A better understanding of the features that define the interplay between cancer cells and immune cells is key to identify new cancer therapies 1 . Yet, focus is often given to those interactions that occur within the primary tumor and its microenvironment, while the role of immune cells during cancer dissemination in patients remains largely uncharacterized 2,3 . Circulating tumor cells (CTCs) are precursors of metastasis in several cancer types [4][5][6] , and are occasionally found within the bloodstream in association with non-malignant cells such as white blood cells (WBCs) 7,8 . The identity and function of these CTC-associated WBCs, as well as the molecular features that define the interaction between WBCs and CTCs are unknown. Here, we achieve the isolation and interrogation of individual CTC-associated WBCs, alongside with corresponding cancer cells within each CTC-WBC cluster, from multiple breast cancer patients and mouse models. Single-cell RNA sequencing reveals a specific pattern of WBCs attached to CTCs, with neutrophils representing the majority of the cases. When comparing the transcriptome profiles of CTCs that were associated to neutrophils with that of CTCs alone, we detect a number of differentially expressed genes that outline cell cycle progression, leading to a higher ability to efficiently seed metastasis. Additionally, we identify cell-cell junction and cytokine-receptor pairs that define CTC-neutrophil clusters, representing key vulnerabilities of the metastatic process. Thus, the association between neutrophils and CTCs fuels cell cycle progression within the bloodstream and expands the metastatic potential of CTCs, providing a rationale for targeting this interaction in breast cancer. 3/28 Main TextCirculating tumor cells (CTCs) are precursors of metastasis in various solid cancers including breast cancer 6 , and are occasionally found in association to white blood cells (WBCs) 7 . The role of CTC-WBC clusters in metastasis development as well as the principles that govern the interplay between CTCs and WBCs during blood-borne metastasis are largely uncharacterized.We first sought to determine the number and composition of CTC-WBC clusters in breast cancer patients and mouse models. We obtained blood samples from 70 patients with invasive breast cancer that discontinued their treatment due to progressive disease, as well as from five different breast cancer mouse models, and we enriched for CTCs using the Parsortix microfluidic device 9 (Extended Data Fig. 1a-e). Live CTCs were stained for cancer-associated cell surface markers EpCAM, HER2, and EGFR or imaged directly for the expression of GFP, as well as labeled for CD45 to identify WBCs (Fig. 1a and Extended Data Fig. 1f). Among 70 patients, 34 (48.6%) had detectable CTCs, with a mean number of 22 CTCs per 7.5ml of blood (Supplementary Tables 1 and 2). While the majority of CTCs were single (88.0%), we also detected CTC clusters (8.6%) and CTC-WBC clusters (3.4%) (Fig. 1b and Extended Data Fig. 1g,h). Similarly, we observed that CTC-...

SummaryThe ability of circulating tumor cells (CTCs) to form clusters has been linked to increased metastatic potential. Yet biological features and vulnerabilities of CTC clusters remain largely unknown. Here, we profile the DNA methylation landscape of single CTCs and CTC clusters from breast cancer patients and mouse models on a genome-wide scale. We find that binding sites for stemness- and proliferation-associated transcription factors are specifically hypomethylated in CTC clusters, including binding sites for OCT4, NANOG, SOX2, and SIN3A, paralleling embryonic stem cell biology. Among 2,486 FDA-approved compounds, we identify Na+/K+ ATPase inhibitors that enable the dissociation of CTC clusters into single cells, leading to DNA methylation remodeling at critical sites and metastasis suppression. Thus, our results link CTC clustering to specific changes in DNA methylation that promote stemness and metastasis and point to cluster-targeting compounds to suppress the spread of cancer.