Tracking of [18F]FDG-labeled natural killer cells to HER2/neu-positive tumors

PET Imaging for Monitoring Cellular and Immunotherapy of Cancer

ABSTRACT

Cancer immunotherapy, including checkpoint blockade and cellular therapy, has become a cornerstone in cancer treatment. However, understanding the factors driving patient response or resistance to these therapies remains challenging. The dynamic interplay between the immune system and tumors requires new approaches for characterization. Biopsies and blood tests provide valuable information, but their limitations have led to increased interest in positron emission tomography (PET)/computed tomography imaging to complement these strategies. The noninvasive nature of PET imaging makes it ideal for monitoring the dynamic tumor immune microenvironment. This review discusses various PET imaging approaches, including immune cell lineage markers, immune functional markers, immune cell metabolism, direct cell labeling, and reporter genes, highlighting their potential in targeted immunotherapies and cell-based approaches. Although PET imaging has limitations, its integration into diagnostic strategies holds promise for improving patient outcomes and accelerating drug development in cancer immunotherapy.

89Zr-leukocyte labelling for cell trafficking: in vitro and preclinical investigations

BACKGROUND

The non-invasive imaging of leukocyte trafficking to assess inflammatory areas and monitor immunotherapy is currently generating great interest. There is a need to develop more robust cell labelling and imaging approaches to track living cells. Positron emission tomography (PET), a highly sensitive molecular imaging technique, allows precise signals to be produced from radiolabelled moieties. Here, we developed a novel leukocyte labelling approach with the PET radioisotope zirconium-89 (89Zr, half-life of 78.4 h). Experiments were carried out using human leukocytes, freshly isolated from whole human blood.

RESULTS

The 89Zr-leukocyte labelling efficiency ranged from 46 to 87% after 30-60 min. Radioactivity concentrations of labelled cells were up to 0.28 MBq/1 million cells. Systemically administered 89Zr-labelled leukocytes produced high-contrast murine PET images at 1 h-5 days post injection. Murine biodistribution data showed that cells primarily distributed to the lung, liver, and spleen at 1 h post injection, and are then gradually trafficked to liver and spleen over 5 days. Histological analysis demonstrated that exogenously 89Zr-labelled human leukocytes were present in the lung, liver, and spleen at 1 h post injection. However, intravenously injected free [89Zr]Zr4+ ion showed retention only in the bone with no radioactivity in the lung at 5 days post injection, which implied good stability of radiolabelled leukocytes in vivo.

CONCLUSIONS

Our study presents a stable and generic radiolabelling technique to track leukocytes with PET imaging and shows great potential for further applications in inflammatory cell and other types of cell trafficking studies.

Molecular Imaging of Innate Immunity and Immunotherapy

The innate immune system plays a key role as the first line of defense in various human diseases including cancer, cardiovascular and inflammatory diseases. In contrast to tissue biopsies and blood biopsies, in vivo imaging of the innate immune system can provide whole body measurements of immune cell location and function and changes in response to disease progression and therapy. Rationally developed molecular imaging strategies can be used in evaluating the status and spatio-temporal distributions of the innate immune cells in near real-time, mapping the biodistribution of novel innate immunotherapies, monitoring their efficacy and potential toxicities, and eventually for stratifying patients that are likely to benefit from these immunotherapies. In this review, we will highlight the current state-of-the-art in noninvasive imaging techniques for preclinical imaging of the innate immune system particularly focusing on cell trafficking, biodistribution, as well as pharmacokinetics and dynamics of promising immunotherapies in cancer and other diseases; discuss the unmet needs and current challenges in integrating imaging modalities and immunology and suggest potential solutions to overcome these barriers.

Advances in NK cell therapy for brain tumors

Despite advances in treatment regimens that comprise surgery, chemotherapy, and radiation, outcome of many brain tumors remains dismal, more so when they recur. The proximity of brain tumors to delicate neural structures often precludes complete surgical resection. Toxicity and long-term side effects of systemic therapy remain a concern. Novel therapies are warranted. The field of NK cell-based cancer therapy has grown exponentially and currently constitutes a major area of immunotherapy innovation. This provides a new avenue for the treatment of cancerous lesions in the brain. In this review, we explore the mechanisms by which the brain tumor microenvironment suppresses NK cell mediated tumor control, and the methods being used to create NK cell products that subvert immune suppression. We discuss the pre-clinical studies evaluating NK cell-based immunotherapies that target several neuro-malignancies and highlight advances in molecular imaging of NK cells that allow monitoring of NK cell-based therapeutics. We review current and ongoing NK cell based clinical trials in neuro-oncology.

Present status and future trends in molecular imaging of lymphocytes

Immune system is emerging as a crucial protagonist in a huge variety of oncologic and non-oncologic conditions including response to vaccines and viral infections (such as SARS-CoV-2). The increasing knowledge of molecular biology underlying these diseases allowed the identification of specific targets and the possibility to use tailored therapies against them. Immunotherapies and vaccines are, indeed, more and more used nowadays for treating infections, cancer and autoimmune diseases and, therefore, there is the need to identify, quantify and monitor immune cell trafficking before and after treatment. This approach will provide crucial information for therapy decision-making. Imaging of B and T-lymphocytes trafficking by using tailored radiopharmaceuticals proved to be a successful nuclear medicine tool. In this review, we will provide an overview of the state of art and future trends for "in vivo" imaging of lymphocyte trafficking and homing by mean of specific receptor-tailored radiopharmaceuticals.

Radionuclide Imaging of Cytotoxic Immune Cell Responses to Anti-Cancer Immunotherapy

Cancer immunotherapy is an evolving and promising cancer treatment that takes advantage of the body’s immune system to yield effective tumor elimination. Importantly, immunotherapy has changed the treatment landscape for many cancers, resulting in remarkable tumor responses and improvements in patient survival. However, despite impressive tumor effects and extended patient survival, only a small proportion of patients respond, and others can develop immune-related adverse events associated with these therapies, which are associated with considerable costs. Therefore, strategies to increase the proportion of patients gaining a benefit from these treatments and/or increasing the durability of immune-mediated tumor response are still urgently needed. Currently, measurement of blood or tissue biomarkers has demonstrated sampling limitations, due to intrinsic tumor heterogeneity and the latter being invasive. In addition, the unique response patterns of these therapies are not adequately captured by conventional imaging modalities. Consequently, non-invasive, sensitive, and quantitative molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using specific radiotracers, have been increasingly used for longitudinal whole-body monitoring of immune responses. Immunotherapies rely on the effector function of CD8⁺ T cells and natural killer cells (NK) at tumor lesions; therefore, the monitoring of these cytotoxic immune cells is of value for therapy response assessment. Different immune cell targets have been investigated as surrogate markers of response to immunotherapy, which motivated the development of multiple imaging agents. In this review, the targets and radiotracers being investigated for monitoring the functional status of immune effector cells are summarized, and their use for imaging of immune-related responses are reviewed along their limitations and pitfalls, of which multiple have already been translated to the clinic. Finally, emerging effector immune cell imaging strategies and future directions are provided.

