A natural killer cell granule protein that induces DNA fragmentation and apoptosis

Purification of a 24-kD protease from apoptotic tumor cells that activates DNA fragmentation

We report the purification of a protease from tumor cells undergoing apoptosis that is involved in activating DNA fragmentation. Initial studies revealed that two inhibitors of serine proteases, N-1-tosylamide-2-phenylethylchloromethyl ketone and carbobenzoxy-Ala-Ala-borophe (DK120), suppressed tumor necrosis factor or ultraviolet (UV) light-induced DNA fragmentation in the U937 histiocytic lymphoma as well as UV light-induced DNA fragmentation in the BT-20 breast carcinoma, HL-60 myelocytic leukemia, and 3T3 fibroblasts. The protease was purified by affinity chromatography with DK120 as ligand and showed high activity on a synthetic substrate preferred by elastase-like enzymes (Ala-Ala-Pro-Val p-nitroanilide), but was inactive on the trypsin substrate, N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester, or the chymotrypsin substrate, Ala-Ala-Pro-Phe p-nitroanilide. The activity of the DK120-binding protease purified from U937 cells undergoing apoptosis was increased approximately 10-fold over that recovered from normal cells. Further purification to homogeneity by heparin-Sepharose affinity chromatography followed by reverse phase high-performance liquid chromatography revealed a single band of 24 kD on a silver-stained sodium dodecyl sulfate gel. In addition to protease activity, the purified enzyme induced DNA fragmentation into multiples of 180 basepairs in isolated U937 nuclei. These findings suggest the 24-kD protease is a novel enzyme that activates DNA fragmentation in U937 cells undergoing apoptosis.

Why we live and why we die

Programmed cell death, or apoptosis, is a tightly-controlled phenomenon that is important in many pathological processes. Several important regulators of apoptosis have now been identified and can be targeted for manipulation.

The detergent Triton X-100 induces a death pattern in human carcinoma cell lines that resembles cytotoxic lymphocyte-induced apoptosis

The detergent Triton X-100 (TX100) was used with the intention to establish a model for necrotic cell death. However, TX100 was found to induce apoptotic and necrotic death in prostate and colon cancer cell lines. Apoptosis was characterized by the typical morphological features and internucleosomal DNA fragmentation. The rapid onset within 60 min and the lack of inhibition by cycloheximide indicated that apoptosis induced by TX100 was not dependent on protein synthesis. Removal of extracellular calcium blocked internucleosomal DNA fragmentation. This pattern of cell death shows a striking similarity to the effect of cytotoxic lymphocytes on their target cells.

Cleavage of poly(ADP-ribose) polymerase by a proteinase with properties like ICE

Recent studies suggest that proteases of the interleukin 1-beta-converting enzyme (ICE)/ced-3 family are involved in initiating the active phase of apoptosis. Here we identify a novel protease resembling ICE (prICE) that is active in a cell-free system that reproduces the morphological and biochemical events of apoptosis. prICE cleaves the nuclear enzyme poly(ADP-ribose) polymerase (PARP) at a tetrapeptide sequence identical to one of two ICE sites in pro-interleukin-1-beta. However, prICE does not cleave purified pro-interleukin-1-beta, and purified ICE does not cleave PARP, indicating that the two activities are distinct. Inhibition of prICE abolishes all manifestations of apoptosis in the extracts including morphological changes, cleavage of PARP and production of an oligonucleosomal ladder. These studies suggest that prICE might be pivotal in initiating the active phase of apoptosis in vitro and in intact cells.

