Serial Transplantation Research Articles

Abstract A004: E2F5 conditional knockout in mammary epithelium drives organotropic metastasis in breast cancer

Abstract Breast cancer ranks as the most commonly diagnosed cancer in women with 90% of associated deaths caused by distant metastasis. The metastatic lesions in brain, lung, liver, bone and lymph nodes in human breast cancer are not arbitrary, but rather are a complex mechanism of extrinsic and intrinsic regulatory factors controlling organ-specific colonization. Specialized mechanisms driving bone, brain and lung cancer have been described. However, the studies of liver and lymph node metastatic drivers are limited despite the prevalence of metastases in these organs. A significant limitation is the lack of genetically engineered mouse models that develop mammary tumors that spontaneously metastasize to liver and lymph nodes. Our previous research identified the role for E2F5 in both mammary gland development and breast cancer. Loss of E2F5 resulted in tumorigenesis after a latency of one year and 70% of mice showed metastatic lesions in lung, lymph node and liver. Transplantation of these spontaneous tumors into FVB MMTV Cre mice recapitulated the metastatic pattern observed in the primary tumors. We propose that E2F5 regulates drivers of metastasis and potentiates organ tropism to lymph nodes and the liver. To address this hypothesis, we conducted serial transplantations of tumors originating from the lymph nodes and liver back into the mammary gland. This enrichment study led to a significant increase in organ-specific metastasis, with 60% and 77% of penetrance observed in two distinct mouse lineages with liver (LVM) and lymph node tropism (LNM). Immunohistochemical studies confirmed a complete epithelial-mesenchymal transition (EMT) phenotype in LVM that is conserved in the cell lines generated. LNM showed a partial EMT with a more epithelial phenotype in cell lines. We observed the presence of an immune enriched environment in primary and metastatic tumors in both lineages. Bulk-RNA sequencing studies uncovered the presence of unique organotropic pathways associated with liver and lymph node metastasis. We observed enriched pathways from the primary tumor as major component of our organotropic drivers with a clear distinction between liver and lymph node enriched lineages. Additionally, a gradual acquisition of distant organ-specific markers and signaling pathways suggests that phenotypic mimicry and tumor plasticity are key for successful events of colonization to liver and lymph nodes in our model. With these results, we unravel potential mechanisms responsible for the organotropic metastases in breast cancer under the absence of the transcription factor E2F5. Citation Format: Jesus A. Garcia Lerena, Briana To, Jing-Ru Jhan, Eran Andrechek. E2F5 conditional knockout in mammary epithelium drives organotropic metastasis in breast cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor-body Interactions: The Roles of Micro- and Macroenvironment in Cancer; 2024 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(22_Suppl):Abstract nr A004.

A p38 MAPK-ROS axis fuels proliferation stress and DNA damage during CRISPR-Cas9 gene editing in hematopoietic stem and progenitor cells

Exvivo activation is a prerequisite to reaching adequate levels of gene editing by homology-directed repair (HDR) for hematopoietic stem and progenitor cell (HSPC)-based clinical applications. Here, we show that shortening culture time mitigates the p53-mediated DNA damage response to CRISPR-Cas9-induced DNA double-strand breaks, enhancing the reconstitution capacity of edited HSPCs. However, this results in lower HDR efficiency, rendering exvivo culture necessary yet detrimental. Mechanistically, exvivo activation triggers a multi-step process initiated by p38 mitogen-activated protein kinase (MAPK) phosphorylation, which generates mitogenic reactive oxygen species (ROS), promoting fast cell-cycle progression and subsequent proliferation-induced DNA damage. Thus, p38 inhibition before gene editing delays G1/S transition and expands transcriptionally defined HSCs, ultimately endowing edited cells with superior multi-lineage differentiation, persistence throughout serial transplantation, enhanced polyclonal repertoire, and better-preserved genome integrity. Our data identify proliferative stress as a driver of HSPC dysfunction with fundamental implications for designing more effective and safer gene correction strategies for clinical applications.

Time Course Analysis of Serial Biopsies Post-Kidney Transplant Reveals a Time Zero Cell State Negatively Associated with Delayed Graft Function

Time Course Analysis of Serial Biopsies Post-Kidney Transplant Reveals a Time Zero Cell State Negatively Associated with Delayed Graft Function

Insight into mammary gland development and tumor progression in an E2F5 conditional knockout mouse model