Visualizing CAR-T cell Immunotherapy Using 3 Tesla Fluorine-19 MRI

PURPOSE

Chimeric antigen receptor (CAR) T cell cancer immunotherapies have shown remarkable results in patients with hematological malignancies and represent the first approved genetically modified cellular therapies. However, not all blood cancer patients respond favorably, serious side effects have been reported, and the treatment of solid tumors has been a challenge. An imaging tool for visualizing the variety of CAR-T cell products in use and being explored could provide important patient-specific data on CAR-T cell location to inform on potential success or failure of treatment as well as off-target toxicities. Fluorine-19 (19F) magnetic resonance imaging (MRI) allows for the noninvasive detection of 19F perfluorocarbon (PFC) labeled cells. Our objective was to visualize PFC-labeled (PFC +) CAR-T cells in a mouse model of leukemia using clinical field strength (3 Tesla) 19F MRI and compare the cytotoxicity of PFC + versus unlabeled CAR-T cells.

PROCEDURES

NSG mice (n = 17) received subcutaneous injections of CD19 + human B cell leukemia cells (NALM6) expressing firefly luciferase in their left hind flank (1 × 106). Twenty-one days later, each mouse received an intratumoral injection of 10 × 106 PFC + CD19-targeted CAR-T cells (n = 6), unlabeled CD19-targeted CAR-T cells (n = 3), PFC + untransduced T cells (n = 5), or an equivalent volume of saline (n = 3). 19F MRI was performed on mice treated with PFC + CAR-T cells days 1, 3, and 7 post-treatment. Bioluminescence imaging (BLI) was performed on all mice days - 1, 5, 10, and 14 post-treatment to monitor tumor response.

RESULTS

PFC + CAR-T cells were successfully detected in tumors using 19F MRI on days 1, 3, and 7 post-injection. In vivo BLI data revealed that mice treated with PFC + or PFC - CAR-T cells had significantly lower tumor burden by day 14 compared to untreated mice and mice treated with PFC + untransduced T cells (p < 0.05). Importantly, mice treated with PFC + CAR-T cells showed equivalent cytotoxicity compared to mice receiving PFC - CAR-T cells.

CONCLUSIONS

Our studies demonstrate that clinical field strength 19F MRI can be used to visualize PFC + CAR-T cells for up to 7 days post-intratumoral injection. Importantly, PFC labeling did not significantly affect in vivo CAR-T cell cytotoxicity. These imaging tools may have broad applications for tracking emerging CAR-T cell therapies in preclinical models and may eventually be useful for the detection of CAR-T cells in patients where localized injection of CAR-T cells is being pursued.

Tissue Trafficking Kinetics of Rhesus Macaque Natural Killer Cells Measured by Serial Intravascular Staining

The in vivo tissue distribution and trafficking patterns of natural killer (NK) cells remain understudied. Animal models can help bridge the gap, and rhesus macaque (RM) primates faithfully recapitulate key elements of human NK cell biology. Here, we profiled the tissue distribution and localization patterns of three NK cell subsets across various RM tissues. We utilized serial intravascular staining (SIVS) to investigate the tissue trafficking kinetics at steady state and during recovery from CD16 depletion. We found that at steady state, CD16+ NK cells were selectively retained in the vasculature while CD56+ NK cells had a shorter residence time in peripheral blood. We also found that different subsets of NK cells had distinct trafficking kinetics to and from the lymph node as well as other lymphoid and non-lymphoid tissues. Lastly, we found that following administration of CD16-depleting antibody, CD16+ NK cells and their putative precursors retained a high proportion of continuously circulating cells, suggesting that regeneration of the CD16 NK compartment may take place in peripheral blood or the perivascular compartments of tissues.

The emerging role of off-the-shelf engineered natural killer cells in targeted cancer immunotherapy

Natural killer (NK) cells are innate lymphocytes that recognize and clear infected and transformed cells. The importance of NK cells in tumor surveillance underlies the development of NK cell therapy as cancer treatment. The NK-92 cell line has been successfully modified to express high-affinity CD16 receptor for antibody-dependent cellular cytotoxicity and/or chimeric antigen receptors (CARs) that can recognize antigens expressed on tumor cells and mediate NK cell activation. Since there is no need for human leukocyte antigen matching or prior exposure to the tumor antigens, NK-92 provides an opportunity for the development of next-generation off-the-shelf cell therapy platforms. CAR-engineered NK-92 cells have demonstrated robust antitumor activity in in vitro and in vivo preclinical studies, propelling the clinical development of CAR NK-92 cells. Preliminary phase 1 data indicate that CAR NK-92 can be safely administered in the clinic. In this review, we provide an overview of recent advances in the research and clinical application of this novel cell immunotherapy.

Preclinical studies of chimeric antigen receptor-modified natural killer cells in cancer immunotherapy: a review

INTRODUCTION

As one of the most efficacious methods of cancer immunotherapy, chimeric antigen receptor-modified immune cells have recently drawn enormous attention. After the great success achieved with CAR-T-cells in cancer treatment both in preclinical setting and in the clinic, other types of immune cells, including natural killer (NK)-cells and macrophages, have been evaluated for their anti-cancer effects along with their potential superiority against CAR-T-cells, especially in terms of safety. First introduced by Tran et al. almost 26 years ago, CAR-NK-cells are now being considered as efficient immunotherapeutic modalities in various types of cancers, not only in preclinical setting but also in numerous phase I and II clinical studies.

AREAS COVERED

In this review, we aim to provide a comprehensive survey of the preclinical studies on CAR-NK-cells' development, with an evolutional approach on CAR structures and their associated signaling moieties. Current NK-cell sources and modes of gene transfer are also reviewed.

EXPERT OPINION

CAR-NK-cells have appeared as safe and effective immunotherapeutic tools in preclinical settings; however, designing CAR structures with an eye on their specific biology, along with choosing the optimal cell source and gene transfer method require further investigation to support clinical studies.

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.

Magneto-Activation and Magnetic Resonance Imaging of Natural Killer Cells Labeled with Magnetic Nanocomplexes for the Treatment of Solid Tumors

Natural killer (NK) cell-based immunotherapy has been considered a promising cell-based cancer treatment strategy with low side effects for early tumors and metastasis. However, the therapeutic efficacy is generally low in established solid tumors. Ex vivo activation of NK cells with exogenous cytokines is often essential but ineffective to generate high doses of functional NK cells for cancer treatment. Image-guided local delivery of NK cells is also suggested for the therapy. However, there is a lack of noninvasive tools for monitoring NK cells. Herein, magnetic nanocomplexes are fabricated with clinically available materials (hyaluronic acid, protamine, and ferumoxytol; HAPF) for labeling NK cells. The prepared HAPF-nanocomplexes effectively attach to the NK cells (HAPF-NK). An exogenous magnetic field application effectively achieves magneto-activation of NK cells, promoting the generation and secretion of lytic granules of NK cells. The magneto-activated HAPF-NK cells also allow an MR image-guided NK cell therapy to treat hepatocellular carcinoma (HCC) solid tumors via transcatheter intra-arterial infusion. Suppressed tumor growth after the treatment of IA infused magneto-activated NK cells demonstrated a potential enhanced therapeutic efficacy of image guided local delivery of magneto-activated HAPF-NK cells. Given the potential challenges of NK cell cancer immunotherapy against established solid tumors, the effective NK cell labeling with HAPF, magneto-activation, and MRI contrast effect of NK cells will be beneficial to enhance the NK cell-therapeutic efficacy in various cancers.

