Immune evasion, infectivity, and fusogenicity of SARS-CoV-2 BA.2.86 and FLip variants.

Evolution of SARS-CoV-2 requires the reassessment of current vaccine measures. Here, we characterized BA.2.86 and XBB-derived variant FLip by investigating their neutralization alongside D614G, BA.1, BA.2, BA.4/5, XBB.1.5, and EG.5.1 by sera from 3-dose-vaccinated and bivalent-vaccinated healthcare workers, XBB.1.5-wave-infected first responders, and monoclonal antibody (mAb) S309. We assessed the biology of the variant spikes by measuring viral infectivity and membrane fusogenicity. BA.2.86 is less immune evasive compared to FLip and other XBB variants, consistent with antigenic distances. Importantly, distinct from XBB variants, mAb S309 was unable to neutralize BA.2.86, likely due to a D339H mutation based on modeling. BA.2.86 had relatively high fusogenicity and infectivity in CaLu-3 cells but low fusion and infectivity in 293T-ACE2 cells compared to some XBB variants, suggesting a potentially different conformational stability of BA.2.86 spike. Overall, our study underscores the importance of SARS-CoV-2 variant surveillance and the need for updated COVID-19 vaccines.

A next-generation intranasal trivalent MMS vaccine induces durable and broad protection against SARS-CoV-2 variants of concern.

As SARS-CoV-2 variants of concern (VoCs) that evade immunity continue to emerge, next-generation adaptable COVID-19 vaccines which protect the respiratory tract and provide broader, more effective, and durable protection are urgently needed. Here, we have developed one such approach, a highly efficacious, intranasally delivered, trivalent measles-mumps-SARS-CoV-2 spike (S) protein (MMS) vaccine candidate that induces robust systemic and mucosal immunity with broad protection. This vaccine candidate is based on three components of the MMR vaccine, a measles virus Edmonston and the two mumps virus strains [Jeryl Lynn 1 (JL1) and JL2] that are known to provide safe, effective, and long-lasting protective immunity. The six proline-stabilized prefusion S protein (preS-6P) genes for ancestral SARS-CoV-2 WA1 and two important SARS-CoV-2 VoCs (Delta and Omicron BA.1) were each inserted into one of these three viruses which were then combined into a trivalent "MMS" candidate vaccine. Intranasal immunization of MMS in IFNAR1-/- mice induced a strong SARS-CoV-2-specific serum IgG response, cross-variant neutralizing antibodies, mucosal IgA, and systemic and tissue-resident T cells. Immunization of golden Syrian hamsters with MMS vaccine induced similarly high levels of antibodies that efficiently neutralized SARS-CoV-2 VoCs and provided broad and complete protection against challenge with any of these VoCs. This MMS vaccine is an efficacious, broadly protective next-generation COVID-19 vaccine candidate, which is readily adaptable to new variants, built on a platform with a 50-y safety record that also protects against measles and mumps.

Neutralizing antibody response to SARS-CoV-2 bivalent mRNA vaccine in SIV-infected rhesus macaques: Enhanced immunity to XBB subvariants by two-dose vaccination.

The evolution of SARS-CoV-2 paired with immune imprinting by prototype messenger RNA (mRNA) vaccine has challenged the current vaccination efficacy against newly emerged Omicron subvariants. In our study, we investigated a cohort of macaques infected by SIV and vaccinated with two doses of bivalent Pfizer mRNA vaccine containing wildtype and BA.5 spikes. Using a pseudotyped lentivirus neutralization assay, we determined neutralizing antibody (nAb) titers against new XBB variants, i.e., XBB.1.5, XBB.1.16, and XBB.2.3, alongside D614G and BA.4/5. We found that compared to humans vaccinated with three doses of monovalent mRNA vaccine plus a bivalent booster, the monkeys vaccinated with two doses of bivalent mRNA vaccines exhibited relatively increased titers against XBB subvariants. Of note, SIV-positive dam macaques had reduced nAb titers relative to SIV-negative dams. Additionally, SIV positive dams that received antiretroviral therapy had lower nAb titers than untreated dams. Our study underscores the importance of reformulating the COVID-19 vaccine to better protect against newly emerged XBB subvariants as well as the need for further investigation of vaccine efficacy in individuals living with HIV-1.

A Nanoparticle-Based Hepatitis C Virus Vaccine With Enhanced Potency.

