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

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.

A highly sensitive lateral flow immunoassay based on a group-specific monoclonal antibody and amorphous carbon nanoparticles for detection of sulfonamides in milk

To meet high-throughput screening of the residues of sulfonamides (SAs) with high sensitivity toward sulfamethazine (SM2) in milk samples, a new highly sensitive lateral flow immunoassay (LFA) based on amorphous carbon nanoparticles (ACNs) was developed. First, a group-specific monoclonal antibody 10H7 (mAb 10H7) that could recognize 25 SAs with high sensitivity toward SM2 (IC₅₀ value of 0.18 ng/mL) was prepared based on H1 as an immune hapten and H4 as a heterologous coating hapten. Then, mAb 10H7 was conjugated to ACNs as an immune probe for LFA development. Under the optimized conditions, the LFA could detect 25 SAs with the cut-off value toward SM2 of 2 ng/mL, which could meet the requirement for detection of SAs. In addition, the LFA developed was also used for screening SAs' residues in real milk samples, with results being consistent with HPLC-MS/MS. Thus, this LFA can be used as a high-throughput screening tool for detection of SAs.

Novel Latex Microsphere Immunochromatographic Assay for Rapid Detection of Cadmium Ion in Asparagus

Recently, concerns about heavy metal cadmium ion (Cd²⁺) residue in asparagus have been frequently reported, and there is an urgent need to develop an effective, sensitive, and rapid detection method for Cd²⁺. In this study, we innovatively combined molecular microbiology to carry out the comparative screening of Cd²⁺ chelators in a green, efficient, and specific way. The knock-out putative copper-transporter gene (pca1Δ) yeast strain with high sensitivity to Cd²⁺ was first used to screen the Cd²⁺ chelator, and the optimum chelator 1-(4-Isothiocyanatobenzyl)ethylenediamine-N,N,N,N′-tetraacetic acid (ITCBE) was obtained. Additionally, a rapid latex microsphere immunochromatographic assay (LMIA) was developed, based on the obtained monoclonal antibody (mAb) with high specificity and high affinity (affinity constant Ka = 1.83 × 10¹⁰ L/mol), to detect Cd²⁺ in asparagus. The 50% inhibitive concentration (IC₅₀) of test strip was measured to be 0.2 ng/mL, and the limit of detection (IC₁₀) for qualitative (LOD, for visual observation) and quantitative detection (LOQ, for data simulation) of the test strip was 2 ng/mL and 0.054 ng/mL, respectively. In all, the developed mAb-based LMIA shows a great potential for monitoring Cd²⁺ in asparagus, even in vegetable samples.

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.

Novel monoclonal antibody-sandwich immunochromatographic assay based on Fe3O4/Au nanoparticles for rapid detection of fish allergen parvalbumin

In this study, a rapid sandwich immunochromatographic assay (ICA) was developed to detect parvalbumin (PV). Firstly, two optimum primary monoclonal antibody (mAb) against PV had been screened out: mAb1 was used as the capture antibody, and mAb2 conjugated to Fe₃O₄/Au nanoparticles (Fe₃O₄/AuNPs) that served as a detection reagent. Using this pair of mAbs, a sandwich ICA strip based on Fe₃O₄/AuNPs was developed. The results showed that the color intensity of test line positively correlated with the PV concentration in the standard or spiked sample. The limit of detection for qualitative (LOD) and quantitative detection (LOQ) were 2 ng/mL and 0.691 ng/mL, respectively. Besides, the detection time of this ICA strip was within 15 min. The recovery rates ranged from 104.0% to 117.4%, within an acceptable level (80-120%). Moreover, the developed assay also showed high cross reaction in different fish species. These results demonstrated that the established test strip has the potential to be used as a rapid screening tool for large scale determination of PV in foodstuffs.