In Vivo Cell Tracking Using PET: Opportunities and Challenges for Clinical Translation in Oncology

Cell therapy is a rapidly evolving field involving a wide spectrum of therapeutic cells for personalised medicine in cancer. In vivo imaging and tracking of cells can provide useful information for improving the accuracy, efficacy, and safety of cell therapies. This review focuses on radiopharmaceuticals for the non-invasive detection and tracking of therapeutic cells using positron emission tomography (PET). A range of approaches for imaging therapeutic cells is discussed: Direct ex vivo labelling of cells, in vivo indirect labelling of cells by utilising gene reporters, and detection of specific antigens expressed on the target cells using antibody-based radiopharmaceuticals (immuno-PET). This review examines the evaluation of PET imaging methods for therapeutic cell tracking in preclinical cancer models, their role in the translation into patients, first-in-human studies, as well as the translational challenges involved and how they can be overcome.

Live cell imaging of highly activated natural killer cells against human hepatocellular carcinoma in vivo

BACKGROUND AIMS

Tracking administered natural killer (NK) cells in vivo is critical for developing an effective NK cell-based immunotherapy against human hepatocellular carcinoma (HCC). Here the authors established a new molecular imaging using ex vivo-activated NK cells and investigated real-time biodistribution of administered NK cells during HCC progression.

METHODS

Ex vivo-expanded NK cells from healthy donors were labeled with a near-infrared lipophilic cytoplasmic dye, and their proliferation, surface receptor expression and cytotoxicity activity were evaluated. Human HCC HepG2 cells were implanted into the livers of NOD.Cg-Prkdc^scid IL2rg^tm1Wjl/SzJ (NSG) mice. The authors administered 1,1'-dioctadecyltetramethyl indotricarbocyanine iodide (DiR)-labeled NK cells intravenously to non-tumor-bearing and intrahepatic HCC tumor-bearing NSG mice. Fluorescent imaging was performed using a fluorescence-labeled organism bioimaging instrument. Single cell suspensions from the resected organs were analyzed using flow cytometry.

RESULTS

The fluorescent DiR dye was nontoxic and did not affect the proliferation or surface receptor expression levels of the NK cells, even at high doses. The administered DiR-labeled NK cells immediately migrated to the lungs of the non-tumor-bearing NSG mice, with increased NK cell signals evident in the liver and spleen after 4 h. NK cells migrated to the intrahepatic tumor-bearing livers of both early- and late-stage HCC mice within 1 h of injection. In early-stage intrahepatic tumor-bearing mice, the fluorescence signal increased in the liver until 48 h post-injection and decreased 7 days after NK injection. In late-stage HCC, the NK cell fluorescence signal was the highest in the liver for 7 days after NK injection and persisted for 14 days. The purity of long-term persistent CD45⁺CD56⁺CD3^- NK cells was highest in early- and late-stage HepG2-bearing liver compared with normal liver 2 weeks after NK injection, whereas highest purity was still observed in the lungs of non-tumor-bearing mice. In addition, Ki-67 expression was detected in migrated human NK cells in the liver and lung up to 72 h after administration. With HepG2 tumor progression, NK cells reduced the expression of NKp30 and NKG2D.

CONCLUSIONS

Administered NK cells were successfully tracked in vivo by labeling the NK cells with near-infrared DiR dye. Highly expanded, activated NK cells migrated rapidly to the tumor-bearing liver, where they persisted for 14 days after administration, with high purity of CD45⁺CD56⁺CD3^- NK cells. Liver biodistribution and persistence of administered NK cells showed significantly different accumulation patterns during HCC progression.

Applications of nuclear-based imaging in gene and cell therapy: probe considerations

Several types of gene- and cell-based therapeutics are now emerging in the cancer immunotherapy, transplantation, and regenerative medicine landscapes. Radionuclear-based imaging can be used as a molecular imaging tool for repetitive and non-invasive visualization as well as in vivo monitoring of therapy success. In this review, we discuss the principles of nuclear-based imaging and provide a comprehensive overview of its application in gene and cell therapy. This review aims to inform investigators in the biomedical field as well as clinicians on the state of the art of nuclear imaging, from probe design to available radiopharmaceuticals and advances of direct (probe-based) and indirect (transgene-based) strategies in both preclinical and clinical settings. Notably, as the nuclear-based imaging toolbox is continuously expanding, it will be increasingly incorporated into the clinical setting where the distribution, targeting, and persistence of a new generation of therapeutics can be imaged and ultimately guide therapeutic decisions.

Cell-Based Tracers as Trojan Horses for Image-Guided Surgery

Surgeons rely almost completely on their own vision and palpation to recognize affected tissues during surgery. Consequently, they are often unable to distinguish between different cells and tissue types. This makes accurate and complete resection cumbersome. Targeted image-guided surgery (IGS) provides a solution by enabling real-time tissue recognition. Most current targeting agents (tracers) consist of antibodies or peptides equipped with a radiolabel for Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT), magnetic resonance imaging (MRI) labels, or a near-infrared fluorescent (NIRF) dye. These tracers are preoperatively administered to patients, home in on targeted cells or tissues, and are visualized in the operating room via dedicated imaging systems. Instead of using these 'passive' tracers, there are other, more 'active' approaches of probe delivery conceivable by using living cells (macrophages/monocytes, neutrophils, T cells, mesenchymal stromal cells), cell(-derived) fragments (platelets, extracellular vesicles (exosomes)), and microorganisms (bacteria, viruses) or, alternatively, 'humanized' nanoparticles. Compared with current tracers, these active contrast agents might be more efficient for the specific targeting of tumors or other pathological tissues (e.g., atherosclerotic plaques). This review provides an overview of the arsenal of possibilities applicable for the concept of cell-based tracers for IGS.

Noninvasive Imaging of Cancer Immunotherapy

Immunotherapy has revolutionized the treatment of several malignancies. Notwithstanding the encouraging results, many patients do not respond to treatments. Evaluation of the efficacy of treatments is challenging and robust methods to predict the response to treatment are not yet available. The outcome of immunotherapy results from changes that treatment evokes in the tumor immune landscape. Therefore, a better understanding of the dynamics of immune cells that infiltrate into the tumor microenvironment may fundamentally help in addressing this challenge and provide tools to assess or even predict the response. Noninvasive imaging approaches, such as PET and SPECT that provide whole-body images are currently seen as the most promising tools that can shed light on the events happening in tumors in response to treatment. Such tools can provide critical information that can be used to make informed clinical decisions. Here, we review recent developments in the field of noninvasive cancer imaging with a focus on immunotherapeutics and nuclear imaging technologies and will discuss how the field can move forward to address the challenges that remain unresolved.

