TLR4-dependent fibroblast-monocyte axis in tumor-draining lymph nodes contributes to metastasis in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is the most aggressive form of BC and despite improvements in neoadjuvant treatment with immune checkpoint blockade (ICB) targeting PD-1, response rates vary, and patients relapse with metastatic disease. Using human samples and mouse models, we show that myeloid cells, particularly monocytes, increase in aggressive TNBC draining lymph nodes (TDLN) showing T-cell suppressive capacity via PD-L1 and iNOS. Tumor-induced TLR4 ligands reprogrammed TDLN fibroblastic reticular cells (FRCs) through TLR4 activation to induce the recruitment of monocytes via CCL2 and CCL7 in vascularized and T-cell areas. Indeed, monocytes expressing the receptor of CCL2/7 CCR2 home to and colocalize with FRC-rich niches. The local inhibition of this recruitment axis at the TDLN site, either through CCR2 or TLR4 inhibition, in combination with PD-1 blocking, resulted in reduced LN and distant lung metastasis in mouse models. A subset of TNBC patients with worse survival was stratified using TLR4 ligands, including TNC, S100A9, and FN1. Our results thus demonstrate a novel mechanism through which metastatic TNBC tumors reprogram the TDLN-niche to facilitate PD-L1-mediated immune evasion that precedes and drives metastasis. We thereby provide an opportunity to improve the efficacy of ICB in TNBC patients and to use the TDLN or our primary tumor signature as predictive biomarkers.