Multi-ancestral origin of intestinal tumors: Impact on growth, progression, and drug efficacy

Background

Data are steadily accruing that demonstrate that intestinal tumors are frequently derived from multiple founding cells, resulting in tumors comprised of distinct ancestral clones that might cooperate or alternatively compete, thereby potentially impacting different phases of the disease process.

Aim

We sought to determine whether tumors with a multi‐ancestral architecture involving at least two distinct clones show increased tumor number, growth, progression, or resistance to drug intervention.

Methods

Mice carrying the Min allele of Apc were generated that were mosaic with only a subset of cells in the intestinal epithelium expressing an activated form of PI3K, a key regulatory kinase affecting several important cellular processes. These cells were identifiable as they fluoresced green, whereas all other cells fluoresced red.

Results

Cell lineage tracing revealed that many intestinal tumors from our mouse model were derived from at least two founding cells, those expressing the activated PI3K (green) and those which did not (red). Heterotypic tumors with a multi‐ancestral architecture as evidenced by a mixture of green and red cells exhibited increased tumor growth and invasiveness. Clonal architecture also had an impact on tumor response to low‐dose aspirin. Aspirin treatment resulted in a greater reduction of heterotypic tumors derived from multiple founding cells as compared to tumors derived from a single founding cell.

Conclusion

These data indicate that genetically distinct tumor‐founding cells can contribute to early intratumoral heterogeneity. The coevolution of the founding cells and their progeny enhances colon tumor progression and impacts the response to aspirin. These findings are important to a more complete understanding of tumorigenesis with consequences for several distinct models of tumor evolution. They also have practical implications to the clinic. Mouse models with heterogenous tumors are likely better for predicting drug efficacy as compared to models in which the tumors are highly homogeneous. Moreover, understanding how interactions among different populations in a single heterotypic tumor with a multi‐ancestral architecture impact response to a single agent and combination therapies are necessary to fully develop personalized medicine.

PD-1 and LAG-3 blockade improve anti-tumor vaccine efficacy

Concurrent blockade of different checkpoint receptors, notably PD-1 and CTLA-4, elicits greater anti-tumor activity for some tumor types, and the combination of different checkpoint receptor inhibitors is an active area of clinical research. We have previously demonstrated that anti-tumor vaccination, by activating CD8 + T cells, increases the expression of PD-1, CTLA-4, LAG-3 and other inhibitory receptors, and the anti-tumor efficacy of vaccination can be increased with checkpoint blockade. In the current study, we sought to determine whether anti-tumor vaccination might be further improved with combined checkpoint blockade. Using an OVA-expressing mouse tumor model, we found that CD8 + T cells activated in the presence of professional antigen presenting cells (APC) expressed multiple checkpoint receptors; however, T cells activated without APCs expressed LAG-3 alone, suggesting that LAG-3 might be a preferred target in combination with vaccination. Using three different murine tumor models, and peptide or DNA vaccines targeting three tumor antigens, we assessed the effects of vaccines with blockade of PD-1 and/or LAG-3 on tumor growth. We report that, in each model, the anti-tumor efficacy of vaccination was increased with PD-1 and/or LAG-3 blockade. However, combined PD-1 and LAG-3 blockade elicited the greatest anti-tumor effect when combined with vaccination in a MycCaP prostate cancer model in which PD-1 blockade alone with vaccination targeting a “self” tumor antigen had less efficacy. These results suggest anti-tumor vaccination might best be combined with concurrent blockade of both PD-1 and LAG-3, and potentially other checkpoint receptors whose expression is increased on CD8 + T cells following vaccine-mediated activation.

Treatment Combinations with DNA Vaccines for the Treatment of Metastatic Castration-Resistant Prostate Cancer (mCRPC)

Simple Summary

The only vaccine approved by FDA as a treatment for cancer is sipuleucel-T, a therapy for patients with metastatic castration-resistant prostate cancer (mCRPC). Most investigators studying anti-tumor vaccines believe they will be most effective as parts of combination therapies, rather than used alone. Unfortunately, the cost and complexity of sipuleucel-T makes it difficult to feasibly be used in combination with many other agents. In this review article we discuss the use of DNA vaccines as a simpler vaccine approach that has demonstrated efficacy in several animal species. We discuss the use of DNA vaccines in combination with traditional treatments for mCRPC, and other immune-modulating treatments, in preclinical and early clinical trials for patients with mCRPC.

