threatening complications, including cancer. PID currently include over 400 different disorders, however, the variety of PID-related cancers is narrow. We discuss here reasons for this clinical phenotype. Namely, PID can lead to cell intrinsic failure to control cell transformation, failure to activate tumor surveillance by cytotoxic cells or both. As the most frequent tumors seen among PID patients stem from faulty lymphocyte development leading to leukemia and lymphoma, we focus on the extensive genomic alterations needed to create the vast diversity of B and T lymphocytes with potential to recognize any pathogen and why defects in these processes lead to malignancies in the immunodeficient environment of PID patients. In the second part of the review, we discuss PID affecting tumor surveillance and especially membrane trafficking defects caused by altered exocytosis and regulation of the actin cytoskeleton. As an impairment of these membrane trafficking pathways often results in dysfunctional effector immune cells, tumor cell immune evasion is elevated in PID. By considering new anti-cancer treatment concepts, such as transfer of genetically engineered immune cells, restoration of anti-tumor immunity in PID patients could be an approach to complement standard therapies.

SNAREs and developmental disorders

Members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family mediate membrane fusion processes associated with vesicular trafficking and autophagy. SNAREs mediate core membrane fusion processes essential for all cells, but some SNAREs serve cell/tissue type-specific exocytic/endocytic functions, and are therefore critical for various aspects of embryonic development. Mutations or variants of their encoding genes could give rise to developmental disorders, such as those affecting the nervous system and immune system in humans. Mutations to components in the canonical synaptic vesicle fusion SNARE complex (VAMP2, STX1A/B, and SNAP25) and a key regulator of SNARE complex formation MUNC18-1, produce variant phenotypes of autism, intellectual disability, movement disorders, and epilepsy. STX11 and MUNC18-2 mutations underlie 2 subtypes of familial hemophagocytic lymphohistiocytosis. STX3 mutations contribute to variant microvillus inclusion disease. Chromosomal microdeletions involving STX16 play a role in pseudohypoparathyroidism type IB associated with abnormal imprinting of the GNAS complex locus. In this short review, I discuss these and other SNARE gene mutations and variants that are known to be associated with a variety developmental disorders, with a focus on their underlying cellular and molecular pathological basis deciphered through disease modeling. Possible pathogenic potentials of other SNAREs whose variants could be disease predisposing are also speculated upon.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

Various Stages of Immune Synapse Formation Are Differently Dependent on the Strength of the TCR Stimulus

Cytotoxic T lymphocytes (CTL) are key players of the adaptive immune system that target tumors and infected cells. A central step to that is the formation of a cell–cell contact zone between the CTL and its target called an immune synapse (IS). Here, we investigate the influence of the initial T cell receptor (TCR) trigger of a cytolytic IS on the distinct steps leading to cytotoxic granule (CG) exocytosis. We stimulated primary CTLs from mouse using lipid bilayers with varying anti-CD3 but constant ICAM concentrations. We fluorescently labeled molecular markers of distinct IS zones such as actin, CD3, granzyme B, and Synaptobrevin2 in CTLs and imaged cytolytic IS formation by total internal reflection fluorescence microscopy (TIRFM). We found that an intermediate anti-CD3 concentration of 10 µg/mL induces the fastest adhesion of CTLs to the bilayers and results in maximal CG fusion efficiency. The latency of actin ring formation, dwell time, and maximum surface area at the IS exhibit different dependencies on the stimulatory anti-CD3 concentrations. The number and surface area of CD3 clusters at the IS seem to show a different dependency to the TCR trigger when compared to their dwell time. Finally, the mode of full CG exocytosis appears to be independent of the TCR trigger.

Molecular regulation of the plasma membrane-proximal cellular steps involved in NK cell cytolytic function

Natural killer (NK) cells, cytolytic lymphocytes of the innate immune system, play a crucial role in the immune response against infection and cancer. NK cells kill target cells through exocytosis of lytic granules that contain cytotoxic proteins, such as perforin and granzymes. Formation of a functional immune synapse, i.e. the interface between the NK cell and its target cell enhances lysis through accumulation of polymerized F-actin at the NK cell synapse, leading to convergence of lytic granules to the microtubule organizing center (MTOC) and its subsequent polarization along microtubules to deliver the lytic granules to the synapse. In this review, we focus on the molecular mechanisms regulating the cellular processes that occur after the lytic granules are delivered to the cytotoxic synapse. We outline how - once near the synapse - the granules traverse the clearings created by F-actin remodeling to dock, tether and fuse with the plasma membrane in order to secrete their lytic content into the synaptic cleft through exocytosis. Further emphasis is given to the role of Ca²⁺ mobilization during degranulation and, whenever applicable, we compare these mechanisms in NK cells and cytotoxic T lymphocytes (CTLs) as adaptive immune system effectors.

