CTL granules: evolution of vesicles essential for combating virus infections

Resistance to granzyme B-mediated cytochrome c release in Bak-deficient cells

Granzyme B (GrB), a serine protease with substrate specificity similar to the caspase family, is a major component of granule-mediated cytotoxicity of T lymphocytes. Although GrB can directly activate caspases, it induces apoptosis predominantly via Bid cleavage, mitochondrial outer membrane permeabilization, and cytochrome c release. To study the molecular regulators for GrB-mediated mitochondrial apoptotic events, we used a CTL-free cytotoxicity system, wherein target cells are treated with purified GrB and replication-deficient adenovirus (Ad). We report here that the Bcl-2 proapoptotic family member, Bak, plays a dominant role in GrB-mediated mitochondrial apoptotic events. A variant of Jurkat cells, deficient in Bak expression, was resistant to GrB/Ad-mediated apoptosis, as determined by lack of membranous phosphatidylserine exposure, lack of DNA breaks, lack of mitochondrial outer membrane permeabilization, and unchanged expression of inner mitochondrial membrane cardiolipin. The resistance of Bak-deficient cells to GrB/Ad cytotoxicity was reversed by transduction of the Bak gene into these cells. The requirement for both Bid and Bak, was further demonstrated in a cell-free system using purified mitochondria and S-100 cytosol. Purified mitochondria from Bid knockout mice, but not from Bax knockout mice, failed to release cytochrome c in response to autologous S-100 and GrB. Also, Bak-deficient mitochondria did not release cytochrome c in response to GrB-treated cytosol unless recombinant Bak protein was added. These results are the first to report a role for Bak in GrB-mediated mitochondrial apoptosis. This study demonstrates that GrB-cleaved Bid, which differs in size and site of cleavage from caspase-8-cleaved Bid, utilizes Bak for cytochrome c release, and therefore, suggests that deficiency in Bak may serve as a mechanism of immune evasion for tumor or viral infected cells.

How melanoma cells evade trail-induced apoptosis

At the doses used clinically, chemotherapy is believed to kill melanoma by a final common 'mitochondrial' pathway that leads to apoptosis. Similarly, several natural defence mechanisms kill melanoma by the same pathways. A corollary to the latter is that survival of melanoma in the host is due to the development of anti-apoptotic mechanisms in melanoma cells. What are these mechanisms? And how might we bypass them to improve the treatment of melanoma?

CD8(+) T lymphocytes mediate Borna disease virus-induced immunopathology independently of perforin

Perforin-mediated lysis of target cells is the major antiviral effector mechanism of CD8(+) T lymphocytes. We have analyzed the role of perforin in a mouse model for CD8(+) T-cell-mediated central nervous system (CNS) immunopathology induced by Borna disease virus. When a defective perforin gene was introduced into the genetic background of the Borna disease-susceptible mouse strain MRL, the resulting perforin-deficient mice developed strong neurological disease in response to infection indistinguishable from that of their perforin-expressing littermates. The onset of disease was slightly delayed. Brains of diseased perforin-deficient mice showed similar amounts and a similar distribution of CD8(+) T cells as wild-type animals. Perforin deficiency had no impact on the kinetics of viral spread through the CNS. Unlike brain lymphocytes from diseased wild-type mice, lymphocytes from perforin-deficient MRL mice showed no in vitro cytolytic activity towards target cells expressing the nucleoprotein of Borna disease virus. Taken together, these results demonstrate that CD8(+) T cells mediate Borna disease independent of perforin. They further suggest that the pathogenic potential of CNS-infiltrating CD8(+) T cells does not primarily reside in their lytic activity but rather in other functions.

