Toward Effective HIV Vaccination

Abstract

We describe murine monoclonal antibodies (mAbs) raised by immunization with an electrophilic gp120 analog (E-gp120) expressing the rare ability to neutralize genetically heterologous human immunodeficiency virus (HIV) strains. Unlike gp120, E-gp120 formed covalent oligomers. The reactivity of gp120 and E-gp120 with mAbs to reference neutralizing epitopes was markedly different, indicating their divergent structures. Epitope mapping with synthetic peptides and electrophilic peptide analogs indicated binary recognition of two distinct gp120 regions by anti-E-gp120 mAbs, the 421-433 and 288 -306 peptide regions. Univalent Fab and single chain Fv fragments expressed the ability to recognize both peptides. X-ray crystallography of an anti-E-gp120 Fab fragment revealed two neighboring cavities, the typical antigen-binding cavity formed by the complementarity determining regions (CDRs) and another cavity dominated by antibody heavy chain variable (V H ) domain framework (FR) residues. Substitution of the FR cavity V H Lys-19 residue by an Ala residue resulted in attenuated binding of the 421-433 region peptide probe. The CDRs and V H FR replacement/silent mutation ratios exceeded the ratio for a random mutation process, suggesting adaptive development of both putative binding sites. All mAbs studied were derived from V H 1 family genes, suggesting biased recruitment of the V gene germ line repertoire by E-gp120. The conserved 421-433 region of gp120 is essential for HIV binding to host CD4 receptors. This region is recognized weakly by the FR of antibodies produced without exposure to HIV, but it usually fails to induce adaptive synthesis of neutralizing antibodies. We present models accounting for improved CD4-binding site recognition and broad HIV neutralizing activity of the mAbs, long sought goals in HIV vaccine development.

Deficient synthesis of class-switched, HIV-neutralizing antibodies to the CD4 binding site and correction by electrophilic gp120 immunogen

“…3a). We previously reported neutralization of HIV strains heterologous to the E-gp120 immunogen by monoclonal IgGs YZ23 and 3A5 with binding specificity directed to a short C LIN peptide mimetic (residues 421–433) [22], and these IgGs also recognized the longer E-C LIN b antigen employed in the present study (Fig. 3b).…”

“…C LIN mutations across HIV subtypes are limited because of the selective pressure for maintenance of CD4 binding activity [22] (supplemental Table S1a, http://links.lww.com/QAD/A552). Synthetic E-C LIN b (supplemental Fig.…”

“…Mice were immunized with recombinant gp120 or E-gp120 under varying conditions (differing immunization routes, immunogen dose, adjuvant and administration schedule; see supplemental Methods section). Purified serum IgM and IgG, monoclonal IgGs [9,22], and IgM and IgG clones containing identical light chain and heavy chain variable domains were prepared [34] (supplemental Methods section). Monoclonal IgGs YZ23 and 3A5 are C LIN -directed nucleophilic antibodies induced by E-gp120 [22].…”

“…Purified serum IgM and IgG, monoclonal IgGs [9,22], and IgM and IgG clones containing identical light chain and heavy chain variable domains were prepared [34] (supplemental Methods section). Monoclonal IgGs YZ23 and 3A5 are C LIN -directed nucleophilic antibodies induced by E-gp120 [22]. IgG and IgM JL427contain C LIN -specific V-domains [10].…”

“…Unexpectedly, the E-gp120 immunogen altered the specificity of the antibody response. About 50% of E-gp120-induced monoclonal IgGs recognized the C LIN , and because of the conserved character of this epitope, HIV strains heterologous to the immunogen were neutralized [22]. Immunization with gp120 devoid of the electrophilic phosphonate, in contrast, mostly induces the synthesis of antibodies to the hypermutable gp120 epitopes that only neutralize strains homologous to the immunogen sequence [23–25].…”