A sequential binding assay with a working range extending beyond seven orders of magnitude

Ultrasonic enhancement of bead-based bioaffinity assays

Ultrasonic radiation forces can be used for non-intrusive manipulation and concentration of suspended micrometer-sized particles. For bioanalytical purposes, standing-wave ultrasound has long been used for rapid immuno-agglutination of functionalized latex beads. More recently, detection methods based on laser-scanning fluorometry and single-step homogeneous bead-based assays show promise for fast, easy and sensitive biochemical analysis. If such methods are combined with ultrasonic enhancement, detection limits in the femtomolar region are feasible. In this paper, we review the development of standing-wave ultrasonic manipulation for bioanalysis, with special emphasis on miniaturization and ultrasensitive bead-based immunoassays.

Fluorescence-microscopy-based image analysis for analyte-dependent particle doublet detection in a single-step immunoagglutination assay

A novel fluorescence-microscopy-based image analysis method for classification of singlet and doublet latex particles is demonstrated and applied to a particle-based immunoagglutination assay for quantification of biomolecules in microliter-volume bulk samples. The image analysis method, verified by flow cytometric agglutination analysis, is based on a pattern recognition algorithm employing Gaussian-base-function fitting which allows robust identification and counting of singlets, doublets, and higher agglomerates of fluorescent microparticles. The immunoagglutination assay is experimentally modeled by a biotin-streptavidin interaction, with the goal of both theoretically and experimentally investigating the performance of a general immunoagglutination-based assay. For this purpose a theoretical model of the initial agglutination kinetics, based on particle diffusion combined with a steric factor determined by the level of specific and nonspecific agglutination, was developed. The theoretical model combined with the experimental data can be used to optimize an agglutination-based assay with regard to sensitivity and dynamic range and to estimate the affinity, receptor surface density, molecular and binding site sizes, and level of nonspecific binding that is present in the assay. The experimental results are in good agreement with the theoretical model, indicating the usefulness of the model for immunoagglutination assay optimization.

Flow cytometric analysis of immunoprecipitates: high-throughput analysis of protein phosphorylation and protein-protein interactions

BACKGROUND

Activation-induced protein phosphorylation can be studied by Western blotting, but this method is time consuming and depends on the use of radioactive probes for quantitation. We present a novel assay for the assessment of protein phosphorylation based on latex particles and flow cytometry.

METHODS

This method employs monoclonal antibodies coupled to latex particles to immobilize protein kinase substrates. Their phosphorylation status is assessed by reactivity with phosphoepitope-specific antibodies. The amount of immobilized protein on the particles was analyzed by direct or indirect immunofluorescence with antibodies to nonphosphorylated epitopes.

RESULTS

The assay allowed measurement of phosphorylation of multiple protein kinase substrates in stimulated T cells, including the zeta chain of the T-cell receptor, ZAP-70, CD3, CD5, SHP-1, and ERK-2, using 1-3 microg of total cell protein per sample. The assay provided high resolution of kinetics of phosphorylation and dephosphorylation. Interactions of protein kinase substrates with associated signaling molecules were demonstrated.

CONCLUSIONS

The novel assay allows high-throughput quantitative measurement of protein modifications during signal transduction.

A flow cytometric immunoassay for beta2-microglobulin in whole blood

Flow cytometers can discriminate a single particle type in an unwashed whole blood sample. Utilizing this capability. we devised a homogeneous bead-immobilized sandwich immunoassay for soluble beta2M (beta2-microglobulin) in whole blood, utilizing an antibody that discriminates soluble from cellular beta2M. A 4 microm bead was chosen that fluoresces only in a FACScan flow cytometer's FL3 channel. thus allowing triggering on this bead to the exclusion of the many blood cell events. The bead was adsorbed with a capture antibody (clone A7801) which binds only to soluble and not to cellular beta2M. This antibody appears to recognize an epitope on beta2M which interfaces to the heavy chain of cellular Class I MHC molecules. The signal antibody (PE conjugate of clone L376, emitting in the FL2 channel) binds to both soluble and cellular beta2M (present in roughly equal amounts in normal blood). The various parameters required for a flow cytometric immunoassay were optimized. The 4 microm sized bead was adequately large to give a near full scale signal at saturation. The relative amounts of signal antibody and capture beads were balanced to give a low blank, minimal 'hook effect', and reasonable event rate on the flow cytometer. The amount of blood added was selected to give a signal near the bottom of the immunassay range for normals with the higher range available for clinical samples. The assay requires no washing, minimizes blood handling, and has a working range (2.5 decades) that is compatible with the biological range of beta2M concentrations with a single blood dilution.

Dual analyte assay based on particle types of different size measured by flow cytometry

Simultaneous flow cytometric assays have been developed for alpha-fetoprotein (AFP) and human chorionic gonadotropin (hCG), with internal determination of sample related non-specific binding (NSB). The assays use particles of 7.5, 6.5 and 5.5 microns diameter coated with, respectively, monoclonal antibodies specific for AFP, hCG or an epitope normally not present in serum. The different particle types were identified simultaneously by light-scatter measurements as their specific immunofluorometric responses were determined. The NSB in the simultaneous assay of AFP and hCG was increased by approximately 30% compared to corresponding single analyte assays. The working range of the dual analyte assays was 0.6-2000 kIU/l for AFP and 6-10,000 IU/l for hCG. No significant interference from the presence of the other analyte was observed in the measurement of either AFP or hCG. The 95% confidence interval for the ratio of dual over single analyte assay results was [0.81, 1.11] for AFP and [0.88, 1.16] for hCG.