Vaccine elicitation and structural basis for antibody protection against alphaviruses.

Alphaviruses are RNA viruses that represent emerging public health threats. To identify protective antibodies, we immunized macaques with a mixture of western, eastern, and Venezuelan equine encephalitis virus-like particles (VLPs), a regimen that protects against aerosol challenge with all three viruses. Single- and triple-virus-specific antibodies were isolated, and we identified 21 unique binding groups. Cryo-EM structures revealed that broad VLP binding inversely correlated with sequence and conformational variability. One triple-specific antibody, SKT05, bound proximal to the fusion peptide and neutralized all three Env-pseudotyped encephalitic alphaviruses by using different symmetry elements for recognition across VLPs. Neutralization in other assays (e.g., chimeric Sindbis virus) yielded variable results. SKT05 bound backbone atoms of sequence-diverse residues, enabling broad recognition despite sequence variability; accordingly, SKT05 protected mice against Venezuelan equine encephalitis virus, chikungunya virus, and Ross River virus challenges. Thus, a single vaccine-elicited antibody can protect in vivo against a broad range of alphaviruses.

Antibody Lineages with Vaccine-Induced Antigen-Binding Hotspots Develop Broad HIV Neutralization.

The vaccine-mediated elicitation of antibodies (Abs) capable of neutralizing diverse HIV-1 strains has been a long-standing goal. To understand how broadly neutralizing antibodies (bNAbs) can be elicited, we identified, characterized, and tracked five neutralizing Ab lineages targeting the HIV-1-fusion peptide (FP) in vaccinated macaques over time. Genetic and structural analyses revealed two of these lineages to belong to a reproducible class capable of neutralizing up to 59% of 208 diverse viral strains. B cell analysis indicated each of the five lineages to have been initiated and expanded by FP-carrier priming, with envelope (Env)-trimer boosts inducing cross-reactive neutralization. These Abs had binding-energy hotspots focused on FP, whereas several FP-directed Abs induced by immunization with Env trimer-only were less FP-focused and less broadly neutralizing. Priming with a conserved subregion, such as FP, can thus induce Abs with binding-energy hotspots coincident with the target subregion and capable of broad neutralization.

Vaccination in a humanized mouse model elicits highly protective PfCSP-targeting anti-malarial antibodies.

Repeat antigens, such as the Plasmodium falciparum circumsporozoite protein (PfCSP), use both sequence degeneracy and structural diversity to evade the immune response. A few PfCSP-directed antibodies have been identified that are effective at preventing malaria infection, including CIS43, but how these repeat-targeting antibodies might be improved has been unclear. Here, we engineered a humanized mouse model in which B cells expressed inferred human germline CIS43 (iGL-CIS43) B cell receptors and used both vaccination and bioinformatic analysis to obtain variant CIS43 antibodies with improved protective capacity. One such antibody, iGL-CIS43.D3, was significantly more potent than the current best-in-class PfCSP-directed antibody. We found that vaccination with a junctional epitope peptide was more effective than full-length PfCSP at recruiting iGL-CIS43 B cells to germinal centers. Structure-function analysis revealed multiple somatic hypermutations that combinatorically improved protection. This mouse model can thus be used to understand vaccine immunogens and to develop highly potent anti-malarial antibodies.

A Potent Anti-Malarial Human Monoclonal Antibody Targets Circumsporozoite Protein Minor Repeats and Neutralizes Sporozoites in the Liver.

Discovering potent human monoclonal antibodies (mAbs) targeting the Plasmodium falciparum circumsporozoite protein (PfCSP) on sporozoites (SPZ) and elucidating their mechanisms of neutralization will facilitate translation for passive prophylaxis and aid next-generation vaccine development. Here, we isolated a neutralizing human mAb, L9 that preferentially bound NVDP minor repeats of PfCSP with high affinity while cross-reacting with NANP major repeats. L9 was more potent than six published neutralizing human PfCSP mAbs at mediating protection against mosquito bite challenge in mice. Isothermal titration calorimetry and multiphoton microscopy showed that L9 and the other most protective mAbs bound PfCSP with two binding events and mediated protection by killing SPZ in the liver and by preventing their egress from sinusoids and traversal of hepatocytes. This study defines the subdominant PfCSP minor repeats as neutralizing epitopes, identifies an in vitro biophysical correlate of SPZ neutralization, and demonstrates that the liver is an important site for antibodies to prevent malaria.

