Shifting foci of hematopoiesis during reconstitution from single stem cells

Regulation of stemness and stem cell niche of mesenchymal stem cells: implications in tumorigenesis and metastasis

Human mesenchymal stem cells (MSCs) derived from adult tissues have been considered a candidate cell type for cell-based tissue engineering and regenerative medicine. These multipotent cells have the ability to differentiate along several mesenchymal lineages and possibly along non-mesenchymal lineages. MSCs possess considerable immunosuppressive properties that can influence the surrounding tissue positively during regeneration, but perhaps negatively towards the pathogenesis of cancer and metastasis. The balance between the naïve stem state and differentiation is highly dependent on the stem cell niche. Identification of stem cell niche components has helped to elucidate the mechanisms of stem cell maintenance and differentiation. Ultimately, the fate of stem cells is dictated by their microenvironment. In this review, we describe the identification and characterization of bone marrow-derived MSCs, the properties of the bone marrow stem cell niche, and the possibility and likelihood of MSC involvement in cancer progression and metastasis.

Antioxidants improve early survival of cardiomyoblasts after transplantation to the myocardium

PURPOSE

We tested the hypothesis that modulation of the microenvironment (using antioxidants) will increase stem cell survival in hypoxia and after transplantation to the myocardium.

PROCEDURES

Rat cardiomyoblasts were stably transfected with a reporter gene (firefly luciferase) for bioluminescence imaging (BLI). First, we examined the role of oxidative stress in cells under hypoxic conditions. Subsequently, stem cells were transplanted to the myocardium of rats using high-resolution ultrasound, and their survival was monitored daily using BLI.

RESULTS

Under hypoxia, oxidative stress was increased together with decreased cell survival compared to control cells, both of which were preserved by antioxidants. In living subjects, oxidative stress blockade increased early cell survival after transplantation to the myocardium, compared to untreated cells/animals.

CONCLUSION

Modulation of the local microenvironment (with antioxidants) improves stem cell survival. Increased understanding of the interaction between stem cells and their microenvironment will be critical to advance the field of regenerative medicine.

Guided by the light: visualizing biomolecular processes in living animals with bioluminescence

Bioluminescence imaging (BLI) exploits the light-emitting properties of luciferase enzymes for monitoring cells and biomolecular processes in living subjects. Luciferases can be incorporated into a variety of non-luminescent hosts and used to track cells, visualize gene expression, and analyze collections of biomolecules. This article highlights recent applications of BLI to studies of mammalian biology, along with the development of novel bioluminescent probes to 'see' cells and molecules in action. Collectively, these efforts are expanding our understanding of living systems and shedding light on the molecular underpinnings of disease.

Timing of bone marrow cell delivery has minimal effects on cell viability and cardiac recovery after myocardial infarction

BACKGROUND

Despite ongoing clinical trials, the optimal time for delivery of bone marrow mononuclear cells (BMCs) after myocardial infarction is unclear. We compared the viability and effects of transplanted BMCs on cardiac function in the acute and subacute inflammatory phases of myocardial infarction.

METHODS AND RESULTS

The time course of acute inflammatory cell infiltration was quantified by FACS analysis of enzymatically digested hearts of FVB mice (n=12) after left anterior descending artery ligation. Mac-1(+)Gr-1(high) neutrophil infiltration peaked at day 4. BMCs were harvested from transgenic FVB mice expressing firefly luciferase (Fluc) and green fluorescent protein (GFP). Afterward, 2.5x10(6) BMCs were injected into the left ventricle of wild-type FVB mice either immediately (acute BMC) or 7 days (subacute BMC) after myocardial infarction, or after a sham procedure (n=8 per group). In vivo bioluminescence imaging showed an early signal increase in both BMC groups at day 7, followed by a nonsignificant trend (P=0.203) toward improved BMC survival in the subacute BMC group that persisted until the bioluminescence imaging signal reached

BACKGROUND

<0.01) and 6 weeks (both BMC groups versus saline; P<0.05) but no significant differences between the 2 BMC groups. FACS analysis of BMC-injected hearts at day 7 revealed that GFP(+) BMCs expressed hematopoietic (CD45, Mac-1, Gr-1), minimal progenitor (Sca-1, c-kit), and no endothelial (CD133, Flk-1) or cardiac (Trop-T) cell markers.

CONCLUSIONS

Timing of BMC delivery has minimal effects on intramyocardial retention and preservation of cardiac function. In general, there is poor long-term engraftment and BMCs tend to adopt inflammatory cell phenotypes.

In vivo imaging of hematopoietic stem cells and their microenvironment

In this review we provide a description of the basic concepts and paradigms currently constituting the foundations of adult stem cell biology, and discuss the role that live imaging techniques have in the development of the field. We focus on live imaging of hematopoietic stem cells (HSCs) as the basic biology and clinical applications of HSCs have historically been at the forefront of the stem cell field, and HSC are the first mammalian tissue stem cells to be visualized in vivo using advanced light microscopy techniques. We outline the current technical challenges that remain to be overcome before stem cells and their niche can be more fully characterized using the live imaging technology.

Trafficking mesenchymal stem cell engraftment and differentiation in tumor-bearing mice by bioluminescence imaging

The objective of the study was to track the distribution and differentiation of mesenchymal stem cells (MSCs) in tumor-bearing mice. The 4T1 murine breast cancer cells were labeled with renilla luciferase-monomeric red fluorescence protein (rLuc-mRFP) reporter gene. The MSCs labeled with firefly luciferase-enhanced green fluorescence protein (fLuc-eGFP) reporter gene (MSCs-R) were isolated from L2G85 transgenic mice that constitutively express fLuc-eGFP reporter gene. To study the tumor tropism of MSCs, we established both subcutaneous and lung metastasis models. In lung metastasis tumor mice, we injected MSCs-R intravenously either on the same day or 4 days after 4T1 tumor cell injection. In subcutaneous tumor mice, we injected MSCs-R intravenously 7 days after subcutaneous 4T1 tumor inoculation. The tumor growth was monitored by rLuc bioluminescence imaging (BLI). The fate of MSCs-R was monitored by fLuc BLI. The localization of MSCs-R in tumors was examined histologically. The osteogenic and adipogenic differentiation of MSCs-R was investigated by alizarin red S and oil red O staining, respectively. The mechanism of the dissimilar differentiation potential of MSCs-R under different tumor microenvironments was investigated. We found that the 4T1 cells were successfully labeled with rLuc-mRFP. The MSCs-R isolated from L2G85 transgenic mice constitutively express fLuc-eGFP reporter gene. When injected intravenously, MSCs-R survived, proliferated, and differentiated in tumor sites but not elsewhere. The localization of GFP(+) MSCs-R in tumor lesions was confirmed ex vivo. In conclusion, the MSCs-R can selectively localize, survive, and proliferate in both subcutaneous tumor and lung metastasis as evidenced by noninvasive bioluminescence imaging and ex vivo validation. The MSCs-R migrated to lung tumor differentiated into osteoblasts, whereas the MSCs-R targeting subcutaneous tumor differentiated into adipocytes.

