PDCD1 and neoplasm: TGF-β is produced in excess by tumour cells themselves, as well as cells of the tumour stroma.121 TGF-β has been shown to impair both innate and adaptive cellular immunity, generating a favourable microenvironment for tumour growth and metastasis, and is deemed an essential component of the tumour’s anti-immune defences.122,123 Following this line of thought, strategies that combine CAR T cell therapy with blocking immune-suppressive axes such as PD-1–PD-L1 or providing important cytokines depleted in the tumour environment, have been investigated.