Abstract
HIV envelope protein (Env) is the sole target of broadly neutralizing antibodies (BNAbs) that are capable of neutralizing diverse strains of HIV. While BNAbs develop spontaneously in a subset of HIV-infected patients, efforts to design an envelope protein-based immunogen to elicit broadly neutralizing antibody responses have so far been unsuccessful. It is hypothesized that a primary barrier to eliciting BNAbs is the fact that HIV envelope proteins bind poorly to the germline-encoded unmutated common ancestor (UCA) precursors to BNAbs. To identify variant forms of Env with increased affinities for the UCA forms of BNAbs 4E10 and 10E8, which target the Membrane Proximal External Region (MPER) of Env, libraries of randomly mutated Env variants were expressed in a yeast surface display system and screened using fluorescence activated cell sorting for cells displaying variants with enhanced abilities to bind the UCA antibodies. Based on analyses of individual clones obtained from the screen and on next-generation sequencing of sorted libraries, distinct but partially overlapping sets of amino acid substitutions conferring enhanced UCA antibody binding were identified. These were particularly enriched in substitutions of arginine for highly conserved tryptophan residues. The UCA-binding variants also generally exhibited enhanced binding to the mature forms of anti-MPER antibodies. Mapping of the identified substitutions into available structures of Env suggest that they may act by destabilizing both the initial pre-fusion conformation and the six-helix bundle involved in fusion of the viral and cell membranes, as well as providing new or expanded epitopes with increased accessibility for the UCA antibodies.
Highlights
Despite the success of antiretroviral drugs for controlling HIV infection, and their potential for preventing the spread of HIV infection, there remains an urgent need for a vaccine that is capable of preventing infection
To identify variant forms of envelope glycoprotein (Env) with enhanced affinities for unmutated common ancestor (UCA) precursors to Membrane Proximal External Region (MPER)-targeted Broadly Neutralizing Antibodies (BNAbs), we chose starting forms of Env that could be well-expressed in the yeast surface display system and exhibited robust binding to mature anti-MPER antibodies
These consisted of gp140 constructs containing the full complement of variable loop regions as well as mutations previously found to enhance expression and binding to mature forms of BNAbs [31] that we will refer to as “dsm” forms of Env
Summary
Despite the success of antiretroviral drugs for controlling HIV infection, and their potential for preventing the spread of HIV infection, there remains an urgent need for a vaccine that is capable of preventing infection. Efforts to develop a vaccine capable of eliciting antibody-mediated immune protection against HIV have focused on the HIV envelope glycoprotein (Env), the major surface glycoprotein on the virus that is the target of all known neutralizing anti-HIV antibodies. A subset of infected individuals develop antibodies to HIV that are capable of neutralizing a broad range of viruses [4], though these responses usually occur too slowly to protect those individuals. Such Broadly Neutralizing Antibodies (BNAbs) have generally been found to target epitopes on the HIV envelope protein that cannot be altered without diminishing function of the protein, such as its roles in binding receptors and fusing viral and cellular membranes. The existence of naturally occurring BNAbs suggests that it should be possible to develop an HIV vaccine by identifying an immunogen capable of eliciting these responses
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