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CUTTING EDGE |
Department of Biology and Therapy of Cancer, Institut National de la Santé et de la Recherche Médicale U 517, Faculty of Medicine and Pharmacy, Dijon, France
| Abstract |
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| Introduction |
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The ability of tumor cells to trigger a specific immune tolerance is a central event in tumor development. It was reported that tumor cells from various origins could express FasL and induce apoptosis of Fas-expressing T cells (4, 5, 6, 7, 8, 9, 10). From these observations, it was proposed that tumor cells could counterattack Fas-expressing activated tumor-infiltrating lymphocytes and escape rejection by the immune system. This Fas-mediated depletion of tumor-infiltrating lymphocytes was suggested to be an essential factor in the inhibition of anti-tumor immune response and in the induction of a specific tolerance to tumor Ags (11). Since these reports have been published, several of the anti-FasL Abs used have been shown to cross-react with unrelated molecules (12, 13). The conditions of the cytotoxicity assays, e.g., the importance of repeating washings in the JAM test, were also demonstrated to influence the results (14). Taking into account these recent developments, we revisited the tumor counterattack hypothesis. Since the role of FasL expression by tumor cells in immune tolerance to tumors was first suggested by studies bearing on human colon cancer cell lines, particularly SW480 and SW620 (4, 8), we examined these cell lines, as well as five other colon cancer cell lines, for their capacity to both express FasL and to kill Fas-sensitive target cells.
| Materials and Methods |
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Colon cancer cell lines SW480, SW620, HT29, Caco-2, HCT116, HCT15, HCT8, and Jurkat T cells were obtained from the American Type Culture Collection (Manassas, VA). The FasL-transfected and mock-transfected Neuro-2a cell lines (15) were a gift from Dr. A. Fontana (Zurich, Switzerland). Soluble FasL was obtained as described previously (15, 16, 17). L1210 cells and its murine Fas transfectant, L1210-Fas cells (kindly given by Dr. P. Goldstein, Marseilles, France), as well as the KFL9 cell line, a human FasL transfectant of human myeloid leukemia K562 cells (a gift from Dr. D. Kaplan, Cleveland, OH) (12), were used.
Modified JAM test
DNA fragmentation was measured using the modified JAM test (14, 18). Target cells were labeled with 2.5 µCi/ml [3H]thymidine (Isotopchim, Ganagobie-Peynius, France) for 20 h at 37°C. The target and effector cells were incubated for 5 h at 37°C at the indicated E:T ratios. The target cells were harvested by five washings and collected onto glass fiber filters (Skatron, Suffolk, U.K.). Each filter was counted in a liquid scintillation beta counter (Wallac, Turku, Finland). Specific cell killing was calculated using the following equation: % specific killing = (S - E/S) x 100, where E and S are the cpm of retained DNA, respectively, in the presence or the absence of effector cells.
Fas-sensitive cell growth during mixed culture
Jurkat, L1210, or L1210-Fas cells (5 x 102 cells/well) were seeded alone or mixed with SW480, SW620, or HT29 cells (5 x 105 cells/well) in a 24-well plate and cultured for 5 days before photographing the mixed culture and counting the floating lymphocytic cells.
Detection of FasL mRNA expression
Total RNA was isolated from colon cancer cell lines using the RNAble kit (Eurobio, Les Ulis, France). PCR was performed on the cDNA using intron-spanning primers (19). RT-PCR products were hybridized with the human full-length FasL DNA probe (Dr. S. Nagata, Osaka Bioscience Institute, Osaka, Japan) which was radiolabeled with the random primed DNA labeling kit (Boehringer Mannheim, Mannheim, Germany).
Immunoblotting analysis and immunoprecipitation
Subconfluent cultured cells were lysed in lysis buffer. Equal amounts of protein (80 µg) were separated by SDS-PAGE using a 10% polyacrylamide gel and electroblotted to polyvinylidene difluoride membranes (Bio-Rad, Ivry sur Seine, France). FasL was detected with G247-4 anti-human FasL mAb (PharMingen, San Diego, CA) and peroxidase-coupled anti-mouse IgG (Jackson Immunoresearch, West Grove, PA) using the enhanced chemiluminescence reaction system (Amersham, Arlington Heights, IL). The specificity of the results was verified by immunoprecipitation with the NOK-1 anti-human FasL (PharMingen) and protein A-Sepharose before immunoblotting analysis.
Confocal laser scanning microscopy analysis
Nonpermeabilized cells were incubated with NOK-1 mAb for 45 min at 4°C and then with secondary anti-mouse biotin-conjugated Ig for 45 min. After addition of FITC-conjugated streptavidin, cells were fixed in 2% paraformaldehyde for 20 min and then analyzed with a confocal laser scanning microscope as reported previously (20). A nonrelevant isotype-matching murine mAb was used as a negative staining control.
