Proinflammatory signaling in Ewing sarcoma is driven by retroelement activity and counteracted by reverse...

Ewing sarcoma (EwS) is a childhood malignancy driven by oncogenic fusion proteins, most commonly EWS::FLI1, and is characterized by paradoxical co-occurrence of inflammation and immunosuppression. Our study shows that LINE, SINE, and LTR/HERV endogenous retroviral elements (EREs) may drive local and systemic inflammation in EwS, and their expression is linked to EWS::FLI1. EREs are not only highly expressed in EwS tumor cells but also disseminated in extracellular vesicles (EVs), selectively targeting blood monocytes and stromal cells and inducing inflammatory responses and immunosuppressive phenotypes. We also demonstrate that some EREs, particularly LINE-1 and HERV-K, retain the ability to encode proteins and to reverse transcribe, coincident with the activation of cGAS-IFN-I, STAT3, and NF-κB antiviral and proinflammatory programs in tumor cells and target monocytes. Treatment with reverse transcriptase (RT) inhibitors abacavir (ABC) and lamivudine (3TC) reduced RT activity, inflammatory signaling, and cytokine release, suggesting a potential strategy for overcoming systemic inflammation and immunosuppression in EwS.