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.

Protection from SARS-CoV-2 Delta one year after mRNA-1273 vaccination in rhesus macaques coincides with anamnestic antibody response in the lung.

mRNA-1273 vaccine efficacy against SARS-CoV-2 Delta wanes over time; however, there are limited data on the impact of durability of immune responses on protection. Here, we immunized rhesus macaques and assessed immune responses over 1 year in blood and upper and lower airways. Serum neutralizing titers to Delta were 280 and 34 reciprocal ID50 at weeks 6 (peak) and 48 (challenge), respectively. Antibody-binding titers also decreased in bronchoalveolar lavage (BAL). Four days after Delta challenge, the virus was unculturable in BAL, and subgenomic RNA declined by ∼3-log10 compared with control animals. In nasal swabs, sgRNA was reduced by 1-log10, and the virus remained culturable. Anamnestic antibodies (590-fold increased titer) but not T cell responses were detected in BAL by day 4 post-challenge. mRNA-1273-mediated protection in the lungs is durable but delayed and potentially dependent on anamnestic antibody responses. Rapid and sustained protection in upper and lower airways may eventually require a boost.

Mucosal adenovirus vaccine boosting elicits IgA and durably prevents XBB.1.16 infection in nonhuman primates.

A mucosal route of vaccination could prevent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication at the site of infection and limit transmission. We compared protection against heterologous XBB.1.16 challenge in nonhuman primates (NHPs) ~5 months following intramuscular boosting with bivalent mRNA encoding WA1 and BA.5 spike proteins or mucosal boosting with a WA1-BA.5 bivalent chimpanzee adenoviral-vectored vaccine delivered by intranasal or aerosol device. NHPs boosted by either mucosal route had minimal virus replication in the nose and lungs, respectively. By contrast, protection by intramuscular mRNA was limited to the lower airways. The mucosally delivered vaccine elicited durable airway IgG and IgA responses and, unlike the intramuscular mRNA vaccine, induced spike-specific B cells in the lungs. IgG, IgA and T cell responses correlated with protection in the lungs, whereas mucosal IgA alone correlated with upper airway protection. This study highlights differential mucosal and serum correlates of protection and how mucosal vaccines can durably prevent infection against SARS-CoV-2.

Robust IgM responses following intravenous vaccination with Bacille Calmette-Guérin associate with prevention of Mycobacterium tuberculosis infection in macaques.

Development of an effective tuberculosis (TB) vaccine has suffered from an incomplete understanding of the correlates of protection against Mycobacterium tuberculosis (Mtb). Intravenous (i.v.) vaccination with Bacille Calmette-Guérin (BCG) provides nearly complete protection against TB in rhesus macaques, but the antibody response it elicits remains incompletely defined. Here we show that i.v. BCG drives superior antibody responses in the plasma and the lungs of rhesus macaques compared to traditional intradermal BCG administration. While i.v. BCG broadly expands antibody titers and functions, IgM titers in the plasma and lungs of immunized macaques are among the strongest markers of reduced bacterial burden. IgM was also enriched in macaques that received protective vaccination with an attenuated strain of Mtb. Finally, an Mtb-specific IgM monoclonal antibody reduced Mtb survival in vitro. Collectively, these data highlight the potential importance of IgM responses as a marker and mediator of protection against TB.

mRNA-1273 protects against SARS-CoV-2 beta infection in nonhuman primates.

B.1.351 is the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant most resistant to antibody neutralization. We demonstrate how the dose and number of immunizations influence protection. Nonhuman primates received two doses of 30 or 100 µg of Moderna's mRNA-1273 vaccine, a single immunization of 30 µg, or no vaccine. Two doses of 100 µg of mRNA-1273 induced 50% inhibitory reciprocal serum dilution neutralizing antibody titers against live SARS-CoV-2 p.Asp614Gly and B.1.351 of 3,300 and 240, respectively. Higher neutralizing responses against B.1.617.2 were also observed after two doses compared to a single dose. After challenge with B.1.351, there was ~4- to 5-log10 reduction of viral subgenomic RNA and low to undetectable replication in bronchoalveolar lavages in the two-dose vaccine groups, with a 1-log10 reduction in nasal swabs in the 100-µg group. These data establish that a two-dose regimen of mRNA-1273 will be critical for providing upper and lower airway protection against major variants of concern.

Analysis of antibody binding specificities in twin and SNP-genotyped cohorts reveals that antiviral antibody epitope selection is a heritable trait.

