DC-SIGNR, a DC-SIGN homologue expressed in endothelial cells, binds to human and simian immunodeficiency viruses and activates infection in trans.
ABSTRACT DC-SIGN, a C-type lectin expressed on the surface of dendritic cells (DCs), efficiently binds and transmits HIVs and simian immunodeficiency viruses to susceptible cells in trans. A DC-SIGN homologue, termed DC-SIGNR, has recently been described. Herein we show that DC-SIGNR, like DC-SIGN, can bind to multiple strains of HIV-1, HIV-2, and simian immunodeficiency virus and transmit these viruses to both T cell lines and human peripheral blood mononuclear cells. Binding of virus to DC-SIGNR was dependent on carbohydrate recognition. Immunostaining with a DC-SIGNR-specific antiserum showed that DC-SIGNR was expressed on sinusoidal endothelial cells in the liver and on endothelial cells in lymph node sinuses and placental villi. The presence of this efficient virus attachment factor on multiple endothelial cell types indicates that DC-SIGNR could play a role in the vertical transmission of primate lentiviruses, in the enabling of HIV to traverse the capillary endothelium in some organs, and in the presentation of virus to CD4-positive cells in multiple locations including lymph nodes.
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ABSTRACT: Colon cancer has always been diagnosed at a late stage, which is associated with poor prognosis. The currently used serum tumor markers CEA and CA19-9 display low sensitivity and specificity and may not have diagnostic value in early stage colon cancer. Thus, there is an urgent need to identify novel serum biomarkers for use in the early detection of colon cancer. In this study, the expression of DC-SIGN and DC-SIGNR in serum was detected by enzyme-linked immunosorbent assay (ELISA). DC-SIGN and DC-SIGNR expression was detected in cancer tissues by immunohistochemistry (IHC). The level of sDC-SIGN was lower in patients than in the healthy controls, while the level of sDC-SIGNR in patients was higher than in the healthy controls. Both sDC-SIGN and sDC-SIGNR had diagnostic significances for cancer patients, and the combined diagnosis of these two markers was higher than both of them alone. Furthermore, there were significant differences between both sDC-SIGN and sDC-SIGNR in stage I/II patients and the healthy controls. Moreover, high sDC-SIGN level was accompanied with the long survival time. Additionally, DC-SIGNR was negative in the cancer foci and matched normal colon tissues but was weakly positive between the cancer foci. DC-SIGN staining was faint in matched normal colon tissues, strong in the tumor stroma and the invasive margin of colon cancer tissues, and negatively correlated with the sDC-SIGN level in serum from the same patient. Interestingly, the percent survival of patients with a DC-SIGN mean density of>0.001219 (the upper 95% confidence interval of matched normal colon tissues) was higher than for all other patients. DC-SIGN and DC-SIGNR are blood-based molecular markers that can potentially be used for the diagnosis of early stage patients. Moreover, expression of DC-SIGN in serum and cancer tissues may affect the survival time for colon cancer patients.PLoS ONE 01/2014; 9(12):e114748. · 3.53 Impact Factor
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ABSTRACT: Lymph nodes (LNs) form the intersection between the vascular and lymphatic systems. Lymphocytes and antigen-presenting cells (APCs) traffic between these systems but the barriers crossed during this trafficking in human LNs are poorly defined. We identified a population of cells in human LNs that lines the boundary between the parenchyma and lymphatic sinuses, consistent with descriptions of marginal reticular cells (MRCs) in murine LNs. Human MRCs are CD141high podoplanin+ CD90+ ICAM1+ VCAM1+ but lack endothelial and haematopoietic cell markers, or alpha-smooth muscle actin. We then examined expression of the enzyme sphingosine-1-phosphate (S1P) lyase (SGPL1) relative to the boundary defined by MRCs. SGPL1 expression was almost exclusively restricted to cells on the parenchymal side of MRCs, consistent with a role in maintaining the S1P gradient between the sinuses and the parenchyma. Surprisingly the cells expressing SGPL1 in the parenchyma were CD68+ APCs. CD68+ APCs generated from human monocytes were able to internalise and irreversibly degrade S1P, and this activity was inhibited by the S1P analogue FTY720. This work provides a map of the key structures at the boundary where human lymphocytes egress into sinuses, and identifies a novel potential mechanism for the activity of S1P analogues in humans.This article is protected by copyright. All rights reservedEuropean Journal of Immunology 05/2014; · 4.52 Impact Factor
