Broadly neutralizing antibodies (bnAbs) against highly variable viral pathogens are much sought-after to treat or protect against global circulating viruses. We have probed the neutralizing antibody repertoires of four HIV-infected donors with remarkably broad and potent neutralizing responses and rescued 17 new monoclonal antibodies (MAbs) that neutralize broadly across clades. Many of the new MAbs are almost 10-fold more potent than the recently described PG9, PG16, and VRC01 bnMAbs and 100-fold more potent than the original prototype HIV bnMAbs1–3. The MAbs largely recapitulate the neutralization breadth found in the corresponding donor serum and many recognize novel epitopes on envelope (Env) glycoprotein gp120, illuminating new targets for vaccine design. Analysis of neutralization by the full complement of anti-HIV bnMAbs now available reveals that certain combinations of antibodies provide significantly more favorable coverage of the enormous diversity of global circulating viruses than others and these combinations might be sought in active or passive immunization regimes. Overall, the isolation of multiple HIV bnMAbs, from several donors, that, in aggregate, provide broad coverage at low concentrations is a highly positive indicator for the eventual design of an effective antibody-based HIV vaccine.

SUMMARY Broadly neutralizing antibodies to HIV are much sought-after (a) to guide vaccine design, both as templates and to inform on the authenticity of vaccine candidates, (b) to assist in structural studies and (c) as potential therapeutics. However, the number of targets on the viral envelope spike for such antibodies is limited. Here, we describe a set of human monoclonal antibodies that define a previously undefined target on HIV Env. The antibodies recognize a glycan-dependent epitope on the prefusion conformation of gp41 and unambiguously distinguish cleaved from uncleaved Env trimers, an important property given increasing evidence that cleavage is required for vaccine candidates that seek to mimic the functional HIV envelope spike. The availability of this set of antibodies expands the number of vaccine targets on HIV and provides reagents to characterize the native envelope spike.

Summary All previously characterized broadly neutralizing antibodies to the HIV-1 envelope glycoprotein (Env) target one of four major sites of vulnerability. Here, we define and structurally characterize a unique epitope on Env that is recognized by a recently discovered family of human monoclonal antibodies (PGT151-158). The PGT151 epitope is comprised of residues and glycans at the interface of gp41 and gp120 within a single protomer and glycans from both subunits of a second protomer and represents a neutralizing epitope that is dependent on both gp120 and gp41. As PGT151 binds only to properly formed, cleaved trimers, this distinctive property, and its ability to stabilize Env trimers, has enabled the successful purification of mature, cleaved Env trimers from the cell surface as a complex with PGT151. Here we compare the structural and functional properties of membrane-extracted Env trimers from several clades with those of the soluble, cleaved SOSIP gp140 trimer.

