P2X7 modulatory web in Trypanosoma cruzi infection.

P2X7 is a member of the purinergic receptors family, with extracellular adenosine triphosphate (ATP) as the main agonist, promoting cations influx and membrane permeabilization that can lead to cell death. We previously proposed that extracellular ATP is involved in thymus atrophy induced by Trypanosoma cruzi infection through the induction of CD4+/CD8+ double-positive cell death and that P2X7 could be involved in this process. To further elucidate this possibility raised by in vitro assays, in this study, we used P2X7-/- mice and observed no difference in thymus atrophy or parasitemia when compared to C57Bl/6. We then decided to investigate other aspects of purinergic receptor interplay that could be better evidenced by the infection and observed that (1) thymocytes from infected and noninfected C57Bl/6 mice express P2X4 and P2X7 receptors (Western blotting), but ATP-induced membrane permeabilization only occurs in thymocytes from infected mice; (2) peritoneal macrophages from noninfected C57Bl/6 mice (P2X4+ and P2X7+) are permeabilized by ATP. Although macrophages from infected C57Bl/6 mice are P2X7- but P2X4+, they are resistant to ATP, either through permeabilization or Ca++ influx (fluorimetry); (3) using noninfected P2X7-/- mice, C57Bl/6 infected mice, and different agonistic stimuli, we observed interesting cross-talks among P2X and P2Y receptors (flow cytometry).

The role of purinergic P2X7 receptors in the inflammation and fibrosis of unilateral ureteral obstruction in mice.

Receptors of the P2X7 type have been demonstrated in granulocytes, monocytes/macrophages, B and T lymphocytes, and have been involved in several cellular mechanisms including those related to inflammation and immunological response. This study attempted to investigate the role of these receptors on the inflammatory and fibrogenic response in the kidneys of unilateral ureteral obstruction (UUO), by using P2X7 knockout mice (-/-). C57Bl6 mice were submitted to left UUO and killed after 7 and 14 days. Histopathology using hematoxylin-eosin, periodic-acid Schiff and Sirius-red staining, immunohistochemistry for macrophages, myofibroblasts, transforming growth factor-beta (TGF-beta)1 and P2X7, and immunofluorescence for apoptotic cells (terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate nick-end labeling) were performed. Protocols were as follows: (1) control; (2) sham; (3) control P2X7 (-/-); (4) sham P2X7 (-/-); (5) UUO wild type (WT); (6) UUO P2X7 (-/-). Myofibroblasts and Sirius-red staining were significantly lower in UUO P2X7 (-/-) mice at days 7 and 14, compared to UUO WT. Kidneys from UUO P2X7 (-/-) mice showed reduced number of inflammatory cells at day 14 but not at day 7, compared to UUO WT. TGF-beta1 was less in UUO P2X7 (-/-) mice at days 7 and 14 when compared to UUO WT. Macrophage infiltration and tubular apoptosis were lower in UUO P2X7 (-/-) at day 14 but not at day 7, compared to UUO WT. P2X7 was expressed only in tubular epithelial cells at day 7 of UUO WT mice. These findings constitute the first evidence that P2X7 receptors are implicated in macrophage infiltration, collagen deposition and apoptosis in response to ureteral obstruction in mice.

Characterization of P2 receptors in thymic epithelial cells.

The presence of P2 receptors was investigated in three distinct preparations of murine thymic epithelial cells (TEC): 2BH4 murine cell line, IT45-R1 rat cell line, and a primary murine cell derived from the Nurse cell lympho-epithelial complex. In all preparations, application of ATP to the extracellular milieu triggered intracellular calcium signals indicating the presence of P2 receptor(s) in these cells. After an initial peak of calcium concentration, a plateau phase that could last more than 10 min was frequently observed. Ion replacement and channel blockage experiments indicated that the initial peak was associated with the release of calcium from intracellular stores, while the plateau phase was associated with an influx from the extracellular medium. ATP and UTP induced similar calcium signals, suggesting the presence of P2Y2 receptors in all three cell types. The murine 2BH4 cells also expressed P2X7/P2Z receptor, since under exposure to millimolar concentrations of ATP, a continuous rise in intracellular calcium concentration was observed and their plasma membranes became permeabilized to the fluorescent dyes Lucifer yellow and ethidium bromide. In addition, this permeabilization phenomenon was blocked by the P2Z-specific antagonist, oxidized ATP. RT-PCR assays confirmed the presence of mRNAs for the P2Y2 molecule in all TEC, while mRNA for the P2X7 molecule was detected only in 2BH4 cells. Our data indicate that P2Y2 purinergic receptors are widely expressed by thymic epithelial cells, whereas the expression of the P2X7 receptor appears to be more restricted, raising the possibility that its expression is related only to a particular epithelial microenvironment within/the thymus.

