Molecular mechanisms of cytolysis by complement and by cytolytic lymphocytes

Structure of the poly-C9 component of the complement membrane attack complex

The membrane attack complex (MAC)/perforin-like protein complement component 9 (C9) is the major component of the MAC, a multi-protein complex that forms pores in the membrane of target pathogens. In contrast to homologous proteins such as perforin and the cholesterol-dependent cytolysins (CDCs), all of which require the membrane for oligomerisation, C9 assembles directly onto the nascent MAC from solution. However, the molecular mechanism of MAC assembly remains to be understood. Here we present the 8 Å cryo-EM structure of a soluble form of the poly-C9 component of the MAC. These data reveal a 22-fold symmetrical arrangement of C9 molecules that yield an 88-strand pore-forming β-barrel. The N-terminal thrombospondin-1 (TSP1) domain forms an unexpectedly extensive part of the oligomerisation interface, thus likely facilitating solution-based assembly. These TSP1 interactions may also explain how additional C9 subunits can be recruited to the growing MAC subsequent to membrane insertion.

Visualization of cytolytic T cell differentiation and granule exocytosis with T cells from mice expressing active fluorescent granzyme B

To evaluate acquisition and activation of cytolytic functions during immune responses we generated knock in (KI) mice expressing Granzyme B (GZMB) as a fusion protein with red fluorescent tdTomato (GZMB-Tom). As for GZMB in wild type (WT) lymphocytes, GZMB-Tom was absent from naïve CD8 and CD4 T cells in GZMB-Tom-KI mice. It was rapidly induced in most CD8 T cells and in a subpopulation of CD4 T cells in response to stimulation with antibodies to CD3/CD28. A fraction of splenic NK cells expressed GZMB-Tom ex vivo with most becoming positive upon culture in IL-2. GZMB-Tom was present in CTL granules and active as a protease when these degranulated into cognate target cells, as shown with target cells expressing a specific FRET reporter construct. Using T cells from mice expressing GZMB-Tom but lacking perforin, we show that the transfer of fluorescent GZMB-Tom into target cells was dependent on perforin, favoring a role for perforin in delivery of GZMB at the target cells' plasma membranes. Time-lapse video microscopy showed Ca++ signaling in CTL upon interaction with cognate targets, followed by relocalization of GZMB-Tom-containing granules to the synaptic contact zone. A perforin-dependent step was next visualized by the fluorescence signal from the non-permeant dye TO-PRO-3 at the synaptic cleft, minutes before the labeling of the target cell nucleus, characterizing a previously undescribed synaptic event in CTL cytolysis. Transferred OVA-specific GZMB-Tom-expressing CD8 T cells acquired GZMB-Tom expression in Listeria monocytogenes-OVA infected mice as soon as 48h after infection. These GZMB-Tom positive CD8 T cells localized in the splenic T-zone where they interacted with CD11c positive dendritic cells (DC), as shown by GZMB-Tom granule redistribution to the T/DC contact zone. GZMB-Tom-KI mice thus also provide tools to visualize acquisition and activation of cytolytic function in vivo.

Targeting tumours by adoptive transfer of immune cells

1. Surgery, radiotherapy and chemotherapy are the most widely used and well-established modalities for treating malignant diseases. Surgery is used to excise solid tumours and radiotherapy/chemotherapy are used for the treatment of liquid tumours and for solid tumours where there is a risk of micrometastases. A major drawback for both radiotherapy and chemotherapy is their lack of specificity for tumour cells. Both these treatments can destroy normal bone marrow cells and result in severe side-effects. 2. The impairment of haemapoiesis due to bone marrow destruction combined with the use of toxins in chemotherapy that inhibit the proliferation of immune cells leaves many patients immunocompromised. This complicates the development of prophylactic (vaccine) strategies for tumours where patients are undergoing conventional therapy. 3. An alternative approach is to expand and activate tumour-specific immune cells in vitro that can then be adoptively transferred back in large numbers. This is defined as adoptive immunotherapy and has the advantage of potentially bypassing the immuno-inhibitory effects of conventional therapies. 4. Transferred immune cells have been shown to mediate tumour regression in patients by both direct and indirect mechanisms. The immune cells used include tumour reactive T lymphocytes and dendritic cells, which elicit tumour specific responses. 5. Many novel cell-based immunotherapeutic strategies developed in murine tumour models are now being applied in human clinical trials. The malignancies targeted include melanoma, chronic myelogenous leukaemia and breast, ovarian, colon and kidney cancers. In the present review, we discuss these novel cell-based strategies and the implications they have for the future treatment of human malignancies.

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.

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.