Syntaxin 11 regulates the stimulus-dependent transport of Toll-like receptor 4 to the plasma membrane by cooperating with SNAP-23 in macrophages

Syntaxin 11 (stx11) is a soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) that is selectively expressed in immune cells; however, its precise role in macrophages is unclear. We showed that stx11 knockdown reduces the phagocytosis of Escherichia coli in interferon-γ–activated macrophages. stx11 knockdown decreased Toll-like receptor 4 (TLR4) localization on the plasma membrane without affecting total expression. Plasma membrane–localized TLR4 was primarily endocytosed within 1 h by lipopolysaccharide (LPS) stimulation and gradually relocalized 4 h after removal of LPS. This relocalization was significantly impaired by stx11 knockdown. The lack of TLR4 transport to the plasma membrane is presumably related to TLR4 degradation in acidic endosomal organelles. Additionally, an immunoprecipitation experiment suggested that stx11 interacts with SNAP-23, a plasma membrane–localized SNARE protein, whose depletion also inhibits TLR4 replenishment in LPS-stimulated cells. Using an intramolecular Förster resonance energy transfer (FRET) probe for SNAP-23, we showed that the high FRET efficiency caused by LPS stimulation is reduced by stx11 knockdown. These findings suggest that stx11 regulates the stimulus-dependent transport of TLR4 to the plasma membrane by cooperating with SNAP-23 in macrophages. Our results clarify the regulatory mechanisms underlying intracellular transport of TLR4 and have implications for microbial pathogenesis and immune responses.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

Unconventional Protein Secretion in Animal Cells

All eukaryotic cells secrete a range of proteins in a constitutive or regulated manner through the conventional or canonical exocytic/secretory pathway characterized by vesicular traffic from the endoplasmic reticulum, through the Golgi apparatus, and towards the plasma membrane. However, a number of proteins are secreted in an unconventional manner, which are insensitive to inhibitors of conventional exocytosis and use a route that bypasses the Golgi apparatus. These include cytosolic proteins such as fibroblast growth factor 2 (FGF2) and interleukin-1β (IL-1β), and membrane proteins that are known to also traverse to the plasma membrane by a conventional process of exocytosis, such as α integrin and the cystic fibrosis transmembrane conductor (CFTR). Mechanisms underlying unconventional protein secretion (UPS) are actively being analyzed and deciphered, and these range from an unusual form of plasma membrane translocation to vesicular processes involving the generation of exosomes and other extracellular microvesicles. In this chapter, we provide an overview on what is currently known about UPS in animal cells.

The cytotoxic T lymphocyte immune synapse at a glance

The immune synapse provides an important structure for communication with immune cells. Studies on immune synapses formed by cytotoxic T lymphocytes (CTLs) highlight the dynamic changes and specialised mechanisms required to facilitate focal signalling and polarised secretion in immune cells. In this Cell Science at a Glance article and the accompanying poster, we illustrate the different steps that reveal the specialised mechanisms used to focus secretion at the CTL immune synapse and allow CTLs to be such efficient and precise serial killers.

Syntaxin 4 mediates endosome recycling for lytic granule exocytosis in cytotoxic T-lymphocytes

Adaptive and innate immunity utilize the perforin-killing pathway to eliminate virus-infected or cancer cells. Cytotoxic T-lymphocytes (CTLs) and natural killer cells mediate this process by releasing toxic proteins at the contact area with target cells known as immunological synapse (IS). Formation of a stable IS and exocytosis of toxic proteins requires persistent fusion of Rab11a recycling endosomes with the plasma membrane (PM) that may assure the delivery of key effector proteins. Despite the importance of the recycling endosomal compartment, the membrane fusion proteins that control this process at the IS remain elusive. Here, by performing knockdown experiments we found that syntaxin 4 (STX4) is necessary for cytotoxic activity and CD107a degranulation against target cells in a similar fashion to syntaxin 11, which is involved in lytic granule (LG) exocytosis and immunodeficiency when it is mutated. Using total internal reflection fluorescent microscopy we identified that STX4 mediates fusion of EGFP-Rab11a vesicles at the IS. Immunoprecipitation experiments in lysates of activated CTLs indicate that endogenous STX4 may drive this fusion step by interacting with cognate proteins: Munc18-3/SNAP23/VAMP7 and/or VAMP8. These results reveal the role of STX4 in mediating fusion of Rab11a endosomes upstream of lytic granules (LGs) exocytosis and further demonstrate the importance of this pathway in controlling CTL-mediated cytotoxicity.