Extrapolating differential scanning calorimetry data for monoclonal antibodies to low temperatures.

For irreversible denaturation transitions such as those exhibited by monoclonal antibodies, differential scanning calorimetry provides the denaturation temperature, Tm, the rate of denaturation at Tm, and the activation energy at Tm. These three quantities are essential but not sufficient for an accurate extrapolation of the rate of denaturation to temperatures of 25 °C and below. We have observed that the activation energy is not constant but temperature dependent due to the existence of an activation heat capacity, Cp,a. It is shown in this paper that a model that incorporates Cp,a is able to account for previous observations like, for example, that increasing the Tm does not always improve the stability at low temperatures; that some antibodies exhibit lower stabilities at 5 °C than at 25 °C; or that low temperature stabilities do not follow the rank order derived from Tm values. Most importantly, the activation heat capacity model is able to reproduce time dependent stabilities measured by size exclusion chromatography at low temperatures.

Mutational fitness landscapes reveal genetic and structural improvement pathways for a vaccine-elicited HIV-1 broadly neutralizing antibody.

Vaccine-based elicitation of broadly neutralizing antibodies holds great promise for preventing HIV-1 transmission. However, the key biophysical markers of improved antibody recognition remain uncertain in the diverse landscape of potential antibody mutation pathways, and a more complete understanding of anti-HIV-1 fusion peptide (FP) antibody development will accelerate rational vaccine designs. Here we survey the mutational landscape of the vaccine-elicited anti-FP antibody, vFP16.02, to determine the genetic, structural, and functional features associated with antibody improvement or fitness. Using site-saturation mutagenesis and yeast display functional screening, we found that 1% of possible single mutations improved HIV-1 envelope trimer (Env) affinity, but generally comprised rare somatic hypermutations that may not arise frequently in vivo. We observed that many single mutations in the vFP16.02 Fab could enhance affinity >1,000-fold against soluble FP, although affinity improvements against the HIV-1 trimer were more measured and rare. The most potent variants enhanced affinity to both soluble FP and Env, had mutations concentrated in antibody framework regions, and achieved up to 37% neutralization breadth compared to 28% neutralization of the template antibody. Altered heavy- and light-chain interface angles and conformational dynamics, as well as reduced Fab thermal stability, were associated with improved HIV-1 neutralization breadth and potency. We also observed parallel sets of mutations that enhanced viral neutralization through similar structural mechanisms. These data provide a quantitative understanding of the mutational landscape for vaccine-elicited FP-directed broadly neutralizing antibody and demonstrate that numerous antigen-distal framework mutations can improve antibody function by enhancing affinity simultaneously toward HIV-1 Env and FP.

Development of high-affinity nanobodies specific for NaV1.4 and NaV1.5 voltage-gated sodium channel isoforms.

Voltage-gated sodium channels, NaVs, are responsible for the rapid rise of action potentials in excitable tissues. NaV channel mutations have been implicated in several human genetic diseases, such as hypokalemic periodic paralysis, myotonia, and long-QT and Brugada syndromes. Here, we generated high-affinity anti-NaV nanobodies (Nbs), Nb17 and Nb82, that recognize the NaV1.4 (skeletal muscle) and NaV1.5 (cardiac muscle) channel isoforms. These Nbs were raised in llama (Lama glama) and selected from a phage display library for high affinity to the C-terminal (CT) region of NaV1.4. The Nbs were expressed in Escherichia coli, purified, and biophysically characterized. Development of high-affinity Nbs specifically targeting a given human NaV isoform has been challenging because they usually show undesired crossreactivity for different NaV isoforms. Our results show, however, that Nb17 and Nb82 recognize the CTNaV1.4 or CTNaV1.5 over other CTNav isoforms. Kinetic experiments by biolayer interferometry determined that Nb17 and Nb82 bind to the CTNaV1.4 and CTNaV1.5 with high affinity (KD ∼ 40-60 nM). In addition, as proof of concept, we show that Nb82 could detect NaV1.4 and NaV1.5 channels in mammalian cells and tissues by Western blot. Furthermore, human embryonic kidney cells expressing holo NaV1.5 channels demonstrated a robust FRET-binding efficiency for Nb17 and Nb82. Our work lays the foundation for developing Nbs as anti-NaV reagents to capture NaVs from cell lysates and as molecular visualization agents for NaVs.

Protective effects of combining monoclonal antibodies and vaccines against the Plasmodium falciparum circumsporozoite protein.

