Lymphocyte granule-mediated apoptosis: matters of viral mimicry and deadly proteases

NK cell-derived extracellular granzyme B drives epithelial ulceration during HSV-2 genital infection

Genital herpes is characterized by recurrent episodes of epithelial blistering. The mechanisms causing this pathology are ill defined. Using a mouse model of vaginal herpes simplex virus 2 (HSV-2) infection, we show that interleukin-18 (IL-18) acts upon natural killer (NK) cells to promote accumulation of the serine protease granzyme B in the vagina, coinciding with vaginal epithelial ulceration. Genetic loss of granzyme B or therapeutic inhibition by a specific protease inhibitor reduces disease and restores epithelial integrity without altering viral control. Distinct effects of granzyme B and perforin deficiency on pathology indicates that granzyme B acts independent of its classic cytotoxic role. IL-18 and granzyme B are markedly elevated in human herpetic ulcers compared with non-herpetic ulcers, suggesting engagement of these pathways in HSV-infected patients. Our study reveals a role for granzyme B in destructing mucosal epithelium during HSV-2 infection, identifying a therapeutic target to augment treatment of genital herpes.

Natural Products of Marine Origin for the Treatment of Colorectal and Pancreatic Cancers: Mechanisms and Potential

Gastrointestinal cancer refers to malignancy of the accessory organs of digestion, and it includes colorectal cancer (CRC) and pancreatic cancer (PC). Worldwide, CRC is the second most common cancer among women and the third most common among men. PC has a poor prognosis and high mortality, with 5-year relative survival of approximately 11.5%. Conventional chemotherapy treatments for these cancers are limited due to severe side effects and the development of drug resistance. Therefore, there is an urgent need to develop new and safe drugs for effective treatment of PC and CRC. Historically, natural sources—plants in particular—have played a dominant role in traditional medicine used to treat a wide spectrum of diseases. In recent decades, marine natural products (MNPs) have shown great potential as drugs, but drug leads for treating various types of cancer, including CRC and PC, are scarce. To date, marine-based drugs have been used against leukemia, metastatic breast cancer, soft tissue sarcoma, and ovarian cancer. In this review, we summarized existing studies describing MNPs that were found to have an effect on CRC and PC, and we discussed the potential mechanisms of action of MNPs as well as future prospects for their use in treating these cancers.

Oxidative and Non-Oxidative Antimicrobial Activities of the Granzymes

Cell-mediated cytotoxicity is an essential immune defense mechanism to fight against viral, bacterial or parasitic infections. Upon recognition of an infected target cell, killer lymphocytes form an immunological synapse to release the content of their cytotoxic granules. Cytotoxic granules of humans contain two membrane-disrupting proteins, perforin and granulysin, as well as a homologous family of five death-inducing serine proteases, the granzymes. The granzymes, after delivery into infected host cells by the membrane disrupting proteins, may contribute to the clearance of microbial pathogens through different mechanisms. The granzymes can induce host cell apoptosis, which deprives intracellular pathogens of their protective niche, therefore limiting their replication. However, many obligate intracellular pathogens have evolved mechanisms to inhibit programed cells death. To overcome these limitations, the granzymes can exert non-cytolytic antimicrobial activities by directly degrading microbial substrates or hijacked host proteins crucial for the replication or survival of the pathogens. The granzymes may also attack factors that mediate microbial virulence, therefore directly affecting their pathogenicity. Many mechanisms applied by the granzymes to eliminate infected cells and microbial pathogens rely on the induction of reactive oxygen species. These reactive oxygen species may be directly cytotoxic or enhance death programs triggered by the granzymes. Here, in the light of the latest advances, we review the antimicrobial activities of the granzymes in regards to their cytolytic and non-cytolytic activities to inhibit pathogen replication and invasion. We also discuss how reactive oxygen species contribute to the various antimicrobial mechanisms exerted by the granzymes.

Potentiation of natural killer (NK) cell activity by methanol extract of cultured cambial meristematic cells of wild ginseng and its mechanism

AIMS

As an alternative strategy to obtain large amounts of ginseng extract with high yield of ginsenosides, we have utilized culture of cambial meristematic cells (CMCs) from wild ginseng. The anti-tumor effects of methanol extract of ginseng CMCs (MEGC) and their action mechanisms were investigated.

MAIN METHODS

Mice were intraperitoneally administered with MEGC, and we explored NK cell activity, suppression of in vivo growth of tumor cells and relevant molecule expression.

KEY FINDINGS

MEGC significantly potentiated NK cell activity and suppressed in vivo growth of B16 melanoma cells. However, we observed no increase in NK cell number and unaltered expression of NK cell-activating (NKG2D) and inhibitory (Ly49, CD94/NKG2A) receptors as well as NK cell activation markers (CD25, CD69, CD119, and CD212) in MEGC-treated group compared to the controls. Instead, MEGC significantly enhanced IL-2 responsiveness in the early effector phase and the constitutive expression of granzyme B.

SIGNIFICANCE

Our data indicate that culture of CMCs is an attractive alternative method for sustainable production of ginseng extracts and clinical use. In addition, we have unraveled a novel mechanism underlying the potentiation of NK cell activity and antitumor effect of ginseng extract, in which it upregulates the constitutive expression of cytotoxic mediator(s) and IL-2 responsiveness.

RNA Viruses: ROS-Mediated Cell Death

Reactive oxygen species (ROS) are well known for being both beneficial and deleterious. The main thrust of this review is to investigate the role of ROS in ribonucleic acid (RNA) virus pathogenesis. Much evidences has accumulated over the past decade, suggesting that patients infected with RNA viruses are under chronic oxidative stress. Changes to the body's antioxidant defense system, in relation to SOD, ascorbic acid, selenium, carotenoids, and glutathione, have been reported in various tissues of RNA-virus infected patients. This review focuses on RNA viruses and retroviruses, giving particular attention to the human influenza virus, Hepatitis c virus (HCV), human immunodeficiency virus (HIV), and the aquatic Betanodavirus. Oxidative stress via RNA virus infections can contribute to several aspects of viral disease pathogenesis including apoptosis, loss of immune function, viral replication, inflammatory response, and loss of body weight. We focus on how ROS production is correlated with host cell death. Moreover, ROS may play an important role as a signal molecule in the regulation of viral replication and organelle function, potentially providing new insights in the prevention and treatment of RNA viruses and retrovirus infections.

Effects of quercetin on exercise potential and exercise-induced cytokines in the horse

The goal of this experiment was to test the hypothesis that quercetin would alter markers of athletic performance as well as markers of exercise-induced inflammation. Six healthy, unfit Standardbred mares (∼500 kg, age 4-5 years) were assigned to one of two treatments (quercetin or water) in an experiment conducted in a crossover fashion. The horses were dosed via a nasogastric tube twice a day (08:00 and 20:00) with either 6 g of quercetin in 2 l tap water or 2 l water for 3.5 d. Two hours after the seventh and final dose, a graded exercise test (GXT) was run. During the test the treadmill remained at a fixed 6% grade. At t=0 the treadmill was started at a speed of 4 m/s. Each following minute the treadmill speed was increased by 1 m/s, up to a maximum speed of 11 m/s. Blood and muscle samples were collected before the initial dosing, before exercise and after the GXT to measure markers of inflammation. There was a significant (P<0.10) increase in run time to fatigue when the horses were dosed with quercetin. The VO2recovery time was shortened significantly when compared to water-treated trials. There were exercise-induced increases (P<0.10) in haematocrit and the plasma concentrations of total protein, glucose, and lactate as well as expression of mRNA for TNF-α, IL-1, interferon-γ, granzyme-B (GrB) in blood. However, quercetin only affected (P<0.10) the expression of mRNA for GrB. Intramuscular levels of IL-1 and GrB also increased significantly with exercise, but there was no effect (P≯0.10) from quercetin treatment. This study provides evidence that quercetin could be useful in enhancing exercise performance, although the mechanism for this enhancement is unclear.