Heat-shocked A20 lymphoma cells fail to induce degranulation of cytotoxic T lymphocytes: possible mechanism of resistance

A20 lymphoma cells were subjected to heat shock for 2 h at 42 and 43 +/- 0.1 degrees C and then evaluated at 37 degrees C for sensitivity to lysis by intact allo-specific cytotoxic T lymphocytes (CTLs), perforin-containing granules isolated from CTLs, and Fas-mediated apoptosis. Heat shock at 42 degrees C caused little change in sensitivity of the lymphoma cell line to lysis by intact CTLs or their isolated cytotoxic granules, but caused increased sensitivity to Fas-mediated apoptosis. However, A20 cells shocked at 43 degrees C declined significantly in sensitivity to lysis by intact CTLs, while remaining very sensitive to perforin granules and to Fas-mediated apoptosis. Expression of the inducible heat shock protein was observed in A20 cells incubated at 43 degrees C, but not in those incubated at 42 degrees C, suggesting a role for heat shock proteins. Furthermore, A20 cells shocked at 43 degrees C did not provoke degranulation and secretion of granzymes by antigen-specific CTLs, although formation of CTL-target conjugates and levels of MHC class I molecules remained unchanged. These observations demonstrate that hyperthermia or febrile conditions may reduce susceptibility of target cells to CTL attack due to failure of antigen presentation and the inability of CTLs to recognize heat stressed targets, thus enabling targets to escape CTL attack.

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.

Modification of membrane permeability by animal viruses

Animal viruses permeabilize cells at two well-defined moments during infection: (1) early, when the virus gains access to the cytoplasm, and (2) during the expression of the virus genome. The molecular mechanisms underlying both events are clearly different; early membrane permeability is induced by isolated virus particles, whereas late membrane leakiness is produced by newly synthesized virus protein(s) that possess activities resembling ionophores or membrane-active toxins. Detailed knowledge of the mechanisms, by which animal viruses permeabilize cells, adds to our understanding of the steps involved in virus replication. Studies on early membrane permeabilization give clues about the processes underlying entry of animal viruses into cells; understanding gained on the modification by viral proteins of membrane permeability during virus replication indicates that membrane leakiness is required for efficient virus release from infected cells or virus budding, in the case of enveloped viruses. In addition, the activity of these membrane-active virus proteins may be related to virus interference with host cell metabolism and with the cytopathic effect that develops after virus infection.

Structure-activity analysis of the antitumor and hemolytic properties of the amphiphilic alpha-helical peptide, C18G

The in vitro antitumor and hemolytic activities of analogs of peptide C18G were compared in order to elucidate important structural features which affect cytotoxicity. The sequence of C18G, a basic peptide which can form an amphiphilic alpha-helix, is a derivative of the carboxyl terminus of human platelet factor IV. The results demonstrate that both amphiphilicity and helicity are essential for peptide activity, and that addition of a negatively charged amino acid results in decreased cell lysis. Whereas peptides exhibiting various degrees of potency did not differ with respect to helical content, an increase in peptide hydrophobicity did correlate with an increase in antitumor and hemolytic activity, as well as susceptibility to inhibition by serum. Higher hydrophobicity could be associated with improved ability to insert into the cell membrane. The position or context of specific residues within an amphiphilic peptide can also be important for activity. Furthermore, an increase in tumoricidal activity is not always accompanied by an increase in hemolytic activity or susceptibility to inhibition by serum. Possible reasons for the lower sensitivity of RBCs versus tumor cells to peptide cytotoxicity are discussed. Finally, compared with structurally idealized amphiphilic alpha-helical peptides, non-idealized peptides can possess higher tumoricidal activity, but are less hemolytic and less susceptible to serum inhibition.

Regulation of perforin gene expression in a T cell hybrid with inducible cytolytic activity

A mouse x rat T cell hybrid (PC60) that does not require interleukin (IL)-2 for proliferation, was used as a model to study regulation of perforin gene expression. Perforin mRNA is barely detectable in non-induced PC60 cells; however, a 30-fold induction is observed after stimulation with IL-1 alone. Peak perforin mRNA levels were reached after 10 h of induction with IL-1, and these levels were maintained for as long as the stimulus was present. IL-2 by itself has no detectable effect. However, in combination with IL-1 it shows the same induction kinetics as IL-1 alone for the first 10 h, subsequently there is synergism (100-fold induction) between IL-1 and IL-2. The induction response was mainly due to increased transcriptional rates of the perforin gene, and require newly synthesized proteins. The half-life of perforin mRNA in this system is about 5 h. In addition, we confirm the existence of two types of mouse perforin mRNA that differ in their 5' untranslated regions, and show evidence that both mRNA are translated in vivo with similar efficiencies.

Resistance of HER2/neu-overexpressing tumor targets to lymphokine-activated-killer-cell-mediated lysis: evidence for deficiency of binding and post-binding events

