Applications of in vivo imaging in the evaluation of the pathophysiology of viral and bacterial infections and in development of countermeasures to BSL3/4 pathogens

Type I interferon shapes brain distribution and tropism of tick-borne flavivirus

Viral tropism within the brain and the role(s) of vertebrate immune response to neurotropic flaviviruses infection is largely understudied. We combine multimodal imaging (cm-nm scale) with single nuclei RNA-sequencing to study Langat virus in wildtype and interferon alpha/beta receptor knockout (Ifnar^-/-) mice to visualize viral pathogenesis and define molecular mechanisms. Whole brain viral infection is imaged by Optical Projection Tomography coregistered to ex vivo MRI. Infection is limited to grey matter of sensory systems in wildtype mice, but extends into white matter, meninges and choroid plexus in Ifnar^-/- mice. Cells in wildtype display strong type I and II IFN responses, likely due to Ifnb expressing astrocytes, infiltration of macrophages and Ifng-expressing CD8+ NK cells, whereas in Ifnar^-/-, the absence of this response contributes to a shift in cellular tropism towards non-activated resident microglia. Multimodal imaging-transcriptomics exemplifies a powerful way to characterize mechanisms of viral pathogenesis and tropism.

Preclinical evaluation of [18F]FDG-PET as a biomarker of lymphoid tissue disease and inflammation in Zika virus infection

Purpose

Zika (ZIKV) is a viral inflammatory disease affecting adults, children, and developing fetuses. It is endemic to tropical and sub-tropical countries, resulting in half the global population at risk of infection. Despite this, there are no approved therapies or vaccines against ZIKV disease. Non-invasive imaging biomarkers are potentially valuable tools for studying viral pathogenesis, prognosticating host response to disease, and evaluating in vivo efficacy of experimental therapeutic interventions. In this study, we evaluated [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG)-positron emission tomography (PET) as an imaging biomarker of ZIKV disease in a mouse model and correlated metabolic tracer tissue uptake with real-time biochemical, virological, and inflammatory features of tissue infection.

Methods

[¹⁸F]FDG-PET/CT imaging was performed in an acute, lethal ZIKV mouse infection model, at increasing stages of disease severity. [¹⁸F]FDG-PET findings were corroborated with ex vivo wholemount-tissue autoradiography and tracer biodistribution studies. Tracer uptake was also correlated with in situ tissue disease status, including viral burden and inflammatory response. Immune profiling of the spleen by flow cytometry was performed to identify the immune cell subsets driving tissue pathology and enhancing tracer uptake in ZIKV disease.

Results

Foci of increased [¹⁸F]FDG uptake were consistently detected in lymphoid tissues—particularly the spleen—of ZIKV-infected animals. Splenic uptake increased with disease severity, and corroborated findings in tissue pathology. Increased splenic uptake also correlated with increased viral replication and elevated expression of pro-inflammatory cytokines within these tissues. ZIKV-infected spleens were characterized by increased infiltration of myeloid cells, as well as increased proliferation of both myeloid and lymphoid cells. The increased cell proliferation correlated with increased tracer uptake in the spleen. Our findings support the use of [¹⁸F]FDG as an imaging biomarker to detect and track ZIKV disease in real time and highlight the dependency of affected tissue on the nature of the viral infection.

Conclusion

[¹⁸F]FDG uptake in the spleen is a useful surrogate for interrogating in situ tissue viral burden and inflammation status in this ZIKV murine model.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-022-05892-9.

Impact of a PMMA tube on performances of a Vereos PET/CT system adapted for BSL-3 environment according to the NEMA NU2-2012 standard

Introduction

A Vereos PET/CT device was adapted to be compatible with the experimentation in large animals within BSL-3 environment. The aim of this study was to investigate the impact of this modification on the performance according to NEMA NU2-2012 standard.

