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105 Mechanisms of HIV-1 Resistance to Neutralization at the CD4 Binding Site: Implications for Vaccine
P. Bouma*, P. Zhang, E. Park, and G. Quinnan
Uniformed Services Univ. of the Hlth. Sci., Bethesda, MD, USA
Background: Induction of virus neutralizing antibodies is a major goal of HIV vaccine development, but profound resistance to neutralization is a common characteristic of primary virus isolates. Neutralization resistance (NR) may occur as a result of changes in cell surface protein interactions, or as a result of enhanced fusogenicity following ligand binding. Differences in neutralization sensitivity between the neutralization sensitive (NS) MN strain envelope (MN-T) and a primary envelope (MN-P) from the same donor have been evaluated. MN-P was about 100-fold more resistant to neutralization by monoclonal antibodies that target the CD4 binding site (CD4bs) and by soluble CD4 (sCD4) than MN-T. The mechanism of this NR was evaluated. Seven mutations were seen to cluster in and around the CD4 binding pocket on the surface of gp120. Two of these mutations have been demonstrated to form chemical bonds with residues in CD4, 4 result in charge alteration and 3 are predicted to change N-linked glycosylation. Other gp120 mutations were located in presumed coreceptor and gp41 binding sites and in the V1/V2 and V3 regions.
Methods: Chimeras containing fragments and site mutants of the MN-P and MN-T clones were constructed. Psuedoviruses made from these constructs were tested for resistance to neutralization by anti-CD4bs reagents.
Results: Introduction of the 7 CD4bs core structure mutations individually or in combination caused only a 2- to 4-fold increase in NR by anti-CD4bs reagents. Introduction of the MN-P V1/V2 sequence into an MN-T clone containing these 7 mutations plus the MN-P gp41 ectodomain enhanced this NR about 8-fold, but the resistance was still substantially less than that of MN-P. Addition of MN-T V3 sequences to this mutant did not further enhance resistance.
Conclusions: The NR of MN-P to CD4bs ligands is only partly due to sequences localized in or near the CD4 binding surface of gp120 and is probably also dependent upon other functional properties of the envelope glycoprotein complex resulting from mutations elsewhere. Further definition of the mechanism of this neutralization resistance phenotype should help clarify the nature of the envelope epitopes that are important for inclusion in HIV vaccine.
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