Binding of perforin to membranes is sensitive to lipid spacing and not headgroup

Cell softness renders cytotoxic T lymphocytes and T leukemic cells resistant to perforin-mediated killing

Mechanical force contributes to perforin pore formation at immune synapses, thus facilitating the cytotoxic T lymphocytes (CTL)-mediated killing of tumor cells in a unidirectional fashion. How such mechanical cues affect CTL evasion of perforin-mediated autolysis remains unclear. Here we show that activated CTLs use their softness to evade perforin-mediated autolysis, which, however, is shared by T leukemic cells to evade CTL killing. Downregulation of filamin A is identified to induce softness via ZAP70-mediated YAP Y357 phosphorylation and activation. Despite the requirements of YAP in both cell types for softness induction, CTLs are more resistant to YAP inhibitors than malignant T cells, potentially due to the higher expression of the drug-resistant transporter, MDR1, in CTLs. As a result, moderate inhibition of YAP stiffens malignant T cells but spares CTLs, thus allowing CTLs to cytolyze malignant cells without autolysis. Our findings thus hint a mechanical force-based immunotherapeutic strategy against T cell leukemia.

Escaping Death: How Cancer Cells and Infected Cells Resist Cell-Mediated Cytotoxicity

Cytotoxic lymphocytes are critical in our immune defence against cancer and infection. Cytotoxic T lymphocytes and Natural Killer cells can directly lyse malignant or infected cells in at least two ways: granule-mediated cytotoxicity, involving perforin and granzyme B, or death receptor-mediated cytotoxicity, involving the death receptor ligands, tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas ligand (FasL). In either case, a multi-step pathway is triggered to facilitate lysis, relying on active pro-death processes and signalling within the target cell. Because of this reliance on an active response from the target cell, each mechanism of cell-mediated killing can be manipulated by malignant and infected cells to evade cytolytic death. Here, we review the mechanisms of cell-mediated cytotoxicity and examine how cells may evade these cytolytic processes. This includes resistance to perforin through degradation or reduced pore formation, resistance to granzyme B through inhibition or autophagy, and resistance to death receptors through inhibition of downstream signalling or changes in protein expression. We also consider the importance of tumour necrosis factor (TNF)-induced cytotoxicity and resistance mechanisms against this pathway. Altogether, it is clear that target cells are not passive bystanders to cell-mediated cytotoxicity and resistance mechanisms can significantly constrain immune cell-mediated killing. Understanding these processes of immune evasion may lead to novel ideas for medical intervention.

Radiotherapy transiently reduces the sensitivity of cancer cells to lymphocyte cytotoxicity

The impact of radiotherapy on the interaction between immune cells and cancer cells is important not least because radiotherapy can be used alongside immunotherapy as a cancer treatment. Unexpectedly, we found that X-ray irradiation of cancer cells induced significant resistance to natural killer (NK) cell killing. This was true across a wide variety of cancer-cell types as well as for antibody-dependent cellular cytotoxicity. Resistance appeared 72 h postirradiation and persisted for 2 wk. Resistance could also occur independently of radiotherapy through pharmacologically induced cell-cycle arrest. Crucially, multiple steps in NK-cell engagement, synapse assembly, and activation were unaffected by target cell irradiation. Instead, radiotherapy caused profound resistance to perforin-induced calcium flux and lysis. Resistance also occurred to a structurally similar bacterial toxin, streptolysin O. Radiotherapy did not affect the binding of pore-forming proteins at the cell surface or membrane repair. Rather, irradiation instigated a defect in functional pore formation, consistent with phosphatidylserine-mediated perforin inhibition. In vivo, radiotherapy also led to a significant reduction in NK cell-mediated clearance of cancer cells. Radiotherapy-induced resistance to perforin also constrained chimeric antigen receptor T-cell cytotoxicity. Together, these data establish a treatment-induced resistance to lymphocyte cytotoxicity that is important to consider in the design of radiotherapy-immunotherapy protocols.