HSV and glycoprotein J inhibit caspase activation and apoptosis induced by granzyme B or Fas

HSV-1 inhibits apoptosis of infected cells, presumably to ensure that the infected cell survives long enough to allow completion of viral replication. Because cytotoxic lymphocytes kill their targets via the induction of apoptosis, protection from apoptosis could constitute a mechanism of immune evasion for HSV. Several HSV genes are involved in the inhibition of apoptosis, including Us5, which encodes glycoprotein J (gJ). Viruses deleted for Us5 showed defects in inhibition of caspase activation after Fas ligation or UV irradiation. Transfected cells expressing the Us5 gene product gJ were protected from Fas- or UV-induced apoptosis, as measured by morphology, caspase activation, membrane permeability changes, or mitochondrial transmembrane potential. In contrast, caspase 3 activation in mitochondria-free cell lysates by granzyme (gr)B was inhibited equivalently by Us5 deletion and rescue viruses, suggesting that gJ is not required for HSV to inhibition this process. However, mitochondria-free lysates from transfected cells expressing Us5/gJ were protected from grB-induced caspase activation, suggesting that Us5/gJ is sufficient to inhibit this process. Transfected cells expressing Us5/gJ were also protected from death induced by incubation with purified grB and perforin. These findings suggest that HSV has a comprehensive set of immune evasion functions that antagonize both Fas ligand- and grB-mediated pathways of CTL-induced apoptosis. The understanding of HSV effects on killing by CTL effector mechanisms may shed light on the incomplete control of HSV infections by the immune system and may allow more rational approaches to the development of immune modulatory treatments for HSV infection.

Dendritic cell vaccines for cancer therapy

Dendritic cells (DC) are professional antigen-presenting cells whose primary function is the initiation of immune response. Based on the finding that the immune system usually fails to identify and kill cancer cells, DC have been recently used as vaccines for stimulation of tumour-specific immunity. This review focuses on pitfalls related to DC-based vaccination against solid tumours and on improvement of this immunotherapeutic approach for routine treatment of cancer disease.

Familial hemophagocytic lymphohistiocytosis: too little cell death can seriously damage your health

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare and fatal disease of early childhood characterized by a non-malignant accumulation of activated T lymphocytes and histiocytes in the reticuloendothelial system. Moreover, immune system derangement, with prominent hypercytokinemia and low or absent cytotoxic T and natural killer (NK) cell activity, is a consistent feature of this autosomal recessive disorder. Recent work has demonstrated that the degree of spontaneous caspase activation in FHL lymphocytes is attenuated in vitro whereas Fas-mediated caspase activation and apoptosis induction remains unmitigated, and FHL can thus be distinguished from the related chronic disorder of immune regulation termed autoimmune lymphoproliferative syndrome or ALPS. However, subsequent studies have identified mutations in the gene encoding perforin, a cytotoxic granule constituent required for apoptotic killing of target cells, in a number of FHL patients. Hence, the underlying defect in FHL may be conceived of as a lack of apoptosis triggering within the immune system, rather than apoptosis resistance per se. These observations represent an important step in our understanding of the pathogenesis of FHL and also serve to emphasize the pivotal role of cellular (perforin-based) cytotoxicity in the regulation of immune homeostasis.

Identification of committed NK cell progenitors in adult murine bone marrow

Natural killer (NK) cells play important roles in innate immunity by lysing tumor and virally infected cells and by producing cytokines including interferon-gamma. While NK cell progenitors have been described in the fetal thymus, NK cell generation from hematopoietic stem cells (HSC) in the bone marrow (BM) occurs throughout life, and in athymic mice and humans. Interleukin (IL)-15 promotes NK development in vitro and is essential for the generation of normal numbers of NK cells in vivo. By characterizing BM cells expressing IL-15 receptor components, we found marked heterogeneity within the IL-2 receptor beta chain(+) (CD122(+)) subset, which included cells uniquely committed to the NK lineage. These CD122(+) NK cell precursors (NKP) are negative for markers used to identify mature NK cells, including NK1.1, DX5 and members of Ly-49 family, and fail to demonstrate natural cytotoxicity against susceptible target cells. In vitro culture of NKP generates mature lytic NK1.1(+) cells at high frequencies, while they do not give rise to T, B, myeloid or erythroid cells under appropriate conditions. NKP lack transcripts associated with early B and T cell differentiation (pTalpha, lambda5 and CD3epsilon), but express a group of genes (IL-15Ralpha, Id2, GATA-3 and Ets-1) and the 2B4 marker, which may define NK cell commitment. We propose that NKP represent the earliest adult BM precursor uniquely restricted to the NK cell lineage.

