The second extracellular loop of CCR5 contains the dominant epitopes for highly potent anti-human immunodeficiency virus monoclonal antibodies.

Six mouse anti-human CCR5 monoclonal antibodies (mAbs) that showed potent antiviral activities were identified from over 26,000 mouse hybridomas. The epitopes for these mAbs were determined by using various CCR5 mutants, including CCR5/CCR2B chimeras. One mAb, ROAb13, was found to bind to a linear epitope in the N terminus of CCR5. Strikingly, the other five mAbs bind to epitopes derived from extracellular loop 2 (ECL2). The three most potent mAbs, ROAb12, ROAb14, and ROAb18, require residues from both the N-terminal (Lys171 and Glu172) and C-terminal (Trp190) halves of ECL2 for binding; two other mAbs, ROAb10 and ROAb51, which also showed potent antiviral activities, require Lys171 and Glu172 but not Trp190 for binding. Binding of the control mAb 2D7 completely relies on Lys171 and Glu172. Unlike 2D7, the novel mAbs ROAb12, ROAb14, and ROAb18 do not bind to the linear peptide 2D7-2SK. In addition, all three mAbs bind to monkey CCR5 (with Arg at position 171 instead of Lys); however, 2D7 does not. Since five of the six most potent CCR5 mAbs derived from the same pool of immunized mice require ECL2 as epitopes, we hypothesize that CCR5 ECL2 contains the dominant epitopes for mAbs with potent antiviral activities. These dominant epitopes were found in CCR5 from multiple species and were detected in large proportions of the total cell surface CCR5. mAbs recognizing these epitopes also showed high binding affinity. A homology model of CCR5 was generated to aid in the interpretation of these dominant epitopes in ECL2.