TME conditions promote CD8+ T cell exhaustion
The TME drives CD8+ T cell exhaustion and significantly impedes their ability to elicit effective anti-tumor responses. Immunosuppressive regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs) facilitate an environment promoting T cell exhaustion. These cells secrete immunosuppressive cytokines, such as IL-10 and TGF-β, which hinder the proliferation and effector function of CD8+ T cells26,27.
Fibroblasts drive immunosuppression through cytokine/chemokine secretion and extracellular matrix (ECM) remodeling, thus critically shaping T cell infiltration and functionality28. The direct interactions between fibroblasts and CD8+ T cells are mediated by immune checkpoint molecule expression. Notably, in response to pro-inflammatory cytokines or oncogenic stimuli, TME fibroblasts express high levels of PD-L1, which directly interacts with PD-1 on CD8+ T cells, and subsequently inhibits their activation, clonal expansion, and cytotoxic potential. Immunohistochemistry has confirmed fibroblast PD-L1/T cell PD-1 co-localization in tumor regions29. Furthermore, the expression of other inhibitory ligands, such as B7-H4 and VISTA, on fibroblasts intensifies immunosuppression and exacerbates T cell exhaustion.30,31. Fibroblasts profoundly influence CD8+ T cell function by altering local cytokine and chemokine levels. These cells secrete various factors, such as TGF-β, which fosters an immunosuppressive environment and induces T cell exhaustion32. Moreover, fibroblasts modify the ECM, creating physical barriers that can impede T cell motility and access to APCs, thereby exacerbating T cell exhaustion and immunological tolerance.
Hypoxia, a TME hallmark, further exacerbates T cell exhaustion. Hypoxic conditions within tumors stabilize hypoxia-inducible factor 1α, a transcription factor that reprograms T cell metabolism, ultimately driving the cells into a state of metabolic insufficiency.33. This metabolic stress impairs T cells’ ability to sustain effector function, and promotes an exhausted phenotype characterized by upregulation of inhibitory receptors, such as PD-1 and CTLA-434,35.
Adenosine, another suppressive factor, is abundant in the TME. Tumor cells and associated stromal cells express ectonucleotidases, such as CD39 and CD73, which convert ATP to adenosine. Adenosine binding to the A2A receptor on T cells triggers signaling cascades that diminish the cells’ cytotoxic activity and proliferative ability36,37. Consequently, the CD8+ T cells have diminished ability to recognize and kill cancer cells.
In addition, the TME is rich in reactive oxygen species (ROS) and other oxidative stress-inducing agents that limit T cell function. ROS contribute to energy dysfunction by impairing TCR signaling and downregulating the expression of important co-stimulatory molecules38. Oxidative stress induces the expression of additional inhibitory receptors and ligands, and consequently perpetuates the cycle of exhaustion39.
The antigenic burden within the TME, characterized by the continued presence of tumor antigens, also plays a critical role in T cell exhaustion. Continual antigen exposure without adequate co-stimulation or assistance from other immune cells leads to progressive loss of T cell function40. This phenomenon is characterized by a hierarchical loss of cytokine production, diminished proliferative potential, and sustained expression of exhaustion markers.
The interplay between lactate accumulation (via tumor glycolysis) and histone deacetylase (HDAC) activation is a novel axis of TME-induced exhaustion. Lactate, a byproduct of Warburg metabolism, directly inhibits histone acetylation at effector gene loci and consequently silences effector gene expression41. Concurrently, targeting HIF-1α abrogates PD-L1-mediated immune evasion by suppressing PD-L1 expression on malignant and myeloid regulatory cells, and ultimately causes tumor rejection42. This dual metabolic-epigenetic suppression suggests that therapies combining LDHA inhibitors (to decrease lactate) with HDAC inhibitors (to restore acetylation) have potential to synergistically rejuvenate exhausted T cells.
Collectively, the TME orchestrates multifaceted, dynamic processes that drive CD8+ T cell exhaustion. Understanding these mechanisms is critical for developing therapeutic strategies that reverse T cell dysfunction and enhance anti-tumor immunity. The interplay among various cellular and molecular components within the TME creates a complex network that perpetuates T cell exhaustion, thus posing major challenges in effective cancer immunotherapy.
Strategies to reverse CD8+ T cell exhaustion in the TME
The TME comprises an intricate network of stromal cells, immunosuppressive entities, metabolic restrictions, and hypoxic conditions, which significantly impede effective anti-tumor immunity. Modulation of these components has been found to reinvigorate exhausted CD8+ T cells and to restore their cytotoxic potential against malignancies (Figure 2).
