Recent advances in cancer therapy have increased survivability of numerous pediatric and adult cancers. By 2023, over 20 million cancer survivors will live in the United States. Unfortunately, the vast majority of these individuals will exhibit some indication of sustained neurological deficiency, clinically called cancer therapyrelated cognitive impairment (CRCI). These impairments include deficits in memory, learning, attention, executive function, information processing, language, and multitasking. Despite the myriad suspected causes of CRCI, one of the main commonalities is a persistent neuroinflammatory state following cancer treatment. How this sustained neuroinflammation is mediated remains a critical gap in our understanding of the underlying biology of CRCI. Our previous work in a preclinical mouse model demonstrated that the commonly used chemotherapeutic agent, methotrexate (MTX), induces tri-glial dysregulation that is dependent on the direct activation of microglia, the resident immune cells of the central nervous system. Systemic MTX administration results in persistent microglial pan activation which promotes astrocyte reactivity and decreased OPC density and differentiation into myelin-forming oligodendrocytes, leading to thinner myelin. MTX-induced aberrant myelination causes persistent cognitive deficits associated with CRCI, including decrements in short-term memory and attention for up to six months post-treatment. Preliminary data from our lab demonstrates that this microglial activation, and consequent dysregulation of oligodendrocyte lineage cells, is time-of-day dependent, suggesting that the sustained microglial activation and clinical deficiency associated with CRCI may be regulated in a circadian manner and thus susceptible to chronomodulation. The objective of this proposal is to determine if MTX chemotherapy drives intrinsic changes to the microglial transcriptome and alters the structure or function of the BBB to sustain the activation of microglia associated with CRCI. Our central hypothesis is that chemotherapy-induced chronic neuroinflammation is modulated by circadian regulation of microglia and the BBB. Our approach to testing this hypothesis is to expose microglia in vitro and in vivo to MTX chemotherapy at different circadian phases and analyze the transcriptional profile of isolated microglia and BBB endothelial cells, as well as assess BBB permeability and integrity. The rationale for this approach is that information gleaned from the results will contribute mechanistic understanding into the intrinsic and microenvironmental modulators of neuroinflammatory microglia following cancer therapy. Upon completion of this proposal, we expect to have identified how circadian modulation dictates microglial activation to chemotherapy. There remains an urgent need to define the underlying mechanisms of neuroinflammation mediating the persistent neurological dysregulation in CRCI in preclinical models of cancer therapy. In the absence of such knowledge, effective therapeutic strategies aimed at mitigating neuroinflammation will remain elusive and subsequent sustained neurological deficits will continue to be a substantive burden to cancer survivors and our healthcare system.