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Infection and Immunity, August 2000, p. 4391-4398, Vol. 68, No. 8
Laboratoire de Mycologie Fondamentale et
Appliquée, INSERM E9915, Faculté de Médecine,
Pôle Recherche, 59037 Lille Cedex, France
Received 31 January 2000/Returned for modification 10 March
2000/Accepted 26 April 2000
The use of molecules derived from
the yeast cell wall has led to the elucidation of both binding and
signaling roles for lectin receptors present on macrophage membrane
(48). Based on the use of mannan or zymosan from
Saccharomyces cerevisiae, two macrophage surface proteins
involved in carbohydrate recognition have been characterized. The
macrophage mannose receptor (MMR) (47), a 175-kDa
calcium-dependent lectin, has been described as a receptor specific for
Both MMR and CR3 are involved in the phagocytosis of unopsonized,
heat-killed S. cerevisiae yeasts by murine macrophages
(18). Binding and phagocytosis of Candida
albicans yeasts also involve the MMR (10, 32), and
C. albicans stimulates macrophage secretory activities
through its mannan and the binding of In contrast to S. cerevisiae mannan, C. albicans
mannan displays in its acid-labile fraction Reagents.
Except for reagents of stated origin, all reagents
were obtained from Sigma-Aldrich Chimie, Saint Quentin Fallavier,
France. The monoclonal antibody (MAb) AF1, a murine immunoglobulin M
(IgM) to C. albicans Yeast cells and oligomannosides.
The VW32 strain of C. albicans (serotype A) (9) and the SU1 strain of
S. cerevisiae (45) were used throughout the
study. Yeast cells were maintained at 4°C. Before experiments, yeast cells were cultured for 24 h at 28°C on Sabouraud dextrose agar. Cell lines and animal-derived cells.
The mouse
macrophage-like cell line J774 (ECACC 85011428) is derived from a tumor
of a female BALB/c mouse (41). Adherent J774 cells were
cultured at 37°C in an atmosphere containing 5% CO2 in
Dulbecco's modified Eagle's medium (DMEM) (Biowhittaker, Verviers,
Belgium) supplemented with glutamine (2 mM), streptomycin (100 µg/ml), penicillin (50 µg/ml), and 10% fetal calf serum.
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
-1,2-Linked Oligomannosides from Candida
albicans Bind to a 32-Kilodalton Macrophage Membrane Protein
Homologous to the Mammalian Lectin Galectin-3
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ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-1,2-linked oligomannoside residues are present, associated with
mannan and a glycolipid, the phospholipomannan, at the Candida albicans cell wall surface.
-1,2-linked oligomannoside
residues act as adhesins for macrophages and stimulate these cells to
undergo cytokine production. To characterize the macrophage receptor
involved in the recognition of C. albicans
-1,2-oligomannoside we used the J774 mouse cell line, which is
devoid of the receptor specific for
-linked mannose residues. A
series of experiments based on affinity binding on either C. albicans yeast cells or
-1,2-oligomannoside-conjugated bovine
serum albumin (BSA) and subsequent disclosure with biotinylated conjugated BSA repeatedly led to the detection of a 32-kDa macrophage protein. An antiserum specific for this 32-kDa protein inhibited C. albicans binding to macrophages and was used to
immunoprecipitate the molecule. Two high-pressure liquid
chromatography-purified peptides from the 32-kDa tryptic digest showed
complete homology to galectin-3 (previously designated Mac-2 antigen),
an endogenous lectin with pleiotropic functions which is expressed in a
wide variety of cell types with which C. albicans interacts
as a saprophyte or a parasite.
![]()
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-mannosides (49). A
-glucan receptor (5)
first identified as a protein of 160 to 180 kDa (6) specific
for zymosan (54) was subsequently shown to be made up of a
ligand-binding 20-kDa subunit which was specific for a
-1,3-heptaglucoside (53). Based on its specificity for
zymosan (43), the
M
2-integrin CR3 (Mac-1,
CD11b/CD18) was also shown to serve as a macrophage
-glucan
receptor. The sugar specificity of CD11b/CD18 was examined using
methylated monosaccharides, and a cation-independent lectin site was
located C-terminal to the I domain of CD11b (55, 57). Both
methylglucosides and methylmannosides, but not mannan, were recognized
by CR3 (58), showing that in contrast to MMR, CR3 could bind
a wide range of individual sugar but not the corresponding polymers.
