Extracellular release of lymphocyte cytolytic pore-forming protein (perforin) after ionophore stimulation

Perforin-2 clockwise hand-over-hand pre-pore to pore transition mechanism

Perforin-2 (PFN2, MPEG1) is a pore-forming protein that acts as a first line of defense in the mammalian immune system, rapidly killing engulfed microbes within the phagolysosome in macrophages. PFN2 self-assembles into hexadecameric pre-pore rings that transition upon acidification into pores damaging target cell membranes. Here, using high-speed atomic force microscopy (HS-AFM) imaging and line-scanning and molecular dynamics simulation, we elucidate PFN2 pre-pore to pore transition pathways and dynamics. Upon acidification, the pre-pore rings (pre-pore-I) display frequent, 1.8 s-1, ring-opening dynamics that eventually, 0.2 s-1, initiate transition into an intermediate, short-lived, ~75 ms, pre-pore-II state, inducing a clockwise pre-pore-I to pre-pore-II propagation. Concomitantly, the first pre-pore-II subunit, undergoes a major conformational change to the pore state that propagates also clockwise at a rate ~15 s-1. Thus, the pre-pore to pore transition is a clockwise hand-over-hand mechanism that is accomplished within ~1.3 s. Our findings suggest a clockwise mechanism of membrane insertion that with variations may be general for the MACPF/CDC superfamily.

The pore conformation of lymphocyte perforin

Perforin is a pore-forming protein that facilitates rapid killing of pathogen-infected or cancerous cells by the immune system. Perforin is released from cytotoxic lymphocytes, together with proapoptotic granzymes, to bind to a target cell membrane where it oligomerizes and forms pores. The pores allow granzyme entry, which rapidly triggers the apoptotic death of the target cell. Here, we present a 4-Å resolution cryo-electron microscopy structure of the perforin pore, revealing previously unidentified inter- and intramolecular interactions stabilizing the assembly. During pore formation, the helix-turn-helix motif moves away from the bend in the central β sheet to form an intermolecular contact. Cryo-electron tomography shows that prepores form on the membrane surface with minimal conformational changes. Our findings suggest the sequence of conformational changes underlying oligomerization and membrane insertion, and explain how several pathogenic mutations affect function.

Giant MACPF/CDC pore forming toxins: A class of their own

Pore Forming Toxins (PFTs) represent a key mechanism for permitting the passage of proteins and small molecules across the lipid membrane. These proteins are typically produced as soluble monomers that self-assemble into ring-like oligomeric structures on the membrane surface. Following such assembly PFTs undergo a remarkable conformational change to insert into the lipid membrane. While many different protein families have independently evolved such ability, members of the Membrane Attack Complex PerForin/Cholesterol Dependent Cytolysin (MACPF/CDC) superfamily form distinctive giant β-barrel pores comprised of up to 50 monomers and up to 300Å in diameter. In this review we focus on recent advances in understanding the structure of these giant MACPF/CDC pores as well as the underlying molecular mechanisms leading to their formation. Commonalities and evolved variations of the pore forming mechanism across the superfamily are discussed. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Human perforin employs different avenues to damage membranes

Perforin (PFN) is a pore-forming protein produced by cytotoxic lymphocytes that aids in the clearance of tumor or virus-infected cells by a mechanism that involves the formation of transmembrane pores. The properties of PFN pores and the mechanism of their assembly remain unclear. Here, we studied pore characteristics by functional and structural methods to show that perforin forms pores more heterogeneous than anticipated. Planar lipid bilayer experiments indicate that perforin pores exhibit a broad range of conductances, from 0.15 to 21 nanosiemens. In comparison with large pores that possessed low noise and remained stably open, small pores exhibited high noise and were very unstable. Furthermore, the opening step and the pore size were dependent on the lipid composition of the membrane. The heterogeneity in pore sizes was confirmed with cryo-electron microscopy and showed a range of sizes matching that observed in the conductance measurements. Furthermore, two different membrane-bound PFN conformations were observed, interpreted as pre-pore and pore states of the protein. The results collectively indicate that PFN forms heterogeneous pores through a multistep mechanism and provide a new paradigm for understanding the range of different effects of PFN and related membrane attack complex/perforin domain proteins observed in vivo and in vitro.

NK Cells Use Perforin Rather than Granulysin for Anticryptococcal Activity

Cytotoxic lymphocytes have the capacity to kill microbes directly; however, the mechanisms involved are poorly understood. Using Cryptococcus neoformans, which causes a potentially fatal fungal infection in HIV-infected patients, our previous studies showed that granulysin is necessary, while perforin is dispensable, for CD8 T lymphocyte fungal killing. By contrast, the mechanisms by which NK cells exert their antimicrobial activity are not clear, and in particular, the contribution of granulysin and perforin to NK-mediated antifungal activity is unknown. Primary human NK cells and a human NK cell line YT were found to constitutively express granulysin and perforin, and possessed anticryptococcal activity, in contrast to CD8 T lymphocytes, which required stimulation. When granulysin protein and mRNA were blocked by granulysin small interfering RNA, the NK cell-mediated antifungal effect was not affected in contrast to the abrogated activity observed in CD8 T lymphocytes. However, when perforin was inhibited by concanamycin A, and silenced using hairpin small interfering RNA, the anticryptococcal activities of NK cells were abrogated. Furthermore, when granulysin and perforin were both inhibited, the anticryptococcal activities of the NK cells were not reduced further than by silencing perforin alone. These results indicate that the antifungal activity is constitutively expressed in NK cells in contrast to CD8 T lymphocytes, in which it requires prior activation, and perforin, but not granulysin, plays the dominant role in NK cell anticryptococcal activity, in contrast to CD8 T lymphocytes, in which granulysin, but not perforin, plays the dominant role in anticryptococcal activity.