Methods

Spatial resolution, sensitivity, count rate performance, accuracies of corrections and image quality were assessed using the NEMA NU2-2012 standards before and after installation of a transparent poly-methyl methacrylate tube of 8 mm thickness, 680 mm diameter and 2800 mm long inside the tunnel of the system. In addition, CT performance tests were performed according to manufacturer standard procedure.

Results

Although the presence of the tube led to a slight decrease in sensitivity, performance measurements were in accordance with manufacturer preconisation ranges and comparable to previous performance published data.

Conclusion

Modifications of Vereos PET/CT system allowing its use in BSL-3 conditions did not affect significantly its performance according to NEMA NU2-2012 standard.

Key points

Question. Does a BSL-3 compatible modification alter Philips Vereos PET/CT performances according to NEMA NU2-2012 standards?

Pertinent findings. Our Vereos PET/CT system was modified by a wall separating BSL-1 and BSL-3 sides and an 8 mm thickness PMMA tube inserted into the bore of the camera in order to extend the BSL-3 containment along the bed movement. The performances of our modified system according to NEMA NU2-2012 standards were not significantly impacted by the modifications and were in accordance with the values prescribed by the manufacturer.

Implications for patients care. Our clinical PET/CT device was modified for human infectious diseases studies in Non-Human Primates. This unusual set up may then provide truly transposable data from preclinical studies into clinical application in infected patients.

A quorum-based fluorescent probe for imaging pathogenic bacteria

Imaging of bacterial infections can be used for a wide range of investigations, including diagnosis and pathogenesis of infections, and molecular probes targeting biological processes during infection have been used...

Nuclear Medicine in Times of COVID-19: How Radiopharmaceuticals Could Help to Fight the Current and Future Pandemics

The emergence and global spread of COVID-19, an infectious disease caused by the novel coronavirus SARS-CoV-2, has resulted in a continuing pandemic threat to global health. Nuclear medicine techniques can be used for functional imaging of (patho)physiological processes at the cellular or molecular level and for treatment approaches based on targeted delivery of therapeutic radionuclides. Ongoing development of radiolabeling methods has significantly improved the accessibility of radiopharmaceuticals for in vivo molecular imaging or targeted radionuclide therapy, but their use for biosafety threats such as SARS-CoV-2 is restricted by the contagious nature of these agents. Here, we highlight several potential uses of nuclear medicine in the context of SARS-CoV-2 and COVID-19, many of which could also be performed in laboratories without dedicated containment measures. In addition, we provide a broad overview of experimental or repurposed SARS-CoV-2-targeting drugs and describe how radiolabeled analogs of these compounds could facilitate antiviral drug development and translation to the clinic, reduce the incidence of late-stage failures and possibly provide the basis for radionuclide-based treatment strategies. Based on the continuing threat by emerging coronaviruses and other pathogens, it is anticipated that these applications of nuclear medicine will become a more important part of future antiviral drug development and treatment.

Peptide-Conjugated Phosphorodiamidate Morpholino Oligomers for In Situ Live-Cell Molecular Imaging of Dengue Virus Replication

Current methods to detect and monitor pathogens in biological systems are largely limited by the tradeoffs between spatial context and temporal detail. A new generation of molecular tracking that provides both information simultaneously involves in situ detection coupled with non-invasive imaging. An example is antisense imaging that uses antisense oligonucleotide probes complementary to a target nucleotide sequence. In this study, we explored the potential of repurposing antisense oligonucleotides initially developed as antiviral therapeutics as molecular probes for imaging of viral infections in vitro and in vivo. We employed nuclease-resistant phosphorodiamidate synthetic oligonucleotides conjugated with cell-penetrating peptides (i.e., PPMOs) previously established as antivirals for dengue virus serotype-2 (DENV2). As proof of concept, and before further development for preclinical testing, we evaluated its validity as in situ molecular imaging probe for tracking cellular DENV2 infection using live-cell fluorescence imaging. Although the PPMO was designed to specifically target the DENV2 genome, it was unsuitable as in situ molecular imaging probe. This study details our evaluation of the PPMOs to assess specific and sensitive molecular imaging of DENV2 infection and tells a cautionary tale for those exploring antisense oligonucleotides as probes for non-invasive imaging and monitoring of pathogen infections in experimental animal models.

