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.

Vaccine-Induced Antibodies that Neutralize Group 1 and Group 2 Influenza A Viruses.

Antibodies capable of neutralizing divergent influenza A viruses could form the basis of a universal vaccine. Here, from subjects enrolled in an H5N1 DNA/MIV-prime-boost influenza vaccine trial, we sorted hemagglutinin cross-reactive memory B cells and identified three antibody classes, each capable of neutralizing diverse subtypes of group 1 and group 2 influenza A viruses. Co-crystal structures with hemagglutinin revealed that each class utilized characteristic germline genes and convergent sequence motifs to recognize overlapping epitopes in the hemagglutinin stem. All six analyzed subjects had sequences from at least one multidonor class, and-in half the subjects-multidonor-class sequences were recovered from >40% of cross-reactive B cells. By contrast, these multidonor-class sequences were rare in published antibody datasets. Vaccination with a divergent hemagglutinin can thus increase the frequency of B cells encoding broad influenza A-neutralizing antibodies. We propose the sequence signature-quantified prevalence of these B cells as a metric to guide universal influenza A immunization strategies.

Maturation Pathway from Germline to Broad HIV-1 Neutralizer of a CD4-Mimic Antibody.

Antibodies with ontogenies from VH1-2 or VH1-46-germline genes dominate the broadly neutralizing response against the CD4-binding site (CD4bs) on HIV-1. Here, we define with longitudinal sampling from time-of-infection the development of a VH1-46-derived antibody lineage that matured to neutralize 90% of HIV-1 isolates. Structures of lineage antibodies CH235 (week 41 from time-of-infection, 18% breadth), CH235.9 (week 152, 77%), and CH235.12 (week 323, 90%) demonstrated the maturing epitope to focus on the conformationally invariant portion of the CD4bs. Similarities between CH235 lineage and five unrelated CD4bs lineages in epitope focusing, length-of-time to develop breadth, and extraordinary level of somatic hypermutation suggested commonalities in maturation among all CD4bs antibodies. Fortunately, the required CH235-lineage hypermutation appeared substantially guided by the intrinsic mutability of the VH1-46 gene, which closely resembled VH1-2. We integrated our CH235-lineage findings with a second broadly neutralizing lineage and HIV-1 co-evolution to suggest a vaccination strategy for inducing both lineages.

Maturation and Diversity of the VRC01-Antibody Lineage over 15 Years of Chronic HIV-1 Infection.

HIV-1-neutralizing antibodies develop in most HIV-1-infected individuals, although highly effective antibodies are generally observed only after years of chronic infection. Here, we characterize the rate of maturation and extent of diversity for the lineage that produced the broadly neutralizing antibody VRC01 through longitudinal sampling of peripheral B cell transcripts over 15 years and co-crystal structures of lineage members. Next-generation sequencing identified VRC01-lineage transcripts, which encompassed diverse antibodies organized into distinct phylogenetic clades. Prevalent clades maintained characteristic features of antigen recognition, though each evolved binding loops and disulfides that formed distinct recognition surfaces. Over the course of the study period, VRC01-lineage clades showed continuous evolution, with rates of ∼2 substitutions per 100 nucleotides per year, comparable to that of HIV-1 evolution. This high rate of antibody evolution provides a mechanism by which antibody lineages can achieve extraordinary diversity and, over years of chronic infection, develop effective HIV-1 neutralization.

Structural Repertoire of HIV-1-Neutralizing Antibodies Targeting the CD4 Supersite in 14 Donors.

The site on the HIV-1 gp120 glycoprotein that binds the CD4 receptor is recognized by broadly reactive antibodies, several of which neutralize over 90% of HIV-1 strains. To understand how antibodies achieve such neutralization, we isolated CD4-binding-site (CD4bs) antibodies and analyzed 16 co-crystal structures -8 determined here- of CD4bs antibodies from 14 donors. The 16 antibodies segregated by recognition mode and developmental ontogeny into two types: CDR H3-dominated and VH-gene-restricted. Both could achieve greater than 80% neutralization breadth, and both could develop in the same donor. Although paratope chemistries differed, all 16 gp120-CD4bs antibody complexes showed geometric similarity, with antibody-neutralization breadth correlating with antibody-angle of approach relative to the most effective antibody of each type. The repertoire for effective recognition of the CD4 supersite thus comprises antibodies with distinct paratopes arrayed about two optimal geometric orientations, one achieved by CDR H3 ontogenies and the other achieved by VH-gene-restricted ontogenies.

