Respiratory tract explant infection dynamics of influenza A virus in California sea lions, northern elephant seals, and rhesus macaques and severe acute respiratory syndrome virus-2 in rhesus macaques

To understand susceptibility of California sea lions and Northern elephant seals to influenza A virus (IAV), we developed an ex vivo respiratory explant model and used it to compare infection kinetics for multiple IAV subtypes. We used a similar approach with explants from rhesus macaques to establish the system and to compare infection kinetics with marine mammals. Trachea, bronchi, and lungs from 11 California sea lions, 2 Northern elephant seals and 10 rhesus macaques were inoculated within 24 hours post-mortem with 6 strains representing 4 IAV subtypes. Explants from all 3 species showed similar IAV infection kinetics with peak viral titers 48-72 hours post-inoculation (hpi) that increased by 2-4 log10 PFU/explant relative to the inoculum. Immunohistochemistry localized IAV infection to apical epithelial cells. These results demonstrate that respiratory tissue explants from marine mammals support IAV infection. In the absence of the ability to perform experimental infections of marine mammals, this ex vivo culture of respiratory tissues mirrors the in vivoenvironment and serves as a tool to study IAV susceptibility, host-range, and tissue tropism. We adapted the explant approach and used it to inoculate tissues from 2 rhesus macaques with severe acute respiratory syndrome virus 2 (SARS-CoV-2). SARS-CoV-2 titers increased by 2-4 log10 PFU/explant relative to the inoculum and peaked 48 or 72 hpi in trachea, bronchi, and kidney of both macaques and in the lung of 1 animal. These results demonstrate that this ex vivo model can define infection dynamics for 2 respiratory viruses of significant public health importance.