Non-invasive bioluminescence imaging for monitoring herpes simplex virus type 1 hematogenous infection

Reporter Transgenes for Monitoring the Antitumor Efficacy of Recombinant Oncolytic Viruses

Accurate measurement of tumor size and margins is crucial for successful oncotherapy. In the last decade, non-invasive imaging modalities, including optical imaging using non-radioactive substrates, deep-tissue imaging with radioactive substrates, and magnetic resonance imaging have been developed. Reporter genes play the most important role among visualization tools; their expression in tumors and metastases makes it possible to track changes in the tumor growth and gauge therapy effectiveness. Oncolytic viruses are often chosen as a vector for delivering reporter genes into tumor cells, since oncolytic viruses are tumor-specific, meaning that they infect and lyse tumor cells without damaging normal cells. The choice of reporter transgenes for genetic modification of oncolytic viruses depends on the study objectives and imaging methods used. Optical imaging techniques are suitable for in vitro studies and small animal models, while deep-tissue imaging techniques are used to evaluate virotherapy in large animals and humans. For optical imaging, transgenes of fluorescent proteins, luciferases, and tyrosinases are used; for deep-tissue imaging, the most promising transgene is the sodium/iodide symporter (NIS), which ensures an accumulation of radioactive isotopes in virus-infected tumor cells. Currently, NIS is the only reporter transgene that has been shown to be effective in monitoring tumor virotherapy not only in preclinical but also in clinical studies.

Bioluminescence Imaging as a Tool for Poxvirus Biology

Bioluminescence imaging, with luciferase as a reporter-encoding gene, has been successfully and widely used for studies to follow viral infection in an organism and to measure therapeutic efficacy of antiviral agents in small animal models. Bioluminescence is produced by the reaction of a luciferase enzyme stably inserted into the viral genome with a defined substrate systemically delivered into the animal. The light emitted is captured allowing the detection of viral infection sites and the quantification of viral replication in the context of tissues of a living animal. The goal of this chapter is to provide a technical background for the evaluation of poxvirus infection in cells and animals through bioluminescence imaging technology using luciferase-expressing recombinant poxviruses.

In-vivo monitoring of infectious diseases in living animals using bioluminescence imaging

Traditional methods of localizing and quantifying the presence of pathogenic microorganisms in living experimental animal models of infections have mostly relied on sacrificing the animals, dissociating the tissue and counting the number of colony forming units. However, the discovery of several varieties of the light producing enzyme, luciferase, and the genetic engineering of bacteria, fungi, parasites and mice to make them emit light, either after administration of the luciferase substrate, or in the case of the bacterial lux operon without any exogenous substrate, has provided a new alternative. Dedicated bioluminescence imaging (BLI) cameras can record the light emitted from living animals in real time allowing non-invasive, longitudinal monitoring of the anatomical location and growth of infectious microorganisms as measured by strength of the BLI signal. BLI technology has been used to follow bacterial infections in traumatic skin wounds and burns, osteomyelitis, infections in intestines, Mycobacterial infections, otitis media, lung infections, biofilm and endodontic infections and meningitis. Fungi that have been engineered to be bioluminescent have been used to study infections caused by yeasts (Candida) and by filamentous fungi. Parasitic infections caused by malaria, Leishmania, trypanosomes and toxoplasma have all been monitored by BLI. Viruses such as vaccinia, herpes simplex, hepatitis B and C and influenza, have been studied using BLI. This rapidly growing technology is expected to continue to provide much useful information, while drastically reducing the numbers of animals needed in experimental studies.

A bright future for bioluminescent imaging in viral research

Bioluminescence imaging (BLI) has emerged as a powerful tool in the study of animal models of viral disease. BLI enables real-time in vivo study of viral infection, host immune response and the efficacy of intervention strategies. Substrate dependent light emitting luciferase enzyme when incorporated into a virus as a reporter gene enables detection of bioluminescence from infected cells using sensitive charge-coupled device (CCD) camera systems. Advantages of BLI include low background, real-time tracking of infection in the same animal and reduction in the requirement for larger animal numbers. Transgenic luciferase-tagged mice enable the use of pre-existing nontagged viruses in BLI studies. Continued development in luciferase reporter genes, substrates, transgenic animals and imaging systems will greatly enhance future BLI strategies in viral research.

