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Journal of Virology, December 2001, p. 11891-11896, Vol. 75, No. 23
AIDS Research Center, National Institute of
Infectious Diseases, Musashi-murayama, Tokyo 208-0011, Japan
Received 11 July 2001/Accepted 5 September 2001
Heterologous prime/boost regimens are AIDS vaccine candidates
because of their potential for inducing cellular immune responses. Here, we have developed a prime/boost regimen leading to rapid control
of highly pathogenic immunodeficiency virus infection in macaques. The
strategy, priming by an env and nef
deletion-containing simian-human immunodeficiency virus (SHIV) proviral
DNA followed by a single booster with a Gag-expressing Sendai virus
(SeV-Gag), efficiently induced virus-specific T cells, which were
maintained for more than 3 months until challenge. While all naive
control macaques showed acute CD4+ T-cell depletion at week
2 after an intravenous SHIV89.6PD challenge, all the macaques
vaccinated with the prime/boost regimen were protected from depletion
and showed greatly reduced peak viral loads compared with controls.
Vaccination with the DNA alone or SeV-Gag alone was not enough to
confer the consistent protection from the depletion, although it led to
efficient secondary CD8+ T-cell responses at week 2 after
challenge. At week 1, a difference in the secondary responses between
the protected and the unprotected macaques was clear; rapid
augmentation of virus-specific CD8+ T cells was detected in
the former but not in the latter. Thus, our results indicate the
importance of rapid secondary responses for reduction in the peak viral
loads and protection from acute CD4+ T-cell depletion.
Cellular immune responses
play a critical role in the control of immunodeficiency virus
infections (8, 25). The importance of
CD8+ T cells in this control has been indicated
in human immunodeficiency virus type 1-infected individuals (7,
15, 22) and in macaque AIDS models (11, 20, 24).
Also, virus-specific CD4+ T-cell responses have
been shown to be essential for effective cytotoxic-T-lymphocyte
responses and for controlling virus infections (3, 18,
23). Therefore, a strategy inducing virus-specific T-cell
responses efficiently can be a promising AIDS vaccine candidate.
For efficient induction of the responses, we previously developed a DNA
vaccine system (19) using FMSIV, which is a chimeric simian-human immunodeficiency virus (SHIV) with ecotropic Friend murine
leukemia virus (FMLV) env in place of SHIV env,
in combination with the FMLV receptor, mCAT1 (1), which is
not normally expressed in primate cells. Vaccination of macaques with
both the FMSIV proviral DNA and mCAT1 expression plasmid DNA allowed
mCAT1-dependent FMSIV replication and induced resistance against
intravenous challenge with a pathogenic strain of simian
immunodeficiency virus, SIVmac239; the macaques vaccinated with FMSIV
DNA plus mCAT1 DNA showed reduced viral loads at both the peak and the
set point compared with controls (19). Further, we
established an efficient antigen expression system using Sendai virus
(SeV), which is a nonsegmented negative-strand RNA virus considered
nonpathogenic for humans and nonhuman primates (13, 14,
21). Intranasal immunization of macaques with a recombinant SeV
vector expressing SIV Gag (SeV-Gag) elicited resistance against
intravenous SIVmac239 challenge, leading to marked reduction in the
set-point plasma viral loads to below or just above the detectable
level, although the primary acute viremia was not controlled (12). In this study, we combined these two systems to
develop a prime/boost vaccine strategy and evaluated its protective
efficacy in a macaque AIDS model using a highly pathogenic
immunodeficiency virus. We used 14 rhesus macaques (Macaca
mulatta) divided into four groups for the evaluation (Table
1).
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.23.11891-11896.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Rapid Appearance of Secondary Immune Responses and Protection
from Acute CD4 Depletion after a Highly Pathogenic Immunodeficiency
Virus Challenge in Macaques Vaccinated with a DNA Prime/Sendai
Virus Vector Boost Regimen
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TABLE 1.