T lymphocyte mediated lysis of mitomycin C treated Tenon's capsule fibroblasts

AIMS

To evaluate the effect of T cell co-culture on mitomycin C treated and untreated Tenon's capsule fibroblasts.

METHODS

IL-2 dependent allogeneic T cells were incubated over a monolayer of mitomycin C treated or control fibroblasts. Fibroblast numbers were evaluated by direct counts using phase contrast microscopy. To determine whether T cell mediated lysis was a consequence of MHC mismatch, co-culture experiments were repeated with autologous T cells. The effect of Fas receptor blockade was established by co-incubation with a Fas blocking (M3) antibody.

RESULTS

T cell co-culture resulted in a dramatic reduction in fibroblast survival compared to mitomycin C treatment alone (p = 0.032). T cell killing required fibroblast/lymphocyte cell to cell contact and was observed in both allogeneic and autologous co-culture experiments. Fas blocking antibodies did not significantly inhibit T cell killing (p = 0.39).

CONCLUSION

T cells augment mitomycin C treated fibroblast death in vitro. Similar mechanisms may contribute to the cytotoxic effect of mitomycin C in vivo and account for the largely hypocellular drainage blebs that are observed clinically.

Evidence for perforin-like activity associated with earthworm leukocytes

Earthworm (Eisenia fetida) coelomic fluid contains several leukocytes (coelomocytes): basophils, acidophils and neutrophils as well as chloragocytes. Small coelomocytes and coelomocyte lysate are cytotoxic for the tumor cell target K562. The expression of a lytic factor was investigated by immunocytochemistry using light and transmission electron microscopy. A rat-anti-mouse-perforin-mAb labeled mainly small coelomocytes (nearly 20%) as visualized by light microscopy. TEM analysis using immunogold showed a homogenous labeling in the cytoplasm of small coelomocytes. The highest number of immunogold particles was estimated in coelomocytes with many small cytoplasmic granules. Coelomocytes with large lysosomal granules were also labeled but less intensely. No antibody binding was observed for chloragocytes either in light or electron microscopy. This suggests that the perforin-like activity is associated with only one cell type and that chloragocytes are responsible for other lytic activities. MALDI-MS revealed calreticulin usually associated with perforin in mammalian cells that mediate lysis (e.g. NK, CTL). Together, results strongly suggest the presence of putative perforin in earthworms. This in turn supports the hypothesis that perforin is a conserved component important in immune defense during evolution.

Preferential interaction of a novel tumor surface protein (p38.5) with naive natural killer cells

A receptor-ligand interaction exclusive to natural killer (NK) cell-mediated recognition and triggering of tumor cell destruction has not yet been identified. In contrast, molecules that are involved in cellular adhesion and regulation of NK cytolysis have been well studied. In this report, a novel tumor surface protein is identified that exhibits characteristics of a recognition structure for naive NK cells. A tagged ligand-cell adsorption technique revealed a 38.5-kD plasma membrane protein (p38.5) from a prototypical NK-susceptible cell line (K562) that preferentially bound to NK cells (CD3(-)CD5(-)CD16(+)) relative to T lymphocytes (CD3(+)CD5(+) CD16(-)). The molecule was purified to apparent homogeneity for further characterization. An amino acid sequence of an 11-mer internal peptide of p38.5 did not exhibit homology to known proteins. Affinity-purified antibody generated against this peptide (anti-p38.5) reacted with a single protein of 38.5 kD on Western blots of whole cell extracts of K562. Flow cytometry and immunoprecipitation studies of surface-labeled tumor cells demonstrated expression of p38.5 on NK-susceptible tumor cell lines (K562, MOLT-4, Jurkat), whereas p38.5 was not detected on NK-resistant tumor cell lines (A549, Raji, MDA-MB-231). Significantly, p38.5 loss variants derived from wild-type Jurkat and Molt-4 cell lines exhibited decreased susceptibility to NK cell-mediated lysis demonstrating a strong association between cell surface expression of p38.5 and cytotoxicity. Purified p38.5 retained preferential binding to NK cells and inhibited NK activity in a dose-dependent manner, thereby providing direct evidence of a role in the lytic process. Binding studies identified a 70-kD membrane protein from NK cells as a possible receptor for the p38.5 tumor ligand. Consistent with cellular adsorption studies, the 70-kD, p38.5 binding protein was not detected on T lymphocytes. Based on studies demonstrating selective binding of p38.5 to NK cells, lack of expression on NK-resistant tumor cell lines and ability of the purified molecule to block cytolysis, we conclude that p38.5 may serve as a recognition/triggering ligand for naive human NK cells.

Low conductance states of a single ion channel are not 'closed'

We have used a polymer-exclusion method to estimate the sizes of the high- and low-conductance states of Staphylococcus aureus alpha-toxin channels across planar lipid bilayers. Despite a > 10-fold difference in conductance between high- and low-conductance states, the size differs by < 2-fold. We conclude that factors other than the dimensions have a strong influence on the conductance of alpha-toxin channels. We also show that the high conductance state is destabilized by the presence of high molecular weight polymers outside the channel, compatible with the removal of channel water as the high conductance state "shrinks" to the low conductance state.

