The puzzle the review addresses is a long-standing one. Cellular senescence is a safeguard program: telomere erosion, oxidative damage, oncogene activation, or mitochondrial dysfunction all push cells into a state of permanent growth arrest. Senescent cells adopt a senescence-associated secretory phenotype, or SASP, releasing inflammatory mediators that should flag them for destruction by cytotoxic T cells and natural killer cells. But in aging tissues, these cells persist and multiply. Previous work showed that senescent cells upregulate PD-L1, which engages PD-1 receptors on immune cells and dampens their attack—mirroring the strategy tumors use. What remained unclear was which upstream signals drive PD-L1 upregulation in senescence. Wang and Qin argue that the DNA damage response, a conserved signaling network activated by genotoxic stress, is the key driver.

The DDR can be engaged by multiple senescence-associated stressors. Telomere dysfunction is the most extensively studied: telomeres, the protective caps at chromosome ends, shorten with each round of DNA replication when telomerase is insufficient. When they become critically short, they lose their protective structure and are recognized by the DNA repair machinery as double-strand breaks, initiating a DDR that becomes persistent in replicative senescence rather than transient. Oxidative stress, which accumulates with age, generates reactive oxygen species that damage nuclear DNA and activate the same signaling. Replication stress, arising from conflicts between DNA replication and transcription or from insufficient replication origins, also engages the DDR. In each case, the response persists and shapes cellular behavior well beyond immediate DNA repair—including the regulation of immune checkpoint molecules.

The review details two routes by which DDR signaling elevates PD-L1. The first is transcriptional, through the cGAS-STING pathway. DNA damage can generate cytosolic DNA, including micronuclei that arise from genomic instability. When a micronuclear envelope ruptures, this DNA is sensed by cGAS, triggering STING activation. STING recruits the kinase TBK1, which phosphorylates IRF3, leading to type I interferon production and subsequent PD-L1 transcription. In senescent cells, the same cascade is engaged by cytoplasmic chromatin fragments—bits of chromatin expelled from the nucleus that the cell mistakes for viral DNA. Beyond cGAS-STING, the central DDR kinases ATM and ATR contribute as well: following DNA damage, the ATM/ATR/Chk1 cascade activates STAT1 and STAT3 signaling and induces IRF1 expression, both of which drive PD-L1 transcription. ATM can also enhance PD-L1 through NF-κB activation. The tumor suppressor p53, another central DDR effector, influences PD-L1 in a context-dependent manner that may vary in senescent cells.

The second route is post-translational and operates independently of transcription. The cell cycle inhibitor p16, a hallmark of senescence, inhibits the kinases CDK4 and CDK6. This inhibition limits ubiquitin-dependent proteasomal degradation of PD-L1, stabilizing the protein on the cell surface even when transcriptional input declines. The two mechanisms cooperate: DDR-driven transcription raises PD-L1 production, while p16-mediated stabilization prevents its breakdown. This dual control may render senescent cells inherently difficult for the immune system to clear, because even if one layer of regulation is disrupted, the other can sustain PD-L1 expression.

At the cell surface, PD-L1 exerts its immunosuppressive effects by binding PD-1 on cytotoxic immune cells. Upon engagement, PD-1 recruits the phosphatases SHP-1 and SHP-2, which dephosphorylate key components of the T cell receptor signaling cascade, dampening T cell activation, proliferation, and cytokine production. The same pathway limits natural killer cell cytotoxic function. In aged lungs, p16-positive alveolar macrophages upregulate PD-L1 through this p16-CDK4/6 mechanism, and these PD-L1-high senescent macrophages create an immunosuppressive microenvironment that favors senescent cell accumulation. Single-cell analyses have revealed a correlation between p16 and PD-L1 expression in both immune and epithelial cells, more pronounced in older mice. Notably, senescent cells that display enhanced MHC class I machinery also express PD-L1, suggesting they can present antigens while simultaneously using PD-L1 to engage PD-1 on T cells—potentially driving T cell exhaustion.

