Identification of critical antibody-binding sites in the HIV-1 gp41 six-helix bundle core as potential targets for HIV-1 fusion inhibitors

Comprehensive profiling of pre-infection antibodies identifies HIV targets associated with viremic control and viral load

Background

High HIV viral load (VL) is associated with increased transmission risk and faster disease progression. HIV controllers achieve viral suppression without antiretroviral (ARV) treatment. We evaluated viremic control in a community-randomized trial with >48,000 participants.

Methods

A massively multiplexed antibody profiling system, VirScan, was used to quantify pre- and post-infection antibody reactivity to HIV peptides in 664 samples from 429 participants (13 controllers, 135 viremic non-controllers, 64 other non-controllers, 217 uninfected persons). Controllers had VLs <2,000 copies/mL with no ARV drugs detected at the first HIV-positive visit and one year later. Viremic non-controllers had VLs 2,000 copies/mL with no ARV drugs detected at the first HIV-positive visit. Other non-controllers had either ARV drugs detected at the first HIV-positive visit (n=47) or VLs <2,000 copies/mL with no ARV drugs detected at only one HIV-positive visit (n=17).

Results

We identified pre-infection HIV antibody reactivities that correlated with post-infection VL. Pre-infection reactivity to an epitope in the HR2 domain of gp41 was associated with controller status and lower VL. Pre-infection reactivity to an epitope in the C2 domain of gp120 was associated with non-controller status and higher VL. Different patterns of antibody reactivity were observed over time for these two epitopes.

Conclusion

These studies suggest that pre-infection HIV antibodies are associated with controller status and modulation of HIV VL. These findings may inform research on antibody-based interventions for HIV treatment.

Peptide Triazole Thiol Irreversibly Inactivates Metastable HIV-1 Env by Accessing Conformational Triggers Intrinsic to Virus-Cell Entry

KR13, a peptide triazole thiol previously established to inhibit HIV-1 infection and cause virus lysis, was evaluated by flow cytometry against JRFL Env-presenting cells to characterize induced Env and membrane transformations leading to irreversible inactivation. Transiently transfected HEK293T cells were preloaded with calcein dye, treated with KR13 or its thiol-blocked analogue KR13b, fixed, and stained for gp120 (35O22), MPER (10E8), 6-helix-bundle (NC-1), immunodominant loop (50-69), and fusion peptide (VRC34.01). KR13 induced dose-dependent transformations of Env and membrane characterized by transient poration, MPER exposure, and 6-helix-bundle formation (analogous to native fusion events), but also reduced immunodominant loop and fusion peptide exposure. Using a fusion peptide mutant (V504E), we found that KR13 transformation does not require functional fusion peptide for poration. In contrast, simultaneous treatment with fusion inhibitor T20 alongside KR13 prevented membrane poration and MPER exposure, showing that these events require 6-helix-bundle formation. Based on these results, we formulated a model for PTT-induced Env transformation portraying how, in the absence of CD4/co-receptor signaling, PTT may provide alternate means of perturbing the metastable Env-membrane complex, and inducing fusion-like transformation. In turn, the results show that such transformations are intrinsic to Env and can be diverted for irreversible inactivation of the protein complex.

Fejerlectin, a Lectin-like Peptide from the Skin of Fejervarya limnocharis, Inhibits HIV-1 Entry by Targeting Gp41

Human immunodeficiency virus type 1 (HIV-1) is mainly transmitted by sexual intercourse, and effective microbicides preventing HIV-1 transmission are still required. Amphibian skin is a rich source of defense peptides with antiviral activity. Here, we characterized a lectin-like peptide, fejerlectin (RLCYMVLPCP), isolated from the skin of the frog Fejervarya limnocharis. Fejerlectin showed significant hemagglutination and d-(+)-galacturonic acid-binding activities. Furthermore, fejerlectin suppressed the early entry of HIV-1 into target cells by binding to the N-terminal heptad repeat of HIV-1 gp41 and preventing 6-HB formation and Env-mediated membrane fusion. Fejerlectin is the smallest lectin-like peptide identified to date and represents a new and promising platform for anti-HIV-1 drug development.

