Novel recombinant engineered gp41 N-terminal heptad repeat trimers and their potential as anti-HIV-1 therapeutics or microbicides

Discovery of a Double-Stapled Short Peptide as a Long-Acting HIV-1 Inactivator with Potential for Oral Bioavailability

Different from most antiretroviral drugs that act as passive defenders to inhibit HIV-1 replication inside the host cell, virus inactivators can attack and inactivate HIV-1 virions without relying on their replication cycle. Herein, we describe the discovery of a hydrocarbon double-stapled helix peptide, termed D26. D26 is based on the HIV-1 gp41 protein lentiviral lytic peptide-3 motif (LLP3) sequence, which can efficiently inhibit HIV-1 infection and inactivate cell-free HIV-1 virions. It was noted that D26 was highly resistant to proteolytic degradation and exhibited a remarkably extended in vivo elimination half-life. Additionally, relative to its linear, nonstapled version, D26 exhibited much higher exposure in sanctuary sites for HIV-1. Amazingly, this lead compound also demonstrated detectable oral absorption. Thus, it can be concluded that D26 is a promising candidate for further development as a long-acting, orally applicable HIV-1 inactivator for the treatment of HIV-1 infection.

Modulating the immune response to SARS-CoV-2 by different nanocarriers delivering an mRNA expressing trimeric RBD of the spike protein: COVARNA Consortium

Vaccines based on mRNA technology have revolutionized the field. In fact, lipid nanoparticles (LNP) formulated with mRNA are the preferential vaccine platform used in the fight against SARS-CoV-2 infection, with wider application against other diseases. The high demand and property right protection of the most potent cationic/ionizable lipids used for LNP formulation of COVID-19 mRNA vaccines have promoted the design of alternative nanocarriers for nucleic acid delivery. In this study we have evaluated the immunogenicity and efficacy of different rationally designed lipid and polymeric-based nanoparticle prototypes against SARS-CoV-2 infection. An mRNA coding for a trimeric soluble form of the receptor binding domain (RBD) of the spike (S) protein from SARS-CoV-2 was encapsulated using different components to form nanoemulsions (NE), nanocapsules (NC) and lipid nanoparticles (LNP). The toxicity and biological activity of these prototypes were evaluated in cultured cells after transfection and in mice following homologous prime/boost immunization. Our findings reveal good levels of RBD protein expression with most of the formulations. In C57BL/6 mice immunized intramuscularly with two doses of formulated RBD-mRNA, the modified lipid nanoparticle (mLNP) and the classical lipid nanoparticle (LNP-1) were the most effective delivery nanocarriers at inducing binding and neutralizing antibodies against SARS-CoV-2. Both prototypes fully protected susceptible K18-hACE2 transgenic mice from morbidity and mortality following a SARS-CoV-2 challenge. These results highlight that modulation of mRNAs immunogenicity can be achieved by using alternative nanocarriers and support further assessment of mLNP and LNP-1 prototypes as delivery vehicles for mRNA vaccines.

Computational design and engineering of self-assembling multivalent microproteins with therapeutic potential against SARS-CoV-2

Multivalent drugs targeting homo-oligomeric viral surface proteins, such as the SARS-CoV-2 trimeric spike (S) protein, have the potential to elicit more potent and broad-spectrum therapeutic responses than monovalent drugs by synergistically engaging multiple binding sites on viral targets. However, rational design and engineering of nanoscale multivalent protein drugs are still lacking. Here, we developed a computational approach to engineer self-assembling trivalent microproteins that simultaneously bind to the three receptor binding domains (RBDs) of the S protein. This approach involves four steps: structure-guided linker design, molecular simulation evaluation of self-assembly, experimental validation of self-assembly state, and functional testing. Using this approach, we first designed trivalent constructs of the microprotein miniACE2 (MP) with different trimerization scaffolds and linkers, and found that one of the constructs (MP-5ff) showed high trimerization efficiency, good conformational homogeneity, and strong antiviral neutralizing activity. With its trimerization unit (5ff), we then engineered a trivalent nanobody (Tr67) that exhibited potent and broad neutralizing activity against the dominant Omicron variants, including XBB.1 and XBB.1.5. Cryo-EM complex structure confirmed that Tr67 stably binds to all three RBDs of the Omicron S protein in a synergistic form, locking them in the "3-RBD-up" conformation that could block human receptor (ACE2) binding and potentially facilitate immune clearance. Therefore, our approach provides an effective strategy for engineering potent protein drugs against SARS-CoV-2 and other deadly coronaviruses.

A variant-proof SARS-CoV-2 vaccine targeting HR1 domain in S2 subunit of spike protein

The emerging SARS-CoV-2 variants, commonly with many mutations in S1 subunit of spike (S) protein are weakening the efficacy of the current vaccines and antibody therapeutics. This calls for the variant-proof SARS-CoV-2 vaccines targeting the more conserved regions in S protein. Here, we designed a recombinant subunit vaccine, HR121, targeting the conserved HR1 domain in S2 subunit of S protein. HR121 consisting of HR1-linker1-HR2-linker2-HR1, is conformationally and functionally analogous to the HR1 domain present in the fusion intermediate conformation of S2 subunit. Immunization with HR121 in rabbits and rhesus macaques elicited highly potent cross-neutralizing antibodies against SARS-CoV-2 and its variants, particularly Omicron sublineages. Vaccination with HR121 achieved near-full protections against prototype SARS-CoV-2 infection in hACE2 transgenic mice, Syrian golden hamsters and rhesus macaques, and effective protection against Omicron BA.2 infection in Syrian golden hamsters. This study demonstrates that HR121 is a promising candidate of variant-proof SARS-CoV-2 vaccine with a novel conserved target in the S2 subunit for application against current and future SARS-CoV-2 variants.

A highly potent and stable pan-coronavirus fusion inhibitor as a candidate prophylactic and therapeutic for COVID-19 and other coronavirus diseases

The development of broad-spectrum antivirals against human coronaviruses (HCoVs) is critical to combat the current coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants, as well as future outbreaks of emerging CoVs. We have previously identified a polyethylene glycol-conjugated (PEGylated) lipopeptide, EK1C4, with potent pan-CoV fusion inhibitory activity. However, PEG linkers in peptide or protein drugs may reduce stability or induce anti-PEG antibodies in vivo. Therefore, we herein report the design and synthesis of a series of dePEGylated lipopeptide-based pan-CoV fusion inhibitors featuring the replacement of the PEG linker with amino acids in the heptad repeat 2 C-terminal fragment (HR2-CF) of HCoV-OC43. Among these lipopeptides, EKL1C showed the most potent inhibitory activity against infection by SARS-CoV-2 and its spike (S) mutants, as well as other HCoVs and some bat SARS-related coronaviruses (SARSr-CoVs) tested. The dePEGylated lipopeptide EKL1C exhibited significantly stronger resistance to proteolytic enzymes, better metabolic stability in mouse serum, higher thermostability than the PEGylated lipopeptide EK1C4, suggesting that EKL1C could be further developed as a candidate prophylactic and therapeutic for COVID-19 and other coronavirus diseases.

