We examine the binding of fluorescent ligands to proteins by analyzing the fluctuation amplitude g(0) of fluorescence fluctuation experiments. The normalized variance g(0) depends on the molecular brightness and the concentration of each species in the sample. Thus a single g(0) measurement is not sufficient to resolve individual species. Titration of the ligand with protein establishes the link between molecular brightness and concentration by fitting g(0) to a binding model and allows the separation of species. We first apply g(0) analysis to binary dye mixtures with brightness ratios of 2 and 4 to demonstrate the feasibility of this technique. Next we consider the influence of binding on the fluctuation amplitude g(0). The dissociation coefficient, the molecular brightness ratio, and the stochiometry of binding strongly influence the fluctuation amplitude. We show that proteins with a single binding site can be clearly differentiated from proteins with two independent binding sites. The binding of fluorescein-labeled digoxigenin to a high-affinity anti-digoxin antibody was studied experimentally. A global analysis of the fluctuation amplitude and the fluorescence intensity not only recovered the dissociation coefficient and the number of binding sites, but also revealed the molecular heterogeneity of the hapten-antibody complex. Two species were used to model the molecular heterogeneity. We confirmed the molecular heterogeneity independently by fluorescence lifetime experiments, which gave fractional populations and molecular brightness values that were virtually identical to those of the g(0) analysis. The identification and characterization of molecular heterogeneity have far-reaching consequences for many biomolecular systems. We point out the important role fluctuation experiments may have in this area of research.

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Background: Organic solvents used for extraction of tacrolimus from whole blood samples lower the apparent affinity of the antibody used in a diagnostic immunoassay, thereby affecting the detection limit. Methods: We used in vitro recombinant antibody engineering to screen and isolate clones from diverse libraries with mutagenic complementarity regions (CDRs) from tacrolimus 1-60-46 hybridoma cell line, with improved binding to tacrolimus in the presence of 10% methanol organic solvent solution. Results: We isolated a number of clones with mutations in variable heavy (VH) CDR 2, variable light (VL) CDR 1, and VL CDR 3 with improved binding. Various combinatorial pairings constructed from these individual mutations contained >10-fold improvements in both the dissociation rate and overall equilibrium affinity constants. Selected clones produced as IgG have increased functional sensitivity, with a 3- to 6-fold reduction in the limit of detection relative to the parental tacrolimus 1-60-46 monoclonal antibody in the Architect® Tacrolimus immunodiagnostic assay. Conclusions: The recent advent of recombinant in vitro antibody display technologies in general, and yeast surface display in particular, allows the flexibility to engineer new or augment specific analytical characteristics, such as affinity, specificity, or stability, into previously isolated and otherwise desirable antibodies to enhance assay performance. These in vitro selections can also be performed under conditions meant to mimic the assay in which the reagent will ultimately be used, to increase the likelihood of successful assay development.

Mouse-human chimeric antibodies (cAbs) against hepatitis C virus (HCV) core, NS3 (nonstructural), NS4, and NS5 antigens were developed as quality control (QC) reagents to replace the use of human sera/plasma for Abbott HCV immunoassays. The cAb retains the mouse monoclonal antibody (MAb) specificity and affinity but still reacts in the existing HCV assay format, which measures human anti-HCV immunoglobulin. Mouse heavy-chain (V H ) and light-chain (V L ) variable regions of anti-HCV core, NS3, NS4, and NS5 antigens were PCR amplified from hybridoma lines and then cloned with human IgG1 heavy-chain (C H ) and light-chain (C L ) constant regions, respectively. A single mammalian expression plasmid containing both heavy-chain and light-chain immunoglobulin genes was constructed and transfected into dihydrofolate reductase (DHFR)-deficient Chinese hamster ovary (CHO) cells. The transfected CHO cells were selected using hypoxanthineand thymidine-free medium and screened by an enzyme immunoassay (EIA). The clone secreting the highest level of antibody was isolated from the CHO transfectants and further subcloned. Each cAb-expressing CHO cell line was weaned into serum-free medium, and the cAb was purified by protein A affinity chromatography. The levels of cAb production for the various CHO cell lines varied from 10 to 20 mg/liter. Purified anti-HCV cAbs were tested with Abbott HCV immunoassays and showed reactivity. Moreover, yeast surface display combined with alanine-scanning mutagenesis was used to map the epitope at the individual amino acid level. Our results suggest that these HCV cAbs are ideal controls, calibrators, and/or QC reagents for HCV assay standardization.Infection with hepatitis C virus (HCV) causes an inflammation of the liver and is the most common chronic blood-borne infection in the United States. According to the U.S. Centers for Disease Control and Prevention, approximately 1.8% of the U.S. population, or 3.9 million Americans, have been infected with the virus. About 35,000 new cases of HCV are estimated to occur in the United States each year. Common routes of infection include needle stick accidents, blood transfusions, and injection drug use. Most individuals acutely infected with HCV become chronically infected. Once a person is chronically infected, the virus is almost never cleared without treatment (20).Abbott HCV immunoassays designed to detect anti-HCV antibodies in patient samples provide a fast and reliable serological diagnostic method. Typically, diagnostic kits contain one or more antibodies as calibrators/positive controls. Traditionally, these controls consist of human plasma and/or serum samples from infected individuals. The quality control (QC) reagents, such as assay sensitivity panels, are human plasma/ serum samples selected for antibodies against HCV core, NS3 (nonstructural), NS4, and NS5 antigen epitopes. However, the use of human serum/plasma has several significant disadvantages, including increasing regulatory concerns about patient sample drawing, sample storage a...

