To Örebro University

PURPOSE: Conflicting data have been reported on the prognostic value of PD-L1 protein and gene expression in breast cancer.

EXPERIMENTAL DESIGN: Medline, Embase, Cochrane Library and Web of Science Core Collection were searched and data were extracted independently by two researchers. Outcomes included pooled PD-L1 protein positivity in tumor cells, immune cells or both, per subtype and per antibody used; and its prognostic value for disease-free and overall survival. A pooled gene expression analysis of 39 publicly available transcriptomic datasets was also performed.

RESULTS: Of the initial 4184 entries, 38 retrospective studies fulfilled the predefined inclusion criteria. The overall pooled PD-L1 protein positivity rate was 24% (95% CI 15 - 33%) in tumor cells and 33% (95% CI 14 - 56%) in immune cells. PD-L1 protein expression in tumor cells was prognostic for shorter overall survival (HR = 1.63; 95% CI 1.07 - 2.46, p=0.02); there was significant heterogeneity (I2 = 80%, pheterogeneity<0.001). In addition, higher PD-L1 gene expression predicted better survival in multivariate analysis in the entire population (HR=0.82, 95% CI 0.74 - 0.90, p<0.001 for OS) and in basal-like tumors (HR=0.64, 95% CI 0.52 - 0.80, p<0.001 for OS), pinteraction 0.005.

CONCLUSION: The largest to our knowledge meta-analysis on the subject informs on PD-L1 protein positivity rates and its prognostic value in breast cancer. Standardization is needed prior to routine implementation. PD-L1 gene expression is a promising prognostic factor, especially in basal-like BC. Discrepant prognostic information might be related to PD-L1 gene expression in the stroma.