Structural studies of human HIV-1 V3 antibodies.

X-ray crystallographic structures of two human, anti-V3 HIV-1 neutralizing antibodies, 447-52D and 2219, show how the two antibodies use different strategies for their cross-reactivity with different V3 sequences. 447-52D recognizes V3 primarily through main-chain hydrogen bonds to the N-terminal side of V3, with the GPGR tip region buried in the antigen-combining site. The side chains on the N-terminal side of V3 are exposed to solvent, allowing for sequence changes in this region, thus explaining how 447-52D can neutralize a wide array of viral isolates. Antibody 2219 contacts a more extensive V3 surface, with more side-chain involvement. However, residues at the GPGR tip are exposed to solvent, with no constraints on residue size, so that binding to unusual tip sequences with larger side chains such as RPRQ is possible.