Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

Beyond target cell death - Granzyme serine proteases in health and disease

Granzymes are a family of small (∼32 kDa) serine proteases with a range of substrate specificities that are stored in, and released from, the cytoplasmic secretory vesicles ('granules') of cytotoxic T lymphocytes and natural killer cells. Granzymes are not digestive proteases but finely tuned processing enzymes that target their substrates in specific ways to activate various signalling pathways, or to inactivate viral proteins and other targets. Great emphasis has been placed on studying the pro-apoptotic functions of granzymes, which largely depend on their synergy with the pore-forming protein perforin, on which they rely for penetration into the target cell cytosol to access their substrates. While a critical role for granzyme B in target cell apoptosis is undisputed, both it and the remaining granzymes also influence a variety of other biological processes (including important immunoregulatory functions), which are discussed in this review. This includes the targeting of many extracellular as well as intracellular substrates, and can also lead to deleterious outcomes for the host if granzyme expression or function are dysregulated or abrogated. A final important consideration is that granzyme repertoire, biochemistry and function vary considerably across species, probably resulting from the pressures applied by viruses and other pathogens across evolutionary time. This has implications for the interpretation of granzyme function in preclinical models of disease.

Evaluating frequency and quality of pathogen-specific T cells

It is generally accepted that enumeration and characterization of antigen-specific T cells provide essential information about potency of the immune response. Here, we report a new technique to determine the frequency and potency of antigen-specific CD8 T cells. The assay measures changes of intracellular Ca²⁺ in real time by fluorescent microscopy in individual CD8 T cells responding to cognate peptides. The T cells form continuous monolayer, enabling the cells to present the peptides to each other. This approach allows us to evaluate the kinetics of intracellular Ca²⁺ signalling that characterizes the quality of T cell response. We demonstrate the usefulness of the assay examining the frequency and quality of cytomegalovirus-specific CD8 T cells from healthy donor and patient after haploidentical stem cell transplantation. The new assay has a potential to provide essential information determining the status of the immune system, disease morbidity, potency of therapeutic intervention and vaccine efficacy.

Characterization of T cell antigen specificity human blood is challenging due to the low clonal frequencies. Here the authors develop a fluorescent microscopy-based method to detect antigen-specific CD8 T cell activation, and apply it to characterize the anti-CMV repertoire.

HIV-1 and hijacking of the host immune system: the current scenario

Human immunodeficiency virus (HIV) infection is a major health burden across the world which leads to the development of acquired immune deficiency syndrome (AIDS). This review article discusses the prevalence of HIV, its major routes of transmission, natural immunity, and evasion from the host immune system. HIV is mostly prevalent in Sub-Saharan Africa and low income countries. It is mostly transmitted by sharing syringe needles, blood transfusion, and sexual routes. The host immune system is categorized into three main types; the innate, the adaptive, and the intrinsic immune system. Regarding the innate immune system against HIV, the key players are mucosal membrane, dendritic cells (DCs), complement system, interferon, and host Micro RNAs. The major components of the adaptive immune system exploited by HIV are T cells mainly CD4+ T cells and B cells. The intrinsic immune system confronted by HIV involves (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G) APOBEC3G, tripartite motif 5-α (TRIM5a), terherin, and (SAM-domain HD-domain containing protein) SAMHD1. HIV-1 efficiently interacts with the host immune system, exploits the host machinery, successfully replicates and transmits from one cell to another. Further research is required to explore evasion strategies of HIV to develop novel therapeutic approaches against HIV.

Regulation of CD8+ T-cell cytotoxicity in HIV-1 infection

Understanding the mechanisms involved in cellular immune responses against control of human immunodeficiency virus (HIV) infection is key to development of effective immunotherapeutic strategies against viral proliferation. Clear insights into the regulation of cytotoxic CD8+ T cells is crucial to development of effective immunotherapeutic strategies due to their unique ability to eliminate virus-infected cells during the course of infection. Here, we reviewed the roles of transcription factors, co-inhibitory molecules and regulatory cytokines following HIV infection and their potential significance in regulating the cytotoxic potentials of CD8+ T cells.

Antibacterial activity and immune responses of a molluscan macrophage expressed gene-1 from disk abalone, Haliotis discus discus

The membrane-attack complex/perforin (MACPF) domain-containing proteins play an important role in the innate immune response against invading microbial pathogens. In the current study, a member of the MACPF domain-containing proteins, macrophage expressed gene-1 (MPEG1) encoding 730 amino acids with the theoretical molecular mass of 79.6 kDa and an isoelectric point (pI) of 6.49 was characterized from disk abalone Haliotis discus discus (AbMPEG1). We found that the characteristic MACPF domain (Val(131)-Tyr(348)) and transmembrane segment (Ala(669)-Ile(691)) of AbMPEG1 are located in the N- and C-terminal ends of the protein, respectively. Ortholog comparison revealed that AbMPEG1 has the highest sequence identity with its pink abalone counterpart, while sequences identities of greater than 90% were observed with MPEG1 members from other abalone species. Likewise, the furin cleavage site KRRRK was highly conserved in all abalone species, but not in other species investigated. We identified an intron-less genomic sequence within disk abalone AbMPEG1, which was similar to other mammalian, avian, and reptilian counterparts. Transcription factor binding sites, which are important for immune responses, were identified in the 5'-flanking region of AbMPEG1. qPCR revealed AbMPEG1 transcripts are present in every tissues examined, with the highest expression level occurring in mantle tissue. Significant up-regulation of AbMPEG1 transcript levels was observed in hemocytes and gill tissues following challenges with pathogens (Vibrio parahemolyticus, Listeria monocytogenes and viral hemorrhagic septicemia virus) as well as pathogen-associated molecular patterns (PAMPs: lipopolysaccharides and poly I:C immunostimulant). Finally, the antibacterial activity of the MACPF domain was characterized against Gram-negative and -positive bacteria using a recombinant peptide. Taken together, these results indicate that the biological significance of the AbMPEG1 gene includes a role in protecting disk abalone through the ability of AbMPEG1 to initiate an innate immune response upon pathogen invasion.

