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CUTTING EDGE |

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Department of Pathology and the Committee on Immunology, and
Department of Medicine, Section of Hematology/Oncology, University of Chicago, Chicago, IL 60637
| Abstract |
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production and CTL
activity. These results suggest that pharmacologic inhibition of Stat6
signaling could potentiate anti-tumor immunity in
vivo. | Introduction |
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in lymph
nodes draining a tumor site correlates with tumor rejection, suggesting
that deficient T cell differentiation into a Th1/Tc1 (type 1) effector
population may comprise another mechanism for failed tumor rejection
(11). Additional evidence supporting an important role for
type 1 T cell responses comes from the observations that neutralization
of endogenous IL-12 prevents spontaneous rejection of immunogenic
tumors (11), that Stat1-deficient mice (defective in IFN
signaling) also fail to reject tumors (12, 13), and that
the inclusion of IL-12 in tumor-specific vaccine strategies can induce
potent rejection of pre-established tumors in vivo (2, 14). The reason why a type 1 T cell response might fail to be generated spontaneously in vivo is not clear. One potential mechanism might be through the dominant activity of factors that antagonize T cell differentiation down a type 1 pathway. In vitro, the cytokine IL-4 has been shown not only to promote CD4+ T cell differentiation into Th2 cells, but also to inhibit the ability of low doses of IL-12 to induce development of Th1 cells (15). More recently, IL-13 has been shown to share some properties with IL-4, but also to exert additional, nonoverlapping activities, which are critical for particular immune-mediated processes in vivo. Specifically, IL-13 has been shown to be necessary for the Th2-directed lung inflammation in mouse models of asthma (16), for clearance of intestinal nematodes (17), and for granuloma formation in response to Schistosoma eggs (18). Based on these collective observations, it is conceivable that the combined activities of both IL-4 and IL-13 contribute to imposing a brake on type 1 T cell responses in mouse models of tumor immunity.
One strategy to minimize the activities of both IL-4 and IL-13 is to eliminate a signaling molecule central to the biochemical response to both cytokines. In addition to sharing one receptor component (19), both IL-4 and IL-13 use Stat6 to mediate signals leading to gene transcription (20). Stat6-deficient mice have been generated by homologous recombination, and have been shown to be severely deficient in the biologic response to both IL-4 and IL-13 (21, 22). In vivo, Stat6-deficient mice are resistant to Leishmania infection (23), are resistant in asthma models (24), and show defective contact hypersensitivity (25), all consistent with diminished type 2-mediated immunity.
To determine whether elimination of IL-4 and IL-13 activities could
potentiate anti-tumor immunity, P1.HTR tumors were implanted into
Stat6-deficient mice. We show that this poorly immunogenic tumor was
spontaneously rejected by Stat6-/- mice but not
by Stat6+/+ control littermates, and that tumor
rejection was associated with increased specific CTL activity and
IFN-
production. These results suggest that pharmacologic inhibition
of Stat6 signaling could provide a strategy for potentiating the immune
response against tumors in vivo.
| Materials and Methods |
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Stat1-deficient mice were generated as described previously (26) and generously provided by Dr. David Levy (New York University, New York, NY). Stat6-deficient mice were obtained from Dr. Michael Grusby (21). In each case, mice were backcrossed for six generations onto DBA/2 mice (The Jackson Laboratory, Bar Harbor, ME). Heterozygous mice were intercrossed to obtain wild-type (+/+), heterozygous (+/-), and homozygous knockout (-/-) animals. All mice were maintained under specific pathogen-free conditions in a barrier facility at the University of Chicago. Mice between 6 and 10 wk of age were used for experiments. PCR of tail DNA was used to identify mice carrying the targeted Stat1 or Stat6 genes. For Stat1, a set of three primers was used for PCR analysis: P1 (5'-GAGATAATTCACAAAATCAGAGAG-3'), P2 (5'-CTGATCCAGGCAGGCGTTG-3'), and P3 (5'-TAATGTTTCATAGTTGGATATCAT-3'). Thirty-five cycles were performed, using an annealing temperature of 50°C. For Stat6, independent primer pairs were used to amplify thewild-type Stat6 gene and the inserted neomycin gene. The Stat6-specific primers were 5'-TGCTGGGCCGAGGCTTCACATTT-3' and 5'-TATCTGTGAGGAGCCATCCTGAC-3', and the neomycin-specific primers were 5'-GCCCGGTTCTTTTTGTCAAGACCGA-3' and 5'-ATCCTCGCCGTCGGGCATGCGCGCC-3'. For each of these, 45 cycles were performed at an annealing temperature of 55°C. PCR products were resolved using 1.52% agarose gels and visualized by ethidium bromide staining. Bands of distinct sizes corresponded to the wild-type and targeted alleles.
