Granzyme M Is a Regulatory Protease That Inactivates Proteinase Inhibitor 9, an Endogenous Inhibitor of Granzyme B

The biological function of Serpinb9 and Serpinb9-based therapy

Recent breakthroughs in discovering novel immune signaling pathways have revolutionized different disease treatments. SERPINB9 (Sb9), also known as Proteinase Inhibitor 9 (PI-9), is a well-known endogenous inhibitor of Granzyme B (GzmB). GzmB is a potent cytotoxic molecule secreted by cytotoxic T lymphocytes and natural killer cells, which plays a crucial role in inducing apoptosis in target cells during immune responses. Sb9 acts as a protective mechanism against the potentially harmful effects of GzmB within the cells of the immune system itself. On the other hand, overexpression of Sb9 is an important mechanism of immune evasion in diseases like cancers and viral infections. The intricate functions of Sb9 in different cell types represent a fine-tuned regulatory mechanism for preventing immunopathology, protection against autoimmune diseases, and the regulation of cell death, all of which are essential for maintaining health and responding effectively to disease challenges. Dysregulation of the Sb9 will disrupt human normal physiological condition, potentially leading to a range of diseases, including cancers, inflammatory conditions, viral infections or other pathological disorders. Deepening our understanding of the role of Sb9 will aid in the discovery of innovative and effective treatments for various medical conditions. Therefore, the objective of this review is to consolidate current knowledge regarding the biological role of Sb9. It aims to offer insights into its discovery, structure, functions, distribution, its association with various diseases, and the potential of nanoparticle-based therapies targeting Sb9.

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.

Decoding the mechanisms of chimeric antigen receptor (CAR) T cell-mediated killing of tumors: insights from granzyme and Fas inhibition

Chimeric antigen receptor (CAR) T cell show promise in cancer treatments, but their mechanism of action is not well understood. Decoding the mechanisms used by individual T cells can help improve the efficacy of T cells while also identifying mechanisms of T cell failure leading to tumor escape. Here, we used a suite of assays including dynamic single-cell imaging of cell-cell interactions, dynamic imaging of fluorescent reporters to directly track cytotoxin activity in tumor cells, and scRNA-seq on patient infusion products to investigate the cytotoxic mechanisms used by individual CAR T cells in killing tumor cells. We show that surprisingly, overexpression of the Granzyme B (GZMB) inhibitor, protease inhibitor-9 (PI9), does not alter the cytotoxicity mediated by CD19-specific CAR T cells against either the leukemic cell line, NALM6; or the ovarian cancer cell line, SkOV3-CD19. We designed and validated reporters to directly assay T cell delivered GZMB activity in tumor cells and confirmed that while PI9 overexpression inhibits GZMB activity at the molecular level, this is not sufficient to impact the kinetics or magnitude of killing mediated by the CAR T cells. Altering cytotoxicity mediated by CAR T cells required combined inhibition of multiple pathways that are tumor cell specific: (a) B-cell lines like NALM6, Raji and Daudi were sensitive to combined GZMB and granzyme A (GZMA) inhibition; whereas (b) solid tumor targets like SkOV3-CD19 and A375-CD19 (melanoma) were sensitive to combined GZMB and Fas ligand inhibition. We realized the translational relevance of these findings by examining the scRNA-seq profiles of Tisa-cel and Axi-cel infusion products and show a significant correlation between GZMB and GZMA expression at the single-cell level in a T cell subset-dependent manner. Our findings highlight the importance of the redundancy in killing mechanisms of CAR T cells and how this redundancy is important for efficacious T cells.

Identification of Human Host Substrates of the SARS-CoV-2 Mpro and PLpro Using Subtiligase N-Terminomics

The recent emergence of SARS-CoV-2 in the human population has caused a global pandemic. The virus encodes two proteases, Mpro and PLpro, that are thought to play key roles in the suppression of host protein synthesis and immune response evasion during infection. To identify the specific host cell substrates of these proteases, active recombinant SARS-CoV-2 Mpro and PLpro were added to A549 and Jurkat human cell lysates, and subtiligase-mediated N-terminomics was used to capture and enrich protease substrate fragments. The precise location of each cleavage site was identified using mass spectrometry. Here, we report the identification of over 200 human host proteins that are potential substrates for SARS-CoV-2 Mpro and PLpro and provide a global mapping of proteolysis for these two viral proteases in vitro. Modulating proteolysis of these substrates will increase our understanding of SARS-CoV-2 pathobiology and COVID-19.

Role of the granzyme family in rheumatoid arthritis: Current Insights and future perspectives

Rheumatoid arthritis (RA) is a complex autoimmune disease characterized by chronic inflammation that affects synovial tissues of multiple joints. Granzymes (Gzms) are serine proteases that are released into the immune synapse between cytotoxic lymphocytes and target cells. They enter target cells with the help of perforin to induce programmed cell death in inflammatory and tumor cells. Gzms may have a connection with RA. First, increased levels of Gzms have been found in the serum (GzmB), plasma (GzmA, GzmB), synovial fluid (GzmB, GzmM), and synovial tissue (GzmK) of patients with RA. Moreover, Gzms may contribute to inflammation by degrading the extracellular matrix and promoting cytokine release. They are thought to be involved in RA pathogenesis and have the potential to be used as biomarkers for RA diagnosis, although their exact role is yet to be fully elucidated. The purpose of this review was to summarize the current knowledge regarding the possible role of the granzyme family in RA, with the aim of providing a reference for future research on the mechanisms of RA and the development of new therapies.

