The entry of human immunodeficiency virus (HIV) into cells requires the sequential interaction of the viral exterior envelope glycoprotein, gp120, with the CD4 glycoprotein and a chemokine receptor on the cell surface. These interactions initiate a fusion of the viral and cellular membranes. Although gpl20 can elicit virus-neutralizing antibodies, HIV eludes the immune system. We have solved the X-ray crystal structure at 2.5 Å resolution of an HIV-1 gp120 core complexed with a two-domain fragment of human CD4 and an antigen-binding fragment of a neutralizing antibody that blocks chemokine-receptor binding. The structure reveals a cavity-laden CD4-gp120 interface, a conserved binding site for the chemokine receptor, evidence for a conformational change upon CD4 binding, the nature of a CD4-induced antibody epitope, and specific mechanisms for immune evasion. Our results provide a framework for understanding the complex biology of HIV entry into cells and should guide efforts to intervene.The human immunodeficiency viruses HIV-1 and HIV-2 and the related simian immunodeficiency viruses (SIV) cause the destruction of CD4 + lymphocytes in their respective hosts, resulting in the development of acquired immunodeficiency syndromeCorrespondence and requests for materials should be addressed to W. A.H. (wayne@convex.hhmi.columbia.edu).Coordinates have been deposited in the Brookhaven Protein Data Bank (accession code 1gc1) and maybe obtained from the authors. HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript (AIDS) 1,2 . The entry of HIV into host cells is mediated by the viral envelope glycoproteins, which are organized into oligomeric, probably trimeric spikes displayed on the surface of the virion. These envelope complexes are anchored in the viral membrane by the gp41 transmembrane envelope glycoprotein. The surface of the spike is composed primarily of the exterior envelope glycoprotein, gp120, associated by non-covalent interactions with each subunit of the trimeric gp41 glycoprotein complex 3,4 . Comparison of the gp120 sequences of different primate immunodeficiency viruses identified five variable regions (V1-V5) (ref. 5 ). The first four variable regions form surface-exposed loops that contain disulphide bonds at their bases 6 . The conserved gp120 regions form discontinuous structures important for the interaction with the gp41 ectodomain and with the viral receptors on the target cell. Both conserved and variable gp120 regions are extensively glycosylated 6 . The variability and glycosylation of the gp120 surface probably modulate the immunogenicity and antigenicity of the gp120 glycoprotein, which is the main target for neutralizing antibodies elicited during natural infection 7 .Entry of primate immunodeficiency viruses into the host cell involves the binding of the gp120 envelope glycoprotein to the CD4 glycoprotein, which serves as the primary receptor. The gp120 glycoprotein binds to the most amino-terminal of the four immunoglobulin-like domains of CD4. S...

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.

The human immunodeficiency virus-type 1 (HIV-1) envelope glycoproteins interact with receptors on the target cell and mediate virus entry by fusing the viral and cell membranes. The structure of the envelope glycoproteins has evolved to fulfill these functions while evading the neutralizing antibody response. An understanding of the viral strategies for immune evasion should guide attempts to improve the immunogenicity of the HIV-1 envelope glycoproteins and, ultimately, aid in HIV-1 vaccine development.

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.

The human immunodeficiency virus HIV-1 establishes persistent infections in humans which lead to acquired immunodeficiency syndrome (AIDS). The HIV-1 envelope glycoproteins, gp120 and gp41, are assembled into a trimeric complex that mediates virus entry into target cells. HIV-1 entry depends on the sequential interaction of the gp120 exterior envelope glycoprotein with the receptors on the cell, CD4 and members of the chemokine receptor family. The gp120 glycoprotein, which can be shed from the envelope complex, elicits both virus-neutralizing and non-neutralizing antibodies during natural infection. Antibodies that lack neutralizing activity are often directed against the gp120 regions that are occluded on the assembled trimer and which are exposed only upon shedding. Neutralizing antibodies, by contrast, must access the functional envelope glycoprotein complex and typically recognize conserved or variable epitopes near the receptor-binding regions. Here we describe the spatial organization of conserved neutralization epitopes on gp120, using epitope maps in conjunction with the X-ray crystal structure of a ternary complex that includes a gp120 core, CD4 and a neutralizing antibody. A large fraction of the predicted accessible surface of gp120 in the trimer is composed of variable, heavily glycosylated core and loop structures that surround the receptor-binding regions. Understanding the structural basis for the ability of HIV-1 to evade the humoral immune response should assist in the design of a vaccine.

