Influence of antibody affinity on the performance of different antibody assays

Two-center validation of assays for the detection of binding and neutralizing anti-factor VIII antibodies

INTRODUCTION

Patients with hemophilia A treated with coagulation Factor VIII (FVIII) products are at risk for developing anti-FVIII antibodies. The ABIRISK Consortium aimed to provide knowledge on the formation and detection of anti-drug antibodies against biopharmaceutical products, including FVIII. Accordingly, standardized and validated assays for the detection of binding (total) and neutralizing antibodies are needed.

AIM

Two-center validation of an ELISA for the detection of total FVIII-binding IgG-antibodies and Nijmegen-Bethesda assays for the quantification of FVIII-neutralizing antibodies according to consensus validation guidelines.

METHODS

Validation of assays at both sites was done according to published recommendations and included preanalytics, the determination of key assay parameters, including cut-points, assay sensitivity, precision, and FVIII interference.

RESULTS

The validated assays reproducibly detected FVIII-binding and -neutralizing antibodies with comparable performance in both laboratories. Floating screening cut-points were established for both assays. Determined mass-based sensitivity of both assays (all values ≤66 ng/mL) complied with the minimum sensitivity for the detection of anti-drug antibodies as recommended by the FDA (<100 ng/mL). Intra- and inter-assay coefficients of variation did not exceed 25%. Assay validation further revealed that pre-analytical heat treatment led to potentially false-positive ELISA results, while up to 0.15 IU/mL, residual FVIII showed no significant impact. Overall, good agreement of results was found for patient samples analyzed at both study sites.

CONCLUSION

Comprehensive validation of different anti-FVIII-antibody assays in two laboratories gave novel insights into the impact of pre-analytical sample treatment as well as the comparability of test results generated by the use of methodically different assays.

IgY Antibodies from Birds: A Review on Affinity and Avidity

IgY antibodies are found in the blood and yolk of eggs. Several studies show the feasibility of utilising IgY for immunotherapy and immunodiagnosis. These antibodies have been studied because they fulfil the current needs for reducing, replacing, and improving the use of animals. Affinity and avidity represent the strength of the antigen-antibody interaction and directly influence antibody action. The aim of this review was to examine the factors that influence the affinity and avidity of IgY antibodies and the methodologies used to determine these variables. In birds, there are few studies on the maturation of antibody affinity and avidity, and these studies suggest that the use of an adjuvant-type of antigen, the animal lineage, the number of immunisations, and the time interfered with the affinity and avidity of IgY antibodies. Regarding the methodologies, most studies use chaotropic agents to determine the avidity index. Studies involving the solution phase and equilibrium titration reactions are also described. These results demonstrate the need for the standardisation of methodologies for the determination of affinity and avidity so that further studies can be performed to optimise the production of high avidity IgY antibodies.

Development of sandwich ELISAs for detecting glucagon-like peptide-1 secretion from intestinal L-cells and their application in STC-1 cells and mice

Certain nutrients stimulate glucagon-like peptide-1 (GLP-1) secretion from the intestinal enteroendocrine L-cells, but due to rapid degradation by the DPP-4 enzyme, >90% is converted to inactive metabolite before reaching the target organs via circulation. Plants are a source of potent bioactive compounds that promote endogenous secretion of GLP-1 from L-cells. To search for the effective bioactive compound from a vast library of natural compounds, a reliable and low-cost assay is required considering the high cost of commercial assays. We developed a low-cost sandwich enzyme-linked immunosorbent assays (s-ELISAs) for detecting 'total', 'sensitive active', and 'wide-range active' GLP-1. The s-ELISAs exhibited high sensitivity with measurement ranges between 0.94~240, 0.98~62.5, and 4.8~4,480 pmol/L, respectively. High precision was observed; i.e., CVs within 5% and 20% for intra- and inter-assay variations, respectively, and excellent recovery of exogenous GLP-1 from assay buffer. The developed s-ELISAs had the same performance as the commercial kits and approximately 80% cheaper cost. For their application, cinnamtannin A2-induced GLP-1 secretion was confirmed in STC-1 cells consistent with our previous findings. The s-ELISAs were further validated by measuring plasma GLP-1 level in mice after oral administration of black soy bean seed coat extract containing cinnamtannin A2.

Bi-epitope SPR surfaces: a solution to develop robust immunoassays

Surface plasmon resonance (SPR)-based immunoassays have numerous applications and require high affinity reagents for sensitive and reliable measurements. We describe a quick approach to turn low affinity antibodies into appropriate capture reagents. We used antibodies recognizing human ephrin type A receptor 2 (EphA2) and a ProteOn XPR36 as a model system. We generated so-called 'bi-epitope' sensor surfaces by immobilizing various pairs of anti-EphA2 antibodies using standard amine coupling. The apparent binding affinities to EphA2 and EphA2 detection sensitivities of the bi-epitope and 'single-epitope' surfaces were then compared. For all antibody pairs tested, bi-epitope surfaces exhibited an ∼ 10-100-fold improvement in apparent binding affinities when compared with single-epitope ones. When pairing 2 antibodies of low intrinsic binding affinities (∼ 10(-8) M) and fast dissociation rates (∼ 10(-2) s(-1)), the apparent binding affinity and dissociation rate of the bi-epitope surface was improved up to ∼ 10(-10) M and 10(-4) s(-1), respectively. This led to an ∼ 100-200-fold enhancement in EphA2 limit of detection in crude cell supernatants. Our results show that the use of antibody mixtures in SPR applications constitutes a powerful approach to develop sensitive immunoassays, as previously shown for non-SPR formats. As SPR-based assays have significantly expanded their reach in the last decade, such an approach promises to further accelerate their development.

Monoclonal antibodies recognizing the surface autolysin IspC of Listeria monocytogenes serotype 4b: epitope localization, kinetic characterization, and cross-reaction studies

Listeria monocytogenes serotype 4b is responsible for a high percentage of fatal cases of food-borne infection. In a previous study, we created 15 monoclonal antibodies (MAbs) against a ∼77 kDa antigen that is associated with the cell surface of live L. monocytogenes serotype 4b cells. Here we report an extensive characterization of these MAbs to further their development as diagnostic reagents. The ∼77 kDa target antigen was identified by mass spectrometry and N-terminal sequencing to be IspC, a novel surface associated autolysin. Epitope localization experiments revealed that each of the 15 MAbs recognized the C-terminal cell-wall binding domain of IspC. The presence of IspC was shown to be highly conserved within L. monocytogenes serotype 4b, as evidenced by a strong reaction between anti-IspC MAbs and all 4b isolates. To determine the range of cross-reactivity with other L. monocytogenes serotypes ELISA was used to test each MAb against multiple isolates from each of the L. monocytogenes serotypes. Of the 15 MAbs, five: M2774, M2775, M2780, M2790 and M2797, showed specificity for L. monocytogenes serotype 4b and only cross reacted with serotype 4ab isolates. The kinetics of the interaction between each of the MAbs and IspC was measured using surface plasmon resonance. The MAbs M2773, M2792, M2775, M2797 and M2781 each had very low dissociation constants (4.5 × 10⁻⁹ to 1.2 × 10⁻⁸ M). While several of these antibodies have properties which could be useful in diagnostic tests, the combined high fidelity and affinity of M2775 for the IspC protein and serotype 4b isolates, makes it a particularly promising candidate for use in the development of a specific L. monocytogenes serotype 4b diagnostic test.