Extended cleavage specificity of a Chinese alligator granzyme B homologue, a strict Glu-ase in contrast to the mammalian Asp-ases

Granzyme B (GzmB) is primarily expressed by mammalian cytotoxic T cells and serves as one of the key components in the defense against viral infection by the induction of apoptosis in virus infected cells. By direct cell to cell contact and delivery into target cells by perforin, cytotoxic T cells activate apoptosis through the action of GzmB by both caspase-dependent and -independent pathways. In search for early ancestors of GzmB we have in the current study identified and characterized a GzmB homologue from a reptile, the Chinese alligator. This enzyme is encoded from the same locus as the mammalian counterparts, the chymase locus. Phage display analysis of the cleavage specificity of the recombinant alligator enzyme (named MCP1A-like) shows that it is a relatively strict Glu-ase, with strong preference for glutamic acid in the P1 position of a substrate. The majority of mammalian GzmB:s are, in marked contrast to the alligator enzyme, relatively strict Asp-ases. The alligator enzyme also showed strong preference for Ala, Pro and Gly in the P2 position and Val in the P3 position indicating that it has a narrow specificity, similar to the mammalian counterparts. Analysis of the three amino acids forming the substrate binding pocket (S1 pocket) in three amphibian homologues to MCP1A-like, from the frogs Xenopus laevis and Xenopus tropicalis, shows that these amphibian enzymes have similar substrate binding pocket as their mammalian counterparts. This finding, together with the apparent lack of GzmB homologs in fish, indicates that the ancestor of GzmB did appear with the amphibians at the base of tetrapod evolution. This study is a first step in a larger effort to understand the evolutionary processes involved in shaping anti-viral immunity in non-mammalian vertebrates.

Granzymes in cardiovascular injury and disease

Chronic inflammation and impaired wound healing play important roles in the pathophysiology of cardiovascular diseases. Moreover, the aberrant secretion of proteases plays a critical role in pathological tissue remodeling in chronic inflammatory conditions. Human Granzymes (Granule secreted enzymes - Gzms) comprise a family of five (GzmA, B, H, K, M) cell-secreted serine proteases. Although each unique in function and substrate specificities, Gzms were originally thought to share redundant, intracellular roles in cytotoxic lymphocyte-induced cell death. However, an abundance of evidence has challenged this dogma. It is now recognized, that individual Gzms exhibit unique substrate repertoires and functions both intracellularly and extracellularly. In the extracellular milieu, Gzms contribute to inflammation, vascular dysfunction and permeability, reduced cell adhesion, release of matrix-sequestered growth factors, receptor activation, and extracellular matrix cleavage. Despite these recent findings, the non-cytotoxic functions of Gzms in the context of cardiovascular disease pathogenesis remain poorly understood. Minimally detected in tissues and bodily fluids of normal individuals, GzmB is elevated in patients with acute coronary syndromes, coronary artery disease, and myocardial infarction. Pre-clinical animal models have exemplified the importance of GzmB in atherosclerosis, aortic aneurysm, and cardiac fibrosis as animals deficient in GzmB exhibit reduced tissue remodeling, improved disease phenotypes and increased survival. Although a role for GzmB in cardiovascular disease is described, further work to elucidate the mechanisms that underpin the remaining human Gzms activity in cardiovascular disease is necessary. The present review provides a summary of the pre-clinical and clinical evidence, as well as emerging areas of research pertaining to Gzms in tissue remodeling and cardiovascular disease.

Escape of tumor cells from the NK cell cytotoxic activity

In recent years, NK cells, initially identified as potent cytotoxic effector cells, have revealed an unexpected complexity, both at phenotypic and functional levels. The discovery of different NK cell subsets, characterized by distinct gene expression and phenotypes, was combined with the characterization of the diverse functions NK cells can exert, not only as circulating cells, but also as cells localized or recruited in lymphoid organs and in multiple tissues. Besides the elimination of tumor and virus-infected cells, these functions include the production of cytokines and chemokines, the regulation of innate and adaptive immune cells, the influence on tissue homeostasis. In addition, NK cells display a remarkable functional plasticity, being able to adapt to the environment and to develop a kind of memory. Nevertheless, the powerful cytotoxic activity of NK cells remains one of their most relevant properties, particularly in the antitumor response. In this review, the process of tumor cell recognition and killing mediated by NK cells, starting from the generation of cytolytic granules and recognition of target cell, to the establishment of the NK cell immunological synapse, the release of cytotoxic molecules, and consequent tumor cell death is described. Next, the review focuses on the heterogeneous mechanisms, either intrinsic to tumors or induced by the tumor microenvironment, by which cancer cells can escape the NK cell-mediated attack.

