Low-dose carboplatin modifies the tumor microenvironment to augment CAR T cell efficacy in human prostate cancer models

Chimeric antigen receptor (CAR) T cells have transformed the treatment landscape for hematological malignancies. However, CAR T cells are less efficient against solid tumors, largely due to poor infiltration resulting from the immunosuppressive nature of the tumor microenvironment (TME). Here, we assessed the efficacy of Lewis Y antigen (LeY)-specific CAR T cells in patient-derived xenograft (PDX) models of prostate cancer. In vitro, LeY CAR T cells directly killed organoids derived from androgen receptor (AR)-positive or AR-null PDXs. In vivo, although LeY CAR T cells alone did not reduce tumor growth, a single prior dose of carboplatin reduced tumor burden. Carboplatin had a pro-inflammatory effect on the TME that facilitated early and durable CAR T cell infiltration, including an altered cancer-associated fibroblast phenotype, enhanced extracellular matrix degradation and re-oriented M1 macrophage differentiation. In a PDX less sensitive to carboplatin, CAR T cell infiltration was dampened; however, a reduction in tumor burden was still observed with increased T cell activation. These findings indicate that carboplatin improves the efficacy of CAR T cell treatment, with the extent of the response dependent on changes induced within the TME.

Processing efficiency in pediatric cancer survivors: A review and operationalization for outcomes research and clinical utility

OBJECTIVE

Childhood cancer and cancer-related treatments disrupt brain development and maturation, placing survivors at risk for cognitive late effects. Given that assessment tools vary widely across researchers and clinicians, it has been daunting to identify distinct patterns in outcomes across diverse cancer types and to implement systematic neurocognitive screening tools. This review aims to operationalize processing efficiency skill impairment-or inefficient neural processing as measured by working memory and processing speed abilities-as a worthwhile avenue for continued study within the context of childhood cancer.

METHODS

A comprehensive literature review was conducted to examine the existing research on cognitive late effects and biopsychosocial risk factors in order to conceptualize processing efficiency skill trends in childhood cancer survivors.

RESULTS

While a frequently reported pattern of neurobiological (white matter) and cognitive (working memory and processing speed) disruption is consistent with processing efficiency skill impairment, these weaknesses have not yet been fully operationalized in this population. We offer a theoretical model that highlights the impacts of a host of biological and environmental factors on the underlying neurobiological substrates of cancer survivors that precede and may even predict long-term cognitive outcomes and functional abilities following treatment.

CONCLUSION

The unified construct of processing efficiency may be useful in assessing and communicating neurocognitive skills in both outcomes research and clinical practice. Deficits in processing efficiency may serve as a possible indicator of cognitive late effects and functional outcomes due to the unique relationship between processing efficiency skills and neurobiological disruption following cancer treatment. Continued research along these lines is crucial for advancing childhood cancer outcomes research and improving quality of life for survivors.

A-34 Comparison of BRIEF, SRS Relationships among Children with Autism and Comorbid ADHD

Objective

Recent investigations have found specific relationships between subscales of the Social Responsiveness Scale (SRS) and the Behavior Rating Inventory of Executive Functioning (BRIEF) among individuals with autism spectrum disorder (ASD). Aim: Examine whether Social-Executive relationships differ among individuals with ASD versus comorbid autism and attention-deficit/hyperactivity disorder (ASD + ADHD).

Method

Parent ratings on the SRS and BRIEF were obtained from the ABIDE-II database. Final sample: 25 ASD, 37 ASD + ADHD (5–12 years). T-tests examined group differences in BRIEF and SRS T-scores. Hierarchical multiple regressions examined the effect of BRIEF Index T-scores (Metacognition, MI; Behavioral Regulation, BRI) on SRS total and subscale T-scores, as well as the potential moderating effect of comorbid ADHD diagnosis.

Results

T-tests revealed that the ASD + ADHD group had higher severity T-scores on the BRIEF MI (T = 3.70, p < 0.01) and SRS Awareness (T = 4.00, p < 0.001) scales. ADHD diagnosis was associated with less severe deficits in social cognition (B = −6.33, p = 0.02). However, ADHD diagnosis did not significantly moderate the relationship between BRIEF and SRS scores. The relationship between BRIEF BRI and SRS Motivation was marginally significant and stronger among children with ADHD (B = 0.81, p = 0.0513).

Conclusions

Overall, similar profile of social-executive relationships were observed among participants with ASD and ASD + ADHD. ADHD diagnosis was associated with better understanding of social situations and ability to interpret social cues appropriately. There is some indication that ADHD diagnosis may be associated with a stronger relationship between behavior regulation and social motivation. Future work should replicate these findings with a larger sample size.

The N170 event-related potential reflects delayed neural response to faces when visual attention is directed to the eyes in youths with ASD

Atypical neural response to faces is thought to contribute to social deficits in autism spectrum disorder (ASD). Compared to typically developing (TD) controls, individuals with ASD exhibit delayed brain responses to upright faces at a face-sensitive event-related potential (ERP), the N170. Given observed differences in patterns of visual attention to faces, it is not known whether slowed neural processing may simply reflect atypical looking to faces. The present study manipulated visual attention to facial features to examine whether directed attention to the eyes normalizes N170 latency in ASD. ERPs were recorded in 30 children and adolescents with ASD as well as 26 TD children and adolescents. Results replicated prior findings of shorter N170 latency to the eye region of the face in TD individuals. In contrast, those with ASD did not demonstrate modulation of N170 latency by point of regard to the face. Group differences in latency were most pronounced when attention was directed to the eyes. Results suggest that well-replicated findings of N170 delays in ASD do not simply reflect atypical patterns of visual engagement with experimental stimuli. These findings add to a body of evidence indicating that N170 delays are a promising marker of atypical neural response to social information in ASD. LAY SUMMARY: This study looks at how children's and adolescents' brains respond when looking at different parts of a face. Typically developing children and adolescents processed eyes faster than other parts of the face, whereas this pattern was not seen in ASD. Children and adolescents with ASD processed eyes more slowly than typically developing children. These findings suggest that observed inefficiencies in face processing in ASD are not simply reflective of failure to attend to the eyes.

