The V1V2 domain and an N-linked glycosylation site in the V3 loop of the HIV-1 envelope glycoprotein modulate neutralization sensitivity to the human broadly neutralizing antibody 2G12

A VRC13-like bNAb response is associated with complex escape pathways in HIV-1 envelope

The rational design of HIV-1 immunogens to trigger the development of broadly neutralizing antibodies (bNAbs) requires understanding the viral evolutionary pathways influencing this process. An acute HIV-1-infected individual exhibiting >50% plasma neutralization breadth developed neutralizing antibody specificities against the CD4-binding site (CD4bs) and V1V2 regions of Env gp120. Comparison of pseudoviruses derived from early and late autologous env sequences demonstrated the development of >2 log resistance to VRC13 but not to other CD4bs-specific bNAbs. Mapping studies indicated that the V3 and CD4-binding loops of Env gp120 contributed significantly to developing resistance to the autologous neutralizing response and that the CD4-binding loop (CD4BL) specifically was responsible for the developing resistance to VRC13. Tracking viral evolution during the development of this cross-neutralizing CD4bs response identified amino acid substitutions arising at only 4 of 11 known VRC13 contact sites (K282, T283, K421, and V471). However, each of these mutations was external to the V3 and CD4BL regions conferring resistance to VRC13 and was transient in nature. Rather, complete resistance to VRC13 was achieved through the cooperative expression of a cluster of single amino acid changes within and immediately adjacent to the CD4BL, including a T359I substitution, exchange of a potential N-linked glycosylation (PNLG) site to residue S362 from N363, and a P369L substitution. Collectively, our data characterize complex HIV-1 env evolution in an individual developing resistance to a VRC13-like neutralizing antibody response and identify novel VRC13-associated escape mutations that may be important to inducing VRC13-like bNAbs for lineage-based immunogens.IMPORTANCEThe pursuit of eliciting broadly neutralizing antibodies (bNAbs) through vaccination and their use as therapeutics remains a significant focus in the effort to eradicate HIV-1. Key to our understanding of this approach is a more extensive understanding of bNAb contact sites and susceptible escape mutations in HIV-1 envelope (env). We identified a broad neutralizer exhibiting VRC13-like responses, a non-germline restricted class of CD4-binding site antibody distinct from the well-studied VRC01-class. Through longitudinal envelope sequencing and Env-pseudotyped neutralization assays, we characterized a complex escape pathway requiring the cooperative evolution of four amino acid changes to confer complete resistance to VRC13. This suggests that VRC13-class bNAbs may be refractory to rapid escape and attractive for therapeutic applications. Furthermore, the identification of longitudinal viral changes concomitant with the development of neutralization breadth may help identify the viral intermediates needed for the maturation of VRC13-like responses and the design of lineage-based immunogens.

Signatures of immune selection in intact and defective proviruses distinguish HIV-1 elite controllers

Increasing evidence suggests that durable drug-free control of HIV-1 replication is enabled by effective cellular immune responses that may induce an attenuated viral reservoir configuration with a weaker ability to drive viral rebound. Here, we comprehensively tracked effects of antiviral immune responses on intact and defective proviral sequences from elite controllers (ECs), analyzing both classical escape mutations and HIV-1 chromosomal integration sites as biomarkers of antiviral immune selection pressure. We observed that, within ECs, defective proviruses were commonly located in permissive genic euchromatin positions, which represented an apparent contrast to autologous intact proviruses that were frequently located in heterochromatin regions; this suggests differential immune selection pressure on intact versus defective proviruses in ECs. In comparison to individuals receiving antiretroviral therapy, intact and defective proviruses from ECs showed reduced frequencies of escape mutations in cytotoxic T cell epitopes and antibody contact regions, possibly due to the small and poorly inducible reservoir that may be insufficient to drive effective viral escape in ECs. About 15% of ECs harbored nef deletions in intact proviruses, consistent with increased viral vulnerability to host immunity in the setting of nef dysfunction. Together, these results suggest a distinct signature of immune footprints in proviral sequences from ECs.

