Cholesterol Constrains the Antigenic Configuration of the Membrane-Proximal Neutralizing HIV-1 Epitope

Generation of a Nonbilayer Lipid Nanoenvironment after Epitope Binding Potentiates Neutralizing HIV-1 MPER Antibody

Establishment of interactions with the envelope lipids is a cardinal feature of broadly neutralizing antibodies (bnAbs) that recognize the Env membrane-proximal external region (MPER) of HIV. The lipid envelope constitutes a relevant component of the full "quinary" MPER epitope, and thus antibodies may be optimized through engineering their capacity to interact with lipids. However, the role of the chemically complex lipid nanoenvironment in the mechanism of MPER molecular recognition and viral neutralization remains poorly understood. To approach this issue, we computationally and experimentally investigated lipid interactions of broadly neutralizing antibody 10E8 and optimized versions engineered to enhance their epitope and membrane affinity by grafting bulky aromatic compounds. Our data revealed a correlation between neutralization potency and the establishment of favorable interactions with small headgroup lipids cholesterol and phosphatidylethanolamine, evolving after specific engagement with MPER. Molecular dynamics simulations of chemically modified Fabs in complex with an MPER-Transmembrane Domain helix supported the generation of a nanoenvironment causing localized deformation of the thick, rigid viral membrane and identified sphingomyelin preferentially occupying a phospholipid-binding site of 10E8. Together, these interactions appear to facilitate insertion of the Fabs through their engagement with the MPER epitope. These findings implicate individual lipid molecules in the neutralization function of MPER bnAbs, validate targeted chemical modification as a method to optimize MPER antibodies, and suggest pathways for MPER peptide-liposome vaccine development.

Influence of membrane on the antigen presentation of the HIV-1 envelope membrane proximal external region (MPER)

The membrane proximal external region (MPER) of the HIV envelope glycoproteins has generated renewed interest after a recent phase I vaccine trial that presented MPER lipid-peptide epitopes demonstrated promise to elicit a broad neutralization response. The antigenicity of MPER is intimately associated with the membrane, and its presentation relies significantly on the lipid composition. This review brings together recent findings on the influence of membranes on the conformation of MPER and its recognition by broadly neutralizing antibodies. Specifically, the review highlights the importance of properly accounting for the balance between protein-protein and membrane-protein interactions in vaccine design.

Viroporin-like activity of the hairpin transmembrane domain of African swine fever virus B169L protein

African swine fever virus (ASFV) is the causative agent of a contagious disease affecting wild and domestic swine. The function of B169L protein, as a potential integral structural membrane protein, remains to be experimentally characterized. Using state-of-the-art bioinformatics tools, we confirm here earlier predictions indicating the presence of an integral membrane helical hairpin, and further suggest anchoring of this protein to the ER membrane, with both terminal ends facing the lumen of the organelle. Our evolutionary analysis confirmed the importance of purifying selection in the preservation of the identified domains during the evolution of B169L in nature. Also, we address the possible function of this hairpin transmembrane domain (HTMD) as a class IIA viroporin. Expression of GFP fusion proteins in the absence of a signal peptide supported B169L insertion into the ER as a Type III membrane protein and the formation of oligomers therein. Overlapping peptides that spanned the B169L HTMD were reconstituted into ER-like membranes and the adopted structures analyzed by infrared spectroscopy. Consistent with the predictions, B169L transmembrane sequences adopted α-helical conformations in lipid bilayers. Moreover, single vesicle permeability assays demonstrated the assembly of lytic pores in ER-like membranes by B169L transmembrane helices, a capacity confirmed by ion-channel activity measurements in planar bilayers. Emphasizing the relevance of these observations, pore-forming activities were not observed in the case of transmembrane helices derived from EP84R, another ASFV protein predicted to anchor to membranes through a α-helical HTMD. Overall, our results support predictions of viroporin-like function for the B169L HTMD.IMPORTANCEAfrican swine fever (ASF), a devastating disease affecting domestic swine, is widely spread in Eurasia, producing significant economic problems in the pork industry. Approaches to prevent/cure the disease are mainly restricted to the limited information concerning the role of most of the genes encoded by the large (160-170 kba) virus genome. In this report, we present the experimental data on the functional characterization of the African swine fever virus (ASFV) gene B169L. Data presented here indicates that the B169L gene encodes for an essential membrane-associated protein with a viroporin function.

