Myeloid precursor cell reprogramming into a myeloid-derived suppressor cell (MDSC) contributes to high mortality rates in mouse and human sepsis. S100A9 mRNA and intracellular protein levels increase during early sepsis and remain elevated in Gr1+CD11b+ MDSCs after pro-inflammatory sepsis transitions to the later chronic anti-inflammatory and immunosuppressive phenotype. The purpose of this study was to determine whether intracellular S100A9 protein might sustain Gr1+CD11b+ MDSC repressor cell reprogramming during sepsis. We used a chronic model of sepsis in mice to show that S100A9 release from MDSCs and circulating phagocytes decreases after early sepsis and that targeting the S100a9 gene improves survival. Surprisingly, we find that intracellular S100A9 protein translocates from the cytosol to nucleus in Gr1+CD11b+ MDSCs during late sepsis and promotes expression of miR-21 and miR-181b immune repressor mediators. We further provide support of this immunosuppression pathway in human sepsis. This study may inform a new therapeutic target for improving sepsis outcome.

The myeloid-related protein S100A9 reprograms Gr1CD11b myeloid precursors into myeloid-derived suppressor cells (MDSCs) during murine sepsis. Here, we show that the immunosuppressive cytokine IL-10 supports S100A9 expression and its nuclear localization in MDSCs to function as immune repressors. To support this new concept, we showed that antibody mediated IL-10 blockade in wild-type mice after sepsis induction inhibited MDSC expansion during late sepsis, and that ectopic expression of S100A9 in Gr1CD11b cells from S100A9 knockout mice switched them into the MDSC phenotype only in the presence of IL-10. Knockdown of S100A9 in MDSCs from wild-type mice with late sepsis confirmed our findings in the S100A9 knockout mice. We also found that while both IL-6 and IL-10 can activate S100A9 expression in naive Gr1CD11b cells, only IL-10 can induce S100A9 nuclear localization. These results support that IL-10 drives the molecular path that generates MDSCs and enhances immunosuppression during late sepsis, and inform that targeting this immune repressor path may improve sepsis survival in mice.

Sepsis-induced myeloid-derived suppressor cells (MDSCs) contribute to immunosuppression associated with sepsis. We reported that the CCAAT enhancer-binding protein C/EBPβ activates microRNA (miR)-21 and miR-181b expressions, which induce transcription factor NFI-A to support the generation and expansion of MDSCs in the bone marrow and spleens of septic mice. Here, using a conditional knockout mouse model lacking C/EBPβ in the myeloid lineage, we find that without C/EBPβ, myeloid progenitor cells could not express miR-21 or miR-181b, and ectopic expression of C/EBPβ in the C/EBPβ-deficient myeloid progenitors activated the expression of the two miRNAs. Moreover, C/EBPβ-reconstituted myeloid cells expressed IL-10 and reduced T cell proliferation and function, similar to control MDSCs that express C/EBPβ. Exogenous expression of miR-21 and miR-181b in the C/EBPβ-deficient myeloid progenitors from septic mice produced similar results. Notably, NFI-A-dependent transactivation of NF-kB MDSC generating pathway was reversed in the C/EBPβ-deficient myeloid progenitors from septic mice. Together, these results support that decreasing C/EBPβ expression prevents MDSC generation and decreases immunosuppression in septic mice, providing a target for sepsis treatment.

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Sepsis-induced immunosuppression increases the risk of chronic infection and reduces survival. Myeloid-derived suppressor cells (MDSCs) expand in the bone marrow and spleen during murine polymicrobial sepsis, contributing to immunosuppression. A better understanding of molecular controls of MDSC production is needed to identify treatment targets. We previously reported that miR-21 and miR-181b couple with transcription factor NFI-A to induce MDSCs during murine sepsis. Here, we expand upon these observations by showing that conditional deletion of the Nfia gene in the myeloid lineage precludes MDSC development. NFI-A-deficient Gr1CD11b myeloid cells are not immunosuppressive and differentiate normally into macrophages and dendritic cells. In contrast, ectopically expressed NFI-A prevents differentiation of these immature Gr1CD11b cells, while converting them into MDSCs. In addition, NFI-A-deficient Gr1CD11b cells decreased, and cells transfected with NFI-A increase expression of miR-21 and miR181b. Our results support a myeloid cell loop in which NFI-A and miR-21 and miR-181b sustain Gr1CD11b MDSC-dependent immunosuppression during sepsis.