Revealing the Fate of Transplanted Stem Cells In Vivo with a Novel Optical Imaging Strategy

A multi-modality imaging strategy to determine the multiple in vivo fates of human umbilical cord mesenchymal stem cells at different periods of acute liver injury treatment

Human umbilical cord mesenchymal stem cells (HUCMSCs) are applied for disease therapy as a new type of drug in many countries. Their effects are not only presented by live cells, but also apoptotic bodies or cell fragments of dead cells. Therefore, it is meaningful to determine the multiple fates of HUCMSCs in vivo. Although various probes combining different imaging modalities have been developed to label and trace transplanted HUCMSCs in vivo, the status of the cells (live, dead, or apoptotic) was not distinguished, and a thorough understanding of the multiple fates of HUCMSCs after transplantation in vivo is lacking. Therefore, a magnetic resonance (MR)/near infrared fluorescent (NIRF)/bioluminescence (BI) multi-modality imaging strategy was developed. Iron oxide nanoparticles (IONPs) were assembled into 100 nm nanoparticles using epigallocatechin gallate as a chemical linker to increase the MR signal and reduce the exocytosis of IONPs for direct cell labeling and longitudinal MR imaging tracking. Fluorescent probes for apoptosis (DEVD-Cy-OH) were also loaded in the above assemblies to monitor the cell status. Meanwhile, the cell surface was labeled with the fluorescent dye Cy7 via bioorthogonal reactions to visualize the NIRF signal. Luciferase was lentivirally transfected into live cells to generate bioluminescence. Such labeling did not affect either the viability, proliferation, migration, differentiation characteristics of HUCMSCs or their therapeutic effects on acute liver injury mice in vivo. The in vivo fates of HUCMSCs were monitored via MR/NIRF/BI multi-modality imaging in acute liver injury mice. Although MR and Cy7 signals aggregated in injured liver for 7 days, the BI signals persisted for less than 24 hours. There was an increase in DEVD-Cy-OH signals in the injured liver, but they were almost at the basal level. That means that HUCMSCs survive in mice for a short time, and the dead form of HUCMSCs accumulated in a large quantity and sustained for a long time, which might contribute to their therapeutic effect.

NIR-II Fluorescence Imaging for In Vivo Quantitative Analysis

In vivo real-time qualitative and quantitative analysis is essential for the diagnosis and treatment of diseases such as tumors. Near-infrared-II (NIR-II, 1000-1700 nm) bioimaging is an emerging visualization modality based on fluorescent materials. The advantages of NIR-II region fluorescent materials in terms of reduced photon scattering and low tissue autofluorescence enable NIR-II bioimaging with high resolution and increasing depth of tissue penetration, and thus have great potential for in vivo qualitative and quantitative analysis. In this review, we first summarize recent advances in NIR-II imaging, including fluorescent probe selection, quantitative analysis strategies, and imaging. Then, we describe in detail representative applications to illustrate how NIR-II fluorescence imaging has become an important tool for in vivo quantitative analysis. Finally, we describe the future possibilities and challenges of NIR-II fluorescence imaging.

Tracking of Stem Cells in Chronic Liver Diseases: Current Trends and Developments

Stem cell therapy holds great promise for future clinical practice for treatment of advanced liver diseases. However, the fate of stem cells after transplantation, including the distribution, viability, and the cell clearance, has not been fully elucidated. Herein, recent advances regarding the imaging tools for stem cells tracking mainly in chronic liver diseases with the advantages and disadvantages of each approach have been described. Magnetic resonance imaging is a promising clinical imaging modality due to non-radioactivity, excellent penetrability, and high spatial resolution. Fluorescence imaging and radionuclide imaging demonstrate relatively increased sensitivity, with the latter excelling in real-time monitoring. Reporter genes specialize in long-term tracing. Nevertheless, the disadvantages of low sensitivity, radiation, exogenous gene risk are inevitably present in each of these means, respectively. In this review, we aim to comprehensively evaluate the current state of methods for tracking of stem cell, highlighting their strengths and weaknesses, and providing insights into their future potential. Multimodality imaging strategies may overcome the inherent limitations of single-modality imaging by combining the strengths of different imaging techniques to provide more comprehensive information in the clinical setting.

NIR light-facilitated bone tissue engineering

In the last decades, near-infrared (NIR) light has attracted considerable attention due to its unique properties and numerous potential applications in bioimaging and disease treatment. Bone tissue engineering for bone regeneration with the help of biomaterials is currently an effective means of treating bone defects. As a controlled light source with deeper tissue penetration, NIR light can provide real-time feedback of key information on bone regeneration in vivo utilizing fluorescence imaging and be used for bone disease treatment. This review provides a comprehensive overview of NIR light-facilitated bone tissue engineering, from the introduction of NIR probes as well as NIR light-responsive materials, and the visualization of bone regeneration to the treatment of bone-related diseases. Furthermore, the existing challenges and future development directions of NIR light-based bone tissue engineering are also discussed. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement.

Endochondral Repair of Jawbone Defects Using Periosteal Cell Spheroids

Recapitulation of the natural healing process is receiving increasing recognition as a strategy to induce robust tissue regeneration. Endochondral ossification has been recognized as an essential reparative approach in natural jawbone defect healing. However, such an approach has been overlooked in the recent development of cell-based therapeutics for jawbone repair. Therefore, this study aimed to explore a bioinspired stem cell-based strategy for jawbone repair by mimicking the mesenchymal condensation of progenitor cells during the early endochondral ossification process. For this purpose, passage 3 of jawbone periosteum-derived cells (jb-PDCs) was cultured in our previously reported nonadherent microwells (200 µm in diameter, 148 µm in depth, and 100 µm space in between) and self-assembled into spheroids with a diameter of 96.4 ± 5.8 µm after 48 h. Compared to monolayer culture, the jb-PDC spheroids showed a significant reduction of stemness marker expression evidenced by flow cytometry. Furthermore, a significant upregulation of chondrogenic transcription factor SOX9 in both gene and protein levels was observed in the jb-PDC spheroids after 48 h of chondrogenic induction. RNA sequencing and Western blotting analysis further suggested that the enhanced SOX9-mediated chondrogenic differentiation in jb-PDC spheroids was attributed to the activation of the p38 MAPK pathway. Impressively, inhibition of p38 kinase activity significantly attenuated chondrogenic differentiation jb-PDC spheroids, evidenced by a significant decline of SOX9 in both gene and protein levels. Strikingly, the jb-PDC spheroids implanted in 6- to 8-wk-old male C57BL/6 mice with critical-size jawbone defects (1.8 mm in diameter) showed an evident contribution to cartilaginous callus formation after 1 wk, evidenced by histological analysis. Furthermore, micro-computed tomography analysis showed that the jb-PDC spheroids significantly accelerated bone healing after 2 wk in the absence of exogenous growth factors. In sum, the presented findings represent the successful development of cell-based therapeutics to reengineer the endochondral bone repair process and illustrate the potential application to improve bone repair and regeneration in the craniofacial skeleton.

Optical and MRI Multimodal Tracing of Stem Cells In Vivo

Stem cell therapy has shown great clinical potential in oncology, injury, inflammation, and cardiovascular disease. However, due to the technical limitations of the in vivo visualization of transplanted stem cells, the therapeutic mechanisms and biosafety of stem cells in vivo are poorly defined, which limits the speed of clinical translation. The commonly used methods for the in vivo tracing of stem cells currently include optical imaging, magnetic resonance imaging (MRI), and nuclear medicine imaging. However, nuclear medicine imaging involves radioactive materials, MRI has low resolution at the cellular level, and optical imaging has poor tissue penetration in vivo. It is difficult for a single imaging method to simultaneously achieve the high penetration, high resolution, and noninvasiveness needed for in vivo imaging. However, multimodal imaging combines the advantages of different imaging modalities to determine the fate of stem cells in vivo in a multidimensional way. This review provides an overview of various multimodal imaging technologies and labeling methods commonly used for tracing stem cells, including optical imaging, MRI, and the combination of the two, while explaining the principles involved, comparing the advantages and disadvantages of different combination schemes, and discussing the challenges and prospects of human stem cell tracking techniques.