Mesenchymal stem cells: emerging therapy for Duchenne muscular dystrophy

Multipotent cells that can give rise to bone, cartilage, fat, connective tissue, and skeletal and cardiac muscle are termed mesenchymal stem cells. These cells were first identified in the bone marrow, distinct from blood-forming stem cells. Based on the embryologic derivation, availability, and various pro-regenerative characteristics, research exploring their use in cell therapy shows great promise for patients with degenerative muscle diseases and a number of other conditions. In this review, the authors explore the potential for mesenchymal stem cell therapy in the emerging field of regenerative medicine with a focus on treatment for Duchenne muscular dystrophy.

In vivo delivery of siRNA to immune cells by conjugation to a TLR9 agonist enhances antitumor immune responses

Efficient delivery of siRNA to specific cell populations in vivo remains a formidable challenge to its successful therapeutic application. We describe a novel siRNA-based approach – synthetically linking siRNA to an oligonucleotide TLR9 agonist – that targets and silences genes in TLR9⁺ myeloid cells and B cells, both of which are key components of the tumor microenvironment. Because Stat3 in tumor-associated immune cells suppresses antitumor immune responses and hinders TLR9-induced immune stimulation, we tested CpG-Stat3siRNA conjugates for anti-tumor effects. When injected locally at the tumor site or systemically through an intravenous route, the CpG-Stat3siRNA conjugates access tumor-associated dendritic cells, macrophages and B cells, inhibit Stat3 expression, leading to activation of tumor-associated immune cells, and ultimately potent anti-tumor immune responses. Our findings demonstrate the potential of TLR agonist-siRNA conjugates for targeted gene silencing coupled with TLR stimulation and immune activation in the tumor microenvironment.

Real-time bioluminescence imaging of polycythemia vera development in mice

Polycythemia vera (PV) is a myeloproliferative disorder involving hematopoietic stem cells. A recurrent somatic missense mutation in JAK2 (JAK2V617F) is thought to play a causal role in PV. Therefore, targeting Jak2 will likely provide a molecular mechanism-based therapy for PV. To facilitate the development of such new and specific therapeutics, a suitable and well-characterized preclinical animal model is essential. Although several mouse models of PV have been reported, the spatiotemporal kinetics of PV formation and progression has not been studied. To address this, we created a bone marrow transplant mouse model that co-expresses mutant Jak2 and luciferase 2 (Luc2) genes. Bioluminescent imaging (BLI) was used to visualize disease cells and analyze the kinetics of PV development in vivo. To better understand the molecular mechanism of PV, we generated mice carrying a kinase inactive mutant Jak2 (Jak2K882E), demonstrating that the PV disease was dependent on constitutive activation of the Jak2 kinase activity. We further showed that the Jak2V617F mutation caused increased stem cell renewal activity and impaired cell differentiation, which was at least in part due to deregulated transcriptional programming. The Jak2V617F-Luc2 PV mice will be a useful preclinical model to characterize novel JAK2 inhibitors for the treatment of PV.

Tissue-specific homing and expansion of donor NK cells in allogeneic bone marrow transplantation

NK cells have potential therapeutic impact in suppressing graft-versus-host disease (GVHD) and enhancing antitumor effects as a cellular therapy for hematologic malignancies. However, few studies have addressed the trafficking and in vivo behavior of NK cells in murine models of bone marrow transplantation (BMT). We investigated NK cell trafficking and survival following allogeneic and syngeneic BMT using a novel bioluminescence-based imaging strategy. Transplantation of luciferase-expressing NK cells revealed CD62L-mediated trafficking to lymphoid organs and trafficking to GVHD target tissues, as evidenced by in vivo and ex vivo bioluminescence imaging. The NK cells persisted for approximately 4 wk after transplantation in allogeneic recipients, but were not detectable in syngeneic recipients. CFSE-labeling studies showed extensive NK cell proliferation in vivo. Transplanted NK cells up-regulated molecules necessary for homing to the lymph nodes, gastrointestinal tract, and skin, yet did not cause clinical GVHD. This expansion and tissue-specific homing was not solely due to the conditioning regimen, as NK cells proliferated and reached lymphoid and GVHD target tissue in unconditioned allogeneic RAG2(-/-) gamma-chain(-/-) recipients. IL-2 enhanced expansion and antitumor activity of NK cells. These results provide significant insight into the behavior and potential therapeutic impact of NK cells in BMT.

Stem cell therapy for the kidney?

The kidney has a remarkable capacity to regenerate after injury, as it is not a terminally differentiated organ. This regenerative potential is somehow incomplete, however, and as the insult continues, progressive and irreversible scarring results in chronic renal disease. Dialysis and organ transplantation are nonspecific and incomplete methods of renal replacement therapy. Stem cells may provide a more efficacious method for both prevention and amelioration of renal disease of many etiologies. Although many reports have claimed the existence of renal-specific stem or progenitor cells isolated and characterized by various methods, the results have been diverse and debatable. The bone marrow stem cells seem to play a minor role in renal regeneration after acute ischemia in mice through transdifferentiation and cell fusion, but their immediate paracrine effects result in considerable improvements in renal function. Therefore, as in stem cell therapy for the heart, bone marrow-derived stem cells show promise in regeneration of the kidney. Although more research is needed in the basic science of renal regeneration, clinical research in animals has demonstrated the versatility of stem cell therapy. The first phase of clinical trials of bone marrow mesenchymal cells in protection against acute kidney injury may begin shortly. This will enable further exploration of stem cell therapy in renal patients with multiple comorbidities.

