By
From the * Sir William Dunn School of Pathology, Oxford OX1 3RE, United Kingdom; and the Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis,
Missouri 63110
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Abstract |
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The mannose receptor (MR) has established roles in macrophage (M) phagocytosis of microorganisms and endocytic clearance of host-derived glycoproteins, and has recently been implicated in antigen capture by dendritic cells (DCs) in vitro. MR is the founder member of a family of homologous proteins, and its recognition properties differ according to its tissue of origin.
Given this heterogeneity and our recent discovery of a soluble form of MR in mouse serum,
we studied the sites of synthesis of MR mRNA and expression of MR protein in normal mouse
tissues. We demonstrate that synthesis and expression occur at identical sites, and that mature
M
and endothelium are heterogeneous with respect to MR expression, additionally describing
MR on perivascular microglia and glomerular mesangial cells. However, MR was not detected
on DCs in situ, or on marginal zone or subcapsular sinus M
, both of which have MR-like
binding activities. We also compared expression of MR to the binding of a recombinant probe
containing the cysteine-rich domain of MR. We show that MR and its putative ligand(s) are
expressed at nonoverlapping sites within lymphoid organs, consistent with a transfer function
for soluble MR. Therefore, in addition to endocytic and phagocytic roles, MR may play an
important role in antigen recognition and transport within lymphoid organs.
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Introduction |
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The mannose receptor (MR)1 was first identified as a
specific uptake system in rat liver Kupffer cells for
mannosylated/N-acetylglucosamine-terminal and fucosylated
neoglycoproteins in vivo (1). Later studies demonstrated
similar carbohydrate-specific binding by hepatic endothelium (2), alveolar macrophages (M) (3), resident and elicited peritoneal M
(4), human monocyte-derived M
(5), and cultured bone marrow-derived M
(6). These were
attributed to the MR by Ab reactivity in Western blots of
purified receptor. MR has also been purified from human
retinal pigment epithelium (7). M
expression of MR appears restricted to mature populations, is downregulated
during classical activation, such as in response to IFN-
(8),
and is upregulated during an alternative form of activation
by IL-4 characterized by enhanced MHC class II (MHCII) expression and reduced proinflammatory cytokine production (9). More recently, MR has been detected on cultured
human dendritic cells (DCs) matured from CD14+ peripheral blood monocytes (10) and cord blood CD34+ hemopoietic progenitors with GM-CSF and IL-4 (11), although it is not known how closely these cells reflect the properties of DCs in situ. Freshly isolated murine Langerhans cells
do not appear to express MR protein, although uptake of
mannose-BSA and a mannan-inhibitable component of zymosan phagocytosis have been documented (12). In contrast, functional MR has been detected on freshly isolated
human Langerhans cells (13). Uncharacterized receptors
with similar binding activity to MR have been detected in
lymph node subcapsular sinus M
of mouse (14) and rat (15) and splenic marginal zone M
of mouse (16) and rat (15).
The early studies on liver and mature M suggested two
major functions for MR, in endocytic clearance of host-
derived glycoproteins and phagocytosis/endocytosis of microorganisms and soluble ligands, and evidence has accrued
in support of both roles. MR mediates uptake of ligands for
the purposes of both homeostasis and immunity. Homeostatic functions include uptake of tissue plasminogen activator (17, 18) and lysosomal hydrolases (3). MR also plays a major role in host defence. It is now widely accepted that
the recognition and phagocytosis of many nonopsonized
microorganisms, including bacteria, fungi, and protozoa by
M
, is mediated by MR, through interactions with polysaccharide components of fungal cell walls such as yeast
mannan, bacterial capsules, and some strains of LPS and lipoarabinomannan (19). Transfection of nonphagocytic COS
cells with MR cDNA is sufficient to confer an ability to
recognize and phagocytose Candida albicans (20) and Pneumocystis carinii (21). Ligation of MR in M
causes intracellular signaling resulting in functional changes, including
increased superoxide anion release (22) and induction of
cytokine synthesis (23). The immunological roles of MR
may extend to specific immunity if the observed MR- mediated uptake of glycoconjugates by cultured human
DCs for efficient presentation to T cells by MHCII (24)
and CD1b (27) prove to have in vivo correlates.