Development of breast cancer is linked to altered regulation of mammary gland developmental processes. A better understanding of normal mammary gland development can thus reveal possible mechanisms of how normal cells are re-programmed to become malignant. E2Fs 1-4 are part of the E2F transcription factor family with varied roles in mammary development, but little is known about the role of E2F5. A combination of scRNAseq and predictive signature tools demonstrated the presence of E2F5 in the mammary gland and showed changes in predicted activity during the various phases of mammary gland development. Testing the hypothesis that E2F5 regulates mammary function, we generated a mammary-specific E2F5 knockout mouse model, resulting in modest mammary gland development changes. However, after a prolonged latency the E2F5 conditional knockout mice developed highly metastatic mammary tumors. Whole genome sequencing revealed significant intertumor heterogeneity. RNAseq and protein analysis identified altered levels of Cyclin D1, with similarities to MMTV-Neu tumors, suggesting that E2F5 conditional knockout mammary glands and tumors may be dependent on Cyclin D1. Transplantation of the tumors revealed metastases to lymph nodes that were enriched through serial transplantation in immune competent recipients. Based on these findings, we propose that loss of E2F5 leads to altered regulation of Cyclin D1, which facilitates the development of metastatic mammary tumors after long latency. More importantly, this study demonstrates that conditional loss of E2F5 in the mammary gland leads to tumor formation, revealing its role as a transcription factor regulating a network of genes that normally result in a tumor suppressor function.

Inflammation-induced epigenetic imprinting regulates intestinal stem cells

It remains unknown whether and how intestinal stem cells (ISCs) adapt to inflammatory exposure and whether the adaptation leaves scars that will affect their subsequent regeneration. We investigated the consequences of inflammation on Lgr5+ ISCs in well-defined clinically relevant models of acute gastrointestinal graft-versus-host disease (GI GVHD). Utilizing single-cell transcriptomics, as well as organoid, metabolic, epigenomic, and invivo models, we found that Lgr5+ ISCs undergo metabolic changes that lead to the accumulation of succinate, which reprograms their epigenome. These changes reduced the ability of ISCs to differentiate and regenerate exvivo in serial organoid cultures and also invivo following serial transplantation. Furthermore, ISCs demonstrated a reduced capacity for invivo regeneration despite resolution of the initial inflammatory exposure, demonstrating the persistence of the maladaptive impact induced by the inflammatory encounter. Thus, inflammation imprints the epigenome of ISCs in a manner that persists and affects their sensitivity to adapt to future stress or challenges.

NFAT5 counters long-term IFN-1 responses in hematopoietic stem cells to preserve reconstitution potential

AbstractHematopoietic stem cells (HSCs) readily recover from acute stress, but persistent stress can reduce their viability and long-term potential. Here, we show that the nuclear factor of activated T cells 5 (NFAT5), a transcription modulator of inflammatory responses, protects the HSC pool under stress. NFAT5 restrains HSC differentiation to multipotent progenitors after bone marrow transplantation and bone marrow ablation with ionizing radiation or chemotherapy. Correspondingly, NFAT5-deficient HSCs fail to support long-term reconstitution of hematopoietic progenitors and mature blood cells after serial transplant. Evidence from competitive transplant assays shows that these defects are HSC intrinsic. NFAT5-deficient HSCs exhibit enhanced expression of type 1 interferon (IFN-1) response genes after transplant, and suppressing IFN-1 receptor prevents their exacerbated differentiation and cell death after reconstitution and improves long-term regeneration potential. Blockade of IFN-1 receptor also prevented the overdifferentiation of NFAT5-deficient HSCs after bone marrow ablation. These findings show that long-term IFN-1 responses to different hematopoietic stressors drive HSCs toward more differentiated progenitors, and that NFAT5 has an HSC-intrinsic role, limiting IFN-1 responses to preserve reconstitution potential. Our identification of cell-intrinsic mechanisms that strengthen the resistance of HSCs to stress could help to devise approaches to protect long-term stemness during the treatment of hematopoietic malignancies.

Inflammation drives tumor growth in an immunocompetent implantable metastasis model.

Nearly 90% of cancer deaths are due to metastasis. Conventional cancer therapeutics including chemotherapy, surgery, and radiotherapy, are effective in treating primary tumors, but may aggravate disseminated tumor cells (DTCs) into regaining a proliferative state. Models isolating the post dissemination environment are needed to address the potential risks of these therapies, however modeling post dissemination environments is challenging. Often, host organisms become moribund due to primary tumor mass before native metastatic niches can evolve. Implantable tissue engineered niches have been used to attract circulating tumor cells independent of the primary tumor. Here, we serially transplant such tissue engineered niches with recruited DTCs in order to isolate the post dissemination environment. After transplantaion, 69% of scaffolds developed overt post-dissemination cancer growth, however 100% of scaffolds did not grow to a life-threatening critical size within twelve weeks. Adjuvant chemotherapy, while initially effective, did not prevent long-term DTC growth in scaffolds. Subjecting these transplanted niches to surgical resection via biopsy punch enhanced CD31, MMP9, Ly6G, and tumor burden compared to control scaffolds. Biopsy punching was able to rescue tumor incidence from prior chemotherapy. This model of serial transplantation of engineered DTC niches is a highly controllable and flexible method of establishing and systematically investigating the post-dissemination niche.

