Membrane channel formation by the lymphocyte pore-forming protein: comparison between susceptible and resistant target cells

Degranulation enhances presynaptic membrane packing, which protects NK cells from perforin-mediated autolysis

Natural killer (NK) cells kill a target cell by secreting perforin into the lytic immunological synapse, a specialized interface formed between the NK cell and its target. Perforin creates pores in target cell membranes allowing delivery of proapoptotic enzymes. Despite the fact that secreted perforin is in close range to both the NK and target cell membranes, the NK cell typically survives while the target cell does not. How NK cells preferentially avoid death during the secretion of perforin via the degranulation of their perforin-containing organelles (lytic granules) is perplexing. Here, we demonstrate that NK cells are protected from perforin-mediated autolysis by densely packed and highly ordered presynaptic lipid membranes, which increase packing upon synapse formation. When treated with 7-ketocholesterol, lipid packing is reduced in NK cells making them susceptible to perforin-mediated lysis after degranulation. Using high-resolution imaging and lipidomics, we identified lytic granules themselves as having endogenously densely packed lipid membranes. During degranulation, lytic granule–cell membrane fusion thereby further augments presynaptic membrane packing, enhancing membrane protection at the specific sites where NK cells would face maximum concentrations of secreted perforin. Additionally, we found that an aggressive breast cancer cell line is perforin resistant and evades NK cell–mediated killing owing to a densely packed postsynaptic membrane. By disrupting membrane packing, these cells were switched to an NK-susceptible state, which could suggest strategies for improving cytotoxic cell-based cancer therapies. Thus, lipid membranes serve an unexpected role in NK cell functionality protecting them from autolysis, while degranulation allows for the inherent lytic granule membrane properties to create local ordered lipid “shields” against self-destruction.

Degranulation enhances presynaptic membrane packing, which protects NK cells from perforin-mediated autolysis

Natural killer (NK) cells kill a target cell by secreting perforin into the lytic immunological synapse, a specialized interface formed between the NK cell and its target. Perforin creates pores in target cell membranes allowing delivery of proapoptotic enzymes. Despite the fact that secreted perforin is in close range to both the NK and target cell membranes, the NK cell typically survives while the target cell does not. How NK cells preferentially avoid death during the secretion of perforin via the degranulation of their perforin-containing organelles (lytic granules) is perplexing. Here, we demonstrate that NK cells are protected from perforin-mediated autolysis by densely packed and highly ordered presynaptic lipid membranes, which increase packing upon synapse formation. When treated with 7-ketocholesterol, lipid packing is reduced in NK cells making them susceptible to perforin-mediated lysis after degranulation. Using high-resolution imaging and lipidomics, we identified lytic granules themselves as having endogenously densely packed lipid membranes. During degranulation, lytic granule-cell membrane fusion thereby further augments presynaptic membrane packing, enhancing membrane protection at the specific sites where NK cells would face maximum concentrations of secreted perforin. Additionally, we found that an aggressive breast cancer cell line is perforin resistant and evades NK cell-mediated killing owing to a densely packed postsynaptic membrane. By disrupting membrane packing, these cells were switched to an NK-susceptible state, which could suggest strategies for improving cytotoxic cell-based cancer therapies. Thus, lipid membranes serve an unexpected role in NK cell functionality protecting them from autolysis, while degranulation allows for the inherent lytic granule membrane properties to create local ordered lipid "shields" against self-destruction.

Degranulation enhances presynaptic membrane packing, which protects NK cells from perforin-mediated autolysis

Natural killer (NK) cells kill a target cell by secreting perforin into the lytic immunological synapse, a specialized interface formed between the NK cell and its target. Perforin creates pores in target cell membranes allowing delivery of proapoptotic enzymes. Despite the fact that secreted perforin is in close range to both the NK and target cell membranes, the NK cell typically survives while the target cell does not. How NK cells preferentially avoid death during the secretion of perforin via the degranulation of their perforin-containing organelles (lytic granules) is perplexing. Here, we demonstrate that NK cells are protected from perforin-mediated autolysis by densely packed and highly ordered presynaptic lipid membranes, which increase packing upon synapse formation. When treated with 7-ketocholesterol, lipid packing is reduced in NK cells making them susceptible to perforin-mediated lysis after degranulation. Using high-resolution imaging and lipidomics, we identified lytic granules themselves as having endogenously densely packed lipid membranes. During degranulation, lytic granule-cell membrane fusion thereby further augments presynaptic membrane packing, enhancing membrane protection at the specific sites where NK cells would face maximum concentrations of secreted perforin. Additionally, we found that an aggressive breast cancer cell line is perforin resistant and evades NK cell-mediated killing owing to a densely packed postsynaptic membrane. By disrupting membrane packing, these cells were switched to an NK-susceptible state, which could suggest strategies for improving cytotoxic cell-based cancer therapies. Thus, lipid membranes serve an unexpected role in NK cell functionality protecting them from autolysis, while degranulation allows for the inherent lytic granule membrane properties to create local ordered lipid "shields" against self-destruction.