Lipid specificity of the immune effector perforin

Perforin is a pore forming protein used by cytotoxic T lymphocytes to remove cancerous or virus-infected cells during the immune response. During the response, the lymphocyte membrane becomes refractory to perforin function by accumulating densely ordered lipid rafts and externalizing negatively charged lipid species. The dense membrane packing lowers the capacity of perforin to bind, and the negatively charged lipids scavenge any residual protein before pore formation. Using atomic force microscopy on model membrane systems, we here provide insight into the molecular basis of perforin lipid specificity.

The Extracellular Milieu of Toxoplasma's Lytic Cycle Drives Lab Adaptation, Primarily by Transcriptional Reprogramming

Evolve and resequencing (E&R) was applied to lab adaptation of Toxoplasma gondii for over 1,500 generations with the goal of mapping host-independent in vitro virulence traits. Phenotypic assessments of steps across the lytic cycle revealed that only traits needed in the extracellular milieu evolved. Nonsynonymous single-nucleotide polymorphisms (SNPs) in only one gene, a P4 flippase, fixated across two different evolving populations, whereas dramatic changes in the transcriptional signature of extracellular parasites were identified. Newly developed computational tools correlated phenotypes evolving at different rates with specific transcriptomic changes. A set of 300 phenotype-associated genes was mapped, of which nearly 50% is annotated as hypothetical. Validation of a select number of genes by knockouts confirmed their role in lab adaptation and highlights novel mechanisms underlying in vitro virulence traits. Further analyses of differentially expressed genes revealed the development of a “pro-tachyzoite” profile as well as the upregulation of the fatty acid biosynthesis (FASII) pathway. The latter aligned with the P4 flippase SNP and aligned with a low abundance of medium-chain fatty acids at low passage, indicating this is a limiting factor in extracellular parasites. In addition, partial overlap with the bradyzoite differentiation transcriptome in extracellular parasites indicated that stress pathways are involved in both situations. This was reflected in the partial overlap between the assembled ApiAP2 and Myb transcription factor network underlying the adapting extracellular state with the bradyzoite differentiation program. Overall, E&R is a new genomic tool successfully applied to map the development of polygenic traits underlying in vitro virulence of T. gondii.

IMPORTANCE It has been well established that prolonged in vitro cultivation of Toxoplasma gondii augments progression of the lytic cycle. This lab adaptation results in increased capacities to divide, migrate, and survive outside a host cell, all of which are considered host-independent virulence factors. However, the mechanistic basis underlying these enhanced virulence features is unknown. Here, E&R was utilized to empirically characterize the phenotypic, genomic, and transcriptomic changes in the non-lab-adapted strain, GT1, during 2.5 years of lab adaptation. This identified the shutdown of stage differentiation and upregulation of lipid biosynthetic pathways as the key processes being modulated. Furthermore, lab adaptation was primarily driven by transcriptional reprogramming, which rejected the starting hypothesis that genetic mutations would drive lab adaptation. Overall, the work empirically shows that lab adaptation augments T. gondii’s in vitro virulence by transcriptional reprogramming and that E&R is a powerful new tool to map multigenic traits.

Metabolic Dynamics of In Vitro CD8+ T Cell Activation

CD8+ T cells detect and kill infected or cancerous cells. When activated from their naïve state, T cells undergo a complex transition, including major metabolic reprogramming. Detailed resolution of metabolic dynamics is needed to advance the field of immunometabolism. Here, we outline methodologies that when utilized in parallel achieve broad coverage of the metabolome. Specifically, we used a combination of 2 flow injection analysis (FIA) and 3 liquid chromatography (LC) methods in combination with positive and negative mode high-resolution mass spectrometry (MS) to study the transition from naïve to effector T cells with fine-grained time resolution. Depending on the method, between 54% and 98% of measured metabolic features change in a time-dependent manner, with the major changes in both polar metabolites and lipids occurring in the first 48 h. The statistical analysis highlighted the remodeling of the polyamine biosynthesis pathway, with marked differences in the dynamics of precursors, intermediates, and cofactors. Moreover, phosphatidylcholines, the major class of membrane lipids, underwent a drastic shift in acyl chain composition with polyunsaturated species decreasing from 60% to 25% of the total pool and specifically depleting species containing a 20:4 fatty acid. We hope that this data set with a total of over 11,000 features recorded with multiple MS methodologies for 9 time points will be a useful resource for future work.