Hepatitis C virus (HCV) is a major human pathogen that causes serious liver disease, including cirrhosis and hepatocellular carcinoma. The primary target cells of HCV are hepatocytes, and entry is restricted by interactions of the envelope glycoproteins, E1 and E2, with cellular receptors. E1 and E2 form noncovalently linked heterodimers and are heavily glycosylated. Glycans contribute to protein folding and transport as well as protein function. In addition, glycans associated with viral envelopes mask important functional domains from the immune system and attenuate viral immunogenicity. Here, we explored the role of N-and O-linked glycans on E2, which is the receptor binding subunit of the HCV envelope. We identified a number of glycans that are critical for viral entry. Importantly, we showed that the removal of several glycans significantly increased the inhibition of entry by sera from HCV-positive individuals. Only some of the glycans that affected entry and neutralization were also important for CD81 binding. Our results show that HCV envelopeassociated glycans play a crucial role in masking functionally important regions of E2 and suggest a new strategy for eliciting highly neutralizing antibodies against this virus.Hepatitis C virus (HCV) is a single-stranded positive RNA flavivirus that causes serious liver disease in humans (2,21,29,47). Approximately 15% of newly infected individuals clear the virus, and an estimated 170 million people worldwide are persistently infected with HCV (11, 31). Persistence probably results from antigenic variability that allows the virus to escape host immunity (39). It is presently unclear what roles the different arms of the immune system play in the outcome of HCV infection. Several groups have shown that the clearance of HCV infection correlates with a strong, early cytotoxic-T-lymphocyte response (11,31,54). Both neutralizing and nonneutralizing antibodies are elicited by natural HCV infection, but their roles in viral clearance remain contested (4, 9, 10, 33). High antibody titers elicited by the vaccination of chimpanzees with recombinant HCV envelope glycoproteins correlate with protection or a delay in disease onset after challenge with live virus (1,16,45). Furthermore, individuals negative for HCV RNA but positive for HCV antibodies are 12 times more likely to clear a second infection than individuals infected for the first time (35). These studies support the premise that neutralizing antibodies are an essential component of protective immunity, but its correlates remain to be identified.The infectious HCV particle is enveloped by a lipid membrane comprising E1-E2 heterodimers that specifically restrict viral tropism to hepatocytes (8,27,30,47,53). Based on analogies to alphaviral envelope glycoproteins as well as structural modeling, it has been postulated that E1 serves as the fusogenic subunit and that E2 acts as the receptor binding subunit of the HCV envelope (22,24,32,42,60). Alternatively, HCV E1 may facilitate E1-E2 folding into a metastable conformat...

e Recently, several broadly neutralizing monoclonal antibodies (bnMAbs) directed to the CD4-binding site (CD4bs) of gp120 have been isolated from HIV-1-positive donors. These include VRC01, 3BNC117, and NIH45-46, all of which are capable of neutralizing about 90% of circulating HIV-1 isolates and all of which induce conformational changes in the HIV-1 gp120 monomer similar to those induced by the CD4 receptor. In this study, we characterize PGV04 (also known as VRC-PG04), a MAb with potency and breadth that rivals those of the prototypic VRC01 and 3BNC117. When screened on a large panel of viruses, the neutralizing profile of PGV04 was distinct from those of CD4, b12, and VRC01. Furthermore, the ability of PGV04 to neutralize pseudovirus containing single alanine substitutions exhibited a pattern distinct from those of the other CD4bs MAbs. In particular, substitutions D279A, I420A, and I423A were found to abrogate PGV04 neutralization. In contrast to VRC01, PGV04 did not enhance the binding of 17b or X5 to their epitopes (the CD4-induced [CD4i] site) in the coreceptor region on the gp120 monomer. Furthermore, in contrast to CD4, none of the anti-CD4bs MAbs induced the expression of the 17b epitope on cell surface-expressed cleaved Env trimers. We conclude that potent CD4bs bnMAbs can display differences in the way they recognize and access the CD4bs and that mimicry of CD4, as assessed by inducing conformational changes in monomeric gp120 that lead to enhanced exposure of the CD4i site, is not uniquely correlated with effective neutralization at the site of CD4 binding on HIV-1.A study (Protocol G) that screened 1,800 HIV-1 donors infected with viruses of different clades revealed that a significant fraction of donors developed broad and potent neutralizing serum responses, in agreement with studies from several laboratories (5-8, 20, 23). The top 1% of Protocol G donors that exhibited the most broad and potent serum neutralizing responses were designated "elite neutralizers." A significant proportion of Protocol G donors, who ranked within the top 5%, had an overall broad and/or potent serum neutralization activity that was mediated by antibodies to a conserved region on the primary entry receptor of the virus, the CD4 binding site (CD4bs) (26). The CD4bs is of particular interest as a potential vaccine target since it is a conserved region whose accessibility, at least to CD4, must be maintained.The first potent broadly neutralizing monoclonal antibody (bnMAb) to this region, MAb b12, was isolated from a phage display library utilizing RNA from an HIV-1-seropositive individual (presumed clade B virus) (1, 2) and neutralized 35% of a 162-virus cross-clade panel (25). However, the observation that b12 interacts with gp120 apparently solely through its heavy chain (34) and the inability, despite extensive efforts, to isolate further antiCD4bs bnMAbs led to doubts as to whether such Abs could be elicited through immunization. An advance came when MAb HJ16 was isolated by immortalization of memory B cells from a c...

