B lymphocyte-derived acetylcholine limits steady-state and emergency hematopoiesis

Autonomic nerves control organ function through the sympathetic and parasympathetic branches, which have opposite effects. In the bone marrow, sympathetic (adrenergic) nerves promote hematopoiesis; however, how parasympathetic (cholinergic) signals modulate hematopoiesis is unclear. Here, we show that B lymphocytes are an important source of acetylcholine, a neurotransmitter of the parasympathetic nervous system, which reduced hematopoiesis. Single-cell RNA sequencing identified nine clusters of cells that expressed the cholinergic α7 nicotinic receptor (Chrna7) in the bone marrow stem cell niche, including endothelial and mesenchymal stromal cells (MSCs). Deletion of B cell-derived acetylcholine resulted in the differential expression of various genes, including Cxcl12 in leptin receptor+ (LepR+) stromal cells. Pharmacologic inhibition of acetylcholine signaling increased the systemic supply of inflammatory myeloid cells in mice and humans with cardiovascular disease.

tiRNA signaling via stress-regulated vesicle transfer in the hematopoietic niche

Extracellular vesicles (EVs) transfer complex biologic material between cells. However, the role of this process in vivo is poorly defined. Here, we demonstrate that osteoblastic cells in the bone marrow (BM) niche elaborate extracellular vesicles that are taken up by hematopoietic progenitor cells in vivo. Genotoxic or infectious stress rapidly increased stromal-derived extracellular vesicle transfer to granulocyte-monocyte progenitors. The extracellular vesicles contained processed tRNAs (tiRNAs) known to modulate protein translation. 5'-ti-Pro-CGG-1 was preferentially abundant in osteoblast-derived extracellular vesicles and, when transferred to granulocyte-monocyte progenitors, increased protein translation, cell proliferation, and myeloid differentiation. Upregulating EV transfer improved hematopoietic recovery from genotoxic injury and survival from fungal sepsis. Therefore, EV-mediated tiRNA transfer provides a stress-modulated signaling axis in the BM niche distinct from conventional cytokine-driven stress responses.

Human prostate cancer bone metastases have an actionable immunosuppressive microenvironment

Bone metastases are devastating complications of cancer. They are particularly common in prostate cancer (PCa), represent incurable disease, and are refractory to immunotherapy. We seek to define distinct features of the bone marrow (BM) microenvironment by analyzing single cells from bone metastatic prostate tumors, involved BM, uninvolved BM, and BM from cancer-free, orthopedic patients, and healthy individuals. Metastatic PCa is associated with multifaceted immune distortion, specifically exhaustion of distinct T cell subsets, appearance of macrophages with states specific to PCa bone metastases. The chemokine CCL20 is notably overexpressed by myeloid cells, as is its cognate CCR6 receptor on T cells. Disruption of the CCL20-CCR6 axis in mice with syngeneic PCa bone metastases restores T cell reactivity and significantly prolongs animal survival. Comparative high-resolution analysis of PCa bone metastases shows a targeted approach for relieving local immunosuppression for therapeutic effect.

Abstract IA23: Impact of metastatic prostate cancer on human bone marrow

Cancer-related mortality due to solid tumor malignancies is overwhelmingly due to the development and progression of metastases. In advanced prostate cancer, metastases most often involve the bone and generally represent incurable disease. It remains unclear what aspects of the bone marrow microenvironment make it hospitable to metastatic dissemination. Similarly, the impact of the metastatic tumor on hematopoiesis and the marrow immune response is poorly understood. We took advantage of rare spinal cord decompression surgeries to profile marrow and metastatic tumors from men with advanced prostate cancer at single-cell resolution. Our analysis contrasts the cellular composition and transcriptional states in matched samples of tumor and liquid bone marrow collected at adjacent vertebral body levels, as well as bone marrow of orthopedic patients without malignancy. Metastatic prostate cancer was associated with hematopoietic suppression and multifaceted immune distortion. There was exhaustion of specific T cell subsets, appearance of inflammatory monocytes and macrophages, and alteration of cytokine profiles. Computational analysis showed association between the presence of specific myeloid subsets and the level of T lymphocyte dysfunction in the tumor fraction. We screened for potential signaling axes that may underlie this interaction. Among them was chemokine CCL20, notably overexpressed by myeloid cells, as was its cognate CCR6 receptor, expressed on T cells. We developed a syngeneic mouse model of bone-metastatic prostate cancer to explore this observation, and demonstrated that disruption of the CCL20-CCR6 axis from either side resulted in significant prolongation of survival. Our results further indicated that this dual overexpression was associated with repressed immune responses. Overall, comparative high-resolution analysis of bone marrow reveals distinct alterations associated with prostate cancer bone metastases that may be amenable to therapeutic targeting with the goal of altering cancer progression.