To Kill But Not Be Killed: Controlling the Activity of Mammalian Pore-Forming Proteins

Pore-forming proteins (PFPs) are present in all domains of life, and play an important role in host-pathogen warfare and in the elimination of cancers. They can be employed to deliver specific effectors across membranes, to disrupt membrane integrity interfering with cell homeostasis, and to lyse membranes either destroying intracellular organelles or entire cells. Considering the destructive potential of PFPs, it is perhaps not surprising that mechanisms controlling their activity are remarkably complex, especially in multicellular organisms. Mammalian PFPs discovered to date include the complement membrane attack complex (MAC), perforins, as well as gasdermins. While the primary function of perforin-1 and gasdermins is to eliminate infected or cancerous host cells, perforin-2 and MAC can target pathogens directly. Yet, all mammalian PFPs are in principle capable of generating pores in membranes of healthy host cells which-if uncontrolled-could have dire, and potentially lethal consequences. In this review, we will highlight the strategies employed to protect the host from destruction by endogenous PFPs, while enabling timely and efficient elimination of target cells.

Lipid order and charge protect killer T cells from accidental death

Killer T cells (cytotoxic T lymphocytes, CTLs) maintain immune homoeostasis by eliminating virus-infected and cancerous cells. CTLs achieve this by forming an immunological synapse with their targets and secreting a pore-forming protein (perforin) and pro-apoptotic serine proteases (granzymes) into the synaptic cleft. Although the CTL and the target cell are both exposed to perforin within the synapse, only the target cell membrane is disrupted, while the CTL is invariably spared. How CTLs escape unscathed remains a mystery. Here, we report that CTLs achieve this via two protective properties of their plasma membrane within the synapse: high lipid order repels perforin and, in addition, exposed phosphatidylserine sequesters and inactivates perforin. The resulting resistance of CTLs to perforin explains their ability to kill target cells in rapid succession and to survive these encounters. Furthermore, these mechanisms imply an unsuspected role for plasma membrane organization in protecting cells from immune attack.

Cytotoxic T lymphocytes (CTLs) eliminate virus-infected and cancerous cells by secreting the pore-forming protein (perforin) and pro-apoptotic serine proteases (granzymes). Here authors show that two mechanisms protect the membranes of CTLs from disruption by perforin and granzymes.

Mechanisms of natural killer cell-mediated cellular cytotoxicity

Cellular cytotoxicity, the ability to kill other cells, is an important effector mechanism of the immune system to combat viral infections and cancer. Cytotoxic T cells and natural killer (NK) cells are the major mediators of this activity. Here, we summarize the cytotoxic mechanisms of NK cells. NK cells can kill virally infected of transformed cells via the directed release of lytic granules or by inducing death receptor-mediated apoptosis via the expression of Fas ligand or TRAIL. The biogenesis of perforin and granzymes, the major components of lytic granules, is a highly regulated process to prevent damage during the synthesis of these cytotoxic molecules. Additionally, NK cells have developed several strategies to protect themselves from the cytotoxic activity of granular content upon degranulation. While granule-mediated apoptosis is a fast process, death receptor-mediated cytotoxicity requires more time. Current data suggest that these 2 cytotoxic mechanisms are regulated during the serial killing activity of NK cells. As many modern approaches of cancer immunotherapy rely on cellular cytotoxicity for their effectiveness, unraveling these pathways will be important to further progress these therapeutic strategies.

