In vitro expansion of CD13+CD33+ dendritic cell precursors from multipotent progenitors is regulated by a discrete fas-mediated apoptotic schedule.

We provide new information on how apoptosis regulates the expansion and survival of dendritic cell (DC) elements during in vitro hematopoiesis. Functionally distinct apoptotic schedules were associated with different phases of DC development when multipotent CD34+ progenitor cells were treated with GM-CSF + TNF +/- SCF (c-kit ligand). During early phases of growth, unselected progenitors underwent apoptosis. During intermediate stages, high levels of apoptosis resulted in the preferential selection of DC precursors, as revealed by the massive expansion of DR+CD33+CD13+ cells. Late apoptosis was associated with the death of mature DCs. Apoptotic events surrounding the earlier periods were related to the exogenous addition of TNF-alpha and appeared to be mediated by fas. In contrast, those events associated with terminally differentiated DCs were fas independent because there was no correlation between fas expression and cell death. The bcl-2 protein family appeared to confer resistance to apoptotic death, as revealed by the high levels of bcl-2 and bclxL during peak DC development and in long-term DC cultures. We demonstrate that activation of distinct apoptotic programs regulates DC development and homeostasis. Although suppression of apoptosis may prolong the survival of late DC elements, an earlier apoptotic schedule appears to be required for the selective expansion of DC elements from multipotent progenitors. Our data also provides insight into the mechanism(s) of myeloid lineage selection by cytokines such as TNF-alpha, which may promote both cell death and survival.