Periostin thorn Stromal Cells Guide Lymphovascular Invasion by Cancer Cells

Cancer cell dissemination to sentinel lymph nodes is associated with poor patient outcomes, particularly in breast cancer. The process by which cancer cells egress from the primary tumor upon interfacing with the lymphatic vasculature is complex and driven by dynamic interactions between cancer cells and stromal cells, includ-ing cancer-associated fibroblasts (CAF). The matricellular protein periostin can distinguish CAF subtypes in breast cancer and is associated with increased desmoplasia and disease recurrence in patients. However, as periostin is secreted, periostin-expressing CAFs are difficult to characterize in situ, limiting our understanding of their specific contribution to cancer progression. Here, we used in vivo genetic labeling and ablation to lineage trace periostin thorn cells and characterize their functions during tumor growth and metas-tasis. Periostin-expressing CAFs were spatially found at periductal and perivascular margins, were enriched at lymphatic vessel periph-eries, and were differentially activated by highly metastatic cancer cells versus poorly metastatic counterparts. Surprisingly, genetically depleting periostin thorn CAFs slightly accelerated primary tumor growth but impaired intratumoral collagen organization and inhibited lymphatic, but not lung, metastases. Periostin ablation in CAFs impaired their ability to deposit aligned collagen matrices and inhibited cancer cell invasion through collagen and across lymphatic endothelial cell monolayers. Thus, highly metastatic cancer cells mobilize periostin-expressing CAFs in the primary tumor site that promote collagen remodeling and collective cell invasion within lymphatic vessels and ultimately to sentinel lymph nodes.Significance: Highly metastatic breast cancer cells activate a population of periostin-expressing CAFs that remodel the extra -cellular matrix to promote escape of cancer cells into lymphatic vessels and drive colonization of proximal lymph nodes.