Lipid vesicle-cell interactions. II. Induction of cell fusion

In Situ Labeling of the Aqueous Compartment of Extracellular Vesicles with Luminescent Gold Nanoclusters

Extracellular vesicles (EVs) are well-known membrane-limited particles secreted by both healthy and cancerous cells. They are considered as biomarkers for early cancer diagnosis and are involved in many pathologies and physiological pathways. They could serve as diagnostic tools in liquid biopsies, as therapeutics in regenerative medicine, or as drug delivery vehicles. Our aim is here to encapsulate luminescent nanoprobes in the aqueous compartment of human EVs extracted from reproductive fluids. The analysis and labeling of the EVs content with easily detectable luminescent nanoparticles could enable a powerful tool for early diagnosis of specific diseases and also for the design of new therapeutics. In this view, gold nanoclusters (AuNCs) appear as an attractive alternative as nontoxic fluorophore probes because of their luminescence properties, large window of fluorescence lifetimes (1 ns-1 μs), ultrasmall size (<2 nm), good biocompatibility, and specific ability as X-ray photosensitizers. Here, we investigated an attractive method that uses fusogenic liposomes to deliver gold nanoclusters into EVs. This approach guarantees the preservation of the EVs membrane without any breakage, thus maintaining compartmental integrity. Different lipid compositions of liposomes preloaded with AuNCs were selected to interact electrostatically with human EVs and compared in terms of fusion efficiency. The mixture of liposomes and EVs results in membrane mixing as demonstrated by FRET experiments and fusion revealed by flux cytometry and cryo-TEM. The resulting fused EVs exhibit typical fluorescence of the AuNCs together with an increased size in agreement with fusion. Moreover, the fusion events in mixtures of EVs and AuNCs preloaded liposomes were analyzed by using cryo-electron microscopy. Finally, the ratio of released AuNCs during the fusion between the fusogenic liposomes and the EVs was estimated to be less than 20 mol % by Au titration using ICP spectroscopy.

Encapsulation of Luminescent Gold Nanoclusters into Synthetic Vesicles

Gold nanoclusters (Au NCs) are attractive luminescent nanoprobes for biomedical applications. In vivo biosensing and bioimaging requires the delivery of the Au NCs into subcellular compartments. In this view, we explore here the possible encapsulation of ultra-small-sized red and blue emitting Au NCs into liposomes of various sizes and chemical compositions. Different methods were investigated to prepare vesicles containing Au NCs in their lumen. The efficiency of the process was correlated to the structural and morphological aspect of the Au NCs' encapsulating vesicles thanks to complementary analyses by SAXS, cryo-TEM, and confocal microscopy techniques. Cell-like-sized vesicles (GUVs) encapsulating red or blue Au NCs were successfully obtained by an innovative method using emulsion phase transfer. Furthermore, exosome-like-sized vesicles (LUVs) containing Au NCs were obtained with an encapsulation yield of 40%, as estimated from ICP-MS.

Cephalin as an efficient fusogen in hybridoma technology: can it replace poly ethylene glycol?

In this study we set up a simple, fast, and highly efficient protocol to fuse cells and produce human hybridoma using non-toxic cephalin as a fusogenic lipid. We compared our proposed method with PEG-mediated fusion, the well-known conventional method. Human lymphoblastoid cells were fused with an F3B6 heteromyeloma cell line using cephalin or PEG as the fusogenic compound. The viability of the cells and their fusion rate were determined microscopically and hybridoma (antigen-specific and non-specific) production yield was calculated following HAT selection and screening. The fusion rates of cells in cephalin and PEG-mediated methods were comparable (25.9+/-5.73% versus 27.3+/-6.07%) while the viability of the cells immediately and after overnight incubation was obviously greater in the cephalin method than in the PEG (p<0.001). Our proposed cephalin-mediated cell fusion method is about five times more efficient than PEG in production of hybridoma clones; thus it may dismiss PEG as the most generalized fusogen in the future.

The how and why of exocytic vesicles

The purpose of this review is to draw the attention of general readers to the importance of cellular exocytic vesiculation as a normal mechanism of development and subsequent adjustment to changing conditions, focusing on red cell (RBC) vesiculation. Recent studies have emphasized the possible role of these microparticles as diagnostic and investigative tools. RBCs lose membrane, both in vivo and during ex vivo storage, by the blebbing of microvesicles from the tips of echinocytic spicules. Microvesicles shed by RBCs in vivo are rapidly removed by the reticuloendothelial system. During storage, this loss of membrane contributes to the storage lesion and the accumulation of the microvesicles are believed to be thrombogenic and thus to be clinically important.

