Perforin pores in the endosomal membrane trigger the release of endocytosed granzyme B into the cytosol of target cells. Nat Immunol. 2011;12:770-777. [PMID: 21685908 PMCID: PMC3140544 DOI: 10.1038/ni.2050]

Gene transcripts associated with muscle strength: a CHARGE meta-analysis of 7,781 persons

Lower muscle strength in midlife predicts disability and mortality in later life. Blood-borne factors, including growth differentiation factor 11 (GDF11), have been linked to muscle regeneration in animal models. We aimed to identify gene transcripts associated with muscle strength in adults. Meta-analysis of whole blood gene expression (overall 17,534 unique genes measured by microarray) and hand-grip strength in four independent cohorts (n = 7,781, ages: 20-104 yr, weighted mean = 56), adjusted for age, sex, height, weight, and leukocyte subtypes. Separate analyses were performed in subsets (older/younger than 60, men/women). Expression levels of 221 genes were associated with strength after adjustment for cofactors and for multiple statistical testing, including ALAS2 (rate-limiting enzyme in heme synthesis), PRF1 (perforin, a cytotoxic protein associated with inflammation), IGF1R, and IGF2BP2 (both insulin like growth factor related). We identified statistical enrichment for hemoglobin biosynthesis, innate immune activation, and the stress response. Ten genes were associated only in younger individuals, four in men only and one in women only. For example, PIK3R2 (a negative regulator of PI3K/AKT growth pathway) was negatively associated with muscle strength in younger (<60 yr) individuals but not older (≥ 60 yr). We also show that 115 genes (52%) have not previously been linked to muscle in NCBI PubMed abstracts. This first large-scale transcriptome study of muscle strength in human adults confirmed associations with known pathways and provides new evidence for over half of the genes identified. There may be age- and sex-specific gene expression signatures in blood for muscle strength.

Protein-lipid interactions and non-lamellar lipidic structures in membrane pore formation and membrane fusion

Pore-forming proteins and peptides act on their targeted lipid bilayer membranes to increase permeability. This approach to the modulation of biological function is relevant to a great number of living processes, including; infection, parasitism, immunity, apoptosis, development and neurodegeneration. While some pore-forming proteins/peptides assemble into rings of subunits to generate discrete, well-defined pore-forming structures, an increasing number is recognised to form pores via mechanisms which co-opt membrane lipids themselves. Among these, membrane attack complex-perforin/cholesterol-dependent cytolysin (MACPF/CDC) family proteins, Bax/colicin family proteins and actinoporins are especially prominent and among the mechanisms believed to apply are the formation of non-lamellar (semi-toroidal or toroidal) lipidic structures. In this review I focus on the ways in which lipids contribute to pore formation and contrast this with the ways in which lipids are co-opted also in membrane fusion and fission events. A variety of mechanisms for pore formation that involve lipids exists, but they consistently result in stable hybrid proteolipidic structures. These structures are stabilised by mechanisms in which pore-forming proteins modify the innate capacity of lipid membranes to respond to their environment, changing shape and/or phase and binding individual lipid molecules directly. In contrast, and despite the diversity in fusion protein types, mechanisms for membrane fusion are rather similar to each other, mapping out a pathway from pairs of separated compartments to fully confluent fused membranes. Fusion proteins generate metastable structures along the way which, like long-lived proteolipidic pore-forming complexes, rely on the basic physical properties of lipid bilayers. Membrane fission involves similar intermediates, in the reverse order. I conclude by considering the possibility that at least some pore-forming and fusion proteins are evolutionarily related homologues. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Natural Killer Cell Immunomodulation: Targeting Activating, Inhibitory, and Co-stimulatory Receptor Signaling for Cancer Immunotherapy

There is compelling clinical and experimental evidence to suggest that natural killer (NK) cells play a critical role in the recognition and eradication of tumors. Efforts at using NK cells as antitumor agents began over two decades ago, but recent advances in elucidating NK cell biology have accelerated the development of NK cell-targeting therapeutics. NK cell activation and the triggering of effector functions is governed by a complex set of activating and inhibitory receptors. In the early phases of cancer immune surveillance, NK cells directly identify and lyse cancer cells. Nascent transformed cells elicit NK cell activation and are eliminated. However, as tumors progress, cancerous cells develop immunosuppressive mechanisms that circumvent NK cell-mediated killing, allowing for tumor escape and proliferation. Therapeutic intervention aims to reverse tumor-induced NK cell suppression and sustain NK cells’ tumorlytic capacities. Here, we review tumor–NK cell interactions, discuss the mechanisms by which NK cells generate an antitumor immune response, and discuss NK cell-based therapeutic strategies targeting activating, inhibitory, and co-stimulatory receptors.

Challenges in carrier-mediated intracellular delivery: moving beyond endosomal barriers

The deployment of molecular to microscale carriers for intracellular delivery has tremendous potential for biology and medicine, especially for in vivo therapies. The field remains limited, however, by a poor understanding of how carriers gain access to the cell interior. In this review, we provide an overview of the different types of carriers, their speculated modes of entry, putative pathways of vesicular transport, and sites of endosomal escape. We compare this alongside pertinent examples from the cell biology of how viruses, bacteria, and their effectors enter cells and escape endosomal confinement. We anticipate insights into the mechanisms of cellular entry and endosomal escape will benefit future research efforts on effective carrier-mediated intracellular delivery. WIREs Nanomed Nanobiotechnol 2016, 8:465-478. doi: 10.1002/wnan.1377 For further resources related to this article, please visit the WIREs website.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

Cytotoxic granule secretion by lymphocytes and its link to immune homeostasis

The granule-dependent cytotoxic activity of T and natural killer lymphocytes has progressively emerged as an important effector pathway not only for host defence but also for immune regulation. The analysis of an early-onset, severe, primary immune dysregulatory syndrome known as hemophagocytic lymphohistiocytosis (HLH) has been decisive in highlighting this latter role and identifying key effectors on the basis of gene mutation analyses and mediators in the maturation and secretion of cytotoxic granules. Studies of cytotoxicity-deficient murine counterparts have helped to define primary HLH as a syndrome in which uncontrolled T-cell activation in response to lymphocytic choriomeningitis virus infection results in excessive macrophage activation and inflammation-associated cytopenia. Recent recognition of late-onset HLH, which occurs in a variety of settings, in association with hypomorphic, monoallelic mutations in genes encoding components of the granule-dependent cytotoxic pathway or even in the absence of such mutations has broadened our view about the mechanisms that underlie the perturbation of immune homeostasis. These findings have led to the development of a model in which disease occurs when a threshold is reached through the accumulation of genetic and environmental risk factors. Nevertheless, validation of this model will require further investigations.