Cells
Two variants of the mastocytoma P815 were used in this study: P1.HTR, a highly transfectable variant of P815 (27), and P511, a subclone of P815 known to express the tumor Ag P1A. HTR.B7-1 and HTR.IL-12 (and a control transfectant expressing empty vector, HTR.C) were generated by transfection of P1.HTR cells as described previously (2, 28). The irrelevant syngeneic tumor L1210, which lacks expression of the known P815 Ags, was used as a control target.
In vivo tumor experiments
Cultured tumor cells were washed three times with Dulbeccos PBS (DPBS),3 and 106 living cells were injected s.c. in 100 µl DPBS via a 27-gauge needle on the left flank. tumor size was assessed twice per week using calipers, the longest and shortest diameters were measured, and a mean was calculated. Data from groups of three to seven mice were analyzed at each time point, and a mean and SE were determined. Measurements continued for 34 wk, after which time the mice were sacrificed and the spleens were removed for restimulation in a mixed lymphocyte-tumor culture (MLTC).
MLTC and cytolytic assay
Splenocytes (5 x 106) from immunized or control mice were stimulated with irradiated (10,000 rad) HTR.B7-1 cells (2.5 x 105) in a volume of 2 ml, and 56 days later effector function was analyzed. For cytolytic assays, unfractionated cells from the MLTC were washed, adjusted to 2 x 106/ml, and titrated in duplicate in V-bottom microtiter plates along with 2000 51Cr-labeled target cells. Supernatants were collected after 4 h, and transferred to 96-well LumaPlates (Packard, Meridien, CT). After overnight incubation to allow drying, radioactivity was measured using a microplate scintillation counter (Packard). Percent specific lysis was calculated using standard methods.
Lymph node stimulations and functional assays
For analysis of cytokine production, mice were injected into
each hind footpad with 106 living HTR.C cells in
50 µl DPBS; control mice received DPBS. After 5 days, the draining
popliteal lymph nodes were harvested and single cell suspensions were
prepared. Cells (106) were incubated in the
presence or absence of 2.5 x 105 irradiated
(10,000 rad) HTR.B7-1 cells, supernatants were harvested after 48
h, and residual cells were removed by centrifugation. IFN-
and IL-4
concentrations were determined using an ELISA with Ab pairs obtained
from PharMingen (San Diego, CA). Concentrations were expressed in units
per milliliter as determined by the respective recombinant cytokines as
standards. For cytolytic assays, mice were injected with
106 HTR.IL-12 cells, and 5 days later, the
draining popliteal lymph nodes were removed and analyzed for
P815-specific cytolytic activity directly ex vivo, without in vitro
expansion.
| Results |
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We have shown previously that the highly transfectable
variant of the P815 mastocytoma, P1.HTR, grows progressively in the
majority of syngeneic DBA/2 mice (28). This growth does
not appear to result from global or tumor-specific immunosuppression,
as mice bearing large pre-established tumors on one flank can reject
B7-1-transfected tumors on the opposite flank (2). To
determine whether augmentation of a type 1 T cell response could enable
rejection of this tumor, transfection to express murine IL-12 p35 and
p40 cDNAs was performed. As shown in Fig. 1
A, IL-12-transfected P1.HTR
tumors were rejected in normal DBA/2 mice but grew progressively in
irradiated mice, supporting rejection by an immunologic mechanism.
Control-transfected tumors grew progressively (data not shown; see
also Fig. 2
). Mice that rejected IL-12
transfectants subsequently rejected challenge with wild-type P1.HTR
cells 1 mo later, consistent with immunologic memory (data not
shown).
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P1.HTR is spontaneously rejected by Stat6-/- mice
Inasmuch as promotion of a type 1 T cell response by IL-12 could
enable rejection of P1.HTR cells, it was conceivable that elimination
of an endogenous inhibitory factor for type 1 differentiation could
have a similar consequence. IL-4 has been shown not only to promote
type 2 T cell responses but also to inhibit T cell differentiation into
type 1 effector cells (15). As IL-13 has been shown to
share some functional properties with IL-4 (29), it was
desirable to use a strategy to eliminate activity of both cytokines in
vivo. To this end, mice deficient in Stat6, which is required for
signaling by both IL-4 and IL-13, were employed as tumor recipients. As
shown in Fig. 2
, although P1.HTR tumors grew progressively in
Stat6+/+ mice, there was slowing of tumor growth
in Stat6+/- mice, and complete rejection in
Stat6-/- littermates. Thus, elimination of
Stat6 signaling on host cells was sufficient to enable spontaneous
rejection of this normally progressively growing tumor.