Epitope reactions can be gauged by relative antibody discriminating specificity (RADS) values supported by deletion, substitution and cysteine bridge formation analyses: potential uses in pathogenesis studies

Background

Epitope-mapping of infectious agents is essential for pathogenesis studies. Since polyclonal antibodies (PAbs) and monoclonal antibodies (MAbs) are always polyspecific and can react with multiple epitopes, it is important to distinguish between specific and non-specific reactions. Relative antibody discriminating specificity (RADS) values, obtained from their relative ELISA reactions with L-amino acid peptides prepared in the natural versus reverse orientations (x-fold absorbance natural/absorbance reverse = RADS value) may be valuable for this purpose.

PAbs generated against the dengue type-2 virus (DENV-2) nonstructural-1 (NS1) glycoprotein candidate vaccine also reacted with both DENV envelope (E) glycoproteins and blood-clotting proteins. New xKGSx/xSGKx amino acid motifs were identified on DENV-2 glycoproteins, HIV-1 gp41 and factor IXa. Their potential roles in DENV and HIV-1 antibody-enhanced replication (AER) and auto-immunity were assessed.

In this study, a) RADS values were determined for MAbs and PAbs, generated in congeneic (H2: class II) mice against DENV NS1 glycoprotein epitopes, to account for their cross-reaction patterns, and b) MAb 1G5.3 reactions with xKGSx/xSGKx motifs present in the DENV-4 NS1, E and HIV-1 glycoproteins and factor IXa were assessed after the introduction of amino acid substitutions, deletions, or intra-/inter-cysteine (C-C) bridges.

Results

MAbs 1H7.4, 5H4.3, 3D1.4 and 1G5.3 had high (4.23- to 16.83-fold) RADS values against single epitopes on the DENV-2 NS1 glycoprotein, and MAb 3D1.4 defined the DENV complex-conserved LX1 epitope. In contrast, MAbs 1G5.4-A1-C3 and 1C6.3 had low (0.47- to 1.67-fold) RADS values against multiple epitopes. PAb DENV complex-reactions occurred through moderately-high (2.77- and 3.11-fold) RADS values against the LX1 epitope. MAb 1G5.3 reacted with xSGKx motifs present in DENV-4 NS1 and E glycoproteins, HIV-1 gp41 and factor IXa, while natural C-C bridge formations or certain amino acid substitutions increased its binding activity.

Conclusions

These results: i) were readily obtained using a standard 96-well ELISA format, ii) showed the LX1 epitope to be the immuno-dominant DENV complex determinant in the NS1 glycoprotein, iii) supported an antigenic co-evolution of the DENV NS1 and E glycoproteins, and iv) identified methods that made it possible to determine the role of anti-DENV PAb reactions in viral pathogenesis.

Seroprotection associated with infant vitamin A supplementation given with vaccines is not related to antibody affinity to Hepatitis B and Haemophilus influenzae type b vaccines

Vaccines are usually assessed by analyses of their safety and immunogenicity to determine the effectiveness of eliciting antibody responses against target organisms. However, it is equally important to establish antibody affinity because of its specific role in protection from infection. Antibody affinity can be determined by comparisons of various antibody concentrations in dose-response curves. During a study on the immunogenicity of a pentavalent vaccine in 888 infants, antibody affinity analyses of the hepatitis B and Haemophilus influenzae type b components were investigated in infants given 15mg RE vitamin A with their vaccination and those who were not given vitamin A. In this paper we present the results of 222 infants; a 25% sub-sample of the original study. Analyses were carried out using dilutions of serum samples from fitted values corresponding to optical densities from antibody detection assays. These were obtained from the ligand binding equation and mid point titres in dose-response curves were then calculated. Vitamin A supplementation had no effect on the midpoint titres of Hepatitis B and H. influenzae type b vaccine derived antibodies. The significant effect of vitamin A supplementation on the Hepatitis B vaccine component observed in a previous seroprotection analysis is probably due to the amount of antibodies since affinity was unaffected.

Effect of different hapten-carrier conjugation ratios and molecular orientations on antibody affinity against a peptide antigen

Hapten-carrier conjugate immunization is an important tool in the generation of hapten-specific antibodies for analytical purposes and in the uncovering of basic vaccine-immunological mechanisms. The affinity of antibodies is known to play an important role for the resulting sensitivity of antibody-based assay systems and in deciding whether a vaccine-induced antibody response will be protective. With ovalbumin as a carrier protein and a peptide (7.2 NY) representing a 19 amino acid sequence from the E. coli-derived Verotoxin 2e as a model hapten we investigated whether it was possible to influence the affinity and titre of antibodies raised against the hapten using different conjugation ratios and orientations. The peptide was coupled to ovalbumin in four conjugation ratios and two molecular orientations - terminal and central - and the conjugates were verified by mass spectrometry. Mice were immunised ten times at two-weeks intervals with low doses of the eight conjugates. Blood samples collected between each immunisation were analysed by ELISA for specific antibody titres and relative affinities. With both types of conjugations, the anti-peptide antibody titres increased in response to increasing conjugation ratios, but central conjugation resulted in markedly higher titres than terminal conjugation. The overall anti-peptide antibody affinities reached approximately similar levels with both orientations, whereas a reversed proportionality was observed between conjugation ratio and antibody affinity for terminal conjugation. Thus, it appears that the molar ratio of a peptide and its carrier may affect the resulting antibody affinities, and that a conjugation ratio between a terminally conjugated peptide and its carrier approaching one will result in relatively high antibody affinities. Furthermore, the molecular orientation of the coupled peptide has a major effect on the anti-peptide antibody titres induced.

Generation of a recombinant Fab antibody reactive with the Alzheimer's disease-related Abeta peptide

A recombinant Fab antibody, designated 1E8-4b, which reacts with the Alzheimer's disease (AD)-related Abeta peptides, Abeta[1-40], Abeta[1-42] and Abeta[1-43] has been developed. The 1E8-4b Fab was constructed by cloning the V(H)C(H1) and V(L)C(L) domains from the parent hybridoma 1E8 antibody, reported previously to recognize these Abeta peptides. Briefly, a C-terminal Flag tag sequence was incorporated into this construct, which was ligated into the vector pHFA2 and expressed in Escherichia coli. Following purification on an M2 anti-Flag affinity column, the 1E8-4b recombinant Fab antibody was shown to bind plaques within sections of brain tissue from CERAD-defined AD patients by immunohistochemistry. ELISA, epitope mapping and immunoblotting confirmed the recognition of the Abeta1-40/42/43] peptides by the 1E8-4b Fab. The 1E8-4b Fab did not recognize APP695 or APP770 which contain the Abeta sequence. The Abeta specificity of the recombinant 1E8-4b Fab antibody was identical to the parent 1E8 monoclonal antibody.

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.

