A simple fluorescent method to determine complement-mediated liposome immune lysis

H+- and Ca2+-induced fusion and destabilization of liposomes

A new liposome fusion assay has been developed that monitors the mixing of aqueous contents at neutral and low pH. With this assay we have investigated the ability of H+ to induce membrane destabilization and fusion. The assay involves the fluorophore 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and its quencher N,N'-p-xylylenebis(pyridinium bromide) (DPX). ANTS is encapsulated in one population of liposomes and DPX in another, and fusion results in the quenching of ANTS fluorescence. The results obtained with the ANTS/DPX assay at neutral pH give kinetics for the Ca2+-induced fusion of phosphatidylserine large unilamellar vesicles (PS LUV) that are very similar to those obtained with the Tb3+/dipicolinic acid (DPA) assay [Wilschut, J., & Papahadjopoulos, D. (1979) Nature (London) 281, 690-692]. ANTS fluorescence is relatively insensitive to pH between 7.5 and 4.0. Below pH 4.0 the assay can be used semiquantitatively by correcting for quenching of ANTS due to protonation. For PS LUV it was found that, at pH 2.0, H+ by itself causes mixing of aqueous contents, which makes H+ unique among the monovalent cations. We have shown previously that H+ causes a contact-induced leakage from liposomes composed of phosphatidylethanolamine and the charged cholesteryl ester cholesteryl hemisuccinate (CHEMS) at pH 5.0 or below, where CHEMS becomes protonated. Here we show that H+ causes lipid mixing in this pH range but not mixing of aqueous contents. This result affirms the necessity of using both aqueous space and lipid bilayer assays to comprehend the fusion event between two liposomes.

Electrostatic forces control the penetration of membranes by charged solutes

Using fluorescent, anionic dyes such as carboxyfluorescein as model solutes, it is shown that the forces allowing such solutes to be retained within sealed lipid vesicles, against a large concentration gradient, can be primarily electrostatic in nature. At temperatures distant from that of the ordered-fluid lipid phase transition a small number of the anionic dye molecules trapped within lipid vesicles are capable of traversing the lipid bilayer and establishing an electrical diffusion potential across the membrane. Further solute movement can then only occur with the concomitant permeation of ions which restore electrical balance. A significant flux of dye can be triggered by (a) increasing the permeability of the membrane to ions (for example by the addition of ionophores such as gramicidin, or by allowing the lipid to approach a phase transition) or by (b) adding lipophilic counterions such as tetraphenylborate or dinitrophenol to the system.

Liposome immune lysis assay (LILA): a simple method to measure anti-protein antibody using protein antigen-bearing liposomes

A new simple immunoassay technique using immune lysis of liposomes was developed to measure antibody against protein antigens. Multilamellar liposomes were composed of dipalmitoylphosphatidylcholine, cholesterol and phosphatidylethanolamine substituted with the hetero-bifunctional cross-linking reagent N-hydroxysuccinimidyl 3-(2-pyridyldithio)propionate (SPDP). The protein antigen (human IgG) was coupled to these liposomes after treatment with SPDP and mild reduction. As a release marker, carboxyfluorescein (CF) was entrapped in the liposomes. The CF release was specific to anti-human IgG antibody and depended on the presence of complement. This technique could detect 10(-15) mol of anti-human IgG antibody or human IgG. The liposomes were stable over 8 months at 4 degrees C under nitrogen gas.

A new method of evaluating complement mediated lysis of liposomes

Immune lysis of liposomes has been utilized in immunoassays for detection of specific antibodies. Membrane immunoassays which have sensitivities comparable with those of radioimmunoassays are easy and rapid to perform. Information obtained so far has been limited by the fact that the method is based on empirical relationships. The present paper describes a simple theoretical model for analysing membrane immunoassay data. According to the model, relative dissociation constants for the antibody-antigen reaction and relative concentrations of high affinity antibodies for different antisera may be determined. Furthermore, the analysis yields the percentage of antibody bound to antigen under certain conditions. The method is also useful in the search for optimum experimental conditions in membrane immunoassay measurements.

pH-induced destabilization of phosphatidylethanolamine-containing liposomes: role of bilayer contact

