Cancers are composed of transformed cells, characterized by aberrant growth and invasiveness, in close relationship with non-transformed healthy cells and stromal tissue. The latter two comprise the so-called tumor microenvironment (TME), which plays a key role in tumorigenesis, cancer progression, metastatic seeding, and therapy resistance. In these regards, cancer-TME interactions are complex and dynamic, with malignant cells actively imposing an immune-suppressive and tumor-promoting state on surrounding, non-transformed, cells. Immune cells (both lymphoid and myeloid) can be recruited from the circulation and/or bone marrow by means of chemotactic signals, and their functionality is hijacked upon arrival at tumor sites. Molecular characterization of tumor-TME interactions led to the introduction of novel anti-cancer therapies targeting specific components of the TME, such as immune checkpoint blockers (ICB) (i.e., anti-programmed death 1, anti-PD1; anti-Cytotoxic T-Lymphocyte Antigen 4, anti-CTLA4). However, ICB resistance often develops and, despite the introduction of newer technologies able to study the TME at the single-cell level, a detailed understanding of all tumor-TME connections is still largely lacking. In this work, we highlight the main cellular and extracellular components of the TME, discuss their dynamics and functionality, and provide an outlook on the most relevant clinical data obtained with novel TME-targeting agents, with a focus on T lymphocytes, macrophages, and cancer-associated fibroblasts.

Approximately half of breast cancers (BCs), historically categorized as human epidermal growth factor receptor 2 (HER2)-negative, have low expression of HER2 defined as an immunohistochemical (IHC) score of 1+ or 2+ with negative in situ hybridization. Retrospective evidence suggest that HER2-low BC does not represent a distinct subtype from a biological and prognostic perspective. Nonetheless, it currently constitutes an essential biomarker to guide treatment selection and its introduction has led to reconsidering the binary classification of HER2 status according to which only patients with HER2-positive BC were thought to derive benefit from anti-HER2 therapies. Trastuzumab deruxtecan has recently been approved by the U.S. Food and Drug Administration for the treatment of patients with HER2-low metastatic BC based on the results of the DESTINY-Breast04 phase III trial, and other antibody–drug conjugates (ADCs) targeting HER2 are showing promising results. Treatment paradigms for both triple-negative and hormone receptor-positive BCs exhibiting HER2-low expression are thus rapidly evolving. Given its therapeutic implications, it is essential to accurately recognize the level of HER2 expression, and the development of more sensitive and reliable methods for HER2 testing and scoring is warranted, especially since the minimum threshold of HER2 expression required for T-DXd efficacy is currently under investigation. Given the signs of activity of T-DXd even in patients with HER2-0 (IHC 0) disease, an evolution in the way we define HER2-low is anticipated. Considering the expansion of the therapeutic armamentarium for BC patients, with several ADCs approaching the clinic, research efforts are needed to clarify whether the expression level of targets can enrich for responders to a given ADC as well as to understand mechanisms of resistance with the goal of optimizing the sequencing of ADCs.