Affinity of IgE antibody to antigen influences allergen-induced histamine release

BACKGROUND

Although affinity of an antibody for an antigen is recognized to be an important factor in determining its biological effects, little is known about the relevance of such affinity of IgE antibodies to the functional response.

OBJECTIVES

To investigate the effect of IgE antibody affinity to Der p 2 on Der p 2-induced histamine release from human basophils.

METHODS

The most probable value of the dissociation constant (Kd) of IgE antibody to Der p 2 was calculated and histamine release by Der p 2-challenged leucocytes was used to evaluate the biological efficacy of the IgE antibody.

RESULTS

The most probable Kd value of IgE antibody to Der p 2 ranged from 5.6 to 177.8 pM in 14 asthmatic patients sensitive to Der p 2. A significant correlation was observed in Der p 2-induced histamine release between the sensitivity and the Kd value for Der p 2-specific IgE antibody (rs = 0.797, P = 0.0040), suggesting that the higher the affinity, the lower the amount of allergen required for the release of a specific level of histamine.

CONCLUSION

Apart from the changes associated with the reactivity, the sensitivity of histamine release is closely related to the affinity of IgE antibody for its antigen.

Correlations between antibody immune responses at different mucosal effector sites are controlled by antigen type and dosage

Monitoring specific secretory immunoglobulin A (IgA) responses in the intestines after mucosal immunization or infection is impeded by the fact that sampling of small intestinal secretions requires invasive methods not feasible for routine diagnostics. Since IgA plasma cells generated after intragastric immunization are known to populate remote mucosal sites as well, secretory IgA responses at other mucosal surfaces may correlate to those in the intestines and could serve as proxy measures for IgA secretion in the gut. To evaluate the practicability of this approach, mice were immunized intragastrically with 0.2, 2, and 20 mg of ovalbumin plus 10 microg of cholera toxin, and the antigen-specific local secretory IgA responses in duodenal, ileal, jejunal, rectal, and vaginal secretions, saliva, urine, and feces, as well as serum IgG and IgA responses were analyzed by enzyme-linked immunosorbent assay. Correlation analysis revealed significant relationships between serum IgG and IgA, urinary IgA, salivary IgA, and secretory IgA in duodenal, jejunal, ileal, and rectal secretions for the 0.2-mg but not for the 20-mg ovalbumin dose. Fecal samples were poor predictors for intestinal antiovalbumin IgA responses, and no correlations could be established for cholera toxin, neither between local anti-cholera toxin levels nor to the antiovalbumin responses. Thus, specific IgA in serum, saliva, or urine can serve as a predictor of the release of specific IgA at intestinal surfaces after intragastric immunization, but the lack of correlations for high ovalbumin doses and for cholera toxin indicates a strong dependency on antigen type and dosage for these relationships.

Immunosensing platforms using spontaneously adsorbed antibody fragments on gold

This paper describes the construction and characterization of miniaturized antigenic immunosurfaces composed of spontaneously adsorbed Fab'-SH fragments on gold. Rabbit Fab'-SH fragments contain a free sulfhydryl group that forms a thiolate bond with a gold substrate as detailed by X-ray photoelectron spectroscopy. This approach creates surfaces of higher epitope density, a factor critical to the early detection of disease, than surfaces composed of adsorbed whole molecule IgG on gold. The viability and specificity of antigenic Fab'-SH immunosurfaces is demonstrated using atomic force microscopy and confocal fluorescence microscopy, and possible explanations for the larger epitope density are discussed.

Indirect enzyme-linked immunosorbent assay for the quantitative estimation of lysergic acid diethylamide in urine

A new antibody to lysergic acid diethylamide (LSD) was used to develop a novel indirect ELISA for the quantification of drug in urine. Evaluation of the new assay with the commercially available LSD ELISA (STC Diagnostics) shows improved performance. The test requires 50 μL of urine, which is used to measure concentrations of drug in the μg/L to ng/L range. The limit of detection was 8 ng/L compared with 85 ng/L in the commercial assay, and analytical recoveries were 98–106%. Our test detected 0.1 μg/L of LSD in urine with an intraassay CV of 2.4% (n = 8) compared with 6.0% for a 0.5 μg/L sample in the commercial assay (n = 20). The upper and lower limits of quantification were estimated to be 7 μg/L and 50 ng/L, respectively. Specificity was evaluated by measuring the extent of cross-reactivity with 24 related substances. Drug determination using the new assay offers both improved sensitivity and precision compared with existing methods, thus facilitating the preliminary quantitative estimation of LSD in urine at lower concentrations with a greater degree of certainty.

Increased content of type-VI collagen epitopes in human osteoarthritic cartilage: quantitation by inhibition ELISA

Type-VI collagen is an integral part of the extracellular cartilage matrix. However, the exact amounts of type-VI collagen in normal and osteoarthritic human cartilage still are not known. In this study, we describe an inhibition enzyme-linked immunosorbent assay that was developed to quantitate type-VI collagen epitopes found in guanidinium chloride extracts from normal and osteoarthritic human cartilage. In 31 cartilage samples from various localizations of healthy adult human knees, type-VI collagen epitopes accounted for approximately 0.40% of the total collagen content. Interestingly, type-VI collagen epitopes increased about 4-fold in osteoarthritic cartilage. A statistically significant increase of type-VI collagen epitopes was found during early stages of the disease, with only a superficial roughening of the cartilage surface and a loss of proteoglycans. Thus, these findings indicate that type-VI collagen is a minor component of normal human articular cartilage and that the amount of type-VI collagen epitopes increases significantly during early stages of osteoarthritis.

Assessment of the functional affinity constant of monoclonal antibodies using an improved enzyme-linked immunosorbent assay

The use of an enzyme-linked immunosorbent assay (ELISA) for the determination of affinity constants implies heterogeneous measurements. Therefore, despite their simplicity, direct solid-phase binding assays are not common. Many investigators have serious, and mostly justified, reservations about the application of solid-phase affinity methods. They refer to problems such as diffusion effects and difficulties in reaching equilibrium due to heterogeneous binding and co-operativity. Accordingly, functional affinity determinations are often described as meaningless. These objections apply to the measurement of the affinity of a monoclonal antibody using the enzyme-linked immunosorbent assay of Beatty et al. (J. Immunol. Methods (1987) 100, 173), which is based on the effect of antibody affinity on the sigmoidal dose response curve. The affinity constant is calculated by mathematical equations, based on the Law of Mass Action and the authors made a number of important assumptions--avoiding the above mentioned problems--in order to justify the use of the Law of Mass Action. By carefully examining these assumptions we have developed an improved ELISA procedure for functional affinity determinations on the basis of a primary coating with the antigen only. the coating conditions were validated by employing gold labelled colloidal particles and physical counting of the bound particles under the scanning electron microscope. Since monovalent binding between human chorionic gonadotropin and its monoclonal antibody could be achieved under equilibrium conditions, the application of the Law of Mass Action and hence of the Beatty formula became possible. We conclude that under these conditions functional affinity determinations are appropriate.

