Additionally, podoplanin-positive CAFs contribute to trastuzumab resistance in HER2-positive breast cancer by secreting immunosuppressive factors, including indoleamine 2,3-dioxygenase 1 (IDO1) as well as tryptophan 2,3-dioxygenase 2 (TDO2) [32]. Here, IDO1 is linked to breast carcinoma.