Correlation of vaccine-elicited systemic and mucosal nonneutralizing antibody activities with reduced acute viremia following intrarectal simian immunodeficiency virus SIVmac251 challenge of rhesus macaques

Abstract

Cell-mediated immunity and neutralizing antibodies contribute to control of human immunodeficiency virus/simian immunodeficiency virus (HIV/SIV) infection, but the role of nonneutralizing antibodies is not defined. Previously, we reported that sequential oral/oral or intranasal/oral (I/O) priming with replication-competent adenovirus type 5 host range mutant (Ad5hr)-SIV recombinants, followed by intramuscular envelope protein boosting, elicited systemic and mucosal cellular immunity and exhibited equivalent, significant reductions of chronic viremia after rectal SIV(mac251) challenge. However, I/O priming gave significantly better control of acute viremia. Here, systemic and mucosal humoral immunity were investigated for potential correlates with the acute challenge outcome. Strong serum binding but nonneutralizing antibody responses against SIV(mac251) were induced in both groups. Antibody responses appeared earlier and overall were higher in the I/O group. Reduced acute viremia was significantly correlated with higher serum binding titer, stronger antibody-dependent cellular cytotoxicity activity, and peak prechallenge and 2-week-postchallenge antibody-dependent cell-mediated viral inhibition (ADCVI). The I/O group consistently displayed greater anti-envelope immunoglobulin A (IgA) antibody responses in bronchoalveolar lavage and a stronger rectal anti-envelope IgA anamnestic response 2 weeks postchallenge. Pre- and postchallenge rectal secretions inhibited SIV transcytosis across epithelial cells. The inhibition was significantly higher in the I/O group, although a significant correlation with reduced acute viremia was not reached. Overall, the replicating Ad5hr-SIV priming/envelope boosting approach elicited strong systemic and mucosal antibodies with multiple functional activities. The pattern of elevated immune responses in the I/O group is consistent with its better control of acute viremia mediated, at least in part, by ADCVI activity and transcytosis inhibition.

FIG. 1.

Immunization of rhesus macaques and challenge outcome. Regimen summarized from reference (60). (A) Three Ad5hr recombinants containing SIV_smH4env/rev, SIV₂₃₉gag, and SIV₂₃₉nef were administered at 5 × 10⁸ PFU/recombinant/dose either orally as enteric coated tablets or intranasally in PBS. SIV gp120 was administered at 100 μg/dose in MPL-SE. Controls received a total Ad5hr vector dose of 1.5 × 10⁹ PFU and MPL-SE only. The SIV_mac251 challenge dose was 10 50% monkey infectious doses. (B) Summary of challenge results (60). *, Both O/O and I/O groups exhibited reduced peak viremia (weeks 1 to 4) compared to controls (P = 0.042 for both). At week 2 postchallenge, viremia of the I/O group was significantly lower than that of the O/O group (P = 0.026). **, Both the O/O and the I/O groups exhibited reduced chronic viremia (weeks 12 to 28) compared to controls (P = 0.048 and 0.038, respectively).

FIG. 2.

Serum IgG anti-SIV_mac251 titers. (A) Geometric mean titers for the two immunization groups and controls over the duration of the study. Arrows indicate Ad5hr-SIV recombinant immunizations, protein boosts, and time of challenge. *, I/O titers are significantly different compared to the O/O group at weeks 26 and 34 (P values of 0.0017 and 0.047, respectively). (B to E) The binding antibody titer of individual animals at the indicated time points is plotted against week 2 viremia. The straight line in each graph is the result of linear regression analysis of the data set, and the correlation coefficients (r) and P values are from Spearman rank analysis.

FIG. 3.

ADCC activity induced by the vaccine regimens. (A) Sera collected at the indicated time points were tested for ADCC activity to SIV_mac251 gp120-coated target cells. The geometric mean ADCC titers of each group are shown. Arrows indicate Ad5hr-SIV recombinant immunizations, protein boosts, and time of challenge. *, The geometric mean ADCC titer of the I/O group is significantly higher than that of the O/O group (P = 0.016). The straight lines in graphs B and C result from linear regression analysis of week 26 ADCC titers versus peak acute viremia of all macaques (B) and immunized macaques only (C). The correlation coefficients (r) and P values are from Spearman rank analysis.

FIG. 4.

Vaccine-induced ADCVI activity. (A) Sera collected at the indicated time points were measured for ADCVI activity. Arrows indicate Ad5hr-SIV recombinant immunizations, protein boosts, and time of challenge. Error bars indicate the standard error of the mean. Inhibition by the I/O group was significantly greater than that by the O/O group at week 26 (*, P = 0.0006) and at week 44 (**, P = 0.0023). (B to E) The straight line in each graph is the result of linear regression analysis of serum ADCVI activity at the indicated time point versus week 2 viremia. The correlation coefficients (r) and P values are from Spearman rank analysis.

FIG. 5.

SIV_mac251 gp120 specific antibodies in mucosal samples. Mucosal samples collected at the indicated time points were assayed for SIV_mac251 gp120 specific IgA (A, C, and E) or IgG (B, D, and F). Titers of each sample were normalized to the total IgA or IgG of the corresponding samples. Values for nasal and rectal secretions were further compared to values prior to immunization and reported as fold increase. The data are reported as geometric means. *, In panel A, the BAL anti-SIV gp120 IgA titer of the I/O group is significantly higher than that of the O/O group at weeks 38 (P = 0.0023) and 43 (P = 0.040).

FIG. 6.

Transcytosis inhibition. Serum (A) and rectal (B) samples collected at the indicated time points were measured for inhibition of SIV transcytosis across a tight epithelial cell barrier. Arrows indicate Ad5hr-SIV recombinant immunizations, protein boosts, and time of challenge. Error bars indicate the standard error of the mean. *, Rectal secretions of the I/O group exhibited greater transcytosis inhibition over weeks 38, 43, and 44 compared to the O/O group (P = 0.0042).