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Center for Immunotherapy of Cancer and Infectious Diseases, University of Connecticut School of Medicine, Farmington, CT 06030
| Abstract |
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| Introduction |
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| Materials and Methods |
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Peritoneal exudate cells (PEC) were obtained from C57BL/6 mice (The Jackson Laboratory, Bar Harbor, ME) injected 515 days earlier with 0.5 ml pristane (2,6,10,14-tetramethylpentadecane). CD11b+ cells were positively selected from mouse peritoneal exudates by the magnetic bead system (Miltenyi Biotec, Auburn, CA). Purifications of >93% CD11b+ (as measured by FACScan analysis) were always achieved. These cells are macrophages as defined by the following: MHC II+, CD11c-, phagocytic.
FITC labeling of proteins
Labeling of proteins with FITC was performed using FITC-conjugation kits and according to the manufacturers recommendations. Briefly, purified hsps or mouse serum albumin (SA, 1 mg/ml) were incubated with FITC in 0.1 M carbonate-bicarbonate buffer for 2 h at room temperature. Free unconjugated FITC was removed by passing the mixture over gel filtration columns. The number of FITC molecules bound per protein molecule was estimated by measuring the optical density at 280, 495, and 490 nm as recommended. Estimates show that between 4 and 5 FITC molecules bind to each protein molecule. All conjugated proteins were analyzed by SDS-PAGE and immunoblotting with the respective anti-hsp mAbs, anti-SA Ab or anti-FITC mAb. Abs used for the immunoblots were mouse mAb SPA-820 (clone N27F3-4 specific for both constitutive hsp73 and inducible hsp72 forms), rat mAb SPA-850 (clone 9G10 specific for gp96), all from StressGen Biotechnologies (Victoria, Canada). Anti-hsp90 Ab was a rabbit mAb (NeoMarkers, Lab Vision, Fremont, CA) specific for hsp84 and hsp86. Anti-SA and anti-FITC Abs were purchased from SIGMA (St. Louis, MO). All FITC-labeled proteins were centrifuged at 100,000 x g before use to remove any particulate matter.
Purification of hsps
hsp70 and gp96 were purified from liver cells of C57BL/6 mice according to previously described methods (19, 20). Supernatants of 100,000 x g centrifugations of cell lysates were subjected to stepwise ammonium sulfate precipitations. Pellets of 50% precipitations were used for hsp70 purification. Pellets of 80% precipitates were resuspended in PBS (with 2 mM MgCl2, 2 mM CaCl2, and 2 mM PMSF) and applied to preequilibrated Con A affinity columns. Con A eluents were exchanged into a phosphate buffer with PD-10 gel exclusion columns and applied to DEAE anion exchange columns. gp96 eluted from these columns as a purified protein and was identified by immunoblotting. Pellets from 50% ammonium sulfate precipitation were resuspended in buffer D (20 mM Tris with 20 mM NaCl, 3 mM MgCl, and 15 mM 2-ME) and applied to ADP-affinity columns as previously described. hsp70 was eluted with ADP-buffer D and applied to DEAE anion exchange columns in a phosphate buffer. hsp70 eluted off these columns as a purified protein shown by a single band on SDS-PAGE and identified in immunoblotting.
hsp90 was purified according to the protocol of Denis (21) with minor modifications. Briefly, 100,000 x g supernatants were obtained from liver cell lysates and applied to a Mono Q column (Mono Q HR 16/10, purchased from Pharmacia Biotech (Uppsala, Sweden) and attached to the BIOCAD Perseptives Biosystems, Cambridge, MA) in 200 mM sodium phosphate buffer and eluted with a gradient salt concentration to 600 mM. hsp90-positive fractions were collected and changed to Tris buffer, pH 8. Hsp90 was reapplied to the MonoQ column in the Tris buffer and eluted with a gradient salt concentration from 0 to 1 M NaCl. The Mono Q column was used according to the recommended conditions. hsp90 was eluted in a pure form as shown by SDS-PAGE and identified by immunoblotting.
Immunization with hsps
Mice were immunized with each of the three hsps as previously described (1).
