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Laboratory of Gene Regulation, Division of Therapeutic Proteins, Center for Biologics Evaluation and Research, Food and Drug Administration, National Institutes of Health, Bethesda, MD 20892
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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1 enhanced
expression of HIV chemokine coreceptors, the role of this host factor
in virus effects was investigated using PBLs cultured in a
nonmitogen-added system in the absence or presence of TGF-1. Most
CD4 cells in such cultures had the phenotype
CD25-CD69-DR-Ki67-
and were CD45RObrightCD45RAdim. Cultured cells
had increased expression of CCR5 and CXCR4 and supported both HIV-1
entry and completion of viral reverse transcription. Virus production
by cells cultured in the presence of IL-2 was inhibited by TGF-1,
and this inhibition was accompanied by a loss of T cells from the
culture and an increase in CD4+ T cell apoptosis. Whereas
R5X4 and X4 HIV-1 infection was sufficient to induce T cell apoptosis,
R5 HIV-1 failed to induce apoptosis of PBLs in the absence of TGF-1
despite the fact that R5 HIV-1 depletes CD4+ T cells in
vivo. Increased apoptosis with HIV and TGF-1 was associated with
reduced levels of Bcl-2 and increased expression of apoptosis-inducing
factor, caspase-3, and cleavage of BID, c-IAP-1, and X-linked
inhibitor of apoptosis. These results show that TGF-1 promotes
depletion of CD4+ T cells after R5 HIV-1 infection by
inducing apoptosis and suggest that TGF-1 might contribute to the
pathogenesis of HIV-1 infection in vivo. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Isolates of HIV-1 that are dependent on the chemokine receptor CCR5 (R5 isolates) are the primary viruses sexually transmitted. However, R5 isolates show weak cytopathic effects on cultured primary human T cells, even though these viruses have been isolated from individuals with CD4+ T cell depletion and are able to induce a rapid loss of CD4+ T cells in the human-PBL-SCID mouse model (11). These observations suggest that depletion of T cells may require both continuous production of R5 HIV or viral proteins and the presence of host factors.
Host factors, such as cytokines, have multiple effects on T
lymphocytes, macrophages, and HIV-1 infection (12, 13, 14, 15).
TGF-1 is a 25-kDa homodimeric protein with important roles in cell
cycle control, differentiation, embryonic development, wound healing,
angiogenesis, and apoptosis (16). TGF-1 is secreted by
several cell types, including hemopoietic, endothelial, and connective
tissue cells. Secreted TGF-1 is cleaved from the propeptide of its
precursor and stored as a complex with the propeptide and latent
TGF-1-binding proteins. In vivo TGF-1 is released from the
complex by thrombospondin-1 or plasmin (16). TGF-1 was
increased in individuals with HIV-1 infection (17, 18) and
induced by HIV-1 from PBMC, macrophages, and astrocytes
(19, 20, 21), but its contribution to viral pathogenesis is
not clear. It has been reported that TGF-1 suppressed T cell
proliferation in vitro (17), enhanced infection of
macrophages by X4 viruses (22), up-regulated CXCR4 on
Langerhans cells (23), and increased CXCR4 and CCR5 on
monocyte-derived dendritic cells (24). It has dual effects
on X4 HIV-1 infection of T cells (25) (stimulating
replication at low doses, while suppressing virus at high doses) and R5
HIV-1 infection of macrophages (26, 27). Although TGF-1
protects T cells from apoptosis under some conditions (28, 29), it can induce T cell apoptosis in the presence of
Ca2+ ionophores (30). Induction of
apoptosis of B lymphocytes (31, 32) and transformed
fibroblasts (33) by TGF-1 has also been reported. In
this study, we have investigated the role of TGF-1 in the depletion
of CD4+ T cells during HIV-1 infection in
vitro.