Molecular characterization of HCMV-specific immune responses: Parallels between CD8(+) T cells, CD4(+) T cells, and NK cells

CD8(+) T cells are important for immunity against human cytomegalovirus (HCMV). The HCMV-specific CD8(+) T-cell response is characterized by the accumulation of terminally differentiated effector cells that have downregulated the costimulatory molecules CD27 and CD28. These HCMV-specific CD8(+) T cells maintain high levels of cytotoxic molecules such as granzyme B and rapidly produce the inflammatory cytokine IFN-γ upon activation. Remarkably, HCMV-specific CD8(+) T cells are able to persist long term as fully functional effector cells, suggesting a unique differentiation pathway that is distinct from the formation of memory CD8(+) T cells after infection with acute viruses. In this review, we aim to highlight the most recent developments in HCMV-specific CD8(+) T-cell differentiation, maintenance, tissue distribution, metabolism and function. HCMV also induces the differentiation of effector CD4(+) T cells and NK cells, which share characteristics with HCMV-specific CD8(+) T cells. We propose that the overlap in differentiation of NK cells, CD4(+) and CD8(+) T cells after HCMV infection may be regulated by a shared transcriptional machinery. A better understanding of the molecular framework of HCMV-specific CD8(+) T-cell responses may benefit vaccine design, as these cells uniquely combine the capacity to rapidly respond to infection with long-term survival.

A High Yield and Cost-efficient Expression System of Human Granzymes in Mammalian Cells

When cytotoxic T lymphocytes (CTL) or natural killer (NK) cells recognize tumor cells or cells infected with intracellular pathogens, they release their cytotoxic granule content to eliminate the target cells and the intracellular pathogen. Death of the host cells and intracellular pathogens is triggered by the granule serine proteases, granzymes (Gzms), delivered into the host cell cytosol by the pore forming protein perforin (PFN) and into bacterial pathogens by the prokaryotic membrane disrupting protein granulysin (GNLY). To investigate the molecular mechanisms of target cell death mediated by the Gzms in experimental in-vitro settings, protein expression and purification systems that produce high amounts of active enzymes are necessary. Mammalian secreted protein expression systems imply the potential to produce correctly folded, fully functional protein that bears posttranslational modification, such as glycosylation. Therefore, we used a cost-efficient calcium precipitation method for transient transfection of HEK293T cells with human Gzms cloned into the expression plasmid pHLsec. Gzm purification from the culture supernatant was achieved by immobilized nickel affinity chromatography using the C-terminal polyhistidine tag provided by the vector. The insertion of an enterokinase site at the N-terminus of the protein allowed the generation of active protease that was finally purified by cation exchange chromatography. The system was tested by producing high levels of cytotoxic human Gzm A, B and M and should be capable to produce virtually every enzyme in the human body in high yields.

Increased expression of perforin, granzyme B, and C5b-9 in villitis of unknown etiology

INTRODUCTION

Villitis of unknown etiology (VUE) is associated with fetal growth restriction. However, the underlying mechanisms of villous injury in placentas with VUE are still largely unknown. We aimed to verify whether apoptosis-related factors are increased in VUE placentas. Furthermore, we determined apoptosis of villous cells.

METHODS

Six placentas with VUE and 3 control placentas were stained using immunohistochemistry with antibodies for CD3, CD4, CD8, CD68, CD163, perforin, granzyme B, granzyme K, and C5b-9. TUNEL assay analysis was also performed with these placentas. The percentage of cells that stained positive, CD163/CD68 ratio, percentage of C5b-9 positive area, and apoptosis index were quantified and compared between the inflammatory lesions of the VUE placentas, non-VUE inflammatory lesions of the VUE placentas, and control placentas.

RESULTS

The percentages of CD3, CD4, CD8 CD68, CD163, perforin, and granzyme B positive cells were significantly higher in the inflammatory lesions of the VUE placentas (p < 0.05). The intravillous CD163/CD68 ratio was higher in the inflammatory lesions compared with the non-inflammatory lesion of the VUE placentas (p < 0.05). The percentage of granzyme K-positive cells was not significantly different between the groups. C5b-9 deposition was higher in the inflammatory lesions of the VUE placentas (p < 0.05). TUNEL-positive cells were significantly higher in the inflammatory lesions of the VUE placentas (p < 0.05).

DISCUSSION

To the best of our knowledge, this is the first report to assess villous injury, especially from a viewpoint of villous apoptosis in VUE placentas. An activated perforin/granzyme pathway and C5b-9 are suggested as possible mechanisms of apoptosis.

Granzyme B expression is enhanced in human monocytes by TLR8 agonists and contributes to antibody-dependent cellular cytotoxicity

FcγRs are critical mediators of mAb cancer therapies, because they drive cytotoxic processes upon binding of effector cells to opsonized targets. Along with NK cells, monocytes are also known to destroy Ab-coated targets via Ab-dependent cellular cytotoxicity (ADCC). However, the precise mechanisms by which monocytes carry out this function have remained elusive. In this article, we show that human monocytes produce the protease granzyme B upon both FcγR and TLR8 activation. Treatment with TLR8 agonists elicited granzyme B and also enhanced FcγR-mediated granzyme B production in an additive fashion. Furthermore, monocyte-mediated ADCC against cetuximab-coated tumor targets was enhanced by TLR8 agonist treatment, and this enhancement of ADCC required granzyme B. Hence we have identified granzyme B as an important mediator of FcγR function in human monocytes and have uncovered another mechanism by which TLR8 agonists may enhance FcγR-based therapies.

Hybrid, multiplexed, functional DNA nanotechnology for bioanalysis

DNA nanotechnology allows for the realization of novel multiplexed assays in bioanalytical sciences.