Human granzyme B is essential for DNA fragmentation of susceptible target cells

We have partially characterized the granules of the human NK cell line, YT-INDY, and assessed granule-mediated lysis and DNA fragmentation of assorted targets. Biochemical studies demonstrated significant quantities of granzyme B (asp-ase) and a heretofore undescribed chymase but no tryptase (i.e., granzyme A or 3) or distinct met-ase. YT-INDY expressed mRNA for granzyme B, perforin and CCPX. The existence of perforin was confirmed by immunoblot. The granules lysed both human and murine NK-sensitive and NK-resistant targets. YT-INDY and NK3.3, two human cytotoxic cells, were also lysed. EGTA reduced lysis by only 50%, suggesting that a perforin-independent lytic pathway is associated with the granules. In addition, 4-(2-aminoethyl) benzenesulfonylfluoride hydrochloride (AEBSF), an inhibitor that selectively blocked the chymase and 3,4-dichloroisocoumarin (DCI), an inhibitor that inactivated both chymase and asp-ase activities, marginally affected lysis. By gel electrophoresis and 125I-labeled deoxyuridine release assay, only murine cells (SP2/0 and YAC-1) underwent DNA fragmentation, and cleavage was completely inhibited by DCI, whereas EGTA, AEBSF and aurintricarboxylic acid (ATA) had no effect. The results, therefore, underscore the central role of granzyme B in granule-mediated DNA fragmentation, emphasize that the protease acts via an ATA-resistant endonuclease pathway and stress that nucleolysis does not invariably accompany granule-mediated cytolysis. Finally, ATA inhibited the asp-ase activity of isolated but not granule-associated granzyme B. ATA, therefore, is not a specific endonuclease inhibitor and results obtained with ATA should be viewed cautiously.

Expression of Fas antigen on keratinocytes in vivo and induction of apoptosis in cultured keratinocytes

Fas antigen, which belongs to a nerve growth factor/tumor necrosis factor receptor superfamily, is a membrane protein that induces apoptosis. In humans, distribution of Fas antigen has been reported on cell lines and lymphocytes. Immunohistochemical studies revealed Fas antigen on the keratinocytes of lesional epidermis in lichenoid drug eruption, erythema multiforme, contact dermatitis, bullous pemphigoid, pemphigus vulgaris, and herpes zoster; it is co-expressed with intercellular adhesion molecule-1. Cultured keratinocytes expressing Fas antigen increased from 8.4% to 34.6% after stimulation with interferon gamma for 24 h. Treatment of interferon-gamma-stimulated keratinocytes with anti-Fas for 48 h resulted in DNA fragmentation and death of 32% of cells, suggesting that Fas antigen may mediate apoptosis. The expression of Fas antigen on keratinocytes in lesional skin suggests that death via Fas antigen may play an important role in the pathogenesis of keratinocyte cytotoxicity.

Conversion of the substrate specificity of mouse proteinase granzyme B

Mouse granzyme B is the prototypic member of a subfamily of serine proteinases expressed in cytolytic lymphocytes. Molecular modelling of granzyme B indicated that the side chain of Arg 208 partially fills the specificity pocket, thus predicting the preference of this enzyme for substrates containing acidic side chains, a feature unique among eukaryotic serine proteinases. Replacement of Arg 208 with glycine results in an enzyme lacking this activity, but which is able to hydrolyze hydrophobic substrates. These results demonstrate unequivocally that the substrate preference of granzyme B is determined by a positive charge in the specificity pocket and also represent one of the few examples of rational and efficient alteration of serine proteinase substrate-specificity following a single amino acid substitution.

A protein produced by a monocytic human cell line can induce apoptosis on tumor cells

A serum-free medium conditioned by U937, a human cell line of monocyte/macrophage origin, was found capable of inducing apoptosis on exponentially growing U937 cells themselves (autocrine suicide). The apoptosis-inducing agent is a macromolecule and possibly a protein (SKT factor), with a relative molecular mass in the range of 18-25 kDa. All human tumor cell lines examined have been induced to apoptosis with high efficiency, whereas non transformed human lymphocytes and monocytes are insensitive to the apoptosis-inducing activity; moreover, partially differentiated U937 are not killed but induced to full maturation. These observations suggest that the SKT factor could possibly be a cytokine with a specific cytotoxic tropism, that resembles in many respects the cytokine tumor necrosis factor (TNF), even though no TNF is detectable in the conditioned medium.

Pharmacological inhibition of programmed lymphocyte death

Programmed cell death (PCD) is a necessary process that helps to regulate the lifespan of lymphocytes and maintain the compartmental balance of lymphoid organs. In addition, PCD is required for the generation and maintenance of self-tolerance. Strategies that inhibit PCD cause profound alterations in the (patho)physiology of the immune system. Here, Guido Kroemer and Carlos Martínez-A. discuss the multiplicity of PCD-inducing pathways, which have been revealed through the use of PCD-inhibitory agents, and analyse the levels at which these agents act.

