PDCD1 and lung carcinoma: Though these associations may seem counterintuitive, as PD-1 expression is often associated with immune exhaustion and consequent tumor progression [38], Du et al. have shown that, in lung carcinoma patients with high PD-1 IHC scores in TCs, the disease progressed rapidly after initiation of anti-PD-1 therapy [39], suggesting that PD-1 expression in TCs could potentially play a determining role in suppressing tumor progression when coupled with PD-L1 or PD-L2 ligands expressed on other TCs.