Abstract A024: Fibroblast-specific inhibition of p38α MAPK reprograms the tumor stroma to overcome therapeutic resistance in pancreatic cancer

Background: Pancreatic ductal adenocarcinoma (PDAC) remains highly resistant to chemoimmunotherapy due in part to its dense, immunosuppressive stroma. Cancer-associated fibroblasts (CAFs), the predominant cell type in the PDAC stroma, promote tumor progression and resistance by depositing aligned extracellular matrix (ECM), secreting pro-inflammatory cytokines, and excluding cytotoxic immune cells. Attempts to broadly deplete CAFs have failed clinically, underscoring the need to understand and selectively reprogram pro-tumorigenic CAF phenotypes. We identified p38α MAPK (p38α) as a central regulator of pro-inflammatory CAF activation and ECM remodeling in PDAC. We demonstrate that p38α MAPK signaling is upregulated in stroma-enriched PDAC and is essential for pancreatic stellate cell activation into pro-inflammatory CAFs, highlighting its functional significance in tumor stroma. Methods: p38α was inhibited or deleted in CAFs using pharmacologic and CRISPR-Cas9 approaches to assess effects on tumor-promoting gene expression, tumor cell proliferation, and invasion in vitro. A tamoxifen-inducible, fibroblast-specific knockout mouse model Col1a2 Cre/ERT2;Mapk14 flox/flox (CAF-p38 KO), was used to evaluate the in vivo impact of stromal p38α deletion on tumor growth, immune infiltration (via flow cytometry and scRNA-seq), and chemotherapy response. Results: Genetic or pharmacologic inhibition of p38α in CAFs resulted in significant downregulation of tumor-promoting and myeloid chemoattractant genes, including IL6, CXCL1, and CXCL8. In functional co-culture assays, p38α-deficient CAFs markedly reduced tumor cell proliferation, invasion, and organization of aligned ECM fibers, indicating impaired pro-tumorigenic stromal support. In vivo, fibroblast-specific CAF-p38KO mice revealed significantly reduced tumor burden and a transcriptional shift within the CAF compartment characterized by suppression of inflammatory signaling pathways and upregulation of ECM remodeling programs, consistent with a reversion to a tumor-restraining phenotype. Flow cytometry confirmed this stromal reprogramming, demonstrating decreased infiltration of myeloid-derived suppressor cells (MDSCs) and increased infiltration of activated intratumoral CD4+ and CD8+ T cells. These findings were corroborated by scRNA-seq, which revealed enhanced expression of T cell activation and effector function gene signatures. Notably, in the context of chemotherapy, CAF-p38KO mice exhibited improved therapeutic response and prolonged survival compared to controls, supporting the therapeutic potential of targeting stromal p38α to overcome treatment resistance in PDAC. Conclusion: These findings establish CAF-intrinsic p38α MAPK as a key driver of pro-tumorigenic stromal remodeling and immune evasion in PDAC. Targeted disruption of p38α reprograms the tumor stroma toward a less fibrotic, immune-permissive state, attenuates tumor progression, and enhances therapeutic efficacy, underscoring its potential as a rational target to overcome chemoimmunotherapy resistance. Citation Format: Camille Acevedo, Samara Singh, Edmond W. Box, Andrew Adams, Sayan Chakraborty, Karthik Rajkumar, Haleh Amirian, Anna Bianchi, Siddharth Mehra, Lucy Min, Fenghua Yuan, Iago C. Silva, Jashodeep Datta, Nagaraj Nagathihalli, Austin Dosch, Nipun Merchant. Fibroblast-specific inhibition of p38α MAPK reprograms the tumor stroma to overcome therapeutic resistance in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research—Emerging Science Driving Transformative Solutions; Boston, MA; 2025 Sep 28-Oct 1; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_3): nr A024.