Apoptosis: programmed cell death in health and disease

Apoptosis is a normal physiological cell death process of eliminating unwanted cells from living organisms during embryonic and adult development. Apoptotic cells are characterised by fragmentation of nuclear DNA and formation of apoptotic bodies. Genetic analysis revealed the involvement of many death and survival genes in apoptosis which are regulated by extracellular factors. There are multiple inducers and inhibitors of apoptosis which interact with target cell specific surface receptors and transduce the signal by second messengers to programme cell death. The regulation of apoptosis is elusive, but defective regulation leads to aetiology of various ailments. Understanding the molecular mechanism of apoptosis including death genes, death signals, surface receptors and signal pathways will provide new insights in developing strategies to regulate the cell survival/death. The current knowledge on the molecular events of apoptotic cell death and their significance in health and disease is reviewed.

Cytotoxic lymphocytes require granzyme B for the rapid induction of DNA fragmentation and apoptosis in allogeneic target cells

We have generated H-2b mice with a homozygous null mutation in the granzyme (gzm) B gene. Gzm B is a neutral serine protease with Aspase activity that is found only in the granules of activated cytolytic T cells, natural killer cells, and lymphokine-activated killer cells. Gzm B-/- mice develop normally and have normal hematopoiesis and lymphopoiesis. In vitro, cytotoxic T lymphocytes (CTL) derived from gzm B-/- animals are able to induce 51Cr release from allotarget cells, but with reduced efficiency. However, gzm B-/- CTL have a profound defect in their ability to induce rapid DNA fragmentation and apoptosis in allogeneic target cells. This defect is kinetic since DNA fragmentation is partially compensated and 51Cr release is completely rescued with long incubation times. We conclude that gzm B serves a critical and nonredundant role for the rapid induction of target cell DNA fragmentation and apoptosis by alloreactive cytotoxic T lymphocytes.

Premature p34cdc2 activation required for apoptosis

Activation of the serine-threonine kinase p34cdc2 at an inappropriate time during the cell cycle leads to cell death that resembles apoptosis. Premature activation of p34cdc2 was shown to be required for apoptosis induced by a lymphocyte granule protease. The kinase was rapidly activated and tyrosine dephosphorylated at the initiation of apoptosis. DNA fragmentation and nuclear collapse could be prevented by blocking p34cdc2 activity with excess peptide substrate, or by inactivating p34cdc2 in a temperature-sensitive mutant. Premature p34cdc2 activation may be a general mechanism by which cells induced to undergo apoptosis initiate the disruption of the nucleus.

Separate metabolic pathways leading to DNA fragmentation and apoptotic chromatin condensation

Apoptosis is the predominant form of cell death observed in a variety of physiological and pathological conditions such as cancer involution, insect metamorphosis, the development of the immune and nervous systems, and embryogenesis. The typical nuclear changes taking place in apoptotic cells include extensive condensation of chromatin and internucleosomal DNA fragmentation into units of 200 base pairs. However, the mechanisms responsible for both chromatin condensation and DNA fragmentation have yet to be elucidated. In this study, micrococcal nuclease and the divalent cations, Ca2+ and Mg2+, were applied to isolated nuclei in an attempt to reconstitute in vitro the digestion of genomic DNA associated with apoptosis. Micrococcal nuclease was found to induce a typical pattern of DNA fragmentation, but did not give rise to chromatin condensation, whereas Ca2+/Mg2+ induced both chromatin condensation and DNA fragmentation in isolated mouse liver nuclei. When the endonuclease inhibitor ZnCl2 was used, the DNA fragmentation induced by Ca2+/Mg2+ in nuclei could be completely inhibited, but chromatin condensation still occurred. For comparison, intact liver cells were treated with valinomycin, a potassium ionophore, which gave rise to an atypical cell death, with chromatin condensation appearing without DNA fragmentation. Our results suggest that endonuclease activation in apoptosis is neither necessary nor sufficient to induce chromatin condensation, and that DNA fragmentation and chromatin condensation may be triggered through separate pathways during apoptosis.