Thyroid hormones as modulators of immune activities at the cellular level

BACKGROUND

Increasing evidence suggests that thyroid hormones, L-thyroxine (T(4)) and 3,3',5-triiodo-L-thyronine (T(3)), are modulators of the immune response. In monocytes, macrophages, leukocytes, natural killer cells, and lymphocytes, a wide range of immune functions such as chemotaxis, phagocytosis, generation of reactive oxygen species (ROS), and cytokine synthesis and release are altered under hypo- and hyperthyroid conditions.

SUMMARY

Hyperthyroidism decreases the proinflammatory activities of monocytes and macrophages, whereas enhancement of phagocytosis and increased levels of ROS may occur during hypothyroidism. The expression of proinflammatory molecules such as macrophage inflammatory protein-1α and interleukin-1β increases in hypothyroidism. However, in Kupffer cells, proinflammatory activities such as the respiratory burst, nitric oxide synthase activity, and tumor necrosis factor-α expression may result from increased T(3) levels. Thyroid hormones also affect natural killer cell activity and cell-mediated immune responses. Still, for many immune cells no clear correlation has been found so far between abnormally high or low T(3) or T(4) levels and the effects observed on the immune responses.

CONCLUSIONS

In this review we outline the contributions of thyroid hormones to different aspects of innate and adaptive immune responses. The relationship between thyroid hormones and immune cells is complex and T(3) and T(4) may modulate immune responses through both genomic and nongenomic mechanisms. Future studies of the molecular signaling mechanisms involved in this cross-talk between thyroid hormones and the immune system may support development of new strategies to improve clinical immune responses.

Human and mouse perforin are processed in part through cleavage by the lysosomal cysteine proteinase cathepsin L

The pore-forming protein perforin is synthesized as an inactive precursor in natural killer (NK) cells and cytotoxic T lymphocytes (CTLs), and becomes active when a short C-terminal peptide is cleaved within acidic lysosome-like cytotoxic granules. Although it was shown more than a decade ago that this cleavage is pH dependent and can be inhibited by the generic cysteine cathepsin inhibitor E-64d, no protease capable of processing the perforin C terminus has been identified. Neither is it known whether a single protease is responsible or the processing has inbuilt redundancy. Here, we show that incubation of human NK cells and primary antigen-restricted mouse CTLs with the cathepsin L (CatL) inhibitor L1 resulted in a marked inhibition of perforin-dependent target cell death and reduced perforin processing. In vitro, CatL preferentially cleaved a site on full-length recombinant perforin close to its C terminus. The NK cells of mice deficient in CatL showed a reduction but not a complete absence of processed perforin, indicating that cysteine proteases other than CatL are also able to process perforin. We conclude that granule-bound cathepsins are essential for processing perforin to its active form, and that CatL is an important, but not exclusive, participant in this process.

Vaccines for Venezuelan equine encephalitis

Arboviruses are capable of causing encephalitis in animals and human population when transmitted by the vector or potentially via infectious aerosol. Recent re-emergence of Venezuelan equine encephalitis virus (VEEV) in South America emphasizes the importance of this pathogen to public health and veterinary medicine. Despite its importance no antivirals or vaccines against VEEV are currently available in the USA. Here we review some of the older and newer approaches aimed at generating a safe and immunogenic vaccine as well as most recent data about the mechanistic of protection in animal models of infection.

The molecular basis for perforin oligomerization and transmembrane pore assembly

Perforin, a pore-forming protein secreted by cytotoxic lymphocytes, is indispensable for destroying virus-infected cells and for maintaining immune homeostasis. Perforin polymerizes into transmembrane channels that inflict osmotic stress and facilitate target cell uptake of proapoptotic granzymes. Despite this, the mechanism through which perforin monomers self-associate remains unknown. Our current study establishes the molecular basis for perforin oligomerization and pore assembly. We show that after calcium-dependent membrane binding, direct ionic attraction between the opposite faces of adjacent perforin monomers was necessary for pore formation. By using mutagenesis, we identified the opposing charges on residues Arg213 (positive) and Glu343 (negative) to be critical for intermolecular interaction. Specifically, disrupting this interaction had no effect on perforin synthesis, folding, or trafficking in the killer cell, but caused a marked kinetic defect of oligomerization at the target cell membrane, severely disrupting lysis and granzyme B-induced apoptosis. Our study provides important insights into perforin's mechanism of action.

NK and NKT cells in aging and longevity: role of zinc and metallothioneins

INTRODUCTION

During aging, dysregulated immune functions occur contributing to increased susceptibility to morbidity and mortality. However, these dysregulations are normally counterbalanced by continuous adaptation of the body to the deteriorative changes occurring over time. These adaptive changes well occur in healthy centenarians.

DISCUSSION

Both innate (natural) and adaptive (acquired) immune responses decline with advancing age. Natural killer (NK) and natural killer T (NKT) cell cytotoxicity, representing one of best models of innate immune response, decreases in aging as well as interferon-gamma (IFN-gamma) production by both activated types of cells. Both NK and NKT cell cytotoxicity and IFN-gamma production increase in very old age with respect to normal aging, especially by NKT cells bearing TCRgammadelta. The role played by zinc and metallothioneins (MT) is crucial because this affects NK and NKT cell development, maturation, and functions. In particular, some MT polymorphisms are involved in maintaining innate immune response and intracellular zinc ion availability in aging with thus a role of MT genetic background to escape some age-related diseases with subsequent healthy aging and longevity.

Dihydrofuro[3,4-c]pyridinones as inhibitors of the cytolytic effects of the pore-forming glycoprotein perforin

Dihydrofuro[3,4-c]pyridinones are the first class of small molecules reported to inhibit the cytolytic effects of the lymphocyte toxin perforin. A lead structure was identified from a high throughput screen, and a series of analogues were designed and prepared to explore structure-activity relationships around the core bicyclic thioxofuropyridinone and pendant furan ring. This resulted in the identification of a submicromolar inhibitor of the perforin-induced lysis of Jurkat T-lymphoma cells.