Structural basis of Toxoplasma gondii perforin-like protein 1 membrane interaction and activity during egress

Intracellular pathogens must egress from the host cell to continue their infectious cycle. Apicomplexans are a phylum of intracellular protozoans that have evolved members of the membrane attack complex and perforin (MACPF) family of pore forming proteins to disrupt cellular membranes for traversing cells during tissue migration or egress from a replicative vacuole following intracellular reproduction. Previous work showed that the apicomplexan Toxoplasma gondii secretes a perforin-like protein (TgPLP1) that contains a C-terminal Domain (CTD) which is necessary for efficient parasite egress. However, the structural basis for CTD membrane binding and egress competency remained unknown. Here, we present evidence that TgPLP1 CTD prefers binding lipids that are abundant in the inner leaflet of the lipid bilayer. Additionally, solving the high-resolution crystal structure of the TgPLP1 APCβ domain within the CTD reveals an unusual double-layered β-prism fold that resembles only one other protein of known structure. Three direct repeat sequences comprise subdomains, with each constituting a wall of the β-prism fold. One subdomain features a protruding hydrophobic loop with an exposed tryptophan at its tip. Spectrophotometric measurements of intrinsic tryptophan fluorescence are consistent with insertion of the hydrophobic loop into a target membrane. Using CRISPR/Cas9 gene editing we show that parasite strains bearing mutations in the hydrophobic loop, including alanine substitution of the tip tryptophan, are equally deficient in egress as a strain lacking TgPLP1 altogether. Taken together our findings suggest a crucial role for the hydrophobic loop in anchoring TgPLP1 to the membrane to support its cytolytic activity and egress function.

The intracellular parasite Toxoplasma gondii infects many hosts including humans. Infected people with a weak immune system can suffer severe disease when the parasite replicates uncontrolled via repeated cycles of cell invasion, intracellular growth, and exit, resulting in cell death. Previous studies showed that T. gondii encodes a pore-forming protein, TgPLP1, which contains an unusual domain that is crucial for efficient exit from both the parasite containing vacuole and the host cell. However, how TgPLP1 recognizes and binds to the appropriate membrane is unclear. Here we use a combination of biochemistry, structural biology, and parasitology to identify a preference of TgPLP1 for specific lipids and show that a loop within the structure of the membrane-binding domain inserts into the target membrane and is necessary for exit from the parasite containing vacuole. Our study sheds light into the determinants of membrane binding in TgPLP1 and may inform the overall mechanism of pore formation in similar systems.

Bispecific T-cell engagers: Towards understanding variables influencing the in vitro potency and tumor selectivity and their modulation to enhance their efficacy and safety

Bispecific molecules redirecting the cytotoxicity of T-cells are a growing class of therapeutics with numerous molecules being tested in clinical trials. However, it has been a long way since the proof of concept studies in the mid 1980's. In the process we have learnt about the impact of different variables related to the bispecific molecule and the target antigen on the potency of this type of drugs. This work reviews the insights gained and how that knowledge has been used to design more potent bispecific T-cell engagers. The more recent advancement of antibodies with this modality into safety studies in non-human primates and as well as in clinical studies has revealed potential toxicity liabilities for the mode of action. Modifications in existing antibody formats and new experimental molecules designed to mitigate these problems are discussed.

Real-time visualization of perforin nanopore assembly

Perforin is a key protein of the vertebrate immune system. Secreted by cytotoxic lymphocytes as soluble monomers, perforin can self-assemble into oligomeric pores of 10-20 nm inner diameter in the membranes of virus-infected and cancerous cells. These large pores facilitate the entry of pro-apoptotic granzymes, thereby rapidly killing the target cell. To elucidate the pathways of perforin pore assembly, we carried out real-time atomic force microscopy and electron microscopy studies. Our experiments reveal that the pore assembly proceeds via a membrane-bound prepore intermediate state, typically consisting of up to approximately eight loosely but irreversibly assembled monomeric subunits. These short oligomers convert to more closely packed membrane nanopore assemblies, which can subsequently recruit additional prepore oligomers to grow the pore size.

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.

