Intratumoral Mediated Immunosuppression is Prognostic in Genetically Engineered Murine Models of Glioma and Correlates to Immunotherapeutic Responses

The extrapolation of preclinical data, using murine models to determine clinical efficacy of novel therapeutics, is confounded by many issues including, but not limited to, immunologically incompetent model systems and clonotypic tumors that do not fully recapitulate the heterogeneous nature of human tumors (1, 2). In addition, many experimental cell lines, by being perpetuated in vitro (in some cases for decades), have significantly diminished immunosuppressive properties when compared with the phenotype and function of cancer cells isolated immediately from human tumors (3). Ideally, screening immunotherapeutic approaches for clinical trial implementation should be selected for robust responses in immuno competent murine model systems in which the murine gliomas correlate with the biology of human malignant gliomas (4–6). The use of genetically engineered mouse models of malignancy has been shown to simulate human responses within the context of clinical trials (7, 8) and thus provides distinct advantages for evaluating immune prognostic factors and immunotherapeutics.

Ligand interaction of the platelet-derived growth factor B (PDGF-B) receptor, which is overexpressed in human malignant gliomas (9, 10), results in activation of prosurvival signaling pathways that promote tumor cell growth (11). Murine model systems have shown that overexpression of PDGF-B in the brain induces grade II/III oligodendroglioma (12). We and others have shown that expression of other genes can cooperate with PDGF-B to enhance tumor formation and promote progression to a higher grade tumor (T. Doucette, Y. Yang, W. Zhang, G. Fuller, D. Suki, D. Fults, et al., manuscript submitted; refs. 13, 14). PDGF-B, upon ligand binding and in a manner dependent on Src kinase activity, rapidly phosphorylates and activates signal transducer and activator of transduction 3 (STAT3), forming p-STAT3 (15). Under normal physiologic conditions, STAT3 activation depends on ligand–receptor interaction, primarily under the control of growth factor receptor tyrosine kinases or cytokine and G-protein receptors with associated Janus kinase-2 (16, 17). However, in cancer cells, these tyrosine kinases are among the most frequently activated oncogenic proteins. STAT3 has been shown to be persistently activated in most human cancers, including gliomas (18). STAT3 activation entails protein phosphorylation at the tyrosine-705 site (p-STAT3), dimerization, nuclear translocation, and subsequent binding to consensus promoter sequences of target genes, thus initiating transcription of many genes, including Bcl-2, implicated in tumorigenesis (19–21). Overall, the STAT3 pathway mediates a transcriptional response favoring tumor survival, proliferation, and angiogenesis (22).

Tumor-expressed STAT3 also induces STAT3 expression in a variety of immune cells, resulting in global immunosuppression (23). STAT3 expression in macrophages inhibits their activation (24) and induces a polarization from the effector M1 phenotype to the immunosuppressive M2 phenotype (25, 26). Moreover, STAT3 expression can reduce the cellular cytotoxicity of natural killer cells and neutrophils, as well as the expression of MHC II, CD80, CD86, and IL-12 in dendritic cells, rendering them unable to activate T cells and to generate antitumor immunity (27). Furthermore, STAT3 has been shown to be a transcriptional regulator of forkhead box protein (FoxP3; ref. 28) and regulatory T cell (Treg) functional activity (29). Finally, STAT3 has been shown to maintain the proliferation and multipotency in glioma cancer stem cells (30), including their immunosuppressive properties (31). Cumulatively, these data indicate that the STAT3 pathway is a key molecular hub in tumor-mediated immunosuppression.

STAT3, and its downstream regulated genes such as Bcl-2, can be blocked with WP1066 (32), an orally bioavailable small molecule inhibitor with excellent central nervous system penetration and minimal dose-limiting toxicity (33). WP1066 can exert direct antitumor activity including the induction of caspase-dependent apoptotic cell death (32, 34, 35) and inhibition of angiogenesis (36). Moreover, WP1066 is a potent inducer of proinflammatory (37) responses and can reverse the functional immunosuppression of macrophages (26) and glioma cancer stem cells (31).