Despite the emergence of new direct-acting antivirals, hepatitis C virus (HCV) chronic infection and its consequent fibrosis and hepatocarcinoma remain a significant burden for public health, thus requiring an effective preventive vaccine. Our group previously showed that a subunit vaccine based on recombinant soluble E2 (sE2) can induce broadly neutralizing antibodies. To improve the immunogenicity of sE2, we designed and produced a fusion protein (sE2-ferritin) comprising sE2 and a ferritin unit in Drosophila S2 cells, which self-assembled into a nanoparticle with sE2 displayed on the surface. The sE2 moiety on the sE2-ferritin nanoparticle not only had nearly natural conformation but also had better affinities than the unfused sE2 to neutralizing antibodies, receptor, and patient serum. Mouse immunization studies showed that sE2-ferritin was more potent than sE2 in inducing anti-HCV broadly neutralizing antibodies. Our results demonstrate that sE2-ferritin is a vaccine candidate superior to previously developed sE2, providing a new possibility for controlling HCV.

Three SARS-CoV-2 spike protein variants delivered intranasally by measles and mumps vaccines are broadly protective.

As the new SARS-CoV-2 Omicron variants and subvariants emerge, there is an urgency to develop intranasal, broadly protective vaccines. Here, we developed highly efficacious, intranasal trivalent SARS-CoV-2 vaccine candidates (TVC) based on three components of the MMR vaccine: measles virus (MeV), mumps virus (MuV) Jeryl Lynn (JL1) strain, and MuV JL2 strain. Specifically, MeV, MuV-JL1, and MuV-JL2 vaccine strains, each expressing prefusion spike (preS-6P) from a different variant of concern (VoC), were combined to generate TVCs. Intranasal immunization of IFNAR1-/- mice and female hamsters with TVCs generated high levels of S-specific serum IgG antibodies, broad neutralizing antibodies, and mucosal IgA antibodies as well as tissue-resident memory T cells in the lungs. The immunized female hamsters were protected from challenge with SARS-CoV-2 original WA1, B.1.617.2, and B.1.1.529 strains. The preexisting MeV and MuV immunity does not significantly interfere with the efficacy of TVC. Thus, the trivalent platform is a promising next-generation SARS-CoV-2 vaccine candidate.

Determinants and Mechanisms of the Low Fusogenicity and High Dependence on Endosomal Entry of Omicron Subvariants.

The rapid spread and strong immune evasion of the SARS-CoV-2 Omicron subvariants has raised serious concerns for the global COVID-19 pandemic. These new variants exhibit generally reduced fusogenicity and increased endosomal entry pathway utilization compared to the ancestral D614G variant, the underlying mechanisms of which remain elusive. Here, we show that the C-terminal S1 mutations of the BA.1.1 subvariant, H655Y and T547K, critically govern the low fusogenicity of Omicron. Notably, H655Y also dictates the enhanced endosome entry pathway utilization. Mechanistically, T547K and H655Y likely stabilize the spike trimer conformation as suggested by increased molecular interactions in structural modeling and enhanced S1 shedding of their reversion mutants K547T and Y655H in viral producer cells. Importantly, the H655Y mutation also determines the low fusogenicity and enhanced dependence on the endosomal entry pathway of other Omicron subvariants, including BA.2, BA.2.12.1, BA.4/5, and BA.2.75. Together, these results uncover mechanisms governing Omicron subvariant entry and provide insights into altered Omicron tissue tropism and pathogenesis. IMPORTANCE Omicron has been shown to predominantly use the endosomal entry pathway, resulting in reduced lung tropism and reduced disease severity; however, the underlying mechanism is not fully understood. In addition, whether the most recent Omicron subvariants, including BA.5 and BA.2.75, use the same pathway as their ancestor for entry is currently not known. In this study, we show that T547K and H655Y mutations in the C terminus of the S1 subunit critically determine the enhanced dependence on the endosomal entry pathway as well as the reduced cell-cell fusion activity of Omicron BA.1, BA.1.1, and other subvariants. Further experiments and molecular modeling suggest that H655Y and K547T stabilize the spike trimer conformation, likely contributing to the decreased fusogenicity and endosomal entry. Our work uncovers novel mechanisms underlying the distinct entry pathway of Omicron subvariants and advances our understanding of their biological characteristics.

Continued evasion of neutralizing antibody response by Omicron XBB.1.16.

The evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to challenge the efficacy of vaccination efforts against coronavirus disease 2019 (COVID-19). The Omicron XBB lineage of SARS-CoV-2 has presented dramatic evasion of neutralizing antibodies stimulated by mRNA vaccination and COVID-19 convalescence. XBB.1.16, characterized by two mutations relative to the dominating variant XBB.1.5, i.e., E180V and K478R, has been on the rise globally. In this study, we compare the immune escape of XBB.1.16 with XBB.1.5, alongside ancestral variants D614G, BA.2, and BA.4/5. We demonstrate that XBB.1.16 is strongly immune evasive, with extent comparable to XBB.1.5 in bivalent-vaccinated healthcare worker sera, 3-dose-vaccinated healthcare worker sera, and BA.4/5-wave convalescent sera. Interestingly, the XBB.1.16 spike is less fusogenic than that of XBB.1.5, and this phenotype requires both E180V and K478R mutations to manifest. Overall, our findings emphasize the importance of the continued surveillance of variants and the need for updated mRNA vaccine formulations.

Neutralization escape of Omicron XBB, BR.2, and BA.2.3.20 subvariants.

New Omicron subvariants continue to emerge throughout the world. In particular, the XBB subvariant, which is a recombinant virus between BA.2.10.1.1 and BA.2.75.3.1.1.1, as well as the BA.2.3.20 and BR.2 subvariants that contain mutations distinct from BA.2 and BA.2.75, are currently increasing in proportion of variants sequenced. Here we show that antibodies induced by 3-dose mRNA booster vaccination as well as BA.1- and BA.4/5-wave infection effectively neutralize BA.2, BR.2, and BA.2.3.20 but have significantly reduced efficiency against XBB. In addition, the BA.2.3.20 subvariant exhibits enhanced infectivity in the lung-derived CaLu-3 cells and in 293T-ACE2 cells. Overall, our results demonstrate that the XBB subvariant is highly neutralization resistant, which highlights the need for continued monitoring of the immune escape and tissue tropism of emerging Omicron subvariants.

Immune Evasion, Infectivity, and Fusogenicity of SARS-CoV-2 Omicron BA.2.86 and FLip Variants.

Evolution of SARS-CoV-2 requires the reassessment of current vaccine measures. Here, we characterized BA.2.86 and the XBB-lineage variant FLip by investigating their neutralization alongside D614G, BA.1, BA.2, BA.4/5, XBB.1.5, and EG.5.1 by sera from 3-dose vaccinated and bivalent vaccinated healthcare workers, XBB.1.5-wave infected first responders, and monoclonal antibody (mAb) S309. We assessed the biology of the variant Spikes by measuring viral infectivity and membrane fusogenicity. BA.2.86 is less immune evasive compared to FLip and other XBB variants, consistent with antigenic distances. Importantly, distinct from XBB variants, mAb S309 was unable to neutralize BA.2.86, likely due to a D339H mutation based on modeling. BA.2.86 had relatively high fusogenicity and infectivity in CaLu-3 cells but low fusion and infectivity in 293T-ACE2 cells compared to some XBB variants, suggesting a potentially differences conformational stability of BA.2.86 Spike. Overall, our study underscores the importance of SARS-CoV-2 variant surveillance and the need for updated COVID-19 vaccines.

Immune evasion and membrane fusion of SARS-CoV-2 XBB subvariants EG.5.1 and XBB.2.3.

Immune evasion by SARS-CoV-2 paired with immune imprinting from monovalent mRNA vaccines has resulted in attenuated neutralizing antibody responses against Omicron subvariants. In this study, we characterized two new XBB variants rising in circulation - EG.5.1 and XBB.2.3, for their neutralization and syncytia formation. We determined the neutralizing antibody titers in sera of individuals that received a bivalent mRNA vaccine booster, BA.4/5-wave infection, or XBB.1.5-wave infection. Bivalent vaccination-induced antibodies neutralized ancestral D614G efficiently, but to a much less extent, two new EG.5.1 and XBB.2.3 variants. In fact, the enhanced neutralization escape of EG.5.1 appeared to be driven by its key defining mutation XBB.1.5-F456L. Notably, infection by BA.4/5 or XBB.1.5 afforded little, if any, neutralization against EG.5.1, XBB.2.3 and previous XBB variants - especially in unvaccinated individuals, with average neutralizing antibody titers near the limit of detection. Additionally, we investigated the infectivity, fusion activity, and processing of variant spikes for EG.5.1 and XBB.2.3 in HEK293T-ACE2 and CaLu-3 cells but found no significant differences compared to earlier XBB variants. Overall, our findings highlight the continued immune evasion of new Omicron subvariants and, more importantly, the need to reformulate mRNA vaccines to include XBB spikes for better protection.