Abstract

Metastatic castration-resistant prostate cancer (mCRPC) is a challenging disease to treat, with poor outcomes for patients. One antitumor vaccine, sipuleucel-T, has been approved as a treatment for mCRPC. DNA vaccines are another form of immunotherapy under investigation. DNA immunizations elicit antigen-specific T cells that cause tumor cell lysis, which should translate to meaningful clinical responses. They are easily amenable to design alterations, scalable for large-scale manufacturing, and thermo-stable for easy transport and distribution. Hence, they offer advantages over other vaccine formulations. However, clinical trials with DNA vaccines as a monotherapy have shown only modest clinical effects against tumors. Standard therapies for CRPC including androgen-targeted therapies, radiation therapy and chemotherapy all have immunomodulatory effects, which combined with immunotherapies such as DNA vaccines, could potentially improve treatment. In addition, many investigational drugs are being developed which can augment antitumor immunity, and together with DNA vaccines can further enhance antitumor responses in preclinical models. We reviewed the literature available prior to July 2020 exploring the use of DNA vaccines in the treatment of prostate cancer. We also examined various approved and experimental therapies that could be combined with DNA vaccines to potentially improve their antitumor efficacy as treatments for mCRPC.

Abstract 2262: Low-dose targeted radionuclide therapy has favorable local and systemic effects on immune populations in a murine prostate cancer model

Introduction: Prostate cancer remains the second leading cause of cancer-related death in American men because of its metastatic, incurable form. Prostate cancer responds poorly to checkpoint blockade likely due to its immunosuppressive microenvironment with relatively few infiltrating CD8+ T cells, limited mutational burden, and activity of myeloid-derived suppressor cells (MDSCs). External beam radiotherapy (EBRT) has been shown to help overcome this suppressive state, but EBRT is infeasible for patients with widely metastatic disease. Our group has developed a compound called NM600 that can be used to deliver radiation to all sites of disease simultaneously in an approach called Targeted radionuclide therapy (TRT). However, the effects of TRT on immune populations within prostate tumors are not yet well described. In this study, we examined the effects of TRT in the form of 90Y-NM600 on mice bearing Myc-CaP prostate tumors.

Methods: 6-week old male FVB mice were implanted subcutaneously with Myc-CaP cells, then given a single intravenous injection of either 50 (“low-dose”) or 250 (“high-dose”) μCi of 90Y-NM600, estimated to deliver 3.1 Gy or 15.5 Gy to 300 mm3 tumors, respectively. Groups of mice (n=3) were then euthanized at several timepoints following TRT administration. Their tumors and spleens were collected for analysis by flow cytometry and Luminex cytokine analysis. A separate group (n=5) were followed for survival to 2000 mm3.

Results: Flow cytometry of splenocytes revealed that high-dose, but not low-dose TRT caused a 6-fold increase in numbers of MDSCs (p = 0.002) at Day 21 compared to baseline. Additionally, low-dose TRT caused a 2-fold (p = 0.005) increase in CD8+ T cells at Day 21 compared to baseline, unlike in the high-dose condition. These CD8+ T cells in the low-dose TRT condition were predominantly proliferating naïve T cells. Flow cytometry of tumor infiltrating lymphocytes showed a 13% increase in CD8+ T cells in both dose conditions at Day 14 (p = 0.03). Returning CD8+ T cells displayed high expression of PD-1, CTLA-4, and LAG-3. Within the tumor, there was an 8-fold increase in chemokines including CXCL1, Rantes, and MIP2 at Day 21, suggesting that there may be further recruitment of tumor infiltrating cells even three weeks following treatment. Mice treated with high-dose TRT (median survival 37 days, p = .029) had significantly improved median survival compared to control (median survival 23 days), unlike low-dose treated mice (median survival 29 days, p =.250).

Conclusions: These data suggest that low-dose TRT is superior for immunomodulation because, although it has little effect on tumor growth, it increases intratumoral CD8+ T cell infiltration while avoiding deleterious systemic immune effects. We hypothesize that immunomodulatory TRT combined with checkpoint blockade will improve anti-tumor efficacy compared to high-dose TRT.

