PROJECT SUMMARY It is critical to monitor inflammation in multiple sclerosis (MS) patients for prognostication and optimization of treatment. In current clinical practice, inflammation is inferred from accumulation of gadolinium (Gd) in acute lesions where the blood brain barrier is disrupted. However, the substantial and long-lasting microglial inflammation in established lesions occurring behind an intact blood brain barrier cannot be detected with conventional MRI. In this proposal we will address this unmet need by exploring the ability of quantitative susceptibility mapping (QSM) to quantify microglial activation in white matter lesions. QSM is a post-processing tool that extracts tissue magnetic susceptibility from gradient echo (GRE) data and is thus highly sensitive to iron. A striking feature of chronically activated microglia within MS lesions and the lesion perimeter is their high iron content, which can be detected by QSM. We hypothesize that iron is a sensitive biomarker for chronic, neurotoxic microglial activation in MS lesions and can be accurately detected with QSM. We further hypothesize that dimethyl fumarate (Tecfidera), a FDA-approved MS treatment, prevents iron uptake by microglia and concurrent tissue damage in chronic lesions in MS patients. We will test our hypotheses in a multipronged approach, by confirming that accumulation of iron is associated with a proinflammatory, cytotoxic phenotype in cultured human microglia and in human MS autopsy tissue and that dimethyl fumarate (Tecfidera) reduces iron uptake and proinflammatory polarization in cultured microglia. Moreover, we will combine imaging of MS autopsy tissue with its histopathological analysis to confirm the accuracy of QSM in detecting iron-positive microglia. We will finally conduct a clinical study in which we test the ability of Tecfidera to prevent iron accumulation (and thus proinflammatory microglial activation) and concomitant tissue damage in white matter lesions of MS patients. In summary, we are characterizing, quantifying and targeting in MS patients a novel pathomechanisms, persistent proinflammatory activation of microglia, which may contribute to neurodegeneration and disease severity. QSM can be easily implemented in clinical practice and may become a routine MRI technique to aid treatment decisions for patients that appear stable on conventional MRI but contain a high burden of lesional microglial activation.