Paracrinally stimulated expansion of early human hemopoietic progenitors by stroma-generated cyclic ADP-ribose.

Both mammalian ectoenzymes CD38 and BST-1, which convert NAD+ into the potent intracellular calcium regulator cyclic ADP-ribose (cADPR), are expressed in the bone marrow microenvironment. Therefore, we investigated the effect of extracellular cADPR on human long-term culture-initiating cells (LTC-IC), the most immature hemopoietic progenitors (HP). A threefold expansion of LTC-IC was elicited by exposure of cord blood mononuclear cells for 24 h either to micromolar concentrations of exogenously added cADPR or to the nanomolar concentrations of cADPR that are produced by CD38-transfected murine stromal cell lines cocultured with the HP. The latter effect was due to connexin 43-mediated release of NAD+ from feeder cells and to subsequent CD38-catalyzed generation of cADPR at their outer surface. The increased [Ca2+]i induced in the feeder cells by autocrinally generated cADPR, however, increased production of interferon-g, a potent hemopoiesis-inhibiting cytokine. Thus, long-term culture (5 wk) of HP over undiluted CD38+ feeders decreased LTC-IC output by 90%, whereas a fivefold increase was observed when HP were cultured over a mixed CD38+/- feeder (1:10), whose ecto-ADP-ribosyl cyclase activity was as low as that expressed by native human stroma. These results demonstrate a new paracrine role for cADPR in the regulation of human hemopoiesis.