Evolving endemic viruses such as influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remain major health threats. Although variant updated vaccines aim to enhance protection, preexisting immunity shapes memory B cell (MBC) responses. To assess how SARS-CoV-2 spike protein variant-based vaccine boosters and infections alter the MBC repertoire, we analyzed MBC responses in the COVAIL vaccine trial, where participants previously vaccinated with SARS-CoV-2 Wuhan-1 spike immunogen were boosted with Wuhan-1, variant, or bivalent spike immunogens. Some participants also experienced a subsequent infection with an Omicron variant. We determined that variant vaccine boosters and SARS-CoV-2 infections led to transiently greater recall of cross-reactive MBCs compared with a Wuhan-1 vaccine booster. Long term, we detected little change in the MBC repertoire after an Omicron vaccine booster, but MBCs evaluated several months after Omicron variant infection had higher neutralization capacity to both Wuhan-1 and BA.1 compared with those from individuals who had not experienced an infection. However, these Wuhan-1/BA.1 cross-reactive MBCs from infected individuals displayed less breadth toward the more distant BA.2.86 lineage than MBCs from uninfected individuals. Thus, SARS-CoV-2 variant boosters and infections differentially shape the long-term MBC repertoire.

For rapidly mutating viruses such as influenza viruses and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), immune memory recalled by antigenically drifted variants primarily comprises antibodies that cross-react to the priming strain rather than de novo elicited responses, a phenomenon termed original antigenic sin or immune imprinting. The composition and functionality of de novo responses elicited by variant exposures remain unclear. Here we isolated and characterized hundreds of recall and de novo neutralizing monoclonal antibodies after sequential exposures to SARS-CoV-2 variants in ancestral-imprinted humans. De novo variant type-specific antibodies used different V(D)J genes that were closer to germline sequence, potently neutralized future variants and targeted distinct receptor binding domain epitopes compared to ancestral cross-reactive (recall) antibodies. Nevertheless, neutralizing responses to the updated 2024-2025 booster were predominantly ancestral cross-reactive. These results reveal the distinct contributions of recall and de novo antibodies to a balanced immune response and underscore the benefit of updated booster vaccines, which augment both subsets.

Broadly neutralizing antibodies (bNAbs) targeting multiple sites of HIV-1 Env vulnerability can be induced by infection, but simultaneous elicitation of bNAbs against multiple epitopes has not been achieved by vaccination. In this study, we designed a dual-epitope vaccine targeting both the fusion peptide (FP) and the V2 apex and evaluated its capacity to induce bNAbs against both epitopes in rhesus macaques. This vaccine combined an FP conjugate with a cocktail of engineered Env trimers with enhanced V2 apex recognition and increased antigen retention in lymph nodes. Macaque immunization with the dual-epitope vaccine elicited >1,000-fold higher autologous tier 2-neutralizing titers than wild-type Env trimers and enhanced heterologous neutralization. Both FP- and V2 apex-monoclonal antibodies were isolated from immunized macaques and showed heterologous neutralization with genetic and structural signatures similar to well-characterized FP and V2 apex bNAbs. These results demonstrate proof of concept for simultaneous vaccine elicitation of neutralizing antibodies against multiple sites of Env vulnerability.

