Characterization of human myeloid progenitors and their differentiation

All hematopoietic cells are derived from the hematopoietic stem cells (HSCs) in a continuous and dynamic process, regulated by a network of transcription factors in different combinations and extrinsic signals from growth factors (GFs) and other factors in the microenvironment. In the initial stages of hematopoiesis HSCs differentiate to progenitor cells with gradually more restricted lineage potential and according to the most accepted model the first restriction in developmental potential results in commitment to either the lymphoid or the myeloid branch, in the latter followed by commitment to the neutrophil/monocyte and erythrocyte/megakaryocyte lineages. The objective of this thesis was to further characterize early human myeloid development. As a first step we established a map over neutrophil and erythroid differentiation-associated changes both on the surface and in gene expression, using in vitro cultures of normal human bone marrow cells. This map identifies lineage-specific markers and describes the expression patterns of suggested crucial regulators of myeloid development. Next we found that the inclusion of the growth factor receptor (GFR) thrombopoietin receptor (TpoR) in a previously presented immunophenotype definition of the human common myeloid progenitor (CMP), granulocyte/monocyte progenitor (GMP) and megakaryocyte/erythrocyte progenitor (MEP), significantly improved the separation of MEPs (TpoR+) from CMPs. Furthermore, it was shown that even though the surface expression of GFRs on these progenitors coincides with the supposed regulating functions of the corresponding GFs, it cannot always be predicted by the gene expression. More specific studies of the proposed CMP revealed that this is a heterogeneous population, composed of a fraction of multipotent cells that are susceptible to extrinsic regulation of lineage fate and a fraction of lineage-polarized or committed cells, which differentiate along their respective lineages regardless of the presence or absence of different GFs. However, it has proven difficult to find surface markers that separate these functional subcompartments and it is possible that they only differ in their expression of transcription factors and other intrinsic molecules.