AbstractSTAT3 regulates the expansion of myeloid-derived suppressor cells (MDSCs) during inflammation, infection and cancer. Hyperactivation of STAT3 in gp130^757F/F mice is associated with protection from experimental colitis. This study determined mechanisms for this protection and compared this to mice with myeloid-specific STAT3-deficiency (LysMcre/STAT3^flox; gp130^757F/F LysMcre/STAT3^flox). Acute and chronic colitis was induced and colons were removed for histological, mRNA and protein analysis. Cell populations from spleen, mesenteric lymph node and colon were analyzed for different myeloid cell populations using flow cytometry. Functions of MDSCs and LPS-stimulated peritoneal macrophages were further characterized by in vitro and in vivo assays. Here we show that the resistance to experimental colitis in gp130^757F/F mice is via myeloid-cell specific STAT3 activation, MDSC expansion and increased production of suppressive and protective cytokines.