Resistance of cloned cytotoxic T lymphocytes to cell-mediated cytotoxicity

To Kill But Not Be Killed: Controlling the Activity of Mammalian Pore-Forming Proteins

Pore-forming proteins (PFPs) are present in all domains of life, and play an important role in host-pathogen warfare and in the elimination of cancers. They can be employed to deliver specific effectors across membranes, to disrupt membrane integrity interfering with cell homeostasis, and to lyse membranes either destroying intracellular organelles or entire cells. Considering the destructive potential of PFPs, it is perhaps not surprising that mechanisms controlling their activity are remarkably complex, especially in multicellular organisms. Mammalian PFPs discovered to date include the complement membrane attack complex (MAC), perforins, as well as gasdermins. While the primary function of perforin-1 and gasdermins is to eliminate infected or cancerous host cells, perforin-2 and MAC can target pathogens directly. Yet, all mammalian PFPs are in principle capable of generating pores in membranes of healthy host cells which-if uncontrolled-could have dire, and potentially lethal consequences. In this review, we will highlight the strategies employed to protect the host from destruction by endogenous PFPs, while enabling timely and efficient elimination of target cells.

Mechanisms of natural killer cell-mediated cellular cytotoxicity

Cellular cytotoxicity, the ability to kill other cells, is an important effector mechanism of the immune system to combat viral infections and cancer. Cytotoxic T cells and natural killer (NK) cells are the major mediators of this activity. Here, we summarize the cytotoxic mechanisms of NK cells. NK cells can kill virally infected of transformed cells via the directed release of lytic granules or by inducing death receptor-mediated apoptosis via the expression of Fas ligand or TRAIL. The biogenesis of perforin and granzymes, the major components of lytic granules, is a highly regulated process to prevent damage during the synthesis of these cytotoxic molecules. Additionally, NK cells have developed several strategies to protect themselves from the cytotoxic activity of granular content upon degranulation. While granule-mediated apoptosis is a fast process, death receptor-mediated cytotoxicity requires more time. Current data suggest that these 2 cytotoxic mechanisms are regulated during the serial killing activity of NK cells. As many modern approaches of cancer immunotherapy rely on cellular cytotoxicity for their effectiveness, unraveling these pathways will be important to further progress these therapeutic strategies.

Pyk2 Controls Integrin-Dependent CTL Migration through Regulation of De-Adhesion

Protein tyrosine kinase 2 (Pyk2) is required for T cell adhesion to ICAM-1; however, the mechanism by which it regulates adhesion remains unexplored. Pyk2 function in murine CTL clones and activated ex vivo CD8(+) T cells was disrupted by pharmacological inhibition, knockdown of expression with small interfering RNA, or expression of the dominant-negative C-terminal domain. We found that Pyk2 is not absolutely required for adhesion of CTL to ICAM-1, but rather delays the initial adhesion. Disruption of Pyk2 function caused cells to display an unusual elongated appearance after 1 h on ICAM-1, consistent with abnormally strong adhesion. Furthermore, the random mobility of CTL on ICAM-1 was severely compromised using all three methods of disrupting Pyk2 function. Live-cell imaging studies revealed that the decreased migration is the result of a defect in the detachment from ICAM-1 at the trailing edge when Pyk2 function is inhibited. Examination of Pyk2 tyrosine phosphorylation in normal polarized cells demonstrated that Pyk2 phosphorylated at Y579 and Y580 preferentially localizes to the leading edge, whereas Y881-phosphorylated Pyk2 is enriched at the trailing edge, suggesting that the tyrosine phosphorylation of Pyk2 is spatially regulated in migrating CTL. Additionally, inhibition of Pyk2 caused cells to form multiple LFA-1-rich tails at the trailing edge, most likely resulting from a defect in LFA-1 release required for forward movement. Our results show that Pyk2 contributes to CTL migration by regulating detachment of CTL at the trailing edge, which could explain why Pyk2 is important for chemotactic and migratory responses.

Paxillin associates with the microtubule cytoskeleton and the immunological synapse of CTL through its leucine-aspartic acid domains and contributes to microtubule organizing center reorientation

The cytoskeletal adaptor protein paxillin localizes to the microtubule organizing center (MTOC) in T cells and, upon target cell binding, is recruited to the supramolecular activation complex (SMAC). We mapped the region of paxillin that associates with both the MTOC and SMAC to the leucine-aspartic acid (LD) domains and showed that a protein segment containing LD2-4 was sufficient for MTOC and SMAC recruitment. Examination of the localization of paxillin at the SMAC revealed that paxillin localizes to the peripheral area of the SMAC along with LFA-1, suggesting that LFA-1 may contribute to its recruitment. LFA-1 or CD3 engagement alone was insufficient for paxillin recruitment because there was no paxillin accumulation at the site of CTL contact with anti-LFA-1- or anti-CD3-coated beads. In contrast, paxillin accumulation was detected when beads coated with both anti-CD3 and anti-LFA-1 were bound to CTL, suggesting that signals from both the TCR and LFA-1 are required for paxillin accumulation. Paxillin was shown to be phosphorylated downstream of ERK, but when we generated a mutation (S83A/S130A) that abolished the mobility shift as a result of phosphorylation, we found that paxillin still bound to the MTOC and was recruited to the SMAC. Furthermore, ERK was not absolutely required for MTOC reorientation in CTL that require ERK for killing. Finally, expression of the LD2-4 region of paxillin substantially reduced MTOC reorientation. These studies demonstrated that paxillin is recruited, through its LD domains, to sites of integrin engagement and may contribute to MTOC reorientation required for directional degranulation.