A Five−Helix−Based SARS−CoV−2 Fusion Inhibitor Targeting Heptad Repeat 2 Domain against SARS−CoV−2 and Its Variants of Concern

The prolonged duration of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic has resulted in the continuous emergence of variants of concern (VOC, e.g., Omicron) and variants of interest (VOI, e.g., Lambda). These variants have challenged the protective efficacy of current COVID-19 vaccines, thus calling for the development of novel therapeutics against SARS-CoV-2 and its VOCs. Here, we constructed a novel fusion inhibitor-based recombinant protein, denoted as 5-Helix, consisting of three heptad repeat 1 (HR1) and two heptad repeat 2 (HR2) fragments. The 5-Helix interacted with the HR2 domain of the viral S2 subunit, the most conserved region in spike (S) protein, to block homologous six-helix bundle (6-HB) formation between viral HR1 and HR2 domains and, hence, viral S-mediated cell–cell fusion. The 5-Helix potently inhibited infection by pseudotyped SARS-CoV-2 and its VOCs, including Delta and Omicron variants. The 5-Helix also inhibited infection by authentic SARS-CoV-2 wild-type (nCoV-SH01) strain and its Delta variant. Collectively, our findings suggest that 5-Helix can be further developed as either a therapeutic or prophylactic to treat and prevent infection by SARS-CoV-2 and its variants.

Supercoiling Structure-Based Design of a Trimeric Coiled-Coil Peptide with High Potency against HIV-1 and Human β-Coronavirus Infection

Hexameric structure formation through packing of three C-terminal helices and an N-terminal trimeric coiled-coil core has been proposed as a general mechanism of class I enveloped virus entry. In this process, the C-terminal helical repeat (HR2) region of viral membrane fusion proteins becomes transiently exposed and accessible to N-terminal helical repeat (HR1) trimer-based fusion inhibitors. Herein, we describe a mimetic of the HIV-1 gp41 HR1 trimer, N3G, as a promising therapeutic against HIV-1 infection. Surprisingly, we found that in addition to protection against HIV-1 infection, N3G was also highly effective in inhibiting infection of human β-coronaviruses, including MERS-CoV, HCoV-OC43, and SARS-CoV-2, possibly by binding the HR2 region in the spike protein of β-coronaviruses to block their hexameric structure formation. These studies demonstrate the potential utility of anti-HIV-1 HR1 peptides in inhibiting human β-coronavirus infection. Moreover, this strategy could be extended to the design of broad-spectrum antivirals based on the supercoiling structure of peptides.

Surface-Functionalized Silica-Coated Calcium Phosphate Nanoparticles Efficiently Deliver DNA-Based HIV-1 Trimeric Envelope Vaccines against HIV-1

Human immunodeficiency virus type 1 (HIV-1) infection remains one of the worst crises in global health. The prevention of HIV-1 infection is a crucial task that needs to be addressed due to the absence of a licensed vaccine against HIV-1. DNA vaccines present a promising alternative approach to combat HIV-1 infection due to their excellent safety profile, lack of severe side effects, and relatively rapid fabrication. Traditional vaccines composed of a monomeric envelope or peptide fragments have been indicated to lack protective efficacy mediated by inducing HIV-1-specific neutralizing antibodies in clinical trials. The immunogenicity and protection against HIV-1 induced by DNA vaccines are limited due to the poor uptake of these vaccines by antigen-presenting cells and their ready degradation by DNases and lysosomes. To address these issues of naked DNA vaccines, we described the feasibility of CpG-functionalized silica-coated calcium phosphate nanoparticles (SCPs) for efficiently delivering DNA-based HIV-1 trimeric envelope vaccines against HIV-1. Vaccines comprising the soluble BG505 SOSIP.664 trimer fused to the GCN4-based isoleucine zipper or bacteriophage T4 fibritin foldon motif with excellent simulation of the native HIV-1 envelope were chosen as trimer-based vaccine platforms. Our results showed that SCP-based DNA immunization could significantly induce both broad humoral immune responses and potent cellular immune responses compared to naked DNA vaccination in vivo. To the best of our knowledge, this study is the first to assess the feasibility of CpG-functionalized SCPs for efficiently delivering DNA vaccines expressing a native-like HIV-1 trimer. These CpG-functionalized SCPs for delivering DNA-based HIV-1 trimeric envelope vaccines may lead to the development of promising vaccine candidates against HIV-1.

Inhibition of viral suppressor of RNAi proteins by designer peptides protects from enteroviral infection in vivo

RNA interference (RNAi) is the major antiviral mechanism in plants and invertebrates, but the absence of detectable viral (v)siRNAs in mammalian cells upon viral infection has questioned the functional relevance of this pathway in mammalian immunity. We designed a series of peptides specifically targeting enterovirus A71 (EV-A71)-encoded protein 3A, a viral suppressor of RNAi (VSR). These peptides abrogated the VSR function of EV-A71 in infected cells and resulted in the accumulation of vsiRNAs and reduced viral replication. These vsiRNAs were functional, as evidenced by RISC-loading and silencing of target RNAs. The effects of VSR-targeting peptides (VTPs) on infection with EV-A71 as well as another enterovirus, Coxsackievirus-A16, were ablated upon deletion of Dicer1 or AGO2, core components of the RNAi pathway. In vivo, VTP treatment protected mice against lethal EV-A71 challenge, with detectable vsiRNAs. Our findings provide evidence for the functional relevance of RNAi in mammalian immunity and present a therapeutic strategy for infectious disease.

