Inactivation and proteolytic degradation of perforin within lytic granules upon neutralization of acidic pH

Small Molecule Inhibitors of Lymphocyte Perforin as Focused Immunosuppressants for Infection and Autoimmunity

New drugs that precisely target the immune mechanisms critical for cytotoxic T lymphocyte (CTL) and natural killer (NK) cell driven pathologies are desperately needed. In this perspective, we explore the cytolytic protein perforin as a target for therapeutic intervention. Perforin plays an indispensable role in CTL/NK killing and controls a range of immune pathologies, while being encoded by a single copy gene with no redundancy of function. An immunosuppressant targeting this protein would provide the first-ever therapy focused specifically on one of the principal cell death pathways contributing to allotransplant rejection and underpinning multiple autoimmune and postinfectious diseases. No drugs that selectively block perforin-dependent cell death are currently in clinical use, so this perspective will review published novel small molecule inhibitors, concluding with in vivo proof-of-concept experiments performed in mouse models of perforin-mediated immune pathologies that provide a potential pathway toward a clinically useful therapeutic agent.

Locked and Loaded: Mechanisms Regulating Natural Killer Cell Lytic Granule Biogenesis and Release

NK cell-mediated cytotoxicity is a critical element of our immune system required for protection from microbial infections and cancer. NK cells bind to and eliminate infected or cancerous cells via direct secretion of cytotoxic molecules toward the bound target cells. In this review, we summarize the current understanding of the molecular regulations of NK cell cytotoxicity, focusing on lytic granule development and degranulation processes. NK cells synthesize apoptosis-inducing proteins and package them into specialized organelles known as lytic granules (LGs). Upon activation of NK cells, LGs converge with the microtubule organizing center through dynein-dependent movement along microtubules, ultimately polarizing to the cytotoxic synapse where they subsequently fuse with the NK plasma membrane. From LGs biogenesis to degranulation, NK cells utilize several strategies to protect themselves from their own cytotoxic molecules. Additionally, molecular pathways that enable NK cells to perform serial killing are beginning to be elucidated. These advances in the understanding of the molecular pathways behind NK cell cytotoxicity will be important to not only improve current NK cell-based anti-cancer therapies but also to support the discovery of additional therapeutic opportunities.

NK cells eliminate Epstein-Barr virus bound to B cells through a specific antibody-mediated uptake

Epstein Barr virus (EBV) causes a highly prevalent and lifelong infection contributing to the development of some malignancies. In addition to the key role played by T cells in controlling this pathogen, NK cells mediate cytotoxicity and IFNγ production in response to EBV-infected B cells in lytic cycle, both directly and through antibody (Ab)-dependent activation. We recently described that EBV-specific Ab-dependent NK cell interaction with viral particles (VP) bound to B cells triggered degranulation and TNFα secretion but not B cell lysis nor IFNγ production. In this report we show that NK cell activation under these conditions reduced B cell transformation by EBV. NK cells eliminated VP from the surface of B cells through a specific and active process which required tyrosine kinase activation, actin polymerization and Ca2+, being independent of proteolysis and perforin. VP were displayed at the NK cell surface before being internalized and partially shuttled to early endosomes and lysosomes. VP transfer was encompassed by a trogocytosis process including the EBV receptor CD21, together with CD19 and CD20. Our study reveals a novel facet of the antibody-dependent NK cell mediated response to this viral infection.

Epstein-Barr virus (EBV) is a member of the herpesvirus family which causes a frequent and lifelong infection. The immune system is unable to fully eliminate the virus, which remains dormant in infected B lymphocytes. EBV reactivation leads to the production of new infective particles, spreading to other cells and favoring its transmission. EBV infection goes generally unnoticed in healthy individuals, though it may occasionally cause a disease termed Infectious Mononucleosis, as well as severe disorders in patients with a defective immune response. Remarkably, EBV has oncogenic potential contributing to the development of some tumors, and has been associated to autoimmune diseases. T lymphocytes and Natural Killer (NK) cells play an essential role in the defense against EBV, killing infected cells when the virus reactivates. Antiviral NK cell functions may be also triggered by antibodies (Ab) recognizing infected cells. In this report we provide the first evidence supporting that NK cells in combination with anti-EBV Ab are able to eliminate the virus attached to the surface of B cells, reducing their infection without killing them.