Target cell death triggered by cytotoxic T lymphocytes: a target cell mutant distinguishes passive pore formation and active cell suicide mechanisms

The role of the target cell in its own death mediated by cytotoxic T lymphocytes (CTL) has been controversial. The ability of the pore-forming granule components of CTL to induce target cell death directly has been taken to suggest an essentially passive role for the target. This view of CTL-mediated killing ascribes to the target the single role of providing an antigenic stimulus to the CTL; this signal results in the vectoral degranulation and secretion of pore-forming elements onto the target. On the other hand, by a number of criteria, target cell death triggered by CTL appears fundamentally different from death resulting from membrane damage and osmotic lysis. CTL-triggered target cell death involves primary internal lesions of the target cell that reflect a physiological cell death process. Orderly nuclear disintegration, including lamin phosphorylation and solubilization, chromatin condensation, and genome digestion, are among the earliest events, preceding the loss of plasma membrane integrity. We have tested directly the involvement of the target cell in its own death by examining whether we could isolate mutants of target cells that have retained the ability to be recognized by and provide an antigenic stimulus to CTL while having lost the capacity to respond by dying. Here, we describe one such mutant, BW87. We have used this CTL-resistant mutant to analyze the mechanisms of CTL-triggered target cell death under a variety of conditions. The identification of a mutable target cell element essential for the cell death response to CTL provides genetic evidence that target cell death reflects an active cell suicide process similar to other physiological cell deaths.

Hypothesis: cytotoxic lymphocyte granule serine proteases activate target cell endonucleases to trigger apoptosis

Upon interaction with target cells, cytotoxic T lymphocytes and natural killer cells vectorially secrete highly specialized cytoplasmic granules containing perforin and a family of serine proteases (granzymes). This granule exocytosis mechanism of cytolysis is of patho-physiological importance, and usually results in target cell DNA fragmentation. Neither perforin nor granzymes possess inherent nuclease activity, but in combination they can induce target cell apoptosis. Perforin forms transmembrane pores in the target cell, thereby enabling granzymes to access target cell substrates. The target cell substrates of granzymes are unknown, but granzyme A binding and cleavage of the nuclear shuttle protein nucleolin in target cells demonstrates that granzymes may act on nuclear substrates. Furthermore, the presence of granzyme B and other granzyme activities in the nucleus of cytotoxic lymphocytes indicates that granzymes can be transported from the cytoplasm to the nucleus. It is hypothesized that perforin enables effector granzymes to enter the target cell cytoplasm and following their transport into the nucleus, granzymes cleave specific target cell nuclear proteins to activate autolytic endonucleases that fragment DNA. In cytotoxic effectors, these nuclear substrates are normally protected from granzymes by endogenous inhibitors.

Target cell death triggered by cytotoxic T lymphocytes: a target cell mutant distinguishes passive pore formation and active cell suicide mechanisms

The role of the target cell in its own death mediated by cytotoxic T lymphocytes (CTL) has been controversial. The ability of the pore-forming granule components of CTL to induce target cell death directly has been taken to suggest an essentially passive role for the target. This view of CTL-mediated killing ascribes to the target the single role of providing an antigenic stimulus to the CTL; this signal results in the vectoral degranulation and secretion of pore-forming elements onto the target. On the other hand, by a number of criteria, target cell death triggered by CTL appears fundamentally different from death resulting from membrane damage and osmotic lysis. CTL-triggered target cell death involves primary internal lesions of the target cell that reflect a physiological cell death process. Orderly nuclear disintegration, including lamin phosphorylation and solubilization, chromatin condensation, and genome digestion, are among the earliest events, preceding the loss of plasma membrane integrity. We have tested directly the involvement of the target cell in its own death by examining whether we could isolate mutants of target cells that have retained the ability to be recognized by and provide an antigenic stimulus to CTL while having lost the capacity to respond by dying. Here, we describe one such mutant, BW87. We have used this CTL-resistant mutant to analyze the mechanisms of CTL-triggered target cell death under a variety of conditions. The identification of a mutable target cell element essential for the cell death response to CTL provides genetic evidence that target cell death reflects an active cell suicide process similar to other physiological cell deaths.

T-cell based cancer immunotherapy: direct or redirected tumor-cell recognition?

In development of strategies for immunotherapy of cancer a new emphasis is emerging, termed T-cell retargeting, which involves artificial redirection of cytotoxic T lymphocytes (CTL) against cancer cells, using bispecific reagents. In this article, Gideon Beun, Cornelis van de Velde and Gert Jan Fleuren evaluate this potential strategy for cellular immunotherapy, and propose how the gap between in vitro results and clinical application might be bridged.