Construction of an HRP-streptavidin bound antigen and its application in an ELISA for porcine circovirus 2 antibodies

A fusion protein SBP-Cap∆41, consisting of Cap∆41 (without 41 amino acids at the N-terminus) protein of porcine circovirus 2 (PCV2) and a streptavidin binding peptide (SBP), was constructed. This fusion protein binds to HRP-labeled streptavidin (HRP-SA) through high affinity between SBP and SA, forming an HRP-streptavidin bound antigen (Hsb-Ag) with both immunoreactivity and enzymatic activity, which can be used in a double-antigen sandwich ELISA for detection of PCV2 antibodies. Comparison of the characteristics of the HSb-Cap∆41 and chemical conjugates of the recombinant Cap∆41 protein showed that the HSb-Cap∆41 based double-antigen sandwich ELISA (HBDS-ELISA) had higher specificity and sensitivity. Use of the HBDS-ELISA detected PCV2-IgG in 9 injected pigs as early as 10 days p.i., 3 days earlier than both a double-antigen sandwich ELISA (DS-ELISA) based on a chemically conjugated antigen, and a commercial indirect ELISA kit.

Screening a Phage Display Library for Two Novel OmpU-Binding Peptides with Adhesion Antagonistic Activity against Vibrio mimicus

Vibrio mimicus is a pathogen that causes ascites disease in fish. We have previously demonstrated that the outer membrane protein U (OmpU) is an important adhesin in V. mimicus. Here eight specific OmpU-binding phage clones, which presented three different OmpU-binding peptides (designated P1, P2, P3), were screened from a commercially available phage displayed 12-mer peptide library using rOmpU protein as target. Then, synthetic OmpU-binding peptides were measured for their adhesion antagonistic activity and binding affinity via adhesion inhibition test and non-competitive ELISA, respectively. The results showed that after co-incubated with the mixture of rOmpU and P3, visible green fluorescence could be observed on the epithelioma papulosum cyprinidi (EPC) cells surface; while the EPC cells co-incubated with the mixture of rOmpU and P1/P2 exhibited little green fluorescence. The average adhesion number of V. mimicus 04–14 isolate before and after treatment with peptide was 21.4 ± 1.5, 20.8 ± 0.8 (irrelevant peptide), 20.2 ± 0.5 (P3), 5.1 ± 0.7 (P1) and 3.4 ± 0.8 (P2), respectively. There was a significant decrease in the adhesive level of 04–14 isolate treated with P1/ P2 compared to the untreated isolate (p<0.01). The affinity constants of P1 and P2 were (6.17 ± 0.19) × 10⁸ L/mol and (1.24 ± 0.56) × 10⁹ L/mol, respectively. Furthermore, protective effects of P1 and P2 on grass carps challenged with V. mimicus were preliminary detected. It was found there was delayed death of fish in the groups treated with P1/P2, and the survival rate of challenged fish improved with the increase of the dose of adhesion antagonistic peptide. Taken together, two novel OmpU-binding peptides, which possessed adhesion antagonistic activity, high affinity and a certain degree of antibacterial activity against V. mimicus, were screened and identified.

A Human Anti-Toll Like Receptor 4 Fab Fragment Inhibits Lipopolysaccharide-Induced Pro-Inflammatory Cytokines Production in Macrophages

The results of clinical and experimental studies suggest that endotoxin/toll-like receptor 4 (TLR4)-mediated proinflammatory and profibrotic signaling activation is critical in the development of hepatic fibrosis. However, studies examining the role of specific TLR4 inhibitor are still lacking. The present study was aimed to prepare a human anti-TLR4 Fab fragment, named hTLR4-Fab01, and to explore its immune activity. We screened the positive clone of anti-human TLR4 phagemid from a human phage-display antibody library using recombinant TLR4 protein, which was used as template cDNA for the amplification of variable regions of the heavy (V_H) chain and light chain (V_L), then coupled with highly conserved regions of the heavy chain domain 1 (C_H1) and the light chain (C_L), respectively. Thus, the prokaryotic expression vector pETDuet-1 of hTLR4-Fab01 was constructed and transformed into Escherichia coli (E. coli) BL21. The characteristic of hTLR4-Fab01 was examined by SDS-PAGE, Western blotting, ELISA, affinity and kinetics assay. Further, our data demonstrate that hTLR4-Fab01 could specifically bind to TLR4, and its treatment obviously attenuated the proinflammatory effect, characterized by less LPS-induced TNF-α, IL-1, IL-6 and IL-8 production in human macrophages. In conclusion, we have successfully prepared the hTLR4-Fab01 with efficient activity for blocking LPS-induced proinflammatory cytokines production, suggesting that the hTLR4-Fab01 may be a potential candidate for the treatment of hepatic fibrosis.

