Effect of altered membrane fluidity on NK cell-mediated cytotoxicity. I. Selective inhibition of the recognition or post recognition events in the cytolytic pathway of NK cells

NK cell-mediated cytotoxicity results from membrane interactions between NK effector and target cells. The role of membrane fluidity in these events is not known. The present study was undertaken to investigate the effect of changes in membrane lipid fluidity of NK effector and NK-sensitive target cells on the lytic pathway of NK cell-mediated cytotoxicity. Fluidity was modulated by various lipids and measured by fluorescence polarization. NK effector cells treated with phosphatidylcholine complexed with polyvinylpyrrolidone (PVP) and bovine serum albumin (BSA) showed increased membrane fluidity. This fluidization of the effector cell membrane resulted in a significant inhibition of cytotoxic activity in the 51Cr-release assay. Single cell analysis revealed that the inhibition was due to a decrease in the frequency of NK target conjugates and reduced killing of conjugated targets. Rigidification of the NK effector cell membranes by treatment with cholesteryl hemisuccinate complexed with PVP and BSA also resulted in inhibition of cytotoxicity. This inhibition was post binding, because binding was increased and lysis was abrogated. Fluidization of K562 target cell membranes caused a slight but insignificant increase in their lysis by NK cells without affecting the binding step. On the other hand, rigidification of K562 membranes decreased the sensitivity of these target cells to lysis. Single cell analysis revealed that this inhibition of NK lysis is post binding, because the frequency of killers was significantly decreased. It was also shown that membrane rigidification of target cells that were programmed for lysis during the lethal hit stage and subsequently separated from effector cells, rendered the programmed cells resistant to killing during the killer cell-independent lysis step. These results demonstrate that fluidization or rigidification of the plasma membrane of either effector or target cells affect different stages of the NK cell-mediated cytolytic events.     

Effect of altered membrane structure on NK cell-mediated cytotoxicity. III. Decreased susceptibility to natural killer cytotoxic factor (NKCF) and suppression of NKCF release by membrane rigidification

We have shown recently that alteration of the membrane fluidity of either effector or target cells results in significant and selective inhibition of NK cell-mediated cytotoxicity (NK CMC). However, the localization of the defective stage in the NK lytic pathway is not known. In the present study, we show that rigidification of the NK-sensitive U937 target cell membrane by lipid modulation reduces its sensitivity to lysis by NK cytotoxic factor (NKCF). This resistance was not due to loss of NKCF binding sites on the target cell because target cells with rigid membranes absorbed more NKCF than control cells. The enhanced ability to absorb NKCF by membrane modification was supported by data showing that NK-resistant Raji cells lacking NKCF-binding sites absorb NKCF after lipid modification. Furthermore, consistent with the lipophilic nature of NKCF, synthetic lipid vesicles absorb NKCF. In contrast to membrane rigidification, membrane fluidization of the target cell did not change the target cell properties. Rigidification of the NK effector cell membrane abrogates it ability to secrete active NKCF when stimulated by target cells or by mitogens. Membrane fluidization of the NK effector cells did not inhibit their ability to release NKCF. The results of these studies demonstrate that inhibition of NK CMC by rigidification of the target cell membrane results in cells that are inhibited in processing bound NKCF to lysis. Inhibition of NK CMC by rigidification of the NK effector cell results in defective trigger for activation of the NKCF release mechanism.     

