In vivo cell tracking with bioluminescence imaging

Design and Biodistribution of PEGylated Core-Shell X-ray Fluorescent Nanoparticle Contrast Agents

Nanoparticle (NP) uptake by macrophages and their accumulation in undesired organs such as the liver and spleen constitute a major barrier to the effective delivery of NPs to targeted tissues for bioimaging and therapeutics. Surface functionalization with polyethylene glycol (PEG) has been demonstrated to be a promising strategy to limit NP sequestration, although its longitudinal stability under physiological conditions and impact on the NP biodistribution have not been investigated with an in vivo quantitative approach. X-ray fluorescence (XRF) imaging has been employed to noninvasively map the in vivo biodistribution of purposely designed molybdenum-based contrast agents, leading to submillimeter resolution, elemental specificity, and high penetration depth. In the present work, we design a stepwise layering approach for NP synthesis to investigate the role of chemisorbed and physisorbed PEG on silica-coated molybdenum-based contrast agents in affecting their in vivo biodistribution, using whole-body XRF imaging. Comparative quantitative in vivo studies indicated that physisorbed PEG (1.5 kDa) did not substantially affect the biodistribution, while the chemisorption route with mPEG-Si (6-9 PEG units) led to significant macroscopic variations in the biodistribution, leading to a reduction in NP uptake by the liver. Furthermore, the results highlighted the major role of the spleen in compensating for the limited sequestration by the liver, microscopically validated with a multiscale imaging approach with fluorophore doping of the silica shell. These findings demonstrated the promising role of XRF imaging for the rapid assessment of surface-functionalized contrast agents with whole-body in vivo quantitative pharmacokinetic studies, establishing the groundwork for developing strategies to identify and bypass undesired NP uptake.

Advances in Tracing Techniques: Mapping the Trajectory of Mesenchymal Stem-Cell-Derived Extracellular Vesicles

Mesenchymal stem-cell-derived extracellular vesicles (MSC-EVs) are nanoscale lipid bilayer vesicles secreted by mesenchymal stem cells. They inherit the parent cell's attributes, facilitating tissue repair and regeneration, promoting angiogenesis, and modulating the immune response, while offering advantages like reduced immunogenicity, straightforward administration, and enhanced stability for long-term storage. These characteristics elevate MSC-EVs as highly promising in cell-free therapy with notable clinical potential. It is critical to delve into their pharmacokinetics and thoroughly elucidate their intracellular and in vivo trajectories. A detailed summary and evaluation of existing tracing strategies are needed to establish standardized protocols. Here, we have summarized and anticipated the research progress of MSC-EVs in various biomedical imaging techniques, including fluorescence imaging, bioluminescence imaging, nuclear imaging (PET, SPECT), tomographic imaging (CT, MRI), and photoacoustic imaging. The challenges and prospects of MSC-EV tracing strategies, with particular emphasis on clinical translation, have been analyzed, with promising solutions proposed.

Doxorubicin-loaded PEGylated liposome modified with ANGPT2-specific peptide for integrative glioma-targeted imaging and therapy

Liposomal nanocarriers are able to carry peptides for efficient and selective delivery of radioactive tracer and drugs into the tumors. Angiopoietin 2 (ANGPT2) is an excellent biomarker for precise diagnosis and therapy of glioma. The present study aimed to design ANGPT2-specific peptides to modify the surface of nanoliposomes containing doxorubicin (Dox) for integrative imaging and targeting therapy of glioma. The targeted ANGPT2 peptides were designed using the molecular operating environment. Peptide-conjugated PEGlated liposomes containing Dox (peptide-Lipo@Dox) were prepared for radionuclide and drug delivery. Glioma cell functions were determined based on cell cycle and viability, apoptosis, cell invasion and migration, and colony-formation assays. The anti-tumor effect of peptide-Lipo@Dox was validated in intracranial U87-MG cell glioma-bearing mice in vivo. The peptides GSFIHSVPRH (GSF) and HSVPRHEV (HSV) showed specific affinity for ANGPT2 and a better cellular uptake in U87-MG cells. Micro-positron emission tomography (PET)/computed tomography (CT) imaging was used to visualize the orthotopic transplantation of glioma in the brain 1 h after injection of radionuclide 68Ga-labeled peptide-Lipo@Dox. Lipo@Dox with peptide modification demonstrated stable Dox loading, small sizes (<40 nm), and enrichment in the tumor region of the mouse brain. Peptide-Lipo@Dox treatment inhibited the Tie-2/Akt/Foxo-1 pathway, thereby inhibiting cell invasion and migration, cell viability, and colony-forming ability of U87-MG cells. Lipo@Dox peptide modification showed a better suppression of glioma development than Lipo@Dox. Thus, the ANGPT2-specific peptides were successfully designed, and the PEGylated liposome modified with ANGPT2-specific peptide served as part of a potent delivery method for integrative glioma-targeted imaging and therapy.

Noninvasive in vivo imaging of macrophages: understanding tumor microenvironments and delivery of therapeutics

Macrophages are pivotal in the body's defense and response to inflammation. They are present in significant numbers and are widely implicated in various diseases, including cancer. While molecular and histological techniques have advanced our understanding of macrophage biology, their precise function within the cancerous microenvironments remains underexplored. Enhancing our knowledge of macrophages and the dynamics of their extracellular vesicles (EVs) in cancer development can potentially improve therapeutic management. Notably, macrophages have also been harnessed to deliver drugs. Noninvasive in vivo molecular imaging of macrophages is crucial for investigating intricate cellular processes, comprehending the underlying mechanisms of diseases, tracking cells and EVs' migration, and devising macrophage-dependent drug-delivery systems in living organisms. Thus, in vivo imaging of macrophages has become an indispensable tool in biomedical research. The integration of multimodal imaging approaches and the continued development of novel contrast agents hold promise for overcoming current limitations and expanding the applications of macrophage imaging. This study comprehensively reviews several methods for labeling macrophages and various imaging modalities, assessing the merits and drawbacks of each approach. The review concludes by offering insights into the applicability of molecular imaging techniques for real time monitoring of macrophages in preclinical and clinical scenarios.

A new effLuc/Kate dual reporter allele for tumor imaging in mice

Genetically engineered mouse models (GEMMs) are instrumental for modelling local and systemic features of complex diseases, such as cancer. Non-invasive, longitudinal cell detection and monitoring in tumors, metastases and/or the micro-environment is paramount to achieve a better spatiotemporal understanding of cancer progression and to evaluate therapies in preclinical studies. Bioluminescent and fluorescent reporters marking tumor cells or their microenvironment are valuable for non-invasive cell detection and monitoring in vivo. Here, we report the generation of a dual reporter allele allowing simultaneous bioluminescence and fluorescence detection of cells that have undergone Cre-Lox recombination in mice. The single copy knock-in allele in the permissive collagen I locus was evaluated in the context of several cancer GEMMs, where Cre expression was achieved genetically or by ectopic virus-mediated delivery. The new reporter allele was also combined with gene-targeted alleles widely used in bone, prostate, brain and pancreas cancer research, as well as with alleles inserted into the commonly used Rosa26 and collagen I loci. This allele is, therefore, a useful addition to the portfolio of reporters to help advance preclinical research.

Exosomes in cancer diagnosis based on the Latest Evidence: Where are We?