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

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

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

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

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

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

Role of V-ATPase a3-Subunit in Mouse CTL Function

CTLs release cytotoxic proteins such as granzymes and perforin through fusion of cytotoxic granules (CG) at the target cell interface, the immune synapse, to kill virus-infected and tumorigenic target cells. A characteristic feature of these granules is their acidic pH inside the granule lumen, which is required to process precursors of granzymes and perforin to their mature form. However, the role of acidic pH in CG maturation, transport, and fusion is not understood. We demonstrate in primary murine CTLs that the a3-subunit of the vacuolar-type (H⁺)-adenosine triphosphatase is required for establishing a luminal pH of 6.1 inside CG using ClopHensorN(Q69M), a newly generated CG-specific pH indicator. Knockdown of the a3-subunit resulted in a significantly reduced killing of target cells and a >50% reduction in CG fusion in total internal reflection fluorescence microscopy, which was caused by a reduced number of CG at the immune synapse. Superresolution microscopy revealed a reduced interaction of CG with the microtubule network upon a3-subunit knockdown. Finally, we find by electron and structured illumination microscopy that knockdown of the a3-subunit altered the diameter and density of individual CG, whereas the number of CG per CTL was unaffected. We conclude that the a3-subunit of the vacuolar adenosine triphosphatase is not only responsible for the acidification of CG, but also contributes to the maturation and efficient transport of the CG to the immune synapse.

Perforin: a key pore-forming protein for immune control of viruses and cancer

Perforin (PFN) is the key pore-forming molecule in the cytotoxic granules of immune killer cells. Expressed only in killer cells, PFN is the rate-limiting molecule for cytotoxic function, delivering the death-inducing granule serine proteases (granzymes) into target cells marked for immune elimination. In this chapter we describe our current understanding of how PFN accomplishes this task. We discuss where PFN is expressed and how its expression is regulated, the biogenesis and storage of PFN in killer cells and how they are protected from potential damage, how it is released, how it delivers Granzymes into target cells and the consequences of PFN deficiency.

Recombinant CD95-Fc (APG101) prevents graft-versus-host disease in mice without disabling antitumor cytotoxicity and T-cell functions

Graft-versus-host disease (GVHD) induced by transplant-derived T cells represents a major complication after allogeneic bone marrow transplantation (BMT). However, these T cells support engraftment, early T-cell immunity, and mediate the graft-versus-tumor (GVT) effect. Cytotoxic effector functions by transplanted T cells are predominantly mediated by the perforin/granzyme and the CD95/CD95L system. APG101, a novel recombinant human fusion protein consisting of the extracellular domain of CD95 and the Fc domain of an IgG1 antibody inhibited CD95L-induced apoptosis without interfering with T-cell function in vitro and was therefore tested for its ability to prevent GVHD in murine BMT models across minor or major histocompatibility barriers. Starting APG101 treatment either 1 day before or 6 days after transplantation effectively reduced clinical GVHD and rescued survival between 60% and 100% if GVHD was CD95L mediated. APG101 did not interfere with the GVT effect, because P815 mastocytoma and most importantly primary Bcr-Abl-transformed B-cell leukemias were completely eradicated by the alloantigen-specific T cells. Phenotype and homing of alloantigen-specific T cells or their perforin/granzyme-mediated cytotoxicity and proliferative capacity were not affected by APG101 treatment suggesting that APG101 therapy might be useful in GVHD prophylaxis without impairing T-cell function and most importantly preserving GVT activity.