Computational modeling of membrane proteins

The determination of membrane protein (MP) structures has always trailed that of soluble proteins due to difficulties in their overexpression, reconstitution into membrane mimetics, and subsequent structure determination. The percentage of MP structures in the protein databank (PDB) has been at a constant 1-2% for the last decade. In contrast, over half of all drugs target MPs, only highlighting how little we understand about drug-specific effects in the human body. To reduce this gap, researchers have attempted to predict structural features of MPs even before the first structure was experimentally elucidated. In this review, we present current computational methods to predict MP structure, starting with secondary structure prediction, prediction of trans-membrane spans, and topology. Even though these methods generate reliable predictions, challenges such as predicting kinks or precise beginnings and ends of secondary structure elements are still waiting to be addressed. We describe recent developments in the prediction of 3D structures of both α-helical MPs as well as β-barrels using comparative modeling techniques, de novo methods, and molecular dynamics (MD) simulations. The increase of MP structures has (1) facilitated comparative modeling due to availability of more and better templates, and (2) improved the statistics for knowledge-based scoring functions. Moreover, de novo methods have benefited from the use of correlated mutations as restraints. Finally, we outline current advances that will likely shape the field in the forthcoming decade.

Defining the interaction of perforin with calcium and the phospholipid membrane

The present study resolves the molecular mechanism behind the key first steps in the action of an essential immune protein, cytotoxic lymphocyte perforin, binding to the plasma membrane of a target cell and initiation of pore formation.

Packing a punch: the mechanism of pore formation by cholesterol dependent cytolysins and membrane attack complex/perforin-like proteins

The bacterial cholesterol dependent cytolysins (CDCs) and membrane attack complex/perforin-like proteins (MACPF) represent two major branches of a large, exceptionally diverged superfamily. Most characterized CDC/MACPF proteins form large pores that function in immunity, venoms, and pathogenesis. Extensive structural, biochemical and biophysical studies have started to address some of the questions surrounding how the soluble, monomeric form of these remarkable molecules recognize diverse targets and assemble into oligomeric membrane embedded pores. This review explores mechanistic similarities and differences in how CDCs and MACPF proteins form pores.

Pumping lipids with P4-ATPases

While accumulating evidence indicates that P4-ATPases catalyze phospholipid transport across cellular bilayers, their kinship to cation-pumping ATPases has raised fundamental questions concerning the underlying flippase mechanism. Loss of P4-ATPase function perturbs vesicle formation in late secretory and endocytic compartments. An intriguing concept is that P4-ATPases help drive vesicle budding by generating imbalances in transbilayer lipid numbers. Moreover, activation of P4-ATPases by phosphoinositides and other effectors of coat recruitment provide a potential mechanism to confine flippase activity to sites of vesicle biogenesis. These developments have raised considerable interest in understanding the mechanism, regulation and biological implications of P4-ATPase-catalyzed phospholipid transport.

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.

Identification and functional expression of four isoforms of ATPase II, the putative aminophospholipid translocase. Effect of isoform variation on the ATPase activity and phospholipid specificity

ATPase II, a vanadate-sensitive and phosphatidylserine-dependent Mg(2+)-ATPase, is a member of a subfamily of P-type ATPase and is presumably responsible for aminophospholipid translocation activity in eukaryotic cells. The aminophospholipid translocation activity plays an important physiological role in the maintenance of membrane phospholipid asymmetry that is observed in the plasma membrane as well as the membranes of certain cellular organelles. While the preparations of ATPase II from different sources share common fundamental properties, such as substrate specificity, inhibitor spectrum, and phospholipid dependence, they are divergent in several characteristics. These include specific ATPase activity and phospholipid selectivity. We report here the identification of four isoforms of ATPase II in bovine brain. These isoforms are formed by a combination of two major variations in their primary sequences and show that the structural variation of these isoforms has functional significance in both ATPase activity and phosholipid selectivity. Furthermore, studies with the phosphoenzyme intermediate of ATPase II and its recombinant isoforms revealed that phosphatidylserine is essential for the dephosphorylation of the intermediate. Without phosphatidylserine, ATPase II would be accumulated as phosphoenzyme in the presence of ATP, resulting in the interruption of its catalytic cycle.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

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.