Extremely Thermostabilizing Core Mutations in Coiled-Coil Mimetic Proteins of HIV-1 gp41 Produce Diverse Effects on Target Binding but Do Not Affect Their Inhibitory Activity

A promising strategy to neutralize HIV-1 is to target the gp41 spike subunit to block membrane fusion with the cell. We previously designed a series of single-chain proteins (named covNHR) that mimic the trimeric coiled-coil structure of the gp41 N-terminal heptad repeat (NHR) region and potently inhibit HIV-1 cell infection by avidly binding the complementary C-terminal heptad repeat (CHR) region. These proteins constitute excellent tools to understand the structural and thermodynamic features of this therapeutically important interaction. Gp41, as with many coiled-coil proteins, contains in core positions of the NHR trimer several highly conserved, buried polar residues, the role of which in gp41 structure and function is unclear. Here we produced three covNHR mutants by substituting each triad of polar residues for the canonical isoleucine. The mutants preserve their helical structure and show an extremely increased thermal stability. However, increased hydrophobicity enhances their self-association. Calorimetric analyses show a marked influence of mutations on the binding thermodynamics of CHR-derived peptides. The mutations do not affect however the in vitro HIV-1 inhibitory activity of the proteins. The results support a role of buried core polar residues in maintaining structural uniqueness and promoting an energetic coupling between conformational stability and NHR-CHR binding.

Probing Vulnerability of the gp41 C-Terminal Heptad Repeat as Target for Miniprotein HIV Inhibitors

One of the therapeutic strategies in HIV neutralization is blocking membrane fusion. In this process, tight interaction between the N-terminal and C-terminal heptad-repeat (NHR and CHR) regions of gp41 is essential to promote membranes apposition and merging. We have previously developed single-chain proteins (named covNHR) that accurately mimic the complete gp41 NHR region in its trimeric conformation. They tightly bind CHR-derived peptides and show a potent and broad HIV inhibitory activity in vitro. However, the extremely high binding affinity (sub-picomolar) is not in consonance with their inhibitory activity (nanomolar), likely due to partial or temporal accessibility of their target in the virus. Here, we have designed and characterized two single-chain covNHR miniproteins each encompassing one of the two halves of the NHR region and containing two of the four sub-pockets of the NHR crevice. The two miniproteins fold as trimeric helical bundles as expected but while the C-terminal covNHR (covNHR-C) miniprotein is highly stable, the N-terminal counterpart (covNHR-N) shows only marginal stability that could be improved by engineering an internal disulfide bond. Both miniproteins bind their respective complementary CHR peptides with moderate (micromolar) affinity. Moreover, the covNHR-N miniproteins can access their target in the context of trimeric native envelope proteins and show significant inhibitory activity for several HIV pseudoviruses. In contrast, covNHR-C cannot bind its target sequence and neither inhibits HIV, indicating a higher vulnerability of C-terminal part of CHR. These results may guide the development of novel HIV inhibitors targeting the gp41 CHR region.

Vaccine effect of recombinant single-chain hemagglutinin protein as an antigen

Vaccination is one of the most effective interventions for preventing the spread of influenza viruses at the population level. Currently most influenza vaccines are produced by using embryonated chicken eggs, but alternative methods that achieve more rapid large-scale production are highly desirable for vaccines against both pandemic and seasonal influenza viruses. The use of recombinant hemagglutinin (HA), a key virus surface protein, as an antigen is an attractive candidate alternative approach, because of the potential for high protein yields and the ease of cloning new antigenic variants. Although fusion of HA with trimerization domains is needed to stabilize the trimeric structure and enhance the immunogenicity of the recombinant HA protein, whether the trimerization domains are immunogenic must be considered. Here, we generated recombinant multimeric HA without trimerization domains by using a short peptide linker, termed a single-chain HA (scHA), and evaluated scHAs as potential antigens for generating vaccines against influenza virus. Using mammalian cells, we succeeded in making three types of recombinant scHAs—two dimeric scHAs and a trimeric scHA. After immunization with aluminium salts in mice, one of the dimeric scHAs induced the greatest HA-specific IgG response among the scHAs and protected against virus challenge as strongly as the typically used trimeric HA containing a trimerization domain. We did not observe IgGs specific for the short peptide linker in mice immunized with the dimeric scHA, although IgGs specific for the trimerization domain occurred in mice immunized with the trimeric HA containing that domain. Furthermore, changing to another adjuvant did not diminish the utility of the dimeric scHA. These results suggest the potential usefulness of dimeric scHA as a vaccine antigen. We believe that single-chain antigens may represent new alternatives for production of recombinant antigen–based vaccines.

Broad-Spectrum Coronavirus Fusion Inhibitors to Combat COVID-19 and Other Emerging Coronavirus Diseases

In the past 17 years, three novel coronaviruses have caused severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and the coronavirus disease 2019 (COVID-19). As emerging infectious diseases, they were characterized by their novel pathogens and transmissibility without available clinical drugs or vaccines. This is especially true for the newly identified COVID-19 caused by SARS coronavirus 2 (SARS-CoV-2) for which, to date, no specific antiviral drugs or vaccines have been approved. Similar to SARS and MERS, the lag time in the development of therapeutics is likely to take months to years. These facts call for the development of broad-spectrum anti-coronavirus drugs targeting a conserved target site. This review will systematically describe potential broad-spectrum coronavirus fusion inhibitors, including antibodies, protease inhibitors, and peptide fusion inhibitors, along with a discussion of their advantages and disadvantages.

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.

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.

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.

Potent MERS-CoV Fusion Inhibitory Peptides Identified from HR2 Domain in Spike Protein of Bat Coronavirus HKU4

The Middle East respiratory syndrome coronavirus (MERS-CoV) emerged in 2012 and caused continual outbreaks worldwide with high mortality. However, no effective anti-MERS-CoV drug is currently available. Recently, numerous evolutionary studies have suggested that MERS-CoV originated from bat coronavirus (BatCoV). We herein reported that three peptides derived from the HR2 region in spike protein of BatCoV HKU4, including HKU4-HR2P1, HKU4-HR2P2 and HKU4-HR2P3, could bind the MERS-CoV HR1-derived peptide to form a six-helix bundle (6-HB) with high stability. Moreover, these peptides, particularly HKU4-HR2P2 and HKU4-HR2P3, exhibited potent inhibitory activity against MERS-CoV S-mediated cell–cell fusion and viral infection, suggesting that these HKU4 HR2-derived peptides could be candidates for futher development as antiviral agents against MERS-CoV infection.