-linked mannose (15,
37), although some of the activities are partly mediated by
-glucan (3). However, cumulative evidence (13, 27, 30, 31) suggests that recognition of C. albicans by
macrophages may also involve a sugar interaction independent of both
-linked mannose and
-linked glucan, leading to the hypothesis of
the existence of an alternative lectin system for C. albicans binding.
bound to the rest of the
molecule by phosphodiester bridges
a special type of sugar consisting
of
-1,2-linked mannopyranose units (16, 51). These
oligomannoses interact with macrophages (13, 31) and
stimulate these cells for tumor necrosis factor alpha (TNF-
)
production (27). Although
-1,2-oligomannoside interaction
with these cells has been shown to initiate signal transduction
(25, 26), the nature of the macrophage molecule responsible
for the binding of
-1,2-oligomannosides has not been characterized
yet. This was the objective of the present study. For this purpose, we
used the murine macrophage J774 cell line, which recognizes
-1,2-oligomannosides but is devoid of MMR expression
(13). Through a series of experiments involving protein
extracts of J774 cells and either S. cerevisiae or C. albicans yeast cells or a neoglycoprotein constructed with C. albicans-derived
-1,2-mannotetraose, we repeatedly
isolated a 32-kDa protein which specifically binds these residues. A
specific antiserum raised against the 32-kDa
-1,2-mannose-binding
protein which inhibited C. albicans recognition by J774
cells was used to immunoprecipitate the 32-kDa molecule. Two major
peptides were sequenced, and both had complete sequence homology to
galectin-3.
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-1,2-oligomannosides
(56), was kindly provided by A. Cassone, Department of
Bacteriology and Medical Mycology, Istituto Superiore di Sanita, Rome,
Italy (2). The anti-MMR rat polyclonal antibody was a gift
from P. D. Stahl, Department of Cell Biology and Physiology,
Washington University School of Medicine, St. Louis, Mo. The
corresponding horseradish peroxidase and fluorescein isothiocyanate
conjugates (i.e., goat anti-mouse IgM and goat anti-rat Ig) were
obtained from Zymed (San Francisco, Calif.).
-1,2-oligomannosides were purified from the cell wall of C. albicans as previously described (9). Briefly,
phosphopeptidomannan was extracted by the method of Kocourek and Ballou
(28). The acid-labile fraction of phosphopeptidomannan was
obtained by hydrolysis in 10 mM HCl for 30 min at 100°C. After
cooling and neutralization with NaOH, acid-stable phosphopeptidomannan
was removed by precipitation with 70% ethanol.
-1,2-oligomannosides
from the acid-labile fraction were separated by gel filtration
chromatography on Bio-Gel P2 (Bio-Rad, Hemel Hempstead, England).
Biotinylation and extraction of the J774 cells. Protein extracts from adherent J774 cells were made from either biotinylated or unlabeled cells. For surface biotinylation, plated cells were washed with 20 mM phosphate-buffered saline (pH 7.2) (PBS) containing 1 mM CaCl2 and 1 mM MgCl2 (buffer A). Then, 100 µg of succinimidyl-6-(biotinamido)hexanoate in 1 ml of buffer A was added. After an incubation of 30 min at 20°C, the cells were washed in buffer A. Biotinylated or unlabeled J774 cells were gently scraped with a rubber policeman and pelleted at 300 × g for 10 min at 20°C. A total of 1.5 × 107 cells were extracted for 20 min in ice with 1 ml of PBS containing 0.5% Triton X-100, 1 µM aprotinin, 1 mM leupeptin, and 1 mM phenylmethylsulfonyl fluoride. Samples containing the extracted material were centrifuged for 10 min at 12,000 × g in a microcentrifuge to remove insoluble material.
In some experiments, membrane-enriched fractions from J774 cells were obtained by a differential centrifugation procedure. J774 cells were subjected to four freeze-thaw cycles in water containing the protease inhibitors mentioned above. The cells were centrifuged for 30 min at 15,000 × g, the resulting supernatants were centrifuged for 1 h at 100,000 × g, and pellets containing the cell membranes were extracted using 0.5% Triton X-100. Membrane protein extracts were then clarified by centrifugation for 10 min at 12,000 × g.Neoglycoprotein.