Laser-induced disruption of systemically administered liposomes for targeted drug delivery

Liposomal formulations of drugs have been shown to enhance drug efficacy by prolonging circulation time, increasing local concentration and reducing off-target effects. Controlled release from these formulations would increase their utility, and hyperthermia has been explored as a stimulus for targeted delivery of encapsulated drugs. Use of lasers as a thermal source could provide improved control over the release of the drug from the liposomes with minimal collateral tissue damage. Appropriate methods for assessing local release after systemic delivery would aid in testing and development of better formulations. We use in vivo bioluminescence imaging to investigate the spatiotemporal distribution of luciferin, used as a model small molecule, and demonstrate laser-induced release from liposomes in animal models after systemic delivery. These liposomes were tested for luciferin release between 37 and 45 degrees C in PBS and serum using bioluminescence measurements. In vivo studies were performed on transgenic reporter mice that express luciferase constitutively throughout the body, thus providing a noninvasive readout for controlled release following systemic delivery. An Nd:YLF laser was used (527 nm) to heat tissues and induce rupture of the intravenously delivered liposomes in target tissues. These data demonstrate laser-mediated control of small molecule delivery using thermally sensitive liposomal formulations.

Lymphoid-tissue-specific homing of bone-marrow-derived dendritic cells

Because of their potent immunoregulatory capacity, dendritic cells (DCs) have been exploited as therapeutic tools to boost immune responses against tumors or pathogens, or dampen autoimmune or allergic responses. Murine bone marrow-derived DCs (BM-DCs) are the closest known equivalent of the blood monocyte-derived DCs that have been used for human therapy. Current imaging methods have proven unable to properly address the migration of injected DCs to small and deep tissues in mice and humans. This study presents the first extensive analysis of BM-DC homing to lymph nodes (and other selected tissues) after intravenous and intraperitoneal inoculation. After intravenous delivery, DCs accumulated in the spleen, and preferentially in the pancreatic and lung-draining lymph nodes. In contrast, DCs injected intraperitoneally were found predominantly in peritoneal lymph nodes (pancreatic in particular), and in omentum-associated lymphoid tissue. This uneven distribution of BM-DCs, independent of the mouse strain and also observed within pancreatic lymph nodes, resulted in the uneven induction of immune response in different lymphoid tissues. These data have important implications for the design of systemic cellular therapy with DCs, and in particular underlie a previously unsuspected potential for specific treatment of diseases such as autoimmune diabetes and pancreatic cancer.

TBI with lung dose reduction does not improve hematopoietic cell homing to BM during allogeneic transplantation

Purpose

To determine the effects of total body irradiation (TBI) dose, fractionation, and lung shielding on hematopoietic stem cell homing to the bone marrow.

Material and Methods

Bone marrow (BM) cells were extracted from tibiae and femurs of B6-GFP mice and were transplanted into B6 mice. Recipient mice had either: 1) no radiation, 2) single dose TBI at 13.6 Gray (Gy), 3) single dose TBI at 13.6 Gy with reduced lung exposure to 0.4 Gy by shielding, 4) split dose TBI at 12 Gy to twice/day over four days, or 5) split dose TBI at 12 Gy to twice/day over four days with reduced lung exposure to 0.36 Gy by shielding. The last radiation exposure preceded tail vein injection by 4–6 hours. Mice were sacrificed after 18 hours.

Results

Homing of GFP positive, lineage negative cells was not significantly improved in any irradiated group compared to control. Homing of GFP positive, lineage negative, Kit positive cells was significantly worse in all irradiated groups.

Conclusion

TBI does not improve the homing of lineage negative donor BM cells to the recipient marrow. Homing of lineage negative, Kit positive donor BM cells was significantly worse following TBI, with or without lung dose reduction.

Comparison of transplantation of adipose tissue- and bone marrow-derived mesenchymal stem cells in the infarcted heart

BACKGROUND

Mesenchymal stem cells hold promise for cardiovascular regenerative therapy. Derivation of these cells from the adipose tissue might be easier compared with bone marrow. However, the in vivo fate and function of adipose stromal cells (ASC) in the infarcted heart has never been compared directly to bone marrow-derived mesenchymal cells (MSC).

METHODS

ASC and MSC were isolated from transgenic FVB mice with a beta-actin promoter driving firefly luciferase and green fluorescent protein double fusion reporter gene, and they were characterized using flow cytometry, microscopy, bioluminescence imaging and luminometry. FVB mice (n=8 per group) underwent myocardial infarction followed by intramyocardial injection of 5x10(5) ASC, MSC, fibroblasts (Fibro, positive control), or saline (negative control). Cell survival was measured using bioluminescence imaging for 6 weeks and cardiac function was monitored by echocardiography and pressure-volume analysis. Ventricular morphology was assessed using histology.

RESULTS

ASC and MSC were CD34(-), CD45(-), c-Kit(-), CD90(+), Sca-1(+), shared similar morphology and had a population doubling time of approximately 2 days. Cells expressed Fluc reporter genes in a number-dependent fashion as confirmed by luminometry. After cardiac transplantation, both cell types showed drastic donor cell death within 4 to 5 weeks. Furthermore, transplantation of either cell type was not capable of preserving ventricular function and dimensions, as confirmed by pressure-volume-loops and histology.

CONCLUSION

This is the first study comparing the in vivo behavior of both cell types in the infarcted heart. ASC and MSC do not tolerate well in the cardiac environment, resulting in acute donor cell death and a subsequent loss of cardiac function similar to control groups.

Imaging survival and function of transplanted cardiac resident stem cells

OBJECTIVES

The goal of this study is to characterize resident cardiac stem cells (CSCs) and investigate their therapeutic efficacy in myocardial infarction by molecular imaging methods.

BACKGROUND

CSCs have been isolated and characterized in vitro. These cells offer a provocative method to regenerate the damaged myocardium. However, the survival kinetics and function of transplanted CSCs have not been fully elucidated.