Theranostics applications of quantum dots in regenerative medicine, cancer medicine, and infectious diseases

Quantum dots (QDs) have attracted attention for their application and commercialization in all industrial fields, including communications, displays, and solar cells, due to their excellent optical properties based on the quantum size effect. In recent years, the development of QDs that do not contain cadmium which is toxic to cells and living organisms, has progressed, and they have attracted considerable attention in the bio-imaging field for targeting molecules and cells. Furthermore, recently, the need for diagnostics and treatment at the single molecule and single cell level in the medical field has been increasing, and the application of QDs in the medical field is also accelerating. Therefore, this paper outlines the frontiers of diagnostic and therapeutic applications (theranostics) of QDs, especially in advanced medical fields such as regenerative medicine, oncology, and infectious diseases.

Multiscale NIR-II Imaging-Guided Brain-Targeted Drug Delivery Using Engineered Cell Membrane Nanoformulation for Alzheimer's Disease Therapy

Effective drug delivery in the central nervous system (CNS) needs to have long blood-circulation half-lives, to pass through the blood-brain barrier (BBB), and subsequently to be taken up by target cells. Herein, a traceable CNS delivery nanoformulation (RVG-NV-NPs) is developed by encapsulating bexarotene (Bex) and AgAuSe quantum dots (QDs) within Lamp2b-RVG-overexpressed neural stem cell (NSC) membranes. The high-fidelity near-infrared-II imaging by AgAuSe QDs offers a possibility of in vivo monitoring the multiscale delivery process of the nanoformulation from the whole-body to the single-cell scale. It was revealed the synergy of acetylcholine receptor-targeting of RVG and the natural brain-homing and low immunogenicity of NSC membranes prolong the blood circulation, facilitate BBB crossing and nerve cell targeting of RVG-NV-NPs. Thus, in Alzheimer's disease (AD) mice, the intravenous delivery of as low as 0.5% of oral dose Bex showed highly effective up-regulation of the apolipoprotein E expression, resulting rapid alleviation of ∼40% β-amyloid (Aβ) level in the brain interstitial fluid after a single dose administration. The pathological progression of Aβ in AD mice is completely suppressed during a 1 month treatment, thus effectively protecting neurons from Aβ-induced apoptosis and maintaining the cognitive abilities of AD mice.

Cu-In-S/ZnS:Gd3+ quantum dots with isolated fluorescent and paramagnetic modules for dual-modality imaging in vivo

Gd³⁺-doped quantum dots (QDs) have been widely used as small-sized bifunctional contrast agents for fluorescence/magnetic resonance (FL/MR) dual-modality imaging. However, Gd³⁺ doping will always compromise the FL of host QDs. Therefore, balancing the Gd³⁺ doping and the optical properties of QDs is crucial for constructing high-performance bifunctional nanoprobes. Additionally, most paramagnetic QDs are synthesized in the organic phase and need to be transferred to the aqueous phase for bioimaging. Herein, ingeniously designed shell-doped Cu-In-S/ZnS:Gd³⁺ QDs have been prepared in the aqueous phase. It has been demonstrated that isolating paramagnetic Gd³⁺ from fluorescent Cu-In-S core via doping Gd³⁺ into ZnS shell not only avoided the decrease of FL quantum yield (QY), but also ensured the water accessibility of paramagnetic Gd³⁺ ions, by which the FL QY and r₁ relaxivity of Cu-In-S/ZnS:Gd³⁺ QDs achieved as much as 15.6% and 15.33 mM^-1·s^-1, respectively. These high-performance QDs with excellent stability, low biotoxicity, and good tumor permeability were successfully applied for in vivo tumor FL/MR dual-modality imaging, and have shown significant potential in the precision detection and diagnosis of diseases.

Biocompatible Iron Oxide Nanoring-Labeled Mesenchymal Stem Cells: An Innovative Magnetothermal Approach for Cell Tracking and Targeted Stroke Therapy

Labeling stem cells with magnetic nanoparticles is a promising technique for in vivo tracking and magnetic targeting of transplanted stem cells, which is critical for improving the therapeutic efficacy of cell therapy. However, conventional endocytic labeling with relatively poor labeling efficiency and a short labeling lifetime has hindered the implementation of these innovative enhancements in stem-cell-mediated regenerative medicine. Herein, we describe an advanced magnetothermal approach to label mesenchymal stem cells (MSCs) efficiently by local induction of heat-enhanced membrane permeability for magnetic resonance imaging (MRI) tracking and targeted therapy of stroke, where biocompatible γ-phase, ferrimagnetic vortex-domain iron oxide nanorings (γ-FVIOs) with superior magnetoresponsive properties were used as a tracer. This approach facilitates a safe and efficient labeling of γ-FVIOs as high as 150 pg of Fe per cell without affecting the MSCs proliferation and differentiation, which is 3.44-fold higher than that by endocytosis labeling. Such a high labeling efficiency not only enables the ultrasensitive magnetic resonance imaging (MRI) detection of sub-10 cells and long-term tracking of transplanted MSCs over 10 weeks but also endows transplanted MSCs with a magnetic manipulation ability in vivo. A proof-of-concept study using a rat stroke model showed that the labeled MSCs facilitated MRI tracking and magnetic targeting for efficient replacement therapy with a significantly reduced dosage of 5 × 10⁴ transplanted cells. The findings in this study have demonstrated the great potential of the magnetothermal approach as an efficient labeling technique for future clinical usage.

Self-illuminating NIR-II bioluminescence imaging probe based on silver sulfide quantum dots

Bioluminescence (BL) imaging has emerged to tackle the potential challenges of fluorescence (FL) imaging including the autofluorescence background, inhomogeneous illumination over a wide imaging field, and the light-induced overheating effect. Taking advantage of the bioluminescence resonance energy transfer (BRET) mechanism between a conventional luciferin compound and a suitable acceptor, the visible light of the former can be extended to photons with longer wavelengths emitting from the latter. Although BRET-based self-illuminating imaging probes have already been prepared, employing potentially cytotoxic elements as the acceptor with the emission wavelengths which hardly reach the first near-infrared (NIR-I) window, has limited their applications as safe and high performance in vivo imaging agents. Herein, we report a biocompatible, self-illuminating, and second near-infrared (NIR-II) emissive probe to address the cytotoxicity concerns as well as improve the penetration depth and spatiotemporal resolution of BL imaging. To this end, NanoLuc luciferase enzyme molecules were immobilized on the surface of silver sulfide quantum dots to oxidize its luciferin substrate and initiate a single-step BRET mechanism, resulting in NIR-II photons from the quantum dots. The resulting dual modality (BL/FL) probes were successfully applied to in vivo tumor imaging in mice, demonstrating that NIR-II BL signals could be easily detected from the tumor sites, giving rise to ∼2 times higher signal-to-noise ratios compared to those obtained under FL mode. The results indicated that nontoxic NIR-II emitting nanocrystals deserve more attention to be tailored to fill the growing demands of preparing appropriate agents for high quality BL imaging.

