PROJECT SUMMARY Obesity is associated with an increased risk of relapse from acute lymphoblastic leukemia (ALL), the most common childhood cancer. We have uncovered a novel mechanism by which fat tissue may make ALL more aggressive and harder to cure. ALL cells found in adipose tissue in mouse models have a higher expression of RANKL (receptor activator of nuclear factor kappa-B ligand) than ALL found in marrow. Exposing ALL cells to adipose tissue ex vivo also increases their expression of RANKL. RANKL is known to contribute to ALL cell expansion in vivo, as well as invasion into bone and the central nervous system (CNS). In the present grant, we will investigate the pathways by which adipose tissue increases ALL expression of RANKL. We will explore this finding using a translational approach, with experiments ranging from bench to mouse to evaluations of clinical samples. We will elucidate which adipose tissue-derived signals are responsible for ALL cell RANKL expression. We will examine whether obesity increases ALL RANKL expression in mice and clinical samples, and explore the relationships between RANKL expression and bone density, CNS invasion, and chemotherapy treatment response. Finally, we will test an FDA-approved treatment, denosumab, in mice to see whether impairing RANKL signaling can improve ALL treatment outcome. These studies may provide a novel strategy for ALL patients to reduce bone loss and fractures, reduce CNS invasion, and potentially improve chemotherapy treatment outcomes.