Novel avian single-chain fragment variable (scFv) targets dietary gluten and related natural grain prolamins, toxic entities of celiac disease

BACKGROUND

Celiac disease (CD) is a chronic, small intestinal inflammatory disease mediated by dietary gluten and related prolamins. The only current therapeutic option is maintenance of a strict life-long gluten-free diet, which implies substantial burden for CD patients. Different treatment regimes might be feasible, including masking of toxic celiac peptides with blocking antibodies or fragments thereof. The objective of this study was therefore to select and produce a recombinant avian single-chain fragment variable (scFv) directed against peptic-tryptic digested gliadin (PT-Gliadin) and related celiac toxic entities.

RESULTS

Gluten-free raised chicken of same age were immunized with PT-Gliadin. Chicken splenic lymphocytes, selected with antigen-coated magnetic beads, served as RNA source for the generation of cDNA. Chicken VH and VL genes were amplified from the cDNA by PCR to generate full-length scFv constructs consisting of VH and VL fragments joined by a linker sequence. ScFv constructs were ligated in a prokaryotic expression vector, which provides a C-terminal hexahistidine tag. ScFvs from several bacterial clones were expressed in soluble form and crude cell lysates screened for binding to PT-Gliadin by ELISA. We identified an enriched scFv motif, which showed reactivity to PT-Gliadin. One selected scFv candidate was expressed and purified to homogeneity. Polyclonal anti-PT-Gliadin IgY, purified from egg yolk of immunized chicken, served as control. ScFv binds in a dose-dependent manner to PT-Gliadin, comparable to IgY. Furthermore, IgY competitively displaces scFv from PT-Gliadin and natural wheat flour digest, indicating a common epitope of scFv and IgY. ScFv was tested for reactivity to different gastric digested dietary grain flours. ScFv detects common and khorasan wheat comparably with binding affinities in the high nanomolar range, while rye is detected to a lesser extent. Notably, barley and cereals which are part of the gluten-free diet, like corn and rice, are not detected by scFv. Similarly, the pseudo-grain amaranth, used as gluten-free alternative, is not targeted by scFv. This data indicate that scFv specifically recognizes toxic cereal peptides relevant in CD.

CONCLUSION

ScFv can be of benefit for future CD treatment regimes.

Identification of a high-affinity monoclonal antibody against ochratoxin A and its application in enzyme-linked immunosorbent assay

Ochratoxin A (OTA) is one of the most commonly occurring mycotoxins produced by some species of Aspergillus and can contaminate cereal and cereal products. A high-affinity anti-OTA monoclonal antibody (mAb) was generated from a hybridoma cell line 2D8 using splenocytes from a BALB/c mouse immunized with synthesized OTA-bovine serum albumin conjugate. The mAb 2D8 is specific with high affinity (3.75 × 10(9) L/M). An indirect competitive ELISA (ic-ELISA) was then developed using this mAb for quantitative determination of OTA in corn and feed samples. Using the optimized conditions, there was good linearity between OTA concentration and competitive inhibition (y = -0.6076x + 0.2441, R(2) = 0.9923) with the working range from 2.4 to 23.6 μg/kg, IC50 at 7.6 μg/kg and lower limit of detection at 1.4 μg/kg. The recovery rates in spiked samples were 91.2-110.3%. Of the 56 corn and feed samples, this ic-ELISA and a commercial kit both found the same 13 samples positive for OTA with good linear correlation between the two methods in OTA quantification (R(2) = 0.9706). We conclude that this ic-ELISA can be used for rapid and quantitative screening of corn and feed samples for the presence of OTA.