Metastable HIV-1 Surface Protein Env Sensitizes Cell Membranes to Transformation and Poration by Dual-Acting Virucidal Entry Inhibitors

Dual-acting virucidal entry inhibitors (DAVEIs) have previously been shown to cause irreversible inactivation of HIV-1 Env-presenting pseudovirus by lytic membrane transformation. This study examined whether this transformation could be generalized to include membranes of Env-presenting cells. Flow cytometry was used to analyze HEK293T cells transiently transfected with increasing amounts of DNA encoding JRFL Env, loaded with calcein dye, and treated with serial dilutions of microvirin (Q831K/M83R)-DAVEI. Comparing calcein retention against intact Env expression (via Ab 35O22) on individual cells revealed effects proportional to Env expression. "Low-Env" cells experienced transient poration and calcein leakage, while "high-Env" cells were killed. The cell-killing effect was confirmed with an independent mitochondrial activity-based cell viability assay, showing dose-dependent cytotoxicity in response to DAVEI treatment. Transfection with increasing quantities of Env DNA showed further shifts toward "High-Env" expression and cytotoxicity, further reinforcing the Env dependence of the observed effect. Controls with unlinked DAVEI components showed no effect on calcein leakage or cell viability, confirming a requirement for covalently linked DAVEI compounds to achieve Env transformation. These data demonstrate that the metastability of Env is an intrinsic property of the transmembrane protein complex and can be perturbed to cause membrane disruption in both virus and cell contexts.

Revisiting the mechanism of enfuvirtide and designing an analog with improved fusion inhibitory activity by targeting triple sites in gp41

OBJECTIVE

To revisit the mechanism of action of enfuvirtide (T20) and based on the newly defined mechanism, design an analogous peptide of T20 with improved antiviral activity.

DESIGN

We compared the inhibitory activity of T20 with that of T1144 on six-helix bundle (6HB) formation at different time after coculture of HIV type 1 (HIV-1) envelope (Env)-expressing Chinese hamster ovary (CHO-Env) cells and CD4-expressing MT-2 cells at 31.5 °C and with that of T20-SF, an analogous peptide of T20 with an additional tryptophan-rich motif, on hemolysis mediated by FP-P, which contains fusion peptide and fusion peptide (FP) proximal region (FPPR), and HIV-1 infection.

METHODS

Inhibitory activity of peptides on 6HB formation was tested in a temperature-controlled cell-cell fusion assay by flow cytometry using 6HB-specific mAb 2G8; on HIV-1 infection and fusion was assessed by p24 and cell-cell fusion assays. Interaction between different peptides or peptide and antibody was evaluated by ELISA.

RESULTS

T20 could inhibit 6HB formation at early, but not late, stage of HIV-1 fusion, whereas T1144 was effective at both stages. T20-SF is much more effective than T20 in binding to FP-P and inhibiting infection of HIV-1, including T20-resistant strains, and FP-P-mediated hemolysis.

CONCLUSION

Results suggest that T20 has a double-target mechanism, by which its N-terminal and C-terminal portions bind to N-terminal heptad repeat and FPPR, respectively. T20-SF designed based on this new mechanism exhibits significantly improved anti-HIV-1 activity because it targets the triple sites in gp41, including N-terminal heptad repeat, FPPR, and fusion peptide. Thus, this study provides clues for designing novel HIV fusion inhibitors with improved antiviral activity.

Development of Protein- and Peptide-Based HIV Entry Inhibitors Targeting gp120 or gp41

Application of highly active antiretroviral drugs (ARDs) effectively reduces morbidity and mortality in HIV-infected individuals. However, the emergence of multiple drug-resistant strains has led to the increased failure of ARDs, thus calling for the development of anti-HIV drugs with targets or mechanisms of action different from those of the current ARDs. The first peptide-based HIV entry inhibitor, enfuvirtide, was approved by the U.S. FDA in 2003 for treatment of HIV/AIDS patients who have failed to respond to the current ARDs, which has stimulated the development of several series of protein- and peptide-based HIV entry inhibitors in preclinical and clinical studies. In this review, we highlighted the properties and mechanisms of action for those promising protein- and peptide-based HIV entry inhibitors targeting the HIV-1 gp120 or gp41 and discussed their advantages and disadvantages, compared with the current ARDs.

Molecular mechanism of HIV-1 resistance to sifuvirtide, a clinical trial-approved membrane fusion inhibitor

