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Research Article

Tumor endothelial marker 1–specific DNA vaccination targets tumor vasculature

John G. Facciponte1, Stefano Ugel1, Francesco De Sanctis1,2, Chunsheng Li1, Liping Wang3, Gautham Nair4, Sandy Sehgal5, Arjun Raj4, Efthymia Matthaiou1, George Coukos1 and Andrea Facciabene1


1Ovarian Cancer Research Center (OCRC), University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA.
2Department of Experimental Medicine and Biochemical Science, University of Perugia, Perugia, Italy.
3Department of Pathology and Laboratory Medicine,
4Department of Bioengineering, and
5Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania, USA.

Address correspondence to: Andrea Facciabene, Ovarian Cancer Research Center, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA. Phone: 215.746.7071; Fax: 215.573.5408; E-mail: facciabe@mail.med.upenn.edu.

Authorship note: John G. Facciponte and Stefano Ugel contributed equally to this work.

First published March 18, 2014
Submitted: October 25, 2012; Accepted: January 16, 2014.

Tumor endothelial marker 1 (TEM1; also known as endosialin or CD248) is a protein found on tumor vasculature and in tumor stroma. Here, we tested whether TEM1 has potential as a therapeutic target for cancer immunotherapy by immunizing immunocompetent mice with Tem1 cDNA fused to the minimal domain of the C fragment of tetanus toxoid (referred to herein as Tem1-TT vaccine). Tem1-TT vaccination elicited CD8+ and/or CD4+ T cell responses against immunodominant TEM1 protein sequences. Prophylactic immunization of animals with Tem1-TT prevented or delayed tumor formation in several murine tumor models. Therapeutic vaccination of tumor-bearing mice reduced tumor vascularity, increased infiltration of CD3+ T cells into the tumor, and controlled progression of established tumors. Tem1-TT vaccination also elicited CD8+ cytotoxic T cell responses against murine tumor-specific antigens. Effective Tem1-TT vaccination did not affect angiogenesis-dependent physiological processes, including wound healing and reproduction. Based on these data and the widespread expression of TEM1 on the vasculature of different tumor types, we conclude that targeting TEM1 has therapeutic potential in cancer immunotherapy.

Introduction

Angiogenesis, the growth of new blood vessels from preexisting vessels, is essential for many physiological processes, such as pregnancy, embryonic development, and wound healing (1). However, uncontrolled or defective angiogenesis also contributes to many pathological processes, including tumor growth and metastasis (2).

Targeting angiogenesis is a promising strategy for the treatment of cancer. Tumor-associated blood vessels differ from vessels of normal tissue both structurally and functionally and can offer unique molecular targets for therapeutic interventions (35). There are several advantages to targeting the vasculature rather than the tumor cells proper. Tumor-associated vascular antigens are broadly expressed on cancers of different histological types, whereas they are generally absent or poorly expressed on most normal tissues (68). Immunotherapy may also elicit a significant bystander effect, because each capillary is thought to support hundreds of tumor cells and adaptive or antibody-mediated immune responses can easily access the tumor vasculature, whereas tumor islets may be compartmentalized and less accessible. Also, adaptive immunotherapy can persist, leaving a long-lasting memory response that protects the host against tumor recurrence. Finally, tumor cells are intrinsically more genetically unstable than vascular endothelial cells, and targeting them directly may promote tumor escape mechanisms (9).

Plasmid DNA offers a safe and promising method for vaccination and is approved for cancer immunotherapy for veterinary use (10). However, the immune responses induced to date by DNA vaccines in humans have been relatively weak compared with conventional vaccines, an issue that is particularly relevant for cancer vaccines because of the limited immunogenicity of tumor antigens. A combination of approaches are being studied to boost immune responses. In vivo electroporation of plasmid DNA or electrogene transfer increases DNA uptake, leading to enhanced expression in treated muscle and a concomitant increase in immune responses to the target antigen (11). The minimized domain of the C fragment of tetanus toxoid (TT) has been used to elicit antigen-specific immune responses (12). For example, DNA vectors encoding the tumor-associated antigen carcinoembryonic antigen (CEA) fused to TT can break tolerance to CEA and elicit significant antigen-specific CD8+ T cell–mediated immune responses (13).

Human tumor endothelial marker 1 (TEM1; also known as endosialin or CD248) is overexpressed in the vasculature of carcinomas and brain tumors as well as in the vasculature and stroma of most sarcomas (1417). Expression of TEM1 is variable among tumor types; it may be expressed on endothelial cells, pericytes, and fibroblasts in both mouse tumors (18) and human tumors (19, 20). Human TEM1 gene expression is also induced by hypoxia under the regulation of HIF-2α (21). In the mouse, TEM1 is overexpressed during embryonic development, but is largely absent in adult tissue (18, 22). TEM1 is one of the most abundantly expressed tumor endothelial or stromal antigens in human tumors, making it a prime candidate for immunological targeting.

We hypothesized that effective DNA vaccination using TT as an adjuvant would be able to break tolerance to TEM1 and generate adaptive immunity targeted to the tumor vasculature. We propose that such vaccination can elicit potent antitumor activity in TEM1-expressing tumors by ablation of the tumor vasculature without affecting normal physiological functions that are dependent on angiogenesis. We tested these hypotheses by genetically fusing mouse Tem1 cDNA with the first domain of fragment C of TT, creating a DNA vaccine (referred to herein as Tem1-TT) that we administered i.m. via electrogene transfer.

Results

DNA vaccination with Tem1-TT breaks tolerance to TEM1. Consistent with previous work in humans and mice, we found that Tem1 was overexpressed in actively growing tumors of different background or histology (TC1, CT26, and LLC models) and was expressed at very low to undetectable levels in mouse normal tissues, as quantified by quantitative RT-PCR (qRT-PCR; Figure 1A and data not shown). In the hypervascularized CT26 mouse colon carcinoma model, Tem1 mRNA was found to localize with, or to be in close juxtaposition to, CD31 mRNA (Figure 1B and Supplemental Figure 1; supplemental material available online with this article; doi: 10.1172/JCI67382DS1), which suggests that TEM1 is expressed by ECs and/or pericytes. These findings support the notion that TEM1 is an overexpressed vasculature-associated antigen in CT26 colon tumors, which can serve as a target for antitumor vascular therapy.

Physiological and plasmid DNA-induced TEM1 expression.Figure 1

Physiological and plasmid DNA-induced TEM1 expression. (A) Tem1 was expressed in tumors in vivo. qRT-PCR was performed with a Tem1 probe. An 18S probe was used as an endogenous control, and samples were normalized with mouse liver. Post–qRT-PCR calculations to analyze relative gene expression were performed by the 2–ΔΔCt method. Error bars denote SD (n = 5). (B) Tem1 RNA colocalized with CD31+ cells. CT26 tumor sections were subjected to immunofluorescence with a CD31 antibody (red) and single-molecule RNA FISH for Tem1 (yellow) or CD31 (cyan). Bright spots in the RNA FISH channels correspond to individual transcript molecules. DAPI staining (purple) shows cell nuclei. Original magnification, ×100. Images shown are 85 μm wide. (C) Expression of the Tem1-TT DNA plasmid. A fusion Tem1-TT construct was generated by fusing codon-optimized Tem1 cDNA (nt 1–2,297; aa 1–765) with the cDNA of the amino terminal domain of the fragment C of TT (819 bp, 273 aa). Expression of Tem1 or TT after transfection of the Tem1-TT construct was tested with CHO cells and assessed by qRT-PCR. Error bars denote SD (n = 3).