Antibody responses to the HIV-1 envelope high mannose patch

Neutralizing antibodies against human immunodeficiency virus subtype 1 (HIV-1) bind to its envelope glycoprotein (Env). Half of the molecular mass of Env is carbohydrate making it one of the most heavily glycosylated proteins known in nature. HIV-1 Env glycans are derived from the host and present a formidable challenge for host anti-glycan antibody induction. Anti-glycan antibody induction is challenging because anti-HIV-1 glycan antibodies should recognize Env antigen while not acquiring autoreactivity. Thus, the glycan network on HIV-1 Env is referred to as the glycan shield. Despite the challenges presented by immune recognition of host-derived glycans, neutralizing antibodies capable of binding the glycans on HIV-1 Env can be generated by the host immune system in the setting of HIV-1 infection. In particular, a cluster of high mannose glycans, including an N-linked glycan at position 332, form the high mannose patch and are targeted by a variety of broadly neutralizing antibodies. These high mannose patch-directed HIV-1 antibodies can be categorized into distinct categories based on their antibody paratope structure, neutralization activity, and glycan and peptide reactivity. Below we will compare and contrast each of these classes of HIV-1 glycan-dependent antibodies and describe vaccine design efforts to elicit each of these antibody types.

Conformational Epitope-Specific Broadly Neutralizing Plasma Antibodies Obtained from an HIV-1 Clade C-Infected Elite Neutralizer Mediate Autologous Virus Escape through Mutations in the V1 Loop

Broadly neutralizing antibodies isolated from infected patients who are elite neutralizers have identified targets on HIV-1 envelope (Env) glycoprotein that are vulnerable to antibody neutralization; however, it is not known whether infection established by the majority of the circulating clade C strains in Indian patients elicit neutralizing antibody responses against any of the known targets. In the present study, we examined the specificity of a broad and potent cross-neutralizing plasma obtained from an Indian elite neutralizer infected with HIV-1 clade C. This plasma neutralized 53/57 (93%) HIV pseudoviruses prepared with Env from distinct HIV clades of different geographical origins. Mapping studies using gp120 core protein, single-residue knockout mutants, and chimeric viruses revealed that G37080 broadly cross-neutralizing (BCN) plasma lacks specificities to the CD4 binding site, gp41 membrane-proximal external region, N160 and N332 glycans, and R166 and K169 in the V1-V3 region and are known predominant targets for BCN antibodies. Depletion of G37080 plasma with soluble trimeric BG505-SOSIP.664 Env (but with neither monomeric gp120 nor clade C membrane-proximal external region peptides) resulted in significant reduction of virus neutralization, suggesting that G37080 BCN antibodies mainly target epitopes on cleaved trimeric Env. Further examination of autologous circulating Envs revealed the association of mutation of residues in the V1 loop that contributed to neutralization resistance. In summary, we report the identification of plasma antibodies from a clade C-infected elite neutralizer that mediate neutralization breadth via epitopes on trimeric gp120 not yet reported and confer autologous neutralization escape via mutation of residues in the V1 loop.

IMPORTANCE

A preventive vaccine to protect against HIV-1 is urgently needed. HIV-1 envelope glycoproteins are targets of neutralizing antibodies and represent a key component for immunogen design. The mapping of epitopes on viral envelopes vulnerable to immune evasion will aid in defining targets of vaccine immunogens. We identified novel conformational epitopes on the viral envelope targeted by broadly cross-neutralizing antibodies elicited in natural infection in an elite neutralizer infected with HIV-1 clade C. Our data extend our knowledge on neutralizing epitopes associated with virus escape and potentially contribute to immunogen design and antibody-based prophylactic therapy.

Association between gp120 envelope V1V2 and V4V5 variable loop profiles in a defined HIV-1 transmission cluster

OBJECTIVE

Variations in the HIV-1 gp120 Env variable loop sequences correlate with virus phenotypes associated with transmission and/or disease progression. We aimed to identify whether signature sequences could be identified in the gp120 Env between acute infection and chronic infection viruses obtained from a group of individuals infected with closely related viruses.

METHODS

To analyse acute infection versus chronic infection viruses, we studied a transmission cluster of 11 individuals, in which six presented during acute infection and five during chronic infection. Multiple HIV-1 gp120 Env clones were sequenced from each patient with predicted amino acid sequences compared between the groups.