Host cell infection with HIV-1 requires fusion of viral and cell membranes. Sifuvirtide (SFT) is a peptide-based HIV-1 fusion inhibitor approved for phase III clinical trials in China. Here, we focused on characterizing HIV-1 variants highly resistant to SFT to gain insight into the molecular resistance mechanism. Three primary substitutions (V38A, A47I, and Q52R) located at the inhibitor-binding site of HIV-1's envelope protein (Env) and one secondary substitution (N126K) located at the C-terminal heptad repeat region of the viral protein gp41, which is part of the envelope, conferred high SFT resistance and cross-resistance to the anti-HIV-1 drug T20 and the template peptide C34. Interestingly, SFT's resistance profile could be dramatically improved with an M-T hook structure-modified SFT (MTSFT) and with short-peptide inhibitors that mainly target the gp41 pocket (2P23 and its lipid derivative LP-19). We found that the V38A and Q52R substitutions reduce the binding stabilities of SFT, C34, and MTSFT, but they had no effect on the binding of 2P23 and LP-19; in sharp contrast, the A47I substitution enhanced fusion inhibitor binding. Furthermore, the primary resistance substitutions impaired Env-mediated membrane fusion and cell entry and changed the conformation of the gp41 core structure. Importantly, whereas the V38A and Q52R substitutions disrupted the N-terminal helix of gp41, a single A47I substitution greatly enhanced its thermostability. Taken together, our results provide crucial structural insights into the mechanism of HIV-1 resistance to gp41-dependent fusion inhibitors, which may inform the development of additional anti-HIV drugs.

Mechanism of HIV-1 Resistance to an Electronically Constrained α-Helical Peptide Membrane Fusion Inhibitor

SC29EK is an electronically constrained α-helical peptide HIV-1 fusion inhibitor that is highly effective against both wild-type and enfuvirtide (T20)-resistant viruses. In this study, we focused on investigating the mechanism of HIV-1 resistance to SC29EK by two approaches. First, SC29EK-escaping HIV-1 variants were selected and characterized. Three mutant viruses, which possessed two (N43K/E49A) or three (Q39R/N43K/N126K and N43K/E49A/N126K) amino acid substitutions in the N- and C-terminal repeat regions of gp41 were identified as conferring high resistance to SC29EK and cross-resistance to the first-generation (T20 and C34) and newly designed (sifuvirtide, MT-SC29EK, and 2P23) fusion inhibitors. The resistance mutations could reduce the binding stability of SC29EK, impair viral Env-mediated cell fusion and entry, and change the conformation of the gp41 core structure. Further, we determined the crystal structure of SC29EK in complex with a target mimic peptide, which revealed the critical intra- and interhelical interactions underlying the mode of action of SC29EK and the genetic pathway to HIV-1 resistance. Taken together, the present data provide new insights into the structure and function of gp41 and the structure-activity relationship (SAR) of viral fusion inhibitors.IMPORTANCE T20 is the only membrane fusion inhibitor available for treatment of viral infection, but it has relatively low anti-HIV activity and genetic barriers for resistance, thus calling for new drugs blocking the viral fusion process. As an electronically constrained α-helical peptide, SC29EK is highly potent against both wild-type and T20-resistant HIV-1 strains. Here, we report the characterization of HIV-1 variants resistant to SC29EK and the crystal structure of SC29EK. The key mutations mediating high resistance to SC29EK and cross-resistance to the first and new generations of fusion inhibitors as well as the underlying mechanisms were identified. The crystal structure of SC29EK bound to a target mimic peptide further revealed its action mode and genetic pathway to inducing resistance. Hence, our data have shed new lights on the mechanisms of HIV-1 fusion and its inhibition.

Genetic Pathway of HIV-1 Resistance to Novel Fusion Inhibitors Targeting the Gp41 Pocket

The peptide drug enfuvirtide (T20) is the only HIV-1 fusion inhibitor in clinical use, but it easily induces drug resistance, calling for new strategies for developing effective drugs. On the basis of the M-T hook structure, we recently developed highly potent short-peptide HIV-1 fusion inhibitors (MTSC22 and HP23), which mainly target the conserved gp41 pocket and possess high genetic barriers to resistance. Here, we focused on the selection and characterization of HIV-1 escape mutants of MTSC22, which revealed new resistance pathways and mechanisms. Two mutations (E49K and L57R) located at the inhibitor-binding site and two mutations (N126K and E136G) located at the C-terminal heptad repeat region of gp41 were identified as conferring high resistance either singly or in combination. While E49K reduced the C-terminal binding of inhibitors via an electrostatic repulsion, L57R dramatically disrupted the N-terminal binding of M-T hook structure and pocket-binding domain. Unlike E49K and N126K, which enhanced the stability of the endogenous viral six-helical bundle core (6-HB), L57R and E136G conversely destabilized the 6-HB structure. We also demonstrated that both primary and secondary mutations caused the structural changes in 6-HB and severely impaired the capability for HIV-1 entry. Collectively, our data provide novel insights into the mechanisms of short-peptide fusion inhibitors targeting the gp41 pocket site and help increase our understanding of the structure and function of gp41 and HIV-1 evolution.

