MERTK inhibition alters the PD-1 axis and promotes anti-leukemia immunity

: MERTK is ectopically expressed and promotes survival in acute lymphoblastic leukemia (ALL) cells and is thus a potential therapeutic target. Here we demonstrate both direct therapeutic effects of MERTK inhibition on leukemia cells and induction of anti-leukemia immunity via suppression of the coinhibitory PD-1 axis. A MERTK-selective tyrosine kinase inhibitor, MRX-2843, mediated therapeutic anti-leukemia effects in immunocompromised mice bearing a MERTK-expressing human leukemia xenograft. In addition, inhibition of host MERTK by genetic deletion (Mertk-/- mice) or treatment with MRX-2843 significantly decreased tumor burden and prolonged survival in immune-competent mice inoculated with a MERTK-negative ALL, suggesting immune-mediated therapeutic activity. In this context, MERTK inhibition led to significant decreases in expression of the coinhibitory ligands PD-L1 and PD-L2 on CD11b+ monocytes/macrophages in the leukemia microenvironment. Furthermore, although T cells do not express MERTK, inhibition of MERTK indirectly decreased PD-1 expression on CD4+ and CD8+ T cells and decreased the incidence of splenic FOXP3+ Tregs at sites of leukemic infiltration, leading to increased T cell activation. These data demonstrate direct and immune-mediated therapeutic activities in response to MERTK inhibition in ALL models and provide validation of a translational agent targeting MERTK for modulation of tumor immunity.