Risk of immunodeficiency virus infection may increase with vaccine-induced immune response.
ABSTRACT To explore the efficacy of novel complementary prime-boost immunization regimens in a nonhuman primate model for HIV infection, rhesus monkeys primed by different DNA vaccines were boosted with virus-like particles (VLP) and then challenged by repeated low-dose rectal exposure to simian immunodeficiency virus (SIV). Characteristic of the cellular immune response after the VLP booster immunization were high numbers of SIV-specific, gamma interferon-secreting cells after stimulation with inactivated SIV particles, but not SIV peptides, and the absence of detectable levels of CD8(+) T cell responses. Antibodies specific to SIV Gag and SIV Env could be induced in all animals, but, consistent with a poor neutralizing activity at the time of challenge, vaccinated monkeys were not protected from acquisition of infection and did not control viremia. Surprisingly, vaccinees with high numbers of SIV-specific, gamma interferon-secreting cells were infected fastest during the repeated low-dose exposures and the numbers of these immune cells in vaccinated macaques correlated with susceptibility to infection. Thus, in the absence of protective antibodies or cytotoxic T cell responses, vaccine-induced immune responses may increase the susceptibility to acquisition of immunodeficiency virus infection. The results are consistent with the hypothesis that virus-specific T helper cells mediate this detrimental effect and contribute to the inefficacy of past HIV vaccination attempts (e.g., STEP study).
- SourceAvailable from: faculty.washington.edu[show abstract] [hide abstract]
ABSTRACT: In Thailand, phase 1/2 trials of monovalent subtype B and bivalent subtype B/E (CRF01_AE) recombinant glycoprotein 120 human immunodeficiency virus type 1 (HIV-1) vaccines were successfully conducted from 1995 to 1998, prompting the first HIV-1 vaccine efficacy trial in Asia. This randomized, double-blind, placebo-controlled efficacy trial of AIDSVAX B/E (VaxGen), which included 36-months of follow-up, was conducted among injection drug users (IDUs) in Bangkok, Thailand. The primary end point was HIV-1 infection; secondary end points included plasma HIV-1 load, CD4 cell count, onset of acquired immunodeficiency syndrome-defining conditions, and initiation of antiretroviral therapy. A total of 2546 IDUs were enrolled between March 1999 and August 2000; the median age was 26 years, and 93.4% were men. The overall HIV-1 incidence was 3.4 infections/100 person-years (95% confidence interval [CI], 3.0-3.9 infections/100 person-years), and the cumulative incidence was 8.4%. There were no differences between the vaccine and placebo arms. HIV-1 subtype E (83 vaccine and 81 placebo recipients) accounted for 77% of infections. Vaccine efficacy was estimated at 0.1% (95% CI, -30.8% to 23.8%; P=.99, log-rank test). No statistically significant effects of the vaccine on secondary end points were observed. Despite the successful completion of this efficacy trial, the vaccine did not prevent HIV-1 infection or delay HIV-1 disease progression.The Journal of Infectious Diseases 01/2007; 194(12):1661-71. · 5.85 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The precise identification of the HIV-1 envelope glycoprotein (Env) responsible for productive clinical infection could be instrumental in elucidating the molecular basis of HIV-1 transmission and in designing effective vaccines. Here, we developed a mathematical model of random viral evolution and, together with phylogenetic tree construction, used it to analyze 3,449 complete env sequences derived by single genome amplification from 102 subjects with acute HIV-1 (clade B) infection. Viral env genes evolving from individual transmitted or founder viruses generally exhibited a Poisson distribution of mutations and star-like phylogeny, which coalesced to an inferred consensus sequence at or near the estimated time of virus transmission. Overall, 78 of 102 subjects had evidence of productive clinical infection by a single virus, and 24 others had evidence of productive clinical infection by a minimum of two to five viruses. Phenotypic analysis of transmitted or early founder Envs revealed a consistent pattern of CCR5 dependence, masking of coreceptor binding regions, and equivalent or modestly enhanced resistance to the fusion inhibitor T1249 and broadly neutralizing antibodies compared with Envs from chronically infected subjects. Low multiplicity infection and limited viral evolution preceding peak viremia suggest a finite window of potential vulnerability of HIV-1 to vaccine-elicited immune responses, although