Respiratory Syncytial Virus (RSV) is the single largest viral cause of pediatric bronchiolitis and pneumonia, with a high worldwide mortality, and there is no safe and effective vaccine. Ever since the encounter with vaccine- enhanced disease (VED) during a formalin-inactivated RSV vaccination trial in the 1960s, it has been enormously challenging to impart both sufficient safety and efficacy in a single vaccine. For RSV-nave children, live-attenuated vaccines are an important focus, because inactivated and subunit vaccines are poor at inducing cell-mediated immunity which contributes to VED. Moreover, live vaccines can induce broad systemic and local immunity. Thus, for RSV-nave individuals a live vaccine approach is an attractive option, provided the vaccine itself is sufficiently safe and cannot revert to a more aggressive phenotype. It has recently been recognized that the viral fusion (F) protein is unstable and readily shifts to the post-fusion conformation during purification or vaccine preparation. As a result, a large proportion of vaccine-induced antibodies (Abs) target the post-fusion form, which is functionally obsolete. To avoid induction of anti- post- fusion F Abs, McLellan et al were able to genetically stabilize the pre-fusion form (referred to as FPRE), thereby greatly increasing neutralizing capacity of anti-F Abs, when given as a protein vaccine. However, subunit vaccines are deemed unsafe for the RSV-nave target population. Thus, to protect RSV-nave children there is a need to place the advantageous pre-fusion F concept in the context of a balanced immune response that does not cause VED, which could be achieved by expressing FPRE from a live-attenuated virus. However, stabilization of FPRE renders it non-functional and a virus solely expressing FPRE is not viable. We previously reported a baculovirus GP64 based complementation system that can overcome this problem, as it uniquely allows generation of infectious F-deleted or F-compromised viruses from cDNA in GP64-expressing cells. These GP64-pseudotyped viruses could be amplified to high titer and were significantly more temperature stable than wildtype RSV. Due to replacement of functional F with trans-complemented GP64, the viruses are infectious but self-limited and cannot spread beyond initially infected cells, thus constituting an attractive live-attenuated platform. We have in vivo preliminary data using a similar single-round RSV vaccine that suggests such vaccines are efficaceous. The goal of this proposal is to exploit the GP64 system to generate a live RSV which solely expresses the pre- fusion F form. Our hypothesis is that replacing the native, functional, F gene with a gene encoding a pre-fusion stabilized F in a live virus, will drive the anti-F response toward the pre-fusion F form, and will induce a balanced response that includes cell-mediated immunity and avoids VED. We will test this hypothesis through two aims: 1) Generate and in vitro characterize a live-attenuated RSV lacking a functional F copy and expressing instead a conformationally stabilized pre-fusion F; and 2) Characterize immune properties of RSV expressing FPRE in vivo to assess safety and efficacy. If successful, the vaccine will exceed previous formulations in inducing a broadly efficaceous yet safe immune response for the RSV-nave target population.
Respiratory syncytial virus (RSV) is responsible for more than 100,000 deaths in children annually. Generating an effective yet safe vaccine has been enormously challenging. This project enables the combining of novel vaccine efficacy improvements with the safety and broad immunogenicity of live-attenuated vaccines, to achieve a vaccine suitable for RSV-nave children.