Cytofluorometric analysis
Cells were detached with 1 mM EDTA in PBS supplemented with 10 mM HEPES, adjusted to 1 x 106 cells/100 µl, and incubated with mouse anti-FasL mAb NOK-1 (PharMingen) for 45 min at 4°C. After washing, the cells were incubated with FITC-conjugated anti-mouse Ig (Amersham) for 30 min at 4°C. A total of 10,000 events was analyzed for each sample with a FACScan (Becton Dickinson, Grenoble, France).
| Results and Discussion |
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Colon cancer cell lines were previously shown to induce
apoptosis of Fas-sensitive target cells such as Jurkat T cells
by using the JAM test (4, 8). We repeated these
experiments with Jurkat and L1210-Fas cells as target cells and
parental L1210 cells as the negative control. However, according to the
recommendations of Böhm et al. (14), we multiplied
the washes to completely detach labeled target cells from tumor cell
monolayer. The JAM test was initially designed for measuring
apoptosis induced by floating killer cells (18).
When adherent tumor cells are tested as effector cells, a part of
radiolabeled target cells attaches to the tumor cell monolayer. These
cells are not recovered on filters and can be falsely considered as
undergoing cell death (14). When repeated washes were
performed, soluble FasL (16), as well as FasL-expressing
KFL9 cells, induced a significant and dose-dependent cytotoxic effect
on L1210-Fas cells without demonstrating any effect on the Fas-negative
parental L1210 cells. In contrast, none of the seven tested colon
cancer cell lines induced significant apoptosis of
Fas-expressing target cells (Fig. 1
). The
same result was obtained with Jurkat cells (data not shown).
Preincubation of colon cancer cell lines with 10 µM of the
metalloproteinase inhibitor KB8301 (PharMingen), which was used to
prevent potential FasL cleavage by cellular metalloproteinase
(21), had no influence on the ability of colon cancer
cells to trigger apoptosis of Fas-expressing target cells (data
not shown).
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When Jurkat cells and L1210-Fas cells were cultured in the
presence of colon cancer cell monolayers, these cells proliferated
vigorously, even at an initial ratio as high as 1 x
103 cancer cells for one Fas-expressing cell.
Cell count demonstrated that coculture with SW480 and SW620 cancer
cells did not inhibit and sometimes even enhanced the growth of Jurkat
and L1210-Fas cells (Fig. 2
A)
as compared with culture growth in the absence of a colon cancer cell
monolayer. Colonies of apparently unaltered Jurkat, L1210,
and L1210-Fas cells did appear above the monolayers of SW480 or SW620
cells, as they appeared when leukemic T cells were cultured alone (Fig. 2
B). When added at the end of this assay, soluble FasL
induced apoptosis of virtually all Jurkat and L1210-Fas cells,
whether these cells were cultured alone or in the presence of colon
cancer cells (data not shown). Thus, coculture of Fas-expressing cells
with cancer cells did not select Fas-resistant cell variants.
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RT-PCR experiments initially suggested that melanoma cells
expressed FasL mRNA (5). However, these results were not
confirmed when intron-spanning primers were used to avoid amplification
of contaminating genomic DNA (22). We used such
intron-spanning primers to detect the expression of FasL mRNA in the
studied colon cancer cells. A cDNA signal of expected size (234 bp) was
amplified in each of the tested colon cancer cell lines (data not
shown). The specificity of the signal detected in the five other cell
lines was confirmed by hybridization with a FasL cDNA probe. These
results confirmed previous data (22) showing that colon
cancer cell lines do express FasL mRNA. Immunoprecipitation with NOK-1
and immunoblotting with G247-4 of cell extracts of KFL9 cells and the
studied colon cancer cells identified the protein as a 40-kDa band
(Fig. 3
). These results demonstrate that
FasL mRNA and protein are expressed in colon cancer cell extracts.
However, expression levels were low when compared with those obtained
with the FasL-transfected KFL9 cell line.
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Differing from professional cytotoxic cells, such as T
lymphocytes and NK cells, tumor cells have no capacity to store FasL in
secretory lysosomes which undergo a polarized delivery to the interface
between the cytotoxic cells and its target cell (23). For
inducing apoptosis, FasL has to be constitutively expressed on
the tumor cell surface where it binds its receptor Fas, which is
located on the surface of target cells. The choice of the Ab is
critical, as only a few anti-FasL mAbs, including NOK-1, were shown
to specifically label FasL on the surface of nonpermeabilized cells
(12, 13). For example, the C-20 polyclonal Ab raised
against peptides from the extracellular domain of FasL was shown to
label the surface of SW480 cells (8), but this Ab is now
known to be nonspecific (12, 13). Using NOK-1 mAb,
confocal microscopy (Fig. 4
) and flow
cytometry (Fig. 5
A) easily
identified FasL protein on the surface of KFL9 cells. Under these
conditions, no protein could be identified on the surface of any of the
studied colon cancer cell lines (Fig. 5
A), even after
pretreatment of the cells with the metalloproteinase inhibitor KB8301
(Fig. 5
B). KFL9 cells were transfected to express relatively
large amounts of FasL (Fig. 3
). Consequently, the failure of the colon
carcinoma cell lines to express FasL may be a representation of the
poor sensitivity of the techniques, especially in comparison to the
sensitivity of the apoptosis assay.
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| Conclusions |
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| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. François Martin, Department of Biology and Therapy of Cancer, Institut National de la Santé et de la Recherche Médicale U 517, Faculty of Medicine and Pharmacy, BP 87900, 21079 Dijon, France. ![]()
3 Abbreviation used in this paper: FasL, Fas ligand. ![]()
Received for publication February 2, 2000. Accepted for publication March 23, 2000.
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