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Institut National de la Santé et de la Recherche Médicale, Unité 430, and Université Pierre et Marie Curie, Paris, France
| Abstract |
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| Introduction |
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| Materials and Methods |
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Seminal fluid from 10 HIV-1-seronegative healthy donors was obtained from the semen bank of Hôpital Tenon (Paris, France). The semen was centrifuged for 10 min at 4°C, and the supernatant was stored at -70°C. Anti-CD4 mAb (FITC-CD4, Leu3a) was obtained from BD Biosciences (Mountain View, CA); anti-CCR5 (PE-CCR5, 2D7) and anti-CXCR4 (PE-CXCR4, 12G5) mAbs were obtained from BD PharMingen (Le Pont de Claix, France). PE-conjugated mAbs against CD11b/CD18 (CR3) and CD11c/CD18 (CR4) and unlabeled anti-CD11b/CD18 mAb MO-1/7E4 were purchased from Immunotech (Beckman Coulter, Villepinte, France). Recombinant RANTES and stromal cell-derived factor 1 (SDF-1) were obtained from R&D Systems Europe (Abingdon, U.K.). Soluble CD16 (sCD16) was a gift from C. Sautes (Institut Curie, Paris, France).
Human epithelial cell line and sources of HIV-1
HT-29 human colorectal epithelial cells line (ATCC HTB-38; American Type Culture Collection, Manassas, VA) was maintained in RPMI 1640 containing 10% FCS and antibiotics. Primary HIV-1VN-44 and HIV-1NDK X4-tropic viruses (gifts from Prof. F. Barré-Sinoussi, Institut Pasteur, Paris, France) were grown in PBL of healthy donors stimulated with PHA and IL-2. Primary R5-tropic HIV-1BaL and HIV-1JRCSF were amplified in monocyte-derived macrophages of healthy donors. Tropism of viruses was determined using U87 cells (provided by Dr. E. Menue, Institut Pasteur) transfected with DNA encoding for human CD4, CCR5, or CXCR4. HIV was quantified in cell culture supernatants by means of the DuPont HIV-p24 ELISA (HIV-1 core profile ELISA; DuPont de Nemours, Les Ulis, France).
Opsonization of HIV-1 and in vitro infection of HT-29 epithelial cells
Seminal fluid and normal human serum obtained from
HIV-seronegative individuals were used as sources of complement. We
estimated levels of C3 fragments in semen to be
10% of those in
serum. C3 and C9 complement fragments levels in semen have been
previously reported to be between 0.3 and 5% of those in blood plasma
(S. E. Bozas, unpublished observations). HIV-1 particles were
opsonized by adding seminal fluid or normal human serum diluted
in veronal buffer supplemented with 0.6 mM CaCl2
and 0.9 mM MgCl2 (VBS++;
v/v) to HIV-1 (10 ng p24) for 1 h at 37°C (opsonized virus). As
negative control for opsonized virus, HIV-1 particles were incubated
with heat-inactivated seminal fluid (HI-SF) or heat-inactivated serum
(HI unopsonized virus). For infection of epithelial cells,
105 HT-29 cells were inoculated with opsonized
X4-tropic (HIV-1VN-44,
HIV-1NDK) or R5-tropic
(HIV-1BaL, HIV-1JRCSF)
strains (10 ng p24) or with similar amounts of the corresponding
unopsonized viral particles for 3 h at 37°C. The cells were then
washed with RPMI 1640, resuspended in fresh RPMI 1640 containing FCS,
and cultured for 3 days. Cell culture supernatants were collected at
24-h intervals, and viral content was monitored by HIV-1 p24 ELISA.