Modulation of P2Z/P2X(7) receptor activity in macrophages infected with Chlamydia psittaci.

Given the role that extracellular ATP (ATP(o))-mediated apoptosis may play in inflammatory responses and in controlling mycobacterial growth in macrophages, we investigated whether ATP(o) has any effect on the viability of chlamydiae in macrophages and, conversely, whether the infection has any effect on susceptibility to ATP(o)-induced killing via P2Z/P2X(7) purinergic receptors. Apoptosis of J774 macrophages could be selectively triggered by ATP(o), because other purine/pyrimidine nucleotides were ineffective, and it was inhibited by oxidized ATP, which irreversibly inhibits P2Z/P2X(7) purinergic receptors. Incubation with ATP(o) but not other extracellular nucleotides inhibits the growth of intracellular chlamydiae, consistent with previous observations on ATP(o) effects on growth of intracellular mycobacteria. However, chlamydial infection for 1 day also inhibits ATP(o)-mediated apoptosis, which may be a mechanism to partially protect infected cells against the immune response. Infection by Chlamydia appears to protect cells by decreasing the ability of ATP(o) to permeabilize macrophages to small molecules and by abrogating a sustained Ca(2+) influx previously associated with ATP(o)-induced apoptosis.

P2Z/P2X7 receptor-dependent apoptosis of dendritic cells.

Macrophages and thymocytes express P2Z/P2X7 nucleotide receptors that bind extracellular ATP. These receptors play a role in immune development and control of microbial infections, but their presence on dendritic cells has not been reported. We investigated whether extracellular ATP could trigger P2Z/P2X7 receptor-dependent apoptosis of dendritic cells. Apoptosis could be selectively triggered by tetrabasic ATP, since other purine/pyrimidine nucleotides were ineffective, and it was mimicked by the P2Z receptor agonist, benzoylbenzoyl ATP, and blocked by magnesium and the irreversible antagonist, oxidized ATP. RT-PCR analysis confirmed the mRNA expression of the P2Z/P2X7 receptor and the absence of P2X1. Caspase inhibitors and cycloheximide had only a partial effect on the apoptosis, suggesting that a caspase-independent mechanism may also be operative. Brief treatment with ATP led to an increase in the intracellular calcium concentration and permeabilization of the plasma membrane to Lucifer yellow, which diffused throughout the dendritic cell cytosol. Other small extracellular molecules may thus attain a similar intracellular distribution, perhaps activating endogenous proteases that contribute to initiation of apoptosis.

Extracellular ATP in the lymphohematopoietic system: P2Z purinoceptors off membrane permeabilization.

The effects of extracellular nucleosides and nucleotides on many organs and systems have been recognized for almost 50 years. The effects of extracellular ATP (ATPo), UTPo, ADPo, and other agonists are mediated by P2 purinoceptors. One of the most dramatic effects of ATPo is the permeabilization of plasma membranes to low molecular mass solutes of up to 900 Da. This effect is evident in several cells of the lymphohematopoietic system and is supposed to be mediated by P2Z, an ATP(4-)-activated purinoceptor. Here, we review some basic information concerning P2 purinoceptors and focus our attention on P2Z-associated phenomena displayed by macrophages. Using fluorescent dye uptake, measurement of free intracellular Ca2+ concentration and electrophysiological recordings, we elucidate some of the events that follow the application of ATP to the extracellular surface of macrophages. We propose a regulatory mechanism for the P2Z-associated permeabilization pore. The presence of P2 purinoceptors in cells of the lymphohematopoietic system makes them potential candidates to mediate immunoregulatory events.