At a biochemical level, polysaccharide recognition has been attributed to cooperative, calcium-dependent binding of the sugar moieties mannose, fucose, and N-acetylglucosamine by several of the eight C-type lectin domains within the ectodomain of MR. Carbohydrate recognition domains 4-8 show affinity for natural ligands comparable to that of MR itself (28). The phagocytic and endocytic activity is mediated by a 45-amino acid cytoplasmic tail and transmembrane domain (20). MR also contains a cysteine-rich domain (CR) with sequence similarity to the plant lectin Ricin B at the NH2 terminus and an adjacent fibronectin type II-like domain (29).
Our recent discovery of ligands of CR in mouse secondary lymphoid organs gave the first indication of a function
for CR, the domain of MR most highly conserved between mice and humans (30). Tissues were probed with a
chimeric probe consisting of CR fused to the Fc region of
human IgG1, CR-Fc. Binding of CR-Fc to spleen marginal metallophilic M and undefined cells in B cell areas, and to lymph node subcapsular sinus M
, was observed in
naive animals, and a time-course study of a secondary immune response indicated apparent migration of CR-binding cells from the subcapsular sinus to sites of developing
germinal centers. This suggested that MR could be directed
to areas where affinity maturation of B cells occurs. We have
recently purified ligands of CR-Fc from spleen and identified among these novel glycoforms of sialoadhesin (Sn) and
CD45 (Martínez-Pomares, L., our unpublished results).
We have also documented the existence of a soluble
form of MR (sMR) which may act as a mobile antigen
capture protein for delivery to the marginal zone of spleen
and lymph node subcapsular sinus, as well as to primary and
secondary B cell follicles (31). sMR is generated by proteolysis of MR from cultured M and is shed into the media
where it retains calcium-dependent mannosyl binding activity. Immunoreactive sMR also occurs naturally in serum.
The roles of MR outlined above have all been assigned to
a functionally homogeneous MR, but Fiete and Baenziger
have recently revealed tissue heterogeneity in MR and a
new lectin activity of CR. They identified a receptor within
rat liver that recognizes and internalizes lutropin hormone
bearing Asn-linked oligosaccharides terminating in SO4-4-GalNAc1,4GlcNAc
1,2Man
(S4GGnM), with structural and antigenic properties similar to MR, although MR
purified from lung did not recognize S4GGnM (32). A
protein with the same properties as this receptor could be
generated from the same cDNA as MR, and the ability to
bind galNAc-4-SO4 appeared to be determined posttranslationally (33). The galNAc-4-SO4 binding site was then
localized to the CR by binding studies of deletion mutants of MR (34).
In addition to heterogeneity within MR, a wider family of molecules with the same basic structure as MR exists. These are the phospholipase A2 receptors (35, 36), DEC-205 (37, 38), and a novel lectin (39). Each has CR and fibronectin type II-like domains, and either 8 or, in the case of DEC-205, 10, C-type lectin-like domains.
More specific methods to detect in situ expression of
MR are required, given the heterogeneity of MR and the
limitations of ligand-binding assays. Several mAbs recognizing human MR have recently been developed. Uccini
et al. (40) demonstrated MR expression in various reticulo-endothelial tissues and in neoplasms of possibly endothelial
origin. MR was detected in resident tissue M, including
those in spleen red pulp, lymph node paracortex, and thymus cortex. Sinus lining cells of spleen and lymph node also
expressed MR and coexpressed M
and endothelial markers. Noorman et al. (41) surveyed MR antigen in human
tissue, with broadly similar results. In mouse, Takahashi
and co-workers surveyed MR protein in a range of tissues
from fetal development to the adult, revealing expression
in M
and some endothelial cells, although a precise definition of most of these cell types was not attempted (42).
