Spin membrane immunoassay: simplicity and specificity

Cellular uptake of electron paramagnetic resonance imaging probes through endocytosis of liposomes

Electron paramagnetic resonance imaging (EPRI) allows detection and localization of paramagnetic spin probes in vivo and in real time. We have shown that nitroxide spin probes entrapped in the intracellular milieu can be imaged by EPRI. Therefore, with the development of a tumor-targetable vehicle that can efficiently deliver nitroxides into cells, it should be possible to use nitroxide spin probes to label and image cells in a tumor. In this study, we assess the potential of liposomes as a delivery vehicle for imaging probes. We demonstrate that liposomes can stably encapsulate nitroxides at very high concentrations (>100 mM), at which nitroxides exhibit concentration-dependent quenching of their EPR signal-a process analogous to the quenching of fluorescent molecules. The encapsulating liposomes thus appear spectroscopically "dark". When the liposomes are endocytosed and degraded by cells, the encapsulated nitroxides are liberated and diluted into the much larger intracellular volume. The consequent relief of quenching generates a robust intracellular nitroxide signal that can be imaged. We show that through endocytosis of nitroxide-loaded liposomes, CV1 cells can achieve intracellular nitroxide concentrations of approximately 1 mM. By using tissue phantom models, we verify that this concentration is more than sufficient for in vivo EPR imaging.

Liposomes and immunoassays

Various aspects of the application of liposomes as a label in immunoassays are reviewed. Methods for the preparation of liposomes, from the basic film method to the more advanced dehydration-rehydration method, are discussed. Furthermore, the markers used in liposome labels, as well as the methods to conjugate liposomes to antigens or antibodies, are summarized. Liposome immunoassays are applied as homogeneous or heterogeneous assays. Homogeneous assays often rely on the lytic activity of complement on antibody-associated liposomes. Another group of homogeneous assays utilizes the inhibitory action of antibodies on the activity of conjugates of mellitin (a bee venom protein) with a hapten. Free mellitin conjugates are able to lyse liposomes effectively. Heterogeneous liposome immunoassays, performed either competitively or non-competitively, resemble more closely standard enzyme linked immunosorbent assays, with the enzyme being replaced by a liposome label. Washing steps are used to separate antigen-specifically bound liposomes from unbound liposomes. All bound liposomes are lysed with a detergent, giving an instantaneous amplification. Flow-injection liposome immunoassays and liposome immunosensors are also described as examples of other possible immunoassay formats.

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.

Enzyme modulator mediated immunoassay (EMMIA)

1. Enzyme modulator mediated immunoassay (EMMIA) is a separation-free (homogeneous) enzyme amplified immunoassay. 2. The assay is based on the ability of an analyte labeled enzyme modulator to modify the activity of an indicator enzyme and on ability of an anti-analyte antibody to abrogate the modifying action of the analyte-labeled enzyme modulator upon its binding to anti-analyte antibody. 3. The principle of EMMIA is elaborated in detail. The stages in the development of an EMMIA are described. 4. Criteria in selecting enzymes and modulators suitable for EMMIA are discussed. 5. Examples of EMMIA's using different kinds of modulators for different classes of analytes are presented.

Liposome immune assay (LIA). Use of membrane antigens inserted into labeled lipid vesicles as targets in immune assays

A new method is described for detection of membrane antigens and antibodies against such structures. By inserting partially purified rat transplantation antigen (RT-1) into iodine or fluorescein-labeled lipid vesicles, a precipitation of the liposome is demonstrated by the use of a specific alloantiserum and a heterologous anti-rat IgG serum. Precipitation of liposomes carrying WF transplantation antigens could be detected at a final dilution of the alloantiserum of 1:3000, which is comparable to that obtained in a 51Cr-release assay. Furthermore, when unpurified membrane proteins from WF splenocytes were incorporated into labeled liposomes, an amount of RT-1 corresponding to 8000 cells could be detected.

Specific anti-thyroxine antisera induced by thyroxine sensitized liposomes

Specific anti-thyroxine rabbit antisera were generated from complete Freund's adjuvant with liposomes consisting of sphingomyelin, cholesterol, dicetylphosphate and 5-N-thyroxine-2,4-dinitrophenyl-phosphatidylethanolamine (T4-Dnp-PE). Spin membrane immunoassay technique was used to measure the sensitivity and specificity of these antisera. Addition of 10 to 70 ng of L-thyroxine produced significant inhibition of immune lysis. Addition of L-3,3',5-triiodothyronine (T3) up to 800 ng showed no cross reaction.