Imaging of lytic granule exocytosis in CD8+ cytotoxic T lymphocytes reveals a modified form of full fusion

Here we imaged the exocytosis of lytic granules from human CD8(+) cytotoxic T lymphocytes using rapid total internal reflection microscopy, Lamp-1 tagged with mGFP to follow the fate of the lytic granule membrane, and granzyme A, granzyme B or serglycin tagged with mRFP to follow the fate of lytic granule cargo. Lytic granules were released by full fusion with the plasma membrane, such that the entire granule content for all three cargos visualized was released on a subsecond time scale. The behavior of GFP-Lamp-1 was, however, more complex. While it entered the plasma membrane in all cases, the extent to which it then diffused away from the site of exocytosis varied from nearly complete to highly restricted. Finally, the diffusion properties upon release of the three cargos examined put an upper limit on the size of the macromolecular complex of granzyme and serglycin that is presented to the target cell.

Extranodal NK/T-cell lymphoma: toward the identification of clinical molecular targets

Extranodal natural killer (NK)/T-cell lymphoma of nasal type (NKTCL) is a malignant disorder of cytotoxic lymphocytes of NK or more rarely T cells associated with clonal Epstein-Barr virus infection. Extranodal NKTCL is rare in Western countries, but in Asia and Central and South America it can account for up to 10% of non-Hodgkin's lymphomas. It is an aggressive neoplasm with very poor prognosis. Although the pathogenesis of extranodal NKTCL remains poorly understood, some insights have been gained in the recent years, especially from genome-wide studies. Based on our own experience and knowledge of the literature, we here review some of the genomic and functional pathway alterations observed in NKTCL that could provide a rationale for the development of innovative therapeutic strategies.

Caspase-3 mediated feedback activation of apical caspases in doxorubicin and TNF-alpha induced apoptosis

Aberrant apoptosis has been associated with the development and therapeutic resistance of cancer. Recent studies suggest that caspase deficiency/downregulation is frequently detected in different cancers. We have previously shown that caspase-3 reconstitution significantly sensitized MCF-7 cells to doxorubicin and etoposide. In contrast to the well established role of caspase-3 as an effector caspase, the focus of this study is to delineate caspase-3 induced feedback activation of the apical caspases-2, -8, -9 and -10A in doxorubicin and TNF-alpha induced apoptosis. Using cell-free systems we show that caspases-9 and 2 are the most sensitive, caspase-8 is less sensitive and caspase-10A is the least sensitive to caspase-3 mediated-cleavage. When apoptosis is induced by doxorubicin or TNF-alpha in an intact cell model, cleavage of caspases-8 and -9, but not caspase-2, was markedly enhanced by caspase-3. Caspase-3 mediated-feedback and activation of caspase-8 and -9 in MCF-7/C3 cells is further supported by an increase in the cleavage of caspase-8 and 9 substrates and cytochrome c release. These data indicate that, in addition to its function as an effector caspase, caspase-3 plays an important role in maximizing the activation of apical caspases and crosstalk between the two major apoptotic pathways. The significant impact of caspase-3 on both effector and apical caspases suggests that modulation of caspase-3 activity would be a useful approach to overcome drug resistance in clinical oncology.

NK cell-based immunotherapies against tumors

Natural killer (NK) cells provide the first line of defence against pathogens and tumors. Their activation status is regulated by pro-inflammatory cytokines and by ligands that either target inhibitory or activating cell surface receptors belonging to the immunoglobulin-like, C-type lectin or natural cytotoxicity receptor families. Apart from non-classical HLA-E, membrane-bound heat shock protein 70 (Hsp70) has been identified as a tumor-specific recognition structure for NK cells expressing high amounts of the C-type lectin receptor CD94, acting as one component of an activating heterodimeric receptor complex. Full-length Hsp70 protein (Hsp70) or the 14-mer Hsp70 peptide T-K-D-N-N-L-L-G-R-F-E-L-S-G (TKD) in combination with pro-inflammatory cytokines enhances the cytolytic activity of NK cells towards Hsp70 membrane-positive tumors. Based on these findings cytokine/TKD-activated NK cells were adoptively transferred in tumor patients. These findings were compared to results of clinical trials using cytokine-activated NK cells.

Posttranscriptional regulation of Fas (CD95) ligand killing activity by lipid rafts

Fas (CD95/Apo-1) ligand-mediated apoptosis induction of target cells is one of the major effector mechanisms by which cytotoxic lymphocytes (T cells and natural killer cells) kill their target cells. In T cells, Fas ligand expression is tightly regulated at a transcriptional level through the activation of a distinct set of transcription factors. Increasing evidence, however, supports an important role for posttranscriptional regulation of Fas ligand expression and activity. Lipid rafts are cholesterol- and sphingolipid-rich membrane microdomains, critically involved in the regulation of membrane receptor signaling complexes through the clustering and concentration of signaling molecules. Here, we now provide evidence that Fas ligand is constitutively localized in lipid rafts of FasL transfectants and primary T cells. Importantly, disruption of lipid rafts strongly reduces the apoptosis-inducing activity of Fas ligand. Localization to lipid rafts appears to be predominantly mediated by the characteristic cytoplasmic proline-rich domain of Fas ligand because mutations of this domain result in reduced recruitment to lipid rafts and attenuated Fas ligand killing activity. We conclude that Fas ligand clustering in lipid rafts represents an important control mechanism in the regulation of T cell–mediated cytotoxicity.

Requirement of the human T-cell leukemia virus (HTLV-1) tax-stimulated HIAP-1 gene for the survival of transformed lymphocytes

Human T cell leukemia virus type 1 (HTLV-1), the cause of adult T cell leukemia (ATL), induces clonal expansion of infected T-cells in nonleukemic individuals and immortalizes T cells in vitro. The resistance against apoptotic stimuli of these cells hints at a viral survival function in addition to a proliferation-stimulating activity. Here we describe the up-regulation of the antiapoptotic HIAP-1/CIAP-2 gene as a consistent phenotype of HTLV-1-transformed and ATL-derived cultures and its stimulation by the viral oncoprotein Tax. Cotransfections revealed a 60-fold increase of HIAP-1 promoter activity mediated by Tax mainly via nuclear factor-kappaB (NF-kappaB) activation. To address the relevance of virally increased HIAP-1 levels for the survival of HTLV-1-transformed cells, its expression was RNA interference (RNAi) suppressed using a lentiviral transduction system. This resulted in a dramatic reduction of cell growth, a strong induction of apoptosis rates, and increased caspases 3/7 activity, which is known to be suppressed by HIAP-1. Thus, the Tax-mediated HIAP-1 overexpression is required to suppress endogenous apoptosis and, therefore, is essential for the survival of HTLV-1-transformed lymphocytes. Moreover, this points to HIAP-1 as an important target of the HTLV-1-mediated NF-kappaB activation.