phenotypic properties of transmitted Envs pose a formidable defense.Proceedings of the National Academy of Sciences 06/2008; 105(21):7552-7. · 9.74 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: We developed an animal model for the male-to-female transmission of human immunodeficiency virus, consisting of an atraumatic vaginal application of simian immunodeficiency virus onto the intact vaginal mucosa of cynomolgus macaques. Different doses of a pathogenic isolate of SIVmac251, with or without seminal plasma, were infused into the vaginas of female macaques. Infection of macaques could be achieved after a single exposure to the virus. Two patterns of infection were underscored with no relation to the virus dose inoculated: in 50% of the monkeys, SIV was persistently recovered and a strong antibody response to SIV was evidenced in blood and vaginal secretions. In the other infected animals, SIV infection was only transiently evidenced and a weak systemic antibody response was detected. It appeared that the presence of seminal plasma may be implicated in this variability only when low doses of virus are inoculated. Sequence analysis of the env gene of SIV revealed that most of the persistently viraemic animals were infected with a viral variant different from that of transiently viraemic macaques.Research in Virology 01/1998; 149(1):53-68.
Risk of Immunodeficiency Virus Infection May Increase with Vaccine-
Induced Immune Response
Matthias Tenbusch,aRalf Ignatius,bVladimir Temchura,aGhulam Nabi,aBettina Tippler,aGuillaume Stewart-Jones,c
Andres M. Salazar,dUlrike Sauermann,eChristiane Stahl-Hennig,eand Klaus U¨berlaa
Department of Molecular and Medical Virology, Ruhr University Bochum, Bochum, Germanya; Institute of Tropical Medicine and International Health, Charité—University
Medicine of Berlin, Berlin, Germanyb; Human Immunology Unit, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Oxford University, Oxford, United
Kingdomc; Oncovir Inc., Washington, DC, USAd; and German Primate Center, Göttingen, Germanye
tive HIV vaccine. In addition, testing the efficacy of HIV vaccines
in clinical studies is time-consuming and costly. Therefore, only
three vaccine strategies have been tested for efficacy in human
volunteers so far. A recombinant protein vaccine based on the
gp120 surface protein (AIDSVAX) did not provide protection (5,
lack of efficacy was attributed to the absence of neutralizing anti-
bodies. To explore the efficacy of cellular immune responses, vol-
unteers were also immunized with adenoviral vectors carrying
to the adenoviral vector the susceptibility to acquisition of HIV
infection was increased (2). The third vaccine strategy tested for
efficacy in human volunteers aimed at the induction of cellular
and humoral immune responses by combining an avipox vector
carrying gag, protease, and env with the AIDSVAX vaccine. The
acquisition of HIV infection was reduced by approximately 30%
(15), providing the first evidence that protection from HIV infec-
tion by vaccination may be possible.
Since such an efficacy is probably too low for general use, we
explored the efficacy of a novel complementary prime-boost im-
munization in nonhuman primates. In our previous study, we
compared the immunogenicities of dendritic cell (DC)-targeting
DNA vaccines in rhesus macaques and could demonstrate that
DNA electroporation results in robust cellular and humoral im-
mune responses even at low doses (23). However, DNA vaccines
encoding DC-targeted antigens induced lower responses than the
nontargeted counterpart and nearly no response if delivered by
conventional intramuscular (i.m.) injection. To further explore
how these different priming strategies influenced the immunoge-
mmune escape mechanisms and the diversity of the circulating
HIV strains pose many hurdles for the development of an effec-
keys were boosted with a virus-like particle (VLP) vaccine to trig-
ger antibody responses to the Env protein of SIV in its native
were induced, vaccinated monkeys were not protected from ac-
contrary, vaccinees with high numbers of vaccine-induced SIV-
ceptible to acquisition of challenge virus infection than poor vac-
MATERIALS AND METHODS
duced by transient transfection of 293T cells by the polyethylenimine
(PEI) method with plasmids Sgpsyn(26) and pcD-SIVgp140-GTM/CD,
which encodes the SIV gp140 ectodomain fused with the transmembrane
domain of the G protein of vesicular stomatitis virus. The transfection
containing 1.5% fetal calf serum (FCS) 18 h after transfection. VLPs con-
taining supernatants were harvested 36 h later, centrifuged at 300 ? g for
10 min, and filtrated through a 0.45-?m filter to remove cellular debris.
