PDCD1 and leukemia: Simultaneous manipulation of the 2 signalling pathways IL-33/ST2 and PD-1/PD-L1, either through blockade in solid tumours or IL-33 stimulation in combination with PD-1 inhibition in leukaemia, achieves significant inhibition of tumour growth primarily through enhanced mobilization and activity of effector cells of innate (NK, NKT) and acquired (CD8+ T ly) immunity (Figure 2).