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ABSTRACT: The endothelium lines blood and lymph vessels and protects underlying tissues against external agents such as viruses, bacteria and parasites. Yet, microbes and particularly viruses have developed sophisticated ways to bypass the endothelium in order to gain access to inner organs. De novo infection of the liver parenchyma by many viruses and notably hepatitis viruses, is thought to occur through recruitment of virions on the sinusoidal endothelial surface and subsequent transfer to the epithelium. Furthermore, the liver endothelium undergoes profound changes with age and in inflammation or infection. However, primary human liver sinusoidal endothelial cells (LSEC) are difficult to obtain due to scarcity of liver resections. Relevant derived cell lines are needed in order to analyse in a standardized fashion the transfer of pathogens across the liver endothelium. By lentiviral transduction with hTert only, we have immortalized human LSECs isolated from a hereditary hemorrhagic telangiectasia (HHT) patient and established the non-transformed cell line TRP3. TRP3 express mesenchymal, endothelial and liver sinusoidal markers. Functional assessment of TRP3 cells demonstrated a high capactiy of endocytosis, tube formation and reactivity to immune stimulation. However, TRP3 displayed few fenestrae and expressed C-type lectins intracellularly. All these findings were confirmed in the original primary LSECs from which TRP3 were derived suggesting that these features were already present in the liver donor. We consider TRP3 as a model to investigate the functionality of the liver endothelium in hepatic inflammation in infection.Biochemical and Biophysical Research Communications 05/2014; 450(1). · 2.28 Impact Factor
DC-SIGNR, a DC-SIGN homologue expressed in
endothelial cells, binds to human and simian
immunodeficiency viruses and activates
infection in trans
Stefan Po ¨hlmann*†, Elizabeth J. Soilleux†‡, Fre ´de ´ric Baribaud*†, George J. Leslie*, Lesley S. Morris§, John Trowsdale‡,
Benhur Lee*, Nicholas Coleman§, and Robert W. Doms*¶
*Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104; and‡Division of Immunology, Department of
Pathology, and§Department of Molecular Histopathology, University of Cambridge, Cambridge CB2 1QP, United Kingdom
Communicated by Malcolm A. Martin, National Institutes of Health, Bethesda, MD, December 29, 2000 (received for review December 22, 2000)
DC-SIGN, a C-type lectin expressed on the surface of dendritic cells
(DCs), efficiently binds and transmits HIVs and simian immunode-
ficiency viruses to susceptible cells in trans. A DC-SIGN homologue,
termed DC-SIGNR, has recently been described. Herein we show
that DC-SIGNR, like DC-SIGN, can bind to multiple strains of HIV-1,
HIV-2, and simian immunodeficiency virus and transmit these
viruses to both T cell lines and human peripheral blood mononu-
clear cells. Binding of virus to DC-SIGNR was dependent on carbo-
hydrate recognition. Immunostaining with a DC-SIGNR-specific
antiserum showed that DC-SIGNR was expressed on sinusoidal
endothelial cells in the liver and on endothelial cells in lymph node
sinuses and placental villi. The presence of this efficient virus
attachment factor on multiple endothelial cell types indicates that
DC-SIGNR could play a role in the vertical transmission of primate
lentiviruses, in the enabling of HIV to traverse the capillary endo-
thelium in some organs, and in the presentation of virus to
CD4-positive cells in multiple locations including lymph nodes.
which cells are susceptible to virus entry and the efficiency with
which entry occurs. Infection of target cells by primary HIV-1
strains depends on the presence of CD4 molecules and chemo-
efficiency of virus entry (7–10). Equally important is simple
attachment of virus to the cell surface. For many cell types, virus
attachment is independent of the presence or absence of the
CD4 receptor and is rate-limiting for virus infection (11–13).