To study tumor-induced immunosuppression in endogenously arising gliomas, we used the RCAS/Ntv-a transgenic mouse system. In this system, a gene is cloned into a modified avian retrovirus (RCAS) that is replication defective in mammalian cells. The vector is introduced into Ntv-a mice that express TVA (avian leukosis virus subtype A receptor, the receptor for RCAS) under control of the Nestin promoter. Nestin-positive cells include glioneuronal precursors, the presumed cells of origin for glial tumors (5, 38). The gene is incorporated into the cell genome and is expressed by the constitutive retroviral promoter, long terminal repeat. This method of somatic cell gene transfer has been used to assess gene overexpression in vivo and to model various brain tumors (39–41). We used this model for PDGF-B and Bcl-2 coexpression. Bcl-2 lies downstream in the STAT3 signaling pathway, making tumors formed by the coexpression of PDGF-B + Bcl-2 relevant to the study of STAT3 biology. Because Ntv-a mice are immunocompetent, they are ideal for the study of immunosuppression induced by these gliomas. Most current murine model systems use either immunodeficient mice with human tumor xenografts or syngeneic clonotypic cell lines, which make it difficult, if not impossible, to appreciate the immunologic influence on tumor biology in the native animal.

We hypothesized that the gliomas formed in Ntv-a mice are immunosuppressive, that the high-grade gliomas would be more immunosuppressive relative to the low-grade gliomas, and that this model of endogenously arising malignant gliomas could be used to test immunotherapeutics. Here, we show that constitutive expression of PDGF-B + Bcl-2 in this model system induces intratumoral p-STAT3 and macrophages, similar to the induction of these cells observed in human malignant gliomas. We also show that this model system can be exploited for testing immunotherapeutics.

The RCAS-Bcl-2 vector was a gift of Dr. Daniel Fults (University of Utah) and the details of its creation are described elsewhere (42). Briefly, this vector was constructed by ligating a PCR-generated cDNA corresponding to the entire coding sequence of human Bcl-2 into the retroviral vector RCASBP. RCAS-PDGF-B was a gift of Dr. Wei Zhang (M. D. Anderson Cancer Center), and the details of its creation are described elsewhere (43).

Live virus was produced using the plasmid versions of the RCAS vectors transfected into DF-1-immortalized chicken fibroblasts (grown in DMEM medium containing 10% FBS; Gibco) in a humidified atmosphere of 95% air/5% CO2 at 37°C), using FuGene6 (Roche).

Bcl-2 expression after infection with RCAS-Bcl-2 was verified by exposing untransfected DF-1 cells (cultured to 50% confluency) for 48 hours to filtered medium conditioned by DF-1/RCAS-Bcl-2–transfected cells. Cells were then fixed with 4% paraformaldehyde in PBS followed by treatment with cold methanol, and immunocytochemical labeling was done by standard methods, using a mouse monoclonal antibody against human Bcl-2 (1:200; Santa Cruz Biotechnology) and goat anti-mouse Alexa Fluor 594 fluorescent conjugate (1:500; Molecular Probes) for detection. After mounting and labeling of cell nuclei with Prolong Gold antifade reagent with DAPI (Molecular Probes), staining was examined using a Zeiss Axioskop 40 microscope. Western blotting was used for the verification of PDGF-B expression from DF-1 cells after infection. Whole-cell lysates were prepared from DF-1 cell cultures 48 hours after infection with virus-containing medium conditioned by DF-1 cells expressing RCAS-PDGF-B. Protein samples (10 μg) were fractionated by SDS-PAGE using gels containing 10% polyacrylamide, transferred to polyvinylidene difluoride membrane, and probed with the anti-HA antibody (1:1,000; F7, Santa Cruz Biotechnology) to detect PDGF-B expression. Secondary antibody used for detection was goat anti-mouse IgG (1:2,500; Pierce). The blots were developed with the ECL Plus Detection kit (GE Healthcare) following the manufacturer protocol.

Creation of the transgenic Ntv-a mouse has been previously described (44). The mice are mixtures of the following strains: C57BL/6, BALB/C, FVB/N, and CD1. To transfer genes via RCAS vectors, transfected DF-1 producer cells (1 × 10⁵ cells in 1–2 μL of PBS) were injected into the right frontal brain lobe of Ntv-a mice from an entry point just anterior to the coronal suture of the skull using a 10-μL gas-tight Hamilton syringe. Mice were injected within 24 to 72 hours after birth because the population of Nestin-positive cells producing TVA receptors diminishes progressively with time. For coinjection of RCAS-PDGF-B and RCAS-Bcl-2, equal numbers of DF-1 cells were injected. The mice were sacrificed 90 days after injection or sooner if they showed neurologic morbidity related to tumor burden, including hydrocephalus or disability. The brains were fixed in formalin, embedded in paraffin, sectioned for immunohistochemical analysis, and analyzed for tumor formation. Histologic verification of tumor formation and determination of low- or high-grade type were carried out by the study neuropathologist (G.N.F.). High-grade tumors were differentiated by the presence of microvascular proliferation, mitotic activity, and necrosis. The animal experiments described in this research were approved by the Institutional Animal Care and Use Committee at The University of Texas M. D. Anderson Cancer Center (protocol 08-06-11632).