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

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

Flow cytometric detection of perforin upregulation in human CD8 T cells

Perforin and granzymes work synergistically to induce apoptosis in target cells recognized by cytotoxic T lymphocytes. While perforin is readily detectable by flow cytometry in resting CD8 T cells, upregulation of perforin in activated cells is thought to require proliferation. However, perforin undergoes numerous conformational changes during its maturation, which may affect the ability of conventional antibodies to recognize newly synthesized perforin. Polychromatic flow cytometry was used to detect perforin and cytokine production following stimulation of ex vivo human CD8 T cells. Two different anti-perforin antibodies, clones B-D48 and deltaG9, were used to discriminate various forms of perforin after cellular activation. We provide evidence for the rapid upregulation of perforin protein, which may contribute to the ability of CD8 T cells to kill multiple targets over time. The deltaG9 clone recognizes the granule-associated conformation of perforin, while the B-D48 clone is able to detect perforin in multiple forms. Finally, we show there is variability in the ability of CD8 T cells to upregulate perforin. Human CD8 T cells are capable of new perforin production immediately following activation. This work defines a novel flow cytometric procedure that can be used to more completely assess the cytotoxic capacity of human CD8 T cells.

ECH, an Epoxycyclohexenone Derivative That Specifically Inhibits Fas Ligand-Dependent Apoptosis in CTL-Mediated Cytotoxicity

CTL eliminate cells infected with intracellular pathogens and tumor cells by two distinct mechanisms mediated by Fas ligand (FasL) and lytic granules that contain perforin and granzymes. In this study we show that an epoxycyclohexenone derivative,(2R,3R,4S)-2,3-epoxy-4-hydroxy-5-hydroxymethyl-6-(1E)-propenyl-cyclohex-5-en-1-one (ECH) specifically inhibits the FasL-dependent killing pathway in CTL-mediated cytotoxicity. Recently, we have reported that ECH blocks activation of procaspase-8 in the death-inducing signaling complex and thereby prevents apoptosis induced by anti-Fas Ab or soluble FasL. Consistent with this finding, ECH profoundly inhibited Fas-mediated DNA fragmentation and cytolysis of target cells induced by perforin-negative mouse CD4+ CTL and alloantigen-specific mouse CD8+ CTL pretreated with an inhibitor of vacuolar type H+-ATPase concanamycin A that selectively induces inactivation and proteolytic degradation of perforin in lytic granules. However, ECH barely influenced perforin/granzyme-dependent DNA fragmentation and cytolysis of target cells mediated by alloantigen-specific mouse CD8+ CTL. The components of lytic granules and the granule exocytosis pathway upon CD3 stimulation were also insensitive to ECH. In conclusion, our present results demonstrate that ECH is a specific nonpeptide inhibitor of FasL-dependent apoptosis in CTL-mediated cytotoxicity. Therefore, ECH can be used as a bioprobe to evaluate the contributions of two distinct killing pathways in various CTL-target settings.

Cytolytic CD4(+)-T-cell clones reactive to EBNA1 inhibit Epstein-Barr virus-induced B-cell proliferation

In the absence of immune surveillance, Epstein-Barr virus (EBV)-infected B cells generate neoplasms in vivo and transformed cell lines in vitro. In an in vitro system which modeled the first steps of in vivo immune control over posttransplant lymphoproliferative disease and lymphomas, our investigators previously demonstrated that memory CD4(+) T cells reactive to EBV were necessary and sufficient to prevent proliferation of B cells newly infected by EBV (S. Nikiforow et al., J. Virol. 75:3740-3752, 2001). Here, we show that three CD4(+)-T-cell clones reactive to the latent EBV antigen EBNA1 also prevent the proliferation of newly infected B cells from major histocompatibility complex (MHC) class II-matched donors, a crucial first step in the transformation process. EBNA1-reactive T-cell clones recognized B cells as early as 4 days after EBV infection through an HLA-DR-restricted interaction. They secreted Th1-type and Th2-type cytokines and lysed EBV-transformed established lymphoblastoid cell lines via a Fas/Fas ligand-dependent mechanism. Once specifically activated, they also caused bystander regression and bystander killing of non-MHC-matched EBV-infected B cells. Since EBNA1 is recognized by CD4(+) T cells from nearly all EBV-seropositive individuals and evades detection by CD8(+) T cells, EBNA1-reactive CD4(+) T cells may control de novo expansion of B cells following EBV infection in vivo. Thus, EBNA1-reactive CD4(+)-T-cell clones may find use as adoptive immunotherapy against EBV-related lymphoproliferative disease and many other EBV-associated tumors.