Effect of lipid composition upon fusion of liposomes with Sendai virus membranes

How the lipid composition of liposomes determines their ability to fuse with Sendai virus membranes was tested. Liposomes were made of compositions designed to test postulated mechanisms of membrane fusion that require specific lipids. Fusion does not require the presence of lipids that can form micelles such as gangliosides or lipids that can undergo lamellar to hexagonal phase transitions such as phosphatidylethanolamine (PE), nor is a phosphatidylinositol (PI) to phosphatidic acid (PA) conversion required, since fusion occurs with liposomes containing phosphatidylcholine (PC) and any one of many different negatively charged lipids such as gangliosides, phosphatidylserine (PS), phosphatidylglycerol, dicetyl phosphate, PI, or PA. A negatively charged lipid is required since fusion does not occur with neutral liposomes containing PC and a neutral lipid such as globoside, sphingomyelin, or PE. Fusion of Sendai virus membranes with liposomes that contain PC and PS does not require Ca2+, so an anhydrous complex with Ca2+ or a Ca2+-induced lateral phase separation is not required although the possibility remains that viral binding causes a lateral phase separation. Sendai virus membranes can fuse with liposomes containing only PS, so a packing defect between domains of two different lipids is not required. The concentration of PS required for fusion to occur is approximately 10-fold higher than that required for ganglioside GD1a, which has been shown to act as a Sendai virus receptor. When cholesterol is added as a third lipid to liposomes containing PC and GD1a, the amount of fusion decreases if the GD1a concentration is low.(ABSTRACT TRUNCATED AT 250 WORDS)     

Incorporation of adenylate cyclase into membranes of giant liposomes using membrane fusion with recombinant baculovirus-budded virus particles

Recombinant transmembrane adenylate cyclase (AC) was incorporated into membranes of giant liposomes using membrane fusion between liposomes and baculovirus-budded virus (BV). AC genes were constructed into transfer vectors in a form fused with fluorescent protein or polyhistidine at the C-terminus. The recombinant BVs were collected by ultracentrifugation and AC expression was verified using western blotting. The BVs and giant liposomes generated using gentle hydration were fused under acidic conditions; the incorporation of AC into giant liposomes was demonstrated by confocal laser scanning microscopy through the emission of fluorescence from their membranes. The AC-expressing BVs were also fused with liposomes containing the substrate (ATP) with/without a specific inhibitor (SQ 22536). An enzyme immunoassay on extracts of the sample demonstrated that cAMP was produced inside the liposomes. This procedure facilitates direct introduction of large transmembrane proteins into artificial membranes without solubilization.     

Differential effects of ouabain on human cell-mediated cytotoxicity. II. Stimulation of monocyte-mediated, spontaneous cytotoxicity against chicken red cell targets.

We have previously shown that ouabain inhibits mitogen-induced cellular cytotoxicity (MICC) and antibody-dependent cellular cytotoxicity (ADCC) against chicken red cell (CRC) targets. We now report that ouabain increases spontaneous killing of CRC targets in the absence of mitogen or antibody. Spontaneous cytotoxicity by fresh mononuclear leukocytes (MNL) was enhanced by ouabain in a dose-dependent fashion and was maximal at a ouabain concentration of 5 × 10^?5M. Removal of phagocytic cells from the MNL effector cell population abrogated ouabain-induced spontaneous cytotoxicity, suggesting that the effector cell activated by ouabain was a monocyte. Ouabain-induced spontaneous cytotoxicity was relatively inefficient compared to MICC or ADCC and was only demonstrated consistently at effector:target cell ratios higher than those routinely employed for MICC and ADCC. Very low concentrations of ouabain (5 × 10^?9M) also enhanced spontaneous cytotoxicity of MNL precultured for 7 days, when added at either Day 0 or Day 6 of preculture. The cell effecting spontaneous cytotoxicity after 7 days of culture has been previously shown to be a monocyte. Thus, ouabain has opposing effects on cell-mediated cytotoxic functions: it inhibits MICC and ADCC against CRC targets, but stimulates spontaneous, monocyte-mediated cytotoxicity against the same targets.     

Modeling degranulation with liposomes: Effect of lipid composition on membrane fusion

