Novel approaches for identification of broadly cross-reactive HIV-1 neutralizing human monoclonal antibodies and improvement of their potency.

Human monoclonal antibodies (hmAbs) that neutralize HIV isolates from different clades at physiologically relevant concentrations (broadly cross-reactive neutralizing antibodies (bcnAbs)) are rare in infected individuals. Only small number of such antibodies have been identified and extensively characterized, but efforts to elicit them in vivo have not been successful. We have recently developed novel approaches, based on sequential (SAP) and competitive (CAP) antigen panning methodologies, and the use of antigens with increased exposure of conserved epitopes, for enhanced identification of bcnAbs to gp120-gp41. Some of the antibodies identified by using these approaches (X5, m6, m9) bind better to gp120-CD4 complexes than to gp120 alone (CD4i antibodies); they exhibit exceptional neutralizing activity and breadth of neutralization as scFvs and on average lower potency as Fabs and IgGs. Other antibodies that compete with CD4 for binding to gp120 (m14, m18) (CD4bs antibodies) are weaker neutralizers but also exhibit broad neutralizing activity although at relatively high concentrations. The anti-gp41 antibodies (m43, m44, m45, m47 and m48) appear to have broad cross-reactivity and bind to a new group of conserved conformational epitopes distinct from those of the bcnAbs 4E10, 2F5 and Z13. Recently, the crystal structures of X5, m14 and m18 have been solved and compared to those of 17b and b12; they all contain long H3s that play a major role in their mechanism of binding. The H3s of X5, m6 and m9, unlike the others known, appear to be very flexible which may be related to the mechanism of their exceptional neutralizing activity. The further characterization of the molecular interactions of the bcnAbs with gp120-gp41 will undoubtedly help in our understanding of the mechanisms of virus neutralization, and in the design of entry inhibitors and vaccines.