Effectual Labeling of Natural Killer Cells with Upconverting Nanoparticles by Electroporation for In Vivo Tracking and Biodistribution Assessment

Natural killer (NK) cells, which are cytotoxic lymphocytes of the innate immune system and recognize cancer cells via various immune receptors, are promising agents in cell immunotherapy. To utilize NK cells as a therapeutic agent, their biodistribution and pharmacokinetics need to be evaluated following systemic administration. Therefore, in vivo imaging and tracking with efficient labeling and quantitative analysis of NK cells are required. However, the lack of the phagocytic capacity of NK cells makes it difficult to establish breakthroughs in cell labeling and subsequent in vivo studies. Herein, an effective labeling of upconverting nanoparticles (UCNPs) in NK cells is proposed using electroporation with high sensitivity and stability. The labeling performance of UCNPs functionalized with carboxy-polyethylene glycol (PEG) is better than with methoxy-PEG or with amine-PEG. The labeling efficiency becomes higher, but cell damage is greater as electric field increases; thus, there is an optimum electroporation condition for internalization of UCNPs into NK cells. The tracking and biodistribution imaging analyses of intravenously injected NK cells show that the labeled NK cells are initially distributed primarily in lungs and then spread to the liver and spleen. These advances will accelerate the application of NK cells as key components of immunotherapy against cancer.

Immune cells as tumor drug delivery vehicles

This review article describes the use of immune cells as potential candidates to deliver anti-cancer drugs deep within the tumor microenvironment. First, the rationale of using drug carriers to target tumors and potentially decrease drug-related side effects is discussed. We further explain some of the current limitations when using nanoparticles for this purpose. Next, a comprehensive step-by-step description of the migration cascade of immune cells is provided as well as arguments on why immune cells can be used to address some of the limitations associated with nanoparticle-mediated drug delivery. We then describe the benefits and drawbacks of using red blood cells, platelets, granulocytes, monocytes, macrophages, myeloid-derived suppressor cells, T cells and NK cells for tumor-targeted drug delivery. An additional section discusses the versatility of nanoparticles to load anti-cancer drugs into immune cells. Lastly, we propose increasing the circulatory half-life and development of conditional release strategies as the two main future pillars to improve the efficacy of immune cell-mediated drug delivery to tumors.

PET Imaging of the Natural Killer Cell Activation Receptor NKp30

Redirecting the immune system in cancer treatment has led to remarkable responses in a subset of patients. Natural killer (NK) cells are innate lymphoid cells being explored as they engage tumor cells in different mechanisms compared with T cells, which could be exploited for treatment of nonresponders to current immunotherapies. NK cell therapies are monitored through measuring peripheral NK cell concentrations or changes in tumor volume over time. The former does not detect NK cells at the tumor site, and the latter is inaccurate for immunotherapies because of pseudoprogression. Therefore, new imaging methods are required as companion diagnostics for optimizing immunotherapies. Methods: In this study, we developed and completed preclinical in vivo validation of 2 antibody-based PET probes specific for NKp30, an activation natural cytotoxicity receptor expressed by human NK cells. Quantitative, multicolor flow cytometry during a variety of NK cell activation conditions was completed on primary human NK cells and the NK92MI cell line. Human renal cell carcinoma (RCC) tumors were stained for the NK cell receptors CD56, NKp30, and NKp46 to determine expression on tumor-infiltrating NK cells. An NKp30 antibody was radiolabeled with ⁶⁴Cu or ⁸⁹Zr and evaluated in subcutaneous xenografts and adoptive cell transfer mouse models. Results: Quantitative flow cytometry showed consistent expression of the NKp30 receptor during different activation conditions. NKp30 and NKp46 costained in RCC samples, demonstrating the expression of these receptors on tumor-infiltrating NK cells in human tumors, whereas tumor cells in one RCC sample expressed the peripheral NK marker CD56. Both PET tracers showed high stability and specificity in vitro and in vivo. Notably, ⁸⁹Zr-NKp30Ab had higher on-target contrast than ⁶⁴Cu-NKp30Ab at their respective terminal time points. ⁶⁴Cu-NKp30Ab delineated NK cell trafficking to the liver and spleen in an adoptive cell transfer model. Conclusion: The consistent expression of NKp30 on NK cells makes it an attractive target for quantitative imaging. Immunofluorescence staining on human RCC samples demonstrated the advantages of NKp30 targeting versus CD56 for detection of tumor infiltrating NK cells. This work advances PET imaging of NK cells and supports the translation of imaging agents for immunotherapy monitoring.

Application of In Vivo Imaging Techniques for Monitoring Natural Killer Cell Migration and Tumor Infiltration

In recent years, the use of natural killer (NK) cell-based immunotherapy has shown promise against various cancer types. To some extent therapeutic potential of NK cell-based immunotherapy depends on migration of NK cells towards tumors in animal models or human subjects and subsequent infiltration. Constant improvement in the pharmacological and therapeutic properties of NK cells is driving the performance and use of NK cell-based immunotherapies. In this review, we summarize the molecular imaging techniques used in monitoring the migration and infiltration of NK cells in vivo at preclinical and clinical levels. A review of pros and cons of each molecular imaging modality is done. Finally, we provide our perception of the usefulness of molecular imaging approaches for in vivo monitoring of NK cells in preclinical and clinical scenarios.

Dynamic Cell Imaging in PET With Optimal Transport Regularization

We propose a novel dynamic image reconstruction method from PET listmode data that could be particularly suited to tracking single or small numbers of cells. In contrast to conventional PET reconstruction our method combines the information from all detected events not only to reconstruct the dynamic evolution of the radionuclide distribution, but also to improve the reconstruction at each single time point by enforcing temporal consistency. This is achieved via optimal transport regularization where in principle, among all possible temporally evolving radionuclide distributions consistent with the PET measurement, the one is chosen with least kinetic motion energy. The reconstruction is found by convex optimization so that there is no dependence on the initialization of the method. We study its behaviour on simulated data of a human PET system and demonstrate its robustness even in settings with very low radioactivity. In contrast to previously reported cell tracking algorithms, our technique is oblivious to the number of tracked cells. Without any additional complexity one or multiple cells can be reconstructed, and the model automatically determines the number of particles. For instance, four radiolabelled cells moving at a velocity of 3.1 mm/s and a PET recorded count rate of 1.1 cps (for each cell) could be simultaneously tracked with a tracking accuracy of 5.3 mm inside a simulated human body.

