Targeting foam cell formation in inflammatory brain diseases by the histone modifier MS-275 Bettina Zierfuss (1,2), Isabelle Weinhofer (2), Agnieszka Buda (2), Niko Popitsch (3), Lena Hess (4), Verena Moos (4), Simon Hametner (5), Stephan Kemp (6), Wolfgang Köhler (7), Sonja Forss-Petter (2), Christian Seiser (4) & Johannes Berger (2)* (1) Department of Neuroscience, Laboratory of Neuroimmunology, Centre de Recherche du CHUM, University of Montreal, Montreal, QC-H2X 0A9, Canada (2) Department of Pathobiology of the Nervous System, Centre for Brain Research, Medical University of Vienna, Vienna, 1090, Austria (3) Institute of Molecular Biotechnology, Vienna, 1030, Austria (4) Division of Cell and Developmental Biology, Center for Anatomy and Cell Biology, Medical University of Vienna, Vienna, 1090, Austria (5) Department of Neuropathology and Neurochemistry, Medical University of Vienna, Vienna, 1090, Austria (6) Laboratory Genetic Metabolic Diseases, Amsterdam UMC, Amsterdam Gastroenterology & Metabolism, Amsterdam Neuroscience, University of Amsterdam, Amsterdam, 1105AZ, The Netherlands (7) Department of Neurology, University of Leipzig Medical Centre, Leukodystrophy Clinic, Leipzig, 04103, Germany In neuroinflammatory demyelinating diseases like multiple sclerosis (MS) and cerebral X-linked adrenoleukodystrophy (CALD) recruited macrophages and resident microglial cells are crucial for the efficient clearance of myelin debris from active lesion sites to promote tissue repair and remyelination. However, upon excessive uptake of lipid-enriched myelin, these phagocytes adopt a foamy morphology associated with a disease-promoting phenotype as described for lipid-laden macrophages in atherosclerotic plaques. The objective of this project was to assess the effect of class I-histone deacetylase (HDAC) inhibition on the formation of such lipid-accumulating, disease-promoting phagocytes upon myelin load in vitro. First, we analyzed the activation and foam cell states of phagocytes by immunohistochemistry on postmortem brain tissue of acute MS (n = 6) and CALD (n = 4) cases. After sustained myelin loading of in vitro differentiated healthy human macrophages (n = 8), we applied RNA-Seq to assess the metabolic shift associated with foam cell formation. In the next step, we investigated the effect of class I HDACs on the set of genes linked to lipid degradation and export by using either the pharmacological inhibitor MS275 (Entinostat), specifically targeting class I HDACs, or by genetic KO of single members of class I HDACs in HAP1 cells. The outcome prompted us to test whether MS-275 treatment would lower the intracellular lipid/myelin content of myelin-laden human foam cells. Our results revealed that enlarged foam cells coincided with a pro-inflammatory, lesion-promoting phenotype in postmortem tissue of acute MS and CALD patients. In vitro, myelin laden foam cells derived from healthy donors upregulated genes linked to the LXRa/PPAR pathways mimicking a program associated with tissue repair. Treating these cells with MS-275 amplified this gene transcription program and significantly reduced lipid and cholesterol accumulation and, thus, foam cell formation in vitro. In conclusion, our findings identify class I-HDAC inhibition with pharmacological compounds like MS275 as a potential novel treatment strategy to prevent disease promoting foam cell formation in neuroinflammation.