A murine-human chimeric IgG antibody against vascular endothelial growth factor receptor 2 inhibits angiogenesis in vitro

Vascular endothelial growth factor receptor 2 (VEGFR2) has been reported to play an important role in angiogenesis and tumorigenesis. A murine anti-VEGFR2 mAb (A8H1) has been established in a previous study. To reduce the incompatibility of the murine mAb for human use, the chimeric anti-VEGFR2-IgG was developed by genetic recombination of the variable regions of the A8H1 antibody and the constant regions of human IgG, and was expressed in Sp2/0 cells transfected with the two recombinant vectors containing the heavy chain and the light chain regions. After screening, clone 2F12 was selected and was found to stably secrete the murine-human chimeric anti-VEGFR2-IgG (coded 2F12). This chimeric IgG maintained the specificity and the affinity of the parental murine antibody against VEGFR2, and effectively identified VEGFR2 expressed on the surface of HUVECs and BEL-7402 cells. Furthermore, the 2F12 antibody demonstrated inhibition of angiogenesis in vitro, such as proliferation, migration, invasion and tube formation of HUVECs. This murine-human chimeric IgG may be considered for further development as an anti-angiogenesis and anti-tumor agent.

A rapid subtractive immunization method to prepare discriminatory monoclonal antibodies for food E. coli O157:H7 contamination

To detect food E. coli O157:H7 contamination rapidly and accurately, it is essential to prepare high specific monoclonal antibodies (mAbs) against the pathogen. Cyclophosphamide (Cy)-mediated subtractive immunization strategy was performed in mice to generate mAbs that react with E. coli O157:H7, but not with other affiliated bacteria. Specificity of 19 mAbs was evaluated by ELISA and/or dot-immunogold filtration assay (DIGFA). Immunogloubin typing, affinity and binding antigens of 5 selected mAbs were also analysed. MAbs 1D8, 4A7, 5A2 were found to have high reactivity with E. coli O157:H7 and no cross-reactivity with 80 other strains of bacteria including Salmonella sp., Shigella sp., Proteus sp., Yersinia enterocolitica, Staphylococcus aureus, Klebsiella pneumoniae, Citrobacter freundii and other non-E. coli O157:H7 enteric bacteria. Their ascetic titers reached 1∶10⁶ with E. coli O157:H7 and affinity constants ranged from 1.57×10¹⁰ to 2.79×10¹⁰ L/mol. The antigens recognized by them were different localized proteins. Furthermore, immune-colloidal gold probe coated with mAb 5A2 could specifically distinguish minced beef contaminated by E. coli O157:H7 from 84 other bacterial contaminations. The Cy-mediated subtractive immunization procedure coupled with hybridoma technology is a rapid and efficient approach to prepare discriminatory mAbs for detection of E. coli O157:H7 contamination in food.

The novel use of a routine quantitative system to analyze the activity, content and affinity of an antibody to hepatitis B core antigen

BACKGROUND

Currently, quantitative measurement of serum markers of HBV infection has been widely used, but commonly used analysis for specific antibodies only involves the measurement of the total antibody activity, and the binding affinity or protein content is rarely analyzed.

OBJECTIVE

To investigate the detailed features of an antibody to hepatitis B core antigen (anti-HBc) during different periods of hepatitis B virus (HBV) infection with a new method of analysis.

STUDY DESIGN

Serum samples were collected from patients that were positive for the anti-HBc antibody. On the basis of the other serological markers in the samples, all patients were divided into a hepatitis B surface antigen (HBsAg)-positive group and an antibody to hepatitis B surface antigen (anti-HBs)-positive group. All samples were diluted 2-, 20- and 200-fold. Anti-HBc quantification was measured with a chemiluminescent microparticle immunoassay; total anti-HBc activity, protein content and affinity were calculated according to a measured value of each dilution. Serum HBV DNA load and alanine aminotransferase (ALT) levels were also measured.

RESULTS

The total anti-HBc activity in the HBsAg-positive group was statistically higher than that in the anti-HBs-positive group (p<0.05). The anti-HBc protein content during active HBV infection was statistically higher than during the convalescence stage (p<0.05), while anti-HBc affinity during HBV infection was lower than during recovery. There were correlations among total activity, affinity, protein content of anti-HBc, and ALT, HBV-DNA (p<0.05).

CONCLUSIONS

It is potentially possible to predict the status of HBV infection by measuring total activity, protein content and affinity of anti-HBc.

