In one embodiment, a single modality cancer immunotherapy regimen that includes a therapeutic composition is provided. Such a therapeutic composition may include a Salmonella strain comprising a plasmid that expresses an shRNA molecule that suppresses the expression of an immunosuppressive target and suppresses tumor growth. In some aspects, the Salmonella strain is an attenuated Salmonella typhimurium strain. In other aspects, the immunosuppressive target is STAT3, IDO1, IDO2, Arginase 1, iNOS, CTLA-4, TGF-β, IL-10, pGE2 or VEGF. In one embodiment, the immunosuppressive target is IDO1 or Arg1 and the shRNA molecule is any one of SEQ ID NO:5-14.