Mouse and human granzyme B have distinct tetrapeptide specificities and abilities to recruit the bid pathway

Identification of a novel caspase cleavage motif AEAD

Infections of many viruses induce caspase activation to regulate multiple cellular pathways, including programmed cell death, immune signaling and etc. Characterizations of caspase cleavage sites and substrates are important for understanding the regulation mechanisms of caspase activation. Here, we identified and analyzed a novel caspase cleavage motif AEAD, and confirmed its caspase dependent cleavage activity in natural substrate, such as nitric oxide-associated protein 1 (NOA1). Fusing the enhanced green fluorescent protein (EGFP) with the mitochondrial marker protein Tom20 through the AEAD motif peptide localized EGFP to the mitochondria. Upon the activation of caspase triggered by Sendai virus (SeV) or herpes simplex virus type 1 (HSV-1) infection, EGFP diffusely localized to the cell due to the caspase-mediated cleavage, thus allowing visual detection of the virus-induced caspase activation. An AEAD peptide-derived inhibitor Z-AEAD-FMK were developed, which significantly inhibited the activities of caspases-1, -3, -6, -7, -8 and -9, exhibiting a broad caspase inhibition effect. The inhibitor further prevented caspases-mediated cleavage of downstream substrates, including BID, PARP1, LMNA, pro-IL-1β, pro-IL-18, GSDMD and GSDME, protecting cells from virus-induced apoptotic and pyroptotic cell death. Together, our findings provide a new perspective for the identification of novel caspase cleavage motifs and the development of new caspase inhibitors and anti-inflammatory drugs.

Granzyme B-activated IL18 potentiates αβ and γδ CAR T cell immunotherapy in a tumor-dependent manner

Interleukin (IL)18 is a potent pro-inflammatory cytokine that is activated upon caspase 1 cleavage of the latent precursor, pro-IL18. Therapeutic T cell armoring with IL18 promotes autocrine stimulation and positive modulation of the tumor microenvironment (TME). However, existing strategies are imperfect since they involve constitutive/poorly regulated activity or fail to modify the TME. Here, we have substituted the caspase 1 cleavage site within pro-IL18 with that preferred by granzyme B, yielding GzB-IL18. We demonstrate that GzB-IL18 is constitutively released but remains functionally latent unless chimeric antigen receptor (CAR) T cells are activated, owing to concomitant granzyme B release. Armoring with GzB-IL18 enhances cytolytic activity, proliferation, interferon (IFN)-γ release, and anti-tumor efficacy by a similar magnitude to constitutively active IL18. We also demonstrate that GzB-IL18 provides a highly effective armoring strategy for γδ CAR T cells, leading to enhanced metabolic fitness and significant potentiation of therapeutic activity. Finally, we show that constitutively active IL18 can unmask CAR T cell-mediated cytokine release syndrome in immunocompetent mice. By contrast, GzB-IL18 promotes anti-tumor activity and myeloid cell re-programming without inducing such toxicity. Using this stringent system, we have tightly coupled the biological activity of IL18 to the activation state of the host CAR T cell, favoring safer clinical implementation of this technology.

PET Imaging for Monitoring Cellular and Immunotherapy of Cancer

ABSTRACT

Cancer immunotherapy, including checkpoint blockade and cellular therapy, has become a cornerstone in cancer treatment. However, understanding the factors driving patient response or resistance to these therapies remains challenging. The dynamic interplay between the immune system and tumors requires new approaches for characterization. Biopsies and blood tests provide valuable information, but their limitations have led to increased interest in positron emission tomography (PET)/computed tomography imaging to complement these strategies. The noninvasive nature of PET imaging makes it ideal for monitoring the dynamic tumor immune microenvironment. This review discusses various PET imaging approaches, including immune cell lineage markers, immune functional markers, immune cell metabolism, direct cell labeling, and reporter genes, highlighting their potential in targeted immunotherapies and cell-based approaches. Although PET imaging has limitations, its integration into diagnostic strategies holds promise for improving patient outcomes and accelerating drug development in cancer immunotherapy.

Granzymes in health and diseases: the good, the bad and the ugly

Granzymes are a family of serine proteases, composed of five human members: GA, B, H, M and K. They were first discovered in the 1980s within cytotoxic granules released during NK cell- and T cell-mediated killing. Through their various proteolytic activities, granzymes can trigger different pathways within cells, all of which ultimately lead to the same result, cell death. Over the years, the initial consideration of granzymes as mere cytotoxic mediators has changed due to surprising findings demonstrating their expression in cells other than immune effectors as well as new intracellular and extracellular activities. Additional roles have been identified in the extracellular milieu, following granzyme escape from the immunological synapse or their release by specific cell types. Outside the cell, granzyme activities mediate extracellular matrix alteration via the degradation of matrix proteins or surface receptors. In certain contexts, these processes are essential for tissue homeostasis; in others, excessive matrix degradation and extensive cell death contribute to the onset of chronic diseases, inflammation, and autoimmunity. Here, we provide an overview of both the physiological and pathological roles of granzymes, highlighting their utility while also recognizing how their unregulated presence can trigger the development and/or worsening of diseases.

A Programmable Peptidic Hydrogel Adjuvant for Personalized Immunotherapy in Resected Stage Tumors

Adjuvant treatment after surgical resection usually plays an important role in delaying disease recurrence. Immunotherapy displays encouraging results in increasing patients' chances of staying cancer-free after surgery, as reported by recent clinical trials. However, the clinical outcomes of current immunotherapy need to be improved due to the limited responses, patient heterogeneity, nontargeted distribution, and immune-related adverse effects. This work describes a programmable hydrogel adjuvant for personalized immunotherapy after surgical resection. By filling the hydrogel in the cavity, this system aims to address the limited secretion of granzyme B (GrB) during immunotherapy and improve the low immunotherapy responses typically observed, while minimizing immune-related side effects. The TLR7/8 agonist imidazoquinoline (IMDQ) is linked to the self-assembling peptide backbone through a GrB-responsive linkage. Its release could enhance the activation and function of immune cells, which will lead to increased secretion of GrB and enhance the effectiveness of immunotherapy together. The hydrogel adjuvant recruits immune cells, initiates dendritic cell maturation, and induces M1 polarized macrophages to reverse the immunosuppressive tumor microenvironment in situ. In multiple murine tumor models, the hydrogel adjuvant suppresses tumor growth, increases animal survival and long-term immunological memory, and protects mice against tumor rechallenge, leading to effective prophylactic and therapeutic responses. This work provides a potential chemical strategy to overcome the limitations associated with immunotherapy.

Recent Advances in the Development of Non-Invasive Imaging Probes for Cancer Immunotherapy

The last two decades have witnessed a major revolution in the field of tumor immunology including clinical progress using various immunotherapy strategies. These advances have highlighted the potential for approaches that harness the power of the immune system to fight against cancer. While cancer immunotherapies have shown significant clinical successes, patient responses vary widely due to the complex and heterogeneous nature of tumors and immune responses, calling for reliable biomarkers and therapeutic strategies to maximize the benefits of immunotherapy. Especially, stratifying responding individuals from non-responders during the early stages of treatment could help avoid long-term damage and tailor personalized treatments. In efforts to develop non-invasive means for accurately evaluating and predicting tumor response to immunotherapy, multiple affinity-based agents targeting immune cell markers and checkpoint molecules have been developed and advanced to clinical trials. In addition, researchers have recently turned their attention to substrate and activity-based imaging probes that can provide real-time, functional assessment of immune response to treatment. Here, we highlight some of those recently designed probes that image functional proteases as biomarkers of cancer immunotherapy with a focus on their chemical design and detection modalities and discuss challenges and opportunities for the development of imaging tools utilized in cancer immunotherapy.

