A cluster of continuous antigenic structures in the transmembrane protein of HIV-1: individual patterns of reactivity in human sera

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

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.

What Do Chaotrope-Based Avidity Assays for Antibodies to HIV-1 Envelope Glycoproteins Measure?

When HIV-1 vaccine candidates that include soluble envelope glycoproteins (Env) are tested in humans and other species, the resulting antibody responses to Env are sifted for correlates of protection or risk. One frequently used assay measures the reduction in antibody binding to Env antigens by an added chaotrope (such as thiocyanate). Based on that assay, an avidity index was devised for assessing the affinity maturation of antibodies of unknown concentration in polyclonal sera. Since a high avidity index was linked to protection in animal models of HIV-1 infection, it has become a criterion for evaluating antibody responses to vaccine candidates. But what does the assay measure and what does an avidity index mean? Here, we have used a panel of monoclonal antibodies to well-defined epitopes on Env (gp120, gp41, and SOSIP.664 trimers) to explore how the chaotrope acts. We conclude that the chaotrope sensitivity of antibody binding to Env depends on several properties of the epitopes (continuity versus tertiary- and quaternary-structural dependence) and that the avidity index has no simple relationship to antibody affinity for functional Env spikes on virions. We show that the binding of broadly neutralizing antibodies against quaternary-structural epitopes is particularly sensitive to chaotrope treatment, whereas antibody binding to epitopes in variable loops and to nonneutralization epitopes in gp41 is generally resistant. As a result of such biases, the avidity index may at best be a mere surrogate for undefined antibody or other immune responses that correlate weakly with protection.

IMPORTANCE

An effective HIV-1 vaccine is an important goal. Such a vaccine will probably need to induce antibodies that neutralize typically transmitted variants of HIV-1, preventing them from infecting target cells. Vaccine candidates have so far failed to induce such antibody responses, although some do protect weakly against infection in animals and, possibly, humans. In the search for responses associated with protection, an avidity assay based on chemical disruption is often used to measure the strength of antibody binding. We have analyzed this assay mechanistically and found that the epitope specificity of an antibody has a greater influence on the outcome than does its affinity. As a result, the avidity assay is biased toward the detection of some antibody specificities while disfavoring others. We conclude that the assay may yield merely indirect correlations with weak protection, specifically when Env vaccination has failed to induce broad neutralizing responses.

The gp41 epitope, QARVLAVERY, is highly conserved and a potent inducer of IgA that neutralizes HIV-1 and inhibits viral transcytosis

Mucosal surfaces are the predominant site of human immunodeficiency virus (HIV)-1 transmission. For prophylactic approaches to effectively prevent HIV infection and subsequent dissemination, the induction of mucosally relevant protective immunity will be critical. Here, we have characterized the antibody (Ab) response generated by a highly conserved gp41epitope, QARVLAVERY, in an optimized immunization model that elicits potent epitope-specific Abs in the serum, vaginal washes, and fecal secretions of immunized mice. Our results show that QARVLAVERY is indeed a potent inducer of IgA and importantly, QARVLAVERY-specific IgA was effective in neutralizing HIV and inhibiting viral transcytosis. Intriguingly, QARVLAVERY also generated an approximate 1:1 ratio of IgG:IgA in the serum of immunized mice, independent of the delivery regimen and produced early systemic IgA, even before IgG. In light of the significantly high IgA induction by QARVLAVERY and the functionality of epitope-specific Abs in the inhibition of HIV infection and transcytosis, QARVLAVERY is an attractive epitope to be considered in mucosal vaccination strategies against HIV.

Monoclonal antibodies to an HIV-1 group O envelope recombinant

Monoclonal antibodies were developed to a recombinant HIV-I group O envelope protein derived from the isolate HAM112. These monoclonal antibodies were characterized for reactivity to a series of overlapping synthetic peptides (29-30 mers) covering gp120 C-terminal and gp41 ectodomain regions of the HIV-1 group O envelope protein. Most of these monoclonal antibodies reacted with peptides spanning sequences analogous to HIV-1 group M epitopes identified from studies in mice and humans. However, several of the antibodies that were nonreactive to individual peptides did react to a mixture of longer peptides from the N-terminal and C-terminal helical regions of the gp41 ectodomain. The monoclonal antibodies described in this study are valuable tools for characterization of antigenic differences between HIV-1 group O and group M viruses.

Synthetic peptides representing sequences within gp41 of HIV as immunogens for murine T- and B-cell responses

Within the gp41 glycoprotein of the human immunodeficiency virus type 1 (HIV-1) there is a relatively conserved region which appears accessible to the immune system during the course of HIV infection and is recognised by antibody from virtually all patients with AIDS. This region has also been shown to function as a target for human T cells. We have examined synthetic peptides spanning this sequence, between residues 572 and 604, with a view to evaluating their potential as immunogens. Peptides 572GIKQLQARILAVERYLKDQQ591 and 579RILAVERYLKDQQLLGGIWGCSGK601 were good immunogens in two different strains of mice while peptide 576LQARILAVERYLKDQQ591 was an inferior immunogen, and peptide 593LGIWGCSGKLIC604 was non-immunogenic unless coupled to a carrier protein. For both antibody and T cell responses it was apparent that sequences that could function as determinants within one peptide could not do so in the context of a different peptide immunogen. It follows that by judicious choice of immunogen sequence it may be possible to direct the immune response towards a desired fine specificity. Unwanted responses by CD4+ T cells isolated from certain peptide-primed animals were also observed. These T cells showed an unusual reactivity in that they were incapable of recognising their determinant AVERYLKDQQ if it was extended at the C-terminal end with the native sequence and as such would not be expected to recognise the native molecule unless processing created the identical C-terminus.

Three-dimensional structure and antigenicity of transmembrane-protein peptides of the human immunodeficiency virus type 1. Effects of a neutralization-escape substitution

A point mutation (Ala-589 to Thr) in the transmembrane protein of the human immunodeficiency virus type 1 (HIV-1) has been shown to decrease the sensitivity of the virus to the neutralizing effect of human HIV-1 specific antibodies [(1990) J. Virol. 64, 3240-3248]. Here 17-residue peptides with the parental and mutant sequences were compared: the parental peptide bound antibodies of sera from HIV-1 infected persons more frequently and with higher affinity than the mutant peptide. However, according to circular dichroism (CD), NMR spectroscopy and molecular modelling the peptides have indistinguishable backbone conformations under a variety of experimental conditions. These techniques showed for both peptides that no ordered helix was present in water solution. However, for both peptides in alcohol-water solutions approximately 60% alpha-helix could be induced. The three-dimensional structures of these peptides provide a basis for understanding how this mutation in the transmembrane protein may affect the interaction with both the outer envelope glycoprotein and with antibodies.