Exosomes are small extracellular vesicles (EVs) derived from various cellular sources and have emerged as favorable biomarkers for cancer diagnosis and prognosis. These vesicles contain a variety of molecular components, including nucleic acids, proteins, and lipids, which can provide valuable information for cancer detection, classification, and monitoring. However, the clinical application of exosomes faces significant challenges, primarily related to the standardization and scalability of their use. In order to overcome these challenges, sophisticated methods such as liquid biopsy and imaging are being combined to augment the diagnostic capabilities of exosomes. Additionally, a deeper understanding of the interaction between exosomes and immune system components within the tumor microenvironment (TME) is essential. This review discusses the biogenesis and composition of exosomes, addresses the current challenges in their clinical translation, and highlights recent technological advancements and integrative approaches that support the role of exosomes in cancer diagnosis and prognosis.

A Novel Luciferase-Based Reporter Gene Technology for Simultaneous Optical and Radionuclide Imaging of Cells

Multimodality reporter gene imaging combines the sensitivity, resolution and translational potential of two or more signals. The approach has not been widely adopted by the animal imaging community, mainly because its utility in this area is unproven. We developed a new complementation-based reporter gene system where the large component of split NanoLuc luciferase (LgBiT) presented on the surface of cells (TM-LgBiT) interacts with a radiotracer consisting of the high-affinity complementary HiBiT peptide labeled with a radionuclide. Radiotracer uptake could be imaged in mice using SPECT/CT and bioluminescence within two hours of implanting reporter-gene-expressing cells. Imaging data were validated by ex vivo biodistribution studies. Following the demonstration of complementation between the TM-LgBiT protein and HiBiT radiotracer, we validated the use of the technology in the highly specific in vivo multimodal imaging of cells. These findings highlight the potential of this new approach to facilitate the advancement of cell and gene therapies from bench to clinic.

Luciferase transduction and selection protocol for reliable in vivo bioluminescent measurements in cancer research

Bioluminescence imaging has become an essential non-invasive tool in cancer research for monitoring various cellular processes and tumor progression in vivo. In this article, we aimed to propose a transduction and selection protocol for reliable in vivo bioluminescent measurements in immunocompetent mouse models. Using two different heterogenous luciferase-expressing cell models, we underlined factors influencing transduction. The protocol was tested through an in vitro luciferase activity assay as well as using in vivo longitudinal monitoring of metastases formation (In Vivo Imaging System®). The data were cross validated with histological assessment. Our results demonstrated stable and proportional in vitro and in vivo bioluminescent signals correlating with actual metastatic burden. Furthermore, ex vivo analysis confirmed the accuracy of bioluminescent imaging in quantifying metastatic surface area. This protocol should ensure reliable and reproducible measurements in cancer research utilizing luciferase-positive cell lines, confirming the validity and accuracy of preclinical studies in immunocompetent models.

Development and validation of Mayaro virus with luciferase reporter genes as a tool for antiviral assays

Arboviruses are etiological agents in an extensive group of emerging diseases with great clinical relevance in Brazil, due to the wide distribution of their vectors and the favorable environmental conditions. Among them, the Mayaro virus (MAYV) has drawn attention since its emergence as the etiologic agent of Mayaro fever, a highly debilitating disease. To study viral replication and identify new drug candidates, traditional antiviral assays based on viral antigens and/or plaque assays have been demonstrating low throughput, making it difficult to carry out larger-scale assays. Therefore, we developed and characterized two DNA-launched infectious clones reporter viruses based on the MAYV strain BeAr 20290 containing the reporter genes of firefly luciferase (FLuc) and nanoluciferase (NLuc), designated as MAYV-firefly and MAYV-nanoluc, respectively. The viruses replicated efficiently with similar properties to the parental wild-type MAYV, and luminescence expression levels reflected viral replication. Reporter genes were also preserved during passage in cell culture, remaining stably expressed for one round of passage for MAYV-firefly and three rounds for MAYV-nanoluc. Employing the infectious clone, we described the effect of Rimantadine, an FDA-approved Alzheimer's drug, as a repurposing agent for MAYV but with a broad-spectrum activity against Zika virus infection. Additionally, we validated MAYV-nanoluc as a tool for antiviral drug screening using the compound EIDD-2749 (4'-Fluorouridine), which acts as an inhibitor of alphavirus RNA-dependent RNA polymerase.

Bioluminescence as a functional tool for visualizing and controlling neuronal activity in vivo

The use of bioluminescence as a reporter for physiology in neuroscience is as old as the discovery of the calcium-dependent photon emission of aequorin. Over the years, luciferases have been largely replaced by fluorescent reporters, but recently, the field has seen a renaissance of bioluminescent probes, catalyzed by unique developments in imaging technology, bioengineering, and biochemistry to produce luciferases with previously unseen colors and intensity. This is not surprising as the advantages of bioluminescence make luciferases very attractive for noninvasive, longitudinal in vivo observations without the need of an excitation light source. Here, we review how the development of dedicated and specific sensor-luciferases afforded, among others, transcranial imaging of calcium and neurotransmitters, or cellular metabolites and physical quantities such as forces and membrane voltage. Further, the increased versatility and light output of luciferases have paved the way for a new field of functional bioluminescence optogenetics, in which the photon emission of the luciferase is coupled to the gating of a photosensor, e.g., a channelrhodopsin and we review how they have been successfully used to engineer synthetic neuronal connections. Finally, we provide a primer to consider important factors in setting up functional bioluminescence experiments, with a particular focus on the genetic model Caenorhabditis elegans, and discuss the leading challenges that the field needs to overcome to regain a competitive advantage over fluorescence modalities. Together, our paper caters to experienced users of bioluminescence as well as novices who would like to experience the advantages of luciferases in their own hand.

Croix B, Leen AM, Bajgain P. A dual-luciferase bioluminescence system for the assessment of cellular therapies

Bioluminescence imaging is a well-established platform for evaluating engineered cell therapies in preclinical studies. However, despite the discovery of new luciferases and substrates, optimal combinations to simultaneously monitor two cell populations remain limited. This makes the functional assessment of cellular therapies cumbersome and expensive, especially in preclinical in vivo models. In this study, we explored the potential of using a green bioluminescence-emitting click beetle luciferase, CBG99, and a red bioluminescence-emitting firefly luciferase mutant, Akaluc, together to simultaneously monitor two cell populations. Using various chimeric antigen receptor T cells and tumor pairings, we demonstrate that these luciferases are suitable for real-time tracking of two cell types using 2D and 3D cultures in vitro and experimental models in vivo. Our data show the broad compatibility of this dual-luciferase (duo-luc) system with multiple bioluminescence detection equipment ranging from benchtop spectrophotometers to live animal imaging systems. Although this study focused on investigating complex CAR T cells and tumor cell interactions, this duo-luc system has potential utility for the simultaneous monitoring of any two cellular components-for example, to unravel the impact of a specific genetic variant on clonal dominance in a mixed population of tumor cells.

In vivo tracking transplanted cardiomyocytes derived from human induced pluripotent stem cells using nuclear medicine imaging

Introduction

Transplantation of human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) is a promising treatment for heart failure. Information on long-term cell engraftment after transplantation is clinically important. However, clinically applicable evaluation methods have not yet been established.

