Subpopulation composition of PD-L1-positive lymphocytes in the primary tumour in luminal breast cancer patients

The relationship between the tumour and the microenvironment is of great interest because it may determine the efficacy of new agents aimed at targeting the anti-tumour immune response, such as immune checkpoint inhibitors (ICI s), which have been used to treat breast cancer. PD -L1 status in immune cells should be examined when prescribing ICI s for breast cancer. This highlights the importance of studying the characteristics of the tumour microenvironment, the main approach being to uncover its heterogeneity. The aim of this study was to investigate the subpopulation composition of PD -L1-positive lymphocytes in the tumour microenvironment, separately in each luminal subtype of BC, and to compare it according to the PD -L1 status of the tumour. Material and Methods. Fifty-two primary tumour samples were obtained from patients with invasive luminal A, luminal B HER2- and luminal B HER2+ subtypes of breast cancer (T1–2N0–1M0). No drug therapy was administered prior to surgery to any patient in this study. Cytotoxic lymphocytes (CTL s), B lymphocytes, T helper lymphocytes, T regulatory lymphocytes and their PD -L1 expression in tumour tissue samples were assessed by flow cytometry, and tumour PD -L1 status was determined by Ventana SP 142 immunohistochemistry. Results. All of the key lymphocyte populations we identified were present in almost all patients. The number of PD -L1-positive Th2 lymphocytes was significantly higher in the luminal A and luminal B HER2- BC samples compared to the luminal B HER2+ cases (р=0.0240 and p=0.0092, respectively). When the proportion of PD -L1-positive cells was calculated, the proportion of PD -L1-positive Th2 lymphocytes and T regulatory lymphocytes was significantly lower in luminal B HER2-compared to luminal A BC. Cytotoxic lymphocytes, Th2 lymphocytes and T-regulatory lymphocytes represented the predominant PD -L1-positive immune cells in the breast cancer microenvironment and were present in higher numbers in PD -L1-positive luminal B HER2-. Conclusions. Different lymphocyte populations, including those expressing PD -L1, can be found in the breast cancer microenvironment and there are differences in their numbers between different luminal breast cancers. This may explain the discordant prognostic and predictive value of the microenvironment in luminal breast cancer when considered as a single molecular subtype.