Neural progenitor cells labeling with microbubble contrast agent for ultrasound imaging in vivo

Tracking adoptive natural killer cells via ultrasound imaging assisted with nanobubbles

The recent years has witnessed an exponential growth in the field of natural killer (NK) cell-based immunotherapy for cancer treatment. As a prerequisite to precise evaluations and on-demand interventions, the noninvasive tracking of adoptive NK cells plays a crucial role not only in post-treatment monitoring, but also in offering opportunities for preclinical studies on therapy optimizations. Here, we describe an NK cell tracking strategy for cancer immunotherapy based on ultrasound imaging modality. Nanosized ultrasound contrast agents, gas vesicles (GVs), were surface-functionalized to label NK cells. Unlike traditional microbubble contrast agents, nanosized GVs with their unique thermodynamical stability enable the detection of labeled NK cells under nonlinear contrast-enhanced ultrasound (nCEUS), without a noticeable impact on cellular viability or migration. By such labeling, we were able to monitor the trafficking of systematically infused NK cells to a subcutaneous tumor model. Upon co-treatment with interleukin (IL)-2, we observed a rapid enhancement in NK cell trafficking at the tumor site as early as 3 h post-infusion. Altogether, we show that the proposed ultrasound-based tracking strategy is able to capture the dynamical changes of cell trafficking in NK cell-based immunotherapy, providing referencing information for early-phase monotherapy evaluation, as well as understanding the effects of modulatory co-treatment. STATEMENT OF SIGNIFICANCE: In cellular immunotherapies, the post-infusion monitoring of the living therapeutics has been challenging. Several popular imaging modalities have been explored the monitoring of the adoptive immune cells, evaluating their trafficking and accumulation in the tumor. Here we demonstrated, for the first time, the ultrasound imaging-based immune cell tracking strategy. We showed that the acoustic labeling of adoptive immune cells was feasible with nanosized ultrasound contrast agents, overcoming the size and stability limitations of traditional microbubbles, enabling dynamical tracking of adoptive natural killer cells in both monotherapy and synergic treatment with cytokines. This article introduced the cost-effective and ubiquitous ultrasound imaging modality into the field of cellular immunotherapies, with broad prospectives in early assessment and on-demand image-guided interventions.

In Vivo Ultrasound Imaging of Macrophages Using Acoustic Vaporization of Internalized Superheated Nanodroplets

Activating patients' immune cells, either by reengineering them or treating them with bioactive molecules, has been a breakthrough in the field of immunotherapy and has revolutionized treatment, especially against cancer. As immune cells naturally home to tumors or injured tissues, labeling such cells holds promise for non-invasive tracking and biologic manipulation. Our study demonstrates that macrophages loaded with extremely low boiling point perfluorocarbon nanodroplets not only survive ultrasound-induced phase change but also maintain their phagocytic function. Unlike observations made when using higher boiling point perfluorocarbon nanodroplets, our results show that phase change occurs intracellularly at a low mechanical index using a clinical scanner operating within the energy limit set by the Food and Drug Administration (FDA). After nanodroplet-loaded macrophages were given intravenously to nude rats, they were invisible in the liver when imaged at a very low mechanical index using a clinical ultrasound scanner. They became visible when power was increased but still within the FDA limits up to 8 h after administration. The acoustic labeling and in vivo detection of macrophages using a clinical ultrasound scanner represent a paradigm shift in the field of cell tracking and pave the way for potential therapeutic strategies in the clinical setting.

Application of Ultrasound to Enhancing Stem Cells Associated Therapies

Pluripotent stem cell therapy exhibits self-renewal capacity and multi-directional differentiation potential and is considered an important regenerative approach for the treatment of several diseases. However, insufficient cell transplantation efficiency, uncontrollable differentiation, low cell viability, and difficult tracing limit its clinical applications and treatment outcome. Ultrasound (US) has mechanical, cavitation, and thermal effects that can produce different biological effects on organs, tissues, and cells. US can be combined with different US-responsive particles for enhanced physical-chemical stimulation and drug delivery. In the meantime, US also can provide a noninvasive and harmless imaging modality for deep tissue in vivo. An in-depth evaluation of the role and mechanism of action of US in stem cell therapy would enhance understanding of US and encourage research in this field. In this article, we comprehensively review progress in the application of US alone and combined with US-responsive particles for the promotion of proliferation, differentiation, migration, and in vivo detection of stem cells and the potential clinical applications.

