From Oncolysis to Adaptive Immunity: Yellow Fever Virus 17D and Ruxolitinib Activate Antitumor Immune Responses in Pancreatic Cancer Models

Background: Pancreatic ductal adenocarcinoma (PDAC) remains one of the most lethal cancers, with a five-year survival rate of less than 12%. Oncolytic viruses are considered a promising immunotherapeutic approach, but their effectiveness is often limited by the innate interferon response, which is the main antiviral barrier in resistant tumors. A combination of an oncolytic virus with JAK/STAT pathway inhibitors has been proposed to overcome this resistance. Methods: In this study, we investigated the oncolytic and immunotherapeutic potential of the attenuated yellow fever vaccine strain YFV 17D in combination with the JAK1/JAK2 inhibitor ruxolitinib in a panel of six PDAC cell lines with diverse genetic profiles and JAK/STAT signaling activities and in a syngeneic immunocompetent PAN02 mouse model. Results: In vitro, we identified three distinct response patterns: synergistic enhancement of viral replication and cytopathic effect, lack of synergism due to absent interferon signaling, and increased viral replication without cytopathic effect. Despite different cell mutation profiles, the cell response patterns were determined by the basal JAK/STAT activity of each cell line rather than by specific KRAS or TP53 mutation. In vivo, the combination therapy significantly extended median survival compared to YFV 17D monotherapy and the control group and induced tumor regression in 62.5% of animals. Since no difference in intratumoral viral RNA was detected between the combination and monotherapy groups, we suggest that the antitumor effect in vivo was mediated not by enhanced viral replication, but by activation of adaptive immunity. This is indirectly suggested by increased intratumoral IL-12, a fourfold increase in CD8+ T cell infiltration, a reduction in CD4+ cells, and the generation of virus-neutralizing antibodies. No systemic cytokine surge, neurovirulence, or viral dissemination was observed, suggesting the safety of the proposed regimen. Conclusions: These findings clarify the immune mechanisms determining the efficacy of YFV 17D combined with ruxolitinib and establish the basis for personalized patient stratification by tumor interferon signaling status.