Using pcDNA3.1 as a DNA plasmid vector, the Tem1 gene sequence was fused to the first domain of the C fragment of the TT sequence (TT 865–1,120) at its carboxyl terminal (see Methods). Expression levels of both sequences were measured by qRT-PCR after transient transfection of CHO cells (Figure 1C). This fusion construct was used to vaccinate healthy mice at 1-week intervals for a total of 3 immunizations. Splenocytes harvested from vaccinated mice were stimulated with a TEM1 peptide library consisting of 15 mers that overlap by 10 aa. The TEM1 library encompassing the entire protein sequence was divided into 4 pools: pool A (sequences spanning aa 1–197), pool B (aa 187–377), pool C (aa 367–557), and pool D (aa 547–765). This peptide-based approach facilitates the detection of CD8+ and CD4+ T cells that are specific for TEM1 sequences in the context of MHC class I and MHC class II, respectively (23). IFN-γ enzyme-linked immunosorbent spot assay (ELISpot) and intracellular staining (ICS) indicated that the majority of CD8+ and CD4+ T cell epitopes in BALB/c mice were contained within pool C (Figure 2A). Spleen-derived CD8+ T cells from vaccinated C57BL/6 mice exhibited reactivity against pool D, whereas CD4+ T cell responses were not detected (Figure 2B). As hypothesized, only the Tem1-TT DNA fusion vaccine was able to break tolerance to TEM1; specific responses were not detected in mice of either strain that were vaccinated with DNA encoding Tem1 or TT adjuvant alone.

Characterization of TEM1 immunodominant epitopes.Figure 2

Characterization of TEM1 immunodominant epitopes. (A and B) Specific peptide pools restricted in H-2d (A) and H-2b (B) haplotypes. 1 × 106 splenocytes from BALB/c (A) or C57BL/6 (B) mice vaccinated with Tem1-TT were cultured with the 4 TEM1 peptide pools and tested by ELISpot and ICS for IFN-γ secretion. (A) Splenocytes from Tem1-TT–vaccinated BALB/c mice recognized pool C (left), which induced CD3+CD4+ (middle) and CD3+CD8+ (right) T cell responses. (B) Splenocytes from Tem1-TT–vaccinated C57BL/6 mice recognized pool D (left). By ICS, there was a specific CD3+CD8+ response (right). Data are mean ± SD (n = 5 per group) from 1 of 3 experiments. (C and D) Characterization of TEM1-derived peptides specific for H-2d (C) and H-2b (D) haplotypes. Peptides from pool C (C) or pool D (D) were divided into 7 minipools of 15 mers, so each individual peptide was present in 2 minipools and could be identified through an experimental matrix design. (C) Reactivity to pool C was present in minipools f, g, and 5. (D) Reactivity to pool D was present in minipools a and 6. Data are from 1 of 3 independent experiments. (E) TEM1696–710 induced IFN-γ secretion and cytotoxic effector activity. Splenocytes from TT-, Tem1-, or Tem1-TT–vaccinated mice was used in peptide-stimulated cultures and tested in an IFN-γ secretion ELISA (left) and CTL assay (right). Tem1-TT exclusively primed and expanded antigen-specific CD8+ T cells that killed ECs in the context of MHC class I/TEM1696–710 peptide, but not control peptides.

To characterize the TEM1 sequences recognized by CD4+ and CD8+ T cells generated by the Tem1-TT vaccine, pools C and D were deconvoluted using a matrix scheme. For the BALB/c strain, pool C was further divided into minipools. TEM1-specific reactivity was found against minipools f, g, and 5 (Figure 2C). IFN-γ ICS analysis revealed that minipools g and 5, corresponding to TEM1516–530 (ITSATHPARSPPYQP), induced a stronger CD8+ T cell response compared with minipools f and 5, corresponding to TEM1511–525 (GHKPGITSATHPARS). Both TEM1516–530 and TEM1511–525 peptides stimulated a CD4+ T cell response (data not shown). We therefore decided to use the TEM1516–530 peptide for subsequent monitoring of the immune response in BALB/c mice. Using the same approach to further divide pool D into minipools for testing the C57BL/6 background, a specific TEM1 response was detected for minipools a and 6, corresponding to the TEM1696–710 peptide (GQSQRDDRWLLVALL) (Figure 2D). Using IFN-γ ICS, this sequence was found to specifically activate CD8+ T cells (data not shown), which indicates that peptide TEM1696–710 contains the immunodominant CD8+ T cell epitope for C57BL/6 mice.

To confirm that the TEM1696–710 peptide mediates target cell recognition and killing by CTL, splenocytes from vaccinated C57BL/6 mice were harvested and tested in mixed leukocyte peptide–stimulated culture (MLPC). MBL2 leukemia cells were used as the stimulatory cells. MLPCs were pulsed with TEM1696–710 or control peptides for 5 days and tested for IFN-γ secretion by ELISA and in a 51Cr cytotoxicity CTL assay. IFN-γ ELISA revealed that Tem1-TT vaccination selectively generated antigen-specific T cells that recognize the TEM1696–710 peptide, but not the control peptides TEM1691–705 or β-gal96–103 (Figure 2E). Splenocytes from vaccinated C57BL/6 mice were also able to recognize unpulsed or TEM1696–710–pulsed H5V ECs, which express endogenous TEM1 protein, whereas MS1 ECs (which do not express TEM1; ref. 24) required TEM1 peptide pulsing to stimulate splenocytes from Tem1-TT–vaccinated mice (Figure 2E). In CTL assays, splenocytes from Tem1-TT–vaccinated mice efficiently lysed all 3 target cell types when pulsed with TEM1 peptide (Figure 2E). Together, these data suggest that Tem1-TT can break tolerance and elicit functional CD8+ T cells that are specific for TEM1 epitopes. Furthermore, ECs can naturally process the endogenous TEM1 antigen and are able to present the cognate TEM1 peptide in an MHC class I–restricted fashion.

Tem1-TT DNA vaccine suppresses tumor growth and induces CD3+ T cell tumor infiltration. To evaluate the ability of Tem1-TT vaccination to control tumor growth in vivo, we tested both prophylactic and therapeutic vaccine approaches using CT26, TC1, and LLC tumor models. Mice were vaccinated 3 times i.m. at weekly intervals, followed by tumor challenge (prophylactic), or tumor cells were inoculated first and then 3–5 days later were given 3 weekly vaccinations (therapeutic). Prophylactic treatment with Tem1-TT induced significant tumor protection compared with single Tem1 and TT constructs in all 3 tumor models (Figure 3A). These results reinforced the notion that the fusion construct Tem1-TT vaccine is essential to break TEM1 tolerance and generate protective immunity.

