Stat3 and C/EBPβ synergize to induce miR‐21 and miR‐181b expression during sepsis

Myeloid‐derived suppressor cells (MDSCs) increase late sepsis immunosuppression and mortality in mice. We reported that microRNA (miR) 21 and miR‐181b expression in Gr1 + CD11b + myeloid progenitors increase septic MDSCs in mice by arresting macrophage and dendritic cell differentiation. Here, we report how sepsis regulates miR‐21 and miR‐181b transcription. In vivo and in vitro binding studies have shown that C/EBPα transcription factor, which promotes normal myeloid cell differentiation, binds both miRNA promoters in Gr1 + CD11b + cells from sham mice. In contrast, in sepsis Gr1 + CD11b + MDSCs miR‐21 and miR‐181b promoters bind both transcription factors Stat3 and C/EBPβ, which co‐imunoprecipitate as a single complex. Mechanistically, transcription factor Rb phosphorylation supports Stat3 and C/EBPβ accumulation at both miRNA promoters, and C/EBPβ or Stat3 depletion by siRNA in sepsis Gr1 + CD11b + MDSCs inhibits miR‐21 and miR‐181b expression. To further support this molecular path for MDSC accumulation, we found that Stat3 and C/EBP binding at miR‐21 or miR‐181b promoter was induced by IL‐6, using a luciferase reporter gene transfection into naive Gr1 + CD11b + cells. Identifying how sepsis MDSCs are generated may inform new treatments to reverse sepsis immunosuppression.