Abstract PO-115: Effects of mesothelin exert on tumor microenvironment in pancreatic ductal adenocarcinoma

Background: High mesothelin (MSLN) expression has been associated with poor prognosis in patients with pancreatic ductal adenocarcinoma (PDAC); however, the effect of MSLN exert on PDAC tumor microenvironment (TME) is largely unknown. Methods: MSLN over-expressed (MSLN-OE) and knocked-down (MSLN-KD) PDAC cells were established to investigate the MSLN-PD-L1 relationship in PDAC. Using an MSLN overexpressed Panc02 (Panc02-MSLN) cell line, we established an orthotopic PDAC mouse model. All mice were euthanized on day 27 after cell inoculation and the tumor-infiltrating leukocytes (TILs) were isolated for mass cytometry (CyTOF) study. The gene expression correlation between MSLN and a panel of immune cell markers was assessed by using a dataset that includes149 PDAC patients in the TCGA-PAAD cohort. Results: MSLN upregulated PD-L1 expression at both mRNA and protein levels in PDAC. MSLN was able to enhance PD-L1 transcription by recruitment of NF-kB P65 to the PD-L1 promoter. The Orthotopic implanted Panc02-MSLN group showed significantly increased tumor size, tumor weight, upregulated PD-L1, and reduced survival when compared with the vector control group. CyTOF analysis of the total CD45+ hematopoietic cells infiltrated to the TME revealed 13 distinct cell subsets. In MSLN-high tumors, there is a significant expansion of neutrophils, eosinophils, PD-1+CD8+, and CD8+NK cells. In addition, pan T cells, regulatory T cells (Treg), CD8+, and PD-1+CD4+ cells were increased. A considerable decrease of tumor-associated macrophages (TAM), dendritic cells, B cells, NK cells, and a trend towards decreased Ly6C+CD8+ cells was also found. Within the TAM subset, a decreased iNOS+M1 to Relm-a+M2 ratio was detected. Furthermore, although there were increased CD8+NK cells, we found more immunosuppressive markers such as PD-1, CTLA4, LAG3, KLRG1, and TIM3, and a decreased activation marker CD69 and transcription factor T-bet. Intriguingly, gene expression correlation analysis with 149 PDAC patients cohort in TCGA dataset showed that MSLN expression negatively correlates with the mRNA levels of the B220 marker of B cells, the F4/80 marker of macrophages, as well as the CD4, CD8, CD69, granzyme B, IL-2 and IFN-γ markers of T-cell activation. Conclusions: We showed here that MSLN could significantly modulate the TME, possibly through the upregulation of PD-L1 expression. Compared with MSLN-low PDAC tumors, MSLN-high tumors recruit diverse immunosuppressive rather than immunoreactive leukocytes into tumor tissues to constitute an immunologically cold TME. Since targeting MSLN or PD-L1 as a monotherapy exhibited only modest objective response rates in the clinic, our findings may provide an alternative strategy by combining the depletion of MSLN with immune checkpoint inhibitors, which could result in improved treatment efficacy in PDAC. Citation Format: Dongliang Liu, Ethan Poteet, Zhengdong Liang, Emily Laplante, Lisa Brubaker, Sadhna Dhingra, Aleksandar Milosavljevic, Changyi Chen, Qizhi Cathy Yao. Effects of mesothelin exert on tumor microenvironment in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2021 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2021;81(22 Suppl):Abstract nr PO-115.