Extracellular ATP in the lymphohematopoietic system: P2Z purinoceptors and membrane permeabilization

The effects of extracellular nucleosides and nucleotides on many organs and systems have been recognized for almost 50 years. The effects of extracellular ATP (ATP(o)), UTP (o), ADP(o), and other agonists are mediated by P2 purinoceptors. One of the most dramatic effects of ATP(o) is the permeeabilization of plasma membranes to low molecular mass solutes of up to 900 Da. This effect is evident in several cells of the lymphohematopoietic system and is supposed to be mediated by P2Z, and APT(4-) -activated purinoceptor. Here, we review some basic information concerning P2 purinoceptors and focus our attention on P2Z-associated phenomena displayed by macrophages. Using fluorescent dye uptake, measurement of free intracellular Ca2+ concentration and electrophysiological recordings, we elucidate some of the events that follow the application of ATP to the extracellular surface of macrophages. We propose a regulatory mechanism for the P2Z-associated permeabilization pore. The presence of P2 purinoceptors in cells of the lymphohematopoietic system makes them potential candidates to mediate immunoregulatory events.

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.

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.

Are there functional gap junctions or junctional hemichannels in macrophages?

The existence of functional gap junctions in migratory cells of the immune system is a controversial issue. In this report, we have focused on one particular cell type, namely the macrophages, because connexin-43, a protein that forms gap junctions, has been described in peritoneal macrophages and a macrophage cell line (J774), by Northern and Western blot analysis. To test whether these cell types expressed functional gap junctions, we assayed dye coupling by intracellular injection of Lucifer Yellow. We observed that nonstimulated macrophages are not coupled among themselves and did not form functional gap junctions with an epithelial cell line, which expresses functional gap junctions formed by connexin-43. Dye coupling was also not detected between macrophages previously activated by lipopolysaccharide or interferon-gamma. We further examined the presence of functional coupling using the more sensitive technique of dual whole cell patch-clamp, and again, did not find electrical coupling between macrophages, consistent with the dye microinjection data. We also examined the possible presence of hemigap junction channels activated by extracellular adenosine triphosphate (ATP) using a dye uptake assay and the whole cell patch-clamp technique. Conditions expected to close gap junction hemichannels (exposure to octanol and low intracellular pH) did not decrease ATP-induced Lucifer Yellow uptake, whereas conditions expected to increase hemichannel opening either did not affect ATP permeabilization (dibutyryl adenosine monophosphate) or decreased it (zero extracellular CA+2). Finally, in experiments using resident macrophages derived from conexin-43 knockout mice, we observed ATP induced dye uptake. Our experimental data thus indicate that macrophages in vitro do not form functional gap junctions and that the permeability pathway activated by extracellular ATP is not formed by a hemigap junction channel.

Expression and characterization of functionally active recombinant perforin produced in insect cells.

A key cytolytic mediator used by killer lymphocytes, perforin (also known as pore-forming protein or cytolysin), has been shown to be capable of undergoing polymerization to form pores in cell membranes and cause osmotic lysis of target cells. Although perforin has been purified from killer lymphocytes and the coding gene has been cloned and sequenced, information concerning the domain structure of the perforin molecule has remained scarce. To overcome the difficulty in obtaining sufficient amounts of perforin and to further assess the functional relevance of the N-terminal portion of the perforin molecule in its lytic activity, we have attempted in the present study to produce recombinant perforins. Three forms of recombinant mouse perforin, a full-length form and two N-terminal truncated forms, have been expressed in insect (Spodoptera frugiperda (Sf9)) cells using recombinant baculovirus. Biochemical and functional characterization showed the purified full-length recombinant perforin to be capable of lysing target cells, inducing Ca2+ influx into target cells, and forming structural pores in target membranes. Significant lytic activities were also detected for the two truncated recombinant perforins lacking, respectively, the N-terminal 21 amino acid residues and 121 amino acid residues. Time course study showed that the latter acted less efficiently than the former. These results suggest the N-terminal portion of the perforin molecule to be an important, but not the only, domain responsible for the lytic function of the perforin molecule.

Characterization of P2Z purinergic receptors on phagocytic cells of the thymic reticulum in culture.