Given the heterogeneity of MR and the existence of
other MR family members, we studied expression of MR
in the normal adult mouse by both in situ hybridization
(ISH) and immunocytochemistry (ICC). In lymphoid organs, we used double ICC to define the phenotype and location of cells expressing MR. We compared MR with
markers of M, DCs, and endothelium, and with the binding of CR-Fc. We found MR by ISH and ICC at identical
locations, in subsets of M
and endothelium; no expression
was seen in Langerhans cells, other DCs, cells that express
putative CR ligands, or in sites in spleen and lymph node
that express mannosyl ligand binding activity. Cells at these
sites therefore may express novel MR-like receptor(s).
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Materials and Methods |
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Animals.
BALB/c and sv/ev129 mice were bred at the Sir William Dunn School of Pathology, and males and females were used at 8-10 wk of age.Abs and Fc Chimeric Protein.
Primary Abs used in this study are described in Table I. MR polyclonal Abs raised against MR purified from the J774e cell line and mAbs F4/80, FA.11, and 3D6 were prepared in-house. CR-Fc, a recombinant protein consisting of the CR of mouse MR fused to the Fc region of human IgG1, was also prepared in our laboratory (30). The ERTR-9 mAb was a gift of Dr. C.D. Dijkstra (Free University, Amsterdam, The Netherlands). Other Abs were purchased as shown. N418 was biotinylated in-house for direct detection. The secondary Abs, biotinylated goat anti-rabbit IgG and biotinylated rabbit anti-rat IgG, were purchased from Sigma Chemical Co. Biotinylated mouse anti-human IgG (Fab')2 was purchased from Jackson ImmunoResearch Labs.
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ICC.
Organs were collected and immersed in OCT compound (BDH Chemicals-Merck) and frozen in dry ice-cooled isopentane. Frozen sections were cut at 5 µm, air-dried for 1 h, and stored atISH.
The probe templates were generated by subcloning regions of MR and Sn cDNA into pBS SK+/ ![]() |
Results |
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The specific recognition of MR may be hampered by
the tissue heterogeneity of MR, the potential cross-reactivity of reagents with other members of the family of proteins
with which it shares homology, and the existence of other
receptors that share a similar ligand recognition profile. We
have used two independent methods to detect MR specifically: ISH to define mRNA and therefore sites of synthesis,
and ICC to define protein expression in a wide range of
organs of normal adult mice. Specificity of mRNA detection was confirmed by performing control ISH with sense
strand probes. The specificity of the polyclonal anti-MR
Ab was examined by Western blotting of tissue lysates.
There was some tissue-specific heterogeneity with respect
to apparent molecular weight in the protein detected, but
in the absence of anti-MR, no signal was detected (not
shown). The treatment of tissues for ICC was mild, allowing double ICC detection of MR with markers of M, DCs,
and endothelium to define expression more closely. Of
particular interest, we used double ICC in lymphoid organs
to compare the expression of MR with that of the putative
CR ligand, Sn, and other ligands of CR-Fc.
MR Expression in Lymphoid Organs: MR and Sn Expression by ISH
Peripheral Lymph Node.MR mRNA expression was seen
in the medullary cords (Fig. 1 A, arrow). The subcapsular sinus was clearly negative (Fig. 1 A, arrowhead), although this
site and the medulla were strongly labeled by the Sn probe
(Fig. 1 B). Control sections hybridized with sense probes of
MR (Fig. 1 C) and Sn had low background (Fig. 1 D). Although Sn is expressed by medullary and subcapsular sinus
M, only the latter bear ligands of CR-Fc (30). Together
these data clearly indicate that MR and its Sn ligand are not
produced concurrently in the lymph node.
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MR expression was observed throughout the red pulp by ISH, but appeared to be absent from the marginal zone and white pulp (Fig. 1 E). The marginal metallophilic zone was readily identified by its high expression of Sn (Fig. 1 F). The control sections for MR and Sn, respectively, probed with sense strand RNAs, had no significant background (Fig. 1, G and H). Although precise anatomical localization of the sites of synthesis was not possible by this method, these data are highly suggestive of MR and Sn synthesis occurring at distinct sites.
Thymus.Discrete cells were labeled with MR probe within the thymus (Fig. 1 I), with very little background in the control (Fig. 1 J). Comparison with expression of Sn is not informative, since thymic Sn is not a ligand of CR-Fc.