Citation Format: Ninib Baryawno, Youmna Kfoury, Nicolas Severe, Shenglin Mei, Karin Gustafsson, Taghreed Hirz, Thomas Brouse, Elizabeth W. Scadden, Anna A. Igolkina, Bryan D. Choi, Nikolas Barkas, John H. Shin, Philip J. Saylor, David T. Scadden, David B. Sykes, Peter V. Kharchenko, as part of the Boston Bone Metastasis Consortium. Impact of metastatic prostate cancer on human bone marrow [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr IA23.

Glucocorticoids Regulate Bone Marrow B Lymphopoiesis After Stroke

RATIONALE

After a stroke, patients frequently experience altered systemic immunity resulting in peripheral immunosuppression and higher susceptibility to infections, which is at least partly attributed to lymphopenia. The mechanisms that profoundly change the systemic leukocyte repertoire after stroke are incompletely understood. Emerging evidence indicates that stroke alters hematopoietic output of the bone marrow.

OBJECTIVE

To explore the mechanisms that lead to defects of B lymphopoiesis after ischemic stroke.

METHODS AND RESULTS

We here report that ischemic stroke triggers brain-bone marrow communication via hormonal long-range signals that regulate hematopoietic B lineage decisions. Bone marrow fluorescence-activated cell sorter analyses and serial intravital microscopy indicate that transient middle cerebral artery occlusion in mice arrests B-cell development beginning at the pro-B-cell stage. This phenotype was not rescued in Myd88^-/- and TLR4^-/- mice with disrupted TLR (Toll-like receptor) signaling or after blockage of peripheral sympathetic nerves. Mechanistically, we identified stroke-induced glucocorticoid release as the main instigator of B lymphopoiesis defects. B-cell lineage-specific deletion of the GR (glucocorticoid receptor) in CD19-Cre loxP Nr3c1 mice attenuated lymphocytopenia after transient middle cerebral artery. In 20 patients with acute stroke, increased cortisol levels inversely correlated with blood lymphocyte numbers.

CONCLUSIONS

Our data demonstrate that the hypothalamic-pituitary-adrenal axis mediates B lymphopoiesis defects after ischemic stroke.

Stress-Induced Changes in Bone Marrow Stromal Cell Populations Revealed through Single-Cell Protein Expression Mapping

Stromal cell populations that maintain hematopoietic stem and progenitor cells (HSPCs) are generally characterized in steady-state conditions. Here, we report a comprehensive atlas of bone marrow stromal cell subpopulations under homeostatic and stress conditions using mass cytometry (CyTOF)-based single-cell protein analysis. We identified 28 subsets of non-hematopoietic cells during homeostasis, 14 of which expressed hematopoietic regulatory factors. Irradiation-based conditioning for HSPC transplantation led to the loss of most of these populations, including the LeptinR⁺ and Nestin⁺ subsets. In contrast, a subset expressing Ecto-5'-nucleotidase (CD73) was retained and a specific CD73⁺NGFR^high population expresses high levels of cytokines during homeostasis and stress. Genetic ablation of CD73 compromised HSPC transplantation in an acute setting without long-term changes in bone marrow HSPCs. Thus, this protein-based expression mapping reveals distinct sets of stromal cells in the bone marrow and how they change in clinically relevant stress settings to contribute to early stages of hematopoietic regeneration.