Imaging and monitoring of granzyme B in the immune response

Significant progress has been made in tumor immunotherapy that uses the human immune response to kill and remove tumor cells. However, overreactive immune response could lead to various autoimmune diseases and acute rejection. Accurate and specific monitoring of immune responses in these processes could help select appropriate therapies and regimens for the patient and could reduce the risk of side effects. Granzyme B (GzmB) is an ideal biomarker for immune response, and its peptide substrate could be coupled with fluorescent dyes or contrast agents for the synthesis of imaging probes activated by GzmB. These small molecules and nanoprobes based on PET, bioluminescence imaging, or fluorescence imaging have proved to be highly GzmB specific and accuracy. This review summarizes the design of different GzmB-responsive imaging probes and their applications in monitoring of tumor immunotherapy and overreactive immune response. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Massively-multiplexed epitope mapping techniques for viral antigen discovery

Following viral infection, viral antigens bind specifically to receptors on the surface of lymphocytes thereby activating adaptive immunity in the host. An epitope, the smallest structural and functional unit of an antigen, binds specifically to an antibody or antigen receptor, to serve as key sites for the activation of adaptive immunity. The complexity and diverse range of epitopes are essential to study and map for the diagnosis of disease, the design of vaccines and for immunotherapy. Mapping the location of these specific epitopes has become a hot topic in immunology and immune therapy. Recently, epitope mapping techniques have evolved to become multiplexed, with the advent of high-throughput sequencing and techniques such as bacteriophage-display libraries and deep mutational scanning. Here, we briefly introduce the principles, advantages, and disadvantages of the latest epitope mapping techniques with examples for viral antigen discovery.

Synthesis and evaluation of 111 In-labeled tetrapeptide-based compounds as single-photon emission computed tomography imaging probes targeting granzyme B

Granzyme B is an attractive target as a biomarker for contributing to improve the treatment with immune checkpoint inhibitor (ICI). In this study, we designed novel 111 In-labeled granzyme B-targeting single-photon emission computed tomography (SPECT) imaging probes, [111 In]IDT and [111 In]IDAT. Nonradioactive In-labeled granzyme B-targeting compounds ([nat In]IDT, [nat In]IDAT) showed the affinity for recombinant mouse granzyme B. [111 In]IDT and [111 In]IDAT were obtained with moderate radiochemical yield and high stability in mouse plasma (>95%). In a biodistribution experiment using tumor-bearing mice, [111 In]IDT and [111 In]IDAT showed moderate accumulation in tumor. Ex vivo autoradiography (ARG) indicated that the accumulation of radioactivity in tumor was correlated to expression of granzyme B confirmed by the immunohistochemical staining. These results indicated that [111 In]IDT and [111 In]IDAT showed the basic properties as granzyme B-targeting SPECT probes.

Quantifying requirements for mitochondrial apoptosis in CAR T killing of cancer cells

Chimeric antigen receptor (CAR) T cell therapy is an FDA-approved treatment for several hematologic malignancies, yet not all patients respond to this treatment. While some resistance mechanisms have been identified, cell death pathways in target cancer cells remain underexplored. Impairing mitochondrial apoptosis via knockout of Bak and Bax, forced Bcl-2 and Bcl-XL expression, or caspase inhibition protected several tumor models from CAR T killing. However, impairing mitochondrial apoptosis in two liquid tumor cell lines did not protect target cells from CAR T killing. We found that whether a cell was Type I or Type II in response to death ligands explained the divergence of these results, so that mitochondrial apoptosis was dispensable for CART killing of cells that were Type I but not Type II. This suggests that the apoptotic signaling induced by CAR T cells bears important similarities to that induced by drugs. Combinations of drug and CAR T therapies will therefore require tailoring to the specific cell death pathways activated by CAR T cells in different types of cancer cells.

Activatable near-infrared probes for the detection of specific populations of tumour-infiltrating leukocytes in vivo and in urine

Tracking the immune microenvironment of tumours is essential for understanding the mechanisms behind the effectiveness of cancer immunotherapies. Molecular imaging of tumour-infiltrating leukocytes (TILs) can be used to non-invasively monitor the tumour immune microenvironment, but current imaging agents do not distinguish TILs from leukocytes resident in other tissues. Here we report a library of activatable molecular probes for the imaging, via near-infrared fluorescence, of specific TILs (including M1 macrophages, cytotoxic T lymphocytes and neutrophils) in vivo in real time and also via excreted urine, owing to the probes' renal clearance. The fluorescence of the probes is activated only in the presence of both tumour and leukocyte biomarkers, which allows for the imaging of populations of specific TILs in mouse models of cancers with sensitivities and specificities similar to those achieved via flow-cytometric analyses of biopsied tumour tissues. We also show that the probes enable the non-invasive evaluation of the immunogenicity of different tumours, the dynamic monitoring of responses to immunotherapies and the accurate prediction of tumour growth under various treatments.

In vivo bioluminescence imaging of granzyme B activity in tumor response to cancer immunotherapy

Cancer immunotherapy has revolutionized the treatment of cancer, but only a small subset of patients benefits from this new treatment regime. Imaging tools are useful for early detection of tumor response to immunotherapy and probing the dynamic and complex immune system. Here, we report a bioluminescence probe (GBLI-2) for non-invasive, real-time, longitudinal imaging of granzyme B activity in tumors receiving immune checkpoint inhibitors. GBLI-2 is made of the mouse granzyme B tetrapeptide IEFD substrate conjugated to D-luciferin through a self-immolative group. GBLI-2 was evaluated for imaging the dynamics of the granzyme B activity and predicting therapeutic efficacy in a syngeneic mouse model of CT26 murine colorectal carcinoma. The GBLI-2 signal correlated with the change in the population of PD-1- and granzyme B-expressing CD8+ T cells in tumors.

Noninvasive interrogation of CD8+ T cell effector function for monitoring tumor early responses to immunotherapy

Accurately identifying patients who respond to immunotherapy remains clinically challenging. A noninvasive method that can longitudinally capture information about immune cell function and assist in the early assessment of tumor responses is highly desirable for precision immunotherapy. Here, we show that PET imaging using a granzyme B–targeted radiotracer named ⁶⁸Ga-grazytracer, could noninvasively and effectively predict tumor responses to immune checkpoint inhibitors and adoptive T cell transfer therapy in multiple tumor models. ⁶⁸Ga-grazytracer was designed and selected from several radiotracers based on non-aldehyde peptidomimetics, and exhibited excellent in vivo metabolic stability and favorable targeting efficiency to granzyme B secreted by effector CD8⁺ T cells during immune responses. ⁶⁸Ga-grazytracer permitted more sensitive discrimination of responders and nonresponders than did ¹⁸F-fluorodeoxyglucose, distinguishing between tumor pseudoprogression and true progression upon immune checkpoint blockade therapy in mouse models with varying immunogenicity. In a preliminary clinical trial with 5 patients, no adverse events were observed after ⁶⁸Ga-grazytracer injection, and clinical responses in cancer patients undergoing immunotherapy were favorably correlated with ⁶⁸Ga-grazytracer PET results. These results highlight the potential of ⁶⁸Ga-grazytracer PET to enhance the clinical effectiveness of granzyme B secretion–related immunotherapies by supporting early response assessment and precise patient stratification in a noninvasive and longitudinal manner.

