SARS-CoV-2 ORF8 modulates the upper respiratory tract inflammatory response to facilitate transmission

The ability of respiratory viruses to exploit host immune responses to promote transmission is a defining feature of pandemics. SARS-CoV-2 remains a major global public health threat because of its persistent evolution and capacity to counteract evolving immune defenses. Although the immune evasion properties of the SARS-CoV-2 Spike protein are well characterized, the contributions of other viral proteins to transmission remain poorly understood. Here, we used the infant mouse model to define the role of the accessory protein ORF8 in SARS-CoV-2 spread. We demonstrate that ORF8 supports efficient upper respiratory tract (URT) infection, infectious virus shedding, and host-to-host transmission. Mice infected with a recombinant SARS-CoV-2 strain lacking ORF8 (rΔORF8) had less infectious virus recovered from URT tissues and nasal secretions and transmitted less efficiently than mice infected with the isogenic ancestral strain rWA-1, which contains an intact ORF8. Recombinant viruses encoding naturally occurring ORF8 mutations exhibited distinct transmission phenotypes, with ORF8-deficient viruses resembling rΔORF8. Infection with ORF8-sufficient viruses induced greater macrophage recruitment, inflammatory cytokine production, and type-I interferon (IFN-I) signaling programs than ORF8-deficient viruses. Intranasal IFNβ supplementation partially restored URT shedding by rΔORF8-infected mice and rescued transmission to contacts, whereas blockade of the type I interferon receptor (IFNAR) in rWA-1-infected index mice reduced contact infection and transmission. Together, these findings demonstrate that ORF8 promotes SARS-CoV-2 transmission by engaging an IFN-I-associated inflammatory and secretory program in the URT that supports virus shedding from the infected host. These data identify ORF8 as a viral determinant of host mucosal responses that promote contagiousness.