The mechanism of pH-induced destabilization of liposomes composed of phosphatidylethanolamine and a charged cholesteryl ester was studied by following the release of encapsulated aqueous contents. The kinetics of release were measured continuously by using the water-soluble fluorophore 8-aminonaphthalene-1,3,6-trisulfonic acid in combination with the water-soluble quencher p- xylylenebis (pyridinium) bromide. With this fluorescence assay, release of contents from liposomes composed of phosphatidylethanolamine and cholesteryl hemisuccinate was shown to be a function of pH, ratio of phosphatidylethanolamine to cholesteryl hemisuccinate, and acyl chain composition of the phosphatidylethanolamine. Leakage was very slow at pH 5.5 and increased dramatically with decreasing pH down to 4.0. Replacing phosphatidylethanolamine by phosphatidylcholine eliminated the effect of pH on leakage. Analysis of the kinetics of release by a mass action model demonstrated that bilayer destabilization and leakage occur subsequent to aggregation. The requirement of bilayer contact for destabilization has been found previously for acidic phospholipid bilayers in the presence of divalent cation and for saturated phosphatidylcholine bilayers below the isothermal phase transition temperature. The phosphatidylethanolamine-containing bilayers examined here satisfy the same requirement.

Particle concentration fluorescence immunoassay (PCFIA): a new, rapid immunoassay technique with high sensitivity

A new solid-phase fluorescence immunoassay technique is described and is exemplified by the detection of murine monoclonal antibodies to human IgG in hybridoma culture supernatants and the detection of murine IgG. The assay is performed in a specially designed 96-well plate. For antibody detection, antigen bound to submicron polystyrene particles is bound to its specific antibody, which is in turn reacted with fluorescein-labeled affinity-purified goat anti-mouse IgG. The reaction is complete in 10 min at ambient temperature. The solid phase is separated from the reaction mixture by filtration, washed and the total particle-bound fluorescence is determined by front-surface fluorimetry. The sensitivity of the technique for antibody detection is equivalent to enzyme-linked immunoabsorbent assay and 2-4 ng/ml for murine IgG detection. It is readily amenable to automation.

Serological specificity of the liposome lysis test for measurement of anti-ganglioside antibodies. A comparison with hemagglutination inhibition

A simple and reliable complement-dependent liposome lysis test for the detection of anti-ganglioside antibodies is described. For sera raised in rabbits against the monosialoganglioside NG-GM3, the sensitivity and specificity of antibody detection was compared with that of the HRBC hemagglutination-inhibition test: the liposome lysis test appears more sensitive. A difference in antigen presentation was also demonstrated.

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

The uptake by murine macrophages of liposomes, exhibiting one of a variety of haptenic groups on their surfaces, was greatly enhanced by the addition of an intact antibody or a lectin specific for the incorporated hapten. The uptake of untreated liposomes was slow and linear over long periods, whereas upon addition of the antibody or lectin, over 30-fold increase in the maximal rate of uptake was observed. The process reached a plateau after 90-120 min. The interaction of the antibody- or lectin-treated liposome with the macrophages apparently resulted in an active endocytosis of soluble fluorescent, intraliposomal marker had a granular intracellular pattern in treated cells. The uptake was sensitive to azide and the liposome constituents could not be detected at the cell surface. The size of the liposomes as well as the state of stimulation of the macrophages (thioglycollate stimulated vs. normal) did not seem to have a major effect on the phagocytic process. The time required to reach the plateau in uptake was independent of liposome composition or antibody concentration and is, apparently, an intrinsic property of the cells. The implication of this phenomenon on the dynamics of the relevant macrophage receptors is discussed.

A simple method to measure anti-glycolipid antibody by using complement-mediated immune lysis of fluorescent dye-trapped liposomes

A simple, reproducible, and micro quantity method is described to measure the antibody against glycolipid antigens. The multilamellar liposomes containing carboxyfluorescein (CF), which is self-quenched at high concentration, are prepared by vortexing the dried lipid films consisting of egg lecithin, cholesterol, phosphatidic acid and Forssman glycolipid antigen. On addition of anti-glycolipid serum plus active complement, liposome lysis occurs, and trapped CF is released. The dilution of CF in the external volume abolishes the quenching, resulting in a high fluorescence signal. Experimental conditions to measure anti-glycolipid antibody is established in this paper.

