Enhanced Immunity and Protective Efficacy against SIV mac251 Intrarectal Challenge Following Ad-SIV Priming by Multiple Mucosal Routes and gp120 Boosting in MPL-SE

Previously, 39% of rhesus macaques primed orally, intranasally, and intratracheally with adenovirus(Ad)-simian immunodeficiency virus (SIV) recombinants and boosted with gp120 in monophosphoryl lipid A-stable emulsion (MPL-SE) remained aviremic or cleared or controlled viremia at the threshold of detection following SIV_mac251 intrarectal challenge (Study B). In contrast, no macaques primed orally and intranasally with Ad-SIV recombinants and boosted with gp120 in Quillaja Saponaria-21 exhibited undetectable viremia post-challenge (Study A). We conducted a detailed comparison of the studies to elucidate the effect of different vaccine regimens on induced immunity associated with the different challenge outcomes. Quantitative viral load comparisons were statistically analyzed. All immune responses were assessed at identical timepoints post-immunization, and cellular immunity was re-evaluated on cryopreserved cells from Study B macaques to match Study A data acquired with frozen cells. Study B exhibited greater protective efficacy, increased levels of p11C and p54m tetramer positive cells and a trend toward enhanced interferon-γ secreting cells in response to Env and Gag peptides, modestly enhanced serum neutralizing antibodies, and greater positivity in anti-gp120 rectal IgA and IgG antibodies. Study A macaques exhibited greater positivity in salivary IgA anti-gp120 antibodies. Thus, the vaccine regimen using oral–intranasal–intratracheal priming and protein boosting in MPL-SE was superior, eliciting greater protective efficacy against pathogenic SIV_mac251 and enhanced SIV-specific immunity, systemically and at rectal sites. The mechanism(s) by which binding antibodies, lacking neutralizing activity against the primary challenge virus, may contribute to protection requires further study.