Broad and Potent Neutralizing Antibodies Recognize the Silent Face of the HIV Envelope.

Broadly neutralizing antibodies (bNAbs) against HIV-1 envelope (Env) inform vaccine design and are potential therapeutic agents. We identified SF12 and related bNAbs with up to 62% neutralization breadth from an HIV-infected donor. SF12 recognized a glycan-dominated epitope on Env's silent face and was potent against clade AE viruses, which are poorly covered by V3-glycan bNAbs. A 3.3Å cryo-EM structure of a SF12-Env trimer complex showed additional contacts to Env protein residues by SF12 compared with VRC-PG05, the only other known donor-derived silentface antibody, explaining SF12's increased neutralization breadth, potency, and resistance to Env mutation routes. Asymmetric binding of SF12 was associated with distinct N-glycan conformations across Env protomers, demonstrating intra-Env glycan heterogeneity. Administrating SF12 to HIV-1-infected humanized mice suppressed viremia and selected for viruses lacking the N448gp120 glycan. Effective bNAbs can therefore be raised against HIV-1 Env's silent face, suggesting their potential for HIV-1 prevention, therapy, and vaccine development.

Isolation and Structure of an Antibody that Fully Neutralizes Isolate SIVmac239 Reveals Functional Similarity of SIV and HIV Glycan Shields.

HIV- and SIV-envelope (Env) trimers are both extensively glycosylated, and antibodies identified to date have been unable to fully neutralize SIVmac239. Here, we report the isolation, structure, and glycan interactions of antibody ITS90.03, a monoclonal antibody that completely neutralized the highly neutralization-resistant isolate, SIVmac239. The co-crystal structure of a fully glycosylated SIVmac239-gp120 core in complex with rhesus CD4 and the antigen-binding fragment of ITS90.03 at 2.5-Å resolution revealed that ITS90 recognized an epitope comprised of 45% glycan. SIV-gp120 core, rhesus CD4, and their complex could each be aligned structurally to their human counterparts. The structure revealed that glycans masked most of the SIV Env protein surface, with ITS90 targeting a glycan hole, which is occupied in ∼83% of SIV strains by glycan N238. Overall, the SIV glycan shield appears to functionally resemble its HIV counterpart in coverage of spike, shielding from antibody, and modulation of receptor accessibility.

Vaccination with Glycan-Modified HIV NFL Envelope Trimer-Liposomes Elicits Broadly Neutralizing Antibodies to Multiple Sites of Vulnerability.

The elicitation of broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope glycoprotein (Env) trimer remains a major vaccine challenge. Most cross-conserved protein determinants are occluded by self-N-glycan shielding, limiting B cell recognition of the underlying polypeptide surface. The exceptions to the contiguous glycan shield include the conserved receptor CD4 binding site (CD4bs) and glycoprotein (gp)41 elements proximal to the furin cleavage site. Accordingly, we performed heterologous trimer-liposome prime:boosting in rabbits to drive B cells specific for cross-conserved sites. To preferentially expose the CD4bs to B cells, we eliminated proximal N-glycans while maintaining the native-like state of the cleavage-independent NFL trimers, followed by gradual N-glycan restoration coupled with heterologous boosting. This approach successfully elicited CD4bs-directed, cross-neutralizing Abs, including one targeting a unique glycan-protein epitope and a bNAb (87% breadth) directed to the gp120:gp41 interface, both resolved by high-resolution cryoelectron microscopy. This study provides proof-of-principle immunogenicity toward eliciting bNAbs by vaccination.

Longitudinal Analysis Reveals Early Development of Three MPER-Directed Neutralizing Antibody Lineages from an HIV-1-Infected Individual.

Lineage-based vaccine design is an attractive approach for eliciting broadly neutralizing antibodies (bNAbs) against HIV-1. However, most bNAb lineages studied to date have features indicative of unusual recombination and/or development. From an individual in the prospective RV217 cohort, we identified three lineages of bNAbs targeting the membrane-proximal external region (MPER) of the HIV-1 envelope. Antibodies RV217-VRC42.01, -VRC43.01, and -VRC46.01 used distinct modes of recognition and neutralized 96%, 62%, and 30%, respectively, of a 208-strain virus panel. All three lineages had modest levels of somatic hypermutation and normal antibody-loop lengths and were initiated by the founder virus MPER. The broadest lineage, VRC42, was similar to the known bNAb 4E10. A multimeric immunogen based on the founder MPER activated B cells bearing the unmutated common ancestor of VRC42, with modest maturation of early VRC42 intermediates imparting neutralization breadth. These features suggest that VRC42 may be a promising template for lineage-based vaccine design.