Mechanisms and kinetics of liposome-cell interactions

Although the possibility of targeting drugs to specific tissues and cells, as well as facilitating their uptake and cytoplasmic delivery has rendered liposomes a versatile drug carrier system with numerous potential applications in medicine, the molecular mechanisms of liposome-cell interactions are not understood well. Here we have reviewed the early and current concepts of liposome-cell interactions, including possible liposome receptors. Uptake of liposomes by cells can be modified by the lipid composition, particularly by the inclusion of steric stabilizers such as PEG-conjugated lipids. Such modifications also alter the circulation time and biodistribution of liposomes, which can thus be tailored for particular applications. The intracellular fate of encapsulated molecules can be modified by the use of pH-sensitive liposomes which can also be sterically stabilized. Cationic liposomes that can undergo lipid mixing with cellular membranes can deliver complexed DNA to cells, but most likely via an endocytotic process. Kinetic analysis of liposome-cell interactions can elucidate the numbers of liposome receptors of several types and the corresponding binding constants. It is likely that liposomes bind to different cell surface receptors on different cells, and that they utilize more than one type of receptor on a particular cell. The kinetic analysis also provides the rate constants of endocytosis and the percentages of liposomes that are bound or endocytosed.

Physical and biological properties of cationic triesters of phosphatidylcholine

The properties of a new class of phospholipids, alkyl phosphocholine triesters, are described. These compounds were prepared from phosphatidylcholines through substitution of the phosphate oxygen by reaction with alkyl trifluoromethylsulfonates. Their unusual behavior is ascribed to their net positive charge and absence of intermolecular hydrogen bonding. The O-ethyl, unsaturated derivatives hydrated to generate large, unilamellar liposomes. The phase transition temperature of the saturated derivatives is very similar to that of the precursor phosphatidylcholine and quite insensitive to ionic strength. The dissociation of single molecules from bilayers is unusually facile, as revealed by the surface activity of aqueous liposome dispersions. Vesicles of cationic phospholipids fused with vesicles of anionic lipids. Liquid crystalline cationic phospholipids such as 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine triflate formed normal lipid bilayers in aqueous phases that interacted with short, linear DNA and supercoiled plasmid DNA to form a sandwich-structured complex in which bilayers were separated by strands of DNA. DNA in a 1:1 (mol) complex with cationic lipid was shielded from the aqueous phase, but was released by neutralizing the cationic charge with anionic lipid. DNA-lipid complexes transfected DNA into cells very effectively. Transfection efficiency depended upon the form of the lipid dispersion used to generate DNA-lipid complexes; in the case of the O-ethyl derivative described here, large vesicle preparations in the liquid crystalline phase were most effective.

The effect of lipid molecular packing stress on cationic liposome-induced rabbit erythrocyte fusion

The effect of curvature stress on the efficiency of cationic liposome-induced fusion between rabbit erythrocytes was studied. Multilamellar cationic liposomes containing 1,2-dioleoyl-3-trimethylammoniumpropane (DOTAP) and different PEs (1,2-dilnoleoyl-sn-glycero-3-phosphoethanolamine (dilin-PE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and lysophosphatidylethanolamine, egg (lyso-PE)) were used to induce cell-cell fusion. It was found that high cell-cell fusion yield (FY) of about 50% could be achieved in sucrose solution by using cationic liposomes containing 50% DOTAP. Cell-cell fusion was assayed by shape criterion and was verified by fluorescence microscopy, using a membrane dye mixing method. The curvature stress, as a result of mixing unsaturated PEs in cationic liposomes, had a significant effect on cell-cell FY which increased with the degree of acyl chain unsaturation, in the order dilin-PE > DOPE > POPE > lyso-PE. Replacement of dilin-PE, DOPE, or POPE by lyso-PE gradually in cationic liposomes lowered the cell-cell FY from 50% to 1%. Furthermore, cationic liposome induced cell lysis, and fusion between cationic liposomes and cells, as assayed by the N-(lissamine rhodamine B sulfonyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt and N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2- dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt (Rh-PE/NBD-PE) energy transfer method, followed the same order as that for cell-cell fusion. Fusion between the negatively charged PS liposomes and cationic liposomes also followed the same order. The electric double layer screening by electrolytes in NaCl-containing solution and phosphate buffered saline (PBS) was found to reduce the cell-cell FY and cell lysis. These findings suggest that liposome-induced cell-cell fusion was achieved by cationic liposomes serving as fusion-bridges among cells.