Abstract Introduction Inducing broadly neutralizing antibodies (bnAb) is a central challenge in HIV vaccine development. The HIV envelope (Env) fusion peptide (FP) is a highly conserved bnAb epitope, yet is inefficiently targeted by antibodies elicited by experimental vaccines. Here, we test how Env trimer design and dosing strategies shape the magnitude and specificity of antibody responses by pairing FP priming with boosts using native-like Env trimers. Escalating-dose (ED) immunization regimens are a recently described approach to deliver antigens over a prolonged period, resulting in sustained increased antigen stability in the lymph node and germinal-center activity, improved immune kinetics, and enhanced recruitment and selection of targeted B-cell populations. The impact of ED regimens on boosting HIV-1 FP-specific responses has not been explored. Methods Here, guinea pigs (n = 5 per group) were primed with FP-rTTHC followed by boosts with prefusion trimers from clade A (BG505), clade C (ConC), or a Triple-Tandem-Trimer (three trimeric units of ConC, linked one after another: ConC-DS-14ln-TTT), delivered either as bolus injections or ED regimens. Results All strategies elicited strong Env-specific responses, but the quality of these responses varied among immunogen designs. Bolus boosts using BG505 or ConC generated the most robust FP-specific recall responses, but neutralization levels ultimately converged across regimens. However, the TTT trimer expanded reactivity to non-FP epitopes, suggesting broader epitope engagement during boosting. Notably, cross-reactive BG505-binding antibodies appeared as early as week 6 after two FP primes, highlighting rapid activation of FP-targeting B cells. Conclusion These findings clarify how trimer architecture and dosing influence the trajectory of epitope targeting, strengthening the case for FP priming as a powerful method for engaging a vulnerable HIV Env epitope and demonstrating that the TTT platform may serve as an effective boost to epitopes beyond FP. Funding Source N/A Topic Categories Vaccines and Immunotherapy (VAC)

Viral entry glycoproteins are often shielded from immune recognition by dense N-linked glycans that limit antibody access to protein epitopes. While glycan-reactive antibodies with unusual architectures have been described, how canonical Y-shaped antibodies engage these glycan-rich surfaces remains poorly defined. Here, we characterize two human antibodies, VRC35 and VRC36, isolated from an HIV-1-infected donor, that recognize diverse glycosylated viral glycoproteins. Cryo-electron microscopy structural analyses of these antibodies in complex with viral entry glycoproteins, including HIV-1 envelope, influenza hemagglutinin, SARS-CoV-2 spike, and the Lassa virus glycoprotein complex, reveal adaptive Fab stoichiometries ranging from single-Fab binding to dimeric and higher-order assemblies are mediated by intra- and inter-IgG interactions that depend on local glycan organization. Dense glycan clustering on HIV-1 and influenza glycoproteins supports multivalent Fab assemblies and correlates with neutralization activity, whereas sparse glycan environments on SARS-CoV-2 and Lassa virus favor weak or heterogeneous engagement without neutralization. Structural and mutational analyses further demonstrate that homotypic Fab-Fab interactions stabilize multivalent engagement and contribute to neutralizing activity. Together, these findings define a structural framework in which viral glycan organization constrains antibody valency and engagement, while somatic hypermutation contributes to the acquisition of homotypic Fab-Fab interactions that facilitate multivalent recognition of viral glycan shields.

Assembly of flaviviruses such as Zika virus (ZIKV), dengue virus, and West Nile virus in the host cell endoplasmic reticulum is driven by the structural envelope (E) and premembrane (prM) proteins. The formation of an infectious virion requires cleavage of prM by the host furin protease during a maturation step that is dependent on a conformational change in virion structure. Here, we demonstrate that the biogenesis of flavivirus particles does not require an intact prM protein or proteolytic activation. The expression of E protein preceded by a truncated version of prM (M-E) was sufficient for the formation of noninfectious ZIKV subviral particles and pseudo-infectious reporter virions. Subviral particles encoded by a ZIKV M-E DNA vaccine elicited a neutralizing antibody response in macaques that was insensitive to the virion maturation state, a feature of flavivirus humoral immunity shown to correlate with protection. M-E vaccines that uniformly present structural features shared with mature virions offer a higher-quality and more broadly applicable approach for vaccination against flaviviruses.

The N terminus of the H3 subtype of influenza virus hemagglutinin is ∼10 residues longer than the N termini of most other hemagglutinins. As conserved, exposed, and linear regions may be good vaccine targets, we investigated the vaccine utility of the extended H3-N terminus. First, we identified antibody 5E10, for which structure and binding analyses revealed recognition of the H3-N terminus. Second, we immunized mice with immunogens incorporating the H3-N terminus, boosted with hemagglutinin trimer, and isolated antibodies from immunogen-elicited B cells that bound both H3-N terminus and hemagglutinin trimer. However, hemagglutinin-complex structures of two such antibodies, 3864-6 and 3864-10, that neutralized H3-influenza strains, revealed only peripheral recognition of the hemagglutinin N terminus. Collectively, these results reveal the N terminus of H3 hemagglutinin to be a suboptimal vaccine target and suggest that-in addition to being conserved, flexible, and accessible-other factors influence the elicitation of potent broadly neutralizing responses.

