It has been shown that PDPN-positive CAF can promote resistance to trastuzumab in Her2-positive breast cancer by secreting the immunosuppressive factors indoleamine 2,3-dioxygenase 1 (IDO1) as well as tryptophan 2,3-dioxygenase 2 (TDO2) [50]. Here, TDO2 is linked to HER2 positive breast carcinoma.