Molecular analysis of HIV-1 gp120 antibody response using isotype IgM and IgG phage display libraries from a long-term non-progressor HIV-1-infected individual

To characterize the variable heavy chain (VH)3 antibody response to HIV-1 g p120, we analyzed a panel of IgM and IgG1 Fab fragments from phage display isotype libraries from a long-term, non-progressor HIV-1-infected individua l. The IgM Fab antibodies isolated had low affinity for gp120, were not res tricted to a particular VH3 germ-line gene, and consisted mainly of unmutat ed VH genes. In contrast, IgG Fab fragments were gp120 specific, with high affinity and extensive somatic mutation; all were clonally related and were derived from a single VH3 germ-line gene (DP50). One IgG Fab (S8) has DP50 VH region nucleotide substitutions identical to those of IgM Fab M025 and uses similar DH and JH segments, suggesting that S8 arose from M025 by isot ype switching. In addition, somatic mutation in the IgG heavy chain third c omplementarity-determining region results in a 100-fold affinity increase f or gp120, which correlates with a similar increase in neutralization capaci ty. These results imply that in vivo IgM to IgG isotype switch and affinity maturation may be important for protection and long-term survival in certa in HIV-1-infected individuals.