Residual cytotoxicity and granzyme K expression in granzyme A-deficient cytotoxic lymphocytes

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.

Widespread discrepancy in Nnt genotypes and genetic backgrounds complicates granzyme A and other knockout mouse studies

Granzyme A (GZMA) is a serine protease secreted by cytotoxic lymphocytes, with Gzma-/- mouse studies having informed our understanding of GZMA's physiological function. We show herein that Gzma-/- mice have a mixed C57BL/6J and C57BL/6N genetic background and retain the full-length nicotinamide nucleotide transhydrogenase (Nnt) gene, whereas Nnt is truncated in C57BL/6J mice. Chikungunya viral arthritis was substantially ameliorated in Gzma-/- mice; however, the presence of Nnt and the C57BL/6N background, rather than loss of GZMA expression, was responsible for this phenotype. A new CRISPR active site mutant C57BL/6J GzmaS211A mouse provided the first insights into GZMA's bioactivity free of background issues, with circulating proteolytically active GZMA promoting immune-stimulating and pro-inflammatory signatures. Remarkably, k-mer mining of the Sequence Read Archive illustrated that ≈27% of Run Accessions and ≈38% of BioProjects listing C57BL/6J as the mouse strain had Nnt sequencing reads inconsistent with a C57BL/6J genetic background. Nnt and C57BL/6N background issues have clearly complicated our understanding of GZMA and may similarly have influenced studies across a broad range of fields.

Therapeutic targeting with DABIL-4 depletes myeloid suppressor cells in 4T1 triple-negative breast cancer model

In many solid tumors including triple-negative breast cancer (TNBC), upregulation of the interleukin-4 receptor (IL-4R) has been shown to promote cancer cell proliferation, apoptotic resistance, metastatic potential, and a Th2 response in the tumor microenvironment (TME). Since immunosuppressive cells in the TME and spleen including myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs) also express the IL-4R, we hypothesized that selective depletion of IL-4R-bearing cells in TNBC would result in the direct killing of tumor cells and the depletion of immunosuppressive cells and lead to an enhanced antitumor response. To selectively target IL-4R+ cells, we employed DABIL-4, a fusion protein toxin consisting of the catalytic and translocation domains of diphtheria toxin fused to murine IL-4. As anticipated, DABIL-4 has potent cytotoxic activity against TNBC cells both in vitro and in vivo. We demonstrate in the murine 4T1 TNBC model that DABIL-4 significantly reduces tumor growth, splenomegaly, and lung metastases. Importantly, we also show that the administration of DABIL-4 results in the selective depletion of MDSCs, TAMs, and regulatory T cells in treated mice, with a concomitant increase in IFN-γ+ CD8 effector T cells in the TME. Since the 4T1 antitumor activity of DABIL-4 was largely diminished in IL-4R knockout mice, we postulate that DABIL-4 functions primarily as an immunotherapeutic by the depletion of MDSCs, TAMs, and regulatory T cells. NanoString analysis of control and treated tumors confirmed and extended these observations by showing a marked decline of mRNA transcripts that are associated with tumorigenesis and metastasis. In conclusion, we demonstrate that DABIL-4 targeting of both tumor and immunosuppressive host cells likely represents a novel and effective treatment strategy for 4T1 TNBC and warrants further study.

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.

Distinct transcriptome profiles of Gag-specific CD8+ T cells temporally correlated with the protection elicited by SIVΔnef live attenuated vaccine

The live attenuated vaccine (LAV) SIVmac239Δnef (SIVΔnef) confers the best protection among all the vaccine modalities tested in rhesus macaque model of HIV-1 infection. This vaccine has a unique feature of time-dependent protection: macaques are not protected at 3–5 weeks post vaccination (WPV), whereas immune protection emerges between 15 and 20 WPV. Although the exact mechanisms of the time-dependent protection remain incompletely understood, studies suggested that both cellular and humoral immunities contribute to this time-dependent protection. To further elucidate the mechanisms of protection induced by SIVΔnef, we longitudinally compared the global gene expression profiles of SIV Gag-CM9+ CD8+ (Gag-specific CD8+) T cells from peripheral blood of Mamu-A*01+ rhesus macaques at 3 and 20 WPV using rhesus microarray. We found that gene expression profiles of Gag-specific CD8+ T cells at 20 WPV are qualitatively different from those at 3 WPV. At 20 WPV, the most significant transcriptional changes of Gag-specific CD8+ T cells were genes involved in TCR signaling, differentiation and maturation toward central memory cells, with increased expression of CCR7, TCRα, TCRβ, CD28 and decreased expression of CTLA-4, IFN-γ, RANTES, granzyme A and B. Our study suggests that a higher quality of SIV-specific CD8+ T cells elicited by SIVΔnef over time contributes to the maturation of time-dependent protection.