Characterization, epitope identification and mechanisms of the anti-septic capacity of monoclonal antibodies against macrophage migration inhibitory factor

Sepsis is characterized by uncontrolled inflammatory responses. Macrophage migration inhibitory factor (MIF) has been shown to play an important role in the progression of sepsis thus is a potential therapeutic target. The aim of this study is to produce IgG anti-MIF monoclonal antibodies (mAbs) with anti-septic abilities in vivo and to determine mechanisms of their function. We generated 8 IgG anti-MIF mAbs with high specificity and 3 of them showed potent protective abilities in murine lethal peritonitis induced by cecal ligation and puncture (CLP). One anti-MIF mAb, F11, showed 100% protection within 72 h after sepsis induction and 72% mice treated with this mAb survived up to 84 h with reduced lung and kidney pathology. F11 treatment also reduced tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) levels in septic mice. We further found that all 8 anti-MIF mAbs recognized the same epitope located in the amino acid residue 1-20 region of the N terminus of the MIF protein. Three of the mAbs, F11 in particular, inhibited tautomerase activity in association with their protective effect on CLP mice. Thus, we have produced anti-MIF mAbs that protected mice from CLP-induced sepsis by recognizing the same epitope domains in MIF. These mAbs are promising candidates for further development of therapeutics against inflammatory diseases.

Deconvolution of antibody affinities and concentrations by non-linear regression analysis of competitive ELISA data

Physiological responses of the adaptive immune system are polyclonal in nature whether induced by a naturally occurring infection, by vaccination to prevent infection or, in the case of animals, by challenge with antigen to generate reagents of research or commercial significance. The composition of the polyclonal responses is distinct to each individual or animal and changes over time. Differences exist in the affinities of the constituents and their relative proportion of the responsive population. In addition, some of the antibodies bind to different sites on the antigen, whereas other pairs of antibodies are sterically restricted from concurrent interaction with the antigen. Even if generation of a monoclonal antibody is the ultimate goal of a project, the quality of the resulting reagent is ultimately related to the characteristics of the initial immune response. It is probably impossible to quantitatively parse the composition of a polyclonal response to antigen. However, molecular regression allows further parameterization of a polyclonal antiserum in the context of certain simplifying assumptions. The antiserum is described as consisting of two competing populations of high- and low-affinity and unknown relative proportions. This simple model allows the quantitative determination of representative affinities and proportions. These parameters may be of use in evaluating responses to vaccines, to evaluating continuity of antibody production whether in vaccine recipients or animals used for the production of antisera, or in optimizing selection of donors for the production of monoclonal antibodies.

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.

Method for assessment of functional affinity of antibodies for live bacteria

A rapid and convenient method for assessment of functional affinity of antibodies against live bacteria is described. When a combination of immunomagnetic separation (IMS) with bioluminescent or fluorescent genetic labelling of the cells was employed, the method showed good correlation with plate count. However, the use of reporter bacteria allowed results to be obtained within 1 h compared with days using conventional methods. Due to its lower detection limit, the bioluminescent assay performed better than the fluorescent assay. Antibody affinities for Escherichia coli O157:H7 and Salmonella enteritidis were examined at different environmental conditions such as pH 3-7, temperature 4-25 degrees C, and sodium chloride concentrations 0-5% and compared with sensitivities of ELISA.

Production and Characterization of Anti‐recombinant Human Erythropoietin (rhEPO) Monoclonal Antibody

Anti-rhEPO McAb is valuable for the determination of recombinant human erythropoietin (rhEPO) levels for diagnosis of renal anemia and for doping control analysis. In this paper, anti-rhEPO hybridoma was prepared by fusion of SP2/0 murine myeloma cells with spleen cells isolated from immunized BALB/c mouse, using an enzyme-linked immunosorbent assay (ELISA) method to screen the positive hybridoma. The purified McAb was characterized by ELISA, SDS-PAGE, and Western-blotting. Experimental results showed that the subclass and the light chain of anti-rhEPO McAb was IgG1 and kappa light chain. The molecular weight of anti-rhEPO McAb was 166,000 Daltons. The affinity constant (K(aff)) of anti-rhEPO McAb with coated antigen was 5.0 x 10(5)L/mol.