The broadly neutralizing HIV monoclonal antibodies (bnMAbs) PG9, PG16, PGT151, and PGT152 have been shown earlier to occasionally display an unusual virus neutralization profile with a non-sigmoidal slope and a plateau at <100% neutralization. In the current study, we were interested in determining the extent of non-sigmoidal slopes and plateaus at <100% for HIV bnMAbs more generally. Using both a 278 panel of pseudoviruses in a CD4 T-cell (U87.CCR5.CXCR4) assay and a panel of 117 viruses in the TZM-bl assay, we found that bnMAbs targeting many neutralizing epitopes of the spike had neutralization profiles for at least one virus that plateaued at <90%. Across both panels the bnMAbs targeting the V2 apex of Env and gp41 were most likely to show neutralization curves that plateaued <100%. Conversely, bnMAbs targeting the high-mannose patch epitopes were less likely to show such behavior. Two CD4 binding site (CD4bs) Abs also showed this behavior relatively infrequently. The phenomenon of incomplete neutralization was also observed in a large peripheral blood mononuclear cells (PBMC)-grown molecular virus clone panel derived from patient viral swarms. In addition, five bnMAbs were compared against an 18-virus panel of molecular clones produced in 293T cells and PBMCs and assayed in TZM-bl cells. Examples of plateaus <90% were seen with both types of virus production with no consistent patterns observed. In conclusion, incomplete neutralization and non-sigmoidal neutralization curves are possible for all HIV bnMAbs against a wide range of viruses produced and assayed in both cell lines and primary cells with implications for the use of antibodies in therapy and as tools for vaccine design.

Challenge studies following passive immunization with neutralizing antibodies suggest that an HIV vaccine could be efficacious were it able to elicit broadly neutralizing antibodies (bNAbs4). To better understand the requirements for activation of B cells producing bNAbs, we generated cell lines expressing bNAbs or their germline-reverted versions (gl-bNAbs) as BCRs. We then tested the abilities of the bNAb-expressing cells to recognize HIV pseudovirions and vaccine candidate proteins by binding and activation assays. The results suggest that HIV Env antigen-expressing, infection-competent virions are poorly recognized by high affinity bNAb-expressing cells, as measured by the inability of antigens to induce rapid increases in intracellular calcium levels. Other antigen forms appear to be highly stimulatory: in particular, soluble gp140 trimers and a multimerized, scaffolded epitope protein. Virions failed to efficiently activate bNAb-expressing B cells owing to delayed or inefficient BCR recognition, most likely caused by the low density of Env spikes. Importantly, B cells carrying gl-bNAb BCRs were not stimulated by any of the tested vaccine candidates. These data provide insight into why many HIV immunogens, and natural HIV infections, fail to rapidly stimulate bNAb responses and suggest that bNAb-expressing cell lines might be useful tools in evaluation of vaccine antigens for infectious diseases. As soluble Env trimers or multimerized scaffolded epitopes are best at activating B cell expressing bNAbs, these antigenic forms should be considered as preferred vaccine components, though they should be modified to better target naïve gl-bNAb B cells.

Broadly neutralizing monoclonal antibodies effective against the majority of circulating isolates of HIV-1 have been isolated from a small number of infected individuals. Definition of the conformational epitopes on the HIV spike to which these antibodies bind is of great value in defining targets for vaccine and drug design. Drawing on techniques from compressed sensing and information theory, we developed a computational methodology to predict key residues constituting the conformational epitopes on the viral spike from cross-clade neutralization activity data. Our approach does not require the availability of structural information for either the antibody or antigen. Predictions of the conformational epitopes of ten broadly neutralizing HIV-1 antibodies are shown to be in good agreement with new and existing experimental data. Our findings suggest that our approach offers a means to accelerate epitope identification for diverse pathogenic antigens.