Gene Expression Profiles, Differentiation and Lineage Commitment of Human Myeloid Progenitors.

With the objective to further elucidate the mechanism behind commitment to erythroid and neutrophil lineages, we isolated by cell sorting common myeloid progenitors (CMPs), granulocyte/monocyte progenitors (GMPs) and megakaryocyte/erythrocyte progenitors (MEPs), based on the surface expression of CD123 (IL3R-α) and CD45RA on human CD34+ bone marrow cells (first described by Manz et al. PNAS, 2002;99:11872). Methylcellulose cultures supporting the growth of myeloid and erythroid progenitors, and real-time RT-PCR mapping the gene expression of Flt3, c-kit, TpoR, GATA-2, GATA-1, SCL, NF-E2, EpoR, ABO, β-globin, GPA, PU.1, C/EBPα, C/EBPε, G-CSFR, proteinase 3 (PR3) and lactoferrin, were used to validate and characterize the progenitors and their progeny. Cell sorted progenitors were labeled with CFDA, SE to track cell division and cultured in suspension to induce neutrophil or erythroid differentiation (SCF+G-CSF for neutrophil and Epo+IL–3+GM–CSF for erythroid culture). After 3–5 days, cells that had gone through 1–8 divisions were sorted and changes in clonogenicity and gene expression were studied. The CMP-population retained some clonogenicity after as many divisions as were tested (at the most six divisions in erythroid and eight in neutrophil culture) and the CMPs differentiated along the lineage defined by the culture system, as evidenced both by the methylcellulose cultures and an increasing expression of GATA-1 and EpoR in erythroid and PU.1, G-CSFR and PR3 in neutrophil cultures, respectively. On the other hand, the GMP-population displayed granulocyte/monocyte (G/M)-differentiation irrespective of the culture system used, although it divided fewer times and lost its clonogenic capacity faster in erythroid culture. Little or no clonogenicity remained after 4–5 divisions in erythroid culture, while some colony-forming capacity remained even after seven divisions in neutrophil culture (maximum number tested). The increased expression of the granulopoiesis-associated genes was also less pronounced in the erythroid culture. The MEP-population dominated by erythroid differentiation capacity retained colony-forming capacity for at least 6–7 divisions in both erythroid and neutrophil cultures, although with a higher overall clonogenicity in erythroid culture. Unexpectedly, however, MEPs were restricted to G/M-differentiation when cultured in neutrophil culture. In cells from erythroid culture the expression of GATA-1, EpoR and β-globin increased, while a corresponding pattern was seen for PU.1, G-CSFR and PR3 in neutrophil culture. Overall, our data support the progenitor classification, based on the surface expression of CD123 and CD45RA, with regard to CMP and GMP populations but question it with regard to the MEP-population. The change in differentiation course for the MEPs in neutrophil culture could be a result of an initially present G/M-potential or a less strict commitment susceptible to cytokine-induced redifferentiation.