Methods

In this study, to noninvasively assess transplanted cell engraftment, human SLC5A5, which encodes a sodium/iodide symporter (NIS) that transports radioactive tracers such as 125I, 18F-tetrafluoroborate (TFB), and 99mTc-pertechnetate (99mTcO4-), was transduced into human induced pluripotent stem cells (iPSCs), and nuclear medicine imaging was used to track engrafted human iPSC-CMs.

Results

To evaluate the pluripotency of NIS-expressing human iPSCs, they were subcutaneously transplanted into immunodeficient rats. Teratomas were detected by 99mTcO4- single photon emission computed tomography (SPECT/CT) imaging. NIS expression and the uptake ability of 125I were maintained in purified human iPSC-CMs. NIS-expressing human iPSC-CMs transplanted into immunodeficient rats could be detected over time using 99mTcO4- SPECT/CT imaging. Unexpectedly, NIS expression affected cell proliferation of human iPSCs and iPSC-derived cells.

Discussion

Such functionally designed iPSC-CMs have potential clinical applications as a noninvasive method of grafted cell evaluation, but further studies are needed to determine the effects of NIS transduction on cellular characteristics and functions.

Theranostic chimeric antigen receptor (CAR)-T cells: Insight into recent trends and challenges in solid tumors

Cell therapy has reached significant milestones in various life-threatening diseases, including cancer. Cell therapy using fluorescent and radiolabeled chimeric antigen receptor (CAR)-T cell is a successful strategy for diagnosing or treating malignancies. Since cell therapy approaches have different results in cancers, the success of hematological cancers has yet to transfer to solid tumor therapy, leading to more casualties. Therefore, there are many areas for improvement in the cell therapy platform. Understanding the therapeutic barriers associated with solid cancers through cell tracking and molecular imaging may provide a platform for effectively delivering CAR-T cells into solid tumors. This review describes CAR-T cells' role in treating solid and non-solid tumors and recent advances. Furthermore, we discuss the main obstacles, mechanism of action, novel strategies and solutions to overcome the challenges from molecular imaging and cell tracking perspectives.

Bioluminescence Imaging and ICP-MS Associated with SPION as a Tool for Hematopoietic Stem and Progenitor Cells Homing and Engraftment Evaluation

Bone marrow transplantation is a treatment for a variety of hematological and non-hematological diseases. For the transplant success, it is mandatory to have a thriving engraftment of transplanted cells, which directly depends on their homing. The present study proposes an alternative method to evaluate the homing and engraftment of hematopoietic stem cells using bioluminescence imaging and inductively coupled plasma mass spectrometry (ICP-MS) associated with superparamagnetic iron oxide nanoparticles. We have identified an enriched population of hematopoietic stem cells in the bone marrow following the administration of Fluorouracil (5-FU). Lately, the cell labeling with nanoparticles displayed the greatest internalization status when treated with 30 µg Fe/mL. The quantification by ICP-MS evaluate the stem cells homing by identifying 3.95 ± 0.37 µg Fe/mL in the control and 6.61 ± 0.84 µg Fe/mL in the bone marrow of transplanted animals. In addition, 2.14 ± 0.66 mg Fe/g in the spleen of the control group and 2.17 ± 0.59 mg Fe/g in the spleen of the experimental group was also measured. Moreover, the bioluminescence imaging provided the follow up on the hematopoietic stem cells behavior by monitoring their distribution by the bioluminescence signal. Lastly, the blood count enabled the monitoring of animal hematopoietic reconstitution and ensured the transplantation effectiveness.

Accurate and Early Metastases Diagnosis in Live Animals With Multimodal X-ray and Optical Imaging

PURPOSE

In vivo optical imaging systems are essential to track disease progression and evaluate therapeutic efficacy in animal studies. However, current approaches are limited by their inability to accurately capture 3-dimensional (3-D) image information. To overcome this hindrance, we adopted x-ray computed tomography (CT) as a prior for 3-D optical image reconstruction and further challenged the multimodal imaging performance with a metastasis model.

METHODS AND MATERIALS

The iSMAART system, an integrated small animal research platform, features coregistered high-quality quantitative optical tomography and CT. In the synergistic dual-modality imaging, CT provides both 3-D anatomy information and animal structure mesh for optical tomography reconstruction, which is performed using bioluminescence projections acquired from 4 orthogonal angles. The multimodal imaging system was challenged with a prostate cancer metastasis model, and a double-blind histopathology diagnosis was obtained to validate the imaging results.

RESULTS

The iSMAART located, visualized, and quantified early tumor metastases at the millimeter scale, and can accurately track deep tumors as small as 1.5 mm in live animals. Tumors metastasized into the liver, diaphragm, and tibia in 4 mice were all successfully diagnosed by the integrated tomographic imaging.

CONCLUSIONS

Instead of roughly comparing surface-light intensities, as traditionally performed in 2-dimensional optical imaging, iSMAART provides accurate tumor imaging and quantitative assessment capabilities with integrated CT and optical tomography for cancer metastasis research. With the powerful 3-D optical/CT imaging capability, iSMAART has the potential to tackle more complex research needs with higher targeting accuracy.

Distribution of Embryonic Stem Cell-Derived Mesenchymal Stem Cells after Intravenous Infusion in Hypoxic-Ischemic Encephalopathy

Systemic administration of mesenchymal stem cells (MSCs) has been reported to improve neurological function in brain damage, including hypoxic-ischemic encephalopathy (HIE), though the action mechanisms have not been fully elucidated. In this study, the cells were tracked live using a Pearl Trilogy Small Animal fluorescence imaging system after human embryonic stem Cell-Derived MSCs (ES-MSCs) infusion for an HIE mouse model. ES-MSC-treated HIE mice showed neurobehavioral improvement. In vivo imaging showed similar sequential migration of ES-MSCs from lungs, liver, and spleen within 7 days in both HIE and normal mice with the exception of lungs, where there was higher entrapment in the HIE 1 h after infusion. In addition, ex vivo experiments confirmed time-dependent infiltration of ES-MSCs into the organs, with similar findings in vivo, although lungs and brain revealed small differences. ES-MSCs seemed to remain in the brain only in the case of HIE on day 14 after the cell infusion. The homing effect in the host brain was confirmed with immunofluorescence staining, which showed that grafted cells remained in the brain tissue at the lesion area with neurorestorative findings. Further research should be carried out to elucidate the role of each host organ's therapeutic effects when stem cells are systemically introduced.

In Vivo Bioluminescence for the Detection of the Fate of Pancreatic Islet Organoids Post-transplantation

Pancreatic islet transplantation is a promising cell replacement treatment for patients afflicted with type 1 diabetes (T1D), which is an autoimmune disease resulting in the destruction of insulin-producing islet β-cells. However, the shortage of donor pancreatic islets significantly hampers the widespread application of this strategy as routine therapy. Pluripotent stem cell-derived insulin-producing islet organoids constitute a promising alternative β-cell source for T1D patients. Early after transplantation, it is critical to know the fate of transplanted islet organoids, but determining their survival remains a significant technical challenge. Bioluminescence imaging (BLI) is an optical molecular imaging technique that detects the survival of living cells using light emitted from luciferase-expressing bioreporter cells. Through BLI, the post-transplantation fate of islet organoids can be evaluated over time in a noninvasive fashion with minimal intervention, thus making BLI an ideal tool to determine the success of the transplant and improving cell replacement therapy approaches for T1D.

