Predictive biomarkers of response to immunotherapy in triple-negative breast cancer – state of the art and future perspectives

Abstract

Background: Immunotherapy by using immune checkpoint inhibitors (ICIs) heralded a new era in the treatment of patients with advanced triple-negative breast cancer (TNBC). Nevertheless, in a substantial proportion of TNBC patients, the clinical outcomes of ICIs treatment remain unpredict-able and proper biomarkers to identify tumors sensitive to immunotherapy are urgently needed. Currently, the most clinically relevant biomarkers used to predict efficacy of ICIs in patients with advanced TNBC remain the immunohistochemical analysis of programmed death-ligand 1 (PD-L1) expression, the assessment of tumor infiltrating lymphocytes (TILs) present in the tumor microen-vironment (TME), and the evaluation of the tumor mutational burden (TMB). Emerging biomarkers related to activation of the transforming growth factor beta signaling pathway, the discoidin domain receptor 1, and thrombospondin-1 as well as other cellular and molecular factors present within TME, have the potential to be utilized as predictors of response to ICIs in the future. Purpose: In this review, we summarize the current knowledge of mechanisms regulating PD-L1 expression, of the predictive value of TILs as well as of associated cellular and molecular components present in the TME in TNBC. Furthermore, TMB and emerging biomarkers with potential value in predicting efficacy of ICIs are discussed, and new therapeutic strategies will be outlined.