Mechanisms of Apoptosis Resistance to NK Cell-Mediated Cytotoxicity in Cancer

Natural killer (NK) cells are major contributors to immunosurveillance and control of tumor development by inducing apoptosis of malignant cells. Among the main mechanisms involved in NK cell-mediated cytotoxicity, the death receptor pathway and the release of granules containing perforin/granzymes stand out due to their efficacy in eliminating tumor cells. However, accumulated evidence suggest a profound immune suppression in the context of tumor progression affecting effector cells, such as NK cells, leading to decreased cytotoxicity. This diminished capability, together with the development of resistance to apoptosis by cancer cells, favor the loss of immunogenicity and promote immunosuppression, thus partially inducing NK cell-mediated killing resistance. Altered expression patterns of pro- and anti-apoptotic proteins along with genetic background comprise the main mechanisms of resistance to NK cell-related apoptosis. Herein, we summarize the main effector cytotoxic mechanisms against tumor cells, as well as the major resistance strategies acquired by tumor cells that hamper the extrinsic and intrinsic apoptotic pathways related to NK cell-mediated killing.

Engineering the serpin α1 -antitrypsin: A diversity of goals and techniques

α₁ -Antitrypsin (α₁ -AT) serves as an archetypal example for the serine proteinase inhibitor (serpin) protein family and has been used as a scaffold for protein engineering for >35 years. Techniques used to engineer α₁ -AT include targeted mutagenesis, protein fusions, phage display, glycoengineering, and consensus protein design. The goals of engineering have also been diverse, ranging from understanding serpin structure-function relationships, to the design of more potent or more specific proteinase inhibitors with potential therapeutic relevance. Here we summarize the history of these protein engineering efforts, describing the techniques applied to engineer α₁ -AT, specific mutants of interest, and providing an appended catalog of the >200 α₁ -AT mutants published to date.

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.

Granzyme B Is an Essential Mediator in CD8+ T Cell Killing of Theileria parva-Infected Cells

There is established evidence that cytotoxic CD8⁺ T cells are important mediators of immunity against the bovine intracellular protozoan parasite Theileria parva However, the mechanism by which the specific CD8⁺ T cells kill parasitized cells is not understood. Although the predominant pathway used by human and murine CD8⁺ T cells to kill pathogen-infected cells is granule exocytosis, involving the release of perforin and granzyme B, there is to date a lack of published information on the biological activities of bovine granzyme B. The present study set out to define the functional activities of bovine granzyme B and determine its role in mediating the killing of T. parva-parasitized cells. DNA constructs encoding functional and nonfunctional forms of bovine granzyme B were produced, and the proteins expressed in Cos-7 cells were used to establish an enzymatic assay to detect and quantify the expression of functional granzyme B protein. Using this assay, the levels of killing of different T. parva-specific CD8⁺ T cell clones were found to be significantly correlated with the levels of granzyme B protein but not the levels of mRNA transcript expression. Experiments using inhibitors specific for perforin and granzyme B confirmed that CD8⁺ T cell killing of parasitized cells is dependent on granule exocytosis and, specifically, granzyme B. Further studies showed that the granzyme B-mediated death of parasitized cells is independent of caspases and that granzyme B activates the proapoptotic molecule Bid.

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.

Emerging challenges in the design of selective substrates, inhibitors and activity-based probes for indistinguishable proteases

Proteases are enzymes that hydrolyze the peptide bond of peptide substrates and proteins. Despite significant progress in recent years, one of the greatest challenges in the design and testing of substrates, inhibitors and activity‐based probes for proteolytic enzymes is achieving specificity toward only one enzyme. This specificity is particularly important if the enzyme is present with other enzymes with a similar catalytic mechanism and substrate specificity but completely different functionality. The cross‐reactivity of substrates, inhibitors and activity‐based probes with other enzymes can significantly impair or even prevent investigations of a target protease. In this review, we describe important concepts and the latest challenges, focusing mainly on peptide‐based substrate specificity techniques used to distinguish individual enzymes within major protease families.

Channel catfish granzyme-like I is a highly specific serine protease with metase activity that is expressed by fish NK-like cells

Here we present the extended cleavage specificity of catfish granzyme-like I, previously identified in fish NK-like cells. This protease has been characterised using substrate phage display and further validated by using a panel of recombinant substrates. A strict preference for Met in the P1 (cleavage) position, indicating metase specificity was observed. A screening of potential in vivo substrates was performed based on the derived P5-P3' consensus: Arg-Val-Thr-Gly-Met(↓)Ser-Leu-Val. Channel catfish caspase 6 was one very interesting potential target identified. This site was present in an adjacent position to the classic caspase activation site (Asp179 in human caspase 6). Cleavage of this site (hence potential activation) by the catfish granzyme-like I could reveal a novel mechanism of caspase 6 activation. This poses an interesting idea that the role of granzyme-like proteases in the activation of caspase dependent apoptosis mechanisms has been conserved for over 400 million years.

Granzyme M has a critical role in providing innate immune protection in ulcerative colitis

Inflammatory bowel disease (IBD) is an immunoregulatory disorder, associated with a chronic and inappropriate mucosal immune response to commensal bacteria, underlying disease states such as ulcerative colitis (UC) and Crohn's disease (CD) in humans. Granzyme M (GrzM) is a serine protease expressed by cytotoxic lymphocytes, in particular natural killer (NK) cells. Granzymes are thought to be involved in triggering cell death in eukaryotic target cells; however, some evidence supports their role in inflammation. The role of GrzM in the innate immune response to mucosal inflammation has never been examined. Here, we discover that patients with UC, unlike patients with CD, display high levels of GrzM mRNA expression in the inflamed colon. By taking advantage of well-established models of experimental UC, we revealed that GrzM-deficient mice have greater levels of inflammatory indicators during dextran sulfate sodium (DSS)-induced IBD, including increased weight loss, greater colon length reduction and more severe intestinal histopathology. The absence of GrzM expression also had effects on gut permeability, tissue cytokine/chemokine dynamics, and neutrophil infiltration during disease. These findings demonstrate, for the first time, that GrzM has a critical role during early stages of inflammation in UC, and that in its absence colonic inflammation is enhanced.

