Broad and Potent Neutralizing Antibody Responses Elicited in Natural HIV-2 Infection

Little is known about the potency, breadth and epitope specificities of neutralizing antibody (Nab) responses elicited in natural HIV-2 infection. Analysis of plasma specimens from 64 HIV-2 chronically-infected subjects in a single round infectivity assay (JC53bl-13/TZM-bl) revealed median reciprocal IC50 Nab titers of 1.7x105, 2.8x104 and 3.3x104 against three primary virus strains, HIV-27312A, HIV-2ST and HIV-2UC1, respectively. A subset of 5 plasma samples was tested on 17 additional HIV-2 strains, and similarly high Nab titers were observed against all but four viruses. These Nab titers were higher, by orders of magnitude, than HIV-1 Nab titers in plasma from chronically infected subjects tested against heterologous primary HIV-1 strains. This finding was confirmed by using purified IgG, HIV-2 primary Envs derived by single genome amplification of plasma vRNA, and viruses grown in human PBMCs and tested for neutralization in human PBMCs. We next characterized 15 mAbs that were isolated from subjects chronically-infected with HIV-2. All 15 mAbs bound to the monomeric HIV-2 gp120 and potently neutralized most primary strains of HIV-2, with median IC50 titers ranging from 0.007 to 0.028 µg/ml. Epitope mapping using cross-competition Env binding, peptide scanning, site-directed mutagenesis, chimeric Env constructions and single-cycle virus neutralization assays revealed three distinct mAb competition groups (CG-I, CG-II and CG-III). CG-I mAbs recognized conformational epitopes centered on the carboxy-terminus of the variable loop 4 (V4). CG-II mAbs bound to linear epitopes in the V3. The epitopes recognized by CG-III mAbs were complex and related to the CD4 or coreceptor binding surfaces. Patient plasmas competed with representative mAbs from each group for binding HIV-2 gp120 in titers that correlated significantly with Nab titers. HIV-2 MPER antibodies were rarely detected, low in titer and did not contribute to the neutralization potency and breadth. These findings indicate that primary HIV-2 strains are highly immunogenic in natural infection and surprisingly sensitive to antibody neutralization. The striking differences in susceptibility to neutralization between primary strains of HIV-2 and HIV-1 suggest fundamental differences in the biophysical properties of the respective Env trimers and mechanisms by which the two viruses persist in vivo in the face of neutralizing antibodies.