A Cellular Taxonomy of the Bone Marrow Stroma in Homeostasis and Leukemia

Stroma is a poorly defined non-parenchymal component of virtually every organ with key roles in organ development, homeostasis, and repair. Studies of the bone marrow stroma have defined individual populations in the stem cell niche regulating hematopoietic regeneration and capable of initiating leukemia. Here, we use single-cell RNA sequencing (scRNA-seq) to define a cellular taxonomy of the mouse bone marrow stroma and its perturbation by malignancy. We identified seventeen stromal subsets expressing distinct hematopoietic regulatory genes spanning new fibroblastic and osteoblastic subpopulations including distinct osteoblast differentiation trajectories. Emerging acute myeloid leukemia impaired mesenchymal osteogenic differentiation and reduced regulatory molecules necessary for normal hematopoiesis. These data suggest that tissue stroma responds to malignant cells by disadvantaging normal parenchymal cells. Our taxonomy of the stromal compartment provides a comprehensive bone marrow cell census and experimental support for cancer cell crosstalk with specific stromal elements to impair normal tissue function and thereby enable emergent cancer.

Abstract B13: Plasma G-CSF levels are predictive of lack of response to zoledronic acid treatment in reducing breast cancer recurrence

Some breast cancer patients have no evidence of metastatic disease at the time of original diagnosis, yet ~30-40% of patients will experience recurrent breast cancer. Over 90% of breast cancer deaths occur due to distant spread of the disease, and bone is the most common site of breast cancer metastasis. Bisphosphonates, such as zoledronic acid (ZA), are used to treat patients with osteolytic disease, including metastatic breast cancer. ZA has been demonstrated to reduce disseminated tumor cells (DTCs) and breast cancer recurrence, but not all patients see this benefit and it is not yet clear what predicts benefit from adjuvant ZA treatment; the mechanism of action for this effect of ZA in reducing breast cancer remains undetermined. Here, we establish that ZA renders osteoclast progenitor cells (OCPs) tumor suppressive, primarily by directing their lineage potential. In instances where the OCP lineage potential was modulated via systemic or tumor-derived granulocyte-colony stimulating factor (G-CSF), OCPs did not differentiate into tumor-suppressive populations but instead into mature osteoclasts. High G-CSF was sufficient to abrogate the tumor-suppressive effect of ZA. Furthermore, we determined that in a subset of patient plasma samples from the AZURE clinical trial where stage II/III breast cancer patients were randomized to receive standard systemic treatment alone with adjuvant ZA treatment, patients who had baseline plasma G-CSF levels >23 pg/mL had reduced disease-free survival as assessed over a 10-year period post treatment initiation compared to patients with baseline plasma G-CSF levels <23 pg/mL (p=0.02). These findings indicate that patients with higher baseline plasma G-CSF levels will not likely observe reduction in breast cancer recurrence with adjuvant ZA treatment, and in fact may have worse prognosis with adjuvant ZA treatment as compared to control treatment. Our data are the first to demonstrate that ZA mediates its tumor-suppressive function via the OCP population and implicate capitalizing on the differentiation potential of the OCPs to maximize patient response to adjuvant ZA treatment in breast cancer risk of recurrence.

Citation Format: Jessalyn M. Ubellacker, Ninib Baryawno, Nicolas Severe, Molly J. DeCristo, Jaclyn Sceneay, Marie-Therese Haider, Catherine S. Rhee, Yuanbo Qin, Ingunn Holen, Walter M. Gregory, Janet E. Brown, Robert E. Coleman, David T. Scadden, Sandra S. McAllister. Plasma G-CSF levels are predictive of lack of response to zoledronic acid treatment in reducing breast cancer recurrence [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B13.