Investigation of the inhibition effect of arachidonic acid on the core structure of the HIV-1 gp41

The gp41 transmembrane domain of the envelope glycoprotein of the human immunodeficiency virus (HIV) modulates the conformation of the viral envelope spike. During the HIV fusion process, C-terminal heptad repeat (CHR, C34) wrap antiparallel to the N-terminal heptad repeat (NHR, N36) helices to form a stable six-helix bundle (6-HB) core structure, which brings the viral and cell membranes into close proximity for fusion. Therefore, inhibiting the formation of 6-HB is considered to be a key activity of an effective HIV-1 fusion inhibitor. The level of arachidonic acid (AA) is increased in HIV infected patients. Our study provides a new insight into the functional role of AA during the formation of HIV-1 gp41 6-HB. Native polyacrylamide gel electrophoresis (N-PAGE), enzyme-linked-immunosorbent serologic assay (ELISA) and circular dichroism (CD) spectroscopy were used to investigate the inhibition of AA for the formation of 6-HB. Molecular docking technique was adopted to explore the underlying mechanism. HIV-1 JR-FL (R5 strain) Envelope was adopted to determine the inhibition effect of AA. AA is shown to interfere with the formation of α-helical complexes of N36 and C34 by N-PAGE, ELISA and CD spectroscopy. The isotherm titration microcalorimetry (ITC) results indicate there is a single class of binding site on N36. ΔH and ΔS are -12.43 kJ mol^-1 and 70.07 J mol^-1 K^-1, respectively, indicating hydrophobic interaction and electrostatic forces are the main acting forces. The molecular docking results manifest that AA interacts with the hydrophobic residues (Trp-571, Leu-568, Val-570 and Leu-576) and ionic interactions occur between Arg-579 and the -COOH of AA. The inhibitory activity of AA on HIV-1 JR-FL is quantified by 50% effective concentration (EC₅₀) and 90% effective concentration (EC₉₀), which are 31.42 ± 1.08 and 133.47 ± 18.10 μg mL^-1, respectively. All the results indicate that AA is able to inhibit the formation of 6-HB but cannot disrupt the preformed 6-HB. Therefore, AA is a potential inhibitor for the viral fusion/entry.

Peptide-Based Membrane Fusion Inhibitors Targeting HCoV-229E Spike Protein HR1 and HR2 Domains

Human coronavirus 229E (HCoV-229E) infection in infants, elderly people, and immunocompromised patients can cause severe disease, thus calling for the development of effective and safe therapeutics to treat it. Here we reported the design, synthesis and characterization of two peptide-based membrane fusion inhibitors targeting HCoV-229E spike protein heptad repeat 1 (HR1) and heptad repeat 2 (HR2) domains, 229E-HR1P and 229E-HR2P, respectively. We found that 229E-HR1P and 229E-HR2P could interact to form a stable six-helix bundle and inhibit HCoV-229E spike protein-mediated cell-cell fusion with IC₅₀ of 5.7 and 0.3 µM, respectively. 229E-HR2P effectively inhibited pseudotyped and live HCoV-229E infection with IC₅₀ of 0.5 and 1.7 µM, respectively. In a mouse model, 229E-HR2P administered intranasally could widely distribute in the upper and lower respiratory tracts and maintain its fusion-inhibitory activity. Therefore, 229E-HR2P is a promising candidate for further development as an antiviral agent for the treatment and prevention of HCoV-229E infection.

Intranasal vaccination of recombinant H5N1 HA1 proteins fused with foldon and Fc induces strong mucosal immune responses with neutralizing activity: Implication for developing novel mucosal influenza vaccines

The highly pathogenic avian influenza (HPAI) H5N1 virus remains a threat to public health because of its continued spread in poultry in some countries and its ability to infect humans with high mortality rate, calling for the development of effective and safe vaccines against H5N1 infection. Here, we constructed 4 candidate vaccines by fusing H5N1 hemagglutinin 1 (HA1) with foldon (HA1-Fd), human IgG Fc (HA1-Fc), foldon and Fc (HA1-FdFc) or His-tag (HA1-His). We then compared their ability to induce mucosal immune responses and neutralizing antibodies in the presence or absence of Poly(I:C) and CpG adjuvants via the intranasal route. Without an adjuvant, HA1-FdFc could elicit appreciable humoral immune responses and local mucosal IgA antibodies in immunized mice, while other vaccine candidates only induced background immune responses. In the presence of Poly(I:C) and CpG, both HA1-Fd and HA1-Fc elicited much higher levels of serum IgG and local mucosal IgA antibodies than HA1-His. Poly(I:C) and CpG could also augment the neutralizing antibody responses induced by these 4 vaccine candidates in the order of HA1-FdFc > HA1-Fc > HA1-Fd > HA1-His. These results suggest that both Fd and Fc potentiate the immunogenicity of the recombinant HA1 protein and that Poly(I:C) and CpG serve as efficient mucosal adjuvants in promoting efficacy of these vaccine candidates to induce strong systemic and local antibody responses and potent neutralizing antibodies, providing a useful strategy to develop effective and safe mucosal H5N1 vaccines.

Site-specific Isopeptide Bridge Tethering of Chimeric gp41 N-terminal Heptad Repeat Helical Trimers for the Treatment of HIV-1 Infection

Peptides derived from the N-terminal heptad repeat (NHR) of HIV-1 gp41 can be potent inhibitors against viral entry when presented in a nonaggregating trimeric coiled-coil conformation via the introduction of exogenous trimerization motifs and intermolecular disulfide bonds. We recently discovered that crosslinking isopeptide bridges within the de novo helical trimers added exceptional resistance to unfolding. Herein, we attempted to optimize (CCIZN17)₃, a representative disulfide bond-stabilized chimeric NHR-trimer, by incorporating site-specific interhelical isopeptide bonds as the redox-sensitive disulfide surrogate. In this process, we systematically examined the effect of isopeptide bond position and molecular sizes of auxiliary trimeric coiled-coil motif and NHR fragments on the antiviral potency of these NHR-trimers. Pleasingly, (IZ14N24N)₃ possessed promising inhibitory activity against HIV-1 infection and markedly increased proteolytic stability relative to its disulfide-tethered counterpart, suggesting good potential for further development as an effective antiviral agent for treatment of HIV-1 infection.