Citation Format: Hemanth Kumar Potluri, Reinier Hernandez, Christopher D. Zahm, Joseph Grudzinski, Christopher Massey, Jamey Weichert, Douglas G. McNeel. Low-dose targeted radionuclide therapy has favorable local and systemic effects on immune populations in a murine prostate cancer model [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2262.

Antibody and fragment-based PET imaging of CTLA-4+ T-cells in humanized mouse models

Imaging of immunotherapy targets using positron emission tomography (PET) can allow for noninvasive monitoring of their dynamic expression and may allow for patient stratification in the future. Therefore, two tracers targeting CTLA-4, one a full antibody and the other a F(ab')₂ fragment, were radiolabeled with ⁶⁴Cu and validated in humanized mouse models. Ipilimumab was digested to develop ipilimumab-F(ab')₂, and both the intact antibody and the fragment were conjugated with NOTA to chelate ⁶⁴Cu for PET. The tracers were administered to both control NBSGW mice and humanized mice (PBL mice, engrafted with human peripheral blood lymphocytes), and PET was conducted out to 48 h post-injection. PET region-of-interest analysis, ex vivo biodistribution studies, and tissue staining were used to confirm that the tracers specifically accumulated in CTLA-4+ tissues. Following injection of tracers (n = 3-5 per group), specific uptake was noted in the salivary gland tissues of the humanized mice. This uptake, a result of graft-versus-host disease onset, was proven to be due to human T-cells through staining of the tissues for human CD3 and CTLA-4. ⁶⁴Cu-NOTA-ipilimumab demonstrated the highest absolute uptake in the salivary glands of PBL mice, peaking at 7.00 ± 2.19 %ID/g. In contrast, ⁶⁴Cu-NOTA-ipilimumab-F(ab')₂ uptake was 2.40 ± 0.86 %ID/g at the same time point. However, the F(ab')₂ agent cleared from circulation more quickly than the intact antibody, providing higher salivary gland-to-blood ratios, which reached 1.78 ± 0.72 at 48 h post-injection, compared to ⁶⁴Cu-NOTA-ipilimumab at 1.19 ± 0.49. Uptake of the tracers in the salivary glands of control mice, and the nonspecific tracer in the PBL mice, was lower at all time points as well. PET imaging with both ⁶⁴Cu-NOTA-ipilimumab and ⁶⁴Cu-NOTA-ipilimumab-F(ab')₂ was able to localize CTLA-4+ tissues. These tracers may thus help elucidate the mechanisms of response to CTLA-4-targeted checkpoint immunotherapy treatments.

TLR Stimulation during T-cell Activation Lowers PD-1 Expression on CD8+ T Cells

Expression of T-cell checkpoint receptors can compromise antitumor immunity. Blockade of these receptors, notably PD-1 and LAG-3, which become expressed during T-cell activation with vaccination, can improve antitumor immunity. We evaluated whether T-cell checkpoint expression could be separated from T-cell activation in the context of innate immune stimulation with TLR agonists. We found that ligands for TLR1/2, TLR7, and TLR9 led to a decrease in expression of PD-1 on antigen-activated CD8⁺ T cells. These effects were mediated by IL12 released by professional antigen-presenting cells. In two separate tumor models, treatment with antitumor vaccines combined with TLR1/2 or TLR7 ligands induced antigen-specific CD8⁺ T cells with lower PD-1 expression and improved antitumor immunity. These findings highlight the role of innate immune activation during effector T-cell development and suggest that at least one mechanism by which specific TLR agonists can be strategically used as vaccine adjuvants is by modulating the expression of PD-1 during CD8⁺ T-cell activation. Cancer Immunol Res; 6(11); 1364-74. ©2018 AACR.

Vaccination with High-Affinity Epitopes Impairs Antitumor Efficacy by Increasing PD-1 Expression on CD8+ T Cells

