ALK and lung adenocarcinoma: Although a significant number of PD-1 positive CD8+ T cells were found in the ALK-positive tumor bed in early lung adenocarcinoma (64), these PD-1 expressing CD8+ T cells were functionally impaired (65) and did not express interferon-γ mRNA, which could upregulate PD-L1 expression in tumor cells (66, 67).