Magnetic Nanobubble Mechanical Stress Induces the Piezo1-Ca2+ -BMP2/Smad Pathway to Modulate Neural Stem Cell Fate and MRI/Ultrasound Dual Imaging Surveillance for Ischemic Stroke

Neural stem cells (NSCs) are used to treat various nervous system diseases because of their self-renewal ability and multidirectional differentiation potential. However, an insufficient ability to track their migration in vivo and poor control over their survival and differentiation efficiency are two major critical challenges for clinical application. Here, it is shown that when magnetic nanobubbles (MNBs), which are assembled from magnetic nanoparticles, are internalized by NSCs, intramembrane volumetric oscillation of the MNBs induces an increase in intracellular hydrostatic pressure and cytoskeleton force, resulting in the activation of the Piezo1-Ca²⁺ mechanosensory channel. This subsequently triggers the BMP2/Smad biochemical signaling pathway, leading to differentiation of NSCs into the neuronal phenotype. Signaling through the Piezo1-Ca²⁺ -BMP2/Smad pathway can be further accelerated by application of an external shear stress force using low-intensity pulsed ultrasound. More importantly, magnetic resonance imaging and ultrasound imaging surveillance of NSCs based on MNB labeling can be leveraged to provide NSC therapeutic outcomes. Both the in vitro and in vivo findings demonstrate that a bubble nanostructure-induced physical force can modulate and control the mechanical signaling pathway regulating stem cell development.

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.

Molecular imaging of cellular immunotherapies in experimental and therapeutic settings

Cell-based cancer immunotherapies are becoming a routine part of the armamentarium against cancer. While remarkable successes have been seen, including durable remissions, not all patients will benefit from these therapies and many can suffer from life-threatening side effects. These differences in efficacy and safety across patients and across tumor types (e.g., blood vs. solid), are thought to be due to differences in how well the immune cells traffic to their target tissue (e.g., tumor, lymph nodes, etc.) whilst avoiding non-target tissues. Across patient variability can also stem from whether the cells interact with (i.e., communicate with) their intended target cells (e.g., cancer cells), as well as if they proliferate and survive long enough to yield potent and long-lasting therapeutic effects. However, many cell-based therapies are monitored by relatively simple blood tests that lack any spatial information and do not reflect how many immune cells have ended up at particular tissues. The ex vivo labeling and imaging of infused therapeutic immune cells can provide a more precise and dynamic understanding of whole-body immune cell biodistribution, expansion, viability, and activation status in individual patients. In recent years numerous cellular imaging technologies have been developed that may provide this much-needed information on immune cell fate. For this review, we summarize various ex vivo labeling and imaging approaches that allow for tracking of cellular immunotherapies for cancer. Our focus is on clinical imaging modalities and summarize the progression from experimental to therapeutic settings. The imaging information provided by these technologies can potentially be used for many purposes including improved real-time understanding of therapeutic efficacy and potential side effects in individual patients after cell infusion; the ability to more readily compare new therapeutic cell designs to current designs for various parameters such as improved trafficking to target tissues and avoidance of non-target tissues; and the long-term ability to identify patient populations that are likely to be positive responders and at low-risk of side effects.

Biomedical nanobubbles and opportunities for microfluidics

The use of bulk nanobubbles in biomedicine is increasing in recent years, which is attributable to the array of therapeutic and diagnostic tools promised by developing bulk nanobubble technologies. From cancer drug delivery and ultrasound contrast enhancement to malaria detection and the diagnosis of acute donor tissue rejection, the potential applications of bulk nanobubbles are broad and diverse. Developing these technologies to the point of clinical use may significantly impact the quality of patient care. This review compiles and summarizes a representative collection of the current applications, fabrication techniques, and characterization methods of bulk nanobubbles in biomedicine. Current state-of-the-art generation methods are not designed to create nanobubbles of high concentration and low polydispersity, both characteristics of which are important for several bulk nanobubble applications. To date, microfluidics has not been widely considered as a tool for generating nanobubbles, even though the small-scale precision and real-time control offered by microfluidics may overcome the challenges mentioned above. We suggest possible uses of microfluidics for improving the quality of bulk nanobubble populations and propose ways of leveraging existing microfluidic technologies, such as organ-on-a-chip platforms, to expand the experimental toolbox of researchers working to develop biomedical nanobubbles.

The use of bulk nanobubbles in biomedicine is increasing in recent years. This translates into new opportunities for microfluidics, which may enable the generation of higher quality nanobubbles that lead to advances in diagnostics and therapeutics.

Options for imaging cellular therapeutics in vivo: a multi-stakeholder perspective

Cell-based therapies have been making great advances toward clinical reality. Despite the increase in trial activity, few therapies have successfully navigated late-phase clinical trials and received market authorization. One possible explanation for this is that additional tools and technologies to enable their development have only recently become available. To support the safety evaluation of cell therapies, the Health and Environmental Sciences Institute Cell Therapy-Tracking, Circulation and Safety Committee, a multisector collaborative committee, polled the attendees of the 2017 International Society for Cell & Gene Therapy conference in London, UK, to understand the gaps and needs that cell therapy developers have encountered regarding safety evaluations in vivo. The goal of the survey was to collect information to inform stakeholders of areas of interest that can help ensure the safe use of cellular therapeutics in the clinic. This review is a response to the cellular imaging interests of those respondents. The authors offer a brief overview of available technologies and then highlight the areas of interest from the survey by describing how imaging technologies can meet those needs. The areas of interest include imaging of cells over time, sensitivity of imaging modalities, ability to quantify cells, imaging cellular survival and differentiation and safety concerns around adding imaging agents to cellular therapy protocols. The Health and Environmental Sciences Institute Cell Therapy-Tracking, Circulation and Safety Committee believes that the ability to understand therapeutic cell fate is vital for determining and understanding cell therapy efficacy and safety and offers this review to aid in those needs. An aim of this article is to share the available imaging technologies with the cell therapy community to demonstrate how these technologies can accomplish unmet needs throughout the translational process and strengthen the understanding of cellular therapeutics.

