A pseudovirus system enables deep mutational scanning of the full SARS-CoV-2 spike.

A major challenge in understanding SARS-CoV-2 evolution is interpreting the antigenic and functional effects of emerging mutations in the viral spike protein. Here, we describe a deep mutational scanning platform based on non-replicative pseudotyped lentiviruses that directly quantifies how large numbers of spike mutations impact antibody neutralization and pseudovirus infection. We apply this platform to produce libraries of the Omicron BA.1 and Delta spikes. These libraries each contain ∼7,000 distinct amino acid mutations in the context of up to ∼135,000 unique mutation combinations. We use these libraries to map escape mutations from neutralizing antibodies targeting the receptor-binding domain, N-terminal domain, and S2 subunit of spike. Overall, this work establishes a high-throughput and safe approach to measure how ∼105 combinations of mutations affect antibody neutralization and spike-mediated infection. Notably, the platform described here can be extended to the entry proteins of many other viruses.

Targeted isolation of diverse human protective broadly neutralizing antibodies against SARS-like viruses.

The emergence of current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs) and potential future spillovers of SARS-like coronaviruses into humans pose a major threat to human health and the global economy. Development of broadly effective coronavirus vaccines that can mitigate these threats is needed. Here, we utilized a targeted donor selection strategy to isolate a large panel of human broadly neutralizing antibodies (bnAbs) to sarbecoviruses. Many of these bnAbs are remarkably effective in neutralizing a diversity of sarbecoviruses and against most SARS-CoV-2 VOCs, including the Omicron variant. Neutralization breadth is achieved by bnAb binding to epitopes on a relatively conserved face of the receptor-binding domain (RBD). Consistent with targeting of conserved sites, select RBD bnAbs exhibited protective efficacy against diverse SARS-like coronaviruses in a prophylaxis challenge model in vivo. These bnAbs provide new opportunities and choices for next-generation antibody prophylactic and therapeutic applications and provide a molecular basis for effective design of pan-sarbecovirus vaccines.

Broadly neutralizing anti-S2 antibodies protect against all three human betacoronaviruses that cause deadly disease.

Pan-betacoronavirus neutralizing antibodies may hold the key to developing broadly protective vaccines against novel pandemic coronaviruses and to more effectively respond to SARS-CoV-2 variants. The emergence of Omicron and subvariants of SARS-CoV-2 illustrates the limitations of solely targeting the receptor-binding domain (RBD) of the spike (S) protein. Here, we isolated a large panel of broadly neutralizing antibodies (bnAbs) from SARS-CoV-2 recovered-vaccinated donors, which targets a conserved S2 region in the betacoronavirus spike fusion machinery. Select bnAbs showed broad in vivo protection against all three deadly betacoronaviruses, SARS-CoV-1, SARS-CoV-2, and MERS-CoV, which have spilled over into humans in the past two decades. Structural studies of these bnAbs delineated the molecular basis for their broad reactivity and revealed common antibody features targetable by broad vaccination strategies. These bnAbs provide new insights and opportunities for antibody-based interventions and for developing pan-betacoronavirus vaccines.

Immunization of cows with HIV envelope trimers generates broadly neutralizing antibodies to the V2-apex from the ultralong CDRH3 repertoire.