Membrane expression of CRs and HIV-1 coreceptors by HT-29 cells
The expression levels of CCR5, CXCR4, CR3, and CR4 by HT-29 epithelial cells were investigated by flow cytometry. The cells were detached by adding versen (1/1000; Life Technologies, Cergy Pontoise, France) to culture plates for 10 min at 37°C, washed twice with PBS containing sodium azide (0.01%) and BSA (0.2%), and then incubated with PE-conjugated anti-CD4, anti-CCR5, anti-CXCR4, and anti-CR3/CR4 mAbs or with unlabeled isotype-matched mAbs for 30 min at 4°C. After washing, the cells were fixed with paraformaldehyde (1%) and analyzed using a FACSCalibur flow cytometer and CellQuest software (BD Biosciences).
Detection of HIV-1 DNA in HT-29 cells
Levels of HIV-1 DNA in HT-29 cells were investigated 3 h
after infection with opsonized or unopsonized virus. DNA was extracted
from washed cellular pellets using the QIAamp DNA mini kit (Qiagen,
Basel, Switzerland) according to the manufacturers instructions. A
semiquantitative PCR of the pol gene was then performed as
previously described (17). PCR was performed with the
oligonucleotides P63 (5'-GCC ATT TAA AAA TCT GAA AAC AGG-3') and P58
(5'-GAC AAA CTC CCA CTC AGG AAT CCA-3') for 37 cycles (94°C for
30 s, 53°C for 30 s, and 72°C for 60 s) in a
reaction volume of 50 µl containing Milli-Q water, DNA templates,
reaction buffer provided by the manufacturer, 1.5 mM
MgCl2, 200 µM of each dNTP, 250 nM of each
primer, and 2.5 U Taq DNA polymerase (Promega France,
Charbonnières, France). To check for the quality of extracted DNA
and the lack of PCR inhibitors, the ubiquitous
-globin gene was
amplified in parallel experiments by PCR as previously described
(18). The final PCR products were visualized under UV
transillumination by means of ethidium bromide staining after
electrophoresis in 2% agarose.
| Results |
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To investigate the ability of HIV to activate complement in
seminal fluid, HIV-1 particles (10 ng/ml p24) were added to seminal
fluid diluted in VBS++ (v/v; final volume, 100
µl) and incubated for 1 h at 37°C. After centrifugation, the
generated C3a Ag was then determined in supernatants by ELISA (OPT EIA
Human C3a ELISA; BD PharMingen, San Diego, CA). HI-SF and 20 mM
EDTA-chelated seminal fluid were used as negative controls. Complement
activation by heat-aggregated Igs (1 mg/ml) in seminal fluid was
used as a positive control. As shown in Fig. 1
, incubation of R5- or X4-tropic
strains of HIV-1 particles with seminal fluid resulted in activation of
complement. X4-tropic (HIV-1VN-44) and R5-tropic
(HIV-1BaL) strains yielded similar levels of
complement activation, reaching values comparable to those obtained
with heat-aggregated Ig. Only low levels of
C3a/C3adesArg Ag were detected when HIV-1 or
heat-aggregated Ig was incubated with heat-inactivated or EDTA-chelated
seminal fluid.
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Ninety-four percent of HT-29 cells expressed a high level of CXCR4
(mean fluorescence intensity (MFI), 154); 35% of the cells were
positive for CCR5 (MFI, 3040), whereas very low (0.1%) expression of
CD4 Ag was detected (Fig. 2
a).
We further observed that 45% of HT-29 epithelial cells expressed CR3
(CD11b/CD18), and 35% expressed CR4 (CD11c/CD18; MFI, 20 and 15,
respectively; Fig. 2
b). There was no expression of CR1
(CD35) or CR2 (CD21; data not shown). Coexpression levels of HIV-1
coreceptors and CR3 on HT-29 cells were further established by
double-staining fluorescence with CCR5 (2D7)/CR3(MO-1) mAbs and
CXCR4(12G5)/CR3 mAbs to be 22 and 35%, respectively (not shown).
