Breast cancer (BC) is a prevalent cancer in women worldwide, with its mechanisms not yet fully understood (1). Immune cells play a crucial role from immunosurveillance in normal breast tissue to the progression of BC, including both primary and metastatic stages. The tumor microenvironment in BC shows an increase in immune cells like CD4+ and CD8+ granzyme B+ cytotoxic T cells, B cells, macrophages, and dendritic cells (DCs) (2). In estrogen receptor (ER)-positive tumors, neutrophils and natural killer (NK) cells are most positively associated, while resting memory T cells and CD8+ T cells are negatively associated. In contrast, ER-negative BC shows strong positive correlations with T regulatory and CD8+ cells, with similar negative correlations as ER-positive cases (3). Notably, even early-stage BC patients display exhaustion in tumor-associated CD8⁺ T cells (4), which are essential for eliminating pathogens and tumors (5). Chronic antigen exposure and inflammation in cancer can lead to CD8⁺ T cell exhaustion or altered differentiation (6), with exhausted T cells showing tumor reactivity and proliferation signatures (7). A higher CD8⁺ T cell exhaustion (TEX) score is linked to poorer disease-free survival in BC (8), highlighting the importance of CD8⁺ effector T cell activation versus exhaustion in BC progression and patient outcomes. Additionally, TEX can predict immunotherapy responses in ER-positive BC, underscoring its potential in treatment strategies (9). Recent studies have extensively explored the mechanisms of CD8⁺ TEX in BC. This article summarizes our current understanding of these mechanisms.

Tex was first observed in murine models of lymphocytic choriomeningitis virus (LCMV) infection (10). It represents T cell dysfunction with a progressive loss of effector functions during chronic infections and cancer (11). Tex cells exhibit elevated inhibitory receptors (PD-1, CTLA-4, TIM-3, TIGIT, LAG-3), reduced antitumor cytokines (IFN-γ, IL-2, TNF) (6, 11, 12), increased tumor-promoting chemokines, altered transcription factors (TCF1, T-bet, TOX), metabolic issues, and decreased proliferation and survival (6, 11).

In BC, immune cell distribution varies by tumor subtype and between stromal and parenchymal regions (2). CD8+ Tex are present in certain ER+ and triple-negative breast cancers (TNBC), creating a unique tumor microenvironment (TME) with higher IFN-γ activity. Increased CD8+ Tex infiltration is linked to poorer overall and relapse-free survival in premenopausal ER+ BC patients (13). Conversely, TNBC exhibits a more immunosuppressive environment compared to HER2+ or luminal types, with more regulatory T cells (Tregs), exhausted CD8+ T cells, and plasma cells (14).

IRs include PD-1, CTLA-4, TIGIT, TIM-3, LAG-3, which are hallmarks of TEX. Exhausted T cells often express high levels of IRs, which act as immunomodulators, limiting immunopathology and promoting tolerance to self-antigens. In BC, the continued expression of inhibitory receptors is critical for TEX.

In addition to cell surface IRs, immunoregulatory cells and cytokines in the TME—including myeloid-derived suppressor cells (MDSCs), TAMs, and Tregs—contribute to TEX. These immunosuppressive cells hinder immune rejection of malignant cells via cytokine secretion, promoting tumor progression and posing challenges to immunotherapy.

CXCR2⁺ MDSCs inversely correlate with infiltrating CD4⁺ and CD8⁺ T cells. These MDSCs facilitate BC progression and metastasis to lungs and lymph nodes, drive EMT via IL-6, and increase levels of immunosuppressive proteins (PD-1, PD-L1, LAG3, CTLA4, TIM-3) on T cells, contributing to their partial exhaustion mediated by IFN-γ (85).

TAMs are predominant immune cells in BC and can polarize into pro-inflammatory M1 or immunosuppressive M2 phenotypes (86–88). Progranulin (PGRN) promotes CD8⁺ TEX by inducing TAM polarization to M2 macrophages, which suppress T cell proliferation and activation via ICAM-1 interactions (89–91). Elevated CHI3L2 levels in TAMs correlate with poor prognosis in BC. Y-box binding protein 1 (YBX1) is positively associated with M2 macrophage infiltration and TEX markers IDO1 and CTLA4 in luminal BC (92).

