Liver transplantation remains the only therapeutic option for many acute and chronic end-stage liver diseases. However, this approach is limited by a serious shortage of donor organs required for transplantation. Hepatocytes have been reported to be generated from cells not originated from liver, such as hematopoietic stem cells, mesenchymal stem cells and most recently embryonic stem cells. However, the frequency of these stem cell-derived hepatocytes is very low in most studies. Therefore, the significance of stem cell contribution to the repair of liver damage is still controversial. To further explore this potential, we used the beta-glucuronidase (GUSB)-null NOD/SCID/MPSVII mouse model for better identification of engrafted human cells. Enriched cord blood primitive cells (lineage depleted cells with high aldehyde dehydrogenase activity, ALDHhiLin−) were transplanted into irradiated NOD/SCID/MPSVII mice. One month after transplantation, carbon tetrachloride (CCl4) was administrated into the mice twice a week for 4 weeks to induce liver damage. In this model, ALDHhiLin− cells efficiently engrafted in the recipient mouse livers as demonstrated by GUSB positive immunohistological staining and the presence of human Alu DNA using PCR. The percentage of human cells in these livers ranged between 3% and 14.2% using quantitative real-time PCR. These engrafted cells improved recovery of the mice from toxic insult, and significantly increased the numbers of surviving mice. Furthermore, human liver-specific a-1-antitrypsin mRNA and albumin protein were expressed in the recipient livers. Interestingly, human vs. murine centromeric fluorescent in situ hybridization analysis on the liver sections demonstrated that most human cells were not fused to mouse cells. However, mouse nuclei were detected in the majority of the albumin-expressing cells, suggesting that fusion had occurred and was responsible for the appearance of donor derived hepatocyte-like cells. With the goal of achieving higher levels of liver reconstitution than had been possible using the adult stem cells, we began studying engraftment of human embryonic stem cells (hESC), which theoretically have the potential to regenerate any tissue. The H1 cell line was cultured on mouse embryonic fibroblasts then allowed to form embryoid bodies (EBs) in suspension culture for 7 days with or without further expansion and differentiation in attached culture for another month. EBs were dissociated into a single cell suspension and transplanted into NOD/SCID/MPSVII mice or NOD/SCID/IL2Rγ−/− mice via the tail vein after 300 RADs sublethal radiation with or without CCl4 administration. Two months post-transplantation, the human EB-derived cells were found to be well engrafted in the NOD/SCID/MPSVII mouse livers, spleens and kidneys, using the clear-cut enzymatic identification method for cells expressing normal levels of beta-glucuronidase in the mice, which are null for the enzyme. Human DNA was also detected in the recipient mouse liver. Most interestingly, human albumin-expressing cells were also found in the livers of engrafted mice. Our data indicate that the progeny of cord blood stem cells can significantly enhance survival of mice with severe liver damage, and that fusion can occur between transplanted and recipient cells. This could be a normal mechanism of liver repair, since hepatocytes exist normally as multinucleate cells. We also demonstrate that the progeny of hESC can be effectively dissociated and transplanted intravenously, then home to the liver and differentiate to the hepatocyte lineage in an immune deficient mouse model of liver damage.

AMD3100 inhibits the interaction between SDF-1 and CXCR4, and rapidly mobilizes hematopoietic progenitors for clinical transplantation. However, the repopulating function of human cells mobilized with AMD3100 has not been characterized in comparison to cells mobilized with granulocyte-colony stimulating factor (G-CSF) in the same donor. Therefore, healthy donors were leukapheresed 4 hours after injection with AMD3100; after 10 days of drug clearance the same donor was mobilized with G-CSF, allowing a paired comparison of repopulation by mobilized cells. Transplantation of mononuclear cells (MNC) or purified CD34+ cells was compared at limiting dilution into NOD/SCID mice. Human AMD3100-mobilized MNC possessed enhanced repopulating frequency in comparison to G-CSF-mobilized MNC from paired donors, and purified CD34+ progenitors were at least as efficient as the G-CSF mobilized cells. The frequencies of NOD/SCID repopulating cells (SRC) were 1 SRC in 8.7 × 106 AMD3100-mobilized MNC compared to 1 SRC in 29.0 × 106 G-CSF-mobilized MNC, and 1 SRC in 1.2 × 105 AMD3100-mobilized CD34+ cells compared to 1 SRC in 1.8 × 105 G-CSF-mobilized CD34+ cells. Hematopoietic differentiation of transplanted progenitors was similar after AMD3100 or G-CSF-mobilization. Thus, AMD3100 mobilized peripheral blood represents a rapidly obtained, highly repopulating source of hematopoietic progenitors for clinical transplantation.