Protease inhibitors selectively block T cell receptor-triggered programmed cell death in a murine T cell hybridoma and activated peripheral T cells

The hypothesis that cytoplasmic proteases play a functional role in programmed cell death was tested by examining the effect of protease inhibitors on the T cell receptor-mediated death of the 2B4 murine T cell hybridoma and activated T cells. The cysteine protease inhibitors trans-epoxysuccininyl-L-leucylamido-(4-guanidino) butane (E-64) and leupeptin, the calpain selective inhibitor acetyl-leucyl-leucyl-normethional, and the serine protease inhibitors diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride, all showed dose-dependent blocking of the 2B4 death response triggered by the T cell receptor complex and by anti-Thy-1. These protease inhibitors enhanced rather than inhibited IL-2 secretion triggered by T cell receptor cross-linking, showing that they did not act by preventing signal transduction. Growth inhibition induced by cross-linking the 2B4 T cell receptor, measured by inhibition of thymidine incorporation, was not generally blocked by these protease inhibitors. All five of these protease inhibitors enhanced rather than blocked 2B4 cell death triggered by dexamethasone, an agent previously shown to have a death pathway antagonistic with that of the TCR. 2B4 cytolysis by the cytotoxic agents staphylococcal alpha-toxin and dodecyl imidazole, and that caused by hypotonic conditions, was not significantly affected by the five protease inhibitors tested. The selected protease inhibitors blocked both the apoptotic nuclear morphology changes and DNA fragmentation induced by T cell receptor cross-linking, and enhanced both these properties induced by dexamethasone in 2B4 cells. The T cell receptor-induced death of activated murine lymph node T cells and human peripheral blood CD4+ T cells was blocked by both cysteine and serine protease inhibitors, showing that the protease-dependent death pathway also operates in these systems.

Origin and differentiation of hepatic natural killer cells (pit cells)

Liver sinusoids contain a population of large granular lymphocytes or natural killer cells, originally termed pit cells. After isolation and purification, these cells were separated into a low-density and a high-density fraction. The liver low-density fraction differs significantly in morphology and function from cells of the blood, whereas the liver high-density fraction shows intermediate properties. In this study we demonstrate that this morphological and functional heterogeneity is based on subsequent steps of differentiation of the large granular lymphocytes within the liver. When cell proliferation was suppressed by sublethal total body irradiation, the life span of the hepatic large granular lymphocytes could be determined: high-density and low-density populations were totally depleted within 1 and 2 wk after irradiation, respectively. By using intravenous asialo-GM1 anti-serum to deplete animals of asialo-GM1-positive cells, we found that the depletion of the asialo-GM1-positive cells preceded the depletion of asialo-GM1-negative hepatic low-density large granular lymphocytes by approximately 1 wk. Direct evidence that the asialo-GM1-positive high-density large granular lymphocytes are precursors of the low-density large granular lymphocytes was given by adoptive transfer experiments with fluorescent-labeled high-density cells. Three days after their injection, labeled large granular lymphocytes were found in the hepatic low-density fraction of the recipient rat, and these cells had developed morphological characteristics of low-density large granular lymphocytes. It is concluded therefore that marginating blood large granular lymphocytes differentiate through high-density large granular lymphocytes into the typical liver specific low-density large granular lymphocytes or pit cells.

Killing by cytotoxic T cells and natural killer cells: multiple granule serine proteases as initiators of DNA fragmentation

The vectorial secretion of the contents of highly specialized cytoplasmic granules is of pivotal importance to the killing by cytotoxic T cells and natural killer cells. The purification and biochemical characterization of some of the granule constituents, in particular the pore-forming protein perforin, had engendered the notion that the killing of cellular targets was largely an osmotic phenomenon analogous to the insult delivered by complement attack. However, the apparent absence of membrane perforation in various examples of lymphocyte-mediated killing, and the observation that perforin alone could not account for apoptosis associated with programmed cell death, suggested that perforin activity represented, at best, only a part of the whole mechanism. More recently, the characterization of a large family of granule serine proteases (granzymes) has provided evidence that these molecules may collaborate in the killing process by inducing a 'suicide' pathway in target cells, resulting in DNA fragmentation. However, the serine proteases are inactive alone, their natural substrates have not been defined and they require access into the target cell cytoplasm via perforin-induced pores to exert their deleterious effects. Thus, we propose that the cytotoxic granule-mediated mechanism comprises at least two interdependent arms, perforin and serine proteases, that together are capable of inflicting cell death by osmotic shock and/or nuclear collapse.