Single-cell perforin and granzyme expression reveals the anatomical localization of effector CD8+ T cells in influenza virus-infected mice

Influenza virus infection activates cytolytic T lymphocytes (CTL) that contribute to viral clearance by releasing perforin and granzymes from cytoplasmic granules. Virus-specific, perforin-dependent CD8(+) CTL were detected in freshly isolated cells from the mouse lung parenchyma but not from the mediastinal lymph nodes (MLN), where they are primed, or from the spleen during primary influenza virus infection. To determine whether this difference was due to the low frequency or incomplete maturation of effector CTL in MLN, we measured expression of perforin, granzymes A, B, and C, and IFN-gamma mRNAs in CD8(+) populations and single cells immediately after isolation from virus-infected mice. Quantitative PCR revealed significant expression of perforin, granzyme A, granzyme B, and IFN-gamma in activated CD8(+) cells from MLN, spleen, and lung parenchyma. Granzyme C expression was not detected. Individual activated or nucleoprotein peptide/class I tetramer-binding CD8(+) cells from the three tissues expressed diverse combinations of perforin, granzyme, and IFN-gamma mRNAs. Although cells from lung expressed granzymes A and B at higher frequency, each of the tissues contained cells that coexpressed perforin with granzymes A and/or B. The main difference between MLN and lung was the elevated frequency of activated CD8(+) T cells in the lung, rather than their perforin/granzyme expression profile. The data suggest that some CTL mature into perforin/granzyme-expressing effector cells in MLN but reach detectable frequencies only when they accumulate in the infected lung.

Involvement of CD27/CD70 interactions in antigen-specific cytotoxic T-lymphocyte (CTL) activity by perforin-mediated cytotoxicity

CD27 molecules are shown to be essential in the regulation of the death, activation and differentiation of T and B cells. However, the influence of CD27 on cytotoxic T-cell function remains obscure. Autologous EBV transformed B-cell lines (LCL), which highly express CD27 ligand CD70, here stimulated T cells and induced the cytotoxic T-lymphocyte (CTL) activity via T-cell antigen receptors (TCR). The cytotoxicity against LCL was diminished when anti-CD70 blocking MoAb was added initially in the culture. Resting T cells killed more CD70-transfected P815 cells than wild type P815 cells in the presence of anti-CD3 MoAb as measured by a 4-h 51Cr release assay, and the cytotoxicity of both of the cell populations completely disappeared in the presence of concanamycin A (CMA). The expression of the perforin by the LCL-induced CTL in the presence of anti-CD70 blocking MoAb was diminished as compared with that without the blockage of CD27/CD70 interactions. The CTL induced by LCL did not kill Fas-transfected WR cells. CD27 signalling in the T cells did not affect Fas ligand (FasL) mRNA expression, LAK activity and IFN-gamma synthesis in humans. Our data demonstrate that CD27/CD70 interactions enhance the cytotoxicity of CTL in the induction phase through enhancement of killing activity induced via the perforin-dependent mechanism, but not via the Fas/FasL system.

Lymphocyte-mediated cytotoxicity

Virtually all of the measurable cell-mediated cytotoxicity delivered by cytotoxic T lymphocytes and natural killer cells comes from either the granule exocytosis pathway or the Fas pathway. The granule exocytosis pathway utilizes perforin to traffic the granzymes to appropriate locations in target cells, where they cleave critical substrates that initiate DNA fragmentation and apoptosis; granzymes A and B induce death via alternate, nonoverlapping pathways. The Fas/FasL system is responsible for activation-induced cell death but also plays an important role in lymphocyte-mediated killing under certain circumstances. The interplay between these two cytotoxic systems provides opportunities for therapeutic interventions to control autoimmune diseases and graft vs. host disease, but oversuppression of these pathways may also lead to increased viral susceptibility and/or decreased tumor cell killing.

