Internal disintegration model of cytotoxic lymphocyte-induced target damage

Granzymes, cytotoxic granules and cell death: the early work of Dr. Jurg Tschopp

Within the powerful legacy left by Jurg Tschopp, we should not forget his early work that helped to elucidate the molecular pathways responsible for the clearance of virus-infected and transformed cells by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. Jurg's skilful biochemical approach formed a firm platform upon which the work of so many other biochemists, cell biologists and immunologists would come to rely. Jurg coined the shorthand term 'granzyme' to denote the individual members of a family of serine proteases sequestered in and secreted from the cytotoxic granules of CTL/NK cells. He was also one of the first to describe the lytic properties of purified perforin and to postulate the synergy of perforin and granzymes, which we now know to underpin target cell apoptosis. Jurg was a major protagonist in the debate that raged throughout the 1980's and early 1990's on the physiological relevance of the 'granule exocytosis' pathway. Ultimately, resolving this issue led Jurg and his colleagues to even greater and impactful discoveries in the broader field of apoptosis research. Jurg Tschopp ranks with other pioneers, particularly Gideon Berke, Chris Bleackley, Pierre Golstein, Pierre Henkart and Eckhard Podack for making seminal discoveries on our understanding of how the immune system eliminates dangerous cells.

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.

Methimazole-induced cell death in rat olfactory receptor neurons occurs via apoptosis triggered through mitochondrial cytochrome c-mediated caspase-3 activation pathway

The administration of methimazole is known to induce cell death in rat olfactory receptor neurons (ORNs). We investigated whether this injury occurs via apoptosis or through necrosis and whether it involves the extrinsic or intrinsic pathway. Rats were intraperitoneally injected with vehicle (control) or 300 mg/kg methimazole. The experimental animals were also administered vehicle or a caspase-3 or caspase-9 inhibitor 30 min earlier. The administration of methimazole induced cell death predominantly in the mature ORNs and partially reduced olfactory sensitivity in the rats; the injured cells were TUNEL-positive and showed a nuclear staining pattern. This insult induced cytochrome c release from the mitochondria and a significant increase in the immunoreactivity of activated caspase-3 and caspase-9 as well as that of cleaved poly-ADP-ribose-polymerase; in addition, it caused a significant increase in the fluorogenic activity of caspase-3 and caspase-9. However, it did not affect the immunoreactivity of activated caspase-8 or the fluorogenic activity of caspase-8. Pretreatment with a caspase-3 or caspase-9 inhibitor nearly completely prevented the morphologic, biochemical, and functional changes induced by methimazole. These findings suggest strongly that methimazole-induced cell death in rat ORNs is predominantly apoptosis; moreover, the majority of this apoptotic cell death is triggered through mitochondrial cytochrome c-mediated caspase-3 activation pathway, and both caspase-3 and caspase-9 inhibitors can prevent methimazole-induced cell death in the ORNs.

Entamoeba histolytica activates host cell caspases during contact-dependent cell killing

Entamoeba histolytica is a human intestinal parasite that causes amoebic colitis as well as liver abscesses. Host tissues are damaged through a three-step process involving adherence, contact-dependent cytolysis, and phagocytosis. These three processes all contribute to the pathogenicity of this parasite. Adherence is provided by the Gal/GalNAc adherence lectin. Host cells are lysed in a contact-dependent fashion. There is evidence that suggests that this contact-dependent killing involves the induction of the host cell's apoptotic machinery. Phagocytosis can then occur, consistent with metazoan apoptotic clearance.

Granzyme B: a natural born killer

A main pathway used by cytotoxic T lymphocytes (CTLs) and natural killer cells to eliminate pathogenic cells is via exocytosis of granule components in the direction of the target cell, delivering a lethal hit of cytolytic molecules. Amongst these, granzyme B and perforin have been shown to induce CTL-mediated target cell DNA fragmentation and apoptosis. Once released from the CTL, granzyme B binds its receptor, the mannose-6-phosphate/insulin-like growth factor II receptor, and is endocytosed but remains arrested in endocytic vesicles until released by perforin. Once in the cytosol, granzyme B targets caspase-3 directly or indirectly through the mitochondria, initiating the caspase cascade to DNA fragmentation and apoptosis. Caspase activity is required for apoptosis to occur; however, in the absence of caspase activity, granzyme B can still initiate mitochondrial events via the cleavage of Bid. Recent work shows that granzyme B-mediated release of apoptotic factors from the mitochondria is essential for the full activation of caspase-3. Thus, granzyme B acts at multiple points to initiate the death of the offending cell. Studies of the granzyme B death receptor and internal signaling pathways may lead to critical advances in cell transplantation and cancer therapy.

Distinct modes of macrophage recognition for apoptotic and necrotic cells are not specified exclusively by phosphatidylserine exposure

The distinction between physiological (apoptotic) and pathological (necrotic) cell deaths reflects mechanistic differences in cellular disintegration and is of functional significance with respect to the outcomes that are triggered by the cell corpses. Mechanistically, apoptotic cells die via an active and ordered pathway; necrotic deaths, conversely, are chaotic and passive. Macrophages and other phagocytic cells recognize and engulf these dead cells. This clearance is believed to reveal an innate immunity, associated with inflammation in cases of pathological but not physiological cell deaths. Using objective and quantitative measures to assess these processes, we find that macrophages bind and engulf native apoptotic and necrotic cells to similar extents and with similar kinetics. However, recognition of these two classes of dying cells occurs via distinct and noncompeting mechanisms. Phosphatidylserine, which is externalized on both apoptotic and necrotic cells, is not a specific ligand for the recognition of either one. The distinct modes of recognition for these different corpses are linked to opposing responses from engulfing macrophages. Necrotic cells, when recognized, enhance proinflammatory responses of activated macrophages, although they are not sufficient to trigger macrophage activation. In marked contrast, apoptotic cells profoundly inhibit phlogistic macrophage responses; this represents a cell-associated, dominant-acting anti-inflammatory signaling activity acquired posttranslationally during the process of physiological cell death.