A Neutralizing Antibody Recognizing Primarily N-Linked Glycan Targets the Silent Face of the HIV Envelope.

Virtually the entire surface of the HIV-1-envelope trimer is recognized by neutralizing antibodies, except for a highly glycosylated region at the center of the "silent face" on the gp120 subunit. From an HIV-1-infected donor, #74, we identified antibody VRC-PG05, which neutralized 27% of HIV-1 strains. The crystal structure of the antigen-binding fragment of VRC-PG05 in complex with gp120 revealed an epitope comprised primarily of N-linked glycans from N262, N295, and N448 at the silent face center. Somatic hypermutation occurred preferentially at antibody residues that interacted with these glycans, suggesting somatic development of glycan recognition. Resistance to VRC-PG05 in donor #74 involved shifting of glycan-N448 to N446 or mutation of glycan-proximal residue E293. HIV-1 neutralization can thus be achieved at the silent face center by glycan-recognizing antibody; along with other known epitopes, the VRC-PG05 epitope completes coverage by neutralizing antibody of all major exposed regions of the prefusion closed trimer.

Virus-like Particles Identify an HIV V1V2 Apex-Binding Neutralizing Antibody that Lacks a Protruding Loop.

Most HIV-1-specific neutralizing antibodies isolated to date exhibit unusual characteristics that complicate their elicitation. Neutralizing antibodies that target the V1V2 apex of the HIV-1 envelope (Env) trimer feature unusually long protruding loops, which enable them to penetrate the HIV-1 glycan shield. As antibodies with loops of requisite length are created through uncommon recombination events, an alternative mode of apex binding has been sought. Here, we isolated a lineage of Env apex-directed neutralizing antibodies, N90-VRC38.01-11, by using virus-like particles and conformationally stabilized Env trimers as B cell probes. A crystal structure of N90-VRC38.01 with a scaffolded V1V2 revealed a binding mode involving side-chain-to-side-chain interactions that reduced the distance the antibody loop must traverse the glycan shield, thereby facilitating V1V2 binding via a non-protruding loop. The N90-VRC38 lineage thus identifies a solution for V1V2-apex binding that provides a more conventional B cell pathway for vaccine design.

Identification of a CD4-Binding-Site Antibody to HIV that Evolved Near-Pan Neutralization Breadth.

Detailed studies of the broadly neutralizing antibodies (bNAbs) that underlie the best available examples of the humoral immune response to HIV are providing important information for the development of therapies and prophylaxis for HIV-1 infection. Here, we report a CD4-binding site (CD4bs) antibody, named N6, that potently neutralized 98% of HIV-1 isolates, including 16 of 20 that were resistant to other members of its class. N6 evolved a mode of recognition such that its binding was not impacted by the loss of individual contacts across the immunoglobulin heavy chain. In addition, structural analysis revealed that the orientation of N6 permitted it to avoid steric clashes with glycans, which is a common mechanism of resistance. Thus, an HIV-1-specific bNAb can achieve potent, near-pan neutralization of HIV-1, making it an attractive candidate for use in therapy and prophylaxis.

Safety, Tolerability, and Pharmacokinetics of the Broadly Neutralizing Human Immunodeficiency Virus (HIV)-1 Monoclonal Antibody VRC01 in HIV-Exposed Newborn Infants.

BACKGROUND: Although mother-to-child human immunodeficiency virus (HIV) transmission has dramatically decreased with maternal antiretroviral therapy, breast milk transmission accounts for most of the 180 000 new infant HIV infections annually. Broadly neutralizing antibodies (bNAb) may further reduce transmission. METHODS: A Phase 1 safety and pharmacokinetic study was conducted: a single subcutaneous (SC) dose of 20 or 40 mg/kg (Dose Groups 1 and 2, respectively) of the bNAb VRC01 was administered to HIV-exposed infants soon after birth. Breastfeeding infants (Dose Group 3) received 40 mg/kg SC VRC01 after birth and then 20 mg/kg/dose SC monthly. All infants received appropriate antiretroviral prophylaxis. RESULTS: Forty infants were enrolled (21 in the United States, 19 in Africa). Subcutaneous VRC01 was safe and well tolerated with only mild-to-moderate local reactions, primarily erythema, which rapidly resolved. For multiple-dose infants, local reactions decreased with subsequent injections. VRC01 was rapidly absorbed after administration, with peak concentrations 1-6 days postdose. The 40 mg/kg dose resulted in 13 of 14 infants achieving the serum 50 micrograms (mcg)/mL target at day 28. Dose Group 3 infants maintained concentrations greater than 50 mcg/mL throughout breastfeeding. CONCLUSIONS: Subcutaneous VRC01 as single or multiple doses is safe and well tolerated in very young infants and is suitable for further study to prevent HIV transmission in infants.

