Comparison of three different methods for radiolabelling human activated T lymphocytes

M. de Rosales R. Direct Cell Radiolabeling for in Vivo Cell Tracking with PET and SPECT Imaging

The arrival of cell-based therapies is a revolution in medicine. However, its safe clinical application in a rational manner depends on reliable, clinically applicable methods for determining the fate and trafficking of therapeutic cells in vivo using medical imaging techniques─known as in vivo cell tracking. Radionuclide imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) has several advantages over other imaging modalities for cell tracking because of its high sensitivity (requiring low amounts of probe per cell for imaging) and whole-body quantitative imaging capability using clinically available scanners. For cell tracking with radionuclides, ex vivo direct cell radiolabeling, that is, radiolabeling cells before their administration, is the simplest and most robust method, allowing labeling of any cell type without the need for genetic modification. This Review covers the development and application of direct cell radiolabeling probes utilizing a variety of chemical approaches: organic and inorganic/coordination (radio)chemistry, nanomaterials, and biochemistry. We describe the key early developments and the most recent advances in the field, identifying advantages and disadvantages of the different approaches and informing future development and choice of methods for clinical and preclinical application.

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.

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.

Imaging of cell-based therapy using 89Zr-oxine ex vivo cell labeling for positron emission tomography

With the rapid development of anti-cancer cell-based therapies, such as adoptive T cell therapies using tumor-infiltrating T cells, T cell receptor transduced T cells, and chimeric antigen receptor T cells, there has been a growing interest in imaging technologies to non-invasively track transferred cells in vivo. Cell tracking using ex vivo cell labeling with positron emitting radioisotopes for positron emission tomography (PET) imaging has potential advantages over single-photon emitting radioisotopes. These advantages include intrinsically higher resolution, higher sensitivity, and higher signal-to-background ratios. Here, we review the current status of recently developed Zirconium-89 (89Zr)-oxine ex vivo cell labeling with PET imaging focusing on its applications and future perspectives. Labeling of cells with 89Zr-oxine is completed in a series of relatively simple steps, and its low radioactivity doses required for imaging does not interfere with the proliferation or function of the labeled immune cells. Preclinical studies have revealed that 89Zr-oxine PET allows high-resolution in vivo tracking of labeled cells for 1-2 weeks after cell transfer both in mice and non-human primates. These results provide a strong rationale for the clinical translation of 89Zr-oxine PET-based imaging of cell-based therapy.

Outcome Prediction and Evaluation by Imaging the Key Elements of Therapeutic Responses to Cancer Immunotherapies Using PET

Cancer immunotherapy (also known as immuno-oncology), a promising anti-cancer strategy by harnessing the body's own immune system against cancer, has emerged as the "fifth therapeutic pilla" in the field of cancer treatment since surgery, chemotherapy, radiation and targeted therapy. Clinical efficacy of several immunotherapies has been demonstrated in clinical settings, however, only a small subset of patients exhibit dramatic or durable responses, with the highest reported frequency about 10-40% from single-agent PD-L1/PD-1 inhibitors, suggesting the urgent need of consistent objective response biomarkers for monitoring therapeutic response accurately, predicting therapeutic efficacy and selecting responders. Key elements of therapeutic responses to cancer immunotherapies contain the cancer cell response and the alternation of inherent immunological characteristics. Here, we document the literature regarding imaging the key elements of therapeutic responses to cancer immunotherapies using PET. We discussed PET imaging approaches according to different response mechanisms underlying diverse immune-therapeutic categories, and also highlight the ongoing efforts to identify novel immunotherapeutic PET imaging biomarkers. In this article, we show that PET imaging of the key elements of therapeutic responses to cancer immunotherapies using PET can allow for more precise prediction, earlier therapy response monitoring, and improved management. However, all of these strategies need more preclinical study and clinical validation before further development as imaging indicators of the immune response.

Single-cell radioluminescence microscopy with two-fold higher sensitivity using dual scintillator configuration

Radioluminescence microscopy (RLM) is an imaging technique that allows quantitative analysis of clinical radiolabeled drugs and probes in single cells. However, the modality suffers from slow data acquisition (15–30 minutes), thus critically affecting experiments with short-lived radioactive drugs. To overcome this issue, we suggest an approach that significantly accelerates data collection. Instead of using a single scintillator to image the decay of radioactive molecules, we sandwiched the radiolabeled cells between two scintillators. As proof of concept, we imaged cells labeled with [¹⁸F]FDG, a radioactive glucose popularly used in oncology to image tumors. Results show that the double scintillator configuration increases the microscope sensitivity by two-fold, thus reducing the image acquisition time by half to achieve the same result as the single scintillator approach. The experimental results were also compared with Geant4 Monte Carlo simulation to confirm the two-fold increase in sensitivity with only minor degradation in spatial resolution. Overall, these findings suggest that the double scintillator configuration can be used to perform time-sensitive studies such as cell pharmacokinetics or cell uptake of short-lived radiotracers.