Induction of rapid histone degradation by the cytotoxic T lymphocyte protease Granzyme A

The cytotoxic T lymphocyte protease granzyme A induces caspase-independent cell death in which DNA single-strand nicking is observed instead of oligonucleosomal fragmentation. Granzyme A is a specific tryptase that concentrates in the nucleus of targeted cells and synergistically enhances DNA fragmentation induced by the caspase activator granzyme B. Here we show that granzyme A treatment of isolated nuclei enhances DNA accessibility to exogenous endonucleases. In vitro and after cell loading with perforin, GrnA completely degrades histone H1 and cleaves core histones into approximately 16-kDa fragments. Histone digestion provides a mechanism for unfolding compacted chromatin and facilitating endogenous DNase access to DNA during T cell and natural killer cell granule-mediated apoptosis.

Granzymes (lymphocyte serine proteases): characterization with natural and synthetic substrates and inhibitors

Natural killer (NK) and cytotoxic T-lymphocytes (CTLs) kill cells within an organism to defend it against viral infections and the growth of tumors. One mechanism of killing involves exocytosis of lymphocyte granules which causes pores to form in the membranes of the attacked cells, fragments nuclear DNA and leads to cell death. The cytotoxic granules contain perforin, a pore-forming protein, and a family of at least 11 serine proteases termed granzymes. Both perforin and granzymes are involved in the lytic activity. Although the biological functions of most granzymes remain to be resolved, granzyme B clearly promotes DNA fragmentation and is directly involved in cell death. Potential natural substrates for Gr B include procaspases and other proteins involved in cell death. Activated caspases are involved in apoptosis. The search continues for natural substrates for the other granzymes. The first granzyme crystal structure remains to be resolved, but in the interim, molecular models of granzymes have provided valuable structural information about their substrate binding sites. The information has been useful to predict the amino acid sequences that immediately flank each side of the scissile peptide bond of peptide and protein substrates. Synthetic substrates, such as peptide thioesters, nitroanilides and aminomethylcoumarins, have also been used to study the substrate specificity of granzymes. The different granzymes have one of four primary substrate specificities: tryptase (cleaving after Arg or Lys), Asp-ase (cleaving after Asp), Met-ase (cleaving after Met or Leu), and chymase (cleaving after Phe, Tyr, or Trp). Natural serpins and synthetic inhibitors (including isocoumarins, peptide chloromethyl ketones, and peptide phosphonates) inhibit granzymes. Studies of substrate and inhibitor kinetics are providing valuable information to identify the most likely natural granzyme substrates and provide tools for the study of key reactions in the cytolytic mechanism.

Protein Kinase C Inhibits CD95 (Fas/APO-1)-Mediated Apoptosis by at Least Two Different Mechanisms in Jurkat T Cells

We have recently reported that activation of protein kinase C (PKC) plays a negative role in CD95-mediated apoptosis in human T cell lines. Here we present data indicating that although the PKC-induced mitogen-activated protein kinase pathway could be partially implicated in the abrogation of CD95-mediated apoptosis by phorbol esters in Jurkat T cells, the major inhibitory effect is exerted through a PKC-dependent, mitogen-activated protein kinase-independent signaling pathway. Furthermore, we demonstrate that activation of PKC diminishes CD95 receptor aggregation elicited by agonistic CD95 Abs. On the other hand, it has been reported that UV radiation-induced apoptosis is mediated at least in part by the induction of CD95 oligomerization at the cell surface. Here we show that activation of PKC also inhibits UVB light-induced CD95 aggregation and apoptosis in Jurkat T cells. These results reveal a novel mechanism by which T cells may restrain their sensitivity to CD95-induced cell death through PKC-mediated regulation of CD95 receptor oligomerization at the cell membrane.

Dipeptidyl peptidase I is required for the processing and activation of granzymes A and B in vivo

Dipeptidyl peptidase I (DPPI) is a lysosomal cysteine protease that has been implicated in the processing of granzymes, which are neutral serine proteases exclusively expressed in the granules of activated cytotoxic lymphocytes. In this report, we show that cytotoxic lymphocytes derived from DPPI-/- mice contain normal amounts of granzymes A and B, but these molecules retain their prodipeptide domains and are inactive. Cytotoxic assays with DPPI-/- effector cells reveal severe defects in the induction of target cell apoptosis (as measured by [(125)I]UdR release) at both early and late time points; this defect is comparable to that detected in perforin-/- or granzyme A-/- x B-/- cytotoxic lymphocytes. DPPI therefore plays an essential role in the in vivo processing and activation of granzymes A and B, which are required for cytotoxic lymphocyte granule-mediated apoptosis.

Cell adhesion and the immune system: a case study using earthworms

In the earthworm's immune system, cell adhesion, which occurs by putative receptors on leukocytes, is essential after recognition of self vs. non-self. Confrontation with foreign antigens is a normal event in the environment, replete with microbial pathogens that pose a threat to survival. To better understand what happens when an effector cell first recognizes a foreign target followed by its adhesion to it, isolated leukocytes, in sufficient quantities to be subjected to various analyses, have been extremely beneficial. In vitro approaches when accompanied by biochemical, immunological, and molecular technologies, have opened up new vistas concerning the immune response of earthworms and other invertebrates. The most recent discovery includes the preliminary identification of cell differentiation (CD) markers that play vital roles in recognitive and adhesive events. Certain leukocyte effectors show characteristics of natural killer (NK) cells that may act differently depending upon their source, whether autogeneic, allogeneic, xenogeneic, or expressed under normal or varying environmental conditions including exposure to xenobiotics. At the level of earthworm evolution, there is apparently a dissociation of phagocytosis from the process of killing by NK-like effectors. There are at least three future challenges. First, it is essential to determine the precise nature of the CD markers with respect to their molecular structure. Second, once their molecular and biochemical characteristics have been defined, the role of these markers in cellular and humoral mechanisms must be clarified in order to define effector cell products and resulting immune responses. Third, there is a need to differentiate between the several lytic factors that have been found in earthworms with respect to molecular structure, and biochemical and functional characterization.