Combinations of monoclonal antibodies (mAbs) against different epitopes on the same antigen synergistically neutralize many viruses. However, there are limited studies assessing whether combining human mAbs against distinct regions of the Plasmodium falciparum (Pf) circumsporozoite protein (CSP) enhances in vivo protection against malaria compared to each mAb alone or whether passive transfer of PfCSP mAbs would improve protection following vaccination against PfCSP. Here, we isolated a panel of human mAbs against the subdominant C-terminal domain of PfCSP (C-CSP) from a volunteer immunized with radiation-attenuated Pf sporozoites. These C-CSP-specific mAbs had limited binding to sporozoites in vitro that was increased by combination with neutralizing human "repeat" mAbs against the NPDP/NVDP/NANP tetrapeptides in the central repeat region of PfCSP. Nevertheless, passive transfer of repeat- and C-CSP-specific mAb combinations did not provide enhanced protection against in vivo sporozoite challenge compared to repeat mAbs alone. Furthermore, combining potent repeat-specific mAbs (CIS43, L9, and 317) that respectively target the three tetrapeptides (NPDP/NVDP/NANP) did not provide additional protection against in vivo sporozoite challenge. However, administration of either CIS43, L9, or 317 (but not C-CSP-specific mAbs) to mice that had been immunized with R21, a PfCSP-based virus-like particle vaccine that induces polyclonal antibodies against the repeat region and C-CSP, provided enhanced protection against sporozoite challenge when compared to vaccine or mAbs alone. Collectively, this study shows that while combining mAbs against the repeat and C-terminal regions of PfCSP provide no additional protection in vivo, repeat mAbs do provide increased protection when combined with vaccine-induced polyclonal antibodies. These data should inform the implementation of PfCSP human mAbs alone or following vaccination to prevent malaria infection.

Impact of temperature on the affinity of SARS-CoV-2 Spike glycoprotein for host ACE2.

The seasonal nature of outbreaks of respiratory viral infections with increased transmission during low temperatures has been well established. Accordingly, temperature has been suggested to play a role on the viability and transmissibility of SARS-CoV-2, the virus responsible for the COVID-19 pandemic. The receptor-binding domain (RBD) of the Spike glycoprotein is known to bind to its host receptor angiotensin-converting enzyme 2 (ACE2) to initiate viral fusion. Using biochemical, biophysical, and functional assays to dissect the effect of temperature on the receptor-Spike interaction, we observed a significant and stepwise increase in RBD-ACE2 affinity at low temperatures, resulting in slower dissociation kinetics. This translated into enhanced interaction of the full Spike glycoprotein with the ACE2 receptor and higher viral attachment at low temperatures. Interestingly, the RBD N501Y mutation, present in emerging variants of concern (VOCs) that are fueling the pandemic worldwide (including the B.1.1.7 (α) lineage), bypassed this requirement. This data suggests that the acquisition of N501Y reflects an adaptation to warmer climates, a hypothesis that remains to be tested.

Reversibility and irreversibility in the temperature denaturation of monoclonal antibodies.

There have been numerous studies of the temperature denaturation of monoclonal antibodies (mAbs) using differential scanning calorimetry (DSC). In general, mAbs are characterized by complex temperature denaturation transitions in which the various domains (CH2, CH3, Fab) give rise to different peaks in the heat capacity function. The complexity and overall irreversibility of the temperature denaturation transition is well known and has limited the number of publications with an in-depth analysis of the data. Here we report that the temperature denaturation of the CH2 domain is reversible and only becomes irreversible after denaturation of the Fab domain, which is intrinsically irreversible. For these studies we have used the HIV neutralizing monoclonal antibody 17b. To account for the experimental heat capacity function, a mixed denaturation model that combines multiple reversible and irreversible transitions has been developed. This model accounts well for the DSC data and for the pH dependence of the heat capacity function of 17b and other monoclonal antibodies for which data is available in the literature. It is expected that a more detailed analysis of the stability of monoclonal antibodies will contribute to the development of better approaches to understand and optimize the structural viability of these therapeutic macromolecules.

SOSIP Changes Affect Human Immunodeficiency Virus Type 1 Envelope Glycoprotein Conformation and CD4 Engagement.

