Effects on CD4 binding of anti-peptide sera to the fourth and fifth conserved domains of HIV-1 gp120.

Antisera to peptides that represent regions within the fourth and fifth conserved domains of the human immunodeficiency virus type 1 (HIV-1) gp120 were tested for recognition of the gp120 glycoprotein and for the ability to interfere with gp120 binding to the CD4 receptor molecule. Antisera to both peptides contained equivalent antibody titers, showed equivalent reactions with denatured gp120 on Western blot, and had group-specific reactivity. Preincubation of gp120 with either anti-peptide sera prebound to a solid phase substantially blocked soluble CD4 binding to gp120. Similarly, preincubation of gp120 with CD4-positive cells substantially diminished recognition of gp120 by both anti-peptide antisera. These results provide serologic evidence that regions near or within the fourth and fifth conserved domains of gp120 are involved in CD4 binding. However, neither anti-peptide sera could block soluble gp120 from binding to CD4-positive cells nor inhibited HIV-1 envelope-mediated syncytium formation or virus infection. These results demonstrate that antisera to regions proximal to the CD4 binding site of gp120 may compete poorly with CD4 for gp120 binding.