'Golden' exosomes as delivery vehicles to target tumors and overcome intratumoral barriers: in vivo tracking in a model for head and neck cancer

Exosomes are promising vectors for anti-tumor therapy, due to their biocompatibility, low immunogenicity, and innate ability to interact with target cells. However, promoting clinical application of exosome-based therapeutics requires elucidation of key issues, including exosome biodistribution, tumor targeting and accumulation, and the ability to overcome tumor barriers that limit the penetration of various nano-carriers and drugs. Here, we examined these parameters in exosomes derived from mesenchymal stem cells (MSC-exo) and from the A431 squamous cell carcinoma line (A431-exo), which both have potential use in cancer therapy. Using our novel technique combining gold nanoparticle (GNP) labeling of exosomes and non-invasive computed tomography imaging (CT), we longitudinally and quantitatively tracked the two intravenously-injected exosome types in A431 tumor-bearing mice. CT imaging up to 48 h and subsequent ex vivo analysis revealed tumor homing abilities of both exosome types, yet there was significantly higher tumor accumulation of MSC-exo as compared to A431-exo. Moreover, MSC-exo demonstrated the ability to penetrate the tumor and distribute throughout its bulk, while non-encapsulated GNPs remained concentrated at the tumor periphery. Histological analysis showed penetration of MSC-exo not only into the tumor tissue, but also into tumor cell cytoplasm. While the proportion of biodistribution between organs at 48 h was similar for both exosome types, more rapid clearance was indicated for A431-exo. Thus, our findings demonstrate an effect of exosome type on tumor targeting abilities and biodistribution, and suggest that MSC-exo may have superior abilities for tumor-targeted therapy.

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.

Bubble Inflation Using Phase-Change Perfluorocarbon Nanodroplets as a Strategy for Enhanced Ultrasound Imaging and Therapy

Phase-change perfluorocarbon microdroplets were introduced over 2 decades ago to occlude downstream vessels in vivo. Interest in perfluorocarbon nanodroplets has recently increased to enable extravascular targeting, to rescue the weak ultrasound signal of perfluorocarbon droplets by converting them to microbubbles and to improve ultrasound-based therapy. Despite great scientific interest and advances, applications of phase-change perfluorocarbon agents have not reached clinical testing because of efficacy and safety concerns, some of which remain unexplained. Here, we report that the coexistence of perfluorocarbon droplets and microbubbles in blood, which is inevitable when droplets spontaneously or intentionally vaporize to form microbubbles, is a major contributor to the observed side effects. We develop the theory to explain why the coexistence of droplets and microbubbles results in microbubble inflation induced by perfluorocarbon transfer from droplets to adjacent microbubbles. We also present the experimental data showing up to 6 orders of magnitude microbubble volume expansion, which occludes a 200 μm tubing in the presence of perfluorocarbon nanodroplets. More importantly, we demonstrate that the rate of microbubble inflation and ultimate size can be controlled by manipulating formulation parameters to tailor the agent's design for the potential theranostic application while minimizing the risk to benefit ratio.

Ultrasound monitoring of magnet-guided delivery of mesenchymal stem cells labeled with magnetic lipid–polymer hybrid nanobubbles

Mesenchymal stem cells labeled with positively charged magnetic lipid–polymer hybrid nanobubbles could be tracked for magnet-guided delivery onto the site of an injured artery using ultrasound.

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.

Listening for the therapeutic window: Advances in drug delivery utilizing photoacoustic imaging

The preclinical landscape of photoacoustic imaging has experienced tremendous growth in the past decade. This non-invasive imaging modality augments the spatiotemporal capabilities of ultrasound with optical contrast. While it has principally been investigated for diagnostic applications, many recent reports have described theranostic delivery systems and drug monitoring strategies using photoacoustics. Here, we provide an overview of the progress to date while highlighting work in three specific areas: theranostic nanoparticles, real-time drug monitoring, and stem cell ("living drug") tracking. Additionally, we discuss the challenges that remain to be addressed in this burgeoning field.

