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Breast disseminated cancer cells (DCCs) can remain dormant in the lungs for extended periods, but the mechanisms limiting their expansion are not well understood. Research indicates that tissue-resident alveolar macrophages suppress breast cancer metastasis in lung alveoli by inducing dormancy. Through ligand-receptor mapping and intravital imaging, it was found that alveolar macrophages express transforming growth factor (TGF)-β2. This expression, along with persistent macrophage-cancer cell interactions via the TGF-βRIII receptor, maintains cancer cells in a dormant state. Depleting alveolar macrophages or losing the TGF-β2 receptor in cancer cells triggers metastatic awakening. Aggressive breast cancer cells are either suppressed by alveolar macrophages or evade this suppression by avoiding interaction and downregulating the TGF-β2 receptor. Restoring TGF-βRIII in aggressive cells reinstates TGF-β2-mediated macrophage growth suppression. Thus, alveolar macrophages act as a metastasis immune barrier, and downregulation of TGF-β2 signaling allows cancer cells to overcome macrophage-mediated growth suppression., Competing Interests: Declaration of interests J.A.A.-G. is a scientific co-founder and scientific advisory board member of and an equity owner in HiberCell and receives financial compensation as a consultant for HiberCell, a Mount Sinai spin-off company focused on the research and development of therapeutics that prevent or delay the recurrence of cancer. J.A.A.-G. is also a consultant for Astrin Biosciences and Chief Mission Advisor for the Samuel Waxman Cancer Research Foundation. M.M. serves on the scientific advisory board of and holds stock from Compugen Inc., Myeloid Therapeutics Inc., Morphic Therapeutic Inc., Asher Bio Inc., Dren Bio Inc., Nirogy Inc., Oncoresponse Inc., Owkin Inc., Pionyr Inc., OSE Inc., and Larkspur Inc. M.M. serves on the scientific advisory board of Innate Pharma Inc., DBV Inc., and Genenta Inc. M.M. receives funding for contracted research from Regeneron Inc. and Boerhinger Ingelheim Inc. M.M. is a named co-inventor on an issued patent for multiplex immunohistochemistry to characterize tumors and treatment responses. The technology is filed through Icahn School of Medicine at Mount Sinai (ISMMS) and is currently unlicensed. This technology was used to evaluate tissue in this study, and the results could impact the value of this technology. M.M. has ownership interest (<5%) in Compugen Inc., Morphic Therapeutic Inc., Myeloid Therapeutics Inc., Asher Bio Inc., Dren Bio Inc., Nirogy Inc., Owkin Inc., Pionyr Inc., and Larkspur Inc. M.M. and J.A.A.-G. declare no ownership interest greater than or equal to 5%., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Tumor-associated macrophages (TAMs) are a phenotypically diverse, highly plastic population of cells in the tumor microenvironment (TME) that have long been known to promote cancer progression. In this review, we summarize TAM ontogeny and polarization, and then explore how TAMs enhance tumor cell migration through the TME, thus facilitating metastasis. We also discuss how chemotherapy and host factors including diet, obesity, and race, impact TAM phenotype and cancer progression. In brief, TAMs induce epithelial-mesenchymal transition (EMT) in tumor cells, giving them a migratory phenotype. They promote extracellular matrix (ECM) remodeling, allowing tumor cells to migrate more easily. TAMs also provide chemotactic signals that promote tumor cell directional migration towards blood vessels, and then participate in the signaling cascade at the blood vessel that allows tumor cells to intravasate and disseminate throughout the body. Furthermore, while chemotherapy can repolarize TAMs to induce an anti-tumor response, these cytotoxic drugs can also lead to macrophage-mediated tumor relapse and metastasis. Patient response to chemotherapy may be dependent on patient-specific factors such as diet, obesity, and race, as these factors have been shown to alter macrophage phenotype and affect cancer-related outcomes. More research on how chemotherapy and patient-specific factors impact TAMs and cancer progression is needed to refine treatment strategies for cancer patients., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision., (Copyright © 2024 Friedman-DeLuca, Karagiannis, Condeelis, Oktay and Entenberg.)

Although the CXCL12/CXCR4 pathway has been prior investigated for its prometastatic and immuno- suppressive roles in the tumor microenvironment, evidence on the spatiotemporal regulation of these hallmarks has been lacking. Here, we demonstrate that CXCL12 forms a gradient specifically around cancer cell intravasation doorways, also known as Tumor Microenvironment of Metastasis (TMEM) doorways, thus facilitating the chemotactic translocation of prometastatic tumor cells expressing CXCR4 toward the perivascular TMEM doorways for subsequent entry into peripheral circulation. Fur- thermore, we demonstrate that the CXCL12-rich micro-environment around TMEM doorways may cre- ate immunosuppressive niches, whereby CD8 + T cells, despite being attracted to these regions, often exhibit reduced effector functions, limiting their efficacy. While the CXCL12/CXCR4 pathway can mini- mally influence the overall composition of immune cell populations, it biases the distribution of CD8 + T cells away from TMEM doorways, justifying its prior-established role as immunosuppressive factor for CD8 + T cells. Our research suggests that the complex interactions between CXCL12 and the various tumor and immune cell types contributes not only to the completion of the initial steps of the metastatic cascade, but also offers an immunological "sanctuary" to prometastatic tumor cells homed around TMEM doorways. Overall, our study enhances our current understanding on the mechanisms, via which CXCL12 orchestrates tumor cell behavior and immune dynamics, potentially guiding future thera- peutic strategies to combat breast cancer metastasis and improve anti-tumor immunity.

The physiology and pathophysiology of the pancreas are complex. Diseases of the pancreas, such as pancreatitis and pancreatic adenocarcinoma (PDAC) have high morbidity and mortality. Intravital imaging (IVI) is a powerful technique enabling the high-resolution imaging of tissues in both healthy and diseased states, allowing for real-time observation of cell dynamics. IVI of the murine pancreas presents significant challenges due to the deep visceral and compliant nature of the organ, which make it highly prone to damage and motion artifacts. Described here is the process of implantation of the Stabilized Window for Intravital imaging of the murine Pancreas (SWIP). The SWIP allows IVI of the murine pancreas in normal healthy states, during the transformation from the healthy pancreas to acute pancreatitis induced by cerulein, and in malignant states such as pancreatic tumors. In conjunction with genetically labeled cells or the administration of fluorescent dyes, the SWIP enables the measurement of single-cell and subcellular dynamics (including single-cell and collective migration) as well as serial imaging of the same region of interest over multiple days. The ability to capture tumor cell migration is of particular importance as the primary cause of cancer-related mortality in PDAC is the overwhelming metastatic burden. Understanding the physiological dynamics of metastasis in PDAC is a critical unmet need and crucial for improving patient prognosis. Overall, the SWIP provides improved imaging stability and expands the application of IVI in the healthy pancreas and malignant pancreas diseases.