Granulocyte radiolabeling using Leukokit® with introduction of a density gradient medium as ancillary material

BACKGROUND

Radiolabeled white blood cells (WBCs) prepared in radiopharmacies are used to detect infectious or inflammatory sites with scintigraphy. Radiolabeling can be performed by using a disposable closed device, Leukokit®. Nevertheless, owing to the high radiosensitivity of lymphocytes, the question of eliminating lymphocytes before granulocyte radiolabeling is still a controversial step. The aim of this study was to assess a new modified Leukokit® with a protocol that allows granulocyte radiolabeling only.

METHODS

Seventy patients (male/female: 40/30, mean age: 61 years) with suspected infectious diseases underwent labeled leukocyte scintigraphy by radiolabeling with a density gradient medium in addition to Leukokit®. Compliance and quality of radiolabeling were checked according to the following criteria: visual inspection, labeling efficiency, cell viability (Trypan blue exclusion test), cell subset recovery test, lymphocyte elimination rate (granulocyte/WBC rate) and sterility test using media fills.

RESULTS

Visual inspection showed that all cell preparations were free of residual cell clumps or fibrin clots. Mean labeling efficiency was 70.4±9.4% compliant with EANM Guidelines for leukocyte labeling. The mean cell viability was 97.7±1.4% (>96%). The mean number of leucocytes injected was 116x106 ±62x106 (>50x106). The mean erythrocyte/WBC ratio was 2.1 ±0.9 (<3) and the removed lymphocyte rate was 97.4±1.6% (>90%). Finally, the three sterility tests were negative and therefore successful.

CONCLUSIONS

Purification of granulocytes with Leukokit® can safely, easily and effectively be performed using a density gradient medium. Moreover, clarification regarding the status of density gradient medium could provide support for its clinical use even if further studies are needed. Since all technical obstacles have been removed, the precautionary principle should apply and lead users to eliminate lymphocytes that are highly radiosensitive cells and whose in vivo fate is uncertain.

In Vivo PET Tracking of 89Zr-Labeled Vγ9Vδ2 T Cells to Mouse Xenograft Breast Tumors Activated with Liposomal Alendronate

Gammadelta T (γδ-T) cells are strong candidates for adoptive immunotherapy in oncology due to their cytotoxicity, ease of expansion, and favorable safety profile. The development of γδ-T cell therapies would benefit from non-invasive cell-tracking methods and increased targeting to tumor sites. Here we report the use of [89Zr]Zr(oxinate)4 to track Vγ9Vδ2 T cells in vivo by positron emission tomography (PET). In vitro, we showed that 89Zr-labeled Vγ9Vδ2 T cells retained their viability, proliferative capacity, and anti-cancer cytotoxicity with minimal DNA damage for amounts of 89Zr ≤20 mBq/cell. Using a mouse xenograft model of human breast cancer, 89Zr-labeled γδ-T cells were tracked by PET imaging over 1 week. To increase tumor antigen expression, the mice were pre-treated with PEGylated liposomal alendronate. Liposomal alendronate, but not placebo liposomes or non-liposomal alendronate, significantly increased the 89Zr signal in the tumors, suggesting increased homing of γδ-T cells to the tumors. γδ-T cell trafficking to tumors occurred within 48 hr of administration. The presence of γδ-T cells in tumors, liver, and spleen was confirmed by histology. Our results demonstrate the suitability of [89Zr]Zr(oxinate)4 as a cell-labeling agent for therapeutic T cells and the potential benefits of liposomal bisphosphonate treatment before γδ-T cell administration.

The Immunoimaging Toolbox

The recent clinical success of cancer immunotherapy has renewed interest in the development of tools to image the immune system. In general, immunotherapies attempt to enable the body's own immune cells to seek out and destroy malignant disease. Molecular imaging of the cells and molecules that regulate immunity could provide unique insight into the mechanisms of action, and failure, of immunotherapies. In this article, we will provide a comprehensive overview of the current state-of-the-art immunoimaging toolbox with a focus on imaging strategies and their applications toward immunotherapy.