Identification of differentially expressed proteins of brain tissue in response to methamidophos in flounder (Paralichthys olivaceus)

Methamidophos (MAP), an organophosphorus pesticide used around the world, has been associated with a wide spectrum of toxic effects on organisms in the environment. In this study, the flounder Paralichthys olivaceus was subjected to 10 mg/L MAP for 72 h and 144 h, and the morphological and proteomic changes in the brain were observed, analyzed and compared with those in the non-exposed control group. Under the light microscope and transmission electron microscope, MAP had evidently induced changes in or damage to the flounder tissues. Gas chromatography analysis demonstrated that the MAP residues were significantly accumulated in the flounder brain tissues. Proteomic changes in the brain tissue were revealed using two-dimensional gel electrophoresis and 27 protein spots were observed to be significantly changed by MAP exposure. The results indicated that the regulated proteins were involved in immune and stress responses, protein biosynthesis and modification, signal transduction, organismal development, and 50% of them are protease. qRT-PCR was used to further detect the corresponding change of transcription. These data may be beneficial to understand the molecular mechanism of MAP toxicity in flounder, be very useful for MAP-resistance screening in flounder culture. According to our results and analyzing, heat shock protein 90 (HSP90) and granzyme K (GzmK) had taken important part in immune response to MAP-stress and could be biomarkers for MAP-stress in flounder.

Substrate specificities of the granzyme tryptases A and K

The physiological roles of the granzymes A and K have been debated, especially concerning their involvement in cytotoxic and inflammatory processes. By performing N-terminal COFRADIC assisted N-terminomics on the homologous human granzymes A and K, we here provide detailed data on their substrate repertoires, their specificities, and differences in efficiency by which they cleave their substrates, all of which may aid in elucidating their key substrates. In addition, the so far uncharacterized mouse granzyme K was profiled alongside its human orthologue. While the global primary specificity profiles of these granzymes appear quite similar as they revealed only subtle differences and pointed to substrate occupancies in the P1, P1', and P2' position as the main determinants for substrate recognition, differential analyses unveiled distinguishing substrate subsite features, some of which were confirmed by the more selective cleavage of specifically designed probes.

Expression of granzyme B sensitizes ALK+ ALCL tumour cells to apoptosis-inducing drugs

Background

The serine protease Granzyme B (GzB) is primarily expressed by cytotoxic T lymphocytes and natural killer cells, and functions in allowing these cells to induce apoptosis in virally-infected or transformed cells. Cancers of both lymphoid and non-lymphoid origin also express GzB, and in some cases this expression has been linked to pathogenesis or sensitizing tumour cells to cell death. For example, GzB expression in urothelial carcinoma was implicated in promoting tumour cell invasion, whereas its expression in nasal-type NK/T lymphomas was found to correlate with increased apoptosis. GzB expression is also a hallmark of the non-Hodgkin lymphoma, anaplastic lymphoma kinase-positive, anaplastic large cell lymphoma (ALK+ ALCL). Given the fact that ALK+ ALCL exhibits high levels of apoptosis and is typically responsive to conventional chemotherapy, we examined whether GzB expression might play a role in sensitizing ALK+ ALCL tumour cells to apoptosis.

Methods

ALK+ ALCL cell lines stably expressing GzB or non-targeting (control) shRNA were generated and apoptosis was examined by anti-PARP western blotting and terminal deoxynucleotidyl transferase dUTP nick end labelling. Both spontaneous apoptosis and apoptosis in response to treatment with staurosporine or doxorubicin were investigated. In order to assess whether additional granzymes might be important in promoting cell death in ALK+ ALCL, we examined whether other human granzymes were expressed in ALK+ ALCL cell lines using reverse-transcriptase PCR and western blotting.

Results

Expression of several GzB shRNAs in multiple ALK+ ALCL cell lines resulted in a significant decrease in GzB levels and activity. While spontaneous apoptosis was similar in ALK+ ALCL cell lines expressing either GzB or control shRNA, GzB shRNA-expressing cells were less sensitive to staurosporine or doxorubicin-induced apoptosis as evidenced by reduced PARP cleavage and decreased DNA fragmentation. Furthermore, we found that GzB is the only granzyme that is expressed at significant levels in ALK+ ALCL cell lines.

Conclusions

Our findings are the first to demonstrate that GzB expression sensitizes ALK+ ALCL cell lines to drug-induced apoptosis. This suggests that GzB expression may be a factor contributing to the favourable response of this lymphoma to treatment.

Serine protease inhibition attenuates rIL-12-induced GZMA activity and proinflammatory events by modulating the Th2 profile from estrogen-treated mice

Estrogen has potent immunomodulatory effects on proinflammatory responses, which can be mediated by serine proteases. We now demonstrate that estrogen increased the extracellular expression and IL-12-induced activity of a critical member of serine protease family Granzyme A, which has been shown to possess a novel inflammatory persona. The inhibition of serine protease activity with inhibitor 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride significantly diminished enhanced production of proinflammatory interferon-γ, IL-1β, IL-1α, and Granzyme A activity even in the presence of a Th1-inducing cytokine, IL-12 from splenocytes from in vivo estrogen-treated mice. Inhibition of serine protease activity selectively promoted secretion of Th2-specific IL-4, nuclear phosphorylated STAT6A, signal transducer and activator of transcription (STAT)6A translocation, and STAT6A DNA binding in IL-12-stimulated splenocytes from estrogen-treated mice. Inhibition with 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride reversed the down-regulation of Th2 transcription factors, GATA3 and c-Maf in splenocytes from estrogen-exposed mice. Although serine protease inactivation enhanced the expression of Th2-polarizing factors, it did not reverse estrogen-modulated decrease of phosphorylated STAT5, a key factor in Th2 development. Collectively, data suggest that serine protease inactivity augments the skew toward a Th2-like profile while down-regulating IL-12-induced proinflammatory Th1 biomolecules upon in vivo estrogen exposure, which implies serine proteases as potential regulators of inflammation. Thus, these studies may provide a potential mechanism underlying the immunomodulatory effect of estrogen and insight into new therapeutic strategies for proinflammatory and female-predominant autoimmune diseases.