CD8(+) T Cell-Mediated Neuronal Dysfunction and Degeneration in Limbic Encephalitis

Autoimmune inflammation of the limbic gray matter structures of the human brain has recently been identified as major cause of mesial temporal lobe epilepsy with interictal temporal epileptiform activity and slowing of the electroencephalogram, progressive memory disturbances, as well as a variety of other behavioral, emotional, and cognitive changes. Magnetic resonance imaging exhibits volume and signal changes of the amygdala and hippocampus, and specific anti-neuronal antibodies binding to either intracellular or plasma membrane neuronal antigens can be detected in serum and cerebrospinal fluid. While effects of plasma cell-derived antibodies on neuronal function and integrity are increasingly becoming characterized, potentially contributing effects of T cell-mediated immune mechanisms remain poorly understood. CD8⁺ T cells are known to directly interact with major histocompatibility complex class I-expressing neurons in an antigen-specific manner. Here, we summarize current knowledge on how such direct CD8⁺ T cell–neuron interactions may impact neuronal excitability, plasticity, and integrity on a single cell and network level and provide an overview on methods to further corroborate the in vivo relevance of these mechanisms mainly obtained from in vitro studies.

Fluorescent dyes as a reliable tool in P2X7 receptor-associated pore studies

Since the nineteenth century, a great amount of different biological structures and processes have been assessed by fluorescent dyes. Along with the uses of these compounds as vital and histological dyes, some fluorescent dyes have become valuable tools for the study of the pore phenomenon in plasma membranes. Some ion channels capable of forming large conductance channels, such as P2X7, TRPV1, VDAC-1 and the maxi-anion channels transiently alter the plasma membrane permeability, producing pores, which permit the passage of molecules of up to 1,000 Da. In this review, we discuss the uses of the fluorescent dyes chosen in diverse studies of this topic up to now.

Cytotoxic CD8+ T cell-neuron interactions: perforin-dependent electrical silencing precedes but is not causally linked to neuronal cell death

Cytotoxic CD8(+) T cells are considered important effector cells contributing to neuronal damage in inflammatory and degenerative CNS disorders. Using time-lapse video microscopy and two-photon imaging in combination with whole-cell patch-clamp recordings, we here show that major histocompatibility class I (MHC I)-restricted neuronal antigen presentation and T cell receptor specificity determine CD8(+) T-cell locomotion and neuronal damage in culture and hippocampal brain slices. Two separate functional consequences result from a direct cell-cell contact between antigen-presenting neurons and antigen-specific CD8(+) T cells. (1) An immediate impairment of electrical signaling in single neurons and neuronal networks occurs as a result of massive shunting of the membrane capacitance after insertion of channel-forming perforin (and probably activation of other transmembrane conductances), which is paralleled by an increase of intracellular Ca(2+) levels (within <10 min). (2) Antigen-dependent neuronal apoptosis may occur independently of perforin and members of the granzyme B cluster (within approximately 1 h), suggesting that extracellular effects can substitute for intracellular delivery of granzymes by perforin. Thus, electrical silencing is an immediate consequence of MHC I-restricted interaction of CD8(+) T cells with neurons. This mechanism is clearly perforin-dependent and precedes, but is not causally linked, to neuronal cell death.

CD8+ T cells and neuronal damage: direct and collateral mechanisms of cytotoxicity and impaired electrical excitability

Cytotoxic CD8(+) T cells are increasingly recognized as key players in various inflammatory and degenerative central nervous system (CNS) disorders. CD8(+) T cells are believed to actively contribute to neural damage in these CNS conditions. Conceptually, one can separate two possible ways that CD8(+) T cells harm neuronal function or integrity: CD8(+) T cells either directly target neurons and their neurites in an antigen- or contact-dependent fashion, or exert their action via "collateral" mechanisms of neuronal damage that might follow destruction of the myelin sheath or glial cells in both the CNS gray and white matter. After introducing clinical examples, in which the putative relevance CD8(+) T cells has been demonstrated, we summarize knowledge on the sequence of initiation and execution of CD8(+) T-cell responses in the CNS. This includes the initial antigen cross-presentation and priming of naive CD8(+) T cells, followed by the invasion, migration, and target-cell recognition of CD8(+) effector T cells in the CNS parenchyma. Moreover, we discuss mechanisms of impaired electrical signaling and cell death of neurons as direct and collateral targets of CD8(+) T cells in the CNS.

