Mark K. Louder

Rational Design of Envelope Identifies Broadly Neutralizing Human Monoclonal Antibodies to HIV-1

Designer Anti-HIV Developing a protective HIV vaccine remains a top global health priority. One strategy to identify potential vaccine candidates is to isolate broadly neutralizing antibodies from infected individuals and then attempt to elicit the same antibody response through vaccination (see the Perspective by Burton and Weiss). Wu et al. (p. 856, published online 8 July) now report the identification of three broadly neutralizing antibodies, isolated from an HIV-1–infected individual, that exhibited great breadth and potency of neutralization and were specific for the co-receptor CD4-binding site of the glycoprotein 120 (gp120), part of the viral Env spike. Zhou et al. (p. 811, published online 8 July) analyzed the crystal structure for one of these antibodies, VRC01, in complex with an HIV-1 gp120. VRC01 focuses its binding onto a conformationally invariant domain that is the site of initial CD4 attachment, which allows the antibody to overcome the glycan and conformational masking that diminishes the neutralization potency of most CD4-binding-site antibodies. The epitopes recognized by these antibodies suggest potential immunogens that can inform vaccine design. A human antibody achieves broad neutralization by binding the viral site of recognition for the primary host receptor, CD4. Cross-reactive neutralizing antibodies (NAbs) are found in the sera of many HIV-1–infected individuals, but the virologic basis of their neutralization remains poorly understood. We used knowledge of HIV-1 envelope structure to develop antigenically resurfaced glycoproteins specific for the structurally conserved site of initial CD4 receptor binding. These probes were used to identify sera with NAbs to the CD4-binding site (CD4bs) and to isolate individual B cells from such an HIV-1–infected donor. By expressing immunoglobulin genes from individual cells, we identified three monoclonal antibodies, including a pair of somatic variants that neutralized over 90% of circulating HIV-1 isolates. Exceptionally broad HIV-1 neutralization can be achieved with individual antibodies targeted to the functionally conserved CD4bs of glycoprotein 120, an important insight for future HIV-1 vaccine design.

Co-evolution of a broadly neutralizing HIV-1 antibody and founder virus

Current human immunodeficiency virus-1 (HIV-1) vaccines elicit strain-specific neutralizing antibodies. However, cross-reactive neutralizing antibodies arise in approximately 20% of HIV-1-infected individuals, and details of their generation could provide a blueprint for effective vaccination. Here we report the isolation, evolution and structure of a broadly neutralizing antibody from an African donor followed from the time of infection. The mature antibody, CH103, neutralized approximately 55% of HIV-1 isolates, and its co-crystal structure with the HIV-1 envelope protein gp120 revealed a new loop-based mechanism of CD4-binding-site recognition. Virus and antibody gene sequencing revealed concomitant virus evolution and antibody maturation. Notably, the unmutated common ancestor of the CH103 lineage avidly bound the transmitted/founder HIV-1 envelope glycoprotein, and evolution of antibody neutralization breadth was preceded by extensive viral diversification in and near the CH103 epitope. These data determine the viral and antibody evolution leading to induction of a lineage of HIV-1 broadly neutralizing antibodies, and provide insights into strategies to elicit similar antibodies by vaccination.

Structure of HIV-1 gp120 V1/V2 domain with broadly neutralizing antibody PG9

Variable regions 1 and 2 (V1/V2) of human immunodeficiency virus-1 (HIV-1) gp120 envelope glycoprotein are critical for viral evasion of antibody neutralization, and are themselves protected by extraordinary sequence diversity and N-linked glycosylation. Human antibodies such as PG9 nonetheless engage V1/V2 and neutralize 80% of HIV-1 isolates. Here we report the structure of V1/V2 in complex with PG9. V1/V2 forms a four-stranded β-sheet domain, in which sequence diversity and glycosylation are largely segregated to strand-connecting loops. PG9 recognition involves electrostatic, sequence-independent and glycan interactions: the latter account for over half the interactive surface but are of sufficiently weak affinity to avoid autoreactivity. The structures of V1/V2-directed antibodies CH04 and PGT145 indicate that they share a common mode of glycan penetration by extended anionic loops. In addition to structurally defining V1/V2, the results thus identify a paradigm of antibody recognition for highly glycosylated antigens, which—with PG9—involves a site of vulnerability comprising just two glycans and a strand.

Broad and potent neutralization of HIV-1 by a gp41-specific human antibody

Characterization of human monoclonal antibodies is providing considerable insight into mechanisms of broad HIV-1 neutralization. Here we report an HIV-1 gp41 membrane-proximal external region (MPER)-specific antibody, named 10E8, which neutralizes ∼98% of tested viruses. An analysis of sera from 78 healthy HIV-1-infected donors demonstrated that 27% contained MPER-specific antibodies and 8% contained 10E8-like specificities. In contrast to other neutralizing MPER antibodies, 10E8 did not bind phospholipids, was not autoreactive, and bound cell-surface envelope. The structure of 10E8 in complex with the complete MPER revealed a site of vulnerability comprising a narrow stretch of highly conserved gp41-hydrophobic residues and a critical arginine or lysine just before the transmembrane region. Analysis of resistant HIV-1 variants confirmed the importance of these residues for neutralization. The highly conserved MPER is a target of potent, non-self-reactive neutralizing antibodies, suggesting that HIV-1 vaccines should aim to induce antibodies to this region of HIV-1 envelope glycoprotein.

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.

Broad HIV-1 neutralization mediated by CD4-binding site antibodies.

