The Chimpanzee SIV Envelope Trimer: Structure and Deployment as an HIV Vaccine Template

Andrabi, Raiees; Pallesen, Jesper; Allen, Joel D.; Song, Ge; Zhang, Jinsong; de Val, Natalia; Gegg, Gavin; Porter, Katelyn; Su, Ching-Yao; Pauthner, Matthias; Newman, Amanda; Bouton-Verville, Hilary; Garces, Fernando; Wilson, Ian A.; Crispin, Max; Hahn, Beatrice H.; Haynes, Barton F.; Verkoczy, Laurent; Ward, Andrew B.; Burton, Dennis R.

Previous mapping studies have defined the HIV core epitope recognized by mature V2-apex bnAbs (Andrabi et al., 2015; Gorman et al., 2016; Landais et al., 2017; Lee et al., 2017; McLellan et al., 2011; Pancera et al., 2013; Walker et al., 2009) . To examine the contributions of V2-apex core epitope glycan and protein residues to binding by V2-apex bnAb iGL Ab versions, we generated MT145K strand C peptide and glycan trimer variants that are known to eliminate interactions of V2-apex bnAbs with the Env trimer (Andrabi et al., 2015; McLellan et al., 2011; Pancera et al., 2013) . Bio-layer interferometry (BLI or octet) binding analyses of the iGL Abs with these trimer variants showed that glycan and/or peptide epitope requirements of precursor Abs were largely similar to the requirements of corresponding mature Abs ( Figure 1D ), suggesting that most contacts with the MT145K V2-apex core epitope are already encoded in the germline configuration for this class of bnAbs. Notably, the mature Abs showed slightly more tolerance to changes within the core protein epitope, particularly for the CAP256.09 bnAb, suggesting that part of the affinity maturation in this class of Abs may be to accommodate variation within the bnAb V2-apex core epitope. Garces et al., 2015; PDB: 5CEZ) . The ribbon representation of V1V2 loop strands that form the trimer apex show a cluster of positively charged lysine-rich peptide regions (HXB2-R166-K171: R or K residues shown as blue spheres) and the two glycans N156 and N160 (depicted in green spheres with lines). The side chains of the positively charged residues intersperse with the side chains of residues from adjacent protomers to form a continuous positively charged surface at the tip of the trimer to provide a minimal V2-apex bnAb epitope. (B) Amino-acid sequence alignment of strand B and C V2 of HIV CRF250, CAP256.SU, chimpanzee SIV MT145 WT, and its V2-modified variant (Q171K), MT145K. Glutamine (Q) at position 171 (shown in red) was substituted with lysine (K) in MT145 Env to gain binding to V2-apex bnAb inferred germline (iGL) Abs. (C) ELISA binding of mature V2-apex bnAbs, PG9, CAP256.09, and CH01 and their iGL versions to WT MT145 (red) and MT145K SOSIP trimers. (D) Octet binding curves (association, 120 s [180-300]; dissociation, 240 s [300-540]) of CAP256 UCA and CH01 iGL Abs and their respective mature Ab versions (CAP256.09 and CH01) to MT145K trimer, its glycan knockout (N160K) variant, K-rich core epitope substituted variants, and the corresponding monomeric gp120. The Abs were immobilized on human IgG Fc capture biosensors and 1uM trimer or gp120 proteins used as analytes. The binding response is shown in nanometers (nm).