394P Immune microenvironment characterization in early-onset breast cancer

further revealed enrichment of tertiary lymphoid structures (TLSs) in responders, with TLS abundance positively correlating with post-treatment CD8 + TRM infiltration.Fibroblast analysis revealed enrichment of tCAF-TWIST1 and mCAF-CTHRC1 in nonresponders, which strongly correlated with Treg abundance and high LRRC15 expression.Spatial mIF confirmed LRRC15 + CAFs in close proximity to CD4 + FOXP3 + Tregs, supporting a fibroblast-Treg immunosuppressive axis mediated by COL1A1/2-CD44 signaling.Macrophage profiling identified SPP1 + macrophages as highly enriched in non-responders.These cells interacted with Tregs via SPP1-CD44, potentially promoting Treg recruitment and impairing cytotoxic T cell function.Bulk RNA-seq and mIF confirmed that SPP1 + macrophages decreased after treatment in responders, whereas they persisted in non-responders and correlated positively with Treg abundance.Conclusions: Integrated single-cell, bulk, and spatial profiling revealed distinct immune-stromal ecosystems.Response was defined by TRM expansion and TLS activation, whereas resistance involved a fibroblast-macrophage-Treg axis driving immunosuppression.