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Journal of Virology, February 2002, p. 1866-1875, Vol. 76, No. 4
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.76.4.1866-1875.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Expression of DC-SIGN by Dendritic Cells of Intestinal and Genital Mucosae in Humans and Rhesus Macaques
Brian Jameson,1 Frédéric Baribaud,2 Stefan Pöhlmann,2 Darlene Ghavimi,2 Frank Mortari,3 Robert W. Doms,2 and Akiko Iwasaki4*
Department of Pathology,1
Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, Connecticut 06520,4
Department of Microbiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104,2
R&D Systems, Minneapolis, Minnesota 554133
Received 17 September 2001/
Accepted 13 November 2001

ABSTRACT
To better understand the role of dendritic cells (DCs) in human
immunodeficiency virus (HIV) transmission at mucosal surfaces,
we examined the expressions of the HIV adhesion molecule, dendritic-cell-specific
ICAM-3 grabbing nonintegrin (DC-SIGN), its closely related homologue
DC-SIGNR, and HIV coreceptors by distinct DC populations in
the intestinal and genital tracts of humans and rhesus macaques.
We also developed monoclonal antibodies (MAbs) specific for
DC-SIGN or DC-SIGNR. In the Peyer's patches, DC-SIGN expression
was detected in the interfollicular regions and in clusters
of cells in the subepithelial dome regions. DC-SIGN expression
was not found on plasmacytoid DCs. DC-SIGNR expression was restricted
to endothelial cells in approximately one-third of the capillaries
in the terminal ileum. In the vaginal epithelium, Langerhans'
cells did not express DC-SIGN, whereas subepithelial DCs in
the lamina propria expressed moderate levels of DC-SIGN. Finally,
the rectum contained cells that expressed high levels of DC-SIGN
throughout the entire thickness of the mucosa, while solitary
lymphoid nodules within the rectum showed very little staining
for DC-SIGN. Triple-color analysis of rectal tissue indicated
that CCR5
+ CD4
+ DC-SIGN
+ DCs were localized just beneath the
luminal epithelium. These findings suggest that DC-SIGN
+ DCs
could play a role in the transmission of primate lentiviruses
in the ileum and the rectum whereas accessibility to DC-SIGN
+ cells is limited in an intact vaginal mucosa. Finally, we identified
a MAb that blocked simian immunodeficiency virus interactions
with rhesus macaque DC-SIGN. This and other specific MAbs may
be used to assess the relevance of DC-SIGN in virus transmission
in vivo.

INTRODUCTION
Worldwide, human immunodeficiency virus (HIV) infection spreads
primarily as a result of sexual exposure through mucosal surfaces.
Identifying cellular factors that influence the efficiency of
HIV transmission at mucosal surfaces is important not only in
understanding the pathogenesis of HIV type 1 (HIV-1) infection,
but also for the development of preventative measures such as
topically applied microbicides. Dendritic cells (DCs) are professional
antigen-presenting cells localized throughout the body that
are ideally positioned to survey incoming microbial pathogens.
DCs are capable of taking up microbial antigens at the sites
of infection and migrating to draining lymph nodes to initiate
antigen-specific T-lymphocyte activation. DCs may also serve
as carriers of HIV-1 from mucosal tissues to secondary lymphoid
organs (
6,
13,
31). A C-type lectin that is highly expressed
on DCs, termed dendritic cell-specific ICAM-3 grabbing nonintegrin
(DC-SIGN), binds with high affinity to the envelope protein
(Env) of HIV-1 and has been posited to play an important role
in this process (
11).
DC-SIGN is a type II integral membrane protein primarily expressed on DCs and on some types of tissue macrophages (3, 12, 33a). Human DC-SIGN has been shown to bind all HIV-1, HIV-2, and simian immunodeficiency virus (SIV) strains examined to date (3, 11, 26) and plays an important role in virus adhesion to DCs. Once bound to DC-SIGN, virus can be transmitted to susceptible target cells expressing CD4 and viral coreceptors (3, 11, 26), providing a molecular explanation for the well-known ability of DCs to enhance HIV infection of T cells in trans (7, 28, 41). A highly similar molecule termed DC-SIGNR (DC-SIGN related) is expressed on certain types of endothelial cells in vivo and also functions as an efficient virus attachment and transmission factor (4, 27).
While DC-SIGN can mediate virus attachment and transmission in vitro (3, 11, 26), its in vivo relevance is uncertain. To assess its in vivo role in viral transmission, the distribution of DC-SIGN, particularly in mucosal tissues, needs to be carefully assessed both in humans and in nonhuman primates that serve as important model systems for HIV transmission and pathogenesis. Previously, DC-SIGN expression was demonstrated in the human cervix, rectum, and uterus (11). In serial tissue sections, the majority of the mucosal DC-SIGN+ cells were shown to coincide with CD4 but not CCR5 staining (11). To more carefully document the expression of this virus attachment factor and to determine if viral coreceptors and DC-SIGN are expressed on the same cells, we utilized double and triple fluorescence labeling and confocal microscopy on sections of mucosal tissues of both humans and rhesus macaques. To unequivocally document DC-SIGN expression, we developed monoclonal antibodies (MAbs) that were specific for either DC-SIGN or DC-SIGNR. Several of these MAbs were able to block DC-SIGN-dependent virus transmission in vitro.
