Cellular senescence is characterized by irreversible cell cycle arrest. The phenomenon of senescence was introduced by Leonard Hayflick >60 y back upon the observation that the proliferation of human fetal fibroblasts is limited to a doubling rate of 40 to 60×.1 More recent studies have delineated the critical role of senescence as a fundamental driver of several age-related diseases. In detail, senescent cells accumulate with aging, causing local and systemic inflammation and contributing to organ dysfunction. Senescence is typically initiated in response to various stress signals, including DNA damage, telomere shortening, oncogenic activation, radiation, oxidative and genotoxic stress, epigenetic changes, chromatin disorganization, perturbed proteostasis, mitochondrial dysfunction, inflammation, and nutrient deprivation.2 Senescent cells remain viable and metabolic active, developing a proinflammatory senescence-associated secretory phenotype represented by a repertoire of inflammatory cytokines and chemokines, proteases, stem cell toxins, reactive metabolites, noncoding nucleotides, and exosomes. Senolytics, agents that eliminate senescent cells by promoting apoptosis, have recently been tested for their potential to halt and improve various age-related conditions.3 Evidence has also accumulated that senescence has detrimental consequences on organ quality, immunogenicity, recovery after ischemia/reperfusion injury (IRI), and thus transplant outcomes.4-7 Clinically, augmented amounts of senescent biliary epithelial cells, for example, have been associated with both increased frequencies of acute cellular rejection and chronic rejections in liver transplant recipients.4,6 Cellular senescence has also been shown to play a pathogenic role in the development of interstitial fibrosis, tubular atrophy, and kidney graft deterioration.5 Our own study has demonstrated that senescence promotes the immunogenicity of organs from older donors. Treating organs before transplantation prolonged cardiac graft survival impressively, with outcomes comparable or even slightly improved compared with those from young donors.7 The work by Stuart J. Forbes and coworkers in Science Translational Medicine contributes to another milestone in the “senescence story” demonstrating that the depletion of senescent cells preserved biliary architecture and regenerative capacities in cold storage preserved livers.8 The authors observed that hepatocytes and cholangiocytes (epithelial cells representing the inner lining of bile ducts) behaved differently in response to cold storage. Although prolonged cold storage did not alter the gross anatomy of the liver, detachment of cholangiocytes into the lumen was observed. Additionally, DNA damage and cellular senescence became obvious in cholangicoytes, whereas apoptosis-related markers were upregulated in hepatocytes. At a molecular level, decoy receptor 2 (DCR2), a member of the tumor necrosis factor receptor superfamily, was identified as a major contributor to these events. Prolonged cold storage increased apoptosis in hepatocytes, but not cholangiocytes, associated with a constitutively high expression of DCR2 compared with a low expression in hepatocytes. In vitro analysis demonstrated that DCR2 expression confers resistance to FasL-related apoptosis in cholangiocytes. Although the augmented expression of DCR2 in cholangiocytes may prevent cell death, it may also promote cellular senescence during cold storage and stress induced by IRI. Notably, the percentage of DCR2 expression increased in cholangiocytes but not in hepatocytes after cold storage (Figure 1). To delineate downstream signaling events of DCR2 in more detail, the authors performed immunoprecipitation and mass spectrometry analysis with DCR2 interacting proteins. Senescence-related proteins, including TRIM28 and MIF, were found uncoupled from DCR2 in cholangiocytes subsequent to cold storage, whereas DCR2 mainly interacted with apoptosis-related elements in hepatocytes. The authors hypothesize that the differing mechanisms of DCR2 coupling/uncoupling from apoptosis- and senescent-related proteins observed in cholangiocytes versus hepatocytes initiate cell-specific senescent and apoptotic responses during static cold storage.FIGURE 1.: Static cold storage induces apoptosis in hepatocyte and senescence in cholangiocytes, resulting in impaired bililary regeneration with the clinical consequence of NAS. DCR2, a member of the TNF receptor superfamily, is constitutively highly expressed in cholangiocytes, compared with a low expression in hepatocytes. The percentage of DCR2 expression increased in cholangiocytes subsequent to cold storage. Created with BioRender.com. DCR2, Decoy receptor 2; NAS, nonanastomic