RESULTS

Cluster analysis of V1V5 Env sequences (n = 215) identified that acute infection viruses had lower potential N-linked glycosylation site (PNGS) densities than viruses from chronic infection, with a higher amino acid length/PNGS ratio. We found a negative correlation between the V1V2 and V4V5 regions for both amino acid length (Pearson P < 0.01) and PNGS numbers (Pearson P < 0.01) during HIV-1 transmission. This association was lost following seroconversion. These findings were confirmed by analysing sequences from the Los Alamos database that were selected and grouped according to timing of transmission. This included acute infection sequences collected 0-10 days (n = 400) and chronic infection sequences 0.5-3 years postseroconversion (n = 394).

CONCLUSION

Our observations are consistent with a structural association between the V1V2 and V4V5 gp120 regions that is lost following viral transmission. These structural considerations should be taken into consideration when devising HIV-1 immunogens aimed at inducing protective antibody responses targeting transmitted viruses.

The HIV-1 gp120 V1V2 loop: structure, function and importance for vaccine development

Although the second variable loop (V2) of the HIV-1 gp120 envelope glycoprotein shows substantial sequence diversity between strains, its functional importance imposes critical conservation of structure, and within particular microdomains, of sequence. V2 influences HIV-1 viral entry by contributing to trimer stabilization and co-receptor binding. It is one of 4 key domains targeted by the broadly neutralizing antibodies that arise during HIV-1 infection. HIV-1 uses V1V2 sequence variation and glycosylation to escape neutralizing antibody. In the Thai Phase III HIV-1 vaccine trial, RV144, vaccine-induced IgG against V1V2 inversely correlated with the risk of HIV-1 acquisition, and HIV-1 strains infecting RV144 vaccine recipients differed from those infecting placebo recipients in the V2 domain. Similarly, non-human primate challenge studies demonstrated an inverse correlation between vaccine-induced anti-V2 responses and simian immunodeficiency virus acquisition. We hypothesize that increased magnitude, frequency and duration of vaccine-induced anti-V2 antibody responses should improve efficacy afforded by pox-protein prime-boost HIV vaccine strategies.

HIV-1 envelope glycoprotein resistance to monoclonal antibody 2G12 is subject-specific and context-dependent in macaques and humans

HIV-1 Envelope (Env) protein is the sole target of neutralizing antibodies (NAbs) that arise during infection to neutralize autologous variants. Under this immune pressure, HIV escape variants are continuously selected and over the course of infection Env becomes more neutralization resistant. Many common alterations are known to affect sensitivity to NAbs, including residues encoding potential N-linked glycosylation sites (PNGS). Knowledge of Env motifs associated with neutralization resistance is valuable for the design of an effective Env-based vaccine so we characterized Envs isolated longitudinally from a SHIV(SF162P4) infected macaque for sensitivity to neutralizing monoclonal antibodies (MAbs) B12, 2G12, 4E10 and 2F5. The early Env, isolated from plasma at day 56 after infection, was the most sensitive and the late Env, from day 670, was the most resistant to MAbs. We identified four PNGS in these Envs that accumulated over time at positions 130, 139, 160 and 397. We determined that removal of these PNGS significantly increased neutralization sensitivity to 2G12, and conversely, we identified mutations by in silico analyses that contributed resistance to 2G12 neutralization. In order to expand our understanding of these PNGS, we analyzed Envs from clade B HIV-infected human subjects and identified additional glycan and amino acid changes that could affect neutralization by 2G12 in a context-dependent manner. Taken together, these in vitro and in silico analyses of clade B Envs revealed that 2G12 resistance is achieved by previously unrecognized PNGS substitutions in a context-dependent manner and by subject-specific pathways.

Forced virus evolution reveals functional crosstalk between the disulfide bonded region and membrane proximal ectodomain region of HIV-1 gp41

Background

The disulfide-bonded region (DSR) of HIV-1 gp41 mediates association with gp120 and plays a role in transmission of receptor-induced conformational changes in gp120 to gp41 that activate membrane fusion function. In this study, forced viral evolution of a DSR mutant that sheds gp120 was employed to identify domains within gp120-gp41 that are functionally linked to the glycoprotein association site.

