Enforced expression of HOXB7 promotes hematopoietic stem cell proliferation and myeloid-restricted progenitor differentiation

Hematopoietic progenitor/stem cells (HPCs/HSCs) purified from human adult p eripheral blood (PB) were triggered into cycling, retrovirally transduced w ith HOXB7 and then functionally assayed in vitro. HPCs were assayed in mult i- and unilineage differentiation cultures in either liquid phase or semiso lid medium, primitive HPCs in the high proliferative potential colony-formi ng cell (HPP-CFC) evaluation system and putative HSCs in Dexter type long-t erm culture (LTC) as LTC initiating cells (LTC-ICs), Control experiments en sured that the exogenous HOXB7 gene was constantly expressed, while the end ogenous one was barely or not transcribed. Enforced expression of the gene markedly modulated the proliferation/differentiation program of the entire HSC/HPC population, Enforced HOXB7 expression exerted a potent stimulatory effect on the proliferation of the primitive HPC and putative HSC subsets, assayed as HPP-CFCs and LTC-ICs respectively. While not modifying the total number of HPCs, exogenous HOXB7 induced an increase of the number of granu lo-monocytic (GM) HPCs [colony-forming unit GM (CFU-GM) CFU-GM, CFU-C and C FU-M, as evaluated by clonogenic assays] and markedly amplified the progeny of both CFU-G and CFU-M, which showed a sustained proliferation through at least 1-2 months (as evaluated in liquid suspension culture). The prolonge d proliferative stimulus induced by HOXB7 transfer into LTC, primitive and GM oriented HPC culture was characterized by persistent proliferation of a discrete population of blast cells and a large pool of differentiated myelo id precursors. Altogether, these results suggest the hypothesis that the pr oliferative stimulus exerted by exogenous HOXB7 in primitive and GM-oriente d HPCs may represent a preleukemic immortalization step. Consistent with th e functional role of HOXB7 in the initial ontogenetic phase, these studies indicate that ectopic HOXB7 expression in early HPCs and HSCs from adult PB stimulates their self renewal, sustained proliferation and myeloid differe ntiation.