The genes for perforin, granzymes A-C and IFN-gamma are differentially expressed in single CD8(+) T cells during primary activation

Here we show that the genes for perforin, the three major T cell granzymes (A-C) and IFN-gamma are differentially expressed during primary activation of naive CD8(+) T cells, kinetically and at the single-cell level. When CD44(low)CD62L(high)CD8(+) lymph node T cells were activated with IL-2 and immobilized antibodies to CD3, CD8 and CD11a, expression of perforin, granzyme B and IFN-gamma mRNAs was induced by day 2, and increased in parallel with perforin-dependent cytolytic activity. Granzyme C and A transcripts were not detected until 1 and 3 days later respectively. Single-cell PCR showed that expression frequencies rose in parallel with total levels of each mRNA, but that individual cells expressed diverse combinations of perforin, granzyme A-C and IFN-gamma mRNAs. These expression patterns indicated that the delayed expression of granzymes A and C was not due to late activation of distinct cell subpopulations. Statistical analysis of the data suggested that each gene was differentially regulated at the single-cell level. Individual naive CD8(+) T cells gave rise over 7 days to clones that expressed all five products at the clonal level, but also expressed diverse combinations at the single-cell level. We conclude that, during primary activation, CD8(+) T cells progressively acquired the ability to express most or all of these genes, and that the variable expression patterns observed among single cells within clones and populations reflected transient rather than heritable differences in expression profile.

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

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

Heterogeneity of channel catfish CTL with respect to target recognition and cytotoxic mechanisms employed

Two types of catfish alloantigen-dependent cytotoxic T cells were cloned from PBL from a fish immunized in vivo and stimulated in vitro with the allogeneic B cell line 3B11. Because these are the first clonal cytotoxic T cell lines derived from an ectothermic vertebrate, studies were undertaken to characterize their recognition and cytotoxic mechanisms. The first type of CTL (group I) shows strict alloantigen specificity, i.e., they specifically kill and proliferate only in response to 3B11 cells. The second type (group II) shows broad allogeneic specificity, i.e., they kill and proliferate in response to several different allogeneic cells in addition to 3B11. "Cold" target-inhibition studies suggest that group II CTL recognize their targets via a single receptor, because the killing of one allotarget can be inhibited by a different allotarget. Both types of catfish CTL form conjugates with and kill targets by apoptosis. Killing by Ag-specific cytotoxic T cells (group I) was completely inhibited by treatment with EGTA or concanamycin A, and this killing is sensitive to PMSF inhibition, suggesting that killing was mediated exclusively by the secretory perforin/granzyme mechanism. In contrast, killing by the broadly specific T cytotoxic cells (group II) was only partially inhibited by either EGTA or concanamycin A, suggesting that these cells use a cytotoxic mechanism in addition to that involving perforin/granzyme. Consistent with the presumed use of a secretory pathway, both groups of CTL possess putative lytic granules. These results suggest that catfish CTL show heterogeneity with respect to target recognition and cytotoxic mechanisms.

Synthesis and biological evaluation of nonylprodigiosin and macrocyclic prodigiosin analogues

Nonylprodigiosin (4) and various of its analogues have been prepared by Suzuki cross-coupling reactions of a well accessible pyrrolyl triflate with (hetero)aryl boronic acid derivatives bearing alkenyl side chains. The resulting alkenes or dienes were subjected to metathesis dimerization or ring-closing metathesis (RCM) reactions, respectively, by using a ruthenium indenylidene complex as the catalyst. The biological activity of the products thus obtained was tested in two different assays monitoring i) the proliferation of murine spleen cells induced by lipopolysaccharides (LPS) and concanavalin A (Con A), and ii) the vacuolar acidification of baby hamster kidney (BHK) cells. Compounds 4 and 21 suppressed Con A-induced T-cell proliferation much more potently than LPS-induced B-cell proliferation. Furthermore, compounds 4 and 26 markedly inhibited vacuolar acidification, although other compounds exhibited no or only marginal effects. Thus, the immunosuppressive activity of prodigiosins toward T-cell proliferation seems to be mediated through cellular targets distinct from vacuolar acidification, and the prodigiosin analogues might be powerful tools to dissect these biological responses. The X-ray crystal structure of the macrocyclic product 25 has been determined, showing that the replacement of one pyrrole ring of the parent compound 4 by a phenyl group does not alter the overall electronic features of the remaining heterocyclic ring system of these alkaloids.

