Post-translational modifications orchestrate CD8+ T cell exhaustion from signal initiation to epigenetic locking

A new review published in Frontiers in Immunology provides a mechanistic framework for understanding how post-translational modifications (PTMs) drive CD8+ T cell exhaustion (搜索)搜索该疾病, tracing the process from initial signal transduction through metabolic reprogramming to terminal epigenetic locking. The authors identify five "key" PTMs—phosphorylation, ubiquitination, acetylation, glycosylation, and lactylation—selected based on their prevalence in the exhaustion literature, their mechanistic relevance to the signal-metabolism-epigenetic axis, and, notably, their therapeutic tractability, as pharmacological modulators for these pathways are already in clinical development or represent promising drug targets.

Phosphorylation as the initiation hub

Phosphorylation is described as the earliest activated modification in the PTM-driven model, converting sustained T cell receptor (TCR) signals into intracellular cascades that lay the foundation for metabolic reprogramming and epigenetic regulation. Persistent TCR signals continuously activate key kinases such as LCK and ZAP-70, promoting upregulation of inhibitory receptors including PD-1 (搜索)搜索该靶点, CTLA-4, and TIM-3 (搜索)搜索该靶点.

At the membrane level, PD-1 (搜索)搜索该靶点 ligand binding triggers phosphorylation of its ITIM (Y223) and ITSM (Y248) motifs, recruiting the phosphatase SHP-2, which dephosphorylates ZAP-70 and blocks downstream TCR signaling. Concurrently, ERK-mediated phosphorylation of PD-1 at Thr234 recruits the deubiquitinase USP5, which removes K48-linked polyubiquitin chains and stabilizes the PD-1 protein, prolonging its membrane retention and enhancing inhibitory signaling.

At the transcriptional level, calcium signaling triggers NFAT dephosphorylation and nuclear entry, followed by re-phosphorylation by GSK-3β and CK1, sustaining PDCD1 and TOX (搜索)搜索该靶点 expression. Cytokine signals are integrated through STAT phosphorylation states, with increased STAT1/3 phosphorylation and decreased STAT5 phosphorylation together driving the transcriptome toward exhaustion.

Phosphorylation also dominates metabolic reprogramming. The PI3K/Akt/mTOR signaling axis undergoes abnormal phosphorylation in exhausted T cells (Tex), leading to failure to activate key glycolytic enzymes and insufficient energy supply. Under energy stress, AMPK is phosphorylated at Thr172, transiently promoting PGC-1α expression to maintain mitochondrial biogenesis, though this pathway is eventually inactivated under persistent stress, leading to mitochondrial dysfunction, ROS accumulation, and metabolic collapse.

Ubiquitination and ubiquitin-like modifications as signal stabilizers

Ubiquitination and ubiquitin-like modifications (UBLs) maintain protein homeostasis and stabilize inhibitory signals, converting transient signals into persistent protein-level changes. E3 ubiquitin ligases FBXO38 and KLHL22 promote PD-1 (搜索)搜索该靶点 polyubiquitination and degradation under normal conditions, but this regulation is inhibited in the tumor microenvironment, leading to PD-1 stabilization and sustained inhibitory signaling. The authors note that the role of FBXO38 in PD-1 regulation has recently been challenged, with a study by Dibus et al. reporting that FBXO38 does not interact with PD-1 and that its deficiency does not affect PD-1 surface levels under steady-state, ex vivo activation, or viral infection conditions.

At the transcriptional level, the E3 ligase KLHL6 catalyzes K48-linked polyubiquitination of TOX (搜索)搜索该靶点, targeting it for degradation. Persistent TCR signaling suppresses KLHL6 via the PI3K-AKT-FOXO1 axis, reducing TOX ubiquitination and promoting its accumulation, which accelerates progenitor-to-terminal exhausted cell differentiation.

The UBLs—SUMOylation, NEDDylation, and UFMylation—extend the network beyond classic ubiquitin degradation. SUMOylation reduces glycolysis and mitochondrial respiration rates, placing cells in a low-metabolic state and representing one important mechanism mediating PD-1 (搜索)搜索该靶点 inhibitor resistance. In lymphoma and pancreatic cancer models, SUMOylation inhibitors such as TAK-981查看药品详情 can reprogram the metabolism of exhausted tumor-infiltrating lymphocytes, restore cytotoxicity, and synergize with immune checkpoint blockade. NEDDylation directly modifies core glycolytic enzymes including LDHA, ENO1, and HK1, determining energy production efficiency and metabolic adaptability. UFMylation maintains ribosome function and endoplasmic reticulum stress tolerance, inhibiting the transformation of CD8+ T cells into an exhausted-like phenotype.

