Histone H1(0) expression is restricted to progenitor cells during human hematopoiesis

The histone H1(0) accumulates in cells with little or no proliferative activity during the terminal phase of differentiation in adult tissues. The hematopoietic cell system is an interesting in vivo model to study the relationship between H1(0) and both the proliferative capacity and differentiation state of cells. Using immunofluorescence techniques, we have analyzed the distribution of histone H1(0) during human hematopoietic differentiation, in normal bone marrow cells and in cell lines representative of cells blocked at early stages of differentiation. In enriched bone marrow cell suspensions, H1(0) was not expressed in any cell population highly engaged into a differentiation pathway. However, more than 50% of cells from blastic population (CD34-positive cells) were expressing H1(0), whereas only 5% of CD34-negative cell population expressed H1(0). We show that H1(0) was also detected in almost all the cell lines studied. These results indicate that histone H1(0) is expressed in immature cells which, although committed, still retain several differentiation potentialities. For normal human hematopoiesis, cells expressing H1(0) belong to a population of cells that are largely quiescent, although having a high proliferative capacity. H1(0) is no longer present in terminally differentiating or differentiated cells with limited or no proliferative potential. Thus, we suggest that H1(0) accumulates in cells with little or no proliferative activity but which are able to resume cell proliferation if required. These results are in keeping with the hypothesis that H1(0) contributes to stabilize a chromatin structure in cells for which integrity and/or longevity are essential.