Metacognition and behavioral regulation predict distinct aspects of social functioning in autism spectrum disorder

Executive function (EF) deficits are common in autism spectrum disorder (ASD), and previous studies suggest that EF may influence or predict social functioning. Thus, EF is a potential treatment target in this population. However, the nature of how specific metacognition and behavioral regulation components of EF may differentially impact social function remains unclear. The goal of the current study was to examine the relationships between sub-components of EF (e.g., working memory, shifting, inhibition, etc.) and social functioning as measured by parent ratings on the Behavior Rating Inventory of Executive Functioning (BRIEF) and the Social Responsiveness Scale (SRS), while controlling for the influence of age, sex, and IQ. A second goal was to examine whether BRIEF scores were predictive of clinician-rated measures of ASD symptoms. Behavioral data were acquired from the Autism Brain Imaging Data Exchange-II database and included 106 children with ASD (ages 5-13). Based on analysis of parent ratings, self-monitoring skills predicted social awareness; shifting ability predicted social cognition; working memory and monitoring skills predicted social communication; initiation predicted social motivation; and shifting ability predicted restrictive and repetitive behaviors among children with ASD. Parent ratings on the BRIEF did not predict clinician-rated measures of ASD symptoms; this requires further study. Overall, the current findings indicate that metacognition and behavioral regulation both contribute to social functioning in ASD, although they each have distinct patterns of influence on different aspects of social functioning. These findings have promising implications for tailoring social interventions for ASD that target specific EF skills.

A genome scale RNAi screen identifies GLI1 as a novel gene regulating vorinostat sensitivity

Vorinostat is an FDA-approved histone deacetylase inhibitor (HDACi) that has proven clinical success in some patients; however, it remains unclear why certain patients remain unresponsive to this agent and other HDACis. Constitutive STAT (signal transducer and activator of transcription) activation, overexpression of prosurvival Bcl-2 proteins and loss of HR23B have been identified as potential biomarkers of HDACi resistance; however, none have yet been used to aid the clinical utility of HDACi. Herein, we aimed to further elucidate vorinostat-resistance mechanisms through a functional genomics screen to identify novel genes that when knocked down by RNA interference (RNAi) sensitized cells to vorinostat-induced apoptosis. A synthetic lethal functional screen using a whole-genome protein-coding RNAi library was used to identify genes that when knocked down cooperated with vorinostat to induce tumor cell apoptosis in otherwise resistant cells. Through iterative screening, we identified 10 vorinostat-resistance candidate genes that sensitized specifically to vorinostat. One of these vorinostat-resistance genes was GLI1, an oncogene not previously known to regulate the activity of HDACi. Treatment of vorinostat-resistant cells with the GLI1 small-molecule inhibitor, GANT61, phenocopied the effect of GLI1 knockdown. The mechanism by which GLI1 loss of function sensitized tumor cells to vorinostat-induced apoptosis is at least in part through interactions with vorinostat to alter gene expression in a manner that favored apoptosis. Upon GLI1 knockdown and vorinostat treatment, BCL2L1 expression was repressed and overexpression of BCL2L1 inhibited GLI1-knockdown-mediated vorinostat sensitization. Taken together, we present the identification and characterization of GLI1 as a new HDACi resistance gene, providing a strong rationale for development of GLI1 inhibitors for clinical use in combination with HDACi therapy.

The preclinical pharmacokinetic disposition of a series of perforin-inhibitors as potential immunosuppressive agents

The cytolytic protein perforin is a key component of the immune response and is implicated in a number of human pathologies and therapy-induced conditions. A novel series of small molecule inhibitors of perforin function have been developed as potential immunosuppressive agents. The pharmacokinetics and metabolic stability of a series of 16 inhibitors of perforin was evaluated in male CD1 mice following intravenous administration. The compounds were well tolerated 6 h after dosing. After intravenous administration at 5 mg/kg, maximum plasma concentrations ranged from 532 ± 200 to 10,061 ± 12 ng/mL across the series. Plasma concentrations were greater than the concentrations required for in vitro inhibitory activity for 11 of the compounds. Following an initial rapid distribution phase, the elimination half-life values for the series ranged from 0.82 ± 0.25 to 4.38 ± 4.48 h. All compounds in the series were susceptible to oxidative biotransformation. Following incubations with microsomal preparations, a tenfold range in in vitro half-life was observed across the series. The data suggests that oxidative biotransformation was not singularly responsible for clearance of the compounds and no direct relationship between microsomal clearance and plasma clearance was observed. Structural modifications however, do provide some information as to the relative microsomal stability of the compounds, which may be useful for further drug development.

Mouse granzyme A induces a novel death with writhing morphology that is mechanistically distinct from granzyme B-induced apoptosis

Human and mouse granzyme (Gzm)B both induce target cell apoptosis in concert with pore-forming perforin (Pfp); however the mechanisms by which other Gzms induce non-apoptotic death remain controversial and poorly characterised. We used timelapse microscopy to document, quantitatively and in real time, the death of target cells exposed to primary natural killer (NK) cells from mice deficient in key Gzms. We found that in the vast majority of cases, NK cells from wild-type mice induced classic apoptosis. However, NK cells from syngeneic Gzm B-deficient mice induced a novel form of cell death characterised by slower kinetics and a pronounced, writhing, 'worm-like' morphology. Dying cells initially contracted but did not undergo membrane blebbing, and annexin-V staining was delayed until the onset of secondary necrosis. As it is different from any cell death process previously reported, we tentatively termed this cell death 'athetosis'. Two independent lines of evidence showed this alternate form of death was due to Gzm A: first, cell death was revealed in the absence of Gzm B, but was completely lost when the NK cells were deficient in both Gzm A and B; second, the athetotic morphology was precisely reproduced when recombinant mouse Gzm A was delivered by an otherwise innocuous dose of recombinant Pfp. Gzm A-mediated athetosis did not require caspase activation, early mitochondrial disruption or generation of reactive oxygen species, but did require an intact actin cytoskeleton and was abolished by latrunculin B and mycalolide B. This work defines an authentic role for mouse Gzm A in granule-induced cell death by cytotoxic lymphocytes.