Metallomics insights into the programmed cell death induced by metal-based anticancer compounds

Since the discovery of cisplatin more than 40 years ago, enormous research efforts have been dedicated to developing metal-based anticancer agents and to elucidating the mechanisms involved in the action of these compounds. Abnormal metabolism and the evasion of apoptosis are important hallmarks of malignant transformation, and the induction of apoptotic cell death has been considered to be a main pathway by which cytotoxic metal complexes combat cancer. However, many cancers have cellular defects involving the apoptotic machinery, which results in an acquired resistance to apoptotic cell death and therefore reduced chemotherapeutic effectiveness. Over the past decade, it has been revealed that a growing number of cell death pathways induced by metal complexes are not dependent on apoptosis. Metal complexes specifically triggering these alternative cell death pathways have been identified and explored as novel cancer treatment options. In this review, we discuss recent examples of metallomics studies on the different types of cell death induced by metal-based anticancer drugs, especially on the three major forms of programmed cell death (PCD) in mammalian cells: apoptosis, autophagy and regulated necrosis, also called necroptosis.

Characterization of bispecific T-cell Engager (BiTE) antibodies with a high-capacity T-cell dependent cellular cytotoxicity (TDCC) assay

The Bispecific T-cell Engager (BiTE) antibody modality is a clinically validated immunotherapeutic approach for targeting tumors. Using T-cell dependent cellular cytotoxicity (TDCC) assays, we measure the percentage of specific cytotoxicity induced when a BiTE molecule engages T-cells, redirects T-cell mediated cytolysis, and ultimately kills target cells. We establish a novel luminescence-based TDCC assay quantified by measuring cell viability via constitutive expression of luciferase. The luciferase-based TDCC assay performance is valid and comparable to an adenosine triphosphate (ATP)-based detection method. We demonstrate that the luciferase-based TDCC assay is an efficient homogeneous assay format that is amenable to both suspension and adherent target cells. The luciferase-based TDCC assay eliminates the need for plate-washing protocols, allowing for higher-throughput screening of BiTE antibodies and better data quality. Assay capacity is also improved by performing serial dilutions of BiTE antibodies in 384-well format with an automated liquid handler. We describe here a robust, homogeneous TDCC assay platform with capacity for in vitro assessment of BiTE antibody potency and efficacy using multiple tumor cell lines and T-cell donors.

ITPR1 protects renal cancer cells against natural killer cells by inducing autophagy

Clear cell renal cell carcinomas (RCC) frequently display inactivation of von Hippel-Lindau (VHL) gene leading to increased level of hypoxia-inducible factors (HIF). In this study, we investigated the potential role of HIF2α in regulating RCC susceptibility to natural killer (NK) cell-mediated killing. We demonstrated that the RCC cell line 786-0 with mutated VHL was resistant to NK-mediated lysis as compared with the VHL-corrected cell line (WT7). This resistance was found to require HIF2α stabilization. On the basis of global gene expression profiling and chromatin immunoprecipitation assay, we found ITPR1 (inositol 1,4,5-trisphosphate receptor, type 1) as a direct novel target of HIF2α and that targeting ITPR1 significantly increased susceptibility of 786-0 cells to NK-mediated lysis. Mechanistically, HIF2α in 786-0 cells lead to overexpression of ITPR1, which subsequently regulated the NK-mediated killing through the activation of autophagy in target cells by NK-derived signal. Interestingly, both ITPR1 and Beclin-1 silencing in 786-0 cells inhibited NK-induced autophagy and subsequently increased granzyme B activity in target cells. Finally, in vivo ITPR1 targeting significantly enhanced the NK-mediated tumor regression. Our data provide insight into the link between HIF2α, the ITPR1-related pathway, and natural immunity and strongly suggest a role for the HIF2α/ITPR1 axis in regulating RCC cell survival.

Granzyme B-activated p53 interacts with Bcl-2 to promote cytotoxic lymphocyte-mediated apoptosis

Granzyme B (GzmB) plays a major role in CTLs and NK cell-mediated elimination of virus-infected cells and tumors. Human GzmB preferentially induces target cell apoptosis by cleaving the proapoptotic Bcl-2 family member Bid, which, together with Bax, induces mitochondrial outer membrane permeabilization. We previously showed that GzmB also induces a rapid accumulation of the tumor-suppressor protein p53 within target cells, which seems to be involved in GzmB-induced apoptosis. In this article, we show that GzmB-activated p53 accumulates on target cell mitochondria and interacts with Bcl-2. This interaction prevents Bcl-2 inhibitory effect on both Bax and GzmB-truncated Bid, and promotes GzmB-induced mitochondrial outer membrane permeabilization. Consequently, blocking p53-Bcl-2 interaction decreases GzmB-induced Bax activation, cytochrome c release from mitochondria, and subsequent effector caspases activation leading to a decreased sensitivity of target cells to both GzmB and CTL/NK-mediated cell death. Together, our results define p53 as a new important player in the GzmB apoptotic signaling pathway and in CTL/NK-induced apoptosis.

Immunohistochemical expression of perforin in lichen planus lesions

BACKGROUND

Lichen planus (LP) is a chronic inflammatory papulosquamous skin disease characterized by epidermal basal cell damage and a particular band-like infiltrate predominantly of T cells in the upper dermis. It is characterized by the formation of colloid bodies representing apoptotic keratinocytes. The apoptotic process mediated by CD8+ cytotoxic T lymphocytes and natural killer cells mainly involves two distinct pathways: the perforin/granzyme pathway and the Fas/FasL pathway. So far, little is known regarding the role of perforin-mediated apoptosis in LP.

AIM

Is to study the expression and distribution of perforin in the epidermis and dermis of lesional LP skin.

MATERIALS AND METHODS

Skin biopsy specimens from lesional skin of 31 patients with LP and 10 healthy persons were analyzed by immunohistochemistry.

RESULTS

Significant accumulation of perforin + cells was found in both epidermis and dermis of LP lesions compared with healthy skin. Perforin expression was significantly upregulated in the epidermis of LP lesions.

CONCLUSION

Accumulation of perforin + cells in the epidermis of LP lesions suggest a potential role of perforin in the apoptosis of basal keratinocytes.

An immunology primer for computational modelers

The immune system is designed to protect an organism from infection and damage caused by a pathogen. A successful immune response requires the coordinated function of multiple cell types and molecules in the innate and adaptive immune systems. Given the complexity of the immune system, it would be advantageous to build computational models to better understand immune responses and develop models to better guide the design of immunotherapies. Often, researchers with strong quantitative backgrounds do not have formal training in immunology. Therefore, the goal of this review article is to provide a brief primer on cellular immunology that is geared for computational modelers.