Results

The HIV-1_AD8 mutant, W596L/K601D, was serially passaged in U87.CD4.CCR5 cells until replication was restored. Whereas the W596L mutation persisted throughout the cultures, a D601H pseudoreversion in the DSR partially restored cell-free virus infectivity and virion gp120-gp41 association, with further improvements to cell-free virus infectivity following a 2nd-site D674E mutation in the membrane-proximal external region (MPER) of gp41. In an independent culture, D601H appeared with a deletion in V4 (Thr-394-Trp-395) and a D674N substitution in the MPER, however this MPER mutation was inhibitory to W596L/K601H cell-free virus infectivity. While cell-free virus infectivity was not fully restored for the revertant genotypes, their cell-to-cell transmission approached the levels observed for WT. Interestingly, the functional boost associated with the addition of D674E to W596L/K601H was not observed for cell-cell fusion where the cell-surface expressed glycoproteins function independently of virion assembly. The W596L/K601H and W596L/K601H/D674E viruses exhibited greater sensitivity to neutralization by the broadly reactive MPER directed monoclonal antibodies, 2F5 and 4E10, indicating that the reverting mutations increase the availability of conserved neutralization epitopes in the MPER.

Conclusions

The data indicate for the first time that functional crosstalk between the DSR and MPER operates in the context of assembled virions, with the Leu-596-His-601-Glu-674 combination optimizing viral spread via the cell-to-cell route. Our data also indicate that changes in the gp120-gp41 association site may increase the exposure of conserved MPER neutralization epitopes in virus.

Neutralizing antibodies to HIV-1 induced by immunization

Most neutralizing antibodies act at the earliest steps of viral infection and block interaction of the virus with cellular receptors to prevent entry into host cells. The inability to induce neutralizing antibodies to HIV has been a major obstacle to HIV vaccine research since the early days of the epidemic. However, in the past three years, the definition of a neutralizing antibody against HIV has been revolutionized by the isolation of extremely broad and potent neutralizing antibodies from HIV-infected individuals. Considerable hurdles remain for inducing neutralizing antibodies to a protective level after immunization. Meanwhile, novel technologies to bypass the induction of antibodies are being explored to provide prophylactic antibody-based interventions. This review addresses the challenge of inducing HIV neutralizing antibodies upon immunization and considers notable recent advances in the field. A greater understanding of the successes and failures for inducing a neutralizing response upon immunization is required to accelerate the development of an effective HIV vaccine.

Molecular evolution of the HIV-1 Thai epidemic between the time of RV144 immunogen selection to the execution of the vaccine efficacy trial

The RV144 HIV-1 vaccine trial (Thailand, 2003 to 2009), using immunogens genetically matched to the regional epidemic, demonstrated the first evidence of efficacy for an HIV-1 vaccine. Here we studied the molecular evolution of the HIV-1 epidemic from the time of immunogen selection to the execution of the efficacy trial. We studied HIV-1 genetic diversity among 390 volunteers who were deferred from enrollment in RV144 due to preexisting HIV-1 infection using a multiregion hybridization assay, full-genome sequencing, and phylogenetic analyses. The subtype distribution was 91.7% CRF01_AE, 3.5% subtype B, 4.3% B/CRF01_AE recombinants, and 0.5% dual infections. CRF01_AE strains were 31% more diverse than the ones from the 1990s Thai epidemic. Sixty-nine percent of subtype B strains clustered with the cosmopolitan Western B strains. Ninety-three percent of B/CRF01_AE recombinants were unique; recombination breakpoint analysis showed that these strains were highly embedded within the larger network that integrates recombinants from East/Southeast Asia. Compared to Thai sequences from the early 1990s, the distance to the RV144 immunogens increased 52% to 68% for CRF01_AE Env immunogens and 12% to 29% for subtype B immunogens. Forty-three percent to 48% of CRF01_AE sequences differed from the sequence of the vaccine insert in Env variable region 2 positions 169 and 181, which were implicated in vaccine sieve effects in RV144. In conclusion, compared to the molecular picture at the early stages of vaccine development, our results show an overall increase in the genetic complexity of viruses in the Thai epidemic and in the distance to vaccine immunogens, which should be considered at the time of the analysis of the trial results.

