Proliferative and anti-apoptotic fractions in maturing hematopoietic cell lineages and their role in homeostasis of normal bone marrow

Recent developments in clinical flow cytometry allow the simultaneous assessment of proliferative and anti-apoptotic activity in the different hematopoietic cell lineages and during their maturation process. This can further advance the flow cytometric diagnosis of myeloid malignancies. In this study we established indicative reference values for the Ki-67 proliferation index and Bcl-2 anti-apoptotic index in blast cells, as well as maturing erythroid, myeloid, and monocytic cells from normal bone marrow (BM). Furthermore, the cell fractions co-expressing both proliferation and anti-apoptotic markers were quantified. Fifty BM aspirates from femoral heads of patients undergoing hip replacement were included in this study. Ten-color/twelve-parameter flow cytometry in combination with a software-based maturation tool was used for immunophenotypic analysis of Ki-67 and Bcl-2 positive fractions during the erythro-, myelo-, and monopoiesis. Indicative reference values for the Ki-67 and Bcl-2 positive fractions were established for different relevant hematopoietic cell populations in healthy BM. Ki-67 and Bcl-2 were equally expressed in the total CD34 positive blast cell compartment and 30% of Ki-67 positive blast cells also showed Bcl-2 positivity. The Ki-67 and Bcl-2 positive fractions were highest in the more immature erythroid, myeloid and monocytic cells. Both fractions then gradually declined during the subsequent maturation phases of these cell lineages. We present a novel application of an earlier developed assay that allows the simultaneous determination of the Ki-67 proliferative and Bcl-2 anti-apoptotic indices in maturing hematopoietic cell populations of the BM. Their differential expression levels during the maturation process were in accordance with the demand and lifespan of these cell populations. The indicative reference values established in this study can act as a baseline for further cell biological and biomedical studies involving hematological malignancies.