Bioluminescence imaging of hematopoietic stem cell repopulation in murine models

In Vivo Observation of Multi‐phase Spatiotemporal Cellular Dynamics of Transplanted HSPCs During Early Engraftment

Hematopoietic stem cell transplantation (HSCT) is commonly used to treat patients with various blood disorders, genetic and immunological diseases, and solid tumors. Several systemic complications following HSCT are critical limiting factors for achieving a successful outcome. These systemic complications are mainly due to the lack of initial engraftment after transplantation. However, the detailed underlying cellular dynamics of early engraftment have not been fully characterized yet. We performed in vivo longitudinal visualization of early engraftment characteristics of transplanted hematopoietic stem and progenitor cells (HSPCs) in the mouse calvarial bone marrow (BM). To achieve this, we utilized an in vivo laser‐scanning confocal microscopy imaging system with a cranial BM imaging window and stereotaxic device. We observed two distinct cellular behaviors of HSPCs in vivo, cluster formation and cluster dissociation, early after transplantation. Furthermore, we successfully identified three cellular phases of engraftment with distinct cellular distances which are coordinated with cell proliferation and cell migration dynamics during initial engraftment.

A Functional Bioluminescent Zebrafish Screen for Enhancing Hematopoietic Cell Homing

To discover small molecules that modulate hematopoietic cell homing after adoptive transfer, we created a transgenic zebrafish expressing firefly luciferase downstream of the ubiquitin promoter (ubi:luc) to serve as a hematopoietic donor. Bioluminescence imaging (BLI) was used to detect and follow ubi:luc hematopoietic cells that homed to the marrow as early as 1 day post-transplant. BLI was able to detect the biological effect of prostaglandin E₂ on early homing/engraftment of donor hematopoietic cells. This system was utilized in a functional screen of small molecules to enhance homing/engraftment. We discovered a phytosterol, ergosterol, that could increase hematopoietic cell homing in zebrafish and mice. In addition, ergosterol increased CXCR4 expression and promoted expansion of Lin⁻SCA-1⁺KIT⁺ cells in vitro. We have demonstrated the utility of in vivo BLI to non-invasively monitor donor hematopoietic cell activity in adult zebrafish as a functional screen for mediators of cellular homing.

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Bioluminescent imaging (BLI) can track engrafting hematopoietic cells

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BLI can be used for screening of enhancers of hematopoietic cell homing

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Using BLI, ergosterol was found to increase hematopoietic cell homing

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Ergosterol affects hematopoietic progenitor migration, growth, and viability in vitro

Hepatocellular Carcinoma Cells Carrying a Multimodality Reporter Gene for Fluorescence, Bioluminescence, and Magnetic Resonance Imaging In Vitro and In Vivo

RATIONALE AND OBJECTIVES

The study aimed to evaluate the feasibility of imaging or tracking hepatocellular carcinoma cells by modifying these cells to carry a multimodality reporter gene, enabling fluorescence, bioluminescence, and magnetic resonance imaging (MRI) in vitro and in vivo.

MATERIALS AND METHODS

HepG2 cells were labeled with the enhanced green fluorescent protein (EGFP)/luciferase2/ferritin-the multimodality reporter gene (labeled HepG2 cells). The labeled and unlabeled HepG2 cells were cultured in vitro and then injected subcutaneously into mice as a hepatoma model in vivo. The expressions of EGFP, luciferase2, and ferritin in HepG2 cell suspensions and hepatoma model were investigated using fluorescence, bioluminescence, and MRI.

RESULTS

Individual HepG2 cells expressing EGFP were identified under blue laser excitation. The linear coefficient between the optical signal intensity of luciferase2 and the number of labeled cells was 0.993. MRI was used to distinguish the T2* signal of 2 × 10⁷ cells/mL between the labeled (6.67 ± 1.88 ms) and unlabeled cells (10.66 ± 2.22 ms) (P = 0.034). In vivo, individual HepG2 cells expressing EGFP in frozen sections were observed. Labeled cells expressing luciferase2 have been detected since the second day after injection, and the bioluminescence increased with the tumor size. The T2* signal was significantly different between the labeled (6.04 ± 1.60 ms) and unlabeled cells (17.06 ± 2.17 ms) (P <0.001).

CONCLUSIONS

A multimodality reporter gene consisting of EGFP, luciferase2, and ferritin was successfully integrated into the HepG2 cell genome via a lentiviral vector and was highly expressed in the daughter cells. These cells could be detected by fluorescence, bioluminescence, and MRI in vitro and in vivo.