Host protease activity classifies pneumonia etiology

Community-acquired pneumonia (CAP) has been brought to the forefront of global health priorities due to the COVID-19 pandemic. However, classification of viral versus bacterial pneumonia etiology remains a significant clinical challenge. To this end, we have engineered a panel of activity-based nanosensors that detect the dysregulated activity of pulmonary host proteases implicated in the response to pneumonia-causing pathogens and produce a urinary readout of disease. The nanosensor targets were selected based on a human protease transcriptomic signature for pneumonia etiology generated from 33 unique publicly available study cohorts. Five mouse models of bacterial or viral CAP were developed to assess the ability of the nanosensors to produce etiology-specific urinary signatures. Machine learning algorithms were used to train diagnostic classifiers that could distinguish infected mice from healthy controls and differentiate those with bacterial versus viral pneumonia with high accuracy. This proof-of-concept diagnostic approach demonstrates a way to distinguish pneumonia etiology based solely on the host proteolytic response to infection.

Multiparameter Longitudinal Imaging of Immune Cell Activity in Chimeric Antigen Receptor T Cell and Checkpoint Blockade Therapies

Longitudinal multimodal imaging presents unique opportunities for noninvasive surveillance and prediction of treatment response to cancer immunotherapy. In this work we first designed a novel granzyme B activated self-assembly small molecule, G-SNAT, for the assessment of cytotoxic T lymphocyte mediated cancer cell killing. G-SNAT was found to specifically detect the activity of granzyme B within the cytotoxic granules of activated T cells and engaged cancer cells in vitro. In lymphoma tumor-bearing mice, the retention of cyanine 5 labeled G-SNAT-Cy5 correlated to CAR T cell mediated granzyme B exocytosis and tumor eradication. In colorectal tumor-bearing transgenic mice with hematopoietic cells expressing firefly luciferase, longitudinal bioluminescence and fluorescence imaging revealed that after combination treatment of anti-PD-1 and anti-CTLA-4, the dynamics of immune cell trafficking, tumor infiltration, and cytotoxic activity predicted the therapeutic outcome before tumor shrinkage was evident. These results support further development of G-SNAT for imaging early immune response to checkpoint blockade and CAR T-cell therapy in patients and highlight the utility of multimodality imaging for improved mechanistic insights into cancer immunotherapy.

Radionuclide Imaging of Cytotoxic Immune Cell Responses to Anti-Cancer Immunotherapy

Cancer immunotherapy is an evolving and promising cancer treatment that takes advantage of the body’s immune system to yield effective tumor elimination. Importantly, immunotherapy has changed the treatment landscape for many cancers, resulting in remarkable tumor responses and improvements in patient survival. However, despite impressive tumor effects and extended patient survival, only a small proportion of patients respond, and others can develop immune-related adverse events associated with these therapies, which are associated with considerable costs. Therefore, strategies to increase the proportion of patients gaining a benefit from these treatments and/or increasing the durability of immune-mediated tumor response are still urgently needed. Currently, measurement of blood or tissue biomarkers has demonstrated sampling limitations, due to intrinsic tumor heterogeneity and the latter being invasive. In addition, the unique response patterns of these therapies are not adequately captured by conventional imaging modalities. Consequently, non-invasive, sensitive, and quantitative molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using specific radiotracers, have been increasingly used for longitudinal whole-body monitoring of immune responses. Immunotherapies rely on the effector function of CD8⁺ T cells and natural killer cells (NK) at tumor lesions; therefore, the monitoring of these cytotoxic immune cells is of value for therapy response assessment. Different immune cell targets have been investigated as surrogate markers of response to immunotherapy, which motivated the development of multiple imaging agents. In this review, the targets and radiotracers being investigated for monitoring the functional status of immune effector cells are summarized, and their use for imaging of immune-related responses are reviewed along their limitations and pitfalls, of which multiple have already been translated to the clinic. Finally, emerging effector immune cell imaging strategies and future directions are provided.

Emerging Canonical and Non-Canonical Roles of Granzyme B in Health and Disease

The Granzyme (Gzm) family has classically been recognized as a cytotoxic tool utilized by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells to illicit cell death to infected and cancerous cells. Their importance is established based on evidence showing that deficiencies in these cell death executors result in defective immune responses. Recent findings have shown the importance of Granzyme B (GzmB) in regulatory immune cells, which may contribute to tumor growth and immune evasion during cancer development. Other studies have shown that members of the Gzm family are important for biological processes such as extracellular matrix remodeling, angiogenesis and organized vascular degradation. With this growing body of evidence, it is becoming more important to understand the broader function of Gzm's rather than a specific executor of cell death, and we should be aware of the many alternative roles that Gzm's play in physiological and pathological conditions. Therefore, we review the classical as well as novel non-canonical functions of GzmB and discuss approaches to utilize these new findings to address current gaps in our understanding of the immune system and tissue development.

Urinary detection of early responses to checkpoint blockade and of resistance to it via protease-cleaved antibody-conjugated sensors

Immune checkpoint blockade (ICB) therapy does not benefit the majority of treated patients, and those who respond to the therapy can become resistant to it. Here we report the design and performance of systemically administered protease activity sensors conjugated to anti-programmed cell death protein 1 (αPD1) antibodies for the monitoring of antitumour responses to ICB therapy. The sensors consist of a library of mass-barcoded protease substrates that, when cleaved by tumour proteases and immune proteases, are released into urine, where they can be detected by mass spectrometry. By using syngeneic mouse models of colorectal cancer, we show that random forest classifiers trained on mass spectrometry signatures from a library of αPD1-conjugated mass-barcoded activity sensors for differentially expressed tumour proteases and immune proteases can be used to detect early antitumour responses and discriminate resistance to ICB therapy driven by loss-of-function mutations in either the B2m or Jak1 genes. Biomarkers of protease activity may facilitate the assessment of early responses to ICB therapy and the classification of refractory tumours based on resistance mechanisms.

Improved Purification of Human Granzyme A/B and Granulysin Using a Mammalian Expression System

Cytotoxic lymphocytes release proteins contained within the cytoplasmic cytolytic granules after recognition of infected or tumor target cells. These cytotoxic granular proteins (namely granzymes, granulysin, and perforin) are key immunological mediators within human cellular immunity. The availability of highly purified cytotoxic proteins has been fundamental for understanding their function in immunity and mechanistic involvement in sepsis and autoimmunity. Methods for recovery of native cytotoxic proteins can be problematic leading to: 1) the co-purification of additional proteins, confounding interpretation of function, and 2) low yields of highly purified proteins. Recombinant protein expression of individual cytolytic components can overcome these challenges. The use of mammalian expression systems is preferred for optimal post-translational modifications and avoidance of endotoxin contamination. Some of these proteins have been proposed for host directed human therapies (e.g. - granzyme A), or treatment of systemic infections or tumors as in granulysin. We report here a novel expression system using HEK293T cells for cost-effective purification of high yields of human granzymes (granzyme A and granzyme B) and granulysin with enhanced biological activity than previous reports. The resulting proteins are free of native contaminants, fold correctly, and remain enzymatically active. Importantly, these improvements have also led to the first purification of biologically active recombinant human granulysin in high yields from a mammalian system. This method can be used as a template for purification of many other secreted cellular proteins and may lead to advances for human medicine.