BCL-2 blocks perforin-induced nuclear translocation of granzymes concomitant with protection against the nuclear events of apoptosis

Cytolytic granule-mediated target cell killing is effected in part through the synergistic action of the membrane-acting protein perforin and serine proteases such as granzymes (Gr) A and B. In this study, we examine the subcellular distribution of granzymes in the presence of perforin and the induction of apoptosis in mouse FDC-P1 myeloid and YAC-1 lymphoma cells that express the proto-oncogene bcl2. Using confocal laser scanning microscopy to visualize and quantitate subcellular transport of fluoresceinated granzyme, we find that granzyme entry into the cytoplasm in the absence of perforin is not impaired in the bcl2-expressing lines. However, perforin-dependent enhancement of granzyme cellular uptake and, importantly, granzyme redistribution to the nucleus were strongly inhibited in the bcl2-expressing lines, concomitant with greatly increased resistance to granzyme/perforin-induced cell death. DNA fragmentation induced by granzyme/perforin was severely reduced in the bcl2-expressing lines, implying that prevention of granzyme nuclear translocation blocks the nuclear events of apoptosis. The kinetics of GrB nuclear uptake and induction of apoptosis were faster than for GrA, whereas YAC-1 cells showed greater resistance to granzyme nuclear uptake and apoptosis than FDC-P1 cells. In all cases, granzyme nuclear accumulation in the presence of perforin correlated precisely with ensuing apoptosis. All results supported the idea that GrA and GrB share a common, specific nuclear targeting pathway that contributes significantly to the nuclear changes of apoptosis.

Cell-associated pentosidine as a marker of aging in human diploid cells in vitro and in vivo

Cellular aging is characterized by alterations at both the morphological and molecular levels, some of which are decreased mitotic rate, increased cytoplasmic vacuolization, and changes in intrinsic cellular constituents (Stanulis-Praeger, 1987. Mech. Ageing Dev. 38, 1-48). In the present investigation, glycoxidation is studied as a marker for cellular aging by measuring cell-associated pentosidine levels in human skin fibroblasts as a function of replicative life span and in human peripheral blood T lymphocytes as a function of chronological age. Fibroblasts were isolated from culture by detachment/centrifugation while lymphocytes were isolated from blood by a Ficoll-Paque/Lympho-Kwik T-Cell Prep technique. Pentosidine levels were measured in acid-hydrolyzed cell pellet suspensions by high-pressure liquid chromatography. Results show that pentosidine was detected in early and late cultured reticular and papillary fibroblasts. Pentosidine, expressed as either protein, DNA, or cell number, significantly (P < 0.0006) increased with in vitro passage and was significantly (P < 0.01) related to cell proliferation as measured by cell density and cell doublings per day during culture. Cell-associated pentosidine was measured in T lymphocytes isolated from healthy, diabetic, and uremic individuals. In healthy controls, levels significantly (P < 0.0003) increased with age. In uremic individuals, a large variation was observed with many values above the 95% confidence intervals determined for controls. Since a previous study showed that plasma pentosidine in healthy subjects does not increase with age, these results suggest that cellular turnover perhaps coupled to a deterioration in cellular anti-glycoxidation defensive mechanisms play a substantial role in explaining increased pentosidine concentrations during cellular aging.

Caspase Dependence of Target Cell Damage Induced by Cytotoxic Lymphocytes

Since the CTL secreted granule protease granzyme B can activate multiple target caspases, it has been proposed that this pathway is responsible for CTL-induced cytolysis of Fas-negative targets. However, target lysis via the granule exocytosis pathway is completely resistant to caspase inhibitors. To test the possibility that granzymes trigger a postcaspase cytoplasmic apoptotic pathway leading to lysis, we have examined the caspase dependence of several cytoplasmic changes associated with apoptotic death. Rapid prelytic phosphatidylserine externalization was induced in Jurkat target cells by both the Fas ligand (FasL)/Fas and the granule exocytosis effector pathways. This was specifically blocked by peptide ketone caspase inhibitors when induced by the former, but not by the latter, pathway. A rapid prelytic loss of target mitochondrial ψ was also induced by both CTL effector pathways, and this was also specifically blocked by caspase inhibitors when induced by the FasL/Fas, but not by the granule exocytosis, pathway. Similarly, target membrane blebbing induced by CTL via the FasL/Fas, but not via the granule exocytosis, effector pathway was specifically blocked by caspase inhibitors. In contrast to the above nonnuclear damage, CTL-induced target staining by the lipid probe FM1–43 reflecting plasma membrane endocytosis was blocked by caspase inhibitors. Thus, when caspase activation is blocked, the granule exocytosis pathway triggers several parameters of target apoptotic damage in addition to lysis, suggesting that granzymes directly trigger a postcaspase cytoplasmic apoptotic death pathway.

A Role for Heat Shock Protein 27 in CTL-Mediated Cell Death

CTL exocytosis of granules containing perforin and granzyme proteases induces apoptotic cell death. Either granzyme A or B can act with perforin to trigger apoptosis. Granzyme B activates a ubiquitous apoptotic cascade induced by caspase cleavage, but the granzyme A pathway is largely unknown. Using affinity chromatography with recombinant mutant inactive granzyme A, we previously isolated two granzyme A-binding proteins, PHAP (putative HLA-associated protein) I and II. PHAP II, a substrate of granzyme A, is degraded within minutes of CTL attack. Two additional cytoplasmic proteins of 27 and 53 kDa bind strongly to the mutant granzyme A column, requiring 6 M urea to elute. Sequencing identified these as the monomer and dimer of hsp27, a small heat shock protein up-regulated by stress and cellular activation. Hsp27 coprecipitates with granzyme A from cytoplasmic lysates and is not a substrate of the enzyme. Hsp27 translocates to the detergent-insoluble fraction of target cells and relocalizes from diffuse cytoplasmic staining to long filamentous fibers, especially concentrated in a perinuclear region, within minutes of CTL attack. Hsp27 may participate in morphologic changes during granule-mediated lysis. Low or absent levels of hsp27 expression in T lymphocytes, even after heat shock, may play a role in CTL resistance to granule-mediated lysis.

Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

Cytotoxic T lymphocytes and natural killer cells together comprise the means by which the immune system detects and rids higher organisms of virus-infected or transformed cells. Although differing considerably in the way they detect foreign or mutated antigens, these cells utilize highly analogous mechanisms for inducing target cell death. Both types of effector lymphocytes utilize two principal contact-dependent cytolytic mechanisms. The first of these, the granule exocytosis mechanism, depends on the synergy of a calcium-dependent pore-forming protein, perforin, and a battery of proteases (granzymes), and it results in penetration by effector molecules into the target cell cytoplasm and nucleus. The second, which requires binding of FasL (CD95L) on the effector cell with trimeric Fas (CD95) molecules on receptive target cells, is calcium independent and functions by generating a death signal at the inner leaflet of the target cell membrane. Exciting recent developments have indicated that both cytolytic mechanisms impinge on an endogenous signaling pathway that is strongly conserved in species as diverse as helminths and humans and dictates the death or survival of all cells.

Induction of differentiation in maxillary adenoid cystic carcinomas by adoptive immunotherapy in combination with chemoradiotherapy

The successful treatment results of a case of maxillary adenoid cystic carcinoma (ACC) and the possibility of differentiation of ACC cells with chemoradioimmunotherapy are described. Combined therapy was applied to two maxillary ACCs. Adoptive immunotherapy consisted of intra-arterial injection of lymphokine-activated killer cells (total 9.0 x 10(8) cells) and recombinant interleukin-2 (1.8 x 10(5) U) and interferon-gamma (1.8 x 10(5) U) was combined with 60Co radiation (50 Gy), 5-fluorouracil (4000 mg) and peplomycin (10 mg). Immunohistochemical staining of the biopsy material obtained during the therapy revealed a marked decrease of proliferating cell nuclear antigen-positive cells and a prominent increase of Lewis Y antigen- and bone morphogenetic protein-2-positive cells. The disappearance of tumour cells and the remodeling of the sinus wall with calcification in the sinus cavity, which had been occupied by the tumour, were observed after therapy in both patients. Adoptive immunotherapy in combination with chemoradiotherapy is useful for the treatment of ACC.

In vitro- and ex vivo-derived cytolytic leukocytes from granzyme A x B double knockout mice are defective in granule-mediated apoptosis but not lysis of target cells

Granzyme (gzm) A and gzmB have been implicated in Fas-independent nucleolytic and cytolytic processes exerted by cytotoxic T (Tc) cells, but the underlying mechanism(s) remains unclear. In this study, we compare the potential of Tc and natural killer (NK) cells of mice deficient in both gzmA and B (gzmAxB-/-) with those from single knockout mice deficient in gzmA (-/-), gzmB (-/-), or perforin (-/-) to induce nuclear damage and lysis in target cells. With the exception of perforin-/-, all in vitro- and ex vivo-derived Tc and NK cell populations from the mutant strains induced 51Cr-release in target cells at levels and with kinetics similar to those of normal mice. This contrasts with their capacity to induce apoptotic nuclear damage in target cells. In gzmAxB-/- mice, Tc/NK-mediated target cell DNA fragmentation was not observed, even after extended incubation periods (10 h), but was normal in gzmA-deficient and only impaired in gzmB-deficient mice in short-term (2-4 h), but not long-term (4-10 h), nucleolytic assays. This suggests that gzmA and B are critical for Tc/NK granule- mediated nucleolysis, with gzmB being the main contributor, while target cell lysis is due solely to perforin and independent of both proteases.

Fas-mediated cytotoxicity induces degradation of vesicular stomatitis virus RNA transcripts and reduces viral titer

Several investigators have recently examined the effect of Fas (CD95)-mediated apoptotic cell death on target cells (TC). The effect of Fas-mediated death on viral RNA within the TC, however, has not been explored. In this study, we investigated the ability of the Fas pathway to mediate pre-lytic degradation of vesicular stomatitis virus (VSV) RNA and TC RNA. We show that engagement of Fas antigen on VSV-infected Jurkat cells induces pre-lytic degradation of VSV RNA transcripts, whereas full-length VSV genome RNA, known to be tightly associated with viral proteins, is not degraded. Cellular RNA, including beta-actin and glyceraldehyde-3-phosphate-dehydrogenase mRNAs, is also degraded by Fas-mediated cytotoxicity. In addition, Fas-mediated cytotoxicity reduced the yield of VSV plaque-forming units (PFU) from Jurkat by an average of 82.0%. An anti-Fas blocking Ab inhibited the RNA degradation and restored the number of VSV PFU to near control levels. These data indicate that the Fas lytic pathway could play a role in the elimination of viruses through degradation of intracellular viral RNA. reserved

DNA fragmentation follows delayed neuronal death in CA1 neurons exposed to transient global ischemia in the rat

Apoptosis is an active, gene-directed process of cell death in which early fragmentation of nuclear DNA precedes morphological changes in the nucleus and, later, in the cytoplasm. In ischemia, biochemical studies have detected oligonucleosomes of apoptosis whereas sequential morphological studies show changes consistent with necrosis rather than apoptosis. To resolve this apparent discrepancy, we subjected rats to 10 minutes of transient forebrain ischemia followed by 1 to 14 days of reperfusion. Parameters evaluated in the CA1 region of the hippocampus included morphology, in situ end labeling (ISEL) of fragmented DNA, and expression of p53. Neurons were indistinguishable from controls at postischemic day 1 but displayed cytoplasmic basophilia or focal condensations at day 2; some neurons were slightly swollen and a few appeared normal. In situ end labeling was absent. At days 3 and 5, approximately 40 to 60% of CA1 neurons had shrunken eosinophilic cytoplasm and pyknotic nuclei, but only half of these were ISEL. By day 14, many of the necrotic neurons had been removed by phagocytes; those remaining retained mild ISEL. Neither p53 protein nor mRNA were identified in control or postischemic brain by in situ hybridization with riboprobes or by northern blot analysis. These results show that DNA fragmentation occurs after the development of delayed neuronal death in CA1 neurons subjected to 10 minutes of global ischemia. They suggest that mechanisms other than apoptosis may mediate the irreversible changes in the CA1 neurons in this model.