Perforin and lymphocyte-mediated cytolysis

We have discussed in the previous sections the recent progress made toward elucidating the regulatory mechanism of perforin gene transcription and the domain structure of the perforin molecule. It appears that the expression of perforin is, at least partially, controlled at the transcription level through the interaction between killer cell-specific cis- and trans- acting factors. One of such cognate pairs, NF-P motif (an EBS-homologous motif) and NF-P2 (a killer cell-specific DNA-binding protein), has been described. The regulatory mechanism of gene transcription, however, is likely to involve multiple factors which act in a coordinated fashion to bring about the most efficient expression of perforin limited strictly to activated killer lymphocytes. Through studies using synthetic peptides and recombinant perforins, it has been suggested that the N-terminal region of the perforin molecule is an important, though not the only, domain responsible for the lytic activity. Further studies are warranted to elucidate the role(s) of other potential amphiphilic structures located in the central portion of the perforin molecule in the overall pore-forming activity. The molecular basis underlying the resistance of killer lymphocytes to perforin-mediated lysis still remains an open question. Preliminary results, however, suggest that the surface protein(s) restricted to killer cells may account for their self-protection against perforin. Based on recent studies using perforin-deficient mice, the involvement of perforin in lymphocyte-mediated cytolysis both in vivo and in vitro has been confirmed. Two functional roles, a direct (lytic) and an indirect (endocytosis enhancer; conduit), both of which may contribute critically to the cell-killing event can be attributed to perforin. The fact that lymphocytes may also employ perforin-independent killing mechanism(s), e.g. Fas-dependent pathway, is beyond the scope of this review. There is, nevertheless, no doubt that these alternative cytolytic mechanisms may also play important roles in immune effector and/or regulatory responses associated with killer lymphocytes. Obviously, we are still a long way from concluding on the functional relevance of each individual cytolytic mechanism seen in different physiopathological situations. Suffice it to say, however, that a wealth of information on lymphocyte-mediated killing has already emerged through the multidisciplinary efforts conducted in our and other laboratories that promise to further dissect this complicated event in the years to come.

Interaction of the hemolytic lectin CEL-III from the marine invertebrate Cucumaria echinata with the erythrocyte membrane

CEL-III is one of four Ca(2+)-dependent galactose/N-acetylgalactosamine (GalNAc)-binding lectins from the marine invertebrate Cucumaria echinata which exhibits hemolytic activity, especially toward rabbit and human erythrocytes. The hemolytic activity of CEL-III was also Ca(2+)-dependent and was found to be inhibited by galactose or GalNAc-containing carbohydrates, suggesting that the hemolysis was caused by CEL-III binding to specific carbohydrates on the erythrocyte membrane by Ca(2+)-dependent lectin activity, followed by partial destruction of the membrane. The activity of CEL-III was highest at 10 degrees C and decreased markedly with increasing temperature, unlike usual enzymatic reactions. The hemolytic activity of CEL-III increased with increasing pH from neutral to 10, but almost no hemolysis was observed below pH 6.5. Immunoblotting analysis of proteins from the erythrocyte membrane after treatment with CEL-III indicated that CEL-III aggregates were irreversibly bound to the membrane. When erythrocytes were incubated with CEL-III in the presence of dextran with molecular masses greater than 4 kDa, lysis was impeded considerably, while a concomitant release of ATP was detected from these osmotically protected cells. It was found that CEL-III released carboxyfluorescein from artificial globoside-containing lipid vesicles, and it is suggested that CEL-III is a novel pore-forming protein with the characteristics of a Ca(2+)-dependent lectin, which may act as a toxic protein to foreign microorganisms.

Zanvil Alexander Cohn 1926-1993

Zanvil Alexander Cohn, an editor of this Journal since 1973, died suddenly on June 28, 1993. Cohn is best known as the father of the current era of macrophage biology. Many of his scientific accomplishments are recounted here, beginning with seminal studies on the granules of phagocytes that were performed with his close colleague and former editor of this Journal, James Hirsch. Cohn and Hirsch identified the granules as lysosomes that discharged their contents of digestive enzymes into vacuoles containing phagocytosed microbes. These findings were part of the formative era of cell biology and initiated the modern study of endocytosis and cell-mediated resistance to infection. Cohn further explored the endocytic apparatus in pioneering studies of the mouse peritoneal macrophage in culture. He described vesicular inputs from the cell surface and Golgi apparatus and documented the thoroughness of substrate digestion within lysosomal vacuoles that would only permit the egress of monosaccharides and amino acids. These discoveries created a vigorous environment for graduate students, postdoctoral fellows, and junior and visiting faculty. Some of the major findings that emerged from Cohn's collaborations included the radioiodination of the plasma membrane for studies of composition and turnover; membrane recycling during endocytosis; the origin of the mononuclear phagocyte system in situ; the discovery of the dendritic cell system of antigen-presenting cells; the macrophage as a secretory cell, including the release of proteases and large amounts of prostaglandins and leukotrienes; several defined parameters of macrophage activation, especially the ability of T cell-derived lymphokines to enhance killing of tumor cells and intracellular protozoa; the granule discharge mechanism whereby cytotoxic lymphocytes release the pore-forming protein perforin; the signaling of macrophages via myristoylated substrates of protein kinase C; and a tissue culture model in which monocytes emigrate across tight endothelial junctions. In 1983, Cohn turned to a long-standing goal of exploring host resistance directly in humans. He studied leprosy, focusing on the disease site, the parasitized macrophages of the skin. He injected recombinant lymphokines into the skin and found that these molecules elicited several cell-mediated responses. Seeing this potential to enhance host defense in patients, Cohn was extending his clinical studies to AIDS and tuberculosis. Zanvil Cohn was a consummate physician-scientist who nurtured the relationship between cell biology and infectious disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of perforin-mediated cardiac myocyte injury in acute murine myocarditis caused by Coxsackie virus B3

We have recently demonstrated that killer cells expressing a cytolytic factor, perforin, infiltrate the hearts of mice with acute viral myocarditis and may play an important role in the mechanism of myocardial damage. To clarify the mechanism of in vivo cardiac myocyte injury mediated by perforin, we investigated the release of perforin molecules from killer cells by immunoelectron microscopy and examined the circular lesions formed by perforin on the membrane of cardiac myocytes. We found that there was massive release of perforin molecules from the killer cells directly onto the surface of the cardiac myocytes. Furthermore, electron microscopy of ultrathin ventricular sections treated with trypsin revealed numerous circular lesions with the characteristics of perforin pores, in the membranes of cardiac myocytes. These findings provide the first direct evidence that killer cells injure cardiac myocytes by releasing perforin and may play a critical role in the myocardial damage occurring in acute viral myocarditis.