Intratumour T cells, their activation status and survival in gastric carcinomas characterised for microsatellite instability and Epstein-Barr virus infection

Gastric carcinomas (GCs) with high microsatellite instability (MSI) or an Epstein-Barr virus (EBV) infection are prevalently poorly differentiated adenocarcinomas with abundant lymphoid infiltration. The aims of the study were to clarify (1) if tumour-infiltrating lymphocytes (TILs) and cytotoxic-activated TILs are associated with a better clinical outcome in patients with GCs characterised for the presence of MSI and EBV; (2) if the nature and the activation status of TILs are involved in tumour cell apoptosis, evaluated using the M30 antibody, directed against a fragment of cytokeratin-18 caspase-cleaved during early steps of epithelial cell apoptosis. The immunophenotype of TILs and the tumour cell apoptosis were analysed with immunohistochemistry in 96 GCs, including 35 MSI GCs, and 61 GCs without MSI [microsatellite stable (MSS)], 17 of which were EBV+. MSI and MSS/EBV+ GCs displayed a significantly higher mean number of cytotoxic-activated TILs and apoptotic tumour cells than MSS/EBV- GCs (CD8+ TILs/HPF, 21.7 and 69.6 vs 6.4; T-cell intracellular antigen (TIA)-1+ TILs/HPF, 16.7 and 32.05 vs 5.2; granzyme B+ TILs/HPF, 7.5 and 8.6 vs 0.8; perforin+ TILs/HPF, 5.9 and 9.2 vs 0.9; and M30 IR tumour cells, 5.9 and 2.9 vs 2.3%). In addition to the most reliable clinico-pathological parameters (lymph node status, depth of tumour invasion and tumour stage), a univariate analysis showed that the presence of CD3+ TILs higher than 14.9 (p=0.01), CD8+ TILs higher than 9.5 (p<0.05) and MSI (p=0.02) were associated with better overall patient survival. Using a Cox regression model, only a high number of CD3+ TILs (p=0.02) and a low tumour stage (p=0.00001) were identified as independent prognostic factors. In conclusion, our study demonstrates that a high number of CD3+ and CD8+ TILs is a characteristic of MSI- and EBV-associated GCs and represents a favourable prognostic factor, independently of the pathogenesis of GCs.

Clinicopathologic and Prognostic Significance of Cytotoxic Molecule Expression in Nodal Peripheral T-Cell Lymphoma, Unspecified

Cytotoxic molecules (CMs) are apoptosis-inducing molecules that are present in azurophilic cytoplasmic granules of T lymphocytes. Expression of TIA-1 and granzyme B was examined for 100 cases of nodal peripheral T-cell lymphoma, unspecified (PTCL-U) to assess clinicopathologic significance of CM. Forty-one were positive for at least one CM. Patients with CM-positive PTCL-U showed younger onset (median, 55 years vs. 64 years, P = 0.01) and less male predominance (male:female ratio, 21:20 vs. 44:15, P = 0.02). CM-positive PTCL-U was significantly associated with several clinical factors to indicate poor prognosis, in comparison with CM-negative PTCL-U, such as poorer performance status (P = 0.006), more frequent B-symptoms (68% vs. 35%, P = 0.002), higher serum lactate dehydrogenase levels (P = 0.003), and more frequent extranodal involvement, particularly bone marrow involvement (33% vs. 9%, P = 0.004). Epstein-Barr virus was mostly found in CM-positive PTCL-U (51% vs. 2%, P < 0.0001). The CM-positive group showed higher distribution of the International Prognostic Index (P = 0.009) and the Prognostic Index for T-cell lymphoma (P = 0.004) scores than CM-negative group. Complete remission rate was 30% for the former but 63% for the latter. Overall survival of CM-positive PTCL-U was significantly lower than that of CM-negative patients (P = 0.004). Multivariate analyses confirmed that CM expression is a significant prognostic factor, independent from other clinical factors or prognostic index scores. These findings suggest that nodal CM-positive PTCL-U show distinct clinicopathologic characteristics among the current category of PTCL-U.

The Pathophysiology of Acute Graft-Versus-Host Disease

Despite improvements in allogeneic stem cell transplantation, acute graft-versus-host disease (GVHD) remains a significant problem after transplantation, and it is still a major cause of post-transplant mortality. Disease progression is characterized by the differentiation of alloreactive T cells to effector cells leading to tissue damage, recruitment of additional inflammatory cell populations and further cytokine dysregulation. To make the complex process of acute GVHD more explicit, the pathophysiology of acute GVHD is often divided into three different phases. This review summarizes the mechanisms involved in the three phases of acute GVHD.

Membrane receptors are not required to deliver granzyme B during killer cell attack

Granzyme B (GzmB), a serine protease of cytotoxic T lymphocytes and natural killer (NK) cells, induces apoptosis by caspase activation after crossing the plasma membrane of target cells. The mechanism of this translocation during killer cell attack, however, is not understood. Killer cells release GzmB and the membrane-disturbing perforin at the contact site after target recognition. Receptor-mediated import of glycosylated GzmB and release from endosomes were suggested, but the role of the cation-independent mannose 6-phosphate receptor was recently refuted. Using recombinant nonglycosylated GzmB, we observed binding of GzmB to cellular membranes in a cell type–dependent manner. The basis and functional impact of surface binding were clarified. GzmB binding was correlated with the surface density of heparan sulfate chains, was eliminated on treatment of target cells with heparinase III or sodium chlorate, and was completely blocked by an excess of catalytically inactive GzmB or GzmK. Although heparan sulfate–bound GzmB was taken up rapidly into intracellular lysosomal compartments, neither of the treatments had an inhibitory influence on apoptosis induced by externally added streptolysin O and GzmB or by natural killer cells. We conclude that membrane receptors for GzmB on target cells are not crucial for killer cell–mediated apoptosis.

Independent roles of perforin, granzymes, and Fas in the control of Friend retrovirus infection

Cytotoxic T-cells (CTL) play a central role in the recovery of mammalian hosts from retroviral infections. However, the molecular pathways that mediate the antiretroviral activity of CTL are still elusive. Here we explore the protective role of the two main cytolytic pathways of CTL, that is, granule exocytosis and Fas/Fas ligand (FasL), in acute and persistent Friend retrovirus (FV) infection of mice. For this purpose, we have used mutant mouse strains with targeted gene defects in one or more components of the two cytolytic pathways including perforin, granzyme A, granzyme B, Fas, and FasL. The important function of CTL in resistance of C57BL/6 (B6) mice to FV is emphasized by the finding that depletion of CD8+ T-cells prior to virus infection resulted in severe splenomegaly and high viral loads in blood and spleen tissue. Analysis of primary FV infection in knockout mice revealed that acute infection was readily controlled in the absence of functional Fas. Most notably in the presence of Fas/FasL each of the three effector molecules of the exocytosis pathway (i.e., perforin, granzyme A, and granzyme B) was capable on its own to mediate suppression of virus replication and protection from leukemia. However, triple knockout mice lacking perforin and the two granzymes were fully susceptible to FV-induced leukemia. In contrast to acute infection the Fas/FasL pathway was mandatory for effective control of FV replication during persistent infection. These findings suggest novel pathways of CTL-mediated viral defense and contribute towards a better understanding of the molecular mechanisms of CTL activity in retroviral infections.

Regulation of FasL expression in natural killer cells

Fas ligand (FasL)-mediated cytotoxicity is initiated in natural killer (NK) cells through ligation of their activating receptors. The CD16 receptor has been shown to induce FasL expression and cytotoxicity in NK cells. In this study, we made the novel observation that FasL expression was upregulated in NKL cells stimulated through 2B4 and LFA-1 activating receptors, implying a role for FasL-mediated cytotoxicity early in the immune response. Coligation with CD94/NKG2A human leukocyte antigen (HLA) class I inhibitory receptor did not block the induced FasL expression; therefore, these opposing pathways appear to function independently. We also showed, however, that FasL-mediated cytotoxicity was downregulated in CD94/NKG2A-expressing LAK cells in response to the HLA-E ligand, suggesting a mechanism by which aberrant cells expressing class I may evade FasL-mediated cytotoxicity. Thus we show for the first time that 2B4, LFA-1, and CD94/NKG2A receptors are involved in modulating FasL expression and, therefore, cytotoxicity mediated by NK cells.