The generation of broadly neutralizing antibodies (bnAbs) to conserved epitopes on HIV Envelope (Env) is one of the cornerstones of HIV vaccine research. The animal models commonly used for HIV do not reliably produce a potent broadly neutralizing serum antibody response, with the exception of cows. Cows have previously produced a CD4 binding site response by homologous prime and boosting with a native-like Env trimer. In small animal models, other engineered immunogens were shown to focus antibody responses to the bnAb V2-apex region of Env. Here, we immunized two groups of cows (n = 4) with two regimens of V2-apex focusing Env immunogens to investigate whether antibody responses could be generated to the V2-apex on Env. Group 1 was immunized with chimpanzee simian immunodeficiency virus (SIV)-Env trimer that shares its V2-apex with HIV, followed by immunization with C108, a V2-apex focusing immunogen, and finally boosted with a cross-clade native-like trimer cocktail. Group 2 was immunized with HIV C108 Env trimer followed by the same HIV trimer cocktail as Group 1. Longitudinal serum analysis showed that one cow in each group developed serum neutralizing antibody responses to the V2-apex. Eight and 11 bnAbs were isolated from Group 1 and Group 2 cows, respectively, and showed moderate breadth and potency. Potent and broad responses in this study developed much later than previous cow immunizations that elicited CD4bs bnAbs responses and required several different immunogens. All isolated bnAbs were derived from the ultralong CDRH3 repertoire. The finding that cow antibodies can target more than one broadly neutralizing epitope on the HIV surface reveals the generality of elongated structures for the recognition of highly glycosylated proteins. The exclusive isolation of ultralong CDRH3 bnAbs, despite only comprising a small percent of the cow repertoire, suggests these antibodies outcompete the long and short CDRH3 antibodies during the bnAb response.

Potent antibody-dependent cellular cytotoxicity of a V2-specific antibody is not sufficient for protection of macaques against SIV challenge.

Fc-mediated antibody effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), can contribute to the containment HIV-1 replication but whether such activities are sufficient for protection is unclear. We previously identified an antibody to the variable 2 (V2) apex of the HIV-1 Env trimer (PGT145) that potently directs the lysis of SIV-infected cells by NK cells but poorly neutralizes SIV infectivity. To determine if ADCC is sufficient for protection, separate groups of six rhesus macaques were treated with PGT145 or a control antibody (DEN3) by intravenous infusion followed five days later by intrarectal challenge with SIVmac239. Despite high concentrations of PGT145 and potent ADCC activity in plasma on the day of challenge, all animals became infected and viral loads did not differ between the PGT145- and DEN3-treated animals. To determine if PGT145 can protect against a neutralization-sensitive virus, two additional groups of six macaques were treated with PGT145 and DEN3 and challenged with an SIVmac239 variant with a single amino acid change in Env (K180S) that increases PGT145 binding and renders the virus susceptible to neutralization by this antibody. Although there was no difference in virus acquisition, peak and chronic phase viral loads were significantly lower and time to peak viremia was significantly delayed in the PGT145-treated animals compared to the DEN3-treated control animals. Env changes were also selected in the PGT145-treated animals that confer resistance to both neutralization and ADCC. These results show that ADCC is not sufficient for protection by this V2-specific antibody. However, protection may be achieved by increasing the affinity of antibody binding to Env above the threshold required for neutralization.

Glycans Function as Anchors for Antibodies and Help Drive HIV Broadly Neutralizing Antibody Development.

Apex broadly neutralizing HIV antibodies (bnAbs) recognize glycans and protein surface close to the 3-fold axis of the envelope (Env) trimer and are among the most potent and broad Abs described. The evolution of apex bnAbs from one donor (CAP256) has been studied in detail and many Abs at different stages of maturation have been described. Using diverse engineering tools, we investigated the involvement of glycan recognition in the development of the CAP256.VRC26 Ab lineage. We found that sialic acid-bearing glycans were recognized by germline-encoded and somatically mutated residues on the Ab heavy chain. This recognition provided an "anchor" for the Abs as the core protein epitope varies, prevented complete neutralization escape, and eventually led to broadening of the response. These findings illustrate how glycan-specific maturation enables a human Ab to cope with pathogen escape mechanisms and will aid in optimization of immunization strategies to induce V2 apex bnAb responses.

A Broadly Neutralizing Antibody Targets the Dynamic HIV Envelope Trimer Apex via a Long, Rigidified, and Anionic ß-Hairpin Structure.