Mouse adaptation of human inflammatory bowel diseases microbiota enhances colonization efficiency and alters microbiome aggressiveness depending on the recipient colonic inflammatory environment

BackgroundUnderstanding the cause vs consequence relationship of gut inflammation and microbial dysbiosis in inflammatory bowel diseases (IBD) requires a reproducible mouse model of human-microbiota-driven experimental colitis.ResultsOur study demonstrated that human fecal microbiota transplant (FMT) transfer efficiency is an underappreciated source of experimental variability in human microbiota-associated (HMA) mice. Pooled human IBD patient fecal microbiota engrafted germ-free (GF) mice with low amplicon sequence variant (ASV)-level transfer efficiency, resulting in high recipient-to-recipient variation of microbiota composition and colitis severity in HMA Il-10−/− mice. In contrast, mouse-to-mouse transfer of mouse-adapted human IBD patient microbiota transferred with high efficiency and low compositional variability resulting in highly consistent and reproducible colitis phenotypes in recipient Il-10−/− mice. Engraftment of human-to-mouse FMT stochastically varied with individual transplantation events more than mouse-adapted FMT. Human-to-mouse FMT caused a population bottleneck with reassembly of microbiota composition that was host inflammatory environment specific. Mouse-adaptation in the inflamed Il-10−/− host reassembled a more aggressive microbiota that induced more severe colitis in serial transplant to Il-10−/− mice than the distinct microbiota reassembled in non-inflamed WT hosts.ConclusionsOur findings support a model of IBD pathogenesis in which host inflammation promotes aggressive resident bacteria, which further drives a feed-forward process of dysbiosis exacerbated by gut inflammation. This model implies that effective management of IBD requires treating both the dysregulated host immune response and aggressive inflammation-driven microbiota. We propose that our mouse-adapted human microbiota model is an optimized, reproducible, and rigorous system to study human microbiome-driven disease phenotypes, which may be generalized to mouse models of other human microbiota-modulated diseases, including metabolic syndrome/obesity, diabetes, autoimmune diseases, and cancer.9-8iLEe2BorBmu_z2q_8UWVideo

Cxcl10 and Cxcr3 regulate self-renewal and differentiation of hematopoietic stem cells

BackgroundThe function of hematopoietic stem cells (HSC) is regulated by HSC internal signaling pathways and their microenvironment. Chemokines and chemokine ligands play important roles in the regulation of HSC function. Yet, their functions in HSC are not fully understood.MethodsWe established Cxcr3 and Cxcl10 knockout mouse models (Cxcr3−/− and Cxcl10−/−) to analyze the roles of Cxcr3 or Cxcl10 in regulating HSC function. The cell cycle distribution of LT-HSC was assessed via flow cytometry. Cxcr3−/− and Cxcl10−/− stem/progenitor cells showed reduced self-renewal capacity as measured in serial transplantation assays. To study the effects of Cxcr3 or Cxcl10 deficient bone marrow microenvironment, we transplanted CD45.1 donor cells into Cxcr3−/−or Cxcl10−/− recipient mice (CD45.2) and examined donor-contributed hematopoiesis.ResultsDeficiency of Cxcl10 and its receptor Cxcr3 led to decreased BM cellularity in mice, with a significantly increased proportion of LT-HSC. Cxcl10−/− stem/progenitor cells showed reduced self-renewal capacity in the secondary transplantation assay. Notably, Cxcl10−/− donor-derived cells preferentially differentiated into B lymphocytes, with skewed myeloid differentiation ability. Meanwhile, Cxcr3-deficient HSCs demonstrated a reconstitution disadvantage in secondary transplantation, but the lineage bias was not significant. Interestingly, the absence of Cxcl10 or Cxcr3 in bone marrow microenvironment did not affect HSC function.ConclusionsThe Cxcl10 and Cxcr3 regulate the function of HSC, including self-renewal and differentiation, adding to the understanding of the roles of chemokines in the regulation of HSC function.

PHF6 suppresses self-renewal of leukemic stem cells in AML

Acute myeloid leukemia is characterized by uncontrolled proliferation of self-renewing myeloid progenitors accompanied by a differentiation arrest. PHF6 is a chromatin-binding protein mutated in myeloid leukemias, and its isolated loss increases mouse HSC self-renewal without malignant transformation. We report here that Phf6 knockout increases the aggressiveness of Hoxa9-driven AML over serial transplantation, and increases the frequency of leukemia initiating cells. We define the in vivo hierarchy of Hoxa9-driven AML and identify a population that we term the “LIC-e” (leukemia initiating cells enriched) population. We find that Phf6 loss expands the LIC-e population and skews its transcriptome to a more stem-like state; concordant transcriptome shifts are also observed on PHF6 knockout in a human AML cell line and in PHF6 mutant patient samples from the BEAT AML dataset. We demonstrate that LIC-e accumulation in Phf6 knockout AML occurs not due to effects on cell cycle or apoptosis, but due to an increase in the fraction of its progeny that retain LIC-e identity. Our work indicates that Phf6 loss increases AML self-renewal through context-specific effects on leukemia stem cells.