Induction of apoptosis by shikonin through coordinative modulation of the Bcl-2 family, p27, and p53, release of cytochrome c, and sequential activation of caspases in human colorectal carcinoma cells

Shikonin is a main constituent of the roots of Lithospermum erythrorhizon that has antimutagenic activity. However, its other biological activities are not well-known. Shikonin displayed a strong inhibitory effect against human colorectal carcinoma COLO 205 cells and human leukemia HL-60 cells, with estimated IC(50) values of 3.12 and 5.5 microM, respectively, but were less effective against human colorectal carcinoma HT-29 cells, with an estimated IC(50) value of 14.8 microM. Induce apoptosis was confirmed in COLO 205 cells by DNA fragmentation and the appearance of a sub-G1 DNA peak, which were preceded by loss of mitochondrial membrane potential, reactive oxygen species (ROS) generation, cytochrome c release, and subsequent induction of pro-caspase-9 and -3 processing. Cleavages of poly(ADP-ribose) polymerase (PARP) and DNA fragmentation factor (DFF-45) were accompanied by activation of caspase-9 and -3 triggered by shikonin in COLO 205 cells. Here, we found that shikonin-induced apoptotic cell death was accompanied by upregulation of p27, p53, and Bad and down-regulation of Bcl-2 and Bcl-X(L), while shikonin had little effect on the levels of Bax protein. Taken together, we suggested that shikonin-induced apoptosis is triggered by the release of cytochrome c into cytosol, procaspase-9 processing, activation of caspase-3, degradation of PARP, and DNA fragmentation caused by the caspase-activated deoxyribonuclease through the digestion of DFF-45. The induction of apoptosis by shikonin may provide a pivotal mechanism for its cancer chemopreventive action.

Activation, differentiation, and migration of naive virus-specific CD8+ T cells during pulmonary influenza virus infection

The low precursor frequency of individual virus-specific CD8(+) T cells in a naive host makes the early events of CD8(+) T cell activation, proliferation, and differentiation in response to viral infection a challenge to identify. We have therefore examined the response of naive CD8(+) T cells to pulmonary influenza virus infection with a murine adoptive transfer model using hemagglutinin-specific TCR transgenic CD8(+) T cells. Initial activation of CD8(+) T cells occurs during the first 3 days postinfection exclusively within the draining lymph nodes. Acquisition of CTL effector functions, including effector cytokine and granule-associated protease expression, occurs in the draining lymph nodes and differentially correlates with cell division. Division of activated CD8(+) T cells within the draining lymph nodes occurs in an asynchronous manner between days 3 and 4 postinfection. Despite the presence of Ag for several days within the draining lymph nodes, dividing T cells do not appear to maintain contact with residual Ag. After multiple cell divisions, CD8(+) T cells exit the draining lymph nodes and migrate to the infected lung. Activated CD8(+) T cells also disseminate throughout lymphoid tissue including the spleen and distal lymph nodes following their emigration from draining lymph nodes. These results demonstrate an important role for draining lymph nodes in orchestrating T cell responses during a local infection of a discrete organ to generate effector CD8(+) T cells capable of responding to infection and seeding peripheral lymphoid tissues.

Killing of target cells by redirected granzyme B in the absence of perforin

Granzyme B (GzmB) is a potent apoptosis-inducing serine protease of cytotoxic lymphocytes. Following receptor-mediated endocytosis, GzmB is supposed to enter the cytosol through perforin-mediated membrane disruption. We investigated whether retargeting of GzmB to Lewis Y positive surface receptors could lead to perforin-independent target cell death. We coupled recombinant GzmB to the Lewis Y-binding antibody dsFv-B3. Targeting of GzmB to Lewis Y positive cells triggered cell death with similar efficacy as dsFv-B3 targeted Pseudomonas exotoxin fragment 38 (PE38). Since GzmB was only weakly inhibited by plasma proteins, GzmB-based immunoconjugates should be useful as a new class of immunotoxins with low immunogenicity utilizing programmed cell death for therapeutic purposes.

Granzyme B and perforin: constitutive expression in human polymorphonuclear neutrophils

Polymorphonuclear neutrophils (PMNs) produce an abundance of bactericidal and cytotoxic molecules consistent with their role as first-line defense against bacterial infection. PMNs, however, also cause efficient cellular cytotoxicity when targeted through Fc receptors to appropriate antibody-coated target cells. Although this so-called antibody-dependent cellular cytotoxicity (ADCC) was described many years ago, the mechanism of killing is still elusive. We now have found that PMNs contain perforin and granzyme B, the 2 molecules known as the cytotoxic entity of natural killer cells and of cytotoxic T lymphocytes as well. Lysates of PMNs were lytic for chicken erythrocytes in a time-, temperature-, and Ca2+-dependent manner. Moreover, apoptosis of Jurkat cells was induced, consistent with the observation that the PMN lysates contain enzymatically active granzyme B. Taken together, our data provide evidence for the presence of perforin and granzyme B within the cytotoxic arsenal of PMNs. (Blood. 2004;103:1099-1104)

Apoptosis induction by interleukin-2-activated cytotoxic lymphocytes in a squamous cell carcinoma cell line and Daudi cells - involvement of reactive oxygen species-dependent cytochrome c and reactive oxygen species-independent apoptosis-inducing factors

Investigation of the induction of apoptosis by cytotoxic lymphocytes has mainly focused on the signalling associated with Fas and its adaptor proteins. The signal pathway via mitochondria, however, has not been sufficiently elucidated in cytotoxic lymphocyte-induced apoptosis. We examined the release of mitochondrial proapoptotic factors by lymphokine-activated killer (LAK) cells in two cell lines. LAK cell-induced DNA fragmentation of the target cells was suppressed to approximately 50% of control levels by the addition of neutralizing monoclonal antibody to Fas and a granzyme B inhibitor. When intracellular reactive oxygen species (ROS) were scavenged, the LAK cell-induced DNA fragmentation was decreased to approximately 60% of the non-treated cell level. Co-cultivation of Daudi cells with LAK cells increased cytosolic and mitochondrial ROS levels. Activation of procaspase-3 and apoptosis by treatment of oral squamous cell carcinoma cells (OSC) with LAK cells was partially inhibited by pretreatment of OSC cells with ROS scavengers and mitochondrial complex inhibitors. Furthermore, cytochrome c and apoptosis-inducing factor (AIF) were released from mitochondria by OSC cell treatment with supernatants of LAK cells. The supernatant-induced cytochrome c release was suppressed by mitochondrial complex inhibitors, but the inhibitors did not inhibit the release of AIF. These results indicate that LAK cells induce target cell apoptosis via not only the Fas/Fas ligand system and granzyme B, but also ROS-dependent cytochrome c and ROS-independent AIF release.

