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Background Targeting the Transforming Growth Factor-β (TGF-β) pathway to reverse the immunologically ‘cold’ tumor microenvironment (TME) of pancreatic ductal adenocarcinoma (PDAC) remains clinically unsuccessful, warranting novel therapeutic strategies.Methods We developed a novel tumor-CD8 T cell co-culture to interrogate the TGF-β signaling pathways that promotes T cell-mediated cytotoxicity. We performed multiplex immunohistochemistry (mIHC) on human PDAC samples to correlate cell-type specific TGF-β pathway activation and CD8 T cell abundance. We employed specific pathway inhibitor and newly generated genetically-engineered mouse models (GEMMs) and confirmed our findings using single-cell RNA sequencing, flow cytometry and mIHC. We performed proteomics and various in vitro assays to establish the molecular mechanisms.Results We identify TGF-β-activated kinase 1 (TAK1 or MAP3K7) as an aberrantly activated kinase in human and mouse PDAC tissues that is associated with T cell dysfunction. Pharmacological inhibition of TAK1 with Takinib, or genetic deletion of MAP3K7 in autochthonous p48-Cre;TP53flox/flox;LSL-KRASG12D GEMM, enhances intratumoral CD4+ and CD8+ effector T cell infiltration and renders immune checkpoint blockade (ICB) effective. Mechanistically, TAK1 inhibition induces DNA damage and cytoplasmic DNA leakage, which activates the cyclic GMP-AMP synthase-Stimulator of Interferon Genes (cGAS-STING) DNA sensing pathway, triggering inflammatory responses that promote adaptive immune cell infiltration. At the molecular level, TAK1 phosphorylates Ephrin Receptor A2 (EphA2) at Serine 897, which in turn phosphorylates RAD51 at Tyrosine 315, a key DNA repair protein involved in homologous recombination.Conclusions We uncover TAK1 as a critical mediator in maintaining genomic integrity and highlights its potential as a therapeutic target to induce an inflamed TME that sensitizes PDAC to ICB.