IMPORTANCE

The deep pocket on the N-trimer of HIV-1 gp41 has been considered an ideal drug target because of its high degree of conservation and essential role in viral entry. Short-peptide fusion inhibitors, which contain an M-T hook structure and mainly target the pocket site, show extremely high binding and inhibitory activities as well as high genetic barriers to resistance. In this study, the HIV-1 mutants resistant to MTSC22 were selected and characterized, which revealed that the E49K and L57R substitutions at the inhibitor-binding site and the N126K and E136G substitutions at the C-terminal heptad repeat region of gp41 critically determine the resistance phenotype. The data provide novel insights into the mechanisms of action of the M-T hook structure-based fusion inhibitors which will help further our understanding of the structure-function relationship of gp41 and molecular pathways of HIV-1 evolution and eventually facilitate the development of new anti-HIV drugs.

P20A inhibits HIV-1 fusion through its electrostatic interaction with the distal region of the gp41 fusion core

We previously identified an HIV-1 fusion inhibitor P20A targeting HIV-1 gp41 6-HB fusion core. Using alanine scanning mutagenesis, we investigated the effect of 6-HB surface residue mutations on the binding affinity between P20A and 6-HB. Substitution of positively or negatively charged residues in the distal region of 6-HB with alanines resulted in significant decrease or increase of its binding affinity to P20A, respectively. The 6-HB with E630K, D632K, or E634K mutation exhibited enhanced binding affinity with P20A, suggesting that P20A blocks HIV-1 fusion through electrostatic interaction with the positively charged residues in the distal region of the gp41 fusion core.

Mechanism of HIV-1 Resistance to Short-Peptide Fusion Inhibitors Targeting the Gp41 Pocket

The deep hydrophobic pocket on the N trimer of HIV-1 gp41 has been considered an ideal drug target. On the basis of the M-T hook structure, we recently developed short-peptide-based HIV-1 fusion inhibitors (MTSC22 and HP23), which mainly target the pocket site and possess highly potent antiviral activity. In this study, we focused on investigating their resistance pathways and mechanisms by escape HIV-1 mutants to SC22EK, a template peptide for MTSC22 and HP23. Two substitutions, E49K and N126K, located, respectively, at the N- and C-heptad repeat regions of gp41, were identified as conferring high resistance to the inhibitors targeting the pocket and cross-resistance to enfuvirtide (T20) and sifuvirtide (SFT). The underlying mechanisms of SC22EK-induced resistance include the following: (i) significantly reduced binding affinity of the inhibitors, (ii) dramatically enhanced interaction of the viral six-helix bundle, and (iii)severely damaged functionality of the viral Env complex. Our data have provided important information for the structure-function relationship of gp41 and the structure-activity relationship of viral fusion inhibitors.

IMPORTANCE

Enfuvirtide (T20) is the only HIV-1 fusion inhibitor in clinical use, but the problem of resistance significantly limits its use, calling for new strategies or concepts to develop next-generation drugs. On the basis of the M-T hook structure, short-peptide HIV-1 fusion inhibitors specifically targeting the gp41 pocket site exhibit high binding and antiviral activities. Here, we investigated the molecular pathway of HIV-1 resistance to the short inhibitors by selecting and mapping the escape mutants. The key substitutions for resistance and the underlying mechanisms have been finely characterized. The data provide important information for the structure-function relationship of gp41 and its inhibitors and will definitely help our future development of novel drugs that block gp41-dependent fusion.

The M-T hook structure is critical for design of HIV-1 fusion inhibitors

CP621-652 is a potent HIV-1 fusion inhibitor peptide derived from the C-terminal heptad repeat of gp41. We recently identified that its N-terminal residues Met-626 and Thr-627 adopt a unique hook-like structure (termed M-T hook) thus stabilizing the interaction of the inhibitor with the deep pocket on the N-terminal heptad repeat. In this study, we further demonstrated that the M-T hook structure is a key determinant of CP621-652 in terms of its thermostability and anti-HIV activity. To directly define the structure and function of the M-T hook, we generated the peptide MT-C34 by incorporating Met-626 and Thr-627 into the N terminus of the C-terminal heptad repeat-derived peptide C34. The high resolution crystal structure (1.9 Å) of MT-C34 complexed by an N-terminal heptad repeat-derived peptide reveals that the M-T hook conformation is well preserved at the N-terminal extreme of the inhibitor. Strikingly, addition of two hook residues could dramatically enhance the binding affinity and thermostability of 6-helix bundle core. Compared with C34, MT-C34 exhibited significantly increased activity to inhibit HIV-1 envelope-mediated cell fusion (6.6-fold), virus entry (4.5-fold), and replication (6-fold). Mechanistically, MT-C34 had a 10.5-fold higher increase than C34 in blocking 6-helix bundle formation. We further showed that MT-C34 possessed higher potency against T20 (Enfuvirtide, Fuzeon)-resistant HIV-1 variants. Therefore, this study provides convincing data for our proposed concept that the M-T hook structure is critical for designing HIV-1 fusion inhibitors.

