TZM-bl and HOS.CD4-R5 cells were obtained from the NIH-AIDS Research and Reference Reagent Program (ARRRP). 293T/17 cells were obtained from the ATCC. MAbs 2G12 [29] , 2F5, 4E10 [42] , b12 [18] , 3D6 [43] and 5F3 [29] were obtained from Polymun Scientific GmbH (Austria), as part of the Collaboration for AIDS Vaccine Discovery program.