Molecular imaging of T cell co-regulator factor B7-H3 with 89Zr-DS-5573a

B7-H3 is a transmembrane protein widely expressed in a variety of cancers and has been shown to play a role in anti-tumor immunity. This study aims to develop a molecular imaging probe to identify B7-H3 expression in tumors and to develop ⁸⁹Zr-DS-5573a as a theranostic that could aid patient selection in clinical Phase I studies. Methods: The anti-B7-H3 humanised monoclonal antibody DS-5573a was labeled with zirconium-89 (⁸⁹Zr-), and assessed for radiochemical purity, immunoreactivity (Lindmo analysis), antigen binding affinity (Scatchard analysis), and serum stability in vitro. In vivo biodistribution and imaging studies were performed with positron emission tomography and magnetic resonance imaging (PET/MRI) studies to identify and quantitate ⁸⁹Zr-DS-5573a tumor uptake in a B7-H3-positive breast cancer model (MDA-MB-231) and a B7-H3-negative murine colon cancer model (CT26). Imaging and biodistribution studies were also performed in MDA-MB-231 tumor-bearing SCID mice in the absence and presence of therapeutic DS-5573a antibody dose (3 mg/kg DS-5573a). Results:⁸⁹Zr-DS-5573a showed high and specific binding to B7-H3-expressing MDA-MB-231 cells (immunoreactivity on day 0, 75.0 ± 2.9%), and low binding to B7-H3-negative CT26 cells (immunoreactivity on day 0, 10.85 ± 0.11%) in vitro. ⁸⁹Zr-DS-5573a demonstrated good serum stability in vitro with 57.2 ± 2.0% of immunoreactivity remaining on day 7. In vivo biodistribution studies showed high uptake of ⁸⁹Zr-DS-5573a in B7-H3-expressing MDA-MB-231 tumor-bearing mice, achieving 32.32 ± 6.55 %ID/g on day 7 post injection in BALB/c nu/nu mice and 25.76 ± 1.79 %ID/g in SCID mice, with minimal evidence of non-specific uptake in normal tissues, and excellent tumor localization on PET/MRI. In a combined imaging/therapy study, receptor saturation was demonstrated in tumors responding to therapy. Conclusion:⁸⁹Zr-DS-5573a demonstrates specific and prolonged targeting of B7-H3-expressing tumors in vivo. Saturation of binding sites was demonstrated in tumors responding to DS-5573a therapy. These results indicate that ⁸⁹Zr-DS-5573a has potential to target B7-H3-expressing tumors in cancer patients. Furthermore ⁸⁹Zr-DS-5573a has the potential to provide important insights into T cell biology through its specific binding to B7-H3.

PET of Adoptively Transferred Chimeric Antigen Receptor T Cells with 89Zr-Oxine

Chimeric antigen receptor (CAR) T cell therapy is a promising clinical approach for reducing tumor progression and prolonging patient survival. However, improvements in both the safety and the potency of CAR T cell therapy demand quantitative imaging techniques to determine the distribution of cells after adoptive transfer. The purpose of this study was to optimize ⁸⁹Zr-oxine labeling of CAR T cells and evaluate PET as a platform for imaging adoptively transferred CAR T cells. Methods: CAR T cells were labeled with 0-1.4 MBq of ⁸⁹Zr-oxine per 10⁶ cells and assessed for radioactivity retention, viability, and functionality. In vivo trafficking of ⁸⁹Zr-oxine-labeled CAR T cells was evaluated in 2 murine xenograft tumor models: glioblastoma brain tumors with intracranially delivered IL13Rα2-targeted CAR T cells, and subcutaneous prostate tumors with intravenously delivered prostate stem cell antigen (PSCA)-targeted CAR T cells. Results: CAR T cells were efficiently labeled (75%) and retained more than 60% of the ⁸⁹Zr over 6 d. In vitro cytokine production, migration, and tumor cytotoxicity, as well as in vivo antitumor activity, were not significantly reduced when labeled with 70 kBq/10⁶ cells. IL13Rα2-CAR T cells delivered intraventricularly were detectable by PET for at least 6 d throughout the central nervous system and within intracranial tumors. When intravenously administered, PSCA-CAR T cells also showed tumor tropism, with a 9-fold greater tumor-to-muscle ratio than for CAR-negative T cells. Conclusion: ⁸⁹Zr-oxine can be used for labeling and imaging CAR T cells while maintaining cell viability and function. On the basis of these studies, we conclude that ⁸⁹Zr-oxine is a clinically translatable platform for real-time assessment of cell therapies.

Noninvasive PET Imaging of T cells

The rapidly evolving field of cancer immunotherapy recently saw the approval of several new therapeutic antibodies. Several cell therapies, for example, chimeric antigen receptor-expressing T cells (CAR-T), are currently in clinical trials for a variety of cancers and other diseases. However, approaches to monitor changes in the immune status of tumors or to predict therapeutic responses are limited. Monitoring lymphocytes from whole blood or biopsies does not provide dynamic and spatial information about T cells in heterogeneous tumors. Positron emission tomography (PET) imaging using probes specific for T cells can noninvasively monitor systemic and intratumoral immune alterations during experimental therapies and may have an important and expanding value in the clinic.

Reporter gene imaging of targeted T cell immunotherapy in recurrent glioma

High-grade gliomas are aggressive cancers that often become rapidly fatal. Immunotherapy using CD8⁺ cytotoxic T lymphocytes (CTLs), engineered to express both herpes simplex virus type 1 thymidine kinase (HSV1-TK) and interleukin-13 (IL-13) zetakine chimeric antigen receptor (CAR), is a treatment strategy with considerable potential. To optimize this and related immunotherapies, it would be helpful to monitor CTL viability and trafficking to glioma cells. We show that noninvasive positron emission tomography (PET) imaging with 9-[4-[¹⁸F]fluoro-3-(hydroxymethyl)butyl]guanine ([¹⁸F]FHBG) can track HSV1-tk reporter gene expression present in CAR-engineered CTLs. [¹⁸F]FHBG imaging was safe and enabled the longitudinal imaging of T cells stably transfected with a PET reporter gene in patients. Further optimization of this imaging approach for monitoring in vivo cell trafficking should greatly benefit various cell-based therapies for cancer.