Biotinylated neoglycoprotein was used
either for fluorescence examination of
-1,2-oligomannoside binding
to J774 membranes or as a probe to detect J774 proteins reacting with
-1,2-oligomannosides by ligand blotting. Unlabeled neoglycoprotein
was used in affinity purification experiments. Both bovine serum
albumin (BSA) and biotinylated BSA (biot-BSA) (1 µmol) (Sigma) were
incubated for 7 days at 37°C with 300 µmol of C. albicans-derived
-1,2-mannotetraose in 2.5 ml of PBS containing
1.59 mmol of NaBH3CN and 1 drop of toluene (19).
The neoglycoproteins were then purified from unbound sugars by gel
filtration chromatography. Under these conditions, the sugar-to-protein
ratio was 6:1 (mole/mole). The presence of
-1,2-oligomannosides
within the neoglycoproteins was examined by blotting with the
anti-
-1,2-oligomannoside MAb AF1.
Fluorescence analysis. J774 cells (105 cells per well) were cultured at 37°C in eight-well Lab-Tek tissue culture chambers (Nunc, Naperville, Ill.). After an 18-h incubation, 150 µl of biotinylated neoglycoprotein in DMEM (25 µM sugar) was added for 30 min at 37°C. The slides were then washed and fixed with 2% Formol. Bound neoglycoprotein was revealed with 200 µl of fluorescein isothiocyanate-conjugated streptavidin (Zymed) (1:500 dilution in PBS) and mounted for microscopy examination.
Affinity purification.
Affinity chromatography experiments
were performed using either live yeast cells or unlabeled
neoglycoprotein coupled to CNBr-activated Sepharose beads. For this
purpose, 5 to 10 mg of
-1,2-oligomannoside conjugated to BSA
(
-1,2-Man-BSA) in 3 ml of coupling buffer (0.1 M sodium carbonate
[pH 8.3], 0.5 M NaCl) was incubated for 2 h at 20°C with 1 ml
of CNBr-activated Sepharose beads. The beads were then centrifuged at
1,500 rpm for 5 min and blocked with 10 ml of 1 M ethanolamine (pH
4)-0.5 M NaCl.
-1,2-Man-BSA-coupled beads were then equilibrated
in PBS.
-1,2-Man-BSA-coupled beads for 1 h at 4°C. After several washes in PBS to remove
unbound material, the bound proteins were desorbed by boiling at
100°C for 5 min with 40 µl of Laemmli sample buffer without
-mercaptoethanol and clarified by centrifugation at 13,000 rpm for 8 min.
Western blotting.
J774 proteins were separated by sodium
dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (10%
polyacrylamide) before being blotted onto a nitrocellulose membrane
(Hybond ECL [Amersham, Little Chalfont, England]) for 1 h at 400 mA in a semidry transfer system. After being stained with 0.1% Ponceau
S in 5% acetic acid to confirm even loading and transfer, the membrane was blocked by incubation for 1 h at 20°C with TNT (10 mM Tris, 100 mM NaCl, 0.1% Tween) containing 5% milk. For experiments
involving affinity-purified biotinylated J774 surface proteins,
purified proteins were detected by probing the blot with alkaline
phosphatase-conjugated streptavidin (1:1,000 dilution) in TNT
containing 0.5% BSA (TNT-BSA). In experiments based on direct
detection of proteins reacting with
-1,2-oligomannosides, the
membranes were probed with biot-
-1,2-Man-BSA followed by alkaline
phosphatase-conjugated streptavidin. In experiments involving antibody
detection, blotted J774 proteins were incubated with a 1:100 dilution
of immune sera for 1 h at 37°C in TNT. After being washed, the
membranes were probed with the corresponding alkaline
phosphatase-conjugated antisera (1:2,000 dilution) in TNT.
Preparation of the anti-32-kDa membrane protein rat serum. Fisher rats were immunized with 200 µl of PBS containing 20 µg of 26- to 35-kDa J774 proteins electroeluted from electrophoretically separated protein extracts together with 200 µl of complete Freund adjuvant. The animals were boosted every 15 days for 2 months with the same antigens but in 200 µl of incomplete Freund adjuvant. Throughout the entire period, the immune response induced in the sera was monitored by Western blotting onto J774 protein extracts.