Metastasis - the systemic spread of cancer - is the leading cause of cancer-related deaths. Although metastasis is commonly thought of as a unidirectional process wherein cells from the primary tumor disseminate and seed metastases, tumor cells in existing metastases can also redisseminate and give rise to new lesions in tertiary sites in a process known as "metastasis-from-metastases" or "metastasis-to-metastasis seeding." Metastasis-to-metastasis seeding may increase the metastatic burden and decrease the patient's quality of life and survival. Therefore, understanding the processes behind this phenomenon is crucial to refining treatment strategies for patients with metastatic cancer. Little is known about metastasis-to-metastasis seeding, due in part to logistical and technological limitations. Studies on metastasis-to-metastasis seeding rely primarily on sequencing methods, which may not be practical for researchers studying the exact timing of metastasis-to-metastasis seeding events or what promotes or prevents them. This highlights the lack of methodologies that facilitate the study of metastasis-to-metastasis seeding. To address this, we have developed - and describe herein - a murine surgical protocol for the selective photoconversion of lung metastases, allowing specific marking and fate tracking of tumor cells redisseminating from the lung to tertiary sites. To our knowledge, this is the only method for studying tumor cell redissemination and metastasis-to-metastasis seeding from the lungs that does not require genomic analysis.

Black, compared to white, women with residual estrogen receptor-positive (ER+) breast cancer after neoadjuvant chemotherapy (NAC) have worse distant recurrence-free survival (DRFS). Such racial disparity may be due to difference in density of portals for systemic cancer cell dissemination, called TMEM doorways, and pro-metastatic tumor microenvironment (TME). Here, we evaluate residual cancer specimens after NAC from 96 Black and 87 white women. TMEM doorways are visualized by triple immunohistochemistry, and cancer stem cells by immunofluorescence for SOX9. The correlation between TMEM doorway score and pro-metastatic TME parameters with DRFS is examined using log-rank and multivariate Cox regression. Black, compared to white, patients are more likely to develop distant recurrence (49% vs 34.5%, p = 0.07), receive mastectomy (69.8% vs 54%, p = 0.04), and have higher grade tumors (p = 0.002). Tumors from Black patients have higher TMEM doorway and macrophages density overall (p = 0.002; p = 0.002, respectively) and in the ER+/HER2- (p = 0.02; p = 0.02, respectively), but not in the triple negative disease. Furthermore, high TMEM doorway score is associated with worse DRFS. TMEM doorway score is an independent prognostic factor in the entire study population (HR, 2.02; 95%CI, 1.18-3.46; p = 0.01), with a strong trend in ER+/HER2- disease (HR, 2.38; 95%CI, 0.96-5.95; p = 0.06). SOX9 expression is not associated with racial disparity in TME or outcome. In conclusion, higher TMEM doorway density in residual breast cancer after NAC is associated with higher distant recurrence risk, and Black patients are associated with higher TMEM doorway density, suggesting that TMEM doorway density may contribute to racial disparities in breast cancer., (© 2023. The Author(s).)

Metastasis is a multistep process that leads to the formation of clinically detectable tumor foci at distant organs and frequently to patient demise. Only a subpopulation of breast cancer cells within the primary tumor can disseminate systemically and cause metastasis. To disseminate, cancer cells must express MenaINV, an isoform of the actin regulatory protein Mena, encoded by the ENAH gene, that endows tumor cells with transendothelial migration activity, allowing them to enter and exit the blood circulation. We have previously demonstrated that MenaINV mRNA and protein expression is induced in cancer cells by macrophage contact. In this study, we discovered the precise mechanism by which macrophages induce MenaINV expression in tumor cells. We examined the promoter of the human and mouse ENAH gene and discovered a conserved NF-κB transcription factor binding site. Using live imaging of an NF-κB activity reporter and staining of fixed tissues from mouse and human breast cancer, we further determined that for maximal induction of MenaINV in cancer cells, NF-κB needs to cooperate with the Notch1 signaling pathway. Mechanistically, Notch1 signaling does not directly increase MenaINV expression, but it enhances and sustains NF-κB signaling through retention of p65, an NF-κB transcription factor, in the nucleus of tumor cells, leading to increased MenaINV expression. In mice, these signals are augmented following chemotherapy treatment and abrogated upon macrophage depletion. Targeting Notch1 signaling in vivo decreased NF-κB signaling activation and MenaINV expression in the primary tumor and decreased metastasis. Altogether, these data uncover mechanistic targets for blocking MenaINV induction that should be explored clinically to decrease cancer cell dissemination and improve survival of patients with metastatic disease., (© 2023. The Author(s).)

