Abstract
Adenoviral (Ad) vectors show promise as platforms for vaccine applications against infectious diseases including HIV. However, the requirements for eliciting protective neutralizing antibody and cellular immune responses against HIV remain a major challenge. In a novel approach to generate 2F5- and 4E10-like antibodies, we engineered an Ad vector with the HIV membrane proximal ectodomain region (MPER) epitope displayed on the hypervariable region 2 (HVR2) of the viral hexon capsid, instead of expressed as a transgene. The structure and flexibility of MPER epitopes, and the structural context of these epitopes within viral vectors, play important roles in the induced host immune responses. In this regard, understanding the critical factors for epitope presentation would facilitate optimization strategies for developing viral vaccine vectors. Therefore we undertook a cryoEM structural study of this Ad vector, which was previously shown to elicit MPER-specific humoral immune responses. A subnanometer resolution cryoEM structure was analyzed with guided molecular dynamics simulations. Due to the arrangement of hexons within the Ad capsid, there are twelve unique environments for the inserted peptide that lead to a variety of conformations for MPER, including individual α-helices, interacting α-helices, and partially extended forms. This finding is consistent with the known conformational flexibility of MPER. The presence of an extended form, or an induced extended form, is supported by interaction of this vector with the human HIV monoclonal antibody 2F5, which recognizes 14 extended amino acids within MPER. These results demonstrate that the Ad capsid influences epitope structure, flexibility and accessibility, all of which affect the host immune response. In summary, this cryoEM structural study provided a means to visualize an epitope presented on an engineered viral vector and suggested modifications for the next generation of Ad vectors with capsid-incorporated HIV epitopes.
Highlights
Viruses and virus-like particles (VLPs) with capsid-incorporated or chemically-attached heterologous epitopes are being explored as vaccine platforms to provide protective immunity against pathogens [1]
A hybrid cryoEM and molecular dynamics approach enabled us to model the conformation of membrane proximal ectodomain region (MPER) presented on an engineered Ad vaccine vector that shows a promising immunological response in mice [7]
There are twelve unique hexon hypervariable region 2 (HVR2) insertion sites within the Ad capsid that result in different MPER conformations, including well resolved individual a-helices at the icosahedral 2fold axis, interacting a-helices, and partially extended forms
Summary
Viruses and virus-like particles (VLPs) with capsid-incorporated or chemically-attached heterologous epitopes are being explored as vaccine platforms to provide protective immunity against pathogens [1]. The potential advantages of viral vaccine vectors include multivalent display of epitopes and the ability of viral particles to stimulate both the adaptive and innate immune systems. The hepatitis B VLP system has been utilized to present GFP, and vectors have been produced that lead to a strong humoral immune response against GFP in rabbits [4]. In another example, a recombinant VLP was designed to display domains from the anthrax toxin receptor. A single administration of this VLP in rats led to a potent immune response against a lethal anthrax toxin challenge [5]
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