Tem1-TT vaccination controls tumor progression.Figure 3

Tem1-TT vaccination controls tumor progression. (A) Tumor control in a prophylactic setting. BALB/c and C57BL/6 mice were immunized 3 times with Tem1, TT, or Tem1-TT vaccines, then injected s.c. with 1 × 105 CT26, TC1, or LLC cells. Data are mean ± SD (n = 10 per group) from 1 of 5 experiments. P < 0.05, Tem1-TT vs. Tem1 and Tem1-TT vs. TT (CT26, TC1, and LLC), pairwise multiple-comparison Tukey test. (B and C) Inhibition of CT26 and TC1 tumor progression and induction of CD3+ T cell infiltration in a therapeutic setting. BALB/c mice were injected s.c. with 1 × 105 CT26 cells (B), and C57BL/6 mice were injected with 1 × 105 TC1 cells (C). Mice were immunized 1 day, 1 week, and 2 weeks later with Tem1, TT, or Tem1-TT vaccine and euthanized 33 (B) or 29 (C) days after tumor challenge. Left: Data points are mean ± SD (n = 10 mice/group) from 1 of 3 experiments. P < 0.05, Tem1-TT vs. Tem1 and Tem1-TT vs. TT (CT26 and TC1), Tukey test. CD3+ T cells in corresponding tumor sections and quantification are also shown. Original magnification, ×20. Data are mean ± SD of a representative experiment (n = 5 per group).

Therapeutic Tem1-TT vaccination conferred significant tumor protection compared with single construct controls in CT26 and TC1 tumor-bearing mice (Figure 3, B and C). Analysis of CD3+ T cell tumor infiltration by immunohistochemistry revealed consistent, heavy infiltration of CD3+ T cells after therapeutic vaccination with Tem1-TT within CT26 (42 ± 5 CD3+ cells/high-powered field [hpf]; P < 0.05 vs. TT) and TC1 tumors (54 ± 11 CD3+ cells/hpf; P < 0.05 vs. TT) compared with the single constructs and saline control (Figure 3, B and C). Cumulatively, these data indicate that the Tem1-TT vaccine induces significant T cell infiltration within tumors and exerts potent antitumor activity.

To demonstrate that the antitumor response by Tem1-TT vaccine is indeed immune mediated, and to determine whether the immune response is cellular or humoral in nature, we performed adoptive transfer of CD3+ T cells or sera from Tem1-TT–immunized mice into TC1 tumor-bearing mice (Supplemental Figure 2). Splenocytes from Tem1-TT–immunized mice were enriched for CD3+ cells and inoculated into naive mice. In simultaneous experiments, the serum from immunized mice was also transferred to naive mice. Mice were then sublethally irradiated and challenged with TC1 tumor. The antitumor response mediated by Tem1-TT vaccine was based on the induction of a CD3+ T cell response, whereas serum had no therapeutic efficacy (Supplemental Figure 2). Therefore, although antibodies may be generated by the Tem1-TT vaccine, the results of our adoptive transfer experiments ruled out any humoral contribution to antitumor efficacy.

Tem1-TT vaccination inhibits tumor vascularization. To determine whether Tem1-TT vaccination controls tumor progression by targeting the tumor vasculature, we used common readouts for measuring the effects on the vasculature, but also measured the functional dynamics of the vasculature in the hypervascularized CT26 model (25). To ensure that tumors of the same size were studied, mice that were immunized with Tem1-TT were sacrificed 10 days later than mice immunized with TT vaccine, and available tumors were harvested. Tumors of similar volume (∼200 mm3) from both groups were assessed for tumor hemoglobin levels by ELISA, as a readout for blood perfusion. Tumors from mice vaccinated with Tem1-TT had significantly decreased hemoglobin levels compared with those of TT-vaccinated mice (Figure 4A), suggestive of reduced red blood cell perfusion in these tumors.

Tem1-TT vaccination inhibits CT26 tumor vascularization.Figure 4

Tem1-TT vaccination inhibits CT26 tumor vascularization. (A) Tem1-TT vaccination reduced tumor hemoglobin content. Tumors at approximately 200 mm3 were excised from TT- or Tem1-TT–vaccinated mice and inspected grossly. Tumors from Tem1-TT–vaccinated mice appeared pale relative to control tumors. Reduced hemoglobin levels in tumors from Tem1-TT–vaccinated mice were observed by ELISA. (B) Tem1-TT vaccine reduces tumor vascularity. Tumors from TT- or Tem1-TT–vaccinated mice were analyzed by Doppler ultrasound. Perfused tumor area and real blood flux are shown, measured and calculated by Doppler image analysis. (C) CT26 tumors from Tem1-TT–immunized mice had significantly decreased CD31 expression compared with TT vaccination. Also note the abnormal blood vessel shape. Original magnification, ×20. (D) CAIX expression was increased in tumors from Tem1-TT–immunized animals. CAIX expression was visualized by immunohistochemistry, and Caix was independently quantified by qRT-PCR in tumors from mice vaccinated with either TT or Tem1-TT. Original magnification, ×20. Data in AD are mean ± SD of a representative experiment (n = 5 per group). Statistical analyses were performed with Student’s t test.

We then assessed the effects of Tem1-TT vaccination on the functional vasculature by measuring tumor vessel blood perfusion using ultrasound for each treatment group when tumors reached ∼600 mm3 in volume. Gas-filled microbubbles were injected i.v. and acted as a contrast agent as they flowed through the tumor blood vessels. Regions of perfusion in the tumors were visualized by ultrasound pulsing and contrast-enhanced Doppler imaging, which enabled us to determine the area of the tumor that was perfused (as a percentage of total tumor area) as well as the red blood cell flux (rate of blood flow) per unit area of the tissue (color-weighted fractional area) (26, 27). Contrast-enhanced images (after perfusion) showed a marked reduction in tumor perfusion (colored regions) in the Tem1-TT–treated group compared with TT-treated controls (Figure 4B). The reductions in both area of perfusion and blood flux in Tem1-TT–vaccinated tumors were statistically significant (P = 0.008 and P = 0.010, respectively; Figure 4B), which suggests that Tem1-TT vaccination disrupts the functional vasculature within the CT26 tumor.

We next assessed the apoptotic index of CT26 tumors by TUNEL assay. Compared with CT26 tumors from TT-immunized mice, cellular apoptosis increased in tumors from Tem1-TT–immunized mice (P = 0.014; Supplemental Figure 3A). We also observed colocalization of these apoptotic cells with the CD31 antigen (Supplemental Figure 3B), indicative of cell death of ECs of the tumor vasculature. Lastly, we also observed a reduction of cells positive for Ki-67, a marker for proliferating cells, within tumors from Tem1-TT– versus TT-immunized mice, which indicates that Tem1-TT vaccination lowers the proliferative capacity of the tumor, a likely consequence of disruption of the tumor vasculature (data not shown).

Next, the microvasculature density (MVD) of the tumors was measured by immunohistochemical staining of CD31. CT26 tumors from Tem1-TT–treated mice displayed reduced MVD compared with TT-treated tumors (Figure 4C). In addition to fewer CD31+ ECs per hpf, structural disorganization of the vasculature was noted: Tem1-TT–treated tumor vessels appear flattened with less luminal space (Figure 4C), suggestive of vessel collapse as well as difficulty assembling new vasculature. Lastly, we tested whether reduced tumor perfusion in Tem1-TT–vaccinated mice was associated with an increase in tumor-associated hypoxia. Using immunohistochemistry, an increase in carbonic anhydrase IX (CAIX), a cellular biomarker of hypoxia, was observed in tumors of Tem1-TT– versus TT-vaccinated mice (Figure 4D). Analysis by qRT-PCR also showed an increase in Caix message levels in tumors from Tem1-TT–treated mice relative to controls (Figure 4D). These data suggest an association between reduced tumor blood perfusion and hypoxia in CT26 tumors from Tem1-TT–vaccinated mice.

