CD4-IGG BINDING THRESHOLD FOR INACTIVATION OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1

The stoichiometry of human immunodeficiency virus type 1 (HIV-1) inact ivation by soluble receptor CD4-IgG hybrid dimers (CD4-IgG) was examin ed. The extent of HIV-1 inactivation was measured in a sensitive plaqu e-forming assay, and the corresponding level of CD4-IgG binding was de termined by immunofluorescence of infected cells, Ninety percent virus inactivation occurred at relatively low levels of CD4-IgG binding (10 % of the saturating level). At even lower binding levels (1.4% of maxi mum binding), virus survival was 44%. Over a broad range of binding co nditions, the survival curve followed a model in which viruses binding more than a threshold level of CD4-IgG were completely inactivated, w hile viruses binding less remained infectious, The data indicate that CD4-IgG binding to 1.4% of gp120 binding sites equals the threshold fo r inactivation, Thus, virus inactivation can begin when 3 CD4-IgG (of similar to 216 gp120 sites) bind per virion.