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*
Division of Medical Virology, Department of Molecular Microbiology and Immunology, Nagasaki University Graduate School of Medical Sciences; and
Department of Pediatrics, Nagasaki University School of Medicine, Nagasaki, Japan
| Abstract |
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| Introduction |
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In contrast to HTLV-I, it is generally considered that HIV-1, another human retrovirus, has been recently introduced to Homo sapiens. Milk-borne infection contributes considerably to vertical transmission of this deadly virus (9); however, whether breast milk components influence HIV-1 infection remains largely unknown.
To address these issues, we started to investigate whether breast milk components can influence replication of the two retroviruses. In this study we demonstrate that a milk protein lactoferrin (Lf) can facilitate replication of HTLV-I by up-regulating viral expression, whereas it can inhibit replication of HIV-1 by interfering with viral fusion/entry.
| Materials and Methods |
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Recombinant human (h) Lf, provided by Agennix (Houston, TX), was derived from Aspergillus awamori and had a purity of 99.4%. Where indicated, hLf was heat-denatured by incubating at 56°C for 1 h. Bovine (b) Lf, provided by Morinaga Milk Industry (Kanagawa, Japan), was derived from bovine milk. In experiments aimed to compare effects of iron-saturated hLf and apolactoferrin, we used human milk-derived hLf reagents that had been purchased from Sigma (St. Louis, MO). PMA, ionomycin, AZT, and mitomycin C (MMC) were also purchased from Sigma.
Cells, virus, and infection
PBMCs were isolated from HTLV-I-seropositive or -seronegative healthy individuals (Nagasaki Red Cross Blood Center, Nagasaki, Japan), as described previously (10). Cord blood mononuclear cells (CBMCs) were provided by the Pre-and Post-Natal Care Unit, Nagasaki University School of Medicine Hospital with consent by donors.
For HTLV-I infection studies, unfractionated PBMCs obtained from HTLV-I carriers were propagated with RPMI 1640 supplemented with 10% heat-inactivated FBS in the presence or absence of Lf and/or AZT. Also, PBMCs obtained from HTLV-I-seronegative healthy donors were infected with purified HTLV-I virions (Advanced Biotechnologies, Columbia, MD) in the presence or absence of Lf. For HTLV-I transmission studies, CBMCs were cocultured with MMC-treated PBMCs obtained from HTLV-I carriers in the presence or absence of Lf. Replication of HTLV-I was monitored by HTLV-I p19 Ag detection ELISA as per manufacturers instruction (Cellular Products, Buffalo, NY).
For HIV-1 infection studies, PBMCs obtained from healthy donors seronegative for both HIV-1 and HTLV-I were infected with HIV-1 AD8 at an approximate multiplicity of infection of 0.05, or PBMCs obtained from an asymptomatic HIV-1 carrier were incubated in the presence or absence of Lf. Replication of HIV-1 was monitored by reverse transcriptase (RT) assays as described previously (10, 11).
Plasmids and transient expression assays
Plasmid pU3R-luc, which contains HTLV-I long terminal repeat (LTR) U3R region followed by the luciferase gene, was a gift of K.-T. Jeang (National Institute of Allergy and Infectious Diseases, Bethesda, MD). Plasmids pHIV-1 LTR-luc and pCMV-luc contain HIV-1 LTR or human CMV major immediate-early promoter (MIEP), respectively, followed by the luciferase gene. Transfections of PBMCs or CBMCs and luciferase assays were performed as described previously (12).
Recombinant vaccinia vectors and fusion assays
Recombinant vaccinia viruses (rVVs) were propagated as described
previously (13, 14). rVV expressing HTLV-I Env (vWR-Env17)
was a gift of H. Shida (Hokkaido University, Hokkaido, Japan)
(15). For fusion assays, BSC-1 cells were infected with
vCB21R (a rVV encoding the lacZ gene under the control of T7
promoter) as well as a rVV expressing the indicated viral Env at a
multiplicity of infection of 10. Primary CD4+ T
cells obtained from healthy volunteers were infected with vTF7-3 (a rVV
expressing T7 RNA polymerase) at a multiplicity of infection of 10
each. After a 10-h culture at 37°C, both fusion effectors (BSC-1
cells) and fusion targets (CD4+ T cells) were
mixed and incubated at 37°C for 6 h in the presence of cytosine
arabinoside, and the mixed cell culture lysates were subjected to
-galactosidase assays. Where indicated, hLf (100 µg/ml) was added
to CD4+ T cell cultures 1 h before and
throughout cocultivation.
