Evidence for rapid selection and deletion of HIV-1 subpopulations in vivo by V3-specific neutralizing antibody: a model of humoral-associated selection.

: Emergence in two chimpanzees of HIV-1 HTLV-IIIB variants resistant to neutralization by the pre-existing antibody is described. Viruses isolated from the HTLV-IIIB gp120 vaccinated and challenged animal were more resistant to neutralization, had more heterogenic genomes and showed more pronounced antigenic drift as determined by serotypic neutralization analysis than viruses isolated from the naive infected animal, indicating immune pressure as the selective mechanism. The earliest selecting antibody population detected in the chimpanzees appeared to be conformationally dependent. This was demonstrated by its ability to neutralize only the most replication-competent sub-populations of the HTLV-IIIB inoculum strain, yet was found to bind a HTLV-IIIB common nonapeptide (IQRGPGRAF) derived from the gp120 isolate-specific 3rd variable domain (V3) also known to induce isolate-specific neutralization in multiple species. Each of the recovered isolates had become resistant to neutralization by both a monoclonal (0.5 beta) and polyclonal HTLV-IIIB gp120 V3-specific antibody by as much as 16 to 256-fold. Amplification of the V3 coding sequence by polymerase chain reaction and subsequent sequence analysis of the neutralization-resistant variants obtained from in vivo infected animals indicated that resistance to neutralization was conferred by changes outside the direct binding site for the selective neutralizing antibody. Further studies indicated that apparent neutralization-resistant variants, yielded after in vitro passage through chimpanzee and human peripheral blood mononuclear cell cultures void of HIV-specific antibody, result from the homogenic amplification of the more replication competent sub-population pre-existing in the original viral stock. These faster replicating sub-populations appear to dominate initially and therefore are the initial prime targets recognized by the chimpanzee humoral immune system upon infection in vivo and thus eliminated. The described finding of virus escape due to a conformationally-induced flexibility of the major neutralization epitope termed "conformational-V3 hypervariability", coupled to the known primary amino acid hypervariability of this epitope (V3) termed "linear-V3 hypervariability", may be all that is required by the virus to survive in an otherwise effective humoral immune system.