INTERACTION OF STEM-CELL FACTOR AND ITS RECEPTOR C-KIT MEDIATES LODGMENT AND ACUTE EXPANSION OF HEMATOPOIETIC-CELLS IN THE MURINE SPLEEN

The phenotypes of mice that harbor a defect in the genes encoding eith er stem cell factor (SCF) or its receptor, c-kit, indicate that this l igand/receptor pair is necessary for maintenance of normal hematopoies is in the adult. Our objective was to determine whether SCF, like eryt hropoietin, is necessary for acute erythroid expansion during recovery from hemolytic anemia. Monoclonal antibody ACK2, which recognizes the murine c-kit receptor, was used to selectively block the hematopoieti c growth-promoting effects of SCF. Mice were treated with phenylhydraz ine on day 0 and day 1 to induce hemolytic anemia and also received no antibody, control IgG, or ACK2 on day 0. The mice were killed on day 3 and the hematocrit (Hct), reticulocyte count, and numbers of erythro id and myeloid hematopoietic progenitor cells (colony-forming unit-ery throid [CFU-E], burst-forming unit [BFU]-E, and CFU-granulocyte-macrop hage [GM]) were quantitated in the femoral marrow and spleen using hem atopoietic colony-forming assays. Induction of hemolytic anemia with p henylhydrazine resulted in a drop in the Hct from approximately 50% to 30%, and an approximate 8- to 10-fold increase in the reticulocyte co unt. The numbers of CFUE increased modestly in the femur, and approxim ately 25-to 50-fold in the spleen, in comparison with normal mice. BFU -E and CFU-GM values did not increase in the femur but expanded 6- to 10-fold in the spleen, in comparison with normal mice. This confirms t hat much of the erythroid expansion in response to hemolytic anemia oc curs in the murine spleen. Neutralizing quantities of the ACK2 antibod y reduced femoral CFU-E, BFU-E, and CFU-GM content to less than half t hat found in phenylhydrazine-treated control mice and nearly totally a blated splenic hematopoiesis. These results suggest that c-kit recepto r function may be required for optimal response to acute erythropoieti c demand and that erythropoiesis in the splenic microenvironment is mo re dependent on SCF/c-kit receptor interaction than is erythropoiesis in the marrow microenvironment. Because expansion of late erythropoies is in the spleen was preferentially blocked, we tested the hypothesis that homing of more primitive hematopoietic cells to the spleen was de pendent on c-kit receptor function. Lethally irradiated mice were inje cted with marrow cells obtained from mice that had received phenylhydr azine plus control IgG or with marrow cells obtained from mice that ha d received phenylhydrazine plus ACK2. In parallel experiments, normal murine marrow cells were treated in vitro with control IgG or with ACK 2 and were injected into lethally irradiated mice. The fraction of BFU -E and CFU-GM retrieved from the marrow and spleen of the recipient mi ce 4 hours later was reduced by approximately 75% when progenitor cell s had been exposed to ACK2, in comparison with control IgG. These data suggest that interaction of SCF with the c-kit receptor affects the h oming behavior of hematopoietic progenitor cells in the adult animal. (C) 1996 by The American Society of Hematology.