Programmed death ligand 1 expression in triple‐negative breast cancer is associated with tumour‐infiltrating lymphocytes and improved outcome

Aims Triple‐negative breast cancer ( TNBC ) patients generally have a poor outcome; there is a pressing need to identify more effective therapeutic strategies. Clinical trials targeting programmed death 1/programmed death ligand 1 ( PD 1/ PDL 1) in melanoma and non‐small‐cell lung cancer have reported high response rates, and tumoral PDL 1 expression has been suggested as a potential biomarker to enrich for patient response to these treatments. There are only very limited data to date reporting the expression of PDL 1 in TNBC . Methods and results PDL 1 immunohistochemistry was performed on 161 primary TNBC s and assessed in the tumour as well as immune cells in the stromal compartment. PDL 1 expression was very common in TNBC , expressed in the tumour cell membrane (64%), cytoplasm (80%) and stromal (93%) cellular compartments. Cytoplasmic tumoral expression of PDL 1 was associated with a lower risk of breast cancer‐specific death [hazard ratio ( HR ) 0.45, P = 0.035] while stromal PDL 1 expression was associated with a lower rate of deaths from all causes ( HR 0.305, P = 0.0042). Membranous expression of PDL 1 was not associated with outcome. While both PDL 1 expression and tumour‐infiltrating lymphocytes were associated with a better outcome, only lymphovascular invasion and high tumour‐infiltrating lymphocytes were independently prognostic for breast cancer‐specific death. Conclusion While PDL 1 expression is frequent in TNBC , it was not independently prognostic. There were differences in outcome depending on the cellular compartment of PDL 1 expression. These data provide further impetus for investigating the utility of immune checkpoint therapies in TNBC , given the clinical significance of tumour‐infiltrating lymphocytes ( TIL s) and PDL 1 expression in this cohort.