Human immune responses to viral infections are highly variable, but the genetic factors that contribute to this variability are not well characterized. We used VirScan, a high-throughput epitope scanning technology, to analyze pan-viral antibody reactivity profiles of twins and SNP-genotyped individuals. Using these data, we determined the heritability and genomic loci associated with antibody epitope selection, response breadth, and control of Epstein-Barr virus (EBV) viral load. 107 EBV peptide reactivities were heritable and at least two Epstein-Barr nuclear antigen 2 (EBNA-2) reactivities were associated with variants in the MHC class II locus. We identified an EBV serosignature that predicted viral load in peripheral blood mononuclear cells and was associated with variants in the MHC class I locus. Our study illustrates the utility of epitope profiling to investigate the genetics of pathogen immunity, reports heritable features of the antibody response to viruses, and identifies specific HLA loci important for EBV epitope selection.

Phage display of environmental protein toxins and virulence factors reveals the prevalence, persistence, and genetics of antibody responses.

Microbial exposures are crucial environmental factors that impact healthspan by sculpting the immune system and microbiota. Antibody profiling via Phage ImmunoPrecipitation Sequencing (PhIP-Seq) provides a high-throughput, cost-effective approach for detecting exposure and response to microbial protein products. We designed and constructed a library of 95,601 56-amino acid peptide tiles spanning 14,430 proteins with "toxin" or "virulence factor" keyword annotations. We used PhIP-Seq to profile the antibodies of ∼1,000 individuals against this "ToxScan" library. In addition to enumerating immunodominant antibody epitopes, we studied the age-dependent stability of the ToxScan profile and used a genome-wide association study to find that the MHC-II locus modulates bacterial epitope selection. We detected previously described anti-flagellin antibody responses in a Crohn's disease cohort and identified an association between anti-flagellin antibodies and juvenile dermatomyositis. PhIP-Seq with the ToxScan library is thus an effective tool for studying the environmental determinants of health and disease at cohort scale.

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.

The genetic architecture of the human immune system: a bioresource for autoimmunity and disease pathogenesis.

Despite recent discoveries of genetic variants associated with autoimmunity and infection, genetic control of the human immune system during homeostasis is poorly understood. We undertook a comprehensive immunophenotyping approach, analyzing 78,000 immune traits in 669 female twins. From the top 151 heritable traits (up to 96% heritable), we used replicated GWAS to obtain 297 SNP associations at 11 genetic loci, explaining up to 36% of the variation of 19 traits. We found multiple associations with canonical traits of all major immune cell subsets and uncovered insights into genetic control for regulatory T cells. This data set also revealed traits associated with loci known to confer autoimmune susceptibility, providing mechanistic hypotheses linking immune traits with the etiology of disease. Our data establish a bioresource that links genetic control elements associated with normal immune traits to common autoimmune and infectious diseases, providing a shortcut to identifying potential mechanisms of immune-related diseases.

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.

Single-cell technologies for monitoring immune systems.

The complex heterogeneity of cells, and their interconnectedness with each other, are major challenges to identifying clinically relevant measurements that reflect the state and capability of the immune system. Highly multiplexed, single-cell technologies may be critical for identifying correlates of disease or immunological interventions as well as for elucidating the underlying mechanisms of immunity. Here we review limitations of bulk measurements and explore advances in single-cell technologies that overcome these problems by expanding the depth and breadth of functional and phenotypic analysis in space and time. The geometric increases in complexity of data make formidable hurdles for exploring, analyzing and presenting results. We summarize recent approaches to making such computations tractable and discuss challenges for integrating heterogeneous data obtained using these single-cell technologies.

Intravenous Bacillus Calmette-Guérin (BCG) Induces a More Potent Airway and Lung Immune Response than Intradermal BCG in Simian Immunodeficiency Virus-infected Macaques.

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is one of the leading causes of death due to an infectious agent. Coinfection with HIV exacerbates M. tuberculosis infection outcomes in people living with HIV. Bacillus Calmette-Guérin (BCG), the only approved TB vaccine, is effective in infants, but its efficacy in adolescents and adults is limited. In this study, we investigated the immune responses elicited by BCG administered via i.v. or intradermal (i.d.) routes in SIV-infected Mauritian cynomolgus macaques (MCM) without the confounding effects of M. tuberculosis challenge. We assessed the impact of vaccination on T cell responses in the airway, blood, and tissues (lung, thoracic lymph nodes, and spleen), as well as the expression of cytokines, cytotoxic effectors, and key transcription factors. Our results showed that i.v. BCG induces a robust and sustained immune response, including tissue-resident memory T cells in lungs, polyfunctional CD4+ and CD8αß+ T cells expressing multiple cytokines, and CD8αß+ T cells and NK cells expressing cytotoxic effectors in airways. We also detected higher levels of mycobacteria-specific IgG and IgM in the airways of i.v. BCG-vaccinated MCM. Although i.v. BCG vaccination resulted in an influx of tissue-resident memory T cells in lungs of MCM with controlled SIV replication, MCM with high plasma SIV RNA (>105 copies/ml) typically displayed reduced T cell responses, suggesting that uncontrolled SIV or HIV replication would have a detrimental effect on i.v. BCG-induced protection against M. tuberculosis.