Recombinant human granzyme A binds to two putative HLA-associated proteins and cleaves one of them

The release of cytotoxic granule contents by cytotoxic T lymphocytes triggers apoptotic target cell death. Cytotoxic granules contain a pore-forming protein, perforin, and a group of serine proteases called granzymes. We expressed human granzyme A in bacteria as a proenzyme capable of in vitro activation by enterokinase. The recombinant activated enzyme has catalytic activity against substrates with Arg, preferably, or Lys at the P1 position, comparable to trypsin. An enzymatically inactive recombinant granzyme A, with the active site Ser mutated to Ala, was produced and used with affinity chromatography to identify potential substrates. Two granzyme A-binding cytoplasmic proteins of molecular mass 33 and 44 kDa were isolated and identified by tryptic fragment sequencing as PHAP I and II, ubiquitous putative HLA-associated proteins, previously coisolated by binding to an HLA class II peptide. PHAP II forms an SDS-stable complex with recombinant mutant granzyme A and coprecipitates with it from cytoplasmic extracts. PHAP II, either purified or in cell lysates, is cleaved by the recombinant enzyme at nanomolar concentrations to a 25-kDa fragment. PHAP II begins to be degraded within minutes of initiation of cytotoxic T lymphocyte attack. PHAP I and II are candidate participants in the granzyme A pathway of cell-mediated cytotoxicity.

Activation of protein kinase C attenuates early signals in Fas-mediated apoptosis

Activation of protein kinase C (PKC) has been reported to inhibit Fas (APO-1, CD95)-mediated apoptosis in different cellular systems. Human Jurkat leukemic T cells express the Fas antigen in the cell membrane and undergo apoptosis upon cross-linking by anti-Fas monoclonal antibodies (mAb). Cleavage of the apoptosis-associated protease CPP32 and its substrate poly(ADP-ribose)polymerase are observed after the engagement of Fas antigen with mAb. In this report, we show that all these effects are substantially inhibited by the activation of PKC with a phorbol ester. Bisindolylmaleimide, an inhibitor of PKC, prevents phorbol ester-induced down-regulation of Fas signaling. Inhibition of Fas-mediated cell death by phorbol ester is also observed in other human leukemic T cell lines. Cross-linking of Fas antigen by mAb results in the rapid increase in tyrosine phosphorylation of several protein substrates which is further elevated in the presence of the protein tyrosine phosphatase inhibitor, orthovanadate. Furthermore, orthovanadate markedly enhances the cell death response to Fas mAb in different human leukemic T cell lines and human T cell blasts. These effects of orthovanadate on early tyrosine phosphorylation and cell death are clearly diminished by PKC activation. These results strongly suggest that tyrosine phosphorylation is involved in Fas signaling in apoptosis and that PKC plays a negative role in Fas-mediated apoptosis by counteracting at a very early stage the signals generated following cross-linking of this receptor.

TRAMP, a novel apoptosis-mediating receptor with sequence homology to tumor necrosis factor receptor 1 and Fas(Apo-1/CD95)

A novel member of the tumor necrosis factor (TNF) receptor family, designated TRAMP, has been identified. The structural organization of the 393 amino acid long human TRAMP is most homologous to TNF receptor 1. TRAMP is abundantly expressed on thymocytes and lymphocytes. Its extracellular domain is composed of four cysteine-rich domains, and the cytoplasmic region contains a death domain known to signal apoptosis. Overexpression of TRAMP leads to two major responses, NF-kappaB activation and apoptosis. TRAMP-induced cell death is inhibited by an inhibitor of ICE-like proteases, but not by Bcl-2. In addition, TRAMP does not appear to interact with any of the known apoptosis-inducing ligands of the TNF family.

Human T-lymphocyte serine proteases (granzymes) 1, 2, and 3 mediated DNA fragmentation in susceptible target cells

We reported the characterization of three serine proteases (granzymes 1, 2, and 3) from human cytotoxic T lymphocytes. In this study, human granzymes 1, 2, and 3 were purified from the cytoplasmic granules of lymphokine activated killer (LAK) cells by gel filtration and cation exchange chromatography. Human perforin was purified by phenyl superose and heparin-agarose chromatography. Each purified granzyme was used with purified perforin to study DNA fragmentation in target cells of both human and murine origin. As measured by agarose gel electrophoresis and [125I]dUrd assay, the granzymes induced oligonucleosomal DNA fragmentation and [125I]dUrd release respectively from various target cells. Murine target cells were generally more susceptible to nuclear DNA release than were human targets. Both enzyme activity and nuclear DNA breakdown were significantly inhibited by 3,4-dichloroisocoumarin (DCI) or by heat inactivation of each granzyme. Perforin alone or granzyme alone failed to fragment nuclear DNA in various target cells. We conclude that human granzymes are an important family of effector molecules that with perforin induce DNA fragmentation in susceptible target cells.