Advances in PET Detection of the Antitumor T Cell Response

Positron emission tomography (PET) is a powerful noninvasive imaging technique able to measure distinct biological processes in vivo by administration of a radiolabeled probe. Whole-body measurements track the probe accumulation providing a means to measure biological changes such as metabolism, cell location, or tumor burden. PET can also be applied to both preclinical and clinical studies providing three-dimensional information. For immunotherapies (in particular understanding T cell responses), PET can be utilized for spatial and longitudinal tracking of T lymphocytes. Although PET has been utilized clinically for over 30 years, the recent development of additional PET radiotracers have dramatically expanded the use of PET to detect endogenous or adoptively transferred T cells in vivo. Novel probes have identified changes in T cell quantity, location, and function. This has enabled investigators to track T cells outside of the circulation and in hematopoietic organs such as spleen, lymph nodes, and bone marrow, or within tumors. In this review, we cover advances in PET detection of the antitumor T cell response and areas of focus for future studies.

(89)Zr-Oxine Complex PET Cell Imaging in Monitoring Cell-based Therapies

PURPOSE

To develop a clinically translatable method of cell labeling with zirconium 89 ((89)Zr) and oxine to track cells with positron emission tomography (PET) in mouse models of cell-based therapy.

MATERIALS AND METHODS

This study was approved by the institutional animal care committee. (89)Zr-oxine complex was synthesized in an aqueous solution. Cell labeling conditions were optimized by using EL4 mouse lymphoma cells, and labeling efficiency was examined by using dendritic cells (DCs) (n = 4), naïve (n = 3) and activated (n = 3) cytotoxic T cells (CTLs), and natural killer (NK) (n = 4), bone marrow (n = 4), and EL4 (n = 4) cells. The effect of (89)Zr labeling on cell survival, proliferation, and function were evaluated by using DCs (n = 3) and CTLs (n = 3). Labeled DCs (444-555 kBq/[5 × 10(6)] cells, n = 5) and CTLs (185 kBq/[5 × 10(6)] cells, n = 3) transferred to mice were tracked with microPET/CT. In a melanoma immunotherapy model, tumor targeting and cytotoxic function of labeled CTLs were evaluated with imaging (248.5 kBq/[7.7 × 10(6)] cells, n = 4) and by measuring the tumor size (n = 6). Two-way analysis of variance was used to compare labeling conditions, the Wilcoxon test was used to assess cell survival and proliferation, and Holm-Sidak multiple tests were used to assess tumor growth and perform biodistribution analyses.

RESULTS

(89)Zr-oxine complex was synthesized at a mean yield of 97.3% ± 2.8 (standard deviation). It readily labeled cells at room temperature or 4°C in phosphate-buffered saline (labeling efficiency range, 13.0%-43.9%) and was stably retained (83.5% ± 1.8 retention on day 5 in DCs). Labeling did not affect the viability of DCs and CTLs when compared with nonlabeled control mice (P > .05), nor did it affect functionality. (89)Zr-oxine complex enabled extended cell tracking for 7 days. Labeled tumor-specific CTLs accumulated in the tumor (4.6% on day 7) and induced tumor regression (P < .05 on day 7).

CONCLUSION

We have developed a (89)Zr-oxine complex cell tracking technique for use with PET that is applicable to a broad range of cell types and could be a valuable tool with which to evaluate various cell-based therapies.

Fluorescence-guided development of a tricistronic vector encoding bimodal optical and nuclear genetic reporters for in vivo cellular imaging

Background

In vivo imaging using genetic reporters is a central supporting tool in the development of cell and gene therapies affording us the ability to selectively track the therapeutic indefinitely. Previous studies have demonstrated the utility of the human norepinephrine transporter (hNET) as a positron emission tomography/single photon emission computed tomography (PET/SPECT) genetic reporter for in vivo cellular imaging. Here, our aim was to extend on this work and construct a tricistronic vector with dual optical (firefly luciferase) and nuclear (hNET) in vivo imaging and ex vivo histochemical capabilities. Guiding this development, we describe how a fluorescent substrate for hNET, 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP⁺), can be used to optimise vector design and serve as an in vitro functional screen.

Methods

Vectors were designed to co-express a bright red-shifted firefly luciferase (FLuc), hNET and a small marker gene RQR8. Genes were co-expressed using 2A peptide linkage, and vectors were transduced into a T cell line, SupT1. Two vectors were constructed with different gene orientations; FLuc.2A.RQR8.2A.hNET and hNET.2A.FLuc.2A.RQR8. hNET function was assessed using ASP⁺-guided flow cytometry. In vivo cellular conspicuity was confirmed using sequential bioluminescence imaging (BLI) and SPECT imaging of transduced SupT1 cells injected into the flanks of mice.