Antitumor vaccines encoding self-antigens generally have low immunogenicity in clinical trials. Several approaches are aimed at improving vaccine immunogenicity, including efforts to alter encoded epitopes. Immunization with epitopes altered for increased affinity for the major histocompatibility complex (MHC) or T-cell receptor (TCR) elicits greater numbers of CD8 T cells but inferior antitumor responses. Our previous results suggested that programmed death 1 (PD-1) and its ligand (PD-L1) increased on antigen-specific CD8 T cells and tumor cells, respectively, after high-affinity vaccination. In this report, we use two murine models to investigate whether the dose, MHC affinity, or TCR affinity of an epitope affected the antitumor response via the PD-1/PD-L1 axis. T cells activated with high-affinity epitopes resulted in prolonged APC:T-cell contact time that led to elevated, persistent PD-1 expression, and expression of other checkpoint molecules, in vitro and in vivo Immunization with high-affinity epitopes also decreased antitumor efficacy in the absence of PD-1 blockade. Thus, APC:T-cell contact time can be altered by epitope affinity and lead to therapeutically relevant changes in vaccine efficacy mediated by changes in PD-1 expression. These findings have implications for the use of agents targeting PD-1 expression or function whenever high-affinity CD8 T cells are elicited or supplied by means of vaccination or adoptive transfer. Cancer Immunol Res; 5(8); 630-41. ©2017 AACR.

DNA vaccines for prostate cancer

DNA vaccines offer many advantages over other anti-tumor vaccine approaches due to their simplicity, ease of manufacturing, and safety. Results from several clinical trials in patients with cancer have demonstrated that DNA vaccines are safe and can elicit immune responses. However, to date few DNA vaccines have progressed beyond phase I clinical trial evaluation. Studies into the mechanism of action of DNA vaccines in terms of antigen-presenting cell types able to directly present or cross-present DNA-encoded antigens, and the activation of innate immune responses due to DNA itself, have suggested opportunities to increase the immunogenicity of these vaccines. In addition, studies into the mechanisms of tumor resistance to anti-tumor vaccination have suggested combination approaches that can increase the anti-tumor effect of DNA vaccines. This review focuses on these mechanisms of action and mechanisms of resistance using DNA vaccines, and how this information is being used to improve the anti-tumor effect of DNA vaccines. These approaches are then specifically discussed in the context of human prostate cancer, a disease for which DNA vaccines have been and continue to be explored as treatments.

Mini-intronic plasmid vaccination elicits tolerant LAG3+ CD8+ T cells and inferior antitumor responses

Increasing transgene expression has been a major focus of attempts to improve DNA vaccine-induced immunity in both preclinical studies and clinical trials. Novel mini-intronic plasmids (MIPs) have been shown to cause elevated and sustained transgene expression in vivo. We sought to test the antitumor activity of a MIP, compared to standard DNA plasmid immunization, using the tumor-specific antigen SSX2 in an HLA-A2-restricted tumor model. We found that MIP vaccination elicited a greater frequency of antigen-specific CD8⁺ T cells when compared to conventional plasmid, and protected animals from subsequent tumor challenge. However, therapeutic vaccination with the MIP resulted in an inferior antitumor effect, and CD8⁺ tumor-infiltrating lymphocytes from these mice expressed higher levels of surface LAG3. Antitumor efficacy of MIP vaccination could be recovered upon antibody blockade of LAG3. In non-tumor bearing mice, MIP immunization led to a loss of epitope dominance, attenuated CD8⁺ cytokine responses to the dominant p103 epitope, and increased LAG3 expression on p103-specific CD8⁺ T cells. Further, LAG3 expression on CD8⁺ T cells was associated with antigen dose and persistence in spite of DNA-induced innate immunity. These data suggest that for antitumor immunization, approaches leading to increased antigen expression following vaccination might optimally be combined with LAG3 inhibition in human trials. On the other hand, mini-intronic vector approaches may be a superior means to elicit LAG3-dependent tolerance in the treatment of autoimmune diseases.

Abstract 2362: Optimizing prostate cancer immunotherapy by reducing vaccine induced PD-1 expression

