Heterogeneous effects of M-CSF isoforms on the progression of MLL-AF9 leukemia

Macrophage colony-stimulating factor (M-CSF) regulates both malignant cells and microenvironmental cells. Its splicing isoforms show functional heterogeneity. However, their roles on leukemia have not been well established. Here, the expression of total M-CSF in patients with hematopoietic malignancies was analyzed. The roles of M-CSF isoforms on the progression of acute myeloid leukemia (AML) were studied by establishing MLL-AF9-induced mouse AML models with high level membrane-bound M-CSF (mM-CSF) or soluble M-CSF (sM-CSF). Total M-CSF was highly expressed in myeloid leukemia patients. Furthermore, mM-CSF but not sM-CSF prolonged the survival of leukemia mice. While sM-CSF was more potent to promote proliferation and self-renew, mM-CSF was more potent to promote differentiation. Moreover, isoforms had different effects on leukemia-associated macrophages (LAMs) though they both increase monocytes/macrophages by growth-promoting and recruitment effects. In addition, mM-CSF promoted specific phagocytosis of leukemia cells by LAMs. RNA-seq analysis revealed that mM-CSF enhanced phagocytosis-associated genes and activated oxidative phosphorylation and metabolism pathway. These results highlight heterogeneous effects of M-CSF isoforms on AML progression and the mechanisms of mM-CSF, that is, intrinsically promoting AML cell differentiation and extrinsically enhancing infiltration of macrophages and phagocytosis by macrophages, which may provide potential clues for clinical diagnosis and therapy.