Antigenic Properties of the HIV Envelope on Virions in Solution

Abstract

The structural flexibility found in human immunodeficiency virus (HIV) envelope glycoproteins creates a complex relationship between antigenicity and sensitivity to antiviral antibodies. The study of this issue in the context of viral particles is particularly problematic as conventional virus capture approaches can perturb antigenicity profiles. Here, we employed a unique analytical system based on fluorescence correlation spectroscopy (FCS), which measures antibody-virion binding with all reactants continuously in solution. Panels of nine anti-envelope monoclonal antibodies (MAbs) and five virus types were used to connect antibody binding profiles with neutralizing activities. Anti-gp120 MAbs against the 2G12 or b12 epitope, which marks functional envelope structures, neutralized viruses expressing CCR5-tropic envelopes and exhibited efficient virion binding in solution. MAbs against CD4-induced (CD4i) epitopes considered hidden on functional envelope structures poorly bound these viruses and were not neutralizing. Anti-gp41 MAb 2F5 was neutralizing despite limited virion binding. Similar antigenicity patterns occurred on CXCR4-tropic viruses, except that anti-CD4i MAbs 17b and 19e were neutralizing despite little or no virion binding. Notably, anti-gp120 MAb PG9 and anti-gp41 MAb F240 bound to both CCR5-tropic and CXCR4-tropic viruses without exerting neutralizing activity. Differences in the virus production system altered the binding efficiencies of some antibodies but did not enhance antigenicity of aberrant gp120 structures. Of all viruses tested, only JRFL pseudoviruses showed a direct relationship between MAb binding efficiency and neutralizing potency. Collectively, these data indicate that the antigenic profiles of free HIV particles generally favor the exposure of functional over aberrant gp120 structures. However, the efficiency of virion-antibody interactions in solution inconsistently predicts neutralizing activity in vitro.

Structural Definition of an Antibody-Dependent Cellular Cytotoxicity Response Implicated in Reduced Risk for HIV-1 Infection

“…These studies indicate that this epitope region remains buried at the interface of virion-associated HIV-1 trimers triggered with soluble CD4, despite the exposure of the coreceptor binding site. This is consistent with previous findings regarding exposure of the MAb A32 epitope on virions in solution and using a cell-cell fusion system (56,57,59) and points to an additional energy component likely provided by a conformational change in the d3d4 region of cell surface CD4, which is required for trimer unfolding during viral entry.…”

“…Since soluble CD4 does not expose the A32 subregion (Fig. 5C) (56), and since we know that it is exposed during cell-to-cell fusion and during viral entry (57), either binding to cell surface CD4 or binding of trimer-CD4 complexes to coreceptor exposes this region. We tested this hypothesis by spinoculating AT-2 inactivated HIV-1 BaL virions onto the surfaces of CEM-Nkr and CEM-Nkr-CCR5 ϩ cells and testing binding of MAbs N5-i5 and 2.2c.…”

A Highly Conserved Residue of the HIV-1 gp120 Inner Domain Is Important for Antibody-Dependent Cellular Cytotoxicity Responses Mediated by Anti-cluster A Antibodies

“…1C and D). Our results are in agreement with previous reports indicating that the highly conserved region recognized by anti-cluster A antibodies is buried inside the Env trimer, where it is not readily accessible for binding in the ligand-free closed state (10,(21)(22)(23)(24)(25)(26)(27). Accordingly, anti-cluster A and HIV ϩ sera recognized more efficiently cells infected with a virus lacking Nef and Vpu and hence exposing Env at higher levels and in its CD4-bound conformation (2,5,28).…”

Blockage of CD59 Function Restores Activities of Neutralizing and Nonneutralizing Antibodies in Triggering Antibody-Dependent Complement-Mediated Lysis of HIV-1 Virions and Provirus-Activated Latently Infected Cells

“…In the case of HIV-1 infection, both nAbs and non-nAbs against Env (gp160 or gp120 plus gp41) are simultaneously elicited, and the levels of these Abs rise in response to the gradual increase in viral load and are maintained at high levels throughout the infection (7,8). Although anti-HIV-1 nAbs have been extensively studied in vitro and in vivo, non-nAbs have received little attention (44,53). Studies have demonstrated that almost all HIV-1-infected individuals develop Abs capable of neutralizing autologous viruses (16).…”