| Abstract: | Broadly neutralizing HIV antibodies (bNAbs) can recognize carbohydrate-dependent epitopes on gp120. In contrast to previously characterized glycan-dependent bNAbs that recognize high-mannose N-glycans, PGT121 binds complex-type N-glycans in glycan microarrays. We isolated the B-cell clone encoding PGT121, which segregates into PGT121-like and 10-1074–like groups distinguished by sequence, binding affinity, carbohydrate recognition, and neutralizing activity. Group 10-1074 exhibits remarkable potency and breadth but no detectable binding to protein-free glycans. Crystal structures of unliganded PGT121, 10-1074, and their likely germ-line precursor reveal that differential carbohydrate recognition maps to a cleft between complementarity determining region (CDR)H2 and CDRH3. This cleft was occupied by a complex-type N-glycan in a “liganded” PGT121 structure. Swapping glycan contact residues between PGT121 and 10-1074 confirmed their importance for neutralization. Although PGT121 binds complex-type N-glycans, PGT121 recognized high-mannose-only HIV envelopes in isolation and on virions. As HIV envelopes exhibit varying proportions of high-mannose- and complex-type N-glycans, these results suggest promiscuous carbohydrate interactions, an advantageous adaptation ensuring neutralization of all viruses within a given strain.Antibodies are essential for the success of most vaccines (1), and antibodies against HIV appear to be the only correlate of protection in the recent RV144 anti-HIV vaccine trial (2). Some HIV-1–infected patients develop broadly neutralizing serologic activity against the gp160 viral spike 2–4 y after infection (3–10), but these antibodies do not generally protect infected humans because autologous viruses escape through mutation (11–13). Nevertheless, broadly neutralizing activity puts selective pressure on the virus (13) and passive transfer of broadly neutralizing antibodies (bNAbs) to macaques protects against simian/human immunodeficiency virus (SHIV) infection (14–24). It has therefore been proposed that vaccines that elicit such antibodies may be protective against HIV infection in humans (10, 25–28).The development of single-cell antibody cloning techniques revealed that bNAbs target several different epitopes on the HIV-1 gp160 spike (29–35). The most potent HIV-1 bNAbs recognize the CD4 binding site (CD4bs) (31, 34, 36) and carbohydrate-dependent epitopes associated with the variable loops (32, 33, 37, 38), including the V1/V2 (antibodies PG9/PG16) (33) and V3 loops (PGTs) (32). Less is known about carbohydrate-dependent epitopes because the antibodies studied to date are either unique examples or members of small clonal families.To better understand the neutralizing antibody response to HIV-1 and the epitope targeted by PGT antibodies, we isolated members of a large clonal family dominating the gp160-specific IgG memory response from the clade A-infected patient who produced PGT121. We report that PGT121 antibodies segregate into two groups, a PGT121-like and a 10-1074–like group, according to sequence, binding affinity, neutralizing activity, and recognition of carbohydrates and the V3 loop. The 10-1074 antibody and related family members exhibit unusual potent neutralization, including broad reactivity against newly transmitted viruses. Unlike previously characterized carbohydrate-dependent bNAbs, PGT121 binds to complex-type, rather than high-mannose, N-glycans in glycan microarray experiments. Crystal structures of PGT121 and 10-1074 compared with structures of their germ-line precursor and a structure of PGT121 bound to a complex-type N-glycan rationalize their distinct properties. |
