Breast Cancer 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.

Abstract B045: Requirement of MenaINV for metastatic colonization of the lungs by breast cancer stem cells

Abstract Background: Breast cancer is the most frequently diagnosed cancer, and the second leading cause of cancer-related death in women. Most breast cancer related deaths are due to metastasis to distant organs. The primary tumor and secondary site microenvironments provide cues that activate cancer cell plasticity programs such as those associated with invasion and stemness, which are needed for dissemination and the establishment of metastatic foci. We demonstrated that in the primary tumor >50% of cancer stem cells (CSCs) co-express the invasive phenotype marker, MenaINV. Given that ∼80% of newly arriving circulating tumor cells to the lung are CSCs, we questioned if extravasation of CSCs to the lung is mediated solely by MenaINV or if other compensatory pathways are at play. We describe here our discovery that CSCs require MenaINV for extravasation to the lung parenchyma. Methods: To visualize breast cancer stem cell extravasation into the lung parenchyma, we expressed the stemness biosensor, SORE6, in MDA-MB-231 breast cancer cells. SORE6 is composed of 6 tandem repeats of the Sox2 and Oct4 response elements placed upstream of destabilized Green Fluorescent Protein (GFP). To distinguish CSCs (green) from non-CSCs, all cells were also labeled with red tracker dye. Breast cancer cells transduced with lenti-SORE6 were FACS sorted into low SORE6-GFP cells (non- CSCs) and high SORE6-GFP (CSCs). Activation of the stem program in FACS sorted cells was validated by qRT-PCR analysis of the widely used stem cell markers CD44, CD133, P63, ALDH1, Sox9, Sox2, and Oct4. The extravasation efficiency of non-CSCs vs. CSCs was assessed in vitro by the extravasation directed transendothelial migration (eTEM) assay, which mimics extravasation by allowing migration of cancer cells through endothelial cells cultured on a porous membrane, as well as in vivo, by intravital imaging. The impact of MenaINV on CSC extravasation was evaluated through CRISPR/Cas9 mediated knockout of MenaINV. In in vivo experiments, a 50/50 mixture of non-CSCs and CSCs expressing either wild type or knocked-out MenaINV were injected into the vasculature of Rag2-/- MacBlue mice. in vivo visualization of cancer cell extravasation was accomplished by implantation of an indwelling lung imaging window prior to the injection of cancer cells, followed by high-resolution intravital imaging. Results: CSCs exhibit significantly higher expression of all stem cell markers and express more MenaINV compared to non-CSCs. Moreover, CSCs show significantly better transendothelial migration in eTME assay (p=0.03) and lung extravasation (p<0.0001) in intravital imaging, compared to non-CSCs. Finally, knocking out MenaINV reduced lung extravasation to the levels comparable to non-CSCs. Conclusions: This study highlights the crucial role of MenaINV in facilitating lung metastasis by CSCs. Thus, targeting MenaINV may represent a promising therapeutic strategy to reduce metastatic burden in breast cancer patients. Citation Format: Mohd Nauman, Yookyung Jung, Burcu Karadal-Ferrena, Camille Duran, Madeline Friedman-DeLuca, Nicole Barth, Prachiben Patel, John Condeelis, David Entenberg, Maja H. Oktay. Requirement of MenaINV for metastatic colonization of the lungs by breast cancer stem cells [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 B045.

Abstract B040: Investigating the impact of tumor EMT states on immune cell infiltration in breast cancer

Abstract Breast cancer is one of the most frequently diagnosed cancers and the second leading cause of women’s cancer death. Although the five-year survival rate for women in the United States with nonmetastatic invasive breast cancer is 90%, this rate drops to less than 30% for those with tumors that have metastasized to distant organs. Epithelial-to-Mesenchymal Transition (EMT) plays a critical role in tumor metastasis by enabling the loss of epithelial characteristics and the gain of mesenchymal traits, leading to increased cell motility, invasion, and resistance to therapy. Consequently, targeting the EMT process may prevent tumor cell migration and improve patient outcomes. Rather than being a binary switch from an epithelial to a mesenchymal state, cells undergoing EMT can assume a spectrum of intermediate states, some of which may reshape the tumor microenvironment by recruiting pro-tumor immunosuppressive cells, such as regulatory T cells, M2-like macrophages, and myeloid-derived suppressor cells. However, the impact of EMT on the immune composition of the tumor microenvironment and the causative mechanisms remain poorly understood. Using multi-omics approaches in a mouse model of triple-negative breast cancer, we assessed the interaction between tumor and immune cells in the tumor microenvironment driven by the EMT. A heterogeneous mouse breast cancer cell line (4T1) was used to isolate five distinct single-cell-derived clonal populations residing in distinct states within the EMT spectrum. These EMT clones, along with the parental 4T1 cell line, were orthotopically transplanted into the mammary fat pads of BALB/c mice. Primary tumors were subsequently harvested and dissociated for single-cell RNA-sequencing (scRNA-seq) and DNA methylation analysis. Lungs were histologically evaluated for metastases. Tumors derived from an intermediate EMT clone exhibited increased tumor growth kinetics and a higher incidence of metastasis compared to epithelial or mesenchymal clones. In scRNA-seq data, tumors derived from different EMT clones largely retained their original EMT states in vivo and recruited distinct immune cell profiles. DNA methylation analysis further revealed that these tumors displayed unique methylation patterns in EMT-related CpGs. This study introduced a model that captures the complexity of the tumor microenvironment across the EMT spectrum. This was accomplished through immunophenotyping using scRNA-seq and identifying unique epigenetic patterns through DNA methylation. Our findings highlight the potential for distinct immune cell populations to be recruited to tumors in different EMT states, thereby influencing survival and metastasis outcomes. Furthermore, our results open new opportunities for developing cancer combination treatment strategies that target specific tumor-immune cell interactions. Citation Format: Hanxu Lu, Meisam Bagheri, Todd Miller, Diwakar Pattabiraman, Brock Christensen. Investigating the impact of tumor EMT states on immune cell infiltration 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 B040.

