Abstract B019: Tumor-derived PTHrP molds an immunosuppressive microenvironment
Authors: Johnson Calvin P. , Ma Boyang , Pitarresi Jason 
Published: January 2024

Abstract
Purpose[JP1] : Molecular phenotyping of PDAC [PJ2] has revealed an aggressive squamous/basal/quasi-mesenchymal[JP3] subtype, clinically presenting as highly metastatic and harboring genetic amplifications of KRAS. Our previous work indicated that a squamous identity gene, Parathyroid Hormone Like Hormone (PTHLH; encoding for the PTHrP protein), is frequently co-amplified with KRAS and skews PDAC towards a squamous identity. Further, PTHrP is secreted by tumor cells and promotes tumor growth and metastasis through autocrine signaling. However, pathological analysis of PTHrP-null tumors indicated that the tumor microenvironment (TME) was re-organized upon loss of PTHrP. Thus, we hypothesized that PTHrP is a tumor-derived factor that shapes the TME through its systemic secretion and mobilization of immunosuppressive myeloid cells. Methods: In vivo experiments used KrasG12D; p53R172H; Pdx1-Cre; YFP (KPCY) and KPCY-PthlhloxP/WT (KPCY-Pthlhhet) for spontaneous tumor formation in immune competent mice. Orthotopic implantation experiments used Pthlh overexpressing KPCY tumor cells (herein KPCY-PthlhOE) or empty vector control cells (KPCY-PthlhEV). Tumor immunophenotyping was done using flow cytometry. Cytokine expression was performed using qPCR and ELISA. Results: In vivo investigation of tumors from KPCY-Pthlhhet mice revealed less MDSC and greater CD8+ T cell infiltration than KPCY controls, demonstrating the necessity of PTHrP for immunosuppression[PJ4]. Orthotopic implantation of KPCY-PthlhOE cell lines reduced overall TME immune cell recruitment, while simultaneously increasing the proportion of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) infiltration. A concomitant decrease in the frequency of T cells in the KPCY-PthlhOE tumors was also observed, showing the sufficiency of PTHrP to organize an immunosuppressive TME[PJ5]. Additionally, a reduction of PD-1+ and CD44high CD8+ T cells revealed they were less activated. Interestingly, recombinant human PTHrP was sufficient to induce MDSC mobilization in vivo.[JP6] To interrogate the mechanism of MDSC trafficking into the TME, cytokine production by was analyzed in vitro. KPCY-PthlhOE exhibited a pronounced increase and decrease of Cxcl1 and Cxcl10 expression, respectively, relative to the KPCY-EV control. Conclusions: The work herein describes a new mechanism for the mobilization and infiltration of immune suppressive MDSC in the tumor microenvironment, initiated by PTHrP. The PTHrP-dependent immunosuppressive TME excludes and inhibits effector immune cells, such as CD8+ T cells. This phenomenon has been described in squamous-like PDAC but may also apply to other squamous adenocarcinomas. [PJ7] Further elucidation of the mechanism by which PTHrP is the genesis of the immune suppressive TME may spur the development of novel treatments for these more clinically severe cancer subtypes.
Citation Format: Calvin P. Johnson, Boyang Ma, Jason Pitarresi. Tumor-derived PTHrP molds an immunosuppressive microenvironment [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Pancreatic Cancer; 2023 Sep 27-30; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(2 Suppl):Abstract nr B019.