Formalin-fixed, paraffin-embedded 4-μm sections of the glioma were first deparaffinized in xylene and rehydrated in ethanol. Endogenous peroxidase was blocked with 0.3% hydrogen peroxide/methanol for 10 minutes at room temperature. Then, the ThermoScientific PTModule (Thermo Fisher Scientific) with citrate buffer (pH 6.0) was used for antigen retrieval. Immunohistochemical staining was done using the Lab Vision Immunohistochemical Autostainer 360 (Thermo Fisher Scientific). The staining was visualized using an avidin-biotin complex technique with diaminobenzidine (Invitrogen) as the chromogenic substrate and hematoxylin as the counterstain. A mouse monoclonal anti-HA antibody (1:50; Santa Cruz Biotechnology) was used to detect the HA epitope tag on the PDGF-B gene. To detect expression of human Bcl-2 expressed by RCAS, we used a primary monoclonal antibody specific for human Bcl-2 (1:100; Santa Cruz Biotechnology). To detect GFAP expression, a rabbit polyclonal anti-GFAP antibody was used (1:500; DAKOCytomation). To detect p-STAT3 expression, we used a rabbit polyclonal anti-p-STAT3 (Tyr705) antibody (1:50; Cell Signaling Technology). To detect expression of the macrophage-restricted cell surface glycoprotein F4/80, a purified anti-mouse F4/80 (1:50; Biolegend) antibody was used. To detect FoxP3 expression, a purified mouse anti-FoxP3 antibody (1:50; Biolegend) was used.

Two independent observers (L.-Y.K., A.W.) quantitatively evaluated p-STAT3, FoxP3, and F4/80 expression by analyzing the tumors with high-power fields (max: 400× objective and 100× eyepiece) of each specimen in the regions, with the highest relative positive staining for that individual specimen. The analysis was reviewed again by the neuropathologist (G.N.F.). The observers examined each tumor in a blinded fashion and in duplicate. Each observer recorded the absolute number of cells with positive staining. The duplicate numbers were then averaged for the final number of cells with positive expression per specimen.

To detect and quantify mitotic activity, formalin-fixed, paraffin-embedded, tumor-bearing tissue sections were immunostained with an antibody against pHH3. The number of positively stained cells in the area of highest tumor cell density were counted in 10 nonoverlapping high-power microscopic fields (magnification × 400) from 5 different tumor-bearing brains from the RCAS-PDGF-B and RCAS-PDGF-B + RCAS-Bcl-2 mice. The mitotic index was calculated as the number of positive cells divided by the number of total cells in each field. The median number of cells counted was 1,795 (range, 616–3,003).

WP1066, which blocks p-STAT3, was synthesized and supplied by Dr. Waldemar Priebe (M. D. Anderson Cancer Center). It was dissolved in dimethylsulfoxide (DMSO; Sigma-Aldrich).

After receiving the injection of the PDGF-B + Bcl-2 constructs as described, littermates were randomized to the treatment or control groups. Twenty-one days after introduction of the glioma-inducing transgenes, treatment was started with WP1066, administered by oral gavage (o.g.) in a vehicle of DMSO/PEG300 (20 parts/80 parts) once per day, every other day schedule (5 days on, 2 days off) for a total of 3 weeks. Fifteen mice per experimental group were used including treatment with the DMSO/PEG300 vehicle alone in the control group. This methodology of using RCAS/Ntv-a mice to determine treatment efficacy has been described previously (45).

Kaplan–Meier product-limit survival probability estimates of overall survival were calculated (46) and log-rank tests (47) were carried out to compare overall survival times between treatment groups and the control arm. Ex vivo or in vitro data are presented as the mean ± standard error of means (SEM). The Student t test was used. Comparisons of proportions were made using the Fisher exact test. A P value below 0.05 was considered statistically significant.