Allosensitisation in NHP results in cross-reactive anti-SLA antibodies not detected by a lymphocyte-based flow cytometry crossmatch.

Xenotransplantation is a potential option for individuals for whom an acceptable human allograft is unavailable. Individuals with broadly reactive HLA antibodies due to prior exposure to foreign HLA are potential candidates for a clinical xenotransplant trial. It remains controversial if allosensitisation results in the development of cross-reactive antibodies against SLA. This may require increased histocompatibility scrutiny for highly sensitised individuals prior to enrollment in a clinical trial. Serum samples were obtained from non-human primates sensitised via serial skin transplantation from maximally MHC-mismatched donor, as reported. Sera from pre- and post-allosensitisation timepoints were assessed in a flow crossmatch (FXM) for IgM and IgG binding to pig splenocytes with or without red blood cell adsorption. Xenoreactive antibodies were eluted from pig splenocytes and screened on a single antigen HLA bead assay. A MHC Matchmaker algorithm was developed to predict potential conserved amino acid motifs among the pig, NHP, and human. Our sensitised NHP model was used to demonstrate that allosensitisation does not result in an appreciable difference in xenoreactive antibody binding in a cell-based FXM. However, antibody elution and screening on single antigen HLA beads suggest the existence of potential cross-reactive antibodies against SLA. The cross-reactive IgG after allosensitisation were predicted by comparing the recipient Mamu alleles against its previous allograft donor Mamu alleles and the donor pig SLA alleles. Our study suggests that allosensitisation could elevate cross-reactive antibodies, but a more sensitive assay than a cell-based FXM is required to detect them. The MHC Matchmaker algorithm was developed as a potential tool to help determine amino acid motif conservation and reactivity pattern.

IκBα controls dormancy in hematopoietic stem cells via retinoic acid during embryonic development

Recent findings suggest that Hematopoietic Stem Cells (HSC) and progenitors arise simultaneously and independently of each other already in the embryonic aorta-gonad mesonephros region, but it is still unknown how their different features are established. Here, we uncover IκBα (Nfkbia, the inhibitor of NF-κB) as a critical regulator of HSC proliferation throughout development. IκBα balances retinoic acid signaling levels together with the epigenetic silencer, PRC2, specifically in HSCs. Loss of IκBα decreases proliferation of HSC and induces a dormancy related gene expression signature instead. Also, IκBα deficient HSCs respond with superior activation to in vitro culture and in serial transplantation. At the molecular level, chromatin regions harboring binding motifs for retinoic acid signaling are hypo-methylated for the PRC2 dependent H3K27me3 mark in IκBα deficient HSCs. Overall, we show that the proliferation index in the developing HSCs is regulated by a IκBα-PRC2 axis, which controls retinoic acid signaling.

Inducible CXCL12/CXCR4–dependent extramedullary hematopoietic niches in the adrenal gland

AbstractAdult hematopoietic stem and progenitor cells (HSPCs) reside in the bone marrow (BM) hematopoietic niche, which regulates HSPC quiescence, self-renewal, and commitment in a demand-adapted manner. Although the complex BM niche is responsible for adult hematopoiesis, evidence exists for simpler, albeit functional and more accessible, extramedullary hematopoietic niches. Inspired by the anecdotal description of retroperitoneal hematopoietic masses occurring at higher frequency upon hormonal dysregulation within the adrenal gland, we hypothesized that the adult adrenal gland could be induced into a hematopoietic-supportive environment in a systematic manner, thus revealing mechanisms underlying de novo niche formation in the adult. Here, we show that upon splenectomy and hormonal stimulation, the adult adrenal gland of mice can be induced to recruit and host functional HSPCs, capable of serial transplantation, and that this phenomenon is associated with de novo formation of platelet-derived growth factor receptor α/leptin receptor (PDGFRα+/LEPR+/–)–expressing stromal nodules. We further show in CXCL12–green fluorescent protein reporter mice that adrenal glands contain a stromal population reminiscent of the CXCL12-abundant reticular cells, which compose the BM HSPC niche. Mechanistically, HSPC homing to hormonally induced adrenal glands was found dependent on the CXCR4–CXCL12 axis. Mirroring our findings in mice, we found reticular CXCL12+ cells coexpressing master niche regulator FOXC1 in primary samples from human adrenal myelolipomas, a benign tumor composed of adipose and hematopoietic tissue. Our findings reignite long-standing questions regarding hormonal regulation of hematopoiesis and provide a novel model to facilitate the study of adult-specific inducible hematopoietic niches, which may pave the way to therapeutic applications.