Cell surface-bound heat shock protein 70 (Hsp70) mediates perforin-independent apoptosis by specific binding and uptake of granzyme B

Cell surface-bound heat shock protein 70 (Hsp70) renders tumor cells more sensitive to the cytolytic attack mediated by natural killer (NK) cells. A 14-amino acid Hsp70 sequence, termed TKD (TKDNNLLGRFELSG, aa450-463) could be identified as the extracellular localized recognition site for NK cells. Here, we show by affinity chromatography that both, full-length Hsp70-protein and Hsp70-peptide TKD, specifically bind a 32-kDa protein derived from NK cell lysates. The serine protease granzyme B was uncovered as the 32-kDa Hsp70-interacting protein using matrix-assisted laser desorption ionization time-of-flight mass peptide fingerprinting. Incubation of tumor cells with increasing concentrations of perforin-free, isolated granzyme B shows specific binding and uptake in a dose-dependent manner and results in initiation of apoptosis selectively in tumor cells presenting Hsp70 on the cell surface. Remarkably, Hsp70 cation channel activity was also determined selectively in purified phospholipid membranes of Hsp70 membrane-positive but not in membrane-negative tumor cells. The physiological role of our findings was demonstrated in primary NK cells showing elevated cytoplasmic granzyme B levels following contact with TKD. Furthermore, an increased lytic activity of Hsp70 membrane-positive tumor cells could be associated with granzyme B release by NK cells. Taken together we propose a novel perforin-independent, granzyme B-mediated apoptosis pathway for Hsp70 membrane-positive tumor cells.

Differential expression of perforin and granzyme B in the liver of patients with chronic hepatitis C

T lymphocytes have been reported to be the predominant inflammatory cells in the liver of patients with chronic hepatitis C. On activation, CD8+ T lymphocytes can exert their cytolytic activity by releasing their granule components, notably perforin and granzyme B. The aim of the present study was to assess whether the granule cytolytic pathway was used by liver-infiltrating CD8+ T lymphocytes. Immunostaining for perforin and granzyme B was performed in 25 patients with chronic hepatitis C, according to the disease activity and their virologic status. Cells stained for perforin and for granzyme B represented 0.15% and 0.10% of the total liver-infiltrating CD8+ T lymphocytes, respectively. Perforin was expressed mainly by activated CD8+ T lymphocytes located in liver lobules. In contrast, granzyme B was expressed mainly by activated CD8+ T lymphocytes located in interface hepatitis and portal tracts. The results were similar in the different groups of patients, whatever the disease activity. In conclusion, this is the first study showing a differential expression of granule components of CD8+ T lymphocytes in the same tissue in vivo. Perforin and granzyme B may be differently expressed by liver-infiltrating CD8+ T lymphocytes, according to their localization in the different specific compartments of the liver, in patients with chronic hepatitis C.

Induction of apoptosis by hydroxydibenzoylmethane through coordinative modulation of cyclin D3, Bcl-X(L), and Bax, release of cytochrome c, and sequential activation of caspases in human colorectal carcinoma cells

DBM (dibenzoylmethane) is a minor constituent of licorice that has antimutagenic activity. However, its other biological activities are not well-known. The structurally related beta-diketones hydroxydibenzoylmethane (HDB) and hydroxymethyldibenzoylmethane (HMDB) were able to induce apoptosis in colorectal carcinoma COLO 205 cells. Thus, the effect of structurally related beta-diketones on cell viability, DNA fragmentation, and caspase activity was assessed. The potency of these compounds on these features of apoptosis were in the order of HDB > HMDB > DBM in colorectal carcinoma COLO 205 cells. Here, we found that HDB-induced apoptotic cell death was accompanied by upregulation of cyclin D3, Bax, and p21 and down-regulation of Bcl-X(L), while HDB had no effect on the levels of Bcl-2 and Bad protein. These results indicate that HDB allows caspase-activated deoxyribonuclease to enter the nucleus and degrade chromosomal DNA and induces DFF-45 degradation. It is suggested that HDB-induced apoptosis is triggered by the release of cytochrome c into cytosol, procaspase-9 processing, activation of caspase-3 and caspase-2, degradation of PARP, and DNA fragmentation caused by the caspase-activated deoxyribonuclease through the digestion of DFF-45. The induction of apoptosis by HDB may provide a pivotal mechanism for its cancer chemopreventive action.

Proteinases and their inhibitors in the immune system

The most important roles of proteinases in the immune system are found in apoptosis and major histocompatibility complex (MHC) class II-mediated antigen presentation. A variety of cysteine proteinases, serine proteinases, and aspartic proteinases as well as their inhibitors are involved in the regulation of apoptosis in neutrophils, monocytes, and dendritic cells, in selection of specific B and T lymphocytes, and in killing of target cells by cytotoxic T cells and natural killer cells. In antigen presentation, endocytosed antigens are digested into antigenic peptides by both aspartic and cysteine proteinases. In parallel, MHC class II molecules are processed by aspartic and cysteine proteinases to degrade the invariant chain that occupies the peptide-binding site. Proteinase activity in these processes is highly regulated, particularly by posttranslational activation and the balance between active proteinases and specific endogenous inhibitors such as cystatins, thyropins, and serpins. This article discusses the regulation of proteolytic processes in apoptosis and antigen presentation in immune cells and the consequences of therapeutic interference in the balance of proteinases and their inhibitors.

Caspase-activation pathways in apoptosis and immunity

Members of the caspase family of cysteine proteases have been firmly established to play key roles in signal transduction cascades that culminate in apoptosis (programmed cell death). Caspases are normally expressed as inactive precursor enzymes (zymogens) that become activated during apoptosis and proceed to dismantle the cell from within. To date, three major apoptosis-associated pathways to caspase activation have been elucidated. Certain caspases, such as caspase-1, also occupy important positions in signaling pathways associated with immune responses to microbial pathogens. In this situation, caspase activation is associated with the maturation of pro-inflammatory cytokines, such as interleukin-1beta (IL-1beta) and IL-18, and not apoptosis per se. Here, we discuss the current understanding of how caspases are activated during apoptosis and inflammation and the roles these proteases play in either context.

Galectin-9 induces apoptosis through the calcium-calpain-caspase-1 pathway

Galectin-9 (Gal-9) induced the apoptosis of not only T cell lines but also of other types of cell lines in a dose- and time-dependent manner. The apoptosis was suppressed by lactose, but not by sucrose, indicating that beta-galactoside binding is essential for Gal-9-induced apoptosis. Moreover, Gal-9 required at least 60 min of Gal-9 binding and possibly de novo protein synthesis to mediate the apoptosis. We also assessed the apoptosis of peripheral blood T cells by Gal-9. Apoptosis was induced in both activated CD4(+) and CD8(+) T cells, but the former were more susceptible than the latter. A pan-caspase inhibitor (Z-VAD-FMK) inhibited Gal-9-induced apoptosis. Furthermore, a caspase-1 inhibitor (Z-YVAD-FMK), but not others such as Z-IETD-FMK (caspase-8 inhibitor), Z-LEHD-FMK (caspase-9 inhibitor), and Z-AEVD-FMK (caspase-10 inhibitor), inhibited Gal-9-induced apoptosis. We also found that a calpain inhibitor (Z-LLY-FMK) suppresses Gal-9-induced apoptosis, that Gal-9 induces calcium (Ca(2+)) influx, and that either the intracellular Ca(2+) chelator BAPTA-AM or an inositol trisphosphate inhibitor 2-aminoethoxydiphenyl borate inhibits Gal-9-induced apoptosis. These results suggest that Gal-9 induces apoptosis via the Ca(2+)-calpain-caspase-1 pathway, and that Gal-9 plays a role in immunomodulation of T cell-mediated immune responses.

Endocytosing the death sentence

A series of recent studies have suggested that endocytosis of the mannose-6-phosphate receptor (MPR)* might play a critical role in delivering the death signal to cells targeted for destruction by the immune system (for review see Barry and Bleackley, 2002). These studies have raised a number of controversial issues regarding the trafficking of proteins from the plasma membrane of the target cell to their substrates in the cytosol. In this issue, Trapani and colleagues examine the death of cells in which endocytosis of the MPR is blocked and show that the death signal is delivered effectively in the absence of MPR endocytosis (Trapani et al., 2002, this issue). How then is the death sentence delivered?

