Astrocyte-specific exosomes as a platform for biomarker discovery in multiple sclerosis. Acute and chronic neuroinflammation are major drivers of MS pathology, which are difficult to quantify in vivo. Thus, there is a significant unmet need for biomarkers that reliably reflect these processes. Astrocytes are highly sensitive indicators of CNS pathology and active drivers of a wide range of pathological processes in MS. Like most cells, astrocytes secrete a class of ultrasmall microvesicles termed exosomes. Because exosomal content changes with the functional state of the parent cell, the cargo of astrocyte exosomes may provide a window into the specific activation states of astrocytes. We have developed a strategy for isolating astrocytespecific exosomes from patient plasma, based on the combined expression of glutamate/aspartate transporter (GLAST) and glial fibrillary acidic protein (GFAP). In this proposal, we are exploring astrocyte-derived exosomes as a platform for biomarker discovery in acute demyelination and secondary progression in MS. In specific aim 1, we will comprehensively validate that GLAST+/GFAP+ exosomes are astrocyte-derived. In specific aim 2, we will explore the utility of plasma-derived GLAST+/GFAP+ exosomes to provide biomarkers for two distinct MS disease states – acute white matter demyelination and MS progression. To this end, we will profile the exosomal miRNA content of MS patients with gadolinium-enhancing white matter lesions and with secondary progressive MS and determine the miRNA signatures that are specific to each condition. Identifying exosome-based biomarkers for lesion formation and MS progression would constitute a major advancement for MS patient care and clinical trials. In follow-up studies, we will validate these biomarkers in larger MS patient cohorts.