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.

Granzyme A activates another way to die

Granzyme A (GzmA) is the most abundant serine protease in killer cell cytotoxic granules. GzmA activates a novel programed cell death pathway that begins in the mitochondrion, where cleavage of NDUFS3 in electron transport complex I disrupts mitochondrial metabolism and generates reactive oxygen species (ROS). ROS drives the endoplasmic reticulum-associated SET complex into the nucleus, where it activates single-stranded DNA damage. GzmA also targets other important nuclear proteins for degradation, including histones, the lamins that maintain the nuclear envelope, and several key DNA damage repair proteins (Ku70, PARP-1). Cells that are resistant to the caspases or GzmB by overexpressing bcl-2 family anti-apoptotic proteins or caspase or GzmB protease inhibitors are sensitive to GzmA. By activating multiple cell death pathways, killer cells provide better protection against a variety of intracellular pathogens and tumors. GzmA also has proinflammatory activity; it activates pro-interleukin-1beta and may also have other proinflammatory effects that remain to be elucidated.

Death by a thousand cuts: granzyme pathways of programmed cell death

The granzymes are cell death-inducing enzymes, stored in the cytotoxic granules of cytotoxic T lymphocytes and natural killer cells, that are released during granule exocytosis when a specific virus-infected or transformed target cell is marked for elimination. Recent work suggests that this homologous family of serine esterases can activate at least three distinct pathways of cell death. This redundancy likely evolved to provide protection against pathogens and tumors with diverse strategies for evading cell death. This review discusses what is known about granzyme-mediated pathways of cell death as well as recent studies that implicate granzymes in immune regulation and extracellular proteolytic functions in inflammation.

Acquisition of murine NK cell cytotoxicity requires the translation of a pre-existing pool of granzyme B and perforin mRNAs

Although activated murine NK cells can use the granule exocytosis pathway to kill target cells immediately upon recognition, resting murine NK cells are minimally cytotoxic for unknown reasons. Here, we showed that resting NK cells contained abundant granzyme A, but little granzyme B or perforin; in contrast, the mRNAs for all three genes were abundant. Cytokine-induced in vitro activation of NK cells resulted in potent cytotoxicity associated with a dramatic increase in granzyme B and perforin, but only minimal changes in mRNA abundance for these genes. The same pattern of regulation was found in vivo with murine cytomegalovirus infection as a physiologic model of NK cell activation. These data suggest that resting murine NK cells are minimally cytotoxic because of a block in perforin and granzyme B mRNA translation that is released by NK cell activation.

Granzyme K Directly Processes Bid to Release Cytochrome c and Endonuclease G Leading to Mitochondria-dependent Cell Death

Granule-mediated cytolysis is the major pathway for killer lymphocytes to kill pathogens and tumor cells. Little is known about how granzyme K functions in killer lymphocyte-mediated cytolysis. We previously showed that human GzmK triggers rapid cell death independently of caspase activation with single-stranded DNA nicks, similar to GzmA. In this study we found that GzmK can induce rapid reactive oxygen species generation and collapse of mitochondrial inner membrane potential (DeltaPsim). Blockade of reactive oxygen species production by antioxidant N-acetylcysteine or superoxide scavenger Tiron inhibits GzmK-induced cell death. Moreover GzmK targets mitochondria by cleaving Bid to generate its active form tBid, which disrupts the outer mitochondrial membrane leading to the release of cytochrome c and endonuclease G. Thus, we showed herein that GzmK-induced caspase-independent death occurs through Bid-dependent mitochondrial damage that is different from GzmA.

Granzyme K cleaves the nucleosome assembly protein SET to induce single-stranded DNA nicks of target cells

Although granzymes (Gzms) A- and B-induced cell death pathways have been defined, little is known about how other orphan Gzms function in CTL-mediated cytotoxicity. GzmK and A are tryptases among all the Gzms of humans and they are closely linked on the same chromosome. In this study, we showed that GzmK can be efficiently delivered into target cells with a cationic lipid protein transfection reagent Pro-Ject. We found human GzmK triggers rapid cell death independently of caspase activation. The features of death are characterized by rapid externalization of phosphatidylserine, nuclear morphological changes and single-stranded DNA nicks. GzmK hydrolyzes the nucleosome assembly protein SET in its recombinant and native forms or in intact cells. Cleavage of SET by GzmK abrogates its nucleosome assembly activity. After GzmK loading, SET and DNase NM23H1 rapidly translocate into the nucleus and SET is cleaved, where the nuclease activity of NM23H1 is activated to nick chromosomal DNA.