Degranulation involves the regulated fusion of granule membrane with plasma membrane. To study the role of lipid composition in degranulation, large unilamellar vesicles (LUVs) of increasing complexity in lipid compositions were constructed and tested for Ca²⁺-mediated lipid and contents mixing. Lipid-mixing rates of LUVs composed of phosphatidylethanolamine (PE) and phosphatidylserine (PS) were strongly decreased by the addition of either phosphatidylcholine (PC) or sphingomyelin (SM), while phosphatidylinositol (PI) had little effect. Complex LUVs of PCPESMPIPS (2427201613, designed to emulate neutrophil plasma membranes) also showed very low rates of both lipid mixing and contents mixing. The addition of cholesterol significantly lowered the Ca²⁺ threshold for contents mixing and increased the maximum rates of both lipid and contents mixing in a dose-dependent manner. Membrane remodeling, which occurs in neutrophil plasma membranes upon stimulation, was simulated by incorporating low levels of phosphatidic acid (PA) or a diacylglycerol (DAG) into complex LUVs containing 50% cholesterol. The addition of PA both lowered the Ca²⁺ threshold and increased the rate of contents mixing in a dose-dependent manner, while the DAG had no significant effect. The interaction of dissimilar LUVs was also examined. Contents-mixing rates of LUVs of two different cholesterol contents were intermediate between the rates observed for the LUVs of identical composition. Thus, cholesterol needed to be present in only one fusing partner to enhance fusion. However, for PA to stimulate fusion, it had to be present in both sets of LUVs. These results suggest that the rate of degranulation may be increased by a rise in the cholesterol level of either the inner face of the plasma membrane or the outer face of the granule membrane. Further, the production of PA can promote fusion, and hence degranulation, whereas the subsequent conversion of PA to DAG may reverse this promotional effect.Abbreviations ANTS 8-aminonaphthalene-1,3,6-trisulfonic acid - DiC₈ 1,2-dioctanoyl-sn-glycerol - DPX p-xylene-bis-pyridinium bromide - LUV large unilamellar vesicle - PA phosphatidic acid - PC phosphatidylcholine - PE phosphatidylethanolamine - PI phosphatidylinositol - PS phosphatidylserine - R18 octadecyl rhodamine - SM sphingomyelin     

Effect of cisplatin containing liposomes formulated by unsaturated chain-containing lipids on gynecological tumor cells

Gynecological tumors are major therapeutic areas of platinum-based anticancer drugs. Here, we report the characterization and in vitro biological assays of cisplatin-containing Egg L-α-phosphatidylcholine liposomes with different amounts of cholesterol. Dynamic light scattering estimated sizes of all obtained liposomes in the 100?nm range that are suitable for in vivo use. On the basis of these data and of the drug loading values, the best formulation has been selected. Stability and drug release properties of platinum-containing liposomes have been verified in serum. The growth inhibitory effects of both liposomal and free drug in a panel of ovarian and breast human cancer cell lines, characterized by a different drug sensitivity, give comparable or better results with respect to free cisplatin drug.     

Studies on the mechanism of natural killer cytotoxicity. II. coculture of human PBL with NK-sensitive or resistant cell lines stimulates release of natural killer cytotoxic factors (NKCF) selectively cytotoxic to NK-sensitive target cells

This investigation has employed the "innocent bystander" type of experimental design to determine whether soluble cytotoxic factor(s) are released during interactions between human peripheral blood lymphocytes (PBL) and NK-sensitive target cells. PBL cocultured with NK-sensitive Molt-4 or K562 target cells in the lower well of a miniaturized Marbrook culture released natural killer cytotoxic factors (NKCF), which diffused across a 0.2-mu Nucleopore membrane and lysed Molt-4 or K562 target cells cultured in the upper chamber. Coculture of PBL with the NK-resistant Raji or WI-L2 cell lines also induced release of NKCF. These factors were selectively cytotoxic to NK-sensitive targets and lysed Molt-4 and, to a lesser extent, K562 cells. However, Raji, WI-L2, and RPMI 1788 cells were all resistant to lysis. In addition, low density fractions from Percoll density gradients that were enriched for NK effector cells also released increased levels of NKCF during coculture with Molt-4 cells. Lysis of Molt-4 and K562 targets was observed after exposure to NKCF for 48 hr and 60 to 70 hr, respectively. Cellfree supernatants containing NKCF were obtained after a short time of incubation (i.e., within 5 hr of coculture of PBL with NK target cells). The factors were nondialyzable, stable at 56 degrees C for 3 hr, and showed partial loss of activity on storage at 4 degrees C or -20 degrees C for 7 days. These data suggest that NKCF may be involved in the lytic mechanism of human NK cell-mediated cytotoxicity.     