Attachment efficiency can be enhanced in vitro by including
polycations in the virus inoculum or by centrifuging virus onto
the cell surface (14). Infection efficiency can also be enhanced
by pulsing dendritic cells (DCs) with virus before the addition of
receptor-positive target cells (15–17). DCs are able to efficiently
transmit bound virus to peripheral blood mononuclear cells
(PBMCs) (16), resulting in a robust infection even though the
DCs themselves are either not infected or infected inefficiently
DC-SIGN, a type II membrane protein with a C-terminal
C-type (i.e., calcium-dependent) lectin domain, has been shown
to be largely responsible for the ability of dendritic cells to
efficiently capture and present HIV-1 to receptor-positive cells
(15). DC-SIGN appears to be a universal attachment factor for
primate lentiviruses that can bind and transmit all HIV-1,
HIV-2, and simian immunodeficiency virus (SIV) strains tested
to date (15, ?). In addition, DC-SIGN also interacts with the
intercellular adhesion molecule-3 receptor expressed on T cells,
thereby contributing to the close interaction between DCs and
T cells required for efficient antigen presentation (22). Because
dendritic cells are among the first cells encountered by HIV-1
relatively large number of interactions between the HIV-1
envelope (Env) protein and cell surface molecules dictate
during sexual transmission, it is possible that virus bound by
DC-SIGN may be ultimately ferried to lymph nodes (a major site
of viral replication) as a consequence of the normal cellular
trafficking of DCs (23, 24). In fact, virus bound to DCs remains
infectious for at least several days (15).
Identification of such a specific and efficient virus attachment
factor raises the possibility that other attachment factors on
relevant cell types may exist. We recently described (25) a
homologue of DC-SIGN, termed DC-SIGNR (for DC-SIGN
related), that exhibits 77% amino acid identity with DC-SIGN.
In this study, we show that DC-SIGNR also functions as a
universal attachment factor for primate lentiviruses that can
bind and transmit multiple HIV-1, HIV-2, and SIV strains to
receptor-positive cell lines and to human PBMCs. Using a
specific antiserum we developed, we found that DC-SIGNR is
expressed on sinusoidal endothelial cells in the liver, endothelial
cells present in lymph node sinuses, and a significant proportion
of capillary endothelial cells in term placenta but was not
expressed at appreciable levels in peripheral blood-derived DCs.
The presence of an efficient virus attachment and presentation
factor in these cell types indicates that DC-SIGNR could influ-
ence vertical transmission and result in enhanced infection of
receptor-positive cell types in lymph nodes.
Materials and Methods
Plasmids. Production and characterization of DC-SIGN wt and
DC-SIGN with a C-terminal AU-1 tag has been described.?For
detection by immunostaining before the development of specific
antiserum, a C-terminal AU-1 antigenic tag was also added to
DC-SIGNR by PCR mutagenesis. All constructs were cloned
into pcDNA3 (Invitrogen) by using the unique BamHI and
EcoRI restriction sites.
Production of DC-SIGNR Antiserum. To produce antiserum specific
for DC-SIGNR, a peptide (DPTTSGIRLFPRDFQ) corre-
sponding to a unique sequence in the cytoplasmic domain of
DC-SIGNR was covalently coupled to keyhole limpet hemocy-
anin. Anti-peptide antiserum was raised in a chicken by three s.c.
Abbeviations: DC, dendritic cell; DC-SIGN, DC-specific, ICAM-3 grabbing, nonintegrin;
DC-SIGNR, DC-SIGN related; PBMC, peripheral blood mononuclear cell; GFP, green fluo-
rescent protein; FACS, flow cytometry.
†S.P., E.J.S., and F.B. contributed equally to this work.
¶To whom reprint requests should be addressed at: Department of Pathology and Labo-
ratory Medicine, University of Pennsylvania, 806 Abramson, Philadelphia, PA 19104.
?S.P., F.B., B.L., G.J.L., M. D. Sanchez, K. Hiebenthal-Millow, J. Mu ¨nch, F. Kirchhoff, and
R.W.D., unpublished work.
The publication costs of this article were defrayed in part by page charge payment. This
article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C.
§1734 solely to indicate this fact.