Both low-grade and high-grade malignant oligodendrogliomas with features of diffuse infiltration, pseudopallisading necrosis, endothelial cell proliferation, and tracking along white matter fibers were observed in animals that were transfected with PDGF-B or PDGF-B + Bcl-2 (Fig. 1A). The gliomas expressed GFAP (Fig. 1B). In mice injected with RCAS-PDGF-B alone, 42% (11/26) developed gliomas and 19% overall (5/26) developed high-grade malignant gliomas. In contrast, when mice were injected with both RCAS-PDGF-B + RCAS-Bcl-2, 82% (27/33) developed gliomas, with 61% overall (20/33) being high-grade malignant gliomas (Table 1). In mice that were transfected only with PDGF-B, 81% survived for 90 days at which time the experiment was terminated. In comparison, in mice that were cotransfected with PDGF-B + Bcl-2, only 44% survived to day 90 (P = 0.002; Fig. 1C). Mice injected with RCAS-Bcl-2 did not develop tumors.

Because a subset of mice that were injected with RCAS-PDGF-B, an inducer of p-STAT3, and the majority of mice injected with RCAS-PDGF-B + RCAS-Bcl-2 developed high-grade malignant gliomas, we determined whether these tumors showed induction of STAT3. The p-STAT3 expression within tumors in each group was measured using immunohistochemistry and expressed as the percentage of p-STAT3–positive cells (Fig. 2A). We observed p-STAT3 expression in all tumors regardless of model system or grade. Next, we quantified the extent of p-STAT3 expression by counting the number of p-STAT3–positive cells. In the tumors induced by PDGF-B alone (Fig. 2B), the percentage of cells that expressed p-STAT3 was 27.9 ± 11.1% in high-grade tumors (range, 11.5%–48.8%, n = 3) and 1.0 ± 0.1% in low-grade tumors (range, 0.8%–1.3%, n = 3). In the PDGF-B + Bcl-2 mice, the percentage of cells that expressed p-STAT3 was 24.1 ± 5.4% in high-grade tumors (range, 0.1%–49.7%, n = 12) and 2.6 ± 1.6% in low-grade tumors (range, 0.6%–7.3%, n = 4). The intensity of staining within tumor groups appeared similar.

Given the robust development of high-grade gliomas with marked lethality in the mice injected with RCAS-PDGF-B + RCAS-Bcl-2, this particular model system was selected for the analysis of prognostic markers both for survival and for treatment studies. The percentage of cells that expressed p-STAT3 was 35.0 ± 4.2% (range, 12.9%–49.7%, n = 8) in mice that died from tumor progression compared with 2.4 ± 1.2% (range, 0.1%–5.4%, n = 4) in long-term survivors that survived past 90 days and were euthanized without succumbing to their tumor (Fig. 2C). Indeed, all mice with less than 10% of cells expressing p-STAT3 survived for the entire 90-day duration, and all mice with cells expressing a p-STAT3 level of more than 10% died (Fig. 2D).

Because STAT3 is known to regulate Tregs and immunosuppressive macrophages, we next evaluated the presence and prognostic influence of intratumoral immuno suppression. Mice that developed high-grade gliomas developed a marked intratumoral influx of macrophages as determined by F4/80 immunohistochemical staining. There was a propensity in the PDGF-B + Bcl-2 model system for the F4/80-positive cells to colocalize with tumor along the infiltrating subependymal/periventricular edge of the tumors and at markedly higher levels in high-grade tumors than in low-grade tumors, which had few, if any, macrophages (Fig. 3A). In the PDGF-B–only mice, macrophage infiltration was 16.2 ± 1.7% (range, 13.2%–19.0%, n = 3) in high-grade tumors and 0.5 ± 0.2% (range, 0.3%–1.0%, n = 3) in low-grade tumors (Fig. 3B). In the PDGF-B + Bcl-2 group, high-grade tumors showed macrophage infiltration of 25.6 ± 3.8% (range, 3.3%–43.7%, n = 12), whereas low-grade tumors had macrophage infiltration of 1.7 ± 0.9% (range, 0.3%–4.0%, n = 5). In the PDGF-B + Bcl-2 mice with high-grade gliomas, increased macrophage infiltration was associated with reduced survival time (Fig. 3C). Among long-term survivors (defined henceforth as mice surviving to 90 days that were euthanized without succumbing to their tumor), infiltration of F4/80-positive macrophages was 11.3 ± 3.8% (range, 3.3%–19.5%, n = 4), whereas in the mice that succumbed to intracranial tumors prior to 90 days, there was a macrophage infiltration of 32.8 ± 3.2% (range, 21.7%–43.7%, n = 8). Indeed, all mice with macrophage infiltration of less than 20% survived for the entire 90-day duration, and all mice with macrophage infiltration of more than 20% died (Fig. 3D).

