HOXB4 But Not BMP4 Confers Self-Renewal Properties to ES-Derived Hematopoietic Progenitor Cells

Background. Achieving transplantation tolerance remains an unresolved clinical challenge. Although bone marrow transplantation (BMT) induces mixed chimerism that establishes transplantation tolerance, the preconditioning regimens required for BMT to succeed are too prohibitive for routine use. Recently, embryonic stem (ES) cells have emerged as a potential alternative cell source to bone marrow cells. However, it remains difficult to efficiently differentiate these cells into hematopoietic cells. Here, we tested whether bone morphogenetic protein-4 (BMP-4)-treated or HOXB4-transduced ES-derived hematopoietic cells engraft permanently inducing long-term mixed chimerism. Methods. Initially, 129 SvJ RI ES cells (H-2(b)) were treated with BMP-4 for 36 hr. The cells were phenotyped and polymerase chain reaction studies were performed. The robustness of mixed chimerism was tested using mixed lymphocyte), cultures and skin grafts. Chimeric MRL (H-2(k)) animals received grafts from 129SvJ (H-2(b) Balb/c (H-2(d))or class II-/- (H-2(b)) donor mice, and graft survival was monitored. Additionally, HOXB4-transduced ES cells were shown to more efficiently differentiate into hematopoietic progenitor cells that engrafted in allogenic and syngeneic recipient mice. Results. BMP-4 treatment induced Sca-1 expression and up-regulated HOXB4, BMP-4, and BMP receptor gene expressions. The cells induced transient mixed chimerism, whereas cells derived from HOXB4-transduced ES cells engrafted long term (>100 days). Conclusions. Although BMP-4 promotes hematopoiesis of ES cells, its impact is only transient, whereas permanent ectopic expression of HOXB4 significantly confers self-renewal and long-term engraftment of ES-derived hematopoietic cells. This strategy could facilitate the establishment of an alternative source of hematopoietic cells that could induce transplantation tolerance.