Immunogenicity of a Prefusion HIV-1 Envelope Trimer in Complex with a Quaternary-Structure-Specific Antibody

Abstract

The HIV-1 envelope trimer (Env) is the target of broadly neutralizing antibodies and is being explored as a vaccine candidate to elicit protective antibodies. One of the most promising antigenic and structural mimics of HIV-1 Env is the SOSIP.664-stabilized soluble trimer from the clade A strain BG505, which is preferentially recognized by broadly neutralizing antibodies. Trimer immunization elicits high-titer neutralization of the autologous tier 2 BG505 strain; however, breadth is limited, and substantial interest has focused on understanding and improving trimer immunogenicity. We sought to improve the antigenic specificity of BG505 SOSIP.664 by reducing recognition of the variable loop 3 (V3) region, which elicits only weakly neutralizing antibodies. To stabilize the trimer in its prefusion closed conformation, we complexed trimeric BG505 SOSIP.664 with the antigen-binding fragment (Fab) of PGT145, a broadly neutralizing quaternary-structure-specific antibody. Compared to the ligand-free trimer, the PGT145 Fab-BG505 SOSIP.664 complex displayed increased melting temperature stability and reduced V3 recognition. In guinea pigs, immunization with the PGT145 Fab-BG505 SOSIP.664 complex elicited ϳ100-fold lower V3-directed binding and neutralization titers than those obtained with ligand-free BG505 SOSIP.664. Both complexed and ligand-free BG505 SOSIP.664 elicited comparable neutralization of the autologous BG505 virus, and in both cases, BG505 neutralization mapped to the outer domain of gp120 for some guinea pigs. Our results indicate that it is possible to reduce immune recognition of the V3 region of the trimer while maintaining the antigenic profile needed to induce autologous neutralizing antibodies. These data suggest that appropriate modifications of trimer immunogens could further focus the immune response on key neutralization epitopes. A s the sole viral antigen on the surfaces of HIV-1 virions, the envelope trimer (Env) mediates virus entry into host cells and is the target of virus-directed neutralizing antibodies (NAbs) (1-3; reviewed in references 4 and 5). Env comprises three gp120 receptor-binding subunits noncovalently associated with three gp41 transmembrane subunits. Similar to the transmembrane subunits of other type 1 fusion proteins, the gp41 subunit transitions between prefusion, intermediate, and postfusion states as part of its entry-related fusion of viral and target cell membranes (2, 6, 7). However, unlike other type 1 fusion machines, HIV-1 Env transitions between at least three distinct prefusion states (8). One of these, the prefusion closed conformation, is the target of most broadly neutralizing antibodies (2,3,5,8,9), and it has been proposed that HIV-1 Env fixed in the prefusion closed state may be a preferred HIV-1 immunogen (2,3,5,9,10).A soluble recombinant glycoprotein mimic of the HIV-1 spike, named BG505 SOSIP.664 for the HIV-1 strain (BG505) and the stabilizing mutations involved (SOSIP.664), was recently described (11). An important antigenic characteristic of t...

Native‐like Env trimers as a platform for HIV‐1 vaccine design

“…The BG505 SOSIP.664 trimers were also tested in guinea pigs both alone and as complexes with the PGT145 bNAb, again leading to the elicitation of autologous Tier-2 NAbs, together with NAbs to heterologous Tier-1 but not Tier-2 viruses (154) (Fig. 3B).…”

Improving the Expression and Purification of Soluble, Recombinant Native-Like HIV-1 Envelope Glycoprotein Trimers by Targeted Sequence Changes

“…These broadly neutralizing antibodies (bNAbs) target relatively conserved regions or motifs that span most of the trimer surface. Accordingly, immunogens that present the epitopes for multiple bNAbs are being developed and evaluated as components of strategies aimed at inducing similar antibodies in test animals and, eventually, humans (3,7,10,(12)(13)(14)(15). While the presence of a bNAb epitope on an immunogen does not guarantee that an appropriate immune response will be elicited, its absence means that such a response is highly unlikely.…”

“…Both factors create challenges when making Env trimers as recombinant proteins for immunogenicity testing and as the substrates for high-resolution structural studies (11,(22)(23)(24). Most Env immunogen development programs involve producing proteins in a secreted (i.e., soluble) form to improve the yield and simplify purification strategies (8,12,25,26). However, the required removal of the membrane-anchoring domain exacerbates the fragility of appropriately cleaved trimers and necessitates the use of stabilization procedures (6,23,27,28).…”

Epitope-Independent Purification of Native-Like Envelope Trimers from Diverse HIV-1 Isolates

“…Many vaccine strategies involve the use of Env-based immunogens aimed at eliciting NAbs with broad cross-reactivity. Because the trimeric form of Env found on the surface of the virus mediates viral entry, and a major goal of vaccine design is to elicit antibodies that block this process, it is expected that immunogens must recapitulate the native structure of this functional Env (1)(2)(3)(4)(5)(6). Diverse Env variants representing the most prevalent HIV-1 clades are currently under evaluation as potential immunogens.…”