CHARACTERIZATION OF MUTANTS OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 THAT HAVE ESCAPED NEUTRALIZATION BY A MONOCLONAL-ANTIBODY TO THE GP120 V2 LOOP

The biologically cloned human immunodeficiency virus type I (HIV-1) RF isolate is sensitive to neutralization by the murine monoclonal antib ody (MAb) G3-4 to a conformationally sensitive epitope in the V2 loop of HIV-1 gp120. To assess how variation in the V2 amino acid sequence affects neutralization by this MAb, we cultured RF in the presence of G3-4 Co select neutralization escape mutants. Three such mutants resis tant to G3-4 neutralization were generated from three independent expe riments. Solubilized gp120 from each of these escape mutants had a red uced affinity for G3-4 and also for two other V2 MAbs that were able t o bind the wild-type RF gp120. PCR sequencing of the entire gp120 of t he wild-type RI: virus and the escape mutants showed that amino acid s ubstitutions had occurred only at two positions, Y177H and L179P, both in V2. Experimental introduction of the Y177H substitution into the R F V2 loop in the context of the NLA-3 molecular clone re created the G 3-4-resistant phenotype. The L179P mutant was not viable. Thus, our fi ndings confirm that the HIV-1 V2 loop contains the conformationally se nsitive neutralization epitope recognized by G3-4 and that a single am ino acid substitution within this region can result in escape variants that arise from immune selection pressure.