Abstract B051: Influence of MUC16 on the immune tumor microenvironment of PDAC
Authors: Rajesh Christabelle , Sagar Satish , Radhakrishnan Prakash 
Published: September 2024

Abstract
Pancreatic ductal adenocarcinoma (PDAC) is an aggressive disease. In most cases, PDAC initiation results from oncogenic KRAS mutations, that act in part by directly increasing levels of a glycoprotein called Mucin-16 (MUC16). MUC16 is virtually absent in the normal pancreas, expressed on tumor cells in &gt;65% of PDAC and is a marker of poor prognosis. The cell autonomous roles of MUC16 involve signaling through the ErbB-1/2/3 axes, and eliciting downstream AKT and GSK-3β activation to promote cell proliferation and survival. PDAC has a very interesting “immune cold” tumor microenvironment (TME) that is devoid of anti-tumor T cells and is instead flush with pro-tumor myeloid cells like macrophages. However, the role of MUC16 in influencing immune cells, particularly macrophages has not been investigated. Methods: We use immunohistochemistry to check for macrophage populations in spontaneous and orthotopic models of PDAC, KPC (LSL-Kras G12D/+ ; LSL-Trp53 R172H/+ ; Pdx-1- Cre) and a derived KPC-MUC16 knockout (KPC-M16KO). Orthotopic implantation with KPC and KPC-M16- cells was done to assess survival. In vitro cytokine screening from M16+/- cells was done to understand cytokine players in MUC16-mediated immunosuppression. Clodronate liposomes (CL) and neutral control liposomes (NL) were used to deplete macrophages in orthotopically implanted KPC and KPC-M16- tumor-bearing mice, and flow cytometry was conducted to profile macrophages and T cells in the pancreas and spleens at the experimental endpoint. Results: We show that KPC-M16KO tumors have decreased M2-like macrophages, and a modest increase in M1-like macrophages, leading to a high M1/M2 ratio suggesting reduced immune suppressed cells in the KPC-M16KO models. KPC-M16KO tumor-bearing mice showed longer survival compared to the KPC group. In addition, we detect low levels of CCL2 and Serpin-E1 in cell lines and tumor tissues from KPC-M16KO models suggesting that they could serve as potential regulators of macrophage infiltration and M2-like polarization downstream of MUC16. Siglec-E (murine homolog of human Siglec-7/9) was also downregulated in KPC-M16KO tumors, suggesting MUC16-Siglec mediated immunosuppression as another mechanism. CL administration (versus NL) resulted in significant macrophage depletion in the spleens of mice with KPC and KPC-M16KO tumors, however, such depletion was not robust in the pancreas. Immune profiling studies showed a high M1/M2 ratio in the spleen and pancreas of mice treated with CL, suggesting a tumor cell-macrophage crosstalk, that could mediate M2-like macrophage states in a macrophage-rich TME. Mice treated with CL have smaller tumors, and KPC-M16KO+CL group had the smallest tumors, indicative that dual disruption of MUC16 and macrophages lowers tumor burden. Conclusions: These results suggest that MUC16 plays a crucial role in modulating the tumor immune microenvironment by increasing the accumulation of immunosuppressive macrophages in PDAC.
Citation Format: Christabelle Rajesh, Satish Sagar, Prakash Radhakrishnan. Influence of MUC16 on the immune tumor microenvironment of PDAC [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research; 2024 Sep 15-18; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl_2):Abstract nr B051.