Modulating Bone Marrow Hematopoietic Lineage Potential to Prevent Bone Metastasis in Breast Cancer

The presence of disseminated tumor cells in breast cancer patient bone marrow aspirates predicts decreased recurrence-free survival. Although it is appreciated that physiologic, pathologic, and therapeutic conditions impact hematopoiesis, it remains unclear whether targeting hematopoiesis presents opportunities for limiting bone metastasis. Using preclinical breast cancer models, we discovered that marrow from mice treated with the bisphosphonate zoledronic acid (ZA) are metastasis-suppressive. Specifically, ZA modulated hematopoietic myeloid/osteoclast progenitor cell (M/OCP) lineage potential to activate metastasis-suppressive activity. Granulocyte-colony stimulating factor (G-CSF) promoted ZA resistance by redirecting M/OCP differentiation. We identified M/OCP and bone marrow transcriptional programs associated with metastasis suppression and ZA resistance. Analysis of patient blood samples taken at randomization revealed that women with high-plasma G-CSF experienced significantly worse outcome with adjuvant ZA than those with lower G-CSF levels. Our findings support discovery of therapeutic strategies to direct M/OCP lineage potential and biomarkers that stratify responses in patients at risk of recurrence.Significance: Bone marrow myeloid/osteoclast progenitor cell lineage potential has a profound impact on breast cancer bone metastasis and can be modulated by G-CSF and bone-targeting agents. Cancer Res; 78(18); 5300-14. ©2018 AACR.

dropEst: pipeline for accurate estimation of molecular counts in droplet-based single-cell RNA-seq experiments

Recent single-cell RNA-seq protocols based on droplet microfluidics use massively multiplexed barcoding to enable simultaneous measurements of transcriptomes for thousands of individual cells. The increasing complexity of such data creates challenges for subsequent computational processing and troubleshooting of these experiments, with few software options currently available. Here, we describe a flexible pipeline for processing droplet-based transcriptome data that implements barcode corrections, classification of cell quality, and diagnostic information about the droplet libraries. We introduce advanced methods for correcting composition bias and sequencing errors affecting cellular and molecular barcodes to provide more accurate estimates of molecular counts in individual cells.

Osteoblasts remotely supply lung tumors with cancer-promoting SiglecFhigh neutrophils

Bone marrow-derived myeloid cells can accumulate within tumors and foster cancer outgrowth. Local immune-neoplastic interactions have been intensively investigated, but the contribution of the systemic host environment to tumor growth remains poorly understood. Here, we show in mice and cancer patients (n = 70) that lung adenocarcinomas increase bone stromal activity in the absence of bone metastasis. Animal studies reveal that the cancer-induced bone phenotype involves bone-resident osteocalcin-expressing (Ocn⁺) osteoblastic cells. These cells promote cancer by remotely supplying a distinct subset of tumor-infiltrating SiglecF^high neutrophils, which exhibit cancer-promoting properties. Experimentally reducing Ocn⁺ cell numbers suppresses the neutrophil response and lung tumor outgrowth. These observations posit osteoblasts as remote regulators of lung cancer and identify SiglecF^high neutrophils as myeloid cell effectors of the osteoblast-driven protumoral response.

Hematopoiesis: Reconciling Historic Controversies about the Niche

The niche, as first conceptualized, was in conflict with the prevailing wisdom that stem cells have internal logic. The niche hypothesis has been indisputably confirmed. Yet, recent findings indicate little plasticity of epigenetically scripted hematopoietic stem/progenitors. Reconciling this conflict requires re-envisioning the niche as an enabler, not designer, of cell fate.

Bone marrow drives central nervous system regeneration after radiation injury

Nervous system injury is a frequent result of cancer therapy involving cranial irradiation, leaving patients with marked memory and other neurobehavioral disabilities. Here, we report an unanticipated link between bone marrow and brain in the setting of radiation injury. Specifically, we demonstrate that bone marrow-derived monocytes and macrophages are essential for structural and functional repair mechanisms, including regeneration of cerebral white matter and improvement in neurocognitive function. Using a granulocyte-colony stimulating factor (G-CSF) receptor knockout mouse model in combination with bone marrow cell transplantation, MRI, and neurocognitive functional assessments, we demonstrate that bone marrow-derived G-CSF-responsive cells home to the injured brain and are critical for altering neural progenitor cells and brain repair. Additionally, compared with untreated animals, animals that received G-CSF following radiation injury exhibited enhanced functional brain repair. Together, these results demonstrate that, in addition to its known role in defense and debris removal, the hematopoietic system provides critical regenerative drive to the brain that can be modulated by clinically available agents.

