Murine Pneumonia Virus Expressing the Fusion Glycoprotein of Human Respiratory Syncytial Virus from an Added Gene Is Highly Attenuated and Immunogenic in Rhesus Macaques.

Human respiratory syncytial virus (RSV) continues to be the leading viral cause of severe acute lower respiratory tract disease in infants and children worldwide. A licensed vaccine or antiviral drug suitable for routine use remains unavailable. Like RSV, Murine pneumonia virus (MPV) is a member of the genus Orthopneumovirus, family Pneumoviridae Humans are not normally exposed to MPV, and MPV is not cross-protective with RSV. We evaluated MPV as an RSV vaccine vector expressing the RSV fusion (F) glycoprotein. The RSV F open reading frame (ORF) was codon optimized, and the encoded RSV F protein was made identical to an early passage of RSV strain A2. The RSV F ORF was placed under the control of MPV transcription signals and inserted at the first (rMPV-F1), third (rMPV-F3), or fourth (rMPV-F4) gene position of a version of the MPV genome that contained a codon-pair-optimized polymerase (L) gene. The recovered viruses replicated in vitro as efficiently as the empty vector, with stable expression of RSV F protein. Replication and immunogenicity of rMPV-F1 and rMPV-F3 were evaluated in rhesus macaques following intranasal and intratracheal administration. Both viruses replicated at low levels in the upper and lower respiratory tracts, maintained stable RSV F expression, and induced RSV-neutralizing serum antibodies at high levels similar to those induced by wild-type RSV replicating to a 5- to 25-fold-higher titer. In conclusion, this study demonstrated that rMPV provides a highly attenuated yet immunogenic vector for the expression of RSV F protein, with potential application in RSV-naive and RSV-experienced populations.IMPORTANCE Human respiratory syncytial virus (RSV) is an important human pathogen that lacks a licensed vaccine or antiviral drug suitable for routine use. We describe here the evaluation of recombinant murine pneumonia virus (rMPV) as a live-attenuated vector that expresses the RSV F protein, the major RSV neutralization antigen, as an experimental RSV vaccine. The rMPV-RSV-F vectors expressing RSV F from the first, third, or fourth gene position were genetically stable and were not restricted for replication in vitro In contrast, the vectors exhibited highly attenuated replication in the respiratory tract of rhesus macaques, maintained stable RSV F expression, and induced RSV-neutralizing serum antibodies at high titers similar to those conferred by wild-type RSV. Given the lack of preexisting immunity to MPV in humans and the lack of cross-neutralization and cross-protection between MPV and RSV, an rMPV-vectored RSV vaccine should be immunogenic in both RSV-naive children and RSV-experienced adults.