In vivo studies provide functional validation. Activating anti-PD-L1 antibodies that engage Fcγ receptors on effector cells can eliminate PD-L1-positive senescent cells and reduce senescence-associated inflammation, both in naturally aged mice and in models of chronic lung inflammation. These findings indicate that PD-L1 is not merely a passive marker of senescence but a functionally relevant checkpoint that actively protects senescent cells from immune elimination—a senolytic vulnerability that could, in principle, be exploited therapeutically.

Recent work has uncovered a second, entirely different function of PD-L1 that operates not at the surface but in the nucleus. A fraction of PD-L1 translocates to the nucleus through an importin-dependent pathway involving the transport factors KPNA2 and KPNA6. Once inside, it binds directly to the cohesin subunit SA1 (STAG1) via its cytoplasmic tail, an interaction required for SA1 nuclear localization and proper telomere maintenance. Depleting either PD-L1 or SA1 produces comparable genomic instability phenotypes—aneuploidy, telomere cohesion defects, and reduced proliferation—and double knockdown shows no synergistic effect, suggesting they act in the same pathway. The circularity is striking: the same telomere dysfunction that triggers DDR and PD-L1 upregulation is also mitigated by PD-L1’s nuclear function. The response is mounted after the damage has occurred and feeds back to limit further damage—a form of damage control that allows a potentially harmful cell to persist while maintaining genomic integrity.

These observations converge on a self-reinforcing circuit. Telomere attrition and other stressors engage a persistent DDR, which upregulates PD-L1 through transcriptional induction and post-translational stabilization. Elevated PD-L1 allows senescent cells to evade immune clearance and accumulate. These cells secrete SASP factors that promote chronic inflammation, and the inflammatory milieu in turn induces DNA damage and telomere dysfunction in neighboring cells—mediated in part by SASP-driven recruitment of neutrophils, which transfer reactive oxygen species to neighboring cells and inflict telomeric damage. Each round amplifies the burden of senescent cells. The system becomes locked in a pro-inflammatory, immunoevasive state: inflammatory signals that should activate immune responses instead reinforce PD-L1-mediated suppression, accounting for the hallmark aging relationship between chronic inflammation and impaired immune function.

Intervention could, in theory, target any node in this loop—dampening DDR signaling, blocking PD-L1, or reducing inflammation—to restore immune clearance and lower senescent cell burden. But significant challenges remain. The causal relationship between DDR signaling and PD-L1 upregulation in vivo has not been fully defined, and whether DDR precedes PD-L1 upregulation in aging tissues or other senescence-associated signals act independently is unclear. The relative contribution of transcriptional versus post-translational mechanisms likely varies across tissues; p16-mediated stabilization has been demonstrated in alveolar macrophages and epithelial cells, but whether it operates with similar efficiency elsewhere remains unknown. Systemic PD-1/PD-L1 blockade carries a risk of autoimmune toxicity, and because the DDR-induced PD-L1 pathway operates in both senescence and cancer, interventions could yield benefits in both contexts or have unintended consequences depending on the cellular environment. Whether these strategies can be safely translated to humans is not yet clear, but the circuit model offers a clear rationale for testing them—and recasts senescence not as a passive endpoint but as an actively maintained, immunoevasive state that might, with the right molecular leverage, be reversed.

Subject of Research: DNA damage response signaling driving PD-L1-mediated immune evasion in cellular senescence and aging

Article Title: DNA damage response as a driver of PD-L1 expression in cellular senescence: implications for immune evasion and immunosenescence

Article References: Wang, Y., & Qin, L. (2026). DNA damage response as a driver of PD-L1 expression in cellular senescence: implications for immune evasion and immunosenescence. Frontiers in Cell and Developmental Biology, 14, Article 1974523. https://doi.org/10.3389/fcell.2026.1974523

Image Credits: AI Generated

DOI: 10.3389/fcell.2026.1974523

Keywords: cellular senescence, DNA damage response, PD-L1, immune evasion, immunosenescence, telomere dysfunction, cGAS-STING, ATM/ATR, p16, SASP, inflammaging, senolytics