Perforin expression in plaque psoriasis: an immunohistochemical study

Psoriasis (PsO) is T-cell-mediated disease resulting from aberrant activation of both innate and adaptive immunity. Perforin is a multi-domain, pore-forming protein. It is located within the cytoplasm of CD 8 cytotoxic T cells (CTLs) and natural killer cells (NK). The aim of this study was to evaluate the immunohistochemical (IHC) expression of perforin in lesional and perilesional skin of chronic plaque psoriatic patient and correlate its expression with the standard clinico-pathological variables. This prospective case-control study was conducted on 50 PsO patients and 30 age- and gender-matched healthy subjects as a control group. There were high-significant differences between lesional and perilesional skin of plaque PsO patients as regards to IHC perforin status and localization (p < 0.001 for both). There was a high-significant difference between positive and negative perforin cases as regards to psoriasis area severity index (PASI) (p < 0.000). There were significant differences between mild and moderate-to-severe intensity of IHC perforin expression as regards to triggering factors and PASI (p = 0.02 and 0.03, respectively). Localization of IHC perforin positive lymphocytes in both epidermis and dermis was significantly associated with higher degree of acanthosis and higher degree of inflammatory infiltrates in comparison with positive cells located in dermis (p = 0.001 for both). Perforin might have a putative signaling in early and late plaque PsO. Plaque psoriatic patients with positive perforin expression could be a candidate for a future target therapy to stop the proposed scenario and achieve a therapeutic response.

Presence of perforin in endomyocardial biopsies of patients with inflammatory cardiomyopathy predicts poor outcome

AIMS

Intramyocardial inflammation is considered an adverse prognostic factor in inflammatory cardiomyopathy (CMi). However, the precise nature of immune system factors relevant for the prediction of long-term course remains elusive. The aim of this study was to analyse the prognostic relevance of perforin in a large cohort of patients with CMi.

METHODS AND RESULTS

We investigated 495 consecutive patients with suspected CMi, undergoing endomyocardial biopsies (EMBs), and examined haemodynamic measurements after a long follow-up period (interquartile range 10.2-37.1 months). In EMBs, myocardial inflammation was assessed by histology and immunohistology. At follow-up, 388 patients (Group I) showed stable mild dysfunction or significant improvement, with LVEF rising from 46.2 ± 14.8% to 64.3 ± 12.3% (P < 0.0001). Lack of improvement of LV function or significant deterioration of LVEF from 42.1 ± 14.2% to 32.3 ± 11.6% (P < 0.0001) was observed in 107 patients (Group II). Multivariable statistical analysis of LVEF and immunohistochemical parameters in all patients revealed that the single most important predictor of LVEF development was detection of perforin in EMBs, with an odds ratio (OR) of 7.922 [95% confidence interval (CI) 4.380-14.326; P < 0.001] for deteriorating LVEF. Importantly, baseline LVEF (OR 0.962), LV end-diastolic diameter (OR 1.847), and other immmunohistochemical parameters (CD3, Mac-1, CD45R0, LFA-1, HLA-1, and ICAM-1) made minor or insignificant contributions to LVEF course in these 495 patients.

CONCLUSIONS

In this EMB-based analysis of the long-term course of CMi we identified, for the first time, that detection of perforin in the myocardium is a key predictor of LVEF course.

Dealing with damage: plasma membrane repair mechanisms

Eukaryotic cells have developed repair mechanisms, which allow them to reseal their membrane in order to prevent the efflux of cytoplasmic constituents and the uncontrolled influx of calcium. After injury, the Ca(2+)-concentration gradient fulfils a dual function: it provides guidance cues for the repair machinery and directly activates the molecules, which have a repair function. Depending on the nature of injury, the morphology of the cell and the severity of injury, the membrane resealing can be effected by lysosomal exocytosis, microvesicle shedding or a combination of both. Likewise, exocytosis is often followed by the endocytic uptake of lesions. Additionally, since plasmalemmal resealing must be attempted, even after extensive injury in order to prevent cell lysis, the restoration of membrane integrity can be achieved by ceramide-driven invagination of the lipid bilayer, during which the cell is prepared for apoptotic disposal. Plasmalemmal injury can be contained by a surfeit of plasma membrane, which serves as a trap for toxic substances: either passively by an abundance of cellular protrusions, or actively by membrane blebbing.

Perforin oligomers form arcs in cellular membranes: a locus for intracellular delivery of granzymes

Perforin-mediated cytotoxicity is an essential host defense, in which defects contribute to tumor development and pathogenic disorders including autoimmunity and autoinflammation. How perforin (PFN) facilitates intracellular delivery of pro-apoptotic and inflammatory granzymes across the bilayer of targets remains unresolved. Here we show that cellular susceptibility to granzyme B (GzmB) correlates with rapid PFN-induced phosphatidylserine externalization, suggesting that pores are formed at a protein-lipid interface by incomplete membrane oligomers (or arcs). Supporting a role for these oligomers in protease delivery, an anti-PFN antibody (pf-80) suppresses necrosis but increases phosphatidylserine flip-flop and GzmB-induced apoptosis. As shown by atomic force microscopy on planar bilayers and deep-etch electron microscopy on mammalian cells, pf-80 increases the proportion of arcs which correlates with the presence of smaller electrical conductances, while large cylindrical pores decline. PFN appears to form arc structures on target membranes that serve as minimally disrupting conduits for GzmB translocation. The role of these arcs in PFN-mediated pathology warrants evaluation where they may serve as novel therapeutic targets.

Cytotoxic cells kill intracellular bacteria through granulysin-mediated delivery of granzymes

When killer lymphocytes recognize infected cells, perforin delivers cytotoxic proteases (granzymes) into the target cell to trigger apoptosis. What happens to intracellular bacteria during this process is unclear. Human, but not rodent, cytotoxic granules also contain granulysin, an antimicrobial peptide. Here, we show that granulysin delivers granzymes into bacteria to kill diverse bacterial strains. In Escherichia coli, granzymes cleave electron transport chain complex I and oxidative stress defense proteins, generating reactive oxygen species (ROS) that rapidly kill bacteria. ROS scavengers and bacterial antioxidant protein overexpression inhibit bacterial death. Bacteria overexpressing a GzmB-uncleavable mutant of the complex I subunit nuoF or strains that lack complex I still die, but more slowly, suggesting that granzymes disrupt multiple vital bacterial pathways. Mice expressing transgenic granulysin are better able to clear Listeria monocytogenes. Thus killer cells play an unexpected role in bacterial defense.