Granzyme B triggers a prolonged pressure to die in Bcl-2 overexpressing cells, defining a window of opportunity for effective treatment with ABT-737

Overexpression of Bcl-2 contributes to resistance of cancer cells to human cytotoxic lymphocytes (CL) by blocking granzyme B (GraB)-induced mitochondrial outer membrane permeabilization (MOMP). Drugs that neutralise Bcl-2 (e.g., ABT-737) may therefore be effective adjuvants for immunotherapeutic strategies that use CL to kill cancer cells. Consistent with this we found that ABT-737 effectively restored MOMP in Bcl-2 overexpressing cells treated with GraB or natural killer cells. This effect was observed even if ABT-737 was added up to 16 h after GraB, after which the cells reset their resistant phenotype. Sensitivity to ABT-737 required initial cleavage of Bid by GraB (gctBid) but did not require ongoing GraB activity once Bid had been cleaved. This gctBid remained detectable in cells that were sensitive to ABT-737, but Bax and Bak were only activated if ABT-737 was added to the cells. These studies demonstrate that GraB generates a prolonged pro-apoptotic signal that must remain active for ABT-737 to be effective. The duration of this signal is determined by the longevity of gctBid but not activation of Bax or Bak. This defines a therapeutic window in which ABT-737 and CL synergise to cause maximum death of cancer cells that are resistant to either treatment alone, which will be essential in defining optimum treatment regimens.

Granzymes, cytotoxic granules and cell death: the early work of Dr. Jurg Tschopp

Within the powerful legacy left by Jurg Tschopp, we should not forget his early work that helped to elucidate the molecular pathways responsible for the clearance of virus-infected and transformed cells by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. Jurg's skilful biochemical approach formed a firm platform upon which the work of so many other biochemists, cell biologists and immunologists would come to rely. Jurg coined the shorthand term 'granzyme' to denote the individual members of a family of serine proteases sequestered in and secreted from the cytotoxic granules of CTL/NK cells. He was also one of the first to describe the lytic properties of purified perforin and to postulate the synergy of perforin and granzymes, which we now know to underpin target cell apoptosis. Jurg was a major protagonist in the debate that raged throughout the 1980's and early 1990's on the physiological relevance of the 'granule exocytosis' pathway. Ultimately, resolving this issue led Jurg and his colleagues to even greater and impactful discoveries in the broader field of apoptosis research. Jurg Tschopp ranks with other pioneers, particularly Gideon Berke, Chris Bleackley, Pierre Golstein, Pierre Henkart and Eckhard Podack for making seminal discoveries on our understanding of how the immune system eliminates dangerous cells.

The structure and function of mammalian membrane-attack complex/perforin-like proteins

The membrane-attack complex (MAC) of complement pathway and perforin (PF) are important tools deployed by the immune system to target pathogens. Both perforin and the C9 component of the MAC contain a common 'MACPF' domain and form pores in the cell membrane as part of their function. The MAC targets gram-negative bacteria and certain pathogenic parasites, while perforin, released by natural killer cells or cytotoxic T lymphocytes (CTLs), targets virus-infected and transformed host cells (1). Remarkably, recent structural studies show that the MACPF domain is homologous to the pore-forming portion of bacterial cholesterol-dependent cytolysins; these data have provided important insight into the mechanism of pore-forming MACPF proteins. In addition to their role in immunity, MACPF family members have been identified as animal venoms, factors required for pathogen migration across host cell membranes and factors that govern developmental processes such as embryonic patterning and neuronal guidance (2). While most MACPF proteins characterized to date either form pores or span lipid membranes, some do not (e.g. the C6 component of the MAC). A current challenge is thus to understand the role, pore forming or otherwise, of MACPF proteins in developmental biology. This review discusses structural and functional diversity of the mammalian MACPF proteins.

Ex vivo culture of chimeric antigen receptor T cells generates functional CD8+ T cells with effector and central memory-like phenotype

The anti-tumor efficacy of adoptively transferred T cells requires their in vivo persistence and memory polarization. It is unknown if human chimeric antigen receptor (CAR)-expressing T cells can also undergo memory polarization. We examined the functional status of CAR CD8(+) T cells, re-directed to Lewis Y antigen (LeY-T), throughout a period of ex vivo expansion. Immediately before culture CD8(+) T cells comprised a mixture of phenotypes including naive (CD45RA(+)/CCR7(+)/CD27(+)/CD28(+)/perforin-), central memory (CM, CD45RA(-)/CCR7(lo)/CD27(+)/CD28(+)/perforin(lo)), effector memory (EM, CD45RA(-)/CCR7(-)/CD27(+)/CD28(+)/perforin(mod)) and effector (Eff, CD45RA(+)/CCR7(-)/CD27(-)/CD28(-)/perforin(hi)) cells. After transduction and expansion culture of peripheral blood mononuclear cells from normal donors or multiple myeloma patients, CD8(+) LeY-T cells polarized to EM- and CM-like phenotype. CD8(+) LeY-T cells differed from starting CD8(+) CM and EM T cells in that CD27, but not CD28, was downregulated. In addition, CD8(+) LeY-T cells expressed high levels of perforin, similar to starting CD8(+) Eff. CD8(+) LeY-T cells also showed hallmarks of both memory and Eff function, underwent homeostatic proliferation in response to interleukin (IL)-15, and showed interferon (IFN)-γ production and cytotoxicity in response to Le-Y antigen on OVCAR-3 (human ovarian adenocarcinoma) cells. This study confirms CD8(+) LeY-T cells have a CM- and EM-like phenotype and heterogeneous function consistent with potential to persist in vivo after adoptive transfer.

Visualizing CTL activity for different CD8+ effector T cells supports the idea that lower TCR/epitope avidity may be advantageous for target cell killing

Time-lapse video microscopy allows analysis of the interaction between individual CTLs and adherent peptide-pulsed targets, from contact, to lymphocyte detachment, APC rounding, phosphatidylserine exposure and finally loss of plasma membrane integrity characteristic of end-stage apoptosis. Using in vitro-stimulated effectors specific for the ovalbumin K(b)OVA(257) (OT-I) and influenza A virus D(b)NP(366) and D(b)PA(224) epitopes, no significant correlation was found between the duration of CTL contact and the time to phosphatidylserine exposure or loss of membrane integrity. Furthermore, there were minimal indications that transgenic T cells specific for the K(b)OVA(257) epitope (TCR) diversity had any effect. However, when the analysis was repeated with D(b)NP(366) and D(b)PA(224)-specific CTLs recovered directly from the lungs of mice with influenza pneumonia, the lower avidity D(b)NP(366)-specific set was found to elute much more quickly. Shorter contact time may allow individual CTLs to lyse more targets, suggesting that lower TCR/epitope avidity may be more beneficial than higher epitope avidity for cell-mediated immunity.

