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

Tumor-draining lymph nodes (TDLNs) are sites of anti-tumor immune priming as well as metastases. Here, we examined how the cellular networks within TDLNs are reorganized in triple-negative breast cancer (TNBC). We found that the frequencies of programmed death ligand 1 high (PD-L1^hi) monocytes increased in TDLNs of metastatic TNBC mouse tumors. Fibroblastic reticular cell (FRC) subtypes heightened the expression of the chemokines CCL2 and CCL7, supporting the homing of CCR2⁺ monocytes. These monocytes suppressed T cells in vitro via PD-L1 and inducible nitric oxide synthase (iNOS). Spatial transcriptomics revealed immunosuppressive FRC-monocyte niches in vascularized and T cell areas. Tumor-associated Toll-like receptor (TLR) 4 ligands induced CCL2 and CCL7 expression by FRCs to promote monocyte recruitment. Localized TLR4 inhibition in combination with anti-programmed cell death protein 1 (αPD-1) therapy reduced monocyte homing and boosted T cell function, ultimately attenuating lung metastases. Monocytes accumulate in human TNBC TDLNs, with evidence of a FRC-monocyte axis, and a TLR4 ligand signature is predictive of poor survival outcomes in TNBC patients. Thus, metastatic TNBC can reprogram lymph nodes (LNs) to facilitate PD-L1-mediated immune evasion and metastasis.