Overview of Strategies to Improve Therapy against Tumors Using Natural Killer Cell

NK cells are lymphocytes with antitumor properties and can directly lyse tumor cells in a non-MHC-restricted manner. However, the tumor microenvironment affects the immune function of NK cells, which leads to immune evasion. This may be related to the pathogenesis of some diseases. Therefore, great efforts have been made to improve the immunotherapy effect of natural killer cells. NK cells from different sources can meet different clinical needs, in order to minimize the inhibition of NK cells and maximize the response potential of NK cells, for example, modification of NK cells can increase the number of NK cells in tumor target area, change the direction of NK cells, and improve their targeting ability to malignant cells. Checkpoint blocking is also a promising strategy for NK cells to kill tumor cells. Combination therapy is another strategy for improving antitumor ability, especially in combination with oncolytic viruses and nanomaterials. In this paper, the mechanisms affecting the activity of NK cells were reviewed, and the therapeutic potential of different basic NK cell strategies in tumor therapy was focused on. The main strategies for improving the immune function of NK cells were described, and some new strategies were proposed.

State of the Art of Natural Killer Cell Imaging: A Systematic Review

Natural killer (NK) cell therapy is a promising alternative to conventional T cell-based treatments, although there is a lack of diagnostic tools to predict and evaluate therapeutic outcomes. Molecular imaging can offer several approaches to non-invasively address this issue. In this study, we systematically reviewed the literature to evaluate the state of the art of NK cell imaging and its translational potential. PubMed and Scopus databases were searched for published articles on the imaging of NK cells in humans and preclinical models. Study quality was evaluated following Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) criteria. We pooled studies as follows: Optical, magnetic resonance imaging (MRI) and nuclear medicine imaging with a total of 21 studies (n = 5, n = 8 and n = 8, respectively). Considering the limitation of comparing different imaging modalities, it appears that optical imaging (OI) of NK cells is very useful in a preclinical setting, but has the least translational potential. MRI provides high quality images without ionizing radiations with lower sensitivity. Nuclear medicine is the only imaging technique that has been applied in humans (four papers), but results were not outstanding due to a limited number of enrolled patients. At present, no technique emerged as superior over the others and more standardization is required in conducting human and animal studies.

The Molecular Imaging of Natural Killer Cells

The recent success of autologous T cell-based therapies in hematological malignancies has spurred interest in applying similar immunotherapy strategies to the treatment of solid tumors. Identified nearly 4 decades ago, natural killer (NK) cells represent an arguably better cell type for immunotherapy development. Natural killer cells are cytotoxic lymphocytes that mediate the direct killing of transformed cells with reduced or absent major histocompatibility complex (MHC) and are the effector cells in antibody-dependent cell-mediated cytotoxicity. Unlike T cells, they do not require human leukocyte antigen (HLA) matching allowing for the adoptive transfer of allogeneic NK cells in the clinic. The development of NK cell-based therapies for solid tumors is complicated by the presence of an immunosuppressive tumor microenvironment that can potentially disarm NK cells rendering them inactive. The molecular imaging of NK cells in vivo will be crucial for the development of new therapies allowing for the immediate assessment of therapeutic response and off-target effects. A number of groups have investigated methods for detecting NK cells by optical, nuclear, and magnetic resonance imaging. In this review, we will provide an overview of the advances made in imaging NK cells in both preclinical and clinical studies.

Natural Killer Cells and Current Applications of Chimeric Antigen Receptor-Modified NK-92 Cells in Tumor Immunotherapy

Natural killer (NK) cells are innate immune cells that can be activated rapidly to target abnormal and virus-infected cells without prior sensitization. With significant advancements in cell biology technologies, many NK cell lines have been established. Among these cell lines, NK-92 cells are not only the most widely used but have also been approved for clinical applications. Additionally, chimeric antigen receptor-modified NK-92 cells (CAR-NK-92 cells) have shown strong antitumor effects. In this review, we summarize established human NK cell lines and their biological characteristics, and highlight the applications of NK-92 cells and CAR-NK-92 cells in tumor immunotherapy.

Real-Time Tracking of Ex Vivo-Expanded Natural Killer Cells Toward Human Triple-Negative Breast Cancers

Introduction

Ex vivo-expanded natural killer (NK) cells are a potential candidate for cancer immunotherapy based on high cytotoxicity against malignant tumor cells. However, a limited understanding of the migration of activated NK cells toward solid tumors is a critical dilemma in the development of effective and adoptive NK cell-based immunotherapy.

Methods

Ex vivo-expanded NK cells from healthy donors were stained with near-infrared fluorophores at different concentrations. NK cell proliferation and cytotoxicity were assessed using a WST-8 assay, while the expression levels of surface molecules were analyzed by flow cytometry. To investigate the biodistribution of NK cells in both normal and tumor-bearing NSG mice, NK cells labeled with ESNF13 were subjected to NIR fluorescence imaging using the Mini-FLARE imaging system. Finally, mice were sacrificed and histopathological tests were performed in resected organs.

Results

The signal intensity of ESNF-stained NK cells was long-lasting at 72 h using concentrations as low as 0.04 µM. At a low dose range, ESNF13 did not affect NK cell purity, expression levels of surface receptors, or cytotoxic functions against MDA-MB-231 cancer cells. Ex vivo-expanded NK cells labeled with ESNF13 had a 4-h biodistribution in non-tumor-bearing NSG mice that mainly localized to the lungs immediately after injection and then fully migrated to the kidney after 4 h. In an MDA-MB-231 tumor-bearing NSG mice with extensive metastasis in both lungs, the fluorescence signal was dominant in both lungs and steady at 1, 2, and 4 h post-injection. In a early phase of tumor progression, administered NK cell migrated to the lungs and tumor sites within 30 min post-injection, the signal dominated the tumor site after 1 h, and remained steady at 4 h.

Conclusion

Optical imaging with NIR fluorophore ESNF13 is a highly sensitive, applicable, and inexpensive method for the real-time tracking of ex vivo-expanded NK cells both in vitro and in vivo. Administered NK cells had different patterns of NK cell distribution and accumulation to the tumor site according to tumor progression in triple-negative breast cancer xenograft models.

Molecular Imaging: A Useful Tool for the Development of Natural Killer Cell-Based Immunotherapies

Molecular imaging is a relatively new discipline that allows visualization, characterization, and measurement of the biological processes in living subjects, including humans, at a cellular and molecular level. The interaction between cancer cells and natural killer (NK) cells is complex and incompletely understood. Despite our limited knowledge, progress in the search for immune cell therapies against cancer could be significantly improved by dynamic and non-invasive visualization and tracking of immune cells and by visualization of the response of cancer cells to therapies in preclinical and clinical studies. Molecular imaging is an essential tool for these studies, and a multimodal molecular imaging approach can be applied to monitor immune cells in vivo, for instance, to visualize therapeutic effects. In this review, we discuss the usefulness of NK cells in cancer therapies and the preclinical and clinical usefulness of molecular imaging in NK cell-based therapies. Furthermore, we discuss different molecular imaging modalities for use with NK cell-based therapies, and their preclinical and clinical applications in animal and human subjects. Molecular imaging has contributed to the development of NK cell-based therapies against cancers in animal models and to the refinement of current cell-based cancer immunotherapies. Developing sensitive and reproducible non-invasive molecular imaging technologies for in vivo NK cell monitoring and for real-time assessment of therapeutic effects will accelerate the development of NK cell therapies.