The thymic microenvironment is under intrinsic and extrinsic control circuits by several elements including hormones, neuropeptides, lymphokines, innervation and cell contact. P2 purinergic receptors have been described in a number of cells including macrophages, thymocytes, and other cells of the immune-inflammatory system. Here, we use the whole-cell patch-clamp technique and dye permeabilization assays to investigate the presence of ionic channels and purinergic receptors in one microenvironmental thymic component, namely the phagocytic cell of the thymic reticulum. At holding potentials ranging from -30 to -60 mV, applications of extracellular ATP in the vicinity of the cell membrane induce a transient and fast-activating inward current followed in most cells by an outward current. The whole event lasts 5-20 s. The inward current has a reversal potential close to 0 mV and the outward current can be ascribed to a Ca2+ -dependent K+ conductance. Both currents are inhibited by Mg2+, suggesting that the phenomenon is mediated by ATP4-. ATP-gamma-S can also induce both inward and outward currents. Exposure of phagocytic cells of the thymic reticulum to 5 mM ATP for 10 min induced permeabilization to lucifer yellow but not to the larger dyes trypan blue and rhodamine-dextran, suggesting a molecular weight cut-off smaller than 900. These observations lead us to conclude that phagocytic cells of the thymic reticulum express P2Z purinergic receptors that can mobilize Ca2+, induce the opening of ionic channels and permeabilize the cell membrane.

A cation non-selective channel induced by extracellular ATP in macrophages and phagocytic cells of the thymic reticulum.

Extracellular ATP4- can bind to P2Z purinergic receptors including depolarization and cytoplasmic membrane permeabilization to small molecular weight solutes in macrophages, thymocytes, mast cells, phagocytic cells of the thymic reticulum and other cell types. An ATP(4-)-induced cation current has been described in whole-cell records of some of these cells but it is currently not clear whether these currents and the phenomenon of membrane permeabilization are a consequence of only one type of P2Z-associated channel/pore or two different phenomena triggered by one or more receptors. Here we use the outside-out patch-clamp technique to describe a single channel associated with this cation current in two murine phagocytic cells: intraperitoneal macrophages and phagocytic cells of the thymic reticulum. Multi channel currents could be readily observed in 77% of the outside-out patches of macrophages. Single channels of 7.8 pS could usually be resolved only in tail currents. Reversal potential measurements and ion replacement experiments indicated a lack of cation selectivity, similarly to what has already been described for the ATP(4-)-induced whole-cell inward current. No large-conductance channels that could explain the permeabilization to small molecular weight studies solutes was observed under our experimental conditions. A single channel of approx. 5 pS was also observed in phagocytic cells of the thymic reticulum under similar conditions. We conclude that the channel here described is the main carrier of cation current usually associated with the binding of ATP4- to P2Z receptors in whole-cell and outside-out patch-clamp experiments.

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.

ATP- and UTP-induced currents in macrophages and macrophage polykaryons.

We have investigated the currents induced by extracellular ATP (ATPo), extra-cellular UTP, and other related compounds in macrophages. At potentials of -20 to -60 mV, a typical response to ATPo puffs consists of a fast-activating inward current followed by a transient outward current. The phenomenon lasts 5-20 s, but for sustained exposure to ATP the inward current persists for up to 10 min (our longest recording time). Both currents are inhibited by Mg2+, suggesting that the phenomenon is mediated by ATP4-. The outward current can be ascribed to a Ca(2+)-dependent K+ conductance, and release of Ca2+ from intracellular stores is at least in part responsible for this current. The inward current has a reversal potential of approximately 0 mV, and it is nonspecific for monovalent cations. UTP, a nucleotide that induces an increase in the cytoplasmic concentration of free Ca2+ but does not permeabilize macrophages, and ATP-gamma-S can also induce inward and outward current similar to those described for ATP, but higher doses are required. Adenosine and AMP produce no detectable effect, whereas ADP induces a small outward current. The implications of these results to the phenomenon of ATPo-induced permeabilization of macrophage membranes to large molecules are discussed.

Channel-forming activity of the perforin N-terminus and a putative alpha-helical region homologous with complement C9.

Cytolytic lymphocytes are endowed with a pore-forming protein called perforin. Recently, a cytolytic domain was located in the first 34 residues of the perforin N-terminus. It has been proposed that the first 19 residues are composed of a 3-domain structure including a putative amphipathic beta-sheet and that the 19 residues are sufficient for cytolytic activity. This model has now been tested by synthesizing peptides covering different portions of the N-terminus, and testing their ability to lyse lipid vesicles or increase the conductance of lipid bilayers or plasma membranes. It was found that the putative beta-sheet is indispensable for lytic activity and that the first 19 residues of the N-terminus are required for optimal lytic activity but that shorter peptides, containing only 16 residues, can form pores in lipid bilayers and cell membranes. A putative amphipathic alpha-helix from the central portion of perforin, homologous to complement C9, is nonlytic to lipid vesicles, but it can form pores in lipid bilayers. Taken together, these results support the model that the perforin N-terminus is important in initial pore formation and that the putative alpha-helical domain may be involved in subsequent perforin polymerization into large pores.

