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Cutting Edge |
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Departments of
*
Neurology and
Microbiology and Immunology and
The Cancer Center, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642; and
Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892
| Abstract |
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, and glioma rejection is compromised in
IL-10-/- hosts. Hence, our findings demonstrate that
IL-10-producing CD4+ T cells can manifest antitumor
functions and suggest that IL-10 may have proinflammatory effects in
disease states. | Introduction |
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and IL-2 (the
"bystander" effect), or by licensing APCs through the CD40R
(1, 2). Consequently, experimental tumor vaccines are
often directed against MHC class I-restricted epitopes and incorporate
Th1-promoting adjuvants (3, 4). However, there is a growing recognition that CD4+ T cells may play alternative roles in tumor immunity independent of CD8+ CTL responses (5). In some instances these antitumor CD4+ T cells do not fall within the Th1 subset but paradoxically produce cytokines that can act as suppressors of Th1-mediated inflammation. For example, tumor-specific CD4+ Th2 cells act in collaboration with more traditional Th1 effectors to reject experimental B16 melanomas in vaccinated C57BL/6 mice (6). The development of effective immunotherapies will be facilitated by an understanding of the entire range of T cell phenotypes that can exert tumoricidal effects and could potentially be harnessed for therapeutic purposes.
Gliomas are the most common and most lethal primary brain tumors of
adults (7). Immunotherapy offers an attractive
experimental approach against these malignancies because it has the
potential to selectively target migrating glioma cells while sparing
adjacent healthy brain tissue. In this study, using a s.c. model of
glioma cell growth in mice, we demonstrate that
CD4+, but not CD8+, T cells
play a critical role in tumor rejection following vaccination with
irradiated tumor cells. Surprisingly, glioma-specific
CD4+ T cells produce IL-10, but neither IL-4 nor
IFN-
, and glioma rejection is compromised in
IL-10-/- hosts. Hence, the types of antitumor
CD4+ T cells should be expanded to include IL-10
producers with a cytokine profile reminiscent of the Tr1 subset
(8).
| Materials and Methods |
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C57BL/6 wild-type mice were obtained from either the National
Cancer Institute (Fredrick, MD) or The Jackson Laboratory (Bar Harbor,
ME). C57BL/6 mice genetically deficient in either IFN-
, IL-4, IL-12
p40, IL-10, MHC class II, or
2-microglobulin
were obtained from The Jackson Laboratory. All mice were housed under
specific pathogen-free conditions.
Glioma cell lines
Cryopreserved GL261 tumor fragments were obtained from the National Cancer Institute Tumor Repository (Frederick, MD) and maintained in monolayer cultures in RPMI 1640 with 10% FCS and standard supplements. CT-2A cells were obtained from Dr. T. N. Seyfried (Boston College, Boston, MA) and grown in DMEM with 10% FCS and standard supplements. Both cells lines were found to be negative for the presence of indigenous murine viruses (standard panel of 12) by the Mouse Antibody Production test (Charles River Laboratories, Wilmington, MA). Glioma cells were harvested from culture by treatment with trypsin, washed three times, counted by trypan blue exclusion, and resuspended in PBS before s.c. or i.p. inoculation.
Priming and challenge of mice with glial tumor cells
Mice were injected i.p. with three weekly doses of 1 x 106 irradiated GL261 cells (15,000 rad) in 0.1 ml of PBS. Control mice were injected with vehicle alone according to the same schedule. One week following the final inoculum all mice were anesthetized with Avertin, shaved across the lower back, and challenged s.c. with 4 x 106 live GL261 cells in 0.1 ml of PBS. In certain experiments, mice were injected i.p. with rat anti-mouse IL-10 (1 mg each of mAbs SXC-1 and SXC-2 per injection; Ref. 9) or control rat IgG (2 mg; Sigma-Aldrich, St. Louis, MO) on days 0, 3, 6, and 9 post challenge. Tumor size was determined as the product of length and width measured by a blinded examiner using calipers.
Cytokine quantification
Supernatants of splenocyte cultures were collected at 24-h intervals and subjected to a standard sandwich ELISA technique using noncompeting pairs of cytokine-specific mAbs as previously described (10). The lower limit of detection of each assay was 30 pg/ml or less.
