Non-neutralizing Antibodies May Contribute to Suppression of SIVmac239 Viremia in Indian Rhesus Macaques

Abstract

The antiviral properties of broadly neutralizing antibodies against HIV are well-documented but no vaccine is currently able to elicit protective titers of these responses in primates. While current vaccine modalities can readily induce non-neutralizing antibodies against simian immunodeficiency virus (SIV) and HIV, the ability of these responses to restrict lentivirus transmission and replication remains controversial. Here, we investigated the antiviral properties of non-neutralizing antibodies in a group of Indian rhesus macaques (RMs) that were vaccinated with vif, rev, tat, nef, and env, as part of a previous study conducted by our group. These animals manifested rapid and durable control of viral replication to below detection limits shortly after SIVmac239 infection. Although these animals had no serological neutralizing activity against SIVmac239 prior to infection, their pre-challenge titers of Env-binding antibodies correlated with control of viral replication. To assess the contribution of anti-Env humoral immune responses to virologic control in two of these animals, we transiently depleted their circulating antibodies via extracorporeal plasma immunoadsorption and inhibition of IgG recycling through antibody-mediated blockade of the neonatal Fc receptor. These procedures reduced Ig serum concentrations by up to 80% and temporarily induced SIVmac239 replication in these animals. Next, we transferred purified total Ig from the rapid controllers into six vaccinated RMs one day before intrarectal challenge with SIVmac239. Although recipients of the hyperimmune anti-SIV Ig fraction were not protected from infection, their peak and chronic phase viral loads were significantly lower than those in concurrent unvaccinated control animals. Together, our results suggest that non-neutralizing Abs may play a role in the suppression of SIVmac239 viremia.

Keywords: SIV; adoptive transfer; anti-FcRn Ab; antibody depletion; immunoadsorption; non-neutralizing antibodies; rhesus macaques (Macaca mulatta).

Figure 1

Characterization of Env-binding Abs from Mamu-B*17+ RMs at the time of first SIV challenge. Sera of immunized Mamu-B*17+ rapid controllers (in green) were compared to sera of non-controllers (in pink) prior to challenge at 1:500 dilution. (A) gp140-specific Ab subclass and isotype titers, and rhesus FcγRIIa/FcγRIIIa affinity were measured by Luminex. Mean log MFI for negative controls was 3.7. (B) gp140-specific responses for each of the three Ab effector functions measured: ADCP, ADCD, and NK degranulation. None of the differences between rapid controllers and non-controllers at the time of SIV challenge from both panels were statistically significant (p > 0.05 by Mann-Whitney U test).

Figure 2

IgG levels and viral loads after the removal of circulating Abs from r05007 and r09062. Ig depletion by immunoadsorption and anti-FcRn Ab infusion of (A) r05007 and (B) r09062. Gray arrows indicate each immunoadsorption procedure. The blue dashed arrow indicates the first anti-FcRn Ab infusion. Viral load limit of detection (LOD) is shown as purple dotted line.

Figure 3

Total and Env-specific binding Abs present in the purified total Ig isolated from the Mamu-B*17+ RMs at 1 μg/ml. At a normalized concentration of 1 μg/ml, a large proportion of Abs present in the purified total IgG bound to SIVmac239 gp140 protein. The purified total Ig had no neutralization activity against SIVmac239 (data not shown).

Figure 4

Outcome of SIV challenge. Two years after vaccination, vaccinees and controls were intrarectally challenged with a low dose of SIVmac239. (A) Kaplan-Meier analysis of SIV acquisition, (B) individual viral loads of Group 1 vaccinees (blue) and the Group 2 (red) RMs. While the difference in acquisition rate was not statistically significant (p > 0.05 by log-rank test), (C) peak, and (D) setpoint viremia were significantly lower for vaccinees than controls (P = 0.0079 and P = 0.0317, respectively). Peak viral loads were defined as the highest viral load measurement within the first 4 weeks post-infection. Chronic phase setpoint was calculated as the geometric mean of viral load values between 8 and 12 weeks post-infection. (E) Geometric mean viral loads for Group 1 and Group 2 animals (P = 0.0003). Of note, we did not identify any correlation between the level of Env-binding Abs on the challenge day and take of the infection. P-values were determined using the Mann-Whitney U test.