In this study, we demonstrate that in the Peyer's patches, DC-SIGN was mainly expressed on major histocompatibility complex (MHC) class II+, nonplasmacytoid DCs in the interfollicular regions and in clusters of cells within the subepithelial dome region. DC-SIGNR was expressed exclusively by the endothelial cells in the Peyer's patches and in villous lamina propria. In the rectum, DC-SIGN was expressed throughout the lamina propria, and DC-SIGN+ CCR5+ CD4+ DCs were present just beneath the luminal epithelium. In the vagina, subepithelial DCs but not Langerhans' cells expressed DC-SIGN. Taken together, our results for both humans and rhesus monkeys suggest that primate lentiviruses may gain access to the DC-SIGN+ DCs most readily in the intestinal tract, while the thick squamous epithelium serves as a barrier for viral access to DC-SIGN+ DCs in the vaginal mucosa. These data are consistent with the observation that the risk of HIV-1 transmission is greater in rectal intercourse than in vaginal exposure (40). Our results also revealed that tissue distribution of DC-SIGN and viral coreceptors is quite similar in humans and in rhesus macaques and further corroborate the validity of this animal model for mucosal HIV-1 transmission studies.

MATERIALS AND METHODS
Tissue samples.
Human vaginal, rectal, and ileum tissues were obtained from
Yale Pathology Critical Technologies program under an institutionally
approved HIC protocol. Tissues were obtained either from archived
frozen blocks of tissues from areas with no pathological involvement
or from autopsy from patients that died of causes unrelated
to intestinal or genital diseases. All specimens used in this
study were of adult origin, and all vaginal tissues were of
premenopausal women. Rhesus macaque tissues were obtained from
the New England Regional Primate Research Center, Southborough,
Mass., supported by Division of Research Resources (National
Institutes of Health) grant RR00168. The tissue usage has complied
with all federal guidelines and institutional policies.
MAbs to DC-SIGN.
MAbs DC4, DC11, and DC28, which recognize the repeat region of DC-SIGN and cross-react with DC-SIGNR, were developed as described previously (3). To obtain antibodies to the lectin domain of DC-SIGN or DC-SIGNR, mice were immunized with 3T3 cells expressing human DC-SIGN or DC-SIGNR, and MAbs were produced as previously described 3; E. J. Soilleux et al., submitted). The characterization of these MAbs is described in Results. The isotypes of the antibodies are as follows: 120506 (immunoglobulin G2a [IgG2a]), 120507 (IgG2b), 120516 (IgG2a), 120518 (IgG2a), 120526 (IgG2a), 120531 (IgG1), 120604 (IgG2b), and 120612 (IgG2a).
Cell lines and flow cytometry analysis.
We engineered T-Rex cells to express either human DC-SIGN (hu DC-SIGN), human DC-SIGNR (hu DC-SIGNR), rhesus macaque DC-SIGN (rh DC-SIGN), pigtailed macaque DC-SIGN (pt DC-SIGN), murine DC-SIGN (mu DC-SIGN), a lectin domain-truncated hu DC-SIGN (
C), or a repeat region-truncated hu DC-SIGN (
repeat) following the manufacturer's instructions (Invitrogen, Carlsbad, Calif.). For fluorescence-activated cell sorter (FACS) analysis, each T-Rex cell line was induced with 0.01 µg of doxycycline per ml overnight, recovered, and washed once with FACS buffer (phosphate-buffered saline supplemented with 3% fetal bovine serum and 0.02% sodium azide). Half a million cells were then stained with each MAb at 10 µg/ml in FACS buffer for 30 min on ice. The samples were washed and incubated with phycoerythrin-conjugated goat anti-mouse Fab fragments (Caltag, Burlingame, Calif.) (1:100) for 30 min on ice and then washed and resuspended in FACS buffer containing 2% paraformaldehyde. The samples were analyzed with a FACScan (Becton Dickinson, San Jose, Calif.) cell analyzer using the CellQuest software for data evaluation. Dead cells were excluded on the basis of their forward and side scatter characteristics.
Multiple-color immunofluorescence staining.
