Breast cancer remodels lymphatics in sentinel lymph nodes
Cancer metastasis into sentinel lymph nodes (LNs) is a crucial determinant of patient mortality. While it is recognized that tumor-induced lymphangiogenesis facilitates metastasis into LNs in murine models, the tumor-induced alterations in human lymphatic vessels remain obscure. Here we utilized single-cell RNA-seq to profile lymphatic endothelial cell (LEC) subsets in paired metastatic and non-metastatic LNs obtained from breast cancer patients. Tumor metastasis leads to a decrease in inflammatory LEC subsets such as subcapsular sinus LECs, while inducing increase of immunosuppressive CD200+ LECs. Matrix Gla protein (MGP) was the most upregulated gene in metastatic LN LECs, and its expression on LECs was VEGF and TGF-b dependent. Silencing MGP in LECs revealed that MGP promotes cancer cell adhesion to LECs. Thus, breast cancer cell metastasis into LNs remodels LEC subsets in human LNs and escalates MGP expression, which potentially facilitates the  dissemination of cancer cells through lymphatics. Overall design: Droplet-based single-cell RNA sequencing (scRNAseq) of human CD45- CD31+ PDPN+ cells in the metastatic and non-metastatic LNs from breast cancer patients. ScRNAseq was performed using 10X Genomics Single Cell 3' solution, according to manufacture's instructions. Libraries were sequenced on HiSeq3000.