METHODS

CSCs were isolated from L2G85 transgenic mice (FVB strain background) that constitutively express both firefly luciferase and enhanced green fluorescence protein reporter gene. CSCs were characterized in vitro and transplanted in vivo into murine infarction models. Multimodality noninvasive imaging techniques were used to assess CSC survival and therapeutic efficacy for restoration of cardiac function.

RESULTS

CSCs can be isolated from L2G85 mice, and fluorescence-activated cell sorting analysis showed expression of resident CSC markers (Sca-1, c-Kit) and mesenchymal stem cell markers (CD90, CD106). Afterwards, 5 x 10(5) CSCs (n = 30) or phosphate-buffered saline control (n = 15) was injected into the hearts of syngeneic FVB mice undergoing left anterior descending artery ligation. Bioluminescence imaging showed poor donor cell survival by week 8. Echocardiogram, invasive hemodynamic pressure-volume analysis, positron emission tomography imaging with fluorine-18-fluorodeoxyglucose, and cardiac magnetic resonance imaging demonstrated no significant difference in cardiac contractility and viability between the CSC and control group. Finally, postmortem analysis confirmed transplanted CSCs integrated with host cardiomyocytes by immunohistology.

CONCLUSIONS

In a mouse myocardial infarction model, Sca-1-positive CSCs provide no long-term engraftment and benefit to cardiac function as determined by multimodality imaging.

Stem cell-mediated accelerated bone healing observed with in vivo molecular and small animal imaging technologies in a model of skeletal injury

Adult stem cells are promising therapeutic reagents for skeletal regeneration. We hope to validate by molecular imaging technologies the in vivo life cycle of adipose-derived multipotent cells (ADMCs) in an animal model of skeletal injury. Primary ADMCs were lentivirally transfected with a fusion reporter gene and injected intravenously into mice with bone injury or sham operation. Bioluminescence imaging (BLI), [(18)F]FHBG (9-(fluoro-hydroxy-methyl-butyl-guanine)-micro-PET, [(18)F]Fluoride ion micro-PET and micro-CT were performed to monitor stem cells and their effect. Bioluminescence microscopy and immunohistochemistry were done for histological confirmation. BLI showed ADMC's traffic from the lungs then to the injury site. BLI microscopy and immunohistochemistry confirmed the ADMCs in the bone defect. Micro-CT measurements showed increased bone healing in the cell-injected group compared to the noninjected group at postoperative day 7 (p < 0.05). Systemically administered ADMC's traffic to the site of skeletal injury and facilitate bone healing, as demonstrated by molecular and small animal imaging. Molecular imaging technologies can validate the usage of adult adipose tissue-derived multipotent cells to promote fracture healing. Imaging can in the future help establish therapeutic strategies including dosage and administration route.

Sensitive in vivo imaging of T cells using a membrane-bound Gaussia princeps luciferase

We developed a new approach to bioluminescent T cell imaging using a membrane-anchored form of the Gaussia luciferase (GLuc) enzyme, termed extGLuc, which we could stably express in both mouse and human primary T cells. In vitro, extGLuc+ cells emitted significantly higher bioluminescent signal when compared to cells expressing GLuc, Renilla luciferase (RLuc) or membrane-anchored RLuc (extRLuc). In vivo, mouse extGLuc+ T cells showed higher bioluminescent signal when compared to GLuc+ and RLuc+ T cells. Application of this imaging approach to human T cells genetically modified to express tumor-specific chimeric antigen receptors (CARs) enabled us to show in vivo CAR-mediated T cell accumulation in tumor, T cell persistence over time and concomitant imaging of T cells and tumor cells modified to express firefly luciferase. This sensitive imaging technology has application to many in vivo cell-based studies in a wide array of mouse models.

Validation of bioluminescent imaging techniques

Bioluminescence imaging (BLI) is frequently cited for its ease of quantification. This fundamental strength of BLI has led to applications in cancer research, cell transplantation, and monitoring of infectious disease in which bioluminescence intensity is correlated with other metrics. However, bioluminescence measurements can be influenced by a number of factors, among them source location, tissue optical properties, and substrate availability and pharmacokinetics. Accounting for these many factors is crucial for accurate BLI quantification. A number of methods can be employed to ensure correct interpretation of BLI results and validate BLI techniques. This chapter summarizes the use of calibrated light-emitting standards, bioluminescence tomography, and post-mortem validation of luciferase expression for validating quantitative BLI measurements.

Impact of mammalian target of rapamycin inhibition on lymphoid homing and tolerogenic function of nanoparticle-labeled dendritic cells following allogeneic hematopoietic cell transplantation

Dendritic cells (DC) play a major role in the pathogenesis of graft-vs-host disease (GvHD). Directed modification of surface molecules on DC that provide instructive signals for T cells may create a tolerogenic DC phenotype that affects GvHD severity. To investigate the impact of the mammalian target of rapamycin (mTOR) inhibitor rapamycin (RAPA) on in vivo migratory capacities, tolerogenic function, and B7 superfamily surface expression on DC following allogeneic hematopoietic cell transplantation (aHCT), we generated a platform for magnetic resonance imaging and bioluminescence imaging based cell trafficking studies. Luciferase transgenic DC were labeled with superparamagnetic iron oxide nanoparticles bound to a murine IgG Ab that allowed for Fc-gammaR-mediated endocytosis. Locally injected luc(+) DC could be tracked within their anatomical context by bioluminescence imaging and magnetic resonance imaging after aHCT, based on stable intracellular localization of superparamagnetic iron oxide-IgG complexes. RAPA preconditioned DC (DC-R) displayed reduced expression of MHC class II, B7-1 (CD80), and B7-2 (CD86) but not B7-H4 whose ligation of T cells has a profound inhibitory effect on their proliferation and cytokine secretion. DC-R of recipient genotype reduced GvHD severity that is compatible with their tolerogenic phenotype. CCR5, CCR7, and CD62L expression was not affected by mTOR inhibition, which allowed for DC-R in vivo trafficking to secondary lymphoid compartments where immunregulation is required. This study is the first to delineate the impact of RAPA on DC migration and tolerogenic function after aHCT. Modification of the DC phenotype by mTOR inhibition may have therapeutic potential in an attempt to reduce GvHD following aHCT.