Through early phase clinical trials we have shown that a plasmid DNA vaccine can elicit antigen-specific CD8 T-cells that can persist for years. However, the persistence of antigen-expressing prostate tumors demonstrates that mechanisms of tumor escape are at play. To investigate these mechanisms we have focused on vaccine strategies using the synovial sarcoma, X breakpoint 2 (SSX2) protein as a model target antigen and utilized mice which express HLA-A2 but not murine MHC-I. We have shown that immunization with a DNA plasmid encoding SSX2 elicits robust CD8+ cells and that altering HLA-A2 affinity can augment this response. High affinity altered peptide ligand (APL)-encoding DNA vaccines elicited a greater frequency of specific T-cells; however, they fared worse in anti-tumor studies due to increased PD-1 expression on T-cells, an effect which could be abrogated with PD-1 blockade. Our current research has focused on why high-affinity epitopes elicit higher PD-1 expression, and whether this might be modulated at the time of antigen presentation. To that aim we have used the OT-1 mouse model, in which T-cells express a T-cell receptor that is specific for the dominant ovalbumin epitope SIINFEKL. We generated APLs of SIINFEKL and used them to stimulate OT-1 splenocytes ex vivo. The high affinity SIINFEKL epitope resulted in high, sustained expression of PD-1 following activation, while lower affinity APLs resulted in less, transient expression. However, lowered PD-1 expression can be indicative of T-cells that receive weaker signals via the T-cell receptor (TCR) and likewise display lowered effector function, therefore we also analyzed cytokine production and the in vivo effects of the APLs. In immunization experiments using B6 mice carrying adoptively transferred OT-1 T-cells the lower affinity peptide again resulted in transient PD-1 expression while the high affinity peptide led to sustained levels of PD-1 expression. One week after vaccination with the low affinity APL, PD-1 on tetramer+ CD8 T-cells had returned to baseline while cells from mice immunized with SIINFEKL maintained PD-1 expression. Furthermore, 38(±8)% of tetramer+ cells from the SIINFEKL immunized group were PD-1+ 4-1BB- indicating that PD-1 expression is maintained even after activation in vivo. We are currently assessing the ability of immunization with each peptide to prevent B16-OVA tumor growth in vivo. To assess the mechanism by which PD-1 is regulated after stimulation with each peptide we used a combination of live cell imaging and RNA sequencing to observe APC/T-cell interactions and T-cell gene expression respectively. This research indicates that while increasing antigen affinity may generate a more robust T-cell response it does not always result in better anti-tumor immunity and provides a model in which we can study the biology underlying PD-1 expression.

Citation Format: Christopher D. Zahm, Douglas G. McNeel. Optimizing prostate cancer immunotherapy by reducing vaccine induced PD-1 expression. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2362.

Advanced Intestinal Cancers often Maintain a Multi-Ancestral Architecture

A widely accepted paradigm in the field of cancer biology is that solid tumors are uni-ancestral being derived from a single founder and its descendants. However, data have been steadily accruing that indicate early tumors in mice and humans can have a multi-ancestral origin in which an initiated primogenitor facilitates the transformation of neighboring co-genitors. We developed a new mouse model that permits the determination of clonal architecture of intestinal tumors in vivo and ex vivo, have validated this model, and then used it to assess the clonal architecture of adenomas, intramucosal carcinomas, and invasive adenocarcinomas of the intestine. The percentage of multi-ancestral tumors did not significantly change as tumors progressed from adenomas with low-grade dysplasia [40/65 (62%)], to adenomas with high-grade dysplasia [21/37 (57%)], to intramucosal carcinomas [10/23 (43%]), to invasive adenocarcinomas [13/19 (68%)], indicating that the clone arising from the primogenitor continues to coexist with clones arising from co-genitors. Moreover, neoplastic cells from distinct clones within a multi-ancestral adenocarcinoma have even been observed to simultaneously invade into the underlying musculature [2/15 (13%)]. Thus, intratumoral heterogeneity arising early in tumor formation persists throughout tumorigenesis.

Abstract 4143: Discrete clones cooperate to promote tumor progression through a non-cell-autonomous mechanism in intestinal cancers

Background: The process of tumor evolution has often been described as a single initiated cell propagating a tumor with subclones of that cell acquiring distinct molecular changes and competing for dominance within the tumor. However, colon tumors are now known to often be derived from multiple initiated progenitor cells. Distinct initiated clones might cooperate or alternatively compete during tumorigenesis, progression, and therapy.

Methods: Mice carrying the Min allele of Apc and expressing an oncogenic form of PI3K in a subset of colonic epithelial cells develop multiple adenomas and adenocarcinomas in the colon. We utilized cell lineage tracing to follow the progeny of individual progenitor cells through tumor establishment, progression, and treatment with GDC 0941, a PI3K inhibitor.

Results: Nearly half (44%; 30/68) of the tumors were derived from at least two progenitor clones, one of which expressed the oncogenic form of PI3K and one of which did not. The presence of both types of clones correlated to an increased frequency of either clone becoming invasive (p = 0.0002). Additionally, the presence of both clones appeared to protect susceptible clones from targeted therapy.