Blocking granule-mediated death by primary human NK cells requires both protection of mitochondria and inhibition of caspase activity

Human GraB (hGraB) preferentially induces apoptosis via Bcl-2-regulated mitochondrial damage but can also directly cleave caspases and caspase substrates in cell-free systems. How hGraB kills cells when it is delivered by cytotoxic lymphocytes (CL) and the contribution of hGraB to CL-induced death is still not clear. We show that primary human natural killer (hNK) cells, which specifically used hGraB to induce target cell death, were able to induce apoptosis of cells whose mitochondria were protected by Bcl-2. Purified hGraB also induced apoptosis of Bcl-2-overexpressing targets but only when delivered at 5- to 10-fold the concentration required to kill cells expressing endogenous Bcl-2. Caspases were critical in this process as inhibition of caspase activity permitted clonogenic survival of Bcl-2-overexpressing cells treated with hGraB or hNK cells but did not protect cells that only expressed endogenous Bcl-2. Our data therefore show that hGraB triggers caspase activation via mitochondria-dependent and mitochondria-independent mechanisms that are activated in a hierarchical manner, and that the combined effects of Bcl-2 and direct caspase inhibition can block cell death induced by hGraB and primary hNK cells.

Functional dissociation of ΔΨm and cytochrome c release defines the contribution of mitochondria upstream of caspase activation during granzyme B-induced apoptosis

Loss of Bid confers clonogenic survival to granzyme B-treated cells, however the exact role of Bid-induced mitochondrial damage--upstream or downstream of caspases--remains controversial. Here we show that direct cleavage of Bid by granzyme B, but not caspases, was required for granzyme B-induced apoptosis. Release of cytochrome c and SMAC, but not AIF or endonuclease G, occurred in the absence of caspase activity and correlated with the onset of apoptosis and loss of clonogenic potential. Loss of mitochondrial trans-membrane potential (DeltaPsim) was also caspase independent, however if caspase activity was blocked the mitochondria regenerated their DeltaPsim. Loss of DeltaPsim was not required for rapid granzyme B-induced apoptosis and regeneration of DeltaPsim following cytochrome c release did not confer clonogenic survival. This functional dissociation of cytochrome c and SMAC release from loss of DeltaPsim demonstrates the essential contribution of Bid upstream of caspase activation during granzyme B-induced apoptosis.

Lack of apoptosis of Sezary cells in the circulation following oral bexarotene therapy

BACKGROUND

Apoptosis of malignant cells has been suggested as an important mechanism of the action of bexarotene in the treatment of cutaneous T-cell lymphoma (CTCL).

OBJECTIVES

Our purpose was to examine the in vivo and in vitro responses of patients with Sézary syndrome treated with oral bexarotene and assess them for apoptosis of the Sézary cells.

METHODS

Six patients with CTCL with circulating Sézary cells, participating in a clinical trial of oral bexarotene (300 mg m(-2) daily) were included in the study. Peripheral blood from the patients was analysed for in vivo and in vitro apoptosis.

RESULTS

None of the six patients demonstrated in vivo apoptosis. In vitro apoptosis of Sézary cells was demonstrated in one patient following exogenous bexarotene.

CONCLUSIONS

Apoptosis is not detectable in the circulation of patients with Sézary syndrome treated with bexarotene.

Critical role of the transcription factor AP-1 for the constitutive and interferon-induced expression of IFI 16

IFI 16 is a member of the HIN-200 family of transcriptional regulators that suppress cell growth, modulate the cell cycle and have been linked to cellular differentiation. We hypothesized that the activity of IFI 16 depends on its level of expression and therefore studied the transcriptional activity of the IFI 16 promoter. A discrete sequence within the 5' untranslated region was required for constitutive activity of the promoter and the functional motif within this region was shown to be a consensus AP-1 site. Interestingly, this AP-1 site was also critical for IFN-induced activation of the promoter and consistent with these observations, treatment of cells with IFNgamma resulted in a rapid and robust induction of AP-1 activity that preceded expression of IFI 16. These experiments define the transcriptional mechanisms of IFI 16 gene regulation and provide evidence suggesting that AP-1 activation may be an important event in IFN signaling.

Mechanisms of interferon mediated anti-viral resistance

Interferons (IFNs) are an important part of immune responses and are believed to protect the host from viral and bacterial pathogens as well as having a role in rejection of malignancies. The well-known anti-viral and cytostatic properties of IFNs have led to the clinical use of these proteins to treat some cancers and viral infections. Extensive research has begun to unravel much of the molecular basis for the biological effects of IFNs, and this information could now be used as a foundation for the development of novel therapeutic strategies that avoid some of the acknowledged shortcomings of cytokine therapies. This review explains the current model of IFN action, during viral infections and the potential for well-established and emerging groups of IFN inducible genes as therapeutic targets is highlighted.

CTL: Caspases Terminate Life, but that's not the whole story

The induction of cell death by cytotoxic T-lymphocytes (CTL) or natural killer (NK) cells is one of the main ways by which higher organisms protect themselves from rogue cells, including those infected by a virus, or posing a risk of cancer. Considering the rapidity of viral replication and spread to uninfected cells, CTL and NK are extremely efficient killers. This is at least partly due to the variety of pathways that these cytolytic lymphocytes (CL) can use to ensure the death of a cell. Primarily, CL utilize two independently initiated pathways involving either ligation of death receptors or perforin mediated trafficking of granzyme B to the target cell cytosol to activate a family of death proteases (caspases) in the target cell. The caspases then orchestrate the orderly dismantling of that cell by cleavage of a set of critical substrates. If caspases are inactivated, due either to mutations in proteins that signal their activation or direct inhibition by a viral gene product, CL can utilize a caspase-independent pathway to ensure the death of the target cell. Here we will discuss the mechanisms by which these stellar killers achieve their goal.

Perforin-independent expression of granzyme B and proteinase inhibitor 9 in human testis and placenta suggests a role for granzyme B-mediated proteolysis in reproduction

Granzyme B (graB) plays a pivotal role in cytotoxic lymphocyte granule-mediated apoptosis through cleavage of intracellular proteins in target cells. Proteinase inhibitor-9 (PI-9) is a potent inhibitor of graB and is highly expressed in cytotoxic lymphocytes. Here, we show by immunohistochemistry that PI-9 is also abundantly expressed in human testicular Sertoli cells and placental syncytial trophoblasts. Postulating that PI-9 protects these tissues from graB-producing auto- or allo-reactive cytotoxic lymphocytes, we also stained sections for graB. Unexpectedly, graB was observed in non-cytotoxic cells in both tissues. In the adult human testis, graB was present in spermatogenic cells within the seminiferous tubule, and this was verified by in-situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR). Immunohistochemical analysis of term placentae demonstrated graB in syncytial trophoblasts, and this was confirmed by RT-PCR on primary trophoblasts from term placenta. Perforin, which is co-produced with graB by activated cytotoxic lymphocytes and is required for graB release into the target cell, was not detected in either testis or placenta. We postulate that, in these organs, graB has a perforin-independent role, involving hydrolysis of extracellular matrix components. In the testis, graB may facilitate migration of developing germ cells, while in the placenta, it may contribute to extracellular matrix remodelling during parturition.

