PDCD1 and head and neck squamous cell carcinoma: There was a 65‐fold increase in IDO1 in 10 PD‐L1+ vs 5 PD‐L1− HPV+HNSCCs (P = .004). IDO1 expression was upregulated and co‐localised in the TME of the validation cohort. IDO1 expression increased and correlated with disease progression in anti‐PD‐1 treated HNSCC patients