Gene and protein characteristics reflect functional diversity of CD56dim and CD56bright NK cells

Recent findings underline the role of NK cell subsets in regulating adaptive immunity. To define characteristics of NK cell subpopulations, purified CD56(dim) and CD56(bright) NK cells were analyzed by using gene chip arrays covering more than 39,000 transcripts. Gene profiling revealed resting NK cells to differ in respect to 473 transcripts with 176 exclusively expressed in CD56(dim) and 130 solely in CD56(bright) NK cells. Results were compared with array analyses using mRNA obtained from activated CD56(dim) and CD56(bright) NK cells. In this approach, NK cell receptors, cytolytic molecules, adhesion structures, and chemokine ligands showed differential expression patterns in the two subpopulations. These data were validated using FACS, RT-qPCR, or cytokine bead array (CBA) techniques. Cytokines produced by CD56(dim) and CD56(bright) NK cells were determined using a protein array covering 79 different bioactive mediators. GDNF, IGFBP-1, EGF, and TIMP-2 were detected in both subsets. In contrast, IGFBP-3 and IGF-1 were mainly produced by CD56(dim), while GM-CSF, TARC, and TGFbeta3 were expressed by CD56(bright) NK cells. In summary, we report new characteristic features of CD56(dim) and CD56(bright) NK cells, further underscoring that they represent independent populations with functionally diverse capabilities. The information on NK cells generated in this study will help to define corresponding NK cell populations in other species that lack CD56 expression on NK cells, such as mice. This will subsequently lead to the establishment of suitable animal models for detailed analysis of NK cell populations in vivo.

Differential expression of the granzymes A, K and M and perforin in human peripheral blood lymphocytes

Granzymes (Gzm) are a group of serine proteases which are stored in the granules of cytotoxic lymphocytes. In humans, five granzymes have been characterized to date at the molecular level. While GzmA and GzmB have been extensively studied, little is known about GzmH, GzmK and GzmM. In this study, we describe the generation of mAbs against human GzmK and GzmM by genetic immunization. The obtained anti-GzmK and anti-GzmM mAbs are not cross-reactive with GzmA, GzmB, GzmM and GzmA, GzmB, GzmK, respectively, and show a granular staining pattern in human lymphocytes. Flow cytometric analysis of peripheral blood lymphocytes revealed that GzmA, GzmM and perforin show a similar distribution. They are expressed in almost all CD16+CD56+ NK cells, CD3+CD56+ NKT cells and gammadelta T cells as well as in 20-30% of all CD3+CD8+ TC cells. Surprisingly, GzmK was not detected in the highly cytotoxic CD16+CD56+ NK cells but was preferentially expressed in lymphocytes of the T cell lineage, staining 20% of CD3+CD8+ TC cells, 50% of CD3+CD56+ NKT cells and 40% of gammadelta T cells, as well as 60% of the small sub-population of CD56bright+ NK cells. Our data suggest that human granzymes are differentially expressed in distinct sub-populations of peripheral blood lymphocytes.

Functional Analysis of Granzyme M and Its Role in Immunity to Infection

Cytotoxic lymphocytes express a large family of granule serine proteases, including one member, granzyme (Grz)M, with a unique protease activity, restricted expression, and distinct gene locus. Although a number of Grzs, including GrzM, have been shown to mediate target cell apoptosis in the presence of perforin, the biological activity of Grz has been restricted to control of a number of viral pathogens, including two natural mouse pathogens, ectromelia, and murine CMV (MCMV). In this article, we describe the first reported gene targeting of GrzM in mice. GrzM-deficient mice display normal NK cell/T cell development and homeostasis and intact NK cell-mediated cytotoxicity of tumor targets as measured by membrane damage and DNA fragmentation. GrzM-deficient mice demonstrated increased susceptibility to MCMV infection typified by the presence of more viral inclusions and transiently higher viral burden in the visceral organs of GrzM-deficient mice compared with wild-type (WT) mice. The cytotoxicity of NK cells from MCMV-infected GrzM-deficient mice remained unchanged and, like WT control mice, GrzM-deficient mice eventually effectively cleared MCMV infection from the visceral organs. In contrast, GrzM-deficient mice were as resistant as WT control mice to mouse pox ectromelia infection, as well as challenge with a number of NK cell-sensitive tumors. These data confirm a role for GrzM in the host response to MCMV infection, but suggest that GrzM is not critical for NK cell-mediated cytotoxicity.

