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Cutting Edge |


Departments of
* Cell Biology and
Pathology, Duke University Medical Center, Durham, NC 27710
| Abstract |
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2-macroglobulin binding and
uptake was readily obtained, the addition of excess CD91 ligand,
activated
2-macroglobulin, or receptor-associated
protein, an antagonist of all known CD91 ligands, did not affect GRP94
cell surface binding, receptor-mediated endocytosis, or peptide
re-presentation. These data identify a CD91-independent, GRP94
internalization pathway that functions in peptide Ag
re-presentation. | Introduction |
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Recently, a member of the low-density lipoprotein
(LDL)3 family of
scavenger receptors, CD91 (
2-macroglobulin
(
2M) receptor; LDL receptor-related protein),
was proposed to serve as the unique receptor responsible for directing
GRP94 into the class-I Ag processing pathways of APCs
(12, 13, 14). The conclusion that CD91 functions as the GRP94
receptor stems from the observation that
2M,
the active form (
2M*) of which is an
established, endogenous ligand for CD91, abrogates GRP94-mediated,
APC-dependent T cell stimulation (12). Although consistent
with a role for CD91 in GRP94-based Ag re-presentation, previous
observations call into question whether the effects of CD91-directed
ligands on GRP94-based peptide re-presentation reflect a direct role of
CD91 in chaperone uptake and processing. Primarily, CD91 is expressed
on a diverse array of cell types, including fibroblasts and
hepatocytes, the majority of which do not function as professional APCs
(15). Additionally, in affinity chromatography and
chemical cross-linking studies of GRP94-interacting proteins, GRP94 was
recovered with a single proteolytic product of CD91; no interactions of
GRP94 with intact CD91 were reported (12).
In the present study, CD91 function in the receptor-mediated
endocytosis and trafficking of GRP94 in APCs was analyzed. It is well
established that ligand binding functions of CD91 are regulated by
receptor-associated protein (RAP) (16, 17, 18), which
efficiently blocks the cell surface binding and uptake of all known
CD91 ligands (16, 17, 18, 19). We report in this work that the
binding of GRP94 to APC cell surface receptors was RAP and
2M*-insensitive. Furthermore, CD91 and its
ligand, Pseudomonas exotoxin, segregated from
receptor-internalized GRP94 in early compartments of the endocytic
pathway. Additionally, re-presentation of GRP94-associated peptides was
2M* insensitive. These data identify a
primary, CD91-independent re-presentation pathway for GRP94-associated
peptides in APCs.
| Materials and Methods |
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C57BL/6 mice (Charles River Breeding Laboratories, Wilmington, MA) were used to prepare thioglycolate-elicited peritoneal macrophages. Macrophages were harvested 45 days postinjection and enriched by adherence selection.
Protein purification and labeling
GRP94 was purified by the method of Wearsch and Nicchitta (20). Pseudomonas exotoxin was obtained from Sigma-Aldrich (St. Louis, MO). Texas Red-, biotin-, and fluorescein-succinymidyl esters (Molecular Probes, Eugene, OR) were used to label proteins according to manufacturers protocols. Abs against Kb-OVA complex (25-D1.16) and CD91 were the kind gifts of Dr. J. Yewdell (National Institutes of Health, Bethesda, MD) and Dr. S. Argraves (University of South Carolina, Charleston, SC), respectively.
2M was purified as previously described
(21). Purified
2M was converted
to the CD91-binding, thiol ester-cleaved derivative
(
2M*) by incubation with 0.2 M
NH4HCO3.
2M* was then labeled with Alexa Fluor
(Molecular Probes). RAP was purified as previously described
(22). Dr. J. Herz (University of Texas Southwestern
Medical Center, Dallas, TX) kindly provided pGEX-RAP expression
vectors.
Cell surface binding and uptake
Receptor-mediated uptake reactions were performed as described previously (23). For confocal microscopy analysis, cells were fixed in 4% paraformaldehyde and mounted under 10% PBS, 90% glycerol, and 1 mg/ml phenylenediamine.
Biotin-GRP94 cell surface receptor binding interactions were analyzed by incubating 106 cells/assay with increasing concentrations of biotin-GRP94 for 30 min on ice. Cells were then washed and resuspended in SDS-PAGE sample buffer, and extracts were prepared for SDS-PAGE. Following transfer to nitrocellulose membranes, biotin-GRP94 levels were determined by ECL, following avidin-HRP detection of biotin-GRP94. Quantification of cell surface-bound biotin-GRP94 was determined against a standard curve prepared from serial dilutions of biotin-GRP94.
Adherent macrophages were incubated at 4°C in RPMI 1640, 0.5%
BSA for 30 min with the following ligands: fluorescein-GRP94 and
Alexa-
2M* in the presence or absence of RAP.