MR Expression in Lymphoid Organs: MR Expression by ICC;
Comparison with Markers of DC, M, and Endothelial Cells
MR antigen was found on medullary M (m), sinus lining M
, and endothelium of the
marginal sinus (ms), but not in T cell areas (t) (Fig. 2 A). No
detectable background staining was found in these areas in
the absence of MR Ab (Fig. 2 B). In contrast, DEC-205 was detected on interdigitating cells throughout the T cell
areas of an adjacent section (Fig. 2 C). By double ICC, Sn
and MR were shown to colocalize in medullary M
(m)
(Fig. 3 A), but only Sn could be detected on the subcapsular sinus M
(Fig. 3 A, arrow). By contrast, no colocalization of CR-Fc and MR could be detected, CR-Fc being
confined to subcapsular sinus M
and some germinal center cells (Fig. 3 B, arrowhead). Scattered cells expressing both MHCII and MR were detected in lymph nodes (Fig.
3 C) in the paracortex bordering the B cell follicle defined
by reactivity to anti-IgM (Fig. 3 D). These cells did not express DC markers DEC-205 or CD11c, nor did they express the M
marker F4/80 (not shown). The elongated
morphology of MR staining cells and location in the marginal sinus are characteristic of lymphatic endothelial cells,
but expression of MR was restricted to a CD31
population, indicating that high endothelial venules do not express MR (Fig. 3 E). MR+ endothelial cells (arrow) did not
coexpress the M
marker macrosialin, detected with mAb
FA.11, although the intimately associated sinus lining M
(arrowhead) expressed both of these markers (Fig. 3 F).
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Single immunostaining of MR in spleen revealed expression in red pulp (rp) M but not in the white
pulp (wp) or marginal zone (mz) (Fig. 2 D), while background staining in the absence of primary Ab was not detected (Fig. 2 E). This is in contrast to the expression of
CD11c by DCs at the border of the white and red pulp
(Fig. 2 F) and other DC subsets of the white pulp that are
detectable with Abs to MHCII and DEC-205 (not shown).
Expression of Sn, a CR-Fc ligand in spleen, was compared
with that of MR by double ICC (Fig. 3 G). Sn alone was
detected in marginal metallophilic M
, and there was additional low level expression in red pulp M
along with strong
expression of MR. In contrast, CR-Fc bound to splenic
marginal metallophilic M
, but not red pulp M
(Fig. 3 H).
Therefore, MR and its putative ligand are expressed at
nonoverlapping sites, separated by a clear region in the
outer marginal zone. The absence of MR expression in this
compartment was confirmed by double staining with
ERTR-9, an mAb that specifically recognizes M
of the
outer marginal zone (Fig. 3 I). Double ICC for MR and macrosialin with FA.11 defined two subsets of MR+ cells,
double-positive M
(Fig. 3 J), and elongated venous sinus endothelial cells which did not express macrosialin (Fig. 3 J, arrow).
ICC revealed two distinct populations of cells
that express MR. These were highly stained flattened M
lying beneath the capsule and along the connective tissue
septa that penetrate the cortex (not shown), and less intensely stained M
with fine processes that were found
throughout the cortex (c) and the corticomedullary junction (cmj) (Fig. 2 G). Staining of M
in the medulla (m)
was very weak or negative (Fig. 2 G). A control section did
not reveal any detectable background staining (Fig. 2 H).
Cells expressing MR were quite distinct from the DEC-205-expressing cortical epithelial cells which have extensive dendrites, and the few rounded interdigitating cells of
the medulla (Fig. 2 I). It seems likely that all MR+ cells in
the thymus are M
. By double ICC it is apparent that most
of them coexpress the M
marker, F4/80 (Fig. 3 K). As in spleen and lymph node, double staining with CR-Fc in
thymus revealed that MR and CR-Fc ligand(s) are expressed by two distinct populations of cells (Fig. 3 L). CR-Fc bound to large undefined cells of the medulla which may
be part of the thymic epithelium.
MR antigen appeared to be confined to
the lymphatic endothelium of interfollicular areas (i) and
was notably absent in follicles (f) (Fig. 2 J). M and DCs in
the interfollicular areas and follicles of an adjacent section
that were identified with FA.11 did not express MR (Fig.