HER2/neu-overexpressing tumor cell lines are relatively resistant to lymphokine-activated killer (LAK) cell cytotoxicity when compared to HER2/neu-nonexpressing lines. HER2/neu+ targets were also resistant to binding by LAK large granular lymphocytes (LGL) as shown by visualization at the single-cell level, a target monolayer binding assay and in "cold" target inhibition experiments. HER2/neu+ LAK-resistant ovarian cell lines demonstrated an absence of ICAM-1 expression while expression of LFA-3, N-CAM, laminin and beta 1 integrins was comparable to that of HER2/neu- targets. In contrast, the HER2/neu+ breast cell line, SKBR-3, which was also resistant to lysis and binding by LAK LGL, demonstrated normal expression of ICAM-1. Anti-ICAM-1 antibodies blocked binding and lysis of HER2/neu- carcinoma targets by LAK cells, further supporting the notion that lack of ICAM-1 expression on HER2/neu+ cells contributes to their resistance. The modest binding and lysis of HER2/neu+ targets by LAK cells was significantly inhibited by anti-LFA-1 antibodies, suggesting the existence of another counter-receptor for LFA-1 on HER2/neu+ targets. The following also supported deficiencies in post-binding events when HER2/neu+ cells resisted the lytic activity of LAK cells: (a) when the relative resistance to effector cell binding was overcome by exogenous lectin. HER2/neu+ cell lines were still resistant to LAK cytolysis, and (b) HER2/neu+ targets were resistant to perforin-containing granule extracts obtained from the CTLL-R8 cytotoxic lymphocyte cell line. These results indicate that deficiency in effector binding as well as post-binding events contributes to the resistance of HER2/neu-overexpressing tumor targets to LAK-cell-mediated lysis.

In vivo and in vitro primed lymphocytes. Correlation of cytochemically detected BLT-specific lymphoid serine protease with cytotoxic activity

We describe the validation of a cytochemical method to detect a cytolytic cell-specific lymphoid serine protease which can be upregulated during viral infection and allogeneic stimulation. The cytolytic cell specificity was ascertained by demonstrating a high correlation between BLT substrate-specific serine protease (SP) activity and cytotoxicity of in vivo and in vitro stimulated lymphocytes. The presence of SP in peripheral blood lymphocytes was compared with their capacity to kill K562 targets in a lectin-dependent cytotoxicity assay. The correlation coefficient was 0.92 and 0.93 at E:T ratios 10:1 and 20:1 respectively. In allogeneic mixed lymphocyte cultures an increase of SP activity in effector lymphocytes was paralleled by an augmentation of cytotoxic capacity towards stimulator target cells. SP+ granules showed intracellular polarization to the effector/target cell interface during conjugate formation. These results together with previous studies suggest that this method provides a sensitive assay which predicts the cytolytic potential present in a lymphocyte population.

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.

Incidence and phenotype restriction of lymphoid BLT-serine protease granules in spontaneously diabetes prone BB rats compared with a normal rat strain

In diabetes prone BB rats a relative increase of serine protease (SP)-positive lymphocytes (39.8 +/- 10%) was observed in peripheral blood at the time of diabetes manifestation (DM) compared with non-diabetic healthy Sprague Dawley control rats (Co: 10.3 +/- 4%), with BB rats at age of premanifestation (PM: 14.7 +/- 4%) and beyond age of expected manifestation (non-diabetic animals, ND: 25.2 +/- 4%). Similar absolute numbers were found in diabetic BB rats in comparison with Sprague Dawley rats. In PM, absolute numbers were lower in comparison with diabetic BB rats. SP granular positivity was found restricted to OX8+ lymphocytes. SP granule-bearing OX8+ lymphocytes were more frequently seen in the BB rat strain (PM: 74.3 +/- 8%; DM: 79.4 +/- 8%; ND: 78 +/- 10%) compared with normal rats (Co: 32.5 +/- 8%). Absolute numbers were lower in PM animals in comparison with DM rats. OX8+ cells were found in a higher relative number in DM animals (49.1 +/- 7%) compared with controls (28.2 +/- 3%), PM (26.3 +/- 5%) and ND (34 +/- 2%) animals. T lymphocytes expressing the W3/25+ marker, invariably negative for SP granules, were present in a higher relative number in ND (49.8 +/- 7%) and the control group (52.3 +/- 10%) compared with PM (31 +/- 8%) and DM (38 +/- 11%) animals. Absolute numbers of the OX39+ lymphocyte subpopulation were decreased in PM and DM-BB rats in comparison with the control group.

Identification and partial characterization of a novel plasma membrane-associated lytic factor isolated from highly purified adherent lymphokine-activated killer cells

We have identified and partially purified a novel cytolytic factor isolated from enriched plasma membranes prepared from highly purified lymphokine-activated killer cells (adherent-LAK. A-LAK cells) and a large granular lymphocytic NK cell leukemia, CRNK-16. The enriched plasma membranes were shown to be physically devoid of lytic granules and contained no detectable pore-forming protein (PFP, perforin) activity. The plasma membrane-associated cytolytic factor (designated M-CTX) was solubilized in biologically active form and was highly lytic to a large panel of target cells in 2- to 4-hr 51Cr release assays. Characteristics of the M-CTX include: (1) it is plasma membrane- not granule-associated: (2) it is not hemolytic and functions in the absence of Ca2+: (3) nucleated target cells are lysed in 2 to 4 hr at 37 degrees C but not at 4 degrees C: (4) it induces apoptotic cell death with nuclear DNA fragmentation and massive membrane blebbing: (5) it is isolated from the plasma membranes of cultured A-LAK cells, a lytically active LGL leukemia (CRNK-16), and fresh spleen cells but not from thymocytes or L929 fibroblasts: and (6) the lytic activity of the partially purified toxin is inactivated by trypsin, serum, and heat, but is not blocked by antibodies that inactivate TNF-alpha, LT or IFN-gamma. Taken collectively, these data suggest that M-CTX may represent a heretofore undescribed membrane-associated toxin possibly involved in contact-mediated cell killing.