Acute respiratory effects in the potato processing industry due to a bioaerosol exposure

The relation between bioaerosol exposure in the potato starch industry and work related respiratory symptoms is described. One group of workers was exposed to high dust concentrations (geometric mean up to 56.0 mg/m3) with low endotoxin and antigen concentrations (geometric mean up to 12.6 ng/m3 and 90 relative antigen units (RAU) per m3). A second group was exposed to low dust concentrations (geometric mean up to 3.9 mg/m3), but the endotoxin and antigen concentrations were high (geometric means of environmental samples up to 72 ng/m3 and 2.9.10(2)RAU/m3). Twenty of the 48 workers had specific IgG4 titres to dust extract. Of these 20 workers, 14 showed an increase in IgG4 titre during the first month of the potato processing season. No specific IgE antibodies to dust extracts were found. Twenty two workers were equipped with a Mini-Wright peak flow meter during a four week period. Two workers showed a work related decline in peak flow values. It was concluded that exposure to dust made airborne during the refining process of potato starch may cause work related respiratory symptoms.

Studies on monoclonal anti-isotypic and anti-idiotypic antibodies against leukemia and myeloma: VI. Purification and relative affinity of monoclonal antibodies

In this study, the anti-idiotypic and anti-isotypic antibodies (McAbs) against IgM of the Patient with B chronic lymphocytic leukemia (B-CLL) were purified from hybridoma ascites by n-Octoic acid precipitation method. The purified McAbs have high purify and high antibody activity as evidenced by immunoelectrophoresis, SDS-PAGE and ELISA. Relative affinity of 11 McAbs was measured by using indirect ELISA and double antibody sandwich ELISA method. It was found that relative affinity of various McAbs to the same antigen was different. 11 McAbs could be divided into two groups by analysing their 50% maximum binding. The relative affinity of 4 McAbs in the culture supernatants was consistent with that of McAbs in the purified ascites. Our experimental results provide an important basis for rational application of these McAbs.

Quantitative and qualitative characterization of virus envelope proteins and specific polyclonal antibodies

A method for simultaneous qualitative and quantitative characterization of virus envelope proteins and virus-neutralizing antibodies is described. The quantitative estimate is based solely on the precipitation of the antibody/antigen (Ab/Ag) complexes at equivalence under equilibrium conditions. The qualitative analysis is performed by determining the diffusion coefficients, i.e., molecular masses of antigen and antibodies. Isolation, purification, labelling of antigen and antibodies and use of any standards are not required. The analyses are carried out directly in crude biological fluids in which antigen and antibodies naturally occur. The results obtained are independent on the avidity of the Ab/Ag system to be analysed. The method was tested by using antigenic subunits of Newcastle disease virus (NDV) and four chicken anti-NDV immune sera of different avidities.

Antibody response to accelerated immunisation with diphtheria, tetanus, pertussis vaccine

From May, 1990, a new schedule of immunisation against diphtheria, tetanus, and pertussis (at 2, 3, and 4 months) replaced the previous more widely spaced schedule. A report that children had lower concentrations of diphtheria and tetanus antibodies a month after an accelerated schedule led us to undertake a controlled study to assess antibody response and the persistence of antibodies a year after immunisation in children receiving vaccine according to widely spaced and accelerated schedules. Concentrations of antibodies to diphtheria and tetanus toxoids and to Bordetella pertussis filamentous haemagglutinin (FHA) were measured by solid-phase radioimmunoassay (SP-RIA). We studied 57 children who received accelerated immunisation at median ages of 11, 16, and 21 weeks and two control cohorts (total n = 82) who received vaccine at median ages of 15, 21, and 45 weeks. 6-8 weeks after the third dose the accelerated-schedule group had lower (p < 0.0001) geometric mean concentrations of antibody to tetanus (0.522 [95% CI 0.383-0.710] vs 3.43 [2.45-4.81] IU/mL), diphtheria (0.266 [0.179-0.396] vs 2.39 [0.616-3.53] IU/mL), and FHA (0.044 [0.030-0.063] vs 0.270 [0.196-0.374] units/mL) than the longer-schedule group. 12 months after the third dose the differences between the groups had narrowed (tetanus 0.197 vs 0.341 IU/mL, p = 0.29; diphtheria 0.100 vs 0.131 IU/mL, p = 0.64; FHA 0.014 vs 0.016 units/mL, p = 0.72). At that time all children had tetanus antibody concentrations above protective levels (0.01 IU/mL); only 2 of 31 in the accelerated-schedule group and 3 of 31 in the longer-schedule group had diphtheria antibody concentrations below the protective level. The use of an accelerated schedule of diphtheria, tetanus, and pertussis vaccination is unlikely to lead to an increase in the proportion of children unprotected against these diseases before the preschool booster.

Calculation of the affinity constant KASS for solid phase antigen. A model system using monoclonal antibodies against the cat allergen Fel d I

In this report a procedure is described to calculate the affinity constant of an antibody for solid-phase Ag. KSP (KASS solid phase) was defined as the reciprocal of the concentration of Ag required for half saturation of the Ab-binding sites, extrapolated to an infinitely small concentration of Ab (semi-saturation plot). Using this procedure, the affinity of IgE antibodies can be measured without interference from 'invisible' IgG antibodies. As a model system, mAbs against the cat allergen Fel d I were used. Serial dilutions of Fel d I-Sepharose were incubated with serial dilutions of mAb Fd1a and Fd1b, with or without rabbit antibodies as 'invisible' antibodies. The Ab-binding capacity of Sepharose-coupled Fel d I was low: 4.15% and 2.13% of the expected value for mAb Fd1a and Fd1b, respectively, and this must be taken into account when calculating KSP. The K values of mAb Fd1a and Fd1b, calculated from the gamma axis intercept of the semi-saturation plot, were 85 (pmol/test)-1 and 65 (pmol/test)-1 respectively. Using the semi-saturation plot, KSP of mAb Fd1a was not affected by the presence of rabbit antibodies against Fel d I, confirming the applicability of the procedure for measuring the KSP of IgE in patient sera. For one cat-allergic patient the KSP of IgE and IgG4 for Fel d I were calculated and found to be 62 (pmol/test)-1 and 147 (pmol/test)-1 for IgE and IgG4 respectively.

Cooperativity in the antibody binding to surface-adsorbed antigen

The binding to surface-adsorbed antigen of monoclonal mouse IgG-antibodies (mAbs), with two different affinities to dinitrophenyl (DNP), was measured by a calibrated ELISA. The concentration-dependence of antibody binding to surface-bound antigen of different epitope densities was analysed using Scatchard plots. The dissociation of bound tritium-labelled antibodies was measured in the presence of unlabelled antibodies in the bulk. At low surface concentration of bound anti-DNP, both high-affinity mAb and low-affinity mAb show a positive cooperativity in the binding reaction to antigen of high epitope density. Using antigen of lower epitope densities, the positive cooperativity is more pronounced for low-affinity clones. At higher surface concentrations of bound anti-DNP, the Scatchard plots indicate a negative cooperativity of binding, which is also implied by the increased dissociation found in the presence of antibodies in solution. The study confirms previous findings that the binding of antibodies to surface-bound antigen not only depends on intrinsic antibody affinity measured in solution. Other factors, such as self-interaction, also affect the heterogeneous binding reaction.