The MACPF/CDC family of pore-forming toxins

Pore-forming toxins (PFTs) are commonly associated with bacterial pathogenesis. In eukaryotes, however, PFTs operate in the immune system or are deployed for attacking prey (e.g. venoms). This review focuses upon two families of globular protein PFTs: the cholesterol-dependent cytolysins (CDCs) and the membrane attack complex/perforin superfamily (MACPF). CDCs are produced by Gram-positive bacteria and lyse or permeabilize host cells or intracellular organelles during infection. In eukaryotes, MACPF proteins have both lytic and non-lytic roles and function in immunity, invasion and development. The structure and molecular mechanism of several CDCs are relatively well characterized. Pore formation involves oligomerization and assembly of soluble monomers into a ring-shaped pre-pore which undergoes conformational change to insert into membranes, forming a large amphipathic transmembrane β-barrel. In contrast, the structure and mechanism of MACPF proteins has remained obscure. Recent crystallographic studies now reveal that although MACPF and CDCs are extremely divergent at the sequence level, they share a common fold. Together with biochemical studies, these structural data suggest that lytic MACPF proteins use a CDC-like mechanism of membrane disruption, and will help understand the roles these proteins play in immunity and development.

A common fold mediates vertebrate defense and bacterial attack

Proteins containing membrane attack complex/perforin (MACPF) domains play important roles in vertebrate immunity, embryonic development, and neural-cell migration. In vertebrates, the ninth component of complement and perforin form oligomeric pores that lyse bacteria and kill virus-infected cells, respectively. However, the mechanism of MACPF function is unknown. We determined the crystal structure of a bacterial MACPF protein, Plu-MACPF from Photorhabdus luminescens, to 2.0 angstrom resolution. The MACPF domain reveals structural similarity with poreforming cholesterol-dependent cytolysins (CDCs) from Gram-positive bacteria. This suggests that lytic MACPF proteins may use a CDC-like mechanism to form pores and disrupt cell membranes. Sequence similarity between bacterial and vertebrate MACPF domains suggests that the fold of the CDCs, a family of proteins important for bacterial pathogenesis, is probably used by vertebrates for defense against infection.

Residual active granzyme B in cathepsin C-null lymphocytes is sufficient for perforin-dependent target cell apoptosis

Cathepsin C activates serine proteases expressed in hematopoietic cells by cleaving an N-terminal dipeptide from the proenzyme upon granule packaging. The lymphocytes of cathepsin C–null mice are therefore proposed to totally lack granzyme B activity and perforin-dependent cytotoxicity. Surprisingly, we show, using live cell microscopy and other methodologies, that cells targeted by allogenic CD8⁺ cytotoxic T lymphocyte (CTL) raised in cathepsin C–null mice die through perforin-dependent apoptosis indistinguishable from that induced by wild-type CTL. The cathepsin C–null CTL expressed reduced but still appreciable granzyme B activity, but minimal granzyme A activity. Also, in contrast to mice with inactivation of both their granzyme A/B genes, cathepsin C deficiency did not confer susceptibility to ectromelia virus infection in vivo. Overall, our results indicate that although cathepsin C clearly generates the majority of granzyme B activity, some is still generated in its absence, pointing to alternative mechanisms for granzyme B processing and activation. Cathepsin C deficiency also results in considerably milder immune deficiency than perforin or granzyme A/B deficiency.

Perforin-mediated target-cell death and immune homeostasis

The granule exocytosis pathway of cytotoxic lymphocytes is crucial for immune surveillance and homeostasis. The trafficking of granule components, including the membrane-disruptive protein perforin, to the immunological synapse leads to the delivery of granule proteases (granzymes) into the target cell and its destruction through apoptosis. Several independent molecular abnormalities associated with defects of either granule trafficking or perforin function can cause cytotoxic lymphocyte dysfunction. In humans, inherited perforin mutations result in severe immune dysregulation that manifests as familial haemophagocytic lymphohistiocytosis. This Review describes recent progress in defining the structure, function, biochemistry and cell biology of perforin.

Endothelial cell dysfunction, injury and death

Vascular endothelial cells (ECs) perform a number of functions required to maintain homeostasis. Inflammation can cause EC injury and death which disrupt these processes and result in endothelial dysfunction. Three common mediators of EC injury in inflammation are macrophage-derived cytokines, such as tumour necrosis factor (TNF); neutrophil-generated reactive oxygen species (ROS) and cytolytic T lymphocytes (CTL). Here we describe the distinct but overlapping biochemical pathways of injury elicited by these different agents.

Cytotoxic T Lymphocytes from Cathepsin B-deficient Mice Survive Normally in Vitro and in Vivo after Encountering and Killing Target Cells

The lysosomal protease cathepsin B has been proposed to protect cytotoxic T lymphocytes from the membrane-disruptive effects of perforin secreted during the execution phase of target cell death. Accordingly, cathepsin B that translocates to the lymphocyte surface upon degranulation has been postulated to cleave and inactivate perforin molecules that diffuse back to the killer cell. We have found that recombinant perforin is cleaved inefficiently by cathepsin B and shows no significant reduction in its lytic activity following co-incubation. Furthermore, purified CD8+ cytotoxic T lymphocytes of cathepsin B-null gene-targeted mice were able to induce normal death of target cells both in vitro and in vivo and to survive the encounter with target cells as efficiently as cathepsin B-expressing killer cells. We conclude that cathepsin B is not essential for protection of cytotoxic lymphocytes from the toxic effects of their secreted perforin.

