IDO1 and sarcoma: A clinical trial of the PD-1 blocker pembrolizumab in combination with cyclophosphamide chemotherapy for sarcoma reported an increased infiltration of CD163-positive macrophages and immunosuppressive effects in treated tumors and that these macrophages expressed indoleamine 2 and 3-dioxygenase 1 (IDO1) to produce kynurenine.