To examine the distribution of DC subsets and their DC-SIGN expression patterns, frozen sections of Peyer's patches, vagina, and rectum from rhesus macaques and humans were stained with a variety of antibodies in a procedure similar to that described previously (18) with minor modifications. Briefly, 6- to 8-µm frozen sections were fixed in cold acetone and blocked with NEN blocking buffer (NEN Life Science Products Inc., Boston, Mass.) containing normal donkey serum. To block endogenous biotin, the sections were further treated with Avidin/Biotin block (Vector Laboratories, Inc., Burlingame, Calif.). Next, endogenous peroxidase activity was quenched with 1% H2O2 for 10 min and primary antibodies against CD11c (Novocastra Laboratories Ltd., Newcastle, United Kingdom), CD123, CD4, CXCR4, CCR5, or HLA-DR/DP/DQ (BD PharMingen, San Diego, Calif.), CD31 or platelet-endothelial cell adhesion molecule-1 (eBioscience, San Diego, Calif.), mouse MAbs that recognize the ectodomains of both DC-SIGN and DC-SIGNR (clones DC4, DC11, or DC28), or mouse MAbs specific to either DC-SIGN (clone 120507; R&D Systems, Minneapolis, Minn.) or DC-SIGNR (clone 120604; R&D Systems) were applied for 1.5 h at room temperature. All mouse MAbs were used at 5 to 10 µg/ml except for 120507, which was applied at the concentration of 0.25 µg/ml. Slides were washed and incubated with biotin-conjugated donkey F(ab")2 anti-mouse IgG (Jackson Immunoresearch Laboratories, Inc., West Grove, Pa.) for 30 min, followed by incubation with streptavidin-horseradish peroxidase conjugate (Zymed Laboratories, San Francisco, Calif.). The antigens were detected with tetramethylrhodamine-Tyramide, FITC-Tyramide, or Cy5-Tyramide (NEN Life Science Products, Inc.), or Alexa 488-Tyramide or Alexa 594-Tyramide (Molecular Probes, Eugene, Oreg.) according to the manufacturer's instructions. In the case of double or triple labeling on the same section, the exact same procedure was carried out in sequence, with the following blocking steps in between: 2% H2O2 for 10 min, followed by Avidin/Biotin block, incubation with mouse IgG (Sigma-Aldrich, St. Louis, Mo.), and finally incubation with Fab goat anti-mouse IgG (Jackson Immunoresearch). At the end of the staining, slides were washed, incubated with 4",6"-diamidino-2-phenylindole (DAPI) (Molecular Probes), and mounted with Fluoromount G (Southern Biotechnology Associates, Inc. Birmingham, Ala.). The stained slides were analyzed by fluorescence microscope (Leitz Orthoplan 2) or by confocal microscopy with a Zeiss LSM510 confocal laser microscope equipped with a 40x objective lens with water.
Blocking SIV transmission by antibodies against DC-SIGN.
Inhibition of DC-SIGN-mediated transmission of SIV was assessed using a T-Rex cell line that expresses rhesus macaque DC-SIGN upon induction with doxycycline. DC-SIGN T-Rex cells and parental T-Rex cells were seeded in 96-well plates, induced with 0.01 µg of doxycycline per ml for 12 h, incubated with DC-SIGN- and DC-SIGNR-specific antibodies or mannan for 30 min, and pulsed with replication-competent SIVmac239 reporter virus harboring the MER Env. After a 3-h incubation, the cells were vigorously washed and cocultivated with CEMx174 target cells. The cultures were lysed 3 days later, and luciferase activity in 20 µl of lysate was determined by using a commercially available kit (Promega, Madison, Wis.).

RESULTS
Generation of DC-SIGN- and DC-SIGNR-specific MAbs.
The fact that many MAbs to DC-SIGN cross-react with the closely
related homologue DC-SIGNR (
3,
4,
27) necessitates the development
of antibodies that are specific for each of these molecules.
To accomplish this, we immunized mice with murine 3T3 cells
overexpressing either human DC-SIGN or DC-SIGNR. Hybridomas
were generated, and the resulting MAbs were analyzed by staining
cell lines expressing DC-SIGN or DC-SIGNR. By FACS analysis,
we found that MAbs 120506, 120507, 120516, and 120531 bound
to human, rhesus macaque, and pigtailed macaque DC-SIGN but
not to DC-SIGNR; that MAb 120604 recognized human DC-SIGNR but
not DC-SIGN; and that MAbs 120518, 120526, and 120612 recognized
both DC-SIGN and DC-SIGNR (Fig.
1). All seven of the MAbs that
bound to DC-SIGN recognized a construct lacking the repeat region
of DC-SIGN (

repeat) (Table
1)
(
3). In contrast, binding to a
construct lacking the lectin binding domain (

C) (Table
1) was
not observed. Expression of this construct was confirmed by
the use of a MAb to the repeat region (
3). None of the MAbs
recognized DC-SIGN efficiently by Western blotting (data not
shown). Therefore, we conclude that these MAbs bind to conformation-dependent
epitopes in the lectin binding domain of DC-SIGN. Finally, none
of the MAbs recognized murine DC-SIGN (Table
1). We chose MAbs
120507 and 120604 as well as MAbs that recognize both DC-SIGN
and DC-SIGNR (DC4, DC11, and DC28 [
3]) to study the distribution
of these molecules in human and rhesus macaque mucosal tissues.
DC subset distribution in human Peyer's patch.
Peyer's patches harbor a significant fraction of gut-associated
lymphoid tissue. While myeloid and lymphoid DCs are known to
localize in the subepithelial dome and the interfollicular regions
of mouse Peyer's patches, respectively (
18), the tissue distribution
of myeloid and plasmacytoid DCs in human Peyer's patches is
not known. Prior to assessing DC-SIGN expression in the Peyer's
patches, we studied the distribution of both myeloid DCs that
express CD11c and plasmacytoid DCs that express interleukin
3R (IL-3R) (
8,
14,
21,
29,
30) by using double immunofluorescence
labeling of frozen sections of the terminal ileum (Fig.
