Prefusion piv f immunogens and their use

An immunogen, comprising:

• a recombinant hPIV4 F ectodomain trimer comprising protomers comprising one or more amino acid substitutions or deletions that stabilize the hPIV4 ectodomain trimer in a prefusion conformation, wherein the one or more amino acid substitutions comprise
• a non-native disulfide bond between I166C and T232C substitutions;
• or
• a non-native disulfide bond between I166C and T232C substitutions, and Y457F and S471I cavity filling substitutions.

An immunogen, comprising:

• a recombinant bovine parainfluenza virus (bPIV)3 F ectodomain trimer or caprine parainfluenza virus (cPIV)3 F ectodomain trimer comprising protomers comprising one or more amino acid substitutions or deletions that stabilize the bPIV3 or cPIV3ectodomain trimer in a prefusion conformation, wherein the one or more amino acid substitutions or deletions comprise
• 463V and/or 474Y cavity filling substitutions;
• and/or
• one or more of the following sets of substitutions to form a disulfide bond to stabilize the hPIV3 ectodomain trimer in a prefusion conformation:
• 162C and 168C;
• 213C and 230C;
• 170C and 242C;
• 216C and 221C;
• 85C and 222C;
• and
• 172C and 238C.