Forced expression of the Ikaros 6 isoform in human placental blood CD34+ cells impairs their ability to differentiate toward the B-lymphoid lineage

Studies in mice suggest that the Ikaros ( Ik ) gene encodes several isoforms and is a critical regulator of hematolymphoid differentiation. Little is known on the role of Ikaros in human stem cell differentiation. Herein, the biological consequences of the forced expression of Ikaros 6 (Ik6) in human placental blood CD34+ progenitors are evaluated. Ik6 is one of the isoforms produced from the Ikaros premessenger RNA by alternative splicing and is thought to behave as a dominant negative isoform of the gene product because it lacks the DNA binding domain present in transcriptionally active isoforms. The results demonstrate that human cord blood CD34+ cells that express high levels of Ik6 as a result of retrovirally mediated gene transfer have a reduced capacity to produce lymphoid B cells in 2 independent assays: (1) in vitro reinitiation of human hematopoiesis during coculture with the MS-5 murine stromal cell line and (2) xenotransplantation in nonobese diabetic–severe combined immunodeficient mice. These results suggest that Ikaros plays an important role in stem cell commitment in humans and that the balance between the different isoforms is a key element of this regulatory system; they support the hypothesis that posttranscriptional events can participate in the control of human hematopoietic differentiation.