Fluorous-phase iron oxide nanoparticles as enhancers of acoustic droplet vaporization of perfluorocarbons with supra-physiologic boiling point

Perfluorocarbon emulsion nanodroplets containing iron oxide nanoparticles (IONPs) within their inner perfluorohexane (PFH) core were prepared to investigate potential use as an acoustically activatable ultrasound contrast agent, with the hypothesis that incorporation of IONPs into the fluorous phase of a liquid perfluorocarbon emulsion would potentiate acoustic vaporization. IONPs with an oleic acid (OA) hydrophobic coating were synthesized through chemical co-precipitation. To suspend IONP in PFH, OA was exchanged with perfluorononanoic acid (PFNA) via ligand exchange to yield fluorophilic PFNA-coated IONPs (PFNA-IONPs). Suspensions with various amounts of PFNA-IONPs (0-15% w/v) in PFH were emulsified in saline by sonication, using 5% (w/v) egg yolk phospholipid as an emulsifier. PFNA-IONPs were characterized with transmission electron microscopy (TEM), transmission electron cryomicroscopy (cryoTEM), and thermogravimetric analysis (TGA) with Fourier transform infrared spectroscopy (FTIR). IONP were between 5 and 10 nm in diameter as measured by electron microscopy, and hydrodynamic size of the PFH nanodroplets were 150 to 230 nm as measured by dynamic light scattering (DLS). Acoustic droplet vaporization of PFH nanodroplets (PFH-NDs) was induced using conversion pulses (100 cycle at 1.1 MHz and 50% duty cycle) provided by a focused ultrasound transducer, and formed microbubbles were imaged using a clinical ultrasound scanner. The acoustic pressure threshold needed for PFH-NDs vaporization decreased with increasing temperature and IONP content. PFH-NDs containing 5% w/v IONP converted to microbubbles at 42 °C at 2.18 MI, which is just above the exposure limits of 1.9 MI allowed by the FDA for clinical ultrasound scanners, whereas 10 and 15% emulsion vaporized at 1.87 and 1.24 MI, respectively. Furthermore, 5% IONP-loaded PFH-NDs injected intravenously into melanoma-bearing mice at a dose of 120 mg PFH/kg, converted into detectable microbubbles in vivo 5 h, but not shortly after injection, indicating that this technique detects NDs accumulated in tumors.

Selective infarct zone imaging with intravenous acoustically activated droplets

Background

Microbubbles (MB) can be compressed to nanometer-sized droplets and reactivated with diagnostic ultrasound; these reactivated MB possess unique imaging characteristics.

Objective

We hypothesized that droplets formed from compressing Definity MB may be used for infarct-enhancement imaging.

Methods

Fourteen rats underwent ligation of their left anterior descending (LAD) artery, and five pigs underwent 90 minute balloon occlusions of their mid LAD. At 48 hours in rats, transthoracic ultrasound was performed at two and four minutes following 200 μL intravenous injections (IVI) of Definity droplets (DD), at which point the MI was increased from 0.5 to 1.5 to assess for a transient contrast enhancement zone (TEZ) within akinetic segments. In pigs, 1.0 mL injections of DD were administered and low frame rate (triggered end systolic or 10 Hz) imaging 2–4 minutes post iVI to selectively activate and image the infarct zone (IZ). Infarct size was defined by delayed enhancement magnetic resonance imaging (DE-MRI) and post-mortem staining (TTC).

Results

Increasing MI to 1.5 (at two or four minutes after IVI) resulted in a TEZ in rats, which correlated with infarct size (r = 0.94, p<0.001). A TEZ was not seen at 2–4 minutes in any rat (n = 8) following Definity MB injections. Fluorescent staining confirmed DD presence within the infarct zone 10 minutes after intravenous injection. In pigs, selective enhancement within the IZ was achieved by using a low frame rate single pulse harmonic mode; IZ size matched the location seen with DE-MRI and correlated with TTC defect size (r = 0.90, p<0.05).

Conclusion

DD formulated from commercially available MB can be acoustically activated for selective infarct enhancement imaging.

Clinical Tracking of Cell Transfer and Cell Transplantation: Trials and Tribulations

Cell therapy has provided unprecedented opportunities for tissue repair and cancer therapy. Imaging tools for in vivo tracking of therapeutic cells have entered the clinic to evaluate therapeutic cell delivery and retention in patients. Thus far, clinical cell tracking studies have been a mere proof of principle of the feasibility of cell detection. This review centers around the main clinical queries associated with cell therapy: Have cells been delivered correctly at the targeted site of injection? Are cells still alive, and, if so, how many? Are cells being rejected by the host, and, if so, how severe is the immune response? For stem cell therapeutics, have cells differentiated into downstream cell lineages? Is there cell proliferation including tumor formation? At present, clinical cell tracking trials have only provided information on immediate cell delivery and short-term cell retention. The next big question is if these cell tracking tools can improve the clinical management of the patients and, if so, by how much, for how many, and for whom; in addition, it must be determined whether tracking therapeutic cells in every patient is needed. To become clinically relevant, it must now be demonstrated how cell tracking techniques can inform patient treatment and affect clinical outcomes.