Abstract 3026: Targeting GPX4 in tumor-associated stromal cells increases inflammatory-cell infiltration

The lack of a T-cell inflamed microenvironment in tumors limits responsiveness to many immunotherapies. T-cell exclusion is often mediated by a dense infiltration of fibroblast-like stromal cells. Up to 55% of triple-negative breast cancers have ‘stroma-rich’ tumors with markedly lower T-cell inflammation. Here we report a therapeutic strategy that can potentially convert stroma-rich tumors into T-cell inflamed tumors by forcing stromal cells to secrete 5-lipoxygenase products which are powerful chemo-attractants for T cells.

We were initially interested in identifying selective inhibitors of stromal-cell function. To achieve this, we used phenotype-based small-molecule screening in which the phenotype of stroma-induced cancer cell migration in vitro was a surrogate for stroma-induced metastasis in vivo. We identified a compound, RSL3 that inhibited this migration. RSL3 was selectively cytotoxic to stromal cells over cancer cells, in comparisons of immortalized cell lines as well as comparisons of patient-derived primary breast cancer cells to cancer-associated fibroblasts (CAFs). We therefore undertook studies to identify its mechanism of action.

RSL3 was recently reported to target the redox enzyme glutathione peroxidase 4 (GPX4). GPX4 metabolizes lipid peroxides so we performed metabolomic profiling of RSL3-treated stroma-cancer co-cultures and found elevated arachidonic acid products of lipoxygenase enzymes. Stromal cells were found to contain >10-fold higher levels of lipoxygenase products than carcinoma cells. Blocking either 5-lipoxygenase (5-LO) or 15-lipoxygenase (15-LO) with selective inhibitors abrogated RSL3’s cytotoxicity to stromal cells. Thus, high lipoxygenase activity in stromal cells increases their susceptibility to GPX4 inhibition.

Because 5-LO products like leukotriene B4 are powerful chemo-attractants for myeloid cells and T cells, we studied the impact of GPX4 knockdown in vivo using xenografts of cancer cells co-injected with stromal cells. GPX4 knockdown resulted in a large increase in myeloid-cell infiltration into tumors but, surprisingly, T-cell infiltration was suppressed. We reasoned that 15-LO products are immunosuppressive based on recent findings that 15-LO gene amplifications are inversely correlated with T-cell infiltration in breast cancers in The Cancer Genome Atlas. Consistent with this, GPX4 inhibition of stroma-cancer co-cultures suppressed T-cell chemotaxis but combined inhibition of GPX4 and 15-LO significantly enhanced T-cell chemotaxis over untreated controls in vitro. We are undertaking in vivo testing of this combination.

In summary, our unbiased chemical biology approach has revealed a therapeutic strategy to promote T-cell inflammation. We envision this to be a priming strategy for stroma-rich cancers to become responsive to a variety of different immunotherapies thereby unleashing their full curative potential.

Citation Format: Shrikanta Chattopadhyay, Cherrie Huang, Ninib Baryawno, Nicolas Severe, Vasanthi Viswanathan, Carlotta Costa, David Scadden, Stuart Schreiber. Targeting GPX4 in tumor-associated stromal cells increases inflammatory-cell infiltration [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3026. doi:10.1158/1538-7445.AM2017-3026

Angiogenin Promotes Hematopoietic Regeneration by Dichotomously Regulating Quiescence of Stem and Progenitor Cells