Safety, tolerability, pharmacokinetics, and immunogenicity of the therapeutic monoclonal antibody mAb114 targeting Ebola virus glycoprotein (VRC 608): an open-label phase 1 study.

BACKGROUND: mAb114 is a single monoclonal antibody that targets the receptor-binding domain of Ebola virus glycoprotein, which prevents mortality in rhesus macaques treated after lethal challenge with Zaire ebolavirus. Here we present expedited data from VRC 608, a phase 1 study to evaluate mAb114 safety, tolerability, pharmacokinetics, and immunogenicity. METHODS: In this phase 1, dose-escalation study (VRC 608), conducted at the US National Institutes of Health (NIH) Clinical Center (Bethesda, MD, USA), healthy adults aged 18-60 years were sequentially enrolled into three mAb114 dose groups of 5 mg/kg, 25 mg/kg, and 50 mg/kg. The drug was given to participants intravenously over 30 min, and participants were followed for 24 weeks. Participants were only enrolled into increased dosing groups after interim safety assessments. Our primary endpoints were safety and tolerability, with pharmacokinetic and anti-drug antibody assessments as secondary endpoints. We assessed safety and tolerability in all participants who received study drug by monitoring clinical laboratory data and self-report and direct clinician assessment of prespecified infusion-site symptoms 3 days after infusion and systemic symptoms 7 days after infusion. Unsolicited adverse events were recorded for 28 days. Pharmacokinetic and anti-drug antibody assessments were completed in participants with at least 56 days of data. This trial is registered with ClinicalTrials.gov, number NCT03478891, and is active but no longer recruiting. FINDINGS: Between May 16, and Sept 27, 2018, 19 eligible individuals were enrolled. One (5%) participant was not infused because intravenous access was not adequate. Of 18 (95%) remaining participants, three (17%) were assigned to the 5 mg/kg group, five (28%) to the 25 mg/kg group, and ten (55%) to the 50 mg/kg group, each of whom received a single infusion of mAb114 at their assigned dose. All infusions were well tolerated and completed over 30-37 min with no infusion reactions or rate adjustments. All participants who received the study drug completed the safety assessment of local and systemic reactogenicity. No participants reported infusion-site symptoms. Systemic symptoms were all mild and present only in four (22%) of 18 participants across all dosing groups. No unsolicited adverse events occurred related to mAb114 and one serious adverse event occurred that was unrelated to mAb114. mAb114 has linear pharmacokinetics and a half-life of 24·2 days (standard error of measurement 0·2) with no evidence of anti-drug antibody development. INTERPRETATION: mAb114 was well tolerated, showed linear pharmacokinetics, and was easily and rapidly infused, making it an attractive and deployable option for treatment in outbreak settings. FUNDING: Vaccine Research Center, US National Institute of Allergy and Infectious Diseases, and NIH.

Chimpanzee Adenovirus Vector Ebola Vaccine.