Structural design of disialoganglioside GD2 and CD3-bispecific antibodies to redirect T cells for tumor therapy

Antibody-based immunotherapy has proven efficacy for patients with high-risk neuroblastoma. However, despite being the most efficient tumoricidal effectors, T cells are underutilized because they lack Fc receptors. Using a monovalent single-chain fragment (ScFv) platform, we engineered tandem scFv bispecific antibodies (BsAbs) that specifically target disialoganglioside (GD2) on tumor cells and CD3 on T cells. Structural variants of BsAbs were constructed and ranked based on binding to GD2, and on competency in inducing T-cell-mediated tumor cytotoxicity. In vitro thermal stability and binding measurements were used to characterize each of the constructs, and in silico molecular modeling was used to show how the orientation of the variable region heavy (VH) and light (VL) chains of the anti-GD2 ScFv could alter the conformations of key residues responsible for high affinity binding. We showed that the VH-VL orientation, the (GGGGS)3 linker, disulfide bond stabilization of scFv, when combined with an affinity matured mutation provided the most efficient BsAb to direct T cells to lyse GD2-positive tumor cells. In vivo, the optimized BsAb could efficiently inhibit melanoma and neuroblastoma xenograft growth. These findings provide preclinical validation of a structure-based method to assist in designing BsAb for T-cell-mediated therapy.

Real-time detection of CTL function reveals distinct patterns of caspase activation mediated by Fas versus granzyme B

Activation of caspase-mediated apoptosis is reported to be a hallmark of both granzyme B- and Fas-mediated pathways of killing by CTLs; however, the kinetics of caspase activation remain undefined owing to an inability to monitor target cell-specific apoptosis in real time. We have overcome this limitation by developing a novel biosensor assay that detects continuous, protease-specific activity in target cells. Biosensors were engineered from a circularly permuted luciferase, linked internally by either caspase 3/7 or granzyme B/caspase 8 cleavage sites, thus allowing activation upon proteolytic cleavage by the respective proteases. Coincubation of murine CTLs with target cells expressing either type of biosensor led to a robust luminescent signal within minutes of cell contact. The signal was modulated by the strength of TCR signaling, the ratio of CTL/target cells, and the type of biosensor used. Additionally, the luciferase signal at 30 min correlated with target cell death, as measured by a (51)Cr-release assay. The rate of caspase 3/7 biosensor activation was unexpectedly rapid following granzyme B- compared with Fas-mediated signal induction in murine CTLs; the latter appeared gradually after a 90-min delay in perforin- or granzyme B-deficient CTLs. Remarkably, the Fas-dependent, caspase 3/7 biosensor signal induced by perforin-deficient human CTLs was also detectable after a 90-min delay when measured by redirected killing. Thus, we have used a novel, real-time assay to demonstrate the distinct pattern of caspase activation induced by granzyme B versus Fas in human and murine CTLs.

Incomplete pneumolysin oligomers form membrane pores

Pneumolysin is a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming proteins that are produced as water-soluble monomers or dimers, bind to target membranes and oligomerize into large ring-shaped assemblies comprising approximately 40 subunits and approximately 30 nm across. This pre-pore assembly then refolds to punch a large hole in the lipid bilayer. However, in addition to forming large pores, pneumolysin and other CDCs form smaller lesions characterized by low electrical conductance. Owing to the observation of arc-like (rather than full-ring) oligomers by electron microscopy, it has been hypothesized that smaller oligomers explain smaller functional pores. To investigate whether this is the case, we performed cryo-electron tomography of pneumolysin oligomers on model lipid membranes. We then used sub-tomogram classification and averaging to determine representative membrane-bound low-resolution structures and identified pre-pores versus pores by the presence of membrane within the oligomeric curve. We found pre-pore and pore forms of both complete (ring) and incomplete (arc) oligomers and conclude that arc-shaped oligomeric assemblies of pneumolysin can form pores. As the CDCs are evolutionarily related to the membrane attack complex/perforin family of proteins, which also form variably sized pores, our findings are of relevance to that class of proteins as well.

The perforin pore facilitates the delivery of cationic cargos

Cytotoxic lymphocytes eliminate virally infected or neoplastic cells through the action of cytotoxic proteases (granzymes). The pore-forming protein perforin is essential for delivery of granzymes into the cytoplasm of target cells; however the mechanism of this delivery is incompletely understood. Perforin contains a membrane attack complex/perforin (MACPF) domain and oligomerizes to form an aqueous pore in the plasma membrane; therefore the simplest (and best supported) model suggests that granzymes passively diffuse through the perforin pore into the cytoplasm of the target cell. Here we demonstrate that perforin preferentially delivers cationic molecules while anionic and neutral cargoes are delivered inefficiently. Furthermore, another distantly related pore-forming MACPF protein, pleurotolysin (from the oyster mushroom), also favors the delivery of cationic molecules, and efficiently delivers human granzyme B. We propose that this facilitated diffusion is due to conserved features of oligomerized MACPF proteins, which may include an anionic lumen.

Co-expression of perforin and granzyme B genes induces apoptosis and inhibits the tumorigenicity of laryngeal cancer cell line Hep-2

Granzyme B and perforin, two of the most important components, have shown anticancer properties in various cancers, but their effects in laryngeal cancer remain unexplored. Here we decided to examine the effects of Granzyme B and perforin in Hep-2 cells and clarify the role of perforin and granzyme B in the tumorigenicity of laryngeal cancer cell line. Hep-2 cells were transfected with pVAX1-PIG co-expression vector (comprising perforin and granzyme B genes), and then the growth and apoptosis of these Hep-2 cells were evaluated. The tumorigenicity of Hep-2 cell line co-expressing perforin and granzyme B genes was tested in BALB/c nu/nu mice. We found that the co-expression of perforin and granzyme B genes could obviously inhibit cell focus formation and induce cell apoptosis in Hep-2 cells. Furthermore, after subcutaneous injection of Hep-2 cells transfected with pVAX1-PIG, an extensive delay in tumor growth was observed in BALB/c-nu/nu mice. Moreover, our studies demonstrated that the anticancer activity of perforin and granzyme B was sustainable in vivo as tumor development by inducing cell apoptosis. Taken together, our data indicate that the co-expression of perforin and granzyme B genes exhibits anticancer potential, and hopefully provide potential therapeutic applications in laryngeal cancer.