The nuclear pore complex, nuclear transport, and apoptosis

The nuclear pore complex (NPC) is the sole gateway between the nucleus and the cytoplasm of interphase eukaryotic cells, and it mediates all trafficking between these 2 cellular compartments. As such, the NPC and nuclear transport play central roles in translocating death signals from the cell membrane to the nucleus where they initiate biochemical and morphological changes occurring during apoptosis. Recent findings suggest that the correlation between the NPC, nuclear transport, and apoptosis goes beyond the simple fact that NPCs mediate nuclear transport of key players involved in the cell death program. In this context, the accessibility of key regulators of apoptosis appears to be highly modulated by nuclear transport (e.g., impaired nuclear import might be an apoptotic trigger). In this review, recent findings concerning the unexpected tight link between NPCs, nuclear transport, and apoptosis will be presented and critically discussed.

CD4 T Cell-Dependent Autoimmunity against a Melanocyte Neoantigen Induces Spontaneous Vitiligo and Depends upon Fas-Fas Ligand Interactions

Better understanding of tolerance and autoimmunity toward melanocyte-specific Ags is needed to develop effective treatment for vitiligo and malignant melanoma; yet, a systematic assessment of these mechanisms has been hampered by the difficulty in tracking autoreactive T cells. To address this issue, we have generated transgenic mice that express hen egg lysozyme as a melanocyte-specific neoantigen. By crossing these animals to a hen egg lysozyme-specific CD4 TCR transgenic line we have been able to track autoreactive CD4+ T cells from their development in the thymus to their involvement in spontaneous autoimmune disease with striking similarity to human vitiligo vulgaris and Vogt-Koyanagi-Harada syndrome. Our findings show that CD4-dependent destruction of melanocytes is partially inhibited by blocking Fas-Fas ligand interactions and also highlights the importance of local control of autoimmunity, as vitiligo remains patchy and never proceeds to confluence even when Ag and autoreactive CD4+ T cells are abundant. Immune therapy to enhance or suppress melanocyte-specific T cells can be directed at a series of semiredundant pathways involving tolerance and cell death.

Addressing the mysteries of perforin function

Perforin is a cytolytic protein stored in secretory granules of CTL and NK cells. It synergizes with proapoptotic serine proteases, granzymes, to deliver the lethal hit to virus-infected or transformed target cells. The mechanism of perforin action has not been described beyond its original characterization in the 1980s, and its role in human disease has remained elusive. This article addresses recent key advances in genetic, clinical and biochemical studies that have reignited the current interest in perforin biology.

Reovirus serotype 1/strain Lang-stimulated activation of antigen-specific T lymphocytes in Peyer's patches and distal gut-mucosal sites: activation status and cytotoxic mechanisms

Intraduodenal priming of mice with reovirus serotype 1/strain Lang (reovirus 1/L) stimulates gut lymphocytes and generates precursor and effector CTLs. Our earlier studies demonstrated that germinal center and T cell Ag (GCT) is a marker which identifies reovirus 1/L-specific precursor CTL and effector CTL in Peyer's patches (PP) of reovirus 1/L-inoculated mice. In this study, we characterized the expression of the activation markers, GCT and CD11c, on reovirus 1/L-stimulated gut lymphocytes and the effector mechanisms involved in reovirus 1/L-specific cytotoxicity. We found that intraduodenal reovirus 1/L inoculation of mice induced the expression of both GCT and CD11c on PP lymphocytes (PPL), intraepithelial lymphocytes (IEL), and lamina propria lymphocytes (LPL), and these activated cells expressed Fas ligand (FasL). The majority of the GCT+ CD11c+ IEL and LPL expressed a phenotype, TCRalphabeta+ Thy-1+ CD8+ similar to that expressed on reovirus 1/L-stimulated PPL. However, splenic lymphocytes expressed GCT but not CD11c after stimulation with reovirus 1/L. Perforin, Fas-FasL, and TRAIL pathways were found to be involved in PPL, IEL, and LPL cytotoxic activity against reovirus 1/L-infected targets. In PPL, perforin and Fas-FasL pathways were more effective than TRAIL. In IEL, all three cytotoxic mechanisms were equally as effective. However, LPL prefer Fas-FasL and TRAIL over perforin. Further, we demonstrated the preferential migration of GCT+ PPL to the intraepithelial compartment and the lamina propria. These results suggest that GCT and CD11c can be used as activation markers for gut lymphocytes and CD11c can also be used to differentiate between activated gut and systemic lymphocytes.

Molecular effects of cyclosporine and oncogenesis: a new model

Cyclosporine A is the most commonly used immunosuppressive agent during organ transplantation. One of the most feared adverse effects of cyclosporine A is the appearance of de novo cancers. The mechanisms that lead to the genesis of such cancers are thought to be only related to a side effect of cyclosporine A: a depressed immune system. Here, we review different molecular effects induced by cyclosporine A (inhibition of DNA repair, synthesis of TGF Beta, induction of apoptosis of activated T cells, inhibition of apoptosis through the inhibition of the opening of the mitochondrial Permeability Transition Pore) and propose that cyclosporine A can promote the genesis and the spread of cancer not only because of immunosuppression but also because of its ability to facilitate DNA mutations accumulation, to diminish the clearance of altered cells and to transform cancer cells into aggressive cancer cells. This new insights into the mechanisms of genesis of cyclosporine A-related cancers should be taken into account to develop preventive strategies or new immunosuppressive strategies.

Characterization of virus-specific CD8(+) effector T cells in the course of HIV-1 infection: longitudinal analyses in slow and rapid progressors

Studies in humans have provided evidence that CD8(+) T cells exhibit distinct phenotypical and functional properties dependent on virus specificity. It is not known how these T-cell phenotypes develop over the course of infection. Dynamics and properties of T cells specific for human immunodeficiency virus (HIV), cytomegalovirus (CMV), and Epstein-Barr virus (EBV) in HIV infection were investigated in relation to viral load. In rapid progressors, HIV-specific CD8(+) T cells were less differentiated early in infection and did not develop a more differentiated phenotype. In slow progressors, perforin expression of HIV-specific CD8(+) T cells slightly increased over time. HIV and EBV loads were detectable in all individuals, while CMV load could not be detected. Thus, in individuals with progressive HIV infection, HIV-specific T cells are less differentiated already early in infection. This apparent block in differentiation may be partly caused by chronic viremia or lack of CD4(+) T-cell help.

