Intranasal bovine/human parainfluenza virus 3 vaccine candidates expressing human metapneumovirus wild-type or pre-fusion F protein elicit protective immunity against human metapneumovirus in hamsters

Human metapneumovirus (HMPV) and human parainfluenza virus 3 (HPIV3) are important pediatric pathogens and need effective vaccines. We developed live-attenuated chimeric bovine/HPIV3 (B/HPIV3) expressing the fusion (F) protein of HMPV subgroup A as a bivalent pediatric intranasal vaccine against HPIV3 and HMPV. To enhance HMPV F immunogenicity, three approaches were evaluated: codon optimization to increase expression, structure-based mutations to stabilize HMPV F in pre-fusion conformation, or replacement of the HMPV F transmembrane/cytoplasmic domains with those of BPIV3 F to increase incorporation in the virus particle. In hamsters, a single intranasal dose of any of the B/HPIV3s expressing HMPV F was as immunogenic and protective against HMPV as a prior HMPV infection. In the upper respiratory tract, these provided greater protection against HMPV replication than the two intramuscular doses of the adjuvanted HMPV pre-F subunit antigens. B/HPIV3 expressing a codon-optimized pre-fusion stabilized HMPV F was advanced to a pediatric clinical study.