Peptide-Induced Fusion of Dynamic Membrane Nanodomains: Implications in a Viral Entry.

Enveloped viruses infect host cells via protein-mediated membrane fusion. However, insights into the microscopic rearrangement induced by the viral proteins and peptides have not yet emerged. Here, we report a new methodology to extract viral fusion peptide (FP)-mediated biomembrane dynamical nanodomain fusion parameter, λ, based on stimulated emission depletion microscopy coupled with fluorescence correlation spectroscopy. We also define another dynamical parameter membrane gradient, defined in terms of the ratio of average lipid diffusion coefficients across dynamic crossover length scales, ξ. Significantly, we observe that λ as well as these mobility gradients are larger in the stiffer liquid-ordered (Lo) phase compared to the liquid-disordered phase and are more effective at the smaller nanodomain interfaces, which are only present in the Lo phase. The results could possibly help to resolve a long-standing puzzle about the enhanced fusogenicity of FP in the Lo phase. Results obtained from the diffusion results have been correlated with the human immunodeficiency virus gp41 FP-induced membrane fusion.

In Vitro Selection and Characterization of HIV-1 Variants with Increased Resistance to LP-40, Enfuvirtide-Based Lipopeptide Inhibitor.

In our previous work, we replaced the TRM (tryptophan-rich motif) of T20 (Enfuvirtide) with fatty acid (C16) to obtain the novel lipopeptide LP-40, and LP-40 displayed enhanced antiviral activity. In this study, we investigated whether the C16 modification could enhance the high-resistance barrier of the inhibitor LP-40. To address this question, we performed an in vitro simultaneous screening of HIV-1NL4-3 resistance to T20 and LP-40. The mechanism of drug resistance for HIV-1 Env was further studied using the expression and processing of the Env glycoprotein, the effect of the Env mutation on the entry and fusion ability of the virus, and an analysis of changes to the gp41 core structure. The results indicate that the LP-40 activity is enhanced and that it has a high resistance barrier. In a detailed analysis of the resistance sites, we found that mutations in L33S conferred a stronger resistance, except for the well-recognized mutations in amino acids 36-45 of gp41 NHR, which reduced the inhibitory activity of the CHR-derived peptides. The compensatory mutation of eight amino acids in the CHR region (NDQEEDYN) plays an important role in drug resistance. LP-40 and T20 have similar resistance mutation sites, and we speculate that the same resistance profile may arise if LP-40 is used in a clinical setting.

Peptide-Based HIV Entry Inhibitors.

The development of peptide-based HIV entry inhibitors has made an important contribution to the stock of anti-HIV drugs. In particular, the peptide-based anti-HIV drugs enfuvirtide and albuvirtide were approved for clinical use by the U.S. FDA and CFDA in 2003 and 2018, respectively. Peptide-based HIV entry inhibitors exert antiviral activity by targeting the early stage of viral infection, i.e., binding of a viral surface protein to the receptor(s) on the host cell and the subsequent fusion between the viral and host cell membranes. Therefore, they are particularly useful for HIV-infected patients who have failed to respond to the highly active antiretroviral drugs (ARD) targeting the late stage of HIV replication, such as reverse transcriptase inhibitors and protease inhibitors. In this chapter, we will focus on the past, current, and future trends in research and development of peptide-based HIV entry inhibitors.

Small-Molecule HIV Entry Inhibitors Targeting gp120 and gp41.

Interrupting early events in the virus life cycle, such as those prior to the formation of provirus, could effectively prevent HIV-1 infection. HIV-1 entry into host cells is mediated by the envelope glycoprotein (Env) trimer, which is composed of three gp120 exterior glycoproteins and three gp41 transmembrane glycoproteins. Hence, the development of novel inhibitors targeting these two glycoproteins could hold the key to early inhibition of HIV-1 infection. Small-molecule entry inhibitors targeting early events in the virus life cycle comprise a well-established class of useful drugs. Many libraries of small-molecule inhibitors have been established to screen potential drug candidates for a variety of targets based on computer docking, FRET, or peptide-linked assay. This chapter reviews the mechanisms of some small-molecule inhibitors targeting HIV-1 gp120 and gp41 and corresponding high-efficiency screening strategies for potential small-molecule inhibitors.

