CD274 and breast cancer: Breast cancer cell‐secreted GM‐CSF activates the STAT5 signaling pathway to stabilize aryl hydrocarbon receptor in lung macrophages by blocking its ubiquitination, which subsequently drives PD‐L1 upregulation via direct binding to the PD‐L1 promoter, thereby promoting Treg differentiation—a pivotal mechanism for establishing the immunosuppressive lung PMN critical to breast cancer metastasis [131].