(769) PTHrP is a novel mediator of immune suppression in pancreatic cancer

Introduction/Rationale: Pancreatic ductal adenocarcinoma (PDAC) has the worst prognosis of common cancers. PTHrP (encoded by PTHLH) defines the most aggressive squamous PDAC subtype. We have shown PTHrP mediates tumor growth and metastasis. The impact of PTHrP on the tumor microenvironment (TME) remains unknown.

Methods: In vivo experiments used KrasG12D; p53R172H; Pdx1-Cre; YFP (KPCY) and KPCY-PthlhloxP/WT (KPCY-PthlhHet) genetically engineered mice or implantable cell lines. Orthotopic implantation experiments used Pthlh overexpressing KPCY tumor cells (herein PTHrPOE), PTHrPOE-Xcl1OE or empty vector control cells (Control). Tumor immunophenotyping by flow cytometry. IF microscopy performed on sample matched tumors. Immune mobilization studies quantified by flow cytometry of whole blood. Cytokine expression was performed using ELISA and cytokine array (Luminex). Transcriptomic analysis of bulk tumor RNA. NK cell killing assays used splenic NK cells co-cultured with target cells, function assessed with flow cytometry.

Results: Analysis of PTHrP-null tumors indicated that the tumor microenvironment (TME) was re-organized upon loss of PTHrP, suggesting PTHrP-mediated TME remodeling contributes to worse overall survival. Orthotopic implantation of (PTHrPOE) show Pthlh was sufficient to ablate dendritic cell (DC) and T cell recruitment to the TME. Transcriptomic and cytokine array analysis of bulk tumor revealed loss of a critical DC chemokine, Xcl1. Ex vivo analysis of NK cells show PTHrPOE cells inhibit NK cell activation and in vivo over expression of Xcl1 in PTHrPOE cells provides a survival benefit and imparts partial tumor control.

Conclusion: The work herein begins to elucidate a novel mechanisms of immune suppression in pancreatic cancer, driven by PTHrP. The PTHrP-dependent immunosuppressive TME excludes and inhibits effector immune cells, such as CD8+ T cells NK cells, and cDC1s. Intervention of PTHrP signaling in the TME may be a potent therapeutic strategy for the treatment of squamous PDAC.