Fluorescence imaging of MACPF/CDC proteins: new techniques and their application

Structural and biochemical investigations have helped illuminate many of the important details of MACPF/CDC pore formation. However, conventional techniques are limited in their ability to tackle many of the remaining key questions, and new biophysical techniques might provide the means to improve our understanding. Here we attempt to identify the properties of MACPF/CDC proteins that warrant further study, and explore how new developments in fluorescence imaging are able to probe these properties.

Mucopolysaccharide diseases: a complex interplay between neuroinflammation, microglial activation and adaptive immunity

Mucopolysaccharide (MPS) diseases are lysosomal storage disorders (LSDs) caused by deficiencies in enzymes required for glycosaminoglycan (GAG) catabolism. Mucopolysaccharidosis I (MPS I), MPS IIIA, MPS IIIB and MPS VII are deficient in the enzymes α-L-Iduronidase, Heparan-N-Sulphatase, N-Acetylglucosaminidase and Beta-Glucuronidase, respectively. Enzyme deficiency leads to the progressive multi-systemic build-up of heparan sulphate (HS) and dermatan sulphate (DS) within cellular lysosomes, followed by cell, tissue and organ damage and in particular neurodegeneration. Clinical manifestations of MPS are well established; however as lysosomes represent vital components of immune cells, it follows that lysosomal accumulation of GAGs could affect diverse immune functions and therefore influence disease pathogenesis. Theoretically, MPS neurodegeneration and GAGs could be substantiating a threat of danger and damage to alert the immune system for cellular clearance, which due to the progressive nature of MPS storage would propagate disease pathogenesis. Innate immunity appears to have a key role in MPS; however the extent of adaptive immune involvement remains to be elucidated. The current literature suggests a complex interplay between neuroinflammation, microglial activation and adaptive immunity in MPS disease.

Membrane interactions and cellular effects of MACPF/CDC proteins

The cell membrane is crucial for protection of the cell from its environment. MACPF/CDC proteins are a large superfamily known to be essential for bacterial pathogenesis and proper functioning of the immune system. The three most studied groups of MACPF/CDC proteins are cholesterol-dependent cytolysins from bacteria, the membrane attack complex of complement and human perforin. Their primary function is to form transmembrane pores in target cell membranes. The common mechanism of action comprises water-soluble monomeric proteins binding to the host cell membrane, oligomerization, and formation of a functional pore. This causes a disturbance in gradients of ions and other molecules across the membrane and can lead to cell death. Cells react to this form of attack in a complex manner. Responses can be general, like removing the perforated part of the membrane, or more specific, in many cases depending on binding of proteins to specific receptors to trigger various signalling cascades.

Perforin: a key pore-forming protein for immune control of viruses and cancer

Perforin (PFN) is the key pore-forming molecule in the cytotoxic granules of immune killer cells. Expressed only in killer cells, PFN is the rate-limiting molecule for cytotoxic function, delivering the death-inducing granule serine proteases (granzymes) into target cells marked for immune elimination. In this chapter we describe our current understanding of how PFN accomplishes this task. We discuss where PFN is expressed and how its expression is regulated, the biogenesis and storage of PFN in killer cells and how they are protected from potential damage, how it is released, how it delivers Granzymes into target cells and the consequences of PFN deficiency.

Immunotoxins constructed with ribosome-inactivating proteins and their enhancers: a lethal cocktail with tumor specific efficacy

The term ribosome-inactivating protein (RIP) is used to denominate proteins mostly of plant origin, which have N-glycosidase enzymatic activity leading to a complete destruction of the ribosomal function. The discovery of the RIPs was almost a century ago, but their usage has seen transition only in the last four decades. With the advent of antibody therapy, the RIPs have been a subject of extensive research especially in targeted tumor therapies, which is the primary focus of this review. In the present work we enumerate 250 RIPs, which have been identified so far. An attempt has been made to identify all the RIPs that have been used for the construction of immunotoxins, which are conjugates or fusion proteins of an antibody or ligand with a toxin. The data from 1960 onwards is reviewed in this paper and an extensive list of more than 450 immunotoxins is reported. The clinical reach of tumor-targeted toxins has been identified and detailed in the work as well. While there is a lot of potential that RIPs embrace for targeted tumor therapies, the success in preclinical and clinical evaluations has been limited mainly because of their inability to escape the endo/lysosomal degradation. Various strategies that can increase the efficacy and lower the required dose for targeted toxins have been compiled in this article. It is plausible that with the advancements in platform technologies or improved endosomal escape the usage of tumor targeted RIPs would see the daylight of clinical success.

Gap junction intercellular communications regulate NK cell activation and modulate NK cytotoxic capacity

Gap junctions (GJs) mediate intercellular communication between adjacent cells. Previously, we showed that connexin 43 (Cx43), the main GJ protein in the immune system, mediates Ag transfer between human dendritic cells (DCs) and is recruited to the immunological synapse during T cell priming. This crosstalk contributed to T cell activation, intracellular Ca(2+) responses, and cytokine release. However, the role of GJs in NK cell activation by DCs and NK cell-mediated cytotoxicity against tumor cells remains unknown. In this study, we found polarization of Cx43 at the NK/DC and NK/tumor cell-contact sites, accompanied by the formation of functional GJs between NK/DCs and NK/tumor cells, respectively. Cx43-GJ-mediated intercellular communication (GJIC) between human NK and DCs was bidirectional. Blockage of Cx43-GJIC inhibited NK cell activation, though it affected neither the phenotype nor the function of DCs. Cx43 knockdown or inhibition using mimetic peptides greatly reduced CD69 and CD25 expression and IFN-γ release by DC-stimulated NK cells. Moreover, blocking Cx43 strongly inhibited the NK cell-mediated tumor cell lysis associated with inhibition of granzyme B activity and Ca(2+) influx. Our data identify a novel and active role for Cx43-GJIC in human NK cell activation and antitumor effector functions that may be important for the design of new immune therapeutic strategies.

Distinct T-cell responses when BCG vaccination is delayed from birth to 6 weeks of age in Ugandan infants

BACKGROUND

In Uganda, the tuberculosis vaccine BCG is administered on the first day of life. Infants delivered at home receive BCG vaccine at their first healthcare facility visit at 6 weeks of age. Our aim was to determine the effect of this delay in BCG vaccination on the induced immune response.