Macrophages are important players involved in the progression of breast cancer, including in seeding the metastatic niche. However, the mechanism by which macrophages in the lung parenchyma interact with tumor cells in the vasculature to promote tumor cell extravasation at metastatic sites is not clear. To mimic macrophage-driven tumor cell extravasation, we used an in vitro assay (eTEM) in which an endothelial monolayer and a matrigel-coated filter separated tumor cells and macrophages from each other. The presence of macrophages promoted tumor cell extravasation, while macrophage conditioned media was insufficient to stimulate tumor cell extravasation in vitro. This finding is consistent with a requirement for direct contact between macrophages and tumor cells. We observed the presence of Thin Membranous Connections (TMCs) resembling similar structures formed between macrophages and tumor cells called tunneling nanotubes, which we previously demonstrated to be important in tumor cell invasion in vitro and in vivo. To determine if TMCs are important for tumor cell extravasation, we used macrophages with reduced levels of endogenous M-Sec (TNFAIP2), which causes a defect in tunneling nanotube formation. As predicted, these macrophages showed reduced macrophage-tumor cell TMCs. In both, human and murine breast cancer cell lines, there was also a concomitant reduction in tumor cell extravasation in vitro when co-cultured with M-Sec deficient macrophages compared to control macrophages. We also detected TMCs formed between macrophages and tumor cells through the endothelial layer in the eTEM assay. Furthermore, tumor cells were more frequently found in pores under the endothelium that contain macrophage protrusions. To determine the role of macrophage-tumor cell TMCs in vivo, we generated an M-Sec deficient mouse. Using an in vivo model of experimental metastasis, we detected a significant reduction in the number of metastatic lesions in M-Sec deficient mice compared to wild type mice. There was no difference in the size of the metastases, consistent with a defect specific to tumor cell extravasation and not metastatic outgrowth. Additionally, with an examination of time-lapse intravital-imaging (IVI) data sets of breast cancer cell extravasation in the lungs, we could detect the presence of TMCs between extravascular macrophages and vascular tumor cells. Overall, our data indicate that macrophage TMCs play an important role in promoting the extravasation of circulating tumor cells in the lungs.

Navigation through the bulk tumour, entry into the blood vasculature, survival in the circulation, exit at distant sites and resumption of proliferation are all steps necessary for tumour cells to successfully metastasize. The ability of tumour cells to complete these steps is highly dependent on the timing and sequence of the interactions that these cells have with the tumour microenvironment (TME), including stromal cells, the extracellular matrix and soluble factors. The TME thus plays a major role in determining the overall metastatic phenotype of tumours. The complexity and cause-and-effect dynamics of the TME cannot currently be recapitulated in vitro or inferred from studies of fixed tissue, and are best studied in vivo, in real time and at single-cell resolution. Intravital imaging (IVI) offers these capabilities, and recent years have been a time of immense growth and innovation in the field. Here we review some of the recent advances in IVI of mammalian models of cancer and describe how IVI is being used to understand cancer progression and metastasis, and to develop novel treatments and therapies. We describe new techniques that allow access to a range of tissue and cancer types, novel fluorescent reporters and biosensors that allow fate mapping and the probing of functional and phenotypic states, and the clinical applications that have arisen from applying these techniques, reporters and biosensors to study cancer. We finish by presenting some of the challenges that remain in the field, how to address them and future perspectives., (© 2022. Springer Nature Limited.)

Metastasis is the systemic manifestation of cancer and the main cause of death from breast cancer. In mouse models of lung metastases, recruitment of classical monocytes from blood to the lung and their differentiation to metastasis-associated macrophages (MAMs) facilitate cancer cell extravasation, survival and growth. Ablation of MAMs or their monocytic progenitors inhibits metastasis. We hypothesized that factors controlling macrophage polarization modulate tumor cell extravasation in the lung. We evaluated whether signaling by Th1 or Th2 cytokines in macrophages affected transendothelial migration of tumor cells in vitro. Interferon gamma and LPS inhibited macrophage-dependent tumor cell extravasation while the Th2 cytokine interleukin-4 (IL4) enhanced this process. We demonstrated that IL4 receptor ( IL4rα) -null mice developed fewer and smaller lung metastasis in E0771-LG mammary cancer models of this disease. Adoptive transfer of wild-type monocytes to IL4rα -deficient mice partially rescued this phenotype. IL4 signaling in macrophages controlled the expression of the chemokine receptor CXCR2, necessary for IL4-mediated tumor cell extravasation in vitro. Furthermore, IL4 signaling in macrophages regulated the transcript abundance of several other genes already causally associated with mammary cancer lung metastasis including Ccl2 , Csf1 , Ccr1 , Hgf and Flt1 . The central role of IL4 signaling in MAMs was confirmed by high-resolution intravital imaging of the lung in mice at the time of metastatic seeding, which showed reduced physical interaction between tumor cells and IL4rα -deficient macrophages. This interaction with wild-type MAMs enhanced tumor cell survival and seeding, which was lost in the IL4rα mice. These data indicate that IL4 signaling in monocytes and macrophages is key during seeding and growth of breast metastasis in the lung, as it regulates pro-tumoral paracrine signaling between cancer cells and macrophages.

Metastatic dissemination in breast cancer is regulated by specialized intravasation sites called "tumor microenvironment of metastasis" (TMEM) doorways, composed of a tumor cell expressing the actin-regulatory protein Mena, a perivascular macrophage, and an endothelial cell, all in stable physical contact. High TMEM doorway number is associated with an increased risk of distant metastasis in human breast cancer and mouse models of breast carcinoma. Here, we developed a novel magnetic resonance imaging (MRI) methodology, called TMEM Activity-MRI, to detect TMEM-associated vascular openings that serve as the portal of entry for cancer cell intravasation and metastatic dissemination. We demonstrate that TMEM Activity-MRI correlates with primary tumor TMEM doorway counts in both breast cancer patients and mouse models, including MMTV-PyMT and patient-derived xenograft models. In addition, TMEM Activity-MRI is reduced in mouse models upon treatment with rebastinib, a specific and potent TMEM doorway inhibitor. TMEM Activity-MRI is an assay that specifically measures TMEM-associated vascular opening (TAVO) events in the tumor microenvironment, and as such, can be utilized in mechanistic studies investigating molecular pathways of cancer cell dissemination and metastasis. Finally, we demonstrate that TMEM Activity-MRI increases upon treatment with paclitaxel in mouse models, consistent with prior observations that chemotherapy enhances TMEM doorway assembly and activity in human breast cancer. Our findings suggest that TMEM Activity-MRI is a promising precision medicine tool for localized breast cancer that could be used as a non-invasive test to determine metastatic risk and serve as an intermediate pharmacodynamic biomarker to monitor therapeutic response to agents that block TMEM doorway-mediated dissemination., (© 2022. The Author(s).)