Results

SupT1/FLuc.2A.RQR8.2A.hNET cells resulted in >4-fold higher ASP⁺ uptake compared to SupT1/hNET.2A.FLuc.2A.RQR8, suggesting that 2A orientation effected hNET function. SupT1/FLuc.2A.RQR8.2A.hNET cells were readily visualised with both BLI and SPECT, demonstrating high signal to noise at 24 h post ¹²³I-meta-iodobenzylguanidine (MIBG) administration.

Conclusions

In this study, a pre-clinical tricistronic vector with flow cytometry, BLI, SPECT and histochemical capabilities was constructed, which can be widely applied in cell tracking studies supporting the development of cell therapies. The study further demonstrates that hNET function in engineered cells can be assessed using ASP⁺-guided flow cytometry in place of costly radiosubstrate methodologies. This fluorogenic approach is unique to the hNET PET/SPECT reporter and may prove valuable when screening large numbers of cell lines or vector/mutant constructs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0097-z) contains supplementary material, which is available to authorized users.

64Cu antibody-targeting of the T-cell receptor and subsequent internalization enables in vivo tracking of lymphocytes by PET

T cells are key players in inflammation, autoimmune diseases, and immunotherapy. Thus, holistic and noninvasive in vivo characterizations of the temporal distribution and homing dynamics of lymphocytes in mammals are of special interest. Herein, we show that PET-based T-cell labeling facilitates quantitative, highly sensitive, and holistic monitoring of T-cell homing patterns in vivo. We developed a new T-cell receptor (TCR)-specific labeling approach for the intracellular labeling of mouse T cells. We found that continuous TCR plasma membrane turnover and the endocytosis of the specific (64)Cu-monoclonal antibody (mAb)-TCR complex enables a stable labeling of T cells. The TCR-mAb complex was internalized within 24 h, whereas antigen recognition was not impaired. Harmful effects of the label on the viability, DNA-damage and apoptosis-necrosis induction, could be minimized while yielding a high contrast in in vivo PET images. We were able to follow and quantify the specific homing of systemically applied (64)Cu-labeled chicken ovalbumin (cOVA)-TCR transgenic T cells into the pulmonary and perithymic lymph nodes (LNs) of mice with cOVA-induced airway delayed-type hypersensitivity reaction (DTHR) but not into pulmonary and perithymic LNs of naïve control mice or mice diseased from turkey or pheasant OVA-induced DTHR. Our protocol provides consequent advancements in the detection of small accumulations of immune cells in single LNs and specific homing to the sites of inflammation by PET using the internalization of TCR-specific mAbs as a specific label of T cells. Thus, our labeling approach is applicable to other cells with constant membrane receptor turnover.

Molecular imaging in tracking tumor-specific cytotoxic T lymphocytes (CTLs)

Despite the remarkable progress of adoptive T cell therapy in cancer treatment, there remains an urgent need for the noninvasive tracking of the transfused T cells in patients to determine their biodistribution, viability, and functionality. With emerging molecular imaging technologies and cell-labeling methods, noninvasive in vivo cell tracking is experiencing impressive progress toward revealing the mechanisms and functions of these cells in real time in preclinical and clinical studies. Such cell tracking methods have an important role in developing effective T cell therapeutic strategies and steering decision-making process in clinical trials. On the other hand, they could provide crucial information to accelerate the regulatory approval process on the T cell therapy. In this review, we revisit the advances in tracking the tumor-specific CTLs, highlighting the latest development in human studies and the key challenges.

SPECT- and PET-based approaches for noninvasive diagnosis of acute renal allograft rejection

Molecular imaging techniques such as single photon emission computed tomography (SPECT) or positron emission tomography are promising tools for noninvasive diagnosis of acute allograft rejection (AR). Given the importance of renal transplantation and the limitation of available donors, detailed analysis of factors that affect transplant survival is important. Episodes of acute allograft rejection are a negative prognostic factor for long-term graft survival. Invasive core needle biopsies are still the “goldstandard” in rejection diagnostics. Nevertheless, they are cumbersome to the patient and carry the risk of significant graft injury. Notably, they cannot be performed on patients taking anticoagulant drugs. Therefore, a noninvasive tool assessing the whole organ for specific and fast detection of acute allograft rejection is desirable. We herein review SPECT- and PET-based approaches for noninvasive molecular imaging-based diagnostics of acute transplant rejection.

(111)Indium-oxine labelling for evaluating the homing process of autologous osteoblasts implanted percutaneously in atrophic nonunion fractures

PURPOSE

The aim of the study was to control the in vivo localisation of implanted cells in cell-based therapies. Labelling cells with (111)indium-oxine is one of the most interesting methods proposed. We evaluated this method in the setting of autologous osteoblast implantation in nonunion fractures.

METHODS

An in vitro study of osteoblasts was conducted after (111)indium-oxine labelling. Radioactivity retention and viability, proliferation and the ability to produce alkaline phosphatase were evaluated in a seven-day culture. In vivo labelling of implanted osteoblastic cells was conducted during a therapeutic trial of atrophic nonunion fractures, with the leakage outside the nonunion site and local uptake evolution at four, 24 and 48 hour being studied.