Visualization of hsp receptors
Incubations of indicated amounts of FITC-labeled proteins and cells were done in the presence of Carnation 1% nonfat dry milk (Nestlé, Glendale, CA) in PBS for 20 min at 4°C. After repeated washing, cells were analyzed by flow cytometry (Becton Dickinson, La Jolla, CA). Cells were also labeled with propidium iodide just before FACScan analysis. Cells staining positive for propidium iodide were gated out of the events. No differences were observed in the binding of hsps to CD11b+ cells from pristaned or nonpristaned mice. For the saturation studies, mean fluorescent intensities were obtained from histogram plots. Competition studies were performed by mixing labeled and unlabeled competitor proteins together before incubation with cells. Excess protein was removed, and mean fluorescence intensities were measured by FACS analysis.
Confocal imaging
Paraformaldehyde-fixed or unfixed cells were labeled with FITC-labeled hsp as above. Labeled cells were visualized using a Zeiss LSM confocal microscope. For internalization studies, unfixed cells were used, and images were recorded at various intervals at the exact same focal plane and view.
| Results and Discussion |
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Homogeneous preparation of hsps gp96, hsp90, and hsp70 or serum
albumin (Fig. 1
A) were labeled
with FITC as described in Materials and Methods. The
labeling was confirmed by SDS-PAGE and immunoblotting with anti-hsp
and anti-FITC Abs (Fig. 1
, B and C). Each hsp
or albumin molecule bound to an average of five FITC molecules. The
resulting 2-kDa increase in the size of the proteins was detectable by
SDS-PAGE (Fig. 1
B). By immunoblotting with an anti-FITC
mAb, conjugation of gp96 to FITC was confirmed (Fig. 1
C).
Similar results were obtained with other hsps (not shown). Because
modification of proteins by conjugation to FITC could result in the
loss of binding of hsp to their putative receptors, we tested whether
the FITC-labeled hsps remained immunogenic. We used the unique ability
of hsp-peptide complexes to generate peptide-specific CTLs as the
functional assay (1). FITC-labeled gp96 was complexed in
vitro to the Kb-binding OVA epitope SIINFEKL and
used to immunize C57BL/6 mice. Spleens were removed from the immunized
mice 1 wk later, stimulated in vitro with the OVA-expressing cell line
E.G7, and tested for cytotoxicity of E.G7 or EL4 cells. It was observed
that FITC-labeled gp96 complexed to SIINFEKL was able to elicit
SIINFEKL-specific CTLs as effectively as unlabeled gp96-SIINFEKL and
thus remained immunogenic (Fig. 2
).
Control preparations of FITC-labeled gp96 uncomplexed to peptide were
not able to generate SIINFEKL specific CTLs. Identical experiments were
conducted with FITC-labeled hsp70 and hsp90 with similar results (data
not shown). The functionally active FITC-labeled hsp preparations were
therefore used for further studies.
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Peritoneum-derived CD11b+ cells can
represent gp96-chaperoned antigenic peptides, suggesting that they
express receptors capable of internalizing hsp-peptide complexes
(10). CD11b+ cells were purified
from peritoneal exudates of mice and were fixed in 2% paraformaldehyde
so as to render them incapable of phagocytosis. Fixed cells were
incubated with FITC-labeled gp96, hsp70, hsp90, or SA. Excess or
unbound protein was removed by extensive washing, and cells were
analyzed by FACS analysis. As shown in Fig. 3
, FITC-labeled hsps could effectively
label CD11b+ cells. Increasing quantities of hsps
used for the incubations labeled an increasing percentage of cells. The
labeling was specific to hsps because SA even at the highest
concentration of protein used did not demonstrate significant labeling
to CD11b+ cells (Fig. 3
). The inability of
FITC-labeled SA to label cells also shows that binding of hsps was not
an artifact of the FITC label. FITC-labeled hsps and SA were also
incubated with Meth A tumor cells (representative of
CD11b- cells). No binding of hsps or SA to
CD11b- cells was observed.