Strategies to revise CD8+ T cell exhaustion in the TME. (A) TME preconditioning. Preconditioning of the TME with radiation or chemotherapy to decrease immunosuppressive cell populations and enhance antigen presentation potentiates the effects of adoptively transferred T cells. (B) TME. TME cells can be targeted to prevent T cell exhaustion. CD8+ T cell activity can be enhanced through strategies targeting Treg-specific pathways such as CTLA-4 or CD25; inhibiting MDSC recruitment and function with agents such as CSF1R inhibitors or chemokine receptor blockers; or reprogramming TAMs from the pro-tumoral M2 phenotype to the anti-tumor M1 state with agents such as CD40 agonists. (C) TME metabolic landscape modulation. Metabolic reprogramming, such as shifting from oxidative phosphorylation to glycolysis in T cells, preventing ATP conversion to adenosine production, or targeting cancer cells through alternative metabolic substrates (e.g., inhibition of lactate dehydrogenase A to decrease lactate production), further enhances CD8+ T cell responses. (D) Gut microbiome targeting. Gut microorganisms such as Bifidobacterium and Akkermansia muciniphila enhance ICI efficacy by promoting T cell priming through microbial metabolites. (E) Oncolytic viruses. Oncolytic viruses designed to selectively infect and lyse tumor cells, thus releasing tumor antigens and inducing local inflammation, provide another innovative approach for reshaping the TME. (F) Adoptive T cell therapies. CAR-T cells with dominant-negative TGF-β receptors and IL-2 or IL-33 secretion ability have shown promise in mitigating TME suppression. (G) Checkpoint inhibitors. Checkpoint inhibitors such as PD-1/PD-L1 blockers primarily neutralize immune checkpoint molecules on the surfaces of T cells, thus preventing them from binding ligands, and subsequently inhibiting T cell function or apoptosis. (H) Targeting hypoxia and VEGF pathways. VEGF inhibitors and inhibitors of hypoxia-inducible factor restore T cell efficacy. Normalizing aberrant tumor vasculature with agents such as VEGF inhibitors can improve oxygenation, subsequently increasing immune cell infiltration and function. ADP, adenosine diphosphate; AMP, adenosine monophosphate; ATP, adenosine triphosphate; CAR-T, chimeric antigen receptor T; CD, cluster of differentiation; CSF1R, colony-stimulating factor 1 receptor; CTLA-4, cytotoxic T-lymphocyte-associated protein 4; HDACs, histone deacetylases; ICIs, immune checkpoint inhibitors; IL-2, interleukin-2; IL-33, interleukin-33; MDSCs, myeloid-derived suppressor cells; MHCI, major histocompatibility complex I; PD-1, cell death protein 1; PD-L1, programmed death-ligand 1; SCFAs, short-chain fatty acids; TAMs, tumor-associated macrophages; TCR, T cell receptor; TGF-β, transforming growth factor beta; TME, tumor microenvironment; Tregs, regulatory T cells; VEGF, vascular endothelial growth factor.
One promising approach involves depletion or reprogramming of immunosuppressive cells, such as Tregs, MDSCs, and TAMs. Tregs, which normally maintain immune homeostasis, are co-opted by tumors, thereby suppressing effector T cell function. Strategies targeting Treg-specific pathways, such as CTLA-4 or CD25, mitigate these suppressive effects and enhance CD8+ T cell activity43. Similarly, inhibition of MDSC recruitment and function through agents such as colony-stimulating factor 1 receptor (CSF1R) inhibitors or chemokine receptor blockers has been found to alleviate T cell suppression44,45. TAMs can be reprogrammed from the pro-tumoral M2 phenotype to the anti-tumor M1 state with agents such as CD40 agonists, thereby fostering an environment conducive to T cell activity46.
Another strategy involves modulating the TME metabolic landscape. Tumors frequently create a metabolically hostile environment by consuming glucose and producing lactic acid, thereby impairing T cell function47. Enhancing nutrient availability by inhibiting cancer cell glycolysis48 or providing alternative metabolic substrates49 has been found to improve T cell function. Additionally, blocking the production of adenosine, a key immunosuppressive TME metabolite, or adenosine-mediated signaling further enhances CD8+ T cell responses50.
Hypoxia-inducible factors induce the expression of immunosuppressive molecules and metabolic alterations that hinder T cell function51. Targeting these pathways with hypoxia-activated prodrugs or inhibitors of hypoxia-inducible factor signaling restores T cell efficacy52. Moreover, normalizing aberrant tumor vasculature with agents such as VEGF inhibitors has been found to improve oxygenation, subsequently increasing immune cell infiltration and function53.
Adoptive T cell therapies, including treatments with CAR-T and TCR-T cells, have immense potential when combined with TME-modulating strategies. Engineering T cells to resist TME-induced exhaustion through the expression of dominant-negative receptors or metabolic reprogramming significantly enhances anti-tumor efficacy54. Furthermore, preconditioning the TME with radiation or chemotherapy to reduce immunosuppressive cell populations and enhance antigen presentation potentiates the effects of adoptively transferred T cells55,56.