Armed therapeutic viruses: strategies and challenges to arming oncolytic viruses with therapeutic genes

Oncolytic viruses are attractive therapeutics for cancer because they selectively amplify, through replication and spread, the input dose of virus in the target tumor. To date, clinical trials have demonstrated marked safety but have not realized their theoretical efficacy potential. In this review, we consider the potential of armed therapeutic viruses, whose lytic potential is enhanced by genetically engineered therapeutic transgene expression from the virus, as potential vehicles to increase the potency of these agents. Several classes of therapeutic genes are outlined, and potential synergies and hurdles to their delivery from replicating viruses are discussed.

IL-18-independent cytotoxic T lymphocyte activation and IFN-gamma production during experimental acute graft-versus-host disease

Acute graft-versus-host disease (GvHD) is a serious complication after allogeneic bone marrow transplantation. Donor-derived T cells infiltrate recipient target organs and cause severe tissue damage, often leading to death of the affected patient. Tissue destruction is a direct result of donor CD8+ T cell activation and cell-mediated cytotoxicity. IL-18 is a novel pro-inflammatory cytokine with potent T(h)1 immune response-promoting and cytotoxic T lymphocyte (CTL)-inducing activity. IL-18 is strongly induced in experimental mouse models and human patients with acute GvHD. However, the precise role of IL-18 in the development of acute GvHD is still unknown. In this study, we have used IL-18-binding protein, a soluble IL-18 decoy receptor, to specifically neutralize IL-18 in vivo and in vitro. Our results demonstrate that IL-18 is induced during GvHD. However, its effect in the induction of GvHD appears to be redundant, since neutralization of IL-18 does not alter any disease parameter analyzed. Our study further shows that IFN-gamma production and CTL induction upon activation by T cell mitogens or by alloantigen does not involve IL-18-mediated amplification, in contrast to lipopolysaccharide-induced IFN-gamma production. We conclude that IL-18 expression correlates with the course of GvHD; however, its effect is dispensable for IFN-gamma and CTL induction for the initiation phase of this disease, most likely due to direct, IL-18-independent, CTL activation.

Cytotoxic cell granule-mediated apoptosis: perforin delivers granzyme B-serglycin complexes into target cells without plasma membrane pore formation

The mechanism underlying perforin (PFN)-dependent delivery of apoptotic granzymes during cytotoxic cell granule-mediated death remains speculative. Granzyme B (GrB) and perforin were found to coexist as multimeric complexes with the proteoglycan serglycin (SG) in cytotoxic granules, and cytotoxic cells were observed to secrete exclusively macromolecular GrB-SG. Contrary to the view that PFN acts as a gateway for granzymes through the plasma membrane, monomeric PFN and, strikingly, PFN-SG complexes were shown to mediate cytosolic delivery of macromolecular GrB-SG without producing detectable plasma membrane pores. These results indicate that granule-mediated apoptosis represents a phenomenon whereby the target cell perceives granule contents as a multimeric complex consisting of SG, PFN, and granzymes, which are, respectively, the scaffold, translocator, and targeting/informational components of this modular delivery system.

Apoptosis in the absence of caspase 3

MCF-7 human breast cancer cells do not express caspase 3, thought by some to be a critical component of the apoptosis cascade. Nonetheless, both mock- and bcl-2-transfected MCF-7 cells undergo apoptosis after treatment with a variety of stimuli, including the DNA-cleaving antimitotic agent, neocarzinostatin (NCS). Transfection with bcl-2 shifts the concentration-response curve to NCS but does not change the phenomenology of apoptosis when it occurs. In both cases, NCS treatment results in condensation and fragmentation of MCF-7 cell nuclei and release of cytochrome c from the mitochondria to the cytosol. This apoptosis is accompanied by decreased levels of Bcl-2 and increased levels of Bax. Using a series of caspase inhibitors with overlapping specificities, enzyme-specific chromogenic substrates, and an antibody specific for activated caspase 7, we have determined that apoptosis in MCF-7 cells proceeds via sequential activation of caspases 9, 7 and 6. P21 is detected only after activation of caspase 7, and P53 is neither expressed at baseline nor up-regulated with apoptosis induction. This pathway bypasses the need for activated caspase 3 in these cells.

Accumulation and activation-induced release of preformed Fas (CD95) ligand during the pathogenesis of experimental graft-versus-host disease

Fas (CD95/APO-1) ligand (FasL)-mediated cytotoxicity has been implicated in tissue destruction in a variety of diseases, including acute graft-vs-host disease (GVHD). In this study, we have analyzed FasL expression and regulation during the course of experimental murine acute GVHD. Although activation-induced FasL-mediated cytotoxicity in control T cells was sensitive to the immunosuppressant cyclosporin A, we observed that functional FasL expression of GVHD T cells became increasingly cyclosporin A unresponsive. This was found to be the result of a massive in vivo accumulation and intracellular storage of FasL protein and its release in a transcription- and protein synthesis-independent manner. Immunohistochemistry analysis of FasL expression in situ revealed accumulation of FasL-expressing cells in the spleen, the liver, and small intestine, with a typical cytoplasmic and granular expression pattern. Thus, we conclude that the release of preformed FasL by infiltrating donor T cells may contribute to recipient tissue damage during the pathogenesis of acute GVHD.

The role of T-cell-mediated mechanisms in virus infections of the nervous system

T lymphocytes play a decisive role in the course and clinical outcome of viral CNS infection. Summarizing the information presented in this review, the following sequence of events might occur during acute virus infection: After invasion of the host and a few initial rounds of replication, the virus reaches the CNS in most cases by hematogeneous spread. After passage through the BBB, CNS cells are infected and replication of virus in brain cells causes activation of the surrounding microglia population. Moreover, local production of IFN-alpha/beta induces expression of MHC antigens on CNS cells, and microglial cells start to phagocytose cellular debris, which accumulates as a result of virus-induced cytopathogenic effects. Upon phagocytosis, microglia becomes more activated; they up-regulate MHC molecules, acquire antigen presentation capabilities and secrete chemokines. This will initiate up-regulation of adhesion molecules on adjacent endothelial cells of the BBB. Transmigration of activated T lymphocytes through the BBB is followed by interaction with APC, presenting the appropriate peptides in the context of MHC antigens. It appears that CD8+ T lymphocytes are amongst the first mononuclear cells to arrive at the infected tissue. Without a doubt, their induction and attraction is deeply influenced by natural killer cells, which, after virus infection, secrete IFN-gamma, a cytokine that stimulates CD8+ T cells and diverts the immune response to a TH1-type CD4+ T cell-dominated response. Following the CD8+ T lymphocytes, tissue-penetrating, TH1 CD4+ T cells contact local APC. This results in a tremendous up-regulation of MHC molecules and secretion of more chemotactic and toxic substances. Consequently an increasing number of inflammatory cells, including macrophages/microglia and finally antibody-secreting plasma cells, are attracted to the site of virus infection. All trapped cells are mainly terminally differentiated cells that are going to enter apoptosis during or shortly after exerting their effector functions. The clinical consequences and the influence of the effector phase on the further course of the infection depends on the balance and fine-tuning of the contributing lymphoid cell populations. Generally, any delay in the recruitment of effector lymphocytes to the tissue or an unbalanced combination of lymphocyte subsets allows the virus to spread in the CNS, which in turn will cause severe immune-mediated tissue effects as well as disease. If either too late or partially deficient, the immune system response may contribute to a lethal outcome or cause autosensitization to brain-specific antigens by epitope spreading to the antigen-presenting system in peripheral lymphoid tissue. This could form the basis for subsequent booster reactions of autosensitized CD4+ T cells--a process that finally will end in an inflammatory autoimmune reaction, which in humans we call multiple sclerosis. In contrast, a rapid and specific local response in the brain tissue will result in efficient limitation of viral spread and thereby a subclinical immune system-mediated termination of the infection. After clearance of virus-infected cells, downsizing of the local response probably occurs via self-elimination of the contributing T cell populations and/or by so far unidentified signal pathways. However, much of this is highly speculative, and more data have to be collected to make decisive conclusions regarding this matter. Several strategies have been developed by viruses to escape T cell-mediated eradication, including interference with the MHC class I presentation pathway of the host cell or "hiding" in cells which lack MHC class I expression. This may result in life-long persistence of the virus in the brain, a state which probably is actively controlled by T lymphocytes. Under severe immunosuppression, however, reactivation of viral replication can occur, which is a lethal threat to the host.