Gene expression characteristics of CD28null memory phenotype CD8+ T cells and its implication in T-cell aging

Accumulation of CD28(null)CD8(+) T cells is considered as one of the hallmarks of aging in the human immune system. However, the precise changes of CD28(null)CD8(+) T cells, compared to those of the precursor CD28(+)CD8(+) memory T cells, have not been determined. In this study, we present an analysis of the global gene expression profiles of CD28(+) and CD28(null) memory phenotype CD8(+) T cells. These two CD8(+) T subsets exhibited an overall similar gene expression profile with only a few dozen genes that were differentially expressed. A wide range of functions, including co-stimulation, effector activity, signaling, and transcription, were possessed by these differentially expressed genes, reflecting significant functional changes of CD28(null) memory phenotype CD8(+) T cells from their CD28(+) counterparts. In addition, CD28(null) memory CD8(+) T cells expressed several natural killer cell receptors and high levels of granzymes, perforin, and FasL, indicating an increasing capacity for cytotoxicity during memory CD8(+) T-cell aging. Interestingly, in vitro culture of these two subsets with interleukin-15 showed that similar gene expression changes occurred in both subsets. Our analysis provides the gene expression portraits of CD28(null) memory phenotype CD8(+) T cells and alteration from their CD28(+) counterparts and suggests potential mechanisms of T-cell aging.

Molecular understanding of oxygen-tension and patient-variability effects on ex vivo expanded T cells

Immunotherapy with ex vivo cultured T cells depends on a large supply of biologically active cells. Understanding the effects of culture parameters is essential for improving the proliferation and efficacy of the expanded cells. Low oxygen tension (5% pO(2)) was previously reported to improve T-cell expansion and alter cellular phenotypic characteristics compared to T cells cultured at 20% pO(2). Here we report the use of DNA-array based transcriptional analysis coupled with protein-level analysis to provide molecular insights into pO(2) and patient-variability effects on expanded primary human T cells. Analysis of seven blood samples showed that reduced pO(2) results in higher expression of genes important in lymphocyte biology, immune function, and cell-cycle progression. 20% pO(2) resulted in higher expression of genes involved in stress response, cell death, and cellular repair. Expression of granzyme A (gzmA) was found to be significantly regulated by oxygen tension with cells at 5% pO(2) having greater gzmA expression than at 20% pO(2). Protein-level analysis of gzmA was consistent with transcriptional analysis. Granzyme K (gzmK) was coexpressed with gzmA, whereas Granzyme B (gzmB) expression was found to precede the expression of both gzmA and gzmK in 15-day cultures. Temporal gene expression patterns for seven blood samples demonstrate that most genes are expressed by all patient samples in similar temporal patterns. However, several patient-specific gene clusters were identified, and one cluster was found to correlate well with cell proliferation and may potentially be used to predict patient-specific T-cell expansion.

Apoptotic pathways are selectively activated by granzyme A and/or granzyme B in CTL-mediated target cell lysis

Purified cytolytic T lymphocyte (CTL) proteases granzyme (gzm)A and gzmB with sublytic dose of perforin (perf) initiate distinct proapoptotic pathways. Their physiological relevance in CTL-mediated target cell apoptosis is elusive. Using ex vivo virus-immune CD8(+) T cells from mice deficient in perf, gzmA and/or gzmB, and the Fas-resistant EL4.F15 tumor target cell, we show that (a) CTL from gzmA(-/-) or gzmB(-/-) mice similarly induced early proapoptotic features, such as phosphatidyl serine (PS) exposure on plasma membrane, Delta Psi(m) loss, and reactive oxygen radical generation, though with distinct kinetics; (b) CTL from gzmA(-/-) but not from gzmB(-/-) mice activate caspase 3 and 9; (c) PS exposure induced by CTL from gzmA(-/-) or gzmB(-/-) mice is prevented, respectively, by caspase inhibitors or by reactive oxygen scavengers without interfering with target cell death; and (d) all gzm-induced apoptotic features analyzed depend critically on perf. Thus, perf is the principal regulator in CTL-mediated and gzm-facilitated intracellular processes. The ability of gzmA and gzmB to induce multiple independent cell death pathways may be the hosts response to circumvent evasion strategies of pathogens and tumors.

Granzyme N, a Novel Granzyme, Is Expressed in Spermatocytes and Spermatids of the Mouse Testis1

We cloned a cDNA for a novel granzyme, granzyme N (Gzmn), from a mouse testes cDNA library. The testes contained two distinct species of Gzmn mRNA, one of which codes for a complete protein of 248 amino acids with three essential residues required for catalytic activity. The Gzmn mRNA was specifically expressed in the testes of adult mice. The Gzmn expression was found to initiate in the testes at 3 wk of age and to become more prominent as the animal reached sexual maturity. In situ hybridization analysis revealed that both spermatocytes and spermatids of the adult mouse testes express Gzmn mRNA. Consistent with these findings, the protein was immunohistochemically detected in the spermatocytes and spermatids, although some of the germ cells showed no positive staining. Gzmn was demonstrated to be a secretory and N-glycosylated protein that exists in two protein forms in the testes extract. In the cryptorchid testes, the expression of Gzmn transcript was drastically reduced on Postoperative Day 10, whereas the protein level was gradually decreased starting on Day 6. The local heating (43 degrees C, 20 min) of the testes did not change the Gzmn expression level at either 8 or 16 h after treatment. These results suggest that Gzmn is not involved in the process of germ cell apoptosis induced by heat shock, but that it may be involved in spermatogenesis in the mouse testes.