Conclusions: Taken together, these data indicate that genetically distinct tumor progenitors can establish a single lesion, and moreover cooperation amongst diverse clonal populations provides a fitness advantage during tumor progression and therapy.

Citation Format: Alyssa A. Leystra, Amanda M. Wisinger, Christopher D. Zahm, Kristina A. Matkowskyj, Chelsie K. Sievers, Alex Schwartz, Dawn M. Albrecht, Linda Clipson, Dustin A. Deming, Michael A. Newton, Richard B. Halberg. Discrete clones cooperate to promote tumor progression through a non-cell-autonomous mechanism in intestinal cancers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4143. doi:10.1158/1538-7445.AM2015-4143

Abstract 2501: Vaccination using epitopes with higher MHC class I affinity elicits CD8+ T cells with increased PD-1 expression

We have been interested in DNA vaccines as treatments for human prostate cancer, and have demonstrated that this approach can elicit antigen-specific CD8+ T-cells in humans that can persist over years. However, the persistence of antigen-expressing tumors demonstrates that mechanisms of tumor escape are at play. Consequently, in preclinical studies we have focused on mechanisms of resistance and efforts to improve the immunological activity of DNA vaccines. As one model antigen, we have used the synovial sarcoma, X breakpoint 2 (SSX2) protein, a cancer-testis antigen expressed in metastatic prostate cancer. We have previously demonstrated that SSX2 encodes two HLA-A2-restricted epitopes, and we have utilized HHD-II mice, in which HLA-A2 and HLA-DR1 (but not murine MHC class I and II) are expressed, as a means to study CD8+ T cells generated by vaccination. We have shown in this system that immunization with a DNA plasmid encoding SSX2 elicits robust CD8+ cells, and anti-tumor responses to syngeneic tumors expressing SSX2. We have recently identified that a DNA vaccine altered to encode peptide epitopes with increased HLA-A2 binding elicited a greater frequency of CD8+ T cells specific for SSX2 when compared to the native vaccine, however these T cells were less effective against tumors due to higher expression of PD-1. In the current study, we investigated the mechanisms underlying this regulation of T-cells following vaccination. Altered peptide ligands (APL) specific for one HLA-A2-restricted epitope were modified at anchor residues to generate peptides of varying affinities for HLA-A2. Groups of HHD-II mice were then immunized with one of five SSX2 APLs with varying HLA-A2 affinity. After one week the resulting antigen-specific CD8+ T cells were assessed for regulatory ligand expression and cytokine expression as an indicator of function. We found that a single immunization with high affinity APLs resulted in an increased percentage of SSX2 tetramer positive T-cells that displayed persistent PD-1 expression compared with lower-affinity APL. Upon ex vivo re-encounter of the native SSX2 peptide, T-cells from mice immunized with higher affinity APLs showed an increased multifunctional Th1 response as evidenced by increased IFNγ, IL2, and TNFα expression when compared to mice immunized with native or low affinity SSX2 peptides, but also higher PD-1 expression. Ongoing studies are further exploring the kinetics and persistence of PD-1 expression following antigen presentation. Notwithstanding, our results have implications for the design of anti-tumor vaccines that seek to incorporate epitopes with altered HLA-A2-binding ability as these may affect the persistence of PD-1 expression on CD8+ T cells following immunization.

Citation Format: Christopher D. Zahm, Douglas McNeel. Vaccination using epitopes with higher MHC class I affinity elicits CD8+ T cells with increased PD-1 expression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2501. doi:10.1158/1538-7445.AM2015-2501

Abstract 1935: Transformation of epithelial cells through recruitment leads to polyclonal intestinal cancers

Background: Intestinal tumors in mice and humans can originate from multiple progenitors. The consequent heterogeneity within these polyclonal tumors probably causes them to be more resistant to chemotherapy. Understanding how these tumors arise is therefore likely to provide insight into more effective prevention and treatment strategies. Polyclonal tumors could form as a result of cooperation between two or more independently initiated cells in close proximity, or else recruitment in which a single initiated cell facilitates the transformation of nearby cells. We tested these two potential mechanisms using murine models of human colorectal cancer that carry clinically relevant gene mutations.

