Imaging the Differential Engraftment of GFP-Expressing Spleen and Bone Marrow Cells in Various Organs after Reduced-Intensity Stem Cell Transplantation in Mice Model.

[Background] Reduced-intensity allogeneic stem cell transplantation is of critical importance in the treatment of hematological malignancies. The object of this experiment is to establish fludrabine-based reduced-intensity allogeneic stem cell transplantation model using donor cells labeled with GFP and image the engraftment process of donor cells in various organs.

[Materials and Methods] GFP transgenic C57/BL6 mice (GFP-Tgm) were used as donors and C57/BL6 × DBA F1 mice (BDF1) were used as recipients. Recipient mice were pretreated with fludarabine (Flu) 150 mg/kg/day × 6 days i.p. and cyclophosphamide (CPA) 150 mg/kg/day × 2 days i.p. On day 0, 10⁷ GFP splenocytes (Group 1) and GFP bone marrow cells (Group 2) were injected in the tail vein. Whole body and intravital imaging were used to visualize the migration of GFP-Tgm cells into various organs including the brain, femur, intestine, liver, lung, ovary, pelvic bone, ribs, skin, skull, spine, spleen and uterus on days-7 and -14. The macro images were obtained with the Olympus OV100 Small Animal Imaging System. GFP-Tgm cell migration, particularly CD34⁺ cells in the various organs was also analyzed by flow cytometry (FACS) using APC-labeled anti-CD34 monoclonal antibodies.

[Results and Discussion] On day-7, the migration of donor GFP-Tgm cells in peripheral lymph nodes, intestine, lung, ovary, skin and uterus were detected in both groups. Migration of GFP-Tgm cells in the femur, pelvic bone, ribs and skull were clearly detected in Group 2, but not in Group 1. On day-14 GFP donor cells were imaged in the lung, ovary, skin and uterus both groups. However, GFP-Tgm cells were no longer imaged in the intestine in Group 2 except in Payer patches. In both groups the GFP-Tgm cells were strongly detected in the femur, pelvic bone, skull and spine on day-14. We also analyzed the migration of CD34⁺ GFP-Tgm cells by FACS analysis. On day-14, the percentage of GFP-Tgm cells in the bone marrow and spleen was, respectively, 14% and 53% in Group 1, and 10% and 13% in Group 2. The percentage of CD34⁺ GFP-Tgm cells among total CD34⁺ cells in the bone marrow was 10% in Group 1 and 14% in Group 2, and in the spleen was 24% and 19%, respectively. Those results suggested that in this model, donor bone marrow and spleen cells, but not purified CD34⁺ cells, have different engraftment kinetics in various organs including the intestine, which is a target organ for graft-versus host disease. These results clearly suggest that caution should be paid to evaluate engraftment kinetics of infused cells, at least in mice model.