IDO1 and neoplasm: This study demonstrated that the hypoxia‐induced EMT of cancer cells helped to educate newly recruited monocytes by secreting CCL22, which led to indoleamine 2,3‐dioxygenase (IDO) upregulation in the monocytes, and IDO+ monocyte‐derived macrophages exerted an immunosuppressive effect on T cells, which thus created a tumour immunosuppressive microenvironment and promoted tumour metastasis.