Single-cell spatiotemporal analysis reveals cell fates and functions of transplanted mesenchymal stromal cells during bone repair

Mesenchymal stromal cells (MSCs) transplantation could enhance bone repair. However, the cell fate of transplanted MSCs, in terms of their local distribution and spatial associations with other types of cells were poorly understood. Here, we developed a single-cell 3D spatial correlation (sc3DSC) method to track transplanted MSCs based on deep tissue microscopy of fluorescent nanoparticles (fNPs) and immunofluorescence of key proteins. Locally delivered fNP-labeled MSCs enhanced tibial defect repair, increased the number of stem cells and vascular maturity in mice. fNP-MSCs persisted in the defect throughout repair. But only a small portion of transplanted cells underwent osteogenic differentiation (OSX+); a significant portion has maintained their expression of mesenchymal stem cell and skeletal stem cell markers (SCA-1 and PRRX1). Our results contribute to the optimization of MSC-based therapies. The sc3DSC method may be useful in studying cell-based therapies for the regeneration of other tissue types or disease models.

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Transplanted marrow stromal cells associated with vessels during bone defect repair

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Small proportion of transplanted cells differentiated into osteogenic cells

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A proportion of transplanted cells maintained expressions of stem cell markers

Lymph Node Mapping for Tumor Micrometastasis

Lymph node mapping for tumor micrometastasis is of great significance for the prevention, prognosis, and treatment of cancer. Currently, the traditional clinical detection methods (computed tomography, magnetic resonance imaging, or positron emission tomography/computed tomography) in clinical lymph node mapping still have some inherent disadvantages, which have prompted the development of various fluorescent probes for lymph node mapping. However, the conventional fluorescent probes such as indocyanine green or methylene blue in lymph node mapping are still accompanied by several problems such as impaired surgical field vision due to dye staining or less accumulation and shorter retention time in the lymph node. In a recent achievement, newly designed nanoparticles are prepared with novel properties that could be attractive for lymph node mapping. In this review, we will provide details on the progress of various nanoparticles for lymph node mapping and emphasize other multivariant properties in different nanoparticles, including strong tumor-targeting affinity and specificity, self-luminescence, and even with the function to kill metastatic cancer cells.

CT/bioluminescence dual-modal imaging tracking of stem cells labeled with Au@PEI@PEG nanotracers and RfLuc in nintedanib-assisted pulmonary fibrosis therapy

Mesenchymal stem cells (MSCs) are promising in idiopathic pulmonary fibrosis (IPF) therapy. However, low survival rate and ambiguous behavior of MSCs after transplantation impede their clinical translation. To this end, we have developed a new strategy to improve the survival rate and monitor the behavior of the transplanted MSCs simultaneously. In our strategy, nintedanib, a tyrosine kinase inhibitor, is employed to protect the human MSCs (hMSCs) from excessive oxidative stress responses and inflammatory environment in the damaged lung. Moreover, by labeling of the transplanted hMSCs with a computed tomography (CT) nanotracer, Au nanoparticles functionalized with polyethylenimine (PEI) and polyethylene glycol (PEG) (Au@PEI@PEG), in combination with red-emitting firefly luciferase (RfLuc), in vivo CT/bioluminescence (BL) dual-modal imaging tracking of the location, distribution, and survival of the transplanted hMSCs in presence of nintedanib were achieved, which facilitates the profound understanding of the role the stem cells play in IPF therapy.

In vivo MRI tracking and therapeutic efficacy of transplanted mesenchymal stem cells labeled with ferrimagnetic vortex iron oxide nanorings for liver fibrosis repair

Mesenchymal stem cells (MSCs) have showed promising effects in the treatment of liver fibrosis. Long-term and noninvasive in vivo tracking of transplanted MSCs is essential for understanding the therapeutic mechanism of MSCs during the therapy of liver fibrosis. In this study, we report the development of a ferrimagnetic vortex iron oxide nanoring (FVIO)-based nanotracer for the long-term visualization of transplanted human MSCs (hMSCs) by magnetic resonance imaging (MRI). The FVIOs were prepared by a hydrothermal reaction followed by hydrogen reduction. To endow the FVIOs with biocompatibility, polyethylene glycol amine (mPEG-NH₂) was covalently coupled on the surface of FVIOs, forming FVIO@PEG nanotracers with high contrast enhancement and intracellular uptake. The hMSCs labeled with FVIO@PEG nanotracers exhibited enhanced MRI contrast than those labeled with a commercial contrast agent, and could be continuously monitored by MRI in liver fibrosis mice for 28 days after transplantation, clearly clarifying the migration behavior of hMSCs in vivo. Moreover, we explored the therapeutic mechanism of the FVIO@PEG labeled hMSCs in the amelioration of liver fibrosis, including the reduction in inflammation and oxidative stress, the inhibition of hepatic fibrosis-caused histopathological damage, as well as the down-regulation of the expression of relevant cytokines. The results obtained in this work may deepen our understanding of the behavior and role of hMSCs in the treatment of liver fibrosis, which is key to the clinical application of stem cells in the therapy of liver diseases.

Near-Infrared-II Quantum Dots for In Vivo Imaging and Cancer Therapy

In vivo fluorescence imaging can perform real-time, noninvasive, and high spatiotemporal resolution imaging to accurately obtain the dynamic biological information in vivo, which plays significant roles in the early diagnosis and treatment of cancer. However, traditional in vivo fluorescence imaging usually operates in the visible and near-infrared (NIR)-I windows, which are severely interfered by the strong tissue absorption, tissue scattering, and autofluorescence. The emergence of NIR-II imaging at 1000-1700 nm significantly breaks through the imaging limitations in deep tissues, due to less tissue scattering and absorption. Benefiting from the outstanding optical properties of NIR-II quantum dots (QDs), such as high brightness and good photostability, in vivo fluorescence imaging exhibits excellent temporal-spatial resolution and large penetration depth, and QDs have become a kind of promising fluorescent biomarkers in the field of in vivo fluorescence imaging. Herein, the authors review NIR-II QDs from preparation to modification, and summarize recent applications of NIR-II QDs, including in vivo imaging and imaging-guided therapies. Finally, they discuss the special concerns when NIR-II QDs are shifted from in vivo imaging applications to further in-depth applications.

Effect of Cell Therapy and Exercise Training in a Stroke Model, Considering the Cell Track by Molecular Image and Behavioral Analysis

The goal of this study is to see how combining physical activity with cell treatment impacts functional recovery in a stroke model. Molecular imaging and multimodal nanoparticles assisted in cell tracking and longitudinal monitoring (MNP). The viability of mesenchymal stem cell (MSC) was determined using a 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assay and bioluminescent image (BLI) after lentiviral transduction and MNP labeling. At random, the animals were divided into 5 groups (control-G1, and experimental G2-G5). The photothrombotic stroke induction was confirmed by local blood perfusion reduction and Triphenyltetrazolium chloride (TTC), and MSC in the G3 and G5 groups were implanted after 24 h, with BLI and near-infrared fluorescence image (NIRF) tracking these cells at 28 h, 2, 7, 14, and 28 days. During a 28-day period, the G5 also conducted physical training, whereas the G4 simply did the training. At 0, 7, 14, and 28 days, the animals were functionally tested using a cylinder test and a spontaneous motor activity test. MNP internalization in MSC was confirmed using brightfield and fluorescence microscopy. In relation to G1 group, only 3% of cell viability reduced. The G2–G5 groups showed more than 69% of blood perfusion reduction. The G5 group performed better over time, with a progressive recovery of symmetry and an increase of fast vertical movements. Up to 7 days, BLI and NIRF followed MSC at the damaged site, demonstrating a signal rise that could be connected to cell proliferation at the injury site during the acute phase of stroke. Local MSC therapy mixed with physical activity resulted in better results in alleviating motor dysfunction, particularly during the acute period. When it comes to neurorehabilitation, this alternative therapy could be a suitable fit.