Multiple mechanisms have been proposed for inducing apoptosis. Apoptosis may occur either by increasing death signals such as through caspases and apoptosis-inducing factors (AIF)2 (34, 35, 36, 37, 38) or by reducing survival signals such as Bcl-2. The protooncogene Bcl-2 was identified as the site of chromosome translocations in B cell follicular lymphomas that result in elevated Bcl-2 expression (39) and encodes an inner mitochondrial membrane protein that blocks apoptosis (40, 41). Increasing evidence indicates that regulation of Bcl-2 expression is a determinant of life or death in normal lymphocytes (42, 43). Other Bcl-2 family members, such as BID, can induce mitochondrial damage through caspase-dependent or -independent pathways (44, 45).
AIF is a flavoprotein and normally confined to mitochondrial intermembrane space. It causes chromatin condensation and DNA fragmentation in a caspase-independent fashion. It induces purified mitochondria to release the apoptogenic proteins cytochrome c and caspase-9 (36). Bcl-2, which controls the opening of mitochondrial permeability transition pores, prevents the release of AIF from mitochondria (36).
Caspases (cysteine-dependent aspartate-specific proteases) are
synthesized as proenzymes (30–50 kDa) and contain an
NH2-terminal domain, a large subunit (
20 kDa),
and a small subunit (10 kDa). Caspases are crucial executioners of
apoptosis, and their functions include inactivation of inhibitors of
apoptosis, disassembly of cell structures, and cleavage of proteins
involved in cytoskeleton regulation (37, 38). Among them,
caspase-3 is a frequently activated death protease. Activation of
caspase-3, the primary activator of apoptotic DNA fragmentation
(46), is mediated either by extrinsic pathways through
delivery of granzyme B or through Fas ligand
(FasL)
FasFas-associated death domain-containing
proteincaspase-8. Activation also occurs through the intrinsic
pathway via mitochondria-released cytochrome c, which
initiates a complex of apoptosis-activating factor-1, ATP, and
caspase-9 (47, 48). Caspases have been implicated in
HIV-1-mediated apoptosis (49), and increased caspase-3
activity has been demonstrated in patients with progressive HIV-1
infection (50) and in cultured primary T cells stimulated
with HIV-1 envelope (51). The inhibitor of apoptosis (IAP)
proteins suppresses cell death by inhibiting the catalytic activity of
caspases (52, 53). IAP proteins include c-IAP-1 and
X-linked inhibitor of apoptosis (XIAP).
We have previously demonstrated that host factors play important roles
in HIV-1 infection by either limiting or enhancing HIV-1 infection in
primary macrophages or T lymphocytes through modulation of chemokine
coreceptor expression (14, 15). During these studies, we
observed that TGF-1 affected chemokine receptor expression on
resting T cells and had a striking effect on HIV-1-induced cell death
following R5 virus infection of primary T lymphocytes.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Human PBLs were isolated from individual healthy donors by
countercurrent centrifugal elutriation. Elutriated PBLs composed of
90% CD3+ cells, 3–5%
CD14+ monocytes, and 5%
CD19+ B cells were cultured in RPMI 1640 medium
supplemented with 10% FBS (HyClone, Logan, UT), penicillin (100 U/ml),
and streptomycin (100 mg/ml) as previously described (14).
TGF-1 (20 ng/ml; R&D Systems, Minneapolis, MN) was added at the
beginning of cultures.
Virus infection
After culture for 10 days in the absence or presence of
TGF-1, PBLs were infected overnight with HIV-1 strains Ba-L, JR-FL,
89.6, or 024 (92UG024) at a dose equivalent to a p24 concentration of
50 ng ml-1. After extensive washing with PBS,
PBLs were cultured in medium containing human rIL-2 (150 U
ml-1; Genzyme, Cambridge, MA) for 10–20 days,
and TGF-1 was added to cultures pretreated with TGF-1. Culture
supernatants were collected and assayed for p24 with an ELISA (Coulter,
Palo Alto, CA).