biliary stricture; TNF, tumor necrosis factor.Further studies were then initiated to comprehend mechanisms in more detail, aiming to identify ligands while elucidating cell-specific signaling events downstream of DCR2. To prove their findings, the authors knocked out DCR2, promoting cholangiocyte proliferation while suppressing the expression of senescence-related markers. Interestingly, depleting senescent cells decreased DCR2 expression and improved preservation of biliary anatomy during cold storage. Notably, the authors also showed that normothermic machine perfusion with senolytics decreased DCR2 abundance in cholangicoytes, preserving the biliary tract of discarded human livers. To mimic clinical scenarios of deceased donations, donation after brain death and donation after circulatory death procurement were modeled, examining the impact on cold storage-induced organ damage. Donation after circulatory death procurement had been linked to a more prominent necrosis in the hepatic parenchyma together with increased senescent markers, whereas functional capacities in cholangiocytes were impaired when compared with the the donation after brain death model. Taken together, cholangiocyte senescence had been identified as a detrimental mechanism during liver cold storage, whereas senolytics preserved the texture and anatomy of the bile duct during cold storage preservation. IRI is in general considered an invertible component of transplantation occurring during organ procurement, transport, and implantation. IRI induces a series of proinflammatory responses, including increased production of reactive oxygen species and inflammatory cytokines, mitochondrial dysfunction, and electrolyte imbalance. All these factors have been implicated in accelerated senescence. Recently, it has also been shown that IRI induces senescence in both cardiomyocytes and interstitial cells in a mouse experimental cardiac infarction model. Treatment with senolytics after IRI improved cardiac function and decreased scar size.9 Although senolytic treatment before IRI had no effects on cholangiocyte proliferation and bile duct morphology in Forbes’s study, aspartate aminotransferase was significantly reduced by senolytic administration. Machine perfusion of solid organs has gained much attention in the past 2 decades. Rather than keeping organs in an icebox, they are continuously and, in most cases, pulsatile perfused at varying temperatures (hypothermic at 4 °C, subnormothermic at 20 °C, or under normothermic conditions at 37 °C). The continuous perfusion provides improved penetration of preservation solution, delivery of oxygen and nutrients within the organ, and removal of toxic metabolites. During machine perfusion, physiological parameters specific to the organ can be measured and biomarkers released in the perfusates can assess viability. More importantly, this approach provides the opportunity to treat and improve organ quality while awaiting transplantation.10 Many different pharmacological and biological approaches, nanotechnologies, or hemadsorption techniques have been applied in several experimental and a few clinical studies. Accumulating evidence, including our own work, has shown that senolytics reduce the burden of senescent cells, alleviate age-related organ dysfunction and inflammation, improve recovery from injury, and thus enhance transplant outcomes. Further clinical and mechanistic studies will need to prove the effectiveness of adding senolytics to the perfusion solution while defining optimized conditions (hypo-, sub-, or normothermic settings), agents, and timing. Moreover, pharmacological and mechanistic interaction between senolytics and immunosuppressive drugs require consideration when administering them to transplant recipients as some senolytics have shown synergistic effects with immunosuppressants. Dasatinib, for example, a tyrosine kinase inhibitor targeting the Src family, interferes with T-cell receptor signaling and glucocorticoid-mediated responses. Calcineurin inhibitors, including tacrolimus, have shown synergistic effects with the senolytic agent Panobinostat. The flavonoids quercetin and fisetin not only have senolytic effects but also operate as mammalian target of rapamycin inhibitors, thus potentially inhibiting T cell–mediated alloimmune responses.11 The evidence seems to accumulate, and there are indications that organ rejuvenation could be more than a pipe dream. Could it be possible to make a significant dent in the discrepancy between demand and supply in organ transplantation that has plagued the field for so long by tapping into available human organs that have been underused? There are certainly many remaining open questions, but there is also a glimmer of hope.