Regulation of the tyrosine kinase Pyk2 by calcium is through production of reactive oxygen species in cytotoxic T lymphocytes

Pyk2 was identified as a Ca(2+)-dependent kinase, however, the regulation of Pyk2 by Ca(2+) in T cells remains controversial. We found that Ca(2+) mobilization preferentially induced Pyk2 phosphorylation in cytotoxic T lymphocytes (CTL). Furthermore, Pyk2 phosphorylation in CTL was not absolutely Ca(2+) dependent but relied on the strength of T cell receptor stimulation. Ionomycin-stimulated Pyk2 phosphorylation did not require calmodulin activity, because phosphorylation was not inhibited by the calmodulin inhibitor W7, and we detected no Ca(2+)-regulated association between Pyk2 and calmodulin. Ca(2+)-stimulated Pyk2 phosphorylation was dependent on Src-family kinase activity, even at the Pyk2 autophosphorylation site. We sought to identify a Ca(2+)-regulated pathway that could trigger Pyk2 phosphorylation in T cells and found that ionomycin stimulated the production of reactive oxygen species and an H(2)O(2) scavenger inhibited ionomycin-induced Pyk2 phosphorylation. Additionally, H(2)O(2) induced strong Erk activation and ionomycin-stimulated Pyk2 phosphorylation was Erk dependent. These data support the conclusion that Ca(2+) mobilization induces the production of reactive oxygen species, which in turn activate the Erk pathway, leading to Src-family kinase-dependent Pyk2 phosphorylation. Our data demonstrate that Pyk2 is not a Ca(2+)-dependent kinase in T cells but instead, increased intracellular Ca(2+) induces Pyk2 phosphorylation through production of reactive oxygen species. These findings are consistent with the possibility that Pyk2 acts as an early sensor of numerous extracellular signals that trigger a Ca(2+) flux and/or reactive oxygen species to amplify tyrosine phosphorylation signaling events.

Regulation of perforin lysis: implications for protein disulfide isomerase proteins

Perforin, a membrane-permeabilizing protein, is important to T cell cytotoxic action. Perforin has potential to damage the T cell in the endoplasmic reticulum (ER), is sequestered in granules, and later is exocytosed to kill cells. In the ER and after exocytosis, calcium and pH favor perforin activity. We found a novel perforin inhibitor associated with cytotoxic T cell granules and termed it Cytotoxic Regulatory Protein 2 (CxRP2). CxRP2 blocked lysis by granule extracts, recombinant perforin and T cells. Its effects lasted for hours. CxRP2 was calcium stable and refractory to inhibitors of granzyme and cathepsin proteases. Through mass spectrometric analysis of active 50-100 kDa proteins, we identified CxRP2 candidates. Protein disulfide isomerase A3 was the strongest candidate but was unavailable for testing; however, protein disulfide isomerase A1 had CxRP2 activity. Our results indicate that protein disulfide isomerases, in the ER or elsewhere, may protect T cells from their own perforin.

CTLs contain and use intracellular stores of FasL distinct from cytolytic granules

CTL lyse target cells through the release of cytolytic granule contents and cell surface expression of Fas ligand (FasL). Current models suggest that FasL is stored in cytolytic granules and that FasL cell surface expression would be subject to the same controls as degranulation. We demonstrate that murine CTLs undergo two waves of FasL cell surface expression after stimulation. The first wave is from a pre-existing pool of FasL, and the second wave requires new protein synthesis. Signaling for FasL expression appears to be finely tuned as a weak signal preferentially induced surface translocation of the stored FasL, whereas a strong signal preferentially triggered the expression of de novo synthesized FasL. The early FasL is differentially regulated from degranulation, as there were multiple circumstances whereby rapid FasL cell surface expression and FasL-dependent killing occurred in the absence of detectable degranulation. Furthermore, we found through confocal microscopy that stored FasL resides in vesicles distinct from cytolytic granules. Our data clearly show that CTL degranulation and FasL lytic mechanisms are fully independent with respect to stored component localization and regulation.

A novel PKC regulates ERK activation and degranulation of cytotoxic T lymphocytes: Plasticity in PKC regulation of ERK

Stimulation of cytotoxic T lymphocyte (CTL) degranulation with plate-bound anti-CD3 Ab leads to two phases of ERK activation: an early PKC-independent phase followed by a later sustained PKC-dependent phase. Herein, we show that a novel PKC (nPKC) mediates the late phase of ERK activation, upstream of Ras in murine T cells. In contrast, when CTL are activated with cross-linked anti-CD3 Ab, which does not trigger CTL degranulation, there is a requirement for conventional PKC (cPKC) for ERK activation. We detect increased novel PKCtheta activation only when CTL are stimulated with plate-bound Ab and not cross-linked Ab. Interestingly, in T cells from mice lacking PKCtheta, sustained ERK activation requires the activity of cPKC, implying that PKCtheta is required for the nPKC pathway that normally mediates sustained ERK activation. CTL lines derived from PKCtheta-deficient mice degranulate and activate ERK normally, and exhibit altered expression of PKC isozymes, which may compensate for the loss of PKCtheta. Taken together, these data demonstrate that normally an nPKC participates in the sustained activation of ERK. However, if the nPKC pathway is compromised, alternate PKC pathways can compensate, suggesting that considerable plasticity exists with respect to PKC regulation of ERK activation in T cells.

A Role for Phosphatidylinositol 3-Kinase in TCR-Stimulated ERK Activation Leading to Paxillin Phosphorylation and CTL Degranulation

PI3K is an important regulator of a number of cellular processes. We examined the contribution of PI3K to mouse CTL signaling, leading to degranulation. We show that TCR-triggered, but not phorbol ester and calcium ionophore-induced, CTL degranulation is dependent on PI3K activity. Although PI3K activity is required for optimal LFA-1-mediated adhesion and cell spreading, this most likely does not account for its full contribution to degranulation. We demonstrate that PI3K is required for TCR-stimulated ERK activation in CTL, which we have shown previously to be required for CTL degranulation. We thus define a pathway through which PI3K most likely regulates degranulation and in which ERK appears to be a key signaling molecule. Furthermore, we identified the cytoskeletal adaptor paxillin as a target of ERK downstream of TCR stimulation. Consistent with a role in degranulation, we demonstrate that paxillin is localized to the microtubule organizing center in resting cells and upon target cell binding is recruited to the contact point with the target cell. These studies demonstrate that PI3K regulates ERK activity leading to CTL degranulation, and identify paxillin as a target of ERK downstream of the TCR. That paxillin is independently phosphorylated by both tyrosine kinase(s) and ERK downstream of the TCR and localized both at the microtubule organizing center and at the target cell contact point suggests an important role for paxillin in CTL-mediated killing.