Characterization of Brain Inflammation, Apoptosis, Hypoxia, Blood-Brain Barrier Integrity and Metabolism in Venezuelan Equine Encephalitis Virus (VEEV TC-83) Exposed Mice by In Vivo Positron Emission Tomography Imaging

Traditional pathogenesis studies of alphaviruses involves monitoring survival, viremia, and pathogen dissemination via serial necropsies; however, molecular imaging shifts this paradigm and provides a dynamic assessment of pathogen infection. Positron emission tomography (PET) with PET tracers targeted to study neuroinflammation (N,N-diethyl-2-[4-phenyl]-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide, [¹⁸F]DPA-714), apoptosis (caspase-3 substrate, [¹⁸F]CP-18), hypoxia (fluormisonidazole, [¹⁸F]FMISO), blood–brain barrier (BBB) integrity ([¹⁸F]albumin), and metabolism (fluorodeoxyglucose, [¹⁸F]FDG) was performed on C3H/HeN mice infected intranasally with 7000 plaque-forming units (PFU) of Venezuelan equine encephalitis virus (VEEV) TC-83. The main findings are as follows: (1) whole-brain [¹⁸F]DPA-714 and [¹⁸F]CP-18 uptake increased three-fold demonstrating, neuroinflammation and apoptosis, respectively; (2) [¹⁸F]albumin uptake increased by 25% across the brain demonstrating an altered BBB; (3) [¹⁸F]FMISO uptake increased by 50% across the whole brain indicating hypoxic regions; (4) whole-brain [¹⁸F]FDG uptake was unaffected; (5) [¹⁸F]DPA-714 uptake in (a) cortex, thalamus, striatum, hypothalamus, and hippocampus increased through day seven and decreased by day 10 post exposure, (b) olfactory bulb increased at day three, peaked day seven, and decreased day 10, and (c) brain stem and cerebellum increased through day 10. In conclusion, intranasal exposure of C3H/HeN mice to VEEV TC-83 results in both time-dependent and regional increases in brain inflammation, apoptosis, and hypoxia, as well as modest decreases in BBB integrity; however, it has no effect on brain glucose metabolism.

Multimodal Tracking of Controlled Staphylococcus aureus Infections in Mice

There is a need to develop diagnostic and analytical tools that allow noninvasive monitoring of bacterial growth and dissemination in vivo. For such cell-tracking studies to hold translational value to controlled human infections, in which volunteers are experimentally colonized, they should not require genetic modification, and they should allow tracking over a number of replication cycles. To gauge if an antimicrobial peptide tracer, 99mTc-UBI29-41-Cy5, which contains both a fluorescent and a radioactive moiety, could be used for such in vivo bacterial tracking, we performed longitudinal imaging of a thigh-muscle infection with 99mTc-UBI29-41-Cy5-labeled Staphylococcus aureus. Mice were imaged using SPECT and fluorescence-imaging modalities at various intervals during a 28 h period. Biodistribution analyses were performed to quantitate radioactivity in the abscess and other tissues. SPECT and fluorescence imaging in mice showed clear retention of the 99mTc-UBI29-41-Cy5-labeled bacteria following inoculation in the thigh muscle. Despite bacterial replication, the signal intensity in the abscess only modestly decreased within a 28 h period: 52% of the total injected radioactivity per gram of tissue (%ID/g) at 4 h postinfection (pi) versus 44%ID/g at 28 h pi (15% decrease). After inoculation, a portion of the bacteria disseminated from the abscess, and S. aureus cultures were obtained from radioactive urine samples. Bacterial staining with 99mTc-UBI29-41-Cy5 allowed noninvasive bacterial-cell tracking during a 28 h period. Given the versatility of the presented bacterial-tracking method, we believe that this concept could pave the way for precise imaging capabilities during controlled-human-infection studies.