VLPs were further purified and concentrated from the conditioned me-
dium by ultracentrifugation through a 20% sucrose cushion at 28,000
buffered saline (PBS) at approximately 1/180 of the volume of the 293T
cell supernatants. The concentration of viral proteins in the final VLP
Received 30 March 2012 Accepted 9 July 2012
Published ahead of print 18 July 2012
Address correspondence to Klaus U¨berla, email@example.com.
C.S.-H. and K.U¨. contributed equally to this work and share last authorship.
Copyright © 2012, American Society for Microbiology. All Rights Reserved.
October 2012 Volume 86 Number 19 Journal of Virology p. 10533–10539jvi.asm.org
preparation was determined by an in-house enzyme-linked immunosor-
bent assay (ELISA) using recombinant SIVgp130 (EVA670; National In-
for AIDS Reagents) and SIVp27 (EVA643) as standards. The endotoxin
ulus amoebocyte lysate (LAL) endpoint assay (Cambrex, Germany). The
VLP preparation was mixed with 2 mg poly-ICLC (Hiltonol; Oncovir,
Washington, DC) in a final volume of 2 ml PBS. Poly-ICLC is a synthetic
double-stranded RNA (dsRNA), which has been stabilized against serum
anoma differentiation-associated gene 5, and augments immune re-
sponses to nontargeted and CD205-targeted protein antigens (4, 21).
Immunization and immune monitoring of macaques. The animals
assigned to this study were colony-bred young adult rhesus monkeys
(Macaca mulatta) of Indian origin and of either sex. These rhesus ma-
caques were housed at the German Primate Center under conditions in
the European Union guidelines on the use of nonhuman primates for
biomedical research. The study was approved by an ethics committee
authorized by the Lower Saxony State Office for Consumer Protection
and Food Safety. Further details on the animal specification, housing,
sample collection, and approval from respective authorities are described
elsewhere (23). Overall, 24 macaques were used in this study, 18 of them
distributed to three vaccine groups of six animals each and another six
monkeys serving as controls. Macaques were typed for the A1*01, B*08,
and B*17 alleles (16). The A1*01-positive macaques were distributed
evenly among the different groups.
As outlined in Table 1, all vaccinees had been immunized twice 8
weeks apart with DNA vaccines encoding SIVp27 capsid fused to either a
single-chain antibody to DEC205 (scDEC-p27) or to a control single-
chain antibody (scISO-p27). Eight and 16 weeks after the second DNA
ing 10 ?g of SIVgp130 and 6.5 ?g of SIV p27 with 2 mg poly-ICLC as the
adjuvant in a total volume of 2 ml. Formulated VLPs were injected sub-
cutaneously close to the inguinal lymph nodes. SIV-specific antibody re-
where (23). SIVgp140 trimers were expressed by transient transfection in
affinity chromatography (IMAC), lectin, and size exclusion chromatog-
raphy purification steps. Protein purity was assessed by reducing and
nonreducing SDS-PAGE. ELISA plates (Greiner Bio-One; Lumitrac 600)
determined at a 1 to 1,000 dilution using a polyclonal rabbit anti-human
For the neutralization assay, 10 ?l of serial dilutions of heat-inactivated
serum was incubated at 37°C in duplicate with 50 ?l (40 tissue culture
key peripheral blood mononuclear cells (PBMCs). After 1 h, the volume
buffer (Promega) and luciferase activity was determined using the Bright
Glo substrate (Promega). Mean luciferase activities of duplicates were
used to determine the 50% neutralization titer. Background neutraliza-
tion titers of preimmune sera from 17 of the 18 vaccine animals were
?1:40. Only the preimmune serum of macaque 12149 had a background
neutralization titer of 1:80. SIV p27CA- and gp130SU-specific IgA anti-
stimulation only the synthetic Gag peptide pool was used.