A promoter element of the human serine esterase granzyme B gene controls specific transcription in activated T cells

The human granzyme B gene encodes a serine protease expressed specifically in cytoplasmic granules of cytotoxic T lymphocytes, released upon effector-target cell interaction. Previous studies have shown that granzyme B mRNA was induced in T lymphocytes after antigenic or mitogenic stimulation. To study the regulation of human granzyme B gene expression during lymphocyte activation we analyzed its 5' flanking region using chloramphenicol acetyl transferase (CAT) reporter gene constructs. We show that a 208-bp fragment (-148 to +60) containing an NF-AT (nuclear factor of activated T cells)-binding site promotes CAT expression in phytohemagglutinin-activated T lymphocytes, in immobilized monoclonal anti-CD3 antibody-activated Jurkat T cell line while it is inactive in unstimulated PEER and Jurkat T cells lines or B Epstein-Barr virus-transformed cell lines.

Proteases and lymphocyte cytotoxic killing mechanisms

A subfamily of serine proteases is uniquely expressed by cytotoxic natural killer lymphocytes and T cells. Protease cleavage of different natural substrates is now implicated in the cytotoxic mechanisms of target cell membrane pore formation, DNA fragmentation and cytostasis.

The calcium-binding protein calreticulin is a major constituent of lytic granules in cytolytic T lymphocytes

Cytolytic T lymphocytes (CTL), natural killer cells, and lymphokine-activated killer (LAK) cells are cytolytic cells known to release the cytolytic protein perforin and a family of proteases, named granzymes, from cytoplasmic stores upon interaction with target cells. We now report the purification of an additional major 60-kD granule-associated protein (grp 60) from human LAK cells and from mouse cytolytic T cells. The NH2-terminal amino acid sequence of the polypeptide was found to be identical to calreticulin. Calreticulin is a calcium storage protein and carries a COOH-terminal KDEL sequence, known to act as a retention signal for proteins destined to the lumen of the endoplasmic reticulum. In CTLs, however, calreticulin colocalizes with the lytic perforin to the lysosome-like secretory granules, as confirmed by double label immunofluorescence confocal microscopy. Moreover, when the release of granule-associated proteins was triggered by stimulation of the T cell receptor complex, calreticulin was released along with granzymes A and D. Since perforin is activated and becomes lytic in the presence of calcium, we propose that the role of calreticulin is to prevent organelle autolysis due to the protein's calcium chelator capacity.

The role of apoptosis in antibody-dependent cellular cytotoxicity

Apoptosis in three lymphoma cell lines has been studied following cytotoxicity induced in vitro by normal human blood lymphocytes utilizing either natural killer (NK) or antibody-dependent cellular cytotoxic (ADCC) mechanisms. Guinea-pig L2C leukaemic lymphocytes, but not the human cell lines Daudi and Jurkat, revealed a degree of time- and temperature-dependent apoptotic death upon simple culture in vitro. NK cytotoxicity at low effector:target ratios (E:T) induced both release of 51Cr and apoptosis. However NK cytotoxicity at higher E:T, and ADCC at all E:T, increased the level of 51Cr release while reducing the level of apoptosis. The findings were consistent with the apoptotic process being cut short by intervention of necrotic death. The same characteristics accompanied ADCC whether the effectors were recruited by Fc gamma regions of antibody coating the targets, or by bispecific antibodies attaching one arm to the targets and the other to Fc gamma receptors type III on effectors. This finding, and the high level of cytotoxicity elicited by the bispecific method, confirm the belief that NK cells, in addition to exerting NK cytotoxicity, represent the principal effectors for ADCC among blood mononuclear cells. Our results suggest that NK cells have both apoptotic and necrotic mechanisms available for killing their targets, but use only the latter for ADCC.

