Dynamics of antibody- and lectin-mediated endocytosis of hapten-containing liposomes by murine macrophages

Glycolipids and Lectins in Endocytic Uptake Processes

A host of endocytic processes has been described at the plasma membrane of eukaryotic cells. Their categorization has most commonly referenced cytosolic machinery, of which the clathrin coat has occupied a preponderant position. In what concerns intra-membrane constituents, the focus of interest has been on phosphatidylinositol lipids and their capacity to orchestrate endocytic events on the cytosolic leaflet of the membrane. The contribution of extracellular determinants to the construction of endocytic pits has received much less attention, depite the fact that (glyco)sphingolipids are exoplasmic leaflet fabric of membrane domains, termed rafts, whose contributions to predominantly clathrin-independent internalization processes is well recognized. Furthermore, sugar modifications on extracellular domains of proteins, and sugar-binding proteins, termed lectins, have also been linked to the uptake of endocytic cargoes at the plasma membrane. In this review, we first summarize these contributions by extracellular determinants to the endocytic process. We thus propose a molecular hypothesis - termed the GL-Lect hypothesis - on how GlycoLipids and Lectins drive the formation of compositional nanoenvrionments from which the endocytic uptake of glycosylated cargo proteins is operated via clathrin-independent carriers. Finally, we position this hypothesis within the global context of endocytic pathway proposals that have emerged in recent years.

Prevention of antibody-mediated elimination of ligand-targeted liposomes by using poly(ethylene glycol)-modified lipids

One of the major obstacles in the development of ligand-targeted liposomes is poor liposome circulation longevity as a result of antibody-mediated elimination of these highly immunogenic carriers. Because studies from our laboratory suggest that it is not possible to reduce the immunogenicity of ligand-conjugated liposomes by using surface-grafted poly(ethylene glycol) (PEG), we investigated the usefulness of PEG in protecting hapten-conjugated liposomes from elimination by an existing immune response that was previously established against the hapten. Using biotin as a model hapten, a strong biotin-specific antibody response was generated in mice by using bovine serum albumin-biotin. When these animals were challenged with liposomes containing biotin-conjugated lipid (1 or 0.1%), these liposomes were rapidly eliminated. Incorporation of PEG-lipids into these liposomes substantially reduced biotin-specific antibody binding as measured using an in vitro antibody consumption assay. However, depending on the hapten concentration, significant reductions in antibody binding through the use of PEG-lipids may not be sufficient to protect these liposomes from rapid elimination in vivo. Complete protection of liposomes was only achieved when the biotin concentration on liposome surface was low (0.1%) and with 5 mol% of either 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(polyethylene glycol)-2000] or 1,2-dipalmatoyl-sn-glycero-3-phosphoethanolamine-n-methoxy(polyethylene glycol)-2000]. The use of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(polyethylene glycol)-2000] (up to 15 mol%) was not effective in protecting liposomes from rapid elimination in vivo, indicating the limited usefulness of this highly exchangeable PEG-lipid. In conclusion, our in vivo and in vitro data indicate that liposomes can be protected from antibody-mediated elimination by using the right type and concentration of PEG-lipids. This result has important implication in the development of ligand-targeted liposomes.

Complement-dependent phagocytosis of liposomes

In this article we describe an in vitro model for complement-dependent phagocytosis of liposomes. We have previously reported that complement-opsonized liposomes are avidly ingested by murine peritoneal or bone marrow-derived cultured macrophages. However, when the liposomes contained certain lipids, including phosphatidylinositol, ganglioside GM1, and sulfogalactosyl ceramide, that have been identified as causing prolonged circulation time in vivo, complement-dependent phagocytosis of the liposomes was greatly suppressed. We identify certain additional factors associated with suppressed complement-dependent phagocytosis, including, liposomal negative charge and liposomal prostaglandin E2 or thromboxane B2. Possible mechanisms responsible for suppression of complement dependent phagocytosis are suggested. We propose that suppression of complement-dependent phagocytosis could be a contributing factor in the promotion of increased circulation time of 'stealth' liposomes and that complement opsonization probably plays a role in vivo in removing liposomes from the circulation.