Identification of an HIV-1 clade A envelope that exhibits broad antigenicity and neutralization sensitivity and elicits antibodies targeting three distinct epitopes

Broadly neutralizing antibodies (bNAbs) PG9 and PG16 were isolated from an International AIDS Vaccine Initiative (IAVI) Protocol G subject infected with human immunodeficiency virus type 1 (HIV-1) clade A. Both antibodies are highly potent and neutralize greater than 70% of viruses tested. We sought to begin immunogen design based on viral sequences from this patient; however, pseudoviruses prepared with 19 envelope sequences from this subject were resistant to neutralization by PG9 and PG16. Therefore, we used a bioinformatics approach to identify closely related viruses that were potentially sensitive to PG9 and PG16. A most-recent common ancestor (MRCA) sequence for the viral envelope (Env) was determined and aligned with 99 subtype A gp160 sequences from the Los Alamos HIV database. Virus BG505.W6M.ENV.C2 (BG505) was found to have the highest degree of homology (73%) to the MRCA sequence. Pseudoviruses prepared with this Env were sensitive to neutralization with a broad panel of bNAbs, including PG9 and PG16. When expressed by 293T cells as soluble gp120, the BG505 monomer bound well to both PG9 and PG16. We further showed that a point mutation (L111A) enabled more efficient production of a stable gp120 monomer that preserves the major neutralization epitopes. Finally, we showed that an adjuvanted formulation of this gp120 protein elicited neutralizing antibodies in rabbits (following a gp120 DNA vaccine prime) and that the antisera competed with bNAbs from 3 classes of nonoverlapping epitopes. Thus, the BG505 Env protein warrants further investigation as an HIV vaccine candidate, as a stand-alone protein, or as a component of a vaccine vector.

Emergence of a new HIV type 1 CRF01_AE variant in Guangxi, Southern China

The distribution of HIV-1 subtypes and genetic characterization of CRF01_AE in Guangxi, southern China were identified. The distribution of HIV-1 genotypes based on gag, pol, and partial env sequences (n=349) was as follows: CRF01_AE (66.5%), CRF08_BC (19.2%), CRF07_BC (7.2%), URF (4.6%), subtype B (1.7%), and subtype B' (0.9%). CRF01_AE predominated in all geographic regions and risk populations and there were multiple introductions of CRF01_AE strains in Guangxi. We found a peculiar CRF01_AE monophyletic lineage distinct from other CRF01_AE viruses, and we designated it "CRF01_AE-v" for convenience. CRF01_AE-v circulating in both heterosexuals and injecting drug users (IDUs) had accounted for 39.7% of CRF01_AE. It showed a selective advantage in the Guangxi population and formed its own characteristic compared with all the CRF01_AE references. Our results suggested that CRF01_AE-v was a new variant of CRF01_AE and it might lead to a new epidemic in Guangxi.

Human immunodeficiency virus type 1 gp120 envelope characteristics associated with disease progression differ in family members infected with genetically similar viruses

The human immunodeficiency virus type 1 (HIV-1) envelope protein provides the primary contact between the virus and host, and is the main target of the adaptive humoral immune response. The length of gp120 variable loops and the number of N-linked glycosylation events are key determinants for virus infectivity and immune escape, while the V3 loop overall positive charge is known to affect co-receptor tropism. We selected two families in which both parents and two children had been infected with HIV-1 for nearly 10 years, but who demonstrated variable parameters of disease progression. We analysed the gp120 envelope sequence and compared individuals that progressed to those that did not in order to decipher evolutionary alterations that are associated with disease progression when individuals are infected with genetically related virus strains. The analysis of the V3-positive charge demonstrated an association between higher V3-positive charges with disease progression. The ratio between the amino acid length and the number of potential N-linked glycosylation sites was also shown to be associated with disease progression with the healthier family members having a lower ratio. In conclusion in individuals initially infected with genetically linked virus strains the V3-positive charges and N-linked glycosylation are associated with HIV-1 disease progression and follow varied evolutionary paths for individuals with varied disease progression.