Prevention of tuberculosis in macaques after intravenous BCG immunization.

Mycobacterium tuberculosis (Mtb) is the leading cause of death from infection worldwide1. The only available vaccine, BCG (Bacillus Calmette-Guérin), is given intradermally and has variable efficacy against pulmonary tuberculosis, the major cause of mortality and disease transmission1,2. Here we show that intravenous administration of BCG profoundly alters the protective outcome of Mtb challenge in non-human primates (Macaca mulatta). Compared with intradermal or aerosol delivery, intravenous immunization induced substantially more antigen-responsive CD4 and CD8 T cell responses in blood, spleen, bronchoalveolar lavage and lung lymph nodes. Moreover, intravenous immunization induced a high frequency of antigen-responsive T cells across all lung parenchymal tissues. Six months after BCG vaccination, macaques were challenged with virulent Mtb. Notably, nine out of ten macaques that received intravenous BCG vaccination were highly protected, with six macaques showing no detectable levels of infection, as determined by positron emission tomography-computed tomography imaging, mycobacterial growth, pathology and granuloma formation. The finding that intravenous BCG prevents or substantially limits Mtb infection in highly susceptible rhesus macaques has important implications for vaccine delivery and clinical development, and provides a model for defining immune correlates and mechanisms of vaccine-elicited protection against tuberculosis.

Antibody-dependent cellular cytotoxicity, infected cell binding and neutralization by antibodies to the SIV envelope glycoprotein.

Antibodies specific for diverse epitopes of the simian immunodeficiency virus envelope glycoprotein (SIV Env) have been isolated from rhesus macaques to provide physiologically relevant reagents for investigating antibody-mediated protection in this species as a nonhuman primate model for HIV/AIDS. With increasing interest in the contribution of Fc-mediated effector functions to protective immunity, we selected thirty antibodies representing different classes of SIV Env epitopes for a comparison of antibody-dependent cellular cytotoxicity (ADCC), binding to Env on the surface of infected cells and neutralization of viral infectivity. These activities were measured against cells infected with neutralization-sensitive (SIVmac316 and SIVsmE660-FL14) and neutralization-resistant (SIVmac239 and SIVsmE543-3) viruses representing genetically distinct isolates. Antibodies to the CD4-binding site and CD4-inducible epitopes were identified with especially potent ADCC against all four viruses. ADCC correlated well with antibody binding to virus-infected cells. ADCC also correlated with neutralization. However, several instances of ADCC without detectable neutralization or neutralization without detectable ADCC were observed. The incomplete correspondence between ADCC and neutralization shows that some antibody-Env interactions can uncouple these antiviral activities. Nevertheless, the overall correlation between neutralization and ADCC implies that most antibodies that are capable of binding to Env on the surface of virions to block infectivity are also capable of binding to Env on the surface of virus-infected cells to direct their elimination by ADCC.

Immune Activation Profiles Elicited by Distinct, Repeated TLR Agonist Infusions in Rhesus Macaques.

TLR agonists are a promising class of immune system stimulants investigated for immunomodulatory applications in cancer immunotherapy and viral diseases. In this study, we sought to characterize the safety and immune activation achieved by different TLR agonists in rhesus macaques (Macaca mulatta), a useful preclinical model of complex immune interactions. Macaques received one of three TLR agonists, followed by plasma cytokine, immune cell subset representation, and blood cell activation measurements. The TLR4 agonist LPS administered i.v. induced very transient immune activation, including TNF-α expression and monocyte activation. The TLR7/8 agonist 2BXy elicited more persistent cytokine expression, including type I IFN, IL-1RA, and the proinflammatory IL-6, along with T cell and monocyte activation. Delivery of 2BXy i.v. and i.m. achieved comparable immune activation, which increased with escalating dose. Finally, i.v. bacillus Calmette-Guérin (BCG) vaccination (which activates multiple TLRs, especially TLR2/4) elicited the most pronounced and persistent innate and adaptive immune response, including strong induction of IFN-γ, IL-6, and IL-1RA. Strikingly, monocyte, T cell, and NK cell expression of the proliferation marker Ki67 increased dramatically following BCG vaccination. This aligned with a large increase in total and BCG-specific cells measured in the lung. Principal component analysis of the combined cytokine expression and cellular activation responses separated animals by treatment group, indicating distinct immune activation profiles induced by each agent. In sum, we report safe, effective doses and routes of administration for three TLR agonists that exhibit discrete immunomodulatory properties in primates and may be leveraged in future immunotherapeutic strategies.