Complement-dependent phagocytosis of liposomes by macrophages: suppressive effects of "stealth" lipids

We have previously reported that complement-opsonized liposomes composed of dimyristoyl phosphatidylcholine and cholesterol are actively phagocytozed by murine peritoneal macrophages and that such complement-induced phagocytosis can be suppressed by the presence of liposomal phosphatidylinositol (Proc. Natl. Acad. Sci. USA 81, 1984). We now report suppressive effects of other liposomal lipids, including monosialoganglioside (GM1) and sulfogalactosylceramide. Complement-dependent phagocytosis was almost completely suppressed by liposomes containing GM1 or phosphatidylinositol and partially suppressed when liposomes contained sulfogalactosylceramide. Although the mechanism of suppression of complement-induced phagocytosis by these liposomal lipids is not yet completely understood, it does not seem to involve the early stages of complement activation resulting in opsonization of liposomes with complement. We conclude that suppression of complement-induced phagocytosis by phosphatidylinositol, GM1, or sulfogalactosylceramide occurs at a step after liposome opsonization.

Endocytosis of liposomes by macrophages: binding, acidification and leakage of liposomes monitored by a new fluorescence assay

The interaction of liposomes with macrophage cells was monitored by a new fluorescence method (Hong, K., Straubinger, R.M. and Papahadjopoulos, D., J. Cell Biol. 103 (1986) 56a) that allows for the simultaneous monitoring of binding, endocytosis, acidification and leakage. Profound differences in uptake, cell surface-induced leakage and leakage subsequent to endocytosis were measured in liposomes of varying composition. Pyranine (1-hydroxypyrene-3,6,8-trisulfonic acid, HPTS), a highly fluorescent, water-soluble, pH sensitive dye, was encapsulated at high concentration into the lumen of large unilamellar vesicles. HPTS exhibits two major fluorescence excitation maxima (403 and 450 nm) which have a complementary pH dependence in the range 5-9: the peak at 403 nm is maximal at low pH values while the peak at 450 nm is maximal at high pH values. The intra- and extracellular distribution of liposomes and their approximate pH was observed by fluorescence microscopy using appropriate excitation and barrier filters. The uptake of liposomal contents by cells and their subsequent exposure to acidified endosomes or secondary lysosomes was monitored by spectrofluorometry via alterations in the fluorescence excitation maxima. The concentration of dye associated with cells was determined by measuring fluorescence at a pH independent point (413 nm). The average pH of cell-associated dye was determined by normalizing peak fluorescence intensities (403 nm and 450 nm) to fluorescence at 413 nm and comparing these ratios to a standard curve. HPTS-containing liposomes bound to and were acidified by a cultured murine macrophage cell line (J774) with a t1/2 of 15-20 min. The acidification of liposomes exhibited biphasic kinetics and 50-80% of the liposomes reached an average pH lower than 6 within 2 h. A liposomal lipid marker exhibited a rate of uptake similar to HPTS, however the lipid component selectively accumulated in the cell; after an initial rapid release of liposome contents, 2.5-fold more lipid marker than liposomal contents remained associated with the cells after 5 h. Coating haptenated liposomes with antibody protected liposomes from the initial release. The leakage of liposomal contents was monitored by co-encapsulating HPTS and p-xylene-bis-pyridinium bromide, a fluorescence quencher, into liposomes. The time course of dilution of liposome contents, detected as an increase in HPTS fluorescence, was coincident with the acidification of HPTS. The rate and extent of uptake of neutral and negatively charged liposomes was similar; however, liposomes opsonized with antibody were incorporated at a higher rate (2.9-fold) and to a greater extent (3.4-fold).(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatidylinositol liposomes opsonized by concanavalin A stimulate phosphatidylinositol turnover in macrophages

We have previously found that concanavalin A binds specifically to inositol and phosphatidylinositol. In the present study we demonstrate that binding of concanavalin A to liposomes containing phosphatidylinositol influences the uptake of such liposomes by macrophages. Although resident mouse peritoneal macrophages normally ingest liposomes only to a slight extent, attachment of concanavalin A to the liposomes caused a marked enhancement of phagocytosis. Furthermore induction of phagocytosis of concanavalin A-opsonized liposomes was associated with increased phosphatidylinositol turnover. We conclude that (a) concanavalin A can serve as an opsonizing agent for liposomes; (b) opsonization results from binding of concanavalin A to phosphatidylinositol on the liposomal surface; and (c) concanavalin A-induced phagocytosis is associated with increased phosphatidylinositol turnover in the macrophages.

