PDCD1LG2 and neoplasm: Furthermore, to evade adaptive immune surveillance, tumour cells can downregulate the expression of MHC by up to 90% and increase the expression of immune-checkpoint ligands such as PD-L1, PD-L2,65 TIGIT and TIM-3, as well as transforming growth factor (TGF)-β and interleukins, all of which cause T-cell exhaustion.66 Some tumour cells can also downregulate or mutate essential enzymes, such as Janus kinases (JAK1 and JAK2).