ABSTRACT Microglia are long-lived, self-replicating, yolk sac-derived immune cells of the central nervous system (CNS) parenchyma that are critical modulators of immunity and neuropathology. Microglia dysfunction contributes to several neurologic diseases, including multiple sclerosis (MS). In our preliminary studies, we have unexpectedly identified microglia circulating with the cerebrospinal fluid (CSF) of patients with MS. We hypothesize that CSF microglia are endogenous CNS cells that exhibit signatures unique to neurologic disease states. We aim to identify any disease-associated characteristics of CSF microglia. We will explore the number and immunologic features of CSF microglia in MS by flow cytometric analysis. To explore the molecular signature of CSF microglia in various disease states, we will perform single cell RNA sequencing on sorted CSF microglial cells. Comparisons will be made between gene expression patterns in CSF microglia between MS in relation to healthy control subjects to determine the health- and disease-specific profile of these cells. Finally, we will pursue immunologic phenotyping both by flow cytometry and transcriptomic analysis in MS patients before and after treatment. This proposal will lay the groundwork for the characterization of a novel cellular population capable of regulating CNS immunity.