Immune clearance of liposomes inhibited by an anti-Fc receptor antibody in vivo

In a study designed to evaluate the potential for in vivo manipulation of the circulation and tissue distribution of injected liposomes, mice were passively injected with antidinitrophenyl (anti-DNP) monoclonal antibodies of the IgG2a or IgG2b subclasses or were immunized with the nitrophenyl hapten bound to a protein carrier. They were then injected i.v. with 125I- and carboxyfluorescein-labeled, DNP-bearing liposomes. Circulation time of the DNP-bearing liposomes was markedly reduced in actively and passively immune mice, with increased deposition of liposomes in the liver. The increased clearance of liposomes could be abrogated by injection of a monoclonal antibody directed against the murine IgG Fc receptor (2.4G2). The results suggest that clearance of ligand-bearing reagent in the face of an immune response may be modified by specific immunologic manipulation in vivo.

Novel fluorescence method to visualize antibody-dependent hydrogen peroxide-associated "killing" of liposomes by phagocytes

We have developed a new methodology to examine effector-cell-mediated immune attack using liposomes as targets. Hydrogen peroxide-associated killing of liposomes was observed with fluorescence intensification microscopy. Liposomes were composed of 98-99 mol % egg phosphatidylcholine and 1-2 mol % dinitrophenyl lipid hapten. Anti-dinitrophenyl IgG antibody was used to opsonize liposomes. Liposomes were loaded with dihydroxymandelic acid (DHMA) and peroxidase. Macrophage- or neutrophil-mediated recognition of liposomes triggers the release of H2O2 and other oxidative products. Upon interaction of H2O2 or OH radical with liposome contents, DHMA dimerizes forming a fluorescent derivative. Our studies indicate that individual living neutrophils and macrophages deposit oxidative products in a heterogenous fashion among bound targets.

Novel fluorescence method to visualize antibody-dependent hydrogen peroxide-associated "killing" of liposomes by phagocytes

We have developed a new methodology to examine effector-cell-mediated immune attack using liposomes as targets. Hydrogen-peroxide-associated killing of liposomes was observed with fluorescence intensification microscopy. Liposomes were composed of 98-99 mol % egg phosphatidylcholine and 1-2 mol % dinitrophenyl lipid hapten. Anti-dinitrophenyl IgG antibody was used to opsonize liposomes. Liposomes were loaded with dihydroxymandelic acid (DHMA) and peroxidase. Macrophage- or neutrophil-mediated recognition of liposomes triggers the release of H2O2 and other oxidative products. Upon interaction of H2O2 or OH radical with liposome contents, DHMA dimerizes forming a fluorescent derivative. Our studies indicate that individual living neutrophils and macrophages deposit oxidative products in a heterogeneous fashion among bound targets.

Enhanced binding of phosphatidylserine-containing lipid vesicle targets to RAW264 macrophages

Phosphatidylserine was found to significantly enhance the binding of phospholipid vesicles to RAW264 macrophages. We have measured the kinetics of non-specific uptake of unilamellar vesicles as a function of phosphatidylserine concentration in these model target membranes. Dimyristoylphosphatidylcholine was the principle component of these phospholipid vesicles. In most experiments, radiolabeled phospholipid and 1 mol% each of both a fluorescent phospholipid and a hapten-containing lipid headgroup were utilized. In the presence of specific anti-hapten antibody phosphatidylserine-containing vesicles are rapidly taken up via phagocytosis. The antibody-independent non-specific uptake of phosphatidylserine-free vesicles was low, as previously reported. However, the presence of 5 mol% phosphatidylserine dramatically enhanced the uptake of phospholipid vesicles by macrophages. This uptake was shown to be principally due to binding to the macrophage surface. Incubation of macrophages in the presence of sodium azide or at 4 degrees C, conditions which are known to inhibit phagocytosis, do not influence the uptake of the lipid vesicles. Fluorescence video-intensification microscopy was used to observe the interaction of carboxyfluorescein-loaded vesicles with macrophages. Fluorescence could not be observed when using phosphatidylserine-free vesicles. However, phosphatidylserine-containing vesicles can be observed bound to the cell periphery. Intracellular fluorescence could not be observed. The binding of phosphatidylserine-containing vesicles was enhanced roughly four-fold over phosphatidylserine because the effect could not be observed with membranes containing 1 mol% or 2.5 mol % phosphatidylserine. In addition, the binding enhancement required the presence of divalent cations in the incubation medium.