Heme oxygenase-1 deficiency leads to disrupted response to acute stress in stem cells and progenitors

An effective response to extreme hematopoietic stress requires an extreme elevation in hematopoiesis and preservation of hematopoietic stem cells (HSCs). These diametrically opposed processes are likely to be regulated by genes that mediate cellular adaptation to physiologic stress. Herein, we show that heme oxygenase-1 (HO-1), the inducible isozyme of heme degradation, is a key regulator of these processes. Mice lacking one allele of HO-1 (HO-1(+/-)) showed accelerated hematopoietic recovery from myelotoxic injury, and HO-1(+/-) HSCs repopulated lethally irradiated recipients with more rapid kinetics. However, HO-1(+/-) HSCs were ineffective in radioprotection and serial repopulation of myeloablated recipients. Perturbations in key stem cell regulators were observed in HO-1(+/-) HSCs and hematopoietic progenitors (HPCs), which may explain the disrupted response of HO-1(+/-) HPCs and HPCs to acute stress. Control of stem cell stress response by HO-1 presents opportunities for metabolic manipulation of stem cell-based therapies.

The E. Donnall Thomas lecture: normal and neoplastic stem cells

Dr. Irving Weissman was the honored E. Donnall Thomas lecturer at the Tandem BMT Meetings, held on February 10, 2007, at Keystone, Colorado. Dr. Weissman has been a major player, and has provided us with enormous insight into many areas of biology, dating back to his high school days in Montana. He led an enormously productive career at Stanford University where he has taught us many lessons involving our understanding of lymphocyte homing, stem cell biology, both of the hematopoietic system and other types of stem cells, and also now, about cancer stem cells. Dr. Weissman has made enormous contributions to this burgeoning field that has provided us new insights and new opportunities for treatment strategies. In addition to a very productive laboratory career, he is also currently the director of both the Stem Cell Institute, as well as the Cancer Center at Stanford University. The following text is a modified transcribed version of the presentation made by Dr. Weissman.

Comparison of different adult stem cell types for treatment of myocardial ischemia

BACKGROUND

A comparative analysis of the efficacy of different cell candidates for the treatment of heart disease remains to be described. This study is designed to evaluate the therapeutic efficacy of 4 cell types in a murine model of myocardial infarction.

METHODS AND RESULTS

Bone marrow mononuclear cells (MN), mesenchymal stem cells (MSC), skeletal myoblasts (SkMb), and fibroblasts (Fibro) expressing firefly luciferase (Fluc) and green fluorescence protein (GFP) were characterized by flow cytometry, bioluminescence imaging (BLI), and luminometry. Female FVB mice (n=70) underwent LAD ligation and intramyocardially received one cell type (5x10(5)) or PBS. Cell survival was measured by BLI and by TaqMan PCR. Cardiac function was assessed by echocardiography and invasive hemodynamic measurements. Fluc expression correlated with cell number in all groups (r(2)>0.93). In vivo BLI revealed acute donor cell death of MSC, SkMb, and Fibro within 3 weeks after transplantation. By contrast, cardiac signals were still present after 6 weeks in the MN group, as confirmed by TaqMan PCR (P<0.01). Echocardiography showed significant preservation of fractional shortening in the MN group compared to controls (P<0.05). Measurements of left ventricular end-systolic/diastolic volumes revealed that the least amount of ventricular dilatation occurred in the MN group (P<0.05). Histology confirmed the presence of MN, although there was no evidence of transdifferentiation by donor MN into cardiomyocytes.

CONCLUSIONS

This is the first study to show that compared to MSC, SkMB, and Fibro, MN exhibit a more favorable survival pattern, which translates into a more robust preservation of cardiac function.

Self-renewal and expansion of single transplanted muscle stem cells

Adult muscle satellite cells have a principal role in postnatal skeletal muscle growth and regeneration. Satellite cells reside as quiescent cells underneath the basal lamina that surrounds muscle fibres and respond to damage by giving rise to transient amplifying cells (progenitors) and myoblasts that fuse with myofibres. Recent experiments showed that, in contrast to cultured myoblasts, satellite cells freshly isolated or satellite cells derived from the transplantation of one intact myofibre contribute robustly to muscle repair. However, because satellite cells are known to be heterogeneous, clonal analysis is required to demonstrate stem cell function. Here we show that when a single luciferase-expressing muscle stem cell is transplanted into the muscle of mice it is capable of extensive proliferation, contributes to muscle fibres, and Pax7(+)luciferase(+) mononucleated cells can be readily re-isolated, providing evidence of muscle stem cell self-renewal. In addition, we show using in vivo bioluminescence imaging that the dynamics of muscle stem cell behaviour during muscle repair can be followed in a manner not possible using traditional retrospective histological analyses. By imaging luciferase activity, real-time quantitative and kinetic analyses show that donor-derived muscle stem cells proliferate and engraft rapidly after injection until homeostasis is reached. On injury, donor-derived mononucleated cells generate massive waves of cell proliferation. Together, these results show that the progeny of a single luciferase-expressing muscle stem cell can both self-renew and differentiate after transplantation in mice, providing new evidence at the clonal level that self-renewal is an autonomous property of a single adult muscle stem cell.

In vivo trafficking and survival of cytokine-induced killer cells resulting in minimal GVHD with retention of antitumor activity

Cytokine-induced killer (CIK) cells are ex vivo-expanded T lymphocytes expressing both natural killer (NK)- and T-cell markers. CIK cells are cytotoxic against autologous and allogeneic tumors. We previously showed that adoptive transfer of allogeneic CIK cells in a murine model caused minimal graft-versus-host disease (GVHD). However, the precise mechanism of reduced GVHD is not fully understood. Therefore, we evaluated the trafficking and survival of luciferase-expressing CIK cells in an allogeneic bone marrow transplant model. The initial trafficking patterns of CIK cells were similar to conventional T cells that induced GVHD; however, CIK cells infiltrated GVHD target tissues much less and transiently. CIK cells accumulated and persisted in tumor sites, resulting in tumor eradication. We evaluated different properties of CIK cells compared with conventional T cells, demonstrating a slower division rate of CIK cells, higher susceptibility to apoptosis, persistent increased expression of interferon gamma (IFN-gamma), and reduced acquisition of homing molecules required for entry of cells into inflamed GVHD target organs that lack expression of NKG2D ligands recognized by CIK cells. Due to these properties, allogeneic CIK cells had reduced expansion and caused less tissue damage. We conclude that CIK cells have the potential to separate graft-versus-tumor effects from GVHD.