Orbital metastases from breast cancer: three case reports

Purpose: to describe rare cases of orbital metastases from breast cancer.Material and Methods. Three cases of Luminal A(1) and B(2) breast cancer with orbital metastases were identified and studied during the period from 01.01.2018 to 01.05.2024. Metastasis to the orbit was detected 5, 2 and 1.5 years after radical mastectomy for breast cancer. Patients received chemotherapy and radiation therapy.Results. Cases of orbital metastasis originating from breast cancer were found to have a low frequency, but may be associated with a long period between cancer diagnosis and initiation of treatment. Early detection of breast cancer and its active treatment with chemotherapy and radiation therapy are important factors in preventing orbital metastasis.Conclusion. A routine follow-up surveillance after radical treatment of breast cancer is important because there is an increase risk of developing orbital metastasis. Comprehensive strategies for treating breast cancer metastases can provide long-term disease control.

Abstract B017: The double-edged sword of cystatin B in luminal-like and triple-negative breast cancers

Abstract The complex protease network of cathepsins and their inhibitors, cystatins, intricately regulates numerous cellular processes, and its dysregulation is frequently implicated in cancer pathogenesis. Abnormal cathepsin activity, subcellular localization, and a disrupted balance between cathepsins and cystatins have been identified in cancer. While diminished cystatin levels typically correlate with more aggressive cancer phenotypes, few findings challenge this paradigm. In this context, our latest study revealed elevated expression of cystatin B (CSTB) in luminal-like breast cancer (BC) patients experiencing relapse within five years post-surgery. Furthermore, computational analysis of publicly available datasets supports our idea of CSTB as a potential negative prognostic biomarker in luminal-like tumors. Intriguingly, a converse relationship is observed in basal-like/triple negative BC patients, where increased CSTB expression seems to be associated with improved prognosis. In addition, the cysteine/cathepsin network intricately intersects with immune functions, adding another layer of complexity. Cathepsins are indeed involved in the induction of an efficient anti-tumor immune response, being key in antigen processing and presentation by antigen-presenting cells and in the cytotoxic activities of NK and T cells. On the other hand, myeloid cells rely on cathepsins for the degradation of ECM proteins that may favor migration and invasion of tumor cells. All these pieces of evidence led to the hypothesis that the functional impact of CSTB may vary depending on the tumor intrinsic characteristics and on its microenvironment and therefore that CSTB might fulfill distinct functional roles in luminal-like and basal-like BC. To start addressing our hypothesis, we silenced CSTB expression in the 4T1 mouse cell line, a TNBC model spontaneously metastasizing to the lungs upon intra-mammary fat pad injection. Preliminary in vivo experiments to investigate the effect on primary tumor growth and metastatic dissemination showed that 4T1 shCSTB lines were indeed characterized by increased tumor growth and metastatic dissemination than the two sh-control lines. Interestingly, expression of stemness- related genes (Nanog, Sox2 and Oct4) were up-modulated upon CTSB silencing, in agreement with a more aggressive phenotype. On the other hand, preliminary in vitro assays with a luminal- like BC cell line support the hypothesis of an opposite effect of CSTB down-modulation in luminal BC subtype. Moreover, CSTB silencing in RAW264.7 macrophagic cell line, impaired M1 in vitro polarization, while it did not appear to affect M2 skewing. Overall, although preliminary, these data suggest that CSTB may affect BC aggressiveness by both interfering with tumor cell-intrinsic features and microenviroment-related traits. Citation Format: Anna Zaccaria, Chiara Ratti, Laura Botti, Sabina Sangaletti, Mario P. Colombo, Claudia Chiodoni. The double-edged sword of cystatin B in luminal-like and triple-negative breast cancers [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 B017.

Abstract A001: Reciprocal activation of breast cancer metastases and the lung epithelium during metastatic outgrowth

Abstract Introduction. During metastasis, tumor cells interact extensively with endogenous cells in distant organs. These interactions change the behavior of these surrounding cell populations, often inducing a more pro-tumor microenvironment. The lung is one of the most common sites of breast cancer (BC) metastasis, and in the lung, alveolar epithelial cells are the most common cell type. My data suggest that the lung alveolar epithelium contributes to metastatic outgrowth, and I hypothesize that BC lung micrometastases activate surrounding lung epithelial cells which, in turn, support the outgrowth of BC metastases within the lung. The overall purpose of these studies is to identify factors secreted by resident lung epithelial cells that could be used as metastasis-specific therapeutic targets and/or agents. Methods. To test this hypothesis, I utilized immunocompetent preclinical BC metastasis models to study lung metastatic outgrowth, the stage at which most patients are diagnosed with metastatic disease. A custom imaging panel was developed to quantify lung wound repair, and single-cell RNA-sequencing (scRNAseq) was performed on mouse lungs with high or low metastatic burden to identify genes in the lung epithelium that produce potentially targetable pro-metastatic secreted factors. No-contact co-culture experiments were used to directly study reciprocal paracrine interactions between lung type II alveolar epithelial (AT2) and BC cells. Results. A wound repair-related phenotype, characterized by chronic inflammation, developed within the lung microenvironment during metastatic outgrowth, including an increase in the number and activation of AT2 cells surrounding metastases as they grow. Single-cell RNA-sequencing of mouse lungs with a high vs. low metastatic burden showed that metastatic outgrowth significantly changed AT2 gene expression resulting in a modified secretome. No-contact co-culture experiments indicated that TNBC cells directly alter AT2 gene expression, while AT2 secreted factors promoted TNBC growth. I investigated the possible mechanism(s) responsible for these effects and discovered that AT2 pulmonary surfactant protein and lipid levels are altered by BC-derived secreted factors. Interestingly, treatment with the naturally derived calf surfactant Infasurf, which contains native surfactant proteins/lipids and is FDA-approved for use in premature infants, inhibited BC proliferation. Conclusion. Low levels of surfactant are commonly associated with pulmonary disease and lung cancer. My BC lung metastasis data suggest that something similar may be occurring in the lungs during metastatic outgrowth. Overall, my studies demonstrate the potential for targeting lung epithelial cells in the metastatic microenvironment, in addition to directly targeting malignant cells, as an effective way to treat and manage BC lung metastases. Citation Format: Jessica L. Christenson, Nicole S. Spoelstra, Jennifer K. Richer. Reciprocal activation of breast cancer metastases and the lung epithelium during metastatic outgrowth [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 A001.

