The Journal of Immunology, 1999, 162: 7543-7548.
Copyright © 1999 by The American Association of Immunologists
Induction of HIV-1 Replication by Allogeneic Stimulation
Hiroyuki Moriuchi1,2,
Masako Moriuchi1 and
Anthony S. Fauci
Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892
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Abstract
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Allogeneic stimulation presents an immunologic challenge during
pregnancy, blood transfusions, and transplantations, and has been
associated with reactivation of latently infected virus such as CMV.
Since HIV-1 is transmitted vertically, sexually, or via contaminated
blood, we have tested the effects of allostimulation on HIV-1
infection. 1) We show that allostimulated lymphocytes are highly
susceptible to acute infection with T cell-tropic or dual-tropic HIV-1.
2) We show that allostimulation has dichotomous effects on replication
of macrophage-tropic HIV-1; it activates HIV expression in already
infected cells but inhibits HIV entry by secreting HIV-suppressive CC
chemokines. 3) We show that allogeneic stimulation of latently
infected, resting CD4+ T cells induced replication of HIV-1
in these cells. These observations suggest that allogeneic stimulation
may play a role in the transmission, replication, and phenotypic
transition of HIV-1.
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Introduction
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Disparity
between MHC Ags elicits anti-allo-MHC immune responses. In contrast
to conventional antigenic stimulation, which requires priming
immunization to elicit potent immune responses, allogeneic responses
can be quite robust with no previous exposure (1, 2). Allogeneic
stimulation also presents an immunologic challenge during pregnancy,
blood transfusion, and transplantation and has been associated with
reactivation of latent human CMV (3), a herpesvirus that is transmitted
vertically, sexually, or through blood transfusion or transplantation
(4). Thus, reactivation of latent virus by allostimulation may be a
useful strategy for viruses including HIV, which is transmitted
vertically, sexually, or via contaminated blood.
Previous studies have demonstrated potentially important relationships
between MHC and the pathogenesis of HIV disease. First, studies using
SIV infection of rhesus macaques as a model for HIV infection showed
that serological responses to class I MHC molecules can confer
sterilizing immunity against challenge with SIV in some or all
immunized animals (5, 6, 7, 8, 9); passive transfer experiments demonstrated
that the protection is mediated, at least in part, by anti-allo-MHC
Abs (10). Second, it has been shown that HIV virions acquire host cell
proteins including class I and class II MHC Ags as they bud from the
cell membrane and that the integration of MHC molecules in the viral
envelope increases infectivity of HIV virions, while anti-allo-MHC
Abs can block the infectivity of the virus (Ref. 11; reviewed in 12). Furthermore, women who were alloimmunized as a therapy for
unexplained recurrent spontaneous abortions developed anti-allo-MHC
Abs that were able to neutralize HIV-1 in vitro (13), and women who
remained uninfected despite obvious repeated exposure to HIV expressed
rare HLA types for their geographic location (14). These studies
indicate that alloimmunized individuals may be able to protect
themselves from HIV infection by immune responses against alloantigens
expressed on HIV-1 virions or HIV-infected cells.
In this study, we investigate the effects of allogeneic stimulation on
replication of HIV in vitro. We show that allogeneic stimulation
renders PBMC highly susceptible to acute infection with T cell
(T)3-tropic or dual-tropic
HIV-1, while it has dichotomous effect on macrophage (M)-tropic HIV-1
infection; it activates HIV-1 expression in already infected cells but
inhibits M-tropic HIV-1 entry by secreting CC chemokines. We also show
that in vitro allostimulation induces replication of HIV in
CD4+ T cells from HIV-infected individuals. Furthermore,
allogeneic stimulation induced reactivation of HIV in latently
infected, resting CD4+ T cells derived from patients who
had been treated with highly active anti-retroviral therapy (HAART)
and in whom plasma viremia was below the level of detectability. Our
data, in addition to the previous studies described above, indicate
that allogeneic stimulation may play a role in the pathogenesis of HIV
disease by influencing viral transmission, replication, and phenotypic
transition.
