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

Liposome-Based Bioassays

This review highlights the potential of using liposomes in bioassays. Liposomes consist of nano- or micro-sized, synthetically constructed phospholipid vesicles. Liposomes can be loaded with a number of reporting molecules that allow a dramatic amplification of the detection threshold in bioassays. Liposome-based sensors bind or react with the biological components of targets through the introduction of properly tailored vectors anchored on their external surface. The use of liposome-based formulations allows the set-up of bioassays that are rapid, sensitive, and often suitable for in-field applications. Selected applications in the field of immunoassays, as well as recognition/assessment of corona proteins, nucleic acids, exosomes, bacteria, and viruses are surveyed. The role of magnetoliposomes is also highlighted as an additional tool in the armory of liposome-based systems for bioassays.

Current trends in the development of the electrochemiluminescent immunosensors

This review presents a general picture of the current trends and developments (2008-2013) related to electrochemiluminescence-based immunosensors. It briefly covers the milestones of qualitative changes in the field of electrochemiluminescent immunosensors; the peculiarities of the electrochemiluminescent immunoassay formats; the basic mechanisms of ECL detection, main features of early and ongoing approaches in electrochemiluminescent immunoassay commercial instruments, and the recent developments in fabrication of solid-state electrochemiluminescent immunosensors. Moreover, systematized data on biomarkers, immunoassay formats, and novel types of electrochemiluminescent label and immobilization support, such as semiconductor nanocrystals, porous noble metals, graphene, TiO2 nanotube arrays, metal-organic composites, multiwall carbon nanotubes, liposomes, photolummonescent carbone nanocrystals are presented as a table. Considerable efforts have also been devoted towards the following two key points: multiplexing analysis (multi-label, and the multianalyte strategies) and integration in microfluidic lab-on-paper devices with capabilities for point-to-care diagnostics. An immuno-like electrochemiluminescent sensor (based on synthetic receptors-molecularly imprinted polymers), as a new alternative to traditional electrochemiluminescent immunoassay is highlighted. Future perspectives and possible challenges in this rapidly developing area are also discussed.

Application of Ganglioside-Sensitized Liposomes in a Flow Injection Immunoanalytical System for the Determination of Cholera Toxin

Cholera, an acute infectious disease associated with water and seafood contamination, is caused by the bacterium Vibrio cholerae, which lives and colonizes in the small intestine and secretes cholera toxin (CT), a causative agent for diarrhea in humans. Based on earlier lateral flow assays, a flow injection liposome immunoanalysis (FILIA) system with excellent sensitivity was developed in this study for the determination of CT at zeptomole levels. Ganglioside (GM1), found to have specific affinity toward CT, was inserted into the phospholipid bilayer during the liposome synthesis. These GM1-sensitized, sulforhodamine B (SRB) dye-entrapping liposomes were used as probes in the FILIA system. Anti-CT antibodies were immobilized in its microcapillary. CT was detected by the formation of a sandwich complex between the immobilized antibody and GM1 liposomes. During the assay, the sample was introduced first into the column, and then liposomes were injected to bind to all CT captured by the antibody in the microcapillary. Subsequently, the SRB dye molecules were released from the bound liposomes via the addition of the detergent octyl glucopyranoside. The released dye molecules were transported to a flow-through fluorescence detector for quantification. The FILIA system was optimized with respect to flow rate, antibody concentration, liposome concentration, and injected sample volume. The calibration curve for CT had a linear range of 10-16 to 10-14 g mL-1. The detection limit of this immunosensor was 6.6 x 10(-17) g mL-1 in 200-microL samples (equivalent to 13 ag or 1.1 zmol).

Liposome-based microcapillary immunosensor for detection of Escherichia coli O157:H7

Our group has previously reported a sandwich-based strip immunoassay for rapid detection of Escherichia coli O157:H7 [Anal. Chem. 75 (2003) 4330]. In the present study, a microcapillary flow injection liposome immunoanalysis (mFILIA) system was developed for the detection of heat-killed E. coli O157:H7. A fused-silica microcapillary with anti-E. coli O157:H7 antibodies chemically immobilized on the internal surface via protein A served as an immunoreactor/immunoseparator for the mFILIA system. Liposomes tagged with anti-E. coli O157:H7 and encapsulating a fluorescent dye were used as the detectable label. In the presence of E. coli O157:H7, sandwich complexes were formed between the immobilized antibodies in the column, the sample of E. coli O157:H7 and the antibody-tagged sulforhodamine-dye-loaded liposomes. Signals generated by lysing the bound liposomes with 30 mM n-octyl-beta-D-glucopyranoside were measured by a fluorometer. The detected signal was directly proportional to the amount of E. coli O157:H7 in the test sample. The mFILIA system successfully detected as low as 360 cells/mL (equivalent to 53 heat-killed bacteria in the 150 microL of the sample solution injected). MeOH (30%) was used for the regeneration of antibody binding sites in the capillary after each measurement, which allowed the immunoreactor/immunoseparator to be used for at least 50 repeated assays. The calibration curve for heat-killed E. coli O157:H7 has a working range of 6 x 10(3)-6 x 10(7)cells, and the total assay time was less than 45 min. A coefficient of variation for triplicate measurements was < or =8.9%, which indicates an acceptable level of reproducibility for this newly developed method.

