The DNA Damage Response Induces PD-L1 Expression and Immune Evasion in Senescent Cells
Senescent cells are constantly created throughout life, but accumulate in aged tissues as a result of a growing age-related failure of the immune system to promptly clear these cells. Evidence suggests that one contribution to this lack of clearance is that senescent cells in aged tissues increasingly employ immune evasion markers. The research community has already mapped a range of immune evasion markers in the context of cancer, and at least some of these are also employed by senescent cells. This suggests that some of the approaches to therapy developed in the cancer research community may also help to enable the aged immune system to better clear lingering senescent cells.
Cellular senescence arrests cell cycle progression in response to diverse stressors. When such cells persist in aging tissues, they acquire a senescence associated secretory phenotype (SASP) that promotes chronic inflammation, tissue dysfunction, and age-related pathology. This raises a key question. If senescent cells display a pro-inflammatory profile that should recruit immune cells for clearance, why do they instead evade elimination and accumulate with age? The PD-1/PD-L1 immune checkpoint is known for its role in tumor immune evasion, and recent evidence indicates that it operates similarly in senescence. Senescent cells upregulate PD-L1, engage PD-1 on cytotoxic T cells and natural killer cells, and thereby dampen their own immune clearance.
The upstream signals that drive PD-L1 upregulation in senescent cells have not been fully defined. We propose that the DNA damage response (DDR), a conserved signaling network rapidly activated by genotoxic stress, acts as a key upstream driver of PD-L1 expression in senescence through both transcriptional and post-translational mechanisms, thereby linking senescence-associated genomic instability to immune checkpoint activation. The DDR can be engaged by multiple senescence-associated stressors, including telomere dysfunction, oxidative damage, and replication stress. DDR signaling drives PD-L1 expression through two distinct routes: transcriptional induction via the cGAS-STING pathway and post-translational stabilization through p16-mediated inhibition of CDK4/CDK6. The DDR-PD-L1 axis thus forms a self-reinforcing circuit that perpetuates senescent cell accumulation and immunosenescence.