THE C-KIT PROTOONCOGENE RECEPTOR IS EXPRESSED ON A SUBSET OF HUMAN CD3(-)CD4(-)CD8(-)(TRIPLE-NEGATIVE) THYMOCYTES

The c-kit receptor is a tyrosine-kinase transmembrane receptor first i dentified as an oncogene in the HZ4-feline leukemia virus and later fo und to be important in hematopoiesis in mice. The ligand for this rece ptor (Steel factor) can stimulate hematopoiesis both in vitro and in v ivo. To study the pattern of c-kit receptor expression in normal human hematopoietic progenitor cells, we prepared a monoclonal antibody (9B 9) against human c-kit receptor by using a synthetic peptide (amino ac ids 476-501) from the extracellular domain of c-kit receptor to immuni ze Balb/c mice. Monoclonal antibody 9B9 bound to recombinant c-kit pro tein, the erythroleukemic line HEL, the megakaryocytic line MEG-O1, an d the murine mast cell line P815. Monoclonal antibody 9B9 also bound t o the surface of the CD7(+)CD3(-)CD4(-)CD8(-) T cell lymphoid cell lin es DU.528 and HSB2T, and also to 1 to 4% of normal bone-marrow cells. The majority (67 +/- 6%) of CD34(+) bone-marrow progenitor cells coexp ressed c-kit receptor. Flow-cytometry analysis of immature CD3(-)CD4(- )CD8(-) (triple-negative) thymocytes indicated 30 +/- 9.5% expressed t he c-kit receptor, and thymidine incorporation assay revealed that the receptor is functional. Indirect fluorescent microscopy of human thym ic tissue, using a monoclonal antibody against Steel factor, revealed its presence on scattered mononuclear cells within the intralobular se ptae and the subcapsular cortex, which are regions where the triple-ne gative thymocytes are also localized. These data provide evidence that the c-kit receptor is present on human hematopoietic bone marrow and intrathymic T cell progenitor cells, and that it likely plays a role i n early T cell lymphopoiesis.