DNA-coated gold nanoparticles for tracking hepatocyte growth factor secreted by transplanted mesenchymal stem cells in pulmonary fibrosis therapy

The identification of paracrine factors secreted by transplanted mesenchymal stem cells (MSCs) during the treatment of idiopathic pulmonary fibrosis (IPF) is essential for understanding the role of MSCs in therapy. Herein, we report a facile and efficient strategy for in vivo tracking the secretion of hepatocyte growth factor (HGF) in MSCs during IPF therapy. In our strategy, a novel nanoflare tracer consisting of gold nanoparticles (AuNPs), complementary sequences and dye-labeled recognition sequences is developed. Briefly, the AuNPs are functionalized with oligonucleotide complementary sequences hybridized to the organic dye-labeled recognition sequences, where the organic fluorophores are in close proximity to the AuNPs. In the absence of targets, the dye and AuNPs are separated from each other, inducing the quenching of the fluorescence signal. However, in the presence of targets, the recognition sequences gradually fall off from the AuNPs, causing the fluorescence signal to rise. In brief, in vivo monitoring of the dynamic expression of HGF mRNA in transplanted MSCs during IPF therapy in the current work may provide new insight into the paracrine process of the transplanted MSCs, thereby advancing the MSC-based IPF therapy toward clinical applications.

Near-Infrared-II Bioimaging for in Vivo Quantitative Analysis

Near-Infrared-II (NIR-II) bioimaging is a newly emerging visualization modality in real-time investigations of biological processes research. Owning to advances in reducing photon scattering and low tissue autofluorescence levels in NIR-II region (1,000-1700 nm), NIR-II bioimaging affords high resolution with increasing tissue penetration depth, and it shows greater application potential for in vivo detection to obtain more detailed qualitative and quantitative parameters. Herein, this review summarizes recent progresses made on NIR-II bioimaging for quantitative analysis. These emergences of various NIR-II fluorescence, photoacoustic (PA), luminescence lifetime imaging probes and their quantitative analysis applications are comprehensively discussed, and perspectives on potential challenges facing in this direction are also raised.

Recent advances in stem cell therapy for neurodegenerative disease: Three dimensional tracing and its emerging use

Neurodegenerative disease is a brain disorder caused by the loss of structure and function of neurons that lowers the quality of human life. Apart from the limited potential for endogenous regeneration, stem cell-based therapies hold considerable promise for maintaining homeostatic tissue regeneration and enhancing plasticity. Despite many studies, there remains insufficient evidence for stem cell tracing and its correlation with endogenous neural cells in brain tissue with three-dimensional structures. Recent advancements in tissue optical clearing techniques have been developed to overcome the existing shortcomings of cross-sectional tissue analysis in thick and complex tissues. This review focuses on recent progress of stem cell treatments to improve neurodegenerative disease, and introduces tissue optical clearing techniques that can implement a three-dimensional image as a proof of concept. This review provides a more comprehensive understanding of stem cell tracing that will play an important role in evaluating therapeutic efficacy and cellular interrelationship for regeneration in neurodegenerative diseases.

Synthesis and Bioapplications of Ag2 S Quantum Dots with Near-Infrared Fluorescence

Quantum dots (QDs) with near-infrared fluorescence (NIR) are an emerging class of QDs with unique capabilities owing to the deeper tissue penetrability of NIR light compared with visible light. NIR light also effectively overcomes organism autofluorescence, making NIR QDs particularly attractive in biological imaging applications for disease diagnosis. Considering latest developments, Ag₂ S QDs are a rising star among NIR QDs due to their excellent NIR fluorescence properties and biocompatibility. This review presents the various methods to synthesize NIR Ag₂ S QDs, and systematically discusses their applications in biosensing, bioimaging, and theranostics. Major challenges and future perspectives concerning the synthesis and bioapplications of NIR Ag₂ S QDs are discussed.

Recent Progress of Near-Infrared Fluorescence in vivo Bioimaging in the Second and Third Biological Window

Near-infrared (NIR) fluorescence bioimaging using above to 1000 nm wavelength region is a promising analytical method on visualizing deep tissues. As compared to the short-wavelength ultraviolet (UV: < 400 nm) or visible (VIS: 400 - 700 nm) region, which results in an extremely low absorption or scattering of biomolecules and water in the body, NIR light passes through the tissues. Various fluorescent probes that emit NIR emission in the second (1100 - 1400 nm) or third (1550 - 1800 nm) biological windows have been developed and used for NIR in vivo imaging. Single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth doped ceramic nanoparticles (RED-CNPs), and organic dye-based probes have been proposed by many researchers, and are used to successfully visualize the bloodstream, organs, and disease-affected regions, such as cancer. NIR imaging in the second and third biological windows is an effective analytical method on visualizing deep tissues.

Sandwich-Type Near-Infrared Conjugated Polymer Nanoparticles for Revealing the Fate of Transplanted Human Umbilical Cord Mesenchymal Stem Cells

Near-infrared conjugated polymer nanoparticles (NIR-CPNs) have been widely used in in vivo imaging fields. However, most of them face the aggregation-induced fluorescence quenching (ACQ) dilemma and serious dye leakage behavior, which impedes the long-term monitoring of transplanted cells in vivo. In the present work, a novel strategy of sandwich-type encapsulation of the conjugated polymer interlayer in the crystalline SiO2 core + shell (SSiO2@SPFTBT@CSiO2) is developed, which works well to avoid the ACQ problem by homogeneously dispersing poly((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5',5″-diyl) (PFTBT) and suppressing intermolecular π-π stacking. Furthermore, the unparalleled nanostructure efficiently stabilizes nanoparticles and successfully achieves long-term biocompatibility without interfering the biological characteristics of stem cells, indicating the potential of SSiO2@SPFTBT@CSiO2 in cell labeling. In addition, the fate of human umbilical cord mesenchymal stem cells (hucMSCs) in a mouse model with acute liver injury was disclosed. We found that the hucMSCs mainly migrated from the lungs to the injured liver and most transplanted hucMSCs were cleared up by the liver at 8 days post-injection. Revelation of the shuttle process and period will benefit in improving the clinical efficacy of hucMSCs, and the sandwich-type encapsulation strategy could also open a new avenue to obtain bright and robust NIR-CPNs for long-term fluorescence imaging.

Whole-Body Fluorescence Imaging in the Near-Infrared Window

Fluorescence imaging is one of the most widely used in vivo imaging methods for both fundamental research and clinical practice. Due to the reduced photon scattering, absorption, and autofluorescence in tissues, the emerging near-infrared (NIR) imaging (650-1700 nm) can afford deep tissue imaging with high spatiotemporal resolution and in vivo report the anatomical structures as well as the physiological activities in a whole-body level. Here, we give a brief introduction to fluorescence imaging in the first NIR (NIR-I, 650-950 nm) and second NIR (NIR-II, 1000-1700 nm) windows, summarize the recently developed NIR fluorophores and their applications in whole-body vascular system imaging, precision cancer theranostics, and regenerative medicine. Finally, the clinical applications and future prospects of in vivo NIR fluorescence imaging are also discussed.