Differential Src family kinase activity requirements for CD3 zeta phosphorylation/ZAP70 recruitment and CD3 epsilon phosphorylation

The current model of T cell activation is that TCR engagement stimulates Src family tyrosine kinases (SFK) to phosphorylate CD3zeta. CD3zeta phosphorylation allows for the recruitment of the tyrosine kinase ZAP70, which is phosphorylated and activated by SFK, leading to the phosphorylation of downstream targets. We stimulated mouse CTLs with plate-bound anti-CD3 and, after cell lysis, recovered proteins that associated with the CD3 complex. The protein complexes were not preformed, and a number of tyrosine-phosphorylated proteins were inducibly and specifically associated with the TCR/CD3 complex. These results suggest that complex formation only occurs at the site of TCR engagement. The recruitment and tyrosine phosphorylation of most proteins were abolished when T cells were stimulated in the presence of the SFK inhibitor PP2. Surprisingly, CD3zeta, but not CD3epsilon, was inducibly tyrosine phosphorylated in the presence of PP2. Furthermore, ZAP70 was recruited, but not phosphorylated, after TCR stimulation in the presence of PP2, thus confirming the phosphorylation status of CD3zeta. These data suggest that there is a differential requirement for SFK activity in phosphorylation of CD3zeta vs CD3epsilon. Consistent with this possibility, ZAP70 recruitment was also detected with anti-CD3-stimulated, Lck-deficient human Jurkat T cells. We conclude that TCR/CD3-induced CD3zeta phosphorylation and ZAP70 recruitment do not absolutely require Lck or other PP2-inhibitable SFK activity, but that SFK activity is absolutely required for CD3epsilon and ZAP70 phosphorylation. These data reveal the potential for regulation of signaling through the TCR complex by the differential recruitment or activation of SFK.

Functional Segregation of the TCR and Antigen-MHC Complexes on the Surface of CTL

As CTL adhere to and lyze their targets, they extract cognate Ag-MHC and represent this on their own cell surface. Whether such self-presented cognate Ag stimulate the TCR of a CTL is uncertain. To analyze this, we examined TCR capping in response to self-presented Ag. We found that OVA peptide-specific OT-1 CTL that were pulsed with cognate peptide Ag did not cap their TCR, implying that the autologously presented MHC-Ag complex does not normally stimulate the TCR. However, this functional separation of the TCR and its ligand on the cell surface was not absolute. Treatment of Ag-pulsed OT-1 CTL with agents that alter cell surface charge, including trypsin, papain, tunicamycin, neuraminidase, and polybrene, allowed Ag-specific TCR capping. The TCR capped together with the restricting MHC molecule on the surface of the cell, implying an interaction between the TCR and cell-associated Ag. Further, the treated CTL underwent a time- and dose-dependent suicidal death that was both Fas- and perforin-dependent. Therefore, our results indicate that the association of the TCR with its MHC-peptide ligand on the surface of a CTL is normally proscribed by biophysical properties of the plasma membrane. Overcoming this restriction allows TCR stimulation and induces CTL effector functions and cell suicide.

Antigen-specific targeting of CD8+ T cells with receptor-modified T lymphocytes

Chimeric receptors that link ligand recognition domains, such as antibody Fv fragments, with TCR signaling domains can redirect T lymphocytes against MHC-unrestricted targets. Such receptor-modified T lymphocytes have shown promise in the treatment of infectious diseases and cancer. We hypothesized that receptor-modified T lymphocytes may also be designed to target antigen-specific T cells. We synthesized chimeric receptors consisting of the extracellular and transmembrane domains of the class I MHC H-2K(b) molecule linked to the signaling domains of either TCR-zeta, CD28 and zeta, or CD28, zeta, and lck. T lymphocytes modified to express these receptors and pulsed with antigenic peptide specifically killed precursor CTL. Cytolysis was efficient, even at effector:target ratios of less than one, and specific, selectively killing antigen-specific precursor CTL among a mixed population of T cells. Cytolysis required activation of the receptor-modified T cells, and did not occur with a signaling-deficient chimeric receptor. In contrast to precursor CTL, differentiated CTL proved resistant to lysis by the receptor-modified T cells. These data demonstrate the feasibility of redirecting T lymphocytes against antigen-specific T cells. Receptor-modified T cells expressing chimeric MHC receptors have potential application in autoimmune and alloimmune diseases.

Dynamic association of CD45 with detergent-insoluble microdomains in T lymphocytes

The receptor-like protein tyrosine phosphatase CD45 is essential for TCR signal transduction. Substrates of CD45 include the protein tyrosine kinases p56(lck) and p59(fyn), both of which have been shown to be enriched in detergent-insoluble microdomains. Here we find that there is a cholesterol-dependent association between CD45 and the raft-associated protein linker for activation of T cells, suggesting that CD45 and linker for activation of T cells may colocalize in lipid rafts. Consistent with this observation, we find that approximately 5% of total CD45 can be detected in Triton X-100-insoluble buoyant fractions of sucrose gradients, demonstrating that CD45 is not excluded from lipid rafts. Upon stimulation of T cells with anti-CD3, there is a reduction in the amount of CD45 found associating with lipid rafts. Our data suggest that CD45 is present in lipid rafts in T cells before activation, perhaps to activate raft-associated p56(lck), allowing membrane-proximal signaling events to proceed. Furthermore, the reduction in CD45 content of lipid rafts after CD3 stimulation may serve to limit the amounts of activated p56(lck) in rafts and thus possibly the duration of T cell responses.

Surface cathepsin B protects cytotoxic lymphocytes from self-destruction after degranulation

The granule exocytosis cytotoxicity pathway is the major molecular mechanism for cytotoxic T lymphocyte (CTL) and natural killer (NK) cytotoxicity, but the question of how these cytotoxic lymphocytes avoid self-destruction after secreting perforin has remained unresolved. We show that CTL and NK cells die within a few hours if they are triggered to degranulate in the presence of nontoxic thiol cathepsin protease inhibitors. The potent activity of the impermeant, highly cathepsin B-specific membrane inhibitors CA074 and NS-196 strongly implicates extracellular cathepsin B. CTL suicide in the presence of cathepsin inhibitors requires the granule exocytosis cytotoxicity pathway, as it is normal with CTLs from gld mice, but does not occur in CTLs from perforin knockout mice. Flow cytometry shows that CTLs express low to undetectable levels of cathepsin B on their surface before degranulation, with a substantial rapid increase after T cell receptor triggering. Surface cathepsin B eluted from live CTL after degranulation by calcium chelation is the single chain processed form of active cathepsin B. Degranulated CTLs are surface biotinylated by the cathepsin B-specific affinity reagent NS-196, which exclusively labels immunoreactive cathepsin B. These experiments support a model in which granule-derived surface cathepsin B provides self-protection for degranulating cytotoxic lymphocytes.