February 27, 2001 ?
vol. 98 ?
immunizations with this antigen. Immune IgY was purified from
the eggs and affinity-purified against the peptide coupled to
N-hydroxysuccinimide-activated Sepharose beads. Bound anti-
bodies were eluted with 100 mM glycine (pH 2.5), and fractions
containing the antibodies were pooled and dialyzed against PBS.
Virus Binding and Transfer Assays. Both assays were performed
essentially as described.?In brief, 293T cells were seeded in T-25
flasks and transiently transfected with DC-SIGNR and DC-
SIGN expression vectors or with the pcDNA3 control plasmid.
One day after transfection, the cells were detached from the
flasks and seeded into 96-well plates. On the following day, the
cells were incubated with p24-normalized virus stocks for 3 h,
vigorously washed with culture medium, and lysed in 0.5%
Triton X-100. The amount of bound viral antigen was quantified
Miami, FL). For the virus transfer assay, the cells were washed
with culture medium and cocultivated with C8166 T cells or
PBMCs. When transfer of luciferase reporter viruses was inves-
tigated, the cultures were lysed 3 days after the start of the
cocultivation and luciferase activity in 30 ?l of lysate was
determined with a commercially available kit (Promega). To
quantify transfer of primary HIV isolates, supernatant was
removed from the cocultures at regular intervals and the content
of viral p24 antigen measured with the p24 ELISA. Transmission
of pseudotyped green fluorescent protein (GFP) reporter virus
was investigated via flow cytometry (FACS). Five days after
infection, cells were harvested in FACS buffer (PBS containing
3% FCS and 0.05% sodium azide), stained with an anti-CD3
antibody (Sigma) that was directly conjugated to phycoerythrin,
and analyzed via FACS. GFP-positive cells were found exclu-
sively in the CD3?population. Viruses were originally obtained
from the National Institutes of Health AIDS Reagent Reposi-
tory except for isolate SPL3, a primary virus strain isolated in the
laboratory of Ron Collman (University of Pennsylvania), that
has not otherwise been characterized.
Tissue Processing and Staining. All tissues were obtained with the
normal postmortem adult human tissues were obtained from the
Department of Histopathology, Addenbrooke’s Hospital, Hills
Road, Cambridge, U.K., as were postmortem tissues from
fetuses at 12 weeks and 36 weeks of gestation and tissue from
histologically normal term placentas. Tissues were fixed in
neutral-buffered 10% formalin before processing for paraffin
wax embedding and sectioning. Rehydrated 5-?m paraffin serial
sections were immunostained with anti-DC-SIGNR antiserum
at a 1:50 dilution. For negative controls, anti-DC-SIGNR anti-
serum at a 1:50 dilution was used in the presence of a 20-fold
molar excess of the peptide DPTTSGIRLFPRDFQ. Further
serial sections were immunostained with rabbit polyclonal anti-
Von Willebrand factor (Dako) at a 1:200 dilution. Sections were
preblocked in 0.5% hydrogen peroxide in Tris-buffered saline
(donkey serum for anti-DC-SIGNR or goat serum for anti-Von
Willebrand factor) for 2 h before the addition of antibody in
TBS?10% BSA. After overnight incubation at 4°C in primary
antibody, sections were washed with TBS and placed in second-
ary biotinylated antibody in TBS?10% BSA for 2 h. After
washing in TBS, the StreptABC?HRP kit (Dako) was used to
form avidin-biotin–horseradish peroxidase complexes. Slides
were developed with diaminobenzidine (Sigma) and counter-
stained with Harris’s hematoxylin (Dako). Slides were dehy-
drated in xylene and mounted in DPX (BDH Laboratory
Supplies, Poole, U.K.).
DC-SIGNR Protein Is Expressed on Endothelial Cells in Placenta, Liver,
and Lymph Nodes. DC-SIGN and DC-SIGNR share 77% amino
acid identity, with the greatest areas of homology being in the
C-type lectin domain and the neck region (25). The C- and
N-terminal regions of this type II membrane protein are con-
siderably more divergent from the DC-SIGN sequence. To
monitor the tissue expression of DC-SIGNR, we affinity-
purified an anti-peptide antiserum raised against a unique
sequence (DPTTSGIRLFPRDFQ) in the cytoplasmic domain
of DC-SIGNR. The resulting antiserum recognized DC-SIGNR
but not DC-SIGN by Western blot (Fig. 1).