Granzymes and caspase 3 play important roles in control of gammaherpesvirus latency

Gammaherpesviruses can establish lifelong latent infections in lymphoid cells of their hosts despite active antiviral immunity. Identification of the immune mechanisms which regulate gammaherpesvirus latent infection is therefore essential for understanding how gammaherpesviruses persist for the lifetime of their host. Recently, an individual with chronic active Epstein-Barr virus infection was found to have mutations in perforin, and studies using murine gammaherpesvirus 68 (gammaHV68) as a small-animal model for gammaherpesvirus infection have similarly revealed a critical role for perforin in regulating latent infection. These results suggest involvement of the perforin/granzyme granule exocytosis pathway in immune regulation of gammaherpesvirus latent infection. In this study, we examined gammaHV68 infection of knockout mice to identify specific molecules within the perforin/granzyme pathway which are essential for regulating gammaherpesvirus latent infection. We show that granzymes A and B and the granzyme B substrate, caspase 3, are important for regulating gammaHV68 latent infection. Interestingly, we show for the first time that orphan granzymes encoded in the granzyme B gene cluster are also critical for regulating viral infection. The requirement for specific granzymes differs for early versus late forms of latent infection. These data indicate that different granzymes play important and distinct roles in regulating latent gammaherpesvirus infection.

Perforin and Fas act together in the induction of apoptosis, and both are critical in the clearance of lymphocytic choriomeningitis virus infection

In this report we questioned the current view that the two principal cytotoxic pathways, the exocytosis and the Fas ligand (FasL)/Fas-mediated pathway, have largely nonoverlapping biological roles. For this purpose we have analyzed the response of mice that lack Fas as well as granzyme A (gzmA) and gzmB (FasxgzmAxB(-/-)) to infection with lymphocytic choriomeningitis virus (LCMV). We show that FasxgzmAxB(-/-) mice, in contrast to B6, Fas(-/-), and gzmAxB(-/-) mice, do not recover from a primary infection with LCMV, in spite of the expression of comparable numbers of LCMV-immune and gamma interferon-producing cytotoxic T lymphocytes (CTL) in all mouse strains tested. Ex vivo-derived FasxgzmAxB(-/-) CTL lacked nucleolytic activity and expressed reduced cytolytic activity compared to B6 and Fas(-/-) CTL. Furthermore, virus-immune CTL with functional FasL and perforin (gzmAxB(-/-)) are more potent in causing target cell apoptosis in vitro than those expressing FasL alone (perfxgzmAxB(-/-)). This synergistic effect of perforin on Fas-mediated nucleolysis of target cells is indicated by the fact that, compared to perfxgzmAxB(-/-) CTL, gzmAxB(-/-) CTL induced (i) an accelerated decrease in mitochondrial transmembrane potential, (ii) increased generation of reactive oxygen species, and (iii) accelerated phosphatidylserine exposure on plasma membranes. We conclude that perforin does not mediate recovery from LCMV by itself but plays a vital role in both gzmA/B and FasL/Fas-mediated CTL activities, including apoptosis and control of viral infections.

Granzymes in cytolytic lymphocytes--to kill a killer?

Granzymes (gzm) are major components of the granules of cytolytic lymphocytes, natural killer and cytotoxic T cells. Their generally accepted mode of action consists of their directed secretion towards a virus-infected or neoplastic target cell and perforin-dependent delivery to the target cell cytosol, where they engage in various actions resulting in target cell apoptosis. Here, based on observations of infection of gzmAxB(-/-) mice with ectromelia virus, mousepox, we propose an additional--and distinct--function for gzmA and B. In this model, gzm constitute one of the first lines of defence of immune cells against virus infection of immune cells themselves. Accordingly, endogenous gzm interfere with viral replication in cytolytic lymphocytes either directly, as a result of their proteolytic activity, leading to destruction of viral proteins, or indirectly, via: (i) processes akin to the caspase cascade when acting as effector molecules in the induction of target cell apoptosis; or (ii) their capacity to induce early inflammatory mediators. We discuss the predictions of the model in the light of available data.

Characterization of murine cathepsin W and its role in cell-mediated cytotoxicity

Cathepsin W is a member of the papain-like family of cysteine proteases. In this report, we have isolated the cDNA for murine CtsW (mCtsW) from a splenocyte library. The deduced 371-amino-acid sequence shares 68% identity with human CtsW and includes the conserved catalytic triad cysteine, histidine, and asparagine found in all members of this family. In addition to the fulllength form of mCtsW, we have isolated an alternatively spliced form of the mRNA that lacks a complete catalytic triad. An S1 nuclease protection assay and a Western blot analysis showed that mCtsW is mainly restricted to the CD8(+) T cell and natural killer cell compartments. In addition, we confirmed that, like its human homologue, mCtsW is localized mainly to the endoplasmic reticulum and its expression is up-regulated upon activation. We also characterized the mCtsW locus using bacterial artificial chromosome clones. The gene consists of 10 coding exons and 9 introns spanning 3.2 kb. To elucidate the physiologic role of this protease, we generated mice deficient in mCtsW. Our data establish that mCtsW is not required for cytotoxic lymphocyte-induced target cell death in vitro. In addition, mCtsW deficiency does not alter the susceptibility of cytotoxic lymphocytes to suicide or fratricide after degranulation. Thus, mCtsW does not have a unique role in target cell apoptosis or cytotoxic cell survival in vitro.