Multicolour In Vivo Bioluminescence Imaging Using a NanoLuc-Based BRET Reporter in Combination with Firefly Luciferase

The ability to track the biodistribution and fate of multiple cell populations administered to rodents has the potential to facilitate the understanding of biological processes in a range of fields including regenerative medicine, oncology, and host/pathogen interactions. Bioluminescence imaging is an important tool for achieving this goal, but current protocols rely on systems that have poor sensitivity or require spectral decomposition. Here, we show that a bioluminescence resonance energy transfer reporter (BRET) based on NanoLuc and LSSmOrange in combination with firefly luciferase enables the unambiguous discrimination of two cell populations in vivo with high sensitivity. We insert each of these reporter genes into cells using lentiviral vectors and demonstrate the ability to monitor the cells' biodistribution under a wide range of administration conditions, including the venous or arterial route, and in different tissues including the brain, liver, kidneys, and tumours. Our protocol allows for the imaging of two cell populations in the same imaging session, facilitating the overlay of the signals and the identification of anatomical positions where they colocalise. Finally, we provide a method for postmortem confirmation of the presence of each cell population in excised organs.

Bioengineering of bacterial pathogens for noninvasive imaging and in vivo evaluation of therapeutics

Critical bacterial pathogens of public health and biodefense concerns were engineered to constitutively express Escherichia coli enzyme thymidine kinase (TK) that allows for noninvasive nuclear imaging via phosphorylation and entrapment of radiolabeled nucleoside analog 1-(2′deoxy-2′-fluoro-β-D-arabinofuranosyl)-5-iodouracil (FIAU). Expression of functional TK was established using a nucleoside analog Zidovudine that impeded the growth of tk-engineered bacteria. Significantly, no observable growth differences were detected for FIAU. High resolution mass spectrometry with Pseudomonas aeruginosa PAO1 and its tk variant (PAO1TK) confirmed FIAU phosphorylation and retention only in PAO1TK. In vitro gamma counting with wild-type PAO1, Acinetobacter baumannii and Burkholderia pseudomallei Bp82 and their tk derivatives with [¹⁸F]FIAU further confirmed that tk variants selectively incorporated the radiotracer, albeit with varying efficiencies. In vitro [¹⁸F]FIAU labeling coupled with in vivo Positron Emission Tomography/Computed Tomography (PET/CT) imaging of PAO1 and PAO1TK confirmed that only PAO1TK can be imaged in mice at sensitivities ≥10⁷ bacteria per infection site. This was further verified by administering [¹⁸F]FIAU to animals infected with PAO1 and PAO1TK. Utility of tk-engineered P. aeruginosa in noninvasive PET/CT imaging for bacterial therapeutic evaluation in animals was demonstrated employing antibiotic ciprofloxacin, underscoring the immediate use of PAO1TK and potentially other engineered pathogens for evaluating experimental therapeutics.

[18F]DPA-714 PET Imaging Reveals Global Neuroinflammation in Zika Virus-Infected Mice

PURPOSE

The association of Zika virus (ZIKV) infection and development of neurological sequelae require a better understanding of the pathogenic mechanisms causing severe disease. The purpose of this study was to evaluate the ability and sensitivity of positron emission tomography (PET) imaging using [18F]DPA-714, a translocator protein (TSPO) 18 kDa radioligand, to detect and quantify neuroinflammation in ZIKV-infected mice.

PROCEDURES

We assessed ZIKV-induced pathogenesis in wild-type C57BL/6 mice administered an antibody to inhibit type I interferon (IFN) signaling. [18F]DPA-714 PET imaging was performed on days 3, 6, and 10 post-infection (PI), and tissues were subsequently processed for histological evaluation, quantification of microgliosis, and detection of viral RNA by in situ hybridization (ISH).