Apoptotic morphology reflects mitotic-like aspects of physiological cell death and is independent of genome digestion

Several hallmarks characterize what has come to be recognized as a common physiological process of cell death. In particular, the two defining characteristics are the apoptotic morphology of cell shrinkage and chromatin condensation originally described by Kerr et al. [(1972) Br. J. Cancer, 26:239-256] and the prelytic digestion of genomic DNA of the dying cell, as noted first by Wyllie [(1980) Nature, 284:555-556] and Russell et al. [(1982) J. Immunol., 128:2087-2094]. Many suicidal stimuli are able to modulate this process; each of these suicidal inducers activates cell death via a specific pathway. While it remains to be established, we hypothesize that a single mechanism of physiological cell death pertains in all cases [Ucker (1991) New Biol., 3:103-109; Ucker et al. (1994) Immunol. Rev., 142:273-299]. The various modulatory processes act afferently on this single effector pathway. We have examined the significance of the hallmarks of physiological cell death in an effort to elucidate critical mechanistic elements of the cell death process. Here we describe our recent studies of genome digestion. Our work has centered on the characterization of a set of fibroblastic cell clones that vary in their ability to undergo genome digestion associated with physiological cell death induced by cytotoxic T lymphocytes (CTL) and other stimuli. Our results demonstrate that genome digestion is dispensable for physiological cell death and that apoptotic morphology is independent of genome digestion. Our data suggest further that apoptotic morphology is reflective of mitotic-like aspects of the cell death process.

Fas antigen expression in liver tissues of patients with chronic hepatitis B

BACKGROUND

Hepatitis B virus-infected cells can be eliminated by the cytotoxic T cell-mediated immune reaction. Fas ligand, recently detected on the surface of cytotoxic T cell, is thought to induce cells to apoptosis by adhering to Fas antigen.

AIMS/METHODS

To evaluate the role of Fas antigen and apoptosis in chronic hepatitis B, we immunohistochemically studied Fas antigen and HBsAg expression in liver samples from patients with hepatitis B virus infection.

RESULTS

In samples from 56 HBV patients, Fas antigen was mainly expressed in the cytoplasm (partly at the membrane) of hepatocytes, and these positive cells were detected especially at the periportal region near "piecemeal necrosis". According to Knodell's HAI scoring system, the scores of periportal inflammation and necrosis (category I) and the scores of intralobular inflammation and necrosis (category II) were similarly higher in Fas antigen-positive cases than in Fas antigen-negative cases (p < 0.01), and there was a positive correlation between these scores and the degree of Fas antigen expression. In normal cases, Fas antigen was not detected. In patients with HBV infection, Fas antigen expression was closely correlated with the activity of the viral hepatitis. HBsAg was expressed by the majority of hepatocytes. However, Fas antigen was expressed by fewer hepatocytes than the number of HBsAg-positive cells.

CONCLUSIONS

These findings suggest that the expression of Fas antigen may not be triggered only by HBV infection, and immunological interaction may be needed for the expression and for apoptosis to occur.

Target cell apoptosis induced by cytotoxic T cells and natural killer cells involves synergy between the pore-forming protein, perforin, and the serine protease, granzyme B

Cytotoxic lymphocytes (CL) comprise two effector cell populations with the ability to eliminate unwanted or harmful cells. Cytotoxic T cells (CTLs) demonstrate both an exquisite specificity and memory in recognising target cell oligopeptides presented within the groove of major histocompatibility complex class I antigens. By contrast, natural killer (NK) cells mediate "innate' immunity against virus-infected cells and surveillance against neoplastic transformation, and do not require presensitisation. Despite recognising target cells in very different ways, CTLs and NK cells both utilise a pore-forming protein, perforin, and a battery of serine proteases as a principal means of inflicting cell death. The action of both types of CL results in death by apoptosis. Recently, we and others have accumulated evidence that perforin and serine proteases synergistically trigger an endogenous pathway of programmed cell death that results in dissolution of the nuclear membrane, chromatin condensation and DNA fragmentation. These changes are secondary to inappropriate activation of p34, a kinase whose activation and migration from the cytoplasm to the nucleus normally controls a cell's entry into mitosis. Therefore, CI, may exert their actions through the derangement of cell cycle control. The downstream molecular targets of perforin/granzyme-mediated apoptosis (especially the physiological ligand/substrate of granzyme B) are still unclear, though candidate molecules with homology to products of cell death genes found in primitive organisms such as the nematode, C. elegans, are currently under investigation.

Occurrence and a possible mechanism of penetration of natural killer cells into K562 target cells during the cytotoxic interaction

The cytotoxic interaction between cloned human Natural Killer (NK) cells and K562 target cells was studied using confocal laser scanning microscopy (CLSM) and conventional fluorescence microscopy. We observed, using fixed as well as living cells, the occurrence of (pseudo) emperipolesis during the interaction. About 30% of conjugated NK cells penetrated, partly or completely, into the target cells (in-conjugation). Virtually all in-conjugated target cells exhibited polymerized actin. Killer cells of in-conjugates were frequently seen approaching the target cell nucleus or aligning along it. If the cytotoxic process was inhibited by the absence of calcium neither actin polymerization nor in-conjugation were observed. A kinetic study showed that in-conjugation starts somewhat later than actin polymerization but still within a few minutes after addition of calcium to conjugates previously formed in the absence of calcium. The presence of cytochalasin D (an inhibitor of actin polymerization) completely inhibited in-conjugation and partly reduced the cytotoxic activity. Zinc ions (endonuclease inhibition) inhibited in-conjugation and decreased the total number of target cells with polymerized actin in a concentration dependent manner. Cytotoxic activity was also reduced but not as efficiently as in-conjugation. Our study demonstrates that in-conjugation represents a significant fraction of the cytotoxic interaction. The results indicate that it may be a consequence of an actin polymerization and endonuclease activity dependent part of a cytotoxic mechanism.