Assessing siRNA pharmacodynamics in a luciferase-expressing mouse

A significant barrier to the successful general development of small-interfering RNA (siRNA) therapeutics is the ability to deliver them systemically to target organs and cell types. In this study, we have developed a mouse strain that will facilitate the evaluation of the efficacy of siRNA delivery strategies. This strain contains robust ubiquitous expression of firefly luciferase from germ line Cre-mediated recombination of the ROSA26-LSL-Luc allele. We show that luciferase is highly and uniformly expressed in all tissues examined. Using this mouse model, we describe a facile assay that enables the assessment of the pharmacodynamics of a systemically delivered siRNA formulation. These mice can also be used as universal donors, enabling the efficient and sensitive monitoring of cell trafficking or tissue transplantation. The primary advantage of this approach is that siRNA efficacy against a nonessential target can be easily evaluated in any tissue. This strain should generally enhance the ability to rapidly screen, compare and optimize various siRNA formulations for tissue-targeted or -enhanced systemic delivery in a preclinical development setting.

Immunosuppressive therapy mitigates immunological rejection of human embryonic stem cell xenografts

Given their self-renewing and pluripotent capabilities, human embryonic stem cells (hESCs) are well poised as a cellular source for tissue regeneration therapy. However, the host immune response against transplanted hESCs is not well characterized. In fact, controversy remains as to whether hESCs have immune-privileged properties. To address this issue, we used in vivo bioluminescent imaging to track the fate of transplanted hESCs stably transduced with a double-fusion reporter gene consisting of firefly luciferase and enhanced GFP. We show that survival after transplant is significantly limited in immunocompetent as opposed to immunodeficient mice. Repeated transplantation of hESCs into immunocompetent hosts results in accelerated hESC death, suggesting an adaptive donor-specific immune response. Our data demonstrate that transplanted hESCs trigger robust cellular and humoral immune responses, resulting in intragraft infiltration of inflammatory cells and subsequent hESC rejection. Moreover, we have found CD4(+) T cells to be an important modulator of hESC immune-mediated rejection. Finally, we show that immunosuppressive drug regimens can mitigate the anti-hESC immune response and that a regimen of combined tacrolimus and sirolimus therapies significantly prolongs survival of hESCs for up to 28 days. Taken together, these data suggest that hESCs are immunogenic, trigger both cellular and humoral-mediated pathways, and, as a result, are rapidly rejected in xenogeneic hosts. This process can be mitigated by a combined immunosuppressive regimen as assessed by molecular imaging approaches.

Molecular-genetic imaging based on reporter gene expression

Molecular imaging includes proteomic, metabolic, cellular biologic process, and genetic imaging. In a narrow sense, molecular imaging means genetic imaging and can be called molecular-genetic imaging. Imaging reporter genes play a leading role in molecular-genetic imaging. There are 3 major methods of molecular-genetic imaging, based on optical, MRI, and nuclear medicine modalities. For each of these modalities, various reporter genes and probes have been developed, and these have resulted in successful transitions from bench to bedside applications. Each of these imaging modalities has its unique advantages and disadvantages. Fluorescent and bioluminescent optical imaging modalities are simple, less expensive, more convenient, and more user friendly than other imaging modalities. Another advantage, especially of bioluminescence imaging, is its ability to detect low levels of gene expression. MRI has the advantage of high spatial resolution, whereas nuclear medicine methods are highly sensitive and allow data from small-animal imaging studies to be translated to clinical practice. Moreover, multimodality imaging reporter genes will allow us to choose the imaging technologies that are most appropriate for the biologic problem at hand and facilitate the clinical application of reporter gene technologies. Reporter genes can be used to visualize the levels of expression of particular exogenous and endogenous genes and several intracellular biologic phenomena, including specific signal transduction pathways, nuclear receptor activities, and protein-protein interactions. This technique provides a straightforward means of monitoring tumor mass and can visualize the in vivo distributions of target cells, such as immune cells and stem cells. Molecular imaging has gradually evolved into an important tool for drug discovery and development, and transgenic mice with an imaging reporter gene can be useful during drug and stem cell therapy development. Moreover, instrumentation improvements, the identification of novel targets and genes, and imaging probe developments suggest that molecular-genetic imaging is likely to play an increasingly important role in the diagnosis and therapy of cancer.

Noninvasive imaging and quantification of epidermal growth factor receptor kinase activation in vivo

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase (RTK) critical in tumor growth and a major target for anticancer drug development. However, thus far, there is no effective system to monitor its activities in vivo. Here, we report a novel approach to monitor EGFR activation based on the bifragment luciferase reconstitution system. The EGFR receptor and its interacting partner proteins (EGFR, growth factor receptor binding protein 2, and Src homology 2 domain-containing) were fused to NH(2) terminal and COOH terminal fragments of the firefly luciferase. After establishing tumor xenograft from cells transduced with the reporter genes, we show that the activation of EGFR and its downstream factors could be quantified through optical imaging of reconstituted luciferase. Changes in EGFR activation could be visualized after radiotherapy or EGFR inhibitor treatment. Rapid and sustained radiation-induced EGFR activation and inhibitor-mediated signal suppression were observed in the same xenograft tumors over a period of weeks. Our data therefore suggest a new methodology where activities of RTKs can be imaged and quantified optically in mice. This approach should be generally applicable to study biological regulation of RTK, as well as to develop and evaluate novel RTK-targeted therapeutics.

Imaging Stem Cell Implant for Cellular-Based Therapies

Stem cell–based cellular therapy represents a promising outlook for regenerative medicine. Imaging techniques provide a means for noninvasive, repeated, and quantitative tracking of stem cell implant or transplant. From initial deposition to the survival, migration and differentiation of the transplant/implanted stem cells, imaging allows monitoring of the infused cells in the same live object over time. The current review briefly summarizes and compares existing imaging methods for cell labeling and imaging in animal models. Several studies performed by our group using different imaging techniques are described, with further discussion on the issues with these current imaging approaches and potential directions for future development in stem cell imaging.

