Summary Epithelial stem cells exhibit heterogeneity, with distinct stem cell populations occupying specific tissue regions. The human skin displays a characteristic undulating structure at the epidermal-dermal junction, which supports mechanical strength and influences the spatial organization of epithelial stem cells. Unlike human skin, mouse skin lacks these undulations, complicating studies into the effects of tissue architecture on stem cell distribution. Here, we leverage the mouse oral mucosa, which possesses an undulating structure similar to human skin, to characterize stem cell division dynamics and long-term fate in vivo . Using a combination of H2B-GFP pulse-chase analysis and lineage tracing with Dlx1-CreER and Slc1a3-CreER models, we demonstrate that slow-and fast-cycling stem cells localize to distinct anatomical regions relative to the undulating structure and maintain their respective compartments during tissue homeostasis. A three-dimensional culture model using micropatterned collagen scaffolds that recapitulate the undulating structures in vitro reveals that the mechanical environment generated by the undulating structures partially induces proliferative heterogeneity in epithelial stem cells. This study proposes tissue undulating surface structure as a common principle as a niche component that defines the localization of compartmentalized stem cell populations across different epithelial tissues. Abstract Figure

Despite objective responses to PARP inhibition and improvements in progression-free survival compared to standard chemotherapy in patients with BRCA-associated triple-negative breast cancer (TNBC), benefits are transitory. Using high dimensional single-cell profiling of human TNBC, here we demonstrate that macrophages are the predominant infiltrating immune cell type in BRCA-associated TNBC. Through multi-omics profiling we show that PARP inhibitors enhance both anti- and pro-tumor features of macrophages through glucose and lipid metabolic reprogramming driven by the sterol regulatory element-binding protein 1 (SREBP-1) pathway. Combined PARP inhibitor therapy with CSF-1R blocking antibodies significantly enhanced innate and adaptive anti-tumor immunity and extends survival in BRCA-deficient tumors in vivo and is mediated by CD8+ T-cells. Collectively, our results uncover macrophage-mediated immune suppression as a liability of PARP inhibitor treatment and demonstrate combined PARP inhibition and macrophage targeting therapy induces a durable reprogramming of the tumor microenvironment, thus constituting a promising therapeutic strategy for TNBC.