IQF characterization of a cathepsin B-responsive nanoprobe for report of differentiation of HL60 cells into macrophages

Tracking of in vivo fates of exogenous cell transplants in terms of viability, migration, directional differentiation and function delivery by a suitable method of medical imaging is of great significance in the development and application of various cell therapies. In this contribution directional differentiation of HL60 cells into macrophages and granulocytes, and a difference in the associated expression level of cathepsin B (Cat B) among the parent and daughter cells is used as a model to guide and evaluate the development of a Cat B-responsive Abz-FRFK-Dnp@PLGA nanoprobe for an optical report of the differentiation process. A well-documented internally quenched fluorescence (IQF) pair coupled with a peptide substrate FRFK of Cat B was synthesized and imbedded in PLGA to form the nanoprobe. The nanoprobe is resistant to leakage when dispersed in water for 10 days. Degradation of the nanoprobe is dominated by Cat B. HL60 cells were then labelled with the Abz-FRFK-Dnp@PLGA nanoprobe to track the differentiation process. Differentiation of labelled HL60 cells into macrophages exhibited a significantly higher fluorescence relative to the granulocytes or the labelled parent cells. The fluorescence difference allows the differentiation process to be followed. The established characterization and assessment procedure is to be used for the development and evaluation of nanoprobes for other imaging modalities.

A Cat B-responsive Abz-FRFK-Dnp@PLGA nanoprobe for an optical report of the differentiation of HL60 cells into macrophages.

Applications of Nanobiomaterials in the Therapy and Imaging of Acute Liver Failure

This review focuses on the therapeutic mechanisms, targeting strategies of various nanomaterials in acute liver failure, and recent advances of diverse nanomaterials for acute liver failure therapy, diagnosis, and imaging. This review provides an outlook on the applications of nanomaterials, especially on the new horizons in acute liver failure therapy, and inspires broader interests across various disciplines. Acute liver failure (ALF), a fatal clinical disease featured with overwhelming hepatocyte necrosis, is a grand challenge in global health. However, a satisfactory therapeutic option for curing ALF is still absent, other than liver transplantation. Nanobiomaterials are currently being developed for the diagnosis and treatment of ALF. The liver can sequester most of nanoparticles from blood circulation, which becomes an intrinsic superiority for nanobiomaterials targeting hepatic diseases. Nanobiomaterials can enhance the bioavailability of free drugs, thereby significantly improving the therapeutic effects in ALF. Nanobiomaterials can also increase the liver accumulation of therapeutic agents and enable more effective targeting of the liver or specific liver cells. In addition, stimuli-responsive, optical, or magnetic nanomaterials exhibit great potential in the therapeutical, diagnostic, and imaging applications in ALF. Therefore, therapeutic agents in combination with nanobiomaterials increase the specificity of ALF therapy, diminish adverse systemic effects, and offer a multifunctional theranostic platform. Nanobiomaterial holds excellent significance and prospects in ALF theranostics. In this review, we summarize the therapeutic mechanisms and targeting strategies of various nanobiomaterials in ALF. We highlight recent developments of diverse nanomedicines for ALF therapy, diagnosis, and imaging. Furthermore, the challenges and future perspectives in the theranostics of ALF are also discussed.

Recent advances of near infrared inorganic fluorescent probes for biomedical applications

Near infrared (NIR)-excitable and NIR-emitting probes have fuelled advances in biomedical applications owing to their power in enabling deep tissue imaging, offering high image contrast and reducing phototoxicity. There are essentially three NIR biological windows, i.e., 700-950 nm (NIR I), 1000-1350 nm (NIR II) and 1550-1870 nm (NIR III). Recently emerging optical probes that can be excited by an 800 nm laser and emit in the NIR II or III windows, denoted as NIR I-to-NIR II/III, are particularly attractive. That is because the longer wavelengths in the NIR II and NIR III windows offer deeper penetration and higher signal to noise ratio than those in the NIR I window. NIR imaging has indeed become a quickly evolving field and, simultaneously, stimulated the further development of new classes of NIR I-to-NIR II/III inorganic fluorescent probes, which include PbS, Ag₂S-based quantum dots (QDs) and rare earth (RE) doped NPs (RENPs) that possess quite diverse optical properties and follow different emission mechanisms. This review summarizes the recent progress on material merits, synthetic routes, the rational choice of excitation in the NIR I window, NIR II/III emission optimization, and surface modification of aforementioned fluorescent probes. We also introduce the latest notable accomplishments enabled by these probes in fluorescence imaging, lifetime-based multiplexed imaging and photothermal therapy (PTT), together with a critical discussion of forthcoming challenges and perspectives for clinic use.

Preparing the Bone Tissue Regeneration Ground by Exosomes: From Diagnosis to Therapy

Bone tissue engineering employs acellular scaffolds or scaffolds, along with cells and growth factors, to provide the mechanical support needed, as well as serve as a delivery vehicle for bioactive molecules to the injury sites. As tissue engineering continues to evolve, it has integrated two emerging fields: stem cells and nanotechnology. A paracrine factor that is found to be responsible for the major regenerative effect in stem cell transplantation is an extracellular vesicle called an 'exosome'. Recent advances in nanotechnology have allowed the 'exosome' to be distinguished from other extracellular vesicles and be polymerized into a well-defined concept. Scientists are now investigating exosome uses in clinical applications. For bone-related diseases, exosomes are being explored as biomarkers for different bone pathologies. They are also being explored as a therapeutic agent where progenitor cell-derived exosomes are used to regenerate damaged bone tissue. In addition, exosomes are being tested as immune modulators for bone tissue inflammation, and finally as a delivery vehicle for therapeutic agents. This review discusses recently published literature on the clinical utilization of exosomes in bone-related applications and the correlated advantages. A particular focus will be placed on the potential utilization of regenerative cell-derived exosomes as a natural biomaterial for tissue regeneration.

A Bioresponsive Near-Infrared Fluorescent Probe for Facile and Persistent Live-Cell Tracking

Molecular imaging significantly transforms the field of biomedical science and facilitates the visualization, characterization, and quantification of biologic processes. However, it is still challenging to monitor cell localization in vivo, which is essential to the study of tumor metastasis and in the development of cell-based therapies. While most conventional small-molecule fluorescent probes cannot afford durable cell labeling, transfection of cells with fluorescent proteins is limited by their fixed fluorescence, poor tissue penetration, and interference of autofluorescence background. Here, a bioresponsive near-infrared fluorescent probe is reported as facile and reliable tool for real-time cell tracking in vivo. The design of this probe relies on a new phenomenon observed upon fluorobenzene-conjugated fluorescent dyes, which can form complexes with cytosolic glutathione and actively translocates to lysosomes, exhibiting enhanced and stable cell labeling. Fluorobenzene-coupled hemicyanine, a near-infrared fluorophore manifests to efficiently staining tumor cells without affecting their invasive property and enables persistent monitoring of cell migration in metastatic tumor murine models at high resolution for one week. The method of fluorobenzene functionalization also provides a simple and universal "add-on" strategy to render ordinary fluorescent probes suitable for long-term live-cell tracking, for which currently there is a deficit of suitable molecular tools.

Stem cell homing, tracking and therapeutic efficiency evaluation for stroke treatment using nanoparticles: A systematic review

BACKGROUND

Stroke is the second leading cause of death worldwide. There is a real need to develop treatment strategies for reducing neurological deficits in stroke survivors, and stem cell (SC) therapeutics appear to be a promising alternative for stroke therapy that can be used in combination with approved thrombolytic or thrombectomy approaches. However, the efficacy of SC therapy depends on the SC homing ability and engraftment into the injury site over a long period of time. Nonetheless, tracking SCs from their niche to the target tissues is a complex process.

AIM

To evaluate SC migration homing, tracking and therapeutic efficacy in the treatment of stroke using nanoparticles

METHODS

A systematic literature search was performed to identify articles published prior to November 2019 that were indexed in PubMed and Scopus. The following inclusion criteria were used: (1) Studies that used in vivo models of stroke or ischemic brain lesions; (2) Studies of SCs labeled with some type of contrast agent for cell migration detection; and (3) Studies that involved in vivo cellular homing and tracking analysis.