Challenge of macaques. The repeated low-dose challenge started 8
weeks after the final immunization. The SIVmac251 stock used for chal-
in vitro titer of 104.650% tissue culture infectious doses (TCID50) per ml
on C8166 cells and 2.88 ? 107viral RNA copies per ml. For virus inocu-
ketamine, xylazine, and atropine and were placed in ventral recumbency
of virus-containing medium was administered atraumatically into the
rectum using a 10-cm cylindrical human urethral catheter with a closed
distal end (CH06; Urotech, Germany). The intrarectal viral exposures
were performed using escalating doses, i.e., with 30 TCID50for the first
seven applications, 60 TCID50for the following six challenges, and 120
TCID50for the final three inoculations. Challenges were performed
weekly and stopped 1 week after viral RNA became detectable in plasma
with levels ?100 copy equivalents/ml, indicating systemic infection (11).
Data and statistical analyses. Differences between the experimental
groups were analyzed by the Kruskal-Wallis analysis of variance
(ANOVA) by ranks test. For analyzing the correlation between immune
responses at the time of challenge and the number of inoculations re-
quired for infection, all 18 animals of the vaccine groups were included.
Differences in the number in inoculations required for infection between
106PBMCs after stimulation with AT2 SIV on the day of challenge and
those with less than 200 IFN-?-secreting cells/106PBMCs were analyzed
by the Mann-Whitney t test.
In a recent study, we explored whether targeting the SIV p27 cap-
sid protein encoded by a DNA vaccine for dendritic cells (23)
would increase immunogenicity in nonhuman primates. Six rhe-
sus monkeys (group A) had been immunized twice by electropo-
ration of a DNA vaccine (scDEC-p27) encoding a fusion protein
of a single-chain antibody to CD205 (DEC205) and the SIV p27
TABLE 1 Efficacy of the different immunization regimens
(n ? 6)DNA vaccine
Dose and route at
wk ?16 and ?8d
VLP boost at
wk 0 and 8e
Mean no. of
inoc. ? SDa
0.1 mg, EP
0.1 mg, EP
1 mg i.m. ? Adj.
1 mg i.m. ? Adj.
s.c. ? Adj.
s.c. ? Adj.
s.c. ? Adj.
8.0 ? 4.6
4.5 ? 2.6
8.3 ? 4.5
7.2 ? 5.9
5.6 ? 0.6
4.3 ? 1
5.9 ? 0.4
5.4 ? 1.3
3.9 ? 1.3
3.7 ? 1.8
4.5 ? 0.8
3.4 ? 1.3
aMean number of inoculations (inoc.) required for infection.
bPeak log10values of viral RNA copies per ml plasma between weeks 1 to 3 after infection.
cMean log10values of viral RNA copy numbers per ml plasma of week 24 and 32 after infection.
dEP, electroporation; i.m., intramuscular injection.
eRhesus monkeys primed by the indicated DNA vaccines as previously described (23) were injected subcutaneously (s.c.) close to the inguinal lymph nodes with SIV VLPs
containing 10 ?g of SIVgp130, 6.5 ?g of SIV p27, and 2 mg of poly-ICLC adjuvant (Adj.).
f—, no boost.
Tenbusch et al.
jvi.asm.org Journal of Virology
group B had received a DNA vaccine (scIso-p27) encoding a fu-
sion protein of a single-chain control antibody also fused to the
SIV p27 capsid protein. Additionally, six monkeys (group C) had
received scDEC-p27 with the TLR3 agonist poly-ICLC (Hiltonol)
further explore how these different priming strategies influenced
were boosted with the same VLP vaccine 8 weeks after the last
DNA immunization and another 8 weeks later. The Gag and Env
content of the VLP preparation was determined by ELISA. Fur-
thermore, the envelope protein gp160 (160 kDa) and the precur-
the capsid protein p27 (27 kDa), were successfully detected in
Western blot analysis, indicating correct incorporation of these
proteins into the VLPs (Fig. 1). The VLPs were injected together
with poly-ICLC as the adjuvant. A summary of the entire immu-
nization regimen for all groups is provided in Table 1.