Human stem cells from adult sources have been shown, in our laboratory and others, to promote tissue repair. Different populations of stem cells have been shown to contribute to the regeneration of muscle, liver, heart, and vasculature, although the mechanisms by which they accomplish this are still not well understood. Stem cells are known, however, to secrete a variety of factors that have both paracrine and autocrine activities. One theory of tissue repair and regeneration by adult stem cells is that they home to hypoxic and/or inflamed areas, and release trophic factors that hasten endogenous repair. These secreted bioactive factors suppress the local immune system, enhance angiogenesis, inhibit fibrosis and apoptosis, and stimulate recruitment, retention, mitosis and differentiation of tissue-residing stem cells. These effects, which are referred to as trophic effects, are distinct from the direct differentiation of stem cells into the tissue to be regenerated. We tested human umbilical cord blood (UCB) derived CD34+, ALDHhiLin− and ALDHloLin− cells following transplantation to NOD/SCID or NOD/SCID/B2M null mice with experimentally induced acute myocardial infarction (AMI). We used combined nanoparticle labeling and whole organ fluorescent imaging to detect homing of the cells to multiple organs 48 hours post transplantation. Long term engraftment and the regenerative effect of cell treatment was assessed four weeks post transplant. There was superior homing of ALDHhiLin− cells to the site of injury, as compared to CD34+ or ALDHloLin− cells at 48 hours post transplantation. At four weeks post transplantation, ALDHhiLin− cells engrafted multiple organs, including the heart, liver and kidney, at higher frequencies than ALDHloLin− cells. We found no donor derived cardiomyocytes and only few endothelial cells of donor origin. However, there was a significant increase in the density of large caliber vessels in the central infarct zone of ALDHhiLin− cell-transplanted mice, as compared to PBS and ALDHhiLin− cell treated groups. Tissue staining in the damaged regions revealed that the transplanted human cells had undergone very few cell divisions after homing to the area of hypoxia or inflammation. It is important to note that, with over 300 mice now analyzed in tissue repair experiments in our group, no adult human stem cell-derived tumors have ever been observed, even though the mouse strains used have no capacity to reject the human cells. The paucity of human cells remaining in the tissue after repair suggest that the tissue improvements that were observed were not the result of generation of transplanted cell-derived endothelial cells or cardiac tissue, but suggest that cytokines secreted from transplanted cells potentiated angiogenic activity and tissue repair by the endogenous murine cells. Our data indicate that adult human stem cells do not become a significant part of the damaged tissue, but rapidly home to and persist only temporarily at a site of hypoxia or inflammation to exert significant trophic effects on tissue repair, and to enhance recovery of the vasculature.

The use of nanometer-sized iron oxide particles combined with molecular imaging techniques enables dynamic studies of homing and trafficking of human hematopoietic stem cells (HSC). Identifying clinically applicable strategies for loading nanoparticles into primitive HSC requires strictly defined culture conditions to maintain viability without inducing terminal differentiation. In the current study, fluorescent molecules were covalently linked to dextrancoated iron oxide nanoparticles (Feridex) to characterize human HSC labeling to monitor the engraftment process. Conjugating fluorophores to the dextran coat for fluorescence-activated cell sorting purification eliminated spurious signals from nonsequestered nanoparticle contaminants. A short-term defined incubation strategy was developed that allowed efficient labeling of both quiescent and cycling HSC, with no discernable toxicity in vitro or in vivo. Transplantation of purified primary human cord blood lineage-depleted and CD34(+) cells into immunodeficient mice allowed detection of labeled human HSC in the recipient bones. Flow cytometry was used to precisely quantitate the cell populations that had sequestered the nanoparticles and to follow their fate post-transplantation. Flow cytometry endpoint analysis confirmed the presence of nanoparticle-labeled human stem cells in the marrow. The use of fluorophorelabeled iron oxide nanoparticles for fluorescence imaging in combination with flow cytometry allows evaluation of labeling efficiencies and homing capabilities of defined human HSC subsets.

