Conformation Matters: Immunity Elicited by the Stabilized Pre-Fusion F Subunit Vaccine DS-Cav1 is Potent and Durable
Open AccessRespiratory syncytial virus (RSV) causes substantial morbidity in children and the elderly, with no vaccine available. A vaccine eliciting a substantial and sustainable boost in neutralizing antibodies is likely required to provide protection in at-risk populations. The fusion (F) glycoprotein is a major vaccine target because it is highly conserved between RSV subtypes A and B, it mediates viral entry, and it displays distinct antigenic sites that elicit highly potent neutralizing antibodies. RSV F exists in two major conformational states: the active pre-fusion (pre-F) and the inactive post-fusion (post-F) conformation. Multiple vaccine trials using post-F as a subunit vaccine resulted in a modest boost in neutralization and the induction of non-neutralizing antibodies and conferred little to no protection from disease. In 2013, the stabilization of pre-F conformation through cavity-filling mutations and disulfide bonds (DS-Cav1) revealed surfaces unique to the pre-F conformation that are targets for highly neutralizing antibodies. To compare and contrast the serological and B cell profiles of pre-F versus post-F subunit vaccine-elicited responses, we assessed the profile of binding and neutralizing antibodies before and after vaccination from subjects enrolled in a phase I pre-F subunit vaccine trial (DS-Cav1, VRC 317) or a phase IIb post-F subunit vaccine trial (MEDI7510, MedImmune D4420C00005). Unlike vaccination with MEDI7510, DS-Cav1 vaccination elicited antibodies that bound to sites exclusive to pre-F in addition to sites shared by both immunogens. As a result of the higher potency of antibodies binding these pre-F exclusive sites, DS-Cav1 elicited a more robust increase in neutralizing activity. We confirmed this profile of B cell and antibody specificity by quantifying RSV F-specific B cells in peripheral blood mononuclear cells (PBMC) using fluorescently labeled, tetramerized pre-F and post-F probes and a 17-color flow cytometry panel. Together, our results implicate that the induction of pre-F exclusive antibodies is the primary mechanism for supranormal induction of neutralizing activity. We further investigated the longevity of the response to DS-Cav1 and found that RSV-specific antibody was sustained above the week 0 baseline for up to ten months. Similarly, DS-Cav1 vaccination activated pre-F-specific IgG+ and IgA+ memory B cells, with pre-F probe-binding frequencies elevated above the week 0 baseline ten months post-vaccination. Durability was maintained regardless of dose, number of vaccinations, or adjuvant, indicating that a single dose of DS-Cav1 in RSV-experienced individuals may confer protection that extends throughout an entire RSV season. Our results provide a proof-of-concept for the use of structure-based vaccine design to optimize the induction of potent, durable anti-viral immunity.
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