Abstract PR001: An immunomodulatory crosstalk between cancer cells and the hepatic microenvironment underlies mutant estrogen receptor-driven breast cancer-to-liver metastasis

Abstract While major advancements have been made in treatment of primary tumors in estrogen receptor-positive (ER+) breast cancer, reducing mortality from metastatic breast cancer (mBC) in treated patients who develop endocrine resistance to first line therapies remains an unmet clinical need. Wild-type ER activity or availability of its cognate ligand 17-beta estradiol (E2) are major factors in primary tumor progression, which are lost/reduced due to endocrine treatments, such as use of selective estrogen degraders (SERDs) and aromatase inhibitors (AIs), respectively. Over time, this leads to selection of rare cancer cells bearing point mutations in ESR1, the gene encoding ER-alpha, which become the dominant population at metastatic sites, and such mutations are now recognized as a frequent driver of endocrine resistance and metastasis, especially in patients with advanced ER+ breast cancer who have been heavily pre-treated with AIs. Analyses of clinical samples have shown a statistically significant association enrichment of ESR1 mutations in liver metastases, compared to the primary tumor. Other distant tissues commonly colonized by breast cancer cells, such as bones, lungs, or brain, although prevalent, do not display this strong association. While this might be due to ease of biopsy sampling in the liver compared to other sites, an alternate and intriguing hypothesis is that the liver offers a fertile soil for metastatic colonization by mutant ER-expressing cells, compared to other distant sites, such as lungs. Using a combination of pre-clinical models, we assessed if mutant ER could drive metastatic colonization at distant sites, such as the liver. We have uncovered a novel link between cancer cell-intrinsic immunomodulatory signaling pathways that are specifically upregulated in mutant ER-expressing breast cancer cells, which represents at least one major molecular mechanism underlying the observed liver tropism of mutant ER-associated, endocrine resistant ER+ breast cancer. We show that disseminated, mutant ER-expressing BC cells engage in crosstalk with the liver stromal and immune populations post-extravasation to further fuel metastatic outgrowth. Citation Format: Sunny Das, Joana Liu Donaher, Lisa Verhoeven, Ranjan Mishra, Ferenc Reinhardt, Sumeet Gupta, Zheqi Li, Kornelia Polyak, Robert A. Weinberg. An immunomodulatory crosstalk between cancer cells and the hepatic microenvironment underlies mutant estrogen receptor-driven breast cancer-to-liver metastasis [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 PR001.

Abstract B035: Reciprocal macrophage - T cell interactions regulate anti-tumor immunity

Abstract Cancer immunotherapy (including immune checkpoint blockade (ICB)) has shown great promise in the treatment of some solid cancers, but responses in breast cancer patients are limited. While breast cancer is generally considered poorly immunogenic, the abundance of CD8+ T cells correlates with clinical response. CD8+ T cells are critical mediators of anti-tumor immunity and the main target for ICB. However, the onset of terminal functional T cell exhaustion poses a major challenge. Although T cell exhaustion has been linked to persistent antigen exposure, it remains unclear how the immune composition of the breast tumor microenvironment (TME) contributes to this dysfunctional T cell state. Breast cancers are heavily infiltrated with tumor-associated macrophages (TAMs) and their abundance correlates with T cell exhaustion and poor prognosis. Previously, we discovered a spatiotemporal co-dependency between exhausted T cells (TEX) and TAMs in mouse models of melanoma and breast cancer. We showed that TEX actively recruit monocytes to the TME and shape their differentiation trajectory into TAMs. Reciprocally, these TAMs ‘capture’ T cells in long-lived synaptic interactions that contribute to functional T cell exhaustion. Our current work is focused on studying the TEX-derived chemokines that regulate these TAM – T cell interactions in the breast TME with the ultimate goal to alleviate immune evasion and improve responsiveness of breast cancer to immunotherapy. Citation Format: Meenakshi Sudhakaran, Sofia Lombardi, Sophie Ayma, Matthew F. Krummel, Kelly Kersten. Reciprocal macrophage - T cell interactions regulate anti-tumor immunity [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 B035.

Abstract C032: The impact of neoadjuvant endocrine therapy (NET) in ER+ breast cancer tumor microenvironment and metastasis

Abstract Breast cancer is the most common cancer in women worldwide and the second leading cause of death. The Estrogen Receptor positive (ER+) subtype constitutes approximately 80% of the yearly 250,000 breast cancer diagnoses in the US. Anti-estrogen endocrine therapy, administered either pre- or post-operatively, is the cornerstone treatment for all stages of breast cancer. Pre- operative or neoadjuvant endocrine therapy (NET) reduces pre-surgical tumor burden, improves surgical outcome, and provides prognostic information. Although the treatment often has a favorable initial response, about 20% of patients eventually develop resistance and recurrent disease, even decades after the primary diagnosis.We hypothesized that resistance in NET is mediated through pro-metastatic changes in the tumor microenvironment affecting either (i) dissemination machinery or (ii) cancer cell phenotype.The main mode of breast cancer hematogenous dissemination to distant sites is through intravasation portals called Tumor Microenvironment of Metastasis (TMEM) doorways. Each TMEM doorway consists of a tumor cell overexpressing the actin-regulatory protein MENA, a Tie2high/ VEGFAhigh macrophage and an endothelial cell, all in direct contact. TMEM doorway activity leads to vascular opening events, during which tumor cells located in areas around TMEM doorways intravasate and disseminate to secondary sites. Therefore, to investigate the impact of NET on cancer cell dissemination, we evaluated its effect on (i) TMEM doorway score (the number of TMEM doorways per tissue area), (ii) TMEM doorway opening, (iii) the number of circulating tumor cells and (iv) the number of disseminated tumor cells. We used the orthotopic MCF7 breast cancer xenograft model grown in NOD/SCID mice and tested two drugs widely used in NET, Fulvestrant (a Selective Estrogen Receptor Degrader) and Tamoxifen (a Selective Estrogen Receptor Modulator). NET did not affect the TMEM doorway score, TMEM doorway activity, the number of CTCs or the number of DTCs. We are now focused on evaluating the effect of NET on the density of cancer cells which are upregulated in programs of invasion, stemness, and dormancy. This “Triple Threat Phenotype (TTP)” gives tumor cells an enhanced ability to disseminate to and colonize distant organs. To this effect we will stain primary MCF7 tumors from NET and vehicle-treated mice for markers of invasiveness (MENA), stemness (SOX9) and dormancy (NR2F1). Detecting any changes in the phenotype of metastatic tumor cells following NET will help us understand underlying mechanisms of endocrine therapy resistance and may lead to new therapeutic targets and improve patient outcomes. Citation Format: Anastasia Aristoteleia Chondronikola, Dimitra Anastasiadou, Aliona Zintiridou, Camille L. Duran, Nicole Barth, Burcu Karadal-Ferrena, Erika Pereira-Zambalde, Lina Ariyan, Suryansh Shukla, George S. Karagiannis, David Entenberg, John S. Condeelis, Maja H. Oktay. The impact of neoadjuvant endocrine therapy (NET) in ER+ breast cancer tumor microenvironment and metastasis [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 C032.