Background: Black race is associated with worse outcome in patients with breast cancer. The distant relapse-free survival (DRFS) between Black and White women with localized breast cancer who participated in National Cancer Institute-sponsored clinical trial was evaluated., Methods: Pooled data were analyzed from 8 National Surgical Adjuvant Breast and Bowel Project (NSABP) trials including 9702 women with localized breast cancer treated with adjuvant chemotherapy (AC, n = 7485) or neoadjuvant chemotherapy (NAC, n = 2217), who self-reported as Black (n = 1070) or White (n = 8632) race. The association between race and DRFS was analyzed using log-rank tests and multivariate Cox regression., Results: After adjustment for covariates including age, tumor size, nodal status, body mass index and taxane use, and treatment (AC vs NAC), Black race was associated with an inferior DRFS in estrogen receptor-positive (ER+; hazard ratio [HR], 1.24; 95% CI, 1.05-1.46; P = .01), but not in ER- disease (HR, 0.97; 95% CI, 0.83-1.14; P = .73), and significant interaction between race and ER status was observed (P = .03). There was no racial disparity in DRFS among patients with pathologic complete response (pCR) (log-rank P = .8). For patients without pCR, Black race was associated with worse DRFS in ER+ (HR, 1.67; 95% CI, 1.14-2.45; P = .01), but not in ER- disease (HR, 0.91; 95% CI, 0.65-1.28; P = .59)., Conclusions: Black race was associated with significantly inferior DRFS in ER+ localized breast cancer treated with AC or NAC, but not in ER- disease. In the NAC group, racial disparity was also observed in patients with residual ER+ breast cancer at surgery, but not in those who had pCR., Lay Summary: Black women with breast cancer have worse outcomes compared with White women. We investigated if this held true in the context of clinical trials that provide controlled treatment setting. Black women with cancer expressing estrogen receptors (ERs) had worse outcome than White women. If breast cancers did not express ERs, there was no racial disparity in outcome. We also observed racial disparity in women who received chemotherapy before their cancer was removed, but only if they had cancer expressing ERs and residual disease on completion of treatment. If the cancer disappeared with presurgical chemotherapy, there was no racial disparity., (© 2022 American Cancer Society.)

Pancreatitis and pancreatic ductal adenocarcinoma (PDAC) are grave illnesses with high levels of morbidity and mortality. Intravital imaging (IVI) is a powerful technique for visualizing physiological processes in both health and disease. However, the application of IVI to the murine pancreas presents significant challenges, as it is a deep, compliant, visceral organ that is difficult to access, easily damaged and susceptible to motion artefacts. Existing imaging windows for stabilizing the pancreas during IVI have unfortunately shown poor stability for time-lapsed imaging on the minutes to hours scale, or are unable to accommodate both the healthy and tumour-bearing pancreata. To address these issues, we developed an improved stabilized window for intravital imaging of the pancreas (SWIP), which can be applied to not only the healthy pancreas but also to solid tumours like PDAC. Here, we validate the SWIP and use it to visualize a variety of processes for the first time, including (1) single-cell dynamics within the healthy pancreas, (2) transformation from healthy pancreas to acute pancreatitis induced by cerulein, and (3) the physiology of PDAC in both autochthonous and orthotopically injected models. SWIP can not only improve the imaging stability but also expand the application of IVI in both benign and malignant pancreas diseases.

Purpose: to develop several digital pathology-based machine vision algorithms for combining TMEM and Mena Calc scores and determine if a combination of these biomarkers improves the ability to predict development of distant metastasis over and above that of either biomarker alone., Methods: This retrospective study included a subset of 130 patients (65 patients with no recurrence and 65 patients with a recurrence at 5 years) from the Calgary Tamoxifen cohort of breast cancer patients. Patients had confirmed invasive breast cancer and received adjuvant tamoxifen therapy. Of the 130 patients, 86 cases were suitable for analysis in this study. Sequential sections of formalin-fixed paraffin-embedded patient samples were stained for TMEM doorways (immunohistochemistry triple staining) and Mena Calc (immunofluorescence staining). Stained sections were imaged, aligned, and then scored for TMEM doorways and Mena Calc . Different ways of combining TMEM doorway and Mena Calc scores were evaluated and compared to identify the best performing combined marker by using the restricted mean survival time (RMST) difference method., Results: the best performing combined marker gave an RMST difference of 5.27 years (95% CI: 1.71-8.37), compared to 3.56 years (95% CI: 0.95-6.1) for the associated standalone TMEM doorway analysis and 2.94 years (95% CI: 0.25-5.87) for the associated standalone Mena Calc analysis., Conclusions: combining TMEM doorway and Mena Calc scores as a new biomarker improves prognostication over that observed with TMEM doorway or Mena Calc Score alone in this cohort of 86 patients.

Pancreatic ductal adenocarcinoma (PDAC) is a highly metastatic disease. Tumors are poorly immunogenic and immunosuppressive, preventing T cell activation in the tumor microenvironment. Here, we present a microbial-based immunotherapeutic treatment for selective delivery of an immunogenic tetanus toxoid protein (TT 856-1313 ) into PDAC tumor cells by attenuated Listeria monocytogenes . This treatment reactivated preexisting TT-specific memory T cells to kill infected tumor cells in mice. Treatment of KrasG12D,p53R172H, Pdx1-Cre (KPC) mice with Listeria -TT resulted in TT accumulation inside tumor cells, attraction of TT-specific memory CD4 T cells to the tumor microenvironment, and production of perforin and granzyme B in tumors. Low doses of gemcitabine (GEM) increased immune effects of Listeria -TT, turning immunologically cold into hot tumors in mice. In vivo depletion of T cells from Listeria -TT + GEM-treated mice demonstrated a CD4 T cell-mediated reduction in tumor burden. CD4 T cells from TT-vaccinated mice were able to kill TT-expressing Panc-02 tumor cells in vitro. In addition, peritumoral lymph node-like structures were observed in close contact with pancreatic tumors in KPC mice treated with Listeria -TT or Listeria -TT + GEM. These structures displayed CD4 and CD8 T cells producing perforin and granzyme B. Whereas CD4 T cells efficiently infiltrated the KPC tumors, CD8 T cells did not. Listeria -TT + GEM treatment of KPC mice with advanced PDAC reduced tumor burden by 80% and metastases by 87% after treatment and increased survival by 40% compared to nontreated mice. These results suggest that Listeria -delivered recall antigens could be an alternative to neoantigen-mediated cancer immunotherapy.