Soluble HIV-specific T cell receptor: expression, purification and analysis of the specificity

We have produced soluble T cell receptor (TCR) derived from a human CD8(+) cytotoxic T lymphocyte (CTL) clone D3 that recognizes the immunodominant HIV Gag peptide SLYNTVATL (SL9) in association with major histocompatibility complex (MHC) class I protein HLA-A2. Drosophila Schneider cells (S2) were used to express genes coding the TCR alpha and beta chains under an inducible promoter. Both chains were labeled with two different tags: a (His)(6) was introduced at the C-terminal end of alpha chain, while beta chain was terminated by c-myc. Since an isolated alpha chain is unstable unless it is associated with a beta chain, this design permits rapid separation of alpha,beta-heterodimer from unpaired beta chain in a single step of Ni-NTA Agarose chromatography yielding 90% pure alpha,beta-TCR. Introduction of the c-myc epitope to the beta chain allows capture of soluble D3 from the culture supernatant by immobilized anti-c-myc antibody, without the need for receptor purification. The TCR specificity was then examined by analyzing the binding of peptide-HLA-A2/tetramer in an ELISA assay. Using this assay, we have also evaluated the binding of monomeric SL9-HLA-A2 complex to the immobilized D3 TCR and determined that the affinity measurement of the D3-SL9-HLA-A2 reaction is similar to that obtained by a biosensor instrument. We propose that the approach described here is generally useful for purification of other soluble TCRs and will allow rapid analysis of their specificity.

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.

Kinetic analysis of antibody-antigen interactions at a supported lipid monolayer

Modified phospholipids possessing carboxyl head groups synthesized from phosphatidylethanolamine were incorporated into supported lipid monolayers on top of a thin gold film. A monoclonal antibody was chemically coupled to the modified lipids in these monolayers and the kinetics of antigen binding were determined by surface plasmon resonance. The binding could be analyzed using a conventional 1:1 binding algorithm and the derived kinetic and affinity constants were almost identical to those reported for the same interaction on a dextran hydrogel-based sensor chip. When an antigen was chemically coupled to a modified lipid monolayer, the binding of a monoclonal antibody to this surface was biphasic. A two-step algorithm describing the formation of a 1:2 antibody:antigen complex was developed which accurately described the data and enabled differentiation of the two binding steps. The binding was assayed varying both the concentration of antibody in solution and the density of antigen on the surface. The affinities determined by Scatchard analysis of equilibrium binding levels were similar to those values obtained from an ELISA.

The influence of binding capacity and affinity on the improved performance of N-terminally extended hCG peptides, determined by ELISA-based procedures

The improvement of peptide-ELISA responses by the use of small synthetic peptides elongated at the N-terminus with an Ata-group or a (Lys)7 extension has been analyzed. For this purpose, binding capacity and affinity were evaluated by specific ELISA procedures. The ELISA experiments on binding capacity, performed with saturating antibody concentrations, revealed a difference of more than three orders of magnitude in binding capacity between the parent peptides and the N-terminally linked peptides, in favor of the latter peptides. Antibody affinity values were determined by a liquid-phase equilibrium method as well as by a solid-phase equilibrium method. N-terminal extension of the peptides had almost no effect on the affinity when equilibrium between the peptide and the antibody was reached in solution. In contrast, solid-phase affinity was greatly enhanced when the N-terminally linked peptides were adsorbed to the polystyrene surface. This enhancement was determined by the N-terminal extension and the peptide amino acid sequence (40 to 600 times higher). Thus, the use of N-terminally extended peptides can greatly increase the performance of a peptide-ELISA through improved surface effects, resulting in higher binding capacity and functional affinity.