Adenovirus L4-100K assembly protein is a granzyme B substrate that potently inhibits granzyme B-mediated cell death

Cytotoxic lymphocytes kill virus-infected target cells and play a critical role in host recovery from viral infections. Granzyme B (GrB) is a cytotoxic lymphocyte granule protease that plays a critical role in mediating cytotoxicity. In these studies, we demonstrate that the adenovirus assembly protein L4--100K (100K) is a GrB substrate that prevents cytotoxic lymphocyte granule-induced apoptosis in infected target cells by potently inhibiting GrB. This inhibition is absolutely dependent on Asp-48 in 100K, found within a classic GrB consensus motif. 100K is the first viral protein described that exclusively targets the GrB pathway. It represents a novel class of viral protease inhibitor, in which an essential, multifunctional viral protein, which is vulnerable to specific proteolysis by GrB, expresses inhibitory function against that protease.

Perforin and interferon-gamma activities independently control tumor initiation, growth, and metastasis

Perforin (pfp) and interferon-gamma (IFN-gamma) together in C57BL/6 (B6) and BALB/c mouse strains provided optimal protection in 3 separate tumor models controlled by innate immunity. Using experimental (B6, RM-1 prostate carcinoma) and spontaneous (BALB/c, DA3 mammary carcinoma) models of metastatic cancer, mice deficient in both pfp and IFN-gamma were significantly less proficient than pfp- or IFN-gamma-deficient mice in preventing metastasis of tumor cells to the lung. Pfp and IFN-gamma-deficient mice were as susceptible as mice depleted of natural killer (NK) cells in both tumor metastasis models, and IFN-gamma appeared to play an early role in protection from metastasis. Previous experiments in a model of fibrosarcoma induced by the chemical carcinogen methylcholanthrene indicated an important role for NK1.1(+) T cells. Herein, both pfp and IFN-gamma played critical and independent roles in providing the host with protection equivalent to that mediated by NK1.1(+) T cells. Further analysis demonstrated that IFN-gamma, but not pfp, controlled the growth rate of sarcomas arising in these mice. Thus, this is the first study to demonstrate that host IFN-gamma and direct cytotoxicity mediated by cytotoxic lymphocytes expressing pfp independently contribute antitumor effector functions that together control the initiation, growth, and spread of tumors in mice.

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.

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.

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.

Perforin-mediated cytotoxicity is critical for surveillance of spontaneous lymphoma

Immune surveillance by cytotoxic lymphocytes against cancer has been postulated for decades, but direct evidence for the role of cytotoxic lymphocytes in protecting against spontaneous malignancy has been lacking. As the rejection of many experimental cancers by cytotoxic T lymphocytes and natural killer cells is dependent on the pore-forming protein perforin (pfp), we examined pfp-deficient mice for increased cancer susceptibility. Here we show that pfp-deficient mice have a high incidence of malignancy in distinct lymphoid cell lineages (T, B, NKT), indicating a specific requirement for pfp in protection against lymphomagenesis. The susceptibility to lymphoma was accentuated by simultaneous lack of expression of the p53 gene, mutations in which also commonly predispose to human malignancies, including lymphoma. In contrast, the incidence and age of onset of sarcoma was unaffected in p53-deficient mice. Pfp-deficient mice were at least 1,000-fold more susceptible to these lymphomas when transplanted, compared with immunocompetent mice in which tumor rejection was controlled by CD8(+) T lymphocytes. This study is the first that implicates direct cytotoxicity by lymphocytes in regulating lymphomagenesis.