Chimeric Antigen Receptor-Engineered NK-92 Cells: An Off-the-Shelf Cellular Therapeutic for Targeted Elimination of Cancer Cells and Induction of Protective Antitumor Immunity

Significant progress has been made in recent years toward realizing the potential of natural killer (NK) cells for cancer immunotherapy. NK cells can respond rapidly to transformed and stressed cells and have the intrinsic potential to extravasate and reach their targets in almost all body tissues. In addition to donor-derived primary NK cells, also the established NK cell line NK-92 is being developed for adoptive immunotherapy, and general safety of infusion of irradiated NK-92 cells has been established in phase I clinical trials with clinical responses observed in some of the cancer patients treated. To enhance their therapeutic utility, NK-92 cells have been modified to express chimeric antigen receptors (CARs) composed of a tumor-specific single chain fragment variable antibody fragment fused via hinge and transmembrane regions to intracellular signaling moieties such as CD3ζ or composite signaling domains containing a costimulatory protein together with CD3ζ. CAR-mediated activation of NK cells then bypasses inhibitory signals and overcomes NK resistance of tumor cells. In contrast to primary NK cells, CAR-engineered NK-92 cell lines suitable for clinical development can be established from molecularly and functionally well-characterized single cell clones following good manufacturing practice-compliant procedures. In preclinical in vitro and in vivo models, potent antitumor activity of NK-92 variants targeted to differentiation antigens expressed by hematologic malignancies, and overexpressed or mutated self-antigens associated with solid tumors has been found, encouraging further development of CAR-engineered NK-92 cells. Importantly, in syngeneic mouse tumor models, induction of endogenous antitumor immunity after treatment with CAR-expressing NK-92 cells has been demonstrated, resulting in cures and long-lasting immunological memory protecting against tumor rechallenge at distant sites. Here, we summarize the current status and future prospects of CAR-engineered NK-92 cells as off-the-shelf cellular therapeutics, with special emphasis on ErbB2 (HER2)-specific NK-92 cells that are approaching clinical application.

Advances in Molecular Imaging Strategies for In Vivo Tracking of Immune Cells

Tracking of immune cells in vivo is a crucial tool for development and optimization of cell-based therapy. Techniques for tracking immune cells have been applied widely for understanding the intrinsic behavior of immune cells and include non-radiation-based techniques such as optical imaging and magnetic resonance imaging (MRI), radiation-based techniques such as computerized tomography (CT), and nuclear imaging including single photon emission computerized tomography (SPECT) and positron emission tomography (PET). Each modality has its own strengths and limitations. To overcome the limitations of each modality, multimodal imaging techniques involving two or more imaging modalities are actively applied. Multimodal techniques allow integration of the strengths of individual modalities. In this review, we discuss the strengths and limitations of currently available preclinical in vivo immune cell tracking techniques and summarize the value of immune cell tracking in the development and optimization of immune cell therapy for various diseases.

Anti‑migratory effect of rapamycin impairs allograft imaging by 18F‑fluorodeoxyglucose‑labeled splenocytes

Tracking lymphocyte migration is an emerging strategy for non‑invasive nuclear imaging of allografts; however, its clinical application remains to be fully demonstrated. In the present study, the feasibility of using rapamycin‑treated 18F‑fluorodeoxyglucose (18F‑FDG)‑labeled splenocytes for the in vivo imaging of allografts was evaluated. C57BL/6 skin was heterotopically transplanted onto non‑obese diabetic/severe combined immunodeficient recipient mice. BALB/c 18F‑FDG‑labeled splenocytes with or without rapamycin pretreatment (designated as FR and FC cells, respectively) were transferred into recipient mice 30 days later. Imaging of radiolabeled cells in the skin grafts was conducted through in vivo dynamic whole‑body phosphor‑autoradiography and histological analysis. Notably, rapamycin impaired the migration of 18F‑FDG‑labeled splenocytes to the graft. At all time points, the radioactivity of allografts (digital light units/mm2) was significantly lower in the group that received FR cells, compared with the group that received FC cells (P<0.01). Furthermore, the peak allograft to native skin ratio was 1.29±0.02 at 60 min for the FR group and 3.29±0.17 at 30 min for the FC group (P<0.001). In addition, the in vivo radioactivity of the allografts was observed to be correlated with the transferred cells, which were observed histologically (r2=0.887; P<0.0001). Although 18F‑FDG‑labeled splenocytes migrated to the allograft, imaging of these cells may not be possible in the presence of rapamycin.

Clinically applicable magnetic-labeling of natural killer cells for MRI of transcatheter delivery to liver tumors: preclinical validation for clinical translation

AIM

To test the hypothesis that MRI can monitor intraportal vein (IPV) transcatheter delivery of clinically applicable heparin-protamine-ferumoxytol (HPF) nanocomplex-labeled natural killer (NK) cells to liver tumor.

MATERIALS & METHODS

Liver tumor rat models underwent catheterization for IPV infusion of HPF-labeled NK cells (NK-92MI cell line). MRI measurements within tumor and adjacent liver tissues were compared pre- and post-NK cell infusion. Histology studies were used to identify NK cells in the target tumors.

RESULTS

For first time, we demonstrated that MRI tracks HPF-labeled NK cells migration within liver following IPV delivery.

CONCLUSION

IPV transcatheter infusion permitted selective delivery of NK cells to liver tissues and MRI allowed tracking NK cell biodistributions within the tumors.

NK-92: an 'off-the-shelf therapeutic' for adoptive natural killer cell-based cancer immunotherapy

Natural killer (NK) cells are increasingly considered as immunotherapeutic agents in particular in the fight against cancers. NK cell therapies are potentially broadly applicable and, different from their T cell counterparts, do not cause graft-versus-host disease. Efficacy and clinical in vitro or in vivo expansion of primary NK cells will however always remain variable due to individual differences of donors or patients. Long-term storage of clinical NK cell lots to allow repeated clinical applications remains an additional challenge. In contrast, the established and well-characterized cell line NK-92 can be easily and reproducibly expanded from a good manufacturing practice (GMP)-compliant cryopreserved master cell bank. Moreover, no cost-intensive cell purification methods are required. To date, NK-92 has been intensively studied. The cells displayed superior cytotoxicity against a number of tumor types tested, which was confirmed in preclinical mouse studies. Subsequent clinical testing demonstrated safety of NK-92 infusions even at high doses. Despite the phase I nature of the trials conducted so far, some efficacy was noted, particularly against lung tumors. Furthermore, to overcome tumor resistance and for specific targeting, NK-92 has been engineered to express a number of different chimeric antigen receptors (CARs), including targeting, for example, CD19 or CD20 (anti-B cell malignancies), CD38 (anti-myeloma) or human epidermal growth factor receptor 2 (HER2; ErbB2; anti-epithelial cancers). The concept of an NK cell line as an allogeneic cell therapeutic produced 'off-the-shelf' on demand holds great promise for the development of effective treatments.