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We found that both X4-tropic (HIV-1NDK and
HIV-1VN-44) and R5-tropic
(HIV-1BaL and HIV-1JRCSF)
strains of HIV were able to infect HT-29 cells (Fig. 3
). A plateau of p24 Ag production was
reached 72 h postinfection. At the peak of infection, the levels
of HIV-1 p24 Ag released after in vitro infection with
HIV-1NDK (1000 pg/ml) and
HIV-1VN-44 (900 pg/ml) were higher than those
achieved with R5-tropic HIV-1BaL (350 pg/ml) and
HIV-1JRCSF (250 pg/ml).
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Infection of HT-29 cells with HIV-1BaL and
HIV-1VN-44 (Fig. 4
)
particles that had been pre-opsonized with complement in seminal fluid
resulted in an enhanced production of HIV-1 p24 Ag in culture cell
supernatants compared with that observed upon infection with the
corresponding unopsonized virus. The enhancement achieved at 72 h
of culture ranged between 1.5- and 2-fold. No enhancement of infection
was observed when viruses were incubated with HI-SF. Semiquantitative
PCR was then used to quantify HIV-1 DNA in HT-29 cells following
infection with opsonized or unopsonized virus. Viral DNA was measured
after 3 h of ongoing infection with HIV-1BaL
and HIV-1VN-44 (Fig. 5
). A greater amount of DNA was present
in cells infected with opsonized virus than in those infected with
unopsonized virus. When complement was heat inactivated in semen before
opsonization, viral DNA levels measured in the cells were comparable to
those detected in the unopsonized virus. Infection experiments of HT-29
cells by HIVNDK and
HIVJRCSF strains, with or without recombinant
soluble CD4 molecule (Intracel, Issaquah, WA), showed no
inhibition of HT-29 cell infection in the presence of soluble rCD4
molecule, thus demonstrating that the CD4 receptor is not involved in
viral entry into HT-29 epithelial cells (data not shown).
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To investigate the role of chemokine receptors expressed by HT-29
cells for viral entry, cells were incubated with RANTES or SDF-1 (2.5
µg) before the addition of virus. SDF-1 inhibited 90% infection of
HT-29 cells with X4-tropic HIV-1NDK and 85%
infection with HIV-1VN-44 (Fig. 6
) in a dose-dependent manner (data not
shown). No significant inhibition of infection was obtained by RANTES
(<10%), with X4-tropic HIV-1NDK and
HIV-1VN-44. Surprisingly, only 20 and 30%
inhibitions were reached for HIV-1BaL and
HIV-1JRCSF, respectively, in the presence of
RANTES. SDF-1 inhibited 72 and 70% infection of HT-29 cells with
R5-tropic HIV-1BaL and
HIV-1JRCSF, respectively.
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To investigate the role of CR3 receptor in the infection of HT-29
epithelial cells, cells were incubated with anti-CR3 mAbs and
soluble sCD16, one of the natural ligands of CR3, before infection with
complement-opsonized HIV-1BaL and
HIV-1VN-44. HT-29 cells
(105 cells) were incubated with anti-CR3 mAbs
(MO-1/7E4; 10 µg), human recombinant sCD16 (5 µg), and IgG1 isotype
as a negative control for 30 min before infection with opsonized or
unopsonized HIV-1VN-44 and
HIV-1BaL. Cells were washed and further cultured
for 3 days. Cell supernatants were collected every 24 h. As shown
in Fig. 7
, the addition of either sCD16
or a mixture of anti-CD11b and anti-CD18 mAbs abolished the
enhancing effect of complement opsonization at 72 h postinfection.
Upon blockage of CR3 receptors, HIV-1 p24 Ag levels measured in culture
supernatants were similar to those observed in the presence of HI-SF
(data not shown).