Evaluation of the natural immunity in pups inoculated with a modified-live canine parvovirus type 2b (CPV-2b) strain

Three pups 2-4 months old were vaccinated subcutaneously with the modified live canine parvovirus, CPV-2b/29-97 strain. During an observation period of two weeks pups remained clinically health, exhibiting a vigorous post-vaccinal active serological response (haemoagglutinating inhibiting antibody titers for CPV-2 ranging from 1:2560 to 1:5120 at 21 days post inoculation). Phagocytosis and killing of Candida albicans exerted by polymorphonuclear cells and monocytes did not undergo significant modifications 3-6 days post vaccination up to 30 days. Antibacterial activity mediated by peripheral blood lymphocytes (Salmonella typhi was used as a target) was slightly, but not significantly decreased 3 days post vaccination. Conclusively, in pups the CPV type 2b vaccine seems to be safe as far as natural immune responses are concerned, while its immunogenicity is preserved.

CD40 activation does not protect chronic lymphocytic leukemia B cells from apoptosis induced by cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTLs) can kill target cells by the granule/exocytosis pathway or the Fas-mediated apoptosis pathway. The sensitivity of chronic lymphocytic leukemia (CLL) B cells to CTL-mediated apoptosis before and after CD40 activation was examined. Resting or CD40-activated CLL cells were found to be equally sensitive to class I–restricted CTL-mediated killing. Despite expressing CD95, the CD40-activated CLL target cells were found to be resistant to apoptosis induced by CH11, an IgM CD95 monoclonal antibody (mAb). Consistent with this, inhibitors of caspases, which are involved in the Fas-induced apoptotic pathway (eg, N-carbobenzoxy-Val-Ala-Asp fluoromethyl ketone [z-VAD-fmk]), were unable to block destruction of CLL target cells by CTL. In addition, preincubation of the effector T cells with the anti-Fas ligand mAb NOK-2 failed to inhibit their subsequent ability to kill CLL target cells. On the other hand, CTL activity was blocked by inhibitors of the granule exocytosis pathway such as ethylene-glyco-tetra-acetic acid or concanamycin A. These results indicate that CD40 activation does not impair the sensitivity of CLL cells to Fas-independent CTL-mediated apoptosis.

CD40 activation does not protect chronic lymphocytic leukemia B cells from apoptosis induced by cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTLs) can kill target cells by the granule/exocytosis pathway or the Fas-mediated apoptosis pathway. The sensitivity of chronic lymphocytic leukemia (CLL) B cells to CTL-mediated apoptosis before and after CD40 activation was examined. Resting or CD40-activated CLL cells were found to be equally sensitive to class I–restricted CTL-mediated killing. Despite expressing CD95, the CD40-activated CLL target cells were found to be resistant to apoptosis induced by CH11, an IgM CD95 monoclonal antibody (mAb). Consistent with this, inhibitors of caspases, which are involved in the Fas-induced apoptotic pathway (eg, N-carbobenzoxy-Val-Ala-Asp fluoromethyl ketone [z-VAD-fmk]), were unable to block destruction of CLL target cells by CTL. In addition, preincubation of the effector T cells with the anti-Fas ligand mAb NOK-2 failed to inhibit their subsequent ability to kill CLL target cells. On the other hand, CTL activity was blocked by inhibitors of the granule exocytosis pathway such as ethylene-glyco-tetra-acetic acid or concanamycin A. These results indicate that CD40 activation does not impair the sensitivity of CLL cells to Fas-independent CTL-mediated apoptosis.

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.

A role for P-glycoprotein in regulating cell death

P-glycoprotein (P-gp) is an energy dependent drug pump responsible for multidrug resistance (MDR) in human cancers. While it is irrefutable that P-gp can efflux xenobiotics out of cells, the biological function of P-gp in multicellular organisms has yet to be firmly established. The question of what, if anything, P-gp does when not effluxing drugs has been raised by recent reports indicating that P-gp may regulate apoptosis, chloride channel activity, cholesterol metabolism and immune cell function. There is now a lively debate regarding the possible role of P-gp in regulating cell differentiation, proliferation and survival.

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.