Project Summary Despite significant advances in understanding immune system abnormalities associated with B-ALL, how the status of the immune system at birth contributes to leukemia development is unknown. The distinguishing feature of this application is the cellular immunity perspective from which this question is addressed. I propose that in addition to B-cell progenitor intrinsic mutagenesis, non-B lineage immune cells also play a prominent role in B-ALL development. B-cell progenitors develop mutations as a direct response to inflammatory stimuli; however, the bone marrow contains other immune cell populations that can alter tumor development in response to inflammation. For example, in solid cancers, low levels of the cytokine IL-10 elicit strong pro-tumor responses of CD8 T cells and neutrophils. Specifically, CD8 T cells lose their ability to detect and lyse tumor cells, whereas neutrophils become elevated in frequency and increase the potential for reactive-oxygen and - nitrogen species (ROS/RNS) to induce cancer-associated mutations. I hypothesize that the anti-tumor mechanisms of IL-10 in increasing CD8 T cell activity and suppressing neutrophils are involved in the protective role of IL-10 in childhood B-ALL. In addition to developing B-ALL at an accelerated rate, Il10-/- TEL- AML1 Cdkn2a-/- mice have more exhausted, PD1+ CD8 memory T cells and are also characterized by higher levels of neutrophils than Il10+/+ TEL-AML1 Ckdn2a-/- mice. Experiments in Aim 1 will extend this observation by testing the impact of Il10 loss on the cytotoxic activity of CD8 memory T cells directed to leukemic B cells and on the ability of leukemic B cells to suppress their own immunogenicity, a mechanism of immune evasion. Additional experiments will also track the extent to which the recruitment of CD8 T cells to leukemias and their subsequent cytotoxic activity requires responsiveness to IL-10. These experiments will provide a biological demonstration for how CD8 T cells can be a factor in IL-10 mediated protection in B-ALL. Studies in Aim 2 will determine if the ROS/RNS produced by neutrophils of Il10-/- TEL-AML1 Cdkn2a-/- mice induce more DNA damage in B progenitors and contribute to decreased B-ALL latency compared to Il10+/+ TEL-AML1 Ckdn2a-/- mice. Activation and ROS/RNS production will be assessed in neutrophils isolated from the Il10-/- and Il10+/+ leukemias. I will then determine how Il10 loss impacts the mutational landscape of leukemia. The expectation is that the total number of mutations and ROS/RNS-associated mutational signatures will be increased in the leukemic B cells that developed in the absence of Il10. Finally, I will determine if the elevation of neutrophils that is characterized in pre-leukemic and leukemic Il10-/- TEL-AML1 Cdkn2a-/- mice is required for the acceleration of B-ALL development. The results of the proposed research will provide new biological insights into how immune defects at birth contribute to the development of childhood B-ALL. Moreover, this work has the potential to identify immune responses that can be targeted for leukemia prevention in children at high risk for B-ALL.