For competition assays final concentrations used were 25 nM for GRP94
and
2M* and 2500 nM for RAP (100-fold molar
excess). After the 30-min binding period, macrophages were washed and
then fixed in 4% paraformaldehyde/PBS. Following fixation, the cells
were rinsed and unreacted paraformaldehyde was quenched with 0.05 M
NH4Cl. Coverslips were then mounted onto slides
in mounting medium. All images were obtained on a Zeiss LSM 410 laser
scanning confocal microscope (Thornwood, NY) using Zeiss LSM version
3.95 software. All image size and contrast adjustments were performed
with PhotoShop (version 4) software (Adobe Software, Palo Alto,
CA).
For study of CD91 binding and RAP competition by FACS, adherent
macrophages were incubated for 30 min at 4°C with fluorescein-GRP94
or
2M*-AF488, in the presence or absence of
RAP. Ligand and RAP concentrations were as indicated above. Following
incubation, cells were rinsed and fixed in 1% paraformaldehyde. Cells
were analyzed for fluorescence by FACS and analysis was performed using
CellQuest (BD Biosciences, San Jose, CA).
Chaperone-based re-presentation
Peptide re-presentation assays were performed with GRP94
complexed with SIINFEKL peptide (by heat shock, 15 min at 50°C)
(24, 25) and subsequently isolated from free peptide by
Sephadex G-75 size exclusion chromatography (Sigma-Aldrich, St.
Louis, MO). This preparation does not contain free peptide
(26). To assay GRP94/peptide re-presentation,
GRP94/peptide complexes and, where indicated,
2M* were incubated with elicited primary
peritoneal macrophages and, following a 3-h incubation at 37°C,
subsequently stained with 25-D1.16 Ab (27). The cells were
then fixed in 2% paraformaldehyde for FACS analysis.
| Results and Discussion |
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CD91 has been identified as the unique receptor responsible for directing GRP94 into MHC class I re-presentation pathways of APCs (12, 13, 14). CD91 is a member of the LDL family of lipoprotein receptors, which bind and internalize an array of ligands, the majority of which are targeted to lysosomes (28). Because little is known regarding the CD91-dependent trafficking of GRP94, the trafficking itineraries of CD91, GRP94, and known CD91 ligands were examined.
To evaluate the subcellular trafficking of CD91 ligands, the
trafficking pattern of GRP94 was compared with that of
Pseudomonas exotoxin, an obligate CD91 ligand
(29). In these experiments, fluor-labeled GRP94 and
Pseudomonas exotoxin were bound to macrophage cell surface
receptors, the cells were washed, and the staining pattern was analyzed
following warming to 37°C. As is evident in Fig. 1
A, receptor-internalized
GRP94 (red channel) and Pseudomonas exotoxin (green channel)
were trafficked to distinct subcellular compartments. The lack of
costaining suggested that GRP94 and CD91 were rapidly segregated upon
internalization to yield distinct trafficking itineraries. To examine
this hypothesis, we determined whether, at early time points,
internalized GRP94 and CD91 colocalized. Surprisingly, and as shown in
Fig. 1
B, GRP94 (red channel) taken up by receptor-mediated
endocytosis did not colocalize with internalized CD91 (green channel).
This is in direct contrast to the colocalization observed of
receptor-internalized GRP94 and IgG (Fig. 1
C), which traffic
to a FcR+rab5a- endosomal
compartment (26). In interpreting these data with respect
to a role for CD91 in chaperone-mediated cross-presentation, it is
important to note that a functional role for CD91 in this process has
been proposed not on the basis of direct trafficking studies but rather
from the observed inhibition of peptide re-presentation by
CD91-specific Abs and by
2M competition
(12). To reconcile these differences, a direct analysis of
CD91 function in GRP94 cell surface binding and uptake was
performed.
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CD91 is expressed on a diverse array of cell types, including, but
not limited to, APCs (15). Previously, we reported that
HepG2, a CD91-positive human hepatoma cell line, did not display GRP94
cell surface binding (23, 30). To examine whether there
was a positive correlation between CD91 expression and cell surface
binding of GRP94, these two parameters were evaluated in an additional
CD91-positive cell line, Chinese hamster ovary (CHO), and in RAW264.7
macrophages, a cell line that is CD91 positive and that binds GRP94
(12, 23, 31, 32). As depicted in Fig. 2
A, both CHO and RAW264.7
cells express nearly identical levels of CD91, findings consistent with
previous reports (14, 31, 32). However, though both cell
types were CD91 positive, only RAW264.7 cells display appreciable cell
surface binding of GRP94; thus, as previously concluded with regard to
HepG2 cells, a positive correlation between CD91 expression and GRP94
cell surface binding could not be demonstrated.