The role of helper lipids in cationic liposome-mediated gene transfer

In the procedure for cationic liposome-mediated transfection, the cationic lipid is usually mixed with a "helper lipid" to increase its transfection potency. The importance of helper lipids, including dioleoylphosphatidylcholine (DOPC) and phosphatidylethanolamine (dioleoyl PE), DO was examined. Freeze-fracture electron microscopy of DNA:cationic complexes containing the pSV-beta-GAL plasmid DNA, the cationic lipid dioleoyl trimethylammonium propane, and these helper lipids showed that the most efficient mixtures were aggregates of ensheathed DNA and fused liposomes. PE-containing complexes aggregated rapidly when added to culture media containing polyanions, whereas PC-containing complexes did not. However, more granules of PC-containing complexes were formed on cell surfaces after the complexes were added to Chinese hamster ovary (CHO) cells in transfection media. Pronase treatment inhibited transfection, whereas dilute poly-L-lysine enhanced transfection, indicating that the attachment of DNA:liposome complexes to cell surfaces was mediated by electrostatic interaction. Fluorescence spectroscopy studies confirmed that more PC-containing complexes than PE-containing complexes were associated with CHO cells, and that more PC-containing complexes were located in a low pH environment (likely to be within endosomes) with time. Cytochalasin-B had a stronger inhibitory effect on PC-containing liposome-mediated than on PE-containing liposome-mediated transfection. Confocal microscopic recording of the fluorescently label lipid and DNA uptake process indicated that many granules of DNA:cationic liposome complexes were internalized as a whole, whereas some DNA aggregates were left out on the cell surfaces after liposomes of the complexes fused with the plasma membranes. For CHO cells, endocytosis seems to be the main uptake pathway of DNA:cationic liposome complexes. More PC-containing granules than PE-containing granules were formed on cell surfaces by cytoskeleton-directed membrane motion, after their respective DNA:liposome complexes attached to cell surfaces by electrostatic means. Formation of granules on the cell surface facilitated and/or triggered endocytosis. Fusion between cationic liposomes and the cell membrane played a secondary role in determining transfection efficiency.

A requirement for membrane-associated phospholipase A2 in platelet cytotoxicity activated by receptors for immunoglobulin G and complement

Platelets are potent antibody- and complement-dependent cytotoxic effector cells. We showed previously that a single platelet can lyse a target cell sensitized with immunoglobulin G (IgG) and complement components up to C3 (C integral of 3b denotes the target cell-bound fragment of complement up to C3; the precise nature of the bound C3 fragment has not been established), and that the complete cytotoxic system capable of specific recognition and lysis resides in platelet membranes. To define the components of platelet membranes required for cytotoxicity, a set of inhibitors of phospholipase A2 (PLA2) that act by different chemical mechanisms was tested. The lytic reaction is blocked at appropriate concentrations of bromophenacylbromide, mepacrine, and manoalide. When platelets are treated with bromophenacylbromide, inhibition of cytolytic activity and that of PLA2 enzymatic activity occur in parallel. Platelets release arachidonate when incubated with target cells bearing IgG and C integral of 3b, confirming that Fc gamma R and complement receptor trigger both PLA2 action and efficient lysis. Inhibition by thimerosal of a reverse reaction, i.e., reacylation catalyzed by acyltransferase, causes increased target cell lysis, presumably by increasing the products of PLA2 action. Platelet cytotoxicity is increased when platelets are pretreated with some products of PLA2: exogenous lysophospholipids and not free arachidonic acid increase cytotoxicity. Electron microscopy suggests that platelets and target cells may fuse, possibly as a result of the formation of lysophospholipids which are well-known membrane fusogens. Fixation with paraformaldehyde does not affect platelet cytotoxicity, suggesting that the complete cytotoxic system resides as a preformed complex in platelet membranes. The results indicate that platelet membrane-associated PLA2, together with receptors for Fc and complement, are required for platelet cytotoxicity.