Metastases are initiated by disseminated tumor cells (DTCs) that colonize distant organs. Growing evidence suggests that the microenvironment of the primary tumor primes DTCs for dormant or proliferative fates. However, the manner in which this occurs remains poorly understood. Here, using the Window for High-Resolution Intravital Imaging of the Lung (WHRIL), we study the live lung longitudinally and follow the fate of individual DTCs that spontaneously disseminate from orthotopic breast tumors. We find that spontaneously DTCs have increased levels of retention, increased speed of extravasation, and greater survival after extravasation, compared to experimentally metastasized tumor cells. Detailed analysis reveals that a subset of macrophages within the primary tumor induces a pro-dissemination and pro-dormancy DTC phenotype. Our work provides insight into how specific primary tumor microenvironments prime a subpopulation of cells for expression of proteins associated with dissemination and dormancy., (© 2022. The Author(s).)

Tissues are heterogeneous with respect to cellular and non-cellular components and in the dynamic interactions between these elements. To study the behaviour and fate of individual cells in these complex tissues, intravital microscopy (IVM) techniques such as multiphoton microscopy have been developed to visualize intact and live tissues at cellular and subcellular resolution. IVM experiments have revealed unique insights into the dynamic interplay between different cell types and their local environment, and how this drives morphogenesis and homeostasis of tissues, inflammation and immune responses, and the development of various diseases. This Primer introduces researchers to IVM technologies, with a focus on multiphoton microscopy of rodents, and discusses challenges, solutions and practical tips on how to perform IVM. To illustrate the unique potential of IVM, several examples of results are highlighted. Finally, we discuss data reproducibility and how to handle big imaging data sets., Competing Interests: Competing interests P.T. receives reagents from Kadmon, InxMed (also consultant), Redx Pharma, Équilibre Biopharmaceuticals and Amplia Therapeutics. Under a licensing agreement between Amplia Therapeutics and Garvan Institute of Medical Research, D.H. and P.T. (consultant) are entitled to milestone payments. All other authors declare no competing interests.

Cancer stem cells (CSCs) play an important role during metastasis, but the dynamic behavior and induction mechanisms of CSCs are not well understood. Here, we employ high-resolution intravital microscopy using a CSC biosensor to directly observe CSCs in live mice with mammary tumors. CSCs display the slow-migratory, invadopod-rich phenotype that is the hallmark of disseminating tumor cells. CSCs are enriched near macrophages, particularly near macrophage-containing intravasation sites called Tumor Microenvironment of Metastasis (TMEM) doorways. Substantial enrichment of CSCs occurs on association with TMEM doorways, contributing to the finding that CSCs represent >60% of circulating tumor cells. Mechanistically, stemness is induced in non-stem cancer cells upon their direct contact with macrophages via Notch-Jagged signaling. In breast cancers from patients, the density of TMEM doorways correlates with the proportion of cancer cells expressing stem cell markers, indicating that in human breast cancer TMEM doorways are not only cancer cell intravasation portals but also CSC programming sites., (© 2021. The Author(s).)

The Tie2 receptor tyrosine kinase is expressed in vascular endothelial cells, tumor-associated macrophages, and tumor cells and has been a major focus of research in therapies targeting the tumor microenvironment. The most extensively studied Tie2 ligands are Angiopoietin 1 and 2 (Ang1, Ang2). Ang1 plays a critical role in vessel maturation, endothelial cell migration, and survival. Ang2, depending on the context, may function to disrupt connections between the endothelial cells and perivascular cells, promoting vascular regression. However, in the presence of VEGF-A, Ang2 instead promotes angiogenesis. Tie2-expressing macrophages play a critical role in both tumor angiogenesis and the dissemination of tumor cells from the primary tumor to secondary sites. Therefore, Ang-Tie2 signaling functions as an angiogenic switch during tumor progression and metastasis. Here we review the recent advances and complexities of targeting Tie2 signaling in the tumor microenvironment as a possible anti-angiogenic, and anti-metastatic, therapy and describe its use in combination with chemotherapy.

Metastasis, accounting for ~90% of cancer-related mortality, involves the systemic spread of cancer cells from primary tumors to secondary sites such as the bone, brain, and lung. Although extensively studied, the mechanistic details of this process remain poorly understood. While common imaging modalities, including computed tomography (CT), positron emission tomography (PET), and magnetic resonance imaging (MRI), offer varying degrees of gross visualization, each lacks the temporal and spatial resolution necessary to detect the dynamics of individual tumor cells. To address this, numerous techniques have been described for intravital imaging of common metastatic sites. Of these sites, the lung has proven especially challenging to access for intravital imaging owing to its delicacy and critical role in sustaining life. Although several approaches have previously been described for single-cell intravital imaging of the intact lung, all involve highly invasive and terminal procedures, limiting the maximum possible imaging duration to 6-12 h. Described here is an improved technique for the permanent implantation of a minimally invasive thoracic optical Window for High-Resolution Imaging of the Lung (WHRIL). Combined with an adapted approach to microcartography, the innovative optical window facilitates serial intravital imaging of the intact lung at single-cell resolution across multiple imaging sessions and spanning multiple weeks. Given the unprecedented duration of time over which imaging data can be gathered, the WHRIL can facilitate the accelerated discovery of the dynamic mechanisms underlying metastatic progression and numerous additional biologic processes within the lung.

Aligned collagen fibers provide topography for the rapid migration of single tumor cells (streaming migration) to invade the surrounding stroma, move within tumor nests towards blood vessels to intravasate and form distant metastases. Mechanisms of tumor cell motility have been studied extensively in the 2D context, but the mechanistic understanding of rapid single tumor cell motility in the in vivo context is still lacking. Here, we show that streaming tumor cells in vivo use collagen fibers with diameters below 3 µm. Employing 1D migration assays with matching in vivo fiber dimensions, we found a dependence of tumor cell motility on 1D substrate width, with cells moving the fastest and the most persistently on the narrowest 1D fibers (700 nm-2.5 µm). Interestingly, we also observed nuclear deformation in the absence of restricting extracellular matrix pores during high speed carcinoma cell migration in 1D, similar to the nuclear deformation observed in tumor cells in vivo. Further, we found that actomyosin machinery is aligned along the 1D axis and actomyosin contractility synchronously regulates cell motility and nuclear deformation. To further investigate the link between cell speed and nuclear deformation, we focused on the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex proteins and SRF-MKL1 signaling, key regulators of mechanotransduction, actomyosin contractility and actin-based cell motility. Analysis of The Cancer Genome Atlas dataset showed a dramatic decrease in the LINC complex proteins SUN1 and SUN2 in primary tumor compared to the normal tissue. Disruption of LINC complex by SUN1 + 2 KD led to multi-lobular elongated nuclei, increased tumor cell motility and concomitant increase in F-actin, without affecting Lamin proteins. Mechanistically, we found that MKL1, an effector of changes in cellular G-actin to F-actin ratio, is required for increased 1D motility seen in SUN1 + 2 KD cells. Thus, we demonstrate a previously unrecognized crosstalk between SUN proteins and MKL1 transcription factor in modulating nuclear shape and carcinoma cell motility in an in vivo relevant 1D microenvironment.