In Vivo (19)F-Magnetic Resonance Imaging of Adoptively Transferred NK Cells

In order to assess the biodistribution, homing, and persistence of adoptively transferred natural killer (NK) cell immunotherapies, there is a need for imaging methodology suitable for use in preclinical studies with relevance to clinical translation. Amongst the available approaches, (19)F-MRI is very appealing for in vivo imaging due to the absence of background signal, enabling clear detection of (19)F labeled cells in vivo. Here we describe a methodology for in vivo imaging of adoptively transferred NK cells labeled with (19)F nano-emulsion, using clinically translatable technology of (19)F/(1)H magnetic resonance imaging.

Novel (89)Zr cell labeling approach for PET-based cell trafficking studies

Background

With the recent growth of interest in cell-based therapies and radiolabeled cell products, there is a need to develop more robust cell labeling and imaging methods for in vivo tracking of living cells. This study describes evaluation of a novel cell labeling approach with the positron emission tomography (PET) isotope ⁸⁹Zr (T_1/2 = 78.4 h). ⁸⁹Zr may allow PET imaging measurements for several weeks and take advantage of the high sensitivity of PET imaging.

Methods

A novel cell labeling agent, ⁸⁹Zr-desferrioxamine-NCS (⁸⁹Zr-DBN), was synthesized. Mouse-derived melanoma cells (mMCs), dendritic cells (mDCs), and human mesenchymal stem cells (hMSCs) were covalently labeled with ⁸⁹Zr-DBN via the reaction between the NCS group on ⁸⁹Zr-DBN and primary amine groups present on cell surface membrane protein. The stability of the label on the cell was tested by cell efflux studies for 7 days. The effect of labeling on cellular viability was tested by proliferation, trypan blue, and cytotoxicity/apoptosis assays. The stability of label was also studied in in vivo mouse models by serial PET scans and ex vivo biodistribution following intravenous and intramyocardial injection of ⁸⁹Zr-labeled hMSCs. For comparison, imaging experiments were performed after intravenous injections of ⁸⁹Zr hydrogen phosphate (⁸⁹Zr(HPO₄)₂).

Results

The labeling agent, ⁸⁹Zr-DBN, was prepared in 55% ± 5% decay-corrected radiochemical yield measured by silica gel iTLC. The cell labeling efficiency was 30% to 50% after 30 min labeling depending on cell type. Radioactivity concentrations of labeled cells of up to 0.5 MBq/10⁶ cells were achieved without a negative effect on cellular viability. Cell efflux studies showed high stability of the radiolabel out to 7 days. Myocardially delivered ⁸⁹Zr-labeled hMSCs showed retention in the myocardium, as well as redistribution to the lung, liver, and bone. Intravenously administered ⁸⁹Zr-labeled hMSCs also distributed primarily to the lung, liver, and bone, whereas intravenous ⁸⁹Zr(HPO₄)₂ distributed to the liver and bone with no activity in the lung. Thus, the in vivo stability of the radiolabel on the hMSCs was evidenced.

Conclusions

We have developed a robust, general, and biostable ⁸⁹Zr-DBN-based cell labeling strategy with promise for wide applications of PET-based non-invasive in vivo cell trafficking.

Natural killer cells: the journey from puzzles in biology to treatment of cancer

Natural Killer (NK) cells are innate immune effectors that are primarily involved in immunosurveillance to spontaneously eliminate malignantly transformed and virally infected cells without prior sensitization. NK cells trigger targeted attack through release of cytotoxic granules, and secrete various cytokines and chemokines to promote subsequent adaptive immune responses. NK cells selectively attack target cells with diminished major histocompatibility complex (MHC) class I expression. This "Missing-self" recognition by NK cells at first puzzled researchers in the early 1990s, and the mystery was solved with the discovery of germ line encoded killer immunoglobulin receptors that recognize MHC-I molecules. This review summarizes the biology of NK cells detailing the phenotypes, receptors and functions; interactions of NK cells with dendritic cells (DCs), macrophages and T cells. Further we discuss the various strategies to modulate NK cell activity and the practice of NK cells in cancer immunotherapy employing NK cell lines, autologous, allogeneic and genetically engineered cell populations.

In vivo monitoring of natural killer cell trafficking during tumor immunotherapy

Natural killer (NK) cells are a crucial part of the innate immune system and play critical roles in host anti-viral, anti-microbial, and antitumor responses. The elucidation of NK cell biology and their therapeutic use are actively being pursued with 200 clinical trials currently underway. In this review, we outline the role of NK cells in cancer immunotherapies and summarize current noninvasive imaging technologies used to track NK cells in vivo to investigate mechanisms of action, develop new therapies, and evaluate efficacy of adoptive transfer.

Focused ultrasound delivers targeted immune cells to metastatic brain tumors

Natural killer (NK) cells are cytotoxic lymphocytes involved in innate immunity. NK-92, a human NK cell line, may be targeted to tumor-associated antigens in solid malignancies where it exhibits antitumor efficacy, but its clinical utility for treating brain tumors is limited by an inability to cross the blood-brain barrier (BBB). We investigated the potential for focused ultrasound (FUS) to deliver targeted NK-92 cells to the brain using a model of metastatic breast cancer. HER2-expressing human breast tumor cells were implanted into the brain of nude rats. The NK-92-scFv(FRP5)-zeta cell line expressing a chimeric HER2 antigen receptor was transfected with superparamagnetic iron oxide nanoparticles before intravenous injection, before and following BBB disruption using focused ultrasound (551.5 kHz focused transducer, 0.33 MPa average peak rarefaction pressure) in the presence of a microbubble contrast agent. Baseline and posttreatment 1.5T and 7T MR imaging was done, and histology used to identify NK-92 cells post-mortem. Contrast-enhanced MRI showed reproducible and consistent BBB disruption. 7T MR images obtained at 16 hours posttreatment revealed a significant reduction in signal indicating the presence of iron-loaded NK-92 cells at the tumor site. The average ratio of NK-92 to tumor cells was 1:100 when NK cells were present in the vasculature at the time of sonication, versus 2:1,000 and 1:1,000 when delivered after sonication and without BBB disruption, respectively. Our results offer a preclinical proof-of-concept that FUS can improve the targeting of immune cell therapy of brain metastases.

