IDO1 and neoplasm: Although there are still few studies in the field of haematological malignancies, the critical effects of some critical immunosuppressive cells, such as myeloid-derived suppressor cells (MDSCs), tumour-associated macrophages (TAMs) and regulatory T cells (Tregs), immune checkpoints and soluble molecules, such as indoleamine 2,3-dioxygenase (IDO) and adenosine and inhibitory cytokines, on the efficacy of CAR T cell therapy have been gradually elucidated (32).