Apoptosis in the human liver during allograft rejection and end-stage liver disease

The contribution of apoptosis (programmed cell death) to cellular damage in human liver disease is unknown. Using the in situ DNA end labelling method (ISEL), evidence was sought of programmed cell death (PCD) in liver tissue from patients with various liver diseases. In particular, the study aimed to determine whether PCD is involved in either the loss of interlobular bile ducts (vanishing bile duct syndrome--VBDS) or the perivenular hepatocyte drop-out, both of which are characteristic of irreversible graft rejection. Large numbers of apoptotic hepatocytes were found in perivenular areas in tissues taken from patients with chronic graft rejection. Significant hepatocyte apoptosis, was not seen in long-term stable allografts, primary biliary cirrhosis, cholestasis, paracetamol-induced fulminant hepatic failure, or fulminant hepatic failure of indeterminate origin (non-A, non-B, non-C hepatitis). Bile ducts rarely stained positively, but mononuclear cells present in the post-transplant tissues were frequently positive, showing nuclear or cytoplasmic staining. The presence of cytoplasmic staining suggested that some mononuclear cells had ingested apoptotic DNA from other cellular sources. PCD may thus contribute to the perivenular hepatocyte loss in chronic rejection. The absence of ductular epithelial cell staining suggests that PCD is not involved significantly in the bile duct loss of VBDS. Furthermore, apoptosis of mononuclear cells implies that PCD may be involved in regulating the inflammatory cell infiltration of graft rejection.

Changes in intracellular calcium concentration and pH of target cells during the cytotoxic process: a quantitative study at the single cell level

This study reports on the changes in intracellular calcium concentration ([Ca2+]in) and intracellular pH ([pH]in) that occur in K562 target cells during interaction with human Natural Killer (NK) cells. The data were obtained using a quantitative fluorescence microscope and fluorescent ratio probes specific for [Ca2+]in (Fura-2-AM) and [pH]in (BCECF-AM). Results demonstrate that two types of target cell response to the attack by an NK cell can be distinguished. The target cell either dies immediately, due to the complete breakdown of the membrane impermeability, or the initial membrane damage (i.e., increased membrane permeability) is repaired and the cell "escapes" immediate death. During both responses an increase of [Ca2+]in takes place in the target cells. In the cells that die immediately, however, [Ca2+]in reaches higher levels (approximately 1,400 nM) than in the cells that restore the initial damage (approximately 700 nM). Changes in target cell [pH]in are also detected during both responses. The direction of the change (acidification or alkalinization) as well as the level of the change depend on extracellular pH ([pH]ex). Also, [pH]in remains changed during the time the cells were followed (10 min). The programming time (i.e., the time from the initiation of the cytotoxic process to the time that a change in the physiological parameter was detected) of the killing process that leads to an immediate target cell death appears to be shortest at [pH]ex 7.3-7.6 (approximately 3 min).

PELs and the perforin and granzyme independent mechanism of CTL-mediated lysis

The central role of CTLs in immunopathology accounts for the increasing interest in deciphering the mechanism whereby they kill at the molecular level. Recent studies show that CTLs have two molecularly distinct lytic mechanisms at their disposal. The first involves the direct effect(s) of the pore-forming protein perforin, possibly in conjunction with granzymes. In recent years, experiments conducted in our laboratory led to an alternative pathway, of receptor-mediated mechanism for CTL killing, involving neither the secretion nor the lytic action of the pore-forming protein perforin or of granzymes. By this mechanism, engagement of a CTL membrane ligand and an apoptosis-inducing target cell surface receptor triggers the disintegration of the CTL-bound target cell. Cross-linking of apoptosis-inducing target cell surface molecules (e.g. Fas), induced upon binding of CTL ligands (e.g. Fas-L), may be required and sufficient to trigger target cell apoptosis. Intracellular lethal signals emanating from the cross-linked intracellular death domain of Fas are postulated.

Induction of lytic pathways in T cell clones derived from wild-type or protein tyrosine kinase Fyn mutant mice

The OVA-reactive CD4+ Th1 clones and alloreactive CD8+ clones derived from wild-type or fyn-/- mice serve as model systems which have allowed us to investigate several aspects of the molecular events associated with T cell-mediated cytotoxicity, including 1) the differential utilization of two distinct cytolytic pathways by CD4+ Th1 clones and CD8+ CTL, 2) a comparison of the pathways of lysis induced by stimulation of the TCR or by alternative stimuli, 3) the requirement of Fyn for derivation of antigen-specific T-cell clones having properties of CD4+ Th1 and CD8+ CTL cells 4) the differential requirement of Fyn in the induction of responses by TCR and the alternative stimuli. Stimulation through the TCR, either by APC bearing relevant antigen or by immobilized anti-CD3 mAb, resulted in comparable levels of target cell lysis by clones from both wild-type and fyn-/- mice. These clones also utilize the Fas pathway to lyse target cells. Thus, Fyn does not appear to be required for expression of the Fas pathway when triggered through the TCR. In contrast, lysis of target cells by T-cell clones lacking Fyn was deficient when stimulated through Thy-1 or Ly-6C (using mAb) or with Con A or phorbol ester as compared to clones derived from wild-type mice. The basis for the defect in response to stimulation through the GPI-linked molecules appears to be a signaling defect which affects all of the functional responses we measured, while the defect in response to Con A stimulation appears to affect lysis but not lymphokine production. Thus, Fyn expression is selectively required for efficient activation of the Fas pathway of lysis through Thy-1, Ly-6C, and by Con A or phorbol ester in these T-cell clones. CD8+ clones derived from fyn-/- mutant mice, like clones derived from wild-type mice, display antigen-specific lysis, and appear to express perforin message and perforin protein. A Ca(++)-dependent (presumably perforin/exocytosis) component and Fas component of lysis was detected in CD8+ clones derived from fyn-/- mutant mice. Thus, Fyn is not required for expression of these components of antigen specific lysis by CD8+ alloreactive CTL clones. It appears that CD8+ clones that use multiple lytic mechanisms may selectively employ the perforin or Fas-based pathway depending on properties of the target cell or stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

Natural killer and lymphokine-activated killer cells require granzyme B for the rapid induction of apoptosis in susceptible target cells

Granzyme (Gzm) B-deficient mice obtained by gene targeting were used to assess the role of Gzm B in the mechanisms used by natural killer (NK) and lymphokine-activated killer (LAK) cells to destroy target cells. Gzm B-/- NK cells, LAK cells, and cytotoxic T lymphocytes (CTL) all are defective in their ability to rapidly induce DNA fragmentation/apoptosis in susceptible target cells. This defect can be partially corrected with long incubation times of effector and target cells. Moreover, Gzm B-/- NK cells (but not CTL or LAK cells) exhibit a defect in 51Cr release from susceptible target cells. This 51Cr release defect in Gzm B-deficient NK cells is also not overcome by prolonged incubation times or high effector-to-target cell ratios. We conclude that Gzm B plays a critical and nonredundant role in the rapid induction of DNA fragmentation/apoptosis by NK cells, LAK cells, and CTL. Gzm B may have an additional role in NK cells (but not in CTL or LAK cells) for mediating 51Cr release.