Single-round virus replication assays
Single-round virus replication assays were performed using replication-incompetent luciferase reporter viruses that had been pseudotyped by Env derived from either T-tropic (X4) HIV-1 HXB2, M-tropic (R5) HIV-1 JR-FL, or amphotropic murine leukemia virus (AMLV), as described previously (13). In brief, 5 x 105 cells were mock-treated or treated with hLf (100 µg/ml) for 1 h at 37°C and then infected with the indicated virus stock (50,000 cpm RT activity). Three hours later, the infected cells were resuspended in RPMI 1640/10% FBS with or without hLf (100 µg/ml), and incubated for 3 days. Luciferase assays were performed using commercially available reagents (Promega, Madison, WI).
Flow cytometric analysis
Cell surface expression of CD69, CD25, CCR5, or CXC chemokine receptor 4 (CXCR4) was determined by staining cells with monoclonal anti-CD69 Ab FITC-conjugate, anti-CD25 Ab FITC-conjugate, anti-CCR5 Ab 2D7 PE-conjugate, or anti-CXCR4 Ab 12G5 PE-conjugate (PharMingen, San Diego, CA), respectively, and by analyzing in FACScan (Becton Dickinson Immunocytometry Systems, Mountain View, CA), as described previously (13).
| Results |
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Effect of Lf on HTLV-I infection was first evaluated in PBMCs
derived from asymptomatic carriers. In preliminary experiments for
selected donors, induction of HTLV-I replication that was determined by
p19 Ag levels in cell-free culture supernatants was detected as soon as
3 days after stimulation with Lf, and peak p19 Ag titers were obtained
by day 7 in most cases (data not shown). Therefore, p19 Ag titers were
determined on day 7 in the subsequent experiments. HTLV-I replication
was enhanced up to 5- to 10-fold by stimulation with hLf or bLf in a
dose-dependent manner (Table I
).
Similarly enhanced HTLV-I replication was obtained in hLf-stimulated
PBMCs that were infected with cell-free HTLV-I virions (Table I
). Heat
denaturation of hLf abrogated its effects on HTLV-I replication (Table II
). Apolactoferrin also induced HTLV-I
replication, although it appeared to have less pronounced effects (3-
to 5-fold induction) as compared with iron-saturated Lf (5- to 10-fold
induction) (data not shown). Because several different sources of Lf
(recombinant, human milk, or bovine milk) had similar effects on HTLV-I
infection as above, the ability of Lf to induce viral replication
appears to be specific. Treatment with AZT (2 µM) resulted in a
markedly decreased expression of p19 Ag (Table III
). Because AZT has no antiretroviral
effect on PBMCs already infected with HTLV-I (16),
profound suppression of p19 expression by AZT would indicate that
induction of HTLV-I expression in the presence of Lf required expansion
of HTLV-I infection. Addition of Lf to the cell culture medium did not
appear to influence cell viability or expression of CD69 or CD25, as
determined by trypan blue exclusion and flow cytometric analysis,
respectively (data not shown), suggesting that Lf effect does not rely
on cell activation/proliferation. Thus, these results suggest that Lf
may induce HTLV-I replication in carriers PBMCs.
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Lf has been shown to display antiviral activity against a diverse
array of viruses including HIV-1, another human retrovirus
(18, 19, 20, 21). As demonstrated by those studies, Lf treatment
inhibited HIV-1 replication in a dose-dependent manner (Table V
). Thus, it appears that a milk protein
Lf has opposite effects on the two human retroviruses that are
transmitted via breast milk.
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A variety of immunomodulatory activities have been attributed to Lf (22), indicating that sequestration of iron is not the only function of this protein. Recently, Lf has been shown to regulate transcription (23); therefore, Lf may exert its function, at least in part, through transcriptional regulation.
To explore the possibility that Lf can transactivate HTLV-I LTR
promoter, transient expression assays were performed. PBMCs or CBMCs
obtained from HTLV-I-uninfected individuals were transfected with
pU3R-luc, and cell lysates were prepared for luciferase assays 2 days
after transfection. Where indicated, transfected cells were stimulated
with hLf for 12 h before harvest. As shown in Fig. 1
, hLf could transactivate HTLV-I LTR but
had little effect on activity of HIV-1 LTR or human cytomegalovirus
(HCMV) MIEP, whereas stimulation with PMA/ionomycin transactivated all
these promoters (Fig. 1
; data not shown). Thus, our results strongly
suggest that Lf can induce HTLV-I replication by up-regulating HTLV-I
LTR promoter activity.