Inhibitory effect of anti-32-kDa antibody. The inhibitory activity of the day 60 rat immune serum was tested on binding of C. albicans blastoconidia to J774 cells (105 cells per well) were cultured at 37°C in eight-well Lab-Tek tissue culture chambers. After 18 h, a 1:100 dilution in DMEM of either preimmune or immune anti-32-kDa polyclonal antibody was added to the culture for 30 min. C. albicans yeast cells were then added for 10 min at a yeast-to-cell ratio of 50:1. The slides were then washed, fixed with 2% Formol, and mounted for microscopy examination.
Sequence homology.
The 32-kDa
-1,2-mannose-binding
protein was purified from J774 cell membrane extract (2 mg of proteins)
by immunoprecipitation using 400 µl of rat immune serum and protein
G-coupled beads. Precipitated material was eluted with 40 µl of
electrophoresis sample buffer, separated by SDS-PAGE (10%
polyacrylamide), and tested for
-1,2-oligomannoside reactivity by
ligand blotting using biot-
-1,2-Man-BSA as described above. After
staining with amido black (0.003% [wt/vol] in 45% methanol-10%
acetic acid-45% water), the corresponding band was cut out and air
dried in a vacuum centrifuge before being subjected to tryptic
digestion (1 µg of trypsin/ml). The resulting peptides were separated
by high-pressure liquid chromatography (HPLC) through a
DEAE-C18 column with acetonitrile-0.1% trifluoroacetic
acid. Two different peptides were sequenced, and homologies to existing
proteins were examined in the Swiss Prot database using the BLAST
Program (National Center for Biotechnology Information).
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RESULTS |
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C. albicans but not S. cerevisiae binds to
a 32-kDa J774 cell protein.
After surface biotinylation of the
J774 cells, the labeled proteins were extracted with Triton X-100 and
incubated with either S. cerevisiae or C. albicans blastoconidia to examine differential binding on these
two yeast species. Elution of bound proteins and disclosure by alkaline
phosphatase-conjugated streptavidin showed a large array of macrophage
proteins from 50 to 100 kDa that bound to both yeast species (Fig.
1). Beside these molecules, a large
amount of material in the 30- to 35-kDa range was predominantly bound
to C. albicans yeast cells (Fig. 1, compare lane 2 and 3). In this material, a 32-kDa protein was repeatedly identified. The
presence of this protein at the cell membrane was confirmed when Triton
X-100 extracts from membrane-enriched fractions of biotinylated J774
cells were used in the same type of experiments (data not shown).
|
C. albicans mannan-derived
-1,2-oligomannosides
conjugated to BSA bind to the 32-kDa protein.
Since a major
difference in surface molecules between C. albicans and
S. cerevisiae lies in the presence of
-1,2-linked
oligomannose residues in the former yeast species, we examined the role
of these residues through the construction of a neoglycoprotein. We
conjugated a
-1,2-linked mannotetraose derived from C. albicans mannan to BSA (
-1,2-Man-BSA). Figure
2A shows the coupling efficiency as
evidenced by the positivity of Western blotting with MAb AF1 specific
for
-1,2-oligomannoside epitopes of
-1,2-Man-BSA (lane 2)
compared to that obtained with BSA alone (lane 1). Identical results
were obtained after conjugation of the mannotetraose to biot-BSA (data
not shown). Using equal amounts of biotinylated proteins either
conjugated or not conjugated, fluorescence experiments performed on
J774 cells showed that compared to biot-BSA, which accumulated at low
levels within cell vesicles, biot-
-1,2-Man-BSA was associated with
the plasma membrane (Fig. 2B). Forty percent of cells exhibited strong
biot-
-1,2-Man-BSA staining at the plasma membrane.
|
-1,2-Man-BSA. Extracts from J774 cells were subjected to
blotting with different biotinylated BSA probes: uncoupled biot-BSA,
biot-BSA coupled to
-methyl mannopyrannoside (biot-
-Man-BSA) and
biot-
-1,2-Man-BSA. biot-BSA did not bind to macrophage extracts (Fig. 2C, lane 1). Both biot-
-Man-BSA and biot-
-1,2-Man-BSA bound to macrophage components of >55 kDa, including poorly defined polydispersed material and a single band around 80 kDa (lanes 2 and 3).
Beside these bands, biot-
-1,2-Man-BSA bound selectively to a
doublet with a molecular mass of 31 to 32 kDa (lane 3).