Glycosylation as a metabolic sensor and coupler

Glycosylation comprises N-glycosylation of membrane proteins and O-GlcNAc glycosylation of intracellular proteins, regulating receptor sensing and metabolic-transcriptional coupling respectively. FUT8-catalyzed core fucosylation of PD-1 (搜索)搜索该靶点 enhances PD-L1 binding, stabilizes inhibitory signaling, and reduces antibody efficacy. Galectin-9 binds specifically to glycosylated TIM-3 (搜索)搜索该靶点, an interaction requiring proper N-glycosylation of the TIM-3 IgV domain, triggering inhibitory signaling that increases PD-1, TIM-3, and LAG-3 expression while reducing IFN-γ production. This signaling axis operates independently of PD-1 and persists even during anti-PD-1 treatment, contributing to immunotherapy resistance.

At the intracellular level, the O-GlcNAc transferase OGT interacts with DNA demethylases TET2 and TET3, regulating their enzyme activity and chromatin localization to affect memory-related gene expression and stemness maintenance. Mannose supplementation can enhance OGT-mediated β-catenin glycosylation, stabilizing TCF-1 expression and helping retain the stemness and plasticity of progenitor-like exhausted T cells.

Acetylation and lactylation as the metabolic-epigenetic bridge

Acetylation uses acetyl-CoA as a donor, positioning it as the critical link between metabolism and epigenetics. In terminally exhausted T cells, ACLY produces acetyl-CoA from citrate, which KAT2A uses to deposit H3K27ac at exhaustion enhancers, reinforcing the program. In contrast, progenitor-like exhausted or memory T cells use ACSS2 to generate acetyl-CoA from acetate, favoring p300 and maintaining effector/stemness gene expression and plasticity. TOX (搜索)搜索该靶点 recruits the HBO1 acetylation complex, promoting histone acetylation and continuously activating downstream exhaustion-related genes in a positive amplification loop.

Lactylation serves as the terminal metabolic stress sensor, directly writing lactate accumulation and acidosis into histones and functional proteins. After lactate enters CD8+ T cells through the monocarboxylate transporter MCT11, it increases histone lactylation levels, catalyzed by p300 and removed by HDAC1-3. Specifically, H3K9la and H3K18la enrich at effector gene promoters (GZMB, PRF1, IFNG), reducing chromatin accessibility and suppressing transcription to promote terminal exhaustion. Lactylation also enhances METTL3 RNA binding, promoting m6A modification of JAK1 mRNA and sustaining STAT3 activation to reinforce immunosuppression.

Notably, lactylation shows cell-state dependence, exerting a protective effect in naive and memory T cells. In resting naive CD8+ T cells, H3K9la enriches in genes maintaining quiescence and stemness such as Bach2, Foxo1, and Klf2, while in memory T cells lactylation marks target stemness-maintaining genes including Lef1, Ccr7, and Il7r.

Unconventional PTMs and therapeutic implications

Beyond the five core modifications, unconventional PTMs including methylation, succinylation, and palmitoylation provide fine-tuning at key regulatory nodes. Methylation functions as a "terminal locker," with the histone methyltransferase SUV39H1 catalyzing deposition of the inhibitory mark H3K9me3 at memory-related genes such as Tcf7. The EZH2 inhibitor Tazemetostat查看药品详情 blocks inhibitory methylation marks, partially reversing the exhausted phenotype of CD8+ T cells, restoring stem cell characteristics, and enhancing the efficacy of adoptive cell therapy.

Succinylation directly converts metabolic intermediates into functional output: high concentrations of fumarate in the tumor microenvironment perform inhibitory succinylation on ZAP-70, blocking downstream TCR signaling, while CPT1A acts as a succinyltransferase of PD-L1, preventing its degradation and strengthening tumor immune escape. Palmitoylation connects membrane signals, mitochondrial metabolism, and epigenetics, with DHHC9-mediated TIM-3 (搜索)搜索该靶点 palmitoylation enhancing its stability and membrane localization to sustain inhibitory signaling.

Collectively, the review positions these PTMs as a coordinated regulatory network in which phosphorylation initiates, ubiquitination stabilizes, glycosylation senses, and acetylation and lactylation lock the exhaustion program into an epigenetically stable state—offering multiple pharmacologically tractable nodes for therapeutic intervention.