Type-specific epitopes targeted by monoclonal antibodies with exceptionally potent neutralizing activities for selected strains of human immunodeficiency virus type 1 map to a common region of the V2 domain of gp120 and differ only at single positions from the clade B consensus sequence.

Only a few monoclonal antibodies (MAbs) have been isolated that recognize conserved sites in human immunodeficiency virus type 1 (HIV-1) Env proteins and possess broad neutralizing activities. Other MAbs directed against targets in various domains of Env have been described that are strongly neutralizing, but they possess limited breadth. One such MAb, 2909, possesses a uniquely potent neutralizing activity specific for a quaternary epitope on SF162 Env that requires the presence of both the V2 and the V3 domains. We now show that replacement of the SF162 V3 sequence with consensus V3 sequences of multiple subtypes led to attenuated but still potent neutralization by 2909 and that the main determinants for the type specificity of 2909 reside in the V2 domain. A substitution at position 160 completely eliminated 2909 reactivity, and mutations at position 167 either attenuated or potentiated neutralization by this antibody. Different substitutions at the same positions in V2 were previously shown to introduce epitopes recognized by MAbs 10/76b and C108g and to allow potent neutralization by these MAbs. Two substitutions at key positions in the V2 domain of JR-FL Env also allowed potent expression of the 2909 epitope, and single substitutions in YU2 V2 were sufficient for expression of the 2909, C108g, and 10/76b epitopes. These results demonstrate that the minimal epitopes for 2909, C108g, and 10/76b differed from that of the clade B consensus sequence only at single positions and suggest that all three MAbs recognize distinct variants of a relatively conserved sequence in V2 that is a particularly sensitive mediator of HIV-1 neutralization.