An effective strategy for recapitulating N-terminal heptad repeat trimers in enveloped virus surface glycoproteins for therapeutic applications

Sequestering peptides derived from the N-terminal heptad repeat (NHR) of class I viral fusion proteins into a non-aggregating trimeric coiled-coil conformation remains a major challenge. Here, we implemented a synthetic strategy to stabilize NHR-helical trimers, with the human immunodeficiency virus type 1 (HIV-1) gp41 fusion protein as the initial focus. A set of trimeric scaffolds was realized in a synthetic gp41 NHR-derived peptide sequence by relying on the tractability of coiled-coil structures and an additional isopeptide bridge-tethering strategy. Among them, (N36M)3 folded as a highly stable helical trimer and exhibited promising inhibitory activity against HIV-1 infection, exceptional resistance to proteolysis, and effective native ligand-binding capability. We anticipate that the trimeric coiled-coil recapitulation methodology described herein may have broader applicability to yield NHR trimers of other class I enveloped viruses and to prepare helical tertiary structure mimetics of certain natural protein-protein interactions for biomedical applications.

Development of Small-molecule HIV Entry Inhibitors Specifically Targeting gp120 or gp41

Human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein surface subunit gp120 and transmembrane subunit gp41 play important roles in HIV-1 entry, thus serving as key targets for the development of HIV-1 entry inhibitors. T20 peptide (enfuvirtide) is the first U.S. FDA-approved HIV entry inhibitor; however, its clinical application is limited by the lack of oral availability. Here, we have described the structure and function of the HIV-1 gp120 and gp41 subunits and reviewed advancements in the development of small-molecule HIV entry inhibitors specifically targeting these two Env glycoproteins. We then compared the advantages and disadvantages of different categories of HIV entry inhibitor candidates and further predicted the future trend of HIV entry inhibitor development.

Structure-based discovery of Middle East respiratory syndrome coronavirus fusion inhibitor

A novel human coronavirus, Middle East respiratory syndrome coronavirus (MERS-CoV), has caused outbreaks of a SARS-like illness with high case fatality rate. The reports of its person-to-person transmission through close contacts have raised a global concern about its pandemic potential. Here we characterize the six-helix bundle fusion core structure of MERS-CoV spike protein S2 subunit by X-ray crystallography and biophysical analysis. We find that two peptides, HR1P and HR2P, spanning residues 998–1039 in HR1 and 1251–1286 in HR2 domains, respectively, can form a stable six-helix bundle fusion core structure, suggesting that MERS-CoV enters into the host cell mainly through membrane fusion mechanism. HR2P can effectively inhibit MERS-CoV replication and its spike protein-mediated cell–cell fusion. Introduction of hydrophilic residues into HR2P results in significant improvement of its stability, solubility and antiviral activity. Therefore, the HR2P analogues have good potential to be further developed into effective viral fusion inhibitors for treating MERS-CoV infection.

MERS-CoV is a novel human coronavirus that has recently caused outbreaks of respiratory illness with high case fatality rate. Here the authors characterize the membrane fusion apparatus of MERS-CoV and develop a peptide that can inhibit virus fusion and replication in vitro.

De novo design of isopeptide bond-tethered triple-stranded coiled coils with exceptional resistance to unfolding and proteolysis: implication for developing antiviral therapeutics

Isopeptide bond-tethered triple-stranded coiled coils of HIV-1 gp41 N-terminal heptad repeat (NHR) peptides have been designed with de novo auxiliaries to guide site-directed trimerized cross-linking. The presence of isopeptide bridges in the rationally designed trimerization motifs provides extraordinary stability to withstand thermal and chemical denaturation. As a result, these ultra-stable and well-folded trimeric coiled coils direct and yield proteolysis-resistant and remarkably potent N-peptide chimeric trimers with HIV-1 fusion inhibitory activities in the low nanomolar range, much more effective than the corresponding unstructured N-peptide monomers and reaching the potency of clinically used T20 peptide (enfuvirtide). Thus, these isopeptide bond-crosslinked de novo coiled coils may also be used as attractive scaffolds for isolating NHR-trimers in other class I enveloped viruses for therapeutic intervention. Furthermore, this isopeptide bridge-tethering strategy could be extendable to the construction of ultra-stable proteins interfering with certain biological processes.

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.

Single-chain protein mimetics of the N-terminal heptad-repeat region of gp41 with potential as anti-HIV-1 drugs

During HIV-1 fusion to the host cell membrane, the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) of the envelope subunit gp41 become transiently exposed and accessible to fusion inhibitors or Abs. In this process, the NHR region adopts a trimeric coiled-coil conformation that can be a target for therapeutic intervention. Here, we present an approach to rationally design single-chain protein constructs that mimic the NHR coiled-coil surface. The proteins were built by connecting with short loops two parallel NHR helices and an antiparallel one with the inverse sequence followed by engineering of stabilizing interactions. The constructs were expressed in Escherichia coli, purified with high yield, and folded as highly stable helical coiled coils. The crystal structure of one of the constructs confirmed the predicted fold and its ability to accurately mimic an exposed gp41 NHR surface. These single-chain proteins bound to synthetic CHR peptides with very high affinity, and furthermore, they showed broad inhibitory activity of HIV-1 fusion on various pseudoviruses and primary isolates.

Peptide fusion inhibitors targeting the HIV-1 gp41: a patent review (2009 - 2014)

INTRODUCTION

As the first peptide HIV fusion inhibitor targeting gp41, enfuvirtide (T20) was approved by the US FDA in 2003 as a salvage therapy for HIV/AIDS patients who failed to respond to the then existing antiretroviral therapeutics. However, its clinical application is limited by its relatively low potency, low genetic barrier to drug resistance and short half-life. Therefore, it is essential to develop new peptide HIV fusion inhibitors with improved antiviral efficacy, drug-resistance profile and pharmaceutical properties.

AREAS COVERED

In this paper, we reviewed the patents, patent applications and related research articles for the development of new peptide fusion inhibitors targeting the HIV-1 gp41 published between 2009 and 2014.

EXPERT OPINION

To improve enfuvirtide's anti-HIV efficacy, drug-resistance profile, half-life and pharmaceutical properties, the best approaches include the addition of the pocket-binding domain (PBD) to the N-terminus of T20 and linking of the M-T hook to the N-terminus of PBD, as well as conjugation of cholesterol, serum albumin-binding motif or gp120-binding fragment with a PBD-containing C-terminal heptad repeat-peptide. Therefore, sifuvirtide from Tianjin FusoGen Pharmaceuticals, Inc., albuvirtide from Frontier Biotechnologies Co., Ltd., cholesterol-conjugated HIV fusion inhibitor from the Institute of Pathogen Biology, Chinese Academy of Medical Science, 2DLT, a bivalent HIV fusion inhibitor/inactivator, and an enfuvirtide/sifuvirtide combination regimen from the New York Blood Center may all have potential as next-generation HIV fusion inhibitors targeting gp41 for clinical use.