RESULTS

A total of 82 articles were identified by indexing in Scopus and PubMed. After the inclusion criteria were applied, 35 studies were selected, and the articles were assessed for eligibility; ultimately, only 25 studies were included. Most of the selected studies used SCs from human and mouse bone marrow labeled with magnetic nanoparticles alone or combined with fluorophore dyes. These cells were administered in the stroke model (to treat middle cerebral artery occlusion in 74% of studies and for photothrombotic induction in 26% of studies). Fifty-three percent of studies used xenogeneic grafts for cell therapy, and the migration homing and tracking evaluation was performed by magnetic resonance imaging as well as other techniques, such as near-infrared fluorescence imaging (12%) or bioluminescence assays (12%).

CONCLUSION

Our systematic review provided an up-to-date evaluation of SC migration homing and the efficacy of cellular therapy for stroke treatment in terms of functional and structural improvements in the late stage.

CT/MR Dual-Modality Imaging Tracking of Mesenchymal Stem Cells Labeled with a Au/GdNC@SiO2 Nanotracer in Pulmonary Fibrosis

Mesenchymal stem cells (MSCs) have shown potential as an innovative treatment for pulmonary fibrosis (PF), due to their capability to ameliorate the inflammation and moderate the deterioration of PF. The fate of the stem cells transplanted into the lung, including survival, migration, homing, and functions, however, has not been fully understood yet. In this paper, we report the development of a computed tomography/magnetic resonance (CT/MR) dual-modal nanotracer, gold/gadolinium nanoclusters overcoated with a silica shell (Au/GdNC@SiO₂), for noninvasive labeling and tracking of the transplanted human MSCs (hMSCs) in a PF model. The Au/GdNC@SiO₂ nanotracer exhibits good colloidal and chemical stability, high biocompatibility, enhanced longitudinal MR relaxivity, and superior X-ray attenuation property. The hMSCs can be effectively labeled with Au/GdNC@SiO₂, resulting in a significantly increased cellular CT/MR imaging contrast, without any obvious adverse effect on the function, including proliferation and differentiation of the labeled stem cells. Moreover, by using the Au/GdNC@SiO₂ nanotracer, the hMSCs transplanted in the lung can be tracked for 7 d via in vivo CT/MR dual-modality imaging. This work may provide an insight into the role the transplanted hMSCs play in PF therapy, thus promoting the stem cell-based regenerative medicine.

Advanced Near-Infrared Light for Monitoring and Modulating the Spatiotemporal Dynamics of Cell Functions in Living Systems

Light-based technique, including optical imaging and photoregulation, has become one of the most important tools for both fundamental research and clinical practice, such as cell signal sensing, cancer diagnosis, tissue engineering, drug delivery, visual regulation, neuromodulation, and disease treatment. In particular, low energy near-infrared (NIR, 700-1700 nm) light possesses lower phototoxicity and higher tissue penetration depth in living systems as compared with ultraviolet/visible light, making it a promising tool for in vivo applications. Currently, the NIR light-based imaging and photoregulation strategies have offered a possibility to real-time sense and/or modulate specific cellular events in deep tissues with subcellular accuracy. Herein, the recent progress with respect to NIR light for monitoring and modulating the spatiotemporal dynamics of cell functions in living systems are summarized. In particular, the applications of NIR light-based techniques in cancer theranostics, regenerative medicine, and neuroscience research are systematically introduced and discussed. In addition, the challenges and prospects for NIR light-based cell sensing and regulating techniques are comprehensively discussed.

Artificially Engineered Cubic Iron Oxide Nanoparticle as a High-Performance Magnetic Particle Imaging Tracer for Stem Cell Tracking

Stem cell therapies are increasingly recognized as the future direction of regenerative medicine, but the biological fate of the administrated stem cells remains a major concern for clinical translation, which calls for an approach to efficiently monitoring the stem cell behaviors in vivo. Magnetic particle imaging (MPI) is an emerging technology for cell tracking; however, its utility has been largely restricted due to the lack of optimal magnetic nanoparticle tracers. Herein, by controlled engineering of the size and shape of magnetic nanoparticles tailored to MPI physics theory, a specialized MPI tracer, based on cubic iron oxide nanoparticles with an edge length of 22 nm (CIONs-22), is developed. Due to the inherent lower proportion of disordered surface spins, CIONs-22 exhibit significantly larger saturation magnetization than that of spherical ones, while they possess similar saturation magnetization but smaller coercivity compared to larger-sized CIONs. These magnetic properties of CIONs-22 warrant high sensitivity and resolution of MPI. With their efficient cellular uptake, CIONs-22 exhibit superior MPI performance for stem cell labeling and tracking compared to the commercialized tracer Vivotrax. By virtue of these advantages, CIONs-22 enable real-time and prolonged monitoring of the spatiotemporal trajectory of stem cells transplanted to hindlimb ischemia mice, which demonstrates the great potential of CIONs-22 as MPI tracers to advance stem cell therapies.

Recent advances toward understanding the role of transplanted stem cells in tissue-engineered regeneration of musculoskeletal tissues

Stem cell-based tissue engineering is poised to revolutionize the treatment of musculoskeletal injuries. However, in order to overcome scientific, practical, and regulatory obstacles and optimize therapeutic strategies, it is essential to better understand the mechanisms underlying the pro-regenerative effects of stem cells. There has been an attempted paradigm shift within the last decade to think of transplanted stem cells as "medicinal" therapies that orchestrate healing on the basis of their secretome and immunomodulatory profiles rather than acting as bona fide stem cells that proliferate, differentiate, and directly produce matrix to form de novo tissues. Yet the majority of current bone and skeletal muscle tissue engineering strategies are still premised on a direct contribution of stem cells as building blocks to tissue regeneration. Our review of the recent literature finds that researchers continue to focus on the quantification of de novo bone/skeletal muscle tissue following treatment and few studies aim to address this mechanistic conundrum directly. The dichotomy of thought is reflected in the diversity of new advances ranging from in situ three-dimensional bioprinting to a focus on exosomes and extracellular vesicles. However, recent findings elucidating the role of the immune system in tissue regeneration combined with novel imaging platform technologies will have a profound impact on our future understanding of how stem cells promote healing following biomaterial-mediated delivery to defect sites.

Recent Progress in NIR-II Contrast Agent for Biological Imaging

Fluorescence imaging technology has gradually become a new and promising tool for in vivo visualization detection. Because it can provide real-time sub-cellular resolution imaging results, it can be widely used in the field of biological detection and medical detection and treatment. However, due to the limited imaging depth (1-2 mm) and self-fluorescence background of tissue emitted in the visible region (400-700 nm), it fails to reveal biological complexity in deep tissues. The traditional near infrared wavelength (NIR-I, 650-950 nm) is considered as the first biological window, because it reduces the NIR absorption and scattering from blood and water in organisms. NIR fluorescence bioimaging's penetration is larger than that of visible light. In fact, NIR-I fluorescence bioimaging is still interfered by tissue autofluorescence (background noise), and the existence of photon scattering, which limits the depth of tissue penetration. Recent experimental and simulation results show that the signal-to-noise ratio (SNR) of bioimaging can be significantly improved at the second region near infrared (NIR-II, 1,000-1,700 nm), also known as the second biological window. NIR-II bioimaging is able to explore deep-tissues information in the range of centimeter, and to obtain micron-level resolution at the millimeter depth, which surpass the performance of NIR-I fluorescence imaging. The key of fluorescence bioimaging is to achieve highly selective imaging thanks to the functional/targeting contrast agent (probe). However, the progress of NIR-II probes is very limited. To date, there are a few reports about NIR-II fluorescence probes, such as carbon nanotubes, Ag2S quantum dots, and organic small molecular dyes. In this paper, we surveyed the development of NIR-II imaging contrast agents and their application in cancer imaging, medical detection, vascular bioimaging, and cancer diagnosis. In addition, the hotspots and challenges of NIR-II bioimaging are discussed. It is expected that our findings will lay a foundation for further theoretical research and practical application of NIR-II bioimaging, as well as the inspiration of new ideas in this field.