Mutations in apoptosis genes: a pathogenetic factor for human disease

Cell death by apoptosis is exerted by the coordinated action of many different gene products. Mutations in some of them, acting at different levels in the apoptosis process, have been identified as cause or contributing factor for human diseases. Defects in the transmembrane tumor necrosis factor receptor 1 (TNF-R1) lead to the development of familial periodic fever syndromes. Mutations in the homologous receptor Fas (also named CD95; Apo-1) are observed in malignant lymphomas, solid tumors and the autoimmune lymphoproliferative syndrome type I (ALPS I). A mutation in the ligand for Fas (Fas ligand; CD95 ligand, Apo-1 ligand), which induces apoptosis upon binding to Fas, was described in a patient with systemic lupus erythematodes and lymphadenopathy. Perforin, an other cytotoxic protein employed by T- and NK-cells for target cell killing, is mutated in chromosome 10 linked cases of familial hemophagocytic lymphohistiocytosis. Caspase 10, a representative of the caspase family of proteases, which plays a central role in the execution of apoptosis, is defect in autoimmune lymphoproliferative syndrome type II (ALPS II). The intracellular pro-apoptotic molecule bcl-10 is frequently mutated in mucosa-associated lymphoid tissue (MALT) lymphomas and various non-hematologic malignancies. The p53, an executioner of DNA damage triggered apoptosis, and Bax, a pro-apoptotic molecule with the ability to perturb mitochondrial membrane integrity, are frequently mutated in malignant neoplasms. Anti-apoptotic proteins like bcl-2, cellular-inhibitor of apoptosis protein 2 (c-IAP2) and neuronal apoptosis inhibitory protein 1 (NAIP1) are often altered in follicular lymphomas, MALT lymphomas and spinal muscular atrophy (SMA), respectively. This article reviews the current knowledge on mutations of apoptosis genes involved in the pathogenesis of human diseases and summarises the gradual transformation of discoveries in apoptosis research into benefits for the clinical management of diseases.

Rat hepatic natural killer cells (pit cells) express mRNA and protein similar to in vitro interleukin-2 activated spleen natural killer cells

Pit cells are liver-specific natural killer (NK) cells that can be divided into high- (HD) and low-density (LD) subpopulations. The characteristics of pit cells were further investigated in this report. LD and HD pit cells express the specific NK-activation markers gp42, CD25, and ANK44 antigen. LD cells and IL-2-activated NK cells have a high mRNA expression of perforin, granzymes, interferon-gamma, and tumor necrosis factor-alpha. LD pit cells, unlike spleen NK cells, have a weak response to IL-2 with regard to proliferation, cytotoxicity, and production of NK-related molecules. The characteristics of HD cells are intermediate between LD and spleen NK cells. These results show that pit cells, especially LD cells, possess characteristics similar to IL-2-activated NK cells. This is the first evidence on a molecular level that pit cells could be considered in vivo activated NK cells.

Host defense, viruses and apoptosis

To thwart viral infection, the host has developed a formidable and integrated defense network that comprises our innate and adaptive immune response. In recent years, it has become clear that in an attempt to prevent viral replication, viral dissemination or persistent viral infection of the cell, many of these protective measures actually involve the induction of programmed cell death, or apoptosis. An initial response to viral infection primarily involves the innate arm of immunity and the killing of infected cells with cytotoxic lymphocytes such as natural killer (NK) cells through mechanisms that include the employment of perforin and granzymes. Once the virus has invaded the cell, however, a second host defense-mediated response is also triggered which involves the induction of a family of cytokines known as the interferons (IFNs). The IFNs, which are essential for initiating and coordinating a successful antiviral response, function by stimulating the adaptive arm of immunity involving cytotoxic T cells (CTLs), and by inducing a number of intracellular genes that directly prevent virus replication/cytolysis or that facilitate apoptosis. The IFN-induced gene family is now known to comprise the death ligand TRAIL, the dsRNA-dependent protein kinase (PKR), interferon regulatory factors (IRFs) and the promyelocytic leukemia gene (PML), all of which have been reported to be mediators of cell death. That DNA array analyses indicate that numerous cellular genes, many as yet uncharacterized, may similarly be induced by IFN, further emphasizes the likely importance that these cytokines have in the modulation of apoptosis. This likelihood is additionally underlined by the elaborate strategies developed by viruses to inhibit IFN-antiviral function and the mechanisms of cell death.

T-cell release of granulysin contributes to host defense in leprosy

A novel mechanism by which T cells contribute to host defense against microbial pathogens is release of the antimicrobial protein granulysin. We investigated the role of granulysin in human infectious disease using leprosy as a model. Granulysin-expressing T cells were detected in cutaneous leprosy lesions at a six-fold greater frequency in patients with the localized tuberculoid as compared with the disseminated lepromatous form of the disease. In contrast, perforin, a cytolytic molecule that colocalizes with granulysin in cytotoxic granules, was expressed at similar levels across the spectrum of disease. Within leprosy lesions, granulysin colocalized in CD4+ T cells and was expressed in CD4+ T-cell lines derived from skin lesions. These CD4+ T-cell lines lysed targets by the granule exocytosis pathway and reduced the viability of mycobacteria in infected targets. Given the broad antimicrobial spectrum of granulysin, these data provide evidence that T-cell release of granulysin contributes to host defense in human infectious disease.

Apoptosis in cardiac transplant rejection

Apoptosis occurs in human cardiac allograft rejection and may occur with all degrees of rejection and even in its absence. The prevalence and severity of apoptosis is determined predominantly by the intensity of macrophage infiltration and may be mediated by NO-related mechanisms. Apoptosis of interstitial, endothelial, and inflammatory cells is also present in heart allografts and may influence the degree and extent of vascular injury contributing to allograft rejection. Ongoing apoptosis of inflammatory cells suggests an immunoregulatory role. Studies of the involvement of NO in myocyte damage and Fas-FasL interactions in peripheral tolerance have raised the exciting possibility that these pathways can be exploited in a beneficial way. Further understanding of the role of apoptosis and the cellular and biochemical mechanisms that are involved in cardiac myocyte death and in inflammatory, endothelial, and interstitial cell death may provide insights into therapeutic modalities to suppress allograft rejection and vasculopathy.

