To Örebro University

Background: Conflicting data have been reported on the prognostic value of PD-L1 expression per immunohistochemistry (IHC) in breast cancer (BC). There is a paucity of data on the role of PD-L1 gene expression (GE).

Methods: 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 positivity in tumor cells, immune cells or both, per subtype and per antibody used; and the prognostic value of PD-L1 positivity for DFS and OS. Heterogeneity was assessed using the Q and I² statistics. A pooled GE analysis of 39 publicly available transcriptomic datasets was also performed.

Results: Of the initial 4184 entries, 38 retrospective studies fulfilled the inclusion criteria. The overall pooled PD-L1 positivity rate in tumor cells was 24%, 33% in immune cells and 25% in both; highest positivity was reported with Dako 28-8 clone. PD-L1 IHC expression in tumor cells was prognostic for shorter DFS (HR = 1.36, 95% CI 1.02 – 1.83, p < 0.04) and OS (HR = 1.66; 95% CI 1.09 – 2.50, p = 0.02); there was significant heterogeneity. PD-L1 IHC expression in immune cells was associated with better DFS (HR = 0.61; 95% CI 0.51 – 0.73, p < 0.001) and OS (HR = 0.53, 95% CI 0.39 – 0.73, p < 0.001) in TNBC. In addition, higher PD-L1 GE predicted better survival in multivariate analysis in the entire population (HR = 0.70, 95% CI 0.60 – 0.82, p < 0.001 for DFS and HR = 0.84, 95% CI 0.75 – 0.93, p = 0.001 for OS) and in basal-like tumors (HR = 0.55, 95% CI 0.38 – 0.80, p = 0.001 for DFS and HR = 0.63, 95% CI 0.50 – 0.79, p < 0.001 for OS), p_interaction 0.124 for DFS and 0.005 for OS.

Conclusions: The largest to our knowledge meta-analysis on IHC PD-L1 expression in BC informs on PD-L1 positivity rates and its prognostic value. Standardization is needed prior to routine implementation. PD-L1 GE is a promising prognostic factor, especially in basal-like BC.