The orphan granzymes of humans and mice

The granzyme/perforin pathway is a central pathway for lymphocyte-mediated killing in both the innate and adaptive immune systems. This pathway is important in a variety of host defenses, including viral clearance and tumor cell killing, and its dysregulation results in several human and rodent diseases. To date, the majority of reports in this field have concentrated on the functions of granzymes A and B. Recent reports, however, suggest that the non-A/non-B 'orphan' granzymes found in both humans and mice are potentially significant. Although the functions of these orphan granzymes have yet to be fully established, initial data suggests their importance in both immune and nonimmune cells.

Cell-mediated cytotoxicity in recovery from poxvirus infections

The availability of mutant and gene targeted knockout mice with defects in components of cellular cytotoxicity mediated by either the Fas or the exocytosis pathway permitted an analysis of their role in recovery from poxvirus infections. Ectromelia (EV), a natural mouse pathogen causing mousepox, the closely related orthopoxviruses cow pox (CPV) and vaccinia virus (VV), each encode serpins that inhibit Fas mediated apoptosis and lysis of target cells. Nevertheless, distinct differences were seen when the three viruses were inoculated into perforin-deficient mice: highly resistant C57Bl/6 mice became susceptible to low doses of EV; resistance to CPV increased whereas there was no effect on VV infections. Absence of the cytolytic granule associated granzymes (gzm) A and B rendered C57Bl/6 mice increasingly more susceptible to EV infections. Lack of both gzms rendered them as susceptible as perforin deficient mice, despite the presence of functionally active perforin. Elevated EV titres in liver and spleen of gzmA x B deficient mice, early after infection and before cytotoxic T cells were detectable, strongly suggests that these two gzms exert an antiviral effect by a mechanism distinct from effector molecules of NK and cytotoxic T cells.

Cell death induced by granzyme C

Although the functions of granzymes A and B have been defined, the functions of the other highly expressed granzymes (Gzms) of murine cytotoxic lymphocytes (C, D, and F) have not yet been evaluated. In this report, we describe the ability of murine GzmC (which is most closely related to human granzyme H) to cause cell death. The induction of death requires its protease activity and is characterized by the rapid externalization of phosphatidylserine, nuclear condensation and collapse, and single-stranded DNA nicking. The kinetics of these events are similar to those caused by granzyme B, and its potency (defined on a molar basis) is also equivalent. The induction of death did not involve the activation of caspases, the cleavage of BID, or the activation of the CAD nuclease. However, granzyme C did cause rapid mitochondrial swelling and depolarization in intact cells or in isolated mitochondria, and this mitochondrial damage was not prevented by cyclosporin A pretreatment. These results suggest that granzyme C rapidly induces target cell death by attacking nuclear and mitochondrial targets and that these targets are distinct from those used by granzyme B to cause classical apoptosis.

Lymphocyte-mediated cytotoxicity

Virtually all of the measurable cell-mediated cytotoxicity delivered by cytotoxic T lymphocytes and natural killer cells comes from either the granule exocytosis pathway or the Fas pathway. The granule exocytosis pathway utilizes perforin to traffic the granzymes to appropriate locations in target cells, where they cleave critical substrates that initiate DNA fragmentation and apoptosis; granzymes A and B induce death via alternate, nonoverlapping pathways. The Fas/FasL system is responsible for activation-induced cell death but also plays an important role in lymphocyte-mediated killing under certain circumstances. The interplay between these two cytotoxic systems provides opportunities for therapeutic interventions to control autoimmune diseases and graft vs. host disease, but oversuppression of these pathways may also lead to increased viral susceptibility and/or decreased tumor cell killing.

Cytotoxic T lymphocytes: all roads lead to death

Cytotoxic T lymphocytes (CTLs) provide potent defences against virus infection and intracellular pathogens. However, CTLs have a dark side--their lytic machinery can be directed against self-tissues in autoimmune disorders, transplanted cells during graft rejection and host tissues to cause graft-versus-host disease, which is one of the most serious diseases related to CTL function. Although this duplicitous behaviour might seem contradictory, both beneficial and detrimental effects are the result of the same effector proteins. So, an understanding of the mechanisms that are used by CTLs to destroy targets and a knowledge of pathogen immune-evasion strategies will provide vital information for the design of new therapies.

Granzymes are the essential downstream effector molecules for the control of primary virus infections by cytolytic leukocytes

Analysis of perforin-deficient mice has identified the cytolytic pathway and perforin as the preeminent effector molecule in T cell-mediated control of virus infections. In this paper, we show that mice lacking both granzyme A (gzmA) and granzyme B (gzmB), which are, beside perforin, key constituents of cytolytic vesicles, are as incapable as are perforin-deficient mice of controlling primary infections by the natural mouse pathogen ectromelia, a poxvirus. Death of gzmAxgzmB double knockout mice occurred in a dose-dependent manner, despite the expression of functionally active perforin and the absence of an intrinsic defect to generate splenic cytolytic T cells. These results establish that both gzmA and gzmB are indispensable effector molecules acting in concert with perforin in granule exocytosis-mediated host defense against natural viral pathogens.