BACKGROUND: The unprecedented 2014 epidemic of Ebola virus disease (EVD) prompted an international response to accelerate the availability of a preventive vaccine. A replication-defective recombinant chimpanzee adenovirus type 3-vectored ebolavirus vaccine (cAd3-EBO), encoding the glycoprotein from Zaire and Sudan species, that offers protection in the nonhuman primate model, was rapidly advanced into phase 1 clinical evaluation. METHODS: We conducted a phase 1, dose-escalation, open-label trial of cAd3-EBO. Twenty healthy adults, in sequentially enrolled groups of 10 each, received vaccination intramuscularly in doses of 2×1010 particle units or 2×1011 particle units. Primary and secondary end points related to safety and immunogenicity were assessed throughout the first 8 weeks after vaccination; in addition, longer-term vaccine durability was assessed at 48 weeks after vaccination. RESULTS: In this small study, no safety concerns were identified; however, transient fever developed within 1 day after vaccination in two participants who had received the 2×1011 particle-unit dose. Glycoprotein-specific antibodies were induced in all 20 participants; the titers were of greater magnitude in the group that received the 2×1011 particle-unit dose than in the group that received the 2×1010 particle-unit dose (geometric mean titer against the Zaire antigen at week 4, 2037 vs. 331; P=0.001). Glycoprotein-specific T-cell responses were more frequent among those who received the 2×1011 particle-unit dose than among those who received the 2×1010 particle-unit dose, with a CD4 response in 10 of 10 participants versus 3 of 10 participants (P=0.004) and a CD8 response in 7 of 10 participants versus 2 of 10 participants (P=0.07) at week 4. Assessment of the durability of the antibody response showed that titers remained high at week 48, with the highest titers in those who received the 2×1011 particle-unit dose. CONCLUSIONS: Reactogenicity and immune responses to cAd3-EBO vaccine were dose-dependent. At the 2×1011 particle-unit dose, glycoprotein Zaire-specific antibody responses were in the range reported to be associated with vaccine-induced protective immunity in challenge studies involving nonhuman primates, and responses were sustained to week 48. Phase 2 studies and efficacy trials assessing cAd3-EBO are in progress. (Funded by the Intramural Research Program of the National Institutes of Health; VRC 207 ClinicalTrials.gov number, NCT02231866 .).

Structure and immune recognition of trimeric pre-fusion HIV-1 Env.

The human immunodeficiency virus type 1 (HIV-1) envelope (Env) spike, comprising three gp120 and three gp41 subunits, is a conformational machine that facilitates HIV-1 entry by rearranging from a mature unliganded state, through receptor-bound intermediates, to a post-fusion state. As the sole viral antigen on the HIV-1 virion surface, Env is both the target of neutralizing antibodies and a focus of vaccine efforts. Here we report the structure at 3.5 Å resolution for an HIV-1 Env trimer captured in a mature closed state by antibodies PGT122 and 35O22. This structure reveals the pre-fusion conformation of gp41, indicates rearrangements needed for fusion activation, and defines parameters of immune evasion and immune recognition. Pre-fusion gp41 encircles amino- and carboxy-terminal strands of gp120 with four helices that form a membrane-proximal collar, fastened by insertion of a fusion peptide-proximal methionine into a gp41-tryptophan clasp. Spike rearrangements required for entry involve opening the clasp and expelling the termini. N-linked glycosylation and sequence-variable regions cover the pre-fusion closed spike; we used chronic cohorts to map the prevalence and location of effective HIV-1-neutralizing responses, which were distinguished by their recognition of N-linked glycan and tolerance for epitope-sequence variation.

Broad and potent HIV-1 neutralization by a human antibody that binds the gp41-gp120 interface.

The isolation of human monoclonal antibodies is providing important insights into the specificities that underlie broad neutralization of HIV-1 (reviewed in ref. 1). Here we report a broad and extremely potent HIV-specific monoclonal antibody, termed 35O22, which binds a novel HIV-1 envelope glycoprotein (Env) epitope. 35O22 neutralized 62% of 181 pseudoviruses with a half-maximum inhibitory concentration (IC50) <50 µg ml(-1). The median IC50 of neutralized viruses was 0.033 µg ml(-1), among the most potent thus far described. 35O22 did not bind monomeric forms of Env tested, but did bind the trimeric BG505 SOSIP.664. Mutagenesis and a reconstruction by negative-stain electron microscopy of the Fab in complex with trimer revealed that it bound to a conserved epitope, which stretched across gp120 and gp41. The specificity of 35O22 represents a novel site of vulnerability on HIV Env, which serum analysis indicates to be commonly elicited by natural infection. Binding to this new site of vulnerability may thus be an important complement to current monoclonal-antibody-based approaches to immunotherapies, prophylaxis and vaccine design.

Immunological and virological mechanisms of vaccine-mediated protection against SIV and HIV.