ADS-J1 inhibits HIV-1 infection and membrane fusion by targeting the highly conserved pocket in the gp41 NHR-trimer

We previously identified a potent small-molecule human immunodeficiency virus type 1 (HIV-1) fusion inhibitor, termed ADS-J1, and hypothesized that it mainly targeted the hydrophobic pocket in the gp41 N-terminal heptad repeat (NHR) trimer. However, this hypothesis has been challenged by the fact that ADS-J1 cannot induce drug-resistance mutation in the gp41 pocket region. Therefore, we show herein that HIV-1 mutants resistant to T2635, a peptide derived from the gp41 C-terminal heptad repeat (CHR) region with pocket-binding domain (PBD), were also resistant to ADS-J1. We also show that pseudoviruses with mutations at positions 64 and 67 in the gp41 pocket region were highly resistant to ADS-J1 and C34, another CHR-peptide with PBD, but relatively sensitive to T20, a CHR-peptide without PBD. ADS-J1 could effectively bind to N36Fd, a mimic of the gp41 NHR-trimer with pocket exposed, and block binding of C34 to N36Fd trimer to form six-helix bundle (6-HB). However, ADS-J1 was less effective in binding to N36Fd trimer with mutations in the gp41 pocket region, such as N36(Q64A)Fd, N36(Q64L)Fd, N36(A67G)Fd, N36(A67S)Fd, and N36(Q66R)Fd, as well as less effective in blocking 6-HB formation between C34 and these mutant N36Fd trimers. These results confirm that ADS-J1 mainly targets the pocket region in the HIV-1 gp41 NHR trimer and suggest that it could be used as a lead for developing small-molecule HIV fusion inhibitors and as a molecule probe for studying the mechanisms of gp41-mediated membrane fusion.

Peptide HIV fusion inhibitors: modifications and conjugations

HIV fusion inhibitors are a group of virus entry preventing drugs aimed at membrane fusion.

F(ab')2 fragment of a gp41 NHR-trimer-induced IgM monoclonal antibody neutralizes HIV-1 infection and blocks viral fusion by targeting the conserved gp41 pocket

Using a recombinant protein N46FdFc that mimics the HIV-1 gp41 N-helix trimer to immunize mice, we identified the first IgM monoclonal antibody 18D3 that specifically bound to the conserved gp41 pocket. Its F(ab')2 fragment potently inhibited HIV-1 Env-mediated cell-cell fusion and neutralized infection by laboratory-adapted and primary HIV-1 isolates with different subtypes and tropism, including the T20-resistant variants. This F(ab')2 fragment can be used to develop a bispecific broad neutralizing monoclonal antibody or HIV-1 inactivator as a novel immunotherapeutic for treatment and prevention of HIV-1 infection.

Identification of a human protein-derived HIV-1 fusion inhibitor targeting the gp41 fusion core structure

The HIV-1 envelope glycoprotein (Env) gp41 plays a crucial role in the viral fusion process. The peptides derived from the C-terminal heptad repeat (CHR) of gp41 are potent HIV fusion inhibitors. However, the activity of these anti-HIV-1 peptides in vivo may be attenuated by their induction of anti-gp41 antibodies. Thus, it is essential to identify antiviral peptides or proteins with low, or no, immunogenicity to humans. Here, we found that the C-terminal fragment (aa 462–521) of the human POB1 (the partner of RalBP1), designated C60, is an HIV-1 fusion inhibitor. It bound to N36, the peptide derived from the N-terminal heptad repeat (NHR) of gp41, and to the six-helix bundle (6-HB) formed by N36 and C34, a CHR-peptide, but it did not bind to C34. Unlike the CHR-peptides, C60 did not block gp41 6-HB formation. Rather, results suggest that C60 inhibits HIV-1 fusion by binding to the 6-HB, in particular, the residues in the gp41 NHR domain that are exposed on the surface of 6-HB. Since 6-HB plays a crucial role in the late stage of fusion between the viral envelope and endosomal membrane during the endocytic process of HIV-1, C60 may serve as a host restriction factor to suppress HIV-1 entry into CD4+ T lymphocytes. Taken together, it can be concluded from these results that C60 can be used as a lead for the development of anti-HIV-1 therapeutics or microbicides for the treatment and prevention of HIV-1 infection, as well as a molecular probe to study the fusogenic mechanism of HIV-1.

Escape from human immunodeficiency virus type 1 (HIV-1) entry inhibitors

The human immunodeficiency virus (HIV) enters cells through a series of molecular interactions between the HIV envelope protein and cellular receptors, thus providing many opportunities to block infection. Entry inhibitors are currently being used in the clinic, and many more are under development. Unfortunately, as is the case for other classes of antiretroviral drugs that target later steps in the viral life cycle, HIV can become resistant to entry inhibitors. In contrast to inhibitors that block viral enzymes in intracellular compartments, entry inhibitors interfere with the function of the highly variable envelope glycoprotein as it continuously adapts to changing immune pressure and available target cells in the extracellular environment. Consequently, pathways and mechanisms of resistance for entry inhibitors are varied and often involve mutations across the envelope gene. This review provides a broad overview of entry inhibitor resistance mechanisms that inform our understanding of HIV entry and the design of new inhibitors and vaccines.

Side chain requirements for affinity and specificity in D5, an HIV-1 antibody derived from the VH1-69 germline segment

Background

Analysis of factors contributing to high affinity antibody-protein interactions provides insight into natural antibody evolution, and guides the design of antibodies with new or enhanced function. We previously studied the interaction between antibody D5 and its target, a designed protein based on HIV-1 gp41 known as 5-Helix, as a model system [Da Silva, G. F.; Harrison, J. S.; Lai, J. R., Biochemistry, 2010, 49, 5464–5472]. Antibody D5 represents an interesting case study because it is derived from the V_H1-69 germline segment; this germline segment is characterized by a hydrophobic second heavy chain complementarity determining region (HCDR2) that constitutes the major functional paratope in D5 and several antibodies derived from the same progenitor.

