Background:  Identifying the antibody specificities that constrain HIV-1 envelope (Env) diversity, limit virus replication, and contribute to neutralization breadth and potency is an important goal of current HIV/AIDS vaccine research.  Here we describe a novel HIV-2 proviral scaffold into which we substituted the complete env V3 region of HIV-1YU2 or HIV-1Ccon to yield the chimeric viruses HIV-2KR.X7YU2 V3 and HIV-2KR.X7Ccon V3. 

 

Methods:  HIV-2/HIV-1 V3 chimeras were constructed and tested for biological function and antigenicity against a panel of HIV-1 and HIV-2 Env-specific ligands.  V3 specific reactivity was assessed in polyclonal plasma samples obtained from patients infected with clade B and clade C HIV-1.

 

Results:  HIV-2/HIV-1 V3 chimeras were found to be infectious, replication competent and sensitive to selective pharmacological inhibitors of virus entry.  V3 chimeric viruses were resistant to neutralization by HIV-1 mAbs directed against the CD4 binding site, coreceptor binding site, and MPER but exhibited striking sensitivity to HIV-1 V3 specific mAbs, 447-52D and F425 B4e8 (IC50 < 0.005 g/ml for each).  Plasma specimens from 11 HIV-1 clade B and 10 HIV-1 clade C chronically infected subjects showed no neutralizing activity against HIV-2 but exhibited high titer V3-specific neutralization against both HIV-2/HIV-1 V3 chimeras with IC50 measurements ranging from 1:50 to greater than 1:40,000.  Neutralization titers of clade B plasmas were as much as 1000-fold lower when tested against the primary HIV-1YU2 virus compared with the HIV-2KR.X7YU2 V3 chimera, demonstrating highly effective shielding of V3 epitopes in the native Env trimer. HIV-2/HIV-1 V3 chimeras were potently neutralized by 14 sera obtained from individuals recently infected by clade C HIV-1, but these titers did not correlate with autologous or heterologous neutralization by the same sera.

 

Conclusions:  We conclude that V3 is highly immunogenic in vivo, eliciting antibodies of significant breadth and neutralizing potential during both acute and chronic HIV-1 infection.  These antibodies constrain HIV-1 Env to structure(s) in which V3 epitopes are concealed prior to CD4 engagement but they do not otherwise contribute to neutralization breadth and potency against most primary virus strains.