Molecular imaging with bioluminescence and PET reveals viral oncolysis kinetics and tumor viability

Viral oncolysis, the destruction of cancer cells by replicating virus, is an experimental cancer therapy that continues to be explored. The treatment paradigm for this therapy involves successive waves of lytic replication in cancer cells. At present, monitoring viral titer at sites of replication requires biopsy. However, repeat serial biopsies are not practically feasible for temporal monitoring of viral replication and tumor response in patients. Molecular imaging provides a noninvasive method to identify intracellular viral gene expression in real time. We imaged viral oncolysis and tumor response to oncolysis sequentially with bioluminescence and positron emission tomography (PET), revealing the kinetics of both processes in tumor xenografts. We demonstrate that virus replication cycles can be identified as successive waves of reporter expression that occur ∼2 days after the initial viral tumor infection peak. These waves correspond to virions that are released following a replication cycle. The viral and cellular kinetics were imaged with Fluc and Rluc bioluminescence reporters plus two 18F-labeled PET reporters FHBG [9-(4-18F-fluoro-3-[hydroxymethyl] butyl) guanine] and FLT (18F-3'-deoxy-3-'fluorothymidine), respectively. Correlative immunohistochemistry on tumor xenograft sections confirmed in vivo results. Our findings show how PET can be used to identify virus replication cycles and for real-time measurements of intratumoral replicating virus levels. This noninvasive imaging approach has potential utility for monitoring viral oncolysis therapy in patients.

In vivo visualization of encephalitic lesions in herpes simplex virus type 1 (HSV-1) infected mice by magnetic resonance imaging (MRI)

Herpes simplex encephalitis (HSE) is one of the most severe viral infections affecting the temporal lobes of the brain. Despite the improvements in diagnosis and antiviral drug treatment, one third of all patients fail to respond to therapy or subsequently suffer neurological relapse and develop long term neurological damage. Magnetic resonance imaging (MRI) is among the appropriate noninvasive tools for early diagnosis of viral central nervous system (CNS) infections. In this chapter we introduce a mouse model for HSE and describe a MRI protocol to characterize the pathogenesis of HSE over time.

The Influence of Hypoxia and pH on Bioluminescence Imaging of Luciferase-Transfected Tumor Cells and Xenografts

Bioluminescence imaging (BLI) is a relatively new noninvasive technology used for quantitative assessment of tumor growth and therapeutic effect in living animal models. BLI involves the generation of light by luciferase-expressing cells following administration of the substrate luciferin in the presence of oxygen and ATP. In the present study, the effects of hypoxia, hypoperfusion, and pH on BLI signal (BLS) intensity were evaluated in vitro using cultured cells and in vivo using a xenograft model in nude mice. The intensity of the BLS was significantly reduced in the presence of acute and chronic hypoxia. Changes in cell density, viability, and pH also affected BLS. Although BLI is a convenient non-invasive tool for tumor assessment, these factors should be considered when interpreting BLS intensity, especially in solid tumors that could be hypoxic due to rapid growth, inadequate blood supply, and/or treatment.

In vivo bioluminescent imaging of influenza a virus infection and characterization of novel cross-protective monoclonal antibodies

Influenza A virus is a major human pathogen responsible for seasonal epidemics as well as pandemic outbreaks. Due to the continuing burden on human health, the need for new tools to study influenza virus pathogenesis as well as to evaluate new therapeutics is paramount. We report the development of a stable, replication-competent luciferase reporter influenza A virus that can be used for in vivo imaging of viral replication. This imaging is noninvasive and allows for the longitudinal monitoring of infection in living animals. We used this tool to characterize novel monoclonal antibodies that bind the conserved stalk domain of the viral hemagglutinin of H1 and H5 subtypes and protect mice from lethal disease. The use of luciferase reporter influenza viruses allows for new mechanistic studies to expand our knowledge of virus-induced disease and provides a new quantitative method to evaluate future antiviral therapies.