Prior to the VLP immunizations, antibodies to Gag were de-
tectable only in groups A and B (Fig. 2A and B). The VLP immu-
of group C seroconverted to Gag after the second VLP boost (Fig.
2B). Env-specific antibodies became detectable in all groups after
the second VLP boost. The induced antibodies also recognized
trimeric SIVmac251 gp140 (Fig. 3A), and low levels of neutraliz-
ing antibodies to the SIVmac251 isolate were also observed 2
close to background levels (data not shown). Low levels of Gag-
and Env-specific IgA antibodies were also detected after the sec-
ond VLP booster immunization in all animals of group A and in
two or three animals of groups B and C (Fig. 3C and D).
Characterization of cellular immune responses prior to the
VLP booster immunization had revealed that the delivery of both
mune responses, although the response to the nontargeted DNA
with the DC-targeting DNA at a 10-fold-higher dose did not give
the first VLP boost, the IFN-? ELISPOT response after stimula-
tion with a pool of Gag peptides was significantly higher in group
B than in group A (Fig. 4). After the VLP booster immunizations
a consistent increase in the IFN-? ELISPOT response after stimu-
ever, stimulation of the PBMCs with AT2-inactivated SIV (AT2
SIV) instead of the Gag peptides clearly revealed a strong booster
effect of the VLP immunization on the IFN-? ELISPOT response
in groups A and B, but not in group C (Fig. 4A and B, right).
Consistent with the poor ELISPOT response after stimulation
intracellular cytokine staining of PBMCs stimulated with these
peptide pools (data not shown). SIV capsid-specific CD8?T cells
were not detected in MamuA1-positive animals after the VLP boost
ing either (data not shown). Characteristic of the cellular immune
fore the strong IFN-? ELISPOT response upon stimulation with
AT2-inactivated SIV, but not peptide pools, and the absence of de-
Because of the lack of validated correlates of protection, only
challenge experiments can reveal whether the immune responses
induced provide protection. We therefore challenged the vacci-
nated and naive control (group D) monkeys intrarectally with
pathogenic SIVmac251. Since heterosexual transmission in HIV-
infected people is frequently due to a single founder virus (6), a
FIG1 Characterization of VLPs by Western blot analysis. Purified VLPs were
1) and an SIV-infected macaque (lane 2).
tions. Shown are antibody titers (geometric means) of vaccine groups (A) and
Enhanced Susceptibility to SIV Infection
October 2012 Volume 86 Number 19 jvi.asm.org 10535
repeated low-dose challenge approach was used. This strategy al-
lows assessing efficacy by (i) numbers of exposures required for
infection, which should reflect rotection from acquisition of in-
fection, (ii) peak viremia, revealing early antiviral effector mech-
suppression of virus replication.
Eight weeks after the second VLP immunization, monkeys
were challenged weekly after prior blood sampling to determine
viral loads. Since SIV infection usually leads to detectable RNA
levels 1 week after exposure, inoculations preceding the first SIV
RNA-positive samples by 1 week were considered to have led to
infection. Numbers of exposures leading to infection varied from
after first challenge (Fig. 5B). Approximately eight inoculations
were required for infection of monkeys of groups A, C, and D
ulations on average (Table 1). A comparison of the means of the
viral load data between groups (Fig. 5A) revealed a more than
10-fold-lower peak viremia in group B with no apparent differ-
ences in set point RNA levels (Table 1). However, neither the
difference in peak viremia nor the difference in numbers of inoc-
ulations required for infection reached statistical significance.
Since group B was also the group with the strongest IFN-?
