Synergistic effects on erythropoiesis, thrombopoiesis, and stem cell competitiveness in mice deficient in thrombopoietin and steel factor receptors

The degree of redundancy between thrombopoietin (Tpo) and steel factor (SF) cytokine pathways in the regulation of hematopoiesis was investigated by generating mice lacking both c-Mpl and fully functional c-Kit receptors. Double-mutant c-Mpl(-/-)Kit(Wv/Wv) mice exhibited reduced viability, making up only 2% of the offspring from c-Mpl(-/-)Kit(Wv/)(+) intercrosses. The thrombocytopenia and megakaryocytopenia characteristic of c-Mpl(-/-) mice was unchanged in c-Mpl(-/-)Kit(Wv/Wv) mice. However, the number of megakaryocytic colony forming units (CFU-Mks) was significantly reduced, particularly in the spleen. While Kit(Wv/Wv) mice, but not c-Mpl(-/-) mice, are anemic, the anemia was more severe in double-mutant c-Mpl(-/-)Kit(Wv/Wv) mice, indicating redundancy between Tpo and SF in erythropoiesis. At the primitive cell level, c-Mpl(-/-) and Kit(Wv/Wv) mice have similar phenotypes, including reduced progenitors, colony forming units-spleen (CFU-Ss), and repopulating activities. All of these parameters were exacerbated in double-mutant mice. c-Mpl(-/-)Kit(Wv/Wv) mice had 8-fold fewer clonogenic progenitor cells and at least 28-fold fewer CFU-Ss. c-Mpl(-/-) mice also demonstrated a reduced threshold requirement for nonmyeloablative transplant repopulation, a trait previously associated only with Kit(W) mice, and the level of nonmyeloablative engraftment was significantly greater in c-Mpl(-/-) Kit(Wv/Wv) double mutants. Thus, c-Mpl(-/-) Kit(Wv/Wv) mice reveal nonredundant and synergistic effects of Tpo and SF on primitive hematopoietic cells.