Reverse vaccinology 2.0: Human immunology instructs vaccine antigen design

Rappuoli, Rino; Bottomley, Matthew J.; D’Oro, Ugo; Finco, Oretta; De Gregorio, Ennio

The trimeric F protein of RSV undergoes a major conformational change from a relatively compact metastable prefusion state to a more elongated highly stable postfusion state (McLellan et al., 2013c) . Because of its exposure on the virion surface, the prefusion structure was an obvious candidate vaccine antigen, but the spontaneous conversion of recombinant prefusion F to the post-fusion state discouraged initial efforts in this direction. However, although an engineered postfusion F antigen elicited NAbs in animals (Swanson et al., 2011) , an alternative study by Magro et al. (2012) using immunized rabbit sera and sero-positive human sera reported that Abs targeting the prefusion F protein accounted for most of the RSVneutralizing activity. The latter highlighted the importance of renewed attempts to make a stable and efficacious prefusion F antigen, which was facilitated by the discovery and structural characterization of new mouse and human RSV-neutralizing mAbs specific for prefusion F. Crucially, a cocrystal structure reported by McLellan et al. (2013b) showed how the potently RSV-neutralizing human Fab D25 binds and traps F in a previously unobserved prefusion conformation. The structure revealed a large quaternary epitope on the prefusion F trimer apex, defining a major site of vulnerability on a novel antigenic structure. Subsequently, based on the crystal structure, McLellan et al. (2013a) were able to stabilize the prefusion F antigen in a trimeric conformation by replacing its transmembrane region with a trimerization domain and by inserting ad hoc mutations to create a novel disulfide bridge and a core structure with improved hydrophobic packing. This new prefusion antigen was able to elicit much higher levels of RSV-neutralizing activity compared with the postfusion F antigen in mouse and NHP models. Although polyinosinic :polycytidylic acid adjuvant was used in combination with the engineered F antigen to stimulate an appropriate neutralizing response and no information was provided regarding their potency in the absence of such a strong adjuvant, this work provides an elegant preclinical demonstration of the feasibility of a structure-based vaccine strategy driven by the analysis of the human natural protective immune response and focused on mimicry of a large conformational epitope. Indeed, this study provides hope that a vaccine containing a prefusion-stabilized F protein would induce a response protective against RSV infection in humans, and clinical trials are ongoing.