Prolonging islet allograft survival using in vivo bioluminescence imaging to guide timing of antilymphocyte serum treatment of rejection

BACKGROUND

Bioluminescence imaging (BLI) is a sensitive and noninvasive method for tracking the fate of transplanted islets. The aim of this study was to investigate whether early detection of rejection by BLI can aid in the timing of antilymphocyte serum (ALS) treatment for prolonging islet graft survival.

METHODS

Transgenic islets (200 per recipient) expressing the firefly luciferase from FVB/NJ strain (H-2q) mice were transplanted under the kidney capsule of streptozotocin-induced diabetic allogeneic Balb/c strain (H-2q) mice. BLI signals and serum glucose levels were measured daily after transplant. Four groups of mice were transplanted: group 1 recipients were untreated controls (n=12), group 2 (n=8) received ALS before transplant, group 3 (n=10) received ALS at a time after transplant when normoglycemic but prompted by a reduction (approximately 30%) in BLI signal intensity for 2 consecutive days, and group 4 (n=5) received ALS after transplant when prompted by blood glucose levels increasing approximately 20% from the normoglycemic baseline (BLI reduction approximately 70%).

RESULTS

The incidence of graft loss from rejection in groups 1, 2, 3, and 4 was 92.3%, 88%, 40%, and 100%, respectively. The mean (+/-SE) time to graft loss in groups 1, 2, 3 and 4 was 22.5+/-4.8, 29.2+/-9.9, 53.5+/-17.9, and 22.1+/-2.4 days, respectively.

CONCLUSIONS

Noninvasive imaging modalities of functional islet mass, such as BLI (but not blood glucose levels), can prompt the appropriate timing of ALS treatment of islet allograft rejection and significantly prolong graft survival or protect the grafts from permanent loss.

Prowling wolves in sheep's clothing: the search for tumor stem cells

The importance of a subset of cells which have ‘stem like’ characteristics and are capable of tumor initiation has been reported for a range of tumors. Isolation of these tumor-initiating cells (TICs) has largely been based on differential cell surface protein expression. However, there is still much debate on the functional significance of these markers in initiating tumors, as many properties of tumor initiation are modified by cell-cell interactions. In particular, the relationship between TICs and their microenvironment is poorly understood but has therapeutic implications, as the microenvironment can maintain tumor cells in a prolonged period of quiescence. However, a major limitation in advancing our understanding of the crosstalk between TICs and their microenvironment is the lack of sensitive techniques which allow thein vivotracking and monitoring of TICs. Application of newin vivocellular and molecular imaging technologies holds much promise in uncovering the mysteries of TIC behavior at the three-dimensional level. This review will describe recent advances in our understanding of the TIC concept and how the application ofin vivoimaging techniques can advance our understanding of the biological fate of TICs. A supplementary resource guide describing TICs from different malignancies is also presented.

In vivo imaging and evaluation of different biomatrices for improvement of stem cell survival

Therapeutic effects from injection of stem cells are often hampered by acute donor cell death as well as migration away from damaged areas. This is likely due to the fact that injected cells do not have the physical and biochemical cues for ordered engrafment. Here we evaluate 3 common biomatrices (Matrigel, Collagen I, Purmatrix) that has the potential of providing suitable scaffolds needed to enhance stem cell survival. The longitudinal fate of transplanted stem cells was monitored by reporter imaging techniques.

Age-related changes of germline stem cell activity, niche signaling activity and egg production in Drosophila

Adult stem cells are important in replenishing aged cells to maintain tissue homeostasis. Aging in turn may exert profound effects on stem cell's regenerative potential, but to date the mechanisms of such stem cell aging are poorly understood, and it is not clear to what extent stem cell aging contributes to tissue or organ aging. Here we show in female Drosophila that germline stem cell (GSC) division rate progressively declines with age, which is accompanied by reduced decapentaplegic (dpp) niche signaling pathway activation within GSCs. Egg production also rapidly declines with age, which is accompanied by both decreased stem cell division and increased incidence of cell death of developing eggs, especially in the oldest females. Genetically increasing dpp expression delays GSC activity decline and transiently increases egg production. We conclude that age-related decline of reproduction is caused by both decreased GSC activity and increased incidence of cell death during oogenesis, while decreased GSC activity is attributed to declined signaling from the regulatory niche. We suggest that niche functional decay may be an important mechanism for stem cell aging and system failure.

Bioluminescence imaging of hematopoietic stem cell repopulation in murine models

Hematopoietic stem cells (HSCs) have been studied for decades in order to understand their stem cell biology and their potential as treatments in gene therapy, and those studies have resulted in tremendous advancement of understanding HSCs. However, most of the studies required the sacrifice of cohorts of the animals in order to obtain data for analysis, resulting in the use of large animal numbers along with difficult long-term studies. The dynamic engraftment and expansion of HSC are not fully observed and analyzed. Until recently, with the development of optical imaging, HSC repopulation can be continuously monitored in the same animal over a long period of time, reducing animal numbers and opening a new dimension for investigation. In this chapter, bioluminescence imaging of murine HSC is described for observing the dynamic repopulation process after transplantation. Photons emitted from transplanted murine HSCs expressing firefly luciferase within the mice can be visualized in light-sealed chamber with a highly sensitive digital camera after injection of substrate D-luciferin. Xenogen IVIS200 imaging system is used to record the process, and other similar imaging systems can also be used for this process.

Tissue-targeted therapy of autoimmune diabetes using dendritic cells transduced to express IL-4 in NOD mice

A deficit in IL-4 production has been previously reported in both diabetic human patients and non-obese diabetic (NOD) mice. In addition, re-introducing IL-4 into NOD mice systemically, or as a transgene, led to a beneficial outcome in most studies. Here, we show that prediabetic, 12-week old female NOD mice have a deficit in IL-4 expression in the pancreatic lymph nodes (PLN) compared to age-matched diabetes-resistant NOD.B10 mice. By bioluminescence imaging, we demonstrated that the PLN was preferentially targeted by bone marrow-derived dendritic cells (DCs) following intravenous (IV) administration. Following IV injection of DCs transduced to express IL-4 (DC/IL-4) into 12-week old NOD mice, it was possible to significantly delay or prevent the onset of hyperglycemia. We then focused on the PLN to monitor, by microarray analysis, changes in gene expression induced by DC/IL-4 and observed a rapid normalization of the expression of many genes, that were otherwise under-expressed compared to NOD.B10 PLN. The protective effect of DC/IL-4 required both MHC and IL-4 expression by the DCs. Thus, adoptive cellular therapy, using DCs modified to express IL-4, offers an effective, tissue-targeted cellular therapy to prevent diabetes in NOD mice at an advanced stage of pre-diabetes, and may offer a safe approach to consider for treatment of high risk human pre-diabetic patients.