Pore-forming peptide of pathogenic Entamoeba histolytica

A polypeptide that causes pore formation in target-cell membranes is implicated in the potent cytolytic activity of pathogenic Entamoeba histolytica. Pore-forming material was purified to apparent homogeneity by a multistep procedure, and its analysis by NaDodSO4/PAGE revealed one peptide of 4-5 kDa under nonreducing or under reducing conditions. Pore-forming activity was measured by depolarization of liposome membrane potential and was found to be optimally expressed at low pH. Active material preferentially inserted into negatively charged lipid vesicles. Treatment of purified amoeba peptide in solution or bound to liposomes with glutaraldehyde revealed oligomers upon NaDodSO4/PAGE, suggesting functionally relevant peptide-peptide interactions. The NH2-terminal amino acid sequence of the amoeba peptide was determined by protein sequencing and revealed a structural similarity to melittin, the membranolytic peptide of bee venom.

T-cell-mediated cytotoxicity

There are two competing theories to explain the mechanism(s) by which cytolytic T lymphocytes kill target cells: granule exocytosis of a pore-forming protein, and contact-induced internal disintegration. Accumulated evidence supports alternative pathways in lymphocytoxicity, possibly reflecting distinct effector functions expressed by different killer cells and cells at different stages of activation.

A microassay for the pore-forming activity of complement, perforin, and other cytolytic proteins based on confocal laser scanning microscopy

A fluorescence microscopic assay for the activity of complement, perforin, and other cytolytic proteins which form transmembrane pores in cellular membranes is described. The assay was worked out and tested with red blood cell membranes (ghosts) and was then applied to intact hemoglobin-free cells. Resealed human erythrocyte ghosts were incubated with complement or perforin. A small polar fluorescent probe (fluorescein-labeled 1-kDa dextran, FD1) which permeates through complement and perforin pores but not through normal cell membranes was added to the samples. The capability of the confocal laser scanning microscope (CLSM) to generate thin optical sections was exploited to visualize and quantitate fluorescence inside single ghosts and thus determine the fraction of ghosts which had become permeable for FD1. The activity of complement or perforin was quantitated by plotting the fraction of permeable cells versus the concentration of the pore-forming protein. The results were in good agreement with those of a conventional hemolytic assay. The CLSM-based assay was then applied to intact hemoglobin-free cells for which only few alternative assays are available. Compared to conventional hemolytic assays for the activity of pore-forming proteins the assay described here can be applied to a large variety of natural and artificial membrane systems. The assay can be performed under nonlysing conditions. Furthermore, the assay is simple, relatively fast, and requires only extremely small amounts of cells and pore-forming proteins.

The regulation of complement on cell surfaces

The regulation of complement at the surface of cells is mediated by both plasma and membrane proteins. These molecules act in concert to prevent the accelerated catabolism of complement proteins and the concomitant lysis of homologous blood cells. The deficiency of either a plasma or membrane protein predisposes to the development of disease.

Human and murine cytotoxic T lymphocyte serine proteases: subsite mapping with peptide thioester substrates and inhibition of enzyme activity and cytolysis by isocoumarins

The active site structures of human Q31 granzyme A, murine granzymes (A, B, C, D, E, and F), and human granzymes (A, B, and 3) isolated from cytotoxic T lymphocytes (CTL) were studied with peptide thioester substrates, peptide chloromethyl ketone, and isocoumarin inhibitors. Human Q31, murine, and human granzyme A hydrolyzed Arg- or Lys-containing thioesters very efficiently with kcat/KM of 10(4)-10(5) M-1 s-1. Murine granzyme B was found to have Asp-ase activity and hydrolyzed Boc-Ala-Ala-Asp-SBzl with a kcat/KM value of 2.3 X 10(5) M-1 s-1. The rate was accelerated 1.4-fold when the 0.05 M NaCl in the assay was replaced with CaCl2. The preparation of granzyme B also had significant activity toward Boc-Ala-Ala-AA-SBzl substrates, where AA was Asn, Met, or Ser [kcat/KM = (4-5) X 10(4) M-1 s-1]. Murine granzymes C, D, and E did not hydrolyze any thioester substrate but contained minor contaminating activity toward Arg- or Lys-containing thioesters. Murine granzyme F had small activity toward Suc-Phe-Leu-Phe-SBzl, along with some contaminating trypsin-like activity. Human Q31 granzyme A, murine, and human granzyme A were inhibited quite efficiently by mechanism-based isocoumarin inhibitors substituted with basic groups (guanidino or isothiureidopropoxy). Although the general serine protease inhibitor 3,4-dichloroisocoumarin (DCI) inactivated these tryptases poorly, it was the best isocoumarin inhibitor for murine granzyme B (kobs/[I] = 3700-4200 M-1 s-1). Murine and human granzyme B were also inhibited by Boc-Ala-Ala-Asp-CH2Cl; however, the inhibition was less potent than that with DCI. DCI, 3-(3-amino-propoxy)-4-chloroisocoumarin, 4-chloro-3-(3-isothiureidopropoxy)isocoumarin, and 7-amino-4-chloro-3-(3-isothiureidopropoxy)isocoumarin inhibited Q31 cytotoxic T lymphocyte mediated lysis of human JY lymphoblasts (ED50 = 0.5-5.0 microM).

