Abstract A35: MERTK inhibition induces an anti-leukemia dendritic cell – T cell axis while TYRO3 inhibition protects by a separate mechanism
Authors: Huelse Justus M , Bhasin Swati S , Jacobsen Kristen M , Thomas Beena E , Chimenti Madison L , Baxter Travon A , Wang Xiaodong , Frye Stephen V , Henry Curtis J , Earp H. Shelton , Bhasin Manoj , DeRyckere Deborah , Graham Douglas K 
Published: December 2022

Abstract
The TAM family receptor tyrosine kinases TYRO3, AXL and MERTK are potential therapeutic targets in a variety of cancers. In our previous studies, MERTK inhibition in the leukemia microenvironment significantly prolonged survival in a syngeneic B-cell acute leukemia (B-ALL) model, implicating MERTK as a promising immuno-oncology target. Strikingly, Mertk-/- mice were almost completely protected against leukemia. Here, we probed the mechanisms of anti-leukemia immunity mediated by MERTK inhibition and evaluated roles for TYRO3 and AXL in the leukemia microenvironment. Single cell RNA sequencing and flow cytometry studies revealed an increase in CD8α+ dendritic cells (DCs) with enhanced antigen-presentation capacity in naïve and leukemia-bearing Mertk-/- mice. These cells were also increased in leukemic wild type (WT) mice treated with the MERTK inhibitor MRX-2843 (currently in phase I/Ib clinical trials). Additionally, CD8+ T cells expressing high levels of the T cell exhaustion marker Tox were decreased in mice treated with MRX-2843, implicating a CD8α+ DC – T cell axis in the anti-leukemia immune response stimulated by MERTK inhibition. Indeed, combined depletion of CD8+ T cells and CD8α+ DCs completely abrogated the anti-leukemia response in Mertk-/- mice, while immunity remained partially intact in mice with selective depletion of CD8+ T cells. Similarly, protection from leukemia was abrogated in Mertk-/- scid mice, which lack functional B and T cells and have defects in DC function. These data demonstrate a critical immunosuppressive role for MERTK in DCs of the leukemia microenvironment. Similar to Mertk-/- mice, B-ALL growth was almost completely prevented in Tyro3-/- mice, while Axl-/- did not impact leukemogenesis, implicating TYRO3, but not AXL, as an additional immunotherapeutic target. However, in contrast to Mertk-/- mice, CD8α+ DCs with enhanced antigen-presentation capacity were not significantly increased in Tyro3-/- mice compared to WT, indicating differences in the underlying mechanisms by which MERTK and TYRO3 contribute to the immunosurveillance of leukemia cells. Indeed, in vivo depletion experiments confirmed differential roles for MERTK and TYRO3 in the leukemia microenvironment. In contrast to Mertk-/- mice, selective depletion of CD8+ T cells completely abrogated protection from leukemia in Tyro3-/- mice, indicating a mechanism less dependent on DCs. Together, these findings reveal novel and distinct mechanistic insights into the immunosuppressive roles for MERTK and TYRO3 in the leukemia microenvironment, demonstrate a critical role for MERTK in DC activity, and validate a similar mechanism mediated by MRX-2843. Thus, these studies provide strong rationale for development of MERTK and/or TYRO3-targeted immunotherapies for treatment of acute leukemia.
Citation Format: Justus M Huelse, Swati S Bhasin, Kristen M Jacobsen, Beena E Thomas, Madison L Chimenti, Travon A Baxter, Xiaodong Wang, Stephen V Frye, Curtis J Henry, H. Shelton Earp, Manoj Bhasin, Deborah DeRyckere, Douglas K Graham. MERTK inhibition induces an anti-leukemia dendritic cell – T cell axis while TYRO3 inhibition protects by a separate mechanism [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 A35.