Interferon-γ–induced membrane PAF-receptor expression confers tumor cell susceptibility to NK perforin-dependent lysis

Perforin is known to display a membranolytic activity on tumor cells. Nevertheless, perforin release during natural killer (NK)–cell activation is not sufficient to induce membrane target-cell damage. On the basis of the ability of perforin to interact with phospholipids containing a choline phosphate headgroup, we identify the platelet-activating factor (PAF) and its membrane receptor as crucial components in tumor cell killing activity of human resting NK cells. We demonstrate for the first time that upon activation, naive NK cells release the choline phosphate–containing lysolipid PAF, which binds to perforin and acts as an agonist on perforin-induced membrane damage. PAF is known to incorporate cell membranes using a specific receptor. Here we show that interferon-γ (IFN–γ) secreted from activated NK cells ends in PAF-receptor expression on perforin-sensitive K562 cells but not on perforin-resistant Daudi cells. In order to prove the capacity of PAF to interact simultaneously with its membrane PAF receptor and with perforin, we successfully co-purified the 3 components in the presence of bridging PAF molecules. The functional activity of this complex was further examined. The aim was to determine whether membrane PAF-receptor expression on tumor cells, driven to express this receptor, could render them sensitive to the perforin lytic pathway. The results confirmed that transfection of the PAF-receptor complementary DNA into major histocompatibility complex class I and Fas-receptor negative tumor cells restored susceptibility to naive NK cells and perforin attack. Failure of IFN-γ to induce membrane PAF receptor constitutes the first described mechanism for tumor cells to resist the perforin lytic pathway.

Interferon-γ–induced membrane PAF-receptor expression confers tumor cell susceptibility to NK perforin-dependent lysis

Perforin is known to display a membranolytic activity on tumor cells. Nevertheless, perforin release during natural killer (NK)–cell activation is not sufficient to induce membrane target-cell damage. On the basis of the ability of perforin to interact with phospholipids containing a choline phosphate headgroup, we identify the platelet-activating factor (PAF) and its membrane receptor as crucial components in tumor cell killing activity of human resting NK cells. We demonstrate for the first time that upon activation, naive NK cells release the choline phosphate–containing lysolipid PAF, which binds to perforin and acts as an agonist on perforin-induced membrane damage. PAF is known to incorporate cell membranes using a specific receptor. Here we show that interferon-γ (IFN–γ) secreted from activated NK cells ends in PAF-receptor expression on perforin-sensitive K562 cells but not on perforin-resistant Daudi cells. In order to prove the capacity of PAF to interact simultaneously with its membrane PAF receptor and with perforin, we successfully co-purified the 3 components in the presence of bridging PAF molecules. The functional activity of this complex was further examined. The aim was to determine whether membrane PAF-receptor expression on tumor cells, driven to express this receptor, could render them sensitive to the perforin lytic pathway. The results confirmed that transfection of the PAF-receptor complementary DNA into major histocompatibility complex class I and Fas-receptor negative tumor cells restored susceptibility to naive NK cells and perforin attack. Failure of IFN-γ to induce membrane PAF receptor constitutes the first described mechanism for tumor cells to resist the perforin lytic pathway.

Perforin is activated by a proteolytic cleavage during biosynthesis which reveals a phospholipid-binding C2 domain

Perforin is a secreted protein synthesized by activated cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. It is a key component of the lytic machinery of these cells, being able to insert into the plasma membrane of targeted cells, forming a pore which leads to their destruction. Here we analyse the synthesis, processing and intracellular transport of perforin in the NK cell line YT. Perforin is synthesized as a 70 kDa inactive precursor which is cleaved at the C-terminus to yield a 60 kDa active form. This proteolytic cleavage occurs in an acidic compartment and can be inhibited by incubation of the cells in ammonium chloride, concanamycin A, leupeptin and E-64. The increased lytic activity of the cleaved form can be demonstrated by killing assays in which cleavage of the pro-piece is inhibited. Epitope mapping reveals that cleavage of the pro-piece occurs at the boundary of a C2 domain, which we show is able to bind phospholipid membranes in a calcium-dependent manner. We propose that removal of the pro-piece, which contains a bulky glycan, allows the C2 domain to interact with phospholipid membranes and initiate perforin pore formation.