Oxidative and Non-Oxidative Antimicrobial Activities of the Granzymes

Cell-mediated cytotoxicity is an essential immune defense mechanism to fight against viral, bacterial or parasitic infections. Upon recognition of an infected target cell, killer lymphocytes form an immunological synapse to release the content of their cytotoxic granules. Cytotoxic granules of humans contain two membrane-disrupting proteins, perforin and granulysin, as well as a homologous family of five death-inducing serine proteases, the granzymes. The granzymes, after delivery into infected host cells by the membrane disrupting proteins, may contribute to the clearance of microbial pathogens through different mechanisms. The granzymes can induce host cell apoptosis, which deprives intracellular pathogens of their protective niche, therefore limiting their replication. However, many obligate intracellular pathogens have evolved mechanisms to inhibit programed cells death. To overcome these limitations, the granzymes can exert non-cytolytic antimicrobial activities by directly degrading microbial substrates or hijacked host proteins crucial for the replication or survival of the pathogens. The granzymes may also attack factors that mediate microbial virulence, therefore directly affecting their pathogenicity. Many mechanisms applied by the granzymes to eliminate infected cells and microbial pathogens rely on the induction of reactive oxygen species. These reactive oxygen species may be directly cytotoxic or enhance death programs triggered by the granzymes. Here, in the light of the latest advances, we review the antimicrobial activities of the granzymes in regards to their cytolytic and non-cytolytic activities to inhibit pathogen replication and invasion. We also discuss how reactive oxygen species contribute to the various antimicrobial mechanisms exerted by the granzymes.

Why do human B cells secrete granzyme B? Insights into a novel B-cell differentiation pathway

B cells are generally believed to operate as producers of high affinity antibodies to defend the body against microorganisms, whereas cellular cytotoxicity is considered as an exclusive prerogative of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). In conflict with this dogma, recent studies have demonstrated that the combination of interleukin-21 (IL-21) and B-cell receptor (BCR) stimulation enables B cells to produce and secrete the active form of the cytotoxic serine protease granzyme B (GrB). Although the production of GrB by B cells is not accompanied by that of perforin as in the case of many other GrB-secreting cells, recent findings suggest GrB secretion by B cells may play a significant role in early antiviral immune responses, in the regulation of autoimmune responses, and in cancer immunosurveillance. Here, we discuss in detail how GrB-secreting B cells may influence a variety of immune processes. A better understanding of the role that GrB-secreting B cells are playing in the immune system may allow for the development and improvement of novel immunotherapeutic approaches against infectious, autoimmune and malignant diseases.

Granzyme B in Inflammatory Diseases: Apoptosis, Inflammation, Extracellular Matrix Remodeling, Epithelial-to-Mesenchymal Transition and Fibrosis

Inflammation is strictly interconnected to anti-inflammatory mechanisms to maintain tissue homeostasis. The disruption of immune homeostasis can lead to acute and chronic inflammatory diseases, as cardiovascular, pulmonary, metabolic diseases and cancer. The knowledge of the mechanisms involved in the development and progression of these pathological conditions is important to find effective therapies. Granzyme B (GrB) is a serine protease produced by a variety of immune, non-immune and tumor cells. Apoptotic intracellular and multiple extracellular functions of GrB have been recently identified. Its capability of cleaving extracellular matrix (ECM) components, cytokines, cell receptors and clotting proteins, revealed GrB as a potential multifunctional pro-inflammatory molecule with the capability of contributing to the pathogenesis of different inflammatory conditions, including inflammaging, acute and chronic inflammatory diseases and cancer. Here we give an overview of recent data concerning GrB activity on multiple targets, potentially allowing this enzyme to regulate a wide range of crucial biological processes that play a role in the development, progression and/or severity of inflammatory diseases. We focus our attention on the promotion by GrB of perforin-dependent and perforin-independent (anoikis) apoptosis, inflammation derived by the activation of some cytokines belonging to the IL-1 cytokine family, ECM remodeling, epithelial-to-mesenchymal transition (EMT) and fibrosis. A greater comprehension of the pathophysiological consequences of GrB-mediated multiple activities may favor the design of new therapies aim to inhibit different inflammatory pathological conditions such as inflammaging and age-related diseases, EMT and organ fibrosis.

SAM50, a side door to the mitochondria: The case of cytotoxic proteases

SAM50, a 7-8 nm diameter β-barrel channel of the mitochondrial outer membrane, is the central channel of the sorting and assembly machinery (SAM) complex involved in the biogenesis of β-barrel proteins. Interestingly, SAM50 is not known to have channel translocase activity; however, we have recently found that this channel is necessary and sufficient for mitochondrial entry of cytotoxic proteases. Cytotoxic lymphocytes eliminate cells that pose potential hazards, such as virus- and bacteria-infected cells as well as cancer cells. They induce cell death following the delivery of granzyme cytotoxic proteases into the cytosol of the target cell. Although granzyme A and granzyme B (GA and GB), the best characterized of the five human granzymes, trigger very distinct apoptotic cascades, they share the ability to directly target the mitochondria. GA and GB do not have a mitochondrial targeting signal, yet they enter the target cell mitochondria to disrupt respiratory chain complex I and induce mitochondrial reactive oxygen species (ROS)-dependent cell death. We found that granzyme mitochondrial entry requires SAM50 and the translocase of the inner membrane 22 (TIM22). Preventing granzymes' mitochondrial entry compromises their cytotoxicity, indicating that this event is unexpectedly an important step for cell death. Although mitochondria are best known for their roles in cell metabolism and energy conversion, these double-membrane organelles are also involved in Ca²⁺ homeostasis, metabolite transport, cell cycle regulation, cell signaling, differentiation, stress response, redox homeostasis, aging, and cell death. This multiplicity of functions is matched with the complexity and plasticity of the mitochondrial proteome as well as the organelle's morphological and structural versatility. Indeed, mitochondria are extremely dynamic and undergo fusion and fission events in response to diverse cellular cues. In humans, there are 1500 different mitochondrial proteins, the vast majority of which are encoded in the nuclear genome and translated by cytosolic ribosomes, after which they must be imported and properly addressed to the right mitochondrial compartment. To this end, mitochondria are equipped with a very sophisticated and highly specific protein import machinery. The latter is centered on translocase complexes embedded in the outer and inner mitochondrial membranes working along five different import pathways. We will briefly describe these import pathways to put into perspective our finding regarding the ability of granzymes to enter the mitochondria.

