Abstract B45: Loss of MerTK and Tyro3, but not Axl, substantially reverses the immune-suppressive tumor microenvironment in a syngeneic pancreatic cancer model
Authors: Kren Nancy , Holtzhausen Alisha , Graham Douglas , Pylayeva-Gupta Yuliya , EARP H SHELTON 
Published: December 2022

Abstract
Tyro3, Axl and MerTK (TAM) are a family of receptor tyrosine kinases (RTKs) that play physiologic roles in dampening innate immune responses, preventing chronic inflammation and autoimmunity. Their roles in potentially promoting tolerance in the immunosuppressive tumor microenvironment (TME) in the context of neoplasia are unclear. Pancreatic Ductal Adenocarcinoma (PDAC) is an aggressive cancer with poor prognosis, a notoriously immune suppressive TME and limited treatment options. Here we evaluate the role of TAM RTKs in the PDAC TME using orthotopic injection of a pancreatic cell line into mertk-/-,axl-/- ,tyro3-/- and mertk-/-/tyro3 double knockout( DKO) mice. The loss of MerTK or Tyro3 in the TME substantially attenuates orthotopic PDAC growth. Moreover, pancreatic tumors implanted in the mertk-/-/tyro3-/- DKO tumors completely regress suggesting that these two RTKs have at least some non-redundant suppressive activities. Furthermore, loss of MerTK or Tyro3 overcame resistance to anti-PD-1 treatment, with most tumors completely regressed. Surprisingly none of these effects are seen in the axl-/- mouse and, in fact ,a metastatic assay showed increased KPC liver metastases in axl-/- mice as compared to WT. In this assay, liver metastases were decreased in tyro3-/- and mertk-/- mice compared to WT mice. This contrasts with data from our lab and others showing that knock down of Axl in the tumor cells themselves diminishes metastatic potential in wild type mice. These data suggest that inhibition of Axl in host infiltrating cells and the tumor cells may have divergent actions in PDAC. Mechanistically, we utilized NanoString to identify significant changes in genes associated with antigen processing and presentation as well as an overall increase in immune response in mertk-/- mice. However, NanoString RNA expression data highlighted distinct signaling pathways in tumor bearing-tyro3-/- mice, again suggesting that the similar endpoint phenotypes (attenuated growth) are brought about by at least some non-redundant mechanisms. The growth attenuation in the mertk-/- and tyro3-/- is lost with antibody depletion of CD8a cells indicating the final common mechanism for MerTK or Tyro3 loss is CD8 cell dependent. These data highlight the fact that while these three receptors are often thought of as similar in action, deletion or inhibition may have distinct mechanisms or effects in the TME. These data also suggest that MerTK or Tyro3 targeted therapy in combination with anti-PD-1 merits exploration in PDAC; those experiments are ongoing with UNC-developed MerTK-selective inhibitors.
Citation Format: Nancy Kren, Alisha Holtzhausen, Douglas Graham, Yuliya Pylayeva-Gupta, H SHELTON EARP. Loss of MerTK and Tyro3, but not Axl, substantially reverses the immune-suppressive tumor microenvironment in a syngeneic pancreatic cancer model [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr B45.