Treg infiltration was determined using immunohistochemical staining for FoxP3. Similar levels of infiltration by FoxP3-positive cells were seen in both high- and low-grade tumors (Fig. 4A). In the PDGF-B–only mice, Treg infiltration was 5.7 ± 0.1% (range, 5.4%–5.9, n = 3) in high-grade tumors and 3.5 ± 0.8% (range, 2.2%–5.0%, n = 3) in low-grade tumors (Fig. 4B). In the PDGF-B + Bcl-2 mice, high-grade tumors had Treg infiltration of 3.6 ± 0.4% (range, 2.0%–6.4%, n = 12), whereas low-grade tumors had Treg infiltration of 2.8 ± 0.3% (range, 2.1%–3.0%, n = 3). In the PDGF-B + Bcl-2 mice with high-grade gliomas, increased infiltration of the tumors by the Tregs was not associated with decreased survival time (Fig. 4C). Among long-term survivors, infiltration of FoxP3-positive cells was 3.6 ± 0.6% (range, 2.9%–5.1%, n = 4), whereas in the mice that succumbed to intracranial tumors prior to 90 days, the FoxP3-positive cells was 3.6 ± 0.7% (range, 2.0%–6.4%, n = 8). There was no correlation between the number of FoxP3-positive cells and survival duration (Fig. 4D).

Because coexpression of RCAS-PDGF-B and RCAS-Bcl-2 in Ntv-a mice induces tumors that express p-STAT3 and have a high incidence of glioma formation, this model was utilized to test the feasibility of ascertaining the in vivo efficacy of WP1066. Treatment with WP1066 (o.g.) commenced on day 21 after the RCAS vector injection. The median survival time for the control group was 57 days, whereas for mice treated with WP1066, the median survival times were more than 90 days (P = 0.07) in comparison with the vehicle control–treated mice (Fig. 5A and summarized in Table 2). For the mice treated by o.g. with WP1066 at a dose of 40 mg/kg (n = 15), 73% survived long term (>90 days) (chi-square test, P = 0.07 relative to the control group in which 40% survived long term), and there was at least a 58% increase in median survival time when the experiment was terminated at 90 days (Fig. 5A). In addition, after 90 days, all 11 of 15 (73.3%) of the surviving mice treated with WP1066 had no evidence of any intracranial tumor on histopathologic examination compared with 5 of 15 (33.3%, P = 0.03) of the control mice. High-grade tumors developed in 1 of 15 (6.7%) of the WP1066-treated mice relative to 4 of 15 (26.7%) in the control mice (P = 0.16). The tumors in the mice treated with WP1066 showed a marked decrease in p-STAT3 expression (3.6 ± 1.6%; range, 0.6%–5.9%, n = 3) compared with the untreated mice (36.3 ± 10.6%; range, 6.1%–51.5, n = 4, P = 0.05; Fig. 5B). Similarly, WP1066 also decreased macrophage infiltration of tumors, with treated mice having 5.0 ± 3.3% F4/80-positive cells (range, 1.6%–11.5%, n = 3) compared with 17.4 ± 4.9% (range, 6.0%–26.9%, n = 4, P = 0.05) in control mice (Fig. 5C). WP1066 did not affect Treg infiltration of tumors, with treated mice having 6.1 ± 3.7% FoxP3-positive cells (range, 0.7%–13.3%, n = 3) compared with 6.1 ± 2.0% (range, 1.6%–12.8%, n = 5, P = 0.99) in control mice (Fig. 5D).