Rabs set the stage for polarity

Cell polarity refers to the asymmetric localization of cellular components that allows cells to carry out their specialized functions, be they epithelial barrier function, transmission of action potentials in nerve cells, or modulation of the immune response. The establishment and maintenance of cell polarity requires the directed trafficking of membrane proteins and lipids - essential processes that are mediated by Rab GTPases. Interestingly, several of the Rabs that impact polarity are present in the earliest eukaryotes, and the Rab polarity repertoire has expanded as cells have become more complex. There is a substantial conservation of Rab function across diverse cell types. Rabs act through an assortment of effector proteins that include scaffolding proteins, cytoskeletal motors, and other small GTPases. In this review we highlight the similarities and differences in Rab function for the instruction of polarity in diverse cell types.

Simultaneous Membrane Capacitance Measurements and TIRF Microscopy to Study Granule Trafficking at Immune Synapses

Whole-cell capacitance measurements allow the direct measurement of exocytosis with high temporal resolution. An added benefit of the whole-cell configuration is the possibility to control the cytosolic free calcium concentration allowing examination of the role of intracellular calcium in a variety of processes. We have coupled this method with imaging of cytotoxic granule release using total internal reflection fluorescence microscopy (TIRFM) to identify the capacitance steps associated with cytotoxic granule release identified by TIRFM. This requires the use of fluorescent granule markers to identify cytotoxic granules and allows characterization of cytotoxic granule fusion and of the behavior of cytotoxic granules at the immune synapse prior to fusion. Combination of these methods enables the study of a number of processes relevant to the function of the immune synapse.

Preparing the lethal hit: interplay between exo- and endocytic pathways in cytotoxic T lymphocytes

Cytotoxic T lymphocytes patrol our body in search for infected cells which they kill through the release of cytotoxic substances contained in cytotoxic granules. The fusion of cytotoxic granules occurs at a specially formed contact site, the immunological synapse, and is tightly controlled to ensure specificity. In this review, we discuss the contribution of two intracellular compartments, endosomes and cytotoxic granules, to the formation, function and disassembly of the immunological synapse. We highlight a recently proposed sequential process of fusion events at the IS upon target cell recognition. First, recycling endosomes fuse with the plasma membrane to deliver cargo required for the docking of cytotoxic granules. Second, cytotoxic granules arrive and fuse upon docking in a SNARE-dependent manner. Following fusion, membrane components of the cytotoxic granule are retrieved through endocytosis to ensure the fast, efficient serial killing of target cells that is characteristic of cytotoxic T lymphocytes.

Endocytosis of Cytotoxic Granules Is Essential for Multiple Killing of Target Cells by T Lymphocytes

CTLs are serial killers that kill multiple target cells via exocytosis of cytotoxic granules (CGs). CG exocytosis is tightly regulated and has been investigated in great detail; however, whether CG proteins are endocytosed following exocytosis and contribute to serial killing remains unknown. By using primary CTLs derived from a knock-in mouse of the CG membrane protein Synaptobrevin2, we show that CGs are endocytosed in a clathrin- and dynamin-dependent manner. Following acidification, endocytosed CGs are recycled through early and late, but not recycling endosomes. CGs are refilled with granzyme B at the late endosome stage and polarize to subsequent synapses formed between the CTL and new target cells. Importantly, inhibiting CG endocytosis in CTLs results in a significant reduction of their cytotoxic activity. Thus, our data demonstrate that continuous endocytosis of CG membrane proteins is a prerequisite for efficient serial killing of CTLs and identify key events in this process.