Vaccination and challenge protocol in macaques
The frequency of specific T cells measured by flow-cytometric analysis
of intracellular interferon-
(IFN-
) induction is regarded as an
index of antigen-specific cellular immune responses, although it does
not always correlate with antigen-specific cytolytic activity (5,
9, 10, 16). Then, we examined the frequencies of the T cells
reactive to the SHIV antigens other than Env and Nef, which are
expected to be induced by FMSIV DNA-SeV-Gag vaccinations. In brief, an
SHIV proviral DNA with env and nef deleted,
SIVGP1 DNA, was obtained by removing the whole FMLV env
region from the FMSIV DNA. COS-1 cells were cotransfected with SIVGP1
DNA and a plasmid DNA expressing vesicular stomatitis virus G protein (pVSV-G; Clontech, Palo Alto, Calif.) to obtain a pseudotyped SHIV
bearing VSV-G, SIVGP1(VSV-G). For the SHIV-specific stimulation, 106 peripheral blood mononuclear cells (PBMC)
were cocultured with 105 herpesvirus
papio-immortalized B-lymphoblastoid cells (26) infected
with SIVGP1(VSV-G) for 6 h (in the presence of GolgiStop [monensin; Pharmingen, San Diego, Calif.] for the last 5 h). For nonspecific stimulation, a VSV-G-pseudotyped MLV, MLVGP(VSV-G), was
used instead of SIVGP1(VSV-G). Then, intracellular IFN-
staining was
performed with a Cytofix-Cytoperm kit (Pharmingen) according to the
manufacturer's protocol. Fluorescein isothiocyanate-conjugated anti-human CD4 (Pharmingen), peridinin chlorophyll protein-conjugated anti-human CD8 (Becton Dickinson, San Jose, Calif.),
allophycocyanin-conjugated anti-human CD3 (Pharmingen), and
phycoerythrin-conjugated anti-human IFN-
(Pharmingen)
antibodies were used. Stained samples were collected by FACScalibur and
analyzed using CellQuest software (Becton Dickinson). Gating was
performed on mononuclear cells and then on CD3+
CD4+ or
CD3+CD8+ subpopulations.
From the ratio of CD3+ CD4+
IFN-
+ or CD3+
CD8+ IFN-
+ cells to
mononuclear cells, the frequency of CD4+
IFN-
+ or CD8+
IFN-
+ T cells per 106
cells was calculated. Then, the frequency of SHIV-specific
IFN-
+ cells was calculated by subtracting the
frequency after the nonspecific stimulation from that after the
SHIV-specific stimulation.
The frequencies of specific T cells in macaque PBMC after vaccination
were examined by flow-cytometric analysis (Fig.
1A). None of the group I macaques showed
SHIV-specific IFN-
induction before SHIV challenge (data not shown).
In all the group II macaques vaccinated with the DNA alone,
SHIV-specific T cells were clearly induced and remained detectable
until challenge. In all the group III macaques vaccinated with SeV-Gag
alone, significant levels of specific T cells were seen at week 2 following vaccination, but their numbers declined to marginal levels
before challenge. Four animals in group IV were given SeV-Gag boosters
at week 12. Before the SeV-Gag booster, two (R007 and R011) received
only FMSIV DNA (with control DNA), while the other two (R005 and R012) were vaccinated with FMSIV DNA plus mCAT1 DNA. In the former two, SHIV-specific T cells were detected faintly before the SeV-Gag booster,
but only a single SeV-Gag vaccination resulted in efficient induction
of SHIV-specific T cells. In the latter two, the DNA vaccination
induced SHIV-specific T cells efficiently and the SeV-Gag booster led
to more vigorous responses. Thus, SHIV-specific T cells expanded
efficiently after a single SeV-Gag booster and were kept at high levels
for more than 3 months until challenge in all the group IV macaques
(Fig. 1B).
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For the challenge, we used a highly pathogenic SHIV89.6PD virus stock provided by Y. Lu (17). Before the present vaccine study, we had confirmed that its intravenous inoculation at the dose of 10 50% tissue culture infective doses caused almost complete depletion of peripheral CD4+ T cells within 2 weeks in rhesus macaques. Then, all 14 macaques (Table 1) were challenged intravenously with 10 50% tissue culture infective doses of SHIV89.6PD. We challenged macaques in group IV with SHIV89.6PD more than 3 months after the SeV-Gag booster to examine its long-term efficacy, although intravenous challenge was performed no more than 6 weeks after the last vaccination in many previous studies.
All the macaques in groups I and III showed almost complete depletion
of peripheral CD4+ T cells at week 2 after
challenge (Fig. 2). In group II, one animal (R022) was protected completely from depletion and a second (R002) was partially protected, but a third (R021) showed depletion. In
marked contrast to these, all four macaques in group IV were completely
protected from CD4+ T-cell depletion (Fig. 2 and
Table 2).
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All the group IV macaques showed greatly reduced plasma SHIV loads compared with those in group I (Fig. 2). The reduction at the peak (week 2) was striking (Table 2), and viremia was undetectable at the set point in group IV. In group II, the peak viral load was greatly reduced in R022 protected from the CD4 depletion, while no reduction was observed in the unprotected R021. The peak level in the partially protected R002 was between the other two. In group III, no reduction in the peak viral loads was observed, but the set-point viral loads declined to undetectable levels in two animals.