IL1R2 in macrophages and BC cells contributes to an immunosuppressive TME. IL1β from TAMs activates IL1R2, increasing PD-L1 levels by promoting YY1 degradation. Inhibiting IL1R2 reduces macrophage recruitment, alters TAM polarization, decreases breast tumor-initiating cell self-renewal, and reduces CD8⁺ TEX (93). Sustained high levels of IL-2 in BC induce CD8⁺ TEX by persistently activating STAT5, which increases tryptophan hydroxylase 1 expression. This enzyme converts tryptophan to 5-hydroxytryptophan (5-HTP), activating the aryl hydrocarbon receptor (AhR) and leading to upregulation of inhibitory receptors and decreased cytokine production, impairing T cell function (94). Low IL-10 expression or loss of IL-10R-STAT3 signaling is linked to increased CD8⁺ TEX and reduced survival in BC patients (95). Conversely, in murine BC models, Heparanase (HPSE)-induced IL-10 upregulation promotes M2 macrophage polarization and TEX (96). Malignant BC cells transfer active TGF-β type II receptor (TβRII) via tumor-derived extracellular vesicles to CD8⁺ T cells, inducing SMAD3 activation and TEX (97). USP8 stabilizes TβRII by deubiquitination, enhancing its expression in plasma membranes and TEVs. USP8 promotes TGF-β/SMAD-induced EMT, invasion, metastasis, and facilitates TβRII⁺ circulating EVs to induce TEX and chemoimmunotherapy resistance (92). RAI14 levels correlate positively with M2 macrophage marker CD163 and TEX marker PD-1, indicating its association with M2 macrophage infiltration and TNBC (98).

Transcriptomic and epigenetic mechanisms play crucial roles in TEX within BC. Epigenetic programming is central to regulating various T cell subsets. In TNBC patients, dysfunctional PD-1⁺ CD8⁺ T cells are enriched with EOMES and nLSD1p. LSD1 promotes TEX by controlling the nuclear localization of EOMES through bivalent post-translational modification at lysine 641, directly affecting T cells (99).

TWIST1, a transcription factor that binds to the PD-L1 promoter and significantly accelerates the exhaustion and death of cytotoxic CD8⁺ T cells in BC (100). Additionally, TWIST1 induces EMT in BC cells via the ITGB1-FAK/ILK signaling pathways and related downstream networks (101). The expression of transcription factor IRF8 facilitates TAM acquisition, antigen presentation of cancer cells, and induction of CTL exhaustion in BC (102). Thymocyte selection-associated high-mobility group box (TOX), involved in TEX during chronic infection and cancer, is associated with better prognosis in BC when highly expressed, as confirmed by survival analysis (103). Bromodomain and extra-terminal (BET) proteins are important in regulating the PD-1/PD-L1 pathway in BC. They mediate interferon-γ (IFN-γ) secretion from activated T cells and IFN-γ signaling in TNBC to induce PD-L1 expression (104).

METTL3 enhances PD-L1 expression in BC cells post-transcriptionally through an m6A-IGF2BP3-dependent mechanism, promoting mRNA stability and affecting the efficacy of tumor immunotherapy (105). Glycosylphosphatidylinositol (GPI) anchor biosynthesis, a common post-translational modification, is elevated in BC patients with severe TEX. Exhausted CD8⁺ T cells exhibit higher levels of GPI anchor biosynthesis than normal CD8⁺ T cells (106).

A comprehensive evaluation of mitochondrial DNA methylation (MTDM) in BC indicates that patients with high MTDM have increased proliferation rates and elevated CD8⁺ TEX, potentially related to the secretion of growth differentiation factor 15 (GDF15) by malignant breast epithelial cells in a high MTDM state (107).

