HOXB4 overexpression mediates very rapid stem cell regeneration and competitive hematopoietic repopulation

Objective. Hox transcription factors have emerged as important regulators o f hematopoiesis. In particular, we have shown that overexpression of HOXB4 in mouse bone marrow can greatly enhance the level of hematopoietic stem ce ll HSC regeneration achieved at late times (> 4 months) posttransplantation . The objective of this study was to resolve if HOXB4 increases the rate an d/or duration of HSC regeneration, and also to see if this enhancement was associated with impaired production of end cells or would lead to competiti ve reconstitution of all compartments. Methods. Retroviral vectors were generated with the GFP reporter gene +/- H OXB4 to enable the isolation and direct tracking of transduced cells in cul ture or following transplantation. Stem cell recovery was measured by limit dilution assay for long-term competitive repopulating cells (CRU). Results. HOXB4-overexpressing cells have enhanced growth in vitro, as demon strated by their rapid dominance in mixed cultures and their shortened popu lation doubling time. Furthermore, HOXB4-transduced cells have a marked com petitive repopulating advantage in vivo in both primitive and mature compar tments. CRU recovery in HOXB4 recipients was extremely rapid, reaching 25% of normal by 14 days posttransplant or some 80-fold greater than control tr ansplant recipients, and attaining normal numbers by 12 weeks. Mice transpl anted with even higher numbers of HOXB4-transduced CRU regenerated up to bu t not beyond the normal CRU levels. Conclusion. HOXB4 is a potent enhancer of primitive hematopoietic cell grow th, likely by increasing self-renewal probability but without impairing hom eostatic control of HSC population size or the rate of production and maint enance of mature end cells. (C) 2001 International Society for Experimental Hematology. Published by Elsevier Science Inc.