Two unique human leukemic T-cell lines endowed with a stable cytotoxic function and a different spectrum of target reactivity analysis and modulation of their lytic mechanisms

We have reported the establishment of two interleukin (IL)-2-dependent human leukemic cell lines (TALL-103/2 [CD3+TCR gamma delta +] and TALL-104 [CD3+ TCR alpha beta +]) which display major histocompatibility complex nonrestricted tumoricidal activity. Whereas TALL-103/2 cells lyse only natural killer cell-susceptible targets, TALL-104 cells display a broad range of tumor target reactivity. In reverse antibody-dependent cell-mediated cytotoxicity (ADCC), lysis by both cell lines is triggered by monoclonal antibodies (mAb) recognizing CD3 and, to a lesser extent, CD2, but not CD8 or CD56 antigens. In conventional cytotoxic assays, the lytic activity of both cell lines is strictly Ca(2+)-dependent. In reverse ADCC, lysis by TALL-103/2 cells is highly dependent on the presence of Ca2+, whereas TALL-104 cells seem to only partially require extracellular Ca2+. The cytoplasm of both cell lines contains azurophilic granules typical of cytotoxic cells. Northern blot analysis demonstrates mRNA expression of pore-forming protein (PFP; perforin) and serine esterases (SE). The magnitude of expression of these transcripts and of lytic activity depends on the doses of IL-2. Upon deprivation of IL-2, TALL-103/2 cells completely lose cytotoxic granules and function within 16 h, whereas TALL-104 cells progressively lose expression of PFP and SE mRNA, as well as killer activity, within 4 wk. Both anti-CD3 mAb and lysable target cells induce efficient BLT-esterase secretion from TALL-103/2 and TALL-104 cells analogous to findings with conventional cytotoxic T lymphocytes. The stable expression of tumoricidal activity over 2 yr in culture renders these cell lines unique and very useful for studies on the regulation of cell-mediated lysis in vitro and in animal models.

Killing of cells by perforin. Resistance to killing is not due to diminished binding of perforin to the cell membrane

Different cell types vary widely in their susceptibility to killing by the pore-forming cytolytic molecule perforin. In particular, the cells responsible for synthesis of perforin, i.e. cytotoxic T lymphocytes (CTL) and natural killer (NK) cells, are very resistant to cytolysis by this molecule. It has previously been suggested that resistance is due, at least in part, to diminished binding of perforin to these cells. The purpose of the present study was to compare binding of perforin to sensitive and resistant cell types. To this end, perforin was biosynthetically labelled prior to purification. The purified labelled protein was then utilized to obtain a direct measure of the amount of perforin bound to cells during attack. Resistant cells (CTL, neutrophils) bound at least as much perforin as did sensitive cells (K562, HL60 etc.), indicating that resistance to perforin involves mechanisms operating after binding of the lytic molecule.

T lymphocytes degrade fibronectin

Mononuclear cells cause disappearance of fibronectin synthesized and released by fibroblasts. This disappearance of extracellular fibroblast fibronectin is accompanied by the appearance of components of 'lower' molecular weight indicating that a fibronectin-degrading enzymatic activity is responsible for the effect. Additional support for the existence of a fibronectin-degrading enzyme is that mononuclear cells degrade iodinated fibronectin attached to a collagen matrix. Furthermore, lymphocytes seem to mediate the degradation of fibronectin whereas monocytes rather act inhibitory.

Interaction of lymphokine-activated killer cells with susceptible targets does not induce second messenger generation and cytolytic granule exocytosis

CTL activation by specific targets leads to a rapid rise of inositol phosphates (InsPs) and of cytoplasmic-free Ca2+ concentration ([Ca2+]i). While these events are considered necessary to trigger granule secretion, Ca2+-independent cytolytic mechanisms have been recently proposed in addition or as an alternative to the classical Ca2+-dependent exocytosis model. We observed that lymphokine-activated killer (LAK) cells, obtained after stimulation with supraoptimal concentrations of IL-2 in short- or long-term cultures, kill susceptible targets in the absence of a [Ca2+]i rise and InsP3 formation. Moreover, LAK cell-mediated lysis was not associated with an increase in cytotoxic granule exocytosis, as evaluated by BLT-esterase release into the culture supernatant. Furthermore, using an antigen-specific CTL clone, which acquires LAK-like activity when cultured in medium containing high IL-2 doses, second messenger generation and cytolytic granule content secretion were not detected during lysis of unrelated target cells, while killing of specific targets triggered both these processes. These findings suggest that two lytic pathways may coexist in the same effector cells: a second messenger-dependent pathway involving degranulation, which is activated after TCR interaction with specific targets, and another pathway, independent of any known second messenger generation, responsible for unrelated target cell lysis.

Mechanisms of lymphocyte-mediated lysis

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells use multiple mechanisms to destroy their target cells. Pore formation resulting in osmotic lysis of the target is one mechanism; the pore-forming protein (perforin) responsible for this activity has been purified. Antigenically and functionally it resembles proteins of the membrane attack complex of complement. The other known mediators of cytotoxicity appear to be closely interrelated. Tumor necrosis factor (TNF), lymphotoxin (LT), and leukalexin are the three members of this group that have been purified, although their mechanisms of action are still unknown. CTLs fragment the DNA of target cells, as do TNF, LT, and leukalexin; this may be one of the mechanisms of action of these mediators. CTLs and NK cells do not self lyse. The basis of this phenomenon is unclear, although recent advances have shed some light on the problem.