Single thrombopoietin dose alleviates hematopoietic stem cells intrinsic short- and long-term ionizing radiation damage. In vivo identification of anatomical cell expansion sites

Hematopoietic stem cells (HSC) are essential for maintaining the integrity of complex and long-lived organisms. HSC, which are self-renewing, reconstitute the hematopoietic system through out life and facilitate long-term repopulation of myeloablated recipients. We have previously demonstrated that when mice are exposed to sublethal doses of ionizing radiation, subsets of the stem/progenitor compartment are affected. In this study we examine the role of thrombopoietin (TPO) on the regenerative capacities of HSC after irradiation and report the first demonstration of efficacy of a single injection of TPO shortly after in vivo exposure to ionizing radiation for reducing HSC injury and improving their functional outcome. Our results demonstrate that TPO treatment not only reduced the number of apoptotic cells but also induced a significant modification of their intrinsic characteristics. These findings were supported by transplantation assays with long-term HSC that were irradiated or unirradiated, TPO treated or untreated, in CD45.1/CD45.2 systems and by using luciferase-labeled HSC for direct bioluminescence imaging in living animals. Of particular importance, our data demonstrate the skull to be a highly favorable site for the TPO-induced emergence of hematopoietic cells after irradiation, suggesting a TPO-mediated relationship of primitive hematopoietic cells to an anatomical component. Together, the data presented here: provide novel findings about aspects of TPO action on stem cells, open new areas of investigation for therapeutic options in patients who are treated with radiation therapy, and show that early administration of a clinically suitable TPO-agonist counteracts the previously observed adverse effects.

Polymeric vector-mediated gene transfection of MSCs for dual bioluminescent and MRI tracking in vivo

MSC's transplantation is a promising cell-based therapy for injuries in regenerative medicine, and in vivo visualization of transplanted MSCs with noninvasive technique is essential for the tracking of cell infusion and homing. A new cationic polymer, poly(ethylene glycol)-block-poly(l-aspartic acid)-grafted polyethylenimine functionalized with superparamagnetic iron oxide nanoparticles (PAI/SPION), was constructed as a magnetic resonance imaging (MRI)-visible non-viral vector for the delivery of plasmids DNA (pDNA) encoding for luciferase and red fluorescence protein (RFP) as reporter genes into MSCs. As a result, the MSCs were labeled with SPION and reporter genes. The PAI/SPION complexes exhibited high transfection efficiency in transferring pDNA into MSCs, which resulted in efficient luciferase and RFP co-expression. Furthermore, the complexes did not significantly affect the viability and multilineage differentiation capacity of MSCs. After the labeled MSCs were transplanted into the rats with acute liver injury via the superior mesenteric vein (SMV) injection, the migration behavior and organ-specific accumulation of the cells could be effectively monitored using the in vivo imaging system (IVIS) and MRI, respectively. The immunohistochemical analysis further confirmed that the transplanted MSCs were predominantly distributed in the liver parenchyma. Our results indicate that the PAI/SPION is a MRI-visible gene delivery agent which can effectively label MSCs to provide the basis for bimodal bioluminescence and MRI tracking in vivo.

Analyzing hematopoietic stem cell homing, lodgment, and engraftment to better understand the bone marrow niche

The existence of a bone marrow (BM) niche--the location in which hematopoietic stem cells (HSCs) reside--was proposed more than 30 years ago. Recent data suggest that the interaction of HSCs with cellular and extracellular components within the BM is critical for HSC regulation. The tracking of immunofluorescently labeled, prospectively isolated HSCs to and within the BM cavity allows the assessment of the regulatory processes involved in (1) homing, which involves transendothelial migration into the BM; (2) lodgment, including transmarrow migration through the extravascular space; and (3) BM reconstitution. Together, such analyses provide a better understanding of the cellular and extracellular components involved in the regulation of HSC quiescence and differentiation. Homing and lodgment of transplanted HSCs, the first critical steps in engraftment, involve multiple interactions between HSCs and the BM microenvironment. Herein, we describe a refined method of analyzing homing efficiency and spatial distribution of HSCs harvested from endosteal and/or central BM regions; we also review alternate methods. Using these techniques, microenvironment modifications within the recipient or surface protein-expression modifications on donor HSCs in animal models provide insights into components influencing the homing, lodgment, and engraftment processes.

Marrow Stromal Cell Infusion Rescues Hematopoiesis in Lethally Irradiated Mice despite Rapid Clearance after Infusion

Marrow stromal cells (MSCs, also termed mesenchymal stem cells) have been proposed as a promising cellular therapy for tissue injury including radiation-induced marrow failure, but evidence for a direct effect is lacking. To assess the effects of MSCs on survival after lethal irradiation, we infused syngeneic MSCs (either as immortalized MSCs clones or primary MSCs) intravenously into wild-type C57/Bl6 mice within 24 hours of lethal total body irradiation (TBI). Mice receiving either of the MSC preparations had significantly improved survival when compared to controls. In vivo imaging, immune histochemistry, and RT-PCR employed to detect MSCs indicated that the infused MSCs were predominantly localized to the lungs and rapidly cleared following infusion. Our results suggest that a single infusion of MSCs can improve survival after otherwise lethal TBI but the effect is not due to a direct interaction with, or contribution to, the damaged marrow by MSCs.

Clonal interrogation of stem cells

Individual stem cells are functionally defined by their self-renewal and differentiation potential. Methods for clonal analysis are essential for understanding stem cells, particularly given the increasing evidence for stem-cell heterogeneity. Stem cells reside within complex microenvironments, making single-cell analysis particularly challenging. Furthermore, simultaneous molecular and functional characterization of single stem cells is not trivial. Here we explore clonal assays applied to stem cell biology and their use in understanding the cellular and molecular basis of stem-cell identity.

