Immunization of rhesus macaques with a DNA prime/modified vaccinia virus Ankara boost regimen induces broad simian immunodeficiency virus (SIV)-specific T-cell responses and reduces initial viral replication but does not prevent disease progression following challenge with pathogenic SIVmac239

Abstract

Producing a prophylactic vaccine for human immunodeficiency virus (HIV) has proven to be a challenge. Most biological isolates of HIV are difficult to neutralize, so that conventional subunit-based antibody-inducing vaccines are unlikely to be very effective. In the rhesus macaque model, some protection was afforded by DNA/recombinant viral vector vaccines. However, these studies used as the challenge virus SHIV-89.6P, which is neutralizable, making it difficult to determine whether the observed protection was due to cellular immunity, humoral immunity, or a combination of both. In this study, we used a DNA prime/modified vaccinia virus Ankara boost regimen to immunize rhesus macaques against nearly all simian immunodeficiency virus (SIV) proteins. These animals were challenged intrarectally with pathogenic molecularly cloned SIVmac239, which is resistant to neutralization. The immunization regimen resulted in the induction of virus-specific CD8(+) and CD4(+) responses in all vaccinees. Although anamnestic neutralizing antibody responses against laboratory-adapted SIVmac251 developed after the challenge, no neutralizing antibodies against SIVmac239 were detectable. Vaccinated animals had significantly reduced peak viremia compared with controls (P < 0.01). However, despite the induction of virus-specific cellular immune responses and reduced peak viral loads, most animals still suffered from gradual CD4 depletion and progressed to disease.

FIG. 1.

Immunization and challenge schedule. rMVA, recombinant MVA; i.d., intradermally; i.r., intrarectally.

FIG. 2.

ICS data for virus-specific CD8⁺-lymphocyte responses in animals vaccinated with DNA/MVA. The frequency of responding CD8⁺ cells against each protein is shown. These frequencies were obtained by adding the frequencies detected with peptide pools containing overlapping peptides from a single protein (after subtraction of the background). CD8⁺ responses after the first MVA boost (prior to infection), 2 weeks postinfection (acute phase of infection), and 12 weeks postinfection (chronic phase of infection) are shown.

FIG. 3.

ICS data for virus-specific CD4⁺-lymphocyte responses in animals vaccinated with DNA/MVA. The frequency of responding CD4⁺ cells against each protein is shown. These frequencies were obtained as described in the legend to Fig. 2. CD4⁺ responses after the first MVA boost (prior to infection), 2 weeks postinfection (acute phase of infection), and 12 weeks postinfection (chronic phase of infection) are shown.

FIG. 4.

Tetramer staining results for Mamu-A*01-positive animals, 80035 and 96135, after the first MVA boost and throughout infection. Percentages shown are for tetramer-specific CD8⁺ T cells. In addition to the tetramers against the Mamu-A*01-restricted epitopes Gag_181-189CM9 and Tat_28-35SL8, tetramers against two more Mamu-A*01-restricted epitopes, Env_235-243CL9 and Env_622-630TL9 (2), were tested postchallenge.

FIG. 5.

Viral load data for all animals up to 28 weeks postinfection, as determined by real-time PCR. (A) Control animals 90069, 90131, 92050, 92080, and 97086 were given empty MVA boosts only before intrarectal infection with SIVmac239. (B) Animals 80035, 81035, 83108, 87081, 87082, 93062, 96135, and 97073 were vaccinated 10 times with DNA and twice with recombinant MVA before intrarectal infection with SIVmac239. (C) Average viral loads of control animals versus vaccinees. A dagger indicates euthanasia of the animal.

FIG. 6.

Postchallenge CD4⁺-T-lymphocyte counts in control and vaccinated animals. A dagger indicates euthanasia of the animal.

FIG. 7.

Postchallenge mortality in vaccinated and control animals through 38 weeks postinfection.