Broadly neutralizing antibodies (bnAbs) to HIV delineate vaccine targets and are prophylactic and therapeutic agents. Some of the most potent bnAbs target a quaternary epitope at the apex of the surface HIV envelope (Env) trimer. Using cryo-electron microscopy, we solved the atomic structure of an apex bnAb, PGT145, in complex with Env. We showed that the long anionic HCDR3 of PGT145 penetrated between glycans at the trimer 3-fold axis, to contact peptide residues from all three Env protomers, and thus explains its highly trimer-specific nature. Somatic hypermutation in the other CDRs of PGT145 were crucially involved in stabilizing the structure of the HCDR3, similar to bovine antibodies, to aid in recognition of a cluster of conserved basic residues hypothesized to facilitate trimer disassembly during viral entry. Overall, the findings exemplify the creative solutions that the human immune system can evolve to recognize a conserved motif buried under a canopy of glycans.

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.

Identification of Common Features in Prototype Broadly Neutralizing Antibodies to HIV Envelope V2 Apex to Facilitate Vaccine Design.

Broadly neutralizing antibodies (bnAbs) directed to the V2 apex of the HIV envelope (Env) trimer isolated from individual HIV-infected donors potently neutralize diverse HIV strains, but strategies for designing immunogens to elicit bnAbs have not been identified. Here, we compared four prototypes (PG9, CH01, PGT145, and CAP256.VRC26.09) of V2 apex bnAbs and showed that all recognized a core epitope of basic V2 residues and the glycan-N160. Two prototype bnAbs were derived from VH-germlines that were 99% identical and used a common germline D-gene encoded YYD-motif to interact with the V2-epitope. We identified isolates that were neutralized by inferred germline (iGL) versions of three of the prototype bnAbs. Soluble Env derived from one of these isolates was shown to form a well-ordered Env trimer that could serve as an immunogen to initiate a V2-apex bnAb response. These studies illustrate a strategy to transition from panels of bnAbs to vaccine candidates.

Design of immunogens to elicit broadly neutralizing antibodies against HIV targeting the CD4 binding site.

A vaccine which is effective against the HIV virus is considered to be the best solution to the ongoing global HIV/AIDS epidemic. In the past thirty years, numerous attempts to develop an effective vaccine have been made with little or no success, due, in large part, to the high mutability of the virus. More recent studies showed that a vaccine able to elicit broadly neutralizing antibodies (bnAbs), that is, antibodies that can neutralize a high fraction of global virus variants, has promise to protect against HIV. Such a vaccine has been proposed to involve at least three separate stages: First, activate the appropriate precursor B cells; second, shepherd affinity maturation along pathways toward bnAbs; and, third, polish the Ab response to bind with high affinity to diverse HIV envelopes (Env). This final stage may require immunization with a mixture of Envs. In this paper, we set up a framework based on theory and modeling to design optimal panels of antigens to use in such a mixture. The designed antigens are characterized experimentally and are shown to be stable and to be recognized by known HIV antibodies.

A Rapid Assay for SARS-CoV-2 Neutralizing Antibodies That Is Insensitive to Antiretroviral Drugs.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike pseudotyped virus (PSV) assays are widely used to measure neutralization titers of sera and of isolated neutralizing Abs (nAbs). PSV neutralization assays are safer than live virus neutralization assays and do not require access to biosafety level 3 laboratories. However, many PSV assays are nevertheless somewhat challenging and require at least 2 d to carry out. In this study, we report a rapid (<30 min), sensitive, cell-free, off-the-shelf, and accurate assay for receptor binding domain nAb detection. Our proximity-based luciferase assay takes advantage of the fact that the most potent SARS-CoV-2 nAbs function by blocking the binding between SARS-CoV-2 and angiotensin-converting enzyme 2. The method was validated using isolated nAbs and sera from spike-immunized animals and patients with coronavirus disease 2019. The method was particularly useful in patients with HIV taking antiretroviral therapies that interfere with the conventional PSV assay. The method provides a cost-effective and point-of-care alternative to evaluate the potency and breadth of the predominant SARS-CoV-2 nAbs elicited by infection or vaccines.

