An unconventional VH1-2 antibody tolerates escape mutations and shows an antigenic hotspot on SARS-CoV-2 spike.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein continues to evolve antigenically, impacting antibody immunity. D1F6, an affinity-matured non-stereotypic VH1-2 antibody isolated from a patient infected with the SARS-CoV-2 ancestral strain, effectively neutralizes most Omicron variants tested, including XBB.1.5. We identify that D1F6 in the immunoglobulin G (IgG) form is able to overcome the effect of most Omicron mutations through its avidity-enhanced multivalent S-trimer binding. Cryo-electron microscopy (cryo-EM) and biochemical analyses show that three simultaneous epitope mutations are generally needed to substantially disrupt the multivalent S-trimer binding by D1F6 IgG. Antigenic mutations at spike positions 346, 444, and 445, which appeared in the latest variants, have little effect on D1F6 binding individually. However, these mutations are able to act synergistically with earlier Omicron mutations to impair neutralization by affecting the interaction between D1F6 IgG and the S-trimer. These results provide insight into the mechanism by which accumulated antigenic mutations facilitate evasion of affinity-matured antibodies.

Identification of a broad sarbecovirus neutralizing antibody targeting a conserved epitope on the receptor-binding domain.

Omicron, as the emerging variant with enhanced vaccine tolerance, has sharply disrupted most therapeutic antibodies. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) belongs to the subgenus Sarbecovirus, members of which share high sequence similarity. Herein, we report one sarbecovirus antibody, 5817, which has broad-spectrum neutralization capacity against SARS-CoV-2 variants of concern (VOCs) and SARS-CoV, as well as related bat and pangolin viruses. 5817 can hardly compete with six classes of receptor-binding-domain-targeted antibodies grouped by structural classifications. No obvious impairment in the potency is detected against SARS-CoV-2 Omicron and subvariants. The cryoelectron microscopy (cryo-EM) structure of neutralizing antibody 5817 in complex with Omicron spike reveals a highly conserved epitope, only existing at the receptor-binding domain (RBD) open state. Prophylactic and therapeutic administration of 5817 potently protects mice from SARS-CoV-2 Beta, Delta, Omicron, and SARS-CoV infection. This study reveals a highly conserved cryptic epitope targeted by a broad sarbecovirus neutralizing antibody, which would be beneficial to meet the potential threat of pre-emergent SARS-CoV-2 VOCs.

Conversion of monoclonal IgG to dimeric and secretory IgA restores neutralizing ability and prevents infection of Omicron lineages.

The emergence of Omicron lineages and descendent subvariants continues to present a severe threat to the effectiveness of vaccines and therapeutic antibodies. We have previously suggested that an insufficient mucosal immunoglobulin A (IgA) response induced by the mRNA vaccines is associated with a surge in breakthrough infections. Here, we further show that the intramuscular mRNA and/or inactivated vaccines cannot sufficiently boost the mucosal secretory IgA response in uninfected individuals, particularly against the Omicron variant. We thus engineered and characterized recombinant monomeric, dimeric, and secretory IgA1 antibodies derived from four neutralizing IgG monoclonal antibodies (mAbs 01A05, rmAb23, DXP-604, and XG014) targeting the receptor-binding domain of the spike protein. Compared to their parental IgG antibodies, dimeric and secretory IgA1 antibodies showed a higher neutralizing activity against different variants of concern (VOCs), in part due to an increased avidity. Importantly, the dimeric or secretory IgA1 form of the DXP-604 antibody significantly outperformed its parental IgG antibody, and neutralized the Omicron lineages BA.1, BA.2, and BA.4/5 with a 25- to 75-fold increase in potency. In human angiotensin converting enzyme 2 (ACE2) transgenic mice, a single intranasal dose of the dimeric IgA DXP-604 conferred prophylactic and therapeutic protection against Omicron BA.5. Thus, dimeric or secretory IgA delivered by nasal administration may potentially be exploited for the treatment and prevention of Omicron infection, thereby providing an alternative tool for combating immune evasion by the current circulating subvariants and, potentially, future VOCs.

Mosaic RBD Nanoparticles Elicit Protective Immunity Against Multiple Human Coronaviruses in Animal Models.

To combat SARS-CoV-2 variants and MERS-CoV, as well as the potential re-emergence of SARS-CoV and spillovers of sarbecoviruses, which pose a significant threat to global public health, vaccines that can confer broad-spectrum protection against betacoronaviruses (ß-CoVs) are urgently needed. A mosaic ferritin nanoparticle vaccine is developed that co-displays the spike receptor-binding domains of SARS-CoV, MERS-CoV, and SARS-CoV-2 Wild-type (WT) strain and evaluated its immunogenicity and protective efficacy in mice and nonhuman primates. A low dose of 10 µg administered at a 21-day interval induced a Th1-biased immune response in mice and elicited robust cross-reactive neutralizing antibody responses against a variety of ß-CoVs, including a series of SARS-CoV-2 variants. It is also able to effectively protect against challenges of SARS-CoV, MERS-CoV, and SARS-CoV-2 variants in not only young mice but also the more vulnerable mice through induction of long-lived immunity. Together, these results suggest that this mosaic 3-RBD nanoparticle has the potential to be developed as a pan-ß-CoV vaccine.