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Materials and Methods
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Establishment of allogeneically stimulated PBMC cultures and acute
HIV infection
PBMCs were isolated from HIV-seronegative healthy donors, as
described previously (15). Unfractionated PBMCs were resuspended in
RPMI 1640 supplemented with 10% heat-inactivated FBS (Life
Technologies, Gaithersburg, MD) at a cell concentration of 2 x
106 cells/ml. For allostimulation cultures, equal numbers
of cells from two different donors were mixed at the same cell
concentration. MHC disparity between donors was confirmed by HLA
typing, which was kindly performed by the HLA laboratory of the
Department of Transfusion Medicine (Warren Grant Magnuson Clinical
Center, National Institutes of Health). After 4 days of culture at
37°C in 5% CO2, a fraction of the cultures was depleted
of CD8+ cells by immunomagnetic beads (Dynal, Lake Success,
NY), as described previously (15). Unfractionated or CD8-depleted PBMC
cultures were infected with either the T-tropic laboratory strain HIV-1
IIIB, the M-tropic laboratory strain HIV-1 Ba-L, or a dual-tropic
primary HIV-1 isolate AU at an approximate multiplicity of infection of
0.05. Viral replication was measured by RT activity in the cell-free
culture supernatants.
Single round virus replication assays and fusion assays
Single round virus replication assays and fusion assays were
performed as described previously (16). Where indicated, neutralizing
Abs to RANTES, macrophage inflammatory protein (MIP)-1
and MIP-1ß
(R&D Systems, Cambridge, MA), or control Ig were added to the cultures.
Isolation of resting CD4+ T cells from HIV-infected
patients and allostimulation
Resting CD4+ T cells were isolated from six
HIV-infected patients as described previously (17). Purity of resting
CD4+ T cells, as determined by staining with anti-CD4,
anti-CD69, and anti-HLA-DR Abs, was >99%. All the patients
had been on HAART and had fewer than 500 HIV-1 RNA copies per
milliliter of plasma, as determined by branched DNA assays (Chiron,
Emeryville, CA). For allogeneic stimulation experiments, unfractionated
PBMCs were isolated from HIV-uninfected donors, treated with mitomycin
C (MMC) (0.25 mg/ml; Sigma, St. Louis, MO) for 30 min to render them
incapable of propagation and infection, and cocultured with equal
numbers of resting CD4+ T cells from HIV-infected patients
at a cell concentration of 2 x 106 cells/ml. Release
of HIV was monitored by p24 Ag ELISA using commercially available kits
(Coulter, Miami, FL).
Flow cytometric analyses and [3H]thymidine
incorporation
Cell surface expression of CD4, HIV coreceptors (CCR5 and
CXCR4), and cellular activation markers (CD69, CD25, and HLA-DR) was
analyzed in unstimulated and allostimulated PBMCs by FACS (FACScan,
Becton Dickinson Immunocytometry Systems, San Jose, CA). PE-conjugated
mAbs to CCR5 and CXCR4 were purchased from PharMingen (San Diego, CA).
PE-conjugated mAbs to CD4, CD8, and CD14; FITC-conjugated mAbs to CD4,
CD69, CD25, and HLA-DR; and isotype controls were obtained from Becton
Dickinson Immunocytometry Systems.
[3H]Thymidine incorporation was measured in unstimulated
and allostimulated PBMCs.
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Results
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Allogeneically stimulated PBMCs are highly susceptible to acute
infection with T- or Dual-Tropic HIV-1
The replication of HIV-1 is closely associated with the state of
cellular activation of susceptible cell populations (reviewed in 18). Allogeneic stimulation has been shown to elicit robust immune
responses without priming (1, 2). In this regard, we first examined the
expression of cell surface activation markers on unstimulated and
allostimulated PBMCs. While unstimulated PBMCs expressed barely
measurable CD69, CD25, and HLA-DR on their surfaces, allostimulated
PBMCs expressed a variable degree of cellular activation markers (Table I
; data not shown). Cellular
proliferation, judged by incorporation of DNA precursor
[3H]thymidine, also confirmed that allogeneic stimulation
potently activated PBMCs (Table I
). Double staining of PBMCs
demonstrated that all subpopulations of PBMCs analyzed, such as
CD4+ T cells (HIV targets), CD8+ T cells
(effectors), and monocytes (HIV targets), were similarly activated
(Table I
; data not shown). HLA disparity between the donors was
confirmed by HLA typing (data not shown). In the experiment shown in
Table I
, CCR5 expression was moderately down-regulated in
allostimulated cells as compared with unstimulated cells; however, this
was not a consistent finding in other experiments (data not shown).