Effects of divalent cations and saccharides on Vibrio metschnikovii cytolysin-induced hemolysis of rabbit erythrocytes

Divalent cations and polysaccharides such as inulin and dextran reversibly inhibited hemolysis of rabbit erythrocytes caused by Vibrio metschnikovii cytolysin. On the basis of the 50% inhibitory doses, the cations were divided into two groups, group I (Cd2+, Cu2+, Ni2+, Sn2+, and Zn2+) and group II (Ba2+, Ca2+, Co2+, Mg2+, Mn2+, and Sr2+). Neither divalent cations nor polysaccharides interfered with the binding of toxins to the erythrocyte membrane. Group I cations disturbed tetramer formation of cytolysin on the cytolysin-lysed erythrocyte membrane, although group II cations and dextran did not affect the process. Erythrocytes treated with cytolysin in the presence of group II cations or dextran lysed after transfer to toxin- and inhibitor (group II cations or dextran)-free buffer at both 37 and 4 degrees C. However, erythrocytes treated in the presence of group I cations lysed at 37 degrees C but not at 4 degrees C, indicating that group I cations block the temperature-dependent lesion (tetramer)-forming step subsequent to the binding of cytolysin to the erythrocytes. The cytolysin-treated erythrocytes swelled in a colloid osmotic manner, and the swelling was preceded by the binding and the lesion-forming steps. It is also suggested that the lysis of the erythrocytes proceeds in a temperature-independent manner and that the cytolysin does not bind to the erythrocytes at 4 degrees C. These findings suggest that the sequence of V. metschnikovii cytolysin-induced hemolysis is defined by three steps: (i) a temperature-dependent binding step, (ii) a temperature-dependent lesion-forming step, and (iii) a temperature-independent lysis step.

Molecular cloning of an inducible serine esterase gene from human cytotoxic lymphocytes

A cDNA clone encoding a human serine esterase gene was isolated from a library constructed from poly(A)+ RNA of allogeneically stimulated, interleukin 2-expanded peripheral blood mononuclear cells. The clone, designated HSE26.1, represents a full-length copy of a 0.9-kilobase mRNA present in human cytotoxic cells but absent from a wide variety of noncytotoxic cell lines. Clone HSE26.1 contains an 892-base-pair sequence, including a single 741-base-pair open reading frame encoding a putative 247-residue polypeptide. The first 20 amino acids of the polypeptide form a leader sequence. The mature protein is predicted to have an unglycosylated Mr of approximately equal to 26,000 and contains a single potential site for N-linked glycosylation. The nucleotide and predicted amino acid sequences of clone HSE26.1 are homologous with all murine and human serine esterases cloned thus far but are most similar to mouse granzyme B (70% nucleotide and 68% amino acid identity). HSE26.1 protein is expressed weakly in unstimulated peripheral blood mononuclear cells but is strongly induced within 6-hr incubation in medium containing phytohemagglutinin. The data suggest that the protein encoded by HSE26.1 plays a role in cell-mediated cytotoxicity.

Heparin inhibits specific glycosyltransferase activities in interleukin 2 activated murine T cells

In order to better understand the role of cell surface glycolipids in T lymphocyte activation, heparin was used to simultaneously modulate the expression of glycolipids and the lytic capacity of lymphocytes activated by interleukin-2. Results presented here show that heparin added at the start of a 3 day culture inhibited the formation of lymphokine activated killer cells by up to 50%. Heparin also has a profound effect on the synthesis of glycolipids during this three day period. Asialo GM1, a useful cell surface marker for subsets of murine cytotoxic cells, is reduced in amount, as are the other two major neutral glycolipids lactosylceramide and asialo GM2. In addition, the synthesis of some gangliosides is affected by heparin treatment. Comparison of the glycosyltransferase activities of untreated and heparin-treated cells shows that the activities of a 2-3-sialyltransferase and a beta 1-3 galactosyltransferase are inhibited dramatically, while a third enzyme, N-acetyl-galactosaminyltransferase is unaffected. The two heparin inhibitable enzymes bind to heparin affinity columns but the galactosaminyltransferase does not. These studies suggest that the proper regulation of the activities of specific glycosyltransferases may be important events in lymphocyte activation.

Identification and sequencing of cDNA clones encoding the granule-associated serine proteases granzymes D, E, and F of cytolytic T lymphocytes

Cytoplasmic granules of cytolytic T lymphocytes contain at least six related serine esterases (granzymes) that are released together with perforin, a pore-forming protein related to complement component C9, during target-cell lysis. Polyclonal antibodies were used to isolate a large number of cDNA clones from an expression library derived from cytolytic-T-cell mRNA. Three distinct full-length cDNA clones coding for granzymes D, E, and F were identified by restriction site mapping and nucleotide sequencing. The three deduced amino acid sequences are highly similar to one another (between 72% and 90% amino acid identities) and to the sequences of granzymes B and C, cathepsin G, and rat mast-cell proteases I and II (between 43% and 57% amino acid identities). Cysteine residues capable of forming intramolecular disulfide bonds are conserved, as are the catalytic-site residues characteristic of serine proteases. Comparison of the cDNA-derived protein sequences with the amino termini of the isolated granzymes provides evidence that they are stored in a fully processed, activated form after removal of the signal peptide and two additional residues (propeptide) at the amino terminus. Immunoelectron microscopic studies demonstrated that granzymes D, E, and F are present in the same morphologically distinct cytoplasmic granules in which perforin has been found previously.

