MafG-Null Mice Show Delayed Leukocyte Engraftment Following Whole Bone Marrow Transplant.

Megakaryocytes, which give rise to platelets, constitute a small population of cells. However, molecular aspects of megakaryocyte development are poorly understood. MafG is the small subunit of the transcription factor, NF-E2, which plays a major role in terminal megakaryocyte differentiation and platelet release. We sought to determine the effect of impaired platelet release due to targeted ablation of the mafG gene following a hematopoietic stem cell transplant on engraftment kinetics. We performed three types transplants:

1. C57Bl/6 Ly5.1 murine bone marrow (mafG^WT/WT bm) into lethally irradiated mafG null hosts expressing green fluorescent protein, (GFP),

2. mafG-null/GFP bm into lethally irradiated C57BL/6 mafG^WT/WThosts, and

3. mafG^WT/WT GFP bm into lethally irradiated C57BL/6 mafG^WT/WT hosts.

We measured peripheral cell counts, the presence of circulating donor-derived cells by flow cytometry, changes in the cellularity of the bone marrow and splenic weight, and megakaryocyte size and concentration on day 5, 7, 14, and 28 post transplant. We found:

1. Platelet engraftment kinetics differed when mafG null animals served as donors or recipients. Specifically, platelets reached their lowest point at day 7 when mafG^WT/WT or mafG null bm was transplanted into wildtype hosts. When mafG^WT/WT bm was transplanted into mafG null hosts, platelets reached a low point on day 5.

2. MafG null hosts transplant with mafG^WT/WT bm had 5-fold lower circulating platelet levels than mafG^WT/WT hosts at day 5.

3. When mafG^WT/WT served as donors, circulating platelets began to increase after day 7, while animals receiving mafG null donor cells had platelet counts that continued to diminish.

4. Splenic hematopoiesis was markedly decreased when mafG^WT/WT bm was transplanted into mafG null hosts. In contrast, when mafG null bm was transplanted into mafG^WT/WT hosts, splenic hematopoiesis was increased 2.5 fold on day 28 compared to animals receiving mafG^WT/WTdonor cells.

5. Mortality rate was much higher in mafG null hosts.

6. Leukocyte engraftment was delayed when mafG^WT/WT bm was transplanted into mafG null hosts.

These results suggest that host factors determine the timing of the platelet nadir following bone marrow transplant. Furthermore, the mafG protein, contributes in previously unrecognized ways to splenic hematopoiesis and successful engraftment, possibly through its function in promoting megakaryocyte development and platelet release.