HIV-1 Subtype C Tier 3 Viruses Have Increased Infectivity Compared to Tier 2 Viruses

A primary concern of an antibody-based HIV-1 therapy is the virus' ability to rapidly escape antibody responses. Therefore, we investigated the relationships between antibody neutralization sensitivity, viral phenotype, and infectivity in 13 subtype C viruses using a HeLa transfectant-based assay. We observed that the seven tier 3 viruses exhibited higher infectivity than the tier 2 viruses, suggesting that higher neutralization resistance did not have a substantial entry cost. There was no relationship between neutralization resistance and susceptibility to entry inhibitors Maraviroc, PSC RANTES, or the fusion inhibitor T20, indicating that neutralization resistance may not alter these inhibitor target sites. By analyzing glycosylation patterns in 82 subtype C viruses, we found that the presence of an N-linked glycan motif at position N413 and its absence at N332 were the most important predictors of neutralization resistance. In a set of 200 subtype C viruses, tier 3 strains were more resistant than tier 2 or 1B viruses to several broadly neutralizing monoclonal antibodies targeting three different epitopes. This suggests that it is unlikely that resistance to antibodies targeting a single epitope drives overall resistance. In the context of an antibody-based intervention, highly resistant viruses with increased infectivity, circulating in the population, could hinder HIV-1 control since entry of tier 3 viruses is not always selected against. Therefore, for any long-term antibody-based intervention to be globally relevant, it must elicit responses that limit the occurrence of resistance.

Antibody Isotype Switching as a Mechanism to Counter HIV Neutralization Escape

Neutralizing antibodies (nAbs) to highly variable viral pathogens show remarkable diversification during infection, resulting in an “arms race” between virus and host. Studies of nAb lineages have shown how somatic hypermutation (SHM) in immunoglobulin (Ig)-variable regions enables maturing antibodies to neutralize emerging viral escape variants. However, the Ig-constant region (which determines isotype) can also influence epitope recognition. Here, we use longitudinal deep sequencing of an HIV-directed nAb lineage, CAP88-CH06, and identify several co-circulating isotypes (IgG3, IgG1, IgA1, IgG2, and IgA2), some of which share identical variable regions. First, we show that IgG3 and IgA1 isotypes are better able to neutralize longitudinal autologous viruses and epitope mutants than can IgG1. Second, detrimental class-switch recombination (CSR) events that resulted in reduced neutralization can be rescued by further CSR, which we term “switch redemption.” Thus, CSR represents an additional immunological mechanism to counter viral escape from HIV-specific antibody responses.

Scheepers et al. show within an HIV-specific antibody lineage that isotypes confer variable ability to neutralize emerging viral escape variants. This suggests that class switching, in addition to somatic hypermutation of immunoglobulin-variable regions, contributes to antibody maturation during infection.

Targeting the N332-supersite of the HIV-1 envelope for vaccine design

Introduction: Broadly neutralizing antibodies (bNAbs) that are able to target diverse global viruses are widely believed to be crucial for an HIV-1 vaccine. Several conserved targets recognized by these antibodies have been identified on the HIV-1 envelope glycoprotein. One such target that shows particular promise for vaccination is the N332-supersite.Areas covered: This review describes the potential of the N332-supersite epitope as an immunogen design platform. We discuss the structure of the epitope and the bNAbs that target it, emphasizing their diverse modes of binding. Furthermore, the successes and limitations of recent N332-supersite immunization studies are discussed.Expert opinion: During HIV-1 infection, some of the broadest and most potent bNAbs target the N332-supersite. Furthermore, some of these antibodies require less affinity maturation than the high levels typical of many bNAbs, making these potentially more achievable vaccine targets. In addition, bNAbs bind this epitope with multiple angles of approach and glycan dependencies, perhaps increasing the probability of eliciting such responses by vaccination. Animal studies have shown that N332-supersite bNAb precursors can be activated by novel immunogens. While follow-up studies must establish whether boosting strategies can drive the maturation of bNAbs from these precursors, the development of targeted N332-supersite immunogens expands our arsenal of potential HIV-1 vaccine candidates.

