A homogeneous immunoassay for the mycotoxin T-2 utilizing liposomes, monoclonal antibodies, and complement

Nanoliposome based biosensors for probing mycotoxins and their applications for food: A review

Mycotoxins are the most common feed and food contaminants affecting animals and humans, respectively; continuous exposure causes tremendous health problems such as kidney disorders, infertility, immune suppression, liver inflammation, and cancer. Consequently, their control and quantification in food materials is crucial. Biosensors are potential tools for the rapid detection and quantification of mycotoxins with high sensitivity and selectivity. Nanoliposomes (NLs) are vesicular carriers formed by self-assembling phospholipids that surround the aqueous cores. Utilizing their biocompatibility, biodegradability, and high carrying capacity, researchers have employed NLs in biosensors for monitoring various targets in biological and food samples. The NLs are used for surface modification, signal marker delivery, and detection of toxins, bacteria, pesticides, and diseases. Here, we review marker-entrapped NLs used in the development of NL-based biosensors for mycotoxins. These biosensors are sensitive, selective, portable, and cost-effective analytical tools, and the resulting signal can be produced and/or amplified with or without destroying the NLs. In addition, this review emphasizes the benefits of the immunoliposome method in comparison with traditional detection approaches. We expect this review to serve as a valuable reference for researchers in this rapidly growing field. The insights provided may facilitate the rational design of next-generation NL-based biosensors.

Development of Liposome Immune Lysis Assay for the Herbicide Atrazine

A homogeneous complement-mediated liposome immune lysis assay (LILA) was developed for determination of the herbicide atrazine. To dispose the antigen on the surface of lipid bilayer the atrazine was conjugated to a dimirystoylphosphatidylethanolamine (DMPE) carrier. Calcein was compared with sulforhodamine 101 as a fluorophore label for entrapping into the antigen-sensitized liposomes. The liposomes were incubated with rabbit anti-atrazine antibodies in the presence of guinea pig complement. Formation of the antigen-antibody complexes on the liposomal surface initiated the lytic action of the complement. As free competing atrazine inhibited the lytic reaction, the amount of calcein released was inversely proportional to the atrazine content in the probe. Concentration and kinetic dependences of the immunoassay were characterized to reach its maximal sensitivity. The developed assay allows detecting atrazine in concentrations up to 0.13 ng mL(-1) in the sample (0.04 ng mL(-1) in the final reaction mixture). The named sensitivity is two orders higher than those for the microplate enzyme-linked immunosorbent assay (ELISA) with the same antibodies which allows us to recommend LILA for environmental monitoring.

Novel assays and sensor platforms for the detection of aflatoxins

The importance of the aflatoxins from food safety and economic standpoints has continued to drive the development of new analytical methods for these mycotoxins. Currently the widely used methods for measurement of aflatoxins fall into two groups, the established chromatographic methods and traditional enzyme-linked immunosorbent assays (ELISAs). Recently substantial progress has been made in the application of new technologies to the monitoring of aflatoxins. In particular, several research groups have developed biosensors for detection ofthe toxins as well as presumptive tests for fungal infection. Biosensors have been developed in a variety of formats including surface plasmon resonance, fiber optic probes, and microbead-based assays. The sensitivity and selectivity of the biosensors and of the presumptive tests has reached the level the where the application of these techniques to the screening of foods warrants further investigation.

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 homogeneous fluorescence polarization assay for detection of antibody to Brucella abortus

A homogeneous fluorescence polarization (FP) assay (FPA) was developed for detection of antibody in bovine sera to Brucella abortus. The assay used O-polysaccharide prepared from B. abortus lipopolysaccharide in the molecular weight range of 20-30 kDa which was conjugated with fluorescein isothiocyanate and used as a tracer. Fluorescence polarization was measured with a FPM-1 fluorescence polarization analyzer. Sample (20 microliters) was added to 2.0 ml of diluent buffer at ambient temperature. A serum blank reading was taken and tracer (10 microliters) to yield approx. 1.5 nM fluorescein equivalents was added. The FP of the tracer was determined after a period of greater than 2 min. A positive reaction was indicated by a significant elevation of the FP reading over the negative control. In a blind study, 9480 bovine sera were tested in addition to sets of four controls which were included with each lot of 100 samples tested. The controls were a strong positive, a weak positive, a negative and a serum derived from a B. abortus strain 19 vaccinated cow. Test sera included 8669 sera from Canadian cattle which were negative by routine serological tests, 561 sera from cows from which B. abortus had been isolated either from tissues or milk and 250 sera from cattle previously vaccinated with B. abortus strain 19 at various times. One lot of O-polysaccharide tracer was used for all tests. The initial cut-off for negative samples in the fluorescence polarization assay was set at 107.2 mP. This resulted in a sensitivity estimate of 98.1 +/- 1.1% and the specificity was 99.8 +/- 0.09%. After decoding the samples and retesting false positive and negative reactions, the sensitivity estimate was 98.5 +/- 1.0% and the specificity was 100%. It became evident that the initial cut-off value was set too high and, using ROC analysis, a cut-off of 90 mP increased the sensitivity to 99.02% while the specificity decreased to 99.96%. Of the 250 sera from vaccinated cattle, 248 were negative giving a point specificity value of 99.2%.

