A global neutralization resistance phenotype of human immunodeficiency virus type 1 is determined by distinct mechanisms mediating enhanced infectivity and conformational change of the envelope complex

We have described previously genetic characterization of neutralization-res istant, high-infectivity, and neutralization-sensitive, low-infectivity mut ants of human immunodeficiency virus type 1 (HIV-1) MN envelope, The distin ct phenotypes of these clones are attributable to six mutations affecting f unctional interactions between the gp120 C4-V5 regions and the gp41 leucine zipper. In the present study we examined mechanisms responsible for the ph enotypic differences between these envelopes using neutralization and immun ofluorescence assays (IFA), Most monoclonal antibodies (MAbs) tested agains t gp120 epitopes (V3, CD4 binding site, and CD4-induced) were 20 to 100 tim es more efficient at neutralizing pseudovirus expressing sensitive rather t han resistant envelope. By IFA cells expressing neutralization sensitive en velope bound MAbs to gp120 epitopes more, but gp41 epitopes less, than neut ralization-resistant envelope. This binding difference appeared to reflect conformational change, since it did not correlate with the level of protein expression or gp120-gp41 dissociation. This conformational change was most ly attributable to one mutation, L544P, which contributes to neutralization resistance but not to infectivity enhancement. The V420I mutation, which c ontributes a major effect to both high infectivity and neutralization resis tance, had no apparent effect on conformation. Notably, a conformation-depe ndent V3 neutralization epitope remained sensitive to neutralization and ac cessible to binding by MAbs on neutralization-resistant HIV-1 envelope. Sen sitivity to sCD4 did not distinguish the clones, suggesting that the phenot ypes may be related to post-CD4-binding effects. The results demonstrate th at neutralization resistance can be determined by distinguishable effects o f mutations, which cause changes in envelope conformation and/or function(s ) related to infectivity, A conformation-dependent V3 epitope may be an imp ortant target for neutralization of resistant strains of HIV-1.