Perforin-mediated target-cell death and immune homeostasis

The granule exocytosis pathway of cytotoxic lymphocytes is crucial for immune surveillance and homeostasis. The trafficking of granule components, including the membrane-disruptive protein perforin, to the immunological synapse leads to the delivery of granule proteases (granzymes) into the target cell and its destruction through apoptosis. Several independent molecular abnormalities associated with defects of either granule trafficking or perforin function can cause cytotoxic lymphocyte dysfunction. In humans, inherited perforin mutations result in severe immune dysregulation that manifests as familial haemophagocytic lymphohistiocytosis. This Review describes recent progress in defining the structure, function, biochemistry and cell biology of perforin.

Cytotoxic T Lymphocytes from Cathepsin B-deficient Mice Survive Normally in Vitro and in Vivo after Encountering and Killing Target Cells

The lysosomal protease cathepsin B has been proposed to protect cytotoxic T lymphocytes from the membrane-disruptive effects of perforin secreted during the execution phase of target cell death. Accordingly, cathepsin B that translocates to the lymphocyte surface upon degranulation has been postulated to cleave and inactivate perforin molecules that diffuse back to the killer cell. We have found that recombinant perforin is cleaved inefficiently by cathepsin B and shows no significant reduction in its lytic activity following co-incubation. Furthermore, purified CD8+ cytotoxic T lymphocytes of cathepsin B-null gene-targeted mice were able to induce normal death of target cells both in vitro and in vivo and to survive the encounter with target cells as efficiently as cathepsin B-expressing killer cells. We conclude that cathepsin B is not essential for protection of cytotoxic lymphocytes from the toxic effects of their secreted perforin.

Peripheral blood cytotoxic lymphocyte gene transcript levels differ in patients with long-term type 1 diabetes compared to normal controls

The purpose of this study was to compare mRNA levels of the cytotoxic lymphocyte (CL) gene products: granzyme B (GB), perforin (P), and fas ligand (FasL) in patients with long-term type 1 diabetes and healthy controls. The objective was to utilize this information to follow patients as they undergo islet cell transplantation at our center and to determine if changes in CL gene transcript levels correlate with graft status. We have measured mRNA levels for CL genes in peripheral blood samples from 65 long-term (>5 years) type 1 diabetes patients and 29 healthy controls. Total RNA was extracted from EDTA anticoagulated peripheral blood samples and reverse transcribed into first-strand cDNA using SuperScript II reverse Transcriptase. Quantitative, real-time PCR was utilized to determine CL gene transcript levels. mRNA levels of P and FasL genes were found to be significantly lower for patients with type 1 diabetes compared to normal controls (p < 0.05). However, there was no significant difference for GB mRNA levels between patients and controls (p > 0.05). The decreased expression of P and FasL in patients with long-term type 1 diabetes might contribute to the inability to maintain normal levels of peripheral tolerance, which is essential for protection from autoimmune disease.

Gene Expression of Perforin by Peripheral Blood Lymphocytes as a Marker of Acute Rejection

BACKGROUND

Previous findings have demonstrated that the expression of cytotoxic effector molecules is increased in acute rejection of renal allografts. In the present study, we serially examined the gene expression of perforin, granzyme B and Fas ligand (FasL) in peripheral blood lymphocytes (PBLs) of renal allograft recipients to assess the potential of their expression as a marker of acute rejection.

METHODS

PBLs were isolated from blood samples taken on days 2, 4, 6, 8, 10 and 12 after transplantation. Competitive PCR was performed to evaluate the abundance of mRNA of perforin, granzyme B and FasL. The mean value + 2 SD of each molecule in the control group was set as a discriminatory level for that particular molecule.

RESULTS

When all measured samples were compared, perforin expression was significantly higher in patients with acute rejection than in the control group (1.84 +/- 3.01 vs. 0.71 +/- 0.48, p = 0.01). The percentage of perforin expression exceeding the discriminatory level was also significantly higher in patients with acute rejection (p = 0.0003). Five patients in the rejection group (5/7, 71.4%) showed perforin expression exceeding the discriminatory level, while only 1 patient in the control group did so (1/8, 12.5%) (p = 0.02). Perforin expressions of days 0 and 1 of rejection crisis were the highest over the study period. No consistent pattern of granzyme B and FasL expression was identified in relation to rejection crisis.

CONCLUSION

Gene expression of perforin by PBLs was upregulated in accordance with acute rejection, thus offering the possibility that it may be utilized as a marker of acute rejection.