2).
We found that all CD11c
+ cells expressed high levels of MHC
class II and were distributed in the subepithelial dome region,
the interfollicular regions, and the villous lamina propria.
The germinal centers also contained large CD11c
+/MHC class II
+ DCs. In contrast, cells expressing IL-3R (CD123) were found
exclusively in the interfollicular regions, and also expressed
moderate levels of MHC class II molecule (Fig.
2C and D). IL-3R
staining was also found on the high endothelial venules as reported
previously (
8,
14). Thus, in the Peyer's patches, CD11c
+ DCs
were found in the subepithelial dome region, germinal center,
and interfollicular regions, whereas plasmacytoid DCs were localized
exclusively in the interfollicular regions.
DC-SIGN is expressed in the dome and interfollicular regions of human Peyer's patches.
Having established the localization of myeloid and plasmacytoid
DC populations in human Peyer's patches, we investigated whether
these subsets of DCs express DC-SIGN. We used MAb DC4, which
recognizes the repeat region of DC-SIGN and also cross-reacts
with DC-SIGNR. Throughout this study we found that MAbs DC4,
DC11, and DC28 gave very similar staining patterns but that
the use of DC4 and DC11 resulted in the most intense staining
with humans and rhesus macaques, respectively. We also used
the DC-SIGN-specific MAb 120507 to demonstrate DC-SIGN expression
by DC subsets in human Peyer's patches (Fig.
3E and F). As shown
in Fig.
3A, all cells that were stained with MAb DC4, except
for the high endothelial venules, were MHC class II
hi, confirming
the earlier report that DC-SIGN is specifically expressed on
DCs (
12). However, not all MHC class II
+ cells in the Peyer's
patch were DC-SIGN
+. MHC class II
+ DC-SIGN
- cells consisted
of B cells and DCs in the germinal center, interfollicular region,
and dome area (Fig.
3A). Most DC-SIGN
+ cells in Peyer's patches
were localized in the interfollicular regions (Fig.
3). We also
observed numerous DC-SIGN
+ cells within the villous lamina propria.
In about half of the Peyer's patches, we detected distinct clusters
of DC-SIGN
+ cells in the subepithelial dome region. The DC-SIGN
+ cell clusters were normally MHC class II
+ (Fig.
3A) but did
not express the CD11c molecule, as demonstrated by confocal
microscopy analysis (Fig.
3E).
To determine if the interfollicular region DC-SIGN
+ cells represented
plasmacytoid DCs, we performed double labeling with antibodies
to IL-3R and DC-SIGN (Fig.
3C) or with DC-SIGN-specific MAb
120507 (Fig.
3F). We did not observe a double-positive cell
population, indicating that plasmacytoid DCs do not express
DC-SIGN, at least in the tissues we have examined here. However,
DC-SIGN
+ cells and IL-3R
+ cells were often found in close proximity
to one another (Fig.
3C and F). Further, we analyzed the expression
of the viral coreceptor CCR5 since the absence of this molecule
is associated with a high degree of protection from HIV infection
(
20,
32). The human Peyer's patches contained very few CCR5
+ cells, and most CCR5
+ cells were found in the villous lamina
propria and not within the Peyer's patches (data not shown).
On the other hand, many CD4
+ T cells were found in the interfollicular
region and in the lamina propria of the villi (Fig.
3D). Within
the interfollicular region, we detected only a few cells that
coexpressed DC-SIGN and CD4. Thus, in human Peyer's patches,
DC-SIGN
+ DCs localize predominantly in the interfollicular region
and in clusters within the subepithelial dome region but do
not express appreciable levels of CD4 or the viral coreceptor
CCR5.
DC-SIGN+ cells are not found within the human vaginal epithelium.
To determine whether Langerhans' cells within the vaginal epithelium express DC-SIGN, we analyzed human and macaque vaginal tissues. In none of the tissues examined did we find DC-SIGN+ cells within the epithelium, despite the abundance of Langerhans' cells expressing Birbeck granules (recognized by the Lag antibody) and MHC class II (Fig. 4D). Instead, DC-SIGN+ cells were detected in the subepithelial lamina propria, and a small portion of these cells coexpressed CCR5 (Fig. 4A). A similar pattern of DC-SIGN expression has been described on subepithelial cells in the cervix, although no CCR5 expression on these cells was detected (11). Some of the DC-SIGN+ cells were found to be in close contact with CD4+ T cells in the lamina propria (Fig. 4B). All DC-SIGN+ cells were MHC class II+ as shown in the Peyer's patches, but only a subset of DCs in the lamina propria expressed DC-SIGN (Fig. 4C). We did not observe any DC-SIGNR-specific staining in the vagina (data not shown), indicating that staining detected by MAb DC4 is DC-SIGN specific in this tissue. Overall, the expression levels of DC-SIGN in the vaginal mucosa appeared to be significantly lower than that found in the gastrointestinal mucosa since we had to use higher magnification and longer exposures to detect DC-SIGN. In summary, Langerhans' cells in the vaginal mucosa did not express DC-SIGN, and only a very small portion of the subepithelial DC-SIGN+ cells expressed CCR5.
