Frontline Science: Myeloid cell-specific deletion of Cebpb decreases sepsis-induced immunosuppression in mice.

Sepsis inflammation accelerates myeloid cell generation to compensate for rapid mobilization of the myeloid progenitors from bone marrow. This inflammation-driven myelopoiesis, however, generates myeloid progenitors with immunosuppressive functions that are unable to differentiate into mature, innate immune cells. The myeloid-derived suppressor cells (MDSCs) expand markedly in the later phases of sepsis, suppress both innate and adaptive immunity, and thus, elevate mortality. Using a murine model with myeloid-restricted deletion of the C/EBPβ transcription factor, we show that sepsis-induced generation of MDSCs depends on C/EBPβ. C/EBPβ myeloid cell-deficient mice did not generate MDSCs or develop immunosuppression and survived sepsis. However, septic mice still generated Gr1CD11b myeloid progenitors at the steady-state levels similar to the control sham mice, suggesting that C/EBPβ is not involved in healthy, steady-state myelopoiesis. C/EBPβ-deficient Gr1CD11b cells generated fewer monocyte- and granulocyte-like colonies than control mice did, indicating reduced proliferation potential, but differentiated normally in response to growth factors. Adoptive transfer of C/EBPβ-deficient Gr1CD11b cells from late septic mice exacerbated inflammation in control mice undergoing early sepsis, confirming they were not immunosuppressive. These results show that C/EBPβ directs a switch from proinflammatory to repressor myeloid cells and identifies a novel treatment target.