Lymphocyte-mediated immunosurveillance of epithelial cancers?

The validity and importance of the immunosurveillance of tumors have been debated ever since Burnet and Thomas outlined several key concepts that underpin our understanding of tumor immunity. With the development of gene-targeting technology, it has been shown that interferon gamma (IFN-gamma) and perforin--key effector molecules of both the innate and adaptive immune systems--contribute to the prevention of of tumors in mice. These findings have rejuvenated interest in the immunosurveillance of tumors, and recent data have provided evidence that the immune system influences the development of some spontaneous epithelial malignancies.

Mechanisms of interferon mediated anti-viral resistance

Interferons (IFNs) are an important part of immune responses and are believed to protect the host from viral and bacterial pathogens as well as having a role in rejection of malignancies. The well-known anti-viral and cytostatic properties of IFNs have led to the clinical use of these proteins to treat some cancers and viral infections. Extensive research has begun to unravel much of the molecular basis for the biological effects of IFNs, and this information could now be used as a foundation for the development of novel therapeutic strategies that avoid some of the acknowledged shortcomings of cytokine therapies. This review explains the current model of IFN action, during viral infections and the potential for well-established and emerging groups of IFN inducible genes as therapeutic targets is highlighted.

Unlocking the secrets of cytotoxic granule proteins

Cytotoxic lymphocytes largely comprise CD8(+) cytotoxic T cells and natural killer cells and form the major defense of higher organisms against virus-infected and transformed cells. A key function of cytotoxic lymphocytes is to detect and eliminate potentially harmful cells by inducing them to undergo apoptosis. This is achieved through two principal pathways, both of which require direct but transient contact between the killer cell and its target. The first, involving ligation of TNF receptor-like molecules such as Fas/CD95 by their cognate ligands, results in mobilization of conventional, programmed cell-death pathways centered on activation of pro-apoptotic caspases. This review concentrates on the second pathway, in which the toxic contents of secretory vesicles of the cytotoxic lymphocyte are secreted toward the target cell, and some toxins penetrate into the target cell cytoplasm and nucleus. In addition to invoking a powerful stimulus to caspase activation, this "granule-exocytosis mechanism" provides a variety of additional strategies for overcoming inhibitors of the caspase cascade that may be elaborated by viruses. The key molecular players in this process are the pore-forming protein perforin and a family of granule-bound serine proteases or granzymes. The molecular functions of perforin and granzymes are under intense investigation in many laboratories including our own, and recent advances will be discussed. In addition, this review discusses the evidence pointing to the importance of perforin and granzyme function in pathophysiological situations as diverse as infection with intracellular pathogens, graft versus host disease, susceptibility to transplantable and spontaneous malignancies, lymphoid homeostasis, and the tendency to auto-immune diseases.

A fresh look at tumor immunosurveillance and immunotherapy

Despite major advances in our understanding of adaptive immunity and dendritic cells, consistent and durable responses to cancer vaccines remain elusive and active immunotherapy is still not an established treatment modality. The key to developing an effective anti-tumor response is understanding why, initially, the immune system is unable to detect transformed cells and is subsequently tolerant of tumor growth and metastasis. Ineffective antigen presentation limits the adaptive immune response; however, we are now learning that the host's innate immune system may first fail to recognize the tumor as posing a danger. Recent descriptions of stress-induced ligands on tumor cells recognized by innate effector cells, new subsets of T cells that regulate tumor tolerance and the development of spontaneous tumors in mice that lack immune effector molecules, beckon a reflection on our current perspectives on the interaction of transformed cells with the immune system and offer new hope of stimulating therapeutic immunity to cancer.

Novel properties of the protein kinase CK2-site-regulated nuclear- localization sequence of the interferon-induced nuclear factor IFI 16

Members of the interferon-induced class of nuclear factors possess a putative CcN motif, comparable with that within proteins such as the simian virus 40 large tumour antigen (T-ag), which confers phosphorylation-mediated regulation of nuclear-localization sequence (NLS)-dependent nuclear import. Here we examine the functionality of the interferon-induced factor 16 (IFI 16) CcN motif, demonstrating its ability to target a heterologous protein to the nucleus, and to be phosphorylated specifically by the CcN-motif-phosphorylating protein kinase CK2 (CK2). The IFI 16 NLS, however, has novel properties, conferring ATP-dependent nuclear import completely independent of cytosolic factors, as well as binding to nuclear components. The IFI 16 NLS is not recognized with high affinity by the NLS-binding importin heterodimer, and transport mediated by it is insensitive to non-hydrolysable GTP analogues. The IFI 16 NLS thus mediates nuclear import through a pathway completely distinct from that of conventional NLSs, such as that of T-ag, but intriguingly resembling that of the NLS of the HIV-1 transactivator protein Tat. Since the IFI 16 CK2 site enhances nuclear import through facilitating binding to nuclear components, this represents a novel mechanism by which the site regulates nuclear-protein import, and constitutes a difference between the IFI 16 and Tat NLSs that may be of importance in the immune response.

Redirecting mouse CTL against colon carcinoma: superior signaling efficacy of single-chain variable domain chimeras containing TCR-zeta vs Fc epsilon RI-gamma

The structurally related TCR-zeta and Fc receptor for IgE (Fc epsilon RI)-gamma are critical signaling components of the TCR and Fc epsilon RI, respectively. Although chimeric Ab receptors containing zeta and gamma signaling chains have been used to redirect CTL to tumors, a direct comparison of their relative efficacy has not previously been undertaken. Here, in naive T lymphocytes, we compare the signaling capacities of the zeta and gamma subunits within single-chain variable domain (scFv) chimeric receptors recognizing the carcinoembryonic Ag (CEA). Using a very efficient retroviral gene delivery system, high and equivalent levels of scFv-zeta and scFv-gamma receptors were expressed in T cells. Despite similar levels of expression and Ag-specific binding to colon carcinoma target cells, ligation of scFv-anti-CEA-zeta chimeric receptors on T cells resulted in greater cytokine production and direct cytotoxicity than activation via scFv-anti-CEA-gamma receptors. T cells expressing scFv-zeta chimeric receptors had a greater capacity to control the growth of human colon carcinoma in scid/scid mice or mouse colon adenocarcinoma in syngeneic C57BL/6 mice. Overall, these data are the first to directly compare and definitively demonstrate the enhanced potency of T cells activated via the zeta signaling pathway.

