Quantitation and visualization of tumor-specific T cells in the secondary lymphoid organs during and after tumor elimination by PET

Positron Emission Tomography Probes for Imaging Cytotoxic Immune Cells

Non-invasive positron emission tomography (PET) imaging of immune cells is a powerful approach for monitoring the dynamics of immune cells in response to immunotherapy. Despite the clinical success of many immunotherapeutic agents, their clinical efficacy is limited to a subgroup of patients. Conventional imaging, as well as analysis of tissue biopsies and blood samples do not reflect the complex interaction between tumour and immune cells. Consequently, PET probes are being developed to capture the dynamics of such interactions, which may improve patient stratification and treatment evaluation. The clinical efficacy of cancer immunotherapy relies on both the infiltration and function of cytotoxic immune cells at the tumour site. Thus, various immune biomarkers have been investigated as potential targets for PET imaging of immune response. Herein, we provide an overview of the most recent developments in PET imaging of immune response, including the radiosynthesis approaches employed in their development.

Role of noninvasive molecular imaging in determining response

The intersection of immunotherapy and radiation oncology is a rapidly evolving area of preclinical and clinical investigation. The strategy of combining radiation and immunotherapy to enhance local and systemic antitumor immune responses is intriguing yet largely unproven in the clinical setting because the mechanisms of synergy and the determinants of therapeutic response remain undefined. In recent years, several noninvasive molecular imaging approaches have emerged as a platform to interrogate the tumor immune microenvironment. These tools have the potential to serve as robust biomarkers for cancer immunotherapy and may hold several advantages over conventional anatomic imaging modalities and contemporary invasive tissue acquisition techniques. Given the key and expanding role of precision imaging in radiation oncology for patient selection, target delineation, image guided treatment delivery, and response assessment, noninvasive molecular-specific imaging may be uniquely suited to evaluate radiation/immunotherapy combinations. Herein, we describe several experimental imaging-based strategies that are currently being explored to characterize in vivo immune responses, and we review a growing body of preclinical data and nascent clinical experience with immuno-positron emission tomography molecular imaging as a putative biomarker for cancer immunotherapy. Finally, we discuss practical considerations for clinical translation to implement noninvasive molecular imaging of immune checkpoint molecules, immune cells, or associated elements of the antitumor immune response with a specific emphasis on its potential application at the interface of radiation oncology and immuno-oncology.

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.

Imaging techniques: new avenues in cancer gene and cell therapy

Cancer is one of the world's most concerning health problems and poses many challenges in the range of approaches associated with the treatment of cancer. Current understanding of this disease brings to the fore a number of novel therapies that can be useful in the treatment of cancer. Among them, gene and cell therapies have emerged as novel and effective approaches. One of the most important challenges for cancer gene and cell therapies is correct monitoring of the modified genes and cells. In fact, visual tracking of therapeutic cells, immune cells, stem cells and genetic vectors that contain therapeutic genes and the various drugs is important in cancer therapy. Similarly, molecular imaging, such as nanosystems, fluorescence, bioluminescence, positron emission tomography, single photon-emission computed tomography and magnetic resonance imaging, have also been found to be powerful tools in monitoring cancer patients who have received therapeutic cell and gene therapies or drug therapies. In this review, we focus on these therapies and their molecular imaging techniques in treating and monitoring the progress of the therapies on various types of cancer.

Increased Understanding of Stem Cell Behavior in Neurodegenerative and Neuromuscular Disorders by Use of Noninvasive Cell Imaging

Numerous neurodegenerative and neuromuscular disorders are associated with cell-specific depletion in the human body. This imbalance in tissue homeostasis is in healthy individuals repaired by the presence of endogenous stem cells that can replace the lost cell type. However, in most disorders, a genetic origin or limited presence or exhaustion of stem cells impairs correct cell replacement. During the last 30 years, methods to readily isolate and expand stem cells have been developed and this resulted in a major change in the regenerative medicine field as it generates sufficient amount of cells for human transplantation applications. Furthermore, stem cells have been shown to release cytokines with beneficial effects for several diseases. At present however, clinical stem cell transplantations studies are struggling to demonstrate clinical efficacy despite promising preclinical results. Therefore, to allow stem cell therapy to achieve its full potential, more insight in their in vivo behavior has to be achieved. Different methods to noninvasively monitor these cells have been developed and are discussed. In some cases, stem cell monitoring even reached the clinical setting. We anticipate that by further exploring these imaging possibilities and unraveling their in vivo behavior further improvement in stem cell transplantations will be achieved.