Immunofluorescence staining and flow cytometry
Cells were stained with mAbs to CD4, CCR5 (2D7), CXCR4 (12G5), CD25, CD69, CD45RO, Ki67, HLA-DR (BD PharMingen, San Diego, CA), or CD45RA (BioSource International, Camarillo, CA) and then subjected to flow cytometry acquisition and analysis with CellQuest software (BD Biosciences, Mountain View, CA) as described previously (14). For analysis of apoptosis, cells were stained with FITC-conjugated Abs to annexin V (54), and propidium iodide was used as exclusion dye.
Detection of viral DNA
Viral DNA was isolated from 10-day cultured PBLs after exposure to HIV-1 overnight and then amplified by the PCR with primers targeted either to the gag region (SK38, 5'-ATAATCCACCTATCCCAGTAG GAGAAAT-3' and SK39, TTTGGTCCTTGTCTTATGTCCAGAATGC-3') or to the long terminal repeat (LTR)-gag region (M667, 5'-GGCTAACTAGGGAACCCACTG-3' and M661, 5'-CCTGCGTCGAGAGAGCTCCTCTGG-3') as described previously (15, 55).
Western blot
Cell aliquots (107 cells) were solubilized in ice-cold lysis buffer containing 1% (v/v) Nonidet P-40, 50 mM Tris.-HCl (pH 8), 150 mM NaCl, 5 mM EDTA, 0.5% sodium deoxycholate, 0.1% SDS, 10 mM sodium fluoride, 10 mM disodium pyrophosphate, 10 µg/ml benzamidine, and 0.5 mM sodium orthovanadate supplemented with the protease inhibitors aprotinin (10 µg/ml), leupeptin (10 µg/ml), and PMSF (1 mM). After 1 h on ice, the lysates were clarified by centrifugation at 10,000 rpm for 15 min at 4°C. Protein concentration of each lysate was measured by using a micro-bicinchoninic acid protein assay kit (Pierce, Rockford, IL). Lysates with equal amounts of proteins were directly resolved by SDS-PAGE (12% bis-Tris gel; NOVEX, San Diego, CA), transferred to polyvinylidene difluoride membrane, blocked with 5% blocking reagent for 1 h, and then probed with a primary Ab against Bcl-2, AIF, BID, c-IAP-1, XIAP, or caspase-3 (dilution 1/1000; Santa Cruz Biotechnology, Santa Cruz, CA) for 1 h. The membranes were washed three times with TBS buffer with 0.1% Tween 20, then incubated with secondary HRP-conjugated Ab (dilution 1/1000). Bound Abs were detected with the chemiluminescence substrate (Boehringer Mannheim, Indianapolis, IN).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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1 on CCR5 and CXCR4 expression
To investigate the effects of TGF-1 on HIV-1 infection of CD4 T
cells under conditions resembling in vivo conditions, we established a
simple system for culturing human PBLs based on the expression of HIV-1
coreceptor CCR5 and CXCR4 and the infectibility by HIV-1 in the absence
of mitogen or cytokine stimulation. After 10 days in culture, the cells
were composed of 56–63% CD4+ cells, 36–42%
CD8+ cells, and <2% CD14+
macrophages and CD19+ B cells. The T cell
activation markers CD25, CD69, and HLA-DR were detectable on <5% of
the largely
CD45RObrightCD45RAdim
CD4+ lymphocytes (Fig. 1
A). Compared with fresh PBLs,
a transition from CD45RA positive to CD45RO positive was observed (data
not shown). All cells were negative for the cell proliferation marker
Ki67. TGF-1 did not induce the expression of CD25, CD69, or HLA-DR
on CD4+ cells, nor did it affect CD4, CD45RA, and
CD45RO expression. TGF-1 increased the expression of CCR5 and CXCR4
(Fig. 1B). The percentage of
CCR5+CD4+ and the mean
fluorescence intensity (MFI) of CCR5 in CD4+
cells was 14% and 137 in TGF-1 cultures compared with 7% and 81 in
medium-only cultures. All CD4+ T cells are
positive for CXCR4. The MFI of CXCR4 in
CD4+CXCR4+ T cells was 1133
in TGF-1 cultures compared with 661 in medium-only cultures. The
effect of TGF-1 on CCR5 and CXCR4 expression was apparent in both
CD4+ and CD8+ T cells (Fig. 1B).