Prevalence and clinical significance of resistance to perforin- and FAS-mediated cell death in leukemia

Killer lymphocytes play a central therapeutic role in graft-versus-leukemia following allogeneic hematopoietic stem cell transplantation (HSCT). The Perforin/Granzyme and FAS/CD95 pathways are of crucial importance in tumor cell elimination by killer cells. In this study, we have examined whether hematological malignancies are resistant to perforin and anti-FAS antibodies. Leukemic cells were studied from 29 patients suffering either from acute or chronic myeloid leukemia (AML or CML), acute or chronic lymphoid leukemia, or non-Hodgkin's lymphoma. An average of 49 vs 5% of specific cell killing was found when using perforin vs anti-FAS antibodies, respectively. Interestingly, resistance towards both perforin and anti-FAS antibodies was found exclusively in leukemic cells from patients with myeloid leukemia. Analysis of leukemic cells from patients with CML, suffering from leukemia relapse after HSCT and given donor lymphocyte infusion (DLI) to induce remission, indicated that the effectiveness of treatment with DLI was not associated with sensitivity of leukemic cells to perforin. In conclusion, resistance towards anti-FAS antibodies is a common phenomenon in leukemia/lymphoma, whereas perforin resistance occurs only in myeloid leukemia. However, as a single parameter, perforin resistance does not appear to be suitable to predict the outcome of DLI.

Assessment of cytotoxic lymphocyte gene expression in the peripheral blood of human islet allograft recipients: elevation precedes clinical evidence of rejection

Studies in nonhuman primates have demonstrated that elevation of the cytotoxic lymphocyte (CL) genes granzyme B, perforin, and Fas ligand in peripheral blood precedes islet allograft rejection. The purpose of this study was to determine whether this approach has utility for prediction of human islet allograft loss. We studied 13 patients who had long-term type 1 diabetes and were treated with steroid-free immunosuppression and given sequential islet cell infusions. All recipients became insulin independent, and eight of them experienced deterioration in glycemic control, followed by reinitiation of insulin therapy. Frequent peripheral blood samples were collected to monitor CL gene mRNA levels with real-time PCR. For the eight back-to-insulin patients, there was a clear elevation of CL gene mRNA levels 25-203 days before the onset of frequent hyperglycemia. Granzyme B was the most reliable indicator of ongoing graft loss. Additional correlations with infection were noted; however, evidence of sensitization in antidonor mixed lymphocyte reaction was observed in seven of eight patients who experienced partial graft loss, whereas this was not seen when upregulated CL gene expression was associated with infection. The results suggest that, when taken into consideration with other clinical parameters, elevated CL gene levels may enable prediction of islet allograft loss.

Quantitative fluorescence measures for determination of intracellular perforin content

We present methodologic details and operating characteristics of a procedure with whole blood for the quantitative assessment of intracellular perforin within distinct lymphocyte subsets. Using this method, we analyzed 20 healthy controls and 2 individuals with an inherited deficiency of perforin. The mean +/- standard deviation perforin contents of natural killer (NK) cells and cytotoxic T cells of healthy controls were 3561 +/- 1157 and 500 +/- 779 relative number of molecules (rMol) of antiperforin antibody bound per cell, respectively. The NK cell perforin contents of individuals with heterozygous and homozygous perforin deficiency (familial hemophagocytic lymphohistiocytosis) were 2260 and 212 rMol of antiperforin antibodies per NK cell. While the homozygous deficiency was found to be associated with negligible antiperforin binding, the heterozygous condition was associated with a level of perforin binding that was below the 15th percentile for healthy individuals. Because 83% of this subject's NK cells were shown to bind to antiperforin antibodies by conventional flow cytometry (relative to the normal range of 81% +/- 25%), quantitative cytometry may be more sensitive than conventional cytometric methods in identifying cytolytic defects.

Cytotoxic lymphocytes and cardiac electrophysiology

It is widely recognized that immune effector mechanisms contribute to cardiac dysfunction in major cardiac pathologies, such as myocarditis and the consequent dilated cardiomyopathy, Chagas' disease and heart transplant rejection. Of the wealth of immune mechanisms known to affect cardiac function, this review will deal with the adverse effects caused by cytotoxic T lymphocytes (CTL, CD4(+) and CD8(+) T lymphocytes), which participate in a broad range of heart pathologies. The interaction between cytotoxic lymphocytes and their target cells can set off two different effector mechanisms: (1) The perforin/granzymes, and (2) The Fas/FasL. In this review, I will discuss these mechanisms, and present experimental evidence showing that both can adversely affect cardiac myocytes in vitro, in a way that can contribute to a decline in the overall cardiac function.

Elevation of cytotoxic lymphocyte gene expression is predictive of islet allograft rejection in nonhuman primates

Hyperglycemia and increased insulin requirements are indicators of ongoing islet allograft rejection, but there are no methods to predict or confirm rejection. Elevation of cytotoxic lymphocyte (CL) gene expression in peripheral blood (PB) has been correlated with renal allograft rejection in humans, but no published study has assessed the utility of monitoring these markers as predictors of rejection before the onset of clinical symptoms. We have established quantitative real-time PCR methods to determine the levels of mRNA transcripts for the CL genes granzyme B (GB), perforin, and fas ligand in blood samples from rhesus and cynomolgus monkeys. Four rhesus monkeys with long-term islet allograft function were studied. Antirejection (anti-CD154) therapy was discontinued, and weekly PB samples were obtained to determine whether the levels of mRNA transcripts for CL genes correlated with and/or were predictive of islet allograft rejection, defined as a loss of C-peptide production. For all monkeys, elevation of CL gene expression preceded rejection by 83--197 days, with GB as the best predictor. Elevated mRNA levels were sustained for 2--2.5 months in three of four animals and 1 month in the other, thus suggesting that the testing of these parameters may have practical applications in clinical islet cell transplantation.

