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Electron-Microscopy-Based Epitope Mapping Defines Specificities of Polyclonal Antibodies Elicited during HIV-1 BG505 Envelope Trimer Immunization.

ABSTRACT: Characterizing polyclonal antibody responses via currently available methods is inherently complex and difficult. Mapping epitopes in an immune response is typically incomplete, which creates a barrier to fully understanding the humoral response to antigens and hinders rational vaccine design efforts. Here, we describe a method of characterizing polyclonal responses by using electron microscopy, and we applied this method to the immunization of rabbits with an HIV-1 envelope glycoprotein vaccine candidate, BG505 SOSIP.664. We detected known epitopes within the polyclonal sera and revealed how antibody responses evolved during the prime-boosting strategy to ultimately result in a neutralizing antibody response. We uncovered previously unidentified epitopes, including an epitope proximal to one recognized by human broadly neutralizing antibodies as well as potentially distracting non-neutralizing epitopes. Our method provides an efficient and semiquantitative map of epitopes that are targeted in a polyclonal antibody response and should be of widespread utility in vaccine and infection studies.

SUBMITTER: Bianchi M

PROVIDER: S-EPMC6104742 | biostudies-literature | 2018 Aug

REPOSITORIES: biostudies-literature

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Project description:We recently showed that mutations in the HIV-1 envelope (Env) destabilize the V3 loop, rendering neutralization-resistant viruses sensitive to V3-directed monoclonal antibodies (MAbs). Here, we investigated the propagation of this effect on other Env epitopes, with special emphasis on V2 loop exposure. Wild-type JR-FL and 19 mutant JR-FL pseudoviruses were tested for neutralization sensitivity to 21 MAbs specific for epitopes in V2, the CD4 binding site (CD4bs), and the CD4-induced (CD4i) region. Certain glycan mutants, mutations in the gp120 hydrophobic core, and mutations in residues involved in intraprotomer interactions exposed epitopes in the V2i region (which overlies the ?4?7 integrin binding site) and the V3 crown, suggesting general destabilization of the distal region of the trimer apex. In contrast, other glycan mutants, mutations affecting interprotomer interactions, and mutations affecting the CD4bs exposed V3 but not V2i epitopes. These data indicate for the first time that V3 can move independently of V2, with V3 pivoting out from its "tucked" position in the trimer while apparently leaving the V2 apex intact. Notably, none of the mutations exposed V2 epitopes without also exposing V3, suggesting that movement of V2 releases V3. Most mutations increased sensitivity to CD4bs-directed MAbs without exposure of the CD4i epitope, implying these mutations facilitate the trimers' maintenance of an intermediate energy state between open and closed conformations. Taken together, these data indicate that several transient Env epitopes can be rendered more accessible to antibodies (Abs) via specific mutations, and this may facilitate the design of V1V2-targeting immunogens.IMPORTANCE Many epitopes of the HIV envelope (Env) spike are relatively inaccessible to antibodies (Abs) compared to their exposure in the open Env conformation induced by receptor binding. However, the reduced infection rate that resulted from the vaccine used in the RV144 HIV-1 vaccine trial was correlated with the elicitation of V2- and V3-directed antibodies. Previously, we identified various mechanisms responsible for destabilizing the V3 loop; here, we determined, via mutation of numerous Env residues, which of these elements maintain the V1V2 loop in an inaccessible state and which expose V1V2 and/or V3 epitopes. Notably, our data indicate that V3 can move independently of V2, but none of the mutations studied expose V2 epitopes without also exposing V3. Additionally, V1V2 can be rendered more accessible to Abs via specific mutations, facilitating the development of engineered V2 immunogens.

Dataset Information

Electron-Microscopy-Based Epitope Mapping Defines Specificities of Polyclonal Antibodies Elicited during HIV-1 BG505 Envelope Trimer Immunization.

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