Ex vivo bioluminescence detection of alcelaphine herpesvirus 1 infection during malignant catarrhal fever

Alcelaphine herpesvirus 1 (AlHV-1), carried by wildebeest asymptomatically, causes malignant catarrhal fever (WD-MCF) when cross-species transmitted to a variety of susceptible species of the Artiodactyla order. Experimentally, WD-MCF can be reproduced in rabbits. WD-MCF is described as a combination of lymphoproliferation and degenerative lesions in virtually all organs and is caused by unknown mechanisms. Recently, we demonstrated that WD-MCF is associated with the proliferation of CD8(+) cells supporting a latent type of infection in lymphoid tissues. Here, we investigated the macroscopic distribution of AlHV-1 infection using ex vivo bioluminescence imaging in rabbit to determine whether it correlates with the distribution of lesions in lymphoid and nonlymphoid organs. To reach that goal, a recombinant AlHV-1 strain was produced by insertion of a luciferase expression cassette (luc) in an intergenic region. In vitro, the reconstituted AlHV-1 luc(+) strain replicated comparably to the parental strain, and luciferase activity was detected by bioluminescence imaging. In vivo, rabbits infected with the AlHV-1 luc(+) strain developed WD-MCF comparably to rabbits infected with the parental wild-type strain, with hyperthermia and increases of both CD8(+) T cell frequencies and viral genomic charge over time in peripheral blood mononuclear cells and in lymph nodes at time of euthanasia. Bioluminescent imaging revealed that AlHV-1 infection could be detected ex vivo in lymphoid organs but also in lung, liver, and kidney during WD-MCF, demonstrating that AlHV-1 infection is prevalent in tissue lesions. Finally, we show that the infiltrating mononuclear leukocytes in nonlymphoid organs are mainly CD8(+) T cells and that latency is predominant during WD-MCF.

Noninvasive biophotonic imaging for studies of infectious disease

According to World Health Organization estimates, infectious organisms are responsible for approximately one in four deaths worldwide. Animal models play an essential role in the development of vaccines and therapeutic agents but large numbers of animals are required to obtain quantitative microbiological data by tissue sampling. Biophotonic imaging (BPI) is a highly sensitive, nontoxic technique based on the detection of visible light, produced by luciferase-catalysed reactions (bioluminescence) or by excitation of fluorescent molecules, using sensitive photon detectors. The development of bioluminescent/fluorescent microorganisms therefore allows the real-time noninvasive detection of microorganisms within intact living animals. Multiple imaging of the same animal throughout an experiment allows disease progression to be followed with extreme accuracy, reducing the number of animals required to yield statistically meaningful data. In the study of infectious disease, the use of BPI is becoming widespread due to the novel insights it can provide into established models, as well as the impact of the technique on two of the guiding principles of using animals in research, namely reduction and refinement. Here, we review the technology of BPI, from the instrumentation through to the generation of a photonic signal, and illustrate how the technique is shedding light on infection dynamics in vivo.

Zebrafish as a new model for herpes simplex virus type 1 infection

The zebrafish (Danio rerio) is rapidly gaining ground as a disease model. However, until now, the use of this species with human pathogens has been restricted to just three bacteria; no studies involving viruses that infect humans are recorded. In this study, the zebrafish was used as a model of herpes simplex virus type 1 (HSV-1) infection of the nervous system. Fish infected using viral culture supernatants showed detectable HSV-1 DNA concentrations 1-4 days after inoculation, indicating that this virus can experimentally infect and persist in this host. The kinetics of infection was dose dependent, especially in the head. Histological immunodetection of HSV-1 glycoproteins confirmed the presence of HSV-1 in the organs studied; infection led to histopathological changes. Moreover, the suppression of the immune system by cyclophosphamide and the antiviral effect of acyclovir were demonstrated. The infection of the encephalon was studied in detail, and the time course of viral colonization recorded. Immunofluorescence studies provided immunoreactive evidence of viral antigens in the encephalon and spinal cord. Viruses cleared from infected brains showed the ability to infect human neuroblastoma cells. This study is the first to demonstrate HSV-1 infection in the zebrafish and manifests the potential use of this species in herpesvirus studies.

Noninvasive bioluminescent imaging of infections

Traditional studies of viral and bacterial infection and pathogenesis have generally relied on animal models that require the sacrifice of infected animals to determine viral or bacterial distributions and titers. The recent application of the in vivo bioluminescence imaging (BLI) to monitor the replication and tropism of pathogens expressing the luciferase (from firefly or Renilla) reporter proteins has been recently developed. This technology do not requires the sacrifice of the experimental animals, where the in vivo bioluminescence emissions in living animals permit the tracking of the infection. It has been demonstrated that the in vivo BLI is comparable to the classical approaches as measurements of in vitro light emission in organs of sacrificed animals. Moreover, molecular techniques such as PCR determinations show parallel results in pathogen quantification, where the concentrations of microbial DNA measured correlated with the magnitude of bioluminescence in vivo, and with the photon flux determined by the in vitro luciferase enzyme assay. These results show that BLI can be used for noninvasive, real-time monitoring of several infections of pathogens in living animals, supplying a new methodology in the study of pathogens in addition to conventional techniques for the characterization of infections.