A two-step mechanism for the binding of the HIV-1 MPER epitope by the 10E8 antibody onto biosensor-supported lipid bilayers

HIV-1 antibodies targeting the carboxy-terminal area of the membrane-proximal external region (ctMPER) are close to exerting viral pan-neutralization. Here, we reconstituted the ctMPER epitope as the N-terminal extremity of the Env glycoprotein transmembrane domain helix and immobilized it onto biosensor-supported lipid bilayers. We assessed the binding mechanism of anti-MPER antibody 10E8 through Surface Plasmon Resonance, and found, through equilibrium and kinetic binding analyses as a function of bilayer thickness, peptide length, and paratope mutations, that 10E8 engages first with the epitope peptide (encounter), limited by ctMPER helix accessibility at the membrane surface, and then inserts into the lipid bilayer assisted by favorable Fab-membrane interactions (docking). This mechanistic information may help in devising new strategies to develop more efficient MPER-targeting vaccines.

African Swine Fever Virus Gene B117L Encodes a Small Protein Endowed with Low-pH-Dependent Membrane Permeabilizing Activity

African swine fever virus (ASFV) is causing a devastating pandemic in domestic and wild swine in Central Europe to East Asia, resulting in economic losses for the swine industry. The virus contains a large double-stranded DNA genome that contains more than 150 genes, most with no experimentally characterized function. In this study, we evaluate the potential function of the product of ASFV gene B117L, a 115-amino-acid integral membrane protein transcribed at late times during the virus replication cycle and showing no homology to any previously published protein. Hydrophobicity distribution along B117L confirmed the presence of a single transmembrane helix, which, in combination with flanking amphipathic sequences, composes a potential membrane-associated C-terminal domain of ca. 50 amino acids. Ectopic transient cell expression of the B117L gene as a green fluorescent protein (GFP) fusion protein revealed the colocalization with markers of the endoplasmic reticulum (ER). Intracellular localization of various B117L constructs also displayed a pattern for the formation of organized smooth ER (OSER) structures compatible with the presence of a single transmembrane helix with a cytoplasmic carboxy terminus. Using partially overlapping peptides, we further demonstrated that the B117L transmembrane helix has the capacity to establish spores and ion channels in membranes at low pH. Furthermore, our evolutionary analysis showed the high conservation of the transmembrane domain during the evolution of the B117L gene, indicating that the integrity of this domain is preserved by the action of the purifying selection. Collectively our data support a viroporin-like assistant role for the B117L gene-encoded product in ASFV entry. IMPORTANCE ASFV is responsible for an extensively distributed pandemic causing important economic losses in the pork industry in Eurasia. The development of countermeasures is partially limited by the insufficient knowledge regarding the function of the majority of the more than 150 genes present on the virus genome. Here, we provide data regarding the functional experimental evaluation of a previously uncharacterized ASFV gene, B117L. Our data suggest that the B117L gene encodes a small membrane protein that assists in the permeabilization of the ER-derived envelope during ASFV infection.

Molecular recognition of a membrane-anchored HIV-1 pan-neutralizing epitope

Antibodies against the carboxy-terminal section of the membrane-proximal external region (C-MPER) of the HIV-1 envelope glycoprotein (Env) are considered as nearly pan-neutralizing. Development of vaccines capable of producing analogous broadly neutralizing antibodies requires deep understanding of the mechanism that underlies C-MPER recognition in membranes. Here, we use the archetypic 10E8 antibody and a variety of biophysical techniques including single-molecule approaches to study the molecular recognition of C-MPER in membrane mimetics. In contrast to the assumption that an interfacial MPER helix embodies the entire C-MPER epitope recognized by 10E8, our data indicate that transmembrane domain (TMD) residues contribute to binding affinity and specificity. Moreover, anchoring to membrane the helical C-MPER epitope through the TMD augments antibody binding affinity and relieves the effects exerted by the interfacial MPER helix on the mechanical stability of the lipid bilayer. These observations support that addition of TMD residues may result in more efficient and stable anti-MPER vaccines.