Dextran sulfate-dependent fusion of liposomes containing cationic stearylamine

The incorporation of the positively charged stearylamine into phosphatidylcholine liposomes was studied by measuring electrophoretic mobilities. Up to a molar ratio SA/PC = 0.5 an increase of the positive zeta potential can be observed. Addition of the negatively charged macromolecule dextran sulfate leads to a change of the sign of the surface potential of the PC/SA liposomes indicating binding of the macromolecule to the surface. This process is accompanied by an increase in turbidity, which is dependent on the molecular weight of the dextran sulfate and the SA concentration (measured by turbidimetry). Using the NBD/Rh and Pyr-PC fluorescence assays the fusion of SA containing liposomes was investigated. A strong influence of the SA content and molecular weight of dextran sulfate on the fusion extent was observed. The fusion extent is proportional to the SA content in the PC membrane and the molecular weight of dextran sulfate. PC/SA/PE liposomes exhibit a higher fusion extent after addition of dextran sulfate compared to PC/SA liposomes indicating that PE additionally destabilizes the bilayer. Freeze-fracture electron microscopy reveals that the reaction products are large complexes composed of multilamellar stacks of tightly packed, straight membranes and aggregated vesicles. The tight packing of the membranes in the stacks (and the narrow contact of the aggregated vesicles) indicates a strong adherence of opposite membrane surfaces induced by dextran sulfate.

The administration of beta carotene to prevent and regress oral carcinoma in the hamster cheek pouch and the associated enhancement of the immune response

In the past four years this laboratory has utilized the hamster cheek pouch tumor model to investigate the anticancer activities of antioxidants, such as beta carotene. These molecules, which have exhibited no evidence of toxicity, have been administered systemically (oral ingestion), and locally to the tumor site in the hamster cheek pouch. The results have been either the inhibition of tumor growth, or the regression of tumor. Adjacent to the degenerating tumors a dense inflammatory infiltrate was observed. Specifically, the cytokines, tumor necrosis factor alpha, and beta, have been immunohistochemically localized to the site of regressed oral carcinoma. Recently, liposomes composed of phosphaditylcholine, phosphaditylserine, and phosphodityelanolamine were combined with beta carotene and injected locally to oral squamous cell carcinoma of the hamster. The results indicated that tumor cells accumulated the liposomes and were lysed while normal mucosal cells did not demonstrate this effect. Therefore antioxidants such as beta carotene can be localized to a tumor site, without a toxic response. Future studies on the anticancer activity of the antioxidants need to focus on the cellular and molecular changes produced in the immune effectors and in the mucosal cells following administration of the antioxidants.

Induction of fusion in aggregated and nonaggregated liposomes bearing cationic detergents

The addition of polyanionic polymers such as poly(aspartic acid) (PASP), DNA or dextran sulfate to liposomes composed of phosphatidylcholine (PC) and cholesterol (CHOL) and bearing the quaternary ammonium detergent [[[(1,1,3,3-tetramethylbutyl)cresoxy]ethoxy]ethyl]dimethy lbe nzylammonium hydroxide (DEBDA[OH]) resulted in liposome aggregation and fusion. Liposome-liposome fusion was studied by using fluorescently labeled liposomes and fluorescence-dequenching (DQ) methods. Addition of monoanions, such as aspartate or acetate, to liposomes bearing DEBDA[OH] caused neither their aggregation nor liposome-liposome fusion. Aggregation of liposomes bearing DEBDA[OH] by the binding pair avidin-biotin did not result in their fusion. Fusion in such aggregated liposomes was observed by the addition of chaotropic anions, such as nitrate or thiocyanate, or by PASP. A variety of other quaternary ammonium detergents behaved similarly to DEBDA[OH] in their ability to confer fusogenic properties upon PC/chol liposomes. The relevance of these findings to the mechanism of liposome-liposome fusion is discussed.