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We then exposed TGF-1-treated or untreated cells to the HIV-1
R5 isolate Ba-L that was produced from primary macrophages and examined
virus entry by monitoring the reverse transcription of HIV-1 RNA. The
amount of gag DNA in TGF-1-treated cells after infection
was increased compared with that in infected control cells (Fig. 2, A and B). The
replication of HIV-1 in quiescent T cells (55, 56, 57) has
been shown blocked at an early phase of reverse transcription,
specifically at an elongation step of first-strand synthesis. To
examine whether reverse transcription was completed in TGF-1-treated
and untreated PBLs, we measured the amount of full-length viral dsDNA
(LTR-gag DNA). The amounts of two LTR forms of viral DNA in
TGF-1-treated cells were either similar to or increased compared
with controls, thus demonstrating that reverse transcription was
completed in these cells and that TGF-1 did not interfere with this
process.
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1
We then tested whether the infection of nonmitogen-stimulated PBLs
by HIV-1 could lead to virus production. PBLs were cultured in medium
containing IL-2 after exposure to HIV-1. Infection of lymphocytes by
all three types of HIV-1 isolates examined, R5 (Ba-L), R5X4 (89.6), and
X4 (024), was highly productive, as revealed by the presence of high
levels of the viral protein p24 in the culture medium (Fig. 2
C). Virus production in TGF-1-treated cells was markedly
reduced relative to untreated infected cells.
Induction of T cell apoptosis by R5 HIV-1 in the presence of
TGF-1
We next examined the effects of virus infection and TGF-1 on
the number of CD3+ T cells in the PBL cultures.
TGF-1 did not affect the growth of uninfected
CD3+ cells cultured in the presence of IL-2 (Fig. 3A). Likewise, infection with
the R5 isolate Ba-L alone also had no effect on
CD3+ cell number. However, in contrast, infection
of TGF-1-treated cells with this virus isolate induced a marked
decrease in the percentage of CD3+ cells. Both
R5X4 and X4 viruses induced depletion of CD3+
cells, and these effects were enhanced by TGF-1. Staining of cells
with Abs to annexin V revealed that the observed virus- and
TGF-1-induced CD3+ cell loss was attributable
to apoptosis (Fig. 3B).
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1 and R5 HIV-1 on apoptosis of
CD4+ T cells
TGF-1 had no effect on the background level of apoptosis (8%)
in uninfected CD4+ cells cultured in the presence
of IL-2 (Fig. 3C), consistent with previous data showing
that TGF-1 does not induce apoptosis in normal human T lymphocytes
(28). Infection with R5 isolates Ba-L or another virus
JR-FL also had no direct effect on apoptosis in these cells. However,
in combination with TGF-1, an increase was seen in the percentage of
apoptotic CD4+ cells to 40% with Ba-L and 24%
with JR-FL (Fig. 3C). The remaining
CD4+ cells accounted for only 15% of the total
cells in Ba-L/TGF-1-treated cultures compared with 40–60%
CD4+ cells under other conditions, which varies
between donors. These results indicate that depletion of
CD4+ cells results from a synergistic interaction
between HIV-1 and the host factor TGF-1. Given that the production
of p24 by TGF-1-treated PBL cultures was less than in untreated
cultures (Fig. 2B), the data indicate that the greater
extent of HIV-1-induced apoptosis of CD4+ cells
observed in the presence of TGF-1 may have limited virus production.
TGF-1 also increased the extent of virus-induced apoptosis of
CD8+ T cells (data not shown), suggesting that a
mechanism of bystander killing may also contribute to the depletion of
lymphocytes.