Granzymes regulate proinflammatory cytokine responses

Granzymes (Grs) are serine proteases mainly produced by cytotoxic lymphocytes and are traditionally considered to cause apoptosis in tumor cells and virally infected cells. However, the cytotoxicity of several Grs is currently being debated, and additional, predominantly extracellular, functions of Grs in inflammation are emerging. Extracellular soluble Grs are elevated in the circulation of patients with autoimmune diseases and infections. Additionally, Grs are expressed by several types of immune cells other than cytotoxic lymphocytes. Recent research has revealed novel immunomodulatory functions of Grs. In this review, we provide a comprehensive overview on the role of Grs in inflammation, highlighting their role in cytokine induction and processing.

Granzyme M: behind enemy lines

The granule-exocytosis pathway is the major mechanism via which cytotoxic lymphocytes eliminate virus-infected and tumor cells. In this pathway, cytotoxic lymphocytes release granules containing the pore-forming protein perforin and a family of serine proteases known as granzymes into the immunological synapse. Pore-formation by perforin facilitates entry of granzymes into the target cell, where they can activate various (death) pathways. Humans express five different granzymes, of which granzymes A and B have been most extensively characterized. However, much less is known about granzyme M (GrM). Recently, structural analysis and advanced proteomics approaches have determined the primary and extended specificity of GrM. GrM functions have expanded over the past few years: not only can GrM efficiently induce cell death in tumor cells, it can also inhibit cytomegalovirus replication in a noncytotoxic manner. Finally, a role for GrM in lipopolysaccharide-induced inflammatory responses has been proposed. In this review, we recapitulate the current status of GrM expression, substrate specificity, functions, and inhibitors.

Granzyme M targets topoisomerase II alpha to trigger cell cycle arrest and caspase-dependent apoptosis

Cytotoxic lymphocyte protease granzyme M (GrM) is a potent inducer of tumor cell death. The apoptotic phenotype and mechanism by which it induces cell death, however, remain poorly understood and controversial. Here, we show that GrM-induced cell death was largely caspase-dependent with various hallmarks of classical apoptosis, coinciding with caspase-independent G2/M cell cycle arrest. Using positional proteomics in human tumor cells, we identified the nuclear enzyme topoisomerase II alpha (topoIIα) as a physiological substrate of GrM. Cleavage of topoIIα by GrM at Leu(1280) separated topoIIα functional domains from the nuclear localization signals, leading to nuclear exit of topoIIα catalytic activity, thereby rendering it nonfunctional. Similar to the apoptotic phenotype of GrM, topoIIα depletion in tumor cells led to cell cycle arrest in G2/M, mitochondrial perturbations, caspase activation, and apoptosis. We conclude that cytotoxic lymphocyte protease GrM targets topoIIα to trigger cell cycle arrest and caspase-dependent apoptosis.

Granzyme M as a novel effector molecule for human cytolytic fusion proteins: CD64-specific cytotoxicity of Gm-H22(scFv) against leukemic cells

Immunotoxins are promising targeted therapeutic agents comprising an antibody-based ligand that specifically binds to diseased cells, and a pro-apoptotic protein. Toxic components from bacteria or plants can trigger a neutralizing immune response, so that human effector molecules are more suitable. In this context, the protease granzyme B has been successfully tested in cytotoxicity assays against different cancer cells in vitro and in vivo. Our aim here was to introduce granzyme M as an alternative and novel component of human cytolytic fusion proteins. We fused it to the humanized single-chain antibody fragment (scFv) H22 which specifically binds to CD64, an FcγRI receptor overexpressed on activated myeloid cells and leukemic cells. We show that the humanized cytolytic fusion protein Gm-H22(scFv) specifically targets the acute myeloid leukemia cell line HL60 in vitro and is cytotoxic with an IC50 between 1.2 and 6.4 nM. These findings were confirmed ex vivo using leukemic primary cells from patients, which were killed by granzyme M despite the presence of the granzyme B inhibitor serpin B9. In conclusion, granzyme M is a promising new cell-death inducing component for hCFPs because it specifically and efficiently kills target cells when fused to a targeting component.