The antigen-binding characteristics of mAbs derived from in vivo priming of avian B cells

In most vertebrates, a primary antibody repertoire is created through the recombination of a diverse set of Ig variable (V), diversity (D), and joining (J) gene segments. In contrast, an avian immune repertoire is generated by gene conversion of rearranged Ig genes during B cell development within the bursa of Fabricius, a lymphoid organ unique to birds. To investigate the properties of antigen-specific Igs created through the process of gene conversion, we have developed a system for the production of avian-derived mAbs. This system was used to produce multiple antibodies after a single immunization with a conserved peptide from the human cystic fibrosis transmembrane conductance regulator gene. Each antibody isolated was found to have arisen independently through a distinct series of gene conversion events. These primary antibodies displayed evidence of diversity in all of the complementarity determining regions of both heavy and light chains, and both the heavy and the light chains contributed to antigen specificity. In the light chains, diversity could be attributed to gene conversion events. The measured affinity constants of two of the antibodies were between 10(8) and 10(9) M-1, and the antibodies were functional in quantitative ELISA as well as immunohistochemical studies of cystic fibrosis transmembrane conductance regulator expression. These data demonstrate that antigen-specific antibodies produced by Ig gene conversion display both high affinity and specificity. In addition, the methods developed here provide the description of a system for the production of mAbs derived from a nonmammalian species.

Real-Time Monitoring of Peptide-Surface and Peptide-Antibody Interaction by Means of Reflectometry and Surface Plasmon Resonance

The performance of immunodiagnostic assays such as ELISA is governed by many different factors. Reflectometry was used to monitor peptide adsorption and the resulting antibody binding activity on a polystyrene surface. Surface plasmon resonance was used to analyze affinity and kinetic parameters of the (immobilized) peptide-antibody interaction. We demonstrate the capability of both the reflectometer and the BIAcore instrument to determine these immunoassay factors independently. When peptidic antigens other than the parent protein antigen were applied, reduced antibody binding activity (10 times lower) and faster dissociation (100 times faster) rather than poor adsorption proved to be the critical factors determining immune reactivity. When the peptides were modified chemically or when their molecular size was increased, antibody binding activity as well as affinity could be improved or even restored.

Vaccine-induced IgG antibodies to the linear epitope on the PorB outer membrane protein promote opsonophagocytosis of Neisseria meningitidis by human neutrophils

The serotype 15 PorB protein of Neisseria meningitidis contains an N-terminal linear immunodominant B-cell epitope located on the putative loop 1 (VR1) region. This epitope has previously been shown to stimulate antibody formation in 74% of the vaccinees after three doses of the Norwegian group B outer-membrane vesicle (OMV) vaccine. In the present study, the purified PorB protein and the 23mer synthetic peptide D63b2 covering VR1 region were immobilized onto N-hydroxysuccinimide-activated matrix and used for affinity purification of the specific IgG antibodies from sera of three selected vaccinees. PorB- and peptide D63b2-specific IgG preparations bound to the PorB protein on immunoblots and reacted with strain 44/76 and OMV complexes expressing the serotype 15 PorB protein, but not with the PorB-deficient mutant, suggesting high specificity for the PorB protein. Both PorB- and peptide D63b2-specific IgG were marginally bactericidal, but enabled strong opsonophagocytosis measured as respiratory burst response of human neutrophils and internalization of opsonized FTTC-labeled meningococci. The data indicate that about 30-57% of the bulk serum opsonic activity for the 44/76 bacteria could be ascribed to linear epitope-specific IgG1, thus contributing to vaccine-induced protection against systemic meningococcal disease via the opsonophagocytic route of pathogen clearance.

Innate natural antibodies. Primary roles indicated by specific epitopes

Two members of a unique class of natural antibodies have been identified in all of a large cohort of sera from clinically normal humans of broad age distribution. By means of a series of 10-12 mer peptides the epitope for each of those antibodies was characterized with regard to amino acid identity and conformation. Similar epitope specificity was revealed for the IgM isotopes of cord blood and early post natal sera and for IgM and IgG of adult sera, suggesting that the class of natural antibodies represented by the two identified in this study includes those genomically coded for at their effector level of maturation in the B cells of the neonate. Assay of series of specimens from each of four clinically normal adults revealed that those two natural antibodies are present at relatively constant titer, unique to each individual, over four to five and a half year periods. Those observations imply that the primary function of that class of natural antibodies may be related to maintenance of homeostasis and the molecular identity of each of the two epitopes suggests a role, for each, as monitor or control in intracellular traffic. The previous identification of those epitopes in a conserved protein of HIV also provides support for the proposition that a secondary function of natural antibodies, arising from fortuitous coincidence of the identity of the epitopes, may be that of early defense against infectious invaders.