Early-onset spontaneous murine mammary tumors show increased lymphocyte representation and reduced MHC antigen presentation

A wild-derived PWK/PhJ genetic background accelerates HER2/neu-driven mammary tumor onset and reshapes the tumor immune microenvironment. (BALBxPWK) F1 NeuT females developed tumors at 10 weeks versus 15 weeks in BALB NeuT mice (p < 0.0001), with more tumor-infiltrating T and B cells but lower expression of cytotoxicity genes Tox, Ltk, Nkg7, and Gzmb. Proteomics found coordinate down-regulation of Stat1, Tapbp, Parp9, and Dtx3l with MHC class I and II proteins, indicating that inherited background, not the oncogene alone, governs antigen presentation and immune recognition in these tumors.