Methods: We generated mosaic mice in which the status of Apc and Pik3ca varied from crypt to crypt in the distal small intestine and colon. Some cells lost one copy of Apc, expressed an activated form of Pik3ca, or both and consequently had a high tumorigenic potential, whereas others were wild type and had an extremely low tumorigenic potential. These mosaics allowed us to address fundamental questions regarding the formation of polyclonal tumors.

Results: The clonal structure of 50 tumors from 11 mice was determined. Cells with low tumorigenic potential were frequently transformed in combination with cells with high tumorigenic potential: 1 out of 8 tumors (13%) from mice expressing the activated form of Pik3ca were heterotypic, containing a mixture of cells with low and high tumorigenic potential; 9 out 14 tumors (64%) from mice that carried only an Apc mutation were heterotypic; and 12 out of 28 tumors (43%) from mice that carried both mutations were heterotypic. Tumor progression does not affect the structure. Many adenomas (9/20; 45%) and adenocarcinomas (13/30; 43%) were polyclonal. Immunohistochemistry was used to visualize the localization and expression level of β-catenin and pAKT to ascertain the impact of the cells with high tumorigenic potential upon those with low tumorigenic potential.

Conclusion: These data indicate that initiated cells recruit cells that would not otherwise form tumors, and that this structure forms and persists even in settings in which one progenitor would be expected to have a large growth advantage over another progenitor. Recruitment might be mediated by Wnt signalling.

Citation Format: Alyssa A. Leystra, Dustin A. Deming, Amanda M. Wisinger, Christopher D. Zahm, Chelsie K. Sievers, Kristina A. Matkowskyj, Dawn M. Albrecht, Michael A. Newton, Richard B. Halberg. Transformation of epithelial cells through recruitment leads to polyclonal intestinal cancers. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1935. doi:10.1158/1538-7445.AM2014-1935

Dynamic tumor growth patterns in a novel murine model of colorectal cancer

Colorectal cancer often arises from adenomatous colonic polyps. Polyps can grow and progress to cancer, but may also remain static in size, regress, or resolve. Predicting which polyps progress and which remain benign is difficult. We developed a novel long-lived murine model of colorectal cancer with tumors that can be followed by colonoscopy. Our aim was to assess whether these tumors have similar growth patterns and histologic fates to human colorectal polyps to identify features to aid in risk stratification of colonic tumors. Long-lived Apc(Min/+) mice were treated with dextran sodium sulfate to promote colonic tumorigenesis. Tumor growth patterns were characterized by serial colonoscopy with biopsies obtained for immunohistochemistry and gene expression profiling. Tumors grew, remained static, regressed, or resolved over time with different relative frequencies. Newly developed tumors demonstrated higher rates of growth and resolution than more established tumors that tended to remain static in size. Colonic tumors were hyperplastic lesions (3%), adenomas (73%), intramucosal carcinomas (20%), or adenocarcinomas (3%). Interestingly, the level of β-catenin was higher in adenomas that became intratumoral carcinomas than those that failed to progress. In addition, differentially expressed genes between adenomas and intramucosal carcinomas were identified. This novel murine model of intestinal tumorigenesis develops colonic tumors that can be monitored by serial colonoscopy, mirror growth patterns seen in human colorectal polyps, and progress to colorectal cancer. Further characterization of cellular and molecular features is needed to determine which features can be used to risk-stratify polyps for progression to colorectal cancer and potentially guide prevention strategies.

Abstract 2717: The clonal architecture of tumors in the mammalian intestines

Background

Evidence is steadily accumulating that tumors can have a polyclonal origin. In fact, a recent review finds that there is evidence for polyclonal origin in at least 24 different human tumors types. Insights into clonality of intestinal tumors have been achieved through the generation of aggregation chimeras from early-stage mouse embryos and statistical analysis combining information about the pattern of chimerism with tumor phenotype. However, this experimental approach is incredibly laborious and very expensive.

Methods

To overcome these significant limitations, we developed a mosaic mouse model that expresses fluorescent proteins (FPs) in a variegated pattern throughout the distal small bowel and colon and have used this model to investigate the clonal origin of intestinal tumors. Briefly, our model is based on the mosaic expression of Cre recombinase in the intestinal epithelium and it can be generated by simple crosses, eliminating the time and cost associated with generating aggregation chimeras. In addition, FPs allow us to use advanced in and ex vivo imaging techniques such as dual-photon excitation microscopy and fluorescent endoscopy to address fundamental questions.

