PD-L1 expression in breast cancer: expression in subtypes and prognostic significance: a systematic review.

To systematically review the literature on the expression of PD-L1 in primary BC, variation of expression between subtypes and effect on overall survival (OS), disease-free survival (DFS), and recurrence-free survival (RFS). Additionally, for studies in the neoadjuvant setting, we have reviewed the ability of PD-L1 to predict pathological complete response (pCR). Articles included in this review were retrieved by searching PubMed (1966-2018) and EMBASE (1980-2018). The following search terms were used: "PD-L1 expression" and "breast cancer" (PubMed234; EMBASE 161). Thirty-seven articles were found relevant to this study. We summarize important findings from these works, and show that the observed PD-L1 expression in the studies varies greatly, with expression rates ranging from 0 to 83% across subtypes. PD-L1 expression in relation to prognosis both in the adjuvant and neoadjuvant chemotherapy setting remains controversial, with studies finding better, worse, or no effect on prognosis. We also show that a wide variety of strategies are used when evaluating PD-L1 immunohistochemically, e.g., different cut-off points, different cell types evaluated, and different perceptions of when a cell is positive for PD-L1 (cytoplasmic vs membrane staining). Further investigation of PD-L1 expression in breast cancer and its effect on prognosis is required. There is little consensus on the methods used to evaluate PD-L1 expression immunohistochemically, and this may contribute to the diverging results found in this study.