We have identified several patient sera showing potent and broad HIV-1 neutralization. Using antibody adsorption and elution from selected gp120 variants, the neutralizing specificities of the two most broadly reactive sera were mapped to the primary receptor CD4–binding region of HIV-1 gp120. Novel antibodies to the CD4-binding site are elicited in some HIV-1–infected individuals, and new approaches to present this conserved region of gp120 to the immune system may result in improved vaccine immunogens.

Analysis of neutralization specificities in polyclonal sera derived from human immunodeficiency virus type 1-infected individuals.

ABSTRACT During human immunodeficiency virus type 1 (HIV-1) infection, patients develop various levels of neutralizing antibody (NAb) responses. In some cases, patient sera can potently neutralize diverse strains of HIV-1, but the antibody specificities that mediate this broad neutralization are not known, and their elucidation remains a formidable challenge. Due to variable and nonneutralizing determinants on the exterior envelope glycoprotein (Env), nonnative Env protein released from cells, and the glycan shielding that assembles in the context of the quaternary structure of the functional spike, HIV-1 Env elicits a myriad of binding antibodies. However, few of these antibodies can neutralize circulating viruses. We present a systematic analysis of the NAb specificities of a panel of HIV-1-positive sera, using methodologies that identify both conformational and continuous neutralization determinants on the HIV-1 Env protein. Characterization of sera included selective adsorption with native gp120 and specific point mutant variants, chimeric virus analysis, and peptide inhibition of viral neutralization. The gp120 protein was the major neutralizing determinant for most sera, although not all neutralization activity against all viruses could be identified. In some broadly neutralizing sera, the gp120-directed neutralization mapped to the CD4 binding region of gp120. In addition, we found evidence that regions of the gp120 coreceptor binding site may also be a target of neutralizing activity. Sera displaying limited neutralization breadth were mapped to the immunogenic V3 region of gp120. In a subset of sera, we also identified NAbs directed against the conserved, membrane-proximal external region of gp41. These data allow a more detailed understanding of the humoral responses to the HIV-1 Env protein and provide insights regarding the most relevant targets for HIV-1 vaccine design.

The gene product Murr1 restricts HIV-1 replication in resting CD4 + lymphocytes

Although human immunodeficiency virus-1 (HIV-1) infects quiescent and proliferating CD4+ lymphocytes, the virus replicates poorly in resting T cells. Factors that block viral replication in these cells might help to prolong the asymptomatic phase of HIV infection; however, the molecular mechanisms that control this process are not fully understood. Here we show that Murr1, a gene product known previously for its involvement in copper regulation, inhibits HIV-1 growth in unstimulated CD4+ T cells. This inhibition was mediated in part through its ability to inhibit basal and cytokine-stimulated nuclear factor (NF)-κB activity. Knockdown of Murr1 increased NF-κB activity and decreased IκB-α concentrations by facilitating phospho-IκB-α degradation by the proteasome. Murr1 was detected in CD4+ T cells, and RNA-mediated interference of Murr1 in primary resting CD4+ lymphocytes increased HIV-1 replication. Through its effects on the proteasome, Murr1 acts as a genetic restriction factor that inhibits HIV-1 replication in lymphocytes, which could contribute to the regulation of asymptomatic HIV infection and the progression of AIDS.

Differential susceptibility to human immunodeficiency virus type 1 infection of myeloid and plasmacytoid dendritic cells.

ABSTRACT Human immunodeficiency virus type 1 (HIV-1) infection of dendritic cells (DCs) plays an important role in HIV-1 transmission and pathogenesis. Here, we studied the susceptibility of ex vivo-isolated CD11c+ myeloid DCs (MDCs) and CD123+ plasmacytoid DCs (PDCs) to HIV-1 infection and the function of these cells early after infection. Both DC subsets were susceptible to CCR5- and CXCR4-using HIV-1 isolates (BaL and IIIB, respectively). However, MDCs were more susceptible to HIV-1BaL infection than donor-matched PDCs. In addition, HIV-1BaL infected MDCs more efficiently than HIV-1IIIB, whereas PDCs were equally susceptible to both isolates. While exposure to HIV-1 alone resulted in only weak maturation of DCs, Toll-like receptor 7/8 ligation induced full maturation in both infected and uninfected DCs. Maturation did not increase HIV-1 replication in infected DCs, and infected DCs retained their ability to produce tumor necrosis factor alpha after stimulation. Both HIV-1 isolates induced alpha interferon production exclusively in PDCs, irrespective of productive infection. In conclusion, PDCs and MDCs were susceptible to HIV-1 infection, but neither displayed functional defects as a consequence of infection. The difference in susceptibility of PDCs and MDCs to HIV-1 may have implications for HIV-1 transmission and DC-mediated transfer of HIV-1 to T cells.

Structural basis for diverse N-glycan recognition by HIV-1–neutralizing V1–V2–directed antibody PG16

HIV-1 uses a diverse N-linked-glycan shield to evade recognition by antibody. Select human antibodies, such as the clonally related PG9 and PG16, recognize glycopeptide epitopes in the HIV-1 V1–V2 region and penetrate this shield, but their ability to accommodate diverse glycans is unclear. Here we report the structure of antibody PG16 bound to a scaffolded V1–V2, showing an epitope comprising both high mannose–type and complex-type N-linked glycans. We combined structure, NMR and mutagenesis analyses to characterize glycan recognition by PG9 and PG16. Three PG16-specific residues, arginine, serine and histidine (RSH), were critical for binding sialic acid on complex-type glycans, and introduction of these residues into PG9 produced a chimeric antibody with enhanced HIV-1 neutralization. Although HIV-1–glycan diversity facilitates evasion, antibody somatic diversity can overcome this and can provide clues to guide the design of modified antibodies with enhanced neutralization.