Abstract GFI1 is a DNA-binding transcription factor that regulates hematopoiesis by repressing target genes through its association with complexes containing histone demethylases such as KDM1A (LSD1) and histone deacetylases (HDACs). To study the consequences of the disruption of the complex between GFI1 and histone-modifying enzymes, we have used knock-in mice harboring a P2A mutation in GFI1 coding region that renders it unable to bind LSD1 and associated histone-modifying enzymes such as HDACs. GFI1P2A mice die prematurely and show increased numbers of memory effector and regulatory T cells in the spleen accompanied by a severe systemic inflammation with high serum levels of IL-6, TNF-α, and IL-1β and overexpression of the gene encoding the cytokine oncostatin M (OSM). We identified lung alveolar macrophages, CD8 T cell from the spleen and thymic eosinophils, and monocytes as the sources of these cytokines in GFI1P2A mice. Chromatin immunoprecipitation showed that GFI1/LSD1 complexes occupy sites at the Osm promoter and an intragenic region of the Tnfα gene and that a GFI1P2A mutant still remains bound at these sites even without LSD1. Methylation and acetylation of histone H3 at these sites were enriched in cells from GFI1P2A mice, the H3K27 acetylation being the most significant. These data suggest that the histone modification facilitated by GFI1 is critical to control inflammatory pathways in different cell types, including monocytes and eosinophils, and that a disruption of GFI1-associated complexes can lead to systemic inflammation with fatal consequences.