STRUCTURAL CHARACTERIZATION OF BROADLY NEUTRALIZING HUMAN MONOCLONAL-ANTIBODIES AGAINST THE CD4 BINDING-SITE OF HIV-1 GP120

The human monoclonal antibodies (mAbs) 15e and 21h are derived from HI V-1-infected individuals. They block CD4 binding, recognize conformati on-dependent discontinuous epitopes on gp120 and neutralize a broad ra nge of laboratory strains and primary isolates of HIV-1. To determine if a structural basis for neutralization could be identified, analysis of these CD4-binding site anti-gp120 human mAbs was performed, common features and differences were identified and a comparison was made wi th F105, a previously reported CD4-binding site anti-gp120 human mAb. The 15e and 21h mAb heavy chains are derived from different V region g enes, i.e. V2-1 and VDP-35, which are members of the VHIV and VHIII fa milies, respectively. Analysis of the genes encoding the heavy chain c omplementarity determining region (CDR) 3 revealed that both mAbs show a long D-H segment of similar size that could arise from D-D fusions of the dxp1/dlr1 and daudi/d22-12 germline DH genes along with use of the J(H6) and J(H5) germline segments. Similarly, the 15e and 21h ligh t chains are derived from different V region genes, i.e. Hum01/012 and Hum1v318, that are members of the VkappaI and V(lambda)IIIa gene fami lies, respectively. These V genes are rearranged with J(kappa1) and J( lambda2) germline genes. For both mAbs, the pattern of replacement mut ations in the V region genes of the heavy and light chains is consiste nt with a process of somatic mutation and antigen-driven clonal select ion. By comparing the CDRs of 15e, 21h and F105, eight positions in th e rearranged heavy chains and two positions in the rearranged light ch ains were found to have identical amino acids. These studies suggest t hat there is no absolute restriction in the use of V region germline g enes and form the foundation for understanding the humoral immune resp onse to the CD4-binding site of gp120.