Immunogens Modeling a Fusion-Intermediate Conformation of gp41 Elicit Antibodies to the Membrane Proximal External Region of the HIV Envelope Glycoprotein

Abstract

The membrane proximal external region (MPER) of the gp41 subunit of the HIV-1 envelope glycoprotein (Env) contains determinants for broadly neutralizing antibodies and has remained an important focus of vaccine design. However, creating an immunogen that elicits broadly neutralizing antibodies to this region has proven difficult in part due to the relative inaccessibility of the MPER in the native conformation of Env. Here, we describe the antigenicity and immunogenicity of a panel of oligomeric gp41 immunogens designed to model a fusion-intermediate conformation of Env in order to enhance MPER exposure in a relevant conformation. The immunogens contain segments of the gp41 N- and C-heptad repeats to mimic a trapped intermediate, followed by the MPER, with variations that include different N-heptad lengths, insertion of extra epitopes, and varying C-termini. These well-characterized immunogens were evaluated in two different immunization protocols involving gp41 and gp140 proteins, gp41 and gp160 DNA primes, and different immunization schedules and adjuvants. We found that the immunogens designed to reduce extension of helical structure into the MPER elicited the highest MPER antibody binding titers, but these antibodies lacked neutralizing activity. The gp41 protein immunogens also elicited higher MPER titers than the gp140 protein immunogen. In prime-boost studies, the best MPER responses were seen in the groups that received DNA priming with gp41 vectors followed by gp41 protein boosts. Finally, although titers to the entire protein immunogen were similar in the two immunization protocols, MPER-specific titers differed, suggesting that the immunization route, schedule, dose, or adjuvant may differentially influence MPER immunogenicity. These findings inform the design of future MPER immunogens and immunization protocols.

Nanobodies Targeting Norovirus Capsid Reveal Functional Epitopes and Potential Mechanisms of Neutralization

“…Trapping the particles in a particular conformation or otherwise inhibiting capsid "breathing" is a common antiviral strategy shared by many neutralizing mAbs, Nanobodies, and drugs against HIV, flaviviruses, picornaviruses, influenza, and others [54][55][56][57][58][59][60]. For example, several neutralizing Nanobodies against poliovirus and respiratory syncytial virus were shown to specifically stabilize either the native or expanded conformation of capsid, preventing it from further rearrangement necessary for the infection process [30,61].…”

Immunogenic properties of a trimeric gp41-based immunogen containing an exposed membrane-proximal external region

“…In a report by Vassell et al (Vassell et al, 2015), authors evaluated immunogenicity of several constructs comprised of MPER with different lengths of HR1 and HR2 in rabbits. Constructs were made with or without two different trimerization domains (GCN4 or foldon).…”

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“…The highly conserved gp41 MPER, in addition to being rich in bnAb epitopes, also plays key roles not only in viral infectivity, but also in membrane fusion and as such is a promising target for an anti-HIV antibody vaccine development [21,71]. However, MPER is weakly immunogenic due to the fact that it's inaccessible to the immune system being masked in the unmodified HIV-1 envelope glycoprotein [73]. Fig 5A&B) showed strong reactivity with MPER specific polyclonal antibodies in plasma from a greater majority of long standing ARV naïve HIV-1 infected participants.…”