Urinary microalbumin measurement using a homogeneous liposomal immunoassay

A homogeneous colorimetric immunoassay which has been developed for urinary microalbumin utilizes complement-mediated immunolysis of liposomes containing the dye, sulphorhodamine B. Unlike a previously described model complement-mediated liposomal assay for serum albumin (Frost et al., 1994) which was competitive, this assay uses a sandwich-type format and Fab' (antialbumin)-coated liposomes to increase the assay sensitivity. The liposomal assay, performed using a Cobas Bio analyser (Roche, Welwyn Garden City, UK), gave an acceptable correlation with a radioimmunoassay (NETRIA, London, UK): r = 0.94; y (liposomal assay) = 1.09 x (radioimmunoassay) - 1.54 mg/1. The imprecisions of the assays were similar and matrix effects due to the use of urine samples were determined to be acceptably small. The assay demonstrates the advantage of using Fab'-coated liposomes in sandwich-type liposomal immunoassays over liposomes coated with intact antibody, which failed to elicit complement-mediated immunolysis.

Liposome immunoassay (LIA) with antigen-coupled liposomes containing alkaline phosphatase

Immunoliposomes were prepared and their immunoassay applications investigated. Liposomes were prepared from cholesterol and phospholipids including maleimidobenzoylphosphatidylethanolamine (MBPE) for conjunction with thiol-containing antigens. Alkaline phosphatase (APase) was entrapped in the liposome and BSA, the antigen, was modified by reaction with 3-(2-pyridyl-dithio)propionyl N-hydroxysuccinimide ester (SPDP) to introduce thiol groups for efficient coupling. BSA-coupled liposomes (immunoliposomes) were incubated with anti-BSA serum, complement, and then with APase substrate. The amount of coupled BSA was affected by the reaction time, the composition of the liposome and the BSA concentration in the reactant. The amount of enzyme released from immunoliposomes as a final result of the immunoreaction increased with increasing concentrations of complement and antibody. The liposome immunoassay offers a relatively rapid and simple testing procedure to quantitatively or qualitatively determine the presence or absence of antibodies, or antigenic materials for diagnostic purposes.

The use of streptavidin-biotin interaction for preparation of reagents for complement-dependent liposome immunoassay of proteins: detection of latrotoxin

We have developed liposome sensitization by a protein, latrotoxin (LT), using immobilization of biotinylated LT via streptavidin with biotinylated phosphatidylethanolamine contained in liposomes. The use of such liposomes in the complement-dependent homogeneous liposome immune lysis assay (LILA) has allowed us to detect in the test sample as little as 2 micrograms/ml of polyclonal and 50-100 ng/ml of monoclonal IgG and IgM antibodies to LT. LT concentration in solution was determined by inhibition of immune lysis by free LT. The sensitivity of the LT assay varied from 1 x 10(-9) to 5-50 x 10(-9) M when antiserum (polyclonal antibodies) and monoclonal antibodies to LT were correspondingly used. The results show that a streptavidin-biotin spacer can be used to immobilize protein antigens on liposomes for a subsequent application in LILA. The suggested technique greatly simplifies the sensitization procedure and extends the applicability of the LILA.

Anti-idiotypic antibodies against a monoclonal antibody specific for the trichothecene mycotoxin T-2

A BALB/c murine monoclonal antibody against the trichothecene mycotoxin T-2 was generated. The antibody, designated HD11, specifically bound T-2 mycotoxin. The binding of HD11 to T-2 conjugated to bovine serum albumin was inhibited by free T-2 toxin but not by the water-soluble heterocyclic guanidines saxitoxin and tetrodotoxin. The T-2 detection limit in an enzyme-linked immunosorbent assay with HD11 was in the nanogram range. The in vitro cytotoxicity of T-2, as measured by the inhibition of radiolabeled leucine uptake of the human epidermoid carcinoma Hep-2 and KB cell lines, was completely reversed by the addition of HD11. Rabbit anti-idiotypic antibodies specific for HD11 were generated and characterized.