Dependence of the lytic activity of the N-terminal domain of human perforin on membrane lipid composition--implications for T-cell self-preservation

The kinetics and thermodynamics of pore formation by the 22-residue N-terminal domain of human perforin-(1-22)-peptide have been studied for a variety of model phospholipid membranes. Peptide binding and aggregation, and cell lysis were monitored through the change in the fluorescence of Trp, or vesicle-encapsulated carboxyfluorescein, respectively. Peptide binding was analyzed in terms of a model that incorporates non-ideal interactions and aggregation in a membrane bilayer. The minimum number of peptide monomers required to form an active pore averaged from four to six, according to the lipid composition of the vesicle. This combined kinetic and thermodynamic approach has provided quantitative information that allows a direct comparison of the binding behavior of the perforin-(1-22)-peptide in different lipid vesicles and affords molecular insight on the factors controlling pore formation. Pore formation is most favorable in thinner membranes with low melting temperatures. No significant difference in activity is observed for different zwitterionic headgroups. Rather, the gel state of the lipid chain, which diminishes the incorporation and aggregation of the perforin-(1-22)-peptide shows the strongest influence. This effect is observed in both the thermodynamic (incorporation isotherm) and kinetic (carboxyfluorescein release) studies. Insertion and aggregation are more facile in membranes with less densely packed lipids. The dependence of pore-forming activity on lipid composition provides important clues to understanding the self-protection mechanism employed by cytotoxic T lymphocytes (CTL) against perforin-mediated lysis.

Lipid structure of cytotoxic granules in living human killer T lymphocytes studied by Raman microspectroscopy

The structures of cytotoxic granules in interleukin-2-activated human killer T lymphocytes have been investigated by Raman microspectroscopy at a single cell level. The Raman spectra of granules share a common feature that lipid Raman bands are much stronger than the Raman bands due to protein, indicating that one of the main components of the granule is lipid. To analyze the lipid structures of individual granules, relationships between Raman spectra and structures have been examined for a series of triacylgycerols with varied degrees of acyl chain unsaturation. Analysis based on the relationships shows that the granulous lipid is characterized by a high content of cis C=C bond, which ranges from about 1.5 C=C bonds per acyl chain in isolated minor granules and to about 2.2 C=C bonds in clustering major granules. The highly unsaturated lipid of major cytotoxic granules is in sharp contrast to the moderately unsaturated (about one C=C bond per acyl chain) plasma membrane lipid. The large difference in lipid unsaturation between the granule and plasma membrane may have relevance to the role of granulous lipid in packaging cytotoxic proteins inside the granule and preventing them from attacking the killer lymphocyte itself.

Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

That some membranes restrict certain lipid species to one side of the bilayer and others to the opposite side has been known for two decades. However, how this asymmetric transbilayer distribution is generated and controlled, how many and what type of membranes are so structured, and even the reason for its existence is just now beginning to be understood. It has been a decade since the discovery of an activity which transports in an ATP-dependent manner only the aminophospholipids from the outer to the inner leaflet of the plasma membrane. This aminophospholipid translocase has yet to be isolated, reconstituted, and identified molecularly. Elevating intracellular Ca2+ allows all the major classes of phospholipids to move freely across the bilayer, scrambling lipids and dissipating asymmetry. The nature of this pathway and its mode of activation by Ca2+ remain to be determined. Though loss of transbilayer asymmetry by blood cells clearly produces a procoagulant surface and increases interactions with the reticuloendothelial system, it remains to be elucidated whether maintenance of blood homeostasis is just one expression of a more general raison d'être for lipid asymmetry. It is these persisting uncertainties and gaps in our knowledge which make the field such an interesting and exciting challenge at the present time.