Accomplishments and challenges in stem cell imaging in vivo

Stem cell therapies have demonstrated promising preclinical results, but very few applications have reached the clinic owing to safety and efficacy concerns. Translation would benefit greatly if stem cell survival, distribution and function could be assessed in vivo post-transplantation, particularly in patients. Advances in molecular imaging have led to extraordinary progress, with several strategies being deployed to understand the fate of stem cells in vivo using magnetic resonance, scintigraphy, PET, ultrasound and optical imaging. Here, we review the recent advances, challenges and future perspectives and opportunities in stem cell tracking and functional assessment, as well as the advantages and challenges of each imaging approach.

Ultrasound Technology for Molecular Imaging: From Contrast Agents to Multimodal Imaging

Ultrasound (US) takes advantage of ultrasound contrast agents (UCAs) to further increase the sensitivity and specificity of monitoring at the cellular level, which has had a considerable effect on the modern molecular imaging field. Gas-filled microbubbles (MBs) as UCAs in the bloodstream generate resonant volumetric oscillations in response to rapid variations in acoustic pressure, which are related to both the acoustic parameters of applied ultrasound and the physicochemical properties of the contrast agents. Nanoscale UCAs have been developed and have attracted much attention due to their utility in detecting extravascular lesions. Ultrasound molecular assessment is achieved by binding disease-specific ligands to the surface of UCAs, which have been designed to target tissue biomarkers in the area of interest, such as blood vessels, inflammation, or thrombosis. Additionally, the development of multimodal imaging technology is conducive for integration of the advantages of various imaging techniques to acquire additional diagnostic information. In this review paper, the present status and the critical issues for developing ultrasound contrast agents and multimodal imaging applications are described. Conventional MB UCAs are first introduced, including their research material, diagnostic applications, and intrinsic limitations. Then, recent progress in developing targeted UCAs and phase-inversion contrast agents for diagnostic purposes is discussed. Finally, we review the present status and the critical issues for developing ultrasound-based multimodal imaging applications and summarize the existing challenges and future prospects.

Artifacts in contrast-enhanced ultrasound: a pictorial essay

Although contrast-enhanced ultrasound (CEUS) has become a widely utilized and accepted modality in much of the world, the associated contrast agents have only recently received approval in the United States. As with all radiological techniques, image artifacts are encountered in CEUS, some of which relate to commonly encountered ultrasound artifacts, while others are unique to this technique. Image artifacts must be recognized when performing and interpreting examinations to improve technique and diagnostic accuracy. In this article, we review artifacts that may be encountered in CEUS, and where possible discuss how to minimize them or mitigate their effect on image quality and interpretation.

Removal of ligand-bound liposomes from cell surfaces by microbubbles exposed to ultrasound

Gas-filled microbubbles attached to cell surfaces can interact with focused ultrasound to create microstreaming of nearby fluid. We directly observed the ultrasound/microbubble interaction and documented that under certain conditions fluorescent particles that were attached to the surface of live cells could be removed. Fluorescently labeled liposomes that were larger than 500 nm in diameter were attached to the surface of endothelial cells using cRGD targeting to αvβ3 integrin. Microbubbles were attached to the surface of the cells through electrostatic interactions. Images taken before and after the ultrasound exposure were compared to document the effects on the liposomes. When exposed to ultrasound with peak negative pressure of 0.8 MPa, single microbubbles and groups of isolated microbubbles were observed to remove targeted liposomes from the cell surface. Liposomes were removed from a region on the cell surface that averaged 33.1 μm in diameter. The maximum distance between a single microbubble and a detached liposome was 34.5 μm. Single microbubbles were shown to be able to remove liposomes from over half the surface of a cell. The distance over which liposomes were removed was significantly dependent on the resting diameter of the microbubble. Clusters of adjoining microbubbles were not seen to remove liposomes. These observations demonstrate that the fluid shear forces generated by the ultrasound/microbubble interaction can remove liposomes from the surfaces of cells over distances that are greater than the diameter of the microbubble.

Molecular imaging in stem cell-based therapies of cardiac diseases

In the past 15years, despite that regenerative medicine has shown great potential for cardiovascular diseases, the outcome and safety of stem cell transplantation has shown controversial results in the published literature. Medical imaging might be useful for monitoring and quantifying transplanted cells within the heart and to serially characterize the effects of stem cell therapy of the myocardium. From the multiple available noninvasive imaging techniques, magnetic resonance imaging and nuclear imaging by positron (PET) or single photon emission computer tomography (SPECT) are the most used clinical approaches to follow the fate of transplanted stem cells in vivo. In this article, we provide a review on the role of different noninvasive imaging modalities and discuss their advantages and disadvantages. We focus on the different in-vivo labeling and reporter gene imaging strategies for stem cell tracking as well as the concept and reliability to use imaging parameters as noninvasive surrogate endpoints for the evaluation of the post-therapeutic outcome.