T-Scan: A Genome-wide Method for the Systematic Discovery of T Cell Epitopes

T cell recognition of specific antigens mediates protection from pathogens and controls neoplasias, but can also cause autoimmunity. Our knowledge of T cell antigens and their implications for human health is limited by the technical limitations of T cell profiling technologies. Here, we present T-Scan, a high-throughput platform for identification of antigens productively recognized by T cells. T-Scan uses lentiviral delivery of antigen libraries into cells for endogenous processing and presentation on major histocompatibility complex (MHC) molecules. Target cells functionally recognized by T cells are isolated using a reporter for granzyme B activity, and the antigens mediating recognition are identified by next-generation sequencing. We show T-Scan correctly identifies cognate antigens of T cell receptors (TCRs) from viral and human genome-wide libraries. We apply T-Scan to discover new viral antigens, perform high-resolution mapping of TCR specificity, and characterize the reactivity of a tumor-derived TCR. T-Scan is a powerful approach for studying T cell responses.

Killer cell proteases can target viral immediate-early proteins to control human cytomegalovirus infection in a noncytotoxic manner

Human cytomegalovirus (HCMV) is the most frequent viral cause of congenital defects and can trigger devastating disease in immune-suppressed patients. Cytotoxic lymphocytes (CD8⁺ T cells and NK cells) control HCMV infection by releasing interferon-γ and five granzymes (GrA, GrB, GrH, GrK, GrM), which are believed to kill infected host cells through cleavage of intracellular death substrates. However, it has recently been demonstrated that the in vivo killing capacity of cytotoxic T cells is limited and multiple T cell hits are required to kill a single virus-infected cell. This raises the question whether cytotoxic lymphocytes can use granzymes to control HCMV infection in a noncytotoxic manner. Here, we demonstrate that (primary) cytotoxic lymphocytes can block HCMV dissemination independent of host cell death, and interferon-α/β/γ. Prior to killing, cytotoxic lymphocytes induce the degradation of viral immediate-early (IE) proteins IE1 and IE2 in HCMV-infected cells. Intriguingly, both IE1 and/or IE2 are directly proteolyzed by all human granzymes, with GrB and GrM being most efficient. GrB and GrM cleave IE1 after Asp³⁹⁸ and Leu⁴¹⁴, respectively, likely resulting in IE1 aberrant cellular localization, IE1 instability, and functional impairment of IE1 to interfere with the JAK-STAT signaling pathway. Furthermore, GrB and GrM cleave IE2 after Asp¹⁸⁴ and Leu¹⁷³, respectively, resulting in IE2 aberrant cellular localization and functional abolishment of IE2 to transactivate the HCMV UL112 early promoter. Taken together, our data indicate that cytotoxic lymphocytes can also employ noncytotoxic ways to control HCMV infection, which may be explained by granzyme-mediated targeting of indispensable viral proteins during lytic infection.

Human cytomegalovirus (HCMV) is the leading viral cause of congenital defects, can trigger disease in immune-compromised patients, and plays roles in cancer development. Cytotoxic lymphocytes kill HCMV-infected cells via releasing a set of five cytotoxic serine proteases called granzymes. However, the killing capacity of cytotoxic cells is limited and multiple T cell hits are required to kill a single virus-infected cell. This raises the question whether cytotoxic lymphocytes can use granzymes to control HCMV infection in a noncytotoxic manner. Here, we show that cytotoxic lymphocytes can also use granzymes to inhibit HCMV replication in absence of cell death. All five granzymes cleave and inactivate both viral immediate-early (IE1/2) proteins, which are essential players for initiating HCMV infection. Our data support the model that cytotoxic cells employ granzymes to dampen HCMV replication prior to accumulation of sufficient hits to kill the infected cell.

Design of a Human Rhinovirus-14 3C Protease-Inducible Caspase-3

The engineering of enzymes for the purpose of controlling their activity represents a valuable approach to address challenges in both fundamental and applied research. Here, we describe and compare different design strategies for the generation of a human rhinovirus-14 (HRV14) 3C protease-inducible caspase-3 (CASP3). We exemplify the application potential of the resulting protease by controlling the activity of a synthetic enzyme cascade, which represents an important motif for the design of artificial signal transduction networks. In addition, we use our engineered CASP3 to characterize the effect of aspartate mutations on enzymatic activity. Besides the identification of mutations that render the enzyme inactive, we find the CASP3-D192E mutant (aspartate-to-glutamate exchange at position 192) to be inaccessible for 3C protease-mediated cleavage. This indicates a structural change of CASP3 that goes beyond a slight misalignment of the catalytic triad. This study could inspire the design of additional engineered proteases that could be used to unravel fundamental research questions or to expand the collection of biological parts for the design of synthetic signaling pathways.

Mitochondrial Entry of Cytotoxic Proteases: A New Insight into the Granzyme B Cell Death Pathway

The mitochondria represent an integration and amplification hub for various death pathways including that mediated by granzyme B (GB), a granule enzyme expressed by cytotoxic lymphocytes. GB activates the proapoptotic B cell CLL/lymphoma 2 (Bcl-2) family member BH3-interacting domain death agonist (BID) to switch on the intrinsic mitochondrial death pathway, leading to Bcl-2-associated X protein (Bax)/Bcl-2 homologous antagonist/killer- (Bak-) dependent mitochondrial outer membrane permeabilization (MOMP), the dissipation of mitochondrial transmembrane potential (ΔΨm), and the production of reactive oxygen species (ROS). GB can also induce mitochondrial damage in the absence of BID, Bax, and Bak, critical for MOMP, indicating that GB targets the mitochondria in other ways. Interestingly, granzyme A (GA), GB, and caspase 3 can all directly target the mitochondrial respiratory chain complex I for ROS-dependent cell death. Studies of ROS biogenesis have revealed that GB must enter the mitochondria for ROS production, making the mitochondrial entry of cytotoxic proteases (MECP) an unexpected critical step in the granzyme death pathway. MECP requires an intact ΔΨm and is mediated though Sam50 and Tim22 channels in a mtHSP70-dependent manner. Preventing MECP severely compromises GB cytotoxicity. In this review, we provide a brief overview of the canonical mitochondrial death pathway in order to put into perspective this new insight into the GB action on the mitochondria to trigger ROS-dependent cell death.

Mechanisms of natural killer cell-mediated cellular cytotoxicity

Cellular cytotoxicity, the ability to kill other cells, is an important effector mechanism of the immune system to combat viral infections and cancer. Cytotoxic T cells and natural killer (NK) cells are the major mediators of this activity. Here, we summarize the cytotoxic mechanisms of NK cells. NK cells can kill virally infected of transformed cells via the directed release of lytic granules or by inducing death receptor-mediated apoptosis via the expression of Fas ligand or TRAIL. The biogenesis of perforin and granzymes, the major components of lytic granules, is a highly regulated process to prevent damage during the synthesis of these cytotoxic molecules. Additionally, NK cells have developed several strategies to protect themselves from the cytotoxic activity of granular content upon degranulation. While granule-mediated apoptosis is a fast process, death receptor-mediated cytotoxicity requires more time. Current data suggest that these 2 cytotoxic mechanisms are regulated during the serial killing activity of NK cells. As many modern approaches of cancer immunotherapy rely on cellular cytotoxicity for their effectiveness, unraveling these pathways will be important to further progress these therapeutic strategies.