Appearance of granule-associated molecules during activation of cytolytic T-lymphocyte precursors by defined stimuli

Lysis of target cells by cytolytic T lymphocytes (CTL) is associated with the exocytosis of cytoplasmic granules. Purified granules from CTL cell lines contain a pore-forming protein (perforin), tree serine esterases, granzyme A (60,000 MW), granzyme B (29,000 MW), and granzyme C (27,000 MW). We have compared the kinetics of appearance of cytolytic activity with that of perforin and granzyme A activity during activation of lymphocytes from normal animals with leukoagglutinin (LA) and recombinant interleukin-2 (rIL-2). Unstimulated lymph node cells do not express any of these activities, which appear between Day 3 and Day 4 of stimulation and increase rapidly to reach a pronounced peak on Day 6. On Day 7 all the activities are considerably lower, even though the cells still proliferate exponentially. There is a good correlation between the kinetics of appearance of all of these activities. Using antisera against perforin and against granzyme C, one can detect positive cytoplasmic granules in a small fraction of cells on Day 3; by Day 5, 80-90% of the cells are stained. This proportion decreases again on Day 7.

Requirements for the construction of antibody heterodimers for the direction of lysis of tumors by human T cells

We constructed a series of MAb heterodimers consisting of the J5 (anti-common acute lymphoblastic leukemia antigen [CALLA]) antibody and antibodies to a variety of structures present on the surface of activated human T cells, including CD3 antigen (T cell receptor-associated glycoproteins), CD2 antigen (T11/E-rosette receptor), CD25 antigen (IL-2 receptor), and the transferrin receptor. We tested the ability of these heterodimers to direct a CD2 + CD3 + CD8 + CD4 - CD25 + transferrin receptor + MHC-restricted human cytolytic T lymphocyte (CTL) clone to lyse a CALLA + human tumor in vitro. Only heterodimers containing an anti-CD3 antibody or activating antibodies to CD2 could direct the clone to lyse these human tumor targets, even when the clone was additionally activated with anti-CD3 or anti-CD2 antibodies. Our findings may have implications in the design of strategies for the use of such reagents in the treatment of human neoplasia.

Mechanistic, functional and immunopharmacological implications of biochemical studies of antigen receptor-triggered cytolytic T-lymphocyte activation

Biochemical events that follow the engagement of cytotoxic T lymphocytes (CTL) with an Ag-bearing target cell (TC) or triggering by the crosslinking of the Ag-receptor (TcR) by immobilized anti-TcR mAb were studied using cloned CTL and a novel CTL activation assay. The approach described here was undertaken to shed light on the molecular mechanisms of "ON", "STOP" and "OFF" signalling that allow CTL to be activated, kill TC and disengage from the target cell after delivery of the "lethal hit" and then to proceed with the destruction of the next Ag-bearing target encountered. Biochemical studies of TcR-regulated and TcR-triggered constitutive exocytosis in CTL provided a detailed description of the molecular requirements for this important phenomenon in T lymphocytes and provided an alternative CTL activation assay; this assay measures the TcR-dependent response in the absence of a TC. These studies also helped to envision CTLs screening activities as a cycle of engagements-disengagements with the TC, where every surrounding cell is treated by the CTL as a potential Ag-bearing TC. Both constitutive and regulated exocytosis in CTL are triggered through a transmembrane signalling pathway which involves protein kinase C and extracellular Ca2+ that, most likely, is translocated through Ca2+ channels. This is followed by the involvement of calmodulin (CaM)-binding proteins, e.g., calcineurin, a CaM-dependent phosphatase, which was shown to be a major CaM-binding protein in murine lymphocytes. Unexpectedly, these biochemical studies demonstrated that the granule exocytosis model of CTL-mediated cytotoxicity cannot account for the mechanism of target cell lysis by CTL, at least in in vitro conditions in the absence of extracellular Ca2+. These results indicate the existence of an extracellular Ca2+-independent, TcR-regulated CTL response and raise the possibility that second messenger(s) other than Ca2+ and/or products of phosphoinositide turnover are involved in T-cell lysis. Predominance of "non-lethal" engagements between some CTL and TC, revealed during time-lapse cinematographic studies, together with comparative studies of TcR-regulated exocytosis of granules and of constitutive exocytosis of gamma-interferon, suggested that TC destruction by CTL may not be their only or even their most important function in vivo. It is possible that CTL, triggered by Ag recognition to exocytose storage granules and to synthesize and constitutively exocytose macrophage-activating factors, in turn promote tumor destruction by macrophages.(ABSTRACT TRUNCATED AT 400 WORDS)

Granzymes, a family of serine proteases released from granules of cytolytic T lymphocytes upon T cell receptor stimulation

The cytolytic potential of T effector cells appears to be intimately related to the presence of proteins stored in specialized cytoplasmic granules. A striking biological property of isolated granules is their lytic activity for a variety of target cells in a nonrestricted manner. Proteins contained within these granules of CTLs are specifically released upon target cell recognition. We have isolated and characterized six granule-associated proteins in two murine CTL lines in addition to the pore-forming and target membrane-disrupting perforin. Six full length cDNA clones have been identified in a CTL-specific cDNA expression library which code for the granule-associated serine esterases, designated as granzymes A to F. Granzymes A and B represent the genuine proteins encoded by the H factor/CTLA-3 cDNA and the CTLA-1/CCPI cDNA, respectively. The covalent amino acid structures of all six granzymes show the hallmarks for serine proteases and are highly related to that of rat mast cell protease I and II and cathepsin G, which have been found in granules of mast cells and neutrophilic granulocytes, respectively. The primary translation products are processed by removal of a hydrophobic signal peptide and a two residue-long propeptide at the amino-terminus. Immuno-electron microscopy shows that granzymes and perforin are stored together within secretory granules of CTLs. Simultaneous release of at least two of these granzymes has been observed during degranulation of a murine CTL line by anti-T3 antibodies. The biological role, particularly the proteolytic events elicited by granzyme A and other granzymes in the context of target cell recognition, are not known at present. It is unlikely that they form a proteolytic activation cascade together with pore-forming proteins analogous to the complement system. The strictly regulated secretion of granzymes and the lack of measurable enzymatic activity in the case of granzymes B, C, E and F towards a variety of synthetic substrates suggest a highly specific function for each of them.

