Reverse vaccinology 2.0: Human immunology instructs vaccine antigen design

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

The quest for an RSV vaccine has met many challenges that have inspired research breakthroughs, illustrating how information from human immunology and structural biology studies can be used in combination to instruct vaccine antigen design. RSV is the main viral cause of severe respiratory tract disease in children worldwide (Nair et al., 2010) and also afflicts elderly and immunocompromised adults (Falsey et al., 2005) . The membrane-anchored fusion glycoprotein F present on the surface of this virus is the major target of NAbs (Anderson et al., 2010). However, although an approved therapeutic mAb treatment (palivizumab) targeting F has existed for nearly two decades (The IMpact-RSV Study Group, 1998), attempts to develop an RSV vaccine using recombinant F antigen have been hampered by its poor behavior in solution and its tendency to undergo large conformational changes. Early epitope-focused rational antigen designs were enabled by the cocrystal structure of the Fab region of an RSV-neutralizing mAb (motavizumab, an affinity-enhanced form of palivizumab) bound to its target F epitope prepared as a synthetic 24-residue peptide that adopted a helix-turnhelix motif in the crystal (McLellan et al., 2010) . However, initial attempts to design an immunogen comprised of a scaffold protein presenting this F epitope with the observed helix-turn-helix conformation were only partially successful: after immunization, conformation-specific anti-F Abs were elicited but lacked the ability to neutralize RSV (McLellan et al., 2011) . Subsequently, Correia et al. (2014) reported a proof-of-principle study, wherein they developed a new structure-based approach for the computational design and optimization of improved biophysically robust scaffold proteins displaying the desired RSV peptide epitope in a conformationally faithful manner that was able to elicit RSV NAbs in nonhuman primates (NHPs). Furthermore, when the RSV peptide epitope scaffold antigen was mounted in multicopy ordered arrays on the surface of a virus-like particle by linking to the hepatitis B core antigen, at least half of the NHPs raised RSV-neutralizing activity comparable to that induced by natural human infection. Thus, careful manipulations to properly present a structurally optimized form of a single well-defined protective epitope resulted in the generation of the desired immune response, demonstrating the feasibility of this novel immune-focusing approach. Nevertheless, variability of the immune response in a diverse human population, together with the risk of the emergence of naturally occurring epitope escape mutants, makes a vaccine approach relying on a single protective epitope potentially less robust compared with immunization strategies based on a full-length antigen, such as the entire ectodomain of the RSV F protein, which would potentially offer a broader immune response.