Given the fact that allostimulation potently activates both HIV target
and effector subpopulations, we next investigated the susceptibility of
allostimulated PBMC to acute HIV-1 infection. Both unfractionated and
CD8-depleted PBMCs were infected with either T-tropic (IIIB), M-tropic
(Ba-L), or dual-tropic (AU) HIV-1, and viral replication was monitored
by RT activity in the infected culture supernatants. Allostimulated
PBMCs became highly susceptible to T-tropic and dual-tropic HIV-1 (Fig. 1
A). Effects on M-tropic HIV-1
were variable: replication of M-tropic HIV-1 was suppressed in some
donor pairs, but enhanced in other donor pairs (Fig. 1
A;
data not shown). Depletion of CD8-positive cells modestly increased
susceptibility of PBMCs to infection with all HIV-1 strains tested
(Fig. 1
B). These results indicate that allostimulation
provides a more favorable environment for T-tropic and dual-tropic
HIV-1 strains, which are prevalent in late stages of HIV disease in
certain patients (19, 20), than for M-tropic HIV-1.

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FIGURE 1. Allostimulation has dichotomous effects on acute HIV-1 infection,
depending on viral phenotypes. Unfractionated PBMCs from 2 pairs of
healthy donors (donors 1 through 4 in Table II ) were incubated
separately or allostimulated with each other for 4 days. A fraction of
PBMCs was depleted of CD8+ cells. Either unfractionated
PBMCs (A) or CD8-depleted PBMCs (B) were
infected with the indicated strains of HIV-1. Peak RT titers on day 8
postinfection are shown. Results are representative of eight
independent experiments.
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Allostimulation induces expression of CC chemokines, which suppress
M-tropic HIV-1 infection
To delineate the mechanisms whereby allostimulation modulates
HIV-1 infection, single round virus replication assays were performed.
Infectability of allostimulated PBMCs was enhanced for all viruses
tested when the allogeneic culture supernatants were removed before
infection; however, when the allogeneic culture supernatants were
maintained throughout the 3 day period of infection, M-tropic virus
infection was significantly suppressed (Fig. 2
A).

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FIGURE 2. Allostimulation-induced CC chemokines inhibit M-tropic HIV-1 infection.
A, Single round virus replication assays. Either
unstimulated (donor 1 and donor 2 in Fig. 1 ) or allostimulated (donors
1 + 2 in Fig. 1 ) PBMCs were infected with replication-incompetent
luciferase reporter virus NL43-luc-R-E-
(21), which had been pseudotyped by the indicated Env, and luciferase
activity in the infected cell lysates was assayed 3 days after
infection. The culture supernatants were either depleted before
infection (sup (-)) or maintained at 1:1 ratio throughout infection
(sup (+)). Results were representative of seven independent
experiments. B, Fusion assays. Either unstimulated
(donor 1 and donor 2 in Fig. 1 ) or allostimulated (donors 1 + 2 in Fig. 1 ) PBMCs were infected with vTF73 (expressing the T7 polymerase
(22)), while BSC-1 cells were infected with vCB21R (encoding the
lacZ gene under the control of the T7 promoter (22)) as
well as a vaccinia vector expressing the indicated Env (22). After
overnight incubation, the two sets of cells were mixed, and
ß-galactosidase activity was assayed for the infected cell lysates 4 h
after mixing. Where indicated, PBMCs were incubated with their own
conditioned medium 30 min before and during mixed cultures. Fusion
index indicates ß-galactosidase activity of test samples relative to
that of control sample in which wild-type vaccinia vector in place of
Env-expressing vector was infected. Results are representative of four
independent experiments. C, Effects of neutralizing Abs
to CC chemokines. The culture supernatants were incubated with either a
mixture of neutralizing Abs to CC chemokines (10 µg monoclonal
anti-RANTES Ab, 20 µg polyclonal anti-MIP-1 Ab, and 20
µg monoclonal anti-MIP-1ß Ab per milliliter of supernatants) or
control Igs. Unfractionated or CD8-depleted PBMCs were preincubated
with the above supernatants for 2 h and infected with
NL43-luc-R-E- that had been pseudotyped by
M-tropic HIV-1 JRFL Env. Results are representative of three
independent experiments using unfractionated PBMCs. Similar results
were obtained for CD8-depleted PBMCs (data not shown).
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To further investigate early events of virus infection, fusion assays
were employed. While allostimulated cells themselves were more
fusogenic with both M- and T-tropic HIV-1 Envs than were unstimulated
cells, the supernatants from allostimulated cultures inhibited
cell-cell fusion mediated by M-tropic Env (Fig. 2
B). These
results indicate that allostimulated PBMCs secrete a soluble factor(s)
that inhibits M-tropic HIV-1 infection at the level of
fusion/entry.