Immunotherapy of malignant disease using chimeric antigen receptor engrafted T cells

Chimeric antigen receptor- (CAR-) based immunotherapy has been under development for almost 25 years, over which period it has progressed from a new but cumbersome technology to an emerging therapeutic modality for malignant disease. The approach involves the genetic engineering of fusion receptors (CARs) that couple the HLA-independent binding of cell surface target molecules to the delivery of a tailored activating signal to host immune cells. Engineered CARs are delivered most commonly to peripheral blood T cells using a range of vector systems, most commonly integrating viral vectors. Preclinical refinement of this approach has proceeded over several years to the point that clinical testing is now being undertaken at several centres, using increasingly sophisticated and therapeutically successful genetic payloads. This paper considers several aspects of the pre-clinical and clinical development of CAR-based immunotherapy and how this technology is acquiring an increasing niche in the treatment of both solid and haematological malignancies.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

In vivo cell tracking via ¹⁸F-fluorodeoxyglucose labeling: a review of the preclinical and clinical applications in cell-based diagnosis and therapy

The rising interest in using functional cells for diagnosis and treatment has created an urgent need for in vivo cell-tracking techniques. Certain advanced techniques, such as those involving reporter genes or nanoparticles, are still awaiting confirmation of their safety and feasibility in human patients. Tracking cells by labeling them with (18)F-fluorodeoxyglucose, a tracer clinically used in positron emission tomography (PET), may be one way to rapidly translate some of these principles from bench to bedside. The preliminary results are exciting, although further development, optimization, and validation are required. Here, several applications of the technique are surveyed: finding inflammatory foci, targeting cancer immunotherapies, tracking transplanted islet cells, and monitoring cardiac stem cells. Advantages, limitations, and prospects of the technique are discussed. These early experiences only highlight the existing need to improve cell-labeling techniques using PET tracers. This method may finally lead to the development of effective and convenient methods for clinical cell-tracking techniques involving PET/computed tomography.

Migration of iron-labeled KHYG-1 natural killer cells to subcutaneous tumors in nude mice, as detected by magnetic resonance imaging

BACKGROUND AIMS

A novel cell line of cytotoxic natural killer (NK) cells, KHYG-1, was examined in vivo for immunotherapy against prostate cancer. The feasibility of using magnetic resonance imaging (MRI) tracking to monitor the fate of injected NK cells following intravenous (i.v.), intraperitoneal (i.p.) and subcutaneous (s.c.) administration was assessed.

METHODS

PC-3M human prostate cancer cells were injected s.c. into the flank of nude mice (day 0). KHYG-1 NK cells were labeled with an iron oxide contrast agent and injected s.c., i.v. or i.p. on day 8. Mice were imaged by MRI on days 7, 9 and 12. Tumor sections were examined with fluorescence microscopy and immunohistologic staining for NK cells.

RESULTS

NK cells were detected in the tumors by histology after all three administration routes. NK cells and fluorescence from the iron label were co-localized. Signal loss was seen in the areas around the tumors and between the tumor lobes in the s.c. group.

CONCLUSIONS

We are the first to label this cell line of NK cells with an iron oxide contrast agent. Accumulation of NK cells was visualized by MRI after s.c. injection but not after i.v. and i.p. injection.

NK cell therapy: targeting disease relapse after hematopoietic stem cell transplantation

As outcomes of hematopoietic stem cell transplantation have improved over time, disease relapse has emerged as the most significant cause of treatment failure. Cellular therapy represents an alternative therapeutic approach, not only to treat or prevent disease relapse after hematopoietic stem cell transplantation, but also conceivably to help patients achieve remission prior to transplantation or as consolidation therapy for high-risk patients with hematologic malignancies. Of the many cellular therapies available, infusion of NK cells may be the most promising approach against malignant or virally infected cells owing to strong innate activity of NK cells in vitro and in vivo. Only limited clinical data exists mostly from feasability studies of inadequate size to demonstrate clinical benefit. Here, we discuss the current status of clinical investigation using NK cell therapy for patients with hematologic malignancies undergoing hematopoietic stem cell transplantation.

Distribution of adoptively transferred porcine T-lymphoblasts tracked by (18)F-2-fluoro-2-deoxy-D-glucose and position emission tomography

INTRODUCTION

Autologous or allogeneic transfer of tumor-infiltrating T-lymphocytes is a promising treatment for metastatic cancers, but a major concern is the difficulty in evaluating cell trafficking and distribution in adoptive cell therapy. This study presents a method of tracking transfusion of T-lymphoblasts in a porcine model by (18)F-2-fluoro-2-deoxy-d-glucose ([(18)F]FDG) and positron emission tomography.

METHODS

T-lymphoblasts were labeled with the positron-emitting tracer [(18)F]FDG through incubation. The T-lymphoblasts were administered into the bloodstream, and the distribution was followed by positron emission tomography for 120 min. The cells were administered either intravenously into the internal jugular vein (n=5) or intraarterially into the ascending aorta (n=1). Two of the pigs given intravenous administration were pretreated with low-molecular-weight dextran sulphate.

RESULTS

The cellular kinetics and distribution were readily quantifiable for up to 120 min. High (78.6% of the administered cells) heterogeneous pulmonary uptake was found after completed intravenous transfusion. The pulmonary uptake was decreased either by preincubating and coadministrating the T-lymphoblasts with low-molecular-weight dextran sulphate or by administrating them intraarterially.

CONCLUSIONS

The present work shows the feasibility of quantitatively monitoring and evaluating cell trafficking and distribution following administration of [(18)F]FDG-labeled T-lymphoblasts. The protocol can potentially be transferred to the clinical setting with few modifications.

Genetic modification of natural killer cells for adoptive cellular immunotherapy

Immunotherapy of cancer is a rapidly developing field; one such development is the manipulation and use of natural killer (NK) cells. These cells with 'killer instincts' are an attractive cell to utilize, as they are directly reactive toward tumor and could potentially activate the endogenous adaptive immune system. Their employment in adoptive cell transfer treatments has yielded important results and discoveries, although effective antitumor responses are limited. To address these limitations, NK cells are the target of a new generation of immunotherapy involving gene transfer. The gene modification of immune cells is a relatively recent technique and some groups have targeted NK cells for gene modification to improve their antitumor efficacy. This review will investigate studies describing the gene modification of NK cells and their encouraging antitumor effects.

Non-invasive cell tracking in cancer and cancer therapy

Cell-based therapy holds great promise for cancer treatment. The ability to non-invasively track the delivery of various therapeutic cells (e.g. T cells and stem cells) to the tumor site, and/or subsequent differentiation/proliferation of these cells, would allow better understanding of the mechanisms of cancer development and intervention. This brief review will summarize the various methods for non-invasive cell tracking in cancer and cancer therapy. In general, there are two approaches for cell tracking: direct (cells are labeled with certain tags that can be detected directly with suitable imaging equipment) and indirect cell labeling (which typically uses reporter genes approach). The techniques for tracking various cell types (e.g. immune cells, stem cells, and cancer cells) in cancer are described, which include fluorescence, bioluminescence, positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI). Non-invasive tracking of immune and stem cells were primarily intended for (potential) cancer therapy applications while tracking of cancer cells could further our understanding of cancer development and tumor metastasis. Safety is a major concern for future clinical applications and the ideal imaging modality for tracking therapeutic cells in cancer patients requires the imaging tags to be non-toxic, biocompatible, and highly specific. Each imaging modality has its advantages and disadvantages and they are more complementary than competitive. MRI, radionuclide-based imaging techniques, and reporter gene-based approaches will each have their own niches towards the same ultimate goal: personalized medicine for cancer patients.