IDO1 and cancer: MDSCs are a heterogeneous population composed of mononuclear and polymorphonuclear myeloid cells, which produce or express various immunomodulatory molecules, such as TGFβ, IL10, IDO, PGE2, ARG1, and PDL1, to promote cancer metastasis directly and indirectly by suppressing anti-tumor immune responses as described above [102,103].