Neutralizing capacity of monoclonal antibodies that recognize peptide sequences underlying the carbohydrates on gp41 of simian immunodeficiency virus

Extensive glycosylation of the envelope spikes of human and simian immunodeficiency virus (HIV and SIV) is an important factor for the resistance of these viruses to neutralization by antibodies. SIVmac239 gp41 has three closely spaced sites for N-linked carbohydrate attachment. Rhesus macaques experimentally infected with mutant versions of SIVmac239 lacking two or three of these carbohydrate sites developed strong serum reactivity against mutated peptide sequences at the site of these glycosylations, as well as high titers of neutralizing activity to the mutant viruses (E. Yuste et al., J. Virol. 82:12472-12486, 2008). However, whether antibodies that recognize these underlying peptides have neutralizing activity has not been directly demonstrated. Here we describe the isolation and characterization of three gp41-specific monoclonal antibodies (4G8, 6G8, and 7D6) from one of these mutant-infected monkeys. All three antibodies reacted with mutant gp41 from viral particles and also with peptides corresponding to mutated sequences. Slight differences in peptide specificities were observed among the three antibodies. Sequence analysis revealed that the heavy chains of all three antibodies were derived from the same germ line heavy-chain segment (IGHV4-59*01), but they all had very different sequences in complementarity-determining region 3. The light chains of all three antibodies were very closely related to one another. All three antibodies had neutralizing activity to mutant viruses deficient in gp41 carbohydrate attachment, but they did not neutralize the parental SIVmac239. These results demonstrate unambiguously that antibodies with specificity for peptide sequences underlying gp41 carbohydrates can effectively neutralize SIV when these carbohydrates are absent. However, the presence of these gp41 carbohydrates effectively shields the virus from antibodies that would otherwise neutralize viral infectivity.

Human immunodeficiency virus type-1 (HIV-1) continues to evolve in presence of broadly neutralizing antibodies more than ten years after infection

Background

The evolution of HIV-1 and its immune escape to autologous neutralizing antibodies (Nabs) during the acute/early phases of infection have been analyzed in depth in many studies. In contrast, little is known about neither the long-term evolution of the virus in patients who developed broadly Nabs (bNabs) or the mechanism of escape in presence of these bNabs.

Results

We have studied the viral population infecting a long term non progressor HIV-1 infected patient who had developed broadly neutralizing antibodies toward all tier 2/3 viruses (6 clades) tested, 9 years after infection, and was then followed up over 7 years. The autologous neutralization titers of the sequential sera toward env variants representative of the viral population significantly increased during the follow-up period. The most resistant pseudotyped virus was identified at the last visit suggesting that it represented a late emerging escape variant. We identified 5 amino acids substitutions that appeared associated with escape to broadly neutralizing antibodies. They were V319I/S, R/K355T, R/W429G, Q460E and G/T463E, in V3, C3 and V5 regions.

Conclusion

This study showed that HIV-1 may continue to evolve in presence of both broadly neutralizing antibodies and increasing autologous neutralizing activity more than 10 years post-infection.

Kinetic mechanism for HIV-1 neutralization by antibody 2G12 entails reversible glycan binding that slows cell entry

Despite structural knowledge of broadly neutralizing monoclonal antibodies (NMAbs) complexed to HIV-1 gp120 and gp41 envelope glycoproteins, virus inactivation mechanisms have been difficult to prove, in part because neutralization assays are complex and were previously not understood. Concordant with recent evidence that HIV-1 titers are determined by a race between entry of cell-attached virions and competing inactivation processes, we show that NMAb 2G12, which binds to gp120 N-glycans with α (1, 2)-linked mannose termini and inhibits replication after passive transfer into patients, neutralizes by slowing entry of adsorbed virions. Accordingly, apparent neutralization is attenuated when a kinetically competing virus inactivation pathway is blocked. Moreover, removing 2G12 from media causes its dissociation from virions coupled to accelerated entry and restored infectivity, demonstrating the reversibility of neutralization. A difference between 2G12 dissociation and infectivity recovery rates implies that the inhibited complexes at virus-cell junctions contain several 2G12's that must dissociate before entry commences. Quantitative microscopy of 2G12 binding and dissociation from single virions and studies using a split CCR5 coreceptor suggest that 2G12 competitively inhibits interactions between gp120's V3 loop and the tyrosine sulfate-containing CCR5 amino terminus, thereby reducing assembly of complexes that catalyze entry. These results reveal a unique reversible kinetic mechanism for neutralization by an antibody that binds near a critical V3 region in the glycan shield of gp120.