Abstract A031: ECM architecture drives distinct evolutionary patterns of tumor immune escape and elimination in breast cancer

Abstract Cancer progression poses a clinical hurdle due to tumor heterogeneity that contributes to treatment resistance and immune evasion. While the adaptive immune system can effectively eliminate cancer in certain instances, tumor escape remains challenging. The immune microenvironment plays an important role in this process. One unanswered question is the role of ECM geometry, also known as Tumor Associated Collagen Signatures (TACS), and its effects on tumor immune recognition. In solid malignancies, ECM geometry has been associated with disease stage and T cell infiltration. Specifically, empirically observed ECM topologies are frequently categorized based on fiber arrangement: random fibers (TACS1), circumferentially aligned fibers (TACS2), and radially arranged fibers (TACS3). Even though a clear negative correlation between TACS and patient survival has been established, the specific roles and extent of TACS in T cell-driven cancer evolution remain uncharacterized. The precise mechanisms underlying how TACS influences cell movement are still not fully elucidated, with divergent opinions on whether and how the ECM mediates immune cell infiltration. At present, we currently lack a physical model relating the impact of TACS on the spatial co-evolution between an adaptive immune repertoire and a heterogeneous population of evading cancer cells. Here, we developed the EVO-ACT (EVOlutionary Agent-based Cancer T cell interaction) model, which is based on the Gillespie algorithm, to study the effects of TACS on tumor evolution and dynamical tumor-T cell interactions. Our analysis of tumor-T cell interactions across three TACS types reveals distinct migration patterns, with TACS3 showing the highest efficiency, TACS1 intermediate, and TACS2 the lowest. TACS impacts T cells more than tumors, with stronger chemokine gradients aiding T cell infiltration. Our model suggests that despite TACS2's limited efficiency, it cannot fully impede T cell infiltration. Varied migration efficiencies lead to diverse T cell infiltration patterns and immunoediting levels. TACS3 shows a benefit to immune recognition and higher survival, contrary to lower survival in TACS3 patients observed clinically in breast cancer. Our results indicate that only after introducing phenotypic changes, such as Epithelial-Mesenchymal Transition (EMT), and noting that EMT occurs exclusively after the TACS2-TACS3 progression, can our findings successfully explain the clinically observed trends. Considering mesenchymal tumor cells' ability to upregulate PD-1/PD-L1, we explore TACS-specific tumor evolution's impact on PD-1/PD-L1 inhibitor responses and patient survival. We observe higher tumor escape rates and lower survival in TACS3 compared to TACS2. Thus, TACS3 alone does not account for lower survival in breast cancer; TACS3-associated late-stage cancer byproducts like EMT and elevated checkpoint expression significantly decrease survival. Citation Format: Yijia Fan, Jason Tomas George. ECM architecture drives distinct evolutionary patterns of tumor immune escape and elimination 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 A031.

Abstract C027: The role of Liver Receptor Homolog 1 (LRH-1) in regulating cancer progression by modulating the immune response

Abstract Breast cancer is a significant global health concern, being the second most common type of cancer and the second leading cause of cancer-related deaths among women in the U.S. Metastatic breast cancer is responsible for most of these deaths, and current FDA-approved immunotherapies are effective only for a small subset of patients with PD-L1-positive tumors. Thus, there is a critical need for new therapeutic targets to enhance the immune response against breast cancer. Elevated plasma cholesterol concentrations are associated with poor breast cancer outcomes. Our previous studies indicated that myeloid immune cells are highly sensitive to changes in cholesterol levels and other proteins involved in regulating cholesterol homeostasis. However, the role of Liver Receptor Homolog 1 (LRH-1), a key regulator of cholesterol homeostasis, in myeloid cells is not well understood. Notably, higher expression of LRH-1 mRNA in breast tumors is associated with longer time to recurrence and improved overall survival. Therefore, we aimed to investigate the role of LRH-1 in myeloid immune cells and its impact on breast cancer progression. Initial analysis indicated that LRH-1 is expressed in myeloid cells, with particularly high levels observed in neutrophils. Therefore, we examined how LRH-1 regulates neutrophil functions important to cancer biology, including migration, NETosis, and phagocytosis, as well as its influence through neutrophils on T cells. We found that LRH-1 inhibited neutrophil migration toward cancer cells. Since tumor infiltrating neutrophils are generally associated with poor prognosis, LRH-1 reducing neutrophil migration could indicate a decreased ability of these immune-suppressive cells to infiltrate tumors. Importantly, neutrophils treated with an LRH-1 agonist showed reduced NETosis, while antagonist led to increased NETosis. LRH-1 agonist treatment also significantly reduced neutrophil phagocytosis, whereas an antagonist had no notable effect. Furthermore, T cells exhibited increased expansion when co-cultured with neutrophils treated with an LRH-1 agonist, and reduced proliferation with neutrophils pretreated with an LRH-1 antagonist or inverse agonist, particularly in CD4+ helper T cells and later divisions. Since neutrophils were previously implicated in invoking recurrence from dormant lesions, we evaluated LRH-1 in this context. The treatment of mice harboring dormant D2.0R lesions with an LRH-1 inverse agonist resulted in sooner recurrence and metastatic outgrowth compared to vehicle treated mice. In summary, our findings suggest that LRH-1 plays a crucial role in regulating various neutrophil functions, including migration, NETosis, phagocytosis, and T cell expansion. The ultimate consequence of these regulations are control of dormant mammary cancer lesions. These results highlight LRH-1 as a potential therapeutic target for cancer treatment or prevention of metastatic recurrence. Funding: National Cancer Institute (ERN: R01CA234025) Department of Defense (ERN: BCRP Era of Hope Award) Citation Format: Yu Wang, Bryan Duong, Natalia Krawczynska, Shruti V. Bendre, Claire P. Schane, Erin Weisser, Lara Kockaya, Yifan Fei, Anasuya Das Gupta, Hashni E. Vidana Gamage, Adam T. Nelczyk, Dhanya Pradeep, Erik R. Nelson. The role of Liver Receptor Homolog 1 (LRH-1) in regulating cancer progression by modulating the immune response [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 C027.

