Ectopic expression of Mef2c into Irf8^−/− BM progenitors induces acute myelomonocytic leukemia in recipient mice. (A) Kaplan-Meier survival curves of mice receiving Irf8^−/− BM cells after infection with retroviral vectors carrying Mef2c/GFP or GFP alone, or uninfected controls, are shown. (B) Blood smear of moribund mouse receiving Mef2c-Irf^−/− BM cells shows leukocytosis composed predominately of immature blastlike monocytic and granulocytic cells. The heterochromatic erythrocytes reflect a moderate anemia, confirmed by hematocrit values between 27 and 42 (normal values, 42-48; Pappenheim stain; 63×/1.4 oil). (C-D) Cytospin of BM cells from mice transplanted with either Mef2c/GFP (C) or GFP (D) Irf8^−/− BM. The accumulation of blastlike cells and immature monocytes is observed in BM with ectopic Mef2c expression, in contrast to controls, which show the typical increase in granulopoiesis and immature erythropoiesis characteristic of Irf8^−/− mice. In addition to juvenile forms, mature banded granulocytes are the prominent form (Pappenheim stain; 63×/1.4 oil). (E-F) Sternal BM of mouse transplanted with Mef2c-Irf8^−/− BM showing considerable hypercellularity associated with left-shifted monocytosis and granulocytosis and reduced numbers of erythroid cells. The sinusoidal vascular system is compressed and obscured and an eruption of leukemic cells through the nutrient foramina, which normally allows vascularization of the BM cavity, is observed, leading to the invasion of leukemic BM cells into the neighboring thoracic musculature (top half of microphotograph; 10×/0.45). Higher magnification clearly shows myeloid blast and immature forms of the invasive cells and the numerous phagocytic cells with an eosinophilic storage phenomenon in their cytoplasm (hematoxylin and eosin stain; 40×/1.3 oil). (G-I) Splenic sections of mice receiving either Mef2c (G,I) or control GFP (H) Irf8^−/− BM. The regular splenic architecture observed in control mice (H) is completely effaced in Mef2c mice because of the myeloid hyperplasia of the red pulp. As in the BM, an abundance of pseudo pseudo-Gaucher cells with intracellular accumulation of para-crystalline substance at different stages of development can be observed. Large storage cells showing hemophagocytosis are also present (I). Pockets of erythroid cells are observed, which compensates for the loss of erythropoiesis in the BM (hematoxylin and eosin stain; 10×/0.45 and 63×/1.4 oil). (J-K) The highly invasive behavior of the leukemic cells is evidenced by infiltration into nonhematopoietic tissue. (J) Similar to the spleen, the normal liver lobular architecture is completely destroyed and periportal and perivenous liver parenchyma is replaced by infiltrating leukemic myeloblasts and immature forms, as well as phagocytic histiocytes (hematoxylin and eosin stain; 40×/1.3 oil). (K). Pulmonary infiltration with hematopoietic cells was observed in all animals analyzed. Focal peribronchial and perivascular accumulation of immature myeloid and phagocytic histocytes is seen, as well as the diffuse thickening of the avelolar septa by leukemic infiltration (hematoxylin and eosin stain; 20×/0.75). (L) Flow-cytometric analysis of cells isolated from BM and PB from Mef2C and control GFP-transduced mice. Expression analysis of CD11b, Gr1, and F4/80 confirmed the prominent monocytic component of the AML induced by Mef2c. Only GFP⁺ cells are shown. The results are representative of all mice analyzed.