Studies on membrane fusion. II. Induction of fusion in pure phospholipid membranes by calcium ions and other divalent metals

The effect of divalent metals on the interaction and mixing of membrane components in vesicles prepared from acidic phospholipids has been examined using freeze-fracture electron microscopy and differential scanning calorimetry. Ca²⁺, and to a certain extent Mg²⁺, induce extensive mixing of vesicle membrane components and drastic structural rearrangements to form new membranous structures. In contrast to the mixing of vesicle membrane components in the absence of Ca²⁺ described in the accompanying paper which occurs via diffusion of lipid molecules between vesicles, mixing of membrane components induced by Ca²⁺ or Mg²⁺ results from true fusion of entire vesicles. There appears to be a “threshold” concentration at which Ca²⁺ and Mg²⁺ become effective in inducing vesicle fusion and the threshold concentration varies for different acidic phospholipid species. Different phospholipids also vary markedly in their relative responsiveness to Ca²⁺ and Mg²⁺, with certain phospholipids being much more susceptible to fusion by Ca²⁺ than Mg²⁺. Vesicle fusion induced by divalent cations also requires that the lipids of the interacting membranes be in a “fluid” state ($\text{T > T}{}_{\mathrm{c}}$). Fusion of vesicle membranes by Ca²⁺ and Mg²⁺ does not appear to be due to simple electrostatic charge neutralization. Rather the action of these cations in inducing fusion is related to their ability to induce isothermal phase transitions and phase separations in phospholipid membranes. It is suggested that under these conditions membranes become transiently susceptible to fusion as a result of changes in molecular packing and creation of new phase boundaries induced by Ca²⁺ (or Mg²⁺).     

Fusion of phospholipid vesicles with planar phospholipid bilayer membranes. II. Incorporation of a vesicular membrane marker into the planar membrane

Fusion of multilamellar phospholipid vesicles with planar phospholipid bilayer membranes was monitored by the rate of appearance in the planar membrane of an intrinsic membrane protein present in the vesicle membranes. An essential requirement for fusion is an osmotic gradient across the planar membrane, with the cis side (the side containing the vesicles) hyperosmotic to the opposite (trans) side; for substantial fusion rates, divalent cation must also be present on the cis side. Thus, the low fusion rates obtained with 100 mM excess glucose in the cis compartment are enhanced orders of magnitude by the addition of 5-10 mM CaCl2 to the cis compartment. Conversely, the rapid fusion rates induced by 40 mM CaCl2 in the cis compartment are completely suppressed when the osmotic gradient (created by the 40 mM CaCl2) is abolished by addition of an equivalent amount of either CaCl2, NaCl, urea, or glucose to the trans compartment. We propose that fusion occurs by the osmotic swelling of vesicles in contact with the planar membrane, with subsequent rupture of the vesicular and planar membranes in the region of contact. Divalent cations catalyze this process by increasing the frequency and duration of vesicle-planar membrane contact. We argue that essentially this same osmotic mechanism drives biological fusion processes, such as exocytosis. Our fusion procedure provides a general method for incorporating and reconstituting transport proteins into planar phospholipid bilayer membranes.     

The action of synthetic surfactants on membranes of tumor cells: II. Titration experiments

A comparison has been made of the effect of increasing concentrations of a number of nonionic, anionic, and cationic surfactants on the cell membranes of the V-2 carcinoma, the Ehrlich ascites carcinoma and tissue culture preparations of human embryonic connective tissue and peripheral blood cells from human monocytic leukemia. The concentrations necessary to produce rupture of the cell membrane have been compared with those which promote uptake of the vital dye, nigrosin. The action of the agents in promoting staining and lysis has been roughly similar with the following exceptions: the V-2 cell membrane is more resistant to the action of SLS than the other cell lines, lauryl pyridinium chloride promotes staining but not lysis of the malignant and ascites lines, while the LLC-HE₁ human connective tissue line is both stained and lysed in the presence of this surfactant.Within the range of 1 to 30 million cells, cell concentration is without much effect on the action of sodium lauryl sulfate against Ehrlich ascites cells. The presence of calcium chloride likewise does not cause much change in the action of this surfactant. Altering the pH from 1 to 9.4 causes little change in the uptake of dye by the cells.In general, when lysis does occur, it is preceded slightly by nigrosin staining.     