The 32-kDa protein recognized by C. albicans and not by
S. cerevisiae binds to
-1,2-oligomannosides in a
calcium-independent way.
Identity between the 32-kDa macrophage
membrane protein that bound to C. albicans yeast cells and
the protein that bound to the
-1,2-oligomannosides, as evidenced
through biot-
-1,2-Man-BSA binding to J774 extracts, was confirmed
by experiments based on affinity purification. Macrophage proteins were
either incubated with C. albicans blastoconidia (Fig.
3A) or passed through a column consisting
of
-1,2-Man-BSA-coupled Sepharose (Fig. 3B). After being
transferred to nitrocellulose, the eluted materials were detected
either with biot-BSA or with biot-
-1,2-Man-BSA. After passage
through C. albicans yeasts (Fig. 3A), biot-BSA revealed mainly two large bands with masses of 80 and 120 kDa. Coupling to
-1,2-mannotetraose resulted in recognition by the neoglycoprotein of
several other proteins eluted from C. albicans. Among them, three proteins with masses of 31 to 32, 37, and 45 kDa could be clearly
identified. Blotting of material eluted from
-1,2-Man-BSA-coupled Sepharose with biot-
-1,2-Man-BSA revealed mainly two groups of molecules, one with a mass of >90 kDa and a well-defined 32-kDa protein. In both experiments, supplementation with divalent cations during either affinity binding or blotting had no effect on the results.
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An antiserum raised against the 32-kDa protein inhibits C. albicans binding to J774 cells.
Since the 32-kDa protein was
one of the main molecules which appeared to be recognized both by
C. albicans and by
-1,2-oligomannosides but not by
S. cerevisiae or by uncoupled BSA, we hypothesized that this
molecule could be involved in the recognition of
-1,2-oligomannosides by the J774 cells. To characterize the J774
32-kDa protein, we prepared a rat antiserum by immunization with this
molecule electroeluted from preparative polyacrylamide gel
electrophoresis. We first examined the antigenic specificity of this
antiserum. For this purpose, material eluted from C. albicans was blotted either with biot-
-1,2-Man-BSA, used as
positive control (Fig. 4A, lane 1), or
with the anti-32-kDa antiserum (lane 2). The antiserum selectively stained a 32-kDa band, which comigrated with the 32-kDa protein, revealed by the neoglycoprotein. We then examined whether the anti-32-kDa antiserum displayed inhibitory activity toward binding of
C. albicans to the J774 cells. J774 cells were incubated in the presence of preimmune serum or anti-32-kDa antiserum and then cocultivated with C. albicans blastoconidia (13).
When the number of cells that bound more than three yeasts was
determined, the presence of the anti-32-kDa antiserum led to a
decreased number of positive cells (5% ± 5% inhibition with the
preimmune serum compared to 37% ± 3% inhibition with the anti-32-kDa
antiserum). However, the antiserum inhibited only part of the
interaction of the yeast with the cells, suggesting that not all lectin
systems involved in the recognition of C. albicans by the
J774 cells were altered by the anti-32-kDa antiserum (Fig. 4B).
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The 32-kDa protein is expressed by mouse peritoneal macrophages and
is different from the MMR.
To verify that the 32-kDa protein was
present at the membrane of primary cells, the anti-32-kDa antiserum was
allowed to react on blots of protein extracts prepared from mouse
peritoneal macrophages. Staining of a 32-kDa molecule, which comigrated
with the 32-kDa protein observed with the J774 cells, demonstrated the
presence of the same antigen on primary macrophages (Fig.
5A). To explore possible
cross-immunological identities between the 32-kDa molecule and the MMR,
blotted membrane extracts from either mouse peritoneal macrophages or
J774 cells were stained with an anti-MMR polyclonal antibody (Fig. 5B).
As expected, the MMR was recognized in macrophage extracts but was
absent in J774 extracts (Fig. 5B, lane MA). In none of the cell
extracts was a 32-kDa molecule stained by the anti-MMR polyclonal
antibody, demonstrating the absence of cross-reactivity between the two
macrophage membrane mannose-binding proteins.
|
Peptides from the J774 32-kDa protein have sequence homology to
galectin-3.