A model for effects of adaptive immunity on tumor response to chemotherapy and chemoimmunotherapy

Complete clinical regressions of solid tumors in response to chemotherapy are difficult to explain by direct cytotoxicity alone, because of low growth fractions and obstacles to drug delivery. A plausible indirect mechanism that might reconcile this is the action of the immune system. A model for interaction between tumors and the adaptive immune system is presented here, and used to examine controllability of tumors through the interplay of cytotoxic, cytostatic and immunogenic effects of chemotherapy and the adaptive immune response. The model includes cytotoxic and helper T cells, T regulatory cells (Tregs), dendritic cells, memory cells, and several key cytokines. Nearly all parameter estimates are derived from experimental and clinical data. Individual tumors are characterized by two parameters: growth rate and antigenicity, and regions of tumor control are identified in this parameter space. The model predicts that inclusion of the immune response significantly expands the region of tumor control for both cytostatic and cytotoxic chemotherapies. Moreover, outside the control zone, tumor growth is delayed significantly. An optimal fractionation schedule is predicted, for a fixed cumulative dose. The model further predicts expanded regions of tumor control when several forms of immunotherapy (adoptive T cell transfer, Treg depletion, and dendritic cell vaccination) are combined with chemotherapy. Outcomes depend greatly on tumor characteristics, the schedule of administration, and the type of immunotherapy chosen, suggesting promising opportunities for personalized medicine. Overall, the model provides insight into the role of the adaptive immune system in chemotherapy, and how scheduling and immunotherapeutic interventions might improve efficacy.

Immuno-PET of Murine T Cell Reconstitution Postadoptive Stem Cell Transplantation Using Anti-CD4 and Anti-CD8 Cys-Diabodies

The proliferation and trafficking of T lymphocytes in immune responses are crucial events in determining inflammatory responses. To study whole-body T lymphocyte dynamics noninvasively in vivo, we generated anti-CD4 and -CD8 cys-diabodies (cDbs) derived from the parental antibody hybridomas GK1.5 and 2.43, respectively, for (89)Zr-immuno-PET detection of helper and cytotoxic T cell populations.

METHODS

Anti-CD4 and -CD8 cDbs were engineered, produced via mammalian expression, purified using immobilized metal affinity chromatography, and characterized for T cell binding. The cDbs were site-specifically conjugated to maleimide-desferrioxamine for (89)Zr radiolabeling and subsequent small-animal PET/CT acquisition and ex vivo biodistribution in both wild-type mice and a model of hematopoietic stem cell (HSC) transplantation.

RESULTS

Immuno-PET and biodistribution studies demonstrate targeting and visualization of CD4 and CD8 T cell populations in vivo in the spleen and lymph nodes of wild-type mice, with specificity confirmed through in vivo blocking and depletion studies. Subsequently, a murine model of HSC transplantation demonstrated successful in vivo detection of T cell repopulation at 2, 4, and 8 wk after HSC transplantation using the (89)Zr-radiolabeled anti-CD4 and -CD8 cDbs.

CONCLUSION

These newly developed anti-CD4 and -CD8 immuno-PET reagents represent a powerful resource to monitor T cell expansion, localization, and novel engraftment protocols. Future potential applications of T cell-targeted immuno-PET include monitoring immune cell subsets in response to immunotherapy, autoimmunity, and lymphoproliferative disorders, contributing overall to preclinical immune cell monitoring.

