Fiscal Year 2020 has seen significant progress toward accomplishing both Specific Aims; some of this progress is detailed here. For Aim 1, the first project focuses on the early development of MS lesions. Previously, we studied two critical aspects of lesion development: the small veins around which white matter lesions form, and the short-to-medium-term outcomes of acute lesions. To understand whether the presence of a central vein may help distinguish MS lesions from their mimickers -- an idea that remains controversial but to which we mostly subscribe -- we previously developed a rapid imaging approach for clinical 3-tesla MRI called FLAIR*. Two studies to assess the utility of FLAIR* for diagnosis and characterization of MS lesions have been published in the last year (9, 10). We found that FLAIR* is able to significantly improve diagnostic confidence in a variety of settings, including in cases with so-called diagnostic red flags, and we collaborated with an international initiative to develop an artificial intelligence-based method to classify and count central vein-positive lesions. Our technique is now in use at more than 30 around the world. With the North American Imaging in MS (NAIMS) cooperative, we have received a U01 grant (Principal Investigators: Daniel Ontaneda from the Cleveland Clinic and Nancy Sicotte from Cedars-Sinai Health Center) to perform a definitive study of the central vein sign as an imaging biomarker for diagnosis of MS; NINDS is the imaging technical lead site for this study. In addition, in the past year, we published a paper showing that a manganese-based contrast agent for MRI can provide important and new information about MS lesion development not available from standard gadolinium-based MRI methods (17). With respect to lesion outcomes, we previously established that there are two spatiotemporal patterns in MS lesions: a centrifugal pattern, in which serum contents leak from the center of the lesion and then proceed outward, over the course of minutes to hours, to fill the entire lesion; and a centripetal pattern, in which serum contents first appear on the periphery of the lesion and then proceed inward. These findings have important implications for understanding lesion development and its association with blood-brain-barrier permeability. In further work, we described how these permeability patterns help to determine the fashion in which acute MS lesions evolve into their chronic counterparts. Among other things, we found that very early events, perhaps occurring within the first month after lesion formation, appear to determine the efficacy of tissue repair, possibly including remyelination. We also showed that we can reliably identify chronically inflamed lesions on clinical MRI systems. In the past year, we have published a study showing that such lesions can be found even in asymptomatic individuals with MS-like changes on their MRI scans (16). With the NINDS Neuroimmunology Clinic, we are conducting a clinical trial to test whether corticosteroids prevent the evolution of acute to chronically inflamed lesions, and a second trial to assess whether the inflammation in these lesions can be abrogated. We have also reported on the long-term effects of failed lesion repair, which can cause diffuse tissue abnormalities and loss (12, 15). We are also focusing on characterization of MS lesions affecting the cerebral cortex, which have proven difficult to detect by MRI (unlike their white matter counterparts). Our approach here has been to evaluate new MRI approaches with potentially higher sensitivity than previously described methods, taking advantage of the 7-tesla research system at NIH and of our collaborations with MRI pulse sequence developers at NIH, in the extramural community, and in industry. In prior years, we described and are routinely using a method that more than doubles the sensitivity for cortical lesion detection, and published a paper (2) and submitted a patent application (13) for a new technique to visualize these lesions on clinical MRI scanners. Additionally under Aim 1, we have continued our work on improving methods for image acquisition and analysis (4, 5, 6) and have used our imaging techniques to study other neurological diseases (14). For Aim 2, we described the imaging and cellular/molecular events in early inflammatory demyelinating lesions that develop in the brains of marmoset monkeys with experimental autoimmune encephalomyelitis (EAE). We previously suggested, using coarse MRI techniques, that the blood-brain barrier becomes locally permeable up to four weeks prior to the onset of demyelination, and we showed that this permeability is associated with a perivascular lymphocytic and mononuclear infiltrate with parenchymal activation of microglia and astrocytes. In the past year, we published a study showing iron may play a role in the repair of these lesions (7), and we published the first radiological-pathological correlative study of spinal cord lesions in marmoset EAE (8). We also collaborated with extramural colleagues to study glial mechanisms of synaptic loss in MS in several preclinical models, including marmoset EAT (18). The results will pave the way toward using the marmoset model in preclinical studies to predict the response of people to novel treatments. Finally, we continue to contribute to review and position papers with various national and international consortia (1, 3, 11).