The development of novel cell-based therapies requires understanding of distinct human hematopoietic stem and progenitor cell populations. We recently isolated reconstituting hematopoietic stem cells (HSCs) by lineage depletion and purification based on high aldehyde dehydrogenase activity (ALDH(hi)Lin- cells). Here, we further dissected the ALDH(hi)-Lin- population by selection for CD133, a surface molecule expressed on progenitors from hematopoietic, endothelial, and neural lineages. ALDH(hi)CD133+Lin- cells were primarily CD34+, but also included CD34-CD38-CD133+ cells, a phenotype previously associated with repopulating function. Both ALDH(hi)CD133-Lin- and ALDH(hi)CD133+Lin- cells demonstrated distinct clonogenic progenitor function in vitro, whereas only the ALDH(hi)CD133+Lin- population seeded the murine bone marrow 48 hours after transplantation. Significant human cell repopulation was observed only in NOD/SCID and NOD/SCID beta2M-null mice that received transplants of ALDH(hi)CD133+Lin- cells. Limiting dilution analysis demonstrated a 10-fold increase in the frequency of NOD/SCID repopulating cells compared with CD133+Lin- cells, suggesting that high ALDH activity further purified cells with repopulating function. Transplanted ALDH(hi)CD133+Lin- cells also maintained primitive hematopoietic phenotypes (CD34+CD38-) and demonstrated enhanced repopulating function in recipients of serial, secondary transplants. Cell selection based on ALDH activity and CD133 expression provides a novel purification of HSCs with long-term repopulating function and may be considered an alternative to CD34 cell selection for stem cell therapies.

In defining ex vivo cultivation strategies for human HSC gene transduction, a key factor is the responsiveness of the most primitive cells to the in vitro conditions with the aim of maintaining viability without inducing terminal differentiation. Important insights have been gained into the molecular basis of cell cycle regulating mechanisms. The major regulatory events leading to cell proliferation and differentiation occur between the G0 and G1 phases of the cell cycle, when the cell commits itself to DNA replication and both positive and negative external signals are integrated into the cell cycle. Evaluating the effect of an ex vivo transduction protocol with respect to this decisive junction between quiescence and cell division remains an elusive goal. Here, we present a novel p27kip1 flow cytometry assay which assesses the earliest molecular responses to a defined clinically applicable transduction protocol. Using p27kip1 as the main flow cytometric marker in combination with CD34 we developed methods to simultaneously assess the molecular events ongoing in individual human cord blood Lin- cells while they were cultured for 72 hours in X-Vivo 15 serum free medium supplemented with Flt3, SCF and TPO on Retronectin (RN) coated plates. The p27kip1 assay is run in four color combinations with Ki67, Cyclin A, PCNA, or CD133. To evaluate whether this protocol had the preferred effect of sustaining viability without inducing terminal differentiation we compared it to a more stimulatory extended protocol of X-Vivo 15 supplemented with SCF, Flt-3, TPO and IGF-1 on RN coated plates. p27kip1 expression in cells subjected to the basic ex vivo transduction protocol was found to be markedly high, ranging from 74.1% (T=24 Hrs) to 68.1% (T=72 Hrs.), whereas the comparable p27kip1 expression in cells subjected to the extended protocol was between 29.8% (T=24 Hrs.) and 18.8% (T=72 Hrs.). At the same time CD34 expression increased from 53.8% (T=24 Hrs.) to 80.5% (T=72 Hrs.) in the basic protocol, while the CD34 expression on cells subjected to the extended protocol was decreased from 87.5% CD34+ (T=24 Hrs.) to 73% CD34+ (T=72 Hrs.). Finally, in cultures subjected to the 72 hr. basic ex vivo transduction protocol we repeatedly found a small, but distinct population of p27kip1 expressing cells with up to 500 times more p27kip1 signal than the main population. We found this population to be 1.5%+/-0 0.3 (n=3) of the total cells in the cultures after 72 Hrs. Gating on this population showed no expression of the hematopoietic marker CD45 or the major stem/progenitor markers CD34, CD133, and CD117, indicating a highly quiescent cell type. In summary, the p27-based flow cytometric technique allows assessment of the cell cycle and differentiative status of individual hematopoietic stem and progenitor cells during periods of ex vivo culture.