Possible involvement of CTL granule proteases in target cell DNA breakdown

We have carried out experiments to test whether the granule exocytosis model for lymphocyte cytotoxicity can account for the rapid target DNA breakdown seen during CTL-induced cytotoxicity. Dense granules isolated from cloned mouse CTL and from rat NK tumor cells cause target DNA breakdown during granule cytolysin-mediated lysis of tumor cells, while the purified granule cytolysin caused lysis without DNA breakdown. When target cells are permeabilized with detergent, granule extracts have the ability to release 125I-DNA from nuclei in the absence of detectable cytolysin activity. This activity formed the basis for a nuclear DNA release (NDR) assay; this activity was a property of dense granules of cytotoxic lymphocytes but generally not of other types of lymphoid cells. NDR activity in NK tumor granules had a pH optimum of 7 and was inhibited by micromolar levels of Zn+2, and could be purified away from the granule cytolysin by column chromatography. NDR activity in CTL dense granules could be inactivated by submillimolar concentrations of the protease inhibitors PMSF and DFP (but not soybean trypsin inhibitor or TLCK). In support of the relevance to CTL cytotoxicity of these findings with the NDR assay, pretreatment of CTL with PMSF in the presence of agents raising the intragranular pH inactivated 125I-DNA release from target cells (but not the 51Cr release). These results suggest that a CTL granule component(s), probably a protease, is required for target DNA breakdown.

Perforin-dependent and -independent pathways of cytotoxicity mediated by lymphocytes

There is little doubt at the present time that both perforin-dependent and -independent pathways are important in mediating the cytotoxicity associated with lymphocytes. The cell distribution of perforin, initially thought to include both CTL and NK cells, now must be viewed with caution because all previous biochemical studies on CTL have been conducted with cell lines propagated in long-term cultures in the presence of T cell growth factors (IL-2 and perhaps some still undefined factors). Under these conditions, CTL are known to assume a broader, NK-like specificity in target cell killing and may thus differ significantly from primary CTL generated in the body. Accordingly, perforin does not seem to be present in primary CTL activated directly through mixed lymphocyte reactions. It remains to be shown how primary CTL lyse target cells in vivo. Initial studies conducted in several laboratories have already provided some clues. It now seems that even in cultured, perforin-containing CTL, the perforin pathway is not an obligatory mechanism required for target cell killing. Other pathways, possibly involving TNF/lymphotoxin-like molecules, may play a direct role in this type of cytotoxicity. Other still unidentified factors now also need to be sought, including membrane polypeptides that may develop cytotoxicity directly upon cell contact and binding. Although from the studies reviewed here it is clear now that perforin has a more limited role in cell killing than originally proposed, it is still intriguing that it should share structural and functional homologies with complement proteins, drawing paradoxical analogies between two systems (the cellular and the humoral immune systems) which have evolved to become specialized to carry out separate immunological tasks. The cloning of the genes for perforin and for all the C proteins that comprise the MAC should reveal important information on how these genes originated and then diverged during evolution. The cellular distribution of other granule products, such as serine esterases, also must be viewed with caution. A serine esterase activity was initially thought to be CTL-specific. This information stimulated an intensive research activity in many laboratories that resulted in both the purification of a serine esterase family and the cloning of several serine esterase transcripts. It is becoming clear from recent evidence that this group of enzymes is not truly CTL-specific and therefore would not be expected to develop any function rendered absolutely necessary for cytolysis.(ABSTRACT TRUNCATED AT 400 WORDS)

Antigen receptor-regulated exocytosis in cytotoxic T lymphocytes

We demonstrate here that T cell receptor for antigen (TCR)-triggered exocytosis in cytotoxic T lymphocytes (CTL) is not constitutive and is regulated through crosslinking of the TCR by antigen or monoclonal anti-TCR antibodies. Morphological and biochemical data using three different biochemical markers of granules and Percoll gradient fractionation analysis are presented, suggesting that TCR-triggered exocytosis is accompanied by the loss of granules from CTL and appearance of intragranular proteins and enzymatic activities in the incubation medium. The strict requirement for crosslinking of the TCR in exocytosis triggering could be bypassed by protein kinase C activators (phorbol esters or bryostatin I and II) acting in synergy with Ca2+ ionophores. It is shown that external Ca2+ is obligatory for both the TCR-triggered and for the PMA/A23187-triggered exocytosis, since Ca2+ chelators and divalent cations that compete with Ca2+ for A23187 can inhibit exocytosis of granules. These data suggest that Ca2+ from intracellular stores is not sufficient to support exocytosis in CTL. Ca2+ channel blockers and calmodulin antagonists significantly inhibited TCR-triggered exocytosis without affecting the basal level of secretion. The described results are consistent with a model in which exocytosis of granules in CTL is triggered by the crosslinking of TCR, transmembrane protein kinase C activation, and external Ca2+ translocation through CTL plasma membrane Ca2+ channels and modulation of activity of Ca2+, calmodulin-dependent enzymes, and cytoskeletal proteins.

