Antibodies from primary humoral responses modulate the recruitment of naive B cells during secondary responses.

Vaccines generate high-affinity antibodies by recruiting antigen-specific B cells to germinal centers (GCs), but the mechanisms governing the recruitment to GCs on secondary challenges remain unclear. Here, using preclinical SARS-CoV and HIV mouse models, we demonstrated that the antibodies elicited during primary humoral responses shaped the naive B cell recruitment to GCs during secondary exposures. The antibodies from primary responses could either enhance or, conversely, restrict the GC participation of naive B cells: broad-binding, low-affinity, and low-titer antibodies enhanced recruitment, whereas, by contrast, the high titers of high-affinity, mono-epitope-specific antibodies attenuated cognate naive B cell recruitment. Thus, the directionality and intensity of that effect was determined by antibody concentration, affinity, and epitope specificity. Circulating antibodies can, therefore, be important determinants of antigen immunogenicity. Future vaccines may need to overcome-or could, alternatively, leverage-the effects of circulating primary antibodies on subsequent naive B cell recruitment.

A broad and potent neutralization epitope in SARS-related coronaviruses.

Many neutralizing antibodies (nAbs) elicited to ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) through natural infection and vaccination have reduced effectiveness to SARS-CoV-2 variants. Here, we show that therapeutic antibody ADG20 is able to neutralize SARS-CoV-2 variants of concern (VOCs) including Omicron (B.1.1.529) as well as other SARS-related coronaviruses. We delineate the structural basis of this relatively escape-resistant epitope that extends from one end of the receptor binding site (RBS) into the highly conserved CR3022 site. ADG20 can then benefit from high potency through direct competition with ACE2 in the more variable RBS and interaction with the more highly conserved CR3022 site. Importantly, antibodies that are able to target this site generally neutralize a broad range of VOCs, albeit with reduced potency against Omicron. Thus, this conserved and vulnerable site can be exploited for the design of universal vaccines and therapeutic antibodies.

High thermostability improves neutralizing antibody responses induced by native-like HIV-1 envelope trimers.

Soluble HIV-1 envelope glycoprotein (Env) immunogens are a prime constituent of candidate vaccines designed to induce broadly neutralizing antibodies. Several lines of evidence suggest that enhancing Env immunogen thermostability can improve neutralizing antibody (NAb) responses. Here, we generated BG505 SOSIP.v9 trimers, which displayed virtually no reactivity with non-neutralizing antibodies and showed increased global and epitope thermostability, compared to previous BG505 SOSIP versions. Chemical crosslinking of BG505 SOSIP.v9 further increased the melting temperature to 91.3 °C, which is almost 25 °C higher than that of the prototype SOSIP.664 trimer. Next, we compared the immunogenicity of a palette of BG505-based SOSIP trimers with a gradient of thermostabilities in rabbits. We also included SOSIP.v9 proteins in which a strain-specific immunodominant epitope was masked by glycans to redirect the NAb response to other subdominant epitopes. We found that increased trimer thermostability correlated with increased potency and consistency of the autologous NAb response. Furthermore, glycan masking steered the NAb response to subdominant epitopes without decreasing the potency of the autologous NAb response. In summary, SOSIP.v9 trimers and their glycan masked versions represent an improved platform for HIV-1 Env based vaccination strategies.

Disassembly of HIV envelope glycoprotein trimer immunogens is driven by antibodies elicited via immunization.

Rationally designed protein subunit vaccines are being developed for a variety of viruses including influenza, RSV, SARS-CoV-2, and HIV. These vaccines are based on stabilized versions of the primary targets of neutralizing antibodies on the viral surface, namely, viral fusion glycoproteins. While these immunogens display the epitopes of potent neutralizing antibodies, they also present epitopes recognized by non-neutralizing or weakly neutralizing ("off-target") antibodies. Using our recently developed electron microscopy polyclonal epitope mapping approach, we have uncovered a phenomenon wherein off-target antibodies elicited by HIV trimer subunit vaccines cause the otherwise highly stabilized trimeric proteins to degrade into cognate protomers. Further, we show that these protomers expose an expanded suite of off-target epitopes, normally occluded inside the prefusion conformation of trimer, that subsequently elicit further off-target antibody responses. Our study provides critical insights for further improvement of HIV subunit trimer vaccines for future rounds of the iterative vaccine design process.