Development of recombinant varicella-zoster viruses expressing luciferase fusion proteins for live in vivo imaging in human skin and dorsal root ganglia xenografts

Varicella-zoster virus (VZV) is a host specific human pathogen that has been studied using human xenografts in SCID mice. Live whole-animal imaging is an emerging technique to measure protein expression in vivo using luminescence. Currently, it has only been possible to determine VZV protein expression in xenografts postmortem. Therefore, to measure immediate early (IE63) and late (glycoprotein E [gE]) protein expression in vivo viruses expressing IE63 or gE as luciferase fusion proteins were generated. Viable recombinant viruses pOka-63-luciferase and pOka-63/70-luciferase, which had luciferase genes fused to ORF63 and its duplicate ORF70, or pOka-gE-CBR were recovered that expressed IE63 or gE as fusion proteins and generated luminescent plaques. In contrast to pOka-63/70-luciferase viruses, the luciferase gene was rapidly lost in vitro when fused to a single copy of ORF63 or ORF68. IE63 expression was successfully measured in human skin and dorsal root ganglia xenografts infected with the genomically stable pOka-63/70-luciferase viruses. The progress of VZV infection in dorsal root ganglia xenografts was delayed in valacyclovir treated mice but followed a similar trend in untreated mice when the antiviral was withdrawn 28 days post-inoculation. Thus, IE63-luciferase fusion proteins were effective for investigating VZV infection and antiviral activity in human xenografts.

Central nervous system infections of herpesvirus family

Herpesviruses are one of the most common groups of pathogens causing central nervous system infections in humans. They mostly cause encephalitis, meningitis, or myelitis in immunocompetent and immunocompromised patients. Children, adults, and the elderly can all be affected. Although contrast-enhanced CT is more widely used for diagnosis, contrast-enhanced MR imaging combined with diffusion-weighted imaging is superior to CT in the detection of early changes and the real extent of the disease, and in assessing prognosis and monitoring response to antiviral treatment. More sophisticated techniques, such as MR spectroscopy and perfusion imaging, can aid in the differential diagnosis of herpesvirus infections from other tumoral, demyelinating, and ischemic processes.

Noninvasive bioluminescence imaging in small animals

There has been a rapid growth of bioluminescence imaging applications in small animal models in recent years, propelled by the availability of instruments, analysis software, reagents, and creative approaches to apply the technology in molecular imaging. Advantages include the sensitivity of the technique as well as its efficiency, relatively low cost, and versatility. Bioluminescence imaging is accomplished by sensitive detection of light emitted following chemical reaction of the luciferase enzyme with its substrate. Most imaging systems provide 2-dimensional (2D) information in rodents, showing the locations and intensity of light emitted from the animal in pseudo-color scaling. A 3-dimensional (3D) capability for bioluminescence imaging is now available, but is more expensive and less efficient; other disadvantages include the requirement for genetically encoded luciferase, the injection of the substrate to enable light emission, and the dependence of light signal on tissue depth. All of these problems make it unlikely that the method will be extended to human studies. However, in small animal models, bioluminescence imaging is now routinely applied to serially detect the location and burden of xenografted tumors, or identify and measure the number of immune or stem cells after an adoptive transfer. Bioluminescence imaging also makes it possible to track the relative amounts and locations of bacteria, viruses, and other pathogens over time. Specialized applications of bioluminescence also follow tissue-specific luciferase expression in transgenic mice, and monitor biological processes such as signaling or protein interactions in real time. In summary, bioluminescence imaging has become an important component of biomedical research that will continue in the future.

Bioluminescence: imaging modality for in vitro and in vivo gene expression

Molecular imaging offers many unique opportunities to study biological processes in intact organisms. Bioluminescence is the emission of light from biochemical reactions that occur within a living organism. Luciferase has been used as a reporter gene in transgenic mice but, until bioluminescence imaging was described, the detection of luciferase activity required either sectioning of the animal or excision of tissue and homogenization to measure enzyme activities in a conventional luminometer. Bioluminescence imaging (BLI) is based on the idea that biological light sources can be incorporated into cells and animal models artificially that does not naturally express the luminescent genes. This imaging modality has proven to be a very powerful methodology to detect luciferase reporter activity in intact animal models. This form of optical imaging is low cost and noninvasive and facilitates real-time analysis of disease processes at the molecular level in living organisms. Bioluminescence provides a noninvasive method to monitor gene expression in vivo and has enormous potential to elucidate the pathobiology of lung diseases in intact mouse models, including models of inflammation/injury, infection, and cancer.