Cytolytic activity of liposomes containing stearylamine

In order to develop the cytotoxic liposome, the cytolytic effect of polycationic liposome was examined. Upon incubation of the stearylamine-containing liposome (stearylamine-liposome) with rabbit erythrocyte, a significant extent of hemolysis was observed. Hemolytic activity of the liposome depends on the amount of stearylamine in the liposome membrane. The plots of the initial rate of hemolysis versus the concentration of stearylamine-liposome showed a sigmoidal curve, suggesting that stearylamine-liposomes act cooperatively on the erythrocyte membrane. Hemolytic activity of stearylamine-liposome was markedly influenced by the composition of hydrocarbon chains of the phospholipids in the liposome membrane, suggesting that the membrane fluidity of stearylamine-liposome is important to evoke the hemolysis. Since the liposomes containing acidic phospholipids inhibited markedly the stearylamine-liposome-caused hemolysis, it is likely that the primary target of stearylamine-liposome is the negatively charged component(s) such as acidic phospholipids on the erythrocyte membrane. Furthermore, stearylamine-liposome induced the release of the intravesicular contents from the liposome made of acidic phospholipids but not from the liposome made of phosphatidylcholine only. These results suggest that stearylamine-liposome interacted with the negative charges of the erythrocyte membrane and eventually damaged the cell. Erythrocytes from rabbit, horse and guinea pig are highly susceptible to stearylamine-liposome but those from man, sheep, cow and chicken are less so.

Peroxidation of liposomes in the presence of human erythrocytes and induction of membrane damage of erythrocytes by peroxidized liposomes

Hemolysis (Kobayashi, T., Takahashi, K., Yamada, A., Nojima, S. and Inoue, K. (1983) J. Biochem. 93, 675-680) and shedding of acetylcholinesterase-enriched membrane vesicles (diameter 150-200 nm) were observed when human erythrocytes were incubated with liposomes of phosphatidylcholine which contained polyunsaturated fatty acyl chains. These events occurring on erythrocyte membrane were inhibited by radical scavengers or incorporation of alpha-tocopherol into liposomes, suggesting that lipid peroxidation is involved in the process leading to membrane vesiculation and hemolysis. The idea was supported by findings that generation of chemiluminescence, formation of thiobarbituric acid reactive substance, accumulation of conjugated diene compounds in liposomes and decrease of polyunsaturated fatty acids in liposomes occurred concomitantly during incubation. Hemolysis was also suppressed by the addition of extra liposomes, insensitive to peroxidation, or of serum albumin even after the completion of peroxidation of liposomes. These results suggest that peroxidized lipids, responsible for vesiculation and hemolysis, may be formed first in liposomes and then gradually transferred to erythrocyte membranes. The accumulation of these lipids peroxides may eventually cause membrane vesiculation followed by hemolysis.

The interaction of Bacillus protoplasts with sonicated phosphatidylcholine liposomes

When protoplasts from Bacillus subtilis are incubated with sonicated liposomes made from egg-yolk phosphatidylcholine, this phospholipid is incorporated into the protoplast membranes. Biochemical, fluorescence and ultrastructural data suggest that incorporation occurs through membrane fusion.

Plasma membrane-mediated leakage of liposomes induced by interaction with murine thymocytic leukemia cells

The interaction of liposomes with BW 5147 murine thymocytic leukemia cells was studied using fluorescent probes (entrapped carboxyfluorescein and fluorescent phosphatidylethanolamine) in conjunction with a Ficoll-Paque discontinous gradient system for rapid separation of liposomes from cells. Reversible liposomal binding to discrete sites on the BW cell surface was found to represent the major form of interaction; uptake of intact liposomal contents by a process such as liposome-BW cell membrane fusion was found to apparently represent a minor pathway of interaction (2%). Liposomal lysis was found to be associated with the process of liposomal binding (perhaps as a result of the binding itself). Lysis was followed by release of the entrapped carboxyfluorescein into the media and its subsequent uptake by the cells. This lysis was shown to be dependent upon discrete membrane-associated sites that have some of the properties of proteins. The results of these studies suggest that liposomal binding to the cells, subsequent lysis of the liposomes and cellular uptake of their contents should be seriously considered in all studies of liposome-cell interactions as an alternate mode of interaction to the four modes (fusion, endocytosis, adsorption and lipid exchange) previously emphasized in the literature.