Development of a robust immunoassay requires the selection of monoclonal antibodies with desired properties. These properties generally include kinetics parameters such as on-rate and off-rate (i.e., binding affinity), and, often times, the ability to form a sandwich with the analyte of interest. We sought to obtain antibodies suitable for development of an immunoassay capable of detecting human neutrophil gelatinase-associated lipocalin (NGAL), a glycosylated lipocalin of 25 kDa expressed in kidney tubules in response to injury that has been shown to be a urinary biomarker capable of diagnosing acute kidney injury. We immunized CAF1/J and RBF/DnJ mouse strains with recombinant NGAL, and a robust immune response, as measured by serum antibody titer, was observed among all CAF1/J mice. Antibodies secreted from mouse B cell-myeloma hybridomas were screened by enzyme immunoassay (EIA) and by surface plasmon resonance using a method we termed hybrid supernatant kinetic screening. Approximately 300 hybrid clones were evaluated by this technique to identify antibodies with the kinetic binding parameters meeting criteria required for further assay development (i.e., rapid association and slow dissociation). This data, along with epitope grouping, cell growth, cell viability, and antibody secretion, were used to identify antibodies for testing in the ARCHITECT assay.

Monoclonal antibodies were developed to a recombinant HIV-I group O envelope protein derived from the isolate HAM112. These monoclonal antibodies were characterized for reactivity to a series of overlapping synthetic peptides (29-30 mers) covering gp120 C-terminal and gp41 ectodomain regions of the HIV-1 group O envelope protein. Most of these monoclonal antibodies reacted with peptides spanning sequences analogous to HIV-1 group M epitopes identified from studies in mice and humans. However, several of the antibodies that were nonreactive to individual peptides did react to a mixture of longer peptides from the N-terminal and C-terminal helical regions of the gp41 ectodomain. The monoclonal antibodies described in this study are valuable tools for characterization of antigenic differences between HIV-1 group O and group M viruses.

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Several adjuvant regimens were assessed for their ability to safely develop a high titer, high affinity antibody response in rabbits. Female, New Zealand White (NZW) rabbits were given five monthly 20 ug immunizations with a 25 KDa recombinant protein using one of four adjuvant programs: Adjulite® Complete and Adjulite® Incomplete Freund’s adjuvants,Alhydrogel® Aluminum hydroxide gel adjuvant in combination with oligodeoxynucleotides containing unmethylated CpG dinucleotides (Alum/CpG),Difco Freund’s adjuvant alternating with MPL+TDM+CWS orDifco Freund’s adjuvant alternating with Quil A supplemented with CpG-ODN. Sera samples taken following all booster immunizations were evaluated for antibody titer and relative affinity in a microtiter enzyme immunoassay by testing for antibody reactivity to limiting amounts of the administered antigen. Animals were also monitored during the immunization period for any reaction at the sites of injection. Results show that the Adjulite® animals elicited a higher average titer compared to animals immunized using any of the alternate strategies. Additionally, Adjulite® rabbits developed an average relative antibody affinity similar to that for the Alum/CpG animals and higher than either of the groups employing Difco Freund’s adjuvant. One Adjulite® rabbit developed a temporary nodule at the site of injection compared to none of the Alum/CpG animals. Two of the five animals using Difco Freund’s adjuvant developed a nodule, both of which progressed to lesions. This study demonstrates that Adjulite® adjuvant can serve as a safe alternative to other adjuvant regimens for the development of a high titer, high affinity antibody response in rabbits.