Abstract A017: HER2 overexpression initiates breast tumorigenesis non-cell-autonomously by altering energy metabolism in the tissue microenvironment

Abstract Breast cancer is the most common cancer globally, with approximately 70% of pre-malignant breast tumors, known as ductal carcinoma in situ (DCIS), and 20% of invasive breast tumors overexpressing human epidermal growth factor receptor 2 (HER2). While the biological role of HER2 in invasive breast cancer has been extensively studied, its impact on breast cancer stem cells (BCSCs) and early-stage tumorigenesis, particularly within the tumor microenvironment, remains unclear. Here, we explore the role of HER2 overexpression in driving cellular events within the tumor microenvironment during the premalignant stage of breast tumorigenesis, using in vitro, in vivo, and ex vivo models. We conducted multi-omics analyses on mammary ducts from a HER2+ breast cancer mouse model at the pre-cancerous stage. Our results showed that HER2/Neu overexpression alters glucose and lipid metabolism and pentose phosphate pathway in mammary epithelial cells. Ex vivo organ culture of mammary ducts confirmed that these metabolic changes lead to increased reactive oxygen species (ROS) levels within the tumor microenvironment suggesting that oxidative stress and subsequent DNA damage are early events in HER2-driven tumorigenesis. Immunostaining of histological sections from mouse mammary glands further revealed that elevated oxidative stress and DNA damage not only in HER2+ cells but also in neighboring HER2- cells. Moreover, whole genome sequencing of tumor cells confirmed that both HER2+ and HER2- tumor cells have similar mutational burden. This suggests a non-cell-autonomous effect, where HER2 overexpression in one cell population affects the tumor microenvironment and adjacent cells, contributing to early-stage cancer development. To investigate the involvement of BCSCs during tumor initiation, we performed mammosphere assays on primary cells from HER2+ mouse models and clinical DCIS samples. Our findings showed that neither BCSCs nor their cells of origin express HER2 in both DCIS samples and murine mammary glands and tumors. This suggests that HER2 overexpression may not directly drive BCSC formation but rather creates a genotoxic microenvironment that facilitates the transformation of HER2- cell populations through a non-cell-autonomous mechanism. In summary, our findings indicate that HER2 overexpression contributes to the creation of a genotoxic tumor microenvironment by altering cellular energy metabolism and increasing ROS levels. This, in turn, may lead to the transformation of BCSCs in HER2- cell populations during early breast tumorigenesis. Understanding the role of HER2 overexpression in shaping the tumor microenvironment could provide new insights into therapeutic and preventive strategies against breast cancer. Citation Format: Sevim B. Gurler, Oliver Wagstaff, Lili Dimitrova, Fuhui Chen, Robert Pedley, William Weston, Ian J. Donaldson, Brian A. Telfer, David Novo, Kyriaki Pavlou, George Taylor, Yaqing Ou, Kaye Williams, Andrew Gilmore, Keith Brennan, Ahmet Ucar. HER2 overexpression initiates breast tumorigenesis non-cell-autonomously by altering energy metabolism in the tissue microenvironment [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 A017.

Abstract A010: Development and characterization of novel models of lobular breast cancer metastases

Abstract Invasive lobular carcinoma (ILC) is the most common special histological subtype of breast cancer, after No Special Type (NST, ductal carcinoma/IDC), and affects 10-15% of all breast cancer patients. ILC is characterized by E-cadherin loss, Estrogen Receptor (ER) positivity, and a distinct single file invasive growth pattern associated with a unique pattern of metastasis. ILC metastasizes to common breast cancer sites like bone and lung, but ILC also spreads to gynecologic tissues, the peritoneum, gastrointestinal tract and central nervous system - with enrichment for orbital and leptomeningeal metastases. In vitro and in vivo models of ILC metastasis have not been extensively developed. Models of spontaneous metastasis from ER+ breast cancer overall are critically limited; ER+ ILC metastases models present only micro-metastatic disease with extremely long latency (>9 months). Together these issues limit research on the unique nature of metastatic ILC. We leveraged mammary intraductal (MIND) xenografting with two ER+ ILC cell lines (MM134 and 44PE) and confirmed prior reports that these lines spontaneously metastasize to bone, brain, ovary, uterus, lung, and adrenal glands with up to 100% incidence. Importantly, we find that supplementation with low dose estradiol (E2) in drinking water rapidly advances metastatic progression, but not primary tumor growth. Using ex vivo bioluminescence imaging (BLI), we find BLI+ lesions are detectable as early as 90 days. At 150 days post-challenge, 100% of mice have BLI+ metastases at multiple sites. Beyond 22 weeks, 100% of mice present with symptomatic and ultimately lethal CNS metastases. BLI+ macro-metastatic lesions were isolated and expanded ex vivo as organotrophic cell line variants. Analysis of RNA-seq data is ongoing to evaluate 1) common factors enriched in metastatic ILC, and 2) organotrophic factors and pathways enriched in specific metastatic sites. In patients with ILC, bone metastases are typically the first site of metastatic development, occur at a higher incidence versus NST, and are associated with increased morbidity. Bone metastases from ILC patients are more osteogenic compared to the characteristically osteolytic lesions of NST. To examine how ILC cells may promote osteoblastic responses, we prepared ILC and NST conditioned media (CM) and applied it to differentiating osteoblasts (OB). We observed increased OB differentiation and increased mineral deposition with ILC CM compared to NST CM, supporting that ILC cell lines have a distinct impact on the bone microenvironment which may manifest in vivo yet be defined in vitro. Collectively, our observations support that ER+ ILC cell lines can recapitulate key features of clinical disease progression, including spontaneous metastasis mimicking clinical phenotypes. We are leveraging these models to better understand the ILC metastatic cascade and adaptation to unique metastatic niches, and to identify shared and unique attributes of ILC metastases that can be exploited for advancing therapeutics. Citation Format: Joseph L. Sottnik, Matthew J. Sikora. Development and characterization of novel models of lobular breast cancer metastases [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 A010.