Human rectal mucosa contains abundant DC-SIGN+ cells.
By far the most remarkable expression of DC-SIGN was found in
the rectal mucosa (Fig.
5). DC-SIGN expression was found throughout
the lamina propria of the rectum, though there were no intraepithelial
DC-SIGN
+ DCs. Moreover, CCR5
+ cells were found to line the subepithelial
region near the lumen of the rectum. DC-SIGN
+ CCR5
+ cells formed
a narrow band of cells close to the lumen. We also noted that
the solitary lymphoid nodules found along the length of the
rectum contained abundant CD4
+ T cells but that DC-SIGN expression
was minimal except on rare cells in the perimeter within these
follicles (Fig.
5D). By confocal microscopy analysis, cells
which expressed all three molecules, CCR5, CD4, and DC-SIGN,
were found near the lumen of the rectum (Fig.
5E). Since MAb
DC4 used in these assays cross-reacted with DC-SIGNR, we examined
whether some of the staining observed represented DC-SIGNR.
We did not detect any DC-SIGNR-specific staining with MAb 120604
in the rectum (data not shown). Moreover, although numerous
capillary endothelial cells were present in the rectal tissues,
we did not detect any staining of endothelial cells with MAb
DC4 (Fig.
5F). In addition, MAbs DC4 and 120507 gave identical
staining patterns (data not shown), supporting the lack of DC-SIGNR
expression by the rectal endothelial vessels. Thus, the rectum
contains numerous DC-SIGN
+ cells in the lamina propria, some
of which coexpress CD4 and CCR5.
DC-SIGN expression in Peyer's patches, vagina, and rectum of rhesus macaque.
Infection of rhesus macaques with SIV or simian-human immunodeficiency
virus represents the most widely used animal model to study
HIV pathogenesis in vivo. A number of studies have utilized
this primate model of HIV to examine viral entry and infection
via mucosal surfaces (
16,
23,
33,
34,
35,
38). This model provides
the best system to study the role of DC-SIGN in virus transmission
and pathogenesis. We have previously shown that rhesus macaque
DC-SIGN binds and transmits all HIV-1, HIV-2, and SIV strains
tested (
3). However, the pattern of DC-SIGN expression in rhesus
macaque tissues is not known. To determine DC-SIGN expression
in rhesus macaques, we performed immunofluorescence staining
of tissue sections using MAbs that recognize rhesus macaque
DC-SIGN. Overall, the distribution of DC-SIGN in rhesus tissues
was very comparable to that found with humans. In rhesus macaque
Peyer's patches, DC-SIGN
+ cells were localized in the interfollicular
regions and sometimes in a cluster within the subepithelial
dome region (Fig.
6A). Although IL-3R expression on rhesus tissues
was very similar to that found with humans (data not shown),
the anti-human CD11c antibodies tested stained only a few cells
within primate Peyer's patches, making it difficult to demonstrate
CD11c
+ DC distribution (data not shown). As in humans, vaginal
epithelium did not contain DC-SIGN
+ Langerhans' cells, whereas
a subset of the subepithelial DCs expressed this molecule (Fig.
6B). Similar to what was observed for humans, the primate rectal
tissue contained abundant DC-SIGN
+ DCs, and the expression pattern
of CCR5 was indistinguishable from that found in the human rectum,
although somewhat fewer CD4
+ DC-SIGN
+ cells were found in the
rhesus macaque rectum than in humans. Here again we observed
DCs that were DC-SIGN
+ CD4
+ and DC-SIGN
+ CCR5
+ just beneath
the luminal epithelium (Fig.
6C and D).
Differential expression of DC-SIGN and DC-SIGNR in Peyer's patches.
Because MAbs DC4, DC11, and DC28 recognized both DC-SIGN and
DC-SIGNR, the staining pattern observed thus far could represent
either of these molecules, though expression of DC-SIGNR thus
far appears to be limited to certain types of endothelial cells
(
4,
27). In order to distinguish staining for DC-SIGN and DC-SIGNR,
we performed double labeling of tissue sections using MAbs specific
for DC-SIGN (120507) or DC-SIGNR (120604) (Fig.
7A and B). Although
there were rare double-positive cells observed inside some of
the endothelial vessels (Fig.
7A), confocal analysis of this
region ruled out the existence of cells that express both DC-SIGN
and DC-SIGNR (Fig.
7B). Similarly, we compared the staining
by MAb DC4, which recognizes the ectodomains of both DC-SIGN
and DC-SIGNR, to DC-SIGN-specific-MAb 120507 (Fig.
7C and D).
Cells which stained specifically for DC-SIGN were dendritic
in morphology and were identical to those stained with MAb DC4.