Pharmacologic modulation of the immune interaction between cytotoxic lymphocytes and ventricular myocytes

Numerous studies have demonstrated that immune effector mechanisms cause serious heart diseases, among which are heart transplant rejection, myocarditis, and the resulting dilated cardiomyopathy, as well as Chagas' disease. Whereas different effectors of the immune system can affect cardiac function, this review primarily focuses on the immune damage caused by cytotoxic T lymphocytes. The immune attack staged by cytotoxic T lymphocytes is carried out by one of two distinct modes of lymphocytotoxicity: (a) secretion of lytic granules containing the pore-forming protein perforin and a family of serine proteases (i.e., granzymes) and (b) interaction between the lymphocyte Fas ligand and the target cell Fas receptor. Ventricular myocytes challenged by the immune system sustain diverse intracellular changes, among which the rise in intracellular calcium ([Ca2+]i) constitutes an important contributor to myocyte dysfunction. Hence, this [Ca2+]i rise, which does not necessarily result in apoptosis, can affect cardiac function directly and indirectly. Importantly, the final outcomes of these perturbations vary markedly and depend on intracellular circumstances such as the magnitude of the absolute rise in [Ca2+]i and its temporal and spatial determinants, the metabolic status of the myocyte, as well as a fine balance between pro-apoptotic and anti-apoptotic factors. In view of the central role of [Ca2+]i rise in immune-mediated myocyte dysfunction and possibly cell death, this review addresses three topics related to the immune assault on the heart: (a) [Ca2+]i rise in affected myocytes; (b) the source for the [Ca2+]i rise; and (c) pharmacologic modification of the immune-mediated [Ca2+]i rise.

Cytotoxic lymphocyte gene expression in peripheral blood leukocytes correlates with rejecting renal allografts

BACKGROUND

We have shown previously that heightened expression of the cytotoxic lymphocyte (CL) effector genes perforin (P), granzyme B (GB), and Fas ligand (FasL), is closely correlated with acute allograft rejection, particularly when two or more target genes are up-regulated.

METHODS

We used quantitative reverse transcription-polymerase chain reaction to analyze CL gene expression from peripheral blood leukocytes (PBLs) and renal allograft biopsies in 31 paired samples of PBLs and renal tissue from 25 renal allograft recipients. Our aims were (1) to determine whether the expression of CL gene expression in PBLs correlates with expression of these genes in renal allograft biopsy tissue and (2) to determine whether CL gene expression in PBLs correlates with the histological diagnosis.

RESULTS

Coordinate gene expression in PBLs and acutely rejecting allografts was found in 9/11 (82%) for P, 07/11 (64%) for GB, and 10/11 (91%) for FasL. Coordinate absence was found in 15/20 (75%) for P, 17/20 (85%) for GB, and 16/20 (80%) for FasL in nonrejecting allografts. Furthermore, up-regulation of any two genes in PBLs correlated with pathological diagnosis of rejection with excellent positive (100%) and negative (95%) predictive values.

CONCLUSION

Coordinate CL gene expression in PBLs and the allograft is usually detected. CL gene expression in PBLs is closely associated with a pathologic diagnosis of rejection. CL gene expression in PBLs may serve as a noninvasive method of monitoring for renal allograft rejection.

Characterization of Actinobacillus actinomycetemcomitans leukotoxin pore formation in HL60 cells

The mechanism of cell death induced by Actinobacillus actinomycetemcomitans leukotoxin (LTX) has been investigated with flow cytometry and patch electrode recording using cultured HL60 cells. The kinetics of propidium iodide (PI) positive staining of HL60 cells was measured as a function of LTX concentration at 37 degreesC. Results showed a concentration-dependent decrease in the tk times. Cell kill was slow at <1 microg/ml LTX concentrations with fewer than 50% of the cells killed after 1 h; at 1 microg/ml, the tk times ranged from approximately 15 to 30 min. At higher concentrations, the tk times decreased rapidly. The rate of cell kill was appreciably slowed at 20 degreesC. HL60 whole cell currents were recorded with patch electrodes. Immediately following exposure to high concentrations of LTX, large currents were recorded suggesting that the membrane potential of these cells had collapsed due to the large conductance increases. At low toxin concentrations, rapid conductance fluctuations were seen suggestive of a limited number of toxin-mediated events. Cells exposed to low concentrations of LTX exhibited these conductance fluctuations for up to 1 h, whereas toxin-insensitive cells were unaffected by long exposures to high concentrations of toxin. Our results are consistent with LTX-induced pores in susceptible cells which overwhelm the ability of the cell to maintain osmotic homeostasis causing cell death.