Tem1-TT vaccination induces T cell responses against tumor-associated antigens as well as TEM1 antigen. Next, we tested whether immunization against TEM1 by Tem1-TT vaccine results in epitope spreading, an immunological phenomenon in which immunotherapy or chemotherapy kills tumor cells and induces cross-presentation of tumor-associated antigens followed by cross-priming (28, 29). Tumor antigen–specific vaccines can also induce epitope spreading, inducing specificity against antigens unrelated to the original vaccine target (30, 31).

We hypothesized that the antivascular effects described above would provide a rich source of dead or dying tumor/stromal cells capable of supporting a corollary cross-priming event that would generate immune responses to antigens other than TEM1. We used the immunodominant AH1 (gp70423–431) peptide in the immunological assays. AH1 is the MHC class I–associated epitope for CT26 tumor, and as such, AH1-specific CD8+ T cells can cure mice of established CT26 tumor (32, 33). AH1 peptide is derived from an endogenous retroviral gene product, gp70, expressed by CT26 cells (32). Splenocytes from CT26 tumor-bearing BALB/c mice vaccinated with Tem1-TT exhibited a specific response against TEM1516–530 but not against the TEM1506–520 control peptide, as expected, but they also exhibited a vigorous response against the AH1 peptide, as measured by IFN-γ ELISpot (Figure 5A). Tumor-bearing mice immunized with control TT vaccine did not respond to any of these peptides (data not shown). As measured by IFN-γ ICS, Tem1-TT vaccine induced both CD4+ and CD8+ T cell responses against TEM1516–530, while the response to AH1 was restricted to CD8+ cells, as expected (Figure 5A). The frequency of the TEM1 CD8+ T cell response from Tem1-TT–vaccinated mice was inversely correlated with tumor volume (Figure 5A), as was the CD4+ T cell response (data not shown). A similar inverse correlation was found between the AH1-specific CD8+ T cell response from Tem1-TT–vaccinated mice and tumor volume (Figure 5A).

Tem1-TT vaccination induces secondary tumor cell–specific cytotoxic T cell Figure 5

Tem1-TT vaccination induces secondary tumor cell–specific cytotoxic T cell responses in tumor-bearing mice. BALB/c (A) and C57BL/6 (B) mice were challenged, vaccinated, and euthanized as in Figure 3, B and C, respectively. 1 × 106 splenocytes from TT-, Tem1-, or Tem1-TT–vaccinated mice were tested for their capacity to respond to the specific TEM1 peptide (TEM1516–530), and to AH1 antigen (A) or the TC1 tumor-associated E7 antigen (B). (A) Tem1-TT–vaccinated splenocytes mounted specific responses against the immunodominant peptide of TEM1 (TEM1516–530) and the AH1 antigen, but not control peptide (TEM1506–520), as measured by ELISpot. Tem1-TT induced CD3+CD4+ and CD3+CD8+ T cell responses against TEM1 antigen. The CD3+CD8+ T cell response from Tem1-TT–vaccinated mice correlated with suppression of tumor growth. ICS confirmed that Tem1-TT immunization induced an AH1-specific CD3+CD8+ T cell response that also correlated with tumor volume. (B) Splenocytes from mice vaccinated with Tem1-TT recognized TEM1696–710 peptide and the E7 antigen, but not control peptide (TEM1691–705). Only Tem1-TT vaccination was able to induce both a TEM1 and an E7 CD3+CD8+ T cell response, both of which correlated with tumor volume. Correlation analysis was performed by Spearman’s rank correlation.

To further understand the potency of the antivascular TEM1 response and the subsequent antitumor AH1 response, we performed adoptive T cell transfer from Tem1-TT–immunized naive or CT26 tumor–bearing mice to recipient mice that were challenged with CT26 tumor. CD3+ T cells from Tem1-TT–immunized naive and tumor-bearing donor mice both significantly increased the survival of recipient mice injected with CT26 (Supplemental Figure 4, A and B). This result showed that the anti-TEM1 immune response generated by Tem1-TT in the non–tumor-bearing donor mice (without concomitant cross-priming and epitope spreading, as in the tumor-bearing mice) was sufficiently potent to control tumor growth, comparable to CD3+ cells adoptively transferred from Tem1-TT–immunized tumor-bearing mice (Supplemental Figure 4, A and B). As expected, adoptively transferred CD3+ cells from Tem1-TT–immunized naive mice exhibited a response against TEM1516–530, but not against AH1 as in the tumor-bearing mice.

To confirm the epitope spreading phenomenon, we analyzed a second tumor model, TC1, a C57BL/6 epithelial lung tumor cell transformed by HPV E6 and E7 viral oncogenes. E7-derived peptide (E749–57) is the immunodominant epitope that can cure mice of established tumors (34). Mice immunized with Tem1-TT or control vaccine were challenged with TC1, as described previously. Splenocytes from Tem1-TT–vaccinated mice displayed CD8+-specific immune responses against TEM1696–710 peptide (but not the control peptide TEM1691–705), as well as against the E749–57 epitope (Figure 5B). As we observed in the CT26 model, the frequency of both TEM1- and E7-specific CD8+ immune responses correlated with TC1 tumor control (Figure 5B). Thus, effective Tem1-TT vaccination elicited a T cell–mediated response against both the tumor vasculature antigen (TEM1) and also against antigens expressed by the tumor cells themselves (gp70 and E7).

Tem1-TT vaccination does not affect wound healing or reproduction. Physiologic angiogenesis is critical for wound healing, and antiangiogenic agents such as bevacizumab have been associated with important safety concerns, including wound healing (3537). We assessed whether wound repair was impaired by Tem1-TT immunization in C57BL/6 mice by wounding animals on their backs after immunization. There was no significant difference in time to wound closure between Tem1-TT– and control-vaccinated groups, despite generation of robust anti-TEM1 responses as measured by IFN-γ ELISpot and ICS (Figure 6, A–C).

Wound healing is not delayed by Tem1-TT immunization.Figure 6

Wound healing is not delayed by Tem1-TT immunization. (A) Time course of wound healing. C57BL/6 mice were vaccinated 5 times with Tem1-TT or TT vaccine. 10 days after the final vaccination, 2 circular wounds were inflicted on the upper back of the mice. The wound area was measured every 2 days. Data are mean ± SD of 2 experiments (n = 10 per group for each experiment). P = 0.726, Tem1-TT vs. TT, pairwise multiple-comparison Tukey test. (B and C) TEM1-specific response. 20 days after the punch biopsies, mice were sacrificed to check the immune response against TEM1 antigen by IFN-γ ELISpot (B) and ICS (C) of bulk splenocytes (1 × 106). Statistical analyses were performed with Student’s t test.

Reproductive organs are sites of de novo angiogenesis, with both the corpus luteum and placenta being heavily dependent on formation of new blood vessels (3840). Antiangiogenic compounds have been associated with defective luteogenesis, reproductive dysfunction, pregnancy loss, and teratogenesis (4143). Previous studies have reported TEM1 expression in several tissues of the mouse embryo (22, 44). Postnatally, although TEM1 expression is downregulated in most organs, some expression persists in the renal glomerulus and in the adult uterus, where the expression pattern varies with the stage of the estrous cycle (45). Immunological targeting of the vascular TEM1 antigen could therefore have significant side effects on the reproductive system.