Experimental Design

Mosaic mice carrying the Min allele of Apc are predisposed to developing tumors throughout the intestinal tract. Many of the tumors (41%) from the distal small bowel and colon contained both green and red neoplastic cells confirming that mosaic mice can be used to detect polyclonal architecture. Dual-photon excitation microscopy was performed on whole tumors that had been optically cleared revealing the sub-surface structure of the epithelium and tumors to a depth of ∼350 μM. This detailed analysis revealed that some tumors can be derived from as many as 4 progenitors.

Mosaic mice were also treated with azoxymethane (AOM) and AOM in the presence the inflammatory agent dextran sodium sulphate (DSS). The length of exposure to DSS was varied across groups to determine the effects of chronic versus acute inflammation. Results demonstrated that the effect of chronic inflammation is subtle, whereas acute inflammation has a significant effect on clonal architecture of the tumors. One possible explanation for these results is that chronic inflammation supplants the need for interactions among different progenitors to establish a tumor, whereas acute inflammation enhances clonal interactions.

Conclusion

Together these data demonstrate that mosaic mice are useful for studying the clonal origin of tumors. We also find evidence that polyclonal architecture is important in multiple biological contexts within the mammalian intestine. Because FP expression is ubiquitous the same approach can be used to study clonal origin of tumors in any tissue in which mosaic Cre expression can be induced. Having a better understand of the complexity of tumor origin could profoundly change our strategies for the prevention and treatment of cancers.

Citation Format: Christopher D. Zahm, Joseph Szulczewski, Terrah J. Paul Olson, Laura Nettekoven, Alyssa Leystra, Dawn M. Albrecht, Linda Clipson, Kevin W. Eliceiri, Micheal A. Newton, RIchard B. Halberg. The clonal architecture of tumors in the mammalian intestines. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2717. doi:10.1158/1538-7445.AM2013-2717

Mice expressing activated PI3K rapidly develop advanced colon cancer

Aberrations in the phosphoinositide 3-kinase (PI3K) signaling pathway play a key role in the pathogenesis of numerous cancers by altering cellular growth, metabolism, proliferation, and apoptosis. Mutations in the catalytic domain of PI3K that generate a dominantly active kinase are commonly found in human colorectal cancers and have been thought to drive tumor progression but not initiation. However, the effects of constitutively activated PI3K upon the intestinal mucosa have not been previously studied in animal models. Here, we show that the expression of a dominantly active form of the PI3K protein in the mouse intestine results in hyperplasia and advanced neoplasia. Mice expressing constitutively active PI3K in the epithelial cells of the distal small bowel and colon rapidly developed invasive adenocarcinomas in the colon that spread into the mesentery and adjacent organs. The histologic characteristics of these tumors were strikingly similar to invasive mucinous colon cancers in humans. Interestingly, these tumors formed without a benign polypoid intermediary, consistent with the lack of aberrant WNT signaling observed. Together, our findings indicate a noncanonical mechanism of colon tumor initiation that is mediated through activation of PI3K. This unique model has the potential to further our understanding of human disease and facilitate the development of therapeutics through pharmacologic screening and biomarker identification.

Clonal structure of carcinogen-induced intestinal tumors in mice

Previous studies have shown that intestinal tumors from Apc(Min)(/+) (Min) mice and familial adenomatous polyposis (FAP) patients are often polyclonal. We sought to determine whether polyclonality is unique to tumors arising from hereditary predispositions or, instead, is a common feature of intestinal tumorigenesis in other pathways to tumorigenesis. Ethylnitrosourea-induced intestinal tumors from mice wild type at the Apc locus and chimeric for the Rosa26 lineage marker were analyzed. Many were overtly polyclonal, being composed of a mixture of Rosa26(+) and Rosa26(-) neoplastic cells. Statistical analyses revealed that polyclonality could be explained by interactions between two initiated clones separated by a very short distance. The frequency of overtly polyclonal tumors and the range of interactions estimated in this model are similar to those observed when analyzing familial tumors from Min mice. Thus, polyclonality does not depend on the familial pathway to tumorigenesis. Interactions between two initiated clones might provide a selective advantage during the early stages of intestinal tumorigenesis.