Increased apoptosis associated with reduced expression of Bcl-2 and increased expression of AIF and caspase-3, and cleavage of BID, c-IAP-1, and XIAP
To study the mechanism by which TGF-1 may induce apoptosis in
HIV-1 infection, we first assessed Bcl-2 expression in cells by Western
blot (Fig. 4). Uninfected PBLs expressed
a high level of Bcl-2. Ba-L-infected PBLs expressed levels of Bcl-2
equivalent to uninfected PBLs, indicating that R5 virus by itself does
not interfere with the expression of Bcl-2. Bcl-2 expression was
reduced substantially in R5 HIV-1-infected TGF-1 cultures compared
with medium, virus alone, and TGF-1 controls.
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1 cultures prompted
us to examine the role of caspase-3 in this event (Fig. 4). A very low
level of the p33 proenzyme of caspase-3 was expressed in untreated,
uninfected cultures. It was slightly increased in R5 HIV-1 cultures,
increased in TGF-1 cultures, and maximally induced in R5
HIV-1/TGF-1 cultures. The cleaved, active form of caspase-3, p20,
was expressed in uninfected, and TGF-1-untreated cultures, in
agreement with a recent report that caspase-3 cleavage can occur in
nonapoptotic activated T lymphocytes (64). The p20 form
was expressed at levels equivalent to controls in R5 HIV-1-infected
cultures without TGF-1, suggesting that virus infection even in the
presence of active caspase-3 was insufficient to induce apoptosis.
TGF-1 treatment increased p33, but reduced the amount of the p20
fragment. On the other hand, high levels of both the p33 and the p20
caspase-3 were expressed in R5 HIV-1/TGF-1 cultures that expressed a
low level of Bcl-2 and were undergoing apoptosis. To test whether
Ba-L/TGF-1-induced apoptosis is caspase dependent, the pan-caspase
inhibitor z-VAD-fmk was added to cultures before infection. Z-VAD-fmk
failed to block Ba-L/TGF-1-induced apoptosis (Fig. 5).
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1 as well as in
TGF-1-treated samples, indicating that full-length c-IAP-1 and XIAP
were unstable even in uninfected cultures (Fig. 4). This phenomenon may
relate to the activation of caspase-3 in uninfected,
non-TGF-1-treated cultures.
We also examined the amount of AIF under these conditions using Western
blot analysis. AIF provides a new molecular link between mitochondrial
membrane permeabilization and nuclear apoptosis. AIF may induce
apoptosis together with caspases. In R5 HIV-1/TGF-1 cultures
undergoing apoptosis (Fig. 4), two bands differing in m.w. were present
on blots, which differed in amount depending on the culture conditions.
TGF-1-treated R5 HIV-1-infected cells showed the strongest induction
of the low m.w. form of AIF. TGF-1 also induced AIF, whereas R5
HIV-1-infected cells showed only a small induction of AIF expression.
Since the apoptotic activity of AIF is independent of caspases, our
data indicate that a caspase-independent pathway was activated in R5
HIV-1/TGF-1 cultures.