Detergent extraction and characterization of tumor hemolytic factor from plasma membranes of oncogene transformed fibroblasts

Cancer cells have the capacity to lyse erythrocytes by a cell-contact-requiring phenomenon. Subcellular fractionation procedures have revealed that the hemolytic principle resides in the cancer cell plasma membrane. In this study we report the detergent extraction of a potent hemolytic factor from the plasma membranes of ras-oncogene-transformed fibroblasts. Ammonium-sulfate partitioning (60-100%) of detergent-extracted proteins was used to enrich hemolytic activity. Tumor membrane Hemolytic Factor (mTHF) was inactivated by treatment with papain, suggesting that it is a protein. mTHF was inhibited by serum, but was unaffected by extremes of temperature and pH, also by metal chelation with EDTA. Surface radio-iodination of tumor cells and isolation of cell organelles was used to characterize the outer plasma membrane localization of mTHF. mTHF retained hemolytic activity when reconstituted into stable phospholipid vesicles. Pre-incubation of mTHF with red cell ghosts led to an abrogation of hemolytic activity. mTHF-induced hemolysis consists of a 2-stage phenomenon: an early binding step, followed by hemolysis after 4 hr.

Cellular cytotoxic function and potential in acute myelogenous leukaemia

Twenty-seven adult AML patients (13 with active disease and 14 in complete remission) were investigated for their cellular cytotoxic potential and function. All AML patients, whether with active disease or in complete remission, showed increased percentage of CD3+ lymphocytes expressing the cytotoxicity-linked cytoplasmic serine esterase, suggesting a higher than normal cytotoxic potential. However, when the cytotoxic function in these patients were analysed in terms of the natural killer and lectin-dependent cellular cytotoxicity, all AML patients, whether with active disease or in complete remission, had impaired target cell lytic activity. This paradox of cytotoxicity is most likely due to the immunosuppressive effect of the serum factor elaborated by leukaemia myeloblasts.

Extracellular ATP in T-lymphocyte activation: possible role in effector functions

We hypothesized that cytolytic T lymphocytes (CTL) may utilize extracellular ATP (ATPo) during the effector phase of the CTL-target cell interactions and that CTL could be the source of ATPo. It is demonstrated here that incubation of CTL with activating ligands [Con A or monoclonal antibody (mAb) to the T-cell antigen receptor (TCR)] results in the extracellular Ca2(+)-independent accumulation of the ATPo. The addition of the ATP-degrading enzymes into the mixture of CTL and target cells results in a strong inhibition of the CTL-mediated, TCR-triggered lethal-hit delivery to the target cell. In a parallel control experiment, the employed enzymes did not affect target cell-induced, TCR-triggered exocytosis of granules from CTL. Thus, the removal of ATPo with enzymes does not interfere with the activation of CTL by the target cell but does block lytic events. Cloned helper T lymphocytes also accumulate ATPo after incubation with anti-TCR mAb or Con A, suggesting the possibility that ATPo, which acts in concert with ectoprotein kinases and/or purinergic receptors, may be of general use as a messenger in cellular interactions of T lymphocytes.

Purification and characterisation of soluble tumour haemolytic factor isolated from oncogene transformed fibroblasts

Numerous studies have shown that intact cancer cells and cell extracts have the capacity to lyse erythrocytes in vitro. The transformation of NIH-3T3 fibroblasts by ras oncogenes has recently been demonstrated to result in tumour cells releasing a haemolytic factor. The purpose of this study has been to purify and further characterise the soluble tumour haemolytic factor (sTHF) produced by mouse fibroblasts transformed by T24 human bladder cancer DNA and by the cloned Harvey murine sarcoma viral oncogene. To this end, transformed fibroblasts were cultivated in serum-free medium. The cell-free supernatant was treated with ammonium sulphate and the precipitate achieved at 60-100% saturation was dialysed and applied to a gel filtration column. A haemolytic factor was eluted with an Mr between 65,000 and 75,000. Zinc chelate and strong anion exchange column chromatography resulted in greater than 3,000-fold enrichment of sTHF. SDS-PAGE of sTHF resulted in a single protein band of 66,000 Da. Soluble THF had no immunological cross-reactivity with known cytokines produced by lymphocytes and macrophages. The pathophysiological role of sTHF in cancer remains to be determined.

Identification of hemolytic granules isolated from human myocardial cells

Human myocardial cells from fresh autopsy material contained granules which possessed hemolytic activity against guinea pig and rabbit erythrocytes. The hemolytic granules, which had a density of 1.02 and a diameter of 200-300 nm, were recovered as a microsome fraction from subcellular homogenates of human myocardial cells by differential centrifugation in 300 mM sucrose containing 0.1 mM PMSF and 10 mM EDTA. The membrane lesions caused by the granules were ring-like structures with an internal diameter of about 10-17 nm, analogous to that caused by perforin- and complement-induced lysis. However, the requirement for divalent cation differed from that for perforin-induced lysis, since the microsome-mediated lysis occurred in the presence of EDTA.