Calcium-dependent plasma membrane binding and cell lysis by perforin are mediated through its C2 domain: A critical role for aspartate residues 429, 435, 483, and 485 but not 491

The lymphocyte pore-forming protein perforin is essential for maintaining immune homeostasis and for effective defense against intracellular pathogens. To date, there have been no reported structure-function studies to substantiate the function of any putative domains of perforin, which have been postulated totally on primary sequence similarities with domains in other proteins. In this report, we have used recently developed modalities for expressing full-length perforin and robust functional assays to investigate one of the hallmarks of perforin function: its absolute dependence on calcium for lipid binding and cell lysis. We provide, for the first time, experimental evidence that the predicted C-terminal C2 motif constitutes a functional domain that is responsible for membrane binding of perforin. Whereas conserved aspartate residues at positions 429, 435, 483, and 485 were essential for calcium-dependent plasma membrane binding and cell lysis, the contribution of Asp-491 was limited. Finally, after experimentally verifying an optimized three-dimensional model, we have made predictions on the impact of two inherited perforin mutations of the C2 domain on calcium-dependent lipid binding and cell lysis.

The functional basis for hemophagocytic lymphohistiocytosis in a patient with co-inherited missense mutations in the perforin (PFN1) gene

About 30% of cases of the autosomal recessive immunodeficiency disorder hemophagocytic lymphohistiocytosis are believed to be caused by inactivating mutations of the perforin gene. We expressed perforin in rat basophil leukemia cells to define the basis of perforin dysfunction associated with two mutations, R225W and G429E, inherited by a compound heterozygote patient. Whereas RBL cells expressing wild-type perforin (67 kD) efficiently killed Jurkat target cells to which they were conjugated, the substitution to tryptophan at position 225 resulted in expression of a truncated ( approximately 45 kD) form of the protein, complete loss of cytotoxicity, and failure to traffic to rat basophil leukemia secretory granules. By contrast, G429E perforin was correctly processed, stored, and released, but the rat basophil leukemia cells possessed reduced cytotoxicity. The defective function of G429E perforin mapped downstream of exocytosis and was due to its reduced ability to bind lipid membranes in a calcium-dependent manner. This study elucidates the cellular basis for perforin dysfunctions in hemophagocytic lymphohistiocytosis and provides the means for studying structure-function relationships for lymphocyte perforin.

NK and NKT cell functions in immunosenescence

Immunosenescence is defined as the state of dysregulated immune function that contributes to the increased susceptibility to infection, cancer and autoimmune diseases observed in old organisms, including humans. However, dysregulations in the immune functions are normally counterbalanced by continuous adaptation of the body to the deteriorations that occur over time. These adaptive changes are likely to occur in healthy human centenarians. Both innate (natural) and adaptive (acquired) immune responses decline with advancing age. Natural killer (NK) and natural killer T (NKT) cells represent the best model to describe innate and adaptive immune response in aging. NK and NKT cell cytotoxicity decreases in aging as well as interferon-gamma (IFN-gamma) production by both activated cell types. Their innate and acquired immune responses are preserved in very old age. However, NKT cells bearing T-cell receptor (TCR) gammadelta also display an increased cytotoxicity and IFN-gamma production in very old age. This fact suggests that NKT cells bearing TCRgammadelta are more involved in maintaining innate and adaptive immune response in aging leading to successful aging. The role played by the neuroendocrine-immune network and by nutritional factors, such as zinc, in maintaining NK and NKT cell functions in aging is discussed.

Discordant regulation of granzyme H and granzyme B expression in human lymphocytes

We analyzed the expression of granzyme H in human blood leukocytes, using a novel monoclonal antibody raised against recombinant granzyme H. 33-kDa granzyme H was easily detected in unfractionated peripheral blood mononuclear cells, due to its high constitutive expression in CD3(-)CD56(+) natural killer (NK) cells, whereas granzyme B was less abundant. The NK lymphoma cell lines, YT and Lopez, also expressed high granzyme H levels. Unstimulated CD4(+) and particularly CD8(+) T cells expressed far lower levels of granzyme H than NK cells, and various agents that classically induce T cell activation, proliferation, and enhanced granzyme B expression failed to induce granzyme H expression in T cells. Also, granzyme H was not detected in NK T cells, monocytes, or neutrophils. There was a good correlation between mRNA and protein expression in cells that synthesize both granzymes B and H, suggesting that gzmH gene transcription is regulated similarly to gzmB. Overall, our data indicate that although the gzmB and gzmH genes are tightly linked, expression of the proteins is quite discordant in T and NK cells. The finding that granzyme H is frequently more abundant than granzyme B in NK cells is consistent with a role for granzyme H in complementing the pro-apoptotic function of granzyme B in human NK cells.

Induction of cytotoxic T lymphocytes by dendritic cells pulsed with murine leukemic cell RNA

Peptide-pulsed dendritic cells can stimulate T cells showing specific cytotoxicity in chronic myelogenous leukemia. We tried to induce a specific cytotoxic T-cell response stimulated by RNA-pulsed dendritic cells in acute myelogenous leukemia. The total RNA of WEHI-3BD+, a myelomonocytic leukemia cell line derived from BALB/c mice, was transfected into dendritic cells induced from bone marrow nucleated cells of BALB/c mice with granulocyte macrophage colony-stimulating factor (GM-CSF) and lipopolysaccharide (LPS) using liposome. RNA-pulsed dendritic cells were injected into the peritoneal cavity of BALB/c mice, and splenic T cells were isolated for antigen-stimulated proliferation and leukemia-specific cytotoxicity assay. Cultured bone marrow nucleated cells expressed dendritic cell markers including MHC class II antigen, CD80, CD86, and CD11c. T cells stimulated by RNA-pulsed dendritic cells showed enhanced proliferation than those stimulated by unpulsed dendritic cells (P = 0.05) and showed dose-dependent specific cytotoxicity against WEHI-3BD+ cells. We concluded total RNA-pulsed dendritic cells could induce a specific T-cell cytotoxicity in acute myelogenous leukemia.