Extraordinary Evasion of Neutralizing Antibody Response by Omicron XBB.1.5, CH.1.1 and CA.3.1 Variants.

Newly emerging Omicron subvariants continue to emerge around the world, presenting potential challenges to current vaccination strategies. This study investigates the extent of neutralizing antibody escape by new subvariants XBB.1.5, CH.1.1, and CA.3.1, as well as their impacts on spike protein biology. Our results demonstrated a nearly complete escape of these variants from neutralizing antibodies stimulated by three doses of mRNA vaccine, but neutralization was rescued by a bivalent booster. However, CH.1.1 and CA.3.1 variants were highly resistant to both monovalent and bivalent mRNA vaccinations. We also assessed neutralization by sera from individuals infected during the BA.4/5 wave of infection and observed similar trends of immune escape. In these cohorts, XBB.1.5 did not exhibit enhanced neutralization resistance over the recently dominant BQ.1.1 variant. Notably, the spike proteins of XBB.1.5, CH.1.1, and CA.3.1 all exhibited increased fusogenicity compared to BA.2, correlating with enhanced S processing. Overall, our results support the administration of new bivalent mRNA vaccines, especially in fighting against newly emerged Omicron subvariants, as well as the need for continued surveillance of Omicron subvariants.

Enhanced neutralization resistance of SARS-CoV-2 Omicron subvariants BQ.1, BQ.1.1, BA.4.6, BF.7, and BA.2.75.2.

The continued evolution of SARS-CoV-2 has led to the emergence of several new Omicron subvariants, including BQ.1, BQ.1.1, BA.4.6, BF.7, and BA.2.75.2. Here, we examine the neutralization resistance of these subvariants against sera from 3-dose vaccinated healthcare workers, hospitalized BA.1-wave patients, and BA.4/5-wave patients. We found enhanced neutralization resistance in all new subvariants, especially in the BQ.1 and BQ.1.1 subvariants driven by N460K and K444T mutations, as well as the BA.2.75.2 subvariant driven largely by its F486S mutation. All Omicron subvariants maintained their weakened infectivity in Calu-3 cells, with the F486S mutation driving further diminished titer for the BA.2.75.2 subvariant. Molecular modeling revealed the mechanisms of antibody-mediated immune evasion by R346T, K444T, F486S, and D1199N mutations. Altogether, these findings shed light on the evolution of newly emerging SARS-CoV-2 Omicron subvariants.

Enhanced evasion of neutralizing antibody response by Omicron XBB.1.5, CH.1.1, and CA.3.1 variants.

Omicron subvariants continuingly challenge current vaccination strategies. Here, we demonstrate nearly complete escape of the XBB.1.5, CH.1.1, and CA.3.1 variants from neutralizing antibodies stimulated by three doses of mRNA vaccine or by BA.4/5 wave infection, but neutralization is rescued by a BA.5-containing bivalent booster. CH.1.1 and CA.3.1 show strong immune escape from monoclonal antibody S309. Additionally, XBB.1.5, CH.1.1, and CA.3.1 spike proteins exhibit increased fusogenicity and enhanced processing compared with BA.2. Homology modeling reveals the key roles of G252V and F486P in the neutralization resistance of XBB.1.5, with F486P also enhancing receptor binding. Further, K444T/M and L452R in CH.1.1 and CA.3.1 likely drive escape from class II neutralizing antibodies, whereas R346T and G339H mutations could confer the strong neutralization resistance of these two subvariants to S309-like antibodies. Overall, our results support the need for administration of the bivalent mRNA vaccine and continued surveillance of Omicron subvariants.

Determinants and Mechanisms of the Low Fusogenicity and Endosomal Entry of Omicron Subvariants.