Efficacy of an adapted granzyme B-based anti-CD30 cytolytic fusion protein against PI-9-positive classical Hodgkin lymphoma cells in a murine model

Tumors develop when infiltrating immune cells contribute growth stimuli, and cancer cells are selected to survive within such a cytotoxic microenvironment. One possible immune-escape mechanism is the upregulation of PI-9 (Serpin B9) within cancer cells. This serine proteinase inhibitor selectively inactivates apoptosis-inducing granzyme B (GrB) from cytotoxic granules of innate immune cells. We demonstrate that most classical Hodgkin lymphoma (cHL)-derived cell lines express PI-9, which protects them against the GrB attack and thereby renders them resistant against GrB-based immunotherapeutics. To circumvent this disadvantage, we developed PI-9-insensitive human GrB mutants as fusion proteins to target the Hodgkin-selective receptor CD30. In contrast to the wild-type GrB, a R201K point-mutated GrB construct most efficiently killed PI-9-positive and -negative cHL cells. This was tested in vitro and also in vivo whereby a novel optical imaging-based tumor model with HL cell line L428 was applied. Therefore, this variant, as part of the next generation immunotherapeutics, also named cytolytic fusion proteins showing reduced immunogenicity, is a promising molecule for (targeted) therapy of patients with relapsing malignancies, such as cHL, and possibly other PI-9-positive malignancies, such as breast or lung carcinoma.

Human NK cell lytic granules and regulation of their exocytosis

Natural killer (NK) cells form a subset of lymphocytes that play a key role in immuno-surveillance and host defense against cancer and viral infections. They recognize stressed cells through a variety of germline-encoded activating cell surface receptors and utilize their cytotoxic ability to eliminate abnormal cells. Killing of target cells is a complex, multi-stage process that concludes in the directed secretion of lytic granules, containing perforin and granzymes, at the immunological synapse. Upon delivery to a target cell, perforin mediates generation of pores in membranes of target cells, allowing granzymes to access target cell cytoplasm and induce apoptosis. Therefore, lytic granules of NK cells are indispensable for normal NK cell cytolytic function. Indeed, defects in lytic granule secretion lead or are related to serious and often fatal diseases, such as familial hemophagocytic lymphohistiocytosis (FHL) type 2–5 or Griscelli syndrome type 2. A number of reports highlight the role of several proteins involved in lytic granule release and NK cell-mediated killing of tumor cells. This review focuses on lytic granules of human NK cells and the advancements in understanding the mechanisms controlling their exocytosis.

Immunotherapeutic targeting of membrane Hsp70-expressing tumors using recombinant human granzyme B

Background

We have previously reported that human recombinant granzyme B (grB) mediates apoptosis in membrane heat shock protein 70 (Hsp70)-positive tumor cells in a perforin-independent manner.

Methodology/Principal Findings

Optical imaging of uptake kinetics revealed co-localization of grB with recycling endosomes (Rab9/11) as early as 5 min after internalization, with late endosomes (Rab7) after 30 min, and the lysosomal compartment (LAMP1/2) after 60 to 120 min. Active caspase-3-mediated apoptosis was induced in mouse CT26 monolayer cells and 3D tumor spheroids, but not in normal mouse endothelial cells. Granzyme B selectively reduced the proportion of membrane Hsp70-positive cells in CT26 tumor spheroids. Consecutive i.v. injections of recombinant human grB into mice bearing membrane Hsp70-positive CT26 tumors resulted in significant tumor suppression, and a detailed inspection of normal mouse organs revealed that the administration of anti-tumoral concentrations of grB elicited no clinicopathological changes.

Conclusions/Significance

These findings support the future clinical evaluation of human grB as a potential adjuvant therapeutic agent, especially for treating immunosuppressed patients that bear membrane Hsp70-positive tumors.

Inhibition of Granzyme B by PI-9 protects prostate cancer cells from apoptosis

BACKGROUND

In order for tumors to grow and proliferate, they must avoid recognition by immune cells and subsequent death by apoptosis. Granzyme B (GrB), a protease located in natural killer cells, initiates apoptosis in target cells. Inhibition of GrB by PI-9, its natural inhibitor, can prevent apoptosis. Here we investigate whether PI-9 protects prostate cancer cells from apoptosis.

METHODS

The expression of PI-9 was quantified by qPCR in several prostate cancer cell lines, and GrB activity was tested in each cell line. PI-9 was overexpressed in LNCaP cells, which lack endogenous PI-9. Apoptosis was induced by natural killer cells in LNCaP cells that either contained or lacked PI-9, and the percent cell death was quantified. Lastly, PI-9 levels were examined by qPCR and immunohistochemistry in prostate tumor tissue.

RESULTS

Prostate cancer cell lines that expressed PI-9 could inhibit GrB. Overexpression of PI-9 protected LNCaP cells from natural killer cell-mediated apoptosis. Examination of the levels of PI-9 in tissue from prostate tumors showed that PI-9 could be upregulated in low grade tumors and stochastically dysregulated in high grade tumors. Additionally, PI-9 was found consistently in high grade prostatic intraepithelial neoplasia and atrophic lesions.

CONCLUSIONS

These results indicate that overexpression of PI-9 can protect prostate cancer cells from apoptosis, and this effect may occur in human prostate tumors. These findings imply that early prostatic inflammation may trigger this increase in PI-9. This suggests that PI-9 upregulation is needed early in tumor progression, before additional protective mechanisms are in place.

Alpha-1 antitrypsin: a potent anti-inflammatory and potential novel therapeutic agent

Alpha-1 antitrypsin (AAT) has long been thought of as an important anti-protease in the lung where it is known to decrease the destructive effects of major proteases such as neutrophil elastase. In recent years, the perception of this protein in this simple one dimensional capacity as an anti-protease has evolved and it is now recognised that AAT has significant anti-inflammatory properties affecting a wide range of inflammatory cells, leading to its potential therapeutic use in a number of important diseases. This present review aims to discuss the described anti-inflammatory actions of AAT in modulating key immune cell functions, delineate known signalling pathways and specifically to identify the models of disease in which AAT has been shown to be effective as a therapy.