A major challenge for the development of a highly effective AIDS vaccine is the identification of mechanisms of protective immunity. To address this question, we used a nonhuman primate challenge model with simian immunodeficiency virus (SIV). We show that antibodies to the SIV envelope are necessary and sufficient to prevent infection. Moreover, sequencing of viruses from breakthrough infections revealed selective pressure against neutralization-sensitive viruses; we identified a two-amino-acid signature that alters antigenicity and confers neutralization resistance. A similar signature confers resistance of human immunodeficiency virus (HIV)-1 to neutralization by monoclonal antibodies against variable regions 1 and 2 (V1V2), suggesting that SIV and HIV share a fundamental mechanism of immune escape from vaccine-elicited or naturally elicited antibodies. These analyses provide insight into the limited efficacy seen in HIV vaccine trials.

Developmental pathway for potent V1V2-directed HIV-neutralizing antibodies.

Antibodies capable of neutralizing HIV-1 often target variable regions 1 and 2 (V1V2) of the HIV-1 envelope, but the mechanism of their elicitation has been unclear. Here we define the developmental pathway by which such antibodies are generated and acquire the requisite molecular characteristics for neutralization. Twelve somatically related neutralizing antibodies (CAP256-VRC26.01-12) were isolated from donor CAP256 (from the Centre for the AIDS Programme of Research in South Africa (CAPRISA)); each antibody contained the protruding tyrosine-sulphated, anionic antigen-binding loop (complementarity-determining region (CDR) H3) characteristic of this category of antibodies. Their unmutated ancestor emerged between weeks 30-38 post-infection with a 35-residue CDR H3, and neutralized the virus that superinfected this individual 15 weeks after initial infection. Improved neutralization breadth and potency occurred by week 59 with modest affinity maturation, and was preceded by extensive diversification of the virus population. HIV-1 V1V2-directed neutralizing antibodies can thus develop relatively rapidly through initial selection of B cells with a long CDR H3, and limited subsequent somatic hypermutation. These data provide important insights relevant to HIV-1 vaccine development.

Effect of a Chikungunya Virus-Like Particle Vaccine on Safety and Tolerability Outcomes: A Randomized Clinical Trial.

Importance: Chikungunya virus (CHIKV) is a mosquito-borne Alphavirus prevalent worldwide. There are currently no licensed vaccines or therapies. Objective: To evaluate the safety and tolerability of an investigational CHIKV virus-like particle (VLP) vaccine in endemic regions. Design, Setting, and Participants: This was a randomized, placebo-controlled, double-blind, phase 2 clinical trial to assess the vaccine VRC-CHKVLP059-00-VP (CHIKV VLP). The trial was conducted at 6 outpatient clinical research sites located in Haiti, Dominican Republic, Martinique, Guadeloupe, and Puerto Rico. A total of 400 healthy adults aged 18 through 60 years were enrolled after meeting eligibility criteria. The first study enrollment occurred on November 18, 2015; the final study visit, March 6, 2018. Interventions: Participants were randomized 1:1 to receive 2 intramuscular injections 28 days apart (20 µg, n = 201) or placebo (n = 199) and were followed up for 72 weeks. Main Outcomes and Measures: The primary outcome was the safety (laboratory parameters, adverse events, and CHIKV infection) and tolerability (local and systemic reactogenicity) of the vaccine, and the secondary outcome was immune response by neutralization assay 4 weeks after second vaccination. Results: Of the 400 randomized participants (mean age, 35 years; 199 [50%] women), 393 (98%) completed the primary safety analysis. All injections were well tolerated. Of the 16 serious adverse events unrelated to the study drugs, 4 (25%) occurred among 4 patients in the vaccine group and 12 (75%) occurred among 11 patients in the placebo group. Of the 16 mild to moderate unsolicited adverse events that were potentially related to the drug, 12 (75%) occurred among 8 patients in the vaccine group and 4 (25%) occurred among 3 patients in the placebo group. All potentially related adverse events resolved without clinical sequelae. At baseline, there was no significant difference between the effective concentration (EC50)-which is the dilution of sera that inhibits 50% infection in viral neutralization assay-geometric mean titers (GMTs) of neutralizing antibodies of the vaccine group (46; 95% CI, 34-63) and the placebo group (43; 95% CI, 32-57). Eight weeks following the first administration, the EC50 GMT in the vaccine group was 2005 (95% CI, 1680-2392) vs 43 (95% CI, 32-58; P < .001) in the placebo group. Durability of the immune response was demonstrated through 72 weeks after vaccination. Conclusions and Relevance: Among healthy adults in a chikungunya endemic population, a virus-like particle vaccine compared with placebo demonstrated safety and tolerability. Phase 3 trials are needed to assess clinical efficacy. Trial Registration: ClinicalTrials.gov Identifier: NCT02562482.