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Crucial transitions in cancer-including tumor initiation, local expansion, metastasis, and therapeutic resistance-involve complex interactions between cells within the dynamic tumor ecosystem. Transformative single-cell genomics technologies and spatial multiplex in situ methods now provide an opportunity to interrogate this complexity at unprecedented resolution. The Human Tumor Atlas Network (HTAN), part of the National Cancer Institute (NCI) Cancer Moonshot Initiative, will establish a clinical, experimental, computational, and organizational framework to generate informative and accessible three-dimensional atlases of cancer transitions for a diverse set of tumor types. This effort complements both ongoing efforts to map healthy organs and previous largescale cancer genomics approaches focused on bulk sequencing at a single point in time. Generating single-cell, multiparametric, longitudinal atlases and integrating them with clinical outcomes should help identify novel predictive biomarkers and features as well as therapeutically relevant cell types, cell states, and cellular interactions across transitions. The resulting tumor atlases should have a profound impact on our understanding of cancer biology and have the potential to improve cancer detection, prevention, and therapeutic discovery for better precision-medicine treatments of cancer patients and those at risk for cancer.Cancer forms and progresses through a series of critical transitions-from pre-malignant to malignant states, from locally contained to metastatic disease, and from treatment-responsive to treatment-resistant tumors (Figure 1). Although specifics differ across tumor types and patients, all transitions involve complex dynamic interactions between diverse pre-malignant, malignant, and non-malignant cells (e.g., stroma cells and immune cells), often organized in specific patterns within the tumor

Cell death is a prominent feature of B cell development. For example, a large population of B cells dies at the pre‐B cell stage presumably due to the failure to express a functional immunoglobulin receptor. In addition, developing B cells expressing antigen receptors for self are selectively eliminated at the immature B cell stage. The molecular signals that control B cell survival are largely unknown. The product of the bcl‐2 proto‐oncogene may be involved as its overexpression inhibits apoptotic cell death in a variety of biological systems. However, the physiological role of the endogenous Bcl‐2 protein during B cell development is undetermined. Here we show a striking developmental regulation of the Bcl‐2 protein in B lymphocytes. Bcl‐2 is highly expressed in CD43+ B cell precursors (pro‐B cells) and mature B cells but downregulated at the pre‐B and immature B cell stages of development. We found that Bcl‐2 expressed by B cells is a long‐lived protein with a half‐life of approximately 10 h. Importantly, susceptibility to apoptosis mediated by the glucocorticoid hormone dexamethasone is stage‐dependent in developing B cells and correlates with the levels of Bcl‐2 protein. Furthermore, expression of a bcl‐2 transgene rescued pre‐B and immature B cells from dexamethasone‐induced cell death, indicating that Bcl‐2 can inhibit the apoptotic cell death of progenitors and early B cells. Taken together, these findings argue that Bcl‐2 is a physiological signal controlling cell death during B cell development.

SUMMARY Ductal carcinoma in situ (DCIS) is a pre-invasive lesion that is thought to be a precursor to invasive breast cancer (IBC). To understand the changes in the tumor microenvironment (TME) accompanying transition to IBC, we used multiplexed ion beam imaging by time of flight (MIBI-TOF) and a 37-plex antibody staining panel to interrogate 79 clinically annotated surgical resections using machine learning tools for cell segmentation, pixel-based clustering, and object morphometrics. Comparison of normal breast with patient-matched DCIS and IBC revealed coordinated transitions between four TME states that were delineated based on the location and function of myoepithelium, fibroblasts, and immune cells. Surprisingly, myoepithelial disruption was more advanced in DCIS patients that did not develop IBC, suggesting this process could be protective against recurrence. Taken together, this HTAN Breast PreCancer Atlas study offers insight into drivers of IBC relapse and emphasizes the importance of the TME in regulating these processes.

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