Lipids from Mycobacterium leprae cell wall suppress T-cell activation in vivo and in vitro

The influence of Mycobacterium leprae cell wall lipids on lymphocyte functions has been investigated in vivo (delayed-type hypersensitivity) and in vitro. The inflammatory response has been earlier evaluated by the mouse footpad oedema model and the delipidated mycobacteria evoked a mild but significant inflammatory response. Herein a higher level of hypersensitivity reaction was observed with delipidated bacilli than with the intact mycobacteria. The lipids obtained from the extract of M. leprae external cell wall were used to prepare liposomes, which have not been shown to be toxic to lymphocytes. The method of lipidic extraction and the sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the lipid fraction did not reveal any trace of proteins. Thin-layer chromatography of this extract detected four different bands with an apolar character, suggestive of mycolic and fatty acids. These same M. leprae liposomes potently suppressed lymph node cells, as well CD4+ and CD8+ T-cell lines, and an antigen-specific T-cell clone (T 4-9) proliferation, even under potent stimulus. Cholesterol-choline liposomes, unrelated to M. leprae liposomes, used as a control in the biological assays showed no significant effect on lymphoblastic activity, which points to the specificity of M. leprae lipids. These data demonstrated that M. leprae cell wall lipids induce immune suppression in mice without causing any membrane alteration in T cells as assessed throughout kinetic studies in vitro. This fact is closely related to the down-regulating effect induced by M. leprae lipids which we have previously observed in macrophage functions in vivo and in vitro. Although this lipidic fraction showed a suppressive action on T lymphocytes in vitro (proliferation) and in vivo (delayed-type hypersensitivity), its possible significance in the establishment of a specific immune response to M. leprae must be further investigated.

P2Z purinoceptor-associated pores induced by extracellular ATP in macrophages and J774 cells

Millimolar concentrations of extracellular ATP (ATPo) can induce the permeabilization of plasma membranes of macrophages and other bone marrow-derived cells to low-molecular-weight solutes, a phenomenon that is the hallmark of P2Z purinoceptors. However, patch-clamp and whole cell electrophysiological experiments have so far failed to demonstrate the existence of any ATPo-induced P2Z-associated pores underlying this permeabilization phenomenon. Here, we describe ATPo-induced pores of 409 +/- 33 pS recorded using cell-attached patch-clamp experiments performed in macrophages and J774 cells. These pores are voltage dependent and display several properties of the P2Z-associated permeabilization phenomenon: they are permeable to both large cations and anions, such as tris(hydroxymethyl)aminomethane, N-methyl-D-glucamine, and glutamate; their opening is favored at temperatures higher than 30 degrees C; they are blocked by oxidized ATP and Mg2+; and they can be triggered by 3'-O-(4-benzoylbenzoyl)-ATP but not by UTP or ADP. We conclude that the pores described in this report are associated with the P2Z permeabilization phenomenon.

Channel formation and [Ca2+]i accumulation induced by perforin N-terminus peptides: comparison with purified perforin and whole lytic granules

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells express the pore forming protein perforin, which contributes to lymphocytotoxicity. The hallmark of perforin action is opening high-conductance transmembrane channels that enable massive influx of Ca2+ ions (deleterious to many cell types), as well as granzymes, which may trigger the apoptotic pathway. To explore the functional domains in the perforin molecule, we investigated in PN71 lymphocytes, the ability of perforin N-terminus synthetic peptides (compared to purified perforin and perforin-containing lytic granules), to cause intracellular Ca2+ ([Ca2+]i) accumulation and open transmembrane channels. To this end, we used the whole cell recording technique and Indo 1 fluorescence to measure membrane currents and [Ca2+]i, respectively. We have demonstrated that the N-terminus peptide Hu-34 (amino acids 1-34) closely resembled perforin action, reflected by [Ca2+]i accumulation and channel activity, while shorter peptides (e.g., Hu-16) generated mostly short-lived channels but no [Ca2+]i elevation. Hence, the first 34 amino acids of the perforin N-terminus sequence are sufficient for the perforin action.