To test whether Tem1-TT immunization affects various aspects of mouse reproduction, female mice were immunized with Tem1-TT or TT vaccine and then mated (see Methods). Tem1-TT immunization had no effect on pregnancy success rate (P = 0.572), time to gestation (P = 0.698), total litter size (P = 0.552), and pup weight at birth (P = 0.409) compared with control immunization (Figure 7A). Furthermore, there were no anatomical or histological abnormalities in embryos from Tem1-TT– or control-immunized groups during early (E10) or late (E19) gestation (Figure 7B). We also assessed the potential effects of Tem1-TT immunization on various organs of vaccinated pregnant mice postpartum and found no abnormality in heart, lungs, liver, uterus, ovary, or placenta (Figure 7C). Similarly, organs from nonpregnant mice, including the uterus and the renal glomeruli, had no abnormalities after Tem1-TT immunization. To ensure that pregnancy did not dampen the effectiveness of Tem1-TT vaccination, we measured TEM1-specific T cell responses (TEM1696–710) by IFN-γ ELISpot and ICS in the same female mice as above that were impregnated and gave birth, as well as in Tem1-TT–immunized mice that did not become pregnant after vaccination (Figure 7E and Supplemental Figure 5). TEM1 responses were roughly equal between pregnant and nonpregnant mice that were Tem1-TT immunized (Figure 7E). The ICS assay similarly showed that the TEM1 response was roughly of equal frequency between pregnant and nonpregnant mice and CD8+ specific (Supplemental Figure 5), as expected with this TEM1 peptide.

Tem1-TT vaccine does not affect reproduction.Figure 7

Tem1-TT vaccine does not affect reproduction. C57BL/6 female mice were vaccinated 5 times with Tem1-TT or TT vaccine followed by mating 1 week later. (A) Tem1-TT did not alter reproduction. Vaccination had no effect on fertility, gestation time, number of pups, or weight of pups (n = 15 per group). Data are representative of 2 independent experiments. Statistical analyses were performed with Student’s t test. (B) Tem1-TT vaccine did not affect embryonic development. H&E staining of embryos of female mice vaccinated with TT and Tem1-TT showed no abnormalities detected for either group. (C) Tem1-TT vaccination did not induce tissue pathology. Histological analysis of lung, heart, liver, spleen, kidney, colon, ovary, uterus, and placenta from pregnant (postpartum) and nonpregnant mice (no placenta) revealed no pathological effects by either vaccine. Original magnification, ×10, except spleen and ovary (×4). (D) Tem1-TT vaccination did not affect the estrus cycle. In a separate independent experiment, sera from immunized mice had synchronized estrus cycles and were evaluated for reproductive hormones. No fluctuations were noted. (E) TEM1-specific T cell responses were unaltered by pregnancy. 1 × 106 splenocytes of vaccinated pregnant and nonpregnant mice were tested against TEM1696–710 peptide or control peptide (TEM1691–705) using IFN-γ ELISpot. Mice were boosted with a single injection of Tem1-TT or TT 45 days after the last immunization, then sacrificed 2 weeks later. There was significant recognition of TEM1696–710 peptide by Tem1-TT– versus TT-vaccinated splenocytes from both pregnant and nonpregnant mice splenocytes (n = 5).

Lastly, because antiangiogenic agents may disrupt luteogenesis (38, 46, 47), we also tested reproductive endocrine function after Tem1-TT vaccination in a separate independent experiment. Mice were vaccinated only, then had their estrous cycle synchronized by medroxyprogesterone, and serum levels of 17β-estradiol (E2), progesterone (P4), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured longitudinally. Patterns of E2, P4, FSH, and LH expression were similar between TT- and Tem1-TT–vaccinated groups (Figure 7D); therefore, Tem1-TT did not modulate hormone levels during estrus. Collectively, Tem1-TT vaccination appears to have no untoward effects on physiological processes in mice that depend on de novo angiogenesis.

Discussion

Tumor vasculature is critical for tumor growth and presents unique immunological epitopes, becoming an attractive target for therapy. Although many antiangiogenic and vascular-disrupting therapeutic approaches have been developed, immunotherapy offers the unique advantage of protective memory. Active immunization against whole tumor–derived ECs has produced encouraging preclinical results, but is not practical (4850). Development of active vaccination targeting vascular epitopes has been previously reported, but safety has not been adequately addressed (31, 51). In some experiments using adoptive therapy of T cells engineered to recognize the vascular growth factor 2 receptor, there was severe dose-dependent toxicity (52). Thus, identification of unique epitopes on the tumor vasculature that are suitable for immunotherapy is of high significance for the tumor immunology field. TEM1 is a surface protein expressed by tumor endothelial and endothelial progenitor cells (53) as well as pericytes and tumor-associated fibroblasts (19, 54, 55) and is found in the vasculature of tumors (4, 14, 5661), but not in normal vessels (57, 58). TEM1 is implicated in vascular cell adhesion, migration, development (62, 63), neoangiogenesis (57, 58), and tumor progression (20), and its overexpression is associated with poor survival (61). Importantly, Tem1–/– mice were previously shown to resist tumor growth, invasiveness, and metastasis, but were otherwise healthy and exhibited normal wound healing (64). Thus, knockout of TEM1 greatly attenuated tumor establishment and progression, without affecting physiologic angiogenesis. However, whether this null phenotype can be reproduced through therapeutic means is not known. We showed here for the first time that effective immunization against TEM1 closely phenocopied the Tem1–/– mouse, severely prohibiting the establishment of tumors and inhibiting the progression of tumor growth without affecting reproduction and/or would healing.

We developed a DNA vaccine that expresses the full-length mouse Tem1 cDNA fused to TT adjuvant. Immunization with the Tem1-TT vaccine reduced tumor vasculature compared with mice given control vaccine, as measured by MVD, functional imaging (ultrasound imaging of blood perfusion and blood flux), and hematocrit levels and indirectly supported by increased levels of CAIX in the tumor, suggestive of localized hypoxia therein. Splenocytes from Tem1-TT–vaccinated mice actively secreted IFN-γ and lysed TEM1-expressing ECs in vitro and specifically induced a cellular immune response that controlled tumor progression. Cumulatively, these findings indicate that vaccination with Tem1-TT specifically targets the TEM1-expressing cells of the vascular system and can functionally disrupt the tumor vasculature.

Breaking tolerance to TEM1 with DNA vaccine required the fusion of Tem1 and TT genes. TT may enhance immunogenicity through several mechanisms; the whole C fragment of TT activates DCs to secrete cytokines involved in CD4+ T cell activation (11, 65). In addition, fragment C contains universal T helper epitopes (p30, p2), which are effective across different MHC class II haplotypes in mice and humans and elicit strong CD4+ responses (66). Peptides within domain 2 of fragment C are strong CD8+ CTL epitopes, which may compete with the fusion peptides. Elimination of domain 2 enhances CTL responses to fused peptides (67), and thus we eliminated it from our TT construct accordingly. In addition, we generated localized inflammation by in vivo electrogene therapy during both prime and boost injections. Electrogene therapy enhances responsiveness (68, 69) and has facilitated translation of DNA vaccines into clinical trials (12, 70, 71). All of these attributes may be contributing to the generation of potent anti-TEM1 immune responses we observed with Tem1-TT immunization.