The reactivities of HIV-1+ human sera with solid-phase V3 loop peptides can be poor predictors of their reactivities with V3 loops on native gp120 molecules

The binding of HIV-1+ human serum antibodies to solid phase-adsorbed V3 loop peptides from the IIIB, SF-2, and MN isolates was compared with the abilities of the same peptides to inhibit binding of the sera to the homologous or heterologous native gp120 molecules. The reactivities of the sera with the solid-phase peptides were found to be poor predictors of the potencies of the peptides as competitors in solution for antibody binding to the V3 loop in situ. Furthermore, the extent of cross-reaction of HIV-1+ human serum antibodies with the three V3 peptides in solid-phase assays was potently influenced by the presence or absence of nonionic detergent. Because the use of solid-phase V3 peptide assays is widespread, and there is no consensus on the use or omission of detergent, there is considerable potential for confusion. These factors should be considered when interpreting data derived from V3 peptide serology when this method is used to dissect the human immune response to HIV-1 infection. It is also shown in quantitative studies of HIV-1+ serum antibody binding to homologous and heterologous gp120s that the general anti-gp120 antibody response is significantly "type specific" for the homologous gp120.

Analyses of affinity distributions within polyclonal populations of antigen-specific antibodies. Evaluation of their accuracy in population detection using monoclonal antibodies

The potential of an ELISA based detection of affinity distributions within polyclonal populations of antigen-specific serum antibodies was assessed by analyzing defined probes composed of monoclonal antibodies (MAb). In a competitive binding ELISA in which the concentration of antigen in the liquid phase and the solid phase was varied, we analyzed mixtures containing defined percentage compositions of MAb exhibiting apparent affinity constants (aK) between 3 x 10(6) and 2 x 10(9) M-1 for Pseudomonas aeruginosa exotoxin A. Our results indicate that the detectability of antibody populations depends on the antigen concentrations in the solid phase and on the affinity distribution of the probe to be analyzed. In wells coated with high antigen concentrations, antibody titers reflected antibody concentrations, whereas at low antigen concentrations antibody titers primarily reflect antibody affinities. Independent of their affinities, subpopulations less than 10% could not be detected. Low affinity antibodies were preferentially underestimated. The degree of distortion depended on the composition of the probe to be analyzed. In general, the higher the absolute and the relative affinity of a population, the stronger was its capacity to interfere with the detection of other populations. As a consequence, the heterogeneity of affinity distributions in polyclonal samples may be substantially underestimated. The experiments reported provide guidelines for an optimal design and an adequate interpretation of ELISA based qualitative analyses of polyclonal antibody samples.

Affinity of anti-HBs antibody after hepatitis B vaccination

A novel method based on mathematical analysis of the dose-response curve obtained by radioimmunoassay was used to determine anti-HBs affinity and concentration in 29 healthy subjects vaccinated with plasma-derived hepatitis B vaccine. Anti-HBs affinity as well as its serum concentration increased with time in most vaccine recipients; however, the pattern of affinity maturation varied widely between individual subjects. In 20 subjects the highest increase in antibody affinity was observed after the third dose of vaccine, whereas in nine cases values of affinity remained low and relatively constant throughout the time of the study.

Microgravimetric determination of precipitable antigens and antibodies in native biological fluids

A method for the exact and simultaneous measurements of immunoprecipitable antigens and antibodies in native biological fluids is described. It is based solely on the microgravimetric analysis of at least two immunoprecipitates of different stoichiometric compositions. The volume ratios of antigen and antibody solutions for the preparation of the immunoprecipitates at equivalence are determined by the two-cross immunodiffusion technique. The proposed procedure has advantages over other methods for quantitative determination of proteins, since the separation, purification and labelling of immune components, as well as the use of standards, are not required. The method was tested by using human serum albumin (HSA) and three rabbit antiHSA immune sera of various avidities. Immunoprecipitates of different stoichiometric compositions were prepared at pH 5.0, 5.5, 7.0 and 8.6. The minimal precision of the method was +/- 0.2 micrograms of protein. The coefficients of variation were 0.14 to 0.38% for HSA and 0.03 to 0.08% for antiHSA antibody.

Application of a sol particle immunoassay to the determination of affinity constants of monoclonal antibodies

The affinity constants (Ka) of monoclonal antibodies (MAb) for binding to their corresponding antigens (Ag), unlabelled and in buffered solution were determined by the following procedure: 1. Incubation of MAb (fixed concentration) with Ag (concentration dilution series). 2. Rapid bound/free separation by adding immobilized second antibody, followed by centrifugation. 3. Determination of free Ag in the supernatant using a gold sol particle agglutination immunoassay (SPIA) in a microtitration plate format. 4. Calculations and interpretation were based on Scatchard and Sips plots. Ka values found by this procedure were found to be similar to those obtained by a radio-immunoassay (RIA) procedure. The present method avoids possible artefacts in Ka values introduced by the procedure or chemical modification due to labelling of MAb or Ag. It enables rapid, simultaneous screening of a considerable number of different MAbs under non-specialized (i.e. RIA) laboratory conditions.

An appraisal of polystyrene-(ELISA) and nitrocellulose-based (ELIFA) enzyme immunoassay systems using monoclonal antibodies reactive toward antigenically distinct forms of human C-reactive protein

The purpose of this study was to compare and contrast two enzyme immunoassay systems: the enzyme-linked immunosorbent assay (ELISA), which utilizes polystyrene microtiter plates as the adsorptive surface and the enzyme-linked immunoflow assay (ELIFA), which utilizes nitrocellulose membranes. The principal parameter under scrutiny was the denaturing or unfolding effects caused by the interaction of the protein with the adsorptive surfaces in each assay system. These effects were monitored by utilizing two conformationally distinct forms of human C-reactive protein (CRP), the native form of CRP and a denatured form (M-CRP), with a corresponding panel of monoclonal antibodies (MAbs) specific to either CRP or M-CRP. The results show that the ELIFA system was less sensitive than the ELISA system but that the ELIFA assay can be completed in less time than the ELISA. Also, adsorption of native CRP to the polystyrene surface in the ELISA system resulted in conformational changes of the adsorbed native CRP protein such that M-CRP reactive determinants were available for binding with anti-M-CRP MAbs, whereas native CRP adsorbed to the nitrocellulose membrane in the ELIFA system resulted in very limited conversion of CRP to M-CRP reactive epitopes. These results have important implications for development of immunoassays and screening of MAbs for proteins whose conformations may be affected by adsorption to various surfaces.

Theoretical analysis of the accuracy of calibrated immunoassays for measuring antibody concentration

The accuracy of calibrated immunoassays for measuring antibody concentration was analysed from a theoretical perspective. The study shows that there are theoretical limits on the accuracy of antibody immunoassays, which are determined by the affinity of the standard and unknown antibodies and the conditions chosen for the assay. As a result of these limits, calibration of an immunoassay with a standard antibody does not automatically ensure accurate measurements of antibody concentration. Extremely large errors may develop in affinity-dependent assays when the affinities of the standard and unknown antibodies are different. Assay conditions and the affinity of the standardizing antibody must be chosen carefully to measure antibody concentration accurately.