Metabolic reprogramming plays a pivotal role in TEX in BC. Metformin reduces tumor hypoxia and PD-L1 expression, enhances T cell function, and boosts the efficacy of immune checkpoint inhibitors, positioning it as a potential adjunct therapy for refractory tumors like TNBC. At high concentrations, metformin suppresses mTOR signaling and decreases transcription factors (T-bet, Eomes, STAT3), potentially limiting T cell proliferation and cytokine secretion. Notably, metformin reduces PD-1 expression in Jurkat cells and improves the PD-1/CD69 ratio in primary T cells, suggesting a potential to restore T cell activation and counter exhaustion (108).

In mouse BC models, a high-fat diet shifts PD-1^- CD8⁺ non-exhausted T cells to PD-1⁺ CD8⁺ exhausted T cells, fostering tumor progression (109). Hypoxia-driven metabolic reprogramming also exacerbates T cell dysfunction and exhaustion. Hypoxia induces HIF1α-dependent epigenetic changes that suppress effector gene expression in T and NK cells, leading to immune dysfunction. Moreover, HIF1α-independent mechanisms promote TEX by upregulating co-inhibitory molecules like TIM-3 or via metabolic stress. Studies show that hypoxia-induced TEX in TNBC models confers resistance to anti-PD1 therapy (110). Additionally, hypoxic stress generates tRNA-derived fragments (tRFs) that bind and displace the 3’ UTR of the oncogenic RNA-binding protein YBX1, inhibiting oncogene stability and suppressing BC progression (111).

Exosomes play a crucial role in intercellular communication, with cancer cell-derived exosomes promoting tumor progression by modulating the tumor microenvironment. Tumor cells secrete PD-L1 via exosomes (exoPD-L1), which binds to PD-1 on CD8⁺ T cells at metastatic sites, inhibiting T cell activation and proliferation, leading to their functional inactivation and immunosuppression (112–115). ICAM1, a glycoprotein involved in immune functions and cancer progression, is secreted in exosomes and regulates the immunosuppressive microenvironment (116). Its interaction with LFA-1 is vital for exoPD-L1 binding to PD-1 (117). Knockdown of ICAM1 in BC cells reduces exosome-induced inhibition of CD8⁺ T cells, suggesting its role in TNBC bone metastasis (116). BC-derived exosomes also suppress CD8⁺ T cell glycolysis via an AKT-mTOR-dependent mechanism, contributing to immune escape (118). In migrating cancer cells, PD-L1 accumulates at the trailing edge, forming migrasomes that can be internalized by adjacent cells, increasing PD-L1 expression and immune suppression, while also releasing chemokines to promote cell migration within the tumor microenvironment (119). The role of migrasomes in CD8⁺ TEX remains understudied.

In summary, TEX is characterized by high expression of IRs, influenced by immune cells and cytokines, with potential mechanisms including transcriptomic, epigenetic, metabolic, and exosome-related pathways ( Figure 1 ).

CD8⁺ TEX is critical in immune evasion and progression of BC, involving upregulation of inhibitory receptors (PD-1, CTLA-4, LAG-3, TIM-3), immunosuppressive cells and cytokines, transcriptional and epigenetic alterations, metabolic reprogramming, and exosome pathways. These interactions within the tumor microenvironment diminish T cell function, facilitating tumor growth and metastasis

Understanding these mechanisms is essential for developing effective immunotherapies. While targeting inhibitory receptors with immune checkpoint inhibitors shows promise, their limited efficacy highlights the need for comprehensive approaches. Future research should elucidate molecular pathways of TEX in BC, including specific transcription factors, epigenetic modifications, and metabolic processes. Exploring combination therapies that address multiple aspects of T cell exhaustion may enhance efficacy. Personalizing immunotherapies based on each BC subtype’s unique immune landscape could further optimize patient outcomes. Investigating the role of exosomes and their impact on CD8⁺ T cell function may reveal new therapeutic targets. Integrating these insights can lead to novel interventions to prevent or reverse CD8⁺ TEX, ultimately improving survival and quality of life for BC patients.