METHODS

We assessed CD4(+) and CD8(+) T-cell responses with a 12-hour whole-blood intracellular cytokine/cytotoxic marker assay, and with a 6-day proliferation assay.

RESULTS

We enrolled 92 infants: 50 had received BCG vaccine at birth and 42 at 6 weeks of age. Birth vaccination was associated with (1) greater induction of CD4(+) and CD8(+) T cells expressing either interferon γ (IFN-γ) alone or IFN-γ together with perforin and (2) induction of proliferating cells that had greater capacity to produce IFN-γ, tumor necrosis factor α (TNF-α), and interleukin 2 together, compared with delayed vaccination.

CONCLUSIONS

Distinct patterns of T-cell induction occurred when BCG vaccine was given at birth and at 6 weeks of age. We propose that this diversity might impact protection against tuberculosis. Our results differ from those of studies of delayed BCG vaccination in South Africa and the Gambia, suggesting that geographical and population heterogeneity may affect the BCG vaccine-induced T-cell response.

Rapid reuptake of granzyme B leads to emperitosis: an apoptotic cell-in-cell death of immune killer cells inside tumor cells

A cell-in-cell process refers to the invasion of one living cell into another homotypic or heterotypic cell. Different from non-apoptotic death processes of internalized cells termed entosis or cannibalism, we previously reported an apoptotic cell-in-cell death occurring during heterotypic cell-in-cell formation. In this study, we further demonstrated that the apoptotic cell-in-cell death occurred only in internalized immune killer cells expressing granzyme B (GzmB). Vacuole wrapping around the internalized cells inside the target cells was the common hallmark during the early stage of all cell-in-cell processes, which resulted in the accumulation of reactive oxygen species and subsequent mitochondrial injury of encapsulated killer or non-cytotoxic immune cells. However, internalized killer cells mediated rapid bubbling of the vacuoles with the subsequent degranulation of GzmB inside the vacuole of the target cells and underwent the reuptake of GzmB by killer cells themselves. The confinement of GzmB inside the vacuole surpassed the lysosome-mediated cell death occurring in heterotypic or homotypic entosis processes, resulting in a GzmB-triggered caspase-dependent apoptotic cell-in-cell death of internalized killer cells. On the contrary, internalized killer cells from GzmB-deficient mice underwent a typical non-apoptotic entotic cell-in-cell death similar to that of non-cytotoxic immune cells or tumor cells. Our results thus demonstrated the critical involvement of immune cells with cytotoxic property in apoptotic cell-in-cell death, which we termed as emperitosis taken from emperipolesis and apoptosis. Whereas entosis or cannibalism may serve as a feed-on mechanism to exacerbate and nourish tumor cells, emperitosis of immune killer cells inside tumor cells may serve as an in-cell danger sensation model to prevent the killing of target cells from inside, implying a unique mechanism for tumor cells to escape from immune surveillance.

Granzyme B degradation by autophagy decreases tumor cell susceptibility to natural killer-mediated lysis under hypoxia

Recent studies demonstrated that autophagy is an important regulator of innate immune response. However, the mechanism by which autophagy regulates natural killer (NK) cell-mediated antitumor immune responses remains elusive. Here, we demonstrate that hypoxia impairs breast cancer cell susceptibility to NK-mediated lysis in vitro via the activation of autophagy. This impairment was not related to a defect in target cell recognition by NK cells but to the degradation of NK-derived granzyme B in autophagosomes of hypoxic cells. Inhibition of autophagy by targeting beclin1 (BECN1) restored granzyme B levels in hypoxic cells in vitro and induced tumor regression in vivo by facilitating NK-mediated tumor cell killing. Together, our data highlight autophagy as a mechanism underlying the resistance of hypoxic tumor cells to NK-mediated lysis. The work presented here provides a cutting-edge advance in our understanding of the mechanism by which hypoxia-induced autophagy impairs NK-mediated lysis in vitro and paves the way for the formulation of more effective NK cell-based antitumor therapies.

Live or let die: posttranscriptional gene regulation in cell stress and cell death

Studies of the regulation of gene expression historically focused on transcription. However, during stress and apoptosis, profound gene expression changes occur more rapidly and globally than is possible by regulating transcription. Posttranscriptional changes in mRNA processing and translation in response to diverse stresses shut down most protein translation to conserve energy and lead to rapid remodeling of the proteome to promote repair. Pre-mRNA splicing and mRNA stability are fundamentally altered under some stress conditions. Stress pathways coordinate a cytoprotective repair response, while simultaneously initiating signaling that can ultimately trigger cell death. How the cell mediates the decision between repair and apoptosis is largely not understood. In some stresses, microRNAs may tip the balance. Here, we review what is known about posttranscriptional gene regulation during stress, focusing on what is still unknown and how new technologies might be used to understand what changes are most physiologically important in different forms of stress and death.

Caveolae internalization repairs wounded cells and muscle fibers

Rapid repair of plasma membrane wounds is critical for cellular survival. Muscle fibers are particularly susceptible to injury, and defective sarcolemma resealing causes muscular dystrophy. Caveolae accumulate in dystrophic muscle fibers and caveolin and cavin mutations cause muscle pathology, but the underlying mechanism is unknown. Here we show that muscle fibers and other cell types repair membrane wounds by a mechanism involving Ca²⁺-triggered exocytosis of lysosomes, release of acid sphingomyelinase, and rapid lesion removal by caveolar endocytosis. Wounding or exposure to sphingomyelinase triggered endocytosis and intracellular accumulation of caveolar vesicles, which gradually merged into larger compartments. The pore-forming toxin SLO was directly visualized entering cells within caveolar vesicles, and depletion of caveolin inhibited plasma membrane resealing. Our findings directly link lesion removal by caveolar endocytosis to the maintenance of plasma membrane and muscle fiber integrity, providing a mechanistic explanation for the muscle pathology associated with mutations in caveolae proteins.

DOI:http://dx.doi.org/10.7554/eLife.00926.001

Cells must be able to rapidly repair damage to their outer membranes. This is particularly important in the case of muscle cells, which are vulnerable to damage, and the failure of these cells to repair their outer membranes leads to the muscle wastage seen in muscular dystrophy. Researchers do not fully understand how cells repair membrane, but one popular theory is that they use the membranes of specialized vesicles to ‘patch’ areas that have been damaged.