Loss of the stress sensor GADD45A promotes stem cell activity and ferroptosis resistance in LGR4/HOXA9-dependent AML

AbstractThe overall prognosis of acute myeloid leukemia (AML) remains dismal, largely because of the inability of current therapies to kill leukemia stem cells (LSCs) with intrinsic resistance. Loss of the stress sensor growth arrest and DNA damage-inducible 45 alpha (GADD45A) is implicated in poor clinical outcomes, but its role in LSCs and AML pathogenesis is unknown. Here, we define GADD45A as a key downstream target of G protein-coupled receptor (LGR)4 pathway and discover a regulatory role for GADD45A loss in promoting leukemia-initiating activity and oxidative resistance in LGR4/HOXA9-dependent AML, a poor prognosis subset of leukemia. Knockout of GADD45A enhances AML progression in murine and patient-derived xenograft (PDX) mouse models. Deletion of GADD45A induces substantial mutations, increases LSC self-renewal and stemness in vivo, and reduces levels of reactive oxygen species (ROS), accompanied by a decreased response to ROS-associated genotoxic agents (eg, ferroptosis inducer RSL3) and acquisition of an increasingly aggressive phenotype on serial transplantation in mice. Our single-cell cellular indexing of transcriptomes and epitopes by sequencing analysis on patient-derived LSCs in PDX mice and subsequent functional studies in murine LSCs and primary AML patient cells show that loss of GADD45A is associated with resistance to ferroptosis (an iron-dependent oxidative cell death caused by ROS accumulation) through aberrant activation of antioxidant pathways related to iron and ROS detoxification, such as FTH1 and PRDX1, upregulation of which correlates with unfavorable outcomes in patients with AML. These results reveal a therapy resistance mechanism contributing to poor prognosis and support a role for GADD45A loss as a critical step for leukemia-initiating activity and as a target to overcome resistance in aggressive leukemia.

Xenotransplantation of Human Hematopoietic Stem Cells into NBSGW Mice: A Basic Model for Preclinical Development of Gene Therapy Approaches

Background. The gene therapy based on hematopoietic cell xenotransplantation is becoming a powerful and universally applied therapeutic strategy in an ever-expanding range of human diseases. One of the current issues in implementing the techniques of genome modification in hematopoietic stem cells (HSCs) into clinical practice is to assure the quality and safety of gene and cell therapy products for human use. This is achieved by animal model testing at the stage of preclinical studies. With this purpose in view, NBSGW mice seem to be a unique and promising model for human HSC engraftment without pre-conditioning.
 Aim. To test the NBSGW mouse model for human HSC engraftment, to optimize the methods of assessing the state of the animals and monitoring the chimerism level for translational preclinical development of HSC-based products for gene and cell therapy.
 Materials & Methods. The xenograft models of NBSGW mice were generated using the samples of the selected peripheral blood CD34+ HSCs from a healthy donor. Serial transplantation was performed by intravenous injection of bone marrow cells from primary recipients with a high chimerism level. Engraftment efficiency was evaluated by flow cytofluorometry (FCF) and droplet digital PCR (ddPCR). Subpopulation pattern of human cell engraftment was assessed by FCF.
 Results. The tested HSC transplantation regimen is characterized by favorable toxicity profile. In the entire study sample of mice, the FCF analysis showed a long-term engraftment of human cells with a high chimerism level (23.5–93.6 %) in the bone marrow of the animals, also after serial transplantation, which was confirmed by ddPCR. The B-lineage differentiation cells predominated in all tested samples (of peripheral blood, bone marrow, and spleen) from mice after primary and serial transplantation. The ddPCR assay can be used as an additional tool for validating the level of human cell engraftment determined by FCF.
 Conclusion. NBSGW mice present a promising reference model for preclinical development of gene and cell therapy products based on human primary HSCs with a modified genome.

Abstract 5469: ITCC-P4, a preclinical proof-of-concept drug testing platform as a tool for pharmacological screening in pediatric tumor models