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SDF-1 inhibited 71 ± 2 and 86 ± 8% of p24 production
by HT-29 cells infected with HI-SF-opsonized
HIV-1BaL and HIV-1VN-44,
respectively. When cells were infected with SF-opsonized virus, the
inhibition of p24 production observed upon preincubation of the cells
with SDF-1 was significantly reduced (53 ± 1% of inhibition for
HIV-1BaL and 54 ± 13% for
HIV-1VN-44). The inhibitory effect of SDF-1 was
significantly different when comparing p24 production in SDF-1-treated
cells infected with HI-SF-opsonized or SF-opsonized virus
(p = 0.01 and p = 0.008 for
HIV-1BaL and HIV-1VN-44,
respectively). In contrast to SDF-1, RANTES was more efficient in
inhibiting infection of HT-29 cells with SF-opsonized
HIV-1VN-44 (41 ± 19%) than with the
corresponding HI-SF-opsonized virus (10 ± 5%; p
= 0.008 for RANTES inhibitory effect on infection with HI-SF-opsonized
vs SF-opsonized virus). No significant difference was observed with
regard to inhibition by RANTES of infection with HI-SF-opsonized
(36 ± 15%) and SF-opsonized (42 ± 2%)
HIV-1BaL (Table I
).
As a control, the inhibitory potency of SDF-1 was assessed on infection
of PBL with unopsonized HIV-1VN-44 and that of
RANTES using macrophages infected with unopsonized
HIV-1BaL. SDF-1 and RANTES inhibited infection by
80 and 95%, respectively (data not shown).
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| Discussion |
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Upon culture in the presence of semen to which X4- or R5-tropic HIV-1 particles had been added, we observed a productive infection of HT-29 epithelial cells occurring from the first day of culture. The production of viral p24 protein in culture supernatants was increased up to 1.5- to 2-fold in the presence of a functional complement system in semen compared with infection observed in the presence of virus alone in the absence of semen. Infection was also demonstrated by assessing the presence of proviral HIV-1 DNA in HT-29 cells following 3 h of incubation with HIV-1 in semen. The amounts of HIV-1 pol DNA measured by semiquantitative PCR were higher in cells infected with R5- or X4-tropic viruses that had been preincubated in semen compared with cells infected with virus in culture medium. This study provides evidence that complement in semen is involved in the enhancement of infection observed when HIV particles are preincubated with semen before their addition to the cell cultures. The role of complement in semen was strongly suggested by the observation that infection with HIV added to heated or EDTA-treated semen was no different from that seen with HIV alone (i.e., diluted in culture medium). Complement components have been detected in all body secretions, including seminal fluid (4, 5). HIV is known to activate complement in serum in the presence or the absence of Abs (12, 19). Here we show that both primary R5-and X4-tropic strains activate complement in the seminal fluid as assessed by the generation of the C3 cleavage fragments C3a/C3adesArg. No C3 cleavage was observed when heat-inactivated and EDTA-chelated seminal fluid was used. Several regions in viral gp120/gp41, such as the conserved C2 region, the carboxyl-terminal flank of the V3 loop, and the transmembrane gp41 domain, have been shown to be involved in complement activation through direct interaction with C3 and/or C1q (20, 21, 22). Activation of complement results in deposition of C3 fragments on the viral surface. HIV-1 particles are, however, resistant to lysis by the complement membrane attack complex C5b-9 (23, 24, 25).
Since the seminal fluid of infected men contains both free HIV-1 particles (2, 3) and complement components (4), it may be speculated that virus in semen becomes opsonized and acquires an increased potential to infect epithelial cells or other target cells in the submucosa, e.g., dendritic cells, resulting in mucosal transmission of the virus. Mucosal transmission of HIV-1 through infection of epithelial cells remains a controversial issue (26, 27, 28, 29). In the present study we report that HT-29 epithelial cells can be infected by R5- and X4-tropic strains of HIV-1. Our result differs from previous reports indicating that HT-29 cells can only be infected by X4-tropic HIV-1 (30).