Abstract C014: Degradation of extracellular trap DNA sustains anti-tumor immune responses in breast cancer

Abstract Increased extracellular DNA (exDNA) levels in the blood are a hallmark of metastatic cancer, arising from tumor lysis, apoptosis, necrosis, and neutrophil extracellular traps (NETs) released by tumor-expanded neutrophils. NETs, web-like chromatin structures extruded by neutrophils to trap pathogens, are highly immunogenic due to their DNA and histone content and their persistent interaction with dendritic cells. Tumor cells can also release extracellular traps. In myeloid leukemia, blasts release nuclear content via traps to activate coagulation or sustain myeloproliferation. Extracellular traps have been found in NPM1 mutant AML patients, with mutant NPM co-localizing with histones along exDNA traps. Forcing trap extrusion in NPM mutant leukemia cells has been used to create dendritic cell-based vaccines that break tolerance against NPMc antigens. This suggests that NETs in the tumor microenvironment (TME) may promote tolerance, with breaking tolerance requiring consistent NET formation and dendritic cell interaction. NETs are also seen in solid tumors, such as triple-negative breast cancer (TNBC), where they aid in cancer cell migration, invasion, and awakening of circulating tumor cells. However, the capacity of tumor cells to directly extrude DNA traps is not fully understood. To investigate this, various human and murine breast cancer cell lines were seeded on poly-D-lysine coated slides, stained with DAPI and Sytoxgreen, and observed via confocal microscopy. Tumor cells extruded DNA traps within 4 hours without stimuli. Notably, trap extrusion correlated with cell line aggressiveness and was abolished by DNase, indicating dsDNA composition. The absence of traps in 24-hour cultures suggests DNA extrusion may help rare tumor cells survive and proliferate. In line, DNase treatment reduced tumor proliferation and increased apoptosis. To assess the significance of DNA traps in vivo, BALB/c mice were injected with the highly metastatic 4T1 clone 5 breast cancer model, with or without DNase treatment, and monitored starting 5 days post-injection and weekly until day 28. Histological analysis at the injection site 5 days post-injection showed that DNase treatment reduced both local trap formation and tumor cell proliferation, as indicated by BrdU incorporation. Moreover, immunohistochemistry revealed that in DNase-treated mice, initial tumor growth was followed by complete tumor regression, accompanied by local and systemic CD8 T cell activation. Correspondingly, although a few metastases were detectable at day 14 in DNase-treated mice, no metastases were observed by day 28. Although it remains unclear whether tumor-derived or immune cell-derived traps are most relevant in our model, the results demonstrate that their presence in the TME promotes tolerance despite the inherent immune adjuvant properties of the traps. This suggests that DNase treatment could be particularly effective in combating early tumor development or relapse when used as part of a combination therapy designed to sustain anti-tumor immune responses. Citation Format: Sabina Sangaletti, Paola Portararo, Laura Botti, Valeria Cancila, Claudio Tripodo, Mario P. Colombo, Claudia Chiodoni. Degradation of extracellular trap DNA sustains anti-tumor immune responses 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 C014.

Abstract C011: The impact of reproductive factors on breast tumor and normal-adjacent tissue immune profile from menarche to menopause

Abstract Background: Reproductive factors and sex hormones are tightly linked to systemic immunity. However, no studies have examined whether reproductive factors and exogenous hormone use modulate the immune microenvironment of breast tissue. Methods: We prospectively evaluated the associations of reproductive factors (age at menarche, parity, age at first birth, age at menopause, and menopausal status) and exogenous hormone use (oral contraceptives, and menopausal hormone therapy (MHT)) with breast tumor and normal-adjacent tissue immune cell markers among 935 breast cancer cases in the Nurses’ Health Studies. We deconvoluted immune cell abundance using CIBERSORTx and derived gene expression signatures of markers for immune checkpoint (PD1, PDL1, and CTLA4), co-regulatory signal and antigen presentation (MHC class I/ II and T cell receptor) and mammary cytokine signaling (IFN gamma, IL2, IL4, IL12, IL13, TGF beta). Linear regression was used with adjustment for potential confounders. Results: Majority of participants had estrogen receptor (ER)-positive breast cancer (81%) and were post-menopausal at diagnosis (72%). Compared to tumors from pre-menopausal women, tumors from post-menopausal women had higher signature scores for tumor pro-inflammatory cytokine signaling (IFN gamma, IL2, IL4, IL12, IL13, TGF beta) and antigen presentation (FDR≤0.05). Several of the inflammatory cytokine signatures showed modest inverse relations with signatures of immune checkpoint markers (ρ=-0.3 to -0.4). Women who reported current use of estrogen only MHT had higher expression of CTLA4 in ER-positive breast tumors. For normal-adjacent tissues of ER-positive tumors, parity was associated with higher CD8+ T cell abundance, higher PDL1 expression and lower cytokine signaling in normal-adjacent tissues at p≤0.05; while breastfeeding was associated with lower abundance of CD8+ T cell and higher expression of cytokine signaling in normal-adjacent tissues of ER-negative tumors (p≤0.05). None of the associations in normal-adjacent tissues met FDR significance. The associations between other reproductive factors and immune cell profiles were in general weak. Conclusions: Our study suggests pre-diagnostic reproductive factors may influence breast tumor immune profiles. Future studies are needed to validate these results. Citation Format: Cheng Peng, Yuxi Liu, Yujing Heng, Clara Bodelon, Daniel Stover, Wendy Y. Chen, Michelle D. Holmes, A. Heather Eliassen, Peter Kraft, Rulla M. Tamimi. The impact of reproductive factors on breast tumor and normal-adjacent tissue immune profile from menarche to menopause [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 C011.