Emerging evidence indicates that the gut microbiome is a critical regulator of anti-tumor immunity and CD8+ T cell efficacy. Bifidobacterium and Akkermansia muciniphila have been shown to enhance the efficacy of ICIs by promoting T cell priming through microbial metabolites (e.g., short-chain fatty acids, SCFAs)57,58. SCFAs epigenetically reprogram exhausted T cells via HDAC inhibition, thereby reactivating effector gene networks59. In contrast, dysbiosis of the gut microbiota, characterized by an overabundance of pro-inflammatory species, may exacerbate T cell exhaustion via IL-17-driven inflammation or catabolism of tryptophan into immunosuppressive kynurenine59. Microbiome-targeting interventions (fecal microbiota transplantation, probiotics, or diet) show therapeutic potential against exhaustion. Further exploration of the microbiome-TME interplay will be crucial for developing precision immunotherapies against CD8+ T cell exhaustion.
Within the immunosuppressive TME, fibroblasts drive T cell exhaustion through the secretion of immunosuppressive cytokines (TGF-β and IL-10) and ECM proteins60,61. These cells establish peritumoral biophysical barriers that impede immune recognition, thus potentiating tumor progression and metastatic spread while compromising therapeutic efficacy. Detailed understanding of these mechanisms has provided critical insights into how the immunosuppressive environment can be altered to reactivate T cell function and improve patient outcomes. Notably, checkpoint inhibitors (PD-1/PD-L1 blockers) effectively disrupt fibroblast-mediated exhaustion mechanisms through neutralizing T cell inhibitory signals29. Furthermore, agents that degrade or inhibit the production of ECM components have been explored to potentially decrease the fibroblast-mediated barrier62. In addition, emerging approaches focus on fibroblast reprogramming toward pro-immunogenic phenotypes via genetic engineering or small-molecule modulation63. Future research should focus on exploring the specific mechanisms through which fibroblasts regulate T cell exhaustion via secreted factors and ECM alteration. Moreover, developing therapeutic strategies targeting fibroblasts, such as inhibiting PD-L1 or other immunosuppressive pathways, and interventions targeting ECM composition to enhance T cell infiltration and activity, will be key areas for future research. By understanding and strategically manipulating this interaction, the immune functions of CD8+ T cells can be enhanced. This approach promises more effective and sustained anti-tumor responses, and may open new avenues for cancer treatment and improving patient prognosis.
The use of oncolytic viruses designed to selectively infect and lyse tumor cells, thereby releasing tumor antigens and inducing local inflammation, is another innovative approach for reshaping the TME. These viruses can be engineered to express immunostimulatory molecules that further augment T cell responses64. Moreover, recent advancements in CAR-T cell engineering have demonstrated significant potential to overcome TME-induced exhaustion. For instance, “armored” CAR-T cells engineered to secrete cytokines such as IL-2 or IL-33 exhibit enhanced persistence and resistance to exhaustion by modulating the immunosuppressive microenvironment65. Furthermore, combinatorial approaches integrating CAR-T cells with dominant-negative TGF-β receptors have shown promise in mitigating TME suppression and preserving T cell effector function66. These innovations highlight the potential of next-generation CAR-T therapies to act synergistically with checkpoint inhibitors and reshape the TME for sustained anti-tumor responses.
Although ICIs and adoptive T cell therapies can restore anti-tumor immunity, they frequently trigger immune-related adverse events, such as colitis, pneumonitis, and dermatitis, owing to systemic T cell activation targeting healthy tissues. For instance, anti-CTLA-4 therapies are associated with higher rates of severe immune-related adverse events than anti-PD-1 agents, thus underscoring the need for precision in targeting exhaustion pathways67.
Despite the crucial role of the TME in driving CD8+ T cell exhaustion, several key issues remain to be addressed. First, the mechanism underlying synergy among immunosuppressive cells (Tregs/MDSCs/TAMs) in driving exhaustion, particularly their cytokine-mediated (IL-10/TGF-β) T cell regulation, must be elucidated. Second, hypoxia/adenosine/ROS affect T cell bioenergetics, and their crosstalk with immunosuppressive networks requires systematic analysis. Third, antigen-driven T cell dysfunction reversal strategies through antigen-presentation machinery and co-stimulation modulation must be prioritized. Fourth, stromal-mediated T cell exclusion mechanisms require mechanistic exploration to inform barrier-disrupting therapies. Pharmacological innovations should target immunosuppressive cell depletion, TME metabolic reprogramming, and hypoxia/oxidative stress alleviation to potentiate combination immunotherapies. Concurrently, gene-edited T cells with microenvironment resistance and TME-remodeling modalities (tumor vaccines/oncolytic viruses) are promising translational frontiers. Multipronged TME modulation strategies are expected to have transformative potential in reversing CD8+ T cell exhaustion and optimizing immunotherapeutic efficacy.
