Loss Of Ikk Beta But Not Nf-Kappa B P65 Skews Differentiation Towards Myeloid Over Erythroid Commitment And Increases Myeloid Progenitor Self-Renewal And Functional Long-Term Hematopoietic Stem Cells

NF-kappa B is an important regulator of both differentiation and function of lineage-committed hematopoietic cells. Targeted deletion of I kappa B kinase (IKK) beta results in altered cytokine signaling and marked neutrophilia. To investigate the role of IKK beta in regulation of hematopoiesis, we employed Mx1-Cre mediated IKK beta conditional knockout mice. As previously reported, deletion of IKK beta in hematopoietic cells results in neutrophilia, and we now also noted decreased monocytes and modest anemia. Granulocyte-macrophage progenitors (GMPs) accumulated markedly in bone marrow of IKK beta deleted mice whereas the proportion and number of megakaryocyte-erythrocyte progenitors (MEP) decreased. Accordingly, we found a significantly reduced frequency of proerythroblasts and basophilic and polychromatic erythroblasts, and IKK beta-deficient bone marrow cells yielded a significantly decreased number of BFU-E compared to wild type. These changes are associated with elevated expression of C/EBP alpha, Gfi1, and PU.1 and diminished Gata1, Klf1, and SCL/Tal1 in IKK beta deficient Lineage-Sca1(+)c-Kit(+) (LSK) cells. In contrast, no effect on erythropoiesis or expression of lineage-related transcription factors was found in marrow lacking NF-kappa B p65. Bone marrow from IKK beta knockout mice has elevated numbers of phenotypic long and short term hematopoietic stem cells (HSC). A similar increase was observed when IKK beta was deleted after marrow transplantation into a wild type host, indicating cell autonomous expansion. Myeloid progenitors from IKK beta-but not p65-deleted mice demonstrate increased serial replating in colony-forming assays, indicating increased cell autonomous self-renewal capacity. In addition, in a competitive repopulation assay deletion of IKK beta resulted in a stable advantage of bone marrow derived from IKK beta knockout mice. In summary, loss of IKK beta resulted in significant effects on hematopoiesis not seen upon NF-kappa B p65 deletion. These include increased myeloid and reduced erythroid transcription factors, skewing differentiation towards myeloid over erythroid differentiation, increased progenitor self-renewal, and increased number of functional long term HSCs. These data inform ongoing efforts to develop IKK inhibitors for clinical use.