The 32-kDa protein was immunoprecipitated from J774
protein extracts with the anti-32-kDa serum, resolved by SDS-PAGE, and subjected to trypsin digestion. Peptides were separated by HPLC. Of
these, two major peptides (Pept17 and Pept19) were sequenced and the
sequences were subjected to homology analysis with the Swiss-Prot
database. Both peptides were 100% homologous to sequences localized in
different portions (positions 166 to 171 and 214 to 224) of galectin-3
(Fig. 6), previously designated
macrophage cell surface antigen Mac-2.
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| |
DISCUSSION |
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The recognition of systemic candidiasis as a major medical problem in hospital patients (40) has generated unprecedented scientific interest in the understanding of its pathophysiology. Basic microbiological studies have addressed signal transduction pathways (4) and gene regulation mechanisms controlling C. albicans characteristics considered essential for virulence, i.e., filamentation (29), adhesion (14), enzyme secretion (7, 17), and phenotypic switching (38). Immunological studies have begun to unravel the complex immunoregulatory circuits by which the immune system directs the predominance of the CD4+ Th1 or Th2 subset (36). However, very little is known yet about which C. albicans molecules, among those encoded by more than 6,000 genes, play a major role in eliciting reactions that protect the host or the microbe.
As a yeast, a characteristic of C. albicans is to devote an
important part of its metabolism to the synthesis of sugars. Among these, mannose residues may represent up to 40% of the cell wall. They
are expressed mainly at the cell surface, associated with mannoprotein
by either O- or N-glycosylation (16). The archetype of such
molecules is the cell wall phosphopeptidomannan (or mannan), the
immunological properties of which have been extensively studied in
C. albicans and S. cerevisiae. Over the last few
years, interest has focused on
-1,2-linked oligomannosides, first
described in 1985 by Shibata et al. as being associated with the
C. albicans cell wall mannan by phosphodiester bridges
(46). It has been shown that an IgM MAb against a
-1,2-linked mannotriose, MAb B6.1, and not an IgM specific for the
mannan acid-stable fraction, MAb B6, protects mice in a model of
systemic candidiasis (21). In an in vitro assay, MAb B6.1,
but not MAb B6, was found to enhance the candidacidal activity of
polymorphonuclear leukocytes in the presence of fresh mouse serum,
suggesting the involvement of mouse complement in the killing
(20).
Both
- and
-methylmannosides are recognized by CR3
(58). In the present study, we demonstrated that
-1,2-oligomannosides derived from C. albicans mannan
specifically bound to a 32-kDa protein expressed both on the J774 cell
line and on peritoneal murine macrophages. This finding emerged from
our (13, 24, 25, 27) and other (30, 31) previous
experiments demonstrating a specific interaction of C. albicans
-1,2-oligomannosides with macrophage membrane. The
identification of the 32-kDa protein as a
-1,2-mannose-binding
protein was based on immunochemical and affinity approaches. These
methods were applied to J774 cell membrane proteins and either yeast
cells (C. albicans or S. cerevisiae) or
-1,2-Man-BSA. When using an anti-MMR antiserum (49), we found no cross-reactivity with this receptor specific for
-linked mannose residues. This was important for determining that the
-1,2-mannose-binding protein was not an MMR breakdown product, since
it has been shown that the MMR was synthesized by J774 cells and
rapidly degraded (12). With an antiserum raised against the
32-kDa
-1,2-mannose-binding protein, we found that the 32-kDa protein was also present on mouse macrophage membranes. Binding of
C. albicans yeast cells to macrophages was altered by this antiserum, although incompletely. These results are not surprising, since we have already shown that
-1,2-oligomannosides account for
only 40% of the C. albicans binding activities to
macrophage (13).
Several other proteins with molecular weights similar to those of the
and
chains of CR3 (58) were copurified when S. cerevisiae and C. albicans yeast cells were used.
However, these high-molecular-weight proteins were not apparent when
neoglycoproteins were used for affinity purification. Conversely, the
32-kDa molecule was never found in experiments in which material devoid
of
-1,2-oligomannosides, such as S. cerevisiae yeasts or
mannan, or the control neoglycoproteins (BSA or
-Man-BSA) was used,
showing the specificity of the recognition.