Engineered antibody fragments for immuno-PET imaging of endogenous CD8+ T cells in vivo

The noninvasive detection and quantification of CD8(+) T cells in vivo are important for both the detection and staging of CD8(+) lymphomas and for the monitoring of successful cancer immunotherapies, such as adoptive cell transfer and antibody-based immunotherapeutics. Here, antibody fragments are constructed to target murine CD8 to obtain rapid, high-contrast immuno-positron emission tomography (immuno-PET) images for the detection of CD8 expression in vivo. The variable regions of two anti-murine CD8-depleting antibodies (clones 2.43 and YTS169.4.2.1) were sequenced and reformatted into minibody (Mb) fragments (scFv-CH3). After production and purification, the Mbs retained their antigen specificity and bound primary CD8(+) T cells from the thymus, spleen, lymph nodes, and peripheral blood. Importantly, engineering of the parental antibodies into Mbs abolished the ability to deplete CD8(+) T cells in vivo. The Mbs were subsequently conjugated to S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid for (64)Cu radiolabeling. The radiotracers were injected i.v. into antigen-positive, antigen-negative, immunodeficient, antigen-blocked, and antigen-depleted mice to evaluate specificity of uptake in lymphoid tissues by immuno-PET imaging and ex vivo biodistribution. Both (64)Cu-radiolabeled Mbs produced high-contrast immuno-PET images 4 h postinjection and showed specific uptake in the spleen and lymph nodes of antigen-positive mice.

Chemical Approach to a Whole Body Imaging of Sialo-N-Linked Glycans

PET and noninvasive fluorescence imaging of the sialo-N-linked glycan derivatives are described. To establish the efficient labeling protocol for N-glycans and/or glycoconjugates, new labeling probes of fluorescence and ⁶⁸Ga-DOTA, as the positron emission nucleus for PET, through rapid 6π-azaelectrocyclization were designed and synthesized, (E)-ester aldehydes. The high reactivity of these probes enabled the labeling of lysine residues in peptides, proteins, and even amino groups on the cell surfaces at very low concentrations of the target molecules (~10⁻⁸ M) within a short reaction time (~5 min) to result in "selective" and "non-destructive" labeling of the more accessible amines. The first MicroPET of glycoproteins, ⁶⁸Ga-DOTA-orosomucoid and asialoorosomucoid, successfully visualized the differences in the circulatory residence of glycoproteins, in the presence or absence of sialic acids. In vivo dynamics of the new N-glycoclusters, prepared by the "self-activating" Huisgen cycloaddition reaction, could also be affected significantly by their partial structures at the non-reducing end, i.e., the presence or absence of sialic acids, and/or sialoside linkages to galactose. Azaelectrocyclization chemistry is also applicable to the engineering of the proteins and/or the cell surfaces by the oligosaccharides; lymphocytes chemically engineered by sialo-N-glycan successfully target the tumor implanted in BALB/C nude mice, detected by noninvasive fluorescence imaging.

Imaging of genetically engineered T cells by PET using gold nanoparticles complexed to Copper-64

Adoptive transfer of primary T cells genetically modified to have desired specificity can exert an anti-tumor response in some patients. To improve our understanding of their therapeutic potential we have developed a clinically-appealing approach to reveal their in vivo biodistribution using nanoparticles that serve as a radiotracer for imaging by positron emission tomography (PET). T cells electroporated with DNA plasmids from the Sleeping Beauty transposon-transposase system to co-express a chimeric antigen receptor (CAR) specific for CD19 and Firefly luciferase (ffLuc) were propagated on CD19(+) K562-derived artificial antigen presenting cells. The approach to generating our clinical-grade CAR(+) T cells was adapted for electro-transfer of gold nanoparticles (GNPs) functionalized with (64)Cu(2+) using the macrocyclic chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, DOTA) and polyethyleneglycol (GNP-(64)Cu/PEG2000). MicroPET/CT was used to visualize CAR(+)EGFPffLucHyTK(+)GNP-(64)Cu/PEG2000(+) T cells and correlated with bioluminescence imaging. These data demonstrate that GNPs conjugated with (64)Cu(2+) can be prepared as a radiotracer for PET and used to image T cells using an approach that has translational implications.

Therapeutic sentinel lymph node imaging

Improving existing means of sentinel lymph node identification in non-small cell lung cancer will allow for molecular detection of occult micrometastases that may cause recurrence in early stage non-small cell lung cancer. Furthermore, targeted application of chemical and biological cytotoxic agents can potentially improve outcomes in patients with lymph node (LN) metastases. "Therapeutic Sentinel Lymph Node Imaging" incorporates these modalities into a single agent thereby identifying which LNs harbor tumor cells and simultaneously eradicating metastatic disease. In this review, we summarize the novel preclinical agents for identification and treatment of tumor bearing LNs and discuss their potential for clinical translation.