Strikingly, the only cells that stained with DC4 but not with
DC-SIGN-specific MAb 120507 were the endothelial cells found
in the interfollicular regions and within villous lamina propria,
confirming DC-SIGNR expression detected by MAb 120604 (Fig.
7C and D; red cells). By double labeling with a MAb specific
for endothelial cells, we determined that about 30% of all endothelial
cells in the Peyer's patches expressed DC-SIGNR (Fig.
8A and B).
Although capillary vessels composed of endothelial cells
were clearly present in the rectal and vaginal tissues, we did
not detect staining specific for DC-SIGNR with MAb 120604 as
described earlier (data not shown). Thus, DC-SIGNR expression
in the human Peyer's patches is limited to endothelial cells,
whereas DC-SIGN expression is restricted to DCs in the tissues
examined.
Lack of DC-SIGNR expression in rhesus macaque Peyer's patches.
To demonstrate DC-SIGNR expression in rhesus Peyer's patches,
we performed immunofluorescence labeling with human DC-SIGNR-specific
MAb 120604. In contrast to results obtained with human tissues,
we were not able to detect DC-SIGNR expression when using MAb
120604 on rhesus macaque tissues (data not shown). We hypothesized
that MAb 120604 may recognize an epitope in human DC-SIGNR that
is not conserved in the rhesus DC-SIGNR homologue. Thus, we
used MAb DC11, which recognizes rhesus DC-SIGN and cross-reacts
with human DC-SIGNR, to determine if DC-SIGNR is expressed on
endothelial cells in rhesus Peyer's patches. As shown in Fig.
8C and D, endothelial cells stained with anti-CD31 antibody
(green) did not stain with MAb DC11 (red). Thus, in rhesus macaque
Peyer's patches, no DC-SIGN or DC-SIGNR staining was detected
on endothelial cells with MAb DC11 and no DC-SIGNR-specific
staining was observed with MAb 120604. However, we have not
been able to clone rhesus macaque DC-SIGNR, and as a consequence
we cannot be certain that our MAbs, which recognize human DC-SIGN
and DC-SIGNR and cross-react with rhesus DC-SIGN, actually bind
to rhesus DC-SIGNR.
Rhesus DC-SIGN-dependent transmission of SIV is blocked by an anti-DC-SIGN MAb.
Our studies showed that DC-SIGN expression in rhesus macaques was similar to that observed with human mucosal tissues. To assess the importance of DC-SIGN for virus transmission and dissemination in vivo, specific antibodies or small molecules that block SIV or simian-human immunodeficiency virus interactions with rhesus DC-SIGN will be needed. To determine if the DC-SIGN-specific MAb 120507 could block transmission of SIV by rhesus DC-SIGN, we used an inducible T-Rex cell line that expresses rhesus DC-SIGN but does not express CD4 or CCR5. Thus, these cells cannot be infected by SIV. The DC-SIGN-positive cells were incubated with MAb 120507, the DC-SIGNR-specific MAb 120604, or the carbohydrate mannan, which has been shown to block HIV binding to human DC-SIGN. A luciferase reporter virus bearing the SIVmac239MER Env protein was then added for 3 h, the cells were washed vigorously to remove unbound virus, and CEMx174 target cells were added. Three days later the extent of virus infection was determined by measuring luciferase activity. As shown in Fig. 9, preincubation with MAb 120507 and mannan, but not with the DC-SIGNR-specific MAb 120604, reduced rhesus DC-SIGN-mediated virus transmission to background levels. Thus, MAb 120507 represents a specific immunological reagent that can be used to prevent SIV interactions with rhesus macaque DC-SIGN.

DISCUSSION
The precise role of DCs in mediating HIV-1 transmission and
pathogenesis at mucosal surfaces is unclear. In vitro, infection
of peripheral blood mononuclear cells by HIV-1 can be strongly
enhanced by including cocultured DCs (
7,
15,
17,
28,
41). While
DCs are often not infected by HIV-1, virus binds to DCs efficiently
and, once bound, can be retained in an infectious state for
a prolonged period of time (
11). Subsequent addition of susceptible
cell types results in infection by virus in
trans (
7,
15,
17,
28,
41). The ability of DCs to bind HIV-1 and to transmit bound
virus to receptor-positive cells has been linked to DC-SIGN
(
2,
3,
11,
26). These in vitro observations, coupled with the
fact that mucosal DCs are among the first cell types encountered
by HIV-1 at mucosal surfaces (
10,
16,
34,
36), raise the possibility
that HIV-1 interactions with DC-SIGN could impact sexual transmission.
To assess the role of DC-SIGN for virus transmission and pathogenesis, it will be important to develop reagents that specifically block virus binding to DC-SIGN. The abilities of such compounds to impact virus transmission could then be assessed in nonhuman primate models such as rhesus macaques, provided that rhesus DC-SIGN functions like human DC-SIGN and is expressed in a manner similar to that of its human homologue. We have recently shown that rhesus macaque DC-SIGN efficiently binds and transmits HIV-1, HIV-2, and SIV strains (3). In this study, we document the expression of DC-SIGN in mucosal tissues from rhesus macaques and identify a MAb that blocks SIV interactions with DC-SIGN in vitro. Our work extends previous studies by using MAbs specific for DC-SIGN and DC-SIGNR, by studying DC-SIGN expression in macaque tissues, and by using double and triple fluorescence labeling and confocal microscopy to correlate DC-SIGN expression with expression of CD4, CCR5, and specific DC subsets. The expressions of DC-SIGN in humans and in rhesus macaques were highly similar.