Granzymes: a family of lymphocyte granule serine proteases

SUMMARY

Granzymes, a family of serine proteases, are expressed exclusively by cytotoxic T lymphocytes and natural killer (NK) cells, components of the immune system that protect higher organisms against viral infection and cellular transformation. Following receptor-mediated conjugate formation between a granzyme-containing cell and an infected or transformed target cell, granzymes enter the target cell via endocytosis and induce apoptosis. Granzyme B is the most powerful pro-apoptotic member of the granzyme family. Like caspases, cysteine proteases that play an important role in apoptosis, it can cleave proteins after acidic residues, especially aspartic acid. Other granzymes may serve additional functions, and some may not induce apoptosis. Granzymes have been well characterized only in human and rodents, and can be grouped into three subfamilies according to substrate specificity: members of the granzyme family that have enzymatic activity similar to the serine protease chymotrypsin are encoded by a gene cluster termed the 'chymase locus'; granzymes with trypsin-like specificities are encoded by the 'tryptase locus'; and a third subfamily cleaves after unbranched hydrophobic residues, especially methionine, and is encoded by the 'Met-ase locus'. All granzymes are synthesized as zymogens and, after clipping of the leader peptide, maximal enzymatic activity is achieved by removal of an amino-terminal dipeptide. They can all be blocked by serine protease inhibitors, and a new group of inhibitors has recently been identified - serpins, some of which are specific for granzymes. Future studies of serpins may bring insights into how cells that synthesize granzymes are protected from inadvertent cell suicide.

Granzyme A and B-deficient killer lymphocytes are defective in eliciting DNA fragmentation but retain potent in vivo anti-tumor capacity

Recent studies have demonstrated that granzymes A and B make an important contribution to the clearance of the orthopoxvirus ectromelia, and in graft versus host disease. To test whether granzymes are generally necessary for lymphocyte-mediated cytotoxicity in vivo, we assessed the cytotoxic capacity of granzyme A and/or B-deficient lymphocytes in several perforin-dependent settings. Splenocytes and allogeneic CTL of granzyme A and/or B-deficient mice were defective for induction of DNA fragmentation, but induced significant membrane damage and target cell death. These results correlated well with the behavior of granzyme A/B-deficient CTL and NK cells in three different perforin-dependent tumor models. In a classical assay of NK cell-mediated rejection, granzyme A and/or B-deficient mice inoculated with RMA-S cells were as susceptible to tumor as wild-type mice. Perforin-deficient mice were also considerably more susceptible to tumor initiation by methylcholanthrene than granzyme A and/or B-deficient mice. Furthermore, rejection of the K1735-melanoma expressing MHC class I and II molecules was mediated by adoptively transferred H-2b anti-k CTL from immunized granzyme A and/or B-deficient mice. In summary, these data suggest that granzymes A and B are not critical for most anti-tumor effector functions of NK cells and CTL that are perforin mediated.

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.

Functional interaction between p53 and the interferon-inducible nucleoprotein IFI 16

Interferons are important in regulating cell growth and differentiation, immune function and initiating anti-viral responses. While the pleotrophic actions of interferons have been well documented, the molecular mechanisms underpinning their biological effects have not been fully characterized. IFI 16 is a member of the interferon-inducible HIN-200 family of nuclear proteins, which we have recently shown can function as a potent transcriptional repressor. A murine member of the HIN-200 family, p202, can indirectly interact with p53 via the p53 binding protein (p53bp) and inhibit p53-mediated transcriptional activation. The binding activity of p202 to p53bp was shown to require the conserved MFHATVAT motif present in all 200 amino acid repeat regions of HIN-200 proteins. Given that IFI 16 contains two MFHATVAT motifs, we sought to determine whether IFI 16 may form a complex with p53 and if so to ascertain the functional significance of this interaction. We demonstrate that IFI 16 can directly bind to the C-terminal region of p53 and augment p53-mediated transcriptional activation without altering the steady state levels of p53. Thus, in addition to its ability to directly regulate gene expression, IFI 16 can also modulate the transcription function of other cellular transcription factors. These findings demonstrate a possible link between gene induction following interferon stimulation and p53-mediated cellular events.

Initiation of apoptosis by granzyme B requires direct cleavage of bid, but not direct granzyme B-mediated caspase activation

The essential upstream steps in granzyme B-mediated apoptosis remain undefined. Herein, we show that granzyme B triggers the mitochondrial apoptotic pathway through direct cleavage of Bid; however, cleavage of procaspases was stalled when mitochondrial disruption was blocked by Bcl-2. The sensitivity of granzyme B-resistant Bcl-2-overexpressing FDC-P1 cells was restored by coexpression of wild-type Bid, or Bid with a mutation of its caspase-8 cleavage site, and both types of Bid were cleaved. However, Bid with a mutated granzyme B cleavage site remained intact and did not restore apoptosis. Bid with a mutation preventing its interaction with Bcl-2 was cleaved but also failed to restore apoptosis. Rapid Bid cleavage by granzyme B (<2 min) was not delayed by Bcl-2 overexpression. These results clearly placed Bid cleavage upstream of mitochondrial Bcl-2. In granzyme B-treated Jurkat cells, endogenous Bid cleavage and loss of mitochondrial membrane depolarization occurred despite caspase inactivation with z-Val-Ala-Asp-fluoromethylketone or Asp-Glu-Val-Asp-fluoromethylketone. Initial partial processing of procaspase-3 and -8 was observed irrespective of Bcl-2 overexpression; however, later processing was completely abolished by Bcl-2. Overall, our results indicate that mitochondrial perturbation by Bid is necessary to achieve a lethal threshold of caspase activity and cell death due to granzyme B.

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.

Proapoptotic functions of cytotoxic lymphocyte granule constituents in vitro and in vivo

Recent advances in our understanding of cytolytic effector mechanisms include the partial characterization of caspase-independent apoptotic pathways triggered by granzymes, a realization of the vital importance of perforin and granzymes in the defence against certain virus infections in vivo and the first description of hereditary immunodeficiency due to disordered perforin expression in humans.