Abstract Cancer remains the main cause of disease-related death in childhood. Pediatric tumors are characterized by a low mutational burden and high intertumoral heterogeneity, with multiple subtypes compared to their adult counterparts. The lack of access to many innovative therapies remains one of the main challenges in the pediatric oncology, especially for the 25% of patients who experience relapses. In this context, the need for the development of a well characterized collection of pediatric models, to provide large scale preclinical testing, is capital for the subsequent identification and prioritization of promising novel therapeutic options. The EU funded “Innovative Therapies for Children with Cancer-Pediatric Preclinical Proof-of-Concept Platform” (ITCC-P4) consortium is a unique public-private collaborative project consisting of academic and industrial partners that aimed at establishing a collection of >400 patient-derived xenograft (PDX) models representing the most common high-risk pediatric cancers. The project involved various aspects of model development including the thorough molecular and pharmacological characterization. XenTech’s participation was focused on the development and preclinical in vivo drug testing of Ewing sarcoma (n=17), hepatoblastoma (n=10), rhabdoid tumors (n=6), synovial sarcoma (n=2), rhabdomyosarcoma (n=2) and other tumors (n=6), as part of overall cohort. PDXs were obtained by transplantation of post-surgery tumor specimens, either by grafting tumor fragments into the interscapular region or subcutaneously in the right flank of nude, NOD-Scid or NOD-Scid gamma mice. Tumor xenografts were amplified by serial transplantation, and tissue samples were retained at early passages for molecular characterization. Fragments from established PDX models where frozen to generate a revivable ITCC-P4 PDX collection. Then, proof-of-concept drug testing was conducted, in a single mouse trial format: each tumor type (n=X PDX models) was treated with a dedicated panel of Standard-of-Care (SoC;n=3) and novel targeted therapies (n=6), or combinations of 2 or 3 novel targeted therapies; for each PDX model n=1 mouse being included per treatment. All molecular and drug-testing data obtained by the different partners are being centralized in the R2 repository (https://r2.amc.nl), providing a powerful tool for data integration, visualization and interpretation of the results. A unique collection of well characterized pediatric PDX models derived from the most relevant pediatric tumor types was enabled by a strong public-private collaborative project. This large cohort is now available for preclinical testing of novel therapeutic agents within a non-for-profit spinoff company, ITCC-P4 gGmbH (www.itccp4.com), offering new perspectives to the identification of promising treatment options for children with cancer. Citation Format: Emilie Indersie, Sophie Branchereau, Brice Fresneau, Christophe Chardot, Didier Surdez, Alexandra Saint-Charles, Maria Eugénia Marques da Costa, Ángel M. Carcaboso, Katia Scotlandi, Massimo Moro, Heinrich Kovar, Jan-Henning Klusmann, Klaus-Michael Debatin, Simon Bomken, Louis Chesler, Chris Jones, Beat Schäfer, Marco Wachtel, Johannes Gojo, Walter Berger, Christina Guttke, Maureen Hattersley, Frédéric Colland, Ashley Strougo, Dennis Gürgen, Jens Hoffmann, Julia Schueler, Pablo M. Aviles, María José Guillén, Aniello Federico, Apurva Gopisetty, Justyna Anna Wierzbinska, Andreas Schlicker, Sara Colombetti, Olaf Heidenreich, Fatima Iradier, Nicole Huebener, Natalie Jäger, Jan Koster, Marcel Kool, Gudrun Schleiermacher, Jan J. Molenaar, Birgit Geoerger, David J. Shields, Hubert N. Caron, Louis F. Stancato, Stefan M. Pfister, Gilles Vassal, Eva-Maria Rief, Olivier Déas. ITCC-P4, a preclinical proof-of-concept drug testing platform as a tool for pharmacological screening in pediatric tumor models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5469.

Abstract 6926: Tracing clonal evolution and immune evasion in glioblastoma progression

Abstract Glioblastoma continues to be a daunting obstacle in oncology research, with its early-stage progression being particularly enigmatic. In a recent published work, we traced the clonal dynamics of glioblastoma evolution by the simultaneous transfer of PDGFB and genetic barcodes into mouse brains. We observed a continuous clonal loss during the acquisition of a malignant phenotype, underlined by the modulation of the levels of c-Myc expression and their functional targets. In this study, we delve deeper into the evolution of glioblastoma by transplanting multiclonal, early-stage glioma cells into multiple immunodeficient NOD-SCID mice. This experimental design allowed us to track the clonal dynamics over serial transplants, revealing how early-stage glioma clones acquire immune-evasive features capable of initiating tertiary tumors in immune-competent hosts. Through barcode sequencing and single-cell RNA sequencing of early-stage gliomas, coupled with bulk RNA sequencing of secondary and tertiary gliomas, we dissected the clonal and transcriptomic landscape of these tumors. We observed that, of the various clones composing primary tumors, just a few of them are predominating in subsequent passages. Furthermore, the clonal composition of secondary tumors derived from the same primary glioma showed partial overlap, indicating a partial predetermination in the development of immune-evasive behavior. Our intra- and inter-clonal transcriptomic analysis across various stages of tumor progression is shedding light on how new functional traits can emerge in gliomas. Further analysis of our data will determine whether it is the result of the expansion of clones already possessing these traits, or if they stem from functional adaptations within the clones themselves. Our insights further reinforce the concept of clonal competition, highlighting the significant influence of immune-system interactions in driving these competitive dynamics. Citation Format: Davide Ceresa, Sebastian A. Garcia Mena, Francesca Piaggio, Appolloni Irene, Daniela Marubbi, Paolo Malatesta. Tracing clonal evolution and immune evasion in glioblastoma progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6926.

