Single cell mapping of B and T cell dynamics in breast cancer lymph node metastasis.

Metastatic breast cancer remains difficult to cure, and the way B and T lymphocytes adapt across metastatic niches especially under therapy remains insufficiently defined. Clarifying compartment specific immune remodeling may help explain resistance to PD-1/PD-L1 blockade and identify actionable targets. We performed an integrated meta-analysis of single cell RNA-seq datasets from normal breast tissue, primary tumors, tumor-draining lymph nodes (TLNs), and peripheral blood mononuclear cells (PBMCs), focusing on B and Tcell states. Immune composition differed notably by compartment. Tumors were enriched for effector CD8 states (CD8 cytotoxic 20.1%; CD8 activated 13.5%), whereas TLNs preserved larger naïve and memory reservoirs (CD4 naïve 40.7%; B naïve 11.4%; B memory 12.0%) and contained a higher B cell fraction than tumors (39.6% vs. 19.5%). Post therapy, PBMCs and TLNs showed increased BTLA-HVEM (TNFRSF14) checkpoint signaling and enhanced MIF-CD74 interactions with a shift from CD44 toward CXCR4, consistent with CXCR4 driven migratory and survival programs. In TLNs, TNFRSF14 signaling was unidirectional (B→T), absent in the reverse direction, and not detected in tumors. Clinically, higher tumor CXCR4 combined with lower TNFRSF14 was associated with shorter progression free survival in TCGA-BRCA, most evident in node positive, early stage disease. To target the BTLA-HVEM checkpoint axis, we performed structure guided