Measurement of affinity of viral monoclonal antibodies by ELISA titration of free antibody in equilibrium mixtures

Description of a non-competitive ELISA based on time course analysis of ligand binding at saturation, and a direct method for calculating the affinity of monoclonal antibodies

We present a time-course saturation ELISA for measuring the equilibrium constant of the monoclonal antibody (mAb) SIM 28 against horse radish peroxidase (HRP). The curves of HRP binding to a series of fixed mAb dilutions were plotted to completion, and the Kt (= Ks) value (time to occupy 50 % of the mAb paratopes) was determined for each mAb dilution and HRP concentration. Analysis of the kinetic mechanism of the reaction by Lineweaver-Burk and Hanes plots showed that the slope and y-intercept were affected, indicating that mAb ligand saturation follows non-competitive inhibition kinetics in this assay format. In this kinetics, the inhibition constant Ki (= Kd) is the time required to double the slope or halve the Vmax of the Lineweaver-Burk plot. The Kt values of the time courses were doubled (2 x Kt) and normalized by dividing by the total reaction time to obtain a unitless factor which, when multiplied by the concentration of HRP, gives the Ki. The affinity constant of mAb SIM 28 was determined from ELISA data (n = 16) by three methods: i) doubling of Kt, ii) Beatty equation (Kaff = (n-1)/2 (n [HRP']t - [HRP]t), and iii) SPR (Biacore) analysis. The calculated affinities (mean ± 95 % confidence limits) were i) 4.6 ± 0.67 × 10-9 M, ii) Kaff = 0.23 ± 0.03 × 109 M-1 (Kd = 4.8 ± 0.81 × 10-9 M), and iii) 4.3 ± 0.57 × 10-9 M, respectively. The similar results obtained with the three different techniques indicate that this time-course saturation ELISA, combined with the double Kt method, is a repeatable and direct approach to mAb affinity determination.

Development of a competitive inhibition kinetic ELISA to determine the inhibition constant (Ki) of monoclonal antibodies

Antibody-antigen interactions are mediated by the same molecular recognition mechanisms as those of an enzyme and its substrate. On this basis, we developed a competitive inhibition kinetic ELISA to measure monoclonal antibody (mAb) inhibition constants. Serially diluted samples of ligand (mAb) and inhibitor (soluble antigen) were incubated to equilibrium in ELISA plates coated with a fixed concentration of antigen (receptor). Plates were washed, and bound mAb measured with antiglobulin-peroxidase. Initial velocity data of receptor-bound mAb at various ligand and inhibitor concentrations were analyzed with enzyme linear competitive inhibition methods by non-linear regression (NLR), linear transformations (Cornish-Bowden, Lineweaver-Burk, Hanes-Woolf, Dixon, Cortés [1/i_0.5 vs. V_i/V_max], Ascenzi [K_s/V_max/K_s,0/V_max vs. [I]]) and NLR IC₅₀ plots, to derive mAb inhibition constants (K_i)_. We obtained similar mAb K_i and K_d values by ELISA and surface plasmon resonance, which confirmed the accuracy of the ELISA method. This competitive inhibition ELISA is a simple (it requires no labeling or prior knowledge of antibody concentration), sensitive (it detects K_i values in the low nanomolar range by conventional colorimetry), and reproducible method with which to calculate mAb inhibition constants.

Development of a sensitive monoclonal antibody-based sandwich ELISA to detect Vip3Aa in genetically modified crops

Objectives

To develop a sensitive monoclonal antibody-based sandwich enzyme-linked immunosorbent assay (ELISA) to detect Vip3Aa in genetically modified (GM) crops and their products.

Results

Vegetative insecticidal proteins (Vips) are secreted by Bacillus thuringiensis (Bt) and are known to be toxic to Lepidoptera species. Vip3Aa family proteins, Vip3Aa19 and Vip3Aa20, were successfully applied in GM crops to confer an effective and persistent insecticidal resistance. A sensitive monoclonal antibody-based sandwich ELISA was developed to detect Vip3Aa in GM crops and their products. Two monoclonal antibodies were raised against the overexpressed and purified His-Vip3Aa20, were purified from mouse ascites and characterized. A sandwich ELISA method was developed using the 2G3-1D7 monoclonal antibody for capture and the biotin-labeled 1F9-1F5 monoclonal antibody for detection of Vip3Aa20. The linear detection range of the method was found to be approximately 31.25–500 pg/ml, with a sensitivity of 10.24 pg/ml.

