PD-1 is an immune checkpoint molecule with an immunoglobulin-like domain expressed on the surface of activated lymphocytes (T and B cells) and myeloid cells.16,17 It binds to its ligand PD-L1 and inhibits lymphocyte activation, is expressed in various cancer tissues, and plays a major role in immune evasion mechanisms of cancer cells.18,19 Nivolumab and pembrolizumab are fully human monoclonal antibodies to PD-1, and atezolizumab is a humanized immunoglobulin monoclonal antibody targeting PD-L1.20–22 PD-1 and PD-L1 inhibitors have shown significant antitumor effects in various cancers, including lung, gastrointestinal, melanoma, and BC, and are used as effective cancer treatment in clinical practice.23–25 Some of these immune checkpoint inhibitors show high efficacy and long-term survival after treatment discontinuation, whereas others show no efficacy at all, and biomarkers that can predict the efficacy are essential. Currently, PD-L1 IHC expression is used as a partner diagnostic biomarker to predict the efficacy of immune checkpoint inhibitors.26 However, no clear guidelines have been established for the type of PD-L1 antibody and the evaluation method. In addition, assessing the PD-L1 expression alone might be insufficient to evaluate the complex tumor immune mechanisms. Conversely, TILs are thought to play an important role in tumor immunity mechanisms in cancer tissues.3 Several retrospective studies have shown that the TIL status is useful in predicting the prognosis and response to drug therapy in cancer, and the assessment of TILs is increasingly becoming important in clinical practice.1,2,27 TILs alone, which reflect information about dynamic tumor immune mechanisms, have also been involved in predicting the response to immune checkpoint inhibitors.28,29 TILs are composed of different types of immune cells, and their different characteristic patterns are associated with the prognosis and prediction of responses to drug therapy.30 For this reason, many molecular biology studies have attempted to elucidate the molecular biological characteristics involved in TILs.31 However, due to the complexity of the TIL pathway, no critical discoveries have been made. In this study, 49 genes were found to be involved in TIL expression, which was significantly correlated with TIL and PD-L1 expressions. Our discovery of a gene set involved in TILs may result in the identification of new predictors of immune checkpoint inhibitor efficacy through further functional analysis. Furthermore, it may shed light on the mechanism of TIL induction and the overall complex tumor immunity.
Due to the complex molecular pathways involved in the immune response in cancer tissues, humans might be having difficulties clarifying this complex mechanism in detail by hand. Graham et al. have used artificial neural network algorithms to identify candidate proliferation-related genes and evaluate their association with clinicopathological features and outcomes in patients with BC.7 This innovative method can simultaneously treat a large number of multimodal data as genetic information (interacting variables and parameters such as DNA copy number abnormalities, RNA transcription, protein expression, etc.), clinicopathological characteristics, and prognostic data for ML to identify one of the predictive markers for treatment response and prognosis in BC.32 In this study, the morphological characteristics of BC, known as TILs, were combined with a large number of genetic information and analyzed using a digital platform with machine learning. This is a novel study that uses digital technology to unravel the complex molecular pathways that control cancer immunity. However, this technology still faces challenges, such as variations in the best computational models and the identification of potential biological mechanisms. Moreover, digital technology has been used in the development of multi-gene profiling tools to guide chemotherapeutic treatment.33 In addition, tools targeting tumor immunity have not yet been useful. The expression pattern of the TIL-related gene set identified in this study allows us to divide invasive BC into two subgroups. We found a significant difference in the prognosis between these two groups, which may be useful for the development of new gene signatures targeting tumor immunity.
In this study, the TIL-associated gene set was functionally associated with ICOS, which belongs to the CD28 family as PD-1 and CTLA-4; however, ICOS is different because it is expressed on activated T cells.34 ICOS binds to its specific ligand, ICOSL, which plays an important role in the differentiation of memory and effector T cells and immune responses.35 Depending on the types of cytokines secreted, the ICOS/ICOSL pathway may be involved in both antitumor effects and tumor growth. ICOS/ICOSL increases both CD4 + ICOS + T cells and CD8 + ICOS + T cells and parallelly increases the effector T-cell to Treg ratio.36–37 Conversely, ICOS/ICOSL signaling has been reported to enhance the immunosuppressive effects by promoting Treg differentiation from CD4 + T cells.38–39 T cells with high ICOS expression have been reported as an important biomarker for predicting the clinical response in patients with cancer treated with anti-CTLA-4 or anti-PD-1 antibodies. Mice treated with anti-ICOS antibodies have been reported to weakly respond to anti-CTLA-4 antibodies, suggesting that CTLA-4, PD-1, and ICOS costimulatory molecules may be intricately intertwined with the treatment of immune checkpoint inhibitors.40 In our analysis of the METEBRIC cohort, ICOS-mRNA expression was found to be significantly higher in BCs with high proliferative potentials, such as high histological grade, hormone receptor negative, and basal type. In this study, higher ICOS expression was associated with a better prognosis in triple-negative BC. Conversely, increased ICOS + Treg in TILs has been reportedly associated with poor prognosis in gastric cancer.41 The mechanisms of tumor growth and antitumor immunity may differ depending on the ICOS/ICOSL expression pattern in cancer tissues. Further studies on the function of ICOS in antitumor immunity mechanisms are required.