Dependence of the lytic activity of the N-terminal domain of human perforin on membrane lipid composition--implications for T-cell self-preservation

The kinetics and thermodynamics of pore formation by the 22-residue N-terminal domain of human perforin-(1-22)-peptide have been studied for a variety of model phospholipid membranes. Peptide binding and aggregation, and cell lysis were monitored through the change in the fluorescence of Trp, or vesicle-encapsulated carboxyfluorescein, respectively. Peptide binding was analyzed in terms of a model that incorporates non-ideal interactions and aggregation in a membrane bilayer. The minimum number of peptide monomers required to form an active pore averaged from four to six, according to the lipid composition of the vesicle. This combined kinetic and thermodynamic approach has provided quantitative information that allows a direct comparison of the binding behavior of the perforin-(1-22)-peptide in different lipid vesicles and affords molecular insight on the factors controlling pore formation. Pore formation is most favorable in thinner membranes with low melting temperatures. No significant difference in activity is observed for different zwitterionic headgroups. Rather, the gel state of the lipid chain, which diminishes the incorporation and aggregation of the perforin-(1-22)-peptide shows the strongest influence. This effect is observed in both the thermodynamic (incorporation isotherm) and kinetic (carboxyfluorescein release) studies. Insertion and aggregation are more facile in membranes with less densely packed lipids. The dependence of pore-forming activity on lipid composition provides important clues to understanding the self-protection mechanism employed by cytotoxic T lymphocytes (CTL) against perforin-mediated lysis.

Trypanosoma cruzi: resistance to the pore forming protein of cytotoxic lymphocytes--perforin

The pore-forming protein perforin is one of the main effector molecules which cytotoxic lymphocytes utilize to kill their targets both in vivo and in vitro. Natural killer cells and cytotoxic T lymphocytes play an important role in host defense against a number of intracellular microorganisms such as virus and protozoan, but the exact way they help control infection is unknown. On the other hand, many microorganisms have evolved successful escape strategies to avoid immune-cell-mediated attack. It is thus necessary to investigate the direct interaction of infectious microorganisms with the lytic machinery of cytotoxic lymphocytes and other cells. In the present work we report the effect of perforin on both a protozoan, Trypanosoma cruzi, and the infected host cell. Epimastigote, amastigote, and trypomastigote forms of T. cruzi, as well as infected macrophages, were assayed for their susceptibility to perforin based on three different criteria. T. cruzi in all three differentiation stages were resistant to purified perforin at doses up to 100-fold larger than that sufficient to kill susceptible tumor cells. No morphological change was observed under electron microscopy. Survival rates and infectivities of the treated parasites in vitro were similar to those of control parasites. Moreover, the measurement of calcium influx using Fura-2 to assess membrane damage revealed that T. cruzi resist perforin attack by avoiding transmembrane pore formation. Resistance to perforin was not transferred to host cells since infected macrophages could be easily destroyed by perforin while intracellular amastigotes remained intact.

Perforin and lymphocyte-mediated cytolysis

We have discussed in the previous sections the recent progress made toward elucidating the regulatory mechanism of perforin gene transcription and the domain structure of the perforin molecule. It appears that the expression of perforin is, at least partially, controlled at the transcription level through the interaction between killer cell-specific cis- and trans- acting factors. One of such cognate pairs, NF-P motif (an EBS-homologous motif) and NF-P2 (a killer cell-specific DNA-binding protein), has been described. The regulatory mechanism of gene transcription, however, is likely to involve multiple factors which act in a coordinated fashion to bring about the most efficient expression of perforin limited strictly to activated killer lymphocytes. Through studies using synthetic peptides and recombinant perforins, it has been suggested that the N-terminal region of the perforin molecule is an important, though not the only, domain responsible for the lytic activity. Further studies are warranted to elucidate the role(s) of other potential amphiphilic structures located in the central portion of the perforin molecule in the overall pore-forming activity. The molecular basis underlying the resistance of killer lymphocytes to perforin-mediated lysis still remains an open question. Preliminary results, however, suggest that the surface protein(s) restricted to killer cells may account for their self-protection against perforin. Based on recent studies using perforin-deficient mice, the involvement of perforin in lymphocyte-mediated cytolysis both in vivo and in vitro has been confirmed. Two functional roles, a direct (lytic) and an indirect (endocytosis enhancer; conduit), both of which may contribute critically to the cell-killing event can be attributed to perforin. The fact that lymphocytes may also employ perforin-independent killing mechanism(s), e.g. Fas-dependent pathway, is beyond the scope of this review. There is, nevertheless, no doubt that these alternative cytolytic mechanisms may also play important roles in immune effector and/or regulatory responses associated with killer lymphocytes. Obviously, we are still a long way from concluding on the functional relevance of each individual cytolytic mechanism seen in different physiopathological situations. Suffice it to say, however, that a wealth of information on lymphocyte-mediated killing has already emerged through the multidisciplinary efforts conducted in our and other laboratories that promise to further dissect this complicated event in the years to come.

