The infant mouse model is used to define the role of the accessory protein ORF8 in SARS-CoV-2 spread and reveal how a SARS-CoV-2 accessory protein can exploit mucosal antiviral responses to increase host contagiousness.
Abstract
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. Importance Efficient host-to-host transmission underlies the success of respiratory viruses. Although SARS-CoV-2 research has largely focused on the Spike protein, accessory proteins can also shape viral fitness and spread. We previously identified ORF8 as a determinant of SARS-CoV-2 transmission. Here we show that ORF8 promotes SARS-CoV-2 infectious viral shedding and transmission by engaging IFN-I-associated inflammatory and secretory responses in the URT. These findings reveal how a SARS-CoV-2 accessory protein can exploit mucosal antiviral responses to increase host contagiousness.
SARS-CoV-2 ORF8 is a rapidly evolving accessory protein that modulates host immunity through both intracellular and extracellular mechanisms. Intracellularly, ORF8 disrupts antigen presentation by reducing cell-surface MHC-I and is linked to endoplasmic reticulum remodeling and altered stress responses. Extracellularly, secreted ORF8 behaves as a virokine that can amplify inflammatory programs in myeloid and dendritic cells, with potential implications for acute severity and tissue-specific pathology. ORF8 is also reported to antagonize type I interferon induction through effects on IRF3-dependent pathways. Clinically, circulating ORF8 has been associated with disease severity, and persistent detection of ORF8 after viral RNA becomes undetectable has been reported in some individuals with post-acute symptoms. However, whether this persistence reflects a direct pathogenic role or instead marks ongoing viral burden and immune activation remains unresolved. In this review, we summarize recent findings on the pleiotropic effects of ORF8 and outline key priorities to clarify when ORF8 acts primarily as an immune-evasion factor, an inflammatory amplifier, or both. Collectively, the evidence reviewed here identifies ORF8 as a promising candidate for further mechanistic studies, whose biomarker potential and therapeutic relevance warrant rigorous evaluation in acute and post-acute COVID-19.
Carolina Schäfer, C. Blamey, Rocío Balbiano et al.· Frontiers in Immunology· 0 citations
SARS-CoV-2 primarily targets epithelial cells in the respiratory and intestinal tracts where its cognate receptor ACE2 and obligate processing enzymes furin and TMPRSS2 are richly expressed. However, compared with severe inflammation and tissue damage in the lungs of a COVID-19 patient, clinical lesions in the intestine are rare, suggesting an effective intestinal mucosal immunity against SARS-CoV-2 infection. Here, we report that MMP7-/-/hACE2 hybrid mice lacking mature enteric α-defensins or cryptdins were more susceptible to SARS-CoV-2 infection in the intestine than K18-hACE2 transgenic mice. The mouse α-defensin cryptdin-5 (Crp5) displayed potent and broad antiviral activity in vitro and in vivo by two distinct mechanisms, (1) directly targeting the RBD of the spike (S) protein to antagonize its interactions with ACE2, thus blocking viral attachment, membrane fusion and cell-to-cell transmission, and (2) binding to the 630 loop of the S protein to induce its multimerization, thereby impairing proteolytic processing, membrane fusion and, ultimately, viral infectivity. Our findings imply that enteric α-defensins help alleviate, as host protective factors, Covid-19 symptoms in the intestine despite higher ACE2 expression in the gut than in the lungs, and that Crp5 may be developed as a broad-spectrum antiviral for the treatment of coronavirus infection irrespective of virus type and variant.
A model of long-term SARS-CoV-2 tissue persistence characterized by organ-specific immune and metabolic signatures is established, providing a platform to investigate mechanisms underlying post-acute sequelae and evaluate potential therapeutic strategies.
Thais Melquiades de Lima, Carlos Eduardo Capelini Eli Lopes, Maria Vitoria Oliveira de Souza et al.· bioRxiv· 0 citations
The emergence of the SARS-CoV-2 pandemic led to the spread of highly transmissible variants, such as the Delta variant, which originated in India, underscoring the urgent need to develop new antivirals, therapeutics, and vaccines. In our previous study, we showed that Membrane-Envelope Virus-like Particles exhibit antigenicity and neutralization activity. Hence, our present study was conducted to evaluate whether the M protein alone can form VLPs that elicit an immune response. Using computational methods, we identified key interacting residues in M-protein that contribute to VLP formation and interact with other structural proteins, including Spike (S), Nucleocapsid (N), and Envelope (E). The SARS-CoV-2-M protein was expressed in Sf-21 insect cells, and the resulting VLPs were purified, analyzed for shape and size, and characterized using DLS, FESEM, and TEM. The purified VLPs were injected into BALB/c mice to evaluate their immune response compared with uninfected controls. The biophysical analysis confirms that the particles are round and have a size of ~ 180–200 nm. The serum levels of IgG, IgM, and IgA were found to be higher in immunized mice than in uninfected mice. Further qRT-PCR analysis demonstrated the levels of IFN-γ, IL-2, and IL-12, indicating a TH1-biased immune response against the M protein. Our study demonstrates that the highly conserved M protein can self-assemble into VLPs and elicit humoral and cellular immune response. Furthermore, our study indicates that while M-protein VLPs elicit significant antibodies and cytokine responses, they do not induce detectable neutralizing activity when given alone.
Akash Kumar, K. K. Inampudi, Vikas Kumar et al.· Virology Journal· 0 citations
A genome engineering technology is used to change a single amino acid in the viruses’ main protease enzyme to match that of circulating Omicron isolates to demonstrate antiviral efficacy of approved drugs and uncover mutants with reduced drug sensitivity.