The influence of epitope density on the estimation of the affinity of antibody for complex antigens

The influence of the epitope density of the antigen on antibody affinity values determined by fluid- and solid-phase immunoassays was assessed. The affinity of the interaction of a panel of monoclonal anti-DNP antibodies of different affinities (as determined by equilibrium dialysis) for DNP-protein conjugates of various hapten substitution ratios was used as the test system. The results obtained showed that the epitope density of the antigen markedly influences the observed affinity values obtained by both experimental approaches. However, the monoclonal antibodies were ranked in affinity terms by both assays in a similar order to that given by equilibrium dialysis. It is concluded that provided due care is exercised in choosing an appropriate epitope density for the test antigen, these methods can be used to provide rapid estimations of average antibody affinities.

Antibody quantitation in human and veterinary medicine

The determination of antibodies is a matter of clinical importance since it may prove or disprove the existence of an immune reaction against a specific foreign or autologous antigen. It is useful to monitor the immune response against a vaccine or the disappearance of injected therapeutic antibodies and in fundamental immunological research or in industrial R & D. However, this determination of this particular analyte cannot end up with a significant mass measurement, as a result of the heterogeneity of the antibodies present in most samples (with the exception of monoclonal antibody research). This heterogeneity has multiple facets concerning the constant region (isotypes) and the variable region (degeneracy of the antibody response). What is measured is a binding activity of the antibodies for the corresponding antigen. Calibration is a challenge impossible to carry out. In all cases, the selection of an antibody calibrator results from a compromise. The expression of the results can be done in a variety of ways, and presently is far from being standardized. The present state of the art is characterized by the absence of reference method, the scarcity of reference preparation both for antibody and antigen and the lack of comparability of the methods. Improvement can be expected from the better knowledge of antigens and of their epitopes, the selection and the preparation of pure antigen molecule or synthetic molecules. There are a few technical possibilities to approach a better standardization. Finally, the results of such standardized methods should be interpreted operationally, without reference to ponderal estimations, for a specific clinical or epidemiological purpose. Each method should be validated by epidemiological studies.

Evaluation and interpretation of data obtained with immunoassays and DNA-DNA hybridization techniques

During the last decade several new analytical techniques have been developed for testing food products and clinical samples. One technique uses sensitive immunoassays such as enzyme-linked immunosorbent assay (ELISA) and latex agglutination. The most important step in developing sensitive immunoassays is the evaluation of the assay for specificity, cross-reactivity and sensitivity. False-negative results can easily be detected by adding known quantities of antigen to the sample. The most appropriate way to detect false-positive results is the specific inhibition of the immunological reaction by addition to the test-sample of either synthetic epitopes or anti-idiotype antibodies. The progress in recombinant DNA techniques now offers opportunities for application as analytical tools in food and clinical microbiology. Methods are being developed to detect microorganisms by their nucleic acid sequence using the so-called hybridization procedure. With this technique, labelled DNA fragments (probes) are hybridized with a complementary base sequence present in the microorganism. Foodborne pathogens can be detected by using a probe with a complementary base sequence which codes for toxin production. DNA-DNA hybridization techniques may replace the traditional cultural techniques for assaying pathogenic micro-organisms. However, more experience with these techniques is needed before further evaluation can be given.

The antigen-limited nature of microtiter ELISAs requires partial depletion of IgG to permit reliable determination of rabbit serum IgA antibody activity

The antigen-limiting nature of microtiter ELISAs predicts that antibodies of minor classes may be underestimated when the same specimen contains large amounts of IgG antibodies specific for the same antigen. Such competitive inhibition can be diagnosed from ELISA titration plots. A method is described to eliminate the negative effects of this competition on the detection of IgA antibodies in rabbit serum. The detectability of rabbit serum antibodies to ovalbumin and bovine serum albumin is increased 10-fold by prior treatment of 1:100 dilutions of serum with 1% Cowan I S. aureus. High concns of S. aureus, e.g. 10%, completely deplete serum IgG without loss of IgA. However, concns higher than 1% do not lead to additional improvement in the detectability of IgA antibodies in the systems studied. The method is rapid, inexpensive and shows no non-specific depletion of IgA from either serum or bronchoalveolar lavage fluid.

Monoclonal antibodies against human haptoglobin

Three monoclonal antibodies: 2.36.71.41, 7.60.66.55, and 18.4.40. 80 to human haptoglobin 2-1 were produced, purified and characterized. The affinity constants ranged within 0.3-2.4 x 10(8) M-1. The monoclonal antibodies 7.60.66.55 and 18.4.40.80 reacted with beta subunit of haptoglobin, showed similar epitope affinities and epitope densities on main haptoglobin types. However, the epitope on the haptoglobin molecule for the monoclonal antibody 18.4.40.80 occupied somewhat more surface than that for the antibody 7.60.66.55. The monoclonal antibody 2.36.71.41 was able to bind both alpha and beta chains of haptoglobin. In ELISA affinity reactions this antibody achieved with haptoglobin 2-2 the plateau phase at absorbance values 15% higher than with haptoglobin 2-1, and 60% higher than with haptoglobin 1-1. End-point titration of the monoclonal antibody 2.36.71.41 against three haptoglobin types showed differences in titer, indicating distinct epitope densities.

Terasaki-ELISA for murine IgE. III. Determination of concentration and functional affinity by sequential equilibrium binding analysis

A simple Terasaki tray-based ELISA technique with a fluorescent detecting system has been used to determine the affinity of murine IgE antibodies. The system was shown to be sensitive enough to measure affinities in the range of 10(-6)-10(-10) M as well as detect IgE antibodies down to a limit of 0.1 ng/ml. The results, expressed as arbitrary fluorescence units (AFU), were compared with those obtained using equilibrium dialysis for several DNP-specific IgE monoclonal antibodies of known affinities yielding KD values. The relationship between KAFU and KD established a conversion factor which could then be used to compute KD from KAFU, provided the detection system remained identical. Based on the equations proposed, an alternative method for the quantitation of murine IgE is described which is independent of the affinity of IgE for the coated antigen.

Definition of Brucella A and M epitopes by monoclonal typing reagents and synthetic oligosaccharides

The paradigm that Brucella A and M epitopes are simultaneously expressed on single cells and within one antigen molecule was reinvestigated by using polysaccharide-specific murine monoclonal antibodies. Monoclonal antibodies were generated to the M antigen of Brucella melitensis 16M. Chemically defined lipopolysaccharides and O polysaccharides from Brucella abortus 1119-3, B. melitensis 16M, and Yersinia enterocolitica O:9 were used to dissect the binding profiles of the B. melitensis antibodies and an additional set of antibodies available from a B. abortus fusion experiment. Binding specificities were rationalized in terms of prototype A- and M-antigen structures, an interpretation supported by competitive binding studies with O polysaccharides and synthetic oligosaccharide analogs of the A and M antigens. Three binding patterns were characterized. Antibodies specific for the A antigen required five contiguous alpha 1,2-linked 4,6-dideoxy-4-formamido-D-mannopyranosyl residues, while antibodies with equal affinities for A or M epitopes were effectively inhibited by alpha 1,2-linked tri- or tetrasaccharides. Specificity for the M epitope correlated with binding of a critical disaccharide element alpha-D-Rha4NFo(1----3)alpha-D-Rha4NFo bracketed by alpha 1,2-linked residues. The binding profiles of Brucella monoclonal antibodies were consistent with the concept of simultaneous expression of A and M epitopes within a single molecule. A epitopes were present in the M antigen, and the discovery of isolated alpha 1,3 linkages in the A antigen suggests that M epitopes occur in all A antigens. Three monoclonal antibodies are proposed as standard reagents for the detection and identification of Brucella A and M antigens.