HLA class I protective alleles in an HIV-1-infected subject homozygous for CCR5-Δ32/Δ32

Homozygosity for a 32 bp deletion in CCR5 (CCR5-Δ32/Δ32) is associated with strong resistance against HIV-1 infection. Several HLA types have been associated to improved viral control and/or delayed progression to AIDS. We report a unique HIV-1 infected individual homozygous for CCR5-Δ32/Δ32 and carrier of HLA-A*2402 and HLA-B*5701. In comparison with earlier data and although a replication competent virus has been isolated, the patient presents better immune status, response to treatment and disease evolution, which may be related to the control exerted by HLA class I restricted T cell immunity. Importantly, the accumulation of protective factors does not warrant a complete protection to HIV infection and the subsequent life-long treatment.

Identification of a gp41 core-binding molecule with homologous sequence of human TNNI3K-like protein as a novel human immunodeficiency virus type 1 entry inhibitor

Human immunodeficiency virus type 1 (HIV-1) gp41 plays a critical role in the viral fusion process, and its N- and C-terminal heptad repeat domains serve as important targets for developing anti-HIV-1 drugs, like T-20 (generic name, enfuvirtide; brand name, Fuzeon). Here, we conducted a yeast two-hybrid screening on a human bone marrow cDNA library using the recombinant soluble gp41 ectodomain as the bait and identified a novel gp41 core-binding molecule, designated P20. P20 showed no homology with a current HIV fusion inhibitor, T-20, but had sequence homology to a human protein, troponin I type 3 interacting kinase (TNNI3K)-like protein. While it could bind to the six-helix bundle core structure formed by the N- and C-terminal heptad repeats, P20 did not interrupt the formation of the six-helix bundle. P20 was effective in blocking HIV-1 Env-mediated syncytium formation and inhibiting infection by a broad spectrum of HIV-1 strains with distinct subtypes and coreceptor tropism, while it was ineffective against other enveloped viruses, such as vesicular stomatitis virus and influenza A virus. P20 exhibited no significant cytotoxicity to the CD4(+) cells that were used for testing antiviral activity. Among the 11 P20 mutants, four analogous peptides with a common motif (WGRLEGRRT) exhibited significantly reduced anti-HIV-1 activity, suggesting that this region is the critical active site of P20. Therefore, this peptide can be used as a lead for developing novel HIV fusion inhibitors and as a probe for studying the membrane-fusogenic mechanism of HIV.

An antibody directed against the fusion peptide of Junin virus envelope glycoprotein GPC inhibits pH-induced membrane fusion

The arenavirus envelope glycoprotein (GPC) initiates infection in the host cell through pH-induced fusion of the viral and endosomal membranes. As in other class I viral fusion proteins, this process proceeds through a structural reorganization in GPC in which the ectodomain of the transmembrane fusion subunit (G2) engages the host cell membrane and subsequently refolds to form a highly stable six-helix bundle structure that brings the two membranes into apposition for fusion. Here, we describe a G2-directed monoclonal antibody, F100G5, that prevents membrane fusion by binding to an intermediate form of the protein on the fusion pathway. Inhibition of syncytium formation requires that F100G5 be present concomitant with exposure of GPC to acidic pH. We show that F100G5 recognizes neither the six-helix bundle nor the larger trimer-of-hairpins structure in the postfusion form of G2. Rather, Western blot analysis using recombinant proteins and a panel of alanine-scanning GPC mutants revealed that F100G5 binding is dependent on an invariant lysine residue (K283) near the N terminus of G2, in the so-called fusion peptide that inserts into the host cell membrane during the fusion process. The F100G5 epitope is located in the internal segment of the bipartite GPC fusion peptide, which also contains four conserved cysteine residues, raising the possibility that this fusion peptide may be highly structured. Collectively, our studies indicate that F100G5 identifies an on-path intermediate form of GPC. Binding to the transiently exposed fusion peptide may interfere with G2 insertion into the host cell membrane. Strategies to effectively target fusion peptide function in the endosome may lead to novel classes of antiviral agents.