2 K). Control sections of Peyer's patch gave no background
signal (not shown).
MR was detected in dermal M, but not in epidermal Langerhans cells (Fig. 2 L). In contrast, F4/80
stained both M
(Fig. 2 M) and Langerhans cells (Fig. 2 M,
arrow). Again, no nonspecific staining was seen in control
sections (not shown). We could not detect MR in isolated
epidermal sheets of normal mice, using either the ICC
method presented here, or the method of Takahashi and co-workers (42; data not shown).
MR Expression in Nonlymphoid Organs
We confirmed by ISH and ICC previous studies demonstrating expression of MR in hepatic endothelium and M
of liver (Kupffer cells), gut, lung, and resident tissue M
of
other organs (not shown). We describe here the novel finding of MR in perivascular microglia of brain and glomerular mesangial cells of kidney.
M and related cells of the brain perform specialized functions in tissue homeostasis, inflammation, and
maintenance of the blood-brain barrier. They are phenotypically, functionally, and morphologically distinct, and
thus deserve special attention in their expression of MR. In
addition, previous studies have suggested a role for an MR
on vascular endothelium in regulating blood-brain barrier
function. We observed that meningeal M
express MR by ISH (Fig. 4 A) and ICC (not shown). Perivascular microglia also express MR, but adjacent vessel endothelium does
not, as shown by ISH (Fig. 4 B) and ICC (Fig. 4 C). Confirmation that these cells are perivascular microglia was deduced from their expression of F4/80 (Fig. 4 D). No signal
was detected in the meninges or brain parenchyma in control sections examined by ISH or ICC (not shown). Like
perivascular microglia, astrocytes are also associated with
vessels, whereas more differentiated microglia are deeper in
the parenchyma. Neither of these cell types appeared to express MR in normal brain (Fig. 4, B and C).
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MR was observed in kidney glomeruli, both
by ISH (Fig. 5 A) and ICC (Fig. 5 B) in repeated experiments. Control sections for ISH (Fig. 5 C) and ICC (Fig. 5 D)
have low background, verifying the authenticity of these
observations. An example of a glomerulus stained for MR
and observed at high magnification indicated that expression is present on the mesangial cells (Fig. 5 E). No expression of M markers F4/80, FA.11, or Sn was observed on
glomeruli, nor did they bind CR-Fc (not shown).
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Discussion |
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We have used two independent methods to examine the
expression of MR mRNA and protein in normal adult
mouse. In all tissues studied, sites of synthesis of mRNA,
examined by ISH, and expression of antigen, detected by
ICC, were identical, suggesting that protein transfer between
cells did not contribute to the staining observed. Both methods demonstrated MR expression by subsets of M and endothelial cells. We confirmed previous studies of mature
M
labeling in mice (42) and humans (40, 41), although a
closer analysis of M
in spleen and lymph node revealed an
unexpected anomaly. An MR-like binding activity has been
described on marginal zone M
in mouse (16) and rat (15)
spleen. Similarly, the subcapsular sinus M
of the lymph
node have also been documented as having an MR-like
binding activity in mice (14) and rats (15), although no specific carbohydrate receptor has been characterized structurally or antigenically in either case. Here we demonstrate
clearly that MR is not responsible for these activities. Double ICC staining of MR with the marginal zone M
marker ERTR-9 confirmed that the cells of the marginal
zone do not express MR. Similarly, we did not detect MR
on subcapsular sinus M
by ISH or ICC. These binding activities, which are described as calcium dependent and of
high affinity for ligands such as a linear
-1,2-linked tetramannose from C. albicans (16), Trypanosoma cruzi amastigotes (14), mannose/fucose/N-acetylglucosamine-BSA (15),
and mannan (43) must therefore be mediated by some additional unknown receptor(s).