Assessment of endothelial colony forming cells delivery routes in a murine model of critical limb threatening ischemia using an optimized cell tracking approach

Background

Endothelial colony forming cells (ECFCs), alone or in combination with mesenchymal stem cells, have been selected as potential therapeutic candidates for critical limb-threatening ischemia (CLTI), mainly for those patients considered as “no-option,” due to their capability to enhance revascularization and perfusion recovery of ischemic tissues. Nevertheless, prior to translating cell therapy to the clinic, biodistribution assays are required by regulatory guidelines to ensure biosafety as well as to discard undesired systemic translocations. Different approaches, from imaging technologies to qPCR-based methods, are currently applied.

Methods

In the current study, we have optimized a cell-tracking assay based on DiR fluorescent cell labeling and near-infrared detection for in vivo and ex vivo assays. Briefly, an improved protocol for DiR staining was set up, by incubation of ECFCs with 6.67 µM DiR and intensive washing steps prior cell administration. The minimal signal detected for the residual DiR, remaining after these washes, was considered as a baseline signal to estimate cell amounts correlated to the DiR intensity values registered in vivo. Besides, several assays were also performed to determine any potential effect of DiR over ECFCs functionality. Furthermore, the optimized protocol was applied in combination with qPCR amplification of specific human Alu sequences to assess the final distribution of ECFCs after intramuscular or intravenous administration to a murine model of CLTI.

Results

The optimized DiR labeling protocol indicated that ECFCs administered intramuscularly remained mainly within the hind limb muscle while cells injected intravenously were found in the spleen, liver and lungs.

Conclusion

Overall, the combination of DiR labeling and qPCR analysis in biodistribution assays constitutes a highly sensitive approach to systemically track cells in vivo. Thereby, human ECFCs administered intramuscularly to CLTI mice remained locally within the ischemic tissues, while intravenously injected cells were found in several organs. Our data corroborate the need to perform biodistribution assays in order to define specific parameters such as the optimal delivery route for ECFCs before their application into the clinic.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02943-8.

Regulatory guidelines and preclinical tools to study the biodistribution of RNA therapeutics

The success of the messenger RNA-based COVID-19 vaccines of Moderna and Pfizer/BioNTech marks the beginning of a new chapter in modern medicine. However, the rapid rise of mRNA therapeutics has resulted in a regulatory framework that is somewhat lagging. The current guidelines either do not apply, do not mention RNA therapeutics, or do not have widely accepted definitions. This review describes the guidelines for preclinical biodistribution studies of mRNA/siRNA therapeutics and highlights the relevant differences for mRNA vaccines. We also discuss the role of in vivo RNA imaging techniques and other assays to fulfill and/or complement the regulatory requirements. Specifically, quantitative whole-body autoradiography, microautoradiography, mass spectrometry-based assays, hybridization techniques (FISH, bDNA), PCR-based methods, in vivo fluorescence imaging, and in vivo bioluminescence imaging, are discussed. We conclude that this new and rapidly evolving class of medicines demands a multi-layered approach to fully understand its biodistribution and in vivo characteristics.

Whole-Body Imaging to Assess Cell-Based Immunotherapy: Preclinical Studies with an Update on Clinical Translation

In the past decades, immunotherapies against cancers made impressive progress. Immunotherapy includes a broad range of interventions that can be separated into two major groups: cell-based immunotherapies, such as adoptive T cell therapies and stem cell therapies, and immunomodulatory molecular therapies such as checkpoint inhibitors and cytokine therapies. Genetic engineering techniques that transduce T cells with a cancer-antigen-specific T cell receptor or chimeric antigen receptor have expanded to other cell types, and further modulation of the cells to enhance cancer targeting properties has been explored. Because cell-based immunotherapies rely on cells migrating to target organs or tissues, there is a growing interest in imaging technologies that non-invasively monitor transferred cells in vivo. Here, we review whole-body imaging methods to assess cell-based immunotherapy using a variety of examples. Following a review of preclinically used cell tracking technologies, we consider the status of their clinical translation.

A Series of Furimazine Derivatives for Sustained Live-Cell Bioluminescence Imaging and Application to the Monitoring of Myogenesis at the Single-Cell Level

Bioluminescence (BL) imaging, which utilizes light emitted through the enzymatic reaction of luciferase oxidizing its substrate luciferin, enables sensitive and noninvasive monitoring of life phenomena. Herein, we developed a series of caged furimazine (FMZ) derivatives by introducing a protective group at the C-3 position and a hydroxy group at the C-6 phenyl ring to realize long-term live-cell BL imaging based on the NanoLuc (NLuc)/NanoKAZ (NKAZ)-FMZ system. The membrane permeability and cytotoxicity of the substrates were evaluated and related to their hydrophobicity. Among the series, the derivative with the bulkiest protective group (adamantanecarbonyl group) and a hydroxy substituent (named Ad-FMZ-OH) showed significantly prolonged and constant BL signal in cells expressing NLuc compared to the native FMZ substrate. This derivative enabled continuous BL imaging at the single-cell level for 24 h. Furthermore, we applied Ad-FMZ-OH to BL imaging of myocyte fusion and succeeded in the consecutive and sensitive monitoring at a single-cell level over a day. In summary, NLuc/NKAZ-caged FMZ derivatives have the potential to be applied to live-cell BL imaging of various life phenomena that require long-term observation.

A noninvasive iRFP713 p53 reporter reveals dynamic p53 activity in response to irradiation and liver regeneration in vivo

Genetically encoded probes are widely used to visualize cellular processes in vitro and in vivo. Although effective in cultured cells, fluorescent protein tags and reporters are suboptimal in vivo because of poor tissue penetration and high background signal. Luciferase reporters offer improved signal-to-noise ratios but require injections of luciferin that can lead to variable responses and that limit the number and timing of data points that can be gathered. Such issues in studying the critical transcription factor p53 have limited insight on its activity in vivo during development and tissue injury responses. Here, by linking the expression of the near-infrared fluorescent protein iRFP713 to a synthetic p53-responsive promoter, we generated a knock-in reporter mouse that enabled noninvasive, longitudinal analysis of p53 activity in vivo in response to various stimuli. In the developing embryo, this model revealed the timing and localization of p53 activation. In adult mice, the model monitored p53 activation in response to irradiation and paracetamol- or CCl4-induced liver regeneration. After irradiation, we observed potent and sustained activation of p53 in the liver, which limited the production of reactive oxygen species (ROS) and promoted DNA damage resolution. We propose that this new reporter may be used to further advance our understanding of various physiological and pathophysiological p53 responses.

Toward Generalizable Trajectory Planning for Human Intracerebral Trials and Therapy

<b><i>Introduction:</i></b> Stereotactic neurosurgical techniques are increasingly used to deliver biologics, such as cells and viruses, although standardized procedures are necessary to ensure consistency and reproducibility. <b><i>Objective:</i></b> We provide an instructional guide to help plan for complex image-guided trajectories; this may be of particular benefit to surgeons new to biologic trials and companies planning such trials. <b><i>Methods:</i></b> We show how nuclei can be segmented and multiple trajectories with multiple injection points can be created through a single or multiple burr hole(s) based on preoperative images. Screenshots similar to those shown in this article can be used for planning purposes and for quality control in clinical trials. <b><i>Results:</i></b> This method enables the precise definition of 3-D target structures, such as the putamen, and efficient planning trajectories for biologic injections. The technique is generalizable and largely independent of procedural format, and thus can be integrated with frame-based or frameless platforms to streamline reproducible therapeutic delivery. <b><i>Conclusions:</i></b> We describe an easy-to-use and generalizable protocol for intracerebral trajectory planning for stereotactic delivery of biologics. Although we highlight intracerebral stem cell delivery to the putamen using a frame-based stereotactic delivery system, similar strategies may be employed for different brain nuclei using different platforms. We anticipate this will inform future advanced and fully automated neurosurgical procedures to help unify the field and decrease inherent variability seen with manual trajectory planning.