DC-SIGNR expression was demonstrated on the endothelium
lining lymph node sinuses, on the endothelium of approximately
half of the capillaries in term placenta, and on the endothelium
lining the sinusoids of the liver (Fig. 2). This immunohistochem-
ical staining was abrogated in the presence of specific peptide
(data not shown). The endothelial identity of these cells was
confirmed by immunostaining serial sections with Von Wille-
brand factor (factor VIII-related antigen) (Fig. 2 and ref. 26).
Similar DC-SIGNR immunostaining was present in 36-week
fetal liver (data not shown). No evidence of expression could be
found on endothelium in multiple other tissues including lung,
spleen, thymus, kidney, heart, pons, medulla, and midbrain (data
not shown). In addition, no evidence of protein expression could
be found on cells of the myeloid lineage, nor have we detected
DC-SIGNR expression in peripheral blood-derived DCs.
DC-SIGNR Binds HIV-1 Isolates. The presence of DC-SIGNR on
endothelial cells, particularly in the lymph node and placenta,
places this molecule in a position where it could impact vertical
transmission of HIV and present virus to CD4?coreceptor-
positive cell types that come into contact with these surfaces. To
determine whether DC-SIGNR supports binding of R5, X4, and
R5X4 HIV-1 virus strains that use CCR5, CXCR4, or either
receptor in conjunction with CD4 to infect cells, respectively
(27), human 293T cells were transiently transfected with DC-
SIGNR or DC-SIGN expression vectors. By using versions of
DC-SIGN and DC-SIGNR that possessed an AU1 antigenic tag
both proteins were expressed at similar levels on the surface of
293T cells, as judged by FACS analysis (? and data not shown).
Cells expressing DC-SIGN or DC-SIGNR were incubated with
p24-normalized virus stocks and extensively washed, and the
ing AU1-tagged versions of DC-SIGN, DC-SIGNR, or pcDNA3 vector alone
were lysed in nonionic detergent. Aliquots of the resulting lysates were
analyzed by SDS?PAGE and Western blotting. Blots were probed with a
against a peptide based on a unique sequence in the cytoplasmic domain
of DC-SIGNR (B).
DC-SIGNR specific antiserum. Human 293T cells transiently express-
Po ¨hlmann et al.
February 27, 2001 ?
vol. 98 ?
no. 5 ?
bound to DC-SIGNR and DC-SIGN expressing cells more
laboratory-adapted NL4–3 strain and the 89.6 R5X4 viral clone
bound to DC-SIGNR and DC-SIGN transfected cells with the
highest efficiencies. However, those viruses also exhibited the
most efficient binding to pcDNA3-transfected control cells.
Thus, like DC-SIGN, DC-SIGNR functions as an efficient
HIV-1 attachment factor.
DC-SIGNR Transmits HIV-1, HIV-2, and SIV to T Cell Lines and PBMCs
with Different Efficiencies. Primate lentiviruses bound to DC-
SIGN can be efficiently transmitted to receptor-positive T cell
lines and to primary T cells (15, ?). We investigated the ability of
DC-SIGNR to transmit virus to C8166 T cells. DC-SIGN and
DC-SIGNR were expressed in 293T cells, the cells incubated
with replication-competent luciferase-reporter viruses, vigor-
the start of the coculture, the cells were lysed and the luciferase
activity in the lysates was quantified. DC-SIGNR-express-
ing cells transmitted HIV-1 NL4–3 and HIV-2 Rod10 about
4-fold and SIVmac239 MER Env about 2.5-fold more efficiently
than DC-SIGN-transfected cells (Figure 4A). However, when
both proteins were cotransfected, the transmission efficien-
cies obtained ranged between those observed for DC-SIGN
and DC-SIGNR, indicating that the proteins did not function
We also investigated whether DC-SIGNR was capable of
transferring HIV isolates to primary cells. The virus-transfer
experiment was carried out as described above, except that the
transfer of six primary HIV-1 isolates of various tropisms to
cultured human PBMCs was analyzed. The supernatants of the
293T?PBMC cocultures were harvested at regular intervals and
assayed for their p24 content with a p24-antigen-capture ELISA.