Cytolytic and ion channel-forming properties of the N terminus of lymphocyte perforin.

Perforin lyses cells by binding to the target cell membrane, where it polymerizes into large nonspecific pores. It is shown here that the first 34 amino acids of the N-terminal region of either human or murine perforin are soluble in aqueous medium and spontaneously insert into membranes. The N-terminal peptides lyse liposomes and nucleated cells, and they form ion channels in planar bilayers, some of which are comparable to those previously described for perforin. The lytic activity of the N-terminal domains does not require calcium, is independent of the lipid headgroup composition, and can be inhibited by heparin. Tumor cells incubated with the N-terminal peptides undergo the same morphological changes as those induced by native perforin. None of the peptides corresponding to the putative membrane-spanning domains from the central region of perforin is cytolytic. Taken together, these results suggest that the N-terminal region is an important part of the pore-forming domain of perforin.

Structural and functional identification of GP57/51 antigen of Trypanosoma cruzi as a cysteine proteinase.

Purified GP57/51, a Trypanosoma cruzi glycoprotein earlier identified as a major antigen in infected humans, was subjected to N-terminal sequence analysis. Alignment of the first 30 amino acids revealed that its N-terminal region is virtually identical to that reported for a cysteine-proteinase isolated from the Tulahuen strain, including the presence of active site cysteine at position 25. The finding of serine at position 24 of GP57/51 (Y strain) has further increased the homology between this protozoan antigen with other members of the eukaryotic family of cysteine proteases, including human cathepsin L. Functional analysis of GP57/51 indicated that the antigen is indeed an active thiol proteinase, which is active across a wide pH range (5-7.5). This was shown using either human IgG or gelatin substrates co-polymerized into polyacrylamide gels prepared for electrophoresis, and also by enzyme assays peformed with the synthetic substrate Z-phe-arg-NMec. The enzyme was activated by thiol containing reagents, and was strongly inhibited by low concentrations of E-64 (IC50 0.1 microM), cystatin (IC50 1 microM), leupeptin (IC50 0.1 microM) and antipain (IC50 0.1 microM). Monoclonal antibodies directed against distinct epitopes of GP57/51 absorbed the hydrolytic activity from purified preparations, demonstrating that the antigenic and enzymatic activities were indeed expressed by the same molecular entities. The subcellular localization of immunoreactive molecules was investigated by electron microscopy; immunogold staining was conspicuously found in vesicles belonging to the endosomal-lysosomal system, in tissue culture trypomastigotes as well as in epimastigotes. The possibility that this highly antigenic protease is actively secreted and/or leaked out of damaged parasites is under investigation; its release to tissues and to the circulation may contribute to pathology, considering that it (i) can degrade proteins across a wide pH range and (ii) stimulates immune T cells from chronic chagasic patients.

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

The assembly of pores by the pore-forming protein (perforin) of cytolytic T lymphocytes (CTLs) and natural killer cells on the membranes of different cell lines was studied. Using the patch clamp technique in the whole cell configuration, we measured the conductance increase induced by perforin in susceptible cell lines as well as in resistant CTL lines (CTLLs). The results showed that although the amplitudes of the first observed conductance steps produced in both cell types were comparable, CTLLs required at least 10-fold higher doses of perforin to form membrane pores. Outside-out patches excised from CTLL-R8, on the other hand, appeared to be more susceptible to channel formation by perforin than intact cells, as lower doses were able to induce conductance increases. Once channels were induced in CTL membranes, however, their conductances (greater than 1 nS) were indistinguishable from the ones obtained in susceptible cell lines. Fluorescence measurements with quin-2 showed that perforin induced rapid increases in the intracellular Ca2+ concentration in susceptible EL4 cells. In marked contrast, a perforin dose 60-120-fold higher than the minimal dose required to elicit Ca2+ changes in EL4 cells was not able to induce any measurable Ca2+ increase in CTLL-R8. The data suggest that the resistance of CTLs to lysis mediated by their own mediator perforin is at least in part due to their ability to avoid pore formation by this protein. The mechanism underlying this phenomenon is not yet understood, but the observation that outside-out patches excised from CTLL-R8 are more susceptible to channel formation by perforin than intact cells raises the possibility that an intracellular mechanism may be involved.