In the present study the HT-29 colorectal cell line was used to mimic the initial contact of HIV with the rectal epithelial cell as it is thought occur during sexual intercourse, especially during penio-anal intercourse. To investigate the receptors involved in viral entry in HT-29 epithelial cells, we first analyzed the membrane expression of CD4, CXCR4, CCR5, CR3 (CD11b/CD18), and CR4 (CD11c/CD18) by HT-29 cells. Most cells expressed CXCR4 (94%), CCR5 (35%), and CR3 (45%). HT-29 cells did not express the CD4 molecule. Our results obtained with the epithelial cell line HT-29 are consistent with previous observations of the expression of CXCR4, CCR5, and CR3 (CD11b/CD18) by mucosal epithelial cells (31, 32, 33). These receptors are also known to be expressed by dendritic cells and macrophages, which have been suggested to be the first targets for HIV-1 in the submucosa upon sexual transmission of the virus (15, 16, 34). Infection of HT-29 cells with HIV-1NDK and HIV-1VN-44 X4-tropic viruses that had not been pre-oposonized with semen was totally inhibited by SDF-1, the natural ligand of CXCR4. Interestingly, SDF-1 also inhibited infection when HT-29 cells were infected with HIV-1JRCSF and HIV-1BaL, two R5-tropic strains. In contrast, RANTES, the natural ligand of the CCR5 receptor, was a poor inhibitor of infection with R5-tropic viruses (HIV-1JRCSF and HIV-1BaL) and, as expected, had no effect on infection with X4-tropic viruses (HIV-1NDK and HIV-1VN-44). Taken together, our results indicate that unopsonized X4- and R5-tropic viruses use CXCR4 as a coreceptor to infect CD4-negative epithelial cells. Infection of HT-29 cells could also implicate additional receptors, such as galactosylceramide (Galcer), that are expressed on HT-29 cells (31, 35, 36). Abs to Galcer induced 4050% inhibition of infection with R5- and X4-tropic unopsonized HIV-1, suggesting the participation of this receptor (data not shown).
Infection with virus that had been opsonized with complement in semen was inhibited by mAbs directed against the region of CR3 (CD11b/CD18) that recognizes the iC3b binding site and by sCD16, a natural ligand of the CR3 receptor. These results demonstrate that the enhancing effect of opsonization by semen was dependent on CR3-mediated entry of opsonized virus in HT-29 cells. These data extend our previous observations on the enhancing effect of complement opsonization in serum on infection of monocytes/macrophages (14). We further observed that inhibition of infection by SDF-1 was decreased in the case of infection with opsonized virus compared with that observed with unopsonized R5- and X4-tropic viruses. In contrast, inhibition induced by RANTES was higher for opsonized X4-tropic HIV-1VN-44 than for the corresponding unopsonized virus. These data suggest that opsonized virus does not interact with CCR5 and CXCR4 on epithelial cells in the same way as the corresponding free unopsonized virus. It is possible that the presence of iC3b on viral particles affects the interactions of HIV with CCR5 and CXCR4 on target cells. HIV-1 would thus turn complement opsonization to its own replicative advantage, resulting in enhanced infection of epithelial cells and possibly of other cell types in the submucosa, thereby promoting the spread of the virus.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Hakim Hocini, Unité dImmunopathologie Humaine, Institut National de la Santé et de la Recherche Médicale, Unité 430, Hôpital Broussais, Pavillon Leriche, 96 rue Didot, 75674 Paris Cedex 14, France. E-mail address: hakim.hocini{at}brs.ap-hop-paris.fr ![]()
3 Abbreviations used in this paper: CR3, complement receptor type 3; HI-SF, heat-inactivated seminal fluid; MFI, mean fluorescence intensity; sCD16, soluble CD16; SDF-1, stromal cell-derived factor 1. ![]()
Received for publication January 11, 2002. Accepted for publication July 17, 2002.
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-induced production of complement component C3 in human intestinal epithelial cells. Clin. Sci. 97:331.[Medline]
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