Background: Vaccines that stimulate systemic and mucosal immunity to a level required to prevent SARS-CoV-2 infection and transmission are an unmet need. Highly protective hepatitis B and human papillomavirus nanoparticle vaccines highlight the potential of multivalent nanoparticle vaccine platforms to provide enhanced immunity. Here, we report the construction and characterization of self-assembling 60-subunit icosahedral nanoparticle SARS-CoV-2 vaccines using the bacterial enzyme lumazine synthase (LuS). Methods and Results: Nanoparticles displaying prefusion-stabilized SARS-CoV-2 spike ectodomains fused to the surface-exposed amino terminus of LuS were designed using structure-guided approaches. Negative stain-electron microscopy studies of purified nanoparticles were consistent with self assembly into 60-mer nanoparticles displaying 20 spike trimers. After two intramuscular doses, these purified spike-LuS nanoparticles elicited significantly higher SARS-CoV-2 neutralizing activity than spike trimers in vaccinated mice. Furthermore, intramuscular DNA priming and intranasal boosting with a SARS-CoV-2 LuS nanoparticle vaccine stimulated mucosal IgA responses. Conclusion: These data identify LuS nanoparticles as highly immunogenic SARS-CoV-2 vaccine candidates and support the further development of this platform against SARS-CoV-2 and its emerging variants.

Endemic human coronaviruses (HCoVs), like HCoV-HKU1, account for ~30% of common cold/year and can cause serious upper and lower respiratory infections, yet no licensed vaccines target HCoVs. In fact, little is known about HCoV-HKU1's antigenic landscape. Thus, we characterized key interactions between HCoV-HKU1 spike (S) with monoclonal antibodies (mAbs) isolated from pre-pandemic HCoV-HKU1 convalescent PBMCs. We isolated 14 mAbs, which bound distinct S regions: receptor binding domain (RBD), N-terminal domain (NTD), and S2 subunit. Structural and functional studies revealed three groups of RBD-specific mAbs targeting diverse footprints within and around the TMPRSS2 receptor binding site, exemplified by: (1) The most potently neutralizing mAb, H501-022 (IC50 = 0.01 μg/mL), which recognizes the TMPRSS2 binding motif, thereby blocking receptor engagement; (2) mAb H501-008 (IC50 = 0.05 μg/mL) that binds a conserved, cross-reactive epitope outside of the TMPRSS2 binding site that is shared with HCoV-OC43; and (3) H501-018 (IC50 = 0.28 μg/mL) that recognizes both "up" and "down" RBD conformations at a distinct, non-overlapping site outside of the TMPRSS2 binding motif, distinguishing itself from H501-022 and H501-008, which bind exclusively to the "up" RBD conformation. These mAbs represent the first type-specific HCoV-HKU1 mAbs isolated from a convalescent donor. Our findings provide molecular insight into HCoV-HKU1 antibody recognition and neutralization mechanisms, importantly highlighting antigenic differences comparing HCoVs and pandemic CoVs - a critical step towards advancing universal CoV vaccine design.

SARS-CoV-2 neutralizing monoclonal antibodies (mAbs) serve as therapeutic agents to prevent or treat SARS-CoV-2 infection and have potential use against related coronaviruses (sarbecoviruses). However, evolving SARS-CoV-2 variants have rendered many mAbs ineffective. Therefore, we sought to identify potent mAbs with greater cross-reactivity; focusing on a donor with cross-reactive serum neutralization that has longitudinal samples spanning vaccine doses and a breakthrough infection. Using single-cell sorting with S-2P and RBD probes; Rapid Assembly, Transfection and Production of Immunoglobulins (RATP-Ig); and 10X Genomics sequencing, we identified multiple B cell lineages with shared genetic origins, neutralization patterns, and epitope target. The mAbs were isolated from multiple timepoints, including several isolated after three vaccine doses with the ancestral sequence. Notably, these mAbs exhibited broad neutralization despite a lack of exposure to newer variants via vaccination or infection. The mAbs neutralized all SARS-CoV-2 variants including the latest, XEC, as well as multiple SARS-CoV and SARS-CoV-related animal viruses. Based on binding competition assays, they mapped to a class I/IV epitope. Cryo-EM studies defined a compact, conserved epitope that avoids variable amino acid positions, consistent with broad cross-reactivity. Continued investigation of these mAbs may prove useful for the development of clinical therapeutics and optimization of vaccine strategies. Vaccines and Immunotherapy (VAC)

