HIV requires multiple gp120 molecules for CD4-mediated infection

Binding of glycoprotein gp120 to the T cell-surface receptor CD4 is a crucial step in CD4-dependent infection of a target cell by the human immunodeficiency virus (HIV). Blocking some or all gp120 molecules on the viral surface should therefore inhibit infection. Consequently, competitive receptor inhibitors, such as soluble synthetic CD4 (sCD4), synthetic CD4 peptides and immunoglobulins, have been investigated in vitro and in vivo, but little is known about the molecular mechanisms of these inhibitors. We have now quantitatively examined blocking by soluble CD4 in the hope of gaining insight into the complex process of viral binding, adsorption and penetration. At low sCD4 concentrations, the inhibition in three HIV strains is proportional to the binding of gp120. The biological association constant (gp120-sCD4 Kassoc) for HIV-2NIHZ is (8.5 +/- 0.5) x 10(7) M-1, whereas Kassoc for HIV-1HXB3 (1.4 +/- 0.2) and HIV-1MN (1.7 +/- 0.1) x 10(9) M-1 are 15-20-fold larger. For all three viral strains, the biological Kassoc from infectivity assays is comparable to the chemical Kassoc. The inhibitory action of sCD4 at high concentrations, however, is not fully explained by simple proportionality with the binding to gp120. Positive synergy in blocking of infection occurs after about half the viral gp120s molecules are occupied, and is identical for all three viral strains, despite the large differences in Kassoc. Our method of measuring the viral-cell receptor Kassoc directly from infectivity assays is applicable to immunoglobulins, to other viruses and to assays using primary or transformed cell lines.