The identity of the 32-kDa
-mannose-binding protein we isolated to
galectin-3 was based on the sequence analysis of two peptides from
tryptic digests of the immunoprecipitated protein. Both peptides showed
total homologies to peptide sequences localized in distant portions
(positions 166 to 171 and 214 to 224) of galectin-3. The identification
of the molecule capable of binding
-1,2-oligomannosides as
galectin-3 was somewhat surprising, since the definition of the family
of galectins is based on the presence of similar carbohydrate recognition domains and affinity for
-galactosides (22),
although some galectins, such as galectin-10, recognize mannose with
high affinity (52). All of the blotting experiments
described here were performed after saturation of nitrocellulose with
nonfat milk. Since binding of
-1,2-oligomannosides to the protein
could be evidenced under these conditions (in the presence of lactose), this suggested that the lectin has higher affinity for
-1,2-oligomannosides than for
-galactosidase contained in the
lactose. An elegant X-ray crystal structure of the galectin-3
carbohydrate recognition domain in complex with lactose and
N-acetyllactosamine has furnished an explanation of the
differences in specificity between galectin-3 and other members of the
galectin family (44). Such fine-interaction studies may be
required to determine which parts of the
-1,2-oligomannosides are
buried by the protein and their influence on the protein structural arrangement. The striking relationship found between the number of
-1,2-linked mannopyranose residues and the ability to induce TNF-
synthesis by macrophages (27) could represent a good model to study structure-activity relationships for binding and signaling properties. A current basic scientific interest in galectin-3 has shown
its subcellular distribution within the nucleus and the cytoplasm, its
secretion through a nonclassical pathway (33), its
association with the cell surface, and the presence of galectin-3 in
intercellular matrices (39). Therefore, much remains to be discovered before the pleiotropic activities of galectin-3 can be
explained. The topics that await clarification range from tumor transformation and metastasis, inflammation, nerve injury, immune cell
stimulation, and pathogen interaction (1).
Galectin-3 associates with several membrane proteins different from its
putative receptor (42). These molecules, expressed on
polynuclear and mononuclear phagocytes and epithelial cells, are
involved in the binding and phagocytosis of different microbes. Such
molecules are represented by the lysosome-associated membrane glycoproteins 1 and 2 (CD107) (11, 23), CD66-a and CD66-b (11), which bind both galectin-3 and bacterial type 1 fimbriae (50), as well as CD98 and CD11b/CD18
(8). Interaction of microorganisms with multimolecular
membrane complexes allows colonization and stimulates signaling
cascades, which promote intracellular accommodation of the pathogen
and/or induction of cytokine release, priming the immune response
(35). Interaction of C. albicans
-1,2-oligomannosides and galectin-3 may participate, along with other membrane proteins, in forming macromolecular edifices responsible for stimulating properties. TNF-
synthesis induced by
-1,2-oligomannosides is dependent on a signal involving tyrosine
phosphorylation similar to those observed following CR3- or
CD66-related stimulations (25). The bacterial
lipopolysaccharide LPS, which interacts with both its specific receptor
CD14 receptor (59) and galectin-3 (34), initiates
a similar signal. Interestingly, we have shown that interaction of live
C. albicans cells and macrophages mediated by
-1,2-oligomannosides also involves a glycolipid, the
phospholipomannan, which is shed by the yeasts during contact
(25).
This series of studies therefore demonstrates that among C. albicans and host molecules triggering a macrophage response by
-1,2-oligomannosides are the phospholipomannan and galectin-3. Galectin-3 is abundantly expressed on polarized epithelial cells and
neutrophils. Both cell types are essential for colonization by C. albicans and limitation of C. albicans infection,
respectively. An investigation of the participation of galectin-3 in
these basic pathophysiological processes is under way.
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ACKNOWLEDGMENTS |
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This work has been supported in part by a grant from Sidaction.
We thank G. Cole for helpful discussions. We gratefully acknowledge P. D. Stahl and A. Cassone for the gift of specific antibodies and F. van den Brûle, D. Soll, and A. Casadevall for their help in the redaction of the manuscript.
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FOOTNOTES |
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* Corresponding author. Mailing address: Laboratoire de Mycologie Fondamentale et Appliquée, INSERM E9915, Faculté de Médecine, Pôle Recherche, Place Verdun, 59037 Lille Cedex, France. Phone: 33 3 20 47 26 29. Fax: 33 3 20 47 26 25. E-mail: tjou{at}worldnet.fr.
Editor: J. M. Mansfield
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