Conclusions

The established ELISA was effective for detecting Vip3Aa family proteins other than Vip3Aa8, and was successfully applied in the detection of Vip3Aa20 and Vip3Aa19 expressed in transgenic maize and cotton.

A kinetic ELISA to determine the immunoreactive fraction of monoclonal antibodies

We developed a two-step ELISA to determine the immunoreactive fraction of monoclonal antibodies in conditions of antigen excess. An antibody aliquot at limiting dilution was incubated in wells coated with increasing amounts of antigen up to concentrations that bind 100% of antibody. At equilibrium, a supernatant aliquot was transferred to a second plate coated with excess of antiglobulin, and the captured antibody was incubated with peroxidase-conjugated anti-IgG. Antibody was quantitated from the enzyme velocity gradient in a kinetic ELISA, and the immunoreactive fraction calculated as (1 - gradient_i/gradient_T) x 100, where _i and _T are the gradients for the free and total antibody fractions. For four distinct monoclonal antibodies (anti-diphtheria toxoid, -cholera toxin, -bovine serum albumin (BSA), and -trinitrophenyl-BSA), measurement of inter-assay variability yielded values ranging from 3.1 to 7.4 (% coefficient of variation), which supports method repeatability. This ELISA is simple, precise, and applicable to mono- and polyclonal antibodies.

Viral species, viral genomes and HIV vaccine design: is the rational design of biological complexity a utopia?

A common logical confusion is prevalent in the whole of biology, namely that biological species are viewed both as an abstract category in an hierarchical classification and as a concrete kind of organism. This is partly due to the fact that the vast majority of living organisms do not have common names that differ from the Latin name of the species to which the organism belongs. However, it is somewhat astonishing that the same confusion exists in virology since every virus has a common name, different from the species name to which the virus belongs, which could be used to refer to the infectious viral entity as a concrete material object. The original 1991 ICTV definition of virus species stated that a virus species is a polythetic class of viruses and thus that a species is a class, namely a conceptual construction of the mind and not a physical, real object located in space and time. In 2013, the ICTV redefined a virus species no longer as a class but as a material object consisting of a monophyletic group of viruses that were all physically part of the species. This new definition is reminiscent of an earlier school of thought known as bionominalism which considered species to be concrete individuals rather than classes. Both bionominalism and the new ICTV definition are based on the logical fallacy of reification which treats abstractions such as classes as if they were concrete physical entities. The implications of this new ontology of virus species for virus taxonomy and for the possibility of incorporating nucleotide metagenomic sequences in the current ICTV classification is discussed.

Application of atomic force microscopy for characteristics of single intermolecular interactions

Atomic force microscopy (AFM) can be used to make measurements in vacuum, air, and water. The method is able to gather information about intermolecular interaction forces at the level of single molecules. This review encompasses experimental and theoretical data on the characterization of ligand-receptor interactions by AFM. The advantage of AFM in comparison with other methods developed for the characterization of single molecular interactions is its ability to estimate not only rupture forces, but also thermodynamic and kinetic parameters of the rupture of a complex. The specific features of force spectroscopy applied to ligand-receptor interactions are examined in this review from the stage of the modification of the substrate and the cantilever up to the processing and interpretation of the data. We show the specificities of the statistical analysis of the array of data based on the results of AFM measurements, and we discuss transformation of data into thermodynamic and kinetic parameters (kinetic dissociation constant, Gibbs free energy, enthalpy, and entropy). Particular attention is paid to the study of polyvalent interactions, where the definition of the constants is hampered due to the complex stoichiometry of the reactions.