Abstract C037: Chemotherapy causes de novo induction of invasive breast cancer cells

Abstract Metastasis is the leading cause of breast cancer (BC) death, and tumor cells must migrate and invade to metastasize. BC cells that express the actin regulatory protein MenaINV have an enhanced ability to migrate and intravasate within the primary tumor, and extravasate at secondary sites. Though chemotherapy improves patient survival, treatment with paclitaxel, a chemotherapy used for BC, leads to upregulation of MenaINV and an increase in metastasis in mice. MenaINV expression can be induced in BC cells through Notch1 signaling with macrophages, which are often recruited to tumors in response to chemotherapy. MenaINV-expressing cells are also resistant to paclitaxel, begging the question of whether paclitaxel increases MenaINV by de novo induction or by selectively killing non-MenaINV-expressing cells. We hypothesized that paclitaxel causes de novo MenaINV induction by increasing macrophage-tumor cell Notch signaling. Understanding this pro-metastatic effect of chemotherapy is crucial to refining treatment strategies. MMTV-PyMT mice bearing spontaneous mammary tumors were used as a model of BC. Mice were treated with paclitaxel or vehicle every 5 days for a total of 3 treatments. To study the role of macrophages in the paclitaxel-mediated MenaINV increase, in addition to paclitaxel, one cohort of mice was co-administered the macrophage-depleting agent clodronate or vehicle. To study the role of Notch signaling, a second cohort of mice was co-administered the Notch inhibitor DAPT or vehicle. Immunofluorescence staining was used to evaluate tumors for: %MenaINV+ area, proportion of apoptotic cells, macrophage density, and macrophage expression of Notch ligands Jag1 and Jag2. Paclitaxel-treated tumors expressed significantly more MenaINV even when tumor cell death did not increase, indicating that chemotherapy increases MenaINV expression by induction. Both macrophages and Notch signaling were needed for this effect to occur. Interestingly, paclitaxel did not increase macrophage infiltration or macrophage expression of Notch ligands, suggesting that paclitaxel may increase MenaINV by acting on tumor cells to promote Notch signaling. Paclitaxel causes de novo MenaINV induction that is macrophage- and Notch-dependent. Studies aiming to further elucidate the mechanism behind paclitaxel-mediated MenaINV induction are ongoing. These results lay the groundwork for novel microenvironment-based therapies to alleviate the pro-metastatic effects of chemotherapy in BC. Citation Format: Madeline Friedman-DeLuca, George S. Karagiannis, Camille L. Duran, Luis Rivera Sanchez, Prachiben P. Patel, Suryansh Shukla, Nicole D. Barth, Ved P. Sharma, Michael Papanicolaou, John S. Condeelis, Maja H. Oktay, David Entenberg. Chemotherapy causes de novo induction of invasive breast cancer cells [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 C037.

Abstract A034: Single cell RNA sequencing of triple negative breast cancer patient-derived xenograft model identifies CRABP1 of cancer-associated fibroblast as a key regulator of breast cancer metastasis

Abstract Triple-negative breast cancer (TNBC) is associated with a high risk of distant metastasis, particularly to the lung and liver. Both intrinsic characteristics of cancer cells and the tumor microenvironment (TME) influence TNBC growth and metastasis. The TME, comprising stromal, immune, and endothelial cells, exhibits heterogeneity both between and within cell types. Identifying the molecular characteristics of TME cells that affect cancer progression is crucial. We aim to investigate this using single-cell RNA sequencing of patient-derived xenograft (PDX) models to explore TME cells and genes involved in TNBC metastasis. We established PDX models by transplanting tumor tissues from 26 TNBC patients into immunodeficient mice. These models were categorized based on their metastatic potential and patient outcomes: non-metastatic (n=5) and metastatic (n=4). Single-cell RNA sequencing of nine PDX tumors revealed significant differences in gene expression of murine stromal cells between metastatic and non-metastatic models. We identified ten genes (Serpinb2, Spp1, Tnc, Thbs1, Timp1, Il11, Mt2, Crabp1, Cck, Mt1) that were highly expressed in the stromal cells of metastatic PDX models. To test if TNBC cells influence these fibroblast genes, we exposed NIH3T3 fibroblasts to conditioned media from the 4T1 TNBC cell line. In three independent experiments, only Crabp1 expression in NIH3T3 cells was consistently increased by 4T1- conditioned media. CRABP1, a retinoic acid-binding protein, is known for its role in differentiation and proliferation by regulating MAPK signaling. Although Crabp1-positive cancer-associated fibroblasts have been noted, its role in TNBC metastasis was previously unexplored. We investigated CRABP1 function using a CRABP1 knockdown NIH3T3 cell line. CRABP1 knockdown led to reduced invasion and migration of 4T1 cells in trans-well assays and decreased invasiveness of 4T1 spheroids in a collagen matrix. Additionally, CRABP1 knockdown NIH3T3 cells showed reduced proliferation in vitro and fewer alpha-SMA positive cancer-associated fibroblasts in co-injected 4T1 tumors in BALB/C mice. In summary, CRABP1, identified through single-cell RNA sequencing of stromal cells in TNBC PDX models, is a potential regulator of TNBC metastasis, affecting cancer cell migration, invasion, and fibroblast proliferation. Citation Format: Woohang Heo, Yujeong Her, Sieun Yang, Dakyung Lee, Rokhyun Kim, Jong-Il Kim, Hyeong-Gon Moon. Single cell RNA sequencing of triple negative breast cancer patient-derived xenograft model identifies CRABP1 of cancer-associated fibroblast as a key regulator of breast cancer metastasis [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 A034.

Abstract C045: The extracellular matrix glycoprotein periostin supports chemotherapy resistance by modulating macrophage recruitment and phagocytosis in triple negative breast cancer

