IDO1 and breast cancer: Other promising avenues include (a) IL-15 agonists to activate NK cells [226]; (b) CXCL12/CXCR4 axis blockade to enhance immune infiltration [227]; (c) TGF-β inhibition, which restores T-cell function and blocks EMT [228,229,230]; and (d) indoleamine 2,3-dioxygenase 1 (IDO1), a tryptophan-catabolizing enzyme, enhances immunosuppression in metastatic BC, with its inhibition reducing TNBC invasiveness [231,232].