Correlation between the composition of multivalent antibody conjugates with colloidal gold nanoparticles and their affinity

Interactions between multivalent preparations of antibodies (conjugated with colloidal gold nanoparticles (GNP) as a carrier system) and a multivalent ligand were investigated. The aim of the present study was to reveal the relationship between the affinity of the conjugate and its composition (i.e., the valency). Surface plasmon resonance was applied to study the affinity and the kinetics of the interaction of multivalent conjugates and multivalent virus (on the example of the plum pox virus (PPV)). Three monoclonal antibodies against PPV were prepared. Five GNP preparations with an average particle size in the range from 5 to 60nm (according to electron microscopy measurements) were obtained. The series of preparations allowed us to synthesize GNP-antibody conjugates with different surface areas for immobilization of antibodies, and, consequently, conjugates with different valencies. It was shown that the affinity of the conjugates changes with size of colloidal carriers (i.e. with the valency of the conjugates). The affinity of the virus-antibody interaction (antibodies with affinities of 1.46.10(-8)M and 1.73.10(-8)M) is one to three orders of magnitude lower (depending on the valency of the conjugate) compared to that of the interactions of the virus with GNP conjugates (conjugates with the affinity varying from 1.69.10(-9) to 7.02.10(-12)M and from 2.39.10(-9) to 2.62.10(-11)M, respectively). An increase in the conjugate size leads to an increase in its affinity. The similar trends were observed for the potato virus X.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

A simple and convenient approach for evaluation of the parameters of ligand-receptor interaction. Receptor blocking index and its application

A new approach for determination of the parameters for ligand-receptor interaction, which is based on so-called dilution coordinates, was developed earlier. Equations that allow evaluation of not only the affinity of ligand-receptor interaction but also of the amount of free (or occupied by corresponding ligand) receptors were suggested. The most important advantage of this approach as compared with well-known methods is the ability to determine the binding parameters for ligand-receptor interaction even for the cases in which ligand and receptor are already present in a mixture and separation of counterparts from each other is technically difficult or even impossible. Due to this reason, the proposed approach can be especially useful for studying interactions between highly-labile biological receptors and corresponding ligands as found in vivo. In the present paper I continue to consider how to determine the binding parameters for a given ligand-receptor interaction if the value of receptor blocking index is determined experimentally.

Selective binding of polychlorinated biphenyl congeners by a monoclonal antibody: analysis by kinetic exclusion fluorescence immunoassay

A previously described monoclonal antibody, S2B1, was highly selective for coplanar (non-ortho-chlorinated) PCB congeners in enzyme immunoassays that measured binding at equilibrium. In the present study, kinetic exclusion fluoroimmunoassay (KinExA) was used to determine the dissociation constants (Kd) and on and off rates (k(on), k(off)) for binding of various PCB congeners to affinity-purified S2B1 IgG and Fab fragments in solution. This method revealed that mono- and di-ortho-chlorinated PCBs were bound by S2B1, but the on rates were slower, and the off rates faster by 6-60-fold, than with congeners that had no ortho chlorines. Although the sensitivity of immunoassays may be improved by using competing haptens that S2B1 binds more weakly than the parent PCB, the KinExA results demonstrate that congener specificity is an intrinsic property of S2B1 and does not require weaker binding haptens. KinExA also provided new information on the percentage of active binding sites, valence, and effects of buffer, solvent, and biotinylation on S2B1. The advantages and drawbacks of KinExA for measuring antibody-ligand binding are described.

Solid supports in enzyme-linked immunosorbent assay and other solid-phase immunoassays

A very large proportion of modern immunoassays involve the use of synthetic solid phases to immobilize one of the reactants. These solid-phase immunoassays (SPIs) therefore involve ligand-receptor interactions that occur within a reaction volume close to the solution/solid phase interface. As a consequence, the immunochemistry/biochemistry of these ligand-receptor interactions differs from that of their counterparts in solution. Furthermore, the immobilization process can significantly alter the biological activity of the reactant; most adsorbed proteins on polystyrene or silicone are partially or largely denatured. Therefore the use of alternative methods of immobilization is attractive but may result in little increase in the amount of total functional reactant. However, all commonly used solid phases do not have the same properties or the same capacity for reactant immobilization or experience the same level of nonspecific binding. Empiricism plays a major role in SPIs. Derivations of mass law equations for measuring the antigen capture of solid-phase antibodies, for determining the affinity of solid phase for protein adsorption, and for estimating antibody affinity are reviewed.