Strategies for Improving Small-Molecule Biosensors in Bacteria

In recent years, small-molecule biosensors have become increasingly important in synthetic biology and biochemistry, with numerous new applications continuing to be developed throughout the field. For many biosensors, however, their utility is hindered by poor functionality. Here, we review the known types of mechanisms of biosensors within bacterial cells, and the types of approaches for optimizing different biosensor functional parameters. Discussed approaches for improving biosensor functionality include methods of directly engineering biosensor genes, considerations for choosing genetic reporters, approaches for tuning gene expression, and strategies for incorporating additional genetic modules.

Generation of a pdmH1N1 2018 Influenza A Reporter Virus Carrying a mCherry Fluorescent Protein in the PA Segment

Influenza A virus (IAV) is a major human pathogen associated with significant morbidity and mortality worldwide. Through serial passage in mice, we generated a recombinant pdmH1N1 2009 IAV, A/Guangdong/GLW/2018 (GLW/18-MA), which encodes an mCherry gene fused to the C-terminal of a polymerase acidic (PA) segment and demonstrated comparable growth kinetics to the wild-type. Nine mutations were identified in the GLW/18-MA genome: PA (I61M, E351G, and G631S), NP (E292G), HA1 (T164I), HA2 (N117S and P160S), NA (W61R), and NEP (K44R). The recombinant IAV reporter expresses mCherry, a red fluorescent protein, at a high level and maintains its genetic integrity after five generations of serial passages in Madin-Darby Canine Kidney cells (MDCK) cells. Moreover, the imaging is noninvasive and permits the monitoring of infection in living mice. Treatment with oseltamivir or baicalin followed by infection with the reporter IAV led to a decrease in fluorescent protein signal in living mice. This result demonstrates that the IAV reporter virus is a powerful tool to study viral pathogenicity and transmission and to develop and evaluate novel anti-viral drugs, inhibitors, and vaccines in the future.

Protocols for the Evaluation of a Lymphatic Drug Delivery System Combined with Bioluminescence to Treat Metastatic Lymph Nodes

Bioluminescence (BL) imaging is a powerful non-invasive imaging modality widely used in a broad range of biological disciplines for many types of measurements. The applications of BL imaging in biomedicine are diverse, including tracking bacterial progression, research on gene expression patterns, monitoring tumor cell growth/regression or treatment responses, determining the location and proliferation of stem cells, and so on. It is particularly valuable when studying tissues at depths of 1 to 2 cm in mouse models during preclinical research. Here we describe the protocols for the therapeutic evaluation of a lymphatic drug delivery system (LDDS) using an in vivo BL imaging system (IVIS) for the treatment of metastatic lymph nodes (LNs) with 5-fluorouracil (5-FU). The LDDS is a method that directly injects anticancer drugs into sentinel LNs (SLNs) and delivers them to their downstream LNs. In the protocol, we show that metastases in the proper axillary LN (PALN) are induced by the injection of luciferase-expressing tumor cells into the subiliac LN (SiLN) of MXH10/Mo-lpr/lpr (MXH10/Mo/lpr) mice. 5-FU is injected using the LDDS into the accessory axillary LN (AALN) to treat tumor cells in the PALN after the tumor cell growth is confirmed in the PALN. The tumor growth and therapeutic effects are evaluated by IVIS. This method can be used to evaluate tumor growth and efficacy of anticancer drugs/particles, radiotherapy, surgery, and/or a combination of these methods in various experimental procedures in the oncology field.

Longitudinal Tracing of Lyssavirus Infection in Mice via In Vivo Bioluminescence Imaging

Bioluminescence imaging (BLI) is a technique that can be employed to quantify biological processes in living cells. When used in small animal models such as mice, BLI can provide both longitudinal and positional information regarding the biological process under investigation. Although perhaps best known for its utility in non-invasively quantifying tumor burden over time in experimental animals, BLI has also been applied in many pathogenesis models to track pathogen burden and responses to therapeutic interventions. In this chapter, we present a BLI-based method for tracing anatomical progression of lyssavirus infection in a mouse model. We also include validation methods to ensure that semiquantitative BLI data correlate well with viral load. Due to the longitudinal nature of this approach, lyssavirus pathogenesis and therapeutic intervention studies can be performed with far fewer animals than more traditional approaches, which typically require euthanasia of large animal groups at every data collection time point.

In Vivo Bioluminescent Imaging of Bone Marrow-Derived Mesenchymal Stem Cells in Mice

Mesenchymal stem cells (MSCs) are multipotent adult stem cells present in multiple tissues, such as bone marrow, adipose tissue, umbilical cord, and amniotic fluid. MSCs can differentiate into multilineage cells under defined conditions in vitro and in vivo. MSCs have been shown to have therapeutic effects on various types of diseases. Noninvasive in vivo monitoring of MSCs is considered one of the important techniques for developing cell therapy. In this protocol, we introduce strategized MSCs derived from bone marrow (BM-MSCs) of knock-in mouse model expressing mCherry-Renilla luciferase (mCherry-RLuc) for noninvasive bioluminescence imaging (BLI) of injected BM-MSCs in vivo.

Non-invasive In Vivo Tracking of Mammalian Cells Stably Expressing Firefly Luciferase

Firefly luciferase (FLuc)-based in vivo optical imaging technology exerts the non-invasive monitoring of transplanted cells in experimental animal models. This chapter introduces an established cell line that stably expresses a retrovirus-delivered FLuc protein gene. The stable expression does not affect the cell morphology, proliferation, migration, and invasion abilities of the parental cells. After implantation, the bioluminescence signal of FLuc cells truly reflects cell proliferation and survival in vivo, which can provide a reliable method for dynamic detection of in vivo cell transplantation.

Direct and Indirect Chemiluminescence: Reactions, Mechanisms and Challenges

Emission of light by matter can occur through a variety of mechanisms. When it results from an electronically excited state of a species produced by a chemical reaction, it is called chemiluminescence (CL). The phenomenon can take place both in natural and artificial chemical systems and it has been utilized in a variety of applications. In this review, we aim to revisit some of the latest CL applications based on direct and indirect production modes. The characteristics of the chemical reactions and the underpinning CL mechanisms are thoroughly discussed in view of studies from the very recent bibliography. Different methodologies aiming at higher CL efficiencies are summarized and presented in detail, including CL type and scaffolds used in each study. The CL role in the development of efficient therapeutic platforms is also discussed in relation to the Reactive Oxygen Species (ROS) and singlet oxygen (1O2) produced, as final products. Moreover, recent research results from our team are included regarding the behavior of commonly used photosensitizers upon chemical activation under CL conditions. The CL prospects in imaging, biomimetic organic and radical chemistry, and therapeutics are critically presented in respect to the persisting challenges and limitations of the existing strategies to date.