The COVID-19 pandemic underscores the need to prepare for future emerging coronavriuses (CoVs) by understanding the principles behind effective CoV vaccine design such as protective immunity and antibody responses. To study which epitopes and subdomains contribute to in vivo protection, we utilized the prefusion-stabilized spike protein of MERS-CoV, MERS S-2P, as a vaccine immunogen. Vaccination with MERS S-2P elicited both receptor-binding domain (RBD)- and non-RBD-specific antibodies, including N-terminal domain (NTD)-specific G2-and CDC2-A2-like antibodies. Intriguingly, the immunogen MERS S-2P_ΔRBD, MERS S-2P with the RBDs removed, protects comparably to S1 and S-2P immunogens against MERS-CoV challenge. Moreover, passive transfer studies of polyclonal IgG from MERS S-2P immunized mice depleted of subdomain-specific antibodies demonstrated that non-RBD antibodies protected more than non-NTD antibodies. Altogether, these findings illustrate that in-vivo protection is not solely driven by RBD-specific antibodies and highlights the importance of targeting non-RBD sites in future CoV vaccine designs.

The receptor-binding domain (RBD) of the SARS-CoV-2 spike is a primary target of neutralizing antibodies and a key component of licensed vaccines. Substantial mutations in RBD, however, enable current variants to escape immunogenicity generated by vaccination with the ancestral (WA1) strain. Here, we produce and assess self-assembling nanoparticles displaying RBDs from WA1 and BA.5 strains by using the SpyTag:SpyCatcher system for coupling. We observed both WA1- and BA.5-RBD nanoparticles to degrade substantially after a few days at 37 °C. Incorporation of nine RBD-stabilizing mutations, however, increased yield ~five-fold and stability such that more than 50% of either the WA1- or BA.5-RBD nanoparticle was retained after one week at 37 °C. Murine immunizations revealed that the stabilized RBD-nanoparticles induced ~100-fold higher autologous neutralization titers than the prefusion-stabilized (S2P) spike at a 2 μg dose. Even at a 25-fold lower dose where S2P-induced neutralization titers were below the detection limit, the stabilized BA.5-RBD nanoparticle induced homologous titers of 12,795 ID50 and heterologous titers against WA1 of 1767 ID50. Assessment against a panel of β-coronavirus variants revealed both the stabilized BA.5-RBD nanoparticle and the stabilized WA1-BA.5-(mosaic)-RBD nanoparticle to elicit much higher neutralization breadth than the stabilized WA1-RBD nanoparticle. The extraordinary titer and high neutralization breadth elicited by stabilized RBD nanoparticles from strain BA.5 make them strong candidates for next-generation COVID-19 vaccines.

A mucosal route of vaccination could prevent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication at the site of infection and limit transmission. We compared protection against heterologous XBB.1.16 challenge in nonhuman primates (NHPs) ~5 months following intramuscular boosting with bivalent mRNA encoding WA1 and BA.5 spike proteins or mucosal boosting with a WA1-BA.5 bivalent chimpanzee adenoviral-vectored vaccine delivered by intranasal or aerosol device. NHPs boosted by either mucosal route had minimal virus replication in the nose and lungs, respectively. By contrast, protection by intramuscular mRNA was limited to the lower airways. The mucosally delivered vaccine elicited durable airway IgG and IgA responses and, unlike the intramuscular mRNA vaccine, induced spike-specific B cells in the lungs. IgG, IgA and T cell responses correlated with protection in the lungs, whereas mucosal IgA alone correlated with upper airway protection. This study highlights differential mucosal and serum correlates of protection and how mucosal vaccines can durably prevent infection against SARS-CoV-2.