Purification of three cytotoxic lymphocyte granule serine proteases that induce apoptosis through distinct substrate and target cell interactions

We recently reported the purification of a lymphocyte granule protein called "fragmentin," which was identified as a serine protease with the ability to induce oligonucleosomal DNA fragmentation and apoptosis (Shi, L., R. P. Kraut, R. Aebersold, and A. H. Greenberg. 1992. J. Exp. Med. 175:553). We have now purified two additional proteases with fragmentin activity from lymphocyte granules. The three proteases are of two types; one has the unusual ability to cleave a tripeptide thiobenzyl ester substrate after aspartic acid, similar to murine cytotoxic cell protease I/granzyme B, while two are tryptase-like, preferentially hydrolyzing after arginine, and bear some homology to human T cell granule tryptases, granzyme 3, and Hanukah factor/granzyme A. Using tripeptide chloromethyl ketones, the pattern of inhibition of DNA fragmentation corresponded to the inhibition of peptide hydrolysis. The Asp-ase fragmentin was blocked by aspartic acid-containing tripeptide chloromethyl ketones, while the tryptase fragmentins were inhibited by arginine-containing chloromethyl ketones. The two tryptase fragmentins were slow acting and were partly suppressed by blocking proteins synthesis with cycloheximide in the YAC-1 target cell. In contrast, the Asp-ase fragmentin was fast acting and produced DNA damage in the absence of protein synthesis. Using a panel of unrelated target cells of lymphoma, thymoma, and melanoma origin, distinct patterns of sensitivity to the three fragmentins were observed. Thus, these three granule proteases make up a family of fragmentins that activate DNA fragmentation and apoptosis by acting on unique substrates in different target cells.

Cytotoxicity with target DNA breakdown by rat basophilic leukemia cells expressing both cytolysin and granzyme A

The noncytotoxic rat mast cell tumor line RBL was transfected with genes for the cytotoxic lymphocyte granule proteins cytolysin (perforin) and granzyme A, giving transfectants with mRNA and protein expression levels comparable with cloned cytotoxic T lymphocytes. Both RBL-cytolysin and RBL-cytolysin-granzyme A transfectants showed extremely potent killing of red cell targets and lysed 20%-60% of EL4 lymphoma targets at an effector-to-target ratio of 30. RBL transfectants expressing only granzyme A were not cytotoxic. Significant EL4 DNA breakdown accompanying lysis was observed only with RBL that was transfected with both cytolysin and granzyme A. These results support the granule-exocytosis model for lymphocyte cytotoxicity and show that effector granzyme A plays a role in target cell DNA breakdown.

Perforin and its role in T lymphocyte-mediated cytolysis

The killing mediated by cytotoxic T lymphocytes (CTL) represents an important mechanism in the immune defence against tumors and virus infections. The lytic mechanism has been proposed to consist of a polarized secretion of granule-stored molecules, occurring on effector-target cell contact. By electron microscopy, membrane deposited, pore-like lesions are detected on the target cell membrane during cytolysis by CTL. These structures resembled strikingly pores formed during complement attack. Granules of CTL isolated by nitrogen cavitation and Percoll gradient centrifugation were shown to retain cytotoxic activity. Further purification of proteins stored in these granules led to the discovery of a membranolytic protein named perforin which was capable of polymerizing into pore-like structures. In addition to this cytolytic protein, a set of serine esterases was found as well as lysosomal enzymes and proteoglycans, whose function are not yet clearly defined. The role of perforin in the cytotoxic process is currently being explored by ablating the active gene in mice.

Identification and functional characterization of a TIA-1-related nucleolysin

We recently reported the molecular cloning of a cytotoxic granule-associated RNA-binding protein designated TIA-1. The ability of recombinant TIA-1 to induce DNA fragmentation in permeabilized cells suggested that this protein is the granule component responsible for inducing apoptosis in cytolytic lymphocyte (CTL) targets. Here we report the characterization of a cDNA encoding a TIA-1-related protein designated TIAR. The deduced amino acid sequence of TIAR reveals it to be a 42-kDa protein possessing three RNA-binding domains and a carboxyl-terminal auxiliary domain. Although the RNA-binding domains of TIA-1 and TIAR share greater than 85% amino acid homology, their carboxyl-terminal auxiliary domains are only 51% homologous. The carboxyl terminus of TIAR contains a lysosome-targeting motif, indicating that TIAR is probably a cytotoxic granule-associated protein. Like TIA-1, purified recombinant TIAR induced DNA fragmentation in permeabilized target cells. Although immunoblotting analysis of post-nuclear supernatants revealed TIA-1 protein to be restricted to CTLs, PCR analysis revealed the expression of TIA-1 and TIAR mRNA transcripts in a wide variety of cell types. Our data suggest that the granules of CTLs contain at least two candidate nucleolysins involved in CTL killing.