The V3 Loop Is Accessible on the Surface of Most Human Immunodeficiency Virus Type 1 Primary Isolates and Serves as a Neutralization Epitope

Abstract

Antibodies (Abs) against the V3 loop of the human immunodeficiency virus type 1 gp120 envelope glycoprotein were initially considered to mediate only type-specific neutralization of T-cell-line-adapted viruses. However, recent data show that cross-neutralizing V3 Abs also exist, and primary isolates can be efficiently neutralized with anti-V3 monoclonal Abs (MAbs). The neutralizing activities of anti-V3 polyclonal Abs and MAbs may, however, be limited due to antigenic variations of the V3 region, a lack of V3 exposure on the surface of intact virions, or Ab specificity. For clarification of this issue, a panel of 32 human anti-V3 MAbs were screened for neutralization of an SF162-pseudotyped virus in a luciferase assay. MAbs selected with a V3 fusion protein whose V3 region mimics the conformation of the native virus were significantly more potent than MAbs selected with V3 peptides. Seven MAbs were further tested for neutralizing activity against 13 clade B viruses in a single-round peripheral blood mononuclear cell assay. While there was a spectrum of virus sensitivities to the anti-V3 MAbs observed, 12 of the 13 viruses were neutralized by one or more of the anti-V3 MAbs. MAb binding to intact virions correlated significantly with binding to solubilized gp120s and with the potency of neutralization. These results demonstrate that the V3 loop is accessible on the native virus envelope, that the strength of binding of anti-V3 Abs correlates with the potency of neutralization, that V3 epitopes may be shared rather than type specific, and that Abs against the V3 loop, particularly those targeting conformational epitopes, can mediate the neutralization of primary isolates.The third variable domain (V3) of the human immunodeficiency virus type 1 (HIV-1) gp120 envelope glycoprotein is critical for the formation of syncytia and for virus entry into target cells (24,55). These functions are mediated by the interaction of the V3 loop with chemokine receptors and are maintained despite the sequence variation that characterizes this region of the virus envelope (18, 51). Indeed, contrary to its name, the V3 loop is characterized by a constant size of 30 to 35 amino acids, a conserved type II ␤-turn at its tip, a disulfide bond at its base, and a net positive charge (26, 28). Conserved features are also suggested by the structure of the V3 loop discerned by nuclear magnetic resonance studies (47, 52), and conserved elements in the V3 crown and stem are mandatory features for coreceptor interactions (9, 50). All of these structural constraints appear to be imposed by the required interaction of the V3 loop with the coreceptors for HIV-1, CXCR4 or CCR5, and suggest that this region of the virus envelope should induce antibodies (Abs) that are crossreactive among isolates and inhibitory to virus infectivity.Initial studies of anti-V3 Abs, induced by brief immunization protocols in animals and tested against a limited number of T-cell-line-adapted (TCLA) strains of the virus, suggested, however, that anti-V3 Abs were type spec...

Aiming to induce broadly reactive neutralizing antibody responses with HIV-1 vaccine candidates

“…The third variable loop (V3) was originally thought to be the main accessible Env neutralizing epitope and would be a successful target for a vaccine [79][80][81][82][83][84]. However, it was subsequently recognized that anti-V3 loop antibodies were highly strain specific [79,[81][82][83][84].…”

“…Similarly, with the difference in neutralization sensitivity between T-cell line adapted (TCLA) HIV-1 and primary isolates that was observed with V3 antibodies [85,86], it was realized that the envelope structure of TCLA viruses was likely quite different than the native HIV-1 trimer of primary isolates [87]. Hope for a practical V3 immunogen has resurfaced with the work of Zolla-Pazner and colleagues who have shown similarity of the V3 loop with chemokine receptor structure, and human anti-V3 mAbs that can neutralize a subset of HIV-1 grown in PBMC [80,81].…”

“…The vulnerable exposed regions of gp120 include the five variable regions, V1-5 (FIGURES 1 & 2). A second potent mechanism of gp120 escape from antibody is both to modify the sequence of these variable region epitopes to which neutralizing antibodies bind, as well as altering the conformation of the variable loops to occlude neutralization epitopes [66][67][68][69][74][75][76][77][78][79][80][81].…”

“…Similarly, with the difference in neutralization sensitivity between T-cell line adapted (TCLA) HIV-1 and primary isolates that was observed with V3 antibodies [85,86], it was realized that the envelope structure of TCLA viruses was likely quite different than the native HIV-1 trimer of primary isolates [87]. Hope for a practical V3 immunogen has resurfaced with the work of Zolla-Pazner and colleagues who have shown similarity of the V3 loop with chemokine receptor structure, and human anti-V3 mAbs that can neutralize a subset of HIV-1 grown in PBMC [80,81].With Korber, we have studied a panel of V3 peptides selected to represent a spectrum of potential higher order V3 motifs of Krachmarov and colleagues have now made human mAbs against non-B V3s and found antibodies that neutralize a subset of non-B HIV-1 strains [90]. The structure of a V3-containing HIV-1 gp120 core has recently been published and provides evidence that the V3 loop functions as a molecular 'hook', not only for binding to coreceptor but also for modulating sub-unit associations within the viral spike of the HIV-1 envelope trimer [93].…”

“…The vulnerable exposed regions of gp120 include the five variable regions, V1-5 (FIGURES 1 & 2). A second potent mechanism of gp120 escape from antibody is both to modify the sequence of these variable region epitopes to which neutralizing antibodies bind, as well as altering the conformation of the variable loops to occlude neutralization epitopes [66][67][68][69][74][75][76][77][78][79][80][81].The third variable loop (V3) was originally thought to be the main accessible Env neutralizing epitope and would be a successful target for a vaccine [79][80][81][82][83][84]. However, it was subsequently recognized that anti-V3 loop antibodies were highly strain specific [79,[81][82][83][84].…”