IDO1 and neoplasm: ATRA decreased the expression of immunosuppressive genes through the downregulation of TGF-β, PD-L1, IL-10, and IDO in MDSCs and upregulated MHC class I homologs MICA and MICB on tumor cells, enhancing NK cell activity (363, 364) and cytotoxicity of T cells (365) in the CD8− and CD4− immune response (362).