Allogeneic Renal Graft Rejection in a Rat Model: In Vivo MR Imaging of the Homing Trait of Macrophages

To evaluate the feasibility of MR imaging to depict the in vivo recruitment of superparamagnetic iron oxide (SPIO)-labeled macrophages and to aid diagnosis of graft rejection in kidney transplantation.This study was approved by the institution's committee on animal research. Eighteen male Lewis rats received a kidney transplant; 12 had an F344 rat donor and six had a Lewis rat donor. Peritoneal macrophages were harvested from thioglycollate-treated Lewis rats, cultured, and labeled with SPIO. After resuspension of macrophages in a concentration of 1 x 10(7) cells per milliliter of Hanks balanced salt solution, 5 x 10(6) of SPIO-labeled macrophages was administered through the tail vein 2 or 5 days after transplantation in each group. The transplanted kidneys were imaged on a 4.7-T MR imager 24 hours after macrophage administration. The Wilcoxon signed rank test was performed for evaluating the differences between the relative signal intensity (SI) before and after SPIO-labeled macrophage administration.A low-SI zone was predominantly noted in the medulla of the transplanted kidneys, and the relative SI decreased significantly from 1.40 to 0.53 (P.001) in the allogeneic transplants following SPIO-labeled macrophage administration 5 days after the allogeneic transplantation. In the syngeneic group, the lower-SI zone was not noted in the grafts. At histopathologic examination, the lower-SI zone corresponded to the distribution of the SPIO-labeled macrophages.This study demonstrates that the homing of intravenously administered SPIO-labeled macrophages can be monitored in the allograft rejection model on in vivo MR images.