Background:  We have previously identified a human scFv antibody, m9, which for a panel of 33 primary isolates exhibited significantly higher inhibitory activity than the potent broadly HIV-1 neutralizing scFv antibody X5. We have also previously found that on average scFv X5 is more potent than Fab X5 and that the potency of Fab X5 is comparable to that of IgG1 b12. We have hypothesized that m9 could exhibit exceptional potency and breadth of neutralization compared to the few known potent broadly neutralizing human monoclonal antibodies.

Methods:  Neutralization assays based on spreading infection in peripheral blood mononuclear cells (PBMC) and on infection of cell lines by pseudovirus or infectious virus for 6 different panels with envelope glycoproteins from about 100 primary isolates of clades A to D, CRFO1, CRFO2, and group O were performed.

Results:  In all panels of isolates m9 exhibited superior activity to any antibody used for comparison including b12, 4E10, 2F5, 2G12, X5, and scFv 17b in assays based on PBMC/replication competent virus. Its activity was especially extraordinary (IC₈₀ on the order of 0.1 µg/mL) for a subpanel of clade C isolates and was significantly increased (10- to 100-fold lower IC₅₀) for cell lines with low CCR5 surface concentration. Possible mechanisms of its exceptional potency and breadth of neutralization include the extreme conservation of its epitope which interacts predominantly with a unique highly flexible heavy chain CDR3 as revealed by the crystal structures of Fab X5 alone and complexed with a gp120 core, and computer modelling and mutagenesis.

Conclusions:  Our findings indicate the existence of antibodies with exceptional potency and breadth of neutralization that could be used for development of therapeutics. Their conserved epitopes may have potential as HIV vaccine immunogens and as targets for therapeutics.