Functional Delineation of a Protein-Membrane Interaction Hotspot Site on the HIV-1 Neutralizing Antibody 10E8

Antibody engagement with the membrane-proximal external region (MPER) of the envelope glycoprotein (Env) of HIV-1 constitutes a distinctive molecular recognition phenomenon, the full appreciation of which is crucial for understanding the mechanisms that underlie the broad neutralization of the virus. Recognition of the HIV-1 Env antigen seems to depend on two specific features developed by antibodies with MPER specificity: (i) a large cavity at the antigen-binding site that holds the epitope amphipathic helix; and (ii) a membrane-accommodating Fab surface that engages with viral phospholipids. Thus, besides the main Fab-peptide interaction, molecular recognition of MPER depends on semi-specific (electrostatic and hydrophobic) interactions with membranes and, reportedly, on specific binding to the phospholipid head groups. Here, based on available cryo-EM structures of Fab-Env complexes of the anti-MPER antibody 10E8, we sought to delineate the functional antibody-membrane interface using as the defining criterion the neutralization potency and binding affinity improvements induced by Arg substitutions. This rational, Arg-based mutagenesis strategy revealed the position-dependent contribution of electrostatic interactions upon inclusion of Arg-s at the CDR1, CDR2 or FR3 of the Fab light chain. Moreover, the contribution of the most effective Arg-s increased the potency enhancement induced by inclusion of a hydrophobic-at-interface Phe at position 100c of the heavy chain CDR3. In combination, the potency and affinity improvements by Arg residues delineated a protein-membrane interaction site, whose surface and position support a possible mechanism of action for 10E8-induced neutralization. Functional delineation of membrane-interacting patches could open new lines of research to optimize antibodies of therapeutic interest that target integral membrane epitopes.

HIV-1 Entry and Prospects for Protecting against Infection

Human Immunodeficiency Virus type-1 (HIV-1) establishes a latent viral reservoir soon after infection, which poses a major challenge for drug treatment and curative strategies. Many efforts are therefore focused on blocking infection. To this end, both viral and host factors relevant to the onset of infection need to be considered. Given that HIV-1 is most often transmitted mucosally, strategies designed to protect against infection need to be effective at mucosal portals of entry. These strategies need to contend also with cell-free and cell-associated transmitted/founder (T/F) virus forms; both can initiate and establish infection. This review will discuss how insight from the current model of HIV-1 mucosal transmission and cell entry has highlighted challenges in developing effective strategies to prevent infection. First, we examine key viral and host factors that play a role in transmission and infection. We then discuss preventive strategies based on antibody-mediated protection, with emphasis on targeting T/F viruses and mucosal immunity. Lastly, we review treatment strategies targeting viral entry, with focus on the most clinically advanced entry inhibitors.

Neutralizing Antibodies Targeting HIV-1 gp41

HIV-1 vaccine research has obtained an enormous boost since the discovery of many broadly neutralizing antibodies (bnAbs) targeting all accessible sites on the HIV-1 envelope glycoprotein (Env). This in turn facilitated high-resolution structures of the Env glycoprotein in complex with bnAbs. Here we focus on gp41, its highly conserved heptad repeat region 1 (HR1), the fusion peptide (FP) and the membrane-proximal external region (MPER). Notably, the broadest neutralizing antibodies target MPER. Both gp41 HR1 and MPER are only fully accessible once receptor-induced conformational changes have taken place, although some studies suggest access to MPER in the close to native Env conformation. We summarize the data on the structure and function of neutralizing antibodies targeting gp41 HR1, FP and MPER and we review their access to Env and their complex formation with gp41 HR1, MPER peptides and FP within native Env. We further discuss MPER bnAb binding to lipids and the role of somatic mutations in recognizing a bipartite epitope composed of the conserved MPER sequence and membrane components. The problematic of gp41 HR1 access and MPER bnAb auto- and polyreactivity is developed in the light of inducing such antibodies by vaccination.