Near-infrared-persistent luminescence/bioluminescence imaging tracking of transplanted mesenchymal stem cells in pulmonary fibrosis

A dual-labeling strategy integrating near-infrared-persistent luminescence and RfLuc-based bioluminescence imaging techniques has been developed to track the transplanted stem cells, deepening the understanding of the role played by stem cells in PF treatment.

Long-term in vivo CT tracking of mesenchymal stem cells labeled with Au@BSA@PLL nanotracers

Human mesenchymal stem cells (hMSCs) transplantation has attracted considerable interest for the treatment of pulmonary injury. Noninvasive and long-term tracking of hMSCs after transplantation in vivo, which is important for our understanding of the stem cell therapy, still remains a big challenge. Herein, we report on the development of a novel gold nanoparticle-based nanotracer to track by CT imaging the transplantation of hMSCs in vivo. Gold nanoparticles (AuNPs) were synthesized on bovine serum albumin (BSA) via an in situ growth method and modified with a poly-l-lysine (PLL) layer, yielding Au@BSA@PLL nanotracers with enhanced biocompatibility and intracellular uptake. Au@BSA@PLL nanotracers were explored for in vitro and in vivo tracking of hMSCs with computer tomography (CT). Our results showed that the endocytosis of Au@BSA@PLL by hMSCs was as high as ∼293 pg per cell. Meanwhile, the nanotracers had a negligible influence on the viability, proliferation, and osteogenic and adipogenic differentiation of the labeled hMSCs. Using a pulmonary fibrosis injury mouse model induced by bleomycin, the labeled hMSCs could be tracked by CT imaging up to 23 d after transplanted in vivo, suggesting the feasibility of Au@BSA@PLL as a potential cellular nanotracer for noninvasive and long-term CT tracking of hMSCs in lung tissue repair.

Perspectives for Ag2S NIR-II nanoparticles in biomedicine: from imaging to multifunctionality

Research on near-infrared (NIR) bioimaging has progressed very quickly in the past few years, as fluorescence imaging is reaching a credible implementation as a preclinical technique. The applications of NIR bioimaging in theranostics have contributed to its increasing impact. This has brought about the development of novel technologies and, simultaneously, of new contrast agents capable of acting as efficient NIR optical probes. Among these probes, Ag₂S nanoparticles (NPs) have attracted increasing attention due to their temperature-sensitive NIR-II emission, which can be exploited for deep-tissue imaging and thermometry, and their heat delivery capabilities. This multifunctionality makes Ag₂S NPs ideal candidates for theranostics. This review presents a critical analysis of the synthesis routes, properties and optical features of Ag₂S NPs. We also discuss the latest and most remarkable achievements enabled by these NPs in preclinical imaging and theranostics, together with a critical assessment of their potential to face forthcoming challenges in biomedicine.

CT/Bioluminescence Dual-Modal Imaging Tracking of Mesenchymal Stem Cells in Pulmonary Fibrosis

Human mesenchymal stem cells (hMSCs), due to their immune regulation and collateral secretion effects, are currently explored for potential therapy of idiopathic pulmonary fibrosis (IPF). Understanding the migration, homing, functions, and survival of transplanted hMSCs in vivo is critical to successful IPF treatment. Therefore, it is highly desired to develop noninvasive and effective imaging technologies to track the transplanted hMSCs, providing experimental basis for improving the efficacy of hMSCs in the treatment of IPF. The rational design and development of a dual-labeling strategy are reported by integrating gold nanoparticle (AuNP)-based computed tomography (CT) nanotracers and red-emitting firefly luciferase (RfLuc)-based bioluminescence (BL) tags for CT/BL multimodal imaging tracking of the transplanted hMSCs in a murine model of IPF. In this approach, the CT nanotracer is prepared by sequential coupling of AuNPs with polyethylene glycol and trans-activator of transcription (TAT) peptide (Au@TAT), and employed it to monitor the location and distribution of the transplanted hMSCs in vivo by CT imaging, while RfLuc is used to monitor hMSCs viability by BLI. This facile strategy allows for visualization of the transplanted hMSCs in vivo, thereby enabling profound understanding of the role of hMSCs in the IPF treatment, and advancing stem cell-based regenerative medicine.

Visualizing the Fate of Intra-Articular Injected Mesenchymal Stem Cells In Vivo in the Second Near-Infrared Window for the Effective Treatment of Supraspinatus Tendon Tears

Mesenchymal stem cells (MSCs) are capable of exerting strong therapeutic potential for the treatment of supraspinatus tendon tear. However, MSC therapy remains underutilized and perhaps underrated due to the limited evidence of dynamic visualization of cellular behavior in vivo. Here, second near-infrared fluorescence imaging with biocompatible PbS quantum dots (QDs) provides a cellular migration map and information on the biodistribution and clearance processes of three densities of intra-articularly injected, labeled MSCs to treat supraspinatus tendon tear in mice. Intra-articular injection avoids entrapment of MSCs by filter organs and reduces the QD-induced organ toxicity. Notably, the MSCs share a similar migration direction, but the moderate density group is somewhat more efficient, showing the longest residence time and highest cell retention rate around the footprint during the repair stage. Furthermore, quantitative kinetic investigation demonstrates that labeled MSCs are cleared by feces and urine. Histomorphometric analysis demonstrates that the moderate density group achieves maximum therapeutic effect and labeled MSCs do not induce any injury or inflammation to major organs, which suggests that administration of too many or few MSCs may decrease their effectiveness. Such an imaging approach provides spatiotemporal evidence for response to MSC therapy in vivo, facilitating the optimization of MSC therapy.

Optical fluorescence imaging with shortwave infrared light emitter nanomaterials for in vivo cell tracking in regenerative medicine

In vivo tracking and monitoring of adoptive cell transfer has a distinct importance in cell‐based therapy. There are many imaging modalities for in vivo monitoring of biodistribution, viability and effectiveness of transferred cells. Some of these procedures are not applicable in the human body because of low sensitivity and high possibility of tissue damages. Shortwave infrared region (SWIR) imaging is a relatively new technique by which deep biological tissues can be potentially visualized with high resolution at cellular level. Indeed, scanning of the electromagnetic spectrum (beyond 1000 nm) of SWIR has a great potential to increase sensitivity and resolution of in vivo imaging for various human tissues. In this review, molecular imaging modalities used for monitoring of biodistribution and fate of administered cells with focusing on the application of non‐invasive optical imaging at shortwave infrared region are discussed in detail.

Triple-modal imaging of stem-cells labeled with multimodal nanoparticles, applied in a stroke model

BACKGROUND

Mesenchymal stem cells (MSCs) have been widely tested for their therapeutic efficacy in the ischemic brain and have been shown to provide several benefits. A major obstacle to the clinical translation of these therapies has been the inability to noninvasively monitor the best route, cell doses, and collateral effects while ensuring the survival and effective biological functioning of the transplanted stem cells. Technological advances in multimodal imaging have allowed in vivo monitoring of the biodistribution and viability of transplanted stem cells due to a combination of imaging technologies associated with multimodal nanoparticles (MNPs) using new labels and covers to achieve low toxicity and longtime residence in cells.