Developmentally regulated changes in glucosidase II association with, and carbohydrate content of, the protein tyrosine phosphatase CD45

Glucosidase II (GII) stably interacts with the external domain of CD45 in a carbohydrate-dependent manner. We have found that the association occurs in immature cells, but is significantly reduced in mature T cells. Using mannose-binding protein (MBP), in both FACS analysis and pull-down assays, we find that MBP can specifically recognize cell surface CD45 from immature, but not mature T cells. Analysis of thymocytes reveals increased MBP binding and GII association with CD45 in double-positive thymocytes compared with either double-negative or single-positive thymocytes. As well, the same pool of CD45 recognized by MBP can also associate with GII. Initial analysis of the basis of the interaction between CD45 and MBP suggests MBP binds two different glycoforms of CD45 based on the differential competition with glucose. Finally, inhibition of GII activity in cells that do not normally express MBP ligands results in significant increases in cell surface MBP ligands, including CD45. Taken together, these data suggest that the glucose content of the cell surface CD45 changes as thymocytes undergo maturation to mature T cells, and may be regulated by GII interactions. Such changes in the cell surface carbohydrate on CD45 may affect the development of thymocytes, perhaps via binding of CD45 on thymocytes to lectins on stromal cells.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Herpes simplex virus type 1 infection of activated cytotoxic T cells: Induction of fratricide as a mechanism of viral immune evasion

Herpes simplex virus type 1 (HSV1), a large DNA-containing virus, is endemic in all human populations investigated. After infection of mucocutaneous surfaces, HSV1 establishes a latent infection in nerve cells. Recently, it was demonstrated that HSV1 can also infect activated T lymphocytes. However, the consequences of T cell infection for viral pathogenesis and immunity are unknown. We have observed that in contrast to the situation in human fibroblasts, in human T cell lines antigen presentation by major histocompatibility complex class I molecules is not blocked after HSV1 infection. Moreover, HSV1 infection of T cells results in rapid elimination of antiviral T cells by fratricide. To dissect the underlying molecular events, we used a transgenic mouse model of HSV1 infection to demonstrate that CD95 (Apo-1, Fas)-triggered apoptosis is essential for HSV1-induced fratricide, whereas tumor necrosis factor (TNF) also contributes to this phenomenon but to a lesser extent. By contrast, neither TRAIL (TNF-related apoptosis-inducing ligand) nor perforin were involved. Finally, we defined two mechanisms associated with HSV1-associated fratricide of antiviral T cells: (a) T cell receptor-mediated upregulation of CD95 ligand and (b) a viral "competence-to-die" signal that renders activated T lymphocytes susceptible to CD95 signaling. We propose that induction of fratricide is an important immune evasion mechanism of HSV1, helping the virus to persist in the host organism throughout its lifetime.

Homeostatic regulation of CD8+ T cells by perforin

To prevent uncontrolled expansion, the massive proliferation of T cells during an acute immune response has to be followed by controlled deletion. Here we show that similar to Fas, perforin is not only an important effector molecule of cytotoxic T lymphocytes (CTL) but also involved in down-regulating peripheral T cells. Mice deficient for both the CTL effector molecule perforin and the apoptosis-inducing Fas ligand spontaneously develop infiltration of highly activated CD8(+) T cells in kidney and liver and die between 5 and 12 weeks of age. Injection of staphylococcal enterotoxin B (SEB) into perforin-deficient mice results in dramatically increased selective expansion and prolonged persistence of CD8(+), but not CD4(+), SEB-reactive T cells. Also, secondary immunization of TCR transgenic perforin-deficient mice with the lymphocytic choriomeningitis virus glycoprotein-derived epitope peptide leads to an increased proliferation of transgenic CD8(+) T cells, that is not explained by failure to deplete professional antigen-presenting cells. These results point to a novel mechanism of T cell homeostasis in which the acquisition of perforin-dependent cytotoxic activity regulates the expansion and persistence of CD8(+) effector T cells in vivo.

Apoptosis of human naive NK cells mediated by a rat IgG2b anti CD2 mAb through a fractricidal ADCC reaction

LO-CD2a/BTI-322, a rat anti human CD2 mAb, shows in vitro and in vivo immunosuppressive properties and induces T-cell depletion resulting partially from an antibody dependent cellular cytotoxicity (ADCC) mediated by NK cells. The aim of this paper is to study the in vitro effect of LO-CD2a/BTI-322 on NK cells, the majority of them also expressing the CD2 molecule. The addition of the mAb to purified naive NK cells induces apoptosis of CD2+ cells. The apoptosis is rapid, Fas ligand independent and completely inhibited by the calcium chelator EGTA, suggesting a fractricidal ADCC reaction and implying that NK cells are not resistant to lysis when used as target cells. At the end of the reaction, the CD2 - remaining cells are still capable of natural cytotoxicity against K562 cells, but at a lower rate than untreated cells.

Regulation of pro-apoptotic leucocyte granule serine proteinases by intracellular serpins

Caspase activation and apoptosis can be initiated by the introduction of serine proteinases into the cytoplasm of a cell. Cytotoxic lymphocytes have evolved at least one serine proteinase with specific pro-apoptotic activity (granzyme B), as well as the mechanisms to deliver it into a target cell, and recent evidence suggests that other leucocyte granule proteinases may also have the capacity to kill if released into the interior of cells. For example, the monocyte/granulocyte proteinase cathepsin G can activate caspases in vitro, and will induce apoptosis if its entry into cells is mediated by a bacterial pore-forming protein. The potent pro-apoptotic activity of granzyme B and cathepsin G suggests that cells producing these (or other) proteinases would be at risk from self-induced death if the systems involved in packaging, degranulation or targeting fail and allow proteinases to enter the host cell cytoplasm. The purpose of the present review is to describe recent work on a group of intracellular serine proteinase inhibitors (serpins) which may function in leucocytes to prevent autolysis induced by the granule serine proteinases.