The antigenicity of tobacco mosaic virus

The antigenic properties of the tobacco mosaic virus (TMV) have been studied extensively for more than 50 years. Distinct antigenic determinants called neotopes and cryptotopes have been identified at the surface of intact virions and dissociated coat protein subunits, respectively, indicating that the quaternary structure of the virus influences the antigenic properties. A correlation has been found to exist between the location of seven to ten residue-long continuous epitopes in the TMV coat protein and the degree of segmental mobility along the polypeptide chain. Immunoelectron microscopy, using antibodies specific for the bottom surface of the protein subunit, showed that these antibodies reacted with both ends of the stacked-disk aggregates of viral protein. This finding indicates that the stacked disks are bipolar and cannot be converted directly into helical viral rods as has been previously assumed. TMV epitopes have been mapped at the surface of coat protein subunits using biosensor technology. The ability of certain monoclonal antibodies to block the cotranslational disassembly of virions during the infection process was found to be linked to the precise location of their complementary epitopes and not to their binding affinity. Such blocking antibodies, which act by sterically preventing the interaction between virions and ribosomes may, when expressed in plants, be useful for controlling virus infection.

Evaluation of the serologic response against two consensus V3 loop peptides from human immunodeficiency virus-1 in Cuban patients

OBJECTIVES

A retrospective study was conducted to evaluate the antibody response of Cuban patients infected with human immunodeficiency virus (HIV)-1 against two consensus peptides from the third variable domain (V3) loop of glycoprotein gp120.

METHODS

The study included sera from 10 individuals at different stages of disease. Two 15-meric synthetic peptides designed from a consensus sequence, belonging to group B or C of HIV-1, were used to determine antibody titers and avidity indexes in an indirect enzyme-linked immunoassay.

RESULTS

A high reactivity against both peptides was detected, with 80% of the sera reacting with at least one of the peptides. The antibody titers and avidity indexes did not correlate with disease progression. Additionally, for one of the patients from whom the virus had been isolated, a higher avidity index was found against the homologous peptide.

CONCLUSIONS

This study showed high reactivity against two consensus peptides from the V3 loop of gp120 among patients with HIV. Large scale studies are needed to determine whether the titers or avidity of anti-V3 antibodies, at the early stages of infection, are predictive of disease progression. Both peptides are candidates for inclusion in experimental vaccines.

Kinetics and energetics of specific intermolecular interactions

Taking into account the energy vs. distance functions of the aspecific (macroscopic) repulsion that usually prevails between antigen (Ag) and antibody (Ab) molecules in polar media, as well as the specific (microscopic) attraction between epitope and paratope of Ag and Ab, it proved possible to determine the kinetic constants (von Smoluchowski, 1917; Hammes, 1978) of Ag-Ab interactions, from the surface properties of Ag, Ab and the aqueous medium. The kinetic constants thus found correlate well with experimentally determined kinetic constants in comparable systems, and confirm the importance of the influence of the concentration of one of the reagents (e.g. the Ab) on the kinetic association constant (Van Regenmortel et al., 1994), which is largely due to steric hindrance. Applying the same energy vs. distance approach to the influence of temperature (T) on Ag-Ab reactions, it ensues that the familiar occurrence of an apparent 'enthalpy-entropy compensation' in aqueous media is in fact the relatively gratuitous outcome of a complex set of effects caused by an increase in T, on the total free energy, the hydration energy and, as a result, on the inter-epitope-paratope distance. A close correlation exists between the outcome of these surface-thermodynamic analyses and experimental results.

Determination of active single chain antibody concentrations in crude periplasmic fractions

We have measured active single chain antibody (scFv) concentrations under mass transfer limitation conditions using surface plasmon resonance on the BIAcore. For the creation of a standard curve scFv 4Dwt, derived from monoclonal antibody (mAb) 4D, directed against human chorionic gonadotropin (hCG), was purified by positive affinity chromatography. Determination of the active antibody fraction after purification was performed using anti-FLAG, reactive against a tag sequence C-terminally fused to the scFv. Two independent experiments showed that the activity remaining represented over 75% of the total amount of purified protein. Calibration curves on high density antigen surfaces showed a linear relationship between antibody concentration and binding rates. Periplasmic fractions of six mutant scFvs, also derived from mAb 4D, revealed a clear difference in the amount of soluble active scFv expressed in the periplasm of E. coli compared with the total amount of antibody present, indicating the necessity of measuring active antibody concentrations. This rapid concentration determination method will be particularly useful for accurately comparing affinity constants, using antibody concentrations determined with the BIAcore, of the many different scFv fragments routinely isolated from phage display libraries.

