Mechanism of flt3 ligand expression in bone marrow failure: Translocation from intracellular stores to the surface of T lymphocytes after chemotherapy-induced suppression of hematopoiesis

The flt3 ligand (FL) is a growth factor for primitive hematopoietic cells. Serum levels of FL are inversely related to the number and proliferative ca pacity of early hematopoietic progenitors. We sought to elucidate the molec ular mechanism underlying this regulation. Expression of FL was examined in peripheral blood (PB) and bone marrow (BM) cells under normal steady-state hematopoiesis and during transient BM failure induced by chemoradiotherapy in 16 patients with hematological malignancies, Using anti-FL antibodies i n Western analysis, flow cytometry and confocal microscopy, we detected hig h levels of preformed FL inside but not on the surface of T lymphocytes in steady-state hematopoiesis. Intracellular FL colocalized with giantin and E RGIC-53, indicating that it is stored within and close to the Golgi apparat us. After chemotherapy-induced hematopoietic failure, FL rapidly translocat ed to the surface of T lymphocytes and the levels of FL released to serum i ncreased approximately 100-fold. Expression of FL mRNA was enhanced only ab out sevenfold; a similar, twofold to sixfold increase in mRNA was observed in the thymus and BM of mice with irradiation-induced aplasia. Upregulation of FL mRNA was delayed when compared with the appearance of cell surface-a ssociated and soluble protein isoforms. The described changes in FL express ion in response to chemotherapy-induced aplasia were observed in all patien ts, irrespective of the diagnosis and treatment regimen. Our data demonstra te that mobilization of preformed FL from intracellular stores rather than de novo synthesis is responsible for increased FL levels in BM failure. (C) 1999 by The American Society of Hematology.