Cytochemical study of liposome and lipid vesicle phagocytosis

Electron microscopy cytochemistry has been used to study the cytoplasmic location of liposomes and lipid vesicles following specific antibody-dependent phagocytosis. The vesicle compositions were 94-99 mol% 'fluid' lipid (egg phosphatidylcholine or dimyristoylphosphatidylcholine at 37 degrees C or 'solid' lipid (dipalmitoylphosphatidylcholine at 37 degrees C). In some cases, 4 mol% phosphatidylserine was included in the vesicle membrane so as to vary the surface charge density. These vesicles undergo specific antibody-dependent phagocytosis by RAW264 macrophages when the lipid membranes contain 1-2 mol% dinitrophenyl lipid hapten in the presence of rabbit anti-dinitrophenyl IgG antibody. Internalized lipid vesicles can be visualized with the electron microscope when ferritin is trapped in the internal aqueous compartments prior to internalization. The lipid vesicles were demonstrated to be internal to the macrophage plasma membranes by selectively staining the plasma membranes with Ruthenium red. The cytoplasmic location of vesicles and liposomes was studied by electron microscopic staining for activities of the following enzymes: (1) acid phosphatase; (2) inorganic trimetaphosphatase; (3) adenosine triphosphatase; and (4) glucose-6-phosphatase. The first two enzymatic activities were found in association with ferritin-containing vesicles after antibody-dependent phagocytosis, showing the formation of vesicle-containing phagolysosomes. Adenosine triphosphatase and glucose-6-phosphatase were primary not associated with the vesicles, suggesting a minimal association of vesicles with plasma membrane, Golgi, endoplasmic reticulum and perinuclear cisternae. Phagosome-lysosome fusion did not appear to depend on the type of target lipid vesicle or liposome, on the 'fluidity' of the target membrane, or the presence of phosphatidylserine in the target membrane.

Effects of negatively charged lipids on phagocytosis of liposomes opsonized by complement

Ingestion of liposomes opsonized by specific antibody plus complement was investigated in vitro. Although the antibodies alone (IgM) did not have an opsonizing effect, in the presence of such antibodies uptake and ingestion of liposomes by mouse peritoneal macrophages was enhanced 5- to 10-fold by addition of complement. Phagocytosis of complement-opsonized liposomes was strongly dependent on the charge of the liposomal lipids. The presence of a negatively charged (i.e., acidic) lipid profoundly suppressed the uptake of the liposomes. Each of three acidic liposomal lipids, phosphatidylserine, phosphatidylinositol and dicetyl phosphate, suppressed liposome uptake. We conclude that opsonization of liposomes with complement greatly stimulates ingestion of liposomes by murine macrophages. However, most of the opsonic enhancement conferred by complement can be prevented by the presence of negatively charged membrane lipids.

Endocytosis of liposomes bound to cell surface proteins measured by flow cytofluorometry

A new technique for the quantification of cellular receptor-mediated endocytosis has been developed based on the analysis by flow cytometry of ligand-bearing liposomes containing the fluorochrome carboxyfluorescein. Carboxyfluorescein encapsulated at high concentrations in protein A-bearing liposomes is self-quenched. Binding and internalization of such liposomes by cells via antibodies directed towards membrane surface determinants results in the release of the liposome-encapsulated carboxyfluorescein into the cytoplasm causing an increase in cell-associated fluorescence. This increase can be quantified on a flow cytofluorometer.

Interactions of liposomes with the reticuloendothelial system. II: Nonspecific and receptor-mediated uptake of liposomes by mouse peritoneal macrophages

Liposomes are taken up as intact vesicles by mouse peritoneal macrophages in a process which is temperature sensitive and is affected by inhibitors of glycolytic metabolism and of microfilament activity. Macrophages take up negatively charge vesicles more readily than positively charged vesicles (2-fold) or neutral vesicles (4-fold). Macrophages take up similar amounts of multilamellar liposomes, reversed phase liposomes and small unilamellar liposomes in terms of lipids, however this corresponds to vastly different numbers of particles and amounts of trapped volume. Coating the liposomes with macromolecular ligands capable of interacting with macrophage surface receptors can markedly promote liposome uptake. Thus, formation of an IgG-antigen complex on the liposome surface results in a 10(2)-fold enhancement of liposome uptake, while coating the vesicles with fibronectin results in a 10-fold augmentation of uptake. Uptake via IgG-mediated and fibronectin-mediated processes seem to be independent since excess unlabelled, IgG-coated liposomes will inhibit the uptake of radioactively-labelled IgG-coated liposomes much more effectively than the uptake of radioactively-labelled fibronectin-coated liposomes. Cell-bound liposomes can readily be visualized on and inside of the macrophages using fluorescence microscopy techniques.