Our approach used full-length antigen rather than peptide, thus bypassing a major limitation of peptide-based cancer vaccines, namely MHC restriction. Genetic fusion of TT to the 3′ end of full-length antigens has been tested previously by us and others (13, 72). Our DNA vaccine elicited CD4+ and CD8+ anti-TEM1 responses in the BALB/c mouse and a CD8+ response in the C57BL/6 mouse. Given the prime/boost approach we used, it is likely that the responses observed are directed against the immunodominant TEM1 epitopes in each mouse strain (11, 73). Other adjuvant formulations may identify alternative dominant and/or subdominant epitopes.

In previous mouse studies, DNA vaccination with the tumor-associated antigens AH1 and E7 induced CD8+ cell–dependent regression of established tumors that expressed those antigens (33, 74). In the studies presented here, fusion of the TEM1 antigen to the TT adjuvant increased the immunogenicity of the target antigen and the efficacy of tumor control. It also generated robust CTL activity against tumor cell proper immunodominant epitopes, such as AH1 and E7. This was not achieved by direct targeting of these antigens, but rather indirectly by targeting the vasculature antigen TEM1. Taken together, our data suggest a model whereby Tem1-TT vaccination induces expansion of TEM1-specific T lymphocytes (Figure 8). These effector T cells target TEM1-expressing cells in the tumor vasculature, including tumor-associated ECs and/or pericytes. This immune attack impairs the tumor vasculature, with increases in local tumor-associated hypoxia and likely inflammation-related mediators, resulting in apoptosis of local tumor cells. These apoptotic bodies are scavenged by macrophages and DCs that process and present additional tumor-associated antigens. This secondary cross-priming event then expands tumor-specific cytotoxic CD8+ T cells, which work hand in hand with the anti-TEM1 immune response to eradicate larger numbers of tumor cells, resulting in improved control of tumor growth. The anti-TEM1 response and the antitumor immune responses against AH1 and E7 significantly correlated with control of tumor growth. Further studies will address the kinetics of the TEM1 and AH1 immune response. Our findings are consistent with the general paradigm of epitope spreading as a mechanism underlying superior immunotherapeutic outcomes and extend this concept to include T cell specificities against tumor-associated vasculature antigens (75).

Proposed mechanism of action of Tem1-TT vaccination.Figure 8

Proposed mechanism of action of Tem1-TT vaccination. (i) Tem1-TT vaccine induces expansion of TEM1-specific CD8+ T cells that target the CT26 tumor vasculature. These cytolytic effectors target and kill TEM1-expressing cells, most likely tumor-associated endothelial cells and/or pericytes. (ii) This event results in reduction of the tumor vasculature, inducing inflammation and tumor hypoxia, which results in localized tumor cell death. (iii) Dead tumor cells are scavenged by APCs, which process relevant tumor-associated antigens (TAA; e.g., AH1 antigen in CT26 tumor model). (iv) This cross-presentation event results in a secondary cross-priming event that expands tumor-specific cytotoxic CD8+ T cells to tumor cell–derived epitopes. (v) Tumor-specific cytotoxic CD8+ T cells lyse and eliminate tumor cells, resulting in control of tumor growth.

The results of our adoptive transfer experiments (Supplemental Figure 2) suggest that tumor control by Tem1-TT vaccination is mediated primarily by CD3+ cells, while serum (containing antibodies) does not play any major therapeutic role. We did not directly measure anti-TEM1 antibodies potentially generated by Tem1-TT, and therefore cannot entirely rule out the possibility that anti-TEM1 antibodies contribute to the observed efficacy of Tem1-TT vaccine in some capacity. Adoptive cell transfer also illustrated the effective generation of memory responses: mice challenged with tumor 6 months after adoptive transfer of CD8+ T cells remained tumor free (data not shown), which illustrated that Tem1-TT generated robust, long-lasting adaptive immune responses. The adoptive transfer experiments with the CT26 model showed that Tem1-TT vaccination generated TEM1-specific CD3+ T cells that, on their own, could directly delay tumor growth and confer survival advantage to mice. These data also showed that Tem1-TT vaccination generated a dually specific T cell response, with a primary TEM1-specific response targeting the tumor vasculature generated by Tem1-TT vaccine, and a secondary response targeting the tumor cell–specific antigen AH1 generated by epitope spreading.

An ideal tumor vasculature antigen is one that is overexpressed in tumors and poorly expressed in peripheral tissues. Expression of TEM1 in the mouse is spatiotemporally regulated. By using a lacZ knockin model, Tem1 gene expression was initially detectable in the kidney of prenatal embryos (45). Postnatally, Tem1 expression decreases in most organs, but persists in the renal glomerulus and in the adult mouse uterus. In mice immunized with Tem1-TT, we did not observe toxicity in association with angiogenic processes, such as reproduction or wound healing. It is perhaps surprising that the Tem1-TT vaccine could mediate tumor vessel destruction without causing significant toxicity during pregnancy and wound healing. However, angiogenesis during tumor development differs significantly from the uterine NK cell–driven angiogenesis seen during implantation and placenta development (76, 77) and from the highly organized, slow growth of wound healing. In addition, tissue specificity and cells involved in tumor vessel recruitment differ from superficial wounds, although many similarities exist (78).

Cumulatively, our data indicated that immunization with a DNA vaccine encoding Tem1-TT induces TEM1-specific cell-mediated immune responses, disrupts the tumor vasculature, and controls tumor growth without affecting important normal physiological functions. Our preclinical data set can serve as a foundation for clinical studies targeting human TEM1 tumor vasculature antigen.

Methods

Mouse strains and cell lines. 8-week-old C57BL/6 (H-2b) and BALB/c (H-2d) mice were purchased from Charles River. Lung tumor cell line TC1 transformed by HPV E6 and E7 viral oncogenes (H-2b), lung carcinoma line LLC (H-2b), colon carcinoma line CT26 (H-2d), leukemia cell line MBL2 (H-2b), and EC lines MS1 (H-2b) and H5V (H-2b) were cultured in RPMI 1640 (Cellgro; catalog no. 10-104-CV) or DMEM (Cellgro; catalog no. 40-101-CV) medium supplemented with 2 mM l-glutamine and 150 U/ml streptomycin plus 200 U/ml penicillin (Cellgro; catalog no. 30-0010CI) and 10% heat-inactivated FBS (Gibco; catalog no. 35-010-CV).

DNA vectors and immunization procedures. The vaccine construct was generated by fusing mouse Tem1 cDNA (nt 1–2,297; aa 1–765) with the cDNA of the aminoterminal domain of fragment C of TT. The TT fragment DNA was introduced at the 3′ end of the Tem1 coding sequence, generating the plasmid pcDNA3.1/Tem1-TT. The codon usage-optimized cDNA of Tem1-TT was synthesized by oligonucleotide assembly (GeneArt; Life Technologies). All constructs were routinely sequenced by the DNA sequencing core facility at University of Pennsylvania, and Tem1 and TT constructs were confirmed with the correct sequences. DNA immunization was performed as described previously (79). Briefly, 50 μg plasmid in saline was injected i.m., and electrogene transfer was performed (2 pulses at 100 mV for 200 ms) immediately after injection. In Figure 3A (prophylactic setting), mice were euthanized when tumors reached approximately 2,000 mm3.

