Liposome spin immunoassay: a new sensitive method for detecting lipid substances in aqueous media

Computational models in immunological methods: an historical review

The utilization of computational models in immunology dates from the birth of the science. From the description of antibody-antigen binding to the structural models of receptors, models are utilized to bring fundamental understandings of the processes together with laboratory measurements to uncover implications of these data. In this review, an historical view of the role of computational models in the immunology laboratory is presented, and short mathematical descriptions are given of fundamental assays. In addition, the range of current uses of models is explored -- especially as seen through papers which have appeared in the Journal of Immunological Methods from volume 1 (1971/1972) to volume 208 (1997). Each paper which introduced a new mathematical, statistical, or computer simulation model, or introduced an enhancement to an instrument through a model in those volumes is cited and the type of computational model noted.

Prevention of nonspecific lysis in liposomal and erythrocyte immunoassay systems by small lipid vesicles and erythrocyte ghosts

Large unilamellar liposomes prepared by the reverse-phase evaporation method (REVs) were made immunoreactive by incorporating dinitrophenylaminocaproyl-phosphatidylethanolamine (DNP-Cap-PE) or 8-(3-carboxypropyl)-theophylline-dipalmitoylphosphatidylethanolamine (Th-DPPE) into the phospholipid bilayer. Specific lysis in the presence of anti-DNP-BSA and goat anti-theophylline serum respectively, was induced by adding guinea pig serum as source for complement to these liposomes. However, specific lysis was found to be compromised by high levels of nonspecific lysis as monitored by the release of the fluorescent aqueous-space marker 6-carboxyfluorescein. Nonspecific lysis could be prevented without affecting specific lysis by pretreatment of complement or incubation of the reaction mixture with small unilamellar liposomes (SUVs). SUVs of various lipid compositions produced the desired effect; however, when the fraction of negative charge in the SUVs was increased to 30 mol%, specific lysis was inhibited as well. In a similar assay system consisting of hemolysin-sensitized sheep red blood cells it was also found that nonspecific lysis could be inhibited by addition of erythrocyte ghosts to the incubation medium, although specific lysis was somewhat depressed. However, SUVs or REVs of a composition similar to sheep erythrocytes were ineffective indicating a more selective nature of complement-mediated immunoreaction with erythrocyte membranes than with synthetic bilayer membranes.

Antibody multivalency effects in the direct binding model for vesicle immunolysis assays

An extension of a previous model of liposome-based immunoassays is presented which incorporates the effects of antibody multivalency in the binding process. Equations based on the distribution of vesicles having both mono- and divalently bound species show the quantitative relationships of the experimental parameters, including vesicle concentration, antigen density on vesicle surfaces, antibody concentration, and antibody affinity (both for the initial binding step and for the subsequent cross-linking step). It is found that in the case of low antibody concentration, the multivalent model can be cast in the form of the previously described monovalent model, replacing the association equilibrium constant with an effective equilibrium constant which is found to depend linearly on the lateral antigen density and on the valency of the binding antibody. Comparisons to certain experiments are made using this more realistic model of complement-mediated vesicle immunoassay. For the case of IgM binding, it is estimated that as few as 1000 antibody molecules can be detected in a typical lytic assay, representing a significant increase in sensitivity over previous predictions.

Homogeneous immunoassay based on dihexadecyl-dimethylammonium bilayer membranes

We describe a rapid, highly sensitive, simple immunoassay based on cationic bilayer membranes prepared from dihexadecyl-dimethylammonium bromide. This detergent was dispersed in water to give a slightly turbid solution which contained lamellar (not vesicular) bilayer membranes with a gel to liquid crystal phase transition (Tc, 28 degrees C). An anionic hapten (N,N'-bis(2,4-dinitrophenyl)-L-lysine) combined electrostatically with this cationic membrane to induce aggregation of membranes, making the solution turbid. This aggregation was suppressed by antibodies to the hapten. The aggregation rate was rapid at a temperature above Tc, and the amount of antibodies was quantified by measuring absorbance or light scattering of the aqueous solution with a detection limit of 0.1 ng/ml of the antibodies. Anionic and zwitterionic bilayer membranes from di(tetradecyl)phosphate and phosphocholine derivatives, respectively, were less sensitive as detectors of the hapten-antibody reaction. Ionic interactions between cationic bilayer membranes and anionic haptens are important for successful assay performance.

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.

Immune reactivity against membranes containing ganglioside GM1 in chronic-progressive multiple sclerosis: observation by spin-membrane immunoassay

A spin-membrane immunoassay has been employed to examine the immune reactivity of whole serum from patients with chronic-progressive multiple sclerosis (CPMS) against liposomes containing ganglioside GM1. Exposure to serum resulted in complement-mediated lysis of the GM1-liposomes. No lysis occurred with liposomes devoid of ganglioside. The mean (+/- S.E.M.) lysis values were 52.6 (+/- 9.8)% for fifteen CPMS patients and 32.9 (+/- 7.2)% for nine controls. The difference between the means was highly significant (student's t-test, P less than 0.0001), indicating increased anti-ganglioside immunity in patients with CPMS.