Upregulation of a lymphoid serine protease in kidney allograft recipients

The presence of a putative, cytotoxicity-linked lymphoid serine esterase (SE) has been studied in 79 kidney graft recipients. Peripheral blood lymphocytes (PBL) bearing an N-alpha-benzyloxy carbonyl-L-lysine thiobenzyl ester (BLT)-specific SE were evaluated by a novel cytochemical staining method. A characteristic of post-allograft patients was an increased presence of SE containing granules in PBL. In 46 patients with stable graft function SE + PBL were 33.41 +/- 10.34% (controls: 26.30 +/- 5.22%, P less than 0.0025), SE + CD4+ 4.32 +/- 3.85% (controls 2.13 +/- 1.52%, P less than 0.0025) and SE + CD8+ T cells 47.68 +/- 18.64% (controls: 28.50 +/- 6.50%, P less than 0.0005). In those graft recipients undergoing a rejection episode a marked upregulation of SE activity could be observed when compared to the stable graft group: SE + PBL were 59.91 +/- 10.89% (P less than 0.0005), SE + CD8+ 74.30 +/- 10.79% (P less than 0.0005) and SE + CD4+ T cells 28.56 +/- 13.50% (P less than 0.0005). In 10 cases this increase of SE activity was observed with a time lag of up to 37 days prior to the onset of clinical or biopsy proven rejections, promptly decreasing in response to methylprednisolone antirejection therapy. In patients with recurrent rejection episodes and subsequent graft loss, a repeating increase of SE activity indicated a failure of therapeutic agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Perforin binding to cells and lipid membranes determined by a simple competition assay

Perforin-mediated lysis consists of at least three steps: perforin binding to the target cell, insertion into the plasma membrane, and polymerization to form pores. Perforin binding, the first step, is critical for pore formation. Accordingly, a competition assay was here established for detecting the perforin-binding activities of nucleated cells and lipid membrane vesicles such as cytoplasts or liposomes. The competition assay has certain advantages over the 51Cr release assay, since no isotope and less perforin are needed for the competition assay, and the perforin-binding activity of liposomes and proteolytic enzyme-treated and fixed nucleated cells can also be detected. The competition assay was used to study the mechanism of resistance of cytolytic T lymphocytes (CTL) to perforin-mediated lysis. The results from this assay indicate that perforin-binding activity is not a function of membrane rigidity, and that there is a direct correlation between the ability of cells to bind perforin and their susceptibility to lysis by perforin, i.e., resistant CTL and their corresponding cytoplasts bind perforin much less effectively than susceptible tumor cells and their cytoplasts. A model is proposed whereby a surface molecule or complex of molecules on CTL interferes with perforin-binding activity, thus protecting CTL from perforin-mediated lysis.

N-deglycosylation of human complement component C9 reduces its hemolytic activity

The effect of enzymatic deglycosylation of human complement component C9 on its hemolytic activity was investigated. Treatment of native C9 (Mr 71,000) with glyocpeptidase F (PNGase F) results in a stepwise decrease of the mol. wt. The formation of an Mr 67,000 peptide which is further converted to Mr 63,000 suggests that there are two N-linked carbohydrate chains per C9 polypeptide. Removal of approximately 88% of the N-linked oligosaccharides results in 80% reduction of the hemolytic activity (CH50). The completely N-deglycosylated Mr 63,000 peptide contains a remaining amount of 25% of the total carbohydrates of native C9. These glycans are assumed to be O-linked and predominantly attached to the C9a part of C9. The electrophoretic mobility of C9 is not affected by endoglycosidase F or H treatments revealing that the two N-linked glycans are of the tri- or tetra-antennary complex type. Cleavage of terminal sialic acids from native C9 by neuraminidase results in an Mr 67,000 product with nearly unaltered hemolytic activity. In contrast to other glycoproteins in which deglycosylation remained without major effects on their functional activity, our findings suggest that the N-linked carbohydrates are required for full expression of hemolytic activity of C9.

In situ observation of lymphocyte-tumor cell interaction in human lung carcinoma

Of thirty surgical specimens of human lung carcinoma examined with electron microscopy, eleven were found to contain killer cells (cytotoxic lymphocytes). Nearly one-third of the killer cells showed the polarization of their cytoplasm in which Golgi apparatus, dense granules or centrioles could be seen. The tumor cells conjugated by the killer cells showed lesions to varying degrees, including loss of cell membranes, alterations of cell organelles, even cell necrosis. The killer cells frequently penetrated into the cytoplasm, even the muclei of the tumor cells. The results of the present study suggest that the lymphocyte-mediated tumor cell lysis may exist in the microenvironment of human lung carcinoma and that some of these cytotoxic lymphocytes may kill their target cells by a similar mechanism of the pore formation or granule exocytosis model, but some different aspects were also observed, as compared with the results of the in vitro studies.