Since it has been demonstrated that chemokine ligands for CCR5, a
coreceptor for M-tropic HIV-1, inhibit entry of M-tropic HIV-1 into
cells and since allostimulated PBMCs secrete a copious amount of CC
chemokines (Table II
), it is very likely
that CC chemokines secreted by these cells suppress M-tropic HIV-1
infection. To test this hypothesis, neutralizing Abs to these CC
chemokines were added to the culture supernatants in single round virus
replication assays of M-tropic HIV-1 infection. As shown in Fig. 2
C, the inhibitory effect of the supernatants was abrogated
by neutralizing CC chemokines. Taken together, these results indicate
that allostimulated PBMCs render themselves relatively refractory to
M-tropic HIV-1 by secreting CC chemokines that are suppressive to
M-tropic HIV-1.
Allostimulation of latently infected, resting CD4+ T
cells induces replication of HIV-1 in these cells
We next examined whether allostimulation can induce HIV expression
from already infected cells. First, we tested the effects of
allostimulation on HIV expression in an in vitro system in which
CD4+ T cells derived from HIV-uninfected individuals were
infected with replication-incompetent luciferase reporter virus
NL43luc-R-E- which had been complemented
with amphotropic murine leukemia virus (AMV) Env (21). When
unstimulated, the infected CD4+ T cells expressed low
levels of luciferase activity; however, when cultured with
CD4+ T cells from a different donor, luciferase expression
from the HIV-1 long terminal repeat was markedly increased (Fig. 3
A). These results indicate
that allostimulation is able to induce HIV expression in already
infected cells.

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FIGURE 3. Allostimulation induces replication of HIV-1 in latently infected
cells. A, Freshly isolated, unstimulated PBMCs from two
healthy donors were infected with
NL43-luc-R-E- that had been complemented
with Env from AMV. A fraction of the infected cells was mixed for
allostimulation on day 2 postinfection. Three days later,
CD4+ T cells were isolated with immunomagnetic beads
(Dynal), and luciferase activity in the infected CD4+ T
cell lysates was determined. Results were representative of five
independent experiments. B, Resting CD4+ T
cells were isolated from six HIV-1-infected individuals and either
unstimulated or allostimulated by coculturing with unfractionated PBMCs
obtained from healthy donors that had been treated with MMC. The
culture supernatants were collected for p24 Ag on days 4, 8, 12, and 16
after allostimulation, and peak p24 titers are shown.
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To confirm that allogeneic stimulation can induce replication of HIV-1
in endogenously infected CD4+ T cells in patients, we
isolated resting CD4+ T cells from HIV-infected individuals
who had been treated with HAART and in whom plasma viremia was
undetectable using branched DNA assays, and cocultured these cells with
MMC-treated PBMC obtained from HIV-uninfected individuals.
Allostimulated cultures released HIV-1 as determined by p24 Ag in 4 of
6 patients, while p24 Ag was undetectable in control cultures with
resting CD4+ T cells from all HIV-infected patients tested
(Fig. 3
B). Recovered virus was replication competent when
passaged into PHA blasts (data not shown). Since MMC treatment renders
the cells incapable of supporting HIV-1 infection (Ref. 23; data not
shown), the detected virus most probably represents ex vivo
reactivation of latent HIV-1 in in vivo-infected cells, not spread of
infection to allogeneic cells in vitro. Thus, allogeneic stimulation
appears to be a potent inducer of HIV-1 replication.
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Discussion
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Activation of the immune system is critical for the initiation and
propagation of HIV-1 replication (reviewed in 24). Common sources
of immune system activation include stimulation with a variety of Ags.
Paramount among these Ags are a wide range of pathogenic microbes (24).
In addition, a number of studies have demonstrated that immunization
with common recall Ags results in a transient increase in plasma
viremia in HIV-infected individuals (24, 25, 26, 27, 28); furthermore, PBMC
isolated from uninfected, recently vaccinated individuals become highly
susceptible to acute HIV infection in vitro (26). These studies suggest
that induction or enhancement of HIV-1 replication in vivo by immune
stimulation from exogenous sources may play an important role in the
pathogenesis of HIV disease.
Alloantigens elicit robust immune responses without a requirement of
previous exposure (1, 2). Allogeneic stimulation can occur through
sexual contacts, pregnancy, lactation, transfusion, or transplantation.
