Development of a Novel Mucosal Vaccine Vector Derived From Human Parainfluenza Virus Type 2 and Its Efficacy as a <scp>SARS‐CoV‐2</scp> Vaccine by Intratracheal Administration Using Transgenic Mice Expressing Human Angiotensin‐Converting Enzyme 2

We created a novel replication-incompetent vector derived from the human parainfluenza virus type 2 (hPIV2). The vector lacks the hemagglutinin-neuraminidase (HN) gene (hPIV2/∆HN) that is essential for virus attachment and release. We also generated the stable packaging cell line expressing hPIV2 HN protein derived from Vero cells (Vero-HN). As SARS-CoV-2 vaccines, we created hPIV2/∆HN-based SARS-CoV-2 vaccines expressing the receptor binding domain of the spike protein (S-RBD) of SARS-CoV-2, and S-RBD with trimer formation domain (S-RBD-FD) with or without antigen 85B (Ag85B) having strong Th1-type cytokine inducing activity. Intratracheal administration of S-RBD-FD/hPIV2/∆HN and S-RBD-FD/Ag85B/hPIV2/∆HN induced the S-RBD-specific neutralizing antibodies in sera of transgenic mice expressing human angiotensin-converting enzyme 2 (hACE2/TG), and also the sera neutralized the pseudo-typed lentivirus expressing the SARS-CoV-2 S protein. In particular, in an S-RBD-FD/Ag85B/hPIV2/∆HN-administered group, high levels of neutralizing antibodies were induced also in the bronchoalveolar lavage fluid (BALF) in the hACE2/TG mice. Further, hACE2/TG mice vaccinated with S-RBD-FD/hPIV2/∆HN or S-RBD-FD/Ag85B/hPIV2/∆HN did not exhibit weight loss and showed a high survival rate after authentic SARS-CoV-2 challenge. These results suggested that S-RBD-FD/Ag85B/hPIV2/∆HN is a vaccine candidate against SARS-CoV-2. Finally, hPIV2/∆HN or Ag85B/hPIV2/∆HN vector will be potentially applicable to vaccine development and/or human gene therapy for respiratory tract diseases.