STRAIN SPECIFICITY AND BINDING-AFFINITY REQUIREMENTS OF NEUTRALIZING MONOCLONAL-ANTIBODIES TO THE C4 DOMAIN OF GP120 FROM HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1

The binding properties of seven CD4-blocking monoclonal antibodies rai sed against recombinant gp120 of human immunodeficiency virus type 1 s train MN (HIV-1MN) and two CD4-blocking monoclonal antibodies to recom binant envelope glycoproteins gp120 and gp160 of substrain IIIB of HIV (LAI) were analyzed. With a panel of recombinant gp120s from seven div erse HIV-1 isolates, eight of the nine antibodies were found to be str ain specific and one was broadly cross-reactive. Epitope mapping revea led that all nine antibodies bound to epitopes located in the fourth c onserved domain (C4) of gp120. Within this region, three distinct epit opes could be identified: two were polymorphic between HIV-1 strains, and one was highly conserved. Studies with synthetic peptides demonstr ated that the conserved epitope, recognized by antibody 13H8, was loca ted between residues 431 and 439. Site-directed mutagenesis of gp120 d emonstrated that residue 429 and/or 432 was critical for the binding o f the seven antibodies to gp120 from HIV-1MN. Similarly, residues 423 and 429 were essential for the binding of monoclonal antibody 5C2 rais ed against gp120 from HIV-1IIIB. The amino acids located at positions 423 and 429 were found to vary between strains of HIV-1 as well as bet ween molecular clones derived from the MN and LAI isolates of HIV-1. P olymorphism at these positions prevented the binding of virus-neutrali zing monoclonal antibodies and raised the possibility that HIV-1 neutr alization serotypes may be defined on the basis of C4 domain sequences . Analysis of the binding characteristics of the CD4-blocking antibodi es demonstrated that their virus-neutralizing activity was directly pr oportional to their gp120-binding affinity. These studies account for the strain specificity of antibodies to the C4 domain of gp120 and dem onstrate for the first time that antibodies to this region can be as e ffective as those directed to the principal neutralizing determinant ( V3 domain) in neutralizing HIV-1 infectivity.