Induction of target cell DNA release by the cytotoxic T lymphocyte granule protease granzyme A

The rapid breakdown of target cell DNA during CTL-mediated lysis has been difficult to explain by the granule exocytosis model of cytotoxicity. The involvement of CTL granule proteases in this process was strongly suggested by experiments in which CTL were pretreated with the serine protease inhibitor PMSF, in combination with agents that raise the pH of acidic intracellular compartments. While PMSF pretreatment alone had little effect on target lysis or DNA breakdown, the combination of PMSF and NH4Cl or monensin profoundly reduced target cell DNA release, while little effect was observed on target lysis, as measured by 51Cr release. CTL granule extracts cause release of 125I-DNA from detergent-permeabilized cells. This nuclear DNA-releasing (NDR) activity is inhibited by serine esterase inhibitors that also inhibit the granule BLT-esterase activity, and is specifically immunoabsorbed by antibodies to the CTL granule protease granzyme A. The NDR activity comigrates with BLT-esterase activity during subcellular fractionation, solubilization, gel filtration, and aprotinin-Sepharose affinity chromatography. SDS-PAGE analysis of the affinity-purified product indicates a molecular mass of 60,000 daltons under non-reducing conditions, which moves to 30,000 daltons upon reduction, consistent with previously reported behavior of granzyme A. When the purified material was reduced and alkylated, both esterase and NDR activities comigrated at 30,000 daltons upon gel filtration. Although fully lytic concentrations of purified LGL granule cytolysin alone failed to induce target cell DNA release, a combination of purified granzyme A and the cytolysin induces substantial DNA release.

Veto function in vitro and in vivo

A CTLp recognizing another cell, called veto cell, is suppressed by that veto cell. The veto cell can itself be a CTL. For the veto function, the TCR of the veto cell is not required, hence the veto function is a backward action of CTL. Since, from the point of view of the veto cell, only self-reactive CTLp are suppressed, the veto function could be a mechanism for maintaining self tolerance of CTL. The characteristics of veto function in vitro and in vivo are discussed, as well as the potential physiological relevance.

N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester (BLT) serine esterase in human cytolytic effector cells and cell line targets

The granules of in vitro primed cytotoxic mouse T cells and cytotoxic cell lines have been shown to contain high levels of N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester (BLT) esterase. The enzyme activity has been suggested to be associated with the cytotoxic capacity of killer cells. We investigated human leucocytes and found that neutrophils, monocytes, cytotoxic T lymphocytes (CTL), natural killer (NK) cells [large granular lymphocytes (LGL)], and interleukin 2 activated killer (LAK) cells, which all display efficient cytotoxic capacity, show only marginal BLT esterase activity. The low BLT esterase activity in human lymphocytes increases about twofold when cells are stimulated in vitro with interleukin 2 (IL-2), phytohaemagglutinin (PHA), or cultured in mixed lymphocyte culture (MLC). Mouse T lymphocytes have about 20 times more BLT esterase activity than human T lymphocytes. The BLT activity in mouse T cells also increases about twofold in MLC. The human leukaemia cell lines (K562, U937, MOLT-4, Jurkat) and the mouse mastocytoma line (P815), which are frequently used as target cells, contain more BLT esterase activity than human resting or activated lymphocytes. We did not find a direct correlation between the cytotoxic capacity and the BLT esterase activity of killer cells.

Mechanisms of lymphocyte-mediated lysis

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells use multiple mechanisms to destroy their target cells. Pore formation resulting in osmotic lysis of the target is one mechanism; the pore-forming protein (perforin) responsible for this activity has been purified. Antigenically and functionally it resembles proteins of the membrane attack complex of complement. The other known mediators of cytotoxicity appear to be closely interrelated. Tumor necrosis factor (TNF), lymphotoxin (LT), and leukalexin are the three members of this group that have been purified, although their mechanisms of action are still unknown. CTLs fragment the DNA of target cells, as do TNF, LT, and leukalexin; this may be one of the mechanisms of action of these mediators. CTLs and NK cells do not self lyse. The basis of this phenomenon is unclear, although recent advances have shed some light on the problem.

CTL: virus control cells first and cytolytic cells second? DNA fragmentation, apoptosis and the prelytic halt hypothesis

It is usually presumed that cytotoxic T lymphocytes (CTL) stop viral replication by lysing infected cells before a full virus yield has been assembled. Unlike complement-mediated lysis, however, CTL induce apoptosis, including fragmentation of target cell DNA. Why should CTL do this? Here, Eric Martz and Donna Howell suggest that since the major function of CTL appears to be control of viruses, CTL may be able to halt viral replication without inducing rapid lysis. It may be more useful to think of CTL as virus control cells rather than as cytolytic cells.

