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Pancreatic ductal adenocarcinoma (PDA) is an aggressive malignancy characterized by a paucity of tumor-proximal CD8 + T cells and resistance to immunotherapeutic interventions. Cancer-associated mechanisms that elicit CD8 + T-cell exclusion and resistance to immunotherapy are not well-known. Here, using a Kras- and p53-driven model of PDA, we describe a mechanism of action for the protumorigenic cytokine IL35 through STAT3 activation in CD8 + T cells. Distinct from its action on CD4 + T cells, IL35 signaling in gp130 + CD8 + T cells activated the transcription factor STAT3, which antagonized intratumoral infiltration and effector function of CD8 + T cells via suppression of CXCR3, CCR5, and IFNγ expression. Inhibition of STAT3 signaling in tumor-educated CD8 + T cells improved PDA growth control upon adoptive transfer to tumor-bearing mice. We showed that activation of STAT3 in CD8 + T cells was driven by B cell- but not regulatory T cell-specific production of IL35. We also demonstrated that B cell-specific deletion of IL35 facilitated CD8 + T-cell activation independently of effector or regulatory CD4 + T cells and was sufficient to phenocopy therapeutic anti-IL35 blockade in overcoming resistance to anti-PD-1 immunotherapy. Finally, we identified a circulating IL35 + B-cell subset in patients with PDA and demonstrated that the presence of IL35 + cells predicted increased occurrence of phosphorylated (p)Stat3 + CXCR3 - CD8 + T cells in tumors and inversely correlated with a cytotoxic T-cell signature in patients. Together, these data identified B cell-mediated IL35/gp130/STAT3 signaling as an important direct link to CD8 + T-cell exclusion and immunotherapy resistance in PDA., (©2020 American Association for Cancer Research.)