Site-Specific Steric Control of SARS-CoV-2 Spike Glycosylation.

A central tenet in the design of vaccines is the display of native-like antigens in the elicitation of protective immunity. The abundance of N-linked glycans across the SARS-CoV-2 spike protein is a potential source of heterogeneity among the many different vaccine candidates under investigation. Here, we investigate the glycosylation of recombinant SARS-CoV-2 spike proteins from five different laboratories and compare them against S protein from infectious virus, cultured in Vero cells. We find patterns that are conserved across all samples, and this can be associated with site-specific stalling of glycan maturation that acts as a highly sensitive reporter of protein structure. Molecular dynamics simulations of a fully glycosylated spike support a model of steric restrictions that shape enzymatic processing of the glycans. These results suggest that recombinant spike-based SARS-CoV-2 immunogen glycosylation reproducibly recapitulates signatures of viral glycosylation.

Autologous Antibody Responses to an HIV Envelope Glycan Hole Are Not Easily Broadened in Rabbits.

Extensive studies with subtype A BG505-derived HIV envelope glycoprotein (Env) immunogens have revealed that the dominant autologous neutralizing epitope in rabbits is located in an exposed region of the heavily glycosylated trimer that lacks potential N-linked glycosylation sites at positions 230, 241, and 289. The Env derived from B41, a subtype B virus, shares a glycan hole centered on positions 230 and 289. To test whether broader neutralization to the common glycan hole can be achieved, we immunized rabbits with B41 SOSIP (gp120-gp41 disulfide [SOS] with an isoleucine-to-proline mutation [IP] in gp41) alone, as well as B41 and BG505 coimmunization. We isolated autologous neutralizing antibodies (nAbs) and described their structure in complex with the B41 Env. Our data suggest that distinct autologous nAb lineages are induced by BG505 and B41 immunogens, even when both were administered together. In contrast to previously described BG505 glycan hole antibodies, the B41-specific nAbs accommodate the >97% conserved N241 glycan, which is present in B41. Single-particle cryo-electron microscopy studies confirmed that B41- and BG505-specific nAbs bind to overlapping glycan hole epitopes. We then used our high-resolution data to guide mutations in the BG505 glycan hole epitope in an attempt to broaden the reactivity of a B41-specific nAb, but we recovered only partial binding. Our data demonstrate that the lack of cross-reactivity in glycan hole antibodies is due to amino acid differences within the epitope, and our attempts to rationally design cross-reactive trimers resulted in only limited success. Thus, even for the immunodominant glycan hole shared between BG505 and B41, the prospect of designing prime-boost immunogens remains difficult.IMPORTANCE A glycan hole is one of the most dominant autologous neutralizing epitopes targeted on BG505 and B41 SOSIP trimer-immunized rabbits. Our high-resolution cryo-electron microscopy (cryoEM) studies of B41 in complex with a B41-specific antibody complex elucidate the molecular basis of this strain-specific glycan hole response. We conclude that even for the immunodominant glycan hole shared between BG505 and B41, the prospect of designing prime-boost immunogens remains difficult.

Highly efficient neutralization by plasma antibodies from human immunodeficiency virus type-1 infected individuals on antiretroviral drug therapy.