Novel stem cell-based therapies require new imaging techniques to enable the visualization and tracking of transplanted cells in vivo for evaluation of homing and engraftment parameters. Here we present in vitro and in vivo data on nanoparticle labeling of umbilical cord blood (UCB) CD34+ and lineage depleted HSC subsets, in addition to labeling of CD34+ human bone marrow, G-CSF and AMD-3100 M-PBSC. For these studies, we used a 24 hr. clinically applicable ex vivo labeling protocol including protamine complexed ferrumoxide nanoparticles conjugated to Alexa 647 dye or Alexa 750 (FE-PRO[647] or Fe-Pro[750]). Cell cultivation was carried out using serum free X-Vivo 15 defined medium with 10 ng/ml rhTPO, rhSCF, and Flt-3-ligand on retronectin. Transplantation of FACS sorted 97.5% pure FE-PRO [647] labeled human UCB-derived CD34+ cells into NOD/SCID/B2M null mice resulted in mean engraftment levels of 66.7%+/−1.0% CD45+ human cells, after 8 weeks, as compared to 41.8%+/−20.4% in control mice that received non-loaded cells. These data indicate that the FE-PRO [647] did not compromise the engraftment capacity of the human HSC (p>0.05). Moreover, transplantation of labeled human UCB-derived CD34+ cells into NOD/SCID/B2M null mice for in vivo tracking using flow cytometry and magnetic resonance imaging allowed visualization of the FE-PRO[647] labeled CD34+ cells in the spleen and marrow of the recipients, up to three weeks post transplantation. In spleens, human CD34+ FE-PRO [647]+ levels decreased from 20.6.0+/−13.4% (N=5) one week post transplantation to undetectable levels after three weeks (N=7). The total human CD45+ engraftment as evaluated in total murine marrow was 18.7+/−11.3% (N=7) after three weeks. All animals in the cohort were positive for CD34+ FE-PRO[647]+ engrafted human cells (0.8+/− 0.2 %, N=7). In vivo imaging of animals transplanted with 2–5 x 105 human CD34+ cells (16.8% Fe-Pro[750]+ labeled) was performed using the Kodak 4000 MM multimodal imaging unit in which the luminescence signal arising from the nano-labeled human cells can be precisely localized by overlaying the images with x-ray pictures of the animals. Surprisingly, asymmetric engraftment was repeatedly observed between right leg tibia-femur and left-leg tibia-femur in a cohort of 8 NOD/SCID mice at various timepoints over a total of 20 days after intravenous transplantation. Human engraftment was subsequently confirmed and correlated to the luminescence signal by flow cytometry of the bones and spleens of the imaged animals, at the same timepoints. These data demonstrate that nano-particles can be used to label repopulating human HSC for subsequent in vivo tracking, without toxicity to the engrafting cells. This technique offers new methods to dynamically image the homing and engraftment of purified human hematopoietic stem cells over the initial three weeks post-transplantation, in live animals.

The use of iron based nano-particles for multi-modal imaging is gaining interest, since it allows high resolution non-invasive in vivo imaging of human hematopoietic homing and engraftment events in xenograft models. The uptake of ferridex nano-particles complexed to cationic protamine sulphate is believed to be non-specific through mechanisms like endocytosis, but this has not been well defined for hematopoietic stem cells (HSC). In defining ex vivo cultivation strategies for manipulation of human HSC, a key factor is the responsiveness of the most primitive cells to the in vitro conditions, with the aim of maintaining viability without inducing terminal differentiation. Here, we present a novel flow cytometry assay which assesses the earliest molecular responses to a defined clinically applicable ex vivo protocol, aimed at facilitating labeling of human stem/progenitor cells using protamine sulphate complexed nano-particles for subsequent in vivo imaging. We used intracellular staining for the cell cycle inhibitor p27kip1, which is present in the highest levels in non-cycling cells, as the primary flow cytometric marker in combination with CD34, CD133 and Alexa 488, 647 and 750 conjugated ferridex nano-particles and the membrane dye PKH26. An assay was developed to simultaneously assess the molecular events occurring in individual human cord blood Lin− or CD34+ cells while they were cultured for up to 72 hours in X-Vivo 15 serum free medium supplemented with Flt3, SCF and TPO on Retronectin (RN) coated plates with or without nano-particles. Co-expression of p27kip1, CD34 or CD133 in the