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2M*, to compete for GRP94 cell surface binding
was examined. Previously, Binder et al. (12) reported that
2M efficiently inhibits re-presentation of
GRP94-associated peptides, as assayed using peptide-dependent
stimulation of T cells as a surrogate for receptor function in GRP94
peptide uptake. This observation is surprising, as CD91 ligands
generally do not cross-compete for binding (28). However,
a second high-affinity
2M* binding site has
been demonstrated on the surface of APCs (33); thus, it
was considered that GRP94 cell surface binding was conferred by this
related activity. Therefore, we performed competition experiments with
highly purified
2M*, the CD91 binding form of
2M, or serum, which contains
2 mg/ml
2M (34). As shown in Fig. 2
2M*,
and DMEM plus 10% serum had no effect on the binding of GRP94
to thioglycolate-elicited macrophages. Indeed, GRP94 was observed to
bind to and be internalized by elicited macrophages in the presence of
200 µg/ml
2M* (Fig. 2
2M* constitutes
1% of total
2M
(35),the
2M* levels (200
µg/ml) used in these experiments are equivalent to a total
2M concentration of 20 mg/ml, a concentration
in vast excess to that necessary for competition at a shared
site(s). Re-presentation of GRP94-associated peptides via a CD91-independent pathway
RAP is a 39-kDa protein that acts as a universal antagonist to all
known CD91 ligands, including
2M* (16, 17, 19). If CD91 functions in the internalization of GRP94, and
as
2M has been reported to block the
interaction of GRP94 with CD91, RAP would be predicted to antagonize
the binding of GRP94 to APCs. Therefore, we tested the effects of RAP
on cell surface binding of GRP94, using an established ligand,
2M*, as a positive control. Fig. 3
B indicates that RAP, at a
100-fold molar excess, did not inhibit GRP94 binding to elicited
macrophages. Importantly, under identical assay conditions,
2M* binding was efficiently blocked by RAP
(Fig. 3
C). The inhibition of GRP94 binding by RAP was not
due to GRP94 displaying a higher affinity for APCs than
2M*, as GRP94 exhibits an overall
Kd of
2 x
10-7 M to elicited macrophages (Fig. 2
B, inset), as compared with a low nanomolar
Kd for
2M*
binding to CD91 (36). The effects of RAP on cell surface
binding of fluorescein-labeled GRP94 and Texas Red-labeled
2M* was also examined by confocal microscopy
(Fig. 3
, D and E). Consistent with the FACS data,
RAP was without effect on GRP94 binding (green channel), whereas
2M* binding (red channel) was efficiently
blocked (Fig. 3
, compare D, minus RAP, with E,
plus RAP). Finally, we tested whether
2M*
directly blocked the re-presentation of GRP94-associated peptides. In
these experiments, it was observed that SIINFEKL peptide, complexed
with GRP94, was re-presented both in the presence of
2M-containing serum and in the presence of
serum supplemented with 100 µg/ml
2M* (Fig. 3
F), a level previously reported to abolish GRP94-dependent
cross-priming of T cells (12).
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3 h) and
GRP94-mediated T cell activation (>20 h) (12), it should
be considered that the reported inhibition of T cell activation by CD91
Ab and
2M may reflect physiological responses
of the cells to prolonged culture with these reagents. For example,
ligand-bound CD91 elicits pronounced cell activation; thus, we suggest
that CD91 Ab and
2M may disrupt the ability of
APC to activate cognate T cells (37, 38, 39).
In summary, the included data indicate that macrophage CD91
displays biochemical properties consistent with its role as a
2M* receptor. However, the lack of a positive
correlation between CD91 expression and GRP94 cell surface binding, the
absence of colocalization between GRP94 and CD91 in the early
trafficking itinerary of the two proteins, the inability of
2M* to inhibit the binding or internalization
of GRP94 or the re-presentation of its associated peptides, and the
observation that RAP, a biological antagonist for CD91 receptor
ligands, is without effect on GRP94 binding argue strongly against a
role for CD91 in the receptor-mediated internalization of GRP94. From
these data, it is equally evident that APCs bear cell surface receptors
that are capable of directing GRP94 into the class I Ag re-presentation
pathway.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Christopher V. Nicchitta, Department of Cell Biology, Duke University Medical Center, Durham, NC 27710. E-mail address: c.nicchitta{at}cellbio.duke.edu ![]()
3 Abbreviations used in this paper: LDL, low-density lipoprotein; RAP, receptor-associated protein;
2M,
2-macroglobulin; CHO, Chinese hamster ovary. ![]()
Received for publication January 11, 2002. Accepted for publication February 27, 2002.
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