Background: Although most patients with diffuse large B-cell lymphoma (DLBCL) achieve complete remission after first-line rituximab-containing immunochemotherapy, up to 40% of patients relapse and require salvage therapy. Among those patients, a substantial proportion remain refractory to salvage therapy due to insufficient efficacy or intolerance of toxicities. A hypomethylating agent, 5-azacytidine, showed a chemosensitizing effect when primed before chemotherapy in lymphoma cell lines and newly diagnosed DLBCL patients. However, its potential to improve outcomes of salvage chemotherapy in DLBCL has not been investigated.Results: In this study, we demonstrated the mechanism of 5-azacytidine priming as a chemosensitizer in a platinum-based salvage regimen. This chemosensitizing effect was associated with endogenous retrovirus (ERV)-induced viral mimicry responses via the cGAS-STING axis. We found deficiency of cGAS impaired the chemosensitizing effect of 5-azacytidine. Furthermore, combining vitamin C and 5-azacytidine to synergistically activate STING could be a potential remedy for insufficient priming induced by 5-azacytidine alone.Conclusions: Taken together, the chemosensitizing effect of 5-azacytidine could be exploited to overcome the limitations of the current platinum-containing salvage chemotherapy in DLBCL and the status of cGAS-STING has the potential to predict the efficacy of 5-azacytidine priming.