Cell tracking using gold nanoparticles and computed tomography imaging

Cell-based therapies utilize transplantation of living cells with therapeutic traits to alleviate numerous diseases and disorders. The use of such biological agents is an attractive alternative for diseases that existing medicine cannot effectively treat. Although very promising, translating cell therapy to the clinic has proven to be challenging, due to inconsistent results in preclinical and clinical studies. To examine the underlying cause for these inconsistencies, it is crucial to noninvasively monitor the accuracy of cell injection, and cell survival and migration patterns. The combination of classical imaging techniques with cellular contrast agents-mainly nanotechnological-based-has enabled significant developments in cell-tracking methodologies. One novel methodology, based on computed tomography (CT) as an imaging modality and gold nanoparticles (AuNPs) as contrast agents, has recently gained interest for its clinical applicability and cost-effectiveness. Studies have shown that AuNPs can be used to efficiently label a variety of cell types, including stem cells and immune cells, without damaging their therapeutic efficacy. Successful in vivo experiments have demonstrated noninvasive, quantitative and longitudinal cell tracking with high sensitivity. This concept has the potential to be used not only as a research tool, but in clinical settings as well. WIREs Nanomed Nanobiotechnol 2018, 10:e1480. doi: 10.1002/wnan.1480 This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Use of Nanoparticle Contrast Agents for Cell Tracking with Computed Tomography

Efforts to develop novel cell-based therapies originated with the first bone marrow transplant on a leukemia patient in 1956. Preclinical and clinical examples of cell-based treatment strategies have shown promising results across many disciplines in medicine, with recent advances in immune cell therapies for cancer producing remarkable response rates, even in patients with multiple treatment failures. However, cell-based therapies suffer from inconsistent outcomes, motivating the search for tools that allow monitoring of cell delivery and behavior in vivo. Noninvasive cell imaging techniques, also known as cell tracking, have been developed to address this issue. These tools can allow real-time, quantitative, and long-term monitoring of transplanted cells in the recipient, providing insight on cell migration, distribution, viability, differentiation, and fate, all of which play crucial roles in treatment efficacy. Understanding these parameters allows the optimization of cell choice, delivery route, and dosage for therapy and advances cell-based therapy for specific clinical uses. To date, most cell tracking work has centered on imaging modalities such as MRI, radionuclide imaging, and optical imaging. However, X-ray computed tomography (CT) is an emerging method for cell tracking that has several strengths such as high spatial and temporal resolution, and excellent quantitative capabilities. The advantages of CT for cell tracking are enhanced by its wide availability and cost effectiveness, allowing CT to become one of the most popular clinical imaging modalities and a key asset in disease diagnosis. In this review, we will discuss recent advances in cell tracking methods using X-ray CT in various applications, in addition to predictions on how the field will progress.

Ultrasound Detection of Regional Oxidative Stress in Deep Tissues Using Novel Enzyme Loaded Nanoparticles

Oxidative stress is a powerful tool that is critical to immune mediated responses in healthy individuals, yet additionally plays a crucial role in development of cancer, inflammatory pathologies, and tissue ischemia. Despite this, there remain relatively few molecular tools to study oxidative stress, particularly in living mammals. To develop an intravenously injectable probe capable of labeling sites of oxidative stress in vivo, 200 nm catalase synthetic hollow enzyme loaded nanospheres (catSHELS) are designed and fabricated using a versatile enzyme nanoencapsulation method. catSHELS catalyze H₂ O₂ to water and oxygen producing microbubbles that can be detected and imaged using a clinical ultrasound system. catSHELS are optimized in vitro to maximize ultrasound signal and their functionality is demonstrated in rat ischemic renal injury model. Ischemic oxidative injury is induced in a single kidney of normal rats by clamping the renal artery for 1 h followed by 2 h of reperfusion. Imaging of both kidneys is performed following the intravenous bolus injection of 10¹² catSHELS of the optimized formulation. There is significant increase in ultrasound signal of the injured kidney relative to controls. This method offers a novel intravenous approach to detect oxidative stress in deep tissues in living animals.

In Vivo Micro-CT Imaging of Human Mesenchymal Stem Cells Labeled with Gold-Poly-L-Lysine Nanocomplexes

Developing in vivo cell tracking is an important prerequisite for further development of cell-based therapy. So far, few computed tomography (CT) cell tracking studies have been described due to its notoriously low sensitivity and lack of efficient labeling protocols. We present a simple method to render human mesenchymal stem cells (hMSCs) sufficiently radiopaque by complexing 40 nm citrate-stabilized gold nanoparticles (AuNPs) with poly-L-lysine (PLL) and rhodamine B isothiocyanate (RITC). AuNP-PLL-RITC labeling did not affect cellular viability, proliferation, or downstream cell differentiation into adipocytes and osteocytes. Labeled hMSCs could be clearly visualized in vitro and in vivo with a micro-CT scanner, with a detection limit of approximately 2×104 cells/μl in vivo. Calculated HU values were 2.27 /pg of intracellular Au as measured with inductively coupled plasma mass spectrophotometry (ICP-MS), and were linear over a wide range of cell concentrations. This linear CT attenuation was observed for both naked AuNPs and those that were taken up by hMSCs, indicating that the number of labeled cells can be quantified similar to the use of radioactive or fluorine tracers. This approach for CT cell tracking may find applications in CT image-guided interventions and fluoroscopic procedures commonly used for the injection of cellular therapeutics.