Although metastatic disease is the primary cause of mortality in cancer patients, the mechanisms leading to overwhelming metastatic burden are still incompletely understood. Metastases are the endpoint of a series of multi-step events involving cancer cell intravasation, dissemination to distant organs, and outgrowth to metastatic colonies. Here we show, for the first-time, that breast cancer cells do not solely disseminate to distant organs from primary tumors and metastatic nodules in the lymph nodes, but also do so from lung metastases. Thus, our findings indicate that metastatic dissemination could continue even after the removal of the primary tumor. Provided that the re-disseminated cancer cells initiate growth upon arrival to distant sites, cancer cell re-dissemination from metastatic foci could be one of the crucial mechanisms leading to overt metastases and patient demise. Therefore, the development of new therapeutic strategies to block cancer cell re-dissemination would be crucial to improving survival of patients with metastatic disease.

Although cancer immunotherapy has resulted in unpreceded survival benefits to subsets of oncology patients, accumulating evidence from preclinical animal models suggests that the immunosuppressive tumor microenvironment remains a detrimental factor limiting benefit for many patient subgroups. Recent efforts on lymphocyte-mediated immunotherapies are primarily focused on eliminating cancer foci at primary and metastatic sites, but few studies have investigated the impact of these therapies on the highly complex process of cancer cell dissemination. The metastatic cascade involves the directional streaming of invasive/migratory tumor cells toward specialized blood vessel intravasation gateways, called TMEM doorways, to the peripheral circulation. Importantly, this process occurs under the auspices of a specialized tumor microenvironment, herewith referred to as "Dissemination Trajectory", which is supported by an ample array of tumor-associated macrophages (TAMs), skewed towards an M2-like polarization spectrum, and which is also vital for providing microenvironmental cues for cancer cell invasion, migration and stemness. Based on pre-existing evidence from preclinical animal models, this article outlines the hypothesis that dissemination trajectories do not only support the metastatic cascade, but also embody immunosuppressive niches, capable of providing transient and localized immunosubversion cues to the migratory/invasive cancer cell subpopulation while in the act of departing from a primary tumor. So long as these dissemination trajectories function as "immune deserts", the migratory tumor cell subpopulation remains efficient in evading immunological destruction and seeding metastatic sites, despite administration of cancer immunotherapy and/or other cytotoxic treatments. A deeper understanding of the molecular and cellular composition, as well as the signaling circuitries governing the function of these dissemination trajectories will further our overall understanding on TAM-mediated immunosuppression and will be paramount for the development of new therapeutic strategies for the advancement of optimal cancer chemotherapies, immunotherapies, and targeted therapies., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Asiry, Kim, Filippou, Sanchez, Entenberg, Marks, Oktay and Karagiannis.)

In primary breast tumors, cancer cells hematogenously disseminate through doorways in the vasculature composed of three-cell complexes (known as Tumor MicroEnvironment of Metastasis) comprising a perivascular macrophage, a tumor cell overexpressing the actin-regulatory protein Mammalian Enabled (Mena), and an endothelial cell, all in direct physical contact. It has been previously shown that once tumor cells establish lymph node metastases in patients, TMEM doorways form in the metastatic tumor cell nests. However, it has not been established if such lymph node-TMEM doorways actively transit tumor cells into the peripheral circulation and on to tertiary sites. To address this question in this short report, we used a mouse model of lymph node metastasis to demonstrate that TMEM doorways: (1) exist in tumor-positive lymph nodes of mice, (2) are restricted to the blood vascular endothelium, (3) serve as a mechanism for further dissemination to peripheral sites such as to the lungs, and (4) their activity can be abrogated by a pharmaceutical intervention. Our data suggest that cancer cell dissemination via TMEM doorways is a common mechanism of breast cancer cell dissemination to distant sites and thus the pharmacological targeting of TMEM may be necessary, even after resection of the primary tumor, to suppress cancer cell dissemination., (Copyright © 2020 Coste, Karagiannis, Wang, Xue, Lin, Skobe, Jones, Oktay, Condeelis and Entenberg.)

Genetically engineered and orthotopic xenograft mouse models have been instrumental for increasing our understanding of thyroid cancer progression and for the development of novel therapeutic approaches in a setting that is more physiologically relevant than the classical subcutaneous flank implants. However, the anatomical location of the thyroid gland precludes a non-invasive analysis at the cellular level of the interactions between tumor cells and the surrounding microenvironment and does not allow a real-time evaluation of the response of tumor cells to drug treatments. As a consequence, such studies have generally only relied on endpoint approaches, limiting the amount and depth of the information that could be gathered. Here we describe the development of an innovative approach to imaging specific aspects of thyroid cancer biology, based on the implantation of a permanent, minimally invasive optical window that allows high-resolution, multi-day, intravital imaging of the behavior and cellular dynamics of thyroid tumors in the mouse. We show that this technology allows visualization of fluorescently tagged tumor cells both in immunocompetent, genetically engineered mouse models of anaplastic thyroid cancer (ATC) and in immunocompromised mice carrying orthotopic implanted human or mouse ATC cells. Furthermore, the use of recipient mice in which endothelial cells and macrophages are fluorescently labeled allows the detection of the spatial and functional relationship between tumor cells and their microenvironment. Finally, we show that ATC cells expressing a fluorescent biosensor for caspase 3 activity can be effectively utilized to evaluate, in real-time, the efficacy and kinetics of action of novel small molecule therapeutics. This novel approach to intravital imaging of thyroid cancer represents a platform that will allow, for the first time, the longitudinal, in situ analysis of tumor cell responses to therapy and of their interaction with the microenvironment.