Asymmetric cell division of mammary stem cells

Somatic stem cells are distinguished by their capacity to regenerate themselves and also to produce daughter cells that will differentiate. Self-renewal is achieved through the process of asymmetric cell division which helps to sustain tissue morphogenesis as well as maintain homeostasis. Asymmetric cell division results in the development of two daughter cells with different fates after a single mitosis. Only one daughter cell maintains "stemness" while the other differentiates and achieves a non-stem cell fate. Stem cells also have the capacity to undergo symmetric division of cells that results in the development of two daughter cells which are identical. Symmetric division results in the expansion of the stem cell population. Imbalances and deregulations in these processes can result in diseases such as cancer. Adult mammary stem cells (MaSCs) are a group of cells that play a critical role in the expansion of the mammary gland during puberty and any subsequent pregnancies. Furthermore, given the relatively long lifespans and their capability to undergo self-renewal, adult stem cells have been suggested as ideal candidates for transformation events that lead to the development of cancer. With the possibility that MaSCs can act as the source cells for distinct breast cancer types; understanding their regulation is an important field of research. In this review, we discuss asymmetric cell division in breast/mammary stem cells and implications on further research. We focus on the background history of asymmetric cell division, asymmetric cell division monitoring techniques, identified molecular mechanisms of asymmetric stem cell division, and the role asymmetric cell division may play in breast cancer.

Contrast agents for photoacoustic imaging: a review of stem cell tracking

With the advent of stem cell therapy for spinal cord injuries, stroke, burns, macular degeneration, heart diseases, diabetes, rheumatoid arthritis and osteoarthritis; the need to track the survival, migration pathways, spatial destination and differentiation of transplanted stem cells in a clinical setting has gained increased relevance. Indeed, getting regulatory approval to use these therapies in the clinic depends on biodistribution studies. Although optoacoustic imaging (OAI) or photoacoustic imaging can detect functional information of cell activities in real-time, the selection and application of suitable contrast agents is essential to achieve optimal sensitivity and contrast for sensing at clinically relevant depths and can even provide information about molecular activity. This review explores OAI methodologies in conjunction with the specific application of exogenous contrast agents in comparison to other imaging modalities and describes the properties of exogenous contrast agents for quantitative and qualitative monitoring of stem cells. Specific characteristics such as biocompatibility, the absorption coefficient, and surface functionalization are compared and how the labelling efficiency translates to both short and long-term visualization of mesenchymal stem cells is explored. An overview of novel properties of recently developed optoacoustic contrast agents and their capability to detect disease and recovery progression in clinical settings is provided which includes newly developed exogenous contrast agents to monitor stem cells in real-time for multimodal sensing.

Engineered exosomes: desirable target-tracking characteristics for cerebrovascular and neurodegenerative disease therapies

As extracellular vesicles secreted by cells, exosomes are intercellular signalosomes for cell communication and pharmacological effectors. Because of their special properties, including low toxicity and immunogenicity, biodegradability, ability to encapsulate endogenous biologically active molecules and cross the blood-brain barrier (BBB), exosomes have great therapeutic potential in cerebrovascular and neurodegenerative diseases. However, the poor targeting ability of natural exosomes greatly reduces the therapeutic effect. Using engineering technology, exosomes can obtain active targeting ability to accumulate in specific cell types and tissues by attaching targeting units to the membrane surface or loading them into cavities. In this review, we outline the improved targeting functions of bioengineered exosomes, tracing and imaging techniques, administration methods, internalization in the BBB, and therapeutic effects of exosomes in cerebrovascular and neurodegenerative diseases and further evaluate the clinical opportunities and challenges in this research field.

Made to Measure: Patient-Tailored Treatment of Multiple Sclerosis Using Cell-Based Therapies

Currently, there is still no cure for multiple sclerosis (MS), which is an autoimmune and neurodegenerative disease of the central nervous system. Treatment options predominantly consist of drugs that affect adaptive immunity and lead to a reduction of the inflammatory disease activity. A broad range of possible cell-based therapeutic options are being explored in the treatment of autoimmune diseases, including MS. This review aims to provide an overview of recent and future advances in the development of cell-based treatment options for the induction of tolerance in MS. Here, we will focus on haematopoietic stem cells, mesenchymal stromal cells, regulatory T cells and dendritic cells. We will also focus on less familiar cell types that are used in cell therapy, including B cells, natural killer cells and peripheral blood mononuclear cells. We will address key issues regarding the depicted therapies and highlight the major challenges that lie ahead to successfully reverse autoimmune diseases, such as MS, while minimising the side effects. Although cell-based therapies are well known and used in the treatment of several cancers, cell-based treatment options hold promise for the future treatment of autoimmune diseases in general, and MS in particular.

A novel thermostable beetle luciferase based cytotoxicity assay

Cytotoxicity assays are essential for the testing and development of novel immunotherapies for the treatment of cancer. We recently described a novel cytotoxicity assay, termed the Matador assay, which was based on marine luciferases and their engineered derivatives. In this study, we describe the development of a new cytotoxicity assay termed 'Matador-Glo assay' which takes advantage of a thermostable variant of Click Beetle Luciferase (Luc146-1H2). Matador-Glo assay utilizes Luc146-1H2 and D-luciferin as the luciferase-substrate pair for luminescence detection. The assay involves ectopic over-expression of Luc146-1H2 in the cytosol of target cells of interest. Upon damage to the membrane integrity, the Luc146-1H2 is either released from the dead and dying cells or its activity is preferentially measured in dead and dying cells. We demonstrate that this assay is simple, fast, specific, sensitive, cost-efficient, and not labor-intensive. We further demonstrate that the Matador-Glo assay can be combined with the marine luciferase-based Matador assay to develop a dual luciferase assay for cell death detection. Finally, we demonstrate that the Luc146-1H2 expressing target cells can also be used for in vivo bioluminescence imaging applications.

Feasibility study of 68Ga-labeled CAR T cells for in vivo tracking using micro-positron emission tomography imaging

Clinical tracking of chimeric antigen receptor (CAR) T cells in vivo by positron emission tomography (PET) imaging is an area of intense interest. But the long-lived positron emitter-labeled CAR T cells stay in the liver and spleen for days or even weeks. Thus, the excessive absorbed effective dose becomes a major biosafety issue leading it difficult for clinical translation. In this study we used ⁶⁸Ga, a commercially available short-lived positron emitter, to label CAR T cells for noninvasive cell tracking in vivo. CAR T cells could be tracked in vivo by ⁶⁸Ga-PET imaging for at least 6 h. We showed a significant correlation between the distribution of ⁸⁹Zr and ⁶⁸Ga-labeled CAR T cells in the same tissues (lungs, liver, and spleen). The distribution and homing behavior of CAR T cells at the early period is highly correlated with the long-term fate of CAR T cells in vivo. And the effective absorbed dose of ⁶⁸Ga-labeled CAR T cells is only one twenty-fourth of ⁸⁹Zr-labeled CAR T cells, which was safe for clinical translation. We conclude the feasibility of ⁶⁸Ga instead of ⁸⁹Zr directly labeling CAR T cells for noninvasive tracking of the cells in vivo at an early stage based on PET imaging. This method provides a potential solution to the emerging need for safe and practical PET tracer for cell tracking clinically.