The entry of human immunodeficiency virus into host cells is mediated by the envelope glycoprotein (Env) trimeric spike, which consists of three exterior gp120 subunits and three transmembrane gp41 subunits. The trimeric Env undergoes extensive conformational rearrangement upon interaction with the CD4 receptor, transitioning from the unliganded, "closed" State 1 to more-open downstream State 2 and State 3 conformations. Changes in "restraining" amino acid residues, such as leucine 193 and isoleucine 423, destabilize State 1 Env, which then assumes entry-competent, downstream conformations. The introduction of an artificial disulfide bond linking the gp120 and gp41 subunits (SOS) in combination with the I559P (IP) change has allowed structural characterization of soluble gp140 (sgp140) trimers. The conformation of these SOSIP-stabilized sgp140 trimers has been suggested to represent the closed native State 1 conformation. Here we compare the impact on the membrane Env conformation of the SOSIP changes with that of the well-characterized changes (L193R and I423A) that shift Env to downstream States 2 and 3. The results presented here suggest that the SOSIP changes stabilize Env in a conformation that differs from State 1 but also from the downstream Env conformations stabilized by L193R or I423A.IMPORTANCE The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer is triggered by receptor binding to mediate the entry of the virus into cells. Most structural studies of Env trimers have utilized truncated soluble gp140 Envs stabilized with the I559P and SOS changes. Here we present evidence indicating that these stabilizing changes have a profound impact on the conformation of Env, moving Env away from the native pretriggered Env conformation. Our studies underscore the need to acquire structural information on the pretriggered Env conformation, which is recognized by most broadly reactive neutralizing antibodies.

Crystal structures of trimeric HIV envelope with entry inhibitors BMS-378806 and BMS-626529.

The HIV-1 envelope (Env) spike is a conformational machine that transitions between prefusion (closed, CD4- and CCR5-bound) and postfusion states to facilitate HIV-1 entry into cells. Although the prefusion closed conformation is a potential target for inhibition, development of small-molecule leads has been stymied by difficulties in obtaining structural information. Here, we report crystal structures at 3.8-Å resolution of an HIV-1-Env trimer with BMS-378806 and a derivative BMS-626529 for which a prodrug version is currently in Phase III clinical trials. Both lead candidates recognized an induced binding pocket that was mostly excluded from solvent and comprised of Env elements from a conserved helix and the ß20-21 hairpin. In both structures, the ß20-21 region assumed a conformation distinct from prefusion-closed and CD4-bound states. Together with biophysical and antigenicity characterizations, the structures illuminate the allosteric and competitive mechanisms by which these small-molecule leads inhibit CD4-induced structural changes in Env.

A novel lipoate attachment enzyme is shared by Plasmodium and Chlamydia species.

Lipoate is an essential cofactor for enzymes that are important for central metabolism and other processes. In malaria parasites, scavenged lipoate from the human host is required for survival. The Plasmodium falciparum mitochondrion contains two enzymes (PfLipL1 and PfLipL2) that are responsible for activating mitochondrial proteins through the covalent attachment of lipoate (lipoylation). Lipoylation occurs via a novel redox-gated mechanism that remains poorly understood. We show that PfLipL1 functions as a redox switch that determines which downstream proteins will be activated. Based on the lipoate redox state, PfLipL1 either functions as a canonical lipoate ligase or as a lipoate activating enzyme which works in conjunction with PfLipL2. We demonstrate that PfLipL2 is a lipoyltransferase and is a member of a novel clade of lipoate attachment enzymes. We show that a LipL2 enzyme from Chlamydia trachomatis has similar activity, demonstrating conservation between intracellular pathogens from different phylogenetic kingdoms and supporting the hypothesis that an early ancestor of malaria parasites once contained a chlamydial endosymbiont. Redox-dependent lipoylation may regulate processes such as central metabolism and oxidative defense pathways.

Long term stability of a HIV-1 neutralizing monoclonal antibody using isothermal calorimetry.

Different factors affect the long term stability of monoclonal antibodies, among them denaturation or partial denaturation that is often followed by aggregation. Isothermal calorimetry is capable of quantifying the kinetics of denaturation/aggregation of an antibody by measuring the heat that is released or absorbed by the process over a period of days or weeks, at temperatures below its denaturation temperature, Tm. The denaturation/aggregation kinetics of the anti-HIV monoclonal antibody VRC07-523LS was measured by isothermal calorimetry at different concentrations in four different formulation buffers. The measurements were performed at ten degrees below Tm, as determined by differential scanning calorimetry. The formation of aggregates was also followed by size exclusion chromatography at 5 °C, 25 °C and 40 °C over a period of 8-36 weeks. It was observed that the rates measured by isothermal calorimetry correlate quantitatively with those measured by size exclusion chromatography. Since isothermal calorimetry experiments are performed over a period of ten days, it can become a valuable tool for a fast prediction of the best formulations.

