Single-cell profile of tumor and immune cells in primary breast cancer, sentinel lymph node, and metastatic lymph node.

PURPOSE: Little is known about the host-tumor interaction in the lymph-node basin at a single cell level. This study examines single cell sequences in breast cancer nodal metastases of a patient with triple-negative breast cancer. METHODS: The primary breast tumor, sentinel lymph node, an adjacent lymph node with metastatic involvement and a clinically normal-appearing lymph node were collected during surgery. Single-cell sequencing was performed on all four specimens. RESULTS: 14,016 cells were clustered into 6 cell subpopulations. Cancer cells demonstrated the molecular characteristics of TNBC basal B subtype and highly expressed genes in the MAPK signaling cascade. Tumor-associated macrophages regulated antigen processing and presentation and other immune-related pathways to promote tumor invasion. CD8 + and CD4 + T lymphocytes concentrated more in sentinel lymph node and mainly stratified into two transcriptional states. The immune-cell amount variation among primary tumor, sentinel and normal lymph nodes showed a similar tendency between the sc-RNA-seq profile of TNBC samples and a previous reported bulk RNA-seq profile of a breast cancer cohort, including all four breast cancer subtype samples. DISCUSSION: Single-cell sequencing analysis suggested that the sentinel lymph node was the initial meeting site of tumor infiltration and immune response, where partial T lymphocytes perform anti-tumor activity, while other T cells exhibit an exhausted state. We proposed a molecular explanation to the well-established clinical principle that the 5-year and 10-year survival outcomes were noninferior between SLND and ALND.