Little is known about the neutralizing antibodies induced in HIV-1 patients on antiretroviral treatment, which constitute an interesting group of individuals with improved B cell profile. Plasma samples from 34 HIV-1 seropositive antiretroviral drug treated (ART) patients were tested for neutralization against a panel of 14 subtype-A, B and C tier 1 and tier 2 viruses in TZM-bl assay. Of the 34 plasma samples, remarkably all the plasma samples were able to neutralize at least one virus while 32 (94 %) were found to neutralize ≥50 % viruses tested. In terms of overall neutralization frequency, approximately 86 %, 68 % and 17 % of the virus/plasma combinations showed 50 % neutralizing activity at 1 > 60, 1 ≥ 200 and 1 ≥ 2000 dilutions respectively. The improvement in neutralizing activity was shown to be associated with ART in two follow up patients. The neutralization of viruses by two representative plasma samples, AIIMS221 and AIIMS265, was exclusively mediated by immunoglobulin G fractions independent of ART drugs and IgG retained cross-reactive binding to recombinant gp120 proteins. We observed a positive trend of neutralization with duration of ART (p = 0.06), however no such correlation was found with clinical and immunological variables like CD4 count (p = 0.35), viral load (p = 0.09) and plasma total IgG (p = 0.46). Our study suggests that the plasma antibodies from ART patients display high neutralizing activity most likely due to an improved B cell function induced by ART despite low antigenic stimulation.

Long-term suppression of HIV-1C virus production in human peripheral blood mononuclear cells by LTR heterochromatization with a short double-stranded RNA.

OBJECTIVES: A region in the conserved 5' long terminal repeat (LTR) promoter of the integrated HIV-1C provirus was identified for effective targeting by a short double-stranded RNA (dsRNA) to cause heterochromatization leading to a long-lasting decrease in viral transcription, replication and subsequent productive infection in human host cells. METHODS: Small interfering RNAs (siRNAs) were transfected into siHa cells containing integrated LTR-luciferase reporter constructs and screened for efficiency of inducing transcriptional gene silencing (TGS). TGS was assessed by a dual luciferase assay and real-time PCR. Chromatin modification at the targeted region was also studied. The efficacy of potent siRNA was then checked for effectiveness in TZM-bl cells and human peripheral blood mononuclear cells (PBMCs) infected with HIV-1C virus. Viral Gag-p24 antigen levels were determined by ELISA. RESULTS: One HIV-1C LTR-specific siRNA significantly decreased luciferase activity and its mRNA expression with no such effect on HIV-1B LTR. This siRNA-mediated TGS was induced by histone methylation, which leads to heterochromatization of the targeted LTR region. The same siRNA also substantially suppressed viral replication in TZM-bl cells and human PBMCs infected with various HIV-1C clinical isolates for ≥3 weeks after a single transfection, even of a strain that had a mismatch in the target region. CONCLUSIONS: We have identified a potent dsRNA that causes long-term suppression of HIV-1C virus production in vitro and ex vivo by heritable epigenetic modification at the targeted C-LTR region. This dsRNA has promising therapeutic potential in HIV-1C infection, the clade responsible for more than half of AIDS cases worldwide.

Vaccine targeting to mucosal lymphoid tissues promotes humoral immunity in the gastrointestinal tract.

Viruses, bacteria, and parasites frequently cause infections in the gastrointestinal tract, but traditional vaccination strategies typically elicit little or no mucosal antibody responses. Here, we report a strategy to effectively concentrate immunogens and adjuvants in gut-draining lymph nodes (LNs) to induce gut-associated mucosal immunity. We prepared nanoemulsions (NEs) based on biodegradable oils commonly used as vaccine adjuvants, which encapsulated a potent Toll-like receptor agonist and displayed antigen conjugated to their surface. Following intraperitoneal administration, these NEs accumulated in gut-draining mesenteric LNs, priming strong germinal center responses and promoting B cell class switching to immunoglobulin A (IgA). Optimized NEs elicited 10- to 1000-fold higher antigen-specific IgG and IgA titers in the serum and feces, respectively, compared to free antigen mixed with NE, and strong neutralizing antibody titers against severe acute respiratory syndrome coronavirus 2. Thus, robust gut humoral immunity can be elicited by exploiting the unique lymphatic collection pathways of the gut with a lymph-targeting vaccine formulation.

High affinity mAb infusion can enhance maximum affinity maturation during HIV Env immunization.