Non-invasive early detection of acute transplant rejection via nanosensors of granzyme B activity

The early detection of the onset of transplant rejection is critical for the long-term survival of patients. The diagnostic gold standard for detecting transplant rejection involves a core biopsy, which is invasive, has limited predictive power and carries a morbidity risk. Here, we show that nanoparticles conjugated with a peptide substrate specific for the serine protease granzyme B, which is produced by recipient T cells during the onset of acute cellular rejection, can serve as a non-invasive biomarker of early rejection. When administered systemically in mouse models of skin graft rejection, these nanosensors preferentially accumulate in allograft tissue, where they are cleaved by granzyme B, releasing a fluorescent reporter that filters into the recipient's urine. Urinalysis then discriminates the onset of rejection with high sensitivity and specificity before features of rejection are apparent in grafted tissues. Moreover, in mice treated with subtherapeutic levels of immunosuppressive drugs, the reporter signals in urine can be detected before graft failure. This method may enable routine monitoring of allograft status without the need for biopsies.

Single-Fluorescent Protein Reporters Allow Parallel Quantification of Natural Killer Cell-Mediated Granzyme and Caspase Activities in Single Target Cells

Natural killer (NK) cells eliminate infected and tumorigenic cells through delivery of granzymes via perforin pores or by activation of caspases via death receptors. In order to understand how NK cells combine different cell death mechanisms, it is important to quantify target cell responses on a single cell level. However, currently existing reporters do not allow the measurement of several protease activities inside the same cell. Here, we present a strategy for the comparison of two different proteases at a time inside individual target cells upon engagement by NK cells. We developed single-fluorescent protein reporters containing the RIEAD or the VGPD cleavage site for the measurement of granzyme B activity. We show that these two granzyme B reporters can be applied in combination with caspase-8 or caspase-3 reporters. While we did not find that caspase-8 was activated by granzyme B, our method revealed that caspase-3 activity follows granzyme B activity with a delay of about 6 min. Finally, we illustrate the comparison of several different reporters for granzyme A, M, K, and H. The approach presented here is a valuable means for the investigation of the temporal evolution of cell death mediated by cytotoxic lymphocytes.

Antigen-specific primed cytotoxic T cells eliminate tumour cells in vivo and prevent tumour development, regardless of the presence of anti-apoptotic mutations conferring drug resistance

Cytotoxic CD8⁺ T (Tc) cells are the main executors of transformed and cancer cells during cancer immunotherapy. The latest clinical results evidence a high efficacy of novel immunotherapy agents that modulate Tc cell activity against bad prognosis cancers. However, it has not been determined yet whether the efficacy of these treatments can be affected by selection of tumoural cells with mutations in the cell death machinery, known to promote drug resistance and cancer recurrence. Here, using a model of prophylactic tumour vaccination based on the LCMV-gp33 antigen and the mouse EL4 T lymphoma, we analysed the molecular mechanism employed by Tc cells to eliminate cancer cells in vivo and the impact of mutations in the apoptotic machinery on tumour development. First of all, we found that Tc cells, and perf and gzmB are required to efficiently eliminate EL4.gp33 cells after LCMV immunisation during short-term assays (1-4 h), and to prevent tumour development in the long term. Furthermore, we show that antigen-pulsed chemoresistant EL4 cells overexpressing Bcl-X_L or a dominant negative form of caspase-3 are specifically eliminated from the peritoneum of infected animals, as fast as parental EL4 cells. Notably, antigen-specific Tc cells control the tumour growth of the mutated cells, as efficiently as in the case of parental cells. Altogether, expression of the anti-apoptotic mutations does not confer any advantage for tumour cells neither in the short-term survival nor in long-term tumour formation. Although the mechanism involved in the elimination of the apoptosis-resistant tumour cells is not completely elucidated, neither necroptosis nor pyroptosis seem to be involved. Our results provide the first experimental proof that chemoresistant cancer cells with mutations in the main cell death pathways are efficiently eliminated by Ag-specific Tc cells in vivo during immunotherapy and, thus, provide the molecular basis to treat chemoresistant cancer cells with CD8 Tc-based immunotherapy.

Granzyme B Disrupts Central Metabolism and Protein Synthesis in Bacteria to Promote an Immune Cell Death Program

Human cytotoxic lymphocytes kill intracellular microbes. The cytotoxic granule granzyme proteases released by cytotoxic lymphocytes trigger oxidative bacterial death by disrupting electron transport, generating superoxide anion and inactivating bacterial oxidative defenses. However, they also cause non-oxidative cell death because anaerobic bacteria are also killed. Here, we use differential proteomics to identify granzyme B substrates in three unrelated bacteria: Escherichia coli, Listeria monocytogenes, and Mycobacteria tuberculosis. Granzyme B cleaves a highly conserved set of proteins in all three bacteria, which function in vital biosynthetic and metabolic pathways that are critical for bacterial survival under diverse environmental conditions. Key proteins required for protein synthesis, folding, and degradation are also substrates, including multiple aminoacyl tRNA synthetases, ribosomal proteins, protein chaperones, and the Clp system. Because killer cells use a multipronged strategy to target vital pathways, bacteria may not easily become resistant to killer cell attack.

Granzyme B PET Imaging as a Predictive Biomarker of Immunotherapy Response

While cancer immunotherapy can produce dramatic responses, only a minority of patients respond to treatment. Reliable response biomarkers are needed to identify responders, and conventional imaging modalities have not proved adequate. Here, we provide a preclinical proof of concept for the use of granzyme B, a downstream effector of tumoral cytotoxic T cells, as an early biomarker for tumors responding to immunotherapy. We designed novel PET imaging probes for the murine and human granzyme B isoforms that specifically and quantitatively bind granzyme B. Immunotherapy-treated mice were imaged prior to therapy-induced tumor volume reduction. Imaging distinguished treated responders from nonresponders with excellent predictive ability. To assess the clinical value of a granzyme B imaging paradigm, biopsy specimens from melanoma patients on checkpoint inhibitor therapy were analyzed. A marked differential in granzyme B expression was observed between treated responders and nonresponders. Additionally, our human probe was able to specifically detect granzyme B expression in human samples, providing a clear candidate for clinical application. Overall, our results suggest granzyme B PET imaging can serve as a quantitatively useful predictive biomarker for efficacious responses to cancer immunotherapy. Cancer Res; 77(9); 2318-27. ©2017 AACR.

Granzyme B enters the mitochondria in a Sam50-, Tim22- and mtHsp70-dependent manner to induce apoptosis

We have found that granzyme B (GB)-induced apoptosis also requires reactive oxygen species resulting from the alteration of mitochondrial complex I. How GB, which does not possess a mitochondrial targeting sequence, enter this organelle is unknown. We show that GB enters the mitochondria independently of the translocase of the outer mitochondrial membrane complex, but requires instead Sam50, the central subunit of the sorting and assembly machinery that integrates outer membrane β-barrel proteins. Moreover, GB breaches the inner membrane through Tim22, the metabolite carrier translocase pore, in a mitochondrial heat-shock protein 70 (mtHsp70)-dependent manner. Granzyme A (GA) and caspase-3 use a similar route to the mitochondria. Finally, preventing GB from entering the mitochondria either by mutating lysine 243 and arginine 244 or depleting Sam50 renders cells more resistant to GB-mediated reactive oxygen species and cell death. Similarly, Sam50 depletion protects cells from GA-, GM- and caspase-3-mediated cell death. Therefore, cytotoxic molecules enter the mitochondria to induce efficiently cell death through a noncanonical Sam50-, Tim22- and mtHsp70-dependent import pathway.