RESULTS

In susceptible ZIKV-infected mice, viral titers in the brain increased from days 3 to 10 PI. Over this span, these mice showed a two- to sixfold increase in global brain neuroinflammation using [18F]DPA-714 PET imaging despite limited, regional detection of viral RNA. No measurable increase in ionized calcium binding adaptor molecule 1 (Iba-1) expression was noted at day 3 PI; however, there was a modest increase at day 6 PI and an approximately significant fourfold increase in Iba-1 expression at day 10 PI in the susceptible ZIKV-infected group relative to controls.

CONCLUSIONS

The results of the current study demonstrate that global neuroinflammation plays a significant role in the progression of ZIKV infection and that [18F]DPA-714 PET imaging is a sensitive tool relative to histology for the detection of neuroinflammation. [18F]DPA-714 PET imaging may be useful in dynamically characterizing the pathology associated with neurotropic viruses and the evaluation of therapeutics being developed for treatment of infectious diseases.

The Promise of Molecular Imaging in the Study and Treatment of Infectious Diseases

Infectious diseases are a major threat to humanity, and it is imperative that we develop imaging tools that aid in their study, facilitate diagnosis, and guide treatment. The alarming rise of highly virulent and multi-drug-resistant pathogens, their rapid spread leading to frequent global pandemics, fears of bioterrorism, and continued life-threatening nosocomial infections in hospitals remain as major challenges to health care in the USA and worldwide. Early diagnosis and rapid monitoring are essential for appropriate management and control of infections. Tomographic molecular imaging enables rapid, noninvasive visualization, localization, and monitoring of molecular processes deep within the body and offers several advantages over traditional tools used for the study of infectious diseases. Noninvasive, longitudinal assessments could streamline animal studies, allow unique insights into disease pathogenesis, and expedite clinical translation of new therapeutics. Since molecular imaging is already in common use in the clinic, it could also become a valuable tool for clinical studies, for patient care, for public health, and for enabling precision medicine for infectious diseases.

Countering Zika Virus: The USAMRIID Response

The United States Army Medical Research Institute of Infectious Diseases (USAMRIID) possesses an array of expertise in diverse capabilities for the characterization of emerging infectious diseases from the pathogen itself to human or animal infection models. The recent Zika virus (ZIKV) outbreak was a challenge and an opportunity to put these capabilities to work as a cohesive unit to quickly respond to a rapidly developing threat. Next-generation sequencing was used to characterize virus stocks and to understand the introduction and spread of ZIKV in the United States. High Content Imaging was used to establish a High Content Screening process to evaluate antiviral therapies. Functional genomics was used to identify critical host factors for ZIKV infection. An animal model using the temporal blockade of IFN-I in immunocompetent laboratory mice was investigated in conjunction with Positron Emission Tomography to study ZIKV. Correlative light and electron microscopy was used to examine ZIKV interaction with host cells in culture and infected animals. A quantitative mass spectrometry approach was used to examine the protein and metabolite type or concentration changes that occur during ZIKV infection in blood, cells, and tissues. Multiplex fluorescence in situ hybridization was used to confirm ZIKV replication in mouse and NHP tissues. The integrated rapid response approach developed at USAMRIID presented in this review was successfully applied and provides a new template pathway to follow if a new biological threat emerges. This streamlined approach will increase the likelihood that novel medical countermeasures could be rapidly developed, evaluated, and translated into the clinic.