In the Peyer's patches, DC-SIGN+ cells were found predominantly in the interfollicular regions, with the exception of aggregates of DC-SIGN+ cells in the subepithelial dome region. It is now clear that there is considerable variation in the localization, phenotype, and function of different DC populations in various tissues. The human DC subsets in the peripheral blood have been categorized on the basis of differential expression of myeloid (CD11c) and plasmacytoid (IL-3R) markers (reviewed in reference 21). We show here that CD11c+ DCs are distributed in the dome and interfollicular regions and in the germinal center of the Peyer's patches, whereas the IL-3R+ plasmacytoid DCs are found exclusively in the interfollicular regions. Although all DC-SIGN+ cells expressed MHC class II molecules, we did not observe expression of DC-SIGN on IL-3R+ plasmacytoid DCs. This finding is consistent with a recent report of the lack of DC-SIGN mRNA expression by peripheral blood-derived plasmacytoid DCs (24). Thus, plasmacytoid DCs are unlikely to participate in HIV infection of CD4+ cells in trans, at least in a DC-SIGN-dependent manner. Further, CD11c+ DCs in the dome region were not found to express DC-SIGN. These data suggest that the DC-SIGN+ cells belong to another subset of DCs that express neither CD11c nor IL-3R within the Peyer's patches. Recently, a careful analysis of human tonsils revealed the presence of the CD11c- CD123- DCs, which expressed MHC class II and CD68 (37). When DC-SIGN expression on CD68+ cells was examined, we indeed observed that many of the dome region DC-SIGN+ cells coexpressed CD68 (unpublished observation). Yet, the majority of DC-SIGN+ cells in the interfollicular regions were CD68- CD11c- IL-3R- (unpublished observation). Future studies must address how these distinct DC subsets mediate virus transmission and immune induction during HIV-1 infection in vivo.
The epithelium covering the Peyer's patches over the dome contains M cells specialized in the uptake of luminal antigens and is known as the follicle-associated epithelium (19). HIV-1 has been shown to gain entry through M cells in rabbit and mouse Peyer's patches, although neither species supports efficient HIV infection (1). Thus, Peyer's patches are potentially the first sites of HIV entry during oral transmission in cases such as mother-to-child breastfeeding (9). Further, Peyer's patches may represent sites of HIV entry following rectal exposure since rectally injected materials have been shown to be processed and presented within Peyer's patches (5). The presence of clusters of DC-SIGN+ DC in the subepithelial dome region raises the possibility that these DCs may be the first to bind HIV-1 that enters the Peyer's patches via M cells or through tears in the mucosa. Since the dome region DC-SIGN+ DCs did not express the viral coreceptors CCR5 or CD4 at detectable levels, they are less likely to be directly infected by the virus but rather participate in infection of nearby CD4+ T cells. Aside from its HIV binding property, DC-SIGN has also been shown to interact with ICAM-3 expressed on resting T cells and mediate DC-T-cell conjugation (12). Thus, it is also possible that DC-SIGN+ DC aggregates in the dome region acts as the gatekeeper capable of capturing, processing, and presenting HIV antigenic peptides and intact virus to CD4+ T cells within this region. Indeed, CD4+ T cells were found in close proximity to the DC-SIGN+ cells of the subepithelial dome region (Fig. 3D).
In the vaginal mucosa, DC-SIGN expression was not detected within the squamous epithelial layer, but it was detected on rare DCs in the subepithelial lamina propria, comparable to what has been reported for the cervix (11). A small percentage of these cells expressed CCR5. In contrast, DC-SIGN+ DCs were abundantly distributed throughout the lamina propria immediately adjacent to the epithelium of the rectal mucosa. Moreover, DC-SIGN+ DCs that coexpressed the HIV-1 coreceptors CCR5 and CD4 formed a narrow band beneath the rectal luminal epithelium. These data are in discordance with previous studies showing that CCR5 is not expressed by mucosal DCs. Hladik and colleagues demonstrated that, unlike peripheral blood-derived DCs, cervicovaginal DCs express undetectable CCR5 but form stable conjugates with T cells which permit productive infection by HIV-1 (15). More recently, Geijtenbeek et al. reported that CCR5 is not expressed in the rectum or uterus (11). These apparent differences may be attributed to the sensitivities of the assays. For instance, we had to amplify the staining signal using the Tyramide amplification system in order to visualize CCR5 on these sections. Since only low levels of CCR5 are needed to support entry by many virus strains, especially if CD4 is expressed at high levels, the levels of CCR5 detected here could be relevant for virus infection (25). Further, as discussed above, we found CCR5 expression to be restricted to the lamina propria immediately beneath the luminal epithelium in the rectum. Thus, the ability to detect CCR5+ cell population in the rectum may also depend on the tissue orientation. By the same token, it is possible that CCR5 is expressed by Peyer's patch DCs below the limit of detection by our immunofluorescence protocol, since CCR5 expression in the gut-associated lymphoid tissue has been demonstrated by a much more sensitive flow cytometry analysis (39). Thus, future studies are needed to examine the virological relevance of the levels of the HIV coreceptors expressed by each DC subset during in vivo infection.
