SIV envelope glycoprotein epitopes recognized by antibodies from infected or vaccinated rhesus macaques

Antiretroviral therapy reveals triphasic decay of intact SIV genomes and persistence of ancestral variants

The decay kinetics of HIV-1-infected cells are critical to understand virus persistence. We evaluated the frequency of simian immunodeficiency virus (SIV)-infected cells for 4 years of antiretroviral therapy (ART). The intact proviral DNA assay (IPDA) and an assay for hypermutated proviruses revealed short- and long-term infected cell dynamics in macaques starting ART ∼1 year after infection. Intact SIV genomes in circulating CD4+T cells showed triphasic decay with an initial phase slower than the decay of the plasma virus, a second phase faster than the second phase decay of intact HIV-1, and a stable third phase reached after 1.6-2.9 years. Hypermutated proviruses showed bi- or mono-phasic decay, reflecting different selective pressures. Viruses replicating at ART initiation had mutations conferring antibody escape. With time on ART, viruses with fewer mutations became more prominent, reflecting decay of variants replicating at ART initiation. Collectively, these findings confirm ART efficacy and indicate that cells enter the reservoir throughout untreated infection.

Timing of retroviral infection influences anamnestic immune response in vaccinated primates

Using simian immunodeficiency virus (SIV) infection of rhesus macaques to model human immunodeficiency virus (HIV) infection of humans, we assessed whether broadly reactive vaccine-induced humoral immunity would remain broadly reactive after viral challenge, and whether there would be significant differences in anamnestic antibody responses if animals were challenged when predominately effector or memory lymphocyte populations were present. Animals immunized over a prolonged period and challenged 11 months after vaccination mounted more broadly reactive and stronger humoral immunity than those rapidly vaccinated and challenged 2 weeks after their final vaccinations. These data suggest that vaccination schedule and the timing of virus challenge should be considered when evaluating future candidate HIV vaccines.

Humoral Immunity and the Evolution of HIV-2

Human immunodeficiency virus type 2 (HIV-2) evolved from the zoonotic transmission of simian immunodeficiency virus (SIV) that naturally infects sooty mangabeys found in West Africa. Using sera from HIV-2-infected humans, we discovered that an hypervariable region (the V4 loop) of HIV-2 induces antibody responses only weakly reactive against itself but strongly reactive against analogous sequences from the V4 loop of strains of SIV. Available sequence data indicates that all strains of HIV-2 have large deletions in the V4 region that truncate an immunodominant neutralizing B cell epitope among strains of SIV. Infection of a macaque with a sequenced clone of HIV-2 similarly elicited antibodies that poorly recognized the V4 loop of HIV-2 but readily bound to analogous SIV sequences. Our data are consistent with a scenario whereby a disparate antibody response directed against the V4 loop may have influenced the selective expansion and survival of HIV-2 in humans.

Characterization of neutralization epitopes of simian immunodeficiency virus (SIV) recognized by rhesus monoclonal antibodies derived from monkeys infected with an attenuated SIV strain

A major limitation in the simian immunodeficiency virus (SIV) system has been the lack of reagents with which to identify the antigenic determinants that are responsible for eliciting neutralizing antibody responses in macaques infected with attenuated SIV. Most of our information on SIV neutralization determinants has come from studies with murine monoclonal antibodies (MAbs) produced in response to purified or recombinant SIV envelope proteins or intact SIV-infected cells for relatively short periods of time. While these studies provide some basic information on the potential immunogenic determinants of SIV envelope proteins, it is unclear whether these murine MAbs identify epitopes relevant to antibody responses elicited in monkeys during infection with either wild-type or attenuated SIV strains. To accomplish maximum biological relevance, we developed a reliable method for the production of rhesus monoclonal antibodies. In the present study, we report on the production and characterization of a unique panel of monoclonal antibodies derived from four individual monkeys inoculated with SIV/17E-CL as an attenuated virus strain at a time when protective immunity from pathogenic challenge was evident. Results from these studies identified at least nine binding domains on the surface envelope glycoprotein; these included linear determinants in the V1, V2, cysteine loop (analogous to the V3 loop in human immunodeficiency virus type 1), and C5 regions, as well as conformational epitopes represented by antibodies that bind the C-terminal half of gp120 and those sensitive to defined mutations in the V4 region. More importantly, three groups of antibodies that recognize closely related, conformational epitopes exhibited potent neutralizing activity against the vaccine strain. Identification of the epitopes recognized by these neutralizing antibodies will provide insight into the antigenic determinants responsible for eliciting neutralizing antibodies in vivo that can be used in the design of effective vaccine strategies.

Neutralising epitopes of simian immunodeficiency virus envelope glycoprotein

Monoclonal and polyclonal antibodies with weak SIV neutralising activity bind to the V2 and V4 regions of gp120 or bind to the amino acids DWNND in gp41. Antibodies with the most potent neutralising activity recognise conformation-dependent epitopes involving the V3 and V4 regions of gp120. Monoclonal antibodies that map to the V3 region of SIVmac failed to neutralise. However, one antibody to SIV AGM neutralised but only in the presence of soluble CD4.

Fine analysis of humoral antibody response to envelope glycoprotein of SIV in infected and vaccinated macaques

To characterize the serological response to SIV envelope, induced by vaccination with different envelope immunogens or by SIV infection, plasma samples from 11 cynomolgus macaques infected with simian immunodeficiency virus (SIV) and from 16 macaques vaccinated with three different recombinant envelope proteins were analyzed by (1) ELISA, using a variety of antigens including overlapping peptides encompassing the entire sequence of the envelope protein of SIV, and (2) competition assays, using neutralizing monoclonal antibodies to SIV gp120. Seven regions of SIV envelope were predicted to be antigenic. Peptides representing four of these, in the second and third variable regions (V2 and V3) and the fourth constant (C4) region of gp120 and the Gnann region of gp41, were recognized by the majority of sera from infected and vaccinated animals. Additional antigenic regions were identified in the first and fourth variable domains (V1 and V4) and the carboxy terminus (C5) of gp120 and in three additional regions of gp41. Most infected and vaccinated animals made antibodies that competed with the binding of the three conformational MAbs. Among the vaccinated groups, antibodies induced by vaccination with precursor glycoproteins (gp140 or gp160) recognized several additional gp120 epitopes when compared with antibodies induced by external glycoprotein gp130. Sera from infected animals showed a more restricted gp120 response (17 of 46 peptides recognized) compared to animals vaccinated with precursor glycoproteins (31 peptides recognized). The converse was true for antibodies to gp41. Sera from animals vaccinated with recombinant gp140, produced in insect cells, were the only group that failed to compete with the binding of conformational MAbs. Finally, the development of antibodies to specific epitopes of gp120 and gp41 revealed differences between long-term survivors and nonsurvivors, implying that responses to specific epitopes may be important in conferring resistance to disease progression.

Hypervariable epitope constructs as a means of accounting for epitope variability

Epitope variability is one of the greatest obstacles to development of synthetic peptide vaccines. Based on a recently described hypervariable epitope (aa 414-434) on the envelope glycoprotein (gp130) to simian immunodeficiency virus (SIVmac142), we have developed a novel approach to account for epitope variability. We have prepared, in a single synthesis, a cocktail of peptides, designated a hypervariable epitope construct (HEC), which collectively represent all the in vivo variability seen in an epitope. The HEC represents permutations of amino acid substitutions found in the epitope and has been able to induce antibodies with enhanced binding to native SIV and broad immunoreactivity to related epitope analogues.