Antigen-specific antibody infusion is known to enhance or suppress germinal center (GC) responses depending on the affinity of the infusion. We hypothesized that infusing monoclonal antibodies (mAbs) of escalating affinity during an immunization regimen may progressively escalate selection pressure on competing B cells, increasing their affinity. To test this, we immunized mice with HIV envelope gp120 and infused CD4 binding-site (CD4bs)-specific mAbs. While mAb infusion reduced somatic hypermutation (SHM) and affinity in most CD4bs-specific B cells, a sub-population was identified with greater SHM and affinity than control. High-throughput sequencing of plasma cells revealed that CD4bs-specific plasma cells possessed elevated SHM after mAb infusion, with phylogenetic tree topology that suggested more rapid differentiation. We therefore conclude, in accordance with other studies, that high-affinity mAb infusion primarily suppresses recruitment of most competing B cells but can increase and expedite affinity maturation of certain epitope-specific B cells.

Immunization of cows with HIV envelope trimers generates broadly neutralizing antibodies to the V2-apex from the ultralong CDRH3 repertoire.

The generation of broadly neutralizing antibodies (bnAbs) to specific HIV epitopes of the HIV Envelope (Env) is one of the cornerstones of HIV vaccine research. The current animal models we use have been unable to reliable produce a broadly neutralizing antibody response, with the exception of cows. Cows have rapidly and reliably produced a CD4 binding site response by homologous prime and boosting with a native-like Env trimer. In small animal models other engineered immunogens previously have been able to focus antibody responses to the bnAb V2-apex region of Env. Here, we immunized two groups of cows (n=4) with two regiments of V2-apex focusing immunogens to investigate whether antibody responses could be directed to the V2-apex on Env. Group 1 were immunized with chimpanzee simian immunodeficiency virus (SIV)-Env trimer that shares its V2-apex with HIV, followed by immunization with C108, a V2-apex focusing immunogen, and finally boosted with a cross-clade native-like trimer cocktail. Group 2 were immunized with HIV C108 Env trimer followed by the same HIV trimer cocktail as Group 1. Longitudinal serum analysis showed that one cow in each group developed serum neutralizing antibody responses to the V2-apex. Eight and 11 bnAbs were isolated from Group 1 and Group 2 cows respectively. The best bnAbs had both medium breadth and potency. Potent and broad responses developed later than previous CD4bs cow bnAbs and required several different immunogens. All isolated bnAbs were derived from the ultralong CDRH3 repertoire. The finding that cow antibodies can target multiple broadly neutralizing epitopes on the HIV surface reveals important insight into the generation of immunogens and testing in the cow animal model. The exclusive isolation of ultralong CDRH3 bnAbs, despite only comprising a small percent of the cow repertoire, suggests these antibodies outcompete the long and short CDRH3 antibodies during the bnAb response.

Deep repertoire mining uncovers ultra-broad coronavirus neutralizing antibodies targeting multiple spike epitopes.

The development of vaccines and therapeutics that are broadly effective against known and emergent coronaviruses is an urgent priority. We screened the circulating B cell repertoires of COVID-19 survivors and vaccinees to isolate over 9,000 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific monoclonal antibodies (mAbs), providing an expansive view of the SARS-CoV-2-specific Ab repertoire. Among the recovered antibodies was TXG-0078, an N-terminal domain (NTD)-specific neutralizing mAb that recognizes diverse alpha- and beta-coronaviruses. TXG-0078 achieves its exceptional binding breadth while utilizing the same VH1-24 variable gene signature and heavy-chain-dominant binding pattern seen in other NTD-supersite-specific neutralizing Abs with much narrower specificity. We also report CC24.2, a pan-sarbecovirus neutralizing antibody that targets a unique receptor-binding domain (RBD) epitope and shows similar neutralization potency against all tested SARS-CoV-2 variants, including BQ.1.1 and XBB.1.5. A cocktail of TXG-0078 and CC24.2 shows protection in vivo, suggesting their potential use in variant-resistant therapeutic Ab cocktails and as templates for pan-coronavirus vaccine design.