IN-VITRO EXPANSION OF CD13(-MEDIATED APOPTOTIC SCHEDULE()CD33(+) DENDRITIC CELL PRECURSORS FROM MULTIPOTENT PROGENITORS IS REGULATED BY A DISCRETE FAS)

We provide new information on how apoptosis regulates the expansion an d survival of dendritic cell (DC) elements during in vitro hematopoies is. Functionally distinct apoptotic schedules were associated with dif ferent phases of DC development when multipotent CD34(+) progenitor ce lls were treated with GM-CSF + TNF +/- SCF (c-kit ligand). During earl y phases of growth, unselected progenitors underwent apoptosis, During intermediate stages, high levels of apoptosis resulted in the prefere ntial 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-a and appeared to be med iated by fas. In contrast, those events associated with terminally dif ferentiated DCs were fas independent because there was no correlation between fas expression and cell death, The bcl-2 protein family appear ed to confer resistance to apoptotic death, as revealed by the high le vels of bcl-2 and bclx(L) during peak DC development and in long-term DC cultures. One demonstrate that activation of distinct apoptotic pro grams regulates DC development and homeostasis. Although suppression o f apoptosis may prolong the survival of late DC elements, an earlier a poptotic schedule appears to be required for the selective expansion o f DC elements from multipotent progenitors, Our data also provides ins ight into the mechanism(s) of myeloid lineage selection by cytokines s uch as TNF-alpha, which may promote both cell death and survival.