Potentiation of a tumor cell susceptibility to autologous CTL killing by restoration of wild-type p53 function

Inactivation of p53 has been implicated in many types of tumors particularly in non-small cell lung carcinoma, one of the most common cancers in which p53 mutation has been frequently identified. The aim of this study was to investigate the influence of p53 status on the regulation of tumor susceptibility to specific CTL-mediated cell death. For this purpose, we used a cytotoxic T lymphocyte clone, Heu127, able to lyse the human autologous lung carcinoma cell line, IGR-Heu, in a HLA-A2-restricted manner. Direct genomic DNA sequencing revealed that IGR-Heu expresses a mutated p53 at codon 132 of the exon 5 which results in the loss of p53 capacity to induce the expression of the p53-regulated gene product p21(waf/CIP1). Initial experiments demonstrated that IGR-Heu was resistant to Fas, TNF, and TRAIL apoptotic pathways. This correlated with the lack of p55 TNFRI, Fas, DR4, and DR5 expression. The effect of wild-type (wt) p53 restoration on the sensitization of IGR-Heu to autologous CTL clone lysis was investigated following infection of the tumor cell line with a recombinant adenovirus encoding the wt p53 (Adwtp53). We demonstrate that the restoration of wt p53 expression and function resulted in a significant potentiation of target cell susceptibility to CTL-mediated lysis. The wt p53-induced optimization of tumor cell killing by specific CTL involves at least in part Fas-mediated pathway via induction of CD95 expression by tumor cells but does not appear to interfere with granzyme B cytotoxic pathway.

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.

Absence of weight loss during Cryptosporidium infection in susceptible mice deficient in Fas-mediated apoptosis

Apoptosis plays a major role in the development of pathogenesis due to a number of microbial infections. Epithelial cells have been previously shown to die through apoptosis during in vitro infection by the Apicomplexan parasite Cryptosporidium parvum. We now test the possibility that Fas (APO-1/CD95)-dependent apoptosis of uninfected cells, due to enhanced expression of the Fas ligand (FasL) on infected cells, may contribute to the pathology of cryptosporidiosis. Expression of the FasL increased by a large amount on the surface of intestinal epithelial cells infected with C. parvum, and the increase was limited exclusively to infected cells. In addition, a significant increase in FasL expression was observed in epithelial cells from the small intestine of mice infected with C. parvum. Finally, whereas wild-type mice depleted of CD4(+) lymphocytes lost weight during C. parvum infection, CD4(+) cell-depleted lpr mice (deficient in Fas function) infected with C. parvum gained weight at the same rate as undepleted wild-type or lpr mice. These results suggest that bystander Fas-dependent apoptosis of uninfected epithelial cells may exacerbate the weight loss associated with cryptosporidiosis.

Study of diverse mechanisms of cell-mediated cytotoxicity in gene-targeted mice using flow cytometric cytotoxicity assay

Cytotoxic lymphocytes kill tumor or virus-infected target cells utilizing two mechanisms-(1) release of lytic granules (containing perforin and granzymes), and (2) Fas ligand (FasL)/Fas or TNF-initiated apoptosis. We have examined mechanisms of target cell lysis by effector T cells from gene-targeted and mutant mice using a new Flow Cytometric Cytotoxicity Assay (FC Assay). Target cells were labeled with PKH67 dye. Cell death was estimated by 7-AAD inclusion and annexin V-PE binding. A direct correlation has been found between the percentage of dead target cells in FC Assay and the results of 111In release cytotoxicity assay when effector T cells from either Pfp -/- (perforin knockout) or gld (non-functional Fas Ligand) mice were used. As shown by the 4 h FC assay, the granule-mediated mechanism was utilized by T cells from gld mice. In contrast, T cells from Pfp -/- mice used death receptor-mediated lysis. Therefore, cytotoxic cells from gene-targeted and mutant mice can serve as valuable tools for studying different mechanisms of cell-mediated cytotoxicity, and the FC assay could be applied irrespective of which cytotoxic effector pathway is involved.

Biology of kidney cells: ontogeny-recapitulating phylogeny

Biology of kidney cells can be used as a model for further understanding of ontogeny-recapitulating phylogeny. The common and species-specific structural and functional relationship between blood capillaries and the environment via a filtration barrier of nephrons is a biological phenomenon resulting from renal cell memory acquired through evolution. Genetically programmed development, a subsequent series of gene expression, and inductive interactions played a key role in differentiation and maintenance of specific activities of kidneys in birds and mammals. Various environmental factors may alter kidney development and specific activities at the levels of gene expression, repression, or derepression, and defensive mechanisms involved in reaction to risk factors are developed. Autoimmunity and cancerogenesis are closely dependent on a variety of environmental agents, such as antigens originating from infections with some viruses and toxins, or irradiation, advanced industrialization, and progress of civilization. As a result of gene mutation, delation, rearrangement, and/or susceptibility to different agents, renal cell memory is altered. Instead of cell-specific activities, the abilities for regeneration, and other genetically programmed activities, the genesis of kidney diseases are common. Balkan endemic nephropathy, as regional disease, is an important example of the role, of environmental agents, at the level of genes. Research programs on molecular genetics will contribute to our efforts both to prevent infections and to elucidate the genesis, diagnosis, prognosis, prevention, and therapy of kidney diseases.