Flavivirus assembly at the endoplasmic reticulum is driven by the structural proteins envelope (E) and premembrane (prM). Here, contrary to the established paradigm for flavivirus assembly, we demonstrate that the biogenesis of flavivirus particles does not require an intact prM nor proteolytic activation. The expression of E preceded by a truncated version of prM (M-E) was sufficient for the formation of non-infectious Zika virus subviral particles and pseudo-infectious reporter virions. Subviral particles encoded by a ZIKV M-E DNA vaccine elicited a neutralizing antibody response that was insensitive to the virion maturation state, a feature of flavivirus humoral immunity shown to correlate with protection. M-E vaccines that uniformly present structural features shared with mature virions offer a higher quality and broadly applicable approach to flavivirus vaccination.

Despite effective countermeasures, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) persists worldwide because of its ability to diversify and evade human immunity. This evasion stems from amino acid substitutions, particularly in the receptor binding domain (RBD) of the spike protein that confers resistance to vaccine-induced antibodies and antibody therapeutics. To constrain viral escape through resistance mutations, we combined antibody variable regions that recognize different RBD sites into multispecific antibodies. Here, we describe multispecific antibodies, including a trivalent trispecific antibody that potently neutralized diverse SARS-CoV-2 variants and prevented virus escape more effectively than single antibodies or mixtures of the parental antibodies. Despite being generated before the appearance of Omicron, this trispecific antibody neutralized all major Omicron variants through BA.4/BA.5 at nanomolar concentrations. Negative stain electron microscopy suggested that synergistic neutralization was achieved by engaging different epitopes in specific orientations that facilitated binding across more than one spike protein. Moreover, a tetravalent trispecific antibody containing the same variable regions as the trivalent trispecific antibody also protected Syrian hamsters against Omicron variants BA.1, BA.2, and BA.5 challenge, each of which uses different amino acid substitutions to mediate escape from therapeutic antibodies. These results demonstrated that multispecific antibodies have the potential to provide broad SARS-CoV-2 coverage, decrease the likelihood of escape, simplify treatment, and provide a strategy for antibody therapies that could help eliminate pandemic spread for this and other pathogens.

The repeated emergence of zoonotic human betacoronaviruses (β-CoVs) dictates the need for broad therapeutics and conserved epitope targets for countermeasure design. Middle East respiratory syndrome (MERS)-related coronaviruses (CoVs) remain a pressing concern for global health preparedness. Using metagenomic sequence data and CoV reverse genetics, we recovered a full-length wild-type MERS-like BtCoV/li/GD/2014-422 (BtCoV-422) recombinant virus, as well as two reporter viruses, and evaluated their human emergence potential and susceptibility to currently available countermeasures. Similar to MERS-CoV, BtCoV-422 efficiently used human and other mammalian dipeptidyl peptidase protein 4 (DPP4) proteins as entry receptors and an alternative DPP4-independent infection route in the presence of exogenous proteases. BtCoV-422 also replicated efficiently in primary human airway, lung endothelial, and fibroblast cells, although less efficiently than MERS-CoV. However, BtCoV-422 shows minor signs of infection in 288/330 human DPP4 transgenic mice. Several broad CoV antivirals, including nucleoside analogs and 3C-like/Mpro protease inhibitors, demonstrated potent inhibition against BtCoV-422 in vitro. Serum from mice that received a MERS-CoV mRNA vaccine showed reduced neutralizing activity against BtCoV-422. Although most MERS-CoV-neutralizing monoclonal antibodies (mAbs) had limited activity, one anti-MERS receptor binding domain mAb, JC57-11, neutralized BtCoV-422 potently. A cryo-electron microscopy structure of JC57-11 in complex with BtCoV-422 spike protein revealed the mechanism of cross-neutralization involving occlusion of the DPP4 binding site, highlighting its potential as a broadly neutralizing mAb for group 2c CoVs that use DPP4 as a receptor. These studies provide critical insights into MERS-like CoVs and provide candidates for countermeasure development.