Blocking of CD4 binding to rgp120 by antisera against subtype C rgp120.

The ability of antibodies to inhibit the binding of CD4-IgG to rgp120 was measured using an assay similar to that described previously [14]. Briefly, rgp120 was captured onto microtiter dishes. Sera were added at a 1∶200 dilution in duplicate and incubated 2 hr, and soluble CD4 was then added without washing. The amount of bound CD4 with and without added anti-rgp120 sera was monitored by the binding of HRP-labeled mouse monoclonal antibody to CD4. Results using rgp120 from the IN98025 isolate were not available due to the inability of this envelope protein to bind CD4-IgG.