ELISPOT responses after stimulation with AT2 SIV, the trend to
cine-induced cellular immune responses are responsible for this
enhancement. We therefore analyzed whether there is a correla-
magnitude of the IFN-? ELISPOT responses after stimulation
R ? ?0.60; P ? 0.0078) (Fig. 6A). Immunized macaques with
more than 200 IFN-?-secreting cells/106PBMCs were approxi-
immunized macaques with ELISPOT responses lower than 200
IFN-? secreting cells/106PBMCs (P ? 0.0057, Mann-Whitney t
test) (Fig. 6B). To explore if the initial immune response induced
correlation analyses were performed with cellular immune re-
sponses observed 2 weeks after the second DNA immunization
(23). However, neither CD8?T cell proliferation, IFN-?
ELISPOT responses to stimulation with AT2-inactivated SIV and
Gag peptide pools, nor tetramer staining at 2 weeks after the sec-
ond DNA immunization correlated with susceptibility to infec-
have been reported to be associated with the course of SIV infec-
and B*17 alleles. While none of the animals had the B*08 allele,
neither the A*01 nor the B*17 allele was correlated with the num-
ber of inoculations required for infection, peak viremia, or set
point RNA levels.
in response to the gp140 trimer of SIVmac251 at week 10 after the first VLP
boost. RLU, relative light units. (B) Neutralizing antibody titers against SIV-
three independent neutralization assays are shown for all immunized ma-
caques. The horizontal bars mark the geometric means of the groups. (C and
D) IgA antibody titers in blood in response to SIVp27CA (C) and gp130 (D).
FIG 4 Cellular immune response in blood after the VLP booster immuniza-
tions. (A) Geometric means of numbers of IFN-?-secreting PBMCs in the
vaccine groups after stimulation with SIV Gag peptides (left) or AT2-inacti-
ESP, Elispots. (B) Values for each of the macaques. Four- and five-digit num-
bers are monkey designations. Monkeys positive for the A*01 and the B*17
MHC alleles are marked by asterisks and plus signs, respectively.
Tenbusch et al.
jvi.asm.orgJournal of Virology
In this study, we boosted a cohort of rhesus macaques, which had
been used previously to study the immunogenicity of DC-target-
ing DNA vaccines (23), with VLPs and analyzed the protective
capacity of this heterologous prime-boost vaccine in a repeated
low-dose challenge experiment. The VLP booster immunization
ulated ELISPOT responses. Significant differences in the cellular
data at the indicated time points after the first challenge. Four- and five-digit numbers are monkey designations. Monkeys positive for the A*01 and the B*17
MHC alleles are marked by asterisks and plus signs, respectively.
FIG 6 Correlation of the IFN-? ELISPOT response with susceptibility to infection. (A) Correlation of the number of inoculations required for infection and
IFN-? ELISPOTs after stimulation of PBMCs with AT2-inactivated SIV on the day of the first challenge. (B) Numbers of inoculations required for infection in
macaques with more or less than 200 IFN-?-secreting cells/106PBMCs after stimulation with AT2-inactivated SIV at the time of the first challenge.
Enhanced Susceptibility to SIV Infection
October 2012 Volume 86 Number 19jvi.asm.org 10537
immune responses observed between the groups after the DNA
differences in susceptibility to infection and viral load after chal-
striking result of our study is the correlation of the magnitude of
the vaccine-induced, SIV-specific IFN-?-secreting cells with the
susceptibility to acquisition of infection.
cellular response to vaccination were more susceptible to acquisi-
cannot be excluded, it seems more likely that vaccine-induced
immune responses increased their susceptibility to acquisition of
infection. This is also supported by the trend to an enhanced sus-
ceptibility to infection of group B, i.e., the vaccine group with the
strongest ELISPOT response. One plausible scenario is that vac-
cine-induced, SIV-specific CD4?T cells in the lamina propria of
the exposed mucosa and/or the draining lymph nodes were acti-
vated by exposure to SIV antigen from the viral inoculum. In the
cific T helper cells may then have favored viral replication and
early spread, thereby promoting the establishment of infection.
This hypothesis is supported by the observation that SIV prefer-
entially infects highly activated CD4?T cells during primary in-
fection (25). In addition to more efficient replication in activated
T cells, HIV also preferentially infects HIV-specific CD4?T cells
booster immunizations with SIV peptide pools did not reveal a
T cell responses as assessed by tetramer staining suggests that the
AT2-inactivated SIV are CD4?T helper cells.