Mechanisms whereby cytotoxic T lymphocytes damage guinea-pig ventricular myocytes in vitro

We studied possible mechanisms whereby cytotoxic T lymphocytes (CTL) damage the myocardium during the immunological rejection of the transplanted heart, by investigating the in vitro interaction between CTL and cardiac myocytes. We utilized the patch-clamp technique to record membrane currents and action potentials from concanavalin A-treated guinea-pig ventricular myocytes conjugated to mouse peritoneal exudate CTL (PEL). PEL-myocyte interaction reduced action potential duration at 50% repolarization (APD50) from 731.7 +/- 57.8 to 195.3 +/- 58.0 ms, action potential amplitude from 134.9 +/- 1.9 to 104.2 +/- 6.2 mV and resting membrane potential (Vm) from -80.9 +/- 0.5 to 72.5 +/- 1.5 mV. These changes were accompanied by generation of delayed afterdepolarizations, indicative of intracellular [Ca2+] overload. The electrophysiological alterations were associated with myocyte shortening (within 28.9 +/- 2.8 min) followed by complete cell destruction (within 43.5 +/- 4.3 min). To determine whether intracellular Ca2+ stores were involved in PEL-induced myocyte damage, the protective effects of ryanodine and caffeine were investigated. While ryanodine (10 microM) delayed the electrophysiological and morphological alterations, caffeine (5 mM) provided significant protection, suggesting that Ca2+ release from intracellular stores contributes to PEL-induced damage to the myocytes. Based on our findings, we suggest that the functional derangements seen in myocyte-lymphocyte conjugates can contribute to the overall decline in cardiac function during heart transplant rejection.

Mechanisms whereby lytic granules from cytotoxic T lymphocytes damage guinea pig ventricular myocytes

During immunological rejection of the transplanted heart, cytotoxic T lymphocytes (CTL) infiltrate the myocardium and by damaging the myocytes contribute to loss of function. To address one important aspect of heart transplant rejection, we investigated in guinea pig ventricular myocytes how CTL-derived lytic granules containing the pore-forming protein perforin reduce the membrane potential (VM) and cause cell damage. The reduction in VM was biphasic; within 8.4 +/- 1.9 min VM was reduced from a control value of -78.4 +/- 1.9 mV to -69.9 +/- 3.5 mV. Subsequently, within 6.7 +/- 2.1 min VM declined to -3.4 +/- 1.2 mV, associated with a progressive contracture. Under whole cell voltage clamp, in myocytes held at their resting potential (VM = -76.2 +/- 0.9 mV), granules induced discrete inward current steps (resembling 'single channel' activity), with a mean amplitude of -86.8 +/- 1.4 pA and open times lasting from seconds up to several minutes. The mean conductance and reversal potential calculated from the linear regression analysis of the I-V relations were 1390 pS and -6.8 mV, respectively. The probable non-selectivity of these 'channels' and the resultant loss of membrane K+ selectivity can account for the reduction in VM. At the same time, opening of these pores leads to Ca2+ overload, resulting in contracture and cell damage.

Cytoplasts from cytotoxic T lymphocytes are resistant to perforin-mediated lysis

Cytotoxic T lymphocytes (CTL) contain a potent cytolytic pore-forming protein (PFP, perforin or cytolysin) localized in their cytoplasmic granules. In the presence of calcium, perforin lyses a variety of target cells (TC) non-specifically. CTL, however, are generally resistant to the lytic effect of perforin. In this work, cytoplasts from CTL and susceptible TC were made by centrifuging cells on a Ficoll density gradient in the presence of cytochalasin B. Characterization by electron microscopy and a serine esterase assay established that both CTL and TC cytoplasts were completely devoid of nuclei and CTL cytoplasts contained essentially no granules. CTL cytoplasts are just as resistant to perforin-mediated lysis as the intact CTL, and both TC and their corresponding cytoplasts are very sensitive to lysis. Furthermore, CTL cytoplasts are less effective than TC cytoplasts in inhibiting hemolysis, a property shared by the respective intact cells. These results indicate that soluble granular components do not confer resistance on CTL, and suggest that the protective agent(s) acts by impeding perforin binding to the CTL membrane.

Plasma membrane-associated proteins with the ability to partially inhibit perforin-mediated lysis

Cytolytic lymphocytes have previously been reported to be resistant to the lytic effects of perforin. In this work, plasma membrane proteins from a CTL cell line were fractionated by HPLC, and the eluted fractions were collected based on their ability to inhibit perforin-mediated hemolysis. Three proteins with inhibitory activity were thus purified, the serine esterase MCSP-3/granzyme F and the histones H2B and H3. A commercial source of H2B was able to potently inhibit perforin-mediated lysis, and it was confirmed by FACS analysis that H2B is in fact present on the surface of cytolytic cells. However, H2B was also found on the surface of perforin-susceptible tumor cell lines, indicating that the histones may partially inhibit perforin-mediated lysis in vitro, but that they do not represent the factor conferring specific resistance on cytolytic lymphocytes. The origin of the surface histones and the possible role of the surface MCSP-3/granzyme F are discussed.

Cell-mediated cytotoxicity: ATP as an effector and the role of target cells

Cell-mediated cytotoxicity involves a number of distinct mechanisms as well as the active participation of the target cell. Recently, several investigators have demonstrated that extracellular ATP can act as a cytotoxic effector.