The role of PD-L1 expression in pituitary tumours: lessons from the current literature.

BACKGROUND Programmed cell death-1 (PD-1) and PD ligand-1 (PD-L1) expression predicts the biological behaviour, aggres-siveness and response to immune checkpoint inhibitors in different cancers. We reviewed the published data on PD-L1 ex-pression in pituitary tumours, from the perspective of its biological role and prognostic usefulness. SUMMARY A literature review focused on PD-L1 expression in pituitary tumours was performed. Six immunohistochemistry-based studies which assessed PD-L1 positivity in pituitary tumours were included, encompassing 704 patients. The cohort consisted of 384 (54.5%) non-functioning tumours and 320 (43.5%) functioning pituitary tumours. PD-L1 expression was posi-tive in 248 cases (35.2%). PD-L1 positivity rate was higher in functioning than in non-functioning tumours (46.3% vs 26.0%; p<0.001), but also higher in growth hormone-secreting tumours (56.7%) and prolactinomas (53.6%) than in thyrotroph (33.3%) or corticotroph tumours (20.6%). While proliferative pituitary tumours showed higher rate of PD-L1 positivity than non-proliferative tumours (p<0.001), no association with invasion or recurrence was found. KEY MESSAGES PD-L1 is expressed in a substantial number of pituitary tumours, predominantly in the functioning ones. PD-L1 positivity rates were significantly higher in proliferative pituitary tumours in comparison to non-proliferative tumours, but no differences were found concerning invasive or recurrent pituitary tumours. More studies following homogeneous and standardised methodologies are needed to fully elucidate the role and usefulness of PD-L1 expression in pituitary tumours.