Autoantigens as Partners in Initiation and Propagation of Autoimmune Rheumatic Diseases

Systemic autoimmune diseases are characterized by specific targeting of a limited group of ubiquitously expressed autoantigens by the immune system. This review examines the mechanisms underlying their selection as immune targets. Initiation of autoimmune responses likely reflects the presentation of antigens with a distinct structure not previously encountered by the immune system, in a proimmune context (injury, malignancy, or infection). Causes of modified structure include somatic mutation and posttranslational modifications (including citrullination and proteolysis). Many autoantigens are components of multimolecular complexes, and some of the other components may provide adjuvant activity. Propagation of autoimmune responses appears to reflect a bidirectional interaction between the immune response and the target tissues in a mutually reinforcing cycle: Immune effector pathways generate additional autoantigen, which feeds further immune response. We propose that this resonance may be a critical principle underlying disease propagation, with specific autoantigens functioning as the hubs around which amplification occurs.

Imaging Granzyme B Activity Assesses Immune-Mediated Myocarditis

RATIONALE

The development of molecular imaging approaches that assess specific immunopathologic mechanisms can advance the study of myocarditis.

OBJECTIVE

This study validates a novel molecular imaging tool that enables the in vivo visualization of granzyme B activity, a major effector of cytotoxic CD8+ T lymphocytes.

METHODS AND RESULTS

We synthesized and optimized a fluorogenic substrate capable of reporting on granzyme B activity and examined its specificity ex vivo in mice hearts with experimental cytotoxic CD8+ T lymphocyte-mediated myocarditis using fluorescence reflectance imaging, validated by histological examination. In vivo experiments localized granzyme B activity in hearts with acute myocarditis monitored by fluorescent molecular tomography in conjunction with coregistered computed tomography imaging. A model anti-inflammatory intervention (dexamethasone administration) in vivo reduced granzyme B activity (vehicle versus dexamethasone: 504±263 versus 194±77 fluorescence intensities in hearts; P=0.002).

CONCLUSIONS

Molecular imaging of granzyme B activity can visualize T cell-mediated myocardial injury and monitor the response to an anti-inflammatory intervention.

Mouse cytotoxic T cell-derived granzyme B activates the mitochondrial cell death pathway in a Bim-dependent fashion

Cytotoxic T cells (Tc) use perforin and granzyme B (gzmB) to kill virus-infected cells and cancer cells. Recent evidence suggests that human gzmB primarily induces apoptosis via the intrinsic mitochondrial pathway by either cleaving Bid or activating Bim leading to the activation of Bak/Bax and subsequent generation of active caspase-3. In contrast, mouse gzmB is thought to predominantly induce apoptosis by directly processing pro-caspase-3. However, in certain mouse cell types gzmB-mediated apoptosis mainly occurs via the mitochondrial pathway. To investigate whether Bim is involved under the latter conditions, we have now employed ex vivo virus-immune mouse Tc that selectively kill by using perforin and gzmB (gzmB(+)Tc) as effector cells and wild type as well as Bim- or Bak/Bax-deficient spontaneously (3T9) or virus-(SV40) transformed mouse embryonic fibroblast cells as targets. We show that gzmB(+)Tc-mediated apoptosis (phosphatidylserine translocation, mitochondrial depolarization, cytochrome c release, and caspase-3 activation) was severely reduced in 3T9 cells lacking either Bim or both Bak and Bax. This outcome was related to the ability of Tc cells to induce the degradation of Mcl-1 and Bcl-XL, the anti-apoptotic counterparts of Bim. In contrast, gzmB(+)Tc-mediated apoptosis was not affected in SV40-transformed mouse embryonic fibroblast cells lacking Bak/Bax. The data provide evidence that Bim participates in mouse gzmB(+)Tc-mediated apoptosis of certain targets by activating the mitochondrial pathway and suggest that the mode of cell death depends on the target cell. Our results suggest that the various molecular events leading to transformation and/or immortalization of cells have an impact on their relative resistance to the multiple gzmB(+)Tc-induced death pathways.

A natural genetic variant of granzyme B confers lethality to a common viral infection

Many immune response genes are highly polymorphic, consistent with the selective pressure imposed by pathogens over evolutionary time, and the need to balance infection control with the risk of auto-immunity. Epidemiological and genomic studies have identified many genetic variants that confer susceptibility or resistance to pathogenic micro-organisms. While extensive polymorphism has been reported for the granzyme B (GzmB) gene, its relevance to pathogen immunity is unexplored. Here, we describe the biochemical and cytotoxic functions of a common allele of GzmB (GzmB^W) common in wild mouse. While retaining ‘Asp-ase’ activity, GzmB^W has substrate preferences that differ considerably from GzmB^P, which is common to all inbred strains. In vitro, GzmB^W preferentially cleaves recombinant Bid, whereas GzmB^P activates pro-caspases directly. Recombinant GzmB^W and GzmB^P induced equivalent apoptosis of uninfected targets cells when delivered with perforin in vitro. Nonetheless, mice homozygous for GzmB^W were unable to control murine cytomegalovirus (MCMV) infection, and succumbed as a result of excessive liver damage. Although similar numbers of anti-viral CD8 T cells were generated in both mouse strains, GzmB^W-expressing CD8 T cells isolated from infected mice were unable to kill MCMV-infected targets in vitro. Our results suggest that known virally-encoded inhibitors of the intrinsic (mitochondrial) apoptotic pathway account for the increased susceptibility of GzmB^W mice to MCMV. We conclude that different natural variants of GzmB have a profound impact on the immune response to a common and authentic viral pathogen.

Granzymes (Gzm) are serine proteases expressed by cytotoxic T cells and natural killer cells, and are important for the destruction of virally infected cells. To date, the function of these molecules has been assessed exclusively in common laboratory mouse strains that express identical granzyme proteins. In wild mouse populations, variants of granzyme B have been identified, but how these function, especially in the context of infections, is unknown. We have generated a novel mouse strain expressing a granzyme B variant found in wild mice (GzmB^W), and exposed these mice to viral infections. The substrates cleaved by GzmB^W were found to differ significantly from those cleaved by the GzmB^P protein, which is normally expressed by laboratory mice. Alterations in substrate specificity resulted in GzmB^W mice being significantly more susceptible to infection with murine cytomegalovirus, a common mouse pathogen. Our findings demonstrate that polymorphisms in granzyme B can profoundly affect the outcome of infections with some viral pathogens.

A colorimetric assay that specifically measures Granzyme B proteolytic activity: hydrolysis of Boc-Ala-Ala-Asp-S-Bzl

The serine protease Granzyme B (GzmB) mediates target cell apoptosis when released by cytotoxic T lymphocytes (CTL) or natural killer (NK) cells. GzmB is the most studied granzyme in humans and mice and therefore, researchers need specific and reliable tools to study its function and role in pathophysiology. This especially necessitates assays that do not recognize proteases such as caspases or other granzymes that are structurally or functionally related. Here, we apply GzmB's preference for cleavage after aspartic acid residues in a colorimetric assay using the peptide thioester Boc-Ala-Ala-Asp-S-Bzl. GzmB is the only mammalian serine protease capable of cleaving this substrate. The substrate is cleaved with similar efficiency by human, mouse and rat GzmB, a property not shared by other commercially available peptide substrates, even some that are advertised as being suitable for this purpose. This protocol is demonstrated using unfractionated lysates from activated NK cells or CTL and is also suitable for recombinant proteases generated in a variety of prokaryotic and eukaryotic systems, provided the correct controls are used. This assay is a highly specific method to ascertain the potential pro-apoptotic activity of cytotoxic molecules in mammalian lymphocytes, and of their recombinant counterparts expressed by a variety of methodologies.