Naturally occurring substitutions of conserved residues in human immunodeficiency virus type 1 variants of different clades are involved in PG9 and PG16 resistance to neutralization

The recently described anti-human immunodeficiency virus type 1 (HIV-1) human mAb PG9 and PG16 are cross-clade broadly neutralizing. Therefore, it can be postulated that the targeted epitope(s) are highly conserved among variants of the entire group M. We analysed the sensitivity to PG9 and PG16 of pseudotyped viruses carrying envelope glycoproteins from the viral quasispecies of three HIV-1 clade CRF01_AE-infected patients. The broad heterogeneity in sensitivity to PG9 and PG16, despite closely genetically related envelope glycoproteins issued from single individuals, allowed us to identify two gp120 cross-clade conserved residues, a lysine at position 168 in the V2 loop and an isoleucine at position 215 in the C2 region, whose substitutions were associated with resistance to PG9 and PG16. By site-directed mutagenesis, we confirmed both in clades B and CRF01_AE that the substitutions K168E and I215M have a major impact on PG9 and PG16 neutralization sensitivity of pseudotyped viruses.

Neutralizing antibodies and control of HIV: moves and countermoves

It is now evident that powerful antibodies directed to conserved regions of HIV-1 envelope protein develop during chronic infection in some individuals and that these antibodies can neutralize a broad array of diverse isolates in vitro, so termed broadly neutralizing antibodies (bNAbs). A great deal of effort is directed internationally at understanding the ontogeny of NAbs during infection as well as in designing and testing immunogens that can elicit bNAbs in animal models and in humans. Given the parrying tactics of Env, multiple approaches, along with high-resolution structural studies, will be needed to reach a degree of understanding sufficient to design an effective vaccine. We discuss and note here some of the most important recent advances in our knowledge of how neutralizing antibodies develop in vivo, the recent discovery of extremely powerful neutralizing monoclonal antibodies isolated from natural infection, enhanced methodologies that have accelerated discoveries on both fronts, and the progress made in eliciting potent NAbs with limited breadth by vaccination.

HIV-1 transmission and viral adaptation to the host

HIV-1 transmission predominantly occurs via mucosal transmission and blood–blood contact. In most newly infected individuals, outgrowth of a single virus variant has been described. This indicates that HIV-1 transmission is a very inefficient process and is restricted by an extensive transmission bottleneck. The transmission rate is directly correlated to the viral load in the donor and the susceptibility of the recipient, which is influenced by factors such as the integrity of mucosal barriers, target cell availability and genetic host factors. After establishment of infection in the new host, the viral population remains very homogenous until the host immune response drives evolution of the viral quasispecies. This review describes our current knowledge on HIV-1 transmission and recent insights in viral adaption to its host.

Protein glycosylation in infectious disease pathobiology and treatment

A host of bacteria and viruses are dependent on O-linked and N-linked glycosylation to perform vital biological functions. Pathogens often have integral proteins that participate in host-cell interactions such as receptor binding and fusion with host membrane. Fusion proteins from a broad range of disparate viruses, such as paramyxovirus, HIV, ebola, and the influenza viruses share a variety of common features that are augmented by glycosylation. Each of these viruses contain multiple glycosylation sites that must be processed and modified by the host post-translational machinery to be fusogenically active. In most viruses, glycosylation plays a role in biogenesis, stability, antigenicity and infectivity. In bacteria, glycosylation events play an important role in the formation of flagellin and pili and are vitally important to adherence, attachment, infectivity and immune evasion. With the importance of glycosylation to pathogen survival, it is clear that a better understanding of the processes is needed to understand the pathogen requirement for glycosylation and to capitalize on this requirement for the development of novel therapeutics.