A single point mutation in HIV-1V3 loop alters the immunogenic properties of rgp120

The results of the study presented in this report show that clones of env d erived from genetically divergent HIV-1 field isolates fall into two major subsets based on the predicted secondary structure of the V3 region in gp12 0. One subset exemplified by the clones A-UG06c, B-RT3.12 and C-UG045 is pr edicted to assume a beta -turn conformation in the V3 loop and comprises th e GPG (X) under bar residues. The other subset exemplified by the clones D- UG23c and D-UG042 (G (X) under barG (X) under bar are deficient in the expr ession of the beta -turn in the loop. Since secondary conformations are hig hly likely to confer antigenic properties in a protein backbone at least fo r B cells, we have used nucleic acid immunization to test the effect of the beta -turn deficiency on the immunogenic potential of rgp 120 encoded in t hese field isolates. As hypothesized, inoculation of BALB/c mice with the e nv plasmid encoding the beta -turn expressing rgp120 molecules resulted in the development of a vigorous antibody response to the homologous V3 loop p eptides. In contrast, immunization with an rgp120 clone deficient in the be ta -turn in the V3 loop showed no evidence of antibody development to the V 3 loop. Instead, the latter clones triggered T cell proliferative responses and markedly increased the level of IL-2 and IFN-gamma production by T cel ls. Significantly, reconstitution of the beta -turn conformation by site-di rected mutagenesis of a single V3 loop residue yielded rgp120 molecules whi ch restored antibody production while diminishing the cell-mediated immune (CMI) responses to the V3 residue. These observations demonstrate the marke d impact of a single amino acid substitution on the immunogenic properties of V3 region in gp120 encoded by divergent HIV-I, field isolates.