All isolates tested replicated more efficiently in PBMCs cocul-
tured with 293T cells expressing DC-SIGN or DC-SIGNR than
in PBMCs cocultured with 293T control cells. DC-SIGNR and
laboratory-adapted NL4–3 strain and the SF162 primary isolate.
However, robust replication of the ADA isolate was only ob-
served after coculture with DC-SIGN-expressing cells. This was
surprising because ADA bound to DC-SIGNR and DC-SIGN
equally well (Fig. 3). However, the binding assay was done under
equilibrium binding conditions, and it is possible that ADA
dissociates from DC-SIGNR more quickly than the other viruses
tested. If so, this could have a more dramatic effect on virus
transmission than on binding under saturating conditions. The
THO26 (Fig. 4B), SPL-3, UG021 and UG024 isolates (data not
shown) were transmitted slightly more efficiently by DC-SIGN
than by DC-SIGNR. To confirm the identity of the cells that
were infected, we also investigated the transmission of a repli-
cation defective GFP reporter virus pseudotyped with the
NL4–3 Env. Five days after the start of the coculture, the cells
were stained for CD3 and GFP-positive cells were quantified via
that only PBMCs, not the DC-SIGN-positive 293T cells, were
infected (data not shown). Thus, like DC-SIGN, DC-SIGNR can
transmit various bound HIV-1 virus strains to receptor-positive
cell lines and to PBMCs. DC-SIGNR transmitted virus more
efficiently than DC-SIGN when C8166 cells were used and more
of human tissues were immunostained with anti-DC-SIGNR chicken serum or
anti-Von Willebrand factor rabbit serum. (A and C) Normal lymph node: the
and D) Normal lymph node: an identical pattern of staining with anti-Von
Willebrand factor is observed in the lymph node sinusoids. Anti-Von Wille-
brand factor also stained high endothelial vessels that did not stain with
anti-DC-SIGNR. (E) Villi from normal term placenta: the endothelium of ap-
proximately half of the capillaries is stained with anti-DC-SIGNR antibody. (F)
Villi from normal term placenta: all capillaries are stained with anti-Von
Willebrand factor. (G) Normal liver: anti-DC-SIGNR antibody stained all sinu-
soids. (H) Normal liver: anti-Von Willebrand factor confirmed the identity of
the sinusoidal lining cells as endothelium.
Expression of DC-SIGNR. Formalin-fixed paraffin-embedded sections
siently transfected with the indicated expression plasmids. The cells were
incubated with p24-normalized virus stocks, vigorously washed, and lysed in
0.5% Triton X-100, and then the content of bound p24 was quantified by
ELISA. The tropism of each virus is indicated (R5, X4, or R5X4). All viruses are
clade B except for the UG021 and UG024 isolates, which are clade D. Data are
four experiments are shown. N.D., not done.
Binding of HIV to DC-SIGNR transfected cells. 293T cells were tran-
www.pnas.org?cgi?doi?10.1073?pnas.051631398 Po ¨hlmann et al.
variably than DC-SIGN when PBMCs were used as targets.
Therefore, some virus strains may be transmitted by DC-SIGNR
to primary cells more efficiently than others.
DC-SIGNR and DC-SIGN Exhibit Comparable Ligand Specificity.EGTA
and mannan (a carbohydrate that binds to the lectin domain)
block virus transmission by DC-SIGN, suggesting a critical role
of the calcium-dependent lectin domain in this process (15, ?). In
agreement with this data, we found that deletion of the lectin
domain in DC-SIGN prevents virus binding and transmission.?
Moreover, washing virus-pulsed cells with trypsin?EDTA blocks
transmission of virus from DC-SIGN-expressing 293T cells to
CD4?coreceptor-positive cells.?We therefore investigated
whether DC-SIGNR-mediated transmission is sensitive to the
same agents. The virus transmission assay was carried out as
described above, but the cells were incubated with EGTA and
mannan before the addition of reporter virus or the cells were
treated with trypsin?EDTA after the incubation with reporter
virus. Addition of EGTA reduced virus transmission by both
DC-SIGNR- and DC-SIGN-expressing cells to a comparable
degree. Preincubation with mannan also reduced virus trans-
mission, although somewhat less efficiently than did EGTA
(Fig. 5). Trypsin?EDTA strongly inhibited virus transfer by
DC-SIGNR and DC-SIGN, indicating that, in both cases, bound
virus remained at the cell surface.