Peptide-Based Dual HIV and Coronavirus Entry Inhibitors.

The continued HIV/AIDS epidemic worldwide and the battle against emerging infectious diseases caused by coronaviruses underscore the need for the development of an ever-expanding repertoire of antiviral drugs. Entry inhibitors are of particular interest because of their potential to be used as therapeutic or prophylactic treatments for blocking viral invasion. HIV and coronaviruses utilize class I fusion proteins to facilitate their entry and membrane fusion. Discovery of a common hexameric coiled-coil fusion complex resulting from the packing of three C-terminal heptad repeat region from the fusion-mediating subunit of viral fusion proteins against trimeric coiled-coil made up by their N-terminal heptad repeat prompted the search for peptides mimicking the heptad repeat regions that could potentially inhibit viral entry. This has led to the development of effective peptides that are specific to the virus that is developed for. In this review, we focus on peptide-based entry dual inhibitors that block fusion process not only of HIV but also coronaviruses through interrupting their fusogenic six-helical bundle core and which hopefully will help to gain insight into the α-helical secondary structure- and coiled-coil superstructure-based strategies to design entry inhibitors with broad-spectrum antiviral activity against enveloped viruses with class I fusion proteins.

Effective Delivery of Nef-MPER-V3 Fusion Protein Using LDP12 Cell Penetrating Peptide for Development of Preventive/Therapeutic HIV-1 Vaccine.

BACKGROUND: There is no effective and safe preventive/therapeutics vaccine against HIV-1 worldwide. Different viral proteins such as Nef, and two regions of Env including; variable loop of gp120 (V3) and membrane proximal external region of gp41 (MPER) are particularly important for vaccine development in different strategies and they are also the primary targets of cellular and humoral immune responses. On the other side, LDP12 is a new cell-penetrating peptide (CPP) which is capable of therapeutic application and cargoes delivery across the cellular membrane. OBJECTIVE: In current study, we designed and produced Nef-MPER-V3 fusion protein harboring LDP12 that has the capability of being used in future vaccine studies. METHODS: The CPP-protein was expressed in E. coli Rosseta (DE3) strain and purified through Ni-NTA column. Characterization of cellular delivery and toxicity of the recombinant protein were evaluated by western blotting and MTT assay. RESULTS: Our results showed that the CPP-protein was successfully expressed and purified with high yield of 5 mg/L. Furthermore, non-cytotoxic effect was observed and specific band (~ 37 KDa) in western blotting indicated the capability of LDP12 to improve the rate of penetration into HEK-293T cells in comparison with a control sample. CONCLUSION: Altogether, the data indicated that LDP12 CPP could be utilized to internalize HIV-1 Nef-MPER-V3 protein into eukaryotic cell lines without any toxicity and represented a valuable potential vaccine candidate and this guarantees the further evaluation towards the assessment of its immunogenicity in mice, which is currently under process.

Polyethylene Glycol 40-Modified Peptide with High Therapeutic Efficacy in Simian-Human Immunodeficiency Virus-Acutely Infected Rhesus Monkeys.

Anti-human immunodeficiency virus type 1 (anti-HIV-1) fusion peptides have been studied for nearly 2 decades, but few candidates have found useful clinical applications. One factor underlying the failure of such agents to reach the clinic is their poor pharmacokinetic properties, and many efforts have been made to overcome this problem. In this study, we modified C34, a peptide inhibitor of HIV-1 fusion, at its conserved glycosylation site using polyethylene glycols (PEGs) of different molecular weights. PEG40-NC, a conjugate of C34 and branched PEG 40 kDa (PEG40), which has been previously shown to improve the pharmacokinetic profiles of proteins, showed a significantly extended half-life (t1/2; 10.39 h in rats), which compensated for decreased in vitro activity (50% effective concentration [EC50] of 18.51 nM). PEG40-NC also showed a mechanism of action similar to that of C34. PEG40-NC monotherapy in acutely simian-human immunodeficiency virus (SHIV)-infected rhesus monkeys significantly suppressed viral load compared with a control treatment. Efficacy was linked to the extended half-life and lymphatic exposure conferred by attached PEG40. These results highlight the potential of further clinical investigations of PEG40-NC in combination with antiretroviral therapy or other anti-HIV agents.IMPORTANCE Poor pharmacokinetics have severely hindered the clinical use of anti-HIV peptides. Different small molecules, such as lipid, cholesterol, and small PEG, were designed to modify peptides to improve their pharmacokinetics. In this study, we incorporated large branched PEG to anti-HIV peptide and obtained a conjugate with extended half-life and improved in vivo efficacy. The strategy we developed in this study can also be applicable for the development of other peptide candidates.

