Fluorescence-linked immunosorbent assay (FLISA) for quantification of antibodies to food antigens

Binding Characteristic of Various Antibody Formats Against Aflatoxins

The application of recombinant antibodies for the analysis of foods and food contaminants is now a major focus, given their capacity to be engineered to tailor their specificity, enhance their stability, and modify their structural formats to fit the desired analytical platform. In this study, human scFv antibody fragments generated against aflatoxin B1 (AFB1) were selected as the model antibody to explore the effect of antibody formats on their binding activity and to evaluate their potential use as immunoreagents for food contaminant analysis. Four human scFv antibody fragments against aflatoxin B1 (AFB1), previously isolated and engineered by chain shuffling, were converted into various formats, that is, scFv-AP fusions, scFv-Fc, and whole IgG molecules. The result indicated that the effects of the antibody format on the binding property varied, depending on the sequence of scFv. For all of the scFv clones, the scFv-AP fusion format showed the highest sensitivity by competitive ELISA, while the effects on the binding activity after conversion to scFv-Fc or IgG format varied, depending on the amino acid sequence of the antibodies. The sAFH-3e3 antibodies that showed the best performance by competitive ELISA were selected for further investigation. The sAFH-3e3 was converted to the scFv-GFP format and tested by fluorescence-linked immunosorbent assay (FLISA), which showed that its binding property was equivalent to those of scFv-Fc and IgG formats. The potential applications of the sAFH-3e3 in a rapid test kit format based on ELISA (scFv-AP) and in a lateral flow immunochromatography assay (LFIA) (IgG) were demonstrated. A comparison of methods for the extraction of AFB1 from matrices for use with these assay formats indicated that PBS and TBST are better than 70% methanol.

Fluorescent labeling of antibody fragments using split GFP

Antibody fragments are easily isolated from in vitro selection systems, such as phage and yeast display. Lacking the Fc portion of the antibody, they are usually labeled using small peptide tags recognized by antibodies. In this paper we present an efficient method to fluorescently label single chain Fvs (scFvs) using the split green fluorescent protein (GFP) system. A 13 amino acid tag, derived from the last beta strand of GFP (termed GFP11), is fused to the C terminus of the scFv. This tag has been engineered to be non-perturbing, and we were able to show that it exerted no effect on scFv expression or functionality when compared to a scFv without the GFP11 tag. Effective functional fluorescent labeling is demonstrated in a number of different assays, including fluorescence linked immunosorbant assays, flow cytometry and yeast display. Furthermore, we were able to show that this split GFP system can be used to determine the concentration of scFv in crude samples, as well an estimate of antibody affinity, without the need for antibody purification. We anticipate this system will be of widespread interest in antibody engineering and in vitro display systems.

Fluorescence linked immunosorbant assays using microtiter plates

Fluorescence methods are widely used in the detection of antibodies and other binding events. However, as a general screening and detection tool in microtiter plates, enzyme linked immunosorbant (ELISA) methods predominate. In this paper we explore all parameters for effective use of fluorescence as a plate based detection method, including which microtiter plates can be used, the most effective means of immobilization, and the use of different fluorescent dyes or fluorescent proteins. These studies indicate that fluorescent immunosorbant assays (FLISA) can be used as effectively as enzymatic method in microtiter plate based screening methods, including the screening of phage antibody selections.

Plastic versus glass support for an immunoassay on metal-coated surfaces in optically dense samples utilizing directional surface plasmon-coupled emission

We compared plastic (polycarbonate) and high-quality glass support materials for gold-coated slides, when performing a model immunoassay against rabbit IgG using fluorescently labeled (AlexaFluor-647) anti-rabbit IgG, and detecting surface plasmon-coupled emission (SPCE) signals. Both, glass and plastic slides were simultaneously coated with a 48-nm layer of gold and protected with a 10-nm layer of silica. The maximum SPCE signal of AlexaFluor-647 was only two- to three-fold smaller on plastic slides than on glass slides. A small difference in the SPCE angles on glass (theta (F) = 55 degrees ) and plastic (theta (F) = 52.5 degrees ) slides was observed and can be explained with a slightly smaller refractive index of the plastic. We have not found any difference in the angle distribution (sharpness of the fluorescence signal at optimal SPCE angle) for the plastic slide compared to the glass slide. The kinetics of binding was monitored on the plastic slide as well as on the glass slide. Optically dense samples, a 4% red blood cell suspension and a 15% hemoglobin solution, are causing a reduction in the immunoassay SPCE signal by approximately 15% and three times, respectively, and the percentage of the reduction is the same for plastic and for glass slides. We believe that plastic substrates can be readily used in any SPCE assay, with only marginally lower total signal compared to high-quality glass slides.