RESULTS

The mean labelling efficiency for osteoprogenitors was 78.8 ± 4.6 %. The intracellular retention was 89.4 ± 2.1 % at three hours and 67.3 ± 4.7 % at 18 hours. The viability assessed at three hours was 93.7 ± 0.6 %. After seven days of culture, morphology and alkaline phosphatase staining were similar for both labelled and unlabelled control cells, although the proliferation rate was decreased in the labelled cells. Some local intraosseous leakage was observed in four of 17 cases. All patients showed uptake at the injection site, with four having no other uptake. Four patients showed additional uptake in the bladder, liver and spleen, while 11 patients had additional uptake in the lungs in addition to the bladder, liver and spleen. The activity ratios (injection site/body) were 48 ± 28 % at four hours, 40 ± 25 % at 24 hours and 35 ± 25 % at 48 hours. After correcting for decay, the activity within the injection site was 82 ± 15 % at 24 hours and 69 ± 11 % at 48 hours compared with the activity measured at four hours. No relationship was found between uptake and radiological bone repair.

CONCLUSIONS

The (111)indium-oxine labelling appears to be a good method for monitoring the behaviour of the osteoblastic cells after their implantation in atrophic nonunion fractures.

Comparison of cell-labeling methods with ¹²⁴I-FIAU and ⁶⁴Cu-PTSM for cell tracking using chronic myelogenous leukemia cells expressing HSV1-tk and firefly luciferase

Cell-tracking methods with molecular-imaging modality can monitor the biodistribution of cells. In this study, the direct-labeling method with ⁶⁴Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (⁶⁴Cu-PTSM), indirect cell-labeling methods with herpes simplex virus type 1-thymidine kinase (HSV1-tk)-mediated ¹²⁴I-2'-fluoro-2'-deoxy-1-β-D-arabinofuranosyl-5-iodouracil (¹²⁴I-FIAU) were comparatively investigated in vitro and in vivo for tracking of human chronic myelogenous leukemia cells. K562-TL was established by retroviral transduction of the HSV1-tk and firefly luciferase gene in the K562 cell. K562-TL cells were labeled with ⁶⁴Cu-PTSM or ¹²⁴I-FIAU. Cell labeling efficiency, viability, and radiolabels retention were compared in vitro. The biodistribution of radiolabeled K562-TL cells with each radiolabel and small-animal positron emission tomography imaging were performed. Additionally, in vivo and ex vivo bioluminescence imaging (BLI) and tissue reverse transcriptase-polymerase chain reaction (RT-PCR) analysis were used for confirming those results. K562-TL cells were efficiently labeled with both radiolabels. The radiolabel retention (%) of ¹²⁴I-FIAU (95.2%±1.1%) was fourfold higher than ⁶⁴Cu-PTSM (23.6%±0.7%) at 24 hours postlabeling. Viability of radiolabeled cells was statistically nonsignificant between ¹²⁴I-FIAU and ⁶⁴Cu-PTSM. The radioactivity of each radiolabeled cells was predominantly accumulated in the lungs and liver at 2 hours. Both the radioactivity of ⁶⁴Cu-PTSM- and ¹²⁴I-FIAU-labeled cells was highly accumulated in the liver at 24 hours. However, the radioactivity of ¹²⁴I-FIAU-labeled cells was markedly decreased from the body at 24 hours. The K562-TL cells were dominantly localized in the lungs and liver, which also verified by BLI and RT-PCR analysis at 2 and 24 hours postinjection. The ⁶⁴Cu-PTSM-labeled cell-tracking method is more efficient than ¹²⁴I-FIAU-labeled cell tracking, because of markedly decrease of radioactivity and fast efflux of ¹²⁴I-FIAU in vivo. In spite of a high labeling yield and radiolabel retention of ¹²⁴I-FIAU in vitro, the in vivo cell-tracking method using ⁶⁴Cu-PTSM could be a useful method to evaluate the distribution and targeting of various cell types, especially, stem cells and immune cells.

Mesenchymal stem cells impair in vivo T-cell priming by dendritic cells

Dendritic cells (DC) are highly specialized antigen-presenting cells characterized by the ability to prime T-cell responses. Mesenchymal stem cells (MSC) are adult stromal progenitor cells displaying immunomodulatory activities including inhibition of DC maturation in vitro. However, the specific impact of MSC on DC functions, upon in vivo administration, has never been elucidated. Here we show that murine MSC impair Toll-like receptor-4 induced activation of DC resulting in the inhibition of cytokines secretion, down-regulation of molecules involved in the migration to the lymph nodes, antigen presentation to CD4(+) T cells, and cross-presentation to CD8(+) T cells. These effects are associated with the inhibition of phosphorylation of intracellular mitogen-activated protein kinases. Intravenous administration of MSC decreased the number of CCR7 and CD49dβ1 expressing CFSE-labeled DC in the draining lymph nodes and hindered local antigen priming of DO11.10 ovalbumin-specific CD4(+) T cells. Upon labeling of DC with technetium-99m hexamethylpropylene amine oxime to follow their in vivo biodistribution, we demonstrated that intravenous injection of MSC blocks, almost instantaneously, the migration of subcutaneously administered ovalbumin-pulsed DC to the draining lymph nodes. These findings indicate that MSC significantly affect DC ability to prime T cells in vivo because of their inability to home to the draining lymph nodes and further confirm MSC potentiality as therapy for immune-mediated diseases.

