PDL1 and PD1 expression by tumor infiltrating lymphocytes in primary breast cancer

Abstract

Background: A high extent of tumor infiltrating lymphocytes (TIL) has been linked with good clinical outcomes in HER2+ and triple negative (TN) breast cancers (BC). Some patients further mount an anti-tumor immune response organized in tertiary lymphoid structures (TLS). The development of escape mechanisms allows the growth and metastatic spread of the tumor. Programmed death-1 (PD1) and its ligand (PDL1) are checkpoint molecules whose interaction dampens the activation of the immune response. The purpose of this study was to characterize PD1 and PDL1 expression in primary BC. Material and methods: To evaluate the extent and organization of TIL infiltration, 146 untreated BC tumors from patients who underwent surgery between 2001 and 2013 at the Institut Jules Bordet were stained with a double immunohistochemical (IHC) labeling protocol using antibodies against CD3 and CD20, pan T and B cell surface markers, respectively. PDL1 and PD1 expression were assessed on the same cases with a second IHC staining. The stained sections were scored by two independent pathologists blinded to clinical data. PDL1 and PD1 positivity were defined as >1% positive cells. Results: Triple negative breast cancer (TNBCs) patients had the greatest extent of TIL (p < 0.0001) and together with HER2+ the highest presence of TLS compared to luminal subtypes. PDL1 expression was detected in 23% of our tumor samples (mostly TNBCs, p < 0.0001) and more frequently on TIL (21%) than tumor (5%) or stromal cells (3%) (p = 0.0002). A positive association was observed between lymphocytic PDL1 expression and TIL (p < 0.0001), TLS presence (p < 0.0001) (both evaluated by anti-CD3 and anti-CD20) and PD1 (p < 0.0001). 19% of the cases were PD1+ (mostly TNBCs; p = 0.0004) and expression was associated to TIL (p < 0.0001) and TLS (p < 0.0001). Lymphocytic PDL1 and PD1 co-expression in the same tissue section was found in 11% of our cases (28% TNBCs). However, morphologically their presence was rarely co-localized. Our results reveal that, at the protein level, PDL1 is chiefly expressed on lymphocytes in TNBCs, and as well as PD1, its presence is associated with high levels of TIL and TLS presence. In the majority of cases PDL1 or PD1 positive, expression was mutually exclusive, with only a few samples co-expressing these molecules. This indicates that these targetable molecules reflect immune activation, in relation to the various lymphocyte subpopulations present, which we are currently investigating.