Increased Neutralization Sensitivity of CD4-Independent Human Immunodeficiency Virus Variants

Naturally occurring human immunodeficiency virus (HIV-1) variants require the presence of CD4 and specific chemokine receptors to enter a cell. In the laboratory, HIV-1 variants that are capable of bypassing CD4 and utilizing only the CCR5 chemokine receptor for virus entry have been generated. Here we report that these CD4-independent viruses are significantly more sensitive to neutralization by soluble CD4 and a variety of antibodies. The same amino acid changes in the HIV-1 gp120 envelope glycoprotein determined CD4 independence and neutralization sensitivity. The CD4-independent envelope glycoproteins exhibited higher affinity for antibodies against CD4-induced gp120 epitopes but not other neutralizing ligands. The CD4-independent envelope glycoproteins did not exhibit increased lability relative to the wild-type envelope glycoproteins. The utilization of two receptors apparently allows HIV-1 to maintain a more neutralization-resistant state prior to engaging CD4 on the target cell, explaining the rarity of CD4 independence in wild-type HIV-1.