AIM

To evaluate the sensitivity of triple-modal imaging of stem cells labeled with MNPs and applied in a stroke model.

METHODS

After the isolation and immunophenotypic characterization of human bone marrow MSCs (hBM-MSCs), our team carried out lentiviral transduction of these cells for the evaluation of bioluminescent images (BLIs) in vitro and in vivo. In addition, MNPs that were previously characterized (regarding hydrodynamic size, zeta potential, and optical properties), and were used to label these cells, analyze cell viability via the 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide assay and BLI analysis, and quantify the internalization process and iron load in different concentrations of MNPs via magnetic resonance imaging (MRI), near-infrared fluorescence (NIRF), and inductively coupled plasma-mass spectrometry (ICP-MS). In in vivo analyses, the same labeled cells were implanted in a sham group and a stroke group at different times and under different MNP concentrations (after 4 h or 6 d of cell implantation) to evaluate the sensitivity of triple-modal images.

RESULTS

hBM-MSC collection and isolation after immunophenotypic characterization were demonstrated to be adequate in hBM samples. After transduction of these cells with luciferase (hBM-MSC_Luc), we detected a maximum BLI intensity of 2.0 x 10⁸ photons/s in samples of 10⁶ hBM-MSCs. Analysis of the physicochemical characteristics of the MNPs showed an average hydrodynamic diameter of 38.2 ± 0.5 nm, zeta potential of 29.2 ± 1.9 mV and adequate colloidal stability without agglomeration over 18 h. The signal of iron load internalization in hBM-MSC_Luc showed a close relationship with the corresponding MNP-labeling concentrations based on MRI, ICP-MS and NIRF. Under the highest MNP concentration, cellular viability showed a reduction of less than 10% compared to the control. Correlation analysis of the MNP load internalized into hBM-MSC_Luc determined via the MRI, ICP-MS and NIRF techniques showed the same correlation coefficient of 0.99. Evaluation of the BLI, NIRF, and MRI signals in vivo and ex vivo after labeled hBM-MSC_Luc were implanted into animals showed differences between different MNP concentrations and signals associated with different techniques (MRI and NIRF; 5 and 20 µg Fe/mL; P < 0.05) in the sham groups at 4 h as well as a time effect (4 h and 6 d; P < 0.001) and differences between the sham and stroke groups in all images signals (P < 0.001).

CONCLUSION

This study highlighted the importance of quantifying MNPs internalized into cells and the efficacy of signal detection under the triple-image modality in a stroke model.

Novel near-infrared II aggregation-induced emission dots for in vivo bioimaging

Near-infrared II fluorescence imaging holds great promise for in vivo imaging and imaging-guided surgery with deep penetration and high spatiotemporal resolution. However, most NIR-II aromatic luminophores suffer from the notorious aggregation-caused quenching (ACQ) effect in the aqueous solution, which largely hinders their biomedical application in vivo. In this study, the first NIR-II organic aggregation-induced emission (AIE) fluorophore (HLZ-BTED), encapsulated as nanoparticles (HLZ-BTED dots) for in vivo biomedical imaging, was designed and synthesized. The NIR-II AIE HLZ-BTED dots showed high temporal resolution, high photostability, outstanding water-solubility and biocompatibility in vitro and in vivo. The HLZ-BTED dots were further used for long-term breast tumor imaging and visualizing tumor-feeding blood vessels, long-term hind limb vasculature and incomplete hind limb ischemia. More importantly, as a proof-of-concept, this is the first time that non-invasive and real-time NIR-II imaging of the gastrointestinal tract in health and disease has been performed, making the AIE dots a promising tool for gastrointestinal (GI) tract research, such as understanding the healthy status of GI peristalsis, diagnosing and evaluating intestinal motility dysfunction, and assessing drug effects on intestinal obstruction.

Cellular Internalization-Induced Aggregation of Porous Silicon Nanoparticles for Ultrasound Imaging and Protein-Mediated Protection of Stem Cells

Nanotechnology employs multifunctional engineered materials in the nanoscale range that provides many opportunities for translational stem cell research and therapy. Here, a cell-penetrating peptide (virus-1 transactivator of transcription)-conjugated, porous silicon nanoparticle (TPSi NP) loaded with the Wnt3a protein to increase both the cell survival rate and the delivery precision of stem cell transplantation via a combinational theranostic strategy is presented. The TPSi NP with a pore size of 10.7 nm and inorganic framework enables high-efficiency loading of Wnt3a, prolongs Wnt3a release, and increases antioxidative stress activity in the labeled mesenchymal stem cells (MSCs), which are highly beneficial properties for cell protection in stem cell therapy for myocardial infarction. It is confirmed that the intracellular aggregation of TPSi NPs can highly amplify the acoustic scattering of the labeled MSCs, resulting in a 2.3-fold increase in the ultrasound (US) signal compared with that of unlabeled MSCs. The translational potential of the designed nanoagent for real-time US imaging-guided stem cell transplantation is confirmed via intramyocardial injection of labeled MSCs in a nude mouse model. It is proposed that the intracellular aggregation of protein drug-loaded TPSi NPs could be a simple but robust strategy for improving the therapeutic effect of stem cell therapy.

Recent Advances in Tracking the Transplanted Stem Cells Using Near-Infrared Fluorescent Nanoprobes: Turning from the First to the Second Near-Infrared Window

Stem cell-based regenerative medicine has attracted tremendous attention for its great potential to treat numerous incurable diseases. Tracking and understanding the fate and regenerative capabilities of transplanted stem cells is vital for improving the safety and therapeutic efficacy of stem cell-based therapy, therefore accelerating the clinical application of stem cells. Fluorescent nanoparticles (NPs) have been widely used for in vivo tracking of the transplanted stem cells. Among these fluorescent NPs, near-infrared (NIR) NPs have greatly improved the sensitivity, tissue penetration depth, spatial and temporal resolutions of the fluorescence imaging-based stem cell tracking technologies due to the reduced absorption, scattering, and autofluorescence of NIR fluorescence in tissues. Here, this review summarizes the recent studies regarding the tracking of transplanted stem cells using NIR NPs and emphasizes the recent advances of fluorescence imaging in the second NIR window (NIR-II, 1000-1700 nm). Furthermore, the challenges and future prospects of the NIR NP-based technologies are also discussed.

Near-Infrared II Dye-Protein Complex for Biomedical Imaging and Imaging-Guided Photothermal Therapy

The development of novel biodegradable and nontoxic fluorophores that integrate diagnosis and therapy for effective cancer treatment has obtained tremendous attention in the past decades. In this report, water-soluble and biocompatible small-molecule near-infrared II (NIR-II) fluorescent dye H2a-4T complexed with fetal bovine serum (FBS) and Cetuximab proteins with excellent optical properties and targeting ability is prepared. High spatial and temporal resolution imaging of hind limb vasculature and the lymphatic system of living mice using H2a-4T@FBS complex is demonstrated in precise NIR-II imaging-guided sentinel lymph node surgery. More importantly, H2a-4T@Cetuximab complex not only exhibits a remarkable cell-killing ability but also achieves highly active tumor targeting efficiency for epidermal growth factor receptor, overexpressing colorectal cancer which is beneficial to in vivo NIR-II fluorescent imaging-guided photothermal therapy of colon tumors. To the best of our knowledge, it is the first time that the concept of light-harvesting complex is exploited for enhancing the NIR-II signals and photothermal energy conversion in molecule-protein complex theranostic agent, making them a promising candidate for future clinical applications in cancer theranostics.