FD-891, a structural analogue of concanamycin A that does not affect vacuolar acidification or perforin activity, yet potently prevents cytotoxic T lymphocyte-mediated cytotoxicity through the blockage of conjugate formation

FD-891 belongs to a group of 18-membered macrolides, and is a structural analogue of a specific inhibitor of vacuolar type H+-ATPase, concanamycin A (CMA). In our previous work, we have shown that CMA specifically inhibits perforin-dependent cytotoxic T lymphocyte (CTL)-mediated cytotoxicity through the degradation and inactivation of perforin, although CMA does not affect Fas ligand (FasL)-dependent cytotoxicity. Here, we show that FD-891 potently prevents not only perforin-dependent but also FasL-dependent CTL-mediated killing pathways by blocking CTL-target conjugate formation. In contrast to CMA, FD-891 was unable to inhibit vacuolar acidification and only slightly decreased the perforin activity in lytic granules. FD-891 blocked granule exocytosis in response to anti-CD3, mainly owing to the lack of CTL binding to immobilized anti-CD3. The conjugate formation was markedly inhibited only when effector cells were pretreated with FD-891. Consistent with these observations, fluorescence-activated cell sorter (FACS) analysis for cell surface receptors revealed that FD-891 significantly reduced the expression of the T-cell receptor (TCR)/CD3 complex. These data suggest that the blockage of conjugate formation and subsequent target cell killing might be at least partly owing to FD-891-induced down-regulation of the TCR/CD3 complex.

Involvement of FK506-sensitive and insensitive granule exocytosis pathways in perforin-dependent target cell lysis mediated by a CD8+ CTL clone

The release of granzyme A and B through granule exocytosis by CD8+ CTL clone OE4 upon T cell receptor (TCR) activation was blocked by FK506 in a dose-dependent manner (IC50 = 3 nM), whereas a significant granzyme release was still detectable even in the presence of excess FK506. In contrast, the production of IFN-gamma was highly sensitive to FK506 (IC50 = 0.01 nM) and could be completely blocked by FK506. Both FK506-sensitive and insensitive granule exocytosis pathways were involved in the actual perforin-dependent killing toward different target cells. The combination of ionomycin and phorbol ester was able to mimic TCR stimulation to induce IFN-gamma production, although the same treatment triggered granule exocytosis inefficiently. Granule exocytosis and IFN-gamma production following TCR activation were profoundly prevented by calphostin C. Thus, these results demonstrate that the granule exocytosis pathway in this CD8+ CTL clone depends on the activation of protein kinase C, and requires either calcineurin-dependent or independent additional signals downstream of TCR activation.

In vivo rapid reduction of alloantigen-activated CD8+ mature cytotoxic T cells by inhibitors of acidification of intracellular organelles, prodigiosin 25-C and concanamycin B

Prodigiosin (PrG) 25-C and concanamycin B (CMB) are immunosuppressants that specifically inhibit the induction of cytotoxic T cells (CTL) without affecting the function of B cells and helper T cells in vivo. Both compounds inhibit acidification of intracellular organelles and induce destruction of cytotoxic granules and degradation of perforin in vitro. Here we show that a single intraperitoneal (i.p.) injection of PrG 25-C, and of CMB, into mice eliminates cytotoxic activity 7 days after alloantigen stimulation (when mature CTL activity has been detected in control mice), with minimal effect on the alloantigen-specific antibody titre in serum. FK506 did not suppress the cytotoxic activity with this administration schedule. Suppression was accompanied by a decrease in the CD8+ population and in perforin expression of spleen cells induced by alloantigen stimulation. The suppression of CTL activity and decrease in CD8+ cell number was detected as early as 7 hr after the injection of compounds. These results suggest that inhibitors of acidification of intracellular organelles suppress CTL activity in vivo by reducing the number of mature CD8+ CTL.

