Endothelial injury is a central driver of systemic IFN-β toxicity and is reversible through Jak inhibition.

Oncolytic viruses (OVs) encoding interferon beta (IFN-β) are under clinical evaluation. IFN-β is an attractive transgene due to its antiproliferative and immunomodulatory properties; however, the systemic effects of sustained IFN-β expression remain poorly defined. Here, we demonstrate that prolonged circulating IFN-β induces dose-dependent toxicity in murine models. Persistent IFN-β exposure led to elevated transaminases, thrombocytopenia, lymphopenia, and reduced hemoglobin. Histopathologic analysis revealed sinusoidal endothelial sloughing, microvascular coagulation with microthrombi, and hepatocellular vacuolation and degeneration, along with bone marrow hemorrhage and cell death. These toxicities were absent in type I interferon receptor knockout mice. Importantly, termination of IFN-β production via inducible caspase-9 in adeno-associated vector-transduced cells reversed toxicity and normalized serum transaminases. In Balb/c mice bearing MPC-11 myeloma tumors which are highly permissive to IFN-β-expressing OVs, treatment induced rapid tumor lysis, elevated plasma IFN-β, and mortality. Importantly, co-administration of ruxolitinib, a clinically approved JAK1/2 inhibitor, dampened IFN-β signaling and rescued mice from lethal toxicity. Collectively, these findings define the pathophysiological consequences of sustained systemic IFN-β exposure and identify ruxolitinib as a potential mitigation strategy to manage IFN-β-mediated toxicity during OV treatment.