Abstract 5438: Circulating tumor cells exhibit a tumor stemness defense phenotype in Head and Neck cancer subjects undergoing cisplatin based chemotherapy

Abstract Introduction: We have proposed that tumor has its own defense mechanism similar to an organism having its own defense at the population level. We also proposed that the center of this tumor defense is the hypoxic niche of cancer stem cells (CSCs). Our lab has previously characterized the hypoxic niche of CSCs, and demonstrated that these CSCs reprogram to a highly inflammatory, aggressive and embryonic stemness phenotype during oxidative stress (1). These reprogrammed CSCs, the tumor stemness defense (TSD) phenotype is transient, and exert altruistic defense against bacterial invasion in SCC-25 cell line (2). Further, VEGF/VEGFR1 autocrine signaling was found in cisplatin induced TSD phenotype (4) and Myc/Hif2alpha stemness pathway was found in TSD phenotype of a leukemia stem cell model (3). We hypothesize that the platinum-induced stress may also activate TSD phenotype in the dormant CSC fraction of head and neck squamous cell carcinoma (HNSCC), which may mobilize to the distant tissue via circulation as circulating tumor cells (CTCs). Methods: To test the hypothesis, we isolated EpCAM+/ABCG2+ CTCs (blood, n=14) and EpCAM+/ABCG2+ CSCs (tumor tissue, n=6/14) of HNSCC subjects under cisplatin treatment. CTCs were collected from peripheral blood mononuclear cells (PBMNC) by the immunomagnetic sorting of EpCAM+ cells, and then cells were expanded in a defined serum free media that we used to maintain the hypoxic CSCs (1). We evaluated the EpCAM+/ABCG2+ CTCs and EpCAM+/ABCG2+ CSCs for TSD phenotype by performing qPCR gene expression, Boyden chamber assay of invasion, clonogenic assay and serial transplantation assay in NOD/SCID mice. Data was compared with the EpCAM+/ABCG2- cell population. We also developed a pre-clinical model of cisplatin-induced reprogramming of HNSCC derived CSCs to understand the molecular mechanisms. Results: In 8/14 patient derived EpCAM+/ABCG2+ CTCs and EpCAM+/ABCG2+ CSCs showed TSD phenotype associated gene expression, high migration/invasion, self sufficiency (ability to form spheroid when grown in serum free media without growth factors), enhanced secretion of protumorigenic growth factors; VEGF, SDF-1, PIGF, and HMGB1. These CTCs and CSCs have shown tumor formation with significant CSC frequency, when injected to NOD/SCID mice. Finally, using a SCC-25 cell line derived pre-clinical model, we demonstrated that cisplatin therapy reprogram CSCs to TSD phenotype. Conclusion: Our work indicates that CTCs with TSD phenotype may indicate the interaction between cisplatin and CSC niche defense. Using the pre-clinical model, the TSD phenotype based CSC niche defense mechanism may be further explored to develop markers for therapy response, metastasis or recurrence in HNSCC.

Abstract 187: Macrophage instead of leukemia cell expressed PD-L1 drives T cell exhaustion and immune evasion of BCRABL1 chronic myeloid leukemia