Liposomes as immunological adjuvants in eliciting antibodies specific to the synthetic polypeptide poly(LTyr, LGlu)-poly(DLAla)--(LLys) with high frequency of site-associated idiotypic determinants

The antibody response to the synthetic polypeptide, poly(LTyr, LGlu)-poly(DLAla)--poly(LLys), [(T,G)-A--L], injected entrapped in liposomes which served as adjuvant has been analyzed. The liposomes used were composed of phosphatidylcholine, cholesterol, dicetylphosphate and DL alpha-tocopherol (molar ratios as 4:3:0.1:0.5) and therefore, were negatively charged. Since the (T,G)-A--L is also negatively charged, no free complexes were formed. The (T,G)-A--L was found to be entrapped inside the enclosed volume of the liposomes, and no (T,G)-A--L antigenic determinants could be detected on the liposomal membranes. Injection of high-responder C3H.SW (H-2b) mice with (T,G)-A--L-bearing liposomes demonstrated that the i.p. and the i.v. routes of immunization were efficient in eliciting (T, G)-A--L specific antibodies, whereas the i.d. injection led to poor antibody responses. The latter route of immunization is the most effective when (T,G)-A--L is injected in complete Freund's adjuvant (CFA). When low doses (0.1 and 1 microgram) of (T, G)-A--L were used for immunization, the liposomes were better adjuvants than CFA. The effectiveness of the liposomes as immunological adjuvants was also shown in their ability to induce high-potential, primed memory cells. The pattern of low (H-2k,a) and high (H-2b) responsiveness to (T,G)-A--L was retained following immunization with (T,G)-A--L entrapped in liposomes, as tested in two pairs of congenic strains. (T,G)-A--L-specific antibodies induced by injection with 1 microgram antigen entrapped in liposomes bear the (T,G)-A--L site-related idiotypic markers of C3H.SW (Igh-1a) mice in a significantly higher frequency than the homologous idiotypes, namely the antibodies elicited in this strain against (T,G)-A--L in CFA. Thus, liposomes may serve as adjuvants for the production of relatively restricted (T,G)-A--L-specific antibodies of high quality.

A simple radioimmunoassay to determine binding of antibodies to lipid antigens

A solid-phase radioimmunoassay to determine the binding of antibodies to lipid antigens is described. Polyvinyl chloride microtiter plates were coated with lipid antigens by placing in each well 25--100 microliters of solutions of the antigens in ethanol and evaporation of the solvent. The wells were than washed with 0.3% gelatine solution to remove loosely bound antigens and to saturate the plastic with an unrelevant protein to prevent unspecific binding of antibodies to the plates. About one-third of [3H] phosphatidylcholine adsorbed in this way (out of 1 nmole) was firmly attached to the plastic and was not washed away during a standard assay. The rest, loosely bound lipid, was washed away in the first 5 washings before the primary antibodies were applied. The technique is rapid and convenient and is as sensitive as a conventional solid-phase radioimmunoassay with protein antigens. It requires minute amounts of antigen. Less than 10(-12) moles of lipid antigen can be detected by this technique.

Anti-ganglioside antibodies in multiple sclerosis

Serological activity against several purified brain gangliosides has been demonstrated in sera of a proportion of multiple sclerosis patients, but not in normal individuals. The activity was determined by the capacity of the sera to bring about complement-dependent lysis of liposomes containing the respective ganglioside in their lipid bilayer. An apparent correlation is indicated between the severity of the disease and the extent of liposome lysis. Cerebrospinal fluid of the patients did not induce lysis, probably due to low antibody concentration.

Antibodies to ganglioside GM1 induce mitogenic stimulation and cap formation in rat thymocytes

Antibodies towards the ganglioside GM1 [galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosyglucosyl ceramide] stimulated DNA synthesis in rat thymocytes. No mitogenic stimulation was observed with the monomeric Fab fragment of anti-GM1, suggesting that cross-linking of the gangliosides or associated components was required for activation by these antibodies. Incubation of thymocytes with anti-GM1 and fluorescein-labeled anti-rabbit IgG at 0 degree C resulted in uniform ring-like or patchy staining that developed into a pronounced cap upon elevation of temperature. The cap had a characteristic uropod form, enriched with intracellular organelles. Sodium azide and cytochalasin B completely inhibited cap formation, while colchicine was without effect. These results imply a possible direct or indirect association between surface gangliosides and submembraneous cytoskeletal assemblies that control modulation of these surface components and may transmit stimuli to the interior of the cell.