A quarter century of granzymes

Granzymes (Grs) were discovered just over a quarter century ago. They are produced by cytotoxic T cells and natural killer cells and are released upon interaction with target cells. Intensive biochemical, genetic, and biological studies have been performed in order to study their roles in immunity and inflammation. This review summarizes research on the family of Grs.

FADD cleavage by NK cell granzyme M enhances its self-association to facilitate procaspase-8 recruitment for auto-processing leading to caspase cascade

Granzyme M (GzmM), an orphan Gzm, is constitutively and abundantly expressed in innate effector natural killer cells. We previously demonstrated that GzmM induces caspase (casp)-dependent apoptosis and cytochrome c release from mitochondria. We also resolved the crystal structure for GzmM and generated its specific inhibitor. However, how GzmM causes casp activation has not been defined. Here we found that casp-8 is an initiator caspase in GzmM-induced casp cascade, which causes other casp activation and Bid cleavage. GzmM does not directly cleave procaspase-3 and Bid, whose processing is casp dependent. Casp-8 knockdown or deficient cells attenuate or abolish GzmM-induced proteolysis of procaspase-3 and Bid. Extrinsic death receptor pathway adaptor Fas-associated protein with death domain (FADD) contributes to GzmM-induced casp-8 activation. GzmM specifically cleaves FADD after Met 196 to generate truncated FADD (tFADD) that enhances its self-association for oligomerization. The oligomerized tFADD facilitates procaspase-8 recruitment to promote its auto-processing leading to casp activation cascade. FADD-deficient cells abrogate GzmM-induced activation of casp-8 and apoptosis as well as significantly inhibit lymphokine-activated killer cell-mediated cytotoxicity. FADD processing by GzmM can potentiate killing efficacy against tumor cells and intracellular pathogens.

Cleavage of von Willebrand factor by granzyme M destroys its factor VIII binding capacity

Von Willebrand factor (VWF) is a pro-hemostatic multimeric plasma protein that promotes platelet aggregation and stabilizes coagulation factor VIII (FVIII) in plasma. The metalloproteinase ADAMTS13 regulates the platelet aggregation function of VWF via proteolysis. Severe deficiency of ADAMTS13 is associated with thrombotic thrombocytopenic purpura, but does not always correlate with its clinical course. Therefore, other proteases could also be important in regulating VWF activity. In the present study, we demonstrate that VWF is cleaved by the cytotoxic lymphocyte granule component granzyme M (GrM). GrM cleaved both denaturated and soluble plasma-derived VWF after Leu at position 276 in the D3 domain. GrM is unique in that it did not affect the multimeric size and pro-hemostatic platelet aggregation ability of VWF, but instead destroyed the binding of VWF to FVIII in vitro. In meningococcal sepsis patients, we found increased plasma GrM levels that positively correlated with an increased plasma VWF/FVIII ratio in vivo. We conclude that, next to its intracellular role in triggering apoptosis, GrM also exists extracellularly in plasma where it could play a physiological role in controlling blood coagulation by determining plasma FVIII levels via proteolytic processing of its carrier VWF.

Intracellular serine protease inhibitor SERPINB4 inhibits granzyme M-induced cell death

Granzyme-mediated cell death is the major pathway for cytotoxic lymphocytes to kill virus-infected and tumor cells. In humans, five different granzymes (i.e. GrA, GrB, GrH, GrK, and GrM) are known that all induce cell death. Expression of intracellular serine protease inhibitors (serpins) is one of the mechanisms by which tumor cells evade cytotoxic lymphocyte-mediated killing. Intracellular expression of SERPINB9 by tumor cells renders them resistant to GrB-induced apoptosis. In contrast to GrB, however, no physiological intracellular inhibitors are known for the other four human granzymes. In the present study, we show that SERPINB4 formed a typical serpin-protease SDS-stable complex with both recombinant and native human GrM. Mutation of the P2-P1-P1′ triplet in the SERPINB4 reactive center loop completely abolished complex formation with GrM and N-terminal sequencing revealed that GrM cleaves SERPINB4 after P1-Leu. SERPINB4 inhibited GrM activity with a stoichiometry of inhibition of 1.6 and an apparent second order rate constant of 1.3×10⁴ M⁻¹s⁻¹. SERPINB4 abolished cleavage of the macromolecular GrM substrates α-tubulin and nucleophosmin. Overexpression of SERPINB4 in tumor cells inhibited recombinant GrM-induced as well as NK cell-mediated cell death and this inhibition depended on the reactive center loop of the serpin. As SERPINB4 is highly expressed by squamous cell carcinomas, our results may represent a novel mechanism by which these tumor cells evade cytotoxic lymphocyte-induced GrM-mediated cell death.