Suitable fusion of N-terminal heptad repeats to achieve covalently stabilized potent N-peptide inhibitors of HIV-1 infection.

N-terminal heptad repeat (NHR)-derived peptide (N-peptide) fusion inhibitors, which are derived from human immunodeficiency virus (HIV) envelope glycoprotein 41 (gp41), are limited by aggregation and unstable trimer conformation. However, they could function as potent inhibitors of viral infection by forming a coiled-coil structure covalently stabilized by interchain disulfide bonds. We previously synthesized N-peptides with potent anti-HIV-1 activity and high stability by coiled-coil fusion and covalent stabilization. Here, we attempted to study the effects of NHRs of chimeric N-peptides by fusing de novo coiled-coil isopeptide bridge-tethered T21 peptides of different NHR lengths. Peptides (T21N23)3 and (T21N36)3 was a more potent HIV-1 fusion inhibitor than (T21N17)3. The site of isopeptide bond formation was precisely controlled and had little influence on N-peptide properties. The N-peptide (T21N36)3, which had a similar conformation as the NHR trimer and interacted well with the C34 peptide, may be useful for screening other C-peptides and small-molecule fusion inhibitors, and for studying the interactions between the NHR trimer and C-terminal heptad repeats.

Presentation of HIV-1 Envelope Trimers on the Surface of Silica Nanoparticles.

Inducing immune responses protecting from HIV infection or at least controlling replication poses a huge challenge to modern vaccinology. An increasingly discussed strategy to elicit a potent and broad neutralizing antibody response is the immobilization of HIV's trimeric envelope (Env) surface receptor on a nanoparticulate carrier. As a conceptual proof, we attached an Env variant (BG505 SOSIP.664) to highly stable and biocompatible silica nanoparticles (SiNPs) via site-specific covalent conjugation or nonspecific adsorption to SiNPs. First, we demonstrated the feasibility of SiNPs as platform for Env presentation by a thorough characterization process during which Env density, attachment stability, and antigenicity were evaluated for both formulations. Binding affinities to selected antibodies were in the low nanomolar range for both formulations confirming that the structural integrity of Env is retained after attachment. Second, we explored the recognition of SiNP conjugates by antigen presenting cells. Here, the uptake of Env attached to SiNPs via a site-specific covalent conjugation was 4.5-fold enhanced, whereas adsorbed Env resulted only in a moderate 1.4-fold increase compared with Env in its soluble form. Thus, we propose SiNPs with site-specifically and covalently conjugated Env preferably in a high density as a promising candidate for further investigations as vaccine platform.

Trimeric heptad repeat synthetic peptides HR1 and HR2 efficiently inhibit HIV-1 entry.

The trimeric heptad repeat domains HR1 and HR2 of the human immunodeficiency virus 1 (HIV-1) gp41 play a key role in HIV-1-entry by membrane fusion. To develop efficient inhibitors against this step, the corresponding trimeric-N36 and C34 peptides were designed and synthesized. Analysis by circular dichroism of monomeric and trimeric N36 and C34 peptides showed their capacities to adopt α-helical structures and to establish physical interactions. At the virological level, while trimeric-C34 conserves the same high anti-fusion activity as monomeric-C34, trimerization of N36-peptide induced a significant increase, reaching 500-times higher in anti-fusion activity, against R5-tropic virus-mediated fusion. This result was associated with increased stability of the N36 trimer peptide with respect to the monomeric form, as demonstrated by the comparative kinetics of their antiviral activities during 6-day incubation in a physiological medium. Collectively, our findings demonstrate that while the trimerization of C34 peptide had no beneficial effect on its stability and antiviral activity, the trimerization of N36 peptide strengthened both stability and antiviral activity. This approach, promotes trimers as new promising HIV-1 inhibitors and point to future development aimed toward innovative peptide fusion inhibitors, microbicides or as immunogens.