CD4(+) and CD8(+) T cells kill intracellular Mycobacterium tuberculosis by a perforin and Fas/Fas ligand-independent mechanism

Cytotoxic effector phenotype and function of MHC-restricted Mycobacterium tuberculosis (MTB)-reactive CD4(+) and CD8(+) T lymphocytes were analyzed from healthy tuberculin skin test-positive persons. After stimulation in vitro with MTB, both CD4(+) and CD8(+) T cells up-regulated mRNA expression for granzyme A and B, granulysin, perforin, and CD95L (Fas ligand). mRNA levels for these molecules were greater for resting CD8(+) than CD4(+) T cells. After MTB stimulation, mRNA levels were similar for both T cell subsets. Increased perforin and granulysin protein expression was confirmed in both in CD4(+) and CD8(+) T cells by flow cytometry. Both T cell subsets lysed MTB-infected monocytes. Biochemical inhibition of the granule exocytosis pathway in CD4(+) and CD8(+) T cells decreased cytolytic function by >90% in both T cell subsets. Ab blockade of the CD95-CD95L interaction decreased cytolytic function for both T cell populations by 25%. CD4(+) and CD8(+) T cells inhibited growth of intracellular MTB in autologous monocytes by 74% and 84%, respectively. However, inhibition of perforin activity, the CD95-CD95L interaction, or both CTL mechanisms did not affect CD4(+) and CD8(+) T cell mediated restriction of MTB growth. Thus, perforin and CD95-CD95L were not involved in CD4(+) and CD8(+) T cell mediated restriction of MTB growth.

Three-color flow cytometric assay for the study of the mechanisms of cell-mediated cytotoxicity

Cytotoxic lymphocytes kill tumor or virus-infected target cells utilizing two mechanisms (1) release of lytic granules (containing perforin and granzymes) and (2) Fas ligand (FasL)/Fas or TNF initiated apoptosis. We have examined mechanisms of target cell lysis using a new Flow Cytometric Cytotoxicity Assay (FC Assay). Target cells were labeled with PKH 67 dye. Cell death was estimated by 7-amino-actinomycin (7-AAD) inclusion and annexin V-PE binding. A strong direct correlation has been found between the percentage of dead target cells in the FC Assay and the results of 51Cr release assay when human LAK and CTL were used as a model system. We have shown that both NK and CTL kill tumor cells mostly by granule-mediated mechanisms, as lysis was blocked by a perforin inhibitor Concanamycin A (Folimycin) but was significantly less sensitive to zVAD-FMK caspase inhibition. The FC assay allows accurate measurement of cell-mediated cytotoxicity as individual target cell death is detected directly.

Nucleocytoplasmic distribution of the ovalbumin serpin PI-9 requires a nonconventional nuclear import pathway and the export factor Crm1

Proteinase inhibitor 9 (PI-9) is a human serpin present in the cytoplasm of cytotoxic lymphocytes and epithelial cells. It inhibits the cytotoxic lymphocyte granule proteinase granzyme B (graB) and is thought to protect cytotoxic lymphocytes and bystander cells from graB-mediated apoptosis. Following uptake into cells, graB promotes DNA degradation, rapidly translocating to the nucleus, where it binds a nuclear component. PI-9 should therefore be found in cytotoxic lymphocyte and bystander cell nuclei to ensure complete protection against graB. Here we demonstrate by microscopy and subcellular fractionation experiments that PI-9 is present in the nuclei of human cytotoxic cells, endothelial cells, and epithelial cells. We also show that the related serpins, PI-6, monocyte neutrophil elastase inhibitor (MNEI), PI-8, plasminogen activator inhibitor 2 (PAI-2), and the viral serpin CrmA exhibit similar nucleocytoplasmic distributions. Because these serpins lack classical nuclear localization signals and are small enough to diffuse through nuclear pores, we investigated whether import occurs actively or passively. Large (approximately 70 kDa) chimeric proteins comprising PI-9, PI-6, PI-8, MNEI, or PAI-2 fused to green fluorescent protein (GFP) show similar nucleocytoplasmic distributions to the parent proteins, indicating that nuclear import is active. By contrast, CrmA-GFP is excluded from nuclei, indicating that CrmA is not actively imported. In vitro nuclear transport assays show that PI-9 accumulates at a rate above that of passive diffusion, that it requires cytosolic factors but not ATP, and that it does not bind an intranuclear component. Furthermore, PI-9 is exported from nuclei via a leptomycin B-sensitive pathway, implying involvement of the export factor Crm1p. We conclude that the nucleocytoplasmic distribution of PI-9 and related serpins involves a nonconventional nuclear import pathway and Crm1p.

Correlation of human CD56+ cell cytotoxicity and IFN-gamma production

The use of an IFN-gamma ELISPOT assay to evaluate cellular immune responses has gained increasing popularity, especially as a surrogate measure for cytotoxic T lymphocyte (CTL) responses. We have compared the IFN-gamma ELISPOT assay and the traditional(51)Cr release assay for detection of human natural killer (NK) cell activity. The cell populations used for evaluation of these assays included freshly isolated and IL-2-activated peripheral blood mononuclear cells (PBMC). CD56-positive cells were demonstrated to be the primary source of the IFN-gamma signal when PBMC were evaluated with NK-sensitive targets in the IFN-gamma ELISPOT assay. IFN-gamma ELISPOT and(51)Cr release assays showed excellent correlation suggesting that NK activity can be reliably evaluated with methods other than the traditional(51)Cr release assays.

Endothelial cell apoptosis in systemic sclerosis is induced by antibody-dependent cell-mediated cytotoxicity via CD95

OBJECTIVE

Apoptosis of endothelial cells is a key event in the pathogenesis of systemic sclerosis (SSc). The aim of the present study was to analyze in vitro the mechanism causing endothelial cell apoptosis in SSc.

METHODS

Human dermal microvascular endothelial cells (HDMEC) or human umbilical vein endothelial cells (HUVEC) were cultured with native or heat-inactivated serum from SSc patients or controls with or without interleukin-2-activated natural killer (NK) cells or peripheral blood mononuclear cells. SSc and control sera were tested for the presence or absence, respectively, of anti-endothelial cell antibodies (AECA) by indirect immunofluorescence. Apoptosis was detected by the TUNEL technique.

