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

The Tip of Brucella O-Polysaccharide Is a Potent Epitope in Response to Brucellosis Infection and Enables Short Synthetic Antigens to Be Superior Diagnostic Reagents

Brucellosis is a global disease and the world’s most prevalent zoonosis. All cases in livestock and most cases in humans are caused by members of the genus Brucella that possess a surface O-polysaccharide (OPS) comprised of a rare monosaccharide 4-deoxy-4-formamido-D-mannopyranose assembled with α1,2 and α1,3 linkages. The OPS of the bacterium is the basis for serodiagnostic tests for brucellosis. Bacteria that also contain the same rare monosaccharide can induce antibodies that cross-react in serological tests. In previous work we established that synthetic oligosaccharides, representing elements of the Brucella A and M polysaccharide structures, were excellent antigens to explore the antibody response in the context of infection, immunisation and cross reaction. These studies suggested the existence of antibodies that are specific to the tip of the Brucella OPS. Sera from naturally and experimentally Brucella abortus-infected cattle as well as from cattle experimentally infected with the cross-reactive bacterium Yersinia enterocolitica O:9 and field sera that cross react in conventional serological assays were studied here with an expanded panel of synthetic antigens. The addition of chemical features to synthetic antigens that block antibody binding to the tip of the OPS dramatically reduced their polyclonal antibody binding capability providing conclusive evidence that the OPS tip (non-reducing end) is a potent epitope. Selected short oligosaccharides, including those that were exclusively α1,2 linked, also demonstrated superior specificity when evaluated with cross reactive sera compared to native smooth lipopolysaccharide (sLPS) antigen and capped native OPS. This surprising discovery suggests that the OPS tip epitope, even though common to both Brucella and Y. enterocolitica O:9, has more specific diagnostic properties than the linear portion of the native antigens. This finding opens the way to the development of improved serological tests for brucellosis.

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.

The impact of TCR-binding properties and antigen presentation format on T cell responsiveness

TCR interactions with cognate peptide-MHC (pepMHC) ligands are generally low affinity. This feature, together with the requirement for CD8/CD4 participation, has made it difficult to dissect relationships between TCR-binding parameters and T cell activation. Interpretations are further complicated when comparing different pepMHC, because these can vary greatly in stability. To examine the relationships between TCR-binding properties and T cell responses, in this study we characterized the interactions and activities mediated by a panel of TCRs that differed widely in their binding to the same pepMHC. Monovalent binding of soluble TCR was characterized by surface plasmon resonance, and T cell hybridomas that expressed these TCR, with or without CD8 coexpression, were tested for their binding of monomeric and oligomeric forms of the pepMHC and for subsequent responses (IL-2 release). The binding threshold for eliciting this response in the absence of CD8 (K(D) = 600 nM) exhibited a relatively sharp cutoff between full activity and no activity, consistent with a switchlike response to pepMHC on APCs. However, when the pepMHC was immobilized (plate bound), T cells with the lowest affinity TCRs (e.g., K(D) = 30 microM) responded, even in the absence of CD8, indicating that these TCR are signaling competent. Surprisingly, even cells that expressed high-affinity (K(D) = 16 nM) TCRs along with CD8 were unresponsive to oligomers in solution. The findings suggest that to drive downstream T cell responses, pepMHC must be presented in a form that supports formation of appropriate supramolecular clusters.

Molecular requirements for polyamines binding to the antispermine monoclonal antibody Spm8-2

The monoclonal antispermine antibody Spm8-2 was obtained by immunizing mice with a thyroglobulin-spermine conjugate. The molecular requirements for polyamines binding to this antibody were investigated by ELISA binding and inhibition tests, using a variety of natural polyamines and synthetic polyamine analogs. Four major structural determinants are important for the binding of polyamines by the antibody: (1) terminal amino groups: N-alkylation of both terminal amino groups of the polyamines leads to an important drop in the affinity for the antibody; (2) number of methylene groups spacing the amino groups: the four carbon chains appear to present the optimum length since the antibody binds polyamines with repeats of the aminobutyl moiety more actively than their homologues with shorter or longer carbon chains; (3) number of amino groups: the affinity of Spm8-2 for free homologous polyamines varied in the following order: pentamines > tetramines > triamines > diamines, showing the importance of the number of positive charges of the polyamines in the antibody-antigen reaction; the importance of charges is further emphasized by the dependence of antibody binding on the ionic strength of the medium; (4) N-acylation of one terminal amino group: the antibody binds more actively N1-acetylspermidine than spermidine or spermine. The binding properties of Spm8-2 suggest the presence of two recognition sequences, one selective for N-acylaminopropyl moieties, the second for the aminobutyl moiety.

Differential recognition of free and covalently bound polyamines by the monoclonal anti-spermine antibody SPM8-2

The reactivity of an anti-spermine MAb (SPM8-2) toward polyamines either free or bound to a solid surface was investigated using equilibrium dialysis and ELISA methods. When polyamines were covalently linked to hydrophilized microtiter plates using carbodiimide, the MAb SPM8-2 reacted both with spermine and spermidine, with a higher affinity for the latter, but did not show any reactivity towards bound putrescine. In contrast, the MAb SPM8-2 reacted with all three polyamines bound to the microtiter plates with glutaraldehyde, with an affinity in the order: putrescine > spermidine > spermine. Equilibrium dialysis and competitive ELISA tests showed that the MAb SPM8-2 exhibited high affinity for free spermine and 50% and 5% cross-reactivity with free spermidine and putrescine respectively. The affinity of the MAb SPM8-2 for putrescine, spermidine and spermine appears to depend on whether the polyamine is free or bound. The antigenicity of the polyamines differs according to the nature of their link to the solid phase. These observations are discussed in the light of the structural modification produced by covalent binding of the polyamines. It is also concluded that when antibodies are used, due care has to be exercised in choosing the appropriate immunoassay for determining the specificity of antibodies directed against small haptens such as the polyamines.

Concentration and avidity of anti-tetanus antibodies in mother-infant pairs: relation to immunization time

The concentration and avidity of anti-tetanus antibodies in two groups of mother-infant pairs were compared. Mothers immunized during pregnancy and their newborns (group A) had significantly higher antibody concentrations than mothers immunized at least a year before their last pregnancy and their newborns (group B) as measured by an indirect enzyme immunoassay (EIA) procedure. Antibody avidity of samples was measured by an inhibition EIA technique and urea denaturation test. Although antibody avidity was higher in group B, the differences were not significant. These findings may represent a secondary antibody response to a protein antigen, when considering that all mothers in both groups had received a primary tetanus vaccination during childhood. In mothers with a history of primary tetanus immunization, a single booster dose of tetanus toxoid during pregnancy is enough to induce protective levels of antibodies with reasonably high avidity in both mother and newborn.

Problems and pitfalls with measurement of antibody affinity using solid phase binding in the ELISA

Current methods of estimation of antibody affinity constants using ELISA assume homogeneous binding of antibody to the solid phase, despite many reports in the literature that this is not true. I have derived theoretical antibody binding curves for solid phase antigen assuming homogeneous antibody binding. I have compared these curves with a set of experimental binding curves of monoclonal antibodies to the serum protein fibronectin. The results conclusively show that while some monoclonal antibodies behave as predicted by theory, others show departures from homogeneous binding which can be explained by various surface effects. I have discussed how these surface effects can cause errors in estimates of either liquid phase or solid phase affinities using the ELISA, and have demonstrated the limitations of methods of affinity ranking.

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.