Interaction of the herbicide atrazine with model membranes. II: Effect of atrazine on fusion of phospholipid vesicles

The effect of atrazine on Ca2+ induced fusion of cardiolipin(CL) and phosphatidylserine (PS) vesicles is studied by Tb3+/dipicolinic acid fluorescence and turbidity measurements. The interaction of herbicide with CL and PS membranes is studied by DPH fluorescence polarization. At low concentrations the pesticide partially inhibits fusion, especially in CL vesicles. Higher concentrations of atrazine decrease inhibition of fusion in CL, while fusion is slightly increased in PS. The Ca2(+)-induced increase of turbidity is not affected by atrazine in both PS and CL aggregation experiments. DPH polarization measurements show a perturbation only of the membrane hydrophobic core of PS, in presence of Ca2+. It is hypothesized that this biphasic effect shown by low and high atrazine concentrations on Ca2(+)-induced fusion of vesicles is due to a different localization of the pesticide in the membrane.     

Studies on the interaction of Sendai virus with liposomal membranes. Sendai virus-induced agglutination of liposomes containing glycophorin

Liposomes constituted with the major sialoglycoprotein of human erythrocytes, glycophorin, were used as models for studies on cell-virus interactions. Liposomes composed of egg yolk phosphatidylcholine, cholesterol and glycophorin were found to interact with the paramyxovirus HVJ to form aggregates. The aggregation process was temperature dependent: it was maximal at 0 degrees C and decreased with increase of the incubation temperature. The activity of viral neuraminidase is also temperature dependent, and it increases with increase of the incubation temperature; release of N-acetylneuraminic acid was negligible at 0 degrees C. Shift-up of the incubation temperature immediately cancelled HVJ-induced agglutination of liposomes. Viruses attached to liposomes seemed to be released into the supernatant when the 'virus-liposome' complex formed at 0 degrees C was incubated at 37 degrees C, possibly as a result of breakdown of the 'binding' site by neuraminidase. The characteristics of the interaction of HVJ with liposomes containing glycophorin appeared to be phenomenologically similar to those of HVJ-cell interaction.     

Effect of interleukin 2 on cytotoxic effectors: II. Long-term culture of NK cells

The present study reports the establishment of an NK (natural killer) cell line, designated as IL2-CE1. This cell line was maintained in active growth in culture with the supplementation of interleukin 2 (IL2). The cells gave cytotoxicity against a variety of murine tumors. There was no detectable cytotoxicity against three human tumors tested. The reactivity obtained with the murine tumors was generally correlated with the sensitivity of these tumor cells to NK cytotoxicity. With few exceptions, the reactivity was much higher for NK-sensitive targets like YAC and RL♂ 1 cells. There was very little cytotoxicity against NK-resistant targets like P815 and HFL/d. The reactivity of the effectors against YAC was susceptible to trypsin treatment and this effect was reversible. In determining the surface markers of IL2-CE1 cells, it was shown that over 90% of the cells had Thy-1.2 antigen despite their resistance to the lytic effect of anti-Thy-1.2 antibody. There were no detectable surface Ig or Fc receptors. Therefore, the IL2-CE1 cells were consistent with being NK cells. Although these cells gave high levels of cytotoxicity against murine tumors, in the tumor neutralization tests the IL2-CE1 cells failed to give any protection against an immunosensitive leukemic line, FBL-3. After the IL2-CE1 cells were cloned, it was found that different NK clones showed variations in their fine specificity. Our finding supports the presence of different populations or subsets of NK cells. Establishment of these NK clones in long-term culture should be helpful for the detailed characterization of the NK cells and aid in the determination of their significance in the immune surveillance against neoplasia.     

Effect of anti-lymphotoxin on cell-mediated cytotoxicity. Evidence for two pathways, one involving lymphotoxin and the other requiring intimate contact between the plasma membranes of killer and target cells.

