A cell-autonomous PD-1/PD-L1 circuit promotes tumorigenicity of thyroid cancer cells by activating a SHP2/Ras/MAPK signalling cascade

The programmed cell death-1 (PD-1) and its ligands PD-L1 and PD-L2 are immune checkpoints. Typically, cancer cells express the PD-Ls that bind PD-1 on immune cells, inhibiting their anti-cancer activity. Recently, PD-1 expression has been found in cancer cells. We analysed expression and functions of PD-1 in thyroid cancer (TC). Human TC specimens (47%), but not normal thyroids, displayed PD-1 expression in epithelial cells, which significantly correlated with tumour stage and lymph-node metastasis. PD-1 overexpression/stimulation promoted TC cell proliferation and migration in culture. PD-1 recruited the SHP2 phosphatase, potentiated its phosphatase activity thus enhancing Ras activation by dephosphorylation of inhibitory tyrosine 32 and triggering the MAPK cascade. PD-1 inhibition decreased, while PD-1 overexpression facilitated, TC cell xenograft growth by affecting cell proliferation. PD-1 circuit blockade in TC, besides restoring anti-cancer immunity, could also directly impair TC cell growth by inhibiting the Ras/MAPK pathway.