Long-Acting HIV-1 Fusion Inhibitory Peptides and their Mechanisms of Action.

The clinical application of HIV fusion inhibitor, enfuvirtide (T20), was limited mainly because of its short half-life. Here we designed and synthesized two PEGylated C34 peptides, PEG2kC34 and PEG5kC34, with the PEG chain length of 2 and 5 kDa, respectively, and evaluated their anti-HIV-1 activity and mechanisms of action. We found that these two PEGylated peptides could bind to the HIV-1 peptide N36 to form high affinity complexes with high α-helicity. The peptides PEG2kC34 and PEG5kC34 effectively inhibited HIV-1 Env-mediated cell-cell fusion with an effective concentration for 50% inhibition (EC50) of about 36 nM. They also inhibited infection of the laboratory-adapted HIV-1 strain NL4-3 with EC50 of about 4-5 nM, and against 47 HIV-1 clinical isolates circulating in China with mean EC50 of PEG2kC34 and PEG5kC34 of about 26 nM and 32 nM, respectively. The plasma half-life (t1/2) of PEG2kC34 and PEG5kC34 was 2.6 h and 5.1 h, respectively, and the t1/2 of PEGylated C34 was about 2.4-fold and 4.6-fold longer than C34 (~1.1 h), respectively. These findings suggest that PEGylated C34 with broad-spectrum anti-HIV-1 activity and prolonged half-life can be further developed as a peptide fusion inhibitor-based long-acting anti-HIV drug for clinical use to treat HIV-infected patients who have failed to respond to current anti-retrovirus drugs.

The Natural-Based Antitumor Compound T21 Decreases Survivin Levels through Potent STAT3 Inhibition in Lung Cancer Models.

Lung cancer is the leading cause of cancer-related deaths worldwide; hence novel treatments for this malignancy are eagerly needed. Since natural-based compounds represent a rich source of novel chemical entities in drug discovery, we have focused our attention on tambjamines, natural compounds isolated from marine invertebrates that have shown diverse pharmacological activities. Based on these structures, we have recently identified the novel indole-based tambjamine analog 21 (T21) as a promising antitumor agent, which modulates the expression of apoptotic proteins such as survivin. This antiapoptotic protein plays an important role in carcinogenesis and chemoresistance. In this work, we have elucidated the molecular mechanism by which the anticancer compound T21 exerts survivin inhibition and have validated this protein as a therapeutic target in different lung cancer models. T21 was able to reduce survivin protein levels in vitro by repressing its gene expression through the blockade of Janus kinase/Signal Transducer and Activator of Transcription-3 (JAK/STAT3)/survivin signaling pathway. Interestingly, this occurred even when the pathway was overstimulated with its ligand interleukin 6 (IL-6), which is frequently overexpressed in lung cancer patients who show poor clinical outcomes. Altogether, these results show T21 as a potent anticancer compound that effectively decreases survivin levels through STAT3 inhibition in lung cancer, appearing as a promising therapeutic drug for cancer treatment.

Dimeric C34 Derivatives Linked through Disulfide Bridges as New HIV-1 Fusion Inhibitors.

C34, a 34-mer fragment peptide, is contained in the HIV-1 envelope protein gp41. A dimeric derivative of C34 linked through a disulfide bridge at its C terminus was synthesized and found to display potent anti-HIV activity, comparable with that of a previously reported PEGylated dimer of C34REG. The reduction in the size of the linker moiety for dimerization was thus successful, and this result might shed some light on the mechanism of the suppression of six-helix bundle formation by these C34 dimeric derivatives. Addition of a Gly-Cys(CH2 CONH2 )-Gly-Gly motif at the N-terminal position of a C34 monomeric derivative significantly increased the anti-HIV-1 activity. This moiety functions as a new pharmacophore, and this might provide a useful insight into the design of potent HIV-1 fusion inhibitors.

The helix-to-sheet transition of an HIV-1 fusion peptide derivative changes the mechanical properties of lipid bilayer membranes.