A rabbit anti-lymphotoxin serum produced against partially purified, antigeninduced, guinea pig lymphotoxin, was used to study the role of lymphotoxin in lymphocyte-mediated cytotoxicity in vitro. The anti-lymphotoxin serum inhibited cytolysis resulting from incubation of ovalbumin-immune guinea pig spleen cells with either mouse (P815 mastocytoma) or guinea pig (line 10 hepatoma) target cells in the presence of soluble ovalbumin. The antiserum also inhibited the cytolysis of ovalbumin-coupled target cells by ovalbumin-immune guinea pig spleen cells. In contrast, the anti-lymphotoxin serum did not inhibit: (a) the lysis of line 10 (strain 2) hepatoma cells by spleen cells from alloimmunized Hartley or strain 13 animals; (b) the lysis of line 10 hepatoma cells by spleen cells from tumor-bearing syngeneic animals; or (c) the lysis of P815-mastocytoma cells by spleen cells from P815-immune guinea pigs. These results support the hypothesis that there are at least two distinct pathways by which immune lymphocytes can destroy target cells in vitro, one which involves secretion of a nonspecific soluble factor, i.e., lymphotoxin, and another which probably requires intimate contact between the plasma membranes of the target and killer cells.     

Effect of cholesterol on the interaction of the HIV GP41 fusion peptide with model membranes. Importance of the membrane dipole potential

Fusion of viral and cell membranes is a key event in the process by which the human immunodeficiency virus (HIV) enters the target cell. Membrane fusion is facilitated by the interaction of the viral gp41 fusion peptide with the cell membrane. Using synthetic peptides and model membrane systems, it has been established that the sequence of events implies the binding of the peptide to the membrane, followed by a conformational change (transformation of unordered and helical structures into beta-aggregates) which precedes lipid mixing. It is known that this process can be influenced by the membrane lipid composition. In the present work we have undertaken a systematic study in order to determine the influence of cholesterol (abundant in the viral membrane) in the sequence of events leading to lipid mixing. Besides its effect on membrane fluidity, cholesterol can affect a less known physical parameter, the membrane dipole potential. Using the dipole potential fluorescent sensor di-8-ANEPPS together with other biophysical techniques, we show that cholesterol increases the affinity of the fusion peptide for the model membranes, and although it lowers the extent of lipid mixing, it increases the mixing rate. The influence of cholesterol on the peptide affinity and the lipid mixing rate are shown to be mainly due to its influence of the membrane dipole potential, whereas the lipid mixing extent and peptide conformational changes seem to be more dependent on other membrane parameters such as membrane fluidity and hydration.     

Mechanism of defective NK cell activity in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. II. Normal antibody-dependent cellular cytotoxicity (ADCC) mediated by effector cells defective in natural killer (NK) cytotoxicity

Our studies and other investigations have shown that NK effector cells can also mediate antibody-dependent cellular cytotoxicity (ADCC) through the use of the Fc gamma receptor on the NK cell membrane. Peripheral blood lymphocytes (PBL) derived from patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex exhibit a poor NK activity due to a defective "trigger" required for activation in the lethal hit stage of the NK lytic pathway. Consequently, it was important to delineate whether the defect in AIDS NK cells affected the ADCC function. By using the 51Cr-release assay, the ADCC cytotoxic activity of AIDS PBL was found to be within the normal range, despite the absence of significant NK activity. Several experiments corroborated that the same effector cells mediate both NK CMC and ADCC. Depletion of Fc gamma R-bearing cells resulted in elimination of both the ADCC and NK cytotoxic functions. Single cell analyses, using one- and two-target cell conjugates, revealed that the frequency of ADCC effector:target conjugates and the frequency of killer cells from AIDS PBL were comparable to the frequencies seen in the normal controls. However, when mixtures of NK and ADCC targets were used to form mixed two-target conjugates, the AIDS effector cells lysed only the bound ADCC target, whereas the normal effector cells lysed both the bound NK and ADCC targets. These results demonstrate clearly that the same NK/K effector cells from AIDS PBL, defective in NK activity, are not impaired in mediating ADCC activity. These findings were supported by the demonstration that AIDS PBL stimulated with ADCC targets, but not with NK targets, released NK cytotoxic factors, postulated mediators of the NK CMC reaction. These findings indicate that the NK/K cells in AIDS are triggered normally for ADCC activity but are not triggered for NK activity. Furthermore, the results indicate that the lytic machinery is not impaired in the AIDS NK/K cells.