Redirected perforin-dependent lysis of colon carcinoma by ex vivo genetically engineered CTL

The redirection of autologous lymphocytes to predefined tumor target Ags has considerable potential for the immunotherapeutic treatment of cancer; however, robust experimental systems for comparing various approaches have not been developed. Herein, we have generated a single chain variable domain anti-carcinoembryonic Ag (CEA) Fcepsilon receptor I gamma-chain fusion (scFv anti-CEA) receptor and demonstrated high-level expression of this chimeric receptor in naive mouse T lymphocytes by retroviral gene transduction. These gene-modified CTL were able to lyse CEA+ targets and secrete high levels of IFN-gamma following Ag stimulation. Depletion studies demonstrated that specific tumor cell cytotoxicity was mediated by gene-modified CD8+ T cells. Importantly, in increasingly stringent tests of efficacy in vivo, transduced CTL were sequentially shown to reject CEA+ colon carcinoma cells in a Winn assay and then reject established s.c. colon carcinoma in scid or syngeneic mice. Furthermore, using gene-targeted and scFv anti-CEA receptor-transduced donor CTL, perforin and IFN-gamma were demonstrated to be absolutely critical for the eradication of colon carcinoma in mice. In summary, we have developed a highly efficient gene transfer system for evaluating chimeric receptor expression in cytotoxic lymphocytes. This series of experiments has revealed the utility of scFv anti-CEA chimeras in providing mouse T cells the capacity to reject colon carcinoma in an Ag- and perforin-specific manner.

Differential tumor surveillance by natural killer (NK) and NKT cells

Natural tumor surveillance capabilities of the host were investigated in six different mouse tumor models where endogenous interleukin (IL)-12 does or does not dictate the efficiency of the innate immune response. Gene-targeted and lymphocyte subset-depleted mice were used to establish the relative importance of natural killer (NK) and NK1.1(+) T (NKT) cells in protection from tumor initiation and metastasis. In the models examined, CD3(-) NK cells were responsible for tumor rejection and protection from metastasis in models where control of major histocompatibility complex class I-deficient tumors was independent of IL-12. A protective role for NKT cells was only observed when tumor rejection required endogenous IL-12 activity. In particular, T cell receptor Jalpha281 gene-targeted mice confirmed a critical function for NKT cells in protection from spontaneous tumors initiated by the chemical carcinogen, methylcholanthrene. This is the first description of an antitumor function for NKT cells in the absence of exogenously administered potent stimulators such as IL-12 or alpha-galactosylceramide.

A direct comparison of cytolytic T-lymphocyte responses to Melan-A peptides in vitro: differential immunogenicity of Melan-A27-35 and Melan-A26-35

In this study we directly compared the in vitro responses of T-cells from normal donors and melanoma patients to Melan-A27-35 and Melan-A26-35. These peptides have been previously used in peptide-based vaccination studies. Following three stimulations with peptide-pulsed antigen-presenting cells in vitro, Melan-A-specific cytolytic T-lymphocytes (CTLs) were generated from seven of 20 subjects; two of the seven subjects responded reproducibly to both Melan-A27-35 and Melan-A26-35, three to only Melan-A27-35 and two to only Melan-A26-35. However, CTLs generated with either Melan-A27-35 or Melan-A26-35 showed cross recognition, and both types of CTL could recognize naturally processed antigen displayed on HLA-A2+ tumour cells. Furthermore, Melan-A-specific CTLs could also be generated by stimulating peripheral blood mononuclear cells with autologous melanoma cells. Our results suggest that some subjects may have a bias in their CTL repertoire which favours the generation of Melan-A27-35 specific CTLs, while others may favour Melan-A26-35 specific CTLs. It is also likely that CTL precursors capable of detecting both peptides may have different affinities to the two Melan-A peptides. Since it is difficult to predict the CTL responses to Melan-A peptide in a given individual, we suggest vaccinating with both Melan-A27-35 and Melan-A26-35 peptides in clinical trials.

Cytosolic delivery of granzyme B by bacterial toxins: evidence that endosomal disruption, in addition to transmembrane pore formation, is an important function of perforin

Granule-mediated cell killing by cytotoxic lymphocytes requires the combined actions of a membranolytic protein, perforin, and granule-associated granzymes, but the mechanism by which they jointly kill cells is poorly understood. We have tested a series of membrane-disruptive agents including bacterial pore-forming toxins and hemolytic complement for their ability to replace perforin in facilitating granzyme B-mediated cell death. As with perforin, low concentrations of streptolysin O and pneumolysin (causing <10% (51)Cr release) permitted granzyme B-dependent apoptosis of Jurkat and Yac-1 cells, but staphylococcal alpha-toxin and complement were ineffective, regardless of concentration. The ensuing nuclear apoptotic damage was caspase dependent and included cleavage of poly(ADP-ribose) polymerase, suggesting a mode of action similar to that of perforin. The plasma membrane lesions formed at low dose by perforin, pneumolysin, and streptolysin did not permit diffusion of fluorescein-labeled proteins as small as 8 kDa into the cell, indicating that large membrane defects are not necessary for granzymes (32 to 65 kDa) to enter the cytosol and induce apoptosis. The endosomolytic toxin, listeriolysin O, also effected granzyme B-mediated cell death at concentrations which produced no appreciable cell membrane damage. Cells pretreated with inhibitors of endosomal trafficking such as brefeldin A took up granzyme B normally but demonstrated seriously impaired nuclear targeting of granzyme B when perforin was also added, indicating that an important role of perforin is to disrupt vesicular protein trafficking. Surprisingly, cells exposed to granzyme B with perforin concentrations that produced nearly maximal (51)Cr release (1,600 U/ml) also underwent apoptosis despite excluding a 8-kDa fluorescein-labeled protein marker. Only at concentrations of >4,000 U/ml were perforin pores demonstrably large enough to account for transmembrane diffusion of granzyme B. We conclude that pore formation may allow granzyme B direct cytosolic access only when perforin is delivered at very high concentrations, while perforin's ability to disrupt endosomal trafficking may be crucial when it is present at lower concentrations or in killing cells that efficiently repair perforin pores.