We examined frequencies of SHIV-specific T cells in PBMC at weeks 1 and
2 after challenge (Fig. 3). Augmented
SHIV-specific CD4+ and CD8+
T cells appeared at week 1 in all the group IV macaques, while no
significant secondary responses were observed at week 1 or 2 in the
group I macaques. In group II, the protected macaque R022 showed
augmented SHIV-specific CD4+ and
CD8+ T cells at week 1. In contrast, the
unprotected macaque R021 showed significant secondary responses of
SHIV-specific CD4+ T cells at week 1, but
SHIV-specific CD8+ T cell responses were delayed.
Thus, poor protection in R021 can be explained by insufficient
augmentation of SHIV-specific CD8+ T cells at
week 1. The group III macaques showed no significant secondary
responses at week 1, but efficient augmentation of SHIV-specific CD8+ T cells appeared at week 2.
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AIDS vaccine strategies have been evaluated in macaque models using pathogenic SIV or SHIV (2). While most macaques develop AIDS a few years after SIV challenge, the pathogenic SHIV model shows acute CD4 depletion a few weeks after challenge. In the latter model, this study showed excellent protection by our env-independent prime/boost vaccination against acute CD4 depletion. Interestingly, a single protein, Gag, was sufficient as the booster antigen for protection. The macaques vaccinated with the prime/boost regimen showed rapid secondary responses of virus-specific T cells after challenge.
At week 1 after challenge, augmented SHIV-specific CD8+ T cells were observed in all the protected macaques but not in any of the unprotected macaques. The group III macaques without SHIV-specific T-cell augmentation at week 1 were not protected from the CD4+ T-cell depletion at week 2, although they showed efficient secondary CD8+ T-cell responses at week 2. The secondary responses may lead to reduction in the set-point viral loads, as has been indicated (4, 6), but the appearance of SHIV-specific CD8+ T cells at week 2 would be too late for a reduction in peak viral loads and protection from acute CD4 depletion. Thus, the turning point determining whether macaques can be protected from acute CD4 depletion is within a week after challenge. This is consistent with a previous study showing that the effects of CD8 depletion on viral loads are greater in macaques treated with anti-CD8 antibody just before SHIV challenge than in those treated at week 1 after challenge (20). Taken together, our results indicate that rapid secondary responses of SHIV-specific T cells, particularly CD8+ T cells, were essential for the marked reduction in peak viral loads and the protection from acute CD4+ T-cell depletion after challenge.
In group IV, SHIV-specific T-cell induction was not efficient before SeV-Gag booster in two macaques (R007 and R011) primed with FMSIV DNA only, but they showed levels of protection similar to those in the other two (R005 and R012) primed with FMSIV plus mCAT1 DNA. In contrast, the group III macaques vaccinated with SeV-Gag alone were not protected from acute CD4+ T-cell depletion and showed delayed secondary responses. Thus, vaccination with FMSIV DNA (without mCAT1 DNA) may be required and sufficient as priming in our prime/boost regimen for the rapid secondary responses and the protection against acute CD4 depletion.
Recently, a DNA vaccine with cytokine augmentation succeeded in preventing SHIV89.6P-induced AIDS in macaques (6). Further, a DNA priming followed by a booster with a recombinant modified vaccinia virus Ankara expressing multiple proteins has been reported to control mucosal SHIV89.6P challenge (4). In vaccinated animals, however, the virus-specific T cells were undetectable in peripheral blood at week 1 after challenge, potentially reflecting the recruitment of specific T cells to the site of infection. The virus-specific secondary responses became detectable later, and the late responses correlated with set-point viral loads but not with peak viral loads. Although there were some differences between experimental conditions in those studies and in ours, the protected macaques in our study showed the fastest detectable secondary responses at week 1 after challenge. The rapid responses may explain the greater reduction in peak viral loads in our study. Thus, our prime/boost regimen showed rapid control of immunodeficiency virus infection, which could contribute to host immune function and help slow the virus epidemic.
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ACKNOWLEDGMENTS |
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We thank Y. Lu for providing SHIV89.6PD, Y. Ami, M. Honda, M. Nakasone, T. Sata, F. Ono, K. Komatsuzaki, K. Oto, K. Mori, R. Mukai, and A. Yamada for assistance in the animal experiments and A. Kato, M. Miyazawa, N. Watanabe, A. Iwamoto, and H. Yoshikura for helpful suggestions.
This work was supported by Health Sciences Research grants from the Ministry of Health, Labour and Welfare in Japan.
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FOOTNOTES |
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* Corresponding author. Mailing address: AIDS Research Center, National Institute of Infectious Diseases, 4-7-1 Gakuen, Musashi-murayama, Tokyo 208-0011, Japan. Phone: 81-42-561-0771. Fax: 81-42-565-3315. E-mail: matano{at}nih.go.jp.
Present address: Toyama Institute of Health, Toyama 939-0363, Japan.
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