Estimation of the avidity of antibodies in polyclonal antisera against Streptococcus pneumoniae type 3 by inhibition ELISA

The reliability of the determination of antibody avidity in polyclonal sera by indirect sandwich ELISA was studied. Binding of IgM and IgG (sub)classes in unpurified serum to Streptococcus pneumoniae type 3 capsular polysaccharide, which was coated onto ELISA plates, was inhibited with different inhibitors. The inhibitor concn at which 50% inhibition of antibody binding to the ELISA coat was achieved, was used as a measure for antibody avidity. As this 50% inhibition value is dependent upon the dilution of the serum and thus upon the initial amount of free antibody, it is necessary to define (a narrow range of) final ELISA absorbance values to which the dilutions of non-inhibited sera have to be adjusted. The shapes of the serum dilution curves have a good correlation with the numerical 50% inhibition values of the antibody avidity. The inhibition ELISA is suitable to compare the avidity values of the different antibody isotypes, but two remarks should be made: (1) antibody heterogeneity should be considered to influence the results and prevent the accurate measurement of absolute numerical avidity values. Because in the ELISA system merely antibody "activity" is measured, comparison of the efficacy of vaccines by means of the 50% inhibition (avidity) value of various antibody (sub)classes can still be performed in a reliable way; (2) results of the determination of the 50% inhibition values of the different antibody (sub)classes showed them to be dependent on the molecular ratio between antibody (sub)class levels. More aspects of the determination should be taken into account, like shapes of simple dilution curves, influences of various inhibitor concns in the diluent and whole (extended) inhibition curves.

Specific and sensitive detection of purified HLA molecules in an ELISA using mouse, rabbit and human anti-HLA antibodies

An ELISA detecting anti-HLA antibodies of rabbit, mouse or human origin was developed using plates coated with HLA molecules purified on affinity columns. The sensitivity of the assay was optimal when coating was performed in PBS, pH 7.8 at 4 degrees C for 6-16 h and using a serum incubation period of 16 h at 4 degrees C. The optimum protein concentration for coating was estimated to be 1 micrograms/ml. With monoclonal anti-HLA sera, antipeptide antibodies from mice or rabbit and human alloantisera, this method appeared to be highly sensitive, very specific and reproducible.

Microplate ELISA for detection of antibodies to DNA in patients with systemic lupus erythematosus: specificity and correlation with Farr radioimmunoassay

A 96-well microplate ELISA for the detection of antibodies to DNA is described. A number of buffers and precoating treatments were used to evaluate the optimal method for coating the plate with DNA. These included pretreatment of the plates with poly-L-lysine or protamine sulfate, and posttreatment with glutaraldehyde, none of which improved the performance of the assay. Whereas bicarbonate and borate coating buffers gave equivalent and satisfactory results, TRIS buffer resulted in very high binding of immunoglobulin to wells not coated with antigen. Sera from groups of patients with autoimmune disease as well as normal sera were tested against plates optimally coated with native E. coli DNA, calf thymus DNA, and heat-denatured DNA. Using native E. coli DNA, virtually none of 35 normal sera had any detectable antibody. With this antigen, as well as with native calf thymus DNA, significant levels of DNA antibody were found only in SLE patients. Most patients with SLE or drug-induced lupus, as well as some patients with rheumatoid arthritis and normal individuals had antibodies that bound to heat-denatured (single-stranded) DNA. Using either native E. coli or calf thymus DNA, a good correlation was found between the amount of DNA antibody detected by ELISA and the Farr-type radioimmunoassay.

Calculation of antibody affinity in homogeneous and heterogeneous systems

Antibody affinity is an important determinant of all antibody-antigen reactions. A new computer program, AFCRV, was developed to calculate binding constants with data from a radioimmunoassay on most microcomputers in the laboratory by using constant-ratio dilution curves. Evaluation of a homogeneous or heterogeneous antibody in the presence of a single antigen can be accomplished.

Effect of antibody affinity on the isotherm of antibody binding to surface-immobilized antigen

The binding of monoclonal antibodies to surface-adsorbed antigen was studied. Mouse IgG antibodies directed against dinitrophenyl groups (DNP) and O6-ethyl-2'-deoxyguanosine with known affinity for the antigen were used. The hapten was coupled to a protein, bovine serum albumin (BSA) or keyhole limpet hemocyanin, and adsorbed to polystyrene or silicone surfaces. Four different DNP-BSA epitope densities were used. Antibodies were incubated with the antigen-coated surface overnight. The bound antibodies were detected either optically by ellipsometry or by enzyme-conjugated anti-mouse IgG antibodies in the common ELISA technique. Absorbance values from ELISA measurements were transformed to surface density through calibration by ellipsometry. The experimental data showed that the binding of a high affinity antibody (Ka = 2.0 X 10(10] was diffusion rate limited after 24 h incubation time. Identical binding isotherms were found for high and low affinity clones of anti-DNP antibodies (Ka = 4.1 X 10(7) and 3.5 X 10(5] when antigen of high epitope density was used. At low epitope density the amount of bound low affinity antibodies decreased. Electron microscopy was used for studies of the distribution of colloidal gold-antibody complexes bound to surface-immobilized antigen. The results of the experiment showed that low affinity antibodies were bound in clusters whereas high affinity antibodies bound as single particles. These findings were related to the ELISA measurements. The results indicate that the binding isotherm of antibody to surface adsorbed antigen is not merely a reflection of the intrinsic antibody affinity measured in solution. Other macromolecular properties of antibodies, e.g., lateral intermolecular interactions and phase separation, affect the heterogeneous binding reaction.

Kinetics of antigen-antibody reactions at solid-liquid interfaces

The kinetics of antigen-antibody reactions is reviewed with special attention paid to the specific properties at solid-liquid interfaces. Theories of possible diffusion limitation in forward reaction rates are compared to experiments. It is found that the intrinsic forward reaction rate in the bimolecular antigen-antibody reaction is normally not limited by diffusion either in solution or at the solid-liquid interface. However, reactions at the solid-liquid interface can be diffusion limited due to depletion of reactants close to the surface. This effect depends on geometry, intrinsic reaction rate and surface concentration of receptor molecules. Normally cell surface reactions are not diffusion limited whereas reactions at artificial surfaces often are limited by diffusion. When not limited by diffusion it is also found that the intrinsic forward and reverse reaction rates are lower for surface reactions compared to reactions in solution. Antigen-antibody reactions at solid-liquid interfaces can often be considered as practically irreversible and limited by mass transport or steric interactions.