To investigate the possible factors that may lead to mitochondria
damage, the expression of BID was examined. BID was cleaved in
Ba-L/TGF-1-treated cultures, but remained intact in other cultures
(Fig. 4), suggesting that BID was activated by cleavage and that
activated BID may also contribute to mitochondrial damage and release
of AIF.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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1 on virus entry and replication in primary T cells. We have
observed that cultured nonmitogen-stimulated PBLs in the presence or
absence of TGF-1 largely retain the phenotype
CD25-CD69-DR-Ki67-,
and express functional coreceptors CCR5 and CXCR4. TGF-1-treated
cells supported viral entry and completion of viral reverse
transcription. Dual tropic R5X4 and X4 HIV-1-depleted
primary T lymphocytes, while R5 HIV-1 induced apoptosis of primary T
lymphocytes in concert with TGF-1. Induction of apoptosis by R5
HIV-1 and TGF-1 was associated with reduced levels of Bcl-2, with
increased amounts of caspase-3 and AIF, and cleavage of BID, XIAP, and
c-IAP-1. Under the culture conditions used in this study,
CD4+ lymphocytes are largely
CD45RObrightCD45RAdim,
retain low expression of the IL-2R (CD25) and the activation markers
CD69 and HLA-DR, and did not express the cell proliferation marker
Ki67. These cells express both CXCR4 and CCR5 and are susceptible to
infection by all three principal types (R5, R5X4, and X4) of HIV-1
isolates. Reverse transcription of viral RNA was completed after
infection under these conditions. Although the T cell activation
markers were detectable in <5% PBLs cultured for 10 days in our
system, the expression of CCR5 was increased compared with uncultured
cells or those cultured for 3 days. This latter observation may
explain, at least in part, the productive infection of
nonmitogen-stimulated T cells by R5 viruses. Culture of PBLs for 10
days also may mimic more closely the physiological conditions that are
thought to allow virus production by CD4+ T cells
in vivo (65). Virus production in our cultures does
require stimulation with IL-2 following infection of PBLs.
Because TGF-1 increased chemokine receptor expression, it was
expected that viral replication would also be increased. However,
incubation of the cells in TGF-1 suppressed the amount of virus
produced. Virus suppression was associated with loss of
CD3+CD4+ cells and with an
increase in cell apoptosis, suggesting that cell death was limiting
virus production. Reduced T cell activation markers in TGF-1
cultures may also contribute to reduced viral production. TGF-1
treatment did not limit early entry events. Instead, there was some
increase in viral entry, as detected by early and late R5 HIV DNA
reverse-transcriptase products, which was associated with the increase
in expression of chemokine receptors. Thus, for R5 viruses, there is
substantial viral replication with minimal apoptosis; however, in the
presence of TGF-1, high levels of apoptosis occur with low viral
replication.
Apoptosis following virus infection depended on the type of virus in
the culture. X4 or dual tropic virus had cytopathic effects on the
cells, which were associated with extensive apoptosis. R5 infection did
not kill T cells under these culture conditions. Addition of TGF-1
to the R5 cultures led to CD4 cell depletion and death by apoptosis,
while TGF-1 alone did not alter the viability of the uninfected
control cells either with or without IL-2.
The mechanism of synergism between TGF-1 and R5 infection in cell
apoptosis appears to involve several pathways and proteins associated
with cell death. Although Bcl-2 was not decreased by Ba-L R5 HIV
infection or TGF-1 alone, infected cells cultured with TGF-1 did
show reduced levels of Bcl-2, suggesting that the cells may be more
susceptible to apoptosis-inducing signals. With regard to caspase-3,
Ba-L R5 virus infection did not induce caspase-3 p33 or p20 directly.
TGF-1 alone increased the amount of total caspase-3 p33, but
surprisingly eliminated the presence of the p20 cleavage fragment.
Virus-infected cells treated with TGF-1 contained high amounts of
p33 and levels of p20 comparable with control infected and noninfected
cells. These cells also contained more AIF than infected or uninfected
cells, indicating that the caspase-independent activity of AIF may also
be activated in the presence of TGF-1. It was interesting that the
induced AIF band was slightly smaller in m.w., consistent with the
cleaved form of AIF described by Daugas et al. (66). The
observation that the pan-caspase blocker z-VAD-fmk failed to block
apoptosis induced by TGF-1 and Ba-L infection suggests the
involvement of caspase-independent pathways. Once generated, cleaved
BID in TGF-1/Ba-L cultures may translocate to mitochondria to induce
release of cytochrome c through a caspase-independent
mechanism, as demonstrated for granzyme B (44). Together,
these results demonstrate that TGF-1-induced apoptosis in
HIV-infected cells involves changes in several molecules, including
caspase, AIF, BID, c-IAP-1, XIAP, and Bcl-2.