Abstract Triple-negative breast cancer (TNBC) accounts for approximately 15–20% of all breast cancer cases and is notoriously aggressive, often developing resistance to standard chemotherapy thus leading to a high rate of metastatic relapse. Evidence suggests that this resistance is linked to changes in the extracellular matrix (ECM) composition, particularly an increase in periostin (POSTN) expression, which is also associated with poor prognosis in breast cancer. Our recent publication showed that the ECM alone can influence macrophage phenotypes. Furthermore, another study from our lab has demonstrated that POSTN mediates resistance to anti-angiogenic therapy by recruiting macrophages in pancreatic neuroendocrine tumors. Notably, POSTN deletion reduced monocyte/macrophage recruitment and enhanced cytotoxic CD8+ T-cell infiltration. Based on these findings, we hypothesized that paclitaxel (PTX), a widely used chemotherapeutic drug for the management of TNBC, may stimulate POSTN production by fibroblasts leading to the recruitment of pro-tumoral macrophages that facilitate tumor growth resulting into therapy resistance. In this study, we showed that PTX induces increased POSTN expression in mouse models of TNBC through immunohistochemistry, and in both 2D and 3D in vitro models via interactions between TNBC cell lines and mammary fibroblasts, as demonstrated by immunofluorescence. An in vitro migration assay confirmed that PTX-induced POSTN enhances the recruitment of human monocytes and macrophages, which display a mixed immunosuppressive and inflammatory phenotype, as revealed by flow cytometry. Not only did these recruited macrophages exhibit a reduced phagocytic capacity against labelled TNBC cells in an in vitro phagocytosis assay, but we also found that POSTN plays a crucial role in inducing SIRPα expression, that may diminish macrophage phagocytic potential. Increased sialylation and Siglec-1 expression are known to be linked to poor prognosis and polarization towards a more immunosuppressive macrophage phenotype in TNBC. Interestingly in our model, we observed that higher concentrations of recombinant human POSTN induce increased Siglec-1 expression on macrophages, a receptor that interacts with sialic acids in the tumor ECM. Further work using a decellularized tissue model that preserves only the ECM from patient-derived tumor biopsies pre- and post-chemo, aims to further explore the correlation between PTX-induced POSTN, ECM sialylation, and Siglec-1 expression on macrophages. Additionally, we have developed POSTN-knockout murine cancer cells and fibroblasts using CRISPR/Cas9, allowing us to investigate how POSTN influences macrophage phenotypes and chemotherapy responses in vivo. Understanding the mechanisms behind chemotherapy resistance in TNBC is crucial for developing effective treatments for this aggressive cancer subtype. By elucidating the intricate interplay between POSTN, macrophages, and chemotherapy response, we aim to identify new therapeutic strategies to improve outcomes for TNBC patients. Citation Format: Ludovica Tarantola, Ioanna Keklikoglou, Oliver M. Pearce. The extracellular matrix glycoprotein periostin supports chemotherapy resistance by modulating macrophage recruitment and phagocytosis in triple negative 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 C045.

NÚMERO DE CIRURGIAS ONCOLÓGICAS NO BRASIL NOS ÚLTIMOS 10 ANOS E O PAPEL DA ANESTESIA NA PREVENÇÃO DA RECORRÊNCIA TUMORAL DO CÂNCER DE MAMA

Cancer represents one of the major global public health challenges, being the second leading cause of death after cardiovascular diseases. In 2020, the World Health Organization (WHO) recorded approximately 20 million new cases and around 10 million cancer deaths worldwide. In Brazil, breast cancer is the most frequent among women, with an estimated 73,610 new cases annually for the period from 2023 to 2025, representing a significant impact on the healthcare system. Oncologic surgery is essential in cancer management, especially for solid tumors, and is often complemented by chemotherapy, radiotherapy, or hormone therapy. However, studies indicate that the perioperative environment, including anesthesia, may influence tumor progression and disease recurrence, particularly in the context of breast cancer. In recent years, evidence has emerged suggesting that anesthesia, especially the anesthetic agents used, may play an important role in modulating the perioperative immune environment, potentially influencing cancer progression and recurrence. Nevertheless, the evidence remains inconsistent, and further studies are needed to clarify the direct impact of anesthesia on breast cancer recurrence and other types of cancer. The aim of this study is to review the epidemiology of oncologic surgery in Brazil over the past 10 years and to discuss the role of anesthesia in this context, focusing on surgical procedures related to breast cancer and the discussion of anesthetics in relation to tumor recurrence.

Abstract B012: Understanding the role of VISTA in regulating T cell function and the therapeutic potential of blocking VISTA in triple negative breast cancer

Abstract Triple-negative breast cancer (TNBC) is the most aggressive and highly metastatic breast cancer subtype with limited effective treatment options. Immunotherapy with immune checkpoint inhibitors (ICIs) targeting checkpoint molecules have been widely used and have shown therapeutic efficacy in treating multiple malignancies in the last decade. In TNBC, ICI targeting PD-1, alone or in combination with chemotherapy has been approved for the treatment of patients with both early and advanced stage of the disease. Although, there has been significant clinical benefit achieved from anti-PD-1 and chemotherapy, treatment efficacy is often limited to a subgroup of patients. The limited response to ICIs has been attributed to the increased infiltration of immunosuppressive cells (including MDSCs, neutrophils and tumor-associated macrophages), more importantly, the upregulation of alternative immune checkpoint receptors/ligands that may further suppress cytotoxic T cells (CD8+ T cells) in the tumor microenvironment (TME). Therefore, it has been suggested that targeting other immune checkpoint molecules may be more effective in bolstering anti-tumor immune response. V- domain Ig suppressor of T-cell activation (VISTA) is one of the immune checkpoint molecules attracting increasing attention in the context of anti-tumor immunity due to its broad expression across various cancer types and increased expression upon development of immune checkpoint resistance. However, the role of VISTA in regulating the anti-tumor immune response and its therapeutic potential in TNBC remains unclear. In our study, we observed that VISTA is expressed in TNBC patients, predominantly in neutrophils and macrophages. In the Py230 murine breast cancer model, VISTA expression increased upon paclitaxel (PTX), anti-PD-L1 and paclitaxel combined with anti-PD-L1 treatment compared to the control group, suggesting its potential role as a resistance mechanism in sustaining or further inhibiting T cells. In the Py230 model, VISTA blockade increased the infiltration and activation of CD8+ T cells, as evidenced by the higher number of CD8+ T cells, increased CD69+CD8+ T cells, and Granzyme B production, indicating that VISTA blockade improved CD8+ T cells mediated anti-tumor immunity. To functionally test the role of VISTA, we stimulated CD8+ T cells and co-cultured them with naïve macrophages or tumor educated macrophages in the presence or absence of VISTA blocking antibody. Tumor educated macrophages showed increased immunosuppressive activity which was abrogated with VISTA blockade. Moreover, we found that the immunosuppressive effect of macrophages on T cell activation needs direct cell-cell interaction between VISTA on macrophages and T cells. In conclusion, this study shows the mechanism of action of VISTA in the regulation of T cells mediated anti-tumor immune response and provides the rationale for targeting VISTA in TNBC. Citation Format: Maidinaimu Abudula, Yuliana Astuti, Meirion Raymant Raymant, Vijay Sharma, Michael Schmid, Ainhoa Mielgo. Understanding the role of VISTA in regulating T cell function and the therapeutic potential of blocking VISTA in triple negative 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 B012.