CD4 mimetics sensitize HIV-1-infected cells to ADCC.

HIV-1-infected cells presenting envelope glycoproteins (Env) in the CD4-bound conformation on their surface are preferentially targeted by antibody-dependent cell-mediated cytotoxicity (ADCC). HIV-1 has evolved a sophisticated mechanism to avoid exposure of ADCC-mediating Env epitopes by down-regulating CD4 and by limiting the overall amount of Env at the cell surface. Here we report that small-molecule CD4-mimetic compounds induce the CD4-bound conformation of Env, and thereby sensitize cells infected with primary HIV-1 isolates to ADCC mediated by antibodies present in sera, cervicovaginal lavages, and breast milk from HIV-1-infected individuals. Importantly, we identified one CD4 mimetic with the capacity to sensitize endogenously infected ex vivo-amplified primary CD4 T cells to ADCC killing mediated by autologous sera and effector cells. Thus, CD4 mimetics hold the promise of therapeutic utility in preventing and controlling HIV-1 infection.

Three easy pieces.

BACKGROUND: Differential scanning calorimetry is a powerful method that provides a complete thermodynamic characterization of the stability of a protein as a function of temperature. There are, however, circumstances that preclude a complete analysis of DSC data. The most common ones are irreversible denaturation transitions or transitions that take place at temperatures that are beyond the temperature limit of the instrument. Even for a protein that undergoes reversible thermal denaturation, the extrapolation of the thermodynamic data to lower temperatures, usually 25°C, may become unreliable due to difficulties in the determination of ΔCp. METHODS: The combination of differential scanning calorimetry and isothermal chemical denaturation allows reliable thermodynamic analysis of protein stability under less than ideal conditions. RESULTS AND CONCLUSIONS: This paper demonstrates how DSC can be used in combination with chemical denaturation to address three different scenarios: 1) estimation of an accurate ΔCp value for a reversible denaturation using as a test system the envelope HIV-1 glycoprotein gp120; 2) determination of the Gibbs energy of stability in the region in which thermal denaturation is irreversible using HEW lysozyme at different pH values; and, 3) determination of Gibbs energy of stability for a thermostable protein, thermolysin.

Temperature stability of proteins: Analysis of irreversible denaturation using isothermal calorimetry.

The structural stability of proteins has been traditionally studied under conditions in which the folding/unfolding reaction is reversible, since thermodynamic parameters can only be determined under these conditions. Achieving reversibility conditions in temperature stability experiments has often required performing the experiments at acidic pH or other nonphysiological solvent conditions. With the rapid development of protein drugs, the fastest growing segment in the pharmaceutical industry, the need to evaluate protein stability under formulation conditions has acquired renewed urgency. Under formulation conditions and the required high protein concentration (∼100 mg/mL), protein denaturation is irreversible and frequently coupled to aggregation and precipitation. In this article, we examine the thermal denaturation of hen egg white lysozyme (HEWL) under irreversible conditions and concentrations up to 100 mg/mL using several techniques, especially isothermal calorimetry which has been used to measure the enthalpy and kinetics of the unfolding and aggregation/precipitation at 12°C below the transition temperature measured by DSC. At those temperatures the rate of irreversible protein denaturation and aggregation of HEWL is measured to be on the order of 1 day-1 . Isothermal calorimetry appears a suitable technique to identify buffer formulation conditions that maximize the long term stability of protein drugs.

Single-Chain Soluble BG505.SOSIP gp140 Trimers as Structural and Antigenic Mimics of Mature Closed HIV-1 Env.