The human cytotoxic T cell granule serine protease granzyme H has chymotrypsin-like (chymase) activity and is taken up into cytoplasmic vesicles reminiscent of granzyme B-containing endosomes

Serine proteases (granzymes) contained within the cytoplasmic granules of cytotoxic T cells and natural killer cells play a variety of roles including the induction of target cell apoptosis, breakdown of extracellular matrix proteins and induction of cytokine secretion by bystander leukocytes. Different granzymes display proteolytic specificities that mimic the activities of trypsin or chymotrypsin, or may cleave substrates at acidic ("Asp-ase") or at long unbranched amino acids such as Met ("Met-ase"). Here, we report that recombinant granzyme H has chymotrypsin-like (chymase) activity, the first report of a human granzyme with this proteolytic specificity. Recombinant 32-kDa granzyme H expressed in the baculovirus vector pBacPAK8 was secreted from Sf21 cells and recovered by Ni-affinity chromatography, using a poly-His tag encoded at the predicted carboxyl terminus of full-length granzyme H cDNA. The granzyme H efficiently cleaved Suc-Phe-Leu-Phe-SBzl (v = 185 nM/s at [S] = 0.217 mM) and also hydrolyzed Boc-Ala-Ala-X-SBzl (X = Phe, Tyr, Met, Nle, or Nva) with slower rates but had little tryptase or Asp-ase activity. Enzymatic activity was inhibited completely by 0.1 mM 3,4-dichloroisocoumarin and 84% by 1.0 mM phenylmethylsulfonyl fluoride. Fluoresceinated granzyme H was internalized in a temperature-dependent manner by Jurkat cells into endosome-like vesicles, suggesting that it can bind to cell surface receptors similar to those that bind granzyme B. This suggests a hitherto unsuspected intracellular function for granzyme H.

CTL granules: evolution of vesicles essential for combating virus infections

Viral strategies for escaping apoptosis have co-evolved with the immune system, resulting in a complex balance of pro- and anti-apoptotic forces in virus-infected cells under attack by cytotoxic T lymphocytes (CTLs). Here, Joseph Trapani and colleagues argue that CTL cytolytic granules are the principal apoptotic means of eliminating viruses and possess multiple independent mechanisms to counter the viral anti-apoptotic machinery.

Perforin-dependent nuclear targeting of granzymes: A central role in the nuclear events of granule-exocytosis-mediated apoptosis?

Programmed cell death, apoptosis, involves very distinctive changes within the target cell nucleus, including margination of the chromatin, DNA fragmentation and breakdown of the nuclear envelope. Cytolytic granule-mediated target cell apoptosis is effected, in part, through synergistic action of the membrane-acting protein perforin and serine proteases, such as granzymes A or B. Recent work using confocal laser scanning microscopy as well as other techniques supports the idea that perforin-dependent translocation of granzymes to the nucleus of target cells plays a central role in effecting the nuclear changes associated with apoptosis. In vitro experiments indicate that granzyme nuclear import follows a novel pathway, being independent of ATP, not inhibitable by non-hydrolysable GTP analogues and involving binding within the nucleus, unlike conventional signal- dependent nuclear protein import. In intact cells, perforin-dependent nuclear entry of granzymes precedes the nuclear events of apoptosis such as DNA fragmentation and nuclear envelope breakdown; prevention of granzyme nuclear translocation through bcl2 overexpression or treatment of target cells with inhibitors of caspase activation blocks these events. Nuclear localization of granzymes thus appears to be central to induction of the nuclear changes associated with cytolytic granule-mediated apoptosis.

BCL-2 blocks perforin-induced nuclear translocation of granzymes concomitant with protection against the nuclear events of apoptosis

Cytolytic granule-mediated target cell killing is effected in part through the synergistic action of the membrane-acting protein perforin and serine proteases such as granzymes (Gr) A and B. In this study, we examine the subcellular distribution of granzymes in the presence of perforin and the induction of apoptosis in mouse FDC-P1 myeloid and YAC-1 lymphoma cells that express the proto-oncogene bcl2. Using confocal laser scanning microscopy to visualize and quantitate subcellular transport of fluoresceinated granzyme, we find that granzyme entry into the cytoplasm in the absence of perforin is not impaired in the bcl2-expressing lines. However, perforin-dependent enhancement of granzyme cellular uptake and, importantly, granzyme redistribution to the nucleus were strongly inhibited in the bcl2-expressing lines, concomitant with greatly increased resistance to granzyme/perforin-induced cell death. DNA fragmentation induced by granzyme/perforin was severely reduced in the bcl2-expressing lines, implying that prevention of granzyme nuclear translocation blocks the nuclear events of apoptosis. The kinetics of GrB nuclear uptake and induction of apoptosis were faster than for GrA, whereas YAC-1 cells showed greater resistance to granzyme nuclear uptake and apoptosis than FDC-P1 cells. In all cases, granzyme nuclear accumulation in the presence of perforin correlated precisely with ensuing apoptosis. All results supported the idea that GrA and GrB share a common, specific nuclear targeting pathway that contributes significantly to the nuclear changes of apoptosis.

Anti-viral strategies of cytotoxic T lymphocytes are manifested through a variety of granule-bound pathways of apoptosis induction

Cytotoxic T cells and natural killer cells together constitute a major defence against virus infection, through their ability to induce apoptotic death in infected cells. These cytolytic lymphocytes kill their targets through two principal mechanisms, and one of these, granule exocytosis, is essential for an effective in vivo immune response against many viruses. In recent years, the authors and other investigators have identified several distinct mechanisms that can induce death in a targeted cell. In the present article, it is postulated that the reason for this redundancy of lethal mechanisms is to deal with the array of anti-apoptotic molecules elaborated by viruses to extend the life of infected cells. The fate of such a cell therefore reflects the balance of pro-apoptotic (immune) and anti-apoptotic (viral) strategies that have developed over eons of evolutionary time.

A new intracellular serine protease inhibitor expressed in the rat pituitary gland complexes with granzyme B

We have cloned a novel serpin (raPIT5a) from a rat pituitary cDNA library which is structurally related to members of the ovalbumin subfamily of serine protease inhibitors. This new cDNA encodes a 374-amino acid protein, designated raPIT5a. raPIT5a was expressed in specific cells in the intermediate and anterior lobes of the pituitary. Recombinant raPIT5a was not secreted suggesting raPIT5a functions to inhibit intracellular proteases. Recombinant raPIT5a formed an SDS-stable complex with human granzyme B, a serine protease which induces apoptosis by activating members of the caspase enzyme family. These data suggest raPIT5a may have a role in regulating granzyme B or related enzymes and apoptosis in the pituitary gland.