Mouse lymphoblasts lose their immunogenicity and susceptibility to specific cytotoxic T lymphocyte lysis during maintenance in culture

This study was undertaken to search for possible mechanisms by which T-cell lines become non-immunogenic and refractory to cellular-mediated lysis during culture. We demonstrate that mouse lymphoblasts (LB) lost their susceptibility to specific cytotoxic T lymphocyte (CTL)-mediated lysis following culture for more than 5 days in the presence interleukin-2 (IL-2), IL-7 but not IL-4. In contrast, the cultured lymphoblasts (CLB) were efficiently lysed by specific antibody and C' and by CTL in the presence of concanavalin A. In addition, CLB did not inhibit cytotoxicity against LB in a cold target competition assay, indicating that CLB and LB differ in the expression of certain surface molecules. Indeed, a significantly lower expression of H-2D class I antigen, the Fas antigen and the adhesion molecules intracelluar adhesion molecule-1 (ICAM-1) and very late activation antigen-4 (VLA-4) was observed on the CLB surface. Consequently, CLB could not form conjugates with specific CTL, a prerequisite for CTL-mediated lysis. In addition, there was a marked decrease in CLB immunogenicity: the cultured cells were unable to stimulate allogeneic spleen cells in mixed lymphocyte culture nor could they induce a cytotoxic response following their injection into allogeneic mice. The reduced immunogenicity enabled the prolonged survival of active CLB in an allogeneic host. We suggest that the extended survival in an allogeneic tumour-bearing host of cultured, hence weakly immunogenic, anti-tumour CTL, will enable them the in vivo implementation of their anti-tumour activity.

Selective regulation of apoptosis: the cytotoxic lymphocyte serpin proteinase inhibitor 9 protects against granzyme B-mediated apoptosis without perturbing the Fas cell death pathway

Cytotoxic lymphocytes (CLs) induce caspase activation and apoptosis of target cells either through Fas activation or through release of granule cytotoxins, particularly granzyme B. CLs themselves resist granule-mediated apoptosis but are eventually cleared via Fas-mediated apoptosis. Here we show that the CL cytoplasmic serpin proteinase inhibitor 9 (PI-9) can protect transfected cells against apoptosis induced by either purified granzyme B and perforin or intact CLs. A PI-9 P1 mutant (Glu to Asp) is a 100-fold-less-efficient granzyme B inhibitor that no longer protects against granzyme B-mediated apoptosis. PI-9 is highly specific for granzyme B because it does not inhibit eight of the nine caspases tested or protect transfected cells against Fas-mediated apoptosis. In contrast, the P1(Asp) mutant is an effective caspase inhibitor that protects against Fas-mediated apoptosis. We propose that PI-9 shields CLs specifically against misdirected granzyme B to prevent autolysis or fratricide, but it does not interfere with homeostatic deletion via Fas-mediated apoptosis.

Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

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

Subcutaneous panniculitic T-cell lymphoma is a tumor of cytotoxic T lymphocytes

Subcutaneous panniculitic T cell lymphoma (SCPTCL) is characterized by primary involvement of the subcutaneous fat in a manner mimicking panniculitis. We studied 16 cases of this lymphoma to define its immunophenotypical profile as well as cellular origin. Involvement of the subcutaneous fat in a lacelike pattern with neoplastic cells rimming individual fat spaces was present in all cases. All 16 cases were of T cell phenotype. Thirteen of the 16 cases were CD8+, whereas three were negative for both CD4 and CD8. Twelve cases were stained for betaF1; of these, eight were betaF1+ and four were betaF1-. Focal staining for CD56 and CD30 was seen in 2 of 13 and two of eight cases, respectively. Intense diffuse positivity for the cytotoxic granular proteins T cell intracellular antigen-1 (TIA-1) and perforin was present in all cases, indicating an origin from cytotoxic T lymphocytes. Ten cases studied for Epstein-Barr viral sequences were negative. Eight of 9 cases with amplifiable DNA showed a clonal TCR gamma gene rearrangement by polymerase chain reaction. Controls included seven cases of benign panniculitis and seven other peripheral T cell lymphomas involving the skin and subcutaneous tissues: two peripheral T cell lymphomas, not otherwise specified (PTL,NOS), four anaplastic large cell lymphomas (ALCL), one T/NK cell lymphoma. The seven cases of panniculitis lacked cytological atypia and were characterized by an admixture of CD4+ and CD8+ cells with interspersed aggregates of L26+ B cells. Only infrequent cells showed staining for TIA-1 and perforin. In the control cases of T cell lymphoma, the infiltrate had a tendency for dermal and sometimes even epidermal involvement, with sheeting out of malignant cells, in contrast to the characteristic subcutaneous localization and rimming of fat spaces noted in SCPTCL. The two PTL, NOS were CD4+ and negative for both TIA-1 and perforin. Although the remaining controls expressed TIA-1 and perforin, in keeping with their cytotoxic T or natural killer (NK) cell origin, histological and other immunophenotypical features allowed distinction from SCPTCL. Five cases of SCPTCL were also stained for apoptosis using a tdt-mediated end labeling kit. All cases showed numerous positive apoptotic bodies, suggesting apoptosis as the mechanism of cell death in these tumors. Our study indicates that SCPTCL constitutes a distinctive clinicopathological entity derived from cytotoxic T lymphocytes and should be differentiated from other benign and malignant lymphoid infiltrates involving the subcutis. The apoptosis seen in these tumors may be mediated by release of cytotoxic granular proteins.

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.

Inactivation of human immunodeficiency virus (HIV)-1 envelope-specific CD8+ cytotoxic T lymphocytes by free antigenic peptide: a self-veto mechanism?

Free peptide has been found to inhibit cytotoxic T lymphocyte (CTL) activity, and veto cells bearing peptide-major histocompatibility complex (MHC) complexes have been found to inactivate CTL, but the two phenomena have not been connected. Here we show that a common mechanism may apply to both. CD8+ CTL lines or clones specific for a determinant of the human immunodeficiency virus (HIV) 1 IIIB envelope protein gp160, P18IIIB, are inhibited by as little as 10 min exposure to the minimal 10-mer peptide, I-10, within P18IIIB, free in solution, in contrast to peptide already bound to antigen-presenting cells (APC), which does not inhibit. Several lines of evidence suggest that the peptide must be processed and presented by H-2Dd on the CTL itself to the specific T cell receptor (TCR) to be inhibitory. The inhibition was not killing, in that CTL did not kill 51Cr-labeled sister CTL in the presence of free peptide, and in mixing experiments with CTL lines of different specificities restricted by the same MHC molecule, Dd, the presence of free peptide recognized by one CTL line did not inhibit the activity of the other CTL line that could present the peptide. Also, partial recovery of activity could be elicited by restimulation with cell-bound peptide, supporting the conclusion that neither fratricide nor suicide (apoptosis) was involved. The classic veto phenomenon was ruled out by failure of peptide-bearing CTL to inactivate others. Using pairs of CTL lines of differing specificity but similar MHC restriction, each pulsed with the peptide for which the other is specific, we showed that the minimal requirement is simultaneous engagement of the TCR and class I MHC molecules of the same cell. This could occur in single cells or pairs of cells presenting peptide to each other. Thus, mechanistically, the inhibition is analogous to veto, and might be called self-veto. As a clue to a possible mechanism, we found that free I-10 peptide induced apparent downregulation of expression of specific TCR as well as interleukin 2 receptor, CD69, lymphocyte function-associated antigen 1, and CD8. This self-veto effect also has implications for in vivo immunization and mechanisms of viral escape from CTL immunity.