Abstract BCR-ABL1 fusion gene is the hall marker of chronic myeloid leukemia (CML). Currently, the immune microenvironment of BCRABL1 CML remains largely unexplored. Our recent investigation in a transgenic mouse model of BCRABL1 CML revealed a continuous increase in CD11b+Ly6Cint PMN-MDSC with the progression of the disease. Concurrently, there was an increase in CD11b+Ly6Chigh M-MDSC and F4/80+ macrophages. However, The levels of CD4+ T cells, CD8+ T cells, CD19+ B cells, and CD49b+ NK cells, which are essential for tumor-killing significantly decreased. This immune phenotype was consistently observed in both primary leukemia induction and following serial transplantation of cells from primary leukemic mice. Intriguingly, only recipient mice subjected to sublethal irradiation developed the disease, while mice with intact immune system remained disease-free, which underscores the vital role of the immune system in the development of CML. The predominant presence of CD11b+Ly6Cint PMN-MDSC in the peripheral blood, bone marrow and spleen implies these cells may be at least partially consisted of CML leukemia cells. To examine the hypothesis, qRT-PCR and immunofluorescence (IF) staining were conducted. In both assays, Ly6G+ cells from BCRABL1 mice coexpressed the BCRABL1 oncogene. Furthermore, when co-cultured with CD4 T cells, the division of T cells was significantly inhibited, implying immune suppression by the Ly6G+ PMN-MDSCs and/or CML leukemia cells. In addition, the immuosuppressive pathway genes including Arg1, Hif1a, Enptd1, TGFβ were significantly upregulated. To investigate the possible role of the PD1/PDL1 axis in the BCRABL1 model, flow cytometry analysis was conducted. Unexpectedly, PDL1 was barely expressed in CML leukemia cells, whereas the expression of PDL1 in M-MDSCs and macrophages significantly increased, accompanied by remarkable upregulation of PD1 expression on CD4 T cells and CD8 T cells during leukemia progression. In other words, PDL1 expression on macrophages, rather than leukemia cells, was found to induce T cell exhaustion. For translational purpose, mice transplanted with the primary CML leukemia cells were treated with chemotherapy drugs ponatinib alone or in combination with the anti-PD1 antibody. Compared with the chemotherapy alone group, the mice in the combinational therapy group exhibited rapid remission and better disease control. Most importantly, the disease relapse occurred at a slower rate after treatment discontinuation, suggesting the value of combinational therapy in achieving treatment free remission. In summary, our study not only revealed the dynamic changes of immune microenvironment in the BCRABL1 CML mouse model, but also revealed the different mechanisms underlying immune evasion of BCR-ABL1 CML leukemia cells, which sheds light on a combinational treatment for achieving the treatment free remission of CML. Citation Format: Xiaocui Lu, Hui Fang, Xuexiu Fang, Atsuko Matsunaga, Kebin Liu, Jorge Cortes, John K. Cowell, Tianxiang Hu. Macrophage instead of leukemia cell expressed PD-L1 drives T cell exhaustion and immune evasion of BCRABL1 chronic myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 187.

Abstract B007: Establishment of patient-derived brain metastasis initiating cells as a novel preclinical model of breast cancer metastatization

Abstract Brain metastasis from breast cancer (BCBM) are lethal tumors occurring in 10-15% of IV stage breast cancers against which only limited therapeutic options are available. A critical hindrance to the design of novel therapeutic approaches is the narrow set of preclinical models to study BCBM pathophysiology, chiefly those resembling the stem-like population which is thought be the main source of BCBM dissemination and progression. We aimed to isolate patient-derived metastasis-initiating cells (MICs) to furnish novel preclinical models of metastatization. MICs were cultured as stem-enriching tumorspheres, which we characterized in term of i) brain/breast/stem markers expression ii) tumorigenicity, iii) growth and in vitro/in vivo self-renewal, iv) adhesion to organ-specific endothelium, v) metastatic potential and vi) drug responsiveness. Further, we transcriptionally profiled MIC-driven organ-specific metastasis. Patient-derived MICs expressed epithelial markers while the expression of brain markers was poor, coherently with their epithelial origins. About 90% of MICs were positive to the expression of the putative stem cell markers CD15, CD24 and CD44, while CD133 and ALDH1A1 were less represented. MICs were grown indefinitely in stem cell medium and successfully formed spheres in serial clonogenic assays, suggesting their endowment with stem-like traits. Similarly, their ability to form tumors when orthotopically injected in mice brain was maintained upon serial transplantation and under in vivo limiting dilution conditions. The representative PR60 MICs showed metastatic potential to mice brain and bones upon either intracardiac or intranipple transplantation. Of note, independently of the route of injection, this metastatization pattern wholly mirrored the clinical course of the patient they had been derived from. PR60-driven brain metastasis also recapitulated patient tumor histology, cytoarchitecture and marker expression. Further confirming their brain-tropism, PR60 MICs preferentially adhered to human brain compared to non-brain endothelium under shear stress mimicking in vivo hemodynamic conditions. Transcriptomically, PR60 MICs displayed high correlation with the BCBM of origin but retain the ability to form intra-nipple tumors resembling patient primitive breast tumor. PR60-derived brain and bone metastasis reveal a distinct expression profile suggesting that PR60 MICs can colonize different organs rewiring their transcriptomic landscape. Compared to the whole tumor of origin, MICs revealed an up-regulation of drug resistance signatures, which are often linked with stem-like traits, and that were functionally validated by a high throughput drug screening. Altogether, we established a robust and versatile MIC model that reliably mirrors the brain metastasis of origin and mimics the different steps of brain metastatic dissemination for in vitro and in vivo studies. This model has the potential to provide further knowledge for future therapy development for BCBM management. Citation Format: Stefania Faletti, Cristina Richichi, Daniela Osti, Elena Ceccacci, Massimiliano Del Bene, Camilla Cerutti, Giovanni Bertalot, Monica Patanè, Bianca Pollo, Giuliana Pelicci. Establishment of patient-derived brain metastasis initiating cells as a novel preclinical model of breast cancer metastatization [abstract]. In: Proceedings of the AACR Special Conference on Brain Cancer; 2023 Oct 19-22; Minneapolis, Minnesota. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_1):Abstract nr B007.