A unique SNARE machinery for exocytosis of cytotoxic granules and platelets granules

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells target infected or transformed cells with perforin-containing cytotoxic granules through immune synapses, while platelets secrete several types of granules which contents are essential for thrombosis and hemostasis. Recent work has culminated in the notion that an exocytic SNARE complex, based on a very similar set of components, is primarily responsible for exocytosis of the diverse granules in these different cell types. Granule exocytosis is, in particular, uniquely dependent on the atypical Q-SNARE syntaxin 11, its interacting partners of the Sec/Munc (SM) family, and is regulated by Rab27a. Mutations in these exocytic components underlie disease manifestations of familial hemophagocytic lymphohistiocytosis (FHL) subtypes, characterized by hyperactivation of the immune system, as well as platelet granule secretion defects. Here we discuss the key discoveries that led to the converging notion of the syntaxin 11-based exocytosis machinery for cytotoxic granules and platelet-derived granules.

New Insights into How Trafficking Regulates T Cell Receptor Signaling

There is emerging evidence that exocytosis plays an important role in regulating T cell receptor (TCR) signaling. The trafficking molecules involved in lytic granule (LG) secretion in cytotoxic T lymphocytes (CTL) have been well-studied due to the immune disorder known as familial hemophagocytic lymphohistiocytosis (FHLH). However, the knowledge of trafficking machineries regulating the exocytosis of receptors and signaling molecules remains quite limited. In this review, we summarize the reported trafficking molecules involved in the transport of the TCR and downstream signaling molecules to the cell surface. By combining this information with the known knowledge of LG exocytosis and general exocytic trafficking machinery, we attempt to draw a more complete picture of how the TCR signaling network and exocytic trafficking matrix are interconnected to facilitate T cell activation. This also highlights how membrane compartmentalization facilitates the spatiotemporal organization of cellular responses that are essential for immune functions.

Exocytosis in non-neuronal cells

Exocytosis is the process by which stored neurotransmitters and hormones are released via the fusion of secretory vesicles with the plasma membrane. It is a dynamic, rapid and spatially restricted process involving multiple steps including vesicle trafficking, tethering, docking, priming and fusion. For many years great steps have been undertaken in our understanding of how exocytosis occurs in different cell types, with significant focus being placed on synaptic release and neurotransmission. However, this process of exocytosis is an essential component of cell signalling throughout the body and underpins a diverse array of essential physiological pathways. Many similarities exist between different cell types with regard to key aspects of the exocytosis pathway, such as the need for Ca(2+) to trigger it or the involvement of members of the N-ethyl maleimide-sensitive fusion protein attachment protein receptor protein families. However, it is also equally clear that non-neuronal cells have acquired highly specialized mechanisms to control the release of their own unique chemical messengers. This review will focus on several important non-neuronal cell types and discuss what we know about the mechanisms they use to control exocytosis and how their specialized output is relevant to the physiological role of each individual cell type. These include enteroendocrine cells, pancreatic β cells, astrocytes, lactotrophs and cytotoxic T lymphocytes. Non-neuronal cells have acquired highly specialized mechanisms to control the release of unique chemical messengers, such as polarised fusion of insulin granules in pancreatic β cells targeted towards the vasculature (top). This review discusses mechanisms used in several important non-neuronal cell types to control exocytosis, and the relevance of intermediate vesicle fusion pore states (bottom) and their specialized output to the physiological role of each cell type. These include enteroendocrine cells, pancreatic β cells, astrocytes, lactotrophs and cytotoxic T lymphocytes. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).

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.

Approaches to Investigating Complex Genetic Traits in a Large-Scale Inbred Mouse Aging Study

Inbred mice are a unique model system for studying aging because of the genetic homogeneity within inbred strains, the short life span of mice relative to humans, and the rich array of analytic tools that are available. A large-scale aging study was conducted on 28 inbred strains representing great genetic diversity to determine, via histopathology, the type and diversity of spontaneous diseases that aging mice develop. A total of 20 885 different diagnoses were made, with an average of 12 diagnoses per mouse in the study. Eighteen inbred strains have had their genomes sequenced, and many others have been partially sequenced to provide large repositories of data on genetic variation among the strains. This vast amount of genomic information can be utilized in genome-wide association studies to find candidate genes that are involved in the pathogenesis of spontaneous diseases. As an illustration, this article presents a genome-wide association study of the genetic associations of age-related intestinal amyloidosis, which implicated 3 candidate genes: translocating chain-associated membrane protein 1 (Tram1); splicing factor 3b, subunit 5 (Sf3b5); and syntaxin 11 (Stx11). Representative photomicrographs are available on the Mouse Tumor Biology Database and Pathbase to serve as a reference when evaluating inbred mice used in other genetic or experimental studies to rule out strain background lesions. Many of the age-related mouse diseases are similar, if not identical, to human diseases; therefore, the genetic discoveries have direct translational benefit.

