Combining a rhesus cytomegalovirus/SIV vaccine with a neutralizing antibody to protect against SIV challenges in rhesus macaques

Abstract

A vaccine is considered essential for controlling the HIV pandemic and ultimately eradicating AIDS. Neutralizing antibodies and MHC-E-restricted CD8+ T cells have shown the ability to protect against the simian counterpart of HIV, SIV, in rhesus macaques. In this study, we provide preliminary evidence that combining these orthogonal antiviral mechanisms can offer increased protection against SIV. Specifically, the replication arrest observed following vaccination with a rhesus cytomegalovirus (RhCMV/SIV)-based vaccine was enhanced by the presence of a passively administered neutralizing antibody at incompletely protective levels. This report encourages studies involving larger cohorts of macaques and alternative methods for administering neutralizing antibodies.

Keywords: HIV; SIV; T cells; neutralizing antibodies; non-human primate (macaque); vaccine.

Figure 1

A neutralization ID50 of 1:300 is the threshold for protection against high-dose (900 FFU) SIVmac239 challenge by antibody K11-LS. (A) The half-life of K11-LS was determined to be 9.5 days after two RMs (A1 and A2) were administered 20 mg/kg and titers were allowed to decay. (B) RMs A3-A6 were administered 20 mg/kg K11-LS on day −10 followed by 10 mg/kg on day −3. *Animals A1 and A2 were administered 20 mg/kg K11-LS and titers were allowed to decay to 1:100 or less prior to a second infusion of 10 mg/kg at day 42 to calculate half-life. All RMs were challenged 3 days after the second K11-LS infusion and every week thereafter until infected (blue arrow). Red lines indicate blood draws. (C) Neutralization ID50s against SIVmac239 PSV were on average 1:296 at time of infection. This excludes outlier animal A1, who became infected after 9 challenges with an ID50 of 1:44, thus, exhibiting an inherent resistance to SIVmac239. Arrows indicate the last challenge before infection. (D) Neutralization ID50 at time of infection for each animal. (E) Viral load in all 6 RMs shown in panel C post first effective 900 FFU SIVmac239 challenge. Arrows indicate weekly 900 FFU challenge.

Figure 2

Administering K11-LS at a dose of 3 mg/kg achieves a neutralization titer between 1:100 and 1:300 within 21 days. RMs were administered 4 mg/kg (Group 1), 3 mg/kg (Group 2), or 2 mg/kg (Group 3) on day 0. 20 days later, the geometric mean ID50s were within the desired range of 1:100–1:200. A second dose of 3 mg/kg K11-LS was administered to all 3 groups on day 49 as the 3 mg/kg group exhibited the least variability between animals. 20 days after the second K11-LS infusion, geometric mean ID50s were 1:153, 1:215, and 1:314 for Groups 1, 2, and 3, respectively. A dose of 3 mg/kg of K11-LS was chosen for subsequent studies as ID50s were within the desired range and were the most consistent between animals.

Figure 3

Durability and induction of both SIV-specific T cell responses and unconventionally restricted T cells elicited by 68-1 RhCMV/SIV vaccine vectors. (A) Longitudinal analysis of the overall SIV-specific CD4+ and CD8+ T cell responses in peripheral blood. Responses were determined by ICS analysis (TNF vs. IFN-γ) using whole open reading frame (ORF) mixes of overlapping 15mer peptides (Gag; Rev./Nef/Tat; 5’-Pol) to stimulate PBMC. The frequency of IFN-γ and/or TNF-positive memory T cell responses to each ORF peptide mix was summed to get the overall responses shown in the figure. Vaccinations are indicated by the arrowhead above the graph. (B) Longitudinal analyses of CD8+ T cell responses to individual MHC-E- (green; left panel) and MHC-II- (blue; right panel)-restricted 15mer supertopes. Responses were determined by ICS as described in (A).

Figure 4

nAb K11-LS facilitates 68-1 RhCMV/SIV vector-mediated protection via replication arrest. (A) The vaccine phase consisted of 12 months during which RMs were administered the RhCMV/SIV vaccine on weeks 0 and 14. During the challenge phase, RMs were administered 3 mg/kg K11-LS (Groups 1 [n=9] and 3 [n=6]) or control antibody (Group 2 [n=9]) 21 to 24 days before SIVmac239 challenge. Animals A3, B3, B7, B8, C1, C3, C4, C5, and C6 received a second dose of 3 mg/kg K11-LS (Groups 1 and 3) or control antibody (Group 2) 39 days after primary challenge. RMs A3, B3, B7, B8, C3, C4, C5, and C6 were challenged a second time on day 77. RM C1, having been found to be infected after the primary challenge, was not re-challenged. (B) At the time of primary challenge (designated as day 0), the neutralization ID50s against SIVmac239 pseudovirus averaged 1:130 and 1:160 for Groups 1 and 3, respectively. Group 2 showed no neutralization activity, as animals received DEN3 control IgG instead of K11-LS. (C) Within Group 1, nAb titers at time of effective challenge were on average 1:164 and 1:95 for SIV protected and SIV non-protected RM, respectively. (D) Plasma viral load (PVL) and SIVmac239 Vif-specific CD8+ T cell responses are shown for infected RMs across all three groups. The x-axis represents days post effective challenge, with day 0 marking the time point at which infection was established, as indicated by the onset of plasma viremia and/or de novo detection of Vif-specific T cell responses. RMs were considered protected if they exhibited only anti-Vif responses without sustained viremia (with or without transient viral blips; shown as red lines), and non-protected if they developed sustained plasma viremia (black lines). In Group 3, four RMs exhibited sustained viremia (non-protected; black lines), while two animals showed no evidence of viral replication or Vif-specific responses and are thus not shown on the graph. No difference was observed in absolute CD4+ and CD8+ T cell levels between Groups 1 and 2 and pre- and post-vaccination.