The genome is relatively stable in chronic myeloid leukemia (CML), but mutation rates increase during progression from chronic phase (CP) to blast crisis (BC). And, 30-50% of CML patients relapse in CP during prolonged treatment with inhibitors of the Bcr-abl tyrosine kinase (TKIs). Relapse usually involves point mutations in the BCRABL oncogene that impair TKI sensitivity, but many patients achieve remission after switching TKI. Generation of reactive oxygen species (ROS) is relatively high in CML bone marrow, suggesting efficient ROS handling may be required for low mutation rates in CP-CML during remission. CML neutrophils (PMNs) may contribute to bone marrow ROS via the phagocyte-specific NADPH oxidase; activated during the innate immune response. Consistent with this, we found aberrant activation of innate immune response pathways in human and murine CML, and emergency (stress) granulopoiesis (EG) was sustained in CML mice. Failure to efficiently terminate EG may favor mutagenesis by enhancing ROS production by CML-PMNs and expansion of leukemia stem cells (LSCs) during this process. In a murine model of CML, we found that episodes of EG accelerated TKI-resistance and BC compared to steady state. Oxr1 is the major mechanism for detoxifying PMN-ROS. Oxr1 has low (Oxr1S) and high efficiency (Oxr1L) isoforms. We found switch from Oxr1L to Oxr1S during EG in CML mice but not Wt controls. Oxr1L also increased in CP relapse. Oxr1-knockdown in the bone marrow of CML mice accelerated TKI-resistance and BC, consistent with a functional role for Oxr1 in leukemogenesis. Half of CML patients with a sustained major molecular response to TKI-treatment (<0.01% IS) remain in remission after TKI-discontinuation. Unfortunately, molecular markers to predict relapse vs sustained therapy free remission (TFR) are unknown. During the current funding period, we performed single cell RNA-Seq on CD34+CD38- cells from a CML patient cohort undergoing therapy discontinuation. We identified interaction of αβT cells and CML-LSCs in patients with relapse post TKI-discontinuation and in our murine CML model. T cell/ LSC interaction was not found with sustained TFR, or in second remission after failed discontinuation. T cell interaction with CML cells were not previously described, but interaction with αβT cells facilitates intercellular pathogen killing by phagocytes during the innate immune response. We found TFR was prolonged in mice transplanted with single LSCs vs αβT cell/LSC doublets, or by treatment with an αβT cell antibody. These results suggest a functional contribution of these T cells to LSC persistence and relapse post TKI-discontinuation. We hypothesize that dysregulation of the innate immune response in CML facilitates mutations that lead to relapse or BC progression via inadequate ROS-handling and/or aberrant interaction of αβT cells with CML-LSCs. The corollary to this hypothesis is that exposure of CML subjects to infectious challenge may enhance the ris...