Making Eosinophils Through Subtle Shifts in Transcription Factor Expression

More than a century after eosinophil granulocytes were baptized by Paul Ehrlich their role in defense mechanisms against parasitic infections and in allergic reactions has been firmly established (1). In contrast to the wealth of functional knowledge about these cells, their developmental origin, relationship to other blood cell types, and the critical transcription factors that determine their fate have long been elusive. Work in the 1990's showed a key role of both GATA and C/EBP transcription factors through experiments in which their expression was enforced in transformed chicken and murine cell lines. In addition, eosino-phil-specific promoters were found to be regulated by an interplay between these factors. In several new papers this concept has been confirmed and applied to the reprogramming of normal human and murine cells. In addition, two studies show that GATA-1–deficient mice lack eosin-ophils, nicely complementing earlier work that demonstrated a lack of eosinophils in C/EBP ␣-deficient mice. In the following these papers will be discussed and interpreted in the light of the earlier results obtained with transformed cell systems. GATA-1 As an Inducer of Eosinophil Formation. In a striking example of cellular engineering, in this issue, Hirasawa et al. (2) have succeeded in efficiently generating eosinophils from human fetal blood cells. To do this, they isolated hematopoietic progenitors from human cord blood (CD34 ϩ fraction), infected them with a retrovirus that encodes GATA-1 (together with GFP as an indicator), and sorted GFP-positive cells 60 h later. The cells were then grown in SCF and GM-CSF for 5 d and subsequently for an additional 8 d under either myeloid conditions (with the same factors) or eosinophil conditions (with IL-5). Surprisingly , perhaps, the GATA-1–transduced cells cultured under myeloid conditions contained eosinophilic granules and expressed eosinophil peroxidase as well as major basic protein but lacked myeloid markers. In contrast, the vector-only transduced cells were predominantly myelomonocytic and lacked eosinophil markers. GATA-2 exerted a similar effect, while dominant negative forms of the factors (produced by fusion to the Drosophila engrailed protein as a transcriptional repressor) completely prevented eosinophil formation in cells grown with IL-5. They also showed that the COOH-terminal zinc finger of GATA-1 (needed for DNA-binding) is required for this effect, while the NH 2-terminal zinc finger (needed for interaction with FOG-1, see below) is not. Using a related approach Heyworth et al. obtained similar results with mouse bone marrow cells (Tarik Enver, personal communication). Hematopoietic progenitors were …