TGF-β signalling, often upregulated in CAFs and tumour cells, drives the EMT in cancer cells [180,181], upregulates PD-L1 expression [182,183], and promotes the differentiation or expansion of regulatory T cells [184], collectively dampening anti-tumour immunity and contributing to resistance to PD-1/PD-L1 blockade. Here, PDCD1 is linked to cancer.