Introducing a new reporter gene, membrane-anchored Cypridina luciferase, for multiplex bioluminescence imaging

Bioluminescence reporter gene imaging is a robust, high-throughput imaging modality that is useful for tracking cells and monitoring biological processes, both in cell culture and in small animals. We introduced and characterized a novel bioluminescence reporter—membrane-anchored Cypridina luciferase (maCLuc)—paired with a unique vargulin substrate. This luciferase-substrate pair has no cross-reactivity with established d-luciferin- or coelenterazine-based luciferase reporters. We compare maCLuc with several established luciferase-based reporter systems (firefly, click beetle, Renilla, and Gaussia luciferases), using both in vitro and in vivo models. We demonstrate the different imaging characteristics of these reporter systems, which allow for multiplexed-luciferase imaging of 3 and 4 separate targets concurrently in the same animal within 24 h. The imaging paradigms described here can be directly applied for simultaneous in vivo monitoring of multiple cell populations, the activity of selected signal transduction pathways, or a combination of both constitutive and inducible reporter imaging.

Noninvasive Radionuclide Molecular Imaging of the CD4-Positive T Lymphocytes in Acute Cardiac Rejection

Heart transplantation (HT) is an effective treatment for end-stage heart disease. However, acute rejection (AR) is still the main cause of death within one year after HT. AR is an acute immune response mediated by T lymphocytes, mainly CD4+ T lymphocytes. This study innovatively develops a radiolabeled probe 99mTc-HYNIC-mAbCD4 for noninvasive visualization of CD4+ T lymphocyte infiltration and detection of AR. The 99mTc-HYNIC-mAbCD4 and its isotype control 99mTc-HYNIC-IgG were successfully prepared and characterized. The specificity and affinity of the probe in vitro were assessed by cell-binding experiments. Binding of 99mTc-HYNIC-mAbCD4 to CD4+ T lymphocytes was higher than that of the macrophages and IgG probe groups, and mAbCD4 was effective in the blockade of the binding reaction. The biodistribution data confirmed the SPECT/CT images, with significantly higher levels of 99mTc-HYNIC-mAbCD4 observed in allografts compared to allograft treatment (10 mg/kg/d Cyclosporin A subcutaneously for 5 consecutive days after surgery), isografts, or in rats which received allografts injected with 99mTc-HYNIC-IgG. Histological examination confirmed more CD4+ T lymphocyte infiltration in the allograft hearts than other groups. In summary, 99mTc-HYNIC-mAbCD4 achieved high affinity and specificity of binding to CD4+ T lymphocytes and accumulation in the transplanted heart. Radionuclide molecular imaging with 99mTc-HYNIC-mAbCD4 may be a potential diagnostic method for acute cardiac rejection.

Non-radioactive imaging strategies for in vivo immune cell tracking

In vivo tracking of administered cells chosen for specific disease treatment may be conducted by diagnostic imaging techniques preceded by cell labeling with special contrast agents. The most commonly used agents are those with radioactive properties, however their use in research is often impossible. This review paper focuses on the essential aspect of cell tracking with the exclusion of radioisotope tracers, therefore we compare application of different types of non-radioactive contrast agents (cell tracers), methods of cell labeling and application of various techniques for cell tracking, which are commonly used in preclinical or clinical studies. We discuss diagnostic imaging methods belonging to three groups: (1) Contrast-enhanced X-ray imaging, (2) Magnetic resonance imaging, and (3) Optical imaging. In addition, we present some interesting data from our own research on tracking immune cell with the use of discussed methods. Finally, we introduce an algorithm which may be useful for researchers planning leukocyte targeting studies, which may help to choose the appropriate cell type, contrast agent and diagnostic technique for particular disease study.

Bioluminescence Imaging In Vivo Confirms the Viability of Pancreatic Islets Transplanted into the Greater Omentum

PURPOSE

The liver is the most widely used site for pancreatic islet transplantation. However, several site-specific limitations impair functional success, with instant blood-mediated inflammatory reaction being the most important. The aim of this study was to develop a preclinical model for placement of the islet graft into a highly vascularized omental flap using a fibrin gel. For this purpose, we tested islet viability by bioluminescence imaging (BLI).

PROCEDURES

Pancreatic islets were isolated from luciferase-positive and luciferase-negative rats, mixed at a 1:1 ratio, placed into a plasma-thrombin bioscaffold, and transplanted in standard (10 pancreatic islets/g wt; n = 10) and marginal (4 pancreatic islets/g wt; n = 7) numbers into the omentums of syngeneic diabetic animals. For the control, 4 pancreatic islets/g were transplanted into the liver using the standard procedure (n = 7). Graft viability was tested by bioluminescence at days 14, 30, 60, and 90 post transplant. Glucose levels, intravenous glucose tolerance, and serum C-peptide were assessed regularly.

RESULTS

Nonfasting glucose levels < 10 mmol/l were restored in all animals. While islet viability in the omentum was clearly detected by stable luminescence signals throughout the whole study period, no signals were detected from islets transplanted into the liver. The bioluminescence signals were highly correlated with stimulated C-peptide levels detected at 80 days post transplant. Glucose tolerance did not differ among the 3 groups.

CONCLUSIONS

We successfully tested a preclinical model of islet transplantation into the greater omentum using a biocompatible scaffold made from autologous plasma and human thrombin. Both standard and marginal pancreatic islet numbers in a gel-form bioscaffold placed in the omentum restored glucose homeostasis in recipients with diabetes. Bioluminescence was shown promising as a direct proof of islet viability.

Application of bioluminescence resonance energy transfer-based cell tracking approach in bone tissue engineering

Bioluminescent imaging (BLI) has emerged as a popular in vivo tracking modality in bone regeneration studies stemming from its clear advantages: non-invasive, real-time, and inexpensive. We recently adopted bioluminescence resonance energy transfer (BRET) principle to improve BLI cell tracking and generated the brightest bioluminescent signal known to date, which thus enables more sensitive real-time cell tracking at deep tissue level. In the present study, we brought BRET-based cell tracking strategy into the field of bone tissue engineering for the first time. We labeled rat mesenchymal stem cells (rMSCs) with our in-house BRET-based GpNLuc reporter and evaluated the cell tracking efficacy both in vitro and in vivo. In scaffold-free spheroid 3D culture system, using BRET-based GpNLuc labeling resulted in significantly better correlation to cell numbers than a fluorescence based approach. In scaffold-based 3D culture system, GpNLuc-rMSCs displayed robust bioluminescence signals with minimal background noise. Furthermore, a tight correlation between BLI signal and cell number highlighted the robust reliability of using BRET-based BLI. In calvarial critical sized defect model, robust signal and the consistency in cell survival evaluation collectively supported BRET-based GpNLuc labeling as a reliable approach for non-invasively tracking MSC. In summary, BRET-based GpNLuc labeling is a robust, reliable, and inexpensive real-time cell tracking method, which offers a promising direction for the technological innovation of BLI and even non-invasive tracking systems, in the field of bone tissue engineering.

Immune cell labelling and tracking: implications for adoptive cell transfer therapies

Background

The understanding of the role of different immune cell subsets that infiltrate tumors can help researchers in developing new targeted immunotherapies to reactivate or reprogram them against cancer. In addition to conventional drugs, new cell-based therapies, like adoptive cell transfer, proved to be successful in humans. Indeed, after the approval of anti-CD19 CAR-T cell therapy, researchers are trying to extend this approach to other cancer or cell types.