Since several viruses including HIV-1 are transmitted sexually,
vertically, or via contaminated blood products or organs, it is
possible that allogeneic stimulation may play a role in viral
transmission or replication. In this regard, a recent study
demonstrated that allogeneic stimulation can induce reactivation of
latent human infection of macrophages with CMV (3), a ubiquitous
pathogen that is the major cause of morbidity and mortality in
transplant patients, as well as a leading cause of congenital birth
defects (4).
The effects of allostimulation on HIV-1 infection, however, may be more
complex. Previous studies suggested that replication of HIV-1 can be
increased after transfusion of blood products (23, 29) or allogeneic
bone marrow transplantation (30). A recent study reported that
pregnancy increases the risk of disease progression for HIV-infected
women (31). These studies suggest that allogeneic stimulation can
induce replication of HIV-1.
However, a number of studies suggest that allogeneic stimulation may
actually elicit protection from HIV-1/SIV infection (5, 6, 7, 8, 9, 13, 14). The
mechanism of protection was not precisely elucidated, but cellular or
humoral immunity against allo-MHC Ags expressed on HIV-1 virions or on
HIV-1-infected cells appears to contribute to protection from HIV-1
infection (10, 11, 12). Recently, CD8+ T cell-derived soluble
factors, which include but are not limited to CC chemokines, RANTES,
MIP-1
, and MIP-1ß, have been associated with the prevention of SIV
infection in macaques by xenogeneic immunization (32) or with
anti-HIV activity induced by alloantigenic stimulation (33). Thus,
allostimulation could have dichotomous effects on HIV-1 infection; it
may induce replication of HIV-1 through cellular activation, while it
may also confer a degree of protection from HIV-1 infection through
anti-allo-MHC immune responses or the generation of soluble
anti-HIV activity.
In the present study we have demonstrated that cellular activation
associated with allogeneic stimulation enhances in vitro replication of
HIV-1. Allostimulation also induces secretion of CC chemokines, RANTES,
MIP-1
, and MIP-1ß, which inhibit entry of M-tropic HIV-1 by
competing for the CCR5 coreceptor (reviewed in 34). We have also
shown that, in the allogeneic system, depletion of CD8+ T
cells modestly increased replication of all HIV-1 stains tested and
that neutralization of HIV suppressive CC chemokines markedly enhanced
the replication of M-tropic HIV-1, but not of T- or dual-tropic HIV-1
in both unfractionated and CD8-depleted PBMCs. These results indicate
that allostimulation-induced anti-HIV activity is mediated by both
CC chemokines and non-CC chemokine factors, and that the net effect of
allostimulation on HIV-1 infection is a balance of positive (cellular
activation) and negative (induction of HIV-suppressive factors)
effects. Differences in the degree and duration of allostimulation may
confer different levels and types of HIV suppressor activity, as
demonstrated by Pinto et al., who have shown that multiple rounds of
allostimulation induced anti-HIV activity against both M- and
T-tropic HIV-1 (33). However, since allostimulation-induced CC
chemokines inhibit only M-tropic HIV-1 and can enhance replication of
T-tropic HIV-1 (35, 36, 37, 38), allostimulation would provide a more favorable
environment for T-tropic HIV-1 than for M-tropic HIV-1. This effect
could lead to a transition from M- to T-tropic phenotype, which is
indicative of disease progression in certain patients (19, 20).
In summary, the present study indicates that allostimulation has
variable effects on HIV-1 infection; however, the balance of effects
favors replication of HIV-1, particularly T- or dual-tropic viruses.
Therefore, allostimulation may play a role in transmission,
replication, and phenotypic transition of HIV-1.
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Acknowledgments
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We thank the Department of Transfusion Medicine (Warren
Grant Magnuson Clinical Center, National Institutes of Health) for
blood samples and HLA typing, L. A. Ehler and S. Mizell
for recruiting HIV-infected patients, N. Landau and E.
A. Berger for reagents, T.-W. Chun for helpful discussions,
and John Weddle for graphic work.
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Footnotes
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1 H. M. and M. M. contributed equally to this project. 
2 Address correspondence and reprint requests to Dr. Hiroyuki Moriuchi, Department of Pediatrics, Nagasaki University School of Medicine, 1-7-1 Sakamoto, Nagasaki 852-8501, Japan. E-mail address: 
3 Abbreviations used in this paper: T, T cell; M, macrophage; HAART, highly active antiretroviral therapy; MIP, macrophage inflammatory protein; MMC, mitomycin C; CXCR4, CXC chemokine receptor 4; AMV, amphotropic murine leukemia virus. 
Received for publication December 16, 1998.
Accepted for publication March 26, 1999.
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