WT1-Targeted Oral Bifidobacterium longum Vaccine Enhances Checkpoint Blockade Efficacy in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) remains largely unresponsive to immune checkpoint inhibitors (ICI) due to its profoundly immunosuppressive tumor microenvironment. To overcome this resistance, we evaluated an oral WT1-targeted cancer vaccine using a recombinant Bifidobacterium longum strain (B. longum 420) engineered to display a partial WT1 antigen. Oral immunization with B. longum 420 activated WT1-associated cellular immunity, increasing IL-12-producing CD45⁺ cells and Th1/cytotoxic cytokine-producing T cells, and enhancing splenocyte cytotoxic activity against Pan02 cells. In a syngeneic Pan02 tumor model, the combination of B. longum 420 with anti-PD-1 and anti-CTLA-4 antibodies resulted in stronger antitumor effects and improved survival compared with each monotherapy, without marked body weight loss. An empty-vector control strain did not further enhance the efficacy of dual checkpoint blockade. Tumor analyses showed increased intratumoral CD8⁺ T cell density and expansion of effector, central memory, and PD-1⁺ Ki-67⁺ proliferating CD8⁺ subsets, together with treatment-dependent differences in intratumoral CD4⁺ T cell subset composition. These findings demonstrate that an oral WT1-targeted Bifidobacterium vaccine can potentiate dual checkpoint blockade and promote a more immunologically active T cell landscape in PDAC. To our knowledge, this is the first demonstration of this combination strategy in a syngeneic pancreatic cancer model.