Retrotransposable elements (RTEs) are deleterious at multiple levels, and failure of host surveillance systems can thus have negative consequences. However, the contribution of RTE activity to aging and age-associated diseases is not known. Here we show that during cellular senescence LINE-1 elements (L1s) become transcriptionally derepressed and activate a type-I interferon (IFN-I) response. The IFN-I response is a novel phenotype of late senescence and contributes to the maintenance of the senescence associated secretory phenotype (SASP). The IFN-I response is triggered by cytoplasmic L1 cDNA, and is antagonized by nucleoside reverse transcriptase inhibitors (NRTIs) that inhibit the L1 reverse transcriptase (RT). Treatment of aged mice with the NRTI lamivudine downregulated IFN-I activation and age-associated inflammation in several tissues. We propose that RTE activation is an important component of sterile inflammation that is a hallmark of aging, and that L1 RT is a relevant target for the treatment of age-associated disorders.

LINE-1s are active human DNA parasites that are agents of genome dynamics in evolution and disease. These streamlined elements require host factors to complete their lifecycles, whereas hosts have developed mechanisms to combat retrotransposition’s mutagenic effects. As such, endogenous L1 expression levels are extremely low, creating a roadblock for detailed interactomic analyses. Here we describe a system to express and purify highly active L1 RNP complexes from human suspension cell culture and characterize the co-purified proteome, identifying 37 high-confidence candidate interactors. These datasets include known interactors PABPC1 and MOV10 and, with in-cell imaging studies, suggest existence of at least three types of compositionally and functionally distinct L1 RNPs. Among the novel findings, UPF1, a key nonsense-mediated decay factor, and PCNA, the polymerase-delta-associated sliding DNA clamp, were identified and validated. PCNA interacts with ORF2p via a PIP box motif; mechanistic studies suggest this occurs during or immediately after target-primed reverse transcription.