Systematic identification of genomic elements that regulate FCGR2A expression and harbor variants linked with autoimmune disease.

BACKGROUND: FCGR2A binds antibody-antigen complexes to regulate the abundance of circulating and deposited complexes along with downstream immune and autoimmune responses. Although the abundance of FCRG2A may be critical in immune-mediated diseases, little is known about whether its surface expression is regulated through cis genomic elements and non-coding variants. In the current study, we aimed to characterize the regulation of FCGR2A expression, the impact of genetic variation and its association with autoimmune disease. METHODS: We applied CRISPR-based interference and editing to scrutinize 1.7 Mb of open chromatin surrounding the FCGR2A gene to identify regulatory elements. Relevant transcription factors (TFs) binding to these regions were defined through public databases. Genetic variants affecting regulation were identified using luciferase reporter assays and were verified in a cohort of 1996 genotyped healthy individuals using flow cytometry. RESULTS: We identified a complex proximal region and five distal enhancers regulating FCGR2A. The proximal region split into subregions upstream and downstream of the transcription start site, was enriched in binding of inflammation-regulated TFs, and harbored a variant associated with FCGR2A expression in primary myeloid cells. One distal enhancer region was occupied by CCCTC-binding factor (CTCF) whose binding site was disrupted by a rare genetic variant, altering gene expression. CONCLUSIONS: The FCGR2A gene is regulated by multiple proximal and distal genomic regions, with links to autoimmune disease. These findings may open up novel therapeutic avenues where fine-tuning of FCGR2A levels may constitute a part of treatment strategies for immune-mediated diseases.

Effect of Passive Administration of Monoclonal Antibodies Recognizing Simian Immunodeficiency Virus (SIV) V2 in CH59-Like Coil/Helical or ß-Sheet Conformations on Time of SIVmac251 Acquisition.

The monoclonal antibodies (MAbs) NCI05 and NCI09, isolated from a vaccinated macaque that was protected from multiple simian immunodeficiency virus (SIV) challenges, both target an overlapping, conformationally dynamic epitope in SIV envelope variable region 2 (V2). Here, we show that NCI05 recognizes a CH59-like coil/helical epitope, whereas NCI09 recognizes a ß-hairpin linear epitope. In vitro, NCI05 and, to a lesser extent, NCI09 mediate the killing of SIV-infected cells in a CD4-dependent manner. Compared to NCI05, NCI09 mediates higher titers of antibody-dependent cellular cytotoxicity (ADCC) to gp120-coated cells, as well as higher levels of trogocytosis, a monocyte function that contributes to immune evasion. We also found that passive administration of NCI05 or NCI09 to macaques did not affect the risk of SIVmac251 acquisition compared to controls, demonstrating that these anti-V2 antibodies alone are not protective. However, NCI05 but not NCI09 mucosal levels strongly correlated with delayed SIVmac251 acquisition, and functional and structural data suggest that NCI05 targets a transient state of the viral spike apex that is partially opened, compared to its prefusion-closed conformation. IMPORTANCE Studies suggest that the protection against SIV/simian-human immunodeficiency virus (SHIV) acquisition afforded by the SIV/HIV V1 deletion-containing envelope immunogens, delivered by the DNA/ALVAC vaccine platform, requires multiple innate and adaptive host responses. Anti-inflammatory macrophages and tolerogenic dendritic cells (DC-10), together with CD14+ efferocytes, are consistently found to correlate with a vaccine-induced decrease in the risk of SIV/SHIV acquisition. Similarly, V2-specific antibody responses mediating ADCC, Th1 and Th2 cells expressing no or low levels of CCR5, and envelope-specific NKp44+ cells producing interleukin 17 (IL-17) also are reproducible correlates of decreased risk of virus acquisition. We focused on the function and the antiviral potential of two monoclonal antibodies (NCI05 and NCI09) isolated from vaccinated animals that differ in antiviral function in vitro and recognize V2 in a linear (NCI09) or coil/helical (NCI05) conformation. We demonstrate that NCI05, but not NCI09, delays SIVmac251 acquisition, highlighting the complexity of antibody responses to V2.