Effector cells isolated from Stat6-/- mice have
increased lytic activity and produce increased levels of IFN-
To determine whether absence of host Stat6 resulted in improved T
cell differentiation into a cytolytic type 1 phenotype, spleen cells
were isolated from all groups of mice after the
Stat6-/- animals had rejected. These were
restimulated in vitro for 5 days with HTR.B7-1 cells, and cytolytic
activity was measured against 51Cr-labeled P511
cells in the presence of unlabeled L1210 cells to eliminate nonspecific
lysis. As shown in Fig. 3
A,
substantially greater tumor-specific cytolytic activity was observed
from Stat6-/- mice compared with
Stat6+/+ littermates. CTL activity from
heterozygous mice was intermediate in magnitude.
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To examine the cytokine profile of tumor-specific effector T cells in
this system, P1.HTR tumors were implanted into the hind footpads of
Stat6+/+ and Stat6-/-
mice, and popliteal lymph node cells were isolated 5 days later. These
were then stimulated in vitro with HTR.B7-1 cells, and supernatants
were assessed for presence of IFN-
and IL-4. As depicted in Fig. 4
A, substantially greater
levels of IFN-
were produced by lymph node cells isolated from
Stat6-/- mice than by those isolated from
Stat6+/+ littermates. In no instance was IL-4
detected at appreciable levels (Fig. 4
B). These results
indicate that the absence of host Stat6 signaling results in improved
generation of type 1 effector cells, but that the presence of Stat6 in
wild-type mice did not result in the induction of detectable
tumor-specific T cells that produce IL-4.
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| Discussion |
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or FasL by tumor cells. It is not yet known whether the antagonistic factor limiting anti-tumor immunity in wild-type mice is IL-4, IL-13, or both. It is of interest that other model systems have not demonstrated improved tumor rejection in the absence of IL-4 (30), suggesting that IL-13 is an attractive candidate. As it appears that functional IL-13 receptors are not expressed by T cells (29), the hypothetical effect of IL-13 would likely be indirect through another cell type. The source of IL-4 and/or IL-13 also is not clear, as we have not detected significant production of Th2 cytokines by tumor-specific T cells in this model. Low levels of basal IL-4 and/or IL-13 production by nontumor-reactive T cells or by non-T cells could provide sufficient activity to mediate this block on type 1 T cell development.
It is of interest that Stat6+/- mice showed a phenotype that was intermediate between those of the Stat6+/+ and Stat6-/- animals, with partial slowing of tumor growth and a modest increase in CTL activity observed. These results are consistent with a Stat6 gene dosage effect. Inasmuch as the Stat6-/- mice were backcrossed six generations onto the DBA/2 background, it is conceivable that a gene other than the targeted Stat6 locus is mediating the increased tumor rejection observed. This is unlikely, as we observed progressive tumor growth in Stat6+/+ littermate controls. However, the possibility that a gene tightly linked to Stat6 is mediating this effect cannot be entirely excluded.
Although multiple experimental tumor models have supported a dominant role for type 1 T cell responses in promoting optimal tumor rejection, some models have demonstrated a contribution of type 2 cytokines as well. This role has been most clearly demonstrated in experiments using GM-CSF-transfected tumor cells as a vaccination strategy, in which eosinophils driven by IL-4 and IL-5 appear to contribute to the effector phase of the immune response (30). However, rejection of many tumor types has been shown to be promoted by IL-12-based treatment strategies (2, 14), and endogenous IL-12 and IFN are important for the spontaneous rejection of immunogenic tumors (11, 13). In the P1.HTR model, there is a clear hierarchy of rejection potential by mouse genetic variants, with Stat6-/- DBA/2 mice being superior to wild-type DBA/2 mice, which in turn are superior to Stat1-/- DBA/2 mice at rejecting tumors. These observations suggest that Stat1 and Stat6 signaling pathways should be examined in patients with advanced cancer for possible alterations, which if perturbed could either contribute to poor tumor rejection or be caused by the presence of large tumor masses. The development of pharmacologic agents that target Stat6-dependent signal transduction could have applicability in the immunotherapy of cancer.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Thomas F. Gajewski, University of Chicago, 5841 South Maryland Avenue, MC2115, Chicago, IL, 60637. ![]()
3 Abbreviations used in this paper: DPBS, Dulbeccos PBS; MLTC, mixed lymphocyte-tumor culture. ![]()
Received for publication August 24, 2000. Accepted for publication October 12, 2000.
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