The discovery of DC-SIGN helps explain the mechanisms by
which DCs make initial contact with resting T cells, a step that
can ultimately lead to T cell activation (28). In addition, DC-
SIGN appears to be largely responsible for the unique ability of
DCs to present virus to cells that express the necessary viral
receptors. DC-SIGN binds to multiple strains of HIV-1, HIV-2,
and SIV (15, ?) and does so in a manner that maintains the
integrity and infectivity of the virus for up to several days (15).
Thus, virus bound to DC-SIGN-positive DCs can take advantage
of the normal cellular trafficking patterns of DCs, perhaps being
ferried by DCs to lymph nodes (15), the major site of HIV
replication in vivo (29).
We have found that DC-SIGNR functions similarly to DC-
SIGN with regards to virus attachment and transmission, as
might be expected given the high degree of homology between
these molecules (25). Like DC-SIGN, DC-SIGNR appears to
function as a universal attachment factor for primate lentivi-
ruses, supporting binding and transmission of all strains of
HIV-1, HIV-2, and SIV examined. However, transmission of
virus strains from DC-SIGNR-expressing cells to PBMCs was
more variable than when virus was transmitted via DC-SIGN-
positive cells. Therefore, DC-SIGNR may be more selective than
NL4–3, HIV-2 Rod10, and SIVmac239 MER Env luciferase reporter viruses were used in the transfer assay described above. Luciferase activity was determined 3
days after the start of the cocultivation. Data are the mean ? SEM of three experiments. (B) Transmission of HIV isolates to PBMCs. The transmission of primary
HIV isolates and the laboratory-adapted NL4–3 virus was investigated as described above. The culture supernatants were removed as indicated (d.p.i., days
postinfection), and their p24 content was measured by ELISA. Similar results were obtained in an independent experiment.
of NL4–3 luciferase reporter virus was analyzed as described above, except
three wash steps was carried out with trypsin?EDTA instead of medium.
EGTA, mannan, and trypsin?EDTA inhibit virus transfer. The transfer
Po ¨hlmann et al.
February 27, 2001 ?
vol. 98 ?
no. 5 ?
DC-SIGN with regards to the virus strains that it can efficiently
bind and transmit. To accurately discern this, however, it will be
important to study virus binding and transmission with multiple
cell types and to correlate expression levels with function. For
example, we have found that DC-SIGN function is tightly linked
to its expression levels, with ?100,000 copies of DC-SIGN per
293T cell being needed for maximal activity.?In our experimen-
tal systems, high levels of DC-SIGN and DC-SIGNR expression
were attained. Thus, it may be important to reduce expression
levels of these attachment factors to identify differences in how
these molecules interact with diverse virus strains.
Most studies of HIV entry have focused on the interactions of
Env, CD4, and coreceptors, and the resulting conformational
changes in Env that lead to membrane fusion. The impressive
abilities of DC-SIGN and DC-SIGNR to increase the efficiency
of virus infection underscore the importance of virus attachment
as a first step in the entry pathway, both in cis and in trans. A
number of molecules can support HIV attachment to both
receptor-positive and receptor-negative cells, including cell sur-
face heparan sulfate proteoglycans and interactions between
LFA (lymphocyte function-associated antigen) and intercellular
adhesion molecule-1 (11, 30). In the case of DCs, DC-SIGN
appears to be largely responsible for virus attachment. Thus,
there are now several examples where a virus receptor on one
cell can function in trans to support infection of an adjoining cell.
In some cases, binding of HIV-1 to CD4-positive cells in vitro
enables virus to infect CD4-negative coreceptor-positive cells in
in trans but do not obviate the need for either CD4 or coreceptor
for viral entry (15). The precise mechanisms by which DC-SIGN
and DC-SIGNR transmit virus are not known beyond the
obvious dependence on carbohydrate recognition. It will there-
fore be important to determine whether interactions between
Env and these attachment factors involve specific carbohydrate
or polypeptide structural motifs.