Randomized transcoronary delivery of CD34(+) cells with perfusion versus stop-flow method in patients with recent myocardial infarction: Early cardiac retention of ⁹⁹(m)Tc-labeled cells activity

BACKGROUND

For transcoronary progenitor cells' administration, injections under flow arrest (over-the-wire balloon technique, OTW) are used universally despite lack of evidence for being required for cell delivery or being effective in stimulating myocardial engraftment. Flow-mediated endothelial rolling is mandatory for subsequent cell adhesion and extravasation.

METHODS

To optimize cell directing toward the coronary endothelium under maintained flow, the authors developed a cell-delivery side-holed perfusion catheter (PC). Thirty-four patients (36-69 years, 30 men) with primary stent-assisted angioplasty-treated anterior MI (peak TnI 151 [53-356]ng/dL, mean[range]) were randomly assigned to OTW or PC autologous ⁹⁹Tc-extametazime-labeled bone marrow CD34(+) cells (4.34 [0.92-7.54] × 10⁶) administration at 6-14 days after pPCI (LVEF 37.1 [24-44]%). Myocardial perfusion (⁹⁹(m)Tc-MIBI) and labeled cells' activity were evaluated (SPECT) at, respectively, 36-48 h prior to and 60 min after delivery.

RESULTS

In contrast to OTW coronary occlusions, no intolerance or ventricular arrhythmia occurred with PC cells' administration (P < .001). One hour after delivery, 4.86 [1.7-7.6]% and 5.05 [2.2-9.9]% activity was detected in the myocardium (OTW and PC, respectively, P = .84). Labeled cell activity was clearly limited to the (viable) peri-infarct zone in 88% patients, indicating that the infarct core zone may be largely inaccessible to transcoronary-administered cells.

CONCLUSIONS

Irrespective of the transcoronary delivery method, only ≈ 5% of native (i.e., non-engineered) CD34(+) cells spontaneously home to the injured myocardium, and cell retention occurs preferentially in the viable peri-infarct zone. Although the efficacy of cell delivery is not increased with the perfusion method, by avoiding provoking ischemic episodes PC offers a rational alternative to the OTW delivery.

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.

Brain ingress of regulatory T cells in a murine model of HIV-1 encephalitis

CD4+CD25+ regulatory T cells (Treg) transform the HIV-1 infected macrophage from a neurotoxic to a neuroprotective phenotype. This was demonstrated previously in a murine model of HIV-1 encephalitis induced by intracranial injection of HIV-1/vesicular stomatitis virus-infected bone marrow macrophages. In this report, relationships between Treg ingress of end organ tissues, notably the brain, and neuroprotection were investigated. Treg from EGFP-transgenic donor mice were expanded, labeled with indium-111, and adoptively transferred. Treg distribution was assayed by single photon emission computed tomography and immunohistochemistry. Treg readily migrated across the blood brain barrier and were retained within virus-induced neuroinflammatory sites. In non-inflamed peripheral tissues (liver and spleen) Treg were depleted. These observations demonstrate that Treg migrate to sites of inflammation where they modulate immune responses.

Cell tracking and therapy evaluation of bone marrow monocytes and stromal cells using SPECT and CMR in a canine model of myocardial infarction

BACKGROUND

The clinical application of stem cell therapy for myocardial infarction will require the development of methods to monitor treatment and pre-clinical assessment in a large animal model, to determine its effectiveness and the optimum cell population, route of delivery, timing, and flow milieu.

OBJECTIVES

To establish a model for a) in vivo tracking to monitor cell engraftment after autologous transplantation and b) concurrent measurement of infarct evolution and remodeling.

METHODS

We evaluated 22 dogs (8 sham controls, 7 treated with autologous bone marrow monocytes, and 7 with stromal cells) using both imaging of 111Indium-tropolone labeled cells and late gadolinium enhancement CMR for up to12 weeks after a 3 hour coronary occlusion. Hearts were also examined using immunohistochemistry for capillary density and presence of PKH26 labeled cells.

RESULTS

In vivo Indium imaging demonstrated an effective biological clearance half-life from the injection site of ~5 days. CMR demonstrated a pattern of progressive infarct shrinkage over 12 weeks, ranging from 67-88% of baseline values with monocytes producing a significant treatment effect. Relative infarct shrinkage was similar through to 6 weeks in all groups, following which the treatment effect was manifest. There was a trend towards an increase in capillary density with cell treatment.

CONCLUSION

This multi-modality approach will allow determination of the success and persistence of engraftment, and a correlation of this with infarct size shrinkage, regional function, and left ventricular remodeling. There were overall no major treatment effects with this particular model of transplantation immediately post-infarct.