cultured cells slowly decreases from 86.1% CD34+p27kip1 (T=0) to 76.7%+/−12.2% (T=72) and from 89.6% CD133+p27kip1+ (T=0) to 54.1%+/−10.4% (T=72). We suggest that this slow decrease represents cells dividing and potentially differentiating over the time course of the ex vivo cultivation period. Assessing uptake of fluorescent conjugated nano-particles over a 72 hr period showed that the uptake of particles in CD34+ and CD133+ cells declined significantly after the first 24 hrs., from 32.5+/−3.7% nano-positive CD34+ cells to 19.2+/−2.9% at 48 hours ex vivo with a more significant decline to only 8.3+/−3.7% nano positive CD34+ cells in the culture after 72 hours ex vivo. The same decline in uptake over time was observed in cultured human CB cells that were positive for CD133. PKH26 co-staining demonstrated that the majority of cells that undergo cell division within the first 24 hours of ex vivo culture are the most likely to uptake the nano-particles. In summary, using a multi color p27kip1 based flow-cytometry assay, we found that human Lin−, CD133+, and CD34+ cells uptake Fe-Pro in a fashion which is not entirely cell cycle independent as previously suggested. These data indicate that cell cycle or metabolic status may influence the ability of human hematopoietic stem and progenitor subsets to uptake the protamine sulphate-complexed nano-particles. These findings emphasize the need to carefully develop ex vivo conditions for nano-particle labeling of primary human stem cells in order to perform accurate in vivo imaging of the most primitive human hematopoietic stem and progenitor cells.

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.

Top of pageAbstract Transient transduction of human hematopoietic stem cells with novel nano-sized iron particles offers a new approach to transient cell modulation in vitro and in vivo. Moreover, nano particles serve a dual purpose by enabling tracking of labeled cells in vivo by means of magnetic resonance imaging. However, defining ex vivo cultivation strategies for the transduction and/or labeling of human HSC using these particles has not yet been well investigated. Here we present in vitro and in vivo data on human cord blood CD34+ HSC subjected to a 24 hr. clinically applicable ex vivo transduction protocol including protamine complexed ferrumoxide nanoparticles conjugated to Alexa 647 dye (FE-PRO[647]). Cell cultivation were carried out in tissue culture-treated 6 well plates coated with the CH-296 fibronectin fragment. We used X-Vivo 15 defined serum free media supplemented with 10 ng/ml rhTPO, rhSCF, and Flt-3-ligand. Initial CFU-GEMM data indicated that the presence of the FE-PRO [647] did not compromise the human HSC ability to generate colonies in vitro. Subsequent transplantation of labeled human cord blood CD34+ into beta2-NOD/SCID mice for in vivo tracking using flow cytometry consistently showed FE-PRO[647] labeled CD34+ cells in the marrow of the recipients three weeks post transplantation. The total human CD45 engraftment as evaluated in total murine marrow was 18.7+/|[minus]|11.3% (N=7), with a CD34+ component of 10.2+/|[minus]|9.0% (N=7). All 7 animals in the cohort were positive for FE-PRO[647] positive engrafted human cells (0.9+/|[minus]| 0.2 %, N=7). Most interestingly, a striking correlation of the total FE-PRO[647] positive cells and the CD34+ HSC was found in the marrow of the recipients (FE-PRO[647]+/CD34+ was 0.8+/|[minus]|0.2, N=7). Though a subset of CD133+ cells was found in all engrafted mice, interestingly enough no correlation of FE-PRO[647] and CD133+ was found. Moreover, correlating the mean fluorescence activity of the initially FE-PRO [647] transduced CB CD34+ cells to the FE-PRO[647]+/ CD34+ cells found in the recipient marrows 3 weeks post transplantation (relative value: 90.5 versus 2.9+/|[minus]|0.7, N=7) indicated that these CD34+ cells had only divided a total of 5 times over the 3 weeks time period. We hypothesize that these CD34+ cells represent long term engrafting human HSC, whereas the majority of the resulting CD34+ engraftment represents committed progenitors, giving rise to short term engraftment. Overall, these data show that nano particles can indeed be used to transiently transduce repopulating human HSC. This has lead us to define conditions for in vivo modulation of human HSC by co-transduction of FE-PRO[647] and the HIVEF1-EGFP lenti viral vector, with the aim of improving the lenti viral transduction efficiency. In vitro and in vivo data will be presented.

