The role of neutrophils in telomere induced senescence via bystander effects

Abstract

Senescence is classically defined as a state of permanent cell-cycle arrest. Senescence can occur in response to various stresses, which have been shown to act mainly through the activation of a DNA damage response (DDR). Senescence is characterised not solely by a cell-cycle arrest but also by increased production of Reactive Oxygen Species (ROS) and the development of a Senescence-Associated Secretory Phenotype (SASP). SASP components include growth factors, cytokines, chemokines and immune modulators and have been shown to contribute to senescence in an autocrine manner but also impact on the tissue microenvironment trough paracrine effects. Several studies have linked the SASP with immune surveillance suggesting that Natural Killer cells, monocytes and T lymphocytes CD4+ can effectively eliminate senescent cells. However, the interaction between neutrophils (the first innate immune responders to infection or injury) and cellular senescence has not yet been investigated. In this thesis, I have shown that neutrophils induce premature senescence in human fibroblasts in a telomere-dependent manner. My data indicates that hydrogen peroxide released by neutrophils damages telomeric DNA, thereby accelerating the rate of telomere shortening and contributing to the early onset of senescence. Consistently, pre-treatment with the antioxidant enzyme catalase, prevents neutrophil-induced telomere shortening and premature senescence. In addition, overexpression of the catalytic subunit of telomerase (hTERT), which maintains telomere length in cultured fibroblasts, is able to bypass neutrophil-induced premature senescence. In accordance with my in vitro results, I have shown that following acute liver injury (using CCl4) which is characterised by neutrophil infiltration, mouse hepatocytes show increased markers of telomere dysfunction, which can be prevented by neutralisation of neutrophils. Importantly, I have found that during the ageing process or after injection with lipopolysaccharide (LPS), mouse livers experience increased neutrophil infiltrations which positively correlate with markers of telomere-dysfunction. Finally, I have shown that senescent cells secrete factors which act as a neutrophil chemoattractant and that neutrophils preferentially induce cell-death in senescent cells but not young cells. These data suggest for the first time that neutrophils play an important role in the immune clearance of senescent cells. viii Altogether, my data propose that neutrophils act as a double-edged sword: on one hand, they can induce senescence by accelerating telomere shortening; on the other hand, they can be recruited to sites where senescent cells are present and accelerate their specific clearance.