THE EFFECT OF LOW-PROFILE SERINE SUBSTITUTIONS IN THE V3 LOOP OF HIV-1 GP120 IIIB LAI ON THE IMMUNOGENICITY OF THE ENVELOPE PROTEIN/

Many microbial antigens contain powerful hypervariable epitopes that f ail to induce broadly protective immunity because they dominate the im mune response at the expense of more conserved but weaker epitopes. If the undesired a cell epitopes are eliminated, the immune system could be focused on the conserved epitopes and produce a stronger antibody response to conserved parts of the protein and thus become a more effi cacious immunogen for a vaccine. We examined this possibility using th e human immunodeficiency virus envelope glycoprotein (gp)120 IIIB/LAI and selectively replaced the amino acids from the vs region and analyz ed the overall immunogenicity of the mutant proteins after nucleic aci d immunization in mice. The most variable residues of the human immuno deficiency virus type 1 gp120 Vs loop sequence were replaced with seri ne, which has a small uncharged hydrophilic side chain and therefore i s likely to be less immunogenic than amino acids found in wildtype vs sequences. The serine substitutions did not affect the ability of solu ble CD4 to bind the mutant molecules compared with wildtype gp120 and monoclonal antibodies against both linear and discontinuous epitopes l ocated in the V1/V2, C1, and C4 regions of the molecule. These data su ggest that the vs loop substitutions did not grossly affect the overal l conformation of the envelope molecule. Immunization of CBA x BALB/c F1 mice with DNA expression plasmids for the wild-type gp120 sequence induced a predominantly IgG, antibody response with end point titers o f 10(4)-5 x 10(4). The antibodies reacted only with conformationally i ntact gp120. Serine replacements targeted to both sides of the vs loop had a major impact on gp120 immunogenicity, with a markedly reduced r esponse in the majority of animals tested. Analysis of the epitope spe cificity of the responses suggests that N-terminal amino acids in the vs loop contribute to the major immunodominant epitope and provides no evidence that their removal enhances immunogenicity of the conserved regions. (C) 1998 Academic Press.