The design of guanidinium-rich transporters and their internalization mechanisms

The ability of a drug or probe to cross a biological barrier has historically been viewed to be a function of its intrinsic physical properties. This view has largely restricted drug design and selection to agents within a narrow log P range. Molecular transporters offer a strategy to circumvent these restrictions. In the case of guanidinium-rich transporters (GRTs), a typically highly water-soluble conjugate is found to readily pass through the non-polar membrane of a cell and for some across tissue barriers. This activity opens a field of opportunities for the use of GRTs to enable delivery of polar and non-polar drugs or probes as well as to enhance uptake of those of intermediate polarity. The field of transporter enabled or enhanced uptake has grown dramatically in the last decade. Some GRT drug conjugates have been advanced into clinical trials. This review will provide an overview of recent work pertinent to the design and mechanism of uptake of GRTs.

Sustained endothelial progenitor cell dysfunction after chronic hypoxia-induced pulmonary hypertension

Circulating endothelial progenitor cells (EPCs) contribute to neovascularization of ischemic tissues and repair of injured endothelium. The role of bone marrow-derived progenitor cells in hypoxia-induced pulmonary vascular remodeling and their tissue-engineering potential in pulmonary hypertension (PH) remain largely unknown. We studied endogenous mobilization and homing of EPCs in green fluorescent protein bone marrow chimeric mice exposed to chronic hypoxia, a common hallmark of PH. Despite increased peripheral mobilization, as shown by flow cytometry and EPC culture, bone marrow-derived endothelial cell recruitment in remodeling lung vessels was limited. Moreover, transfer of vascular endothelial growth factor receptor-2+/Sca-1+/CXCR-4+-cultured early-outgrowth EPCs failed to reverse PH, suggesting hypoxia-induced functional impairment of transferred EPCs. Chronic hypoxia decreased migration to stromal cell-derived factor-1alpha, adhesion to fibronectin, incorporation into a vascular network, and nitric oxide production (-41%, -29%, -30%, and -32%, respectively, vs. normoxic EPCs; p < .05 for all). The dysfunctional phenotype of hypoxic EPCs significantly impaired their neovascularization capacity in chronic hind limb ischemia, contrary to normoxic EPCs cultured in identical conditions. Mechanisms contributing to EPC dysfunction include reduced integrin alphav and beta1 expression, decreased mitochondrial membrane potential, and enhanced senescence. Novel insights from chronic hypoxia-induced EPC dysfunction may provide important cues for improved future cell repair strategies.

Hepatic parenchymal replacement in mice by transplanted allogeneic hepatocytes is facilitated by bone marrow transplantation and mediated by CD4 cells

The lack of adequate donor organs is a major limitation to the successful widespread use of liver transplantation for numerous human hepatic diseases. A desirable alternative therapeutic option is hepatocyte transplantation (HT), but this approach is similarly restricted by a shortage of donor cells and by immunological barriers. Therefore, in vivo expansion of tolerized transplanted cells is emerging as a novel and clinically relevant potential alternative cellular therapy. Toward this aim, in the present study we established a new mouse model that combines HT with prior bone marrow transplantation (BMT). Donor hepatocytes were derived from human alpha(1)-antitrypsin (hAAT) transgenic mice of the FVB strain. Serial serum enzyme-linked immunosorbent assays for hAAT protein were used to monitor hepatocyte engraftment and expansion. In control recipient mice lacking BMT, we observed long-term yet modest hepatocyte engraftment. In contrast, animals undergoing additional syngeneic BMT prior to HT showed a 3- to 5-fold increase in serum hAAT levels after 24 weeks. Moreover, complete liver repopulation was observed in hepatocyte-transplanted Balb/C mice that had been transplanted with allogeneic FVB-derived bone marrow. These findings were validated by a comparison of hAAT levels between donor and recipient mice and by hAAT-specific immunostaining. Taken together, these findings suggest a synergistic effect of BMT on transplanted hepatocytes for expansion and tolerance induction. Livers of repopulated animals displayed substantial mononuclear infiltrates, consisting predominantly of CD4(+) cells. Blocking the latter prior to HT abrogated proliferation of transplanted hepatocytes, and this implied an essential role played by CD4(+) cells for in vivo hepatocyte selection following allogeneic BMT.

CONCLUSION

The present mouse model provides a versatile platform for investigation of the mechanisms governing HT with direct relevance to the development of clinical strategies for the treatment of human hepatic failure.

Molecular imaging: reporter gene imaging

Non-invasive in-vivo molecular genetic imaging developed over the past decade and predominantly utilises radiotracer (PET, gamma camera, autoradiography), magnetic resonance and optical imaging technology. Molecular genetic imaging has its roots in both molecular biology and cell biology. The convergence of these disciplines and imaging modalities has provided the opportunity to address new research questions, including oncogenesis, tumour maintenance and progression, as well as responses to molecular-targeted therapy. Three different imaging strategies are described: (1) "bio-marker" or "surrogate" imaging; (2) "direct" imaging of specific molecules and pathway activity; (3) "indirect" reporter gene imaging. Examples of each imaging strategy are presented and discussed. Several applications of PET- and optical-based reporter imaging are demonstrated, including signal transduction pathway monitoring, oncogenesis in genetic mouse models, endogenous molecular genetic/biological processes and the response to therapy in animal models of human disease. Molecular imaging studies will compliment established ex-vivo molecular-biological assays that require tissue sampling by providing a spatial and a temporal dimension to our understanding of disease development and progression, as well as response to treatment. Although molecular imaging studies are currently being performed primarily in experimental animals, we optimistically expect they will be translated to human subjects with cancer and other diseases in the near future.