Metastasis, a process that requires tumor cell dissemination followed by tumor growth, is the primary cause of death in cancer patients. An essential step of tumor cell dissemination is intravasation, a process by which tumor cells cross the blood vessel endothelium and disseminate to distant sites. Studying this process is of utmost importance given that intravasation in the primary tumor, as well as the secondary and tertiary metastases, is the key step in the systemic spread of tumor cells, and that this process continues even after removal of the primary tumor. High-resolution intravital imaging of the tumor microenvironment of breast carcinoma has revealed that tumor cell intravasation exclusively occurs at doorways, termed "Tumor MicroEnvironment of Metastasis" (TMEM), composed of three different cell types: a Tie2 high /VEGF high perivascular macrophage, a Mena overexpressing tumor cell, and an endothelial cell, all in direct contact. In this review article, we discuss the interactions between these cell types, the subsequent signaling events which lead to tumor cell intravasation, and the role of invadopodia in supporting tumor cell invasion and dissemination. We end our review by discussing how the knowledge acquired from the use of intravital imaging is now leading to new clinical trials targeting tumor cell dissemination and preventing metastatic progression., (Copyright © 2020 Elsevier GmbH. All rights reserved.)

Intravital imaging, the direct visualization of cells and tissues within a living animal, is a technique that has been employed for the better part of a century. The advent of confocal and multiphoton microscopy has dramatically improved the power of intravital imaging, making it possible to obtain optical sections of tissues non-destructively. This review discusses the various techniques used for intravital imaging, describes how intravital imaging provides information about cellular and tissue dynamics not possible to be garnered by other techniques, and details several ways in which intravital imaging is making a direct impact on the clinical care of patients. © 2019 International Society for Advancement of Cytometry., (© 2019 International Society for Advancement of Cytometry.)

Metastasis causes ~90% of breast cancer mortality. However, standard prognostic tests based mostly on proliferation genes do not measure metastatic potential. Tumor MicroEnvironment of Metastasis (TMEM), an immunohistochemical biomarker for doorways on blood vessels that support tumor cell dissemination is prognostic for metastatic outcome in breast cancer patients. Studies quantifying TMEM doorways have involved manual scoring by pathologists utilizing static digital microscopy: a labor-intensive process unsuitable for use in clinical practice. We report here a validation study evaluating a new quantitative digital pathology (QDP) tool (TMEM-DP) for identification and quantification of TMEM doorways that closely mimics pathologists' workflow and reduces pathologists' variability to levels suitable for use in a clinical setting. Blinded to outcome, QDP was applied to a nested case-control study consisting of 259 matched case-control pairs. Sixty subjects of these were manually scored by five pathologists, digitally recorded using whole slide imaging (WSI), and then used for algorithm development and optimization. Validation was performed on the remainder of the cohort. TMEM-DP shows excellent reproducibility and concordance and reduces pathologist time from ~60 min to ~5 min per case. Concordance between manual scoring and TMEM-DP was found to be >0.79. These results show that TMEM-DP is capable of accurately identifying and scoring TMEM doorways (also known as MetaSite score) equivalent to pathologists., Competing Interests: J.S.C., J.G.J., M.H.O., and T.E.R. had equity interest (including patents) in MetaStat, Inc. J.A.S. had equity interest in MetaStat, Inc. J.S.C., J.G.J., T.E.R., D.E., and J.A.S. served as consultant/advisory board members for MetaStat, Inc. The rest of the authors declare no conflict of interest.

Cancer cells metastasize from primary tumors to regional lymph nodes and distant sites via the lymphatic and blood vascular systems, respectively. Our prior work has demonstrated that in primary breast tumors, cancer cells utilize a three-cell complex (known as tumor microenvironment of metastasis, or TMEM) composed of a perivascular macrophage, a tumor cell expressing high levels of the actin-regulatory protein mammalian enabled (Mena), and an endothelial cell as functional "doorways" for hematogenous dissemination. Here, we studied a well-annotated case-control cohort of human invasive ductal carcinoma of the breast and metastatic lymph nodes from a separate breast cancer cohort. We demonstrate that in primary breast tumors, blood vessels are always present within tumor cell nests (TCNs) and tumor-associated stroma (TAS), while lymphatic vessels are only occasionally present in TCN and TAS. Furthermore, TMEM doorways not only exist in primary tumors as previously reported but also in lymph node metastases. In addition, we show that TMEM intravasation doorways are restricted to the blood vascular endothelium in both primary tumors and lymph node metastases, suggesting that breast cancer dissemination to distant sites from both primary tumors and metastatic foci in lymph nodes occurs hematogenously at TMEM doorways. TMEMs are very rarely detected at lymphatic vessels and do not confer clinical prognostic significance, indicating they are not participants in TMEM-associated hematogenous dissemination. These findings are consistent with recent observations that hematogenous dissemination from lymph nodes occurs via blood vessels., Competing Interests: The authors declare no conflict of interest.

Macrophages represent a heterogeneous group of cells, capable of carrying out distinct functions in a variety of organs and tissues. Even within individual tissues, their functions can vary with location. Tumor-associated macrophages (TAMs) specialize into three major subtypes that carry out multiple tasks simultaneously. This is especially true in the context of metastasis, where TAMs establish most of the cellular and molecular prerequisites for successful cancer cell dissemination and seeding to the secondary site. Perivascular TAMs operate in the perivascular niche, where they promote tumor angiogenesis and aid in the assembly of intravasation sites called tumor microenvironment of metastasis (TMEM). Streaming TAMs co-migrate with tumor cells (irrespective of the perivascular niche) and promote matrix remodeling, tumor cell invasiveness, and an immunosuppressive local microenvironment. Premetastatic TAMs are recruited to the premetastatic niche, where they can assist in tumor cell extravasation, seeding, and metastatic colonization. The dynamic interplay between TAMs and tumor cells can also modify the ability of the latter to resist cytotoxic chemotherapy (a phenotype known as environment-mediated drug resistance) and induce chemotherapy-mediated pro-metastatic microenvironmental changes. These observations suggest that future therapeutics should be designed to target TAMs with the aim of suppressing the metastatic potential of tumors and rendering chemotherapy more efficient., (©2019 Society for Leukocyte Biology.)

