The antiviral effect of soluble recombinant CD4 on HIV-1 provides the first example of receptor-mediated viral inactivation. We have investigated the mechanism of inactivation by asking how many rCD4 molecules must bind to inactivate each virus. Our approach was to simultaneously determine the fraction of viral gpl2O receptors bound to soluble rCD4 and the fraction of viruses surviving at each level of receptor saturation. The extent of viral inactivation was measured in a sensitive HIV-1 plaque forming assay described in previous annual reports. When we titered rCD4 against virus and measured virus survival, we found 50% inactivation at rCD4 concentrations 1/4 to 1/2 of the affinity constant. These results gave the first indication of viral inactivation by sub-saturating concentrations of rCD4. Second, recombinant gpl2O was added as a competitor to a fixed amount of virus and rCD4. When sufficient gpl2O was added, it competed for most of the free rCD4 and rescued the virus from inactivation by rCD4. Under these conditions, using the Scatchard equation, we calculated the fractional occupancy of viral gpl2O receptors. In addition, by plotting log virus survival against fraction of receptors occupied, we have determined how many receptors must bind rCD4 for one inactivation event to occur per virus. Our results showed that viral inactivation occurs when just 1/10th of viral gpl2O receptors have bound rCD4. These results allow a clear distinction between two models of inactivation: competitive saturation of gpl2O to prevent cell surface binding is virtually ruled out. Instead, they reveal a potent new mechanism for HIV-1 inactivation that is triggered by rCD4 binding and proceeds via a noncompetitive mechanism that may interfere with some essential step in viral physiology other than the cell binding step.
