image: Figure 1. Schematic of the cancer-immunity cycle.
Credit: ©Science China Press
CD8+ cytotoxic T lymphocytes (CTLs) serve as central effectors in cancer immunotherapy by directly eliminating tumor cells. However, current clinical therapies face significant limitations. These challenges arise from the multi-step coordination of the cancer-immunity cycle, where defects in any step, such as antigen presentation escape, Treg-mediated suppression, or metabolic/mechanical abnormalities in the tumor microenvironment (TME)—can lead to immune failure. A deeper understanding of T cell fate determination mechanisms is critical for designing novel strategies to overcome immunotherapy resistance.
Prof. Bo Huang from the School of Basic Medicine published a review in Science Bulletin titled "T Cell-Based Cancer Immunotherapy: Opportunities and Challenges". This work explores recent advances and challenges in T cell immunotherapy, emphasizing core regulatory factors influencing T cell differentiation and function, key transcription factors governing T cell fate, and the role of metabolic pathways and metabolites in shaping the epigenetic programs of tumor-infiltrating T cells.
I. The Cancer-Immunity Cycle
The cancer-immunity cycle theory posits that T cells exert antitumor functions through coordinated steps rather than in isolation. The cycle begins with the activation of naïve T cells by antigen-presenting cells (APCs), primarily dendritic cells (DCs) in tumor-draining lymph nodes (TdLNs). Activated T cells proliferate, migrate, and infiltrate tumor sites to recognize and kill malignant cells. Antigens released from dead cells trigger subsequent rounds of antigen presentation and T-cell stimulation, sustaining immune responses during tumor evolution (Figure 1).
II. Key Steps in T Cell-Mediated Antitumor Immunity
1. Tumor Antigen Processing and Presentation
Antigen processing and presentation are essential for adaptive immunity, as T cells cannot recognize free antigens directly. The specialized APCs like classical type 1 DCs (cDC1s) can cross-present exogenous tumor antigens on MHC-I molecules, effectively activating CD8+ T cells and boosting anti-tumor immune responses. Tumor antigens, including tumor-associated antigens (TAAs) and tumor-specific antigens (TSAs), are limited by poor immunogenicity and immune evasion. CAR-T therapy in solid tumors is hindered by the scarcity of tumor-specific targets and "on-target, off-tumor" toxicity. TCR-T cells can recognize low-density antigens using strategies such as MS immunopeptidomics, AI prediction, and high-throughput techniques (e.g., T-Scan, ELISpot). Exploring non-canonical antigens (e.g., RNA splice variants, bacterial proteins) may offer new avenues for cancer vaccines and adoptive cell therapies.
2. Four Signals for T Cell Activation and Differentiation
Dendritic cells (DCs) activate CD8+ T cells via MHC-I-mediated antigen presentation. A simplified framework for effective T cell activation and differentiation in antitumor immunity involves four integrated signals: TCR stimulation (Signal 1), co-stimulation (Signal 2), cytokine signaling (Signal 3), and metabolic cues (Signal 4) (Figure 2).
3. T Cell Migration and Tumor Infiltration
Activated effector T cells (Teff) migrate to tumors via chemokine and integrin signaling, navigating along extracellular matrix (ECM) fibers. However, physical barriers (e.g., cancer-associated fibroblasts [CAFs], dense ECM) and metabolic stressors (e.g., hypoxia, high lactate) restrict T cell infiltration and function. Tumors are classified as immune-inflamed, immune-excluded, or immune-desert, each with distinct immunotherapy responses. Strategies to enhance infiltration include CAF-targeting CAR-T cells, ECM-degrading enzymes, nano drug delivery systems, and metabolic modulators (e.g., VEGF inhibitors, STING agonists). However, toxicity risks necessitate careful evaluation. Overcoming infiltration barriers is pivotal for optimizing cancer immunotherapy.
4. Cytotoxic Mechanisms of CD8+ T Cells
CTLs eliminate cancer cells through direct and indirect mechanisms. Direct killing relies on the immune synapse (IS), where CTLs release perforin and granzymes upon TCR-antigen recognition. Perforin pores enable granzyme entry, inducing apoptosis. IS optimization enhances CAR-T efficacy, though trogocytosis-mediated antigen loss may compromise function. CTLs also secrete supramolecular attack particles (SMAPs) or activate Fas-FASL pathways. Bystander effects kill adjacent tumor cells but risk off-target toxicity. Tumor FAS expression predicts CAR-T efficacy. IFNγ and TNFα from CTLs amplify antitumor immunity, though IFNγ may promote immune escape. Cytokine release syndrome (CRS) underscores the need for safer therapeutic regimens.
5. Mechanobiology in T Cell Function
Mechanical forces regulate T-cell activation, migration, and effector functions (Figure 3). During IS formation, TCR-pMHC interactions are mechanically tuned, influencing signaling and self-tolerance. Techniques like atomic force microscopy (AFM) and high-throughput single-cell assays dissect these forces. Mechanical stress at the IS enhances perforin activity and pore formation. Soft tumor cells evade killing due to reduced stiffness impairing perforin efficiency. TME stiffness also drives exhaustion via Piezo1-mediated Osr2 activation, suppressing cytotoxic genes. Modulating T cell mechanobiology may advance therapies for T cell leukemia and solid tumors.
6. Transcriptional Regulation of CD8+ T Cell Fate
CD8+ T cell fate is governed by TCR signals, co-stimulatory molecules, and cytokines, with transcription factor (TF) networks dictating effector vs. memory differentiation (Figure 4). AP-1, NFAT, IRF4, and BACH2 cooperatively or antagonistically regulate exhaustion and memory formation. AP-1 (JUN/FOS) and IRF4 drive effector differentiation via T-bet/BLIMP1 but also promote exhaustion. BACH2 sustains stem-like memory states via AKT/FOXO1. TCF1/LEF1 and Wnt signaling are critical for memory T cell maintenance.
In the TME, hypoxia-inducible factors (HIFs) promote glycolysis and exhaustion. Tumor metabolites (e.g., (R)-2-hydroxyglutarate) and ionic stress (high Na+/K+) exacerbate exhaustion via NFAT5. Immunosuppressive cells (Tregs, MDSCs) further suppress T cell function through transcriptional interference.
7. Metabolic and Epigenetic Regulation of T-Cell Differentiation
Metabolic pathways and epigenetic modifications critically shape T cell function, memory formation, and exhaustion. Glycolysis, TCA cycle activity, NAD+/NADH ratios, lactate, and ketones influence histone acetylation and DNA methylation. Memory T cells (Tmem) rely on oxidative phosphorylation and fatty acid oxidation, while exhausted T cells (Tex) exhibit metabolic reprogramming and irreversible epigenetic "scarring." Targeting epigenetic enzymes (e.g., TET2, DNMT3A) may enhance CAR-T efficacy but requires caution due to genomic instability.
III. Conclusion
The advancement of immunotherapy faces challenges rooted in the complexity of human immunity and multifaceted barriers within the cancer-immunity cycle. These include neoantigen loss, metabolic stress, physical barriers, immunosuppressive cells, and T-cell exhaustion. A deeper understanding of T cell differentiation, strategies to overcome immune barriers, and insights into systemic immune health will provide the theoretical and technical foundation for effective, personalized T cell-based therapies.