How to assess the binding strength of antibodies elicited by vaccination against HIV and other viruses

ABSTRACT

Vaccines that protect against viral infections generally induce neutralizing antibodies. When vaccines are evaluated, the need arises to assess the affinity maturation of the antibody responses. Binding titers of polyclonal sera depend not only on the affinities of the constituent antibodies but also on their individual concentrations, which are difficult to ascertain. Therefore an assay based on chaotrope disruption of antibody-antigen complexes was designed for measuring binding strength. This assay works well with many viral antigens but gives differential results depending on the conformational dependence of epitopes on complex antigens such as the envelope glycoprotein of HIV-1. Kinetic binding assays might offer alternatives, since they can measure average off-rate constants for polyclonal antibodies in a serum. Here, potentials and fallacies of these techniques are discussed.

KEYWORDS:

• Vaccine
• antibody
• virus
• HIV Env
• neutralization
• avidity
• chaotrope
• surface plasmon resonance
• affinity
• kinetic constants

Financial & competing interests disclosure

The author’s work in this area is supported by the NIH grant R37AI36082. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Key issues

• Chaotrope-based antibody-binding assays have been used widely for assessing the strength of the binding of antiviral antibodies but are not mechanistically understood.

• Binding to continuous epitopes is more resistant to chaotrope treatment than binding to discontinuous epitopes.

• Binding to important quaternary neutralization epitopes on HIV-1 Env trimers is particularly vulnerable to chaotrope disruption.

• The effects of chaotropes and other ions in the Hofmeister series are complex; these effects are the subject of continuing advanced studies in protein chemistry.

• New B-cell and plasma-cell cloning techniques allow the dissection of antibody responses and facilitate detailed studies of paratope–epitope interface structures as well as of binding kinetics, thermodynamics, and stoichiometry.

• Surface plasmon resonance and biolayer interferometry measure the binding kinetics of monoclonal antibodies but can also give some biophysically interpretable information about the binding strength of polyclonal antibodies.

• Pseudovirus-based neutralization assays allow the screening of post-vaccination sera against multiple viral variants/mutants and thereby the determination of the salient neutralization epitopes recognized by vaccine-elicited antibodies.

• Neutralization and binding titers have great predictive value in vaccine analyses, provided the antigens used in the assays are similar to functional entry-mediating proteins on the surfaces of circulating viruses.

• Chaotropic effects on proteins in general and antibody–antigen interactions in particular may become better understood through developments in physical chemistry, while their usefulness in routine immunological assays recedes.