We found endothelium to be heterogeneous with respect to expression of MR. MR was detected on endothelial cells of spleen red pulp and liver, whereas blood vessel and high endothelial cells were negative. However, lymphatic endothelium appeared to express MR widely, consistent with a constitutive function, possibly endocytosis. Our finding contrasts with that in humans, in which lymphatic endothelium appeared negative, although coexpression of MR with endothelial markers CD31, VE-cadherin, and von Willebrand factor was observed in sinus lining cells of the spleen and lymph node (40). There may be additional phenotypic differences between human and murine endothelial cells. In contrast to humans, we noted the absence of CD31 expression by lymphatic endothelium and sinus lining cells of murine lymph node. Further studies are needed to establish the functional significance of heterogeneity in MR expression by selected vascular and lymphatic endothelium in different species.
MR has been implicated in T cell immunity, after the discovery of its expression on cultured human blood monocyte-derived DCs (24) and on DCs expanded from cord blood hemopoietic progenitors (11). Isolated DCs use MR to endocytose mannosylated ligands for presentation to T cells by MHCII (24) and CD1b (27). MR-mediated antigen uptake confers a greatly enhanced efficiency of presentation to T cells, of the order of 100 (25) and 200-10,000-fold (26). MR may be a marker of immature DCs, since it is downregulated in vitro by inflammatory stimuli (10). However, we found no expression of MR on DCs in vivo in thymus, lymph node, spleen, and Peyer's patch of normal mice. In particular, the CD11c+ cells of the spleen, which are thought to represent an immature population of myeloid-derived DCs, did not express MR. Likewise, we did not observe expression of MR by resting Langerhans cells of skin epidermis. This observation is consistent with the study by Reis e Sousa et al. (12), in which MR could not be detected on lysates of purified murine Langerhans cells by Western blotting, although a mannose-specific uptake by these cells was identified. Similarly, ICC studies in human tissue did not detect expression of MR in Langerhans cells (40, 41), although freshly isolated Langerhans cells did express functional MR (13). We did detect a subpopulation of MR+ cells of lymph nodes in the T cell areas bordering the B cell follicles which express MHCII, but these are unlikely to represent a known population of DCs, as they did not express DEC-205 or CD11c (not shown). Further studies are required to determine whether MR is expressed by DCs after immunization, and to characterize the mannose-specific binding activity of Langerhans cells, which may be due to a distinct receptor. The apparent lack of expression of MR on resting murine DCs in situ should be cautionary for those working on cultured DC populations.
We compared expression of MR with that of the putative
endogenous ligand(s) of the CR, those that bind CR-Fc.
Previously we hypothesized that a soluble form of MR or
MR+ cells may interact with CR-Fc binding cells of spleen
marginal metallophilic M, lymph node subcapsular sinus
M
, and germinal center cells (31). This would allow
transfer of MR-bound carbohydrate antigen to cells strategically positioned at sites of generation of B cell responses to
carbohydrate antigens. Here, we show that cells that bind
CR-Fc in spleen and lymph node do not coexpress MR; indeed, the receptor and the ligand(s) are at spatially distinct sites within these organs, consistent with a transfer function via sMR. Although we did not detect sMR bound to the
subcapsular sinus M
or marginal metallophilic M
, it may
be present at levels below detection or may depend on immune stimulation. Intriguingly, we also observed scattered
CR-Fc binding cells in the thymic medulla, where a role in
capture of antigen-laden sMR would be unexpected. Thymic epithelial cells synthesize a variety of glycoprotein hormones (44), and our recombinant protein may recognize one
of these in the thymus. Our CR-Fc, like the CR-Fc prepared
by Fiete and co-workers (34), binds to bovine lutropin hormone, a glycoprotein bearing terminal galNAc-4-SO4 (Linehan, S.A., and L. Martínez-Pomares, unpublished data).
We have made a wider survey of MR expression than
had previously been undertaken, including brain and kidney. We identified MR expression in perivascular microglia of murine brain by ISH and ICC. Perivascular microglia lie on the parenchymal side of arterioles, and MR at
this location may be appropriately placed to endocytose
glycoproteins that have traversed the blood-brain barrier.