Exosome-like silica nanoparticles: a novel ultrasound contrast agent for stem cell imaging

Ultrasound is critical in many areas of medicine including obstetrics, oncology, and cardiology with emerging applications in regenerative medicine. However, one critical limitation of ultrasound is the low contrast of target tissue over background. Here, we describe a novel cup-shaped silica nanoparticle that is reminiscent of exosomes and that has significant ultrasound impedance mismatch for labelling stem cells for regenerative medicine imaging. These exosome-like silica nanoparticles (ELS) were created through emulsion templating and the silica precursors bis(triethoxysilyl)ethane (BTSE) and bis(3-trimethoxysilyl-propyl)amine (TSPA). We found that 40% TSPA resulted in the exosome like-morphology and a positive charge suitable for labelling mesenchymal stem cells. We then compared this novel structure to other silica structures used in ultrasound including Stober silica nanoparticles (SSN), MCM-41 mesoporous silica nanoparticles (MSN), and mesocellular foam silica nanoparticles (MCF) and found that the ELS offered enhanced stem cell signal due to its positive charge to facilitate cell uptake as well as inherently increased echogenicity. The in vivo detection limits were <500 cells with no detectable toxicity at the concentrations used for labelling. This novel structure may eventually find utility in applications beyond imaging requiring an exosome-like shape including drug delivery.

Genetic engineered molecular imaging probes for applications in cell therapy: emphasis on MRI approach

Recent advances in stem cell-based regenerative medicine, cell replacement therapy, and genome editing technologies (i.e. CRISPR-Cas 9) have sparked great interest in in vivo cell monitoring. Molecular imaging promises a unique approach to noninvasively monitor cellular and molecular phenomena, including cell survival, migration, proliferation, and even differentiation at the whole organismal level. Several imaging modalities and strategies have been explored for monitoring cell grafts in vivo. We begin this review with an introduction describing the progress in stem cell technology, with a perspective toward cell replacement therapy. The importance of molecular imaging in reporting and assessing the status of cell grafts and their relation to the local microenvironment is highlighted since the current knowledge gap is one of the major obstacles in clinical translation of stem cell therapy. Based on currently available imaging techniques, we provide a brief discussion on the pros and cons of each imaging modality used for monitoring cell grafts with particular emphasis on magnetic resonance imaging (MRI) and the reporter gene approach. Finally, we conclude with a comprehensive discussion of future directions of applying molecular imaging in regenerative medicine to emphasize further the importance of correlating cell graft conditions and clinical outcomes to advance regenerative medicine.

Stem Cell Imaging: Tools to Improve Cell Delivery and Viability

Stem cell therapy (SCT) has shown very promising preclinical results in a variety of regenerative medicine applications. Nevertheless, the complete utility of this technology remains unrealized. Imaging is a potent tool used in multiple stages of SCT and this review describes the role that imaging plays in cell harvest, cell purification, and cell implantation, as well as a discussion of how imaging can be used to assess outcome in SCT. We close with some perspective on potential growth in the field.

In vitro methods to study bubble-cell interactions: Fundamentals and therapeutic applications

Besides their use as contrast agents for ultrasound imaging, microbubbles are increasingly studied for a wide range of therapeutic applications. In particular, their ability to enhance the uptake of drugs through the permeabilization of tissues and cell membranes shows great promise. In order to fully understand the numerous paths by which bubbles can interact with cells and the even larger number of possible biological responses from the cells, thorough and extensive work is necessary. In this review, we consider the range of experimental techniques implemented in in vitro studies with the aim of elucidating these microbubble-cell interactions. First of all, the variety of cell types and cell models available are discussed, emphasizing the need for more and more complex models replicating in vivo conditions together with experimental challenges associated with this increased complexity. Second, the different types of stabilized microbubbles and more recently developed droplets and particles are presented, followed by their acoustic or optical excitation methods. Finally, the techniques exploited to study the microbubble-cell interactions are reviewed. These techniques operate over a wide range of timescales, or even off-line, revealing particular aspects or subsequent effects of these interactions. Therefore, knowledge obtained from several techniques must be combined to elucidate the underlying processes.

Pre- and postmortem imaging of transplanted cells

Therapeutic interventions based on the transplantation of stem and progenitor cells have garnered increasing interest. This interest is fueled by successful preclinical studies for indications in many diseases, including the cardiovascular, central nervous, and musculoskeletal system. Further progress in this field is contingent upon access to techniques that facilitate an unambiguous identification and characterization of grafted cells. Such methods are invaluable for optimization of cell delivery, improvement of cell survival, and assessment of the functional integration of grafted cells. Following is a focused overview of the currently available cell detection and tracking methodologies that covers the entire spectrum from pre- to postmortem cell identification.