Results

Here we explore side chain requirements for affinity and specificity in D5 using phage display. Two D5-based libraries were prepared that contained diversity in all three light chain complementarity determining regions (LCDRs 1–3), and in the third HCDR (HCDR3). The first library allowed residues to vary among a restricted set of six amino acids (Tyr/Ala/Asp/Ser/His/Pro; D5-Lib-I). The second library was designed based on a survey of existing V_H1-69 antibody structures (D5-Lib-II). Both libraries were subjected to multiple rounds of selection against 5-Helix, and individual clones characterized. We found that selectants from D5-Lib-I generally had moderate affinity and specificity, while many clones from D5-Lib-II exhibited D5-like properties. Additional analysis of the D5-Lib-II functional population revealed position-specific biases for particular amino acids, many that differed from the identity of those side chains in D5.

Conclusions

Together these results suggest that there is some permissiveness for alternative side chains in the LCDRs and HCDR3 of D5, but that replacement with a minimal set of residues is not tolerated in this scaffold for 5-Helix recognition. This work provides novel information about this high-affinity interaction involving an antibody from the V_H1-69 germline segment.

Approaches for identification of HIV-1 entry inhibitors targeting gp41 pocket

The hydrophobic pocket in the HIV-1 gp41 N-terminal heptad repeat (NHR) domain plays an important role in viral fusion and entry into the host cell, and serves as an attractive target for development of HIV-1 fusion/entry inhibitors. The peptide anti-HIV drug targeting gp41 NHR, T-20 (generic name: enfuvirtide; brand name: Fuzeon), was approved by the U.S. FDA in 2003 as the first HIV fusion/entry inhibitor for treatment of HIV/AIDS patients who fail to respond to the current antiretroviral drugs. However, because T20 lacks the pocket-binding domain (PBD), it exhibits low anti-HIV-1 activity and short half-life. Therefore, several next-generation HIV fusion inhibitory peptides with PBD have been developed. They possess longer half-life and more potent antiviral activity against a broad spectrum of HIV-1 strains, including the T-20-resistant variants. Nonetheless, the clinical application of these peptides is still limited by the lack of oral availability and the high cost of production. Thus, development of small molecule compounds targeting the gp41 pocket with oral availability has been promoted. This review describes the main approaches for identification of HIV fusion/entry inhibitors targeting the gp41 pocket and summarizes the latest progress in developing these inhibitors as a new class of anti-HIV drugs.

A critical HA1 neutralizing domain of H5N1 influenza in an optimal conformation induces strong cross-protection

The highly pathogenic avian influenza (HPAI) H5N1 viruses, especially the laboratory-generated H5N1 mutants, have demonstrated the potential to cross the species barrier and infect mammals and humans. Consequently, the design of an effective and safe anti-H5N1 vaccine is essential. We previously demonstrated that the full-length hemagglutinin 1 (HA1) could induce significant neutralizing antibody response and protection. Here, we intended to identify the critical neutralizing domain (CND) in an optimal conformation that can elicit strong cross-neutralizing antibodies and protection against divergent H5N1 strains. We thus constructed six recombinant proteins covering different regions of HA1 of A/Anhui/1/2005(H5N1), each of which was fused with foldon (Fd) and Fc of human IgG. We found that the critical fragment fused with Fd/Fc (HA-13–263-Fdc, H5 numbering) that could elicit the strongest neutralizing antibody response is located in the N-terminal region of HA1 (residues 13–263), which covers the receptor-binding domain (RBD, residues 112–263). We then constructed three additional recombinants fused with Fd plus His tag (HA-13–263-Fd-His), Fc only (HA-13–263-Fc), and His tag only (HA-13–263-His), respectively. We found that the HA-13–263-Fdc, which formed an oligomeric conformation, induced the strongest neutralizing antibody response and cross-protection against challenges of two tested H5N1 virus strains covering clade 1: A/VietNam/1194/2004 (VN/1194) or clade 2.3.4: A/Shenzhen/406H/06 (SZ/406H), while HA-13–263-Fc dimer and HA-13–263-Fd-His trimer elicited higher neutralizing antibody response and protection than HA-13–263-His monomer. These results suggest that the oligomeric form of the CND containing the RBD can be further developed as an effective and safe vaccine for cross-protection against divergent strains of H5N1 viruses.

Unusual biophysics of intrinsically disordered proteins

Research of a past decade and a half leaves no doubt that complete understanding of protein functionality requires close consideration of the fact that many functional proteins do not have well-folded structures. These intrinsically disordered proteins (IDPs) and proteins with intrinsically disordered protein regions (IDPRs) are highly abundant in nature and play a number of crucial roles in a living cell. Their functions, which are typically associated with a wide range of intermolecular interactions where IDPs possess remarkable binding promiscuity, complement functional repertoire of ordered proteins. All this requires a close attention to the peculiarities of biophysics of these proteins. In this review, some key biophysical features of IDPs are covered. In addition to the peculiar sequence characteristics of IDPs these biophysical features include sequential, structural, and spatiotemporal heterogeneity of IDPs; their rough and relatively flat energy landscapes; their ability to undergo both induced folding and induced unfolding; the ability to interact specifically with structurally unrelated partners; the ability to gain different structures at binding to different partners; and the ability to keep essential amount of disorder even in the bound form. IDPs are also characterized by the "turned-out" response to the changes in their environment, where they gain some structure under conditions resulting in denaturation or even unfolding of ordered proteins. It is proposed that the heterogeneous spatiotemporal structure of IDPs/IDPRs can be described as a set of foldons, inducible foldons, semi-foldons, non-foldons, and unfoldons. They may lose their function when folded, and activation of some IDPs is associated with the awaking of the dormant disorder. It is possible that IDPs represent the "edge of chaos" systems which operate in a region between order and complete randomness or chaos, where the complexity is maximal. This article is part of a Special Issue entitled: The emerging dynamic view of proteins: Protein plasticity in allostery, evolution and self-assembly.