A group of proteins called caveolins have also been implicated in membrane repair but, again, the details have not been worked out. These proteins are best known for their role in the formation of caveolae — small pouches formed by invaginated sections of the plasma membrane. Now, Corrotte et al. have obtained evidence that membrane repair relies not on patching, but on endocytosis (the process by which substances are taken into the cell in small vesicles that ‘pinch’ from the plasma membrane) of these caveolae pouches.

Corrotte et al. treated cells with streptolysin O, a toxin that forms pores in the membrane that cannot be repaired using patches, and found that this led to the formation of small membrane-derived vesicles that looked just like caveolae. Further tests confirmed that these vesicles contained caveolar proteins, and that they removed the toxin from the plasma membrane by endocytosis. Similar effects were seen in response to mechanical damage caused by tiny glass beads. Moreover, blocking the expression of caveolar genes prevented cells from repairing membrane damage.

Based on their findings, Corrotte et al. propose an alternative model for the repair process; namely that cellular damage triggers an influx of calcium ions, which causes vesicles called lysosomes to release chemicals that promote the formation of caveolae. These then remove the damaged area through endocytosis, restoring the integrity of the membrane. The results offer new insights into why mutations in caveolar proteins are associated with muscle disorders, including muscular dystrophy and cardiac dysfunction.

DOI:http://dx.doi.org/10.7554/eLife.00926.002

The impact of ageing on natural killer cell function and potential consequences for health in older adults

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Roles are emerging for natural killer (NK) cells beyond removing transformed cells.

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These include immune regulation and the elimination of senescent cells.

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Human ageing is associated with a decline in NK cell function.

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We propose some aspects of human ageing are due in part to reduced NK cell function.

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These include reduced vaccination efficacy and delayed resolution of inflammation.

Forming the first line of defence against virally infected and malignant cells, natural killer (NK) cells are critical effector cells of the innate immune system. With age, significant impairments have been reported in the two main mechanisms by which NK cells confer host protection: direct cytotoxicity and the secretion of immunoregulatory cytokines and chemokines. In elderly subjects, decreased NK cell activity has been shown to be associated with an increased incidence and severity of viral infection, highlighting the clinical implications that age-associated changes in NK cell biology have on the health of older adults. However, is an increased susceptibility to viral infection the only consequence of these age-related changes in NK cell function? Recently, evidence has emerged that has shown that in addition to eliminating transformed cells, NK cells are involved in many other biological processes such as immune regulation, anti-microbial immune responses and the recognition and elimination of senescent cells, novel functions that involve NK-mediated cytotoxicity and/or cytokine production. Thus, the decrease in NK cell function that accompanies physiological ageing is likely to have wider implications for the health of older adults than originally thought. Here, we give a detailed description of the changes in NK cell biology that accompany human ageing and propose that certain features of the ageing process such as: (i) the increased reactivation rates of latent Mycobacterium tuberculosis, (ii) the slower resolution of inflammatory responses and (iii) the increased incidence of bacterial and fungal infection are attributable in part to an age-associated decline in NK cell function.

Immunohistochemical expression of granzyme B and perforin in discoid lupus erythematosus

Discoid lupus erythematosus (DLE) is a chronic photosensitive dermatosis characterized by scarring and atrophy. Granzyme B is a serine protease found in the cytoplasmic granules of cytotoxic lymphocytes and natural killer cells. Perforin permits delivery of the cytotoxic granzymes A and B into target cells to induce apoptosis and cause target cell death. The current study investigated the expression of granzyme B and perforin in 25 cases of DLE and in 10 cases of normal skin by immunohistochemistry and correlated their expression with the clinicopathological features in the studied DLE group. Both granzyme B and perforin were expressed in DLE with absent expression in normal skin. They were parallelly expressed in DLE where granzyme B was associated with features of chronicity such as old age (p = 0.05) and long duration of the disease (p = 0.05). Perforin expression in DLE was associated with male gender (p = 0.04) and outdoor workers (p = 0.04). Finally, expression of both granzyme B and perforin in dermal lymphocytic inflammatory infiltrate in DLE may indicate the cytotoxicity of the infiltrate. The parallel expression of both molecules may refer to the cooperative relationship between them to enhance cytotoxicity. Higher expression of granzyme B than perforin may indicate the presence of other pathways for granzyme B release independent from perforin.

Visualization of cytolytic T cell differentiation and granule exocytosis with T cells from mice expressing active fluorescent granzyme B

To evaluate acquisition and activation of cytolytic functions during immune responses we generated knock in (KI) mice expressing Granzyme B (GZMB) as a fusion protein with red fluorescent tdTomato (GZMB-Tom). As for GZMB in wild type (WT) lymphocytes, GZMB-Tom was absent from naïve CD8 and CD4 T cells in GZMB-Tom-KI mice. It was rapidly induced in most CD8 T cells and in a subpopulation of CD4 T cells in response to stimulation with antibodies to CD3/CD28. A fraction of splenic NK cells expressed GZMB-Tom ex vivo with most becoming positive upon culture in IL-2. GZMB-Tom was present in CTL granules and active as a protease when these degranulated into cognate target cells, as shown with target cells expressing a specific FRET reporter construct. Using T cells from mice expressing GZMB-Tom but lacking perforin, we show that the transfer of fluorescent GZMB-Tom into target cells was dependent on perforin, favoring a role for perforin in delivery of GZMB at the target cells' plasma membranes. Time-lapse video microscopy showed Ca++ signaling in CTL upon interaction with cognate targets, followed by relocalization of GZMB-Tom-containing granules to the synaptic contact zone. A perforin-dependent step was next visualized by the fluorescence signal from the non-permeant dye TO-PRO-3 at the synaptic cleft, minutes before the labeling of the target cell nucleus, characterizing a previously undescribed synaptic event in CTL cytolysis. Transferred OVA-specific GZMB-Tom-expressing CD8 T cells acquired GZMB-Tom expression in Listeria monocytogenes-OVA infected mice as soon as 48h after infection. These GZMB-Tom positive CD8 T cells localized in the splenic T-zone where they interacted with CD11c positive dendritic cells (DC), as shown by GZMB-Tom granule redistribution to the T/DC contact zone. GZMB-Tom-KI mice thus also provide tools to visualize acquisition and activation of cytolytic function in vivo.