Microbubble embedded with upconversion nanoparticles as a bimodal contrast agent for fluorescence and ultrasound imaging

Bimodal imaging offers additional imaging signal thus finds wide spread application in clinical diagnostic imaging. Fluorescence/ultrasound bimodal imaging contrast agent using fluorescent dyes or quantum dots for fluorescence signal has emerged as a promising method, which however requires visible light or UV irradiation resulting in photobleaching, photo blinking,auto-fluorescence and limited tissue penetration depth. To surmount these problems, we developed a novel bimodal contrast agent using layer-by-layer assembly of up conversion nanoparticles onto the surface of microbubbles. The resulting microbubbles with average size of 2 μm provide enhanced ultrasound echo for ultrasound imaging and upconversion emission upon near infrared irradiation for fluorescence imaging. The developed bimodal contrast agent holds great potential to be applied in ultrasound target technique for targeted diseases diagnostics and therapy.

In Vivo Transfection and Detection of Gene Expression of Stem Cells Preloaded with DNA-carrying Microbubbles

PURPOSE

To determine whether (a) stem cells loaded with DNA-carrying microbubbles (MBs) can be transfected in vivo, (b) the cells remain alive to express the gene, and (c) gene expression is sufficiently robust to be detected in vivo.

MATERIALS AND METHODS

The study was approved by the Institutional Animal Care and Use Committee. Cationic MBs were prepared, characterized, and loaded with pLuciferase green fluorescent protein (GFP) plasmid. Loading was confirmed with SYBR Gold staining (Life Technologies, Carlsbad, Calif). C17.2 cells were loaded with the DNA-carrying MBs. Two hundred thousand cells suspended in 20 μL phosphate-buffered saline were mixed with 200 μL Matrigel (BD Biosciences, San Jose, Calif) and injected in both flanks of eight nude mice. One of the Matrigel (BD Biosciences) injections contained 50 000 cells pretransfected in vitro by using lipofectamine as a positive control. Nine flanks were exposed to 2.25-MHz ultrasonic pulses at 50% duty cycle for 1 minute at 1 W/cm(2) (n = 3) or 2 W/cm(2) (n = 6), and six flanks served as the negative control. Two days later, bioluminescent images were acquired in each mouse every 3 minutes for 1 hour after the intraperitoneal injection of d-luciferin (Perkin Elmer, Waltham, Mass). Differences between groups were assessed by using the nonparametric Kruskal-Wallis test with Wilcoxon rank sum tests for follow-up comparisons. Mice were then killed, plugs were explanted, and alternate sections were stained with hematoxylin-eosin or stained for GFP expression.

RESULTS

Mean DNA-loaded MB diameter ± standard deviation was 2.87 μm ± 1.69 with the DNA associated with the MB shell. C17.2 cells were associated with 2-4 MBs each, and more than 90% were viable. Peak background subtracted bioluminescent signal was fourfold higher when cells were exposed to 2 W/cm(2) pulses as compared with 1 W/cm(2) pulses (P = .02) and negative controls (P = .002). Histologic examination showed cells within the Matrigel (BD Biosciences) with robust GFP expression only after 2 W/cm(2) ultrasound exposure and lipofectamine transfection.

CONCLUSION

Stem cells loaded with DNA-carrying MBs can be transfected in vivo with ultrasonic pulses and remain alive to demonstrate robust gene expression.

Seeing stem cells at work in vivo

Stem cell based-therapies are novel therapeutic strategies that hold key for developing new treatments for diseases conditions with very few or no cures. Although there has been an increase in the number of clinical trials involving stem cell-based therapies in the last few years, the long-term risks and benefits of these therapies are still unknown. Detailed in vivo studies are needed to monitor the fate of transplanted cells, including their distribution, differentiation, and longevity over time. Advancements in non-invasive cellular imaging techniques to track engrafted cells in real-time present a powerful tool for determining the efficacy of stem cell-based therapies. In this review, we describe the latest approaches to stem cell labeling and tracking using different imaging modalities.

Biogenic gas nanostructures as ultrasonic molecular reporters

Ultrasound is among the most widely used non-invasive imaging modalities in biomedicine, but plays a surprisingly small role in molecular imaging due to a lack of suitable molecular reporters on the nanoscale. Here, we introduce a new class of reporters for ultrasound based on genetically encoded gas nanostructures from microorganisms, including bacteria and archaea. Gas vesicles are gas-filled protein-shelled compartments with typical widths of 45-250 nm and lengths of 100-600 nm that exclude water and are permeable to gas. We show that gas vesicles produce stable ultrasound contrast that is readily detected in vitro and in vivo, that their genetically encoded physical properties enable multiple modes of imaging, and that contrast enhancement through aggregation permits their use as molecular biosensors.