Until now, only one study using a varicella-zoster virus-based
loads in macaques following vaccination (22). In this study, vac-
cinated monkeys were challenged intravenously with a single in-
termediate dose of SIV, which resulted in infection of all animals.
Thus, this experimental set-up could reveal whether vaccine-in-
duced immune responses increased viral loads once infection was
cination increases the susceptibility to acquisition of infection.
Still, this is important since the mechanisms mediating reduction
in virus replication after establishment of infection might differ
from those favoring acquisition of infection. This is the notion
susceptibility to infection had the lowest peak viremia. Further-
more, the numbers of SIV-specific, IFN-?-secreting cells in
PBMCs stimulated with AT2 SIV correlated with susceptibility to
infection. Thus, the vaccine-induced immune responses may on
the one hand increase the susceptibility to acquisition of immu-
ute to suppression of virus replication once infection has been
The concern that vaccine-induced immune responses may in-
viously based on the results of the STEP study (2). Vaccinated
volunteers with preexisting antibody titers in response to the ad-
enoviral vector vaccine were more likely to acquire HIV infection
than nonvaccinated individuals. Since the higher rate of infection
hypothesized that anamnestic adenovirus-specific CD4?T lym-
phocyte responses were responsible for the enhancement of
HIV-1 acquisition in adenovirus-seropositive subjects (1, 19).
However, subsequent studies provided evidence against this hy-
pothesis (7, 10). A nonhuman primate study modeling the STEP
study also suggested enhanced susceptibility to acquisition of SIV
infection in macaques infected with adenovirus prior to adenovi-
ral vector immunization against SIV (14). Since the trend to en-
hanced susceptibility to infection was not observed in macaques
infected with adenovirus prior to immunization with a control
adenoviral vector, SIV-specific immune responses seem to be re-
sponsible for enhanced susceptibility (14).
In the light of the present study the following hypothesis ex-
plaining the results of the STEP study gains credibility. The pre-
existing humoral immunity to adenoviruses may modulate the
HIV-specific immune response induced by the adenoviral vector
vaccine in the STEP study leading to an increased number of mu-
cosal HIV-specific T helper cells (24). Upon exposure to HIV an-
tigen present in the viral inoculum, these mucosal HIV-specific T
helper cells get activated and thus increase the susceptibility to
the STEP study, the magnitude of numbers of antigen-specific,
IFN-?-secreting cells after vaccination was indeed greater in in-
fected than in noninfected vaccine recipients (8). The association
of the magnitude of vaccine-induced IFN-? ELISPOT responses
with susceptibility to immunodeficiency virus infection in natu-
protection from HIV infection (19). It should also be noted that
not all virus-specific CD4?T cell responses seem to be detrimen-
tal. During acute HIV infection, virus-specific T cells with cyto-
lytic activity are associated with better control of viremia (20).
reduction of viral load in the phase after peak viremia (12). It
remains to be determined whether the different effects attributed
response. Alternatively, the same virus-specific CD4?T cells
might increase susceptibility to acquisition of infection if present
at the time of exposure and contribute to control of virus replica-
tion once infection has been established.
(2004.107.2), the European Commission FP6 program (DEC-VAC,
LSHP-CT-2005-018685, and EUROPRISE, LSHP-CT 2006-037611), the
German Research Foundation (TRR60/1), and the H. W. & J. Hector
Foundation. The SIV peptides (EVA7066,1-72, SIV gag overlapping pep-
supported by the EC FP6/7 Europrise Network of Excellence, and NGIN
consortia and the Bill and Melinda Gates GHRC-CAVD Project. The
AT-2 SIV was kindly provided by Jeffrey Lifson through the EU Pro-
gramme EVA Centre for AIDS Reagents, NIBSC, UK (AVIP contract
A.M.S. is the chief executive officer of Oncovir, Inc., which provided
the poly-ICLC for the study. He was not involved in the acquisition and
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sion of memory CD4 T cells with a mucosal homing phenotype that are
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