The Transcription Factor IRF8 is a Key Transcription Factor for Basophil Development

Basophils are the rarest granulocytes circulating in the peripheral blood. They play critical roles in anti-parasite Th2-type immune responses and chronic allergic disorders. The developmental pathway for basophils has been recently demonstrated; myeloid progenitors pass through common myeloid progenitors, granulocyte-monocyte progenitors, granulocyte-committed progenitors (GPs), and basophil-committed progenitors (BaPs) in the bone marrow. BaPs then give rise to mature basophils. However, our understanding of how this pathway is regulated remains still elusive. Interferon Regulatory Factor-8 (IRF8), a hematopoietic cell-specific IRF transcription factor, is essential for the development of monocytes, dendritic cells, and eosinophils, while it inhibits neutrophil differentiation. Its role in the development of basophils has yet to be analyzed.

In this study, we investigated whether IRF8 has any role in the development of the basophil lineage. We found that Irf8^–/– mice displayed a severe reduction of basophil counts in the bone marrow, peripheral blood and spleen compared to wild-type (WT) mice. Irf8^–/– mice retained GPs but lacked BaPs. Cell transfer experiments revealed that the defect of basophil development in Irf8^–/– mice resides in bone marrow cells. We utilized IRF8-GFP chimera knock-in mice to examine IRF8 protein expression in the basophil lineage at a single cell level. We found that GPs, but not BaPs and mature basophils, expressed IRF8. Furthermore, purified Irf8^–/– GPs failed to efficiently give rise to basophils in vitro. These results indicate that IRF8 acts at the stage of GPs in a cell-intrinsic manner. To understand the mechanism by which IRF8 promotes basophil development, we performed transcriptome analysis of purified GPs from WT and Irf8^–/– mice by microarray. Because IRF8 is no more expressed in BaPs, we envisaged that IRF8 acts by inducing downstream transcription factors in GPs. The expression of several transcription factor genes such as Gata2 and Spib was reduced in Irf8^–/– GPs compared to WT GPs. Analysis of DNA motifs in the promoter regions of genes downregulated in Irf8^–/– GPs predicted that GATA transcription factor(s) may act downstream of IRF8. Indeed, retroviral transduction of GATA2, known to be essential for basophil development, into Irf8^–/– hematopoietic progenitor cells rescued basophil differentiation in vitro. On the other hand, Spib^–/– mice showed no obvious defects in basophil development. Taken together, these results suggest that the IRF8-GATA2 axis in GPs critically regulates basophil development.