RT-PCR
Total RNA was isolated from tumor biopsy specimens using RNAzol
RNA isolation solvent (Tel-Test, Friendswood, TX) and transcribed into
cDNA using reverse transcriptase. RT-PCR was performed with
primers specific for cytokines,
-actin, and the CD3
chain (BD
PharMingen, San Diego, CA).
| Results |
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We successfully stimulated protective immunity against the murine
glioma, GL261, by priming syngeneic C57BL/6 mice with three i.p.
injections of irradiated tumor cells in PBS. The vaccine was similarly
effective whether GL261 cells were obtained from cell lines propagated
in vitro (Fig. 1
A) or from
s.c. tumors serially passaged in nu/nu mice (data not
shown). One hundred percent of GL261-vaccinated mice rejected tumors
within 1 wk of challenge. In contrast, control mice inoculated with
either vehicle alone or with irradiated cells from an independently
derived C57BL/6 glial tumor cell line (CT-2A) experienced progressive
GL261 glioma growth up to the time of sacrifice. Furthermore, mice
primed with irradiated GL261 cells were not protected against challenge
with live syngeneic B16 melanoma cells or CT-2A cells (data not shown).
Collectively these results suggest that vaccination with GL261 cells
generates an immune response specific for an Ag expressed by the GL261
tumor, and not an Ag universally expressed on gliomas or transformed
cells.
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Flow cytometric analysis of the cells contained in GL261 gliomas
undergoing rejection revealed infiltration by both
CD8+ and CD4+ T cells, as
well as CD45+MHC class II+
cells that could serve as APCs (our unpublished observations).
We next assessed the relative contributions of CD4/CD8 cell subsets to
the antitumor response. C57BL/6 wild-type, MHC class II-deficient, and
MHC class I (
2-microglobulin)-deficient mice
were primed with irradiated GL261 cells or vehicle alone and
subsequently challenged with live GL261 cells. In contrast to their
wild-type counterparts, none of the MHC class II-deficient mice mounted
a protective anti-glioma response, suggesting that tumor-specific
CD4+ T cells are required for accelerated glioma
rejection (Fig. 1
B). In fact, tumor growth rates in
GL261-primed as well as mock-primed MHC class II-deficient mice were
comparable to those observed in immunodeficient nu/nu and
RAG2-/- mice (our unpublished
observations). In contrast, 100% of vaccinated
2-microglobulin-/-
mice successfully rejected their tumors during a 25-day observation
period. Therefore, we concluded that glioma rejection can proceed in
the absence of CD8+ T cells. Consistent with the
results of these in vivo studies, CD8+ T cells
purified from the spleens of GL261-vaccinated immunocompetent mice
failed to lyse glioma targets, as opposed to allogeneic lymphoblasts,
in 51Cr release assays (data not shown).
GL261-primed CD4+ T cells selectively produce the cytokine IL-10 upon challenge with glioma cells ex vivo
To investigate the mechanism underlying the anti-glioma
effects of CD4+ T cells, we harvested spleens
from vaccinated mice and measured cytokine production in response to
tumor challenge ex vivo. Surprisingly, whole splenocytes as well as
highly purified CD4+ T cells cocultured with
fresh APCs failed to produce cytokines that have been shown to have
antitumor effects in other experimental systems, including IFN-
,
IL-4, and TNF-
(data not shown). In contrast,
CD4+ T cells isolated from the spleens of both
primed wild-type or
2-microglobulin-/-
mice produced large quantities of IL-10 upon stimulation with mitomycin
C-treated GL261 cells (Fig. 2
A). We did not detect IL-10
in supernatants from GL261 cells cultured either alone or in
combination with splenocytes from primed MHC class II-deficient mice
(data not shown).
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We next questioned whether IL-10 is expressed within gliomas
undergoing rejection in vaccinated hosts. Tumors were removed from
GL261- or mock-primed C57BL/6 mice on day 11 following challenge and
RNA was isolated for RT-PCR with primers specific for a panel of
cytokines and the housekeeping gene,
-actin. CD3
chain mRNA was
also measured in an attempt to assess the extent of T cell
infiltration. At the time of harvest, gliomas were beginning to shrink
in the vaccinated cohort, whereas they were continuing to grow in the
control group. Messenger RNA encoding IL-10, but not IL-4, TGF
or
IFN-
was up-regulated in the gliomas from vaccinated relative to
control mice (Fig. 2
B). This was true despite low levels of
CD3
mRNA, suggesting that few infiltrating T cells were responsible
for significant intratumor IL-10 expression.
