A Broadly Neutralizing Antibody Targets the Dynamic HIV Envelope Trimer Apex via a Long, Rigidified, and Anionic β-Hairpin Structure

Abstract

SummaryBroadly neutralizing antibodies (bnAbs) to HIV delineate vaccine targets and are prophylactic and therapeutic agents. Some of the most potent bnAbs target a quaternary epitope at the apex of the surface HIV envelope (Env) trimer. Using cryo-electron microscopy, we solved the atomic structure of an apex bnAb, PGT145, in complex with Env. We showed that the long anionic HCDR3 of PGT145 penetrated between glycans at the trimer 3-fold axis, to contact peptide residues from all three Env protomers, and thus explains its highly trimer-specific nature. Somatic hypermutation in the other CDRs of PGT145 were crucially involved in stabilizing the structure of the HCDR3, similar to bovine antibodies, to aid in recognition of a cluster of conserved basic residues hypothesized to facilitate trimer disassembly during viral entry. Overall, the findings exemplify the creative solutions that the human immune system can evolve to recognize a conserved motif buried under a canopy of glycans.

High-Throughput Protein Engineering Improves the Antigenicity and Stability of Soluble HIV-1 Envelope Glycoprotein SOSIP Trimers

“…3). The sensitivity of PGT145 binding is consistent with its known absolute specificity for native-like trimers (21,22). Of the three individual mutagenesis strategies, the introduction of cysteine pairs was the most detrimental to trimer antigenicity.…”

“…We first used three bNAbs, PGT151, PGT145, and PG9, as probes to detect both the expression and conformational integrity of Env trimers. These antibodies bind conformationally sensitive epitopes that are either dependent on (PGT151 and PGT145) or influenced by (PG9) the formation of a structurally intact trimer (5,20,21). PGT151 binds at the interface between gp120 and gp41, whereas PGT145 and PG9 recognize the trimer apex (Fig.…”

Glycans Function as Anchors for Antibodies and Help Drive HIV Broadly Neutralizing Antibody Development

“…Broadly speaking, a lysine-rich set of residues on V2 come together in space around the 3-fold axis at the “top” of the spike to constitute a basic region that is recognized by acidic residues (aspartate and sulfated tyrosines) at the tip of a long HCDR3 (heavy chain complementarity-determining region 3 of the heavy chain) of the bnAb. In order to access the basic region, the HCDR3, and to some extent the other CDRs, of the bnAb must penetrate the glycan shield of Env and particularly interact with and/or accommodate a conserved glycan at N160 and, to a lesser extent, a second nearby glycan at N156 or rarely N173 (Amin et al, 2013; Gorman et al, 2016; Lee et al, 2017; McLellan et al, 2011; Pancera et al, 2013; Walker et al, 2009). Detailed studies have combined structure, nuclear magnetic resonance (NMR) and mutagenesis analysis to characterize glycan recognition by the V2 apex bnAbs PG9 and PG16 (McLellan et al, 2011; Pancera et al, 2013).…”

Protection against a mixed SHIV challenge by a broadly neutralizing antibody cocktail

“…Signature residues were resolved using a phylogenetically corrected strategy using large panels of M group or C clade viruses (B. Korber et al, submitted), and contact sites were found by structural modeling using structures of PGT145 and PGT122 complexed with HIV-1 Env and of PGDM1400 and PGT121 Fabs (13, 23–25) (Fig. S7).…”

“…The key N160 PNGS for PGDM1400 was variable in only 3/10 individuals, but a critical R166 signature was either variable or lost in 5/10 individuals, and the key K/R169 positive charge was either variable or lost in 10/10 individuals (Fig. 8; Table S1) (23). These findings in a random sample of HIV-1 infected individuals suggest that diverse resistance profiles evolve over time in HIV-1-infected humans.…”