RNA isolation and qRT-PCR. Total RNA was isolated from 100–500 mg frozen organs or tumors with TRIzol reagent (Invitrogen; catalog no. 15596-018). After treatment with RNase-free DNase (Invitrogen; catalog no. AM2222), RNA was reprecipitated, quantified by spectrophotometry, and analyzed for RNA integrity by gel or Bioanalyzer. Total RNA was reverse transcribed using the Superscript First-Strand Synthesis Kit (Invitrogen; catalog no. 10928-042) for RT-PCR under conditions described by the supplier. qRT-PCR was performed with inventoried mouse Tem1 probe (ABI Biosystems; catalog no. Mm00547485) as well as 18S probe as an endogenous control (4310893E-0710037). Samples for Tem1 expression in mouse organs were normalized using mouse liver as a calibrator. RNA isolated from tumor was used to set up qRT-PCR, with mouse Caix as a probe, as well as 18S probe as an endogenous control. To monitor TEM1 expression in transfected cells, CHO cells were transfected with 1 μg Tem1-TT plasmid or single constructs in a 6-well dish. After 48 hours of incubation, whole-cell lysates were harvested, and total mRNA was extracted using TRIzol. The Tem1 and TT RNA present in the cell lysates was detected by qRT-PCR with custom primers for optimized Tem1 and optimized TT, as designed by ABI Biosystems by PrimerExpress. Post–qRT-PCR analysis to quantitate relative gene expression was performed by the comparative Ct method (2–ΔΔCt). Error bars denote SD (n = 3) unless otherwise indicated.

RNA FISH. Frozen tissue was sectioned onto poly-l-lysine–coated coverglasses (BD Biocoat; catalog no. 354085), fixed in formaldehyde, permeabilized, and stored in 70% ethanol. The single molecule RNA-FISH protocol was carried out as described previously (80). Samples were hybridized to the FISH probes overnight at 37°C in 2× SSC with 10% (v/v) formamide (Ambion; catalog no. AM9342) and 10% (w/v) dextran sulfate (Sigma-Aldrich; catalog no. D8906). For Tem1 and CD31, probe mixtures consisted of 48 20-mer DNA oligonucleotides (Biosearch Technologies) each, designed to tile their respective coding sequences, and coupled to Cy5 (Tem1) and Cy3 (CD31). After washing with PBS to remove unbound probe and storage for a few days, samples were stained for CD31 by overnight incubation with biotin-conjugated rat anti-mouse CD31 (BD Biosciences — Pharmingen; clone MEC13.3) at 4°C in PBS (1:250 dilution) and a 20-minute incubation with FITC-labeled streptavidin (BD Biosciences — Pharmingen; catalog no. 554060) at 4°C in PBS. Samples were mounted in a glucose oxidase–based 2× SSC antifade buffer (Sigma-Aldrich; catalog no. G2133) and imaged on a conventional wide-field epifluorescence microscope (Nikon Eclipse Ti) with a ×100 1.4 NA oil immersion objective. Image stacks were acquired with 0.35-μm vertical spacing and collapsed into single 2D images by maximum projection.

Synthetic peptides. A mouse TEM1 peptide library of 151 peptides generated as 15 mers overlapping by 10 aa was synthesized by Mimotopes and dissolved in DMSO at approximately 20 mg/ml. Pools, minipools, and individual peptides were used at 2 μg/ml. kb-restricted TEM1691–705 peptide (ESGLAGQSQRDDRWL), TEM1696–710 peptide (GQSQRDDRWLLVALL), E7 peptide (RAHYNIVTF), β-gal (DAPIYTNV) and Ld-restricted TEM1506–520 peptide (PDLPFGHKPGITSAT), TEM1516–530 peptide (ITSATHPARSPPYQP), and AH1 peptide (SPSYVYHQF) were all synthesized by JPT. The 151 15-mer peptides were divided into pools A–D, with approximately 40 peptides each (pool D contained fewer peptides), to identify immunoreactive peptides (Figure 2, A and B). Once immunoreactive pools were identified, peptides from pools C and D (Figure 2, C and D) were divided into 7 minipools of 15 mers, so each individual peptide was present in 2 minipools and could be identified through an experimental 7×7 matrix design (23).

ELISpot. 96-well MAIP plates (Millipore; catalog no. N4510) were coated overnight with a 2.5 μg/ml solution of rat anti-mouse IFN-γ (BD Biosciences — Pharmingen; catalog no. 551216). Bulk splenocytes were plated at 1 × 106 cells/well in triplicate and incubated for 20 hours at 37°C with 2 μg/ml peptides. After incubation, plates were washed with PBS and 0.05% Tween 20 (Bio-Rad; catalog no. 170-6531) and incubated overnight at 4°C with anti-mouse biotin-conjugated anti–IFN-γ antibody (BD Biosciences — Pharmingen; catalog no. 551506). The following day, streptavidin-alkaline phosphatase conjugate (BD Biosciences — Pharmingen; catalog no. 554065) was added for 30 minutes. Plates were developed by adding nitroblue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate (Pierce), and spots were then counted using an automated ELISpot reader (Autoimmun Diagnostika GmbH).

ICS for IFN-γ. 5 × 106 mouse splenocytes in 1 ml RPMI with 10% FCS were incubated with the indicated peptides (final concentration of each peptide, 1 μg/ml) and 1 μg/ml brefeldin A (BD Biosciences — Pharmingen) at 37°C for 12–16 hours as previously described (81). Cells were washed, stained with surface antibodies, fixed, permeabilized, and incubated with FITC-labeled IFN-γ antibody (BD Biosciences — Pharmingen; catalog no. 554411). Cells were fixed with 1% formaldehyde solution in PBS and analyzed on a FACSCanto flow cytometer using Flowjo software (Treestar).

MLPCs. MLPCs were set up with 5 × 106 splenocytes from vaccinated mice stimulated in vitro with 1 μg/ml specific peptide. The culture was incubated for 5 days in DMEM with 10% FBS at 37°C in 5% CO2.

ELISA. 105 splenocytes from MLPCs were restimulated for 24 hours in triplicate with an equal number of target cells, supernatants were harvested, and IFN-γ secretion was measured by ELISA (Biolegend). Serum from vaccinated mice was evaluated by ELISA to check the level of P4 (Enzo; catalog no. ADI-900-011), E2 (Enzo; catalog no. ADI-900-008), LH (Uscnk), and FSH (Uscnk). Mouse hemoglobin ELISA kit was obtained from Kamiya Biomedicals. Briefly, 100 mg fresh tumor was lysed with 200 μl RIPA buffer. Mouse hemoglobin quantification was performed according to the manufacturer’s protocol with 2 μg/ml tumor lysate. Reading results were interpolated to test sample values from fitted calibration curve of mHgb serial dilutions. Each value represents the mean of triplicates from a mouse tumor. Representative results of 3 independent measurements are shown from 5 mice per group.

Cytotoxicity assay. Cytotoxicity was measured by 51Cr release assay. MLPCs were incubated with MBL2 cells, MS1 cells, or H5V cells that were unpulsed or pulsed with the TEM1696–710 peptide. Target cells were labeled with 100 μCi 51Cr (Amersham) before being mixed with effector cells. Assays were performed in triplicate, and supernatants were harvested to measure 51Cr released. Specific lysis was calculated from triplicate samples as (experimental cpm – spontaneous cpm)/(maximal cpm – spontaneous cpm) and expressed as a percentage.

