PD-L1 expression and CD8-positive T cells are associated with favorable survival in HER2-positive invasive breast cancer.

Programmed cell death 1 (PD-1) and its ligand (PD-L1) are key physiologic suppressors of the cytotoxic immune reaction. However, to date, the combination of PD1/PD-L1 expression and tumor-infiltrating lymphocytes (TILs) and antigen-presenting cells has been only minimally reported in breast carcinoma, in particular in relation to HER2-positive cases. The goal of this study was to evaluate both cellular tumoral immune reaction and PD-L1/PD1 distribution in HER2-positive cases, as well as any associations with clinical outcome using conventional chemotherapy combined with HER2 blocking. Multicolor immunohistochemical multiplex assays simultaneously demonstrating PD1, PD-L1, and CD8 or PD-L1, CD3, and CD163 were performed on tissue microarrays (TMA) representing 216 pretreatment cases of HER2-positive invasive breast carcinoma. PD-L1 expression was identified in 38 cases (18%), including 12 cases (6%) with PD-L1 labeling of tumor cells and 26 cases (12%) with PD-L1 labeling of immune cells only. Ten of 12 cases with PD-L1 staining of tumor cells showed staining of associated immune cells as well. With this assay method, PD1 was detectable in many fewer cases (6 cases or 3%). PD-L1 expression was positively associated with high Nottingham grade, negative ER and PR, the absence of lymph node metastasis, and high levels of CD8+ cells. The overall survival by univariate analysis was positively associated with lower tumor stage, the absence of lymph node metastasis, PD-L1 expression, and high levels of CD8+ cells. Therefore, our data suggest cytotoxic immune reaction mediated by CD8-positive T cells and PD-L1 expression may predict a better outcome in patients with HER2-positive breast carcinoma managed with conventional chemotherapy and HER2-blocking therapy. These findings recommend clinical trials utilizing checkpoint blocking immunotherapy in some form for HER2-positive breast cancer.