MicroRNA-146a acts as a guardian of the quality and longevity of hematopoietic stem cells

During inflammation and infection, hematopoietic stem and progenitor cells (HSPCs) are stimulated to proliferate and differentiate into mature immune cells, especially of the myeloid lineage. MicroRNA-146a (miR-146a) is a critical negative regulator of inflammation. Deletion of miR-146a produces effects that appear as dysregulated inflammatory hematopoiesis, leading to a decline in the number and quality of hematopoietic stem cells (HSCs), excessive myeloproliferation, and, ultimately, to exhaustion of the HSCs and hematopoietic neoplasms. In the absence of miR-146a, LPS-mediated serial inflammatory stimulation accelerates the effects of aging. The chronic inflammatory stress on HSCs in deleted mice involves a molecular axis consisting of upregulation of TRAF6 leading to excessive activity of NF-κB and overproduction of the cytokine IL-6. At the cellular level, we show that the defects are attributable to both an intrinsic problem in the miR-146a-deficient HSCs and extrinsic effects of miR-146a-deficient lymphocytes and non-hematopoietic cells. This study has identified a microRNA, miR-146a, to be a critical regulator of HSC homeostasis during chronic inflammatory challenge in mice and has provided a molecular connection between chronic inflammation and the development of bone marrow failure and myeloid malignancies. This may have implications for human hematopoietic malignancies, such as myelodysplastic syndrome, which frequently displays downregulated miR-146a expression