Munc18-2 is required for Syntaxin 11 Localization on the Plasma Membrane in Cytotoxic T-Lymphocytes

Cytotoxic T‐lymphocytes (CTL) kill their targets by cytolytic granule secretion at the immunological synapse. The Sec/Munc protein, Munc18‐2, and its binding partner Syntaxin 11 (STX11) are both required for granule secretion, with mutations in either leading to the primary immunodeficiency, Familial Haemophagocytic Lymphohistiocytosis (FHL4 and 5). Understanding how Munc18‐2 and STX11 function in CTL has been hampered by not knowing the endogenous localization of these proteins. Using a novel FHL5 Munc18‐2 mutation that results in loss of protein, cytotoxicity and degranulation together with CTL from an FHL4 patient lacking STX11, enabled us to localize endogenous STX11 and Munc18‐2 in CTL. Munc18‐2 localized predominantly to cytolytic granules with low levels associated with the plasma membrane where STX11 localized. Importantly, while Munc18‐2 localization is unaffected by the absence of STX11 in FHL4 CTL, STX11 is lost from the plasma membrane in FHL5 CTL lacking Munc18‐2. These findings support a role for Munc18‐2 in chaperoning STX11 to the plasma membrane where the final fusion events involved in secretion occur.

Behavior and Properties of Mature Lytic Granules at the Immunological Synapse of Human Cytotoxic T Lymphocytes

Killing of virally infected cells or tumor cells by cytotoxic T lymphocytes requires targeting of lytic granules to the junction between the CTL and its target. We used whole-cell patch clamp to measure the cell capacitance at fixed intracellular [Ca2+] to study fusion of lytic granules in human CTLs. Expression of a fluorescently labeled human granzyme B construct allowed identification of lytic granule fusion using total internal reflection fluorescence microscopy. In this way capacitance steps due to lytic granule fusion were identified. Our goal was to determine the size of fusing lytic granules and to describe their behavior at the plasma membrane. On average, 5.02 ± 3.09 (mean ± s.d.) lytic granules were released per CTL. The amplitude of lytic granule fusion events was ~ 3.3 fF consistent with a diameter of about 325 nm. Fusion latency was biphasic with time constants of 15.9 and 106 seconds. The dwell time of fusing lytic granules was exponentially distributed with a mean dwell time of 28.5 seconds. Fusion ended in spite of the continued presence of granules at the immune synapse. The mobility of fusing granules at the membrane was indistinguishable from that of lytic granules which failed to fuse. While dwelling at the plasma membrane lytic granules exhibit mobility consistent with docking interspersed with short periods of greater mobility. The failure of lytic granules to fuse when visible in TIRF at the membrane may indicate that a membrane-confined reaction is rate limiting.

VAMP8-dependent fusion of recycling endosomes with the plasma membrane facilitates T lymphocyte cytotoxicity

VAMP8 is associated with the recycling endosome compartment rather than with cytotoxic granules and is required for a fusion step between recycling endosomes and the plasma membrane that brings syntaxin-11 to the immune synapse for cytotoxic granule exocytosis.

Cytotoxic T lymphocytes (CTLs) eliminate infected and neoplastic cells through directed release of cytotoxic granule contents. Although multiple SNARE proteins have been implicated in cytotoxic granule exocytosis, the role of vesicular SNARE proteins, i.e., vesicle-associated membrane proteins (VAMPs), remains enigmatic. VAMP8 was posited to represent the cytotoxic granule vesicular SNARE protein mediating exocytosis in mice. In primary human CTLs, however, VAMP8 colocalized with Rab11a-positive recycling endosomes. Upon stimulation, these endosomes rapidly trafficked to and fused with the plasma membrane, preceding fusion of cytotoxic granules. Knockdown of VAMP8 blocked both recycling endosome and cytotoxic granule fusion at immune synapses, without affecting activating signaling. Mechanistically, VAMP8-dependent recycling endosomes deposited syntaxin-11 at immune synapses, facilitating assembly of plasma membrane SNARE complexes for cytotoxic granule fusion. Hence, cytotoxic granule exocytosis is a sequential, multivesicle fusion process requiring VAMP8-mediated recycling endosome fusion before cytotoxic granule fusion. Our findings imply that secretory granule exocytosis pathways in other cell types may also be more complex than previously appreciated.