Spin membrane immunoassay for use in meningococcal serology

A modified and improved spin membrane immunoassay has been developed for detecting complement-activating antibodies to Neisseria meningitidis capsular polysaccharide antigens. The polysaccharides were incorporated in the membranes of large unilamellar vesicles prepared by the reverse-phase evaporation method and filled with the water-soluble spin label tempocholine chloride. Upon addition of group-specific antisera and complement, the lipid membrane was damaged and the spin label leaked out. This process was monitored by electron spin resonance spectroscopy. A satisfactory assay was developed for polysaccharides of group A and C, whereas in the case of the B system the assay was more labile. The method is rapid and has a sensitivity comparable to that of radioimmunoassay. When studying paired sera from five recruits vaccinated with an A + C polysaccharide vaccine, significant rises in titers to both A and C polysaccharides were observed in all the postvaccination sera.

Immunochemical determination of Forssman and blood group A-active glycolipids in human gastric mucosa by inhibition assay of liposome lysis

A simple liposome immunoassay, liposome immune-lysis inhibition (LILI) assay, is described for quantitative determination of individual glycolipid antigens. Liposomes containing fluorogenic marker, 4-methylumbelliferyl phosphate, were prepared from sphingomyelin, cholesterol, dicetylphosphate and standard glycolipid. Release of trapped markers from these liposomes by antibody and complement (liposome lysis) was inhibited by preincubating the antibody with test glycolipid incorporated into inhibitor liposomes. Based on the competitive inhibition, it was possible to quantitate each glycolipid antigen in less than picomolar amounts. The sensitivity and specificity of the assay were examined with purified glycolipid standards. LILI assay has been applied for the determination of Forssman glycolipid and blood group A-active glycolipid in human gastric mucosa and cancer tissues.

Radioimmunoassay of glycosphingolipids: application for the detection of forssman glycolipid in tissue extracts and cell membranes

Development of a radioimmunoassay for detecting glycosphingolipids has been difficult, primarily because of transfer of radiolabeled glycolipid antigen to the unlabeled antigen pool. This difficulty has been overcome by the use of a radiolabeled glycolipid-polymer. Thus, an assay system has been developed for measuring picomolar quantities of Forssman hapten glycolipid. This assay is based on competition for rabbit anti-Forssman antibodies between Forssman glycolipid and a radiolabeled Forssman polyacrylic hydrazide polymer. Antigen-antibody complexes are removed quickly and efficiently by binding to formalin-fixed Staphylococcus aureus and subsequent centrifugation. One nanogram of Forssman glycolipid can be readily detected both in plasma membrane preparations and in purified glycolipid fractions. The isoantigenic expression of Forssman glycolipid in human gastrointestinal tissues has been reported previously (Hakomori et al., 1977). Using the radioimmunoassay, the Forssman status of several additional cases has been determined quantitatively.

Spin membrane immunoassay: simplicity and specificity

The many disadvantages of radioimmunoassay (RIA) have stimulated attempts to develop non-radioactive assays. These include spin immunoassay (SIA), which is simple, specific, and requires no separation procedures but is much less sensitive than RIA. The membrane immunoassay (MIA) described here is more sensitive than the SIA. Serum is prepared as for RIA. The MIA employs liposomes sensitized with epsilon-dinitrophenylated aminocaproyl phosphatidylethanolamine. It records liposome lysis induced by specific anti-Dnp antibodies, and complement which is monitored by the release of trapped spin labels (N-(2,2,6,6-tetramethylpiperidinyl-1-oxyl)-choline chloride). The sensitized liposomes are stable and give reproducible results for up to 4 weeks. The system's sensitivity is limited by the antibody's affinity (Ka approximately 10(8) M-1) rather than the sensitivity of the electron spin resonance spectrometer (approximately 1 X 10(-7) M). The inhibition of liposome lysis is hapten specific: (epsilon-Dnp-aminocaproic acid,epsilon-Dnp-lysine) greater than alpha-Dnp-glycine; o-nitroaniline and epsilon-dansyl-lysine are ineffective. Inhibition is quantitative without augmentation.

Immunochemical determination of ganglioside GM2, by inhibition of complement-dependent liposome lysis

Immunochemical quantitative determination of a lipid antigen, ganglioside GM2, has been developed, based on the inhibition of the immune lysis of liposomes containing the antigen in their lipid bilayer. It has been shown that the full expression of the antigenicity of the competing lipid requires its dispersion in accessory lipids. The assay of inhibition of liposome lysis can be used also for the establishment of the antigenic similarity of structurally related lipid antigens.

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

A simple inexpective method is described to study the kinetics of complement-mediated immune lysis of liposomes containing sheep red blood cell lipid antigens. It is based on the fact that trapping the fluorescent molecule 1-aminonaphthalene-3,6,8-trisulfonate and the dynamic quencher, alpha, alpha'-dipyridinium p-xylene dibromide within the liposome inner volume results in an extinguished fluorescence signal. On addition of helmolysin plus active complement, liposome lysis occurs. The exit of the fluorophore and quencher and their subsequent dilution in the external volume abolishes the quenching, resulting in a high fluorescence signal. The details of the method are described as well as the initial kinetic results.