The use of novel nano-sized iron particles and magnetic imaging techniques are ideal for studies of homing and trafficking of long-term repopulating, human hematopoietic stem cells (HSC). However, defining ex vivo cultivation strategies for the transduction of nano-particles into the HSC involves the use of strictly defined in vitro culture conditions with the aim of maintaining viability without inducing terminal differentiation. Here we present data on in vitro uptake and retainment of nanoparticles in defined human stem cell subsets. We have chosen to use protamine sulphate-complexed Ferridex nano-particles conjugated to an Alexa 647 dye (FE-PRO[647]) to load the cells ex vivo prior to analysis, as these particles are biodegradable and, eventually, will be metabolized by the cells. This makes these particles very well suited for loading of primary cells for transplantation and subsequent imaging of the transplanted cells by MRI. Whereas most cell line studies performed show a uniform 60-75% uptake of the FE-PRO within the first 24 hours in vitro, our study of primary human cord blood (CB) lineage depleted (lin-) cells shows that the overall load of Fe-Pro [647] particles declines from 61.4+/|[minus]|0.9 (n=2) after 24hrs to a level of 39.7+/|[minus]|0.4 (n=2) after 72 hours in vitro. This indicates that the cells ability to uptake the FE-Pro [647] is declining with a prolonged ex vivo cultivation. Evaluating the FE-Pro [647] uptake based on the primary stem cell markers CD34 and CD133, however, reveals a different picture. Most notably, after 72 hours in culture the two stem cell populations have a very uniform level of the G0 determinant p27kip1, indicating that the cells are highly quiescent, but only 8.3+/|[minus]|3.7% CD34 and 5.1+/|[minus]|4.5% (n=2) CD133 cells contain the FE-Pro [647]. This points towards the idea that quiescent stem cells after subjection to ex vivo cultivation might not easily uptake the particles, possibly due to their low metabolic activity. Based on these data we next reduced the ex vivo protocol to a single 24 hr. exposure of the cells to the FE-PRO [647] particles. Using this modified ex vivo protocol we have tested a number of defined stem cell subsets for their ability to uptake and retain FE-PRO [647]. FE-PRO [647] uptake in CB CD34 was 17%, with only 15.1% of these displaying the p27kip1 high quiescent phenotype, indicating that the majority of the transduced cells were actively cycling ex vivo. Currently in vivo data are being collected from cohorts of beta-2 NOD/SCID mice. Using the clinically interesting cell sources of AMD3100 and G-CSF mobilized CD34 cells, we found FE-PRO [647] uptake over 24 hrs. ex vivo to be 7% (G-CSF) and 10.3% (AMD3100), indicating that in these cell sources, targeting of the nanoparticles to the quiescent repopulating stem cells might be even more challenging. Overall our data shows that the use of nanoparticles to transiently transduce human HSC is indeed feasible, using a clinically applicable protocol.

Interactions between stromal derived factor-1 (SDF-1 or CXCL12), and its receptor CXCR4 regulate hematopoietic stem and progenitor cell retention in the bone marrow. AMD3100, a bicyclam molecule that selectively blocks the interaction between CXCL12 and CXCR4, has recently been used in clinical trials to rapidly mobilize hematopoietic progenitor cells. However, the functional properties of human stem and progenitor cells mobilized with this agent are not well characterized. Here, we directly compared the NOD/SCID repopulating function of CD34+ cells rapidly mobilized (4 hours) by AMD3100 versus CD34+ cells mobilized after 5 days of G-CSF treatment. A total of 7 HLA-matched sibling donors were leukapheresed after a single injection of 240ug/kg AMD3100. After 1 week of drug clearance, the same donor was mobilized with G-CSF, allowing a paired comparison of the repopulating function of cells mobilized by the two agents. Total CD34+ cells mobilized by AMD3100 treatment averaged 1.2±0.4x106 CD34+ cells/kg (range 0.4–2.1x106 CD34+ cells/kg), as compared to G-CSF treatment at 3.2±0.9x106 CD34+ cells/kg (range 1.7–5.7 x106 CD34+ cells/kg). Leukapheresis total mononuclear cell (MNC) fraction or purified CD34+ cells (>90% purity), were isolated and transplanted into sublethally irradiated NOD/SCID mice at varying doses. BM, spleen, and peripheral blood of mice were harvested 7–8 weeks post-transplantation and analyzed by flow cytometry for the presence or absence of engrafting human cells. Low frequency human engraftment events (<0.2% human cells) were confirmed