Regulation of stem and progenitor cell populations is critical in the development, maintenance, and regeneration of tissues. Here, we define a novel mechanism by which a niche-secreted RNase, angiogenin (ANG), distinctively alters the functional characteristics of primitive hematopoietic stem/progenitor cells (HSPCs) compared with lineage-committed myeloid-restricted progenitor (MyePro) cells. Specifically, ANG reduces the proliferative capacity of HSPC while simultaneously increasing proliferation of MyePro cells. Mechanistically, ANG induces cell-type-specific RNA-processing events: tRNA-derived stress-induced small RNA (tiRNA) generation in HSPCs and rRNA induction in MyePro cells, leading to respective reduction and increase in protein synthesis. Recombinant ANG protein improves survival of irradiated animals and enhances hematopoietic regeneration of mouse and human HSPCs in transplantation. Thus, ANG plays a non-cell-autonomous role in regulation of hematopoiesis by simultaneously preserving HSPC stemness and promoting MyePro proliferation. These cell-type-specific functions of ANG suggest considerable therapeutic potential.

Proximity-Based Differential Single-Cell Analysis of the Niche to Identify Stem/Progenitor Cell Regulators

Physiological stem cell function is regulated by secreted factors produced by niche cells. In this study, we describe an unbiased approach based on the differential single-cell gene expression analysis of mesenchymal osteolineage cells close to, and further removed from, hematopoietic stem/progenitor cells (HSPCs) to identify candidate niche factors. Mesenchymal cells displayed distinct molecular profiles based on their relative location. We functionally examined, among the genes that were preferentially expressed in proximal cells, three secreted or cell-surface molecules not previously connected to HSPC biology-the secreted RNase angiogenin, the cytokine IL18, and the adhesion molecule Embigin-and discovered that all of these factors are HSPC quiescence regulators. Therefore, our proximity-based differential single-cell approach reveals molecular heterogeneity within niche cells and can be used to identify novel extrinsic stem/progenitor cell regulators. Similar approaches could also be applied to other stem cell/niche pairs to advance the understanding of microenvironmental regulation of stem cell function.

Abstract B11: Targeting mesenchymal cells in the tumor stroma by GPX4 inhibition

Fibroblast-like mesenchymal cells are the most abundant component of tumor stroma and have been shown to promote tumor progression. However, there are no therapeutic agents that target these cells. Developing therapeutics against mesenchymal cells has been challenging because they lack clear “druggable” targets. Here we used a phenotypic screening approach to identify therapeutic targets in bone-marrow mesenchymal stem cells (MSCs), precursors of cancer-associated fibroblasts, that are reported to induce a metastatic phenotype in breast cancer cells (Karnoub et al, Nature 2007). Using the in vitro phenotype of enhanced migration of breast cancer cells (GFP-labeled MDA-MB-231; MDA) induced by MSCs (3-fold faster than MDA alone), we identified inflammatory signaling and glutathione peroxidase 4 as potential targets in MSCs.

To validate this screening model, we first confirmed that MSCs can enhance metastasis using orthotopic xenografts of luciferase-labeled MDA cells co-injected with primary human MSCs in NOD-SCID mice. Mice with MSC+MDA tumors had 5-fold greater thoracic bioluminescence (corresponding to lung metastasis) than mice with MDA tumors alone. We then performed gene-expression profiling of MSC+MDA co-cultures in vitro compared to cells grown alone. The top pathway upregulated in co-cultures was the interferon pathway, suggesting that an inflammatory response follows MSC-MDA interactions. Using publicly available gene-expression datasets, we prioritized transcripts that are expressed in patient stroma and correlate with poor survival in a meta-analysis of 20 whole-tumor datasets. To determine if these transcripts are necessary for MSC-induced metastatic behavior, we performed shRNA knockdown and measured effects on in vitro migration of MSC+MDA co-cultures compared to normal endothelial cells. Knockdown of 9 genes was specific for MSC+MDA migration but the effects were weak. This suggested functional redundancy and indicated that targeting individual upregulated genes is insufficient to block the phenotype.

