Differences in affinity of anti-CD4 monoclonal antibodies predict their effects on syncytium induction by human immunodeficiency virus.

A panel of 20 anti-CD4 monoclonal antibodies (mAb) was ranked in terms of affinity, using an inhibition radioimmunoassay. The ability of these antibodies to inhibit the induction of syncytia by human immunodeficiency virus (HIV) and to prevent binding of the HIV envelope glycoprotein 120 (gp120) to CD4 was also measured. Syncytium inhibition correlated strongly with affinity (P less than 0.001) but only weakly with inhibition of gp120 binding (P = 0.038). Some antibodies partially blocked binding of gp120 to CD4 but did not inhibit syncytia, and some antibodies inhibited syncytia but only weakly blocked binding of gp120. These results suggest that the syncytium inhibition assay is highly affinity-dependent, and that epitopes on CD4 concerned with virus binding are distinct from those involved in syncytium formation.