Human and mouse granzyme M display divergent and species-specific substrate specificities

Cytotoxic lymphocyte protease GrM (granzyme M) is a potent inducer of tumour cell death and a key regulator of inflammation. Although hGrM (human GrM) and mGrM (mouse GrM) display extensive sequence homology, the substrate specificity of mGrM remains unknown. In the present study, we show that hGrM and mGrM have diverged during evolution. Positional scanning libraries of tetrapeptide substrates revealed that mGrM is preferred to cleave after a methionine residue, whereas hGrM clearly favours a leucine residue at the P1 position. The kinetic optimal non-prime subsites of both granzymes were also distinct. Gel-based and complementary positional proteomics showed that hGrM and mGrM have a partially overlapping set of natural substrates and a diverged prime and non-prime consensus cleavage motif with leucine and methionine residues being major P1 determinants. Consistent with positional scanning libraries of tetrapeptide substrates, P1 methionine was more frequently used by mGrM as compared with hGrM. Both hGrM and mGrM cleaved α-tubulin with similar kinetics. Strikingly, neither hGrM nor mGrM hydrolysed mouse NPM (nucleophosmin), whereas human NPM was hydrolysed efficiently by GrM from both species. Replacement of the putative P1′–P2′ residues in mouse NPM with the corresponding residues of human NPM restored cleavage of mouse NPM by both granzymes. This further demonstrates the importance of prime sites as structural determinants for GrM substrate specificity. GrM from both species efficiently triggered apoptosis in human but not in mouse tumour cells. These results indicate that hGrM and mGrM not only exhibit divergent specificities but also trigger species-specific functions.

Efficacy of alpha1-antitrypsin augmentation therapy in conditions other than pulmonary emphysema

Up to now alpha 1-antitrypsin (AAT) augmentation therapy has been approved only for commercial use in selected adults with severe AAT deficiency-related pulmonary emphysema (i.e. PI*ZZ genotypes as well as combinations of Z, rare and null alleles expressing AAT serum concentrations <11 μmol/L). However, the compassionate use of augmentation therapy in recent years has proven outstanding efficacy in small cohorts of patients suffering from uncommon AAT deficiency-related diseases other than pulmonary emphysema, such as fibromyalgia, systemic vasculitis, relapsing panniculitis and bronchial asthma. Moreover, a series of preclinical studies provide evidence of the efficacy of AAT augmentation therapy in several infectious diseases, diabetes mellitus and organ transplant rejection. These facts have generated an expanding number of medical applications and patents with claims for other indications of AAT besides pulmonary emphysema. The aim of the present study is to compile and analyze both clinical and histological features of the aforementioned published case studies and reports where AAT augmentation therapy was used for conditions other than pulmonary emphysema. Particularly, our research refers to ten case reports and two clinical trials on AAT augmentation therapy in patients with both AAT deficiency and, at least, one of the following diseases: fibromyalgia, vasculitis, panniculitis and bronchial asthma. In all the cases, AAT was successfully applied whereas previous maximal conventional therapies had failed. In conclusion, laboratory studies in animals and humans as well as larger clinical trials should be, thus, performed in order to determine both the strong clinical efficacy and security of AAT in the treatment of conditions other than pulmonary emphysema.

α1-Antichymotrypsin inactivates staphylococcal cysteine protease in cross-class inhibition

Staphylococcal cysteine proteases are implicated as virulence factors in human and avian infections. Human strains of Staphylococcus aureus secrete two cysteine proteases (staphopains A and B), whereas avian strains express staphopain C (ScpA2), which is distinct from both human homologues. Here, we describe probable reasons why the horizontal transfer of a plasmid encoding staphopain C between avian and human strains has never been observed. The human plasma serine protease inhibitor α(1)-antichymotrypsin (ACHT) inhibits ScpA2. Together with the lack of ScpA2 inhibition by chicken plasma, these data may explain the exclusively avian occurrence of ScpA2. We also clarify the mechanistic details of this unusual cross-class inhibition. Analysis of mutated ACHT variants revealed that the cleavage of the Leu383-Ser384 peptide bond results in ScpA2 inhibition, whereas hydrolysis of the preceding peptide bond leads to ACHT inactivation. This evidence is consistent with the suicide-substrate-like mechanism of inhibition.

Proprotein convertase PC7 enhances the activation of the EGF receptor pathway through processing of the EGF precursor

Although the processing profile of the membrane-bound epidermal growth factor precursor (pro-EGF) is tissue-specific, it has not been investigated at the cellular level nor have the cognate proteinases been defined. Among the proprotein convertases (PCs), only the membrane-bound PC7, the most ancient and conserved basic amino acid-specific PC family member, induces the processing of pro-EGF into an ∼115-kDa transmembrane form (EGF-115) at an unusual VHPR(290)↓A motif. Because site-directed mutagenesis revealed that Arg(290) is not critical, the generation of EGF-115 by PC7 is likely indirect. This was confirmed by testing a wide range of protease inhibitors, which revealed that the production of EGF-115 is most probably achieved via the activation by PC7 of a latent serine and/or cysteine protease(s). EGF-115 is more abundant at the cell surface than pro-EGF and is associated with a stronger EGF receptor (EGFR) activation, as evidenced by higher levels of phosphorylated ERK1/2. This suggests that the generation of EGF-115 represents a regulatory mechanism of juxtacrine EGFR activation. Thus, PC7 is distinct from the other PCs in its ability to enhance the activation of the cell surface EGFR.