A short sequence on the gp41 envelope protein of HIV-1 is integral to infection by the virus. Without this sequence, termed the fusion peptide (FP), the virus is far less effective at fusing with the cellular membrane. One of the interesting features of the isolated FP is that it transitions between an α-helical conformation and a ß-sheet conformation in lipid bilayer membranes as a function of lipid composition and concentration, and the transition correlates with fusion. To better understand how the conformations of the FP impact lipid bilayer membranes, a variant of the FP that does not strongly promote fusion, termed gp41rk, was studied. Circular dichroism spectroscopy, dynamic light scattering, small-angle neutron scattering (SANS) and neutron spin echo spectroscopy (NSE) were used to relate the conformation of gp41rk to the structure and mechanical properties of lipid bilayer membrane vesicles composed of a 7:3 molar ratio mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol). At a peptide-to-lipid ratio (P/L) of 1/200, it adopts an α-helical conformation, while gp41rk is a ß-sheet at a P/L of 1/50 in the unilamellar vesicles. SANS reveals that the lipid bilayer membrane becomes thicker when gp41rk adopts a ß-sheet conformation, which indicates that the high-concentration state of the peptide increases the order of the lipid acyl chains. At the same time, NSE demonstrates that the bilayer becomes more rigid, demonstrating that the ß-sheet conformation, which correlates with fusion for the native FP sequence, stiffens the bilayer. The results have implications for the function of the FP.

Structural Insights into the Mechanisms of Action of Short-Peptide HIV-1 Fusion Inhibitors Targeting the Gp41 Pocket.

The deep hydrophobic pocket of HIV-1 gp41 has been considered a drug target, but short-peptides targeting this site usually lack potent antiviral activity. By applying the M-T hook structure, we previously generated highly potent short-peptide fusion inhibitors that specifically targeted the pocket site, such as MT-SC22EK, HP23L, and LP-11. Here, the crystal structures of HP23L and LP-11 bound to the target mimic peptide N36 demonstrated the critical intrahelical and interhelical interactions, especially verifying that the hook-like conformation was finely adopted while the methionine residue was replaced by the oxidation-less prone residue leucine, and that addition of an extra glutamic acid significantly enhanced the binding and inhibitory activities. The structure of HP23L bound to N36 with two mutations (E49K and L57R) revealed the critical residues and motifs mediating drug resistance and provided new insights into the mechanism of action of inhibitors. Therefore, the present data help our understanding for the structure-activity relationship (SAR) of HIV-1 fusion inhibitors and facilitate the development of novel antiviral drugs.

In situ depot formation of anti-HIV fusion-inhibitor peptide in recombinant protein polymer hydrogel.

Most peptide drugs have short half-lives, necessitating frequent injections that may induce skin sensitivity reactions; therefore, versatile prolonged-release delivery platforms are urgently needed. Here, we focused on an oxidatively and thermally responsive recombinant elastin-like polypeptide with periodic cysteine residues (cELP), which can rapidly and reversibly form a disulfide cross-linked network in which peptide can be physically incorporated. As a model for proof of concept, we used enfuvirtide, an antiretroviral fusion-inhibitor peptide approved for treatment of human immunodeficiency virus (HIV) infection. cELP was mixed with enfuvirtide and a small amount of hydrogen peroxide (to promote cross-linking), and the soluble mixture was injected subcutaneously. The oxidative cross-linking generates a network structure, causing the mixture to form a hydrogel in situ that serves as an enfuvirtide depot. We fabricated a series of enfuvirtide-containing hydrogels and examined their stability, enfuvirtide-releasing profile and anti-HIV potency in vitro. Among them, hydrophobic cELP hydrogel provided effective concentrations of enfuvirtide in blood of rats for up to 8 h, and the initial concentration peak was suppressed compared with that after injection of enfuvirtide alone. cELP hydrogels should be readily adaptable as platforms to provide effective depot systems for delivery of other anti-HIV peptides besides enfuvirtide. STATEMENT OF SIGNIFICANCE: In this paper, we present an anti-HIV peptide delivery system using oxidatively and thermally responsive polypeptides that contain multiple periodic cysteine residues as an injectable biomaterial capable of in situ self-gelation, and we demonstrate its utility as an injectable depot capable of sustained release of anti-HIV peptides. The novelty of this work stems from the platform employed to provide the depot encapsulating the peptide drugs (without chemical conjugation), which consists of rationally designed, genetically engineered polypeptides that enable the release rate of the peptide drugs to be precisely controlled.

