Triple-Negative Breast Cancer: Molecular Subtypes; Immune Escape; Limitations of Current Immunotherapy; and the BTLA/HVEM/CD160 Axis as an Emerging Target

Triple-negative breast cancer is an aggressive and heterogeneous type of invasive breast cancer (BC) in which the cancer cells lack estrogen and progesterone receptors, as well as expression of the human epidermal growth factor 2 protein. This cancer tends to grow and spread faster than other BC subtypes, and is associated with a poor prognosis due to early visceral and neurological recurrences. Multidisciplinary management includes surgery, chemotherapy, radiation therapy, and immunotherapy with targeted immune checkpoint inhibitors (ICIs). The introduction of ICIs has improved outcomes in patients with TNBC, particularly in the metastatic and neoadjuvant settings. Despite these advances, a significant proportion of patients either do not respond to treatment or develop resistance to it. TNBC mortality remains high, underscoring the urgent need to identify novel prognostic and predictive biomarkers to overcome resistance to immunotherapy. Following a brief overview of the clinical features and established biomarkers of TNBC, the current review focuses on immune checkpoint proteins (ICPs) beyond PD-1 and PD-L1, and on the potential use of soluble ICPs rather than the well-established membrane-bound assays. These soluble ICPs are produced through the alternative splicing of messenger (m)RNA or the cleavage/shedding of membrane-bound proteins. This is followed by an overview of current treatment and novel predictive targets in TNBC. Additionally, the involvement of the B- and T-lymphocyte attenuator (BTLA)/herpes virus entry mediator (HVEM)/CD160 pathway and its role in the pathogenesis of BC and TNBC are reviewed, highlighting the potential use of BTLA and HVEM as biomarkers.