Filamin (280-kDa actin-binding protein) is a caspase substrate and is also cleaved directly by the cytotoxic T lymphocyte protease granzyme B during apoptosis

We used yeast two-hybrid screening to identify the cytoskeletal protein filamin as a ligand for the proapoptotic protease granzyme B, produced by cytotoxic T lymphocytes. Filamin was directly cleaved by granzyme B when target cells were exposed to granzyme B and the lytic protein perforin, but it was also cleaved in a caspase-dependent manner following the ligation of Fas receptors. A similar pattern of filamin cleavage to polypeptides of approximately 110 and 95 kDa was observed in Jurkat cells killed by either mechanism. However, filamin cleavage in response to granzyme B was not inhibited by the caspase inhibitor z-Val-Ala-Asp-fluoromethylketone at concentrations that abolished DNA fragmentation. Filamin staining was redistributed from the cell membrane into the cytoplasm of Jurkat cells exposed to granzyme B and perforin and following ligation of Fas receptors, coincident with the morphological changes of apoptosis. Filamin-deficient human melanoma cells were significantly (although not completely) protected from granzyme B-mediated death compared with isogenic filamin-expressing cells, both in clonogenic survival and (51)Cr release assays, whereas death from multiple other stimuli was not affected by filamin deficiency. Thus, filamin is a functionally important substrate for granzyme B, as its cleavage may account at least partly for caspase-independent cell death mediated by the granzyme.

Lennert's lymphoma: a variant of cytotoxic T-cell lymphoma?

We studied 10 cases of Lennert's lymphoma (lymphoepithelioid lymphoma) to evaluate the cellular origin of the neoplastic cells. There were six men and four women, aged 38 to 75 years (median, 56 yrs; mean, 59 yrs). The lymphoma cells tended to remain confined to lymph nodes, and extranodal involvement was rare. The mean overall survival was 42.2 months, which is relatively good compared with other peripheral T-cell lymphomas. Morphologically, the lymph node was occupied by small to large clusters of epithelioid cells interspersed with medium to large atypical lymphoid cells. In seven cases, large atypical lymphoid cells resembling Hodgkin's or Reed-Sternberg cells were observed. The phenotypes of these neoplastic cells were CD3+ CD4- CD8+ in five cases, CD3+ CD4+ CD8- in four cases, and CD3+ CD4- CD8- in one case. TIA-1 was positive by immunohistochemical staining in seven cases, whereas four cases were positive for granzyme B. Clonal rearrangement of the T-cell receptor gene was confirmed in all cases by either Southern blot hybridization or a polymerase chain reaction-based denature gradient gel electrophoresis method. Epstein-Barr virus was negative by in situ hybridization in all but one case. Lennert's lymphoma was formerly known as a CD4+ helper T-cell neoplasm. Our results suggest that, at least in some cases, the neoplastic cells are of cytotoxic T-cell origin.

Excessive apoptosis in low risk myelodysplastic syndromes (MDS)

The paradox of peripheral cytopenias despite a normo/hypercellular marrow in MDS has been ascribed to excessive intramedullary hematopoietic cell apoptosis. Programmed cell death (PCD) in early disease might be triggered by the BM microenvironment, mediated either through inhibitory cytokines such as tumor necrosis factor alpha (TNF-alpha) or fas/fas ligand signaling or through a relative deficiency in hematopoietic growth factors. Intrinsic cellular defects giving rise to abnormalities in cell-cell or cell-stromal interaction, cell signaling or cell cycling may also underlie hematopoietic progenitor apoptosis. Alternatively, an early 'hit' in the multistep pathogenesis of MDS may result in a higher proliferative rate of the neoplastic clone. Increased apoptosis may thus represent a homeostatic process to control cell numbers. This paper shall summarize current evidence implicating a role for increased PCD in low risk MDS, outline possible etiologic factors and suggest potential therapeutic mechanisms whereby excessive hematopoietic progenitor cell apoptosis might be circumvented.

Crystal structure of the caspase activator human granzyme B, a proteinase highly specific for an Asp-P1 residue

Granzyme B is the prototypic member of the granzymes, a family of trypsin-like serine proteinases localized in the dense cytoplasmic granules of activated natural killer cells and cytotoxic T lymphocytes. Granzyme B directly triggers apoptosis in target cells by activating the caspase pathway, and has been implicated in the etiology of rheumatoid arthritis. Human granzyme B expressed in a baculovirus system has been crystallized without inhibitor and its structure has been determined to 3.1 A resolution, after considerably improving the diffraction power of the crystals by controlled humidity changes. The granzyme B structure reveals an overall fold similar to that found in cathepsin G and human chymase. The guanidinium group of Arg226, anchored at the back of the S1-specificity pocket, can form a salt bridge with the P1-Asp side chain of a bound peptide substrate. The architecture of the substrate binding site of granzyme B appears to be designed to accommodate and cleave hexapeptides such as the sequence Ile-Glu-Thr-Asp-/Ser-Gly present in the activation site of pro-caspase-3, a proven physiological substrate of granzyme B. These granzyme B crystals, with fully accessible active sites, are well suited for soaking with small synthetic inhibitors that might be used for a treatment of chronic inflammatory disorders.

Dependence of granzyme B-mediated cell death on a pathway regulated by Bcl-2 or its viral homolog, BHRF1

The molecular pathways responsible for apoptosis in response to granzyme B have remained unresolved. Here we present data supporting the notion that granzyme B-mediated cell death is largely dependent on a pathway that is inhibitable by Bcl-2 or its viral analog BHRF1. We used a panel of stably transfected FDC-P1 mouse myeloid cell lines to show that overexpression of functional, wild-type Bcl-2 or BHRF1 rescued cells from granzyme B-mediated apoptosis, whereas mutated (Gly145-->Glu) Bcl-2, or wild-type Bcl-2 directed to the plasma membrane conferred no protection. Overexpression of Bcl-2 resulted in inhibition of multiple parameters of apoptosis in response to purified perforin and granzyme B, including DNA fragmentation, changes in light scatter profile indicating cell shrinkage and increased refractivity, loss of mitochondrial membrane potential and inhibited colony formation in clonogenic assays. Nevertheless, when exposed to cytotoxic lymphocytes, FDC-P1 and YAC-1 cells overexpressing Bcl-2 remained susceptible to death imparted by cytolytic granules, irrespective of whether the granules contained granzyme B. Thus, alternative granzyme B-independent pathways can be activated by intact lymphocytes to overcome Bcl-2-like inhibitors of apoptosis, enabling CTLs to overcome potential viral blocks to granzyme B-mediated cell death.

Direct tumor lysis by NK cells uses a Ras-independent mitogen-activated protein kinase signal pathway

Destruction of tumor cells is a key function of lymphocytes, but the molecular processes driving it are unclear. Analysis of signal molecules indicated that mitogen-activated protein kinase (MAPK)/extracellular regulated kinase 2 critically controlled lytic function in human NK cells. We now have evidence to indicate that target ligation triggers a Ras-independent MAPK pathway that is required for lysis of the ligated tumor cell. Target engagement caused NK cells to rapidly activate MAPK within 5 min, and PD098059 effectively blocked both MAPK activation and tumoricidal function in NK cells. Target engagement also rapidly activated Ras, detected as active Ras-GTP bound to GST-Raf-RBD, a GST fusion protein linked to the Raf protein fragment containing the Ras-GTP binding domain. However, Ras inactivation by pharmacological disruption with the farnesyl transferase inhibitor, FTI-277, had no adverse effect on the ability of NK cells to lyse tumor cells or to express MAPK activation upon target conjugation. Notably, MAPK inactivation with PD098059, but not Ras inactivation with FTI-277, could interfere with perforin and granzyme B polarization within NK cells toward the contacted target cell. Using vaccinia delivery of N17 Ras into NK cells, we demonstrated that IL-2 activated a Ras-dependent MAPK pathway, while target ligation used a Ras-independent MAPK pathway to trigger lysis in NK cells.