Applications of bioluminescence imaging to the study of infectious diseases

Bioluminescence imaging (BLI) has emerged as a powerful new method to analyse infectious diseases in animal models. BLI offers real-time monitoring of spatial and temporal progression of infection in the same animal, as opposed to euthanizing a cohort of animals and quantifying colony or plaque forming units at multiple time points. Pathogens or mice are engineered to express genetically encoded luciferase enzymes from bacteria, insects, or the sea pansy. The seminal study showing the feasibility of detecting microbially generated luminescence within a living mouse was published by Contag and colleagues in 1995, using Salmonella typhimurium transformed with the lux operon from Photorhabdus luminescens. Following this, they and others performed many studies of infection by bioluminescent Gram-negative and Gram-positive bacteria. Viruses can also be engineered to encode luciferase. Our laboratory has used bioluminescent reporter viruses to follow HSV and vaccinia pathogenesis; others have used an alphavirus or novirhabdovirus. Recently, even eukaryotic parasites Plasmodium, Leishmania and Toxoplasma have been transformed with luciferase and yielded unique insights into their in vivo behaviour. We expect that both the range of organisms and the molecular events able to be studied by BLI will continue to expand, yielding important insights into mechanisms of pathogenesis.

ICP47 mediates viral neuroinvasiveness by induction of TAP protein following intravenous inoculation of herpes simplex virus type 1 in mice

Herpes simplex virus type 1 (HSV-1) expresses an immediate-early protein, ICP47, that blocks the major histocompatibility complex class I antigen presentation pathway by binding to the transporter associated with antigen presentation (TAP). The result is the virus' evasion of the immune system. Although the interaction between ICP47 and TAP has been examined in vitro, this paper is the first to report their interaction in vivo. In C57BL/6 adult female mice, ICP47-defective virus (Delta ICP47, F strain) was less able to invade the organs studied than was wild-type HSV-1 F strain, showing that ICP47 influences general invasiveness. However, the neuroinvasiveness of the Delta ICP47 virus was recovered in TAP-deficient mice, indicating that the TAP-ICP47 interaction is specific to neural tissues. HSV-1 F strain showed no significant differences in their invasiveness in TAP-deficient and wild-type mice. Therefore, although ICP47 appears to be essential for invasion, the presence of TAP appears not to be crucial. Western blotting showed TAP1 expression to increase by at least fourfold in the brains and adrenal glands of infected mice. This suggests that TAP plays an important role in the host defense system. This increased expression may be particularly important in the encephalon since the baseline protein levels of this organ are low (ratio adrenal protein level/encephalon protein level > 100). However, Delta ICP47 virus provoked no significant increase in the brain TAP1 levels of wild-type mice because it could not invade this organ. These results suggest that ICP47 plays a role in infection, and that TAP1 production is regulated during viral challenge.

Illuminating cellular physiology: recent developments

Bioluminescent methods are gaining more and more attention among scientists due to their sensitivity, selectivity and simplicity; coupled with the fact that the bioluminescence can be monitored both in vitro and in vivo. Since the discovery of bioluminescence in the 19th century, enzymes involved in the bioluminescent process have been isolated and cloned. The bioluminescent reactions in several different organisms have also been fully characterized and used as reporters in a wide variety of biochemical assays. From the 1990s it became clear that bioluminescence can be detected and quantified directly from inside a living cell. This gave rise to numerous possibilities for the in vivo monitoring of intracellular processes non-invasively using bioluminescent molecules as reporters. This review describes recent developments in the area of bioluminescent imaging for cell biology. Newly developed imaging methods allow transcriptional/translational regulation, signal transduction, protein-protein interaction, oncogenic transformation, cell and protein trafficking, and target drug action to be monitored in vivo in real-time with high temporal and spatial resolution; thus providing researchers with priceless information on cellular functions. Advantages and limitations of these novel bioluminescent methods are discussed and possible future developments identified.