Comparative interaction kinetics of two recombinant Fabs and of the corresponding antibodies directed to the coat protein of tobacco mosaic virus

Two recombinant Fab fragments, 57P and 174P, recognizing peptide 134-146 of the coat protein of tobacco mosaic virus have been cloned, sequenced and expressed in Escherichia coli. They differ by 15 amino acid changes in the sequence of their variable region. The interaction kinetics of the Fabs with the wild-type and four mutant peptides have been compared using a BIAcoreTM biosensor instrument. The recombinant Fab 174P had the same reactivity as the Fab fragment obtained by enzymatic cleavage of monoclonal antibody 174P. The two recombinant Fabs recognized the various peptides in the same ranking order but Fab 174P consistently dissociated somewhat faster from the peptides compared to Fab 57P. The two whole antibodies showed the same relative differences in reactivity as the two recombinant Fabs. The location of amino acid changes was visualized on a model structure of the Fab. Differences in dissociation rates of the two antibodies are most likely due to changes located at the periphery of the antigen-combining site and/or at the interface between the light and heavy chain domains. Our results demonstrate the feasibility of detecting very small differences in binding affinity by the biosensor technology, which is a prerequisite for assessing the functional effect of limited structural changes.

Binding kinetics of monoclonal antibody using antigen-beta-galactosidase hybrid protein: application to measurement of peptide antigenicity

A simple method for determination of binding kinetics of a solid-phase antibody using antigen-beta-galactosidase hybrid protein was evaluated. To minimize conformational change of the antigen binding site of the antibody when directly binding to a microtiter plate, the microtiter plate was precoated with protein A. The binding and free antigen concentrations were directly obtained from the beta-galactosidase activity. This method can be used for analyses of the equilibrium dissociation constant (KD), and the association (Kass) and dissociation (Kdiss) rate constants. Peptide antigenicity was also analyzed by competitive ELISA using this method. Since both antigen-beta-galactosidase and the peptide used are localized in the fluid-phase, the proper affinity constant (KA) of the peptide can be estimated from the KD value of the antigen-beta-galactosidase-antibody interaction, and from the IC50 value of the peptide.

Mapping of viral conformational epitopes using biosensor measurements

Earlier electron microscopy studies of the location of various antigenic sites in tobacco mosaic virus indicated that epitopes specific for the quaternary structure and absent in dissociated viral subunits (so-called neotopes) were present along the entire length of the virus particle. In contrast, epitopes expressed in both intact particles and dissociated subunits (so-called metatopes) were found only at the one extremity of the particle containing the 5' end of the RNA. In the present study, the binding properties of antibodies to neotopes and metatopes were studied with the BIAcore. From the results of capture assays with viral subunits and on the basis of binding stoichiometry calculations, it was possible to demonstrate the presence of neotope and metatope specificities on additional parts of the viral surface where they had not been identified before by classical immunoassays. In two site binding assays it was also found that a neotope specificity could be induced in dissociated viral subunits by the binding of a first antimetatope antibody. The results clearly demonstrated the superiority of the biosensor technology for mapping conformational epitopes in viral proteins.

Early high-affinity neutralizing anti-viral IgG responses without further overall improvements of affinity

Affinity maturation of IgG antibodies in adaptive immune responses is a well-accepted mechanism to improve effector functions of IgG within 2 weeks to several months of antigen encounter. This concept has been defined mainly for IgG responses against chemically defined haptens. We have evaluated this concept in a viral system and analyzed neutralizing IgG antibody responses against vesicular stomatitis virus (a close relative of rabies virus) with a panel of monoclonal antibodies obtained early (day 6 or 12) and late (day 150) after hyperimmunization. These neutralizing IgG antibodies recognize a single major antigenic site with high affinities (Ka of 10(8)-10(10) liter.mol-1) and with rapid on-rates already on day 6 of a primary response and with no evidence for further antigen dose- and time-dependent overall improvement of affinity. This type of IgG response is probably representative for viruses or bacterial toxins which are crucially controlled by neutralizing antibodies.

