Comparison of Human Stem Cell Homing after Intravenous or Intra-Femoral Transplantation Using Multimodal In Vivo Imaging of Repopulating Human CD34+ Cells Labeled with Far-Red Fluorescent-Conjugated Nanoparticles.

The use of novel nano-sized iron particles and magnetic imaging techniques are ideal for studies of homing and trafficking after labeling and transplantation of long-term repopulating, pluripotent human hematopoietic stem cells (HSC). Whereas the use of luciferase as a reporter for in vivo imaging requires transfection or viral transduction of the target cells to generate a measurable signal, we present an in vivo imaging system based upon the measurement of deep tissue penetrating, near far-red Alexa 750 nm organic dye conjugated to nano-sized ferum oxide particles (FE [750]), transiently introduced into highly purified human hematopoietic stem/progenitor subsets through complexing to the cationic agent protamine sulphate (Pro). Previous results from our group demonstrate that we can track the FE-Pro [750] labeled cells for a minimum of 30 days post transplantation using flow cytometry, before the signal diminishes due to cell division. We used a Kodak 4000MM multimodal imaging unit, which allows a precise anatomical localization of the signal measured through overlaying of the high resolution luminescent profile with x-ray images. NOD/SCID Beta2M null mice were transplanted using intravenous (IV) or intra femural (IF) injection with 1 x 10⁵ or 2 x 10⁵ human cord blood CD34+ cells labeled with the FE-Pro[750] nano particles. The animals were imaged directly after the injections to confirm successful transplantation, and then were subsequently imaged over a period of 8 days (cohort 1), 20 days (cohort 2) or 30 days (cohort 3). At the end point of each time period, animals were sacrificed and flow cytometry was performed to assess and confirm the location of the human engraftment in right and left leg bones as well as in spleens. Our imaging data shows that the human stem cells transplanted IF reside in the injection site for up to 10 days post transplantation, before the dilution of the signal becomes evident, with migration to the spleen at that time point indicating active engraftment, but without noticeable spreading of labeled cells to the non-injected leg. IV injected animals showed an initial strong repopulation of the spleens, with subsequent however asymmetric homing to the femur-tibiae of the legs over 8 days post transplantation, indicating a delayed homing as compared to the more direct IF delivery of the transplantation dose. Flow cytometry results confirmed the asymmetric homing to the femur-tibia bones of IV transplanted animals with one mouse in particular showing a 0.6% CD45+/Fe-Pro[750]^low engraftment in the left femur-tibia whereas the right femur-tibia showed a stronger 1.3% CD45+/Fe-Pro[750]^low engraftment at day 8. In conclusion, we present a novel system for imaging of human hematopoietic stem cell homing and engraftment post transplantation using dye conjugated nano-particles. This system allow an unprecedented capacity to observe and assess the in vivo dynamics of the engraftment process with high resolution, following intravenous or intrafemoral injection of different purified human stem cell populations.