Characterization of CD4-Induced Epitopes on the HIV Type 1 gp120 Envelope Glycoprotein Recognized by Neutralizing Human Monoclonal Antibodies

The entry of human immunodeficiency virus (HIV-1) into target cells typically requires the sequential binding of the viral exterior envelope glycoprotein, gp120, to CD4 and a chemokine receptor. CD4 binding exposes gp120 epitopes recognized by CD4-induced (CD4i) antibodies, which can block virus binding to the chemokine receptor. We identified three new CD4i antibodies from an HIV-1-infected individual and localized their epitopes. These epitopes include a highly conserved gp120 β-strand encompassing residues 419-424, which is also important for binding to the CCR5 chemokine receptor. All of the CD4i antibodies inhibited the binding of gp120-CD4 complexes to CCR5. CD4i antibodies and CD4 reciprocally induced each other's binding, suggesting that these ligands recognize a similar gp120 conformation. The CD4i antibodies neutralized laboratory-adapted HIV-1 isolates; primary isolates were more resistant to neutralization by these antibodies. Thus, all known CD4i antibodies recognize a common, conserved gp120 element overlapping the binding site for the CCR5 chemokine receptor.