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.

Imaging the immune response to monitor tumor immunotherapy

The goal of cancer immunotherapy is to promote antitumor immunity, and novel approaches include vaccination, adoptive transfer of tumor-reactive T cells, and administration of monoclonal antibodies and small molecules that target immune regulatory pathways. The molecular and cellular events responsible for tumor rejection are not completely defined and correlative studies have been used to help understand the mechanisms and extent of immune activation and tumor regression with these approaches. The real-time monitoring of immune responses to immunotherapy has been challenging as specific cell subsets may be difficult to define, and molecular pathways have evolved functionally diverse outcomes in different cells and in different tissues. Recently, improvements in optics and digital imaging have led to novel imaging techniques that make it possible to track the migration of individual immune cells ex vivo and in vivo, and to detect the dynamic interactions between T cells and antigen-presenting cells or tumor cells within complex microenvironments, including lymphoid tissue and established tumors. This review will explain some of the more established imaging techniques and discuss their role in monitoring the immune response in patients treated with various tumor immunotherapy approaches.

Rapid dissemination of Francisella tularensis and the effect of route of infection

Background

Francisella tularensis subsp. tularensis is classified as a Category A bioweapon that is capable of establishing a lethal infection in humans upon inhalation of very few organisms. However, the virulence mechanisms of this organism are not well characterized. Francisella tularensis subsp. novicida, which is an equally virulent subspecies in mice, was used in concert with a microPET scanner to better understand its temporal dissemination in vivo upon intranasal infection and how such dissemination compares with other routes of infection. Adult mice were inoculated intranasally with F. tularensis subsp. novicida radiolabeled with ⁶⁴Cu and imaged by microPET at 0.25, 2 and 20 hours post-infection.

Results

⁶⁴Cu labeled F. tularensis subsp. novicida administered intranasally or intratracheally were visualized in the respiratory tract and stomach at 0.25 hours post infection. By 20 hours, there was significant tropism to the lung compared with other tissues. In contrast, the images of radiolabeled F. tularensis subsp. novicida when administered intragastrically, intradermally, intraperitoneally and intravenouslly were more generally limited to the gastrointestinal system, site of inoculation, liver and spleen respectively. MicroPET images correlated with the biodistribution of isotope and bacterial burdens in analyzed tissues.

Conclusion

Our findings suggest that Francisella has a differential tissue tropism depending on the route of entry and that the virulence of Francisella by the pulmonary route is associated with a rapid bacteremia and an early preferential tropism to the lung. In addition, the use of the microPET device allowed us to identify the cecum as a novel site of colonization of Francisella tularensis subsp. novicida in mice.

Noninvasive imaging of cell-mediated therapy for treatment of cancer

Cell-mediated therapy (immunotherapy) for the treatment of cancer is an active area of investigation in animal models and clinical trials. Despite many advances, objective responses to immunotherapy are observed in a small number of cases, for certain tumor types. To better understand differences in outcomes, it is critical to develop assays for tracking effector cell localization and function in situ. The fairly recent use of molecular imaging techniques to track cell populations has presented researchers and clinicians with a powerful diagnostic tool for determining the efficacy of cell-mediated therapy for the treatment of cancer. This review highlights the application of whole-body noninvasive radioisotopic, magnetic, and optical imaging methods for monitoring effector cells in vivo. Issues that affect sensitivity of detection, such as methods of cell marking, efficiency of cell labeling, toxicity, and limits of detection of imaging modalities, are discussed.