The rapid spread and strong immune evasion of the SARS-CoV-2 Omicron subvariants has raised serious concerns for the global COVID-19 pandemic. These new variants exhibit reduced fusogenicity and increased endosomal entry pathway utilization compared to the ancestral D614G variant, the underlying mechanisms of which remain elusive. Here we show that the C-terminal S1 mutations of the BA.1.1 subvariant, H655Y and T547K, critically govern the low fusogenicity of Omicron. Notably, H655Y also dictates the enhanced endosome entry pathway utilization. Mechanistically, T547K and H655Y likely stabilize the spike trimer conformation, as shown by increased molecular interactions in structural modeling as well as reduced S1 shedding. Importantly, the H655Y mutation also determines the low fusogenicity and high dependence on the endosomal entry pathway of other Omicron subvariants, including BA.2, BA.2.12.1, BA.4/5 and BA.2.75. These results uncover mechanisms governing Omicron subvariant entry and provide insights into altered Omicron tissue tropism and pathogenesis.

Evasion of neutralizing antibody responses by the SARS-CoV-2 BA.2.75 variant.

The newly emerged BA.2.75 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant contains 9 additional mutations in its spike (S) protein compared to the ancestral BA.2 variant. Here, we examine the neutralizing antibody escape of BA.2.75 in mRNA-vaccinated and BA.1-infected individuals, as well as the molecular basis underlying functional changes in S. Notably, BA.2.75 exhibits enhanced neutralization resistance over BA.2 but less than the BA.4/5 variant. The G446S and N460K mutations of BA.2.75 are primarily responsible for its enhanced resistance to neutralizing antibodies. The R493Q mutation, a reversion to the prototype sequence, reduces BA.2.75 neutralization resistance. The impact of these mutations is consistent with their locations in common neutralizing antibody epitopes. Further, BA.2.75 shows enhanced cell-cell fusion over BA.2, driven largely by the N460K mutation, which enhances S processing. Structural modeling reveals enhanced receptor contacts introduced by N460K, suggesting a mechanism of potentiated receptor utilization and syncytia formation.

Evasion of Neutralizing Antibody Response by the SARS-CoV-2 BA.2.75 Variant.

The newly emerged BA.2.75 SARS-CoV-2 variant exhibits an alarming 9 additional mutations in its spike (S) protein compared to the ancestral BA.2 variant. Here we examine the neutralizing antibody escape of BA.2.75 in mRNA-vaccinated and BA.1-infected individuals, as well as the molecular basis underlying functional changes in the S protein. Notably, BA.2.75 exhibits enhanced neutralization resistance over BA.2, but less than the BA.4/5 variant. The G446S and N460K mutations of BA.2.75 are primarily responsible for its enhanced resistance to neutralizing antibodies. The R493Q mutation, a reversion to the prototype sequence, reduces BA.2.75 neutralization resistance. The mutational impact is consistent with their locations in common neutralizing antibody epitopes. Further, the BA.2.75 variant shows enhanced cell-cell fusion over BA.2, driven largely by the N460K mutation, which enhances S processing. Structural modeling revealed a new receptor contact introduced by N460K, supporting a mechanism of potentiated receptor utilization and syncytia formation.

Durability of the Neutralizing Antibody Response to mRNA Booster Vaccination Against SARS-CoV-2 BA.2.12.1 and BA.4/5 Variants.

The recent emergence of the SARS-CoV-2 BA.4/5 and BA.2.12.1 variants has led to rising COVID-19 case numbers and concerns over the continued efficacy of mRNA booster vaccination. Here we examine the durability of neutralizing antibody (nAb) responses against these SARS-CoV-2 Omicron subvariants in a cohort of health care workers 1-40 weeks after mRNA booster dose administration. Neutralizing antibody titers fell by ~1.5-fold 4-6 months and by ~2.5-fold 7-9 months after booster dose, with average nAb titers falling by 11-15% every 30 days, far more stable than two dose induced immunity. Notably, nAb titers from booster recipients against SARS-CoV-2 BA.1, BA.2.12.1, and BA.4/5 variants were ~4.7-, 7.6-, and 13.4-fold lower than against the ancestral D614G spike. However, the rate of waning of booster dose immunity was comparable across variants. Importantly, individuals reporting prior infection with SARS-CoV-2 exhibited significantly higher nAb titers compared to those without breakthrough infection. Collectively, these results highlight the broad and stable neutralizing antibody response induced by mRNA booster dose administration, implicating a significant role of virus evolution to evade nAb specificity, versus waning humoral immunity, in increasing rates of breakthrough infection.