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 Arg/Lys just prior to 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 Env.

The remarkable diversity, glycosylation and conformational flexibility of the human immunodeficiency virus type 1 (HIV-1) envelope (Env), including substantial rearrangement of the gp120 glycoprotein upon binding the CD4 receptor, allow it to evade antibody-mediated neutralization. Despite this complexity, the HIV-1 Env must retain conserved determinants that mediate CD4 binding. To evaluate how these determinants might provide opportunities for antibody recognition, we created variants of gp120 stabilized in the CD4-bound state, assessed binding of CD4 and of receptor-binding-site antibodies, and determined the structure at 2.3 Å resolution of the broadly neutralizing antibody b12 in complex with gp120. b12 binds to a conformationally invariant surface that overlaps a distinct subset of the CD4-binding site. This surface is involved in the metastable attachment of CD4, before the gp120 rearrangement required for stable engagement. A site of vulnerability, related to a functional requirement for efficient association with CD4, can therefore be targeted by antibody to neutralize HIV-1.The human immunodeficiency virus type 1 (HIV-1) crossed from chimpanzees to humans early in the twentieth century and has since infected ~1% of the world's adult population 1,2 . ThisCorrespondence and requests for materials should be addressed to P.D.K. (pdkwong@nih.gov). Author Contributions T.Z. and P.D.K. carried out structure-based stabilization, SPR analyses and structural determinations; L.X. and G.J.N. constructed gp120 substitutions and developed and implemented a high-throughput gp120-production system suitable for crystallization; B.D. and R.W. carried out ITC characterizations; A.J.H., M.B.Z. and D.R.B. provided b12, b3, b6, b11 and b13, and mutant b12 binding; D.V.R. and J.A. provided D1D2-Igαtp and associated SPR analyses; S.-H.X., X.Y. and J.S. provided OD1 and preliminary design and antigenic analyses; and M.-Y.Z. and D.S.D. provided m6, m14 and m18. All authors contributed to the manuscript preparation.Author Information Coordinates and structure factors have been deposited in the Protein Data Bank and may be obtained from the authors (accession codes 2nxy-2ny6 for the nine variant gp120 molecules with CD4 and 17b; accession code 2ny7 for the b12-gp120 complex). Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. spread and the absence of an effective vaccine are to a large degree a consequence of the ability of HIV-1 to evade antibody-mediated neutralization 3-5 . On HIV-1, the only viral target available for neutralizing antibodies is the envelope spike, which is composed of three copies of the gp120 exterior envelope glycoprotein and three gp41 transmembrane glyco-protein molecules 6,7 . Genetic, immunological and structural studies of the HIV-1 envelope glycoproteins have revealed extraordinary diversity, manifest in a variety of immunodominant loops, as well as multiple overlapping mechanisms of humoral evasion, including se...

For efficient entry into target cells, primary macrophage-tropic and laboratory-adapted human immunodeficiency viruses type 1 (HIV-1) require particular chemokine receptors, CCR-5 and CXCR-4, respectively, as well as the primary receptor CD4 (refs 1-6). Here we show that a complex of gp120, the exterior envelope glycoprotein, of macrophage-tropic primary HIV-1 and soluble CD4 interacts specifically with CCR-5 and inhibits the binding of the natural CCR-5 ligands, macrophage inflammatory protein (MIP)-1alpha and MIP-1beta (refs 7, 8). The apparent affinity of the interaction between gp120 and CCR-5 was dramatically lower in the absence of soluble CD4. Additionally, in the absence of gp120, an interaction between a two-domain CD4 fragment and CCR-5 was observed. A gp120 fragment retaining the CD4-binding site and overlapping epitopes was able to interact with CCR-5 only if the V3 loop, which can specify HIV-1 tropism and chemokine receptor choice, was also present on the molecule. Neutralizing antibodies directed against either CD4-induced or V3 epitopes on gp120 blocked the interaction of gp12O-CD4 complexes with CCR-5. These results suggest that HIV-1 attachment to CD4 creates a high-affinity binding site for CCR-5, leading to membrane fusion and virus entry.