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.

Detection of lytic antimycobacterial antibodies by immune lysis of liposomes sensitized to tuberculin

Two procedures were used in order to incorporate purified protein derivative tuberculin (PPD) from M. tuberculosis, strain H37Rv, into calcein-containing liposomes: formation of multilamellar vesicles (MLV) in a PPD solution or exposure of preformed MLV to this solution. Immune lysis of these PPD-sensitized MLV was studied in the presence of a hyperimmune anti-M. tuberculosis sheep serum using a specific pathogen-free rabbit serum as a source of complement. A 50% release of encapsulated calcein was observed spectrofluorometrically after 30 min and remained unchanged up to 2 h. The release of calcein in the absence of complement or of anti-H37Rv serum or by liposomes which did not contain PPD never exceeded 1-2%. Liposomes formed in PPD solution were more sensitive to anti-H37Rv serum than preformed liposomes exposed to PPD. Trials with human sera from ten tuberculous patients revealed the presence of specific lytic immunoglobulins. In the presence of sera from skin test negative, non-tuberculous subjects, calcein release was significantly lower. This opens the way to a new method for the study of the humoral immunity in tuberculosis.

The use of a fluorescent lipid as a non-covalent bound tracer of integral membrane proteins

Solubilized and affinity purified major histocompatibility class I and class II antigens (MHC-I, MHC-II) were incorporated into liposomes, which contained a fluorescent lipid, by adsorption of the detergent to Biobeads SM-2. The fluorescent lipid was used as a non-covalently bound tracer to detect antibody-mediated agglutination of liposomes with MHC antigens. After incorporation of the membrane proteins into liposomes of different lipid composition, phosphatidylcholine was chosen as a lipid matrix. The use of a fluorescent lipid as a tracer of membrane proteins was demonstrated by the detection of human beta 2-microglobulin association with mouse MHC-I. Furthermore the method was used to measure the amount of liposomes containing two different membrane proteins.

A sensitive method to introduce membrane-bound proteins into recipient cells based on affinity enrichment of lipid vesicles to the recipient cell prior to fusion

A sensitive analytical procedure for studying membrane-bound structures has been developed. Membrane glycoproteins inserted into liposomes were transferred to recipient cells by use of a lectin, concanavalin A, bound to the cells as a bridge to generate proximity between the recipient cell and the glycoprotein-containing liposome, prior to exposure to the fusing agent, poly(ethylene glycol). Partially purified histocompatibility antigen from rats was introduced into the membrane of human lymphocytes. After treating the cells with poly(ethylene glycol) under fusion conditions, some of the antigen present in the preparation could not be eluted with alpha-methyl mannoside and EDTA, indicating that incorporation in the cell membrane had taken place. This antigen remained exposed on the lymphocyte surface for approximately 1 h as demonstrated by sensitivity of the lymphocytes to the lytic effect of an antiserum to the histocompatibility antigen in the presence of complement. Some of the lectin molecules seemed to be internalized in the cells but no induction of cell mitosis was observed. The described method gives an opportunity to work with small amounts of membrane proteins inserted into liposomes, introducing them into recipient cells for analysis of their biological activities.

Lectin-mediated binding of liposome-inserted membrane proteins to red blood cells. A method to detect binding of antibodies to purified rat histocompatibility antigen or binding of insulin to the insulin receptor

Partially purified membrane proteins such as the rat RT-1 histocompatibility antigen or the insulin receptor of porcine liver were inserted into liposomes. These liposomes then bound efficiently to Con A-coated red blood cells. After attachment of the liposomes to the cells, cell-liposome conjugates could be separated from free liposomes by centrifugation. Binding of FITC-labeled specific antibodies or insulin to membrane proteins inserted into the liposomes could then be analyzed with a cell flow cytofluorometer. The amount of RT-1 antigen that became associated with the cells depended on the composition and concentration of phospholipids during liposome formation. No association of RT-1 antigen to the cells was obtained in the presence of 50 mM alpha-methyl mannoside. The technique allows detection of micrograms amounts of the histocompatibility antigen associated with the red blood cells. It was possible to detect binding of insulin to cells to which approximately 9 ng (3 X 10(-14) mol) insulin receptor/10(6) cells had been attached. This amount of membrane protein was close to the detection limit of the method.

Inhibition of antibody binding to viable cells by liposomes containing purified rat spleen glycoprotein

The capacity of liposomes with inserted RT-1 histocompatibility antigen to bind anti-RT-1 antibodies varies depending on their lipid composition and mode of preparation. The binding capacity of liposomes prepared by dialysis is different from that of liposomes prepared by gel filtration or of small membrane protein micelles. A new assay for analysis of membrane antigens has been developed. Liposomes are used to compete for antibody binding with in vitro cultured cells while binding of antibody to the same adherent liposomes is simultaneously assayed.