The cytolytic T lymphocyte and its mode of action

While the binding step of cytolytic T lymphocyte (CTL) target cell interaction resulting in conjugate formation is a well-characterized event, there seems to be more than one mechanism whereby lymphocytes kill the target. In recent years, infliction of complement (C)-like "holes" (I.D. 10-20 nm) on the target cell membrane, believed to be produced by the Ca2+-dependent lytic protein(s) perforin/cytolysin of secretory lytic granule origin has been proposed to be the mechanism of lymphocytotoxicity. More recent evidence, however, suggests that Ca2+-dependent exocytosis of lytic granules (where detectable) is not involved in lymphocyte-mediated cytolysis. Furthermore, neither formation of C-like "holes" in targets exposed to CTL, nor higher-than-background levels of lytic granules, perforin or BLT-esterases, have been detected in highly potent, peritoneal exudate CTL (PEL) derived directly from the animal or in cytocidal PEL-hybridomas. Hence exocytosis of perforin and formation of the above pores may apply to certain effector cells, particularly those grown in vitro in IL-2, but not to in vivo primed CTL such as PEL. On the other hand, work from this laboratory with Ca2+ probes has shown that lysis induced by CTL such as PEL-not involving lytic granules, perforin or formation of the above "holes"-is preceded by a marked prelytic elevation of cytosolic Ca2+ in the target. CTL-induced target cell membrane perturbation--a direct result of receptor-mediated effector-to-target interaction or through a membrane-bound or secreted effector component(s)--may be responsible for triggering the prelytic influx of Ca2+ from external sources, or its mobilization from internal stores in the target. We propose that CTL-induced, persistent elevation of cytosolic Ca2+, above a critical level, rather than formation of 10-20 nm pores, is responsible for the catastrophic prelytic events observed in the target, such as bleb formation, metabolic exhaustion and DNA degradation, ultimately leading to lysis.

Phosphorylcholine acts as a Ca2+-dependent receptor molecule for lymphocyte perforin

Large granular lymphocytes and cytolytic T-lymphocytes (CTL) contain numerous cytoplasmic granules thought to be responsible, at least in part, for the cytolytic activity of these effector cells. Isolated granules are lytic for a variety of target cells and the granule proteins are specifically released upon target-cell interaction. Major proteins in mouse CTL granules are a family of seven serine proteases designated granzymes A to G, and a pore-forming protein called perforin (cytolysin). Purified perforin is cytolytic in the presence of Ca2+ and shows ultrastructural, immunological and amino-acid sequence similarities to complement component C9. Despite these similarities, perforin and C9 are clearly distinct in their mode of target-cell recognition. Whereas C9 insertion is absolutely dependent on a receptor moiety assembled from the complement proteins C5b, C6, C7, and C8 on the target-cell membrane, no requirement for a receptor molecule has been reported for perforin. Here, we demonstrate that phosphorylcholine acts as a specific, Ca2+-dependent receptor molecule for perforin.

The cellular immunotherapy of cancer: current and potential uses of interleukin-2

The potential for immune-mediated destruction of neoplasms was suggested nearly one century ago. Despite this, no "magic bullet" has yet been identified. Nevertheless, the physiology of cell-mediated immune reactions has been well characterized in molecular, cellular, and clinical studies of allograft and microbial immunity. Extensive studies performed in laboratory animal models have documented the in vitro and in vivo destruction of various neoplastic tissues by immune cells. This destruction can be directed against autologous, syngeneic, or allogeneic tumors in several systems with varying degrees of "tumor specificity". Two approaches exist towards utilizing these immune reaction in vivo. The first involves providing the tumor bearer with immunostimulatory agents, either specific or nonspecific, designed to activate and amplify the destructive potential of the individual's endogenous immune cells able to recognize and destroy autologous tumor. The second approach provides immune cells with antitumor capacity to a tumor-bearing individual, these cells having been activated exogenously. A number of successful regimens involving these two approaches, and combinations of them, have been delineated in animal tumor models. These experimental studies lay a strong foundation for initiating clinical trials of these concepts for patients with cancer. This review summarizes the diverse experimental studies in animals leading to clinical trials, presents recent data from ongoing clinical trials directly testing the potential for cellular immunotherapy, and then presents some of the major challenges facing further development and application of this potential therapeutic approach.

Lysis by RNK-16 cytotoxic lymphocyte granules. Rate assays and conditions to study control of cytolysis

Dense subcellular granules of cytolytic lymphocytes can mediate rapid lysis of erythrocytes or nucleated cells. The granules contain several different proteases and proteoglycans that regulate cytolysis. We describe a rate assay that we have already used to demonstrate the requirement for serine proteases in granule-mediated lysis. In this assay, 51Cr-labeled erythrocytes are lysed by limiting concentrations of granules from RNK-16 tumor cells. Cytolysis is initiated by the addition of calcium (1 mM final concentration) and stopped at 0.5-1 min intervals by acidification to pH 6.0. The effects of the granule protein concentration, temperature, the concentration of erythrocytes, pH, and the concentration of calcium on the rate of lysis are reported. A preliminary mathematical approach is described and suggested as a means to differentiate 'lag' or activation times from the initial burst of lysis. With this rate assay, we have found four classes of protease inhibitors that block granule-mediated lysis (Hudig et al. (1987) Biochem. Biophys. Res. Commun. 149, 882). The utility of the rate assays is underscored by the observation that reversible protease inhibitors only showed rates of cytolysis whereas irreversible protease inhibitors stopped cytolysis completely. Rate assays are essential for future analyses of the complex physiological regulation of granule-mediated cytotoxicity by proteases, endogenous protease inhibitors and proteoglycans.