RESULTS

Native sera alone had no effect. Apoptosis induction was observed on HDMEC, but not on HUVEC, in the presence of AECA-positive SSc sera and activated NK cells, and could be inhibited by an anti-Fas ligand antibody. Inhibition of the perforin/granzyme pathway with concanamycin A had no effect on apoptosis induction in this in vitro model. Immunofluorescence analysis of cryosections from SSc skin showed Fas (CD95) expression by endothelial cells, supporting the in vitro findings.

CONCLUSION

The results suggest that endothelial cell apoptosis in SSc is induced by antibody-dependent cell-mediated cytotoxicity via the Fas pathway. These data not only provide insight into the pathogenesis of SSc, but also may open new ways to rational therapy for this disease.

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.

Nonlymphoid cultured cells possess a system controlling cellular compatibility

We show that various nonlymphoid cultured cells can activate the production of cytotoxic factors in response to direct contact with cells of a different kind. Accumulation of cytotoxic factors in the medium was detected 1 h after contact of K562 and L929 cells or after contact of L929 cells with purified membranes of K562 cells. TNF-alpha or immunologically related proteins, or both, but not Fas-ligand or lymphotoxin, were also accumulated in membranes of K562 and L929 cells shortly after these cells had been allowed to contact each other. The cytotoxic factors expressed by nonlymphoid cells trigger apoptosis of target cells. These observations strongly suggest that nonlymphoid cells possess molecular mechanisms controlling cellular compatibility.

Annexin V used for measuring apoptosis in the early events of cellular cytotoxicity

BACKGROUND

Current cytotoxic assays, including Cr release and fluorescent assays, do not directly measure the proportion of target cells which are killed by apoptosis. Cell-mediated cytotoxicity induced by CTLs and NK cells is mainly regulated by the perforin-granzyme, the Fas ligand (Fas L), and the Tumor Necrosis Factor (TNF)-alpha pathways. Perforin generates pores in the membrane of target cells, allowing granzyme B to enter and initiate apoptosis. The other effectors, Fas L and TNF-alpha act by an apoptosis mechanism, leading to DNA fragmentation. A three color flow cytometric method to measure cell-mediated cytotoxicity induced by CTLs or NK cells is described.

METHODS

The fluorochromes used are: PKH-26, a stable membrane dye for the labeling of the effector cells, annexin V-FITC which allows the direct evaluation of early apoptotic cells and propidium iodide which distinguishes membrane permeabilized and late apoptotic cells.

RESULTS

By eliminating through gating PKH-26 positive effector cells, we obtain a direct estimation of the percentage of target cells in the early stages of apoptosis as well as the percentage of target cells dying after late apoptosis and membrane permeabilization. The cytotoxic activity of IL-2 stimulated PBL against K562, Jurkat and KYM-1 was evaluated.

CONCLUSIONS

This rapid and novel assay permits the discrimination of the cell death mechanisms occurring during a cytotoxic response and to precisely evaluate the contribution of apoptosis in the early phases of cell-mediated cytotoxicity.

Concanavalin A hepatotoxicity in mice: tumor necrosis factor-mediated organ failure independent of caspase-3-like protease activation

Several models of tumor necrosis factor (TNF)/TNF-receptor 1 (TNF-R1)-dependent liver injury in mice were investigated with respect to caspase-3-like protease activation representing a pivotal mechanism of apoptotic cell death. Injection of TNF or T-cell-activating agents (i.e., agonistic anti-CD3 antibody or staphylococcal enterotoxin B [SEB]) into galactosamine (GalN)-sensitized mice caused TNF/TNF-R1-dependent liver injury. Intravenous concanavalin A (Con A) alone induced TNF-mediated hepatotoxicity dependent on both TNF-R1 and TNF-R2. Hepatic caspase-3-like proteases were activated in GalN/TNF, GalN/anti-CD3, or GalN/SEB-treated mice, but not in Con A-treated mice. Consistently, the broad-spectrum caspase inhibitor, benzoyloxycarbonyl-val-ala-asp-fluoromethylketone (zVADfmk), prevented TNF-mediated hepatotoxicity in all GalN-dependent models, but failed to protect against Con A. Under transcriptional arrest, however, Con A induced TNF-R1-dependent, but not TNF-R2-dependent, activation of caspase-3-like proteases, and zVADfmk prevented animals from Con A-mediated liver injury under this condition. Histological analysis revealed distinct differences between Con A- and GalN/Con A-induced liver injury regarding apoptotic morphology of hepatocytes. We conclude that impaired transcription induces a switch of Con A hepatotoxicity toward a caspase-3-like protease-dependent pathway. The observation that the functional state of the transcriptional machinery decides whether TNF-driven hepatocyte apoptosis involves activation of caspase-3-like proteases or alternative signaling pathways in vivo might be of relevance for the immunopathology of the liver.

Expression and purification of recombinant human granzyme B from Pichia pastoris

Granzyme B is a cytotoxic lymphocyte granule serine proteinase that is pivotal in the induction of target cell apoptosis. Here we describe the expression of recombinant human granzyme B in Pichia pastoris as a chimeric zymogen comprising the alpha-factor signal sequence, a prodomain including an enterokinase cleavage site, and the mature granzyme B sequence followed by a hexahistidine tag. Inactive zymogen is purified from the medium by immobilized cobalt chelate affinity chromatography and then activated by enterokinase (final yield is approximately 1 mg per liter). The recombinant enzyme resembles native granzyme B in size and glycosylation, hydrolyzes the substrate Boc-Ala-Ala-Asp-thiobenzyl ester with equivalent efficiency (K(m) 82 microM; k(cat) 12 s(-1)), processes procaspase-3 to subunit form, and is inhibited by the cognate serpin PI-9. It efficiently induces DNA degradation and apoptosis of human cells. The availability of recombinant human granzyme B will facilitate further investigation of its structure and role in immune effector cells.

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

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

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.