Epitope masking and immunodominance--complications in the selection of monoclonal antibodies against HCG

Spleen cells from Balb/c mice immunized with HCG were subjected to the normal hybridoma procedures. The resulting MCA's (from 78 clones) were evaluated in radioimmunoassay, haemagglutination and enzyme immunoassay with whole HCG. The RIA analysis was extended to include HCG subunits and intact LH. The alpha-chain of HCG was found to be strongly immunodominant, as shown by the very high frequency (46 of 78) of MCA's directed at the alpha-chain epitopes. These antibodies would bind luteinizing hormone as well as HCG. Only 1 of the 78 clones was specific for the B subunit of HCG, in RIA. This clone was later found to be positive for LH in EIA, thus making the derivation of antibodies specific for this B-epitope extremely difficult. Most MCA's were found to be applicable in the three types of assay systems (48 of 78), but some were compatible in just one or two of the systems. These differences are believed to be due to epitope masking resulting from the way in which the antigen was handled and presented in the different systems.

The effects of immunoglobulin isotype and antibody affinity on complement-mediated inhibition of immune precipitation and solubilization

We have studied complement-mediated prevention of precipitation (PIP) and solubilization of immune complexes (IC) formed with DNP19-BSA and murine monoclonal antibodies (MCAs) of different isotypes and affinities. PIP was effective for IC formed with IgG1 and IgM antibodies, but not for IgA MCA. For IgG1 MCAs, affinity appeared to exert a minor effect on PIP, and IC formed in antibody excess or at equivalence were retained in solution more readily than those formed in antigen excess. For IgM MCAs affinity and antigen-antibody ratio did not affect PIP. As PIP did not occur in Mg-EGTA, it was concluded that PIP was entirely classical pathway dependent. Solubilization of IC containing IgG1 MCAs occurred more rapidly and to a greater extent with low affinity antibodies and an inverse relationship between affinity and the extent of solubilization was observed. Complexes formed with IgG1 MCAs were solubilized relatively poorly when formed in antigen excess. In contrast, affinity and antigen-antibody ratio did not influence the rate and extent of solubilization of IC containing IgM MCAs. IC formed with IgG2b were solubilized rapidly whereas those formed with IgG2a or IgA were solubilized poorly. The relative contributions of the classical and the alternative pathways to solubilization varied with each antibody and the effect of antigen-antibody ratio on these relative contributions was inconsistent.

The effect of divalent and univalent binding on antibody titration curves in solid-phase ELISA

This paper describes the influence of antigen coating concentration, epitope density per antigen molecule and anti-immunoglobulin reagents on antibody titration curves in solid-phase ELISA. Based on results obtained with fluorescein as the hapten and monoclonal anti-fluorescein antibody, which were confirmed in another antigen-antibody system, it is concluded that: (a) Antibody titration curves are independent of antigen-coating concentration in a limited range of concentrations only. (b) The complex between one antibody and two epitopes ('divalent binding') is more stable than the complex between one antibody and one epitope ('univalent binding). The ratio between divalent and univalent binding depends on the epitope density per antigen molecule and on the antigen-coating concentration. (c) The prozone phenomenon can be explained by an increased instability of plate bound antibodies due to a shift from divalent to univalent binding. (d) In solid-phase ELISA a correct evaluation of the antiserum specificity can be performed only if it is ascertained that all target antigens are coated under saturating conditions.

Linear response curves from an ELISA assay: measurement of anti-myelin IgG and IgM during experimental allergic neuritis

An ELISA method for generating linear dose-response curves for measuring anti-myelin IgG and IgM is described. Linearity is achieved without logarithmic transformations. This test is used to measure plasma levels of anti-myelin antibodies in rabbits with EAN. Results are expressed as relative concentrations of antibody in arbitary mass units with 95% confidence limits fitted by inverse prediction. In the 8 animals studied, appearance of anti-myelin antibody always preceded onset of clinical signs and neither anti-myelin IgG nor IgM could be detected in any animal pre-inoculation. Five out of 8 animals displayed steady and progressive increases in anti-myelin IgG with the remaining 3 animals showing plateaus in IgG levels 21-30 days post-inoculation. Increases in anti-myelin IgG were generally parallelled by increases in disease severity. However, 2 animals showed recovery and relapse from EAN with no change in plasma levels of anti-myelin IgG. Patterns of production of anti-myelin IgM varied with some animals displaying early peaks while others showed gradual increases.

Measurement of monoclonal antibody affinity by non-competitive enzyme immunoassay

Enzyme-linked immunoadsorbent assay (EIA) has widespread use for the measurement of antibody concentration. The affinity constant (Kaff) of the antibody has an effect upon the quantification by EIA. It is thus important to be able to measure Kaff by solid-phase EIA. Based upon the Law of Mass Action and using serial dilutions of both antigens (coating the plate) and antibody, Kaff has been measured by EIA. A microtiter plate was coated with antigen (Ag) and then incubated with monoclonal antibody (Ab). The plate was sequentially incubated with a second enzyme-antibody conjugate (EAC) and with the enzyme substrate. The amount of Ab adherent to Ag on the plate [Ag Ab] and [Ag2 Ab] was reflected by the enzyme product measured by OD. The use of serial dilutions of Ab resulted in a sigmoid curve of OD versus logarithm of total Ab added to the well. Comparison of the OD at the upper plateau (OD-100) for different antibodies was a reflection of the relative number of epitopes on the Ag that were identified by the different antibodies, provided excessive EAC was used. [Ab]t and [Ab']t were the measurable total antibody concentrations in the wells at OD-50 and OD-50' for plates coated with [Ag] and [Ag'], respectively. [Ag] and [Ag'] were not true antigen concentrations, but were a measure of antigen density on the plate. For [Ag'] = [Ag]/2, Kaff = 1/2(2[Ab']t-[Ab]t. Using five different anti-CEA antibodies and different proportions of CEA in the coating solution, Kaff was measured. Kaff determined by EIA correlated well with Kaff measured by soluble phase inhibition assay. This EIA method of estimation of Kaff is simple, rapid, and reliable.

A monoclonal-based, two-site enzyme immunoassay of human insulin

A procedure is described for the efficient production of insulin-specific monoclonal antibodies, which involves primary and secondary immunization of BALB/c mice in the hind footpads with bovine or porcine insulin and fusion of lymphocytes from popliteal lymph nodes with a P3x63 murine myeloma line. With this protocol, over 200 positive hybrids were obtained from four separate fusions. Dissociation constants of 31 purified monoclonals, cross-reacting with human insulin, were determined by two different methods and ranged between 4 X 10(-10) and 2 X 10(-6) mol/l. 24 monoclonals were biotinylated, paired in all possible combinations and tested by ELISA for their capacity to simultaneously bind to human insulin in a two-site assay. More than 40 monoclonal pairs were found which formed a sandwich with the hormone. The development of a simple and rapid one-step enzyme immunoassay is described, which involves a first monoclonal bound to the wells of a microtiter plate and a second monoclonal conjugated to alkaline phosphatase. With this assay, insulin can be determined in a range between 0.08 and 7.5 ng/ml in 3-4 h.