The most common cause of cancer related mortality is metastasis, a process that requires dissemination of cancer cells from the primary tumor to secondary sites. Recently, we established that cancer cell dissemination in primary breast cancer and at metastatic sites in the lung occurs only at doorways called Tumor MicroEnvironment of Metastasis (TMEM). TMEM doorway number is prognostic for distant recurrence of metastatic disease in breast cancer patients. TMEM doorways are composed of a cancer cell which over-expresses the actin regulatory protein Mena in direct contact with a perivascular, proangiogenic macrophage which expresses high levels of TIE2 and VEGF, where both of these cells are tightly bound to a blood vessel endothelial cell. Cancer cells can intravasate through TMEM doorways due to transient vascular permeability orchestrated by the joint activity of the TMEM-associated macrophage and the TMEM-associated Mena-expressing cancer cell. In this manuscript, we describe two methods for assessment of TMEM-mediated transient vascular permeability: intravital imaging and fixed tissue immunofluorescence. Although both methods have their advantages and disadvantages, combining the two may provide the most complete analyses of TMEM-mediated vascular permeability as well as microenvironmental prerequisites for TMEM function. Since the metastatic process in breast cancer, and possibly other types of cancer, involves cancer cell dissemination via TMEM doorways, it is essential to employ well established methods for the analysis of the TMEM doorway activity. The two methods described here provide a comprehensive approach to the analysis of TMEM doorway activity, either in naïve or pharmacologically treated animals, which is of paramount importance for pre-clinical trials of agents that prevent cancer cell dissemination via TMEM.

Circulating tumor cells (CTC) seed cancer metastases; however, the underlying cellular and molecular mechanisms remain unclear. CTC clusters were less frequently detected but more metastatic than single CTCs of patients with triple-negative breast cancer and representative patient-derived xenograft models. Using intravital multiphoton microscopic imaging, we found that clustered tumor cells in migration and circulation resulted from aggregation of individual tumor cells rather than collective migration and cohesive shedding. Aggregated tumor cells exhibited enriched expression of the breast cancer stem cell marker CD44 and promoted tumorigenesis and polyclonal metastasis. Depletion of CD44 effectively prevented tumor cell aggregation and decreased PAK2 levels. The intercellular CD44-CD44 homophilic interactions directed multicellular aggregation, requiring its N-terminal domain, and initiated CD44-PAK2 interactions for further activation of FAK signaling. Our studies highlight that CD44 + CTC clusters, whose presence is correlated with a poor prognosis of patients with breast cancer, can serve as novel therapeutic targets of polyclonal metastasis. SIGNIFICANCE: CTCs not only serve as important biomarkers for liquid biopsies, but also mediate devastating metastases. CD44 homophilic interactions and subsequent CD44-PAK2 interactions mediate tumor cluster aggregation. This will lead to innovative biomarker applications to predict prognosis, facilitate development of new targeting strategies to block polyclonal metastasis, and improve clinical outcomes. See related commentary by Rodrigues and Vanharanta, p. 22 . This article is highlighted in the In This Issue feature, p. 1 ., (©2018 American Association for Cancer Research.)

Hypoxia is linked to metastasis; however, how it affects metastatic progression is not clear due to limited consensus in the literature. We posit that this lack of consensus is due to hypoxia being studied using different approaches, such as in vitro, primary tumor, or metastasis assays in an isolated manner. Here, we review the pros and cons of in vitro hypoxia assays, highlight in vivo studies that inform on physiological hypoxia, and review the evidence that primary tumor hypoxia might influence the fate of disseminated tumor cells (DTCs) in secondary organs. Our analysis suggests that consensus can be reached by using in vivo methods of study, which also allow better modeling of how hypoxia affects DTC fate and metastasis., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Black race compared to white race is associated with more advanced stage and biologically aggressive breast cancer. Consequently, black patients are more frequently treated with neoadjuvant chemotherapy (NAC) than white patients. However, it is unclear how distant recurrence-free survival (DRFS) of black patients treated with NAC, compares to DRFS of black patients treated with adjuvant chemotherapy (AC). We evaluated the association between race, distant recurrence, and type of chemotherapy (AC or NAC) in localized or locally advanced breast cancer. We evaluated DRFS in 807 patients, including 473 black, 252 white, 56 Hispanic, and 26 women of other or mixed race. The association between AC or NAC and DRFS was examined using multivariate Cox proportional hazard models that included race, age, stage, estrogen receptor (ER) and triple negative (TN) status. When the black and white subjects were pooled for the analysis the features associated with worse DRFS included stage III disease and age < 50 years, but not ER-negative disease, TN disease, the use of NAC, or black race. However, in the analysis stratified by race NAC was associated with worse DRFS compared to AC in black (HR 2.70; 95% CI 1.73-4.22; p < 0.0001), but not in white women (HR 1.29, 95% CI 0.56-2.95; p = 0.36). Black patients treated with NAC had worse DRFS than black patients treated with AC, or white patients treated with either NAC or AC. These findings need to be validated in a large-scale observational study and the effect of NAC on the breast cancer microenvironment in black women needs to be further evaluated.

Tumor-associated macrophages (TAMs) are critical for tumor metastasis. Two TAM subsets support cancer cell intravasation: migratory macrophages guide cancer cells toward blood vessels, where sessile perivascular macrophages assist their entry into the blood. However, little is known about the inter-relationship between these functionally distinct TAMs or their possible inter-conversion. We show that motile, streaming TAMs are newly arrived monocytes, recruited via CCR2 signaling, that then differentiate into the sessile perivascular macrophages. This unidirectional process is regulated by CXCL12 and CXCR4. Cancer cells induce TGF-β-dependent upregulation of CXCR4 in monocytes, while CXCL12 expressed by perivascular fibroblasts attracts these motile TAMs toward the blood vessels, bringing motile cancer cells with them. Once on the blood vessel, the migratory TAMs differentiate into perivascular macrophages, promoting vascular leakiness and intravasation., (Copyright © 2018 The Francis Crick Institute. Published by Elsevier Inc. All rights reserved.)

This corrects the article DOI: 10.1038/nature20609.