Bjorkman: Improving Potency and Breadth of Natural bNAbs
An effective vaccine against HIV remains elusive and AIDS continues to be a major source of morbidity and mortality worldwide. The failure of HIV vaccine trials has elevated the importance of pursuing an alternative form of prevention, namely the concept of using anti-HIV monoclonal antibodies as a passive form of prevention.
It is thought that broadly neutralizing antibodies (bNAbs) or a vaccine that elicits them would be effective in interfering with transmission. bNAbs may also function as adjuncts to conventional therapies. However, only a small number of bNAbs have been characterized to date and the immunological basis for their breadth and potency is poorly understood. Developing an understanding of the structural underpinnings of neutralization by bNAbs is critical to enhancing their activity, which could make passive transfer methods a realistic and affordable option in the developing world.
This grant to Dr. Pamela Bjorkman at the California Institute of Technology will be focused on determining/interpreting structures of new bNAbs in order to combine features of different bNAbs into a single bNAb. This newly created combination bNAb will be used to make reagents with greater breadth and potency against HIV. This work will be done in collaboration with Dr. Nussenzweig at Rockefeller University.
1. Characterize HIV gp120s from bNAb donors to determine if/how Abs force HIV into a defective state
2. Use structural approaches to define the essential features of broad and potent HIV neutralization
3. Produce natural antibody variants with enhanced activity
4. Combine activities from two or more bNAbs to make single reagents with increased breadth
We are taking a multi-pronged approach to developing improved broadly neutralizing antibodies (bNAbs) against HIV-1 – including screening for new bNAbs, understanding bNAb–virus co-evolution, using structural approaches to define essential features of broad and potent HIV neutralization, and designing bNAbs with increased potency and breadth. We defined the critical sequence characteristics of the VRC01-class of bNAbs, and now understand their germline gene usage. We previously used structure-based design to create NIH45-46G54W, a VRC01-like antibody with superior potency and/or breadth compared with other bNAbs. We then designed/validated more effective variants of NIH45-46G54W using analyses of the NIH45-46/gp120 structure and sequences of NIH45-46G54W-resistant HIV-1 clones. We have designed a NIH45-46 variant to thwart resistance from NIH45-46G54W due to mutations in a V5/loop D gp120 consensus sequence. This modification restores neutralization to strains carrying consensus NIH45-46-resistance mutations, thus effectively targeting a common route of HIV escape. Our antibody analyses were facilitated by development of new software, named "Antibody Database", to identify critical residues on Env whose natural variation affects antibody activity. Using Antibody Database, we determined that 8ANC195, a bNAb of unknown specificity, recognizes a new glycan-dependent epitope on HIV-1 gp120.
We also isolated more potent variants of PGT121, a glycan-dependent bNAb that binds complex-type
N-glycans in glycan microarrays. The B-cell clone encoding PGT121 segregated into PGT121- and 10-1074-like groups distinguished by sequence, binding affinity, carbohydrate recognition and neutralizing activity. 10-1074 exhibited remarkable potency and breadth but no detectable binding to protein-free glycans. Crystal structures of unliganded PGT121, 10-1074, and their likely germline precursor revealed that differential carbohydrate recognition mapped to a cleft on the antibody VH domain that was occupied by a complex-type N-glycan in a "liganded" PGT121 structure. Although PGT121 binds complex-type
N-glycans, PGT121 recognized high-mannose-only HIV envelopes in isolation and on virions. As HIV-1 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. Indeed, a combination of 10-1074 with our two new NIH45-46G54W mutants was able to control HIV-1 infection in humanized mice.