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

Objective. It has been demonstrated that high concentration of the transcription factor PU.1 (encoded by Sfpil) 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 Sfpil (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. Materials and Methods. 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. Results. Lymphoid and myeloid cells derived from cultured Sfpil(BN/BN) fetal liver cells had reduced levels of PU.1 expression and activity. B-cell development was intrinsically blocked in cells isolated from Sfpil(BN/BN) mice. In addition, myeloid development was impaired in Sfpil(BN/BN) fetal liver. However, neonatal Sfpil(BN/BN) mice had a dramatic expansion and infiltration of immature myeloid cells. Conclusion. 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. (c) 2007 International Society for Experimental Hematology. Published by Elsevier Inc.