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.

Immune function in mice lacking the perforin gene

Mice lacking the perforin gene were generated by using targeted gene disruption in embryonal stem cells. When infected with lymphocytic choriomeningitis virus (LCMV), perforin-less (-/-) mice showed clear signs of having mounted an immune response based on activation of CD8 T cells but were unable to clear the LCMV infection. This failure to eliminate virus was accompanied by a failure to generate spleen cells capable of lysing LCMV-infected fibroblasts in vitro. Spleen cells from LCMV-infected -/- mice were able to lyse hematopoietic target cells after exposure to phorbol 12-myristate 13-acetate and ionomycin, provided the target cells expressed the Fas antigen. Spleen cells from -/- mice also responded to alloantigen in mixed leukocyte culture by blastogenesis and proliferation. The resulting cells were able to lyse hematopoietic target cells, although not as well as spleen cells from +/+ littermates sensitized in the same manner. However, lysis by -/- cells was again seen only if the target cells expressed Fas antigen. We conclude that perforin-less -/- mice retain and express the Fas lytic pathway as expressed in vitro but that this pathway is insufficient to clear an LCMV infection in vivo.

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.

Infection of lymphocytes by a virus that aborts cytotoxic T lymphocyte activity and establishes persistent infection

For viruses to establish persistent infections in their hosts, they must possess some mechanism for evading clearance by the immune system. When inoculated into adult immunocompetent mice, wild-type lymphocytic choriomeningitis virus (LCMV ARM) induces a CD8(+)-mediated cytotoxic T lymphocyte (CTL) response that clears the infection within 7-14 d (CTL+ [P-]). By contrast, variant viruses isolated from lymphoid tissues of persistently infected mice fail to induce a CTL response and are thus able to establish a persistent infection in adult mice (CTL- [P+]). This report compares the interaction of CTL+ (P-) and CTL- (P+) viruses with cells of the immune system. Both types of virus initially bind to 2-4% of CD4+ and CD8+ T lymphocytes and replicate within cells of both subsets. The replication of CTL- (P+) and CTL+ (P-) viruses in lymphocytes in vivo is similar for the first 5 d after initiating infection. Thereafter, in mice infected with CTL- (P+) variants, lymphocytes retain viral genetic information, and infectious virus can be recovered throughout the animals' lives. In contrast, when adult mice are infected with wild-type CTL+ (P-) LCMV ARM, virus is not recovered from lymphocytes for greater than 7 d after infection. A CD8(+)-mediated anti-LCMV CTL response is induced in such mice. Clearance of infected lymphocytes is produced by these LCMV-specific CTLs, as shown by their ability to lyse lymphocytes expressing LCMV determinants in vitro and the fact that depletion of CD8+ lymphocytes before infection with CTL+ (P-) viruses results in levels of infected lymphocytes similar to those found in undepleted CTL- (P+)-infected mice. Hence, CTL-mediated lysis of T lymphocytes carrying infectious virus is a critical factor determining whether virus persists or the infection is terminated.

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.

T cell-T cell killing is induced by specific epitopes: evidence for an apoptotic mechanism

Epstein-Barr virus-specific cytotoxic T lymphocyte clones were shown to be an effective target for their own lysis when incubated in the presence of their specific epitopes but not in the presence of irrelevant epitopes. The mode of cell killing appeared to be by apoptosis and was prevented by previously described inhibitors of the process. Degranulation, as measured by serine esterase activity, was involved in this form of T cell-T cell killing. This is the first report of T cell-T cell killing by apoptosis and is only observed in the presence of a specific epitope. This result may be of significance in the use of peptide-based vaccines.

Extracellular ATP as a trigger for apoptosis or programmed cell death

Extracellular ATP is shown here to induce programmed cell death (or apoptosis) in thymocytes and certain tumor cell lines. EM studies indicate that the ATP-induced death of thymocytes and susceptible tumor cells follows morphological changes usually associated with glucocorticoid-induced apoptosis of thymocytes. These changes include condensation of chromatin, blebbing of the cell surface, and breakdown of the nucleus. Cytotoxicity assays using double-labeled cells show that ATP-mediated cell lysis is accompanied by fragmentation of the target cell DNA. DNA fragmentation can be set off by ATP but not the nonhydrolysable analogue ATP gamma S nor other nucleoside-5'-triphosphates. ATP-induced DNA fragmentation but not ATP-induced 51Cr release can be blocked in cells pretreated with inhibitors of protein or RNA synthesis or the endonuclease inhibitor, zinc; whereas pretreatment with calmidazolium, a potent calmodulin antagonist, blocks both DNA fragmentation and 51Cr release. The biochemical and morphological changes caused by ATP are preceded by a rapid increase in the cytoplasmic calcium of the susceptible cell. Calcium fluxes by themselves, however, are not sufficient to cause apoptosis, as the pore-forming protein, perforin, causes cell lysis without DNA fragmentation or the morphological changes associated with apoptosis. Taken together, these results indicate that ATP can cause cell death through two independent mechanisms, one of which, requiring an active participation on the part of the cell, takes place through apoptosis.