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FITC-gp96 (100 µg/ml final concentration) was mixed with
increasing quantities (1001000 µg/ml final concentration) of
competitor (unlabeled) gp96, hsp90, hsp70, or SA, followed by
incubation of the mixtures with fixed CD11b+
cells. As shown in Fig. 5
, increasing
quantities of unlabeled gp96 were able to increasingly inhibit binding
of FITC-gp96 to CD11b+ cells such that at the
highest concentration tested (unlabeled:labeled ratio of 10:1),
unlabeled gp96 could inhibit >50% of the binding of FITC-gp96.
Unlabeled SA showed insignificant and untitratable competition with
FITC-gp96 under the circumstances (p < 0.005).
Interestingly, unlabeled hsp90 was also able to inhibit in a titratable
manner, the binding of FITC-gp96 to CD11b+ cells
quite effectively such that, at the highest concentration of hsp90
tested (unlabeled hsp90:labeled gp96 ratio of 10:1), it could reduce
the binding of gp96 by nearly 90%. This observation suggests that gp96
and hsp90 share a common receptor. Unlabeled hsp70 did not inhibit
binding of FITC-gp96 to CD11b+ cells at the scale
that gp96 or hsp90 did. At the highest hsp70 concentration tested
(unlabeled hsp70:labeled gp96 ratio of 10:1), the degree of inhibition
of binding of FITC-gp96 to CD11b+ cells was
almost identical with the inhibition observed by SA. Nonetheless, the
inhibition by unlabeled hsp70 was titratable, whereas inhibition by
unlabeled SA was not. This leads us to suggest ambiguously that
although hsp70 may interact with C11b+ cells
through a distinct receptor, it may at high concentrations interact
with the gp96/hsp90 receptor as well. There may thus exist a mechanism
of cross-talk among the various hsps and their receptors.
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To visualize the binding of hsps to APCs and the early downstream
consequences of such binding, paraformaldehye-fixed
CD11b+ cells incubated with FITC-gp96 were
analyzed by confocal microscopy. Peripheral staining of
CD11b+ cells with FITC-gp96 was observed (Fig. 6
A). All
CD11b+ cells were found to stain; i.e., no
heterogeneity in the population was observed. No staining was observed
when FITC-gp96 was incubated with P815 cells, an observation consistent
with the FACS data in Fig. 3
, that CD11b- cells
do not interact with hsp. Also, no staining of
CD11b+ cells was observed with FITC-SA. The
specificity of the interaction of hsps with APC observed by FACS
analysis was thus confirmed by confocal analysis. Similar to gp96,
incubation of FITC-hsp90 or FITC-hsp70 led to peripheral staining of
CD11b+ cells but not to
CD11b- P815 cells (Fig. 6
B).
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In addition to the above, the experiments shown here hint to some other novel aspects of hsp-APC interaction. They suggest that gp96 and hsp90 share a receptor and that hsp70 may have a distinct receptor, but it may also interact with the gp96/hsp90 receptor at relatively higher concentrations. This result is consistent with the close phylogenetic relationship between gp96 and hsp90 and with the lack of homology between them and hsp70 (9, 25, 26). Conserved regions in gp96 and hsp90 are most likely to be responsible for binding to this receptor. Secondly, our present results suggest that there is no obvious heterogeneity in the CD11b+ population with respect to binding any of the three hsps. This is in contrast to the suggestion from our previous study in which only a small subpopulation of CD11b+ cells were able to re-present gp96-chaperoned peptides (10). The differences between the two studies may lie in the events further downstream of binding such that binding of hsps to the cells may be necessary but not sufficient for re-presentation of hsp-chaperoned peptides. This remains to be examined. Structural characterization of the hsp receptor(s) is the obvious next step, which will provide a degree of finality to these suggestions and, hopefully, open a new chapter in our understanding of the role of APCs in priming of specific T cell responses.
Note added in proof. A receptor for heat shock protein gp96 has recently been identified as CD91 (27).
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Pramod K. Srivastava, University of Connecticut School of Medicine, MC1601, Farmington, CT 06030-1920. ![]()
3 Abbreviations used in this paper: hsp, heat shock proteins; PEC, peritoneal exudate cells; SA, mouse serum albumin. ![]()
Received for publication April 25, 2000. Accepted for publication June 19, 2000.
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