Biochemistry and biophysics of HIV-1 gp41 - membrane interactions and implications for HIV-1 envelope protein mediated viral-cell fusion and fusion inhibitor design

Human immunodeficiency virus type 1 (HIV-1), the pathogen of acquired immunodeficiency syndrome (AIDS), causes ~2 millions death every year and still defies an effective vaccine. HIV-1 infects host cells through envelope protein - mediated virus-cell fusion. The transmembrane subunit of envelope protein, gp41, is the molecular machinery which facilitates fusion. Its ectodomain contains several distinguishing functional domains, fusion peptide (FP), Nterminal heptad repeat (NHR), C-terminal heptad repeat (CHR) and membrane proximal extracellular region (MPER). During the fusion process, FP inserts into the host cell membrane, and an extended gp41 prehairpin conformation bridges the viral and cell membranes through MPER and FP respectively. Subsequent conformational change of the unstable prehairpin results in a coiled-coil 6-helix bundle (6HB) structure formed between NHR and CHR. The energetics of 6HB formation drives membrane apposition and fusion. Drugs targeting gp41 functional domains to prevent 6HB formation inhibit HIV-1 infection. T20 (enfuvirtide, Fuzeon) was approved by the US FDA in 2003 as the first fusion inhibitor. It is a 36-residue peptide from the gp41 CHR, and it inhibits 6HB formation by targeting NHR and lipids. Development of new fusion inhibitors, especially small molecule drugs, is encouraged to overcome the shortcomings of T20 as a peptide drug. Hydrophobic characteristics and membrane association are critical for gp41 function and mechanism of action. Research in gp41-membrane interactions, using peptides corresponding to specific functional domains, or constructs including several interactive domains, are reviewed here to get a better understanding of gp41 mediated virus-cell fusion that can inform or guide the design of new HIV-1 fusion inhibitors.

Targeting HIV-1 gp41-induced fusion and pathogenesis for anti-viral therapy

HIV gp41 is a metastable protein whose native conformation is maintained in the form of a heterodimer with gp120. The non-covalently associated gp41/gp120 complex forms a trimer on the virus surface. As gp120 engages with HIV's receptor, CD4, and coreceptor, CXCR4 or CCR5, gp41 undergoes several conformational changes resulting in fusion between the viral and cellular membranes. Several lipophilic and amphiphilic domains have been shown to be critical in that process. While the obvious function of gp41 in viral entry is well-established its role in cellular membrane fusion and the link with pathogenesis are only now beginning to appear. Recent targeting of gp41 via fusion inhibitors has revealed an important role of this protein not only in viral entry but also in bystander apoptosis and HIV pathogenesis. Studies by our group and others have shown that the phenomenon of gp41-mediated hemifusion initiates apoptosis in bystander cells and correlates with virus pathogenesis. More interestingly, recent clinical evidence suggests that gp41 mutants arising after Enfuvirtide therapy are associated with CD4 cell increase and immunological benefits. This has in turn been correlated to a decrease in bystander apoptosis in our in vitro as well as in vivo assays. Although a great deal of work has been done to unravel HIV-1 gp41-mediated fusion mechanisms, the factors that regulate gp41-mediated fusion versus hemifusion and the mechanism by which hemifusion initiates bystander apoptosis are not fully understood. Further insight into these issues will open new avenues for drug development making gp41 a critical anti-HIV target both for neutralization and virus attenuation.

Mucosal antibodies induced by tandem repeat of 2F5 epitope block transcytosis of HIV-1

Induction of mucosal antibodies to prevent HIV infection is an important strategy for the HIV-1 prophylaxis. Here we report an epitope-vaccine based antigen that was able to elicit mucosal antibodies capable of blocking HIV-1 transcytosis. Because the ELDKWA epitope of neutralizing antibody 2F5 plays a crucial role in transcytosis, a series of immunogens that contain tandem copies of ELDKWA were prepared. Mice were immunized with these immunogens intranasally, and received intraperitoneal+intranasal boosters. The immunogens that contained more ELDKWA epitopes elicited higher level of mucosal ELDKWA-epitope specific IgAs and systemic IgGs. Although the antisera from the immunized mice exhibited mild neutralizing potency to HIV-1 isolates HXB2 and JRFL, the affinity purified mucosal ELDKWA-epitope specific antibodies could block the transcytosis of cell-free CNE3 (a primary isolate of subtype CRF01_AE) in human tight epithelial models.

A recombinant vaccine of H5N1 HA1 fused with foldon and human IgG Fc induced complete cross-clade protection against divergent H5N1 viruses

Development of effective vaccines to prevent influenza, particularly highly pathogenic avian influenza (HPAI) caused by influenza A virus (IAV) subtype H5N1, is a challenging goal. In this study, we designed and constructed two recombinant influenza vaccine candidates by fusing hemagglutinin 1 (HA1) fragment of A/Anhui/1/2005(H5N1) to either Fc of human IgG (HA1-Fc) or foldon plus Fc (HA1-Fdc), and evaluated their immune responses and cross-protection against divergent strains of H5N1 virus. Results showed that these two recombinant vaccines induced strong immune responses in the vaccinated mice, which specifically reacted with HA1 proteins and an inactivated heterologous H5N1 virus. Both proteins were able to cross-neutralize infections by one homologous strain (clade 2.3) and four heterologous strains belonging to clades 0, 1, and 2.2 of H5N1 pseudoviruses as well as three heterologous strains (clades 0, 1, and 2.3.4) of H5N1 live virus. Importantly, immunization with these two vaccine candidates, especially HA1-Fdc, provided complete cross-clade protection against high-dose lethal challenge of different strains of H5N1 virus covering clade 0, 1, and 2.3.4 in the tested mouse model. This study suggests that the recombinant fusion proteins, particularly HA1-Fdc, could be developed into an efficacious universal H5N1 influenza vaccine, providing cross-protection against infections by divergent strains of highly pathogenic H5N1 virus.

In vitro selection and characterization of HIV-1 variants with increased resistance to sifuvirtide, a novel HIV-1 fusion inhibitor

Sifuvirtide, a novel fusion inhibitor against human immunodeficiency virus type I (HIV-1), which is more potent than enfuvirtide (T20) in cell culture, is currently under clinical investigation for the treatment of HIV-1 infection. We now report that in vitro selection of HIV-1 variants resistant to sifuvirtide in the presence of increasing concentrations of sifuvirtide has led to several specific mutations in the gp41 region that had not been previously reported. Many of these substitutions were confined to the N-terminal heptad repeat region at positions 37, 38, 41, and 43, either singly or in combination. A downstream substitution at position 126 (N126K) in the C-terminal heptad repeat region was also found. Site-directed mutagenesis studies have further identified the critical amino acid substitutions and combinations thereof in conferring the resistant genotypes. Furthermore, the mutant viruses demonstrated variable degrees of cross-resistance to enfuvirtide, some of which are preferentially more resistant to sifuvirtide. Impaired infectivity was also found for many of the mutant viruses. Biophysical and structural analyses of the key substitutions have revealed several potential novel mechanisms against sifuvirtide. Our results may help to predict potential resistant patterns in vivo and facilitate the further clinical development and therapeutic utility of sifuvirtide.