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To explore the mechanism whereby HIV-1 infection is inhibited by
Lf, we next investigated effects of Lf on viral fusion/entry, another
critical step during viral replicative cycle. First, Env glycoproteins
derived from HIV-1 or HTLV-I were expressed on the cell surfaces using
rVV, and fusogenicity of these cells with CD4+ T
cells was determined. As shown in Fig. 2
, Lf markedly down-regulated fusogenicity with HIV-1 Env, irrespective of
their coreceptor usage. Flow cytometric analyses also demonstrated that
expression of CCR5 or CXCR4 was not significantly modified by
stimulation with Lf (data not shown). Lf also down-regulated
fusogenicity with HTLV-I Env, although to a lesser degree.
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| Discussion |
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Induction of HTLV-I replication by Lf is mediated, at least in part, by
transcriptional activation of HTLV-I LTR promoter. Lf induction of
HTLV-I LTR activity is specific, because Lf had little effect on the
HIV-1 LTR or HCMV MIEP activity. Recently, it has been shown that Lf is
transferred to the nucleus, directly binds to DNA, and is capable of
activating transcription (23); however, no Lf-inducible
promoter has actually been identified yet. Therefore, this may be the
first study to identify a promoter that is transactivated by Lf. The
precise mechanism whereby Lf transactivates HTLV-I LTR remains
undetermined; it may directly transactivate HTLV-I LTR promoter, or may
induce expression of host factor(s) that in turn transactivate the
promoter. Interestingly, Lf induction of HTLV-I LTR promoter activity
is more prominent in CBMCs than in PBMCs (Fig. 1
). Although sample
numbers are still small, these results may indicate that immature
newborn cells may contain larger amounts of host factor(s) that are
required for Lf-mediated effect than do adult cells. Such possibilities
are currently under investigation.
Lf has been shown to have antiviral activity. Recently, Harmsen et al. showed that Lf inhibits HIV-1 and HCMV infections (18), probably at the level of viral entry (absorption or penetration). By performing two assays (rVV-based fusion assays and single-round viral replication assays), we have confirmed that Lf could inhibit HIV-1 entry into cell. We have also shown that Lf had similar effects on HTLV-I Env-mediated cell-cell fusion, although to a lesser degree. Thus, Lf appears to have the opposite effects on HTLV-I infection, induction of viral expression, and suppression of viral entry. Therefore, we hypothesize that the ability of Lf to enhance viral infection at later step(s) may overcome its negative effect on earlier step(s) in case of HTLV-I; however, infection with other viruses (e.g., HIV-1, CMV) may be simply suppressed by its inhibitory effect on viral entry. Further investigation is needed to clarify this issue.
Lf is a major milk protein, and the amounts of Lf are 5
7 mg/ml in
colostrum, 1
3 mg/ml in mature milk, 1.5
2.2 mg/ml in tears, and
0.5
1 mg/ml in seminal fluid (24, 25). Therefore, we
believe that the concentration used in this study (up to 100 µg/ml)
is physiologically relevant even after dilution in the gastrointestinal
and genital tracts. Although the precise portal of entry for HTLV-I or
HIV-1 across mucosal surfaces in infants is unknown (9),
it is not unreasonable to hypothesize that Lf actually exerts its
effects on viral transmission or infection in vivo. Finally, it is
important to rule out the possibility that other components or
contaminants rather than Lf contain the relevant activity. However,
based upon our observations that several different sources
(recombinant, human milk, or bovine milk) of Lf had similar effects, it
is reasonable to assume that Lf did actually exhibit activities
demonstrated in this study.
In summary, this study suggested that a milk protein Lf enhances replication and transmission of HTLV-I and suppresses infection with HIV-1, the two milk-borne pathogens, and may have implications for host-pathogen interactions during transmission via milk or other biological secretions.
| Acknowledgments |
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| Footnotes |
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2 M.M. and H.M. contributed equally to this project. ![]()
3 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. ![]()
4 Abbreviations used in this paper: HTLV-I, human T-cell leukemia virus type I; Lf, lactoferrin; hLf, human lactoferrin; bLf, bovine lactoferrin; MMC, mitomycin C; CBMCs, cord blood mononuclear cells; RT, reverse transcriptase; LTR, long terminal repeat; MIEP, major immediate early promoter; rVV, recombinant vaccinia virus; AMLV, amphotropic murine leukemia virus; CXCR4, CXC chemokine receptor 4; HCMV, human cytomegalovirus. ![]()
Received for publication October 19, 2000. Accepted for publication January 8, 2001.
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, suppress HIV-1 replication in monocytes/macrophages. Proc. Natl. Acad. Sci. USA 93:15431.
B potently upregulates the promoter activity for RANTES, a chemokine that inhibits HIV-1 infection. J. Immunol. 158:3483.[Abstract]
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