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The expression of Moloney murine leukemia virus and vectors derived from it is restricted in undifferentiated mouse embryonal carcinoma and embryonal stem (ES) cells. We have developed a retroviral vector, the murine embryonic stem cell virus (MESV), that is active in embryonal carcinoma and ES cells. MESV was derived from a retroviral mutant (18,19). In addition, MPSV and PCMV contain point mutations within the leader sequences that are also major determinants in the expanded permissiveness of these vectors, at least in EC cells (13,15). In this study we have analyzed the expression of PCMV vectors in ES cells. We found that the enhancer region of PCMV was functional in ES cells. However, sequences located within the 5' untranslated region of the viral genome completely abolished viral expression in ES cells. Replacement of this region by functionally equivalent sequences obtained from the dl-587rev virus (20) allowed LTR-mediated expression of retroviral genomes in ES cells, independent of the chromosomal site of integration. MATERIALS AND METHODSCell Culture and Viral Infections. The ES cell line CCE was obtained from M. Evans (Department of Genetics, Cambridge University, Cambridge, U.K.) at passage 10, grown on mitomycin-treated primary fibroblast feeder layers, and frozen at passage 11. The undifferentiated state of the cells was monitored by immunofluorescence as described (17). For viral infections ES cells were grown on gelatin-coated plates in buffalo-rat-liver-cell-conditioned medium (21). Infection of fibroblasts and ES cells was performed as described (17). Neomycin-resistant colonies were counted after 12-14 (NIH 3T3 and CCE) or 8-9 days (ES.D3) in selective medium.For DNA and RNA analysis of neomycin-resistant (neoR) NIH 3T3 and ES cells, cells were infected with viruscontaining cell supernatant and selected in G418-containing medium (0.2 mg/ml). The neoR clones were pooled after 10-12 days in selective medium and expanded. For DNA and RNA analysis of infected and nonselected cells, NIH 3T3 and CCE cells were infected six times with virus supernatants during a 1-week period. Cells were then kept in culture for 3 days before analysis.

Ectopic retroviral expression of homeobox B4 (HOXB4) causes an accelerated and enhanced regeneration of murine hematopoietic stem cells (HSCs) and is not known to compromise any program of lineage differentiation. However, HOXB4 expression levels for expansion of human stem cells have still to be established. To test the proposed hypothesis that HOXB4 could become a prime tool for in vivo expansion of genetically modified human HSCs, we retrovirally overexpressed HOXB4 in purified cord blood (CB) CD34 ؉ cells together with green fluorescent protein (GFP) as a reporter protein, and evaluated the impact of ectopic HOXB4 expression on proliferation and differentiation in vitro and in vivo. When injected separately into nonobese diabetic-severe combined immunodeficient (NOD/SCID) mice or in competition with control vector-transduced cells, HOXB4-overexpressing cord blood CD34 ؉ cells had a selective growth advantage in vivo, which resulted in a marked enhancement of the primitive CD34 ؉ subpopulation (P ‫؍‬ .01). However, high HOXB4 expression substantially impaired the myeloerythroid differentiation program, and this was reflected in a severe reduction of erythroid and myeloid progenitors in vitro (P < .03) and in vivo (P ‫؍‬ .01). Furthermore, HOXB4 overexpression also significantly reduced B-cell output (P < .01). These results show for the first time unwanted side effects of ectopic HOXB4 expression and therefore underscore the need to carefully determine the therapeutic window of HOXB4 expression levels before initializing clinical

hematopoiesis ͉ HOX genes ͉ microarray ͉ self-renewal ͉ embryoid bodies