Nicotinamide and 3-aminobenzamide interfere with receptor-mediated transmembrane signaling in murine cytotoxic T cells: independence of Golgi reorientation from calcium mobilization and inositol phosphate generation

The two competitive inhibitors of ADP-ribosylation, nicotinamide and 3-aminobenzamide, have been reported to interfere with TNF-induced cell apoptosis, and there is evidence that they inhibit killer-induced target cell lysis as well. There are very few drugs known to specifically interfere with target apoptosis induced by killer cells. We therefore sought to explore the effects these inhibitors have on CTL-mediated cell lysis. Here we show that TcR-mediated transmembrane signaling in CTL, measured by Ca2+ mobilization and generation of inositol phosphates, is inhibited by nicotinamide. The possibility that all cell functions are suppressed by the drug is excluded by the finding that constitutive secretion of BLT serine esterase is not inhibited, whereas stimulated secretion of this enzyme is suppressed. We also show that nicotinamide does not interfere with CTL target cell binding or reorientation of the Golgi apparatus toward the target binding site. It is concluded that nicotinamide inhibits transmembrane signaling in CTL and thereby interferes with delivery of the lethal hit to targets.

Cognate peptides induce self-destruction of CD8+ cytolytic T lymphocytes

Cytotoxic T lymphocytes (CTLs) have been shown to be relatively resistant to cytolytic attack by other CTLs. We show here, however, that cloned CTLs, in the absence of other cells, are destroyed by exposure to their cognate peptides (defined as those that in association with major histocompatibility complex proteins are recognized by the antigen-specific receptor of the T cell). Destruction is proportional to peptide concentration and can be prevented by a second peptide that competes with the cognate peptide for presentation by the class I major histocompatibility complex proteins of the CTLs. The speed and extent of peptide-induced changes in the appearance of CTLs suggest that the destruction may be due primarily to self-recognition and self-destruction of individual CTLs (suicide) rather than to the destruction of some CTLs by others of the same clone in the same culture (fratricide). This effect may also take place in vivo because the appropriately timed injection of a cognate peptide into ovalbumin-immunized mice appeared to deplete their spleens of primed anti-ovalbumin CTLs. The results point to a possible physiologic mechanism for postthymic elimination of cytolytic T cells that recognize their own peptides in association with their own major histocompatibility complex protein. The results also raise the possibility that cognate peptides might eventually prove therapeutically useful for eliminating CTL clones that cause pathological cell destruction, as in some autoimmune diseases and some viral infections.

The lysis of cytotoxic T lymphocytes and their blasts by cytotoxic T lymphocytes

After binding to their targets, cytotoxic T lymphocytes (CTL) deliver a lethal hit signal, ultimately leading to target cell lysis, and then can recycle to lyse additional targets, without themselves being destroyed. If non-specific secreted lytic mediators are involved in such lysis. CTL survival would not be expected unless the effectors are immune to CTL-mediated lysis. Therefore the lytic susceptibilities of alloimmune peritoneal exudate lymphocytes (PEL), containing up to 50% CTL, and of the cytolytic PEL blasts (PEB), obtained by culturing with interleukin-2 (IL-2), were examined. 51Cr-labelled BALB/c (H-2d) anti-EL4 (H-2b) (d alpha b) PEL were lysed 88%, 78%, and 48% by C3H/eb (H-2k) anti-P815 (H-2d) (k alpha d) PEL, C57BL/6 (H-2b) anti-P815 (b alpha d) PEL and b alpha d PEB, respectively. Similarly, b alpha d PEL were lysed 82% and 21% by d alpha b PEL and PEB, respectively. b alpha d PEB were lysed 82%, 28-39% and 39-51% by k alpha d PEL, b alpha d PEL and b alpha d PEB, respectively, b alpha d PEB were lysed 29-55% by d alpha b PEL. Furthermore, the CTL-containing populations were no less susceptible to lysis than normal lymphocytes. Since the majority (80-90%) of cells in these two types of CTL-containing populations can be directly and specifically lysed by appropriately immunized PEL CTL, we conclude that both the lytic granule and perforin lacking (PEL) and containing (PEB) CTL are not a priori immune to CTL-mediated lysis. These findings are in accord with theories proposing lysis to be induced by receptor-mediated contact between effector CTL and target cells, and challenge those suggesting the involvement of secreted lytic mediators.

Resistance to the pore-forming protein of cytotoxic T cells: comparison of target cell membrane rigidity

Cytotoxic T lymphocytes (CTL) release from their granules a 70 kDa protein, called PFP, perforin or cytolysin, which inserts into the target cell plasma membrane in its monomeric form. Here it polymerizes into a macromolecular complex forming pores as large as 20 nm. Although purified PFP/perforin can effectively lyze all target cells tested. CTL are refractory to lysis. The mechanism underlying the resistance of CTL is currently unknown. This study represents a search for membrane structural properties that could confer resistance to CTL against PFP/perforin-mediated lysis. The fluorescent dye merocyanine 540 was used to measure the lipid head group packing of CTL and several target cells, and 1-[4-(trimethylamine)phenyl]-6-phenylhexa-1,3,5-triene was used to estimate the fluidity of the membrane hydrocarbon region. The resistance against PFP/perforin-mediated lysis was determined by the 51Cr release assay. A comparison of the membrane rigidity with cell resistance led to the conclusion that the membrane lipid structure cannot account for the unusually high resistance of CTL. In particular, the resistant CTL line CTLL-2 has a lipid head group packing that is looser than that of Yac-1, and the sensitive target cells Jy-25 and EL-4 have membrane acyl chains that are less fluid than those of the effector CTLL-R8.

Extracellular ATP as a possible mediator of cell-mediated cytotoxicity

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells lyse target cells by a specialized mechanism that involves the release of soluble 'killer' mediators. While several possible mediators have been demonstrated, it is likely that others have yet to be identified. For more than two decades it has been known that exogenous ATP is lytic to many cell types. In this article Francesco Di Virgilio and colleagues propose that extracellular ATP may be a mediator of cytotoxic cell-dependent lysis.

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.