Ultra-high-field MRI real-time imaging of HSC engraftment of the bone marrow niche

The bone marrow (BM) undergoes extensive remodeling following irradiation damage. A crucial part of restoring homeostasis following irradiation is the ability of hematopoietic stem cells (HSCs) to home to and engraft specialized niches within the BM through a remodeling BM vascular system. Here we show that a combination of ultra-high-field strength magnetic resonance imaging (17.6 T, MRI) coupled with fluorescent microscopy (FLM) serves as a powerful tool for the in vivo imaging of cell homing within the BM. Ultra-high-field MRI can achieve high-resolution three-dimensional (3D) images (28 × 28 × 60 μm(3)) of the BM in live mice, sufficient to resolve anatomical changes in BM microstructures attributed to radiation damage. Following intra-arterial infusion with dsRed-expressing BM cells, labeled with superparamagnetic iron oxides, both FLM and MRI could be used to follow initial homing and engraftment of donor HSC to a limited number of preferred sites within a few cell diameters of the calcified bone-the endosteal niche. Subsequent histology confirmed the fidelity and accuracy of MRI to create non-invasive, high-resolution 3D images of donor cell engraftment of the BM in living animals at the level of single-cell detection.

Factors Affecting Residence Time of Mesenchymal Stromal Cells (MSC) Injected into the Myocardium

The therapeutic mechanism of mesenchymal stromal/stem cells (MSC) for the treatment of acute myocardial infarction is not well understood. Our goal was to get insights into this mechanism by analyzing the survival kinetics of allogeneic and syngeneic cell transplants under different tissue conditions. Two MSC cell banks, stably and equally expressing the luciferase reporter construct, were developed for these studies and injected directly to the myocardium of Lewis rat recipients under syngeneic or allogeneic transplantation conditions. Cell survival was monitored by real-time fashion for up to 2 weeks, using optical imaging device (IVIS, Xenogen Corp.). We found that both syngeneic and allogeneic grafts reduced significantly in size during the first week of transplantation, either in the normal or in the late infarcted heart (5 days after MI) and allotransplants became always smaller than syngeneic grafts during this period. Low dose of cyclosporine A treatment had a benefit on both allo- and syngeneic graft sizes, suggesting that multiple mechanisms play a role in early graft reduction. The MSC characteristic factors IL-6, IL-8, MCP-1, and VEGF were well above the control level in the heart tissue at 4 days after cell injection, suggesting that the peak therapeutic effect of MSC can be expected during the first week of the administration. Although allogeneic cells induced immunoglobulin production, their biological effects (cell survival, factor productions) are very similar to the syngeneic transplants and therefore they could deliver the same therapeutic effect as the syngeneic cells. Finally, freshly infarcted tissue (30 min) supported better the survival of MSC than late postischemic tissue (5 days) but only “off the shelf” allogeneic cell transplants fits with this treatment strategy.

Noninvasive bioluminescent imaging demonstrates long-term multilineage engraftment of ex vivo-expanded CD34-selected umbilical cord blood cells

The use of umbilical cord blood (UCB) grafts for hematopoietic stem cell transplantation (HSCT) is a promising technique that permits a degree of human leukocyte antigen mismatch between the graft and the host without the concomitant higher rate of graft-versus-host disease that would be observed between an adult marrow graft and a mismatched host. A disadvantage to the use of UCB for HSCT is that immune reconstitution may be significantly delayed because of the low stem cell dose available in the graft. Ex vivo expansion of UCB CD34 cells would provide a greater number of stem cells; however, there are persistent concerns that ex vivo-expanded CD34 cells may lose pluripotency and the ability to contribute meaningfully to long-term engraftment. To address this issue, we transduced CD34-selected UCB cells with a lentiviral construct expressing luciferase, and determined homing and engraftment patterns in vivo by noninvasive bioluminescent imaging in sublethally irradiated NOD/SCID/IL-2Rgamma(-/-) (NSG) mice. Graft contribution to multilineage commitment was also confirmed by analysis of primary and secondary transplants by flow cytometry and immunohistochemistry. Our results demonstrate that, other than a mild delay at the onset of engraftment, there were no significant differences in lineage repopulation or in long-term or secondary engraftment between culture-expanded and unexpanded UCB CD34-selected cells. The results suggest that multipotent stem cells can be expanded ex vivo and can contribute meaningfully to long-term hematopoietic engraftment.

Recent advances in small-animal cardiovascular imaging

Because of the development of gene knockout and transgenic technologies, small animals, such as mice and rats, have become the most widely used animals for cardiovascular imaging studies. Imaging can provide a method to serially evaluate the effect of a particular genetic mutation or pharmacologic therapy (1). In addition, imaging can be used as a noninvasive screening tool for particular cardiovascular phenotypes. Outcome measures of therapeutic efficacy, such as ejection fraction, left ventricular mass, and ventricular volume, can be determined noninvasively as well. Furthermore, small-animal imaging can be used to develop and test new molecular imaging probes (2,3). However, the small size of the heart and rapid heart rate of murine models create special challenges for cardiovascular imaging.