Calcium and inositolphosphates in the activation of T cell-mediated cytotoxicity

Reports from a number of laboratories have shown that mAbs against the T3-Ti receptor complex cause an increase in cytosolic-free Ca2+ [( Ca2+]i) and the hydrolysis of phosphatidylinositolbisphosphate (PIP2) in CTLs. In the present report we show that activation of CTLs by their specific targets causes: (a) release of Ca2+ from intracellular stores; (b) transient formation of inositol trisphosphate (InsP3); and (c) an increased permeability to Ca2+ of CTL plasma membrane. Killing of unrelated targets could be induced by cocentrifugation of the unrelated targets with CTLs in the presence of A23187 or PMA. We conclude that: (a) activation of CTLs by specific antigens triggers the generation of the same intracellular mediators generated by stimulation of lymphocytes with anti-T3-Ti receptor antibodies and/or with polyclonal mitogens; and (b) intracellular signals that mediate the delivery of the lethal hit by CTLs are indistinguishable from those that induce cell proliferation.

Resistance of cytotoxic T lymphocytes to lysis by a clone of cytotoxic T lymphocytes

To investigate how cytotoxic T lymphocytes (CTL) avoid killing themselves when they destroy target cells, we compared 20 different cell lines as target cells, including several CTL cell lines, for their susceptibility to lysis by CTL. Variations in recognition of this diverse set of target cells was circumvented by attaching to all of them a monoclonal antibody to the antigen-specific receptor of a cloned CTL cell line (clone 2C) and using the 2C cell line as the standard aggressor or effector cell. All of the nine tumor cell lines and the four noncytolytic T-helper cell lines tested as targets were highly susceptible to lysis by the aggressor CTL, but seven cytotoxic T-cell lines (six CTL and one T-helper cell line with cytotoxic activity) were largely resistant. These results, and the use of the lectin Con A as an alternative means for triggering CTL activity, point clearly to a level of resistance that could enable CTL to avoid their own destruction when they lyse target cells. The resistance of the cytolytic T cells did not appear to be accompanied by a similar resistance to complement-mediated lysis, indicating that mechanisms of CTL-mediated and complement-mediated lysis are not identical.

Dissociation of membrane binding and lytic activities of the lymphocyte pore-forming protein (perforin)

Granules isolated from CTL and NK cells contain a cytolytic pore-forming protein (PFP/perforin). At low temperatures (on ice), PFP binds to erythrocyte membranes without producing hemolysis. Hemolysis occurs when the PFP-bound erythrocytes are warmed up to 37 degrees C, which defines a temperature-dependent, lytic (pore-formation) step distinct from the membrane-binding event. Ca2+ and neutral pH are required for both membrane binding and pore formation by PFP. Serum, LDL, HDL, and heparin inhibit the hemolytic activity of PFP by blocking its binding to lipid membranes. Lysis by PFP that has bound to erythrocyte membranes is no longer susceptible to the effect of these inhibitors. The hemolytic activities associated with intact granules and solubilized PFP show different requirements for Ca2+ and pH, indicating that cytolysis produced by isolated granules may involve an additional step, possibly fusion of granules with membranes. It is suggested that three distinct Ca2+- and pH-dependent events may be involved during cell killing by CTL and NK cells: fusion of cytoplasmic granules of effector cells with their plasma membrane, releasing PFP from cells; binding of the released PFP to target membranes; and insertion of monomers and the subsequent formation of lytic pores in the target membrane. The serum-mediated inhibition of membrane binding by PFP could prevent the accidental injury of bystander cells by cell-released PFP, but would allow cytolysis to proceed to completion once PFP has bound to the target membrane.

The pore-forming protein (perforin) of cytolytic T lymphocytes is immunologically related to the components of membrane attack complex of complement through cysteine-rich domains

Structural, functional and immunological similarities between the ninth component of complement (C9) and the lymphocyte pore-forming protein (PFP, perforin) have recently been described (8-10). PFP is shown here to be immunologically related to all other components of the membrane attack complex (MAC) of human complement, namely, C5b-6, C7, C8, and C9. Polyclonal antibodies raised against purified human C5b-6, C7, C8, or C9 react with other components of the MAC and with mouse lymphocyte PFP. The antigenic epitopes shared by human complement proteins and mouse lymphocyte PFP are limited to cysteine-rich domains. Only complement proteins that have been reduced and alkylated elicit the production of crossreactive antibodies when used as immunogens. The nonreduced forms of complement components or lymphocyte PFP neither react with these antibodies nor give rise to crossreactive antibodies. The homologous domains of complement proteins and lymphocyte PFP may play related functions in their attachment to lipid membranes and assembly of membrane lesions.

Identification and characterization of a pore-forming protein of human peripheral blood natural killer cells

We show here that human peripheral blood NK cells contain a pore-forming protein (PFP) with an Mr of 70,000-72,000 that assembles structural lesions (with an average internal diameter of 150-170 A) and forms functional channels. The PFP was isolated by affinity chromatography from human NK cells, using a specific anti-C9 antiserum as the immunoadsorbent. The NK cells were isolated from PBL by positive or negative selection by indirect rosetting using a panel of monoclonal antibodies directed against different NK and T cell surface antigens. PFP was identified in NK cells freshly isolated and isolated from cultured PBL, both stimulated with interleukin 2, but not in NK cell-depleted lymphocytes. In planar bilayers, the channels formed by the NK cell-derived PFP are highly voltage resistant, with most channels persisting in the open state once they have inserted into the bilayer. The unit conductances of these channels range 0.3-1 nS in 0.1 M NaCl. The channels show poor selectivity for monovalent and divalent ions. The PFP is also released from human NK cells stimulated with the calcium ionophore A23187, suggesting that this protein, like the one produced by murine CTL lines, may be similarly secreted during cell-mediated killing. Its identification in primary human NK cell cultures indicates that this protein may play an active role in NK cell-mediated killing.