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 typical course of HIV infection for a majority of untreated individuals is persistent viral replication and a gradual loss of CD4 + T cells. One of the consequences of ongoing HIV replication is increased immune activation, aff ecting all major cell populations of the immune system ( 1 -3 ). Within the B cell population, HIV infection has been associated with numerous perturbations ( 4 ), many of which have been attributed to changes in the distribution of B cell subpopulations found in the peripheral blood. These changes include increased frequencies of activated and terminally diff erentiated B cells expressing low levels of CD21 that have been associated with ongoing viral replication ( 5, 6 ), a decreased frequency of memory B cells that is not reversed by antiretroviral therapy ( 7 ), and an increased frequency of immature/transitional B cells that has been associated with CD4 + T cell lymphopenia ( 8, 9 ).The eff ects of immune activation in persistent viral infections have recently been shown to include virus-specifi c T cell exhaustion. After the original description in chronic lymphocyte choriomeningitis virus (LCMV) infection in mice ( 10 ), observations of virus-specifi c CD4 + and CD8 + T cell exhaustion have recently been extended to 12 ). Although PD-1 was the fi rst inhibitory receptor associated with virus-specifi c T cell exhaustion, recent fi ndings suggest that exhaustion may result

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...