A fusion PET-MRI system with a high-resolution research tomograph-PET and ultra-high field 7.0 T-MRI for the molecular-genetic imaging of the brain

We have developed a positron emission tomography (PET) and magnetic resonance imaging (MRI) fusion system for the molecular-genetic imaging (MGI) of the in vivo human brain using two high-end imaging devices: the HRRT-PET, a high-resolution research tomograph dedicated to brain imaging on the molecular level, and the 7.0 T-MRI, an ultra-high field version used for morphological imaging. HRRT-PET delivers high-resolution molecular imaging with a resolution down to 2.5 mm full width at half maximum (FWHM), which allows us to observe the brain's molecular changes using the specific reporter genes and probes. On the other front, the 7.0 T-MRI, with submillimeter resolution images of the cortical areas down to 250 mum, allows us to visualize the fine details of the brainstem areas as well as the many cortical and subcortical areas. The new PET-MRI fusion imaging system will provide many answers to the questions on neurological diseases as well as cognitive neurosciences. Some examples of the answers are the quantitative visualization of neuronal functions by clear molecular and genetic bases, as well as diagnoses of many neurological diseases such as Parkinson's and Alzheimer's. The salient point of molecular-genetic imaging and diagnosis is the fact that they precede the morphological manifestations, and hence, the early and specific diagnosis of certain diseases, such as cancers.

PET (positron emission tomography) imaging of biomolecules using metal-DOTA complexes: a new collaborative challenge by chemists, biologists, and physicians for future diagnostics and exploration of in vivo dynamics

Recently, PET has been paid a great deal of attention as a non-invasive imaging method. In this review, the recent advances of PET using biomolecules, such as peptides, monoclonal antibodies, proteins, oligonucleotides, and glycoproteins will be described. So far, PET of biomolecules has been mainly used for diagnosis of cancers. The biomolecules have been conjugated with the DOTA ligand, labeled with radiometals as the beta+ emitter, and targeted to specific tumors, where they have enabled visualization of even small metastatic lesions, due to the high sensitivity of the PET scanners. Some of the biomolecules have been used not only for PET diagnosis, but also for radiotherapeutic treatments by simply changing the radiometals to beta(-) emitters. Collaborative work between chemists, biologists, and physicians will be important for the future of biomolecule-based targeting and diagnosis.

Tracking the elusive lymphocyte: methods of detection during adoptive immunotherapy

Adoptive immunotherapy is an attractive cancer treatment modality due to its capacity to target primary and metastatic lesions with large numbers of tumor-reactive, cytotoxic lymphocytes. The inability of fully armed lymphocytes to traffic into sites of tumor has been proposed as a causal factor for the minimal success observed clinically with this type of immunotherapy. The study of lymphocyte trafficking during adoptive immunotherapy has been limited, despite the existence of a variety of tracking methods. In murine models that simulate adoptive immunotherapy, the use of congenic mice and cell tracking dyes can be used to elucidate lymphocyte trafficking behavior. The continued development of novel technologies will further contribute to this expanding area of research.

T cell homing to tumors detected by 3D-coordinated positron emission tomography and magnetic resonance imaging

A general hindrance to progress in adoptive cellular therapy is the lack of detailed knowledge of the fate of transferred cells in the body of the recipient. In this study, we present a novel technique for tracking of 124I-labeled cells in situ, which combines the high spatial resolution of magnetic resonance imaging with the high sensitivity and spatial accuracy of positron emission tomography. We have used this technique, together with determination of tissue radioactivity, flow cytometry, and microscopy, to characterize and quantitate the specific accumulation of transferred CD8+ T cells in tumor tissue in a mouse model. Transgenic CD8+ T cells, specific for the ovalbumin peptide SIINFEKL, were adoptively transferred to recipients carrying a subcutaneous tumor of the ovalbumin-expressing malignant melanoma cell line B16-OVA. The number of SIINFEKL-specific CD8+ cells in the tumor tissue was determined by flow cytometry each day for 8 consecutive days after adoptive transfer. From low levels 1 day after injection, their number gradually increased until day 5 when an average of 3.3x10(6) SIINFEKL-specific cells per gram tumor tissue was found. By applying the combined positron emission tomography/magnetic resonance imaging technique we were able to determine the position of the transferred, 124I-labeled SIINFEKL-specific T cells in 3 dimensions in recipient mice, and could demonstrate a highly significant accumulation of the 124I label in and around the subcutaneous B16-OVA tumors compared with normal tissue. Accumulation of 124I was significantly higher in B16-OVA than in B16 tumors not expressing the OVA antigen.