In Situ Bioorthogonal Metabolic Labeling for Fluorescence Imaging of Virus Infection In Vivo

Optical fluorescence imaging is an important strategy to explore the mechanism of virus-host interaction. However, current fluorescent tag labeling strategies often dampen viral infectivity. The present study explores an in situ fluorescent labeling strategy in order to preserve viral infectivity and precisely monitor viral infection in vivo. In contrast to pre-labeling strategy, mice are first intranasally infected with azide-modified H5N1 pseudotype virus (N₃ -H5N1p), followed by injection of dibenzocyclooctyl (DBCO)-functionalized fluorescence 6 h later. The results show that DBCO dye directly conjugated to N₃ -H5N1p in lung tissues through in vivo bioorthogonal chemistry with high specificity and efficacy. More remarkably, in situ labeling rather than conventional prelabeling strategy effectively preserves viral infectivity and immunogenicity both in vitro and in vivo. Hence, in situ bioorthogonal viral labeling is a promising and reliable strategy for imaging and tracking viral infection in vivo.

Introduction

Infectious diseases are a major cause of morbidity and mortality worldwide and in the USA. Overall costs and morbidity are expected to continue to rise due to increasing rates of drug-resistant pathogens, use of invasive techniques such as implants, as well as immunosuppressive and cancer therapies. Tomographic molecular imaging techniques enable rapid visualization and monitoring of molecular processes noninvasively and promise unparalleled opportunities for field of infectious diseases. These technologies are an emerging field of research, overcome several fundamental limitations of current tools, and could have a broad impact on both basic research and patient care. Beyond diagnosis and monitoring disease, these technologies could also provide a uniform cross-species platform for animal studies, allow unique insights into understanding disease pathogenesis, and expedite bench-to-bedside translation of new therapeutics. Finally, since molecular imaging is readily available for humans, validated tracers could also become valuable tools for clinical applications and for enabling personalized medicine for infectious diseases.

18F-FDG PET imaging for identifying the dynamics of intestinal disease caused by SFTSV infection in a mouse model

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging disease that causes fever, enteritis, thrombocytopenia, and leucopenia and can be fatal in up to 30% of cases. However, the mechanism of severe disease is not fully understood. Molecular imaging approaches, such as positron-emission tomography (PET), are functional in vivo imaging techniques that provide real-time dynamics of disease progression, assessments of pharmacokinetics, and diagnoses for disease progression. Molecular imaging also potentially provides useful approaches to explore the pathogenesis of viral infections. Thus, the purpose of this study was to image the pathological features of SFTSV infection in vivo by PET imaging. In a mouse model, we showed that 18F-FDG accumulations clearly identified the intestinal tract site as a pathological site. We also demonstrated that 18F-FDG PET imaging can assess disease progression and response to antiserum therapy within the same individual. This is the first report demonstrating a molecular imaging strategy for SFTSV infection. Our results provide potentially useful information for preclinical studies such as the elucidation of the mechanism of SFTSV infection in vivo and the assessment of drugs for SFTS treatment.

Development of a Pseudomonas aeruginosa Agmatine Biosensor

Agmatine, decarboxylated arginine, is an important intermediary in polyamine production for many prokaryotes, but serves higher functions in eukaryotes such as nitric oxide inhibition and roles in neurotransmission. Pseudomonas aeruginosa relies on the arginine decarboxylase and agmatine deiminase pathways to convert arginine into putrescine. One of the two known agmatine deiminase operons, aguBA, contains an agmatine sensitive TetR promoter controlled by AguR. We have discovered that this promoter element can produce a titratable induction of its gene products in response to agmatine, and utilized this discovery to make a luminescent agmatine biosensor in P. aeruginosa. The genome of the P. aeruginosa lab strain UCBPP-PA14 was altered to remove both its ability to synthesize or destroy agmatine, and insertion of the luminescent reporter construct allows it to produce light in proportion to the amount of exogenous agmatine applied from ~100 nM to 1mM. Furthermore it does not respond to related compounds including arginine or putrescine. To demonstrate potential applications the biosensor was used to detect agmatine in spent supernatants, to monitor the development of arginine decarboxylase over time, and to detect agmatine in the spinal cords of live mice.