It is likely that the IL-10 is expressed by infiltrating
CD4+ T cells, because we detected it in the tumor
beds of primed wild-type and
2-microglobulin-/-,
but not MHC class II-/-, C57BL/6 mice (Fig. 2
C). IL-10 mRNA was also not detected in GL261 cells that
were freshly harvested from in vitro lines or from tumors grown in
immunodeficient mice.
IL-10 plays a physiological role in glioma rejection
To definitively determine which cytokines actively participate in
glioma rejection in vaccinated mice, we primed and challenged C57BL/6
mice deficient in either IL-12, IL-4, IFN-
, or IL-10 with GL261
cells. By day 10 post challenge, tumors were significantly smaller on
vaccinated IL-12-/-,
IFN-
-/-, and
IL-4-/- mice than the tumors borne by
their unvaccinated counterparts (p < 0.005,
Students t test). In fact, vaccinated
IL-12-/-, IFN-
-/-,
and IL-4-/- mice rejected GL261 tumors at a
comparable rate to identically treated immunocompetent animals (Table I
and data not shown). By contrast,
glioma rejection was significantly delayed in primed
IL-10-/- mice as well as in wild-type mice
treated with neutralizing Abs against IL-10.
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| Discussion |
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However, IL-10 is a cytokine with pleiotropic effects and has been shown in some instances to paradoxically augment tumor immunity. For example, introduction of the IL-10 gene into several rodent tumor cell lines, including mammary and pulmonary adenocarcinomas, melanomas, and lymphomas, unexpectedly increased their immunogenicity and rendered them highly susceptible to rejection (21, 22, 23, 24). Book et al. (25) reported that rat 9L gliomas transfected with IL-10 cDNA are heavily infiltrated with inflammatory cells and grow at a significantly slower rate than control gliomas (transfected with empty vector) following intracranial implantation. In addition, transgenic mice expressing IL-10 under the control of an MHC class II (as opposed to IL-2) promoter manifest enhanced resistance to the growth of a mastocytoma cell line that is tumorigenic in wild-type mice (26). Systemic IL-10 acts as an adjuvant when administered in conjunction with experimental tumor vaccines against melanomas, sarcomas, and colorectal and lung carcinomas in animals (21, 24, 27).
In all of the above instances the tumoricidal effects of IL-10 appeared to be T cell and/or NK cell dependent because they are abrogated in immunodeficient (T cell and/or NK cell depleted) mice. In fact, a number of recent studies demonstrate that IL-10 directly enhances NK cell, as well as CD8+ T cell, cytotoxicity in vitro. NK cells pretreated with IL-10, either alone or in combination with other cytokines such as IL-2, IL-18, and IL-12, are more efficient at lysing YAC-1 cells and tumor targets (28, 29). IL-10 might play a similar role in our model because GL261 gliomas are susceptible to lysis by activated NK cells ex vivo (data not shown). IL-10 may also exert indirect effects on tumor growth by down-regulating MHC expression on the tumor cells, modulating apoptotic pathways, stimulating the production of chemokines, and/or suppressing angiogenesis (23, 30, 31, 32).
Most of the previous reports demonstrating a contributory role of IL-10 in tumor rejection used highly artificial systems in which pharmacological doses of the cytokine were administered either systemically or locally via genetically engineered tumor cells. To our knowledge, this study represents the first time that IL-10-producing CD4+ T cells have been demonstrated to exert antitumor effects. Most importantly, the anti-glioma CD4+ T cells do not appear to be typical Th2 cells, because production of IL-4 could not be detected in the spleen or in tumor-infiltrating lymphocytes from vaccinated animals. Thus the IL-10-producing CD4+ T cells that we have described have certain properties in common with Tr1 cells that have been generated in vivo in the presence of IL-10 and are generally characterized as regulatory cells that dampen rather than promote inflammatory responses (8). Our findings anticipate the complexities that will arise with the use of cytokine modulating agents as therapeutic tools in oncology. Lastly, these results raise the possibility that IL-10 may have proinflammatory effects in certain autoimmune diseases.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Benjamin M. Segal, Departments of Neurology, Microbiology, and Immunology and The Cancer Center, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642. E-mail address: Benjamin_segal{at}urmc.rochester.edu ![]()
Received for publication September 26, 2001. Accepted for publication November 2, 2001.
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