Importance of extended protease substrate recognition motifs in steering BNIP-2 cleavage by human and mouse granzymes B

Background

Previous screening of the substrate repertoires and substrate specificity profiles of granzymes resulted in long substrate lists highly likely containing bystander substrates. Here, a recently developed degradomics technology that allows distinguishing efficiently from less efficiently cleaved substrates was applied to study the degradome of mouse granzyme B (mGrB).

Results

In vitro kinetic degradome analysis resulted in the identification of 37 mGrB cleavage events, 9 of which could be assigned as efficiently targeted ones. Previously, cleavage at the IEAD₇₅ tetrapeptide motif of Bid was shown to be efficiently and exclusively targeted by human granzyme B (hGrB) and thus not by mGrB. Strikingly, and despite holding an identical P4-P1 human Bid (hBid) cleavage motif, mGrB was shown to efficiently cleave the BCL2/adenovirus E1B 19 kDa protein-interacting protein 2 or BNIP-2 at IEAD₂₈. Like Bid, BNIP-2 represents a pro-apoptotic Bcl-2 protein family member and a potential regulator of GrB induced cell death. Next, in vitro analyses demonstrated the increased efficiency of human and mouse BNIP-2 cleavage by mGrB as compared to hGrB indicative for differing Bid/BNIP-2 substrate traits beyond the P4-P1 IEAD cleavage motif influencing cleavage efficiency. Murinisation of differential primed site residues in hBNIP-2 revealed that, although all contributing, a single mutation at the P3′ position was found to significantly increase the mGrB/hGrB cleavage ratio, whereas mutating the P1′ position from I₂₉ > T yielded a 4-fold increase in mGrB cleavage efficiency. Finally, mutagenesis analyses revealed the composite BNIP-2 precursor patterns to be the result of alternative translation initiation at near-cognate start sites within the 5′ leader sequence (5′UTR) of BNIP-2.

Conclusions

Despite their high sequence similarity, and previously explained by their distinct tetrapeptide specificities observed, the substrate repertoires of mouse and human granzymes B only partially overlap. Here, we show that the substrate sequence context beyond the P4-P1 positions can influence orthologous granzyme B cleavage efficiencies to an unmatched extent. More specifically, in BNIP-2, the identical and hGrB optimal IEAD tetrapeptide substrate motif is targeted highly efficiently by mGrB, while this tetrapeptide motif is refractory towards mGrB cleavage in Bid.

Granzyme B releases vascular endothelial growth factor from extracellular matrix and induces vascular permeability

The formation of unstable, leaky neovessels underlies the pathogenesis of many chronic inflammatory diseases. Granzyme B (GZMB) is an immune-derived serine protease that accumulates in the extracellular matrix (ECM) during chronic inflammation and is capable of cleaving fibronectin (FN). Vascular endothelial growth factor (VEGF) is a potent vascular permeabilizing agent that is sequestered in the ECM through its interaction with FN. As GZMB levels are elevated in chronic inflammatory diseases that are associated with increased vascular permeability, the role of GZMB in the regulation of VEGF bioavailability and vascular permeability were assessed. GZMB was added to either VEGF bound to FN or VEGF bound to endothelial cell (EC)-derived ECM. Supernatants containing released VEGF were assessed to determine VEGF activity by treating EC and evaluating VEGF receptor-2 (VEGFR2) phosphorylation. GZMB released VEGF from both FN and from EC-derived matrix, whereas GZMB inhibition prevented FN cleavage and VEGF release. GZMB-mediated VEGF release resulted in significant phosphorylation of VEGFR2. The role of GZMB-mediated VEGF release in altering vascular permeability was also assessed in vivo using Miles/Evans blue permeability assay. GZMB induced a significant VEGF-dependent increase in vascular permeability in vivo that was reduced in the presence of an anti-VEGF-neutralizing antibody. Inflammatory-mediated vascular leakage was also assessed in GZMB-KO mice using a delayed-type hypersensitivity model. GZMB-KO mice exhibited reduced microvascular leakage compared with C57\B6 controls. GZMB increases vascular permeability in part through the proteolytic release of ECM-sequestered VEGF, leading to VEGFR2 activation and increased vascular permeability in vivo. These findings present a novel role for GZMB as a modulator of vascular response during chronic inflammation.

Identification of Serpinb6b as a species-specific mouse granzyme A inhibitor suggests functional divergence between human and mouse granzyme A

The granzyme family serine proteases are key effector molecules expressed by cytotoxic lymphocytes. The physiological role of granzyme (Gzm) A is controversial, with significant debate over its ability to induce death in target cells. Here, we investigate the natural inhibitors of GzmA. We employed substrate phage display and positional proteomics to compare substrate specificities of mouse (m) and human (h) GzmA at the peptide and proteome-wide levels and we used the resulting substrate specificity profiles to search for potential inhibitors from the intracellular serpin family. We identified Serpinb6b as a potent inhibitor of mGzmA. Serpinb6b interacts with mGzmA, but not hGzmA, with an association constant of 1.9 ± 0.8 × 10(5) M(-1) s(-1) and a stoichiometry of inhibition of 1.8. Mouse GzmA is over five times more cytotoxic than hGzmA when delivered into P815 target cells with streptolysin O, whereas transfection of target cells with a Serpinb6b cDNA increases the EC50 value of mGzmA 13-fold, without affecting hGzmA cytotoxicity. Unexpectedly, we also found that Serpinb6b employs an exosite to specifically inhibit dimeric but not monomeric mGzmA. The identification of an intracellular inhibitor specific for mGzmA only indicates that a lineage-specific increase in GzmA cytotoxic potential has driven cognate inhibitor evolution.

Are all granzymes cytotoxic in vivo?

Granzymes are serine proteases mainly found in cytotoxic lymphocytes. The most-studied member of this group is granzyme B, which is a potent cytotoxin that has set the paradigm that all granzymes are cyototoxic. In the last 5 years, this paradigm has become controversial. On one hand, there is a plethora of sometimes contradictory publications showing mainly caspase-independent cytotoxic effects of granzyme A and the so-called orphan granzymes in vitro. On the other hand, there are increasing numbers of reports of granzymes failing to induce cell death in vitro unless very high (potentially supra-physiological) concentrations are used. Furthermore, experiments with granzyme A or granzyme M knock-out mice reveal little or no deficit in their cytotoxic lymphocytes’ killing ability ex vivo, but indicate impairment in the inflammatory response. These findings of non-cytotoxic effects of granzymes challenge dogma, and thus require alternative or additional explanations to be developed of the role of granzymes in defeating pathogens. Here we review evidence for granzyme cytotoxicity, give an overview of their non-cytotoxic functions, and suggest technical improvements for future investigations.