Broad coverage of neutralization-resistant SIV strains by second-generation SIV-specific antibodies targeting the region involved in binding CD4.

Both SIV and SHIV are powerful tools for evaluating antibody-mediated prevention and treatment of HIV-1. However, owing to a lack of rhesus-derived SIV broadly neutralizing antibodies (bnAbs), testing of bnAbs for HIV-1 prevention or treatment has thus far been performed exclusively in the SHIV NHP model using bnAbs from HIV-1-infected individuals. Here we describe the isolation and characterization of multiple rhesus-derived SIV bnAbs capable of neutralizing most isolates of SIV. Eight antibodies belonging to two clonal families, ITS102 and ITS103, which target unique epitopes in the CD4 binding site (CD4bs) region, were found to be broadly neutralizing and together neutralized all SIV strains tested. A rare feature of these bnAbs and two additional antibody families, ITS92 and ITS101, which mediate strain-specific neutralizing activity against SIV from sooty mangabeys (SIVsm), was their ability to achieve near complete (i.e. 100%) neutralization of moderately and highly neutralization-resistant SIV. Overall, these newly identified SIV bnAbs highlight the potential for evaluating HIV-1 prophylactic and therapeutic interventions using fully simian, rhesus-derived bnAbs in the SIV NHP model, thereby circumventing issues related to rapid antibody clearance of human-derived antibodies, Fc mismatch and limited genetic diversity of SHIV compared to SIV.

T Cells Specific for a Mycobacterial Glycolipid Expand after Intravenous Bacillus Calmette-Guérin Vaccination.

Intradermal vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) protects infants from disseminated tuberculosis, and i.v. BCG protects nonhuman primates (NHP) against pulmonary and extrapulmonary tuberculosis. In humans and NHP, protection is thought to be mediated by T cells, which typically recognize bacterial peptide Ags bound to MHC proteins. However, during vertebrate evolution, T cells acquired the capacity to recognize lipid Ags bound to CD1a, CD1b, and CD1c proteins expressed on APCs. It is unknown whether BCG induces T cell immunity to mycobacterial lipids and whether CD1-restricted T cells are resident in the lung. In this study, we developed and validated Macaca mulatta (Mamu) CD1b and CD1c tetramers to probe ex vivo phenotypes and functions of T cells specific for glucose monomycolate (GMM), an immunodominant mycobacterial lipid Ag. We discovered that CD1b and CD1c present GMM to T cells in both humans and NHP. We show that GMM-specific T cells are expanded in rhesus macaque blood 4 wk after i.v. BCG, which has been shown to protect NHP with near-sterilizing efficacy upon M. tuberculosis challenge. After vaccination, these T cells are detected at high frequency within bronchoalveolar fluid and express CD69 and CD103, markers associated with resident memory T cells. Thus, our data expand the repertoire of T cells known to be induced by whole cell mycobacterial vaccines, such as BCG, and show that lipid Ag-specific T cells are resident in the lungs, where they may contribute to protective immunity.

The V2 loop of HIV gp120 delivers costimulatory signals to CD4+ T cells through Integrin α4ß7 and promotes cellular activation and infection.

Acute HIV infection is characterized by rapid viral seeding of immunologic inductive sites in the gut followed by the severe depletion of gut CD4+ T cells. Trafficking of α4ß7-expressing lymphocytes to the gut is mediated by MAdCAM, the natural ligand of α4ß7 that is expressed on gut endothelial cells. MAdCAM signaling through α4ß7 costimulates CD4+ T cells and promotes HIV replication. Similar to MAdCAM, the V2 domain of the gp120 HIV envelope protein binds to α4ß7 In this study, we report that gp120 V2 shares with MAdCAM the capacity to signal through α4ß7 resulting in CD4+ T cell activation and proliferation. As with MAdCAM-mediated costimulation, cellular activation induced by gp120 V2 is inhibited by anti-α4ß7 monoclonal antibodies (mAbs). It is also inhibited by anti-V2 domain antibodies including nonneutralizing mAbs that recognize an epitope in V2 that has been linked to reduced risk of acquisition in the RV144 vaccine trial. The capacity of the V2 domain of gp120 to mediate signaling through α4ß7 likely impacts early events in HIV infection. The capacity of nonneutralizing V2 antibodies to block this activity reveals a previously unrecognized mechanism whereby such antibodies might impact HIV transmission and pathogenesis.