Noncytotoxic inhibition of cytomegalovirus replication through NK cell protease granzyme M-mediated cleavage of viral phosphoprotein 71

Granzyme M (GrM) is highly expressed in cytotoxic granules of NK cells, which provide the first line of defense against viral pathogens. GrM knockout mice show increased susceptibility toward murine CMV infection. Although GrM is a potent inducer of cell death, the mechanism by which GrM eliminates viruses remains elusive. In this paper, we show that purified human GrM in combination with the perforin-analog streptolysin O (SLO) strongly inhibited human CMV (HCMV) replication in fibroblasts in the absence of host cell death. In a proteomic approach, GrM was highly specific toward the HCMV proteome and most efficiently cleaved phosphoprotein 71 (pp71), an HCMV tegument protein that is critical for viral replication. Cleavage of pp71 occurred when viral lysates were incubated with purified GrM, when intact cells expressing recombinant pp71 were challenged with living cytotoxic effector cells, and when HCMV-infected fibroblasts were incubated with SLO and purified GrM. GrM directly cleaved pp71 after Leu(439), which coincided with aberrant cellular localization of both pp71 cleavage fragments as determined by confocal immunofluorescence. In a luciferase reporter assay, cleavage of pp71 after Leu(439) by GrM completely abolished the ability of pp71 to transactivate the HCMV major immediate-early promoter, which is indispensable for effective HCMV replication. Finally, GrM decreased immediate-early 1 protein expression in HCMV-infected fibroblasts. These results indicate that the NK cell protease GrM mediates cell death-independent antiviral activity by direct cleavage of a viral substrate.

Orphan granzymes find a home

Cytotoxic lymphocytes are armed with granules that are released in the granule-exocytosis pathway to kill tumor cells and virus-infected cells. Cytotoxic granules contain the pore-forming protein perforin and a family of structurally homologues serine proteases called granzymes. While perforin facilitates the entry of granzymes into a target cell, the latter initiate distinct apoptotic routes. Granzymes are also implicated in extracellular functions such as extracellular matrix degradation, immune regulation, and inflammation. The family of human granzymes consists of five members, of which granzyme A and B have been studied most extensively. Recently, elucidation of the specific characteristics of the other three human granzymes H, K, and M, also referred to as orphan granzymes, have started. In this review, we summarize and discuss what is currently known about the biology of the human orphan granzymes.

Cleavage of survivin by Granzyme M triggers degradation of the survivin-X-linked inhibitor of apoptosis protein (XIAP) complex to free caspase activity leading to cytolysis of target tumor cells

Granzyme M (GzmM) is a chymotrypsin-like serine protease that preferentially cuts its substrates after Met or Leu. GzmM is constitutively expressed in activated innate effector natural killer (NK) cells. GzmM-induced cell death is consistent with the kinetics of cytotoxicity of NK cells. These suggest that GzmM may play an important role in innate immunity. Our previous work demonstrated that GzmM induces caspase-dependent apoptosis. However, it is unknown about how GzmM causes caspase activation. Here, we showed that the inhibitor of the apoptosis gene family member Survivin is a physiological substrate for GzmM. GzmM hydrolyzes Survivin at Leu-138 to remove the last four C-terminal residues. The truncated form (sur-TF) is more rapidly hydrolyzed through proteasome-mediated degradation. In addition, Survivin is in complex with X-linked inhibitor of apoptosis protein (XIAP) to inhibit caspase activation as an endogenous inhibitor. Survivin cleavage by GzmM abolishes the stability of the Survivin-XIAP complex and enhances XIAP hydrolysis, which amplifies caspase-9 and 3 activation of target tumor cells. The noncleavable L138A Survivin overexpression can significantly inhibit GzmM-mediated XIAP degradation, caspase activation, and GzmM- and NK cell-induced cytotoxicity. Moreover, Survivin silencing promotes XIAP degradation and enhances GzmM-induced caspase activation as well as GzmM- and NK cell-induced cytolysis of target tumor cells.

Granzyme M: characterization with sites of post-translational modification and specific sites of interaction with substrates and inhibitors

Granzymes kill cells in a variety of ways. They induce mitochondrial dysfunction through caspase dependent and caspase-independent pathways and destroy DNA and the integrity of the nucleus. For gaining a better understanding of the molecular function of granzyme M and its NK cell specificity, structural characterization of this enzyme by molecular modeling as well as its detailed comparison with other granzymes is presented in this study. The study includes mode of action of granzyme M using cationic binding sites, substrate specificity, post-translational structural modification and its functional relationship and interaction of the enzyme with inhibitor in an attempt to explore how the activity of human granzyme M is controlled under physiological conditions. It is concluded from the present study that the post-translational modification, including Oglycosylation of serine, phosphorylation of serine and threonine and myristoylation of glycine, play an important role in the interaction of enzyme with the cell surface membrane and regulate protein trafficking and stability. Phosphorylated serine and threonine also plays a role in tumor elimination, viral clearance and tissue repair. In Gzm M there are cationic sites, cs1 and cs2 that may participate in binding of Gzm M to the cell surface, thereby promoting its uptake and eventual release into the cytoplasm. Gzm M shows apoptotic activity both by caspase dependent and independent pathways. Modeling of inhibitors bound to the granzyme active site shows that the dimer also contributes to substrate specificity in a unique manner by extending the active-site cleft.