The examination of the relevance of mucosal DCs for the trans infection of T cells must also take into consideration the level of DC-SIGN expressed by these cells. Not only are the tissue distributions of DC-SIGN+ cells different in the vaginal and rectal mucosae, but also the levels of DC-SIGN expression by DC in these tissues appeared to differ considerably. We have recently demonstrated that the efficiency of HIV-1 binding and transmission is strongly dependent on the level of DC-SIGN expression (26). Although we were not able to estimate the level of DC-SIGN expressed on mucosal DCs due to the nonquantitative nature of the immunofluorescence technique, such measurements may prove useful in predicting the efficiency with which DCs in different mucosal sites can transmit HIV-1 to T cells in vivo.
Since most MAbs against DC-SIGN also detect DC-SIGNR, it was important to distinguish whether the staining was specific for DC-SIGN or DC-SIGNR. In previous studies, DC-SIGNR has been detected on endothelial cells in placenta, liver, and lymph nodes (4, 27). By producing a DC-SIGNR-specific MAb, we were able to show that DC-SIGNR is also expressed on about one-third of the capillary endothelial cells in the human ileum. In contrast, no DC-SIGNR-specific staining was observed in rhesus tissues. The MAb DC11, which recognizes the ectodomain of human and rhesus DC-SIGN and cross-reacts with human DC-SIGNR, did not stain rhesus endothelial cells. In fact, double labeling of rhesus Peyer's patches with MAb DC11 and DC-SIGN-specific MAb 120507 resulted in an identical staining pattern (data not shown), suggesting that MAb DC11 detects only DC-SIGN in rhesus tissues. Thus, either the rhesus DC-SIGNR homologue is distinct in the regions recognized by MAb 120604 and MAb DC11, it is not expressed in the tissues we examined, or no homologue for DC-SIGNR exists in rhesus macaques.
If DC-SIGN plays a role in virus transmission, then the accessibility of DC-SIGN+ DCs within different mucosal exposure sites would likely influence the efficiency of this process. During vaginal exposure, virus must somehow cross the epithelial layer to reach DC-SIGN+ DCs in the subepithelial lamina propria. It is interesting that progesterone treatment, which results in thinning of the vaginal epithelium, has been shown to contribute to a higher incidence of vaginal SIV transmission in rhesus macaques (22). Hormonal influences and microbial flora within the vaginal mucosa that result in thinning of the epithelium may potentially enhance the ability of HIV to gain access to the DC-SIGN+ cells in the lamina propria. In contrast, DC-SIGN+ CD4+ CCR5+ DCs in the rectal mucosa are separated from the lumen by only a single columnar epithelium, which should allow virus access to DC-SIGN+ DCs more easily. This hypothesis is corroborated by the fact that the HIV-1 transmission risk is greater in anal intercourse than in vaginal coitus in women (40).
In summary, DCs expressing DC-SIGN were distributed similarly in the mucosal surfaces of humans and rhesus macaques. The physical barriers that exist between the lumen and the closest DC-SIGN+ DCs were greatest in the vaginal mucosa and least in the rectum. An intriguing finding, however, is that the DC-SIGN+ DCs located near the lumen of the rectum were also positive for CD4 and CCR5. These DCs may not only ferry HIV to the draining lymph nodes but also become infected within the rectal mucosa and form a local viral factory in DC-T-cell conjugates. Further understanding of this viral adhesion molecule may provide insight into its potential use in novel preventative microbicidal agents against HIV transmission.

ACKNOWLEDGMENTS
We thank Ken Price, Tracey Crisp, and Jim David for their assistance
in the production and characterization of the monoclonal antibodies
and Charles Dela Cruz for editorial assistance.
A.I. was supported by a Burroughs Wellcome Fund Career Award in Biomedical Sciences and by the American Foundation for AIDS Research. F.B. was supported by a fellowship from the Swiss National Science Foundation (grant 823A-611772). S.P. was supported by a fellowship from the Deutsche Forschungsgemeinshaft. R.W.D. was supported by NIH grants AI35383 and 40880, a Burroughs Wellcome Fund Transnational Research Award, and an Elizabeth Glaser Scientist Award from the Pediatric AIDS Foundation.

FOOTNOTES
* Corresponding author. Mailing address: Department of Epidemiology and Public Health, 60 College St., LEPH 716, New Haven, CT 06510. Phone: (203) 785-2919. Fax: (203) 785-7552. E-mail:
akiko.iwasaki{at}yale.edu.


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Journal of Virology, February 2002, p. 1866-1875, Vol. 76, No. 4
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.76.4.1866-1875.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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