Biological activities of granzyme K are conserved in the mouse and account for residual Z-Lys-SBzl activity in granzyme A-deficient mice

Tryptase-like activities of T and NK cells contribute to the induction of target cell apoptosis, but only granzyme A (GzmA) has been shown to exhibit Z-Lys-SBzl esterase activity in murine T cells. GzmA-deficient mice exhibit residual Z-Lys-SBzl hydrolyzing activity and almost normal levels of lymphocyte-mediated cytotoxicity. Here we report the cloning and biochemical characterization of recombinant mouse granzyme K (GzmK). The purified murine protein shows Z-Lys-SBzl hydrolyzing activity and is inhibited by bikunin, the light chain of inter-alpha-trypsin inhibitor, like the human homolog. We conclude that GzmK expressed by GzmA-deficient T cells accounts for the remaining Z-Lys-SBzl activity. Functional similarities between GzmA and GzmK may explain the subtle immunological deficits observed in GzmA-deficient mice.

cDNA cloning and expression of a novel serine protease in the mouse brain

A cDNA for a novel serine protease, termed brain type granzyme K (B-GRK) was cloned from the mouse brain. The cDNA codes a protein similar to granzyme K (GRK) but completely different at the N-terminus. Genomic Southern and PCR analysis of the gene suggests B-GRK is the alternative transcription form of GRK. B-GRK and GRK have a different organ-specific expression pattern: B-GRK is expressed in the brain, while GRK is expressed in the spleen. The recombinant fusion protein was detected in the neuro2a cells transfected with a plasmid containing B-GRK sequence. The mRNA for B-GRK/GRK was detected in cerebral cortex, hippocampus and diencephalon of the mouse brain. In situ hybridization for B-GRK/GRK revealed that several regions in the forebrain and hypothalamus express the mRNA. Developmental analysis showed that in the prenatal stage, the mRNA was expressed also in pituitary and pineal body in addition to the brain.

Perforin-dependent nuclear targeting of granzymes: A central role in the nuclear events of granule-exocytosis-mediated apoptosis?

Programmed cell death, apoptosis, involves very distinctive changes within the target cell nucleus, including margination of the chromatin, DNA fragmentation and breakdown of the nuclear envelope. Cytolytic granule-mediated target cell apoptosis is effected, in part, through synergistic action of the membrane-acting protein perforin and serine proteases, such as granzymes A or B. Recent work using confocal laser scanning microscopy as well as other techniques supports the idea that perforin-dependent translocation of granzymes to the nucleus of target cells plays a central role in effecting the nuclear changes associated with apoptosis. In vitro experiments indicate that granzyme nuclear import follows a novel pathway, being independent of ATP, not inhibitable by non-hydrolysable GTP analogues and involving binding within the nucleus, unlike conventional signal- dependent nuclear protein import. In intact cells, perforin-dependent nuclear entry of granzymes precedes the nuclear events of apoptosis such as DNA fragmentation and nuclear envelope breakdown; prevention of granzyme nuclear translocation through bcl2 overexpression or treatment of target cells with inhibitors of caspase activation blocks these events. Nuclear localization of granzymes thus appears to be central to induction of the nuclear changes associated with cytolytic granule-mediated apoptosis.

Granzyme A initiates an alternative pathway for granule-mediated apoptosis

Granzyme (gzm) B-deficient cytotoxic lymphocytes (CTL) have a severe defect in the rapid induction of target cell apoptosis that is almost completely corrected by prolonged incubation of the CTL effectors and their targets. We show in this report that perforin-dependent, gzmB-independent cytotoxicity is caused by gzmA (or tightly linked genes). CTL deficient for gzmA and gzmB retain normal perforin function, but these CTL have a cytotoxic defect in vivo that is as severe as perforin-deficient CTL. Collectively, these results suggest that perforin provides target cell access and/or trafficking signals for the gzms, and that the gzms themselves deliver the lethal hits. The gzmA pathway appears to function independently from gzmB and may therefore provide a critical "back-up" system when gzmB is inhibited in the target cell.

How do cytotoxic lymphocytes kill their targets?

CD8+ cytotoxic lymphocytes, natural killer cells and lymphokine-activated killer cells depend primarily on the perforin/granzyme system to kill their targets, while CD4+ T cells utilize Fas and other mechanisms to induce cell death. The molecular mechanisms used by these pathways to induce target cell apoptosis may converge on common death substrates.

Cytotoxic T cell responses to intracellular pathogens

Host defense against intracellular pathogens is thought to require cytotoxic T cells. Recent studies have investigated the impact of host cell lysis and cytokine production by cytotoxic T lymphocytes on the fate of intracellular pathogens. The identification of two mechanisms of lysis induced by cytotoxic T lymphocytes--the granule exocytosis pathway and the Fas-FasL interaction--have provided new insight into the role of cytotoxic T lymphocyters in immunity to infection.