Ultrastructural and functional studies of the interaction between IL-12 and IL-2 for the generation of lymphokine-activated killer cells

IL-12 promotes generation of LAK activity in short-term-cultured NK cells, but information on the structure and function of IL-12-induced LAK cells is not yet available. The latter issues have been here investigated with emphasis on interactions between IL-12 and IL-2. Peripheral blood mononuclear cells (MNC) exposed to IL-12 for 5-7 days displayed a decrease in the amount and density of the matrix of large granular lymphocyte (LGL)-associated granules. In cells cultured with IL-12 and IL-2 for 5-7 days, empty vacuoles were predominant and the electron-dense matrix was scanty. In MNC incubated with IL-2 for 5-7 days, most granules were loaded with electron-dense matrix. IL-12 and IL-2 displayed an additive effect on LAK cell cytotoxicity until approximately 48 h in culture which was followed by a sharp decline. Immunocytochemical and biochemical studies demonstrated that MNC cultured for 5-7 days with IL-12 and IL-2 displayed downregulated perforin expression and upregulated granzyme B expression. Fas ligand expression was virtually undetectable in MNC cultured for 5-7 days with or without cytokines. It appears that perforin downregulation plays a major role in the reduced cytotoxicity of MNC cultured with IL-12 and IL-2 for 5-7 days.

Functional differences between memory and naive CD8 T cells

To determine how murine memory and naive T cells differ, we generated large numbers of long-lived memory CD8(+) T cells and compared them to naive cells expressing the same antigen-specific receptor (T cell receptor; TCR). Although both populations expressed similar levels of TCR and CD8, on antigen stimulation in vitro memory T cells down-regulated their TCR faster and more extensively and secreted IFN-gamma and IL-2 faster than naive T cells. Memory cells were also larger, and when freshly isolated from mice they contained perforin and killed target cells without having to be restimulated. They further differed from naive cells in requiring IL-15 for proliferation and in having a greater tendency to undergo apoptosis in vitro. On antigen stimulation in vivo, however, they proliferated more rapidly than naive cells. These findings suggest that, unlike naive T cells, CD8 memory T cells are intrinsically programmed to rapidly express their effector functions in vivo without having to undergo clonal expansion and differentiation.

Lymphokine-activated killer cells induced in vivo in mice showing IL-2 toxicity have cytoplasmic granules containing perforin and its hemolytic activity

Cytoplasmic granules of LAK cells isolated from mice showing side effects of recombinant human IL-2 (rhIL-2) display BLT-serine esterase activity and calcium-dependent hemolytic activity and seem to be involved in the mechanisms of LAK cell-mediated rhIL-2 toxicity. In this report, the hemolytically active substance was partially purified from the LAK granules and shown to be mouse perforin. Mice were implanted with miniosmotic pumps filled with a dose of rhIL-2 that induced known toxicity. The LAK granules were isolated from LAK cell-rich subcutaneous tissue around the infusion sites. The hemolytically active substance in the granule preparation was solubilized by 2 M NaCl and passed through a butyl Sepharose column and then a DEAE Toyopearl column. In immunoblotting, the hemolytically active fractions reacted with an anti-mouse perforin mAb and the reaction depended on the activity. The molecular sizes of the perforin-positive bands were 66 kDa and 51 kDa under reducing and nonreducing conditions, respectively. These results confirmed the existence of mouse perforin and its hemolytic activity in the LAK granules of rhIL-2-treated mice, as has been shown for granules of cytotoxic T or NK cell clones in vitro.