by PCR for P17H8 alpha-satellite human DNA sequences. Injection of 1–40x106 MNC or 0.5–5x105 CD34+ cells produced consistent human engraftment and allowed limiting dilution analysis using Poisson statistics to be performed on paired samples of AMD3100 and G-CSF leukapheresis products from 3 individual patients. The calculated frequencies of NOD/SCID repopulating cells (SRC) were 1 SRC in 11.5x106 AMD3100-mobilized MNC (n=50) compared to 1 SRC in 44.8x106 G-CSF-mobilized MNC (n=55). For purified CD34+ populations, the overall frequency of repopulating cells was 1 SRC in 1.0x105 AMD3100-mobilized CDC34+ cells (n=53) compared to 1 SRC in 3.1x105 G-CSF-mobilized CD34+ cells (n=45). These data correspond to a 3–4-fold increase in overall repopulating function demonstrated by AMD3100 mobilized cells. Multilineage hematopoietic differentiation of transplanted CD34+ cells was similar for AMD3100 and G-CSF-mobilized CD34+ cells, with equivalent production of myelo-monocytic cells (CD33+CD14+), immature B-lymphoid cells (CD19+CD20+), and primitive repopulating (CD34+CD133+CD38−) cells 7–8 weeks post-transplantation. These studies indicate that human AMD3100-mobilized MNC and purified CD34+ cells possess enhanced repopulating capacity, as compared to G-CSF mobilized counterparts from the same donor. Thus, AMD3100 mobilized peripheral blood represents a rapidly obtained and highly functional source of repopulating hematopoietic stem cells for clinical transplantation procedures.

In defining ex vivo cultivation strategies for human HSC gene transduction, a decisive factor to consider is the responsiveness of the most primitive cells to the employed in vitro conditions with the aim of maintaining viability without inducing terminal differentiation. Important insights have been gained into the molecular basis of cell cycle regulating mechanisms in the last few years. The major regulatory events leading to cell proliferation and differentiation occur between the G0 and G1 phases of the cell cycle, at which point the cell commits itself to DNA replication and both positive and negative external signals are integrated into the cell cycle. Nonetheless, evaluating the effect of an ex vivo transduction protocol with respect to this decisive junction between quiescence and cell division remains an elusive goal. Here, we present a novel p27kip1 flow cytometry assay which assesses the earliest molecular responses to a defined clinically applicable ex vivo transduction protocol. Using p27kip1 as the main flow cytometric marker in combination with CD34 we developed methods to simultaneously assess the molecular events ongoing in individual cord blood Lin- cells while they were cultured for 72 hours in X-Vivo 15 serum free medium supplemented with Flt3, SCF and TPO on Retronectin (RN) coated plates. The p27kip1 assay is run in four color combinations with p21waf, Cyclin E, Cyclin A, PCNA, or CD133. To evaluate whether the employed ex vivo protocol had the preferred effect of sustaining viability without inducing terminal differentiation we compared it to a more stimulatory extended protocol of X-Vivo 15 supplemented with SCF, Flt-3, TPO and IGF-1 on RN coated plates. p27kip1 expression in cells subjected to the basic ex vivo transduction protocol was found to be markedly high, ranging from 74.1% (T=24 Hrs) to 68.1% (T=72 Hrs.), whereas the comparable p27kip1 expression in cells subjected to the extended protocol was found to be between 29.8% (T=24 Hrs.) and 18.8% (T=72 Hrs.). At the same time CD34 expression rose from 53.8% (T=24 Hrs.) to 80.5% (T=72 Hrs.) in the basic protocol, while the CD34 expression on cells subjected to the extended protocol was decreased from 87.5% CD34+ (T=24 Hrs.) to 73% CD34+ (T=72 Hrs.). Finally, in cultures subjected to the 72 hr. basic ex vivo transduction protocol we have repeatedly found a small, but distinct population of p27kip1 expressing cells with up to 500 times more p27kip1 signal than the main population, as evaluated by fluorescence intensity. This population is 1.6% of the total cells in the cultures after 72 Hrs, and has not been observed in either the 24 or the 48 hrs. time point evaluations, nor in the cultures subjected to the extended protocol. Upon gating on this population we found no co-expression of the major stem/progenitor and haematopoietic markers CD34, CD133, CD117 or CD45, indicating a highly quiescent cell type. In summary, this technique allows assessment of the cell cycle and differentiative status of individual hematopoietic stem and progenitor cells during periods of ex vivo culture.