Intranasal or airborne transmission-mediated delivery of an attenuated SARS-CoV-2 protects Syrian hamsters against new variants.

Detection of secretory antibodies in the airway is highly desirable when evaluating mucosal protection by vaccines against a respiratory virus, such as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We show that intranasal delivery of an attenuated SARS-CoV-2 (Nsp1-K164A/H165A) induces both mucosal and systemic IgA and IgG in male Syrian hamsters. Interestingly, either direct intranasal immunization or airborne transmission-mediated delivery of Nsp1-K164A/H165A in Syrian hamsters offers protection against heterologous challenge with variants of concern (VOCs) including Delta, Omicron BA.1, BA.2.12.1 and BA.5. Vaccinated animals show significant reduction in both tissue viral loads and lung inflammation. Similarly attenuated viruses bearing BA.1 and BA.5 spike boost variant-specific neutralizing antibodies in male mice that were first vaccinated with modified vaccinia virus Ankara vectors (MVA) expressing full-length WA1/2020 Spike protein. Together, these results demonstrate that our attenuated virus may be a promising nasal vaccine candidate for boosting mucosal immunity against future SARS-CoV-2 VOCs.

Differences in New Variant of Concern Replication at Physiological Temperatures In Vitro.

Using multiple cell types and isolates of Delta and Omicron variants of SARS-CoV-2, we report differences in virus production, replication, and infectivity in vitro. Ancestral and Delta SARS-CoV-2 variant exhibit reduced virus production and replication at 34°C compared to 37°C while Omicron replication is balanced between temperatures.

Spike protein-independent attenuation of SARS-CoV-2 Omicron variant in laboratory mice.

Despite being more transmissible, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant only causes milder diseases in laboratory animals, often accompanied by a lower viral load compared with previous variants of concern. In this study, we report the structural basis for a robust interaction between the receptor-binding domain of the Omicron spike protein and mouse ACE2. We show that pseudovirus bearing the Omicron spike protein efficiently utilizes mouse ACE2 for entry. By comparing viral load and disease severity among laboratory mice infected by a natural Omicron variant or recombinant ancestral viruses bearing either the entire Omicron spike or only the N501Y/Q493R mutations in its spike, we find that mutations outside the spike protein in the Omicron variant may be responsible for the observed lower viral load. Together, our results imply that a post-entry block to the Omicron variant exists in laboratory mice.

E4orf1 Suppresses E1B-Deleted Adenovirus Vaccine-Induced Immune Responses.

As demonstrated by the recent COVID pandemic, vaccines can reduce the burden arising from infectious agents. Adenoviruses (Ads) with deletion of the early region 1B55K (ΔE1B Ad) are currently being explored for use in vaccine delivery. ΔE1B Ads are different from Ads with deletions in early region 1 and early region 3 (ΔE1/E3) used in most Ad vaccine vectors in that they contain the Ad early region 1A (E1A), and therefore the ability to replicate. Common to almost all Ads that are being explored for clinical use is the Ad early region 4 (E4). Among the E4 genes is open reading frame 1 (E4orf1), which mediates signals through the PI3-kinase/Akt pathway that is known to modulate immune responses. This suggests that E4orf1 might also modulate immune responses, although it has remained unexplored in ΔE1B Ad. Here, we show that cells infected with an E1B55K and E4orf1-deleted (ΔE41) Ad exhibited reduced levels of phosphorylated Akt (Ser473 and Thr308)) and expressed different intrinsic innate immune cytokines from those induced in cells infected with an E4orf1-containing, ΔE1B parental Ad that exhibited elevated levels of phosphorylated Akt. Rhesus macaques immunized with a ΔE41 Ad that expressed rhFLSC (HIV-1BaL gp120 linked to rhesus CD4 D1 and D2), exhibited higher levels of rhFLSC-specific interferon γ-producing memory T-cells, higher titers of rhFLSC-specific IgG1 binding antibody in serum, and antibodies able to mediate antibody-dependent cellular cytotoxicity (ADCC) with greater killing capacity than the ΔE1B Ad. Therefore, E4orf1, perhaps by acting through the PI3-kinase/Akt pathway, limits intrinsic innate and system-wide adaptive immune responses that are important for improved ΔE1B Ad-based vaccines.