There are several distinctive and novel findings in this study. The first is that WP1066 shows efficacy in a heterogeneous, orthotopic glioma model system. Utilizing the RCAS/Ntv-a system, we showed that the histopathologic characterization of the tumors recapitulated many of the fundamental characteristics seen in human high-grade gliomas, including pseudopallisading necrosis, infiltration along white matter tracts, and diffuse infiltration. Previous preclinical studies of the effects of WP1066 on gliomas were confined to a clonotypic glioma in a nonorthotopic position in an immunosuppressed murine model (35). Second, WP1066 is exerting a therapeutic effect on oligodendrogliomas, which has not been previously described. Finally, we are not aware of any previous attempts at correlating a novel, murine model system, including tumor-mediated immunosuppression mechanisms, with human immune prognostic biology in the context of an immunotherapeutic strategy. Specifically, we showed that the expression in vivo of STAT3 in the RCAS-PDGF-B and RCAS-PDGF-B + RCAS-Bcl-2 models of glioma correlated with glioma grade and prognosis, similar to the findings in human glioma patients (18). In addition, the murine gliomas also showed an influx of macrophages that correlated with grade and prognosis seen in human glioma patients (48–51). These tumor-associated macrophages have previously been shown to enhance tumorigenesis, invasion, and angiogenesis in the tumor microenvironment (26, 52–55). There was a propensity of the macrophages to distribute in the subependymal region in the murine models, likely related to the mechanism of induction of tumor, in contrast to the localization of macrophages in regions of pseudopallisading necrosis commonly seen in human gliomas. Finally, we found FoxP3-positive cells were present, albeit low at 6% in the murine glioma microenvironment they did not correlate with grade and were not prognostic. These findings are comparable with observations in human patients with oligodendrogliomas (56). It is possible that the FoxP3 expression was occurring within the cancer cells (57); however, we confined our counts to those cells that had a lymphocyte morphology. These data, in conjunction with another study showing that the immune infiltration in spontaneous mouse astrocytomas evolved along with the stages of tumor development (58), indicate that these murine model systems may be suitable for the study of immunotherapeutic approaches because many features of tumor-mediated immunosuppression observed in these systems seem similar to those seen in human glioma patients.

The STAT3 inhibitor WP1066 exerted a therapeutic benefit, inhibited intratumoral p-STAT3 expression, and suppressed intratumoral macrophage infiltration that correlated with treatment response. Although FoxP3 expression is under transcriptional control of STAT3 (28, 59) and WP1066 has been previously shown to inhibit Tregs in the systemic circulation (29, 60), we did not find a decrease in the level of FoxP3 expression during treatment with WP1066. The failure of WP1066 to further decrease the numbers of FoxP3-expressing cells in the oligodendroglial like tumors in this animal model system may simply reflect the low baseline levels. Although p-STAT3 is not ubiquitously expressed in all tumor cells in the murine model system, there is nonetheless marked in vivo efficacy. This discrepancy can be resolved by the activity of WP1066 exerting direct effects both on tumor cells expressing p-STAT3 and on the immune cells expressing p-STAT3, which restrains their antitumor activity including recognition of tumor-associated and tumor-specific antigens. The inhibition of p-STAT3 restores antitumor immune clearance (27, 61) and therefore we suspect that WP1066 exerts a bystander effect on p-STAT3 negative–expressing cells by eradicating those tumor cells that can by immunologically recognized. This contention is supported by the fact that therapeutic efficacy of WP1066 is lost in nude models systems and with in vivo depletions of the CD4 and CD8 subpopulations (unpublished data).

A confounding problem in the use of these model systems is that tumor incidence is not 100% and thus a randomization schema is necessary. Furthermore, it is unpredictable which mice develop tumors, the exact onset of tumor development, and the pathologic grade at the time of treatment initiation. Finally, because the transgenes are introduced in the neonatal period, onset of treatment also needs to be stratified and/or randomized on the basis of age because most facilities would be unlikely to be able to accommodate sufficient breeding pairs to have adequate numbers of animals for entry into treatment groups. This did not pose a significant hardship in the current studies in which only 2 experimental cohorts were utilized; however, for much more complex immunotherapeutic efficacy trials, especially those involving combinational approaches, this will introduce complexity in the design. A recent study by Singh et al. proposed that the use of genetically engineered murine models might more reliably predict human clinical outcomes (8). In the context of their study, a correlation was made with chemotherapeutics. We now propose that these genetically engineered murine models may also be suitable for the testing of immunotherapeutics based, in part, on the biology recapitulating many of the hallmark features of high-grade gliomas, on the correlative nature of the immunosuppressive biology and, specifically, the intratumoral biology.

W. Priebe and A.B. Heimberger hold patents and have a financial interest in the development of WP1066.

We thank Audria Patrick and David M. Wildrick, PhD, for editorial assistance.

Grant Support: This work was supported by The Brain Tumor Society, the Anthony Bullock III Foundation (A.B.H.), the Mitchell Foundation (A.B.H.), the Dr. Marnie Rose Foundation (A.B.H.), and the U.S. National Institutes of Health grants CA120813-03, A177225-01, P50 CA093459 (A.B.H.), P50CA127001 (G.R., A.B.H.), and P50 CA093459 (W.P.). The funding organizations had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.