Delivery of liposome membrane-associated sterols through silastic membranes

The transport of sterols incorporated into the lecithin bilayer of small unilamellar liposomes through a model membrane was studied. A two-chamber diffusion cell containing liposomes with incorporated [4-14C)cholesterol or beta-[4-14C]sitosterol in the donor chamber and liposomes with unlabeled cholesterol in the receiver chamber was used. The permeability coefficients of the sterols through silastic rubber membranes which served as a model membrane were measured. The permeability for cholesterol incorporated into liposomes in a phosphatidyl choline/cholesterol molar ratio of 1 : 1, produced by sonication for 1 h, and subsequent centrifugation at 100 000 X g for 1 h, was 1.6 . 10(-8) cm sec-1. Dilution of the liposome suspension did not change the permeability coefficient significantly. The permeability coefficient of sitosterol incorporated into liposomes was about 4-times smaller than that of cholesterol. These results suggest that the sterols were delivered to the silastic membrane by the intact liposomes and that free solute was not involved in the transport to the membrane to a significant degree. The large differences in the permeability coefficients between cholesterol and sitosterol indicate that an aqueous interfacial barrier was crossed by the sterol during the delivery to the membrane.

Fusion of liposome membranes by the n-alkyl bromides

It has been found that the n-alkyl bromides are capable of inducing the fusion of unilamellar liposomes. These compounds can bring about fusion of liposomes composed of either pure phosphatidylcholine or phosphatidycholine+phosphatidic acid. Fusion of unilamellar liposomes gives rise to multilamellar structures, the morphology of which has been examined by negative staining and freeze-fracture techniques. It has been shown by microelectrophoresis that the n-alkyl bromides have no effect on the surface charge of liposomes, and fusion has been further characterized by use of light scattering and differential scanning calorimetry, the latter indicating that true mixing of the fatty acyl chains occurs upon fusion. Finally, fusion occurs at n-alkyl bromide levels below that required to saturate the aqueous phase of the system.

Lysophosphatidylcholine in liposomal membranes: enhanced permeability but little effect on transfer of a water-soluble fluorescent marker into human lymphocytes

In an attempt to enhance delivery of liposome contents into cells, we tested the effect of lysophosphatidylcholine on transfer of the fluorescent dye, carboxyfluorescein, from small unilamellar and large multilamellar vesicles to human lymphocytes. Dioleoyl phosphatidylcholine and dioleoyl phosphatidylcholine-lysophosphatidylcholine small unilamellar vesicles with varying lipid ratios were prepared and characterized. In the presence of lysophosphatidylcholine, small unilamellar vesicles were slightly smaller and more leaky than those made without lysophosphatidylcholine. Lysophosphatidylcholine induced less leakage in large multilamellar vesicles. It did not show any appreciable effect on transfer of liposome contents, whether included as part of the liposomal bilayer (of unilamellar or multilamellar vesicles) or added exogenously together with small unilamellar dioleoyl phosphatidylcholine vesicles.

On the mode of liposome-cell interactions. Biotin-conjugated lipids as ultrastructural probes

An efficient method for labeling and visualizing phospholipids at the ultrastructural level is described. Biotin was coupled to the amines of appropriate phospholipids via the N-hydroxysuccinimide ester. The biotinylated lipid could be specifically labeled by ferritin-avidin conjugates and detected by transmission electron microscopy. The lipid derivatives were analyzed and evaluated in terms of their resemblance to the original lipid. Although differing in some aspects from the parent lipid molecules, the biotinyl derivatives still retain the basic characteristics of lipids vis-a-vis their orientation and position in the membrane bilayer. The latter property renders the biotinylated lipid qualitatively suitable for tracing the fate of the lipid component(s) of liposomes during their interaction with biological membranes of various cell types. Using this system, we propose that the extent and pattern of the liposome-cell interaction depends, at least in part, on the cell type employed. This observation may be due to intrinsic variations in cell surface structure and properties relative to those of the liposome.

Interaction of phospholipid vesicles with rat hepatocytes in primary monolayer culture