One-day kinetics of myocardial engraftment after intracoronary injection of bone marrow mononuclear cells in patients with acute and chronic myocardial infarction

OBJECTIVE

To investigate the kinetics of myocardial engraftment of bone marrow-derived mononuclear cells (BMNCs) after intracoronary injection using 99mTc-d,l-hexamethylpropylene amine oxime (99mTc-HMPAO) nuclear imaging in patients with acute and chronic anterior myocardial infarction.

DESIGN

Nuclear imaging-derived tracking of BMNCs at 2 and 20 h after injection in the left anterior descending (LAD) coronary artery.

SETTING

Academical cardiocentre.

PATIENTS

Five patients with acute (mean (SD) age 58 (11) years; ejection fraction range 33-45%) and five patients with chronic (mean (SD) age 50 (6) years; ejection fraction range 28-34%) anterior myocardial infarction.

INTERVENTIONS

A total of 24.2 x 10(8)-57.0 x 10(8) BMNCs (20% labelled with 700-1000 MBq 99mTc-HMPAO) were injected in the LAD coronary artery.

RESULTS

At 2 h after BMNC injection, myocardial activity was observed in all patients with acute (range 1.31-5.10%) and in all but one patient with chronic infarction (range 1.10-3.0%). At 20 h, myocardial engraftment was noted only in three patients with acute myocardial infarction, whereas no myocardial activity was noted in any patient with chronic infarction.

CONCLUSIONS

Engraftment of BMNCs shows dynamic changes within the first 20 h after intracoronary injection. Persistent myocardial engraftment was noted only in a subset of patients with acute myocardial infarction.

Distribution of 99mTc-labeled lymphocytes in control and inflamed rats

We studied the in vivo fate of CD8(+) lymphocytes, of naive (CD8(+)CD45RC(bright)) or memory (CD8(+)CD45RC(dim)) phenotype, injected in syngeneic rats, after their sorting and labeling with [(99m)Tc] HM-PAO. By using the scintigrafic method we showed that memory CD8(+) lymphocytes were able to recirculate into liver and lungs. The same method was also successfully used to in vivo study the homing of total blood lymphocytes obtained from inflamed rats.

Monitoring of a new approach of immunotherapy with allogenic (111)In-labelled NK cells in patients with renal cell carcinoma

The transfusion of allogenic, in vitro expanded natural killer cells (NKC) is a novel therapy option in oncology. To date, however, the biodistribution and kinetics of allogenic NKC have not been investigated. Therefore, in this study three patients with renal cell carcinoma received 3-7 x 10(8) NKC labelled with indium-111 oxine with a tenfold excess of unlabelled cells during NKC therapy. Whole-body scintigrams were obtained (0.5-144 h) in the anterior and posterior views. Scintigrams were analysed using a region of interest technique, and single-photon emission tomography (SPET) studies of the abdomen were performed. Results were compared to those obtained with polymerase chain reaction (PCR) of the peripheral blood (determination of foreign DNA, nested PCR, limit of detection 0.01%). Shortly after transfusion of NKC, more than 50% of the activity was accumulated in the lungs. We observed redistribution effects from lungs to liver, spleen and bone marrow. No significant loss of activity could be detected. In two of four large metastases, tracer accumulation could be proven by SPET. As confirmed by scintigrams and PCR, the fraction of circulating transfused cells was low at all times. Long-term activity retention might be caused either by survival of the allogenic cells, as confirmed by PCR (up to 3 days p.i.), or by phagocytosis of labelled cellular fragments. However, PCR data and uptake in metastases indicated long survival of a portion of allogenic NKC. Such long survival and low retention of the cells in the lung are requirements for an effective immunotherapeutic approach.

Ex vivo cell labeling with 64Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) for imaging cell trafficking in mice with positron-emission tomography

We have used copper-64-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (64Cu-PTSM) to radiolabel cells ex vivo for in vivo positron-emission tomography (PET) imaging studies of cell trafficking in mice and for eventual application in patients. 2-[18F]-Fluoro-2-deoxy-d-glucose (FDG) cell labeling also was evaluated for comparison. 64Cu-PTSM uptake by C6 rat glioma (C6) cells increased for 180 min and then stabilized. The labeling efficiency was directly proportional to 64Cu-PTSM concentration and influenced negatively by serum. Label uptake per cell was greater with 64Cu-PTSM than with FDG. However, both 64Cu-PTSM- and FDG-labeled cells showed efflux of cell activity into supernatant. The 64Cu-PTSM labeling procedure did not interfere significantly with C6 cell viability and proliferation rate. MicroPET images of living mice indicate that tail-vein-injected labeled C6 cells traffic to the lungs and liver. In addition, transient splenic accumulation of radioactivity was clearly detectable in a mouse scanned at 3.33 h postinfusion of 64Cu-PTSM-labeled lymphocytes. In contrast, the liver was the principal organ of tracer localization after tail-vein administration of 64Cu-PTSM alone. These results indicate that in vivo imaging of cell trafficking is possible with 64Cu-PTSM-labeled cells. Given the longer t(1/2) of 64Cu (12.7 h) relative to 18F (110 min), longer cell-tracking periods (up to 24-36 h) should be possible now with PET.