These specialized M also express class A scavenger receptors and take up modified low density lipoprotein injected
into the blood or cerebral ventricles (45). Those authors
also showed that horseradish peroxidase, a known ligand of
MR, can be endocytosed by perivascular microglia (45). In
another study, liposomes labeled with mannose passed
through the murine blood-brain barrier more efficiently
than those labeled with fucose or galactose (46). Similarly,
the ependymal cell layer lining the cerebral ventricles regulates solute transport between the cerebrospinal fluid and brain tissue, and in rat this can be dissociated by mannose-
but not glucose- or galactose-BSA (47). However, we
found that neither the ependymal cells nor the endothelial
cells of the blood-brain barrier expressed MR. Astrocytes
and more differentiated microglia of the parenchyma do
not express MR in normal brain. Both of these cell types
have a tendency to upregulate various M
markers when cultured in vitro or stimulated in vivo, so a definitive study of their phenotype requires further in situ analysis.
We also demonstrated expression of both MR mRNA
and protein in glomerular mesangial cells of the kidney in
situ. The glomerulus is the site at which blood is first filtered in the kidney. MR mRNA and protein have been
observed on in vitro-cultured mouse mesangial cells stimulated with the inflammatory cytokines TNF- and IL-1
,
but were absent from unstimulated cells (48). An endocytic
role for MR on mesangial cells is consistent with clearance of the MR ligand COOH-terminal propeptide of type 1 procollagen labeled with nondegradable 125I-tyramine-cellobiose, in which 20% of the label was found in the kidneys
while 70% was recovered from liver (49). Glomerular mesangial cells share some features of the reticulo-endothelial system, including the ability to phagocytose apoptotic cells
(50, 51). Cultured human mesangial cells also express components of NADPH oxidase (52) and Fc
RIII and Fc
RI
chain (53). However, murine mesangial cells lacked all of the
M
markers used in this study apart from MR (not shown),
and are not believed to share a common lineage with hemopoietic and endothelial cells, which can both be generated from embryonic mesodermal cells (54, 55). Another cell
type that is not hemopoietic or endothelial, but has been reported to express MR, is retinal pigment epithelium (7). Although the expression of MR in myeloid cells appears to be
regulated by the transcription factors PU.1 and Sp1 (56), the
detection of MR in mesangial cells and retinal pigment epithelium suggests that other transcription factors must be involved in these distantly related cell types.
In conclusion, we have characterized murine MR expression in situ in subsets of M and endothelial cells, but
not DCs, describing novel expression in perivascular microglia and renal mesangial cells. We demonstrate that
MR-like binding activities of spleen marginal zone M
and lymph node subcapsular sinus M
, and possibly Langerhans cells, in situ are not due to MR. The expression pattern of MR in lymphoid organs is consistent with a
model of antigen capture by MR and transfer to sites of
anticarbohydrate immunity by a soluble form of MR that
may recognize cells at these sites by their expression of
ligands of the cysteine-rich domain of MR. Overall, the MR
is widely expressed by distinct cell types involved in potential clearance functions. Further studies are needed to investigate the regulation of MR expression by these cells
and the posttranslational modification of MR protein in different tissue microenvironments, as well as to characterize other MR-like activities.
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Footnotes |
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Address correspondence to Siamon Gordon, Sir William Dunn School of Pathology, South Parks Road, Oxford OX1 3RE, UK. Phone: 44-1865-275534; Fax: 44-1865-275515; E-mail: Christine.Holt{at}pathology.oxford.ac.uk
Received for publication 26 March 1999.
We thank Dr. Ann Harris and Dr. Colm Reid (Institute of Molecular Medicine, Oxford, UK) for assistance with setting up the ISH protocol, and Dr. Paul Crocker (Dundee University, Dundee, UK) for providing Sn cDNA. We thank Dr. Christine Dijkstra (Free University, Amsterdam, The Netherlands) for providing ERTR-9 mAb. We are also grateful to Mrs. Liz Darley for preparation of tissue, and Mr. Lance Tomlinson for photography.
This work was supported by grants from the Arthritis Research Campaign and the Medical Research Council, UK.
Abbreviations used in this paper
CR, cysteine-rich domain of MR;
DC, dendritic cell;
ICC, immunocytochemistry;
ISH, in situ hybridization;
MHCII, major histocompatibility complex class II;
M, macrophage(s);
MR, mannose receptor;
Sn, sialoadhesin;
sMR, soluble MR.
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References |
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