Abstract
Invasive lobular breast carcinoma (ILC) shows specific stromal features, and a high tumor-infiltrating lymphocyte (TIL) content being associated with poor patient prognosis. Here, we reveal the underlying mechanism by performing single-cell analysis, immunohistochemistry, deconvolution of bulk RNA-sequencing in a large female ILC series and functional assays. We show that E-cadherin (CDH1)-loss in breast cancer cells prevents differentiation of FAP+ inflammatory cancer-associated fibroblasts (iCAF) into FAP+ myofibroblastic CAF, leading to iCAF accumulation in ILC. In turn, FAP+ iCAF attract TILs into the tumor center, shaping their spatial organization. Subsequently, CDH1-inactivated ILC cancer cells promote immune escape through a lack of retention and activation of ITGAE-expressing resident memory CD8 + T lymphocytes (TRM). Hence, our study uncovers reciprocal interactions between CDH1-inactivated cancer cells, FAP+ iCAF and CD8 + TRM, providing insights into the ILC stromal reaction and revealing why and how TILs are associated with poor prognosis in ILC patients, a mechanism generalizable to other CDH1-inactivated cancer types.