Recombinant human metapneumovirus F proteins and their use

An immunogen, comprising:

• a recombinant human metapneumovirus (HMPV) F ectodomain trimer stabilized in a prefusion conformation by one or more amino acid substitutions in protomers of the trimer, the amino acid substitutions comprising cysteine substitutions at HMPV F positions 84 and 249 that form a non-natural inter-protomer disulfide bond, and cysteine substitutions at HMPV F positions 140 and 147 that form a non-natural intra-protomer disulfide bond;
• and
• wherein the protomers of the recombinant HMPV F ectodomain trimer are single chain proteins comprising one or more amino acid substitutions to remove the F1/F2 protease cleavage site and wherein the F2 polypeptide and F1 ectodomain are linked by a heterologous peptide linker;
• and
• wherein the amino acid numbering is according to the reference HMPV F sequence set forth as SEQ ID NO:
• 7.