Inhibition of gp120/CD4 binding by α-defensin-2. (A) Interference of α-defensin-2 with the reciprocal binding of CD4 and gp120_BaL in ELISA. The plates were coated with sCD4 at 5 μg/mL and gp120_BaL at 2 μg/mL, respectively; α-defensin-2 (7.3 μM) was preincubated with either sCD4 or gp120_BaL immobilized to plastic and then removed by washing before the addition of the respective ligands (gp120_BaL or sCD4, both at 1 μg/mL) to the liquid phase. Binding of gp120_BaL was revealed using mAb 2G12 and binding of sCD4 using mAb DB81, which binds more efficiently to CD4 when it is complexed with gp120 (S. Burastero and P.L., manuscript in preparation). (B) Competition of α-defensin-2 with recombinant HIV-1 gp120_BaL for binding to plastic-immobilized sCD4 in ELISA. The plates were coated with sCD4 at 5 μg/mL; α-defensin-2 was added prior to Leu3a or gp120_BaL and kept in the wells throughout the reaction period. Error bars indicate SD of mean values obtained from 3 repeated assays.