Enfuvirtide (T20)-Based Lipopeptide Is a Potent HIV-1 Cell Fusion Inhibitor: Implications for Viral Entry and Inhibition.

The peptide drug enfuvirtide (T20) is the only viral fusion inhibitor used in combination therapy for HIV-1 infection, but it has relatively low antiviral activity and easily induces drug resistance. Emerging studies demonstrate that lipopeptide-based fusion inhibitors, such as LP-11 and LP-19, which mainly target the gp41 pocket site, have greatly improved antiviral potency and in vivo stability. In this study, we focused on developing a T20-based lipopeptide inhibitor that lacks pocket-binding sequence and targets a different site. First, the C-terminal tryptophan-rich motif (TRM) of T20 was verified to be essential for its target binding and inhibition; then, a novel lipopeptide, termed LP-40, was created by replacing the TRM with a fatty acid group. LP-40 showed markedly enhanced binding affinity for the target site and dramatically increased inhibitory activity on HIV-1 membrane fusion, entry, and infection. Unlike LP-11 and LP-19, which required a flexible linker between the peptide sequence and the lipid moiety, addition of a linker to LP-40 sharply reduced its potency, implying different binding modes with the extended N-terminal helices of gp41. Also, interestingly, LP-40 showed more potent activity than LP-11 in inhibiting HIV-1 Env-mediated cell-cell fusion while it was less active than LP-11 in inhibiting pseudovirus entry, and the two inhibitors displayed synergistic antiviral effects. The crystal structure of LP-40 in complex with a target peptide revealed their key binding residues and motifs. Combined, our studies have not only provided a potent HIV-1 fusion inhibitor, but also revealed new insights into the mechanisms of viral inhibition.IMPORTANCE T20 is the only membrane fusion inhibitor available for treatment of viral infection; however, T20 requires high doses and has a low genetic barrier for resistance, and its inhibitory mechanism and structural basis remain unclear. Here, we report the design of LP-40, a T20-based lipopeptide inhibitor that has greatly improved anti-HIV activity and is a more potent inhibitor of cell-cell fusion than of cell-free virus infection. The binding modes of two classes of membrane-anchoring lipopeptides (LP-40 and LP-11) verify the current fusion model in which an extended prehairpin structure bridges the viral and cellular membranes, and their complementary effects suggest a vital strategy for combination therapy of HIV-1 infection. Moreover, our understanding of the mechanism of action of T20 and its derivatives benefits from the crystal structure of LP-40.

HIV-1 gp41-targeting fusion inhibitory peptides enhance the gp120-targeting protein-mediated inactivation of HIV-1 virions.

Protein- or peptide-based viral inactivators are being developed as novel antiviral drugs with improved efficacy, pharmacokinetics and toxicity profiles because they actively inactivate cell-free human immunodeficiency virus type 1 (HIV-1) virions before attachment to host cells. By contrast, most clinically used antiviral drugs must penetrate host cells to inhibit viral replication. In this study, we pre-treated HIV-1 particles with a gp120-targeting bispecific multivalent protein, 2Dm2m or 4Dm2m, in the presence or absence of the gp41-targeting HIV-1 fusion inhibitory peptides enfuvirtide (T20), T2635, or sifuvirtide (SFT). HIV-1 virions were separated from the inhibitors using PEG-6000, followed by testing of the residual infectivity of the HIV-1 virions. 2Dm2m and 4Dm2m exhibited significant inactivation activity against all HIV-1 strains tested with EC50 values at the low nanomolar level, whereas none of the gp41-targeting peptides showed inactivation activity at concentrations up to 250 nM. Notably, these three peptides significantly enhanced protein-mediated inactivation against cell-free HIV-1 virions, including HIV-1 laboratory-adapted and primary HIV-1 strains, as well as those resistant to T20 or T2635 and virions released from reactivated latently HIV-1-infected cells. These results indicate that the gp120-targeting bispecific multivalent proteins 2Dm2m and 4Dm2m have potential for further development as HIV-1 inactivator-based antiviral drugs for use in the clinic, either alone or in combination with a gp41-targeting HIV-1 fusion inhibitor such as T20, to treat patients with HIV-1 infection and AIDS.