Structural Rearrangements Maintain the Glycan Shield of an HIV-1 Envelope Trimer After the Loss of a Glycan

The HIV-1 envelope (Env) glycoprotein is the primary target of the humoral immune response and a critical vaccine candidate. However, Env is densely glycosylated and thereby substantially protected from neutralisation. Importantly, glycan N301 shields V3 loop and CD4 binding site epitopes from neutralising antibodies. Here, we use molecular dynamics techniques to evaluate the structural rearrangements that maintain the protective qualities of the glycan shield after the loss of glycan N301. We examined a naturally occurring subtype C isolate and its N301A mutant; the mutant not only remained protected against neutralising antibodies targeting underlying epitopes, but also exhibited an increased resistance to the VRC01 class of broadly neutralising antibodies. Analysis of this mutant revealed several glycans that were responsible, independently or through synergy, for the neutralisation resistance of the mutant. These data provide detailed insight into the glycan shield’s ability to compensate for the loss of a glycan, as well as the cascade of glycan movements on a protomer, starting at the point mutation, that affects the integrity of an antibody epitope located at the edge of the diminishing effect. These results present key, previously overlooked, considerations for HIV-1 Env glycan research and related vaccine studies.

Molecular Basis of Unusually High Neutralization Resistance in Tier 3 HIV-1 Strain 253-11

Understanding the mechanisms used by HIV-1 to evade antibody neutralization may contribute to the design of a high-coverage vaccine. The tier 3 virus 253-11 is poorly neutralized by subtype-matched and subtype C sera, even compared to other tier 3 viruses, and is also recognized poorly by V3/glycan-targeting monoclonal antibodies (MAbs). We found that sequence polymorphisms in the V3 loop and N-linked glycosylation sites contribute only minimally to the high neutralization resistance of 253-11. Interestingly, the 253-11 membrane-proximal external region (MPER) is rarely recognized by sera in the context of the wild-type virus but is commonly recognized in the context of an HIV-2 chimera, suggesting steric or kinetic hindrance of binding to MPER in the native envelope (Env). Mutations in the 253-11 MPER, which were previously reported to increase the lifetime of the prefusion Env conformation, affected the resistance of 253-11 to antibodies targeting various epitopes on HIV-1 Env, presumably destabilizing its otherwise stable, closed trimer structure. To gain insight into the structure of 253-11, we constructed and crystallized a recombinant 253-11 SOSIP trimer. The resulting structure revealed that the heptad repeat helices in gp41 are drawn in close proximity to the trimer axis and that gp120 protomers also showed a relatively compact disposition around the trimer axis. These observations give substantial insight into the molecular features of an envelope spike from a tier 3 virus and into possible mechanisms that may contribute to its unusually high neutralization resistance.IMPORTANCE HIV-1 isolates that are highly resistant to broadly neutralizing antibodies could limit the efficacy of an antibody-based vaccine. We studied 253-11, which is highly resistant to commonly elicited neutralizing antibodies. To further understand its resistance, we made mutations that are known to delay fusion and thus increase the time that the virus spends in the open conformation following CD4 binding. Interestingly, we found that these mutations affect the 253-11 envelope (Env) spike before CD4 binding, presumably by destabilizing the trimer structure. To gain further information about the structure of the 253-11 Env trimer, we generated a recombinant 253-11 SOSIP trimer. The crystal structure of the SOSIP trimer revealed that the gp41 helices and the gp120 protomers were drawn in toward the center of the molecule compared to most solved HIV-1 Env structures. These observations provide insight into the distinct molecular features of a tier 3 envelope spike.

Shape-selective dependence of room temperature ferromagnetism induced by hierarchical ZnO nanostructures

We report on the room temperature ferromagnetism of various highly crystalline zinc oxide (ZnO) nanostructures, such as hexagonally shaped nanorods, nanocups, nanosamoosas, nanoplatelets, and hierarchical nano "flower-like" structures. These materials were synthesized in a shape-selective manner using simple microwave assisted hydrothermal synthesis. Thermogravimetric analyses demonstrated the as-synthesized ZnO nanostructures to be stable and of high purity. Structural analyses showed that the ZnO nanostructures are polycrystalline and wurtzite in structure, without any secondary phases. Combination of electron paramagnetic resonance, photoluminescence, and X-ray photoelectron spectroscopy studies revealed that the zinc vacancies (VZn) and singly ionized oxygen vacancies (VO(+)) located mainly on the ZnO surface are the primary defects in ZnO structures. A direct link between ferromagnetism and the relative occupancy of the VZn and VO(+) was established, suggesting that both VZn and VO(+) on the ZnO surface plays a vital role in modulating ferromagnetic behavior. An intense structure- and shape-dependent ferromagnetic signal with an effective g-value of >2.0 and a sextet hyperfine structure was shown. Moreover, a novel low field microwave absorption signal was observed and found to increase with an increase in microwave power and temperature.