MAbs were purified on Protein G columns (GE Healthcare) and biotinylated using the EZ-Link NHS-PEO solid phase biotinylation kit (Pierce). The competition between unlabeled and biotinylated mAbs for binding to immobilized Env antigens was measured by ELISA. Briefly, unlabelled competitor mAbs were added at different concentrations. After 1 h biotinylated mAbs were added at a concentration corresponding to the 70-80% of the maximal OD level. After incubation for 1 h, plates were washed and bound biotinylated mAb was detected using APlabeled streptavidin (Jackson Immunoresearch). The percentage of inhibition was tested in triplicates and calculated as follow: (12[(ODsample-ODneg ctr)/(ODpos ctr -ODneg ctr)])6100.