Our data for the first time reveal that TGF-1 is sufficent for R5
virus to induce apoptosis in primary T lymphocytes. The major producer
of TGF-1 in PBMC is reported to be the monocyte (29).
In the present study, monocytes were depleted from PBMC by elutriation,
so it is unlikely that high levels of endogenous TGF-1 were secreted
by purified PBLs. However, R5 HIV-1 isolates in vivo infect both
CD4+ T cells and monocytes/macrophages, and thus,
infected monocytes/macrophages could be induced to secrete TGF-1.
HIV along with secreted TGF-1 would then work in concert to reduce
the expression of Bcl-2, increase the expression of caspase-3 and AIF,
and potentiate cleavage of BID, c-IAP-1, and XIAP in T cells.
Other pathways involving Fas and FasL have been implicated in
HIV-induced cell death. In our system, TGF-1 and R5 virus-infected
cells did not show increased expression of Fas or FasL on PBLs (data
not shown), in accordance with a previous report that found that X4
viruses did not induce Fas or FasL (5, 67). The role of
Fas/FasL in depletion of CD4+ has also been
questioned by the lack of FasL expression in PBMCs from HIV-infected
patients and by depletion of CD4+ cells in the
presence of Fas-signaling defects (5, 68, 69). Other
reports suggest that HIV-1-infected macrophages express increased FasL
(70, 71), and that cultured T cells expressed moderate
cell surface Fas.
We believe that there are at least two possible pathways leading to T
cell apoptosis in vivo in R5 HIV-1 infection. One pathway is mediated
by increased secretion of TGF-1, which may act in synergy with
viruses or viral factors to reduce the survival signal (Bcl-2) and
increase intracellular death signals (caspase-3, AIF, BID). The second
pathway would involve HIV-1-induced FasL in macrophages that may
transduce death signals to T cells through cell surface Fas. A
reduction in survival signals in the presence of strong death signals
may induce continuous apoptosis-mediated depletion of T cells, which
might contribute to the impairment and eventual collapse of the immune
system, especially if Th cell renewal is impaired. Since R5X4 HIV-1,
like X4 HIV-1, depleted T cells, transition from R5 HIV-1 to R5X4 HIV-1
or to X4 HIV-1 is another mechanism of in vivo depletion of
CD4+ T cells in some patients. Further
characterization of this apoptotic process should facilitate our
understanding of the pathogenesis of AIDS and may lead to the
development of therapeutic interventions that target disease-enhancing
viral as well as host factors.
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Acknowledgments |
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Footnotes |
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2 Abbreviations used in this paper: AIF, apoptosis-inducing factor; FasL, Fas ligand; IAP, inhibitor of apoptosis; LTR, long terminal repeat; MFI, mean fluorescence intensity; XIAP, X-linked IAP.
Received for publication March 7, 2001. Accepted for publication July 10, 2001.
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 E. Guan, J. Wang, G. Roderiquez, and M. A. Norcross Natural Truncation of the Chemokine MIP-1beta /CCL4 Affects Receptor Specificity but Not Anti-HIV-1 Activity J. Biol. Chem., August 23, 2002; 277(35): 32348 - 32352. [Abstract] [Full Text] [PDF]
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 A. M. Krackhardt, S. Harig, M. Witzens, R. Broderick, P. Barrett, and J. G. Gribben T-cell responses against chronic lymphocytic leukemia cells: implications for immunotherapy Blood, June 17, 2002; 100(1): 167 - 173. [Abstract] [Full Text] [PDF]
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J. Wang, E. Guan, G. Roderiquez, V. Calvert, R. Alvarez, and M. A. Norcross Role of Tyrosine Phosphorylation in Ligand-independent Sequestration of CXCR4 in Human Primary Monocytes-Macrophages J. Biol. Chem., December 21, 2001; 276(52): 49236 - 49243. [Abstract] [Full Text] [PDF]
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