Reduction in PU.1 activity results in a block to B-cell development, abnormal myeloid proliferation, and neonatal lethality

Abstract

It has been demonstrated that high concentration of the transcription factor PU.1 (encoded by Sfpi1) promotes macrophage development, whereas low concentration induces B-cell development in vitro. This has led to the hypothesis that lower levels of PU.1 activity are required for B cell than for macrophage development in vivo. We utilized an allele of Sfpi1 (termed BN) with a mutation in the first coding exon, which resulted in a reduction of PU.1 expression in order to test this hypothesis. Using gene targeting in embryonic stem cells, two ATG-start site codons of PU.1 were mutated, resulting in reduced PU.1 expression originating from a third start codon. Mice were assayed for phenotypic abnormalities using fluorescence-activated cell sorting, microscopy, and colony-forming ability. In addition, isolated cells were tested for their differentiation potential in vitro and in vivo. Lymphoid and myeloid cells derived from cultured Sfpi1(BN/BN) fetal liver cells had reduced levels of PU.1 expression and activity. B-cell development was intrinsically blocked in cells isolated from Sfpi1(BN/BN) mice. In addition, myeloid development was impaired in Sfpi1(BN/BN) fetal liver. However, neonatal Sfpi1(BN/BN) mice had a dramatic expansion and infiltration of immature myeloid cells. Contrary to our original hypothesis, high levels of PU.1 activity are required to induce both myeloid and B-cell development. In addition, neonatal mice homozygous for the hypomorphic allele acquire a myeloproliferative disorder and die within 1 month of age.