Expression and distribution of the Kit receptor in bovine bone marrow cells

OBJECTIVE: To characterize the expression and distribution of the Kit receptor in bovine bone marrow cells (BMC) and to define the function of its ligand, stem cell factor (SCF). ANIMALS: Six 7- to 70-day-old healthy male Holstein-Friesian calves. PROCEDURES: Expression and distribution of the Kit receptor were assessed by use of flow cytometry with monoclonal antibodies (mAb) against the bovine Kit protein. Using Giemsa-stained centrifuged preparations, the histologic appearance of Kit receptor positive (Kit+) BMC were evaluated. Semisolid cultures supplemented with granulocyte colony-stimulating factor (G-CSF) and SCF were used to measure the colony formation capacity of Kit+ BMC. RESULTS: The Kit receptor was expressed on approximately 18% of total BMC. Most of Kit+ BMC did not coexpress lineage markers, but a small subset of this population did coexpress CD3. The Kit+CD3- BMC were a heterogeneous cell population comprising blast-like cells such as myeloblasts, promyelocytes, rubriblasts, and prorubricytes. Conversely, Kit+CD3+ BMC had a lymphocyte-like appearance. Kit+ BMC formed colonies in semisolid culture with G-CSF, whereas Kit- BMC failed to grow. Addition of SCF to G-CSF resulted in superadditive enhancement in colony numbers and size. CONCLUSIONS: The Kit receptor is expressed primarily on immature blood cells in bovine bone marrow, and Kit+ BMC contain hematopoietic progenitor cells that are reactive to G-CSF. In addition, SCF synergizes with G-CSF to stimulate colony formation by these cells. Our results suggest that the Kit receptor and its ligand, SCF, are involved in early stages of granulopoiesis in calves.