Main body

This review focuses on the different approaches to non-invasively monitor the biodistribution, trafficking and fate of immune therapeutic cells, evaluating their efficacy at preclinical and clinical stages. PubMed and Scopus databases were searched for published articles on the imaging of cell tracking in humans and preclinical models.

Conclusion

Labelling specific immune cell subtypes with specific radiopharmaceuticals, contrast agents or optical probes can elucidate new biological mechanisms or predict therapeutic outcome of adoptive cell transfer therapies. To date, no technique is considered the gold standard to image immune cells in adoptive cell transfer therapies.

Next Generation Imaging Techniques to Define Immune Topographies in Solid Tumors

In recent years, cancer immunotherapy experienced remarkable developments and it is nowadays considered a promising therapeutic frontier against many types of cancer, especially hematological malignancies. However, in most types of solid tumors, immunotherapy efficacy is modest, partly because of the limited accessibility of lymphocytes to the tumor core. This immune exclusion is mediated by a variety of physical, functional and dynamic barriers, which play a role in shaping the immune infiltrate in the tumor microenvironment. At present there is no unified and integrated understanding about the role played by different postulated models of immune exclusion in human solid tumors. Systematically mapping immune landscapes or "topographies" in cancers of different histology is of pivotal importance to characterize spatial and temporal distribution of lymphocytes in the tumor microenvironment, providing insights into mechanisms of immune exclusion. Spatially mapping immune cells also provides quantitative information, which could be informative in clinical settings, for example for the discovery of new biomarkers that could guide the design of patient-specific immunotherapies. In this review, we aim to summarize current standard and next generation approaches to define Cancer Immune Topographies based on published studies and propose future perspectives.

Reporter gene comparison demonstrates interference of complex body fluids with secreted luciferase activity

Reporter gene assays are widely used to study cellular signaling and transcriptional activity. Few studies describe the use of reporter genes for studying cellular responses on complex body fluids, such as urine and blood. Selection of the optimal reporter gene is crucial for study outcome. Here, we compared the characteristics of five reporter genes (Firefly luciferase, stable- and unstable Nano luciferase, secretable Gaussia luciferase and Red Fluorescent Protein) to study complex body fluids. For this comparison, the NFκB Response Element (NFκB-RE) and Smad Binding Element (SBE) were identically cloned into the five different reporter vectors. Reporter characteristics were evaluated by kinetic and concentration-response measurements in SW1353 and HeLa cell lines. Finally, reporter compatibility with complex body fluids (fetal calf serum, knee joint synovial fluid and human serum) and inter-donor variation were evaluated. Red Fluorescent Protein demonstrated poor inducibility as a reporter gene and slow kinetics compared to luciferases. Intracellularly measured luciferases, such as Firefly luciferase and Nano luciferase, revealed good compatibility with complex body fluids. Secreted Gaussia luciferase appeared to be incompatible with complex body fluids, due to variability in inter-donor signal interference. Unstable Nano luciferase demonstrated clear inducibility, high sensitivity and compatibility with complex body fluids and therefore can be recommended for cellular signaling studies using complex body fluids.

Safe harbor-targeted CRISPR-Cas9 homology-independent targeted integration for multimodality reporter gene-based cell tracking

Imaging reporter genes provides longitudinal information on the biodistribution, growth, and survival of engineered cells in vivo. A translational bottleneck to using reporter genes is the necessity to engineer cells with randomly integrating vectors. Here, we built homology-independent targeted integration (HITI) CRISPR-Cas9 minicircle donors for precise safe harbor-targeted knock-in of fluorescence, bioluminescence, and MRI (Oatp1a1) reporter genes. Our results showed greater knock-in efficiency using HITI vectors compared to homology-directed repair vectors. HITI clones demonstrated functional fluorescence and bioluminescence reporter activity as well as significant Oatp1a1-mediated uptake of the clinically approved MRI agent gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid. Contrast-enhanced MRI improved the conspicuity of both subcutaneous and metastatic Oatp1a1-expressing tumors before they became palpable or even readily visible on precontrast images. Our work demonstrates the first CRISPR-Cas9 HITI system for knock-in of large DNA donor constructs at a safe harbor locus, enabling multimodal longitudinal in vivo imaging of cells.

Use of Bacterial Luciferase as a Reporter Gene in Eukaryotic Systems

Reporter gene assays are powerful tools for monitoring dynamic molecular changes and for evaluating the responses that occur at the genetic elements within cells in response to exogenous molecules. In general, various protein systems can be used as reporter genes, including luciferases. Here, the present protocol introduces a unique reporter gene system for monitoring molecular events in cells using bacterial luciferase (lux), which can generate blue-green light suitable for gene reporter applications with the highest cost performance. The protocol also guides the assay conditions and necessary components for using of lux gene (lux) as a eukaryotic reporter system. The lux system can be applied to monitor variety of molecular events inside mammalian cellular systems.

Noninvasive in vivo cell tracking using molecular imaging: A useful tool for developing mesenchymal stem cell-based cancer treatment

Mounting evidence has emphasized the potential of cell therapies in treating various diseases by restoring damaged tissues or replacing defective cells in the body. Cell therapies have become a strong therapeutic modality by applying noninvasive in vivo molecular imaging for examining complex cellular processes, understanding pathophysiological mechanisms of diseases, and evaluating the kinetics/dynamics of cell therapies. In particular, mesenchymal stem cells (MSCs) have shown promise in recent years as drug carriers for cancer treatment. They can also be labeled with different probes and tracked in vivo to assess the in vivo effect of administered cells, and to optimize therapy. The exact role of MSCs in oncologic diseases is not clear as MSCs have been shown to be involved in tumor progression and inhibition, and the exact interactions between MSCs and specific cancer microenvironments are not clear. In this review, a multitude of labeling approaches, imaging modalities, and the merits/demerits of each strategy are outlined. In addition, specific examples of the use of MSCs and in vivo imaging in cancer therapy are provided. Finally, present limitations and future outlooks in terms of the translation of different imaging approaches in clinics are discussed.

In Vivo Tracking of Extracellular Vesicles by Nuclear Imaging: Advances in Radiolabeling Strategies

Extracellular vesicles (EVs) are naturally secreted vesicles that have attracted a large amount of interest in nanomedicine in recent years due to their innate biocompatibility, high stability, low immunogenicity, and important role in cell-to-cell communication during pathological processes. Their versatile nature holds great potential to improve the treatment of several diseases through their use as imaging biomarkers, therapeutic agents, and drug-delivery vehicles. However, the clinical translation of EV-based approaches requires a better understanding of their in vivo behavior. Several imaging technologies have been used for the non-invasive in vivo tracking of EVs, with a particular emphasis on nuclear imaging due to its high sensitivity, unlimited penetration depth and accurate quantification. In this article, we will review the biological function and inherent characteristics of EVs and provide an overview of molecular imaging modalities used for their in vivo monitoring, with a special focus on nuclear imaging. The advantages of radionuclide-based imaging modalities make them a promising tool to validate the use of EVs in the clinical setting, as they have the potential to characterize in vivo the pharmacokinetics and biological behavior of the vesicles. Furthermore, we will discuss the current methods available for radiolabeling EVs, such as covalent binding, encapsulation or intraluminal labeling and membrane radiolabeling, reporting the advantages and drawbacks of each radiolabeling approach.