Novel antigens for RSV vaccines

Abstract

Respiratory syncytial virus (RSV) remains a leading global cause of infant mortality and adult morbidity. Infection, which recurs throughout life, elicits only short-lived immunity. The development of a safe and efficacious vaccine has, thus far, been elusive. Recent technological advances, however, have yielded promising RSV vaccine candidates that are based on solving atomic-level structures of surface glycoproteins interacting with neutralizing antibodies. The class I fusion glycoprotein, F, serves as the primary antigenic component of most vaccines, and is the target of the only licensed monoclonal antibody product used to reduce the frequency of severe disease in high-risk neonates. However, success of prior F-based vaccines has been limited by the lack of understanding how the conformational rearrangement between a metastable prefusion F (pre-F) and a stable postfusion F (post-F) affected the epitope content. Neutralizing epitopes reside on both conformations, but those specific to pre-F are far more potent than those previously identified and present on post-F. The solution of the pre-F structure and its subsequent characterization and stabilization illustrates the value of a structure-based approach to vaccine development, and provides hope that a safe and effective RSV vaccine is possible.

Figure 1. Schematic drawing of RSV particles

RSV is an enveloped virus that buds from cells as filopodia-like filaments (A) that mediate cell-to-cell fusion or can be released as cell-free pleomorphic particles that range from filamentous to spherical (B). The shape of the virus particle is determined, in part, by the integrity of the matrix protein layer immediately under the membrane. This layer also influences the organization and stability of membrane-associated proteins. The F glycoprotein, in particular, is known to exist on the virion membrane in both metastable pre-F (functional) and stable post-F (non-functional) conformations. The pre-F (red) is thought to be more ordered and have a greater density in the filaments, while the post-F (blue) is more abundant in the spherical particles and continues to accumulate over time, as the matrix layer becomes fragmented. The conformational change in F is unidirectional ending in the nonfunctional post-F protein. The structure of G (green) is not known, but the N- and C-terminal glycan domains are envisioned to be extended or folded over, exposing the cysteine noose (highlighted in red) in the central conserved domain apically or laterally. The pentameric ion channel, SH, is present in the cholesterol-rich lipid microdomains in which virus assembly and budding occurs, but is in relatively low abundance on the surface of virus particles.

Figure 2. Surface representation of RSV F glycoprotein

The RSV F glycoprotein exists in a prefusion (pre-F) conformation prior to an extensive rearrangement that occurs spontaneously or as part of the membrane fusion process that allows entry of the viral nucleocapsid into the target cell. Pre-F and post-F are approximately 11 nm and 16 nm high, and as shown in Figure 1, the density of pre-F and post-F vary depending on the age and condition of the virus. About 50% of the surface of pre-F is preserved in the post-F conformation. While only the pre-F conformation is functional, there are epitopes preserved on post-F (sites I, II, and IV) that are associated with NT activity. However, the most neutralization-sensitive sites recognized by the most potent neutralizing antibodies are only present on pre-F. These include site Ø (red) and at least two other sites (orange) that have not yet been given a numerical designation.