UNLABELLED: Similar to other type I fusion machines, the HIV-1 envelope glycoprotein (Env) requires proteolytic activation; specifically, cleavage of a gp160 precursor into gp120 and gp41 subunits creates an N-terminal gp41 fusion peptide and permits folding from an immature uncleaved state to a mature closed state. While the atomic-level consequences of cleavage for HIV-1 Env are still being determined, the uncleaved state is antigenically distinct from the mature closed state, and cleavage has been reported to be essential for mimicry of the mature viral spike by soluble versions of Env. Here we report the redesign of a current state-of-the-art soluble Env mimic, BG505.SOSIP, to make it cleavage independent. Specifically, we replaced the furin cleavage site between gp120 and gp41 with Gly-Ser linkers of various lengths. The resultant linked gp120-gp41 constructs, termed single-chain gp140 (sc-gp140), exhibited different levels of structural and antigenic mimicry of the parent cleaved BG505.SOSIP. When constructs were subjected to negative selection to remove subspecies recognized by poorly neutralizing antibodies, trimers of high antigenic mimicry of BG505.SOSIP could be obtained; negative-stain electron microscopy indicated these to resemble the mature closed state. Higher proportions of BG505.SOSIP-trimer mimicry were observed in sc-gp140s with linkers of 6 or more residues, with a linker length of 15 residues exhibiting especially promising traits. Overall, flexible linkages between gp120 and gp41 in BG505.SOSIP can thus substitute for cleavage, and sc-gp140s that closely mimicked the vaccine-preferred mature closed state of Env could be obtained. IMPORTANCE: The trimeric HIV-1 envelope glycoprotein (Env) is the sole target of virus-directed neutralizing antibody responses and a primary focus of vaccine design. Soluble mimics of Env have proven challenging to obtain and have been thought to require proteolytic cleavage into two-component subunits, gp120 and gp41, to achieve structural and antigenic mimicry of mature Env spikes on virions. Here we show that replacement of the cleavage site between gp120 and gp41 in a lead soluble gp140 construct, BG505.SOSIP, with flexible linkers can result in molecules that do not require cleavage to fold efficiently into the mature closed state. Our results provide insights into the impact of cleavage on HIV-1 Env folding. In some contexts such as genetic immunization, optimized cleavage-independent soluble gp140 constructs may have utility over the parental BG505.SOSIP, as they would not require furin cleavage to achieve mimicry of mature Env spikes on virions.

Enthalpy screen of drug candidates.

The enthalpic and entropic contributions to the binding affinity of drug candidates have been acknowledged to be important determinants of the quality of a drug molecule. These quantities, usually summarized in the thermodynamic signature, provide a rapid assessment of the forces that drive the binding of a ligand. Having access to the thermodynamic signature in the early stages of the drug discovery process will provide critical information towards the selection of the best drug candidates for development. In this paper, the Enthalpy Screen technique is presented. The enthalpy screen allows fast and accurate determination of the binding enthalpy for hundreds of ligands. As such, it appears to be ideally suited to aid in the ranking of the hundreds of hits that are usually identified after standard high throughput screening.

CD4-mimetic small molecules sensitize human immunodeficiency virus to vaccine-elicited antibodies.

UNLABELLED: Approaches to prevent human immunodeficiency virus (HIV-1) transmission are urgently needed. Difficulties in eliciting antibodies that bind conserved epitopes exposed on the unliganded conformation of the HIV-1 envelope glycoprotein (Env) trimer represent barriers to vaccine development. During HIV-1 entry, binding of the gp120 Env to the initial receptor, CD4, triggers conformational changes in Env that result in the formation and exposure of the highly conserved gp120 site for interaction with the coreceptors, CCR5 and CXCR4. The DMJ compounds (+)-DMJ-I-228 and (+)-DMJ-II-121 bind gp120 within the conserved Phe 43 cavity near the CD4-binding site, block CD4 binding, and inhibit HIV-1 infection. Here we show that the DMJ compounds sensitize primary HIV-1, including transmitted/founder viruses, to neutralization by monoclonal antibodies directed against CD4-induced (CD4i) epitopes and the V3 region, two gp120 elements involved in coreceptor binding. Importantly, the DMJ compounds rendered primary HIV-1 sensitive to neutralization by antisera elicited by immunization of rabbits with HIV-1 gp120 cores engineered to assume the CD4-bound state. Thus, small molecules like the DMJ compounds may be useful as microbicides to inhibit HIV-1 infection directly and to sensitize primary HIV-1 to neutralization by readily elicited antibodies. IMPORTANCE: Preventing HIV-1 transmission is a priority for global health. Eliciting antibodies that can neutralize many different strains of HIV-1 is difficult, creating problems for the development of a vaccine. We found that certain small-molecule compounds can sensitize HIV-1 to particular antibodies. These antibodies can be elicited in rabbits. These results suggest an approach to prevent HIV-1 sexual transmission in which a virus-sensitizing microbicide is combined with a vaccine.