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Session 50 Poster Presentations
Neutralizing Antibodies
Session Day and Time: Wednesday 1:30 - 3:30 pm
Room: Hall D


420
Defined Sequence Variants of Simian Immunodeficiency Virus Strain 239 Reveal Global Effects on Sensitivity to Antibody-mediated Neutralization
W. Johnson*1,2, H. Sanford1,2, J. Lifson3, P. Parren4, D. Burton5, J. Robinson6, R. Desrosiers1,2
1New England Reg Primate Res Ctr, Southborough, MA; 2Harvard Med Sch, Boston, MA; 3Sci Applications Intl Corp -Frederick, MD; 4Genmab, The Netherlands; 5Scripps Res Inst, La Jolla, CA; and 6Tulane Univ Med Sch, New Orleans, LA

Background: Primary isolates of HIV and SIV are highly resistant to antibody-mediated neutralization. We used SIV239, a molecularly cloned pathogenic SIV, to investigate the factors that determine the neutralization-resistant phenotype.

Methods: We studied a collection of genetically altered derivatives of the neutralization-resistant SIV239. The variant strains differed from SIV239 only at defined sites in env. These included variants lacking multiple N-glycans, a variant bearing a 100 a.a. deletion of the V1-V2 loops (SIVDV1V2), and SIVmac316, which bears 8 amino acid substitutions that confer increased replicative capacity for macrophages. We used 57 anti-env monoclonal antibodies, which represented a minimum of 9 distinct competition groups in gp120, to determine the degree to which each genetic modification influenced neutralization-sensitivity. A subset of these MAbs was tested in a virion-capture assay to study binding of neutralizing antibodies to native envelope spikes.

Results: None of the 57 MAbs efficiently neutralized SIVmac239. Three (3) distinct genetic variants, SIVmac316, SIV-M5 (a quintuple N-glycan attachment site mutant) and SIVDV1V2 independently displayed global increases in breadth and degree of neutralization-sensitivity. While the genetic alterations in these 3 variants mapped largely to the N-terminal half of gp120, many of the MAbs that neutralized the variant strains recognized epitopes in the C-terminal half of gp120. Neutralization of these 3 strains was observed with MAbs from 8 of the 9 competition groups as well as an anti-gp41 MAb. Virion-capture assays with 4 MAbs and 2 of the virus strains revealed a direct correlation of ability to capture virions with ability to neutralize.

Conclusions: Of the 57 MAbs tested, none neutralized SIV239. However, most of these neutralized multiple SIV239 variants, indicating that the epitope sequences are intact in the SIV239 env gene. Genetic alterations gave rise to global increases in sensitivity, suggesting that resistance is not simply due to localized shielding of epitopes by N-glycans or variable loops, but rather reflects some underlying physical property of the fully assembled, native envelope complex. Results of virion-capture binding assays suggest that the ability to neutralize correlates directly with the ability to bind envelope spikes on the surface of virions.