Immunohistochemical tumor analysis. CAIX, CD31, and CD3 were detected by immunohistochemistry in solid tumors. Tumors were embedded in OCT medium and immediately snap frozen in dry ice. Sections (6 μm thick) were stained for mouse CAIX (R&D Systems; catalog no. AF2344, DAB chromogen), CD31 (BD Biosciences — Pharmingen; clone 390, catalog no. 558737, DAB chromogen), and CD3 (BD Biosciences — Pharmingen; clone 15.5-2C11, catalog no. 550275, DAB chromogen) with hematoxylin as a counterstain. For TUNEL assay, frozen sections (6 μm thick) of the OCT-embedded tissue were air dried for 30 minutes under the hood and fixed with 2% paraformaldehyde in PBS for 30 minutes at room temperature. Each slice was treated with 100 μl of 3% H2O2-methanol solution for 20 minutes at room temperature and washed 3 times in PBS. To cover the nonspecific binding sites, each slice was treated with 10% normal goat serum blocking solution for 1 hour at room temperature. Tissue sections were incubated overnight at 4°C with rat anti-mouse CD31 primary antibody (1:200 dilution; BD Biosciences — Pharmingen; catalog no. 550274). After washing 3 times in PBS, the slices were incubated with Texas Red goat anti-rat IgG secondary antibody (1:500 dilution; Vector Laboratories; catalog no. TI-9400) at room temperature for 1 hour. To identify the apoptotic dead cells in situ, TUNEL was performed with Cell Death Detection Kit (Roche Applied Science; catalog no. 11684809910) according to the manufacturer’s instructions. Images of the slides were taken using a Nikon Ti fluorescence microscope and a Leica TCS SP8 confocal microscope.

Ultrasound analysis of tumor vasculature. Lipid-coated microspheres filled with gas (Definity Lantheus Medical Imaging Inc.) were used as contrast agent with power Doppler imaging to visualize the regions of perfusion in the tumor. In brief, CT26 tumor-bearing mice (tumor volume, ∼600 mm3) were anesthetized and injected with 0.02 ml octafluoropropane gas (Definity Lantheus Medical Imaging), and power Doppler imaging was performed using a broadband 7- to 15-MHz probe (HDI500 SonoCT; Philips). Power Doppler images were acquired at a frame rate of 0.5 Hz to minimize microbubble destruction by the imaging ultrasound pulses. The Doppler signal from the inflowing contrast agent was visible in the images in color superimposed on the grayscale image of the tumor. Contrast-enhanced power Doppler imaging was performed as described previously (26). The power Doppler images at peak enhancement were analyzed to determine percentage area of the tumor with flow (percentage perfused area). The perfused tumor area for each treated group was calculated as the ratio of the area resolution by the contrast agent to the total tumor area in a single plane. Color-weighted fractional area of the colored pixels within the region of interest was measured as described previously (82). The color of each pixel in the contrast-enhanced power Doppler image measures the fractional volume of the contrast flowing through the pixel, and the color-weighted fractional area (product of color level and fraction area covered by colored pixels) measures the contrast volume per unit area of tumor (blood flux) (83).

Histologic evaluation of embryo and tissues in vaccinated mice. Several organs (liver, lungs, small and large bowel, kidneys, heart, testis, brain, and spleen) and embryos at different time points (E10 and E19) were collected to assess for possible treatment toxicity. Tissues were fixed in 10% buffered formalin and embedded in paraffin. Histopathologic evaluation was performed on standard H&E sections.

Reproduction and wound-healing assay. To test whether Tem1-TT immunization affects mouse pregnancy by interfering with prenatal angiogenic processes, 6- to 8-week-old female mice were immunized 5 times as previously described (79). 1 week after the last immunization, mice were mated for 48 hours with individually housed males. Coitus was monitored the following morning and confirmed by the presence of a vaginal plug. Half the mice were used to measure time to gestation, pup weight at birth, and total litter size. The other pregnant mice were used to check for gross abnormalities of the embryos at E10 and E19 at the whole-animal level as well as at the organ level. For ELISpot and ICS (Figure 7E and Supplemental Figure 5), 45 days after the last immunization, the pregnant mice above that gave birth to pups (for time to gestation and pup weight at birth) and nonpregnant mice were boosted with a single injection of Tem1-TT or TT DNA. 2 weeks later, mice were sacrificed to assess the immune response against TEM1 antigen in both assays.

We also performed a separate independent experiment to measure the effects of Tem1-TT on the estrus cycle. Mice were immunized once per week for 4 weeks. To synchronize the estrus cycle, female mice were then treated s.c. every day with 2 mg medroxyprogesterone (Sigma-Aldrich; catalog no. M6013), beginning 3 days after the final vaccination (84). After the last drug treatment, mice were sacrificed every 12 hours for 5 days. Serum was collected (3 mice per group pooled per time point) to check blood hormone levels.

The wound-healing assay used in this study was done according to previously described methods (85). Briefly, 7 days after the last immunization with DNA vaccine, mice were anesthetized, hair was removed, and skin was cleaned with an aseptic wipe. 2 circular defects were outlined using a fine-tipped marking pen. The defect was created by elevating the skin and panniculus carnosus in the center of the outlined defect using forceps, followed by excision of the outlined area using microdissecting scissors. The wound area was sealed with Tegaderm dressing (Nexcare; catalog no. 55379) to prevent infection. Wound area was measured every day and calculated with ImageJ software. Average time to wound closure was monitored and considered complete when a scar was formed without any visible scab left.

Adoptive cell transfer. Tumor-free mice (C57BL/6 and BALB/c) and CT26 tumor-bearing mice were vaccinated 3 times; mice were sacrificed 1 week after the last vaccination, and splenocytes were used to magnetically isolate the lymphocyte population (CD3+ T cells) (Miltenyl Biotec). Serum was also collected from vaccinated mice by intracardiac bleeding. Isolated lymphocytes and serum were injected in TC1 or CT26 tumor-bearing mice (challenged the day before adoptive transfer) that were sublethally irradiated (4–5 Gy) 3 days before adoptive transfer (i.v. and i.p., respectively). Serum was administered twice weekly for a total of 6 injections to another group of tumor-bearing mice.

Statistics. 2-tailed Student’s t test was used to compare data sets where indicated. Single-step multiple-comparison Tukey test was performed for repeated measurements (Figure 3) at different time points. For CD3+ T cell quantification in tumor sections (Figure 3, B and C), statistical analysis for the indicated comparisons was performed using Student’s t test. Correlation between immune response by Tem1-TT treatment and tumor volume was analyzed with Pearson’s rank correlation test (Figure 5), in which negative r was generated (inverse correlation); when P values are below 0.050, one variable tends to decrease, while the other increases. All P values presented are 2-sided. The log-rank test was performed for the survival curves in Supplemental Figures 2 and 4. A P value less than 0.05 was considered significant.

Study approval. All animal studies were approved by the IACUC and University Laboratory Animal Resources at the University of Pennsylvania. Mice were treated in accordance with University of Pennsylvania guidelines.

Supplemental data

View Supplemental data

Acknowledgments

This project was supported by the Basser Research Center for BRCA, the Alliance for Cancer Gene Therapy, the NIH Director’s New Innovator Award (1DP2OD008514), and the Pennsylvania Department of Health (4100051725).

Footnotes

Conflict of interest: The authors have declared that no conflict of interest exists.

Citation for this article:J Clin Invest. 2014;124(4):1497–1511. doi:10.1172/JCI67382.

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