miR-155 promotes FLT3-ITD-induced myeloproliferative disease through inhibition of the interferon response.

FLT3-ITD+ acute myeloid leukemia (AML) accounts for ∼25% of all AML cases and is a subtype that carries a poor prognosis. microRNA-155 (miR-155) is specifically overexpressed in FLT3-ITD+ AML compared with FLT3 wild-type (FLT3-WT) AML and is critical for the growth of FLT3-ITD+ AML cells in vitro. However, miR-155's role in regulating FLT3-ITD-mediated disease in vivo remains unclear. In this study, we used a genetic mouse model to determine whether miR-155 influences the development of FLT3-ITD-induced myeloproliferative disease. Results indicate that miR-155 promotes FLT3-ITD-induced myeloid expansion in the bone marrow, spleen, and peripheral blood. Mechanistically, miR-155 increases proliferation of the hematopoietic stem and progenitor cell compartments by reducing the growth-inhibitory effects of the interferon (IFN) response, and this involves targeting of Cebpb. Consistent with our observations in mice, primary FLT3-ITD+ AML clinical samples have significantly higher miR-155 levels and a lower IFN response compared with FLT3-WT AML samples. Further, inhibition of miR-155 in FLT3-ITD+ AML cell lines using CRISPR/Cas9, or primary FLT3-ITD+ AML samples using locked nucleic acid antisense inhibitors, results in an elevated IFN response and reduces colony formation. Altogether, our data reveal that miR-155 collaborates with FLT3-ITD to promote myeloid cell expansion in vivo and that this involves a multitarget mechanism that includes repression of IFN signaling.

Exosome-delivered microRNAs modulate the inflammatory response to endotoxin.

MicroRNAs regulate gene expression posttranscriptionally and function within the cells in which they are transcribed. However, recent evidence suggests that microRNAs can be transferred between cells and mediate target gene repression. We find that endogenous miR-155 and miR-146a, two critical microRNAs that regulate inflammation, are released from dendritic cells within exosomes and are subsequently taken up by recipient dendritic cells. Following uptake, exogenous microRNAs mediate target gene repression and can reprogramme the cellular response to endotoxin, where exosome-delivered miR-155 enhances while miR-146a reduces inflammatory gene expression. We also find that miR-155 and miR-146a are present in exosomes and pass between immune cells in vivo, as well as demonstrate that exosomal miR-146a inhibits while miR-155 promotes endotoxin-induced inflammation in mice. Together, our findings provide strong evidence that endogenous microRNAs undergo a functional transfer between immune cells and constitute a mechanism of regulating the inflammatory response.