Antigenic and immunosuppressive properties of a trimeric recombinant transmembrane envelope protein gp41 of HIV-1.

The transmembrane envelope (TM) protein gp41 of the human immunodeficiency virus-1 (HIV-1) plays an important role during virus infection inducing the fusion of the viral and cellular membranes. In addition, there are indications that the TM protein plays a role in the immunopathogenesis leading to the acquired immunodeficiency syndrome (AIDS). Inactivated virus particles and recombinant gp41 have been reported to inhibit lymphocyte proliferation, as well as to alter cytokine release and gene expression. The same was shown for a peptide corresponding to a highly conserved domain of all retroviral TM proteins, the immunosuppressive domain. Due to its propensity to aggregate and to be expressed at low levels, studies comprising authentic gp41 produced in eukaryotic cells are extremely rare. Here we describe the production of a secreted, soluble recombinant gp41 in 293 cells. The antigen was purified to homogeneity and characterised thoroughly by various biochemical and immunological methods. It was shown that the protein was glycosylated and assembled into trimers. Binding studies by ELISA and surface plasmon resonance using conformation-specific monoclonal antibodies implied a six-helix bundle conformation. The low binding of broadly neutralising antibodies (bnAb) directed against the membrane proximal external region (MPER) suggested that this gp41 is probably not suited as vaccine to induce such bnAb. Purified gp41 bound to monocytes and to a lesser extent to lymphocytes and triggered the production of specific cytokines when added to normal peripheral blood mononuclear cells. In addition, gp41 expressed on target cells inhibited the antigen-specific response of murine CD8+ T cells by drastically impairing their IFNγ production. To our knowledge, this is the first comprehensive analysis of a gp41 produced in eukaryotic cells including its immunosuppressive properties. Our data provide another line of evidence that gp41 might be directly involved in HIV-1 immunopathogenesis through modulation of the cytokine release and active inhibition of immune responses.

A Lipopeptide HIV-1/2 Fusion Inhibitor with Highly Potent In Vitro, Ex Vivo, and In Vivo Antiviral Activity.

Peptides derived from the C-terminal heptad repeat (CHR) region of the human immunodeficiency virus type 1 (HIV-1) fusogenic protein gp41 are potent viral entry inhibitors, and currently, enfuvirtide (T-20) is the only one approved for clinical use; however, emerging drug resistance largely limits its efficacy. In this study, we generated a novel lipopeptide inhibitor, named LP-19, by integrating multiple design strategies, including an N-terminal M-T hook structure, an HIV-2 sequence, intrahelical salt bridges, and a membrane-anchoring lipid tail. LP-19 showed stable binding affinity and highly potent, broad, and long-lasting antiviral activity. In in vitro studies, LP-19 efficiently inhibited HIV-1-, HIV-2-, and simian immunodeficiency virus (SIV)-mediated cell fusion, viral entry, and infection, and it was highly active against diverse subtypes of primary HIV-1 isolates and inhibitor-resistant mutants. Ex vivo studies demonstrated that LP-19 exhibited dramatically increased anti-HIV activity and an extended half-life in rhesus macaques. In short-term monotherapy, LP-19 reduced viral loads to undetectable levels in acutely and chronically simian-human immunodeficiency virus (SHIV)-infected monkeys. Therefore, this study offers an ideal HIV-1/2 fusion inhibitor for clinical development and emphasizes the importance of the viral fusion step as a drug target.IMPORTANCE The peptide drug T-20 is the only viral fusion inhibitor in the clinic, which is used for combination therapy of HIV-1 infection; however, it requires a high dosage and easily induces drug resistance, calling for a new drug with significantly improved pharmaceutical profiles. Here, we have developed a short-lipopeptide-based fusion inhibitor, termed LP-19, which mainly targets the conserved gp41 pocket site and shows highly potent inhibitory activity against HIV-1, HIV-2, and even SIV isolates. LP-19 exhibits dramatically increased antiviral activity and an extended half-life in rhesus macaques, and it has potent therapeutic efficacy in SHIV-infected monkeys, highlighting its high potential as a new viral fusion inhibitor for clinical use.