We documented expression of DC-SIGNR on endothelial
cells in human placenta, lymph node sinuses, and hepatic
sinusoids but not on peripheral blood-derived DCs. Expression
of DC-SIGNR in other tissues has not yet been demonstrated,
although the development of additional antiserum or mAbs may
reveal expression in other locations. If the virus attachment and
transmission functions of DC-SIGNR require high levels of
expression, as we have observed for DC-SIGN, then the mere
presence of DC-SIGNR or DC-SIGN in a given cell type may not
necessarily mean that an additional site at which virus can be
efficiently captured and presented has been identified. Thus,
quantitative measures of expression rather than just measure-
ments of expression per se may be required.
What might the consequences of DC-SIGNR expression on
endothelial cells be for HIV-1 infection? The expression of
DC-SIGNR in lymph node and placenta is intriguing. Lymph
nodes represent the major site of HIV replication in vivo (29),
and the presence of DC-SIGNR on the surface of endothelial
cells in lymph node sinuses represents an obvious mechanism by
which cell-free virus can be transmitted to CD4-positive cells
that come into contact with these surfaces. If DC-SIGNR binds
to intercellular adhesion molecule-3 and other cell surface
receptors, interactions between T cells and the endothelial cell
surface may occur more frequently, increasing the likelihood of
virus transmission. We have also documented DC-SIGNR ex-
pression in a significant proportion of capillaries in villi of term
placenta and, in a separate study (E.J.S., J.T., and N.C., unpub-
lished results), have found that DC-SIGN is expressed on
decidual macrophages and fetal Hofbauer cells, a macrophage-
like cell type in the placenta that supports HIV infection (32).
The presence of these attachment factors on either side of the
trophoblast layer that separates the maternal and fetal circula-
tion could serve to concentrate virus at this site and influence
vertical transmission. Expression of DC-SIGNR on hepatic
sinusoidal endothelial cells is of less obvious relevance to HIV
pathogenesis, although there is at least one report that these cells
may be infected by HIV-1 in vitro (33). In addition, hepatic
endothelial cells may also be involved in antigen presentation
(21, 34), perhaps presenting opportunities to transmit virus to
circulating CD4-positive cells.
In summary, DC-SIGNR joins DC-SIGN as a specific virus
attachment factor that could have profound effects on the
tropism and pathogenesis of HIV-1, HIV-2, and SIV. The
discovery of these proteins stresses the need to reexamine virus
attachment in general, not only to cells bearing CD4 and
appropriate coreceptors but also to cells that frequently come
into contact with these targets of virus infection. Perhaps other
highly specific high-affinity virus attachment proteins will be
identified. Finally, DC-SIGN and DC-SIGNR, which appear to
share similar attachment mechanisms, potentially represent an-
We thank Dr. D. G. D. Wight, Department of Histopathology, Adden-
brooke’s Hospital, Hills Road Cambridge, U.K. for advice about liver
histology and Dr S. M. Rushbrook and Mrs K. Bird for assistance with
immunohistochemistry. We also thank Kirsten Hiebenthal-Millow and
Frank Kirchhoff for providing luciferase-reporter virus proviral ge-
nomes and Victor Holubowsky and Farida Shaheen for generation and
quantification of virus stocks. This work was supported by Grant
P30-AI45008 of the Viral?Cell?Molecular Core of the Penn Center for
AIDS Research and by National Institutes of Health Grants R01 35383
and R01 40880 to R.W.D. This work was also supported by a Burroughs
Wellcome Fund Translational Research Award to R.W.D. R.W.D. is a
recipient of an Elizabeth Glaser Scientist Award from the Pediatric
AIDS Foundation. F.B. was supported by a fellowship from the Swiss
National Science Foundation (Grant 823A-61172). E.J.S. was supported
by a Medical Research Council (MRC) clinical training fellowship and
from the MRC and Cancer Research Campaign. S.P. was supported by
a fellowship from the Deutsche Forschungsgemeinschaft (DFG).
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