The target cell nucleus is not required for cell-mediated granzyme- or Fas-based cytotoxicity

The requirement for target cell nuclei in the two apoptotic death pathways used by cytotoxic lymphocytes was tested using model effector systems in which the granzyme and Fas pathways of target damage are isolated. Mast cell tumors expressing granzymes A and B in addition to cytolysin/perforin lysed tumor target cells about 10-fold more efficiently than comparable effector cells without granzymes. Enucleated cytoplast targets derived from these cells were also lysed with a similar 10-fold effect of granzymes. In contrast to cytoplasts, effector granzyme expression did not influence lysis of red cell targets. The Fas pathway was assessed using the selected cytotoxic T lymphocyte hybridoma subline d11S, which lysed target cells expressing Fas but not those lacking Fas. Similarly, cytoplasts derived from Fas+ but not Fas- cells were also readily lysed by these effector cells. Thus, neither the nucleus itself nor the characteristic apoptotic nuclear damage associated with the two major cell death pathways used by cytotoxic lymphocytes are required for cell death per se.

The CTL's kiss of death

The potent and specific lytic activity of CTLs can occur by at least two distinct pathways. In the secretion and perforin-mediated pathway, the direct effect(s) on the target cell membrane of the pore-forming agent perforin, probably in conjunction with granzymes, also secreted from the CTLs, causes the target's demise. Intercytoplasmic transfer of granzymes is believed to be involved in inducing target apoptosis. In the Fas-mediated pathway, engagement of a CTL membrane ligand with an apoptosis-inducing target cell surface receptor, such as the FasL with Fas, triggers programmed disintegration of the CTL-bound target; secretion of granzymes and pore formation by perforin are not involved in this receptor-mediated mechanism. Despite the fundamental differences in their onset for both pathways, the downstream sequence of events that culminate in target cell apoptosis appears to be similar. Further studies will resolve this enigma.

Perforin: structure and function

Perforin is a cytolytic mediator produced by killer lymphocytes, and is stored in and released by cytoplasmic granules. The protein is partially homologous to the terminal components of the membrane attack complex of complement and produces pores of up to 20 nm in diameter on target membranes. Its genomic and protein structures have recently been unraveled, and its function elucidated through the availability of genetically engineered, perforin-deficient mice. Here Chau-Ching Liu, Craig M. Walsh and John Ding-E Young briefly outline certain biochemical and molecular features of perforin, and discuss the still-evolving issues concerning the relevance of perforin and Fas in cell killing.

Granzymes: exogenous proteinases that induce target cell apoptosis

Cytotoxic lymphocytes mediate immunity against viruses and surveillance against neoplastic transformation. They kill target cells by multiple mechanisms, but utilize a pore-forming protein, perforin, and a family of serine proteinases as their principal means of inflicting cell death. Recent studies have demonstrated that perforin and serine proteinases synergistically trigger an endogenous pathway of apoptosis resulting in dissolution of the target cell nuclear membrane and DNA fragmentation. These changes may be secondary to inappropriate activation of p34cdc2 kinase and the subsequent derangement of cell cycle control. As discussed by Mark Smyth and Joseph Trapani, the immediate molecular targets of perforin/granzyme-mediated apoptosis are still unclear, though candidate molecules with homology to cell death gene products from primitive organisms are currently under close scrutiny.

Synergistic roles of granzymes A and B in mediating target cell death by rat basophilic leukemia mast cell tumors also expressing cytolysin/perforin

We have studied the cytotoxic activity of rat basophilic leukemia (RBL) cells transfected with cDNAs for the cytotoxic T lymphocyte (CTL) granule components, cytolysin (perforin), granzyme A, and granzyme B. With red cell targets, cytolysin expression conferred potent hemolytic activity, which was not influenced by coexpression of granzymes. With tumor targets, RBL cells expressing cytolysin alone were weakly cytotoxic, but both cytolytic and nucleolytic activity were enhanced by coexpression of granzyme B. RBL cells expressing all three CTL granule components showed still higher cytotoxic activities, with apoptotic target death. Analysis of the cytotoxic activity of individual transfectant clones showed that cytolytic and nucleolytic activity correlated with granzyme expression but was independent of cytolysin expression within the range examined. A synergism between granzymes A and B was apparent when the triple transfectant was compared with RBL cells expressing cytolysin and one granzyme. These data implicate granzymes as the major mediators of tumor target damage by cytotoxic lymphocytes.

Execution and suicide: cytotoxic lymphocytes enforce Draconian laws through separate molecular pathways

With the ability to analyze cytotoxicity in animals deficient in effector molecules, the debate over the biological significance of individual effector pathways is finally being settled. For CD8+ cytotoxic T lymphocytes, the two primary cytotoxic pathways are mediated by the Fas ligand and perforin. The dichotomy between the two killing pathways is mirrored in the dichotomy between their biological roles: the primary function of the Fas ligand is the control of normal cell renewal by inducing programmed cell death of actively proliferating cells via the Fas pathway. This type of cell death is part of the normal endogenous homeostatic mechanism responsible for maintaining rapidly changing cell populations such as clonally expanding and contracting T cells. In contrast, the biological function of perforin and associated granule proteins is the killing and elimination of parasitized, non-compliant cells that arise as part of pathophysiological processes and may resist lysis pathways signalled to induce apoptosis. Thus, the main function of perforin and granzymes is the maintenance of immune surveillance against both exogenous and endogenous hazards.