Structure-activity relationships for the interaction between cyclosporin A derivatives and the Fab fragment of a monoclonal antibody

The crystallographic structure of a complex between cyclosporin A and the Fab fragment of monoclonal antibody R45-45-11 has been solved to 2.65 A resolution (Altschuh et al., 1992a, Science 256, 92; Vix et al., 1993, Proteins 15, 339), yielding a precise three-dimensional picture of interacting surfaces. In order to evaluate the contribution of observed contacts to the energy of interaction, we have measured the effect on binding affinity of minor chemical modifications of CS. The equilibrium binding constant of the Fab fragment for a set of cyclosporin analogs was obtained by measuring in a biosensor instrument the dependence of complex formation on Fab concentration, at constant analog concentrations. Data were analysed using Scatchard plots. Differences in binding energy resulting from cyclosporin modifications discriminated between two types of contact areas. The first type displays adaptability to structural modifications of cyclosporin at the cost of a small decrease in binding energy, and contacting residues in the antibody form the periphery of the combining site. The second type does not accommodate structural changes and corresponds in cyclosporin to three residues whose modifications drastically decrease binding energy with the antibody. The corresponding contact residues in the antibody form the core of the antibody combining site.

Measurement of kinetic binding constants of viral antibodies using a new biosensor technology

Association (ka) and dissociation (kd) rate constants of three monoclonal antibodies raised against tobacco mosaic virus were determined using a biosensor technique based on surface plasmon resonance (BIAcore, Pharmacia). Dissociation rates were constant over the 4-400 nM antibody concentration range whereas apparent association rates decreased over this range probably due to an increased saturation level of the antigen. Affinity constants K calculated from the ratio of ka/kd were in reasonable agreement with values obtained under equilibrium conditions by two standard methods based on enzyme immunoassay.

Interaction of cyclosporin A with an Fab fragment or cyclophilin. Affinity measurements and time-dependent changes in binding

Different conformers of the immunosuppressant cyclosporin A have been observed in structural studies of the isolated molecule and of its complex with cyclophilin or with an Fab fragment. The factors that control this conformational change are not well understood. Variations in the amount of complex formed with cyclophilin or with the antibody were measured as a function of time after adding cyclosporin to the proteins, using the Pharmacia BIAcore biosensor instrument. Up to 1 hour was needed to reach maximum complex formation in solution, which is likely to reflect the time needed for a conformational transition of cyclosporin. The equilibrium affinity constant of both proteins for cyclosporin has been measured.

Tobacco mosaic virus: a model antigen to study virus-antibody interactions

For more than 50 years, tobacco mosaic virus (TMV) has been used as a model system for studying various aspects of virus-antibody interactions. Distinct epitopes called neotopes and cryptotopes have been identified in intact TMV particles and dissociated viral protein respectively and a correlation has been found to exist between the location of continuous epitopes and the extent of segmental mobility along the viral polypeptide chain. The occurrence of bivalent antibody binding was shown to influence the observed affinity of TMV antibodies and kinetic measurements of antibody binding to viral peptides made it possible to analyze the mechanism of binding of monoclonal antibodies. It seems likely that the TMV model will continue to yield a rich harvest of immunochemical data relevant to many viral systems.

Measurement of affinity of viral monoclonal antibodies using Fab'-peroxidase conjugate. Influence of antibody concentration on apparent affinity

The binding affinity of a monoclonal antibody to tobacco mosaic virus (TMV) was studied using a Fab'-peroxidase conjugate. Measurement of the enzymatic activity allowed the determination of the amount of free antibody present after ultracentrifugation of virus-antibody complexes at equilibrium. The method was very sensitive and allowed measurements over a 1000-fold range of antibody concns. The calculated affinity constant decreased about 25 fold when the antibody concn used in the binding assay was increased from 30 ng/ml to 35 micrograms/ml.

Operational aspects of antibody affinity constants measured by liquid-phase and solid-phase assays

The association constant of monoclonal antibodies (Mabs) to tobacco mosaic virus has been determined in solution and solid-phase binding assays. The ELISA equilibrium titration method developed by Friguet et al. (1985) was found to be suitable for large antigens such as viruses. In the case of intact IgG antibody, it gave equilibrium constant (K) values ca 30% lower than those obtained by classical solution-phase assay while in the case of Fab', the same values were obtained in both assays. Solid-phase binding assays gave higher K values than solution-phase assays by a factor which varied with the Mab tested (1.5- to 5.4-fold higher). Furthermore, in solution-phase assay, K values were found to depend on the antibody concentration used in the assay. These results confirm the operational nature of antibody affinity constants and indicate that in order to compare the affinity of different Mabs in a meaningful way, it is necessary to use a single technique under standardized conditions.