We studied the interaction of positively and negatively charged unilamellar and multilamellar phospholipid vesicles (liposomes) with rat-liver parenchymal cells in primary monolayer culture. Radioactive liposomal phosphatidylcholine was taken up more rapidly and to a larger extent from unilamellar than from multilamellar vesicles. No significant difference in uptake characteristics was observed between vesicles of different charge. The presence of serum greatly reduced uptake of liposomal phosphatidylcholine of both unilamellar and multilamellar vesicles. This serum effect was independent of surface charge of the vesicles. When cells were allowed to take up radioactive liposomal phospholipid and then incubated further in absence of vesicles, part of the radioactivity associated with the cells was released into the medium, most of it as water soluble degradation products. When cells were preincubated with vesicles containing horseradish peroxidase and then, after removal of the vesicles, further incubated, peroxidase activity could be demonstrated in the culture medium, part of it only after addition of Triton X-100. These observations were taken to indicate that part of the phospholipid taken up the cells represented vesicles binding to the cell surface rather than having been internalized. Vesicle-entrapped [125I]albumin was taken up by the cells and rapidly hydrolyzed as indicated by the appearance of radioactivity soluble in trichloroacetic acid within minutes after starting the incubation. No uptake of free albumin could be demonstrated. The kinetics of albumin uptake and release of trichloroacetic acid-soluble radioactivity from the cells suggest that, initially, liposomes are internalized predominantly by endocytosis, while during prolonged incubation fusion of the liposomal membrane with the plasma membrane gradually contributes more substantially to the overall uptake process. The significance of these findings is emphasized with special reference to the use of liposomes as intravenous carriers of enzymes or drugs.

Liposome accumulation in regions of experimental myocardial infarction

The uptake of liposomes bearing positive, negative, or no net charge on their membrane and containing a radioactive tracer, [99mTc]diethylenetriamine pentaacetic acid, was studied in 12 intact dogs 24 hours after the induction of myocardial infarction, and compared to the relative regional myocardial blood flow determined from radioactive microspheres. Positively charged and neutral liposomes concentrate in infarcted regions against a flow gradient, while negative liposomes are passively distributed according to regional blood flow. Because positively charged and neutral lipisomes concentrate in infarct areas and have the ability to incorporate pharmacologic agents in their aqueous or lipid phase, they may serve as vehicles for drug delivery to infarct zones of low flow.

Lipid vesicle-cell interactions. I. Hemagglutination and hemolysis

The interaction of lipid vesicles (liposomes) of several different compositions with erythrocytes has been investigated. Lecithin liposomes, rendered positively charged with stearylamine, exhibit potent hemagglutination activity in media containing low concentrations of electrolytes. The hemagglutination titer is found to be a linear function of the zeta potential of the lipid vesicles. Hemagglutination is reduced when the surface potential of the cells is made more positive by pH adjustment or enzyme treatment. Similarly, hemagglutination is reduced by increasing concentrations of electrolytes. Hemagglutination is examined theoretically and is shown to be consistent with vesicle-cell interactions that are due to only electrostatic forces. Vesicles containing lysolecithin in addition to lecithin and stearylamine cause lysis of erythrocytes, provided the lipids of the vesicles are above the crystal-liquid crystal phase transition temperature. In addition, hemolysis requires close juxtaposition of the vesicle to the cell membrane; vesicles precoated with antibodies exhibit severely diminished hemolytic activities, only a small fraction of which can be attributed to a reduction in hemagglutination titer. Evidence is presented indicating that a single vesicle is sufficient to lyse one cell. With regard to hemagglutination and hemolysis, lipid vesicles of simple composition mimic paramyxoviruses such as Sendai virus.

Lipid vesicle-cell interactions. III. Introduction of a new antigenic determinant into erythrocyte membranes

The introduction of a new antigenic determinant, 2,4-dinitrophenyl-aminocaproyl-phosphatidylethanolamine (DNP-Cap-PE), into the surface membranes of intact human erythrocytes is described. Fresh cells were incubated in the presence of liposomes composed of 10% DNP-Cap-PE, 5% stearylamine, 20% lysolecithin, and 65% lecithin. Such liposome-treated erythrocytes are shown to be susceptible to immune lysis by anti-DNP serum in the presence of complement. Uptake of DNP-Cap-PE by erythrocyte membranes is also demonstrated by immunofluorescence using indirect staining with rabbit anti-DNP serum followed by fluroescein-conjugated goat anti-rabbit IgG and by electron microscopy using ferritin-conjugated antibody. Antigen uptake did not occur at low temperatures or from vesicles lacking lysolecithin and stearylamine. Fluorescence microscopy shows that the antigen-antibody complexes are free to diffuse over the cell surface, eventually coalescing into a single area on the cell membrane. Electron microscopy suggests that a substantial proportion of the lipid antigen is incorporated by fusion of vesicles with the cell membrane. There are indications that vesicle treatment causes a small proportion of cells to invaginate.