We then performed a small-molecule screen on the MSC+MDA migration phenotype, counter-screening hits on endothelial cell migration. Only one compound, RSL3, showed a large and selective inhibition of MSC+MDA migration. RSL3 was selectively toxic to MSCs with no effect on MDA cells. The target of RSL3 was recently identified to be glutathione peroxidase 4 (GPX4) that metabolizes lipid peroxides. To identify lipid mediators of RSL3 toxicity, we profiled lipid levels in RSL3-treated MSC+MDA co-cultures and found that the top changes were lipoxygenase products of arachidonic acid. Pre-treatment with zileuton or PD146176, that inhibit 5- and 15-lipoxygenase respectively, abrogated RSL3's toxic effects on MSCs. We found that MSCs contain >10-fold higher lipoxygenase products than MDA cells, highlighting an inflammatory role of MSCs. Fibroblasts from different tissues (lungs, spleen, breast, skin) were similarly sensitive to RSL3, identifying a previously unrecognized vulnerability in these mesenchymal cells.

Since RSL3 is a tool compound with poor plasma stability, we used GPX4 shRNA knockdown (KD) to determine in vivo effects of depleting MSCs in MSC+MDA xenografts. Unexpectedly, we found accelerated tumor growth in GPX4 KD tumors compared with controls. However, this is consistent with previous reports showing that mesenchymal cells restrain tumor growth and can have both pro- and anti-tumor effects. Histological examination revealed increased myeloid cell infiltration into GPX4 KD tumors reflecting increased inflammation mediated by lipoxygenase products like leukotriene B4.

In summary, phenotypic screening has identified GPX4 inhibition as a novel approach to target mesenchymal cells. This approach may be particularly effective at recruiting immune/ inflammatory cells into tumors with the exciting possibility of synergizing with cancer immunotherapy.

Citation Format: Shrikanta Chattopadhyay, Cherrie Huang, Ninib Baryawno, Nicolas Severe, Vasanthi Viswanathan, Zarko Boskovic, Siddhartha Mukherjee, Jeff Gentry, Ben Wittner, Sridhar Ramaswamy, Alykhan Shamji, David Scadden, Stuart Schreiber. Targeting mesenchymal cells in the tumor stroma by GPX4 inhibition. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr B11.

Ischemic stroke activates hematopoietic bone marrow stem cells

RATIONALE

The mechanisms leading to an expanded neutrophil and monocyte supply after stroke are incompletely understood.

OBJECTIVE

To test the hypothesis that transient middle cerebral artery occlusion (tMCAO) in mice leads to activation of hematopoietic bone marrow stem cells.

METHODS AND RESULTS

Serial in vivo bioluminescence reporter gene imaging in mice with tMCAO revealed that bone marrow cell cycling peaked 4 days after stroke (P<0.05 versus pre tMCAO). Flow cytometry and cell cycle analysis showed activation of the entire hematopoietic tree, including myeloid progenitors. The cycling fraction of the most upstream hematopoietic stem cells increased from 3.34%±0.19% to 7.32%±0.52% after tMCAO (P<0.05). In vivo microscopy corroborated proliferation of adoptively transferred hematopoietic progenitors in the bone marrow of mice with stroke. The hematopoietic system's myeloid bias was reflected by increased expression of myeloid transcription factors, including PU.1 (P<0.05), and by a decline in lymphocyte precursors. In mice after tMCAO, tyrosine hydroxylase levels in sympathetic fibers and bone marrow noradrenaline levels rose (P<0.05, respectively), associated with a decrease of hematopoietic niche factors that promote stem cell quiescence. In mice with genetic deficiency of the β3 adrenergic receptor, hematopoietic stem cells did not enter the cell cycle in increased numbers after tMCAO (naive control, 3.23±0.22; tMCAO, 3.74±0.33, P=0.51).

CONCLUSIONS

Ischemic stroke activates hematopoietic stem cells via increased sympathetic tone, leading to a myeloid bias of hematopoiesis and higher bone marrow output of inflammatory Ly6C(high) monocytes and neutrophils.