Specific killing of cytotoxic T cells and antigen-presenting cells by CD4+ cytotoxic T cell clones. A novel potentially immunoregulatory T-T cell interaction in man

Mycobacterial antigens not only stimulate Th cells that produce macrophage-activating factors, but also CD4+ and CD8+ CTL that lyse human macrophages. The mycobacterial recombinant 65-kD hsp was previously found to be an important target antigen for polyclonal CD4+ CTL. Because of the major role of 65-kD hsp in the immune response to mycobacterial as well as autoantigens, we have studied CTL activity to this protein at the clonal level. HLA-DR or HLA-DQ restricted, CD4+CD8- T cell clones that recognize different peptides of the M. leprae 65-kD hsp strongly lysed EBV-BLCL pulsed with specific but not irrelevant peptide. No bystander lysis of B cells, T cells, or tumor cells was seen. Target cell lysis could not be triggered by PMA + Ca2+ ionophore alone and depended on active metabolism. Interestingly, these CD4+ CTL also strongly lysed themselves and other HLA-class II compatible CD4+ (TCR-alpha/beta or -gamma/delta) or CD8+ CTL clones in the presence of peptide, suggesting that CTL are not actively protected from CTL-mediated lysis. Cold target competition experiments suggested that EBV-BLCL targets were more efficiently recognized than CD4+ CTL targets. These results demonstrate that hsp65 peptide-specific HLA class II-restricted CD4+ T cell clones display strong peptide-dependent cytolytic activity towards both APCs, and, unexpectedly, CD4+ and CD8+ CTL clones, including themselves. Since, in contrast to murine T cells human T cells express class II, CTL-mediated T cell killing may represent a novel immunoregulatory pathway in man.

Influenza peptide-induced self-lysis and down-regulation of cloned cytotoxic T cells

Virus-specific cytotoxic T-cell (Tc) clones can lyse target cells in vitro in the presence of their specific peptide epitopes. The lytic potency of murine influenza nucleoprotein (NP)-specific Tc clones was investigated after observing that target cell killing was reduced in the presence of high (greater than 0.2 microM) concentrations of specific NP peptide antigen. Following incubation of Tc for 16 hr in the presence of a range of peptide concentrations, two effects were observed; (i) a peptide dose-dependent mortality of Tc, which has been attributed to self-lysis by clonal Tc in the presence of specific peptide; (ii) and a reduced ability to specifically lyse NP-expressing target cells whilst retaining lectin-dependent lytic activity in the surviving Tc. This functional down-regulation was reversible after 24 hr incubation in the absence of peptide. Toxic effects were excluded, since inhibition of specific target lysis by Tc was mediated only be pretreatment with specifically recognized peptide.

Expression of mRNAs for pore-forming protein and two serine esterases in murine primary and cloned effector lymphocytes

The cDNAs encoding several proteins present in the granules of cytolytic effector lymphocytes have now been cloned. These include the cytolytic pore-forming protein (PFP) or perforin, and at least six serine esterases (SE), also called granzymes. The cDNA probes for PFP, SE-1, and SE-2 are used here to study the expression of these proteins in murine primary effector lymphocytes. Among the stimuli effective in inducing the expression of PFP, SE-1, and SE-2 were recombinant interleukin-2, the lectin concanavalin A in the presence of phorbol esters, and allogeneic cells in mixed lymphocyte cultures. Some correlation was seen between the levels of PFP and SE mRNAs and cytotoxicity measured in a standard 51Cr release assay. We also examined a panel of 13 cloned cytotoxic T lymphocyte (CTL) lines and found that mRNAs for PFP and SE-2 were expressed in all CTL lines, including some that were previously considered not to produce PFP. Twelve of the 13 CTL lines also proved to possess the mRNA for SE-1. One thymoma cell line, TIMI.4, did not express mRNA for PFP, although it expressed mRNA for SE-1 and SE-2.

Anti-idiotypic antibodies derived against C8, C9 and perforin bind homologous restriction factor

A pore-forming protein (PFP/perforin/cytolysin), stored in the cytoplasmic granules of cytolytic lymphocytes, lyses a variety of target cells but not the cytolytic lymphocytes. In the complement (C) system, a C8-binding protein (C8bp) or homologous restriction factor (HRF) has been described that protects cells against lysis mediated by homologous C. C8bp/HRF is known to bind to C8 and C9 and has also been suggested to protect lymphocytes against perforin-mediated lysis. Here, using an anti-idiotypic antibody approach, several polyclonal antisera were raised against IgGs that are specific for mouse perforin, and human C8 and C9. These anti-idiotypic antisera were shown to react against an overlapping epitope(s) on C8bp/HRF as indicated by the following evidence: (i) all three types of antisera reacted against partially purified C8bp/HRF and against a 65 kDa protein band in cell lysates; reactivity was only observed against disulfide-reduced antigens; (ii) the three antibodies react with a protein band in normal erythrocytes (E) but not with type III E of patients with paroxysmal nocturnal hemoglobinuria or with a mutant B lymphoblastoid cell line, both of which cell types are known to be deficient in C8bp/HRF; and (iii) the three antibodies compete with each other for binding to C8bp/HRF. Type III E and the C8bp/HRF-deficient mutant lymphoblastoid cell line, however, are as susceptible to perforin-mediated lysis as type I E and wild-type lymphoblastoid cell line, respectively, indicating that C8bp/HRF does not play a role in protecting cells against perforin-mediated lysis. These paradoxical findings suggest that perforin may share with C8 and C9 the same domain(s) that bind to C8bp/HRF and yet, unlike C8 and C9, perforin is not inactivated by this type of putative interaction. Since C8 and C9 are now readily available, the anti-idiotypic approach described here provides a convenient protocol for production of antisera specific for C8bp/HRF.

Characterization of the inhibitory effect of lysolipids on perforin-mediated hemolysis

The ability of lysolipids to inhibit the lytic activity of perforin from cytotoxic T lymphocytes was investigated. Sublytic concentrations of various lysolipids were incorporated into the membranes of sheep red blood cells (RBC) and the cells were then lysed with purified perforin. Lysophosphatidylcholines (lysoPC) can effectively block perforin-mediated lysis at micromolar concentrations. This is in marked contrast to phosphorylcholine, the putative calcium-dependent receptor for perforin, which inhibits lysis only at greater than or equal to millimolar concentrations. Unlike the inhibitory action of lipids, the lysolipids do not show a strict dependence on headgroup composition, as lysophosphatidylserine (lysoPS) is just as effective as lysoPC. All the lysoPC tested, ranging from lysolauroyl PC to lysostearoyl PC, are good inhibitors, with lysomyristoyl PC being the most effective. Binding of lysoPC to RBC is reversible; the inhibition by lysoPC can be removed with bovine serum albumin (BSA), and washing RBC that had been pretreated with lysoPC leads to a loss of inhibition. Binding of perforin to membranes is temperature-independent and precedes a temperature-dependent, insertion/pore-formation stage; hemolysis experiments that take advantage of this fact indicate that lysoPC acts mostly by blocking perforin binding to the RBC membranes.