Vaccination with Live Attenuated Simian Immunodeficiency Virus (SIV) Protects from Mucosal, but Not Necessarily Intravenous, Challenge with a Minimally Heterologous SIV

Abstract

Few studies have evaluated the impact of the viral challenge route on protection against a heterologous simian immunodeficiency virus (SIV) challenge. We vaccinated seven macaques with a live attenuated SIV that differed from SIVmac239Δnef by 24 amino acids, called m3KOΔnef. All animals were protected from an intrarectal SIVmac239 challenge, whereas only four animals were protected from subsequent intravenous SIVmac239 challenge. These data suggest that immune responses elicited by vaccination with live attenuated SIV in an individual animal can confer protection from intrarectal challenge while remaining insufficient for protection from intravenous challenge.

Importance: Our study is important because we show that vaccinated animals can be protected from a mucosal challenge with a heterologous SIV, but the same animals are not necessarily protected from intravenous challenge with the same virus. This is unique because in most studies, either vaccinated animals are challenged multiple times by the same route or only a single challenge is performed. An individually vaccinated animal is rarely challenged multiple times by different routes, so protection from different challenge routes cannot be measured in the same animal. Our data imply that vaccine-elicited responses in an individual animal may be insufficient for protection from intravenous challenge but may be suitable for protection from a mucosal challenge that better approximates human immunodeficiency virus (HIV) exposure.

FIG 1

Animals vaccinated with m3KOΔnef are partially susceptible to intravenous (IV) challenge with SIVmac239. gag and nef qRT-PCRs were performed on plasma samples at the indicated times after intravenous SIVmac239 challenge. (A) Three animals with evidence of full-length nef. (B) Four animals without replicating full-length nef. Data for each animal are shown on independent plots. The horizontal dotted line at 50 copies/ml (copy equivalents [ceq] per milliliter) represents the limit of detection of the assay.

FIG 2

Minimal differences in T cell responses elicited following intrarectal and intravenous SIVmac239 challenges. IFN-γ ELISPOT assays were performed by using peptides for epitopes that could be targeted in animals that were M3 positive (A) and M3 negative (B). T cell responses were measured 3 weeks after intrarectal (IR) challenge, 3 weeks after intravenous challenge, and 9 weeks after intravenous challenge, with concanavalin A (ConA) being used as a positive control at all time points. Of note, at the time of the assay, it was unknown that the Nef_194–203LW10 peptide contained the Nef_196–203HW8 epitope used for Fig. 3B. A response was considered positive if the number of SFCs per 10⁶ PBMCs exceeded our threshold of the no-stimulation average plus 2 times the standard deviation for nonstimulated wells or 50 SFCs per 10⁶ PBMCs, whichever was greater. This threshold of 50 SFCs per 10⁶ PBMCs is represented with a horizontal dotted line. Each bar color represents a different peptide and shows the average of data for two duplicate wells following subtraction of the average for the nonstimulated wells.

FIG 3

Animals vaccinated with SIVmac239Δnef are resistant to intravenous challenge with SIVmac239. (A) gag qRT-PCR was performed on plasma samples at the indicated times following intravenous SIVmac239 challenge, and data were graphed. Full-length nef and Δnef qRT-PCRs were performed at select time points for each animal. The limit of detection of the assay is shown with a horizontal dotted line. (B) IFN-γ ELISPOT assays were performed with two epitopes (Nef_103–111RM9 and Nef_196–203HW8) at days 21 and 63 post-intravenous challenge. An additional epitope present in SIVmac239Δnef, Env_338–346RF9, is shown as a positive control, as is concanavalin A (ConA). A response was considered positive if the number of SFCs per 10⁶ PBMCs exceeded our threshold of the no-stimulation average plus 2 times the standard deviation of data for nonstimulated wells or 50 SFCs per 10⁶ PBMCs, whichever was greater. This threshold of 50 SFCs per 10⁶ PBMCs is represented with a horizontal dotted line. Each shape represents a different animal and is the average of data for two duplicate wells following subtraction of the average for the nonstimulated wells.