The advent of immune-checkpoint inhibitors (ICI) in modern oncology has significantly
improved survival in several cancer settings. A subgroup of women with breast cancer
(BC) has immunogenic infiltration of lymphocytes with expression of programmed death-ligand
1 (PD-L1). These patients may potentially benefit from ICI targeting the programmed
death 1 (PD-1)/PD-L1 signaling axis. The use of tumor-infiltrating lymphocytes (TILs)
as predictive and prognostic biomarkers has been under intense examination. Emerging
data suggest that TILs are associated with response to both cytotoxic treatments and
immunotherapy, particularly for patients with triple-negative BC. In this review from
The International Immuno-Oncology Biomarker Working Group, we discuss (a) the biological
understanding of TILs, (b) their analytical and clinical validity and efforts toward
the clinical utility in BC, and (c) the current status of PD-L1 and TIL testing across
different continents, including experiences from low-to-middle-income countries, incorporating
also the view of a patient advocate. This information will help set the stage for
future approaches to optimize the understanding and clinical utilization of TIL analysis
in patients with BC.
